Study On Construction Of Surface Active Sites And Mechanism Of Water Splitting For Hydrogen Evolution Of Nano-ZnS-based Photocatalysts

Photocatalytic water splitting for hydrogen evolution is a potential sustainable energy technology to solve the energy crisis and environmental pollution.However,the development of this technology is limited by the activity,stability,and cost of photocatalysts.Zn S is one of the typical low-cost semiconductor nano-photocatalysts with fast photoelectric response and strong photogenerated electron reduction ability,but its photocatalytic activity and photoresponse range are not ideal,and there is photocorrosion.The construction of surface defects or metal monatomic active sites can effectively improve the activity and stability of semiconductor nano-photocatalysts.Therefore,the surface atomic structure of nano-Zn S semiconductor was regulated and three types of surface active sites models were constructed in this study.The photocatalytic mechanism of different active sites was studied,and the structure-activity relationship between the surface active sites and the photocatalytic performance was elucidated.It provides new ideas for the design and development of efficient,low-cost and stable photocatalysts.The main research contents and results of this paper are as follows:（1）Anionic vacancy（V_S）and cationic vacancy(V_Zn)were simultaneously constructed on the surface of Zn S by“occlusional coprecipitation”,and the concentration of the two vacancies was regulated by using antisolvent to control the heterogeneous nucleation rate,thus a defect Zn S（M-Zn S）with both photocatalytic activity and stability was obtained.Experimental and theoretical studies indicate that the two vacancy defects on the surface of M-Zn S can not only serve as active sites for photocatalytic reaction,but also regulate the band structure and dynamics process of photogenerated carriers,enhancing the photo-absorption ability of Zn S and the separation of photogenerated carriers.Increasing the distribution density of surface vacancy defects can fully expose and activate the catalytic activity of Zn and S atoms on the surface of M-Zn S,which reduces the activation energy of photocatalytic water splitting and increases the reaction rate of photocatalytic hydrogen evolution.The average hydrogen evolution rate（AHER）of the optimal sample M₃-Zn S is 9.41 times higher than that of pristine Zn S;under visible light（λ>400 nm）,the AHER is 576.07μmol?g^-1?h^-1,while the pure Zn S is unresponsive.The introduction of V_Zn and the strong interaction between unsaturated atoms at vacancy-pairs(V_pairs)upregulate the position of valence band,thus weakening the oxidation ability of photogenerated hole and inhibiting photo-corrosion.After the cyclic reaction for 45 h under the irradiation of300 W Xe-lamp,the AHER of M₃-Zn S remained and there was no sign of photo-corrosion.Therefore,the photocatalytic activity and stability are significantly improved by making full use of the advantages of anion and cation vacancies with different properties.（2）Taking advantage of the spatial confinement effect of V_Zn and V_S on the surface of M-Zn S and the strong interaction between active atoms and Cu²⁺,a nano-Zn S（Cu-Zn S）with two surface monoatomic active sites of Cu₁-at-V_Zn and Cu₁-at-V_S was obtained.The d-band interaction between Cu single-atoms（Cu-SAs）and the metal Zn atoms on the surface of support Zn S can promote the kinetic process of H intermediate to H₂ transformation.The interaction between Cu-SAs and p-band electron of nonmetallic S atom in Cu₁-at-V_Zn configuration is not only the key to anchoring Cu-SAs,but also leads to photogenerated electrons rearrangement in local structure to activate intrinsic Zn atom.This makes the adjacent Zn atom become the unsaturated state Zn^+δwith high activity,while the Cu-SA in the Cu₁-at-V_Zn configuration becomes the unsaturated state Cu^+δwith high activity.The electron-rich Zn^+δand Cu^+δnot only reduce the energy barrier of water splitting and become the adsorption sites of H intermediates,which promote the reduction of H intermediates to H₂ by photogenerated electrons.The AHER of the optimal sample Cu1.25-Zn S is 17.20 times of that of pure Zn S,and increases to 27.13 times after photoactivation;under visible light（λ>400 nm）,the AHER is 2.46 mmol?g^-1?h^-1;the apparent quantum yield（AQY）is significantly increased.Moreover,the unsaturated Zn^+δand Cu^+δcan be restored to the initial state without light.（3）Combining M-Zn S with Cu-Zn S and regulating the content of Cu-SAs and concentration of vacancies on the surface of Zn S,a Cu-SAs Zn S with high concentration of surface defects（Cu-M-Zn S）was obtained.Using M-Zn S with different concentration of surface vacancy as precursors,the strong interaction between Cu²⁺and active S atoms impels Cu to fill the V_Zn,and different concentration of V_S and V_pairs was leaved,thus the content of Cu-SAs and concentration of vacancies on the surface of Cu-M-Zn S were regulated simultaneously.Cu-SAs play a dominant role in photoelectron trapping effect and is the active site of photocatalytic water splitting.Defects increase the number of surface active atoms of nano-Zn S and regulate the energy band structure,mainly play a role in activating H₂O and promoting photo-absorption.The synergistic effect between them further improves the photocatalytic performance.The AHER of the optimal sample in Cu-M-Zn S is 27.79 times higher than that of pure Zn S;the AHER under visible light（λ>400 nm）is 3.77 mmol·g^-1·h^-1;the AQY is further increased.Therefore,nano-Cu-M-Zn S not only maintains the high activity and photoactivation of Cu-Zn S,but also improve the visible light absorption capacity.In addition,for the effect of the interaction between Cu-SAs and defect on the photocatalytic performance of Cu-M-Zn S,the d-band center theory is no longer applicable,but the change of p-band electron needs to be taken into account.It is more reasonable to use the change of the distance between the d-band center and the p-band center(?_d-p)to measure the catalytic performance.The results show that the decrease of?_d-p indicates the improvement of the activity of the catalyst and its comprehensive performance for the adsorption and desorption of H intermediate.