Synthesis And Optimized Performance Of Two Dimensional Tungsten-containing Semiconductor Photocatalytic Materials

Two-dimensional semiconductor nanomaterials have been widely applied in photoelectricity,biomedicine,catalysis and energy storage due to their unique structure and properties.In terms of photocatalysis,two-dimensional nanomaterials having the larger specific surface area than bulk materials can expose more active sites which are conducive to the adsorption and activation of reactant molecules.In addition,the photocatalytic efficiency can be greatly improved by shortening the transfer path of photogenerated charge and facilitating the separation of photoinduced electrons and holes.Some tungsten-containing semiconductors,such as Bi₂WO₆ and WO₃·H₂O,are widely used in the photocatalytic field due to their availability,high ability to absorb visible light and good stability.However,single-component semiconductor materials usually have shortcomings such as narrow light absorption range,easy recombination of photogenerated carriers,and insufficient redox potentials,which limit their photocatalytic applications.In this dissertation,based on the synthesis of Bi₂WO₆ and WO₃·H₂O nanosheets,the Bi₂WO₆ and WO₃·H₂O nanosheets were modified by metal loading and constructing Z-scheme heterostructure with CdS to obtain highly efficient nanocomposite photocatalysts.The extended light absorption range,improved charge carriers separation efficiency and increased surface catalytic activity of the photocatalyst were achieved by the complementary advantages and the interface synergy,and thereby the photocatalytic performance was greatly improved.The main research contents of this dissertation are as follows:1.Ultrathin two-dimensional Bi₂WO₆ nanosheets were synthesized by a solvothermal method,and the crystal phase and structure were charactered.Then,Pd nanoparticles were uniformly loaded on the Bi₂WO₆ nanosheets by ethanol reduction,and the loaded Pd amount was tuned to be 0.25,0.5,and 1.0 wt.%,respectively.The chemical composition,morphology,surface structure and other physicochemical properties of the as-prepared photocatalysts were systematically analyzed by various characterization methods.The catalytic activity and product selectivity towards the photocatalytic selective oxidation of aromatic alcohols to the corresponding aldehydes were tested under a simulated sunlight.The results showed that the introduction of Pd significantly improved the performance of Bi₂WO₆ nanosheets for the oxidation of aromatic alcohols and the Pd-Bi₂WO₆ with Pd loading of 0.5 wt.%exhibited the best photocatalytic performance.The optimized Pd-Bi₂WO₆ photocatalyst has enhanced light absorption ability and higher photogenerated carrier generation and separation efficiency.The Schottky junction formed at the interface between Pd and Bi₂WO₆ promotes the separation of photoinduced charge carriers in Bi₂WO₆ nanosheets.At the same time,Pd nanoparticles act as active sites to effectively adsorb and activate reactant molecules,improving the surface catalytic reaction kinetics.2.Au nanocrystals have distinct surface plasmon resonance?SPR?property in visible light region,which can enhance the utilization of visible light.Therefore,based on the previous research work,we anchored Au-Pd bimetallic nanoparticles on the Bi₂WO₆nanosheets further improve the photocatalytic performance.To understand the structure-property correlation of photocatalysts,we have characterized the composition,structure and related physicochemical properties of the materials.The bimetallic Au-Pd decorated Bi₂WO₆ nanosheets presented further enhanced visible-light absorption and photoelectrochemical response,indicating that the Au-Pd modification is more conducive to the utilization of visible light and charge carrier separation.By optimizing the metal composition and content,the Au?0.25?Pd?0.25?-Bi₂WO₆ photocatalyst loaded with 0.25wt.%Au and 0.25 wt.%Pd exhibited the higher activity than other comparative catalysts in the selective oxidation of aromatic alcohol.Upon the light irradiation,the photogenerated electrons in Bi₂WO₆ nanosheets could be transferred to the Au-Pd nanoparticles and the electrons could be generated from LSPR effect of Au.These photoinduced electrons were transfered to Pd further increases the electron density of Pd surface,improving its adsorption and activation ability to O₂.The cooperation of the photogenerated holes accumulated on Bi₂WO₆ nanosheets and the superoxide radicals?·O₂^-?generated on metal Pd achieved the selective oxidation of alcohols.Therefore,in the optimized AuPd-Bi₂WO₆ photocatalyst,the electronic coupling in bimetallic Au-Pd nanoparticles and the interfacial interaction of metal-semiconductor synergistically enhance the phocatalytic efficiency.3.We synthesized optimally the solid phase Z-scheme WO₃·H₂O/Pd/CdS heterostructure photocatalyst through a three-step process.Two-dimensional WO₃·H₂O nanosheets were first synthesized by a hydrothermal method.CdS nanoparticles were deposied on the surface of WO₃·H₂O nanosheets to obtain WO₃·H₂O/CdS binary heterostructures.A small amount?about1wt.%?of Pd nanoparticles was introduced into the WO₃·H₂O/CdS heterostructures with an optimal molar ratio,where the Pd nanoparticles were sandwiched between the WO₃·H₂O nanosheets and the CdS nanoparticles,to construct the WO₃·H₂O/Pd/CdS Z-scheme photocatalyst.The WO₃·H₂O/Pd/CdS ternary heterostructures exhibited the remarkably enhanced photocatalysis towards the selective oxidation of benzylamines to imines,compared with the other photocatalysts of WO₃·H₂O,CdS,WO₃·H₂O/CdS,WO₃·H₂O/Pd and WO₃·H₂O/CdS/Pd.The wavelength-dependent photocatalytic efficiency of WO₃·H₂O/Pd/CdS is in good agreement with its light harvesting ability at the corresponding wavelengths.Photoelectrochemical analysis verified the greatly enhanced charge carrier separation in WO₃·H₂O/Pd/CdS,and the photocatalytic H₂ evolution from water splitting confirmed the more efficient Z-schematic charge flow in this photocatalyst.Therefore,the enhanced photocatalysis of WO₃·H₂O/Pd/CdS could be attributed to its unique advantages of extended light harvesting region,spatial separation of electron and holes,and elevated redox potentials.Pd NPs located between WO₃·H₂O and CdS not only acted as an electron mediator to accelerate the interfacial charge separation but also provided a linker to facilitate the oxidative N-C coupling of benzylamine through the cooperation of holes accumulated in WO₃·H₂O nanosheets and·O₂^-produced in CdS NPs.This work highlights a rational design for the highly efficient photocatalysts and presents some insights into the Z-scheme photocatalytic mechanism for selective organic transformations.