Electrochemical behavior, Raman characterization, and

Surface Morphology/Composition and Photoelectrochemical Behavior of Metal−Semiconductor Composite Films. Norma R. de Tacconi, Catherine A. Boyles, ...
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J. Phys. Chem. 1993,97, 1042-1049

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Electrochemical Behavior, Raman Characterization, and Photoelectrochemistry of Cuprous Thiocyanate-Polypyrrole Bilayers and Films N. R. de Tacconi,*JY. Son, and K. Rajeshwar' Department of Chemistry, Box 19065, The University of Texas at Arlington, Arlington, Texas 76019-0065 Received: July 17, 1992; In Final Form: November 6, 1992

This study describes new types of bilayers on electrode surfaces which combine an organic semiconductor (polypyrrole, PPy) and an inorganic counterpart (cuprous thiocyanate, CuSCN). Two bilayer structures, namely, Cu/CuSCN/PPy and GC/PPy/CuSCN, were compared using voltammetry and in situ Raman spectroelectrochemistry techniques. Aqueous media containing pyrrole and KSCN were found to be unsuitable for growing thick and adherent layers of polypyrrole either at copper or at platinum support electrodes. On the other hand, tetrahydrofuran solutions containing pyrrole and a supporting electrolyte were effective for electrosynthesis of the Cu/PPy system. The latter was modified into a Cu/CuSCN/PPy bilayer by electrochemical reaction with thiocyanate ions. This system showed interesting photoelectrochemical transients arising from the unidirectional flow of minority carriers (electrons) from p-CuSCN to polypyrrole. The transients are contrasted with those observed under chopped white light irradiation of the Cu/CuSCN/electrolyte interphase, wherein a trap-dominated photoresponse was seen. The GC/PPy/CuSCN bilayer showed high ohmic resistance and barrier to CuSCN film formation, presumably because charge transport in this system has to be mediated by an underlying (resistive) PPy layer.

Introduction Since the studies by Murray, Meyer, and co-workers' about a decade ago, electrodes containing polymeric bilayers have been extensively studied in recent years. Much of this interest has stemmed from the rectifying and charge-trapping characteristics of these structures, and, as reviewed else where?^^ a variety of redox and conducting polymers have been used as the bilayer components. This paper describes our attempts to assemble chemical microstructures on electrodes4comprising an organic (polypyrrole, PPy) and an inorganic (cuprous thiocyanate, CuSCN) semiconductor. Three different electrodearrangements will be presented: (a) Cu/CuSCN,PPy, i.e., a mixture of PPy and CuSCN supported on a copper electrode, (b) Cu/CuSCN/ PPy, and (c) GC/PPy/CuSCN, b and c serving as examples of new types of bilayers. Thus, in our hands, PPy functions either as a polymer matrix in which a semiconductor is formed [case a] or as an electron acceptor in the sandwich structures b and c. Polypyrrole is an organic semiconductorwith an energy bandgap (due to t h e r 4 interband transition) variouslyreportedas 3.1,5-' 3.2; or 3.4 eV9in the reduced redox state. Cuprous thiocyanate is a p-type semiconductor with a gap energy of 3.6 eV.I0 The use of semiconductors in bilayer electrodes facilitates the superposition of interesting optical effects on the electrical properties of these structures. Thus photoelectrochemical transients were observed for the Cu/CuSCN/PPy structure, which could be attributed to the photoassisted reduction of the (partially oxidized)PPy by the photogenerated minority carriers (electrons) in the CuSCN film. We note related studies of photoeffects, albeit of different origin, in polypyrrole films and multilayer structures containing chromophores such as [Ru(bp~)~]2+ (bpy = 2,Y-bipyridine)II or donor-acceptor charge-transfer complexes.12 Another point of note in the experiments to be described, in what follows below, concerns the nature of the support electrode at which the electropolymerization of pyrrole is carried out. It has been commonplace to use inert anodematerials (e.g., platinum, glassy carbon) for this purpose. The use of reactive metals (such To whom correspondence should be addressed. Permanent address: INIFTA, Universidad Nacional de La Plata, C. C. 16, SUC.4, (1900) La Plata, Argentina.

' Visiting Scientist.

0022-3654/ 58/2097- 1042$04.OO/0

as copper, as in this study) complicates the situation because metal dissolution often is facileat the polymerization potential." However, the use of organic media was found to be effective for the electrosynthesis of PPy films in such cases.14 Recent studiedS-l7have described the electrodeposition of PPy films at iron and aluminum electrodes in aqueous and organic media. The polymer film formation was found to be highly dependent on the choice of the anion and the s0lvent.'~J6 As in a companion article on CuCSN film formation at the copper anodes,l* we found the Raman scattering probe to be particularly useful for characterizing the various films and structures in this study. In this sense, we believe that the data presented in this article join the growing body of evidence which attest to the usefulnessof spectroscopicprobes in electroanalytical scenarios.19

Experimental Section The working electrodes were copper disks (geometric area 0.1 cm2) sealed in Kel-F. They were mechanically polished to a mirror finish with alumina powder (Buehler) down to 0.05 pm on a wet microcloth. Platinumand glassy carbon (GC) electrodes (Bioanalytical Systems) were also used as working electrodes. These were polished following the procedure described for the copper electrodes. The counter electrode was a platinum sheet placed parallel to the surface of the working electrode at a distance of ca. 3 mm. The reference electrode was Ag/AgCI/3 M KCl. Both aqueous and nonaqueous (tetrahydrofuran, THF) media were used in this study. THF was distilled under N2 using CaH2 before experiments. Aqueous solutionswere made up with double distilled water (Corning Megapure). Solutions were deoxygenated with a stream of Nz (Air Products, 99.999%) before and between experiments. Experiments were carried out at room temperature. All the chemicals were ACS reagent grade. Pyrrole (Aldrich, 99.9%) was purified using an activatedAl20, (Aldrich, 150 mesh) column. Other chemicals including tetrabutylammonium hexafluorophosphate (NBu4PF6), KSCN, and KCI were used without further purification. The electrodeposition of polypyrrole (PPy) was performed under both potentiostatic and potentiodynamic conditions. The Q 1993 American Chemical Society

CuSCN/PPy Bilayers and Films

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E I V vs Ag I AgCl F'igure 1. Cyclic voltammogram of a polycrystalline copper electrode in 0.1 M KSCN. Potential scan rate: 5 mV/s.

thickness of the PPy films was estimated by assuming that a charge of 8 mC cm-* during the anodic polymerization leads to a layer 20 nm thick.2O Raman spectra were obtained with the 488-nm line of an Ar+ laser on a Spex Ramalog instrument equipped with a Model 1680 doublemonochromator and a cooled photomultiplier tube (Model R928) operated in the photon-counting mode. Details of the spectral measurements and the electrochemical setup are given elsewhere.7J8 Photoelectrochemical measurements used a 75-Wxenon arc lamp source and a water filter (3 cm long) with fused silica windows to reduce infrared heating at the electrode surface.

R d t a .nd Discussion EketrOeBemicdFormrtion of Cu/CIISCN. Figure 1 contains a cyclic voltammogram at 5 mV/s for a copper electrode in 0.1

M aqueous thiocyanate. The voltammetric scan was initiated at -0.80 V in the positive-going direction up to +O. 10V. Formation of a CuSCN film manifests as a sharp peak at -0.35 V. The latter is associated with the visible appearance of a white film on theelectrodesurface whose presenceprovides a diffusional barrier to metal (copper) dissolution. Thus the CuSCN acts as a passivating layer for the copper surface.18,21+22 The formation of Cu20 is thermodynamically possible whenever the anodic limit of the scan is chosen positive of the domain wherein CuSCN film formation occurs.23 The cathodic scan in Figure 1 shows a peak at -0.61 V which is a composite of the contribution from more than one species (compare the areas of the two peaks in Figure 1). Thus the long tail on the forward scan together with the structure of the cathodic wave are diagnostic of the presence of oxides on the electrode surface. This is in spite of the fact that the formation of a CuSCN film shifts the oxide formation regime toward more positive potentials.21 At the termination of the scan in Figure 1, the film electroreduction processes culminate in the formation of metallic copper on the electrode surface.

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Figure 2. Cyclic voltammograms at copper for repeated cycling in 0.1 M KSCN + 0.1 M pyrrole. Scan rate: 100 mV/s.

Electropolymerizationof Pyrroleat Copper Anodes in Aqueous Thiocyanate Solutions and Synthesis of Cu/C&N,PPy. The growth of PPy at Cu electrodes in aqueous media containing "aggressive" anions such as perchlorate or chloride is inhibited by the dissolution of copper at the (positive) potentials needed for pyrrole polymerization. On the other hand, the use of an anion such as SCN- brings about the formation of a passive layer which in turn facilitates PPy film growth. It is also pertinent to note that the growth of an inner oxide film at the copper surface (see above) did not appear to perturb the PPy formation because the oxide layer is inert in SCN--containing media.1° Figure 2 containscyclic voltammogramsat copper in aqueous 0.1 M pyrrole and 0.1 M KSCN. Cycling in the potential window between -0.80 and +1.0 V promotes the formation of a CuSCN film (peak Ia)l8 and the polymerization of pyrrole (loop IIa) during the anodic portion of the scan. Correspondingly, cathodic waves IIc and ICwere observed on the return cycle. The latter at -0.69 V is assignable to the electroreduction of the CuSCN film.18 On the other hand, copper oxide formation and electroreduction also play a contributory role in the IIa and IIc potential regions. The potential sweep rate (100 mV/s) was chosen to be rather high in theseexperiments (compare with Figure 1) in order todiminish the dissolution of copper at the positive potentials. It is important tostress that theelectropolymerizationofpyrrole does occur at the Cu/CuSCN electrode because the CuSCN film formation precedes the potential regime for pyrrole electropolymerization. Thus we view the SCN- ions to play a dual role of being incorporated into the PPy film (as dopants?) as well as reacting with the (underlying) copper support surface as the electropolymerization reaction proceeds. Figure 3 contains a Raman spectrum acquired ex situ after repetitive cycling as in Figure 2. The electrode was taken out of the electrochemical cell and washed with distilled water. The spectrum is compared with that of a PPy film (0.4 pm) on a platinum electrode at -0.13 V with Clod- as the dopant anion. The spectral patterns are similar for both electrodes in relation to the PPy film bands. Nonetheless, the PPy film on the copper electrode is thinner and possesses, in addition, two new bands at

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1044 The Journal of Physical Chemistry, Vol. 97, No. 5, 1993 ,,

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Raman shift / cm-1 Figure 3. (a) Ex situ Raman spectrum of a copper electrode after potential cycling as in Figure 2. (b) In situ Raman spectrum at -0.13 V of a PPy film grown on platinum in 0.1 M NaC104 solution. For comparison, the spectrum of a Cu/CuSCN electrode in 0.1 M KSCN is also shown in the u(CS) and v(CN) regions.

749 and 2172 cm-1 that correspond to the stretching vibrations v(CS) and v(CN) of CuSCN in the PPy film.I8 For comparison, the v(CS) and v(CN) signatures for a Cu/CuSCN electrode in 0.1 M KSCN are also included in the figure. The 2067-cm-1 band corresponding to v(CN) of (free) SCN- in solutionb8is notably absent in spectrum a. Assignments of the PPyvibrational bands are also indicated in spectrum b in Figure 3; these are based on previous work.24~25 Taken as a unit, the data in Figures 2 and 3 are consistent with the formation of Cu/CuSCN,PPy, that is, a copper support on which CuSCN and PPy are anodically formed in an interspersed manner. Figure 4 compares the Raman spectra obtained in situ at different potentials for PPy films in the reduced redox state and under partially oxidized conditions. The spectra are shown for a PPy film (0.9 pm thick) grown on a platinum electrode using C104- as the dopant anion (Figure 4a), a PPy film at platinum with SCN- as the dopant (Figure 4b), and a PPy film on copper with SCN- as the supporting electrolyte anion (Figure 4c). The PPy films in the experiments in parts b and c of Figure 4 were ca. 2 orders of magnitude thinner than when C104-was used as the anion in the electropolymerizationprocedure (cf. Figure 4a). The electrooxidation of SCN- to (SCN)2 probably accounts for the lower polymerization efficiency; the SCN-/(SCN)2 couple is reported to lie at 0.79 V26which is close to the onset potential for pyrrole oxidation. The three sets of spectra in Figure 4 share the characteristic bandsof PPy and exhibit a similar evolution with potential. Thus, the v(C=€) band shifts to higher values, becomes broader, and decreases in intensity as the potential is made more positive, and the PPy film becomes more and more doped. Similar trends have been reported in other s t ~ d i e sfor ~~ PPy ~ ~films , ~ grown ~ on inert electrodes. However, two points are worthy of note in the spectrain Figure4b,c. First, both the freeSCN-anions in solution and those that function as the dopants for PPy show their v(CN) bands at 2067 cm-I. On the other hand, the higher frequency band due to the 'bound" SCN (i.e., as CuSCN)18 is prominent only at potentials more positive than ca. -0.50 V (Figure 4c). Note that this feature is absent in the spectrum at -0.75 V in Figure 4c because of the electroreduction of the CuSCN film (cf. Figure 2). Finally, we note that the v(C10) vibrational band (which has been reported at ca. 1100 cm-I; cf. ref 27) expected

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Figure 4. Effect of potential on the Raman spectra of PPy films grown on (a) a platinum electrode using clod- as counterion, (b) a platinum electrode using SCN-solutions, and (c) a copper electrode using SCNsolutions.

for the C104-dopant anions is absent in the spectra in Figure 4a. This is probably a consequence of the interference from the PPy absorption envelope. Electropolymerizationof Pyrrole at Copper in Organic Media and Synthesis of Cu/CuSCN/PPy. Preliminary potential step experimentsusing 0.1 M LiC104and 0.1 M pyrrole in acetonitrile revealed inhibition of PPy film growth. This is rationalizable on the basis of the known proclivity of the copper(1) ion to exist in nitrile solvents as tetrahedral complexes of the type [Cu(CHpCN)4]+,28Thus, copper corrosion is favored in this solvent at the expense of PPy film growth. On the other hand, similar film growth experiments (i.e., potential step from rest to +0.90 V) but with THF containing 0.1 M NBudPF6 and 0.2 M pyrrole yielded well-formed rising current transients (not shown) diagnostic of film growth.29 Further, better film adherence and a lower rate of dissolution of the copper support were obtained under potentiodynamic conditions. Figure 5 contains cyclic voltammograms at copper (Figure Sa) and platinum (Figure 5b) electrode in THF with composition as specified above. The systematic increase in the voltammetric charge envelope on repeated cycling is an unmistakable signature for PPy film growth and is a consequence of the film becoming thicker as polymerizationproceeds under theseconditions(mainly at potentials 20.85 V). Figure 6a contains an in situ Raman spectrum of the film grown at the copper electrode under these conditions. The characteristic bands for PPy (c.f. Figures 3 and 4) are swamped by the THF vibrations; only the most prominent PPy feature at

CuSCN/PPy Bilayers and Films

The Journal of Physical Chemistry, Vol. 97, No. 5, 1993 1045

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E I V vs Ag I AgCl Figure 5. Cyclic voltammograms obtained during the growth of a PPy film on copper and platinum electrodes (a and b) using 0.1 M NBu4PF6 + 0.2 M pyrrole in THF. Voltammograms corresponding to the copper electrode in 0.1 M NBudPF6 in T H F are included in the bottom of the figure (a'). Scan rate: 100 mV/s.

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Raman shift I cm-1 Figure 6. In situ Raman spectrum (a) of a copper electrode in 0.1 M NBu4PF6 + 0.2 M pyrrole in T H F at 0.20 V (conditions of Figure 5 ) . A spectrum of 0.1 M NBu4PF6 comparison in b.

+ 0.2 M pyrrole is also included for

1596cm-1 is discernible in Figure 6a. For comparison, a Raman spectrum of the solution,THF + 0.1 M NBu9F6 0.2 M pyrrole, is also included in Figure 6b. The following assignments are possible:30J 914-916 cm-I, contributions from PFs- stretch and THFring breathqmodes; 1030,1164cm-l,THFringstretching; and 1472cm-', thev(CH3) and u(CH2) vibrations from the NBu4+ cation. After the PPy film was grown in THF as above, the Cu/PPy electrode was disconnected at potentials positive of +0.40 V, washed with distilled water, and immersed in 0.1 M KSCN aqueous solution. The voltammetric behavior obtained is shown

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in Figure 7a and is compared with that of a bare copper electrode in the same electrolyte (Figure 7b). Apart from the characteristics peaks related to the formation and electroreduction of the CuSCN film, there is an additional envelope of charge due to the electroreduction and reoxidation of the PPy layer (compare with the voltammograms of Figure 7b). Interestingly enough, the current peaks related to the formation and electroreduction of CuSCN film appear superimposedon the potentialdomain (-0.80 V to +0.40 V) of PPy redox and increase during the cycling procedure, as it should if a growing CuSCN layer is being formed at the inner interface between copper and the polymer. Raman spectra show the characteristic peaks of PPy, but they do not reveal the v(CN) band at 2172 cm-I characteristic of CuSCN; only the band due to the free SCN- (from the 0.1 M KSCN solution) is visible (Figure 8). The PPy overlayer, 0.3 Mm thick, clearly hinders the appearance of the Raman signal from the CuSCN that grows underneath. Two aspects of the data in Figures 7 and 8 provide conclusive evidence for a layered structure, Cu/@uSCN/PPy. First, the PPy "top" component of the bilayer masks the Raman signature from the underlying CuSCN layer. Second, the close similarity of the voltammetry profiles obtained for this structure with that for the Cu/CuSCN system (cf. Figure 7) shows that the CuSCN layer is largely confined to the immediate vicinity of the copper surface. That is, formation of CuSCN within PPy and/or away from the copper surface would have yielded a distorted voltammetric profile such as that observed for the GC/PPy/CuSCN system (cf. Figure lob below). It is interesting that the cathodic charge envelope in the PPy redox process is much smaller than the anodic counterpart (Figure 7a). This is attributable to the rectifying characteristics of the underlying CuSCN semiconductor layer or alternatively is due to the redox properties of the PPy layer itself. Toenabledistinction

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1046 The Journal of Physical Chemistry, Vol. 97, No. 5, 1993

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E I V vs Ag I AgCl Figure 7. (a) Cyclic voltammograms on repeated cycling of a Cu/PPy electrode in 0.1 M KSCN. The PPy film was initially grown on copper from a T H F solution containing 0.1 M NBu4PFs and 0.2 M pyrrole (cf. Figure sa). For comparison, the cyclic voltammogram for a copper electrode in 0.1 M KSCN is also included in b. Scan rate: 100 mV/s.

between these two possibilities, similar experiments as for a Cu/ PPy electrode were performed but with platinum as the support instead. Thus the PPy layer was first grown in THF and then the Pt/PPy interface was exposed to 0.1 M KSCN. The results are shown in Figure 9. The very fast switch in the current flow, when the potential limit is reached and the scan reversed, is indicative of facile SCN- transport at the polymer/solution interphase. Scan reversal voltammetry profiles of this sort have been profitably used for analyses of oxide electrodes.32 The symmetry of the anodic and cathodic voltammetry envelopes in Figure 9 is diagnostic of the fact that the transport of SCN- is facile in both directions as the polymer is switched from the oxidized to the reduced redox ~tate.~3-"Thus the asymmetry in the PPy redox process observed in Figure 7a is directly attributable to the presence of the (underlying) CuSCN layer, and this aspect will be addressed later using photoelectrochemical probes. However, it must be noted that the ratio of the charge consumed during the repetitive anodic and cathodic cycles was roughly the same (Q./Qc = 0.96-1.05). In other words, the residual portion of thecathodic charge in the polypyrroledomain was compensated in theelectroreduction regimeof the CuSCN layer. This suggests that the polypyrrole reduction proceeds to completion only when the inner CuSCN layer is reduced to metallic copper.

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E I V vs Ag I AgCl Figure 9. Cyclic voltammograms for a Pt/PpY interface in aqueous 0.1 M KSCN. The PPy film initially was grown on platinum from a T H F solution (composition specified in Figure 5b). Scan rate: 20 mV/s.

Electrosyntbesis of CC/PPy/CUSCN. Previous work has s h o ~ that n ~copper ~ ~ may ~ ~be electrodeposited on a PPy layer.

CuSCN/PPy Bilayers and Films

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Raman shift / cm-1 Figure 11. Comparison of in situ Raman spectra at -0.25 V for (a) GC/PPy/CuSCN and (b) Cu/CuSCN/PPy(SCN-) structures.

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E I v vs Ag I AgCl Figure 10. (a) Cathodic voltammetry scan for a 1W2M CuCl2 solution in 0.1 M KCI at a PPy covered GC electrode. The dashed curve is the corresponding scan for a bare GC electrode. (b) Cyclic voltammetric behavior of a GC/PPy/Cu structure in 0.1 M KSCN. Scan rate: 25 mV/s.

Thus it was of interest to see whether a GC/PPy/Cu structure could be transformed to GC/PPy/CuSCN by electrochemical oxidation of the copper outer layer in a KSCN medium. Tothisend,a PPyfilm (ca.0.8” thick) waselectrosynthesized on the GC support from 0.1 M pyrrole + 0.1 M KCI solution. The resulting voltammogram was typical of the behavior expected for PPy (not shown). Copper was then electrodeposited on the PPy layer from 0.01 M CuC12 0.1 M KCI; the cathodic scan in this electrolyte is shown in Figure 10a and is also compared with the corresponding curve for copper electrochemistry at a naked GC surface. A negative shift of the waves and an attenuation of the current flow are seen in the PPy-covered GC case; these differences undoubtedly arising from the presence of an intervening PPy layer. Since the latter has a microporous structure, the copper is electrodeposited both on the exterior surfaces as well as within the polymer pores. The wave at the most negative potential (which is absent in the bare GC case) is attributable to the electroreduction of the PPy itself. We note in passing that the voltammetry data for the bare GC electrode are consistent with previous work in this laboratory and elsewhere.31 After the copper film was formed and the GC/PPy/Cu structure established (the PPy/Cu interphase admittedly is not sharp; see above), the electrode was transferred to 0.1 M KSCN and repetitively cycled to yield the voltammograms in Figure lob. Some important differences relative to the approach using a copper support (cf. Figure 7a) are to be noted. First, the voltammograms in Figure 10b are skewed, diagnostic of a nonnegligible ohmic resistance component to the current flow. Second, the anodic CuSCN wave shows a systematic decrease on repeated cycling (contrast with the increaseobserved in Figure

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Figure 12. Photoelectrochemical transients for (a) Cu/CuSCN/PPy and (b) Cu/CuSCN in 0.1 M KSCN. The applied (bias) potential was -0.40 V, and the white light illumination was manually interrupted as shown.

7a). Note also that the reduction wave for CuSCN is broad and not well defined as in the earlier Cu/CuSCN/PPy case (Figure 7). Figure 11a contains an in situ Raman spectrum of the resultant structure from Figure lob. For comparison, the corresponding spectrum for the Cu/CuSCN/PPy(SCN-) structure is reproduced in Figure 11b. While both spectra are similar in the PPy regime, the GC case shows, interestingly enough, the w(CN) feature attributable to the CuSCN top layer. The barrier to CuSCN formation and electroreduction in the GC/PPy/CuSCN case is understandable on the basis that the current flow is limited by a partially reduced (rather resistive) PPy layer at these potentials. Thus contrary to the previous instance (Figure 7a), the CuSCN formation potential regime is in the “wrong”location relativeto the PPy redox electrochemistry. Pbotoeleetrocbedstry of Cu/CuSCN/PF‘y. Figure 12 compares photoelectrochemical transients for Cu/CuSCN/PPy (Figure 12a) and Cu/CuSCN (Figure 12b). These data were acquired at a fixed potential (-0.40 V) during repeated on-off cycles of white light irradiation. The photocurrent transients are cathodic, signalingptype behavior for both systems.19 The shapes ofthe transients in the twocasts, however, arc markedly different. While the Cu/CuSCN/PPy system shows a “normal” behavior with the photocurrents rapidly attaining saturation, the transients

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1048 The Journal of Physical Chemistry, Vol. 97, No. 5, 1993

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diffusion control of Ru(NH3)b3+. The formation of a CuSCN film is signaled by the sharp peak at -0.28 V (cf. Figure 1). A small anodic hump is unmistakable at -0.07 V and is due to the reoxidation of R u ( N H J ) ~ ~(compare + with Figure 13a). The cathodic scan, which resembled that at the Pt electrode, was reversed at -0.50 V at the onset of the electroreduction of the CuSCN film (cf. Figure 1). Note that the CuSCN formation peak is now reduced in magnitude during the second anodic scan (labeled '2") because of the smaller copper surface available for fresh reaction. The chopped white light illumination of the electrode surface was initiated during the second cathodic scan. Note that the transients evolve in shape from 'spikelike" toward the type shown in Figure 12a as the potential is scanned through the flat-band location and well beyond. Importantly,at an applied bias potential comparable to that in Figure 12, the transients are similar in shape to those of Figure 12a and different from that seen in Figure 12b. Thus Ru(NH3)b3+performs a similar function as PPy in that it scavenges rapidly the photogenerated electrons in the p-CuSCN layer. Thus the photogenerated charge is not allowed to accumulate (and recombine with holes) as in Figure 12b. The kinetics of the electron capture must indeed be fast in the two cases such that photoreduction of the CuSCN layer (to CuO) is effectively suppressed.

Concluding Remarks

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E I V vs Ag I AgCl Figure 13. (a) Cyclic voltammograms for a IO-) M solution of Ru(NH3)63+in 0.1 M KSCN at a platinum electrode. (b) Cyclic voltammograms over two cycles for a copper electrode in the solution as in a. Chopped white light was superimposed on the second cathodic scan as described in the text. Scan rate: 5 mV/s.

Three types of microstructures have been assembled at copper electrodes using anodic electrosynthesisof CuSCN and PPy. The first comprises a mixture of these two semiconductors atop the copper support surface. Of the two bilayer structures examined, Cu/CuSCN/PPy was found to be better in quality than the GC/ PPy/CuSCN counterpart. The former also showed interesting photoelectrochemical behavior arising from unidirectional photoassisted charge transfer from the CuSCN layer to the PPy acceptor moiety. Further studies of this structure are in progress.

Acknowledgment. This research was supported, in part, by the Texas Higher Education Coordinating Board via grants administered by the Advanced Technology and the Energy Research Applications Programs.

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