Encapsulation, Compensation, and Substitution of Catalyst Particles during Continuous Growth of Carbon Nanotubes

Xiang, Rong; Li, Guohua; Qian, Weizhong; Zhang, Qiang; Wang, Yao; Wei, Fei; Li, Qi; Cao, Anyuan

doi:10.1002/adma.200602468

Cited by 73 publications

(60 citation statements)

References 19 publications

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“…Details of the growth 3 processes can be found in our previous work. 17,22 The lengths of as-grown CNT arrays were measured by SEM (JSM-7000 and JSM-7401), and average diameters were determined by TEM (JOEL 2010). First, it is worth clarifying the concept of diffusion limit that is to be discussed below.…”

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confidence: 99%

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Growth Deceleration of Vertically Aligned Carbon Nanotube Arrays: Catalyst Deactivation or Feedstock Diffusion Controlled?

et al. 2008

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Feedstock and byproduct diffusion in the root growth of aligned carbon nanotube arrays is discussed. A non-dimensional modulus is proposed to differentiate catalyst-poisoning controlled growth deceleration from one which is diffusion controlled. It is found that, at current stage, aligned multi-walled carbon nanotube arrays are usually free of feedstock diffusion resistance while single-walled carbon nanotube arrays are already suffering from a strong diffusion resistance. The method presented here is also able to predict the critical lengths in different CVD processes from which carbon nanotube arrays begin to meet strong diffusion resistance, as well as the possible solutions to this diffusion caused growth deceleration. * To whom correspondence should be addressed.Fei Wei, weifei@flotu.org, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China; tel, 86-10-62788984; fax, 86-10-62772051.Shigeo Maruyama, maruyama@photon.t.u-tokyo.ac.jp, Department of Mechanical Engineering, The University of Tokyo, 7-3-1 Hongo, Japan; tel, fax, Vertically aligned carbon nanotube (CNT) arrays grown on flat substrates [1][2][3][4][5][6][7] , in which all the nanotubes are of similar orientation and length, offer an ideal platform to study CNT growth mechanisms and kinetics. Since 1996 1 , various chemical vapor deposition (CVD) methods, including floating catalytic CVD 2 , plasma enhanced CVD 3 and thermal CVD 4 have been proposed to synthesize aligned multi-walled carbon nanotube (MWNT) arrays. Lately, alcohol catalytic CVD 5 (ACCVD), water assisted CVD 6 , microwave plasma CVD 7 , etc. are used to produce vertically aligned single-walled carbon nanotube (SWNT) arrays. These processes usually involve different catalysts, carbon sources and operation parameters, resulting in products with different morphologies and qualities. However, none of these CNT growth processes can overcome the gradual deceleration and eventual termination of growth. The ability to understand and thereby to overcome the underlying deactivation mechanisms becomes one of the most critical steps to develop nano-scale tubes into real macroscopic materials. Recently, many groups have affirmed the root growth mode of their vertically aligned CNTs, indicating that the feedstock molecules have to diffuse through the thick CNT array, reach the substrate where catalysts are located, and then contribute to the CNT growth. [8][9][10][11][12][13] In this bottom-up growth process, the diffusion resistance of the feedstock from the top to the root arises as an obstruction, and can act as a unique decelerating growth mechanism. Existence of a feedstock diffusion resistance means that concentration of the carbon source at the CNT root should be lower than the bulk concentration. Previously, Zhu et al.14 fitted experimentally-obtained film thicknesses with the square root of growth time, and stated that the growth deceleration is attributed to the strong diffusion limit of feedstock to the CNT root. However, Hart et al. 15 claimed later tha...

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“…Vertically aligned SWNTs were synthesized on Co/Mo dip-coated 21 quartz substrates at 800 °C from ethanol as a carbon source. MWNT arrays were grown on quartz substrates at 800 °C with simultaneous feeding of cyclohexane and ferrocene 22 . Details of the growth processes can be found in our previous work.…”

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Growth Deceleration of Vertically Aligned Carbon Nanotube Arrays: Catalyst Deactivation or Feedstock Diffusion Controlled?

et al. 2008

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“…To clarify this point, we started the CVD with 12 C ethanol and switch to 13 C one during the growth. The resulting material are VA-SWNTs with 12 C-13 C junctions on a flat quartz substrate, as shown in Fig. 1a.…”

Section: Results and Disscusionmentioning

confidence: 99%

“…[8][9][10][11][12] In this bottom-up growth process, the diffusion resistance of the feedstock from the top to the root arises as an obstruction, and can act as a unique decelerating growth mechanism. At the same time, since feedstock molecules go through CNT "channels", bulk reaction during this diffusion process may also play a role on the real chemical reaction on the catalyst surface.…”

Section: Introductionmentioning

confidence: 99%

Feedstock Diffusion and Decomposition in Aligned Carbon Nanotube Arrays

Xiang

Einarsson

Shiomi

et al. 2012

Journal of Heat Transfer

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Feedstock diffusion and decomposition in the root growth of aligned carbon nanotube (CNT) arrays is discussed. A nondimensional modulus is proposed to differentiate catalystpoisoning controlled growth deceleration from one which is diffusion controlled. It is found that, at current stage, aligned multi-walled carbon nanotube (MWNT) arrays are usually free of feedstock diffusion resistance. However, for single-walled carbon nanotube (SWNT) arrays, since the inter-tube distance is much smaller than the mean free path of carbon source (ethanol here), high diffusion resistance is significantly limiting the growth rate. The method presented here is also able to predict the critical lengths in different chemical vapor deposition (CVD) processes from which CNT arrays begin to meet this diffusion limit, as well as the possible solutions to this diffusion caused growth deceleration. The diffusion of carbon source inside of an array becomes more important when we found ethanol undergoes severe thermal decomposition at the reaction temperature. This means, in a typical alochol CVD, hydrocarbons and radicals decomposed from ethanol may collide and react with the outer walls of SWNTs before reaching catalyst particles. We found when flow rate is low and ethanol is thoroughly decomposed, the produced SWNTs contain more soot structures than the SWNTs obtained at higher ethanol flow rate. Understanding the mass transport and reaction inside a SWNT array is helpful to synthesize longer and cleaner SWNTs.

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“…[20] Typical The spheres, sometimes along with a quartz substrate as reference, were first put in the central part of the tube and heated to 800 °C. The feedstock solution was then injected by a motorized syringe pump into the reactor with an atmosphere of 90% Ar and 10% H 2 .…”

Section: Methodsmentioning

confidence: 99%

Large Area Growth of Aligned CNT Arrays on Spheres: Towards Large Scale and Continuous Production

Xiang

Qian

et al. 2007

Chemical Vapor Deposition

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Table of Contents: A novel strategy for the large scale and continuous production of aligned carbon nanotube arrays using millimeter-diameter spheres as growth substrates is reported. The present technique is more productive than the conventional process on flat wafers because of the higher available growth surface and the good fluidity of the spherical substrates. It can be adapted for the industrial production and application of aligned carbon nanotube arrays with lengths up to millimeter.

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Encapsulation, Compensation, and Substitution of Catalyst Particles during Continuous Growth of Carbon Nanotubes

Cited by 73 publications

References 19 publications

Growth Deceleration of Vertically Aligned Carbon Nanotube Arrays: Catalyst Deactivation or Feedstock Diffusion Controlled?

Growth Deceleration of Vertically Aligned Carbon Nanotube Arrays: Catalyst Deactivation or Feedstock Diffusion Controlled?

Feedstock Diffusion and Decomposition in Aligned Carbon Nanotube Arrays

Large Area Growth of Aligned CNT Arrays on Spheres: Towards Large Scale and Continuous Production

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