Advanced Synthesis Techniques and Characterization of Functional Semiconductor Nanomaterials
Wong, Michael S.
Doctor of Philosophy
Semiconductor materials are used in several modern day applications ranging from photovoltaic devices to environmental remediation. The electronic, optical, catalytic and physical properties of semiconductor nanomaterials can be precisely tuned by altering their size, shape and composition. It is thus imperative to develop simplified cost-effective techniques to synthesize functional semiconductor nanomaterials with structural and morphological control. The overall goal of this thesis is to design new synthetic schemes for well-characterized semiconductor nanomaterials and subsequently demonstrate their potential in photovoltaic and photocatalytic applications. Shape control of semiconductor nanomaterials is crucial for photovoltaic applications. Longer armed cadmium selenide (CdSe) tetrapods have demonstrated enhanced performance in hybrid solar cells. Conventional long arm tetrapod syntheses necessitate multiple injections of flammable phosphorous based chemicals. A new non-phosphorous route to long CdSe tetrapods with arm lengths > 70 nm is demonstrated by manipulating the “greener” selenium precursor temperature in the presence of a quaternary ammonium salt as the shape directing agent. Another interesting shape is the hollow morphology that provides the advantage of higher surface-to-volume ratio. However this shape for CdSe is much less investigated in photovoltaic applications. A novel molten-droplet synthesis strategy is developed to synthesize quantum confined CdSe HNPs based on the slow heating of a low melting point cadmium salt, elemental Se, alkylammonium surfactant in octadecene solvent with no external ligand. This generic technique is shown to be applicable for a variety of metal chalcogenide compositions. Further, photovoltaic device characterization of HNPs in a hybrid solar cell indicate that HNPs have improved electron transport characteristics compared to standard CdSe quantum dots. Hybrid photovoltaic device fabrication is based on low cost colloidal solution-based techniques. A new insight to understanding nanoparticle solvent interactions is provided using coarse-grained computational models and experimental characterization of oleate-capped NPs in various solvents. Solvent polarity was shown to strongly affect NP hydrodynamic diameter, colloidal stability and aggregation behavior. Photocatalytic removal of organic contaminants using semiconductor nanomaterials provides a low-cost, environmentally clean alternative for the utilization of renewable energy sources. Most photocatalytic environmental remediation techniques are oxidative and result in either partial or complete mineralization of the contaminant. A less explored reductive photocatalytic approach to organohalide removal has been demonstrated without necessitating an external co-feed of hydrogen (H2). Hydrodechlorination (HDC) of trichloroethene (TCE) as the test reaction. Bifunctional palladium-based titanium dioxide (TiO2) reduction catalysts were synthesized for the photocatalytic TCE HDC reaction with simultaneous in-situ H2 generation by photocatalytic water splitting. Extension of this reductive photocatalytic approach to other groundwater contaminants could simplify future remediation efforts.