Interfacial properties of epitaxial gold nanocrystals supported on rutile titanium dioxide

摘要

Interfacial science is a fascinating field in materials science. From grain boundaries in bulk materials to interfaces in thin films, the properties of interfaces can lead to novel functionality in materials. The effect of interfaces on a material???s properties becomes amplified as the proportion of interfaces increases in relation to the volume of the material, for example in supported nanocrystals (NCs). While interfaces in bulk materials and thin films have been well characterized, atomic level characterization of NC interfaces is limited. The aim of this thesis is to develop a general set of methods to comprehensively study NC interfaces including interfacial atomic structure, interfacial energy and interfacial line tension. We have selected Au-TiO2 interface that has significant attention for its role in the remarkably low temperature catalytic oxidation of carbon monoxide to carbon dioxide. One of the best methods to study interfaces in bulk materials and thin films is cross sectional Transmission Electron Microscopy (TEM). Particularly with the advent of aberration corrected electron microscopy the experimental power to probe interfaces has increased manifold. However it is difficult to prepare NCs in the cross sectional viewing geometry for TEM without damaging the NCs using conventional specimen preparation methods. To enable this work, firstly a specimen preparation method was developed to support epitaxial Au NCs in the cross sectional geometry for TEM investigations without damaging the NCs. The interfacial atomic structure was investigated using aberration corrected Scanning Transmission Electron Microscopy (STEM) with a spatial resolution of ~1??. Interfacial energy of the NCs has been measured from cross sectional STEM images of NCs. It is found that Au NCs with the epitaxial relationship Au(111)[-110] || TiO2 (110)[001] have the largest adhesion to TiO2 (110) with an interfacial energy of 0.61??0.05 J/m2 (assuming ??TiO2(110) = 0.33 J/m2 and ??Au(111) = 1.283 J/m2). The stability of this epitaxy is attributed to the nucleation of Au atoms in the missing titanium row of a (1x2) TiO2 (110) reconstruction ??? resulting in an interfacial reconstruction of Au, Ti and O atoms which lowers the interfacial energy and enhances the adhesion of Au NCs to TiO2 (110). It has been found that smaller Au NCs dewet more than bigger Au NCs on TiO2 (110). The dewetting is attributed to the effect of interfacial line tension. In order to measure interfacial line tension, the Wulff-Kaishew was modified to incorporate the effect of interfacial line tension on NC shapes. The lower limit of interfacial line tension was measured to be 0.85??0.24 eV/?? (1.36??0.38 x 10-9 N) (assuming ??TiO2(110) = 0.33 J/m2 and ??Au(111) = 1.283 J/m2) for NCs with the epitaxial relationship of Au(111)[-110] || TiO2 (110)[001].Since TEM/STEM studies are limited to individual NCs, the formation of epitaxial Au NCs was also probed using Reflection High Energy Electron Diffraction (RHEED) of Au NCs on flat TiO2 (110) supports in order to obtain structural information averaged from a larger number of NCs. The RHEED study confirms the epitaxial relationship of Au NCs as Au(111)[-110] || TiO2 (110)[001], irrespective of the details of the starting TiO2 (110) surface structure and the annealing atmosphere. On reconstructed (1x2) TiO2 (110) surfaces, the onset and completion of epitaxy formation occurred at much lower temperatures than unreconstructed TiO2 (110) surfaces. This shows that Au prefers to nucleate and grow as epitaxial NCs over (1x2) reconstructed TiO2 (110) in agreement with TEM/STEM images of NCs.