Electrodeposi tion of thin-film rare-eart h-me ta1 oxocupra tes ~~ ~~ Robert Janes, Paul M. S. Monk,* Robert D. Partridge and Simon B. Hall Department of Chemistry, Manchester Metropolitan University, Chester Street, Manchester, UK M1 5GD Cathodic deposition from an aqueous solution containing the simple nitrate salts of Nd3 + and Cu2+ yields insoluble thin-film oxyhydroxide layers. Variation of the electrode substrate, solute concentrations (absolute and relative, one to another), applied cathodic potential and cell geometry allows the production of films with a wide range of precursor compositions. Conditions were optimised to allow production of a solid thin film which, on firing at 940 "C,yielded Nd,CuO,. Since the electrodeposited material comprises intimate mixtures of oxide, subsequent firing to yield the desired semiconductor requires a relatively low firing temperature.The compositions of the products formed are accounted for in terms of solubility constants and a reaction front between the electrogenerated material and cations in the bulk solution. There is presently great intere~tl-~ in fabricating high-quality superconducting thin films. A much-quoted potential appli- cation is the manufacture of passive microwave devices, e.g. for use as filters, resonators or antennae.',, Conventional techniq~es',~of forming such films include laser ablation' and cathodic or electron-beam sputtering.6 A major problem with this method is the requirement for expensive volatile precur- sors. Alternative methods include ion-beam sputtering and laser ablation, both of which require solid targets of the starting material: in some cases this is an unattainable goal for the materials of interest in this study.Relatively few workers have prepared thin-film superconduc- tors using electrodeposition. Weston et uL7 prepared Y:Ba :Cu :0 with dimethylformamide (DMF) as solvent. Their work has apparently not been developed further. Wieckowski and coworker^^*^ used aqueous solutions for electrodeposition of Y :Ba :Cu: 0 but quote no XRD data. These authors suggest that the films are of poor quality and it may be for this reason that no other workers use water as solvent. Noufi et uL1'-12 prepared T1: Ba :Ca :Cu :0 and Y :Ba :Cu :0 using dimethyl sulfoxide (Me,SO) as solvent but the films were of poor quality, contained sulfur from the Me,SO, were 'difficult to fire' and did not allow for high critical currents.Finally Abolmaali and Talbot13 prepared YBa2Cu307-, by using isopropyl alcohol and and applying a potential of -423 V to the working electrode. The form of electrodeposition used in this present study yields insoluble mixtures of oxide and hydroxide (usually of indeterminate ratio and hence are termed 'oxyhydroxide' here) following electron-transfer reactions to the nitrate counter ion. Following high-temperature firing, the solid products on the electrode are potentially superconducting. One mechanism postulated for the electrodeposition of nickel o~yhydroxide'~ involves the following series of reactions as a mechanism for the electrodeposition: SNi(OH), +5Ni0 +5H20 (3) Oxyhydroxide products will ensue if eqn.(3) does not occur quantitatively. Also, a stoichiometry of eight electrons yielding 5 NiO units will only be achieved in full if convective control is maintained. The technique of electrodeposition as a means of composi- tional control in d-block oxide materials is well established, and has been successfully employed to prepare films of mate- rials such as tungsten trioxide," cobalt oxide,16 and molyb- denum oxides17 as well as a large range of mixed-metal oxides." Recently, a series of novel (electrochromic) films have been produced comprising as many as four transition metal oxides, e.g.W :Co :Cr :Mo or W :Co :Ni :Zn oxides, either series of mixture being deposited cathodically on platinum." The compositions of the electrodeposited product are con- trolled by the deposition voltage, V,, and ratios of solute in solution. Other variables include temperature, ionic strength, solvent permittivity (adjusted by addition of organic solvent to the deposition solution) and ligation. For example, the compositions of the tungsten-nickel oxide formed by elec- trodeposition varies between W :Co =1:2.10 (when V,= -0.7 V) to 1 :1.05 (at V,= -1.3 V)." The oxyhydroxide products of electrodeposition are usually amorphous and are therefore assumed to possess a greater homogeneity, e.g. than do materials formed using bulk pro- cedures such as grinding of solid-state components, since all the precursors are wholly soluble.This follows since the mixing of solutes occurs on an atomic scale. In this work, we describe the formation of neodymium- copper phases, and show how the adjustment of physicochemi- cal parameters allows for the preparation of well-known mate- rials such as neodymium cuprate, Nd2Cu0,. Experimental Cu( NO3), and Nd(N03)3 precursors were Aldrich reagents and used in aqueous solution at a concentration of 0.010 mol dm -3 unless stated otherwise. All water was doubly distilled. Solutions were purged with N, gas prior to electrochemistry. Only results using potentiostatic (three-electrode) deposition are quoted here. The working electrode was silver and the counter electrode was platinum (both had an area of cu.2 cm2; the counter electrode was the larger). The reference electrode was always a saturated calomel electrode, SCE. All applied potentials, V,, here are cited with respect to SCE. The depos- ition current was in the range 50-100 mAcm-,, the value depending on the voltage applied. All electrochemical pro- cedures were performed on a PAR 273A potentiostat interfaced to an IBM PC-clone computer operating PAR 270 electro-chemistry software. Solutions were agitated gently using the nitrogen gas flow during deposition to maintain convective control. In all cases the cathodic limit employed represents the formation of molecular hydrogen at the cathode. No results are quoted here in which hydrogen gas did form at the cathode.Films were rinsed with water following deposition. Still solu- tions did not allow solid films to form: flocculent metal oxyhydroxide products were observed sedimenting from the electrode. J. Muter. Chern., 1996, 6(2), 183-186 183 UV-VIS spectroscopy used indium-tin oxide-coated glass (Pilkington 20 SZ sq-') as the optically transparent electrode substrate. Films were dried in a desiccator prior to firing, and then warmed in an oven at cu. 110 "C to remove occluded moisture. Such films were then fired in air in a conventional furnace for 10 min at 940 "C. X-Ray powder diffractograms (XRDs) were obtained using a Philips PW 3020 diffractometer operating with Cu-Kcr radiation. Measurements of energy dispersion by X-rays (EDX) were obtained using a Cambridge 250 electron microscope.The elemental compositions of deposited (and fired) films were determined using a Philips PU 7450 inductively coupled plasma (ICP) instrument. Films were usually dissolved in 1.0 mol dm-3 nitric acid before analysis, although concentrated acid was used sometimes to effect complete dissolution. ICP standards were matrix matched, i.e. Nd3+ standards were prepared in solutions containing Cu2+. Repeated determi- nation of composition using known solutions showed the reproducibility of the ICP technique was satisfactory (to & 1.0%), provided the ionic strength and deposition tempera- ture remained constant. ICP also showed that the film composi- tions were remarkably invariant at fixed deposition potential, V,.Results Film formation and appearance XRD analysis indicated that all electrodeposited precursor films were either amorphous or insufficiently crystalline for XRD diffraction peaks to be obtainable. Precursor films did not form during electrochemistry with still solutions, i.e. any solid generated did not adhere to the electrode. By stirring the solution, and thus increasing the availability of the electroactive species at the electrode-solution interface, electrodeposition occurred much closer to the elec- trode substrate and gave denser and thus more robust films, as discussed below. Films appeared smooth and even under standard laboratory illumination. The films often looked slightly gelatinous prior to firing.For this reason, it is possible to term the film formation process 'electrochemical gel formation' rather than electrodeposition. Electron-beam microscopy of dried and/or fired films revealed a relatively uneven texture but without any pinholes or significant discontinuities. The material chosen as the working electrode substrate also affected the adhesivity of deposited films, both before and after firing. The range of possible substrates is limited since high- temperature firing is necessary following deposition. Films on steel, platinum and IT0 were fragile and only weakly attached. In contrast, films electrodeposited on silver were considerably more robust. Film composition Initially, the composition of deposited films were determined as a function of deposition voltage, V,, whilst using a con- stant deposition-solution composition ([Nd3+] =[Cu" ]= 10mmol dm-3).The mole fractions of Nd and Cu in the deposited film changed with deposition voltage as shown in Fig. 1: as voltage (V,)becomes more negative, the mole fraction of Nd increases. Next, films were electrodeposited from solutions made with various concentration ratios of Nd and Cu, i.e. keeping [Cu2+]= 10 mmol dm-3 but increasing the Nd3+ concen- tration to a limit of 0.1 mol dmP3 (Fig. 2). Increasing the concentration of the Nd3+ ion increases its availability at the electrode-solution interface and thus enhances its chance of 184 J. Muter. Chem., 1996,6(2), 183-186 0.2 i 00 0,.a IIIIIIIIII 0!4 Ow T ' l!2 ' 1!4 ' 1!6 I l!S I 2' ' deposition voltageN Fig. 1 Plot of mole fraction against deposition voltage for solid mixed- metal oxide films. Copper and neodymium ions were both used as aqueous nitrate salts at a concentration of 10mmol dm-3. No additional inert electrolyte was employed. The electrode substrate was silver metal. 1.0 0.8 rnz 3.-0.6 8-0 c 0.4 *8 0)-0.2 0.0. l~l[l~l]l 2 4 6 8' molar ratio of Nd:Cu Fig. 2 Plot of mole fraction of Nd3+/Cu2+ in solution against the mole ratio within solid mixed-metal oxide films: solution-phase [Cu2+] was 10mmol dm-3 throughout while [Nd3+] was varied. No additional inert electrolyte was employed. The electrode substrate was silver metal.-, 1.4; ---, 1.6; -.-.-, 1.8 V. being in the mixture of oxides electrodeposited, so the mole fraction of Nd3+ increases even at constant V,. Further analyses involved electrodeposition from solutions with constant Nd :Cu mole ratios but with differing absolute concentrations. Results showed that as absolute concentration increases the mole fraction of deposited Cu increases and the mole fraction of Nd decreases (Fig. 3). By controlling all the deposition variables which affect film composition, it is possible to 'tailor' films to have the required stoichiometry. The target material, 'neodymium cuprate', of composition Nd2Cu0, may be prepared using a silver electrode held at a potential of -1.6 V while immersed in an aqueous solution comprising [Nd(NO,),] =0.125 mol dm-3 and [Cu(NO,),] =0.025 mol dm-, at 25 "C.[In fact, while the ideal composition of x(Nd) is 0.66, the mole fraction normally lies in the range 0.66 f0.005; the extremes of composition with these conditions may be considered to be 0.660<x(Nd)<0.670.1 Deposition time All films described so far were deposited for a fixed length of time (120 s). It is possible that a finite iR drop existed across 0.9 0.8 '" 0.7 8 8 0.6 c0.-c.8t 0.5 0) -0.4 f ', Nd 0.3 0 0 0.2 0.1 10 0.02 0.04 0.06 0.08 0.10 concentration/mmol dm3 Fig. 3 Plot of mole fraction against concentration for solid mixed- metal oxide films. Copper and neodymium ions were both used as aqueous nitrate salts.No additional inert electrolyte was employed. The electrode substrate was silver metal and the applied potential was -1.8 V US. SCE. the incipient thin film. The value of R will slowly increase since films grow thicker with time, thus altering the actual potential across the the solution-film interface. Accordingly, films were deposited for varying lengths of time (in the range 5 s to 10min) and the ratio of neodymium to copper ascer- tained. The variation in this ratio with deposition time was within experimental error in all cases, i.e. variation in the product iR was always small implying that films were gel-like prior to firing, rather than compact oxide. Film firing Films were fired in air at 940 "C for 10min. EDX (Fig.4) and ICP showed the post-annealed Nd :Cu ratio was maintained at 2: 1. The XRD pattern (Fig. 5) was identical to traces obtained for bulk samples of neodymium cuprate, NdzCu04, prepared using conventional sol-gel techniques. Investigation of mechanism The model of electrochemical gel formation employed here is given by eqn. (1) and (2) and uses nickel as an example. Solid Nd li cu Fig. 4 EDX spectrum of electrodeposited neodymium cuprate (Nd2Cu04) on silver after drying i1 0.20 ' 215 ' 30 ' 35 ' do ' 45 ' ! 0' 2Wegrees Fig. 5 XRD of electrodeposited neodymium cuprate (Nd2Cu0,) on silver metal. *indicates peaks due to the silver substrate. Inset: XRD of a Nd2Cu0, sample prepared using a sol-gel procedure.films were not produced when the component salts were sulfate or halide; only in the presence of nitrate were these films formed. Films could be formed in the presence of both C1- and NO,-. When ammonium ion was added to solution (as the nitrate salt) then the resultant films were much darker, occasionally appearing pink or purple. Fig. 6 shows the differ- ences by means of the (transmission) electronic spectra of films deposited with and without ammonium ions in solution: a new peak appears at ca. 400 nm which we ascribe to the formation of an underlayer (the 'ringing' effects are caused by thin-layer interference). Solutions containing NH4+ but no nitrate did not allow for film formation. These results are taken to support the idea that adsorption of ammonium ion can play a part in the electrodeposition mechanism here.There was little perceptible change in the visual appearance of the films when the ionic strength of the solution was increased by addition of extra nitrate ion, as KNO,. That nitrite can be electroreduced to form ammonium ion is well established;" and the electroreduction of nitrate to nitrite is readily achieved." Ammonium ion has not been positively identified as the product of reaction (1). The poor sensitivity of standard analytical tests is thought to be the cause of this. For example, no NH4+ was detected chemically, even after the deliberate addition of small amounts of NH4N0, to the solution. NH4+ complexation with cations is thought 300 500 700 Ilnm Fig.6 UV-VIS spectra of neodymium cuprate thin films on ITO. The deposition solution contained [Nd(NO,),] =0.05 mol dm-, and [Cu(NO,),] =0.025 mol drn-,; (a) no NH,NO, in solution; [NH,NO,] =0.1 (b),0.3 (c),0.5 mol dm-, (d). J. Muter. Chem., 1996,6(2), 183-186 185 to be unlikely since no new spectroscopic bands were formed after deliberate additions of ammonium ion to the solution. That hydroxide ion was electrochemically generated at the electrode was demonstrated by electrolysing KN03 solution in the presence of universal indicator. The indicator suggests that the pH at the solution-electrode interface was ca. 10-11. Discussion The pH at the electrode-solution interface is very high since hydroxide is electrogenerated.If efficient mixing within the depletion layer is assumed to occur but without any precipi- tation (i.e. without product loss), then a simple calculation reveals a pH of ca. 13 at the electrode-solution interface within the ‘depletion region’. (This simple model assumes a typical depletion region thickness of 50 pm and a current of 100 mA cm-2; higher currents will have the effect of raising the pH.) However, the solubility constants of the metal hydroxide species used here are small (ref. 20 cites K, {Nd(OH),} = and K, {Cu(OH),) = lo-”) so the electrode reaction (1) is assumed to be followed promptly by reaction (2),with precipi- tation of (mixtures of) solid oxyhydroxide phase(s) which, following firing, represent mixed-metal oxide product.K, for each hydroxide is so small that the amounts of solute ion remaining in solution will be quite small and with the pH remaining high. Conversely, the pH in the bulk solution is always low (in the range 3-6) since Cu2+ cations hydrolyse coordinated water, leading ultimately to increased proton concentrations. The concentration of solute in the solution bulk is not particularly changed from that used initially because the deposited films do not comprise much material. So, after generation of the first modicum of solid oxyhydroxide, the reaction between OH- and M”+ occurs at a front which falls at an indeterminate distance d from the film-solution interface (and probably the value of d increases from zero during deposition).The actual position of d depends on many physicochemical parameters, as follows. The pH derives from the concentration of hydroxide generated, cOH-,itself the quotient of the amount of OH- formed (obtained via the current flowing, I) and the volume of the depletion region around the electrode (d x electrochemical electrode area). The thickness of the depletion region depends on the rate of solution stirring and will follow an approximate form of the Levich equation. This explains why superior films are formed if the solution is stirred: the reaction front resides nearer to the electrode than when using a still solution; and the solid formed at the front has a greater chance of adhering when d is small. The Faradaic current flowing will be 8F(d [NO,-]/dt); the factor of 8 arising from the number of electrons involved in equation (1).Additionally, the current depends in a compli- cated way on the nature of the electrode-solution interface. One measure of the facility of electron transfer across the double layer is the so-called exchange current, i,, which may be treated as the rate constant of charge uptake by electroactive materials residing at the electrode-solution interface. Compendia of exchange-current data at silver (or other sub- strates) do not exist for these solutions. i, may depend markedly on the electrolyte concentration. In the absence of published data, we have measured i, (for comparative purposes) using silver and then platinum as the electrode substrate, and using identical electrolyte solutions.i, for reaction (1) was measured as follows: i, (Ag)=7.0 A m-2 and i, (Pt)=6.5 x lop2A m-2. The hundred-fold increase in i, accounts for the observation that a silver substrate produces more compact (mechanically robust) films than does platinum. It is important to deduce why the ratio of ions within the solid product is a function of the voltage applied for deposition, 186 J. Mater. Chem., 1996, 6(2), 183-186 V,. The rate of reaction (1) depends strongly on potential (following an exponential law from Butler-Volmer consider- ations) so current follows a exp(K)cci relationship; cOH-is directly proportional to i. Both Cu2+ and Nd3+ in the depletion region react with electrogenerated OH- and it will be assumed that the rates of hydroxide formation and precipitation are the same for each, being ‘instantaneous’.The amount of each ion in solution is independent of V,and K,, the solubility constant, is constant for all the solutions, but K, values for each solid differ significantly. So, the amount of hydroxide available (which depends on V,) changes and the OH-is partitioned between the two ions. Clearly, at low concentrations of OH- (less negative V,),more of the relatively soluble Nd (as aquo ion) will remain in solution, i.e. the mole fraction of copper in the solid will be larger at more positive potentials. This result is in accord with that found experimentally. It follows from the above that adjustments to the ratios of solute ions present in solution will affect the ratios of ions in the solid product. Also, if the relative amounts of each ion in solution remain the same but their overall concentrations increase, the precipitation of the least soluble metal hydroxide (in this case Cu2+)will be preferred.Thus by suitable choice of experimental variables, it is possible to prepare films of Nd,Cu,,-O having any composi- tion. We have shown that electrodeposition can yield well known and previously studied phases such as Nd2Cu0,, which are identical to samples made by other, more familiar routes. We wish to thank the EPSRC for a research studentship (R.D.P.). References 1 Science and Technology of Thin-Film Superconductors 2, ed. R. D. McConnell and R.Noufi, Plenum Press, New York, 1990. 2 M. Cyrot and D. Pavuna, Introduction to Superconductivity and High-T,Materials, World Scientific, Singapore, 1992. 3 D. B. Chrisey and G. K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley, Chichester, 1994. 4 R. R. Romanovsky, NASA/Lewis Centre, Cleveland, OH, 1990. 5 D. Dijkamp, T. 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