ZnCdO2 monolayer - A complex 2D structure of ZnO and CdO monolayers for photocatalytic water splitting driven by visible-light.

Abstract

ZnO monolayer possesses band structure matching the conditions of water splitting for hydrogen generation but cannot well response to the visible light, while CdO one, contrariwise, have obvious optical absorption in the visible light range but no satisfactory band edges for the water splitting to produce hydrogen. Here, we predict a two-dimensional ZnCdO2 structure comprising of ZnO and CdO ones to achieve their strengths. The band structures, optical properties, carrier mobility, and the strain engineering for ZnCdO2, ZnO and CdO monolayers are investigated by using the first-principles hybridization functional calculations. The results demonstrate that the two-dimensional ZnCdO2 structure is a promising candidate for water splitting to produce hydrogen. All the structures show a direct band energy gap and the character remains unchanged under the considered biaxial strains. All the conduction band minimums are suitable for water splitting reaction even under the -4% to +4% strain. Moreover, the valence band maximum of ZnCdO2 monolayer matches the conditions of the water-splitting reaction under the -2% to +4% strain. Interestingly, the unsatisfactory valence band maximum of CdO monolayer can be overcome by strain larger than +2%. As expected, the enhanced optical absorption in the visible light range is observed for the ZnCdO2 monolayer. Additionally, the mobilities of the hole and the electron are significantly different for the three monolayers, implying that the low recombination ratio of the photogenerated carrier pairs is available, which is also beneficial for the photocatalytic performance. Therefore, ZnCdO2 monolayer and CdO monolayer (with tensile strain larger than 2%) is a promising candidate for the water splitting to produce hydrogen under the irradiation of the solar light.