The interplay between vanadium and strontium doping in the photoelectrochemical performance of combinatorially deposited single- and dual-doped sodium tantalate thin films

The rational design of photoanode materials for photoelectrochemical (PEC) water splitting is a critical aspect towards making this technology available for large-scale renewable energy transition. In this respect, we present here the combinatorial tailoring of the PEC performance of sodium tantalate (NaTaO3)-based ternary and quaternary oxide thin films grown by chemical beam vapor deposition. The incorporation of the selected dopants, namely vanadium and strontium, mainly at the B-site of the orthorhombic perovskite NaTaO3 structure, enhanced the distortion of the TaO6 octahedrons with consequent benefits on the charge delocalization. Mott-Schottky analysis combined with optical characterization, revealed important alterations of the band structure with an increase in the donor density and band gap narrowing, the latter being more significant in V/Sr dual-doped oxides (from 4.5 eV in pristine NaTaO3 to a minimum of 3.6 eV in codoped NaTaO3). Despite an anodic shift of the onset potential for water oxidation, the V-doping and especially V/Sr codoping were found to be the most effective in improving the PEC activity under HgXe light irradiation, with the best performance observed with Sr and V at optimal doping concentrations of 2 and 3 at.%, respectively. Electrochemical impedance spectroscopy and transient open circuit potential measurements shed the light on the synergistic effects of V and Sr doping in boosting charge separation. Our results demonstrate that V/Sr codoping is a viable route to enhance PEC performance of NaTaO3, mainly due to an increased light-harvesting efficiency, suppression of trap states, and efficient charge transfer to the photoanode/electrolyte interface.