High H2 uptake by alkali-doped carbon nanotubes under ambient pressure and moderate temperatures

COMPUTATION OF INTERACTION POTENTIAL OF ADSORBATES ON ZIGZAG SWCNTs — APPLICATION TO FUNCTIONALIZATION AND HYDROGEN STORAGE

Nature of the interaction potential of different adsorbates on different zigzag single-walled carbon nanotubes is investigated. The intermolecular potentials for H2 absorbed in carbon nanotubes (5, 0), (6, 0), (7, 0), (8, 0), (9, 0), and (10, 0) are computed and sketched. This study is extended to N2 adsorbed on (4, 0) and BH3 adsorbed on (10, 0) tubes. The equilibrium positions of the adsorbates obtained from the potential model serve as an initial guess in designing the CNT + adsorbate complex in the simulation cell and this process considerably reduces the computation time. Further, the hydrogen storage capacity of CNT(10,0) + BH3 complex is calculated. The estimated storage capacity of this system is in the range 6–12 wt.%.

Incorporating magnesium and calcium cations in porous organic frameworks for high-capacity hydrogen storage

We propose incorporating a bi-functional group consisting of magnesium or calcium cations and a 1,2,4,5-benzenetetroxide anion (C6H2O4(4-)) in porous materials to enhance the hydrogen storage capacity. The C6H2O4M2 bifunctional group is highly stable and polarized, and each group provides 18 (M = Mg) or 22 (M = Ca) binding sites for hydrogen molecules with an average binding energy of ca. 10 kJ mol(-1) per hydrogen molecule based on RIMP2/ TZVPP calculations. Two porous materials (PAF-Mg or PAF-Ca) constructed with the bi-functional groups show remarkable improvement in hydrogen uptakes at normal ambient conditions. At 233 K and 10 MPa, the predicted gravimetric uptakes are 6.8 and 6.4 wt% for PAF-Mg and PAF-Ca respectively. This work reveals that fabricating materials with large numbers of binding sites and relatively low binding energies is a promising approach to achieve high capacity for on-board storage of hydrogen.

TRANSPORT PROPERTIES OF TEMPLATE-SYNTHESIZED GOLD AND CARBON NANOTUBE MEMBRANES

We discuss the fabrication of gold and carbon nanotubes prepared by the template method. The gold nanotubes are prepared via electroless deposition of Au onto the pore walls of a nanoporous polycarbonate filtration membrane. Carbon nanotube membranes (CNMs) are prepared by doing chemical vapor deposition of carbon within the pores of a microporous alumina template. The pores in these filtration membranes act as templates for the nanotubes. We have shown that by controlling the Au deposition time, Au nanotubes that have effective inside diameters of molecular dimensions (<1 nm) can be prepared. These membranes are a new class of molecular sieves and can be used to separate molecules based on size. Furthermore, we describe fundamental investigations of electro osmotic flow (EOF) in carbon nanotube membranes. In addition, an electrochemical derivatization method was used to attach carboxylate groups to the nanotube walls; this enhances the anionic surface charge density, resulting in a corresponding increase in the EOF rate in the CNM.

Tetrahydrofuran-induced K and Li doping onto poly(furfuryl alcohol)-derived activated carbon (PFAC): influence on microstructure and H2 sorption properties

We have doped poly(furfuryl alcohol)-derived activated carbon (PFAC) with two alkali metals, potassium (K) and lithium (Li), by previously reacting the metals with naphthalene in the presence of tetrahydrofuran (THF), followed by introducing them to pristine PFAC. The THF molecule causes a minor alteration of the microstructure of PFAC as confirmed by Raman spectra, X-ray diffraction, and pore textural analysis. Raman spectra and X-ray diffraction indicated a slight localized ordering toward the stacking defects of disordered carbon, as in PFAC, which can be attributed to the movement of THF molecules within the internal planes of graphene sheets. Pore textural analysis confirmed the lowering of the specific surface area and pore volume of both K- and Li-doped PFACs (BET SSA, 1378 m(2)/g (PFAC); 1252 m(2)/g (K-PFAC), 1081 m(2)/g (Li-PFAC)). Volumetric hydrogen adsorption measurements at temperatures of 298, 288, 273, and 77 K and pressures of up to 1 bar indicated the enhanced adsorption potential imposed by the presence of alkali metals, which can be reconfirmed by the elevated heats of adsorption of metal-doped PFACs (Li-PFAC, -(10-11) kJ/mol; K-PFAC, -(16-19) kJ/mol) compared to that of pristine PFAC (-9.6 kJ/mol).

OPTIMIZATION OF CNTs PRODUCTION USING FULL FACTORIAL DESIGN AND ITS ADVANCED APPLICATION IN PROTEIN PURIFICATION

Carbon nanotubes (CNTs) have been successfully synthesized by using in-house fabricated Double Stage Chemical Vapor Deposition (DS-CVD) technique, using acetylene (C2H2) and hydrogen (H2) as the precursor gases. The purity, morphology and the structure of CNTs were then characterized using Field Emission Scanning Microscope (FESEM), Transmission Electron Microscope (TEM) and Thermogravimetric Analysis (TGA). The effects of the process parameters were examined whereby the experimental design of the investigation was conducted using Design Expert® Version 6.0.8. The statistical analysis reveals that the optimized conditions for the best CNTs yield production at 850°C reaction temperature, 60 min reaction time, with gas flow rates at 40 and 150 ml/min for C2H2 and H2 , respectively. The CNTs produced were successfully used as column chromatographic media. Due to its nanosized structured dimension, CNTs' have tremendously large surface area and that lead to highly efficient protein purification. Skim latex protein has been used as the model protein and we aim to recover useful proteins and enzymes from this known wasteful material. During the purification, the process parameters such as pH and ionic strength of the running buffer were optimized to enhance protein purification. Results reveal that CNTs behave efficiently as a hydrophobic interaction chromatographic media.

QUANTUM CONDUCTANCE AND ELECTRONIC PROPERTIES OF LOWER DIAMONDOID MOLECULES AND DERIVATIVES

Diamondoids and their derivatives have found major applications as templates and as molecular building blocks in nanotechnology. An ab initio method we calculated the quantum conductance and the essential electronic properties of two lower diamondoids (adamantane and diamantane) and three of their important derivatives (amantadine, memantine and rimantadine). We also studies two artificial molecules that are built by substituting one hydrogen ion with one sodium ion in both adamantane and diamantane molecules. Most of our results are based on an infinite Au two-probe system constructed by ATK and VNL software, which comprise TRANSTA-C package. By changing various system structures and molecule orientations in linear Au and 2 × 2 Au probe systems, we found that although the conductance of adamantane and diamantane are very small, the derivatives of the lower diamondoids have considerable conductance at specific orientations and also showed interesting electronic properties. The quantum conductance of such molecules will change significantly by changing the orientations of the molecules, which approves that residues like nitrogen and sodium atoms have great effects on the conductance and electronic properties of single molecule. There are obvious peaks near Fermi energy in the transmission spectrums of artificial molecules, indicating the plateaus in I–V characteristics of such molecules.

Carbon-tuned bonding method significantly enhanced the hydrogen storage of BN-Li complexes

Through first-principles calculations, we found doping carbon atoms onto BN monolayers (BNC) could significantly strengthen the Li bond on this material. Unlike the weak bond strength between Li atoms and the pristine BN layer, it is observed that Li atoms are strongly hybridized and donate their electrons to the doped substrate, which is responsible for the enhanced binding energy. Li adsorbed on the BNC layer can serve as a high-capacity hydrogen storage medium, without forming clusters, which can be recycled at room temperature. Eight polarized H(2) molecules are attached to two Li atoms with an optimal binding energy of 0.16-0.28 eV/H(2), which results from the electrostatic interaction of the polarized charge of hydrogen molecules with the electric field induced by positive Li atoms. This practical carbon-tuned BN-Li complex can work as a very high-capacity hydrogen storage medium with a gravimetric density of hydrogen of 12.2 wt%, which is much higher than the gravimetric goal of 5.5 wt % hydrogen set by the U.S. Department of Energy for 2015.

Strategy for carrier control in carbon nanotube transistors

Carbon nanotubes exhibit remarkable mechanical and electronic properties and are, therefore, being regarded as a new functional material for next generation electronics. Nevertheless, several obstacles still exist for an application in industry. The control of carriers in carbon nanotubes is of critical importance prior to an industrial application in transistors. As carbon nanotubes exhibit p-type behavior under ambient conditions, it is difficult to convert them from a p- to an n-type transistor. Also, doping control is a critical issue for applying traditional CMOS technology. Here, we discuss various approaches for preparing operating carbon nanotube transistors: i) impurity doping that employs conventional and interstitial insertion of group III or V materials, ii) chemical doping that induces charge transfer between chemicals and CNTs, iii) carrier control that utilizes the work function difference between metal and CNTs, iv) electrostatic doping that controls the carrier type by using a gate bias, and v) ambipolarity that does not use chemical doping. Advantages and drawbacks of these approaches will be discussed extensively in the text.

Capture of trace sulfur gases from binary mixtures by single-walled carbon nanotube arrays: a molecular simulation study

Adsorption of H(2)S and SO(2) pure gases and their selective capture from the H(2)S-CH(4), H(2)S-CO(2), SO(2)-N(2), and SO(2)-CO(2) binary mixtures by the single-walled carbon nanotubes (SWNT) are investigated via using the grand canonical Monte Carlo (GCMC) method. It is found that the (20, 20) SWNT with larger diameter shows larger capacity for H(2)S and SO(2) pure gases at T = 303 K, in which the uptakes reach 16.31 and 16.03 mmol/g, respectively. However, the (6,6) SWNT with small diameter exhibits the largest selectivity for binary mixtures containing trace sulfur gases at T = 303 K and P = 100 kPa. By investigating the effect of pore size on the separation of gas mixtures, we found that the optimized pore size is 0.81 nm for separation of H(2)S-CH(4), H(2)S-CO(2), and SO(2)-N(2) binary mixtures, while it is 1.09 nm for the SO(2)-CO(2) mixture. The effects of concentration and temperature on the selectivity of sulfide are also studied at the optimal pore size. It is found that the concentration (ppm) of sulfur components has little effect on selectivity of SWNTs for these binary mixtures. However, the selectivity decreases obviously with the increase of temperature. To improve the adsorption capacities, we further modify the surface of SWNTs with the functional groups. The selectivities of H(2)S-CO(2) and SO(2)-CO(2) mixtures are basically uninfluenced by the site density, while the increase of site density can improve the selectivity of H(2)S-CH(4) mixture doubly. It is expected that this work could provide useful information for sulfur gas capture.

Direct electrochemistry and voltammetric determination of midecamycin at a multi-walled carbon nanotube coated gold electrode

Macrolide antibiotics generally shows slow electron-transfer rate and produces insensitive redox peaks at conventional electrodes. In this paper, we studied the electrochemical behavior of midecamycin, one of macrolide antibiotics, at a multi-walled carbon nanotube (MWNT) modified gold electrode. It was found that MWNT could adsorb midecamycin and promote its direct electron-transfer. Hence midecamycin exhibited a more sensitive anodic peak at the modified electrode. The electrochemical process showed the feature of a mixed-control system of diffusion and adsorption. Under the optimized conditions (i.e. pH 7.0 phosphate supporting electrolyte, 5 μl 0.5 mg ml⁻¹ multi-walled carbon nanotube suspension for Ø = 2.0 mm electrode, accumulation at -0.8 V for 150 s), the anodic peak current was linear to midecamycin concentration in the range of 5 × 10⁻⁷ to 2 × 10⁻⁵M, with a correlation coefficient of 0.998. For a 5×10⁻⁶ M midecamycin solution, ten repetitive measurements gave a relative standard deviation of 2.2%. This method was successfully applied to the determination of midecamycin in medicine tablet and the recovery was 97.5-104.0%.

Nitrogen-promoted self-assembly of N-doped carbon nanotubes and their intrinsic catalysis for oxygen reduction in fuel cells

Nitrogen atoms were found to exhibit a strong ability to promote the self-assembly of nitrogen-doped carbon nanotubes (NCNTs) from gaseous carbons, without an assistance of metal atoms. On the basis of this discovery, pure metal-free CNTs with a nitrogen-doping level as high as 20 atom % can be directly synthesized using melamine as a C/N precursor. This offers a novel pathway for carbon nanotube synthesis. Furthermore, the metal-free and intact characteristics of the NCNT samples facilitate a clear verification of the intrinsic catalytic ability of NCNTs. The results show that the NCNTs intrinsically display excellent catalytic activity for oxygen reduction in fuel cells, comparable to traditional platinum-based catalysts. More notably, they exhibit outstanding stability, selectivity, and resistance to CO poisoning, much superior to the platinum-based catalysts.

Rapid, Room Temperature, High-Density Hydrogen Adsorption on Single-Walled Carbon Nanotubes at Atmospheric Pressure Assisted by a Metal Alloy

Laser-generated, carbon single-walled nanotubes (SWNTs) adsorb hydrogen in a matter of minutes at room temperature and atmospheric pressure in the presence of a Ti-6Al-4V metal alloy. The unusual hydrogen adsorption properties are activated when the SWNTs are sonicated in nitric acid with a Ti-6Al-4V probe. The process cuts the SWNTs and introduces ∼15-40 wt% metal alloy into the previously pure single-walled nanotube material. Subsequent hydrogen adsorption occurs in two separate sites with a maximum adsorption capacity of ∼7 wt% on a total sample weight basis. Approximately 2.5 wt% hydrogen is evolved at 300 K while the remainder desorbs between 475-850 K. The pure metal alloy adsorbs ∼ 2.5 wt% H2, and evolvesydrogen with increasing temperature in a manner similar to the alloy-doped SWNTs. However, it is clear from studies presented here that the SWNT fraction is quite active in H2 uptake, adsorbing as much as 7 % on a SWNT weight basis.

Large Cryogenic Storage of Hydrogen in Carbon Nanotubes at Low Pressures

We report (6 wt %) storage of H2 at T=77 K in processed bundles of single-walled carbon nanotubes at P=2 atmospheres. The hydrogen storage isotherms are completely reversible. D2 isotherms confirm this anomalous low-pressure adsorption and further reveal the effects of quantum mechanical zero point motion. We propose that our post-synthesis treatment of the sample not only improves access for hydrogen to the central pores within individual nanotubes, but also may create a roughened tube surface with an enhanced binding energy for hydrogen. Such an enhancement is needed to understand the strong adsorption at low pressure. We obtain an experimental isosteric heat qst=125 ± 5 meV for processed SWNT materials.

Strategies for post-synthesis alignment and immobilization of carbon nanotubes

Carbon nanotubes (CNTs) have developed into a standard material used as a building block for nanotechnological developments. Based on the unique properties that make CNTs useful for many different applications in nanotechnology, optics, electronics, and material science, there has been a rapid development of this research area and many different applications have emerged in the past few years. Frequently, the alignment and immobilization of CNTs play an important role for many applications and different strategies, in particular post-synthesis approaches, can be applied. Recent developments of different techniques to immobilize and align carbon nanotubes are discussed and classified into three main categories: chemical immobilization and alignment, physical immobilization and alignment, and the use of external fields for these purposes. Many of the techniques involve multiple steps and may also cross these rather crudely defined boundaries. As such, the techniques are classified according to their most important or unique step.

Quantum dynamics of hydrogen interacting with single-walled carbon nanotubes: multiple H-atom adsorbates

In a previous paper [J. L. McAfee and B. Poirier, J. Chem. Phys. 130, 064701 (2009)], using spin-polarized density functional theory (DFT), the authors reported a binding energy of 0.755 eV, for a single hydrogen atom adsorbed on a pristine (unrelaxed) (5,5) single-walled carbon nanotube (SWNT) substrate. A full three-dimensional (3D) potential energy surface (PES) for the SWNT-H system was also developed, and used in a quantum dynamics calculation to compute all rovibrational bound states, and associated equatorial and longitudinal adsorbate migration rates. A highly pronounced preference for the latter migration pathway at ambient temperatures was observed. In this work, we extend the aforementioned study to include multiple H-atom adsorbates. Extensive DFT calculations are performed, in order to ascertain the most relevant dynamical pathways. For two adsorbates, the SWNT-H-H system is found to exhibit highly site-specific binding, as well as long-range correlation and pronounced binding energy enhancement. The latter effect is even more pronounced in the full-hydrogenation limit, increasing the per-adsorbate binding energy to 2.6 eV. To study migration dynamics, a single-hole model is developed, for which the binding energy drops to 2.11 eV. A global 3D PES is developed for the hole migration model, using 40 radial × 18 cylindrical ab initio geometries, fit to a Fourier basis with radially dependent expansion coefficients (rms error 4.9 meV). As compared with the single-adsorbate case, the hole migration PES does not exhibit separate chemisorption and physisorption wells. The barrier to longitudinal migration is also found to be much lower. Quantum dynamics calculations for all rovibrational states are then performed (using a mixed spectral basis/phase-space optimized discrete variable representation), and used to compute longitudinal migration rates. Ramifications for the use of SWNTs as potential hydrogen storage materials are discussed.

Toward a realistic density functional theory potential energy surface for the H5+ cluster

The potential energy surface of H(5)(+) is characterized using density functional theory. The hypersurface is evaluated at selected configurations employing different functionals, and compared with results obtained from ab initio CCSD(T) calculations. The lowest ten stationary points (minima and saddle-points) on the surface are located, and the features of the short-, intermediate-, and long-range intermolecular interactions are also investigated. A detailed analysis of the surface's topology, and comparisons with extensive CCSD(T) results, as well as a recent ab initio analytical surface, shows that density functional theory calculations using the B3(H) functional represent very well all aspects studied on the H(5)(+) potential. These include the tiny energy difference between the minimum at 1-C(2v) configuration and the 2-D(2d) one corresponding to the transition state for the proton transfer between the two equivalent C(2v) minima, and also the correct asymptotic behavior of the long-range interactions. The calculated binding energy and dissociation enthalpies compare very well with previous benchmark coupled-cluster ab initio data, and with experimental data available. Based on these results the use of such approach to perform first-principles molecular dynamics simulations could provide reliable information regarding the dynamics of protonated hydrogen clusters.

Nanoparticle fractionation using an aligned carbon nanotube array

A technique utilizing the capillary assisted sieving capability of carbon nanotubes (CNTs) to achieve fractionation of nanoparticles of small size distribution is presented. By dipping aligned CNT arrays into a solution comprising different sized quantum dots (QDs), size-selective gradient decoration of QDs onto CNTs is achieved. The fractionating capability of CNTs is also demonstrated for poly-dispersed manganese doped zinc sulfide nanoparticles and QDs of varying sizes and chemical compositions, which we attribute to the size-selective sieving effect of CNTs. By controlling the terminating point for the flow of QDs across the CNT array, a QD size specific CNT/QD hybrid structure is achieved.

Interaction of small gold clusters with carbon nanotube bundles: formation of gold atomic chains

We use first-principles density functional theory to simulate the interaction of bundles of semiconducting (10, 0) and metallic (6, 6) carbon nanotubes (CNTs) with small gold clusters (Au(n), n = 3, 5) inserted in their interstitial spaces. We find that gold clusters spontaneously evolve to form atomic chains along the axis of nanotubes and induce weak metallicity in the semiconducting nanotubes through charge transfer. We further show that a similar structural evolution of Pt(3) clusters occurs in the interstitial spaces of a (10, 0) CNT bundle. Our calculations show that these structural changes, along with interesting changes in the electronic structure, occur at moderate pressures that are readily achievable in a laboratory, and should be relevant to devices that make use of gold-nanotube contacts.

Enhanced photoresponse of CdS/CMK-3 composite as a candidate for light-harvesting assembly

Two typical carbon materials (ordered mesoporous carbon and carbon nanotube) were chosen as scaffolds in combination with semiconductor quantum dots (SQDs) for making light-harvesting assemblies. The effects of interfacial morphology on photoelectric performance of the carbon-based heterostructures have been investigated in detail. The enhanced photoresponse shows a strong dependence on the interfacial morphology as a result of direct interfacial contacts between SQDs and carbon materials, which plays a major role in increasing charge generation at the interface and transport pathways for photoinduced electron transfer. The methodology to enhance the photoresponse through tuning interfacial morphology proves to be a potent alternative in fabricating photochemical energy conversion systems.

Gas phase fullerene anions hydrogenation by methanol followed by IRMPA dehydrogenation

The characterization in the gas phase of the mechanisms responsible for hydride formation can contribute to the development of new materials for hydrogen storage. The present work provides evidence of a hydrogenation-dehydrogenation catalytic cycle for C(60)(*-) anions in the gas phase using methanol vapor at room temperature as hydrogen donor. The involvement of methanol in the reaction is confirmed by experiments using CD(3)OD and CD(3)OH. C(60) hydride anions with up to 11 hydrogen atoms are identified via elemental composition analysis using FT-ICR mass spectrometry. For the longer reaction times, partial conversion of the C(60) hydride ions into oxygen containing ion products occurs. Dehydrogenation using infrared multiphoton activation with a CO(2) laser restores the C(60)(*-) anions.