Gaseous adsorption of carbon tetrachloride onto carbon nanofiber arrays prepared by template-assisted synthesis

Adsorption of carbon tetrachloride onto aligned carbon nanofiber (CNF) arrays, prepared by a template-assisted synthesis, in vapor phase is conducted in the present study. Porous structure analysis indicated that various pore size distributions of the CNF arrays are found to vary with their tubular sizes. The increasing tubular size is accompanied by a decreasing micropore fraction as well as a vapor-phase adsorption capacity. Freundlich and Dubinin–Radushkevich models were employed to analyze the equilibrium adsorption data. The surface accessibility of CNF arrays, i.e., adsorption capacity per surface area, was found to decrease with the pore size, according to these models. It is suggested on the basis of the present work that the micropore fraction of CNFs plays an important role in determining the adsorption coverage. Both the equilibrium constant and free energy for the vapor-phase adsorption increase with the micropore proportion, indicating that the micropores act as a high-energy site for adsorption of carbon tetrachloride.     

How the shape of catalyst nanoparticles determines their crystallographic orientation during carbon nanofiber growth

A theoretical model is presented that explains spontaneous changes in the crystalline orientation of nanoparticles. The spontaneous changes in crystalline orientation are attributed to the crystal anisotropy of the surface energy of nanocrystalline particles. We consider an important specific case of the chemical vapor deposition growth of carbon nanofibers, where previous studies have shown that both the catalyst nanoparticle shape and the nanofiber growth rate change with changes in the chemical potential of diluted carbon. Energetic considerations of the nanoparticle’s free surface and its interfacial energy with the nanofiber during these shape changes are shown to force a reorientation of the nanoparticle crystallographic axes at a critical growth rate. The model therefore reveals the mechanism by which the shape and crystallographic orientation of the catalyst nanoparticle are linked to the nanofiber growth rate. The model suggests a new way, based upon measurable geometry of nanoparticles during in situ growth experiments, to estimate the role of chemisorption in the attraction of the graphene film to the curved catalyst surface and the anisotropy energy of this interface.     

Control of catalyst particle crystallographic orientation in vertically aligned carbon nanofiber synthesis

Vertically aligned carbon nanofibers (VACNF) have been synthesized where the crystallographic orientation of the initial catalyst film was preserved in the nanoparticle that remained at the nanofiber tip after growth. A substantial percentage of catalyst particles (75%), amounting to approximately 200 million nanofibers over a 100 mm Si wafer substrate, exhibited a sixfold symmetry attributed to a cubic Ni(1 1 1)∥Si(0 0 1) orientation relationship which was verified by X-ray diffraction studies. The Ni catalyst films were prepared by rf-magnetron sputtering under substrate bias conditions to yield a single (1 1 1) film texture. The total energy of the Ni thin film was estimated by calculating the sum of the surface free energy and strain energy. The total film energy was minimized by the evolution of the plane of lowest surface free energy, the (1 1 1) texture. This result was in agreement with X-ray diffraction measurements. The preferred orientation present in the Ni catalyst film prior to nanofiber growth was preserved in the Ni catalyst particles throughout the VACNF growth process. The Ni catalyst particles at the nanofiber tips were not pure single crystals but rather consisted of a mosaic structure of Ni nanocrystallites embedded within Ni catalyst nanoparticles (200-400 nm). The tip-located nanoparticles exhibited a faceted, crystal morphology with the faceting transferred to the underlying carbon nanofiber during the growth process. The possibility of precisely and accurately controlling VACNF growth velocity over macroscopic wafer dimensions with uniformly aligned catalyst particles is discussed.     

Catalytic growth of macroscopic carbon nanofiber bodies with high bulk density and high mechanical strength

Carbon nanofibers (CNF) are non-microporous graphitic materials with a high surface area (100-200 m²/g), high purity and tunable surface chemistry. Therefore the material has a high potential for use as catalyst support. However, in some instances it is claimed that the low density and low mechanical strength of the macroscopic particles hamper their application. In this study we show that the bulk density and mechanical strength of CNF bodies can be tuned to values comparable to that of commercial fluid-bed and fixed-bed catalysts. The fibers were prepared by the chemical decomposition of CO/H₂ over Ni/SiO₂ catalysts. The resulting fibers bodies (1.2 μm) were replicates of the Ni/SiO₂ bodies (0.5 μm) from which they were grown. The bulk density of CNF bodies crucially depended on the metal loading in the growth catalyst. Over 5 wt% Ni/SiO₂ low density bodies (0.4 g/ml) are obtained while 20 wt% Ni/SiO₂ leads to bulk densities up to 0.9 g/ml with a bulk crushing strength of 1.2 MPa. The 20 wt% catalysts grow fibers with diameters of ∼22 nm, which grow irregularly in space, resulting in a higher entanglement and a concomitant higher density and strength as compared to the thinner fibers (∼12 nm) grown from 5 wt% Ni/SiO₂.     

Fabrication and electrical conductivity of suspended carbon nanofiber arrays

We demonstrate a simple, efficient and novel self-assembly based method to fabricate arrays of suspended polymeric nanofibers of polyacrylonitrile and SU-8 negative photoresist by electrospinning on micro-fabricated posts of resorcinol–formaldehyde (RF) gel. The suspended electrospun nanofibers together with the RF gel posts were subsequently pyrolyzed in an inert atmosphere to yield large area monolithic structures of suspended glassy carbon nanofibers (CNF) integrated on RF gel derived carbon posts. The electrospun nanofibers self-assemble to connect the posts owing to a stronger electric field on their tips, obviating the need for positioning and integration of carbon nanowires with the underlying microstructures and paving the way for fabricating novel carbon based micro and nanoscale devices. The fabrication technique also allowed measurements of electrical conductivity of a single suspended CNF between carbon electrodes using I–V characteristics and comparison of the carbon nanowire conductivities for the CNF derived from different polymer precursors.     

Growth and high current field emission of carbon nanofiber films with electroplated Ni catalyst

We report high current-density field emission from carbon nanofiber (CNF) films synthesized using electroplated Ni catalysts on gold-buffer layers via hot-filament chemical vapor deposition. High-density thick CNFs which had a solid structure without hollow cores and many protrusions on the outside of CNF body were formed. The protrusions consisted of buckled small graphitic sheets, and some protrusions had very small tip radius to which we attribute good field emission from CNF films. The maximum emission current of 3.67 mA was measured from the area of 4.9 × 10^{− 3} cm², corresponding to the current density of 750 mA/cm², at the electric field of 12.5 V/μm. There was a distinctive hysteresis in emission–current curves measured while ramping up and down the bias-voltage. The deviations between up- and down-sweep emission currents, and the slope change in Fowler–Nordheim curves were most prominent in medium-voltage and -current regime. Moreover, the emission–current hysteresis showed dependence on the pressure during measurement and the voltage-sweep speed. We propose that adsorbate-enhanced field emission and adsorbate desorption during field-emission measurement were responsible for the observed emission behavior.     

Hydrogenolysis of sorbitol to glycols over carbon nanofiber supported ruthenium catalyst

Sorbitol hydrogenolysis was carried out over a carbon nanofiber supported ruthenium catalyst prepared by incipient wetness impregnation. The carbon nanofiber supported ruthenium catalyst was shown to have an attracting behavior when compared with a commercial activated carbon supported ruthenium catalyst, especially in terms of selectivity to glycols. The preferable hydrogen partial pressure for sorbitol hydrogenolysis was ca. 8.0 MPa, lower than that usually reported in previous works. Slightly soluble calcium hydroxide, which was used as a basic promoter, remarkably increased the selectivity to glycols, as compared with the soluble sodium hydroxide. The variation of product selectivity with catalyst amount indicated that glycerol was the initial C3 polyol product while propylene glycol was derived from glycerol. The parametric investigation was further focused on the intrinsic features of sorbitol hydrogenolysis.     

Seeding strategies for the deposition of high density network of nanoporous Au cluster catalyst on glassy carbon electrodes

Recent developments demonstrated the feasibility of functionalized nanoporous Au (NPG) for application in catalysis. This work employed a variety of seeding strategies aimed at facilitating the synthesis of ultra-high density or nearly continuous cluster network on glassy carbon (GC) surfaces of quality and structure achievable on metal substrates only. The NPG synthetic process involving electrodeposition of Au_xAg_(1?x) alloy and its subsequent de-alloying was performed on seeds generated by electrodeposition (Cu, Ag) and electroless (Pd, Au) approaches. The outcome of the densification process was assessed by electrochemical characterization and scanning electron microscopy. While some improvement was observed on Cu and Ag seeds, significantly higher nucleation density was obtained on Au-seeded GC surfaces where the deposit featured most typical de-alloying behavior leading to highest surface area evolution. The best overall quality of NPG film manifested by high density networks of overlapping, uniform, and very small clusters nearing structural continuity was achieved on Pd-seeded GC samples.     

Hydrogenation of phenol in scCO2 over carbon nanofiber supported Rh catalyst

A catalyst of Rh nanoparticles supported on a carbon nanofiber, 5 wt.% Rh/CNF, with an average size of 2–3 nm has been prepared by a method of incipient wetness impregnation. The catalyst presented a high activity in the ring hydrogenation of phenol in a medium of supercritical CO₂ (scCO₂) at a low temperature of 323 K. The presence of compressed CO₂ retards hydrogenation of cyclohexanone to cyclohexanol under the reaction conditions used, and this is beneficial for the formation of cyclohexanone, increasing the selectivity to cyclohexanone. But the selectivity to cyclohexanone is very low at the completion of reaction in the absence of CO₂, at low CO₂ pressures, and in the presence of pressurized N₂ instead of CO₂. That is, high selectivity to cyclohexanone can be achieved with CO₂ species at higher pressures but not with the application of an inert hydrostatic pressure on the liquid substrate phase.     

3-Orders-of-magnitude density control of single-walled carbon nanotube networks by maximizing catalyst activation and dosing carbon supply

Tailoring the density of random single-walled carbon nanotube (SWCNT) networks is of paramount importance for various applications, yet it remains a major challenge due to the insufficient catalyst activation in most growth processes. Here we report on a simple and effective method to maximise the number of active catalyst nanoparticles using catalytic chemical vapor deposition (CCVD). By modulating short pulses of acetylene into a methane-based CCVD growth process, the density of SWCNTs is dramatically increased by up to three orders of magnitude without increasing the catalyst density and degrading the nanotube quality. In the framework of a vapor-liquid-solid model, we attribute the enhanced growth to the high dissociation rate of acetylene at high temperatures at the nucleation stage, which can be effective in both supersaturating the larger catalyst nanoparticles and overcoming the nanotube nucleation energy barrier of the smaller catalyst nanoparticles. These results are highly relevant to numerous applications of random SWCNT networks in next-generation energy, sensing and biomedical devices.     

Crystallization behavior of carbon nanofiber/linear low density polyethylene nanocomposites

Crystallization behavior of LLDPE nanocomposites is reported in the presence of three types of carbon nanofibers (CNFs) (MJ, PR‐19, and PR‐24). During nonisothermal crystallization studies, all three crystalline melting peaks for LLDPE matrix were observed in the presence of PR‐19 nanofibers (up to 15 wt % content), but only the high‐ and low‐temperature peaks were observed in the presence MJ nanofibers. The broad melting peak at low‐temperature became bigger, suggesting an increase in the relative content of thinner lamellae in the presence of MJ nanofibers. TEM results of nanocomposites revealed transcrystallinity of LLDPE on the surface of CNFs, and a slightly broader distribution of lamellar thickness. STEM studies revealed a rougher surface morphology of the MJ nanofibers relative to that of PR nanofibers. Also, BET studies confirmed a larger specific surface area of MJ nanofibers relative to that of PR nanofibers, suggesting that the larger and the rougher surface of MJ nanofibers contributes toward the different crystallization behavior of MJ/LLDPE nanocomposites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Prospects for a high octane gasoline synthesis catalyst

The thermodynamics of synthesis reactions relevant to the production of alcohols, aldehydes, olefins, paraffins and aromatics lead us to conclude that the direct synthesis of aromatics will be favoured at temperatures around 650 K. We also note that the exothermicity of such a synthesis will be some 20% lower than that in conventional Fischer Tropsch synthesis.     

Dual‐bed catalyst system for the direct synthesis of high density aviation fuel with cyclopentanone from lignocellulose

In this article, we demonstrated an integrated process for the direct production of tri(cyclopentane) with cyclopentanone which can be obtained from lignocellulose. The reaction was carried out in a dual‐bed continuous flow reactor. In the first bed, cyclopentanone was selectively converted to 2,5‐dicyclopentylcyclopentanol over the Pd‐MgAl‐HT (hydrotalcite) catalyst. Under solvent‐free and mild conditions (443 K, 0.1 MPa H₂), high carbon yield (81.2%) of 2,5‐dicyclopentylcyclopentanol was achieved. Subsequently, the 2,5‐dicyclopentylcyclopentanol was further hydrodeoxygenated to tri(cyclopentane) in the second bed. Among the investigated catalysts, the Ni‐Hβ‐DP prepared by deposition‐precipitation (DP) method exhibited the highest activity for the hydrodeoxygenation step. By using Pd‐MgAl‐HT as the first bed catalyst and Ni‐Hβ‐DP as the second bed catalyst, tri(cyclopentane) was directly produced at high carbon yield (80.0%) with cyclopentanone as feedstock. This polycycloalkane has high density (0.91 kg/L) and can be used as additive to improve the density and volumetric heating value of bio‐jet fuel. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2754–2761, 2016     

Catalytic CVD synthesis of double and triple-walled carbon nanotubes by the control of the catalyst preparation

We report the influence of catalyst preparation conditions for the synthesis of carbon nanotubes (CNTs) by catalytic chemical vapour deposition (CCVD). Catalysts were prepared by the combustion route using either urea or citric acid as the fuel. We found that the milder combustion conditions obtained in the case of citric acid can either limit the formation of carbon nanofibres (defined as carbon structures not composed of perfectly co-axial walls or only partially tubular) or increase the selectivity of the CCVD synthesis towards CNTs with fewer walls, depending on the catalyst composition. It is thus for example possible in the same CCVD conditions to prepare (with a catalyst of identical chemical composition) either a sample containing more than 90% double- and triple-walled CNTs, or a sample containing almost 80% double-walled CNTs.     

Important parameters for the catalytic nanoparticles formation towards the growth of carbon nanotube aligned arrays

Presented here is a systematic study on the experimental parameters involved in the formation of catalytic nanoparticles from homogeneous Ni films deposited by dc sputtering towards carbon nanotube (CNT) production on Si/SiO₂. We have found a critical temperature and time for the thermal and reduction pre-treatment processes to obtain catalyst nanoparticles with the appropriate size and high density suitable for CNT growth. From such nanoparticles, densely-packed aligned CNT arrays were successfully grown at 750–800°C by thermal CVD.     

Water transport control in carbon nanotube arrays

Based on a recent scaling law of the water mobility under nanoconfined conditions, we envision novel strategies for precise modulation of water diffusion within membranes made of carbon nanotube arrays (CNAs). In a first approach, the water diffusion coefficient D may be tuned by finely controlling the size distribution of the pore size. In the second approach, D can be varied at will by means of externally induced electrostatic fields. Starting from the latter strategy, switchable molecular sieves are proposed, where membranes are properly designed with sieving and permeation features that can be dynamically activated/deactivated. Areas where a precise control of water transport properties is beneficial range from energy and environmental engineering up to nanomedicine.     

Biomimetic synthesis and characterization of carbon nanofiber/hydroxyapatite composite scaffolds

Three dimensional electrospun carbon nanofiber (CNF)/hydroxyapatite (HAp) composites were biomimetically synthesized in simulated body fluid (SBF). The CNFs with diameter of ∼250 nm were first fabricated from electrospun polyacrylonitrile precursor nanofibers by stabilization at 280 °C for 2 h, followed by carbonization at 1200 °C. The morphology, structure and water contact angle (WCA) of the CNFs and CNF/HAp composites were characterized. The pristine CNFs were hydrophobic with a WCA of 139.6°, resulting in the HAp growth only on the very outer layer fibers of the CNF mat. Treatment in NaOH aq. solutions introduced carboxylic groups onto the CNFs surfaces, and hence making the CNFs hydrophilic. In the SBF, the surface activated CNFs bonded with Ca²⁺ to form nuclei, which then easily induced the growth of HAp crystals on the CNFs throughout the CNF mat. The fracture strength of the CNF/HAp composite with a CNF content of 41.3% reached 67.3 MPa. Such CNF/HAp composites with strong interfacial bondings and high mechanical strength can be potentially useful in the field of bone tissue engineering.     

Fabrication and characterization of carbon nanotube arrays using sandwich catalyst stacks

A novel synthesis of carbon nanotubes for field-emitter arrays with a uniform field emission current is reported. Microwave plasma chemical vapor deposition and a unique structure of a sandwich catalyst stack are used to grow vertically aligned carbon nanotubes with a high density, uniform length and diameter. After being etched in a H₂/N₂-microwave plasma, the overall field emission current density from the prepared emitter arrays is 1.2 A/cm² at an electric field of 6.5 V/μm with stable and uniform emission characteristics. The threshold field is 3.2 V/cm, defined at an emission current density of 10⁻⁶ A/cm².