Construction Of Novel Heterojunctions For Visible Light Photocatalysis

Photocatalysis is an advanced technology that converts inexhaustible solar energy into storable chemical energy,which can also be used for pollutant degradation.In the photocatalytic process,the photocatalyst plays a decisive role.Single photocatalysts generally have disadvantages such as low light absorption efficiency,high electron-hole pair complexation rate and weak redox ability,which limit their applications.Among various modification strategies for catalysts,constructing heterojunctions by compounding two or more semiconductors can not only enhance the light absorption range of catalysts,but also reduce the efficiency of photogenerated electron-hole complexes,thus enhancing photocatalytic efficiency.However,the construction of an efficient heterojunction not only needs to meet the above requirements,but also needs to preserve the redox ability of photogenerated electrons and holes in the heterojunction catalytic system.Therefore,the design of efficient heterojunction systems remains a hot topic of current research.In this paper,a series of heterojunction catalysts with excellent redox ability were constructed by modulating the energy bands of semiconductor materials and their structures,and good photocatalytic performance was obtained,and their catalytic mechanisms were investigated in detail.The details of the study are as follows:（1）The S-doped g-C₃N₄/ZnIn₂S₄S-scheme heterojunction catalytic system was constructed to achieve efficient hydrogen production by visible light-driven photocatalysis.By modulating the energy band structure of g-C₃N₄through nonmetallic S doping,the heterojunction type of ZnIn₂S₄/g-C₃N₄is changed from I-type to S-scheme heterojunction,which overcomes the drawback that I-type heterojunction cannot effectively separate photogenerated electron-hole pairs,while the composite retains good redox ability.The prepared heterojunctions have dense nanoflower structures,which can effectively promote the interfacial transfer of charges.ZnIn₂S₄/S-doped g-C₃N₄exhibited excellent photocatalytic hydrogen production performance under visible light,and its hydrogen production rate(9.28 mmol h^-1g^-1)was higher than that of g-C₃N₄(2.35 mmol h^-1g^-1),ZnIn₂S₄(2.99 mmol h^-1g^-1)and g-C₃N₄/ZnIn₂S₄(4.96mmol h^-1g^-1)by a factor of 3.9,3.1 and 1.9.In addition,the heterojunction can achieve efficient visible light degradation of tetracycline（degradation rate of 96%）and total water resolution.It is shown that the electrons in the contact between ZnIn₂S₄and S-doped g-C₃N₄in the composite are spontaneously rearranged and the energy band edges are bent,forming an interfacial electric field with ZnIn₂S₄pointing toward S-doped g-C₃N₄,and the electrons in the conduction band of S-doped g-C₃N₄under light are driven by the electric field to compound with the holes in the valence band of ZnIn₂S₄,thus forming an S-scheme heterojunction..（2）Based on Bi_m+1Fe_m-3Ti₃O_3m+3（m=4,5,6）,S-scheme heterojunctions with two-dimensional nanosheet structures were constructed to achieve efficient degradation of tetracycline.Bi_m+1Fe_m-3Ti₃O_3m+3（m=4,5,6）with nanosheet structures were synthesized by a simple hydrothermal method with in situ growth of g-C₃N₄nanosheets on the surface to form 2D/2D S-scheme heterojunctions.The degradation rates of g-C₃N₄/Bi_m+1Fe_m-3Ti₃O_3m+3（m=4,5,6）were 86%（m=4）,82%（m=5）and 81%（m=6）for tetracycline under visible light for 2h,respectively.The main reason for the difference in performance is that the difference in the number of layers m leads to a difference in the spontaneous polarization intensity of Bi_m+1Fe_m-3Ti₃O_3m+3（m=4,5,6）and therefore to a difference in its ability to separate photogenerated carriers.By studying g-C₃N₄/Bi_m+1Fe_m-3Ti₃O_3m+3（m=4）,it can be found that the visible photodegradation rate of tetracycline in the composites is significantly enhanced compared to g-C₃N₄and Bi_m+1Fe_m-3Ti₃O_3m+3（m=4）,which is due to the formation of S-scheme heterojunctions that provide the materials with stronger redox and carrier separation capabilities,thus promoting the efficient generation of reactive radicals.The visible photodegradation performance of g-C₃N₄/Bi_m+1Fe_m-3Ti₃O_3m+3（m=4）for tetracycline was maximized by p H adjustment at p H=5（2h,92%）.（3）CoP/ZnIn₂S₄Schottky heterojunction nanorods were constructed by in situ growth to achieve efficient visible photocatalytic hydrogen production.The CoP is self-assembled from nanowire-structured precursors,which exhibit a sparse hexagonal rod-like structure.ZnIn₂S₄grows uniformly and tightly on the CoP surface,giving it a larger contact area with CoP and avoiding possible charge blockage during interfacial charge transfer.As evidenced by BET and pore size analysis,CoP/ZnIn₂S₄is rich in mesopores and has a larger specific surface area,which provides more active sites for photocatalytic reactions.The formation of Schottky heterojunctions also greatly reduces the complexation of photogenerated electron-hole pairs.Together,the above results lead to a 14.3-fold improvement in the photocatalytic hydrogen production performance of CoP/ZnIn₂S₄(11.70 mmol h^-1g^-1)compared to ZnIn₂S₄(0.82 mmol h^-1g^-1).