Fundamental studies on growth, interface development, and surface of complex architectures combining oxides nanostructures, noble metal nanoparticles, and carbon nanotubes

Abstract

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.