Graphitic Walls Research Articles

Highly Crystalline and Conductive Nitrogen‐Doped Mesoporous Carbon with Graphitic Walls and Its Electrochemical Performance

We present a rational and simple methodology to fabricate highly conductive nitrogen-doped ordered mesoporous carbon with a graphitic wall structure by the simple adjustment of the carbonization temperature of mesoporous carbon nitride without the addition of any external nitrogen sources. By simply controlling the heat-treatment temperature, the structural order and intrinsic properties such as surface area, conductivity, and pore volume, and the nitrogen content of ordered graphitic mesoporous carbon can be controlled. Among the materials studied, the sample heat-treated at 1000 °C shows the highest conductivity, which is 32 times higher than that for the samples treated at 800 °C and retains the well-ordered mesoporous structure of the parent mesoporous carbon nitride and a reasonable amount of nitrogen in the graphitic framework. Since these materials exhibit high conductivity with the nitrogen atoms in the graphitic framework, we further demonstrate their use as a support for nanoparticle fabrication without the addition of any external stabilizing or size-controlling agent, as well as the anode electrode catalysts. Highly dispersed platinum nanoparticles with a size similar to that of the pore diameter of the support can be fabricated since the nitrogen atoms and the well-ordered porous structure in the mesoporous graphitic carbon framework act as a stabilizing and size-controlling agent, respectively. Furthermore the Pt-loaded, nitrogen-doped mesoporous graphitic carbon sample with a high conductivity shows much higher anodic electrocatalytic activity than the other materials used in the study.

Synthesis of thin-walled carbon nanocages and their application as a new kind of nanocontainer

Carbon nanocages (CNCs) with a hollow core and a thin wall of a few graphitic layers have a wide range of applications. However, the preparation of such a material remains a great challenge. In this study, we report the preparation of thin-walled CNCs through the pyrolysis of a mixture composed of pyridine and liquid iron pentacarbonyl, subsequent heat treatment in the presence of NH4Cl at a low temperature, and finally washing and filtering in water. The finding that pyridine is completely mutually soluble with liquid iron pentacarbonyl in any proportion stands out to be a crucial advantage for reducing the thickness of the graphitic wall formed on a metal core. To obtain hollow CNCs, heat-treatment proves to be simple and effective in removing the metal core and, more importantly, maintaining the thin graphitic wall when compared with conventional boiling in a strong oxidant, such as HNO3. The thin-walled hollow CNCs are demonstrated to be suitable nanocontainers for encapsulating iodine, as iodine can easily get into and out of the CNCs when it is heated at a relatively low temperature. Considering the large internal space and thin graphitic wall, the hollow CNCs could be widely used in many fields, such as biology, medicine and chemistry.

Anomalous decline of water transport in covalently modified carbon nanotube membranes

Carbon nanotube membranes have been shown to rapidly transport liquids; but progressive hydrophilic modification--contrary to expectations--induces a drastic reduction of water flow. Enhanced electrostatic interaction and the disruption of the mechanically smooth graphitic walls is the determinant of this behavior. These results have critical implications in the design of nanofluidic devices.

High rate capability of ordered mesoporous carbon with platelet graphitic pore walls for lithium ion anodes

An ordered mesoporous carbon (OMC) with controllable molecule crystal and platelet graphitic pore walls, which is directionally grown on the internal pore walls of SBA-15 or anchors at liquid/silica interfaces by molecule engineering, has been investigated as lithium ion anodes. It is found that the OMC exhibits high kinetics, rate and cycling performance. The i 0 and D Li are almost constant after 50 cycles. OMC shows a high reversible specific capacity of 153.9 mAh g -1 at the current density as high as 3500 mA g -1. The excellent electrochemical performance could be attributed to the presence of the porosity and platelet graphitic pore walls.

Pore-Width-Dependent Preferential Interaction of sp2Carbon Atoms in Cyclohexene with Graphitic Slit Pores by GCMC Simulation

The adsorption of cyclohexene with two sp2and four sp3carbon atoms in graphitic slit pores was studied by performing grand canonical Monte Carlo simulation. The molecular arrangement of the cyclohexene on the graphitic carbon wall depends on the pore width. The distribution peak of the sp2carbon is closer to the pore wall than that of the sp3carbon except for the pore width of 0.7 nm, even though the Lennard-Jones size of the sp2carbon is larger than that of the sp3carbon. Thus, the difference in the interactions of the sp2and sp3carbon atoms of cyclohexene with the carbon pore walls is clearly observed in this study. The preferential interaction of sp2carbon gives rise to a slight tilting of the cyclohexene molecule against the graphitic wall. This is suggestive of a π-π interaction between the sp2carbon in the cyclohexene molecule and graphitic carbon.

An ordered hydrophobic P6mm mesoporous carbon with graphitic pore walls and its application in aqueous catalysis

A hydrophobic ordered mesoporous carbon with hexagonal arrays of rods was synthesized by a nanocasting process by using silica SBA-15 as a template and 2,3-dihydroxynaphthalene as a fused-aromatic carbon precursor. Impregnation of 2,3-dihydroxynaphthalene and its subsequent conversion into carbon occurred inside the mesopores of the silica template through a dehydration reaction between the surface silanols and hydroxyl groups of the carbon source and mild carbonization under inert atmosphere. After silica removal, X-ray scattering, transmission electron microscopy, pore size analysis and Raman spectroscopy showed that the resulting material was a negative replica of the silica template with a 2D-hexagonal P6mm ordered structure, possessing a large surface area (724 m 2 g −1), a monomodal pore size distribution (3.4 nm) and a relative hydrophobic surface with graphitic pore walls. These features give the system substantial advantages to play a beneficial role in aqueous organometallic catalysis. The material appeared to be an excellent mass transfer promoter to enhance the overall reaction rate of the palladium-catalyzed cleavage reaction of water-insoluble allylundecylcarbonate (Tsuji–Trost reaction).

PH-Tunable Ion Selectivity in Carbon Nanotube Pores

The selectivity of ion transport in nanochannels is of primary importance for a number of physical, chemical, and biological processes ranging from fluid separation to ion-channel-regulated cellular processes. Fundamental understanding of these phenomena requires model nanochannels with well-defined and controllable structural properties. Carbon nanotubes provide an ideal choice for nanofluidic studies because of their simple chemistry and structure, the atomic scale smoothness and chemical inertness of the graphitic walls, and the tunability of their diameter and length. Here, we investigate the selectivity of single and, for the first time, binary salt mixtures transport through narrow carbon nanotubes that act as the only pores in a silicon nitride membrane. We demonstrate that negatively charged carboxylic groups are responsible for the ion rejection performance of carbon nanotube pores and that ion permeation of small salts can be tuned by varying solution pH. Investigation of the effect of solution composition and ion valences for binary electrolytes with common cation in a pressure-driven flow reveals that the addition of slower diffusing multivalent anions to a solution of faster diffusing monovalent anions favors permeation of the monovalent anion. Larger fractions and valences of the added multivalent anions lower the rejection of the monovalent anion. In some cases, we observe negative rejection at low monovalent ion content.

Generation of multiwalled carbon nanotubes from iron–phthalocyanine polymer and their novel dielectric properties

Large-scale production of high dielectric multiwalled carbon nanotubes using iron–phthalocyanine polymer as only carbon source at ambient pressure in nitro atmosphere under 800 °C have been reported here. The dielectric properties of the synthesized carbon tubes were measured at 2–18 GHz. Furthermore, well-ordered mesostructured carbons with graphitic framework walls were conveniently obtained by continuing heating the carbon tubes to 1000 °C, the mesoporous carbons processed high BET surface of 276 m 2 g −1 and narrow pore size dispersion. SEM, TEM, XRD and N 2 adsorption/desorption methods were employed to figure the detail structure information of materials.

Characterization of activated carbons from peach stones through the mixed geometry model

A good characterization of the pore size distribution of activated carbons is of crucial importance for gas separation and gas storage applications of microporous materials. It has been recently shown that characterization methods of activated carbons can be improved by considering the porous space as represented by a mixture of pores of different geometries, slit and triangular geometry. The triangular geometry was included as a necessity of considering the simultaneous interaction of an adsorbed molecule with three graphite walls to account for high observed values of the isosteric heat of adsorption at low pressures. Places where such interaction with three graphitic walls occurs can be clearly seen in some high resolution digitalized TEM images of activated carbons published in the literature. Many lignocellulosic substances have been used in the literature as the precursor material in the preparation of activated carbons. In the present work we use peach stones, which are very abundant in the region of Cuyo (Argentina), as well as in any other regions in the world where peaches are extensively cultivated, processed and canned. Different samples of activated carbons from this precursor are obtained through different processing methods. N 2 adsorption isotherms at 77 K are measured for these samples and fitted by using the mixed geometry model, obtaining the corresponding PSDs. Finally these characterization results are correlated to the preparation procedures for the different samples. The analysis of results reveals that the mixed geometry model provides a more consistent picture of the sample characteristics than the pure slit geometry model, reinforcing the conclusions of previous studies. The same analysis allows to determine clearly some general trends relating the preparation method with the morphological and adsorptive characteristics of the AC.

Synthesis of highly nitrogen-doped hollow carbon nanoparticles and their excellent electrocatalytic properties in dye-sensitized solar cells

Hollow carbon nanoparticles that have been highly doped with nitrogen (N-HCNPs) are directly prepared by a facile one-pot method based on the detonation-assisted chemical vapor deposition of dimethylformamide without the use of metal catalysts. The N-HCNPs exhibit uniform core-shell microstructures with inner cavities encapsulated by graphitic walls, possessing a narrow size distribution of 10–25 nm. The nitrogen content in N-HCNPs is as high as 20.8% atom ratio, and the nitrogen bonds display pyridine-, pyrrole-, and graphite-like configurations. Defects and dislocations are present in the graphene layers due to highly incorporated nitrogen, leading to the creation of micropores on the carbon shell and a large BET surface area of 454 m2 g−1. The unique N-HCNPs with interconnected hierarchical porous structures and nitrogen-containing defects show excellent electrocatalytic activity for triiodide reduction in dye-sensitized solar cells, superior to conventional platinum catalysts.

Patterned growth of ultra long carbon nanotubes. Properties and systematic investigation into their growth process

High aspect ratio carbon nanotube arrays were grown using a hydrogen/water assisted catalytic chemical vapor deposition method using ethylene as precursor which achieves a growth rate of 25 to 30 μm carbon nanotube length per minute. The influence of the growth parameters determining the growth rate and the growth height of the carbon nanotubes arrays was studied in detail. Water and hydrogen concentration were varied systematically and it was found that they have to be in a precise ratio in order to establish ultralong growth together with high quality (no contamination with carbonaceous side products) of the arranged carbon nanotubes. For given growth conditions, the hydrogen content in the feed gas stream controls the carbon nanotube length, the purity level and the number of graphitic walls of carbon nanotubes grown. It was found that water acts as a weak oxidant keeping the mixed aluminium/iron catalyst active for ultrahigh length growth. Its concentration should be established very precisely in the process to achieve a maximum growth rate of carbon nanotubes. Furthermore a straightforward, non-costly mask technique is presented to achieve highly structured carbon nanotube growth. Finally the field emission characteristics of such ultrahigh, well arranged and structured carbon nanotube blocks was studied and this revealed low turn-on electric and threshold field values of 0.34 V μm−1 and 0.71 V μm−1 respectively.

Converting carbon nanofibers to carbon nanoneedles: catalyst splitting and reverse motion

Carbon nanoneedles (CNNs) were grown using a plasma-enhanced chemical vapor deposition process in which the source gas (C(2)H(2)) was turned off 10 min before the NH(3) flow and plasma were turned off. It is demonstrated that tubular carbon nanofibers (CNFs) grow while the source gas is on. However, once the source gas is turned off, the Ni catalyst at the top of each CNF splits to form a small catalyst that remains at the top of the tube and a larger catalyst that travels down the interior of the tube. We postulate that the motion of the bottom (larger) catalyst is driven by etching of the graphitic walls and 'cups' inside the CNF. This process, combined with slowing growth of the CNFs and etching of the material above the bottom catalyst, converts the carbon nanofibers to the final nanoneedle shape.

Influence of Synthesis Conditions and Heat Treatment on the Structure of Ti3SiC2-Derived Carbons

We investigate the characterization and adsorption modeling of Ti3SiC2-derived carbons (Ti3SiC2-DCs) prepared by chlorination at three temperatures (600, 800, and 1000 degrees C) as well as samples heat-treated at 1100 degrees C for one and three day periods. The modeling exploits our recent finite Wall thickness model-based density functional theory approach (Nguyen, T. X.; Bhatia, S. K. Langmuir 2004, 20, 3532), Utilizing experimental adsorption isotherms OF Ar at 87 K for the characterization. In general, characterization results show that the carbon wall structure of carbide-derived carbons (CDCs) is precursor-inherited, and Ti3SiC2-DCs have predominantly graphitic walls whose helium density is close to the true density of graphite (2.27 g/cm(3)), while SiC-derived carbons (SiC-DCs) have predominantly diamond-like walls whose helium density approaches the true density of diamond (3.52 g/cm(3)). This precursor-inherited character is also seen to give rise to two distinct evolutions of the Pore Structure of Ti3SiC2-DCs and SiC-DCs under chlorination temperature or post-heat-treatment conditions. In Particular, both pore enlargement and opening are observed for Ti3SiC2-DCs, but only pore opening, especially in the region of smallest pores (<5 angstrom), is found for SiC-DCs. For adsorption modeling results, the ordering of pore network accessibility, Ar at 87 K < CH4 at (313K and 333K) < CO2 at or above 273 K, has been specifically found for the Ti3SiC2-DC samples prepared by chlorination at 800 degrees C, providing high CO2/CH4 selectivity by the carbon. Heat treatment of this sample at 1100 degrees C for a 1 day period gives rise to significant improvement of pore network accessibility but subtle change in pore bodies. Finally, shrinkage is found for all investigated CDCs and the I day heat-treated sample, but swelling is found for the 3 day heat-treated carbon sample, under high-pressure CO2 adsorption conditions.

Effects of a Nonrigid Graphene Surface on the LH Associative Desorption of H Atoms and on the Deexcitation of Nascent H2 Molecules Colliding with Model Walls of Carbonaceous Porous Material

A planar slab of 200 C atoms bound by the Brenner potential is used to study the Langmuir-Hinshelwood (LH) recombination of two physisorbed H atoms on a graphene sheet and to simulate afterward successive collisions of the nascent H(2) molecule with pore walls of a carbonaceous dust grain of the interstellar medium. The study is based on successive propagations of classical trajectories for the 200 C + 2 H atoms. The characteristics of H(2) molecules formed by the LH reaction on the flexible surface are found to differ but negligibly from those formed on a rigid one. Collisions of those H(2) molecules with graphitic pore walls are studied next. Reflection from and "trapping" onto the surface is observed and discussed. The most important energy transfer is from the molecule vibration to its rotation. This conversion mediates the transfer of the molecule internal energy to its translation or to surface heating. It is found that a single H(2)-surface impact has little effect on the internal energy of the molecules. The grain absorbs on the average but a very weak energy. Several impacts are required to appreciably cool the molecule. The molecule cooling is accompanied by a significant increase of its translational energy. The swifter the molecules are or get, the larger the number of their impacts on the surface they undergo per unit time and the more efficiently cooled they get.

Synthesis and characterization of mesoporous carbon nano-dendrites with graphitic ultra-thin walls and their application to supercapacitor electrodes

Mesoporous carbon nano-dendrites (MCNDs) with ultra-thin graphitic walls are synthesized by controlling the highly exothermic segregation reaction of silver acetylide into a carbon skeleton and silver vapor. The dendroid acetylides were quickly warmed to 150 °C emitting a brilliant flash of reddish orange light with a thunderous sound indicative of the sudden jump of the local temperature to higher than 2000 °C. This sudden heating boils off the silver from the main body, leaving carbon skeletons, the MCNDs. Raman spectra clearly indicate that the skeletons consist of mainly single-layer graphene walls. SEM and TEM images as well as EELS spectra show that the main bodies with ∼50 nm radii branches every 100–150 nm and is composed of cells with graphene walls. The MCNDs showed a BET surface area of 1610 m 2/g. Cyclic voltammetry of a supercapacitor with MCND electrodes showed good rectangular curves, even at a scanning rate of 400 mV/s and a peak current density higher than 40 A/g, suggesting applicability for high current and high-speed charge–discharge capacitors for motor vehicles. These properties are attributed to the dendroid branching structure of the main body that allows fast and massive ion transport through the open channels between branches.

Hollowed carbon capsule based Pt–Fe/carbon electrodecatalysts prepared by chemical vapor infiltration method

A new type of hollowed carbon capsule (HCC) material has been fabricated and utilized as a support for Pt–Fe catalyst by means of chemical vapor infiltration (CVI) method. The supported Pt–Fe/HCC catalyst was found to possess cavities with sizes around 20–40 nm and well-dispersed Pt–Fe alloy particles embedded on the internal graphitic carbon walls and hence can be used as potential electrodecatalyst for fuel cells. Preliminary performance tests by cyclic voltammetry revealed that the supported Pt–Fe/HCC catalyst has electrochemical properties compatible to a common commercial catalyst. The formation mechanism of the HCC material is also proposed.

Surface area and thermal stability effect of the MgO supported catalysts for the synthesis of carbon nanotubes

Surface area (SA) of the oxide support was found to play a major role on the morphology, quality of the single wall carbon nanotubes (SWNTs) growth. Fe–Co bimetal was supported onto MgO powders with different surface areas, and used for the SWNTs synthesis from the pyrolysis of CH4 through radio frequency catalytic chemical vapor deposition (RF-CCVD). The high surface area MgO showed specificity towards the growth of SWNTs even at the very high metal loadings of 17.4%. As the MgO surface areas decreased, the agglomeration of the active metal species on the support, nanotubes with wider diameter distributions and more graphitic walls were obtained. This surface area effect suggested that the high surface area networks were responsible for a more effective confinement of the active metal nanoparticles that impeded their migration during the SWNTs thermal growth process. Therefore, by adjusting the surface area or chemical morphology of the catalyst systems, SWNTs with narrower diameter distribution and specific chiralities can be grown.

3D Aperiodic Hierarchical Porous Graphitic Carbon Material for High‐Rate Electrochemical Capacitive Energy Storage

Electrochemical capacitors: A hierarchical porous graphitic carbon material, composed of macroporous ion-buffering microreservoirs, ion-transporting channels, and localized graphitic wall structures, is presented (see images; top: 3D skeleton, bottom: carbon platelet). The properties of this new material combine to overcome the electrode kinetic problems normally found in electrochemical capacitors, thus resulting in an excellent high-rate energy-storage performance. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2008/z702721_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Adsorption of Water in Finite Length Carbon Slit Pore: Comparison between Computer Simulation and Experiment

The effects of surface dimensions and topology on the adsorption of water on a graphite surface at 298 K were investigated using the grand canonical Monte Carlo (GCMC) simulation. Regarding the surface topology, we specifically considered the functional group and its position on the surface. The hydroxyl group (OH) is used as a model for the functional group. For describing the interaction of water, we used the potential model proposed by Muller et al., and the simulated isotherms of water in slit pores are found to depend on the position and concentration of the functional group. The onset of adsorption shifts to lower pressure when the concentration of functional group increases or when the functional group is positioned at the center of the graphene surface. The configuration of a group of functional groups also affects the adsorption isotherm. In all cases investigated, we have found that the hysteresis loop always exists, and the loop size depends on the concentration of the functional group and its position. Finally, we tested the molecular model of water adsorption on a functional graphite pore against the experimental data of a commercial activated carbon. The agreement is found to be satisfactory when the model porous solid is composed of pores having width in the range between 10 and 20 A and functional groups positioned at the center of the graphitic wall.

Isotherm and heat of adsorption in porous solids with defective pores—adsorption of argon and nitrogen at 77 K in Saran activated carbon

The isotherm and isosteric heat of a porous solid are studied in terms of the local isotherms and isosteric heats of individual pores with defective walls, rather than graphitic walls as commonly assumed in the literature. We point out the incorrect formulas that have been used in the literature, and present a correct formula to calculate the isosteric heat for a porous solid. The correct formula is illustrated with a direct Monte Carlo (MC) simulation of systems of two pores of different sizes, and finally we apply our theory to experimental data of argon and nitrogen adsorption at 77 K on S600H and S84 Saran charcoals to derive their pore size distributions (PSD). We show that the PSD derived from the fitting either the isotherm only or the heat curve only may not be reliable. It is necessary to utilize both the isotherm and heat curves in the derivation of a more reliable PSD. We also show that it is essential to use defected walls of carbon pores to model adsorption in pores as the model using graphitic walls can not describe isotherm and heat of adsorption adequately.