Designed Synthesis And Photocatalytic Performance Of Mesoporous Bismuth-based Oxide Materials

Bismuth-based oxide materials with unique electronic structure,suitable bandgap width,easy synthesis,low cost and abundant raw materials have attracted extensive attention in recent years,and have been widely used in the field of photocatalysis and photocatalysis with great application prospects.However,the spontaneous recombination of photogenerated electron-hole pairs greatly limits the improvement of photoconversion efficiency and photocatalytic performance.Mesoporous materials have high specific surface area,large pore volume,controllable pore structure and size,and abundant adsorption and active sites,which are conducive to promoting the separation of photogenerated carriers and enhancing the mass transfer efficiency,significantly improving their performance in the fields of energy storage and conversion,adsorption separation,biomedicine,especially photocatalysis.Therefore,the design and synthesis of bismuth-based oxide materials with mesoporous structures and the development of suitable synthesis methodology are of great significance for the development and application expansion of mesoporous materials,as well as the improvement of catalytic and energy storage properties of bismuth-based oxide materials.However,at present,there are relatively few relevant research reports,and the applicable synthesis strategies are also very limited.The main reasons may be based on the following:（1）The extremely easy hydrolysis of Bi³⁺precursors leads to their fast hydrolytic condensation rates,which is difficult to control and easily leads to the appearance of phase separation;（2）The uncontrollable hydrolytic condensation rate of Bi³⁺precursors also easily leads to the weakening of the interfacial interaction force between them and traditional mesoporous templates,which is difficult to realize self-assembly and form mesoscopic structure;（3）The strong growth orientation of bismuth-based oxide materials generally destroys the existing mesoscopic structure during the crystallization process,leading to the collapse of mesoporous structure;（4）The common surfactant templates are usually pyrolyzed at temperatures below 350°C,which are rapidly decomposed during the high temperature crystallization process and cannot support the pore channel,thus inevitably damage to the pore structure.Therefore,based on the above problems,a series of related works have been carried out in this thesis focusing on the design,preparation and synthesis methodology development of mesoporous bismuth-based oxide materials.Various kinds of mesoporous bismuth-based oxides have been synthesized,and the specific surface area,pore structure and size,defect types,composition and crystal structure of the materials have been systematically regulated and studied.The appropriate photocatalytic reaction was selected according to its structural properties,and the structure-activity relationship between the physicochemical properties and catalytic properties of the materials was analyzed and clarified.1.We propose a micellar interface modulation self-assembly strategy for the designand synthesis of a series of mesoporous bismuth oxychloride-based materials with tunable composition and stoichiometric ratios,and the controlled tuning of Bi₂O₃ to Bi₁₂O₁₇Cl₂,Bi₁₂O₁₇Cl₂/Bi OCl and Bi OCl with large specific surface area and mesoporous structure by regulating the amount of CTAC.In this strategy,the introduction of nitric acid effectively retards the hydrolysis rate of Bi³⁺precursors.Pluronic P123 and cetyltrimethylammonium chloride（CTAC）form composite micelles driven by solvent polarity to act as mesoporous templates,which are regarded as S⁺（relying on quaternary ammonium cations and PEO moieties to hydrated H⁺via hydrogen bonding to provide positive charge）.Thus,the mesoporous template assembles with Bi³⁺precursors through the Coulomb force of S⁺X^-I⁺interaction.Moreover,the positively charged composite micelles are able to induce and modulate the Cl^-ions to involve in the slow hydrolysis and condensation of Bi³⁺precursors at the hydrophilic-hydrophobic interface of the composite micelles to form mesoscopic structures.The mesoporous heterojunction Bi₁₂O₁₇Cl₂/Bi OCl-O_V photocatalyst obtained by UV light-induced approach not only exhibit well-defined mesoporous structure and close contact heterogeneous structure,but also possess abundant surface oxygen vacancies,which can efficiently separate photogenerated electron-hole pairs,enhance mass transfer of reactants as well as provide plenty of adsorption and active sites.Thanks to the unique structural properties of the photocatalyst,great activity was demonstrated in the photocatalytic degradation of pharmaceuticals（83%degradation efficiency for 10 mg/L carbamazepine within 30 min and 99%degradation efficiency for 10 mg/L ciprofloxacin within 10 min）,and the mechanism and possible pathways of photocatalytic degradation were analyzed in depth.2.Based on the above work,we further designed a template-free chelating ionexchange strategy to realize the construction of Bi OCl ultrathin nanosheet materials with both mesoporous structure and single-crystalline properties to facilitate the separation of photogenerated carriers by a template-free chelated ion-exchange strategy.In this strategy,acetic acid and ammonia are used as chelating agent and ionization promoter,respectively,to generate a large number of acetate ions to chelate Bi³⁺precursors to regulate their hydrolytic condensation kinetics and control the nucleation rate and growth direction of the products,which contribute to the formation of ultra-thin structure.In addition,the strong chelating effect of acetate ions on Bi³⁺precursors also allow the introduction of acetate ions into the solid products during the hydrothermal process and sufficient ion exchange with Cl^-as the reaction proceeded,which causes large lattice mismatch and leads to the release of strain,resulting in void-like mesopores and the eventual formation of mesoporous single-crystalline Bi OCl nanosheets.Moreover,the prepared sample exhibits excellent photocatalytic conversion（99%）,selectivity（98%）and stability as a catalyst for the photocatalytic selective oxidation of aromatic alcohols.Various characterization results and density functional theory calculations confirm that the two-dimensional ultra-thin sheet structure,high-density mesopores,abundant oxygen vacancies,and free grain boundary of single-crystalline structure together contribute to the significant improvement in catalytic performance.3.In order to shorten the charge carrier transfer path in bulk single-crystal bismuth-based oxide materials and achieve efficient separation of photogenerated electron-hole pairs,we propose a polymer-confined regulation self-assembly strategy to fabricate three-dimensional mesoporous single-crystalline Bi VO₄ materials using the cationic polymer polyethyleneimine（PEI）as a porogenic agent.The acetate ions,as an“adhesion agent”,not only effectively modulate and modify the hydrolytic condensation kinetics of various metal precursors,but also interact with the metal precursors and PEI through ligand bonding and hydrogen bonding,respectively,to realize the co-assembly of metal precursors and PEI to form mesoscopic structures.In this process,the regulation of confinement effect of metal oligomers was achieved by adjusting the molecular weight M_w of PEI（300-70,000）,which enables the controlled synthesis of a series of mesoporous single-crystalline Bi VO₄ materials with tunable pore structure and pore volume.Moreover,the strategy is also highly universal,enabling the synthesis of a wide range of mesoporous bismuth-based oxide materials（Bi₂Mo O₆ and Bi₂WO₆）.The prepared Meso-Bi VO₄-1800 photocatalysts exhibit excellent catalytic selective aromatic alcohols oxidation activity（99%conversion,99%selectivity and high stability）,mainly attributed to the three-dimensional mesoporous structure,which provides plenty of adsorption sites and enhances the mass transfer efficiency of the catalytic reaction,and moreover,the single-crystalline structure and abundant vanadium defects,which contribute to broaden the light absorption and response range,and promote the separation of photogenerated carriers.