Fabrication And Photocatalytic Activities For CO₂ Reduction Of BiVO₄-based Z-scheme Nanocomposites

Solar-driven photocatalysis is one of the avenues to convert CO₂ into high value-added solar fuels and chemical feedstocks.It is of great significance to develop efficient photocatalytic systems to facilitate CO₂ conversion into solar fuels.Z-scheme heterostructured photocatalytic systems is capable of not only enhancing charge separation,but also well reserving the sufficient thermodynamic reaction capacities of the electrons and holes.In this case,the spatial separated electrons and holes could initiate CO₂ reduction and water oxidation half-reactions,respectively.BiVO₄ has been regarded as the oxidative semiconductor in the Z-scheme heterojunction due to the deep valence band,which is favorable for facilitating the rate-determining step of the overall CO₂ conversion,namely,the water oxidation half-reaction.However,the photocatalytic activities of BiVO₄-based heterojunctions are still unsatisfactory mainly due to:1)the sluggish charge separation of BiVO₄ and the reductive counterpart,along with the weak interfacial interactions;2)the inevitable charge transfer resembling that taking place in the general type-II heterojunction;3)the light absorption range of the two constituents of the conventional BiVO₄-based Z-scheme heterojunction mostly overlaps and lacks of surface catalytic active sites.Hence,the following works were carried out in this thesis aiming to improve the photocatalytic activities of CO₂ reduction of BiVO₄-based Z-scheme heterojunctions via construction of dimension-matched Z-scheme interface followed by introducing energy platform,development of novel Z-scheme heterojunction by modifying metal phthalocyanines with abundant catalytic active sites and distinct visible light absorption range from BiVO₄.Moreover,the mechanism of charge transfer and separation,along with the improvement of photocatalytic activities have also been clarified.In terms of the weak interfacial interactions between BiVO₄ and the reductive counterpart,along with the insufficient driving force of Z-scheme charge transfer,a hydroxyl induced assembly strategy has been developed to construct dimension-matched 2D/2D g-C₃N₄/BiVO₄ nanocomposites.The Z-scheme interfacial charge transfer and separation is greatly facilitated by the increased contact areas and shortened transport distance of photogenerated charge carriers.Further,a cascade Z-scheme heterojunction has been designed and fabricated by introducing（001）Ti O₂as the energy platform,which blocks the unexpected type-II charge transfer pathway.The sheet-like Ti O₂ could not only meet the requirements of energy level matching for CO₂reduction but also match the dimension of g-C₃N₄,aiming to maximize the Z-scheme transfer and separation and further improve the photocatalytic activities for CO₂reduction.The photocatalytic yield of CO₂ conversion to CO for the optimal（001）Ti O₂-g-C₃N₄/BiVO₄ nanocomposite is 5.2μmol g^-1 h^-1 under visible light irradiation,without any sacrificial agents.In terms of the light absorption range of two constituents of conventional g-C₃N₄/BiVO₄ heterojunctions mostly overlaps and lacks of surface catalytic active sites,a novel Z-scheme photocatalytic system with wide spectrum response and abundant catalytic active sites has been proposed by integrating ZnPc with BiVO₄.The ZnPc/BiVO₄ nanocomposites exhibit superior charge separation and photocatalytic activities.The mechanism of Z-scheme charge transfer has been evidenced by experimental results and fundamental studies.Moreover,it has been verified that the central Zn²⁺in ZnPc could accept the excited electrons from the ligand and then provide a catalytic function for CO₂ reduction.The photocatalytic yield of CO₂ conversion to CO for the optimal ZnPc/BiVO₄ nanocomposite is 3.85μmol g^-1 under visible light irradiation for 4 h,without any sacrificial agents.In terms of the self-aggregation of ZnPc leading to the limited loading amounts of ZnPc and the unmatched light harvesting capacity between the two constituents,a strengthened Z-scheme charge transfer and separation has been achieved by increasing the optimized amount of highly dispersed ZnPc via the functionalized graphene-modulated assembly.The photocatalytic yield of CO₂ conversion to CO for the optimal ZnPc/G/BiVO₄ nanocomposite is 15.01μmol g^-1 under visible light irradiation for 4 h,without any sacrificial agents.It is crucial for the adsorption and activation of CO₂ molecules toward efficient CO₂ conversion.Controllably induce highly dispersed Cu Pc assembly with BiVO₄ to fabricate Cu Pc/BiVO₄ nanocomposite is expected to further improve the activities of the targeted photocatalytic CO₂ reduction.Cu Pc/Au-BiVO₄ nanocomposites with wide visible light responses are rationally designed and fabricated by pre-depositing ultrafine Au on BiVO₄,and followed by the assembly of Cu Pc via the interaction between Au and N atom of Cu Pc.The accelerated Z-scheme charge transfer is mainly attributed to the ultrafine Au interfacial modulation,which is dependent on the directional electron transfer of BiVO₄ and the great increase in the optimal loading amount of assembled Cu Pc with high dispersion.The photocatalytic yield of CO₂ conversion to CO for the optimal Cu Pc/BiVO₄ and Cu Pc/Au-BiVO₄nanocomposite are 8.96μmol g^-1 and 23.50μmol g^-1 under visible light irradiation for 4 h,respectively,without any sacrificial agents.This work provides new avenues for the design of highly efficient BiVO₄-based Z-scheme heterojunction for producing solar fuels,and diversifies new methods for clarify the mechanism of Z-scheme charge transfer.