Synthesis and characterization of coaxial silver/silica/polypyrrole nanocables

Double‐shelled coaxial nanocables of silver nanocables with SiO₂ and polypyrrole (PPy; Ag/SiO₂/PPy) were synthesized by a simple method. The thickness of the outer PPy shell could be controlled by the amount of pyrrole monomer. The silver nanocables encapsulated in the interior of the hollow PPy nanotubes were obtained by the removal of the midlayer SiO₂. By the silver‐mirror reaction, flowerlike Ag nanostructures could be formed on the surface of the Ag/SiO₂/PPy multilayer nanocable. The application of the as‐prepared Ag/SiO₂/PPy–Ag composites in surface‐enhanced Raman scattering (SERS) was studied with Rhodamine B (Rh B) as a probe molecule. We found that the composites could be used as SERS substrates and that they exhibited excellent enhancement ability. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Modelling the surface adsorption of methane on carbon nanostructures

Methane (CH₄) adsorption is investigated on both graphite and in the region between two aligned single-walled carbon nanotubes, which we refer to as the groove site. We exploit the Lennard–Jones potential function and the continuous approximation to determine surface binding energies between a single CH₄ molecule and graphite and between a single CH₄ and two aligned single-walled carbon nanotubes. Our modelling indicates that for a CH₄ molecule interacting with graphite, the binding energy of the system is minimized when the CH₄ carbon is 3.83 Å above the surface of the graphitic carbon, while the binding energy of the CH₄–groove site system is minimized when the CH₄ carbon is 5.17 Å away from the common axis shared by the two aligned single-walled carbon nanotubes. Our results confirm the current view that for larger groove sites, CH₄ molecules in grooves are likely to move towards the outer surfaces of one of the single-walled carbon nanotubes. Our results are computationally efficient and are in good agreement with experiments and molecular dynamics simulations, and show that CH₄ adsorption on graphite and groove surfaces is more favourable at lower temperatures and higher pressures.     

一维氧化铝基纳米材料的制备及其形成机理

采用二次阳极氧化法制备得到孔径为60 nm的氧化铝模板。将氧化铝模板放入NaOH溶液中腐蚀约4 min左右,制备得到了氧化铝纳米管、纳米线的混合物。将沉积有InSb纳米线的氧化铝模板放入NaOH溶液中进行腐蚀5～8 min后,制备得到了InSb/Al₂O₃纳米同轴电缆。在制备基础上,讨论了氧化铝纳米管、纳米线及InSb/Al₂O₃纳米同轴电缆的形成机理。     

Carbon nanotubes as a template for mild synthesis of magnetic CoFe2O4 nanowires

Hundred nanometers outer diameter multi-walled carbon nanotubes have been used as suitable host template for synthesizing CoFe₂O₄ nanowires encapsulated inside nanotubes under mild conditions, i.e. 100 °C and atmospheric pressure, with a high filling yield of the nanotubes, using an aqueous nitrate precursor solution and the confinement effect provided by the surrounding walls. The formation of caps near the tube tips at the beginning of the nitrate decomposition led to consider each nanotube as a closed nanoreactor, in which the reaction conditions could be far different from the macroscopic conditions outside the tube. The structure of the CoFe₂O₄ nanowires could be continuously changed from a disordered hair-like dendritic structure at 100 °C to highly crystallized domains when increasing the temperature. A material with high coercivity at room temperature for small particles of about 25 nm in diameter was obtained by submitting the nanowires to an Ar treatment at 550 °C for 2 h.     

Carbon nanotubes filled with metallic nanowires

A facile method is proposed to use LaNi₂ hydrogen storage alloy as a catalyst precursor to produce metallic nickel filled carbon nanotubes. Multi-walled carbon nanotubes filled with long continuous nickel nanowire with several microns in length are synthesized through chemical vapor deposition at low temperature (550 °C). It is more efficient to fill Ni nanowires into nanotubes after the oxidation treatment of LaNi₂ alloy at low temperatures, while the oxidation treatment at high temperature results in the forming of herringbone carbon nanofibers with tips of Ni nanoparticles. The metallic Ni nanowires inside the cores of carbon nanotubes could not be eliminated during the purification process in concentrated hydrochloric acid. The analysis of transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray diffraction (XRD) reveals that the metallic nickel nanowires filled inside carbon nanotubes exist as a single crystalline with fcc structure.     

Preparation of ZnO nanoparticles localized on SiC@SiO2 nanocables by a physical templating method

Impregnation of interlaced SiC@SiO₂ nanocables (SiC NWs sheathed by SiO₂ coatings) into a ZnO sol at 0 °C yielded, after thermal treatment up to 600 °C under argon, SiC@SiO₂@ZnO nanostructures. These novel nanostructures consist in SiC@SiO₂ nanocables covered by numerous agglomerated ZnO nanoparticles. The latter are less than 5 nm in diameter. This result is to our knowledge the first example of a physical templating technique involving SiC-based nanowires. Moreover, we have obtained localized ZnO nanoparticles. This localization can be of interests for a further study of their physical properties. When a similar experiment was conducted with pure SiC nanowires, there was no interaction between the nanowires (NWs) and the solution, resulting in the formation of agglomerated ZnO NPs embedded into the 3D NWs network.     

Double-walled carbon nanotubes: Quantitative purification assessment,balance between purification and degradation and solution filling as an evidence of opening

We compared the effect of different oxidizing agents on purification, functionalization and opening of double-walled carbon nanotubes. The oxidative treatments were realized in nitric acid solutions at different concentrations (3 M or 15 M), in a mixture of two oxoacids (conc. HNO₃/conc. H₂SO₄) or in sulphuric acid solutions of KMnO₄ or K₂Cr₂O_7. Most of these treatments were very efficient for purification (removal of residual catalytic metal nanoparticles and/or of disorganized carbon) but also caused secondary reactions such as shortening of the nanotubes, creation of functionalized amorphous carbon deposits and covalent functionalization of the outer wall. Secondary treatments were undertaken in order to remove functionalized carbon deposit by washing with sodium hydroxide solutions or by heat treatment in air. A partial filling in solution was obtained with uranyl nitrate, in order to evidence the opening of carbon nanotubes. Effects of purification and filling treatments were characterized both qualitatively by TEM and HRTEM, AFM and Raman spectroscopy, and quantitatively by elemental chemical analysis and chemical titrations.     

Synthesis of titanate nanotubes and its processing by different methods

In the present studies, titanate nanotubes are synthesized using hydrolysis of commercial titania (TiO₂) nano-particle powder. The coatings of titanate nanotubes are fabricated using electro deposition method and these were subsequently processed under different conditions. These include (i) processing in atmospheric conditions, (ii) processing in vacuum, (iii) processing in Ar-gas RF-plasma, and (iv) processing in activated hydrogen (H₂) and methane (CH₄) gas mixtures using hot filament chemical vapour deposition (HF-CVD) method. It is observed that depending upon the type and processing parameters, the nanotube coatings exhibit interesting variations in the evolution and precipitation of materials phases. Typically, heating above temperatures (T_s) ∼600 °C render dehydration process leading to collapse of nanotubular structure, however, the nature of collapse is characteristically different in each case. The treatment in activated hydrogen and carbon led to the precipitation of carbon nanotubes and nanowires, while in other cases, crystallization or amorphization took place. The samples are characterized by using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy. The results are to be discussed in terms of reduction in thermal reaction barrier due to presence of activated particles.     

Single-step synthesis of germanium nanowires encapsulated within multi-walled carbon nanotubes

We report the single-step synthesis of Ge nanowires encapsulated within multi-walled carbon nanotubes (MWCNTs) from a phenyltrimethylgermane (C₆H₅Ge(CH₃)₃) precursor, using a simple chemical vapor deposition (CVD) method. The MWCNT/germanium nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) measurements. TEM analysis reveals that the nanowires consist of well crystallized Ge cores which are completely encapsulated by the sheath-like MWCNTs, the latter corresponding to a layer thickness of 5-10 nm. SEM images, corresponding to various stages of nanowire growth, indicate that MWCNT growth occurs at Ge nanoparticles and that the growing MWCNTs carry Ge as nanowires away from the nanoparticles. By optimizing the CVD parameters, nanowires can be produced with uniform length and diameter in the range 6-10 μm and 200-300 nm, respectively.     

Fabrication of Y-junction carbon nanotubes by reduction of carbon dioxide with sodium borohydride

Y-junction carbon nanotubes with a bamboo-shaped structure have been synthesized by reduction of CO₂ with NaBH₄ at 700 °C. The X-ray power diffraction pattern indicates that the products are hexagonal graphite, and transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images reveal the morphology and structure of carbon nanotubes. The effects of reaction temperature on the growth of the Y-junction carbon nanotubes were also discussed in the paper. Reduction of supercritical CO₂ with sodium borohydride is a promising green chemical method for economically producing Y-junction carbon nanotubes.     

Multiwall carbon nanotubes continuously filled with micrometre-length ferromagnetic α-Fe nanowires

Carbon nanotubes filled with continuous crystalline nanowires of nanometre-scale diameter and micrometre-scale length of the ferromagnetic phase α-Fe were produced with a new chemical vapour deposition method. We report a new two-stage approach, a perturbed-vapour method of synthesis followed by a post-synthesis heat treatment that produces multiwall carbon nanotubes filled with at least 19 micrometre-length nanowires of α-Fe. Previously reported synthesis routes use steady-state conditions to guarantee nanowire continuity but result only in small (less than one-micrometre length) nanowires comprising isolated or mixed phases of either α-Fe, Fe₃C, or γ-Fe. Here flower-like clusters of carbon nanotubes continuously filled with α-Fe were produced by perturbation of a laminar ferrocene (Fe(C₅H₅)₂) vapour flow in a conventional horizontal chemical vapour deposition reactor. Single-phase filling was achieved by a post-synthesis annealing at 500 °C for 15 h in Ar flow. Electron microscopy studies revealed the high quality of the structural integrity of both nanotubes and encapsulated nanowires. These nanostructures possess a high coercivity of 580 Oe and a very high saturation magnetization of 189.5 emu/g comparable with bulk α-Fe.     

Carbothermal Synthesis of the Nanostructures of Al2O3 and ZnO

Nanostructures of Al₂O₃ and ZnO have been synthesized by a carbothermal route involving the reaction of the metal or the metal oxide with carbon. In the case of Al₂O₃, nanowires and nanotubes are obtained starting with Al metal and active carbon or graphite. ZnO nanowires are obtained by the reaction of zinc oxalate or ZnO with active carbon or multiwalled carbon nanotubes. The Al₂O₃ and ZnO nanostructures obtained have been characterized by X-ray diffraction, electron microscopy and photoluminescence spectroscopy. These nanostructures are likely to be of use as catalyst supports and in other technological applications.     

The synthesis of carbon nanotubes at low temperature via carbon suboxide disproportionation

Carbon nanotubes were synthesized via a novel route using an iron catalyst at the extremely low temperature of 180 °C. In this process, carbon suboxide was used as carbon source, which changed to freshly formed free carbon clusters through disproportionation. The carbon clusters can grow into nanotubes in the presence of Fe catalyst, which was obtained by the decomposition of iron carbonyl Fe₂(CO)₉ at 250 °C under nitrogen atmosphere. The products were characterized with XRD, TEM, HRTEM and Raman spectroscopy.     

Fabrication of aluminum carbide nanowires by a nano-template reaction

Large quantities of aluminum carbide nanowires have been prepared by the in situ synthesis of carbon nanotubes within Al powder and heat treatment of the obtained nanotube/Al powder. Scanning and transmission electron microscopy, selected area diffraction, and X-ray powder diffraction have been used to characterize the heat-treated composite powders and the initial one. The results showed that the Al₄C₃ nanowires with diameters ranging from 7 to 25 nm and lengths ranging from 1 to 5 μm are single-crystal. Transmission electron microscopy images indicated that growth of the Al₄C₃ nanowires occurs initially by the formation of a thin, uniform carbide coating and that further growth proceeds by inward growth of this coating with a concomitant consumption of the carbon nanotube until a solid Al₄C₃ nanowire is formed. Reactive wetting kinetics between nanotubes and Al were believed to be responsible for the growth mechanism of Al₄C₃ nanowires.     

Effects of buffer layer materials and process conditions on growth mechanisms of forming networks of SWNTs by microwave plasma chemical vapor deposition

This study investigates the growth mechanism of IC compatible processes and to the feasibility of synthesizing networks of single-walled carbon nanotubes (SWNTs) at lower temperatures (610 °C) on Si wafer using microwave plasma chemical vapor deposition (MPCVD) with CH₄ and H₂ as source gases. The effects of the buffer layer materials (ZnS–SiO₂, Al₂O₃, AlON, and AlN ) and process conditions on growth of carbon nanostructures with Co as catalyst were also examined, where the buffer layers and Co catalyst were deposited in sequence by physical vapor deposition (PVD), followed by H-plasma pretreatment before deposition of carbon nanostructures. Additionally, the morphologies and bonding structures of carbon nanostructures were characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and Raman Spectroscopy. Analytical results demonstrate that networks of SWNTs are more favorable to be synthesized by selecting proper buffer layer material (e.g., AlON), and under higher temperatures, thinner catalyst thickness (e.g., 5 nm) and lower CH₄ / H₂ ratio (e.g., 5 / 100 sccm/sccm). The networks of SWNTs can be fabricated at temperatures as low as 610 °C by manipulating these parameters. In conclusion, the growth mechanism determines the conditions for the formation of nano-sized extrusions on catalyst particles surface.     

Carbon nanowire formation in electron-energy controlled plasma produced by CH4/Ar rf discharges with small coaxial electrodes

Carbon nanowires are formed on a surface of nanoparticles made of diamond-like carbon produced on a nickel plate placed in CH₄–Ar plasma. The dissociation of CH₄ is controlled by changing the mixing position of CH₄ along the Ar plasma produced in a small coaxial tube electrode. When CH₄ is introduced in low electron temperature T_e region, we observed an appearance of nanowires on the surface of microparticles. On the other hand, carbon nanowalls and/or nanoplatelets are formed when the CH₄ is mixed in the high T_e region.     

Electrospinning-derived YVO4:Er/Yb nanowires and nanotubes with temperature sensitivity of their up-converted emission

YVO₄:Er/Yb nanowires and nanotubes phosphors were prepared by electrospinning technique following calcination at different heating rates. The obtained nanowires are of 80–200 nm in diameter and are single crystals or polycrystalline in phase. The outer diameter of the obtained nanotubes is 50–200 nm with a wall thickness of 20–30 nm. The up-conversion properties of YVO₄:Er³⁺/Yb³⁺ nanowires and nanotubes were investigated. All samples were found to produce a green up-converted emission. The temperature-sensing features of YVO₄:Er³⁺/Yb³⁺ nanowires and nanotubes were investigated through the fluorescence intensity ratio approach, and the sensitivity of temperature was determined over the temperature range of 30–390 °C. The temperature sensing sensitivities of the mixed YVO₄:Er³⁺/Yb³⁺ nanowire samples and nanotubes exceeded the single YVO₄:Er³⁺/Yb³⁺ nanowire samples because of higher Yb³⁺-doped concentrations. The morphology influence on optical properties was studied at a fixed doping rare earth concentration and size. The temperature sensitivities of YVO₄:Er/Yb samples depended on the Yb ion concentrations rather than the morphology. The significant temperature sensitivity indicates its potential for use as an optical temperature sensing probe.     

Kinetic study of carbon nanotube synthesis over Mo/Co/MgO catalysts

The kinetics of carbon nanotube (CNT) synthesis by decomposition of CH₄ over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH₄, as well as to the formation of Mo₂C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH₄ pressure, indicating that the dissociation of CH₄ is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation rate of the graphene network is smaller than the carbon deposition rate by decomposition of CH₄.     

Effects of deposition temperature and time on HfC nanowires synthesized by CVD on SiC-coated C/C composites

HfC nanowires were synthesized on SiC-coated carbon/carbon composites via a catalyst-assisted chemical vapor deposition (CVD) process from the HfCl₄–CH₄–H₂ system. The effects of deposition temperature (1273, 1323, 1373 and 1423 K) and time (20, 40, 60 and 90 min) on the formation and microstructure of HfC nanowires were investigated. The results showed that the diameter of HfC nanowires increased with the deposition temperature increasing; both the density and thickness of HfC nanowire films increased with the deposition time prolonging. The growth of HfC nanowires followed the bottom-type vapor–liquid–solid (VLS) mechanism.     

SnO2 Nanowire Arrays and Electrical Properties Synthesized by Fast Heating a Mixture of SnO2 and CNTs Waste Soot

SnO₂ nanowire arrays were synthesized by fast heating a mixture of SnO₂ and the carbon nanotubes waste soot by high-frequency induction heating. The resultant SnO₂ nanowires possess diameters from 50 to 100 nm and lengths up to tens of mircrometers. The field-effect transistors based on single SnO₂ nanowire exhibit that as-synthesized nanowires have better transistor performance in terms of transconductance and on/off ratio. This work demonstrates a simple technique to the growth of nanomaterials for application in future nanoelectronic devices.