Chemical Role of Holes and Electrons in ZnO Photocatalysis

Abstract

The role of the electronic properties of a semiconductor in heterogeneous catalysis and electrochemistry was experimentally investigated on single-crystal ZnO. It was shown quantitatively that the availability of electrons and holes at the surface is dominant in the mechanism of a heterogeneously catalyzed reaction. Chemical rate measurements as well as in situ solid-state measurements were carried out in an aqueous medium for the reaction: HCOOH+O2→H2O2+CO2, photocatalyzed by ZnO. Two new experimental electrochemical methods for semiconductor surface reactions were developed: ``current doubling'' and measurement of ``unfilled'' electronic surface states by the capacitance. They were devised to characterize reactive sorbed species. The detailed catalytic mechanism was based on studies of individual reaction steps under three solid-state surface conditions: (1) only holes reacting, (2) only electrons reacting, and (3) both holes and electrons reacting. The first two correspond to the ZnO being an electrochemical anode and cathode, respectively; the last corresponds to ordinary catalysis when the hole current is balanced by the electron current so that the net current is zero. It was shown that the over-all catalyzed reaction is not simply the sum of individual oxidation and reduction reactions which occur with only holes or only electrons, respectively, but involves a surface intermediate formed when both holes and electrons are present.