Effects of nitrogen-doped carbon nanotubes on the discharge performance of Li-air batteries

Carbon nanotubes (CNTs) and nitrogen-doped carbon nanotubes (N-CNTs) were synthesized using a floating catalyst chemical vapor deposition method and characterized by scanning electron microscopy (SEM), transmission electron microscopy, Raman and X-ray photoelectron spectroscopy. The study found that the as-prepared CNTs and N-CNTs showed different discharge capacity as cathode materials in Li-air battery. To further study the reason why N-doping improves the electrochemical performance exceptionally, the discharge products on the two kinds of nanotubes were detected by SEM, XRD and Raman. SEM study showed, for the first time, that more uniform distribution of discharge products on the surface of CNTs arising from N-doping affected the boost of discharge capacity, a result which was discussed in detail. In comparison to non-doped CNTs, nitrogen doping was considered to be a promising way to improve the performance of carbon based cathode material for Li-air batteries.     

Synthesis of multiwall carbon nanotubes by electric arc discharge in liquid environments

This work reports the experimental results from the production of multiwall carbon nanotubes (MWCN) synthesized by an electric arc discharge performed in liquid environments between pure graphite electrodes. Both liquid nitrogen and deionised water were suitable for a successful synthesis of this form of carbon aggregation. We report a successful synthesis of MWCN by arc discharge submerged in deionised water. Electron microscopy observations of both the reaction products and the surface of the as-synthesized raw material showed the presence of structural degradation of the MWCN, which probably operates after their growth at the cathode. The degradation is tentatively ascribed to a combination of overheating and high current density experienced by the as-synthesized MWNT, which can be caused by the loose structure of the as-deposited material. The damage appeared to be less severe in water environments, probably owing to the better cooling capacity of water relative to liquid nitrogen.     

Encapsulation of TiC AND HfC crystallites within graphite cages by arc discharge

Fine crystallites of titanium and hafnium carbides encapsulated within graphite cages were formed by an arc discharge between a metal/carbon composite anode and a graphite cathode. Soot particles grown in various places within the arc reaction chamber were studied by transmission electron microscopy. Encapsulated TiC crystallites ranging from 30 to 150 nm in size were found in carbonaceous materials formed around a cathode (called “slag-like hard deposit” and “cathode soot” on it). Short, single-walled carbon nanotubes were also observed, extruding from carbon layers with complicated structures which surround TiC crystallites. For hafnium whose vapor pressure is lower than that of carbon at high temperatures, the formation of HfC crystallites (20 to 80 nm in diameter) was limited within a slag-like deposit, and they were also encapsulated in graphite cages.     

Effect of the liquid environment on the formation of carbon nanotubes and graphene layers by arcing processes

The influence of the liquid environment on the formation of different carbon nanostructures by arcing processes, in particular liquid nitrogen (LN₂) and de-ionised water (DI-H₂O), has been investigated. Large structural differences between the nanomaterials produced in the two cases are shown: the use of DI-H₂O induces a large number of pentagon–heptagon defects in the nanotubes, compared to the use of LN₂. A large amount of graphene layers has been observed by scanning electron microscopy and transmission electron microscopy in the case of arc discharge in DI-H₂O. Micro-Raman analysis confirms the electron microscopy results.     

Direct and large scale electric arc discharge synthesis of boron and nitrogen doped single-walled carbon nanotubes and their electronic properties

Boron and nitrogen co-doped single-walled carbon nanotubes (BN-SWCNTs) were directly synthesized at large scale using an electric arc discharge method. X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and UV-vis-NIR spectroscopy were performed to investigate structure and properties of BN-SWCNTs. These results show that the band gaps of SWCNTs have been tuned greatly with B and N doping.     

Influence of magnetic field parallel to the arc on the formation of carbon nano-materials by arc discharge in water

A magnetic field parallel to the carbon arc in water was used to increase the directionality of moving carbon particles and generate carbon nano-materials. The products were analyzed and compared with those obtained without a magnetic field. The study shows that the purity and quality of multi-walled carbon nanotubes in the cathode deposition formed by an arc in water with a magnetic field were both improved, and cylinder-like carbon structures were also found in the deposition. The influence of the magnetic field on the formation of the products was analyzed.     

Production of carbon nanotubes from coal

We have demonstrated that the production of carbon nanotubes in large quantities is possible with inexpensive coal as the starting carbon source by the arc discharge technique. It has been found that a large amount of carbon nanotubes of good quality can be obtained in the cathode deposits in which carbon nanotubes are present in nest-like bundles. For the growth of carbon nanotubes, the buffer gas pressure in the reactor is one of the crucial factors. The mineral matter in raw coals may also play an important part in the formation process of carbon nanotubes.     

Synthesis and characterization of double-walled carbon nanotubes from multi-walled carbon nanotubes by hydrogen-arc discharge

Double-walled carbon nanotubes (DWNTs) were synthesized in a large scale by a hydrogen arc discharge method using graphite powders or multi-walled carbon nanotubes/carbon nanofibers (MWNTs/CNFs) as carbon feedstock. The yield of DWNTs reached about 4 g/h. We found that the DWNT product synthesized from MWNTs/CNFs has higher purity than that from graphite powders. The results from high-resolution transmission electron microscopy observations revealed that more than 80% of the carbon nanotubes were DWNTs and the rest were single-walled carbon nanotubes (SWNTs), and their outer and inner diameters ranged from 1.75 to 4.87 nm and 1.06 to 3.93 nm, respectively. It was observed that the ends of the isolated DWNTs were uncapped and it was also found that cobalt as the dominant composition of the catalyst played a vital role in the growth of DWNTs by this method. In addition, the pore structures of the DWNTs obtained were investigated by cryogenic nitrogen adsorption measurements.     

Co-synthesis, purification and characterization of single- and multi-walled carbon nanotubes using the electric arc method

We report the simultaneous synthesis of single-walled (SWNT) and multi-walled (MWNT) carbon nanotubes using the electric arc method. Two distinct deposits are formed when an electric arc is struck between a Co/Ni impregnated carbon electrode (anode) and a graphite electrode (cathode). The cobweb-like deposit harvested from the walls of the growth chamber (chamber deposit) contained SWNT bundles while the MWNTs were present in the deposit formed at the tip of the cathode (cathode deposit). Energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and Raman spectroscopy of each deposit confirmed the presence of respective carbon nanotubes along with carbonaceous impurities. A systematic purification procedure for separating SWNT bundles from the rest of the impurities is discussed. The optimal yield of purified SWNT bundles is ∼25 wt.% of the as-prepared chamber deposit.     

Two possible emission mechanisms involved in the arc discharge method of carbon nanotube preparation

By investigating the morphologies and microstructures of the cathode deposits prepared by self-sustained arc discharge between graphite rods, we consider that there are two electron emission mechanisms occurring on the cathode: field emission and thermionic emission. The former occurs mainly on the edge of the growing surface, by which we can explain the formation of the outer hard shell of the cathode deposit; while the latter occurs mainly on the growing surface except for the edge area and it is the main cause for the growth of carbon nanotubes.     

The effect of carbon microfiber substrate pretreatment on the growth of carbon nanomaterials

The floating catalyst method was used to obtain carbon nanotubes, carbon submicrotubes and other carbon nanomaterials by changing the pretreatment conditions of carbon microfiber substrate. The morphology and microstructure of the obtained products were characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The results showed that on untreated carbon microfiber surface, only some carbon particles and several carbon nanotubes were deposited. However, after carbon microfibers were boiled in the solution of H₂SO₄/HNO₃ and were immersed in the solutions of Fe(NO₃)₃/xylene, Fe(NO₃)₃/acetone, ferrocene/acetone and Fe(NO₃)₃/ferrocene/acetone, the obtained products were a high-density carbon nanotubes, carbon nanotubes with many carbon particles, carbon submicrotubes and a mixture of carbon nanomaterial, respectively. Thus the pretreatment of the carbon microfiber substrate greatly influenced the morphology and microstructure of the synthesized products.     

Influence of carbon structure of the anode on the production of graphite in single-walled carbon nanotube soot synthesized by arc discharge using a Fe–Ni–S catalyst

We investigated the production of the graphite contained in the soot of single-walled carbon nanotubes (SWCNTs) synthesized using the arc discharge method with a poorly graphitized carbon (PGC) rod in comparison to a graphite rod. A PGC rod was produced using a mixture of coal tar and carbon black and was heat treated to 1000 °C. The rod was packed with a mixture of iron (Fe), nickel (Ni), sulfur (S), and PGC and used for the production of SWCNT soot using arc discharge. From the results of X-ray diffraction and electron microscopy, the amount of graphite in the SWCNT soot synthesized by PGC rod was lower than that by graphite rod. The production of graphite in the soot was found to be dependent on the carbon structure of the anode and the current density of arc discharge.     

化学气相沉积法碳纳米管的制备及性能研究

开发以煤气为碳源采用化学气相沉积法制备单壁碳纳米管，并对其作为超级电容器电极的电化学性能进行研究。通过电子扫描电镜、电子透射电镜、拉曼光谱以及X射线衍射等手段对产品的形态、结构及性质进行表征分析，并对单壁碳纳米管的生长机理进行讨论；分别通过循环伏安、恒压充放电实验对单壁碳纳米管电极进行电化学性能分析。     

The synthesis of carbon nanoballs and its electrochemical performance

Carbon nanoballs were prepared in the presence of acetylene with coke powder as carbon source by arc discharge technique. The arc plasma was diagnosed in situ by optical emission spectroscopy (OES) in the formation process of carbon nanoballs. Field emission scanning electron microscopy (FE-SEM), field emission scanning and transmission electron microscope (STEM) equipped with energy dispersive X-ray (EDX), X-ray diffraction (XRD), and Raman spectroscopy are used to characterize carbon nanoballs. The electrochemical performance of carbon nanoballs as an electrode was measured on a multi-channel battery test system to analyze the electrochemical response. The FE-SEM and STEM results show that carbon nanoballs are the main products in acetylene medium. Many carbon nanoballs are sintered together with a few carbon nanotubes (CNTs) inserted into the sintered carbon nanoballs except for a few carbon nanoballs that exist as a single ball. The STEM results show that the diameter of carbon nanoball is mainly in the range of 50–100 nm. The XRD analysis shows that the graphitization of carbon nanoballs is relatively high. The charge–discharge curves of carbon nanoballs show that the cell electrode has a high reversible capacity and the capacity retention could reach 73.7%, which might contribute to the conductivity of CNTs inserted into the sintered carbon nanoballs.     

Controllable synthesis of single- and double-walled carbon nanotubes from petroleum coke and their application to solar cells

Single- and double-walled carbon nanotubes (SWCNTs and DWCNTs) have been controllably synthesized by an arc discharge in different atmosphere using petroleum coke as carbon source. The morphology and properties of two kinds of carbon nanotubes (CNTs) synthesized with Fe as catalyst were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, UV–visible spectroscopy, inductively coupled plasma optical emission spectrometer, thermogravimetric analysis and infrared spectroscopy. In the He gas atmosphere only SWCNTs were found to be synthesized by arc discharge in contrast to the case in Ar gas atmosphere in which only DWCNTs were formed, In addition, properties of solar cells based on both kinds of CNTs and n-type Si are examined under illumination of light emission diode (LED). It is found that the performance of solar cells depends significantly on the type of CNTs, i.e., SWCNTs-based solar cells show better performance under LED illumination with wavelengths in the range of 400–940 nm than the case of DWCNTs which exhibit high performance under illumination of the 1310 nm infrared light.     

Direct observation of the formation of linear C chain/carbon nanotube hybrid systems

Multi-wall carbon nanotubes (MWCNTs) containing linear C chains have been synthesised by arc discharge in liquid nitrogen. The experimental conditions used allow one to obtain nanotubes with a very thin innermost diameter, as evidenced by the radial breathing mode (RBM) features in the Raman spectra. A correlation between the RBM and the features of the C chains is reported, which gives a direct indication of how these linear carbon chain/carbon nanotube hybrid systems form. C chains are inserted only in CNTs having the innermost diameter equal to 0.7 nm, as expected.     

Catalyst-free synthesis of multi-walled carbon nanotubes from carbon spheres and its implications for the formation mechanism

A route to producing multi-walled carbon nanotubes (MWCNTs) was reported, in which polyacrylonitrile microspheres (PANMSs) were assembled into one-dimensional strings of carbon spheres and transformed to carbon nanotubes at 1000 °C. It was found that the diameters of the MWCNTs are uniform and correlated with the size of the PANMSs used. Structures of intermediate products obtained by stopping the reaction have been examined by high-resolution transmission electron microscopy. The observations indicate that the formation of MWCNTs follows a different mechanism from the well-studied vapor–liquid–solid mode. It involves a direct self-assembly and solid-state structural transformation of PANMSs under the promotion of nitrogen atoms as shown by the X-ray photoelectron spectra of the resulting samples. On the basis of these observations, a sphere-string-tube mechanism was proposed for the MWCNT formation.     

Highly compressible carbon nanowires synthesized by coating single-walled carbon nanotubes

Carbon nanowires with a diameter of 40–60 nm were synthesized by coating single-walled carbon nanotubes (SWCNTs) in an intermittent, one-stage DC arc discharge process. Transmission electron microscopy shows that these nanowires consist of a SWCNT with an amorphous carbon coating, whose thickness depends on the time of arc discharge. The mechanical properties of blocks of these nanowires were tested by load–unload cyclic compression and static force thermomechanical experiments. The results show that carbon nanowire blocks exhibit better compressive behavior than pure SWCNTs blocks, and carbon nanowires show a typical nonlinear strain–temperature response due to the amorphous carbon layer. A mechanism of adsorption-controlled growth of amorphous carbon on the SWCNTs in the vapor phase is proposed for the formation of the nanowires.     

Influence of the carbon surface on cathode deposits in non-aqueous Li–O2 batteries

Cup-stacked carbon nanotubes (CSCNT) with different surface properties were used for the non-aqueous Li–O₂ battery cathodes, and then examined at high magnification to understand how the discharge products were deposited on the cathode. As-prepared CSCNT based cathode had many reactive edges consisting of truncated conical graphene layers. After discharge, discharge products with average particle size 50 nm covered a nanotube, resulting in a layer-like texture. On the other hand, a heat-treated CSCNT based cathode was composed of edges terminated by graphitization of several graphene layers. After discharge, the size of the products was almost the same but the products were agglomerated, forming a bulky morphology. It was, thus, found that the carbon surface structure was closely related with the morphology of the cathode deposits after discharge. First principles calculations also indicated that no terminated edges acted as preferential active sites in adsorbing and storing the reaction species. It was, therefore, concluded that the active edges of the carbon surface were indispensable for controlling the morphology of cathode deposits and improving the battery performance.     

High-efficient synthesis of double-walled carbon nanotubes by arc discharge method using chloride as a promoter

With trace halide as a promoter in an iron sulfide catalyst, relatively perfect structural integrity double-walled carbon nanotubes (DWNTs) have been synthesized in large quantity and high yield by arc discharge method. Scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Raman spectroscopy were used for structural characterization and yield determination. The results revealed that halide was a crucial factor for selectively synthesizing high yield DWNTs. The detailed experimental parameters were systematically investigated. The possible role of halide in growth of DWNTs was discussed as well.