Morphology Regulation Of Transition Metal Oxides By Post-etching Method And Its Photoelectric Catalytic Performance Research

The problem of energy shortage crisis and environmental pollution has become increasingly serious since the industrial age.Based on concept of green chemistry,clean energy has been widely used in the field of chemistry and chemical industry.Luminous and electric energy are typical sustainable energy,and its related catalytic system has been deeply studied in recent years.In the field of photoelectric catalysis,transition metal oxide（TMO）crystal catalysts have excellent application prospects.According to enormous efforts,nano-sized TMO crystals have shown considerable catalytic activity.The generation of active species in the reaction on the basis of structure-activity relationship between the photoelectrocatalytic process and catalysts is mainly affected by the morphology and surface active sites of the catalyst.Constructing catalyst crystals that are more conducive to specific catalytic reactions is an available strategy to enhance the efficiency of catalytic reactions.Compared with the"bottom-up"process for preparing regular morphologies,the"top-down"strategy can generally construct new crystal planes or hierarchical structures on a given crystal,and the optimal structure can be designed through the regulation of reaction conditions to prepare novel catalysts.In this study,the morphology and microstructure of the two kinds of common TMO crystals were precisely controlled by the post-etching method.The activity enhancement of crystal catalysts in photoelectric catalysis before and after etching was studied.This paper mainly focuses on the following two aspects:（1）Nano-cubic cobalt tetroxide was prepared as a model crystal（NCCo）by hydrothermal method,and sulfuric acid was selected as an etchant to generate new crystal planes on the pristine crystal surface and obtain the low surface state.Transmission electron microscopy and X-ray photoelectron spectroscopy showed that micro{111}facets were formed on the cube surface after etching,and the surface Co²⁺ratio was significantly increased.Combined with ion concentration measurement and DFT theoretical calculation,it is determined that sulfuric acid molecules mainly improve the exposure ratio of Co²⁺by attacking surface Co³⁺during the etching process.The oxygen evolution reaction（OER）performance were carried out to evaluate the catalysts activity,and the overpotential,Tafel slope and impedance of the etched cube were significantly reduced.The post-etching method was applied to various morphologies of Co₃O₄samples subsequently,and the electrocatalytic properties of the samples before and after the etching were evaluated.The obtained etched samples all showed excellent OER performance.The overpotential(η₁₀)is only 268 m V,and the Tafel slope is only 74 m V dec^-1,which is better than most pure cobalt tetroxide catalysts in the current research.（2）Based on the above post-etching method theory,hexagonal rod-shaped hematite（HRFe）crystals with mainly exposed{110}facet were prepared by hydrothermal method,and the organic oxalic acid was selected as the etchant for etching at different time under milder conditions.Transmission electron microscopy showed that only the axial length of the HRFe crystals changed and the new{001}facets were generated after etching.X-ray photoelectron spectroscopy demonstrated that the proportion of lattice Fe ^IIincreased slightly after etching.The ion concentration experiment proved that the electron transfer through the complexation of oxalate with lattice Fe ^IIIwas the main reason for the increase of lattice Fe ^II.The photodegradation dye performance were used to assess the samples activity before and after etching,and the sample etched for 24h（E-24h Fe）had the best photodegradation performance,and the degradation efficiency of the system under visible light irradiation for 35 minutes could reach 98%.