Theoretical studies on the form and effect of N-doping in an ZnGa2O4 photocatalyst

In the present work, hybrid density functional theory calculations were employed to analyze the electronic structures of ZnGa2O4 with different N impurity concentrations. The electronic transition energies in N doped ZnGa2O4 with a molar ratio N/O of similar to 3% were 3.35 and 1.43 eV for substituted and interstitial N-doping, respectively, which were much smaller than the value of 4.25 eV of pure ZnGa2O4. In 2N-doped ZnGa2O4 with a molar ratio N/O of similar to 6%, three models with different N-doping sites (2N(s), 2N(i), and N-s + N-i) are analyzed. The calculated result indicated that the N impurity atoms preferred to form an N-N dimer rather than two apart N atoms in all three models. The electronic transition energies decreased by about 1.52, 0.91, and 2.51 eV for 2N(s)-, 2N(i)-, and N-s + N-i-doped ZnGa2O4, respectively, which mainly originated from the N-N pi* states in the midgap. The half-filled impurity levels, which originated from the single electron of N-doping, were passivated due to the interaction of the two impurity atoms. The defect formation energies indicated that the oxygen vacancy would promote the introduction of a nitrogen impurity. Urea was recommended as a nitrogen source to easily achieve 2N(s)-doping forms. Based on the present calculations, 2N(s)-doped ZnGa2O4 was considered as the better photocatalyst due to a smaller band gap for visible light response and a suitable redox couple for overall water splitting. Our work provided a theoretical explanation for the origin of the enhanced visible light photocatalytic activity of N-doped ZnGa2O4.