J . Chem. Soc., Faraday Trans. 1, 1982,18, 2219-2283 Stabilization of Cuprous Oxide Photocathode in Aqueous Thiocyanate Solution by Aliphatic Alcohols K. TENNAKONE,* W. G. D. DHARMARATNA AND S. C. JAYEWARDENA Department of Physics, Ruhuna University College, Matara, Sri Lanka Received 16th November, 198 1 Long-chain normal aliphatic alcohols are found to suppress photocorrosion and increase the efficiency of a cuprous oxide photoelectrochemical cell with aqueous thiocyanate solution as the electrolyte. It is suggested that alcohols inhibit corrosion by surface adsorption. Photoelectrochemical cells with semiconductor electrodes seem to be one of the most promising methods for solar-energy conversion. However, they are plagued with instability resulting from photocorrosion, and methods of suppressing corrosion have attracted much attenti0n.l l1 In this work we describe our observations of the stabilizing influence of adsorbed normal aliphatic alcohols on a photocathode of cuprous oxide in aqueous thiocyanate solution.The mechanism of stabilization is discussed. EXPERIMENTAL Cu,O plates are made by heating 7 x 11 cm copper sheets in an oven to 850 "C. The outer film of CuO is removed by etching in dilute acetic acid. As the cell performs better in the back-wall mode, the anode used is a copper window (same dimensions as the cathode) coated with cupric sulphide. To obtain a reproducible dark open-circuit voltage, the anode is prepared by the following procedure. The cleaned copper electrode is immersed in a 5% solution of Na2S and then left overnight in a saturated solution of H2S.Two electrodes, separated 1 cm from each other, are supported in a glass vessel (12 x 8 x 3 cm). The electrolyte is an alkaline solution of KCNS (0.5 mol dmP3 KCNS, 0.2 mol dma3 NaOH). RESULTS AND DISCUSSION Cu,O is a p-type semiconductor of band-gap Eg = 2.3 eV. When a semiconductor surface in contact with an electrolyte is illuminated, the electron-hole pairs created by photons (hv > E,) are separated by the surface-barrier electric field. In a p-type material, electrons tunnel across the barrier into the electrolyte and holes carry the positive current in the interior of the semiconductor.l2? l3 Electrons tunnelling into the solution cause chemical reductions near the semiconductor surface. Unless a redox system with good electron-accepting properties is present in the solution, the semiconductor is reduced.The photoreduction of a Cu,O cathode is rapid in most electrolytes. An acidic medium dissolves Cu,O or reduces it instantly to metallic copper. In an alkaline medium, the familiar redox systems either produce a weak photo- response or attack the electrode in the dark. After testing several electrolytes we found 22792280 c U 2 0 PHOTOCATHODE STAB I LI Z ATION 2 2 4 E + l .. 1 .5 .o .5 .o 0 10 20 30 40 50 60 tlmin FIG. 1.-Decay of the short-circuit photocurrent: (a) without alcohol, (b) 10-2 mol dmP3 ethanol, (d) lop3 mol dmP3 propan-1-01, (e) loP3 mol dm-3 pentan-1-01, ( f ) hexan-1-01, (s) mol dm-3 methanol, (c) mol dmP3 mol dmP3 octan-1-01. (Intensity of illumination z 10 W m-2, plate area = 60 an2.) > E L- 0 2 4 6 8 10 12 14 16 tlmin FIG.2.-Time development of the open-circuit voltage: (a) without alcohol, (b) lo-* mol dm-3 methanol, ( c ) lo-* mol dmP3 ethanol, ( d ) mol dm-3 propan-1-01, (e) mol dm-3 pentan-1-01, ( f ) rnol dm-3 octan-1-01. (Intensity of illumination x 10 W m-*, plate area = 60 cm2,) mol dmP3 hexan-1-01, ( g ) that a very high photoresponse can be obtained with a solution of KCNS. Neutral or alkaline solutions of KCNS do not react with Cu,O. However, if the short-circuited cell is exposed to light, the Cu20 plate deteriorates rapidly, forming a grey deposit of CuO and CuCNS. The time dependence of the short-circuit current is indicated in fig. 1 (a). The cell decays to complete extinction in ca.25 min (intensity of illumination x 10 W m-2 from a tungsten-filament lamp). Normal aliphatic alcohols (NAAs) have a remarkable effect on the stability of the cell. When minute quantities of NAAs (< mol dm-3 for methanol or ethanol, < lop3 rnol dm-3 for higher NAAs) are added to the electrolyte, the lifetime of the cell is increased from 25 min to several hours (fig. 1). The effectiveness of NAAs as stabilizing agents increases progressively with the chain length. The slow decay of the photocurrent (fig. 1) is almost entirely due to deterioration of the copper sulphideK. TENNAKONE, W. G. D. DHARMARATNA A N D S. C. JAYEWARDENA 2281 anode. If the anode is replaced, the cell recovers to practically the same efficiency. This process can be repeated for ca.10 h, when it shows a 25% drop in the starting photocurrent. In the absence of NAAs the open-circuit voltage of the cell also decays with time (fig. 2). Long-chain NAAs fully stabilize the saturation voltage. It is seen that the higher the chain length, the shorter is the time taken to reach saturation. Again NAAs are found to increase the efficiency of the cell. Fig. 3 gives ZV against Z 91 I N 3 2 1 I I I 1 I 0 10 20 30 40 50 60 //PA cm-2 FIG. 3.-ZVagainst Zcurves: (a) without alcohol, (6) mol dm-3 ethanol, (d) rnol dm-3 propan-1-01, (e) lop3 mol dmp3 pentan-1-01, (f) lop3 mol dmp3 hexan-1-01, (g) mol dm-3 octan-1-01. Efficiencies at the maximum power point are (a) 0.28, (6) 0.45, (c) 0.48, ( d ) 0.52, (e) 0.56, (f) 0.61 and (s) 0.65. (Intensity of illumination sz 10 W m-3r plate area = 60 cm2.) mol dm-3 methanol, (c) curves.It is apparent that long-chain NAAs are more effective. An explanation for the above facts will not be given. KCNS solution acts as a redox couple owing to the existence of CNS- and (CNS); ions. Possible reactions occurring at the electrodes are as follows. At the photocathode (CNS); ions accept electrons to yield CNS- ions, i.e. (CNS); + e -+ 2CNS-. Near the anode CNS- ions discharge electrons producing CNS free radicals, which combine with CNS- ions in the solution to regenerate (CNS); ions, i.e. CNS--e -+ CNS CNS + CNS- -+ (CNS);. Photocorrosion is probably due to the presence of anodic regions in the Cu,O surface. Uneven illumination, difference in surface light absorption coefficients and other non-uniformities create regions of different potential in the photocathode; 74 FAR 12282 c U 2 0 PHOTOCATHODE ST A B I LI ZATION short-circuiting across these regions deteriorates the anodic regions by the following mechanism.In the anodic region, CNS- ions discharge electrons yielding CNS free radicals, which sometimes, instead of combining with (CNS)- to regenerate (CNS);, react with Cu,O to form cuprous thiocyanate and cupric oxide, i.e. C U , ~ + CNS + CuO + CuCNS. The fact that the deposit formed on the photocathode is a mixture of CuCNS and CuO supports the above hypothesis. Two other observations favour this argument. When a metallic copper electrode is used as the anode, it quickly corrodes with deposition of CuCNS. The other observation is that if a circular patch of light is focused into the cathode, the dark region just outside the periphery of the illuminated patch corrodes immediately leaving the bright region untarnished.The illuminated region is cathodic: the anodic region in the immediate vicinity is the dark area around the periphery. Note that photocorrosion in an anodic region destroys the photosensitivity, making it even more anodic; this enhances corrosion which then spreads throughout the surface. Our observations strongly suggest that NAAs prevent photocorrosion by adsorption. Traces of NAAs arrest the deposition of CuCNS and CuO on the photocathode. A concentration of pentan-1 -01, hexan- 1-01 or octan- 1-01 of < mol dmP3 is sufficient for this purpose. There is no evidence that the adsorbed NAAs are chemically changed.If the purity of the electrolyte is maintained the NAA layer (probably a monolayer) remains active almost indefinitely. Addition of soap or other detergent at once restores photocorrosion. Obviously this is caused by removal of the adsorbed layer by the detergent. The adsorbed NAA probably inhibits photocorrosion because it prevents the direct contact of CNS free radicals with the anodic regions. Electrons, however, could easily tunnel across the NAA layer. The long-chain NAAs are better adsorbed14 and produce more effective barriers against ions and free radicals. Although NAAs completely suppress photocorrosion leading to the formation of CuO and CuCNS, at higher intensities of illumination a certain degree of photoreduction of Cu,O to metallic copper takes place.The adsorbed NAA possibly screens only larger ions and free radicals. Hydrogen ions may perhaps penetrate the barrier and reduce Cu,O to metallic copper. Again, as expected the reduction of Cu,O to metallic copper occurs more slowly with long-chain NAAs. We mentioned earlier that the open-circuit voltage of the cell decays in the absence of NAAs. This observation can be explained as being due to short-circuiting across anodic and cathodic regions, which quickly causes the anodic regions to corrode and expand. In the presence of NAAs the enhancement of anodic regions by corrosion is stopped, and the cell attains saturation voltage. The same effect explains why the efficiency is increased by NAAs. When the short-circuiting in the cathode is arrested, the photocurrent is driven almost entirely through the external circuit.The slow decay of the photocurrent results from a deterioration of the copper sulphide anode. This is an oxidation process that deposits Cu,O. Since Cu,O is a very bad electron acceptor, once this oxide layer is formed CNS- ions fail to deliver electrons efficiently to the anode. We have not been able to find a better material for the anode. Familiar stable electrodes have electrode potentials which are either too low or too high compared with that of the photocathode. Another observation we cannot explain is that only NAAs inhibit photocorrosion in Cu,O. Branched aliphatic alcohols have the opposite effect; traces of theseK. TENNAKONE, w. G. D. DHARMARATNA AND s.c. JAYEWARDENA 2283 compounds accelerate corrosion. Aromatic alcohols are ineffective, but they do not accelerate corrosion. The above cell is not suitable as a practical device for the conversion of solar energy. In the present form the efficiency does not exceed 1 %. Also it may not be completely regenerative, as the redox action of KCNS involves several steps. Nevertheless, our investigations suggest that agents which are adsorbed at the photoelectrodes might inhibit corrosion in more efficient practical photoelectrochemical cells. A. Heller, K. C. Change and B. Miller, J. Electrochem. Soc., 1977, 124, 697. A. J. Bard and M. S. Wrighton, J. Electrochem. 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