In-situ Construction And Photocatalytic CO₂ Reduction Performance Of Metal Oxide-based Heterojunction Photocatalysts

Photocatalytic conversion of carbon dioxide（CO₂）into carbon-based fuels by utilizing renewable solar energy is considered to be a promising strategy for achieving“carbon neutralization”and alleviating the energy crisis.However,unitary semiconductor photocatalyst suffers from low photocatalytic CO₂ reduction activity due to the fast recombination of photogenerated electrons and holes.Herein,Zn O and Ti O₂,as typical metal oxides,are selected as the main research objects in this thesis.Graphene-based cocatalyst loading and step-scheme（S-scheme）heterojunction building via an in-situ construction strategy,are proposed to promote the photogenerated charge separation,suppress the recombination of photogenerated electrons and holes as well as improve the performance of photocatalytic CO₂reduction reaction.The research is mainly composed of the following four chapters:Firstly,compared with the high cost and scarcity restriction of noble metal cocatalysts,graphene is considered as the most promising substitute for noble metal cocatalysts due to its excellent conductivity,large work function and earth abundance.However,intimate interfacial contact between semiconductor photocatalysts and graphene cocatalysts is important for the transfer and separation of photogenerated charge carriers.Herein,in order to prepare high-quality graphene cocatalyst with robust contact with photocatalysts,few-layer graphene was in-situ grown on the surface of Zn O via a chemical vapor deposition（CVD）method using benzene as the precursor.The optimized composite sample demonstrated improved photocatalytic CO₂ reduction performance than pristine Zn O,ascribing to the intimate interfacial contact and Schottky junction between Zn O and graphene.Meanwhile,the photothermal effect of graphene andπ-πconjugation interaction between graphene and CO₂ molecules also contributed to the performance enhancement.This work not only provides a feasible approach for the in-situ growth of graphene,but also develops an efficient photocatalyst for CO₂ reduction.Secondly,hollow spheres（HS）become promising candidates for photocatalytic CO₂ reduction due to manifold advantages including reduced charge carrier diffusion distance,improved light scattering and larger specific surface area.Based on the first work,we develop a CVD strategy to in-situ grow ultrathin N-doped graphene（NG）layer on Ti O₂ HS with large area and intimate interfacial contact using pyridine as the precursor.The optimized Ti O₂/NG HS nanocomposite achieved total CO₂ conversion rates（the sum yield of CO,CH₃OH and CH₄）of 18.11μmol g^-1 h^-1,which is about 4.6times higher than blank Ti O₂ HS.Experimental results demonstrated that intimate interfacial contact and abundant pyridinic N sites can effectively facilitate separation and transport of photogenerated charge carriers,realizing enhanced photocatalytic CO₂ reduction performance.Furthermore,this work further proved that the feasibility of in-situ growing graphene-based cocatalyst on photocatalyst surface via CVD method.Thirdly,based on the Ti O₂ HS in the second work,hierarchical S-scheme Ti O₂@Zn In₂S₄ core-shell hollow spheres heterojunction is fabricated by growing Zn In₂S₄ nanosheets on the outer surface of Ti O₂ HS via in-situ chemical bath method for photocatalytic CO₂ reduction.The optimized sample exhibited much higher CO₂photoreduction conversion rates（the sum yield of CO,CH₃OH and CH₄）than the blank control,ie.,Zn In₂S₄ and Ti O₂.The improved photocatalytic performance can be attributed to the inhibited recombination of useful photogenerated charge carriers induced by S-scheme heterojunction.The improvement is also attributed to the large specific surface areas and abundant active sites.Meanwhile,S-scheme photogenerated charge transfer mechanism is testified by in-situ irradiated XPS,work function calculation,and electron paramagnetic resonance measurements.Fourthly,particulate photocatalysts are faced with some problems,such as agglomeration,light shielding and difficulty in reuse.Therefore,it is necessary to prepare the recyclable thin-film photocatalyst for photocatalytic CO₂ reduction.Based on the superiority of the aforementioned CVD method and the S-scheme heterojunction charge transfer,the recyclable S-scheme Ti O₂/g-C₃N₄ film photocatalyst was successfully constructed by in-situ growing g-C₃N₄ on the surface of the vertically-oriented Ti O₂ nanowire arrays on Ti foil with intimate interfacial contact through a vapor deposition polymerization strategy.Benefiting from the effective charge separation by S-scheme charge transfer,intimate contact by the in-situ growth as well as the ingenious structure,the composite,readily recyclable,displays exciting performance in photocatalytic CO2 reduction.It is beyond doubt that the recyclable S-scheme heterojunction film photocatalyst promises practical applications.