Synthesis of carbon nanotubes over gold nanoparticle supported catalysts

The synthesis of carbon nanotubes (CNTs) through the catalytic decomposition of acetylene was carried out over gold nanoparticles supported on SiO₂-Al₂O₃. Monodispersed gold nanoparticles with 1.3-1.8 nm in diameter were prepared by the liquid-phase reduction method with dodecanethiol as protective agent. The carbon products formed after acetylene decomposition consist of multi-walled carbon nanotubes with layered graphene sheets, carbon nanofilaments (CNFs), and carbon nanoparticles encapsulating gold particles. The observed CNTs have outer diameters of 13-25 nm under 850 °C. The influence of several reaction parameters, such as kind of carriers, reaction temperature, gas flow rate, was investigated to search for optimum reaction conditions. The CNTs were observed at a relatively low temperature (550 °C). The silica-alumina carrier showed higher activity for the formation of CNTs than others used in the screening test. With increasing temperature, the CNTs showed cured structures having thick diameters and inside compartments. When Au content on the support was over 5 wt.%, the gold nanoparticles coagulated to form large ones >20 nm in diameter and became encapsulated with graphene layers after decomposition of acetylene.     

Thermal synthesis of Cu@carbon spherical core-shell structures from carbonaceous matrices containing embedded copper particles

A Cu@carbon (Cu@C) spherical core-shell structure has been synthesized from carbonaceous matrices containing embedded copper particles by thermal treatment at 600 °C under an argon atmosphere. The matrices are prepared by reducing CuCl₂ with vitamin C (VC) in the existence of polyacrylamide (PAM) at 180 °C, accompanied by the partial carbonization of VC. It is found that small Cu nanoparticles with diameters of ∼3 nm homogeneously disperse in the carbonaceous matrices. Subsequent thermal treatment at 600 °C under an argon atmosphere leads to the diffusion of these Cu nanoparticles and further carbonization of carbonaceous matrices to form a Cu@C core-shell structure. The core-shell ratio can be controlled by varying the dose of CuCl₂, VC, or PAM. It is found that the carbon shell can effectively shield the metallic Cu core from oxidation in the mixed solution of dilute hydrogen peroxide and nitric acid.     

Structural, electrical and magnetic properties of carbon-nickel composite thin films

Carbon-nickel composite thin films (600 nm thick) were prepared by dc magnetron sputtering of Ni and C at several temperatures (25-800 °C) on oxidized silicon substrates. By transmission electron microscopy it was found that the composite consisted of Ni (or Ni₃C) nanoparticles embedded in a carbon matrix. The metallic nanoparticles were shaped in the form of globular grains or nanowires (of the aspect ratio as high as 1:60 in the sample prepared at 200 °C). The carbon matrix was amorphous, or graphite-like depending on deposition temperature. At low deposition temperatures T_S (25-400 °C) the Ni₃C nanoparticles were of hcp phase. Samples prepared at T_S ? 600 °C contained ferromagnetic fcc Ni nanoparticles. A correlation was found between the structural, electrical and magnetic properties of the composites. To characterise the films, dependences, such as resistivity vs. temperature, current vs. voltage, differential conductivity vs. bias voltage, and magnetoresistivity, were determined. For example, the tunneling effect was found in samples in which the metallic nanoparticles were separated by 2-3 nm thick amorphous carbon. When the metallic nanoparticles were connected by graphite-like carbon regions (having a metallic conductivity, in contrast to a-C), the temperature coefficient of the resistivity became slightly positive. An anisotropic magnetoresistivity of ∼0.1% was found in the sample that contained ferromagnetic columnar fcc Ni. Zero magnetoresistivity was found in the sample in which the metallic nanoparticles were of non-magnetic hcp phase.     

Formation of LiNi0.5Co0.5VO4 nano-crystals by solvothermal reaction

LiNi_0.5Co_0.5VO₄ nano-crystals were solvothermally prepared using a mixture of LiOH·H₂O, Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O and NH₄VO₃ in isopropanol at 150–200 °C followed by 300–600 °C calcination to form powders. TGA curves of the solvothermal products show weight losses due to evaporation and decomposition processes. The purified products seem to form at 500 °C and above. The products analyzed by XRD, selected area electron diffraction (SAED), energy dispersive X-ray (EDX) and atomic absorption spectrophotometer (AAS) correspond to LiNi_0.5Co_0.5VO₄. V–O stretching vibrations of VO₄ tetrahedrons analyzed using FTIR and Raman spectrometer are in the range of 620–900 cm⁻¹. A solvothermal reaction at 150 °C for 10 h followed by calcination at 600 °C for 6 h yields crystals with lattice parameter of 0.8252 ± 0.0008 nm. Transmission electron microscope (TEM) images clearly show that the solvothermal temperatures play a more important role in the size formation than the reaction times.     

Preparation and characterization of carbon membranes made from poly(phthalazinone ether sulfone ketone)

Carbon membranes were prepared from a novel polymeric precursor of poly(phthalazinone ether sulfone ketone) (PPESK), of which the changes of microstructure and chemical compositions during pyrolysis from 500 °C to 950 °C were monitored by thermal gravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It has been found that the weight loss of the PPESK precursor up to 800 °C is about 43.0 wt%. After the heat treatment, the typical chemical structure of the PPESK precursor disappears, at the same time a graphite-like structure with more aromatic rings is formed. The interlayer spacing (i.e., d value) decreases from 0.471 nm to 0.365 nm as the pyrolysis temperature increases. The gas permeation performance of carbon membranes has been tested using pure single gases including H₂, CO₂, O₂ and N₂. For the carbon membrane obtained by carbonizing the PPESK precursor at 800 °C, the maximum ideal permselectivities for H₂/N₂, CO₂/N₂ and O₂/N₂ gas pairs could reach 278.5, 213.8 and 27.5, respectively.     

Nanodiamond crystallites embedded in carbon films prepared by thermionic vacuum arc method

Nanostructured carbon films with thicknesses of 100 and 200 nm have been deposited from pure vapour carbon plasma using an original thermionic vacuum arc method. Silicon single crystalline wafers, glass and stainless steel held at 400 °C were used for substrates. The films consist of diamond nanoparticles of 5 nm diameter on the average embedded in a disordered graphite matrix as revealed by HRTEM, XPS and visible Raman measurements. The graphitic cluster diameters La range from 1.5 to 2.3 nm. Thicker films (200 nm) on stainless steel exhibit the largest clusters.     

Template assisted fabrication of TiO2 and WO3 nanotubes

Anodic aluminum oxide (AAO) templates with diameters of 200–500 nm were generated by anodizing a commercial aluminum (Al) substrate (99.7%) in 1 vol% phosphoric acid (H₃PO₄), with an applied voltage of 195 V. Titania and tungsten oxide nanotubes (NTs) were successfully grown on AAO template by the sol–gel process. Thermal gravimetric analyzer (TGA) curves showed that gel can be transfered to nanocrystalline particles after 19% weight loss of water molecule by evaporation. The results showed that the nanocrystalline TiO₂ NTs presented at 200 °C, and grains grew as temperature increased. At a temperature of 550 °C, the (101), (103), (004), (112), (200), (105), and (211) planes of anatase TiO₂ were detected clearly, whereas tungsten oxide NTs are amorphous after heat treatment at 200 °C or 300 °C. But the (110), (111), (002), (022), (222), and (004) planes of γ-WO₃ phase can be observed obviously after the heat treatment at 400 °C.     

Tin dioxide/carbon nanotube composites with high uniform SnO2 loading as anode materials for lithium ion batteries

SnO₂/multi-walled carbon nanotube (MWCNT) composites were prepared by the solvothermal method and subsequent heat treatment at 360 °C. The samples were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Results on the higher SnO₂ content composite sample indicate that a uniform layer of SnO₂ nanocrystals with crystal size around 5 nm was deposited on the surface of the carbon nanotubes. The composite demonstrates a reversible lithium storage capacity of 709.9 mAh g⁻¹ at the first cycle and excellent cyclic retention up to 100 cycles as anode for lithium ion batteries.     

Thermophysical properties of densified pitch based carbon/carbon materials—II. Bidirectional composites

Bidirectional carbon/carbon composites were developed using high-pressure impregnation/carbonization technique with PAN based carbon fabric as reinforcement and coal tar and synthetic pitches as matrix precursors. Microstructure of these composites has been evaluated using scanning electron microscope and polarized light optical microscope. Thermophysical properties i.e. thermal conductivity and specific heat have been evaluated both at room temperature and between 40 and 300 °C. The temperature dependence of thermal diffusion, specific heat and thermal conductivity has been studied and correlated with microstructure of carbon/carbon composites. It is found that the specific heat of carbon/carbon composites shows increase with temperature with an inverse slope in the temperature range of 150-200 °C. Accordingly, though the thermal conductivity decrease with temperature due to increased phonon interactions, it shows reversible action between 150 and 200 °C.     

TEM and electron tomography studies of carbon nanospheres for lithium secondary batteries

The structure of carbon nanospheres of 100-200 nm diameter, which showed superior high-speed charge-discharge behavior as the negative electrode in a lithium ion battery, was investigated with XRD, SEM and TEM with an electron tomography attachment. Observation of carbon 0 0 2 lattice images, as well as electron diffraction patterns, illustrated that heterogeneous microtexture was formed as the polyhedronization of the particle proceeded with heat-treatment. The outside region of the particle heat-treated at 2800 °C has stacking structure of aromatic layers with some distribution of d₀₀₂, while the center region consisted of non-graphitic. Structure defects seemed to be concentrated along the ridgelines of the polyhedronized particles after heat-treatment. The electron tomography technique clarified the morphology of the graphitized particles, although the images should be understood with other crystallographic measurements. A slice image computed in the 3D-reconstruction process showed the inner texture of the graphitized particles more clearly than the conventional TEM bright-field image.     

Nanoscale fine-tuning of porosity of carbide-derived carbon prepared from molybdenum carbide

Micro- and mesoporous carbide-derived carbon (CDC) was synthesised from molybdenum carbide (Mo₂C) powder by gas phase chlorination in the temperature range from 400 to 1200 °C. Analysis of XRD results show that C(Mo₂C), chlorinated at 1200 °C, consist mainly on graphitic crystallites of mean size, L_a = 9 nm and L_c = 7.5 nm. The first-order Raman spectra showed the graphite-like absorption peak at ∼1587 cm⁻¹ and the disorder-induced (D) peak at ∼1348 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1855 m² g⁻¹ and total pore volume up to 1.399 cm³ g⁻¹ were obtained. Sorption measurements showed the presence of both micro- and mesopores after chlorination at 400-900 °C and only mesopores after chlorination at 1000°-1200 °C. Stepwise formation of micro- and mesopores was achieved and the peak pore size can be shifted from 0.8 nm up to 4 nm by increasing the chlorination temperature.     

Synthesis of nanosized vanadium pentoxide/carbon composites by spray pyrolysis for electrochemical capacitor application

Nanostructured vanadium pentoxide/carbon (V₂O₅/carbon) composite powders with enhanced specific capacitance were synthesized by the spray pyrolysis technique. Electrochemical properties were examined by the cyclic voltammetry technique. Following analysis of powders sprayed at different temperatures, composite powders obtained at an optimum temperature of 450 °C yielded a maximum specific capacitance of 295 F g⁻¹ in 2 M KCl electrolyte at a 5-mV s⁻¹ scan rate. The weight percentage of carbon-related species was 2.7 wt% in this V₂O₅/carbon composite, as detected by thermogravimetric analysis (TGA) and confirmed by transmission electron microscope energy dispersive spectroscopy (TEM-EDS) analysis. Following initial X-ray diffraction (XRD) characterization, scanning electron microscope (SEM), TEM and high-resolution TEM (HRTEM) imaging revealed a specific morphology of spherical shell agglomerates of V₂O₅ nanorods and nanoribbons, with each shell comprising a network of these one- and two-dimensional nanoparticles in an amorphous carbon matrix. The V₂O₅ network was not fully dense, and the majority of the nanorod sizes were in the range of 50-150 nm, with additional long nanoribbons extending from the outsides of the spherical shells. The specific surface area was 18 m² g⁻¹ for the composite powders, and the pore size distribution revealed that the majority of pores had diameters in the range of 40-50 Å, which was relatively larger than the pore diameters obtained at 500 °C and would be beneficial for electrochemical performance. The enhancement of the specific capacitance in V₂O₅/carbon composites was attributed to the distribution of amorphous carbon throughout the V₂O₅ and the particular open nanostructure.     

Super growth of vertically aligned SWCNTs using self-assembled nanoparticles from CoCrPtOx ultra-thin film

The self-assembly of catalytic nanoparticles by the decomposition of an as-deposited oxidized CoCrPt thin film is investigated, and the feasibility of its use in fabricating vertically aligned SWCNT films at a low synthesis temperature (∼600 °C) by microwave plasma CVD is described. The XPS results indicate that small nanoparticles with the diameters of 3-3.5 nm were formed in the explosion associated with the reduction of PtO₂ in the CoCrPtO_x film. Cr₂O₃ is employed to inhibit the agglomeration of nanoparticles and Co is typically involved in the dissolution and precipitation of carbon species for SWCNT growth. These small, self-assembled catalytic nanoparticles obtained from the CoCrPtO_x ultra-thin film can be used to fabricate an extremely dense and highly oriented SWCNT film on a silicon wafer at a temperature of ∼600 °C.     

Development of carbon nanofibers from aligned electrospun polyacrylonitrile nanofiber bundles and characterization of their microstructural, electrical, and mechanical properties

Carbon nanofibers with diameters of 200-300 nm were developed through stabilization and carbonization of aligned electrospun polyacrylonitrile (PAN) nanofiber bundles. Prior to the oxidative stabilization in air, the electrospun PAN nanofiber bundle was tightly wrapped onto a glass rod, so that tension existed during the stabilization. We also investigated several carbonization procedures by varying final carbonization temperatures in the range from 1000 to 2200 °C. The study revealed that: (1) with increase of the final carbonization temperature, the carbon nanofibers became more graphitic and structurally ordered; (2) the carbon nanofiber bundles possessed anisotropic electrical conductivities, and the differences between the parallel and perpendicular directions to the bundle axes were over 20 times; and (3) the tensile strengths and Young's moduli of the prepared carbon nanofiber bundles were in the ranges of 300-600 MPa and 40-60 GPa, respectively.     

Continuous synthesis of metal nanoparticles in supercritical methanol

Metal nanoparticles were synthesized continuously in supercritical methanol (scMeOH) without using reducing agents at a pressure of 30 MPa and at various reaction temperatures ranging 150-400 °C. Wide angle X-ray diffraction (WAXD) analysis revealed that metallic nickel (Ni) nanoparticles were synthesized at a reaction temperature of 400 °C while mixtures of nickel hydroxide (α-Ni(OH)₂) and metallic Ni were produced at lower reaction temperatures of 250-350 °C. In contrast, metallic silver (Ag) nanoparticles were produced at reaction temperatures above 150 °C while metallic cupper (Cu) nanoparticles were produced at reaction temperatures above 300 °C. Mixtures of copper oxide (CuO and Cu₂O) and metallic Cu were produced at lower reaction temperatures of 250 °C. Scanning electron microscopy (SEM) showed that the particles size and morphology changed drastically as the reaction temperature increased. The average diameters of Ni, Cu and Ag particles synthesized at 400 °C were 119 ± 19 nm, 240 ± 44 nm, and 148 ± 32 nm, respectively. The scMeOH acted both as a reaction medium and a reducing agent. A possible reduction mechanism in scMeOH is also presented.     

Ultrafine SnO2 dispersed carbon matrix composites derived by a sol-gel method as anode materials for lithium ion batteries

Ultrafine crystalline SnO₂ particles (2-3 nm) dispersed carbon matrix composites are prepared by a sol-gel method. Citric acid and hydrous SnCl₄ are used as the starting constituents. The effect of the calcination temperatures on the structure and electrochemical properties of the composites has been studied. Structure analyses show that ultrafine SnO₂ particles form and disperse in the disordered carbon matrix in the calcination temperature range of 500-800 °C, forming SnO₂/C composites, and the carbon content shows only a slight increase from 35.8 wt.% to 39.1 wt.% with the temperature. Nano-Sn particles form when the calcination temperature is increased to 900 °C, forming a SnO₂/Sn/C composite, and the carbon content is increased to 49.3 wt.%. Electrochemical testing shows that the composite anodes provide high reversible cycle stability after several initial cycles, maintaining capacities of 380-400 mAh g⁻¹ beyond 70 cycles for the calcination temperature of 600-800 °C. The effect of the structure feature of the ultrafine size of SnO₂ and the disordered carbon matrix on the lithium insertion and extraction process, especially on the reversible behavior of the lithium ion reaction during cycling, is discussed.     

Ultraviolet irradiated ozone treatment of a metal catalyst for the large-scale synthesis of single-walled carbon nanotubes with small, uniform diameters

Large-scale synthesis of single-walled carbon nanotubes with small diameters and narrow distribution was performed using catalytic decomposition of C₂H₂ at 800 °C by introducing an ultraviolet irradiated ozone (UV-ozone) treatment on an as-prepared Fe-Mo/MgO catalyst (APC). The UV-ozone treatment effectively suppressed metal migration and the agglomeration of the Fe-Mo catalyst on the MgO support material at high temperature (800 °C). During UV-ozone treatment, active oxygen species were adsorbed onto the APC and generated hydroxyl groups. The hydroxyl groups prevented the formation of large catalytic metal nanoparticles at 800 °C by acting as a surfactant. We also investigated whether the Mo species prevented the metal sintering of iron species into the MgO lattice.     

Solvothermal syntheses of hollow carbon microspheres modified with -NH2 and -OH groups in one-step process

Homogeneous and monodisperse hollow carbon microspheres (HCMS) functionalized with amino and hydroxyl groups are synthesized by decomposing 2,4,6-tribromophenol/ferrocene mixture in the presence of ammonia via a one-step solvothermal process at 250 °C for 24 h. The effect of experimental conditions on the morphology of carbon microspheres has been investigated systematically. The surface of the HCMS is modified with amino and hydroxyl groups through the synthesis process as confirmed by infra-red spectroscopy and X-ray photoelectron spectroscopy. The pore-size distribution and the specific area of macropores are measured by nitrogen adsorption-desorption and mercury intrusion porosimetry. The as-synthesized HCMS has the high Brunauer-Emmett-Teller surface area of 289 m²/g and macropores whose diameter is mostly larger than 100 nm. To investigate the chemical reactivity of functionalized groups on the surface of HCMS, Au and Ag nanoparticles are successfully loaded onto HCMS by direct reduction of HAuCl₄ or AgNO₃ without adding any reducing agent.     

Dry catalytic partial oxidation of diesel-fuel distillates into syngas

Volatile compounds distilled below 205 °C from diesel fuel are reformed into synthesis gas by dry catalytic partial oxidation using porous membrane reactors, eliminating complex liquid-fuel injectors and fuel-air mixers, greatly simplifying reformers for applications with solid-oxide fuel cells and NO_x traps. For distillates utilizing 20 wt% of the diesel fuel, 88 mol% of the carbon is converted into CO and 75 mol% of the hydrogen into H₂. Rationale is as follows: Long-chain n-alkanes such as n-hexadecane, with normal boiling point, 286.5 °C, but autoignition temperature, 205 °C, are the least thermally stable hydrocarbons in diesel fuel. If attempts are made to vaporize diesel fuel under oxygen-lean conditions without precautions, long-chain n-alkanes crack at autoignition temperatures forming radicals that initiate polymerization. By eliminating more troublesome compounds by distillation, and by effusing cooler air through porous ceramic membranes to react radicals with oxygen, carbon deposition is largely suppressed. A perovskite catalyst, fed pre-heated air at >900 °C, provides a reservoir of mobile lattice oxygen to react with adsorbed carbon. In continuous runs of 72 h, carbon deposition was negligible in the reactor, on the catalyst, and in the exhaust, except for minor graphite deposited onto walls near the catalytic hot zone.     

Preparation of microporous sorbents from cedar nutshells and hydrolytic lignin

Carbon nutshells and hydrolytic lignin were used as starting materials for the preparation of microporous active carbons. Optimum parameters for cedar nutshell carbonization have been selected (temperature of carbonization 700-800 °C, rate of heating less than 3 °C/min) for the preparation of microporous carbons (average pore width 0.56 nm). The textural characteristics of microporous carbons made from nutshell are similar to those of a ‘Coconut’ carbon molecular sieve, but the latter has both a higher CO₂ adsorption capacity and a higher coefficient of N₂/O₂ separation. The influence of carbonization and steam-activation parameters on the microtexture and molecular-sieve properties of granular carbons made from hydrolytic lignin was also investigated. A low rate of heating (less 3 °C/min) promotes the formation of micropores with average sizes around 0.56-0.58 nm at carbonization temperature 700 °C. At the same carbonization temperature the average sizes of micropores were 0.7-0.78 nm at rates of heating more than 3 °C/min. The activation of lignin-char with steam at 800 °C resulted in the formation of active carbons with more developed micropore volume (0.3-0.35 cm³ g⁻¹) and with micropores of widths around 0.6-0.66 nm which are able to separate He from a He-CH₄ mixture. The size of the micropores was varied as a function of burn off value.