Study On Preparation And Performance Of Non-precious Metal-based Photocatalyst For Hydrogen Evolution

Under the circumstance of the fast development of economy and industry,the utilization of new energy has become the research focus of scientific researchers.As a new type of clean energy with zero pollution,renewable,light weight,high energy density and abundant reserves,hydrogen energy has attracted wide attention.Photocatalytic decomposition of water to produce hydrogen,using solar energy in nature as the driving force and water as the reactant,is an effective strategy to supply hydrogen energy,alleviate environmental pollution and energy consumption.However,the traditional single semiconductor material has two major constraints under the excitation of light:narrow optical absorption range and low carrier separation and migration efficiency.In this paper,two non-precious photocatalysis materials(reducing semiconductor g-C3N4 and oxidizing semiconductor TiO2)were designed and modified reasonably,and the structure-activity relationship between photo-absorption range,photo-carrier transfer efficiency,photocatalytic reaction mechanism and photocatalytic hydrogen evolution activity was systematically studied by theoretical simulation.The following are the main contents:1.The relationship between asymmetric insertion of pyridine ring into g-C3N4 and photocatalytic hydrogen evolution activity was studied.Experiment and theoretical calculation simulation confirm that the asymmetric insertion of pyridine ring breaks the symmetric periodic structure of the heptanazin ring of g-C3N4,and then suppresses the carrier recombination efficiency of the original g-C3N4 and enhances the optical response range without changing its long-range structure.UV-visible absorption spectra and valence band spectra confirmed that the insertion of pyridine ring broadened the optical absorption range.Fluorescence spectra,photoelectrochemical tests and the femtosecond transient absorption spectra further confirmed that the photogenerated charge transfer efficiency of the catalyst was significantly improved before and after the insertion of pyridine ring.Compared with the original g-C3N4,the photocatalytic hydrogen evolution rate of DPCN embedded in pyridine ring increased by 6.3 times.At the same time,the quantum efficiency of DPCN catalyst at wavelength of 420 nm is 14.6%.This work provides a new idea for the asymmetric insertion of organic rings into carbon nitride and the construction of efficient and stable photocatalysts.2.The relationship between the enhanced light absorption range of g-C3N4 modified by cyano and cyanamide functional groups and the carrier separation and transfer and the photocatalytic hydrogen evolution was studied.The g-C3N4(CN-HT)modified by cyano and cyanamide functional groups was successfully prepared by one-step molten salt method,which broke the heptanazin ring structure of the original g-C3N4,adjusted its electronic band structure,and widened the light response range of the catalyst.At the same time,cyano and cyanamide functional groups,as strong electron-absorbing groups,can selectively separate and migrate photo-generated carriers within the molecule.In addition,the increased specific surface area and improved hydrophilicity and hydrophobicity effectively promote the photocatalytic decomposition of water to produce hydrogen.The photocatalytic hydrogen evolution rate of CN-HT prepared by molten salt method is 88.29 μmol h-1,14.2 times that of pristine g-C3N4.The experimental and theoretical results show that the CH-HT catalyst prepared by molten salt method can promote the effective charge transfer and change the electronic structure and energy band position of the catalyst,which provides a new strategy for constructing efficient intramolecular heterojunction catalysts.3.The relationship between heterogeneous homojunction promoting the separation and transfer of photogenerated carriers and photocatalytic hydrogen evolution was studied.We successfully prepared TiO2 heterogeneous catalysts(a-Ti/H-Ti)by wet chemical method and room temperature stirring two-step method.Combining the theoretical calculation work function value and the experimental UPS value,it can be found that the heterogeneous homogeneous junction has a suitable work function and a perfect energy band matching.In-situ XPS spectra and ESR technical means confirmed that heterogeneous homogeneous catalysts follow the typeⅡ photocatalytic reaction mechanism.Fluorescence spectra,photoelectrochemical tests and surface photovoltage spectra suggested that the homogeneous catalyst effectively promoted the carriers’ separation and transfer.The experimental results of transient absorption spectra further confirmed that the a-Ti/H-Ti catalyst has a fast photogenerated charge transfer rate.As prepared a-Ti/H-Ti catalyst has excellent photocatalytic hydrogen evolution rate(29.63 mmol g-1 h-1)under full spectrum irradiation,which is 145 times of commercial rutile phase TiO2.At the same time,a-Ti/H-Ti catalyst showed excellent quantum efficiency(45.6%)at wavelength of 365 nm.This work is committed to highlighting the importance of the concept of"heterogeneous and homogeneous",and provides a new and effective strategy for the preparation of high-efficiency photocatalysts.4.The relationship between MXene as a cocatalyst to modify the a-Ti/H-Ti homojunction catalyst to promote carriers’ transportation and photocatalytic hydrogen evolution was studied.The ternary MXTi catalyst was synthesized by coupling MXene with the homogenous catalyst a-Ti/H-Ti with the best performance in the previous chapter through a simple self-assembly strategy.The research shows that MXene,as an efficient cocatalyst,can accelerate the dynamics of photogenerated carrier transport.XPS spectra and ESR techniques confirmed that the ternary heterojunction photocatalyst followed the Type Ⅱ reaction mechanism.Fluorescence spectra and photoelectrochemical tests further confirmed that the introduction of MXene can improve the transportation of photogenerated carriers.At the same time,as-prepared ternary catalyst MXTi has excellent photocatalytic hydrogen evolution rate(38.7 mmol g-1 h-1)under full spectrum irradiation.This work highlights the synergy of homogeneous engineering and co-catalyst modification of ternary heterojunction with MXene as carrier directional transport,providing a new perspective for the preparation of high-efficiency photocatalysts.5.The relationship between the mechanism of double S-type photocatalytic reaction and photocatalytic hydrogen evolution was studied.A new self-assembled structure(labelled TSP)was successfully synthesized by inserting two-dimensional graphite carbon nitride nano sheets between anatase nanoparticles and rutile TiO2 nanorods.The research shows that this self-assembly structure with a double S-type photogenerated charge transfer mechanism not only overcomes the limitations of large contact barrier of traditional heterostructure,but also improves the photogenerated charge transfer rate.At the same time,as-prepared TSP catalyst showed excellent photocatalytic hydrogen evolution rate of 62.37 mmol g-1 h-1 under full spectrum irradiation,and showed high apparent quantum efficiency(45.9%)at wavelength of 365 nm.Through in situ XPS,ESR analysis,femtosecond transient absorption(fs-TA)spectroscopy,transient surface photovoltage(TPV)and other non-in-situ characterizations,we have confirmed that unique double S-type photocatalytic reaction mechanism can well retain the strong redox ability and promote the charge transfer.The double S-type design proposed in this work will provide a new way to develop photocatalysts with high oxidation-reduction ability,enhanced exciton splitting ability and fast charge transfer.