Identifier

etd-07052011-104558

Degree

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Document Type

Dissertation

Abstract

Noble metals on metal oxide surfaces play crucial roles in many catalysis applications. Due to extreme complexity of the interfaces and catalytic reaction processes, basic science studies of model systems are needed to guide the construction and optimization of new industrial catalysts. We have employed ultra-high vacuum techniques to investigate the morphology, electronic structure, and surface reactivations of clean metal-oxide/metal nanocluster interfaces in ideal conditions. The systems we chose are Ag/TiO2, a popular photocatalyst, and Cu/ZnO, recently a hot candidate for liquid fuel production. The main characterization techniques that we have employed include Scanning Tunneling Microscopy (STM), Photoemission Spectroscopy (PES), Electron Energy Loss Spectroscopy (EELS), as well as Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LLED) for surface quality determination. Ag on TiO2(110) grows in a manner that produces 3D hemispherical clusters with uniform sizes in the several nanometer range. The preparation of the surface is found to control the nucleation sites and cluster sizes. Our photoemission data show that the initial coverage of Ag has very different properties when compared with the larger clusters at higher coverage, or that of “bulk” Ag. The optical properties of larger Ag clusters show strong collective excitations at ~ 3.8eV, in the UV part of the solar spectrum. We have found that when we encapsulated the titania supported Ag clusters in a thin titania overcapping layer, the nanoclusters retain their structural integrity and we find that new excitations are induced that better match the solar spectrum. Cu on ZnO(10-10) is another sample of 3D nanoclusters that we have grown and in this case the motivation is the catalytic reduction of carbon dioxide to methanol. In this case we find that multiple Cu-oxide phases form, depending on oxidation treatment, and electrochemical measurements show that Cu(I) species of Cu2O is particularly active on ZnO(10-10). Along with additional studies of Cu on TiO2(110) and Au on ZnO(10-10), we observed a diversity of metal growth behaviors on the different oxide substrates. The preferred nucleation site, cluster size, and the shape of clusters are all different for a different metal/oxide combination.

Date

2011

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Sprunger, Phillip T.

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