Preparation And Interaction Mechanism Of Indium Compound Modified ZnO-based Photocatalysts

Photocatalytic technology utilizes solar light to convert light energy into efficient chemical energy through three procedures:catalyst light absorption,carrier excitation and transport to the surface for catalytic reaction,which has obvious advantages in solving environmental pollution and hydrogen energy generation.Zinc oxide was considered to be a promising photocatalyst due to the low price and non-toxicity.However,ZnO only exhibits strong absorption in the UV region and photogenerated carriers easily recombine,thus limiting its efficient application.Based on the excellent electrical conductivity of In and the indium compounds（In（OH）₃,In OOH and In₂O₃）can form a heterojunction interfacial electric field with ZnO to effectively prevent carrier recombination.In this paper,the synthesis of ZnO-based highly active photocatalysts with indium compounds was achieved through morphological modulation,structural design and chemical combination strategies,as well as revealing the photocatalytic mechanism through theoretical calculations and experimental validation,which provides new ideas for the construction of diverse photocatalytic heterojunctions and beneficial to promote the further development of photocatalytic technology.The main research results included:（1）The fabrication and mechanism investigation on multiple morphological ZnO/In（OH）₃ composite photocatalysts.The results show that hexagonal,hexagonal pyramidal and spindle-shaped ZnO/In（OH）₃ composites were prepared by simple hydrothermal temperature adjustment.In which,the photodegradation efficiency for methylene blue by the spindle-shaped ZnO/In（OH）₃ was 82%,which was higher than that of the hexagonal pyramidal（73%）,hexagonal composite（59%）and pure ZnO（23.1%）.The heterojunctions formed by the high exposure（01（?）0）facets and the retained polar tip（0001）and（000（?））facets of spindle-shaped ZnO associated with the cubic phase In（OH）₃ were the most preferred.The excellent photocatalytic performance of the composites was mainly due to the introduction of In（OH）₃ to form Type-II electron transport paths at the interface of ZnO for easy separation of photoexcited electrons and holes.（2）The preparation and mechanism study on the yolk-shell structured photocatalyst assembled by ZnO/In₂O₃ heterojunction.The results show that the visible light hydrogen production efficiency of the composite catalyst was 240.41μmol·g^-1·h^-1in the triethanolamine sacrificial agent system,which was 26.7 times higher than that of ZnO alone.The yolk-shell structure obtained using the Ostwald ripening effect achieves an effective contact of heterojunction components,as well as the ability to strengthen the utilization of multiple reflections of incident light.The photocatalytic mechanism shows that a Z-scheme charge transfer mechanism exists between ZnO and In₂O₃.The energy band of ZnO bends downward after contact with In₂O₃,and the corresponding In₂O₃energy band bends upward,so that the e^-in the ZnO conduction band migrates to the In₂O₃ valence band to recombine with h⁺,making the electron reduction occurred in the In₂O₃ conduction band area.（3）The preparation and mechanism of ZnO quantum dots and In₂O₃ nanofiber composite catalysts were studied.The results showed that the efficiency of visible light hydrogen production was 355.26μmol·g^-1·h^-1,and the photocatalytic reduction of heavy metal Cr（VI）reached 92%.More importantly,ZnO QDs can use the fluorescence burst effect to self-detect the Cr（VI）concentration,which overcomes the complicated process of the traditional chromogenic spectroscopic method.On the basis of heterojunction differential charge density calculations,Z-scheme and Type-II heterojunctions coexist between ZnO and In₂O₃,in which the Z-scheme charge transport pathway was dominant.the excited electrons in the conduction band of In₂O₃ can be transferred to the conduction band of ZnO（Type-II）,meanwhile the ZnO conduction band electrons can migrate to the valence band of In₂O₃ driven by the interfacial electric field（Z-scheme）,The photocatalytic electron reduction reaction mainly happens on the In₂O₃ surface where the conduction band potential was high.（4）The preparation and mechanism of the visible-light responsive In₂O₃（ZnO）_msolid-solution catalysts were investigated.Results showed that the In₂O₃（ZnO）_m（m=3,4,5 and 7）catalysts all exhibited good visible light hydrogen production efficiency,especially the In₂O₃（ZnO）₄ sample with a high hydrogen production capacity of 818.71μmol·g^-1·h^-1.The surface-responsive hydrogen production experiments showed that the triethanolamine sacrificial agent（TEOA）had the greatest effect on the efficiency,and the relationship between TEOA concentration and hydrogen yield was in accordance with the Langmuir-Hinshelwood（L-H）kinetic model.The good hydrogen production effect of this solid solution originates from the layered structure of In₂O₃（ZnO）_m to accelerate the transfer of photogenerated carriers along the In₂O₃ plate to the surface reaction,and the first principle calculation reveals that there exist stable H adsorption sites on the Zn atoms of In₂O₃（ZnO）_m,where the charge transfer between Zn and O can realize the hydrogen escape.