Using oxide nanostructures to control carrier transport properties

Selecting suitable material(s) for water splitting is an intricate dilemma, as materials with high solar-to-hydrogen (STH) conversion are typically not stable in aqueous environment and/or are scarce, whereas stable and abundant materials often exhibit unacceptable performance for commercialization [1]. For example, Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ is an abundant n-type semiconductor that has excellent stability in neutral and alkaline electrolytes, but so far the reported STH conversion efficiency has not exceeded 3{\%} [2]. A major factor hampering the performance Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$, is the high charge recombination rate inside the semiconductor. Thus, there is a need to develop methods and designs to reduce charge recombination. Recently we showed that by joining different oxides we were able to control the charge recombination rate [3]. The control mechanism relies on the formation of dipole-like electric fields at the interface which, depending on the field direction, attract or repel minority carriers from the interface. Here we investigate the built-in electric field generated at the interfaces of Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$/TiO$_{\mathrm{2}}$ and Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$/Cu$_{\mathrm{2}}$O using first-principles methods. The results show how electronic band alignment and defects doping at the interface determine the direction and strength of the built-in field. Our understanding of the oxide nanostructures can be employed for designing and improving the performance of water-splitting photoelectrodes. [1]: A. Fujishima, et al., \textit{Surf. Sci. Rep.}, \textbf{2008}, 6, 515-582. [2]: D. K. Bora, et al., \textit{Energy Environ. Sci.}, \textbf{2013}, 6, 407-425. [3]: B. Iandolo, et al., \textit{Nano Lett.},~\textbf{2016},~\textit{16}, 2381--2386.