RESUMO
The article gives a comprehensive overview of hydrogen storage in carbon nanostructures, including experimental results and theoretical calculations. Soon after the discovery of carbon nanotubes in 1991, different research groups succeeded in filling carbon nanotubes with some elements, and, therefore, the question arose of filling carbon nanotubes with hydrogen by possibly using new effects such as nano-capillarity. Subsequently, very promising experiments claiming high hydrogen storage capacities in different carbon nanostructures initiated enormous research activity. Hydrogen storage capacities have been reported that exceed the benchmark for automotive application of 6.5 wt% set by the U.S. Department of Energy. However, the experimental data obtained with different methods for various carbon nanostructures show an extreme scatter. Classical calculations based on physisorption of hydrogen molecules could not explain the high storage capacities measured at ambient temperature, and, assuming chemisorption of hydrogen atoms, hydrogen release requires temperatures too high for technical applications. Up to now, only a few calculations and experiments indicate the possibility of an intermediate binding energy. Recently, serious doubt has arisen in relation to several key experiments, causing considerable controversy. Furthermore, high hydrogen storage capacities measured for carbon nanofibers did not survive cross-checking in different laboratories. Therefore, in light of today's knowledge, it is becoming less likely that at moderate pressures around room temperature carbon nanostructures can store the amount of hydrogen required for automotive applications.
Assuntos
Hidrogênio/química , Modelos Moleculares , Nanotecnologia/métodos , Nanotubos de Carbono/química , Adsorção , Simulação por Computador , Fontes de Energia Elétrica , Eletroquímica/métodos , Hidrogênio/isolamento & purificação , Ligação de Hidrogênio , Substâncias Macromoleculares , Modelos Químicos , Conformação Molecular , Nanotubos de Carbono/isolamento & purificaçãoRESUMO
The high-pressure carbon monoxide (HiPco) technique for producing single-wall carbon nanotubes (SWNTs) is analyzed with the use of a chemical reaction model coupled with flow properties calculated along streamlines, calculated by the FLUENT code for pure carbon monoxide. Cold iron pentacarbonyl, diluted in CO at about 30 atmospheres, is injected into a conical mixing zone, where hot CO is also introduced via three jets at 30 degrees with respect to the axis. Hot CO decomposes the Fe(CO)5 to release atomic Fe. Then iron nucleates and forms clusters that catalyze the formation of SWNTs by a disproportionation reaction (Boudouard) of CO on Fe-containing clusters. Alternative nucleation rates are estimated from the theory of hard sphere collision dynamics with an activation energy barrier. The rate coefficient for carbon nanotube growth is estimated from activation energies in the literature. The calculated growth was found be about an order of magnitude greater than measured, regardless of the nucleation rate. A study of cluster formation in an incubation zone prior to injection into the reactor shows that direct dimer formation from Fe atoms is not as important as formation via an exchange reaction of Fe with CO in FeCO.
Assuntos
Monóxido de Carbono/química , Cristalização/métodos , Ferro/química , Modelos Químicos , Nanotubos de Carbono/química , Catálise , Simulação por Computador , Cristalização/instrumentação , Substâncias Macromoleculares , Modelos Moleculares , Conformação Molecular , Nanotubos de Carbono/isolamento & purificação , Pressão , Reologia/métodos , Temperatura , VolatilizaçãoRESUMO
The high-pressure carbon monoxide (HiPco) process for producing single-wall carbon nanotubes (SWNTs) uses iron pentacarbonyl as the source of iron for catalyzing the Boudouard reaction. Attempts using nickel tetracarbonyl led to no production of SWNTs. This paper discusses simulations at a constant condition of 1300 K and 30 atm in which the chemical rate equations are solved for different reaction schemes. A lumped cluster model is developed to limit the number of species in the models, yet it includes fairly large clusters. Reaction rate coefficients in these schemes are based on bond energies of iron and nickel species and on estimates of chemical rates for formation of SWNTs. SWNT growth is measured by the conformation of CO2. It is shown that the production of CO2 is significantly greater for FeCO because of its lower bond energy as compared with that of NiCO. It is also shown that the dissociation and evaporation rates of atoms from small metal clusters have a significant effect on CO2 production. A high rate of evaporation leads to a smaller number of metal clusters available to catalyze the Boudouard reaction. This suggests that if CO reacts with metal clusters and removes atoms from them by forming MeCO, this has the effect of enhancing the evaporation rate and reducing SWNT production. The study also investigates some other reactions in the model that have a less dramatic influence.
Assuntos
Monóxido de Carbono/química , Cristalização/métodos , Ferro/química , Modelos Químicos , Nanotubos de Carbono/química , Níquel/química , Carbono/química , Dióxido de Carbono/química , Simulação por Computador , Metais/química , Modelos Moleculares , Nanotecnologia/métodos , Nanotubos de Carbono/classificação , Nanotubos de Carbono/isolamento & purificação , Pressão , VolatilizaçãoRESUMO
Diagnostics of carbon arc plasma by optical emission spectroscopy during the synthesis of carbon nanotubes is reviewed. Spatial distributions of temperature and C2 radicals in different plasmas are presented. The influence of gas pressure, anode composition, and reaction environment is discussed. Mechanisms of carbon nanotube formation are reviewed, with an emphasis on surface diffusion processes and catalytic effects.
Assuntos
Cristalização/métodos , Gases/química , Nanotubos de Carbono/química , Nanotubos de Carbono/isolamento & purificação , Análise Espectral/métodos , Carbono/química , Cristalografia/métodos , Difusão , Grafite/química , Temperatura Alta , Conformação Molecular , Nanotecnologia/métodos , Nanotubos de Carbono/classificação , Tamanho da Partícula , Soluções/química , Propriedades de SuperfícieRESUMO
The chemical stability of fluoronanotubes in selected solid inorganic matrixes has been studied by initially mixing and mechanically grinding the components and subsequently heating them at temperatures ranging from 35 to 600 degrees C. The inorganic compounds selected for matrixes included halides (KBr, KI, Lil, LiBr, LiCl, NaCl, Znl2), oxides (Li2O, Fe2O3, PbO, MnO), lithium peroxide (Li2O2), potassium superoxide (KO2), sulfides (Li2S and ZnS), zinc selenide (ZnSe), lithium nitride (Li3N), and aluminum phosphide (AIP). Solid products, resulting from the proceeding chemical reactions, were analyzed by X-ray diffraction, Raman spectroscopy, and SEM/EDX elemental analysis. Gaseous and volatile products were identified with the help of the TGA/MS technique. Experimental data presented in this paper provide clear evidence that fluoronanotubes are not chemically inert toward the solid matrixes studied and exhibit significant oxidative properties in the redox reactions occurring under various temperatures, depending on the nature of the inorganic compound.
Assuntos
Cristalização/métodos , Flúor/química , Teste de Materiais/métodos , Nanotubos de Carbono/química , Flúor/isolamento & purificação , Fluorocarbonos/síntese química , Fluorocarbonos/química , Compostos Inorgânicos/química , Conformação Molecular , Nanotecnologia/métodos , Nanotubos de Carbono/isolamento & purificação , Oxirredução , Espectroscopia de Infravermelho com Transformada de Fourier , Análise Espectral Raman , TemperaturaRESUMO
Well-aligned nitrogen-doped multiwall carbon nanotube arrays have been successfully grown over large areas on quartz and silicon wafers by floating-catalyst chemical vapor deposition at low temperatures (600 degrees C). These nitrogen-including nanotubes, derived from pyridine-ferrocene mixtures, have smaller outer diameters but larger inner diameters compared with carbon nanotubes grown from a xylene-ferrocene mixture under similar conditions. The N-doped nanotubes exhibit bamboo-like structures in the core. Elemental analysis and electron energy loss spectroscopy analysis show that the as-prepared nanotubes contain as much as 2.62 wt.% N, with most of the N concentrated in the inner few shells of the nanotube. Such large-scale arrays of well-aligned N-doped nanotubes on silicon wafers have a current density as high as 23.8 mA/cm2 at an applied electric field of 17 V/micron, which can be further improved by patterning the tubes and coating the silicon substrate with a conductive thin metal film for the fabrication of flat panel displays.
Assuntos
Cristalização/métodos , Nanotecnologia/métodos , Nanotubos de Carbono/química , Nitrogênio/química , Condutividade Elétrica , Eletroquímica/métodos , Temperatura Alta , Substâncias Macromoleculares , Teste de Materiais/métodos , Conformação Molecular , Nanotubos de Carbono/isolamento & purificação , Quartzo/química , Silício/química , Propriedades de Superfície , TemperaturaRESUMO
Individual, free-standing, vertically aligned multiwall carbon nanotubes or nanofibers are ideal for sensor and electrode applications. Our plasma-enhanced chemical vapor deposition techniques for producing free-standing and vertically aligned carbon nanofibers use catalyst particles at the tip of the fiber. Here we present a simple purification process for the removal of iron catalyst particles at the tip of vertically aligned carbon nanofibers derived by plasma-enhanced chemical vapor deposition. The first step involves thermal oxidation in air, at temperatures of 200-400 degrees C, resulting in the physical swelling of the iron particles from the formation of iron oxide. Subsequently, the complete removal of the iron oxide particles is achieved with diluted acid (12% HCl). The purification process appears to be very efficient at removing all of the iron catalyst particles. Electron microscopy images and Raman spectroscopy data indicate that the purification process does not damage the graphitic structure of the nanotubes.
Assuntos
Cristalização/métodos , Nanotecnologia/métodos , Nanotubos de Carbono/química , Nanotubos de Carbono/isolamento & purificação , Catálise , Compostos Férricos/química , Temperatura Alta , Ferro/química , Substâncias Macromoleculares , Conformação Molecular , Oxirredução , Pressão , Controle de Qualidade , Propriedades de Superfície , Temperatura , VolatilizaçãoRESUMO
Pyrene was introduced into cavities in functionalized single-walled and multiple-walled carbon nanotubes to be used as a molecular probe in the study of encapsulation. The solubility of these materials in common organic solvents allowed solution-phase absorption and emission spectroscopic measurements. The results, which are consistent with the formation of pyrene excimer, are explained in terms of high local pyrene concentrations and perhaps pyrene microcrystals inside the carbon nanotube cavities. The fluorescence decay results show that there is significant quenching of pyrene excited states by the hosting carbon nanotubes.
Assuntos
Cristalização/métodos , Composição de Medicamentos/métodos , Nanotecnologia/métodos , Nanotubos de Carbono/química , Pirenos/química , Materiais Revestidos Biocompatíveis/síntese química , Materiais Revestidos Biocompatíveis/química , Espectroscopia de Ressonância Magnética/métodos , Teste de Materiais/métodos , Conformação Molecular , Nanotubos de Carbono/isolamento & purificação , Espectrometria de Fluorescência/métodosRESUMO
We have previously shown that high-purity multiwalled carbon nanotubes (pristine MWNTs) can be prepared from a mixture of xylene-ferrocene (99 at% C:1 at% Fe) inside a quartz tube reactor operating at approximately 700 degrees C. In a similar process, approximately 3 g of melamine (C3H6N6) was introduced during the growth of MWNTs to prepare nitrogen-doped nanotubes. The structural and electronic properties of nitrogen-doped MWNTs were determined by scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and thermopower measurements. The individual nitrogen-doped nanotube exhibits a bamboo-like structure and comprises 6-16 tube walls, as evidenced by HRTEM studies. The EELS measurements yielded an average nitrogen content of approximately 5 at% in the doped tubes. The thermoelectric power data of nitrogen-doped MWNTs remained negative even after exposure to oxygen for an extended period of time, suggesting that nitrogen doping of MWNTs renders them n-type, consistent with scanning tunneling spectroscopic studies on similar nanotubes.
Assuntos
Cristalização/métodos , Nanotecnologia/métodos , Nanotubos de Carbono/química , Nitrogênio/química , Condutividade Elétrica , Eletroquímica/métodos , Substâncias Macromoleculares , Teste de Materiais/métodos , Conformação Molecular , Nanotubos de Carbono/isolamento & purificação , Propriedades de SuperfícieRESUMO
Carbon nanotubes were produced from either a template or the polymer-filled pore systems of mesoporous silicates of various structures and dimensions by heat treatment in the absence of air. Successful synthesis was done when the template molecules contained little or no oxygen. For SBA-15 material, where the structure-directing molecule used for synthesis of mesoporous silicate was polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer, no carbon nanostructures were formed. A peculiar carbon nanostructure was generated from the template for pore expanded MCM-41. To demonstrate carbon nanotube formation from polymer in the mesoporous silicates, the mesopores of MCM-41, MCM-48, and SBA-15 silicates were filled with divinyl-benzene polymer and then graphitized at 1300 K. The polymer was successfully transformed into carbon nanotubes for the MCM class silicate but not the SBA-15 silicate.
Assuntos
Cristalização/métodos , Nanotecnologia/métodos , Nanotubos de Carbono/química , Nanotubos de Carbono/isolamento & purificação , Silicatos/química , Catálise , Teste de Materiais , Metais/química , Conformação Molecular , Permeabilidade , Porosidade , Propriedades de Superfície , Temperatura , Elementos de Transição/químicaRESUMO
We report the pressure dependence for the radial (omega R) and tangential (omega T) band frequencies in debundled single-walled carbon nanotubes (SWNTs) derived from laser-synthesized SWNT bundles. As previously described, a chemical procedure was used to prepare debundled SWNTs from as-prepared, large SWNT bundles. The normalized pressure coefficient for omega R in the debundled sample was compared with the corresponding value in the bundled sample to quantify the strength of van der Waals interactions between tubes in these nanotube materials. Furthermore, the pressure dependences for the radial (omega R) and tangential (omega T) band frequencies in debundled tubes were also compared with corresponding dependences predicted for isolated SWNTs, obtained with generalized tight binding molecular dynamic (GTBMD) simulations described in our previous work. The results presented here collectively suggest that the van der Waals interaction is still strong in the debundled sample studied here, which contained predominantly small bundles of SWNTs rather than isolated tubes.
Assuntos
Cristalização/métodos , Teste de Materiais/métodos , Nanotubos de Carbono/química , Nanotubos de Carbono/classificação , Análise Espectral Raman/métodos , Catálise , Metais/química , Conformação Molecular , Nanotecnologia/métodos , Nanotubos de Carbono/isolamento & purificação , Pressão , Semicondutores , Propriedades de SuperfícieRESUMO
Single-walled carbon nanotubes prepared by disproportionation of CO over Co-Mo/SiO2 catalysts have been characterized by Raman spectroscopy, using several excitation energies. By varying the reaction temperature, different ranges of nanotube diameter were obtained. The average diameter of a single-walled nanotube produced at 750 degrees C was 0.9 nm, while it increased up to about 1.5 nm when the synthesis was conducted at 950 degrees C. The analysis of the Raman spectra obtained with a range of laser excitation energies not only gives a definite description of the single-walled nanotubes diameters but also helps differentiate the metallic or semiconducting character of the samples. This analysis can be done by comparing the experimental data with calculated gap energies as a function of nanotube diameter as well as comparing the relative intensity of bands centered at 50-60 cm-1 lower than the tangential G mode. The analysis of this feature, which can be fitted with a Breit-Wigner-Fano line, offers a method for distinguishing between metallic and semiconducting single-walled carbon nanotubes.
Assuntos
Cristalização/métodos , Teste de Materiais/métodos , Nanotubos de Carbono/química , Nanotubos de Carbono/classificação , Análise Espectral Raman/métodos , Monóxido de Carbono/química , Catálise , Cobalto/química , Temperatura Alta , Metais/química , Conformação Molecular , Molibdênio/química , Nanotecnologia/métodos , Nanotubos de Carbono/isolamento & purificação , Semicondutores , Dióxido de Silício/química , Propriedades de Superfície , TemperaturaRESUMO
We report the production and characterization of polymer nanocomposites with single-walled carbon nanotubes having improved mechanical properties and exceptional nanotube alignment. High-pressure carbon monoxide nanotubes (HiPco) were efficiently distributed in polystyrene (PS) and polyethylene (PE) with a twin-screw compounder. Nanotube concentrations were 1, 5, 10, and 20 wt% in PE composites and 0.7 wt% in PS composites. PE composites were melt-spun into fibers to achieve highly aligned nanotubes. Polarized Raman spectroscopy shows that the degree of alignment increases with decreasing fiber diameter and decreases with increasing nanotube loading. The orientation distribution function of a 1 wt% HiPco/PE composite had a full width at half-maximum of approximately 5 degrees. The elastic modulus increases up to 450% relative to PE fibers for 20 wt% nanotube loading at an intermediate fiber diameter of 100 microns.
Assuntos
Cristalização/métodos , Nanotubos de Carbono/química , Nanotubos de Carbono/isolamento & purificação , Polietilenos/química , Poliestirenos/química , Anisotropia , Elasticidade , Manufaturas , Modelos Químicos , Conformação Molecular , Nanotecnologia/instrumentação , Nanotecnologia/métodos , Polímeros/química , Estresse MecânicoRESUMO
The ionic surfactant-assisted dispersion of single-walled carbon nanotubes in aqueous solution has been studied by Raman and fluorescent spectroscopy during ultrasonic processing. During the process, an equilibrium is established between free individuals and aggregates or bundles that limits the concentration of the former that is possible. This equilibrium is a function of free sodium dodecyl sulfate concentration. At surfactant concentrations below this value, fluorescence is shifted to a lower energy due to an increase in micropolarity from water association at the nanotube surface. The mechanism of dispersion is postulated as the formation of gaps or spaces at the bundle ends in the high shear environment of the ultrasonicated solution. Surfactant adsorption and diffusion then propagate this space along the bundle length, thereby separating the individual nanotube. The former is found to be controlling, with the use of a derived kinetic model for the dispersion process and extraction of the characteristic rate of nanotube isolation.
Assuntos
Nanotecnologia/métodos , Nanotubos de Carbono/isolamento & purificação , Nanotubos de Carbono/efeitos da radiação , Sonicação , Tensoativos/química , Adsorção , Coloides/química , Coloides/efeitos da radiação , Difusão , Modelos Químicos , Nanotubos de Carbono/química , Nanotubos de Carbono/classificação , Espectrometria de Fluorescência , Análise Espectral Raman , Propriedades de Superfície , Tensoativos/efeitos da radiaçãoRESUMO
Double-walled carbon nanotubes (DWNTs) may be interesting in many applications since the outer wall would provide an interface with the rest of the system, without modifying the inner wall. CNT-Fe/Fe3C-Al2O3 composite powders containing carbon nanotubes (CNTs) (65% of which are DWNTs) are prepared by reduction of an oxide solid solution in a H2-CH4 gas mixture. The powders and CNTs are studied by both local and macroscopical techniques. The influence of the reducing atmosphere composition and of the dwell time at 1050 degrees C is studied. There is a 6-fold increase in CNT content upon the increase in the CH4 content from 3 to 30 mol.%, but the formation of undesirable carbon nanofibers can also be promoted. A CH4 content of 12-18 mol.% is adapted for the particular iron content in these powders. Increasing the dwell time at 1050 degrees C results in the formation of CNTs with more walls.
Assuntos
Cristalização/métodos , Temperatura Alta , Hidrogênio/química , Metano/química , Nanotubos de Carbono/química , Óxido de Alumínio/química , Cristalografia/métodos , Ferro , Manufaturas , Microscopia Eletrônica , Microscopia Eletrônica de Varredura , Conformação Molecular , Nanotecnologia/métodos , Nanotubos de Carbono/classificação , Nanotubos de Carbono/isolamento & purificação , Pós , Controle de Qualidade , Propriedades de Superfície , TermogravimetriaRESUMO
A generalization of the Anderson model that includes pseudo-Jahn-Teller impurity coupling is proposed to describe distortions of an endohedral impurity in a carbon nanotube. Within mean-field theory, spontaneous axial symmetry breaking is found when the vibronic coupling strength g exceeds a critical value. The effective potential is found to have O(2) symmetry, in agreement with numerical calculations. For metallic zigzag nanotubes endohedrally doped with transition metals in the dilute limit, the low-energy properties of the system may display two-channel Kondo behavior; however, strong vibronic coupling is seen to exponentially suppress the Kondo energy scale.
Assuntos
Modelos Químicos , Nanotubos de Carbono/química , Modelos Moleculares , Nanotubos de Carbono/isolamento & purificaçãoRESUMO
Molecular dynamics of hydrogen trapped in single-walled carbon nanotube bundles was analyzed by nuclear magnetic resonance. The chemical shift of hydrogen was about 5.1 ppm at 293 K, which is similar to that of water. The relaxation time, T1, was about 0.1-0.2 s. Values in this work are comparable to those for hydrogen loaded in silica and a-Si.
Assuntos
Cristalização/métodos , Hidrogênio/química , Espectroscopia de Ressonância Magnética/métodos , Nanotecnologia/métodos , Nanotubos de Carbono/química , Adsorção , Nanotubos de Carbono/isolamento & purificação , Porosidade , Prótons , Propriedades de SuperfícieRESUMO
Plasma-enhanced chemical vapor deposition is used to grow vertically aligned multiwalled carbon nanofibers (MWNFs). The graphite basal planes in these nanofibers are not parallel as in nanotubes; instead they exhibit a small angle resembling a stacked cone arrangement. A parametric study with varying process parameters such as growth temperature, feedstock composition, and substrate power has been conducted, and these parameters are found to influence the growth rate, diameter, and morphology. The well-aligned MWNFs are suitable for fabricating electrode systems in sensor and device development.
Assuntos
Carbono/química , Cristalização/métodos , Nanotecnologia/métodos , Nanotubos de Carbono/química , Carbono/isolamento & purificação , Fibra de Carbono , Gases/química , Microscopia Eletrônica de Varredura , Conformação Molecular , Nanotecnologia/instrumentação , Nanotubos de Carbono/isolamento & purificação , Pressão , Controle de Qualidade , Análise Espectral Raman , Propriedades de Superfície , Temperatura , VolatilizaçãoRESUMO
For the first time, multiwalled carbon nanotubes (MWNTs) could be formed selectively in a high yield, free of any disordered carbon by-product, from the catalytic decomposition of acetylene at 600 degrees C on a CoxMg(1-x)O solid solution. Starting from 1 g of catalytic substrate, 4 g of pure MWNTs were obtained after its dissolution in boiling concentrated HCl, without any additional purification in strongly oxidizing medium, as is required for other methods of nanotube production. In situ reduction of CoO by dihydrogen liberated from acetylene decomposition allows highly divided metal particles to be continuously produced as synthesis proceeds. This is undoubtedly the reason for the good performance of the catalyst and for the ability to produce nanotubes in a narrow diameter range, namely from 10 to 15 nm. With the use of acetylene instead of methane, the synthesis proceeds at low temperature, which prevents the growth of carbon shells, in which the metal particles are generally embedded, decreasing their activity. Because of the very low specific surface area of the catalyst support, the amount of disordered carbon by-product formed is negligible.