Electrochemistry of double-wall carbon nanotubes encapsulating C60 and their spectral characterization

Electrochemistry of double-wall carbon nanotubes (DWCNTs) encapsulating C₆₀ (C₆₀@DWCNT) have been studied by preparing a C₆₀@DWCNT modified electrode, and three pairs of reversible electro-reduction waves corresponding to electron transfer reactions of C₆₀ inside DWCNTs have been obtained in a mixed solvent of toluene and acetonitrile (4:1, v:v) containing tetrabutylammonium cation as supporting electrolyte, which indicates that DWCNTs act as molecular wires to allow electrical communication between the underlying electrode and the redox-active guest C₆₀. The influencing factors on the electrochemistry of C₆₀@DWCNT modified electrodes have been investigated. The results suggest that the voltammetric behavior of C₆₀@DWCNT is dependent on the nature of the supporting electrolyte and the solvent system. In addition, spectral characterization of the C₆₀@DWCNT modified electrodes before and after electrochemical scanning reveals interaction between C₆₀ and DWCNT and verifies the reduction of C₆₀ encapsulated in DWCNTs. C₆₀ molecules inside DWCNTs retains their redox activity, and can also act as an electron-transfer mediator to electrocatalyze the reduction of halohydrocarbon.     

Effect of various adsorbates on electronic states of the thin diamond-like carbon films

The influence of the adsorbed impurity molecules onto energy spectrum of electronic states of the DLC films deposited on SiO₂/Si substrates by direct ion beam from hydrocarbon IC plasma was studied by charge-based deep level transient spectroscopy (Q-DLTS). The strong sensitivity of Q-DLTS spectra to the presence of the vapor water and alcohol at room temperature was found. The principle is that adsorption of any molecules on the surface of the DLC film results in a change of energy spectrum of the electronic states (or trapping centers) at the DLC film surface. For example, a new peak appeared in Q-DLTS spectra in presence of the vapor water and electron state density increased in several orders. Moreover, the effect of different adsorbed molecules (species) on the surface electron states was different and independent, so that different molecules can be detected separately. It was shown to differentiate a few thousand molecules of virtually any impurity adsorbed by DLC film's surface. Such strong surface phenomena of the thin DLC films may be exploited in novel sensitive and selective chemical sensor devices.     

Nitrogen doping effects on the structure behavior and the field emission performance of double-walled carbon nanotubes

The nitrogen (N) doping effect and field emission properties of double-walled carbon nanotubes (DWCNTs) were investigated. Diameter transformation and defect generation in the N-doped DWCNTs mainly depend on the amount of nitrogen employed. By applying N-doping into DWCNTs (1.5 N at.%), the average diameters of the DWCNTs were increased from 1.7 to 2.4 nm, and the crystallinity (I_G/I_D) was decreased from 13.5 to 5. Field emission properties were enhanced by the N doping into DWCNTs. The turn-on field, corresponding to a current density of 0.1 μA/cm², was about 0.9 V/μm for the N-doped DWCNTs (1.5 N at.%). The field enhancement factor of the N-doped DWCNTs was higher than that of the undoped DWCNTs. It was found that the field emission properties were controlled by pyridine-like N in the graphite due to N-doping.     

Activated carbons with metal containing bentonite binders as adsorbents of hydrogen sulfide

Wood-based activated carbon was ground and mixed with 10% bentonite binders containing either iron, zinc or copper cations adsorbed within the interlayer space and/or on the external surface of bentonite flakes. To better understand the role of transition metals, carbon was also impregnated with iron, zinc and copper salts. The structure of materials after modification was determined using nitrogen adsorption. The modification resulted in a decrease in porosity, especially in micropore volume, as a result of combined mass dilution effect and adsorption/re-adsorption of metals in small pores. Introduction of bentonite binders containing adsorbed metal increased the capacity of carbon for hydrogen sulfide only in the case of material containing copper. Copper also significantly increases the performance of carbon as an H₂S adsorbent when impregnation is applied whereas the effects of other metals used in this study are much less pronounced. It is likely that copper present in the small pores acts as a catalyst for oxygen activation causing hydrogen sulfide oxidation. As a result of this process, elemental sulfur is formed which, when present in small pores, is oxidized to weakly adsorbed SO₂. The SO₂ is removed from the surface when continuous reaction with hydrogen sulfide occurs. Thus, even though binding carbon with spent bentonites after copper adsorption increases the capacity of carbon toward H₂S removal, the formation of SO₂, another undesirable pollutant, does detract somewhat from the procedure.     

Spectral properties of single-walled carbon nanotubes encapsulating fullerenes

Single-walled carbon nanotubes (SWCNTs) with diameter ranged from 1.22 to 1.6 nm filled with C₆₀, C₇₀ and C₆₀H₂₈ molecules (peapods), as well as double-walled carbon nanotubes (DWCNTs) derived from peapods, were studied by HRTEM, UV-vis-NIR and Raman spectroscopy. Suspensions with accurate concentration were used for spectroscopic studies to enable quantitative comparison of different substances. Filling of the SWCNTs with C₇₀ molecules resulted in a reduced van der Waals interaction between the tubes in a bundle. The DWCNTs have lower intensity of the van Hove bands and weaker photoluminescence. Raman spectra at 633 and 1064 nm excitation wavelengths reveal that RBM frequencies of C₆₀ and C₇₀ peapods are equally downshifted compared to empty tubes. It was found that filling of the nanotubes with C₆₀ and C₇₀ caused spectral shifts of absorption bands: thin tubes display red shifts, while thick ones show blue shifts. DWCNTs and C₆₀H₂₈@SWCNTs do not show any shifts. All the results suggest that the filling of nanotubes with fullerenes alters the average diameter of the electron cloud around SWCNT framework; namely, it increases for thin SWCNTs, and decreases for thick ones. Our attempts to structurally assign thick nanotubes using reported extrapolations from data for thin tubes were unsuccessful.     

Transport properties of double-walled carbon nanotubes and carbon boronitride heteronanotubes

Using the density functional theory combined with the nonequilibrium Green's function, the transport properties of double-walled carbon nanotubes (DWCNTs) and carbon boronitride (CBN) heteronanotubes were investigated. As the hopping length increases, the conductance of DWCNTs shows a dramatic variation that is independent of the intertube space. The transport of the CBN heterojunctions also displays abnormal behavior when the hopping length is changed, which is very different from the behavior of DWCNTs. The currents of the forward in the CBN heterojunctions are about 3–15 times as large as those of the back under lower bias voltages. The negative differential resistance (NDR) effect occurs in the CBN heterojunctions, and the peak-to-valley ratio in the additional NDR regions is about 2–4 for the current–voltage relationship. The hopping length and BN parts have a great influence on the transport of the double-walled nanodevices.     

Transformation of C70 peapods into double walled carbon nanotubes

X-ray diffraction studies comparing the transformation of C₆₀ and C₇₀ peapods into double walled carbon nanotubes are presented. The structures of the as-formed DWCNTs are strikingly similar, showing that they are not dependent on the nature of the fullerene precursor. High temperature X-ray diffraction measurements of C₇₀ peapods below the coalescence temperature show that confined C₇₀ molecules in large tubes undergo an orientational transition to free rotations. Fast re-orientations of C₇₀ molecules allow cyclo-addition between adjacent fullerenes to form, in good agreement with the mechanism of coalescence proposed in the literature for C₆₀ molecules.     

The effect of phase separation in Fe/Mg/Al/O catalysts on the synthesis of DWCNTs from methane

Double-walled carbon nanotubes (DWCNTs) were prepared by methane decomposition on Fe/Al/Mg/O catalysts with a fixed iron loading of 1.5%. Increasing Al/Mg ratio in the catalyst resulted in the formation of a new MgAl₂O₄ phase, which was characterized by XRD. The size of the MgO crystallites in the support was decreased, due to the phase separation, from 35 nm to 20 nm in the Al/Mg ratio range of 0:1-4:1. At an Al/Mg ratio of 1:200, this effect prevented the sintering of iron on the MgO support and resulted in the synthesis of high-purity DWCNTs in high-yield. Very high-Al/Mg ratio induced the formation of the MgAl₂O₄ phase, which became another catalyst support material. This had a negative effect on the synthesis of DWCNTs due to its acidity and hardness. Simultaneously maintaining MgO as the dominant catalyst support and decreasing its particle size by the phase separation effect are important for good metal dispersion and, consequently, the yield and purity of DWCNTs.     

Surface adsorption of comb polymers by Monte Carlo simulations

This paper reports off-lattice Monte Carlo simulations of highly-branched comb homopolymers weakly adsorbed on a flat, featureless surface showing only covolume and dispersion interactions with the adsorbate. A minimal coarse-grained model, described by hard spheres connected by harmonic springs, was employed. The interaction energy of the adsorbed combs and linear chains is first discussed as a function of the molecular mass and of the number of beads in contact with the surface. The molecular size is then investigated as a function of backbone length and branching density at a fixed arm size. The apparent swelling exponents of the adsorbed combs are larger than those of the corresponding linear chains, and much larger than that of the free molecules. This result indicates a surface-induced stiffening of the comb backbone, further studied through the persistence length l_pers. It is found that l_pers increases upon adsorption over the free-molecule value, more so the larger is the branching density. Finally, the thickness of the adsorbed layer, the surface-induced molecular anisotropy and the molecular aspect ratio are investigated as a function of branching density and molecular mass.     

Improved field emission properties of double-walled carbon nanotubes decorated with Ru nanoparticles

The field emission properties of double-walled carbon nanotubes (DWCNTs) were remarkably improved by decorating their surface with ruthenium (Ru) metal nanoparticles. The Ru nanoparticles were attached effectively on the surface of DWCNTs via a chemical procedure. The Ru-decorated DWCNTs showed lower turn-on voltage, higher emission current density, and improved emission uniformity compared with pristine DWCNTs. The effect of Ru nanoparticles on the work function and density of states was evaluated by the first-principles calculation. The enhanced field emission properties of Ru-DWCNTs were mainly attributed to the Ru nanoparticles which increased the field enhancement factor and the density of emission sites. Our results indicate that the Ru-decorated DWCNTs can be used as an effective field emitter for various field emission devices.     

One-step fabrication of high quality double-walled carbon nanotube thin films by a chemical vapor deposition process

High-quality double-walled carbon nanotubes (DWCNTs) thin-films have been fabricated in one-step by the catalytic chemical vapor deposition gas-flow reaction process with acetone as a carbon source in an argon flow. The DWCNTs film is formed through the self-assembly of the DWCNTs in the gas flow, which is achieved by controlling the gas rates in the synthesis reaction. The DWCNT film is self-supported and consists of preferentially aligned high-quality DWCNT bundles. Raman spectral analysis shows a low intensity ratio of the D band and the G band with I_D/I_G being 0.025 indicating a high-quality of DWCNTs at a macroscopic scale. Property measurements show that the DWCNT film is mechanically robust and highly electrically conductive. The formation of high-quality DWCNTs can be attributed to the reaction in the argon environment that is inert and does not attack the DWCNTs at the high synthesis temperature (1170 °C). This one-step fabrication process is feasible for large-scale productions of high-quality DWCNTs films with promising structural and functional applications.     

Growth of double-walled carbon nanotubes from silicon oxide nanoparticles

Double-walled carbon nanotubes (DWCNTs) were synthesized by a metal-catalyst-free chemical vapor deposition method using silicon oxide nanoparticles as a catalyst. The diameters and lengths of the DWCNTs are in the ranges of 3–5 nm and 1–5 μm, respectively. The amount of DWCNTs produced is about 70%, while the remainder is single-walled carbon nanotubes. A heat treatment of the SiO₂/Si substrate used was found to be crucial for controlling the size of the catalyst nanoparticles, and hence for the growth of the DWCNTs. Flat or cone-shaped caps were observed for the DWCNTs, indicating that the growth of the DWCNTs from the non-metal catalyst follows a vapor–solid–solid mechanism. Energy dispersive spectroscopy and X-ray photoelectron spectroscopy characterizations confirmed that no metal impurity exists in the obtained DWCNT samples.     

Hydrogenation of but-1-yne on platinum catalyst: Role of reversible adsorbed hydrocarbon deposit

By using rectangular pulses of but-1-yne carried by H₂ or H₂-He mixtures over a platinum catalyst, it was possible to visualize the build-up of an adsorbed hydrocarbon deposit which is a necessary condition for getting a selective catalyst (on platinum, but-1-ene selectivity is 90%). Such a deposit is reversibly adsorbed since it desorbs as hydrogenated butyne molecules when the butyne pressure drops at the end of the pulse. Moreover it was shown, during stationary phase, that the sudden fall in the selectivity at high but-1-yne conversion, is due to the desorption of such reversibly adsorbed molecules.     

Differentiation of chemical reaction activity of various carbon nanotubes using redox potential: Classification by physical and chemical structures

The present study systematically examined the kinetics of a hydroxyl radical scavenging reaction of various carbon nanotubes (CNTs) including double-walled and multi-walled carbon nanotubes (DWCNTs and MWCNTs), and carbon nano peapods (AuCl₃@DWCNT). The theoretical model that we recently proposed based on the redox potential of CNTs was used to analyze the experimental results. The reaction kinetics for DWCNTs and thin MWCNTs agreed well with the theoretical model and was consistent with each other. On the other hand, thin and thick MWCNTs behaved differently, which was consistent with the theory. Additionally, surface morphology of CNTs substantially influenced the reaction kinetics, while the doped particles in the center hollow parts of CNTs (AuCl₃@DWCNT) shifted the redox potential in a different direction. These findings make it possible to predict the chemical and biological reactivity of CNTs based on the structural and chemical nature and their influence on the redox potential.     

载体硅胶表面氟改性对Phillips铬系催化剂诱导期内引发乙烯聚合行为的影响

通过密度泛函理论考察了载体表面氟改性对吸附不同数目甲醛分子的Phillips催化剂诱导期内引发乙烯聚合反应的影响。结果表明,当乙烯还原六价铬酸酯形成的二价铬前驱体模型上吸附两分子甲醛时,其位阻效应阻碍了任何反应的进一步发生;当二价铬前驱体模型上吸附一分子甲醛时,只能通过先形成铬金属五元环进而发生乙烯二聚反应和易位反应,但是氟改性对两者的影响很小;当二价铬前驱体模型上吸附的甲醛分子完全脱附后,则可以进一步环增长生成铬金属七元环,并且氟改性对这一步反应有促进作用;而氟改性对铬金属七元环进一步开环生成1-己烯则是不利的。研究还表明,氟改性对于三价铬-烷基聚合活性中心模型上的链增长是有利的。     

Synthesis of High‐Quality,Double‐Walled Carbon Nanotubes in a Fluidized Bed Reactor

Double‐walled carbon nanotubes (DWCNTs) were synthesized in a packed bed reactor (PBR) and a fluidized bed reactor (FBR) by cracking CH₄ on a Fe/MgO catalyst. It is observed that the dominant carbon product changes drastically from DWCNTs to multi‐walled CNTs along the axial direction of PBR. The studies indicated that the high concentration of H₂ from the high conversion of CH₄ causes the quick reduction and sintering of the iron catalyst and inhibits the nucleation of DWCNTs. Based on these results, the batch or continuous feeding mode of small amounts of catalyst was adopted in a FBR to maintain a high space velocity of CH₄ and to inhibit the negative effect of excess H₂. Finally, a DWCNT product with a specific surface area of 950 m²/g and a purity of 98 %, was obtained.     

Prediction of a highly sensitive molecule sensor for SOx detection based on TiO2/MoS2 nanocomposites: a DFT study

First principles calculations within density functional theory have been carried out to investigate the adsorptions of SO_x (x?=?1, 2) molecules on TiO₂/MoS₂ nanocomposites in order to fully discover the gas sensing capabilities of TiO₂/MoS₂ composite systems. The van der Waals interactions were included to obtain the most stable geometrical structures of TiO₂/MoS₂ nanocomposites with adsorbed SO_x molecules. SO_x molecules preferentially interact with the doped nitrogen and fivefold coordinated titanium sites of the TiO₂ anatase nanoparticles because of their higher activities in comparison with the other sites. The results presented include structural parameters such as bond lengths and bond angles and energetics of the systems such as adsorption energies. The variation of electronic structures are discussed in view of the density of states and molecular orbitals of the SO_x molecules adsorbed on the nanocomposites. The results show that the adsorption of the SO_x molecule on the N-doped TiO₂/MoS₂ nanocomposite is energetically more favorable than the adsorption on the undoped one, implying that the nitrogen doping helps to strengthen the interaction of SO_x molecules with TiO₂/MoS₂ nanocomposites. These calculated results thus provide a theoretical basis for the potential applications of TiO₂/MoS₂ nanocomposites in the removal and sensing of harmful SO_x molecules.     

Characterization of n-hexadecylalkylamine-intercalated graphite oxides as sorbents

Adsorption properties of graphite oxides hydrophobized by n-hexadecylamine, (C16)_xGO, were investigated using pyrene molecules as a model of nonionic organic contaminants. A large quantity of pyrene (28.5 mg/g) was adsorbed from a water-ethanol mixture (1:2) containing 2 mM of pyrene when (C16)_0.6GO was used. The isotherm of pyrene adsorption was better described by Freundlich equation rather than Langmuir equation, which indicated a single adsorption mechanism was involved. The change in the amount of adsorbed pyrene as a function of amine content in GO was very similar to that which occurs upon introduction of pyrene into (C16)_xGO films from chloroform solution, as determined by X-ray measurements. This suggests that pyrene molecules were adsorbed not only on the outer surface but also within the interlayer space of the intercalation compound. Swelling of the intercalation compound by ethanol can render the interlayers more organophilic and make access to hexadecylamine molecules bonded to the graphite oxide layer easier for pyrene molecules, especially in (C16)_xGOs with lower amine contents.     

The performance of two adsorption ice making test units using activated carbon and a carbon composite as adsorbents

The available adsorption working pairs applied to adsorption refrigeration system, which utilize activated carbon as adsorbent, are mainly activated carbon-methanol, activated carbon-ammonia, and composite adsorbent-ammonia. The adsorption properties and refrigeration application of these three types of adsorption working pairs are investigated. For the physical adsorbents, consolidated activated carbon showed best heat transfer performance, and activated carbon-methanol showed the best adsorption property because of the large refrigerant amount that can be adsorbed. For the composite adsorbents, the consolidated composite adsorbent with mass ratio of 4:1 between CaCl₂ and activated carbon, showed the highest cooling density when compared to the granular composite adsorbent and to the merely chemical adsorbent. The physical adsorption icemaker that employs consolidated activated carbon-methanol as working pair had the optimum coefficient of refrigeration performance (COP), volume cooling power density (SCP_v) and specific cooling power per kilogram adsorbent (SCP) of 0.125, 9.25 kW/m³ and 32.6 W/kg, respectively. The composite adsorption system that employs the consolidated composite adsorbent had a maximum COP, SCP_v and SCP of 0.35, 52.68 kW/m³ and 493.2 W/kg, respectively, for ice making mode. These results are improved by 1.8, 4.7 and 14 times, respectively, when compared to the results of the physical adsorption icemaker.     

Preparation of double-walled carbon nanotubes from fullerene waste soot by arc-discharge

Fullerene waste soot (FWS) was used as raw material to fabricate double-walled carbon nanotubes (DWCNTs) by arc-discharge in a mixture of Ar and H₂ (2:1, v/v) at 300 Torr. The results of transmission electron microscope and Raman spectroscopy indicate that the high quality FWS-derived DWCNTs can be synthesized by arc-discharge method.