CF<Subscript>4</Subscript> plasma-induced grafting of fluoropolymer onto multi-walled carbon nanotube powder

Grafting of fluoropolymer onto multi-walled carbon nanotube (MWCNTs) powder by CF₄ plasma treatment was investigated in this study. In order to achieve a uniform treatment of powder, a rotating barrel was designed and fixed between the two discharge electrodes. The influence of various plasma parameters, such as power and treatment time, on the fluorination of MWCNT surface was systematically analyzed by X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results revealed that a successful fluorination of MWCNT powder with a maximum of fluorine content of 12% could be achieved by using our plasma equipment. Our work provides a new way for the homogeneous fluorination of MWCNT powder and is valuable for industrial production. PACS 52.50.Dg; 52.40.Mj; 52.59. Ye; 52.77.-j     

Enhancement of the electrical properties of carbon nanotubes with Ar–N2 plasma treatment

Optical emission spectroscopy and Langmuir probe are used to investigate the low pressure inductively coupled Ar–N₂ plasmas as function of rf power, filling pressure and Ar content in N₂ discharge. It is observed that the active species generation, dissociation fraction and electron temperature significantly depends on discharge parameters and may be used to optimize the plasma reactor. Mixture of SWCNTs and MWCNTs are treated for different treatment time (0–120 min) at optimum discharge conditions. Changes induced in the elemental composition, surface morphology, crystallographic structure, and structural disorder in the plasma irradiated CNTs are analyzed by EDX, FTIR, SEM, XRD and Raman spectroscopy, respectively. Ar–N₂ mixture plasma treatment of CNTs lead to a significant increase in the electrical conductivity, modify the microstructure and induce structural disorder and cause a transition of crystalline phase from well crystalline to an amorphous structure.     

Effects of different carbon precursors on synthesis of multiwall carbon nanotubes: Purification and Functionalization

Cyclohexanol and xylene were used as carbon precursors, for synthesis of multiwall carbon nanotubes (MWCNTs) arrays in a CVD system at temperature of 750 °C, using nitrogen as carrier gas and ferrocene as catalyst. Different characterization methods were employed to compare the MWCNTs structure synthesized by these two precursors. All scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA) and Raman spectroscopy results illustrated that using cyclohexanol could significantly reduce formation of amorphous carbon and catalyst particles in the as-grown CNTs. The less amorphous carbon can be attributed to in situ oxidation in presence of oxygen atom of cyclohexanol. Characterizations showed that MWCNTs with high purity could be obtained using cyclohexanol as carbon precursor. The as-grown MWCNTs were purified by oxidation and acid treatment. Characterization of the purified MWCNTs using HNO₃/H₂SO₄ (1/3 or 1/1), 8 M HCl or 8 M HNO₃ was carried out. The results showed that 8 M HNO₃ could be considered as the best chemical to obtain more pure MWCNTs, less amorphous and metal particles and less damaged MWCNTs. The Raman spectroscopy results demonstrated that HNO₃/H₂SO₄ (1/3) treatment could more disorder the MWCNTs structure and this was attributed to the bigger destroying effect of this acid treatment. Furthermore, the TEM analysis of MWCNTs before and after acid treatment revealed that acid treatment could remove encapsulated catalyst particles. The FTIR analysis illustrated that purification of the MWCNTs with nitric acid could connect the functional groups onto the outer surface of MWCNTs and this resulted in more dispersion of the MWCNTs in water.     

Effects of corona discharge on the surface structure, morphology and properties of multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were modified by corona discharge and then heat treated in the air. The influences of corona discharge parameters such as treatment time and processing power were investigated. The results of energy dispersive X-ray analysis (EDX) and thermogravimetric analysis (TGA) indicated the introduction of oxygen-containing functional groups onto the surface of the MWCNTs after heat treatment. The water contact angle tests showed that the hydrophobicity of the MWCNTs was decreased to some extent. The static water contact angle was reduced from 146° to 122° when the time of the corona discharge treatment reached 3 min at the processing power of 200 W. The enhanced thermomechanical and mechanical properties of epoxy nanocomposites filled with the corona discharge treated MWCNTs were observed. The modified MWCNTs conferred better properties on the composites than the pristine MWCNTs because of the improved dispersion of MWCNTs in matrix and the enhanced interfacial interaction between the treated MWCNTs and epoxy.     

Rapid and enhanced functionalization of MWCNTs in a dielectric barrier discharge plasma in presence of diluted CO<Subscript>2</Subscript>

Multiwalled carbon nanotubes (MWCNTs) have been functionalized in dielectric barrier discharge (DBD) plasma in presence of a mixture of carbon dioxide and a diluent gas at room temperature and atmospheric pressure. He, Ar, and N₂ were examined as the diluent gases. Temperature-programmed desorption was used to investigate the influence of various plasma parameters and type of diluent gas as well as the amount of diluent in the plasma gas. It is found that the quantity of functional groups on the surface of MWCNTs is a maximum when He is used as diluent gas. It also passes through a maximum when He content is 60%. The presence of He improves the reactivity of the plasma, which leads to an increase in the quantity of functional groups. However, high percentages of He decrease the CO₂ content, which in turn decreases the number of functional groups. FTIR and Raman spectroscopy showed the presence of Oxygen-containing functional groups on MWCNTs surfaces.     

Functionalization of multi-walled carbon nanotubes (MWCNTs) with nitrogen plasma for photovoltaic device application

Multi-walled carbon nanotubes (MWCNTs) placed under nitrogen (N₂) and argon (Ar) microwave plasma in order to functionalize covalently their side walls with nitrogen containing groups. X-ray photoelectron spectroscopy (XPS) study shows surface modification of the MWCNTs with imine, amine, nitride and amide groups grafted on the side walls. Due to the functional groups, homogenous distribution of MWCNTs in solvent could be obtained. For photovoltaic device fabrication MWCNTs film was casted over n-Si wafer and poly(3-octylthiophene) solution was infiltered. Devices with functionalized MWCNTs show short circuit current density (J_sc), open circuit voltage (V_oc), fill factor (FF) and power conversion efficiency (η) as 1.8 mA/cm², 0.20 V, 24% and 0.086%, respectively. In the composite film functionalized MWCNTs facilitate photo induced charge separation and efficient holes transportation, suppressing recombination of photo generated charges.     

Amino functionalization of carbon nanotube surfaces with NH3 plasma treatment

NH₃ plasma treatment of carbon nanotube (CNT) surfaces was performed with the purpose of incorporating amino groups onto the surface. Amino groups incorporated onto the CNT surface were indentified and quantified using chemical derivatization with pentafluorobenzaldehyde and subsequent characterization with X-ray photoelectron spectroscopy (XPS). The amount of incorporated amino groups reached a maximum value with increasing plasma power. The incorporation of amino groups was seriously affected by the degradation of the CNT surface during the plasma treatment, which became very serious at high plasma power, as verified with optical emission spectroscopy (OES) and FT-IR analyses. The type of species present in the plasma discharge also seems to be important for amino group functionalization; partially decomposed ammonia species are considered to be more favorable than fully decomposed atomic species.     

Controllable purification, cutting and unzipping of multi-walled carbon nanotubes with a microwave method

A rapid microwave-assisted method was developed for the purification, cutting and unzipping of arrays of multi-walled carbon nanotubes (MWCNTs) using a mixture of KMnO₄ and H₂SO₄. To harness the extent of treatment, MWCNT products were fully characterized at different reaction times by UV-visible and Raman spectroscopies as well as scanning and transmission electron microscopies. The results show that the carbon nanoparticles and the amorphous carbon which coated the MWCNTs were removed after about 10 minutes. The excessive oxidation of MWCNTs then leads to cutting and unzipping of graphitic walls. Moreover, while the catalyst residues outside the MWCNTs were rapidly extracted up to 10 minutes, the removal of catalyst residues inside the MWCNTs did not begin before 20 minutes. This method can be considered as an efficient route for the purification, cutting and unzipping of MWCNTs due to its fast and controllable procedure.     

Optimization of cuprous oxide nanocrystals deposition on multiwalled carbon nanotubes

Cu (I) phenyl acetylide was used as a source of copper to achieve a homogeneous distribution of Cu₂O nanocrystals (10–80 nm) decorated on multiwalled carbon nanotubes (MWCNTs) having an average diameter of 10 nm. Pristine MWCNTs were first oxygen-functionalized by treating them with a mixture of concentrated (H₂SO₄/HNO₃ : 3/1) acids and the products were characterized by X-ray powder diffraction, transmission and scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and thermogravimetric analysis. An easy, efficient and one-step impregnation method was followed to produce copper-containing nanoparticles on the MWCNTs. The copper-treated MWCNTs dried at room temperature were seen to be well decorated by copper-containing nanoparticles on their outer surface. The MWCNTs were then heat-treated at 400 °C in a nitrogen atmosphere to produce a homogeneous distribution of cuprous oxide nanocrystals on their surface. By varying the ratio of copper to oxygen-functionalized MWCNTs, Cu₂O nanocrystals decorated on MWCNTs with different copper content can be obtained.     

Photocatalytic inactivation of Gram-positive and Gram-negative bacteria by reactive plasma processed nanocrystalline TiO2 powder

Nanocrystalline TiO₂ powder is synthesized using a novel reactive plasma process in which the precursor TiH₂ powder is oxidized through thermal plasma in-flight route to generate nanocrystalline TiO₂ powder. The synthesized powder consists of nano-sized TiO₂ particles, both anatase and rutile phases, in which anatase is the predominant phase. An additional feature of the plasma synthesized TiO₂ powder is the higher surface concentration of Ti³⁺ state and hydroxyl group that enhance its photocatalytic activity. The photocatalytic inactivation of Gram-positive Enterococcus and Gram-negative Klebsiella bacteria is studied using the plasma synthesized TiO₂ nanopowder with 365 nm ultraviolet (UV) light. The mechanism behind the photocatalytic disinfection of bacteria is discussed. The plasma synthesized TiO₂ nanopowder catalyst is found effective in killing Enterococcus and Klebsiella. The results corroborated that the plasma synthesized TiO₂ powder can be used for waste water treatment and water purification.     

Surface cleaning effect of NCM powder and improvement of lithium ion battery on the thermal stability and life cycle employing dielectric barrier discharge technique

Mixed NCM (nikel, cobalt, and manganese) powder was treated by a reactive gas from dielectric barrier discharge (DBD) to prepare a cathode material in Lithium ion (Li-ion) battery. The DBD was mainly sustained using N₂ gas at atmospheric pressure, and NF₃, SF₆, and H₂ was fed into the discharge to create the reactive gas. Compare to the non-treated sample, impurities on the surface of the NCM powder were significantly removed in a 5 min when the reactive gas was blow into the powder and mixed properly. F atom content on the surface was increased depending on the time duration of mixing, which form a LiF layer on the surface. Desirable LiF layer suppress a heat flow effectively , resulting a decreasing of exothermic temperature inside the Li-ion battery. Additionally, the treatment of NCM powder employing DBD technique was also contributed to electrochemical performance, which was confirmed by c-rate testing.     

Effect of plasma treatment on electrical conductivity and Raman spectra of carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were treated with a radio-frequency discharge. We found that MWCNTs showed opposite trends in electrical conductivity when treated with oxygen and hydrogen plasmas. MWCNTs showed enhanced electrical conductivity when placed at cathode with oxygen plasma treatment, whereas MWCNTs treated at positive column did not show such a trend. In contrast, the conductivity of MWCNTs dropped sharply with hydrogen plasma treatment. The measured conductivity trends of MWCNTs are correlated with observed Raman spectral shift. The possible mechanisms of the change in electrical conductivity in plasma-treated MWCNTs are discussed.     

Effects of dielectric barrier discharge in air on morphological and electrical properties of graphene nanoplatelets and multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) have been functionalized by dielectric barrier discharge (DBD) in air. The extent of functionalization of MWCNTs and GNPs reaches a maximum at the delivered discharge energy of 720 and 240 J mg⁻¹, respectively. Further exposure to plasma leads to reduction of functional groups from the surface of the treated nanomaterials. It has also been demonstrated that DBD plasma does not produce dramatic structural changes in MWCNTs, while flakes of the treated GNPs become thinner and smaller in the lateral size. Conductive thin films, obtained by drop casting a solution of the treated nanomaterials in N-methyl-1-pyrrolidone on poly(methyl methacrylate) substrate, show significantly lower sheet resistance.     

The effect of discharge power density on polyethylene terephthalate film surface modification by dielectric barrier discharge in atmospheric air

Polyethylene terephthalate (PET) films are modified using non-equilibrium plasma generated by DBD in atmospheric air, and the effects of discharge power density on the surface modification are studied. It is found that increasing discharge power density can induce more effective treatment of PET films, because this leads to a faster decrease in water contact angle and a faster increase in surface energy due to more creation of polar groups and more obvious etching occurring on the PET surface. So the treatment time needed to achieve the same level of surface treatment can be reduced by increasing the discharge power density.     

Modification of polyethylene powder with an organic precursor in a spiral conveyor by hollow cathode glow discharge

Hexamethyldisiloxane (HMDSO) films were deposited on polyethylene (PE, (C₂H₄)_n) powder by hollow cathode glow discharge. The reactive species in different HMDSO/Ar plasmas were studied by optical emission spectroscopy (OES). Increasing the HMDSO fraction in the gas mixture additional compounds like CH_x, OH, SiC and SiO can be identified. After deposition the formed silicon and carbon containing groups (C–O, C=O, SiC and SiO) on the PE powder surface have been analyzed by X-ray photoemission spectroscopy (XPS). Changes in wettability depending on the HMDSO fraction were investigated by contact angle measurements (CAM). The free surface energy of the PE powder decreases with increasing HMDSO fraction in the process gas and encapsulation of the powder particles occurs. An aging effect of the plasma treated PE surface was observed depending on the process gas composition. The higher the HMDSO fraction the less is the aging effect of the plasma treated PE surface.     

Influence of air-oxidation on electric double layer capacitances of multi-walled carbon nanotube electrodes

In this work, air-oxidized multi-walled carbon nanotube (MWCNT) electrodes have been prepared from catalytically grown MWCNTs of high purity and narrow diameter distribution. The experimental results show that air-oxidation modifies the intrinsic structure of individual MWCNTs and a little improves the dispersity of the MWCNTs. The specific capacitances of the electrodes in electric double layer capacitors (EDLCs) based on oxidized MWCNTs are obviously improved through air-oxidation. The specific capacitance of 50 F/g is obtained in the air-oxidized MWCNTs at 600 °C on a single cell device with 35 wt% H₂SO₄ as an electrolyte. This is probably increased BET specific surface area and mesopore volume of the oxidized MWCNT electrode materials of EDLCs. These properties are, therefore highly desirable for the development of electrochemical capacitors with high power and long cycle life.     

Nanotitania-coated multi-walled carbon nanotube composite by facile colloidal processing route for photocatalytic applications

Multi-walled carbon nanotubes (MWCNTs) and titanium dioxide nanocomposites (MWCNTs/TiO₂) were fabricated by a simple novel colloidal processing route and tested as a photocatalyst for degradation of methylene blue under UV irradiation. The novel idea behind this work is to make MWCNTs and TiO₂ nanoparticle suspensions separately highly oppositely charged and utilize the electrostatic force of attraction between two entities to deposit nanotitania onto MWCNTs surface. Particle charge detector, scanning electron microscopy, transmission electron microscope, energy dispersive X-rays, X-rays diffraction (XRD), and Raman spectroscopy were used to characterize the composite. XRD and Raman spectroscopic analysis showed the crystalline structure of deposited TiO₂ over MWCNTs surface structure as anatase phase. It was found that MWCNTs/TiO₂ composite structure have much higher photocatalytic activity compared to TiO₂ nanoparticles. The composite material developed may find potential applications in the degradation of organic pollutants in aqueous medium under UV irradiation.     

Metallic nanoparticles on plasma treated carbon nanotubes: Nanohybrids

Multi-wall carbon nanotubes (MWCNTs) were decorated with metal clusters by thermal evaporation. Transmission electron microscopy (TEM) shows that the nature and extent of metal coverage can be varied by plasma treating the MWCNT surface. The metal clusters on oxygen plasma treated arc-discharge MWCNTs have a more dense distribution than the clusters evaporated on as-synthesized arc-discharge MWCNTs. In contrast, the plasma treatment did not affect the cluster distribution on CVD MWCNTs. Analyses of the valence band and the core levels by X-ray photoelectron spectroscopy suggest poor charge transfer between gold clusters and MWCNTs; on the contrary suggest good charge transfer between Ni clusters and MWCNTs.     

Investigations on Plasma Assisted Surface Cleaning of Aluminum in an Oxygen Glow Discharge

Aluminum plates which are contaminated with hydrocarbon containing compounds (oil, grease) were treated in an Ar-, O₂- and Ar/O₂-plasma generated by a dc-discharge. Besides a variation of the macroscopic discharge parameters (power, pressure) also different modes (anomalous glow regime, hollow cathode regime) have been studied. In most cases the contaminated substrates served as cathodes of an anomalous glow discharge where the combined influence of chemically reactive radicals and energetic ions was very effective. During the plasma process the discharge was monitored by means of power variation and optical emission spectroscopy. The spectral line intensities of several species (etchants and products) have been measured as functions of the process duration. Simultaneously the removal of the contamination layers was observed by means of in-situ ellipsometry. The temporal change of the ellipsometric parameters ψ and δ during the cleaning process was taken as a measure for the state of the surface. The combination of spectroscopic and ellipsometric measurements which indicate the progress of the plasma treatment gave interesting results on the mechanisms at the surface. In addition, surface analysis was used to assess the quality of the substrates treated.