Catalytic Chemical Vapor Deposition Method Research Articles

Optical and Field Emission Properties of Thin Single-Crystalline GaN Nanowires

Thin high-quality gallium nitride (GaN) nanowires were synthesized by a catalytic chemical vapor deposition method. The synthesized GaN nanowires with hexagonal single-crystalline structure had thin diameters of 10-50 nm and lengths of tens of micrometers. The thin GaN nanowires revealed UV bands at 3.481 and 3.285 eV in low-temperature PL measurements due to the recombination of donor-bound excitons and donor-acceptor pairs, respectively. The blue shifts of UV bands in the low-temperature PL measurement were observed, indicating quantum confinement effects in the thin GaN nanowires which have smaller diameters than the exciton Bohr radius, 11 nm. For field emission properties of GaN nanowires, the turn-on field of GaN nanowires was 8.5 V/microm and the current density was about 0.2 mA/cm(2) at 17.5 V/microm, which is sufficient for the applications of field emission displays and vacuum microelectronic devices. Moreover, the GaN nanowires indicated stronger emission stability compared with carbon nanotubes.

Synthesis and structural characterization of thin multi-walled carbon nanotubes with a partially facetted cross section by a floating reactant method

Here we describe synthesis of very unusual multi-walled carbon nanotubes through a catalytic chemical vapor deposition method using a floating reactant method and subsequent thermal treatment up to 2600 °C in a large quantity. Main characteristics of these nanotubes are (1) relatively wide distribution of diameters ranging from 20 to 70 nm and linear, long macro-morphology (aspect ratio >100), (2) highly straight and crystalline layers, (3) high purity through removal of metallic impurity, (4) very low interlayer spacing (0.3385 nm) and low R value ( I D/ I G = 0.0717), (5) high G′ intensity over intensity of G band (G′/G = 0.85) and strongly negative magnetoresistance value of −1.08% at 77 K and 1 T. The unusual microstructure of thin multi-walled carbon nanotubes with a partially facetted cross-sectional shape caused by thermal treatment is mainly ascribed to abrupt density changes (from 1.89 to 2.1 g/cm 3) within a confined nanosized space, accompanying with the phase separation.

Moisture-Resistive Properties of SiNx Films Prepared by Catalytic Chemical Vapor Deposition below 100°C for Flexible Organic Light-Emitting Diode Displays

Silicon nitride (SiNx) films were deposited on Si and polycarbonate (PC) substrates at temperatures below 100°C by a catalytic chemical vapor deposition (Cat-CVD) method. By adding H2 to source gases, SiH4 and NH3, it was possible to prevent the deterioration of film qualities in low-temperature deposition processes. H atoms produced from H2 are effective for increasing the film densities and improving passivation properties. The water vapor transmission rate of SiNx films on PC substrates deposited at 80°C was lower than 0.3 g/m2day; the detection limit for a cup method. It is concluded that the Cat-CVD method with H2 dilution is a promising technique for preparing highly moisture-resistive SiNx films at low temperatures.

High‐Yield Catalytic Synthesis of Thin Multiwalled Carbon Nanotubes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic production of carbon nanotubes over first row transition metal oxides supported on montmorillonite

Clay-carbon nanotube composites were prepared by employing the catalytic chemical vapor deposition method (CCVD) over different transition metal oxides supported on montmorillonite. Various analytical techniques including SEM, TEM, XRD and DTA/TGA were used for the characterization of the final composite materials. The morphology, quality and structure of the produced nanotubes is shown to be dependent on the type of transition metals.

Catalytic activity of Fe, Co and Fe/Co supported on Ca and Mg oxides, hydroxides and carbonates in the synthesis of carbon nanotubes

Abstract Catalytic chemical vapour deposition (CCVD) is one of the methods used for large scale production of carbon nanotubes (CNT). In this paper the results of the experiments conducted to study the role of Ca and Mg oxides, hydroxides and carbonates as supports for iron, cobalt and a mixture of iron-cobalt catalysts, in the production of CNT by CCVD method are reported. Acetylene and ethylene were used as the two different sources of carbon and N2 as the carrier gas. The syntheses reactions were conducted at 700 °C under ambient pressure in a horizantal tubular furnace. Almost all the support-catalyst mixtures were found to be active in the formation of CNT. The quantity and quality of the tubes produced in different synthesis reactions were however, significantly different. Out of the several supported catalysts used calcium carbonate and magnesium oxide were found to give the best yield of carbon deposit when they were used as supports for a mixture of cobalt and iron. The yield of carbon deposit was found to be very poor when these supports had only Fe. Cobalt on the otherhand showed better activity than iron. A simple method of purification of crude carbon deposit is also described. The density and the quality of nanotubes were analysed by TEM. The CNT produced were found to be multiwall in nature.

High-Yield Catalytic Synthesis of Thin Multiwalled Carbon Nanotubes

We have synthesized thin multiwalled carbon nanotubes (t-MWNTs) using a catalytic chemical vapor deposition (CCVD) method with FeMoMgO catalyst. The number of tube walls were 2∼6 with the corresponding diameters of 3∼6 nm. We obtained high production yield of over 3000 wt % compared to the weight of the supplied catalyst. These t-MWNTs revealed the intermediate structural characteristics between single- and multiwalled carbon nanotubes (SWNTs and MWNTs). Critical issues for high yield synthesis of t-MWNTs were further discussed in terms of reduction procedure, growth time, and gas species.

Hydrogen Storage in High Surface Area Carbon Nanotubes Produced by Catalytic Chemical Vapor Deposition

Carbon nanotubes, mostly single- and double-walled, are prepared by a catalytic chemical vapor deposition method using H2−CH4 atmospheres with different CH4 contents. The maximum hydrogen storage at room temperatures and 10 MPa is 0.5 wt %. Contrary to expectations, purification of the carbon nanotube specimens by oxidative acid treatments or by heating in inert gas decreases the hydrogen storage. Decreasing the residual catalyst content does not necessarily lead to an increase in ASH. Moreover, increasing the specific surface area does not necessarily increase the hydrogen storage capacity. There seems to be a correlation between the pore volume at low pore diameters (<3 nm) and the hydrogen storage capacity. Contribution from nanoscale disordered carbon to the hydrogen storage cannot be ruled out.

Raman study of SWNTs grown by CCVD method on SiC

Single-walled carbon nanotubes (SWNTs) were grown on both the Si- and C-faces of a 6H–SiC substrate by catalytic chemical vapor deposition (CCVD) using ethanol as the carbon source. Raman scattering measurements showed that uniform SWNTs grew on the Si face. Furthermore, SWNTs grown on the Si face showed larger intensity ratio of G-band to D-band (G/D ratio) than those grown on the C face. These results mean that growth of high quality SWNTs is promoted on the Si face in SiC substrates. This is probably attributed to the more reactive nature of the C face with the catalyst compared to the Si face.

Large-Scale Synthesis of Long Double-Walled Carbon Nanotubes

Long double-walled carbon nanotubes (DWNTs) could be synthesized in a production rate of about 0.5 g per h by an improved catalytic chemical vapor deposition method using xylene solution as carbon feedstock. The argon flowing rate affects the growth of DWNTs sensitively. Raman spectra and HRTEM images show that the DWNTs having inner diameters of about 1.0−2.0 nm dominate in the samples.

Generation of single-walled carbon nanotubes from alcohol and generation mechanism by molecular dynamics simulations.

Recent advances in high-purity and high-yield catalytic chemical vapor deposition (CVD) generation of single-walled carbon nanotubes (SWNTs) from alcohol are comprehensively presented and discussed on the basis of results obtained from both experimental and numerical investigations. We have uniquely adopted alcohol as a carbon feedstock, and this has resulted in high-quality, low-temperature synthesis of SWNTs. This technique can produce SWNTs even at a very low temperature of 550 degrees C, which is about 300 degrees C lower than the conventional CVD methods in which methane or acetylene is typically used. We demonstrate the excellence of the proposed alcohol catalytic CVD method for high-yield production of SWNTs when Fe-Co on USY-zeolite powder was used as a catalyst. At optimum CVD conditions, a SWNT yield of more than 40 wt % was achieved over the weight of the catalytic powder within the reaction time of 120 min. In addition to the advantages for mass production, this method is also suitable for the direct synthesis of high-quality SWNTs on Si and quartz substrates when combined with the newly developed liquid-based "dip-coat" technique to mount catalytic metals on the surface of substrates. This method allows easy and costless loading of catalytic metals without the need for any support or underlayer materials that were usually required in previous studies for the generation of a sufficient quantity of SWNTs on an Si surface. Finally, the result of molecular dynamics simulation for the SWNT growth process is presented to obtain a fundamental insight into the initial growth mechanism on the catalytic particles.

Direct Synthesis of Single-Walled Carbon Nanotubes on Silicon and Quartz-Based Systems

A newly developed technique of synthesizing single-walled carbon nanotubes (SWNTs) directly on the surface of Si and quartz substrates is introduced in this report. This technique adopted a liquid-based dip-coating method to mount a very small amount of catalyst metals on the surface of substrates using Mo/Co bimetallic acetate solution. The merits of this approach lie in its easy, costless, and geometry-flexible nature compared with conventional sputtering and deposition approaches. We used the alcohol catalytic chemical vapor deposition (ACCVD) method that can produce relatively high-quality SWNTs even at low temperatures down to 600°C. This low-temperature process contributes to the prevention of the agglomeration of catalytic metals on the surface and chemical reaction between catalytic metal and silicon, which helps us to eliminate any kind of intermediating support materials. Thereby synthesized SWNTs on Si and quartz substrates under various CVD conditions are characterized by means of SEM, TEM, Raman scattering, and optical absorbance measurements. The underlying reasons our experimental procedure and choice of catalyst worked for the synthesis of SWNTs are discussed through comparative studies. At the end of this report, some possible applications of this technique are stated.

Fluorescence spectroscopy of single-walled carbon nanotubes synthesized from alcohol

Near-infrared fluorescence measurements were performed on single-walled carbon nanotubes (SWNTs) catalytically synthesized from alcohol under various experimental conditions (alcohol catalytic CVD method, ACCVD). The chirality distribution was determined by measuring the fluorescence emitted from separated SWNTs as a function of excitation wavelength. Compared with HiPco SWNTs, chiralities of the ACCVD sample were distributed predominantly in the higher chiral angle region, close to the so-called armchair structure. This tendency toward higher chiral angles was more pronounced for smaller diameter nanotubes. The reason for the armchair-rich chirality distribution is discussed based on the initial cap structure satisfying the ‘isolated pentagon rule’.

Synthesis of two dimensional quasi-aligned carbon nanotube bundles by a catalyticchemical vapour deposition method—an inside story

A simple and novel method was developed for efficient synthesis of 2D quasi-alignedmultiwalled carbon nanotubes by a catalytic chemical vapour deposition technique. TheCNT alignment and in depth analysis of structures has been studied by scanning electronmicroscopy and transmission electron microscopy. The growth rate and the purity of theproduct have been found to be affected by the flow rates of the gases as well as the natureof the support used.

Carbon nanostructures produced by CCVD with induction heating

The catalytic chemical vapor deposition (CCVD) method in which the outer furnace is replaced by the high frequency induction heating (IH) has been used for synthesis of carbon nanostructures. Using different catalysts, various types of carbon nanofibers were obtained, with the absence of the catalyst particles at their tip as a common characteristic. The IH mode combined with the CCVD method seems attractive for the synthesis of carbon nanostructures, allowing a significant decrease of energy consumption and of the overall reaction time as compared with the heating mode with outer furnace.

Molecular dynamics simulations of formation process of single-walled carbon nanotubes

Formation process of single-walled carbon nanotubes (SWNTs) by the catalytic chemical vapor deposition method is studied by molecular dynamics simulations. The random deposition process of carbon atoms to a nano-scale metal catalyst is realized with the multi-body potentials based on density functional theory calculations of small metal-carbon and metal-metal clusters A nanotube cap structure is generated when pieces of the hexagonal network structure emerges from the metal-carbon binary cluster and then coalesces. Furthermore, the tendency of graphitization of carbon atoms is compared for Ni, Co, Fe metal clusters. The Co cluster has a pseudo crystal structure where metal atoms are regularly allocated and embedded in the hexagonal carbon network. On the other hand, carbon atoms cover the entire surface of the Fe cluster. This implies stronger interaction between the graphitic lattice and Co atoms, hence higher graphitization ability. Finally, the mechanism of chirality determination by the stability of nanotube cap structure is discussed in order to explain the recent experimental finding that the near-armchair nanotubes are more dominantly produced for small diameter nanotubes.

Parametric Study of Single-Walled Carbon Nanotubes Using Alcohol Catalytic Chemical Vapor Deposition

ABSTRACTSingle-walled carbon nanotubes (SWNTs) were synthesized both in powder form and on substrates using alcohol catalytic chemical vapor deposition method (ACCVD). A detailed parametric study was conducted. As-grown SWNTs were analyzed using transmission electron microscopy (HRTEM), field-emission scanning electron microscopy (FE-SEM) and Raman spectroscopy. Nucleation and growth of SWNTs occurred within seconds after introduction of the alcohol vapor. The results show that high quality SWNTs with a narrow diameter distribution without amorphous carbon can be grown using Co acetate catalyst doped with Fe acetate above 750°C. Defective nanotubes were observed at low (450°C) and high (950°C) temperatures with the optimum temperature being 850°C. In brief, our work has taken a step towards optimizing SWNTs growth conditions using ACCVD.

A model for the structure and growth of carbon nanofibers synthesized by the CVD method using nickel as a catalyst

Carbon nanofibers were synthesized at 450–800 °C by the catalytic CVD method using alumina plate-supported nickel as catalyst and acetylene as carbon source. It was found that Ni/alumina catalyst exhibited a large catalytic effect on the growth of carbon nanofibers at the temperatures between 550 and 700 °C. TEM observation revealed that most of the carbon nanofibers synthesized at 550 °C had a coil-like shape, and many thick platelet nanofibers were found in the product at 700 °C. A growth model was proposed to explain the structural diversity of the carbon nanofibers. Although the carbon nanofibers showed low crystallinity, they can be easily graphitized at 2500 °C.

Synthesis and properties of carbon nanospheres grown by CVD using Kaolin supported transition metal catalysts

Carbon spheres with diameters between 400 and 2000 nm were synthesized in large quantities by catalytic chemical vapor deposition (CCVD) method using Kaolin supported transition metal salts (M=Fe, Co, Ni, etc.) as catalysts. The reaction conditions for the synthesis of carbon spheres in different sizes are described. The reactivity of the carbon spheres in various organic solvents is discussed. The as-synthesized carbon spheres are composed of unclosed graphene layers with the interlayer distances 0.33–0.35 nm. These carbon spheres have been characterized by SEM, TEM, HRTEM, XRD, Raman, ESR and SQUID magnetization techniques. From the ESR and SQUID the metallic nature of the carbon spheres is described.

気相成長カーボンナノチューブ・ナノファイバーの合成とその構造解析

There are many ways to produce carbon nanotubes and nanofibers, such as arc discharge, laser vaporization, chemical vapor deposition (CVD), etc. The most promising mass production-wise method is the CVD that uses metal catalytic particles, and is known as catalytic CVD (CCVD). Its advantage lies in the ease of controlling the growth conditions to obtain nanotubes with different layers, diameters, and morphologies. In this paper, we briefly introduce the CCVD method and show various nanotubes and fibers obtained from the method.