Study Of Synthesis Of Zn_1-xCd_xS Nanomaterials And Its Photocatalytic H₂ Production Performance

Photocatalytic hydrogen production from water is a green technology that can solve the problem of environmental pollution and energy shortage by using solar energy.Finding highly efficient catalysts represents the key to the photocatalytic technology.CdS is one of the most important semiconductor material for photocatalysis,especially one-dimensional(1D)CdS nanorods with wurtzite structure have attracted many scholars'attention.Previous studies indicate that(001)facets of rod-like CdS with serve as the primary catalytic plane,and two dimensional(2D)CdS nanosheets or nanoplates with dominant(001)facets show much higher activities.In addition,considering the combination with ZnS to form Zn_1-xCd_xS solid solutions is an effictive way to improve the photocatalytic activities of CdS,so the design of 2D Zn_1-xCd_xS solid solution with dominant(001)facets are of great significance to improve its photocatalytic performance.However,very few related works have been reported up to now.Therefore,preparation of 2D Zn_1-xCd_xS solid solution with dominant(001)facets are of great significance.In this work,2D Zn_1-xCd_xS with dominant(001)facets were sucssesfuly prepared via a solvothermal methods by using ethylenediamine(EDA)and ethylene glycol(EG)as solvents,the effects of reaction temperature,value of x,Zn/Cd source concentration,EDA/EG ratio,and reaction time on the morphology and it's photocatalytic activity were investigated.For the first time,direct imaging of the nano-scalled electrical field in Zn_0.5Cd_0.5S were pereformed by using off-axis electron holography,confirming their electrostatic asymmetry.Furthermore,the appropriate band gap and potential of conduction/valence band makes InNbO₄ to be an ideal material to form heterojunction with Zn_0.5Cd_0.5S.Therefore,the synthesis of InNbO₄ nanoparticles by polyacrylamide sol gel method and their photocatalytic properties were also studied.The main research contents and results are as follows:1.The effects of different x values at different temperatures on morphology,crystal structure,and photocatalytic performance of the as-prepared Zn_0.5Cd_0.5S samples were preliminarily explored.The results show that all the samples prepared under 160-220 ? are Zn_1-xCd_xS solid solution with WZ structure,and the morphology of the samples changed from 1D nanorods to the mixtures of 2D nanoplates and 1D nanorods when the reaction temperature is raised to 200 ?.No significant difference in morphology was observed between the Zn_1-xCd_xS samples with different x values.Photocatalytic experiments show that the Zn_1-xCd_xS samples with x=0.5 exhibits highest catalytic activity,and the samples prepared at higher temperature exhibits higher photocatalytic activity.In addition,higher concentration of the Zn/Cd source has negetive effect on the formation of 2D plate-like particles and catalytic activity.2.The effects of different EDA/EG volume ratios on the morphology,crystal structure,and photocatalytic performance of the as-prepared Zn_0.5Cd_0.5S samples under different temperatures were studied.The results show that when the volume of EDA was about 40 mL and the the temperature were 200-220 ?,the as-prepared samples are mainly the mixtures of 2D nanoplates and 1D nanorods.When the volume of EDA was about 10-30 mL,the as-prepared samples are mainly rod-like particles,and the width of nanorods increases from 5 nm to 10 nm as the temperature increases from 160 to 220 ?.However,when the volume of EDA was about 50 mL,the as-prepared samples were mainly irregular particles.The results of photocatalytic experiments show that the samples containing nanoplates show relatively higher photocatalytic activity.3.The effects of different reaction times on the morphology,crystal structure,and photocatalytic performance of the as-prepared Zn_0.5Cd_0.5S samples were studied,and the temperature,volume of EDA,and concentration of the Zn/Cd source was set at 220 ?,40 mL,and 0.5 mmol,respectively.The results showed that the nanoplates were formed within 6 h,the proportion of nanoplates in the samples increased,while nanorods decreased with the extension of the reaction time.The optimal reaction time for the synthesis of Zn_0.5Cd_0.5S nanoplates is 60-72 hours,in which the as-prepared sample are mainly nanoplates,which show the highest photocatalytic activities,i.e.,the average hydrogen generation rate up to 610?mol?h^-1?mg^-1 under visible light irradation,which further confirmed that the formation of nanoplates can improve its photocatalytic activity.4.The intrinsic mechanism of high photocatalytic activity of the nanoplates for hydrogen production was studied.The pure 2D Zn_0.5Cd_0.5S nanoplates were obtained after the rod-like particles were separated.1D nanorods and 2D nanoplates samples were systematically characterized by element mapping,HAADF-STEM,BET,UV-Vis,ESR,PL,Mott Schottky plots,photocurrent and off-axis electron holography,and the relationship between these characteristics and photocatalytic activity was analyzed.The HR-TEM results show that the nanorods are mainly WZ structure,and a small part of zinc blende structure(ZB)exists at the two sides of the nanorods to form twin structure,which arranged as WZ-ZB-WZ.The nanoplates are mainly WZ structure with(001)facets as the main exposure surface.The results of photocatalytic experiments show that the nanoplate samples exhibit high photocatalytic activity and photocurrent,and their photocatalytic activity were hardly affected by the concentration of sacrificial agent.Theoretical calculation and off-axis electron holography show that the direction of the electrostatic field in the Zn_0.5Cd_0.5S is the[001]direction of the crystal,leading to much larger polar surface area of the nanoplate compared with the rod like particles,so that the photoinduced electrons can participate in the reaction quickly without jumping back to the VB and then recombination with the holes.Therefore,the nanoplates exhibits super high photocatalytic activity which were hardly affected by the concentration of sacrificial agent.This result can be applied to other ionic crystal materials with built-in electric field.5.Nanosized InNbO₄ was prepared via an apolyacrylamide sol-gel route.The obtained InNbO₄ nanoparticles exhibits excellent photocatalytic H₂ evolution rates up to68.1?mol?h^-1?mg^-1,much better than the reported counterparts obtained via solid-state reaction or doping treatments.The particle sizes,bandgap energy,and the nano-voids are modulated by using different chelating agents,including tartaric acid,citric acid,and acetic acid.Based onthe HAADF-STEM characterizations,it is interesting to discover that the photocatalytic performance shows obvious dependence on the voids size.Nano-voids(2�8 nm)can provide more active sites and thus enhance the photocatalytic perfor-mance,while lattice-scale voids(<2 nm)may become a carrier trapping defects and thus decrease the photocatalytic performance.