Two-dimensional anisotropic monolayers NbOX2 (X = Cl, Br, I): Promising candidates for photocatalytic water splitting with high solar-to-hydrogen efficiency

Motivated by the recent experimental synthesis of two-dimensional (2D) NbOI2 which possesses a moderate bandgap and outstanding absorption of sunlight, using the first-principles calculations, we conduct a thorough study of the geometric configuration, electronic structures, and photocatalytic properties for NbOX2 (X = Cl, Br, I) monolayers. These NbOX2 monolayers have been demonstrated to be dynamically, thermally, and mechanically stable. The significant anisotropic mechanical properties of NbOX2 monolayers are reflected by the calculated Young's modulus and Poisson's ratio. Our results indicate that these NbOX2 materials unfold semiconductor characters with indirect bandgaps of 1.886, 1.909, and 1.813 eV, respectively. Among these monolayers, it is found that the NbOBr2 system exhibits a favorable photocatalytic activity in an acidic condition (pH = 0), and the NbOI2 monolayer can act as a potential photocatalyst for spontaneous photocatalytic water splitting under a neutral environment (pH = 7). Furthermore, the response of bandgap and band edge positions of NbOX2 monolayers to the exerting in-plane strain (–6% to 6%) are investigated. These NbOX2 monolayers also show strong light absorption from the visible to ultraviolet region and anisotropic high carrier transport. Particularly, the high solar-to-hydrogen efficiency of the NbOCl2 (1% tensile strain), NbOBr2, and NbOI2 monolayers are predicted to be 14.11% (pH = 0), 16.34% (pH = 0), and 17.05% (pH = 7), respectively. Therefore, we expect the NbOX2 monolayers to be promising candidates for highly efficient photocatalytic water splitting.