CNx Nanotubes Research Articles

N-Doped carbon nanotubes enriched with graphitic nitrogen in a buckypaper configuration as efficient 3D electrodes for oxygen reduction to H2O2

Herein, a series of N-doped carbon nanotube (CNx) samples were obtained by modifying the synthesis temperature. Consequently, the proportion of graphitic nitrogen (Ngraph) in the samples was systematically increased as a function of temperature. This allowed evaluation of the role of the CNx graphitic nitrogen in the oxygen reduction reaction (ORR). A correlation between the Ngraph content and the ORR onset potential was observed, which shifted to more positive potentials with an increase in kinetic current density (jk); this showed that Ngraph played a significant catalytic role in the ORR. The samples with high Ngraph content favored the two-electron pathway for the ORR not only in basic media (pH = 13) but also in neutral media (pH = 7), representing an attractive alternative for wastewater remediation through the on-site generation of H2O2. The energetic calculations showed that the formation of H2O2 must be favorable in the presence of graphitic nitrogen sites. Finally, the performance of the buckypaper arrangement was evaluated, and the CNx buckypaper showed a higher cathodic current peak as compared to CNx traditional ink dispersions. Overall, this study not only sheds light on the role of Ngraph in the ORR, but also demonstrates that CNx buckypaper is an efficient 3D electrode for electrocatalytic applications.

One-step chemical vapor deposition synthesis and supercapacitor performance of nitrogen-doped porous carbon-carbon nanotube hybrids.

Novel nitrogen-doped carbon hybrid materials consisting of multiwalled nanotubes and porous graphitic layers have been produced by chemical vapor deposition over magnesium-oxide-supported metal catalysts. CNx nanotubes were grown on Co/Mo, Ni/Mo, or Fe/Mo alloy nanoparticles, and MgO grains served as a template for the porous carbon. The simultaneous formation of morphologically different carbon structures was due to the slow activation of catalysts for the nanotube growth in a carbon-containing gas environment. An analysis of the obtained products by means of transmission electron microscopy, thermogravimetry and X-ray photoelectron spectroscopy methods revealed that the catalyst's composition influences the nanotube/porous carbon ratio and concentration of incorporated nitrogen. The hybrid materials were tested as electrodes in a 1M H2SO4 electrolyte and the best performance was found for a nitrogen-enriched material produced using the Fe/Mo catalyst. From the electrochemical impedance spectroscopy data, it was concluded that the nitrogen doping reduces the resistance at the carbon surface/electrolyte interface and the nanotubes permeating the porous carbon provide fast charge transport in the cell.

Graphitic and pyridinic nitrogen in carbon nanotubes: energetic and polarization aspects

The incorporation of nitrogen atoms into carbon nanotube (CNT) walls occurs mainly via bonding with three or two carbon atoms, and the obtained configurations are referred to as graphitic and pyridinic nitrogen forms. Here, we evaluate the energy of formation of these nitrogen defects in an armchair (6,6) CNT and the static polarizability of the obtained nitrogen-containing carbon (CNx) nanotubes using a dispersion-corrected hybrid functional. The calculations showed that the graphitic nitrogen atoms prefer to be in the pentagonal rings located at a nanotube cap. The CNx nanotubes with such nitrogen impurities have enhanced polarizability as compared to their nondoped counterparts. The formation of the pyridinic nitrogen defect requires ∼7.1 eV; however, if the CNT already contains a vacancy, this energy reduces to ∼0.2 eV. The presence of pyridinic nitrogen atoms in CNx nanotubes should not increase the polarization response. Our results suggest that the electromagnetic properties of CNx nanotubes can be tuned by interconverting between graphitic and pyridinic nitrogen forms.

Field emission properties of aligned CNx nanotube arrays synthesized by pyrolysis of a ferrocene/acetonitrile aerosol at different temperatures

Nitrogen-containing carbon (CNx) nanotubes have been grown vertically on silicon substrates by decomposition of a ferrocene/acetonitrile aerosol at temperatures 750, 800, 850, and 900 °С. A CNx nanotube array, produced at 850 °С, had the greatest height ∼220 μm and the smallest nitrogen content ∼3.8 at%. The content of graphitic nitrogen form, referred to a nitrogen atom with three carbon neighbors in a hexagonal lattice, increased gradually with the synthesis temperature. Field electron emission measurements showed that the CNx nanotube arrays have better characteristics than aligned non-doped carbon nanotubes. To reveal effect of nitrogen on the filed threshold, we calculated current–voltage dependencies for CNx nanotube models containing different amount of graphitic and pyridinic nitrogen species. Influence of the synthesis temperature on the height and total nitrogen content in CNx nanotube array and relation of these parameters with the threshold of field emission appearance.

Changes of the electronic structure of the atoms of nitrogen in nitrogen-doped multiwalled carbon nanotubes under the influence of pulsed ion radiation

With the use of X-ray photoelectron spectroscopy (XPS) there have been investigated the changes of the chemical state of nitrogen atoms in the structure of nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNTs) resulting from the impact of pulsed ion beam at various parameters of the beam (energy density, number of pulses). It has been established that irradiation with the pulsed ion beam leads to a reduction of the total amount of nitrogen in CNx nanotubes. It has been shown that a single pulse irradiation of ion beam at the energy densities of 0.5, 1, 1.5J/cm2 leads to restructuring of the nitrogen from pyridinic and pyrrolic configuration to graphitic state. Complete removal of nitrogen (pyridinic, pyrrolic, graphitic) embedded in the structure of the walls of CNx nanotubes occurs at ten pulses and 1.5J/cm2.

Activity and active sites of nitrogen-doped carbon nanotubes for oxygen reduction reaction

Nitrogen-doped carbon (CNx) nanotubes were synthesized by thermal decomposition of ferrocene/ethylenediamine mixture at 600–900 °C. The effect of the temperature on the growth and structure of CNx nanotubes was studied by transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. With increasing growth temperature, the total nitrogen content of CNx nanotubes was decreased from 8.93 to 6.01 at.%. The N configurations were changed from pyrrolic-N to quaternary-N when increasing the temperature. Examination of the catalytic activities of the nanotubes for oxygen reduction reaction by rotating disk electrode measurements and single-cell tests shows that the onset potential for oxygen reduction in 0.5 M H2SO4 of the most effective catalyst (CNx nanotubes synthesized at 900 °C) was 0.83 V versus the normal hydrogen electrode. A current density of 0.07 A cm−2 at 0.6 V was obtained in an H2/O2 proton-exchange membrane fuel cell at a cathode catalyst loading of 2 mg cm−2.

Iron nanoparticles in aligned arrays of pure and nitrogen-doped carbon nanotubes

Arrays of aligned carbon nanotubes (CNTs) and nitrogen-doped carbon (CNx) nanotubes have been grown on silicon substrates as the result of thermolysis of ferrocene/toluene and ferrocene/acetonitrile mixture. The microstructure of materials was studied by transmission and scanning electron microscopy, and X-ray diffraction was used to control the carbon and iron forms. The composition and properties of iron nanoparticles developed in the CNT and CNx nanotube samples were determined from Mössbauer spectroscopy data. The total iron content in CNx nanotubes was found to be considerably higher than that in CNTs. Three forms of iron nanoparticles α-Fe, γ-Fe, and Fe3C were detected in CNTs and only two last of them in CNx nanotubes. In the interior of CNT channels the α-Fe and Fe3C nanoparticles were observed to be coupled by a strong exchange interaction and to exhibit magnetic behavior at room temperature.

Catalytic Twist‐Spun Yarns of Nitrogen‐Doped Carbon Nanotubes

AbstractThe treatment of free‐standing sheets of multiwalled carbon nanotubes (MWNTs) with a NH3/He plasma results in self‐supporting sheets of aligned N‐doped MWNTs (CNx). These CNx sheets can be easily twist spun in the solid state to provide strong CNx yarns that are knottable, weavable, and sewable. The CNx yarns exhibit tunable catalytic activity for electrochemically driven oxygen reduction reactions (ORR), as well as specific capacitances (up to 39 F·g−1) that are 2.6 times higher than for the parent MWNTs. Due to a high degree of nanotube alignment, the CNx yarns exhibit specific strengths (451 ± 61 MPa·cm3·g−1) that are three times larger than observed for hybrid CNx/MWNT biscrolled yarns containing 70 wt.% CNx in the form of a powder. This difference in mechanical strength arises from substantial differences in yarn morphology, revealed by electron microscopy imaging of yarn cross‐ sections, as well as the absence of a significant strength contribution from CNx nanotubes in the biscrolled yarns. Finally, the chemical nature and abundance of the incorporated nitrogen within the CNx nanotubes is studied as function of plasma exposure and annealing processes using X‐ray photoelectron spectroscopy and correlated with catalytic activity.

Metal-catalyst-free Growth and Characterization of Nitrogen-doped Carbon Nanotubes

Nitrogen-doped carbon (CNx) nanotbues were synthesized by detonation-assisted chemical vapor deposition with carbon nanotubes (CNTs) as catalysts and melamine as carbon/nitrogen sources. CNTs exhibit high catalytic activity for the metal-catalyst-free synthesis of CNx nanotubes. TEM, EDS, Mapping, XPS, Raman and TG were performed to characterize the synthesized CNx nanotubes. CNx nanotubes exhibit compartmentalized bamboo-like structure, and contain high concentration of nitrogen (ca. 17at%), which is homogeneously distributed within compartment layers and tube walls. The pyridine-like and graphitic N are incorporated into the graphitic network. The nitrogen doping induces the decrease of the graphitization degree and results in lower oxidation-resistance temperature. The synthesis of CNx nanotubes without metal catalysts will facilitate the applications of CNx nanotubes.

The Synthesis of Nitrigen Doped Carbon Nanotube with Different Catalyst

Bamboo-like nitrogen-doped carbon (CNX) nanotubes have been synthesized by chemical vapor deposition at high reaction temperature of 850°C. The diethylamine has been used as carbon source and Fe/SBA-15 and Ni/SBA-15 have been used as molecular sieve catalyst. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) observations showed that the diameter and the wall thickness of CNx nanotubes were increased with the change of catalyst from Fe/SBA-15 to Ni/SBA-15.

Growth of carbon nanotubes and nitrogen-doped carbon nanotubes on “carbon nanotube seeds”

Carbon and nitrogen-doped carbon(CNx) nanotubes were synthesized by detonation-assisted chemical vapor deposition with CNTs as seeds and paraffin as the carbon source and melamine as the carbon/nitrogen source.Transmission electron microscopy and electron energy loss spectroscopy were performed to characterize both materials.CNx nanotubes exhibited a compartmentalized bamboo-like structure and contained a high concentration of nitrogen(17.3 at %),while the new pure CNTs had hollow cores.The role of the original CNTs added to the raw materials could be ascribed to their curvature and open tip edges,which adsorbed Cn/CN species and assembled them into carbon and CNx nanotubes.

Theoretical studies of the CNx nanotube with four-nitrogen divacancy (4ND) defects

The formation energies, electronic properties, and applications in hydrogen storage of four-nitrogen divacancy (4ND) defects in a series of CNTs are investigated by density functional theory (DFT) calculations. We find that the formation energies of 4ND defects in the (n, 0) CNT are always smaller than those of the (n, n) CNT, suggesting that the formers might be easily formed. For (n, n) CNTs, the formation energies of 4ND defect are nearly independent of the tube diameters for s, while the values increase monotonically with the increased tube diameters in (n, 0) CNTs. Furthermore, the incorporation of 4ND structures to the CNT introduces the acceptor states to the band structures, resulting in the decrease of the band gaps of pure CNTs in various ways. By studying the hydrogen adsorption on the Ca-dispersed CNx nanotube with 4ND structures, we suggest that up to five H2 molecules can be bound to per Ca atom with the adsorption energy of −0.263eV. The present results indicate that the Ca-dispersed CNx nanotube with 4ND defects might be a promising candidate as hydrogen storage media.

X-Ray Absorption Spectra of N2 Molecules Embedded into CNx Nanotubes as a Marker of Orientation Ordering of Array

Arrays of aligned multi-walled nitrogen-doped carbon (CNx) nanotubes were synthesized by the catalytic chemical vapor deposition method using acetonitrile and a mixture of acetonitrile and toluene taken in equal ratio as sources for nanotube growth. The structure of arrays was investigated by means of scanning electron microscopy (SEM) and angle-resolved X-ray absorption spectroscopy near the CK- and NK-edges. The width of angular distribution of CNx nanotubes in the arrays was determined from the Fourier analysis of SEM images, and the ordering of graphitic layers of nanotubes was evaluated from angular dependence of the π*/σ*-intensity ratio near the CK-edge. It was shown that relative intensity of π*-resonance corresponding to N2 molecules embedded into CNx nanotubes can be used as a marker for definition of the deviation of straight CNx nanotubes from a normal direction.

Chemical functionalization of pyridine-like and porphyrin-like nitrogen-doped carbon (CN x ) nanotubes with transition metal (TM) atoms: a theoretical study

It is well known that chemical functionalization of carbon nanotubes (CNTs) can offer an effective route to modify their properties, thus significantly widening their application areas. In the present work, through spin-polarized density functional theory (DFT) calculations, we have systemically studied the chemical functionalization of pyridine-like and porphyrin-like nitrogen-doped carbon (CNx) nanotubes with 13 different transition metals (TMs = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pt, Pd, and Au). Particular attention is paid to searching for the most stable configurations, calculating the corresponding binding energies, and exploring the effects of the chemical functionalization of TMs on the electronic properties of the two CNx nanotubes. We find that (1) due to the strong interaction between the d orbitals of TMs and the p orbitals of N atoms, the binding strengths of TMs with the two CNx nanotubes are significantly enhanced when compared to the pure CNTs; (2) the electronic properties of CNx nanotubes can be effectively modified in various ways, which are strongly dependent on the adsorbed TMs.

Facile Decoration of Platinum Nanoparticles on Carbon-Nitride Nanotubes via Microwave-Assisted Chemical Reduction and Their Optimization for Field-Emission Application

This study reports a simple, efficient method of using Pt nanoparticles to decorate CNx nanotubes with controllable particle size and population density. CNx nanotubes, synthesized by the chemical vapor deposition of an imidazole-ferrocene mixture at 750 °C, were dispersed in 2-(2-methoxyethoxy)-ethanol solvent together with H2PtCl6. The reduction of Pt(IV) to Pt(0) was achieved with NaBH4 and microwave irradiation, separately. The NaBH4-mediated reduction process results in Pt nanoparticles of small size decorating the CNx surface with low density. However, microwave-assisted chemical reduction leads to larger nanoparticles with a high population density. It is observed that Pt-anchoring on the CNx strands is mainly governed by pyridinic-N (pN) units. The XPS peak intensities of Pt(II) and Pt(IV) are very low when Pt nanoparticles are grown via a microwave-irradiation process, as compared to the NaBH4 reduction process. This suggests that NaBH4 treatment leads to the incomplete reduction of Pt ions. In c...

Electrode microwave discharge: Areas of application and recent results of discharge physics

The first paper on the electrode microwave discharge (EMD) appeared in 1996. Presently many problems of EMD physics and applications have already been solved. Several examples of EMD application are discussed: diamond growth, deposition of CNx films and nanotubes, deposition of metal films (Cu, Al), deposition of TiN and TiO2 films, generation of O2(a1Δ), and EMD as a plasma cathode. Results of EMD experiments and modeling give rise to the assumption that an EMD consists of a self-sustained domain (near-electrode plasma region with overcritical plasma density) which is surrounded by a region of a non-self-sustained discharge (ball shaped region with undercritical plasma density). We assumed that the layer of charge separation and of induced electrostatic field originated at the outer EMD boundary was one of the reasons for the abrupt decrease of the plasma density which leads to the formation of a compact plasma structure. Recent modeling results of the strongly nonuniform electrode microwave plasma based on a quasi static, 1D spherically symmetric model showed that such a layer can be generated at the point where a sudden increase of the total ionization rate takes place.

Controllable growth of highly N-doped carbon nanotubes from imidazole: a structural, spectroscopic and field emission study

Here we report the highest N-doping level achieved so far for carbon nitride (CNx) nanotubes by using a new precursor imidazole (as a dual supplier of carbon and nitrogen) together with ferrocene (as a catalyst). A controllable growth of aligned CNx nanotubes was achieved by simple CVD in a temperature range of 700–950 °C leading to nitrogen doping from 12.1 to 25.7 at%. The highest doping level was achieved at 850 °C, which is attributed to the abundance of C–N fragments at this decomposition temperature. Nitrogen doping level and the type of N-moieties were determined by XPS and EELS analyses. HRTEM analysis suggests that N-substituted graphitic units (gN) give rise to fullerenic curvature into the cylindrical graphite layers (leading to curled side-wall morphology), whereas pyridinic units (pN) break the graphitic continuity (leading to dangling bonds on the side walls). It is observed that the abundance of N-substituted graphitic units (gN) enhances the electrical conductivity of individual nanotubes by donating additional electrons to the CNT network. The field emission (FE) characteristics show that the increase of N doping from 19.6 at% to 25.7 at% effectively reduces the turn-on field (for 10 µA cm−2) from 1.92 V µm−1 to 0.88 V µm−1, whereas the threshold field (for 10 mA cm−2) decreases from 4.05 V µm−1 to 2.32 V µm−1. However, at high growth temperature (950 °C), the N-doping level decreases to 12.1 at%, thereby degrading the FE performance.

Electrocatalysts: Facile Construction of Pt-Co/CNx Nanotube Electrocatalysts and Their Application to the Oxygen Reduction Reaction (Adv. Mater. 48/2009).

Pt-Co alloyed nanoparticles can be facilely immobilized onto CNx nanotubes due to the incorporated nitrogen, report Yanwen Ma, Zheng Hu, and co-workers on p. 4953. The as-prepared electrocatalysts exhibit good performance for oxygen reduction reactions in acidic media arising from the high dispersion and alloying effect of Pt-Co nanoparticles, as well as the intrinsic catalytic capacity of CNx nanotubes, which is significant for the development of fuel cells.

Facile Construction of Pt-Co/CNx Nanotube Electrocatalysts and Their Application to the Oxygen Reduction Reaction.

A straight forward method for immobilizing Pt-Co alloyed nanoparticles onto nitrogen-doped CNx nanotubes is presented. The as-prepared electrocatalysts exhibit good performance for oxygen reduction reaction in acidic medium arising from the high-dispersion and alloying effect of the Pt-Co nanoparticles and the intrinsic catalytic capacity of the CNx nanotubes.

Simultaneous adsorption of Cd 2+ and phenol on modified N-doped carbon nanotubes: Experimental and DFT studies

Carbon nanotubes are novel materials that have been investigated for diverse applications, but only a few studies have been focused on environmental issues. In this work, we report on the efficient adsorption of phenol and cadmium ions on N-doped carbon nanotubes (CNx), which have been modified in air at different temperatures. The pristine and modified CNx nanotubes were characterized by SEM, TGA, elemental analysis and their surface areas were also determined. The adsorption experiments of toxic pollutants were carried out in batch reactors at 25 °C. The characterization of modified CNx by these techniques showed an increase in oxygen content and surface area in comparison with the pristine CNx tubes. The individual adsorption capacity was 0.10 and 0.07 mmol/g for phenol and Cd 2+, respectively. The experimental data of the competitive adsorption of phenol and Cd 2+ revealed that the cadmium removal was favored as the phenol concentration increased, whereas the phenol adsorption capacity was slightly affected at any cadmium concentration. These results suggest that modified CNx tubes have a great potential in environmental applications as adsorbents of organic and inorganic compounds in aqueous phases. In addition, first-principles calculations were carried out in order to elucidate the mechanism of Cd 2+ adsorption on CNx.