Electron Microscopy Study On The Pressure-induced Phase Transformation And Deformation Change In TiO₂ Nanocrystals

High pressure can form high-pressure new phases with structures and properties that are different from normal,which is an important means to search for new functional materials and discover new laws in construction.Titanium dioxide（TiO₂）is a common inorganic multifunctional material with mature technology,low cost,and no toxicity.It is currently the most promising green and environmentally friendly material with extensive applications in various fields such as environment,energy,and industry.As a kind of semiconductors,the band-gap excitation region of TiO₂ at atmospheric pressure is within the ultraviolet range,which means that only a small portion of solar radiation can be utilized by TiO₂,which is not conducive to the development of TiO₂ applications.Recent studies have shown that nano TiO₂-II high-pressure phase has good catalytic properties in the visible light range,making it a multifunctional coating material with both hardness and catalytic ability with promising application prospects.However,at present,the mechanism of high-pressure phase formation in nano TiO₂ is still not well understood,and the relationship between high-pressure phase transition and influencing factors is not clear.There are many factors that affect the high-pressure phase transition of nano TiO₂.Research has shown that factors such as sample size,morphology,interface,defects,and deformation can all affect the pressure point and sequence of its high-pressure phase transition.There is no theory that can uniformly explain the high-pressure phase transition affected by these factors.In addition to nano TiO₂,high-pressure phase transitions influenced by these factors have also been reported in other nano metal oxides such as VO₂,Y₂O₃,and Mn₃O₄.Studying the micro mechanism of TiO₂ phase transition under high pressure is of great significance for exploring the relationship between high-pressure phase transition of metal oxides and influencing factors,and understanding the reasons for the formation of high-pressure phase in nano TiO₂.The commonly used characterization techniques in high-pressure research,such as in-situ Raman spectroscopy and synchrotron radiation,have low spatial resolution and cannot characterize the microscopic mechanisms of phase transitions.Electron microscopy can characterize microstructures,but it is limited by high-pressure devices and cannot achieve in-situ characterization.It can only be indirectly observed by comparing and characterizing samples under different pressures.However,the role of electron microscopy characterization in high-pressure research is relatively limited.Electron microscopy reports on depressurized TiO₂ mostly rely on morphology characterization and lattice characterization to verify in-situ results,and cannot reflect changes in microstructure.In response to the above issues,we have improved and perfected the existing methods for electron microscopy preparation and characterization of high-pressure relief samples,and used these methods to study the changes in microstructure during TiO₂ phase transition and deformation under high pressure,achieving the following results.1.The experiment first analyzed the main reasons limiting the development of electron microscopy characterization in high-pressure science,and proposed targeted solutions and related work.（1）In response to the current lack of high-pressure in-situ high-pressure electron microscopy characterization technology,we plan to use a comparative method to characterize the microstructure of samples under different pressure relief.We used a diamond anvil to pressurize spherical rutile TiO₂with an average diameter of 32 nm,and after unloading to normal pressure at 30GPa,the transmission electron microscopy characterization results showed that the rutile transformed into a high-pressure phase of TiO₂-II and underwent deformation.This high-pressure phase can be stably stored under normal pressure conditions,providing a prerequisite for using comparative characterization methods to study the microstructure changes of TiO₂ phase transformation and deformation under different pressures.（2）We have tested and improved existing methods such as using methanol ethanol mixture as pressure medium when decompressed washing dispersion method,no pressure medium when decompressed crushing dispersion method,and no pressure medium when decompressed dual-beam electron microscopy method to address the issue of unstable quality in various high-pressure unloading electron microscopy sample preparation methods.The test results show that the recovery rate of the sample is high after using the non pressure medium hard pressing method for pressure relief.The use of dual-beam electron microscopy to extract samples from it has a large range of thin areas,good repeatability,and is suitable for microstructure characterization.The experiment also improved the recovery method of the pressure relief sample in the high-pressure experiment of the pressure transmission medium.The experimental results showed that the UV cured resin as the pressure transmission medium had good hydrostatic performance under high pressure,and did not produce significant interference in high-pressure in-situ Raman measurement.It can be cured by UV light in the diamond anvil,achieving the embedding of the sample in the sample cavity at the center of the gasket.The surface of the sample after embedding is flat and not easily broken when combined with the gasket,making it suitable for further sample preparation as a block.The cured sample can be observed better under transmission electron microscopy,better achieving microstructure characterization of the sample under hydrostatic pressure relief.（3）In response to the current problem of limited characterization of high-pressure depressurized samples using scanning electron microscopy,we attempt to apply the transmission Kikuchi diffraction technique from scanning electron microscopy to the characterization of depressurized samples.The transmission Kikuchi diffraction results of the depressurized sample indicate that it can effectively characterize the structure of grains larger than～20 nm in the depressurized sample,and obtain a more intuitive distribution map of the sample’s morphology,size,orientation,grain boundaries,stress,and statistical information of samples in a larger area.This information can provide orientation and grain boundary screening for further research by transmission electron microscopy,this plays an important role in improving the characterization efficiency and quality of depressurized samples.The research on the above technical methods provides a technical guarantee for the study of microstructure changes during titanium dioxide phase transformation and deformation under high pressure in the article.At the same time,these methods can also be extended to the study of other high-pressure nanomaterials,improving the role of electron microscopy characterization in high-pressure nanomaterial research.2.The deformation mechanism of TiO₂ under high pressure was studied by transmission electron microscope.The comparative study of the initial nano anatase samples and the pressurized 30 GPa pressure relief samples by transmission electron microscope showed that the pressurized TiO₂-II grains had obvious deformation.The high-resolution diagram showed that there were a large number of[001]direction stacking faults and deformation twins in the grains,and the lens shaped lamellar structure deformation twin bands were formed in the submicron grains;Fan shaped multiple deformation twins are formed in nanocrystalline grains.The results show that TiO₂ can undergo obvious deformation under high pressure,and the microscopic mechanism of deformation is similar to that of metal,mainly including deformation twins and stacking fault slip.The formation of deformation twins has obvious size effect,which is also reflected in the characterization of transmission Kikuchi diffraction.These results provide a new entry point for the study of the size effect of TiO₂ high-pressure phase transition,and also provide a method for the preparation of twins TiO₂-II high-pressure phase.3.Using transmission Kikuchi diffraction and transmission electron microscopy,the pressure relief nano anatase samples after 10,20 and 30 GPa were studied.The changes of TiO₂-I mesophase and deformation twins in grains during the phase transition from nano anatase to TiO₂-II were found for the first time.According to the experimental results,we proposed the microstructure change process of TiO₂nanocrystals with a diameter of～100 nm under high pressure:first,a 60°deformation twin with{112}plane TB was formed in the[110]direction of the anatase phase.With the increase of pressure,anatase was transformed into TiO₂-I and twinned at 60°in the[010]direction.Finally,TiO₂-I was transformed into TiO₂-II,forming 90°twins in the[010]direction.These results more intuitively show the phase transition process and size effect of TiO₂ under high pressure,provide a supplement to the microstructure of phase transition and deformation behavior of nano-TiO₂ under high pressure,and help to understand the mechanism of phase transition of TiO₂ under high pressure.