Study On Preparation And Hydrogenation Performance Of MoS₂ Based Composite Photocatalysts

Since the beginning of the industrial revolution,the traditional fossil energy of coal,oil,and natural gas has been gradually consumed.The high consumption brought severe energy crisis problems and ecological environment deterioration,which led to seeking new sustainable green energy.Recently,hydrogen has been considered the favorite green energy and is expected to solve these problems.At present,Photocatalytic hydrogen production technology is a feasible method.However,it has encountered problems,such as a narrow spectrum response range,easy recombination of excited electron-hole pairs,absence of active sites,and low catalytic hydrogen evolution rate.In view of the existing problems of photocatalysts,researchers often use ion doping,heterostructure construction and adhesion of cocatalysts to modify them.Therein,cocatalyst mainly plays the role of improving the charge separation efficiency of photocatalyst and increasing the catalytic active site,which also belongs to the category of photocatalyst.MoS₂ photocatalyst with the suitable optical bandgap and the facile exposed S edges activity is a hope to overcome the difficulties encountered.However,the MoS₂ photocatalyst still has problems with few active sites and high recombination of electron-hole pairs.Firstly,the MoS₂ photocatalyst was modified by doping non-metallic（P,N）and constructing a heterostructure with a WO₃ semiconductor to increase the catalytic activity and improve the charge separation efficiency.However,it has been found that MoS₂ as a cocatalyst can show better catalytic performance than that it as the photocatalyst.Secondly,a series of MoS₂ based cocatalysts with high catalytic activity was designed by doping different N concentrations,carbon intercalation,and interface construction.Finally,the hydrogen performance of their combination with g-C₃N₄ photocatalyst for photocatalytic water splitting has been explored.The main results of this dissertation are as follows:1.The P-MoS₂@WO₃ heterogeneous photocatalyst was prepared by a two-step hydrothermal reaction and the results show that non-metallic P doping and Z-scheme heterostructure improve the hydrogen evolution performance of the MoS₂ photocatalyst.The analysis reveals that the P doped atoms provide the active sites for H⁺reduction,and the heterostructure constructs a direct Z-scheme transfer channel.The conduction band electrons of WO₃ have been transferred to the valence band of P-MoS₂ through the heterostructure,reducing the electron back migration of the conduction band in the P-MoS₂ photocatalyst.The prepared P-MoS₂@WO₃ composite has good photocatalytic hydrogen evolution performance(73.8μmolh^–1g^–1)and cycle stability（16 h）.2.The N-MoS₂@WO₃ heterogeneous photocatalyst was prepared by hydrothermal reaction and sol-gel method.The results indicate that the doped non-metallic N atoms improve the electron attraction ability and effectively trigger the base plane’s catalytic activity.The construction of the Z-scheme heterostructure reduces the recombination rate of electron-hole pairs in N-MoS₂,thus improving the hydrogen evolution performance of the N-MoS₂@WO₃ composite.The hydrogen evolution rate of prepared0.5N-MoS₂@0.5WO₃ composite is 102.5μmolh^–1g^–1.In addition,theoretical calculations show that N doping(（35）G_H*≈0.19 e V)provides more excellent catalytic activity for MoS₂ than P doping(（35）G_H*≈0.25 e V).3.The N doped MoS₂ cocatalyst(MoN_1.2xS_2-1.2x)was prepared by the solid-phase reduction method,and the outcomes show that the photocatalytic performance of MoN_1.2xS_2-1.2x@g-C₃N₄ composite is improved.The analysis reveals that the constructed heterostructure induces the conduction band electrons of g-C₃N₄ transfer to the conduction band of MoN_1.2xS_2-1.2x cocatalyst.The transformation enhances the separation efficiency of electron-hole pairs,and the N doped atoms provide abundant active sites for the H⁺reduction.In addition,the hydrogen evolution rate of the prepared5 wt%MoN_1.2xS_2-1.2x@g-C₃N₄ composite exhibits the H₂ evolution rate of 360.4μmolh^–1g^–1 with good stability（16 h）,which is about 3.5 times that of the 0.5N-MoS₂@0.5WO₃composite.The result indicates that MoS₂-based material is more suitable as the cocatalyst than that it as the photocatalyst.4.The carbon-intercalated MoS₂ cocatalyst（C-MoS₂）was prepared through hydrothermal and high-temperature carbonization processes.The results show that the photocatalytic performance of the C-MoS₂@g-C₃N₄ composite is improved.The analysis reveals that the carbon intercalation enhances the conductivity of the C-MoS₂cocatalyst and accelerates the electron separation efficiency from g-C₃N₄ to C-MoS₂cocatalyst.Furthermore,the intercalation of carbon enlarged the interlayer spacing and inhibited the stacking of MoS₂ along the c-axis direction,thus enhancing the catalytic activity of edge S atoms.Additionally,the prepared 5 wt%C-MoS₂@g-C₃N₄composite exhibits the H₂ evolution rate of 157.1μmolh^–1g^–1 with excellent cycle stability（16 h）.5.The MoN/MoS₂ cocatalyst was prepared by replacing partial S anions in the MoS₂lattice with N anions generated by NH₃ decomposition at high temperature.The findings demonstrate that the photocatalytic performance of the MoN/MoS₂@g-C₃N₄composite is improved.The analysis reveals that the MoN/MoS₂ cocatalyst provides abundant active sites and excellent electrical conductivity for H⁺reduction and electron transfer and improves the separation efficiency of electron-hole pairs in the g-C₃N₄photocatalyst.In addition,the hydrogen evolution rate of the prepared 5 wt%MoN/MoS₂@g-C₃N₄ composite is 545.8μmolh^–1g^–1 and exhibits good stability（12 h）.The obtained results are about 1.5 and 3.5 times higher than that of 5 wt%MoN_1.2xS_2-1.2x@g-C₃N₄(360.4μmolh^–1g^–1)and 5 wt%C-MoS₂@g-C₃N₄(157.1μmolh^–1g^–1)composites,respectively.The results indicate that the MoN/MoS₂ heterostructure cocatalyst with dual-function co-modification has better catalytic hydrogen evolution performance.