Scanning Tunneling Microscope Imaging And Oxygen Adsorption Of Oxidation-reduction Catalysts

Phthalocyanine porphyrins and their derivatives have a good prospect in the fields of molecular information storage and molecular electronics by virtue of their unique structure and function.In recent years,molecular self-assembly can be carried out in a"top-down"way.The difference in molecular structure of materials,the interaction force between molecules and substrates,and the interaction between molecules lead to the different shapes of self-assembled structures of two-dimensional materials,which is also the most effective way to construct nanomaterials and structures.As the technology of scanning tunneling microscope（STM）to study the self-assembly process of nanomaterials through tunneling effect becomes more and more mature,the research of STM in surface interface reaction,molecular dynamics and other fields becomes more and more extensive.Based on STM,we selected phthalocyanine compounds and their derivatives as research objects to study the molecular self-assembly process under the action of non-covalent bonds.The growth structure of molecules was observed by changing the different growth modes of molecules,the choice of solvent,the concentration of solution and temperature.We can study catalytic oxygen reduction（ORR）of growth materials by in-situ electrochemical scanning tunneling microscopy（ECSTM）.Oxygen adsorption and desorption could be observed by changing the electrode voltage.It has been found that single-atom catalysts of transition metal-based COF materials have superior performance over precious metals.In this paper,non-pyrolytic method and soft template method are used to carry out heteroatomic doping and loading,so as to change the properties of graphene,construct a new three-dimensional structural material,further improve the catalytic activity of oxygen reduction of the material,and test its application in rechargeable zinc-air batteries.The main research contents,innovations and results of this dissertation are as follows:（1）COF_BTC-Fe self-assembly was performed on highly oriented pyrolytic graphite（HOPG）and Cu（111）substrates by microfluidic method.Soluble COFs were synthesized by microwave synthesis method,which has the characteristics of fully conjugated two-dimensional continuous rigid structure,periodic and abundant Fe-N-C charge center,excellent ORR catalytic performance and excellent processability.By microfluidic method,the solution containing COF_BTC-Fe is added uniform Ly to the highly oriented pyrolytic graphite substrate to form the self-assembly structure of COF material on HOPG.Different influence parameters are changed,including substrate,self-assembly mode,solvent,concentration and temperature.The structural appearance under different influence parameters is observed by scanning tunneling microscope.To find out the most suitable self-assembly method:selecting noctylbenzene as solvent,preparing COF_BTC-Fe solution with concentration of 10^-5 mol/L,and using microfluidic method to complete self-assembly on HOPG substrate at room temperature.Combined with STM,SEM and other characterization methods,it was proved that the grown structure was COF_BTC-Fe.（2）Electrochemical scanning tunneling microscope（ECSTM）was applied to observe the adsorption mechanisms of oxygen reduction under acidic conditions.The HOPG-COF_BTC-Fe material obtained in the first work was transferred to the electrolytic cell,the sweep speed was set at 100 m V/s,and the cyclic voltammetry（CV）scanning was carried out under the acidic condition of oxygen saturation and concentration of 0.1 M HCl O₄.The voltage of oxidation peak and reduction peak was found to be 0.2 V and-0.3 V.Set the bias voltage to the value of the range class in the interval.By observing structural changes before and after oxygen reduction by ECSTM,the core ions of highly ordered COF_BTC-Fe layer showed uniform high contrast in oxygen saturated electrolyte,which could be attributed to the formation of COF_BTC-Fe-O₂ complex.Results showed that the contrast could change significantly when ORR reaction was triggered by changing electrode potential.The difference of material changes before and after oxygen reduction reaction was further proved by theoretical simulation.（3）A three-dimensional oxygen reduction catalyst with definite structure was prepared.By soft template method,melamine and polythiocyanuric acid were used as dopants to polymerize and self-assemble on graphene oxide（GO）.The samples obtained by freeze-drying were then reacted in a tube furnace to obtain 3D hierarchical porous nitrosulfur-co-doped graphene（3D N/SG）.Then 3D N/SG was used as the carrier.Loaded with COF_BTC-Fe,the product 3D N/SG@COF_BTC-Fe was formed in a high-pressure hydrothermal kettle at high temperature with three-dimensional nitrogen and sulfur co-doped graphene loaded with COF_BTC-Fe.The physical characterization of the catalyst was conducted by SEM,TEM,nitrogen desorption,Raman spectroscopy.A porous layered structure with many mesoporous and large pores was showed by the morphology of the catalyst.Cyclic voltammetry,alternating-current impedance and other electrochemical performance tests show that the ORR half-wave potential reaches 0.87 V in basic 0.1 M KOH and the electrochemical specific area can reach 172.09 m² g^-1.Further,when 3D N/SG@COF_BTC-Fe is used in rechargeable zinc-air batteries,the discharge power density reaches 163.6 m W cm^-2.