First‒Principles Prediction of High Solar‒to‒Hydrogen Efficiency in TaOX 2 and Janus TaOXY (X, Y = Cl, Br, I) Monolayers

Abstract Many photocatalytic water splitting catalysts exhibit generally low energy conversion efficiencies due to insufficient sunlight absorption, stringent redox overpotential requirements, and rapid photogenerated carrier recombination. Based on first-principles calculations, this study systematically investigates the structural, optical, and electronic properties of two-dimensional TaOX 2 and its Janus TaOXY (X, Y = Cl, Br, I) monolayers. Our calculations reveal the band edges of these mechanically and optically anisotropic indirect-bandgap semiconductors (1.778–1.844 eV) span the redox potentials for water splitting, satisfying the requirements for photocatalytic water splitting. Importantly, the Janus structures generate an internal electric field due to the symmetry breaking of the top and bottom surfaces, which effectively suppresses the recombination of photogenerated carriers. Consequently, the carrier utilization efficiency of TaOIBr, which has the lowest utilization among the Janus structures (46.21 %) is significantly higher than the maximum value (36.2 %) observed for the TaOBr2 at pH = 0. Except for TaOCl 2 , which does not satisfy the requirement for overall water splitting, and TaOI 2 , whose STH efficiency at pH = 0 is 9.12 %, below the 10% commercial STH benchmark, the remaining TaOX2 exceeds the benchmark within their applicable pH ranges. In particular, even after accounting for internal electric field correction, the Janus TaOXY monolayer can still achieve an STH efficiency of over 20 % within certain pH ranges. Overall, this work provides valuable theoretical insights for the design of novel photocatalysts and highlights the promising application of two-dimensional TaOX 2 and Janus TaOXY materials as catalysts in energy conversion technologies.