Synthesis Of Core-shell Structured Pt（Au）/TiO₂@Graphene Material Catalysts And Their Performances For The Photocatalytic Reduction Of CO₂

As consuming of fossil fuel,global climate change caused by the rapid growth of carbon dioxide(CO2)concentrations in our atmosphere is regarded as one of the greatest crises in this century.Scientists have long dreamed of mimicking photosynthesis of green plants,by using the energy in sunlight to knit together hydrocarbon fuels from water and CO2,which can meet the needs of energy shortage and environmental improvement.However,the fuel-conversion efficiency of photocatalytic reduction of CO2 is limited because of the low performance of photocatalysts.Among the previous reported photocatalysts,TiO2 is the most widely applied semiconductor materials because of its fitting band gap and position of conduction band,which can meet the redox potential of photocatalytic reactions.Hence,TiO2 is one of the most promising materials for industrial application.Based on the reaction nature of photocatalytic CO2 reduction with H2O,in this thesis,we regarded the ultrafine anatase TiO2 nanocrystals with coexposed {001} and{101} facets as the fountain of the photogenerated charges primitively.The surface characteristics and morphology of TiO2 nanocrystals were modified by depositing Pt nanoparticles(NPs)and wrapping reduced graphene oxide(rGO)sheets,which can increase the light absorption efficiency,improve photogenerated charge separation efficiency and enhance the enrichment of photogenerated electrons and reactant molecules(CO2,H2O)on the surface of the catalysts.Finally,the aims to improve the performance of the photocatalytic CO2 reduction and to control the selectivity of products were come true by the adjusting of control factors.The innovation contents are showed as follow:For the reaction of photocatalytic CO2 reduction with H2O,the product of one CH4 molecule requires eight electrons,eight hydrogen protons and one CO2 molecule.In order to enhance photogenerated charge separation efficiency and increase enrichment of photogenerated electrons and reactant molecules on the surface of catalysts,in this thesis,TiO2 nanocrystals with coexposed {001}-51%and {101}-4%facets were prepared and used as support.Subsequently,Pt NPs supported on anatase TiO2 nanocrystals were prepared by the gas bubbling-assisted membrane reduction-precipitation(GBMR)method.Graphite oxide(GO)was prepared by the traditional Hummers method from graphite power,and the core-shell-structured(Pt/TiO2)@rGO photocatalysts of Pt/TiO2 nanocrystals wrapped by rGO sheets were synthesized by a self-assembly method of a simple and pure chemical bonding reactions.In(Pt/TiO2)@rGO photocatalysts,anatase TiO2 nanocrystals are the fountain of photogenerated electrons,and the coexposed {101} and {001} facets undertake a spade work for separation of electrons and holes.Pt nanoparticles(NPs)deposited on the TiO2 nanocrystals can gather and transfer the stimulated electrons originated from anatase TiO2 nanocrystals.The surface residual hydroxyl and extended π bonds of wrapping rGO sheets can improve the adsorption and activation capabilities for CO2 reactant.The TiO2-nanocrystal(core)-Pt(mediator)-rGO(shell)nanojunction is not only favorable to promote the vectorial electron transfer of TiO2→Pt→rGO and enhance the separation efficiency of photogenerated electrons and holes,but also the surface residual hydroxyl and extended π bond of wrapping rGO sheets can improve the adsorption and activation capabilities for CO2 reactant.(Pt/TiO2)@rGO core-shell photocatalysts exhibited excellent performance for the multi-electron process of selective photocatalytic CO2 conversion to CH4.Among the prepared catalysts,(Pt/TiO2)@rGO-2 catalyst shows the highest photocatalytic activity and selectivity for CO2 conversion,i.e.,the formation rate of CH4 is 41.3 μmol g-1 h-1 which has a 25-fold increase in comparison with commercial P25,and the selectivity of CO2 conversion to CH4 product is 99.1%.Its apparent quantum efficiency for CH4 product is 1.93%which has a 2-fold increase in comparison with photosynthesis of green plants.To enhance the visible light absorption efficiency,in this thesis,we prepared TiO2 nanocrystals with coexposed {001}-51%and {101}-4%facets as support.And Au NPs supported on anatase TiO2 nanocrystals were prepared by the gas bubbling-assisted membrane reduction-precipitation(GBMR)method.The ternary core-shell-structured photocatalysts of Au/TiO2 nanocrystals wrapped by C3N4 sheets were synthesized by a self-assembly method of a simple and pure chemical bonding reactions.Au nanoparticles(NPs)deposited on the TiO2 nanocrystals can enhance the absorbition efficiency for the visible light because of its surface plasmon resonance(SPR)effect.As a semiconductor,C3N4 not only has photocatalytic performance and absorption of visible light,but also the surface residual hydroxyl and extended π bond of C3N4 sheets can improve the adsorption and activation capabilities for CO2 reactant.Therefore,the introduction of Au and C3N4 can improve the absorption light range and increase the surface utilization rate for visible light.In addition,the semiconductor heterojunction between C3N4 and TiO2 using Au NPs as the mediator,can enhance the separation efficiency of photogenerated electrons and holes.(Au/TiO2)@C3N4 core-shell photocatalysts exhibit excellent performance of photocatalytic CO2 conversion to CO and CH4 under visible light irradiation(420～780 nm).Among the prepared catalysts,(Au/TiO2)@C3N4-5 catalyst shows the highest photocatalytic activity and selectivity for CO2 conversion,i.e.,the formation rate of CH4 is 25.4 μmol g-1 h-1.The formation rate of CH4 over(Au/TiO2)@C3N4-5 catalyst has a 28-fold increase in comparison with commercial P25.The formation rate of CO over(Au/TiO2)@C3N4-5 catalyst is 32.7μmol g-1 h-1,which has a 54-fold increase in comparison with commercial P25.In addition,the roles of TiO2 monocrystal,Au nanoparticles and C3N4 shell in photocatalytic CO2 conversion to CO and CH4 were researched systematically,and the elemental steps for forming products were proposed.It is found that the enrichment of activated CO2,photogenerated electrons and hydrions are crucial to improve the selectivity of CH4 product.In conclusion,in this thesis,we report a novel fabrication of ternary core-shell-structured Pt(Au)/TiO2@Graphene photocatalysts,which is beneficial to improve the charge separation,absorption efficiency of visible light and facilitate the CO2 adsorption and activation.The formation rate and selectivity of products can be controlled by these photocatalysts.In addition,the systematic study of ternary core-shell structured photocatalysts will stimulate more theoretical basis for development of high efficiency photocatalyst.