Study On Defect Regulation,Photocatalytic And Electrocatalytic Properties Of Transition Metal Compounds

Catalytic technology is one of the effective ways to realize energy conversion.Among them,catalytic water splitting has attracted researchers’attention and has been a hot research.Based on the different energy sources,they are mainly divided into photocatalysis and electrocatalysis.Photocatalytic technology uses solar energy to directly decompose water into H2 and O2,or generates free radicals(·OH,·O2-,etc.)to degrade pollutants;electrocatalytic technology uses electrical energy to convert water into H2 and O2.Precious metals and their compounds are usually excellent photo-and electro-catalytic materials.However,their high prices and scarcity restrict wide applications.Transition metal compounds have become promising candidate materials due to comparable activity and abundance,nevertheless,they should also overcome challenges such as activity,limited active sites,and stability.Therefore,it is urgent to develop new strategies to optimize the crystal and electronic structure of the materials to enhance their activity and stability.Defects can be adopted to regulate the crystal and energy band structure,promote charge redistribution,increase the number of surface coordinated unsaturated active sites of the catalyst,optimize the adsorption energy of reactants intermediate,and improve the conductivity of materials,which makes defect-engineering a feasible and effective catalytic performance control strategy.In recent years,many researchers have adopted defect engineering to improve the catalytic performance of materials,and have achieved some impressive results.However,the types of defects(such as the low-electron-density defect,lattice distortion defect,crystal/amorphous interface defect),the mechanical effect of the defects on the catalytic process(such as the regulation of the oxygen vacancy defect on the OER mechanism),as well as the qualitative and quantitative relationship between the defects and the properties of the materials,are not clear enough,and should be further investigated.Therefore,it is extremely important to select some suitable materials for defect design,use advanced test techniques for characterization,and explore the influence and mechanism of defects on the physical properties of materials.Considering some unclear scientific issues in the research about defects of catalysts,this thesis starts by exploring the relationship between defects and the catalytic activity of materials.Using transition metal oxides and phosphides as model materials,different kinds of defects have been engineered in materials through atom doping and heat treatment.The defects and surface electronic structures were characterized with high-resolution transmission electron microscopy(HRTEM),positron annihilation spectroscopy(PAS),X-ray absorption spectroscopy(XAS),and X-ray photoelectron spectroscopy(XPS).Combining the photocatalytic and electrocatalytic performance measurement results,the influence of the corresponding defects on the catalytic reaction mechanism is discussed.The specific content of this thesis is as follows:Chapter one briefly introduces the research background of this thesis.The mechanisms of photocatalytic and electrocatalytic reactions are reviewed at first,then the types and synthetic methods of the defects in transition metal compounds catalysts,as well as their influences on catalytic activity are discussed.Finally,unresolved scientific problems in this field are summarized.In the work of chapter two,PAS technology has been used to reveal the qualitative and quantitative relationship between the low-electron-density defect and the photo(electro)chemical properties,and the internal regulation mechanism is also discussed.The titanium-doped Na0.5Bi2.5Ta2O9 single crystal nanosheets were synthesized by the modified molten salt method.The types of defect in the titanium-doped samples were systematically studied by PAS and XPS,and a qualitative and quantitative relationship between the low-electron-density defect and the properties of the material has been demonstrated for the first time by studying the generation of·O2-,photocurrent,photodegradation rate,etc.More importantly,it was found that the correlation was not only valid for Na0.5Bi2.5Ta2O9 but also was universal for various materials and photochemical processes,which has been overlooked in previous reports.Considering the principles of fluorescence lifetime spectroscopy(PL)and PAS,we proposed the mechanism of low-electron-density defects on the photochemical process.In the third chapter,we introduce the mechanism of lattice oxygen activation and improvement of OER performance in(Bi0.5Co0.5)2O3 by tuning oxygen vacancy defects.For the proposed adsorption evolution mechanism(AEM)route,the universal scaling relation between the adsorption energy of OOH*and OH*leads to a limited OER efficiency based on the "volcano curve".A possible solution to bypass the scaling relation is to avoid the formation of the OOH*intermediates in the OER process with the participation of lattice oxygen from the catalyst.By adjusting the oxygen vacancy defect concentration,the Fermi level and the hybridization between M 3d and O 2p of transition metal oxide are changed,so that it is possible to activate the lattice oxygen and improve the conductivity of the material.In the work,tetrahedral(Bi0.5Co0.5)2O3 with different oxygen vacancy concentrations was synthesized by co-precipitation method combined with heat treatment under different atmospheres.Compared with oxygen-vacancy-poor(Bi0.5Co0.5)2O3,oxygen-vacancy-rich(Bi0.5Co0.5)2O3 exhibited a much lower Tafel slope(43 mV/dec),15 times higher mass activity,18 times higher turnover frequency,excellent long-term stability in alkaline solutions,superior to those of the commercial benchmark OER catalyst IrO2.With the help of density functional first-principles(DFT)calculations,XAS,XPS,and other characterization technologies,as well as chemical probe experiments,the influence of oxygen vacancy defect on the transformation of the OER mechanism from AEM to LOM was revealed.In the fourth chapter,by utilizing ions with different electronegativity,the surface lattice distortion defects have been introduced into FeCoP through Ru doping,and their influence on the surface electronic structure and the adsorption energy of reaction intermediates were studied.The Ru-doped FeCoP samples were synthesized via the hydrothermal method followed by phosphating treatment.The local lattice distortion defect on the sample surface was characterized by HRTEM,and the strong electronic interaction among different ions was revealed by XAS and XPS.Electrochemical measurements showed that the electrochemical effective active areas and active sites were significantly increased,the charge transfer resistance was reduced,and the hydrogen evolution kinetics was remarkably accelerated.The Tafel slope value is 32.1 mV/dec,which is close to the commercial catalyst Pt.By comparing XPS,HRTEM of catalysts before and after the electrocatalytic test,it is revealed that the lattice distortion defects induced by doping atoms are the reasons for the increased activity of HER and OER.The optimized FeCoRuP shows excellent performance,the overpotential of HER is 45 mV@10 mA·cm-2,the overpotential of OER is 214 mV@20 mA·cm-2,and the potential of overall water splitting is only 1.47 mV@10 mA·cm-2,together with excellent long-term durability(110 hours),the current density has almost few degradations,which has potential commercial application prospects.Based on the research of lattice distortion defects induced by cation doping in the above chapter,in the fifth chapter,we explore the function of crystalline/amorphous interface defects on the surface of CoP on the electrocatalytic performance.Firstly,different F-doped CoP samples were synthesized and systematically optimized by altering the fluorination temperature and dosage.Then,cry stall ine/amorphous interface defects on the crystal surface were observed by HRTEM,which were beneficial to increase the number of surface-exposed active sites.XAS and XPS explored the variations of the electronic localization and interaction among elements after the introduction of F ion.Electrochemistry tests showed that the crystalline/amorphous interface defects efficiently increased the charge transfer ability.The Tafel slope of F-doped CoP is 34 mV/dec,which is much better than the commercial benchmark catalyst IrO2(87 mV/dec),and the overpotential is 220 mV@10 mA·cm-2,meanwhile,HER performance is also improved.Finally,the sixth chapter is a summary of the research conclusions,innovations,and shortcomings of the thesis,as well as the prospects of future research.