Template Assisted Synthesis of Carbonized Polymer Nanowires

Abstract

Nanomaterials synthesis is a highly focused area now-a-days due to great significance of nanomaterials in terms of their vast applications including nano-sensors, nano-catalysts, nano-electro-mechanical systems (NEMS) and memory devices etc. One of the facile and low cost techniques to synthesize nanomaterials is the use of templates. A template acts like a scaffold to restrict the in-filtered material and produces nano-scopic replica of it. Since the synthesis takes place in the confined nanospaces, hence high surface area nanostructures can be achieved. By using this type of synthetic strategy, nanostructures can be tailored according to the prescribed requirement through the control of the design of templates. The present thesis presents the work on the synthesis of carbonized polymer nanowires through template mediated technique using different carbon precursors including poly (vinyl) alcohol (PVA), poly (vinyl) chloride (PVC) and chitosan. For synthesis of carbon nanowires, from poly(vinyl) alcohol, carbonization was performed at temperatures of 300°C, 400°C, 500°C, 600°C, 700°C ,800°C and 900°C in the absence of any catalyst. Scanning electron microscopy (SEM) revealed the diameter of the nanowires in the range of 90-120 nm. Detailed characterization using SEM, thermal gravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy was done confirming the formation of nano-wires and graphitization of amorphous carbon resulting from the PVA pitch. The nanowires formed at temperatures of up to about 600oC were rigid, while those formed at 700oC were quite flexible. At 800oC and 900oC, the nanowires formed were again rigid but of shorter length due to fragmentation of the wires produced. It could be due to the formation of graphitic clusters / crystallites. The work was further extended to develop carbon nanostructures through the PVC thermolysis by infilteration of the synthetic polymer into the nano-pores of the templates. Successful formation of carbon nanowires was realized at 100oC, 200oC, 2 300oC, 400oC, 500°C and 600oC in inert atmosphere. The synthesized nanostructures were characterized for the similar techniques as in the case of PVA. Initiation of aromatization was detected at temperatures as low as 100oC. Up to 200oC, nanofibers were formed. Pure graphitic regions were identified at higher temperatures and SEM showed the formation of nano-tubular structure at 300oC and 400oC. Further increase in temperature up to 600oC caused shrinkage in the diameter of nanotubes resulting in nanowires formation. Apart from using synthetic polymers as carbon precursors, a natural polymer chitosan was also used. It was first irradiated to reduce its molecular weight. Low molecular weight facilitated the infiltration of the polymer into the nano-spaces of the template. Pyrolysis was done at 100°C, 250°C, 300°C, 400°C, 500°C and 600°C. The nanotubes developed at 250°C mainly consisted of the saccharide structure. Drastic increase in weight loss at 300°C collapsed the saccharide structure and extensive aromatization took place as indicated by FTIR spectroscopy. The formation of carbon nanotubes was obvious as demonstrated by Raman spectroscopy and SEM results.