Date of Graduation

2015

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

James P Lewis

Committee Co-Chair

Alan D Bristow

Committee Member

Cheng Cen

Committee Member

Christopher Matranga

Committee Member

Tudor Stanescu

Abstract

Heterostructures of semiconductor metal oxides and metals have been studied extensively due to the synergistic photocatalytic activity they display. Gold-Zinc oxide (Au-ZnO) heterostructured photocatalysts are one of the most widely studied photocatalysts due to the high photocatalytic activity of ZnO and the ability of Au to transfer photoexcited electrons away from ZnO, which leads to enhanced carrier separation and improved catalytic activity. Au heterostructuring is also known to alter the electronic structure of ZnO leading to better photochemical band alignment and a decreased work function which both contribute to enhanced catalytic activity. The use of Au to form heterostructures with ZnO also opens the possibility of using its plasmonic activity for engineering the photophysics of this system through plasmonic heating or optical field enhancement. As such, it is interesting to synthesize a variety Au-ZnO heterostructures and correlate the catalytic activity of these materials with structural details such as nanoparticle morphology, lattice dynamics at the interface, and the general crystallinity of the two materials forming the heterostructure.;In this work, I have synthesized, characterized, and investigated two types of Au-ZnO heterostructured catalysts and evaluated their activity for carbon dioxide (CO2) conversion and waste-water remediation applications. The Au-ZnO catalysts were characterized using Transmission Electron Microscopy, Scanning Electron Microscopy, X-Ray Diffractometry, Raman spectroscopy, UV-Vis absorption spectroscopy and Fourier Transform Infrared spectroscopy. Initial efforts focused on simple heterostructures made by soaking nanocrystalline ZnO in a Au salt with a subsequent heat treatment to reduce the Au to its metallic state. This resulted in ∼20 nm Au particles dispersed on ZnO at ∼5 wt % loading. Plasmonic excitation of the Au nanoparticles also heats the ZnO substrate it is grown on up to ∼600 °C and drives the catalytic conversion of CO2 and hydrogen (H2) to methane (CH4), carbon monoxide (CO), and water (H2O) on the ZnO portion of the heterostructure. In addition, I have also investigated methods to increase the Au-ZnO ratio and to improve the degree of contact between these two materials. For these efforts, ZnO nanopyramids were grown on Au seeds using a wet-chemical method. Electron microscopy was used to characterize an unusual and large lattice expansion for ZnO at the Au interface in these samples. Optical measurements of these samples point to subtle electronic structure changes caused by the Au that improve band alignment for the production of radicals from photoexcited carriers in the ZnO. The improved production of radicals in turn leads to an enhanced photodegradation of organic contaminants in water. In addition to experimental work, computational calculations were performed in order to calculate vibrational modes, phonon spectral density and plasmon relaxation time constants. Two Au clusters (Au20 and Au55) which contain 20 and 55 of Au atoms respectively were used for the calculations.

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