| Solar energy with its advantages of safety,cleanliness,wide distribution range and stable energy input has received widespread attention and importance in the field of new energy,and scientists are committed to developing solar energy into a safe,clean,economically viable and sustainable industrial technology.Among them,the conversion of solar energy into chemical energy is one of the current research hotspots,i.e.,the energy conversion from light energy to chemical energy is achieved by photocatalysts absorbing solar radiation to generate photogenerated carriers for redox reactions.The conversion of solar energy to chemical energy through photocatalysis is mainly applied in the fields of photocatalytic decomposition of water for hydrogen evolution,organic pollutant degradation,nitrogen fixation for ammonia synthesis,carbon dioxide reduction and organic synthesis,which brings new ideas and methods to solve the energy crisis and environmental pollution.problems.In recent years,researchers have done great efforts on the design of photocatalytic materials and the improvement of photocatalytic performance.However,the efficiency of photocatalysis has always been the core issue that restricts its practical application,among which the separation,transport and utilization of photogenerated carriers are the main factors that limit the efficiency of photocatalysis.In promoting the separation and transport of photogenerated carriers,researchers usually construct energy band-matched heterostructures according to the photocatalytic reaction system.Based on the difference of different semiconductor energy band structures,a spontaneous built-in electric field is formed at the composite interface through the potential energy difference,and the spontaneous movement of photogenerated carriers from higher to lower energy levels is realized through"potential",thus improving the separation efficiency of photocatalytic carriers.However,in this case,electrons can only passively rely on potential difference transport,and the regulation range is limited.If the active electron transport system can be constructed by compounding electron-giving materials,it can greatly promote carrier separation and transport,thus providing a new way to improve photocatalytic efficiency.In terms of carrier utilization,previous studies have improved the utilization of photogenerated carriers by loading noble metal catalysts onto the surface of photocatalysts,modulating the crystal surface structure to increase the active surface and the synergistic effect of different crystal surfaces to regulate the redox ability of photogenerated carriers and optimize the adsorption energy at the reaction interface.However,the expensive and scarce reserves of noble metal catalysts greatly limit the practical industrial applications of photocatalysis,so the construction of non-precious metal-based heterogeneous structures to enhance the effective utilization of carriers is of great significance to improve the photocatalytic efficiency.Therefore,in this thesis,Z-scheme heterostructures and non-precious metal-based double heterostructures with active electron transport properties were constructed to improve the separation,transport and utilization of photogenerated carriers for the efficient photocatalytic decomposition of aquatic hydrogen and photocatalytic oxidation of benzyl alcohol,respectively.The main research contents and results are as follows:The way to enhance photogenerated carriers separation and transport:(1)The electron donors 5,10,15,20-tetra(4-aminophenyl)porphyrin(TAPPP)and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine(TATTBD)were designed and selected for in situ growth on the surface of ZnIn2S4(ZIS)via Schiff base reaction to construct donor-acceptor type covalent organic polymer(COP)/ZIS heterostructure was constructed.The advantage of this COP-ZIS organic-inorganic composite heterostructure is that the porphyrin group in COP has active electron transport capability,and the built-in electric field at the interface between COP and ZIS promotes electron transport by potential difference,and the two synergistically enhance the effective separation and transport of photogenerated carriers in COP-ZIS.The effect of COP-ZIS heterostructures with different COP mass ratios on the performance of photocatalytic decomposition of aquatic hydrogen was investigated.4%COP-ZIS heterostructures achieved a good hydrogen evolution rate(0.95 mmol·g-1·h-1),which was three times higher than that of pure ZIS,and the heterostructures were demonstrated by photoelectrochemical tests.The structure-built electric field promoted the separation and transport of carriers.The high hydrogen evolution rate(5.04 mmol·g-1·h-1)was achieved by loading 1 wt%Pt co-catalyst onto the surface of the COP-ZIS heterostructure,which was 5 and 16 times higher than the hydrogen evolution rate of COP-ZIS and pure ZIS,respectively,indicating that the introduction of the co-catalyst improved the COP-ZIS carriers utilization.The electron spin resonance(EPR)spectra and theoretical calculations(DFT)demonstrate the complexation of the electron(e-)at the lowest unoccupied molecular orbital(LUMO)energy level of COP with the hole(h+)in the valence band(VB)of ZIS and the active electron transport properties of the porphyrin group in COP,indicating the successful construction of the Z-scheme heterostructure.(2)Crystalline porphyrin-based covalent organic framework materials(COF)were further synthesized by designing and selecting N,N,N,N-tetra(4-formylphcnyl)-1,4-phenylenediamine(NFPBD)and TAPPP under Schiff base reaction,and the framework structure of COF can serve as an effective carriers transport channel as well as the active electron transport properties of porphyrin groups.The porphyrin-based COF-ZIS heterostructure was constructed by further growing ZIS on the COF surface,and the built-in electric field of the heterostructure promoted carriers separation and transport by potential difference.The effects of COF-ZIS heterostructures with different COF mass ratios on the performance of photocatalytic decomposition of aquatic hydrogen were investigated.The results showed that the 4%COF-ZIS heterostructure achieved a good hydrogen evolution rate(0.64 mmol·g-1·h-1),which was 2.6 times higher than that of pure ZIS,and the separation and transport of photogenerated carriers were promoted by the built-in electric field of the heterostructure as demonstrated by photoelectrochemical tests.Loading 2 wt%Pt onto the surface of the COF-ZIS heterostructure achieved an efficient hydrogen evolution rate(2.70 mmol·g-1·h-1),which was 2.3 and 6.0 times higher than that of COF-ZIS and pure ZIS,respectively,and the introduction of Pt improved the utilization of COF-ZIS photogenerated carriers.The successful construction of Z-scheme COF-ZIS heterostructures was demonstrated by EPR analysis with the complexation of e-in theLUMO energy level of COF with h+on the VB of ZIS.The way to enhance improve the utilization of photogenerated carriers:(1)The Ni(OH)2/CdS-MoS2 double heterostructure was constructed by designing and choosing to modify the non-precious metal catalyst Ni(OH)2 on the surface of the CdS-MoS2 heterostructure.The effect on the CdS-MoS2 photocatalytic benzyl alcohol oxidation reaction was investigated by regulating the Ni(OH)2 loading.The results showed that the 20%Ni(OH)2/CdS-MoS2 double heterostructure had the highest photogenerated carriers utilization,achieving efficient benzyl alcohol conversion(94.2%)and benzaldehyde selectivity(99.1%).The Ni(OH)2/CdS-MoS2 double heterostructure system was demonstrated by photoelectrochemical tests to facilitate improved carriers separation and transport,and the Ni(OH)2 and monomeric Ni synergistically catalyzed the promotion of benzyl alcohol oxidation by X-ray photoelectron spectroscopy(XPS)and the oxidation performance of monomeric Ni-loaded CdS-MoS2 benzyl alcohol.(2)Ti3C2,a non-precious metal catalyst,was modified on the surface of ZIS/TiO2 heterostructure to construct a Ti3C2-ZIS/TiO2 double heterostructure.The effects on the performance of ZIS/TiO2 photocatalytic benzyl alcohol oxidation and decomposition of aquatic hydrogen were investigated by regulating the Ti3C2 loading.The results showed that 5%Ti3C2-ZIS/TiO2 had the most excellent benzyl alcohol conversion(92.7%)and benzaldehyde selectivity(99.0%).In addition,5%Ti3C2-ZIS/TiO2 achieved an efficient photocatalytic decomposition hydrogen evolution rate(1154 μmol·g-1·h-1),which was 26,4 and 2 times higher than the hydrogen evolution rates of TiO2,ZIS and ZIS/TiO2,respectively,as demonstrated by photoelectrochemical tests that the built-in electric field and Ti3C2 loading synergistically improves the separation,transport and utilization of photogenerated carriers.The formation of Z-scheme and Schottky heterojunctions between ZIS and TiO2 and between Ti3C2 and ZIS,respectively,was demonstrated by EPR tests.the Ti3C2-ZIS/TiO2 double heterostructure achieved efficient photocatalytic benzyl alcohol oxidation and decomposition of aquahydrogen.Overall,this thesis investigates the effects of active electron transporting Z-scheme heterostructures and non-precious metal-based double heterojunctions on the separation,transport and utilization of photogenerated carriers,and finally achieves efficient photocatalytic performance.The design of novel organic-inorganic Z-scheme heterostructures and non-precious metal-based double heterostructures is provided as a guideline. |
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