Laser Irradiation Induced Structural Modification Of Bismuth Oxyhalide In Liquid And Its Catalytic Applications

Bismuth oxyhalide(BiOX,X=C1,Br,I)has been widely used in photocatalytic applications,such as degradation of organic pollutants and water splitting,owing to its high catalytic activity,environment-friendly and low-cost characteristics.However,BiOX prepared by traditional methods presents some drawbacks,such as large sizes,insufficient specific surface area,and difficulty in effective separation of photogenerated carriers.In addition,the band position of BiOX could not meet the potential requirements of various photocatalytic reactions.Therefore,it is significant to modify the structure of BiOX in terms of size,morphology,exposed facets,and defects,so as to change its electronic and band structure for improving its photocatalytic activity.Among various structural modification methods,Laser in Liquids technique has the locally extreme thermodynamic conditions of high temperature and high pressure and the non-equilibrium dynamic process of rapid quenching.It shows unique technical characteristics in the preparation of highly active metastable colloidal nanoparticles and the reconstruction of nanocrystalline structure.Meanwhile,the technique can also realize the introduction of defects and the alloying reaction.In view of the above analysis,this dissertation explored new method of laser in liquid technique for BiOX structural reconstruction,defect introduction and heteroatom doping,and investigated their phtocatalytic performance.Owing to the semiconductor properties of BiOX,we designed catalysts of BiOX and Fe-doped BiOCl nanosheets loaded with Pd nanoparticles by photoreduction method,and evaluated their heterogeneous catalytic performance in the field of organic synthesis.Moreover,we successfully extended the laser in liquid technique to hematite(?-Fe2O3)material for preparing Mn-doped a-Fe2O3 with different exposed facets,as well as analyze the structure-property relationship.The main results are summarized as follows:1.A general"top-down"strategy of preparing ultrathin BiOX nanosheets was established by pulsed laser irradiation of micron-sized suspended bulk BiOX(X=Cl,Br,I)in deionized water.Meanwhile,the interaction of laser and material realized the defect introduction and band gap regulation of BiOX with the thickness of less than 10 nm.Ultrathin BiOX nanosheets have larger specific surface area than that of bulk BiOX,providing more adsorption sites for the reactant molecules.Oxygen vacancies are introduced in ultrathin BiOCl and BiOBr nanosheets,while the self-doping of iodine is found in ultrathin BiOI nanosheets.The introduction of these defects makes the bandgap narrow,which facilitates the visible light utilization of BiOCl and the positive shift of valence-band position with stronger oxidation ability of photogenerated holes in BiOBr and BiOI.When Rhodamine B(RhB)was used as the probe molecule,these ultrathin BiOX nanosheets exhibited excellent visible-light photocatalytic activity.2.Exploring novel catalyst supports with unique performance creates more opportunity for transforming organic chemicals in heterogeneous catalysis.In this work,BiOCl is first presented as an effective support for Pd nanoparticles(NPs)toward application in the coupling reaction of benzaldehyde.After the reaction time of 5 h,Pd/BiOCI as the photocatalyst exhibited excellent catalytic performance,in which 100%conversion of benzaldehyde was obtained with 97.9%selectivity toward benzoin ethyl ether.Cycling test proved that the Pd/BiOCI catalyst has good catalytic stability without reduced catalytic activity of coupling reaction.The catalytic performance of Pd/BiOBr and Pd/BiOI was evaluated with a reaction time of 3 h.Pd/BiOBr and Pd/BiOI displayed an enhanced catalytic performance with conversion of 95.6%and 100%,and selectivity of 98.1%and 100%toward benzoin ethyl ether,respectively.The coupling mechanism reveals that the Cl-ion dissociates from BiOCl and forms nucleophile with Pd nanoparticles,which faciliates the coupling reaction of benzaldehyde.The electronegativity of halogens also has an important influence on the activity of catalysts,where their catalytic activities are ranked as:BiOI>BiOBr>BiOCl.3.The strong metal-support interaction is of great significance for supported catalysts in heterogeneous catalysis.So the effect of doping support on catalytic performance and reaction pathway is well worth studying.In this work,a-Fe2O3/BiOCl composites were irradiated by laser irradiation in liquid with the laser wavelength of 355 nm.Fe could be introduced into the lattice of BiOCl to replace Bi atoms for one-step doping.The doping amount of Fe depends on the content of Fe2O3 precursor in the laser irradiatedprocess.Combined with photoreduction method,Pd NPs were dispersedly loaded on Fe-doped BiOCl(Fe-BiOCl)nanosheets.Pd/Fe-BiOCl catalyst changed the catalytic reaction path to achieve the hydrogenation of benzaldehyde,while Pd/BiOCI exhibited the catalytic performance on the coupling reaction of benzaldehyde.Pd/Fe-BiOCl catalyst displayed good catalytic performance with conversion of 100%and selectivity of 99.5%toward the hydrogenation product of benzyl alcohol.DFT calculations reveal that the stabilization of cl-ions by stronger Bi-Cl bonds through doping of Fe ions in substituting Bi sites in BiOCl prevents Cl-ions from participating in the coupling reaction,thus hydrogenation becomes the dominant reaction.4.Engineering exposed active facets by doping impurities can dramatically modify the morphology and physicochemical properties of nanocrystalline hematite.In this work,through the combination of laser ablation in liquid and hydrothermal treatment techniques,faceted Mn-doped a-Fe2O3 nanocrystals(NCs)were prepared by adjusting the doping level of elemental Mn.With the increase of Mn doping level,the hematite crystal evolved sequentially from isotropic polyhedral nanoparticles(NPs)to {116}-faceted saucershaped nanosheets(NSs),and then to {001?-faceted hexagonal NSs.Electrochemical stripping tests and DFT calculations revealed that the Mn-doped a-Fe2O3 NCs show a facet-dependent adsorption ability toward Pb(II),Cd(),and Hg(?)heavy-metal ions;that is,{001?-faceted hexagonal NSs exhibit high and selective adsorption toward Pb2+ ions,while {116}-faceted saucer-shaped NSs present strong and selective adsorption toward Cd2+ and Hg2+ ions.Our experimental and computational results not only provide new insights into the facet-related properties,but also guide the design of crystals with exposed active facets for specific applications.