A Study On The Growth And Growth Mechanism Of Single-Walled Carbon Nanotubes From Silica Nanoparticles

As one-dimensional nanomaterials, carbon nanotubes (CNTs) especially single-walled carbon nanotubes (SWNTs) are attractive for vast potential applications in electronics, optical electronics, field effect transistor, chemical and physical sensors, catalyst supports, composite materials, fuel cell and etc due to their unique structure and outstanding physical and chemical properties. Metal catalysts particularly Fe family elements and their alloys have been regarded as indispensable for the CNTs growth. Since 2006, a variety of other metal catalysts such as Au, Ag, Cu, Pt, Pd, Mn, Mo, Cr, Sn, Mg and Al have been found to be active for catalytic growth of SWNTs. However, on the other hand, the unavoidable metal species and different form of amorphous carbons remained in the CNTs products would result in obvious drawbacks for both intrinsic properties and their applications particularly in nanoelectronics devices and biology and medicine. It has been until now an intractable problem to remove metal catalysts completely from CNT product without introducing defects and contaminations. The successful growth of SWNTs using metal-free catalysts can not only provide valuable implications for investigating their intrinsic properties and materials for applications, but also help to understand the growth mechanism of SWNTs in-depth which accordingly will facilitate the controllable synthesis of CNTs with specific structures.In this thesis, by employing different catalyst precursors and appropriate methods, silica nanoparticles were acquired as catalyst. SWNTs were successfully prepared by CVD using methane or ethanol as carbon feedstock. The morphology and the structure of the catalysts and the as-grown CNTs were characterized by SEM, AFM, TEM, Raman, TG and so on. The relationship among the density , the structure of SWNTs , catalysts and growth conditions were studied and simultaneously the mechanism of the metal-free catalytic growth of SWNTs was discussed. The results will be helpful for the exploitation of novel catalysts, better selection of suitable and effective catalysts for SWNTs growth and the expansion the SWNTs application fields.(1) By employing sol-gel method and tetraethyl orthosilicate(TEOS, Si(OC2H5)4)as precursor under acid or alkaline conditions, SiO₂ sol was first acquired followed by post-heat treatment to convert to well dispersed SiO₂ NPs. The dispersion of the SiO₂ NPs on wafer is influenced by immersion time of the wafer in solution. The size of the SiO₂ NPs on the substrate varied by the immersion time, which has been confirmed by atomic force microscope (AFM) and scanning electron microscopy (SEM). The influence of formative SiO₂ NPs for SWNTs growth were also discussed in-depth. The results indicate that SiO₂ reveals high effective activity for the SWNTs growth, besides the SWNTs density was decided by the density of SiO₂ NPs dispersed on wafer.(2) We have demonstrated that silica(SixOy) is a kind of effective catalyst for the synthesis of SWNTs. The SWNTs grown from SixOy NPs which were produced from thermo-decomposition of POSS have uniform diameter, less defects, better conductivity and structural uniformity than that from Fe/Mo catalyst. Moreover, it is found that more than 80% of the SWNTs from POSS are metallic. Selective growth of metallic-enriched SWNTs from POSS will provide a way to generate metallic SWNT for electronics applications.(3)The preparation of aligned and/or patterned carbon nanotubes is of paramount importance in electron field emitters in panel displays, molecular-filtration membranes, single-molecular transistors in microelectronics and many other applications. We have developed a direct photolithographic approach to generate patterned SWNTs on substrate by using POSS as catalyst precursor, which will be useful for some devices application of SWNTs.