Preparation And Photo/electrocatalytic Properties Of Transition Metal Selenides

The world is now facing the problems of environmental pollution and energy shortage.Directly using green solar energy to degrade organic pollutants and electrolyze water to produce H₂through photocatalytic reactions is one of the most promising long-term solutions.Semiconductor photocatalysis is considered to be a technology that can effectively solve environmental pollution and energy shortages.It is an economical,clean and safe technology.At present,the mainstream semiconductor photocatalysts include TiO₂,g-C₃N₄,and bismuth-based semiconductor materials.Because TiO₂only absorbs ultraviolet light,the band gap of g-C₃N₄is large,and the bismuth-based semiconductor photogenerated carriers are recombined quickly,which results in low energy absorption and utilization.Therefore,it is urgent to develop a catalyst with strong visible light absorption ability.In the aspect of electrocatalytic H₂production,the high price and scarcity of precious metals greatly limit the large-scale commercial application.Therefore,non noble metal electrocatalysts for large-scale electrochemical hydrogen production will be developed in the future.Transition metal selenides may replace the main photocatalysts and expensive metal based photocatalysts.Therefore,different transition metal selenides were prepared by simple hydrothermal method and low-cost method and raw materials for visible light photocatalytic degradation of organic pollutants and electrocatalytic hydrogen evolution.In the field of photocatalysis:the advantage of using a single selenide as visible light photocatalytic degradation pollutant is that it does not need to be modified by doping,and it does not need to change the position of CB and VB in the catalyst,which reduces the cost to a certain extent and enriches the types of photocatalyst materials.In the aspect of electrocatalytic hydrogen preparation:because of its good catalytic performance in both acidic and alkaline conditions,because of its three different phases,it has different crystal structure,which affects the electrocatalytic activity of materials.Different methods have been reported to prepare transition metal selenides and use them to catalyze hydrogen evolution.However,it is still a big challenge to synthesize selenides with certain morphologies by simple hydrothermal method.In this paper,three different transition metal selenides were prepared by hydrothermal method.The crystal structure and morphology of the samples were characterized by XRD,SEM,EDS,XPS,BET and UV-Vis,and the growth mechanism of some materials was analyzed.The photocatalytic activity of Cu₂Se was evaluated by photodegradation of MB dye under simulated visible light.The electrocatalytic hydrogen evolution performance of the other two materials was tested by electrochemical workstation.The main results are divided into three parts:1.Using ascorbic acid(AC)as reducing agent and?-cyclodextrin(?-CD)as surfactant,large-size Cu₂Se thin hexagonal tablets were prepared by convenient and green way.The optical properties,morphology and crystal structure of the samples were measured by XRD,SEM,TEM,UV-Vis and XPS.The N-T-H(nucleation triangle hexagon)mechanism for the formation of Cu₂Se HSS(Cu₂Se hexagonal sheets)was proposed.The results show that the final morphology of Cu₂Se HSs is determined by the growth rate of?-CD and different crystal planes.The visible light catalytic performance of the prepared Cu₂Se HSs was studied by catalyzing methylene blue(MB).Under the synergy of H₂O₂,it was found that Cu₂Se HSs degraded 98.2%of MB within1 h.After 4 cycles of degradation,the degradation efficiency was still as high as 92.1%.2.In this study,sea urchin-like CoSe₂nanoparticles were synthesized by a simple one pot hydrothermal method using ascorbic acid for the first time.Then CoSe₂was immobilized on graphene oxide nanosheets with a few layers to form CoSe₂/GO conductive network(CSGN).It is found that different amount of graphene oxide can make CoSe₂show different catalytic activity.The results show that the Tafel slope of CSGN-30 is very low(38.2 mV/dec)when the doping amount is 30 mg,which is much lower than that of pure CoSe₂(98 mV/dec),and also lower than that of most reported CoSe₂based catalysts.Compared with pure CoSe₂,CSGN-30 showed faster kinetics and better electrocatalytic activity in hydrogen evolution.It is found that the overpotential of CSGN-30 is 184 mV at a current density of 10 mA/cm²,which is much smaller than that of pure CoSe₂(308 mV).Even after 1500 CV cycles,CSGN-30showed good cycle stability.3.Flower like?-Ni(OH)₂ precursor was synthesized from Ni(NO)₃,NH₄F and urea by a simple hydrothermal method(140?,12 h).Then,three kinds of nickel selenides with different stoichiometric ratios(NiSe₂,Ni₃Se₄,Ni_0.85Se)were synthesized by hydrothermal method.All of them showed similar flower like morphology,and the formation process was simply analyzed.The hydrogen evolution performance of the prepared samples was tested in 1 m KOH solution.Through the analysis of LSV curve,the results showed that the overpotential of NiSe₂reached 205 mV when the current density was 10 mA/cm².The potential of polarization curve is slightly higher than that of Ni₃Se₄and Ni_0.85Se,and Tafel slope is higher.The results show that the material has a strong catalytic active site,and provides an overpotential of 83 mV at a current density of 10 mA/cm²,with a Tafel slope of 43 mV/dec.Even after 1500 cycles of CV cycle,the LSV curve only has a displacement of about 8 mV.After 28 hours of current cycle,it still presents a more stable trend.