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Structural and Optical Characterization of Semiconductors for Solar Water Splitting

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dc.contributor.advisor Khalifah, Peter G en_US
dc.contributor.author Malingowski, Andrew Charles en_US
dc.contributor.other Department of Chemistry. en_US
dc.date.accessioned 2017-09-20T16:52:02Z
dc.date.available 2017-09-20T16:52:02Z
dc.date.issued 2014-12-01 en_US
dc.identifier.uri http://hdl.handle.net/11401/77131 en_US
dc.description 165 pg. en_US
dc.description.abstract Finding a clean and renewable source of energy that can compete commercially with fossil fuels is one of the great challenges facing the world today. Photoactive semiconductors that can be used to split water into hydrogen and oxygen gas by absorbing visible light are one means of harvesting the sun's abundant energy and storing it as chemical fuel. Although several photoactive semiconductor systems have demonstrated water splitting, one is yet to be found that has a high enough H2 production efficiency to be a commercially viable source of renewable energy. Water splitting using a photoactive semiconductor is a complex process requiring the optimization of several factors if the reaction is to proceed. This research focuses on understanding the bulk properties of photoactive semiconductors, such as structure and light absorption and their relationship to overall H2/O2 production efficiency. InTaO4 was the first photoactive semiconductor reported to show overall water splitting activity without an applied bias when exposed to visible light. The material's H2/O2 production efficiency was reported to increase greatly when doped with Ni. Much research has been done attempting to maximize the efficiency of this compound via doping or other methods. However before this work no studies have closely focused on the material's bulk structure or light absorption. The valence state of the Ni substituted into the InTaO4 lattice was determined to be 2+ using X ray absorption near-edge spectroscopy and magnetic susceptibility measurements. The synthesis was re examined resulting in higher quality single phase samples. X ray and neutron diffraction were used to construct a new partial phase diagram for the In2O3 Ta2O5 NiO system and (In1 xNi2x/3Tax/3)TaO4. A Ni solubility limit of x &#8776; 0.18 was determined for Ni doped InTaO4. Diffuse reflectance measurements showed that the 3.96 eV band gap of undoped InTaO4 was only slightly reduced (< 0.2 eV) with Ni doping and that the material's visible light absorption was due to a weak 3A2g &#8594; 3T1g internal d &#8594; d transition that is commonly seen in Ni containing oxide compounds when Ni2+ is octahedrally coordinated. Oxynitrides are a promising family of compounds for visible light water splitting because of their smaller band gaps compared to oxides, advantageously placed band edges and chemical stability. The perovskite like compound LaTiO2N was demonstrated to be capable of reducing H+ to H2 using methanol as a sacrificial electron donor and to be capable of oxidizing water to O2 in the presence of Ag+, making it an interesting candidate for further study. The color of LaTiO2N powder made by nitriding different lanthanum oxides varies from orange (La2TiO5 precursor) to red (La4Ti3O12 precursor) to brown (La2Ti2O7 precursor) as the La content of the starting lanthanum oxide decreases. The structure of LaTiO2N was examined using X ray and neutron powder diffraction and the red compound was found to have a slightly larger unit cell than the other two colors. Diffuse reflectance was used to examine the compounds' visible light absorption and the band edge of the red sample was found to be 2.06 eV, approximately 0.2 eV lower than those of the orange and brown samples, 2.23 eV and 2.20 eV respectively. Additional absorption below the band edge was also seen in the red and brown samples and was attributed to (1) Ti3+ and (2) free carriers in the LaTiO2N sample. The optical properties of thin films of LaTiO2N grown on both La2Ti2O7 and La5Ti5O17 single crystal substrates were examined using spectroscopic ellipsometry. The onset of absorption is very rapid with a visible light absorption coefficient greater than 105 cm-1 by 2.45 eV. The absorption above the band gap was modeled and both the oxygen and lower lying nitrogen valence states can be clearly seen. The measured spectrum was found to agree with the absorption spectrum of model superstructures with partial O/N ordering calculated using DFT. The O/N ordering on the anion site was separately investigated using pair distribution function analysis on X ray and neutron diffraction data collected on the powdered LaTiO2N samples. en_US
dc.description.sponsorship This work is sponsored by the Stony Brook University Graduate School in compliance with the requirements for completion of degree. en_US
dc.format Monograph en_US
dc.format.medium Electronic Resource en_US
dc.language.iso en_US en_US
dc.publisher The Graduate School, Stony Brook University: Stony Brook, NY. en_US
dc.subject.lcsh Chemistry en_US
dc.subject.other ellipsometry, neutron diffraction, photoelectrolysis, semiconductor, water splitting, X-ray diffraction en_US
dc.title Structural and Optical Characterization of Semiconductors for Solar Water Splitting en_US
dc.type Dissertation en_US
dc.mimetype Application/PDF en_US
dc.contributor.committeemember Grey, Clare en_US
dc.contributor.committeemember White, Michael en_US
dc.contributor.committeemember Kowach, Glen. en_US

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