Chemical Vapor Decomposition Research Articles

Synthesis and hydrogen storage properties of carbon nanotubes

Multi-walled carbon nanotubes (MWNT) have been synthesized by chemical vapor decomposition of acetylene over rare-earth (RE) based AB 2 alloy hydride catalysts. The AB 2 alloy hydride catalysts have been prepared by hydrogen decrepitation technique and characterized by scanning electron microscopy. The advantage of this novel method of obtaining catalysts has been discussed. The as-grown carbon nanotubes were purified by acid and heat treatments and characterized using powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, thermo gravimetric analysis and Raman spectroscopy. Hydrogen adsorption measurements were carried out on as-prepared and purified MWNT in the temperature range of 143–373 K and pressure range of 10–100 bar using a high pressure hydrogen adsorption setup and the results have been discussed. A maximum hydrogen storage capacity of 3.5 wt% is obtained for purified MWNT prepared with DyNi 2 alloy hydride catalyst at 143 K and 75 bar.

Effect of Purity and Substrate on Field Emission Properties of Multi-walled Carbon Nanotubes

Multi-walled carbon nanotubes (MWNT) have been synthesized by chemical vapour decomposition (CVD) of acetylene over Rare Earth (RE) based AB2(DyNi2) alloy hydride catalyst. The as-grown carbon nanotubes were purified by acid and heat treatments and characterized using powder X-ray diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Thermo Gravimetric Analysis and Raman Spectroscopy. Fully carbon based field emitters have been fabricated by spin coating a solutions of both as-grown and purified MWNT and dichloro ethane (DCE) over carbon paper with and without graphitized layer. The use of graphitized carbon paper as substrate opens several new possibilities for carbon nanotube (CNT) field emitters, as the presence of the graphitic layer provides strong adhesion between the nanotubes and carbon paper and reduces contact resistance. The field emission characteristics have been studied using an indigenously fabricated set up and the results are discussed. CNT field emitter prepared by spin coating of the purified MWNT–DCE solution over graphitized carbon paper shows excellent emission properties with a fairly stable emission current over a period of 4 h. Analysis of the field emission characteristics based on the Fowler–Nordheim (FN) theory reveals current saturation effects at high applied fields for all the samples.

A multi-step strategy for cutting and purification of single-walled carbon nanotubes

A multi-step procedure has been developed to cut and purify single-walled carbon nanotubes (SWCNTs) from the pristine soot synthesized by catalytic chemical vapor decomposition, which contains significant amount of multiwalled carbon nanotube (MWCNT) impurities. The procedure involves refluxing in dilute HNO 3 solution followed by (NH 4) 2S 2O 8/H 2SO 4 ultrasonication and a novel ammonia heat treatment step. The metal impurities are removed in the HNO 3 refluxing step. In the (NH 4) 2S 2O 8/H 2SO 4 treatment step, the SWCNTs are cut uniformly into micrometer long, while the MWCNTs are selectively cut into very short curled pieces. These short MWCNT pieces and the amorphous carbon impurities are partly removed in the (NH 4) 2S 2O 8/H 2SO 4 treatment step, and most of them are finally removed by the final ammonia heat treatment. The ammonia heat treatment step also recovers the structures of SWCNTs by healing defects on the sidewall. The final SWCNT products have purity higher than 95%, a length distribution between 1 and 2 μm, and very few defects.

Electron field emitters based on multiwalled carbon nanotubes decorated with nanoscale metal clusters

The present work describes the field emission characteristics of nanocrystalline metal clusters decorated multi walled carbon nanotubes (MWNT) field emitters fabricated over flexible graphitized carbon cloth. MWNT have been synthesized by catalytic chemical vapour decomposition (CCVD) of acetylene over Rare Earth (RE) based AB2 (DyNi2) alloy hydride catalyst. Fine powders of RE based AB2 alloy hydride catalysts have been prepared by hydrogen decrepitation technique. The as-grown carbon nanotubes (CNTs) are purified by acid and heat treatments and characterized using XRD, SEM and TEM. Purified MWNT have been decorated with nanocrystalline Pd, Pt and Pt–Ru metal clusters using a simple chemical reduction method. The characterization of metal decorated CNTs were done by using XRD, SEM, TEM, HRTEM and EDX. Nanocrystalline Pd, Pt and Pt–Ru metal clusters decorated MWNT field emitters have been fabricated over graphitized carbon fabric using spin coating method. The field emission characteristics have been studied using an indigenously fabricated set up and the results are discussed. The samples show excellent emission properties with a fairly stable emission current over a period of 4 h. As the presence of the graphitic layer provides strong adhesion between the nanotubes and carbon cloth, the use of graphitized carbon cloth as substrate opens new possibilities for CNT field emitters.

Laser-induced synthesis of nanostructured thin films

We present a number of important characteristics of laser techniques for preparation of nanostructured thin films with foci on laser ablation (LA) and laser-assisted chemical vapor decomposition (LCVD). Size-related features and structures of the deposited layers are associated with process parameters. Whenever possible, the performance of the deposits is compared to that of films prepared by other methods ( e.g., physical vapor synthesis such as gas deposition, sputtering, etc.). The LA and LCVD methods allow a wide variety of materials to be prepared. A number of promising applications emerge when one combines this versatility with the potential for making certain types of nanostructured layers. These possibilities are also discussed.

Size and structure of nanoparticles formed via ultraviolet photolysis of ferrocene

Iron nanoparticles enclosed in carbon shells were formed by laser-assisted chemical vapor decomposition of ferrocene (Fe(C5H5)2) vapor in Ar gas atmosphere. The particle size dependence on the total ambient gas pressure and on laser fluence of the pulsed ArF excimer laser was examined and, e.g., an effective size decrease of the iron core was observed at elevated laser fluences. Characterizations of the iron and carbon microstructures were performed by x-ray diffraction and transmission electron microscopy, while relative iron deposition rates were measured by x-ray fluorescence spectroscopy. Both α-Fe and γ-Fe phases were found for the single crystalline iron cores, surrounded by graphitic (inner) and amorphous (outer) carbon layers. The temperature rise of the laser-excited particles was also determined by optical spectroscopy of the emitted thermal radiation, which allowed an estimation of the iron loss of the nanoparticles due to evaporation. The estimated and measured iron losses are in good agreement.

Carbon Felt/Carbon Nanotubes/Pani as pH Sensor

AbstractThis work proposes the use of the composite carbon felt/carbon nanotube/Polyaniline as an alternative for applications as a pH sensor device. The carbon felt/carbon nanotube is an electronic conductivity material that was obtained from polymer felt (poliacrilonitrile felt) using oxidation and carbonization processes. The cup-stacked and bamboo-like tubes were grown on the fibers of carbon felt by chemical vapor decomposition method. The sensor was obtained by incorporating polyaniline (Pani) on the nanotubes present on the fibers of carbon felt/carbon nanotubes composite. The measuring process uses an EGFET (Extended Gate Field Effect Transistors) configuration, which is a derivation of the ISFET (Ion Sensitive Field Effect Transistor) - that is basically a chemical semiconductor sensor. The drain-current versus source-drain voltage is presented for varying pH concentrations from 2 up to 12.

Removal of nickel ions from water by multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were produced by chemical vapor decomposition using acetylene gas in the presence of Ferrocene catalyst at 800 °C, and then oxidized with concentrated nitric acid at 150 °C. Both (as-produced and oxidized) CNTs were characterized by TEM, Boehm titration, N 2-BET and cation exchange capacity techniques. The adsorption capacity for nickel ions from aqueous solutions increased significantly onto the surface of the oxidized CNTs compared to that on the as-produced CNTs. The effects of adsorption time, solution pH and initial nickel ions concentrations on the adsorption uptake of Ni 2+ for both the as-produced and oxidized CNTs were investigated at room temperature. Both Langmuir and Freundlich isotherm models match the experimental data very well. According to the Langmuir model the maximum nickel ions adsorption uptake onto the as-produced and oxidized CNTs were determined as 18.083 and 49.261 mg/g, respectively. Our results showed that CNTs can be used as an effective Ni 2+ adsorbent due to the high adsorption capacity as well as the short adsorption time needed to achieve equilibrium.

Carbon nanotube/felt composite electrodes without polymer binders

In this work we have investigated the suitability of composite electrodes consisting of cup-stacked and bamboo-like carbon nanotubes (CNT) synthesized directly onto a carbon felt for both lithium storage and double-layer capacitance applications. The CNT/felt composite electrode was prepared using catalytic chemical vapor decomposition on the carbon felt. The microstructure of the electrodes was characterized by scanning electron microscopy. Electrochemical characterization of the CNT/felt, either submitted or not to acid treatment for extraction of the catalytic particles used during the CNT growth, was carried out using 1 mol L −1 LiPF 6 in mixtures of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and propylene carbonate. The carbon nanotubes loading and the type of CNT, whether open or closed, on the felt were the most significant factors regarding the electrochemical properties of the composite. With respect to the application of the composite to lithium storage, an anomalous behavior in the reversible specific capacity as a function of the current was detected. The capacity was found to be large at higher current values. The best reversible specific capacity was found for the open-CNT/felt (275 mAh g −1 at 0.16 A g −1, and 200 mAh g −1 at 0.82 A g −1), on an area of 0.634 mm 2. The double-layer capacitance of the CNT decreased with increasing current. In the case of the open-CNT with a CNT loading of 13.93 mg, the composite provided 40.3 μF cm 2 or about 12 F g −1 at 10 mA of polarization current using 1 mol L −1 LiPF 6 in mixtures of ethylene carbonate and dimethyl carbonate. For the closed-CNT with a CNT loading of 9.3 mg, the double-layer capacitance was 30 F g −1 at 20 mA in 1 M H 2SO 4.

Synthesis and characterization of turbostratic carbons prepared by catalytic chemical vapour decomposition of acetylene

The turbostratic mesoporous carbon blacks were prepared by catalytic chemical vapour decomposition (CCVD) of acetylene using Ni/MgO catalysts prepared by co-precipitation. The relationship between deposition conditions and the nanostructures of resultant carbon black materials was investigated. It was found that the turbostratic and textural structures of carbon blacks are dependent on the deposition temperature and nickel catalyst loading. Higher deposition temperature increases the carbon crystallite unit volume V nano and reduces the surface area of carbon samples. Moreover, a smaller V nano is produced by a higher Ni loading at the same deposition temperature. In addition of the pore structure and the active metal surface area of the catalyst, the graphitic degree or electronic conductivity of the carbon support is also a key issue to the activity of the supported catalyst. V nano is a very useful parameter to describe the effect of the crystalline structure of carbon blacks on the reactivity of carbon blacks in oxygen–carbon reaction and the catalytic activity of carbon-supported catalyst in ammonia decomposition semi-quantitatively.

Surface oxygen complexes as governors of neopentane sorption in multiwalled carbon nanotubes

Multiwalled carbon nanotube (MWCNT) samples were obtained by purification and ball-milling of a MWCNT sample synthesized by catalytic chemical vapor decomposition. These samples were oxidized with KMnO 4–H 2SO 4 solution. A heat treatment was employed to decompose the oxygenated groups created by the oxidation treatment. The samples were characterized by transmission electron microscopy (TEM), adsorption of nitrogen, pore size distribution, and by thermogravimetric analysis combined with mass spectroscopy (TG-MS). The dynamics of 2,2-dimethyl-propane (neopentane) sorption was studied by frequency-response (FR) spectroscopy. The FR spectra obtained clearly demonstrate the effect of (a) the ball-milling, (b) the oxidative treatment and (c) the heat treatment on the sorption uptake processes involved in these treated samples.

Synthesis, Properties and Applications of Helical Carbon Nanotubes

Helical carbon nanotubes were produced on silica‐supported Co catalysts by chemical vapour decomposition of acetylene. Coiled nanotubes were examined. They have been found to be of various shapes, diameter, and pitch. Some of them are extremely long, up to 5 µm, with regular helices. The average outer diameter of coils are about 30–50 nm, the pitch is in the range 50–200 nm and the length about 1–1.4 µm. The helix‐shaped windings of the helical carbon nanotubes reveal characteristic mechanical resonances, which are determined by the elastic modulus, mass, shape, and dimensions.

Macroscopic carbon nanofibers for use as photocatalyst support

To reduce the drawbacks related to the use of powders or immobilized catalysts in gas- and liquid-phase applications, a new material for the use as photocatalyst support was obtained by chemical vapor decomposition at 700 °C of an ethane–hydrogen mixture over a woven glass microfiber supported nickel catalyst, consequently leading to carbon nanofibers with macroscopic shaping, consisting in woven glass microfibers supporting a dense network of entangled 40 nm diameter carbon nanofibers. This material could be directly used after synthesis without any subsequent purification treatment due to the high yield and totally selective carbon nanofiber production. This shape memory design results in the direct use of the carbon nanofiber–woven glass microfiber composite as support without any post-synthesis shaping. The presence of hydrophilic oxygenated groups located at the outer surface of the carbon nanofibers allowed the sol–gel preparation of a woven glass microfiber–carbon nanofiber supported TiO 2 (20 wt.%) catalyst, using tetraisopropoxide as precursor. This new photocatalyst was totally stable under UV irradiation.

Synthesis and characterisation of carbon nanofibres with macroscopic shaping formed by catalytic decomposition of C 2H 6/H 2 over nickel catalyst

Carbon nanofibres composites with macroscopic shaping were successfully synthesised by chemical vapour decomposition (CVD) using a low loading in nickel as catalyst (≤1 wt.%). A high carbon nanofibres yield, i.e. 100 wt.% weight gain per hour of synthesis with respect to the initial catalyst weight was obtained, which significantly lowered the synthesis cost while the direct macroscopic shaping rendered more reliable the applications of these materials in conventional catalytic processes.

Nanostructure characterization of tungsten-containing nanorods deposited by electron-beam-induced chemical vapour decomposition

Electron-beam-induced chemical vapour decomposition was performed in a scanning transmission electron microscope using a precursor of tungsten carbonyl (W(CO)6). The self-supporting nanorods were grown from the edges of a C film with widths that depend on the electron-beam scanning speed used in the fabrication process. The nanostructure of as-deposited nanorods has been characterized in detail using energy-dispersive X-ray spectroscopy, selected-area electron diffraction, microdiffraction and high-resolution transmission electron microscopy. A mixture of nanocrystallites and amorphous phases was observed for all beam scanning speeds used for deposition. High-resolution transmission electron microscopy demonstrated that the size of nanocrystallites in as-deposited nanorods ranges between 1.5 and 2.0 nm. The direct evidence of the presence of pure W nanocrystallites in as-deposited nanorods was revealed by microdiffraction.

Capacitance properties of ordered porous carbon materials prepared by a templating procedure

The electrochemical performance of carbon materials with a highly ordered nanoporous structure is investigated in two-electrode supercapacitors. The materials were prepared by a templating procedure using a silica matrix (type MCM-48 or SBA-15) with an organized porosity in which carbon was inserted, either by chemical vapor decomposition of propylene or by impregnation with a sucrose solution followed by carbonisation. After the removal of silica, a micro-mesoporous carbon residue is recovered which displays an uniform pore size distribution. Such a well-defined nanostructure is interesting for a fundamental study of the double layer capacitance behavior. The performance of supercapacitors built with electrodes prepared from the templated carbon was tested in acidic, alkaline and organic electrolyte solutions. High values of capacitance in aqueous and organic media were obtained with a rectangular shape of the voltammograms over a wide range of scan rates indicating a quick charge propagation. Especially, the templated carbons prepared by the impregnation of sucrose in MCM-48 display high capacitance values due to the formation of an adequate micro-mesoporous network during their formation. A marked shift of capacitance drop at higher values of frequency is clearly observed for the materials rich in mesopores; the mesopores make easier the diffusion of the ions to the active surface.

Synthesis of single–walled carbon nanotubes using MgO as a catalyst support

Single-walled carbon nanotubes were synthesized by thermal decomposition of methane over a metal catalyst supported on MgO. MgO as a catalyst support was easily removed by dissolving in acid, and the content of SWNTs in purified samples was up to 90% Cobalt can catalytically promote production of SWNTs more strongly than ferrite during the methane-based chemical vapor decomposition processing Some products with a very large diameter of about 5 nm expect to he well used in basic and application research.

Formation and emission spectroscopy of laser-generated nanoparticles

Fe nanoparticles, with both fcc and bcc structures and with a C shell that protects against oxidation, were generated by the laser-assisted photolytic chemical vapor decomposition of ferrocene (FeCp2). Amorphous W and WN0,3 nanoparticles were formed by laser ablation (LA) of solid W in Ar and in N2 ambient, respectively. Laser-assisted chemical vapor deposition of W yielded crystalline W nanoparticles (β phase) from a WF6/H2/Ar gas mixture. ArF excimer laser was used as the radiation source in all the experiments. Measurements and analysis of the emitted blackbody-like radiation from the laser heated particles were performed and dominant cooling processes such as evaporation and heat transfer by the ambient gases were identified. The particles could be heated up to the boiling and melting point of Fe and W, respectively. Lognormal particle size distributions were found for Fe/C and W nanoparticles generated by vapor decomposition or deposition processes respectively, and then modeled at low particle concentration (with no coagulation). The thickness of the C shell was practically independent of the laser fluence, while the size of the Fe core could be varied for the Fe/C particles. The LA yielded no lognormal-type distribution for the amorphous WN0,3 particles.

Preparation of multi-walled carbon nanotube array electrodes and its electrochemical intercalation behavior of Li ions

In this work, multi-walled carbon nanotube array electrodes were prepared by chemical vapor decomposition (CVD) in nano-sized porous alumina membranes (the diameter of the pore is about 55 nm). The intercalation behavior of Li + in the array electrodes was also primarily investigated. The importance of selection of current collectors for the study of Li +-intercalation processes in carbon nanotube array electrodes was stressed. Since carbon nanotube array electrodes can give high current density due to its high surface area and ordered electrode configuration, which may be used in some fields such as chemical sensors and micro-battery.

A Temperature Window for Chemical Vapor Decomposition Growth of Single-Wall Carbon Nanotubes

Carbon single-wall nanotubes (SWNTs) grow efficiently from methane on alumina-supported metal catalysts within a fairly narrow temperature window from 680 to 850 °C. An abrupt onset in SWNT growth occurs at the low temperature side of the window, and SWNTs produced at these lower temperatures appear to be relatively free of amorphous or nanocrystalline carbon impurities. Raman spectroscopy shows that SWNT yield drops dramatically at the high-temperature side of the window where most previous chemical vapor decomposition (CVD) studies have been performed. The turn-on at the low-temperature side appears to be controlled by the thermodynamics of SWNT growth, while the turn-off at high temperatures is associated with competitive deposition of amorphous and nanocrystalline carbon. The existence of a temperature window for SWNT growth has not been reported elsewhere, and has important general consequences for CVD growth of SWNTs.