Doping of TiO2 as a tool to optimize the water splitting efficiencies of titania-hematite photoanodes

Simple metal oxides such as hematite and titania draw tremendous interest as materials for photoelectrochemical (PEC) water splitting photoelectrodes to produce hydrogen as a clean and sustainable energy carrier. However, the high recombination rates of the photogenerated charges limit their application. Herein, we report on highly efficient and stable composite titania-hematite photoanodes prepared by combining doped TiO2 nanoparticles with amorphous iron oxide and subsequent annealing. Studying the effect of various TiO2 doping strategies, by in-depth structural and chemical characterization, carried out through a multiple technique approach, showed that doping of TiO2 allows subtle tuning of the phase composition, microstructure and surface topography of the photoanodes. When the photoanodes were prepared by combining Ta-doped TiO2 nanoparticles and amorphous iron oxide nanoparticles and subsequently annealed, remarkable photocurrents of up to 2.2 mA cm-2 at 1.23 V in 1 M NaOH under 1.5 AM simulated solar illumination were obtained. The high photocurrents, which were traced back to Ta-doping, were elucidated by rutile-hematite heterojunction energetics and the blocking layer formation. In addition to showing promise for a sustainable and cost-effective generation of an energy carrier, the presented strategies can also be expanded to other material combinations opening doors for new modified semiconductors or heterojunction photoanodes.