Abstract
Nanostructures of transition metal oxides, such as zinc oxide, have attracted considerable interest for solar-energy conversion and photocatalysis. Both applications are sensitive to the transport and trapping of photoexcited charge carriers. The probing of electron trapping has recently become possible using time-resolved element-sensitive methods, such as X-ray spectroscopy. However, valence-band-trapped holes have so far escaped observation. Herein we use X-ray absorption spectroscopy combined with a dispersive X-ray emission spectrometer to probe the charge carrier relaxation and trapping processes in zinc oxide nanoparticles after above band-gap photoexcitation. Our results, supported by simulations, demonstrate that within 80 ps, photoexcited holes are trapped at singly charged oxygen vacancies, which causes an outward displacement by ~15% of the four surrounding zinc atoms away from the doubly charged vacancy. This identification of the hole traps provides insight for future developments of transition metal oxide-based nanodevices.
Highlights
Nanostructures of transition metal oxides, such as zinc oxide, have attracted considerable interest for solar-energy conversion and photocatalysis
The transient XANES (Fig. 3d) shows positive features below the edge at 9.662 and 9.667 keV and a strong minimum at 9.671 keV, which is the position of the maximum absorption in the ground state spectrum
It was found that zinc vacancies do not lead to measurable distortions of the local lattice structure, while the Zn K-edge XANES spectra show two weak pre-edge features, which were assigned to a small concentration of oxygen vacancies
Summary
Nanostructures of transition metal oxides, such as zinc oxide, have attracted considerable interest for solar-energy conversion and photocatalysis Both applications are sensitive to the transport and trapping of photoexcited charge carriers. Devices for solar-energy conversion[1] and photocatalysis[2] based on transition metal oxide (TMO) nanoparticles (NPs) and mesoporous films are amongst the most promising routes for addressing our escalating energy demands. These rely on sunlight to generate charge carriers and it is crucial to understand the room temperature (RT). Second is its instability in aqueous solution arising from photocorrosion under UV irradiation[7]
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