摘要:

THE ROYAL SOCIETY OF CHEMISTRY INFORMATION FOR AUTHORS Journal of Materials Chemistry The Royal Society of Chemistry welcomes submission of manuscripts intended for pub- lication in two forms, Articles and Materials Scientific Editor Staff Editor Chemistry Communications. These should Professor Anthony R. West Department of Chemistry University of Aberdeen Meston Walk Science Park Mrs. Janet M. Leader The Royal Society of Chemistry Thomas Graham House describe original work of high quality dealing with the synthesis, structures, properties and applications of materials, particularly those associated with advanced technology. Aberdeen AB9 2UE, UK Cambridge CB4 4WF, UK Assistant Editor: Mrs. F. J. O'Carroll Editorial Secretary: Miss J. E. Chapman Graphics Designer: Ms.C. Taylor-Reid Articles Full papers contain original scientific work Materials Chemistry Editorial Board that has not been published previously. How- ever, work that has appeared in print in a short Anthony R. West (Aberdeen) (Chairman) C. Richard A. Catlow (London) David A. Dunmur (Sheffield) H. Monty Frey (Reading) John W. Goodby (Hull) David A. Rice (Reading) Rodney P. Townsend (Bebington) Allan E. Underhill (Bangor) Graham Williams (Swansea) 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, John D. Wright (Canterbury) Science Park, Milton Road, Cambridge CB44WF, UK.International Advisory Editorial Board M. A. Alario- Franco (Madrid) D. Kohl (Giessen) K. Bechgaard (Copenhagen) J. D. Birchall (Runcorn) M. Lahav (Rehovot) A. J. Leadbetter (Daresbury) Materials Chemistry Communications D. Bloor (Durham) A. K. Cheetham (Santa Barbara) P. M. Maitlis (Sheffield) J. S. Miller (Wilmington) Mate ria Is C hem ist ry Com m u n ica t ions con tain novel scientific work in short form and of such E. Chiellini (Pisa) P. S. Nicholson (Hamilton) importance that rapid publication is war- M. G. Clark (Wembley) M. Nygren (Stockholm) ranted. The total length is rigorously restric- P. Day (London) D. Demus (Halle) B. Dunn (Los Angeles) W. J. Feast (Durham) A. Fukuda (Tokyo) D.Gatteschi (Florence) A. M.Glass (Murray Hill) V. Percec (Cleveland) C. N. R. Rao (Bangalore) M. Ratner (Evanston) J. Rouxel (Nantes) R. Roy (University Park, PA) J. L. Serrano (Zaragoza) J. N. Sherwood (Glasgow) 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. , J. B. Goodenough (Austin) J. Simon (Paris) Submission of a Materials Chemistry Com- G. W. Gray (Poole) J. F. Stoddart (Birmingham) munication can be made either to The Editor, A. C. Griffin (Cambridge) S-i. Hirano (Nagoya) P. Hodge (Manchester) H.lnokuchi (Okazaki) W. Jeitschko (Munster) 0. Kahn (Orsay) S. Takahashi (Osaka) G. J. T. Tiddy (Bebington and Salford) B. J. Tighe (Birmingham) Yu. D. Tretyakov (Moscow) J. W. White (Canberra) R. J. P. Williams (Oxford) R. Xu (Changchun) 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- Journal of Materials Chemistry (ISSN 0959-9428) is published twelve 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, tetchworth, Herts SG6 1 HN, UK. NB Turpin Transactions Ltd., distributors, is wholly owned by The Royal Society of Chemistry. 1992 Annual subscription rate EC (inc. UK) f280.00, USA $599.00, Canada €331 .OO (plus GST), Rest of World f315.00. Customers should make payments by cheque in stirling payable on a UK clearing bank or in US dollars payable on a US 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.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, 1992. 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. 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. Professor A. R. West, Scientific Editor Tel.: Aberdeen (0224) 27291 8 Fax: (0224) 272938 Telex: 73458 UNIABN G Mrs. J. M. Leader, Staff Editor Tel.: Cambridge (0223) 420066 E-Mail (JANET): RSCl @UK.AC.RL.GB Fax: (0223) 420247 or 423623 Telex: 818293 ROYAL G 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/JM99202FX001

出版商:RSC    

年代:1992

数据来源: RSC

ISSN:0959-9428    

DOI:10.1039/JM99202BX003

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

i Conference Diary 1992 January 6-8 Fluoropdymers '92 Manchester, UK Dr E. Smith or Dr S.Dunn, Department of Chemistry, UMIST, Manchester M60 lQD, UK January 7-10 16th Annual conference on Composites and Advanced Ceramics Coca Beach, Florida,USA Dr Me1I. Mendelson, Mail Stop 1B33,Rocketdyne Div/Rockwell International, 6633 Canoga Avenue, Canoga Park, CA 91303, USA. Tel.: 818/718-4659; FAX: 818/118-3600 February 3-5 19th Australian Polymer Symposium Fmantle, Australia Dr Graeme George, Secretary RACI Polymer Division,ChemistryDepartment, University ofQueensland, St. Luua QLD 4067, Australia March 24-26 Fifth International Conference on Fibre Reinforced Composites Newcastle, UK conference Secretariat, FRC'92, The Plastics and Rubber Institute, 11Hobart Place, London SWlW OHL, UK March 30-31 Kautsdrukel&ische Pdymersysteme Bad Nauheim, Germatly GDCh-Geschiiftsst, Abteilung Tagungen, Postfach 90 0440,D-6000Frankfurtam Main,Gemany A@ 5-10 ACS Spring Meeting San Francisco, USA ACS, International Activities Office, 155 16th StreetNW,Washington DC20036, USA April 5-17 Nato AS1 on Multifunctional Mesoporous Inorganlc Solids sintm,Portugal ProfessorC&ar A.C.Sequeira, Instituto Superior TBcnico, Av. Rovisco Pais, 1096Lisboa codex, Portugal April 6-10 ICMCIR International Conference on Metallurgical Coatings and Thin Films San Diego, USA S.V.Krishnaswamy,Westinghouse Scienceand Technology Center, 13 10 Beulah Road, Pittsburgh, PA 15235,USA April 12-15 2nd European Conference on the Applications of Polar Melcctrics London,UK Ms K.J. Humphrey, Alcan International Lki, Banbury Laboratories, southam Road, Banbury, Oxfordshire OX167SP April 13-15 Regularities, Classifications and Predictions of Advanced Materials coma, Italy John R. Rodgers, National Research CouncilCanada, M-55,Room-275,Ottawa, Canada K1A OS2. Tel.: 613 993 3294. FAX: 613 952 8246. Internet: jn@vm.nrc.ca April 24-28 Fourth World Biomaterials Congress Berlin, Germany ProfessorDr Ulrich M. Gms, Institute of Pathology, Steghtz Clinic, FU Berlin,Hindenburgdamm 30, D-lo00 Berlin 45, Germany. Tel.: 030/ 798 22 96; FAX: 030/ 798 41 41. May 10-15 6th International Meeting on Lithium Batteries Miinster, Germany ProfessorDr J. 0.Besenhard, Departmentof Inorganic Chemistry, University of Miinster, Wfielm-KlemmSmsse 8, W4400 Miinster, Germany May 12-13 Flfth National Conference on Self-Formation, Physics,Technology and Application Vius, Lithuania Dr S.Janusonis, Semicmdutor Physics Institute, A Gostauto 11,232600Vilnius, Lithuania May 17-21 7th International Conference on Relation Between Homogeneous and Heterogeneous Catalysis Tokyo,Japan Professor Yasuhim Iwasawa, Chairman ofWSHHC,Department of Chemistry, The University of Tokyo, Hongo, Tokyo 113,Japan May 20-22 Fourteenth International Conference on Advances in Stabilization and Controlled Degradation of Polymers Lucerne,Switzerland In Europe: Dr N.C.Billingham,MOLS, University ofSussex, Brighton BNI SQJ, UK In the USA: Professor A.V.Patsis, Materials Laboratory, CSB 209, State University of New York, New Paltz, New Yok 12561,USA June 21-26 14th International Liquid Crystal Conference Pisa, My ProfessorE.Chielhi, Dept of Chemisuy, University of Pisa, Via Risorgimento 35,56100 Pisa,Italy July 1-15 International School of Materials Sdence and Technology: Solid State Ionics for Gas Sensors and Electrochromics Sidy, Italy W.Weppler, Max-Planck-Institute, W-7OOO Stuugart 80,Gemany. FAX: xx49-711-687 4371 July 5-12 EICS IV (European Gordon Conference) (Organometallic Compounds in Catalysis and in the Synthesis of Advanced Materials) Sheffield, UK ProfessorP.M.Maitlis FRS,Department of Chemistry, The University, SheBField S37HF 11 July 12-17 July 13-18 July 14-16 August 12-18 August 16-21 August 19-21 August 23-28 August 24-28 August 3 1 -September4 September 7 -11 September9-11 September 14-17 September16-18 September 16-18 September 17-20 Sptember 29-30 October25-30 October November 9-13 2nd International Sympium on Electrochemical Impedence Spectroscopy Santa Barbara, USA D.D.Macdonald, Penn State University, University Park, PA 16802, USA 34th IUPAC International Symposium on Macromolecules Prague, Czechoslovakia IUPAC Maao '92Secretariat, Instituteof Macromolecular chemistry, Czechoslovak Academy of Sciences, Heyrwkeho nam.188m16206 Prague 6,Czechoslovakia Speciality Polymers '92 London,UK conferenceOrganhr, !Speciality Polymers '92,ButtenvorthScientificLimited,PoBox 63, Westbury House, Bury Street, Gddfd, Surrey GU2 5BH, UK ICSM '92: International Conference on Sdence and lkchndogy ofSynthetic Metals Giiteborg, Sweden S.Stafstriim/IcsM'92,Departmentof Physics, IFM,Linkijping University, S-58183 Linkijging,Sweden. Tel.: + 46-13 281352; FAX:+613 137568; E-Mail: IFM::SST@SEWC51 (BITNET) SST@IFM.LKJ.SE (DQERNET) 5th International Conference on the Sdence and Technology of Wrconia Melbourne,Australia S.P.S. Badwal, CSIRO, Clayton, Victoria 3168, Australia. FAX: xx-61-3-5441128. 3rd International Symposium on Organic Materials for Nonlinear Optics Oxford, UK Elaine Wellingham, conference Secretariat, Field End House, Bude Close, Nailsea, Bristol BS19 2FQ ACS Autumn Meeting Washington, USA ACS, Internatid Activities Office,155 16th sveet NW,Washington DC20036,USA European Conference on Molecular Electronics Padua, Italy professor R Bozio, EaE 92,Department of Physical Chemistry, 2 ViaLoxedan, 1-35131 Padua, Italy Macromdecules '92 Canterbwy, UK DrA Amass, Department of Chemical Engineering and Allplied Chemistry, University of Aston, Aston Triangle, Birmingham €347ET, UK 2nd International Workshop on the Crystrrl Growth ofOrganic Materials (CGOM.2) Glasgow, UK Drs.D. Pugh and K.J. Roberts,Department of Pureand Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL. Tel.: 041-552-4400, FAX:041-552 5664 Fourth Meeting on Fire Retardant Polymers Freihq, Germany professor DrRolfMulhaupt, Institut fur Makromolekulare Chemie, Stefan-Meier Strasse 31, D-7800Freiburg I.Br., Gemany Applied Optics and Optoelectronics York,UK MeetingsMice,TheInstitute of Physics, 47 Belgrave !3quare, Lonh SWlX 8QX Sixth International Conference on Plastics in Telecommunications -in Parallel with Electrical, Optical and Acoustical hopertics of Polymers 111 Landon, UK ConferenceOffice,Plastics and RubberInstitute, 11 Hobart Place,London SW1W OHL Thermal Degradation of Polymers: Techniques, Mechanisms and Stabilisation: Polymer Degradation Discussion Group Annual Meeting Cambridge, UK ProfessorN.S.Allen,Department of Chemistry, Manchester Polytechnic, John Dalton Building, Chester Street, Manchester M1 SCD, UK Implant Retrieval Symposium StCharles, Illinois, USA Society for Bimaterials, Post Mice Box 717, Algonquin, IL60102 USA.Tel.:708/658-2909, FAX: 708/658-2921. Fractals and Disordered Systems Hamburg, Germany A. Bunk Institutefor Theoretical Physics, W-2a)OHamburg 36,Germany. FAX: xx49-4O-4123-6571. International Symposium on Chemistry and Physics of Molecular Bad Magnetic Materials Shuzenji, Japan ProfessorHiipl Iwamura, Department of chemistry,Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan FAX:81 3 38142627 6th Chinese National Conference on Mid State!Ionics Fujian Province,China Wang Wenji, Department of Chemistry, Fuzhou, Fujian, 350002, China 3rd Man Conference on Solid State Ionics VananaSiIndia S.Chandra, Department of Physics, Vanarasi Hindu University, Varanasi-221005, India.FAX: xx-91-542-31209. ... 111 ivvvi by a referee from such a paper is not available for citation until the paper is published. 1.4 Policy The primary criterion for acceptance of a contribution for publication is that it should advance scientific knowledge significantly. Papers that do not contain new experimental results may be considered for publication only if they either reinterpret or summarise known facts or results in a manner presenting an advance in chemical knowledge. Papers in interdisciplinary areas are acceptable if the chemical content is considered satisfactory. Papers reporting results regarded as routine or trivial are not acceptable in the absence of other, desirable attributes.Although short papers are acceptable, the Society strongly discourages the fragmentation of a substantial body of work into a number of short publications; such fragmentation is likely to be grounds for rejection. The length of an article should be commensurate with its scientific content; however, authors are allowed every latitude (consistent with reasonable brevity) in the form in which their work is presented. Figures and flow-charts can often save space as well as clarify complicated arguments, and should not be excised unless they are unhelpful or really extrava- gant. If a paper as a whole is judged suitable for the Journal, minor criticisms should not be unduly emphasised. It is the responsibility of the Editor to ensure the use of reasonably brief phraseology, and to assist the author to present his work in the most appropriate format.However, referees should not hesitate to recommend rejection of papers which appear incurably badly composed. It should be clearly understood that referees’ reports are made in confidence to the Editor, at whose discretion comments will be transmitted to the author. To assist the Editor, referees are requested to indicate which comments are designed only for consideration, as distinct from those which, in the referee’s view, require specific action or an adequate answer before the paper is accepted. Referees may ask for sight of supporting data not submitted for publication, or for sight of a previous paper which has been submitted but not yet published.Such requests must be made to the Editor, not directly to the author. 1.4.1 Authentication of new compounds. Referees are asked to assess, as a whole, the evidence in support of the homogeneity and structure of all new compounds. No hard and fast rules can be laid down to cover all types of compounds, but the Society’s policy is that evidence for the unequivocal identification of new compounds should wherever possible include good elemental analytical data; for example, an accurate mass measurement of a molecular ion does not provide evidence of purity of a compound and must be accompanied by independent evidence of homogeneity. Low-resolution mass spectrometry must be treated with even more reserve in the absence of firm evidence to distinguish between alternative molecular formulae.Where elemental analytical data are not available, appropriate evidence which is convincing to an expert in the field may be acceptable. Spectroscopic information necessary to the assignment of structure should normally be given. Just how complete this information should be must depend upon the circumstances; the structure of a compound obtained from an unusual reaction or isolated from a natural source needs much stronger supporting evidence than one derived by a standard reaction from a precursor of undisputed structure. Referees are reminded of the need to be exacting in their standards but at the same time flexible in their admission of REFEREEING PROCEDURE AND POLICY (1992) evidence.It remains the Society’s policy to accept work only of high quality and to permit no lowering of stand- ards. 1.5 Titles and summaries Referees should comment on titles and summaries with the following points in mind. Titles of papers are used out of context by several organisations for current awareness purposes. To enable such systems to serve chemists adequately, titles must be written around a sufficient number of scientific words carefully chosen to cover the important aspects of the paper. Summaries should preferably be self-contained, so that they can be understood without reference to the main text. 1.6 Speed of Refereeing The Editorial Boards are anxious to maintain and to reduce further if possible the publication times now being achieved.In this connection, referees should submit their reports with the minimum of delay, or return manuscripts immediately to the Editor if long delay seems inevitable. 1.7 Suggestion of Alternative Referees The Editor welcomes suggestions of alternative referees competent to deal with particular subject areas. Such suggestions are particularly helpful in cases where referees consider themselves ill-equipped (in terms of specialist knowledge) to deal with a specific paper, and in highly specialized or new areas of research where only a limited number of experts may be available. If, in such a case, the alternative and the original referee work in the same institution, the manuscript may be passed on directly after informing the Editor.1.8 Short Papers and Letters ‘Short Papers’ are published in J. Chem. Research. They are intended for the description of essentially complete pieces of work which can be described in two printed pages or less. They are NOT preliminary communications, nor in any way an alternative to Chemical Communications, for which there are additional criteria of novelty and urgency. The quality of material contained in a short paper should be the same as that in a full paper. Investigations arising out of some larger project but not prosecuted to the same degree are particularly appropriate for this format. A short paper should not normally exceed in length about 8 pages of typescript, including figures, tables, etc.It should comprise a one-sentence abstract and discussion, but adequate experimental details are required. As a consequence of its length, it appears in full in Part S with no microform version in Part M. ‘Letters’, published only in Dalton Transactions, are a medium for the expression of scientific opinions and views normally concerning material published in that journal; it is intended that contributions in this format should be published rapidly. The letters section is for scientific discussion, and is not intended to compete with media for the publication of more general matters such as Chemistry in Britain. Only rarely should a Letter exceed one printed column in length (about 1-2 pages of typescript). Where a letter is polemical in nature, and if it is accepted, a reply will be solicited from other parties implicated, for consideration for publication alongside the original letter.1.9 Relationship with Communications Journals In cases where a preliminary report of the work described has appeared (for example in Chemical Communications), referees vii REFEREEING PROCEDURE AND POLICY (1992) should alert the editor to any excessive and unnecessary repetition of material; this can arise in connection with communications journals in which the restrictions on length and the reporting of experimental data are less severe than those of Chemical Communications. Furthermore, the acceptability of the full paper must be judged on the basis of the significance of the additional information provided, as well as on the criteria outlined in the foregoing sections.2.0 Contributions to Chemical Communic-ations Chemical Communications is intended as a forum for preliminary accounts of original and significant work, in any area of chemistry that is likely to prove of wide general appeal or exceptional specialist interest. Such preliminary reports should be followed up in most cases by full papers in other journals, providing detailed accounts of the work. It is Society policy that only a fraction of research work warrants publication in Chemical Communications, and strict refereeing standards should be applied. The benefit to the reader from the rapid publication of a particular piece of work before it appears as a full paper must be balanced against the desirability of avoiding duplicate publication.The needs of the reader, not the author, must be considered, and priority in publication should not be allowed to determine acceptability. Acceptance should be recommended only if, in the opinion of the referee, the content of the paper is of such urgency that rapid publication will be advantageous to the progress of chemical research. The length of Communications is strictly limited; only in exceptional circumstances should it exceed one printed page (two-and-a-half to three A4 pages of typescript) and referees should be particularly critical of manuscripts longer than this. Communications do not contain extensive spectroscopic or other experimental data, but referees may ask for sight of such data before reaching a decision.The refereeing procedure for Communications is the same as that for full papers, except that rapidity of reporting is crucial in order to maintain rapid publication. The Journals Committee functions as the Editorial Board of Chemical Communications and as such acts as final arbiter in cases of dispute. 3.0 Communications submitted to The Analyst and J. Anal. At. Spectrom. Criteria for acceptance of communications submitted to The Analyst and J. Anal. At. Spectrom. are similar to those for contributions to Chemical Communications, except that they should be concerned specifically with analytical chemistry. However communications to The Analyst and J.Anal. At. Spectrom. are not subjected to refereeing in the usual way; a decision whether or not to publish rests with the Editor, who may or may not obtain advice from a referee. 4.0 Communications submitted to Perkin, Dalton or Faraday Transactions or J. Mater. Chem. Criteria for acceptance of Communications submitted to Perkin, Dalton or Faraday Transactions or J. Mater. Chem. are similar to those for contributions to Chemical Communications, except that the work will be of more specialist interest. For Perkin and Dalton Communications inclusion of key experi- mental data is expected. Assessment is carried out by a small nucleus of referees, consisting largely of members of the appropriate Editorial Boards. 5.0 Contributions to Mendeleev Communic- ations Mendeleev Communications, published jointly by the Royal Society of Chemistry and the USSR Academy of Sciences, is a sister publication to Chemical Communications, containing preliminary reports of the same type, in any area of chemistry.The majority of contributions are from Soviet authors. Assessment involves two stages of refereeing. Manuscripts submitted to the Soviet Editorial Office are refereed initially by a Soviet scientist. If found acceptable they are then reviewed by Western scientists chosen by the Royal Society of Chemistry. A favourable recommendation at this stage, from one referee, is sufficient authority for acceptance. If the recommendation is unfavourable, however, a second RSC referee is consulted; two unfavourable reports are required for rejection.Manuscripts submitted to the UK Editorial Office undergo this two-stage refereeing process in reverse. 6.0 X-Ray Crystallographic Work 6.1 Crystallographic papers are of two types: (A) The majority, which contain definitive data on completely refined determinations. (B) A minority which include brief accounts of structures containing feature(s) of unusual interest and where the structure solutions are clear but where (for any of a variety of reasons) the full refinement has not been completed. These are then regarded as preliminary publications, at least so far as the X-ray results are concerned. Both types of publication are appropriate for Communic- ations; only those of type (A) should normally appear as full papers.6.2 It is often appropriate (but not obligatory) for papers of type (A) to contain the information in their titles that an X-ray structure determination has been carried out. Papers of type (B) need not do so if the X-ray determination forms only a minor part. Summaries should always contain this information unless the paper is of type (B) and the structure determination is not a main point of the communication. 6.3 All papers containing crystallographic determinations will be refereed by two referees, one a structural chemist. If the editor considers it advisable, the paper may also be sent to a crystallographer for comment. Referees will not normally be expected to check values of structural parameters for publication (e.g.bond lengths and angles against atomic co- ordinates; this will be done after publication by CCDC or Bonn), but should still pay attention to the quality of the experimental crystallographic work. However their primary concern should be such new chemistry as is involved in the structure. 6.4 On occasions Communications will contain preliminary accounts [type (B)] of crystal structures of unusual chemical interest. By ‘preliminary’ is meant that the data have not yet been fully refined. Sufficient supplementary data must be provided for the referee to judge whether the ‘not-fully-refined’ structure does indeed prove the desired point, and care should be taken by the referees to ensure that the authors do not overstate the case they have-for example by reporting bond lengths to very high degrees of apparent precision when they have poor R-factors.Such papers will always be refereed by a professional crystallographer. Authors must indicate in the paper or the supplementary data the justification for publishing without full refinement and referees should comment on whether the case for publication is convincing. ... Vlll ix APPENDIX IUPAC Publications on Nomenclature and Symbohsm 1.O Compilations A one-letter notation for amino-acid sequences (1968) 1.1 Nomenclature of Organic Chemistry, a 550-page Abbreviations and symbols for the description of the hardcover volume published in 1979, available from Pergamon, conformation of polypeptide chains (1969) Oxford.Nomenclature of peptide hormones (1974) Section A: Hydrocarbons Recommendations for the nomenclature of human im- Section B: Fundamental heterocyclic systems munoglobulins Section C: Characteristic groups containing carbon, hy- Protein data bank. A computer-based archival file for drogen, oxygen, nitrogen, halogen, sulphur, macromolecular structures (1977) selenium, and tellurium Nomenclature of multiple forms of enzymes (1976) Section D: Organic compounds containing elements not Nucleotides and nucleic acids exclusively those referred to in the title of Abbreviations and symbols for nucleic acids, polynuc- Section C leotides and their constituents (1970) Section E: Stereochemistry Lipids Section F: General principles for the naming of natural Nomenclature of lipids (1976) products and related compounds Section H: Isotopically modified compounds Nomenclature of steroids (1967) Nomenclature of quinones with isoprenoid side chains 1.2 Nomenclature of Inorganic Chemistry, a 278-page (1973) hardcover volume published in 1990, available from Blackwell Tentative rules for the nomenclature of carotenoids (1970).Scientific Publications, Oxford. Amendments (1974) Chapter 1: General aims, functions and methods Nomenclature of tocopherols and related compounds Chapter 2: Grammar (1973) Chapter 3: Elements, atoms, and groups Carbohydrates, e tc. Chapter 4: Formulae Tentative rules for carbohydrate nomenclature. Part 1 Chapter 5: Names based on stoichiometry (1969) Chapter 6: Neutral molecular compounds Nomenclature of cyclitols (1973) Chapter 7: Names for ions, substituent groups and Phosphorus-containing compounds radicals, and salts Nomenclature of phosphorus-containing compounds of Chapter 8: Oxoacids and derived anions biochemical importance (1976) Chapter 9: Co-ordination compounds Miscellaneous Chapter 10: Boron hydrides and related compounds Trivial names of miscellaneous compounds of importance in biochemistry (1965) 1.3 Biochemical Nomenclature and Related Documents, a Nomenclature and symbols for folic acids and related 220-page softcover manual published in 1978 by The compounds (1965) Biochemical Society for IUB, and available from the Nomenclature for vitamins B-6 and related compounds Biochemical Society Book Depot, PO Box 32, Commerce (1973) Way, Colchester, Essex C02 8HP.The contents are as Nomenclature of corrinoids (1973) follows: General 1.4 Compendium of Analytical Nomenclature, a 280-page Nomenclature of organic chemistry. Section E: Stereo-hardcover volume published in 1987, available from Blackwell chemistry (1974) Scientific Publications, Oxford. The contents are as follows: Nomenclature of organic chemistry. Section F: Natural Presentation of the Results of Chemical Analysis products and related compounds (1976) Solution Thermodynamics (activity coefficients, equilibria, Nomenclature of organic chemistry. Section H: Isotopically PHI modified compounds (1977) Recommendations for Terminology to be used with Isotopically labelled compounds: common biochemical Precision Balances practice Recommendations for Nomenclature of Thermal Analysis Recommendations for measurement and presentation of Recommendations for Nomenclature of Titrimetric An- biochemical equilibrium data (1 976) alysis Abbreviations and symbols for chemical names of special Electrochemical Analysis interest in biological chemistry (1965) Analytical Separation Processes (precipitation, liquid- Abbreviations and symbols: a compilation (1976) liquid distribution, zone melting and fractional crystallis- Citation of bibliographic references in biochemical ation, chromatography, ion exchange) journals (1971) Spectrochemical Analysis (radiation sources, general Amino acids, peptides and proteins atomic emission spectroscopy, flame spectroscopy, X-ray Nomenclature of a-amino acids (1974) emission spectroscopy, molecular methods) Symbols for amino-acid derivatives and peptides (1971) Recommendations for Nomenclature of Mass Spec-Rules for naming synthetic modifications of natural trometry peptides (1966) Recommendations for Nomenclature of Radiochemical Abbreviated nomenclature of synthetic polypeptides or Methods polymerized amino acids (1971) Surface Analysis (including photoelectron spectroscopy) X xi surface science and chemical spectroscopy (Pure Appl. Chem., 1991,63,887).2.2.2 Analytical Nomenclature, symbols, units, and their usage in spectro- chemical analysis. Part VII, Molecular absorption spectroscopy, UV and visible (Pure Appl.Chem., 1988, 60, 1449). Part X, Preparation of materials for analytical atomic spectroscopy (Pure Appl. Chem., 1988,60,1461). Recommendations for publication of papers on a new analytical method based on ion exchange or ion-exchange chromato- graphy (Pure Appl. Chem., 1980,52,2555). Recommendations for presentation of data on compleximetric indicators. 1. General (Pure Appl. Chem., 1979,51, 1357). Recommendations for publishing manuscripts on ion-selective electrodes (Pure Appl. Chem., 1981,53, 1907). Recommendations on use of the term amplification reactions (Pure Appl. Chem., 1982,54,2553). Recommendations for the usage of selective, selectivity, and related terms in analytical chemistry (Pure Appl.Chem., 1983, 55, 553). Nomenclature for automated and mechanised analysis (Pure Appl. Chem., 1989,61, 1657). Nomenclature for sampling in analytical chemistry (Pure Appl. Chem., 1990,62, 1193). 2.2.3 Clinical Physicochemical quantities and units in clinical chemistry with special emphasis on activities and activity coefficients (Pure Appl. Chem., 1984,56,567). Quantities and units in clinical chemistry (Pure Appl. Chem., 1979,51,2451). Quantities and units in clinical chemistry: nebulizer and flame properties in flame emission and absorption spectrometry (Pure Appl. Chem., 1986,58, 1737). List of quantities in clinical chemistry (Pure Appl. Chem., 1979, 51,2481). Proposals for the description and measurement of carry-over effects in clinical chemistry (Pure Appl.Chem., 1991,63, 301). 2.2.4 Colloids and Surface Chemistry Definitions, terminology, and symbols in colloid and surface chemistry. I (Pure Appl. Chem., 1972, 31, 577). 11, Hetero- geneous catalysis (Pure Appl. Chem., 1976, 46, 71). Part 1.14: Light scattering (provisional) (Pure Appl. Chem., 1983,55,93 1). Reporting experimental pressure-area data with film balances (Pure Appl. Chem., 1985,57,621). Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Pure Appl. Chem., 1985,57,603). Reporting data on adsorption from solution at the solid/ solution interface (Pure Appl. Chem., 1986,58,967). Manual on catalyst characterization (Pure Appl.Chem., 1991, 63, 1227). 2.2.5 Electrochemistry Nomenclature for transfer phenomena in electrolytic systems (Pure Appl. Chem., 1981,53, 1827). Electrode reaction orders, transfer coefficients, and rate constants-amplification of definitions and recommendations for publication of parameters (Pure Appl. Chem., 1980,52,233). Classification and nomenclature of electroanalytical techniques (Pure Appl. Chem., 1976,45,81). Recommendations for sign conventions and plotting of electrochemical data (Pure Appl. Chem., 1976,45, 131). Electrochemical nomenclature (PureAppl. Chem., 1974,37,499). Recommendations on reporting electrode potentials in non-aqueous solvents (Pure Appl. Chem., 1984,56,461). Definition of pH scales, standard reference values, measurement of pH and related terminology (PureAppl.Chem., 1985,57,531). Interphases in systems of conducting phases (Pure Appl. Chem., 1986,58,437). The absolute electrode potential: an explanatory note (Pure Appl. Chem., 1986,543,955). Electrochemical corrosion nomenclature (Pure Appl. Chem., 1989,61, 19). Terminology in semiconductor electrochemistry and photo- electrochemical energy conversion (Pure Appl. Chem., 1991,63, 569). Nomenclature, symbols, definitions and measurements for electrified interfaces in aqueous dispersions of solids (Pure Appl. Chem., 1991,63,895). 2.2.6 Kinetics Symbolism and terminology in chemical kinetics (provisional) (Pure Appl. Chem., 1981,53,753). 2.2.7 Photochemistry Recommended standards for reporting photochemical data (Pure Appl.Chem., 1984,56,939). Glossary of terms used in photochemistry (Pure Appl. Chem., 1988,60, 1055). 2.2.8 Quantum Chemistry Expression of results in quantum chemistry (Pure Appl. Chem., 1978,50, 75). 2.2.9 Reactions Nomenclature for organic chemical transformations (Pure Appl. Chem., 1989,61, 725). System for symbolic representation of reaction mechanisms (Pure Appl. Chem., 1989,61,23). Detailed linear representation of reaction mechanisms (Pure Appl. Chem., 1989,61, 57). 2.2.10 Rheological Properties Selected definitions, terminology, and symbols for rheological properties (Pure Appl. Chem., 1979,51, 1215). 2.2.11 Spectroscopy Recommendations for publication of papers on methods of molecular absorption spectrophotometry in solution (Pure Appl.Chem., 1978,50,237). Recommendations for the presentation of infrared absorption spectra in data collections. A, Condensed phases (Pure Appl. Chem., 1978,50,23 1). Definition and symbolism of molecular force constants (Pure Appl. Chem., 1978,50,1709). Nomenclature and conventions for reporting Mossbauer spectroscopic data (Pure Appl. Chem., 1976,45,211). Recommendations for the presentation of NMR data for publication in chemical journals. A, Proton spectra (Pure Appl. Chem., 1972,29,625).B, Spectra from nuclei other than protons (Pure Appl. Chem., 1976,45,217). Presentation of Raman spectra in data collections (Pure Appl. Chem., 1981,53, 1879). Names, symbols, definitions and units of quantities in optical spectroscopy (Pure Appl. Chem., 1985,57,105). A descriptive classification of the electron spectroscopies (Pure Appl. Chem., 1987,59,1343). Presentation of molecular parameter values for IR and Raman intensity (Pure Appl. Chem., 1988,60, 1385). Recommendations for EPR/ESR nomenclature and conven- tions for presenting experimental data in publications (Pure Appl. Chem., 1989,61,2195). Nomenclature, symbols, units and their usage in spectro- chemical analysis -VIII. Nomenclature system for X-ray spectroscopy (Pure Appl. Chem., 1991,63,735). Recommendations for nomenclature and symbolism -for mass spectroscopy (Pure Appl. Chem., 1991,63,1541). 2.2.12 Thermodynamics A guide to procedures for the publication of thermodynamic data (Pure Appl. Chem., 1972,39,395). Assignment and presentation of uncertainties of the numerical results of thermodynamic measurements (Pure Appl. Chem., 1981,53, 1805). Notation for states and processes; significance of the word ‘standard’ in chemical thermodynamics and remarks on commonly tabulated forms of thermodynamic functions (Pure Appl. Chem., 1982,54,1239). xii

ISSN:0959-9428    

DOI:10.1039/JM99202BP007

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992,2(1), 13-18 Hydrothermal Synthesis and Characterization of Piezoelectric Lithium Tetraborate, Li,B,O, Crystals Kullaiah Byrappa and Kottamangalam V. K. Shekar Department of Geology and The Mineralogical Institute, University of Mysore, Manasagangotri, Mysore-570 006, India Lithium tetraborate, Li,B,O,, single crystals have been obtained for the first time by a hydrothermal method. Hydrothermal experiments were carried out at relatively low temperature and pressure conditions (T= 230 "C and P= 100 atmt). Solubility measurements were carried out by a weight-loss method and were found to be positive. The crystals obtained were characterized by various techniques, e.g. XRD, morphology, wet chemical analysis, DTA and IR spectroscopy. The advantages of a hydrothermal technique over earlier methods used in the growth of lithium tetraborate crystals are discussed.Keywords: Lithium tetraborate ; Piezoelectricity ; Hydrothermal synthesis Berlinite, a-quartz, lithium niobate (LN), lithium tantalate (LT) and zinc oxide thin films are being used widely for surface acoustic wave (SAW) devices. Except berlinite, these materials have some disadvantages in electromechanical coup- ling or delay. Recently, lithium tetraborate (LBO), as a new piezoelectric and non-ferroelectric crystal, has attracted great attention in research and industry for its application in SAW and bulk acoustic wave (BAW) devices, because it has a lower temperature coefficient of delay than LN and LT and a higher electromechanical coupling constant than wquartz.Since berlinite poses several major problems in its growth as large single crystals, LBO is becoming an attractive prospect as a piezoelectric crystal. LBO occurs naturally as diomignite, i.e. colourless crystals in fluid inclusions in the mineral spodumene. It occurs as very-fine-grained and loosely attached crystals and is of no economic significance, unlike its synthetic counterpart. Although an Li2O-B2O3 phase diagram was reported by Sastry and Hummel' as far back as 1959, single crystals were not obtained until 1977 by Garret et aL2 The raw materials for the growth of LBO in large quantities are abundant, and the ready availability of isotopically pure 6Li, loB and "B as starting materials make LBO growth both practical and economical.In this work, the hydrothermal synthesis of LBO is reported for the first time and the advantages and disadvantages of earlier reported methods of LBO growth and characterisation are discussed. Synthesis Before the work of Garret et d2only small crystals or grains of LBO had been obtained. This may have been because of the rather corrosive nature of the Li20-B203 melt towards the platinum crucible. Garret et al. used nickel crucibles. However, this method was rather cumbersome because of the complexity involved using a graphite susceptor and inert atmosphere. The only method of LBO crystal growth to date has been the melt technique, particularly the Czochralski (CZ) More recently large crystals (diameter 5.1 cm, length 10.2cm) of high-quality LBO have been obtained successfully for the first time by the Bridgman technique.6 At about the same time the preparation of LBO thin films by a sol-gel method was rep~rted.~However, these techniques t 1 atm= 101 325 Pa have many disadvantages.For example, the LBO crystals obtained by the CZ method always show 'coring' as the main macrodefect, thereby hindering wide industrial application of LBO in the manufacture of SAW and BAW devices. Coring is the growth of a continuous or intermittent opaque region parallel to the growth direction at the central core of the cry~tal~.~and is related to the growth rate, the deviation of the melt composition from stoichiometry (e.g.becoming Li rich) and the H20 or O2 content in the melt of LBO. In the Bridgman growth of LBO the authors6 have observed a similar opaque core and considered this defect to be a cluster of holes and a cellular or dendritic structure. A few white inclusions and evidence of constitutional supercooling in the holes or on the walls of cells have been observed also. Similarly, cracking by thermal stress occurs frequently in CZ growth owing to the anisotropic expansion coefficient of LBO. No cracks due to thermal stress occur in Bridgman growth of LBO for various orientations since the temperature gradient in the LBO crystal is smaller. However, if the residual melt on the top of an as-grown crystal in (100) or (1 10) directions changes into a polycrystalline phase by fast cooling, cracks in the LBO often occur in the top region.Similarly the sol- gel technique has some disadvantages, although this method does have several advantages, viz. high purity, low processing temperature, precise composition control and the formation of bulk monoliths, fibres and coating films. The important conclusions drawn from the work on the sol-gel method7 is that a larger amount of water is necessary for the crystalliz- ation of LBO thin gel films and also for the suppression of the formation of undesirable crystalline phases. This has prompted the present authors to attempt to synthesize LBO crystals by a hydrothermal technique. Obviously this will help not only in the crystallization of LBO crystals at lower temperatures, but also in solving many of the problems encountered in other methods.For example, the corrosive nature of the Li20-B203 melt towards platinum crucibles2 can be avoided by using a nickel crucible with a graphite susceptor and an inert-gas atmosphere. Robertson and Young, however, did not experience any difficulties with platinum crucibles'. To avoid such controversies, Teflon liners have been used by the present authors so that incorporation of impurities could be avoided totally. The experiments on the hydrothermal synthesis of LBO were carried out using Morey-type autoclaves (length, 15 cm; diameter, 3.5 cm, fabricated at the Mineralogical Institute, University of Mysore) and Teflon liners of capacity 25 cm'.The use of Teflon liners has helped in overcoming the corrosive J. MATER. CHEM., 1992, VOL. 2 nature of the Li20-B203 melt. The starting materials such e.g. HC1 and HN03 did not yield good crystals. Furthermore, as LiB02 (Analytical grade, Merck), or LiOH (Excelar grade, the molarity of HC02H was varied in the range 1-Glaxo) and boric acid (Excelar grade, Glaxo) were taken in 3.5 rnol dmP3 and it was found that good results were Teflon liners. The molar ratio of the starting materials was obtained in 1.5 mol dm-3 HC02H solution. These results will selected by theoretical calculations. A suitable mineralizer be considered in detail later. Ideal experimental conditions in solution of a specific molarity was added to the Teflon liner the hydrothermal synthesis of LBO crystals can be written as and the entire mixture was stirred well until a homogeneous follows: LiB02, 1.7 g; B203,2.3 g; 1.5 mol dm-3, 8 cm3; T= and relatively viscous solution was obtained.The percentage 250 "C; P= 100 bar; 60% fill for 8 days duration. fill varied in the range 50-60%. Various different mineralizers The crystallization was carried out in all experiments were tried, e.g. HCI, HC02H, H2S04 and HN03. The exper- through spontaneous nucleation. In order to control the imental conditions are given in Table 1. It was found that nucleation centres, several effective measures have been HC02H only acts as a suitable mineralizer; other solvents adopted in the present work which are discussed in detail Table 1 Experimental conditions for the growth of Li,B,O, crystals nutrient composition T/ "C Plbar Oh fill timeldays size/mm remarks LiBO,, 1.668 g B,03, 2.332 g HCO,H(3 rnol dm-3), 8 cm3 250 120 70 8 4-14 bulk crystals of tetragonal and regular shape LiBO,, 1.668 g Bz03, 2.332 g HCO,H(2.5 moldm-3), 7.5 cm3 250 115 65 9 1-8 pol ycr ystalline and wedge-shaped crystals LiBO,, 1.668 g B,03, 2.332 g HCO,H(2 rnol dm-3), 7 cm3 250 100 60 8 3-10 wedge-shaped crystals LiBO,, 1.668 g B,03, 2.332 g HCO,H(1.5 mol dm-3), 6.5 cm3 250 100 55 7 1-8 wedge-shaped crystals LiBO,, 1.668 g Bz03,2.332 g HCO,H(1.5 rnol dm-3), 6 cm3 250 90 50 8 0.5-2 well developed tetragonal crystals LiBO,, 3.2 g 250 70 45 7 0.2-1 small tetragonal HCO,H(l.S moldm-3), 3.8 cm3 B2°3, 4.0 g crystals LiBO,, 2.0 g 250 80 48 7 0.4- 1 well developed HCO,H(l.5 mol dm-3), 5 cm3 B203, 4.0 g tetragonal crystals LiOH, 0.8 g 250 I00 60 7 0.2-1 twinned HCO,H(l.S rnol dm3), 7 cm3 B2°3, 3.2 g te tragonal crystals LiOH, 0.8 g HCO,H(1.5 moldrnp3), 7 cm3 B203, 3-2 g 260 120 60 7 0.1-0.6 small tetragonal crystals Li,CO,, 1.0 g 260 120 60 8 0.1-0.5 poor crystal HCI(5 rnol dm-3), 7 cm3 B2°3, 3.0 g aggregates Li,C03, 1.Og 250 100 60 8 0.08-0.2 small crystal HCl(4 mol dm-3), 7 cm3 B2°3, 3.0 g aggregates Li,CO,, 1.0 g HCl(3 mol dm-3), 7 cm3BzO3, 3.0 g 250 100 60 8 0.08-0.2 small crystal aggregates Li,CO,, 1.0 g 250 100 60 6 0.0 1-0.04 crystal HCl(2 mol dmP3), 7 cm3 B2°3, 3.0 g aggregates Li,CO,, 1.Og H,SO,(2 mol dm-3), 7 cm3 B203, 3.0g 250 100 60 6 0.01-0.2 very-fine-grained crystals Li,CO,, 1.0 g H2S0,(1 rnol dm-3), 7 cm3 B2°3, 3*0g 250 90 60 7 0.01-0.5 crystalline powder Li,CO,, 1.0 g 250 90 60 7 0.05-0.3 pol ycrystalline HN03(2 rnol dm-3), 7 cm3Bz0333.0g mass Li,CO,, 1.0 g 250 90 60 8 0.05-0.4 pol ycrystalline HN0,(1.5 rnol dm-3), 7 cm3 BZ03, 3.0 g mass Li,C03, 1.0 g HN03(l rnol dm-j), 7 cm3 B203, 3-og 250 90 60 8 0.1-0.4 transparent pol ycrystalline mass J.MATER. CHEM., 1992, VOL. 2 later. The pressure in the growth system was internal and was calculated in most of the experiments based on P, V and T relationships. In three initial experiments the pressure was measured using a Bourdon gauge fixed to the autoclave (LRA, Leco corporation, Tempress Division, USA) in order to main- tain 100 bar with reference to the volume (molar ratio of the starting materials) in the subsequent experiments using Morey autoclaves.As such, the crystals obtained under the said conditions are highly transparent and vary in size from 5-6mm. The crystals are transparent and the faces are well developed (Fig. 1). In general the morphology of the hydro- thermally grown LBO is very interesting and varies widely depending upon the experimental conditions, particularly starting materials and percentage fill. Even the size of the crystals varies widely (1-12 mm) depending on these param- eters. As the pressure in the system was decreased slowly (to 40atm) the size of crystals increased.In some experiments well developed and perfect tetragonal crystals (Fig. 2) of small size were obtained and in others wedge-shaped, larger and irregular crystals were obtained. Table 2 gives the starting materials and the corresponding morphology. As is evident from Table2, the morphology varies significantly from a polycrystalline form to a single-crystal form as the molarity of HC02H and the internal pressure change. This has been clearly represented in the form of schematic diagrams [Fig. 3(a)-(e)]. The crystals become less transparent with increasing size. LBO crystals can be obtained within the molar ratio Li20 :B203= 1:7-9. Chemical analysis of the LBO shows that the ratio Li20 :B203is higher than the theoretical ratio (69.2mol% B203 experimental vs.66.6 mol% B203 theoretical) as in the case of ref. 8. Fig. 1 Li,B,O, crystal; scale bar= 1 cm Fig. 2 Li2B40, crystal, magnification x 40 In the present work, emphasis is placed on the control of nucleation centres since crystallization was carried out by spontaneous nucleation. In order to reduce the number of nucleation centres the temperature of the crystallization reac- tor was increased very slowly at a rate of 10 "C h-' up to 100 "C,and beyond this at 5 "Ch-l. The nutrient materials were held at 250 "C for a period of 3 days without any fluctuations in the temperature. From the fourth day onwards temperature fluctuations of ca.20 "C were introduced period- ically in order to reduce the nucleation centres. By this means the small crystallites that had developed previously dissolved and only the larger crystallites remained. This resulted in the growth of larger crystals (12 mm x 10 mm x 10 mm). Unfortu- nately, as the size of the crystal increased, the quality in terms of morphology and the transparency decreased. Some of the larger crystals became slightly irregular, although most of them retained the overall tetragonal morphology [Fig. 4(a) and (b)].Twinning was quite common and the surface mor- phology of LBO showed very interesting features, particularly etch pits, steps and block structures. Detailed morphological studies, including the surface morphology of the as-grown surfaces of the hydrothermally grown LBO crystals will be published separately. The crystal faces showed striations.Furthermore, the larger crystals showed cracking owing to thermal stress during the sudden quenching of the autoclaves after the experiments. We are now attempting to study the effect of cooling on thermal cracking. Striations and thermal cracking are commonly noticed in LBO crystals obtained by the CZ and Bridgman techniques. The greatest advantage of hydrothermal growth is that thermal stress either during growth or during quenching, can be overcome easily. Further- more, the hydrothermal method helps in growing purer and larger single crystals. This work has demonstrated clearly the advantages of growing LBO crystals devoid of all macro- defects.Even the separately crystallized LBO crystals with an experimental duration of 8-10 days have yielded crystals of 12 mm x 12 mm x 10 mm size, with higher growth rates. A careful study of growth parameters and seeding will make this technique a more attractive one for LBO. It is well known that both LBO and berlinite are the most suitable piezoelectric materials, having high SAW coupling coefficients for high frequency and temperature compensation. It is appropriate to mention here, that the growth of berlinite has been shown to be extremely difficult, particularly with a view to obtaining large single crystals of high quality. This is because of the difficulties associated with the highly corrosive nature of P205, i.e. its high reaction susceptibility and the negative temperature coefficient of solubility.Hence, the successful growth of high-quality LBO by a hydrothermal technique would solve the existing problems with respect to techniques adopted so far, and the use of Teflon liners and the availability of isotopically pure starting materials helps the growth of high-purity crystals. Furthermore, the temperature and press- ure conditions involved in the growth of LBO permit very simple autoclave designs and higher practicability. In the present work the authors have studied the solubility of LBO under hydrothermal conditions with reference to the molarity, percentage fill, pressure, etc., in different solvents.The solubility was found to be positive. The solubility measurements were carried out by a weight-loss method in different solvent media. Figs. 5(a)-(g) show the solubility curves for LBO crystals with varying molarity (pH) of LiOH, HC02H, pressure (YO fill) and temperature. As is evident from Fig. 5(a)-(g), the ideal temperature for the growth of LBO crystals is within the range 200-260 "C [Fig. 5(a)].Similarly, the ideal molarity range of the mineralizer in the growth of LBO crystals is 1.5-1.8 mol dm-3 [Fig. 5(b)]. J. MATER. CHEM., 1992, VOL. 2 Table 2 Morphology of L2B407 crystals starting materials HC02H/cm3 (mol dm-3) Yo fill T/ "C remarks 70 65 250 250 polycrystalline and ball-like crystalsQ polycrystalline wedge-shaped crystalsb 60 55 50 250 250 250 wedge-shaped crystals' wedge-shaped crystalsd tetragonal single crystals' Li BO /g B2°3/g 1.668 1.668 2.332 2.332 8 (3)7.5 (2.5) 1.668 1.668 1.668 2.332 2.332 2.332 7 (2)6.5 (1.8) 6 (1.5) "See Fig.3(4; bFig. 3(b); 'Fig. 3(c); dFig. 3(4; 'Fig. 3(e) (c) (d) (e 1 Fig. 3 Schematic diagrams of Li2B407 crystals Characterization The LBO crystals obtained by the hydrothermal method were subjected to X-ray diffraction (XRD), chemical analysis, differential thermal analysis (DTA) and IR spectroscopy. A crystal of ca. 0.2 mm x 0.2 mm x 0.2 mm was selected to obtain XRD intensities in a single-crystal Enraf-Nonius CAD-4 diffractometer, with radiation ;1 (Mo-Ka)= 0.07107 nm. Analy- sis of the reflections reveals that the structure is of tetragonal space group 14, Cd, a = 9.47(3) A, c = 10.26(8)A.These values are in good agreement with the earlier report.' XRD was employed to identify the products of all the experiments. The interpretation of the diffraction patterns revealed no additional phases in the product. Li2B407 crystals were obtained throughout the present experimental range except in experiments where the boric acid was in surplus. Deviation in the stoichiometry resulted in the crystallization of additional phases, e.g. Li4B7OI2 Cl, Li3B508(OH)2, LiB305, Li3B03, H2LiB509, Li6B409 and two unidentified new phases. The structure of LBO consists of two three-dimensional inter- locking networks extending throughout the crystal.The basic unit of the network is a group of two boron atoms tetra- hedrally co-ordinated by oxygen, and two boron atoms tri- angularly co-ordinated by oxygen. The atoms in the group are arranged as a twisted double ring. Li2B407 crystals were subjected to wet chemical analysis using an induction coupled plasma spectrophotometer (ICP, Labtam 8400R, Australia. Specpure internal standards (Law- rence and Mayo, UK) were used. The results of the wet Fig. 4 Li2B407 crystals chemical analysis for a representative sample of Li2B407 crystals were B203 = 80.03, Li20= 19.52, sum = 99.55 wt.% The DTA curves were recorded using a Stanton-Redcroft DTA apparatus (DTA 673-4) for some representative samples of LBO. They did not show any phase transition and melted completely at 917 "C.The IR spectrum of a characteristic sample of LBO was recorded using a Spectrophotometer (Specord 75 IR, GDR) within the range 4000-400 cm- (Fig. 6). The basic structural unit of borates in general consists of [BO3I3- triangles and [B0415-tetrahedra to form an interlocking network. The [BO3I3-show D3h symmetry exhibiting the following vibrations: A\ (vl) and A';(v2); E(v3 and v4). v1 corresponds to the valence vibrations and v2 corresponds to the defor- mational vibrations. v3 represents the symmetric valence vibration and v4 the asymmetric deformational vibration. The absorption E is very active in the IR spectrum with a decrease in symmetry from triangular (D3h), and the splitting of the vibrations v3 and v4 and the symmetric vibration v1 occur.The fundamental vibrational frequencies for [BO,] -are J. MATER. CHEM., 1992, VOL. 2 17 90 c -q1 -70 h80 s 3 5 60 v 50 -0,ci mN.-m, --I 1 40 30 --10 50 100 150 200 250 0.5 1.0 1.5 2.: 2.5 90 -V°C [HCO,H]/mol dm-(c(c )) (d ) h I70 -sI s 32-v -50 40, m, .-m, 7 1-I-I 30 -50 100 150 200 250 Ti"C lot I I I 0.5 1.0 1.5 2.0 2.5 3.0 [HCO,H]/mol dm-3 (e 1 60 -50 h s $ *V,40 0, m,.--I 30 -'20 P/bar -(9) 10 -60 1. I 0.5 1.0 1.5 2.0 2.5 -[LiOH]/mol dm-3 50 h sg 40-Y ci5 30-J -20 10 -200 400 600 800 1000 P/bar Fig. 5 Solubility curves for Li,B,O, crystals.(a) 1.5 mol dm-3 (HC02H, P= 100 bar, (b) T=250 "C;P= 100 bar; (c) T=400 "C, P=200 bar; (d) 1.5 mol dm-3 LiOH, P= 100 bar; (e) T=400 "C, P=200 bar; (J) 1.5 mol dm-3 HC02H, T=400 "C;(g) 1.5 mol dm-3 LiOH, T=400 "C 3600 3000 2200 1600 1200 800 400 v/cm-' Fig. 6 IR spectrum of Li2B,0, crystals v1 = 1000-1 100 cm-'; v2 =720-750 cm-'; v3 = 1200-1350 cm-'; v4=620-650 cm-'. For the distorted [B0415- tetrahedra showing Td symmetry the characteristic vibrations arelo A, (v,)=926, E(v2)=480, E(v3, v4)=930 and 320 cm-'. In the IR spectrum v3 and v4 only are very active, and a fall in the Td symmetry causes a splitting of the absorption bands. The spectra recorded match well with spectra reported pre- viously.11.'2 Conclusions A hydrothermal technique has been successfully employed in the growth of LBO crystals.The use of a hydrothermal method helps to overcome several existing problems related to the growth of LBO crystals by Czochralski or Bridgman techniques. LBO crystals were obtained by spontaneous nucleation and the crystal size varied from 5 to 12 mm. Hydrothermally grown LBO crystals were characterized by J. MATER. CHEM., 1992, VOL. 2 several techniques showing that the hydrothermal method is highly successful for the growth of larger LBO crystals. The authors wish to thank Professor S. Gali at the University of Barcelona for XRD work, Dr. Norio Ohnishi of National Electrotechnical Laboratory, Tsukuba, Japan for his encour- agement and Miss K.Vasundhara and Miss M. Radhika for their assistance in preparing this article. References I B. S. R. Sastry and F. A. Hummel, J. Am. Ceram. SOC., 1958, 41, 7. 2 J. D. Garrett, M. Natarajan-Iyer and J. E. Greedan, J. Cryst. Growth, 1977, 41, 225. 3 D. S. Robertson and I. M. Young, J. Muter. Sci., 1982, 17, 1729. 4 M. Adachi, Jpn. J. Appl. Phys., 1985, 24, Suppl. 24-3, 72. 5 R. Komatsu, T. Suetsugu and M. Ono, J. Cryst. Growth, 1988, 15, 12. 6 S. Fan, G-S. Shen, W. Wand, J-L. Li and X-H. Le, J. Cryst. Growth, 1990,99, 81 1. 7 H. Yamashita, T. Yoko and S. Sakka, J. Muter. Sci., Lett., 1990, 9, 796. 8 D. S. Robertson and I. M. Young, J. Muter. Sci.,1982, 17, 1729. 9 J. Krogh-Moe, Acta Crystallogr., 1962, 15, 190. 10 V. N. Apolonov and D. G. Koshug, Vetsnik, Moscow State University, Geol. Ser. 4, 1990, 3, 55. 11 R. A. Nyquist and R. 0. Kagel, in Infrared Spectra of Inorganic Compounds, Academic Press, New York, 1971, p. 57. 12 S. D. Ross, in Borates, The Infrared Spectra of Minerals, London, 1974, p. 205. Paper 1/00805F; Received 20th February, 199 1

ISSN:0959-9428    

DOI:10.1039/JM9920200013

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992, 2(1), 19-21 Impact of a-Particles on the Nearest-neighbour Environment in Cd,Hg, -,Te Surface Layers Dmitriy 1. Bidnyk" and Sergey P. Kostenkob a Rodon Manufacturing Unified, 225 Gagarin St, 284006 lvano-Frankovsk, USSR Albedo 0.39 Research and Production Centre, 63 Kirov St, 270045 Odessa, USSR a-Particle irradiation of the gapless semiconductors, cadmium mercury telluride in particular, increases the stability of their surface structures and allows variation in the relative concentrations of the surface cadmium and mercury atoms in the parent substrates. a-Particles are specific for changing the nearest-neighbour interaction pattern. The direct electronic interaction becomes predominant owing to the negligible size and high (+ 2) electric charge of the a-particle giving rise to electron density redistribution between Hg 5d and Te 5p bands.Keywords: or-Particle irradiation ; Surface; Cadmium mercury telluride ; Electron density redistribution The important cadmium mercury telluride (CMT) alloy con- tinues to remain popular with photodetector engineers. Some years ago, however, an alternative material, cadmium zinc telluride (CZT), which presented certain new merits, was introduced.' The first and foremost of these merits is the longer stability of CZT over CMT. Note that CZT preparation routes have been derived from the CMT procedures, and CMT and CZT alloys with identical band gaps are rather similar in their kinetic behaviour. CZT has not initiated any revolutionary changes.This may be related to the thermodynamic equilibrium processes involved in both CMT and CZT production. Substitution of Zn for Hg modifies the interaction pattern from a rather deformational one to an electronic one owing to the smaller ionic radius of zinc. The equilibrium state of a system of matrix atoms, impurit- ies, and defects in a solid is determined by their interaction,2 which in turn specifies the inherent physical and chemical f~nctions.~The defect interactions have been detailed rather widely: the direct electronic interaction, the indirect inter- actions of the dipolar impurities, the deformational contri- bution, et~.~-~It is of interest to investigate the interaction of defects with substantial dipole moments.This phenomenon may be a superposition of the above interactions. For example, the possibility of electronic or deformational phenomena is determined by the value of the ionic charge in comparison with ionic size. That is, the pure electronic interaction is modified by ionic size which must tend to zero in contrast to the maximum available charge. For this purpose He2+ ions (a-particles) are considered most suitable because He2 + sur-face-layer interactions (interparticle spacings are negligibly small) are dominated by the interband contributions. The dipoles interact in spin fashion (no dihedral influence whatso- ever)' and change the effective charges on the atoms. This involves quantum-mechanical electronic-density and free-car- rier surface redistributions and the macroscopic 'chemical contraction' of the crystal with augmented anion radii.' Finally, there is one more piece of evidence which supports the use of a-investigation of the defect and impurity electronic interactions in Cd,Hg, -,Te subsurface layers.He+ -Hg, is to date the only system known with unit probability for the collision-averaged charge exchange, for both helium isotope^.^ The absolute probability of ca. unity charge exchange may result merely from the various interaction regions within the small interparticle separations in the system. The above made us turn to CMT once again. The primary goal of the present study was to elucidate the impact of He2+ ions on Cd,Hg, -,Te matrix atom interactions in the surface layers. Experimental The sample pieces were sliced from a Cdo.,Hg,.,Te wafer and 238Puwas a-irradiated with an activity of 3.3 CiT and particle flow density 5 x lo7 s-' cm-2.He2+ exposure varied from 0.5 to 4 h. Both the parent and the irradiated samples were investigated by Auger electron spectroscopy (AES) with the apparatus 09 IAS-10-005 (Ryazan, USSR). We shall expand further on how the AES technique is applied for relating the processes at surfaces and interfaces with the behaviour in the bulk material. As this method is destructive, the research effort concentrated on modifying the sample as little as possible (low-energy primary electron beam, emission current, etc.). Energies of the focused electron beam as low as ca.20 eV started the desorption of atomic and molecular ions and neutral particles from the In other words, the electron beam could initiate and control certain ion-molecular reactions. This finding points to the possibility of inducing some chemical restructuring in specific surface sites.I2 The electron beam stimulates a notable outward diffusion of the matrix atoms towards the electron flux and their build- up across the sample surface.13 This led us to the repeated irradiation of one surface site with in situ secondary Auger analysis. The total irradiation dose was thus increased, involv- ing specific variations of the surface chemistry. The experimental conditions for all such Auger measure- ments were essentially identical, yet from the relative changes in the spectra it is possible to deduce the interconversion of surface structures owing to the electron impact.Systematic alteration of background noise, by the anticipated 'worst approximation', did not affect the relative intensity or location of the various peaks of the Auger spectra of samples subjected to electron beam irradiation. The investigated range of primary energies varied from 2 to 5 keV. Irradiation time was 1-15 min. Since the same surface site of the sample was electron-irradiated repeatedly (5-6 times in some cases) the maximum duration was 100 min. The spectra were measured with the 'amplitude modu- lations 0.8-6 eV, response time was 0.001-0.3 s. t Ci =3.7 x 10'' Bq. Results and Discussion Fig.l(b) presents the Auger spectrum of the initial Cd, .,Hg, .,Te surface, which demonstrates the familiar surface features: deficiency of mercury in tandem with tellurium atoms predominant in the matrix framework with built-in cad-mium. Fig.2 presents Auger spectra of the a-bombarded Cd, .,Hg, .,Te surface. The initial pattern of a-particle impact on the CMT surface involves both He2 +-Hg quasi-molecule formation and cadmium molecule polarization. The Te elec- tronic structure is preserved, unaffected except for the valence- state configurations which gain in valence-band density. This is consistent with our previously detailed" Te (NOO) Auger transitions for a variety of matrices: CdTe, Cd,Hg, -,Te,Cd,Zn, -,Te. Peak mercury amplitudes within +the range 58-64 eV manifest He -Hg quasi-molecule formation.Now consider the details of the above phenomenon. The Auger atomic mercury N6045045 and N704504s transitions involving 4f and 5d electrons, of 80.55 and 77.75 eV,16 respect- ively, may depend upon the matrix to attain energies within the 70-90eV17 range. Brief irradiation of the surface layer (spectrum 1, Fig. 2) gives rise to several Auger transitions characteristic of mercury, with a pronounced N7045045 Auger peak. The peak positions are at 61.7, 62.2 and 64.3 eV. To verify the atomic mercury origin of the above Auger transitions HgTe wafers were also investigated [Fig. l(a)]. The corresponding N6045045 and N7045045 maxima were, respectively, 71.8 and 79.6 eV, and 73.3 and 82.9 eV.The Auger peak at 31 eV Te valence electrons provides evidence that the top layer consists of Te atoms. Variation of the a-irradiation brings about redistribution of the subsurface atoms. This is considered to result from the electronic density redistribution in the system, which involves energetic changes in each atom. Since both the strength and the nature of the xl i Te 79.6 eV Te71.aI 59.7, Te (MNN) x0.5 1 I Cd Te(N4023023) 31eV 4d 5p 5p , 'Te ,I I I 100 200 300 400 500 60(energylev Fig. 1 Auger spectra of the initial surfaces: (a)HgTe; (b) Cd,.,Hg, ,5Te J. MATER. CHEM., 1992, VOL. 2 xl 0 Te Cd Hg 5d and Te 5p electron band splitting He-Hg-He Te I I I I I I 100 200 300 400 500 60C energy/eV Fig.2 Auger spectra of the Cd,.,Hg,.,Te surface after a-irradiation with small (a)and medium (b) doses atomic interactions as well as the free-carrier concentrations can be deduced from the electronic energy distributions the possibility opens up for a-particle controllable interaction. Besides the peaks at 59.7 and 61.2eV, Auger peaks have been observed at 58.2 and 60.9 eV, 101.5, 108.3, and 11 1.6 eV. Energies of the latter Hg series varied within +4eV. The abundance of electronic configurations is due to surface defects such as mercury vacancies, and provides evidence for the distinctly non-spherical distribution of charge around the cations. The possible nature of p- and d-rehybridization by the nearest neighbours accounts for the instability of the surface phases. a-Particles provoke drastic change in the matrix interaction patterns primarily owing to the polarization of the atoms which start interacting in a dipolar manner.A single a-particle thus co-ordinates both mercury and cadmium atoms. The Hg-He2 + -Cd quasimolecule inserts itself as a built-in unit into the tellurium framework. Mild doses (spec- trum 2, Fig. 2) cause the antipolarizing 5p tellurium electrons to compensate for a-particle effects. The resulting system is quite stable against perturbative treatment. Significant He2 +-irradiation gives rise to He-Hg quasi-molecules owing to an electron injection from the 4fI4 Hg shell into the He2+ 1s' orbital. The s-f hybridization involved therein may choose one of two routes: (Hg) 4f14+ls0 (He2')+(HgHe') 4fI3 1s' (1) or (Hg) 4f14+1s0 (He2')+(HgHe) 4f12 ls2 (2) Across the interparticle spacings Ri (where the vector radius Ri determines the position of the ith implanted particle) the interaction energy is an oscillating angular function, common for ordinary dipolar interactions.The minimization of Ri(a-irradiation increased) gives rise to the dominant interband J. MATER. CHEM., 1992, VOL. 2 J4P-l Fig. 3 Photoluminescence spectra (77 K) for a freshly cleaved Cd,,,Hg,,,Te surface (a)and after 6 months (b) I> 113 1.5 1.7 1.9 VPm Fig. 4 Photoluminescence spectra (77 K) for the @-irradiated Cd,,,Hg,,,Te sample, freshly cleaved (a)and after 6 months (b) interaction.The dipoles interact as spins (with null angular dependence), with ferroelectric clusters being predominant at low temperatures. In eqn. (1) the a-particle co-ordinates two mercury atoms. In eqn. (2) the aggregation of He-Hg-He quasimolecules predominates. Cd atoms are forced down into the bulk, 5d Hg bands split, Te atoms become polarized, and result in a drastic increase in the surface Te valence electrons. The Hg Auger series comprises a pronounced 78.4eV peak which is essentially the same as an N7045045 transition. Correlation of this particular fact with cadmium (MNN) amplitude con- traction suggests the surface vacancies to be filled by bulk Hg in exchange for the expelled Cd (atomic redistribution). Hg does not desorb, owing to the increased interaction between the nearest neighbours which is governed by the degree of polarization.Electron-beam-induced in situ weak desorption of the Hg surface atoms from a-irradiated Cd,Hg, -xTe supports the increase in stability. The photoluminescence spectra (77 K) from unirradiated and a-irradiated samples, as-taken and after 6 months storage, illustrate the above conclusion (Fig. 3 and 4). By promoting the predominant type of hybridization (s-p, s-d, or s-f) we can control the nearest-neighbour environment which is regulated by the electronic interaction of the polarized atoms. The layers with new Cdo .lsHgO .ssTe stoichiometry in local volumes resulting from the a-irradiated parent Cdo .,Hgo .,Te may also be considered advantageous.References 1 S. Rolland, Appl. Phys. Rev., 1989, 24, 795. 2 V. K. Dugaev and P. P. Petrov, Ukr. Fiz. Zh. (Russ. Ed.), 1988, 33, 1403. 3 B. F. Chmelka and A. Pines, Science, 1989, 246, 71. 4 A. I. Gusev, Success. Chem., Acad. Sci. USSR, 1988, LVII(lO), 1595. 5 V. F. Elesin, Dokl. Akad. Nauk SSR, Ser. Fiz., 1988, 298, 1377. 6 L. P. Hoshl, P. Moravec, V. Prosser, V. Szocs and R. Grill, Phys. Status Solidi B, 1988, 145, 637. 7 V. A. Terekhov, V. M. Kashkarov, Yu. A. Teterin, I. M. Papenko and A. P. Domashevskaya, Fiz. Tekh. Poluprovodn. Leningrad 1986, 20, 1658. 8 R. D. Chambers, Zd. Chvatal and R. Templeton-Knight, J. Muter. Chem., 1991, 1, 59. 9 0. P. Bochkova, I. A. Ivakin, 0. V. Oginets, V. N. Ostrovsky, Yu. A. Piotrovsky, Yu. N. Sergeev, Yu. A. Tolmachev and A. V. Kuligin, Opt. Spectrosk., 1988, 65, 786. 10 T. E. Madey, Vacuum, 1987, 37, 31. 11 L. Sanche and L. Parenteau, Phys. Rev. Lett., 1987, 59, 136. 12 T. E. Madey, A. L. Johnson and S. A. Joyce, Vacuum, 1988,38, 579. 13 R. A. Vladimirskii, V. B. Lifshitz and V. A. Paiuk, Poverkhnost, 1987, 112. 14 R. Allenspach and M. Landolt, Surf. Sci., 1986, 171, L479. 15 V. L. Avgustimov, D. I. Bidnyk, S. P. Kostenko, 0.G. Maximova, S. 1. Radautsan, I. M. Tiginyanu and A. K. Shkolny, Dokl. Akad. Nauk SSSR, Ser. Fiz., 1990, 313, 330. 16 Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, ed. D. Briggs and M. P. Seach, Wiley, New York, 1983, p. 598. 17 T. Taguchi, T. Terada and 0.Ohno, in Defects in Semiconductors, ed. H. J. von Bardeleben, Materials Science Forum Volumes, Trans Tech Publications, Switzerland, 1986, pp. 10-12, 24 1-246. Paper 1/01634B; Received 9th April, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200019

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992,2(1), 23-29 Effects of High-temperature Sintering on SnO, Sensor Response to Reducing Gases Gary S. V. Coles and Geraint Williams Department of Electrical and Hectronic Engineering, University of Wales, Swansea SA2 8PP, UK Sensors based on undoped polycrystalline tin(rv) oxide sintered at different temperatures in the range 800-1600 "C have been comprehensively characterised with respect to their response to mixtures of carbon monoxide, methane or hydrogen in air. Results obtained at an operating temperature of 400 "C show that increasing the sintering temperature leads to a gradual decline in methane response, but enhanced H, and CO sensitivity, which reaches a maximum after firing at 1375 "C. If the sensors are maintained at a lower temperature (280 "C), then the characteristics obtained differ considerably. A general decrease in sensitivity to all three reducing gases is observed upon increasing the firing temperature until a point is reached where a sensor sintered at 1500 "C displays a remarkable change in the mechanism of detection of carbon monoxide.This sensor exhibits a significant increase in resistance (i.e. p-type behaviour) upon exposure to a low concentration of CO in air but responds conventionally to H,-containing environments while remaining insensitive to methane. A further reduction in operating temperature to 175 "C leads to a similar p-type sensor action in the presence of hydrogen. Keywords: Semiconductor gas sensor; Tin dioxide ; High-temperature sintering 1.Introduction Tin@) oxide (SnO,) is widely used as the basis of solid-state sensors capable of detecting a variety of toxic and flammable The active sensing element usually consists of a sintered polycrystalline mass of the oxide as exemplified by the many forms of the Figaro gas sensor produced in Japan. These behave as gas-sensitive resistors in which oxygen species are adsorbed from the surrounding air and become ionised on the surface. When reducing gases are present oxidation occurs with the subsequent release of electrons back into the conduction band leading to the observed decreases in resist- ance. The fabrication procedure adopted involves heat treat- ment of the SnO, along with any other additives such as PdC1, or Th02,215+6 which are initially dispersed in an aque- ous slurry.This sintering process yields a sensor body of suitable mechanical strength and also confers thermal stability, which is essential considering the elevated temperatures (300-400 "C) at which these devices are operated. In order to achieve the desired thermal stability, the sintering temperature (&)used must be significantly higher than the sensor operating temperature (To).Typical values of quoted in the literature for commercially available devices lie in the range 600-700 0C.2*697However, it is widely known that tin oxide alone sinters poorly at these temperatures. Values of & exceeding 1100 "C, which marks the Tamman temperature of the mater- ia1,8 are required to achieve accelerated adhesion between neighbouring crystallites.To improve low-temperature inter- granular cementing, binders such as tetraethyl orthosilicate (TEOS),9 Mg06 or alumina" are often incorporated prior to heat treatment. These binders may significantly alter the gas- sensing characteristics of the material, for example in the case of TEOS, which decomposes at elevated temperatures forming Si-0 bridges between the SnO, grains, the presence of the binder confers a marked increase in sensitivity to flammable gases.' Considering the importance of the heat-treatment step in the overall fabrication procedure, comparatively few studies have been performed on the influence of sintering temperature on the characteristics of Sn0,-based gas sensors.Research carried out by Borand" on pressed pellets of polycrystalline SnO, annealed in the 400-900 "C range showed that maxi- mum CO sensitivity along with the shortest response time occurred at a sintering temperature of 700 "C. However, for a tin oxide combustion-monitoring device Sasaki and co-workersI2 found that a sintering temperature of 1300 "C gave the most desirable characteristics. Previous studies at S~ansea'~.'~ have centred on the role of additives in conferring selectivity to Sn0,-based sensors. The sintering temperatures employed during the course of these investigations were chosen in line with other research described in the literature15-17 and taking into account the chemical nature of the additives used.The work described here concentrates on the effect of high-temperature sintering, in the range 800-1600 "Cupon the response of polycrystalline undoped SnO, sensors to mixtures of carbon monoxide, methane or hydrogen in air over a range of operating temperatures. 2. Experimental Tin(1v) oxide was prepared via the controlled hydrolysis of an aqueous solution of tin(1v) chloride (analytical grade) by urea at 90 "C.The gelatinous precipitate obtained was washed thoroughly with distilled water until the chloride concen- tration in the filtrate became negligible. After drying, heat treatment of the a-stannic acid in air at 800 "Cfor 2 h ensured complete conversion to tin@) oxide. A fine homogeneous powder was obtained by grinding the oxide in a ball mill for 30 min.Sensors were prepared by applying an aqueous paste of SnO, across the platinum contact pads of an alumina sub- strate (supplied by Rosemount Engineering) as described in previous publication^.'^,'^ For sintering temperatures of 800 or 1000 "C, the whole tin dioxide/substrate assembly was placed directly in the furnace. However, owing to the inability of the substrate to withstand temperatures exceeding 1000 "C, heat treatment of the SnO, between 1100 and 1600 "C was performed on the free powder. The pretreated oxide was then applied to the substrate and fired at 1000°C in the usual manner. All sintering times were of 2 h duration at the specified temperature with additional 2 h heating and cooling ramps.Full details of the procedures adopted for determining sensor resistance and blending mixtures of CO, CH4 or H2 in an oxygen-nitrogen supporting gas are given elsewhere. l8 3. Results and Discussion 3.1 Sensor Behaviour in Air A study of the sensor d.c. resistance uersus working tempera- ture (To)relationship in clean dry air shows that increasing the SnO, sintering temperature leads to a substantial rise in resistance. Fig. 1 illustrates the change in sensor resistance observed at two different values of To for SnO, samples sintered between 800 and 1600 "C. The greatest increases in resistance are exhibited upon sintering the material at temp- eratures in excess of 1400 "C. This result appears to conflict somewhat with the findings of Murakami et ~1.'~who observed a fall in sensor resistance at sintering temperatures in excess of 1300 "C which they ascribe to the formation of 'necks' between separate oxide particles.However, the observed differences may arise because in the present case the Sn02 powder is sintered before the sensors are produced. Arrhenius-type treatments of the data obtained for several sensors employing a range of sintering temperatures are shown in Fig. 2. A curious characteristic of these plots is the gradual appearance of an inflection in the resistance-temperature curve between 230 and 350 "C as 5 is increased. Such behaviour has been observed in previous studies on the electrical conductance of pressed porous pellets of SnO, sintered at 1000 "CI7 and attributed to a change in the adsorbed oxygen species present on the sensor surface.How- ever, the inflection manifests itself at a temperature which is significantly higher than 160 "C determined for the O2--20-?1 I I r 10 12 14 16 Ts/102"C Fig. 1 Variation of sensor resistance as a function of SnOz sintering temperature at To =(a) 400 and (b) 280 "C J. MATER. CHEM., 1992, VOL. 2 I18 16 14 E @v -c 12 1C t I I 1.2 1.6 2.0 2.4 103~/T Fig. 2 Arrhenius-type plots of In R uersus T-' for a series of sensors fabricated from SnO, samples preheated at the following tempera- tures: (i) 800, (ii) 1125, (iii) 1250, (iv) 1440, (v) 1500 and (vi) 1580 "C transformation on the basis of electron paramagnetic reson- ance studies." The slopes of the In R versus T-' plots for working temperatures exceeding 350 "C vary only marginally with sintering temperature (q)as shown in Table 1.The high- temperature activation energy determined for an SnO, sensor sintered at 1000 "C corresponds well with the findings of McAleer et all7 From their results these authors deduce that a surface state associated with adsorbed oxygen is located at 1.1 eV below the conduction band. Many authors have observed that the SnOz grain size increases with increasing values of '& and that a rise in sensor resistance also ensues.8*12*20 have assigned this increase Murakami et ~1.'~ in resistance to the elimination of shallow donor levels as the sintering temperature is increased.However, it may also be possible that an enlargement in the crystallite size leads to a decrease in the number of intergrain boundaries thus restricting the flow of carriers through the sintered mass of material. Table1 Effect of sintering temperature on the slopes of Arrhenius plots obtained in the 320-500 "C region for undoped SnO, sensors ~ ~~ ~ sintering temperature/ "C Arrhenius slope/eV 800 0.95 1000 0.99 1125 0.90 1250 0.90 1375 0.85 1440 0.74 1500 0.91 1580 0.95 J. MATER. CHEM., 1992, VOL. 2 25 3.2 Response to Reducing Gas at High Operating Temperature (To=400 "C) Fig. 3(a)-(c) illustrates the variation of sensor resistance as a function of contaminant gas concentration for a series of SnOz samples sintered in the range 800< T,< 1600 "C.The main characteristics of sensor response to each reducing gas tested will be discussed in turn. CO Response Sensors sintered at 800 or 1000 "C appear to possess little or no response to carbon monoxide at operating temperatures equal to or exceeding 400 "C. However, increasing T, beyond 1100 "C appears to confer a degree of CO sensitivity which reaches a maximum at ca. 1250 "C. A sensor fabricated from a SnO, sample pretreated at this temperature exhibits a 66% resistance drop upon exposure to a 1% v/v CO-air mixture. Further increasing T, diminishes the CO signal to a certain extent, yet the response even at the highest sintering tempera- tures employed remains significant. CH4 Response The use of the lowest sintering temperatures yield sensors possessing high sensitivity to methane.An increase in T, leads to a gradual decline in the CH4 signal, with the consequence that the SnO, sample sintered at the highest temperature I I I Igenerates the least sensitive element. 8 10 12 14 16 H2 Response T,/lb "C The magnitude of resistance changes observed upon exposure Fig.4 A plot of Ro/R (where R,=sensor resistance in air and R= to hydrogen inclusions for sensors employing &<1000 "C resistance is a 1% v/v contaminant gas-air mixture) ueisus sintering appear relatively insubstantial compared to the response temperature (T,)for a series of sensors maintained at To=400 "C exhibited in the presence of an equal concentration of methane.However, increasing the temperature of sintering initiates a substantial hydrogen sensitivity which reaches a maximum air mixture) is plotted as a function of sintering temperature. for materials that had been fired at 1375 "C. The use of higher Therefore, if an undoped tin oxide sensor of this type is to values of & leads to the desensitisation of the H2 signal. be operated at a relatively high temperature (To=400 "C) The trends described above are adequately represented by then the greatest degree of selectivity to a certain gas, namely Fig. 4 where the ratio R$R (where Ro=sensor resistance in methane, is achieved by employing a sintering temperature in clean air and R =sensor resistance in a 1 'YOv/v contaminant-the range 800-1000 "C.However, should sensitivity to a range I (viii) 1o6 1o6 (vii):4S lo5 1o5 p: 1o4 (ii) 1o4 10' J 10' 1o2 1o3 1o4 w21 (PPm Fig. 3 Logarithmic plots of resistance versus reducing gas concentration at a working temperature of 400 "C, for a series of sensors sintered at temperatures of (i) 800, (ii) 1000, (iii) 1125, (iv) 1250, (v) 1375, (vi) 1440, (vii) 1500 and (viii) 1580 "C. The contaminant gases used in each case are (a) CO, (b)CH, and (c) H, of reducing gases be the main requirement, the use of & in the range 1250-1400 "C would be most advantageous. The logarithmic plots of sensor resistance uersus contami-nant gas concentration illustrated in Fig. 3(a)-(c) indicate that a power-law relationship is obeyed by the majority of sensors.However, sensor response, especially to carbon monoxide, appears in several cases merely to approach asymptotically the power-law relationship at high contaminant levels. Similar characteristics have been reported by others2' during studies of the steady-state gas response of Taguchi semiconductor gas sensors. The power-law coefficient (/I)varies considerably with sintering temperature as can be ascertained from the data presented in Table2(a). Again the general trends in the magnitude of j3 mirror those observed for the variation of CO, CH, and H2 sensitivity with & as discussed previously. 3.3 Response to Reducing Gas at Normal Operating Temperature (To= 280 "C) Identical experiments to those described in section 3.2 were performed at a lower working temperature of 280 "C.This temperature was established previously'' as the optimum required for maximum sensitivity to carbon monoxide and hydrogen. The plots of sensor resistance versus contaminant gas concentration obtained for the series of sensors fabricated from Sn02 samples pre-heated over a range of temperatures are shown in Fig. 5(a)-(c). Sensor properties at To=280 "C differ in several ways to those exhibited at a high working temperature. CO Response A significant decline in carbon monoxide sensitivity is observed upon increasing the temperature of sintering from 800 to 1375 "C. Sensors fabricated from Sn02 heat treated at temperatures 3 1440 "C display p-type behaviour upon expo- sure to CO inclusions of less than lo3 ppm.The most mag- nified resistance rises are observed when a sintering temperature of 1500 "C is employed. Increasing the CO con-centration above lo3 ppm in this case causes diminution and eventual cessation of the resistance increments observed. Table 2 Variation of b, the power-law slope as a function of sintering temperature for undoped SnO, sensors maintained at two different working temperatures upon exposure to inclusions of CO, CH, or H, in air (a) To= 400 "C 800 0 0.49 0.30 1000 0 0.54 0.35 1125 0.10 0.37 0.45 1250 0.32 0.47 0.54 1375 0.25 0.34 0.54 1440 0.36 0.36 0.45 I500 0.22 0.33 0.39 1580 0.18 0.36 0.44 (b) T0=280 "C 800 0.38 0.46 0.95 1000 0.40 0.35 0.86 1125 0.25 0.39 0.68 1250 0.19 0.43 0.47 1375 0.15 0.22 0.53 1440 -0.10" 0.12 0.34 1500 -0.72" 0.06 0.51" 1580 -0.14" 0.04 0.22 ~~ ~ " In cases where two types of behaviour are exhibited by the resistance versus gas concentration plots, the slope obtained in the 102-103 ppm region is displayed.J. MATER. CHEM., 1992, VOL. 2 CH, Response A decline in methane sensitivity with increasing sintering temperature is displayed at the lower operating temperature, in accordance with the results attained at To = 400 "C. How- ever, at pretreatment temperatures of 1500 "C or above, CH, response becomes negligible. H2 Response The use of the lowest sintering temperatures confers a substan-tial hydrogen sensitivity, which decreases as & is raised.Curiously, a sensor fabricated from an Sn02 sample sintered at 1500 "C exhibits conventional n-type behaviour upon expo- sure to hydrogen concentrations of less than lo3 ppm, yet as H2 levels are increased further, sensor resistance rises signifi- cantly. This anomaly is not displayed by the sensor sintered at 1580 "C. Table2(b) shows the effect of sintering temperature on j3, the power-law coefficient calculated from the plots illustrated in Fig. 5(a)-(c), at a sensor working temperature of 280 "C. Again the variation of j3 with sintering temperature is very substantial, especially in the case of CO, where a switch from a positive to a negative power-law coefficient is observed at T,= 1440 "C.It can be seen that the magnitude of the power- law slope for H2 gas is ca. a factor of two greater than for CO or CH, at T, < 1125 "C. This finding concurs with pre- viously published results.21 3.4 Gas-Sensing Properties of Polycrystalline SnOz Sintered at 1500 "C A detailed study of the characteristics of the sensor, which displayed remarkable n-type and p-type behaviour depending upon the conditions employed, was undertaken. The vast discrepancies in sensor response to reducing gases at the two operating temperatures utilised previously (see sections 3.2 and 3.3) indicate that this parameter is crucial in determining its properties. The results of an exhaustive study of the effect of the variation of To upon sensor response are represented in Fig.6(a)-(c)where sensor resistance is plotted as a function of contaminant gas concentration using logarithmic axes. If maintained at temperatures of 360 "C or above, the sensor experiences a drop in resistance upon exposure to CO, CH, or H2 inclusions in air, as would be expected for this type of device. Decreasing To below 360 "C produces several effects. First, the resistance changes observed upon exposure to methane become negligible at working temperatures of 280 "C or less. However, secondly and more striking is the change in the mechanism of detection of CO exhibited by the sensor, where substantial increases in resistance are observed in the presence of CO-air mixtures. The magnitude of this p-type response reaches a maximum at To = 230 "C, where a 16-fold rise in sensor resistance is experienced in a 1% v/v CO atmosphere.Lowering the working temperature further leads to a severe reduction in the size of the p-type effect. A similar 'reverse sensitivity' is also exhibited upon exposure to hydrogen-containing environments at working temperatures of 280 "C or less. In this case the most enhanced increase in resistance is observed at the lowest value of To used. The dynamic response of the sensor maintained at To < 280 "C appears substantially slower than the n-type signal observed at high operating temperatures. In the latter instance, the final resistance reading is reached 30 s or less after exposure to the reducing gas.However, at T0=280 "C the resistance uersus time profiles plotted in Fig. 7 show that the final signal is attained only ca. 5 min after introduction of the CO-air mixture. Recovery of the original resistance in air appears even slower, taking up to 30min when the sensor J. MATER. CHEM., 1992, VOL. 2 \(vii)/(iii) 1o2 1o3 1o4 1o2 1o3 1o4 1o2 1o3 1o4 [COI (PPm) m-41(PPm) w21(PPm) Fig. 5 As Fig. 3 except that a working temperature of 280 "C was employed. The reducing gases tested were (a)CO, (b)CH4 and (c) H2 I 1081 II Fig. 6 Logarithmic plots of resistance response to reducing gas inclusions for a sensor prepared from SnOz sintered at 1500 "C,and maintained at working temperatures of (i) 450, (ii) 400, (iii) 360, (iv) 320, (v) 280, (vi) 230 and (vii) 175 "C.The contaminant gases used in each case are (a) CO, (b) CH, and (c) H, has been exposed previously to carbon monoxide concen- trations of 0.5% v/v or greater. The reproducibility of the phenomenon for several sensors prepared from different batches of Sn02 sintered at 1500 "C was investigated. These experiments revealed that this p-type behaviour was displayed in all cases, but the magnitude of the observed resistance increases for a fixed CO or H2 concentration varied significantly between tin oxide batches. In a separate investigation, the effect of different oxygen partial pressures on sensor response at operating temperatures of 280 "C or less were studied. Fig. 8(a) illustrates the change in the ratio R/Ro (defined previously) for a 1% v/v CO inclusion as the oxygen partial pressure (Po,) was decreased from 1 to low4atm.? A steady decline in the p-type response is observed as Po2 is lowered until at an oxygen level of atm, sensor resistance becomes less than that observed in supporting gas only, i.e.a conventional n-type detection mechanism. The results of a similar study of the oxygen dependence of H2 response at To= 175 "C are also included in Fig. 8(a). Here the resistance increment observed upon dosing with 1% v/v H2 inclusions remains constant as Po, is decreased from 1 to lop2atm. However, further reduction of Po, results in the curtailment of the signal until resistance + 1 atmz101 325 Pa. J. MATER. CHEM., 1992, VOL.2 a CO/air air1 1 6 c: I. --.-s4 m. 2 0 I I I I I 0 5 10 15 20 25 time/m i n Fig.7 Dynamic response of a sensor fabricated from SnO, pretreated at 1500°C to different CO concentrations in air. The operating temperature used is 280 "C. (a)2 x lo2, (b) 5 x lo2,(c) lo3,(d) 2 x lo3ppm 10 1 1o6 Ai L? GG 10 1 0-' m. 12 a? 1o5 1 o-2 1L I I I I J 1o4 1 o4 1 o4 1 o-2 lo-' 1 1 o2 1 o3 1 o46,/atm [gas1 (PPm) Fig. 8 (a) A plot of sensor response (represented by the ratio of resistance in supporting gas to the resistance exhibited in the presence of a 1% v/v reducing gas inclusion) versus oxygen partial pressure for an undoped SnO, sample sintered at 1500 "C upon exposure to (i) CO at 280 "C and (ii) H2at 175 "C.(b) Sensor resistance response to reducing gas inclusions when the oxygen partial pressure in the base gas is fixed at atm. An operating temperature of 280 "C was employed changes become insignificant when the oxygen partial pressure law relationship exists. A value of 0.29 was determined for /?, is 10-4atm. the power-law slope from a logarithmic plot of sensor resist- Fig. 8(b) shows the sensor response characteristics at To = ance uersus Po,. 280 "C when Po, in the base gas is fixed at lov4 atm and the How exactly the observed change in the Sn02 gas detection reducing gas concentration is varied. As can be seen from the mechanism arises is unclear. Other researcher~~~.~~have resistance uersus [CO] or [H2] plots, sensor behaviour has observed similar phenomena during their investigation of reverted to conventional n-type, while sensitivity to CH4 Tho2- or Zr02-doped tin oxide sensors sintered at 600 or remains negligible. The resistance response of the sensor to 800 "C, respectively.An increase in sensor resistance occurs oxygen in the absence of reducing gas revealed that a power- as the Tho2-added Sn02 sample is exposed to appropriate J. MATER. CHEM., 1992, VOL. 2 concentrations of hydrogen at temperatures of 220 "C or lower,22 whereas the Zr0,-doped material acts similarly in .~~the presence of ammonia.23 Kanefusa et ~1 tentatively suggest that this negative sensitivity is caused by a change in adsorption rates or physical nature of the adsorbates on the sensor surface under these conditions.It may be possible that preheating undoped Sn02 at a temperature of 1500 "C modi- fies the sensor surface to a considerable extent thus allowing such changes to occur. Previously reported studies of the effects of thermal pretreat- ment on the properties of Sn0224*25have concentrated for the most part on the catalytic activity of tin oxide annealed in the 200-800 "C region in mediating a range of oxidation reactions. However, a more fundamental investigation by Goodman and Gregg' shows that significant changes occur to Sn02 upon high-temperature sintering. These authors report that the specific surface area of stannic oxide is markedly reduced upon increasing the calcination temperature from 250 to 1440 "C, but this is only accompanied by a minimal change in pore volume.However, at a temperature of CQ. 1550 "C the pore volume suddenly falls to zero, an occurrence that is liable to have a critical influence on the sensing properties of a device fabricated from such a material. 4. Conclusions Our studies have revealed that sensor characteristics vary considerably with the Sn02 pretreatment temperature. The use of T,=800 or 1000 "C confers greatest methane sensitivity at high sensor working temperatures and maximum CO or H2 response for To=280 "C. If a sensor is to be maintained at ca. 400 "C then the greatest hydrogen and carbon monoxide sensitivity is obtained by utilising sintering temperatures in the 1250- 1400 "C range. A sensor prepared from a tin dioxide sample fired at 1500 "C displayed some remarkable properties.If this type of device is operated at 400 "C then the resistance of the sensor decreases in the presence of CO, CH4 or H2. However, the use of lower working temperatures leads to negative sensitivity to CO and H2, i.e. sensor resistance increases significantly upon exposure to the reducing gas. Such anomalous behaviour may have important selectivity implications. A sensor main- tained at 280 "C displays p-type response to CO, conventional n-type detection of H2 and negligible sensitivity to CH4. Therefore, a single sensor capable of discerning between different reducing gases if operated at several pre-set tempera- tures can be generated merely by utilising a high-temperature sintering step prior to fabrication. Further investigations are underway to ascertain how the high-temperature pretreatment modifies Sn02 sensor proper- ties to the extent that a completely different gas-sensing mechanism is exhibited at working temperatures lower than 300 "C.The p-type carbon monoxide or hydrogen response occurs even though the resistance-temperature and resist- ance-oxygen partial pressure characteristics of the material appear similar to tin dioxide sensors sintered at lower tempera- tures which act as conventional n-type detectors. It is also known that the phenomenon is only exhibited in the presence of oxygen-containing environments when Po, exceeds 10-atm. Therefore, fundamental gas-adsorption studies are required to deduce the exact nature of the mechanism respon- sible for increased sensor resistivity upon exposure to carbon monoxide or hydrogen.References 1 J. Watson, Sensors Actuators, 1984, 5, 29. 2 K. Ihokura, New Muter. New Processes, 1981, 1, 43. 3 H. Windischmann and P. Mark, J. Electrochem. SOC., 1979, 126(4), 627. 4 G. Heiland, Sensors Actuators, 1982, 2, 343. 5 N. Taguchi, Br. Pat., 1 282 993, 1 288 009, 1 280 809, 1970. 6 M. Nitta and M. Haradome, J. Electron. Muter., 1979,8(5), 571. 7 N. Yamazoe, Y. Kurokawa and T. Seiyama, J. Chem. SOC. Jpn., Chem. Lett., 1982, 1899. 8 J. F. Goodman and S. J. Gregg, J. Chem. SOC., 1960, 1162. 9 S. Yasunaga, S. Sunahara and K. Ihokura, Sensors Actuators, 1986, 9, 133. 10 K.Ihokura, K. Tanaka and N. Murakami, Sensors Actuators, 1983, 4, 607. 11 E. Borand, Sensors Actuators, 1983, 4, 613. 12 N. Murakami, K. Tanaka, K. Sasaki and K. Ihokura, Anal. Chem. Symp. Ser., 1983, 17, 165. 13 G. S. V. Coles, K. J. Gallagher and J. Watson, Sensors Actuators B, 1985, 7, 89. 14 G. S. V. Coles, G. Williams and B. Smith, Sensors Actuators, 1991, 3, 7. 15 L. N. Yannopoulos, Sensors Actuators, 1987, 12, 77. 16 H. Torvela, P. Romppainen and S. Leppavuori, Sensors Actuators, 1988, 14, 19. 17 J. F. McAleer, P. T. Moseley, J. 0.W. Norris and D. E. Williams, J. Chem. SOC.,Faraday Trans. I, 1987,83, 1323. 18 G. S. V. Coles, G. Williams and B. Smith, J. Phys. D., Appl. Phys., 1991, 24, 633. 19 S-C. Chang, J. Vac. Sci. Technol., 1980, 17(1), 366. 20 S. Novel, C. Pijolat, R. Lalauze, M. Loesch and C. Combes, Sensors and their Applications I V Conference, Programme and Abstracts, 1989, p. 118. 21 P. K. Clifford and D. T. Tuma, Sensors Actuators, 1982/83, 3, 233. 22 S. Kanefusa, M. Nitta and M. Haradome, J. Appl. Phys., 1979, 50(2), 1145. 23 S. Kanefusa and M. Haradome, Solid State Electronics, 1984, 27(6), 533. 24 M. J. Fuller and M. E. Warwick, J. Catal., 1973, 29, 441. 25 M. Itch, H. Hattori and K. Tanabe, J. Catal., 1976, 43, 192. Paper 1/01881G; Received 22nd April, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200023

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992, 2(1), 31-35 Predominant Defects in Semiconductor Isovalent Solid Solutions: 9Pbl-y(SexTe1-x)y Pbl-y(SxTe1-x)y and Pbl -y(SxSe1-x)y V. P. Zlomanov, V. N. Demin and A. M. Gas'kov Moscow State University, Inorganic Chemistry Department, Moscow V-234, USSR lsovalent ternary solid solutions of A1"BV'materials such as Pb, -y(SxSel-x)y, Pb, -,(S,Te, -x)y and Pb, -,(Se,Te, -x)y can been used as lattice-matched heterostructures for IR lasers and detectors. The solid/vapour equilibrium has been studied by the method of rapid quenching of crystals of solid solutions prepared by three-zone furnace annealing at fixed T, Ppb and PpbX. Hall-coefficient and lattice-constant measurements on quenched samples were analysed as functions of T and Ppb.The curves of n, p as functions of T and Ppb were used to determine the degree of ionization of defects and to calculate the constants of Frenkel disordering in the form: K= KO exp ( -AH/k, T). Keywords: Ternary solid solution ; Defect; Semiconductor Isovalent solid solutions Pbl -y(Se,Tel -x)y, Pbl -,(S,Te, -JY and Pb, -,,(SXSe1 -x)y are direct energy-gap semiconductors with the rock-salt structure and are used as lattice-matched heterostructures for IR lasers and detectors. The composition of the ternary Pbl -,(Se,Te, -JY solid solutions is characterized by two parameters: x and y. x gives the effective lead selenide content. It determines the energy gap, the density of states and the lattice constant. y is the atomic fraction of chalcogen. The variation of y changes the type of conductivity and the charge-carrier concentration.Excess of metal atoms gives rise to electronic (n-type) conduc- tivity whereas excess of chalcogen atoms causes hole (p-type) conductivity. The deviation from stoichiometry (6) of the solid solution is defined by the difference between the metal-to- non-metal atomic ratio in the real and stoichiometric crystal: for the cubic rock-salt crystal structure: 6 =(1-y)/y -1=(n-p)M/4pNA where M is the molar mass, p is the density and NA is Avogadro's number. The non-stoichiometry leads to neutral and charged lattice defects. These may be donors (D),e.g. D",D, and acceptors/ vacancies (A), e.g. A", A, where ",' and * refer to neutral, negative (electron) and positive (hole) charges.The concen- trations of neutral and charged defects depend upon the parameters of the 'crystal/vapour' equilibrium [temperature (T) and partial pressures (Pi)] and are given by the constants of defect equilibria. The reasons for using the Frenkel mechan- ism of atom disordering are given by the results of studies of density, lattice constants and self-diffusion coefficients of lead for PbS, PbSe, PbTe, Pbl -,Sn,Te as a function of the degree of deviation from stoichiometry.'-* However, the study of self-diffusion of chalcogens in Pb -,Sn,Te and PbSexTe -showed that the defect formation in materials AIVBV1 was complicated and could not be completely explained within the limits of the Frenkel me~hanism.~-'~ It appears that the formation of vacancies in an anionic sublattice and association of defects are required for the interpretation of the results.The degree of ionization of point defects has not been solved unambiguously. Quantum-mechanical calculations predict double ionizati~n.'~-'~ Ion-implantation studies of Pb and Te into PbTe16 have shown that each lead atom gives one electron into the conduction band, while ion implantation of tellurium does not change the concentration of carriers. The dependence of the carrier concentration (n,p) on the partial pressure Ppb, Px2,17-21the magnetic susceptibility and paramagnetic resonance data' all favour single ionization of point defects. However, the data for Pbl -,Sn,Se1*22 indicate double ionization of atomic defects.In this paper we report on the type of predominant defects and their degree of ionization from the data on vapour(v)/ solid(s) equilibria. Hall-coefficient and lattice-constant measurements on quenched samples were analysed as func- tions of T, fpb and Ppbx. The data were used to calculate the constants of defect equilibria using the gradient method and an optimization program. The solid/vapour equilibrium has been studied by means of the method of freezing the equilibrium at fixed T, Ppb and PpbX (x=s, Se, Te) by quenching. The conditions (T, Ppb, PpbX) were chosen from P-T-x-y phase diagrams of the systems Pb-S-Te, Pb-S-Se and Pb-Se-Te.23 The partial pressures Ppb and PpbX during annealing were fixed by heating in a three-zone furnace (Fig.1) elementary lead or tellurium and a source Pb0.51X0.49 placed at different ends of the ampoule. The composition and temperature of the source Pb0.51Te0.49 corresponded to the three-phase equilibrium SpbTe+L+V. The partial pressure PpbX over it would corre- Se(Pb) PbSe, -xTex length of ampoule Fig. 1 Schematic diagram of the three-zone furnace annealing ampoule 32 J. MATER. CHEM., 1992, VOL. 2 spond to PpbX for solid solutions of Pbl-,(Se,Tel-,),, Pbl -y(S,Te, -x)y and Pb, -,(S,Se, -,),; they were calculated 19.0 ' using the Raoult's law approximation and are given in Table 1. 19'5 The fixed partial pressure Ppb or PTe(Pse)always exceeds --18.5 IP~~n(P~,3.24The time (tanflea,)for achieving solid/vapour equi-Elibrium was determined by the dependence of log n, p(7') on 218.0 tannea,and was 650, 500, 320 and 170 h for temperatures of s 873, 923, 973 and 1023 K, respectively. From mass-transport < 17.5 Y calculations of Pbo.51Xo.4924 El)it was shown that it is possible 2 17.0 to neglect alteration of source composition for masses of the source rn >1 g.The experimental study of source composition 16.5 after the end of annealings confirmed the results of calcu-I I I I Ilations. After the annealing procedure ampoules were 16.0 ' -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5quenched in ice water. The majority carrier concentration, the deviation from stoichiometry 6 and parameter x were log [PPb/Pal as a function ofcalculated from the Hall effect at 77 K and lattice constant Fig.2 Carrier concentration in Pbl -y(Seo.08Teo.92)y measurements of the quenched samples. Note that the results Ppb (lines calculated by the model of singly charged defects) of analyses of crystals annealed under the same conditions do not differ within the limits of accuracy of the methods. 19.5 I I I I I The isothermal dependences of charge carrier concentration ,1of log Ppb are given in Fig. 2-4. For each Ppb the mean values 19.0 of the 2-3 samples are given. Under low Ppb values crystals show p-type conductivity; the concentration of holes decreases with increasing Ppb; a change in the conductivity type is observed at some Ppb(P=n). Over a range of Ppb the depen-dence p=f(P,,, T) may be described (Table 2) as lOg(p)=lOgA+B 10g(Ppb)+E/kBT (2) The range of reproducible change in the charge concentration 16.5 -8 3K1 1'"" t'"" ,,$and also the p= n transition conditions are given in Table 3.The solid PbX1X2/vapour equilibrium may be described in I I I I I -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5terms of quasichemical reactions (3)-( 10) with constants log [PPb/Pal as a function ofTable 1 Temperature dependence of partial pressure Ppbx and con-Fig. 3 Carrier concentration in Pbl -y(So.05Teo.95)y stants KpbX,& log P(K)= -A/T+ B Ppb (lines calculated by the model of singly charged defects) PPbSe PPbTe KPbSeTe 19.5PbSe,Te, --x X A B A B A B 19.0 -* 0.90 11080 12.12 10170 10.48 10990 11.96 0.70 10980 11.91 10580 11.34 10860 11.74 18.5 0.30 10480 11.06 11080 12.30 10900 11.93 0.15 10180 10.48 11160 12.37 11020 12.09 s0.08 10030 10.05 11180 12.42 11090 12.23 17.5 Y PbS,Te, -, PPbTe 'PbS KPbSTe El)2 17.0 0.0 11190 12.47 ----10.05 11190 12.45 11570 11.32 11204 12.38 16.5 0.10 11190 12.43 11570 11.62 11075 11.94 0.15 11190 12.40 11570 11.80 11249 12.31 16.0111111"1'11 I 111 0.20 11190 12.37 11570 11.92 11592 12.64 -6.0-5.5 -5.0-4.5 -4.0-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.o --11570 12.62 --log [PPbIPal Fig.4 Carrier concentration in Pb, -y(So,05Seo.95)yas a function of Ppb (lines calculated by the model of singly charged defects) 0.05 I1088 12.16 11474 12.26 11108 12.11 0.10 11084 12.13 11528 11.56 11128 12.08 K =KO exp(-AH/kBT).*'The model proposed to describe the 0.30 11055 12.03 11655 12.04 11235 12.03 equilibria is based on the assumptions of random distribution0.50 11043 11.88 11678 12.26 11360 12.07 0.80 11166 11.48 11622 12.46 11531 12.27 of non-metal atoms, Frenkel disorder in the cation sublattice 0.90 11269 11.18 11604 12.51 11571 12.38 and single or double ionization of atomic defects.P&,= Pb: +v&,+H,; KF=[Pbr][v&,] (3) Table 2 Coefficients of eqn. (2) Pbr =Pbf' +ze'+ E,; K,= [Pbf+]n"/[Pb?] (4) log A B E v&,=vF,+zh' +Eb; Kb= [VF,]p'/[v&,] (5) Pb, -y(Seo.08Teo,92)y 30.18 +1.3 -1.01+0.09 -1.48+0.14 O=e'+h' +Ei; Ki=np (6) Pbl -y(So.05Teo.95)y 25.55 +1.88 -0.89+0.26 -0.63+0.16 Pb' =Pbf +Hpb,$ Kpb,v=[Pbr]/Ppb (7)Pb, -y(So.05Seo.95)y 27.4+1.4 -0.54+0.09 -0.86+0.14 O-SXY,=Xg vgb +Hx2,"; J.MATER. CHEM., 1992, VOL. 2 Table 3 Vapour/crystal equilibrium data 1 873 -2.02 to -1.82 3.6 x 1017 2 923 -1.34 to -1.16 4.8 x 1017 3 973 -0.59 to -0.50 7.9 x 1017 4 1023 -0.01 to +0.11 1.4 x 10" 1 823 -2.17 to -2.28 5.0 x 1017 2 923 -1.42 to -1.49 4.0 x 1017 3 973 -0.67 to -0.76 1.0 x 10'' 4 1015 -0.08 to -0.16 1.0 x 10" 1 823 -2.9 to -3.1 5.0 x 10" 2 923 -1.5 to -1.8 6.0 x 10l8 3 1073 -0.5 to -0.6 1.0 x 1019 Kxy =[v&](1-x)/P:: (8) Pb&,= Pbf' +Vii +HF; KF,=[Pbf'][Vba] (9) PbX,'X: --x =xPbX',' +(1 -x)PbX2"; KpbXlX2 =P&.,X2 +P~\~~' (10) where an asterisk, z+ and z- indicate neutral, positive and negative interstitial atoms (Pbi) or their vacancies (vpb), and H and E are energies of reactions (3)-(7).Reactions (3)-( 10) with the condition of electroneutrality, p +z[Pbff] =n +z[V&] give rise to a set of six equations with six unknown concen- trations n, p, [Pbf'], [Pb?], [V&], [v&,], provided the equilibrium constants K and the values of Ppb(PSe) and PpbX are known. The solutions of these equations for the cases of single (z= 1) and double (z=2) ionization have the following form. z= 1: z=2: p4 +P3KX2,vKPbXPPbX/2KF'PPb(1 -x, -PKiKX2,vKPbXPPbX/2KFfpPb(1 --[KX2,vKPbXPPbXPPbl(1 -x)-112/KF'=o (13) The temperature dependences of the intrinsic constant Ki were obtained by means of the least-squares values of Ki, calculated over the range 800-1 100 K according to Ki =[1 +akBT/Eg( 1030T3(m,*mz/m#T)]4~?2~ x exP(-Jq/kB T) (14) where E,"is the energy gap at T=0 K, E&T) is the temperature dependence of the energy gap;26,27 m,*and mt are the effective masses of electrons and holes.The K, and Kb constants were calculated from the con- ditions K: =KE =2(K0)1/2 and the values E, =Eb=0.14 eV that follow from semiconductor statistic^.^' The constants Kx,v, Kpb,V, KF and KF, were estimated from the Ki, K, and &, values and experimental data on n,p, Ppb and PpbX using the relations (12) and (13). The solution for p and the experimentally determined concentration of holes ~(n)3oohas 3.5 x 10l6 3.7 x 10" 8.1 ~10'~ 1.0 x 1017 5.7 x 10l8 1.7 x 1017 1.0 xi017 5.4 x 10l8 1.6 x 1017 1.0 x 1017 6.0 x 10l8 LO x 1017 7.9 x 10l6 --8.0 x 10l6 5.0 x 10'' 8.0 x 10l6 1.8x 1017 4.3 x 1017 4.0 x 10l8 1.2 xi017 5.0 x 10" 1.2 x 1017 1.0 x 1017 3.0 x 1017 9.0 x 10'' 2.5 x 10l8 1.0 x 1017 1.0 x 1019 1.0 x10" 1.0 x 1019 3.0 x 1017 the form: P(n)300 =P-Ki/p (15) where Ki is taken for the annealing temperature. The constants were calculated using the Fletcher-Rivs gradient method in accordance with the multidimensional optimization program.The values of the four adjustable parameters K:e, HTe,KE and HF were estimated uia minimization of the sum: F(P)=[(Pcalc -~exp)/~expI' (16) where pcal and pexp are the calculated and experimental concentrations of carriers.Note that only the concentration range p= 1 x 1017-5 x 1OI8 cm-3 of the experimental curves log p(n)=log Ppb (Fig. 2) was taken into account. The upper limit of the carrier concentration was caused by the ineffec- tiveness of the sample quenching, especially for p-type conduc- tivity owing to the high speed of decomposition of supersaturated solid solutions. The lower limit is bound up with the effect of compensation of charge carriers near to the stoichiometric composition. Kpb,V and KF were calculated from: KPb.V =KF'Kf(KX2,VKPbXPPbW 1-'( -(17) KF= KFfKiKa-'Kbl (18) The calculated values of constants of quasichemical reactions are given in Table 4. The accuracy of the model in explaining the experimental data is limited by the error in estimating the dependence of the carrier concentration upon T, Ppb, Ppbx.The results of modelling log n, p us.log Ppb for Pbl -,,(Seo.08Teo.92)y for single and double ionization are shown in Fig. 5. The relative error, A=log(pc,,c-pex,), is less than 2% for the model of single ionization in all the cases and is greatest near the p-n transition. The double-ionization model describes the results for the p samples with an error of A=4%, but for the n samples the double-ionization model does not fit the experi- ment adequately, the error being 13-16%. The values of Ppb and T for the p-n transition may also be characteristic of the model, since these experimental points were not used in the equilibrium constant calculations.The temperature dependence Ppb(p =n) for stoichiometric crystals is expressed by the equation: z= 1: PPb(P =n, =KTe,,vKPbXK,PPbX/(KiKF')O.S(l (19) 34 J. MATER. CHEM., 1992, VOL. 2 Table 4 Constants of the defect equilibria in solid solutions PbX,X, PbSe0.08Te0.92 PbS0.05Te0.95 PbS0.05Se0.95 log (Ko/cm-3) H/eV log (Ko/cm-3) HIeV log (Ko/cm-3) H/eV z= 1 4 1.84 0.69 4 1.02 0.58 38.42 0.44 46.30 2.69 46.15 2.56 46.48 2.56 46.90 2.28 46.75 2.26 48.92 2.40 21.22 0.14 20.81 0.14 20.17 0.14 2 1.22 0.14 20.81 0.14 20.17 0.14 17.44 0.15 18.80 0.39 - - - 37.43 0.22 - - - - 37.74 0.28 11.71 -1.00 10.18 -.1.07 32.63 -.I .04 z=2 59.24 3.56 18.00 2.46 24.98 18.15 -0.66 0.34 lgS t 19.0 -m IE 18.5 -0--.h Q 18.0 -c-vY 17.5 --0 16.5 i17.0 .'I 1 Fig. 5 Carrier concentration in Pb, -y(Seo.08Teo.92)y after annealing at 923 K as a function of Ppb (lines calculated by the models of singly and doubly charged defects).(-) doubly charged defect model; (---) singly charged defects; .,experiment z=2: PPb(P =n, =KTe,,,KPbXK,PPbX/KiK~'5(1 -x) (20) Fig. 6 shows the Ppb and T values at the p-n transition in crystals PbSeo.08Teo.92, obtained experimentally and calcu- lated according to eqn. (19) and (20). The single-ionization model fits the experiment. Data on the type of predominant defects and their equilib- rium constants are helpful in predicting the synthesis con- ditions: annealing, doping and growth characteristics of the solid solutions with well defined composition and properties.The lines in Fig. 2 give the predicted (calculated) T,Ppb, PpbX and carrier concentrations for the model of single-ionization of atomic defects. The temperature dependences of Ppb for crystals with fixed deviation from stoichiometry are rep-resented in Fig. 7. The curves obtained, together with lines ekvand P$, give (Ppb-T), sections of the P-T-x phase diagram and are of practical importance for the technology of Pb,-,(X~-,X~), solid solutions. The model of the defect equilibria and data on constants presented in this work may need certain adjustments as new data on the nature of point defects in semiconductor A"BV' crystals appear.II .-- 0.0 - (d -0.3 - Q? -0.6 -Y 0)- -1.2 -0.9 L-1.5 I I 1 I 1 I 0.90 0.95 1.00 1.05 1.10 1.15 103~1~ Fig. 6 The temperature dependence of Ppb(p=n) for Pb, ~y(Seo,08Teo,9,)y: upper line, 2=2; lower line, z= 1 I I I en-2 -d? Y --r3 2.0 10 -4 -lgP(Pb),v+l+s i'l \-tme I I I I-6 0.8 0.9 1.o 1.1 1.2 1.3 103~/T Fig. 7 Ppb us. 1/T diagram for Pbl -y(Se,,08Teo,9,), References 1 A. V. Novoselova and V. P. Zlomanov, Curr. Top. Mater. Sci., 1981, 7,643. 2 H. Gobrecht and A. Richter, J. Phys. Chem. Solids, 1965, 26, 1889. 3 C. Belin and R. Pellegrin, Rev. Appl. Phys., 1971, 6, 415. 4 M. Schenk, H. Berger, A. Klimakov and U. Menzel, Proc. 6th J. MATER. CHEM., 1992, VOL.2 35 Int. Symp. of High-Purity Materials in Science and Technology, 18 V. P. Zlomanov, 0.V. Matveev and A. V. Novoselova, Sou. Dresden, May 1985, Poster Abs. N2, p. 452. Vestn. Mos. Uniu. Ser. 2. Chem., 1967, 5, 81. 5 H. Berger, A. Lehman and M. Schenk, Cryst. Res. Tech., 1985, 19 V. P. Zlomanov and A. M. Gas’kov, Growth of Semiconductors, 20, 579. Crystals and Films, Nauka, Novosibirsk, 1984, part 2, p. 116-133. 6 G. Vitali, E. Fainelli and G. Petrocco, J. Appl. Phys., 1978, 15, 20 T. V. Zajchuk, J. L. Harif and P. V. Kovtunenko, Znorg. Mater., 315. 1986, 22, 507. 7 C. L. Miloslavow and S. I. V. Jas’kow, Znorg. Mater., 1983, 19, 21 I. H. Avetisov, J. L. Harif and P. V. Kovtunenko, Inorg. Mater., 45. 1986, 22, 354. 8 M. S. Selzer and J.B. Wagner, J. Chem. Solids, 1963, 24, 1525. 22 K. R. Mendibaev, V. P. Zlomanov, V. N. Vigdorovich and A. M. 9 Y. N. Simirski and L. P. Firsova, Znorg. Mater., 1984, 20, 293. Sokolov, Znorg. Mater., 1989, 25, 793. 10 L. P. Firsova, Y. N. Simirski and A. V. Novoselova, Sou. Vestn. 23 A. V. Novoselova, V. P. Zlomanov, A. M. Gas’kov, V. N. Demin Mos. Univ. Ser. 2. Chem., 1985, 26, 312. and L. A. Kuznetsova, Russ. J. Inorg. Chem., 1986,31, 1701. 11 H. G. Tang, B. Lunn and D. Shaw, J. Mater. Sci., 1981, 16, 3508. 24 V. P. Zlomanov, A. M. Gas’kov and I. M. Malinski, Znorg. Mater., 12 L. P. Firsova and G. P. Simirskaya, Znorg. Mater., 1986, 22, 172. 1990, 26, 623. 13 B. A. Volkov, V. V. Osipov and 0.A. Pankratov, Sou. Phys. Teq. 25 F. A. Kroger, The Chemistry of Imperfect Crystals, North-Semi., 1980, 14, 1387. Holland, Amsterdam, 1964. 14 N. I. Parada, Phys. Rev. B, 1971, 3, 2042. 26 R. S. Allgaier and W. W. Scanlon, Phys. Reu., 1958, 111, 1029. 15 G. W. Pratt, J. Nonmetals, 1973, 1, 103. 27 D. M. Finlayson and I.A. Johnson, Phys. Status Solidi, 1975, 16 K. H. Gresslehner and L. Palmetshofer, J. Appl. Phys., 1980, 51, 716, 395. 4735. 17 A. M. Gas’kov, 0.V. Matveev, V. P. Zlomanov and A. V. Novo-selova, Znorg. Mater., 1969, 5, 161 1. Paper 1/O 1942B; Received 24th April, 199 1

ISSN:0959-9428    

DOI:10.1039/JM9920200031

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992, 2(1), 37-41 Co-ordination Compounds on the Surface of Laponite: Tri-2-pyridylamine Complexes Stephen P. Bond," Carl E. Hall,bCraig J. McNerlin," William R. McWhinnie" and David J. Waltonb a Department of Chemical Engineering & Applied Chemistry, Aston University, Aston Triangle, Birmingham B4 7ET, UK Department of Applied Physical Sciences, Coventry Polytechnic, Priory Street, Coventry CVI 5FB, UK Freshly prepared [Co(tripyam),](ClO,), (tripyam=tri-2-pyridylamine) contains some low-spin isomer (,E ground state) 'frozen' into the solid; this reverts to the 4T form over 3 months. Ion exchange of orange [Co(tripyam),]*+ from aqueous solution onto laponite gives a pink clay, the exchanged cation being [C~(tripyam),(H,O),]~+ with bident ate p y rid y Iamin e Iig a nd s.Isomeric [C u (t r ipya m ),I (C I04),(trident ate Iig a nds) and [C u (t ripya m ),( CI04),] (bidentate ligands) are absorbed onto laponite as [C~(tripyam),(H,O),]~+ with bidentate ligands; however, the copper clay is thermochromic undergoing a reversible change from green to blue at 100°C. The temperature of the colour change may be increased by employing copper(I1) complexes of substituted tri-2-pyridylamines. The thermochromism is a function of the variable denticity of the ligands. Orange [Co(tripyam),12+ (terdentate ligands) may be exchanged using a novel microwave method which accelerates the ion-exchange reaction, but not the aquation reaction; however, over 1 year the sorbed complex aquates to [C~(tripyam),(H,O),]~+. Although low-spin [Fe(tripyam)J2+ (terdentate ligands) is stable on laponite, in general the clay surface has greater affinity for the bis(bidentate) complex species.Cyclic voltammetric studies of acetonitrile solutions show an order of ease of oxidation of cobalt(1r) complexes: [Co(bipy),12+ > [Co(6-dmdpa),12+ >[Co(tripyam),12+ where bipy =2,2'-bipyridyl and 6-dmdpa =(6-methyl-2-pyridy1)-di-(2-pyridy1)amine. Clay(1aponite)-modified platinum electrodes dipped into acetonitrile solutions of [Co(bipy)J3+ or [Co(tripyam),12+ give very similar E,,, values to those obtained for the free complexes in acetonitrile solution. However, electrodes prepared from laponite pre-exchanged with pink [C~(tripyam),(H,O),]~+ or [Co(bipy),I3+ were rigorously electro-inactive.Keywords: Tri-2-pyridylamine; Laponite ; Cobalt complex; Copper complex; Thermochromic clay Davison' has demonstrated that tri~-2,2'-bipyridylcobalt(111) tris-2,2'-bipyridyl and bis(tri-2-pyridylamine) complexes, but ions exchanged onto hectorite may be reduced chemically with pyridyl conjunction in the former but not the latter (NaBH,) and electrochemically to the cobalt(1) complex. systems, it was of interest to study the clay-exchanged He~torite/[Co(bipy)~]~and NaBH, together reduce nitro- bis(tri-2-pyridylamine) complexes. This paper reports some+ benzene to aniline in excellent yield, the clay-supported cata- new co-ordination chemistry of metal(11)-tri-2-pyridylamine lyst offering the twin advantage of ease of separation and complexes on laponite, a synthetic lithium-magnesium reusability.Large complex cations exchanged into the inter- trioctahedral smectite clay, the closest natural analogue of lamellar regions of the clay effectively pillar the material and which is hectorite. Some electrochemical studies of CMEs +it has been speculated that this fact might facilitate catalyst- based on [C~(tripyam)~]' are also presented briefly. substrate interaction in the above reaction. Others2 have demonstrated the affinity of [Co(bipy),]' + for clay colloids, and recently3 a quantitative examination of a [Co(bipy),12 +/bentonite clay-modified electrode (CME) sys- Experimental and Results tem was reported. The electrochemically produced Co' species Laponite was obtained from Laporte Industries Limited.was shown to be an effective catalyst for the dehalogenation Metal salts were commercial specimens and were used as of both 4,4'-dibromodiphenyl and chloroalkanes present in obtained. organised assemblies achieved in the presence of 1 -hexadecyl trimethylammonium bromide. In order to obtain more information on the sorbed complex Tri-2-pyridylamine. This was synthesised by a literature species and to extend the range of complexes considered, we (Found: C, 72.6; H, 4.93; N, 22.6%. C15H12N4 meth~d.~ have resurrected some complexes of tri-2-p~ridylamine.~ The requires C, 72.6; H, 4.87; N, 22.6%). M.p. 129 "C (literatureg ligand tri-2-pyridylamine (tripyam) co-ordinates either as a m.p. 130 "C.) bidentate or as a vicinal (tripodo-) terdentate base., In the bis-terdentate complex, [C~(tripyam),](ClO,)~, which exhibits spin cross-over beha~iour,~the 295 K structure6 (pseudo- (6-Methyl-Zpyridyl)di-(2-pyridyl)arnine.This oil was syn-'high spin') shows Co-N bond lengths in the range 2.100(2)- thesised as reported previously." (Found: c, 73.3; H, 5.51; N, 2.152(2)8, and N-Co-N angles from 84.86(7) to 86.09(8)". 21.4%.C16H14N4 requires C, 73.3; H, 5.34; N, 21.4%.) By contrast, the low-spin Fe" complex deviates only trivially from true octahedral ~ymmetry.~ The behaviour of the corre- sponding copper(I1) system, Cu(tripyam),(ClO,), is quite com- (4-MethyI-2-pyridyl)di-(2-pyridyl)amine.This semicrystalline plex, with interchangeable bi- and ter-dentate solid was also prepared as previously." (Found: C, 73.2; H, Given that an M(pyridyl-)6 environment pertains to both 5.40; N, 21.4%.) Synthesis of Complexes Bis(tri-2-pyridyEamine)cobaEt(11)Diperchlorate. This was pre-pared as reported previ~usly.~ (Found: C, 47.4; H, 3.04; N, 14.8%.C30H24C12C~N808 requires C, 47.7; H, 3.20; N, 14.9%.) This orange material was dissolved in water in a flask which was stoppered and placed on a mechanical shaker for 1 week, giving a pink solution, Amax/nm 500, cf. 502 for an aqueous solution of [C0(py)~(H~0)~]~ + where py =pyridine. An orange solid, [C~(tripyam),](ClO,)~ 3H20 was isolated from the aqueous solution. (Found C, 44.8; H, 3.33; N, 14.0%. C30H30C12CON8011requires C, 44.6; H, 3.71; N, 13.9%.) Conductivity measurements in acetonitrile confirmed that the perchlorate groups were ionised in that solvent.Bis[(6-methyE-2-pyridyl)di-(2-pyridyl)amine]cobaIt(11) Diper-chlorate Monohydrate. This was prepared using ethanol rather than triethylorthoformate' ' as a solvent. Thus (6-methyl-2- pyridyl)di-(2-pyridyl)amine (10.5 g) in 10 cm3 ethanol was added to Co(C104),*6H20 (0.32g) in 10cm3 ethanol and refluxed for 30 min to give a brown-orange precipitate. The solid was separated and washed with ethanol to give an orange solution and a green residue. On concentration, the orange solution gave an orange solid which was separated, washed with ethanol, and dried. The green residue and orange product gave identical C, H, and N analyses and both had A (molar conductivity) =335 s mol-'dm3 in CH3CN.Infrared (IR) spectra were similar and indicated ionic perchlorate in each case. The orange material had peff=4.91pg. {Found (green): C, 47.8; H, 4.06; N, 14.0%; (orange): C, 47.7; H, 4.08; N, 14.0%. [C0(6-mpdpa)~](ClO.+)~ H20, C32H30C1&ON805 requires C, 47.9; H, 4.00; N, 14.0%.) Bis(tri-2-pyridylamine)iron(11)Diperchlorate Monohydrate. This was prepared by reaction of 10.35 g Fe(C104)2*6H20 in triethylorthoformate (10 cm3) with tri-2-pyridylamine (0.7 g) in ethanol (10 cm3) with passage of dinitrogen. A red-orange solid precipitated immediately and was separated and washed with ethanol. (Found: C, 46.9; H, 3.22; N, 14.6%. C30H28FeN809 requires C, 46.9; H, 3.38; N, 14.6%).The previously reported complex was anhydrous4 but the Mossbauer parameters of the new material are identical [6 = 0.62 & 0.05 mm s-' us. Na2Fe(CN)5(NO)-2H20] with those for [Fe(tripyam), ](ClO,), (6 =0.63f 0.05 mm s-') reported in an earlier paper.I2 However, the Mossbauer spectrum also showed a resonance (5% of total intensity) for which 6= 1.34f0.05 mm s-', A=2.53+0.05 mm s-', corresponding to high-spin iron(r1). Tris(di-2-pyridyl)iron(11)diperchlorate is known to be high spin and has 6= 1.27k0.05, A= 2.51+_0.05mm s-', hence the impurity is likely to be the tris(bidentate tri-2-pyridy1)amine iron@) species. In support of this, if the above experiment is repeated without the passage of dinitrogen, a pale-green complex is found, Fe(tripyam), (C104)2*3H20 with bidentate ligands.(Found C, 52.0; H, 4.06; N, 16.0%. C45H42C12FeN12011 requires C, 51.2; H, 4.00; N, 15.9Yo.) Synthesis of Copper(@ Complexes. Blue [Cu(tripyam),] (C104)2 and yellow-green [Cu(tripyam),(ClO,),] were pre- pared as reported previ~usly.~ Reactions were also carried out between 4- and 6-methyl-di-(2-pyridyl)aminesand cop- per@) perchlorate. Although an earlier method was followed, the products isolated here were hydroxo-bridged dimers of the type [CuL(OH)],(ClO,), where L is 6-methyl-di-(2-pyridyl)-amine. {Found (4-methyl): C, 43.3; H, 3.12; N, 12.2%. C32H30C12CU2N,@10 requires C, 43.4; H, 3.40; N, 12.6%. Found (6-methyl): C, 42.2; H, 3.22; N, 11.9%. C32H32C12CU2N@ll (i.e.monohydrate) requires C, 41.7; H, J. MATER. CHEM., 1992, VOL. 2 3.69; N, 12.1 YO.)Conductivity measurements in acetonitrile confirmed that the complexes were 2 :1 electrolytes based on the dimeric formula, thus [CU,L,(OH)~](C~O~)~, molA dm-3)=278 S mol-' dm3 cf. [COL~](C~O~)~, molA dm-3)=280 S mol-' dm3. Exchange of Complexes onto Laponite Laponite RD was used in the sodium form. Method 1. Laponite (5 g) was added to deionised water (100 cm3) and treated with [C~(tripyam)~](ClO,), (0.75 g) dissolved in deionised water (150 cm3). The mixture was placed in a conical flask which was sealed and set on a mechanical shaker for 1 week. The clay assumed a pink colouration, the supernatant liquid, initially orange, was colourless.The clay was separated, washed with de-ionised water, and air dried. Other metal complexes were exchanged onto laponite using a similar procedure. Method 2. Laponite (1 g) was added to absolute ethanol (10 cm3) and 0.12 g [C~(tripyam)~](ClO,), was added. The mixture was placed in a Teflon reaction vessel and sealed tightly. The vessel was placed in a Sharp Carousel I1 R-84- 801 650 W microwave oven and was subjected to five bursts of 1 min of microwave energy (maximum power) with a delay of 1 min between bursts. On cooling, the vessel was opened and the orange clay was separated and washed with ethanol, then air dried. Other complex ions were exchanged using the same procedure. Some observations on the products are summarised in Table 1.Physical Measurements Spectra. Diffuse reflectance spectra of clays were measured with a Pye-Unicam SP.800 instrument using MgO as a reference. Some EPR spectra were measured with a JEOL PE-1X type spectrometer. Measurements at variable tempera- ture were provided by Dr. K. D. Sales of Queen Mary and Westfield College. IR spectra were recorded for KBr discs on Nujol mulls using a Perkin-Elmer 17 10 FTIR instrument. X-Ray Powder Diflraction. X-Ray powder diffraction (XRD) patterns were recorded with a Philips X-ray diffractometer using Co-Kcr radiation. Some data for the basal spacing [d(OO l)] of complex-ion-exchanged laponite are incorporated in Table 1. Electrochemical Measurements. Cyclic voltammetry measure- ments were made on Princeton Research Model 273 or 362 scanning potentiostats linked to a Rikadenki XY recorder. Potentials are cited relative to SCE.In all cases the supporting electrolyte was 0.1 mol dm- tetrabutylammonium perchlor- ate (TBAP) in dry CH3CN solvent, degassed with N2 prior to scanning. For all voltammetry, the supporting electrolyte was 0.1 mol dm -TBAP in CH3CN. Clay-modified electrodes were made by sonication of laponite (10 g) in 100 cm3 deion- ised water for 30 min. The resultant gel was centrifuged at 4000 rpm for 3 h to remove large particles. Platinum dichlor- ide (0.05 g) and polyvinylalcohol (0.66 g) were added to a 40 :60 ethanol-de-ionised water mixture (50 cm3) and refluxed for ca. 4 h to form a homogeneous mixture to which 5 g of the clay gel was added. Reflux was continued for 1 h.A platinum plate (1 cm xl cm) which had been cleaned by sonication was treated on one side with a few drops of the clay-PVA-Pt mixture which was allowed to dry before the reverse surface was treated similarly. The electrode was then J. MATER. CHEM., 1992, VOL. 2 exchanged complex CCu(tripy am)2l(Clo4)2 (blue isomer) CCu(tripyam),l(CIO,),(green isomer) [Cu(4-mpdpa)(OH)](C104) & 0.1 A. [Co(tripyam),](C10,), Table 1 Some properties of complex cation-exchanged laponite colour diffuse room higher reflectance temperature temperature ( "C) d(O01)/k maximum/nm ~~~~~ green blue (100) 15.1 ca. 420sh reversible green blue (100) 15.1 ca. 420sh reversible green blue (145) ca.15.5 reversible green-brown orange-brown (21 5) ca. 15.5 irreversible pink 15.1 ca. 500 orange 15.1 ca. 440 vbr orange 15.1 red-orange 15.1 ca. 420 sh, ca. 500 sh 13.5 crystals, A,,, =460 nm from ref. 4. dipped into a 0.1 mol dm-3 solution of the desired complex for 1 h, washed well with distilled water, and air dried. Some numerical data are given in Table2. To examine possible differences between the electrochemical response of CMEs dipped into solutions of redox active cations and those prepared from clay specimens previously ion exchanged, two electrodes were prepared with clay that had initially been exchanged with complex cation i.e. lap~nite/[Co(tripyam)~]~-+ (prepared by Method 1) and laponite/[Co(bipy),] (prepared+ by Method 1, complex synthesised following Burstall and Nyholm' 3).Some typical voltammograms are illustrated in Fig. 1, 2. Miscehzneous. Magnetic measurements were taken at room temperature by the Gouy method. Conductivity measurements were made on lop3mol dm-3 solutions using a Mullard bridge in conjunction with a bright platinum-dip electrode. New 57Fe Mossbauer data were provided by Dr. F. J. Berry (University of Birmingham). Chemical isomer shift data cited in the text are relative to Na,[Fe(CN),(NO)] *2H20to facili- tate comparison with earlier data; to convert to iron metal standard, subtract 0.257 mm s-Discussion The spin cross-over ("TH~E) behaviour of [Co(tripyam),] (C104), has been do~umented.~~" When freshly prepared, the value of peff is 4.84~~,~a value somewhat lower than values normally observed for relatively symmetrical cobalt(I1) octa- Table 2 Electrochemical data for cobalt complexes in solution and as components of CMEs (laponite) comp1ex CCo(tripy am)2l(C104)2 solution (Me CN) CME (laponite): dip-coated CCo(6-mPdPa),I(C104)zsolution (Me CN) CME (laponite): dip coated CCO(biPY)31(c104)3solution (Me CN) CME (laponite): dip coated intercalated species [C~(bipy)~]~+ (CME/laponite) [Co( tr~pyam),(H,O),]~ (CME/laponi te) + El ,,/V (us. S.C.E.) 0.50 0.49 0.39 no satisfactory CV 0.2 1 0.205 electroinactive electroinactive -0.5 0 +a5 +in +Fig.1 [Co(tripyam),]' immobilised on a laponite-modified elec- trode, prepared by dipping the electrode into an acetonitrile solution of the complex.(a) 500; (b) 200; (c) 100; (d) 50 pV s-' hedral complexes (i.e. >5 pB). This fact, together with the observation of weak IR spectral bands similar to those of low-spin [Fe(tripyam)2](C104)2 in addition to a more domi- nant set similar to high-spin [Ni(tripyam)2](C104)2, led Kulas- ingamI4 to suggest that some low-spin isomer was trapped in the initially prepared specimen. However, subsequent vari- able-temperature measurements5*" showed Curie-Weiss behaviour down to 200 K, a transformation to the low-spin form occurred between 190 and 143K." The freshly prepared [C~(tripyarn),](ClO,)~ used in this work was subjected to EPR analysis. A weak anisotropic signal was seen at room temperature centred on g=2, but after 1 month the signal had become broad and isotropic.After 3 months the complex was EPR silent at room temperature but after it had been J. MATER. CHEM., 1992, VOL. 2 (b1 Fig. 2 Voltammograms from a laponite-modified electrode, laponite pre-exchanged with [Co(bipy),13+. (a) 20 pA cm-', 100 mV s-'; (b)50 pA cm-' 200 mV s-' cooled a broad resonance was seen to arise at 190 K (full-width at half-maximum, FWHM =493 Hz). As cooling con- tinued, the intensity of the signal increased and the linewidth narrowed, Maximum intensity was reached at 145 K. At 105 K, g=2.1285 and FWHM =239 Hz. Thus, it is confirmed that the correct material had been prepared and Kulasingam's astute observation is also confirmed: it appears that some low-spin isomer is 'frozen' into the freshly prepared solid but, on passage of time, a true room-temperature equilibrium favouring the state is reached. Attempts to exchange [Co(tripyam),](ClO,), onto laponite raised further problems.A conventional solution-contact method (Method 1) gave a pink clay with a reflectance spectrum (Table 1) distinct from that of the orange [Co(tripyam),12 +. The difficulty of oxidising this cobalt(1r) complex has been mentioned previously;" thus, a change of terdentate to bidentate ligands was a more likely explanation. [Co(tripyam),](ClO,), in water gave, on shaking for 1 week, a pink solution. Comparison of the visible spectrum for this solution with that of [C~(pyridine),(H,O),]~ and the diffuse + reflectance spectrum of the pink clay (experimental section) leaves little doubt that the exchanged cation is diaquo{bis[tri- 2-pyridylaminecobalt(r1)]>with bidentate amine ligands.It appears that the clay surface has a greater affinity for this ion than for the bis(terdentate) species. The ease of aquation reflects the fact that two Co-N bonds of [Co(tripyam),] (ClO,), are significantly longer at 2.152(2)A than the other four at 2.100(2) A.6 The copper(I1) perchlorate complexes of tri-2-pyridylamine may be prepared in two isomeric forms, i.e. blue [Cu(tri- pyam),](ClO,), with terdentate ligands and, yellow-green [C~(tripyam),(ClO,),]~ with bidentate ligands, but aqueous solutions of both contain [Cu(tripyam),(H,O),]' with+ bidentate ligands and this is the species which exchanges onto laponite.The observed basal spacings, d(O0l), are very similar +for [M(tripyam)2(H20)2]2 (where M =Co, Cu) exchanged clays (Table 1). This raised the question of whether the bis(terd- entate) form of these complexes could be accommodated by the clay. Accordingly the low-spin [Fe(tripyam),](C10,)2 was prepared. The contamination with a hitherto unrecognised tris[tri-2-pyridylamine]iron(11)complex with bidentate ligands has been detailed. The complex with terdentate ligands exchanged onto laponite unchanged but gave a basal spacing similar to those observed for [M(tripyam)2(H20)2]2 + in which bidentate ligands are present. It is possible that the terdentate co-ordinated ligand complexes lock into the pseudo-hexagonal rings of the silicate layers.When the copper-complex-exchanged clays were heated to 100 "C a greenjblue change occurred corresponding to a bidentatej terdentate change. This change is completely reversible. The temperature at which the colour change occurs may be altered by turning to other ligands, e.g. 4- and 6-methyl-di-(2-pyridyl)amine.Some new complexes of cop- per@) were serendipitously prepared ([CuL(OH)]i +) and exchanged onto laponite. A reversible colour change occurred at 145 "C for the 4-methyl ligand, and an irreversible change occurred at 215 "C for the 6-methyl ligand. Thus, the point is illustrated that thermochromic clays may be prepared by exchange with metal complexes of ligands with variable denticity, and the temperature of the colour change may be tuned.The change is not instantaneous, but the robustness of the materials suggests that they could find application as hazard warnings of excess temperature in or on appropriate equipment. The use of microwave heating in chemistry laboratories can produce remarkable increases in the rates of organicI5 and organometallic'6 reactions and very remarkable heating of some inorganic substrates may OCCU~.'~ It was demonstrated recently that the process of ion exchange onto clay may be greatly accelerated. For example, lithium-exchanged laponite may be produced in 5 min in the microwave oven1* compared with several weeks using the classic method of Posner and Quirk.lg Furthermore, the rapidity of the microwave method may reveal chemistry which could be missed during the passage of time required by conventional exchange methods.l8 In this paper it has been shown that microwave heating (method 2) can greatly accelerate the exchange process. Thus, when laponite is treated with ethanolic [Co(tripyam),](ClO,), in the microwave oven for 5 min, an orange clay is produced, the diffuse reflectance spectrum of which is consistent with the presence of [Co(tripyam),12 [bis(terdentate) ligands]. + However, on storing this material for 12 months the aquation reaction to give pink [C~(tripyam),(H,O),]~ [bis(bidentate)+ ligands] occurs slowly on the clay surface.This strengthens the view that the clay has a greater affinity for the bis(bident- ate) form, [M(tripyam),(H20),l2 +. The microwave experi- ment has apparently greatly accelerated the exchange reaction, but not the aquation reaction. In comparison with [Co(bipy)J2 +,the corresponding com- plex of tri-2-pyridylamine, [Co(tripyam),I2 +,was much more resistant to oxidation, indeed the synthesis of cobalt(w) tri-2- pyridylamine complexes has not yet been achieved. Yet, by contrast the preparation of cobalt(m) complexes of substituted tri-Zpyridylamines was relatively easy.' The opportunity was taken to compare the cyclic voltammetric behaviour of these complexes for acetonitrile solutions of the cobalt(@ complexes both in free solution and as CME containing the compounds. The solvent for all electrochemistry was acetonitrile and solutions of the cobalt(I1) complexes in acetonitrile were carefully monitored by visible absorption spectroscopy to ensure that no change of ligand denticity occurred. The preparation of the CME followed a procedure developed by Ghosh and co-workers20*21 which involved a preliminary deposition of the clay film on a platinum electrode followed J.MATER. CHEM., 1992, VOL. 2 by dipping the prepared electrode in an acetonitrile solution of the complex. Using an acetonitrile solution of [Co(tripyam),12 (terdentate ligands), the electrode assumed + the characteristic orange colour of the bis(terdentate) complex. In order to compare the electrochemical behaviour of the pink bis(bidentate) complex, it was necessary to prepare the electrode from the pre-exchanged and rigorously washed clay.Electrodes were also prepared from bis[6-methyl-di-(2-pyridyl)amine]cobalt(r~) ([Co(6-dmdpa),l2 '1, dipped in acetonitrile solution, and tris(2,2'-bipyridyl)cobalt(111) {[C~(bipy)~]~both dipped and pre-exchanged. The El /2+>, values obtained from the cyclic voltammograms of the solu- tions are detailed in Table 2. The Co2 redox process was + investigated and the data confirm that the ease of oxidation is in the order: + +[Co(bipy),] + >[C0(6-dmdpa)~]' >[Co(tripyam),] , thus the difficulty of synthesising [Co(tripyarn),l3 is+ expressed more quantitatively. The greater basicity expected for the 2-pyridyl group bearing the 6-methyl substituent should lead to greater CT bond strength compared to that for the co-ordinated unsubstituted 2-pyridyl groups.However, the difference may be more significant for Co"' than for Co". The dipped CME, prepared from [Co(tripyam),12 and+ [Co(bipy),13+ gave good-quality cyclic voltammograms with EIl2 data in close agreement with the results obtained from solution measurements. Despite a number of electrode preparations, the cyclic voltammograms obtained from [C0(6-dmdpa)~]~+/CME were unsatisfactory. The attempt to compare the redox behaviour of the bis-(terdentate) [Co(tripyam),12' and the bis(bidentate) [C~(tripyarn),(H,O),]~+ was of necessity confined to CME studies; also the bidentate species was only available pre- exchanged on laponite.The resulting CME was electroinactive (Table 2). King et have demonstrated that cations, bound electro- statically to any exchange site on a smectite clay, are rigorously electro-inactive. The electroactivity which characterises CMEs was attributed to cations which are surface bound, in excess of the clay cation-exchange capacity, by an ion-pair mechan- ism. A recent helpful review23 concludes that the electroactiv- ity of a CME will depend on factors such as those detailed by King et and also on the method of film preparation. A CME was therefore prepared using the method detailed in the experimental section with laponite which had been pre- exchanged with [Co(bipy), J3 +;this electrode was also rigor- ously electro-inactive. We therefore conclude that, using our conditions of electrode preparation, the electroactivity is dependent on sorbed ion pairs from dipping the CME in acetonitrile solutions of the complex.Unfortunately therefore, it is not possible using this methodology to study the redox behaviour of any complex which can exist on the clay exchange sites, but not in acetonitrile solution. S.P.B. thanks SERC for a studentship. We thank Dr. K. D. Sales (Queen Mary and Westfield College, London) and Dr. F. J. Berry (University of Birmingham) for variable-tempera- ture EPR data and Mossbauer data, respectively. References I N. Davison and W. R. McWhinnie, Znorg. Chim. Acta, 1987,131, 9.2 W. E. Rudzinski and A. J. Bard, J. Electroanal. Chem., 1986,199, 323. 3 C. Shi, J. F. Rusling, Z. Wang, W. S. Willis, A. M. Winiecki and S. L. Suib, Langmuir, 1989, 5, 650. 4 W. R. McWhinnie, G. C. Kulasingam and J. C. Draper, J. Chem. SOC.A, 1966, 1199. 5 P. F. B. Barnard, A. T. Chamberlain, G. C. Kulasingam, W. R. McWhinnie and R. C. Dosser, J. Chem. SOC., Chem. Commun., 1970, 520. 6 E. S. Kucharski, W. R. McWhinnie and A. H. White, Aust. J. Chem., 1978,31, 2647. 7 E. S. Kucharski, W. R. McWhinnie and A. H. White, Aust. J. Chem., 1978, 31, 53. 8 J. C. Lancaster and W. R. McWhinnie, Znorg. Chim. Acta, 1971, 5, 515. 9 J. P. Wibaut and G. L. C. La Bastide, Res. Trau. Chim., 1933, 52, 493. 10 J. C. Lancaster and W. R. McWhinnie, J. Chem. SOC. (C), 1970, 2435. 11 D. F. B. Barnard, J. C. Lancaster, M. E. Fernandopulle and W. R. McWhinnie, J. Chem. SOC.,Dalton Trans., 1973,2172. 12 W. R. McWhinnie, R. C. Poller and M. Thevarasa, J. Chem. SOC. (A), 1967, 1671. 13 F. H. Burstall and R. S. Nyholm, J. Chem. SOC.,1952, 3570. 14 G. C. Kulasingam, Ph.D. Thesis, University of London, 1967. 15 R. N. Gedye, F. E. Smith and K. C. Westaway, Can. J. Chem., 1988,66, 17. 16 M. Ali, S.P. Bond, S. A. Mbogo, W. R. McWhinnie and P. W. Watts, J. Organomet. Chem., 1989, 371, 11. 17 D. R. Baghurst and D. M. P. Mingos, J. Chem. SOC., Chem. Commun., 1988, 829. 18 S. P. Bond, A. Gelder, J. Homer, W. R. McWhinnie and M. C. Perry, J. Mater. Chem., 1991, 1, 327. 19 A. M. Posner and J. P. Quirk, Proc. R. SOC. London, A, 1964, 278, 35. 20 P. K. Ghosh and A. J. Bard, J. Am. Chem. SOC.,1983,105,5691. 21 P. K. Ghosh, A. W.-H. Man and A. J. Bard, J. Electroanal. Chem., 1984, 169, 3 15. 22 R. D. King, D. G. Nocera and T. J. Pinnavaia, J. Electroanal. Chem., 1987,236,43. 23 A. Fitch, Clays Clay Miner., 1990, 38,391. Paper 1/02894D, Received 17th June, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200037

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992, 2(1), 43-47 Di(aryIethynyI)bis(trimethyIph0sphine)pIatinum(I I):A New Series of Liquid-crystalline Materials Takeshi Kaharu, Hiroshi Matsubara and Shigetoshi Takahashi* The Institute of Scientific and Industrial Research, Osaka University, lbaraki, Osaka 567, Japan Various derivatives of the title complexes have been synthesized and demonstrated to form stable mesomorphic phases. They provide a new family of thermotropic liquid-crystalline complexes containing a carbon-metal a-bond and a tertiary phosphine co-ordinated to a transition metal. Keywords: Liquid crystal; Metallomesogen ; Platinum alkynyl complex; Platinum phosphine complex Studies of liquid-crystalline materials incorporating transition metals form a rapidly expanding field, because such new materials are expected to have not only the intrinsic properties of organic liquid-crystalline compounds but also unique optical, magnetic and electronic properties due to the tran- sition metals. Various transition-metal-containing liquid crys- tals have already been reported;' however, most of them are co-ordinated complexes, i.e.Werner-type metal complexes, and organometallic mesogens, especially those possessing a a-bond between carbon and transition metal are rare. As far as we know, cyclo-metallated palladium complexes with sev- eral types of ligand2 and some metal carbonyl complexes3 have been shown to form thermotropic liquid crystals. We have found that trans-di(arylethynyl)bis(trimethylphos-phine)platinum(II) complexes (l),which are the constituent unit of rod-like metal-poly(yne) polymers forming lyotropic liquid crystal^,^ exhibit thermotropic liquid-crystalline proper- ties and we have recently reported5 preliminary results. PMe3 R10~C02~C~CI -bt-C~C pMe3 1 The platinum alkynyls 1 provide a rare example of metallo- mesogens having a metal-carbon a-bond, and the first example of thermotropic liquid-crystalline molecules contain- ing tertiary phosphines co-ordinated to a transition metal.Among the known organotransition-metal complexes hav- ing a a carbon-metal bond, metal alkyls and alkenyls are, in general, thermally unstable and sensitive to moisture and air, whereas metal alkynyls are comparatively stable, therefore the alkynyls are good candidates for the principal structure of metallomesogens.Transition-metal alkynyls are usually prepared by the reaction of metal halides with alkynylation reagents such as lithium acetylides and Grignard reagents. Such methods, however, cannot be employed for the prep- aration of metallomesogens because the alkynylation reagents may react with polar groups such as carbonyl, ester, and cyano groups, which are common constituents of liquid- crystalline molecules. We previously found a new method6 for preparing transition-metal alkynyls directly from alk- 1 -ynes by the catalysis of copper(1) chloride in an amine. This method may be applicable to the synthesis of metallomesogens since under the reaction conditions polar groups on R are inactive for copper(1) halide catalysts and amines.By this method we can prepare platinum monoalkynyls from the reaction of dichlorobis(trialky1phosphine)platinum with an alk-1-yne in the molar ratio of 1 : 1 cu catalyst 3(R3P)2MCl2 + HC=CR (R,P),M(CI)(CECR) (1)amine Alk- 1 -ynes having polar groups are also not prepared easily owing to the reason above. The method (reaction 2)7 for carbon-carbon coupling, which we previously found using a homogeneous catalyst consisting of palladium and copper complexes, can be suitable for the preparation of arylethynes bearing polar groups. RX + HCiCSiMe, Pd-Cu catalyst -RCECSiMe, + HX (2)mine I The combination of the above two synthetic methods has provided a convenient route to the new platinum-containing metallomesogens and made it possible for us to perform systematic studies on the new mesogen.Here we wish to report the synthesis and mesomorphic properties of the new liquid-crystalline complexes 1 in detail. ~oR2 Results and Discussion Synthesis and Characterisation of Complex 1 The synthetic procedure leading to the target complex 1 is outlined in Scheme 1. The free mesogenic ligand 4 having a terminal ethynyl group was obtained in good yield from the coupling reaction of compound 2 with trimethylsilylacetylene (Me,Si-C-CH) using a palladium ~atalyst,~ followed by the elimination of the Me3Si protecting group with tetra-n- butylammonium fluoride.8 Complex 7 was prepared from the reaction of dichlorobis(trimethy1phosphine)platinum (5) with one equimolar 4-alkoxyphenylethyne (6)in the presence of copper(1) chloride as a catalyst in triethylamine,6 and identified by NMR.The 31P{'H) NMR spectrum of 7 showed a signal at 6 =ca. -9.6 ppm (upfield from an external triphenylphos- phine reference) with satellites due to the coupling (Jpl--p= 2340 Hz) with lg5Pt nuclei (I= 1/2, natural abundance 34%); in the 'H NMR spectrum the methyl signal of the trimethyl- phosphine ligand appeared at 6= 1.7 ppm as a triplet due to the coupling with 31Pnuclei (vide infra) along with satellites due to the coupling with I9'Pt nuclei. The target complex 1 was formed from the reaction of 7 with 4 in the presence of CuCl as a catalyst in diethylamine at room temperature.Complex 1 was purified by column chromatography and recrystallization, and obtained as a yellow crystalline solid. The identification was made by elemental analysis, and IR and 'H NMR spectra, which were consistent with the pro- posed structure. The IR spectrum of 1 showed absorption bands at 2125 and 1740 cm-' which are assigned to vCrC and vc=o, respectively. Square-planar complexes of Pt" have 4 PMe,I CI-Pt-CI +H-CEC PMe, PMe3 5 6 7 1 Scheme 1 Reagents and conditions: (a)(i) SOCl,, DMF, ClCH,CH,Cl, reflux; then (ii) p-iodophenol, pyridine, 4-N,N-dimethylaminopyridine catalyst; (b) (CH3)3Si-C=CH, PdCI,(PPh,),-CuI catalyst, NEt,, room temp.; (c) n-Bu,NF, THF, -78 "C; (d) CuCl catalyst, NEt, reflux; (e) 4, CuCI catalyst, HNEt,, room temp.two possible configurations: cis and trans. The trans-configur- ation of complex 1 has been confirmed by NMR, i.e. in the 'H NMR spectrum (Fig. 1) the methyl groups of PMe, ligand appeared at 6 =1.8 ppm with an apparent 1 :2 :1 triplet split- ting due to 'virtual coupling' with 31Pn~clei,~indicating that the two trimethylphosphine ligands mutually occupy trans positions of the central platinum atom. The aromatic protons were observed in the range 6 =6.7-8.2 ppm as three pairs of AAXX' pattern, and the a-methylene protons of the alkoxy groups in R' and R2 appeared at 6=4.0 and 3.9 ppm, respectively, with a 1 :2 :1 triplet splitting. The signals of b-methylene protons of the alkoxy groups were observed around 6= 1.8 ppm partially overlapped with the signals of the PMe, ligand.The ,'P{lH} NMR spectra of 1 exhibited one signal in the range of 6= -14.1 to -15.3 ppm with attendant satellites due to the coupling (Jpt-p=2300 Hz) with Ig5Pt nuclei, also indicating trans-configuration around the central platinum atom because cis-configuration must exhibit two pairs of phosphorus signals. In the 13C{1H} NMR spectrum of 1 (R' =R2=n-C5H1,), resonances due to the two sp carbons bonded to the platinum atom appeared at 6= 104.4 and .11, I,, I. I,. I.,l'~"l"'i"'~''"l~ Il~TII7,. ,m 8.5 8.0 7.5 7.0 6.54.5 4.0 3.52.5 2.0 1.5 1.0 0.5 0.0 6 Fig. 1 'H NMR spectra of complex lp J. MATER. CHEM., 1992,VOL. 2 107.8 ppm as a triplet (Jc-pt=15.0 Hz) with attendant satel- lites, and the methyl groups of the PMe, ligand appeared at 6 =15.4 ppm as a virtual triplet (Jc--p=19.5 Hz).The trans configuration suggests that complex 1 has an extended mol- ecular structure which may be desirable for liquid-crystalline materials, Thermal Properties Thermal properties of compounds 3 and 4 are reported in Tables 1 and 2. Compounds 4 having a terminal ethynyl group displayed mesogenic properties, except those compounds with n =3 or 5. For most of them an enantiotropic nematic phase along with a monotropic smectic A phase was observed, though compound 4e (n=8) exhibited enantiotropic N and SA phases, and 4h (n= 14) exhibited monotropic N and SA phases. The identification of mesophases has been made on the basis of optical textures.The nematic phase was identified by the appearance of droplets, which exhibited a typical nematic schlieren texture on cooling from the isotropic liquid. In the SAphase, a focal conic texture was observed. Compounds 3 (n=7, 8, and 10) which are the precursors of compounds 4 also exhibited a monotropic SA phase, but not an N phase. In comparison with 4, compound 3 formed thermodynamically unstable mesophases, because it has a sterically much larger trimethylsilyl group on the terminal position of the molecule. The transition temperatures of complex 1 were summarized in Table 3. The temperatures were determined on heating on the second scan with a differential scanning calorimeter. For complexes la, lb and lp, however, the first scan data were adopted because their clearing point was so high that they began to decompose.Their gradual decomposition starting at ca. 220-230 "C was detected by thermogravimetric analysis. As can be seen, most of 1 showed N phases, and the derivatives with R' =CI2H25 (n= 12) having a long terminal alkyl group R2 exhibited enantiotropic SA phases. The SA phases were identified by the appearance of batonnets on cooling from Table 1 Phase-transition temperature (/"C) for compound 3 c~,,,,-0~co2~c~c-si~cH3~,3 compound n C SA I 3a 3 -132.3 3b 5 * 107.5 3c 6 * 104.6 3d 7 83.9 (-72.9) 3e 8 * 90.0 65.9)(9 3f 10 ' 77.7 (-50.0) 3g 12 * 66.5 3h 14 * 70.4 Table 2 Phase-transition temperature (/"C) for compound 4 C,,H2fl+1-O~CO~-@~C-H 4 compound n C SA N I 4a 3 -135.1 4b 5 -83.8 4c 6 -75.0 * 79.5 4d 7 * 71.6 (-58.6) * 77.6 4e 8 * 58.6 64.2 * 84.1 4f 10 -73.7 (-68.7) * 85.7 4g 12 81.4 (-72.7) -84.7 (a4h 14 85.8 75.2) 81.6 J.MATER. CHEM., 1992, VOL. 2 Table 3 Phase-transition temperature (/ "C) for complex 1 be as sterically large as an iodine atom. Therefore it may be supposed that the lateral PMe3 substituents diminish the C"H,,,-O~CO,~C~C-PI(PMe3,-CrC efficiency of the molecular packing in layers. However, the complexes 1, despite having a structure with a very large complex n m C SA N I lateral substituent, show a trend to exhibit mesomorphic properties, and especially to form nematic phases. This may la 3 8 * 220.8 * 244.5 be due to the rigid rod-like structure and the rather strong lb 5 8 -174.3 * 225.6 intermolecular force often seen for a long n-conjugation lc 6 8 -171.6 * 215.7 system such as poly(yne) and poly(ene).Id 7 8 169.7 -208.0 le 8 8 -173.4 * 205.6 If 10 8 * 176.8 198.8 Experimental1g 12 8 * 174.9 * 189.6 (alh 14 8 -173.7 171.3) * 183.7 Elemental microanalyses were done by the Material Analysis li 12 5 170.3 -199.8 Center, I.S.I.R., Osaka University. IR spectra were obtained 1j 12 6 173.6 -196.6 with a Hitachi 295 Infrared Spectrophotometer. 'H NMR (alk 12 10 173.9 170.3) -183.0 11 12 11 * 174.9 175.6 * 182.1 spectra were recorded on a Brucker WM-360 instrument in lm 12 12 * 173.7 * 177.3 * 179.7 CDC13 with tetramethylsilane as an internal standard, and In 12 14 173.6 * 178.7 -31PNMR spectra on a JEOL FX-100 instrument at 40.3 MHz lo 12 16 * 170.4 * 176.0 in CD2C12 with PPh3 as an external standard.Transition 1P 5 5 * 196.4 244.8 temperatures were measured, and mesomorphic properties 1q 10 10 175.9 190.9 were observed using an Olympus BH-2 polarising microscope in conjunction with a Mettler FP52 heating stage, FP5 control the isotropic liquid. The batonnets coalesced to give focal unit, and a Shimadzu DSC-50 differential scanning calor- imeter. The rate of heating or cooling was fixed at 5 "C min- ' conic fan textures, and on heating homeotropic texture could under an argon atmosphere. Thermogravimetric analyses were be observed. It is known that the number of carbon atoms in a terminal performed on a Shimadzu TGA-50 thermogravimetric ana- alkoxy group for a homologous series affects the behaviour lyser at a heating rate of 5 "C min-' under an argon of nematic-to-isotropic transitions.In the present case, com- atmosphere. plexes with longer alkoxy chains showed lower nematic-to- isotropic transition temperatures and resulted in a narrower Synthesis of 4-(Trimethylsilylethynyl)phenyl temperature range of mesophases. 4-Octyloxybenzoate (3e) In order to obtain a relationship between molecular struc- ture and mesomorphic properties we have also synthesized 4-Iodophenyl 4-octyloxybenzoate (2e) (12.83 g, 28 mmol), complexes 8-11 by a similar method. Complexes 8-10 showed dichlorobis(tripheny1phosphine)palladium (200 mg, 0.28 no mesophases (m.p./ "C: 8, 151.6; 9, 136.3; 10, 220.0), whereas mmol), and triphenylphosphine (150 mg, 0.57 mmol) were 11showed a nematic phase, though the transition temperature dissolved in a mixture of triethylamine (120 cm3) and toluene was as high as ca.200 "C, showing that the Pt complexes of (80 cm3) under an atmosphere of nitrogen. Copper(1) iodide a two-ring system exhibit no mesophases. An increase of (53 mg, 0.28 mmol) and trimethylsilylacetylene (6.0 cm3, structural anisotropy (an increase in the length :width ratio) 42mmol) were added to the solution with stirring. After in the complexes consisting of three- or more-ring systems reaction at room temperature for 5 h, the precipitate of results in the formation of mesophases.This suggests that the triethylammonium iodide was filtered off and washed with part of a three-ring system involving a platinum atom may benzene. The combined filtrate was evaporated under reduced be regarded as the mesogenic group for complex 1. pressure. The resultant product was purified by column chro- matography on silica gel using dichloromethane-hexane (1 :1) as eluent, followed by recrystallization from hexane. A white PMe3C8Hl,0~C02~EC-~~ crystalline poduct was obtained: yield 11.69 g (98%). v,,,/ cm-' (Nujol) 2150 (CEC), 1600 (Ar) and 1265 (C-0-C); 8 0-0-Table 4 Yields and elemental analysis of compounds 3 and 4 ~ ~~C8H1+ \ / C02 \ 1 CEC-Y-CEC-CGH~~ PMe3 calcd. (YO) found (YO) 9 compound n yield (YO) C H C H PMe3 C8H 70-@PC -(7-CECeOC8H17 3a 3 84 71.55 6.86 71.72 6.84 3b 5 99 72.59 7.42 72.62 7.58PMe3 3c 6 90 73.05 7.66 72.99 7.5210 3d 7 93 73.49 7.89 73.43 7.69 3e 8 98 73.89 8.1 1 73.81 8.23 -~&~C8Hl 70~cO2~cZc @&l,, 31 10 99 74.62 8.50 74.50 8.67 PMe3 3g 12 83 75.27 8.84 75.32 8.58 3h 14 89 75.84 9.15 76.07 9.2211 4a 3 80 77.12 5.75 76.88 5.92Thermotropic liquid-crystalline molecules generally have a 4b 5 60 77.90 6.54 77.68 6.38 rod-like structure and even small lateral substituents can 4c 6 90 78.23 6.88 78.33 6.7 1 perturb the structure, thus causing a significant depression in 4d 7 81 78.54 7.19 78.5 1 6.99 clearing point.Even a lateral substituent as small as a fluorine 4e 8 83 78.83 7.48 78.61 7.40 atom strongly affects the thermal stability of mesophases." 4f 10 83 79.33 7.99 79.40 7.78 4g 12 92 79.77 8.43 79.52 8.16Judging from a CPK molecular model, the trimethylphosphine 4h 14 81 80.14 8.8 1 80.34 8.93ligand attached to the central metal is roughly estimated to J.MATER. CHEM., 1992, VOL. 2 Table 5 Yields and elemental analyses of complex 1 C~H,,,-O~cO,~CC-P1(PMe,),-CIC complex n m yield (YO) C la 3 8 85 56.13 lb 5 8 66 57.07 lc 6 8 89 57.51 Id 7 8 86 57.94 le 8 8 71 58.37 If 10 8 27 59.17 1glh 12 14 8 8 49 86 59.92 60.64 li 12 5 61 58.77 1j lk 12 12 6 10 71 35 59.17 60.64 11 12 11 83 60.98 lm 12 12 64 61.31 In 12 14 92 61.95 lo 12 16 42 62.56 1P 5 5 53 55.24 Iq 10 10 84 59.92 8H 0.26 [s, 9 H, Si(CH3)3], 0.90 (t, 3 H, J=7 Hz, CH,CH3), 1.30-1.50 (m, 10H, CH2CH3), 1.82 (m, 2H, J=7Hz, OCH,CH,), 4.04 (t, 2 H, J=7 Hz, CH20C6H4), 6.96 (d, 2 H, J=9Hz, Ar), 7.16 (d, 2H, J=9Hz, Ar), 7.51 (d, 2 H, J= 9 Hz, Ar) and 8.12 (d, 2 H, J=9 Hz, Ar).(Found: C, 73.81; H, 8.23%. Cak. for C~6H3403Si: c, 73.89; H, 8.1 1%). Other 4-(trimethylsilylethyny1)phenyl derivatives 3a-h were similarly prepared and are summarized in Table 4. Synthesis of 4-Ethynylphenyl4-Octyloxybenzoate(4e) To the protected acetylene compound (3e) (1 1.64 g, 28 mmol) in dry THF (130 cm3) was added 1.O mol dm -THF solution of Bu,NF (56 cm3, 56 mmol, 2 equiv.) at -78 "C. The reaction mixture was maintained at -78 "C until the starting material was consumed (checked by thin-layer chromatography), then the reaction was quenched by addition of a large excess of solid ammonium chloride.After evaporation of THF under reduced pressure, the residue was poured into water and extracted with ethyl acetate. The combined organic layer was washed with saturated aqueous NaCl and dried over anhy- drous sodium sulfate. After concentration of the solution, the resultant product was purified by chromatography on silica gel using dichloromethane-hexane( I : I) as eluent. Recrys- tallization from hexane gave the pure product of 4e: yield 8.08 g (83%). v,,,/cm-' (Nujol) 3290 (-C-H), 1740 (C=O), 1600 (Ar), and 1270 (C-0-C); 6 0.89 (t, 3 H, J=7Hz, CH2CH3), 1.30-1.50 (m, 10H, CH2CH3), 1.82 (m, 2H, J= 7 Hz, OCHZCH,), 3.07 (s, 1 H, GC-H), 4.04 (t, 2 H, J= 7 Hz, CH20C6H4), 6.97 (d, 2 H, J=9 Hz, Ar), 7.18 (d, 2 H, J=9 Hz, Ar), 7.54 (d, 2 H, J=8 Hz, Ar), and 8.12 (d, 2 H, J= 9 Hz, Ar).(Found: C, 78.61; H, 7.40%. Calc. for C23H2603: C, 78.83; H, 7.48%). Other 4-ethynylphenyl derivatives, 4a-h, were similarly prepared and are summarized in Table 4. Synthesis of Chloro(4-octyloxyphenylethyny1)-bis( trimethy1phosphine)pla tinum (7c) A mixture of dichlorobis(trimethylphosphine)platinum(II) (2.35 g, 5.6 mmol) and 4-octyloxyphenylethyne (1.29 g, 5.6 mmol) in a mixture of triethylamine (70 cm3) and toluene (50cm3) was allowed to react in the presence of copper(1) calcd. (YO) found (YO) H P C H P 6.36 7.24 55.99 6.35 7.00 6.6 1 7.01 57.12 6.37 6.92 6.73 6.89 57.71 6.60 7.09 6.85 6.79 58.06 6.66 6.83 6.97 6.69 58.07 7.20 6.40 7.18 6.49 59.06 7.26 6.36 7.39 6.3 1 59.63 7.59 6.2 1 7.58 6.13 60.39 7.43 6.35 7.08 6.59 58.57 6.98 6.4 1 7.18 6.49 59.03 6.89 6.2 1 7.58 6.13 60.65 7.47 6.27 7.68 6.05 60.58 7.61 6.22 7.77 5.97 61.16 7.63 5.87 7.94 5.81 61.90 7.94 5.6 1 8.1 1 5.66 62.78 8.31 5.38 6.89 7.3 I 55.27 6.69 7.36 7.39 6.3 1 59.78 7.37 6.40 chloride as a catalyst at 90 "C for 10 h under nitrogen atmosphere. The solvent was removed by rotary evaporation, and the crude product was purified by column chromatogra- phy on alumina, followed by recrystallization from dichloro- methane-hexane (1 :2).A yellow crystalline product was obtained: yield 2.76 g (80%); m.p. 134.2 "C. v,,,/cm-' (Nujol) 2120 (C-C), 1600 (Ar), and 1240 (C-0-C); SH 0.88 (t, 3 H, J=7Hz, CH2CH3), 1.28-1.45 (m, 10H, CH2CH3), 1.65 [m, 18 H, JPPH=4 Hz, JPt-H=28 Hz, P(CH3),], 1.75 (m, 2 H, OCH,CH,), 3.91 (t, 2 H, J=7 Hz, CHZOC~H~), 6.75 (d, 2 H, J=9 Hz, Ar), and 7.21 (d, 2 H, J=9 Hz, Ar); dP-9.66 (JP-pt= 2340 Hz). Other mono(ethyny1)-platinum complexes were similarly prepared. Synthesis of trans-{[4-(4-Octyloxybenzoyloxy)-phenylethynyl](4-octyloxyphenylethynyl)-bis(trimethylphosphine)platinum} (le) Chloro(4-octyloxyphenylethynyl)bis(trimethylphosphine) plat-inum (7c) (0.61 g, 1 mmol) and 4-ethynylphenyl 4-octyloxy- benzoate (0.35 g, 1 mmol) were dissolved in diethylamine (60 cm3) under an atmosphere of nitrogen.Copper(1) chloride as a catalyst was added to the solution. The mixture was stirred for 2 h at room temperature. The resultant solution was concentrated in uucuo and the residue was purified by column chromatography on alumina using benzene as an eluent, followed by recrystallization from dichloromethane- hexane( 1 :2). A yellow crystalline product was obtained, yield 0.65 g (71%). v,,,/cm-' (Nujol) 2100 (CzC), 1740 (C=O), 1600 (Ar), and 1250 (C-0-C); 6, 0.87-0.91 (dt, 6H, J= 7 Hz, CH2CH3), 1.30-1.51 (m, 20 H, CH2CH3), 1.86-1.73 [m, 22 H, Jp-H=4 Hz, CHZCH20, P(CH3)3], 3.92 (t, 2 H, J= 7 Hz, CHZOC~H~), 4.04 (t, 2 H, J=7 Hz, CH20CsH4), 6.75 (d, 2 H, J=9 Hz, Ar), 6.95 (d, 2 H, J=9 Hz, Ar), 7.04 (d, 2 H, J=9 Hz, Ar), 7.24 (d, 2 H, J=9Hz, Ar), 7.35 (d, 2 H, J= 8Hz, Ar), and 8.12 (d, 2H, J=9Hz, Ar); S, -15.29 (JP--Pt= 2300 Hz).(Found: C, 58.07; H, 7.20; P, 6.40%. Calc. for C45Ha04PZPt: C, 58.37; H, 6.97; P, 6.69%). Other bis(ethyny1)platinum complexes were similarly pre- pared and are summarized in Table 5. J. MATER. CHEM., 1992, VOL. 2 47 This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area from the Ministry of Education, Science and Culture, Japan. References 5 N. Hagihara, Macromolecules, 1979, 12, 1016; S. Takahashi, H. Morimoto, Y. Takai, K. Sonogashira and N. Hagihara, Mol. Cryst. Liq. Cryst. Lett., 1981, 72, 101; S.Takahashi, Y. Takai, H. Morimoto, K. Sonogashira and N. Hagihara, Mol. Cryst. Liq. Cryst. Lett., 1982, 82, 139; S. Takahashi, Y. Takai, H. Morimoto and K. Sonogashira, J. Chem. SOC., Chem. Commun., 1984, 3. T. Kaharu, H. Matsubara and S. Takahashi, J. Mater. Chem., 1991, 1, 145. 1 2 3 A-M. Giroud-Godquin and P. M. Maitlis, Angew. Chem. Int. Ed. Engl., 1991, 30,375. M. Ghedini, S. Armentano, R. Bartolino, F. Rustichelli, G. Torquati, N. Kirov and M. Petrov, Mol. Cryst. Liq. Cryst., 1987, 151, 75; P. Espinet, J. Etxebarria, M. Marcos, J. Perez, A. Remon and J. L. Serrano, Angew. Chem., Int. Ed. Engl., 1989, 28, 1065; P. Espinet, E. Lalinde, M. Marcos, J. Perez and J. L. Serrano, Organometallics, 1990, 9, 555. D. W. Bruce, E. Lalinde, P. Styring, D. A. Dunmur and P. M. Maitlis, J. Chem. SOC., Chem. Commun., 1986, 581; M. A. Esteruelas, L. A. Oro, E. Sola, M. B. Ros and J. L. Serrano, 6 7 8 9 10 K. Sonogashira, Y. Fujikura, T. Yatake, N. Toyoshima, S. Takahashi and N. Hagihara, J. Organomet. Chem., 1978, 145, 101. S. Takahashi, Y. Kuroyama, K. Sonogashira and N. Hagihara, Synthesis, 1980, 627. R. M. Williams, D. J. Aldous and S. C. Aldous, J. Chem. SOC., Perkin Trans. 1, 1990, 171. J. M. Jenkins and B. L. Show, Proc. Chem. SOC., 1963,278; E. W. Randall and D. Shaw, Mol. Phys., 1965, 10,41. e.g. K. J. Toyne, in Thermotropic Liquid Crystals, ed. G. W.Gray, Wiley, New York, 1987, p. 28. J. Chem. SOC., Chem. Commun., 1989, 55. 4 S. Takahashi, E. Murata, M. Kariya, K. Sonogashira and Paper 1/03276C; Received 2nd July, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200043

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

J. MATER. CHEM., 1992,2(1), 49-55 Investigation of the Local Structure around Iron dispersed in Vinyl Chloride-Vinylidene Chloride (VC-VdC) Copolymer Coatings on Mild Steel using Glancing-angle X-ray Absorption Spectroscopy Stefania Pizzini,” Kevin J. Roberts,*”+ Ian S. Dring,b Peter J. Moreland,b Richard J. Oldmanb and James Robinson“ “Department of Chemistry, Strathclyde University, 295 Cathedral Street, Glasgow G7 7 XL, UK blCl Chemicals and Polymers, The Heath, Runcorn, Cheshire WA74QD, UK “Department of Physics, University of Warwick, Coventry CV4 7AL, UK The chemistry occurring in vinyl chloride-vinylidene chloride polymer coatings on mild steel has been investigated by probing the local environment around Fe ions using glancing-angle X-ray absorption spectroscopy.Measurements made as a function of penetration depth reveal that close to the air interface the Fe species are present as octahedrally co-ordinated Fe”’ and have a local structure similar to that observed in disordered y-FeOOH. Although this structure is found to be independent of the pH of the coating formulation, Fe ion transport from the metal substrate is apparently enhanced at acidic pH formulations. Deeper into the coating the Fe has a structure typical of a mixed tetrahedral-octahedral environment such as Fe,O,. Keywords: Copolymer coating ; Glancing-angle X-ray absorption spectroscopy; /on transport The first commercial water-borne paints made as alternatives to conventional solvent-borne paints were based generally upon acrylic polymers or copolymers.Their high permeability to water and oxygen resulted in poor anti-corrosive perform- ance and made them unsuitable as competitive primers. How- ever, in the 1980s a new water-borne vinyl chloride-vinylidene chloride (VC-VdC) copolymer (Haloflex 202$)was developed’ which gave an excellent anti-corrosive performance consistent with the outstanding barrier properties of the chlorine-con- taining polymer.’ Through the use of ethylene oxide-propyl- ene-ethylene oxide block copolymers and other formulation additives to aid colloid stability and latex particle coalescence, these properties were transposed to a paint with an exceptional anti-corrosive performance. However, the chemical nature of these chlorine-containing vinyl (acrylic) copolymers (Fig.I) leads to a dehydrochlorination process at alkaline pH. The latex is therefore formulated into a paint at pH 4-6,compared with the traditional range of pH 8-9, whereby the dehydrochlorination process is significantly suppressed. The acidically formulated paints exhibit some interesting anti-corrosive properties as detailed el~ewhere.~ For example, the extent of flash rusting during the ‘wet’ film condition decreases with decreasing pH from 7 to 4.Associated with this reduction in pH is the formation of a darkening effect at the metal/polymer interface in the dry film. Moreland and Padget related the enhanced protective performance of the paint to the presence of this thin 1-2 pm interfacial layer arising from an insignificant steel substrate corrosion process during ‘wet’ painting.Transmission electron microscopy (TEM) studies4 indicate that this layer is composed of platelets of ca. 0.05-0.2 pm in size dispersed between ca. 0.2 pm latex particles. Beyond this interfacial layer the same studies reveal that often the latex particles are ‘decorated’ (Fig. 2). X-Ray diffraction (XRD) measurements showed this interfacial layer to include a phase which is isostructural with the mineral pyroaurite [Mg,Fe2(0H),6C03.4H20] and in which divalent substrate cations substitute for Mg. The properties of anion exchange and buffering capacity expected from this type of t Also at SERC Daresbury Laboratory, Warrington WA4 4AD, UK $ ‘Haloflex’ is a trademark belonging to the ICI group of companies.HCI H H H CCI H4\0 OR ...I # A B C Fig. 1 Schematic formula of the acrylate-modified vinyl chloride- vinylidene chloride copolymer (A =vinyl chloride; B =vinylidene chloride; C =acrylate ester) compound are consistent with a good anti-corrosive perform- ance and hence Moreland and Padget3 considered its presence was possibly playing a key role. Information on the local co-ordination and the oxidation state of Fe species dispersed in such polymer coatings can be obtained from X-ray Absorption Spectroscopy (XAS) measurements. XAS (see Fig. 3) of condensed samples exhibit an oscillatory structure which can extend up to lOOOeV or central atom I I ---I I I 0 backscattering atoms 0 100 200 v1-total Fig.3 EXAFS spectroscopy: (a) the absorption process generates photoelectron waves (A) which are backscattered (B) by the nearest neighbours and which give rise to interference which varies with the energy of the incident photons; (b) this interference is observed as a periodic modulation on the high-energy side of the absorption thresh- old; (c) the intensity of the emitted photoelectric wave essentially represents a Fourier series of backscattered contributions from the various near-neighbour shells. Owing to the short-range nature of the process this is damped rapidly by the crystal lattice which results in EXAFS oscillations, typically, containing significant contributions from only the first three co-ordination shells of the structure. Fourier transformation of the fine structure yields a radial distribution func- tion from the absorbing atom and identification of the characteristic phase shift of the backscattered wave can be used to identify the atom type more, beyond the absorption-edge threshold.In metals and semiconductors the features appearing close to the absorption edge have been attributed to transitions to localised electronic states. The strong oscillations just beyond the absorption edge (XANES) can be explained in terms of the multiple scatterings of photoelectrons by the atoms in a local cluster around the absorbing atom, and can give information on the geometrical arrangement of the near neighbours of the absorbing atom, The structure observed from 30-40eV beyond the edge, the extended X-ray absorption fine structure (EXAFS), is due to the interference between the outgoing photoelectron wave and the wave backscattered from neighbour atoms (e.g.ref. 5 and 6). Interference effects are determined by the distance, the chemical type and the number of atoms around the absorbing atom. Since only elastically scattered electrons can interfere, and the elastic mean free path of electrons is short, the analysis of the EXAFS spectra provides information on the local atomic structure of the absorbing atom. Previous investigations7 have shown the potential of XAS as a probe of the structural and chemical properties of the Fe species transported through polymer coatings.In studies of polymer coating on vacuum evaporated Fe thin films, Oldman7 found that for a polymer formulation prepared at pH 2, the Fe species incorporated in the polymer were found to be present as Fe" ions with a local structure similar to hydrated FeCl,. For the pH 4.5 formulation, an Fe"' com-pound was found and the nearest-neighbour structure indi- cated the presence of a structure typical of an oxide. EXAFS investigations on some related systems have been reported in the literature. Barrett and co-w~rkers~~~ demonstrated the J. MATER. CHEM., 1992, VOL. 2 enhanced surface sensitivity afforded by the glancing angle XAS technique by investigating the early stages of the thermal corrosive oxidation of stainless steel in an atmospheric environment.The uncorroded surface showed an oxide layer in which Fe is present as Fe304. After oxidation for 4 min at 1000°C the material was shown to develop an iron-rich protective layer in which iron is present predominantly as Fe,O,. Kerkar et al." investigated in situ Fe thin films passivated in aqueous solutions and found that their structure was typical of disordered y-FeOOH. In this paper we describe the results of XAS measurements carried out on Haloflex polymer coatings of different thick- nesses and pH formulations, coated onto mild-steel substrates. The aim of this investigation is to clarify the structural properties of iron species which become dispersed in the polymer after coating onto 'real' surfaces such as mild steel plates.XAS measurements above the Fe K-absorption-edge, carried out for several incident angles corresponding to increasing penetration of the X-ray beam in the sample, allow the local structure of Fe species to be obtained as a function of depth. An important stage of this work has been the acquisition and analysis of XAS data recorded from bulk Fe oxides and oxyhydroxides. A comparison of the XAS spectra with model crystal structures has helped the interpretation of the data and the identification of bulk-like Fe species dispersed in the polymer coatings. Materials and Methods Model Compounds The model compounds for this investigation were an Fe foil 5pm thick and FeO, a-Fe203, Fe304, a-FeOOH and y-FeOOH powders. FeO crystallises with an NaC1-like cubic structure.Each Fe" ion is surrounded by six oxygens in octahedral co-ordination." The structure of a-Fe203 (haematite) can be regarded as a slightly distorted hexagonal close packing (h.c.p.) of 0 ions with the metallic cations in some of the octahedral interstices.12 At low and medium temperatures Fe,O, (mag-netite) has an inverse cubic spinel structure (Fe;' Fe;' Fe;' Oi-). Oxygens are in an almost perfect cubic close packing (c.c.P.) with the metal ions lying in tetrahedral and octahedral interstices. Fe"' ions at A sites are tetrahedrally co-ordinated to oxygen and Fe" and Fe"' ions at B sites are octahedrally co-ordinated to oxygen.' a-FeOOH is ortho- rhombic with 0 and OH in an almost perfect h.c.p. and each Fe atom is surrounded by an octahedron of 0 atoms.14 y-FeOOH is also orthorhombic and its structure is built up of well defined layers parallel to the (100) face.Each layer is made up of octahedra surrounding Fe atoms and linked together by sharing corners. The octahedra are nearly regular and have four corners occupied by 0 atoms and two by OH groups.l5 The powders for the EXAFS measurements were finely ground, dispersed in polypropylene and pressed into 1.3 cm diameter discs. To provide a suitable model for iron films oxidised in an aqueous environment we compared our data with that pro- duced by Kerkar et a1.I' in which a thin Fe film was electrochemically passivated in an aqueous solution contain- ing 0.1 mol dm-3 of sodium perchlorate at a potential of +0.8 V us.SCE. VC-VdC Polymer Coatings The substrates for the polymer coatings were mild-steel blocks (nominal composition C, 0.08-0.13%; Mn, 0.3-0.6%; P, J. MATER. CHEM., 1992, VOL. 2 0.04%; S, 0.05%; balance Fe) of 45 mm x 10mm x 1 mm in size, polished using diamond paste down to 0.25 pm diameter. The steel surfaces were coated with Haloflex 202 latex films (containing no additives or pigments) of different thickness and pH. The data presented here refer to the conditions, sample (1) pH 1.5, thickness 50 pm; sample (2)pH 1.5, thick-ness 2 pm; sample (3) pH 7.0, thickness 2pm. The pH 1.5 represented the as-received latex and maximum corrosion activity, while pH 7 represented a much milder corrosion condition. A thickness of 50 pm is typical for paint coating, but a coating of 2 pm allowed more opportunity for measure- ments at the metal/polymer interface. The pH of the nominally acidic formulations was adjusted using ammonium hydroxide solution.The coatings were allowed to dry for 7 days in a dust-free environment before the measurements were carried out. XAS Measurements XAS measurements of the model compounds and polymer coatings were carried out above the Fe K-absorption edge on stations 8.1 and 9.2, respectively, of the Synchrotron Radiation Source (SRS) at Daresbury Laboratory. Double- crystal order-sorting Si (220) and (1 11) monochromators, respectively, were used to obtain spectra in angular scanning mode.The XAS spectra of the model compounds were recorded in transmission geometry [Fig. 4(a)] with the exception of the electrochemically passivated film which was recorded in situ by Kerkar et a!." using fluorescence detection (see ref. 16). To achieve surface sensitivity, the polymer coatings were examined using glancing angle XAS (see e.g. ref. 17, 18) with depth-dependent information provided by varying the glanc- ing angle. Owing to the low concentration of Fe expected in the polymer films, the spectra for these samples were recorded using fluorescence detection as this mode provides optimum signal-to-noise for data acquisition. The X-ray beam was collimated by fine slits and, in order to have a precise alignment with respect to the X-ray beam direction, the samples were mounted on a precision goniometer." The experimental set-up is shown schematically in Fig.qb). XANES spectra of the three Haloflex polymer-coated samples were recorded at incident grazing angles 4 which varied between 0.1 and 10" with the fluorescence signal monitored by a wide-angle ionisation chamber positioned above the sample surface. EXAFS spectra of polymer coating (2)were recorded for monochromator \ collimator slit Fig. 4 The experimental facilities for the collection of X-ray absorption spectra on the SRS:(a) transmission geometry used for the analysis of bulk model compounds where the X-ray intensity transmitted through the sample (IT)is measured with respect to the incident beam intensity (Io);(b) fluorescence geometry where the X-ray fluor- escence yield of a dilute analyte (I,) is measured with respect to I, at low 9 # =0.1 O, using a multi-element Ge solid-state detector.The energy resolution of the detector was such that Fe fluorescence could be accurately discriminated from the background signal. Since Fe species are highly dispersed at the surface of the polymer films, acquisition times of ca. 5 h had to be used to obtain EXAFS data of acceptable quality. Data Analysis The analysis of the experimental EXAFS spectra were carried out using standard procedures. The spectra were background subtracted using a smooth cubic-spline function to fit the atom-like background above the absorption edge. The near- edge background-subtracted spectra were normalised to the absorption step height, which was obtained by extrapolating the background absorption before and after the edge.The EXAFS function ~(k)is defined as: where k is the photoelectron wavevector, p(k)is the absorption coefficient and po is the background absorption i.e. the absorption of an isolated atom. In the so-called plane-wave approximation the EXAFS function can be expressed by: where the summation is made over i co-ordination shells at an average distance ri from the absorbing (central) atom and Niis the number of atoms at distance ri.Fi(k)is the magnitude of the backscattering amplitude for each type of atom. A is the electron mean free path, ci is root-mean-square deviation around the average distance ri and it appears in a Debye- Waller-like factor describing structural and thermal disorder.$i is a phase-shift term introduced by the fact that the wavefunction of the ejected photoelectron is modified by the potentials of the absorbing and backscattering atoms. The central Fe atom and backscattering atoms (Fe, 0) phase shifts were calculated ab initio and were not iterated in the least-squares fitting process. Fits to the experimental EXAFS data were obtained using EXCURVE90 (see e.g. ref. 20) which is a least-squares fitting program based on the curved-wave theory. For each neighbouring atom shell, the parameters allowed to vary were the distances from the central atom (ri),the co-ordination numbers (NJ,the disorder param- eter (20i2)and an inner potential-energy shift (E,) which is used to define the photoelectron wavevector k according to the relation k =[(2rn/h2)(E-Eo)]'/2 (3) where E is the kinetic energy of the photoelectron, rn is its mass and h is Planck's constant.In the fitting of the model compounds, the neighbour-shell distances were allowed to vary with respect to the crystallo- graphic values while the co-ordination numbers were fixed to the crystallographic ones. The accuracy in the determination of the structural param- eters is 1-3% for the shell distances and 10-30% for the co- ordination numbers. Inaccuracy in the determination of the co-ordination numbers results largely from the strong corre- lation with the disorder term.Results and Discussion EXAFS and XANES Studies of Model Compounds Fig. 5 shows the experimental and least-squares-fitted EXAFS spectra for a-Fe,03, FeO, Fe30,, a-FeOOH and y-FeOOH. 52 J. MATER. CHEM., 1992, VOL. 2 10 Table 1 The co-ordination numbers, N, neighbour-shell distances and 5 Debye- Waller factors' are compared with the crystallographic radial 5 distribution functions 2s 0 As 0 siteb atom Ncryst rcrysl/A N r/A 2a21A2 x x -5-5 a-Fe,O,-1 0 0 3 I .960 3 1.94 0.010-1 0 0 3 2.088 3 2.10 0.012I I -1 5 46810 4 6810 Fe 1 2.885 1 2.87 0.0 12 klA-' Fe 3 2.996 3 2.96 0.0 12klA -' Fe 3 3.364 3 3.36 0.015 4 6 0 3 3.382 3 3.34 0.015 4 0 3 3.601 3 3.64 0.015 2 Fe 6 3.700 6 3.64 0.0202 0 3 3.807 3 3.74 0.020 v2s 0 $0 Fe 1 3.985 1 3.99 0.020 -2 x -2 Fe304-4 T 0 4 1.886 1.3 1.91 0.0204 -6 0 0 6 2.058 4.0 2.07 0.034 4 0 Fe 6 2.968 4.0 2.99 0.03 1 -4 6 8 10 4 6 8 10 12 0 Fe 6 3.480 8.0 3.47 0.030 klA-' klA-' T Fe 12 3.480-I --T 0 12 3.493 10 T Fe 4 3,635 1.3 1.91 0.020 0 0 2 3.563 5 0 0 6 3.659 T 0 12 4.5272 so 0 0 12 4.675 x T 0 12 4.748 -5 T 0 12 5.397 0 Fe 12 5.140 8 5.14 0.024 -1 0 T Fe 12 5.452 7 5.44 0.0264 6 8 10 1-2 0 Fe 8 5.452 k/A-' T Fe 12 5.935Fig.5 Experimental (-) and least-squares-fitted (---) EXAFS 0 Fe 12 5.935spectra for (a) a-Fe,O,; (b)FeO; (c) Fe,O,; (d) a-FeOOH and (e) y-FeOOH FeO 0 6 2.166 6 2.15 0.035 The results of the least-squares fits are summarised in Table 1 Fe 12 3.063 12 3.06 0.022 0 8 3.752 8 3.73 0.026where they are compared with the crystallographic radial Fe 6 4.332 6 4.39 0.0 16 distribution functions, 0 24 4.843 The Fe K-near-edge spectra recorded for Fe metal and for Fe 24 5.306 24 5.24 0.030 the model oxides and oxyhydroxides are shown in Fig.6. For Fe 12 6.126 12 6.23 0.015 each compound, the energy separation of the measured pre- a-FeOOH edge feature (A) and the edge-feature (B, corresponding to the 0 2 1.953 3 1.94 0.007 steepest point up the absorption edge) are summarised in 0 1 1.954 Table 2. The amplitudes of feature A (C for Fe foil), normalised 0 2 2.089 3 2.10 0.007 to edge step height are also reported. 0 1 2.093 Fe 2 3.010 2 2.99 0.010The results summarised in Table2, show that the energy Fe 2 3.28 1 2 3.22 0.010 separation of the edge features A and B and the height of the Fe 4 3.459 4 3.39 0.010 pre-edge feature A in the near-edge spectra of Fe oxides and 0 2 3.589 4 3.55 0.012 oxyhydroxides can be used as fingerprints of the oxidation 0 2 3.666 0 1 3.753 2 3.74 0.015state of the Fe cation and of the local geometry of the Fe 0 1 3.767species, For Fe203, ol-Fe00H and y-FeOOH, where Fe is y-FeOOHpresent as Fe", the energy separation between A and B is ca.0 1 I .905 1.91 0.00310eV. In FeO, where iron is present as Fe", and in Fe304 0 2 2.023 2 1 1.99 0.003where both Fe"' and Fe" are present, the energy separation is 0 1 2.027 1 2.03 0.003 ca. 7 eV. The pre-edge feature A arises from the ls+3d dipole 0 2 2.133 2 2.13 0.003 forbidden transition," which in Fe compounds becomes par- Fe 4 3.070 4 3.04 0.015 tially allowed due to the hybridisation with p orbitals of oxygen Fe 2 3.080 2 3.08 0.015 atoms.The amplitude of the pre-edge feature is dependent on 0 2 3.622 the geometry of the co-ordination to oxygen atoms. It is 0.005-0 2 2 3.686 3.6870.008 for FeO, Fe,O, and both forms of FeOOH, where Fe 0 Fe 2 3.870 2 3.97 0.015 is in an octahedral environment. In Fe304, where the mixing of p and d states is enhanced by the tetrahedral co-ordination, a Calculated from the least-squares fits to the Fe K-edge spectra. In the amplitude of feature A is greater (0.016). This is in agreement the least-squares fits, the co-ordination numbers were fixed to the with the observation of Dring et dZ2 crystallographic values.T =tetrahedral site, 0=octahedral site. In Fig. 7(b) the first derivative functions of the near-edge Depth-dependent XANES Studies of Polymer Coatings on spectra are compared with the derivative near-edge spectra Mild Steel recorded for Fe304, y-FeOOH and for the thin Fe film Fig. 7(a) shows the near-edge spectra recorded for polymer passivated in aqueous perchlorate solution. lo coating (1) on mild steel, for incident angles 0.1, 1, 5 and 10 '. The near-edge spectra for polymer coating (2) and (3) J. MATER. CHEM., 1992, VOL. 2 f t I -20 0 20 40 60 80 100 energy/eV Fig.6 Fe K-near-edge spectra measured for the model compounds (a)a-FeOOH; (b) y-FeOOH, a-Fe203; (d)FeO; (e) Fe304 and 0Fe Table 2 Energy separation of features A and B and height of feature A in the near-edge spectra of the model compounds (Fig.5) compound ion E, -E,/eV A height/k Cb Fe Fe 0.46 FeO Fen 7.5 0.008 Fe304 Fez03 FeII/Fem Feu1 7.2 10.4 0.016 0.008 a-Fe00H Feu1 10.0 0.007 y-FeOOH Feu1 10.4 0.005 a Normalized to the edge-stop height. Position of the inflection point at the absorption edge. formulations were recorded for angles increasing from 0.1 to 5 ", In Fig. 8 the heights of the pre-edge feature A in these near-edge spectra are plotted as a function of incident angle. These are compared with the height of the pre-edge feature modelled by summing in several proportions the spectra of Fe metal and y-FeOOH. A qualitative analysis of the near-edge spectra recorded for the polymer coatings deposited on steel may therefore give some preliminary information on the valency and the co- ordination geometry of Fe species.The near-edge spectra recorded for polymer coating (1) as a function of incident angle (Fig. 7) indicate that the local environment of Fe species dispersed in the polymer film changes with depth, i.e. Fe species close to the air interface have a different local co- ordination from that exhibited by the Fe species close to the steel substrate. The spectra recorded for angles 0.1, 1 and 2 O show some resemblance to those of or-FeOOH and y-FeOOH and this indicates that, closer to the surface, Fe might be octahedrally co-ordinated to oxygen atoms.For an incident angle of 0.1 ",the energy separation between the edge-features A and B is 10.4 eV. A comparison with the results in Table 2 suggests that iron may be present in the surface coatings as Fe"'. Significant changes in the near-edge spectra are observed for incident angles greater than 2", i.e. where X-rays probe Fe species closer to the steel substrate. These changes are seen better in the first-derivative spectra [Fig. 7(b)].Note in particular the variation in the near-edge structure at ca. 8 eV beyond the absorption edge. In this energy region, the deriva- tive spectrum for an incident angle of 10" shows features typical of Fe,O, and this indicates that, close to the steel substrate, Fe species might be present in a mixed tetrahedral- octahedral environment.The presence of tetrahedral sites close to the steel substrate is also supported by the enhance- ment of the amplitude of the pre-edge feature A in the spectra recorded for increasing incident angles. In summary, the near- edge spectra suggest that close to the air interface, Fe species -20 0 20 40 60 80 100 energ y/eV (b 1 1 . EKm . . 1I.,, -10 0 10 20 30 40 energylevFig. 7 (a) Fe K-near-edge spectra for polymer coating (1) on mild steel measured for incident angles (i) 0.1; (ii) 1; (iii) 2; (iv) 5 and (v)lo";(b) the first derivatives of the spectra in (a)(iii)-(vii) are compared with the derivative spectra of y-FeOOH (i); Fe304 (viii); and with a spectrum for the Fe film passivated in aqueous perchlorate solution (ii) I 1 0.64 Jt I I I I l o 0.2 0.1 0.6 o a 1.0 4/degrees Fig.8 Plotted height of the pre-edge feature A, shown in Fig. 6 and 7, in the Fe K-near-edge spectra recorded for the polymer ccating (2) (m) and (3) (@) at various values of 4. These are compared with the height of A obtained adding-in different proportions the spectra of metallic Fe and y-FeOOH dispersed-polymer coating (1) (i.e. 50 pm thick, formulated at a pH of 1.5) are FeIn cations with an octahedral environment typical of FeOOH. Closer to the steel interface Fe species are present as a mixture of octahedral and tetrahedral sites. The near-edge spectra recorded for small incident angles (<0.7') for polymer coating (2) (the spectra are presented elsewhere'*) are very close to those measured for similar angles for the 50 pm thick film.This indicates that the Fe species close to the polymer/air interfaces are present as Fe"' cations in an octahedral environment and that this does not depend on coating thickness. When the X-ray penetration depth is increased, i.e. the measurements made at greater incident angles (4= 1 and 1.5 "), the near-edge spectra change dramatically and resemble closely the spectrum of metallic Fe. The height of the pre-edge feature A increases as the penetration depth is increased and for 4 = 1O this reaches the value found for metallic iron. This indicates that for this angle, the X-ray beam penetrates the steel substrate. Fig. 8 shows that for 4=0.7-1" the height of the edge feature (A or C) crosses the value expected for metallic Fe, indicating that the X-ray beam starts penetrating the substrate.The X-ray path from the surface to the steel substrate and back is ca. 180 pm for d, =0.95" and 120 pm for d, =0.7". From these data, and using tabulated values of the mass absorption coefficient of Fe23 it can be calculated that the average Fe loading in this coating is between 1.7 and 2.5 wt.%. Fig. 8 shows that for the thin polymer coating formulated at a pH of 7, sample (3), the height of the pre-edge feature reaches the value expected for Fe metal for a lower angle than for the film with pH 1.5. This indicates that the substrate is reached by X-rays for a smaller incident angle, and therefore that the Fe concentration in this coating is lower than in the film with acidic formulation.EXAFS Studies of the Environment around Iron close to the Metal/Polymer Interface More detailed information on the local structure of Fe species dispersed in polymer coatings close to the air interface was obtained from the EXAFS data recorded for coating (1) in comparison with those obtained for the Fe film passivated in sodium perchlorate aqueous solution. lo The first derivative near-edge spectrum [Fig. 7(b)] shows that the passivated Fe film is very similar to the spectra recorded for polymer coatings (1) and (2) for the smallest incident angles. Kerkar et a1." showed that the structure of the passivated iron film can be regarded as a disordered y-FeOOH.The similarities in the near-edge spectra suggest that Fe species dispersed in the polymer surface coatings, close to the air interface, might have a similar structure. The results of the EXAFS data, shown in Fig. 9, are in agreement with the conclusions from the near-edge data. The results of the least-squares fits to the EXAFS data (Table 3) for the surface coatings prepared at pH 1.5 show that Fe is octahedrally co-ordinated to oxygen. The structural param- eters for the nearest-neighbour oxygen shells are, however, not sufficient to identify the structure in which Fe is involved. These distances are characteristic of the octahedral co-ordi- nation in both crystalline a-FeOOH, y-FeOOH and Fe203 (Table 1).The results of the analysis for the Fe co-ordination shells at greater distances, the presence of a nearest-neighbour shell at 3.89 A and perhaps more significantly the absence of Fe-Fe correlations in the range 3.3-3.5 8, indicate, however, that Fe species might be involved in a compound similar to y-FeOOH. This, however, does not imply that the layer formed at the metal/polymer interface has the same passiv- ation properties as that observed by Kerkar et a/." on electrochemically treated steel, only that the local structures of the two layers appear to be rather similar. It is noteworthy that the co-ordination numbers calculated for the distant shells are largely reduced with respect to those typical of bulk iron oxyhydroxides. This is clearly shown by the Fourier transforms of the spectra of sample 2 and of y-FeOOH (Fig. 10).The reduction in amplitude of the near- J. MATER. CHEM., 1992, VOL. 2 6 3 %o5 x -3 -6 4 6 8 10 kla - I 4 6 8 10 k1A-l Fig. 9 Experimental (-) and least-squares-fitted fluorescence EXAFS spectra: (a) polymer coating (2); (b) Fe film passivated in perchlorate solution Table 3 Co-ordination numbers N, neighbour-shell distances r and disorder terms 20' calculated from the least-squares fits of the fluorescence EXAFS data of polymer coating (2) for q5 =0.1O and for the Fe film passivated in aqueous solution Fe dispersed in polymer coating 0 1.o 1.86 0.004 0 3.0 2.00 0.003 0 1.o 2.08 0.004 Fe I .5 2.96 0.020 Fe 1.1 3.89 0.020 0 I.2 1.85 electrochemically passivated Fe film 0.006 0 2.0 1.94 0.003 0 2.4 2.06 0.004 Fe 1.4 3.00 0.017 Fe 1.o 3.80 0.013 1.2 E 0.9 i;c K2 0.6 0.0 0 2 4 6 ria Fig. 10 Fourier transform of the Fe K-edge EXAFS spectrum of polymer coating (2) (-) in comparison with y-FeOOH (---) J.MATER. CHEM., 1992, VOL. 2 neighbour shells beyond the first could be due to either structural disorder or the amorphous nature (small particle size) of the iron compound dispersed in the polymer. The results of this study, coupled with those of Moreland and Padget3 show that the chemistry at the polymer/metal interface is far from straightforward. Moreland and Padget,3 based on TEM investigations, concluded that a compound isostructural to pyroaurite, observed close to the steel sub- strate, would be responsible for the corrosion inhibition properties of the Haloflex 202 polymer coatings.In pyroaurite both Fe and Mg cations occupy octahedral sites. By compari- son, the results obtained here show that, whereas close to the air interface Fe species are present in an octahedral environ- ment, close to the substrate interface Fe is present in a mixed tetrahedral-octahedral environment typical of Fe304. The variation in the structure and composition of the Fe species in (1) from a disordered y-FeOOH, close to the air interface, to an Fe304-like structure, deeper in the film, can be compared with the reaction pathways of Fe in aqueous solution postulated by Misawa et a1.24These authors pro- posed several reaction pathways which depend on pH and generally involve Fe"-Fe"' mixed-valence intermediates. In acidic solutions the following pathway is proposed: Fe2++(FeI@, Fe:I)(" -2X)+ green complex +Fe 0, (OH) -2x amorphous ferric oxyhdroxide +(aging)+ (a-Fe00H) The results of the XAS measurements on the polymer coating (1) are consistent with this reaction pathway.The presence of Fe" was observed by Oldman7 and a mixed-valence Fe"-Fe"' compound and an amorphous ferric oxyhydroxide, possibly y-FeOOH, were observed in this work. The mixed-valence species is found close to the polymer/steel interface where the oxygen supply is scarce whilst the final corrosion product (before subsequent ageing leading to the formation of crystal- line a-FeOOH), an amorphous Fe" oxyhydroxide, possibly of y-form, is found close to the air/polymer interface where the oxygen supply is greater.However, the chemical pathways proposed by Misawa et for the transition from Fe" to Fe"' also allow for the precipitation of a pyroaurite-type material. Since both amorphous and crystalline materials are present it is highly likely that more than one Fe species is formed, which can engage in anion exchange and buffering. Further ageing should lead to crystalline a-FeOOH but this stage is not reached in the case of Fe dispersed in the dry polymer coatings. Our studies did not show any significant role played by chloride, which is a natural component of the Haloflex latex, in the formation of the interfacial film although earlier work7 in a different kind of experiment which involved total dissol- ution of Fe, showed formation of hydrated FeC12.Conclusions The results of the investigation of VC-VdC polymer coatings on mild steel shows that glancing-angle X-ray absorption measurements made as a function of incident angle allow depth-specific structural information on the local structure of Fe within polymer coatings to be obtained. Close to the air interface Fe species are present as Fe"' and have a local structure typical of disordered y-FeOOH. The local environ- ment of Fe is very similar to that of Fe in thin films passivated in aqueous solutions.Deeper in the polymer, Fe is present in a compound characterised by the mixed tetrahedral-octa- hedral co-ordination typical of Fe304. The identity of the oxyhydroxide compound close to the air interface is not found to depend on the pH of the polymer, as similar spectra were recorded for polymer coating formulations prepared at pH 1.5 and pH 7. The concentration of Fe species transported in the pH 7 formulation is found to be considerably smaller than that found in the acidic formulations. The Authors thank SERC and ICI Chemicals and Polymers for the financial support of this work, SERC Daresbury Laboratory for provision facilities and beam time on the SRS and M. Kerkar for his assistance in recording the in situ XAS spectra of the passivated iron film.References 1 A. J. Burgess, D. Caldwell and J. C. Padget, J. Oil Colour Chem. Assoc., 1981, 64, 175. 2 J. C. Padget and P. J. Moreland, J. Coating Technol., 1982, 55, 39. 3 P. J. Moreland and J. C. Padget, ACS Syrnp. Ser. 322, Polymeric Materials for Corrosion Control, ed. R. A. Dickie, F. L. Floyd, American Chemical Society, Washington, DC, 1986. 4 P. J. Moreland, I. M. Fraser and G. T. Finlan, personal communi- cation, 199 1. 5 E. A. Stern and S. M. Heald, in Handbook on Synchrotron Radiation, ed. E. E. Koch, North-Holland, Amsterdam, 1983. 6 D. E. Koningsberger and R. Prins, in X-Ray Absorption: Prin- ciples, Applications, Techniques of EXAFS, SEXAFS and XANES, Wiley, New York, 1988. 7 R. J. Oldman, J.Phys. Paris Colloque, 1986, CS 47, 321. 8 N. T. Barrett, P. N. Gibson, G. N. Greaves, K. J. Roberts and M. Sacchi, Physica B 1989, 158, 690. 9 N. T. Barrett, P. N. Gibson, G. N. Greaves, P. Mackle, K. J. Roberts and M. Sacchi, J. Phys. D, 1989, 22, 542. 10 M. Kerkar, J. Robinson and A. J. Forty, Faraday Discuss. Chern. SOC.1990, 31. 11 E. R. Jette and F. Foote, J. Chem. Phys, 1933, 1, 29. 12 R. L. Blake, R. E. Hessevick, T. Zoltai and L. W. Finger, Am. Mineral., 1966, 51, 123. 13 M. E. Fleet, Acta Crystallogr. Sect. B 1981, 37, 917. 14 A. Szytula, A. Burewicz, Z. Dimitrijewic, S. Krasnicki, H. Rzany, J. Todorovic, A. Wanic and W. Wolski, Phys. Status Solidi, 1968, 26, 429. 15 H. Christensen and A. N. Christensen, Acta Chem. Scand. Ser. B, 1978, 32, 87. 16 J. B. Hastings, in EXAFS Spectroscopy, Techniques and Appli- cations, ed. B. K. Teo and D. C. Joy, Plenum Press, New York, 1980, p. 171. 17 S. P. Pizzini, K. J. Roberts, G. N. Greaves, N. T. Barrett, I. D. Dring and R. J. Oldman, in Synchrotron Light: Applications and Related instrumentation 11, World Scientific, Singapore, New Jersey, London, Hong Kong, 1990, p. 67. 18 S. Pizzini, PhD Thesis, University of Strathclyde, 1990. 19 S. Pizzini, K. J. Roberts, G. N. Greaves, N. Harris, P. Moore, E. Pantos and R. J. Oldman, Rev. Sci. Instrum., 1989, 60,2525. 20 S. J. Gurman, J. Phys. C, 1988, 21, 3699. 21 L. A. Grunes, Phys. Rev. B, 1983,27, 21 11. 22 I. S. Dring, D. H. Hall, R. J. Oldman, J. L. Casci, W. N. E. Meredith and R. P. Tooze, Physica B, 1989, 167. 23 International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol. IV. 24 T. Misawa, K. Hashimoto and S. Shimodaira, Corros. Sci., 1974, 14, 131. Paper 1/03376J; Received 4th July, 1991

ISSN:0959-9428    

DOI:10.1039/JM9920200049

出版商:RSC    

年代:1992

数据来源: RSC