Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

Thesis (Ph.D.)--Texas Christian University, 2006. / Title from dissertation title page (viewed Sept. 12, 2006). Includes abstract. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: Brian A. Korgel. Vita. Includes bibliographical references.

Upconversion fluorescent nanocrystals, Yb-Er, Yb-Ho and Yb-Tm co-doped LaF₃ were chemically synthesized. The average particle size was 4.4 nm with a narrow size distribution of ± 0.3 nm. Under the 980 nm NIR excitation, the green, red and blue emission bands from these nanocrystals were observed, respectively. These nanocrystals have potential applications as bio-probes and displays. / Singapore-MIT Alliance (SMA)

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

No description available.

Submitted to the Faculty of Science, School of Chemistry at University of the Witwatersrand, in partial fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 05 June 2017. / The colloidal method has extensively been used to synthesize ternary and quaternary copper sulfides and selenides. Although tellurides form part of the chalcogenides, little has been reported on them particularly the synthesis of these nanostructures. Achieving high-quality nanocrystals through colloidal synthesis requires thorough monitoring of parameters such as time, solvent, precursor as they affect nucleation and growth of the nanocrystals. Herein, we report on the colloidal synthesis of ternary CuInTe2 and quaternary CuIn1-xGaxTe2 nanostructured semiconductor materials. A typical synthesis of CuInTe2 entailed varying reaction temperature. At temperatures below 250 °C, no formation of CuInTe2 was seen. At 250 °C formation of CuInTe2 could be observed with the formation of binary impurities. A change in the sequence in which precursors were added at 250 °C yielded pure CuInTe2. Applying different surfactants aided in achieving differently structured morphologies of CuInTe2 nanocrystals. Morphology varied from rods, cubes, nanosheets etc. Different morphologies resulted in different optical properties with the high optical band gap of 1.22 eV measured for 1D rods. Different precursors were employed in the synthesis of quaternary CuIn1-xGaxTe2. Precursor 2 (entailed the use of Cu (acac)2, In (acac)3 and Ga(acac)3) yielded pure CuIn1-xGaxTe2 phase with no formation of impurities. Variation in reaction time influenced the optical properties of the quaternary CuIn1-xGaxTe2 with high band gap obtained at low reaction time (30 min). A change in Ga and In concentration resulted in reduced lattice parameters a and c with lowest values obtained with the highest Ga concentration. However, achieving the intended concentration proved challenging due to the loss of the material during synthesis. Increasing the Ga concentration resulted in a high optical band gap. Conducting the reaction with Hexadecylamine (HDA) resulted in a relatively high optical band though the formation of impurities was evident. The obtained band gap can be attributed to small sized particles as evident from TEM results. Heterojunction ZnO/CIT and ZnO/CIGT solar cell devices were fabricated through a simple solution approach. The performance of ZnO/CIGT device was superior to that of ZnO/CIT in which efficiency increased from 0.26-0.78%. In the ZnO/CIT device, high Voc of 880 mV was recorded while 573.66 mV was measured for ZnO/CIGT device. Chemical and thermal treatments were performed on the ZnO/CIGT devices. The efficiency increased from 0.78 1.25% when the device was chemically treated with a short-chain EDT ligand. A high conversion efficiency of 2.14% was recorded for devices annealed at 300 °C. High annealing temperatures resulted in poor device performance with the lowest efficiency of 0.089% obtained at annealing temperatures of 500 °C attributed to the leaching out of In and Ga into the ZnO layer. / LG2017

This thesis describes the investigation of the properties of cellulose nanocrystals (CNCs) in water and at interfaces in the presence of different water-soluble polymers and surfactants. The potential of producing hydrogels, emulsions, and foams using both CNCs and surfactants and polymers is extensively explored herein. Interactions between CNCs and polymers were studied by measuring adsorption of polymers on CNC-coated surface in quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) instruments. Hydroxyethyl cellulose, hydroxypropyl guar, and locust bean gum adsorbed onto CNC-coated surfaces, whereas dextran did not adsorb. Gelation of CNC dilute dispersions was found for the samples added with adsorbing polymers, whereas the introduction of non-adsorbing polymers showed no such change of rheological behaviors of CNC dilute dispersions. The further addition of negative surfactant SDS or non-ionic surfactant Triton X-100 disrupted the gels whereas cationic surfactant CTAB did not. These behaviors illustrate the complexities associated with including CNC dispersions in formulated water-based products where polymers and surfactants are commonly used as well. The adsorption of cationic surfactants on CNC particle surfaces and the associated change of CNC hydrophobicity were investigated. Surfactant-modified CNCs were then employed as emulsifying agents to determine the effects of stabilizing oil-water interface with CNCs after surfactant addition. Emulsion stability was substantially enhanced with the introduction of surfactants. Based on the chemistry of cationic surfactants, and the extent CNC surface hydrophobicity increases after surfactant binding, either oil-in-water or water-in-oil emulsions were successfully produced. This in situ surfactant adsorption method thus offers a simple way of modifying surface hydrophobicity of CNCs and allows fine tuning of CNC-based emulsion properties. Adsorbing polymers were used together with CNCs to prepare stable emulsions. The introduction of polymers facilitated the production of emulsion droplets with enhanced stability and smaller diameters. Both polymer-coated CNCs and the extra polymers partitioned at the interface and worked as the emulsifiers in a synergistic manner, leading to a reduction in CNC coverage on the emulsion droplet surfaces. Furthermore, reversible thermogelation of the emulsion was obtained when thermosensitive polymers were added. No noticeable emulsion coalescence occurred after multiple cycles of heating and cooling treatments of the emulsion gels. Freeze-drying and air-drying of these emulsion gels produced oil powders containing oil content as high as 94 wt. %. Finally, highly stable wet foams were successfully produced using CNCs and the water-soluble polymer, methyl cellulose. The effect of CNC and methyl cellulose concentration on the stability of air-water interfaces was elucidated. Both foamability and foam stability were greatly improved by adding CNCs to methyl cellulose solutions. The CNC particles helped to retain fluid in the films and plateau borders between bubbles, increasing bulk viscosity, and impeding water drainage. We also demonstrated that adding various monomers to CNCs- methyl cellulose wet foams did not lead to noticeable foam breaking. The successful production of macroporous structures with tailored chemistry and properties was achieved by subsequent polymerization of the monomers added to the foam. / Thesis / Doctor of Philosophy (PhD)

This thesis describes the investigation of the properties of cellulose nanocrystals (CNCs) in water and at interfaces in the presence of different water-soluble polymers and surfactants. The potential of producing hydrogels, emulsions, and foams using both CNCs and surfactants and polymers is extensively explored herein. Interactions between CNCs and polymers were studied by measuring adsorption of polymers on CNC-coated surface in quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) instruments. Hydroxyethyl cellulose, hydroxypropyl guar, and locust bean gum adsorbed onto CNC-coated surfaces, whereas dextran did not adsorb. Gelation of CNC dilute dispersions was found for the samples added with adsorbing polymers, whereas the introduction of non-adsorbing polymers showed no such change of rheological behaviors of CNC dilute dispersions. The further addition of negative surfactant SDS or non-ionic surfactant Triton X-100 disrupted the gels whereas cationic surfactant CTAB did not. These behaviors illustrate the complexities associated with including CNC dispersions in formulated water-based products where polymers and surfactants are commonly used as well. The adsorption of cationic surfactants on CNC particle surfaces and the associated change of CNC hydrophobicity were investigated. Surfactant-modified CNCs were then employed as emulsifying agents to determine the effects of stabilizing oil-water interface with CNCs after surfactant addition. Emulsion stability was substantially enhanced with the introduction of surfactants. Based on the chemistry of cationic surfactants, and the extent CNC surface hydrophobicity increases after surfactant binding, either oil-in-water or water-in-oil emulsions were successfully produced. This in situ surfactant adsorption method thus offers a simple way of modifying surface hydrophobicity of CNCs and allows fine tuning of CNC-based emulsion properties. Adsorbing polymers were used together with CNCs to prepare stable emulsions. The introduction of polymers facilitated the production of emulsion droplets with enhanced stability and smaller diameters. Both polymer-coated CNCs and the extra polymers partitioned at the interface and worked as the emulsifiers in a synergistic manner, leading to a reduction in CNC coverage on the emulsion droplet surfaces. Furthermore, reversible thermogelation of the emulsion was obtained when thermosensitive polymers were added. No noticeable emulsion coalescence occurred after multiple cycles of heating and cooling treatments of the emulsion gels. Freeze-drying and air-drying of these emulsion gels produced oil powders containing oil content as high as 94 wt. %. Finally, highly stable wet foams were successfully produced using CNCs and the water-soluble polymer, methyl cellulose. The effect of CNC and methyl cellulose concentration on the stability of air-water interfaces was elucidated. Both foamability and foam stability were greatly improved by adding CNCs to methyl cellulose solutions. The CNC particles helped to retain fluid in the films and plateau borders between bubbles, increasing bulk viscosity, and impeding water drainage. We also demonstrated that adding various monomers to CNCs- methyl cellulose wet foams did not lead to noticeable foam breaking. The successful production of macroporous structures with tailored chemistry and properties was achieved by subsequent polymerization of the monomers added to the foam. / Thesis / Doctor of Philosophy (PhD)

No description available.