Magnetic and high resolution TEM studies of nanographite derived from nanodiamond

The time evolution of graphitization was analyzed based on the structural and magnetic properties of nanodiamond samples annealed at 1600 °C for various time intervals. High resolution TEM and XRD show that the nanodiamond particles are converted to spherical onions for short annealing time intervals, and then they are completely transformed to polyhedral nanographite through the annealing for 120 min. The in-plane orbital susceptibility χ_orb (300 K) analyzed on the basis of Kotosonov’s equation remains fairly constant in the range −5.5 × 10⁻⁶ to −5.9 × 10⁻⁶ emu/g, suggesting that the coherent scattering region (CSR) and the Fermi energy are independent of the annealing times. It also indicates the presence of serious defects affecting the electronic structure of nanographite. As the annealing time increases from 2 to 120 min, the Pauli paramagnetic susceptibility decreases. The broad ESR signal is associated with the spins localized on the edge states of the π-electron graphite network. The ESR linewidth becomes nine times less as the heat-treatment time rises from 2 to 120 min. The relatively large linewidth is associated with the scattering of π-electrons by the edge phonon modes and magnetic interaction between the edge-localized spins. During continuous annealing the nanoparticle structure changes from more defective to less defective and the number of edge-localized spins in a particle drops from ca. 2 to ca. 1 despite the CSR size and the Fermi energy remain practically the same.     

Thermal desorption of deuterium from modified carbon nanotubes and its correlation to the microstructure

The process of deuterium desorption from single-wall carbon nanotubes (SWNTs) modified by atomic (D) and molecular (D₂) deuterium treatment was investigated in an ultrahigh vacuum environment using thermal desorption mass spectroscopy (TDMS). Microstructural and chemical analyses of SWNT material, modified by this deuterium interaction, were performed by means of a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results disclose characteristic features in the TDMS spectra of deuterium evolved from the SWNT material, which can be correlated to the microstructure of nanocarbon material modified by D-treatment. The TDMS spectra of deuterium originating from the large diameter rope type nanotube structures, resulting from a prolonged low-pressure (D + D₂) gas mixture treatment, exhibit three overlapping desorption peaks: a dominant one with a desorption activation energy (E_des) of approx. 2.86 eV and lower intensity peaks at E_des of ∼1.50 and 2.46 eV. On the other hand, the TDMS spectra of deuterium taken from the “coral reef”-like carbon nanostructures, obtained after prolonged treatment of SWNTs to a high-pressure (D + D₂) gas mixture produced at high temperature, reveal the coexistence of four superimposed desorption peaks with E_des ranging from 1.23 to 4.4 eV. A dominant desorption peak with E_des ≈ 4.4 eV, can be attributed to bulk diffusion of D trapped within this nanocapsule bulk structure.     

Two structural types of carbon bi-filaments

The structure of carbon bi-filaments synthesized on nickel wire by a hot filament-assisted CVD (HF-CVD) technique has been investigated by TEM, HRTEM and SAED. Two main types of bi-filament have been found. The first type consists of elongated dense regions (“rods”) formed by bundles of graphene layers. The “rods” are inclined at an angle to the filament axis and appear to be arranged around the filament axis with pseudo-rotational symmetry. Along these bi-filaments (diameter 140-260 nm) lens-like cavities of different sizes are observed. The Ni catalytic particles have lenticular shape with pseudo-rotational symmetry and possess either a fcc or a hcp lattice. The top and bottom parts of a catalyst particle are usually terminated by {1 1 1} planes, (or {0 0 0 1} for hexagonal lattice), while its inclined part is formed by stepped terraces parallel to (1 1 1). The structural organization of bi-filaments as well as their defects are determined by time dependent surface structure of the catalyst particles and by an oscillatory process of carbon concentration on nickel (1 1 1) or (0 0 0 1) facets. The second type of bi-filament consist of two subfilaments, semi-circular in cross section, connected together by flat sides with pseudo-mirror symmetry. The diameter of the complete filaments is 65-75 nm. Nanometre-sized catalyst particles are distorted pyramids and usually have a hcp lattice. The basic structure of these bi-filaments consists of elongated regions (“rods”) formed by bundles of graphene layers. In this case the “rods” form a layer structure.     

Enhanced electrochemical performance of nano-sized LiFePO4/C synthesized by an ultrasonic-assisted co-precipitation method

A simple and effective method, the ultrasonic-assisted co-precipitation method, was employed to synthesize nano-sized LiFePO₄/C. A glucose solution was used as the carbon source to produce in situ carbon to improve the conductivity of LiFePO₄. Ultrasonic irradiation was adopted to control the size and homogenize the LiFePO₄/C particles. The sample was characterized by X-ray powder diffraction, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). FE-SEM and TEM show that the as-prepared sample has a reduced particle size with a uniform size distribution, which is around 50 nm. A uniform amorphous carbon layer with a thickness of about 4-6 nm on the particle surface was observed, as shown in the HRTEM image. The electrochemical performance was demonstrated by the charge-discharge test and electrochemical impedance spectra measurements. The results indicate that the nano-sized LiFePO₄/C presents enhanced discharge capacities (159, 147 and 135 mAh g⁻¹ at 0.1, 0.5 and 2 C-rate, respectively) and stable cycling performance. This study offers a simple method to design and synthesis nano-sized cathode materials for lithium-ion batteries.     

Microstructural and electrochemical characterization of RuO2/CNT composites synthesized in supercritical diethyl amine

A simple and efficient route to decorate carbon nanotubes (CNTs) with nanocrystalline RuO₂ has been developed. In this method, RuCl₃ · 3H₂O was oxidized into RuO₂ by oxygen in supercritical diethyl amine, and the produced RuO₂ deposited on CNTs, resulting in RuO₂/CNT nanocomposites. The as-prepared composites were structurally and morphologically characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy (TEM). TEM images showed that RuO₂ nanoparticles attached on CNTs had uniform shape and a narrow particle size distribution. The loading content and the size of RuO₂ particles on CNTs could be tuned by changing the mass ratio of RuCl₃ · 3H₂O/CNT. Electrochemical measurements by cyclic voltammetry demonstrated a substantial increment of the specific capacitance of CNTs due to a pseudocapacitance originated from the deposited RuO₂ nanoparticles.     

Partially graphitized carbon filaments from as-synthesized silica/surfactant composite

The carbonization behavior of surfactants templated within mesoporous silica is studied in detail. Cetyltrimethylammonium bromide and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) are used as the structure-directing agents for MCM-41 and SBA-15 synthesis, respectively. Thermal treating the as made silica/surfactant composites under argon flow at 900 °C produces partially graphitized carbon filaments as a result of the carbonization of the surfactants within the mesopores. Furthermore, the carbon materials derived from P123 in SBA-15 yield a more developed graphite structure than the carbon obtained from CTAB in MCM-41, as evidenced by the narrower X-ray Bragg reflections in the powder XRD and larger I_G/I_D ratio in the Raman spectra.     

Nanographite structures formed during annealing of disordered carbon containing finely-dispersed carbon nanocapsules with iron carbide cores

Disordered carbon containing finely-dispersed carbon nanocapsules with iron carbide cores were synthesized by a modified method in which low-current plasma discharge was generated in liquid ethanol with ultrasonic irradiation. The structure of nanographite forms prepared by the annealing at 900 °C for 2 h of disordered carbon containing finely-dispersed carbon nanocapsules was studied. Transmission electron microscopy (TEM) studies of the powder sample after annealing revealed most part of disordered carbon was transformed into nanographite ribbons, hollow polyhedral graphitic cages and thick carbon shells with the turbostratic structure of carbon layers. TEM observations of the carbon layers revealed stacking defects. Selected-area diffraction and fast Fourier transforms of digitized images revealed that carbon inter-layer spacings vary from 3.4 to 3.5 Å. XRD analysis showed that annealing of the powder sample at 900 °C for 2 h resulted in the decompositions of iron carbide cores and a well-defined broad carbon peak (0 0 2) centered at 2θ ≈ 25.9° (d₀₀₂ = 3.44 Å) was detected. The growth of the I_D/I_G ratio and shift of the D peak to a lower wavenumber may indicate increase in size the graphite clusters and ordering carbon structure, i.e. appearance of nanographite structures.     

Characterization and magnetic properties of carbon-coated cobalt nanocapsules synthesized by the chemical vapor-condensation process

Carbon-coated cobalt nanocapsules were synthesized by the chemical vapor-condensation process with cobalt carbonyl (Co₂(CO)₈) used as precursor and carbon monoxide (CO) as carrier gas. The characterization and magnetic properties of carbon-coated cobalt nanocapsules were investigated systematically. The transmission electron microscope (TEM) images showed that the as-prepared nanoparticles consist of a metal core and an amorphous carbon shell. X-ray diffraction and TEM selected area diffraction revealed the presence of f.c.c. Co phase, h.c.p. Co phase, and minority Co₂C, Co₃C phases. The saturation magnetization at room temperature of the nanocapsules is 146.9 Am² kg⁻¹, which is 90% of the bulk ferromagnetic element counterpart. The coercive force at room temperature of the nanocapsules is 0.12 T, while the ratio of remnant to saturation magnetization M_r/M_s is about 0.4. The saturation magnetization and the coercive force increase with increasing the decomposition temperature, mainly due to the increase of the size of the magnetic particles. The decomposition of the cobalt carbonyl (Co₂(CO)₈) and CO gas can decrease efficiently the oxygen content in nanocapsules. The metallic Co nanoparticles completely coated by carbon can resist the dilute acid erosion as well as the oxidation. The thermal stability of the Co nanocapsules is also studied.     

A thermogravimetric analysis/mass spectroscopy study of the thermal and chemical stability of carbon in the Pt/C catalytic system

Vulcan XC72 carbon powder and Pt/Vulcan XC72 catalytic powder were characterized by transmission electron microscopy (TEM) and their reactivity under controlled atmospheres was studied as a function of the temperature. Under air atmosphere, production of water was detected by thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) measurement at m/z 18, which evidenced that hydrogenated surface functions were present on the carbon substrate. Under argon atmosphere, the comparison of TGA-MS measurements performed at m/z 18 and m/z 44 with TEM and XRD results, together with XPS measurements, indicated that platinum surface oxides are rather Pt(OH)₂ than PtO or PtO₂ species. Such reactive surface species is involved in the degradation mechanism of carbon support under air and inert atmospheres. Under H₂(3%)/Ar atmosphere, hydrocarbon production coming from “reforming” reactions of the carbon support started at very low temperatures (below 373 K). TEM images of the same catalytic powder region before and after thermal treatment at 423 K under reducing atmosphere clearly displayed consumption of the carbon substrate. The reaction products may not only affect the intrinsic properties of the support but also the catalytic properties of platinum particles: reaction products could poison the anodic catalyst.     

Influence of SiAlON addition on the microstructure development of hot-pressed ZrB2–SiC composites

The impact of SiAlON on densification behavior and microstructure of the ZrB₂-SiC composite was investigated. ZrB₂, SiC, and SiAlON were used as the initial materials to produce ZrB₂-SiC composite by hot pressing at 1900 °C. A fully dense composite was obtained having ~99.9% relative density. High-resolution X-ray diffraction (HRXRD) assessment verified the in-situ formation of ZrC, and the presence of residual carbon, SiAlON, and ZrB₂ and SiC phases in the as-sintered ceramic. Furthermore, the thermodynamic calculations confirmed the results attained by HRXRD. In addition, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized for the microstructural investigation. SEM fractographs indicated the impact of SiAlON on the hindering of grain growth and the formation of flaky phases (graphitized carbon or solidified liquid phase) at the grain boundaries. TEM studies revealed the presence of a transparent glassy phase at the particle interfaces. A significant impact of liquid phase sintering was also affirmed in the clean interfaces.     

Formation of filamentous carbon during methane decomposition over Co-MgO catalysts

The carbon formation during methane decomposition was investigated at 900 °C over the 48 wt% Co-MgO catalysts as a function of the calcination temperature T_c used in their preparation. Examination of the carbonaceous deposits by transmission electron microscopy revealed three kinds of structures: shapeless tangles, shell-like materials, and carbon filaments. In another set of experiments, the structural characteristics of the calcined catalysts were investigated using temperature-programmed reduction (TPR) and X-ray diffraction (XRD). Co₃O₄, Co₂MgO₄, and (Co, Mg)O (solid solution of CoO and MgO) were identified for T_c≤700 °C, Co₃O₄ and (Co, Mg)O for T_c=800 °C and only (Co, Mg)O for T_c=900 °C. It was found that the metal particles originated from the reduction of the solid solution favored the formation of filamentous carbon. A possible explanation is proposed.     

Density measurement of fine aerosol fractions from wood combustion sources using ELPI distributions and image processing techniques

Aerosols from combustion sources are of high concern since they present a risk for health and environment. Particle size distribution of aerosols and in particular number size distribution are easily and quickly obtained using an Electrical Low Pressure Impactor (ELPI). However, this technique is depending of aerosol density; ρ, which may lead to biased particle size distributions. Aerosol density from combustion sources is usually not well known and depends on several parameters. Aerosol density cannot be measured with usual methods since there is generally not enough matter collected on each stage of the ELPI. Our approach uses electronic microscopy to evaluate ρ at each impaction stage in order to increase the accuracy of the number size distributions resulting from the ELPI measurements.Particles were collected on glass substrates deposited on each impaction stages. Images were obtained using a scanning electron microscope and image processing tools were applied.This method was first tested with silica particles resulting from a combustion process which have a constant density found to be comprised between 2.2 and 2.4 g cm⁻³ for stages 2 (57 and 95 nm) and 3 (95 and 158 nm), respectively. Once validated, this method was used to determine the density of wood combustion aerosols. The results match well for fly ashes from wood combustion with densities varying from 1.1 to 3.0 g cm⁻³ for particles of mean equivalent diameter ranging from 69 to 157 nm, respectively.     

Heat-treatment of two heavy petroleum products containing vanadium and nickel

A raw coke very rich in vanadium (2–3%) and an asphaltene with a high content in nickel were studied by means of a high resolution transmission electron microscope, a scanning transmission electron microscope equipped with X-ray energy dispersive analyser and by X-ray diffraction. Vanadium diffuses out from the raw-coke during heat-treatment and forms small crystals of V₂O₃ trapped inside of carbon pores. After further heat-treatment vanadium carbide is formed which causes a catalytic transformation of the coke into a thermally stable partially graphitized carbon. In the asphaltene, nickel forms cubic crystals of metal at about 400°C. In both cases the metallic impurities are not uniformly distributed throughout the solid. All metallic matter becomes concentrated in a few particles, whereas most of the carbon is metal free. This appears to be common for samples of a low metallic content.     

Nano-structured carbon obtained by chlorination of NbC

New carbon materials have been synthesized by chlorinating niobium carbide at different temperatures. During the reaction, the volatile niobium chloride, mainly NbCl₅, is formed and eliminated leaving the carbon in the shape of spherical particles. TEM examination of the remaining particles revealed that at 400 °C and 700 °C they are composed of a NbC core with an amorphous carbon shell outside. The NbC cores are very small (∼20 nm) at 700 °C and they are not observed at 900 °C. Carbon particles without the NbC core present a more ordered structure composed of concentric wavy graphene layers. This structure seems intermediate between carbon onions and carbon blacks. The 900 °C sample presents a very high BET surface area (1292 m² g⁻¹) and the lower temperature samples exhibit a hysteresis phenomenon that can be attributed to the existence of large pores in the interspace between the NbC core and the carbon shell.     

Carbon fiber paper for fuel cell electrode

Carbon fiber paper (CFP) has many advantages to be used for fuel cell electrode. In this presentation, CFP was prepared from pitch-based carbon fiber through impregnation with resin, molding, and heat-treatment. Effects of heat-treatment on the properties and structure of resultant CFP were studied by means of electrical and mechanical property measurement, X-ray diffraction, and SEM. The results showed that the electrical resistance and tensile strength were decreased at higher heat-treatment temperature, and d₀₀₂ became smaller, while L_a and L_c got larger. CFP with thickness of 0.3 mm, bulk density of 0.47 g/cm³ and specific resistance of 200 μΩ m was produced after heat-treatment at 2773 K.     

Solvated structure of C60 nanowhiskers

This work characterizes the structure of C₆₀ nanowhiskers prepared by the liquid-liquid interfacial precipitation method in the C₆₀-saturated m-xylene and isopropyl alcohol system. Transmission electron microscopy and X-ray diffraction measurement show that the C₆₀ nanowhiskers had a hexagonal structure with cell dimensions a = 2.407 nm and c = 1.018 nm which is different from pristine C₆₀. The structure of the C₆₀ nanowhiskers in solution is different from that of the solvated structure reported for the C₆₀ nanotubes but similar to that reported for the C₆₀ bulk crystal solvated with m-xylene. X-ray diffraction analysis also showed a shift to fcc structure after solvent evaporation. The C₆₀ nanowhiskers prepared using toluene as solvent also showed a similar solvated structure, and a more rapid structural change into fcc upon drying was again observed.     

Synthesis and electrochemical properties of Sn87Co13 alloys by NaBH4 and sodium naphthalenide reduction methods

Sn₈₇Co₁₃ alloys are prepared by two different reduction methods and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and electrochemical cycling. One method, using NaBH₄ as a reducing agent, obtains aggregated particles with particle sizes from 20 to 200 nm. The second method, using sodium naphthalenide as a reducing agent, shows a well-dispersed nanoalloy coated with amorphous carbon, with a particle size of 15 nm. Although electrochemical results shows that the charge capacity of the two alloys is quite similar, 662 mAh/g, the capacity retention of the nanoalloy prepared using sodium naphthalenide was 427 mAh/g, which is two times higher after 30 cycles than the bulk analogue obtained using NaBH₄. This is due to the uniform particle size and amorphous carbon layer that effectively reduces anisotropic volume expansion and also minimizes particle aggregation and pulverization that causes a direct electrical disconnection with the copper current collector.     

Reaction of layered carbon fluorides CxF (x = 2.5-3.6) and hydrogen

The layered carbon fluorides C_xF (x = 2.5, 2.8, 3.6), generally classified as fluorine-graphite intercalation compounds, were heat-treated in hydrogen gas. These fluorides are more reactive with hydrogen compared to (CF)_n and (C₂F)_n. Reduction of C_xF to graphite-like carbon starts at about 573 K, and proceeds gradually along with the elevation of temperature. Fluorine atoms in C_xF are eliminated as HF in the reduction process without being substituted by hydrogen atoms. Systematic difference was not found in the average crystallite sizes of the carbon material prepared from C_xF by the reduction with hydrogen and that by the pyrolysis in vacuum. On the other hand, interlayer distance and fluorine content of the former are smaller than those of the latter. In the case that the C_xF precursor maintains a large particle size, the reduced carbon as well as the pyrolytically prepared carbon possesses a foam-like shape due to the exfoliation during the heat treatment.     

Nanoscale artifacts in RuO4-stained poly(styrene)

This research studies the effects of RuO₄ exposure on various samples of poly(styrene) (PS). Transmission electron energy-loss spectroscopy (EELS) shows a decrease in the 7 eV π-π∗ transition characteristic of aromatic rings in PS indicating that RuO₄ covalently alters aromatic character. Imaging and selected-area electron diffraction show that a layer of RuO₂ solid forms on the surface of bulk PS specimens exposed to RuO₄. High-resolution TEM imaging (HREM) shows that RuO₂ nanocrystals consistently condense on specimen surfaces, independent of the chemical nature of the specimen below. These nanocrystals modulate contrast at length scales on the order of 2-5 nm in TEM images and limit resolution at nanometer length scales.     

Growth of multiwalled carbon nanotubes during the initial stages of aerosol-assisted CCVD

We report a study of the initial stages of growth of aligned multiwalled carbon nanotubes (MWNT) synthesised by catalytic chemical vapour deposition (CCVD) of liquid aerosol obtained from toluene/ferrocene solution. A special experimental procedure has been developed to stop the process after short durations (30 s to 2 min). Two different pyrolysis temperatures are considered: 800 and 850 °C. Both scanning and transmission electron microscopy (SEM, TEM) coupled to energy-dispersive X-ray (EDX) analyses are used in order to determine the location of catalyst particles and to examine their chemical nature, morphology and size distribution when nanotubes start to grow. During the early stages (30 s), we observe the formation of a layer of catalyst particles on silicon substrates before the growth of nanotubes. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements indicate the occurrence of iron oxide (γ-Fe₂O₃ or Fe₃O₄). In addition, XPS analysis reveals the formation of graphite-like carbon, demonstrating that iron oxide particles catalyse the decomposition of toluene vapour. SEM and TEM observations show that these particles are most often located at the nanotube root, suggesting a base growth mechanism responsible for the formation of aligned nanotube when prolonging growth time (2 min).