Controlling the carrier density in niobium oxynitride BaNbO2N via cation doping for efficient photoelectrochemical water splitting under visible light

Although niobium oxynitrides such as BaNbO2N possess desirable properties for solar energy conversion (e.g., narrower band gaps compared to their tantalum counterparts), their photoelectrode performance is generally inferior, probably because of the excessively high donor density derived from the reduced species (e.g., Nb4+) generated during their synthesis via nitridation with NH3. In this study, a series of BaNbO2N particles doped with various cations were synthesized to improve the photoelectrochemical (PEC) water splitting performance of porous BaNbO2N photoanodes via donor density control in the semiconductor bulk. Doping lower-valent cations (Ti4+, Zr4+) into Nb5+ sites decreased the donor density, as indicated by the Mott–Schottky plots, and also suppressed the generation of Nb4+ species to some extent. In contrast, doping with higher-valent cations (W6+, Mo6+) increased the donor density and accelerated Nb4+ generation. The porous photoanodes of BaNbO2N doped with lower-valent cations showed a larger photocurrent density than that of the undoped photoanodes, whereas those fabricated with higher-valent cations exhibited smaller values over the entire applied potential range. The optimized Ti4+-doped BaNbO2N photoanodes exhibited superior performance compared to their tantalum counterparts, demonstrating PEC water splitting with a relatively high quantum efficiency under visible light after appropriate loading of a cocatalyst. These results strongly suggest that the donor density of undoped BaNbO2N was excessively high because of the inevitable reduction of Nb5+ during the nitridation process, and the appropriate reduction of donor density by lower-valent cation doping can provide both a suitable electron conductivity and a sufficient hole diffusion length, thus substantially improving the performance.