ZnO surface decomposes by light in a reversible persistent process creating surface oxygen vacancies

What is it about?

In this paper, we show that the surface of ZnO decomposes upon exposure to white light, giving away its oxygen, creating surface oxygen vacancies. Exposure to oxygen reverses the process, filling the vacancies with oxygen. This explains several peculiar "persistent" phenomena in ZnO, such as persistent photoconductivity, and persistent photovoltage. It also explains some well-known gas sensing properties of ZnO and some of its merits as a photochemical catalyst.

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Why is it important?

Unlike recent theoretical work suggesting that the oxygen vacancy is a deep donor, the current work clearly suggests that when an oxygen vacancy is situated on the ZnO surface, it is actually a shallow donor. This is in no contradiction to any of the above theories, as these theories were based on calculations of a bulk-situated vacancy. However, we show the persistent photoconductivity\photovoltage are clearly surface phenomena associated with surface oxygen exchange with the ambient.