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 CK-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 [similar]160 nm to [similar]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 π* 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 sp2 to more sp3 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−1en 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 π* band may open up possibilities to influence the electronic transport properties of graphite by manipulation of defects during the preparation of the nanographite

X-ray diffraction line profile analysis of nanocrystalline graphite

The structure evolution to nanocrystalline graphite produced by ball milling in n-dodecane has been studied by Fourier analysis of broadened X-ray diffraction line profiles according to double-Voigt method. The Fourier analysis gave size and strain distributions of the coherently diffracting domains (X-ray crystallite size) and root-mean-square-strain (rmss) and their average values. The precursor graphite was defined by average crystal sizes of about hundreds of nanometers, measured along the in-plane and out-of-plane directions, and low rmss value of 0.38 × 10-3. During milling, the average crystallite sizes of graphite decreased to about 6 and 43 nm along the out-of-plane and in-plane directions, respectively. Correspondingly, the rmss of milled graphite increased to 6.54 × 10-3. Analysis of the out-of-plane to in-plane crystallite size ratios showed that the crystallites became progressively thinner and flatter. A linear relationship between rmss and reciprocal crystallite size along the stacking axis revealed that size of disordered boundary regions gradually increased at the expense of ordered crystalline regions. A model describing crystalline-nanocrystalline transformation of graphite along different crystallographic axis was formulated and used to discuss the experimental data. It was concluded that a distortion-controlled process is responsible for the crystalline-nanocrystalline transformation of graphite milled in n-dodecane

Isothermal evaporation of ethanol in a dynamic gas atmosphere

Optimization of evaporation and pyrolysis conditions for ethanol are important in carbon nanotube (CNT) synthesis. The activation enthalpy (ΔHǂ), the activation entropy (ΔSǂ), and the free energy barrier (ΔGǂ) to evaporation have been determined by measuring the molar coefficient of evaporation, kevap, at nine different temperatures (30-70°C) and four gas flow rates (25-200 mL/min) using nitrogen and argon as carrier gases. At 70°C in argon, the effect of the gas flow rate on kevap and ΔGǂ is small. However, this is not true at temperatures as low as 30°C, where the increase of the gas flow rate from 25 to 200 mL/min results in a nearly 6 times increase of kevap and decrease of ΔGǂ by ~5 kJ/mol. Therefore, at 30°C, the effect of the gas flow rate on the ethanol evaporation rate is attributed to interactions of ethanol with argon molecules. This is supported by simultaneous infrared spectroscopic analysis of the evolved vapors, which demonstrates the presence of different amounts of linear and cyclic hydrogen bonded ethanol aggregates. While the amount of these aggregates at 30°C depends upon the gas flow rate, no such dependence was observed during evaporation at 70°C. When the evaporation was carried out in nitrogen, ΔGǂ was almost independent of the evaporation temperature (30-70°C) and the gas flow rate (25-200 mL/min). Thus the evaporation of ethanol in a dynamic gas atmosphere at different temperatures may go via different mechanisms depending on the nature of the carrier gas

Polymorphic transformation of iron-phthalocyanine and the effect on carbon nanotube synthesis

Organometallic compounds such as phthalocyanine are useful precursors for carbon nanotube formation by pyrolysis because they can supply both carbon and metal catalysts needed for synthesis. Prior milling of iron-phthalocyanine (FePc, FeC32H16N8) is investigated and shown to affect the sublimation temperature of the precursor and the nanotube diameter. Without prior milling, a sublimation temperature of 600-650 °C is necessary to produce a sufficient amount of vapors prior to pyrolysis. At that temperature, there is also some decomposition. Milled FePc sublimates at the highest rate at 400-450 °C, where no decomposition occurs. The lower temperature shift of the maximum of the sublimation rate appears to be due to changes in polymorphs upon milling. Carbon K-edge near-edge X-ray absorption fine structure, infrared spectroscopies, and X-ray diffraction analysis show that packing of the phenyl subunits of FePc is modified upon milling and an α-like polymorph is produced. Upon heating, the milled material undergoes polymorphic transformation to a mixture of a and β forms and a third unidentified phase. Above 550 °C, this mixture transforms entirely to the β polymorph. During pyrolysis of the FePc vapors at 900 °C, multiwalled carbon nanotubes (MWCNT) with different diameters are produced between milled and non-milled samples. Transmission electron microscopy shows the average diameter of the MWCNTs produced from the non-milled and milled FePc precursor is about 40-100 nm and 15-50 nm, respectively. It is suggested that the decrease in nanotube diameter caused by the milling of the precursor is due to presence of higher concentrations of un-decomposed FePc molecules with fixed C/Fe atomic ratio in the gas-phase prior to pyrolysis. These results show the importance on the choice of materials for CNT synthesis since small changes in the structure of precursors affect nanotube formation kinetics

Mechanism of carbon nanotube dispersion and precipitation during treatment with poly(acrylic acid)

Carbon nanotubes have been shown to be easily dispersed within an acidic aqueous solution of poly(acrylic acid) but precipitate when the pH is increased. Transmission electron microscopy showed that the nanotubes were more exfoliated under the acidic condition but highly aggregated under the basic condition. Carbon K-edge NEXAFS spectroscopy showed that the carbon nanotubes did not chemically react with poly(acrylic acid) during the dispersion or precipitation and that the dispersion mainly involved physical adsorption of poly(acrylic acid) onto the nanotubes. Together with the carbon K-edge NEXAFS spectra, the cobalt L3,2-edge NEXAFS spectra suggested that under the basic condition, the cobalt impurity within the nanotubes strongly reacted with poly(acrylic acid) resulting in complex formation. Cobalt reduces the adsorption of poly(acrylic acid) onto the nanotubes, which then reduced the nanotube dispersion and resulted in the precipitation

Developing saponite supported cobalt–molybdenum catalysts for upgrading squalene, a hydrocarbon from the microalgae Botryococcus braunii

The long chain hydrocarbons derived from the microalgae Botryococcus braunii are potential source of liquid biofuels for oil refineries. However, there is room for catalyst development on treating these new oil sources since they differ from mineral oils. In this work, the cobalt-molybdenum catalysts supported on the phyllosilicate saponite (Al2O3-SiO2) have been used for upgrading squalene (C30H50) a hydrocarbon related to B. braunii oil. The saponite supported catalysts were synthesised with varying aluminium to silicon (Al:Si) ratios of 1:2, 1:10, 1:20, 1:30 and 1:50 using a non-hydrothermal method. The cobalt and molybdenum (Co:Mo) ratio remained unchanged at 5:1 during the synthesis. Characterisation of the saponite supported catalysts was carried out using X-ray diffraction (XRD), nitrogen gas adsorption, X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. It appears that the catalysts with the Al:Si ratios of 1:2 and 1:10 incorporate aluminium into tetrahedral and octahedral sites. When the Al:Si ratios are reduced from 1:20 to 1:50, the aluminium occupies octahedral sites only. Catalytic upgrading of squalene using these catalysts was carried out at the temperature of 400°C with Formier gas (5% H2:95% N2). Nuclear magnetic resonance (NMR) and gas chromatography-mass spectrometry (GC-MS) analysis showed the formation of pentacyclic triterpenes with a double bond. In addition, the 1:2 saponite supported catalyst has hydrogenation, hydrocracking and esterification abilities by virtue of its capacity to adsorbed exchangeable protons and carboxylate groups. Removal of the adsorbed exchangeable protons caused structural collapse during the reaction for some catalysts. Nevertheless the 1:20, 1:30 and 1:50 saponite supported catalysts remained stable. Our results provide an understanding of reactions of saponites with squalene and the potential of the saponite supported catalysts in biofuel upgrading

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