Chemical Role of Holes and Electrons in ZnO Photocatalysis

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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.