The observation of the interaction between two coupled effects of surface transfer doping (STD) and Burstein-Moss (B-M) shift on the electrical, optical, and electronic properties of ${\mathrm{V}}_2{\mathrm{O}}_5$, a layered van der Waals oxide semiconductor, is reported. Studies on bulk crystals, thin films, and powder samples show that, as a result of its high electron affinity, stoichiometric ${\mathrm{V}}_2{\mathrm{O}}_5$ exposed to humid air exhibits the phenomenon of surface transfer doping, in which a surface-absorbed water layer injects electrons into the conduction band (CB) of ${\mathrm{V}}_2{\mathrm{O}}_5$, leading to degenerate $n$-type doping, formation of a surface electron accumulation layer, and a corresponding B-M shift of the surface Fermi level of 0.25–0.6 eV well into the CB edge. In nonstoichiometric oxide (${\mathrm{V}}_2{\mathrm{O}}_{5-x}$), the presence of high density of bulk oxygen vacancy (${\mathrm{V}}_{\mathrm{O}}$) defects leads to electron doping, an increased B-M-induced $E_{\mathrm{F}}$ shift of 0.4–1.0 eV to higher energy in the bulk. In contrast, the surface $E_{\mathrm{F}}$ level remains pinned at lower energy owing to the switch in the STD effect, leading to reverse electron transfer from the oxide to the absorbed water layer, and causing the formation of an electron depletion layer on the surface. The implications of these effects for the wide variations in the reported band gap, doping, and catalysis of ${\mathrm{V}}_2{\mathrm{O}}_5$ are discussed.

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• Doping effects