| dc.identifier.uri | http://hdl.handle.net/11401/77146 | |
| 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 | |
| dc.format.medium | Electronic Resource | en_US |
| dc.language.iso | en_US | |
| dc.publisher | The Graduate School, Stony Brook University: Stony Brook, NY. | |
| dc.type | Dissertation | |
| dcterms.abstract | Semiconductors must absorb energy from solar photons and subsequently transport photogenerated electrons and holes to surfaces in order to drive the H2 and O2 formation half-reactions of solar water splitting. The hexagonal wurtzite (GaN)1-x(ZnO)x semiconductor solid solution has produced promising results for visible light solar water splitting, but has yet to achieve an efficiency high enough for practical applications, and the origin of the efficiency limit remains unknown. In this work, a detailed investigation of the average and local structure of these materials has been conducted in order to gain insights into how to improve water splitting efficiencies. Our analysis of (GaN)1-x(ZnO)x samples up to x ~ 0.67, has led to the discovery of previously unrecognized types of compositional and structural defects. A new high zinc content Ga2O3(ZnO)16 (2:16) precursor, was developed to prepare higher zinc content oxynitrides. The use of this precursor results in the production of arrays of nanorods with favorable diameters and band gaps for visible light solar water splitting. Optical measurements of these 2:16 precursor samples have shown to be sensitive to small amounts of free carriers whose presence is indicative of compositional defects. Complementary quantitative phase analysis by thermogravimetric analysis, on nanorod samples ranging from xv = 0.08 - 0.52, suggests a substantial quantity of cation vacancies (~3%) may be present. A structural defect in the form of a common zinc-blende (cubic) intergrowth was discovered from transmission electron microscopy measurements. This defect was found to have a uniform size throughout many particles. The abundance of this defect is dependent on both the precursor used during synthesis and the overall zinc content. Further supporting evidence for this cubic intergrowth phase is provided by 14N solid state NMR experiments. Samples synthesized from the novel Ga2O3(ZnO)16 precursor show no evidence of cubic intergrowths, while samples synthesized from a popular ZnGa2O4 precursor show varying amounts of cubic intergrowth with changes in zinc content and reaction conditions. A new method has been developed for using powder diffraction techniques to quantify the amount of this cubic zinc-blende intergrowth. This intergrowth can be modeled in Rietveld refinements using large c-axis superstructures. However, intergrowths such as in this work cannot be seen as explicit diffraction peaks, but are hidden as partial intensities in the existing wurtzite diffraction pattern peaks. Supercells consisting of 80-100 total layers are created with layers of hexagonally close packed atoms and layers of cubic closed packed atoms. Ideal supercell models were investigated and have shown to improve fits to the data as compared to using a typical wurtzite unit cell model. This supercell method for modeling intergrowths is easily applicable to other polymorph systems. It allows for quantification of the amount of intergrowth present using a bulk average structural technique. The local environment of atoms in (GaN)1-x(ZnO)x have been investigated through solid state NMR and pair distribution function studies. 71Ga NMR studies suggest inhomogenous local cation environments that are dependent upon the percentage of zinc in samples. Modeling of neutron pair distribution function data confirms clustering on the local scale and provides a quantitative measure of the differences in bond lengths locally, changes that result from differences in the local composition and bond strengths. Quantitative fitting of the first (nearest-neighbor) PDF peak (~2.0Å) finds that the Ga-N bond distances shrink upon introduction of ZnO into GaN. At low Zn contents, the Zn-O bond lengths are lengthened substantially and relax to nearly the expected average value at the halfway point of the GaN/ZnO solid solution (x = 0.50). The compositional and structural deviations from an average wurtzite structure, as shown through defects and local disorder in this work, stress the importance of conducting an in-depth analysis of these and all materials with potential uses in water splitting. In particular, the defects found in this work may allow for their systematic influence on band gap and efficiency for photoactivity to be resolved. | |
| dcterms.abstract | Semiconductors must absorb energy from solar photons and subsequently transport photogenerated electrons and holes to surfaces in order to drive the H2 and O2 formation half-reactions of solar water splitting. The hexagonal wurtzite (GaN)1-x(ZnO)x semiconductor solid solution has produced promising results for visible light solar water splitting, but has yet to achieve an efficiency high enough for practical applications, and the origin of the efficiency limit remains unknown. In this work, a detailed investigation of the average and local structure of these materials has been conducted in order to gain insights into how to improve water splitting efficiencies. Our analysis of (GaN)1-x(ZnO)x samples up to x ~ 0.67, has led to the discovery of previously unrecognized types of compositional and structural defects. A new high zinc content Ga2O3(ZnO)16 (2:16) precursor, was developed to prepare higher zinc content oxynitrides. The use of this precursor results in the production of arrays of nanorods with favorable diameters and band gaps for visible light solar water splitting. Optical measurements of these 2:16 precursor samples have shown to be sensitive to small amounts of free carriers whose presence is indicative of compositional defects. Complementary quantitative phase analysis by thermogravimetric analysis, on nanorod samples ranging from xv = 0.08 - 0.52, suggests a substantial quantity of cation vacancies (~3%) may be present. A structural defect in the form of a common zinc-blende (cubic) intergrowth was discovered from transmission electron microscopy measurements. This defect was found to have a uniform size throughout many particles. The abundance of this defect is dependent on both the precursor used during synthesis and the overall zinc content. Further supporting evidence for this cubic intergrowth phase is provided by 14N solid state NMR experiments. Samples synthesized from the novel Ga2O3(ZnO)16 precursor show no evidence of cubic intergrowths, while samples synthesized from a popular ZnGa2O4 precursor show varying amounts of cubic intergrowth with changes in zinc content and reaction conditions. A new method has been developed for using powder diffraction techniques to quantify the amount of this cubic zinc-blende intergrowth. This intergrowth can be modeled in Rietveld refinements using large c-axis superstructures. However, intergrowths such as in this work cannot be seen as explicit diffraction peaks, but are hidden as partial intensities in the existing wurtzite diffraction pattern peaks. Supercells consisting of 80-100 total layers are created with layers of hexagonally close packed atoms and layers of cubic closed packed atoms. Ideal supercell models were investigated and have shown to improve fits to the data as compared to using a typical wurtzite unit cell model. This supercell method for modeling intergrowths is easily applicable to other polymorph systems. It allows for quantification of the amount of intergrowth present using a bulk average structural technique. The local environment of atoms in (GaN)1-x(ZnO)x have been investigated through solid state NMR and pair distribution function studies. 71Ga NMR studies suggest inhomogenous local cation environments that are dependent upon the percentage of zinc in samples. Modeling of neutron pair distribution function data confirms clustering on the local scale and provides a quantitative measure of the differences in bond lengths locally, changes that result from differences in the local composition and bond strengths. Quantitative fitting of the first (nearest-neighbor) PDF peak (~2.0Ã…) finds that the Ga-N bond distances shrink upon introduction of ZnO into GaN. At low Zn contents, the Zn-O bond lengths are lengthened substantially and relax to nearly the expected average value at the halfway point of the GaN/ZnO solid solution (x = 0.50). The compositional and structural deviations from an average wurtzite structure, as shown through defects and local disorder in this work, stress the importance of conducting an in-depth analysis of these and all materials with potential uses in water splitting. In particular, the defects found in this work may allow for their systematic influence on band gap and efficiency for photoactivity to be resolved. | |
| dcterms.available | 2017-09-20T16:52:05Z | |
| dcterms.contributor | Khalifah, Peter G | en_US |
| dcterms.contributor | White, Michael | en_US |
| dcterms.contributor | Parise, John | en_US |
| dcterms.contributor | Stephens, Peter. | en_US |
| dcterms.creator | Reinert, Alexandra Audrey | |
| dcterms.dateAccepted | 2017-09-20T16:52:05Z | |
| dcterms.dateSubmitted | 2017-09-20T16:52:05Z | |
| dcterms.description | Department of Chemistry. | en_US |
| dcterms.extent | 176 pg. | en_US |
| dcterms.format | Application/PDF | en_US |
| dcterms.format | Monograph | |
| dcterms.identifier | http://hdl.handle.net/11401/77146 | |
| dcterms.issued | 2014-12-01 | |
| dcterms.language | en_US | |
| dcterms.provenance | Made available in DSpace on 2017-09-20T16:52:05Z (GMT). No. of bitstreams: 1
Reinert_grad.sunysb_0771E_12191.pdf: 4061607 bytes, checksum: 6fee63916a708b39ed7b737ebed2c1ef (MD5)
Previous issue date: 1 | en |
| dcterms.publisher | The Graduate School, Stony Brook University: Stony Brook, NY. | |
| dcterms.subject | defect analysis, (GaN)1-x(ZnO)x, photoelectrolysis, semiconductor, visible light absorption | |
| dcterms.subject | Chemistry | |
| dcterms.title | Quantification and control of inhomogeneity in wurtzite (GaN)1-x(ZnO)x semiconductors for visible light absorption | |
| dcterms.type | Dissertation | |
