Interaction of thiols with n-type cadmium sulfide and n-type cadmium selenide in aqueous solutions: Adsorption of thiolate anion and efficient photoelectrochemical oxidation to disulfides

Organic thiols, RSH = cysteamine, 2-mercaptoethanol, penicillamine, or cysteine, effectively suppress photoanodic decomposition of n-CdS and n-CdSe in aqueous solutions and undergo efficient controlled potential photoelectrochemical oxidation to the corresponding disulfides, RSSR. Photovoltage (the extent to which oxidation occurs at potentials negative of the thermodynamic potential) of greater than 700 mV is observed for certain thiols upon excitation of the semiconductor anode with light of energy greater than the band gap. As electrodes for preparative electrosynthesis, both illuminated n-CdS and n-CdSe offer significant electrical energy savings compared to conventional electrochemical oxidation in the dark at a Pt electrode. The oxidation of RSH to RSSR at n-CdS and n-CdSe has been studied under various conditions: high current efficiencies, typically measured to be greater than 98%, are found at high and low pH for all RSH studied except glutathione which is oxidized with about 60% current efficiency. Attempted controlled potential oxidation of glutathione at n-CdS leads to decomposition of the electrode. No S/Se exchange occurs at the surface of n-CdSe in the efficient photoelectrochemical oxidation of RSH to RSSR, as determined by Auger spectroscopy. The output parameters at n-CdS and n-CdSe depend on the nature of the functional groups on the RSH. The best energy conversion performance is achieved with a 1 M solution of cysteamine at pH 11 with an efficiency for conversion of 501.7-nm light (30 mW/cm²) to electricity of 14.5% (without electrode decomposition) with a photovoltage of 670 mV when n-CdS is used as a photoanode. In contrast, a 1 M glutathione solution at pH 11 yields an efficiency of only 0.6% (with electrode decomposition) with a photovoltage of 510 mV under the same conditions. Both capacitance-voltage and photocurrent-voltage measurements show that the flat-band potentials, F_FB, of n-CdS and n-CdSe are shifted to more negative values (by up to 0.7 V) by the strong adsorption of RS^-. Unfortunately, a simple Langmuir model does not explain the concentration dependence of adsorption of RS^- and detailed comparisons of binding properties cannot be made.