Fluorinated Single-walled Carbon Nanotubes Research Articles

Charge Photogeneration in Composites of Fluorinated Carbon Nanotubes and Semiconducting Polymer P3HT

Spectroscopic and photovoltaic properties of composites of purified and subsequently fluorinated single‐walled carbon nanotubes (F‐SWCNTs) with conjugated polymer poly(3‐hexylthiophene) (P3HT) are tested. Adding cyclohexanone into o‐dichlorobenzene solution of P3HT and F‐SWCNTs significantly affects the composite morphology and promotes P3HT/F‐SWCNT nanofilament formation, as evidenced from atomic force microscopy (AFM) images of spin‐coated composite films. Also, nanofilament formation enhances quenching of P3HT photoluminescence by F‐SWCNTs. The performance of P3HT‐based organic photovoltaics (OPV) devices with separated semiconducting SWCNTs and F‐SWCNTs as the acceptor component of the active layer is comparable. Light‐induced electron paramagnetic resonance (EPR) signal intensity in P3HT/F‐SWCNT composite films and frozen solutions grows with increase in F‐SWCNT content, which is a signature of photoinduced electron transfer. Dramatic change in SWCNT electronic structure upon fluorination is also evidenced by UV–vis– near infra red optical absorption spectra, from which the bandgap of about 1.0 eV is derived for F‐SWCNTs. Overall, the experimental results confirm that fluorination efficiently converts metallic SWCNTs into semiconducting ones, and F‐SWCNTs can be used for as an electron acceptor component in OPV devices, in combination with polymer donors. Presently, the performance of P3HT/F‐SWCNT devices is limited by F‐SWCNT aggregation into bundles, which decreases P3HT/F‐SWCNT interface area.

Direct Intertube Cross-Linking of Carbon Nanotubes at Room Temperature

Carbon nanotubes (CNTs) have long been regarded as an efficient free radical scavenger because of the large-conjugation system in their electronic structures. Hence, despite abundant reports on CNT reacting with incoming free radical species, current research has not seen CNT itself displaying the chemical reactivity of free radicals. Here we show that reactive free radicals can in fact be generated on carbon nanotubes via reductive defluorination of highly fluorinated single-walled carbon nanotubes (FSWNTs). This finding not only enriches the current understanding of carbon nanotube chemical reactivity but also opens up new opportunities in CNT-based material design. For example, spacer-free direct intertube cross-linking of carbon nanotubes was previously achieved only under extremely high temperature and pressure or electron/ion beam irradiation. With the free radicals on defluorinated FSWNTs, the nanotubes containing multiple radicals on the sidewall can directly cross-link with each other under ambient temperature through intertube radical recombination. It is demonstrated that carbon nanotube fibers reinforced via direct cross-linking displays much improved mechanical properties.

Spectroscopic study of electrochemically modified fluorinated single-wall carbon nanotubes

The changes in electrochemical and spectral characteristics of the fluorinated single-wall carbon nanotubes (F-SWCNTs) array upon electrochemical treatment were investigated. Nanotubes were characterized using cyclic voltammetry (CV), galvanostatic charge/discharge, UV–vis-NIR, Raman spectroscopy, X-ray photoemission spectroscopy (XPS), Electron Spin Resonance (ESR) spectroscopy, and scanning electron microscopy (SEM). Based on the comparative analysis of the obtained data the effect of the electrochemical treatment on the defectiveness and functionalization degree of the SWCNTs was revealed. It was shown that the addition of one oxygen-containing group was accompanied by the removal of approximately two fluorine atoms. At the same time, the defluorination leads to the partial healing of the defects formed on the nanotubes walls in the course of fluorination. The results obtained in this work in combination with our previous data on the electrochemical defluorination of the F-SWCNTs demonstrate the efficiency of the electrochemical approach for the controlled nanotube functionalization.

Characterization of Fluorinated Multi-Walled and Single-Walled Carbon Nanotubes Using High Resolution XPS and EDX

Pristine and fluorinated multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) were characterized using energy dispersive X-ray spectra (EDX) and X-ray photoelectron spectroscopy (XPS), respectively. The fluorine percentages of fluorinated multi-walled carbon nanotubes (F-MWCNTs) and fluorinated single-walled carbon nanotubes (F-SWCNTs) were 10.42% and 9.67% respectively by EDX. The absorption and de-absorption of fluorine properties were studied using high resolution C 1s and F 1s core level XPS and valence band spectra. The fluorine can be completely dissociated from F-MWCNTs, but partially dissociated from SWCNTs. There was 5.79% fluorine in atomic percent remaining associated with the F-SWCNTs when annealing the nanotubes to 500 °C measured by EDX. The results of F 1s core level XPS indicated that the binding energy of fluorine associated on SWCNTs was shifted from 687.0 eV to 688.3 eV after annealing the nanotubes to 500 °C. The results of valence band spectra showed that the binding energy of F 2p and F 2s shifted from 7.5 eV and 31.0 eV to 8.8 eV and 32.5 eV respectively in SWCNTS. However, the two peaks disappeared in annealed MWCNTs.

Synthesis of phthalocyanine tert-butyl ligand conjugates with fluorinecontaining single-walled carbon nanotubes having mobile ether bonds

The covalently bonded phthalocyanine conjugates with fluorinated single-walled carbon nanotubes (F-SWCNTs) were synthesized and characterized by Raman and IR spectroscopy and TGA/DTG.

Distinct Infrared Spectral Signatures of the 1,2- and 1,4-Fluorinated Single-Walled Carbon Nanotubes: A Molecular Dynamics Study

Fluorinated single-walled carbon nanotubes (F-SWNTs) form impor- tant intermediates in SWNT sidewall functionalization, leading to a variety of materials and biological applications. By simulating the infrared (IR) signals for the 1,2- and 1,4-addition structures, in which fluorine atoms are arranged in ortho or para positions, respectively, on the aromatic skeleton of the (10,10) SWNT surface, we identify peaks that are unique to each structure. Our full molecular dynamicssimulationsshowthatthe(-C(sp 3 )-C(sp 3 )-)collectivevibrationalpeak at 400 cm -1 is optically active only in the 1,2-isomer, while the 1300 cm -1 band arising due to the F-C(sp 3 ) stretching motion coupled with the neighboring C(sp 2 ) atoms is seen in the IR spectrum of only the 1,4-isomer. The reported results suggest simple and clear experimental means for distinguishing between the two fluorinated structures and provide a valuable tool for controlled SWNT sidewall functionalization. SECTION Nanoparticles and Nanostructures

Demonstration of covalent sidewall functionalization of single wall carbon nanotubes by NMR spectroscopy: Side chain length dependence on the observation of the sidewall sp3 carbons

Carboxylic acid-functionalized single walled carbon nanotubes (SWNTs) prepared via the reaction of an amino acid, NH2(CH2) n CO2H where n = 1 (glycine, GLY), 5 (6-aminohexanoic acid, AHA), 10 (11-aminoundecanoic acid, AUDA), with fluorinated single walled carbon nanotubes (F-SWNTs) have been characterized by MAS 13C NMR spectroscopy. The ease of observing the aliphatic CH2 groups and the resolution of the signal are dependent on the length of the amino acid’s aliphatic chain. We have proposed that where substituent chains are short (making NMR data collection difficult) chemical modification to extend the chain length should alleviate analysis problems. In this regard, we have investigated the esterification of the carboxylic acid termini. The amino acid-functionalized SWNTs were esterified with an appropriate alcohol to ensure parity of the overall substituent length, i.e., GLY-SWNT (C1) + 1-dodecanol (C12) = DOD-GLY-SWNT (1), AHA-SWNT (C5) + 1-octanol (C8) = OCT-AHA-SWNT (2), and AUDA-SWNT (C10) + 1-propanol (C3) = PRO-AUDA-SWNT (3). The 13C NMR shift for the sp3 nitrogen-substituted carbon atoms of the SWNT sidewall is observed at δ ≈ 75 ppm. Increasing the length of SWNT sidewall functional groups enhances the ability to observe the sidewall sp3 carbon. The methylene carbon signal intensity is less attenuated in the dipolar dephasing spectrum of the ester-functionalized SWNTs than their associated amino acid derivatives, suggesting more motional freedom of the side chain in the solid state. The confirmation of the dipolar dephasing spectral effects was assisted by the characterization of the ester of AUDA-SWNT with 1,3-propanediol: PPD-AUDA-SWNT (4).

Covalent sidewall functionalization of single-walled carbon nanotubes by amino acids

Single-walled carbon nanotubes (SWNTs) with amino acids covalently attached to their side walls, viz., “nanotube-aminoacids,” have been prepared starting from colloidal solutions of fluorinated SWNTs (F-SWNTs) and amino acids in o-dichlorobenzene and heating at 80–150 °C in the presence of pyridine. The syntheses were carried out with the F-SWNTs of approximately 2: 1 (C: F) stoichiometry and several natural α-aino acids with both pro-tected and unprotected carboxyl groups, such as glycine ethyl ester hydrochloride, L-serine ethyl ester hydrochloride, l-cysteine, and trans-4-hydroxy-l-proline. The nanotube-aminoacids have been characterized by Raman and FTIR spectroscopy, atomic force, scanning, and transmission electron microscopies, and thermal gravimetric analysis (TGA). Based on TGA data, the degree of sidewall functionalization in the synthesized SWNT derivatives was estimated to be in the range from one of 32 to one of 8 carbon atoms, depending on the amino acid nature and reaction conditions used. The amino acid-functionalized SWNTs, prepared in this work by simple and inexpensive one-step method, can be valuable precursors for peptide synthesis and targeted drug delivery, design and fabrication of nanocomposites and fibers, and other biomedical and engineering applications.

Sidewall Covalent Functionalization of Single Wall Carbon Nanotubes Through Reactions of Fluoronanotubes with Urea, Guanidine, and Thiourea

Sidewall covalent functionalization of carbon nanotubes is necessary to achieve smaller bundles and individuals, link to other functional moieties, and aid in better dispersion in composites. In the present study, we developed a one-step functionalization method which uses fluorinated single wall carbon nanotubes (F-SWNTs) as starting materials in the reactions with urea, thiourea, or guanidine. Through these reactions, the derivatives with terminal amide and heteroamide groups on the nanotube sidewalls have been prepared. The nanotubes also contain some residual fluorine generating bifunctional derivatives. These derivatives were characterized by Raman spectroscopy, Fourier Transform infrared (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Compared to fluorinated nanotubes, the urea-functionalized SWNTs (U-F-SWNTs) have shown among the three derivatives the high...

Attachment of Functionalized Single-Walled Carbon Nanotubes (SWNTs) to Silicon Surfaces

Single-walled carbon nanotubes (SWNTs) were functionalized by direct fluorination and subsequent reaction with 6-aminohexanoic acid for water-soluble carboxylic acid functionalized SWNTs (AHA-SWNTs). Both of the compounds were used as precursors to attach SWNTs to APTES coated silicon surfaces. AHA-SWNTs in aqueous solution were reacted with APTES self-assembled monolayers (SAMs) with coupling reagents N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS). The surface coverage is a function of concentration of AHA-SWNTs, solvent and coupling method. While for the fluorinated SWNTs (F-SWNTs), direct addition of F-SWNTs to preformed APTES SAMs at 90 degrees C shows essentially no reaction, in contrast to the one-pot reaction of F-SWNTs with APTES molecules in the presence of SWNTs on a silicon substrate. This reaction route provides a convenient method to attach SWNTs to silicon surfaces.

Synthesis, Characterization, and Carbon Dioxide Adsorption of Covalently Attached Polyethyleneimine-Functionalized Single-Wall Carbon Nanotubes

The reaction between fluorinated single-wall carbon nanotubes (F-SWNTs) and branched (M(w) = 600, 1800, 10000, and 25000 Da) or linear (M(w) = 25000 Da) polyethyleneimine (PEI) yields the covalent attachment of the polymer to the sidewalls of the nanotubes. The resulting PEI-functionalized SWNTs (PEI-SWNTs) were characterized by solid-state (13)C NMR, Raman spectroscopy, X-ray photoelectron spectroscopy, UV-vis spectroscopy, atomic force microscopy, transmission electron microscopy, and thermal gravimetric analysis studies. As expected, the number of polymer molecules per SWNT is larger for low molecular weight PEI than for high molecular weight PEI. However, above 1800 Da, the number of polymer molecules per SWNT does not vary as much. This is supported by Raman spectral data that shows the D:G ratio is relatively insensitive of the molecular weight for M(w) > 1800 Da. The PEI-SWNTs are shown to have solubility in aqueous media of up to 0.4 mg x mL(-1). Solid-state (13)C NMR shows the presence of carboxylate substituents that have been attributed to carbamate formation as a consequence of the reversable CO(2) absorption to the primary amine substituents of the PEI. Desorption of CO(2) is accomplished by heating under argon at 75 degrees C, while the dependence of the quantity of CO(2) absorbed on temperature and the molecular weight of the PEI is reported. Under the conditions investigated the maximum absorption of 9.2% w/w is observed for PEI(25000)-SWNT at 27 degrees C. The possible CO(2) absorption applications of the PEI-SWNTs is discussed.

Solid-State NMR Analysis of Fluorinated Single-Walled Carbon Nanotubes: Assessing the Extent of Fluorination

Fluorinated single-walled carbon nanotubes (F-SWNTs) have been characterized by magic angle spinning 13C NMR spectroscopy and the results correlated with Raman, IR, and X-ray photoelectron spectroscopy measurements. The 13C NMR shift for the sp3 fluorine-substituted (CF) carbon atoms of the SWNT sidewall is observed at δ = 83.5 ppm. This apparently unusual shift compared to those of most other tertiary alkyl fluorides is confirmed to be due to the CF moieties from ab initio calculations on an 80-carbon fragment of the 5,5 (armchair) SWNT and is in good agreement with the predominance of 1,2-addition rather than 1,4-addition of fluorine. The lack of observable scalar 13C−19F coupling for the CF carbon signal over a wide range of spinning speeds and at two different field strengths apparently results from interaction between 19F−19F and 13C−19F dipolar couplings and from magnetization exchange between the 13C doublet components caused by fluorine spin diffusion. The assignment of the 83.5 ppm peak is furthe...

Nanocomposite Fiber Systems Processed from Fluorinated Single-Walled Carbon Nanotubes and a Polypropylene Matrix

Processing composites of carbon nanotubes into nanotube continuous fibers (NCFs) is an effective way of manipulating the anisotropic properties of the single-walled carbon nanotubes (SWNTs) as it becomes less difficult to transform the SWNTs into an aligned configuration when they are confined in a small diameter fiber. This helps to take fuller advantage of the high mechanical properties of the SWNT materials in the axial direction. However, in creating nanocomposite fiber systems, the issues of dispersion and nanotube−matrix interaction and adhesion become of utmost importance when improved mechanical properties are anticipated. In addressing these issues in this work it has been found that sidewall chemical functionalization can be an effective tool for improving both the dispersion and interaction between the nanotube and the matrix. This work evaluates the effect of sidewall functional groups on fluorinated single-walled carbon nanotubes (F-SWNTs) as a precursor for improved interfacial adhesion in a thermoplastic matrix (polypropylene, PP) via partial defluorination of the F-SWNTs. The partial removal of functional groups from the F-SWNTs during melt processing with PP by shear mixing provides the opportunity for in situ direct covalent bonding between the nanotubes and the matrix during melt processing which ultimately results in better mechanical reinforcement of the composite. The studies conducted herein demonstrate that in comparison with PP composites filled with purified nanotubes (P-SWNTs), improved dispersion, interfacial adhesion, and mechanical properties are achieved for F-SWNT-loaded matrixes due to chemical functionalization.

Thermo-physical properties of epoxy nanocomposites reinforced by carbon nanotubes and vapor grown carbon fibers

In this study, the thermo-physical properties of epoxy nanocomposites reinforced by fluorinated single wall carbon nanotubes (FSWCNT) and vapor grown carbon fibers (VGCF) were investigated. A sonication technique using a suspension of FSWCNT and VGCF in acetone was utilized to process nanocomposites in anhydride-cured epoxy. The viscoelastic properties of the nanocomposites were measured with dynamic mechanical analysis. The glass transition temperature decreased approximately 30°C with an addition of 0.14vol.% (0.2wt.%) FSWCNT. The depression in Tg is attributed to non-stoichiometric balance of the epoxy matrix caused by the fluorine on single wall carbon nanotubes. The correct amount of the anhydride curing agent needed to achieve stoichiometry was experimentally determined by DMA measurements. After adjusting the amount of the anhydride curing agent for stoichiometry, the storage modulus of the epoxy at room temperature increased 0.63GPa with the addition of only 0.21vol.% (0.30wt.%) of FSWCNT, a 20% improvement compared with the anhydride-cured neat epoxy. For VGCF, the storage modulus at room temperature increased 0.48GPa with the addition of only 0.94vol.% (1.5wt.%) and then reached a plateau for larger amounts of VGCF. To understand the influence of VGCF on thermo-physical properties, the microstructure of the nanocomposites was interrogated using transmission electron microscopy (TEM). This study discusses the chemical effects of fluorine on matrix properties and the effect of stoichiometric balance on the thermo-physical properties of nanocomposites.

Thermolysis of Fluorinated Single-Walled Carbon Nanotubes: Identification of Gaseous Decomposition Products by Matrix Isolation Infrared Spectroscopy

The thermal decomposition of fluorinated single-walled carbon nanotubes (F-SWNTs), known to result in pristine SWNTs, has been investigated by freezing the gaseous products formed at temperatures between 50 and 500 degrees C under high vacuum in an argon matrix at 10-20 K and analyzing the trapped species by IR spectroscopy. The major products of F-SWNT decomposition are carbonyl fluoride (COF2) below 300 degrees C and CF4 above 300 degrees C. For comparison, graphite fluoride is stable thermally up to 300 degrees C under these conditions, and the major gas-phase species at temperatures below 500 degrees C are CF4 and the CF3 radical. F-SWNTs are thermally less stable than graphite fluoride, and etching of the nanotubes is observed at lower thermolysis temperatures.

Nanocomposites from biobased epoxy and single-wall carbon nanotubes: synthesis, andmechanical and thermophysical properties evaluation

The synthesis, and thermophysical and mechanical properties of anhydride-curedbiobased epoxy containing diglycidyl ether of bisphenol F (DGEBF) epoxy andepoxidized linseed oil (ELO) reinforced with fluorinated single-wall carbon nanotubes(FSWCNT) are reported. Sonication was used to disperse FSWCNT in thebiobased glassy epoxy network, resulting in great improvement of the modulus ofnanocomposites containing extremely small amounts of FSWCNT. The glasstransition temperature of the obtained nanocomposites decreased by approximately30 °C after the addition of 0.20 wt% (0.16 vol%) FSWCNT, without adjusting the amount of theanhydride curing agent. This was because of the non-stoichiometry of the epoxy matrix,caused by the fluorine on the single wall carbon nanotubes. The adequate amount ofthe anhydride curing agent needed to achieve stoichiometry was experimentallydetermined by dynamic mechanical analysis (DMA). The storage modulus of theepoxy at room temperature, which is below the glass transition temperature of thenanocomposites, increased up to 0.44 GPa with the addition of only 0.24 wt% (0.20 vol%) ofFSWCNT, representing an up to 14% improvement from the modulus of the biobasedELO neat epoxies. The fracture toughness of the neat biobased ELO epoxies wasalso improved by approximately 43% upon addition of FSWCNT. The excellentimprovement of the modulus was achieved without sacrificing the fracture toughness.

Thermo-physical and impact properties of epoxy nanocomposites reinforced by single-wall carbon nanotubes

The thermo-physical properties and the impact strength of diglycidyl ether of bisphenol F (DGEBF) epoxy nanocomposites reinforced with fluorinated single-wall carbon nanotubes (FSWCNT) are reported. A sonication technique was used to disperse FSWCNT in the glassy epoxy network resulting in nanocomposites having large improvement in modulus with extremely small amount of FSWCNT. The glass transition temperature decreased approximately 30 °C with an addition of 0.2 wt% (0.14 vol%) FSWCNT, without adjusting the amount of the anhydride curing agent. This was because of non-stoichiometry of the epoxy matrix that was caused by the fluorine on the single-wall carbon nanotubes. The correct amount of the anhydride curing agent needed to achieve stoichiometry was experimentally examined by dynamic mechanical analysis (DMA). The storage modulus of the epoxy at room temperature (which is below the glass transition temperature of the nanocomposites) increased up to 0.63 GPa with the addition of only 0.30 wt% (0.21 vol%) of FSWCNT, representing an up to 20% improvement compared with the neat epoxy. The Izod impact strength slightly decreased when the amount of FSWCNT was increased to 0.3 wt%. The excellent improvement in the storage modulus was achieved without sacrificing impact strength.

Experimental and computational investigations of the properties of fluorinated single-walled carbon nanotubes.

Fluorination of single-walled carbon nanotubes by reaction with elemental fluorine at elevated temperatures provides fluorinated single-walled carbon nanotubes (F-SWNT), which have the highest degree of functionalization (up to F/C = 1/2) of any derivatized carbon-nanotube material reported to date. Also, F-SWNTs have received more scrutiny than any other functionalized carbon nanotubes. This Minireview covers experimental and computational investigations of F-SWNTs with a focus on the nature and the strength of the C-F linkage.

Solvation of Fluorinated Single-Wall Carbon Nanotubes in Alcohol Solvents

Highly purified single-wall carbon nanotubes (SWNTs) were fluorinated to form “fluorotubes”, which were then solvated as individual tubes in various alcohol solvents via ultrasonication. The solvation of individual fluorotubes was verified by dispersing the tubes on a mica substrate and examining them with atomic force microscopy (AFM). Elemental analysis of the tubes reveals that light sonication in alcohol solvents does not remove significant amounts of the fluorine. While these solutions are metastable, they will persist long enough (over a week) to permit solution-phase chemistry to be carried out on the fluorotubes. For example, the solvated fluorotubes can be precipitated out of solution with hydrazine to yield normal, unfluorinated SWNTs, or they can be reacted with sodium methoxide to yield what are apparently methoxylated SWNTs. These reaction products have been examined with elemental analysis and a variety of spectroscopies and microscopies.