Synthesis of CNTs From Hydrocarbons Resources by CCVD

1. Introduction

Carbon nanotube is one of the most promising candidates of nanomaterials which own wonderful mechanical, electrical, and thermal properties. With one hundred times the steel’s tensile strength, thermal conductivity better than all materials except the purest diamond, and electrical conductivity contact to the copper with ballistic transport of electrons present the ability to carry much higher currents without heat generation. Carbon nanotube can be categorize to single-walled carbon nanotube and multi- walled carbon nanotubes, when SWCNT has one rolled layer of graphene sheet to form cylindrical shape, MWCNT consist of multiple cylindrical rolled graphene sheets (Fig. 1). General carbon nanotube synthesis methods are Arc discharge, laser ablation, and chemical vapor deposition (CVD). However, CVD is the most convenient method to grow all kinds of CNTs and the best choice to produce large amount of CNTs at relatively low cost and with mild growth conditions. The use of hydrocarbon resources for the production of high-value chemical and materials such as carbon nanotubes obtain obvious impact of sustainable development. In this direction, several research groups have explored the use of hydrocarbons as a carbon sources for CNTs synthesis.

2. Synthesis of CNTs by Catalytic Chemical Vapor Deposition

Recently, Catalytic chemical vapor deposition (CCVD) is the most familiar technique to grow all kind of CNTs. Several motives can explain this favoring. Firstly, consider the technical operation it is easy to perform the reaction between a catalyst and a carbon precursor; it only requires an oven designed with a tubular reactor such as quartz tube, and a few of gas flow controllers in order to feed the require gases. Secondly, a numerous of parameters can be contrasted and investigated from the scientists, not only during the catalyst treatment but also the CNTs growth, which influence the quality, purity, and yield of the CNTs. The mainly vital parameters for CNTs synthesis by CCVD technique is the temperature. In CCVD, energy is donated to hydrocarbons to break them into reactive radical objects in the temperature range approximately from 500-900°C, sometimes more. These reactive species diffuse down to a catalyst surface where they remain bonded. As a result, CNTs are formed. The commonly used energy source is resistive heating.

Generally, there are two processing system patterns for CCVD to synthesis CNTs are horizontal and vertical system. In this literature we will demonstrate a usual horizontal system due to it is far reaching for most scientists. I horizontal system there are two techniques for utilization floating and fixed-bed catalyst technique, displays in Fig. 2. Floating catalyst mainly apply when a mixture of reactants and catalyst are present in the gas phase in the reactor at a promoted temperature during CCVD process. The catalyst in the gas phase experience transformation cause by the redox gases and or the elevated temperature and form solid phase nano particles where the CCVD reaction occurs. This method enclose on obstacle in preventing the nano particles and reactants from coalescence which is when the solid catalyst nanoparticles hold on the reactor surfaces, they could have adequate residence time for CNTs growing. In other words, any unreacted gas precursor and solid catalyst nanoparticles which they have not capable to react and holds on the reactor surfaces at adequately elevated temperatures are swept away from reactor with the carrier gases and unused reactants. Therefore, the result practically will reduce the productivity and process efficiency.

In the fixed bed process, the catalyst which is in the solid phase is set in boats and places inside the reactor then the reactant with carrier which they are in the gas phase are introduced at an elevated temperature where the reaction takes place in CCVD. The efficiency and productivity of CNTs growth in this system are limited by heterogeneous contact between the gas and solid reactant and ascent of the reactor gradients temperature. Because of increase growing nanotubes it cover the catalyst nanoparticles surfaces, the diffusion of carbon precursor to the catalyst nanoparticles will decrease. Therefore, the effectiveness of the catalyst nanoparticles surfaces will reduce.

2.1. Catalyst

Catalysts play an essential role for synthesis of CNTs in the CCVD and hence enhance the suitable characteristics of desired catalyst will be improved the attained CNTs quality and process yield as well. Transition metals in the figure of nanoparticles such as Iron (Fe), Cobalt (Co), Nickel (Ni) and their alloys have been specified as the most effective catalysts for CNTs synthesis. These catalysts can Growth CNTs in three steps according to Vapor-Liquid-Solid (VLS) mechanism: Firstly, a gas precursor produces carbons which adsorb and dissociate on the surface of the catalyst particles to form elementary carbon atoms. Secondly, the carbon atoms dissolve in the bulk of the nanoparticles to form liquid metastable carbide and diffuse within the particles. Finally, solid carbons precipitate at the rear side of the nanoparticles to form carbon nanotubes.

Solid organometallocenes such as nickelocene, cobaltocene, and ferrocene are extensively used as a catalyst for utilizing CNTs, because they deliver solid metal nanoparticles which effectively catalyze the hydrocarbons decomposition. Experimentally, the catalyst particle size is in charge of CNTs diameter. Figure 3 (c) summarize that catalyst particle diameter plays a significant role for determining the structure the utilized nano carbon. Catalyst nano particles with 1 nm diameter mainly utilized SWCNTs [ ], while MWCNTs are utilized from catalyst nanoparticles with diameter 0f 10 – 50 nm as well as the number of MWCNTs layers increased with the particles diameter. In other hand, another nano carbon structure named nano-onion utilized when catalyst nanoparticles with diameter exceed 50 nm.

Attaining hydrocarbons decomposition on the catalyst surface unaccompanied and preventing the aerial decomposition is the Key of obtain pure CNTs. Furthermore, alloys have been proven to gain higher catalytic activity comparing with pure metals. Despite considering the Fe, Co, and Ni metals nanoparticles the effective catalyst for CNTs synthesis, other metals such as Cu, Au, Ag, Pd, and pt as well were discovered to be catalysts for CNTs growth from a variety of hydrocarbons. Therefore, this is unlocked field of research to utilize different CNTs technique with variety hydrocarbons by adjusting temperature and pressure.

2.2. Hydrocarbons decomposition and Thermodynamics

In order to understand the reaction in the CCVD, the main aspect which should be taken in our consideration is thermodynamics. The reaction fulfill to the creation of solid carbon nanotubes have be thermodynamically usefulness under the selected temperature and pressure conditions. Gibbs free energy (ΔG) is the key for extraction this information. In the term of the pyrolysis of hydrocarbons ΔG depends on the reactivity of the hydrocarbon for example; whereas methane’s decomposition is thermodynamically preferable above 600 °C because it is the most stable hydrocarbon molecule, ΔG for ethylene, acetylene or benzene is already negative at 200 °C figure 3a). The expert found that CNTs are frequently synthesized by CCVD using methane precursor at temperature of above 850 °C due to the slow reaction rate, while CVD synthesis of CNTs using acetylene is often carried out at temperature of 500-750 °C and for ethylene is 650-850 °C. [ , , , ]

High quality of SWCNTs is usually utilized by CO decomposition over metal nanoparticles, but this reaction is thermodynamically limited above 600 °C. Therefore, the reaction required high pressure about (10-30 bar) to substitute the equilibrium for growing feasible yields of SWCNTs. [ , , ] Magrez et al. established possibility of growing high quality of MWCNTs when added stoichiometric amounts of CO₂ and C₂H₂ by CCVD at 400 °C[ ]. The solid carbon formation happens by oxidative dehydrogenation of acetylene as a substitute of dehydrogenation or pyrolysis. Thermodynamically, the reaction of CO₂ addition is favorable. While numerous reaction paths are feasible, the solid carbon formation accompanying with CO and H₂ is favored. It is quite clear now also possible to accomplish the attained knowledge to discover reaction conditions which gives the greatest solid carbon formation to obtain highest CNTs yield.

Figure 3. Thermodynamic data calculated with the ChemKin database. a) Gibbs free energies of formation for various carbon precursors. The energies are normalized to the number of carbon atoms in the precursor and correspond to its pyrolysis. b) Gibbs free energies of typical reactions: CO disproportionation, water gas shift, oxidative dehydrogenation of acetylene, and pyrolysis of ethanol. The energies are normalized to the number of solid carbon atoms.

2.3. Carbon Precursor for Catalytic Chemical Vapor Deposition

The carbon precursor plays an important role in the growth, characteristics and properties of CNTs, because of their own binding energy, type and role of reactive groups and thermodynamic properties. Concerning gaseous carbon precursors, the CNTs growth efficiency depends strongly on the concentration and reactivity of gas phase intermediates produced simultaneously with reactive radical species as a result of hydrocarbon pyrolysis. Therefore, it is expected that the most capable intermediates, which have the ability of physisorption or chemisorption on the catalyst surface to initiate CNTs growth, suppose to be produced in the gas phase. A comparison of produced CNT characterizations showed that there is a relationship between chemical structures of hydrocarbons and the CNTs formation [5,7,17,49,62,64,74–77]. Hernadi et al. [49] affirmed that unsaturated hydrocarbons have much higher yield and deposition rate than saturated gases. Besides, saturated hydrocarbon gases manage to produce highly graphitized filaments with fewer walls compared to unsaturated gases. Consequently, they suggested that saturated hydrocarbons are favored for SWCNTs growth and unsaturated hydrocarbons for MWCNTs. However, SWCNTs have been obtained from a highly diluted unsaturated hydrocarbon [10,19,23,38,40,47,75,76,78,79]. The growth of clean SWCNTs was observed at relatively low temperatures using alcohols with various catalysts [19,25,30,64,81–85]. The authors concluded that alcohols are much better carbon sources for SWNTs than hydrocarbons and this is likely due to the ability attributed to OH radicals to etch away amorphous carbon deposits.

General experiences show that low temperature CVD about 600–900°C grow MWCNTs, while high temperature at 900–1200°C reaction favors utilizing SWCNTs. The results indicate that SWCNTs have a higher energy of formation due to small diameter and high curvature which tolerate the high strain energy. Therefore, SWCNTs grow from only selected hydrocarbons such as carbon monoxide, and methane which have an equitable stability at higher temperature, whereas common effective precursor for MWCNTs such as acetylene, benzene, and xylene are unstable at higher temperature which lead to deposit a large quantity of amorphous carbon.

Hata et al. synthesized a highly efficient of impurity-free SWCNTs by water assisted ethylene on substrate in CVD method (Hata et al., 2004). It was reported that controllable rate of steam into the CVD reactor operated as mild oxidizer leads to selective remove of amorphous carbon without harmful the CNTs growth. Controlling relative rate of ethylene and water steam was essential to minimize catalyst’s poison. These studies ascertain prove that carbon precursors play an important part in CNTs growth. Thus, by accurate selection of carbon precursor and water vapor rate, not only the catalyst’s lifetime but also CNTs growth could be crucially maximized and therefore both quality and yield of CNTs could be improved.

Hydrocarbons such as carbon monoxide, methane, ethylene, acetylene, benzene, and xylene are typically the most commonly used CNT precursors. Among the essentially experiences of synthesis CNTs by CVD technique are that MWCNTs ware grown from the decomposition of benzene at 1100°C [] and acetylene at 700°C []. Both of these reports were used iron nanoparticles as the catalyst. As well as, MWCNTs were also grown from many other hydrocarbon precursors including cyclohexane [] and fullerene []. As well as, MWNTs were utilized from supercritical toluene at 600 °C and using ferrocene as growth catalysts, the toluene serves not only as the carbon source for nanotube formation but also as the solvent.

On the other hand, synthesis of high purity SWCNTs at low temperature was reported when Fe-Co supported on zeolite utilized as a catalyst in alcoholic CVD and since then, ethanol consider as the most common universal CNTs precursor in the CVD method. Particular aspect of ethanol for growing CNTs with nearly free from amorphous carbon due to the effect of OH radical which operate as the mild oxidizer. Afterward, vertically aligned SWCNTs were grown as well on Mo-Co supported on coated silicon substrate. Lately, the articles have been shown discontinuous supply rates of acetylene to ethanol CVD apparently assists ethanol to increase the catalytic activity and therefore enhances the CNTs growth rate.

Apart from the well-defined chemical reagents described above, CNTs have also been successfully and systematically synthesized from domestic fuels such as kerosene (Pradhan et al., 2002), liquefied petroleum gas (Qian et al., 2002) and coal gas (Qiu et al., 2006).