Effects of Substitution of Aliovalent N^3– and Cl^– Ions in Place of O^2– in ZnO: Properties of ZnO_1–x–yN_xCl_y (x, y = 0.0–0.5)

Aliovalent anion substitution in metal oxides and other inorganic materials has a remarkable effect on the electronic structures of these materials as recently demonstrated in N- and F-cosubstituted ZnO. We have examined ZnO where O is partially substituted with N and Cl and found significant changes in the bond lengths by single-crystal X-ray crystallography in the case of ZnO_0.6N_0.2Cl_0.2. Moderate substitution of N and Cl brings the band gap of ZnO down to 1.93 eV and renders ZnO coloured. These materials are photocatalytically active for hydrogen production. Complete substitution of O by N and Cl results in Zn₂NCl, a colourless insulator. First-principles calculations have shown how the presence of a band originating from N-2p states occupying the top of the valence band is responsible for the reduction in the band gap in ZnO. The calculations have also thrown light on the stability and electronic structures of the N- and Cl-cosubstituted ZnO compounds.