Nitrogen-doped Multi-walled Carbon Nanotubes Research Articles

Fragmentation characteristics of undoped and nitrogen-doped multiwalled carbon nanotubes in aqueous dispersion in dependence on the ultrasonication parameters

Sonication techniques are commonly used to disagglomerate carbon nanotube bundles into more or less uniformly distributed individual tubes. The present study discusses the statistical length distribution of multiwalled carbon nanotubes (MWCNTs) as affected by varied sonication time, sonication amplitude and overall energy input applied. Undoped and nitrogen-doped multiwall carbon nanotubes (N-MWCNTs) were investigated, and the influence of nitrogen in the carbon structure on the length-reduction behavior during sonication was assessed. Both kinds of MWCNTs were synthesized using the same process, an Aerosol Assisted Chemical Vapor Deposition (AACVD), utilizing cyclohexane and acetonitrile as precursors. X-ray photoelectron spectroscopy (XPS) clarified the chemical bonding in the CNTs and proved the incorporation of nitrogen in the carbon lattice. A drastically shortening of the initial length, i.e. the carpet length, after short times was shown. It was also found that such incorporation of nitrogen results in distinctly modified length reduction behavior during ultrasonication. The N-MWCNTs showed a much more pronounced length-reduction with higher ultrasonication amplitude than the undoped MWCNTs. This can be explained by the weaker chemical bond between C and N in a CNT-lattice, as compared to that among the carbon atoms only. On the other hand, sonication time has no particular effect on the fracture of N-MWCNTs when compared to the undoped MWCNTs.

Electrochemical response of nitrogen-doped multi-walled carbon nanotubes decorated with gold and iridium nanoparticles toward ferrocyanide/ferricyanide redox system

Novel composite films consisting of nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) were fabricated by means of chemical vapor deposition technique and decorated with gold (AuNP) and iridium (IrNP) nanoparticles possessing diameters of 12.5 and 2.7 nm, respectively. The electrochemical responses of fabricated composite films, further denoted as N-MWCNTs/MNPs (M: Au and Ir), toward ferrocyanide/ferricyanide, [Fe(CN)6]3−/4− redox couple was probed by means of cyclic voltammetry and electrochemical impedance spectroscopy techniques. The findings demonstrate that both N-MWCNT/MNP composite films exhibit greater electrochemical response and sensitivity toward [Fe(CN)6]3−/4− compared to unmodified N-MWCNTs. The results verify that the N-MWCNT/MNP composite films are extremely promising for application in electrochemical sensing.

Electrochemical analysis of ascorbic acid, dopamine, and uric acid on nobel metal modified nitrogen-doped carbon nanotubes

Simultaneous oxidation of ascorbic acid (AA), dopamine (DA), and uric acid (UA) in phosphate puffer solution (pH 7.0) on films consisting of nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) decorated with 2.7nm rhodium (RhNPs), 2.6nm palladium (PdNPs), 2.7nm iridium (IrNPs), 2.7nm platinum (PtNPs), and 14nm gold (AuNPs) nanoparticles show a lower limit of detection towards simultaneous oxidation of AA (7.9, 6.8, 6.1, 3.1, and 0.9μM, respectively), DA (1.1, 0.8, 0.7, 0.6, and 0.3μM, respectively), and UA (4.1, 3.6, 3.1, 2.1, 0.4μM, respectively). The findings demonstrate that the film’s lower limit of detection towards simultaneous oxidation of AA, DA, and UA tends to decrease with the order: N-MWCNTs>N-MWCNTs/RhNPs>N-MWCNTs/PdNPs>N-MWCNTs/IrNPs>N-MWCNTs/PtNPs>NMWCNTs/AuNPs. The findings suggest that N-MWCNTs/MNPs can be considered as potential electrode material for electrochemical simultaneous analysis of AA, DA, and UA.

Cement Pastes and Mortars Containing Nitrogen-Doped and Oxygen-Functionalized Multiwalled Carbon Nanotubes

Cement pastes and mortars based on ordinary Portland cement containing nitrogen-doped multiwalled carbon nanotubes (MWCNT-Nx) or oxygen-functionalized multiwalled carbon nanotubes (MWCNT-Ox) are investigated. To incorporate MWCNTs into the cementitious matrix, the as-produced carpets are dispersed over periods of 1 and 2 hours in distilled water at pH levels of 1 and 7. The cement pastes are prepared by adding 0.1 wt% of MWCNTs to cement powder, followed by characterization with SEM and X-ray diffraction (XRD) at an early age (first hours of hydration). The mortars are mechanically characterized during the hydration process for a period of 28 days. SEM characterization of cement pastes revealed that the carbon nanotubes are well incorporated in the cementitious matrix, with the hydrated cement grains interconnected by long carbon nanotubes. XRD characterizations demonstrated that, during the hydration of cement pastes, different peaks emerged that were associated with ettringite, hydrated calcium silicate, and calcium hydroxide, among other structures. Results of the compressive strength measurements for mortars simultaneously mixed with MWCNT-Nx and MWCNT-Ox reached an increment of approximately 30% in compressive strength. In addition, density functional theory calculations were performed in nitrogen-doped and oxygen-functionalized carbon nanotubes interacting with a cement grain.

Highly Active and Durable Non-Precious Metal Catalyst for the Oxygen Reduction Reaction in Acidic Medium

Polymer electrolyte fuel cells exhibit high potentials at the cathode during start-stop cycles in automotive applications, which leads to carbon support corrosion, and concomitant loss of electrocatalytic activity. In this study, carbon nanomaterials (CNM), predominantly composed of nitrogen doped multi-walled carbon nanotubes (N-MWCNTs) with encapsulated cobalt nanoparticles, were synthesized in-situ by the solid-state pyrolysis (SSP) of melamine and cobalt Oxide (Co3O4). The best formulation of the catalyst exhibited an ORR activity of of 2.3 mA cm−2 at 0.75 V vs. RHE (4.6 mA mg−1). The role played by cobalt to complete the active site was demonstrated as follows: Upon complexing the cobalt site with bipyridine, the ORR onset potential decreased by ∼90 mV. The stability of the above non-precious metal (NPM) catalyst was studied through accelerated stress tests (ASTs) designed to mimic load cycling and start-stop cycling protocols, wherein the catalyst was exposed to high anodic potentials (up to 1.5 V vs. RHE) in an acidic medium. In rotating disk electrode mode, the ORR polarization curve shifted to more negative values by about 20 mV and 14 mV, respectively, after the load cycling and start-stop cycling AST protocols, suggesting high stability. Similar stability was observed in fuel cell mode.

Temperature Dependence of Sensors Based on Silver-Decorated Nitrogen-Doped Multiwalled Carbon Nanotubes

Vapor sensors are easily fabricated onto alumina substrates using foils of silver-decorated nitrogen-doped multiwalled carbon nanotubes (CNX-MWNTs-Ag) as active sensing material. The vapor sensors are tested using carbon disulfide, acetone, ethanol, and chloroform vapors. The CNX-MWNTs are produced by chemical vapor deposition process and then decorated with 14 nm Ag nanoparticles (Ag-NPs). The samples are characterized using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Our results demonstrate that Ag-decorated CNX-MWNTs exhibit a better response and sensitivity when compared with pristine CNX-MWNTs based sensors, making them promising candidates for air-pollutants environmental monitoring. The temperature effect on the sensor performance is also studied; we found that the detection mechanism could be tuned from physisorption, at room temperature, to chemisorption at higher working temperature. Finally, first-principles density functional calculations are carried out to understand the interactions between the systems involved in the sensors, finding good agreement between experimental results and the theoretical approach.

Nitrogen/sulfur co-doped helical graphene nanoribbons for efficient oxygen reduction in alkaline and acidic electrolytes

Nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNTs) with bamboo structures have been unzipped via a helical mechanism, and nitrogen-doped graphene oxide nanoribbons (CNx-GONRs) with a multifaceted microstructure have been obtained. CNx-GONRs have then been exposed to nitrogen and/or sulfur precursors at high temperatures to obtain doped graphene nanoribbons. The effects of single-doping, two-step-doping, and co-doping time and temperature on the concentration of the dopants and their functional groups have been investigated by X-ray photoelectron spectroscopy (XPS). The oxygen reduction reaction (ORR) activity of the doped helical GNRs have been measured in both alkaline and acidic electrolytes, and compared with the state-of-the-art platinum/carbon (Pt/C) electrocatalysts. The nitrogen/sulfur co-doped graphene nanoribbons (CNx/CSx-GNRs) with helically unzipped structures have shown highly efficient ORR activity that is comparable to Pt/C in onset potential, exchange current density, four electron pathway selectivity, kinetic current density, and methanol tolerance. Such helical CNx/CSx-GNRs will greatly contribute to the development of precious metal-free carbon nanomaterials as electrocatalysts in the cathode of metal-air batteries, and alkaline and proton-exchange membrane fuel-cells.

Enhanced Adsorption of Hydroxyl- and Amino-Substituted Aromatic Chemicals to Nitrogen-Doped Multiwall Carbon Nanotubes: A Combined Batch and Theoretical Calculation Study.

A large effort is being made to develop nanosorbents with tunable surface chemistry for enhanced adsorption affinity and selectivity toward target organic contaminants. Heteroatom N-doped multiwall carbon nanotubes (N-MCNT) were synthesized by chemical vapor deposition of pyridine and were further investigated for the adsorptive removal of several aromatic chemicals varying in electronic donor and acceptor ability from aqueous solutions using a batch technique. Compared with commercial nondoped multiwall carbon nanotubes (MCNT), N-MCNT had similar specific surface area, morphology, and pore-size distribution but more hydrophilic surfaces and more surface defects due to the doping of graphitic and pyridinic N atoms. N-MCNT exhibited enhanced adsorption (2-10 folds) for the π-donor chemicals (2-naphthol and 1-naphthalmine) at pH ∼6 but similar adsorption for the weak π-donor chemical (naphthalene) and even lower adsorption (up to a 2-fold change) for the π-acceptor chemical (1,3-dinitrobenzene). The enhanced adsorption of 2-naphthol and 1-naphthalmine to N-MCNT was mainly attributed to the favored π-π electron-donor-acceptor (EDA) interaction between the π-donor adsorbate molecule and the polarized N-heterocyclic aromatic ring (π-acceptor) on N-MCNT. The proposed adsorption enhancement mechanisms were further tested through the pH effects on adsorption and the density function theory (DFT) calculation. The results show for the first time that the adsorptive interaction of π-donor aromatic compounds with carbon nanomaterials can be facilitated by N-doping.

A biosensor based on Coriolopsis gallica laccase immobilized on nitrogen-doped multiwalled carbon nanotubes and graphene oxide for polyphenol detection

The use of nanomaterials allows the design of ultrasensitive biosensors with advantages in the detection of organic molecules. Catechol and catechin are molecules that occur naturally in fruits, and their presence in products like dyes and wines affects quality standards. In this study, catechol and catechin were measured at the nanoscale by means of cyclic voltammetry. The oxidation of Coriolopsis gallica laccase immobilized on nitrogen-doped multiwalled carbon nanotubes (Lac/CNx-MWCNT) and on graphene oxide (Lac/GO) was used to measure the concentrations of catechol and catechin. Nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNT) were synthesized by spray pyrolysis and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). Covalently bonded hybrids with laccase (Lac/CNx-MWCNT and Lac/GO) were generated. Catalytic activity of free enzymes determined with syringaldazine yielded 14 584 UmL−1. With Lac/CNx-MWCNT at concentrations of 6.4 mmol L−1 activity was 9326 U mL−1, while enzyme activity measured with Lac/GO at concentration of 6.4 mmol L−1 was 9 234 U mL−1. The Lac/CNx-MWCNT hybrid showed higher stability than Lac/GO at different ethyl alcohol concentrations. The Lac/CNx-MWCNT hybrid can measure concentrations, not previously reported, as low as 1 × 10−8 mol L−1 by measuring the electric current responses.

Integrating NiCo Alloys with Their Oxides as Efficient Bifunctional Cathode Catalysts for Rechargeable Zinc-Air Batteries.

The lack of high-efficient, low-cost, and durable bifunctional electrocatalysts that act simultaneously for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is currently one of the major obstacles to commercializing the electrical rechargeability of zinc-air batteries. A nanocomposite CoO-NiO-NiCo bifunctional electrocatalyst supported by nitrogen-doped multiwall carbon nanotubes (NCNT/CoO-NiO-NiCo) exhibits excellent activity and stability for the ORR/OER in alkaline media. More importantly, real air cathodes made from the bifunctional NCNT/CoO-NiO-NiCo catalysts further demonstrated superior performance to state-of-the-art Pt/C or Pt/C+IrO2 catalysts in primary and rechargeable zinc-air batteries.

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.

Helical and Dendritic Unzipping of Carbon Nanotubes: A Route to Nitrogen-Doped Graphene Nanoribbons.

Bamboo structured nitrogen-doped multiwalled carbon nanotubes (CN(x)-MWCNTs) have been successfully unzipped by a chemical oxidation route, resulting in nitrogen-doped graphene nanoribbons (CN(x)-GNRs) with a multifaceted microstructure. The oxidation of CN(x)-MWCNTs was carried out using potassium permanganate in the presence of trifluoroacetic acid or phosphoric acid. On the basis of the high resolution transmission electron microscopy studies, the bamboo compartments were unzipped via helical or dendritic mechanisms, which are different from the longitudinal unzipping of open channel MWCNTs. The product graphene oxide nanoribbons were simultaneously reduced and doped with nitrogen by thermal annealing in an ammonia atmosphere. The effects of the annealing temperature, time, and atmosphere on the doping level and types of the nitrogen functional groups have been investigated. X-ray photoelectron spectroscopy results indicate that a wide range of doping levels can be achieved (4-9 at %) simply by changing the annealing conditions. Pyridinic and pyrrolic nitrogen functional groups were the dominant species that were attached to the edges of the CN(x)-GNRs. The GNRs, with a faceted structure and pyridinic and pyrrolic groups on their edges, have abundant nitrogen sites. These active sites could play a vital role in enhancing the electrocatalytic performance of GNRs.

Electrochemical preparation of nitrogen-doped graphene quantum dots and their size-dependent electrocatalytic activity for oxygen reduction

Here we report a remarkable transformation of nitrogen-doped multiwalled carbon nanotubes (MWCNTs) to size selective nitrogen-doped graphene quantum dots (N-GQDs) by a two-step electrochemical method. The sizes of the N-GQDs strongly depend on the applied anodic potential, moreover increasing potential resulted in a smaller size of N-GQDs. These N-GQDs display many unusual size-dependant optoelectronic (blue emission) and electrocatalytic (oxygen reduction) properties. The presence of N dopants in the carbon framework not only causes faster unzipping of MWCNTs but also provides more low activation energy site for enhancing the electrocatalytic activity for technologically daunting reactions like oxygen reduction. The smaller size of N-GQDs has shown better performance as compared to the large N-GQDs. Interestingly, N-GQDs-3 (size = 2.5±0.3 nm, onset potential = 0.75 V) show a 30-mV higher positive onset potential shift compared to that of N-GQDs-2 (size = 4.7±0.3 nm, onset potential = 0.72 V) and 70 mV than that of N-GQDs-1 (size = 7.2±0.3, onset potential = 0.68 V) for oxygen reduction reaction (ORR) in a liquid phase. These result in the size-dependent electrocatalytic activity of N-GQDs for ORR as illustrated by the smaller sized N-GQDs (2.5±0.3 nm) undoubtedly promising metal-free electrocatalysts for fuel cell applications.

Magnetoresistance and electrical resistivity of N‐doped multi‐walled carbon nanotubes at low temperatures

AbstractThe magnetoresistance and electrical resistivity of nitrogen‐doped multi‐walled carbon nanotubes (N‐MWCNTs) were studied in the temperature range of 1.6–100.3 and 1.6–286 K, respectively, using a standard four‐probe technique. The possible mechanisms of the observed effects are discussed in detail.

Molecular-confinement of polysulfides within mesoscale electrodes for the practical application of lithium sulfur batteries

Nitrogen-doped porous carbon (NPC) and multi-wall carbon nanotubes (MWCNT) have been frequently studied to immobilize sulfur in lithium–sulfur (Li–S) batteries. However, neither NPC nor MWCNT itself can effectively confine the soluble polysufides if cathode thickness e.g. sulfur loading is increased. In this work, NPC was combined with MWCNTs to construct an integrated host structure to immobilize sulfur at a relevant scale. The function of doped nitrogen atoms was revisited and found to effectively attract sulfur radicals generated during the electrochemical process. The addition of MWCNT facilitated the uniform coating of sulfur nanocomposites to a practically thickness and homogenized the distribution of sulfur particles in the pristine electrodes, while NPC provided sufficient pore volume to trap the dissolved polysulfides species. More importantly, the difficulty of electrode wetting, a critical challenge for thick sulfur cathodes, is also mitigated after the adoption of MWCNT, leading to a high areal capacity of ca. 2.5mAh/cm2 with a capacity retention of 81.6% over 100 cycles.

Biotin molecules on nitrogen-doped carbon nanotubes enhance the uniform anchoring and formation of Ag nanoparticles

An efficient method for anchoring silver nanoparticles (Ag-NPs) on the surface of nitrogen-doped multi-walled carbon nanotubes (CNx-MWCNTs) is reported. The process involves the attachment of biotin molecules on the surface of CNx-MWCNTs (both, pristine and acid treated) that act as a reducing agent for AgNO3, thus generating an efficient and homogeneous coating of Ag-NPs (∼3nm in diameter). The reduction of AgNO3 on either pristine CNx-MWCNTs or acid treated CNx-MWCNTs (without biotin) results in Ag-NPs of large diameters and size distribution, in addition to a low anchoring efficiency. We confirmed that the use of biotin substantially improves the Ag-NPs anchoring efficiency, especially on acid treated CNx-MWCNTs. In order to elucidate the mechanism whereby Ag-NPs strongly bind to the surface of CNx-MWCNTs, density functional theory (DFT) calculations were carried out. These revealed the existence of covalent bonds established between one side of the biotin molecule and the CNx-MWCNT surface through oxygen atoms, leaving accessible the exposed sulfur atoms at the other end, which further provided an excellent interaction with the Ag-NPs via S–Ag bonds. Finally, we demonstrate that these Ag-NPs coated CNx-MWCNTs could be used as efficient sensors of CS2.

Magnetic and Electrical Properties of Nitrogen‐Doped Multiwall Carbon Nanotubes Fabricated by a Modified Chemical Vapor Deposition Method

Chemical vapor deposition (CVD) is a preferential method to fabricate carbon nanotubes (CNTs). Several changes have been proposed to obtain improved CNTs. In this work we have fabricated nitrogen‐doped multiwall carbon nanotubes (N‐MWCNTs) by means of a CVD which has been slightly modified. Such modification consists in changing the content of the by‐product trap. Instead of acetone, we have half‐filled the trap with an aqueous solution of NaCl (0–26.82 wt.%). Scanning electron microscope (SEM) characterization showed morphological changes depending upon concentration of NaCl included in the trap. Using high resolution transmission electron microscopy several shape changes on the catalyst nanoparticles were also observed. According to Raman spectroscopy results N‐MWCNTs fabricated using pure distillate water exhibit better crystallinity. Resistivity measurements performed on different samples by physical properties measurement Evercool system (PPMS) showed metallic to semiconducting temperature dependent transitions when high content of NaCl is used. Results of magnetic properties show a ferromagnetic response to static magnetic fields and the coercive fields were very similar for all the studied cases. However, saturation magnetization is decreased if aqueous solution of NaCl is used in the trap.

Enhanced electrocatalytic activity of nitrogen-doped multi-walled carbon nanotubes towards the oxygen reduction reaction in alkaline media

Nitrogen-doped carbon nanotube materials derived from dicyandiamide and cyanamide are highly active electrocatalysts for oxygen reduction reaction in alkaline media.

Self-Assembly Synthesis of Decorated Nitrogen-Doped Carbon Nanotubes with ZnO Nanoparticles: Anchoring Mechanism and the Effects of Sulfur

Hybrid systems consisting of ZnO nanoparticles (ZnO-NPs) anchored on the surface of nitrogen-doped multiwalled carbon nanotubes (CNX-MWNTs) have been synthesized. The anchoring process consists of a self-assembly method involving the mixing of CNX-MWNTs in a solution with N,N-dimethylformamide, zinc acetylacetonate, and thiophene. Thiophene is used as a capping agent for controlling the size and distribution of ZnO-NPs, as well as an anchoring element between the NPs and the nanotube walls. Scanning and transmission electron microscopy characterization revealed that the ZnO-NPs are homogeneously deposited on the surface of CNX-MWNTs. X-ray powder diffraction analysis demonstrated that the ZnO-NPs exhibit a Wurtzite-type crystal structure with an average particle diameter of 5 nm. We also show that the ZnO-NPs do not exhibit a preferential growth direction with respect to the nanotube surface, and their formation is simply controlled by the concentration of the passivating agent. Density functional theory (DFT) calculations confirm that sulfur (from thiophene) is an effective passivating agent for ZnO by preferentially binding low-coordinated Zn atoms. However, the ZnO-NPs could be chemically bonded to the nanotubes through oxygen atoms close to the nitrogenated sites of the tubes. Our results also demonstrate that isolated and sulfur passivated ZnO-NPs become magnetic and exhibit half-metallicity (electronic states with only one spin component are present at the Fermi level). Sulfur-passivated ZnO retains these properties even after forming ZnO/CNX-MWNT hybrid materials.

Electrochemical reduction of CO2 on SnO2/nitrogen-doped multiwalled carbon nanotubes composites in KHCO3 aqueous solution

Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) have been prepared from n-propylamine precursor with Co0.1Mg0.9MoO4 catalyst. Then, SnO2 nanoparticles are decorated on the surface of N-MWCNTs through wet-chemical method. The cyclic voltammetry curves of the SnO2/N-MWCNTs composites in CO2 saturated 0.1molL−1 KHCO3 aqueous solution show that SnO2 will not be reduced to Sn2+ or Sn if the cathode potential is higher than −1.3V vs. Ag/AgCl. The electrochemical reduction of CO2 produces formate in the aqueous solution during the potentiostatic electrolysis and its faradaic efficiency is up to 46%, whereas that on the N-MWCNTs is lower than 10%.