Construction Of P-n-type Heterojunction And Control Morphology To Improve The Photocatalytic Hydrogen Evolution Performance Of Co-based Semiconductors

With the increasing demand for energy in countries around the world,and the emergence of the global greenhouse effect caused by excessive CO₂emissions,researchers around the world are working hard to explore new clean and sustainable energy sources to replace fossil energy.Such as:wind energy,tidal energy and hydrogen energy.Hydrogen energy is a clean and sustainable energy with high energy density,its energy density is as high as 140 MJ/kg.In recent years,many researchers have paid great attention to the research of semiconductor photocatalytic water splitting technology to produce hydrogen.However,most semiconductors themselves have some defects,such as high recombination rate of photo-generated electron-hole pairs and wide band gap.Therefore,researchers have explored many methods to improve these problems:through band regulation,catalyst loading,and construction of heterojunctions.Here we mainly study the hydrogen evolution activity of the photocatalytic system in the presence of the transition metal element Co.The main research contents are as follows:（1）The flower spherical composite catalyst CoNi₂S₄/CdWO₄is synthesized by a simple hydrothermal method,and TBA is added to adjust the morphology of the catalyst during the synthesis process.The results after characterization by BET test show that the spherical bimetallic sulfide CoNi₂S₄/CdWO₄has the largest specific surface area.Generally speaking,a large specific surface area can provide more active sites for hydrogen evolution,thereby further improving the photocatalytic hydrogen evolution activity of the flower spherical catalyst CoNi₂S₄/CdWO₄.（2）Further explore the photocatalytic hydrogen evolution activity of the bimetallic sulfide CuCo₂S₄.The bimetallic sulfide CuCo₂S₄and the single metal sulfide CoS₂and Cu₃₁S₁₆are tested for hydrogen evolution,respectively.The experimental results show that the bimetallic sulfide CuCo₂S₄has more excellent photocatalytic hydrogen evolution activity.And in the electrochemical test results,it can be observed that CuCo₂S₄can more effectively reduce the recombination of photogenerated carriers.Therefore,the photocatalytic hydrogen evolution activity of the bimetallic sulfide is more excellent.（3）The transition metal sulfide CoS₂and the rare earth perovskite Fe La O₃are combined to form a p-n type heterojunction.After a series of XRD,SEM,TEM,XPS,UV-vis,PL and electrochemical characterization tests,it is found that the p-n-type heterojunction is formed.The heterojunction composite catalyst can greatly reduce the recombination of photo-generated electron and hole pairs,and is also more conducive to accelerating the electron transfer rate on the catalyst surface,thereby further promoting the photocatalytic hydrogen evolution activity of the catalyst.（4）A simple hydrothermal method is used to synthesize a composite of transition metal element Cooxide（CoFe₂O₄）and NiMoO₄,and the two are calcined to form a traditional p-n heterojunction.Through the hydrogen evolution activity test and experimental characterization results,it can be seen that the composite catalyst CoFe₂O₄/NiMoO₄exhibits excellent photocatalytic hydrogen evolution activity,and the separation of the internal photo-generated electrons and holes is improved.The composite catalyst CoFe₂O₄/NiMoO₄provides more possibilities for the photocatalytic decomposition of water and hydrogen evolution.To sum up,all the research contents are to improve the photocatalytic hydrogen evolution activity of the catalysts by studying the morphology control of the bimetallic sulfide of the transition metal element Coand constructing a p-n heterojunction.After adjusting the morphology,the composite catalyst has a larger specific surface area,which can provide more active sites for hydrogen evolution,thereby improving the photocatalytic hydrogen evolution activity.Constructing a p-n type heterojunction can improve the recombination of photogenerated electrons and holes in the catalyst,thereby further reducing the loss of photogenerated electrons in the catalyst,accelerating the electron transfer rate on the catalyst surface,and being more conducive to the proton reduction reaction on the catalyst surface,thus improving the photocatalytic hydrogen evolution activity of the composite catalyst.This provides a certain reference significance for future experiments of photocatalytic water decomposition and hydrogen evolution.In the sixth chapter of this article,we have summarized all the research contents and existing shortcomings,summarized the innovations of the studied photocatalytic hydrogen evolution experiments,and put forward new insights for photocatalytic hydrogen evolution.