Effect of purification of carbon nanotubes on their electrocatalytic properties for oxygen reduction in acid solution

The oxygen reduction reaction has been investigated on acid-treated single-walled (SWCNT) and multi-walled carbon nanotubes (MWCNT) modified glassy carbon (GC) electrodes in acid media using the rotating disk electrode (RDE) method. Different acids were used for the carbon nanotube (CNT) purification. A systematic study was carried out to elucidate whether the metal catalyst impurities of CNTs play a role in the electroreduction of oxygen on the CNT modified GC electrodes. The surface morphology of the carbon nanotube samples was examined by transmission electron microscopy and the concentration of metal catalysts in the CNT materials was determined by energy dispersive X-ray spectroscopy. The acid-treated MWCNTs were also characterised by Raman and X-ray photoelectron spectroscopies. Aqueous suspensions of SWCNTs and MWCNTs used for GC surface modification were prepared in the presence of Nafion. The RDE results indicated that the acid-treated CNT modified GC electrodes are less active catalysts for oxygen reduction than as-received CNTs which could be explained by the absence of metal catalysts on the surface of purified CNTs.     

Design of carbon nanotube-polymer frameworks by electropolymerization of SWCNT-pyrrole derivatives

Single-walled carbon nanotubes (SWCNTs) have been functionalized by electropolymerizable pyrrole groups following covalent and non-covalent strategies. The covalent pyrrole grafting was carried out by ester formation between pyrrole alcohol and chemically oxidized SWCNT. The strong π-interactions between pyrene and SWCNT were exploited for the non-covalent adsorption of a new pyrene-pyrrole derivative on the pristine nanotube surface. The pyrrole-ester-SWCNT was solubilized in THF and electropolymerized by controlled potential electrolysis at 0.95 V. The pyrene-pyrrole SWCNT was spread on an electrode surface and electropolymerized in its adsorbed state at 0.95 V in CH₃CN. The reinforced nanostructured polypyrrole SWCNT-framework was investigated with scanning electron microscopy.     

Electrochemical characterization of single-walled carbon nanotubes for electrochemical double layer capacitors using non-aqueous electrolyte

Single-walled carbon nanotubes (SWCNTs) were investigated by cyclic voltammetry and electrochemical impedance spectroscopy in a non-aqueous electrolyte, 1 M Et₄NBF₄ in acetonitrile, suitable for supercapacitors. Further, in situ dilatometry and in situ conductance measurements were performed on single electrodes and the results compared to an activated carbon, YP17. Both materials show capacitive behavior characteristic of high surface area electrodes for supercapacitors, with the maximum full cell gravimetric capacitance being 34 F/g for YP17 and 20 F/g for SWCNTs at 2.5 V with respect to the total active electrode mass. The electronic resistance of SWCNTs and activated carbon decreases significantly during charging, showing similarities of the two materials during electrochemical doping. The SWCNT electrode expands irreversibly during the first electrochemical potential sweep as verified by in situ dilatometry, indicative of at least partial debundling of the SWCNTs. A reversible periodic swelling and shrinking during cycling is observed for both materials, with the magnitude of expansion depending on the type of ions forming the double layer.     

Different behaviors of single and multi wall carbon nanotubes for studying electrochemistry and electrocatalysis of choline oxidase

This work presents a detailed comparison between single and multi wall carbon nanotubes (SWCNTs & MWCNTs) in an effort to understand which could be a better supporting material for studying the electrochemistry and electrocatalysis of enzymes. Choline oxidase (ChOx) was chosen as a model enzyme for evaluation of the electrodes’ performance. The enzyme was adsorbed on either SWCNT or MWCNT modified electrode, in the presence of a typical room temperature ionic liquid (RTIL), and its electron transfer and electroanalytical response toward choline was investigated. RTIL/MWCNTs/GC electrode was uniformly covered by ChOx. Besides, higher electrical conductivity, better reversibility of the ChOx redox reaction and higher electron transfer rate of the enzyme indicated more facile and rapid rate of electron transfer. On the other hand, RTIL/SWCNTs/GC electrodes showed higher amount of enzyme loading, higher enzyme–substrate affinity, lower detection limit, better sensitivity and wider linear range. Consequently, MWCNTs are preferable for kinetic study of ChOx, while SWCNTs are more convenient for biosensing applications.     

Self-assembled nano-arrays of single-walled carbon nanotube-octa(hydroxyethylthio)phthalocyaninatoiron(II) on gold surfaces: Impacts of SWCNT and solution pH on electron transfer kinetics

The construction by sequential self-assembly process of reproducible, highly stable and pH-responsive redox-active nanostructured arrays of single-walled carbon nanotubes (SWCNTs) integrated with octa(hydroxyethylthio)phthalocyaninatoiron(II) (FeOHETPc) via ester bonds on a gold surface (Au-Cys-SWCNT-FeOHETPc) is investigated and discussed. The successful construction of this electrode is confirmed using atomic force microscopy and X-ray photoelectron spectroscopy as well as from the distinct cyclic voltammetric and electrochemical impedance spectroscopic profiles. The Au-Cys-SWCNT-FeOHETPc electrode exhibited strong dependence on the reaction of the head groups and the pH of the working electrolytes, the surface pK_a is estimated as 7.3. The high electron transfer capability of the Au-Cys-SWCNT-FeOHETPc electrode over other electrodes (Au-Cys-SWCNT or the Au-Cys-FeOHETPc or the Au-FeOHETPc) suggests that SWCNT greatly improves the electronic communication between FeOHETPc and the bare gold electrode. The electron transfer rate constant (k_app) of Au-Cys-SWCNT-FeOHETPc in pH 4.8 conditions (∼1.7 × 10⁻² cm⁻² s⁻¹) over that of the electrode obtained from SWCNT integrated with tetraaminophthalocyninatocobalt(II) (Au-Cys-SWCNT-CoTAPc) (5.1 × 10⁻³ cm⁻² s⁻¹) is attributed to the possible effect of the central metal on the phthalocyanine core and substituents on the peripheral positions of the phthalocyanine rings. We also prove that aligned SWCNT arrays exhibit much faster electron transfer kinetics to redox-active species in solutions compared to the randomly dispersed (drop-dried) SWCNTs.     

High-performance biosensors based on enzyme precipitate coating in gold nanoparticle-conjugated single-walled carbon nanotube network films

Single-walled carbon nanotube (SWCNT) network films with high network density were prepared by vacuum-filtering a suspension of SWCNTs, and used as a host of enzyme precipitate coating of glucose oxidase (EPC-GOx). EPC-GOx was fabricated into the SWCNT network films in a two-step process of enzyme precipitation and crosslinking. High GOx loading in a form of EPC expedited the generation of electrons while the good connectivity of conductive SWCNTs in the network structure increased the electron transfer rate. According to amperometric measurements, the sensitivities of GOx/SWCNT electrodes, governed by both generation and transfer of electrons, were significantly enhanced by filling up the open pores of SWCNT films with the EPC-GOx when compared to the approaches of covalent-attachment (CA) and enzyme coating (EC) with no step of enzyme precipitation. For example, the sensitivities of CA, EC and EPC-GOx were 0.039, 0.140, and 5.72 μA mM⁻¹, respectively. High sensitivity of EPC-GOx was maintained under iterative uses for 10 days. The deposition of gold nanoparticles into SWCNT films has resulted in high-performance glucose sensors with a remarkable sensitivity of 24.5 μA mM⁻¹, which can be explained by further expedited electron transfer due to deposited gold nanoparticles.     

Polyaniline/single-wall carbon nanotube (PANI/SWCNT) composites for high performance supercapacitors

PANI/SWCNT composites were prepared by electrochemical polymerisation of polyaniline onto SWCNTs and their capacitive performance was evaluated by means of cyclic voltammetry and charge-discharge cycling in 1 M H₂SO₄ electrolyte. The PANI/SWCNT composites single electrode showed much higher specific capacitance, specific energy and specific power than pure PANI and SWCNTs. The highest specific capacitance, specific power and specific energy values of 485 F/g, 228 W h/kg and 2250 W/kg were observed for 73 wt.% PANI deposited onto SWCNTs. PANI/SWCNT composites also showed long cyclic stability. Based upon the variations in the surface morphologies and specific capacitance of the composite, a mechanism is proposed to explain enhancement in the capacitive characteristics. The PANI/SWCNT composites have demonstrated the potential as excellent electrode materials for application in high performance supercapacitors.     

Fabrication of Carbon Nanotube/SiO2 and Carbon Nanotube/SiO2/Ag Nanoparticles Hybrids by Using Plasma Treatment

Based on plasma-treated single wall carbon nanotubes (SWCNTs), SWCNT/SiO₂ and thiol groups-functionalized SWCNT/SiO₂ hybrids have been fabricated through a sol–gel process. By means of thiol groups, Ag nanoparticles have been in situ synthesized and bonded onto the SiO₂ shell of SWCNT/SiO₂ in the absence of external reducing agent, resulting in the stable carbon nanotube/SiO₂/Ag nanoparticles hybrids. This strategy provides a facile, low–cost, and green methodology for the creation of carbon nanotube/inorganic oxides-metal nanoparticles hybrids.     

Spectroscopic characterization of the electrochemical functionalization of single-walled carbon nanotubes in aqueous and organic media

An electrochemical method of functionalizing of single-walled carbon nanotubes (SWCNTs) in aqueous and organic electrolytes by the electrolysis of bromide solutions at anodic potentials is proposed. Cyclic voltammeter studies point to a substantial increase in the SWCNTs electrode capacitance after its electrochemical treatment, which is associated with an increase in the electrode working surface accessible to solution. This effect is attributed to the formation of functional groups on the SWCNTs surface. This conclusion is confirmed by analyzing the electrochemically processed SWCNTs by the methods Raman, UV–Vis–Nir and X-ray photoelectron spectroscopies. The electrolysis of potassium bromide solutions in both aqueous and nonaqueous media is shown to produce functional groups containing oxygen and, to a smaller extent, bromine. The dependence of the degree of SWCNT functionalization (the number of chemically bound foreign atoms per the number of carbon atoms) on the electrolyte nature is observed experimentally. The degree of functionalization reached in dimethyl sulfoxide is comparable with that of the original SWCNTs, whereas in aqueous solutions, the high density of functional groups (one O atom per 3–4 C atoms) is observed.     

An easy method for direct metal coordination reaction on unoxidized single-walled carbon nanotubes

The transition metal copper (II) ion (Cu²⁺) was effectively coordinated with a single-walled carbon nanotube (SWCNT) to produce a SWCNT–Cu²⁺ complex by a metal coordination reaction. Since the complex was very reactive towards the carboxylic acid group, the chemical functionalization of SWCNTs was easy to accomplish. This approach was used to functionalize the surface of the SWCNTs with stearic acid or ethylenediaminetetraacetic acid for tuning of the relative hydrophobicity and hydrophilicity of the surface, respectively. The mild reaction conditions used for metal coordination of the SWCNTs minimized the defects that result from chemical modification of SWCNT. Thus, the electrical properties of unmodified SWCNTs were preserved. Various analytical techniques, including Fourier transform infrared spectroscopy, thermal gravimetric analysis, ultraviolet–visible spectroscopy, and water sorption isotherm measurements, were used to characterize the surface properties of the functionalized SWCNTs. Functionalization of SWCNTs by metal coordination reaction effectively modified the SWCNT surface, while conserving the excellent physical properties of the SWCNTs. The surface properties of the SWCNTs were easily tuned by introduction of the functional groups required for specific applications.     

Differential scanning calorimetry analysis of an enhanced LiNi0.8Co0.2O2 cathode with single wall carbon nanotube conductive additives

The replacement of traditional conductive carbon additives with single wall carbon nanotubes (SWCNTs) in lithium metal oxide cathode composites has been shown to enhance thermal stability as well as power capability and electrode energy density. The dispersion of 1 wt% high purity laser-produced SWCNTs in a LiNi_0.8Co_0.2O₂ electrode created an improved percolation network over an equivalent composite electrode using 4 wt% Super C65 carbon black; evidenced by additive connectivity in SEM images and an order of magnitude increase in electrode electrical conductivity. The cathode with 1 wt% SWCNT additives showed comparable active material capacity (185–188 mAh g⁻¹), at a low rate, and Coulombic efficiency to the cathode composite with 4 wt% Super C65. At increased cycling rates, the cathode with SWCNT additives had higher capacity retention with more than three times the capacity at 10C (16.4 mA cm⁻²). The thermal stability of the electrodes was evaluated by differential scanning calorimetry after charging to 4.3 V and float charging for 12 h. A 40% reduction of the cathode exothermic energy released was measured when using 1 wt% SWCNTs as the additive. Thus, the results demonstrate that replacing traditional conductive carbon additives with a lower weight loading of SWCNTs is a simple way to improve the thermal transport, safety, power, and energy characteristics of cathode composites for lithium ion batteries.     

Immobilization of the Laccases from Trametes versicolor and Streptomyces coelicolor on Single‐wall Carbon Nanotube Electrodes: A Molecular Dynamics Study

In this work, we investigate the immobilization of laccases from Trametes versicolor (TvL) and the small laccase (SLAC) from Streptomyces coelicolor on single‐wall carbon nanotube (SWCNT) surfaces. SLAC may potentially offer improved adsorption on the electrode, thus improving bioelectrocatalytic activity via direct electron transfer (DET). Laccase immobilization on SWCNTs is achieved non‐covalently with a molecular tether (1‐pyrene butanoic acid, succinimidyl ester) that forms an amide bond with an amine group on the laccase surface while the pyrene coordinates to the SWCNT by π–π stacking. In our approach, density functional theory calculations were first used to model the interaction energies between SWCNTs and pyrene to validate an empirical force field, thereafter applied in molecular dynamics (MD) simulations. In the simulated models, the SWCNT was placed near the region of the (type 1) Cu(T1) atom in the laccases, and in proximity to other regions where adsorption seems likely. Calculated interaction energies between the SWCNTs and laccases and distances between the SWCNT surface and the Cu(T1) atom have shown that SWCNTs adsorb more strongly to SLAC than to TvL, and that the separation between the SWCNTs and Cu(T1) atoms is smaller for SLAC than for TvL, having implications for improved DET.     

Improved field emission stability from single-walled carbon nanotubes chemically attached to silicon

ABSTRACT: Here we demonstrate the simple fabrication of a single walled carbon nanotube (SWCNT) field emission electrode which shows excellent field emission characteristics and remarkable field emission stability without requiring post treatment. Chemically functionalized SWCNTs were chemically attached to a silicon substrate. The chemical attachment led to vertical-alignment of SWCNTs on the surface. Field emission sweeps and Fowler-Nordheim plots showed that the Si-SWCNT electrodes field emit with a low turn-on electric field of 1.5 V mu m-1 and high electric field enhancement factor of 3965. The Si-SWCNT electrodes were shown to maintain a current density of > 740 mu A cm -1 for 15 hr with negligible change in applied voltage. The results indicate that adhesion strength between SWCNTs and substrate is a much greater factor in field emission stability than previously reported.     

A comparison of three carbon nanoforms as catalyst supports for the oxygen reduction reaction

Defective graphene nanosheets (GNSs), single-walled carbon nanotubes (SWCNTs), and herringbone graphite nanofibres (GNFs) were used as Pd₃Pt₁ catalyst supports for an oxygen reduction reaction (ORR). Raman spectroscopy and cyclic voltammetry analyses revealed oxygen-containing functional groups and physical defects on the surfaces of the SWCNTs, GNFs, and synthesised GNSs. Mass-transfer-corrected Tafel diagrams obtained in an O₂-saturated electrolyte showed that the SWCNTs with a high curvature allowed for more surface Pt atoms; thus, these Pd₃Pt₁ catalysts are the first SWCNT system to promote the ORR. These catalysts, however, were slower than the GNS-supported catalysts after 0.875 V (vs. SCE; saturated calomel electrode). In terms of the kinetic current density, the highest mass activity was found for the Pd₃Pt₁/GNS composites. Additionally, according to rotating-ring disk electrode (RRDE) measurements, the H₂O production efficiencies for the Pd₃Pt₁/GNS, Pd₃Pt₁/SWCNT, and Pd₃Pt₁/GNF systems were 70.35%, 66.7%, and 9.58%, respectively. Among these carbon supports, Pd₃Pt₁ on GNS showed the greatest efficiency and durability for producing H₂O via an approximate four-electron pathway; this efficiency was ascribed to metal-support interaction.     

Adsorption of single-ringed N- and S-heterocyclic aromatics on carbon nanotubes

Adsorption of single-ringed N- and S-heterocyclic aromatics on single-walled carbon nanotubes (SWCNTs) was examined to explore the potential of using carbon nanotubes (CNTs) as drug carriers and environmental adsorbents. Adsorbates included pyrimidine, 2-aminopyrimidine, 4,6-diaminopyrimidine, thiophene, benzene and aniline. Adsorbents included pristine SWCNTs, oxidized SWCNTs, and nonporous graphite. Adsorption of N- and S-heterocyclic aromatics was significantly enhanced by non-hydrophobic interactions. Particularly, the -NH₂-substituted compounds exhibited much stronger (up to 2 orders of magnitude) adsorption affinities to oxidized SWCNTs than benzene, even though they are much less hydrophobic. The π-π coupling or electron donor-acceptor (EDA) interactions are likely adsorption-enhancement mechanisms for all six compounds. The lone-pair electrons of the N heteroatoms and the -NH₂ group can enable n-π EDA interactions with SWCNT surfaces. Lewis acid-base interactions are another significant adsorption-enhancement mechanism for the -NH₂-substituted compounds (and possibly for pyrimidine) on SWCNTs. For the N-heterocyclic aromatics, adsorption affinity is highly dependent on the O-functionality of the SWCNT surface and on solution pH, due to the speciation reactions of both adsorbates and SWCNT surface O-functional groups, indicating that selective adsorption of N-heterocyclic aromatics is possible by combining the surface functionality of CNTs and solution chemistry.     

Modifying the electronic properties of single-walled carbon nanotubes using designed surfactant peptides

The electronic properties of carbon nanotubes can be altered significantly by modifying the nanotube surface. In this study, single-walled carbon nanotubes (SWCNTs) were functionalized noncovalently using designed surfactant peptides, and the resultant SWCNT electronic properties were investigated. These peptides have a common amino acid sequence of X(Valine)(5)(Lysine)(2), where X indicates an aromatic amino acid containing either an electron-donating or electron-withdrawing functional group (i.e. p-amino-phenylalanine or p-cyano-phenylalanine). Circular dichroism spectra showed that the surfactant peptides primarily have random coil structures in an aqueous medium, both alone and in the presence of SWCNTs, simplifying analysis of the peptide/SWCNT interaction. The ability of the surfactant peptides to disperse individual SWCNTs in solution was verified using atomic force microscopy and ultraviolet-visible-near-infrared spectroscopy. The electronic properties of the surfactant peptide/SWCNT composites were examined using the observed nanotube Raman tangential band shifts and the observed additional features near the Fermi level in the scanning tunneling spectroscopy dI/dV spectra. The results revealed that SWCNTs functionalized with surfactant peptides containing electron-donor or electron-acceptor functional groups showed n-doped or p-doped altered electronic properties, respectively. This work unveils a facile and versatile approach to modify the intrinsic electronic properties of SWCNTs using a simple peptide structure, which is easily adaptable to obtain peptide/SWCNT composites for the design of tunable nanoscale electronic devices.     

Evidence of sidewall covalent functionalization of single-walled carbon nanotubes and its advantages for composite processing

Single-walled carbon nanotubes (SWCNTs) were functionalized in a three-step procedure. The first step is a radical reaction creating a covalent bond between the carbon nanotube surface and grafted p-methoxyphenyl functional groups. In a second step, a deprotection of the methoxy functions generates free alcohol groups and in the final step an esterification is done in order to install a double bond for further polymerization. Evidence that functionalization has actually occurred on the SWCNT sidewalls is furnished through investigations involving several complementary techniques (visual dispersion tests, transmission electron microscopy, thermal gravimetric analysis and adsorption volumetry). We show that surface properties of SWCNTs are changed throughout the chemical treatments and that the obtained level of functionalization is low. Incorporation of functionalized SWCNTs in a polymer (poly(methyl methacrylate)) matrix was done through an in situ polymerization process. Observations of the obtained composites using scanning and transmission electron microscopy illustrate that interactions between the SWCNT surface and the polymer matrix are improved.     

Gas sensors based on thick films of semi-conducting single walled carbon nanotubes

A comparative study was made of sorted semi-conducting single walled carbon nanotube (SWCNT) films and unsorted SWCNT films for gas sensing applications. The transmission line method is used to monitor separately the SWCNTs film resistance and the contact resistance between electrodes and the SWCNTs, thus revealing that the sensing mechanism mainly relies on a modification of the tube conductivity during gas exposure. The fabricated sensors demonstrate a detection limit of 20 ppb NO₂ and 600 ppb NH₃ mainly attributed to experimental setup limitations. Moreover, semi-conducting nanotubes happened to be 2.5 times more sensitive to NH₃ than unsorted ones, thus proving that selectivity can be improved by sorting the SWCNTs. The temperature dependence of the sensor sensitivity was studied, and a good agreement was found between experimental results and the Langmuir adsorption model.     

Using oxidation to increase the electrical conductivity of carbon nanotube electrodes

Recent studies have demonstrated that significantly low sheet resistance (Rs) (<100 Ω/sq; comparable to ITO) were achieved in single-walled carbon nanotube (SWCNT) films treated with HNO₃ followed by thionyl chloride. Here we show that H₂SO₄ can effectively reduce the Rs of SWCNT electrodes. H₂SO₄ treatment generates defects (COOH and SO₃H functionalities) on SWCNTs and the produced chemical functionalities are beneficial for enhancing the electrical conductivity in SWCNT electrodes. It is plausible that the H₂SO₄p-dopes the SWCNTs and the attachment of chemical functionalities helps to stabilize p-doping owing to their electron-deficient property.     

Electrochemical doping of pure single-walled carbon nanotubes used as supercapacitor electrodes

Pure single-walled carbon nanotubes (SWCNTs) with little bundling show an excellent capacitor performance attributable to the intrinsic nature of the SWCNTs, and possess unusual electrochemical properties characterized by a butterfly shaped cyclic voltammogram which differs from those for conventional activated carbon electrodes. Electrochemical doping in semi-conductor nanotubes occurred at the interface between the electrolyte and the SWCNT surface. In situ measurements showed a remarkable increase of electric conductivity with the polarization from the flat band potential. Because of the potential dependence, the capacitance of the SWCNT electrodes was higher at the higher charging potentials.