Construction And Properties Of Nb₂CT_x（MXene） Modified Sulfide Visible Light Photocatalyst For Hydrogen Production

Photocatalytic water cracking for hydrogen production has become an ideal solution to overcome the energy and environmental threats posed by the continued use of fossil fuels.Among many photocatalysts,metal sulfide has been widely studied for its excellent visible light absorption and suitable band edge position.However,the single metal sulfide semiconductor material as a photocatalyst has the disadvantages of fast photoelectron recombination rate and photocorrosion.In order to achieve efficient photocatalytic hydrogen production,the design of photocatalysts becomes the key.MXene,a novel transition metal carbon/nitride,is considered a good candidate cocatalyst due to its excellent metallic conductivity,anisotropic carrier transport,and tunable surface chemistry and electronic properties.In this thesis,the metal sulfide and Nb₂CT_x are taken as the research object.By utilizing their respective advantages and combining different composite methods,the semiconductor composite catalyst Nb₂CT_x/sulfide is constructed to achieve high efficiency photolysis water hydrogen production under visible light irradiation.Specific research contents are as follows:（1）A hierarchical structure of CdS@Nb₂O₅/Nb₂CT_x ternary composite catalyst was prepared for photocatalytic hydrogen production.The results show that the hydrogen evolution rates of the optimized composites are 1501.7（Na₂S/Na₂SO₃）and 2715.8（lactic acid）μmol g^-1h^-1,and the apparent quantum efficiency（AQE）at 420 nm is 12.4%and 26.1%,respectively.Its hydrogen evolution performance is 10.9 times higher than that of pure CdS,and it still maintains 87%activity after 5 cycles of experiment.This enhancement results from excellent light absorption properties,tight interfacial contacts,fast charge transfer channels,and abundant active sites.The structural characterization and mechanism analysis show that an S-scheme is formed between CdS and Nb₂O₅/Nb₂CT_x,andSchottky junctions are formed between CdS（Nb₂O₅）and Nb₂CT_x.Thanks to the synergistic effect between S-scheme andSchottky junction,photocatalytic activity and stability are enhanced.（2）By using the polycrystalline structure of CdS,the cubic/hexagonal CdS/Nb₂CT_xheterojunction is constructed,which has suitable band structure and excellent electron reduction ability,and improves the photocatalytic performance of CdS.When the loading capacity of Nb₂CT_x is 6 wt%,the results show that the photocatalytic hydrogen evolution rate is 4220.9μmol g^-1 h^-1,and the corresponding AQE at 420 nm is 19.4%.The structural characterization and mechanism analysis show that there are cubic/hexagonal CdS heterojunction and CdS/Nb₂CT_x heterojunction in the composites.The synergistic effect of phase junction and heterojunction improves the photocatalytic activity and delays the photocorrosion.（3）Two-dimensional（2D）Zn In₂S₄ nanosheets were prepared with the help of sodium citrate,exposing more（110）crystal faces to provide more active sites.The surface of Nb₂CT_xwas modified by using the adjustable surface chemical and electronic properties of MXene,and2D Nb₂CT_x with surface-OH terminal was prepared.On this basis,the 2D/2D Zn In₂S_4/Nb₂CT_xheterojunction were prepared.Using triethanolamine as sacrificant,when the mass fraction of few-layer Nb₂CT_x is 6 wt%,the composite has the best visible light photocatalytic hydrogen production activity,up to 3524.5μmol g^-1 h^-1,and AQE at 420 nm is 13.6%.Its hydrogen evolution rate is 10.2 times higher than original Zn In₂S₄.After 5 rounds of cycle test,the activity remained 91%.Energy band structure analysis combined with density functional theory（DFT）calculation shows that Nb₂CT_x adsorbed by-OH on the surface has low work function and high Fermi level.As a hole cocatalyst,it can effectively improve the photoactivity of Zn In₂S₄/Nb₂CT_x heterojunction and delay the photocorrosion of Zn In₂S₄.