¹⁷O Solid-state NMR Studies On Structure Of Nanooxides

Inorganic oxide nanocrystals have large surface area and plenty of structural defects,and their properties can be better tuned in comparison to their bulk counterparts,therefore they have potential applications in catalysis,gas sensing,laser emission and energy storage.The specific exposed facets and lattice defects(such as oxygen vacancy)of the oxide nanocrystals can remarkably influence their properties and applications.It is very important to investigate the structure details of the nanocrystals on an atomic scale,in order to develop structure-property relationships and rationally design nanostructures with desired properties.In this dissertation,we have applied17O NMR spectroscopy,accompanied with controlled 17O-isotopic labeling,to characterize surface structure of TiO2 and ZnO nanocrystals with tailored exposed facets,and partially reduced TiO2 with abundant oxygen vacancies.In chapter 1,the preparation method for faceted TiO2 and ZnO nanocrystals,the influence of the exposed facets of TiO2 nanocrystal on its performance in various applications,surface local environment of specific exposed facets of these two oxides,and the influence of self-structural modifications on the properties of nanocrystals,as well as the solid-state NMR studies on TiO2 and ZnO are reviewed.In chapter 2,17O NMR spectroscopy,combined with surface-selective isotopic labeling,are applied to distinguish TiO2 nanocrystals preferentially exposing {001}and {101} facets,respectively.In combination with DFT calculations,we show that the oxygen ions on the {001} and {101} facets of anatase titania nanocrystals are associated with distinct 17O chemical shifts and these facets exhibit different 16/17O exchange reactivity.Distinct surface defects are found to be associated with these two types of exposed facets.Surface reconstruction occurs on exposed {001} facet,while step edges are observed on exposed {101} facet.Water is adsorbed in different modes on the two types of facets.In chapter 3,surface oxygen and hydrogen species of partially reduced anatase TiO2(ReATiO2)nanoparticles have been studied by 17O and 1H solid-state NMR spectroscopy,and compared to the as-prepared anatase TiO2(ATiO2).The large amount of oxygen vacancy(OV)and Ti3+ in ReATiO2 leads to more hydroxyl groups during thermal treatment in water moisture.1H→17O CP-MAS and 2D HETCOR spectra confirm the close proximity between certain oxygen and proton sites and these results agree with DFT calculation results of the chemical shifts of bridging and terminal hydroxyl oxygen species.In chapter 4,ZnO nanorods with dominantly exposed {1010} facets are prepared,surface selectively labeled with 17O and investigated with 17O solid state NMR spectroscopy.For the first time,four 17O NMR resonances arising from different oxygen species have been observed.Their shifts and ratio are influenced by the content of the adsorbed water,the method with which 17O are labeled,and loading of Au nanoparticles.Various NMR pulse sequences,such as single pulse with decoupling,REDOR and 2D MQMAS are combined to assign these different resonances.In summary,we have applied 17O solid-state NMR spectroscopy,with various pulse sequences,in combination with DFT calculations,to characterize faceted TiO2 and ZnO,and partially reduced TiO2.The results presented in this study open up methods for characterizing other faceted or partially reduced nanocrystalline oxides and related materials.