Controlling the carrier density in niobium oxynitride BaNbO2N via cation doping for efficient photoelectrochemical water splitting under visible light. Issue 23 (9th November 2021)

• Record Type: Journal Article
• Authors: Iwai, Takafumi; Nakada, Akinobu; Higashi, Masanobu; Suzuki, Hajime; Tomita, Osamu; Abe, Ryu
• Is Part Of: Sustainable energy & fuels. Volume 5:Issue 23(2021)
• Page Start: 6181
• Page End: 6188
• Publication Date: 2021-11-09
• Subjects: Renewable energy sources -- Periodicals; Fuel cells -- Periodicals; Electric batteries -- Periodicals; Electrochemistry -- Periodicals; 660.297
• DOI: 10.1039/d1se01272j
• Languages: English
• ISSNs: 2398-4902

Abstract

Abstract : Lower-valent cation doping enables appropriate reduction of undesirably high donor density in BaNbO2 N, providing both a suitable electron conductivity and hole diffusion length with high photoelectrochemical performance for water splitting. Abstract : Although niobium oxynitrides such as BaNbO2 N 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., Nb 4+ ) generated during their synthesis via nitridation with NH3 . In this study, a series of BaNbO2 N particles doped with various cations were synthesized to improve the photoelectrochemical (PEC) water splitting performance of porous BaNbO2 N photoanodes via donor density control in the semiconductor bulk. Doping lower-valent cations (Ti 4+, Zr 4+ ) into Nb 5+ sites decreased the donor density, as indicated by the Mott–Schottky plots, and also suppressed the generation of Nb 4+ species to some extent. In contrast, doping with higher-valent cations (W 6+, Mo 6+ ) increased the donor density and accelerated Nb 4+ generation. The porous photoanodes of BaNbO2 N 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 Ti 4+ -doped BaNbO2 N 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 BaNbO2 N was excessively high because of the inevitable reduction of Nb 5+ 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.