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.     

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.     

Carbon Nanotube Growth on Calcium Carbonate Supported Molybdenum-Transition Bimetal Catalysts

A comparison of different catalyst systems (Fe–Mo, Co–Mo or Ni–Mo nanoparticles supported on calcium carbonate) has been performed in order to optimize the carbon nanotube (CNT) growth. The influences of the reaction temperature, metal loading and carbon source on the synthesis of CNTs were investigated. Dense CNT networks have been synthesized by thermal chemical vapor deposition (CVD) of acetylene at 720 °C using the Co–Mo/CaCO₃ catalyst. The dependence of the CNT growth on the most important parameters was discussed exemplarily on the Co catalyst system. Based on the experimental observations, a phenomenological growth model for CVD synthesis of CNTs was proposed. The synergy effect of Mo and active metals was also discussed.     

Characteristics of carbon nanotubes grown by mesh-inserted plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition (CVD) has the advantages of low temperature and vertical growth in synthesizing carbon nanotubes (CNTs), but has generally produced stubby CNTs, probably due to an ion bombardment effect. To suppress the ion bombardment, a metal mesh with the same electrical potential as that of the cathode was placed just above the substrate on the cathode. The anode was electrically grounded while the cathode and the mesh were both negatively biased, causing no plasma to occur below the mesh. The substrate was therefore separated from the plasma by the mesh so that the ion bombardment was suppressed. CNTs were grown on a 2 nm-thick Invar catalyst with different DC plasma powers of 0–112 W at 500 °C, 3.3 torr for 10 min, using C₂H₂ (28 sccm) and NH₃ (172 sccm). Compared to CNTs grown with no mesh, these CNTs showed smaller diameters and greater lengths. As the plasma power decreased, the CNTs grown with mesh were thinner and longer and resembled those grown at a higher temperature by thermal CVD. Etching these CNTs by N₂ plasma reduced their population density and considerably improved their field emission characteristics.     

The formation mechanisms of multi-wall carbon nanotubes over the Ni modified MCM-41 catalysts

Multi-wall carbon nanotubes (MWCNTs) were grown by thermal chemical vapor deposition (thermal CVD) of CH₄ by using Ni-MCM-41 as the catalyst. Methane pyrolysis has been performed in a quartz tube reactor over the catalyst surface to form carbon atoms via dehydrogenation process. The migration and rearrangement of the surface carbon atoms result in the formation of MWCNTs. Transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to determine the morphologies and structures of CNTs, and Raman spectroscopy was exploited to analyze their purity with the relative intensity between the D-band (Disorder band) in the vicinity of 1,350 cm⁻¹ which is characteristic of the sp³ structure and G-band (Graphitic band) in vicinity of 1,580 cm⁻¹ which is characteristic of the sp² structure. In addition, the controlling factors of methane pyrolysis such as the catalyst composition; the reaction temperature, and the methane flow rate on the formation of MWCNTs were investigated to optimize the structure and yield of MWCNTs. SEM/TEM results indicate that the yield of the CNTs increases with increasing Ni concentration in the catalyst. The optimized reaction temperature to grow CNT is located between 640 and 670 °C. The uniform and narrow diameter MWCNTs form at lower flow rate of methane (∼30 sccm), and non-uniform in diameter and disorder structure of MWCNTs are observed at higher flow rate of methane. This is consistent with Raman analysis that the relative intensity of I _D/I _G increases with increasing methane flow rate. The formation mechanisms of the MWCNTs on the Ni-MCM-41 catalyst have been determined to be a Tip-Growth mode with a nanoscale catalyst particle capsulated in the tip of the CNT.     

Synthesis of narrow-diameter carbon nanotubes from acetylene decomposition over an iron-nickel catalyst supported on alumina

Carbon nanotubes (CNTs) were synthesized by the catalytic decomposition of acetylene over 40Fe:60Al₂O₃, 40Ni:60Al₂O₃ and 20Fe:20Ni:60Al₂O₃ catalysts. High density CNTs of 20 nm diameter were grown over the 20Fe:20Ni:60Al₂O₃ catalyst, whereas low growth density CNTs of 40 and 50 nm diameter were found over 40Fe:60Al₂O₃ and 40Ni:60Al₂O₃ catalysts. Smaller catalyst particles enabled the synthesis of highly dense, long and narrow-diameter CNTs. It was found that a homogeneous dispersion of the catalyst was an essential factor in achieving high growth density. The carbon yield and the quality of CNTs increased with increasing temperature. For the 20Fe:20Ni:60Al₂O₃ catalyst, the carbon yield reached 121% after 90 min at 700 °C. The CNTs were grown according to the tip growth mode. Based on reports regarding hydrocarbon adsorption and decomposition over different faces of Ni and Fe, the growth mechanism of CNTs over the 20Fe:20Ni:60Al₂O₃ catalyst are discussed.     

Structural evolution and stability studies of heterostructures comprised of carbon nanotubes decorated with nickel/nickel oxide core/shell nanoparticles

The single-step synthesis of heterostructures, denoted as CNC heterostructures, comprised of carbon nanotubes (CNTs) decorated with nickel (Ni)/nickel oxide (NiO) core/shell nanoparticles through a direct nucleation approach is reported. This approach allowed for a single-parameter and well-controlled structural evolution of Ni/NiO core/shell nanoparticles on CNTs. It was possible to control the size (∼12–52 nm), shape, spatial density, and chemical composition of nanoparticles formed on CNTs by changing the nanoparticle growth time (15 min to 15 h). It was observed that growth time of 15 h led to the reaction between phosphine-based stabilizers and Ni core forming solid and hollow Ni₁₂P₅ nanoparticles directly on CNTs. Thermal stability of CNC heterostructures (in N₂-rich atmosphere) dispersed on a silicon wafer as well as subsequent surface migration of nanoparticles were studied by annealing heterostructures at different temperatures (125–750 °C). In comparison to as-produced CNTs, the surface characteristics, oxidative resistance or thermal stability, and changes in heterostructure compositions were studied using electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The heterostructures showed greater oxidative resistance as compared to as-produced CNTs.     

A possible approach exploring the melting state of catalysts during the low-temperature growth of carbon nanotubes

A method to qualitatively examine the physical state of catalysts has been demonstrated by simultaneously supplying Ti species for carbon nanotube (CNT) growth. Under a TiCl₄/CH₄/H₂/N₂ plasma ambience, a continuous TiCN film was deposited on the Si substrate in the absence of catalysts; however, for the Ni-coated Si substrate, one-dimensional CNTs rather than TiC or TiCN nanofibers were grown through the catalytic vapor growth process, with the amorphous Ti-containing layers coated on their surfaces. The obtained coaxial CNT structure and the electron diffraction pattern analysis of catalysts support the factor that Ti atoms hardly dissolve into the catalyst and pass through it (or diffuse along the surface). Selective dissolution and diffusion between the Ti and C atoms in Ni nanoparticles suggest that catalysts during low-temperature growth of CNTs (580 °C) are not melted, instead they should be in solid state.     

A molecular level model for the nucleation of a single-wall carbon nanotube cap over a transition metal catalytic particle

Recent in-situ video rate TEM studies have revealed that the base growth of single-wall carbon nanotubes (SWCNT) in thermal chemical vapour deposition (CVD) is accompanied by a considerable deformation of the Ni catalyst nanoparticle and the creation of a subsurface carbon layer. In this paper we argue that these effects may be produced by the adsorption – on the catalyst nanoparticle – of cyclopentadienyl ions formed in gas phase during C₂H₂ pyrolysis. These ions can, at the same time, facilitate the nucleation of a SWCNT cap by the provision of “ready made” pentagons. To this end we have performed semi-empirical quantum mechanical calculations with the ZINDO method. The results support the above proposed mechanism. We also suggest that this mechanism could also explain the increased rate of SWCNT production in plasma enhanced CVD, where these ions are expected to be present in higher concentrations during C₂H₂ pyrolysis.     

Carbon nanotubes field emission devices grown by thermal CVD with palladium as catalysts

The effects of palladium (Pd) catalyst film thickness and ammonia (NH₃) in thermal chemical vapor deposition (CVD) growth of carbon nanotubes (CNTs) are systematically compared per the resulting morphologies, Raman spectra and field emission characteristics. The CNT field emitters were tested under identical experimental configurations. Field emission characteristics were described with Fowler-Nordheim field emission theory. Experimental results demonstrate that thermally grown CVD CNTs configured as diode field emitters exhibit low turn-on fields and high emission current density. The work is extended to include the study of gated field emitters or field emission triode, important to achieving high-resolution, full gray-scale imaging for field emission, flat-panel displays. The gated device was fabricated utilizing single-mask, self-aligned gate electrode with conventional integrated-circuit (IC) fabrication process. The CNT-triode showed gate-controlled modulation of emission current where higher gate voltage gives rise to higher anode currents. The triode fabrication process using silicon-on-insulator (SOI) wafers is discussed.     

化学气相沉积法制备碳纳米管过程中NH3对其微观结构和石墨化程度的影响

以二茂铁为催化剂前驱体,C₂H₂为碳源,在NH₃和N₂气氛中在Al₂O₃多孔基体上化学气相沉积制备出了一致取向的碳纳米管阵列.采用拉曼散射、扫描电子显微镜、高分辨电子显微镜、X射线衍射和元素分析等技术对产物进行表征.实验结果表明,以NH₃为载气时,所制备的碳纳米管为竹节形结构,并且有一定量的N掺杂在其中,其质量含量大约在3.46%,碳纳米管的石墨层间距在0.384 nm左右,实验产物中有Fe₃C和Fe₂N或Fe₃N的生成,NH₃在催化剂表面的吸附导致催化剂的表面性质的改变以及N的掺杂使生成的碳纳米管结构有一些缺陷,石墨化程度有所降低.碳纳米管的管径随着反应温度的增加而增大.     

Large-scale synthesis of n-type gallium nitride nanowires using NiI2-decorated carbon nanotubes as a reactant

This study demonstrates the first example of the use of NiI₂-filled carbon nanotubes (CNTs) for the synthesis of GaN nanowires (NWs). Large quantities of single crystal and n-type GaN NWs were synthesized after NiI₂-decorated CNTs reacted with Ga₂O₃ in NH₃. Comparatively few short GaN NWs (<1 μm) were synthesized in the absence of CNTs, and GaN NWs were found to be synthesized with a reasonable yield using graphite as a reactant. Therefore, CNTs play no role as a template in the growth of NW, but this growth is assisted by NiI₂ nanocrystals via a vapour-liquid-solid mechanism in which the presence of carbon materials facilitates the reduction of Ga₂O₃ to Ga₂O and Ga, thus providing a constant Ga source during the growth of the NW. Furthermore, the use of NiI₂-filled single wall carbon nanotubes results in a higher NW yield at a low growth temperature (600 °C), indicating that NiI₂-filled single wall CNTs can serve as an effective reactant for the synthesis of GaN NWs.     

Catalyst‐Assisted Growth of Single‐Crystalline Hafnium Carbide Nanotubes by Chemical Vapor Deposition

Single‐crystalline hafnium carbide (HfC) nanotubes were synthesized by a one‐step catalyst‐assisted chemical vapor deposition (CVD) method. The typical nanotubes had uniform diameters of ~60 nm and wall thicknesses of ~15 nm and preferentially grew along [201]. From HRTEM/EELS analysis, the growth mechanism based on carbon nanotubes (CNT) tip growth and CNT‐templated reaction was proposed for explaining the formation of HfC nanotubes. According to the mechanism, CNTs were first formed by diffusion of C atoms on the surface of solid Ni catalyst particles. Then, gaseous Hf species reacted with C atoms from CNTs to form HfC nanotubes. During the entire growth process, Hf atoms did not participate in the catalytic reaction. Thus, this process was distinguished from the conventional vapor–liquid–solid process.     

Hot filament enhanced CVD synthesis of carbon nanotubes by using a carbon filament

The low temperature synthesis of single-walled carbon nanotubes (CNTs) on a Si substrate has been reported. Single-walled CNTs were grown from a C₂H₂ and H₂ mixture by a hot-filament enhanced CVD method using a carbon filament. The catalyst was silica-supported iron–cobalt prepared by sol–gel method. We observed the influence of the catalyst material, C₂H₂ concentration, growth pressure and substrate temperature on the formation of the CNTs. By optimizing the catalyst, both single-walled CNTs and multiwalled CNTs could be synthesized, depending on the reaction conditions. The formation of single-walled CNTs occurred when the carbon supply was kept low, i.e. low C₂H₂ concentration and low reaction pressure. The diameter of tubes decreased with increasing the substrate temperature. By optimizing the growth conditions, a small diameter of 0.65 nm single-walled CNTs, estimated from Raman scattering spectrum, was achieved even the low substrate temperature as 660 °C.     

High-pressure pyrolysis of melamine route to nitrogen-doped conical hollow and bamboo-like carbon nanotubes

Nitrogen-doped conical hollow and bamboo-like carbon nanotubes (CNTs) have been prepared by pyrolysis of melamine with NaN₃–Fe–Ni and Ni catalysts at high temperature and high pressure, respectively. The conical hollow CNTs with an average diameter of about 70 nm and a length up to 5 μm account for ∼50% of the product, whose N/C atomic ratios are about 0.27. The conical bamboo-like CNTs with the diameter of ∼65 nm and length of 1–4 μm and wall thickness of 10–20 nm account for ∼95% of the product, whose N/C atomic ratios are up to 0.18. The control experiments show that NaN₃ plays a key role in keeping high nitrogen content and high conversion ratio in the CNTs. The possible growth mechanisms have been discussed on the base of the experimental observation. The strategy provides an alternative route to nitrogen-doped CNTs and other carbon nitride materials.     

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.     

The role of nitrogen in carbon nanotube formation

To examine the role of nitrogen, Co- and Ni-coated substrates were pretreated with three different gas compositions to compare the pretreated catalyst surfaces; the Fe, Co and Ni foils were subjected to carbon nanotube (CNT) growth experiments with CH₄/H₂ and CH₄/N₂ as source gases; the catalyst pretreatment plus the CNT growth experiments on Co- and Ni-coated Si substrates were carried out using both microwave plasma chemical vapor deposition and electron cyclotron resonance chemical vapor deposition (ECR-CVD) under different nitrogen-containing gases. The results show that the role of nitrogen may be summarized as follows: by comparing with hydrogen plasma, the bombardment energy of nitrogen plasma is greater. Therefore, the presence of nitrogen during CNT growth can keep the front catalyst surface clean and active to prolong surface passivation to enhance carbon bulk diffusion. The higher temperature due to higher bombardment energy of nitrogen plasma can promote agglomeration effects during catalyst pretreatment and the initial stage of CNT growth to produce larger size nano-particles. The presence of nitrogen is a favorable condition for formation of the bamboo-like CNTs, but not a necessary condition. Another favorable condition for formation of the bamboo-like CNTs is to deposit CNTs by ECR-CVD.     

Fex-Co1-x bimetallic catalysts for highly efficient growth of carbon nanotubes on carbon fibers

This paper aims to develop a catalyst for highly efficient growth of carbon nanotubes (CNTs) on carbon fibers (CFs) with improved tensile strength by chemical vapor deposition (CVD). The effect of the composition of Fe_x-Co_1-x catalysts on the morphology of the synthesized CNTs and the tensile properties of the CFs at different stages of the CVD process were explored. It was found that the bimetallic catalysts possessed better catalytic effect than monometallic catalysts at low temperature. The detailed study of the tensile properties of CFs at different stages of CVD process manifested that the composition of catalysts affected the catalytic activity. When the atomic fraction of Fe was 50%, the bimetallic catalyst among Fe_x-Co_1-x family was considered to hold the highest catalytic efficiency due to the least damage to the fibers and the ability to generate more carbon atoms to repair and reinforce CFs. The corresponding tensile strength of the final CNTs-grafted CFs was 11.53% higher than that of the desized CFs.     

Temperature gradient chemical vapor deposition of vertically aligned carbon nanotubes

We present temperature gradient chemical vapor deposition (TG CVD) for producing vertically aligned (VA-) carbon nanotubes (CNTs). Independent heaters on the gas inlet and catalyst substrate sides of a cold-wall, vertical CVD reactor can modulate the gas temperature gradient to lead to controlled thermal histories of acetylene precursor. Our growth results reveal that such a precursor thermal history can play a significant role in the growth and structural features of the resultant VA-CNTs. We find several gas thermal zones particularly important to the VA-CNT growth by evaluating the precursor dwell time in different zones. Thermal treatment of the acetylene precursor at 600–700 °C is found crucial for the synthesis of VA-CNTs. When this thermal zone is conjoined in particular with a zone >700 °C, efficient growths of single-walled and double-walled VA-CNTs can be achieved. These gas thermal zones can contribute to VA-CNT growths by mixing various secondary hydrocarbons with acetylene, corroborated by the results of our reacting flow simulation. Our findings emphasize the influence of gas-phase reactions on the VA-CNT growth and suggest that our TG CVD approach can be practically utilized to modulate complex gas-phase phenomena for the controlled growth of VA-CNTs.     

Growth evolution of rapid grown aligned carbon nanotube forests without water vapor on Fe/Al2O3/SiO2/Si substrate

This paper presents the growth evolutions in terms of the structure, growth direction and density of rapid grown carbon nanotube (CNT) forests observed by scanning and transmission electron microcopies (SEM/TEM). A thermal CVD system at around 700 °C was used with a catalyst of Fe films deposited on thin alumina (Al₂O₃) supporting layers, a very fast raising time to the growth temperature below 25 °C/s, and a carbon source gas of acetylene diluted with hydrogen and nitrogen without water vapor. Activity of Fe catalyst nanoparticles was maintained for 5 min during CVD process, and it results in CNT forests with heights up to 0.6 mm. SEM images suggest that the disorder in CNT alignment at the initial stage of CNTs plays a critical role in the formation of continuous CNT growth. Also, the prolonged heating process leads to increased disorder in CNT alignment that may be due to the oxidation process occurring at the Fe nanoparticles. TEM images revealed that both double- and few-walled CNTs with diameters of 5-7 nm were obtained and the CNT density was controlled by thickness of Fe catalytic layer. The number of experiments at the same conditions showed a very good repeatability and reproducibility of rapid grown CNT forests.