Nguyen H. Tran

Photoemission and absorption spectroscopy of carbon nanotube interfacial interaction.

Element-specific techniques including near edge X-ray absorption fine structure, extended X-ray absorption fine structure and X-ray photoemission spectroscopy for the characterization of the carbon nanotube interfacial interactions are reviewed. These techniques involve soft and hard X-rays from the laboratory-based and synchrotron radiation facilities. The results provided information of how the nano-particles of catalysts are involved in the initial stage of nanotube growth, the nanotube chemical properties after purification, functionalization, doping and composite formation.

Nanocrystalline order of zinc oxide thin films grown on optical fibers

The crystallographic orientation of polycrystalline zinc oxide films grown on optical fibers using single-source chemical vapor deposition (SS CVD) of basic zinc acetate have been studied. The films have been characterized using near-edge x-ray absorption fine structure. For the SS CVD ZnO growth on planar substrates, the film orientation can be varied from randomly oriented to highly c-axis oriented. In contrast, the films grown on optical fibers were either randomly oriented or a,b-axis oriented, depending on growth conditions. The correlations between growth conditions and the crystallographic properties of the films on fibers were discussed. The results suggest that factors such as curvature may have an effect on the crystallinity of film growth.

Unoccupied electronic structure of ball-milled graphite

Changes in electronic and vibrational structure of well characterised macrocrystalline graphite milled by a planetary ball-mill are investigated by Raman spectroscopy and Near Edge X-ray Absorption Fine structure (NEXAFS) measurements at the C K-edge. The electronic structure changes at the surface and in the sub-surface of the particles are examined by comparing two-different NEXAFS detection modes: total fluorescence yield (TFY) and partial electron yield (PEY) respectively. When the in-plane crystallite sizes of graphite are decreased to nanosized (from approximately 160 nm to approximately 9 nm), a new spectral structure appears in TFY at 284.1 eV which is not present in the macrocrystalline graphite. This feature is assigned to electronic states associated with zigzag edges. Further the TFY shows a shift of the main graphite pi* band from 285.5 to 285.9 eV, attributed to breaking the conjugation and hence the electron localization effect during milling, The TFY spectra also show strong spectral features at 287.5 and 288.6 eV, which suggest that the local environment of carbon atoms changes from sp(2) to more sp(3) due to physical damage of the graphite sheets and formation of structures other than aromatic hexagons. Complementary Raman spectroscopic measurements demonstrate an up-shift of the graphite G band from 1575 to 1583 cm(-1)en route to nanosize. The changes in TFY NEXAFS and Raman spectra are attributed to modification of the sub-surface electronic structure due to the presence of defects in the graphite crystal produced during milling. The discovery of the strong spectral feature at 284.1 eV in nanographite and the 0.4 eV up-shift of the pi* band may open up possibilities to influence the electronic transport properties of graphite by manipulation of defects during the preparation of the nanographite.

Dispersion of silicate nano-plates within poly(acrylic acid) and their interfacial interactions

Abstract Dispersion of the silicate nano-plates of clays within polymers for nanocomposite formation generally involved interactions of the polymers with the plate surfaces. In this paper, we provide preliminary evidence showing dependency of the dispersion on the interactions of the plate lattices with polymers. Experiments have been carried out on the dispersion of the natural clay, montmorillonite (≈0.5 g) within the aqueous solutions of poly(acrylic acid) at varying temperatures and the products analysed by X-ray diffraction and related techniques. In the product from reaction at 60 ˚C, the silicate plates are dispersed with fully extended chains of poly(acrylic acid) intercalated within the interlayer spaces between unexfoliated plates. Photoemission spectroscopy showed that during the process sodium ions are removed from the silicate surface. In the product from reaction at 85 ˚C or above, the silicate plates are partly exfoliated. During the process, Fe2+ ions within the clay lattice are oxidised by the acidic poly(acrylic acid) solution, which suggests strong reaction of poly(acrylic acid) with the silicate plates facilitating exfoliation. Interestingly, the Al and Mg concentrations in the lattice remain virtually unchanged during the reactions.

Influence of bond defects on coiling of graphite

Abstract The effect of annealing at 1400 ˚C in argon on the bond structure of graphite ball milled for 100 h at 400 rpm in polar (water) and in non-polar (n-dodecane) liquids was investigated primarily by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and transmission electron microscopy (TEM). Carbon K-edge NEXAFS allows the distortion of bonds in the hexagonal lattice to be investigated. It is shown that in-plane sp2 bonds are strained and distorted after ball milling because sp3 bonds are introduced. Not surprisingly, annealing of the milled product restores sp2 bonds but at the same time, coiling and formation of tube-like structures takes place. It is well established that graphite is not formed on annealing, and hence the results shown here demonstrate that the loss of sp3 carbons on annealing must proceed via a different mechanism by which they are formed by milling.

Mechanism Of Silica Nano-Plate Formation From Lucentite

The mechanism of formation of silica nano-plates by exfoliation of a phyllosilicate magnesium containing clay, Lucentite, in an aqueous solution of poly(acrylic acid) has been studied. Fourier transform infrared spectroscopy and Mg K-edge near edge X-ray adsorption fine structure (NEXAFS) analysis shows that non-surface (bulk) Mg ions were not chemically involved in the poly(acrylic acid)/clay intercalation, but were substantially involved in the exfoliation resulting in the silica nano-plates. During intercalation, O K-edge NEXAFS shows that surface defects were formed which represent additional structural branches on the surface. During exfoliation, these increased significantly. Si L3,2-edge NEXAFS measurements shows that this occurred by migration of SiO4 groups within the exfoliated silica plates.

Formation mechanism and surface property of exfoliated silica nano-plates

The silica nano-plates formed by exfoliation of the synthetic clay Lucentite in the poly(acrylic acid) aqueous solution has been examined using near edge X-ray adsorption fine structure (NEXAFS). Mg K - edge NEXAFS analysis shows that non surface (bulk) Mg ions were not chemically involved in the poly(acrylic acid)/clay intercalation, but were substantially involved in the exfoliation resulting in the silica nano-plates. During intercalation, O K - edge NEXAFS shows structural branches on the plate surfaces were formed. During exfoliation, these increased significantly. Si L3,2 - edge NEXAFS shows this occurred by migration of SiO4 groups within the exfoliated silica plates.

Mechanism of exfoliation of clays

The exfoliated structures of lamellar clays offer potentials as precursor for formation of nano‐structured materials. We explored the Synchrotron Radiation soft X‐ray techniques and nuclear magnetic resonance to study the exfoliation of phyllosilicate clays by polymers. Experiments were carried out in dispersions containing approximately 1% weight phyllosilicate in 5% aqueous solution of poly(acrylic acid) at different temperatures. The clays were exfoliated as the reaction was performed at 85°C. X‐ray photoemission spectroscopy indicated that the exfoliated structures were consisted of virtually pure silica nano‐plates. 29Si nuclear magnetic resonance and oxygen K‐edge near edge X‐ray absorption fine structure indicated that the surface of the plates was terminated by high concentrations of the silanol groups, which created structural branches. The formation of the branches created a steric effect that inhibited the stacking of the plates, which eventually resulted in the exfoliation.

Microstructural Changes upon Milling of Graphite in Water and Subsequent MWCNT Formation During High Temperature Annealing

The method of preparing carbon nanotube (CNT) by milling of graphite particles in water followed by high temperature annealing is proposed and the mechanism discussed. Transmission electron microscopy (TEM) and X‐ray diffraction (XRD) line broadening analysis reveal that cleavage of the graphite particles occurs preferentially along the out‐of‐plane π bonds. Carbon K‐edge near edge X‐ray absorption fine structure (NEXAFS) of the milled graphite shows an increased sp3 character of the C=C bonds, but no major bonds rupture in the graphene sheets. The annealing at 1400 °C for 4 h of the milled graphite in argon results in formation of multiwalled carbon nanotubes accompanied with a number of coiled and twisted stacks of graphene sheets. The increased structural disorder of the milled graphite and presence of iron contaminations facilitate the rolling up of the cleaved graphene sheets during annealing.

Carbon nanotube formation from milled iron-phthalocyanine

Organometallics supply carbon and metal catalyst needed for carbon nanotube synthesis. It is shown that experiments involving prior milling of iron-phthalocyanine (FePc, FeC32H16N8) before pyrolysis at 800 °C in argon produces carbon nanotubes with diameters ranging from 5 to 15 nm. Under the same conditions, the diameters of nanotubes produced from non-milled FePc range from 20 to more than 50 nm. Milling decreases the onset of sublimation of FePc from about 450 to 200 °C and also reduces the activation energy barrier of sublimation at 360 °C from 287 to 193 kJ/mol. This appears to be due to changes in molecular packing of the phthalocyanine precursor. It is suggested that the decrease in nanotube diameter is due to greater homogeneity in the gas phase on pyrolysis after milling, which leads to more systematic capture of carbon species during the catalytic growth of the carbon nanotubes.