Heterojunction of Zinc Blende/Wurtzite in Zn_1-xCd_xS Solid Solution for Efficient Solar Hydrogen Generation: X-ray Absorption/Diffraction Approaches

In the past decade, inorganic semiconductors have been successfully demonstrated as light absorbers in efficient solar water splitting to generate chemical fuels. Pseudobinary semiconductors Zn_1-xCd_xS (0 ≤ x ≤ 1) have exhibited a superior photocatalytic reactivity of H₂ production from splitting of water by artificial solar irradiation without any metal catalysts. However, most studies had revealed that the extremely high efficiency with an optimal content of Zn_1-xCd_xS solid solution was determined as a result of elevating the conduction band minimum (CBM) and the width of bandgap. In addition to corresponding band structure and bandgap, the local crystal structure should be taken into account as well to determine its photocatalytic performance. Herein, we demonstrated the correlations between the photocatalytic activity and structural properties that were first studied through synchrotron X-ray diffraction and X-ray absorption spectroscopy. The crystal structure transformed from zinc blende to coexisted phases of major zinc blende and minor wurtzite phases at a critical point. The heterojunction formed by coexistence of zinc blende and wurtzite phases in the Zn_1-xCd_xS solid solution can significantly improve the separation and migration of photoinduced electron-hole pairs. Besides, X-ray absorption spectra and UV-vis spectra revealed that the bandgap of the Zn_0.45Cd_0.55S sample extended into the region of visible light because of the incorporation of Cd element in the sample. These results provided a significant progress toward the realization of the photoelectrochemical mechanism in heterojunction between zinc blende and wurtzite phases, which can effectively separate the charge-carriers and further suppress their recombination to enhance the photocatalytic reactivity.