Construction And Photocatalytic Properties Of M_xMo_1-xS₂ And Its Heterojunction

With the deepening of the world industrialization process,environmental pollution is becoming more and more serious,especially in the water area.It is one of the green and pollution-free methods widely used at present to deoxidize pollutants by photocatalysis technology which produces active groups when semiconductor materials are irradiated in water and is used for oxidation-reduction reaction.Mo S₂is widely used in the field of photocatalysis due to its excellent physicochemical properties.However,the lower forbidden band width of Mo S₂and the fast compounding of photogenerated carriers and other defects lead to unsatisfactory photocatalytic effects.In order to improve the deficiency of Mo S₂material,this study firstly improved the defects of rapid photogenerated carrier recombination by doping transition metals（Co and Ni）,and improved the photocatalytic efficiency of Mo S₂.Secondly,the heterojunction composite was constructed with Bi OI and Bi₂WO₆to further improve the photocatalytic performance.The composition,morphology and size of the photocatalyst were analyzed by XRD,SEM and TEM.UV Vis DRS,PL and EIS methods were used to study the optical absorption capacity,photogenerated carrier recombination efficiency and interfacial electron migration rate of the composites.The photocatalytic performance of the catalyst was observed according to the dye degradation experiment.The active groups in the photocatalytic reaction were analyzed by free radical capture test.The electronic properties of the materials were calculated and analyzed by first principles.Finally,the mechanism of photocatalysis was analyzed by combining the experimental results and calculation results.M_xMo_1-xS₂（M=Co,Ni）photocatalyst was prepared by one-step hydrothermal method.XRD results are consistent with Mo S₂when the doping ratio is lower than 15%.Doping reduces the photogenerated carrier recombination efficiency of Mo S₂and improves the electron migration rate and visible light utilization efficiency.The prepared M_0.05Mo_0.95S₂（M=Co,Ni）had excellent degradation effect on the dye solution.At 240 min,the photocatalytic degradation efficiency of Co_0.05Mo_0.95S₂and Ni_0.05Mo_0.95S₂on 150 mg/L Rh B was76.22%and 91.15%,respectively.The reaction rate constants of 0.00596 min^-1and 0.01010 min^-1are 8.76 times and 14.85 times of Mo S₂,respectively.The degradation efficiency of Ni_0.05Mo_0.95S₂is 1.69 times of that of Co_0.05Mo_0.95S₂.The active groups in the degradation process are holes（h+）.The physical and chemical properties of M_0.05Mo_0.95S₂（M=Co,Ni）remained stable after the cycle test.Constructing intrinsic and doping models based on density generalized function theory.The first-principles calculation shows that Co and Ni doping can produce impurity levels in the band structure of Mo S₂,and the impurity levels reduce the band gap by affecting the conduction band minimum and valance band maximum.Doping will disturb the uniform charge distribution in Mo S₂,form a local potential difference in the semiconductor,and promote carrier transfer.The work function calculation results show that the work function of doping system decreases,and the work function of Ni doping system is the lowest among the three systems,and the lower work function is beneficial to the photogenerated carrier transfer inside the semiconductor,which is beneficial to improve the photocatalytic efficiency.BiOI/Ni_0.05Mo_0.95S₂and Bi₂WO₆/Ni_0.05Mo_0.95S₂photocatalysts were prepared by microwave-assisted method and multistage hydrothermal method,respectively.After the heterojunction structure is formed,the optical absorption efficiency of the composites is increased,among which 4%Bi₂WO₆/Ni_0.05Mo_0.95S₂has the best optical absorption effect,and the photogenerated carrier recombination rate and band gap are the lowest.The photocatalytic degradation efficiency of Rh B was improved by both composites.At 300 min,the decolorization rate of Rh B reached95.08%by 5%Bi OI/Ni_0.05Mo_0.95S₂,and the reaction rate constant k=0.01037min^-1.It was 1.095 times of Ni_0.05Mo_0.95S₂(k=0.00947 min^-1).The decolorization rate of 4%Bi2WO6/Ni_0.05Mo_0.95S₂reached 98.65%at 300 min,and the reaction rate constant k=0.01605 min^-1was 1.69 times that of Ni_0.05Mo_0.95S₂(k=0.00947min^-1).During the degradation of 5%Bi OI/Ni_0.05Mo_0.95S₂and 4%Bi₂WO₆/Ni_0.05Mo_0.95S₂,h⁺,·OH,O₂^-·all participated in the reaction,but the main active group was h+.The physicochemical properties of the composites are stable after cyclic test.In addition,from the first-principles calculation results of the heterogeneous structure models of Bi OI,Bi OI（001）,Bi₂WO₆,Bi₂WO₆（001）,Ni_0.05Mo_0.95S₂,Bi OI/Ni_0.05Mo_0.95S₂and Bi₂WO₆/Ni_0.05Mo_0.95S₂.The decrease of the forbidden band width of the heterogeneous structure and the increase of the visible light utilization efficiency due to the phenomenon of energy level hybridization between the composites.Due to the work function difference between Bi OI（Bi₂WO₆）and Ni_0.05Mo_0.95S₂,the electron migration between the heterostructural interfaces makes the heterostructural charge transfer follow the Z-scheme mechanism,and finally the internal electric field is formed between the interfaces,which effectively promotes the photogenerated carrier separation and improves the photocatalytic efficiency.Comprehensive experiments and first-principles results show that the Z-scheme heterojunction formed by Bi OI（Bi₂WO₆）and Ni_0.05Mo_0.95S₂can effectively highlight the photocatalytic efficiency.