Preparation And Application Of Heterostructure Photocatalysts Based On Covalent-Organic Framework

Energy crisis and environmental pollution have been now the intractable global problems.Hence,the conversion of solar energy into chemical energy through photocatalytic H₂evolution has evolved into a potential and effective technology to obtain alternative energy sources.Covalent organic frameworks?COFs?have exhibited promising H₂evolution photocatalysts owing to their tunable light harvesting and excellent charge transport properties.However,the rapid recombination of photogenerated charges for single-component COFs inhibits its development of photocatalysis.To improve the photocatalytic H₂evolution activity of semiconductor-based photocatalysts,an effective strategy is constructing rational heterostructure to facilitate the transfer of photogenerated charges.Compared with traditional heterostructure systems,direct Z-scheme system has provoked widespread interest of researchers because of efficaciously separating charge carriers and substantially enhancing the efficiency of photocatalytic activities.In this work,considering the photocatalytic H₂production performance of covalent organic framework material?Tp Pa-X-COF?,hence Tp Pa-1-COF and black titanium oxide firstly were selected to build a traditional type II heterojunction photocatalyst.Subsequently,Tp Pa-2-COF with excellent hydrogen production performance and?-Fe₂O₃with good visible light absorption effect were employed to construct Z-scheme heterojunction photocatalyst.Ultimately,we apply them to photocatalyst H₂production respectively.The main research contents are as follows:?1?The Ti O₂was synthesized by solvothermal method using butyl titanate and hydrofluoric acid solution,then and black titanium oxide was prepared by high temperature thermal reduction method.Ultimately a series of Ti O_2?OVs?/Tp Pa-1-COF type II heterojunction photocatalyst were synthesized by adding Ti O_2?OVs?into the reaction system of Tp Pa-1-COF using a one-pot method.The structure and morphology were analyzed by a series of physical characterization.UV-vis spectroscopy and flat band potential analysis analyzed the band structures of Ti O_2?OVs?and Tp Pa-1-COF.The electrochemical impedance spectra,photocurrent-time profiles,photoluminescence spectrum and surface photovoltage spectroscopy showed that the Ti O_2?OVs?/Tp Pa-1-COF composite catalyst can effectively promote the separation and transport of photogenerated electrons and holes.Based on these,the photocatalytic performance of a series of Ti O_2?OVs?/Tp Pa-1-COF composite catalysts for hydrogen production is investigated.The results show that the hydrogen production rate for Ti O_2?OVs?/Tp Pa-1-COF?6:4?can reach 11.86 mmol·g^-1·h^-1,which is about 6 times that of Tp Pa-1-COF.?2?The?-Fe₂O₃was synthesized by solvothermal method using Fe Cl₃and CH₃COONa,and then a series of?-Fe₂O₃/Tp Pa-2-COF Z-scheme heterojunction photocatalyst were synthesized by adding?-Fe₂O₃into the reaction system of Tp Pa-2-COF by a one-pot method.The structure and morphology were analyzed by a series of physical characterization.UV-vis spectroscopy and flat band potential analysis analyzed the band structures of?-Fe₂O₃and Tp Pa-2-COF,indicating that they can form Z-scheme heterojunctions.It was proved by electron paramagnetic resonance spectroscopy that?-Fe₂O₃and Tp Pa-1-COF formed a Z-scheme system.The electrochemical impedance spectra,photocurrent-time profiles,photoluminescence spectrum and surface photovoltage spectroscopy showed that the?-Fe₂O₃/Tp Pa-2-COF composite catalyst can more effectively promote the separation and transport of photogenerated electrons and holes.Based on these,the photocatalytic performance of a series of?-Fe₂O₃/Tp Pa-2-COF composite catalysts for hydrogen production is investigated.The results show that the hydrogen production rate for?-Fe₂O₃/Tp Pa-2-COF?3:7?can reach 3.77 mmol·g^-1·h^-1with no precious metal co-catalyst,which is about 53 times that of Tp Pa-2-COF.