Reduction of nitroaromatics on cadmium sulfide: further probing the electrochemical model of semiconductor photocatalysis

Abstract

The photoreduction of nitroarene compounds is a typical multielectron process that is examined here at cadmium sulfide electrodes and particle suspensions. Electrochemistry in the dark and photoluminescence indicate that reduction is mediated by surface states (sulfur vacancies) approximately located at 0.55 eV below the conduction band. In keeping with the known reduction mechanism, phenylhydroxylamine intermediates are detected at both electrodes and in suspensions. A focus of the present work was to use the nitroarenes reduction as a probe reaction to confirm the common bases of photoelectrochemistry and photocatalysis. Superimposition photocurrents (due to oxidation of hole scavengers) and dark currents identifies one potential at which the net observed current is zero and then establishes the operative photocatalytic condition. Rates of reduction on electrodes at open circuit compare very well with currents calculated from the intersection of photocurrent and dark current confirming that the photocatalytic process can be predicted from electrochemical measurements. Analysis of reaction rates in CdS suspensions suggests that a significant portion of reduction occurs on dark sites of the particles.