6504 J. Phys. Chem. zyxwvu 1993,97, 6504-6508 Anodic Growth and Interphasial Photoelectrochemistry of Cadmium Sulfide zyx Thin Films As Probed by Laser Raman Spectroscopy N. R. de Tacconi'gt and K. Rajeshwar' Department of Chemistry and Biochemistry. The University of Texas at Arlington, Box 19065, Arlington, Texas 76019-0065 Received: February 19, I993 The anodic growth and interphasial photoelectrochemistry of CdS films were studied by cyclic voltammetry and laser Raman spectroscopy in aqueous sulfide electrolytes. The potential regimes and sulfide levels in the electrolyte were carefully chosen to avoid interference from oxide and hydroxide growth at the cadmium electrode surface. The use of the 488-nm Ar+ laser line was effective in generating Raman signals in the resonance scattering mode via absorption of the excitation light by the incipient CdS semiconductor layer. At 0.1 M sulfide, sulfur (predominantly SS) was detected via its Raman signature as a CdS photocorrosion product. The evolution of the Raman bands (attributable to these species and to CdS) as a function of time and potential was seen to reflect a complex interplay of several concurrent processes including photocorrosion, film regeneration, and desorption of the photogenerated sulfur from the CdS surface. On the other hand, an increase of the sulfide concentration to 0.5 M resulted in the absence of Raman signals due to sulfur at laser outputs ranging from 20 to 200 mW. The experiments described herein also serve to underline the utility of laser resonance Raman spectroscopy as an in situ tool for molecular-level tailoring of the variables in a photoelectrochemical system such that photogenerated carrier (electron or hole) transfer to an electrolyte species may be promoted at the expense of the electrode corrosion pathway. Introduction Since the early studies by Miller and Hellerl and by Peter,2 the anodic electrosynthesis of CdS thin films has been extensively in~estigated.~-IO A variety of electrochemical probes have been employed for the analysis of the Cd/CdS/aqueous sulfide interphaseincluding,for example, hydrodynamic~ o l t a m m e t r y , ~ ~ ~ ~ cyclic/linear sweep ~oltammetry,4.~~9~.~.~~ analysis of charging transients,*5 capacitance measurements,4q8 chronoamperometry zyxwvut ," and chronopotentiometry.8a The photoelectrochemical properties of these films (CdS is an n-type semiconductor with an optical bandgap of -2.4 eV) have also been profitably used to gain mechanistic insights into the film growth and surface chem- istry.3~8b~9bJ0 However, as pointed out by us in a recent review," measurement of charge, current, or potential alone only provides data with limited information content in terms of molecular details concerning the electrochemical system under study. On the other hand, spectroscopic probes in general, and Raman spectroelec- trochemistry in particular, can be a useful complement in this regard." Thus vibrational spectroscopies have been employed for the in situ study of surface films on electrodesand for identifying surface and bulk participant species in electrochemical systems.I2-l6 Photoelectrochemical systems have also been examined in situ by the Raman scattering probe.I7J8 In previous studies from this laboratory, we demonstrated the use of this technique for the study of organic (polypyrr~le)'~ and inorganic (cuprous thio- cyanate)20asemiconductor films and microstructures/bilayers containing a combination of these electrode materials.20b A preliminary study also showed the feasibility of the Raman spectroelectrochemical probe as a useful monitor of anodic thin film growth.9a We develop this aspect in more detail and also explore the postdeposition surface chemistry of CdS thin films in this paper. In particular, our use of suprabandgap laser excitation energy [the 488-nm (2.54-eV) Ar+ linesimultaneously excites electron-hole pairs in the anodic CdS film] affords a dynamic in situ monitor of the interphasialphotoelectrochemistry. These experiments open a route to optimization of hole transfer to the solution at the expense of the film photocorrosion process. f Visiting Scientist. Permanent address: INIFTA, Universidad Nacional de La Plata, C. C. 16, SUC. 4 (1900) La Plata, Argentina. 0022-3654/93/2097-6504$04.00/0 __ We thus show that a sulfur (corrosion) layer does not form at the CdS surface even at relatively high photon fluxes as long as the sulfide concentration in the electrolyte is 2-03 M. While this is not a new finding, the in situ Raman monitor of sulfur affords a molecular-level probe of the photocorrosion, as demonstrated below for CdS, and contrasts with the (largely) "chemistry insensitive" or ex situ techniques that have been employed thus far to assess the extent of this process. Experimental Section The working electrode was a polycrystalline cadmium disc (Johnson-Matthey, 0.10 cm2). Before each experiment the electrode was mechanically polished with alumina of decreasing size down to 0.05 pm to yield mirrorlike surfaces. The electrode was then chemically polished by a 1:l mixture of glacial acetic acid and 30% hydrogen peroxide for 5-10 s and washed with water. The CdS films were grown in the dark, either in 0.1 or 0.5 M sulfide solutions. The cell was shielded from ambient light by a black box. The CdS single crystal (Cleveland Crystals) electrode was polished with alumina (0.05 pm) etched for zyx 5 s in 20% HCl and then rinsed with water. Anhydrous Na2S (Alfa Products) was used as received. The sulfide solutions were prepared just before running each exper- iment by dissolving Na2S in deoxygenated double distilled water (Coming Megapure). The Raman spectroelectrochemicalmeasurements employed the 488-nm line of an Ar+ laser and a Spex Ramalog instrument equipped with a Model 1680double monochromator and a cooled photomultiplier tube (Model R928) operated in the photon- counting mode. The experimental resolution was 5 cm-'. More details of the spectral measurements are given e l s e ~ h e r e . ' ~ . ~ ~ A holographic edge filter (Physical Optics Corporation) was added for Rayleigh line rejection. All potentials quoted in this work are with respect to a Ag/ AgC1/3 M KCl reference. The laser source was operated at a nominal output of 20 mW in the experiments described below, except in instances where the light intensity was a variable. Q 1993 American Chemical Society