Structural evolution and growth mechanism of hierarchial heterostructures comprised of carbon nanotubes decorated with nanoparticles

Abstract

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.