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.     

Fluffy carbon nanotubes produced by shearing vertically aligned carbon nanotube arrays

Fluffy carbon nanotubes (CNTs), which are cotton-like macroscopic structures, are obtained by simple high-speed shearing of vertically aligned CNT (VACNT) arrays. The fluffy CNTs are composed of CNT bundles with a diameter of several micrometers, and have an extremely low apparent density of 3-10 g/L. A requisite for their formation is the alignment of CNTs in the initial array. The shear between the rotor and the arrays tears the arrays along the axial direction and this results in their dispersion into low density fluffy CNTs.     

Differences in cytocompatibility and hemocompatibility between carbon nanotubes and nitrogen-doped carbon nanotubes

The cytocompatibility and hemocompatibility of multiwalled carbon nanotubes and N-doped multiwalled carbon nanotubes grown on carbon papers by chemical vapor deposition were investigated. These materials were characterized using contact-angle measurements, cell- and platelet-adhesion assays, and hemolytic-rate testing, revealing significant effects of nitrogen doping in carbon nanotubes. The results showed that mouse fibroblast cells and mouse adipose-derived stem cells cultured on N-doped multiwalled carbon nanotubes displayed the higher cell-adhesion strength, viability, proliferation, and stretching than those on multiwalled carbon nanotubes without N doping and carbon paper, indicating that N-doped multiwalled carbon nanotubes possessed good cytocompatibility. No toxicity reactions were observed during the culturing period. It also displayed the lowest hemolytic rate.     

Synthesis of aligned carbon nanotubes

Vertically aligned carbon nanotubes (ACNTs) are bundles of carbon nanotubes oriented perpendicular to a substrate, and horizontally aligned CNTs are parallel to the substrate. Their dense and orderly arrangement, along with outstanding physical and chemical properties, enables ACNTs to be used in various fields. The methods of synthesising ACNTs can be classified into single-step and double-step techniques. Thermal pyrolysis and flame synthesis are the common single-step methods, and both are relatively simple. The double-step methods, including catalyst coating and chemical vapour deposition, provide more control over the catalyst morphology. This review explores different methods used for ACNT growth, the process parameters that determine the morphology of ACNTs and the applications of structured ACNTs.     

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.     

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 vertically aligned, double-walled carbon nanotubes from highly active Fe-V-O nanoparticles

Monodispersed Fe-V-O nanoparticles were prepared by a liquid-phase synthesis to be used as catalysts for carbon nanotube (CNT) growth. Vertically aligned, dense CNTs have been grown from the highly active Fe-V-O nanoparticles by chemical vapor deposition. Diameter distribution of CNTs (3.7 ± 0.6 nm) was consistent with that of the original nanoparticles (3.1 ± 0.5 nm), and the value was smaller than those of other reported vertically aligned CNTs from as-prepared nanoparticles. TEM study showed that the CNTs consisted mainly of double-walled CNTs (single: 14%, double: 74%, and triple: 12%). The CNT diameter increased to 4.4 ± 0.8 nm as the growth temperature was increased from 810 to 870 °C. Energy dispersive X-ray spectroscopy of nanoparticles before and after the CNT growth revealed that the V content decreased from 7.2 to 2.7 at.%, suggesting that the segregation of Fe and V played an important role for the high activity of the Fe-V-O nanoparticles.     

Industrial compatible re-growth of vertically aligned multiwall carbon nanotubes by ultrafast pure oxygen purification process

Reproducible high-yield purification process of multiwalled carbon nanotubes (CNTs) was developed by thermal annealing in ultrapure oxygen. The optimized condition involves thermal annealing via a PID controlled heater in high purity oxygen at temperature of 450 °C for 180 s, which burns out amorphous carbon and reactivate nickel catalyst. Controlling of the purification temperature is important for high yield CNTs, as excessive high annealing temperature results in deformation of the CNTs. Unlike hazardous wet purification treatments, purified CNTs remained vertically aligned and offer the possibility of tailored CNTs length through layer-by-layer process. The purification process is glass compatible and can be adapted to industrial synthesis of CNTs.     

Interplay between exchange interaction and magnetic anisotropy for iron based nanoparticles in aligned carbon nanotube arrays

In this work, we investigate magnetic properties of iron based nanoparticles (NP) intercalated into carbon nanotube (CNT) aligned arrays synthesized by injection chemical vapor deposition. We have analyzed the temperature (T) and the ferrocene concentration (C_F) dependences of the macroscopic magnetic parameters. From these experiments a weaker interaction between magnetic moments of NP was obtained for low C_F values. The random anisotropy model for the experimental data analysis was applied and micromagnetic parameters were evaluated. The law of the approach to magnetic saturation (LAS) was analyzed using the general expression with the correlation function C(r = x/R_a) of magnetic axes, R_a being the magnetic anisotropy correlation length. We obtained that, while for C_F = 0.5% C(r) is a step-like (C(r < 10) = 1, C(r > 10) = 0), for C_F ⩾ 1% C(r) decays rapidly on a short range, (2-3)R_a. Such extended correlations for C_F = 0.5% could be associated with the dominant role of the coherent anisotropy, which is caused by the influence of the alignment of CNT. When the aligned CNTs for C_F = 0.5% are destroyed into powder, the LAS is changed to H^−1/2, which means the dominant role of the exchange mechanism.     

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.     

Important parameters for the catalytic nanoparticles formation towards the growth of carbon nanotube aligned arrays

Presented here is a systematic study on the experimental parameters involved in the formation of catalytic nanoparticles from homogeneous Ni films deposited by dc sputtering towards carbon nanotube (CNT) production on Si/SiO₂. We have found a critical temperature and time for the thermal and reduction pre-treatment processes to obtain catalyst nanoparticles with the appropriate size and high density suitable for CNT growth. From such nanoparticles, densely-packed aligned CNT arrays were successfully grown at 750–800°C by thermal CVD.     

Tribological properties of vertically aligned carbon nanotube arrays

Carbon nanotubes (CNTs) are a promising material for the fabrication of biomimetic dry adhesives. The dimensions of single CNTs are in the range of those of terminal elements of biological dry hairy adhesion systems, such as the setal branches on the toe of the gecko. Here, the tribological properties of densely packed arrays of vertically aligned and up to 1.1 mm long multi-walled CNTs (VACNTs) synthesized by chemical vapor deposition are examined. The coefficient of friction μ is as high as 5–6 at the first sliding cycle, and decreases down to stable values between 2 and 3 at the fourth to fifth sliding cycles. Such high values of μ can only be explained by the strong contribution of adhesion induced by applied shear force. After the tests, wear-induced deformations of the VACNT surface are observed, which strongly depend on the amount of normal force applied during the friction experiments. Interestingly, the plastic deformation of the VACNTs does not significantly affect μ after a preconditioning by a few sliding cycles. However, a strong decrease of μ during the initial wear cycles has to be taken into account for the development of applications, such as non-slip surfaces and pick-and-place techniques for manufacturing.     

Iron particles in carbon nanotubes

Nanometer-size iron-rich particles in carbon nanotubes have been studied by transmission electron microscopy with and without in situ and ex situ heating. Several remarkable results were found; a high temperature phase (γ-Fe) of iron stable at low temperatures and preferential presence of iron and iron carbide in carbon nanotubes. Based upon these experimental results, thermodynamics of the Fe-C phase diagram and its kinetics were used to explain the non-uniform distribution of iron and iron carbide, which also yielded a deeper insight into the formation of carbon nanotubes. Some of the results also allowed describing the role of the graphitic structure in retaining the high temperature phase (γ-Fe) of iron at low temperatures. Furthermore, methods have been demonstrated with which γ-Fe can be produced in carbon nanotubes intentionally or in a large quantity. Selected area electron diffraction patterns of iron inside nanotubes demonstrated the crystallographic relationship of the iron to the nanotube axis along with phase changes of the iron. This paper summarizes the findings and draws further conclusions on the particle shape inside multiwalled carbon nanotubes.     

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.     

Surface properties of vertically aligned carbon nanotube arrays

This study focuses on the structural changes of vertically aligned carbon nanotube (CNT) arrays while measuring their adhesive properties and wetting behaviour. CNT forests grown by chemical vapor deposition with a height of ~ 100 µm, an outer CNT diameter of ~ 10 nm and a density of the order of ~ 10¹⁰ CNTs/cm² show an average adhesion of 4 N/cm² when pressed against a glass surface. The applied forces lead to the collapse of the regular CNT arrays which limits their reusability as functional dry adhesives. Goniometric water contact angle (CA) measurements on CNT forests show a systematic decrease from an initial value of ~ 126° to a final CA similar to highly orientated graphite. Environmental scanning electron microscopy shows that this loss of hydrophobicity is due to an evaporation induced compaction of CNTs together with the loss of their vertical alignment. We observe the formation of cellular patterns for controlled drying.     

The production of horizontally aligned single-walled carbon nanotubes

The current progress on the production of aligned single-walled carbon nanotubes (SWCNTs), particularly the horizontally aligned ones, is reviewed. There are two main categories for the alignment of SWCNTs: the post synthesis assembly and the in situ growth approaches. The post synthesis assembly approach mainly involves dispersing SWCNTs in solutions and aligning SWCNTs using spin-coating, Langmuir–Blodgett assembly, mechanical shearing, or blown bubble film techniques. The in situ growth approach produces aligned SWCNTs directly during their growth using controlled chemical vapor deposition and arc discharge techniques. The latter approach has the advantage of avoiding the defects generated during the post treatment methods, and may also be combined with other growth controls such as structure selectivity of SWCNTs and direct device patterning for scale up applications.