Structure-Property Relationship of Oxygen-Doped Two-Dimensional Gallium Selenide for Hydrogen Evolution Reaction Revealed from Density Functional Theory

Two-dimensional (2D) gallium selenide (GaSe) is known for its inert surface and wide bandgap, limiting its application as a photocatalytic material for the hydrogen evolution reaction (HER). Partial substitution of Se with O atoms can improve its catalytic efficiency. This work discovered that the surface activity of the substitutional O-doped single-layer GaSe surfaces (GaSe_1-xO_x, for x ≤ 22%) and their bandgap sizes are dependent on the detailed atomic configuration of the dopants, as revealed from density functional theory. For GaSe_1-xO_x at low O contents, where all O atoms are favorably separated by at least one-Ga-Se-Ga-unit, the surface activity for the HER is insignificantly improved by increasing dopant concentration. By contrast, when more O dopants are available and arranged in adjacent positions (O-Ga-O), the hydrogen adsorption efficiency of GaSe_1-xO_x increases and their bandgaps are reduced with increasing dopant concentration. These important features are attributed to weakening of the Ga-O covalent interaction in these more localized dopant arrangements, which in turn strengthens the O-H bonds. This weakened Ga-O covalent bond also descends the conduction band minimum toward the Fermi level, resulting in bandgap reduction and thus favoring visible-light absorption. Optimal atomic configurations (all having localized O-dopant arrangements) have been identified, and they exhibit almost thermoneutral hydrogen adsorption free energy ΔG_H and small bandgaps (2.09-2.21 eV), making them promising materials to perform an efficient HER. Fine-tuning the Ga-O interaction by applying tensile strength T_S parallel to the 2D surface of up to 1% further reduces their bandgaps to 1.95-2.05 eV. Our theoretical predictions suggest that controlling the atomic configuration of dopants provides opportunities for engineering single-layered GaSe_1-xO_x materials with surface reactivity and bandgaps that suit photocatalytic water splitting.