Preparation And Propertiesof Sulfur-deficient Sulfide Photocatalysts Active In Visible Light

Recently, semiconductor materials has attraced tremendous attention because oftheir potential applications in the field of energy conversion and pollution treatment.Metal sulfide has become one of the most important semiconductor materials owning to its excellent visible-light response, appropriate bandgap structure, Earth-abundant raw materials, low-cost operation, etc.Nevertheless, metal sulfides, such as Cd S,Zn In₂S₄ and Cd In₂S₄ still suffer obstacleslikethe rapid combination of photoinduced electron-hole pair, small specific surface area and tendentious corrosion in photo-electric reaction, etc. Therefore, it is desirable to find out an effective route improve the activity and stability of this kind of semiconductors.Herein, firstly, Cd S was treated by a two-step method, Na BH₄ solid-phase reduction and surface modification by HF, resulting in the doping of sulfur defects and fluorine ions. Similarly, sulfur-deficient Zn In₂S_4-xand Cd In₂S_4-xwere successfully prepared using the Na BH₄ solid-phase reduction method. It seems that the reduction approach is universal to induce sulfur defects into the matrix of some metal sulfides.To achieve the detailed properties after the modification, XRD, sorption isotherm,Raman, XPS, UV-vis DRS, PL and photoelectrochemistry analysis were carried out to investigate the crystal phase structure, specific surface area, surface chemical states,types and content of sulfur defects, optical absorption property, bandgap structure and separation efficiency of photoinduced carriers on the as-prepared catalysts.And the photocatalytic performance of the samples was systematically determined by the water splitting for hydrogen production under visible light（λ>420nm）.The main research contents and results are as follows:（1） The introduction of sulfur defects decreases the conduction band minimum（CBM） of Cd S, contributing to the narrowed bandgap, enhanced optical absorption,improved separation efficiency of surface photoinduced carriers and increased photoreactivity toward H₂ production over the Cd S-R2. Treated at 275 oC, the Cd S-R2 achieves its maximum photocatalytic performance. The corresponding H₂-evolution rate reachs 4.2 mmol h^-1.g^-1, which is 3.3 times as high as the pristine Cd S. However,the Cd S-R2 encounters the poor photocatalytic stability, and is deactivated slowly after 4 h reaction. In this regard, the unstable performance of the Cd S-R2 can be significantly improved by the modification fluorine ions. Moreover, the introductionof fluorine ions has a positive effect on the efficient separation of the surface photogenerated carriers on the Cd Sx. Compared with the Cd Sx, the F-modified Cd Sx displays an enhanced H₂-production rate of 6.0 mmol?h^-1?g^-1 with an increasment of42.9 % and maintains the high stability after 20 h photocatalytic reaction.（2） After the heat-treatment with Na BH₄, the Zn-S and In-S bonds in the lattice of Zn In₂S₄ was partially destructed, resulting in the generation of a sulfur-defectrelated state, a decrease in the CBM, improved visible-light absorption, a remarkably increased separation efficiencyof photogenerated charge carriers and a consequently enhanced photocatalytic activity. It shows that the optimal H₂-evolution rate of 13.2mmol h^-1.g^-1 was achieved on the Zn In₂S₄ treated at 275 o C, 2.9 times as high as that of the pristine one. In addition, the catalyst exhibits the good photocatalytic stability after 15 h reaction.（3） The bandgap structure of Cd In₂S₄ can also be slightly configured by regulating the content of the sulfur in the bulk by the Na BH₄ solid-phase reduction method. Similar improved photoelectric properties were obtained as mentioned above.The result indicates that the optimum heat-treatment temperature for Cd In₂S_4-xis300℃.The Cd In₂S_4-x displays an enhanced and stable photocatalytic activity toward water splitting. The H₂-production rate over the Cd In₂S_4-x(1.1mmol·h^-1·g^-1) is twice as high as that on the pure Cd In₂S₄, even if the sulfur-deficient sample was evaluated after 18 h photocatalytic reaction.