Haolan Wang

Properties of carbon onions produced by an arc discharge in water

A simple method to fabricate high-quality nanoparticles including spherical carbon onions and elongated fullerene-like nanoparticles similar to nanotubes in large quantities without the use of vacuum equipment is reported. The nanoparticles are obtained in the form of floating powder on the water surface following an arc discharge between two graphite electrodes submerged in water. High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy images confirm the presence of spherical carbon onions with diameters ranging from 4 to 36 nm. The specific surface area of the floating powder was found to be very large, 984.3 m2/g, indicating that the material is promising for gas storage. From the surface area measurements, the mean particle diameter was calculated to be 3.7 nm. This value is close to the lower limit of the carbon onions observed in HRTEM. However, closer HRTEM observations also reveal that some carbon onions are not well crystallized. The large specific surface area c...

Characterisation of carbon nano-onions using Raman spectroscopy

Characteristics of the Raman spectrum from carbon onions have been identified in terms of the position of the G peak and appearance of the transverse optic phonon peaks. Five new peaks were observed in the low wavenumber region, at about 1100, 861, 700, 450 and 250 cm−1. The origins of these peaks are discussed in terms of the phonon density of states (PDOS) and phonon dispersion curves of graphite. The curvature of the graphene planes is invoked to explain the relaxation of the Raman selection rules and the appearance of the new peaks. The Raman spectrum of carbon onions is compared with that of highly oriented pyrolytic graphite (HOPG). The strain of graphene planes due to curvature has been estimated analytically and is used to account for the downward shift of the G peak.

Large-scale synthesis of single-walled carbon nanohorns by submerged arc

We report on the synthesis of single-walled carbon nanohorns (SWNHs) by arc discharge between two graphite electrodes submerged in liquid nitrogen. The product in its powder form was found to consist of spherical aggregates with sizes in the range of 50–100 nm. The nanohorns are characterized by transmission electron microscopy (TEM), Raman and electron energy loss spectroscopies and surface area measurements. TEM observations revealed that the internal structure of the aggregates could be described as a mixture of ‘dahlia-like’ and ‘bud-like’. A structural transition above 300 ◦ Cw as inferred from the Raman data. Here we show that a simple technique that does not require relatively expensive laser and vacuum equipment provides an economical alternative for the synthesis of SWNHs.

Fabrication of inorganic molybdenum disulfide fullerenes by arc in water

Closed caged fullerene-like molybdenum disulfide (MoS2) nano-particles were obtained via an arc discharge between a graphite cathode and a molybdenum anode filled with microscopic MoS2 powder submerged in de-ionized water. A statistical study of over 150 polyhedral fullerene-like MoS2 nano-particles in plan view transmission electron microscopy revealed that the majority consisted of 2–3 layers with diameters of 5–15 nm. We show that the nano-particles are formed by seamless folding of MoS2 sheets. A model based on the agglomeration of MoS2 fragments over an extreme temperature gradient around a plasma ball in water is proposed to explain the formation of nano-particles. 2002 Elsevier Science B.V. All rights reserved.

Structure of carbon onions and nanotubes formed by arc in liquids

Since carbon nanotubes and onions were discovered, many methods have been proposed for their production. For applications the main requirements are low capital cost, high purity of the produced material, simplicity of technique, and its potential for scale up. Recently a cathodic arc between two graphitic electrodes immersed in liquids has been demonstrated to be a simple method to produce carbon nanoparticles such as nanotubes and onions. In this paper high-resolution transmission electron microscopy is employed to examine the shape of the nanoparticles and the purity of the final material produced under various conditions. In this study we have used an arc discharge in two different liquids--liquid nitrogen and distilled water--and we have changed the grade of the carbon electrodes. The variety in structure, shape, and size of the produced particles is discussed in line with a model proposed to describe the physical process.

Enhanced supercapacitors from hierarchical carbon nanotube and nanohorn architectures

Supercapacitors fill the power and energy gap between electrolytic capacitors and batteries. The energy density for commercial supercapacitors is currently limited to ∼5 Wh kg−1. Enhancing the energy and power density of supercapacitors is of great interest as it would open up a much wider range of applications. In this work, thin film supercapacitors made of random networks of single-walled carbon nanotubes (SWNTs) were enhanced by the use of carbon nanoparticles of a size ideal to fill the pores in the SWNT network. These nanoparticles, termed carbon nanohorns (CNHs), provide a much enhanced surface area, whilst maintaining high permeability and porosity. We demonstrate the hierarchical use of carbon nanostructures in a controlled fashion, allowing an enhancement provided by both types of materials, high power density by the SWNTs and high energy density from the CNHs. SWNT films serve as an ideal template onto which CNHs are deposited, with a good size match, adhesion and charge transfer between particles of a single chemical species. This combination results in an enhanced specific capacitance and a reduced equivalent series resistance (ESR) compared to a capacitor made of either individual component. Additionally, the good binding properties of the hybrid material and the high electrical conductivity of the SWNTs permit the removal of both the binder and the charge collector, paving the way for thinner and lighter supercapacitors. These electrodes allow the fabrication of supercapacitors with novel properties. As an example, we demonstrate a semitransparent supercapacitor. These results demonstrate the possibilities that may be available for the enhancement of electrodes by tailoring and combining relevant materials hierarchically in multiple scales. Much potential remains in further enhancement through tailored hierarchical nanostructuring.

Pressure effects on nanotubes formation using the submerged arc in water method

Synthesis of multi-walled carbon nanotubes (MWCNTs) by an arc discharge between two graphite electrodes submerged in water under controlled pressure (from 400 to 760 Torr) is reported. Transmission and scanning electron microscopy investigations of the arc discharge product collected from the bottom of the reactor revealed high concentrations of MWCNTs at all pressures. Dynamic light scattering (DLS) on suspensions containing MWCNTs showed that the mean diameter of the nanotubes increases with decreasing pressure. Raman spectroscopy analysis reveals that the relative amount of disordered carbon is significantly less in the low-pressure samples. Furthermore, the yield of the deposit was found to be independent of the pressure. These results suggest that the physical properties of MWCNTs formed by the submerged arc can be controlled by varying the pressure.

Structural investigation of MoS2 core-shell nanoparticles formed by an arc discharge in water

The intense environment of an electric arc has been used to create MoS2 core–shell particles. Instead of running the arc in a vacuum chamber under reduced pressure conditions, an arc discharge in water was employed. The arc was run between a carbon cathode and a Mo hollow rod anode whose interior was packed with MoS2 powder. High-resolution electron microscopy and image simulations have been used to characterize the structure of the resultant material which was seen to consist of agglomerates of polyhedral MoS2 cages with filled cores. A correlation between the growth conditions and the observed nanostructure is presented.

Template-free electrochemical nanofabrication of polyaniline nanobrush and hybrid polyaniline with carbon nanohorns for supercapacitors

Polyaniline (PANI) nanobrushes were synthesized by template-free electrochemical galvanostatic methods. When the same method was applied to the carbon nanohorn (CNH) solution containing aniline monomers, a hybrid nanostructure containing PANI and CNHs was enabled after electropolymerization. This is the first report on the template-free method to make PANI nanobrushes and homogeneous hybrid soft matter (PANI) with carbon nanoparticles. Raman spectroscopy was used to analyze the interaction between CNH and PANI. Electrochemical nanofabrication offers simplicity and good control when used to make electronic devices. Both of these materials were applied in supercapacitors and an improvement capacitive current by using the hybrid material was observed.

Nanocarbon based supercapacitors with reduced internal resistance

Supercapacitors, also known as electrical double layer capacitors, are promising candidates to meet the increasing power demands upon energy storage systems. They have an important role in complementing or replacing batteries in the energy storage field, possessing advantages such as high power density, rapid charge/discharge (few seconds), >100 000 cycle life, and intrinsic safety (they contain no heavy metals and have a reduced likelihood of catastrophic failure). The performance of a supercapacitor depends on charge capacitance, operating voltage and internal resistance. In this paper, particular attention is given to methods of decreasing the internal resistance of supercapacitors because for intended use in high current applications, high internal resistances cause unacceptable power loss and voltage drop. We describe two steps to decrease the equivalent series resistance (ESR) of nanocarbon based supercapacitors. First, a thin, conductive carbon layer was coated on a copper current collector to decrease the contact resistance between the active electrodes and the copper. The carbon layer formed on the copper has a rough surface, which increases both the contact area and adhesion ability between the electrode material and the copper surface. In addition, the carbon layer prevents oxidation and corrosion of the metal layer, and therefore prevents ESR increase after extended cycling, thus prolonging the supercapacitor life. Second, multiwall carbon nanotubes (CNTs) were added to the nanocarbon to further decrease the ESR of the device. CNTs have good electrical conductivity and a readily accessible surface area. Scanning electron microscope images show that the added CNTs cover the surface of the nanocarbon particles and bridge the gap between the particles. After surface treatment of the copper and adding CNTs to the active electrodes, the supercapacitor have a significantly reduced ESR and highly enhanced capacitance. Evaluation of capacitor performance by different techniques, such as voltammetry, charge/discharge characteristics is also discussed.