Catalytic activity of Fe, Co and Fe-Co-MCM-41 for the growth of carbon nanotubes by chemical vapour deposition method

Iron, cobalt and a mixture of iron and cobalt incorporated mesoporous MCM-41 molecular sieves were synthesised by hydrothermal method and used to investigate the rules governing their nanotube producing activity. The catalysts were characterised by XRD and N₂ sorption studies. The effect of the catalysts has been investigated for the production of carbon nanotubes at an optimised temperature 750 °C with flow rate of N₂ and C₂H₂ is 140 and 60 ml/min, respectively for a reaction time 10 min. Fe-Co-MCM-41 catalyst was selective for carbon nanotubes with low amount of amorphous carbon with increase in single-walled carbon nanotubes (SWNTs) yield at 750 °C. Formation of nanotubes was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Transmission electron microscope and Raman spectrum was used to follow the quality and nature of carbon nanotubes formed and the graphitic layers and disordered band, which shows the clear evidence for the formation of SWNTs, respectively. The result propose that the diameter of the nanotubes in the range of 0.78-1.35 nm. Using our optimised conditions for this system, Fe-Co-MCM-41 showed the best results for selective SWNTs with high yield when compared with Fe-MCM-41 and Co-MCM-41.     

Quality improvement of single-walled carbon nanotubes by doping B in Fe/MgO catalyst

We demonstrate that the quality of the as-grown single-walled carbon nanotubes (SWCNTs) can be effectively improved by the addition of the B ingredient in the Fe/MgO catalyst. The as-grown SWCNTs were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The SWCNTs prepared by the pure Fe/MgO catalyst have relatively low graphite crystallinity and are coated by much amorphous carbon. The intensity ratio of the D- and G-bands (I_D/I_G) in Raman spectra is relatively high (0.098 for laser 532 nm and 0.075 for laser 785 nm). The SWCNTs grown from the Fe/MgO catalyst doped with 0.1 part of B have more regular graphite structure with little amorphous carbon. The I_D/I_G values reduced remarkably (0.041 for laser 532 nm and 0.040 for laser 785 nm). The effect would be attributed to the inhibitory action of the doped B on the formation of radical hydrocarbon species for the formation of SWCNTs.     

Selective growth, characterization, and field emission performance of single-walled and few-walled carbon nanotubes by plasma enhanced chemical vapor deposition

Single-walled carbon nanotubes (SWCNTs) and few-walled carbon nanotubes (FWCNTs) have been selectively synthesized by plasma enhanced chemical vapor deposition at a relative low temperature (550 °C) by tuning the thickness of iron catalyst. The parametric study and the optimization of the nanotube growth were undertaken by varying inductive power, temperature, catalyst thickness, and plasma to substrate distance. When an iron film of 3-5 nm represented the catalyst thickness for growing FWCNT arrays, SWCNTs were synthesized by decreasing the catalyst thickness to 1 nm. The nanotubes were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Electron field emission properties of the nanotubes indicate that the SWCNTs exhibit lower turn-on field compared to the FWCNTs, implying better field emission performance.     

Green luminescence from triphenylphosphine functionalized single-wall carbon nanotubes

In a simple wet chemical process, purified single-wall carbon nanotubes (SWCNTs) are treated with triphenylphosphine (Ph₃P) at room temperature. The functionalized material is characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. HRTEM micrograph clearly reveals that triphenylphosphine nanocrystals of nearly uniform size are attached to the surfaces of SWCNTs. The hybrid structure shows remarkable green luminescence with peak emission at around 500 nm, under UV excitation. The photoluminescence may be attributed to charge transfer from the electron-donating phosphorous atoms to the carbon nanotubes.     

Raman spectroscopy of SWNTs produced by a XeCl excimer laser ablation at high temperatures

Synthesis of single-wall carbon nanotubes (SWNTs) was carried out by an ablation method using a XeCl excimer laser. It was irradiated onto a graphite target containing Co and Ni at the temperatures of 1073, 1173, 1273, 1373, 1473, 1523 and 1623 K under the atmosphere (0.1 MPa) of Ar gas with the flow rate of 12 ml/min. The measurement by a scanning/transmission electron microscope and Raman spectroscopy found the formation of SWNTs with the diameter of about 1.3 nm and the length of about 2 μm in ablated carbonaceous soot. The ratio of peak intensity of 1590 cm⁻¹ (G band) to that of 1335 cm⁻¹ (D band) in the high frequency Raman spectra increased with increasing the ambient temperature. The radial breathing mode (RBM) in the low frequency Raman spectra shows that the mean diameter of SWNTs increased with increasing the ambient temperature.     

Standing or lying C70s encapsulated in carbon nanotubes with different diameters

Single-walled carbon nanotubes (SWNTs) encapsulating C₇₀s, so-called C₇₀ peapods, were synthesized in high yield by a vapor-phase doping method. Raman spectra, high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) measurement indicate that the tube diameter is one of the important factors to determine the orientation of C₇₀ molecules inside the SWNTs. SWNTs with different diameters give different alignment of C₇₀ molecules. The lying orientation is favorable over the standing orientation in thin nanotube, i.e. 1.36 nm nanotubes, whereas the standing orientation is favorable in thick nanotubes, i.e. 1.49 and 1.61 nm nanotubes.     

Diameter-dependent thermal-oxidative stability of single-walled carbon nanotubes synthesized by a floating catalytic chemical vapor deposition method

In this paper, purified single-walled carbon naotubes (SWCNTs) with three different diameters were synthesized using a floating catalytic chemical vapor deposition method with ethanol as carbon feedstock, ferrocene as catalyst, and thiophene as growth promoter. The thermal-oxidative stability of different-diameter SWCNTs was studied by using thermal analysis (TG, DTA), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis. The results indicate that small diameter SWCNTs (∼1 nm) are less stable and burn at lower temperature (610 °C), however, the larger diameter SWCNTs (∼5 nm) survive after burning at higher temperature (685 °C), the oxidation rate varies inversely with the tube diameter of SWCNTs, which may be concluded that the higher oxidation-resistant temperature of larger diameter SWCNTs can be attributed to the lower curvature-induced strain by rolling the planar graphene sheet for the larger diameter, so small tubes will become thermodynamically unstable.     

Zinc oxide catalyzed growth of single-walled carbon nanotubes

We demonstrate that zinc oxide can catalyze the growth of single-walled carbon nanotubes (SWNTs) with high efficiency by a chemical vapor deposition process. The zinc oxide nanocatalysts, prepared using a diblock copolymer templating method and characterized by atomic force microscopy (AFM), were uniformly spaced over a large deposition area with an average diameter of 1.7 nm and narrow size distribution. Dense and uniform SWNTs films with high quality were obtained by using a zinc oxide catalyst, as characterized by scanning electron microscopy (SEM), Raman spectroscopy, AFM, and high-resolution transmission electron microscopy (HRTEM).     

Position-controlled synthesis of single-walled carbon nanotubes on a transparent substrate by laser-induced chemical vapor deposition

The synthesis of single-walled carbon nanotubes (SWCNTs) on a transparent substrate with multiple-catalyst layer (Fe/Al/Cr: 0.5/15/500 nm) using laser-induced chemical vapor deposition is reported. Ethylene (C₂H₄) mixed with hydrogen (H₂) and a continuous wave Nd:YVO₄ laser (532 nm) were used as the precursor gas and the irradiation source, respectively. It was found that the density and quality of the SWCNT dots varied sensitively to laser irradiance and chamber pressure. From subsequent micro-Raman analyses at different excitation sources (488, 514, 633, and 785 nm), the diameters of the SWCNTs were estimated to be within the range of 0.8-2 nm and that the SWCNT dots were composed of both semiconducting and metallic SWCNTs. It is demonstrated that an array of SWCNT dots can be fabricated at precisely controlled positions of a transparent substrate at room temperature with no need of catalysis patterning.     

Thermally oxidized aluminum as catalyst-support layer for vertically aligned single-walled carbon nanotube growth using ethanol

Characteristics and role of Al oxide (Al-O) films used as catalyst-support layer for vertical growth of single-walled carbon nanotubes (SWCNTs) were studied. EB-deposited Al films (20 nm) were thermally oxidized at 400 °C (10 min, static air) to produce the most appropriate surface structure of Al-O. Al-O catalyst-support layers were characterized using various analytical measurements, i.e., atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and spectroscopy ellipsometry (SE). The thermally oxidized Al-O has a highly roughened surface, and also has the most suitable surface chemical states compared to other type of Al-O support layers. We suggest that the surface of thermally oxidized Al-O characterized in this work enhanced Co catalyst activity to promote the vertically aligned SWCNT growth.     

Flexible polyethylene terephthalate (PET) electrodes based on single-walled carbon nanotubes (SWCNTs) for supercapacitor application

Flexible polyethylene terephthalate (PET) electrodes based on pristine single-walled carbon nanotubes (SWCNTs) and acid-treated single-walled carbon nanotubes (A-SWCNTs) were prepared by spray coating technique. Flexible A-SWCNTs electrodes showed enhanced electrochemical properties compared to the pristine SWCNTs electrodes. The electrochemical properties of the flexible A-SWCNTs electrodes were optimized with various types of aqueous electrolytes including sulfuric acid (H₂SO₄), sodium sulfate (Na₂SO₄), potassium chloride (KCl), sodium hydroxide (NaOH), and potassium hydroxide (KOH). The electrochemical performance of the A-SWCNTs electrodes as a function of bending to 30° were evaluated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) measurements in 1 M H₂SO₄. The specific capacitance value of the unbent A-SWCNTs electrode was 67 F g^?1, which decreased to 63 F g^?1 (94% retention) after 1000 GCD cycles. Interestingly, the specific capacitance of the unbent A-SWCNTs electrode with application of the 1000 GCD cycles was retained even after 500 bending to 30° with 6000 GCD cycles.     

Exploration of bimetallic Pt-Pd/C nanoparticles as an electrocatalyst for oxygen reduction reaction

In this study, carbon supported Pt and Pt-Pd were synthesized as oxygen reduction reaction electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs). Pt and Pt-Pd nanoparticles have been synthesized by reduction of metal precursors in presence of NaBH₄. Various techniques such as X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) were utilized to study the prepared samples. Furthermore, electrochemical properties of the prepared samples were evaluated from cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry and electrochemical impedance spectroscopy (EIS). The results showed, the crystallite size of electrocatalysts (Pt and Pt-Pd) is below 10 nm. The higher catalytic activity was detected for Pt-Pd/C electrocatalyst for oxygen reduction reaction (ORR). In addition, it is believed that the better performance of electrocatalyst is related to the synergic effect between Pt and Pd nanoparticles, weakening of the OO bond on Pd-modified Pt nanoparticles in ORR, uniform dispersion of Pd and Pt on the carbon support and higher electrochemical active surface area (EAS) of Pt-Pd/C electrocatalyst.     

Coating of carbon nanotubes on flexible substrate and its adhesion study

The primary goal of this project was to develop a flexible transparent conductor with 100 Ω/sq and 90% transmittance in the wavelength range of 400-700 nm on a flexible substrate. The best result achieved so far was 110 Ω/sq at 88% transmittance using purified single-walled carbon nanotubes (SWNTs) coated on a polyethylene naphthalate (PEN) substrate. The secondary goal was to simplify the overall coating procedure; we successfully reduced the process from five (prior art method) to three steps utilizing a sonication method. We also found that the use of metallic SWNTs significantly improved the conductivity and transmittance compared with the use of mixed SWNTs, i.e., unseparated SWNTs. Furthermore, a possible adhesion mechanism between SWNTs and the surface of PEN was studied; we concluded that a π-π stacking effect and a hydrophobic interaction are the major contributing factors for SWNTs to adhere to the surface of the substrate.     

Comparison of different types of polypyrimidine/CNTs/Pt hybrids in fuel cell catalysis

In this paper, we mainly studied the preparation of platinum-containing composite materials with carbon nanotubes wrapped by polypyrimidine-conjugated polymers and the performance of the composites. The polymer-based carbon nanotubes/Pt catalysts were prepared successfully and confirmed by infrared spectroscopy, XPS, XRD, and TEM images. The performance of polypyrimidine/multi-walled carbon nanotubes (MWCNTs)/Pt and polypyrimidine/double-walled carbon nanotubes (DWCNTs)/Pt was compared with the polypyrimidine/single-walled carbon nanotubes (SWCNTs)/Pt. The amount of the loaded Pt on the polypyrimidine/MWCNTs and polypyrimidine/DWCNTs was calculated to be 50.5 wt% and 52.7 wt% respectively. The effective specific surface area of the polypyrimidine/MWCNTs/Pt (45.7 m²/g) and polypyrimidine/DWCNTs/Pt (42.47 m²/g) was observed by electrochemical cyclic voltammetry. These studies strongly imply that the MWCNTs were better candidates than DWCNTs and SWCNTs in the application of polypyrimidine/CNT materials as catalyst for fuel cells.     

Silicon carbide nanocrystals growth on Si(1 0 0) and Si(1 1 1) from a chemisorbed methanol layer

SiC nanocrystals are grown at high temperature on Si(1 0 0) and Si(1 1 1) surfaces starting from a chemisorbed layer of methanol. The decomposition of this layer allows to have a well defined amount of carbon to feed SiC growth. Nanocrystals ranging from 10 nm to 50 nm with density from 100 μm⁻² to 1500 μm⁻² are obtained, and the total volume of produced SiC corresponds to carbon provided by the chemisorbed organic layer. Large differences in nanocrystal size and density, as well as in surface roughness, are observed depending on substrate orientation. The internal structure, crystallinity and epitaxy of nanocrystals grown on Si(1 0 0) are studied using cross-sectional transmission electron microscopy (XTEM), methanol adsorption and surface evolution using scanning tunnelling microscopy (STM). The joint application of XTEM and STM techniques allows a complete characterization of the geometry and chemical composition of these nanostructures.     

From crystalline V2O5 to nanostructured vanadium oxides using aromatic amines as templates

Nanoneedles, nanorods of B-VO₂, and vanadium oxide nanotubes with high crystallinity were synthesized via a one-step hydrothermal treatment using crystalline V₂O₅ as a precursor and aromatic amines (C₆H₅-(CH₂)_n-NH₂ with n=0, 1, 3) as structure-directing templates. Samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), thermal analysis, nitrogen adsorption/desorption isotherms and infrared spectroscopy. Nanoneedles, 0.5-5 μm in length and about 50 nm in average diameter and VO₂(B) nanorods about 20-100 nm wide and up to 2.5 μm long, have been obtained. The inner and the outer diameters of the vanadium oxide nanotubes vary, respectively, between 15-25 and 70-100 nm with a length up to 4 μm.     

High-field electron emission of carbon nanotubes grown on carbon fibers

Carbon nanotubes with uniform density were synthesized on carbon fiber substrate by the floating catalyst method. The morphology and microstructure were characterized by scanning electron microscopy and Raman spectroscopy. The results of field emission showed that the emission current density of carbon nanotubes/carbon fibers was 10 μA/cm² and 1 mA/cm² at the field of 1.25 and 2.25 V/μm, respectively, and the emission current density could be 10 and 81.2 mA/cm² with the field of 4.5 and 7 V/μm, respectively. Using uniform and sparse density distribution of carbon nanotubes on carbon fiber substrate, the tip predominance of carbon nanotubes can be exerted, and simultaneously the effect of screening between adjacent carbon nanotubes on field emission performance can also be effectively decreased. Therefore, the carbon nanotubes/carbon fibers composite should be a good candidate for a cold cathode material.     

Field emission properties of carbon nanotubes grown on silicon nanowire arrays

Carbon nanotubes are synthesized on the silicon nanowire arrays which are fabricated on silicon substrate by chemical vapor depositing SiCl₄ and H₂ gases in the presence of Au catalysts. The silicon nanowires are single-crystal with lengths up to 100 μm and diameters ranging from 50 to 500 nm. The tangled carbon nanotubes are grown directly from the surface of Si nanowires. The field emission properties of the carbon nanotubes are investigated at the gap of 200 μm. The low turn on and threshold fields are obtained. The stabilization of the emission currents is also presented.     

Fabrication of silver nanoparticles ring templated by plasmid DNA

Silver nanoparticles ring was successfully fabricated by electrostatic assembling 4-aminothiophenol (4-ATP) capped silver nanoparticles on predefined extended circular plasmid pBR322 DNA. The silver nanoparticles ring which was about 1.5 μm in length, and about 2.2 nm in height can be obtained by adjusting the reaction time. The normal Raman scattering spectra reveal that the 4-ATP has contacted with the silver nanoparticles by forming a strong Ag-S bond. The AFM data show that the assembly of 4-ATP capped silver nanoparticles on DNA is ordered.     

Poly(3-methylthiophene)-based porous silicon substrates as a urea-sensitive electrode

Poly(3-methylthiophene) (P3MT)-based porous silicon (PS) substrates were fabricated and characterized by cyclic voltammetry, scanning electron microscopy, and auger electron spectroscopy. After doping urease (Urs) into the polymeric matrix, sensitivity and physicochemical properties of the P3MT-based PS substrate was investigated compared to planar silicon (PLS) and bulk Pt substrates. PS substrate was formed by electrochemical anodization in an etching solution composed of HF, H₂O, and ethanol. Subsequently, Ti and Pt thin-films were sputtered on the PS substrate. Effective working electrode area (A_eff) of the Pt-deposited PS substrate was determined from a redox reaction of Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple in which nearly reversible cyclic voltammograms were obtained. The i_p versus v^1/2 plots showed that A_eff of the PS-based Pt thin-film electrode was 1.62 times larger than that of the PLS-based electrode.Electropolymerization of P3MT on both types of electrodes were carried out by the anodic potential scanning under the given potential range. And then, urease molecules were doped to the P3MT film by the chronoamperometry. Direct electrochemistry of a Urs/P3MT/Pt/Ti/PS electrode in an acetonitrile solution containing 0.1 mol/L NaClO₄ was introduced compared to a P3MT/Pt/Ti/PS electrode at scan rates of 10 mV s⁻¹, 50 mV s⁻¹, and 100 mV s⁻¹.Amperometric sensitivity of the Urs/P3MT/Pt/Ti/PS electrode was ca. 1.67 μA mM⁻¹ per projected unit square centimeter, and that of the Urs/P3MT/Pt/Ti/PLS electrode was ca. 1.02 μA mM⁻¹ per projected unit square centimeter in a linear range of 1-100 mM urea concentrations. 1.6 times of sensitivity increase was coincident with the results from cyclic voltammetrc analysis.Surface morphology from scanning electron microscopy (SEM) images of Pt-deposited PS electrodes before and after the coating of Urs-doped P3MT films showed that pore diameter and depth were 2 μm and 10 μm, respectively. Multilayered-film structures composed of metals and organics for both electrodes were also confirmed by auger electron spectroscopy (AES) depth profiles.