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Raman imaging and spectroscopy of individual single-wall carbon nanotubes
Year of Dissertation:
Single-wall carbon nanotubes (SWNT) are unique one-dimensional materials that are promising for many potential applications in various important areas. Their vibrational properties reflect the electron and phonon confinement as well as the structures of the tubes. Resonant Raman spectroscopy has been proven to be an exceedingly powerful tool for the characterization of the vibrational and electronic properties of SWNTs. This thesis focuses on the study of Raman spectroscopy of individual single carbon nanotubes. Single tube spectroscopy allows probing the structure dependent properties of SWNTs. A beam-scanning confocal Raman microscope system capable of large-area Raman imaging is first developed for characterizing SWNTs at the single tube level. Raman images and first-order Raman spectra of nanotubes, consisting of both semicoducting and metallic nanotubes, are systemically studied at room temperature in ambient air. The diameter of the nanotubes is determined from their radial breathing mode (RBM) frequency. A broad diameter distribution is observed for nanotubes synthesized by chemical vapor deposition. The tangential G mode Raman spectra of individual metallic nanotubes are found to exhibit a broad distribution of line shapes, which is attributed to shift of the Fermi level due to O2 adsorption. The doping dependence of Raman spectra of metallic tubes is further studied by both electrostatic gating and electrochemical gating. Significant changes in the G band Raman spectra of nanotubes are observed, suggesting the effect of doping on electron-phonon interaction. The observation of a gradual evolution of G band spectrum from a semiconducting type to the broad BWF type reveals evidence of phonon interaction between two G band modes. Raman imaging and Raman spectra of isolated SWNTs and single-layer graphenen are investigated at both room temperature and low temperature. The temperature-induced Raman spectral change of individual nanotubes is observed to be tube diameter dependent, which can be ascribed to the temperature dependence of carbon-carbon bond force constant in SWNTs and the nanotube curvature effect. At last, second-order Raman modes between 1650 and 2000 cm-1 of small-diameter SWNTs are characterized under different excitation wavelength. Excitation wavelength dependent Raman spectra of the same nanotube reveals that frequencies of the overtone M band and combination iTOLA mode of a single tube are insensitive to excitation energy, which is in contrast to the dispersive behavior observed in the ensemble measurement. It is also discovered that the relative intensity of these second-order modes depends on the chirality and family type of a nanotube.
Preparation of Mesoporous Silica and Its Applications in Hydrogen Storage Materials
Year of Dissertation:
The present study investigated the synthesis of mesoporous silica nanospheres (MSNs) and the application of mesoporous silica in hydrogen storage applications. MSNs with particle sizes ranging from ca. 25 to 150 nm are synthesized via sol-gel chemistry with 80 °C isothermal water bath. Initial pH values of the reactant solution were used as control parameters to tune both the particles and pores size of the products. We modified the syntheses conditions and synthesized MSNs using a microwave-assisted heating approach. The introduction of microwave-assisted heating results in better crystallized MSNs. After enhanced hydrogen release properties of pretreated AB were study, some AB/MSN nanocomposites were prepared and the hydrogen release properties of these composites were evaluated. It was found that the AB/MSN nanocomposites had faster hydrogen release kinetics. A critical loading level of ca.0.15 (weigh ratio of AB to MSNs) was found. When the loading level of the nanocomposites is below the critical level, the first two moles hydrogen would be released simultaneously from AB at temperature below 90 °C. Additional, the critical loading level still exists in the AB/mesoporous silica nanocomposite materials of different mesopore sizes, and their critical loading level is affected by the total surface area of the mesopores.
EMERGING ORGANIC CONTAMINANTS IN SURFACE AND GROUND WATERS OF NEW YORK
Year of Dissertation:
The first study was about monitor estrogens (estrone, 17a-estradiol, 17b-estradiol, and estriol) in three headwater streams within a concentrated animal feed operation (CAFO) site on a monthly base for a year. In general, estrogen concentrations in the streams are low (<1 ng/l), and appeared to increase in spring, likely due to the mobilization of estrogens from soils upon snow melting/precipitation. Estrogens were detected in the streams during dry periods, indicating the contribution of estrogens from groundwater. The low concentrations of estrogens in stream water were probably the result of the long residence time (~8 months) of the manure in the lagoons where the majority of the estrogens were degraded during storage. The second study was designed to distinguish between unsewered areas and septic systems application as two possible sources of nitrogen to coastal groundwater by analyzing groundwater samples for pharmaceutical residuals. Groundwater samples were taken through piezometers at shoreline sites in unsewered areas in Northport Harbor and in sewered areas adjacent to Manhasset Bay, both in western Long Island Sound. The frequent detection of the anticonvulsant compound carbamazepine in groundwater samples of Northport (unsewered), together with the fact that few pesticides associated with lawn applications were detected, suggest that wastewater input and atmospheric input are the likely sources of nitrogen in Northport groundwater. High concentrations of nitrogen were also detected in Manhasset (unsewered) groundwater. The low detection frequency of carbamazepine, however suggests that the sewer system effectively intercept nitrogen from wastewater there.
Synthesis and Characterization of a Novel Polyacetal & Design and Preparation of Superhydrophobic Photocatalytic Surfaces
Year of Dissertation:
Polyacetal polymers are thermoplastic resins that play an important role in industry because of numerous industrial applications including automobile; household appliance; etc. The first part of this thesis (Chapter 2) is about the synthesis of a new acetal copolymer that exhibits superior thermal stability. The second part of this thesis (Chapter 3) is about the preparation and applications of TiO2-based polymer nanocomposite films, where the reactive oxygen species (ROS) are generated on the solid surface. Catalytic nanocomposite films are an active area of research because of their potential uses for environmental remediation and chemical synthesis. Furthermore, to enhance surface functionality, superhydrophobic surfaces are prepared using catalyst particles, where the ROS could be generated at the solid-liquid-gas interphase. These works are presented in the third part of this thesis (Chapters 4 and 5). Acetal copolymers represent a family of well-established engineering thermoplastics serving a broad range of important industrial applications including replacement for metals. Their structure consists of oxymethylene units with a low concentration of co-monomer units. By interrupting the facile hemiacetal hydrolysis reaction that can propagate along the macromolecular chain, these co-units function as a "stopper" against degradation of the main block, -(CH2O)n-. The copolymer can also be blended with additives such as stabilizers and reinforcements more easily than the homopolymer due to more flexible polymer chains. Previous approaches have incorporated the "stopper" through cationic copolymerization of cyclic acetals such as ethylene oxide, dioxolane and dioxepane. The first part of this thesis describes the first synthesis of an eight-member ring acetal, 6-methyl-1, 3-dioxocane (MDOC), and its cationic copolymerization with trioxane initiated by boron trifluoride dibutyl etherate. The copolymerization process was monitored in situ using proton NMR. Incorporation of MDOC led to the insertion of the "stopper" unit, "-[CH2CH2CH(CH3)CH2CH2)O]-", thus synthesizing the new acetal copolymer. A superior copolymer thermal stability with a ~ 20oC increase in degradation onset temperature compared with end-capped polyoxmethylene was observed. Both TGA and DSC data indicated the random placement of the "stopper" in the copolymer likely due to efficient transacetalization because of the higher basicity and flexibility of the stopper unit compared with co-units comprising 2 to 4 carbons in length. DSC thermo-grams showed a melting curve of a polymer with melting point lower, as expected, than that of oxymethylene homopolymer. No homopolymer in the copolymer samples was in indicated by TGA. The new acetal copolymer, poly(6-methyl-1,3-dioxocane-co-trioxane), which has a "stopper" co-unit with five carbon atoms along the backbone, contains the longest reported stopper co-unit, potentially leading to improved elongation, and toughness and better compatibility with a range of additives compared to acetal homopolymers.. The second part of this thesis is focused on the design and preparation of photocatalytic surfaces. The use of TiO2 as a semiconducting heterogeneous photocatalyst for the photodegradation of organic pollutants has been extensively investigated as the material is non-toxic, inexpensive, and chemically stable over a wide pH range. Chapter 3 presents a novel lamination fabrication method that enables pre-formed TiO2 nanoparticles to become partially embedded in the surface of a thermoplastic polymer film. In this way, the particles are strongly adhered to the surface while remaining accessible to the aqueous solution. By modifying the fabrication conditions (e.g. temperature, pressure, polymer melt viscosity, etc.), the morphology of the hierarchical TiO2-polymer surface can be controlled and thus the rate of photocatalytic reactions can be increased. In addition, the fraction of TiO2 particles that become fully embedded in the polymer surface, and so inaccessible to photocatalysis reactions, can be reduced through lamination process control, thereby reducing costs. Nanocomposite films were characterized (XPS, SEM, AFM, TGA) and tested by photo-oxidizing a Rhodamine B solution under either a UV lamp or natural sunlight. The morphology of the surface was correlated with both fabrication conditions and photocatalysis rate. This environmentally friendly technique is compatible with any type of TiO2 catalyst particle and so the wavelength response of the photocatalysis can be improved as particles that retain photocatalytic activity at longer wavelengths become commercially available. The wide variety of thermoplastic polymers that are compatible with the process will facilitate their introduction into a wide range of applications including waste water treatment and water purification. In Chapter 4 and Chapter 5, a general approach is presented to incorporating particles into a superhydrophobic surface that catalyze the formation of reactive oxygen species. Superhydrophobic photocatalytic surfaces are prepared using hydrophilic TiO2 nanoparticles and hydrophobic Silicon-Phthalocyanine photosensitizer particles. A stable Cassie state was maintained, even on surfaces fabricated with hydrophilic TiO2 particles, due to significant hierarchical roughness. A triple phase photogenerator is designed and fabricated. By printing the surface on a porous support, oxygen could be flowed through the plastron resulting in significantly higher photooxidation rates relative to a static ambient. Photooxidation of Rhodamine B and BSA were studied on TiO2-containing surfaces and singlet oxygen was trapped on surfaces incorporating Silicon-Phthalocyanine photosensitizer particles. Catalyst particles could be isolated in the plastron to avoid contamination by the solution. This approach may prove useful for water purification and medical devices where isolation of the catalyst particle from the solution is necessary and so Cassie stability is required.
Development of Responsive Nano/Microgels for Materials Application
Year of Dissertation:
Abstract Development of Responsive Nano/Microgels for Materials Application By Ting Zhou Adviser: Professor Shuiqin Zhou Stimulus-responsive polymer microgels swell and shrink reversibly upon exposure to various environmental stimuli such as change in pH, temperature, ionic strength or magnetic fields. Therefore, they become ideal candidates for biomaterial applications. This work covers the general areas of responsive microgels and their application on controlled and targeted drug release. According to the therapy purpose, this work can be classified to two parts. The first part of this thesis (chapter 3-5) focused on the development of biocompatible microgels-based drug delivery systems as anticancer drug carrier. These microgels are constructed from thermo-responsive and/or pH-responsive biocompatible materials, such as, oligo(ethylene glycol) and chitosan. The effects of pH values and temperatures on drug release behaviors of these stimulus-responsive microgels have been discussed. In chapter 5, hybrid ZnO quantum dots (QDs) encapsulated pH and temperature dual-responsive core-shell structure microgels has been prepared, which can not only be applied as targeting drug release system, but also can act as optical sensor for imaging in therapeutic application. The latter part of this thesis (chapter 6, 7) investigated the synthesis, functionalization and characterization of glucose-responsive microgels for diabetes therapy purpose. CdS QDs immobilized glucose-sensitive microgels exhibit fluorescence quenching in the physiologically important glucose concentration range 1-25 mM, which shows promise for a continuous non-invasive in vivo glucose sensing system. In another chapter, core-shell microgels with the P(NIPAM-AAm-PBA) microgel as core and the P(MEO5MA) gel layer as shell were prepared for biocompatibility purpose. The presence of P(MEO5MA) shell could prevent the glucose-sensitive core network from swelling due the hydrogen bonding between oxygens from P(MEO5MA) side chains and glucose molecules, resulting in a shift of glucose sensitivity of core-shell microgels to higher glucose concentration in comparison with the free parent core microgels. Therefore, the set point of glucose sensitivity of microgels could be adjusted possibly and result in potential biomedical applications.