Investigation on the Formation Mechanism of Double-Layer Vertically Aligned Carbon Nanotube Arrays via Single-Step Chemical Vapour Deposition
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The mechanism for the formation of double-layer vertically aligned carbon nanotube arrays (VACNTs) through single-step CVD growth is investigated. The evolution of the structures and defect concentration of the VACNTs are tracked by scanning electron microscopy (SEM) and Raman spectroscopy. During the growth, the catalyst particles are stayed constantly on the substrate. The precipitation of the second CNT layer happens at around 30 min as proved by SEM. During the growth of the first layer, catalyst nanoparticles are deactivated with the accumulation of amorphous carbon coatings on their surfaces, which leads to the termination of the growth of the first layer CNTs. Then, the catalyst particles are reactivated by the hydrogen in the gas flow, leading to the precipitation of the second CNT layer. The growth of the second CNT layer lifts the amorphous carbon coatings on catalyst particles and substrates. The release of mechanical energy by CNTs provides big enough energy to lift up amorphous carbon flakes on catalyst particles and substrates which finally stay at the interfaces of the two layers simulated by finite element analysis. This study sheds light on the termination mechanism of CNTs during CVD process.
KeywordsSynthesis Vertically aligned carbon nanotube arrays CVD Double-layer
Carbon nanotubes (CNTs) have been widely investigated since the report by Iijima in 1991 , due to their unique structures and superior properties [2, 3, 4, 5, 6]. The practical application of CNTs depends on the development of synthesis approaches. Most CNT production, which is mainly unorganized CNTs and has limited properties, is currently mainly utilized in bulk composite materials and thin films. In contrast, organized CNT architectures, such as horizontally aligned CNT arrays [7, 8, 9, 10, 11, 12, 13, 14] and vertically aligned CNT (VACNT) arrays, have superior properties which promise new functionalities and applications [15, 16]. In a VACNT array, all the CNTs are perpendicular to the surface and aligned very well, forming a forest-like structure. Due to their unique structures, VACNTs have been suggested for applications in energy-absorbing, thermal management, highly specular absorbing, electromagnetic shielding coatings, super strong fibres, novel nano-composites, desalination membranes, and high-performance electrodes [17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28].
The main technique to grow VACNTs is chemical vapour deposition (CVD). CVD is considered as the most advantageous method for the synthesis of VACNTs due to the fact that the growth condition can be easily controlled, and the CVD process can be integrated into the standard lithographic methods, which is suitable for chip fabrication. Typically, the CNTs in a VACNT array, if using a predeposited metal film as the catalysts, are continuous from the bottom to the top. However, the growth of multilayer VACNTs, in which the CNTs are not continuous, has also been reported [18, 29, 30]. Stacked multiple layers of VACNTs formed through multistep CVD processes were reported by Ajayan et al. . Multilayer aligned CNTs were also synthesized by Martine et al. using an aerosol-assisted catalytic CVD process .
Multilayer VACNTs have diverse applications such as acting as composite reinforcements, p-n junctions for electronic devices, and allowing the fabrication of complex multilayer nanotube structures . However, for practical applications, single-step CVD methods, which are simple and cost effectively compared to multisteps, are preferred. Zhang et al. reported the growth of double-layered VACNT arrays via single-step CVD method . Although it was proposed that the growth of the double-layered VACNTs by single-step CVD should originate from the deactivation and reactivation of the catalysts, experimental evidence and better understanding on the mechanism are still lacked.
This work systematically investigated the evolution of the structure of VACNTs with the growth duration and proposed the mechanism for the formation of double-layered VACNTs through single-step CVD. There are amorphous carbon flakes between the top and the bottom CNT layers, which should be responsible for the termination of the first CNT layer. Besides, by controlling the growth parameters, the height of the bottom layer in the double-layered VACNTs can reach millimetre scale. These results shed new light on the termination of growth of CNTs and the formation mechanisms of multilayer VACNTs via single-step CVD.
VACNTs were grown in a quartz tube furnace via CVD method. Si wafers coated with silicon oxide (~1 μm) were used as substrates. A thin layer of Fe (~2 nm) supported with Al2O3 (~10 nm) was used as the catalysts. The furnace temperature was ramped to 750 °C in 10 min with flow of Ar (140 sccm) and H2 (20 sccm). Then C2H4 (35 sccm) was introduced as the carbon source for the growth of CNTs. Growth time of 15, 30, 60, and 120 min were applied to obtain different samples. The growth was terminated by turning off C2H4 gas and cooling down the furnace under the protection of H2 and Ar.
3 Results and Discussion
Amorphous carbon flakes deposited on the surfaces of catalyst particles, and Al2O3 substrate will be formed on the bottom of the first layer of VACNTs when the growth of the first layer is terminated, as proved by the results of Electron microprobe. After termination of the growth, the carbon source and the forming gas including hydrogen is continuing aerating. Hydrogen could etch the accumulated carbon on the surface of catalyst nanoparticles randomly  and bring carbon source gas into catalyst particles again, turning the deactivated particles into reactivated. Therefore, the catalyst particles function again for the growth of the second VACNT layer, leading to the nucleation and the elongation of the second layer of VACNTs. The extrusion of the new CNT layer is required to release enough mechanical energy to overcome the adhesion energy between amorphous carbon flakes and catalyst/Al2O3 substrate and then lift the amorphous carbon flakes up, leading to the existence of the carbon flakes at the interface of the top and the bottom CNT layers.
Finite element analysis (FEA) was used to simulate lifting process of amorphous carbon flakes by the extrusion of the second VACNTs growth in the Supplementary Information. Through FEA, it is estimated that 225 nN is the maximum force requested for a CNT to lift up the amorphous carbon film in the same system. It is previously reported that a CNT could bear about 500 nN compression stress during growth, demonstrating our lifting process reasonable in this system . In contrast, the catalyst particle will stay on the surface of the substrate. In the experiments, when the growth time is 15 min, the catalyst is not totally deactivated and no termination of growth happens. However, for growth time of 30 min, both one-layered VACNTs and double-layered VACNTs were observed, indicating the termination and the precipitation of the second-layer CNTs happens at around 30 min growth in the process.
In summary, we investigated the evolution of the structure of VACNTs with growth time and studied the mechanism for the growth of double-layered VACNTs via single-step CVD process. SEM characterization showed that the termination of the first CNT layer and the precipitation of the second CNT layer happened at around 30 min. Raman spectroscopy analysis showed that the top layer and the bottom layer have similar decreasing quality from the top to the bottom, which indicates that both layers experienced the process of degradation of catalyst particles. Interestingly, as shown by SEM and element analysis, there are carbon flakes at the interface of the two CNT layers. Based on all these observations, a mechanism is proposed, which includes the deactivation of catalyst by accumulation of amorphous carbon, the termination of growth of the first CNT layer, the reactivation of catalyst particles by hydrogen gas, and the precipitation and growth of second CNT layer while lifting the amorphous carbon flakes up. The release of mechanical energy by CNTs provides big enough energy to lift up amorphous carbon coatings on catalyst particles and substrates simulated by finite element analysis. This work helps to get a better understanding of the growth termination of CNTs during a CVD process and may be valuable for the mass production of VACNTs. Besides, the structures of the carbon flakes at the interfaces may benefit the construction of novel three-dimensional carbon structures.
The work is supported by NSFC (51422204, 51372132), National Basic Research Program of China (2013CB934200), SRFDP (20120002120038), TNLIST Cross-discipline Foundation, and BNLMS Cross-discipline Foundation.
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