More than protection: the function of TiO2 interlayers in hematite functionalized Si photoanodes

Abstract

Worldwide significant efforts are ongoing to develop devices that store solar energy as fuels. In one such approach, solar energy is absorbed by semiconductors and utilized directly by catalysts at their surfaces to split water into H₂ and O₂. To protect the semiconductors in these photo-electrochemical cells (PEC) from corrosion, frequently thin TiO₂ interlayers are applied. Employing a well-performing photoanode comprised of 1-D n-Si microwires (MWs) covered with a mesoporous (mp) TiO₂ interlayer fabricated by solution processing and functionalized with α-Fe₂O₃ nanorods, we studied here the function of this TiO₂ interlayer by high-energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy, along with X-ray emission spectroscopy (XES) and standard characterization techniques. Our data reveal that the TiO₂ interlayer not only protects the n-Si MW surface from corrosion, but that it also acts as a template for the hydrothermal growth of α-Fe₂O₃ nanorods and improves the photocatalytic efficiency. We show that the latter effect correlates with the presence of stable oxygen vacancies at the interface between mp-TiO₂ and α-Fe₂O₃, which act as electron traps and thereby substantially reduce the charge recombination rate at the hematite surface.