摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of Materials Chemistry Scientific Editor Staff Editor Professor Anthony R. West Mrs. Janet M. Leader Department of Chemistry The Royal Society of Chemistry University of Aberdeen Thomas Graham House M esto n Wa I k Science Park Aberdeen AB9 2UE, UK Cambridge CB4 4WF, UK Assistant Editor: Mrs. F. J. O’CarrolI Editorial Secretary: Miss J. E. Chapman Materials Chemistry Editorial Board Anthony R. West (Aberdeen) (Chairman) C. Richard A. Catlow (London) David A. Rice (Reading) David A. Dunmur (Sheffield) Rodney P. Townsend (Bebington) H. Monty Frey (Reading) Allan E. Underhill (Bangor) John W. Goodby (Hull) Graham Williams (Swansea) John D. Wright (Canterbury) International Advisory Editorial Board M.A. Alario- Franco (Madrid) D. Kohl (Aachen) K. Bechgaard (Copenhagen) M. Lahav (Rehovot) J. D. Birchall (Runcorn) A. J. Leadbetter (Daresbury) D. Bloor (Durham) P. M. Maitlis (Sheffield) A. K. Cheetham (Oxford) J. S. Miller (Wilmington) E. Chiellini (Pisa) P. S. Nicholson (Hamilton) M. G. Clark (Wembley) M. Nygren (Stockholm) P. Day (Grenoble) V. Percec (Cleveland) D. Demus (Halle) C. N. R. Rao (Bangalore) B. Dunn (Los Angeles) M. Ratner (Evanston) W. J. Feast (Durham) J. Rouxel (Nantes) A. Fukuda (Tokyo) R. Roy (University Park, PA) D. Gatteschi (Florence) J. L. Serrano (Zaragoza) A. M. Glass (Murray Hill) J. N. Sherwood (Glasgow) J. B. Goodenough (Austin) J. Simon (Paris) G. W. Gray (Poole) J. F. Stoddart (Sheffield) A. C. Griffin (Cambridge) S.Takahashi (Osaka) S-i. Hirano (Nagoya) G. J. T. Tiddy (Bebington and Salford) P. Hodge (Manchester) B. J. Tighe (Birmingham) H. lnokuchi (Okazaki) Yu. D. Tretyakov (Moscow) W. Jeitschko (Munster) R. J. P. Williams (Oxford) 0. Kahn (Orsay) R. Xu (Changchun) Journal of Materials Chemistry (ISSN 0959-9428) is published six times a year by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Transactions Ltd., Blackhorse Road, Letchworth, Herts SG6 1 HN, UK. NB Turpin Transactions Ltd., distributors, is wholly owned by The Royal Society of Chemistry. 1991 Annual subscription rate EC (inc. UK) fl75.00,USA $395.00, Rest of World f195.00.Customers should make payments by cheque in stirling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Air freight and mailing in the USA by Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11 003. USA Postmaster: send address changes to Journal of Materials Chemistry, Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11 003. Second Class postage paid at Jamaica, NY 11 431. All other dispatches outside the UK by Bulk Airmail within Europe, Accelerated Surface Post outside Europe. PRINTED IN THE UK. @ The Royal Society of Chemistry, 1991. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, photographic, recording, or otherwise, without the prior permission of the publishers.Professor A. R. West, Scientific Editor Mrs. J. M. Leader, Staff Editor Tel.: Aberdeen (0224) 27291 8 Tel.: Cambridge (0223) 420066 Fax: (0224) 272938 E-Mail (JANET): Telex: 73458 UNIABN G RSCI @UK.AC.RL.GB Fax: (0223) 420247 or 423623 Telex: 818293 ROYAL G INFORMATION FOR AUTHORS The Royal Society of Chemistry welcomes submission of manuscripts intended for pub- lication in two forms, Articles and Materials Chemistry Communications. These should describe original work of high quality dealing with the synthesis, structures, properties and applications of materials, particularly those associated with advanced technology.Articles Full papers contain original scientific work that has not been published previously. How- ever, work that has appeared in print in a short form such as a Materials Chemistry Com- munication is normally acceptable. Four copies of Articles including a top copy with figures etc. should be sent to The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB44WF, UK. Materials Chemistry Communications Materials Chemistry Communications contain novel scientific work in short form and of such importance that rapid publication is war-ranted. The total length is rigorously restric- ted to two pages of the double-column A4 format.The manuscript will be returned for reduction if this length is exceeded. For a Communication consisting entirely of text and ten references, with no figures, equations or tables, this corresponds to approximately 1600 words plus an abstract of up to 40 words. Submission of a Materials Chemistry Com- munication can be made either to The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK, or viaa member of the Interna- tional Advisory Editorial Board. In the latter case, the top copy of the manuscript includ- ing any figures etc., together with the name of the person to whom the Communication is being submitted, should be sent simultan- eously to the Editor at the Cambridge address. Authors may wish to contact the Board mem- ber to ensure that he is available to arrange review of the manuscript within reasonable time. In order to avoid delay in publication, proofs of Communications are not sent to authors unless this is specifically requested. Full details of the form of manuscripts for Articles and Materials Chemistry Communi- cations, conditions for acceptance etc. are given in issue number one of Journal of Materials Chemistry published in January of each year, or may be obtained from the Staff Editor. There is no page charge for papers published in Journal of Materials Chemistry. Fifty reprints are supplied free of charge. Any author who is publishing in Journal of Materials Chemistry for the first time is entitled to a free copy of the issue in which the paper appears.

ISSN:0959-9428    

DOI:10.1039/JM99101FX013

出版商:RSC    

年代:1991

数据来源: RSC

ISSN:0959-9428    

DOI:10.1039/JM99101BX015

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ISSN 0959-9428 JMACEP(4) 489-713 (1991) Journal of Materials Chemistry Synthesis, structures, properties and applications of materials, particularly those associated with advanced technology CONTENTS 489 FEATURE ARTICLE. Photoemission in the study of oxide superconductors M. S. Golden, R.G. Egdell and W. R. Flavell 503 Microstructural studies on polypyrrole L. Madsen, B. N.Zaba, M. van der Sluijs, A. E. Underhill and K. Carneiro 507 Semi-crystalline alkali-metal salt complexes with poly(oligooxyethyleneoxy-1,2-phenylene)s J. D. Hague and P. V. Wright 51 1 X-Ray diffraction characterization of iridium dioxide electrocatalysts A. Benedetti, S. Polizzi, P. Riello, A. De Battisti and A. Maldotti 517 Scanning tunnelling microscopy study of osmium-containing electroactive metallopolymer [Os(bipy),(PVP),,C1]Cl films on polycrystalline graphite electrodes N.M.D Brown, H. X. You, R. J. Forster and J. G. Vos 525 Characterization of conducting polymer-quartz composites S. P. Armes, S. Gottesfeld, J. G. Beery, F. Garzon, C. Mombourquette, M. Hawley and H. H. Kuhn 53 1 Copper-cobalt hydroxysalts and oxysalts: Bulk and surface characterization P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lo Jacono and G. Moretti 539 Magnesium oxide as a support material for dehydrogenation catalysts D.E. Stobbe, F.R. van Buren, P. E. Groenendijk, A. J. van Dillen and J. W. Geus 545 Phase relations in the system BiO,,,-SrO-CuO at 1123 K K. T. Jacob and T. Mathews 551 Low-temperature vapour deposition of high-purity iridium coatings from cyclooctadiene complexes of iridium.Synthesis of a novel liquid iridium chemical vapour deposition precursor J. B. Hoke, E. W. Stem and H. H. Murray 555 Synthesis and crystal structures of the layered I-111-V Zintl phases, K,In,X,, where X=As, Sb T. L. T. Birdwhistell, C. L. Klein, T. Jeffries, E. D. Stevens and C. J. O’Connor 559 Preparation of novel intercalation compounds of silver and copper in layered perovskites, ALaNb,O, T. Matsuda, T. Fujita and M. Kojima 563 Nuclear magnetic resonance spectroscopy investigation of thermal transformation sequences of alumina hydrates. Part 1.-Gibbsite, y-Al(OH), R. C. T. Slade, J. C. Southern and I. M. Thompson 569 Reinvestigation of the nickel phosphine catalysed electrochemical synthesis of poly(2,5-pyridine). X-Ray crystal structures of [Ni,Br,(p-5-BrC,H,N-CZ, N), (PPh,),] and [PtBr(5-BrC,H,N-CZ) (PPh,),] N.W.Alcock, P. N.Bartlett, V. M. Eastwick-Field, G. A. Pike and P. G. Pringle 577 Zirconium compounds as coatings on polystyrene latex and as hollow spheres N.Kawahashi, C. Persson and E. Matijevik 583 Second-harmonic generation properties of some co-ordination compounds based on pentanedionato ligands R. C. B. Copley, C. Lamberth, J. Machell, D. M. P. Mingos, D. M. Murphy and H. Powell 591 Second-harmonic generation from mixed crystals of p-nitroaniline with substituted benzenes studied by the powder method R.Matsushima, H. Takeshita and N.Okamoto 595 Synthesis and properties of a new family of phases, Li,XYO,: X =Al, Ga; Y =Si, Ge C.K. Lee and A. R. West 597 La, -,Sr,CuO, -d: Structural, magnetic and transport measurements on antiferromagnets, insulators and superconductors M. J. Rosseinsky, K. Prassides and P. Day 61 1 Molecular engineering of liquid-crystalline polymers by living polymerization. Part 13.-Synthesis and living cationic polymerization of (S)-(-)-2-methylbutyl 4-(w-vinyloxy)alkoxybiphenyl-4-carboxylatewith undecanyl and hexyl alkyl groups V. Percec, Q. Zheng and M. Lee 621 Sol-gel synthesis of WO, thin films P. Judeinstein and J. Livage 629 Effect of composition of polymer backbone on spectroscopic and electrochemical properties of ruthenium(I1) bis(2,2’-bipyridyl)-containing 4-vinylpyridine/styrene copolymers D. Leech, R.J. Forster, M. R.Smyth and J.G. Vos 63 7 Transformation of schoepite into uranyl oxide hydrates of the bivalent cations Mgz+, Mn2+ and Ni2+ R. Vochten, L. Van Haverbeke and R.Sobry 643 Synthesis, characterization and biodegradation test of nylon 2/6 and nylon 2/6/6 K. E. Gonsalves, X.Chen and T. K. Wong 649 The Bi,O,-Sm,O, system: Phase diagram and electrical properties P. Conflant, C. Follet-Houttemane and M. Drache 655 Aromatic ether-ketone-'X polymers. Part 2.-EK-Imide copolymers. C. J. Borrill and R. H. Whiteley 663 Use of phenylarsine in the atmospheric pressure metal organic chemical vapour deposition of GaAs on Si(100) N. R. Dennington, A. C. Wright and J. 0. Williams 667 Crystal structures of chiral smectogenic 4-octylbiphenyl-4-y1 p-[(S)-1-methylheptyloxy]benzoate and poctylpheny14-[(S)- I-methylheptyl- oxy]biphenyl-4-carboxylate K.Hori and Y. Ohashi sh 673 Variation with composition of the intrinsic sensitivity of halogen-substituted styrene copolymers to electron-beam radiation P.C. Miller Tate and R. G. Jones 677 Synthesis, structure and electrical properties of Sr,CuO,(CO,), an oxide carbonate related to perovskite T. G. N. Babu, D. J. Fish and C. Greaves 681 Sol-gel synthesis of Zr(HPO&.H,O H. Benhamza, P. Barboux, A. Bouhaouss, F-A. Josien and J. Livage 685 Reorientational motions of hydrogenic species in 1Ztungstophosphoric acid 14-hydrate: A neutron scattering study R. C. T. Slade, G. P. Hall, H. A. Pressman and I. M. Thompson 691 Synthesis and characterisation of [( q-C,Me,)Ru(p,q-C,Me,)Ru(q-C5Me5)]+[A]-;[A]-=TCNE, TCNQ, C3[C(CN)z]3.Crystal structure of the one-dimensional salt [( q-C,Me,)Ru(p,q-C,Me,)Ru( q-C,Me,)] '[TCNE] -D. O'Hare, J. Brookes and D. J. Watkin MATERIALS CHEMISTRY COMMUNICATIONS 699 Electrochemical probing of the sol-gel-xerogel evolution P. Audebert, P. Griesmar and C. Sanchez 701 Low-temperature vapour deposition of high-purity copper coatings from bis[N-(fluoroalkyl)salicylaldiminato]copper chelates J. B. Hoke and E. W. Stern 703 Liquid-crystalline zinc and nickel I ,4,8,11,15, I8,22,25-octaalkylphthalocyaninates:Beneficial effect of the zinc ion on mesophase stabilities M. J. Cook, S. J. Cracknell and K. J. Harrison 705 Chemical intercalation of magnesium into solid hosts P. G. Bruce, F. Krok, J. Nowinski, V. C. Gibson and K. Tavakkoli 707 Correlation between absolute configuration of benzylic chiral centre and sign of spontaneous polarization of chiral dopants for ferroelectric liquid crystals T. Kusumoto, A. Nakayama, K-i. Sato, T. Hiyama, S. Takehara, M. Osawa and K. Nakamura 709 Book Reviews: J. A. Hunter; K. C. Waugh; H. Block; J. Klinowski 7 I3 Corrigendum Note: Where an asterisk appears against the name of one or more authors, it is included with the authors' approval to indicate that correspondence may be addressed to this person.

ISSN:0959-9428    

DOI:10.1039/JM99101FP041

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

Cumulative Author Index 199 1 Abe M., 483 Davies T. W., 361 Horner P. J., 271 Madsen L., 503 Pringle P. G., 569 Adams D. M., 487 Davis M. E., 79 Howe S.D., 29 Maffi S., 259 Pugh C., 217 Adams J. M., Adams P. N., 43 141 Day P., 597 De Battisti A., 191, 51 1 Howlin B., 29 Hudson M. J., 375 Maireles-Torres P., 3 19 Maitlis P. M., 251, 255 Quill K., 141 Ramasesha S. K., 477 Alagna L., 3 19 Alcock N. W., 569 Allen Sir Geoffrey, 1 Allen G. C., 69, 73 Demus D., 347 Dennington N. R., 663 Dent A. J., 103 Desiraju G. R., 201 Hunt S. E., 251 Hunter J. A., 709 Hursthouse M. B., Imaeda K., 37 139 Maldotti A., 51 1 Male S. E., 69 Malitesta C., 259 Mandal K. C., 301 Rao C. N. R., 299 Rappaport M., 339 Rastomjee C. S., 451 Rayment T., 299 Alonso P.J., Annen M. J., 197 79 Dickens P. G., 105, 137,415 Dodd S.M., 11 Ingletto G., 437 Inokuchi H., 37 . Manterfield M. M., Marcos M., 197 255 Richardson R. M., Riello P., 51 1 121 Apblett A. W., 143 Aragon-Santamaria P., Arhancet J. P., 79 Armes S. P., 525 Audebert P., 699 Audiere J. P.. 475 409 Doi T., 169 Drache M., 649 Dragone R., 531 Dray G. R., 485 Dunmur D. A., 251, 255 Dunn B., 265 Inomiya M., 483 Inoue M., 213 Inoue M. B., 213 Inversi M., 531 Irvine J. T. S., Ishikawa M., 387 147, 289 Marsden J. R., 251 Maruyama Y., 37 Mateus C. A. S., 289 Mathews T., 545 MatijeviC E., 87, 577 Matsubara H., 145 Rodriguez-Castellon E., Rosenberg M. F., 447 Roser S. J., 121 Rosseinsky D. R., 487 Rosseinsky M. J., 597 Rotella F., 175 319 Babu T. G. N., 677 Ball R.G. J., Barbieri A., 191 Barboux P., 681 Barley S. H., 481 Barrer R. M., 305 Barron A. R., 143 Bartlett P. N., 569 Battaglin G., 191 Beery J. G., 525 Benedetti A,, 51 1 Benhamza H., 681 Bettinelli M., 437 Bicelli L. P., 259 Binks J. H., 289 Birdwhistell T. L. T., 555 Blau W. J., 245 Block H., 709 Bond S. P., 327 Borrill C. J., 655 Boscolo Boscoletto A., 191 Bouhaouss A,, 681 Brambley D. R., 401 Brock T., 151 Brookes J., 691 Brown I. T., 69 Brown N. M. D., 469; 517 Bruce D. W., 251, 255 Bruce L.A., 423 Bruce P. G., 705 Byrne H. J., 245 Cahen D., 339 Campillos E., 197 Cardin D. J., 245 Carneiro K., 503 Catlow C. R. A.. 233 Centonze D., 259 Chambers R. D., 59 Chatakondu K.. 205 Cheatham L. K., 143 Cheetham A. K., 113, 223, Chen C-Y., 79 Chen X., 643 Cho C.G., 217 Christy A. G., 487 Chvatal Z., 59 Clarke J. H. R., 487 Clement R., 475 Cogle T. J., 289 Conflant P., 649 Cook M. J., 121, 703 Copley R. C. B., 583 Cox P. A,, 51, 451 Cracknell S. J., 703 105, 415 297 Dyer A., 43 Eastwick-Field V. M., 569 Eddy M. M., 223 Edge S., 103 Edwards D. J., 223 Egdell R. G., 63,451, 489 Epstein A. J., 479 Esteruelas M. A., 251 Faber J., 175 Fernando Q., 213 Fierro G., 531 Fish D. J., 677 Fitch A. N., 461 FitzGerald E. T., 51 Flavell W. R., 63, 451, 489 Flint C. D., 437 Follet-Houttemane C., 649 Formstone C. A., 51, 205 Forster R. J., 517, 629 Friend R. H., 485 Fujita T., 559 Fujiyasu H., 357 Garzon F., 525 Gelder A., 327 Geus J. W., 539 Giatti A,, 191 Gibb T. C., 23 Gibson V. C., 705 Gier T. E., 153 Gilbert A., 303, 481 Glasser F.P., 305 Golden M. S., 63, 489 Golden S. J., 63 Gonsalves K. E., 643 Goodby J. W., 5, 307 Gottesfeld S., 525 Courier D., 265 Greaves C., 17,677 Green M. L. H., 205 Grey C. P., 113 Griesmar P., 699 Grimes R. W., 461 Grins J., 239 Groenendijk P. E., 539 Grossel M. C., 223 Hague J. D., 507 Hall G. P., 685 Hardin S., 423 Harrison K. J., 121, 703 Harrison W. T. A., 153 Hawley M., 525 Hayter J. B., 181 Higashi N., 365 Hirao M., 293 Hirst P. R., 281, 429 Hitterman R. L., 175 Ishikawa Y., 483 Ito H., 387 Iwasawa N., 37 Jacob K. T., 477, 545 Jarman R. H., 113, 297 Jeffries T., 555 Jiang M. R. M., 11 Jimenez-Lopez A., 3 19 Jin J. Y., 457 Johnson B. F. G., 485 Johnson O., 223 Johnstone R. A. W., 457 Jones A. C., 139 Jones M.N., 447 Jones R. G., 401,673 Jorgensen J. D., 175 Josien F-A., 681 Judeinstein P., 621 Jutson J.A., 73 Kaduk J.A., Kaharu T., 145 Kakkar A. K., 485 Kamiya K., 387 Kane J., 447 Kathirgamanathan P., 103, Kawaguchi T., 387 Kawahashi N., 577 Kellar E. J. C., 331 Kemp J. P., 451 Khan M. S., 485 Kimura K., 293 Klein C. L., 555 Klinowski J., 709 Kojima M., 559 Kordas G., 97, 175, 181 Korgul P., 239 Krok F., 705 Krongauz V., 331 Kuhn H. H., 525 Kurmoo M., 51 Kusabayashi S., 169 Kusumoto T., 707 Kuzuya M., 387 Lacroix P., 475 Lamberth C., 583 Lee C. K., 149, 595 Lee G. R., 381 Lee M., 611 Leech D., 629 Legge C. H., 303 Le Lagadec R., 251 Lewis J., 485 Li H-X., 79 113, 297 KIll P-O., 239 141 Matsuda T., 559 Matsushima R., 591 McKeown N. B., 121 McLean R. S., 479 McWhinnie W. R., 327 Milburn G. H. W., 155 Miller J.S., 479 Miller Tate P. C., 401, 673 Mingos D. M. P., 583 Mitchell G. R., 303, 481 Mohr K., 347 Mombourquette C., 525 Moon B. M., 97 Moore G. A,, 175, 181 Moretti G., 129, 531 Mori T., 37 Morris M., 43 Motevalli M., 139 Muramatsu H., 357 Murphy D. M., 583 Murray H. H., 551 Musicanti M., 129 Mutlu M., 447 Mutlu S., 447 Nakamura K., 707 Nakamura T., 357 Nakanishi Y., 357 Nakano C., 37 Nakayama A,, 707 Nalini V., 201 Narayan K. S., 479 Nardella A., 129 Nebesny K. W., 213 Nishihata Y., 169 Niwa M., 365 Noguchi A., 387 Nowinski J., 705 O’Brien P., 139 Ocaiia M., 87 O’Connor C. J., 555 OConnor P., 103 OHare D., Ohashi Y., 667 Okamoto N., 591 Olivera-Pastor P., 319 Olsson P-O., 239 Orr R., 255 Osawa M., 707 O’Sullivan T. P., 393 Owen J. J., 113 Percec V., Perry M. C., 327 Persson C., 577 Pike G.A., 569 Polizzi S., 51 1 5 1, 205, 69 1 21 7, 61 1 Sabbatini L., 259 Saito G., 37 Sakurai Y., 169 Salamon M.B., 181 Salmon L., 265 Sanchez C., 699 Sankar G., 299 Santos-Delgado M. J., 409 Sat0 K-i., 707 Savadogo O., 301 Schafer W., 347 Schwartz M., 339 Scolnik Y., 339 Serrano J. L., 197 Sherrington D. C., 151, 371 Silver J., 29 Simmons J. M., 121 Sinclair D. C., 147 Singh N., 441 Slade R. C. T., Slaney A. J., 5 Slater P. R., 17 Smith R. I., 91 Smyth M. R., 629 Sobry R., 637 Sola E., 251 Sonoda K., 483 Southern J. C., 563 Stacey J. M., 251 Stern E. W., 551, 701 Stevens E. D., 555 Stobbe D. E., 539 Stucky G. D., 153 Takahashi S., 145 Takehara S., 707 Takematsu M., 365 Takenaka S., 169 Takeshita H., 591 Takeuchi Y., 357 Tamatani A., 169 Tavakkoli K., 705 Taylor S.E., 393 Templeton-Knight R., 59 Terauchi H., 169 Thompson D. P., 239 Thompson I. M., 563, 685 Thompson W. C., 305 Thomson A. J., 121 Tilley R. J. D., 155 Tomlinson A. A. G., 319 Turney T. W., 423 Twyman J. M., 205 Uchida M., 483 Underhill A. E., 103, 141, 281, 361, 429,441, 563, 685 Crayston J. A., 381 Crennell S. J., 113, 297 Currie D. B., 295 Hiyama T., 707 Hoang M., 423 Hodes G., 339 Liddell K., 239 Livage J., 621, 681 Lo Jacono M., 129, 531 Polo-Diez L. M., 409 Porta P., 129, 531 Postle S. R., 223 Vadgama P., 447 van Buren F. R., 539 503 Dalas E., 473 Hoffman D., 87 Lomas L., 475 Powell A. V., 137 van der Sluijs M., 503 Daniel M. F., 121 Hoke J. B., 551, 701 Lukes P., 29 Powell H., 583 van Dillen A. J., 539 Daolio S., 191 Homer J., 327 Maceira-Vidan A., 409 Prassides K., 597 Van Haverbeke L., 637 Davey A.P., 245 Hori K., 667 Machell J., 583 Pressman H. A., 429, 685 Vazquez C., 479 1 Verweij P. D., 371 Watkin D. J., 691 West B. C., 281 Wong T. K., 643 You H. X., 469, 5 17 Vivien D., 265 Waugh K. C., 709 Whitcombe M. J., 303 Workman A. D., 375 Zaba B. N., 503 Vochten R.. 637 Wedler W., 347 Whiteley R. H., 271, 655 Wright A. C., 663 Zambonin P. G., 259 Volin K. J., 175 Weissflog W., 347 Williams G., 331 Wright P. V., 507 Zaschke H., 347 Vos J. G., 517, 629 Weller M. T., 1 I, 295 Williams J. O., 663 Yee G.T.. 479 Zhang X., 233 Walsh J. R.. 139 West A. R., 91, 147, 149, Wiseman P.J., 205 Yitzchaik S., 331 Zheng C., 163 Watanabe H., 483 157, 163, 595 Wittmann F., 485 Yokoyama M., 293 Zheng Q., 611 11 Conference Diary 1991 July 1-4 ICIM 91 (Second International Conference on Inorganic Membranes) Montpellier, France Professor L.Cot, c/o ICIM2-9 1, ENSCM, 8 rue de 1'Ecole Normale, 34053 Montpellier Cedex 1 I France Tel.: 33 67540085. FAX: 33 67635970 July 2-5 International Symposium: Supported Reagent Chemistry York, UK DrJohn F. Gibson, The Royal Society of Chemistry, Burlington House, London WlV OBN, UK July 3-5 Understanding Self-Assembly and Organisation in Liquid Crydals (Joint British Liquid Crystal Sodety and Statistical Mechanics and Thermodynamics Group of the RSC) Leeds, UK Dr J.R Henderson, School of Chemistry, University of Leeds, Leeds Is2 9JT July 7-12 10th International Conference on the Chemistry of the Organic Solid State University of British Cdumbia, Vancouver, Canada Cunference Secretariat, ICCOSS X,c/o Venue West Ltd,M5-375Water Street, Vancouver, B.C. Canada V6B 505. Tel.: (604) 681-5226. FAX: (604) 681-2503 July 7-12 7th International Conference on Surface and Cdldd Science Compikgne, France Secretariat ofthe 7th ICSCS, c/o Wagons-Lits Tourisme, BP 244, F-92307 Levallois-Perret Cedex, France July 9-10 Polymers in Extreme Environments Nottingham, UK Conference Department C/129, The Plastics and Rubber Institute, 11 Hobart Place,London SW 1 W OHL, UK July 9-13 The VIth International Conference on the Chemistry atSelenium and Tellurium Osaka, Japan Professor Noboru Sonoda,Osaka University, Dept of Apphed Chemistry, Faculty of Engineering, Suita, Osaka 565, Japan.Tel.: (81) 6-877-5111 Ext. 4276. FAX: (81) 6-876-4754 July 15-17 Advanced Inorganic Materials Ambleside, UK DrJulia Wates, Akm Chemicals Ltd,Hollingwolth Road, Littleborough, Lancs. FAX: 070673628. July 15-18 Rheology of Polymer Melts Prague, Czechoslovakia PMM Secretariat, c/o Institute of Macromoleadar Chemistry, Czechoslovak Academy of Sciences, 16206 Prague, Czechoslovakia July 17-19 DCEM 11,Deposition and Characterisation of Electronic Materials (ASSCGkSC Dalton) Manchester, UK DrM. E.Pemble, Department ofChemistry, UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, UK or Mrs E.S. Wellingham, Field End House, Bude Close,Nailsea, Bristol BS19 2FQ. UK July 21 -26 Polymer Surfaces and Interfaces I1 Dumam, UK Professor W. J. Feast, Department of Chemistry, University of Durham,South Road, Durham DH13LE. UK July 22-24 EUROMAT 91: The 2nd European Conference on Advanced Materials and Processes Cambridge, UK Euramat 91, Conference Department, TheInstitute of Metals, 1Carltm House Terrace, London SWlY 5DB, UK August 17-22 33rd IUPAC Congress Budapest, Hungary E. Pungor, c/o Hungarian Academy of Sciences, H-111 1Budapest, Gellert ter 4, Hungary August 19-22 International Topical Conference on Optical ProbesofConjugated Polymers Snowbird, Utah, USA Department of conferences and Institutes, Division ofContinuing Education, 2174 Annex, University of Utah, Salt Lake City, UT 84112, USA.Tel.: (801) 581-5809. FAX:(801) 581-3165 August 20-24 International Conference on Pdytypes, Modulated Structures and Quasicrystals (MOSPQQ '91) Balatonszeplak, Hungary Secretary ofMOSPOQ '9 1,Roland Jktvos Physical Society, PO Box 433, H-1371 Budapest, Hungary August 25-30 ACS Autumn Meeting New Yo&, USA ACS, International Activities Office, 155 16th Street NW,Washington DC 20036, USA August 26-30 FiAh International Conference on Langmuir-Blodgett Films Pans, France Annie Ruaudel-Teixier, Chairman, Servicede Chimie Mol&ulaire, Bit. 125, C.E.N. Saclay, 91 191, Gif-sur-Yvette Cedex, France. Tel: 69-08-54-55. FAX: 69-08-79-63 August 26-30 Summer European Liquid Crystal Conference Vius University, Lithuania Dr P.Adomenas, 1991 SELCC, Department of Chemistry, Vilnius university, Naugarduko 24,232006 Vilnius, Lithuania. Tel.: Vilnius 661553. Telex: 261212 VUSU. FAX: (007-012-2) 613473 ... 111 September 1-6 XM International Symposium on Maaocyclic Chemistry (ISMC 1991) Sheffield, UK DrNorma A. Stoddart, Department of Chemistry, The University, Sh&ield S3 7HF, UK. Tel.: (0742) 768555 Ex. 4522. FAX: (0742) 739826 September 2-4 Crystalline Structures and Defects in Ceramics Ljubljana, Yugoslavia M. Dmfenik, Institute Jozef Stefan, Yu-61001 Ljubljana, PO Box 100,Yugoslavia September 4-6 IWASES-I1 (International Workshop on Auger Spectroscopy and EJectronic Structure) Lund, Sweden Dr (2-0.Almbladh, Department of Theoretical physics, Salvegatan 14 A, S-22362Lund, Sweden E-mail:COA@SELDC52.BlTNET. FAX: 46-(0)46-104710 September 4-6 PolymerStabilization: Mechanisms and Applications Birmingham, UK Professor N. S. Allen, Depament of Chemistry,Manchester Polytechnic, John Dalton Building, Chester Street, Manchester M15GD September 8-13 International Symposium: Zeolite Chemistry and Catalysis Prague, Czechoslovakia Dr B. Wichterlovfi (Zeolite Chmisuy and Catalysis), The J. Heyrwsky Institute of Physical Chemistry and Electrochemistry, Dolejskova 3,182 23 Prague 8, Czechoslovakia. Tel.: (422) 858 412Q (422) 815 35%. Telex: 122018 ATOM C. FAX: (422) 858 4569. September 9-11 6th Biennial Polymer Conference Leeds, UK Meetings Office, The Institute of Physics, 47 Belgrave Square, London SW 1X 8QX, UK September 9-14 Physics of PolymerNetworks Ballenstedt, Germany Organizing Committee, Physics of Polymer Networks, Technical University of Merseburg, Dept of Physics, Merseburg, DDR-4200, Gemany September 18-20 Recycling of Polymers Marbella, Spain W.Heitz, PWpp-Universitiit Marburg FB14 Physikalische Chemie, Polymere, Hans-Meenveinstrasse, D-3550 Marburg, Germany September 22-25 5th Sensors and their Applications Conference Edinburgh. UK Meetings Office, The Institute of Physics, 47 Belgrave !3quare, London SWlX SQX, UK September 23-27 7th International Symposium on EJectrets Potsdam, Germany Professor Dedev Geiss, Academy of Sciences, Institute of Polymer Chemistry 'En& Correns', Kantstrasse 55, 1530TeltowSeehof, Germany September 30-EurosensorsV October 2 Rome, Italy Professor A.D'Amico, Universita' di Roma 'TOR VERGATA, Diprtimento di Ingegneria Elettmica, Via Orazio Raimondo, 140173Roma, Italy September 30-Speciality Polymers '91 October 2 Mainz, Germany conference Organizer, SP'91, Buttemoh Scientific Ltd, PO Box 63, Westbury House, Bury Street, Guildford, Surrey GU2 SBH, UK Sepmber 30-MacromdeculeMetal Complexes IV October 5 Siena, Italy Professor Roland0 Bahucci, I)lparumento di Chimica, Universita di Siena, Piano dei Mantellini 44,53100 Siena, Italy October 6-11 Sixth International Workshop on Glasses and Ceramics from Gels Seville, Spain Dr Luis Esquivias Fedriani (WorkshopChairman), Depmrtamento de Estrudura y Propiedades de 10s Materiales, Apartado, 40-1 1510herto Real (Cddiz), Spain October 13-17 Unconventional Photoactive Solids (UPS) Symposium on Molecular Systems Okazaki, Japan Tadaoki Mitani,Institute for Molecular Science, Myaiaiji, Okazaki 444,Japan October 14-18 ECASIA 91 (4th European Conference on Applications of Surface and Interface Analysis) Budapest, Hungary ECASIA 91.MTA ATOMIKI, Pf. 51, H-4001 Debrecen, Hungary. Tel.: (36)-52-16181. Telex: 72210 (atom h). FAX: (36)-52-16 1 8 1 October 14-18 Oleftn and Vinyl Polymerhation and Functionalization Hangzhou, China Professor Kun-Yuan Qlu, Secretary-General of Symposium 91 Hangzhou, Chemistq Department. Peking University, Beijing 10087 1, China October 20-26 Solid State Ionics 8th International Conference Lake Louise, Canada Dr P.S.Nicholson, McMaster University, JHE-249, Hamilton, Ontario, Canada L8S 4L7. Tel.: (416) 529-7575. FAX: (416) 529-5994 iv October 23 -25 Polymers in a Marine Environment III London, UK Man Bufton, Conference Organiser, The Institute of Marine Engineers, The Memorial Building, 76 Mark Lane, London EC3R 7JN, UK October 28- Europhysics Conference on Macmolecular physics: PolymerNetworks November 2 Wemigerode, Germany S. Wartewig, 'Carl Schorlanmer' Technical University Leuna-Merseburg, Department of Physics, DDR-4200 Merseburg, Germany November 4-8 EMRS 1991 Fall Meeting: 15th International Symposium on the Scientific Basis for Nuclear Waste Management Strasbourg, France I? Siffert, EMRS 1991Fall Meeting, C T.T., B. I? 20,67037Strasbourg, Cedex, France November 1 1 -15 5th International Kyotoconferenceon New Aspeds of Organic Chemistry (IKCOC-5) (Session 2: Organic Synthesis for Materials Science) Kyoto. JapanProfessor YoshikiOhshim, Chairmanof IKCOC-5, Dept of Applied Chemistry, Osaka University, Ymadaoka 2-1, Suita, Osaka 565, Japan November 30-International Sympodum on New Polymers December 1 Kyoto, Japan Professor Toshinobu Higashim, Depattment of Polymer Chemistry, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606,Japan December 1-6 Electrical, Optical, and Magnetic PropertiesdOrganic Sdid State Materials (MRS Symposium) Boston,USA D. J. Sandman, GTE Laboratories Incorporated, 40Sylvan Road, Waltham, MA02254, USA.Tel.: (617) 466-4216 December 2-6 Cellulose '91 New Orleans, USA Dr Noelie R. Bertcmiere, USDA ARS Southern Regional Research Center, 1100 Robert E. Lee Boulevard, PO Box 1%87, New Orleans, LA 70179-0687, USA December 16-17 Protein Engineering: Structure Prediction and Ugand Interactions London, UK DrNeera Brokakoti, c/o Roche Products Ltd,BroadwaterRoad,Welwyn Garden City, Hens AL7 3AY, UK Conference Diary 1992 January 6-8 Fluoropdymers '92 Manchester, UK Dr E. Smith or Dr S. 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Birchall, ICI Advanced Materials, PO Box 11, The Heath, Runcorn, Cheshire WA7 4QE, UK D. Bloor, School of Engineering and Applied Science, University of Durham, South Road, Durham DH1 3LE, UK A. K. Cheetham, Chemical Crystallography Laboratory, 9 Parks Road, Oxford OX1 3PD, UK E. Chiellini, Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35,561 00 Pisa, ttaly M. G. Clark, GEC Hirst Research Centre, East Lane, Wembley, Middlesex HA9 7PP, UK P. Day, lnstitut Max Von Laue -Paul Langevin, Avenue des Martyrs, 156 X -38042 Grenoble Cedex, France D. Demus, Sektion Chemie, WB Physikalische Chemie, Martin-Luther-Universitat, Halle-Wittenberg, 4010 , Germany B. Dunn, Department of Materials Science and Engineering, School of Engineering and Applied Science, 405 Hilgard Avenue, Los Angeles, California 90024-1 595, USA W. J.Feast, IRC in Polymer Science and Technology, University of Durham, Durham DH1 3LE, UK A. Fukuda, Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 0-Okayama, Meguro-ku, Tokyo 152, Japan D.Gatteschi, Laboratorio di Chimica lnorganica dello Stato Solido, Via Maragliano 75/77, 501 44 Firenze, Italy A. M. Glass, Materials Physics Research Laboratory, AT & T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974, USA J. B. Goodenough, Center for Materials Science and Engineering, University of Texas at Austin, ETC 5.160, Austin, TX 7871 2, USA G. W. Gray, R & D Admin, BDH Ltd, Broom Road, Poole, Dorset BH12 4NN, UK A.C. Griffin, Polymer Synthesis Unit, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK S-i. Hirano, Department of Applied Chemistry, Faculty of Engineering, Nagoya University, Furo-Cho, Chikusa-Ku, Nagoya 464, Japan P. Hodge, Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK H. Inokuchi, Institute for Molecular Science, Myodaiji, Okazaki 444, Japan W. Jeitschko, Anorganisch-Chemisches Institut, Westfalische Wilhelms-Universitat, Wilheim-Klemm-Str. 8, D-4400 Munster, Germany 0. Kahn, Laboratoire de Chimie Inorganque, Universitb de Paris-Sud, Bat. 420,91405 Orsay, France D. Kohl, 2 Physikalisches lnstitut Der Rhein-Westf. Techn.Hochschule Aachen, Temperlergraben 55, D-5100 Aachen, Germany M. Lahav, Department of Structural Chemistry, The Weizrnann Institute of Science, Rehovot 761 00, Israel A. J. Leadbetter, Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK P. M. Maitlis, Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK J. S. Miller, Central Research and Development Department, Experimental Station, Du Pont, PO Box 80328, Wilmington, Delaware 19880-0328, USA P. S. Nicholson, Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L85 4L7 M. Nygren, Inorganic Chemistry, Arrhenius Laboratory, University of Stockholm, S-106 91 Stockholm, Sweden V. Percec, Department of Macromolecular Science, School of Engineering, Case Western Reserve University, Cleveland, Ohio 44106, USA C.N. R. Rao, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 56001 2, India M. Ratner, Northwestern University, Department of Chemistry, 2145 Sheriian Road, Evanston, Illinois 60208-31 13, USA J. Rouxel, lnstitut de Physique et Chimie des Materiaux, 2 Rue de la Houssiniere, 44072 Nantes Cedex 03, France R. Roy, 102 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802-4801 , USA J. L. Serrano, Facultad de Ciencias, Departamento de Quimica Organics, Universidad de Zaragoza, 50009 Zaragoza, Spain J. N. Shewood, Department of Pure and Applied chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 lXL, UK J. Simon, ESPCI, CNRS -URA 958 -10, Rue Vauquelin 75231, Paris Cedex 05, France J. F. Stoddart, Department of Chemistry, University of Sheffield, Sheff ield S3 7HF, UK S. Takahashi, The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567, Japan G. J. T. Xddy, Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral L63 3JW, UK and Department of Chemistry and Applied Chemistry, University of Safford, Salford M5 4WT, UK B. J. Tighe, Department of Chemistry, University of Aston, Gosta Green, Birmingham 84 7ET, UK Yu. D.Tretyakov, Department of Inorganic Chemistry, Facutty of Chemistry, Moscow State University, Leninskie Gory, MOSCOWV-234, USSR R. J. P. Williams, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK R. Xu, Department of Chemistry, Jilin University, Changchun, P.R. China

ISSN:0959-9428    

DOI:10.1039/JM99101BP043

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(4), 489-502 FEATURE ARTICLE Photoemission in the Study of Oxide Superconductors Mark S. Golden," Russell G. Egdellb and Wendy R. Flavell*" a Centre for High Temperature Superconductivity, Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 ZAY, UK Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK " Department of Chemistry, UMIST, PO Box 88, Manchester M60 IQD, UK Photoemission has the potential to yield important information about the electronic structure of high-temperature superconducting oxides. However, the surface specificity of the technique, combined with the chemical complexity of these oxides can lead to considerable problems in data analysis and interpretation.Here we review photoemission results for c-axis-oriented thin films of Bi,Sr,CaCu,O,-based materials, where the surfaces are prepared for photoemission studies by in situ oxygen annealing. The current status of photoemission studies of high-T, oxides is surveyed, highlighting areas of controversy. Keywords: Oxide superconductor; Photoemission; Feature article 1. Introduction Following the discovery of high-temperature superconduc- tivity in a number of families of complex metal oxides, there have now been at least 500 investigations of these materials by electronic spectroscopies such as photoemission (XPS and UPS), inverse photoemission (IPES), X-ray absorption, Auger electron spectroscopy and electron energy loss.A great deal of attention has been focussed on photoemission studies in particular, as the technique has the potential to give detailed information about the filled band structure of these materials. Currently, there is considerable interest in detailed studies of the density of states (DOS) at the Fermi energy, both in terms of the chemical character of these states, and the way in which they evolve through the superconducting transition tempera- ture, T,. These studies are valuable as, at the very least, they place constraints on possible theories of superconductivity in these oxides. One important feature of photoemission is that it is intrin- sically surface sensitive. The photoelectron flux generated within the material is attenuated as it leaves the sample in accordance with a Beer-Lambert-type law, characterised by an electron mean-free-path length of only ca.10-15 A.' Thus the technique may be used powerfully to probe processes occurring at the surfaces of the new materials, for example reaction with atmospheric gases, or surface segregation of dopant or impurity atoms. For complex metal oxides of this type, it is common for the surface composition and electronic structure to be quite distinct from that of the bulk, owing to perturbations caused by the bulk termination created by the surface.2 This feature of the technique gives photoemission studies an added importance, since, in any working device, contacts and interfaces will have to be made to the supercon- ductor surface.It seems likely that the intrinsic surface proper- ties of the materials will ultimately determine the scale of their application. Unfortunately, the surface sensitivity of photoemission leads to a number of problems in data interpretation, and has led to a number of continuing controversies within the literature. In particular, it has now been evident for some time that all families of oxide superconductors are to some extent subject to atmospheric degradation reactions when in contact with air containing water ~apour.~-" Although the extent of the problem varies from material to material, the nett result is always the creation of a number of extraneous insulating phases at the surface. Given the extreme surface sensitivity of photoemission, this adventitious contamination must have a strong influence on the spectra obtained.It is clear that very careful attention must be paid to surface preparation and Cleaning before meaningful spectroscopic measurements can be undertaken. The surface preparation methods used to date fall into two categories. The first of these involves the creation of a new surface in ultra-high vacuum (UHV), using techniques such as cleavage, abrasion or ion milling. Alternatively, existing external surfaces may be studied, using in situ oxygen annealing to reverse surface degradation reactions. This is the procedure that we have favoured in most of our work in this area.4,11,12 The normal preparative conditions for ceramic, thin-film and single-crystal materials all involve extended oxygen annealing treatments, and the in situ cleaning procedure mimics the conditions of these.Whereas the electronic structure revealed by cleaved surfaces may be more representative of the bulk, annealed materials provide a more accurate model for the surfaces typically encountered in device manufacture. In a previous paper4 we reviewed our results using in situ oxygen annealing for a range of systems, and compared them to results reported elsewhere in the literature at the time. The aim of the present paper is to update this review, drawing attention to results obtained since publication of our earlier paper. Studies published elsewhere in this period will be surveyed, and areas of controversy highlighted. 2.Surface Ageing Reactions of SuperconductorSurfaces Like many other oxides, high-temperature superconductors are susceptible to reaction with atmospheric C02 and H20 to give surface carbonate and hydroxide Extraneous features arising from this atmospheric degradation are always evident in spectra taken from 'as-presented', un- cleaned oxide superconductor surfaces. It is therefore rather surprising that there have been so few studies dedicated to the investigation of these reactions by photoemission.3~13-19 The degradation reaction of YBa2Cu307 is perhaps the best stUdied,3-6.10,14,15,17-20 with the dominant reaction now established as 2YBa2Cu307+ 3C02 +Y,BaCuO, + 3BaC03+ 5CuO + at the synthesis temperature of 950 0C,496 possibly changing to 2YBa2Cu307 +4C0,-+4BaC03 +Y2Cu205 +4CuO+3O2 at lower temperatures.6 These reactions are strongly catalysed by the presence of water ~apour.~ The presence of degradation products at the surfaces of these oxides has a very strong effect on both core-level and valence-level photoemission ~pectra.~.~ In the case of core- level spectra, the effect is probably most dramatic on the shape of the 0 1s core-level peak; this has contributed con- siderably to controversy in the literature concerning the detailed interpretation of this peak shape, and is considered further in sections 3 and 4.In the valence band spectrum, contaminant-related features appearing at 9-10 eV below the Fermi energy EFhave also been the subject of heated debate; these are considered further below and in section 5.The build-up of the insulating contaminant phases may be rapid on the photoemission depth scale under ambient conditions; a contaminant layer of ca. 12 A thick develops on the surface of YBa2Cu307 ceramics after only ca. IOmin of air exp~sure.~ However, the degradation of some of the other oxide superconductors such as YBa2Cu40810 and Bi,Sr,CaCu208 (where the dominant products appear to be SrC03 and CaC0313*21 are apparently not so rapid. Superconductor/Ag composite materials, such as (YB~,CU~O~)~-xAg, exhibit enhanced resistance to degra- dation." The possibility of passivating the surfaces of these oxides using Ag, Cu or Au overlayers has now been investi- gated in a number of photoemission studies of precious-metal deposition on single-crystal surfaces.22-26 These studies indicate a contrast between the surface reactivity of Bi,Sr,CaCu208 (BSCCO) and related materials such as Bi2Sr2Cu06, and that of YBa2Cu307-, (YBCO) and its homologue EuBa2Cu307 -,.In the case of BSCCO materials, deposited Ag,,, Cu2, or Au23,24 causes only weak reaction with the Bi-0 (001) cleavage plane (the preferred cleavage plane for the BCSCO structure). However, similar deposition on YBCO materials causes more extensive disruption of the surface electronic stru~ture,~~,~~,~~ and the metallicity of EuBa,Cu307 -,in the surface region is essentially de~troyed.,~ As the degradation reactions are catalysed by water vapour, important information about the initial stages of the reaction may be gained by valence-band photoemission studies of water adsorption.Very little experimental work has been done in this area. Early studies of polycrystalline La, -,Sr,Cu04 indicated that water is dissociatively adsorbed at room temperature,I6 while studies of polycrystalline YBa2Cu307-, indicated that water is chemisorbed at low temperatures, becoming dissociated when the surface is warmed to 300 K.'4315These findings are in line with what might be expected from studies of other perovskites, such as SrTi03.27 One study of water adsorption on single-crystal Bi2Sr2CaCu208(001) has been carried out using synchrotron radiation (Fig. 1).Again, the reactivity of the Bi-0 planes making up the natural cleavage face of the (001) surface appears to be very low. At a temperature of 90K and low coverage, water remains essentially physisorbed (rather than chemisorbed), as evidenced by the absence of any bonding shift of the 3a, molecular orbital of the adsorbed water (Fig. I). (In cases of chemisorption of water on metal oxides, the 3a, level is shifted to higher binding energy relative to the Ib, and 1b2 levels by as much as 1.3 eV.13) Studies of adsorption on single-crystal YBa,Cu307 -,surfaces, and further studies of BSCCO materials at a range of temperatures J. MATER. CHEM., 1991, VOL. 1 12 8 4 EF binding energylev Fig. 1 Photoemission spectra of (a) water-dosed and (b) clean Bi,Sr2CaCu20, (001) at 90 K, recorded at near-normal emission using 33eV photon energy.(c) The difference spectrum (2 L H20-clean) was obtained by normalising the intensity away from the adsorbate-induced features. Vertical ionization energies for gas- phase water3' are also shown, aligned at the 1b2 peak positions. Note the absence of any bonding shift of the 3a, level away from its position relative to Ib, in gas-phase water (from Flavell et a1.I3) are necessary to investigate this apparent difference in re-activity further. However, there appear to be several plausible explanations for the difference. The (001) surface of Bi,Sr2CaCu208 cleaves easily, resulting in a surface termin- ated by relatively defect-free, planar Bi-0 1a~ers.I~This is in contrast to other less anisotropic high-T, materials such as YBa2Cu307-x, where stepped (and hence highly reactive) surfaces may be produced.28 By analogy with the low surface reactivity of the ruthenates of bismuth and lead (Bi,Ru207 and Pb2Ru207-J towards water,29 it may also be possible that the surface Bi"' cations possess stereochemically active sp-hybrid lone pairs of electrons protruding from the surface.This would make the surface particularly inert towards a species such as H20, which would tend to be adsorbed via its oxygen atom at a Lewis-acid site. 3. Surface Preparation for Photoemission Studies Although photoemission may be a useful tool in investigation of the surface reactions of superconducting oxides, it is clear that any adventitious degradation products must be removed from the surface before photoemission can be used to probe the intrinsic band structure of the materials.In our previous review, we surveyed the cleaning techniques which have been applied to these oxide surface^.^ These techniques fall into two categories. The first involves the creation of a new surface in UHV. One common UHV cleaning technique which might be used to achieve this is argon-ion bombardment. Unfortu- nately, as with all complex oxides, this has the effect of irreversibly changing the surface cation ratios, and in the case of copper oxide superconductors reduces surface copper cat- ions to CU'.~ Other techniques that have been used to create new surfaces include surface abrasion (of ceramic^^,^^-^^, single crystal^^'.^^ or thin films3,), fracture of ceramic bar^,^^.^^ and peeling or cleavage of single ~rystals.~,~~-~~ In some cases, the new surface created has been found to degrade rapidly, and to minimise this surface preparation has been carried out at low temperatures. The two most important instances where this has been necessary to date are studies of Nd2-,Ce,Cu04 and YBa2Cu307-,.In the case of Nd2 -,Ce,CuO,, several authors have now found it necessary to cool the samples to temperatures between 20 and 100 K J. MATER. CHEM., 1991, VOL. 1 before cleaving single crystals38 or scraping ceramic^.^^-^^ In the case of YBa,Cu307 --x, some authors have reported degra- dation of cleaved single-crystal surfaces even at quite low temperatures (ca.50 K).4 In some recent studies, YBa2Cu3O7--x crystals have been cleaved at temperatures as low as 8 K, in order to maintain a representative ~urface.~~~~~ 49 1 . .The exact cause of surface deterioration in this case remains rather uncertain, and will be discussed further in section 5. Interestingly, the same authors have reported the (001) surface of Bi2Sr2CaCu208 to be considerably more table:^^^^ in line with the observations of the previous section. We have previously discussed the advantages and disadvan- tages of methods of surface preparation which involve the creation of a new ~urface.~Perhaps the most significant disadvantage is that all these techniques are destructive, and are thus rather difficult to apply to plate-like single crystals, or thin films.Most of our own work has been conducted using an alternative preparation technique, designed to elimin- ate this problem. This involves reversing the effects of atmos- pheric degradation by in situ annealing under conditions chosen to mimic those used in the synthesis of the samples. Our experiments have been conducted in a two-chamber VG Escalab Mark I1 spectrometer. Samples are mounted on platinum stubs using platinum clasp wires and transferred to the spectrometer preparation chamber, which is filled to 1 bar with pure oxygen. Samples are then subject to an annealing and slow cooling cycle by r.f. induction, using a Radyne 400 kHz, 1.5 kW generator.The process has been described in more detail el~ewhere.~ The efficacy of this cleaning procedure may be gauged by monitoring the C 1s:O 1s intensity ratio in XPS. The efficiency of the procedure varies from substrate to substrate; in BSCCO- based materials, we have found that it is possible to reduce the C 1s signal below levels of dete~tability,~.'~,~~,~~whilst in the case of ceramic YBa,Cu,O,-,, it may only be possible to reduce the ratio to below 1:100.4," The technique has been particularly successful when applied to thin films of the Bi2.1Sr1.7Ca0.85-xYxcu208 (Y-BSCCO) and related techniques have been applied by other authors to thin 523 526 529 532 535 538 binding energylev Fig.2 The effect of sample cleaning by oxygen annealing on the 0 Is peak shape in Al-Kcr XPS (hv=1486.6eV).(a) Biz.lSr,.,Cao.425Yo.,25CU208+6thin film before cleaning by in situ oxygen annealing. (b) As in (a), after cleaning, revealing the intrinsic peakshape due to the superconductor (from Golden et ~1.~') contact must be made for electrical measurements. However, there may be significant differences in chemical composition and electronic structure between cleaved surfaces and post- annealed external surfaces. This may be due to intrinsic segregation of one cation to the surface (e.g. Sr in Laz -,S~,CUO~~) or purely due to the apparently very different films of YBa2Cu30,- +x52-55 and Nd, -,C~,CUO~.~~.~~ reactivities Contaminated surfaces display a strong, high-binding energy 0 1s component.This is chemically shifted by ca. 2-3 eV to higher binding energy relative to the 0 1s peak at 528-529 eV associated with the superconductor phase. For all the substrates which we have studied (including YBa,Cu307 -x,ll La, -,SrxCu04,4 and Bi,Sr,Ca,-'Cu,,04+2,,"), the high-binding-energy peak shows enhanced intensity in grazing-emission experiments, thus demonstrating its surface origin. This feature is also found to increase in intensity as a function of length of exposure of the sample to the ambient atm~sphere.~ As an example, the 0 1s peak shape for a sample of Y-BSCCO with x=O.425 is shown in Fig. 2, to illustrate the effect of the oxygen cleaning procedure. The contaminant- related high-binding-energy component of the peak shape is completely removed during oxygen annealing, leaving a single, slightly asymmetric peak.In the case of YBCO, a small shoulder remains in this region after cleaning, which may be intrinsic to the superconductor itself." The detailed structure of the 0 1s envelope in XPS in superconducting oxides continues to excite debate31,32,39,54*58-63 and will be discussed further in section4. However, we would maintain that the observation of appreciable structure beyond 531 eV is merely indicative of a significantly contaminated surface. Oxygen-annealed surfaces are of immediate interest in relation to technological application of oxide superconductors, as they represent 'intrinsic' free surfaces of samples after furnace preparation, and it is generally to these surfaces that of equilibrated annealed surfaces and freshly cleaved surface^.^ This effect has been noted previously for the perovskite SrTi03.27,64 In general, we have found surfaces prepared in this way to be stable in UHV for prolonged periods (in some cases, several days).4. Core-level Studies The major interest in core-level photoemission lies in the ability of the technique to distinguish different valence states and chemical environments of atoms, using the resulting chemical shift differences in binding energy. One of the most important questions is then how much core-level studies can tell us about the formal valency of copper in the Cu-0, planes of the copper oxide superconductors (or the valency of bismuth in the Bi-based materials).We will discuss this in section 4.2. First, however, we will consider studies of the 0 1s core levels, as here our conclusions are clearly influenced by comments in the previous section. 4.1 0 1s Core Levels In our earlier paper, we surveyed 0 Is core-level studies carried out up to late 1989.4 The discussion here will centre on work which has appeared since this date, although the controversy surrounding the shape of the 0 1s peak does not appear to have been in any way resolved. Many published spectra show two components of variable intensity ratio for 0 1s (Fig. 2),4l3l with some as-presented surfaces displaying more complex structure.54 It is widely accepted that structure at ca.528-529eV is intrinsic to the oxide superconductor, whereas much of the intensity at ca. 531 eV can be attributed to an extrinsic, surface component,23,2426,32.36.39,43,45$54,65-7 1 whose size is dependent on a variety of surface treatment^,^' and is enhanced in grazing emissi~n.~,~'*~~ The exact origins of this surface component have not been defined precisely; possible suggestions for the YBa2Cu307-x material are BaC0333491 formed as a result of atmospheric degradation, Ba(OH)2,39 Ba04*39 formed by the decomposition of BaC03 during surface treatment, B~CUO~,~~or a BaCu0,-like phase.54 In the case of BSCCO materials, the most likely candidates appear to be SrC03,4*21*71 CaC03,21 or SrO formed by decomposition of SrC03 during surface treat- ment4v7' However, the surface contaminant feature appears to be present in spectra of uncleaned surfaces of less well J.MATER. CHEM., 1991, VOL. 1 removed from the surface, leaving a single feature (e.g.Fig. 2), this peak is generally found to be somewhat asymmet-ric.4y16,24v39,43,45,49*66Calculations show that the strong Cu 3d-0 2p covalency in the Cu-02 planes may give rise to several possible final states (in the case of CuO correspond- ing to 'well screened' 101s' Cu 3d9 Lo> and 'poorly screened' 101s' Cu 3d" L1>(where L refers to ligand oxygen, and annderscore denotes holes), with the latter appearing at ca. 1.5 eV higher binding energy than the main Thus, weak high-binding-energy structure may be intrinsic to cuprate phases. 4.2 Cu 2p Core Levels By comparison, the interpretation of Cu 2p features from copper oxide superconductors is reasonably uncontroversial, studied materials; these include Nd2-xCexCu04,36*45*68,69and the lineshapes of these peaks may be used as an indication (Tlo~5Pbo~s)Sr2(Cal-xThx)Cu20,, 67 and Ba, -,RbxBi03.46 In these cases, the nature of the contamination has been less well identified.Most oxide superconductors contain oxygen in a number of different lattice sites. It might be expected that these would give rise to 0 1s signals at differing binding energies. However, calc~lation~~and O-Ka X-ray emission spectra73 have shown that distinct oxygen sites differ in binding energy by at most 1.5eV, so that in the normal XPS experiment, with resolution of ca.1 eV, these contributions to the intrinsic '528 eV' super- conductor peak are not distinguishable, and a single, rather broad peak is ~een.~',~~However, in recent studies by Parmigiani et ~l.,~'using a monochromated Al-Kcc source and low analyser pass energy to study oxygen-annealed Bi2Sr2CaCu208 single crystals, a resolution of 0.35 eV was obtained, enabling two components of the bulk superconduc- tor feature to be resolved. The studies indicate that oxygens close to the Bi-0 planes have a slightly higher binding energy than oxygen atoms in Sr-0 and Cu-0, layers,61 roughly in accord with calc~lation.~~Interestingly, when crystals are re-annealed in a high overpressure of oxygen (12 atmt rather than 1 atm), a new feature which is enhanced in grazing emission appears at 531.2 eV.The authors attribute this to surface oxygen. Specific surface oxygen species, such as superoxide and peroxide have been proposed previously to explain structure at higher binding energy than the bulk superconductor fea- t~re.~*~'Recent studies by Qiu et al. on the reaction of alkali metals with oxygen have indicated that assignment of this structure to peroxides may be possible, but that superoxide appears at substantially higher binding energy (ca. 535 eV).59*74 An alternative interpretation of 0 1s core-level structure is given by Balzarotti et a1.31,62,63He re, reduction in intensity of the 531 eV feature on vacuum annealing YBa2Cu30,-, is attributed to stepwise loss, first of contaminant oxygen, but predominantly of the chain oxygen atom^.^',^^ Re-annealing in oxygen is found to restore the 531 eV peak.In the case of Bi2Sr2CaCu208, these authors contribute a 53 1.5 eV peak to oxygen in the Bi-0 and Sr-0 planes, and a third compo- nent, centred at 533.5 eV, to adsorption of adventitious CO in UHV. Detailed measurements from single crystals as a function of photoemission take-off angle are necessary to resolve these controversies. However, we would point out that parallel measurements of the Cu 2p core levels from our own oxygen annealed surfaces (showing low intensity at 531 eV) indicate that these surfaces are not oxygen deficient (section 4.2, be lo^).^^^^^^' Even when extraneous surface phases or species can be 1 atmx101 325 Pa.of the formal oxidation state of the Cu-02 planes in both hole-doped4,23.24.26.31,33,35,39,50,51,62,6S,67,7S-78 and electron- doped materials.36,43,45,68*69As an example, in Fig. 3 we show the Cu 2p3,, multiplet for c-axis oriented thin films of Biz.lSrl.7Cao.8s-xYxcu208 (Y-BSCCO) for x=O (metallic,m) and x =0.85 (non-metallic, n) corn position^.^^^^^ The x= 0 material is a hole-doped superconductor, with a formal copper valency of 2.3, whereas the x=0.85 material has a corresponding Cu valency of 1.9,i.e. essentially a Cu" material. The major features of both spectra are similar to those of CUO,~~and show the familiar double-peaked structure com- mon to all high-TT, copper oxides.This arises from strong m n 929 933 937 941 945 949 binding energy/eV Fig.3 Al-Ka XPS profiles in the Cu 2p,,, region for Bi2,1Srl,7YxCal-xCu208+d thin films for metallic (m, x=O) and non- metallic (n, x=0.85) samples. The difference spectrum (d, m-n) is obtained after normalisation of the areas of the complete 2p,,, structure and alignment of Bi 4f or 0 1s core-level peaks. AI-Ka,., satellite structure has been subtracted. The horizontal lines indicate FWHM, and the vertical line indicates the peak maximum in the metallic (m) spectrum (from Golden et ~1.") J. MATER. CHEM., 1991, VOL. 1 Cu/O mixing in the initial state, allowing for valence electron transfer from the 0 2p to the Cu 3d levels during photoionis- ation.In CuO, the main peak at ca. 933 eV is due to the well screened 12p"d"L' > final state, and the satellite centred at 493 significant intensity shift to lower binding energy, with the difference spectrum of Fig. 3 (d; m-n) showing a peak at 945 eV. In the case of YBa2Cu307-,, which is prone to oxygen ca. 942 eV% due tothe unscreened 12p '3d9L0 > final ~tate.~,~~ loss, much lower satellite intensities may be obtained,35 for The satellite is subject to final-statemultiget splitting into a total of eight closely spaced states,80 giving a complex peak structure. The observation of more than one final state in XPS indicates that the core-valence Coulomb interaction in the final state is sufficiently large to localise the electrons on copper.This behaviour is characteristic of the oxides at the end of the first transition-metal series, and reflects the fact that electron correlation in the initial state is strong; CuO, NiO, COO, FeO and MnO are magnetic insulators. Obviously in the case of copper oxide superconductors, the initial state is itinerant, rather than localised, but core-level studies show that correlation forces are nevertheless large, and the oxides can generally be made to undergo a metal-to-non-metal transition by quite subtle changes in chemical doping." [Note that core-level studies of this type give us information primar- ily about the final-state configurations; the distribution of the itinerant electrons in the initial state (e.g.whether the holes in p-type materials are on copper or oxygen) cannot be probed; only aforrnal Cu oxidation state is obtained.] In the case of a Cu' material, such as Cu20, the only accessible final state is 12p13d"L0 >, and no satellite feature is observed.79 Converselzfor C$", e.g. in N~CUO~,~'-'~ and L~CUO,,'~" three states are accessible: 12p '3d"L2 >, )2p '3d9L1 > and )2p '3d8L0 >. XPS shows thacn thiscase, G"' is-manifest mainly as the 'two-ligand-hole' state, 12p '3d"L >, appearing at the high-binding-energy side of the main933 eV peak seen in the CuO spectrum. From Fig. 3, we can see that in going from the metallic (m) Bi2~lSrl,7Cao~8sCu208+ato the non-metallic (n), fully Y-sub- stituted material Bi2~lSrl~7Yo~85C~208+6, we see a transfer in spectral weight from the high-binding-energy side of the 12p '3d"L" > peak at 933 eV to the low-binding-energy side, together-with an overall narrowing of this peak.We have previously shown that this is consistent with the formal change in copper oxidation state, and hence the absence of the 12p'3d''L2> final state in the case of the non-metallic material?' Parallel effects are observed in valence-band photoemission (section 5). Similar trends have been found for YBa2Cu307-,, where the formal oxidation state is >2 in the superconducting state with x <0.5, and <2 in the non-metallic Itstate with x >0.5.4,26,39,5',65,75.77 now seems clear that core-level XPS is able to provide a measure of the valence- band hole concentration in hole-doped materials.Lindberg et have pointed out that the mean-free-path length of Cu 2p electrons is extremely small, (as low as 6 in Bi2Sr2CaCu208) owing to their low kinetic energy, so that the Cu 2p spectrum is a very sensitive test of surface stoichiometry. Low intensity in the satellite feature at 942eV may often be indicative of some surface reduction to as no satellite feature is expected for Cu20. In the case of hole-doped materials, which should give 'Cu"-like' and 'Cu"'-like' final states, this interpretation is not entirely without pitfalls. This is because Cu"' compounds, unlike Cu" materials, also appear to give very low satellite inten~ity.~~.~'~'~ Thus, increasing hole concentration may in some cases lead to decreasing satellite inten~ity.'~' Combined XES and XPS studies78 have indicated that the 'Cu"'-like' (2p '3d9L1 > contribution to the satellite feature is a low-intensity peak centred at 945 eV.Thus in the case of Y-BSCCO, where we have a nett Cu valency change from 2.3 to 1.9, we see a minimal change in satellite intensity (in fact a slight increase, as the non-metallic material is very close to a Cu" oxide)." However, there is a example by heating in uacu0,4*51i75indicative of significant reduction to Cu'. (On heating in uacuo, the composition obtained is essentially YBa2Cu306, corresponding to CU'.~~, ref. 51.) In a recent paper, Fowler et associated low satellite intensity with poor-quality YBCO surfaces showing large amounts of reduction, and Yeh et al.have in turn correlated this with poor superconducting transition tempera- tures in thin films.35 In principle, interpretation of satellite structure should be more straightforward in the case of electron-doped materials, such as Nd,-,Ce,CuO,, as the only formal oxidation states involved here are Cu' and Cu". The satellite intensity should decrease regularly with the amount of hole doping, i.e. with increasing Ce concentration. Studies of this material tend to indicate that the satellite intensity in Ce-doped compositions is indeed lower than that in Cu0.36,43,45,68369As-grown samples of this material do not superconduct, and must undergo a reducing annealing cycle before a superconducting transition is obtained.In general, as-grown, unreduced samples appear to show a satellite intensity which decreases with increasing Ce-doping leve1.36,43*45 However, the decrease is less marked than that which would be expected for the occupation of the Cu 3d orbitals by all doped electron^.^^,^^^^^ After reduction the satellite intensity is more in keeping with the model of one doped electron per introduced Ce,36,43,45 although the presence of extraneous Cu'-containing surface phases caused by the reduction cannot be ruled o~t.~~'~~ 5. Valence-level Studies Photoemission has been very widely used to study the elec- tronic structure of the filled density of states in oxide supercon- ductors in the valence-band region and close to the Fermi level.Particular interest has focussed on the nature of satellite structure in valence-region photoemission (which reflects elec- tron correlation), and on the electronic states close to the Fermi energy (as these are the states primarily involved in superconducting behaviour). Experiments have been carried out using both conventional, fixed-energy photon sources (noble-gas discharge lamps and soft X-ray guns), and tuneable synchrotron radiation. Changes in intensity of spectral features with changing photon energy reflect changes in atomic ionis- ation cross-sections and are of value in helping to identify the atomic nature of states responsible for the emission. Synchro- tron studies are particularly valuable here, as resonant enhancement of these cross-sections at core excitation thresh- olds may be exploited using tuneable energy radiation.How- ever, the superior resolution attainable with a noble-gas discharge lamp has meant that studies using conventional sources are very important in monitoring slight changes occurring at the Fermi energy when a sample is cooled through the superconducting transition temperature T,. Valence-region studies using either synchrotron radiation or a discharge lamp source generally involve low-energy electrons. The momentum resolution possible is then generally sufficient to locate a transition to within a small fraction of the typical dimension of a Brillouin zone. This opens up the possibility of mapping electronic structure using angle- resolved photoemission, in cases where high-quality single crystals are available.Measurement of the extent of dispersion along various zone directions allows for detailed comparison with theoretical models of electronic structure. In the case of the oxide superconductors, both angle-integrated and angle- resolved photoemission have been used extensively. 5.1 General Features of the Valence-band Region In order to illustrate the general features of the valence band structure of these materials, in Fig. 4 we show valence- region spectra for oxygen-annealed, c-axis-oriented Bi2~3Srl~3Cal~oC~2.008+b thin films (a single-phase, two-layer '(2212)' BSCCO-material with a T, of 85 Kg5). These are taken with three different conventional laboratory sources, He I (hv=21.2 eV), He I1 (hv=40.8 eV) and Al-Ka (hv= 1486.6 eV).The He I spectra are dominated by a steeply rising background of secondary electron emission beyond ca. 10 eV binding energy, whereas the higher-energy sources allow the obser- vation of shallow core-level structure (in this case Sr 4p at ca. 18 eV binding energy). The main valence band region, which is composed of 0 2p and Cu 3d levels (ca. 1.5-7 eV binding energy), changes very little in shape on raising the photon energy from the UV to the soft X-ray range. Over this photon energy range, the 0 2p:Cu 3d cross-section ratio decreases from ca. 0.35 to 0.08.4,11*75The Al-Ka spectrum thus reflects predominantly the Cu 3d density of states, and indicates that 15 1'0 5 6 binding energy/eV Fig.4 Valence-region photoemission spectra of c-axis-oriented thin- film Biz,3Srl &al,oCuz.oO, (001) ['(2212)'-phase material] excited with (a) Al-Kcr (hv= 1486.6 eV), (b) He I1 (hv=40.8 eV) and (c) He I (hv=21.2 eV) radiation. Note the increase in the intensity of the 3.4 eV feature with decreasing photon energy. The Fermi level was established from measurements on a clean Ni stub. All spectra are taken at room temperature and normal emission J. MATER. CHEM., 1991, VOL. 1 there is a considerable copper contribution across the whole width of the valence band. This is in contrast to the profiles from d" perovkites containing elements from earlier in the transition series: where it is possible to distinguish well separated 0 2p and M nd bands, which undergo dramatic intensity fluctuations in a comparable e~periment.~~~~ It is clear that in the cuprate superconductors, there is considerable covalent mixing between copper and oxygen.The only signifi- cant change that is discernible is the decrease in intensity of the feature at 3.4 eV as the photon energy is raised, suggesting that this feature has predominantly 0 2p character. This observation is consistent with our previous studies of -x4311975YBa2Cu307 and a range of BSCCO-based material^.^^'^^^^*^^ A small, but distinct, density of states at the Fermi energy is evident as a sharp cut-off in both the He I and the He I1 spectra of Fig. 4. This small feature can be seen more clearly in Fig.5, where we show expanded valence band regions for the same BSCCO thin film, recorded using He I (hv=21.2 eV) and Ne I (hv= 16.8 eV) radiation. With the use of these low- energy photon sources, four distinct valence band features are resolved at ca. 1.6, 3.4, 4.7, and 5.7 eV binding energy. These energies are in good agreement with those observed in studies of single-crystal Bi2Sr2CaCu208 (001),13*22,42*87-93indicating that our oxygen-annealed c-axis-oriented thin-film material shows essentially the same density of states in the valence- band region as a cleaved single crystal. The relative intensity of the valence-band shoulder at 1.6 eV binding energy decreases with decreasing photon energy. However, since the electron mean-free-path length in the solid changes fairly rapidly over this kinetic energy range,94 this effect may not be attributable solely to cross-section changes.The nature of the density of states at the Fermi energy is discussed in more detail in section 6. 86420-2 binding energy/eV Fig. 5 Valence-level spectra excited within (a) Ne I (hv = 16.8 eV) and (b) He I (hv= 21.2 eV) radiation. Note the clear DOS at EF and the decrease in the intensity of the ca. 1.6eV feature with decreasing photon energy (see text). Spectra are normalised to the height of the valence band maximum. The Fermi level was established from measurements on a cleaned Ni stub J. MATER. CHEM., 1991, VOL. 1 5.2 Resonant Photoemission Studies In principle, resonant photoemission using a synchrotron source may be used to investigate the atomic character of the main valence band more closely.The most important reson- ances explored here are that at the 02p+O 2s threshold (at ca. 18-20 eV), and the Cu 3p-C~ 3d transition at ca. 74 eV. In fact, the relative intensity changes in the features in this part of the spectrum as a function of photon energy, even around resonant thresholds, appear rather muted. This again points to a very strong mixing of 02p and Cu 3d character in the main valence band. However, resonant photoemission 495 summarise the current situation. In superconducting oxides, resonant Cu 3p enhancement of satellite intensity is observed at 12.3- 12.9 eV binding energy in La, -xSrxCu04,105 YBa2C~307-x,53,105-115 Bi2Sr2CaC~208,87.98*116-119 Bi2Sr2C~06,93 Nd2-x Ce Cu04,38,44,98~104~120 and x Pb2Sr2Y1 -,Ca,Cu, -xAgx03121 and is generally assumed to be analogous to the behaviour of the 12.9 eV satellite of CuO.The observation of further structure at ca. 9.5eV binding energy has been a contentious issue.4 This additional structure could simply be part of the Cu 3d8 multiplet, by analogy with cuo~4,99,100,103However, alternative assignments have been in the region of the 02s tends to reinforce our conclusions regarding the 3.4 eV feature of the valence band of fig. 4 and 5. We reviewed the use of resonance photoemission as applied to oxide superconductors in our earlier paper.4 Since this time, a number of groups have investigated the behaviour of valence-band photoemission around a wide range of resonant thresholds in addition to 0 2s and Cu 3p.383a395-98 In particu- lar, a number of rare-earth 4d+4f thresholds have been used to explore the rare-earth (RE) partial density of states con- tributing to the valence-band emission from Nd, -xCexCu04 and its homolog~es.~~*~*~~ These show quite a large contri- bution to the valence bands of these materials from hybridised RE 4f-0 2p states, in addition to structure corresponding to proposed, including 3d1'LL, where LL denotes two holes on core excitation thre~hold~~.~'.~~ different oxygen atoms.'ITResonantphotoemission experi- ments in this spectral range gave very conflicting results; in general very little res~nance,~'~,~~~,~~~ or no resonance at 10.111 was observed at the Cu 3p threshold, while in some cases very large resonances were seen at the 02s threshold123 (for a full discussion see ref.4). The observation of time- and temperature-dependent inten- sity changes in this part of the spectrum has led to the suggestion that a strong features at 9.5 eV is associated with some type of surface contamination. A feature has been observed in this region of the spectrum for the Bi-based superconductor BaPb, -xBix03; in this case the intensity of the feature increases with time after the initial surface prep- In the In experiments by Ark0 and co-workers on cleaved arati~n.~~specific multiplet lines of localised 4f electron~.~~*~'~~ EuBa2Cu307-x124-126 and YB~,CU~O,-~,~'~ the feature was case of Pr, .85Ceo.l,Cu04, the hybridised component is par- ticularly intense and very close to the top of the valence band.98 Studies of Pb2Sr2PrCu308 and PrBa,Cu,O, -x show a similar Pr 4f contribution to the valence band density of states, with hybridisation in the latter being particularly strong;96 it is suggested that this Pr-0 or Pr-Cu hybridisation may be important in the disruption of superconductivity in this material.96 One of the most widespread applications of resonance photoemission has been to the study of the 'satellite' structure appearing below the main valence band, at binding energy >8 eV.This is characteristic of transition-metal oxides toward the right-hand side of the Periodic Table, and arises in a similar way to the satellite structure that we saw in the Cu 2p core levels (section 4.2).In valence-band photoemission, new final states arise where the photohole is screened by transfer of electron density from surrounding atoms. Thus in CuO, the main valence-band emission in fact arises from states of the type 13d9L1 >,and a 'satellite' structure corresponding to 13d8> unscreened final states appears between 10.5 and 12.9 eV below EF.99*100The energy shift is due to strong on- site Coulomb repulsion. [Similarly in the case of NiO, ionis- ation of Ni 3d electrons gives an unscreened Ni 3d7 feature at ca. 10 eV binding energy, the screened 3d peak ( (3d8L1>) being centred at 2 eV."1.102] In the square-planar co-ordi- nation (D4h)symmetry (corresponding to the CuO, planes in the high-T, superconductors) the d8 states span singlet and triplet irreducible representations.The CuO satellites show found to appear as single crystals cleaved at 20 K warmed to room temperature. This led to the interesting suggestion that the 9.5 eV feature is associated with adsorbed or interstitial oxygen derived from the decomposing bulk crystal, as the crystal is warmed from 20 K.'06 However, this possibility has been refuted by other workers who find YBCO surfaces to be relatively stable to oxygen Perhaps a more obvious possibility is that a time-dependent increase in intensity at 9.5eV may be associated with adsorption of H20 or other contaminants from the residual vacuum.Studies of polycrys- talline YBa2Cu30714 and La, ,8Sro.2C~0416 have identified the presence of an OH derived peak at 9.0-9.4eV binding energy, following reaction with H20 (section 2). In the case of YB~,CU,O,-~, the authors note that water exposures as low as 0.1 Lt can modify the valence-region ~pectrum.'~ In Fig. 1 we showed results for the physisorption of H20 on Bi,Sr2CaCu208 (OOl), where it can be seen that water adsorp- tion does indeed modify the spectral features in this range.13 The overall effect is somewhat similar to that noted by Balzarotti et al. for Bi2Sr,CaCu208 (OOl), and attributed to adventitious CO ad~orption.~, The presence of spectral features at ca.9-10eV binding energy from Nd, -xCexCu0444,45*57and ErBa2CU40865 have also been associated with contaminated or degraded sample surfaces. The feature has been found to increase in intensity on mild sputtering of YBa2Cu307 -x ceramic surfaces,127 but as this will tend to increase the reactivity of the surface, it does not allow us to distinguish between oxygen loss/readsorption, and strong resonant enhancement at the Cu 3p threshold.99~100~103 extrinsic contamination. (In the model of Gunnarsson et this is ultimately related to their strong d8 character.) The triplet states are expected to resonate less strongly than the singlet states,1oo the nett effect being that the satellite structure at 12.5 eV in CuO shows the strongest resonant enhancement.99~100*103 The main Cu 3d valence band shows antiresonance intensity variations, centred around 3 eV binding energy.99 Provided that correlation effects in the superconductor materials are similar to those in CuO, we expect to see similar resonance behaviour in the high-T, materials. In our earlier paper,4 we discussed this issue at some length, and the controversies then surrounding it.Here we merely attempt to In summary, it now appears likely that although a weak resonantly enhanced Cu 3d-derived satellite is found at 10 eV binding energy,lo6 the majority of the intensity in this region is not intrinsic to the superconductor itself, but is derived from an oxygen-containing surface species. However, more exotic mechanisms, such as that recently proposed by Kasow- ski et al., involving two-electron processes128 cannot be ruled out until control over sample quality and surface stoichi- ometry is substantially improved.t I L=1.32~10-~mbars. 5.3 Effectsof Chemical Doping on Valence-band Photoemission One question which is currently attracting considerable atten- tion is the way in which the electronic structure of supeycon- ducting oxide changes with chemical doping level. In particular, the position of the valence band edge relative to the Fermi energy as a function of composition is of importance in allowing us to infer whether or not a rigid band model is applicable to these oxides. Fig. 6 shows the changes in the He I (hv=21.2 eV) valence band spectra which occur on crossing the metal-to-non-metal transition for c-axis-oriented thin films of the Bi2.1Sr,.,Cao.8s -xYxCU208+b (Y-BSCCO) ~ystem.~~,’~The corresponding changes in the Cu 2p core- level spectra were discussed in section 4.2.Spectra for x=O and 0.25 show a clear density of states at the Fermi energy. The metal-to-non-metal transition occurs at ca. x = 0.425,’l and the x=O.85 sample shows no appreciable intensity in this region. The onset of the main part of the valence band (between 1.0 and 1.5 eV below EF) moves away from the Fermi energy as the system becomes non-metallic. This is consistent with a simple interpretation of the electronic struc- ture of this p-type material in terms of the progressive filling of holes in the valence band to form a magnetic insulator.The progressive, rather than sudden, disappearance of the density of states at the Fermi energy is also consistent with this type of m0de1.l~’ In the simplest Mott-Hubbard model, the band concerned would be a lower Hubbard band of Cu 3d character. However, we have seen in section 5.1 that the 0 2p and Cu 3d levels overlap in energy very strongly, and it ...... ... .. EF 10 8 6 4 2 0 -2 binding energylev Fig. 6 He I (hv= 2 1.2 eV) valence-level photoemission of Biz,lSr,,,Y,Ca, -,CuzO8 thin films, for a series of compositions spanning the metal-to-non-metal transition in this system. Values of x: (a) 0,(b) 0.25,(c) 0.425, (d) 0.85. The Fermi level was established from measurements on a cleaned Ni stub (from Golden et ~1.’~) J.MATER. CHEM., 1991, VOL. 1 appears that the band gap in these materials may be better described as of the charge-transfer type; in other words, the doped holes go into 0 2p orbitals (see section 6). An alternative model begins with normal Fermi-liquid states consisting of 0 2p and Cu 3d energy bands. In this model, the density of states at EF arises on doping through a Kondo-type many-body resonance of Cu 3d character resulting in renormalised heavy-electron bands being formed in the vicinity of the Fermi level or through creation of new impurity states. The basic difference between the two approaches is that in the Kondo-resonance or impurity-state approach, the states filled by doping do not pre-exist in the insulator, and the Fermi level is pinned by creation of these states on doping.In the alternative, rigid-band approach, the doping creates holes in pre-existing states of predominant 0 2p character, which are split off from the top of the 0 2p valence band by hybridisation with Cu 3d8 states, as described in cluster calculation^.'^^^'^^ The movement of the valence band relative to the Fermi level should in principle allow for the two models to be distinguished. However, this issue is currently somewhat con- troversial. Our conclusion regarding the Y-BSCCO system discussed above (i.e.that hole doping is not in itself responsible for the appearance of structure above the main valence band edge) is reinforced by the experiments of Fujimori et ~l.,’~~ Itti et and Fukuda et ~2E.l~~However, no valence-band l~~shift was observed in the work of Matsuyama et ~1.These authors conclude that hole-doping produces new impurity states at the Fermi level, created by strong hybridisation between doped 0 2p hole orbitals and empty Cu 3d orbitals. These authors also point out that the dominant 0 2p character of the states at EF (see section 6) tends to preclude the possibility of these states arising owing to a Kondo resonance, as a Kondo-like state should have dominant Cu 3d ~haracter.’~~ The experimental situation is no less confusing in the case of other oxide superconductor systems. Some interesting results have been obtained for the Bi-based systems.Shifts of the valence-band edge to lower binding energy on doping have been reported for Ba, -xRbxBi03,66 Ba, -xKxBi03135 and BaPb, -,Bi,03.34 However, in the case of BaPb, -,Bi,03, this shift is rather small, and the authors argue that the Fermi level is pinned in new electronic states created at the Fermi level by doping.34 By contrast, the shift observed in the related Ba,-,M,BiO, systems is roughly in accord with that pre- dicted on the basis of a one-electron rigid-band m~del.~~,~~’ However, since this model fails to describe the electronic structure of the parent compound BaBi03 (which is a semicon- ductor),66 this conclusion must be treated with caution. It is tempting to suggest that comparison of the effect of doping on hole-and electron-doped superconductors should lead to some rationalisation of experimental data.However, here again, quite conflicting results have In thebeen ~btained.~~-~~@.~’ electron-doped material Nd2-,Ce,Cu04, a simple Mott-Hubbard approach would imply that the doped electrons occupy the upper Hubbard band of Cu 3d character. In this case, we would expect the valence-band edge-Fermi level separation to be greater than that found in the hole-doped material La2-,Sr,Cu04, and to increase with doping. Experimentally the situation is compli- cated by the sensitivity of surfaces of the Nd-compound to degradation, so that Fermi-level emission has not been observed in all ~tudies.~’ An increase in the separation of the valence-band edge from EFwith doping is seen in combined photoemission and inverse photoemission measurements by Reihl et aL3’ and interpreted in terms of rigid-band behaviour.However, this shift is not observed by Suzuki et who interpret their data in terms of the creation of a narrow band J. MATER. CHEM., 1991, VOL. 1 of impurity states in the band gap on doping. In a comparative study of Nd, -,Ce,CuO, and La, -,Sr,CuO,, Namatame et .~~1 observe a larger valence band-Fermi energy separation in the former, but argue that the separation is not large enough to be consistent with a model where the doped electrons enter the d" conduction band. They argue that the doped electrons are introduced into impurity levels within the band gap.In another comparison of these hole-and electron- .~~doped materials, Allen et ~1 report that the position of the Fermi energy in Nd,-,Ce,CuO, is virtually identical to that in La, -,Sr,CuO,, and that in both cases the doping produces new states filling the charge-transfer gap. This leads the authors to suggest that these gap-filling states of the doped materials obey a Luttinger-type sum rule at EF,central to the Fermi-liquid theory of metals. The controversy, particularly surrounding the electron- doped materials is unlikely to be resolved until sample quality and problems of surface degradation in these materials can be better controlled. 5.4 Angle-resolved Valence-band Studies Controversy surrounding the electronic structure of these materials has led to a number of detailed angle-resolved measurements, aimed at studying any dispersion of the states close to the Fermi level. These latter measurements are discussed in more detail in section 6.In addition, there have been a number of valence-band studies using angle-resolved photoemission, and the rather interesting electronic properties of these highly correlated materials have led to some re- investigation of the magnetic insulators CuO, NiO, COO and MnO by both angle-resolved and angle-integrated photo- emission.100,103,136-140 There have been two main approaches to understanding the electronic structure of magnetic insulators. In Hubbard- type models, we think of localised states of d" ions, with itinerant behaviour inhibited by electron-repulsion effects.On the other hand, it has been suggested that these materials can be understood in band-theory terms when the correct ground- state magnetic order is incl~ded.'~~~'~~~'~~ In these spin- dependent band models, the band gap arises essentially owing to exchange splitting of the d orbitals (giving an energy difference between spin-up and spin-down electrons on a given ion), combined with crystal-field effects. If these effects are not included, one-electron band theory calculations predict a metallic ground state for these oxides. However, the band gap predicted by spin-dependent calculations for MnO and NiO is generally almost an order of magnitude smaller than the 3-4eV found experimentally. The prediction of a gap at all relies on the metals possessing a half-filled d sub-band; this is not the case for COO, which is predicted to be a metal, even though the experimental gap is similar to that found in MnO and Ni0.129 The existence of the gap in these calcu- lations is also dependent on ground-state antiferromagnetic order; however, experiment shows that the gap does not disappear above the magnetic ordering temperature.A general feature of one-electron band-structure calculations is that the calculated DOS has to be rigidly shifted to higher binding energy for a comparison to be made with experiment, with this effect being generally attributed to ~orrelation.~ However, failings are also found in the localised approach using cluster calculations. In particular, these models are not capable of treating d band dispersion, although this effect has now been observed in ARPES of COO and Ni0.138-140 The observation of band dispersion has been widely cited as supporting the band-theory approach to the structure of these oxides and high-T, materials. However, the observed d-band dispersions are generally less than those predicted by band 497 calculations.140 This leaves us in a rather unfortunate situation where neither the 'localised' nor the 'itinerant' approach appears to describe the electronic structure of magnetic insu- lators satisfactorily. has suggested that the way forward may be to introduce the effect of a Hubbard 'U' into the band model via a Hartree-Fock calculation.In such a calcu- lation, the energy of the empty states include extra Coulomb terms. This type of approach is used by Anisimov et ~1.,'~' who introduce an unoccupied-states potential correction in the local spin-density functional formalism. This model repro- duces the experimental band gaps for NiO, MnO, FeO and COO, and produces an insulating ground state for COO and FeO. A number of ARPES studies of the high-T, materials have now been carried OU~.~~.~~,~~~,~~-~~,~~~ In general, similar features are observed to those for the magnetic insulators. Dispersion of valence band states is observed, particularly in the basal a-b planeg2 (for Bi,.oSrl.8Cao.8Lao.3C~2.108+~, dispersionless behaviour is found in the perpendicular direc- tiongo) but the experimental dispersion is generally less than that predicted by band the~ry.~',~~-~* Again, it is found to be necessary to introduce a rigid downward shift of the DOS calculated by band theory to make comparison with experi- ment.4,92*106*143*1uAs we have discussed above, various corre- lation-related satellite features also appear in the valence- band photoemission, which are not reproduced by a band- theory approach.Thus, the situation appears to be very similar to that found for the magnetic insulators, and further theoretical work is needed to produce a model which can reproduce both the observed band dispersions and the satellite structure due to electron correlation. 6. Fermi-level Studies The transition of cuprate materials from a normal metallic state into a superconducting state depends ultimately on the behaviour of electrons close to the Fermi energy in the normal state.Thus there has been a great deal of interest in Fermi- level structure in photoemission, and the way in which this evolves on going into a superconducting state. We begin by discussing the normal state properties of this structure. 6.1 The Appearance and Size of the DOS at EFin the Normal State In most photoemission studies from oxide superconductors, a clear density of states at EF is observed at room temperature, whether the material is in ceramic, thin-film, or single-crystal f~rm.~?'~'The exceptions to this appear to be andNd2 -,C~,CUO~~~ YBa,Cu,O, -x.4 We discussed the possible surface deterioration of both of these materials in section 2.The non-appearance of a DOS at EFin the case of many room-temperature studies of YBa2Cu307 -x has been a particularly controversial topic., A reproducible Fermi-level DOS has been obtained most successfully for YBCO and its homologues by cleaving single crystals of the material at low temperat~re.~~*~~*~~~-~~~,~~~,~~~This led to the controversial suggestion, which we discussed earlier, that the bulk crystal decomposes losing oxygen (and hence becoming non-metallic) on warming from 20 K; this oxygen then is readsorbed on the surface, giving rise to structure in valence-band photoemis- sion at 9.5 eV and in the 0 1s core-level peak. However, as we pointed out above, there are grounds for suspecting that both the last two effects may be attributable to extrinsic contamination, related to the obviously very high reactivity of this surface.The issue is further confused by the fact that a number of authors have now obtained a finite DOS at E, from YBa2Cu307 -,at room or above the superconducting transition temperature of ca. 91 K.147 Interestingly, the '124' system, YBa2Cu40y appears to be considerably more stable, and a DOS at EF is observed at room temperature. 149 The low stability of these surfaces has meant that the majority of studies of the states close to the Fermi level have been carried out on the considerably more stable BSCCO systems. Our own findings (for example, in fig.5) of a small, but well defined DOS at EF in spectra of these materials is in broad agreement with those of many other groups (see ref. 4). The size of the Fermi edge discontinuity has attracted some attention. Interpreting our own data'2*21v49-'1 in terms of a very simple model that ignores variation in ionisation matrix elements and densities of accessible final states across the occupied valence band,150 the height of the Fermi-edge discontinuity implies that the cut-off in the density of states corresponds to ca. 0.5 states per eV cell for the BSCCO thin films shown in fig. 4 and 5 (T,=85 K). We have previously shown that the size of the Fermi-energy discontinuity is correlated with the superconducting transition temperature as expected in BCS the~ry.~.~' Even taking into account the simplicity of the model used, this is rather lower than typical estimates of 2-3 states per eV cell from band structure calculations.15' (Note, though, that these calculations reflect hypothetical electronic structure at 0 K, where the disconti- nuity is a step function; in photoemission we measure the centroid of the Fermi-level cut-off, which corresponds to half this value, assuming a Fermi-liquid model is appropriate.) Our observations are consistent with those of other workers who have attempted to quantify their spectra in this way; for example, Ark0 et al.lo6 observe a DOS at EF from YBa2Cu306.9 cleaved at low temperature which is two to five times lower than that expected from band structure calculations.A more accurate comparison of the intensity of the observed Fermi level structure with theory is rather difficult. However, very recent ARPES measurements from YBa2C~306.9147 indi-cate that the density of states close to EF below the supercon- ducting transition temperature is consistent with that predicted by BCS theory, assuming a Fermi-liquid description for the normal state. These ARPES measurements are dis- cussed further below. 6.2 Resonant Photoemission of the Fermi-level States Resonant photoemission at the Cu 3p and 0 2s thresholds has been used to investigate the atomic character of the states close to the Fermi level. The data relating to the Cu 3p resonance is relatively uncontroversial. A number of groups have now observed very little or no enhancement of these states at the Cu 3p threshold in Bi2Sr2CaCu208, YBa2Cu,07--x and La, -xSrxCu04.87,106~116~118~119Both Shen et ~1."~and Takahashi et al."' argue that the absence of either resonance or antiresonance behaviour at the Cu 3p threshold indicates that the states at EF have low Cu 3d character.However, List and ~o-workers~~.~~~use cross-section arguments to reach the conclusion that these states have ca. 35% Cu3d character in Bi,Sr2CaCu208 and ca. 20% Cu 3d character in YB~&U,O~-~. The authors argue that the non-appearance of intensity fluctuations at the Cu 3p threshold is not unusual, as an equivalent resonance has not been observed in Ni metal.'" We would point out that this is not the conclusion of Thuler et al., who observe antireson- ance at the Ni3p threshold for the states at EF in both Ni metal and NiO.lS2 In Cu metal, CuO and Cu20, where the 3d band lies at higher binding energy, these authors observe clear antiresonance at the band maximum, ca.3 eV binding energy.99 We would therefore argue that the absence of any J. MATER. CHEM., 1991, VOL. 1 resonant or antiresonant behaviour at the Cu 3p threshold precludes the possibility of the states at EF having appreciable Cu 3d character. Resonance photoemission data at the 02s threshold (ca. 18-22 eV) has aroused considerably more argument. Early observation of resonant enhancement of two Fermi-level features near the 0 2s threshold"' in Bi2Sr2CaCu208 led to their assignment as states having high 0 2p character.This conclusion was disputed by other groups who argued that 18 eV enhancement does not represent the true 0 2s threshold resonance beha~iour."~.' ', This initial confusion seems to have been caused to some extent by the misassignment of the Sr 4p and 02s core levels in the original paper by Takahashi et al."' More recently, Wells et al. reported anomalous resonant enhancement of features at 0.3,0.65 and 1.6 eV below the Fermi energy near 18 eV photon energy." Photoemission final-state effects were proposed as a possible origin." How-ever, recent work has demonstrated unambiguously that a significant part of this structure is attributable to Sr 4p and 0 2s core level peaks excited by a small second-order compo- nent of the exciting radiation (i.e.with energy twice that of the nominal selected photon energy).ls4 In further preliminary work carried out using a monochromator having a very low second-order contribution, this anomalous enhancement appears to be ab~ent.'~' It is clear that the issue of oxygen resonance at EF still remains an open q~estion,''~ and much more experimental work is needed to clarify this problem. However, studies of the empty states above EF by inverse ph~toemission,"~~'~~soft X-ray ab~orption''~ and electron energy-loss spectroscopies'60*'61 tend to indicate that there is indeed significant 0 2p character around the Fermi level in these hole-doped oxides. 6.3 Angle-resolved Measurements of Fermi-level States At the date of our last review, there had been very few investigations by ARPES of the states close to the Fermi leve1.56,'7v92.117 However, a number of high-quality angle-resolved studies have now been reported for BiZSr2CaCu,08,41.47,48,161~162YBa2Cu307 and-x409146*147 Nd2 -xCe,Cu04.56*'7 In general, these studies have revealed dispersive behaviour for the states close to the Fermi energy, with Fermi-level crossings at k values that are in general agreement with the predictions of band the-ory.40,47,56,57,146,162,161However, dispersion of the experimen- tal bands is considerably smaller than the theoretical di~persion.~~,~~,~~,'~~The deviation from band-theory predic- tions away from the Fermi level is attributed to strong correlation effects.Although no detailed resonance photo- emission data are yet available for the Fermi-level states in Nd, -,CexCu04, ARPES measurements in this case indicate that a band which disperses through the Fermi level at the r point has predominant Cu 3d character (in contrast to the dominant 0 2p character indicated for the hole-doped mater- ial~),~~,'~opening up the possibility of a Kondo-type reson- ance state. ARPES measurements for YBa2C~306.940.'46 appear to show very little intensity at the r point. This may go some way toward explaining the anomalously low DOS at EF often observed in studies of this material. 6.4 Measurements of the Superconducting Gap The possibility of measuring the changes in structure which occur close to the Fermi energy (on an energy scale of order k,T,) has led to an enormous effort to improve the resolution attainable in valence-band photoemission, and has also meant that such changes have to date only been observed in oxides with relatively high transition temperatures, T, J.MATER. CHEM., 1991, VOL. 1 (Bi2Sr2CaCu208 and YBa2C~306.9). In these studies, the superior resolution attainable with a conventional gas-discharge lamp has been particularly important; a resolution of 13.5 meV has recently made possible a study of the super- conducting transition in the conventional A1 5 superconductor Nb,Al( T,= 18.6 K).63 Thus, there seems no intrinsic reason why this type of study should not ultimately be possible for all the oxide superconductors.On cooling a normal metal, one expects to see merely a sharpening of the Fermi edge, in accordance with the Fermi- Dirac distribution f~ncti0n.l~~ However, in a BCS-like super- conducting state an energy gap in the quasiparticle density of states opens around EF,and in photoemission one expects spectral weight to be pulled below the Fermi energy as shown schematically in Fig. 7. Spectral profiles which conform to this picture have now been observed by several groups.47,48,147,153,162,165,166The data may be interpreted consistently in terms of an energy gap A = 24-30 meV, corre- sponding to 2A(0)/kBT, z 6.8-8, larger by almost a factor of two than the weak coupling value of BCS theory.One crucial experiment involves angle-resolved photo- emission above and below the superconducting transition temperature, as measurements at different points on the Fermi surface should allow for the detection of any basal plane anisotropy in the size of the superconducting gap. This places constraints on theories of the superconducting pairing mech- I I I I I I I I I I I I A 0.2 0.1 0 -0.1 -0.2 binding energy/eV Fig. 7 Schematic band shapes in photoemission spectra: (a) normal metal at 300 K; (b) normal metal at 0 K; (c) superconductor at 0 K assuming a BCS-like gap with A = 30 meV. Broadening effects due to limited experimental resolution are not included. In (c) empty states which could in principle be seen in inverse photoemission (if resolution could be improved) are shown by the broken curve (from Egdell et a1.4) anism, as models based on d-wave pairing predict gap ani- sotropy in the a-b plane.Recent ARPES measurements for Bi2Sr2CaCu208 have revealed a gap of A =24 meV (& 5 meV) for four distinct points in the rXYZ plane, indicating no gap anisotropy in the a-b plane.48 Wells et al. have used Au deposition on the (001) surface of this oxide in combination with ARPES above and below the superconducting transition temperature to enable them to distinguish the atomic charac- ter of the dispersing bands at the Fermi energy.41 They assign the band-crossing along the TM direction to states having Bi-0 character (i.e.to the surface Bi-0 plane of this layer material). The Bi-0 planes are thus fully metallic (at least when the crystal is oxygen annealed), and contribute to the DOS at EF.This is in line with our own observation of a Doniach-Sunjic lineshape in the Bi 4f and Pb 4f core-level features from Bi2Sr2CaCu208 materials and Pb-stabilised Bi2Sr2Ca2Cu3010, implying a significant Bi/Pb 6p partial DOS at EF.12’49--51A band crossing along TX and TY arises from the Cu-0 states.41 These observations are in general agreement with band-structure calc~lations.~~ Rather different results are obtained for surfaces of crystals which are not properly oxygen annealed, implying that, in these cases, the surface Bi-0 plane is not metallic and superc~nducting.~~ Data for YBa2C~306.9 have been recently reported, and represent the first gap study on this mate~ia1.l~’ (In previous work, the DOS at EF tended to disappear on warming the crystal above the superconducting transition, as we discussed in section 6.1.) Here, the BCS gap is measured in two distinct Fermi surfaces, one originating from the Cu02 planes, and the other from the hybridisation of the planes and the chains of the YBCO structure.Identical gaps of A=25 k2.5 meV are found in both cases.147 In the light of our ideas about electron pairing, these results are perhaps rather unexpected, and pave the way for much further detailed investigation by high-resolution photo- emission. 7.Summary The measurements referred to in the last section show us that photoemission has the potential to yield important infor- mation about the electronic structure of high-temperature superconducting oxides.The fact that so much controversy surrounds the interpretation of many of the experimental data is intimately connected to the currently indifferent quality of the available samples and our rather poor control over the degradation of these samples on the photoemission depth scale, both in atmosphere and in UHV. 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Kimizuka, T. Akahane, T. Chiba, S. Kimura, F. Minami, K. Siratori, M. Taniguchi, S. Ogawa and S. Suga, Phys. Rev. B, 1990,42, 7580. Z-X. Shen, J. W. Allen, P. A. P. Lindberg, D. S. Dessau, B. 0. Wells, A. Borg, W. Ellis, J. S. Kang, S-J. Oh, I. Lindau and W. E. Spicer, Phys. Rev. B, 1990,42, 1817. Z-X. Shen, P. A. P. Lindberg, C. K. Shih, W. E. Spicer and I. Lindau, Physica C, 1989, 162-164, 13 11. Z-X. Shen, C. K. Shih, 0.Jepson, W. E. Spicer, I. Lindau and J. W. Allen, Phys. Rev. Lett., 1990, 64, 2442. V. I. Anisimov, M. A. Korotkin and E. Z. Kurmaev, J. Phys. Condensed Matt., 1990, 2, 3973. 150 151 152 153 154 155 156 157 158 159 160 R. G. Egdell and M. D. Hill, J. Phys.C, 1983, 16,6221. C. Guillot, Y. Ballu, J. Paigne, J. Lecante, K. P. Jain, P. Thiry, R. Pinchaux, Y. Petroff and L. M. Falicov, Phys. Rev. Lett., 1977, 39, 1632. M. R. Thuler, R. L. Benbow and Z. Hurych, Phys. Rev. B, 1983, 27, 2082. R. Manzke, T. Buslaps, R. Claessen and J. Fink, Europhys. Lett., 1989, 9, 477. W. R. Flavell, J. H. Laverty, D. S-L. Law, R. Lindsay, C. A. Muryn, C. F. J. Flipse, G. N. Raiker, P. L. Wincott and G. Thornton, Phys. Rev. B, in the press. P. L. Wincott et al., unpublished work, to be presented at the 3rd International Conference on Materials and Mechanisms of Superconductivity, Kanazawa, Japan, July 1991. B. 0. Wells, Z-X. Shen, D. S. Dessau and W. E. Spicer, Phys. Rev. B, in the press. R. Claessen, R. Manzke, H. Cartensen, B. Burandt, T. Buslaps, M. Skibowski and J. Fink, Phys. Rev. B, 1989,39, 7316. T. J. Wagener, Y-J. Hu, M. B. Jost and J. H. Weaver, Phys. Rev. B, 1990,42, 1041. P. Kuiper, M. Grioni, G. A. Sawatzky, D. B. Mitzi, A. Kapitul- nik, A. Santaniello, P. de Padova and P. Thiry, Physica C, 1989, 157,260. N. Nucker, H. Romberg, X. X. Xi, J. Fink, B. Gegenheimer and Z. X. Zhao, Phys. Rev. B, 1989,39, 6619. 142 R. Bottner, N. Schroeder, E. Dietz, U. Gerhardt, W. Assmus 161 J. Fink, N. Nucker, H. Romberg, M. Alexander, P. Adelmann, and J. Kowalewsi, Phys. Rev. B, 1990, 41, 8679. J. Mante, R. Claessen, T. Buslaps, S. Harm, R. Manzke and M. 143 M. A. Korotkin, V. I. Anisimov, S. M. Butorin, V. R. Galakhov Skibowski, J. Less-Common Met., 1990, 164-165, 967. 144 145 146 and E. Z. Kurmaev, Mater. Lett., 1990, 10, 34. G. Drager, F. Werfel-and J. A. Leiro, Phys. Rev., 1990,41,4050. P. A. P. Lindberg, Z-X. Shen, W. E. Spicer and I. Lindau, Sur-Sci. Rep., 1990, 11, 1. J. C. Campuzano, L. C. Smedskjaer, R. Benedek, G. Jennings and A. Bansil, Phys. Rev. B, 1991, 43, 2788. 162 163 164 C. G. Olson, R. Liu, A-B. Yang, D. W. Lynch, A. J. Arko, R. S. List, B. W. Veal, Y. C. Change, P. Z. Jiang and A. P. Paulikas, Science, 1989, 245, 73 1. M. Grioni, D. Malterre, B. Dardel, J-M. Imer, Y. Baer, J. Muller, J. L. Jorda and Y. Petroff, Phys. Rev. B, 1991,43, 1216. G. K. L. Cranstoun, R. G. Egdell, M. D. Hill and R. Samson, 147 148 J. C. Campuzano, G. Jennings, N. River, A. J. Arko, R. S. List, B. W. Veal and A. P. Paulikas, Phys. Rev. Lett., submitted. P. D. Johnson, S. L. Qiu, L. Jiang, M. W. Ruckman, M. Strongin, S. L. Hulbert, R. F. Garrett, B. Sinkovic, N. V. Smith, R. J. Cava, C. S. Jee, D. Nichols, E. Kaczanoxicz, R. E. Salomon 165 166 J. Electron. Spectrosc. Relat. Phenom., 1984, 33, 23. Y. Chang, M. Tang, R. Zanoni, M. Onellion, R. Joynt, D. L. Huber, G. Margaritondo, P. A. Morris, W. A. Bonner, J. M. Tarascon and N. G. Stoffel, Phys. Rev. B, 1989, 39,4740. J-M. Imer, F. Patthey, B. Dardel, W.-D. Schneider, Y. Baer, Y. 149 and J. E. Crow, Phys. Rev. B, 1987, 35, 881 1. R. Itti, T. Miyatake, K. Ikeda, K. Yamaguchi, N. Koshizuka Petroff and A. Zettl, Phys. Rev. Lett., 1989, 62, 336. and S. Tanaka, Phys. Rev. B, 1990, 41, 9559. Paper 1/01714D; Received 9th April, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910100489

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(4), 503-506 Microstructural Studies on Polypyrrole Lars Madsen/ Bogumil N. Zaba,*b Marijke van der Sluijs/, Allan E. Underhill,b Kim Carneiroa a Danish Institute for Fundamental Metrology, Lyngby, Denmark Institute for Molecular and Biomolecular Electronics, University College of North Wales, Bangor, UK Scanning tunnelling microscopy has been used in an attempt to determine the surface roughness of polypyrrole films, prepared by electrodeposition on platinum electrodes. Films of nominal thickness 0.7 and 15 pm were used and compared with bare platinum surfaces using both scanning tunnelling microscopy and conventional scanning electron microscopy. The surface area for the thicker polypyrrole films was found to be ca. 2.5 times that of the underlying platinum surface.This should be contrasted with our previous, much higher, estimate of this parameter (40) from electrochemical measurements (J. Phys. D, 20, 1411). This discrepancy is discussed in terms of the accessibility of internal spaces to electrochemically reactive species. Keywords: Conducting polymer; Scanning tunnelling microscopy; Surface roughness; Modified electrode Conducting polymers are under consideration for a variety of applications in devices such as batteries, sensors and photovoltaic cells. In many of these applications, the polymer is in contact with electrolyte and must function as a charge- transferring electrode. It has previously been pointed out that the properties of conducting polymers are strongly influenced by their relatively high surface areas.’ In our previous work2 we have studied the polymer-electrolyte interface and have shown that electron-transfer reactions are enhanced at a polypyrrole-covered electrode compared with a bare platinum electrode.We ascribed this enhancement to two possible effects: (1) an increase in surface area caused by a greater degree of surface roughness in polypyrrole as compared to bare platinum; (2) an electrocatalytic effect, which enhances the rate of the electron-transfer reaction. By fitting values for model circuit parameters to impedance data over a wide range of frequencies, we were able to estimate the surface area effect. We estimate that the surface area of the polypyrrole is 40 times greater than that of the platinum.In order to evaluate the accuracy of such an estimate it is necessary to use an independent method to measure the surface area (or at least the ratio of the surface area of polypyrrole to that of platinum). In the present paper we present scanning electron micrographs and scanning tunnel- ling microscopy data from which we can estimate indepen- dently surface areas to compare with estimates based on electrochemical parameters. In addition the results presented here show that ‘thin’ polypyrrole films (<1 pm thick) are in fact patchy on the platinum surface, with areas of bare platinum exposed between islands of polymer. STM is a new powerful technique for investigating surfaces of conductors and semiconductor^.^ When a very sharp tip is brought close (ca.1 nm) to a surface there will be a significant overlap of the electronic wavefunctions of the tip and sample. Provided that there is a small voltage applied between the tip and sample a tunnelling current will start to flow. This current, is extremely sensitive to the distance between the tip and sample. This current is used, via a feed- back mechanism to keep the tip-sample distance constant. By using a piezoceramic it is possible to control the motion of the tip to within 0.01 A. It is now possible to scan the tip across the sample, and at the same time measure the vertical displacement, performed by the feedback mechanism, perpen- dicular to the sample. In this way we obtain a topographic map of the surface..~Yang et ~1 have published STM data on polypyrrole formation, but their data, which is at very high resolution and presented in the form of reconstituted images does not allow the type of area calculation which we make below. Fan and Bard’ have also presented some STM data, which is in a similar form to our own, and which is in good agreement with our data, but again, they did not attempt area calcu- lations of the type in which we are interested. Experimental The films prepared for this study were electropolymerised onto a platinum surface. The platinum electrodes used were wires of 0.5 mm diameter which were polished with 0.075 p alumina. The pyrrole monomer was distilled and dissolved in water at a concentration of 0.25 moldm-3 together with tetraethylammonium toluene-p-sulphonate at the same con- centration to act as the counterion during polymerisation.Polymerisation was carried out at constant potential (0.6 V with respect to a saturated calomel electrode) using a EG&G Model 273 potentiostat. A.c. impedance measurements were carried out as described in ref. 2 with potassium ferricyanide as the electroactive species in solution. Nominal polymer film thicknesses were calculated from the empirical relationship between charged passed (Q) in C cm-2 and film thickness (d) in pm first given by Jacobs et aL6 d=2.8Q. In the experiments reported here, the STM used was of our own design, consisting of an inchworm motor (Burleigh) for coarse adjustment and a piezoelectric tubescanner (EBL Com- pany) for fine movement.The tip was of electrochemically etched tungsten. The polypyrrole film samples were examined by STM directly on the Pt-wire surface. Results Fig. 1 shows SEM images and STM scans of the bare Pt-wire surface, which we used as the underlying electrode for polypyr- role film formation. The dislocations seen in the STM scans are reflected in the regular lines seen in the SEM image. The dislocations, which are highly regular, are of ca. 20nm in height, corresponding to some 60-70 platinum atoms. Such features cannot be resolved on the SEM and hence appear only as thin lines even at the highest magnification. Fig. 2 shows similar images and scans for an electrode on which a thin film (of ca.1 pm thickness) had been produced. It is apparent from both imaging techniques that a film of this thickness is in fact discontinuous on the surface. Areas J. MATER. CHEM., 1991, VOL. 1 Fig. 1 (a) Scanning electron micrograph of a clean, polished platinum-wire surface. The area of the specimen in view is 50pmx50p. (b) STM image of the same wire; the scale is in nm Fig. 2 Scanning electron micrographs of a ‘thin film’ of polypyrrole on a platinum surface. (a) Side view; (b) top view. Both views are of 50 pm x 50 pm areas. (c) and (d) show STM images of a similar polypyrrole film; the scale is in nm of bare platinum are seen as flat, featureless regions. Rising seen with a polypyrrole growth rising (within this field of from these flat regions are much rougher areas which in the view) to some 10 nm.These small features are therefore of a SEM images can be seen to have the characteristic features similar size to the dislocations in the underlying platinum. of growing polypyrrole films.7 Fig. 2(a) and 2(b) show these Fig. 2(d) shows an adjacent region in which the height of the features as side and top views respectively. Fig. 2(c) shows the polypyrrole film above the platinum surface has risen to ca. STM scans in which a flat area of bare platinum is clearly 70 nm above the adjacent platinum surface. J. MATER. CHEM., 1991, VOL. 1 In Fig. 3 similar images are again shown, this time for a thicker (10-20 pm) polypyrrole film. Both the SEM images and the STM scans now show no bare platinum surface. It appears that the growing polypyrrole structures have coalesced into a continuous, but very rough sheet.Again Fig. 3(a) and 3(b) show views from the side and top respect- ively. It is apparent that the features growing are as high as 10-15 pm. This is beyond the dynamic range of the STM tip and such gross features cannot be scanned. Nevertheless, Fig. 3(c) shows the surface of a thick polypyrrole film in an area which lacks such outgrowths. The large features that can be seen, which are some 50-60 nm in height, clearly rise from a surface which is much rougher than that seen in Fig. 2(c) or Fig. 2(d).The background from which the larger features rise is characterised by continuous fluctuations on the 10-15 nm scale.It is apparent from Fig. 3(c) and 3(d)that on an atomic scale, the film height can increase very abruptly (by ca. 10-50 nm in this case). In order to quantitate the surface area of the polypyrrole samples from the STM images, the digitised data for tip height were analysed in a portion of the image. Portions were chosen such that for a perfectly flat sample, the area (as defined by the x and y coordinates of the tip movement) would have been 2500 nm2. Fig. 4(a) shows a typical result for a bare platinum surface adjacent to an area in which a polymer film is growing. A portion of the bare platinum area is marked out for area determination. The actual surface-area determined from a point-by-point analysis of the heights was 2754 nm2, giving a roughness ratio value of 1.10. Fig.4(b) shows a similar marked segment from an area in which polypyrrole was obviously growing on the electrode. Here the results of an area analysis showed the surface area to be 6009 nm2, giving a roughness ratio value of 2.40. For 14 such segments taken from two differing electrodes, the average surface area found was 6700 nm2, corresponding to a surface roughness value of 2.68 & 0.26 (standard deviation). Discussion Our previous electrochemical characterisation of the polypyr- role-electrolyte interface suggested that the surface area of a thick film of polypyrrole was 40 times that of a bare platinum surface. This estimate was based on the values obtained for the double-layer capacitance and the Warburg Impedance,* both of which are linearly dependant on the surface area at which the electrochemical reaction takes place.For a thin film (0.7 pm) there was a much smaller apparent increase in surface area. The present SEM and STM experiments support the notion of an increased surface area. It is now apparent that the films which we treated previously as ‘thin’ were in fact partially covering the platinum surface. It is not surprising, therefore, that the increase in surface area for these films was small. There is an obvious discrepancy between the surface area increase as measured electrochemically (x 40) and that found by the STM (x2.2). This is probably due to the fact that the electrochemical ‘probe’, potassium ferricyanide, a small inorganic ion, is able to enter the ‘internal’ water-filled spaces of the polymer, whereas the STM tip is able to measure only the surface features which are directly beneath it (see Fig. 5).There remains an interesting feature of the electrochemical analysis which cannot be explained by surface-area effects alone. This is the effect of the polypyrrole coating on the charge-transfer resistance for the electrochemical reaction, Fig. 3 Scanning electron micrographs of a thick film of polypyrrole on a platinum surface. (a) Side view; (b) top view. Both views are of 50 pm x 50 pm areas. (c) and (d)show STM images of a similar polypyrrole film; the scale is in nm J. MATER. CHEM., 1991,VOL.1 Fig. 4 STM images of electrode surfaces selected for area calculation. The area marked with a heavy line was calculated. (a) Bare platinum surface; (b) polypyrrole-covered surface. In each case a perfectly flat surface would have given a surface area of 2500 nm2 Fig.5 Illustration of STM tip movement over the surface of a polypyrrole film. The tip cannot enter the ‘internal’ surfaces shown at (4,(b)and (4 which falls by some three to four orders of magnitude for a thick film compared with bare platinum, i.e. by a factor much greater than the surface-area increase, however this is meas- ured. This is evidence that the charge-transfer reaction is catalysed by the polypyrrole. It remains to be seen how general this catalytic effect might be. References 1 M. S. Wrighton, Science, 1986, 231, 32. 2 M. J. van der Sluijs, A. E. Underhill and B. N. Zaba, J. Phys. D, 1987, 20, 1411. G. Binning, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett., 1982, 49, 57. 4 R. Yang, D.F. Evans, L. Christensen and W. A. Hendrickson, J. Phys. Chem., 1990,94, 61 17. 5 F-R. Fan and A. J. Bard, J. Electrochem. Soc., 1989,136,3216. 6 R. M. C. Jacobs, L. J. J. Janssen and E. Branderecht, Rec. Trau. Chim. Pays-Bas, 1984,103,275. 7 R. Qian, J. Qiu and D. Shen, Synth. Met., 1987, 18, 13. 8 J. R. Macdonald, Electround. Chem. Interface Electrochem., 1974, 129,115. Paper 0/04572A;Received 1 1 th October, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100503

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(4), 507-509 Semi-crystalline Alkali-metal Salt Complexes with Poly(o1igooxyethyleneoxy-I,2=phenylene)s Jonathan D. Hague and Peter V. Wright* School of Materials, University of Sheffield, Hadfield Building, Mappin Street, Sheffield S I 3JQ UK A series of poly(oligooxyethyleneoxy-l,2-phenylene)shaving the formula where n=2, 3 and 5, have been synthesised with molecular weights of ca. 10000. Although the pure polymers are fully amorphous, thermal analysis and X-ray scattering show that the polyether with n=2 forms a well organised complex with NaSCN, with a stoichiometry of three oxygens per Na+ ion. The melting temperature of the new complex (187 "C) is similar to that of the analogous complex with poly(ethy1ene oxide) (PEO). Comparison with the PEO-NaSCN complex suggests that the new polymer may enclose the Na+ ions to form a 2, helix with 18 bonds per repeat.A plausible 2, helical structure for this new polyether complex has been computed. A Crystalline complex between the polyether with n= 5 and NaSCN is also observed but no organised complexes with the n=3 system or with Nal, LiCIO, or KSCN were obtained with any of the polyethers. Keywords: Polyether-alkali salt complex; Crystallinity; Helical structure; Computed conformation Salt complexes with high-molecular-weight poly(ethy1ene oxide) (PEO) are well known as amorphous phase materials for ionic conduction in so-called 'polymer electrolyte^"-^ and we have studied the organised phases of these materials with a variety of inorganic and organic anion^.^-^^ How-ever, while there have been several reports of amorphous complexes based on high-polymer ligands other than PE0,14 we are not aware of organised complexes based on other synthetic high-polymer ligands, with the notable exception of the sodium-ion complexes with poly(ethy1ene imine).159" Chatani and co-workers have reported crystallographic structures for PEO-NaI17 and PEO-NaSCN." With Na' (and presumably with Li' and Ag'), PEO forms a 2' helix with a repeating unit in which six EO units having conformations (~tgttgttg)~enclose two cations (ie.the stoichi- ometry of the crystalline phase, x =[ether oxygen]/[M '1 = 3).Here, the gauche conformation (g and gx +120") is adopted by the C-C bonds and the trans (tz0")by the C-0 bonds.In order to accommodate external anions of a range of sizes, which are stacked alongside the helix and which interact with the enclosed cations, this helical 'spring' can adjust its fibre repeat distance from ca. 7.1 A (phen-olate") to ca. 8.4A (perchlorate2'). The thickness of the aromatic nucleus (3.2-3.6 A) is readily accommodated so that radical anions, giving electronic condu~tivity,~,'~ or mesogenic and other aromatic anions8*'0-12~21may be stacked alongside the helix with their planes approximately normal to the helix axis. In order to explore further the possibilities of molecular organisation in these high-polymer complexes with stacked appendages, we have embarked upon a study of the complexation behaviour of polyethers, having substituents attached to, or included in, the sequence of co-ordinating atoms along the chain.In this paper, we report on the preparation and complexation of poly(oligooxyethy1eneoxy-1,2-phenylene) chains having the formulae 0 I where n=2, 3 and 5. These polymers are coded P2EOP, P3EOP and PSEOP, respectively. If the 21 helix of PEO- Na' is the linear analogue of the cyclic ether 18-crown-6, the corresponding structure for P2EOP-Na' may be analogous to dibenzo-18-crown-619 and could also be a 2, helix in which successive aromatic rings protrude on either side at each 180" of turn. For PSEOP, a helix analogous to benzo-18-crown-6 having 1' symmetry with an aromatic ring protruding on one side only within each fibre repeat should be formed by complexation with salts of the smaller cations mentioned above.P3EOP was prepared in order to investigate the possibility of forming complexes analogous to the crystalline PEO-K+ complexes.' Although the latter have an unknown structure they have crystalline stoichiometry x =4. P3EOP also incorporates four oxygens per repeat and may form an analogous crystalline complex. Experimental Diethylene glycol, triethylene glycol, pentaethylene glycol (99Y0 purity) and catechol were supplied by Aldrich and the ditoluene-p-sulphonates of the glycols were prepared by reac- tion with twice the molar proportions of toluene-p-sulphonyl chloride in pyridine. All monomers were recrystallised.Poly(o1igooxyet h yleneox y-1,2-phenylene)s were prepared from solvent-free equimolar mixtures of an ethylene glycol ditoluene-p-sulphonate and catechol, with NaOH in slight stoichiometric excess (2.1 molar proportion). These compo- nents were finely ground and intimately mixed under dry nitrogen and transferred to a reaction tube. The mixture was heated and pumped under vacuum at 120 "C for ca. 48h. The 508 reaction mixture was first extracted with chloroform and filtered to separate the polyether products from the sodium toluene-p-sulphonate product. The high-molecular-weight polymer was then separated from low-molecular-weight and cyclic material by extracting the latter and precipitating the polymer in boiling ethanol. The yields of high polymer were 30-50%. All three of the polyether materials were amorphous highly viscous liquids and their molecular structures were character- ised using 'H NMR, transmission IR spectroscopy and gel- permeation chromatography, and the thermal behaviour was investigated using differential thermal analysis.The 'H NMR data are in good agreement with those of the corresponding cyclic ethers,22 with small differences between the chemical shifts for the methylene protons in the ring and in the cyclic molecules. The NMR data suggest that the polymers were more than 96% pure. This was supported by the IR data for the linear polyethers which differed slightly from the corre- sponding cyclic spectra23 only in the band at 1050cm-' which appeared at lOOOcm-' in the cyclic (attributed by Peder~en~~to CH2 wag or twist).Molecular size was investi- gated using gel-permeation chromatography with Ultrastyra- gel columns (Waters Associates) and tetrahydrofuran solvent. Complexes with alkali salts were prepared by dissolving the salt and polymer in acetonitrile. In view of the established stoichiometry of the crystalline complexes of the smaller cations with PEO (x =[ether oxygen]/[M+] =3), mixtures with x=2, 3 and 5 were prepared. The complexes were analysed by WAXS, DTA and hot-stage polarised-light microscopy. Discussion The three poly(oligooxyethy1eneoxy-1,2-phenylene)s were fully amorphous. Their glass-transition temperatures, as deter-mined by DTA, were found to be 6, -13 and -26 "C for P2EOP, P3EOP and PSEOP, respectively.The polyethers eluted in tetrahydrofuran at GPC peak maximum elution volumes corresponding to polystyrene standards of molecular weight 7000- 10 000. However, significant fractions of material having 'polystyrene molecular weights' >20 000 were also present in 'most probable' distributions. A crystalline product was precipitated from acetonitrile solutions of P2EOP in the presence of NaSCN over a period of ca. 1 h. The WAXS spectra of P2EOP-NaSCN (x=3), PEO-NaSCN (x=3) and NaSCN pure salt are shown in Fig. l(a), (b)and (c). These spectra demonstrate that a novel crystalline polymer complex has been formed. Mixtures having a greater NaSCN content (x <3) showed WAXS tracings with reflections characteristic of the pure salt, supporting the assumed stoichiometry x =3 for the P2EOP-NaSCN crystal- line lattice.In Fig. l(d), (e) and (f),DTA traces of P2EOP-NaSCN for x =2, 3 and 5, respectively, are shown. The traces in Fig. l(d), (e) and (f)show well defined endotherms at 169, 177 and 187 "C, respectively; hot-stage polarised-light microscopy indi- cated that these endotherms correspond to order-isotropic transitions. (Dibenzo- 18-crown-6-NaSCN melts at 230 0C.23) These results thus confirm the formation of a novel complex, and it is of interest that these melting temperatures are well within the temperature range observed for PEO-NaSCN and PEO-NaI complexes.6 In the latter cases also the complexes with slight salt deficiencies gave higher melting temperatures which were attributed6 to a pronounced propensity for lamel- lar thickening in the salt-deficient samples.The failure of the P2EOP-NaSCN sample with x=2 to undergo lamellar thick- ening owing to salt-stabilised crystal surfaces may also account for the lower melting endotherm (169 "C) in this sample. A J. MATER. CHEM., 1991, VOL. 1 I I I I I 1 I I30 0 30 60 90 120 150 180 Tl°C Fig. 1 (a) Wide-angle X-ray scattering (WAXS) spectrum for P2EOP- NaSCN, x =3; (b)WAXS spectrum for PEO-NaSCN, x =3; (c)WAXS spectrum for NaSCN. (d)Differential thermal analysis (DTA) tracing for P2EOP-NaSCN, x =2; DTA tracing for P2EOP-NaSCN, x =3; DTA tracing for P2EOP-NaSCN, x =5. (e)DTA tracing for PSEOP- NaSCN, x=4.(h) and (i) are DTA tracings for pure P5EOP and P2EOP, respectively distribution of lamellar dimensions is suggested by the broader endotherms of the lower melting compositions. Fig. l(d) also reveals an endotherm at 28 "C indicating the presence of excess of NaSCN when x=2, and supporting the stoichi- ometry x =3 for P2EOP-NaSCN. The lower-temperature region of the DTA traces for P2EOP-NaSCN reveals a glass- transition temperature at 25-30 "C and a first-cycle endo- therm at ca. 60 "C. The latter is reminiscent of a similar endotherm in PEO-salt complexes, but in P2EOP-NaSCN this cannot be ascribed to the uncomplexed polyether which is amorphous. This endotherm, which is removed by thermal cycling, may perhaps correspond to disorganised, less stable regions in the solution-deposited sample.However, whereas P2EOP-NaSCN x =3 powdered samples are difficult to shape, the salt-deficient x=5 material is malleable in the presence of some solvent and may be rolled into oriented films or filaments. However, we have been unable as yet to obtain fibre photographs of sufficient quality for structural determination. The similar stoichiometry and melting temperatures of the PEO-NaSCN and P2EOP-NaSCN systems strongly suggest that the two complexes may have similar molecular and unit- cell structure, in accord with our molecular-design strategy J. MATER. CHEM., 1991, VOL. 1 Fig. 2 A computed helical model for P2EOP-Na' being a plausible structure for P2EOP-NaSCN, after the structure for PEO-NaSCN (Form I) determined by Chatani and co-worker~.'~ (This proposed structure for P2EOP-NaSCN is not based on any crystallographic experimentation.) Filled spheres are carbon atoms; small open, shaded spheres are oxygen atoms; (hydrogen atoms are not shown).The cations would be located near the helix axis for development of the 2, helix of the PEO-Na' systems. Using the computational procedures for helical parameters of Miya~awa,~~a plausible 21 helix for P2EOP-NaSCN with a similar fibre repeat distance (7.32 A) to that of PEO-NaSCN is readily computed, as shown in Fig. 2. The oxygen-to-axis distances (2.3-2.5 A) are within the range observed for PEO- Na' systems. The conformations of the nine skeletal bonds of the half-repeat are ttgttgttcis, the last being the fixed C-C bond of the 1,2-phenylene ring.In this sequence, small tor- sional deviations from conformational-energy minima are required to compensate for the fixed, planar configuration of the aromatic C-C bond. This fixed C-C bond would exert a constraint on the ability of the helix to expand along its axis. This may be a contributory factor in the failure which we have encountered to prepare P2EOP-Na' complexes with the larger I -anion. However, preliminary DTA investi- gations indicate that P2EOP-Na' complexes with various phenolate anions. SiddiquiIg observed that PEO-sodium phenolate has a short fibre repeat distance of 7.1 A which is similar to that of PEO-NaSCN. All attempts to prepare organised alkali-metal salt com- plexes with P3EOP mixtures were unsuccessful.For Na' and Li', this is in accord with expectation for the 2, helix having x =3. However, mixtures with KSCN (which forms crystalline complexes with PEO having x=4) were also prepared with P3EOP. The structure of PEO-KSCN is unknown but if PEO-K+ simply forms a 2, helix, analogous to PEO-Na' but with greater radius and fibre repeat distance, 8 A,20,25the failure of P3EOP-KSCN to crystallise may, at first sight, be surprising and offer some support to other models.20 A limited quantity (x>4) of KSCN dissolved in P3EOP so as to raise the glass-transition temperature from -13 to 25 "C. However, an organised phase in the PSEOP-NaSCN sys- tem may be readily observed by polarised-light microscopy.DTA evidence for this phase, which melts at 134 "C, is shown in Fig. l(g) and a distinctive WAXS pattern may be observed. Although it seems likely that the organised phase may have a stoichiometry x =3 with a PEO-Na+-like helix, salt separ- ation was observed in mixtures having x<4. Further work is required to establish the stoichiometry of this phase or to obtain evidence for a 'lop-sided' 1, helical adduct. Further investigations of organised, linear high-polymer- salt adducts having structures analogous to cyclic ('crown') oligomers are in progress. We are grateful to the University of Sheffield and Unilever for a research grant (J. D. H.), and to Mr. P. Hempstead, Department of Chemistry, University of Sheffield for assist- ance in computer drawing.References 1 D. E. Fenton, J. M. Parker and P. V. Wright, Polymer, 1973, 14, 589. 2 P. V. Wright, Br. Polym. J., 1975, 7, 319. 3 M. B. Armand, J. M. Chabagno and M. Duclot, in Fast Zon Transport in Solids, ed. P. Vashisha, J. N. Mundy and G. K. Shenoy, North Holland, New York, 1979, pp. 131-136. 4 Polymer Electrolyte Reuiews, ed. J. R. MacCallum and C. A. Vincent, Elsevier, London, 1987, vol. 1. 5 Polymer Electrolyte Reviews, ed. J. R. MacCallum and C. A. Vincent, Elsevier, London, 1989, vol. 2. 6 C. C. Lee and P. V. Wright, Polymer, 1982, 23, 681. 7 D. R. Payne and P. V. Wright, Polymer, 1982, 23,690. 8 J. A. Siddiqui and P. V. Wright, Polym. Commun., 1987,28, 7.9 J. A. Siddiqui and P. V. Wright, Polym. Commun., 1987, 28, 89. 10 B. Mussarat, K. Conheeney, J. A. Siddiqui and P. V. Wright, Br. Polym. J., 1988, 20,293. 11 P. V. Wright, Polymer, 1989, 30,1179. 12 P. V. Wright, in Polymer Electrolyte Reviews, ed. J. R. MacCallum and C. A. Vincent, Elsevier, London, 1989, vol. 2, ch. 2. 13 J. A. Siddiqui and P. V. Wright, Faraday Discuss. Chem. SOC., 1989, 88, 113. 14 J. M. G. Cowie, ref. 4, p. 69. 15 C. S. Harris, D. F. Shriver and M. A. Ratner, Macromolecules, 1986, 19, 987. 16 C. K. Chiang, G.T.Davies, C.A. Harding and T.Takahashi, Macromolecules, 1985, 18, 827. 17 Y. Chatani and S. Okamura, Polymer, 1987, 28, 1815. 18 Y. Chatani, S. Okamura and Y. Fujii, Polym. Preprints, 1989, 30(1), 404. 19 J. A. Siddiqui, PhD Thesis, University of Sheffield, 1989. 20 T. Hibma, Solid State Zonics, 1983, 9, 10, 1101. 21 J. Patel, P. V. Wright, R. Orr, D. W. Bruce, D. A. Dunmur and P. M. Maitlis, Handbook of British Liquid Crystal Society Annual Conference, Bristol, April 1990, poster P23. 22 P. Live and S. I. Chan, J. Am. Chem. SOC., 1976,98, 3769. 23 C. J. Pedersen, J. Am. Chem. SOC.,1967, 89, 7017. 24 H. Sugeta and T. Miyazawa, Biopolymers, 1967, 5, 673. 25 J. M. Parker, P. V. Wright and C. C. Lee, Polymer, 1981, 22, 1305. Paper 0/04608F; Received 15th October, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100507

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(4), 511-515 511 X-Ray Diffraction Characterization of Iridium Dioxide Electrocatalysts Alvise Benedetti,*" Stefan0 Polizzi," Pietro Riello," Achille De Battistib and Andrea Maldottib a Dipartimento di Chimica Fisica, CaIle Larga S. Marta 2137, 1-30123 Venezia, Italy Dipartimento di Chimica dell'Universita and Centro di Studio sulla Fotochimica e Reattivita degli Stati Eccitati C.C. del C.N.R., via L. Borsari 46, 1-44100 Ferrara, Italy An X-ray diffraction (XRD) line-broadening analysis of coatings of pyrolytic iridium dioxide supported on amorphous silica microbeads is reported. The influence of the temperature of pyrolysis and of the annealing treatments on the average crystallite sizes of the IrO, and Ir phases have been studied in detail.The values obtained for IrO, are lower than those obtained from an analgous RuO, system. However, the analysis suggests that in both cases the crystalline phase grows within an amorphous microcrystalline environment where impurities are mostly segregated in the amorphous phase and then released to the atmosphere. The lattice parameters are also calculated. Compared with the reference values, slightly greater values are obtained both for IrO, and for Ir. Keywords: X-Ray diffraction ; Line broadening; Iridium dioxide; Electrocatalyst The industrial interest of pyrolytic oxides of ruthenium and iridium has stimulated many works on the electrochemistry of these synthetic materials since the early 1970s. The literature on fundamental and industrial aspects has been thoroughly discussed in several and the influence of annealing temperatures of ruthenium oxide electrodes on the microstruc- tural features was recognized by Pizzini et a!.in one of the first papers.' The same aspect was implicitly taken into account in other pioneering where the electrochemi- cal charging mechanism of this material has been studied. Analogous investigations were carried out later on iridium dioxide, for which precursor paths and microstructural fea- tures have also been discu~sed.~-'~ Generally, the microstruc- tural texture of RuO, and IrO, affects their electrochemical properties in different ways. The surface area or the roughness factor substantially reflects the crystallite size which is related to the 'degree of dispersion' of the material.In this sense, not only the catalytic activity, but also the resistance to corrosion under applied potential may be modified by changing the microstructure of the oxides. This aspect, directly correlatable with the preparation path, is probably the most widely considered in the literature. 192,9 Moreover, the presence of lattice distortions can also exert a basic effect on the chemical behaviour of the material: a large number of microstructural defects can be related, in principle, to a larger number of active sites. In this context the analysis of the line-broadening of XRD peaks has proven to be extremely useful for the microstructural characterization of Crystalline phases.However, the simplified approaches generally used in the characterization of Ru02 and IrOz films,'T6 which are based on the measurement of the apparent crystallite size calculated by the Scherrer equa- tion, have limited the potential of this approach. In the present work a careful XRD line-broadening analysis on coatings of pyrolytic iridium dioxide supported on amorph- ous silica microbeads is presented. In a previous study, the formation of iridium metal, together with the expected oxide was discussed on the basis of XRD phase identification and thermoanalytical data." In the same work an analysis of the effect of temperature of pyrolysis and temperature and dur- ation of the annealing treatments was also carried out. The complexity of the transformations that the film undergoes during the different thermal treatments"." has prompted a more detailed XRD investigation.In particular, a Fourier line-broadening analysis of XRD peak profiles, and, if possible, the Warren-Averbach (W-A) method" allow a careful microstructural analysis of the different phases in order to obtain average crystallite sizes, crystallite size distributions and lattice distortions possibly present. Usually this approach, for which a complete knowl- edge of the peak profiles on a large portion of reciprocal space is necessary, is limited by the presence of very broadened peak envelopes and/or of an important or complex back- ground. In order to overcome this problem a 'profile fitting technique' has been used, in which one fits the experimental envelopes of peaks with analytical functions [one for the background and two for each profile (Kal +KQ,)]utilizing a minimization routine. Previous tests on different materials 16v1 showed that it is possible to obtain very reliable results in spite of some simplifications and theoretical assumptions.Experimental and Methodology Samples Preparation The samples investigated were prepared by painting amorph- ous silica microbeads (spherosil) with isopropyl alcohol solu- tions of hydrated iridum(Ir1) chloride, followed by evaporation of the solvent under controlled conditions." The final coatings were obtained by pyrolysis at 673 K for 2 h, and by successive annealing at 673, 773 and 873 K for periods of 15 h and at 673 K for 30 and 58 h, respectively.Both pyrolysis and annealing were performed under dry oxygen. In such a way we were able to analyse the influence of the temperature and of the time of annealing on the microstructural features of the developed components. Wide-angle X-Ray Scattering (WAXS) Profiles were collected using a Philips vertical goniometer connected to a highly stabilized generator. Cu-Ka Ni-filtered radiation, a graphite monochromator and a proportional counter with a pulse-height discriminator were used. A step- by-step technique was employed; steps were of 0.05"with an accumulation counting time of 100 s per angular abscissa. Line-broadening Analysis For each peak, two pseudo-Voigt functions (linear combi- nation of a Gaussian profile with a Cauchian one) related by the well known crystallographic constraints on peak intensit- ies, angular positions and half-width at half-maximum were used in order to take account of the Ka,-Ka2 doublet.16 For the background, a straight line or a polynomial of the third degree was used, depending on the angular range under consideration.The minimization was carried out using a modified version of the Simplex method.18 The optimized pseudo-Voigt functions relevant to the Ka, of the experimental broadened profile were deconvoluted from the instrumental and spectral effects in order to obtain the corrected Fourier transforms A(L)(Lis the variable in direct space).Ig According to Warren and Averbach the coefficients A(L)(or the Fourier transforms) are the products of size coefficients A&) and distortion coefficients Ad&): 4L)=As(ww) where A&) is independent of the peak order and A&) is dependent upon the order of the diffraction peak.If at least two orders of reflections of the same plane family are known, by means of the following expression: In A (L,l/dhkr2)=In A&) -27r2(~2(L))L2/dhk12(1) where hkl are the Miller indices, (c2(L))is the squared microstrain averaged over all distances L, and dhkr is the interplanar spacing, it is possible to separate the crystallite size contribution from that of the lattice distortion. Further- more, it has been shown that the volume-weighted crystalline size distribution, p,(L), obeys the following relation P,(L) LCd2As(L)/dL2I (2) while the volume-weighted average crystallite size (D),calcu-lated perpendicular to the (hkl)planes is given by m W (3) IJ 0 J. MATER.CHEM., 1991, VOL. 1 Only the peaks in the range between 10 and 50" in 28 and the 202 Ir02 peak at ca. 73" were examined. For the other profile envelopes it was not possible to obtain an unambiguous and stable solution for the very broadened constituent reflec- tions. Moreoever, owing to the variable values of the unit cell parameters of the crystalline phases considered (see Table 3, later), it was not possible to fix, a priori, some 'known' parameters in the fitting procedure, at the position of the maximum of the peaks, in order to diminish the number of parameters to minimize.In order to describe the decreasing background scattering of the amorphous silica, a polynomial of the third degree was considered for the 28 range ca. 24-45". This simplification was necessary in the fitting procedure because no suitable scaling factor of the 'true' experimental amorphous back- ground, scattered by the microbeads, could be found. Even if this fact could be explained by interference effects, the contri- bution of a non-crystalline phase containing Ir cannot be excluded. For narrower ranges a straight line was found sufficient to reproduce the background. Results The spectra of the five different samples are characterized by a big halo centred at about 28=22", due to the amorphous silica, and by the presence of two different crystalline phases, Ir02 (rutile structure) and Ir.As already described in our previous paper, the higher the annealing temperature the smaller the amount of iridium present, whereas the amount of iridium was almost constant for different times of isothermal treatment. lo In Fig. 1 we report, as an example, the X-ray pattern of the sample annealed at 673 K for 30 h. From the figure, and inset, it is evident that the presence of the background and the overlaps makes the use of the 'profile-fitting technique' necessary in order to apply Fourier X-ray analysis. In this way, it has been possible to study the 110, 101, 200 and 202 reflections relevant to the Ir02 phase and the 111 and 200 reflections relevant to the Ir phase.In Fig. 2 the fitting of the 10 20 30 40 50 2e/o Fig. 1 XRD pattern of the sample annealed at 673 K for 30 h. The inset shows the best-fitted envelope of the 200 IrO, and the 111 Ir profiles with the corresponding residuals J. MATER. CHEM., 1991, VOL. 1 i6 N e X u) -C 3 $4.L .-u) Q,c. .-C 2 24 30 36 42 261" Fig. 2 Experimental data (dots) and fitting (continuous line) of the envelope of the amorphous component; the 110, 101 and 200 Ir02 and 11 1 Ir profiles relevant to the sample annealed at 673 K for 58 h. Residuals are reported below IrOz 110, 101, 200 and the Ir 111 profiles, together with the amorphous component, is shown for the sample annealed at 673 K for 58 h.The index used was Rwp ={Twi[Ifit(28i)-zobs(28i)12 /icwizobs(28i)2 Yl' where wi= 1/[Iob&?8i)]is the weight assigned to the ith observation, Zfit(20i)and IObs(2Oi)are the calculated and observed profile intensities at 28,. The obtained R,, values (<2.0%), indicate that the fitted analytical envelope repro- duces the experimental X-ray pattern in a satisfactory way. The W-A method has been applied to the 101 and 202 IrOz peak profiles. In Fig. 3 the relevant Fourier coefficients for the sample annealed at 873 K for 15 h are shown. The inset shows the corresponding Warren-Averbach diagram. The slopes of the straight lines in this diagram are related to the strain parameter, according to eqn. (1): negative values indicate the presence of strain.The slopes shown in Fig. 3 are 0.3 .. h-A 0.9. -q 011.5:- I 1. 0.15 0.3 0.45 1ldkl 0 40 80 LIA Fig. 3 A(L) Fourier coefficients for the 101-202 pair of reflections for the sample annealed at 873 K for 15 h. Inset: W arren-Averbach diagram in which the logarithms of A(L) are reported as functions of l/dhkr2(AL=3.56 A) nearly zero within the errors involved in this procedure, so that this kind of disorder can be considered negligible. Similar results have been obtained for the other samples. This confirms that no strain is present in the Ir02 crystalline phase. Table 1 shows the values of the volume-weighted average crystallite sizes obtained from each peak. The Fourier analysis of profiles 110 and 200 is further evidence of the absence of strain.In fact, for these two profiles, although all the broaden- ing has been attributed only to the crystallite size, the resulting average crystallite sizes are similar to the ones obtained by applying the W-A method. The fact that the crystallite sizes calculated from the different analysed profiles of the same sample are very similar, indicates that Ir02 crystallites can be considered equiaxial in three-dimensional space. The last column of Table 1 reports the average of the volume-weighted average crystallite sizes of the four profiles, D.The duration of the annealing treatment has no effect on the Ir02 crystallite size. As far as the effect of the annealing temperature is concerned, the increase of (D), from 673 to 873 K can be considered larger than the relevant estimated error (10-15%).The role of the different pyrolysis temperature is shown in Fig. 4. The volume-weighted crystallite size distribution obtained according to eqn. (2) for the 101 reflection for the sample annealed at 873 K [curve (a)] and that obtained directly by pyrolysis of the precursor salt at 873 K [curve (b)] are reported. The different pyrolysis temperature affects the crystallite size quite significantly. The behaviour of the iridium crystalline phase under heat treatment is rather different. In Table2 the (D), values are reported. Both the time of isothermal annealing and the annealing to higher temperature have the effect of increasing the iridium crystallite size.The difference obtained for the two considered crystallographic directions could be due to a preferential direction of growth of the crystallites. However, owing to the lack of further peaks for analysis and to the large amorphous contribution the problem has no definite solution. The application of the fitting procedure also has the advan- tage that it supplies the values of the position of the peak maximum with very high precision, allowing one to obtain accurate lattice parameters. Table 3 shows the values of the IrOz and Ir unit-cell parameters, obtained with a computer J. MATER. CHEM., 1991, VOL. 1 Table 1 Average crystallite sizes calculated from different hkl profiles relevent to the IrO, phase average crystallite size (D>,/A annealing temperature/K time/h W-A pair 101-202 110 200 D/A 673 15 37 41 43 40 673 30 40 42 36 39 673 58 38 48 43 43 773 15 41 46 51 46 873 15 54 55 45 51 thermal and the annealing treatment, neither crystalline phase is easily reproducible owing to the presence of substitutional ions in the structure.Discussion and Conclusions The relatively low (D),values of the IrO, crystallites yield a larger dispersion of the crystalline part of the IrO, catalyst compared to that obtained for an analogous RuO, system." In this case, an average crystallite size up to 1500 A was found under the same preparation conditions. However, in order to propose a possible explanation of this different behaviour, it is necessary to consider other aspects of anal- ogous IrO, systems reported in previous papers.Chemical analysis," for instance, indicates that larger amounts of residual water and chlorine species are present in L/A pyrolitic iridium oxide films compared with ruthenium oxide Fig. 4 Volume-weighted crystalline size distributions p,(L),calculated films prepared at the same temperature from the respective from the 101 reflection, relevant to the sample annealed at 873 K for hydrated chlorides." Thermal analysis confirms this result. lo 15 h [curve (a) (D), =51 A] and to the sample obtained directly by Hydrogen profiles obtained by the 1H('5N,ay)'2C nuclear pyrolysis of the precursor salt at 873 K [curve (b) (0),=80 A] reaction for RuO, and IrO, films" indicate that hydrogen species are stored in larger amounts in the IrO, samples. The Table 2 Volume-weight average crystallite sizes calculated from the average oxygen stoichiometry, found by Rutherford backscatt- 111 and 200 profiles relevant to the Ir phase ering spectrometry was 2.5 for IrO, and 1.8 for RUO,~~~~~.average crystal-Moreover, in these systems, the RuO, and the IrO, crystalline lite size phases grow from a pre-existing amorphous phase,14 and for (D>,lA the RuO, with lattice parameters closely related to the tem- perature of annealing and chloride ~ontent:~." the higher the annealing temperature/K time/h 111 200 temperature the closer the lattice parameters are to the refer- ence values.On the basis of these results, it seems reason- 673 15 94 72 able to assume that the crystalline rutile phase, common to 673 30 115 72 673 58 124 87 both RuO, and IrO,, grows within an amorphous microcrystal- 773 15 113 79 line environment where impurities are mostly segregated in 873 15 153 133 the amorphous phase and then released into the surround- ings. This stage seems to be quite slow in the case of the iridium dioxide films and powders, thus preventing further Table 3 Unit-cell parameters relevant to the tetragonal IrO, and growth of IrO, crystallites, e.g. during the annealing treat- cubic Ir phases ments. Only direct pyrolysis at higher temperatures allows IrO, the achievement of larger crystallite size as shown in Fig.4. annealing For this system, the different values of lattice parameters are temperature/K time/h a,/A c,/A Ir a,/A influenced by the temperature and the annealing time, how- ever, no direct influence of the chloride ions has been detected. 673 15 4.544(3) 3.175(5) 3.859(3) As has already been outlined, as far as the Ir phase is 673 30 4.505(3) 3.183(5) 3.853(3) concerned, we have not been able to evaluate the disorder 67 3 58 4.497(4) 3.178(2) 3.846(2) 4.533(6) 3.165(3) 3.853(4) influence, nor consequently to clarify the role of the impurities. 773 15 873 15 4.522(6) 3.162(3) 3.850(3) In any case, the variation of the unit-cell parameters seems Lit.20 4.4983 3.1544 3.8394 to suggest a 'true' decrease of a, as a function of the annealing treatment.The results obtained on the microstructure of IrO, films refinement programme for the different samples. In general, are also interesting from the electrochemical point of view. In compared with the reference values," slightly greater values fact, as mentioned in the introduction, the electrocatalytic are obtained both for the IrO, and for Ir. In any case, when activity of oxide electrodes, and their instability in relation to the time or the temperature are increased the values approach corrosion processes taking place under polarization, are linked the standard ones. This fact could suggest that, after the by the ion-insertion characteristics of these materials. Accord- J. MATER. CHEM., 1991, VOL. 1 ing to the evidence obtained in this work, the IrO, crystallites show no strain, which should minimize ion-exchange processes across them, so limiting the ions’ mobility.The intergranular region, on the other hand, is likely to be quite large and, owing to its defective nature, could allow larger diffusion coefficients for small ions such as protons, that are involved in the charge-storage mechanism. IrO, electrodes prepared by the above-mentioned method in fact show large charge- storage capacity, compared with RuOz e1ectr0des.l~ We are grateful to Professor G. Fagherazzi for his constructive criticism and to Mr. L. Bertoldo for assistance during exper- imental work. This research work was supported by CNR (Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate) and by Minister0 dell’universita e della Ricerca Scientifica (MURST 40%).References 1 S. Trasatti and G. Lodi, in Electrodes of Conductive Metal Oxides, ed. S. Trasatti, Elsevier, Amsterdam, 1980, part A, p. 301. 2 S. Trasatti and G. Lodi, in Electrodes of Conductive Metal Oxides, ed. S. Trasatti, Elsevier, Amsterdam 1981, part B p. 521. 3 A. Nidola, in Electrodes of Conductive Metal Oxides, ed. S. Trasatti, Elsevier, Amsterdam, 1981, Part B p. 627. 4 D. M. Novak, B. V. Tilak and B. E. Conway, in Modern Aspects of Electrochemistry, ed. B. E. Conway and J. Bockris, Plenum Press, New York, 1982, vol. 14, p. 195. 5 S. Pizzini, G. Buzzanca, C. Mari, L. Rossi and S. Torchio, Mnter. Res. Bull., 1972, 7, 449. 6 G. Lodi, G. Zucchini, A.De Battisti, E. Sivieri and S. Trasatti, Mater. Chem., 1978, 3, 179. 7 S. Trasatti and G. Buzzanca, J. Electroanal. Chem., 1971, 29, 1. 8 G. Lodi, E. Sivieri, A. De Battisti and S. Trasatti, J. Appl. Electrochem., 1978, 8, 135. 9 C. Angelinetta, S. Trasatti, L. J. Atanososka, Z. S. Minevski and R. T. Anatososki, Mater. Chem. Phys., 1989, 22, 231. 10 G. Lodi, A. De Battisti, A. Benedetti, G. Fagherazzi and J. Kristof, J. ElectroanaI. Chem., 1988, 256,441. 11 G. Lodi, A. De Battisti, G. Bordin, C. De Asmundis and A. Benedetti, J. Electroanal. Chem., 1990, 277, 139. 12 G. Battaglin, A. Carnera, G. Della Mea, G. Lodi and S. Trasatti, J. Chem. SOC., Faraday Trans. 1, 1985,81, 2995. 13 G. Lodi, G. L. Wucchini, A. De Battisti, A. Giatti, G. Battaglin and G. Della Mea, Sur-Sci, submitted. 14 I. D. Belova, T. V. Varlamova, B. Sh. Galyamov, Yu. E. Rogin- skaya, R. R. Shifrina, S. G. Prutchenko, G. I. Kaplan and M. A. Sevostyanov, Mater. Chem. Phys., 1988, 20, 39. 15 B. E. Warren, and X-Ray Difraction, Addison- Wesley, Reading, MA. 1969, p. 264. 16 S. Enzo, S. Polizzi and A. Benedetti, 2. Kristallogr., 1985, 170, 275. 17 A. Benedetti, G. Fagherazzi, S. Enzo and M. Battagliarin, J. Appl. Crystallogr., 1988, 21, 543. 18 J. A. Nelder and R. Mead, Comput. J., 1965, 7, 308. 19 S. Enzo, G. Fagherazzi, A. Benedetti and S. Polizzi, J. Appl. Crystallogr., 1988, 21, 536. 20 Joint Committee on Powder Diffraction Standards, 1988, Powder Diffraction File (Swarthmore, Pennsylvania: International Centre for Diffraction Data) no. 6-0598 and 15-870. 21 G. Lodi, C. Bighi and C. De Asmundis, Mater. Chem., 1976, 1, 177. Paper 0105257D; Received 22nd November, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100511

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991,1(4), 517-523 Scanning Tunnelling Microscopy Study of Osmium-containing Electroactive Metallopolymer [Os(bipy),(PVP),,CI]CI Films on Polycrystalline Graphite Electrodes Norman M. D. Brown,*a Hong Xing You/ Robert J. Forsterband Johannes G. Vosb a Surface Science Laboratorx Department of Applied Physical Sciences, University of Ulster at Coleraine, Co. Londonderrx BT52 ISA, Northern Ireland, UK School of Chemical Sciences, Dublin City Universitx Dublin 9, Ireland Scanning tunnelling microscopy (STM) has been applied to the study, in air, of the topography of Os(bipy),CI,- loaded poly(4-vinylpyridine) (PVP) (bipy=2,2'-bipyridyl) films supported on polycrystalline graphite. Two markedly different topographical features are observed on the sample surfaces before and after electrochemical cycling in aqueous sulphuric acid electrolytes.These features are characterized, respectively, by ordered domains with rows of fibrillar structure and ordered domains with rows of granular structure. Electrochemical cycling in the sulphuric acid electrolyte therefore clearly has a profound effect on the topographies of the electroactive polymer fi Ims. Keywords: Scanning tunnelling microscopy; Electrochemical cycling; Modified electrode; Electroactive polymer; Topography 1. Introduction Metallopolymer-modified electrodes have shown potential applications in electrocatalysis,' photoelectrochemistry2 and macromolecular electronic^.^ The useful applicability of the metallopolymer-modified electrodes in many of these areas will be determined by the mechanism of charge transport through the polymer film.This electrical charge transport has not only a strong bearing on the proper design of the polymer microstructure but also on the topography of the polymer surfaces concerned. Thus, the transverse motion of a charge carrier can be prevented by the physical intervention, for example, of topographical wells: i.e. a local surface pit. In order to explain the electronic properties of these polymer- modified electrodes fully, it is essential to have a clear under- standing of the structures of the conducting modifying polymers. For particularly regular polymer structures, X-ray or elec-tron diffraction can be used to analyse both the relative position of the atoms in the molecular repeat units, i.e.the unit cell, and the arrangement of the unit cell along the molecular chains. Spectroscopic techniques, such as infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR) and X-ray photoelectron spectroscopy (XPS), can also be used collectively to probe the microstructure of the polymer molecules and can give information on the conformation and packing of the polymer chains involved and the chemical states of the atoms or ions present. However, these techniques do not provide sufficient information to determine in detail the structure of the polymer matrix ~oncerned.~ The general topographies of polymers can be viewed comp- lementarily with an optical or an electron microscope at low or moderately high resolution, i.e.from micrometres to several thousand Angstroms, respectively. On the other hand, some other electron and ion microprobe techniques, for instance low-energy electron diffraction (LEED), Auger electron spec- troscopy (AES) or scanning Auger microscopy (SAM), employing the high-energy electron beams to excite Auger emission of electrons, may cause radiation damage to the polymer surfaces of interest.' This problem has restricted, to some degree, the application of the above microprobe tech- niques in the studies of polymer surfaces. Thus, resort is made to novel or other complementary techniques for the structure determination of polymers. Scanning tunnelling microscopy (STM), since its emergence in 1982,6 has proved to be a powerful tool in surface science, uniquely in the determination of surface topography at ultra- high resolution, i.e. in scales from nanometres to a few Angstroms, whether in vacuum, air, or liquid environments.At the outset, scanning tunnelling microscopy was applied mainly to metal and semiconductor surfaces. Recently, how- ever, scanning tunnelling microscopy has been turned directly to the study of polymers and other organic With the ultra-low electron energy used (a few eV at most) and without mechanical contact, the STM technique is non-destructive and comparatively flexible. Hence, it appears to be ideal for the surface study of polymers or organic films in certain circumstances.The main limitations of the STM technique lie essentially in the need for the sample to be electrically conductive and the sample surface to be reasonably flat. In general, polymers and other organic films are poor electrical conductors. Therefore, they are excluded from STM studies if they are not conductive intrinsically and sufficiently or if a preliminary surface metallization is not performed. Vacuum deposition of thin layers (<I0 nm) of conductive material (Pt/C or Pt/Ir/C) on the polymer surface or the organic film~~*"7'~ can solve, to a certain degree, this conduc- tivity problem with a loss, because of the overlayer, in the ultimate spatial resolution attainable. Given these restrictions, only a few STM studies have so far been done on conductive polymers per se,l1-I4 and on the polymer films deposited on graphite and gold substrates by Langmuir-Blodgett (LB) technique^.'^-'* In this paper, scanning tunnelling microscopy is applied to the study, in air under ambient conditions, of the osmium- containing metallopolymer films { [Os(bipy),(PVP) oC1] C1) coated on polycrystalline graphite electrodes.The topograph- ical images, obtained from the samples before and after the electrochemical cycling in a sulphuric acid electrolyte solution, show two striking features. These are, first, ordered domains with the unidirectional rows of fibrillar structure, typically on the sample surfaces prior to the electrochemical cycling and, secondly, ordered domains with aligned rows of granular structure, particularly for samples following the aqueous sulphuric acid cycling.The experimental results presented amply demonstrate the applicability of scanning tunnelling microscopy to the topographical study of the surfaces of the conducting polymer-modified electrodes, revealing the micro- structure of the polymer overlayer at ultra-high resolution. Such methods can clearly be extended to the working electrode in an aqueous environment. 2. Experimental 2.1 Sample Preparation The polycrystalline graphite electrodes of 7 mm diameter mounted in Teflon shrouds, used in the experiments for the polymer overlayer growth, were cut sections of polycrystal- line graphite rod (Agar Aids Ltd., Essex) prepared by mechan- ical polishing using a 0.5 pm alumina slurry on a felt pad followed by thorough washing with distilled water and methanol.The poly(4-vinylpyridine) (PVP) was prepared from freshly distilled 4-vinylpyridine by bulk polymer-isation under nitrogen at 70-75 "C using 2,2'-azoisobutyro- nitrile as initiator. The crude polymer was purified there- after by repeated precipitation in diethyl ether from methanol. The molar mass, as determined by viscometry in absolute ethanol, was found to be 4.3 x lo5 g. The Os(bipy),Cl, complex was prepared by standard laboratory procedures. The modifying metallopolymer was prepared, as reported for the corresponding ruthenium-containing polymer, by refluxing Os(bipy),Cl, with a 10-fold excess of PVP in ethanol.Finally, the adherent electrode coatings were ob- tained by evaporating a few mm3 of a 1% solution of the osmium-containing modifying polymer in ethanol onto the electrode surface in a solvent-vapour saturated chamber followed by air drying to give a film ca. 4000 A thick. The detailed sample preparations and the electrochemical pro- cedures and equipment are described elsewhere." The mol- ecular structure of the thus prepared osmium-containing metallopolymer ([0~(bipy)~(PVP),,Cl]Cl}is shown in Fig. 1. In the study of electron transport in the osmium-containing metallopolymer or any redox polymer,' using chronocoulome- try or cyclic voltammetry, the redox polymer-modified elec- trode will normally undergo electrochemical cycling.In use therefore, the coated electrode has its potential controlled on one side and is wetted on the other by the solvent of the electrochemical cell and any supporting electrolyte. Sulphuric acid (H2S04)solutions with concentrations in the range 0.2- L N J Fig. 1 Molecular structure of the osmium-containing metallopolymer [Os(bipy),(PVP),,Cl]CI (N-N =2,2'-bipyridyl) J. MATER. CHEM., 1991, VOL. 1 1.0 mol dm-3 were chosen in this case as the supporting electrolyte. The electrochemical cells were of conventional design and were thermostatted to &1 "C.19 2.2 STM Instrumentation The scanning tunnelling microscope used (W. A. Technology Ltd., Cambridge) consists of a scanning head suitable for operation in either air or vacuum, an electronic control unit, a Tandon 386 microcomputer with VGA colour monitor and frame store facility, and a monochrome monitor for image display. The tip used is produced from a 0.5 mm diameter tungsten wire (99.99%) which was etched, by a routine electro- chemical method, in an aqueous 1 mol dmP3 KOH solution.20 A more detailed description of this STM instrument can be found elsewhere.20'2' The STM images presented were obtained in the constant tunnel current mode, from the osmium-containing metallopolymer-modified polycrystalline graphite electrodes before and after electrochemical cycling in sulphuric acid electrolytes of varying concentrations.The bias voltages were in the range 700-1600 mV, with the tunnel current set between 0.6 and 1.2 nA.The typical scanning speeds used in the X or Y direction scans were between 0.1 and 5 nms-'. A typical sample surface, on inspection through an optical microscope, generally looked very smooth and shiny before the electrochemical cycling, whereas it was flat and grey in appearance after cycling. Here we present the STM results for samples, treated in sulphuric acid electrolyte solutions of 0.2 and 0.4 mol dm-3, reproducible STM images could be obtained over a considerable area. For the samples exposed to the sulphuric acid electrolytes at higher concentrations, the surfaces became less dense and visibly swollen. The STM images obtained from these were unsatisfactory because of the swollen nature of the surfaces and are not considered further.3. Results 3.1 Scanning Tunnelling Microscopy The STM images obtained from osmium-containing polymer- modified polycrystalline graphite electrodes before electro- chemical cycling are shown in Fig. 2. These images were recorded at a bias voltage of 900 mV (with the sample positive) and at a tunnel current of 0.8 nA. Fig. 2(a), typical of a larger area (here scanned over 800 Ax800 A), illustrates some straight fibrils running through the scanned area with the same general orientation. Moreover, it can also be seen from Fig. 2(a) that there are other branch-like fibrils running in a zigzag manner beside the straight fibrils. Some of these branch- like fibrils extend one end in another direction and then line up parallel with the straight fibrils.Some extend close to the sides of the straight fibrils and finally join them. The smaller- scale scan expanded from the left part of the straight fibrils in Fig. 2(a) is shown in Fig. 2(b). As can be seen clearly, there are several straight fibrils on the surface with a side-by-side distance or a fibrillar width of ca. 20 A. Fig. 2(c) shows another frequently observed feature, i.e. a series of steps, on the sample surfaces before electrochemical cycling.The steps exhibit rough and uneven edges with an edge-to-edge distance of ca. 20 8, and with step heights in the range 8-9 8,. The STM images of the samples, after being exposed to the 0.4 mol dm-3 sulphuric acid electrolyte, shown in Fig. 3 and 4, are typical of those observed for such samples.These images were recorded at a bias voltage of 1400 and 1350 mV, respect- ively, and at a tunnel current of 1.0 nA, over the scanned areas of 800 8, x 800 8, and 1000 8, x 1000 A. As shown, J. MATER. CHEM., 1991, VOL. 1 Fig. 2 STM images of an osmium-containing metallopolymer-modi- fied polycrystalline graphite electrode before electrochemical cycling in the sulphuric acid electrolyte, recorded at a bias voltage of 900 mV and a tunnel current of 0.8 nA. (a) A large-scale image (image size: 800 A x 800 A x 68 A); (b)the smaller-scale scan expanded from the left side of (a)showing rows of the rod-like structure (image size: 276 8, x 276 A x 54 A); (c) a series of the steps (image size: 106 A x 106 Ax16 A) Fig.3(a)and 3(b)are actually the same image, but to emphasize the featured area which is rather diffuse in Fig. 3(a), image contrast enhancement processing was used for Fig. 3(b). In this, the image intensity data values of each pixel are adjusted up or down so that, in the displayed image, there is an equal number of pixels of each intensity. A smaller scale scan zoomed from the featured area in Fig. 3(a)is shown in Fig. 3(c). It can be seen clearly that the featured area in Fig. 3(a)consists of rows with a granular structure which sit side-by-side aligned in one direction. The distance between two such granules from centre to centre was, typically, ca. 40 A, with the granules showing considerable size uniformity, i.e. ca. 40 8, x 40 A or ca.40 8, x 30 A. The side-by-side distance of the rows apparent in Fig. 3(c) was found to be, typically, ca. 40 A, the same as the granular centre-to-centre distance visible in the longitudi- Fig. 3 STM images of an osmium-containing metallopolymer-modi- fied polycrystalline graphite electrode after exposure to a 0.4 mol dm- aqueous sulphuric acid electrolyte, recorded at a bias voltage of 1400 mV and a tunnel current of 1.0 nA. (a)A large-scale image showing rows of the granular structure (image size: 800 8,x 800 Ax81 A); (b) the same image as (a) but with the image contrast enhanced; (c) the smaller-scale scan ex anded from the featured area of(a) (image size: 313 A x 313 AX 24 1) nal direction. Fig. 4 shows the overlapping layer structure (or wide fibril") frequently observed on the sample surfaces having been exposed to the 0.4 mol dmP3 sulphuric acid electrolyte.Fig. 4(b) is a high magnification image of the middle of Fig.4(a), showing the fine step structure. As seen in Fig. 4(b), the steps have relatively smooth and even edges. These edges lie approximately parallel to the diagonal of the image, i.e. at an angle of ca. 135" with respect to the X axis. The step heights found vary from ca. 30 A for the first and second steps (counting from the upper right-hand corner), ca. 40 A for the third and ca. 60 A for the fourth. Correspond- ingly, the respective edge-to-edge distances of the steps have different values as well, i.e. ca. 60, ca. 90 and ca. 120 A.In these experiments, the step height and the step edge-to-edge distances were found to change with the specific local tip Fig. 4 STM images of an osmium-containing metallopolymer-modi- fied polycrystalline graphite electrode after exposure to a 0.4 mol dm-3 aqueous sulphuric acid electrolyte, recorded at a bias voltage of 1350 mV and a tunnel current of 1.0nA. (a) A large-scale image showing the overlapping layer structure (image size: 1000 Ax1000 Ax236 A); (b) the high magnification image of (a) taken from the middle showing steps (image size: 423 A x 416 A x 151 A) location on the sample surface, but the corresponding param- eters of the similar features identified were frequently close to the above values or within the range of the values given.Occasionally, but consistently, the step heights found were only half of the above step heights. Typical STM images of samples examined after exposure to the 0.2 mol dmP3 sulphuric acid electrolyte are shown in Fig. 5. These images were recorded at a bias voltage of 1200 mV and at a tunnel current of 1.0 nA. From Fig. 5(a), two distinct features can be seen, i.e. the granular structure of the left part of the image and the overlapping layer structure of the remainder of the image. To show these topographical features more clearly, two high magnification images from the left and the right halves of Fig. 5(a) are shown in Fig. 5(b) and 5(c), respectively. In Fig. 5(b) it can be seen that the granular structure runs roughly parallel in the same general direction, with typical granule sizes of ca.80 81 x 80 81 or ca. 70 8, x 6081, respectively. The centre-to-centre distances of adjacent granules are, typically, ca. 90 81 both laterally and longitudinally. In Fig. 5(c), there are marked steps lying approximately parallel to the diagonal of the image, i.e. at an angle of ca. 135", as before, with respect to the X axis. The step heights are measured to be ca. 20 or ca. 30 A,respectively, with an edge-to-edge distance of ca. 80 8,. In these experiments, this mixture of granular structure and overlapping layer structure was frequently observed on the surfaces which had been treated in the 0.2mol dm- sulphuric acid electrolyte. Some of the structured regions on the surfaces were wholly composed of granular structured domains, and by contrast, others were totally composed of domains showing the overlap- ping layer structure.These two types of differently structured J. MATER. CHEM., 1991, VOL. 1 Fig. 5 STM images of an osmium-containing metallopolymer-modi- fied polycrystalline graphite electrode after exposure to a 0.2 mol dmP3 aqueous sulphuric acid electrolyte, recorded at a bias voltage of 1200 mV and a tunnel current of 1.0 nA. (a) A large-scale image (image size: 700 8,x 700 8, x 96 A); (b)the high-magnification image taken from the left half of (a) showing the granular structure (image size: 301 A x 301 8, x 74 A); (c)the high-magnification image taken from the right half of (a) showing steps (image size: 301 A x 340 A x 66 A) regions in some locations on the sample surface had a clear demarcation line while in other locations this line was not so obvious.For example, some of the granules lay side-by-side individually, while others overlapped each other and formed straight rows or wide fibrils. 3.2 Electrochemistry A cyclic voltammogram obtained for [0~(bipy)~(PVP)~~Cl]Cl in 0.4 mol dm-3 H2S04 is shown in Fig. 6. Upon varying the electrolyte concentration the same behaviour for the homo- geneous charge-transfer process within the complex-loaded layer was observed, as reported in an earlier paper22 in which J. MATER. CHEM., 1991, VOL. 1 li Fig. 6 A cyclic voltammogram of a polycrystalline graphite electrode coated with the [Os(bipy),(PVP )loCl]Cl polymer films.Electro-lyte 0.4 mol dm-3 HJO,, scan rate 100 mV s-', coverage 3 x lop8mol ~m-~,estimated layer thickness 4300 a detailed discussion of the electrochemistry of these modified electrodes in H2S04 and in other solvents is given. However, it should be emphasized here that the charge transport and kinetic parameters obtained for the osmium- containing metallopolymer show a large dependence on the nature and concentration of the electrolyte.22 The activation parameters obtained suggest strongly that at high sulphuric acid concentrations a substantial amount of swelling of the polymer matrix occurs. Cyclic voltammetry experiments sug- gest that in 1.0 mol dm-3 H2S04 polymer movement is rate determining; at lower electrolyte concentrations the process is controlled by ion movement. This suggests that at higher electrolyte concentrations the distance between the redox active osmium sites increases.The STM data elaborated here are believed to be consistent with these electrochemical obser- vations. After electrochemical cycling the surface structures of the layers have clearly been altered. These changes become more pronounced with treatments involving the increase of electrolyte concentrations. 4. Discussion Although artefacts can be observed in the STM imaging of organic films because of the attendant interaction of a flexible or poorly conducting organic film with the STM tip,7 the observed topographies described here are believed to be real for the following reasons.First, the features identified on the osmium-containing metallopolymer-modified polycrystalline graphite electrodes are consistently imaged in a large number of locations on a given sample surface and from a succession of samples, irrespective of the scan directions, scan speeds, bias voltage (within reasonable limits) or bias directions (Note, 521 there are some sample surface positions where the local conductivity is too poor for adequate STM image quality). Secondly, the row-like structures lie at various different angles, since a given sample has structured domains with different local alignments, and thirdly, the topographical features pre- sented here do not arise from the polycrystalline graphite substrate used because the known thickness of the polymer films coated on the carbon substrates, as indicated earlier, is ca.4000 A.Moreover, STM images of the uncoated polycrys- talline graphite substrate do not show any topographical features of similar appearance over the same range of scanned area scales. As a result, the topographical contributions of the substrate itself need not be considered further. The row-like structures of fibrils and granules seem to be the common topographical features observed on the surfaces of the sample electrodes studied. Similar structures have also been observed on the other polymer Therefore, such structures may be a consequence of local polymer ordering or ~ystallization'~*'~-'~ arising in the films, reflecting in turn the allowed molecular pa~king.'~,'~,'~ Now, in the imaging experiments described here, the ordered structures could be traced over several hundred or thousand Angstroms longitudinally and at least several hundred Angstroms laterally, especially for the samples imaged after the electrochemical cycling indicated.At the same time, the orientations of the row-like structures with respect to the X axis remained constant over distances of at least hundreds of Angstroms. Here, it is believed that the ordered row-like structures on the surfaces of the sample electrodes either before or after the electrochemical cycling in a sulphuric acid electrolyte may reflect, at least in part, the more ordered or crystalline regions of the polymer film^.'^,^^ Furthermore, the array of the branch-like fibrils illustrated in Fig.2(a) is attri- buted to an amorphous region typical of those which are generally assumed to surround the more crystallinedomain^.^^.^^ Assuming then that there are local crystalline regions on the surfaces of the polymer films, in principle it might be expected that individual pendent pyridine rings would be observed. In spite of much effort in the ordered areas of both the fibrillar and the granular rows, no topographical details as fine as a pyridine ring were resolved. This difficulty of imaging smaller scale molecular features is probably made more difficult by the inherent atacticity of the polyvinylpyri- dine matrix and the random loading of the osmium centres, but note that better ultimate resolution may be attainable in vacuum.On the other hand, in a study of the metallopolymer of Ru(bipy), bound to poly-4-~inylpyridine,~' a structural model was proposed in which a metallopolymer strand containing metal sites would exhibit a rod-like shape, especially at high metallation levels, because of the stereochemical and electro- static demands of the then present poly(pyridy1) ruthenium units. For the metallopolymer derived from Os(bipy),Cl, bound to poly-4-vinylpyridine, the diameter of the pendent osmium-containing unit is ca. 12-14 A. If the polymer back- bone is included, a total diameter is given in that direction of ca. 14-16 A.This would naturally be a minimum figure without consideration of the presence of chloride ions, hydrated or otherwise (diameter of the hydrated C1- ion is ca.7 A,26 that of free Cl- ion is 1.84 A). Replacement of chloride ions with sulphate ions during the electrochemical cycling in the electrolyte specified could also contribute to the effective diameter of the osmium complex polymer chains. Here, the spacing of rows of the straight fibrillar structure indicated in Fig. 2(a) and 2(b)is ca. 20 A,commensurate with the transverse dimension of the osmium-loaded polymer fea- tures outlined above. It is therefore tempting to reach the conclusion that the rows of straight fibrillar structure may represent the metallopolymer strands containing the active osmium sites.But considering the changes in the spacings of rows of granular structure or the separations of the wide fibrils after the electrochemical cycling used, it is also possible that the row-like structures are polymer backbones or a bundle of a few polymer chains, as suggested by the other authors.13-15,17,18 In regard to the terrace steps observed, the corresponding theoretical or experimental crystalline parameters available for comparison with the observed step heights shown in Fig. 2(c), 4(b) and 5(c) appear not to be known. However, if the osmium groups present are here assumed to somehow interlink with the immediate subsurface layer, the increase in the thickness as a result of the presence of the complexed osmium grouping would be ca.6-7 8,, the radius of the osmium pendent unit. This is very close to the measured step heights of 8-9 8, in Fig. 2(c). On this basis, the steps shown possibly represent extra polymer layers. On the other hand, the steps on the sample surfaces [see Fig. 4(b) and Fig. 5(c)], after electrochemical cycling, are found to have increased in step height and step edge-to-edge distance. However, there still remain certain regularities, for example, the step heights were usually multiples of ca. 20 or ca. 30 8,. The exact reason for such an increase is not clear at present. The concomitant exchange of chloride ions with sulphate ions may be signifi- cant. Moreover, the similar topographical features identified, especially the approximately parallel edges, of the steps seen are supportive of their common origin. Another significant change in the topography on the sample surfaces after electrochemical cycling used is the appearance of rows of nearly the same size as the observed granular structure, typically ca.40 8, x 40 8, or ca. 80 8, x 80 A, respect-ively [see Fig. 3(b) and Fig. 5(b)].Therefore, certain relation- ships appear to exist not only between the spacings of the row-like structures and the granular size but also between the granular size and the granular centre-to-centre distance. This is thought to be, in some way, related to the electrochemically induced process or processes which, in the presence of the sulphuric acid electrolyte, bring about the observed topo- graphical changes in the polymer matrix.Upon immersion of the polymer layer in sulphuric acid two processes are expected to occur. With the high acid concentrations, protonation of the free pyridine rings occurs immediately since the pK, of PVP is 3.5.25 Such a process is expected to alter the structure of the layer as the number of ionic sites will be greatly increased. A second significant change, as indicated above, will be the replacement of chloride ions by sulphate ions. These changes are expected to affect the layer structure significantly. It follows therefore that the layer granular structures observed might be caused by clus- tering of the osmium pendent units. It should be noted in this regard that XPS studies of the same materials, to be reported elsewhere,27 provide further evidence in support of these arguments. For example, the sulphate loading in the dried samples reflects the acid concentration used in the electrolyte, just as does the parallel development of protonated nitrogen centres.Furthermore, in the case of the ruthenium-containing metal- lopolymer [R~(bipy)~(PVP),(N0,)1 the+-coated ele~trode,’~ equilibrium spatial separation of the ruthenium centres is calculated to be 13.5 8, on the assumption of a rod-like structure. Also, in the study of a series of osmium and ruthenium-containing metallopolymers showing redox behav- iour, the suggestion is made that the average separation between the metal centres will be ca. 15 Accordingly, the granular structure observed here would not represent simply the redox or metal centres, disregarding the fact that the J.MATER. CHEM., 1991, VOL. 1 separation of the metal centres is connected with the nature of the polymer backbone and the metal-to-polymer ratio. In the images described, the granular sizes and the granular centre-to-centre distances were found to depend to a certain extent on the concentrations of the sulphuric acid electrolyte used. This behaviour is thought with some confidence to reflect the effect of the electrochemical cycling taking place in the osmium-containing metallopolymer.22 Now, there are some significant difficulties associated with deciding whether the granular structure shown in Fig. 3(b) and Fig. 5(b) rep-resents the redox clusters related to the osmium pendent units, or say the packing of the redox centres or something else.18 For example, there are few serious studies on the surface electronic properties and the crystalline structure of osmium-containing or analogous metallopolymers.Nevertheless, it is clear at this juncture that the granular structure observed arises from the effects of the electrochemical cycling in or on the surface of the osmium-containing metallopolymer-modi- fied polycrystalline electrodes concerned. 5. Conclusions Topographical images have been obtained from the osmium- containing metallopolymer-modified polycrystalline graphite electrodes by scanning tunnelling microscopy in air at room temperature. Before electrochemical cycling in a sulphuric acid electrolyte, rows of the fibrillar structure and significant steps of consistent size observed were found to be the main topographical features present on the sample surfaces.After electrochemical cycling, the topography of the sample surfaces was found to have changed in two ways. First, rows of distinct granules are observed, separately from an overlapping layer structure on the sample surface. Secondly, the steps observed are changed in both step height and step edge-to-edge distance. This change in topography is assumed to be associated with the attendant electrochemical cycling in the sulphuric acid electrolyte. It is anticipated that this investigation may allow a better understanding of the correlation between the sample processing history, the topography of the osmium-containing metallopolymer-modified electrode and the performance of the latter.In pursuit of this understanding, further work is in hand and the possibility of in situ electrode surface imaging is being addressed. One of the authors (H.X.Y.) thanks the University of Ulster for a postgraduate research studentship. Thanks are also due to the IRCSS at the University of Liverpool and the European Science Foundation for their support. The author would like to thank Dr. G. Taggart for helpful discussions, and Mr. C. Anderson and Mr. B. Meenan for technical assistance. References 1 C. E. D. Chidsey and R. W. Murray, Science, 1986, 231, 25. 2 D. A. Buttry and F.C. Anson, J. Am. Chem. SOC., 1982, 104, 4824. 3 R. W. Murray, A. G. Ewing and R. A. Durst, Anal. Chem., 1987, 59, 379A. 4 T. J. Lewis, in Polymer Surfaces, ed. D. T. Clark and W. J. Feast, Wiley-Interscience, New York, 1978, ch. 4, p. 65; D. T. Clark, in Polymer Surfaces, ed. D. T. Clark and W. J. Feast, Wiley- Interscience, New York, 1978, ch. 16, p. 309. D. Briggs, M. Hearn, G. Beamson and I. Fletcher, Spectrosc. World, 1990, 2, 11. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett., 1982, 49, 57. E. Occhiello, G. Marra and F. Garbassi, Polym. News, 1989, 14, 198. D. P. E. Smith, H. Horber, Ch. Gerber and G. Binning, Science, 1989, 245, 43. J. MATER. CHEM., 1991, VOL. I 523 9 J. K. Spong, H. A. Mizes, L.J. LaComb Jr., M. M. Dovek, J. E. Frommer and J. S. Foster, Nature (London), 1989, 338, 137. 18 H. Sotobayashi, T. Schilling, and B. Tesche, Langmuir, 1990, 6, 1246. 10 11 H. Fuchs, W. Schrepp and H. Rohrer, Surf.Sci., 1987, 181, 391. D. H. Reneker and B. F. Howell, J. Vac. Sci. Technol., 1988, A6, 19 R. J. Forster, A. J. Kelly, J. G. Vos and M. E. G. Lyons, J. Electroanal. Chem., 1989, 270, 365. 553. 20 N. M. D. Brown and H. X. You, Surf. Sci., 1990,233, 317. 12 R. Yang, K. M. Dalsin, D. F. Evans, L. Christensen and W. A. 21 N. M. D. Brown and H. X. You, Surf.Sci., 1990,237, 273. 13 Hendrichson, J. Phys. Chem., 1989,93, 51 1; 1990,94, 61 17. L. Christensen, W. A. Hendrickson, R. Yang and D. F. Evens, Polym. Prep., 1989, 30(2), 100. 22 23 R. J. Forster and J. G. Vos, Macromolecules, 1990, 23, 4372. R. B. Seymour and C. E. Carraher, in Polymer Chemistry, An Instruction, Marcel Dekker, New York, 2nd edn., 1988, ch. 2, 14 D. H. Reneker, J. Schneir, B. Howell and H. Harary, Polym. p. 19. Commun., 1990, 31, 167. 24 J. Guillet, in Polymer Photophysics and Photochemistry, An 15 T. R. Albrecht, M. M. Dovek, C. A. Lang, P. Grutter, C. F. Quate, S. W. J. Kuan, C. W. Frank and R. F. W. Pease, J. Appl. Introduction to the Study of Photoprocesses in Macromolecules, Cambridge University Press, Cambridge 1985, ch. 2, p. 28. Phys., 1988,64, 1178. 25 J. M. Calvert and T. J. Meyer, Inorg. Chem., 1982, 21, 3978. 16 J. P. Rabe, M. Sano, D. Batchelder and A. A. Kalatchev, J. 26 Y. Marcus (ed.), Zon Solvation, Wiley-Interscience, New York, 17 Microsc., 1988, 152, 573. M. M. Dovek, T. R. Albrecht, S. W. J. Kuan, C. A. Lang, R. 27 1985, p. 117. N. M. D. Brown, B. J. Meenan and J. G. Vos, to be published. Emch, P. Grutter, C. W. Frank, R. F. W. Pease and C. F. Quate, J. Microsc., 1988, 152, 229. Paper 0/05460G; Received 4th December, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100517

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(4), 525-529 Characterization of Conducting Polymer-Quartz Composites Steven P. Armes,*a S. Gottesfeld,*J. G. Beery,* F. Garzon,bC. Mombourquette,*M. Hawley," and H. H. Kuhd a School of Chemistry and Molecular Sciences, University of Sussex, Brighton, BN I 9QJ, UK " Materials Science and Technology Division, Electronics Research Group, Los Alamos National Laboratory, Los Alamos, NM 87545. USA Los Alamos National Laboratory, Los Alamos, NM 87545, USA Milliken Research Corporation, P.0. Box 1927, M-405, Spartanburg, SC 29304, USA We have characterized several polypyrrole-quartz and polyaniline-quartz composites over a range of con- ducting-polymer loading levels by thermogravimetric analysis (TG), Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), scanning tunnelling microscopy (STM) and conductivity measure- ments.It is shown that the conducting polymer overlayers are remarkably thin and uniform; film thicknesses were determined independently by TG and RBS and are in close agreement. The film thickness of one of the polypyrrole-quartz samples was used to obtain the first direct measurement of the conductivity of the polypyrrole component (ca.35 Q-' cm-'). Keywords: Polypyrrole; Conducting polymer; Organic-inorganic composite It is well known that the successful commercial exploitation of organic conducting polymers, such as polyacetylene, poly- pyrrole, or polyaniline, has been frustrated by these materials' poor environmental stability and/or intractability.Previous workers have reported many novel organic and inorganic composites and thin films containing a conducting-polymer component.' Recently, the Milliken Research Corporation reported a new technique that involves the in situ deposition of polypyrrole or polyaniline onto various textile substrates such as nylon or polye~ter.~-~ Although not yet fully under- stood from a fundamental viewpoint, this process has been ~atented,~and it now seems likely that the much-vaunted potential of conducting polymers as materials with novel electronic applications will finally be realized. These conducting polymer-textile composites have pre- viously been characterized by various techniques, including SEM, resistance measurements and X-ray photoelectron spec- troscopy (XPS).2-4 One problem that emerged during these earlier studies was the difficulties encountered in estimating the mass and average thickness of the deposited conducting polymer overlayer.The latter parameter is essential if com- posite conductivities (Q-' cm-') rather than resistances (Q/ 0)are to be determined. By using thermally stable quartz or glass fibres rather than conventional textile materials as the substrate, it was shown that the mass loading of the con- ducting polymer could be directly determined by TG.4 In this report we have characterized several polypyrrole- quartz and polyaniline-quartz composites by TG, RBS, and SEM. The combined use of these techniques has enabled us to determine accurately the average overlayer thicknesses of the conducting polymer coatings by two independent methods and to confirm their remarkable uniformity.By using these coating thickness values, and by measuring the resistance of a single filament of these conducting polymer-quartz com-posites, we have succeeded in directly determining the intrinsic conductivity of the conducting polymer overlayer. Finally, STM techniques were used to examine the nano- morphology and structure of the conducting-polymer overlayer. Experimental Preparation of the Conducting Polymer-Quartz Samples The quartz fabric used in these studies is Astroquartz 11, which was purchased as one batch (J. P. Stevens Company). According to the suppliers' specifications, it has a density of 2.20 g cm-, and an average fibre diameter of 9 pm, and it consists of at least 99.9% silicon dioxide.The quartz fibres were coated with polypyrrole or polyaniline according to previously described procedure^.^ For polypyrrole, this pro- cess utilized FeC1, as an oxidant, 5-sulphosalicylic acid as a polymerization retarder (which forms a less reactive 1:1 complex with the Fe"' species), and napthalene- 1,5-disulphonic acid (used as the disodium salt) as an additional doping agent. The reaction was carried out in aqueous media. For polyani- line, the oxidant used was (NH,),S20s in toluene-p-sulphonic acid solution. Characterization of the Conducting Polymer-Quartz Samples Scanning electron microscopy images were obtained using a Philips 505 instrument.Owing to the conductive nature of the samples, no gold sputter-coating techniques were necessary for sample preparation. The conducting polymer-quartz samples were simply affixed to aluminium sample stools using a conductive carbon paste. Scanning tunnelling microscopy studies were carried out using a Nanoscope I1 instrument. Full experimental details will be published elsewhere.6 Thermogravimetric analyses were run in an oxygen atmos- phere to ensure complete combustion using a Perkin-Elmer TGA-7 instrument at a scan rate of 20 "C min-'. Rutherford backscattering spectrometry experiments were run using a 2.2 MeV He' source at the Ion Beam Facility, Los Alamos National Lab~ratory.~ Briefly, the experimental conditions were: ion-beam current =60 nA; total charge focused on sample =40 pC; and single backscattering detector placed at 13" to the incident He' beam.Conductivity measurements were carried out as follows: a single 9 pm filament was carefully removed from a polypyr- role-quartz composite (sample 3) with the aid of an optical microscope and was placed on the surface of a strip of double- sided adhesive tape mounted on a glass slide. Four electrical contacts were made to the filament using silver epoxy glue, which was allowed to cure at room temperature overnight. A low current (clpA) was passed between the two outermost contacts using a Keithley 227 current source and the resulting potential difference between the two inner contacts was meas- ured using a Keithley 177 digital microvoltmeter.Ohmic contacts to the silver epoxy spots were made using four needle pressure contacts in conjunction with the optical microscope. In a control experiment, no current or voltage could be detected between four similarly spaced silver epoxy spots with no polypyrrole-quartz filament between them. Thus, the adhesive tape substrate makes no contribution to the meas- ured electrical conductivity of the filament. Results and Discussion Some of the conducting polymer-quartz samples studied in this work have been characterized previously by TG and resistance measurements by workers at Milliken Research C~rporation.~We have repeated the thermogravimetric analy- ses, and in our control experiments, we found that the untreated quartz fabric exhibits a weight loss of 0.4 0.1YO under the combustion conditions (oxygen atmosphere).This weight loss is attributable to surface moisture and/or certain lubricants and binders present on the fabric' and is well within the manufacturer's specification (0.25-0.90%). Thus, the observed weight losses of the conducting polymer-quartz composites require a small correction to obtain the actual weight of volatilized conducting polymer. Assuming that the conducting polymer overlayer is com- pletely uniform and of known density and that the quartz fibres are all long, thin cylinders with the same cross-sectional area, we may easily derive an equation that allows us to calculate the average thickness of this overlayer from the TG data.From simple geometric considerations, then, we have 6=u{[ 1+($) (3lli.I) where M1, p1and M2, p2 are the masses and densities of the quartz substrate and the conducting polymer overlayer, respectively, a is the mean radius of the quartz filament and 6 is the thickness of the conducting polymer overlayer (see Fig. 1). For the quartz fibre, we have p1 =2.20 g cm-3 and a = 4500 nm & YO.^ For chloride-doped bulk polypyrrole powder, p2 has been estimated to be 1.50 g~m-~ from flotation Fig. 1 Schematic representation of a cylindrical quartz fibre (of length L and radius a coated with a uniform overlayer of conducting polymer of thickness 6 J. MATER. CHEM., 1991, VOL.1 measurements in chlorinated solvent^.^ This value was used to calculate the film thicknesses of the polypyrrole-quartz composites. However, XPS studies of these materials by Gregory et al. suggest that the conducting polymer overlayer contains fewer aj? defects than does the conventional bulk polypyrrole po~der.~ In addition, recent neutron-scattering experiments by Mitchell et aE. on deuterated thin films of electrochemically synthesized polypyrrole indicate that the toluene-p-sulphonate doped material is significantly more ordered than films doped with small inorganic anions such as SO:-or CIO;.'o We have shown that the polypyrrole overlayer on both textile and quartz substrates is almost exclusively doped with similar aromatic sulphonate anions (5-sulphosalicylate and/or napthalene- 1,5-disulphonate) by energy dispersive analytical X-ray (EDAX) techniques.6 Thus, if these observed increases in local molecular order result in more efficient polymer chain packing, then it is likely that the assumed value of 1.50 g cm-3 for p2 may be an underestimate of the true density of the polypyrrole overlayer. This would naturally result in an overestimate of 6 by both TG and RBS techniques.For the polyaniline-quartz composite (sample 6) p2 was taken to be 1.50 g ~m-~,which is the density reported by Stilwell and Park for polyaniline sulphate in sulphuric acid media by flotation measurements." The ratio M2/M1 in eqn. (1) is obtained directly from the TG data. Calculated values of the overlayer thickness using eqn.(1) for all the conducting polymer-quartz composites are presented in Table 1. In the preparation of our polypyrrole-quartz composites, the conducting polymer loading was varied simply by increas- ing the concentrations of monomer and oxidant (and, hence, conducting polymer) relative to the weight of the quartz textile ~ubstrate.~From Table 1, it is evident that the sheet resistance of the polypyrrole-quartz composites decreases non-linearly with increasing thickness of the polypyrrole coating. This relationship is depicted in Fig. 2. RBS has been used by the semiconductor industry for many years to characterize thin film samples. l2 Efficient backscatter- ing mechanisms require substrate atoms with relatively heavy nuclei, so this technique is suitable for probing the silicon atoms in the quartz fibre but not the lighter atoms in the conducting polymer coatings.It is rather unfortunate that neither the RBS nor the TG techniques can be applied to the characterization of more technologically interesting textile substrates such as polyesters or nylons. However, by studying the quartz-fibre based materials, we hoped to gain consider- able insight into the synthesis-structure-property relation-ships of conducting polymer-textile composites in general. Our RBS results for the polypyrrole-quartz samples 1, 2, 4 and 5 are presented in Fig. 3. A similar RBS spectrum was obtained for the polyaniline-quartz sample 6 (not shown). In all five samples, the shift in the silicon edge caused by the conducting polymer overlayer is essentially parallel to the silicon edge of the uncoated quartz fabric.This is direct evidence for the remarkable uniformity of the conducting Table 1 sample ~~ sample number sheet resistance (SZji7) corrected TG weight loss (YO) 6,,'/nm 6,,,d/nm pol ypyrrole-quartz 1 800" 1.34 45f3 47 f4 2 210" 2.15 72+4 68f5 3 120 2.24 80f3 7525 4 75" 2.94 99f4 94+5 5 25" 3.78 128f4 120f5 pol yaniline-quartz 6 - 10.41 28f3368 f8 "From ref. 4; bweight loss of each sample corrected by subtracting the average weight loss of uncoated quartz (0.4 f0.1%); 'assuming a quartz fibre diameter of 9.0pm, density of quartz fibre to be 2.20gcm-3 and the density of both polypyrrole and polyaniline overlayers to be 1.50 g cm-3; derror bars estimated from computer simulation of the energy shift spectrum ?! J. MATER.CHEM., 1991, VOL. 1 900 800 700 0 $ 600. s a .-3; 500 in c Q, 400 in 0,c..-v)8 300 20c 1oc c 0 20 40 60 80 100 120 140 Fig. 2 Variation of composite sheet resistance (Q/O) with conducting polymer overlayer thickness (6) for polypyrrole-quartz samples 1-5 (6 values calculated from TG data) I 1.20 1.25 1.30 1.35 energy/MeV Fig. 3 RBS spectra of polypyrrole-quartz composites of various coating thicknesses: (-..-) sample 5; (---) sample 4; (--.-) sample 2; (-..-) sample 1; and (-) bare quartz fabric polymer overlayers.Moreover, it is a welcome confirmation that the assumptions inherent in the calculation of the 6 values from the TG data using eqn (I) are, indeed, justified. Computer simulations of the silicon edge shifts enable us to calculate approximate overlayer thicknesses from the RBS data. These values are presented in Table 1. We wish to emphasize the close agreement between 6 values of the polyp- yrrole-quartz samples determined independently using the RBS and TG techniques. However, the 6 values calculated from the TG and RBS data for the polyaniline-quartz com-posite (sample 6) are substantially different (368+_ 8 nm and 28 f3 nm, respectively). Close examination of this particular sample using an optical microscope showed that loosely bound macroscopic aggregates of bulk polyaniline powder are randomly distributed throughout the quartz fibres. Obvi- ously, the presence of this powder affects the TG results for the sample and results in a considerable overestimate of 6.On the other hand, this bulk powder is not detected by the RBS technique, which only 'sees' the polyaniline coating that adheres directly to the quartz fibre. This surface-polymerized overlayer is clearly very thin and appears to be uniform when examined by SEM. Compared with the polypyrrole deposition process, we have generally found it much more difficult to eliminate the extraneous bulk-polymerized powder in order to prepare solely submicronic coatings of polyaniline on quartz or textile substrates.The use of NaVO, rather than (NH4)2S208 as a chemical oxidant for the aniline polymeriz- ation should be beneficial in this regard5. The conductivity of a single filament of a polypyrrole-quartz composite (sample 3 in Table I) was calculated using the relation o=l/RA (2) where R is the resistance in ohms (calculated from the current and voltage readings), A is the cross-sectional area of the filament, and 1 is the length of the filament between the inner two silver epoxy contacts. Assuming that the single filament is uniformly coated with polypyrrole (see Fig. 1) then A is given by A =2na6 (3) where a and 6 have their previously defined meanings. For sample 3, 6 2575 nm (from Table I), 1=0.80 cm (from the sample geometry), and we measure R to be ca.1.1 MR. Taking into account the various experimental uncertainties, we calcu- late the conductivity of the polypyrrole coating to be 35 f7 i2-l cm-'. This value is slightly higher than chloride- doped bulk polypyrrole powder prepared using FeCl, as the chemical oxidant', but it is similar to the conductivity of polypyrrole toluene-p-sulphonate powder prepared using iron(@ toluene-p-sulphonate in methanol or aqueous media (23-46 K cm -1).1491 As we have already noted, EDAX (and XPS4) studies indicate that aromatic sulphonate anions are incorporated as dopant anions in the polypyrrole-quartz composites in prefer- ence to chloride anions derived from the FeC1, oxidant.6 This observation explains the higher conductivity of the polypyr- role coating and its improved air ~tability.~ We wish to emphasize that the conductivity measurements in the present work were made on an 8 month old polypyrrole-quartz composite which had been stored under ambient conditions.Thus, it is likely that the conductivity of the freshly prepared sample would have been somewhat higher than 35 0-l cm-'.16 We examined the morphology of the polypyrrole-quartz and polyaniline-quartz composites by SEM. The thinnest polypyrrole (and polyaniline) overlayer (sample 1 in Table 1) seemed to be smooth and featureless (see Fig.4). However, the thicker polypyrrole coatings become less uniform, with some distinctly globular features reminiscent of bulk polypyr- role being observed (see Figs 5 and 6).We have reported similar features on polypyrrole-polyester textile com- posites6 Recently, D. F. Evans et al. observed similar morpho- logical changes in electrochemically synthesized polypyrrole and polythiophene films of increasing thickness by STM' We believe that, in the present work, this change in mor- phology can be rationalized by either one or both of the following two hypotheses. First, the substrate surface could be playing an important role in influencing the order and morphology of the conducting polymer overlayer. Obviously, this effect would be reduced for increasing coating thicknesses and for sufficiently large values of 6. The overlayer mor-phology would eventually be expected to revert to that of conventional, chemically synthesized polypyrrole.Secondly, it is possible that, for the thickly coated samples, the textile substrate surface area is insufficient to fully accommodate the Fig. 4 SEM image of a PolyPyrrole-quartz composite (sample 1; 6= 47 f4 nm by RBS) Fig. 5 SEM image of a PolYPYrrole-quartz composite (sample 4;6 = 94+ 5 nm by RBS) Fig. 6 SEM image of a polypyrrole-quartz composite (sample 5; 6 = 120k5 nm by RBS) amount of deposited polypyrrole. If this were the case, we might expect that some globular solution-polymerized pyrrole would be deposited in addition to and on top of the surface- polymerized pyrrole. Finally, the high resolution of the STM enables us to examine the morphology of these suPPosedlY smooth, feature- less PolYPYrrole overlayers in much greater detail- A typical STM constant current-height image of sample 3 is shown in J.MATER. CHEM., 1991, VOL. 1 Fig. 7. The polypyrrole coating appears to be made up of partially ordered aggregates of nanoparticulates (average diameter 5-10 nm). We have observed similar nanomorphol- ogies on other polypyrrole and polyaniline textile composites (polyester and nylon substrates), sterically stabilized polypyr- role and polyaniline colloids, and electrochemically synthe- sized polyaniline films.6 Conclusions We have shown that both TG and RBS can be used to determine the average coating thickness (6) of very thin (45-130 nm) polypyrrole overlayers on quartz fibres. The results obtained from these two independent methods are in close agreement.Furthermore, the latter technique provides direct evidence of the remarkable uniformity of these overlayers. The polyaniline-quartz sample we examined contained deposits of bulk polyaniline powder, which invalidated the measurement of the coating thickness 6 by thermogravimetry. However, the RBS technique enabled us to determine 6 to be ca.28nm. This overlayer was also smooth and uniform, as evidenced by SEM studies. It is likely that the observed powdery deposits could be minimized or even eliminated by using a more appropriate chemical oxidant, such as NaVO,, for the aniline polymerization. Our scanning electron microscopy studies confirm that, in some cases, these conducting polymer coatings can be remark- ably smooth and uniform.However, it seems that thicker coatings (6x125 nm) are much less uniform, with a distinctly globular morphology being observed. This change in mor- phology could be caused by a reduced ordering effect of the substrate. Alternatively, the globular deposits could simply be solution-polymerized pyrrole. The high-resolution of the STM enables us to examine the nanomorphology of the conducting polymer overlayers. In the case of the polypyrrole-quartz composites, the polypyrrole coating is made up of partially ordered aggregates of very small particulates (5-10 nm diameter). Similar morphologies have been observed in other conducting polymer systems. Finally, we have measured the electrical resistance of an isolated 9 pm diameter filament abstracted from a polypyr- role-quartz composite. Since the coating thickness 6 of this filament is known from our TG and RBS experiments, we have been able to determine, for the first time, the conductivity of the polypyrrole overlayer to be ca.35 R-' cm-'. Fig. 7 Constant current-height STM image of a polypyrrole-quartz composite (sample 3; 6 =75 k5 nm by RBS). Applied bias voltage 1600 mV; current 0.14 nA J. MATER. CHEM., 1991, VOL. 1 In conclusion, we have established a reliable characteriz- ation methodology for conducting polymer-quartz com-posites. We believe that our results have significant implications for the polypyrrole-textile and polyaniline-tex- tile composites currently being produced on a commercial basis by Milliken Research Corporation.These latter systems are important because they have the potential to overcome the various technological problems that have previously plag- ued the commercial development of conducting polymers. S. P. A. wishes to thank SERC for the travel grant that enabled this collaborative project to be carried out. This work was funded by the U.S. Department of Energy Advanced Industrial Concepts Division. References 1 Proc. Int: Con$ Synth. Met. (ZCSM '88) ed. M. Aldissi, Synth. Met., 1989, 27-29, and references therein. 2 R. V. Gregory, W. C. Kimbrell, and H. H. Kuhn, Synth. Met., 1989, 28, 823. 3 R. V. Gregory, W. C. Kimbrell, and H. H. Kuhn, Proc. A.C.S. Div. Polym.Chem. 1989, 30, 165. 4 R. V. Gregory, W. C. Kimbrell and H. H. Kuhn, Proc. 3rd Znt. SAMPE Electron. ConJ, 1989, 570. 5 H. H. Kuhn and W. C. Kimbrell, U.S. Pat. 4803 096, 1989. 6 S. P. Armes, M. Hawley, S. Gottesfeld, J. Beery and M. Aldissi, Lungmuir, in the press. 7 J. R. Tesmer, D. M. Parkin and C. J. Maggiore, MRS Bull., 1989, 12(6), 101. 8 T. Bettencourt, Technical Products Department, J. P. Stevens Company, personal communication. 9 S. P. Armes, M. Aldissi, G. C. Idzorek, P. W. Keaton, L. J. Rowton, G. L. Stradling, M. T. Collopy and D. B. McColl, J. Coll. Znterface Sci.,1991, 141(1), 119. 10 G. R. Mitchell, F. J. Davis, R. Cywinski and A. C. Hannon, Polym. Commun. 1989, 30, 98. 11 D. E. Stilwell and S. M. Park, J. Electrochem. SOC., 1988, 135, 248 1. 12 W. K. Chu, J. W. Mayer and M. A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978. 13 S. P. Armes, Synth. Met. 1987, 20, 367. 14 S. P. Armes, unpublished results. 15 J. A. Walker, L. R. Warren and E. F. Witucki, Am. Chem. SOC. Polym. Prep. 1987, 28(2), 256. 16 S. P. Armes and M. Aldissi, Polymer, 1990, 31, 569. 17 T. H. Chao and J. March, J. Polym. Sci.Polym. Chem., 1988, 26, 743. 18 R. Yang, D. F. Evans, L. Christensen and W. A. Hendrickson, J. Phys. Chem., 1990,94, 61 17. Paper 0/05588C; Received 12th December, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100525

出版商:RSC    

年代:1991

数据来源: RSC