Preparation, Performance And Catalytic Mechanism Of Bismuth Vanadate Composite Photocatalys

Photo（electro）catalysis（meaning photocatalysis and photoelectrocatalysis）technology based on semiconductor materials can directly convert solar energy into chemical energy with high energy density,which is one of the ideal ways to generate renewable clean energy and wastewater treatment.Among them,the design of efficient and stable semiconductor photo（electro）catalytic materials and the study of the mechanism of photo（electro）catalysis action are the key steps and important foundation to realize this process.In view of this,the main research of this thesis is focused on enhancing the catalytic efficiency of BiVO₄.The cocatalytic mechanism of MXene and nano-Ag is discussed in detail through first principles calculations combined with experimental characterization,and the differences in catalytic efficiency and stability between them are compared,and the specific conclusions obtained are as follows.（1）BiVO₄ nanosheets were prepared by hydrothermal method by using bismuth nitrate pentahydrate and sodium orthovanadate dodecahydrate as raw materials.To further enhance the photocatalytic efficiency of BiVO_4,BiVO₄-Ag composite photocatalysts with different Ag contents were prepared by photodeposition process.With the preferred addition of 40 mg of silver nitrate,BVO-Ag40 achieved 78.3%and85.4%removal of Cr（VI）and MB,respectively,within 120 min,which were 3.48 and1.40 times higher than that of pristine BiVO₄,and the removal rate of pollutants was still maintained above 80%after five cycles.The cocatalytic mechanism of nano-Ag was discussed in detail in conjunction with the trapping agent experiments as well as theoretical calculations.Firstly,the introduction of nano-Ag leads to the appearance of interstitial states in the forbidden band of BiVO₄ and shifts the VBM of BiVO₄-Ag to the Fermi energy level,which leads to the reduction of the band gap value,while the interstitial states can also act as electron acceptors to improve the separation and migration efficiency of photogenerated carriers.Secondly,Schottky heterojunction will be formed at the interface of BiVO₄ and nano-Ag,and the built-in electric field formed by the heterojunction will accelerate the carrier migration.Finally,the LSPR effect of nano-Ag will enhance the light absorption of BiVO₄ and improve the light utilization.（2）BiVO₄-Ag-FeOOH photoanodes were successfully prepared by sequentially compounding nano-Ag particles and FeOOH cocatalysts on the surface of BiVO₄ by photodeposition and in-situ self-hydrolytic deposition,where the photocurrent density and ABPE of BiVO₄-Ag-FeOOH photoanodes in 0.2 M Na₂SO₄ electrolyte are 3.19m A/cm²（1.23 V vs.RHE）and 0.8%,which are 3.18 times（1.01 m A/cm²）and 4.35times（0.19%）higher than that of pristine BiVO₄.The quencher experiments confirm the role of FeOOH in promoting the kinetics of surface water oxidation.The main role of nano-Ag is to form Schottky heterojunctions with the semiconductor and promote carrier separation through the built-in electric field introduced by the Schottky heterojunctions.（3）After calculating the band structure,work function and adsorption energy of Ti₃C₂T_x MXene with different surface functional groups,it is found that Ti₃C₂O₂ and Ti₃C₂（OH）₂ have higher work function and smaller adsorption energy than Ti₃C₂F₂.Therefore,they are more suitable as cocatalysts in photocatalytic synergistic redox reactions.And the surface functional groups of Ti₃C₂T_x MXene can be successfully modified by hydrothermal composite process.The photocatalytic performance test showed that the removal efficiencies of Cr（VI）and MB by BVO-TC9 within 120 min was 83.6%and 88.7%,respectively,which were 3.67 and 2.17 times higher than the photocatalytic efficiency of pristine BiVO₄.The cocatalytic mechanism of MXene can be summarized as follows:The Schottky heterojunction between BiVO₄ and Ti₃C₂T_xMXene promotes the directional separation and migration of photogenerated carriers and avoids recombination.In addition,the 2D-2D stacking structure of BiVO₄ and Ti₃C₂T_x MXene can shorten the carrier migration distance,and Ti₃C₂T_x MXene can enhance the light absorption intensity of BiVO₄ and extend the light absorption range.（4）Cyclic voltammetry deposition was used to form continuous electron transport channels by uniformly dispersing Ti₃C₂T_x MXene nanosheets inside the porous BiVO₄substrate,and FeOOH with oxygen vacancies was introduced by in situ self-hydrolytic deposition to accelerate the hole separation and improve the water oxidation kinetics at the electrode-electrolyte interface.The results show that the constructed BiVO₄-MXene-FeOOH composite photoanode has a current density of up to 4.95 m A/cm² and a negative shift of the starting potential by 250 m V.The synergistic effect of MXene and FeOOH is the main reason for the high efficiency and stability:when BiVO₄ is compounded with MXene,the energy band bending causes a negative shift of the E_CBof BiVO₄ and lowers the starting potential,the built-in electric field generated by the interfacial dipole forces the electrons to transfer to MXene and finally reach the cathode through the conducting framework formed by cyclic voltammetry deposition.At the same time,MXene also strengthens the interfacial bonding of BiVO₄ and FeOOH,thus reducing the resistance to carrier migration.In addition,the O vacancied FeOOH improves the adsorption of H₂O molecules and accelerates the kinetics of water oxidation.