Controllable Preparation And Catalytic Performance Of Two-dimensional Nanomaterials

With the rapid growth of the sustainable energy economy,fossil fuels cannot satisfy the increasing global energy demands any more due to their limited reserves as well as series of environmental problems they cause.As a result,it is particularly important to look for renewable clean energy sources to serve as substitutes.Hydrogen fuel,which can be produced through water splitting,is one of clean renewable energy sources.Water electrolysis,composed of hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）,is considered as the most promising method for water decomposition due to not only the abundant reserve of water in nature,but also it is simple controllable products（hydrogen in HER and oxygen in OER）.Catalysts will play an important role in water electrolysis which will reduce the kinetic potential,promote the production of hydrogen and improve the reaction efficiency.Therefore,it is important to find stable and efficient oxygen evolution catalysts.Herein,we choose spinel Co₃O₄ as water electrolysis catalyst and choose oxygen evolution reaction as label reaction.We prepare precursor through solvothermal method first,and then we obtain the Co₃O₄ powder through annealing of precursor in the air.Besides,we adopt gold and urea to modify our sample in order to improve the catalytic performance.Structural characterizations and electrochemical tests are also conducted for each sample.The main work of this thesis can be summarized as follows:1.The precursor is obtained by solvothermal.Then the precursor obtained is annealed to obtain Co₃O₄ nanosheets.The crystal phases are confirmed by X-ray Diffraction（XRD）,and they are alpha-Co（OH）₂ and Co₃O₄.Co₃O₄ and its precursor are two-dimensional sheets confirmed by SEM.TEM confirms the results of XRD and SEM.The binding energy of each element can be inferred through XPS results.These materials are then subjected to OER catalytic test using a CHI660 electrochemical workstation.During the test,the color of the precursor changes from green to brown.Confirmed by XRD and SEM,the brown product is CoOOH.When the current density reachs 10 mA·cm^-2,the overpotentials of alpha-Co（OH）₂,Co₃O₄ and CoOOH are 328mV,287 mV and 233.5 mV,respectively.The Co₃O₄ nanosheets have better catalytic performance than the precursor,while the CoOOH obtained after electrochemical oxide has better catalytic performance than Co₃O₄ nanosheets.2.Nanowires and nanosheets precursors are obtained by solvothermal method with urea modified.Both precursors are followed by annealing to obtain Co₃O₄ nanowires and nanosheets,respectively.The OER test shows that the catalytic performances of these two catalysts are similar.When the current density was 10 mA·cm^-2,the overpotential of the Co₃O₄ nanosheets are about 80 mV lower than that of the urea-modulated catalysts,indicating that the Co₃O₄ nanosheets exhibit higher catalytic performance than adjusted materials.3.In order to improve the catalytic activity of Co₃O₄ nanosheets,Au doping is considered in this thesis.The Au/alpha-Co（OH）₂ nanosheets are obtained.The Au/alpha-Co（OH）₂ nanosheets are then annealed at 400℃for 5 minutes to obtain Au/Co₃O₄.The mapping diagram of TEM confirms that Au nanoparticles are small in size and distribute uniformly.The CHI660 electrochemical workstation is adopted to perform OER catalytic tests.The color of the Au/alpha-Co（OH）₂ changes rapidly from green to brown and the crystal phase also changes during the test.The sample obtained after the electrochemical oxide of the precursor exhibits the best catalytic performance.Of all the Co₃O₄ nanomaterials synthesized in this thesis,the two-dimensional Co₃O₄nanosheets exhibit the highest catalytic performance.