Gas adsorption studies of CO2 and N2 in spatially aligned double-walled carbon nanotube arrays

Gas adsorption studies (CO₂ and N₂) over a wide pressure range on vertically, highly aligned dense double-walled carbon nanotube arrays of high purity and high specific surface area are reported. At high pressures, the adsorption capacity of these materials was found to be comparable to those of metal organic frameworks and mesoporous molecular sieves. These highly aligned CNT arrays were chemically modified by treating with oxygen plasma and structurally modified by decreasing the diameter of individual carbon nanotubes. Oxygen plasma treatment led to grafting of a large number of C–O functional groups onto the CNT surface, which further increased the gas adsorption capacity. It was found that gas adsorption is dependent on tube diameter and increases with decrease of the individual CNT diameter in the CNT bundles. As results of our studies we have found that at lower pressure regimes, plasma functionalized carbon nanotubes exhibit better adsorption characteristics whereas at higher pressures, lower diameter carbon nanotube structures exhibited better gas adsorption characteristics.     

A simple way to CNx/carbon nanotube intramolecular junctions and branches

Large quantities of CN_x/carbon nanotube intramolecular junctions were successfully synthesized on a silicon substrate via the chemical vapor deposition method by pyrolysis of ferrocene and melamine. The nanotubes have two apparently different sections, one made of carbon with an empty hollow cylinder structure, and the other made of carbon nitride with a bamboo-like structure. Three- and four-terminal CN_x/carbon nanotube intramolecular branches were also observed, and such multi-terminal structures offer potential applications for future nanodevices. A simple model is suggested for the synthesis of these CN_x/carbon nanotube intramolecular junctions and branches.     

Understanding the electrical response and sensing mechanism of carbon-nanotube-based gas sensors

Gas sensors based on carbon nanotube field effect transistors (CNFETs) have outstanding sensitivity compared to existing technologies. However, the lack of understanding of the sensing mechanism has greatly hindered progress on calibration standards and customization of these nano-sensors. Calibration requires identifying fundamental transistor parameters and establishing how they vary in the presence of a gas. This work focuses on modeling the electrical response of CNTFETs in the presence of oxidizing (NO₂) and reducing (NH₃) gases and determining how the transistor characteristics are affected by gas-induced changes of contact properties, such as the Schottky barrier height and width, and by the doping level of the nanotube. From the theoretical fits of the experimental transfer characteristics at different concentrations of NO₂ and NH₃, we find that the CNTFET response can be modeled by introducing changes in the Schottky barrier height. These changes are directly related to the changes in the metal work function of the electrodes that we determine experimentally, independently, with a Kelvin probe. Our analysis yields a direct correlation between the ON – current and the changes in the electrode metal work function. Doping due to molecules adsorbed at the carbon-nanotube/metal interface also affects the transfer characteristics.     

Adsorption of Pb(II) ions from aqueous solutions by carbon nanotubes oxidized different methods

This study investigated the heavy metal adsorption of the carbon nanotubes (CNTs) oxidized different methods. Besides the conventional ultrasonication method, the UV-light used as an oxidation agent. The two oxidation methods compared with each other by Pb(II) adsorption. The characterizations of oxidized CNTs were analyzed by FTIR, XRD, DTG, SEM and total acidity capacity analysis. The adsorption capacities of carbon nanotubes were compared with using Langmuir and Freundlich isotherms. Two different kinetic theories were applied to experimental data. These theories are pseudo-second order and intra-particle diffusion models. The adsorption results can be compared using non-linear Langmuir isotherm parameters. For single wall carbon nanotubes (SWCNTs), theoretical adsorption capacity value (Q_m) of UV-light method is 511.99 mg/g and ultrasonication method is 342.36 mg/g. The UV-light increased the surface acidity of the carbon nanotubes more than ultrasonication. After the adsorption experiments, it is apparently seen that the UV-light oxidation method is a useful method for heavy metal adsorption.     

Synthesis of carbon nanotubes grown by hot filament plasma-enhanced chemical vapor deposition method

We prepared two kinds of catalytic layers onto n-typed silicon substrate—nickel by r.f.-magnetron sputtering and iron (III) nitrate metal oxide by spin coating. For iron (III) nitrate metal oxide 0.5 mol of ferric nitrate nonahydrate [Fe₂(NO₃)₃·9H₂O] ethanol solution was coated onto silicon by spin coater at different rotation speeds (rev./min). Carbon nanotubes were synthesized on both Ni and iron (III) nitrate metal oxide layers by the HFPECVD (hot filament plasma-enhanced chemical vapor deposition) method. We used ammonia (NH₃) and acetylene (C₂H₂) for the dilution gas and a carbon precursor for the growth of the carbon nanotubes, respectively. We could observe the relationship between the catalytic cluster density and the nanotube density with scanning electron microscopy (SEM) images. The density of carbon nanotubes on iron (III) nitrate metal oxide was controlled by the rev./min of the spin coater. Transmission electron microscopy (TEM) image shows multi-walled carbon nanotube where the catalyst was found in the tip of the carbon nanotube. Electron dispersive X-ray spectrometry (EDS) peaks for CNT's tip show that it was constituted with nickel and iron, respectively. Raman spectroscopy of nanotubes shows D-band and G-band peaks approximately 1370 and 1590 cm⁻¹.     

Targeted removal of bioavailable metal as a detoxification strategy for carbon nanotubes

There is substantial evidence for toxicity and/or carcinogenicity upon inhalation of pure transition metals in fine particulate form. Carbon nanotube catalyst residues may trigger similar metal-mediated toxicity, but only if the metal is bioavailable and not fully encapsulated within fluid-protective carbon shells. Recent studies have documented the presence of bioavailable iron and nickel in a variety of commercial as-produced and vendor “purified” nanotubes, and the present article examines techniques to avoid or remove this bioavailable metal. First, data are presented on the mechanisms potentially responsible for free metal in “purified” samples, including kinetic limitations during metal dissolution, the re-deposition or adsorption of metal on nanotube outer surfaces, and carbon shell damage during last-step oxidation or one-pot purification. Optimized acid treatment protocols are presented for targeting the free metal, considering the effects of acid strength, composition, time, and conditions for post-treatment water washing. Finally, after optimized acid treatment, it is shown that the remaining, non-bioavailable (encapsulated) metal persists in a stable and biologically unavailable form up to two months in an in vitro biopersistence assay, suggesting that simple removal of bioavailable (free) metal is a promising strategy for reducing nanotube health risks.     

Hydrogen adsorption on single-walled carbon nanotubes studied by core-level photoelectron spectroscopy and Raman spectroscopy

We have investigated the adsorption of atomic hydrogen on vertically aligned carbon nanotube (CNT) films using in situ synchrotron-radiation-based core-level (CL) photoelectron spectroscopy and Raman spectroscopy. From C 1s CL spectra, we identified a CL peak component due to C-H bonds of carbon atoms in single-walled carbon nanotubes (SWCNTs). We also found the suppression of π-plasmon excitation, indicating that the hydrogen adsorption deforms the bonding structure. Raman spectra of the SWCNT film indicated that the radial-breathing-mode intensities of SWCNTs decreased due to the adsorption-induced bonding-structure deformation. Moreover, the decrease for small-diameter SWCNTs was more severe than that for large-diameter SWCNTs. Our results strongly suggest that the hydrogen adsorption, which induces the structure deformation from sp² to sp³-like bonding, depends on the diameter of SWCNTs.     

Effect of the nanostructure and surface chemistry on the gas adsorption properties of macroscopic multiwalled carbon nanotube ropes

The NO₂ adsorption properties of macroscopic multiwalled carbon nanotube (MWCNT) ropes and acid treated MWCNT ropes, obtained by the floating catalyst chemical vapour deposition process, have been examined. The structural characterisation shows that these ropes constitute bundles of MWCNTs. This bundled structure is found to control the electrical and gas adsorption properties of the material. The electrical resistance of these ropes decreases upon exposure to NO₂ with a high sensitivity even at room temperature. The adsorption of the NO₂ onto MWCNT bundles is found to be more stable with temperature in comparison to isolated MWCNTs revealing the complex nature of the adsorption process. These adsorption sites, which are created within the bundles of carbon nanotubes, are more stable requiring higher desorption energy. The surface of the MWCNT ropes is also modified with acid treatment, which increases the response to NO₂ by a factor 100% due to increased polar interactions between the gas molecules and the existing functionalised surface. These results suggest the possibility of using these macroscopic MWCNT ropes as low cost gas sensing materials.     

Characterization of single wall carbon nanotubes by nonane preadsorption

The preferential blocking of the interior adsorption sites of single walled carbon nanotubes (SWNTs) by n-nonane is demonstrated. Following adsorption of n-nonane and evacuation for 24 h at 323 K, it was found that interior sites with diameters less than ∼14 Å remained filled with n-nonane, blocking the physical adsorption of N₂ on these sites at 77.3 K. We demonstrate that “nonane blocking” is a very useful technique for nanotube porosity characterization.     

Unambiguous atomic structural determination of single-walled carbon nanotubes by electron diffraction

We introduce a novel non-dimensional “intrinsic layer-line spacing” concept for electron diffraction analysis of single-walled carbon nanotubes (SWCNTs). Accordingly, we develop a unique method for direct determination of chiral indices (n, m) of the carbon nanotubes from their electron diffraction patterns (EDPs). The new method is totally calibration-free. Errors due to the nanotube inclination are specified. The tilt angle of the carbon nanotube with respect to the incident electron beam is simultaneously evaluated, thus the effect of the tube tilting is compensated for in the (n, m) determination. Several effective procedures are proposed to cross-check the results by using abundant information contained in the diffraction patterns. The efficiency of the method is demonstrated on both simulated and experimental diffraction patterns from single-walled nanotubes. The technique can be extended to structural analysis of nanotubes of structure similar to carbon nanotubes, such as boron nitride nanotubes.     

Recent advances in hybrids of carbon nanotube network films and nanomaterials for their potential applications as transparent conducting films

The use of carbon nanotubes (CNTs) as transparent conducting films is one of the most promising aspects of CNT-based applications due to their high electrical conductivity, transparency, and flexibility. However, despite many efforts in this field, the conductivity of carbon nanotube network films at high transmittance is still not sufficient to replace the present electrodes, indium tin oxide (ITO), due to the contact resistances and semi-conducting nanotubes of the nanotube network films. Many studies have attempted to overcome such problems by the chemical doping and hybridization of conducting guest components by various methods, including acid treatment, deposition of metal nanoparticles, and the creation of a composite of conducting polymers. This review focuses on recent advances in surface-modified carbon nanotube networks for transparent conducting film applications. Fabrication methods will be described, and the stability of carbon nanotube network films prepared by various methods will be demonstrated.     

Carbon nanotubes horizontal interconnects with end-bonded contacts,diameters down to 50 nm and lengths up to 20 μm

This work describes a novel approach to fabricate horizontal nanotube interconnects with dimensions comparable to state-of-the-art copper interconnects. The interconnects consist of carbon nanotubes bundles with wall density ≈10¹³ cm⁻², wire lengths of tens of micrometers and wire diameters scalable to 50 nm. The nanotubes are first grown vertically, inside vias with diameters ranging from 300 to 200 nm, and then flipped on the horizontal direction. Symmetrical contacts are made at the tips of the nanotubes in the so-called end-bonded geometry via a metallization process with a key dry-etch step. The quality of the contacts and the nanotubes is evaluated from the electrical measurements by extracting the specific contact resistivity and the carbon nanotube resistivity, respectively. The measured contact resistivity is 3.9 × 10⁻⁸ Ω cm² with Pd/Au contacts. This is the lowest value ever reported so far for nanotubes contacted in an end-bonded geometry. The nanotube resistivity is as low as 1.1 mΩ cm, a value among the best reported to date and only two decades higher than that of copper.     

Synthesis of multi-walled carbon nanotubes by catalytic chemical vapor deposition using Cr2 − xFexO3 as catalyst

Chromium oxide and iron oxide solid solution was used as a catalyst for multi-walled carbon nanotubes synthesis by the catalytic chemical vapor deposition technique. The catalyst was prepared by the solution combustion synthesis method. Natural gas (NG) was employed as a carbon source for the carbon nanotube growth instead of methane, which is typically used. The carbon nanotube synthesis was carried out under H₂/NG and Ar/NG atmospheres at 950 °C. The Cr_2 − xFe_xO₃ catalyst was capable to produce carbon nanotubes only in H₂/NG atmospheres. Partial elimination of the catalyst after the synthesis was possible using a concentrated solution of HNO₃.     

Hall Measurements on Carbon Nanotube Paper Modified With Electroless Deposited Platinum

Carbon nanotube paper, sometimes referred to as bucky paper, is a random arrangement of carbon nanotubes meshed into a single robust structure, which can be manipulated with relative ease. Multi-walled carbon nanotubes were used to make the nanotube paper, and were subsequently modified with platinum using an electroless deposition method based on substrate enhanced electroless deposition. This involves the use of a sacrificial metal substrate that undergoes electro-dissolution while the platinum metal deposits out of solution onto the nanotube paper via a galvanic displacement reaction. The samples were characterized using SEM/EDS, and Hall-effect measurements. The SEM/EDS analysis clearly revealed deposits of platinum (Pt) distributed over the nanotube paper surface, and the qualitative elemental analysis revealed co-deposition of other elements from the metal substrates used. When stainless steel was used as sacrificial metal a large degree of Pt contamination with various other metals was observed. Whereas when pure sacrificial metals were used bimetallic Pt clusters resulted. The co-deposition of a bimetallic system upon carbon nanotubes was a function of the metal type and the time of exposure. Hall-effect measurements revealed some interesting fluctuations in sheet carrier density and the dominant carrier switched from N- to P-type when Pt was deposited onto the nanotube paper. Perspectives on the use of the nanotube paper as a replacement to traditional carbon cloth in water electrolysis systems are also discussed.     

Poly(ethylene succinate)/single-walled carbon nanotube composites: a study on crystallization

An investigation on the crystallization of composites based on poly(ethylene succinate) and unmodified single-walled carbon nanotube was made in this report. Both isothermal and non-isothermal modes were studied along with subsequent melting behavior using differential scanning calorimetry. Crystal morphology was then explored using X-ray scattering and infrared spectroscopy. It was observed during isothermal crystallization that carbon nanotube (CNT) could contribute to the crystallization rate through heterogeneous nucleation. Furthermore, nanotubes enhanced the crystallinity within low and high undercooling rather than medium undercooling. Similar findings were obtained in non-isothermal crystallization mode. At lower cooling rates, the crystallization rate was more strongly influenced by the nanotubes, while at higher cooling rates the crystallinity was affected to the greater extent. The onset of the cold crystallization of polymer remained unaffected in presence of the nanotube, while its extent was reduced. X-ray diffraction together with infrared spectroscopy found that the polymer crystalline morphology was of α type, and no transition from α to β occurred in presence of the CNT.     

Novel approaches to developing carbon nanotube based polymer composites: fundamental studies and nanotech applications

The adsorption of several types of conducting polymers on carbon nanotubes is investigated by electrical transport measurements. We report the optoelectronic properties occurring in single-walled carbon nanotubes (SWNTs) conjugated polymer, poly(3-octylthiophene), composites. Al/polymer-nanotube composite/indium-tin oxide diodes show photovoltaic behavior proposing that the main reason for this increase is the photoinduced electron transfer at the polymer/nanotube interface. Interesting results were obtained in the case of poly(o-anisidine) (POAS)-multi-walled nanotubes (MWNTs) composites where the increment of monolayers results in a significant improvement of the specific conductivity. POAS-coated MWNTs thin films demonstrated their potentiality as a new class of materials for inorganic vapors detection for environmental applications.     

Observation of single-walled carbon nanotubes by photoemission microscopy

Single-walled carbon nanotube networks grown on SiO₂ pillars were studied by means of scanning photoemission microscopy. The individual nanotubes or nanotube bundles growing from the pillar tops were observed in C 1s images. Band bending near catalytic Fe/nanotube contacts in an end-bonded configuration was studied by measuring C 1s spectra along the tube axes. Within our experimental resolution, no band bending was observed. This implies that the depletion width is less than the spatial resolution of the scanning photoemission microscope (90 nm) or that the amount of the band bending is less than 0.1 eV.     

Thermal,mechanical, and rheological properties of biodegradable polybutylene succinate/carbon nanotubes nanocomposites

Biodegradable poly(butylene succinate)/carbon nanotubes nanocomposites were prepared by melt mixing process, and the influence of carbon nanotubes on the properties of the nanocomposites was investigated. Differential scanning calorimetry showed that crystallization temperature (T_c) increase with increasing carbon nanotube content. Improvement of tensile modulus was observed by the addition of carbon nanotubes compared with pure poly(butylene succinate). Electrical conductivity showed that conductivity of polybutylene succinate/carbon nanotube composites increased with addition of carbon nanotube content. The storage moduli of polybutylene succinate/carbon nanotube composites are higher than the neat polybutylene succinate. The processability of polybutylene succinate/carbon nanotubes composites was improved and more pronounced in higher content of carbon nanotubes. POLYM. COMPOS., 31:1309–1314, 2010. © 2009 Society of Plastics Engineers     

碳纳米管吸附水体污染物的研究进展

碳纳米管是一种新型纳米材料,可分为单壁碳纳米管和多壁碳纳米管两种类型,具有独特的物理和化学性质,其应用研究已成为当前的研究热点。文章介绍了碳纳米管的结构和性质,综述了碳纳米管对水体中金属离子、无机非金属离子以及有机物脂肪烃、芳烃、胺、酚等物质的吸附研究进展,并展望了碳纳米管在吸附方面的应用前景。