Origin of photocatalytic activity of nitrogen-doped germanium dioxide under visible light from first principles

The large intrinsic band gap of GeO2 hinders its potential application as a photocatalyst under visible-light irradiation. Here, we perform first-principles calculations to investigate the origin of the experimentally observed visible-light photocatalytic activity of GeO2 induced by N doping. Four possible defects (N-doping, N+H codoping, Ge vacancy and O vacancy) for the redshift of N-doped GeO2 are tentatively put forward. The lowest formation energy indicates that N+H codoped GeO2 is the most stable and easiest to form. N-doping at an oxygen site induces gap states, which are made up primarily of N 2p-derived states with some O 2p contributions, and the lowest-energy empty states is in the middle of the gap. For Ge vacancy model, O 2p states form the intermediate band, while Ge 4s orbitals localized in the gap contribute to the impurity level in O vacancy model. These gap levels lead to a reduced effective band gap, which is one of the potential interest for photocatalytic applications. Electronic transitions from these localized states induce a redshift to the visible region of the optical absorption edge. The calculated optical properties for N-doped GeO2 show a significant visible light absorption at about 400–600 nm, in close agreement with the experimental result. This work indicates that N-doped GeO2 would be a promising photocatalyst with favorable photocatalytic activity in the visible region.