Fabrication of carbon nanomaterials using thermal plasma jet and their applications

Abstract

Recently, carbon nanomaterials are widely studied and used in various applications including electronic devices, solar cells, and nanobiomedicine. Carbon nanomaterials have been mainly determined by their shapes: zero-dimensional fullene, one-dimensional carbon nanotube, and two-dimensional graphene. In addition to listed materials, there are various carbon nanomaterials such as graphene quantum dots (GQDs), onion-type carbon, nanodiamond, amorphous carbon, and carbon dots. Each of carbon nanomaterials has different physical and chemical properties by their structure. Characteristics of carbon materials have been utilized in the need of application. However, the commercialization of carbon nanomaterials stayed for a long time due to diffculty in the technology of mass production. We have developed mass production of carbon nanomaterials using thermal plasma jet and utilized the product for application. Graphene has generated tremendous interest over the past decade because of its extraordinary properties and potential applications. In this work, dispersible graphene flakes were successfully fabricated via a one-step process using a thermal plasma jet system. The graphene flakes fabricated by injection of ethylene gas as a carbon source (500 sccm) were very pure, contained no oxygen, and were few layered. Although their average size was larger than 100 nm, they were well-dispersed in organic solvents by sonication. The production rate based on the collected amount was approximately 1.5 g/h. As a representative application, thin films of the graphene flakes were fabricated on fluorine-doped tin oxide (FTO) glass using three deposition techniques. The resulting dye-sensitized solar cell with a graphene flake/FTO counter electrode exhibited a power conversion efficiency of 9.03%, which was similar to the efficiency of the solar cell with a conventional Pt/FTO counter electrode. Therefore, our graphene flake/FTO electrode could be used as a substitute for the conventional Pt/FTO counter electrode for DSSCs as graphene flakes are much less expensive than Pt. In addition to this specific application, dispersible graphene could be used in the fabrication of composites as well as various energy storage, sensor, and electronic devices. Graphene is a zero band gap semiconductor, which reduces its electronic and optoelectronic properties almost impossible to use for device applications. However, the GQDs, which are graphene sheets smaller than 100 nm, possess strong quantum confinement and edge effects. The former effect allows the bandgap of GQDs to be controlled by modifying their size, and the latter effect causes the GQDs to be dispersed in common solvents. We reported a size-controllable method for GQDs by a gas phase collision reaction of carbon atoms using thermal plasma jet. A carbon tube was generated by injecting a large amount (2,500 sccm) of ethylene gas continuously into Ar plasma. A carbon tube was attached to an anode and ethylene gas was flowed continuously as a carbon source into a torch through a gas flow meter. The production rate of GQDs was about 4 g/h, and the average size of GQDs was controlled by varying the length of the carbon tube attached. However, there is no quantum confinement effects to control the band gap of GQDs by modulating their size. Thus, we focused on strong edge effects of GQDs. When a graphene sheet is cut along zigzag lines, carbene edges having two unshared valence electrons at each edge carbon atom are made, while along armchair lines carbyne edges having carbon triple bonds are made. Carbene and carbyne edges have polar and nonpolar characters, respectively. 90 degree corners are made when armchair and zigzag lines are encountered, while 120 degree corners are made when the same type lines are encountered. Therefore, hexagonal GQDs are made when the same type cutting lines are encountered, at all corners, while rectangular GQDs are made when different type cutting lines are encountered. We have separated three kinds of GQDs: Zigzag GQDs having only carbene edges were dispersed in polar solvents and had basically hexagonal shapes. Armchair GQDs having only carbyne edges were dispersed in nonpolar solvents and had also basically hexagonal shapes. Hybrid GQDs having both carbyne and carbene edges in each dot were dispersed in both polar and nonpolar solvents and had rectangular shapes. The photoluminescence and photoluminescence excitation spectra of hybrid GQDs responded to the combination of the spectra of zigzag and armchair GQDs. The absolute quantum yields of three kinds of GQDs are relatively very high. For a 2.5 L/min injection rate of ethylene gas, the production rate of GQDs is about 4 g/hour. The relative abundance of armchair, zigzag and hybrid GQDs is 96.9, 2.7, and 0.4%, respectively. The producted GQDs were used in nanobiomedicine such as bioimaging and photodynamic therapy. Onion-type carbon and nanodiamond, before discovery of graphene and carbon nanotube, were one of the most promising carbon nanomaterials in the past. However, research of onion-type carbon stayed for a long time due to high-cost fabrication methods. We have fabricated onion-type carbon by a gas phase collision reaction of carbon atoms using thermal plasma jet. A carbon atomic beam was generated by injecting a large amount of ethylene gas continuously into Ar plasma. A carbon tube was attached to an anode and ethylene gas was flowed continuously as a carbon source into a torch through a gas flow meter. The production rate of onion-type carbon was about 36 g/h and the average size was 15 – 20 nanometers, showing a lattice like the structure of the onion. The produced onion-type carbon was well dispersed in organic solvent and absorbed all light from ultraviolet to near-infrared region. The onion-type carbon was used for photothermal therapy, which destroys cancer cells by generating heat with light irradiation using nanoparticles, due to its low-toxicity and the heat generated by light irradiation. In the absence of light, the dispersed solution at concentration up to 0.5 mg/ml did not show obvious toxicity to cells. In vitro photothermal effect of onion-type carbon led to cells destruction after 10 min of laser irradiation at 2 W/cm2.