Synthesis of double-walled carbon nanotubes from coal in hydrogen-free atmosphere

Double-walled carbon nanotubes (DWNTs) were synthesized from coal in large quantity by arc-discharge method in hydrogen-free atmosphere, which were systematically examined using scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The results show that the as-synthesized DWNTs have an outer diameter of 1.0–5.0 nm with an interlayer spacing in the tube walls of ca. 0.41 nm. The possible mechanism involved in the formation process of DWNTs is proposed and discussed in term of the unique chemical composition of coal and the process parameters adopted in the study.     

Diameter dependence of interwall separation and strain in multiwalled carbon nanotubes probed by X-ray diffraction and Raman scattering studies

We have made a systematic study on the diameter dependent spectral features in X-ray diffraction (XRD) and Raman scattering studies of multiwalled carbon nanotubes (MWCNTs) of various diameters in the range 5?100 nm. High resolution transmission electron microscopy (HRTEM) imaging reveals a systematic decrease in the interwall separation from 3.8 Å down to 3.2 Å as the diameter of nanotubes increases from 5.8 nm to 63.2 nm. Analysis of the XRD patterns shows an exponential decrease in d₀₀₂ interlayer spacing with increasing tube diameter, in close agreement with the HRTEM results. Further, XRD profile line width shows inverse diameter dependence and exponential increase in intensity as the diameter of the MWCNTs increases. Raman spectra of different diameter nanotubes show different evolutions of metallic and semiconducting components in the G-band, as found from spectral deconvolution. The frequency and full width at half maximum (FWHM) of the semiconducting component of the G^? band gradually decreases as the tube diameter increases. Ratio of intensities of G^? band to D-band first shows a sharp fall as the tube diameter increases from 7 nm to 15 nm and then slowly increases with increasing diameter. On the other hand, G′ mode frequency shows large up shift when average diameter is increased from 7 nm to 15 nm and then saturates for higher diameter tubes. Analysis of Raman and XRD data reveals that the lowest diameter (7 nm) MWCNTs have features similar to those of the single walled nanotubes, while the spectral features are distinctly different for higher diameter MWCNTs due to the interaction among tube walls that is very significant for large diameter MWCNTs. Observed diameter dependence of the spectral features is explained in terms of nanotube curvature and atomic vibrations involving interaction among the walls in MWCNTs. The present study demonstrates the power of XRD for nondestructive evaluation of diameter distribution and interwall separation in MWCNTs with wide range of diameters.     

Hydroxy-interlayered vermiculite genesis in Jiujiang late-Pleistocene red earth sediments and significance to climate

To understand the paleoclimatic significance of the red earth sediments in Jiujiang, southern China, hydroxy-interlayered vermiculite (HIV) in the deposits was investigated using X-ray diffraction (XRD), chemical extraction, and high resolution transmission electron microscopy (HRTEM). The XRD results indicated that the clay mineral assemblages of the sediments are mainly illite, kaolinite, HIV, vermiculite, with minor illite-HIV mixed-layer clays. The 1.41-nm reflection did not expand after Mg saturation and glycerol solvation, but it shifted to 1.38 nm and 1.20 nm when the samples were heated to 350 °C and 550 °C, respectively. The 1.41-nm spacing showed partial collapse after K⁺ saturation at 80 °C in association with the reinforcement of the 1.0 nm reflection. Collapse of the HIV (001) spacing was related to the release of Al, Mg, and Ca from the interlayer after KCl extraction. The Al was probably present as hydroxy-Al in HIV interlayers before being released. The HRTEM image revealed 1.4-nm lattice fringes interstratified with illite 1.0 nm fringes and two illite lattice fringes merged into one HIV fringe. This suggests that the HIV clays in Jiujiang soils resulted from illite weathering. The shift of a small X-ray diffraction reflection between 1.0 and 1.4 nm for an air-dried and glycerol-solvated sample to 0.997 nm after heating at 550 °C indicated the presence of an irregularly interstratified illite–HIV mineral. The common occurrence of HIV minerals in the Jiujiang red earth sediments suggests a climate with frequent wet and dry cycles prevailed during late-Pleistocene time.     

Diamond-like carbon sintered compacts formed by spark plasma sintering

A spark plasma sintering (SPS) method was utilized for the novel production of diamond-like carbon (DLC) compacts. Two amorphous carbon powders with different particle sizes (45 μm and 24 nm diameter) were employed as starting materials for the sintering experiments. The carbon powders were sintered using a SPS system at various sintering temperatures and holding times. The structural properties of the sintered compacts were evaluated using X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). Disk-shaped compacts were obtained by sintering the powder with a particle diameter of 45 μm, although the compacts were very brittle and easily broken. However, sintering of the 24 nm diameter powder particles at temperatures of 1473 to 1573 K with a holding time of 300 s led to the successful production of sintered compacts without breakage. Reflection peaks related to graphite structure were observed in XRD patterns of the compacts sintered from the 24 nm diameter particles. HRTEM analysis revealed that the compacts sintered at 1473 K with a holding time of 300 s had an amorphous structure and consisted of 34% sp³ carbon bonding. Evaluation of the structural properties indicated that sintered compacts with DLC structure could be created by the SPS method with 24 nm diameter amorphous carbon particles.     

Electron microscope study of the formation of graphitic nanostructures in nickel-loaded wood char

We investigated the graphitization of carbonized larch wood chars impregnated with aqueous solution of nickel acetate, using scanning electron microscopy (SEM), both in secondary and transmission modes, and high-resolution transmission electron microscopy (HRTEM). Heat treatment of the chars at 500 °C brought about homogeneous distribution of metallic Ni particles about 5 nm in size in the amorphous carbon matrix. Graphitization sporadically started at this temperature, and some of the Ni particles are aggregated. SEM observations on chars heat-treated at 900 °C suggested that graphitic nanoshells about 50–200 nm in diameter, formed by catalytic effects of the Ni particles, grow in a “meandering” manner inside the amorphous carbon matrix. Some graphitic protrusions are found to grow outwards. Upon removal of the residual amorphous carbon matrix, long chains of the graphitic nanoshells exhibited a three-dimensionally intertwined structure, while transmission SEM showed that the interior of the shells is empty. HRTEM images exhibited not only stacked graphitic layers, but also cross-sectional contrasts expected from the hexagonal net of the graphite structure. These findings are discussed from the viewpoints of processing parameters, such as the use of aqueous solutions and atmosphere, specific to the catalytic graphitization of lignocellulosic materials.     

Core–shell BaMoO4@SiO2 nanospheres: Preparation,characterization, and optical properties

Core–shell BaMoO₄@SiO₂ nanospheres were prepared in reverse microemulsions and exhibited enhanced photoluminescence (PL) intensity as compared to that of the uncoated BaMoO₄. Characterization was performed using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDX), and X-ray powder diffraction (XRD). It was found that the silica shell could increase the PL intensity, but the shell is not the thicker the better. The PL emission can be decomposed into three individual Gaussian components: two UV emissions at 308 nm and 369 nm and a visible emission at 448 nm. Such short emission wavelengths can be attributed to quantum size effect of the small BaMoO₄ cores (～16 nm).     

Template free synthesis of highly ordered mullite nanowhiskers with exceptional photoluminescence

An efficient method to produce highly ordered mullite nanowhiskers using B₂O₃-doped molten salt synthesis was reported in this work. The morphology and optical properties of the obtained nanowhiskers were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) spectroscopy. The results show that highly ordered mullite nanowhiskers with uniform morphology in large scale are obtained at 1000 °C for 3 h in air. The structure of mullite is Al-rich and single crystalline. The diameter is in the range of 90–110 nm and length up to 20–30 μm. The reaction mechanism of the highly ordered mullite nanowhiskers is attributed to local concentration gradient. Due to the Al-rich structure, mullite nanowhiskers demonstrate strong photoluminescence (PL) emission at 399 nm, 452 nm and 468 nm, implying mullite nanowhiskers can be applied as a promising candidate in optical and electronic fields.     

Synthesis of mesoporous niobium oxophosphate using niobium tartrate precursor by soft templating method

Mesoporous niobium oxophosphate (NBP) with high specific surface area (427 m²/g) and narrow pore size distribution (3–15 nm) has been prepared using aqueous solution of niobium tartrate complex, diammonium hydrogen phosphate as precursors and cationic surfactant, tetradecyl trimethylammonium bromide (TTBr) as an organic template. The synthesized material has been characterized by simultaneously recorded thermogravimetric and differential thermal analysis (TG–DTA), fourier transformed infrared spectroscopy (FTIR), high and low angle X-ray diffraction (XRD), N₂ adsorption–desorption isotherms, high resolution transmission electron microscopy (HRTEM), ³¹P solid state magic angle spinning nuclear magnetic resonance (³¹P MASNMR) spectroscopy and energy dispersive X-ray (EDX) analysis. HRTEM and XRD confirm the formation of mesoporous structure and HRTEM image indicates the formation of wormhole-like mesopores in the synthesized sample. The interaction of pyridine with niobium oxophosphate has been investigated by means of adsorption infrared spectroscopy (FTIR) which shows that both Brönsted and Lewis acid sites are present on the surface of niobium oxophosphate.     

Onion-like fullerenes synthesis from coal

Onion-like fullerenes (OLFs) were synthesized in high yields from coal by radio frequency plasma. The morphologies and structures of the products were characterized by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) technique. Results reveal that OLFs can be prepared from coal with high purity. The particles display a clear polyhedral or quasi-spherical morphology with hollow center, having an average diameter ranging from 10 nm to 35 nm and high degree of graphitization.     

Carbon nanotubes: Amino functionalization and its application in the fabrication of Al-matrix composites

Al-matrix composites reinforced with amino-functionalized multiwalled carbon nanotubes (fCNTs) have been fabricated using the powder metallurgy process. Using this method fCNTs (1.5 wt.%) were dispersed in Al powder by high energy ball milling. Al–fCNTs composites (1.5 wt.%) were fabricated by the consolidation of powders at 550 MPa followed by sintering at 620 °C under a vacuum of 10^? 2 Torr for 2 h. Functionalization of the nanotubes was carried out by ball milling multi-walled carbon nanotubes (MWCNTs) in the presence of ammonium bicarbonate. It was observed that the dispersion of fCNTs in Al-matrix was much higher than those of non-functionalized MWCNTs. Microhardness measurements showed that a microhardness value of about 400 kg/mm² could be obtained for Al-matrix composites loaded with 1.5 wt.% fCNTs. Microstructure observations using scanning electron microscopy (SEM) and high-resolution electron microscopy (HRTEM) confirmed that the sintered composites had a good dispersion of fCNTs in Al matrix and they do not agglomerate with each other. Further, the HRTEM characterization of these composites revealed the formation of a thin transition layer of Al₄C₃ between fCNTs and Al matrix, which is believed to be responsible for load transfer from Al matrix to fCNTs. A thorough characterization of MWCNTs and fCNTs synthesized in the present work was carried out using XRD, SEM, TGA, HRTEM, FTIR, SIMS and Raman spectroscopy.     

Formation of dicalcium phosphate dehydrate coating on carbon fibers with in-situ grown graphene nanosheet interlayer

A dicalcium phosphate dehydrate (DCPD) coating was successfully fabricated on carbon fiber with in-situ grown graphene (IGN) interlayer. The obtained coating was analyzed using XRD, SEM, XPS, Raman, TEM and Thermo-gravimetric analysis(TGA). The in-vitro bioactivity of the obtained coating was investigated by simulated body fluid (SBF) immersion test. The results showed that the IGN interlayer grew homogenously on the carbon fiber with a wavy shape. The IGN interlayer was 40–90 nm in length and less than 10 nm in thickness. The DCPD exhibited a flake shape with a size of 10–50 nm. The IGN could bond with the DCPD tightly and form a uniform distribution within the DCPD coating. TGA test revealed that the carbon fiber with IGN interlayer was more effective for DCPD attachment and deposition than pure carbon fiber. The SBF tests showed that the DCPD coating could induce the formation of a cloud shaped apatite layer. The DCPD coated carbon fiber with IGN interlayer would be possible for potential application in tissue engineering.     

Interface evolution of the Cf/leucite composites derived from Cf/geopolymer composites

In this paper, effect of heat treatment temperature on the interface structure of the carbon fiber reinforced geopolymer composites was investigated by transmission electron microscopy (TEM), selected-area diffraction (SAD) analysis and high-resolution transmission electron microscopy (HRTEM). In the composite treated at 1100 °C, carbon fiber showed a good bond with the leucite matrix and no interface reaction layer was observed, while due to the thermal mismatch between fiber and matrix, microcrack which was perpendicular to the fiber axial direction can be seen in the matrix. With increase in heat treatment temperature to 1200, 1300 and 1400 °C, interface reaction occurred and reaction layers with thickness of 50, 100 and 1000 nm, respectively, were detected. The interface layer was formed by the reaction between Si–O groups in the matrix and C element in the fiber. Especially for the sample treated at 1400 °C, serious interface reaction led to the formation of β-SiC and property of carbon fiber was greatly degraded.     

Controlled synthesis,efficient purification,and electrochemical characterization of arc-discharge carbon nano-onions

Arc-produced carbon nano-onions (A-CNOs), with a hollow core surrounded by multilayered sp² carbon shells, possess unique structural and electronic properties. While nanodiamond derived CNOs (5–7 nm) are attracting significant attentions, for A-CNOs (20–50 nm) controlling the growth and separating them from carbon impurities are the major challenges that impede further study and application of these materials. We have addressed these issues; first by designing an in-house automated underwater arc discharge apparatus to control the arc plasma that produces homogeneous A-CNOs (diameter 25–35 nm) with minimal carbon impurities. Secondly, for further purification we have developed a very efficient method by utilizing the strong preferential adsorption of polyoxometalates. A-CNO growth and purification were investigated using thermogravimetric analysis (TGA), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and X-ray diffraction (XRD). Furthermore, the electrochemical properties and electrocatalytic activities of purified A-CNOs were investigated with various redox species including neurotransmitter molecules. Compared to glassy carbon (GC) electrode, A-CNO showed excellent electrochemical performances including larger faradaic currents and facilitated electron-transfer kinetics. Controlled synthesis, efficient purification and the excellent electrocatalytic activities of A-CNOs reported herein will enable the utilization of these materials for various applications including biosensing, fuel-cell catalysts and energy-storage devices.     

Preparation of pillared carbon thin films from the reduction of silylated graphite oxide by UV light irradiation and their size selective electrical response to various molecules

Silylated graphite oxide thin films were reduced by UV light irradiation using a super high pressure Hg lamp at 95 °C. The reduction of silylated graphite oxide was completed within 24 h and a new pillared carbon with an interlayer spacing of 0.81 nm was obtained. Pillared carbons with larger interlayer spacings of about 1.13 nm were also obtained from graphite oxide silylated with octyltrichlorosilane and then with 3-aminopropyltriethoxysilane for 1.5–6 h, when they were reduced by UV light irradiation and heating at 200 °C under vacuum. Selective electrical response during exposure to gaseous vinylene carbonate, acetonitrile, ozone and hydrogen molecules has been achieved by changing the pillar density in the resulting pillared carbon films.     

Microstructural evolution of multi-walled carbon nanotubes in the presence of mixture of silicon and silica powders at high temperatures

Microstructural evolution of multi-walled carbon nanotubes (MWCNTs) in the presence of mixture of silicon and silica powders in a coke bed is studied in the temperature range of 1000–1500 °C by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and thermogravimetry–differential scanning calorimetry (TG–DSC). The results showed that a thin amorphous SiO₂ coating was formed on the surface of MWCNTs at the temperature below 1300 °C. With the increase of the treated temperature, the coating became thicker, 3–7 nm in thickness at 1400 °C and a maximum of 10 nm at 1500 °C. Meanwhile, SiC nanowires and SiC nanocrystals around Ni catalyst at the tip of MWCNTs were formed at 1400 °C and 1500 °C, which were related to the vapor–vapor (V–V) and vapor–liquid–solid (V–L–S) reactions between SiO (g) and CO (g) or C (s), respectively. The oxidation resistance of all the treated MWCNTs was better than that of as-received ones. The oxidation peak temperature reached 804.2 °C for the treated MWCNTs, much higher than 652.2 °C for as-received ones.     

Nanostructured alumina particles synthesized by the Spray Pyrolysis method: microstructural and morphological analyses

Ultrafine particles or nanoparticles (ranging from a few nanometers to 100 nm) are of considerable interest for a wide variety of applications, ranking from catalyst to luminescence ceramics, due to their unique and improved properties primarily determined by size, composition and structure. This study presents the preparation and characterization of nanostructured spherical alumina particles by the Spray Pyrolysis method for the application in reinforcements of metal-matrix composites (MMCs). Synthesis procedure includes aerosol formation ultrasonically from alumina nitrate water solution and its decomposition into a tubular flow reactor at 700 °C. The obtained particles are spherical, smooth, amorphous and in non-agglomerated state. Microstructural and morphological analyses were carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM/EDS) and analytical and high resolution transmission electron microscopy (TEM/HRTEM).     

Heteroepitaxial nucleation and growth of graphene nanowalls on silicon

Heteroepitaxial nucleation of {0 0 2} graphene sheets on {1 1 1} facets of plasma treated (1 0 0) silicon by direct-current plasma enhanced chemical vapor deposition in methane–hydrogen gas mixtures is confirmed by high-resolution transmission electron microscopy. Lattice mismatch by 12% is compensated by tilting the graphene {0 0 2} with respect to silicon {1 1 1} and matching the silicon lattice with fewer graphene layers. The interlayer spacing of graphene sheets near the silicon surface is 0.355 nm, which is larger than that of AB stacked graphite and confirmed as AA stacked graphitic phase. Subsequent growth of standing graphene nanowalls is characterized by scanning electron microscopy and Raman scattering (633 and 514 nm excitation). The Raman peaks of D-band, G-band, and 2D-band are discussed in correlation with SEM images of graphene nanowalls. A strong Raman peak corresponding to silicon–hydrogen stretch vibration is detected by 633 nm excitation at the early stage of graphene nucleation, indicating the silicon substrate etched by hydrogen plasma. With these analyses, the growth mechanism is also proposed in this paper.     

Onion-like carbon nanoparticles generated by multiple laser irradiations on laser-ablated particles

A one-step process to synthesize onion-like carbon nanoparticles at room temperature under atmospheric pressure was developed. Periodic pulsed laser irradiation of carbon nanoparticles confined in the cavity of a carbon target rod induces drastic changes to their structure, changing them from amorphous to ordered concentric graphitic shells. The size distribution of gas-borne nanoparticles measured by a scanning mobility particle sizer showed the mean mobility diameter decreased from 83 nm to 18 nm as a result of restructuring of carbon agglomerates. Polyhedral and unagglomerated nanoparticles with shell structures composed of multiple graphene sheets were observed by a transmission electron microscopy.     

Intercalation of various organic molecules into pillared carbon

Intercalation of various organic molecules into pillared carbon prepared from the pyrolysis of graphite oxide silylated with methyltrichlorosilane for three times was investigated. Liquid n-alkylamine molecules with various alkyl chain lengths were intercalated into the resulting pillared carbon and the interlayer spacing increased with increasing in the alkyl chain length, until it became a constant value of 2.24 nm for chains having more than six carbon atoms. This suggests that the length of the pillar is 1.9 nm. Various organic molecules including non-polar xylene isomers and alcohol molecules can also penetrate into pillared carbon. The n-hexadecylamine molecules with a higher melting point than room temperature were intercalated into pillared carbon simply by mixing them with pillared carbons in hexane, though the interlayer spacing was smaller. The space between the layers of pillared carbon was saturated with n-hexadecylamine molecules when 1.8 molecules per 40 carbon atoms were added. The n-hexadecylamine molecules occupied 51% of the micropore volume of pillared carbon. For the intercalation of organic molecules into pillared carbon, the shorter axis of them should be smaller than 0.87 nm.     

Magnetic and EPR studies of edge-localized spin paramagnetism in multi-shell nanographites derived from nanodiamonds

Prolonged (up to 2 h) heat treatment at 1600 °C of nanodiamond particles (5 nm) leads to their conversion to the mixture of quasi-spherical carbon onions and multi-shell polyhedral nanographites. Structural and magnetic properties of two (A and B) series of nanographite samples obtained at various annealing intervals were studied. XRD data show that both multi-shell nanographite samples have practically the same crystalline structures. HRTEM evidences that the most of particles obtained by short time (7 min) annealing have a spherical like shape whereas the long time (~ 2 h) annealing leads to the majority of particles having a polyhedral shape with a hollow inside. Electronic and magnetic properties of these nanocarbons were investigated by magnetic susceptibility and EPR. Annealing results in entire transformation of the EPR signal of nanodiamond to new EPR signals of various line shapes and widths. These signals are extremely sensitive to ambient oxygen. Concentrations for all EPR active spins vary from ~ 1 × 10¹⁹ spins/g (7 min) to ~ 2 × 10¹⁹ spins/g (2 h). Temperature dependences of EPR spectra' parameters were obtained for vacuum-processed samples within the range 4?600 K. Intensity vs. T plots may be well-fitted by the combination of Curie–Weiss and temperature-independent Pauli susceptibility contributions. The latter one increases on heat treatment. Significant extension of electron spin-lattice relaxation time on decreasing temperature was found.