Construction Of TpPa-1-COF And WO₃ Z-Scheme Heterostructures And Performance Of Photocatalytic Hydrogen Evolution

Hydrogen energy is an ideal energy carrier due to its high energy density,green and pollution-free advantages.Solar-driven photocatalytic water splitting to produce hydrogen technology is an ideal way to obtain hydrogen energy.The focus of photocatalytic hydrogen evolution research lies in the development of high-efficiency photocatalysts.In recent years,Covalent Organic Frameworks（COFs）,as emerging crystalline porous materials,have shown outstanding photocatalytic hydrogen evolution activity,especially the two-dimensional（2D）structure of COFs,due to their conjugated flakes.Layer structure,outstanding specific surface area,and excellent visible light absorption and utilization capabilities have become emerging materials in photocatalytic hydrogen evolution.However,COFs catalyst still has the problem of high recombination rate of photogenerated electrons and holes,which limits the further improvement of its catalytic activity.In response to this scientific problem,this paper constructs a Z-Scheme heterojunction based on COFs and achieves a substantial increase in catalytic activity.The main research contents are as follows:The Z-Scheme heterojunction of TpPa-1-COF and WO₃ was constructed for the first time,which promoted the effective separation of COFs photo-generated charges and improved the photocatalytic hydrogen evolution activity.Firstly,the monoclinic phase WO₃ was synthesized by solvothermal method.By adding different proportions of WO₃ to the TpPa-1-COF synthesis system,a series of WO₃@TpPa-1-COF composite materials were prepared in situ.X-ray powder diffraction analysis,Fourier transform infrared spectroscopy and other structural characterization analysis proved the successful synthesis of WO₃@TpPa-1-COF.The morphological characterization results show that the heterojunction mat erial is a core-shell structure of TpPa-1-COF coated WO₃.The band gap and conduction band valence band of WO₃@TpPa-1-COF material are determined by UV-visible diffuse reflection and flat-band potential analysis,which proves that it meets the conditions of photocatalytic hydrogen evolution thermodynamically.Photocatalytic activity test shows that the hydrogen evolution yield of pure TpPa-1-COF is 2.27 mmol·h^-1·g^-1,while the WO₃（10%）@TpPa-1-COF hybrid material is 16.65 mmol·h^-1·g^-1,which is about 7.33 times than that of TpPa-1-COF alone.X-ray photoelectron spectroscopy is used to analyze the change of element electron density,thus confirming that the electron transfer of the material follows Z-Scheme transmission.The results of transient photocurrent response,electrochemical impedance spectroscopy,photoluminescence spectroscopy,surface photovoltage and time-resolved fluorescence spectroscopy show that the construction of WO₃（10%）@TpPa-1-COF Z-Scheme heterostructure can effectively inhibit photogeneration The recombination of electrons and holes can increase the rate of photocatalytic hydrogen evolution.The WO₃@TpPa-1-COF/rGO composite material was prepared by the photoreduction method,and the electrons on the semiconductor conduction band of the hydrogen-producing component were further enriched and conducted.In the test,adding H₂Pt Cl₆ can realize the anchoring of Pt nanoparticles on graphene.On the basis of WO₃@TpPa-1-COF heterojunction,H₂Pt Cl₆ and GO are first added to the reaction solution for photocatalytic hydrogen production,using light-induced pre-reduction to obtain WO₃@TpPa-1-COF/rGO composite material.By optimizing the loading ratio of rGO,the best hydrogen production effect was obtained.The characterization of the structure proved that the WO₃@TpPa-1-COF/rGO material was successfully synthesized.Ultraviolet-visible diffuse reflectance spectroscopy and flat-band potential test results show that WO₃@TpPa-1-COF/rGO material has good visible light absorption and meets the thermodynamic requirements of photocatalytic water splitting and hydrogen evolution.The photocatalytic activity test shows that the photocatalytic hydrogen evolution rate of WO₃（10%）@TpPa-1-COF/rGO（30%）is 26.73 mmol·h^-1·g^-1,which is 1.6 times than that of WO₃（10%）@TpPa-1-COF and 11.73 times than that of TpPa-1-COF alone.Further analysis of electrochemical impedance spectroscopy,photocurrent response,photoluminescence spectroscopy,and time-resolved fluorescence spectroscopy showed that rGO,as a medium for en riching electrons on the conduction band of TpPa-1-COF material,can further separate the photogenerated in the heterojunction.The charge is transferred more efficiently,which greatly improves the photocatalytic activity.