Effects of different valence states of Mo and point vacancies on magneto-optical properties of ZnO

Abstract

Experimental studies have been conducted to characterize the physical properties of Mo-doped ZnO. However, they have not considered the effects of different valence states and point vacancies on the magneto-optical performance of the system. The present study adopted the plane-wave ultrasoft pseudopotential + U method within the framework of spin density functional theory. The formation energy, electronic structure, and optical properties of ZnO systems with coexisting different valence states of Mo and point vacancies were calculated. Results showed that the formation energy of all doping systems was negative under Zn-rich and O-rich conditions, but the formation energy was relatively lower under O-rich conditions than under Zn-rich conditions. Compared with Mo6+ and Mo5+ when the concentration of O vacancies or Zn vacancies was the same, Mo6+-doped ZnO had the lowest formation energy, was the easiest to dope, and had the most stable system structure. Zn34Mo5+O36 and Zn34Mo6+O36 systems exhibited ferromagnetism, and their Curie temperature was above room temperature. The Zn34Mo6+O36 system had the largest magnetic moment, which is beneficial for designing and preparing new types of diluted magnetic semiconductors. The band gap widths of the Zn34Mo5+O36 (3.04 eV) and Zn34Mo6+O36 (2.92 eV) systems were both narrower than that of the pure ZnO system. By comparison, the band gap widths of the Zn35Mo5+O35 and Zn35Mo6+O35 systems were 4.07 and 3.94 eV, respectively. Although the band gap width widened, under light excitation conditions, the valence band electrons could absorb the photons of lower energy to transition to the impurity energy level, and then transition further from the impurity energy level to the conduction band. During the hierarchical transition, visible light and infrared light effects still occurred. The range of photon energy response of all doping systems expanded, and the absorption spectrum red-shifted in the light-absorption edge within the low-energy (0–3.2 eV) range. In particular, the Zn35Mo6+O35 system exhibited the strongest polarization ability, photoelectric field strength, and charge-binding ability in the low-energy region. Moreover, the dielectric and absorption peaks of the imaginary part of the dielectric function were relatively large, and the red-shift of the absorption spectrum was relatively substantial. Therefore, the Zn35Mo6+O35 system is useful in designing and preparing novel ZnO-based photocatalysts.