Design,Preparation,and Photocatalytic Degradation Activity Of Micro-Nano Structured Biox(X=Br,Cl)-Based Heterojunctions

Semiconductor photocatalysts are capable to convert solar energy into chemical energy to achieve the degradation of pollutants through redox reactions.Photocatalytic technology is considered as one of the most effective methods of solving environmental pollution problems due to its low cost,high efficiency,and green advantages.As a photocatalyst,bismuth halide oxide has attracted much attention because of its special layered crystal structure and its good photocatalytic activity.However,the low utilization of visible light and fast recombination rate of photogenerated carriers of bismuth halide oxide lead to poor photocatalytic activity and thus limit its further application.Improving the photocatalytic performance of bismuth halide oxide by means of element doping,noble metal deposition,and heterojunction construction has become a research hotspot at home and abroad.In this thesis,bismuth halide oxide-based heterostructures are studied.Firstly,bismuth halide oxide is self-assembled into different grades microspheres by using different crystal growth guidance agents with ionic liquid method or solvothermal method.Then,other bismuth-based semiconductors or TiO₂ are compounded on the surface of bismuth halide oxide microspheres through in situ reaction to prepare microspheres materials loaded on nanosheet and nanoparticle bismuth halide oxide-based heterostructures;Or bismuth halide oxide-based heterostructures catalytic materials are prepared by coating the ZnO nanorods,which are synthesized by precipitation method,with chemically adsorb bismuth halide oxide nanosheets.Their phase composition,surface chemical state,microstructure,light absorption properties and photocatalytic properties were investigated by various characterizations.The photocatalytic mechanisms of the bismuth halide-based heterojunctions were revealed based on the density functional theory（DFT）calculations and experiments.The main obtained results were as follows:（1）Study on the Bi₂WO₆/BiOBr heterojunction photocatalysts.Bi₂WO₆ nanosheet/BiOBr microsphere heterojunction photocatalystsare in situ prepared by a two-step solvothermal method bismuth nitrate,sodium tungstate and cetyltrimethylammonium bromide as raw materials.DFT calculations show that the heterojunction interface formed by Bi₂WO₆（010）/BiOBr（001）is stable with an interface-formation energy of-0.36e V.The charges of Br,O,and Bi atoms at the heterojunction interface change and reconfigure forming a built-in electric field between Bi₂WO₆ and BiOBr.After Bi₂WO₆ and BiOBr contacted with each other,their energy band shift and the energy band gap（E_g）is narrowed forming a type II or type Z heterojunction.The experimental results show that E_g of Bi₂WO₆/BiOBr is smaller than that of BiOBr and Bi₂WO₆,which is beneficial to improve the photocatalytic activity;the specific surface area is slightly larger than that of BiOBr and Bi₂WO₆.Under the irradiation of visible light after 120 min,the degradation rates of Rh B,K₂Cr₂O₇and TC catalyzed by Bi₂WO₆/BiOBr heterojunction are 94.4%,79.5%and82.03%,respectively.The degradation rate of tetracycline hydrochloride（TC）by Bi₂WO₆/BiOBr heterojunction is 1.9 times higher than that of Bi₂WO₆ and its main photocatalytic active species are h⁺and·O₂^-.The catalytic activity of Bi₂WO₆/BiOBr photocatalyst only decreased 4.95%after 5 cycles,which is better than that of BiOBr and Bi₂WO₆.The photogenerated carriers of Bi₂WO₆/BiOBr heterojunction migrate through a"Z-scheme"route,which lead to improvement of the photocatalytic performance.（2）Study on the Bi₂WO₆/BiOCl heterojunction photocatalysts.Bi₂WO₆/BiOCl photocatalysts are prepared by a two-step solvothermal method using bismuth nitrate,sodium tungstate,and ammonium chloride as raw materials.DFT calculation results show that the interface forming energy between Bi₂WO₆（010）and BiOCl（001）is-1.46 e V,which is more stable than interfaces of Bi₂WO₆（010）and BiOCl（001）.The charges of Cl,O and Bi atoms at the heterojunction interface change and reconfigure,forming a built-in electric field between Bi₂WO₆and BiOCl,while E_g of the heterojunction is shifted leading to a narrowing of the band gap and form a typeⅠheterojunction.The experimental results show that the prepared Bi₂WO₆/BiOCl heterojunctions are flower-like microspheres with a diameter of about 1μm.When the theoretical molar ratio of Bi₂WO₆ and BiOCl is 1:3.5,E_g of Bi₂WO₆/BiOCl heterojunction is lower than those of BiOCl and Bi₂WO₆ and its specific surface area is much larger than those of BiOCl and Bi₂WO₆.Under the irradiation of visible light,the degradation rates of Rh B and K₂Cr₂O₇ catalyzed by Bi₂WO₆/BiOCl heterojunction are 94.4%and79.5%,respectively,which are higher than those of BiOCl and Bi₂WO₆.After five cycles of experiments,the photocatalytic activities of Bi₂WO₆/BiOCl composite decrease by less than 2.7%,indicating a good stability.It is found that the main active species in the photocatalytic degradation of Rh B by Bi₂WO₆/BiOCl heterojunction is·O₂^-.The photogenerated carriers of Bi₂WO₆/BiOCl heterojunction migrate through a"Z-scheme"route to achieve an enhanced photocatalytic performance.（3）Study on the Bi VO₄/BiOBr heterojunction photocatalysts.Bi VO₄/BiOBr heterojunction photocatalysts were prepared by a solvothermal-hydrothermal method using bismuth nitrate,ammonium metavanadate,and cetyltrimethylammonium bromide as raw materials.DFT calculation results show that the interface between Bi VO₄（001）and BiOBr（001）is stable with interface-forming energy of-0.90 e V.The charges of V,O,and Bi atoms at the heterojunction interface change and reconfigure,forming a built-in electric field of Bi₂WO₆ region pointing to the BiOBr region,while the energy band of heterojunction is shifted to form a type II or type Z heterojunction.The experimental results show that the E_g of the Bi VO₄/BiOBr heterojunction is between that of Bi VO₄ and BiOBr,and its specific surface area is larger than those of BiOBr and Bi VO₄.The degradation rates of rhodamine B（Rh B）,potassium dichromate（K₂Cr₂O₇）,and TC by Bi VO₄/BiOBr photocatalysts are 97.80%,92.70%,and 56.20%,respectively,which were much higher than that of Bi VO₄and in general agreement with BiOBr,but their photocatalytic rate constants were higher than that of BiOBr.The photocatalytic stability of Bi VO₄/BiOBr heterojunction is better than those of BiOBr and Bi VO₄.The photogenerated carriers of Bi₂WO₆/BiOBr heterojunction migrate through a"II"route,thus achieving the improvement of photocatalytic performance.（4）Study on the ZnO/BiOBr heterojunction photocatalysts.ZnO/BiOBr heterojunction photocatalysts are prepared by chemical precipitation-solvothermal method using bismuth nitrate,zinc acetate,and cetyltrimethylammonium bromide as raw materials.ZnO nanorods were first synthesized by chemical precipitation method and then BiOBr nanosheets were wrapped onto ZnO nanorods by solvothermal method.The experimental results show that E_g and specific surface area of ZnO/BiOBr heterojunction are between those of the pure component ZnO and BiOBr.When the theoretical molar ratio of ZnO and BiOBr is 1.25:1,the degradation rates of Rh B,K₂Cr₂O₇,and TC catalyzed by ZnO/BiOBr heterojunction are 97.92%,59.95%,and 90.81%respectively,which are higher than that of pure BiOBr and ZnO.Furthermore,the photocatalytic stability of ZnO/BiOBr heterojunction is also better than that of pure BiOBr and ZnO.It is found that the main active species for the photocatalytic degradation of Rh B and K₂Cr₂O₇ by ZnO/BiOBr heterojunction is h⁺,while the main active substance for the degradation of TC is·O₂^-.The photogenerated carriers of ZnO/BiOBr heterojunction migrate through a"Z-scheme"route.（5）Study on the TiO₂/BiOBr heterojunction photocatalysts.TiO₂/BiOBr heterojunction photocatalysts are prepared using bismuth nitrate,Ti（SO₄）₂,d-glucose,and cetyl-3-methylimidazolium bromide salt as raw materials through a two-step solvothermal-hydrothermal method.The experimental results show that the prepared TiO₂/BiOBr heterojunctions are flower-like microspheres with a diameter of about 2μm.When the theoretical molar ratio of TiO₂ and BiOBr is 1.5:1,E_g of the synthesized TiO₂/BiOBr heterojunction photocatalysts is smaller than that of the pure TiO₂ and BiOBr,and its specific surface area is between that of the pure TiO₂ and BiOBr.Under the irradiation of visible light,the degradation rates of Rh B,K₂Cr₂O₇ and TC catalyzed by TiO₂/BiOBr heterojunction are 96.2%,74.3%and 77.9%,respectively,which are higher than that of the pure BiOBr and TiO₂.It is found that the main active species in the photocatalytic degradation of Rh B,K₂Cr₂O₇and TC by TiO₂/BiOBr heterojunction is·O₂^-and the second one is h⁺.The photogenerated carriers of TiO₂/BiOBr heterojunction migrate through a"Z-scheme"route,thus achieving improved photocatalytic performance.（6）Among the prepared bismuth halide oxide based heterojunction photocatalysts,Bi₂WO₆/BiOBr,TiO₂/BiOBr,ZnO/BiOBr and Bi₂WO₆/BiOCl heterojunction form Z-type heterojunction,and Bi VO₄/BiOBr is II type heterojunction.Ignoring the factor of illumination time,ZnO/BiOBr heterojunction has the best degradation effect on Rh B,while TiO₂/BiOBr heterojunction has better degradation rate on K₂Cr₂O₇ than ZnO/BiOBr heterojunction;Bi₂WO₆/BiOBr heterojunction and Bi VO₄/BiOBr heterojunction have the highest degradation efficiency for K₂Cr₂O₇ and TC respectively,which is superior to Bi₂WO₆/BiOCl heterojunction.