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Anomalous Photocatalytic Activity of Nanocrystalline γ‑Phase Ga2O3 Enabled by Long-Lived Defect Trap States

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journal contribution
posted on 12.04.2017, 00:00 by Vahid Ghodsi, Susi Jin, Joshua C. Byers, Yi Pan, Pavle V. Radovanovic
Semiconductor photocatalysis has emerged as an efficient and sustainable advanced oxidation process for wastewater treatment and other environmental remediation and forms the basis for water splitting and solar-to-fuel conversion. Nanocrystalline metal oxides are particularly promising photocatalysts because of their efficiency, stability, and low toxicity. However, the influence of the crystal structure and defects on the photocatalytic activity of these polymorphic materials is still poorly understood. In this work we investigated the structural dependence of the photocatalytic activity of nanocrystalline Ga2O3. We demonstrate that metastable cubic-phase γ-Ga2O3 prepared from colloidal nanocrystals exhibits an anomalously high photocatalytic activity, which rapidly decreases upon thermally induced transformation to monoclinic β-Ga2O3. Using steady-state and time-resolved photoluminescence measurements we showed that the reduction in photocatalytic activity upon annealing is accompanied by a decrease in native defect (i.e., oxygen vacancy) concentration and interactions. Trapping charge carriers in defect-induced states in γ-Ga2O3 nanocrystals results in a reduced rate of charge recombination and enhanced interfacial charge transfer, which has been unambiguously confirmed by comparative measurements using In3+-doped Ga2O3. These phenomena are enabled by the unique character of defect states in γ-Ga2O3 nanocrystals which have much longer lifetime than typical metal oxide surface states. Using various scavengers, we demonstrated that reactive radicals (OH and O2•–) formed by photogenerated charge carriers play a key role in the mechanism of photocatalytic degradation by Ga2O3. The results of this work demonstrate how manipulation of the location and electronic structure of defect sites in nanostructured metal oxides can be effectively used to control charge carrier separation and enhance photocatalytic activity, without a detriment to high surface-to-volume ratio.