Doctor of Philosophy / Department of Chemistry / Kenneth J. Klabunde / First, a new tiara Pd(II) thiolate complex-[Pd(SC[subscript]12H[subscript]25)[subscript]2][subscript]6 has been synthesized and fully characterized. Then the complex was further used as a single-source precursor to prepare nearly monodisperse palladium sulfide (PdS) nanoparticles through the high-temperature-induced decomposition in diphenyl ether. Secondly, the influence of dodecanethiol on the product distribution upon heating a Pd(II)-octylamine system was investigated. The molar ratio of octylamine to Pd(II) was fixed at 20:1, and the concentration of dodecanethiol was changed systematically. Without thiol ligand, only aggregated Pd(0) particles were obtained due to the reduction of Pd(II) by octylamine. When the molar ratio of dodecanethiol to Pd(II) was increased to 0.5, highly monodisperse sulfurized palladium nanoparticles with diameter 7.55 [plus or minus] 0.73 nm were generated. When the molar ratio reached to 2, only a thiolate complex-[Pd(SC[subscript]12H[subscript]25)[subscript]2][subscript]6 was found as the final product. Thirdly, we report a facile method to prepare nearly monodisperse Pd nanoparticles by heating Pd(II) ions in 4-tert-butyltoluene solvent, in the presence of oleylamine and trioctylphosphine (TOP) ligands. It has been found the concentration of TOP ligand was highly pivotal for the formation of Pd nanoparticles. Without TOP, only aggregated Pd particles were obtained due to the reduction of Pd(II) by oleylamine. When the molar ratio of TOP to Pd(II) was less than two, well-protected Pd nanoparticles were obtained. However, when the molar ratio reached to two, only Pd(II)-TOP coordination complexes were obtained as the final product. Fourthly, we present a novel way to synthesize cadmium selenide (CdSe) nanoparticles from a heterogeneous system only containing cadmium oxide, trioctylphosphine, and trioctylphosphine selenide. Last, the formation of magnesium oxychloride (Mg[subscript]x(OH)[subscript]yCl[subscript]z•nH[subscript]2O) nanorods from the system MgO-MgCl[subscript]2-H[subscript]2O was investigated. By changing the amounts of the starting materials, short nanorods (< 1 micron) or long nanorods (up to 20 micron) could be obtained readily with the aspect ratio in the range of 10-70. The resulting magnesium oxychloride nanorods could be further transformed to magnesium hydroxide (Mg(OH)[subscript]2) nanorods by treating with NaOH.

Gallium arsenide (GaAs), cadmium selenide (CdSe), and cadmium telluride (CdTe) are semiconductor materials with remarkable opto-electronic properties that make them suitable for a wide variety of applications including, light emitting diodes (LEDs), mobile phones, tablets, and solar panels. Due to the growing demand and the short lifespan of these electronic devices, a remarkable amount of electronic waste (e-waste) has been produced in the last decades. An important fate of e-waste is landfill disposal; therefore, there is an increasing concern about the release of toxic elements into the landfill environment and the potential risks for human health and the environment. Among the elements constituting GaAs, CdTe, and CdSe, tellurium (Te) has gained increasing attention in recent years. Tellurium is a scarce element on the earth’s crust, and a shortage in its supply might compromise the development of new advanced technology, particularly in the energy and defense fields. For these reasons, the US Department of Energy and the European Union regard Te as a critical element, and have urged the need to develop efficient and cost-effective processes to recover Te from waste streams. This thesis dissertation explored different aspects related to the fate and impact of the widely used semiconductor materials, GaAs, CdSe and CdTe in municipal mixed solid waste (MSW) landfills. Furthermore, it investigated the removal of the Te oxyanions, tellurite (Teᴵⱽ, TeO₃²⁻) and tellurate (Teⱽᴵ, TeO₄²⁻), from aqueous streams and the recovery of this strategic metalloid as biogenic, elemental tellurite (Te⁰) nanoparticles (NPs). In the first part this work, the dissolution of GaAs was evaluated under a range of redox conditions, pH levels, ionic strength, and the presence of organic constituents commonly found in landfills. Our results indicated that aerobic conditions and mildly alkaline pH conditions favor the dissolution and release of high levels of soluble arsenic (As) and gallium (Ga) to the synthetic leaching solutions. The rate of As and Ga dissolution in long-term exposure experiments was initially constant but later progressively decreased due to the formation of a passivating layer on the surface of GaAs. The leaching behavior of CdSe and CdTe was also investigated under different pH and redox conditions in solutions simulating landfill leachates. CdTe and CdSe were subjected to two different standardized leaching tests, the federal Toxicity Characteristic Leaching Procedure (TCLP) and the California Waste Extraction Test (WET). CdTe showed a very high leaching potential in both tests and the concentrations of Cd released were 1500- and 260-times higher than the regulatory limit (1 mg Cd L⁻¹), respectively. In contrast, CdSe was relatively stable and dissolved selenium (Se) in both leaching tests was below the regulatory threshold (1 mg Se L⁻¹). Tests performed under different pH and redox conditions confirmed a marked enhancement in CdTe and CdSe dissolution both under acidic pH and aerobic conditions, which is consistent with thermodynamic predictions. Due to the high leaching potential observed for CdTe in the previous batch experiments, leaching studies were designed to investigate the potential release of soluble Cd and Te from a commercially available CdTe thin-film solar panel under different chemical and biogeochemical conditions commonly found in landfills. The solar panel was subjected to two standardized batch leaching tests (i.e., TCLP and WET), and to a continuous column test designed to investigate the dissolution of CdTe under conditions simulating the acidic- and the methanogenic circumneutral phases of a MSW landfill. A negligible amount of Cd and Te was measured in the synthetic leachate of both batch tests. On the other hand, a significant amount of Cd and Te was released from the panel to the synthetic leachate of the column simulating an acid landfill after 30 days (73% and 21% of the total Cd and Te, respectively). In contrast, the amount of Cd or Te detected in the effluent of the column operated at circumneutral pH values was negligible. The marked difference in the leaching behavior of CdTe in both columns is related to the different aqueous pH and redox conditions promoted by the microbial communities in the columns. The microbial toxicity of the soluble species that can be released from CdTe and CdSe was also assessed as a part of this work. The toxicity of cadmium (Cdᴵᴵ), selenite (Seᴵⱽ), selenate (Seⱽᴵ), Teᴵⱽ, and Teⱽᴵ was evaluated in bioassays with different microbial targets, including acetoclastic and hydrogenotrophic methanogenic populations in a mixed microbial culture, similar to those involved in the stabilization of organic waste stabilization in a landfill, and the bioluminescent marine bacterium, 𝐴𝑙𝑖𝑖𝑣𝑖𝑏𝑟𝑖𝑜 𝑓𝑖𝑠𝑐ℎ𝑒𝑟𝑖 (Microtox® test). The acetoclastic methanogens were most sensitive to the presence of the various soluble species, with the toxicity decreasing in the following order: Cdᴵᴵ, Teᴵⱽ, Teⱽᴵ, Seᴵⱽ, while Seⱽᴵ was only toxic at non-environmentally relevant concentration. Hydrogenotrophic methanogens were highly inhibited by Cdᴵᴵ and Seᴵⱽ, but Teᴵⱽ and Teⱽᴵ only had a moderate toxic impact. The bacterium 𝐴. 𝑓𝑖𝑠𝑐ℎ𝑒𝑟𝑖 was very sensitive to inhibition by Cdᴵᴵ and Seᴵⱽ, and Teᴵⱽ. In the last part of this work, the potential recovery of insoluble, elemental Te⁰ NPs from aqueous solutions containing soluble Teᴵⱽ or Teⱽᴵ was investigated in batch- and continuous flow bioreactors inoculated with a methanogenic granular mixed culture. In the batch experiments, the capacity of the culture to catalyze the reduction of the Te oxyanions and to produce Te⁰ NPs internally and externally to the cells was demonstrated. The granular sludge was found to contain enough endogenous substrates to provide the electron equivalents required to reduce both Te oxyanions and the reduction rates were only modestly increased by an exogenous electron-donor (e-donor) such as H₂. The effect of several redox mediators (RM), namely, anthraquinone-2,6-disulfonate (AQDS), hydroxocobalamin, riboflavin, and lawsone, was also tested. Riboflavin and lawsone caused a remarkable increase of the rate of Teᴵⱽ and Teⱽᴵ reduction, respectively, and also enhanced the fraction of Te recovered as extracellular Te⁰ NPs. The morphology and localization of the Te⁰ NPs were also impacted by the presence of a particular RM and e-donor in the system, suggesting that NP production can be tailored for a particular application. Finally, the feasibility of utilizing upflow anaerobic sludge bed (UASB) bioreactors to reduce Teᴵⱽ oxyanions to non-toxic Te⁰ NPs was also investigated. Two reactors were supplied with ethanol as the external e-donor source to promote the biological reduction of Teᴵⱽ. Riboflavin, a redox mediator, was supplied to one of the reactors to enhance Teᴵⱽ bioreduction. Continuous formation of Te⁰ NPs using an UASB was found to be feasible and remarkably improved by addition of riboflavin at the low Teᴵⱽ:riboflavin molar ratio of 4:1. This flavonoid enhanced the conversion rate of Teᴵⱽ and reduced the toxic impact of Teᴵⱽ towards the methanogenic consortium. Overall, the evidence found in this work indicates that recycling of decommissioned devices containing GaAs, CdTe, or CdSe is desirable to prevent the potential environmental release of toxic metals and metalloids in MSW landfills, but also to allow the recovery of critical resources. Microbial processes offer potential for the removal and recovery of soluble metals and metalloid ions leached from decommissioned semiconductor materials. In particular, this study demonstrated the feasibility of utilizing continuous UASB bioreactors for the removal of Teᴵⱽ from aqueous streams and the recovery of this valuable metalloid as biogenic Te⁰ NPs.

Master of Science / Department of Chemistry / Emily McLaurin / Cu-doped ZnSe/CdSe core/shell nanocrystals were synthesized using the growth doping method. Upon shell growth, the nanocrystals exhibit dual emission. The green luminescence peak is assigned as band edge emission and the broad, lower energy red peak is due to Cu dopant. Although, the oxidation state of Cu in the nanocrystals is debated, the emission is explained as recombination of a hole related to Cu²⁺ with an electron from the conduction band. The emission changed in the presence of dodecanethiol. Generally, the band edge emission intensity decreases and the Cu emission intensity increases. One explanation is the thiol acts as a hole trap, preventing hole transfer to the conduction band. Samples were obtained with varying amounts of Cd²⁺. In the presence of larger amounts of Cd²⁺, the nanocrystals had “thicker shells”, and both the band edge and Cu emission were less sensitive to thiol. The sensitivity likely decreased because the shelled, larger nanocrystals have fewer surface defects resulting in more available electrons.

Este trabalho foi desenvolvido em duas etapas: i) síntese, caracterização e aplicação de pontos quânticos de CdS, CdSe e CdTe em células solares. ii) modificação da superfície dos pontos quânticos de CdSe através de troca de ligante, seguida de caracterização e aplicação em células solares. Os pontos quânticos foram sintetizados utilizando acetatos de cádmio, selênio, telúrio e enxofre como precursores e ácido oleico como agente de estabilização. Na segunda etapa o ácido oleico foi substituído por ligantes com maior afinidade eletrônica pelos sítios de Cd2+: ácido 3-mercaptopropiônico, 4-ácido-mercaptobenzóico e ácido 11-mercaptoundecanóico. As amostras foram caracterizadas por UV-Vis, fluorescência, microscopia eletrônica de transmissão, difratometria de raios-X e voltametria cíclica. Adicionalmente, testes de solubilidade, análises de TGA e de RMN foram realizadas para confirmar a troca de ligante. Através dos resultados, verificou-se que todos os pontos quânticos sintetizados são adequados para sensibilização de TiO2 em dispositivos fotovoltaicos. No entanto, os pontos quânticos de CdSe e CdTe apresentaram fatores que evidenciam maior confinamento quântico, sendo que a maior estabilidade do éxciton foi obtida para o CdSe. Através das análises de RMN foi possível verificar que não existe apenas uma confirguração espacial preferencial para a adsorção do ligante sobre a superfície deste ponto quântico enquanto que curvas de corrente versus potencial e de eficiência de conversão de fóton incidente mostraram que a eficiência do dispositivo é fracamente dependente do ligante. Contudo, a troca de ligantes favorece a solubilidade em solventes com diferentes polaridades, inclusive água, o que amplia as possibilidades de aplicação dos pontos quânticos sintetizados neste trabalho. / This work was developed in two stages: i) synthesis, characterization and application of CdS, CdSe and CdTe quantum dots to assemble solar cells, ii) surface modification, characterization and application of CdSe quantum dots to assemble solar cells. The quantum dots were synthesized by using cadmium acetate, Se, S or Te as precursors and oleic acid as stabilizing agent. In the second stage the oleic acid capping layer was replaced by other ligands with higher electron affinity to Cd2+: 3-mercaptopropionic acid, 4-mercaptobenzoic acid and 11-mercaptoundecanoic acid. The samples were characterized by UV-Vis, fluorescence, transmission electron microscopy, x-ray diffractometry and cyclic voltammetry. Additionally, solubility tests, TGA analysis and NMR were performed to evaluate the CdSe surface modification. The results showed that all quantum dots synthesized are adequate to sensitize TiO2 in photovoltaic devices. However, CdSe and CdTe quantum dots presented better quantum confinement and the exciton generated in CdSe presented the higher stability. NMR analysis provided information about the non-preferential orientation for adsorption of the ligands on the CdSe surface, meanwhile measurements of current vs. potential and incident photon current efficiency showed a weak dependence of photovoltaic device efficiency with the nature of the ligand. On the other side, the surface modification favors the solubility in solvents with different polarizabilities, including water, widening the range for applications of the quantum dots synthesized in this work.

Photovoltaic cells and fluorescence sensing are two important areas of research in chemistry. The combination of photon-activated electron donors with electron acceptors provides a strong platform for the study of optical devices. A series of four oligothiophene-ruthenium complexes has been synthesized. Variation in oligothiophene length and bipyridine substitution allowed comparison of these variables on electronic properties. The longer oligothiophenes display lower energy absorption and emission compared to the shorter ones. Aromatic conjugation appears more complete with para-, rather than meta-, substitution. Oligothiophenes and Ru(bpy)32+ are highly fluorescent individually, but fluorescence is quenched when connected. Bonds of carbon to fluorine are among the strongest single bonds. Single bonds between carbon and hydrogen are also very strong and are ubiquitous. The ability to manipulate these bonds is of great interest to chemists. Two tungsten metal complexes, [6 (perfluorophenyl)bipyridyl] tetracarbonyltungsten and [6-(phenyl)bipyridyl]tetracarbonyltungsten, were prepared for mechanistic C-F and C-H bond activation studies, respectively. These compounds were synthesized through Stille and Suzuki coupling of commercial reagents. Ligands were then bound to tungsten to form the tetracarbonyl complexes.

Magic-sized nanocrystals (MSNs) are nanocrystals with a single size distribution. They have narrow spectral features that do not exhibit inhomogeneous line broadening. This enabled us to analyze homogeneous line broadening of CdSe and CdTe MSNs. In solution, we observed two aggregated configurations of CdSe and CdTe MSNs. Sub-peaks within MSN excitonic peaks were caused by these two aggregated configurations and surface states. A two-dimensional photoluminescence spectrum of a mixture of CdTe 427 nm and 500 nm MSNs gave direct evidence of Förster resonant energy transfer (RET) between the two sizes of MSNs. Normalized experimental overlap between donor emission and acceptor absorption spectra was on the order predicted by theory, confirming that there is sufficient overlap for RET to take place in this system. Additionally, within both aggregated configurations, the two sizes of MSNs were within sufficient distance from one another for RET to occur.

Magic-sized nanocrystals (MSNs) are nanocrystals with a single size distribution. They have narrow spectral features that do not exhibit inhomogeneous line broadening. This enabled us to analyze homogeneous line broadening of CdSe and CdTe MSNs. In solution, we observed two aggregated configurations of CdSe and CdTe MSNs. Sub-peaks within MSN excitonic peaks were caused by these two aggregated configurations and surface states. A two-dimensional photoluminescence spectrum of a mixture of CdTe 427 nm and 500 nm MSNs gave direct evidence of Förster resonant energy transfer (RET) between the two sizes of MSNs. Normalized experimental overlap between donor emission and acceptor absorption spectra was on the order predicted by theory, confirming that there is sufficient overlap for RET to take place in this system. Additionally, within both aggregated configurations, the two sizes of MSNs were within sufficient distance from one another for RET to occur.

Este trabalho foi desenvolvido em duas etapas: i) síntese, caracterização e aplicação de pontos quânticos de CdS, CdSe e CdTe em células solares. ii) modificação da superfície dos pontos quânticos de CdSe através de troca de ligante, seguida de caracterização e aplicação em células solares. Os pontos quânticos foram sintetizados utilizando acetatos de cádmio, selênio, telúrio e enxofre como precursores e ácido oleico como agente de estabilização. Na segunda etapa o ácido oleico foi substituído por ligantes com maior afinidade eletrônica pelos sítios de Cd2+: ácido 3-mercaptopropiônico, 4-ácido-mercaptobenzóico e ácido 11-mercaptoundecanóico. As amostras foram caracterizadas por UV-Vis, fluorescência, microscopia eletrônica de transmissão, difratometria de raios-X e voltametria cíclica. Adicionalmente, testes de solubilidade, análises de TGA e de RMN foram realizadas para confirmar a troca de ligante. Através dos resultados, verificou-se que todos os pontos quânticos sintetizados são adequados para sensibilização de TiO2 em dispositivos fotovoltaicos. No entanto, os pontos quânticos de CdSe e CdTe apresentaram fatores que evidenciam maior confinamento quântico, sendo que a maior estabilidade do éxciton foi obtida para o CdSe. Através das análises de RMN foi possível verificar que não existe apenas uma confirguração espacial preferencial para a adsorção do ligante sobre a superfície deste ponto quântico enquanto que curvas de corrente versus potencial e de eficiência de conversão de fóton incidente mostraram que a eficiência do dispositivo é fracamente dependente do ligante. Contudo, a troca de ligantes favorece a solubilidade em solventes com diferentes polaridades, inclusive água, o que amplia as possibilidades de aplicação dos pontos quânticos sintetizados neste trabalho. / This work was developed in two stages: i) synthesis, characterization and application of CdS, CdSe and CdTe quantum dots to assemble solar cells, ii) surface modification, characterization and application of CdSe quantum dots to assemble solar cells. The quantum dots were synthesized by using cadmium acetate, Se, S or Te as precursors and oleic acid as stabilizing agent. In the second stage the oleic acid capping layer was replaced by other ligands with higher electron affinity to Cd2+: 3-mercaptopropionic acid, 4-mercaptobenzoic acid and 11-mercaptoundecanoic acid. The samples were characterized by UV-Vis, fluorescence, transmission electron microscopy, x-ray diffractometry and cyclic voltammetry. Additionally, solubility tests, TGA analysis and NMR were performed to evaluate the CdSe surface modification. The results showed that all quantum dots synthesized are adequate to sensitize TiO2 in photovoltaic devices. However, CdSe and CdTe quantum dots presented better quantum confinement and the exciton generated in CdSe presented the higher stability. NMR analysis provided information about the non-preferential orientation for adsorption of the ligands on the CdSe surface, meanwhile measurements of current vs. potential and incident photon current efficiency showed a weak dependence of photovoltaic device efficiency with the nature of the ligand. On the other side, the surface modification favors the solubility in solvents with different polarizabilities, including water, widening the range for applications of the quantum dots synthesized in this work.

Este trabalho foi desenvolvido em duas etapas: i) síntese, caracterização e aplicação de pontos quânticos de CdS, CdSe e CdTe em células solares. ii) modificação da superfície dos pontos quânticos de CdSe através de troca de ligante, seguida de caracterização e aplicação em células solares. Os pontos quânticos foram sintetizados utilizando acetatos de cádmio, selênio, telúrio e enxofre como precursores e ácido oleico como agente de estabilização. Na segunda etapa o ácido oleico foi substituído por ligantes com maior afinidade eletrônica pelos sítios de Cd2+: ácido 3-mercaptopropiônico, 4-ácido-mercaptobenzóico e ácido 11-mercaptoundecanóico. As amostras foram caracterizadas por UV-Vis, fluorescência, microscopia eletrônica de transmissão, difratometria de raios-X e voltametria cíclica. Adicionalmente, testes de solubilidade, análises de TGA e de RMN foram realizadas para confirmar a troca de ligante. Através dos resultados, verificou-se que todos os pontos quânticos sintetizados são adequados para sensibilização de TiO2 em dispositivos fotovoltaicos. No entanto, os pontos quânticos de CdSe e CdTe apresentaram fatores que evidenciam maior confinamento quântico, sendo que a maior estabilidade do éxciton foi obtida para o CdSe. Através das análises de RMN foi possível verificar que não existe apenas uma confirguração espacial preferencial para a adsorção do ligante sobre a superfície deste ponto quântico enquanto que curvas de corrente versus potencial e de eficiência de conversão de fóton incidente mostraram que a eficiência do dispositivo é fracamente dependente do ligante. Contudo, a troca de ligantes favorece a solubilidade em solventes com diferentes polaridades, inclusive água, o que amplia as possibilidades de aplicação dos pontos quânticos sintetizados neste trabalho. / This work was developed in two stages: i) synthesis, characterization and application of CdS, CdSe and CdTe quantum dots to assemble solar cells, ii) surface modification, characterization and application of CdSe quantum dots to assemble solar cells. The quantum dots were synthesized by using cadmium acetate, Se, S or Te as precursors and oleic acid as stabilizing agent. In the second stage the oleic acid capping layer was replaced by other ligands with higher electron affinity to Cd2+: 3-mercaptopropionic acid, 4-mercaptobenzoic acid and 11-mercaptoundecanoic acid. The samples were characterized by UV-Vis, fluorescence, transmission electron microscopy, x-ray diffractometry and cyclic voltammetry. Additionally, solubility tests, TGA analysis and NMR were performed to evaluate the CdSe surface modification. The results showed that all quantum dots synthesized are adequate to sensitize TiO2 in photovoltaic devices. However, CdSe and CdTe quantum dots presented better quantum confinement and the exciton generated in CdSe presented the higher stability. NMR analysis provided information about the non-preferential orientation for adsorption of the ligands on the CdSe surface, meanwhile measurements of current vs. potential and incident photon current efficiency showed a weak dependence of photovoltaic device efficiency with the nature of the ligand. On the other side, the surface modification favors the solubility in solvents with different polarizabilities, including water, widening the range for applications of the quantum dots synthesized in this work.

Made available in DSpace on 2014-06-12T15:51:31Z (GMT). No. of bitstreams: 2 arquivo5719_1.pdf: 2389208 bytes, checksum: fd3ee770e606c50de4354c931bb7ce22 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2011 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Nanocristais fluorescentes de semicondutores, quantum dots (QDs) vêm sendo obtidos para diversas aplicações por vários grupos de pesquisa em todo o mundo. A maioria das rotas sintéticas encontradas na literatura refere-se à produção desta classe de nanomateriais em meio orgânico e condições de alta temperatura. A síntese em meio aquoso consiste em uma metodologia simples para a obtenção de nanocristais com estrutura core-shell de CdSe/CdS altamente fluorescentes em regime de confinamento quântico. Estes QDs dispersos em água foram sintetizados através da adição de sal de cádmio, selênio reduzido e ácidos orgânicos com radicais tióis. Tais compostos orgânicos nesse caso são fundamentais para a estabilização das partículas coloidais, além de fornecerem íons sulfeto, que participam da formação da camada de passivação. A caracterização dos QDs obtidos foi realizada através de medidas espectroscópicas e análises estruturais. Foram feitos também planejamentos quimiométricos e avaliações temporais da luminescência para se saber quais os melhores parâmetros e quando os QDs apresentam a melhor intensidade de luminescência após serem sintetizados. Devido sua elevada luminescência, estabilidade em meio aquoso e baixa fotodegradação, os QDs de CdSe/CdS foram utilizados para a marcação de fluorescência de cromossomos metafásicos. Apesar do grande interesse e crescente aumento na produção de QDs, os mesmos apresentam muitas vezes, metais de elevada toxicidade, tais como o Cádmio. Assim, o tratamento dos resíduos oriundos da produção de quantum dots, que ainda é alvo de poucos trabalhos, é aqui abordado mais especificamente com relação ao tratamento através de processos oxidativos avançados, de rejeitos oriundos da síntese dos QDs de CdSe/CdS, objetos do presente trabalho