Zn_0.3Cd_0.7S Nanorods Loaded With Noble-metal-free Ni₃C Co-catalyst Enhancing Photocatalytic Hydrogen Evolution

With the development of social economy and industry,environmental and energy problems are becoming more serious.In recent years,the scientific community has paid particular attention to the development of renewable energy in nature.For example,the use of wind and tidal energy to generate electricity,and the use of photolysis water to generate hydrogen is one of the most promising solutions in the development of renewable energy.With the continuous in-depth study of the principle of photocatalysis,it has been found that the rapid recombination of photo-generated carriers inside the catalyst is one of the main obstacles to the improvement of catalytic performance.For this problem,researchers have tried a variety of strategies,such as:growing heterojunctions,supporting cocatalysts,and building nano-twin structures.Among them,the co-catalyst is a relatively novel and feasible method,which has significantly improved the separation efficiency and migration ability of the charge.Most of the highly effective cocatalysts currently studied are noble metals（Pt,Pd,Au）,and noble metals have been proven to be the most efficient cocatalysts.However,since noble metal cocatalysts are expensive and scarce,this has led to their large-scale application.There are big restrictions.Therefore,the synthesis of a non-precious metal co-catalyst has become a hot research direction.Before synthesizing the cocatalyst,it is necessary to determine a host material with excellent performance.Due to the superiority of the ZnCdS solid solution nanomaterial itself in the field of photocatalysis（adjustable band gap and better carrier separation rate）,Studies on supported non-noble metal cocatalysts for photolysis of water have not been reported.Among many reported cocatalysts,metal carbides,as cocatalysts for photolysis of water,have proven to be an efficient and low-cost alternative to precious metals.Based on the above research status,we prepared a Ni₃C/ZnCdS photocatalytic composite nanomaterial by a low-temperature and simple colloidal synthesis method and studied its photocatalytic performance.In this paper,we first explored the synthesis of ZnCdS nanorods based on colloidal synthesis.Using metal-diethyldithiocarbamic acid（DDTC）xerogels as reaction precursors,tetrapod-shaped Zn_0.3Cd_0.7S nanorods were successfully prepared.Due to their special nano-structures（homojunction）compared with other morphologies of ZnCdS nanomaterials,it has a higher photocatalytic performance.After the successful synthesis of tetrapod-shaped Zn_0.3Cd_0.7S nanorods,Ni₃C nanoparticles were grown on Zn_0.3Cd_0.7S nanorods by solvent heat method.Named Ni₃C/Zn_0.3Cd_0.7S nanorods composite photocatalytic material.X-ray diffraction（XRD）,Ultraviolet-visible light absorption spectrum（UV-Vis）,X-ray photoelectron spectroscopy（XPS）,etc.have jointly confirmed the successful synthesis of Ni₃C/Zn_0.3Cd_0.7S composite nanomaterials.From the transmission electron microscopy image（TEM）,we know that Zn_0.3Cd_0.7S is a monodisperse nanorod,and its average size is about 30 nm.It can be proved that Ni₃C nanoparticles were successfully grown on Zn_0.3Cd_0.7S nanorods and dispersed uniformly.We used a photocatalytic activity evaluation system to test the hydrogen production performance of the sample.The test found that the hydrogen production of the Zn_0.3Cd_0.7S sample was 517.4μmolg^-1h^-1,and the Zn_0.3Cd_0.7S-1%Ni₃C sample had the highest hydrogen production rate.It is 3310.1μmolg^-1h^-1.It can be seen that the addition of a certain amount of cocatalyst Ni₃C can significantly improve the photocatalytic performance of Zn_0.3Cd_0.7S nanorods,up to 6.4 times.In order to further explore the reasons for improving the photocatalytic performance,UV-vis and band gap calculation methods were used to find that the absorption edge of the Zn_0.3Cd_0.7S-1%Ni₃C sample changed significantly and the band gap was significantly reduced to 2.46 eV.Further with the help of XPS valence band analysis and Mott-Schottky measurement,it was found that loading 1%Ni₃C nanoparticles changed the energy band structure of Zn_0.3Cd_0.7S nanorods,that is,the position of the bottom of the conduction band was significantly positive Shift,the position of the top of the valence band has a slight positive shift,thereby reducing the band-gap.The bandgap structure of the Zn_0.3Cd_0.7S and Zn_0.3Cd_0.7S-1%Ni₃C samples was also measured using a more accurate UPS analysis.The band structure was found to be consistent with the previous test results（XPS and MS）,which reduced the band-gap and enhanced visible light absorption.A series of carrier separation tests were conducted:photoluminescence spectrum（PL）,photo-current（i-t）,and impedance（EIS）.It can be clearly seen that loading 1%Ni₃C cocatalyst can significantly improve the electron-hole separation efficiency of the sample;H₂-evolution kinetics:The polarization curves and Tafel slopes also show that the use of co-catalyst can increase the active site and reduce the hydrogen evolution potential.We also evaluated the effect of the addition of Ni₃C cocatalyst on the stability of the sample.Through repeated experiments（15 hours of visible light irradiation）,it was found that the hydrogen production of the Zn_0.3Cd_0.7S sample was reduced by 35%,while the hydrogen production of the Zn_0.3Cd_0.7S-1%Ni₃C sample was only reduced by 20%,which was a significant improvement on the stability of the photocatalyst.In addition,comparing XPS data before and after the cycling experiments found that the valence state of Ni element did not change,which also proved that Ni₃C as a co-catalyst itself has good stability.And by analyzing the content of Ni element in the composite before and after the cyclic experiment,it is shown that the shedding of Ni₃C nanoparticles is the reason that the hydrogen production performance of the Zn_0.3Cd_0.7S-1%Ni₃C sample is reduced.In the research of this paper,a simple and efficient synthesis process of Ni₃C/Zn_0.3Cd_0.7S nanorods is summarized,which makes the photocatalytic hydrogen production performance of Ni₃C/Zn_0.3Cd_0.7S nanorod composites compared to pure the Zn_0.3Cd_0.7S nanorods were increased by 6.4 times,while significantly enhancing the stability of pure Zn_0.3Cd_0.7S nanorod photocatalytic materials.In addition,by studying the mechanism of improving the photocatalytic performance of Zn_0.3Cd_0.7S nanorods by loading a certain amount of Ni₃C nanoparticles,this also provides new research ideas and directions for the use of cocatalysts to replace precious metals,and has important research value.