Structure Of Nitrogen-doped Carbon Nanotubes Research Articles

Preparation of MoS2 nanosheets/nitrogen-doped carbon nanotubes/MoS2 nanoparticles and their electrochemical energy storage properties

Inferior electrical conductivity, huge volume swell, shuttle effect and slow redox reactions in Li-S batteries restrict their practical applications. In this study, molybdenum disulfide nanosheets anchored on nitrogen-doped carbon nanotubes encapsulating molybdenum disulfide nanoparticles (MoS2/NC/MoS2 NTs) were prepared by heat treatment, ammonia etching, high-temperature sulfurization, and solvent-thermal in-situ growth using polypyrrole-coated molybdenum trioxide nanorods (MoO3 NRs) as templates. The hollow structure of nitrogen-doped carbon nanotubes and the high specific surface area are conducive to increasing sulfur loading, increasing the contact area of electrolyte, accelerating the electron transport, and shortening the lithium ion transport route. The MoS2 nanosheets on the outer layer of the nitrogen-doped carbon tubes and the MoS2 particles wrapped inside can anchor polysulfides by chemisorption, slowing down the shuttle effect and enhancing the energy storage performance of lithium-sulfur batteries. The MoS2/NC/MoS2/S nanotubes (MoS2/NC/MoS2/S NTs) maintain a capacity of 498.9 mAh g−1 over 500 cycles at 1.0 A g−1, with an average single-cycle capacity decay rate of 0.061 %. The density functional theory calculations, the adsorption experiments, and the X-ray photoelectron spectroscopy results all confirm that the MoS2/NC/MoS2 NTs are able to anchor lithium polysulfides through strong adsorption, thus effectively mitigating the shuttle effect of polysulfides. The synthesis of MoS2/NC/MoS2/S NTs in this study is of important significance for the development of advanced lithium-sulfur batteries.

An electrochemical sensor based on synergistic enhancement effects between nitrogen-doped carbon nanotubes and copper ions for ultrasensitive determination of anti-diabetic metformin

Metformin (MET) is the primary medicine for type II diabetes, which produces carcinogenic byproducts during chlorine disinfection, so the detection of MET in aqueous environment is crucial. In this work, an electrochemical sensor based on nitrogen-doped carbon nanotubes (NCNT) has been constructed for ultrasensitive determination of MET in the presence of Cu(II) ions. The excellent conductivity and rich π-conjugated structure of NCNT facilitate the electron transfer rate of fabricated sensor and benefit the adsorption of cation ions. Cu(II) ions can chelate with MET to form MET-Cu(II) complex, which are easily accumulated on the surface of NCNT through cation-π interaction. Attributing to the synergistic enhancement effects of NCNT and Cu(II) ions, the fabricated sensor exhibits excellent analytical performances with a low detection limit of 9.6 nmol L−1, high sensitivity of 64.97 A mol−1 cm−2 and wide linear range of 0.3–10 μmol L−1. The sensing system has been successfully applied for rapid (20 s) and selective determination of MET in real water samples with satisfactory recoveries (90.2 %–108.8 %). This study provides a robust strategy for MET detection in aqueous environment and holds great promise for rapid risk assessment and early warning of MET.

Regulation of impedance matching feature and electronic structure of nitrogen-doped carbon nanotubes for high-performance electromagnetic wave absorption

In this study, we developed a facile method to fabricate three-dimensional (3D) structures composed of FeNi alloy nanoparticles encapsulated in N-doped carbon nanotubes that grafted on the SiO2 spheres (FexNiy@[email protected]2) for electromagnetic wave (EMW) absorption. The experimental results suggest that the impedance matching characteristic can be tuned by the introduction of SiO2 spheres in the 3D structure. Density functional theory (DFT) calculations showed that the introduction of Ni improved the polarization and conductive losses of the FexNiy@[email protected]2. As a result, the optimal 3D structure exhibits excellent EMW absorption property with a reflection loss and effective absorption bandwidth are -49.39 dB and 4.32 GHz, respectively, even though the matching thickness is only 1.6 mm, superior to most magnetic carbon-based composites. Thus, our current approach opens up an effective way to the development of low-cost, high-performance EMW absorbers.

Electronic structure of nitrogen-doped carbon nanotubes under stress

Due to the demand for miniaturization and low energy consumption of electronic device, field-effect transistors based on carbon nanotubes have received extensive attention. However, the electronic properties of carbon nanotubes, especially when subjected to stress and doping, are not very clear. In this study, first-principles calculations are used to investigate the deformation of carbon nanotubes under the action of nitrogen doping and tensile stress, as well as the changes in electronic energy bands. The results show that nitrogen atom doping will reduce the tensile strength of carbon nanotubes and increase the conductivity of carbon nanotubes, while stress helps to open the band gap. Our research has guiding significance for the prediction of electronic properties of carbon nanotubes under doping and stress.

Structural evolution of nitrogen-doped carbon nanotubes: From synthesis and oxidation to thermal defunctionalization

Mechanism of the oxidation and thermal defunctionalization of nitrogen-doped carbon nanotubes (N-CNTs) was studied. TGA-MS, low temperature nitrogen adsorption, Raman spectroscopy, TEM and XPS were used to trace the changes in the morphology, surface properties and defectiveness of N-CNTs during HNO3 treatment. The first hour of treatment resulted in the intensive oxidation of N-CNTs due to the transformation of surface nitrogen groups into oxidized pyridone ones. Complete destruction of segmented structure of N-CNTs and a decrease in both oxygen and nitrogen content was observed after prolonged oxidation. The changes in the defectiveness were confirmed by the increase in ID1/IG and the decrease in ID3/IG ratio in Raman spectra. The thermal stability of nitrogen species in N-CNTs was found to increase in the following sequence: pyridone, pyridine and graphitic nitrogen. Decomposition of pyridone nitrogen starts at 200–400°С resulting in the formation of pyridine and pyrrolic nitrogen species. Above 500 °C these two nitrogen species transform into graphitic nitrogen which decomposes at temperatures above 1000°С. NO release in different temperature ranges was associated with both the destruction of nitro-, nitrito-, nitroso-groups and oxidation of HCN. The mechanism of functionalization and thermal destruction of oxygen- and nitrogen-containing groups was proposed.

Effect of synthesis catalyst on structure of nitrogen-doped carbon nanotubes and electrical conductivity and electromagnetic interference shielding of their polymeric nanocomposites

Different catalysts including Co, Fe, and Ni were used to synthesize nitrogen-doped carbon nanotubes (N-CNTs) by chemical vapor deposition technique. Synthesized N-CNTs were melt mixed with a polyvinylidene fluoride (PVDF) matrix using a small scale mixer at different concentrations ranging from 0.3 to 3.5 wt%, and then compression molded. The characterization techniques revealed significant differences in the synthesis yield and the morphological and electrical properties of both N-CNTs and nanocomposites depending on the catalyst type. Whereas Co and Fe resulted in yields comparable to industrial multiwalled CNTs, Ni was much less effective. The N-CNT aspect ratio was the highest for Co catalyst, followed by Ni and Fe, whereas nitrogen content was the highest for Ni. Raman spectroscopy revealed lowest defect number and highest N-CNT crystallinity for Fe catalyst. Characterization of N-CNT/PVDF nanocomposites showed better dispersion for N-CNTs based on Co and Fe as compared to Ni, and the following order of electrical conductivity and electromagnetic interference shielding (from high to low): Co > Fe > Ni. The superior electrical properties of (N-CNT)Co nanocomposites were ascribed to a combination of high synthesis yield, high aspect ratio, low nitrogen content and high crystallinity of N-CNTs combined with a good state of N-CNT dispersion.

Effect of the manufacturing parameters on the structure of nitrogen-doped carbon nanotubes produced by catalytic laser-induced chemical vapor deposition

Nitrogen-containing carbon nanotubes (CNx-NTs), with a relatively high level of nitrogen doping were prepared by the catalytic laser-induced CVD method. The nanotubes were catalytically grown directly on a silicon substrate from C2H2/NH3 gaseous precursors. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) give firm evidence for the nitrogen doping. As determined by XPS, the N concentration for the prepared CNx-NTs increases from 3.6 to 30.6 at.% with increasing ammonia concentration and pressure. TEM images indicate that the nanotubes are bamboo like. As the nitrogen content increases, there is a transition from the bamboo shape with few defects and little distortion to a corrugated structure with a much larger number of defects. Raman spectroscopy revealed that with increasing nitrogen concentration, there is more disorder and defects, together with an increase in ID/IG ratio. By energy-filtering TEM, a higher N concentration was found on the outer amorphous nanolayer than in the compartment core of the nanotubes.

Nitrogen-doped carbon nanotubes as catalysts for oxygen reduction reaction

The aligned nitrogen-doped carbon nanotubes (NCNT) with bamboo-like structure are synthesized via thermal chemical vapor deposition using melamine and urea as different nitrogen precursors. Meanwhile, ferrocene is used as catalyst and carbon precursor. The resulting NCNT with melamine (M-NCNT) have shown superior ORR performance in terms of limiting current density and number of electrons transferred. Further characterizations by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy illustrated higher nitrogen content and more defects in M-NCNT compared to that in NCNT with urea (U-NCNT), which indicate the important role of the nitrogen precursor in nitrogen content and structure of NCNT. It is concluded that higher nitrogen content and more defects of NCNT lead to high performance of ORR.

Electrocatalytic activity of nitrogen doped carbon nanotubes with different morphologies for oxygen reduction reaction

Nitrogen doped carbon nanotubes (NCNTs) were synthesized by a single step chemical vapor deposition technique using either ferrocene or iron(II) phthalocyanine as catalyst and pyridine as the carbon and nitrogen precursor. Variations in surface morphology and electrocatalytic activity for oxygen reduction reaction (ORR) were observed between the NCNTs synthesized using different catalysts. The structural and chemical characterizations were carried out using transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrochemical activity of NCNTs was evaluated with rotating ring disc electrode (RRDE) voltammetry. Structural characterization suggested more defects formed on the NCNTs synthesized from ferrocene (Fc-NCNTs) which led to a rugged surface morphology compared to the NCNTs synthesized from iron(II) phthalocyanine (FePc-NCNTs). Based on the RRDE voltammetry study, Fc-NCNTs demonstrated much higher activity for ORR than FePc-NCNT. Evidences from the structural and chemical characterizations illustrate the potential impact of catalyst structure in shaping the surface structure of NCNTs and the positive effect of surface defects on ORR activity. These results showed that potential improvements on ORR activity of NCNTs could be achieved by tailoring the surface structure of NCNTs by using catalysts with different structures.

Highly Active Nitrogen-Doped Carbon Nanotubes for Oxygen Reduction Reaction in Fuel Cell Applications

One of the main challenges in the commercialization of low temperature fuel cells is the slow oxygen reduction reaction (ORR) kinetics and the high cost and scarcity of platinum (Pt)-based catalysts. As a result, alternative non-noble electrocatalysts to Pt materials for ORR is needed to realize the practical application of fuel cells. In this study, nitrogen-doped carbon nanotubes (NCNTs) were synthesized as a non-noble electrocatalyst for the ORR using ethylenediamine (EDA-NCNT) and pyridine (Py-NCNT) as different nitrogen precursors by a single-step chemical vapor deposition (CVD) process. The resulting EDA-NCNT has shown similar ORR performance compared to platinum on carbon support in terms of onset and half-wave potentials. Moreover, EDA-NCNT showed superior ORR performance in terms of limiting current density, number of electrons transferred, and H2O selectivity. The effects of nitrogen content on ORR performance of NCNT were investigated by comparing EDA-NCNT with Py-NCNT. The ORR performance of Py-NCNT was inferior compared to EDA-NCNT in terms of onset and half-wave potentials, limiting current density, number of electrons transferred, and H2O selectivity. Further material characterizations by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy illustrated a higher nitrogen content and more defects in EDA-NCNT compared to that of Py-NCNT which indicates the important role of the nitrogen precursor on nitrogen content and structure of NCNT. By combining the results of ORR activity and material characterization, it is concluded that higher nitrogen content and more defects of NCNT lead to high ORR performance.

Effect of the experimental parameters on the structure of nitrogen-doped carbon nanotubes produced by aerosol chemical vapour deposition

We describe the systematic study of multi-walled carbon nanotubes with different nitrogen doping produced by aerosol chemical vapor deposition. Benzylamine:toluene mixtures of 0:100, 5:95, 10:90, 25:75, 50:50, 75:25 and 100:0 were thermally decomposed at 800–900 °C under argon at atmospheric pressure, whereby the nitrogen content of the bulk material was varied between 0 and 2.2 at%. We also show how the presence of nitrogen in the precursor changed the nanotube morphology, i.e. nitrogen decreased the number of kinks incorporated into the carbon nanotubes, decreased their length and diameter and increased the proportion of ‘bamboo’ shaped nanotubes. Furthermore, due to the nitrogen doping, the oxidation resistance of the nanotube material was decreased. With concentrations above 10% benzylamine the increase of the reaction temperature had no significant effect on the quality of the nanotubes, however, at higher temperatures the nitrogen content was decreased. We demonstrate the control over the nanotube geometry, the nitrogen content and oxidation resistance of the nanotubes, and show that these properties are interlinked.