Local Structure,Lattice Thermal Expansion And Related Properties In Nano Metal-oxides

Due to its unique physical and chemical properties,nanomaterials have been widely used in materials engineering,energy conversion,biochemistry,information technology,etc.In recent years,the regulation of nanomaterials has gradually gone deep into the atomic-scale level,while the miniaturization and integration of nanodevices also impose higher requirements on their thermal stability.As a result,it is of great significance to study the local structure,lattice thermal expansion and their coupling with the related properties in nanomaterials.This dissertation focuses on the thermal expansions and local structural distortions in several typical nano metal-oxides.The thermal expansion property has been controlled through size,morphology and surface modification,etc.In addition,the coupling of atomic arrangement,local structure and macroscopic performance has been investigated,and the underlying physical mechanisms have been revealed.First,by controlling the morphology,size and crystal facet,the coefficient of thermal expansion(CTE)of TiO2 could be adjusted from positive to negative.The coupling between thermal expansion and local structural distortion was revealed by X-ray pair distribution function.The dissociative adsorption of water occurring selectively on {001} and nearby high-index surfaces is verified to be responsible for the anomalous thermal expansion.Further DFT calculations of the hydrated(001)surface indicate that the weakening of the O-sp2 hybridization is the electronic mechanism of the NTE behavior along a-axis for nanosized TiO2.The humidity induced effect on SnO2 sensor has been directed by structure-property relationship.SnO2 nanoparticles were used for ethonal senor fabrication.The sensing response of ethanol was found to be efficiently activated by adsorbing trace of water but inhibited as humidity increases.This result is closely related to the interactions of water and ethanol on the surface,which was clearly revealed by lattice parameters evolution and local structure distortion of Sn02.Further DFT calculations demonstrated the coupling mechanism of local structural distortion and ethanol sensing,which provides a new insight into the cross interference of humidity on SnO2 sensors,and also offers clues for further optimizating the gas-sensing properties.The thermal expansion properties of nanomaterials could be also adjusted by specific nanostructures.SnO2 nanowires composed entirely of twin crystals were prepared by hydrothermal method.A near zero thermal expansion(ZTE)was achieved from the twin-crystal nanowires,which is highly correlated to the twin boundaries.Combining X-ray PDF,Raman scatterings and first-principles calculations,the key role of the local structural distortion in phonon vibration was revealed.This study provides a good example of tunable thermal expansion with nanostructured modifications,which might provide a new route of controlling thermal expansion property.Finally,the thermal expansion and local structural evolution of nanosized ceriawere investigated.A reversible tetragonal(P42/nmc)to cubic(Fm-3m)phase transition was found in nanosized ceria,which triggers a negative thermal expansion(NTE)and in the temperature range of-25 ?-75 ?.Local structure investigations determined by neutron pair distribution function(nPDF)and Raman scatterings reveal the distortion of oxygen sublattice in the tetragonal structure.First-principles calculations clearly show the pivotal role of oxygen vacancies in stabilizing the tetragonal ceria.Further experiments evidence a charge transfer between oxygen vacancies and 4f orbitals in ceria,which enhances the conductivity in tetragonal ceria.This is the first time to report the phase transition in ceria,which will provide a unique perspective for ceria applications.