Date of Award

2020-01-01

Degree Name

Doctor of Philosophy

Department

Material Science and Engineering

Advisor(s)

Ramana V. Chintalapalle

Abstract

From September 2012 to May 2015, a phenocam monitored the seaward edge of a protected mangrove forest. Calculated GCC revealed seasonal greening patterns of a mangrove species, Rhizophoa apiculata, and an overall increase in the GCC, suggesting mangrove expansion. In comparing temperature and precipitation effects, it was found that this particular mangrove species had a greening optima at temperatures between 28°C and 28.5°C, and greening and canopy development response lag time of 10 weeks in response to precipitation. Tree saplings were monitored and showed to grow by 50%, mostly during a three month period during the rainy season. The establishment of new saplings was recorded, illustrating the seaward expansion of this mangrove community. Digital repeat photography appeared to be highly advantageous and an invaluable tool in assessing and monitoring mangrove phenology, growth and expansion.

New sets of GFO compounds were synthesized by varying the iron (Fe)-doping amount (i.e., Ga2–xFexO3; x= 0.00 – 0.30) following the standard high-temperature (Tsint: 1200 oC) solid-state chemical reaction method. XRD studies of the sintered compounds provided evidence for the Fe3+ substitution at Ga3+ site without any secondary phase formation. Rietveld refinement of XRD patterns reveal that the GFO compounds crystallized in monoclinic crystal symmetry. X-ray photoelectron spectroscopy (XPS) data revealed that at lower concentrations of doping, Fe exhibited mixed chemical valence states, whereas single chemical valence state was evident for higher Fe content. Local structure and chemical bonding analyses using X-ray absorption near edge structure (XANES) revealed that the Fe occupied octahedral and tetrahedral sites similar to Ga in parent Ga2O3 lattice without considerable changes in the local symmetry. Raman spectroscopy also confirmed the crystalline nature of the GFO compounds. Morphology of the GFO compounds was characterized by the presence of rod-shaped particle features employing SEM. The EDS confirmed the chemical stoichiometry of the GFO compounds, where the atomic ratio of the constituted elements was in accordance with the calculated concentration values.

Optical absorption spectra revealed a significant red shift in the optical band gap with Fe doping. Origin of the significant red shift is attributed to the strong sp-d exchange interaction originated from the 3d5 electrons of Fe3+. Coupled with optical band gap red shift, electrocatalytic studies of GFO compounds revealed that doped Ga2O3 compound exhibited electrocatalytic activity in contrast to intrinsic Ga2O3. Fe doped samples demonstrated appreciable electrocatalytic activity towards the generation of H2 through electrocatalytic water splitting. Electrocatalytic activity of the GFO compounds is attributed to cumulative effect of different mechanisms such as doping resulted new catalytic centers, enhanced conductivity, and electron mobility. Hence, a new pathway in which electrocatalytic behavior of the GFO compounds resulted due to Fe chemical states, red shift in optical band gap was explored for the very first time. The implications derived from this work may be applicable to a large class of compounds and further options may be available to design functional materials for electrocatalytic energy production.

Language

en

Provenance

Received from ProQuest

File Size

138 pages

File Format

application/pdf

Rights Holder

Swadipta Roy

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