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.

Electrodeposition of Nanometer-Sized Ferric Oxide Materials in Colloidal Templates for Conversion of Light to Chemical Energy

“…The addition of formate to the electrolyte resulted in an approximate doubling of the magnitude of the photocurrent. It is known that the one electron oxidation of formate produces a highly energetic radical that can inject electrons into the conduction band of some semiconductors, effectively doubling the current [45,46]. Kennedy and Frese have previously suggested that Small photocurrents observed at hematite single crystal and thin film electrodes have previously been attributed to short minority carrier (hole) diffusion lengths [6].…”

Semiconductor Electrochemistry

“….J Holes which accumulate in the surface in this way can cause oxidation processes which are otherwise very unlikely. As already mentioned in Section IIIB.2a, at a surface of a ZnO or CdS electrode, when illuminated by suitable light and anodically polarized, many organic molecules are oxidized to radicals, in reactions such as [Morrison andFreund (1967), (1968), Markham and Upret (1965), Haberkorn (1967)] …”

“…There seem to be some exceptions to this rul.e J, Morrison and Freund (1967), (1968) The quantitative analysis is often complicated by parallel reactions or by more or less regular variations in the surface properties of the semiconductor.. Figure 23 gives an example of a current-voltage curve obtained at a Ge electrode for the oxidation of V+2 ions. [Mauerer (1965).J Since the oxidation occurs by injection of electrons into the conduction band, the rate should be equal at n-and p-type electrodes and independent of the applied voltage.…”

Hybrid materials utilizing polyelectrolyte-derivatized carbon nanotubes and vanadium-mixed addenda heteropolytungstate for efficient electrochemical charging and electrocatalysis

“…Our recent studies also showed that an increase of the current intensities was also seen when the ink suspension was prepared using CNTs that had already been modified with PDDA and [P 2 W 17 VO 62 ] 8− . It is reasonable to expect that current increases were caused by so-called current doubling historically observed in the presence of different organic molecules [58,59]. As the observed effect led to the pseudocapacitance increase, the ink suspension was prepared as a rule with addition of diethyl ether.…”