Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.119
Publication date: 6th July 2018
The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Herein, we map the correlation between morphological and functional heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity dependent current-voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space-charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. In addition, we elucidate the effects of oxygen and water surface adsorption on band alignment, interfacial charge transfer, and charge carrier transport by using complementary Kelvin probe measurements and photoconductive atomic force microscopy on this material. By observing variations in surface potential, we show that adsorbed oxygen acts as an electron trap state at the surface of bismuth vanadate, whereas adsorbed water results in formation of a dipole layer without inducing interfacial charge transfer. The apparent change of trap state density under dry or humid nitrogen, as well as under oxygen-rich atmosphere, proves that surface adsorbates influence charge carrier transport properties in the material. The finding that oxygen introduces electronically active states on the surface of bismuth vanadate may have important implications for understanding local functional characteristics of water splitting photoanodes and their effects on the macroscopic performance, and for devising strategies to passivate interfacial trap states, and elucidating important couplings between energetics and charge transport in reaction environments.
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