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Theses and Dissertations - Department of Metallurgical and Materials Engineering

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Browsing Theses and Dissertations - Department of Metallurgical and Materials Engineering by Author "Bakker, Martin G."

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    Fundamental studies on growth, interface development, and surface of complex architectures combining oxides nanostructures, noble metal nanoparticles, and carbon nanotubes
    (University of Alabama Libraries, 2013) Shi, Wenwu; Chopra, Nitin; University of Alabama Tuscaloosa
    Nanoscale architectures/heterostructures integrated with multiple functional components with controlled morphology, interface, and phase purity hold great potential for fabrication of photocatalysts with high efficiency, stimuli-responsive drug delivery system, and sensitive and selective chemical/biological sensor. The most urgent tasks are fundamental studies on growth, interface development, and surface of different types of heterostructures. This comprehensive knowledge could directly contribute to rational selection of building components and design of heterostructures with improved properties and/or multifunctionality. In this dissertation, we selected three types of heterostructures: copper oxide (CuO) - cobalt oxide (Co3O4), carbon nanotubes (CNTs)-noble metal nanoparticles, and zinc oxide (ZnO)-noble metal nanoparticles-graphene. Growth mechanisms, interfacial development and interaction, as well as surface chemistry of three systems were studied. Their applications in different directions were also evaluated. In chapters 2 and 3, oxides heterostructures based on CuO nanowires were grown from direct oxidation of copper substrate decorated/coated with Co3O4 using surfactant-free methods. Aligned CuO nanowires were coated with cobalt nitrate and then annealed at high temperature. Tuning of annealing conditions (temperature, duration, and atmosphere) could lead to Co3O4 nanoparticles with controlled distribution density and morphologies on CuO nanowires. Alternative complete dry method combining scalable sputtering of cobalt and annealing of core-shell nanowires was also demonstrated. Effect of sputtering duration on the morphologies and thickness of cobalt shell, mechanical properties of core-shell nanowires, and photocatalytic activity of annealed nanowires were studied. For both methods, heterostructures combined with CuO and Co3O4 showed enhanced photodegradation activity under a low power lamp with visible light illumination. This is mainly due to the improved charge separation at the interface. In chapters 4 and 5, growth of carbon nanotubes (CNTs) by chemical vapor deposition were optimized through an efficient statistic method (Taguchi method). Several important parameters with multiple levels were considered to achieve an optimal condition. Ternary plots incorporating three most important parameters for different catalysts were attained, which provide direct information on effects of each parameter on the final quality of CNTs. The as-prepared CNTs were decorated with noble metal nanoparticles by a direct nucleation method, and then incorporated inside a temperature sensitive hydrogel, poly N-isopropylacrylamide (PNIPAAm). Due to the plasmonic properties of noble metal nanoparticles and good thermal conductivity of carbon nanotubes, nanocomposite hydrogel appeared to be light sensitive and hold potential for stimuli-response releasing. Model molecules (methyl orange, MO and methylene blue, MO) were loaded on nanocomposite hydrogel and released in controllable and programmable manner stimulated by both temperature and visible light. In chapters 6 to 8, mechanism of simultaneous growth of ZnO nanowires of Zn nanostructures was firstly studied. ZnO nanowires were decorated with noble metal nanoparticles (Au, Pt, and Pd) with tunable distribution density to form nanowire-nanoparticles heterostructures and utilized for surface enhanced Raman scattering (SERS). The sensitivity of SERS could also be improved by adjusting distance of adjacent ZnO nanowires decorated with gold nanoparticles by shrinking of a polymer substrate induced by local laser. In order to improve the selectivity of SERS, growth mechanism of graphene on plasma oxidized gold nanoparticles was fundamentally studied. The same condition was applied to grow tubular graphene shell embedded with noble nanoparticles by using nanowire-nanoparticles as a sacrificial template. Acid treatment was used to remove amorphous carbon from tubular structures and introduce extra carboxyl functional groups for specific linking and sensing. A proof-of-concept experiment was performed by linking biotin with graphene surface and then conjugating with streptavidin. Both fluorescence and Raman spectroscopies confirmed the successful linking, which suggests this could be further utilized for fabrication of sensitive and selective chemical/biological sensor.
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    Multifunctional heterostructures comprised of carbon and metal nanostructures: growth mechanisms, plasmonic modeling, and applications
    (University of Alabama Libraries, 2013) Wu, Junchi; Chopra, Nitin; University of Alabama Tuscaloosa
    Noble metal nanoparticles were synthesized by either nucleation in solution or dewetting from thin metal films, and further oxidized to create a thin surface oxide shell. A detailed analysis of surface oxidation of noble metal nanoparticles is presented in this dissertation. This study allowed for utilizing these nanoparticles with controlled surface oxide to result in the growth of graphene shells around noble metal nanoparticles in a chemical vapor deposition process. Oxidation kinetics of noble metal nanoparticles was studied by combining electron microscopy and x-ray photoelectron spectroscopy techniques. This was further correlated with the growth of graphene shells and thicker oxide shell resulted in larger number of graphene layers. In regard to explore their applications, graphene shells encapsulated nanoparticles were demonstrated as a unique plasmonic substrates and catalytic substrates. Plasmonic modeling was done by discrete dipole approximation, simulated and explored the optical properties of graphene shells encapsulated noble metal nanostructures. This approach of graphene shells growth around noble metal nanoparticles was further exploited to understand the role of catalytic noble metal morphology and thus, detailed investigation of the CVD growth of graphene shells around segmented nanowire system was conducted. It was observed that graphene shells were grown around metal nanowires. However, the melting of the nanowires during the growth process must be carefully controlled. This further lead to complex nanowire heterostructures and their incorporation into polymer for bio-applications as demonstrated in this dissertation.
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    Structural evolution and growth mechanism of hierarchial heterostructures comprised of carbon nanotubes decorated with nanoparticles
    (University of Alabama Libraries, 2011) Shi, Wenwu; Chopra, Nitin; University of Alabama Tuscaloosa
    Novel hybrid materials composed of hydrogels and heterostructured 1-D nanostructures such as carbon nanotubes (CNTs) coated with nanoparticles are critical for development of multi-functional analytical systems and biological applications. In this thesis, CNT-nickel/nickel oxide (Ni/NiO) core/shell nanoparticle (CNC) heterostructures were prepared in a simple and single step synthetic approach. The high surface-to-volume ratio and aspect ratio of chemical vapor deposition (CVD)-grown CNTs (average diameter ~ 46±16.4 nm) allowed to uniformly coat with Ni/NiO core/shell nanoparticles (average diameter ~ 12±2 nm). The crystal structure, morphology, and phases of CNC heterostructures were characterized using high resolution transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman Spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and X-ray diffraction (XRD). Single parameter controlled structural and morphological evolution of heterostructures was also evaluated. With the increase of reaction time, distribution density of nanoparticles on CNTs decreased and different shapes of nanoparticles also emerged. When reaction time extended to 15 hrs, due to the interaction between nickel and phosphine based stabilizers, phosphide nanoparticles on CNTs were also synthesized. Thermal stability of prepared heterostructures was evaluated in a N2-rich atmosphere. It was found that high temperature will result nanoparticles migration from CNTs to flat substrate. Meanwhile, decoration of nanoparticles effectively extended the stability range of CNTs from ~ 400 °C to temperatures greater than 600°C. Subsequently, as-produced CNC heterostructures were incorporated into poly vinyl alcohol (PVA) hydrogel. These CNC heterostructure-PVA hydrogels were rigorously characterized for their chemical functionality, morphology, and water absorbing capacity using spectroscopic (FTIR and UV-vis transmittance), SEM, and swelling/shrinking studies. CNC heterostructure-PVA hydrogel was utilized for separating and concentrating chemical species from a mixture. The approach also demonstrated that these hybrid materials can also selectively concentrate L-histidine or histidine-tagged green fluorescent protein (GFP) in the solution. Finally, a cycled magnetic field was applied to control the releasing speed of molecules loaded in the PVA hydrogel. Such selective and multi-component hydrogels can be very useful for developing advanced chemical and biological sensors.
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    Understanding the growth of graphene encapsulated noble metal nanoparticles: morphological and structural evolution studies, growth mechanisms, and characterization
    (University of Alabama Libraries, 2011) Wu, Junchi; Chopra, Nitin; University of Alabama Tuscaloosa
    The major goal of this work was to study the morphological evolution of noble metal nanoparticles such as gold, palladium, and platinum nanoparticles as a function of single-parameter variation in simple synthesis approach. As a next step, these nanoparticles were plasma oxidized to result in surface oxidized noble metal nanoparticles. These noble metal nanoparticles were further utilized for the growth of graphene shells in a chemical vapor deposition method resulting in graphene encapsulated noble metal nanoparticles. In regard to morphological evolution of noble metal nanoparticles, systematic studies were performed, where single growth parameter (temperature, metal salt concentration, surfactant type or concentration, seed amount, or growth duration) was varied while other parameters were kept constant. Gold nanoparticles were synthesized by both single-step method and seed-growth method while palladium and platinum nanoparticles were synthesized at high temperature by alcohol reduction. The size, shape, crystallinity, and sample heterogeneity for the nanoparticles were characterized by high-resolution transmission electron microscopy. Single parameter systematic studies allowed for fundamentally understanding the growth and evolution of noble metal nanoparticles. As temperature increased, nanoparticles size increased due to the decrease of absolute value of volume Gibbs free energy. With the existence of surfactant (e.g. hexadecyltrimethylammonium bromide), stabilizer molecules bind to nanoparticles surface with affinity to different facets. When synthesis temperature higher than boiling point of water, the annealing process resulted in rupture of surfactant from weak binding facets, and boosted anisotropic growth of nanoparticles. Subsequently, oxidation behavior of gold, palladium, and platinum were studied by X-ray photoelectron spectroscopy. Gold oxide, palladium oxide, and platinum oxide were found after plasma oxidation. Noble metal nanoparticles were plasma oxidized for 30 min, and then further utilized for chemical vapor deposition (CVD) of graphene shells. These graphene encapsulated noble metal nanoparticles were thoroughly characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Aggregation of noble metal nanoparticles was observed after graphene growth. Raman spectra showed D-, G-band for graphene encapsulated gold, palladium and platinum nanoparticles after CVD growth. Raman chemical mapping indicates large area growth of graphene encapsulated nanoparticles.