Green Preparation And Photo/Electrocatalytic Properties Of Molybdenum And Cobalt Selenides

In recent years,the problems of environmental pollution and energy shortage have received increasing attention with the development of science and technology and the rapid growth of population.Photocatalytic technology,which uses photocatalysts to break down organic pollutants into small molecules such as water and carbon dioxide under sunlight,has become an important method to solve environmental problems.Therefore,the development of economical and environmentally friendly semiconductor photocatalysts has become the key to realize this technology.Since the conventional photocatalysts Ti O₂ and g-C₃N₄ are affected by factors such as limited absorption spectra or wide band gaps,the development of a photocatalyst with suitable band gaps,easy preparation,excellent performance,and cheap and environmentally friendly is needed for large-scale commercial applications.Electrochemical hydrolysis,including hydrogen precipitation reaction（HER）and oxygen precipitation reaction（OER）,is an important approach to solve the energy problem.For the OER process,the mainstream Ru/Ir-based compounds have low overpotential and Tafel slope,but their application is limited by the high cost and scarcity of materials.Therefore,it is important to develop OER electrocatalysts with low price,good electrocatalytic activity and excellent stability.Transition metal selenides have attracted much attention due to their low forbidden band width and good electrical conductivity,and their good physicochemical properties,easily adjustable electronic structure,special layered structure,and low cost make them widely used in fields such as photo,electrocatalysis.However,since most of the preparation methods of selenides are complicated and the simple and effective hydrothermal methods often require highly toxic or explosive raw materials such as hydrazine hydrate or sodium borohydride as reducing agents,it remains a great challenge to develop a new synthetic route for the preparation of transition metal selenides by hydrothermal methods.In this thesis,two catalyst materials,MoSe₂ and Co₉Se₈,were prepared by hydrothermal method using melamine-oxalate polymer as reducing agent,surfactant and template.The materials were characterized by XRD,SEM,FESEM,EDS,XPS,Raman,BET,and UV-Vis to analyze the physical phase,morphological structure,chemical composition,specific surface area,and physicochemical properties of the band gap.The prepared materials were used in photocatalytic degradation performance tests or electrocatalytic OER tests to analyze their catalytic activities.The main results of the study are as follows:1.Multilayered MoSe₂ nanosheets were successfully prepared by a hydrothermal method.XRD,EDS,TEM,FESEM,BET,XPS and UV-Vis spectroscopy were used to characterize the composition,morphology,specific surface area and band gap of the product.In addition,the photodegradation experiments of Rhodamine B and methylene blue were carried out by using such MoSe₂ nanosheets.The degradation of Rhodamine B reached 94.4%in 40 minutes,while the degradation of methylene blue reached 95.1%in50 minutes,respectively.After four cycles of degradation,it still maintained a high catalytic activity.In addition,the formation mechanism and photocatalytic mechanism of MoSe₂ nanosheets were explored,and the TAN growth mechanism was proposed to explain the important role of melamine-oxalate polymer as reducing agent,surfactant and template in the synthesis of MoSe₂.2.MoSe₂ nanomaterials were successfully prepared by one-pot hydrothermal method according to different raw material ratios.The material phases,phase structure and chemical composition were characterized using XRD,FESEM,EDS and Raman.The prepared samples were used for the electrochemical oxygen precipitation reaction（OER）test in alkaline environment,and it was found that the sample MS1.6 exhibited superior catalytic activity,possessing the lowest overpotential（260 m V）and the smallest Tafel slope（117 m V/dec）when reaching a current density of 10 m A/cm².The best sample MS1.6 was tested for stability,and the current density basically maintained a stable trend after 55 h of current cycling,proving that the material has relatively excellent durability.3.Sea urchin-like Co₉Se₈ crystals were successfully prepared by the green hydrothermal method.The samples were characterized observationally by means of XRD,SEM,EDS and XPS tests.The prepared Co₉Se₈ crystals were used for photocatalytic degradation of Rh B solution,and the experimental results showed that the degradation efficiency of Co₉Se₈ crystals could reach 98.1%within 35 min in the presence of 1000μL H₂O₂.The sample was tested for cyclic degradation,and it was found that the sample still had good degradation efficiency after four times of degradation,indicating that the sample has certain cyclic stability.In addition,the sample was also tested for degradation of MB solution,and it was found that the sample reached a degradation efficiency of 91.1%after 70 minutes of light exposure,which proves that it has the ability of dual pollutant degradation.