Study On Design Of BiOX-based Novel Photocatalytic Material Systems For Enhanced Catalysis

Semiconductor photocatalysis technology is one of the most promising approaches to address the problems of energy shortage and environmental pollution,which has the advantages of economy,high efficiency and environmental protection.Bismuth-based semiconductor materials have been widely concerned in the field of dealing with organic pollutants and high toxic heavy metal pollution due to the non-toxic,abundant element reserves,excellent photoelectric properties and easy control of the electronic structure.Among them,Bi OX-based materials possess the two-dimensional（2D）layered structure,that the inner electric field along the[001]direction could effectively promote the separation of photoinduced electrons and holes,which is an ideal photocatalyst.However,current research of Bi OX still faces some challenges,such as unclear morphology growth and assembly mechanism,single modification means,complicated construction process of heterostructure,large resistance of heterogeneous interface and single built-in electric field,which greatly restricts their practical application in environmental pollution removal.To address the above problems,three typical Bi OX-based photocatalysts with Sillén structure,Bi OCl,Bi OBr and Bi₄Ta O₈Cl,were chosen as the research targets.Solid solution engineering,rare earth doping,heterojunction design and facet induced selective growth strategy were adopted to optimize the morphology,band structure,dimensional coupling mode,built-in electric field configuration and charge transfer path,so as to construct Bi OX-based novel photocatalytic material systems with excellent visible light photocatalytic oxidation and reduction performance.Lastly,the structure-activity relationship and carrier separation transport enhancement mechanism of each Bi OX-based photocatalytic system is systematically explored to provide some references for the rational design and development of photocatalysts for efficient solar energy conversion.The main research achievements of this dissertation are summarized as follows:（1）The growth and assembly mechanism of Bi OCl_0.5Br_0.5 was investigated by optimizing the morphology via solvothermal method,and a series of 3D Bi OCl_xBr_1-x（x=0-1）solid solutions with large specific surface area and continuously adjustable band gap were synthesized.The results show that the cationic polyacrylamide and cetyltrimethylammonium bromide not only participate in the reaction as chlorine source and bromine source,but also act as the surfactant to adjust the thickness of solid solutions nanosheets,which will self-assembled into flower-like structure along the（110）direction with the assistance of glycol.The special morphology displays rich specific surface area(>50 m²·g^-1),which could provide more reaction active sites to adsorb and remove the target contaminants.The ultra-thinner nanosheet makes photo-generated carriers easier to transport to the surface for photocatalytic reaction,which inhibits the internal recombination of photogenerated electrons and holes.Meanwhile,the appropriate pore structure also helps to accelerate the mass transfer process.Moreover,the controllable ratio of Cl and Br enables the continuous red shift of the solid solutions absorption edge from 360 to 430 nm,and changes the band structure of solid sloutions.The visible light utilization efficiency and the redox ability of Bi OCl_xBr_1-x reach the optimal equilibrium at x=0.5,that its photocatalytic activity for tetracycline（TC）degradation is 35.2 and 3 times higher than pure Bi OCl and Bi OBr respectively,and the reduction rate of Cr（VI）is 13.4 and 4 times of those of Bi OCl and Bi OBr respectively.（2）A series of La³⁺-doped Bi OCl microspheres which contained in-situ deposited Bi nanoparticles were synthesized via a facile one-step solvothermal pathway.The La³⁺doping,oxygen vacancies（OVs）and surface plasmon resonance（SPR）of Bi synergistically enhance the photocatalytic performance of Bi OCl.The Bi nanoparticles obtained by in-situ reduction of Bi OCl make the lattice match between each other,which is conducive to carrier transfer.At the same time,the SPR effect of Bi is demonstrated by surface enhanced Raman scattering,and the introduction of Bi could narrow the band gap of Bi OCl from 3.33 to 2.91 e V,along with the creation of OVs on the surface of Bi OCl.Besides,La³⁺,which matches the radius and charge of Bi³⁺,was selected as the doping ion.La³⁺doping further reduces the band gap to 2.79 e V,increases the concentration of OVs,and improves the visible light absorption.Moreover,La³⁺doping could not only increase the specific surface area and reduce the crystallite size of Bi OCl,but also as the efficient scavenger to trap photoinduced electrons,which is in favour of the separation of photogenerated electrons and holes.When the doping amount of La³⁺is 4.5 mol%,the composite photocatalyst exhibits the optimal visible light catalytic activity for TC degradation and Cr（VI）reduction,which are 35.9 and 40times higher than pure Bi OCl,respectively.（3）A simple and environmentally friendly one-step self-melting salt technique has been developed to prepare 0D Ce O₂ quantum dots/2D Bi OX（X=Cl,Br）nanosheets heterostructures with self-built Ce⁴⁺/Ce³⁺redox centers.Bi（NO₃）₃·5H₂O and Ce（NO₃）₃·6H₂O possess relatively low melting points,which can provide a molten ionic liquid reaction circumstance to make the faster nucleation and growth rate of Bi OX（X=Cl,Br）than Ce O₂.Combined with its high anisotropy,Bi OX（X=Cl,Br）will grow into 2D nanosheets,thus obtaining unique 0D/2D coupling heterojunction via one-step process.The construction of the tight heterojunction improves the visible light absorption ability,and broadens the spectral response range to 550 nm.In addition,the cyclic reactions of Ce⁴⁺/Ce³⁺generated in the heterogeneous interface with photogenerated electrons and O₂ further promotes the separation and transfer of space charges.As expected,the optimal visible light photocatalytic degradation rates for TC in as-prepared heterojunction systems are 11.2 and 3.7 times of Bi OCl and Bi OBr respectively,and the reduction efficiency of Cr（VI）is 14.0 and 5.9 times higher than those of pure Bi OX（X=Cl,Br）respectively,which is also superior to the the conventional 0D Ce O₂/2D Bi OX（X=Cl,Br）heterojunctions prepared by multistep method.This work solves the problem of cumbersome processes for the synthsis of current Bi OX-based heterojunctions.Furthermore,the successful construction of bismuth molybdate or carbon nitride-based heterojunctions via one-step molten salt procedure proves the generality of this strategy.（4）A novel S-O bonded 0D Ag₂S/2D Bi₄Ta O₈Cl oriented p-n heterojunction photocatalyst was designed and synthesized through the facet engineering.The selective growth of p-type Ag₂S nanoparticles on the（100）facet of n-type Bi₄Ta O₈Cl nanosheets was realized by using the surface junction characteristics,that the photoinduced electrons and holes will separately migrate to different facets of single crystal Bi₄Ta O₈Cl nanosheets under light irradiation.The unique double built-in electric field modulation（derived from the synergism of facet junction and p-n heterojunction）delivers powerful driving force for impactful spatial photocarrier separation along the cascade path.More strikingly,experimental and theoretical analyses co-disclose that the interfacial S-O bond could serve as efficient atomic-level interfacial channel,which is conducive to steering and encouraging the vectorial migration of photocharges between Ag₂S and Bi₄Ta O₈Cl.Furthermore,in this novel S-O bonded 0D Ag₂S/2D Bi₄Ta O₈Cl oriented p-n heterostructure system,the photocatalytic oxidation reaction is mainly concentrated on the（001）facet of Bi₄Ta O₈Cl nanosheets and the surface of Ag₂S nano particles,while the photocatalytic reduction reaction is primarily focused on the（100）facet of Bi₄Ta O₈Cl nanosheets,so as to providing separate reduction and oxidation sites.As expected,the visible light photocatalytic degradation rate of TC over the as-prepared S-O bonded 0D Ag₂S/2D Bi₄Ta O₈Cl oriented p-n heterojunction is 49.2 and 2.4 times of Ag₂S and Bi₄Ta O₈Cl respectively,and its photocatalytic reduction activity of Cr（VI）is5.3 and 2.6 times higher than those of Ag₂S and Bi₄Ta O₈Cl respectively,which is also superior to the conventional 0D Ag₂S/2D Bi₄Ta O₈Cl random heterojunction and the mixed Ag₂S/Bi₄Ta O₈Cl heterojunction with only a single built-in electric field.This study solves the drawbacks of high interfacial energy barriers and single built-in electric field in traditional heterojunctions,further accelerating charge separation and transfer dynamics through multiple electric field tuning and interface covalent bond bridging.