Origin of the enhanced photocatalytic activities of semiconductors:: A case study of ZnO doped with Mg2+

A series of Zn1-xMgxO samples with dopant content ranging from x = 0 to 0.10 were prepared by a novel theological phase reaction route. All Zn1-xMgxO samples were investigated by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, inductive coupled plasma optical emission spectroscopy, infrared and UV-vis absorption spectroscopy, and the Barrett-Emmett--Teller technique. The effects of Mg2+ doping in ZnO on the electronic structures and photogradation of methylene blue dye solution were investigated experimentally and theoretically. All Zn1-xMgxO samples exhibit high photoactivities comparable to Degussa P-25, which first increased with the Mg doing content up to x = 0.05, and then slightly decreased with further doping of Mg to x = 0.10. Density function theory calculations revealed that the substitutions of Mg for Zn ions in the wurtzite ZnO structure largely affected the conduction band, but left the valence band nearly unchanged. The bottom of the conduction band shifted toward higher energies and the contribution of Mg 3s orbitals to the conduction band became more pronounced with increasing Mg content, which explains the enhanced photocatalytic activities. The state density of interstitial Mg in ZnO showed a set of shallow acceptor levels above the valence band. These shallow levels could act as the trapping or recombination centers for photoinduced electrons and holes, accounting for the slightly decreased photodegradation efficiency with further increasing Mg content to x = 0.10. The optimal doping content for photocatalytic performance was determined to be x = 0.05, which is the consequence of the balance of two competing doping effects from lattice substitution and interstitial occupations on the electronic structures.