Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.071
Publication date: 6th July 2018
The transformation of solar energy into chemical energy stored in the form of hydrogen, through photoelectrochemical water splitting is a promising method that has the important advantages of being environment friendly and free from carbon dioxide emission. Metal oxides are promising candidates for photoanode but the strong electron-hole recombination may explain their low efficiency. It has been recently proposed to use the spontaneous electric field of a ferroelectric compound to separate photogenerated charges in photoanode. In our laboratory we have been developing model oxide thin films for solar water splitting prepared by oxygen assisted molecular beam epitaxy for several years, in order to understand the relevant parameters to improve photoanodes performance. In this context, in order to unravel the role of ferroelectricity we have been investigating epitaxial TiO2 films and TiO2/BaTiO3 heterojonctions deposited on Pt(001) where BaTiO3 is the prototypical ferroelectric material. We have studied the growth, the electronic structure, the ferroelectric properties and the photoelectrochemical performance (photocurrent and impedance spectroscopy) as a function of the position of the ferroelectric layer in the film (above, in the middle and below) and of the orientation of ferroelectric polarization. Thereby, we show that the TiO2 films adopt a TiO2-B (001) structure for thickness lower than 3 nm and an anatase (001) structure for higher thicknesses, while the BaTiO3 films adopt a tetragonal (001) structure with a natural electrical polarization perpendicular to the surface. We show that the performance of the TiO2 photoanode can be improved by a polarized layer of BaTiO3, the best improvement is when the ferroelectric film is below the TiO2 film and when the electrical polarization is downward polarized. We explain this finding by the presence of an internal electric field which favors the separation of photo-generated charges, and explain how the electronic structure is modified by the electrical polarization in each case.
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