Role of iron catalyst particles density in the growth of forest-like carbon nanotubes

Carbon nanotubes (CNTs) were fabricated by Chemical Vapour Depositon using a C₂H₂/H₂ mixture. They were grown on Si/SiO₂ substrate with Fe film as catalyst, deposited using thermal evaporation technique. The aim of this work is to emphasize the role of the Fe catalyst and the C₂H₂/H₂ flow rate ratio to grow vertically aligned CNTs. Fe metal samples with the deposition times ranging from 1 min to 16 min were deposited and CNTs were grown with different C₂H₂/H₂ flow rate ratio, from 5/95 to 30/70 by thermal CVD at 750 ^oC. Results show that CNTs were not vertically aligned with the longest catalyst deposition time for all flow rate ratios, while CNTs were always vertically aligned for deposition time less than 4 min and vertically aligned only for a C₂H₂ flow rate greater than 20% for the 7 min catalyst deposition time. Morphological and structural information about CNTs and Fe metal clusters were provided by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). An accurate balance between the Fe metal clusters density and the C₂H₂/H₂ flow rate ratio favours to achieve of a good vertical alignment     

Aligned carbon nanotubes fabricated by thermal CVD at atmospheric pressure using Co as catalyst with NH3 as reactive gas

Co is used as a catalyst for chemical vapor deposition (CVD) of vertically aligned multi-walled carbon nanotubes (CNTs) in a tube furnace at atmospheric pressure. C₂H₂ and NH₃ were used for the carbon feedstock and reaction control, respectively. The CVD process parameters determine the chemical properties of the Co particles and subsequently the morphologies and field emission behavior of CNTs as they strongly depend upon the catalyst condition. The flow rate ratio of NH₃ to C₂H₂ is shown to be central to the synthesis of vertically aligned CNTs. Repeatable synthesis of vertically aligned CNTs at atmospheric pressure in a tube furnace is cost effective for large area deposition of such structures which may be used, for example, in vacuum field emission devices.     

Growth of high-quality carbon nanotubes on free-standing diamond substrates

Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5 kW microwave plasma chemical vapor deposition (CVD) system using CH₄ as precursor. Ni and Ni-9%W-1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5 μm thickness and consisting of ∼20 nm diameter tubes were observed to grow in a thermal CVD system using C₂H₂ as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2-0.3 in Raman spectra.     

Fluidized-bed synthesis of sub-millimeter-long single walled carbon nanotube arrays

The rapid growth method for vertically aligned, single walled carbon nanotube (SWCNT) arrays on flat substrates was applied to a fluidized-bed, using ceramic beads as catalyst supports as a means to mass produce sub-millimeter-long SWCNT arrays. Fe/Al₂O_x catalysts were deposited on the surface of Al₂O₃ beads by sputtering and SWCNTs were grown on the beads by chemical vapor deposition (CVD) using C₂H₂ as a feedstock. Scanning electron microscopy and transmission electron microscopy showed that SWCNTs of 2–4 nm in diameter grew and formed vertically aligned arrays of 0.5 mm in height. Thermogravimetric analysis showed that the SWCNTs had a catalyst impurity level below 1 wt.%. Furthermore, they were synthesized at a carbon yield as high as 65 at.% with a gas residence time as short as <0.2 s. Our fluidized-bed CVD, which efficiently utilizes the three-dimensional space of the reactor volume while retaining the characteristics of SWCNTs on substrates, is a promising option for mass-production of high-purity, sub-millimeter-long SWCNT arrays.     

Three-terminal CNTs gas sensor for N2 detection

A novel three-terminal gas sensor was fulfilled by utilizing the vertically aligned carbon nanotubes (CNTs) mat. Carbon nanotubes were synthesized by thermal chemical vapor deposition (thermal CVD) at 700 °C under C₂H₂ gas flow rate of 30 sccm. Upon exposure to a with and without N₂ environment at the room temperature of 25 °C, the electrical resistance of as-made devices was found to increase and to return back, respectively. Compared to a low bias one, the sensitivity increased when applying a high source drain bias voltage. Furthermore, the device became more sensitive for N₂ detection by applying a negative gate voltage. It was concluded that the alteration of free holes concentration in the CNTs mat played the major mechanism for the N₂ gas detection.     

Synthesis of single-walled carbon nanotubes produced using a three layer Al/Fe/Mo metal catalyst and their field emission properties

Single-walled carbon nanotubes (SWCNTs) were synthesized on SiO₂/Si substrates by thermal chemical vapor deposition using an Al/Fe/Mo triple layer catalyst, methane (CH₄) as the carbon source, and a mixture of Ar/H₂ (10% H₂) as the carrier gas. The effects of volume ratio of CH₄ to Ar/H₂ (10% H₂), pretreatment time, growth temperature, and Al underlayer thickness on SWCNT growth were studied. The pretreatment time in Ar/H₂ and Al underlayer thickness were found to be crucial for a high-yield of high-purity SWCNTs, since they both governed the size of the catalyst nanoparticles. The optimum growth conditions were found to be a pretreatment time of 20 min, growth time of 10 min, growth temperature of 900 °C, and CH₄/Ar/H₂ flow rates of 50/900/100 sccm, with a catalyst composed of Al (2 nm)/Fe (1 nm)/Mo (0.5 nm). The SWCNTs grown under these conditions have excellent field emission characteristics with low turn-on and threshold fields of 2.4 and 4.3 V/μm, respectively, and a current density of 38.5 mA/cm² at 5 V/μm.     

Single stage production of carbon nanotubes using microwave technology

Carbon nanotubes (CNTs) were produced by gas phase single stage tubular microwave chemical vapor deposition (TM–CVD) using ferrocene as a catalyst and acetylene (C₂H₂) and hydrogen (H₂) as precursor gasses. The effect of the process parameters such as microwave power, radiation time, and gas ratio of C₂H₂/H₂ was investigated. The CNTs were characterized using scanning and transmission electron microscopy (TEM), and by thermogravimetric analysis (TGA). Results reveal that the optimized conditions for CNT production were 900 W reaction power, 35 min radiation time, and 0.6 gas ratio of C₂H₂/H₂. TEM analyses revealed that the uniformly dispersed vertical alignment of multiwall carbon nanotubes (MWCNTs) have diameters ranging from 16 to 23 nm. The TGA analysis showed that the purity of CNT produced was 98%.     

Growth and characterization of vertically aligned centimeter long CNT arrays

Record-long (21.7 mm) vertically aligned MWNT arrays were synthesized using water-assisted thermal CVD process. The catalyst lifetime was maintained for 790 min at optimized experimental condition. The growth of the centimeter long CNT was observed by real time photography at different growth conditions. The growth length increased linearly with increasing growth time followed by a sudden growth end. The ratio of ethylene and H₂ concentration as well as the water and ethylene concentration was studied and optimized which led to prolong catalyst lifetime. Transmission electron microscope images confirmed that most CNTs were double wall and the number of wall distribution was uniform along different height position. Raman spectra showed that the I_D/I_G ratio remained constant at the 3 studied positions along the CNT. TGA demonstrated negligible impurity incorporation in the CNT array.     

Growth of conformal single-walled carbon nanotube films from Mo/Fe/Al2O3 deposited by electron beam evaporation

We discuss growth of high-quality carbon nanotube (CNT) films on bare and microstructured silicon substrates by atmospheric pressure thermal chemical vapor deposition (CVD), from a Mo/Fe/Al₂O₃ catalyst film deposited by entirely electron beam evaporation. High-density films having a tangled morphology and a Raman G/D ratio of at least 20 are grown over a temperature range of 750-900 °C. H₂ is necessary for CNT growth from this catalyst in a CH₄ environment, and at 875 °C the highest yield is obtained from a mixture of 10%/90% H₂/CH₄. We demonstrate for the first time that physical deposition of the catalyst film enables growth of uniform and conformal CNT films on a variety of silicon microstructures, including vertical sidewalls fabricated by reactive ion etching and angled surfaces fabricated by anisotropic wet etching. Our results confirm that adding Mo to Fe promotes high-yield SWNT growth in H₂/CH₄; however, Mo/Fe/Al₂O₃ gives poor-quality multi-walled CNTs (MWNTs) in H₂/C₂H₄. An exceptional yield of vertically-aligned MWNTs grows from only Fe/Al₂O₃ in H₂/C₂H₄. These results emphasize the synergy between the catalyst and gas activity in determining the morphology, yield, and quality of CNTs grown by CVD, and enable direct growth of CNT films in micromachined systems for a variety of applications.     

Effects of bimetallic catalyst composition and growth parameters on the growth density and diameter of carbon nanotubes

Multi-wall carbon nanotubes (MWNTs) were synthesized by catalytic decomposition of acetylene over Fe, Ni and Fe-Ni bimetallic catalysts supported on alumina under various controlled conditions. The growth density and diameter of CNTs were markedly dependent on the activation time of catalysts in H₂ atmosphere, reaction time, reaction temperature, flow rate of acetylene, and catalyst composition. Bimetallic catalysts were apt to produce narrower diameter of CNTs than single metal catalysts. For the growth of CNTs at 600 ‡C under 10/100 seem flow of C₂H₂/H₂ mixture, the narrowest diameter about 20 nm was observed at the reaction time of 1 h for 20Fe : 20Ni : 60Al₂O₃ catalyst, but at that of 1.5 h for 10Fe : 30Ni : 60Al₂O₃ catalyst. It was considered that the diameter and density of CNTs decreased with the increase of the growth time mainly due to hydrogen etching. The growth of CNTs followed the tip growth mode.     

Mass production of aligned carbon nanotube arrays by fluidized bed catalytic chemical vapor deposition

A parametric study investigating the impacts of loading amount of active phase, growth temperature, H₂ reduction, space velocity, and apparent gas velocity on the intercalated growth of vertically aligned carbon nanotube (CNT) arrays among lamellar catalyst was performed. A series of Fe/Mo/vermiculite catalysts with Fe/vermiculite ratio of 0.0075-0.300 were tested. Metal particles were dispersed among the layers of vermiculite after H₂ reduction. Uniform catalyst particles, with a size of 10-20 nm and a density of 8.5 × 10¹⁴ m⁻², were formed among the vermiculite layers at 650 °C. CNTs with high density synchronously grew into arrays among the vermiculites. With the increasing growth temperature, the alignment of CNTs intercalated among vermiculites became worse. Moreover, intercalated CNTs were synthesized among vermiculite layers in various flow regimes. The as-grown particles were with a size of 1-2 mm when the fluidized bed reactor was operated in particulate fluidization and bubbling fluidization, while the size of the as-grown products decreased obviously when they grown in the turbulent fluidized bed. Based on the understanding of the various parameters investigated, 3.0 kg/h of CNT arrays were mass produced in a pilot plant fluidized bed reactor.     

Effect of Temperature Gradient Direction in the Catalyst Nanoparticle on CNTs Growth Mode

To improve the understanding on CNT growth modes, the various processes, including thermal CVD, MP-CVD and ECR-CVD, have been used to deposit CNTs on nanoporous SBA-15 and Si wafer substrates with C₂H₂ and H₂ as reaction gases. The experiments to vary process parameter of ΔT, defined as the vector quantities of temperature at catalyst top minus it at catalyst bottom, were carried out to demonstrate its effect on the CNT growth mode. The TEM and TGA analyses were used to characterize their growth modes and carbon yields of the processes. The results show that ΔT can be used to monitor the temperature gradient direction across the catalyst nanoparticle during the growth stage of CNTs. The results also indicate that the tip-growth CNTs, base-growth CNTs and onion-like carbon are generally fabricated under conditions of ΔT > 0, <0 and ~0, respectively. Our proposed growth mechanisms can be successfully adopted to explain why the base- and tip-growth CNTs are common in thermal CVD and plasma-enhanced CVD processes, respectively. Furthermore, our experiments have also successfully demonstrated the possibility to vary ΔT to obtain the desired growth mode of CNTs by thermal or plasma-enhanced CVD systems for different applications.     

Low-Temperature Growth of Carbon Nanotubes on Bi- and Tri-metallic Catalyst Templates

Low temperature growth process of carbon nanotubes (CNTs) over bi-metallic (Co–Fe) and tri-metallic (Ni–Co–Fe) catalysts on Si/Al/Al₂O₃ substrates is carried out from acetylene precursor using hydrogen, ammonia or nitrogen as a carrier in a low pressure chemical vapor deposition system. Using the tri-metallic Ni–Co–Fe catalyst template, vertically aligned CNTs of ~700 nm length could be grown already at 450 °C within 10 min using ammonia as a carrier. Within the same period of time, on bi-metallic Co–Fe catalyst templates, ~250 nm long aligned nanotubes emerged already at 400 °C in nitrogen carrier. At low temperatures most of the catalyst materials were elevated from the support by the grown nanotubes indicating tip growth mechanism. The structure of catalyst layers and nanotube films was studied using scanning and transmission electron microscopy and atomic force microscopy.     

Synthesis of tall carpets of vertically aligned carbon nanotubes by in situ generation of water vapor through preheating of added oxygen

Dense millimeter-tall carpets of vertically aligned carbon nanotubes (VACNTs) were grown using thermal chemical vapor deposition (CVD) from ethylene and hydrogen gases with two or three independently controlled hot zones while introducing controlled flows of oxygen. Through preheating, oxygen and hydrogen reacted through a multi-step reaction to form water, enabling the growth of tall CNT carpets. This process showed a large tolerance for variations of O₂, H₂, and C₂H₄. The measured water vapor produced was half the theoretical maximum. The residence time strongly affected the decomposition of the gases. The simplicity and robustness of this CVD process provides a simpler alternative to direct addition of water vapor for manufacturing tall carpets of aligned CNTs with a high level of control.     

The Influences of H2 Plasma Pretreatment on the Growth of Vertically Aligned Carbon Nanotubes by Microwave Plasma Chemical Vapor Deposition

The effects of H₂ flow rate during plasma pretreatment on synthesizing the multiwalled carbon nanotubes (MWCNTs) by using the microwave plasma chemical vapor deposition are investigated in this study. A H₂ and CH₄ gas mixture with a 9:1 ratio was used as a precursor for the synthesis of MWCNT on Ni-coated TaN/Si(100) substrates. The structure and composition of Ni catalyst nanoparticles were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The present findings showed that denser Ni catalyst nanoparticles and more vertically aligned MWCNTs could be effectively achieved at higher flow rates. From Raman results, we found that the intensity ratio of G and D bands (I _D/I _G) decreases with an increasing flow rate. In addition, TEM results suggest that H₂ plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles. As a result, the pretreatment plays a crucial role in modifying the obtained MWCNTs structures.     

Alcohol-assisted rapid growth of vertically aligned carbon nanotube arrays

Millimeter-to-centimeter scale vertically aligned carbon nanotube (VACNT) arrays are widely studied because of their immense potential in a range of applications. Catalyst control during chemical vapor deposition (CVD) is key to maintain the sustained growth of VACNT arrays. Herein, we achieved ultrafast growth of VACNT arrays using Fe/Al₂O₃ catalysts by ethanol-assisted two-zone CVD. One zone was set at temperatures above 850 °C to pyrolyze the carbon source and the other zone was set at 760 °C for VACNT deposition. By tuning synthesis parameters, up to 7 mm long VACNT arrays could be grown within 45 min, with a maximal growth rate of ∼280 μm/min. Our study indicates that the introduction of alcohol vapor and separation of growth zones from the carbon decomposition zone help reduce catalyst particle deactivation and accelerate the carbon source pyrolysis, leading to the promotion of VACNT array growth. We also observed that the catalyst film thickness did not significantly affect the CNT growth rate and microstructures under the conditions of our study. Additionally, the ultralong CNTs showed better processability with less structural deformation when exposed to solvent and polymer solutions. Our results demonstrate significant progress towards commercial production and application of VACNT arrays.     

Anodic titanium oxide: A new template for the synthesis of larger diameter multi-walled carbon nanotubes

Carbon nanotubes (CNTs) with larger diameter were synthesized over anodic titanium oxide (ATO) template by CVD method using acetylene as carbon source. The porous titanium oxide was obtained by anodization of titanium metal in a mixture of 1 M H₂SO₄ + 0.5% HF electrolyte at a constant applied potential of 40 V. The XRD analysis of anodized titanium revealed that rutile and anatase forms of TiO₂ are formed due to anodization. Further, SEM analysis was used to follow the development of pores on titanium surface. The TEM analysis revealed that the formed CNTs are straight and hollow with uniform wall thickness as well as larger diameter (70–80 nm). HRTEM study showed that the formed CNTs are multi-walled and their wall thickness is around 2–3 nm. Further, the structural features of the formed CNTs were studied by XRD. Raman spectroscopy was used to study the degree of graphitization of CNTs. The Lewis acid sites of TiO₂ present in the internal surface of the pores play an important role in the catalytic decomposition of acetylene and hence the formation of CNTs. When increasing the carbon deposition time, the wall thickness of CNTs is not increased significantly, indicating that the decomposition of acetylene is due to Lewis acid sites of TiO₂ and not due to thermal decomposition. Further, the morphology of CNTs formed over ATO template was compared with that of CNTs formed on Co electrodeposited ATO. There is no significant difference in morphology as well as wall thickness was observed between the CNTs grown over ATO with and without Co catalyst. But, still further investigations are necessary to study the structural differences between the CNTs grown over ATO with and without Co catalyst.     

Influence of Ni Catalyst Layer and TiN Diffusion Barrier on Carbon Nanotube Growth Rate

Dense, vertically aligned multiwall carbon nanotubes were synthesized on TiN electrode layers for infrared sensing applications. Microwave plasma-enhanced chemical vapor deposition and Ni catalyst were used for the nanotubes synthesis. The resultant nanotubes were characterized by SEM, AFM, and TEM. Since the length of the nanotubes influences sensor characteristics, we study in details the effects of changing Ni and TiN thickness on the physical properties of the nanotubes. In this paper, we report the observation of a threshold Ni thickness of about 4 nm, when the average CNT growth rate switches from an increasing to a decreasing function of increasing Ni thickness, for a process temperature of 700°C. This behavior is likely related to a transition in the growth mode from a predominantly “base growth” to that of a “tip growth.” For Ni layer greater than 9 nm the growth rate, as well as the CNT diameter, variations become insignificant. We have also observed that a TiN barrier layer appears to favor the growth of thinner CNTs compared to a SiO₂ layer.     

Influences of NH3 pre-treatment and radicals on the growth of carbon nanotubes

A triple-layered catalyst (Al/Fe/Mo) undergoes considerable restructuring of surface morphology during NH₃ annealing prior to carbon nanotube (CNT) growth. The diameter (or density) of Al_xO_y–Fe clusters formed during the annealing is found to be dependent on the concentration ratio of NH₃ to H₂O present inside the chamber, which is confirmed by in-situ mass spectroscopy. The different diameter clusters then affect the types of CNTs (i.e. single or multi-walled CNTs) during the growth. Here, a growth model is also presented, where hydrocarbon radicals (C₅H₉, C₆H₉, and C₆H₁₃) generated from C₂H₂ pyrolysis (~ 800 °C) can be used as effective precursors to synthesize CNTs.     

Effects of the catalyst pretreatment on CO2 sensors made by carbon nanotubes

A novel CO₂ sensor was made by carbon nanotubes (CNTs). The CNTs were synthesized by catalystic thermal chemical vapour deposition at 700 °C. Prior to the synthesis, the Fe catalysts were pretreated by H₂ plasma for different times. Two terminal resistance of the as-grown CNTs mat was measured under different CO₂ concentrations. It was found that without the catalyst pretreatment, the sensitivity was about 4% when the CNTs mat was exposed to 800 mTorr CO₂ concentration. However, with various catalyst pretreatment times of 5, 10, 15 and 20 min, the sensitivity was 3.69%, 6.27%, 9.54%, and 12.1%, respectively. The Raman spectroscopy showed the I_D/I_G decreased from 0.668 to 0.539 as the catalyst pretreatment time increased. The XPS also showed the correlation of surface chemical components with the Raman spectroscopy. The Fe catalyst H₂ plasma pretreatment affected both the graphitization and surface binding sites of CNTs.