Synthesis of Carbon Nanotubes from Iron-containing Proteins

Abstract

The electronic property of carbon nanotubes (CNTs) depends on the diameter and chirality of the CNTs. Especially, single-walled CNTs (SWCNTs) are classified into metals and semiconductors depending on their chirality. Particularly in semiconductors, the band gap of the SWCNTs is inversely proportional to the diameter of SWCNTs. However, SWCNTs are grown as mixture of various chirality, approximately 67% of semiconductors and 33% of metals at room temperature. Therefore, research has been made recently in the selective synthesis of SWCNTs with certain electronic properties. The mechanism of growth of CNTs by catalytic chemical vapor deposition (CVD) is summarized as follows. The carbon precursor is decomposed and diffused on the surface of the nanostructured transition metal nanoparticles such as Co, Ni, or Fe at high temperature between 600 and 1000 °C, and the increase and saturation of C content in metal solution cause to grow the CNTs. Thus, the structure of CNTs is dependent on the growth parameters such as reaction temperature, catalytic nanoparticles, reaction gas, etc. Among them, the metal nanoparticles are considered as a crucial factor for the selective and controlled growth of SWCNTs from the theoretical and experimental studies on in-situ transmission electron microscopy (TEM). The catalytic nanoparticles obtained from sputtering or evaporation are formed by heat treatment of thin metal film on a substrate. Thus, the nanoparticles are usually polydisperse in size and randomly distributed over the whole substrate. Also, when the nanotemplate method and sol-gel reaction are used to make the nanoparticles, they are formed with narrow distribution but the diameter of the particles cannot be smaller than 3 nm with current technology.Here, the iron-containing proteins, hemoprotein were used as the catalyst for the growth of CNTs. Hemoproteins contains defined number of iron atoms in themselves, thus, it is possible to control the size of the nanoparticles if the assembled number of hemoproteins could be precisely controlled. Therefore, in this thesis, the feasibility of the catalytic ability of the hemoproteins, hemoglobin (Hb), myoglobin (Mb), and cytochrome c (Cyt c) on the growth of CNTs was investigated. The proteins were strongly adsorbed onto amin-terminated self-assembled monolayer (SAM) surfaces by the peptide bonds between the carboxyl groups of the proteins and the amine group of the SAMs and initiated the growth of highly graphitized multi-walled CNTs (MWCNTs) with an average diameter of 5.63 nm. Also, the nanoparticles with a diameter of 2.41 nm obtained from spin-coated Hb after the heat treatment at 800 ℃ were used to synthesize SWCNTs with an average diameter of 1.22 nm. Although they can initiate the growth of SWCNTs as well as MWCNTs, they are not able to be controlled precisely in this manner. Thus, we polymerized Hb molecules using cross-linking agent, glutaraldehyde in order to form the iron nanoparticles with narrow diameter distribution. After the polymerization, polymerized Hb (polyHb) was fractionated using size-exclusion chromatography (SEC). Likewise, deposited polyHb was oxidized at 800 ℃ and 1.22 nm of the iron nanoparticles with a narrow diameter distribution were obtained. From these nanoparticles, 1.11 nm of SWCNTs with a narrow diameter distribution were also synthesized. Based on the accurate separation of polyHb by size, it is expected that the nanoparticles derived from separated polyHb will initiate the exact same size of SWCNTs. Furthermore, regulating the number of assembled Hb in polyHb could be possible.