Characterization of fluorinated multiwalled carbon nanotubes by x-ray absorption spectroscopy

The C 1s and F 1s x-ray absorption spectra of fluorinated multiwalled carbon nanotubes with different fluorine contents and reference compounds (highly oriented pyrolytic graphite crystals and “white” graphite fluoride) were measured using the equipment of the Russian-German beamline at the BESSY II storage ring with a high energy resolution. The spectra obtained were analyzed with the aim of characterizing multiwalled carbon nanotubes and their products formed upon treatment of the nanotubes with fluorine at a temperature of 420°C. It was established that, within the probing depth (～15 nm) of carbon nanotubes, the process of fluorination occurs uniformly and does not depend on the fluorine concentration. The interaction of fluorine atoms with multiwalled carbon nanotubes in this case proceeds through the covalent attachment of fluorine atoms to graphene layers of the graphite skeleton and is accompanied by a change in the hybridization of the 2s and 2p valence electron states of the carbon atom from the trigonal (sp ²) to tetrahedral (sp ³) hybridization.     

Comparative X-ray absorption investigation of fluorinated single-walled carbon nanotubes

The C 1s and F 1s X-ray absorption spectra of pristine and fluorinated single-walled carbon nanotubes with different fluorine contents and nanodiamond as a reference compound have been measured with the aim of characterizing single-walled carbon nanotubes and their products formed upon treatment of the nanotubes with molecular fluorine at a temperature of 190°C. The spectra obtained have been analyzed by thoroughly comparing with the previously measured spectra of highly oriented pyrolytic graphite and fluorinated multiwalled carbon nanotubes and the spectrum of nanodiamond. It has been established that the fluorination of single-walled and multiwalled carbon nanotubes leads to similar results and is characterized by the attachment of fluorine atoms to carbon atoms on the lateral surface of the nanotube with the formation of the σ(C-F) bonds due to the covalent mixing of F 2p and C 2p _z π valence electron states.     

Characterization of fluorinated multiwalled carbon nanotubes with X-ray absorption,photoelectron and emission spectroscopies

The combined investigation of the chemical bond formation in fluorinated multiwalled carbon nanotubes with 15 wt.% fluorine concentrations (MWCNTs + F 15 wt.%) using X-ray absorption, emission and photoelectron spectroscopy at C 1s and F 1s thresholds is presented. All measurements were performed at BESSY II. The analysis of the soft X-ray and photoelectron spectra point to the formation of covalent chemical bonding between fluorine and carbon atoms in the fluorinated nanotubes. Based on results of this combined study the depth dependent effects are discussed.     

Effect of the cesium and potassium doping of multiwalled carbon nanotubes grown in an electrical arc on their emission characteristics

The effect of cesium and potassium atoms deposited onto multiwalled carbon nanotubes grown in an electrical arc on their emission characteristics was studied. The current–voltage characteristics of the field electron emission of specimens with cesium or potassium doped multiwalled carbon nanotubes of this type were revealed to retain their linear character in the Fowler–Nordheim coordinates within several orders of magnitude of change in the emission current. The deposition of cesium and potassium atoms was shown to lead to a considerable increase in the emission current and a decrease in the work function φ of studied emitters with multiwalled nanotubes. The work function was established to decrease to φ ~ 3.1 eV at an optimal thickness of coating with cesium atoms and to φ ~ 2.9 eV in the case of doping with potassium atoms. Cesium and potassium deposition conditions optimal for the attainment of a maximum emission current were found.

     

Anti-prismatic modifications of single-walled carbon nanotubes and their electronic properties: Regular adsorption of fluorine atoms on graphene surfaces of nanotubes

The regular adsorption of fluorine atoms on the surfaces of single-walled carbon nanotubes along their cylindrical axes leads to a modification of cylindrical carbon skeletons of these single-walled carbon nanotubes into carbon skeletons that have a nearly “anti-prismatic” shape (anti-prismatic modifications). In the faces of these modified single-walled carbon nanotubes, there can arise quasi-one-dimensional isolated carbon conjugated subsystems (tracks) with different structures. Model fragments of nanotubes of the (n, 0) type that contain up to 360 carbon atoms and their derivatives with regularly adsorbed fluorine atoms on the graphene surface have been calculated using the semiempirical PM3 method. It has been found that the main properties of the single-walled carbon nanotubes modified by the above method are determined by the character of the conjugation of the electrons in isolated carbon tracks, which is close to the character of the conjugation of the electrons in the initial single-walled carbon nanotubes.     

Prismatic modifications of single-walled carbon nanotubes and their electronic properties: Regular adsorption of fluorine atoms on graphene surfaces of nanotubes

The regular adsorption of fluorine atoms on surfaces of single-walled carbon nanotubes along their axes can lead to a modification of cylindrical carbon cores of these single-walled carbon nanotubes to carbon cores that have a nearly prismatic shape (prismatic modification). In faces of these modified single-walled carbon nanotubes, there can arise quasi-one-dimensional isolated carbon conjugated subsystems (tracks) with different structures. It has been established that the main characteristics of the single-walled carbon nanotubes thus modified are rather close to the corresponding characteristics of the related isostructural polymer conjugated systems (such as cis-polyenes, polyphenylenes, poly(periacenes), or polyphenantrenes). Fragments of model nanotubes of the (n, n) and (n, 0) types that contain up to 360 carbon atoms and their derivatives doped with fluorine atoms have been calculated using the semiempirical parametric method 3.     

XPS study of fluorinated carbon multi-walled nanotubes

Arc-produced carbon multi-walled nanotubes (MWNTs) were fluorinated at 420 °C in a flow of diluted F₂ gas containing small admixture of HF gas. Fluorinated materials (F-MWNTs) with 10–55 wt.% fluorine content were studied by XPS. It was shown that fluorination begins at the external layers of nanotubes and the reaction front propagates inside the multi-layer particles in concert with structural deterioration of graphene layers. The C₂F stoichiometry still allows MWNT wall integrity, similar to known for SWNTs. The fluorine contents in the product can noticeably exceed this higher fluorine limit for tube stability. The position of the F 1s line at 688.2 eV does not depend on the fluorine concentration. Nearly covalent C–F bonds dominate the F-MWNT samples, with a small quantity (2–9%) of ionic bonds also present. Fluorinated carbon tends to spatially separate from non-fluorinated carbon.     

Depth distribution of the fluorine concentration during radiative carbonization of PVDF

The XPS integral intensity of the F1s line and its satellite is measured during the long-term radiative carbonization of PVDF (polyvinylidene fluoride). A model is proposed that describes the effect of the fluorine depth distribution on the shape and intensity of the F1s spectra. A comparison of the experimental data with the model calculations provides estimates for the concentration inhomogeneity during the radiative carbonization of PVDF, for the photoelectron escape depth, and for the probability of a single energy loss by a photoelectron in its motion towards the surface. A technique determining the fluorine concentration is presented. It is based on the occurrence of chemical shifts of the C1s line towards larger bond energies for the carbon atoms chemically bonded to one or two fluorine atoms.     

Temperature-induced transformations in hydrogenated and fluorinated single-wall carbon nanotubes studied by Raman scattering

Raman spectra of hydrogenated and fluorinated single-wall carbon nanotubes (SWCNTs) are measured at ambient temperature before and after heat treatment. The spectra of the as-prepared hydrogenated SWCNTs show a giant structureless band in the visible region that screens the Raman peaks related to the carbon atom vibrations. The onset of this strong band follows the excitation laser line, which is typical of hot luminescence. The intensity of the luminescence background decreases exponentially with the annealing time, while the dependence of the luminescence decay time constant on the annealing temperature is of the Arrhenius type with the activation energy E _a = 465 ± 44 meV. The luminescence background in the Raman spectra of the fluorinated SWCNTs is comparable with the Raman peak intensity and decreases exponentially with the annealing time. The dependence of the decay time constant on the temperature is again of the Arrhenius type with the activation energy E _a = 90 ± 8 meV. The appearance of hot luminescence is related to the upshift of the fundamental energy gap in functionalized SWCNTs and the structural disorder induced by random binding of hydrogen or fluorine atoms. The luminescence background disappears upon annealing in vacuum or in air after removal of hydrogen (fluorine), while the annealed samples still demonstrate large structural disorder.     

Electron emission features of the derivatives of radiation carbonization of poly(vinylidene fluoride)

It has been studied how photoelectron and CKVV spectra of partially crystalline poly(vinylidene fluoride) (PVDF) are modified during a long-term degradation of its surface under soft X-rays (AlK _α), which is accompanied by a flow of secondary electrons having different energies, and upon exposure to a unfocused beam of 600 eV Ar⁺ ions. In both cases, the surface layer of the sample is enriched with carbon owing to defluorination. The shape of the electron emission spectra of the carbonized layer depends on an external effect; that is, whether soft X-ray photons or ions are used for defluorination. In the case of bombardment with Ar⁺, there is clear evidence for the dominance of the sp2 bonds between carbon atoms, as can be seen from the specific shape of the C KVV band and the C1s spectrum. The most surprising result of this study is that both photons and ions produce the same depth gradient of residual fluorine at an equal fluorine concentration in the carbonized surface layer. The reason for this is not clear and needs further investigation.     

Imaging the elastic properties of coiled carbon nanotubes with atomic force microscopy

Coiled carbon nanotubes were produced catalytically by thermal decomposition of hydrocarbon gas. After deposition on a silicon substrate, the three-dimensional structure of the helix-shaped multiwalled nanotubes can be visualized with atomic force microscopy. Helical structures of both chiralities are present in the nanotube deposits. For larger coil diameters ( >170 nm), force modulation microscopy allows one to probe the local elasticity along the length of the coil. Our results agree with the classical theory of elasticity. Similar to the case of straight nanotubes, the Young modulus of coiled multiwalled nanotubes remains comparable to the very high Young modulus of hexagonal graphene sheets.     

Sidewall fluorination and hydrogenation of single-walled carbon nanotubes: a density functional theory study

The fluorination and hydrogenation reactions on (6, 6) and (10, 0) single-walled carbon nanotubes (SWCNTs) have been examined via computing the reaction energy for the chemisorption. The examined nanotubes have comparable lengths and diameters, with or without Stone-Wales defects on the sidewall. The two fluorine or hydrogen atoms are anchored to the external walls of the SWCNTs. The computed chemisorption energies of these virtual reactions reveal that the fluorination and hydrogenation of the nanotubes are moderately sensitive to the nanotube chirality and the sidewall topology, and the (10, 0) SWCNT with Stone-Wales defect can be easily fluorinated and hydrogenated.      

Multifunctional Hybrids Based on 2D Fluorinated Graphene Oxide and Superparamagnetic Iron Oxide Nanoparticles

Carbon‐based nanomaterials have garnered a lot of attention in the research of yesteryear. Here this study reports a composite based on fluorinated graphene oxide—a multifunctional subsidiary of graphene; and iron oxide nanoparticles as a contrast agent for magnetic resonance imaging (MRI). Extensive structural and functional characterization is carried out to understand composite behavior toward biotoxicity and its performance as a contrast agent. The electron withdrawing fluorine group decreases the charge transfer to iron oxide increasing the magnetic saturation of the composite thus enhancing the contrast. The interaction of paramagnetic and superparamagnetic systems yields a superior contrast agent for MRI and fluorescent imaging.     

Spectroscopic Studies on Pure and Histidine-Functionalized Multiwalled Carbon Nanotubes

Electron paramagnetic resonance, Fourier transform infrared, and ultraviolet-visible studies were carried out for pure and histidine-functionalized multiwalled carbon nanotubes. Electron paramagnetic resonance absorption spectral data were found to be the best fit for the Gaussian lineshape. The g-values indicate the presence of magnetic impurities in the samples and the interaction between the localized electrons and delocalized electrons in the nanotubes trapped at defects or magnetic ion sites. The electron spin concentration decreases with increasing concentration of histidine, which implies that the unpaired electrons undergo a reduction process in the histidine-functionalized multiwalled carbon nanotubes. Fourier transform infrared study confirms the presence of functional groups in pure and histidine-functionalized multiwalled carbon nanotubes. Ultraviolet-visible study reveals the formation of a charge transfer complex in histidine-functionalized multiwalled carbon nanotubes.     

Effect of thermal motion of residual gas molecules on the degradation of the field emission cathode based on carbon nanotubes

The rate of degradation of carbon nanotubes in an electron field emission cathode is calculated taking into account thermal motion of the residual gas atoms. As the degradation mechanism, we consider the sputtering of the surface of carbon nanotubes by ions formed as a result of ionization of residual gas molecules by electron impacts. It is shown that the allowance for the initial thermal motion of atoms, the ionization of which is a source of ions, considerably reduces the degradation rate as compared to the results of approximate calculations based on the assumption of motionless atoms.     

Modification of the structural and electronic properties of graphene by the benzene molecule adsorption

A survey of the literature data on the adsorption of benzene on graphene or carbon nanotubes indicates that the distance between the graphene sheet and benzene molecule is determined from weak van der Waals forces (∼3.40 Å). In our theoretical study, it was found that the benzene/graphene structure (in a specific configuration with carbon atoms located at the atop positions, stacked directly on the top of each other) forms strong covalent bonds, if the distance between the graphene and benzene is about 1.60 Å. Such a short distance corresponds to about a half of the usual separation between the graphite layers. It was also shown that at such a short distance the carbon atoms of the benzene molecule move towards the graphene sheet, whereas the hydrogen atoms move in a different direction, thus breaking the benzene planar structure.     

Effect of carbon nanotubes on the anode performance of natural graphite for lithium ion batteries

A comparative investigation was carried out on carbon black and multiwalled carbon nanotubes as conductive additives in spherical natural graphite for lithium ion batteries. Scanning electron microscopy images showed that carbon nanotubes interlaced graphite particles in series to form a three-dimensional network. The constant current charge-discharge experiments showed that carbon nanotubes were more effective in improving reversible capacity and cycle stability. The reversible capacity was improved to 366 mAh/g and the cycle stability was improved effectively when carbon nanotubes were used. The research is of potential interest to the application of carbon nanotubes as conductive additives in anode materials for high-power lithium ion batteries.     

Effective electron drift mobility in multiwalled carbon nanotubes

We introduce the concept of effective electron drift mobility for multiwalled carbon nanotubes. This effective quantity is calculated under a quantum-box approach to conceive quantum transport in a given multiwalled carbon nanotube. In addition, effective motion time is determined.     

多壁碳纳米管束储氢机理的X射线吸收谱研究

采用X射线吸收精细结构光谱探索性地研究了多壁碳纳米管束.在多壁碳纳米管束不同入射角的X射线吸收精细结构光谱中，观察到C—H σ^*共振峰强度随入射角的变化而发生变化.在常温常压下出现C—H键可能与多壁碳纳米管束中存在缺陷有关，缺陷数量越大C—H σ^*共振峰的强度越大.光谱中C—C π^*和C—C σ^*共振峰强度的变化趋势都不同于C—H σ^*共振峰，这有力地证明了在常温常压条件下氢原子是吸附在多壁碳纳米 关键词： X射线吸收精细结构光谱 碳纳米管 储氢 化学吸附     

Structural, curvature and electronic properties of Rh adsorption on armchair single-walled carbon nanotube

This paper systematically studies the rolling effects of the (n, n) single-wall carbon nanotubes (SWCNT) with different curvatures on Rh adsorption behaviours by using density functional theory. The outside charge densities of SWCNTs are found to be higher than those inside, and the differences decrease with the increase of the tube radius. This electronic property led to the discovery that the outside adsorption energies are higher than the inside ones, and that the differences are reduced with the increase of the tube radius. Partial density of states and charge density difference indicate that these strong interactions induce electron transfer between Rh atoms and SWCNTs.