One-Pot Colloidal Synthesis Of Zn_xCd(1-x)S Nanoparticles With Nano-twin Structure For Photocatalytic Hydrogen Evolution

With the development of economy and industry,environmental pollution and energy shortages have become severe problems in recent years.Hydrogen energy has become a new type of ideal renewable energy because of its non-toxicity,good combustion performance,and high utilization rate.Photocatalytic H2 evolution from water splitting by sunlight is one of the most promising solutions in the development of renewable energy.Metal sulfides are the most potential visible light catalytic materials among many catalytic materials.Among them,Zn_xCd_1-xS solid solution has attracted much attention as a high-efficiency photocatalyst for the photocatalytic decomposition of water to produce hydrogen under visible light.In recent years,with the continuous research on the principle of photocatalysis,the rapid recombination of photogenerated carriers into the catalyst is considered to be one of the main obstacles to the improvement of catalytic performance.Faced with this challenge,the researchers tried a variety of strategies,such as building heterojunctions,using co-catalysts,and so on.Among them,the construction of nano-twin structure in the catalyst is a relatively novel way,which could efficiently improve the separation efficiency and migration ability of electron/hole?e/h?pairs.The existing reported Zn_xCd_1-xS nanomaterial with nano-twin structure exhibit excellent visible light H2 evolution performance.However,the current methods for preparing Zn_xCd_1-xS nanomaterials with nano-twin structure are relatively simple,and there are some shortcomings,such as the use of high-pressure equipment,complicated experimental procedures,and poor safety.At this work,we explored the synthesis of Zn_xCd_1-xS nanomaterials with nano-twin structures by colloidal synthesis routine.This synthesis routine has the advantages of strong controllability,simple operation,and supply of an environment without water and oxygen.After repeated attempts to adjust various synthesis conditions,the Zn_0.6Cd_0.4S nanoparticles?NPs?with nano-twin structure using thiourea-metal complex gels as precursor successfully prepared by colloidal synthesis routine.The formation of Zn_0.6Cd_0.4S solid solution was confirmed by X-ray diffraction pattern?XRD?,ultraviolet-visible absorption spectroscopy?UV-Vis?,inductively coupled plasma spectrometer?ICP?.The transmission electron microscopy?TEM?images showed that the prepared Zn_0.6Cd_0.4S NPs are spherical or short-rod monodisperse nanoparticle with an average size of about 6 nm;the apparent zigzag shape from the high-resolution transmission electron microscopic?HRTEM?images and the corresponding selected area electron diffraction?SAED?pattern can be demonstrated the formation of nanotwin structure.In further research,we explored the effects of two factors,precursor and solid solution composition,on the formation of the nano-twin structure in Zn_xCd_1-xS NPs.First,we prepared two comparative samples of Zn_0.6Cd_0.4S-S and Zn_0.6Cd_0.4S-exan using sulfur powder and ethyl xanthate as new precursors,respectively.Although the XRD spectrum,Zn/Cd ratio and particle size distribution of the three samples were similar,no nano-twin structure was found in the HRTEM images of Zn_0.6Cd_0.4S-S and Zn_0.6Cd_0.4S-exan samples.UV-Vis spectroscopy tests showed that there was no significant difference in the band gap size of them.The ESR and EIS tests showed that the carrier separation of the Zn_0.6Cd_0.4S-twin sample was significantly better than that of the two comparative samples.Moreover,we found that the overpotential of H2 reduction reaction on Zn_0.6Cd_0.4Stwin NPs surface is lower than that on Zn_0.6Cd_0.4S-S and Zn_0.6Cd_0.4S-exan surface by the polarization curve of H2 evolution reaction?HER?.Therefore,the photocatalytic reduction reaction is more likely to occur on the Zn_0.6Cd_0.4S-twin NPs surface.Finally,the H2 evolution tests of three samples shown that the Zn_0.6Cd_0.4S-twin NPs achieve the highest H2 evolution rate 1926 ?mol/h/g,which is nearly four or five times more than the H2 evolution rate of Zn_0.6Cd_0.4S-exan NPs?479 ?mol/h/g?and Zn_0.6Cd_0.4S-S NPs?408 ?mol/h/g?.These experimental results confirmed that the nano-twin structure is only present in the sample using the thiourea-metal complex gels as the precursor.We suspect that the dense metal ion concentration in the gels precursor and the complexing agent containing the amino group may be the key reason for the formation of nano-twin structures in colloidal synthesis route.After then,we prepared different compositions of Zn_xCd_1-xS NPs using thioureametal complex gels.XRD,ICP,and UV-Vis results confirmed the formation of Znx Cd1-xS solid solution.The TEM images showed that there was no significant difference in the morphology of the different Zn/Cd radio.The ESR spectra and EIS test shown that the samples with x=0.4,0.6 and 0.8 formed the nano-twin structure and with a nanotwin concentration sequence of x=0.6 > x=0.4 > x=0.8.However,the samples of x=0,0.2,and 1 did not form a nano-twin structure in NPs.Also,the H2 evolution performance of different Zn/Cd radios of Zn_xCd_1-xS NPs shown that the Zn_xCd_1-xS NPs with nano-twin structure?such as x=0.6,x=0.4?have superior H2 evolution rate than the NPs without nano-twin structure?such as x=0.2,x=0,x=1?under visible light irradiation.After the ultraviolet light irradiation was used to exclude the difference in light absorption properties of different samples,the sequence of the H2 evolution rate under UV light of the Zn_xCd_1-xS NPs?x=0.6 > x=0.4 > x=0.8 > x=0.2 > x=0 > x=1?is consistent with the extent of spatial separation of e/h pairs characterized by ESR spectra and EIS tests.These experiments demonstrate that the concentration of the nano-twin structure can be adjusted by adjusting the Zn/Cd ratio in the Zn_xCd_1-xS NPs.This adjustability is presumed to be caused by the difference in the self-generating structure of ZnS and Cd S under mild experimental conditions.This research not only provides a simple and efficient synthesis process of Znx Cd1-xS NPs with nano-twin structure but also proposes a new idea and direction for the further application of colloidal synthesis in the synthesis of sulfide nanomaterials.