Electronic Structure of (Ga1-xZnx)N1-xOx Photocatalyst for Water Splitting by Hybrid Hartree-Fock Density Functional Theory Methods

GaN:ZnO solid solutions have been identified as a promising system for photocatalytic or photoelectrochemical water splitting under visible-light irradiation. However, the origin of their activity at longer wavelength with respect to the parent materials (GaN and ZnO) absorbing in the UV spectrum is still matter of debate. Previous theoretical studies were based on standard GGA or GGA+U calculations which largely underestimated the band gap values of the two semiconductors and are thus not best suited methods for the required analysis. The present is a hybrid density functional study (B3LYP) which provides more accurate description of the electronic structure of the parent semiconductors and is thus also more reliable for the evaluation of the mixed GaN:ZnO system. For small concentrations of ZnO in GaN, local inhomogeneity of Zn or O concentration must be invoked to observe a red-shift of the absorption edge. For larger concentrations, some random alloy distributions, enthalpically more expensive but entropically more favorable, are found to present reduced band gap values because of a positive shift of the N 2p states from the GaN component interfacing the ZnO fragments as a consequence of the repulsive interaction with the Zn 3d states.