Synthesis Of VO₂ Micro-nanostructures And Investigations Of Their Phase Transitions Under Pressure

Vanadium dioxide(VO2)is a typical strongly correlated material with different polymorphs,which undergo reversible first-order metal-insulator transition at 68℃accompanied by crystallographic structure transition and drastic changes in magnetic and optical properties,exhibits promising applications in optics,sensors and optical data storage medium.Nanosized VO2 samples usually show novel physical and chemical properties that differ greatly from their bulk counterparts,and these properties are largely dependent on their morphology and size.Pressure,an important tuning parameter,can reduce the distance between atoms and tune the overlap degree of electronic orbit effectively.Combining high pressure technology with nanoscience and nanotechnology on the study of the structure and properties transition of VO2 will not only provide further insight into electronic correlation,but also give a new approach to explore new structures and properties.However,size-controlled synthesize of VO2 micro-nanostructures is still a challenge,and the pressure/temperature induced metallization mechanism and structural phase transition process are still unclear.Based on these questions,we carried out size-controlled synthesize of VO2 micro-nanorods and nanoparticles,and investigated the pressure/temperature induced phase transitions and metallization on these materials.The research contents and results are as follows:1.Size-controlled synthesize of one dimensional VO2 nanorods and nanoparticles with different polymorphs were carried out by hydrothermal reaction combined with subsequent calcinations.The effects of hydrothermal reaction condition on the size and structure of resulted samples were investigated.We find that the reactant concentration and molar ratio play key roles on the size distributions and the uniformity of VO2(M)rods,respectively.VO2(M1)micro/nano-rods with sized diameter size from 120-1000 nm were synthesized by adjusting the synthesis conditions.Our results provide a facile method to control the size and structure of VO2 micro-nanostructures.2.In situ temperature-dependent Raman and X-ray absorption fine structure(XAFS)studies have been carried out on free-standing monoclinic vanadium dioxide(VO2(M))micron-sized rods and nanoparticles.The size of VO2 samples has great effect on structural transition temperature and process.The structural transition from M1 to R phase is completed at about 50℃,which is much lower than that of nanoparticles,and an intermediate M2 phase were observed during the phase transition.These results suggest that more nucleating defects are formed in VO2(M1)micro-sized rods,which serve as nucleation site of R phase and reduce the phase transition temperature.Moreover,partial transformation from M1 to R phase,would produces tensile stress on the untransformed regions of M1 pahse,leading to the occurrence of the structural transition from M1 to M2 phase.Our results provide insight into the structural phase transition of VO2,and find a new way for reducing M-R phase transition temperature.3.The pressure induced structural transition and metallization of VO2(M1)were investigated,and a new VO2 structure,which are stable under ambient condition,was synthesized.Upon compression,the structure transition sequence follows M1→(11.4 GPa)M1’ →(26.8 GPa)M1’ + X →(59 GPa)X,upon decompression,the structure transition follows X →(20 GPa)L.M1 and L phase is semiconductor,X phase is metal,M1’ phase is semiconductor at low pressure region but being metallic above 43.1 GPa.Further analysis suggests that the pressure-induced metallization(PIM)of the M1’ phase is associated with electron-electron correlations,while the PIM from the M1’ to the X phases is relate to structural phase transitions.Moreover,we realized the reversible switching between the new L phase and M1 phase in VO2 by calcination at 400 ℃.These results are not only helpful to understand the pressure-induced metallization mechanism of VO2,but also instructive for exploring new structures and realizing structure switching between these structures.4.We have firstly investigated the pressure-induced phase transitions and metallization on VO2(M1)nanoparticles.Upon compression,the structure transition sequence follows M1→(14.9 GPa)M1’ →(32GPa)M1’ + X →(63.1 GPa)X,upon decompression,the structure transition follows X →(20 GPa)L.This results are in general agreement with that of bulk VO2.The phase transition pressure,bulk modulus for M1 and M1’ are higher than those of their bulk counterparts due to their nano-size effects.Nano-sized M1’ phase is semiconductor at low pressure region and being metallic above ?37 GPa,?6℃ below that of bulk counterparts.The quenced sample still maintains its nanoparticle morphology,so it is the first time for synthesizing VO2(L)nanostructures.These results pave a way for obtaining new nanostructures.5.We have firstly investigated the pressure-induced phase transitions and metallization on w-doped VO2(R)nanoparticles.The R→(13.5GPa)O →(31.3GPa)O + Mx →(56.1 GPa)Mx phase transition are found upon compression.Upon decompression,the Mx phase transformed into L phase,which is in general agreement with that of pure VO2(M1)nanoparticles.Mx phase shows poor metallicity due to the local structure distortion upon doping.L phase being metallic without structure transition due to the high energy barrier transforming into R phase.The IMT of L phase may results from the enhancement of electron concentration due to the presence of W donors,which shifted the Fermi level toward the conduction band.Present results demonstrate that structure transition is not the key factor in driving metal-insulator transition,and gives further insight into the IMT transition on strongly correlated materials.