The Mechanism of Photocatalysis on Semiconductor Powders in a Liquid Medium. New Experimental Methods and Their Results

Abstract

AbstractTwo new methods are described for following the reactions occurring on semiconductor powders, when illuminated in a solution of reactants. The reaction between nitroaromatics and hydrazine gives gaseous nitrogen as one of the products, which can be measured manometrically. The rate with CdSe as the catalyst is strongly dependent on the structure of the nitroaromatic, giving faster reactions for the better electron acceptors. The more polar the solvents the lower the yields. The reaction between K4Fe(CN)6 and air in aqueous solution is measured by electrochemical reduction of the product K3Fe(CN)6. Rates are strongly dependent on heat treatments given to the semiconductor before reaction and the dependence of rate on CdSe heat treatment is very similar for both the nitrobenzene/hydrazine reaction in methanol and the K4Fe(CN)6 oxidation in water. Electrochemical measurements with CdSe electrodes in benzene show the current onset to be at a potential similar to the first reduction potential of the nitroaromatics used, which explains the strong rate dependence on the reduction potential of the nitroaromatic. A mechanism is proposed for the n‐type case of a rapid reaction occurring between the minority carriers and hydrazine, by which the powder charges up negatively and the electron transfer from the charged powder to the nitroaromatic in solution is rate determining. With p‐type semiconductors, which give slower rates, the minority carrier reaction with the nitroaromatics is rate determining and only little positive charge accumulates on the particles.