Substitutional Mo doping in a Ta₃N₅ photoanode: mitigating native defects through engineering and enhancing water-splitting performance

Ta₃N₅, with its 2.1 eV bandgap and favorable band edge positions, is a promising compound for solar water splitting. However, its performance is limited by defective states introduced during high-temperature nitridation, particularly those based on reduced Ta species that act as electron recombination centers and can pin the Fermi level. Increasing electron density to extend the conduction band may suppress the formation of these states. Here, we introduce a combined theoretical and experimental study on Mo doping in Ta₃N₅, aiming to inhibit structural defects and enhance photoelectrochemical activity. Theoretical calculations reveal that Mo doping in Ta₃N₅ not only decreases the bandgap but also transforms the material from an indirect to a direct bandgap semiconductor. This transformation is attributed to the ability of Mo⁴⁺ ions, with comparable ionic radii and oxidation states for substitutional doping. This substitution introduces neutralizing acceptor states, effectively mitigating the formation of reduced Ta³⁺/Ta⁴⁺ states and nitrogen vacancies. As a result, charge carrier transport is enhanced, and recombination is suppressed. Additionally, the refractive index increases from 2.65 to 2.89 upon Mo doping, demonstrating improved optical performance for photoelectrochemical applications. Experimental results demonstrate a 4.3-fold enhancement in photoelectrochemical activity, alongside a 150 mV cathodic shift in the onset potential with substitutional Mo doping in Ta₃N₅. Moreover, the substitutional Mo doping does not induce lattice strain. These findings suggest that precise Mo doping in Ta₃N₅ has the potential to drive the development of innovative photoelectrochemical systems for practical solar fuel applications.