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.     

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.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

Kinetic study of carbon nanotube synthesis over Mo/Co/MgO catalysts

The kinetics of carbon nanotube (CNT) synthesis by decomposition of CH₄ over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH₄, as well as to the formation of Mo₂C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH₄ pressure, indicating that the dissociation of CH₄ is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation rate of the graphene network is smaller than the carbon deposition rate by decomposition of CH₄.     

Investigation in the role of hydrogen on the properties of diamond films grown using Ar/H2/CH4 microwave plasma

The transition of diamond grain sizes from micron- to nano- and then to ultranano-size could be observed when hydrogen concentration is being decreased in the Ar/CH₄ plasma. When grown in H₂-rich plasma (H₂ = 99% or 50%), well faceted microcrystalline diamond (MCD) surface with grain sizes of less than 0.1 μm are observed. The surface structure of the diamond film changes to a cauliflower-like geometry with a grain size of around 20 nm for the films grown in 25% H₂-plasma. In the Ar/CH₄ plasma, ultrananocrystalline diamond (UNCD) films are produced with equi-axed geometry with a grain size of 5-10 nm. The H₂-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature. The induction of the grain growth process, either by using H₂-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries.     

Effects of CF4 plasma on the field emission properties of aligned multi-wall carbon nanotube films

We present a simple method to functionalize the surface and to modify the structures of aligned multi-wall carbon nanotube (CNT) arrays grown on silicon substrates using CF₄ plasma produced by reactive ion etching (RIE). Field emission (FE) measurements showed that after 2 min of plasma treatment, the emission currents were enhanced compared with as-grown CNTs; however, extended treatment over 2 min was found to degrade the FE properties of the film. Scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy have been employed to investigate the mechanism behind the modified FE properties of the CNT film. The FE enhancement after 2 min of etching could be attributed to favorable surface morphologies, open-ended structures and a large number of defects in the aligned CNT films. On the other hand, deposition of an amorphous layer comprising carbon and fluorine during extended CF₄ plasma treatment may hamper the field emission of CNT films.     

Deep desulfurization of fuels catalyzed by surfactant-type decatungstates using H2O2 as oxidant

Oxidation of dibenzothiphene (DBT) in model oil with H₂O₂ using surfactant-type decatungstates Q₄W₁₀O₃₂ (Q = (CH₃)₃NC₁₆H₃₃, (CH₃)₃NC₁₄H₂₉, (CH₃)₃NC₁₂H₂₅ and (CH₃)₃NC₁₀H₂₁) as catalysts was studied. The surfactant-type decatungstates were synthesized and characterized. The suitable reaction condition of deep desulfurization was suggested: n(DBT):n(catalyst):n(H₂O₂) = 1:0.01:3, 60 °C for 0.5 h, under which the DBT conversion can reach 99.6% with [(CH₃)₃NC₁₆H₃₃]₄W₁₀O₃₂ as catalyst. The length of carbon chains of quaternary ammonium cations played a vital role in the catalytic activity of surfactant-type decatungstates, that is, the longer the carbon chain of quaternary ammonium cation of a catalyst was, the better the activity of this catalyst showed. [(CH₃)₃NC₁₆H₃₃]₄W₁₀O₃₂ exhibited the best catalytic performance and can be recycled for six times without significant decrease in catalytic activity. Using benzothiphene (BT) and 4,6-dimethyldibenzothiphene (4,6-DMDBT) as substrates in model oil, surfactant-type decatungstates also showed high catalytic activity. During desulfurization process, BT conversion can reach 99.6% at 3.25 h, while 99.4% of 4,6-DMDBT conversion reached at 1.25 h, with the temperature of 60 °C under atmospheric pressure. The sulfone can be separated from the oil using N,N-dimethylformamide (DMF) as an extractant, and the sulfur content can be lowered from 1000 to 4 ppm. For real diesel, the sulfur removal can reach 93.5% after five times extraction.     

Microstructural and electrochemical characterization of RuO2/CNT composites synthesized in supercritical diethyl amine

A simple and efficient route to decorate carbon nanotubes (CNTs) with nanocrystalline RuO₂ has been developed. In this method, RuCl₃ · 3H₂O was oxidized into RuO₂ by oxygen in supercritical diethyl amine, and the produced RuO₂ deposited on CNTs, resulting in RuO₂/CNT nanocomposites. The as-prepared composites were structurally and morphologically characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy (TEM). TEM images showed that RuO₂ nanoparticles attached on CNTs had uniform shape and a narrow particle size distribution. The loading content and the size of RuO₂ particles on CNTs could be tuned by changing the mass ratio of RuCl₃ · 3H₂O/CNT. Electrochemical measurements by cyclic voltammetry demonstrated a substantial increment of the specific capacitance of CNTs due to a pseudocapacitance originated from the deposited RuO₂ nanoparticles.     

A new regime for high rate growth of nanocrystalline diamond films using high power and CH4/H2/N2/O2 plasma

In this work, we report high growth rate of nanocrystalline diamond (NCD) films on silicon wafers of 2 inches in diameter using a new growth regime, which employs high power and CH₄/H₂/N₂/O₂ plasma using a 5 kW MPCVD system. This is distinct from the commonly used hydrogen-poor Ar/CH₄ chemistries for NCD growth. Upon rising microwave power from 2000 W to 3200 W, the growth rate of the NCD films increases from 0.3 to 3.4 μm/h, namely one order of magnitude enhancement on the growth rate was achieved at high microwave power. The morphology, grain size, microstructure, orientation or texture, and crystalline quality of the NCD samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, and micro-Raman spectroscopy. The combined effect of nitrogen addition, microwave power, and temperature on NCD growth is discussed from the point view of gas phase chemistry and surface reactions.     

Synthesis of carbon nanowalls by plasma-enhanced chemical vapor deposition in a CO/H2 microwave discharge system

Synthesis of carbon nanowalls was performed by a plasma-enhanced chemical vapor deposition in a CO/H₂ microwave discharge system. At the optimum CO/H₂ feed ratio of 46 sccm/4 sccm, the aligned carbon nanowalls with thickness of about several tens of nanometers were able to be synthesized. The extremely high growth rate of 1 μm min^− 1 was obtained in our system with a relatively low microwave discharge power of 60 W for a growth area of 1 cm². An optical emission spectroscopy was performed to clarify the characteristics of a CO/H₂ discharge system.     

Growth and characteristics of carbon nanotubes obtained under different C2H2/H2/NH3 concentrations

Aligned carbon nanotubes were synthesized under a combination of 20 different C₂H₂/H₂/NH₃ compositions at 700 °C using a thermal chemical vapor deposition method. Thin film Fe was used as the catalyst, which was pretreated with H₂ or NH₃ prior to the growth of carbon nanotubes. The use of different pretreatment gases results in little difference in the growth and characteristics of the carbon nanotubes except that the carbon nanotubes grown on H₂ treated catalysts have smaller diameters. The growth rate of the CNTs does not depend on the NH₃ concentration but on the ratio of NH₃/C₂H₂. There is a critical NH₃/C₂H₂ ratio that is independent of the C₂H₂ concentration and at which the peak growth rate occurs. The critical value was found to be 4.7 ± 1.2. Microstructural analysis indicates that the carbon nanotubes obtained at higher NH₃ concentrations contain defects and disorder. Field emission tests show that the carbon nanotubes exhibit a turn-on field of 2.36 V/μm and a maximum current density of 1.91 mA/cm². The field emission properties were found to be stable after 15 test cycles.     

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.     

Conversion of CO2 into CH4 by methane-producing bacterium FJ10 under a pressurized condition

In this study, fresh water methane-producing bacterium (MPB), strain FJ10, which used H₂ as an electron donor and CO₂ as an electron acceptor, was isolated and chosen as the primary methanogen for the conversion of CO₂ into CH₄. Improvements to culture medium to increase methane production were investigated using a fractional factorial design in 32 experiments with six variables under consumption of H₂/CO₂ at ratios of 4 and 1. The tested nutrient compositions were NaCl, NH₄Cl, FeSO₄, MgCl₂, H₂PO₄ and yeast extract. Experimental results indicate that yeast extract was essential for the growth of strain FJ10 to impact the conversion of CO₂ into CH₄. Strain FJ10 generated maximum CH₄ production with 5.0 g/l of yeast extract. Moreover, optimal culture conditions for methane production by strain FJ10 were 40 °C and pH 8. Approximately 22-25% of CO₂ conversion into CH₄ was achieved at an H₂/CO₂ ratio of 4 and roughly 2.5-6% of CO₂ conversion into CH₄ was obtained at an H₂/CO₂ ratio of 1 under different pressurized conditions of 1 atm, 50 atm and 100 atm. Under 100 atm, about 6780 μM CH₄ was produced with an H₂/CO₂ ratio of 4 and 4240 μM CH₄ was produced with an H₂/CO₂ ratio of 1 under the steady state condition. The kinetic model for H₂/CO₂ utilization and CH₄ production under different pressures was verified by experimental data. Model predictions are in good agreement with experimental results. The experimental and modeling approaches in this study can be applied to evaluate the conversion of CO₂ into CH₄ as an energy source by geo-microorganisms in geological sequestration.     

Defect structure for the ultra-nanocrystalline diamond films synthesized in H2-containing Ar/CH4 plasma

The modification on the microstructure of diamond films due to the addition of H₂ species into the Ar/CH₄ plasma was investigated. While the Ar/CH₄ plasma produced UNCD films with equiaxed grains (about 5 nm in size), the (Ar-H₂)/CH₄ plasma produced acicular-shaped grains (about 5 × 20 nm in size). Transmission electron microscopy studies indicate that these acicular-shaped grains actually are agglomerates of diamond flakes, which contain stacking faults lying on the (111) lattice plane. Presumably, the incorporation of H₂ species in the plasma leads to partial etching of hydrocarbons adhered onto the diamond clusters, such that the C₂- (or active carbon) species contained in the plasma can attach to the diamond surface anisotropically, leading to diamond flakes. The incorporation of H₂ in Ar plasma can also suppress the formation of i-carbons, an allotropic phase of diamonds. The critical proportion of H₂ in Ar plasma for inducing the changes in the granular structure is around 0.03%. The proportion of grain boundaries was thus reduced and the electron field emission properties of the materials were thus degraded. However, the suppression of the film electrical conductivity without sacrificing the smooth surface characteristic has the applications as high-thermal-conductivity heat spreaders and substrates for surface-acoustic-wave devices.     

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.     

Electro-catalytic oxidation of CO on Pt catalyst supported on carbon nanotubes pretreated with oxidative acids

Characteristics of nanosized Pt electro-catalyst deposited on carbon nanotubes (CNTs) were studied with CO-stripping voltammogram and chronoamperometry measurements. The CNTs were pretreated by oxidation in HNO₃, mixed HNO₃ + H₂SO₄ and H₂SO₄ + K₂Cr₂O₇ solution, respectively, to enable surface modification. Well-homogenized Pt particles (average size: ≈3 nm) were loaded onto the pretreated CNT samples by a modified colloidal method. TEM, BET, FTIR and XRD techniques were used to characterize the physicochemical properties of the pretreated CNT samples. In the electro-oxidation of CO, all the Pt/CNT samples showed lower on-set as well as peak potentials than the conventional Pt/XC-72 electro-catalyst, indicating that the Pt/CNT samples were more resistant to CO poisoning and could be superior anode electro-catalyst for the proton exchange membrane fuel cells (PEMFCs). Moreover, we found that the pretreatment of CNTs in mixed HNO₃ + H₂SO₄ solution was very beneficial for the performance enhancement of Pt/CNT electro-catalyst; the catalyst obtained as such gave the lowest peak potential and the highest catalytic activity for the electro-oxidation of CO. Larger amount of oxygen-containing functional groups, higher percentage of mesopores, and higher graphitic crystallinity of the pretreated CNTs were considered crucial for the performance enhancement, e.g., by strengthening the interaction between Pt nanoparticles and the CNT support and enhancing the mass diffusion in the electro-chemical reaction.     

The performances of higher alcohol synthesis over nickel modified K2CO3/MoS2 catalyst

Ni modified K₂CO₃/MoS₂ catalyst was prepared and the performance of higher alcohol synthesis catalyst was investigated under the conditions: T = 280–340 °C, H₂/CO (molar radio) = 2.0, GHSV = 3000 h^− 1, and P = 10.0 MPa. Compared with conventional K₂CO₃/MoS₂ catalyst, Ni/K₂CO₃/MoS₂ catalyst showed higher activity and higher selectivity to C₂⁺OH. The optimum temperature range was 320–340 °C and the maximum space-time yield (STY) of alcohol 0.30 g/ml h was obtained at 320 °C. The selectivity to hydrocarbons over Ni/K₂CO₃/MoS₂ was higher, however, it was close to that of K₂CO₃/MoS₂ catalyst as the temperature increased. The results indicated that nickel was an efficient promoter to improve the activity and selectivity of K₂CO₃/MoS₂ catalyst.     

Enhancement of pyrolysis gasoline hydrogenation over Pd-promoted Ni/SiO2-Al2O3 catalysts

Pyrolysis gasoline upgrading by hydrogenation and ring opening was investigated over highly loaded Ni catalysts supported on amorphous silica-alumina and incorporating promoters as Pd, seeking a higher aromatic reduction of this feedstock in order to meet stringent fuel regulations. The effect of Ni loading and Pd component on the activity of those systems was evaluated in a fixed bed reactor under the following operating conditions: T = 573 and 673 K, H₂:PyGas molar ratio = 10, P = 5.0 MPa, WHSV = 4 h⁻¹. The catalyst properties, measured by several characterization techniques (ICP-AES, XRD, N₂ adsorption-desorption isotherms, TPR, H₂-TPD, CO chemisorption, XPS, FTIR spectroscopy of adsorbed pyridine and NH₃-TPD), were related to their catalytic activity and selectivity. Interestingly, the increase in Ni loading from 24.4 to 33.2 Ni wt.% has a negative effect on both hydrogenation and ring opening activities, as it causes a drop in the BET surface area and a decrease in metal-support interaction, with a negative bearing on catalyst stability. On the other hand, the addition of Pd has a positive effect for hydrogenation, linked with the higher electronegativity of Pd⁰ species compared to those of Ni⁰, as well as with a greater stability of Pd-promoted catalysts during on-stream conditions. A linear correlation has been found between the total amount of desorbed H₂, as determined from H₂-TPD experiments on freshly reduced catalysts, and the initial turnover frequency.     

Functionalization of carbon nanotubes by an effective intermittent microwave heating-assisted HF/H2O2 treatment for electrocatalyst support of fuel cells

A method is developed to effectively functionalize carbon nanotubes (CNTs) by intermittent microwave heating (IMH)-assisted HF/H₂O₂ solution treatment. CNTs functionalized by IMH-assisted HF/H₂O₂ solution treatment (CNTs-HF/H₂O₂) are characterized by high oxygen-containing groups and high graphitization degree, as compared with CNTs treated by HF (CNTs-HF) or by IMH-assisted H₂O₂ solution treatment without prior HF treatment (CNTs-H₂O₂). Pt supported on CNTs-HF/H₂O₂ (Pt/CNTs-HF/H₂O₂) has an average particle size of 2.8 nm, smaller than 2.9 nm for Pt supported on CNTs-HF, 3.3 nm for Pt supported on CNTs-H₂O₂ and 4.0 nm for Pt supported on pristine CNTs. Pt/CNTs-HF/H₂O₂ electrocatalysts display a high electrochemical surface area, high Pt utilization efficiency, a superior electrocatalytic and mass activity for the O₂ reduction reaction (ORR) with respect to other catalyst samples in the present study. The results demonstrate the efficiency and effectiveness of the IMH-assisted HF/H₂O₂ solution methods for the functionalization of CNTs, and the method could be easily scaled-up to treat CNTs in large quantities.     

Oxidization of carbon nanotubes through hydroxyl radical induced by pulsed O2 plasma and its application for O2 reduction in electro-Fenton

The oxidation of carbon nanotubes (CNTs) by hydroxyl radical produced by pulsed O₂ plasma in a gas-liquid hybrid discharge reactor was conducted with the goal of enhancing their solubility and improving the yield of H₂O₂ in electro-Fenton. Data from the characterization experiments showed that oxygen bearing groups (COH, COO, COOH, CO) were formed on the surface of CNTs. The possible mechanism indicated that introduction of oxygen bearing groups onto CNTs could be attributed to the attacks by hydroxyl radical. The oxidized CNTs were easily dispersed in ethanol. The H₂O₂ yield on the original CNTs was 102 mg/L at −0.85 V after 90 min; in contrast, H₂O₂ yield on CNTs-15 reached 146 mg/L under the same conditions, resulting from the enhancement of the accessibility of O₂ on CNTs. In the electro-Fenton, the removal of methyl orange on the original CNTs was around 40%, and it increased to 95% on CNTs-15.