Double layer capacitance of high surface area carbon nanospheres derived from resorcinol–formaldehyde polymers

Micro-mesoporous bimodal carbon nanospheres with high surface areas were synthesized by a combined use of surfactant templating technique and BaO₂ chemical activation one. Starting spherical nanopolymer/surfactant composites were prepared by the NaOH-catalyzed reaction of resorcinol (R) and formaldehyde (F) in the presence of cetyltrimethylammonium bromide (CTAB) as a core template and 1,3,5-trimethylbenzene (TMB) and tert-butanol (t-BuOH) as cosurfactants. After pretreatment with hydrochloric acid, the composite materials were calcined at 1000 °C in N₂ coexistent with varying weight ratios of BaO₂ to RF polymer ranging from 0 to 11. It produced micro-mesoporous bimodal carbon nanospheres of 124–143 nm diameter, with specific surface areas as high as 1884 m² g⁻¹ or up to 3301 m² g⁻¹, in contrast to microporous ones with smaller surface areas obtained at low BaO₂-loadings. The electrochemical double layer capacitance of the resulting nanocarbons in 0.5 M H₂SO₄ showed a marked increase with specific surface areas, up to as high as 219 F g⁻¹ for the highest surface area carbon material.     

Synthesis and magnetic properties of Cu-doped ZnO nanowire arrays

Arrays of Cu-doped ZnO nanowires were successfully fabricated by electrodeposition of Zn²⁺ and Cu²⁺ into anodic aluminum oxide template and post-oxidation annealing in air atmosphere. The transmission electron microscopy result shows that the nanowires are uniform, about 100 nm in diameter and with the aspect ratio of up to 40. Selected area electron diffraction and X-ray diffraction results indicate that the nanowires are in hexagonal wurtzite structure. Magnetization measurements show that the Zn_1−xCu_xO (x = 0.07 and 0.11) nanowires exhibit room-temperature ferromagnetism and the enhancement of the ferromagnetism is revealed for the Zn_0.93Cu_0.07O nanowires annealed in vacuum.     

Pt nanowires prepared via a polymer template method: Its promise toward high Pt-loaded electrocatalysts for methanol oxidation

Platinum nanowires were prepared via a template-synthesis method by electrodeposition of platinum within pores of a track-etched polycarbonate (PCTE) membrane, followed by chemical etching to separate the Pt nanowires from the polymer. The structure and morphology of the Pt nanowires were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM), revealing a polycrystalline phase with nanowire dimension up to 6 μm long and ca. 47 ± 9.8 nm of diameter. The unsupported Pt nanowires showed the better electrochemical mass activities over the methanol electro-oxidation than supported or unsupported Pt nanoparticles under the high Pt content-loaded conditions that is typically required for direct methanol fuel cells. This enhancement could be rationalized by its unique physicochemical and electrical properties arising from the inherent anisotropic one-dimensional (1D) nanostructure, such as charge transfer facilitation by reducing number of particle interfaces and more efficient use of Pt by alleviating fraction of embedded catalysts.     

Carbon/platinum nanotextured films produced by plasma sputtering

Platinum loaded carbon layers were synthesized by a two-step plasma sputtering process. Two hundred nanometers thick columnar (columns with an average diameter of 20 nm) carbon films having a large open porosity were formed in the first step. Using the same plasma system, the films were subsequently loaded with platinum. SEM, TEM and Rutherford backscattering spectrometry analysis show that platinum diffuses into the carbon layer and forms nano-sized particles (mean diameter ca. 3 nm) along and around the carbon nanocolumns and down to the film/support interface. Optimized catalytic layers were formed at low plasma pressure operation (<1 Pa) and had an upper platinum loading limit of about 0.1 mg cm⁻².     

Generation of carbon nanofilaments on carbon fibers at 550 °C

Employing a relatively new method, in which carbon structures are grown from fuel rich combustion mixtures using palladium particles as catalyst, multi-scale diameter nanometer - micrometer filament structures were grown from ethylene/oxygen mixtures at 550 °C on commercial PAN micrometer carbon fibers. The filaments formed had a diameter roughly equal to the palladium particle size. At sufficiently high metal loadings (>0.05 wt.%) a bimodal catalyst size distribution formed, hence a bimodal filament size distribution was generated. Relative short, densely spaced nanofilaments (ca. 10 nm diameter), and a slightly less dense layer of larger (ca. 100 nm diameter) faster growing fibers (ca. 10 μm/h) were found to exist together to create a unique multi-scale structure. A protocol was developed such that only nano-scale fibers or a mixture of nano and sub-micron fibers could be produced. No large range order was evident in the filaments. This work demonstrates a unique ability to create a truly ’multi-scale’ carbon structure on the surface of carbon fibers. This fiber structure potentially can enhance composite material strength, ductility and energy absorption characteristics.     

Modified carbon nanoparticle-chitosan film electrodes: Physisorption versus chemisorption

Surface functionalised carbon nanoparticles of ca. 8 nm diameter co-assemble with chitosan into stable thin film electrodes at glassy carbon surfaces. Robust electrodes for application in sensing or electrocatalysis are obtained in a simple solvent evaporation process. The ratio of chitosan binder backbone to carbon nanoparticle conductor determines the properties of the resulting films. Chitosan (a poly-d-glucosamine) has a dual effect (i) as the binder for the mesoporous carbon composite structure and (ii) as binding site for redox active probes. Physisorption due to the positively charged ammonium group (pK_A ≈ 6.5) occurs, for example, with anionic indigo carmine (a reversible 2e⁻-2H⁺ reduction system in aqueous media). Chemisorption at the amine functionalities is demonstrated with 2-bromo-methyl-anthraquinone in acetonitrile (resulting in a reversible 2e⁻-2H⁺ anthraquinone reduction system in aqueous media). Redox processes within the carbon nanoparticle-chitosan films are studied and at sufficiently high scan rates diffusion of protons (buffer concentration depended) is shown to be rate limiting. The chemisorption process provides a much more stable interfacial redox system with a characteristic and stable pH response over a pH 2-12 range. Chemisorption and physisorption can be employed simultaneously in a complementary binding process.     

Self-organized carbon connections between catalyst particles on a silicon surface exposed to atmospheric-pressure Ar + CH4 microplasmas

Ag nanoparticles and Fe-coated Si micrograins were separately deposited onto Si(1 0 0) surfaces and then exposed to an Ar + CH₄ microplasma at atmospheric pressure. For the Ag nanoparticles, self-organized carbon nanowires, up to 400 nm in length were produced, whereas for the Fe-coated Si micrograins carbon connections with the length up to 100 μm were synthesized on the plasma-exposed surface area of about 0.5 mm². The experiment has revealed that long carbon connections and short nanowires demonstrate quite similar behavior and structure. While most connections/nanowires tended to link the nearest particles, some wires were found to ‘dissolve’ into the substrate without terminating at the second particle. Both connections and nanowires are mostly linear, but long carbon connections can form kinks which were not observed in the carbon nanowire networks. A growth scenario explaining the carbon structure nucleation and growth is proposed. Multiscale numerical simulations reveal that the electric field pattern around the growing connections/nanowires strongly affects the surface diffusion of carbon adatoms, the main driving force for the observed self-organization in the system. The results suggest that the microplasma-generated surface charges can be used as effective controls for the self-organized formation of complex carbon-based nano-networks for integrated nanodevices.     

Synthesis of tellurium nanotubes by galvanic displacement

Tellurium nanotubes with controlled diameter and wall thickness were synthesized by galvanic displacement of cobalt nanowires and their temperature dependent field effect transistor and magnetoresistance properties were systematically investigated. The nanotube diameter was slightly larger than the sacrificial cobalt nanowire diameter with a wall thickness of range from 15 to 30 nm depending on the diameter of cobalt nanowires. Te nanotubes show p-type semiconducting property with the field effect carrier mobility of approx. 0.01 cm²/V s which is relatively lower than other 1D nanostructure. Low mobility might be attributed to porous morphology with small grain size (<10 nm). Temperature dependent mobility also exhibiting a Conwell-Weisskopf relationship to temperatures below 250 K, indicating that the dominant scattering sites are ionized impurity centers. Unique MR behavior was observed from nanotube with a maximum magnetoresistance ratio of 37% at 260 K.     

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.     

Use of amorphous monodispersed spinel film electrode in photo-electrochemical cells

A high purity amorphous spinel film (CdCr₂S₄) electrode composed of monodispersed grains has been successfully deposited on an ordinary glass and conducting glass (FTO) substrates from aqueous medium using a simple, inexpensive, reproducible chemical bath method at ambient temperature. The as-deposited film composed of small elongated particles of 100-150 nm in length and 10-20 nm in diameter aggregated into large monodispersed spherical clusters of 100-400 nm in diameter. Furthermore, the influence of this CdCr₂S₄ film electrode on photo-electrochemical properties was studied and reported.     

Vertically aligned carbon nanotube arrays grown on a lamellar catalyst by fluidized bed catalytic chemical vapor deposition

Large amount of vertically aligned carbon nanotube (CNT) arrays were grown among the layers of vermiculite in a fluidized bed reactor. The vermiculite, which was 100-300 μm in diameter and merely 50-100 μm thick, served as catalyst carrier. The Fe/Mo active phase was randomly distributed among the layers of vermiculite. The catalyst shows good fluidization characteristics, and can easily be fluidized in the reactor within a large range of gas velocities. When ethylene is used as carbon source, CNT arrays with a relatively uniform length and CNT diameter can be synthesized. The CNTs in the arrays are with an inner diameter of 3-6 nm, an outer diameter of 7-12 nm, and a length of up to several tens of micrometers. The as-grown CNTs possess good alignment and exhibit a purity of ca. 84%. Unlike CNT arrays grown on a plane or spherical substrate, the CNT arrays grown in the fluidized bed remain their particle morphologies with a size of 50-300 μm and the good fluidization characteristics were preserved accordingly.     

Continuous production of single-walled carbon nanotubes using a supported floating catalyst

Using catalytic decomposition, a technique for the production of singe-walled carbon nanotubes (SWCNTs) is reported with a production rate up to 6 g h⁻¹ after purification, and scaling capability up to 220 g h⁻¹. This is achieved by injection of pre-prepared alumina supported catalyst powder into a modified vertical floating reactor. The product is collected in several cyclones connected in series. Wide range Raman studies (laser excitations from λ = 1064 to 488 nm) and temperature programmed oxidation measurements of the samples collected from the different cyclones show that SWCNTs were separated in situ by tube diameter. This is attributed to the different residual times of the catalyst in the reaction zone depending on particle diameter. A series of computational fluid dynamics calculations of the flow and heat transfer in the reactor, as well as modeling of catalyst particle transport reveals the parametrical dependence of the process.     

Structure and properties of polyacrylonitrile/single wall carbon nanotube composite films

Polyacrylonitrile (PAN)/single wall carbon nanotube (SWNT) composite films have been processed with unique combination of tensile strength (103 MPa), modulus (10.9 GPa), electrical conductivity (1.5×10⁴ S/m), dimensional stability (coefficient of thermal expansion 1.7×10⁻⁶/°C), low density (1.08 g/cm³), solvent resistance, and thermal stability. PAN molecular motion above the glass transition temperature (T_g) in the composite film is significantly suppressed, resulting in high PAN/SWNT storage modulus above T_g (40 times the PAN storage modulus). Rope diameter in the SWNT powder was 26 nm, while in 60/40 PAN/SWNT film, the rope diameter was 40 nm. PAN crystallite size from (110) plane in PAN and PAN/SWNT films was 5.3 and 2.9 nm, respectively. This study suggests good interaction between PAN and SWNT.     

Transport properties of conducting amorphous carbon-poly(vinyl chloride) composite

The dc electrical conductivity of composites of poly(vinyl chloride) filled with amorphous carbon (a-C) flakes has been studied from room temperature to 1.2 K. The dc conductivity shows percolative behavior and as a function of a-C content, follows the scaling law σα(p − p_c)^t where p_c is percolation threshold and t the critical exponent with values 0.0165 and 3.1, respectively. The value of t shows evidence for non-universal value of critical exponent. In the temperature range 1.2 K to 50 K-60 K all the samples show the thermal fluctuation induced tunneling of the charge carriers through the insulating layer of polymer separating amorphous carbon aggregates. At high temperatures the differential thermal expansion of PVC and amorphous carbon plays a crucial role in conduction mechanism that leads to positive temperature coefficient of resistivity near to room temperature.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

Preparation and electrochemical performances of ZnO nanowires as anode materials for Ni/Zn secondary battery

ZnO nanowires were synthesized by a hydrothermal route without any substrate or template. Structure analyses through XRD, SEM, TEM and HRTEM indicated that ZnO nanowires had high purity and perfect crystallinity, and grew along [0 0 0 1]. The diameter was 50-80 nm, the length was about several micrometers and length-diameter ratio was more than 100. As electrode materials of Ni/Zn batteries, ZnO nanowires showed the obviously improved cycle stability, average discharge capacity of 609 mAh g⁻¹, higher discharge voltage/lower charge voltage. Slow rate cyclic voltammetry showed that electrochemical activity of ZnO nanowires was superior to that of the conventional ZnO. The improvements of electrochemical performance were ascribed to the unique nanowire structure. During the charging/discharging cycles, nanowires were broke, grew in diameter, and changed into nanorods. Nanowires lying parallel to the anodes could suppress the growth of dendrite clusters perpendicular to the anodes.     

The spatial uniformity and electromechanical stability of transparent, conductive films of single walled nanotubes

We have prepared thin films of arc discharge single walled nanotubes by vacuum filtration. For film thicknesses greater than 40 nm, the films are of high optical quality; the optical transmission varies by <2% over the film area when measured with a spatial resolution of 4 μm. However, the films become spatially non-uniform for film thickness below 40 nm. The in-plane DC conductivity correlates with the uniformity, increasing from ∼3800 S/m for a 10 nm thick film to ∼2-2.5 × 10⁵ S/m for films of thickness >40 nm. Conductive atomic force microscopy maps show reasonably uniform current flow out of the plane of the film. For all thicknesses, the optical transmittance scales with film thickness as expected for a thin conducting film with optical conductivity of 1.7 × 10⁴ S/m (λ = 550 nm). For films with t > 40 nm the ratio of DC to optical conductivity was σ_DC/σ_Op = 13.0, leading to values of transmittance and sheet resistance such as T = 80% and R_s = 110 Ω/□ for the t = 40 nm film. Electromechanically, these films were very stable showing conductivity changes of <5% and <2% when cycled over 2000 times in compression and tension respectively.     

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.     

Silicon nanowires as negative electrode for lithium-ion microbatteries

The increasingly demand on secondary batteries with higher specific energy densities requires the replacement of the actual electrode materials. With a very high theoretical capacity (4200 mAh g⁻¹) at low voltage, silicon is presented as a very interesting potential candidate as negative electrode for lithium-ion microbatteries. For the first time, the electrochemical lithium alloying/de-alloying process is proven to occur, respectively, at 0.15 V/0.45 V vs. Li⁺/Li with Si nanowires (SiNWs, 200-300 nm in diameter) synthesized by chemical vapour deposition. This new three-dimensional architecture material is well suited to accommodate the expected large volume expansion due to the reversible formation of Li-Si alloys. At present, stable capacity over ten to twenty cycles is demonstrated. The storage capacity is shown to increase with the growth temperature by a factor 3 as the temperature varies from 525 to 575 °C. These results, showing an attractive working potential and large storage capacities, open up a new promising field of research.     

Uniaxially oriented carbon monoliths as supercapacitor electrodes

Cylindrical carbon monoliths of 7 mm in diameter and certain heights (1, 2, 3, 4 and 5 mm) are studied as model electrodes for supercapacitors. The monoliths show a narrow microporous structure with average micropore size of 0.73 nm and specific surface area of 1086 m² g⁻¹. The monoliths show straight walls and channels, both arranged along the cylinder axis. The former account for a remarkable electrical conductivity (6.5 S cm⁻¹ at room temperature). The latter allow a rapid ionic transport between the electrolyte bulk and the carbon walls and account for a high specific capacitance at high current density. The cell capacitance and resistance increase linearly with the monolith height according to C = (1.78 ± 0.06)h and ESR = (0.08 ± 0.01)h + (1.67 ± 0.04), respectively. The contribution of the electrolyte resistance, monolith resistance and monolith/collector resistance to ESR is discussed. The cell response time or constant time increases with the monolith height but according to a power dependence, τ = (4.5 ± 0.2)h^{(1.61 ± 0.03)}. The carbon of the monoliths show in KOH electrolyte a specific capacitance of 150 F g⁻¹ and a capacitance per surface area of 14 μF cm⁻².