Modification And Properties Of Bismuth-based Two-dimensional Photocatalytic Materials

Photocatalytic technology can transform green and sustainable solar energy into high value-added chemical energy,which has great potenti al in solving the problems of energy shortage and ecological pollution.In the past few decades,the development of highly active photocatalysts with wide spectral response has attracted great interest.Bismuth based two-dimensional photocatalysis materials present the characteristics of visible light response,easy modification and preparation,diverse morphology and environmental friendliness.It shows good application prospects in the fields of photolysis of water,CO₂ conversion,nitrogen fixation and organic synthesis.However,due to the limited visible light response range and the easy recombination of photogenerated carriers,the single component bismuth based photocatalysts exhibit low quantum efficiency.In this paper,Bi₂Mo O₆ and Bi OI were selected as the research objects to improve the optical response range and photo-generated carriers separation efficiency.The effects of band engineering,microstructure and surface defect states on the electronic structure of two-dimensional semiconductor materials were studied by element doping,heterostructure construction,morphology design and surface modification.The control mechanism of effective charge concentration,electron hole separation efficiency and photocatalysis performance were studied.The spe cific research work is as follows:(1)One-step hydrothermal method was used to prepare defect-rich indium-doped Bi₂Mo O₆ photocatalyst.The results show that as the amount of indium doped increases,the Bi₂Mo O₆ crystal structure was distorted and oxygen va cancy defects were formed.When In^(3-x)+replaced the Bi site,a p-type doping effect was generated and an acceptor energy level was formed above the valence band,which moves the edge of the valence band upward,resulting in a narrower forbidden band widt h and expanding the optical response range of Bi₂Mo O₆.In addition,low-cost indium doping can effectively improve the photo-generated carrier separation efficiency and reduce the carrier transmission impedance(interface transmission impedance was reduced by 92.97%).After 30 minutes of visible light irradiation,the photo-reduction efficiency of In_0.15-Bi₂Mo O₆ reached 98%,and the apparent reaction rate was 0.0626 min^-1.Compared with the pure Bi₂Mo O₆ sample,the performance of In_0.15-Bi₂Mo O₆ for photocatalytic reduction of Cr(VI)was improved by 13.94 times.(2)The In₂O_3-x(OH)_y in-situ modified In₂O_3-x(OH)_y/Bi₂Mo O₆ two-dimensional S-scheme heterojunction can be prepared by controlling the dehydroxylation of indium precursor with lowering the heat treatment temperature.The results show that In ₂O_3-x(OH)_y was deposited on the surface of Bi₂Mo O₆ to form a two-dimensional heterojunction.The surface state formed by the residual hydroxyl group made the shoulder band absorption in the range of 500-700 nm,which expands the visible light absorption range and optimizes the dynamic process of interface charge transfer.Theoretical calculation and active radical characterization results show that charge transfer in the prepared heterojunction follows the S-scheme mechanism,which improves the redox ability of In₂O_3-x(OH)_y/Bi₂Mo O₆ heterojunction.Under visible light irradiation,the performance of 10wt%In₂O_3-x(OH)_y/Bi₂Mo O₆ photocatalyst for Cr(VI)reduction was 4.1 and 3.2 times of pure Bi₂Mo O₆ and In₂O_3-x(OH)_y,respectively.And the photocatalytic degradation of RHB was 2.7 and 3.7 times higher than that of Bi₂Mo O₆ and In₂O_3-x(OH)_y,respectively.Two-dimensional S-scheme heterojunction displays excellent performance in photocatalytic reduction of Cr(VI)and degradat ion of organic pollutants.(3)Porous Bi₅O₇I nanosheets were constructed by I element removal process,which improved the in-plane carrier separation efficiency of two-dimensional materials.The experimental results show that the proportion of bismuth nitr ate can regulate the oxidation environment of hydrothermal reaction conditions,so that some iodine ions and bismuth ions were oxidized.In the process of heat treatment,high valence bismuth with strong oxidation can further oxidize iodine ion to form iod ine element and sublimate,thus realizing pore forming effect not.The results show that the charge transfer impedance of the two-dimensional porous structure is smaller and recombination probability of the in-plane charge carriers was reduced.The photocu rrent density of the samples annealed at 450 ^oC was 23 times higher than that of pure Bi OI nanosheets and 1.53 times higher than that of the samples annealed at 500 ^oC.After four cycles,the photocatalytic degradation performance of Rhodamine B was mainta ined above 93%,which showed good stability of prepared photocatalyst.(4)Ag₂O/Bi₅O₇I photocatalyst was prepared by in-situ photodeposition of Ag₂O.The results show that the deposition of p-type Ag₂O not only expands the photoresponse range of the composite system,but also optimizes the separation efficiency of photogenerated electron-hole pairs,which makes the photogenerated electrons quickly consumed through the porous structure,reducing the reduction probability of Ag ₂O by photoelectrons,and improves the stability of the composite system.The unique porous structure of two-dimensional Bi₅O₇I substrate can provide a large number of reactive sites,accelerate the reaction and mass transfer process,so that the charge on the two-dimensional substrate is quickly consumed,thus reducing the electrostatic impedance of photogenerated charge in the cross-interface transmission process.The results of photoelectrochemical experiments and Bisphenol A degradation experiment showed that 25%Ag₂O/Bi₅O₇I had the best photocatalytic degradation effect on BPA under visible light irradiation,and the apparent reaction rate was 0.082 min^-1,which was 7.02and 4.21 times higher than that of pure Ag₂O and Bi₅O₇I,respectively.After four cycles,no Ag was detected and the degradation rate of bisphenol A was more than 90%after 1h visible light irradiation.The intermediates in the degradation process of bisphenol A were analyzed by liquid chromatography-mass spectrometry and two reasonable photodegradation pathways,demethylation and?-cleavage,were deduced.