| The continuous consumption of fossil fuels has led to a serious energy and environmental crisis,which has seriously hindered the sustainable development of mankind.Over the past few decades,significant progress has been made in sustainable and clean energy sources such as hydrogen,solar and biomass energy.Hydrogen energy(H2),as a clean,economical and renewable energy source,has the highest energy density(142 MJ/kg)among all chemical fuels,and is considered a promising alternative to fossil fuels.Photocatalysis technology has attracted the attention of the academic community as a green way without secondary energy consumption.However,due to the limited light absorption range,low separation and migration of photocarriers,it is difficult to perform well in photocatalytic hydrogen evolution.To achieve efficient,stable and low-cost photocatalytic decomposition of water,it is necessary to use highly active cocatalysts composed of inexpensive and abundant elements on earth.Overall,the cocatalysts have the following three functions:(1)Trapping carriers and inhibiting carrier recombination;(2)Adjusting the active site of hydrogen adsorption to catalyze hydrogen production by reducing the activation energy;(3)Inhibiting the photocorrosion of photocatalysts.Transition metal dichalcogenides(TMDs)exhibit excellent properties of unconventional and amorphous materials due to their special physicochemical properties and adjustable atomic arrangement,which have attracted widespread attention.TMDs nanomaterials can exhibit different crystal phases(including 1T phase,2H phase,amorphous phase,etc.).The regulation of their crystal structure makes it possible to optimize photocatalytic performance.Based on this,the paper developed conventional synthesis methods to achieve the controllable synthesis of TMDs ideal crystal phase,and explored the influence on the photocatalytic hydrogen evolution performance.Combined with theoretical analysis and characterizations,the internal mechanism of crystal phase engineering of transition metal dichalcogenides on photocatalytic hydrogen evolution was investigated.The main research contents and results are as follows:(1)In view of the easy recombination of photogenerated electron-hole pairs of TiO2 nanocrystalline sheets and lack of effective active sites on the surface,which limit their performance in photocatalytic decomposition of water,the photocatalytic hydrogen evolution reaction was regulated by loading CoSe2 additive particles with different crystal phases on the surface of TiO2 nanocrystalline sheets.The internal mechanism of the influence of different CoSe2 crystals on their relative catalytic performance was studied.In the actual electrocatalytic and photocatalytic hydrogen evolution reactions,although the two crystalline phases of CoSe2 could carry out the electrocatalytic hydrogen evolution and serve as a promoter to improve the photocatalytic hydrogen evolution activity,they showed different crystal phase dependent activities in the electrocatalytic and photocatalytic hydrogen evolution reaction.o-CoSe2 which had a suitable hydrogen adsorption free energy,had higher electrocatalytic hydrogen evolution activity because it was more suitable for surface hydrogen proton adsorption.Meanwhile,the efficient photogenerated electron of cCoSe2 could transfer more electrons to the active surface and thus performed better in photocatalytic hydrogen evolution reaction.The different characteristics of CoSe2 with different crystal phase structures have different performance in electrocatalytic and photocatalytic reactions.The rate-determining step is the adsorption and conversion of surface hydrogen,while the key in photocatalytic reactions is the efficient charge transfer between TiO2 and CoSe2.(2)2D g-C3N4 nanosheets have limited photocatalytic hydrogen evolution efficiency due to their limited photocarrier separation,migration efficiency and surface protonreducing activity.In the experiment,NiSe2 was used as a photocatalytic hydrogen evolution cocatalyst supported on the surface of 2D g-C3N4 in the experiments.The results showed that the hydrogen evolution rate of the composite photocatalyst supported NiSe2 with different crystalline phases was significantly improved,and there existed no photocatalytic hydrogen evolution activity dependence on crystal phase.Different from previous reports,NiSe2 with different crystal structure showed different stability during photocatalytic hydrogen production.In order to investigate the underlying mechanism of this phenomenon,electrochemical and electron microscopic analysis were carried out.It was found that mNiSe2 was stable during the catalytic process,and p-NiSe2 was easily converted into NiOOH,so m-NiSe2 had better stability than p-NiSe2.The above results indicate the important impact of crystal phase is not only limited to activity,but also provides a basis for the regulation of stability.(3)On the basis of previous work,we selected three kinds of NiSx cocatalyst particles with different crystal phases,and developed a general method to support them on metal oxide(TiO2),metal sulfide(CdS)and metal-free nitride(2D g-C3N4)by simple mechanical grinding method.Starting from 2D g-C3N4 composites with monomer NiSx and loaded NiSx,it was found that the photocatalytic hydrogen evolution activity could be improved by all three composites,and the order of efficiency is c-NiS2/CN>h-NiS/CN>t-NiS/CN.The same activity was observed in two other semiconductor carriers.Combined with theoretical calculation and characterization analysis,it was found that c-NiS2/CN material exhibited excellent activity due to its better charge separation and transport efficiency. |
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