Study of Solid State Photocatalysts and other Energy Materials using Synchrotron Radiation
This work presents a spectroscopic and theoretical study of several energy materials using synchrotron-based techniques. Two classes of materials are studied: solids that have reported photocatalytic properties, and lithium compounds that are thought to form during the cycling of modern battery electrodes. An overview of synchrotron soft X-ray spectroscopic techniques is presented, along with the theory and procedures associated with performing such measurements. These measurements are compared to density functional theory (DFT) calculations, as implemented by the WIEN2k package, along with a description of the DFT method. Calculated electronic structure is shown to be a useful aid in interpreting the results of X-ray emission and X-ray near-edge absorption measurements (XES and XANES), allowing conclusions about the physical structure and properties of the materials to be reached. Two photocatalytic systems are outlined, the first of which is a solid solution of GaN and ZnO (GaN:ZnO) that exhibits an unexpected reduction in band gap. By carefully comparing common hybridized features from O, N and Zn core emission lines, a binding energy picture of the valence and conduction bands of GaN:ZnO is constructed, allowing its band gap reduction to be described as a consequence of heterojunctions between predominantly GaN and ZnO regions within the solid solution. This description attempts to resolve controversy in the literature regarding the origin of the band gap reduction, as well as to rule out a hypothesized oxynitride superlattice structure as the explanation. The second photocatalytic system studied is a carbon nitride derivative, poly(triazine imide) (PTI) that displays high crystallinity and that could be very inexpensive to produce due to its elemental abundance. Through resonant excitation, two inequivalent N sites in PTI can be probed by X-ray emission spectroscopy, indicating the material is not a conjugated polymer like other reported carbon nitrides. The band gap of the system is observed to decrease in response to disordered Li loading, an e ect that is con rmed by DFT calculation. Several potential disorder models of the Li loading of PTI are investigated with DFT force minimization in order to choose a structural candidate capable of producing calculated X-ray spectra that agree with our measurements. The presented lithium study attempts to use a modern soft X-ray absorption facility to characterize the Li surface by-products inherent to the charge-discharge cycling of a battery electrode. A survey of potential Li compounds was performed using Li K-edge XANES will be compared to DFT calculations and X-ray Raman Scattering measurements performed by collaborators in the future. Correlating measurements of the survey compounds with charge-cycled electrode measurements will be an area for future work.
DegreeMaster of Science (M.Sc.)
DepartmentPhysics and Engineering Physics
CommitteeXiao, Chijin; Tse, John; Grosvenor, Andrew; Koustov, Sasha
Copyright DateSeptember 2012