Preparation Of Two-dimensional Material Heterojunction For Photocatalytic Hydrogen Production

Hydrogen（H₂）is generally considered to be the most promising renewable and clean energy source for future industry,owing to its many advantages such as high combustion calorific value as well as easy transportation and storage,which is expected to replace existing fossil fuels as the main source of energy supply to the world.Photocatalytic decomposition of water technology is one of the most environmentally friendly and safe methods of hydrogen production,therefore,the preparation of efficient and stable photocatalysts is the key to realize the industrialization of hydrogen production by photocatalytic technology.At present,a large number of semiconductor photocatalytic materials,such as Ti O₂,Ag₃PO₄,Cd S,etc.,have been widely applied to photocatalytic hydrogen production technology after long and continuous exploration.However,conventional semiconductor materials have common drawbacks such as wide band gap,small optical response range,easy charge compounding and secondary pollution,which hinder the industrialization process.Two-dimensional（2D）photocatalytic materials are a new class of semiconductor materials which feature atomic-scale thickness,large interfacial contact area,low density and layered structure,and exhibit superior carrier mobility and unique direct band gap.More importantly,2D photocatalytic materials possess extremely high stability and activity.Based on these advantages incomparable to traditional semiconductor photocatalytic materials,2D photocatalytic materials have shown great potential as highly efficient photocatalysts,causing a technological revolution and bringing breakthroughs to many fields such as medicine,materials,and electronics technology.Nevertheless,monomeric 2D photocatalyst materials are limited by high photogenerated electron complexation rate,which makes it difficult to attain industrial conversion efficiency.Increasing electron transfer efficiency by constructing heterojunctions is a simple and effective method to enhance photocatalytic performance.Therefore,in this thesis,two representative 2D photocatalyst materials,BP and g-C₃N₄,were selected as substrates to introduce two-dimensional shaped COFs materials（TpPa-1-COF）,Cd S and WO₃ to construct heterojunction composite photocatalysts,respectively.The main research of this thesis is as follows:（1）2D-2D BP/TpPa-1-COF p-n type heterojunction materials were constructed by a facile hydrothermal method.Under visible light irradiation（λ≧420 nm）,the 2D-2D BP/TpPa-1-COF p-n type heterojunction composite photocatalyst produced hydrogen at a rate of up to 456.7μmol·h^-1·g^-1.The 2D-2D structure increases the contact area of the interfaces,effectively increasing the photogenerated electron transport channels between the components and achieving the electron-hole separation,which ultimately enhances the photocatalytic performance.In addition,the construction of p-n type heterojunctions facilitates the enhancement of the photogenerated charge transfer rate.The construction of heterojunctions was demonstrated to enhance the photocatalytic activity by relevant characterizations,and a possible reaction mechanism was proposed.（2）2D-2D BP/CdS p-n type heterojunction materials were prepared by a simple and efficient hydrothermal method for the first time.The 8%BP/CdS exhibited the best hydrogen production rate(508.74μmol·h^-1·g^-1),which was 26 times that of 2D BP and twice that of 2D Cd S,respectively,and it still showed excellent stability after three cycles in the cycling test.The 2D BP and 2D Cd S with 2D morphology can be prepared simply and effectively by ultrasound-assisted exfoliation.The 2D-2D structure increases the contact area between the components,and the tight interfacial contact effectively improves the problems of high photogenerated electron-hole pair complexation and photocorrosion of the photocatalyst.The XRD characterization,photocatalytic performance and electrochemical tests have proved that the photoresponse range is successfully expanded by constructing heterojunction,and the band location matching accelerates the carrier separation and migration.（3）2D-2D g-C₃N₄/WO₃ direct Z-type heterojunction composites were constructed by in-situ growth method with suitable energy band structure of WO₃ and g-C₃N₄.In particular,the hydrogen production rate of 40%g-C₃N₄/WO₃ is as high as 898.33μmol·h^-1·g^-1,which is 6 times higher than that of g-C₃N₄.The close interfacial contact between WO₃ and g-C₃N₄ was ensured by in-situ growth method,as well as the oxidation-reduction ability of the photocatalyst was improved by the optimized photogenic electron transport path,which effectively improved the photocatalytic performance of the heterojunction.The valence bands positions of WO₃ and g-C₃N₄were calculated respectively through the characterization of DRS and valence band spectrum,and the reaction mechanism of direct Z-type heterojunction was proposed.