Diameter-controlled and nitrogen-doped vertically aligned single-walled carbon nanotubes

Diameter controlled and vertically aligned single-walled carbon nanotubes were synthesized from pure and mixed ethanol/acetonitrile feedstock. With increasing acetonitrile concentration in the feedstock, nitrogen incorporation into the sp² carbon network increased until saturating at approximately one atomic percent. The incorporation of nitrogen correlates with a significant diameter reduction from a mean diameter of 2.1 nm down to 0.7 nm. Heteroatom-mediated diameter control is independent of catalyst preparation and represents a versatile tool for the direct synthesis of tailored single-walled carbon nanotubes.     

Structural and morphological control of aligned nitrogen-doped carbon nanotubes

Nitrogen-doped carbon nanotubes (CN_x-NTs) were prepared using a floating catalyst chemical vapor deposition method. Melamine precursor was employed to effectively control nitrogen content within the CN_x-NTs and modulate their structure. X-ray photoelectron spectroscopy (XPS) analysis of the nitrogen bonding demonstrates the nitrogen-incorporation profile according to the precursor amount, which indicates the correlation between the nitrogen concentration and morphology of nanotubes. With the increase of melamine amount, the growth rate of nanotubes increases significantly, and the inner structure of CN_x-NTs displayed a regular morphology transition from straight and smooth walls (0 at.% nitrogen) to cone-stacked shapes or bamboo-like structure (1.5%), then to corrugated structures (3.1% and above). Both XPS and CHN group results indicate that the nitrogen concentration of CN_x-NTs remained almost constant even after exposing them to air for 5 months, revealing superior nitrogen stability in CNTs. Raman analysis shows that the intensity ratio of D to G bands (I_D/I_G) of nanotubes increases with the melamine amount and position of G-band undergoes a down-shift due to increasing nitrogen doping. The aligned CN_x-NTs with modulated morphology, controlled nitrogen concentration and superior stability may find potential applications in developing various nanodevices such as fuel cells and nanoenergetic functional components.     

NH2+ implantations induced superior hemocompatibility of carbon nanotubes

NH₂⁺ implantation was performed on multiwalled carbon nanotubes (MWCNTs) prepared by chemical vapor deposition. The hemocompatibility of MWCNTs and NH₂⁺-implanted MWCNTs was evaluated based on in vitro hemolysis, platelet adhesion, and kinetic-clotting tests. Compared with MWCNTs, NH₂⁺-implanted MWCNTs displayed more perfect platelets and red blood cells in morphology, lower platelet adhesion rate, lower hemolytic rate, and longer kinetic blood-clotting time. NH₂⁺-implanted MWCNTs with higher fluency of 1 × 10¹⁶ ions/cm² led to the best thromboresistance, hence desired hemocompatibility. Fourier transfer infrared and X-ray photoelectron spectroscopy analyses showed that NH₂⁺ implantation caused the cleavage of some pendants and the formation of some new N-containing functional groups. These results were responsible for the enhanced hemocompatibility of NH₂⁺-implanted MWCNTs.     

Boron- and nitrogen-doped multi-wall carbon nanotubes for gas detection

The response of pristine, nitrogen and boron doped carbon nanotube (CNT) sensors to NO₂, CO, C₂H₄ and H₂O at ppm concentrations was investigated at both room temperature and 150 °C. N-doped CNTs show the best sensitivity to nitrogen dioxide and carbon monoxide, while B-doped CNTs show the best sensitivity to ethylene. All tubes (including undoped) show strong humidity response. Sensing mechanisms are determined via comparison with density functional calculations of gas molecule absorption onto representative defect structures in N and B-doped graphene. N-CNTs show decreased sensitivity with temperature, and detection appears to occur via gas physisorption. B-CNTs appear to react chemically with many of the absorbed species as shown by their poor baseline recovery and increasing sensitivity with temperature. This limits their cyclability. Overall gas sensitivity is as good or better than post-growth functionalised nanotubes, and used in combination, CNTs, N-CNTs and B-CNTs appear highly promising candidates for cheap, low power, room temperature gas sensing applications.     

Self-organization of nitrogen-doped carbon nanotubes into double-helix structures

Self-organization of nitrogen-doped carbon nanotube (N-CNT) double helices was achieved by chemical vapor deposition (CVD) with Fe–Mg–Al layered double hydroxides (LDHs) as the catalyst precursor. The as-obtained N-CNT double helix exhibited a closely packed nanostructure with a catalyst flake on the tip, which connected the two CNT strands on both sides of the flake. A mechanism for the self-organization of N-CNTs into double-helix structures with a moving catalyst head is proposed. Effective carbon/nitrogen sources, high-density active catalyst nanoparticles, space confinement, and the precise chiral match between the two CNT strands are found to be crucial for the N-CNT double helix formation. The morphologies of N-CNTs can be well tuned between bamboo-like and cup-stacked structures, and a CNT/N-CNT heterojunction can be constructed by changing the carbon feedstock from C₂H₄ to CH₃CN during CVD growth. N-CNT double helices with a length of 10–36 μm, a screw pitch of 1–2 μm, a CNT diameter of 6–10 nm, and a N-content of 2.59 at.% can be synthesized on the LDH catalysts by the efficient CVD growth.     

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

Arrays of aligned carbon nanotubes (CNTs) and nitrogen-doped carbon (CN_x) 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 CN_x nanotube samples were determined from Mössbauer spectroscopy data. The total iron content in CN_x 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 CN_x 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.     

Growth of nitrogen-doped carbon nanotubes and fibers over a gold-on-alumina catalyst

Nitrogen-doped carbon nanotubes (N-CNTs) were synthesized by chemical vapor decomposition of a N,C-precursor over a supported Au catalyst. Large quantities of carbon nanotubes with a compartmentalized structure containing ca. 5 at.% N were produced over a 1.5% Au/δ-Al₂O₃ catalyst with a mean diameter of Au particles equal to 2.8 nm under continuous flow conditions at 800 °С and 1 bar of the reaction gas mixture containing pyridine (Py) vapor (5 vol.%), Н₂ (10–20 vol.%) and Ar (balance). The majority of the N-CNTs obtained after 10 min have outer diameters (ODs) of 13–45 nm and closed tips without any encapsulated gold particles of the catalyst which indicates the “base-growth” mechanism of N-CNT formation. Carbon deposits synthesized for 30–135 min contain carbon fibers with OD values up to several micrometers, formed by self-assembling of N-CNTs, and individual N-CNTs. X-ray photoelectron spectra provide evidence for various chemical states of nitrogen (pyridinic, “quaternary” (graphitic) and pyrrolic nitrogen) in the N-CNTs, and these are discussed.     

氮掺杂石墨烯/碳纳米管/无定形炭复合材料制备及其电化学性能

碳基复合材料被认为是超级电容器广泛应用最有前景的电极材料之一。本文使用氧化石墨烯（GO）、硝酸钴［Co(NO₃)₂］、三聚氰胺为原料,利用钴对高温下热解碳源的催化作用,制备得到了氮掺杂石墨烯/碳纳米管/无定形炭（NC）复合材料,并测试了其电化学性能。探究了金属和三聚氰胺添加量对碳基复合材料结构和性能的影响,研究发现,在添加量分别为0.02mmol和0.3g时,制得的样品具有大比表面积（380.5m²/g）和高掺氮质量分数（6.29%）,并在三电极系统中体现出优异的电化学性能,电流密度为0.5A/g时样品的比电容为137.1F/g,5A/g时比电容为113.5F/g,保持率为88.5%,具有优异的倍率性能,在循环5000圈后样品的容量保持率为104%,具有良好的循环稳定性,这归因于三维结构可以加快充放电过程中的离子转移和氮掺杂可提高材料润湿性和贡献部分赝电容,为超级电容器电极材料的制备提供了理论借鉴。     

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.     

In situ synthesis of iron-filled nitrogen-doped carbon nanotubes and their magnetic properties

Iron-filled nitrogen-doped carbon (Fe@CN_x) nanotubes were prepared by an in situ chemical vapor deposition with ferrocene as catalyst and ethylenediamine as carbon and nitrogen sources. The as-grown products were characterized using an X-ray diffractometer, transmission electron microscope, X-ray photoelectron spectroscopy, thermogravimetric analyzer and vibrating sample magnetometer. It was found that the “bamboo-like” N-doped carbon nanotubes were filled with 41.1 wt% of Fe nanoparticles. The coercivity and saturation magnetization of Fe@CN_x nanotubes were much larger than those of carbon nanotubes and CN_x nanotubes.     

Effect of nitrogen atomic percentage on N+-bombarded MWCNTs in cytocompatibility and hemocompatibility

N⁺-bombarded multi-walled carbon nanotubes (N⁺-bombarded MWCNTs), with different nitrogen atomic percentages, were achieved by different N ion beam currents using ion beam-assisted deposition (IBAD) on MWCNTs synthesized by chemical vapor deposition (CVD). Characterizations of N⁺-bombarded MWCNTs were evaluated by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectroscopy, and contact angle. For comparison, the in vitro cytocompatibility of the N⁺-bombarded MWCNTs with different N atomic percentages was assessed by cellular adhesion investigation using human endothelial cells (EAHY926) and mouse fibroblast cells (L929), respectively. The results showed that the presence of nitrogen in MWCNTs accelerated cell growth and proliferation of cell culture. The higher nitrogen content of N⁺-bombarded MWCNTs, the better cytocompatibility. In addition, N⁺-bombarded MWCNTs with higher N atomic percentage displayed lower platelet adhesion rate. No hemolysis can be observed on the surfaces. These results proved that higher N atomic percentage led N⁺-bombarded MWCNTs to better hemocompatibility.     

Tailoring the structure and nitrogen content of nitrogen-doped carbon nanotubes by water-assisted growth

Vertically aligned nitrogen-doped carbon nanotubes (NCNTs) were synthesized by the pyrolysis of acetonitrile in the presence of ferrocene catalyst (2 wt.%) with water assistance. Herein, we demonstrated that water could be a special tool to shape the structure of the NCNTs and it can be injected as much as 33.3 wt.% to grow NCNTs. More significantly, the nitrogen content of NCNTs and their tubular microstructures including tube diameter and wall thickness can be finely tailored by water introduction, which may further modulate their film electrical conductivity and electrocatalytic activities for oxygen reduction reactions.     

High-pressure pyrolysis of melamine route to nitrogen-doped conical hollow and bamboo-like carbon nanotubes

Nitrogen-doped conical hollow and bamboo-like carbon nanotubes (CNTs) have been prepared by pyrolysis of melamine with NaN₃–Fe–Ni and Ni catalysts at high temperature and high pressure, respectively. The conical hollow CNTs with an average diameter of about 70 nm and a length up to 5 μm account for ∼50% of the product, whose N/C atomic ratios are about 0.27. The conical bamboo-like CNTs with the diameter of ∼65 nm and length of 1–4 μm and wall thickness of 10–20 nm account for ∼95% of the product, whose N/C atomic ratios are up to 0.18. The control experiments show that NaN₃ plays a key role in keeping high nitrogen content and high conversion ratio in the CNTs. The possible growth mechanisms have been discussed on the base of the experimental observation. The strategy provides an alternative route to nitrogen-doped CNTs and other carbon nitride materials.     

Controlled platinum nanoparticles uniformly dispersed on nitrogen-doped carbon nanotubes for methanol oxidation

This work presents the synthesis of platinum nanoparticles (Pt NPs) and their subsequent deposition on the nitrogen-doped carbon nanotubes, which have been directly grown on a carbon cloth (CNT-CC electrode). The CNT-CC electrode provides a fast electron-transfer path to the carbon cloth, resulting in energy-loss reduction and enhancing catalytic activity of Pt NPs. The N-dopants in CNT serve as the defect sites to enhance nucleation of Pt particles. The reduction of the Pt precursor salt was carried out in the ethylene glycol solution at an elevated temperature. In order to control the Pt NP size, the pH of the reaction solution was controlled by the addition of NaOH. Zeta potential measurements of the as-prepared sample indicate that a higher zeta potential results in a smaller particle size, due to a stronger electrostatic repulsion between NPs. This serves a powerful tool for size control of the Pt nanoparticle. The Pt NPs dispersed on the CNT-CC have an average size of 2.81 nm (Pt/CNT-CC) prepared using 15 mM NaOH, with high uniformity under electron microscopy. Cyclic voltammetry measurements of the electrocatalytic activity of the Pt/CNT-CC for methanol oxidation indicate that it exhibits excellent electrocatalytic activity and are ideal for direct methanol fuel cell applications.     

Effects of nitrogen-doped carbon nanotubes on the discharge performance of Li-air batteries

Carbon nanotubes (CNTs) and nitrogen-doped carbon nanotubes (N-CNTs) were synthesized using a floating catalyst chemical vapor deposition method and characterized by scanning electron microscopy (SEM), transmission electron microscopy, Raman and X-ray photoelectron spectroscopy. The study found that the as-prepared CNTs and N-CNTs showed different discharge capacity as cathode materials in Li-air battery. To further study the reason why N-doping improves the electrochemical performance exceptionally, the discharge products on the two kinds of nanotubes were detected by SEM, XRD and Raman. SEM study showed, for the first time, that more uniform distribution of discharge products on the surface of CNTs arising from N-doping affected the boost of discharge capacity, a result which was discussed in detail. In comparison to non-doped CNTs, nitrogen doping was considered to be a promising way to improve the performance of carbon based cathode material for Li-air batteries.     

Selective oxidation of glycerol over nitrogen-doped carbon nanotubes supported platinum catalyst in base-free solution

In this communication, N-doped multiwall carbon nanotube (N-MWCNT) supported Pt NPs (1.8 nm) were prepared via a facile routine under microwave irradiation and tested in the selective oxidation of glycerol in an aqueous base-free solution. Characterizations confirmed that N-MWCNTs could improve the dispersion of Pt through strengthened metal-support interactions and donate its electron to metallic Pt. This electron-enriched Pt NPs on the surface of N-MWCNTs is active and stable for the selective oxidation of glycerol.     

PtRu nanoparticles supported on nitrogen-doped multiwalled carbon nanotubes as catalyst for methanol electrooxidation

Nitrogen-doped carbon nanotubes (N-CNT) obtained by plasma treatment were compared to the conventional acid-treated carbon nanotubes (O-CNT) as catalyst support for platinum-ruthenium (PtRu) nanoparticles in the anodic oxidation of methanol in direct methanol fuel cells. PtRu catalysts were prepared by an impregnation-reduction method from chloride precursors with metal loadings of 20 wt.%, and were characterised by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical methods. Voltammetry and chronoamperometry studies showed that the performance of PtRu/N-CNT was significantly higher compared to PtRu/O-CNT and also to the commercial E-TEK PtRu/C catalyst, indicating that N-CNT are an interesting support material for fuel cell electrocatalyst. Nitrogen plasma treatment produced pyridinic and pyrrollic species on the CNT surface, which acts as the anchoring sites for the deposition of PtRu particles. A mechanism for the deposition of PtRu on N-CNT is tentatively proposed and discussed.     

Synthesis of nitrogen-doped horn-shaped carbon nanotubes by reduction of pentachloropyridine with metallic sodium

Nitrogen-doped horn-shaped carbon nanotubes (CNTs) have successfully been prepared by reducing pentachloropyridine with metallic sodium at 350 °C. A typical CNT has an open-end diameter of ∼2 μm, a close-end diameter of ∼0.3 μm, a wall thickness of ∼30 nm, and a length up to 8 μm. TEM observation indicates that the CNTs account for ∼30% of the products, and the rest is solid and hollow carbon nanospheres (CNSs) with a diameter of about 50-290 nm. Elemental analysis shows that the N/C atomic ratio of the carbon nanostructures is about 0.0208. XRD and HRTEM measurements reveal that the CNTs are amorphous. To understand the growth process and refine the growth condition, various control experiments have been finished. At last, a sodium-catalysis-reduction solid-liquid-solid growth mechanism of the CNTs has been suggested on the basis of the experiments.     

The effect of temperature on the morphology and chemical surface properties of nitrogen-doped carbon nanotubes

One of the most effective ways to tune the physical and chemical properties of CNTs is to dope them with a foreign element, such as nitrogen. Nitrogen atoms that are incorporated into the CNT structure can drastically change the properties of CNTs. The properties of nitrogen-doped carbon nanotubes (N-CNTs) originate from their structure; thus, it is practical to create a desired structure during synthesis. We have investigated the effects of synthesis parameters, mainly temperature, on various characterizations of N-CNTs, such as their morphologies, dimensions (diameter and length), defects, nitrogen inclusions and thermal stability. The results revealed strong correlations between the synthesis parameters and the properties of the synthesized N-CNTs. XPS characterization indicated that the percentage of nitrogen inclusion decreased with increasing synthesis temperature up to 850 °C and then increased at 950 °C. Raman spectroscopy showed a decrease in the number of defects in the N-CNT structure with increasing synthesis temperature. Finally TGA demonstrated a trend of increasing thermal stability of N-CNTs with increasing synthesis temperature, up to 850 °C then a reduced thermal stability at 950 °C. Based on these results, one can control the properties of N-CNTs and obtain materials with the desired characteristics by choosing the appropriate synthesis conditions.