Scanning Tunneling Microscopic Studies of SiO2 Thin Film Supported Metal Nano- Clusters

다국어 초록 (Multilingual Abstract)

This dissertation is focused on understanding heterogeneous metal catalysts supported on oxides using a model catalyst system of SiO_(2) thin film supported metal nano-clusters. The primary technique applied to this study is scanning tunneling microscopy (STM).
The most important constituent of this model catalyst system is the SiO_(2) thin film, as it must be thin and homogeneous enough to apply electron or ion based surface science techniques as well as STM. Ultra-thin SiO_(2) films were successfully synthesized on a Mo(112) single crystal. The electronic and geometric structure of the SiO_(2) thin film was investigated by STM combined with LEED, Auger electron spectroscopy (AES), and Xray photoelectron spectroscopy (XPS).
The relationship between defects on the SiO_(2) thin film and the nucleation and growth of metal nano-clusters was also investigated. By monitoring morphology changes during thermal annealing, it was found that the metal-support interaction is strongly dependent on the type of metal as well as on the defect density of the SiO_(2) thin film. Especially, it was found that oxygen vacancies and Si impurities play an important role in the formation of Pd-silicide.
By substituting Ti atoms into the SiO_(2) thin film network, an atomically mixed TiO_(2)-SiO_(2) thin film was synthesized. Furthermore, these Ti atoms play a role as heterogeneous defects, resulting in the creation of nucleation sites for Au nano-clusters. A marked increase in Au cluster density due to Ti defects was observed in STM. A TiO_(2)-SiO_(2) thin film consisting of atomic Ti as well as TiOx islands was also synthesized by using higher amounts of Ti (17 %). More importantly, this oxide surface was found to have sinter resistant properties for Au nano-clusters, which are desirable in order to make highly active Au nano-clusters more stable under reaction conditions.

목차 (Table of Contents)

• ABSTRACT = iii
• TABLE OF CONTENTS = viii
• LIST OF FIGURES = x
• INTRODUCTION = 1
• Model Catalysts = 1
• Oxide Thin Films = 3
• Nucleation and Growth of Metal Particles on Oxide Surfaces = 5
• Metal-Support Interactions = 10
• Interactions between Noble Metals and SiO_(2) at Elevated Temperature = 13
• EXPERIMENTAL = 25
• Ultra-High Vacuum Chamber = 25
• Surface Analysis Methods = 29
• Experimental Preparation = 38
• RESULTS AND DISCUSSION = 41
• Mo(112) Single Crystal = 41
• SiO_(2) Thin Film = 51
• Metal Nano-Clusters Supported on SiO_(2) Thin Films = 69
• TiO_(2)-SiO_(2) Mixed Oxide Thin Film = 101
• SUMMARY = 128
• REFERENCES = 130
• VITA = 140