Nitrogen-rich Carbon Nanotubes Research Articles

Growing-fruits-type nanoarchitectonics of nickel‑vanadium layered double hydroxide on branches of nitrogen-rich carbon nanotube for high performance supercapacitors

An innovative structural design leverages the distinctive properties inherent in each material of three type electrode material for supercapacitors. First, polyaniline nanotubes (PN) are prepared controllably. Subsequently, the PN are carbonized into nitrogen-rich carbon nanotubes (NCN) to exploit their high electrical conductivity. Finally, a nickel‑vanadium layered double hydroxide (NiV-LDH) is grown on the surface of NCN “branches” to fabricate a nitrogen-rich carbon nanotubes/nickel‑vanadium layered double hydroxide composite material (NCN/NiV-LDH). The NCN/NiV-LDH exhibits an impressive specific capacitance of 1018.2 F g−1 at a current density of 2 A g−1, and demonstrates favorable rate capability as the capacitance value remains at 702.2 F g−1 even when the current density is tripled. Moreover, the assembled asymmetric supercapacitor comprising NCN/NiV-LDH and active carbon (AC) as the positive and negative electrodes, respectively, achieves a high energy density of 108.25 Wh kg−1, even under a power density of 7500 W kg−1.

Constructing Directional Electrostatic Potential Difference via Gradient Nitrogen Doping for Efficient Oxygen Reduction Reaction.

Nitrogen doping has been recognized as an important strategy to enhance the oxygen reduction reaction (ORR) activity of carbon-encapsulated transition metal catalysts (TM@C). However, previous reports on nitrogen doping have tended to result in a random distribution of nitrogen atoms, which leads to disordered electrostatic potential differences on the surface of carbon layers, limiting further control over the materials' electronic structure. Herein, a gradient nitrogen doping strategy to prepare nitrogen-deficient graphene and nitrogen-rich carbon nanotubes encapsulated cobalt nanoparticles catalysts (Co@CNTs@NG) is proposed. The unique gradient nitrogen doping leads to a gradual increase in the electrostatic potential of the carbon layer from the nitrogen-rich region to the nitrogen-deficient region, facilitating the directed electron transfer within these layers and ultimately optimizing the charge distribution of the material. Therefore, this strategy effectively regulates the density of state and work function of the material, further optimizing the adsorption of oxygen-containing intermediates and enhancing ORR activity. Theoretical and experimental results show that under controlled gradient nitrogen doping, Co@CNTs@NG exhibits significantly ORR performance (Eonset=0.96V, E1/2=0.86V). At the same time, Co@CNTs@NG also displays excellent performance as a cathode material for Zn-air batteries, with peak power density of 132.65mAcm-2 and open-circuit voltage (OCV) of 1.51V. This work provides an effective gradient nitrogen doping strategy to optimize the ORR performance.

Metal-Free Catalytic Conversion of Veratryl and Benzyl Alcohols through Nitrogen-Enriched Carbon Nanotubes

Nitrogen-rich carbon nanotubes NCNT700 and NCNT800 were prepared using the chemical vapor deposition method (CVD). The catalysts were characterized via high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analysis. Both the catalysts were found to have an inverted cup-stack-like morphology. The XPS analysis revealed that the catalysts are rich in pyridinic sites with variable amounts of nitrogen on their surface. The NCTN700, with a higher nitrogen content and more pyridinic sites on its surface, was found to be a good catalyst for the oxidation of benzyl and veratryl alcohols into respective aldehydes. It was observed that toluene and 4-methyl veratrole were also produced in this reaction. The amount of toluene produced was as high as 21%, with 99% conversion of benzaldehyde in the presence of NCNTs-700. The mechanistic pathway was revealed through DFT studies, where the unusual product formation of aromatic alkanes such as toluene and 4-methyl veratrole was explained during the reaction. It was astonishing to observe the reduced product in the reaction that proceeds in the forward direction in presence of a peroxide (tert-butyl hydroperoxide, TBHP). During the computational analysis, it was revealed that the reduced product observed in the reaction did not appear to proceed through a direct disproportionation reaction. Rather, the benzyl alcohol (the reactant) used in the reaction may undergo oxidation by releasing the hydrogen radicals. The hydrogen atoms released during the oxidation reaction appear to have been trapped on pyrrolic sites on the surface of catalyst and later transferred to the reactant molecules to produce toluene as a side product.

Cu nanoclusters on N-doped carbon nanotubes as efficient electrocatalyst for oxygen reduction reaction

Earth-rich transition metal-based catalysts are considered to be the most promising candidates to alternate noble metal-based electrocatalysts for oxygen reduction reaction (ORR). Nevertheless, the practical applications have been hampered due to their poor electrocatalytic behaviors and stability. Here, we report the facial thesis of Cu nanoclusters supported on N-doped carbon nanotubes by a simple grinding and heat treatment method, which presented a superior electrocatalytic performance than commercial Pt/C. In alkaline media, an onset potential (E0) and half-wave potential (E1/2) of 0.98 V and 0.88 V (RHE) have been achieved, respectively. In addition, the synthesized catalysts present a good tolerance and long-term stability to methanol, and exhibit less than 7% H2O2 formation. It is inferred that the enhanced electrocatalytic behavior could be attributed to the formation of Cu-NX active sites, which are coupled into the nitrogen-rich carbon nanotubes. This low-cost, extremely durable and stable electrocatalyst showed a great potential as an efficient cathode material for a variety of renewable energy conversion technologies, such as fuel cells and metal-air cells.

Wall thickness-tunable AgNPs-NCNTs for hydrogen peroxide sensing and oxygen reduction reaction

The nitrogen-rich carbon nanotubes doped with well-dispersed fine silver nanoparticles (AgNPs-NCNTs) were fabricated by carbonization of the silver-polydopamine (AgNPs-PDA) multilayers. The wall thickness of the nanotube is tunable by varying the numbers of the layer-by-layer alternate deposition of Ag and PDA on electrospun polystyrene (PS) fibers. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrate the tunable tubular structure by various AgNPs-PDA layers and the uniform distribution of fine silver nanoparticles with the size of about 6 nm in the nanotube. The composition of the nanocomposites was characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) demonstrate that the electrochemical properties of AgNPs-NCNTs depend on the numbers of AgNPs-PDA layers. The composite material shows excellent electrocatalytic activity to H2O2 reduction, with a low limit of detection (LOD) of 0.03 μM (S/N ≥ 3) and a sensitivity of 1004.9 μA mM−1 cm−2. Meanwhile, the AgNPs-NCNTs present a high catalytic activity to O2 reduction reaction with near 4-electron transfer process. The related catalysis mechanisms were proposed and validated by rotating disk electrode and dissolved oxygen analysis. This catalyst shows high stability for oxygen reduction over 500 cycles.

Cobalt nanoparticles encapsulated in nitrogen-rich carbon nanotubes as efficient catalysts for organic pollutants degradation via sulfite activation

The activation of sulfite by heterogeneous catalysts displays a great potential in the development of new sulfate radials based technologies for wastewater treatment. Herein, cobalt nanoparticles embedded in N-doped carbon nanotubes (Co@NC) were prepared by a simple pyrolysis method. Due to the synergistic effects of the cobalt nanoparticles and N-doped carbon nanotubes, the Co@NC catalyst intrinsically shows an outstanding efficiency, excellent reusability and high stability in the catalytic oxidation of methyl orange (MO) in the presence of sulfite and dioxygen. The structure and efficiency of the catalyst was significantly affected by the content of cobalt and pyrolysis temperature. Several quenching experiments and electron paramagnetic resonance were carried out to investigate the catalytic mechanism. It is found that hydroxyl and sulfate radicals worked together to degrade MO in the system. The formation and decomposition of peroxymonosulfate may be an important route of these reactive radicals production. The effect of different anions, bicarbonate concentration, initial solution pH and dye types on the performance of the catalyst was also studied. This study can open a new approach for design and preparation of encapsulated cobalt in carbon materials as effective catalysts for pollutants degradation via sulfite activation.

Metal-Free Oxidation of Glycerol over Nitrogen-Containing Carbon Nanotubes.

Nitrogen rich carbon nanotubes have been used as a metal free catalyst for the conversion of glycerol into dihydroxyacetone using tert-butyl hydroperoxide as an oxidant. Pyridine nitrogen groups embedded in a carbon matrix are identified as active sites for the reaction. Computational studies have demonstrated that oxidation of pyridine groups to pyridine oxime followed by hydrogen abstraction from secondary alcohol is likely responsible for the oxidation process.

Cobalt‐Embedded Nitrogen‐Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values

Despite being technically possible, splitting water to generate hydrogen is still practically unfeasible due mainly to the lack of sustainable and efficient catalysts for the half reactions involved. Herein we report the synthesis of cobalt-embedded nitrogen-rich carbon nanotubes (NRCNTs) that 1) can efficiently electrocatalyze the hydrogen evolution reaction (HER) with activities close to that of Pt and 2) function well under acidic, neutral or basic media alike, allowing them to be coupled with the best available oxygen-evolving catalysts-which also play crucial roles in the overall water-splitting reaction. The materials are synthesized by a simple, easily scalable synthetic route involving thermal treatment of Co(2+) -embedded graphitic carbon nitride derived from inexpensive starting materials (dicyandiamide and CoCl2 ). The materials' efficient catalytic activity is mainly attributed to their nitrogen dopants and concomitant structural defects.

Cobalt‐Embedded Nitrogen‐Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values

AbstractDespite being technically possible, splitting water to generate hydrogen is still practically unfeasible due mainly to the lack of sustainable and efficient catalysts for the half reactions involved. Herein we report the synthesis of cobalt‐embedded nitrogen‐rich carbon nanotubes (NRCNTs) that 1) can efficiently electrocatalyze the hydrogen evolution reaction (HER) with activities close to that of Pt and 2) function well under acidic, neutral or basic media alike, allowing them to be coupled with the best available oxygen‐evolving catalysts—which also play crucial roles in the overall water‐splitting reaction. The materials are synthesized by a simple, easily scalable synthetic route involving thermal treatment of Co2+‐embedded graphitic carbon nitride derived from inexpensive starting materials (dicyandiamide and CoCl2). The materials’ efficient catalytic activity is mainly attributed to their nitrogen dopants and concomitant structural defects.

Ab-initio calculations for a realistic sensor: A study of CO sensors based on nitrogen-rich carbon nanotubes

The use of nanoscale low-dimensional systems could boost the sensitivity of gas sensors. In this work we simulate a nanoscopic sensor based on carbon nanotubes with a large number of binding sites using ab initio density functional electronic structure calculations coupled to the Non-Equilibrium Green's Function formalism. We present a recipe where the adsorption process is studied followed by conductance calculations of a single defect system and of more realistic disordered system considering different coverages of molecules as one would expect experimentally. We found that the sensitivity of the disordered system is enhanced by a factor of 5 when compared to the single defect one. Finally, our results from the atomistic electronic transport are used as input to a simple model that connects them to experimental parameters such as temperature and partial gas pressure, providing a procedure for simulating a realistic nanoscopic gas sensor. Using this methodology we show that nitrogen-rich carbon nanotubes could work at room temperature with extremely high sensitivity.

Realistic calculations of carbon-based disordered systems

Carbon nanotubes rank amongst potential candidates for a new family of nanoscopic devices, in particular for sensing applications. At the same time that defects in carbon nanotubes act as binding sites for foreign species, our current level of control over the fabrication process does not allow one to specifically choose where these binding sites will actually be positioned. In this work we present a theoretical framework for accurately calculating the electronic and transport properties of long disordered carbon nanotubes containing a large number of binding sites randomly distributed along a sample. This method combines the accuracy and functionality of ab initio density functional theory to determine the electronic structure with a recursive Green's functions method. We apply this methodology on the problem of nitrogen-rich carbon nanotubes, first considering different types of defects and then demonstrating how our simulations can help in the field of sensor design by allowing one to compute the transport properties of realistic nanotube devices containing a large number of randomly distributed binding sites.

Designing Real Nanotube-Based Gas Sensors

Using a combination of density functional theory and recursive Green's functions techniques, we present a full description of a large scale sensor, accounting for disorder and different coverages. Here, we use this method to demonstrate the functionality of nitrogen-rich carbon nanotubes as ammonia sensors as an example. We show how the molecules one wishes to detect bind to the most relevant defects on the nanotube, describe how these interactions lead to changes in the electronic transport properties of each isolated defect, and demonstrate that there are significative resistance changes even in the presence of disorder, elucidating how a realistic nanosensor works.