Showing papers by "Malcolm L. H. Green published in 2006"

Purification and opening of carbon nanotubes using steam.

Abstract: Purification and opening of carbon nanotubes has been carried out by treatment of as-made single-wall carbon nanotubes (SWNTs) with pure steam at 1 atm pressure. Treated samples have been characterized by high-resolution transmission electron microscopy and IR and Raman spectroscopy. Comparison between the steam purification and the standard nitric acid purification treatment shows that steam is less aggressive toward damage to the tubular nanotube wall structure and forms fewer functional groups.

Correlation of structural and electronic properties in a new low-dimensional form of mercury telluride.

Abstract: Using high resolution electron microscopy and first principles quantum mechanical calculations we have explored the fundamental physics and chemistry of the semiconductor, HgTe grown inside single wall carbon nanotubes. This material forms a low-dimensional structure based on a repeating Hg2Te2 motif in which both atom species adopt new coordination geometries not seen in the bulk. Density-functional theory calculations confirm the stability of this structure and demonstrate conclusively that it arises solely as a consequence of constrained low dimensionality. This change is directly correlated with a modified electronic structure in which the low-dimensional form of HgTe is transformed from a bulk semimetal to a semiconductor.

MCM-41 supported Cu-Mn catalysts for catalytic oxidation of toluene at low temperatures

Abstract: Complete catalytic oxidation of toluene was investigated on Cu−Mn doped mesoporous and microporous catalysts, i.e., Cu−Mn/MCM-41, Cu−Mn/β-zeolite, Cu−Mn/ZSM-5 (where SiO2/Al2O3 is either 25 or 38), and Cu−Mn/porous silica, in the presence of excess oxygen. The result shows that mesoporous catalysts have exhibited the highest catalytic activity among these catalysts above. The less amount of coke formation due to the unique mesoporous structures could play a key role in the high activity on the mesoporous catalyst. In addition, the bimetallic Cu−Mn−MCM-41 supported catalyst shows higher oxidation activity than either single metal catalyst, i.e., Cu−MCM-41 and Mn−MCM-41. The highly dispersed Cu−Mn mixed oxides on mesoporous structures probably provide active sites for the complete oxidation of toluene on these mesoporous catalysts.

Influence of double promotion on HDS catalysts prepared by urea-matrix combustion synthesis

Abstract: The stringent environmental regulations in the US, Japan and Europe are requiring significant improvement in the quality of transportation fuels. A new strategy based on urea-matrix combustion method for the synthesis of alumina-supported molybdate-based mixed oxides (i.e., bimetallic and trimetallic oxides) has been applied. This permits to control the alumina–mixed oxide interaction and therefore the HDS catalytic behaviour. The oxidic and sulfurized states of the HDS catalysts were characterized by X-ray diffraction (XRD), laser Raman spectroscopy (LRS), temperature-programmed reduction (TPR) and high resolution transmission electron microscopy (HRTEM). Catalyst performance was evaluated using a tubular fixed-bed reactor and the hydrodesulfurization of thiophene under normal pressure as model reaction. It has been found that Ni-promoted alumina-supported MoO 3 catalyst precursor presented a non well-ordered structure of Ni–Mo phase supported on alumina surface. However, when cobalt was added to Ni–Mo precursor the β-isomorph stability was significantly improved and the formation of alumina-supported β-Co 0.5 Ni 0.5 MoO 4 was observed. The activation treatments markedly affect the catalyst structure and hence the HDS catalytic performance. The catalyst series pretreated in H 2 S–H 2 was 2–3 times more active than those C 4 H 4 S–H 2 -pretreated catalysts and ca. 2–10 times more active than the pre-reduced samples. A significantly greater HDS activity of H 2 S–H 2 -pretreated Co 0.5 Mg 0.5 MoS x /γ-Al 2 O 3 catalyst was observed, which is attributed to the fact that both promoters are into the same network interacting directly with the molybdenum. This feature hinders not only the segregation of cobalt sulfide, but also the formation of long MoS 2 slabs.

Thermal stability and reactivity of metal halide filled single-walled carbon nanotubes.

Abstract: Thermal stability and reactivity to oxidation of several nanocomposite systems obtained by encapsulation of metal halides in single-walled carbon nanotubes are studied. Thermogravimetric analysis coupled with Raman spectroscopy allows insight into the various contributing factors, such as charge transfer, strain, and defect formation, and establishing a hierarchy of reactivity for the systems studied (AgX@SWCNTs, with X = Br, I; SWCNTs = arc discharge and HiPCO). The activation energy for oxidation decreases considerably after filling, indicating that filled nanotubes are more amenable to controlled modifications based on chemical reactivity than the originating empty nanotubes. The complete removal of the carbon shell at high temperatures does not preserve the nanowire morphology of the encapsulated halides; these are freed on surfaces in the form of nanoparticles arranged in 1D patterns. Metallic nanoparticles were obtained after hydrogen reduction of the halides, and growth of silicon nanowires in the footprint of the originating nanocomposites was demonstrated from such Co seeds. MX@SWCNTs (M = Ag, Co) can thus be used as environmentally stable nanoscale containers that allow the deliverance of catalytic nanoparticles in a prepatterned and aligned way.

Migratory insertion in N-heterocyclic carbene-containing Fe carbonyl complexes: an experimental and theoretical study.

Abstract: The compound [Fe(η-C5H5)(CO)2(Me)] reacts thermally with N-heterocyclic carbenes (L) to give both alkyl, [Fe(η-C5H5)(L)(CO)(Me)], and acyl, [Fe(η-C5H5)(L)(CO)(COMe)], derivatives. The reaction temperature has been shown to affect the product distribution. The alkyl and acyl derivatives exist in an equilibrium that is more easily perturbed than in the tertiary phosphine analogues. DFT studies on the reactivity of [Fe(η-C5H5)(CO)2(Me)] with PH3 and dihydroimidazole-2-ylidene (IH) have shown that CO exchange is energetically favoured for IH, and energetically disfavoured for PH3. The products of CO-induced migratory insertion, [Fe(η-C5H5)(L)(CO)(COMe)], are more stable than the parent alkyl, [Fe(η-C5H5)(L)(CO)(Me)], compounds. This stabilisation is larger when L = IH than when L = PH3. Stabilisation of the transition state by agostic interactions was seen in both instances, but this was significantly more pronounced for L = IH.

Crystal-encapsulation-induced band-structure change in single-walled carbon nanotubes: Photoluminescence and Raman spectra

Abstract: We report studies of the modification of the band structure of single-walled carbon nanotubes through encapsulation of the inorganic material manganese ditelluride MnTe2. Using photoluminescence excitation mapping we show that this leads to a global reduction of their first and second band gap energies by a similar percentage up to 3.8%. We interpret this as due to the additional screening which causes a change in the internal dielectric constant and a possible lowering of the carbon-carbon transfer integrals. The shifts increase with increasing tube diameter due to the increased quantity of filled materials.

‘Green’ derivatization of carbon nanotubes with Nylon 6 and L-alanine

Abstract: Amide derivatives of L-alanine and e-caprolactam were readily obtained on diamine-functionalized oxidized single-walled and pristine multi-walled carbon nanotubes through a one-step direct amidation reaction, which employs thermal activation at 160–200 °C instead of chemical activation, avoids the use of organic solvents, and requires a few hours only for completion. In the case of e-caprolactam, amino groups attached to the nanotubes initiated polymerization into Nylon 6. The functionalized nanotubes were characterized by infrared and Raman spectroscopy, scanning and transmission electron microscopy, atomic force microscopy, thermal gravimetric analysis and differential scanning calorimetry.

Cobalt N-heterocyclic carbene alkyl and acyl compounds: synthesis, molecular structure and reactivity

Abstract: The N-heterocyclic-carbene containing cobalt carbonyl compound [Co(IMes)(CO)3(Me)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene), 1, has been synthesised by tertiary phosphine displacement from [Co(PPh3)(CO)3(Me)]. Subsequent carbonylation afforded the acyl derivative [Co(IMes)(CO)3(COMe)], 2. Similarly, the compound [Co(IMes)(CO)3(COEt)], 3, has been synthesised. The compounds 2 and 3 have been shown to react with dihydrogen to form the cobalt hydride compound [Co(IMes)(CO)3(H)], 4. The molecular structures of compounds 1 and 2 have been determined.

Effects of KI encapsulation in single-walled carbon nanotubes by Raman and optical absorption spectroscopy.

Abstract: The effect of KI encapsulation in narrow (HiPCO) single-walled carbon nanotubes is studied via Raman spectroscopy and optical absorption. The analysis of the data explores the interplay between strain and structural modifications, bond-length changes, charge transfer, and electronic density of states. KI encapsulation appears to be consistent with both charge transfer and strain that shrink both the C-C bonds and the overall nanotube along the axial direction. The charge transfer in larger semiconducting nanotubes is low and comparable with some cases of electrochemical doping, while optical transitions between pairs of singularities of the density of states are quenched for narrow metallic nanotubes. Stronger changes in the density of states occur in some energy ranges and are attributed to polarization van der Waals interactions caused by the ionic encapsulate. Unlike doping with other species, such as atoms and small molecules, encapsulation of inorganic compounds via the molten-phase route provides stable effects due to maximal occupation of the nanotube inner space.

Rationalizing the catalytic performance of γ-alumina-supported Co(Ni)–Mo(W) HDS catalysts prepared by urea-matrix combustion synthesis

Abstract: A comparative study of the influence of Co (or Ni) promoter loadings and the effect of different sulfurizing agents and sulfurizing temperatures on the structure, morphology and catalytic performance of Mo- or W-based hydrodesulfurization (HDS) catalysts was carried out. Catalyst performance using a tubular fixed-bed reactor and the HDS of thiophene as a model reaction was evaluated. The oxidic and sulfurized states of the HDS catalysts were characterized by laser Raman spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and high resolution transmission electron microscopy (HRTEM). It has been found that the urea-matrix combustion (UMxC) synthesis is a simple tool for preparing supported catalysts in a short period of thermal treatment. Several consecutive stages such as urea melting, metal precursor dissolution and chemical reactions take place before and upon combustion process. The C4H4S/H2-activated Co- (or Ni-) promoted MoS2 (or WS2) catalysts present a strong synergistic effect (SE) when the Co (or Ni)/Mo (or W) molar ratio is near to 0.5, whereas the C4/C 4 = molar ratios display a weak antagonistic effect. Alumina-supported Ni–W catalyst showed an optimal SE 2.5 times higher than those for Co (or Ni)-promoted Mo HDS catalysts. The kinetic parameters for thiophene-HDS reaction were also determined, suggesting that the C–S bond cleavage reaction for alumina-supported Co(Ni)–Mo HDS catalysts and H2 activation reaction for Ni-promoted WS2catalysts play an important role in the rate-limiting step.

B(C6F5)3 adducts of transition metal acyl compounds

Abstract: The transition metal acyl compounds [Co(L)(CO)3(COMe)] (L = PMe3, PPhMe2, P(4-Me-C6H4)3, PPh3 and P(4-F-C6H4)3), [Mn(CO)5(COMe)] and [Mo(PPh3)(η5-C5H5)(CO)2(COMe)] react with B(C6F5)3 to form the adducts [Co(L)(CO)3(C{OB(C6F5)3}Me)] (L = PMe31, PPhMe22, P(4-Me-C6H4)33, PPh34, P(4-F-C6H4)35), [Mn(CO)5(C{OB(C6F5)3}Me)] 6 and [Mo(η5-C5H5)(PPh3)(CO)2(C{OB(C6F5)3}Me)], 7. Addition of B(C6F5)3 to a cooled solution of [Mo(η5-C5H5)(CO)3(Me)], under an atmosphere of CO gave [Mo(η5-C5H5)(CO)3(C{OB(C6F5)3}Me)] 8. In the presence of adventitious water, the compound [Co{HOB(C6F5)3}2{OP(4-F-C6H4)3}2] 9, was formed from [Co(P(4-F-C6H4)3)(CO)3(C{OB(C6F5)3}Me)]. The compounds 4 and 9 have been structurally characterised. The use of B(C6F5)3 as a catalyst for the CO-induced migratory-insertion reaction in the transition metal alkyl compounds [Co(PPh3)(CO)3(Me)], [Mn(CO)5(Me)], [Mo(η5-C5H5)(CO)3(Me)] and [Fe(η5-C5H5)(CO)2(Me)] has been investigated.

Redshift and optical anisotropy of collective π -volume modes in multiwalled carbon nanotubes

Abstract: A combined study concerning localized electron energy-loss spectroscopy (EELS) and modeling of collective $\ensuremath{\pi}$-volume modes in multiwalled carbon nanotubes (MWCNT) is presented The changing line width and eigenfrequency of the $\ensuremath{\pi}$-volume mode can be ascribed solely to optical anisotropy and ``cylindrical anisotropy'' Optical anisotropy results from the weighting of various nearly degenerate and nondegenerate states allowed for the ${\mathbf{E}}_{\ensuremath{\Vert}c}$ and ${\mathbf{E}}_{\ensuremath{\perp}c}$ polarizations Cylindrical anisotropy arises from a lowering of the symmetry arising from the nanotube geometry The eigenfrequency of the $\ensuremath{\pi}$-volume mode corresponds to polarization eigenmodes of graphite, and not to new maxima in the joint density of states, since momentum transfer $\mathrm{\ensuremath{\Delta}}{\mathbf{q}}_{\ensuremath{\pi}}\ensuremath{\rightarrow}0$ Results are also included from multiwalled hexagonal-boron nitride nanotubes (MWBNNT) An accurate description of the $\ensuremath{\pi}$-volume mode in multiwalled nanotubes has not been attempted so far, and is essential to resolve coupled MWCNT $\ensuremath{\pi}$-surface features, which are usually obscured in spectra obtained in penetrating-beam geometry Volume mode-extracted EEL spectra demonstrate eigenfrequency modification of coupled $\ensuremath{\pi}$-surface features in the presence of a MWCNT dielectric filling It was found, owing to dielectric screening effects and smearing of the dipole mode, that aloof-beam EELS which is conventionally applied to surface plasmon investigations, cannot give this information

Force and energy dissipation variations in noncontact atomic force spectroscopy of composite carbon nanotube systems

Abstract: UHV dynamic force and energy dissipation spectroscopy in noncontact atomic force microscopy were used to probe specific interactions with composite systems formed by encapsulating inorganic compounds inside single-walled carbon nanotubes. It is found that forces due to nanoscale van der Waals interaction can be made to decrease by combining an Ag core and a carbon nanotube shell in the silver-filled single-walled carbon nanotube (Ag@SWNT) system. This specific behavior was attributed to a significantly different effective dielectric function compared to the individual constituents, evaluated using a core-shell model. Energy dissipation measurements showed that by filling dissipation increases. Aside from minimized Joule dissipation, such an effect was attributed to two short-range mechanisms due to metal filling: softening of C-C bonds resulting in a more deformable nanotube cage, and an increased mechanical damping of the nanotube's bending oscillation mode. Thus, filled and unfilled nanotubes can be discriminated based on force and dissipation measurements. These findings have two different implications for potential applications: tuning the effective dielectric properties and tuning the interaction force for molecular absorption by appropriately choosing the filling with respect to the nanotube.

Urea–matrix combustion method: A versatile tool for the preparation of HDS catalysts

Abstract: Urea-matrix combustion method is an effective new approach for preparing highly active Ni-W, Co-Mo and Ni-Mo HDS catalysts. The improved activity is mainly associated with a very efficient interaction between the well-dispersed oxidic precursors on the alumina surface. This is reflected in a strong synergistic effect that markedly increases the C-S bond cleavage reaction whereas a less pronounced antagonistic effect affects the relative rate of hydrogen transfer reactions.

Opening and reversible filling of single-walled carbon nanotubes with various materials.

Abstract: Single-walled carbon nanotubes have been advocated as perfect candidates for the sustainable miniaturisation of electronic and mechanical nanoscale devices. The encapsulation of selected compounds within the inner hollow cavity of SWNTs allows controlled preparation of nano-meter size "nanowires" and "nanocables" with purpose-tailored physical properties. Therefore is crucial to have control of opening and closing their tips. In a previous study we showed that molten metal hydroxide [MOH (M=Cs, Na)] is filled into the carbon nanotubes and can be easily washed out with water leaving opened nanotubes. Following this approach we have explored the use of milder ways to open SWNTs that can be easily scalable for the production of large amount of opened SWNTs. The opened tubes have then successfully been filled in solution with various inorganic and organic materials.

Transmission of a plasmon e-field through carbon nanotube walls

Abstract: Multiwalled carbon nanotubes (MWCNTs) are seen to possess exceptional electronic and optical properties, and might allow the development of photonic devices based on their collective modes. MWCNTs would thus represent an exciting medium for use in nanocomposites. In this article, the transmission and attenuation of an E-field coupled to a surface plasmon mode, through the walls of double-walled and MWCNTs, is investigated and examined within the context of optical field enhancement. The surface mode under investigation originates from silver filling of the nanotubes. Highly spatially resolved electron energy-loss spectroscopy is used to estimate the silver Mie-plasmon transmission coefficient, which dictates the sensing volume of the silver filling. The eigenfrequency of this mode is also examined, with regards to possible hybridisation of the Mie- and the graphitic π-surface plasmon. While a change in the Mie-plasmon energy could not be observed, the graphitic π-surface plasmon exhibited an unusual red-shift. It appears that variation of the nanotube aspect ratio and thus of the filling diameter would allow engineering of the sensing volume, and to some degree, of the graphitic π-surface plasmon eigenfrequency.

Image Restoration of One-Dimensional HgTe Crystals Formed within Single Walled Carbon Nanotubes

Abstract: Restored high resolution transmission electron microscopy (HRTEM) images have been recorded from 1D semiconductor crystals formed within narrow diameter (ca. 1.4 nm) single walled carbon nanotubes (SWNTs). Two unique projections were obtained from separate crystal fragments encapsulated within separate nanotubes that has facilitated the reconstruction of the three dimensional arrangement of atoms within the two encapsulated fragments.