Theoretical insight into the roles of cocatalysts in the Ni-NiO/β-Ga₂O₃ photocatalyst for overall water splitting

The formation and stability of Ni_n and (NiO)_n (n = 1-4) clusters on the β-Ga₂O₃ surface have been studied by means of first-principles density functional theory calculations. It is found that the optimum interaction of the Ni_n and (NiO)_n clusters with the surface requires different surface sites. This optimizes the formation of interfacial bonds between the atoms from clusters and the coordinatively unsaturated atoms from the surface. The stability of the adsorbed Ni clusters increases with the number of Ni atoms. In a Ni_n/Ga₂O₃ system, as the Ga unoccupied states overlap with the unoccupied Ni state, the excited electrons transferred from Ga to Ni participate in the proton reduction reaction. Our calculations show that (NiO)_n clusters strongly adsorb on the Ga₂O₃ surface due to the negative adsorption energies within -1.9 eV to -3.7 eV. For (NiO)_n/Ga₂O₃, occupied states from the (NiO)_n cluster may accept the holes from O atoms in the Ga₂O₃ surface to take part in the photocatalytic water oxidation reaction.