Reactivity of polymeric carbon chains reduced from poly(tetrafluoroethylene)

Electrochemical reduction of poly(tetrafluoroethylene) (Teflon), which contains very long linear carbon atom chains, yields in the presence of Li⁺ a solid mixture of lithium fluoride and polymeric carbon. The latter is probably in the sp state and is very chemically reactive. Its chains begin to bind into a solid carbon skeleton only after separation from the salt sheaths, so that chemical bonds are formed between certain carbon atoms of neighbouring chains. The form of the skeleton is much influenced by physico-chemical conditions during the separation process. The porous carbon layer consisting from sporadically bound carbon chains was found to be plastic as long as the skeleton does not become more rigid by the formation of additional chemical bonds. Three different carbonaceous materials were prepared from the same mixture (C, LiF) by different technologies. Differences between them were studied by elementary chemical analysis, nitrogen adsorption, wide-angle and small-angle X-ray scattering, and electron microscopy. The obtained results enable to discuss the course of formation of the carbon skeleton.     

A contribution of radial electron density distribution measurements to the study of PTFE-Carbon

The sp-hybridized bonds in elemental carbon expected as primary product of low temperature electrochemical reduction of poly(tetrafluoroethylene) were proved by determination of C-C bond length from radial electron density distribution measurements. A value of (0.139 ± 0.002) nm was found for the C-C bonding distance for the carbon phase in the “as received” reduction products. This value, lower than 0.142 nm for graphitic sp₂ bonds, can be explained only by the presence of a remainder (about 5–20%) of sp-hybridized bonds with a length of about 0.120 nm, whereas the majority of very reactive sp carbon chains was transformed to six-membered rings (sp₂ bonds). On the contrary, a higher bonding distance of (0.145 ± 0.001)nm was found for carbons isolated from the reduction products, an evidence for the formation of a portion of tetrahedral sp₃ bonds in edge-to-edge cross-linkages. Interlayer distances of 0.342 nm, constituted at 950°C, were unusually low in comparison with polymeric carbons prepared by pyrolysis.     

The kinetics of sputtered deposited carbon on silicon: a phenomenological model

A phenomenological model based on the rate equations for the carbon sputter deposition on Si surface is proposed. The processes of carbon adsorption, formation of SiC and transition from sp² to sp³ sites induced by low energy ion bombardment are included. The calibration of the model was performed with the experimental results. The amorphous carbon films were deposited by magnetron sputtering of graphite with Ar⁺ ions. The energy of ions bombarding the growing film was varied by applying a bias voltage on the substrate. It is shown that the non-monotonous kinetics of film growth is determined by the variations of surface composition at different stages of growth.     

Multi-wall carbon nanotubes by catalytic decomposition of carbon monoxide on Ni/MgO

The redox behavior of the catalyst and the catalytic decomposition of carbon monoxide (CO) were investigated in the synthesis process of multi-wall carbon nanotubes (MWCNT) using Ni/MgO catalyst. The surface morphology of the heated Ni layer was observed by TEM to confirm the formation of NiO particles (50 nm or less) and NiO (222). The chemical reaction behavior of the catalyst in CO the atmosphere was displayed via TG-DSC analysis, and the reduction of NiO was revealed due to the mass decrease of 2.71 wt% and the exothermic peak at around 400°C. The deposition of carbon was identified with an increase in mass and the exothermic peak near 600°C. Ni (111) and carbon (002) facets was taken place in a diffraction pattern of carbon deposited catalyst, indicating the reduction in NiO and the graphitic carbon deposition. The crystallinity of the graphitic carbon was analyzed as the ratios of 0.998 for I_D/I_G and 0.26 for sp³/sp² in Raman and photoelectron spectra. The encapsulated Ni in MWCNT was observed through TEM-EDS, verifying the activation of the catalyst by CO.     

Palladium‐Catalyzed Regioselective γ‐Mono‐ and Diarylation of Acrylamide Derivatives with Aryl Halides

Palladium‐catalyzed direct mono‐ and diarylations involving C(sp³) H cleavage at the γ‐position of acrylamides are described. The monoarylation products can be obtained with ortho‐substituted aryl halides. Single crystal X‐ray diffraction has shown that the double bond has shifted towards the introduced aryl group to afford the γ‐arylated β,γ‐unsaturated amide products. A second arylation occurs when less sterically hindered aryl halides are employed. This chemistry offers a novel disconnection for the synthesis of γ‐arylated compounds.     

Synthesis of highly transparent ultrananocrystalline diamond films from a low-pressure,low-temperature focused microwave plasma jet

This paper describes a new low-temperature process underlying the synthesis of highly transparent ultrananocrystalline diamond [UNCD] films by low-pressure and unheated microwave plasma jet-enhanced chemical vapor deposition with Ar-1%CH₄-10%H₂ gas chemistry. The unique low-pressure/low-temperature [LPLT] plasma jet-enhanced growth even with added H₂ and unheated substrates yields UNCD films similar to those prepared by plasma-enhanced growth without addition of H₂ and heating procedure. This is due to the focused plasma jet which effectively compensated for the sluggish kinetics associated with LPLT growth. The effects of pressure on UNCD film synthesis from the microwave plasma jet were systematically investigated. The results indicated that the substrate temperature, grain size, surface roughness, and sp ³ carbon content in the films decreased with decreasing pressure. The reason is due to the great reduction of H _α emission to lower the etching of sp ² carbon phase, resulting from the increase of mean free path with decreasing pressure. We have demonstrated that the transition from nanocrystalline (80 nm) to ultrananocrystalline (3 to 5 nm) diamond films grown via microwave Ar-1%CH₄-10%H₂ plasma jets could be controlled by changing the pressure from 100 to 30 Torr. The 250-nm-thick UNCD film was synthesized on glass substrates (glass transition temperature [T _g] 557°C) using the unique LPLT (30 Torr/460°C) microwave plasma jet, which produced UNCD films with a high sp ³ carbon content (95.65%) and offered high optical transmittance (approximately 86% at 700 nm).     

Liquid/solid two‐phase carbonization of low‐molecular‐weight chloro‐/bromo‐hydrocarbons

A novel liquid/solid two‐phase reaction has been discovered that enables destruction of a series of low‐molecular‐weight chloro‐/bromo‐hydrocarbons to carbon‐based materials. The solid phase is anhydrous potassium hydroxide and the liquid phase is a benzene or tetrahydrofuran solution of halide and contains a certain amount of tetrabutyl ammonium bromide (TBAB) as phase transfer catalyst. The structure of the carbon‐based materials have been characterized by elemental analysis, Fourier transform infrared (FT‐IR), FT‐Raman, and X‐ray photoelectron spectroscopies, and their morphologies have been examined by wide‐angle X‐ray diffraction and transmission electron microscopy. The results indicate that the products are amorphous nanoparticles and contain mainly elemental carbon. They consist of sp, sp², and sp³ carbon atoms simultaneously and can be regarded as carbyne analogues. This work provides a convenient method for synthesizing new carbon‐based materials in relatively high yields. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1510–1515, 2000     

Preparation of grafted polytetrafluoroethylene fibers and adsorption of bilirubin

BACKGROUND: A successful hemoperfusion technique requires that the adsorbent for bilirubin should have a high specificity, adsorption capacity and adsorption rate, blood compatibility and no toxicity. Compared with polymer microbeads, polytetrafluoroethylene (PTFE) fibers have many advantages. The aim of the work reported here was to prepare a new polytetrafluoroethylene‐graft‐poly(glycidyl methacrylate)‐block‐polyethyleneimine (PTFE‐g‐PGMA‐b‐PEI) adsorbent for bilirubin based on PTFE fibers by the ⁶⁰Co radiation‐induced graft polymerization of GMA followed by the chemical modification of the epoxy groups on the PTFE‐g‐PGMA fibers with PEI. In addition, the adsorption properties of this novel adsorbent for bilirubin were examined. RESULTS: The highest content of amino groups obtained on the PTFE‐g‐PGMA‐b‐PEI fibers was 1.87 mmol g^?1. The maximum adsorption capacity of the grafted fibers was 9.6 mg g^?1 at pH = 6.5. Bilirubin adsorption on these fibers obeyed the Langmuir model. Also, these fibers possessed the ability to selectively adsorb bilirubin in the presence of bovine serum albumin. CONCLUSION: The PTFE‐g‐PGMA‐b‐PEI fibers have a high adsorption capacity for bilirubin and excellent adsorption properties. In addition, this new adsorbent is inexpensive, easy to prepare and has no toxicity. So the PTFE‐g‐PGMA‐b‐PEI fibers as a biomedical adsorbent are promising for the removal of bilirubin through the hemoperfusion technique. Copyright © 2009 Society of Chemical Industry     

Metal‐Free Synthesis of 3‐Arylquinolin‐2‐ones from N,2‐Diaryl‐ acrylamides via Phenyliodine(III) Bis(2,2‐dimethylpropanoate)‐ Mediated Direct Oxidative C−C Bond Formation

Treatment of N,2‐diarylacrylamides with the organoiodine(III) compound phenyliodine(III) bis(2,2‐dimethylpropanoate) [PhI(O₂C‐t‐Bu)₂] and boron trifluoride etherate (BF₃⋅Et₂O) resulted in a direct and selective oxidative C(sp²)−C(sp²) bond formation leading to a convenient assemblage, under mild conditions, of the biologically important 3‐arylquinolin‐2‐one skeleton. Differing from the five‐membered oxindole products from oxidative cyclizations mediated by transition metals, this metal‐free approach realized a direct annulation of the N‐arylacrylamide into a six‐membered 3‐arylquinolin‐2‐one skeleton.

     

Surface amorphisation of TiZrN coatings by laser carburisation

ABSTRACT

The enhancement of wear resistance by laser carburisation on TiZrN coatings was investigated in terms of bond state. Graphite paste was used to cover the TiZrN coatings and then pulsed laser ablation was used to solidify the carbon. Tribometer test showed that the friction coefficient was reduced from 0.64 to 0.17 after laser carburisation. The doped carbon was analysed as a mixture with sp²C and sp³C bonds using an X-ray photoelectron spectroscopy depth profile. The sp²/sp³ ratio was 2.29 at the surface and increased up to 2.91 in the depth direction. To verify the effect of the hybrid bonds on the atomic order, annealing was carried out to the carburised specimens. As the ratio increased to 3.28, graphitisation resulting in transition from sp³ to sp² was confirmed, and variation in the lattice structure was demonstrated by a reduction in the lattice constant (4.45?Å to 4.40?Å) using Rietveld refinement.     

AES and core level photoemission in the study of a-C and a-C:H

The state of the art in the use of two probes of the occupied electron states, namely C KVV Auger emission and C 1s photoemission, in the study of a-C and a-C:H is reviewed, with particular attention to the issue of deriving the sp² fraction. The local character of the two probes justifies decomposition of the relative spectra into an sp² and an sp³ component. While however decomposition of the C 1s spectrum relies upon a theoretical basis and allows accounting for disorder effects in the amorphous state, no theory is available to support C KVV spectrum decomposition which has therefore to rely upon a purely empirical basis. In addition, the introduction of disorder related effects is not straightforward for this spectrum. A real validation of the sp² fraction measurement is lacking for both techniques, though there are indications that both allow qualitative or even semi-quantitative (C 1s spectrum) understanding of the electronic structure of amorphous carbon systems. Beside the sp² fraction evaluation, other pieces of information, concerning the spatial organization of the sp² sites, are possibly extracted from these spectra.     

Hydrogen adsorption on single-walled carbon nanotubes studied by core-level photoelectron spectroscopy and Raman spectroscopy

We have investigated the adsorption of atomic hydrogen on vertically aligned carbon nanotube (CNT) films using in situ synchrotron-radiation-based core-level (CL) photoelectron spectroscopy and Raman spectroscopy. From C 1s CL spectra, we identified a CL peak component due to C-H bonds of carbon atoms in single-walled carbon nanotubes (SWCNTs). We also found the suppression of π-plasmon excitation, indicating that the hydrogen adsorption deforms the bonding structure. Raman spectra of the SWCNT film indicated that the radial-breathing-mode intensities of SWCNTs decreased due to the adsorption-induced bonding-structure deformation. Moreover, the decrease for small-diameter SWCNTs was more severe than that for large-diameter SWCNTs. Our results strongly suggest that the hydrogen adsorption, which induces the structure deformation from sp² to sp³-like bonding, depends on the diameter of SWCNTs.     

Bulk quantity and physical properties of boron nitride nanocapsules with a narrow size distribution

A new synthesis route for the formation of boron nitride (BN) nanocapsules by means of a substitution process using single wall carbon nanotubes as templates, with yields of >95% is presented. It is also shown that these BN nanocapsules can act as ideal reference samples for the determination of the relative sp² to sp³ configuration in BN species, a value that is crucial for the physical properties of these nanostructures.     

Comparative investigation of the formation of polytetrafluoroethylene nanoparticles on different solid substrates through the adsorption of tetrafluoroethylene

The effects of different solid substrates, including carbon nanofibers (CNFs), activated carbon, alumina, silica, molecular sieves, and poly(N‐vinylpyrrolidione) (PVP), were compared for the high‐pressure synthesis of polytetrafluoroethylene [PTFE or (CF₂)_n] nanoparticles via the adsorption of thermally synthesized tetrafluoroethylene (C₂F₄) as the monomer. Scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis (TGA) were used for the characterization of the PTFE nanoparticles on different solid substrates. The results demonstrate that the average diameters of the PTFE nanoparticles were about 90 nm for the CNFs, 130 nm for PVP, 150 nm for alumina, and about 200 nm for silica. Also, TGA showed that the amounts of PTFE nanoparticles synthesized on each solid substrate were 3.53 ± 0.09% for CNFs, 2.31 ± 0.10% for PVP, 2.11 ± 0.12% for silica, and 0.97 ± 0.16% for alumina. Depending on the active surface area and the morphology of nanomaterials, such as CNFs, different capacities were evaluated for each solid support in the formation of the PTFE nanoparticles. The quantities and the size of the synthesized PTFE nanoparticles relied on the characteristics of the solid substrate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Clustering in nanostructured carbon: Evidence of electron delocalization

The electronic properties of disordered carbon-based materials can be discussed in terms of the clustering of the sp² carbon phase and delocalization of the electron wave function. In smooth amorphous carbon thin films this results in a mixed phase material of conductive sp² clusters embedded in an electrically insulating sp³ matrix. The delocalization of the electron wave function associated with the sp² clusters is shown to play an important role in understanding many of the electronic and optical properties of the films. It is demonstrated that the extent of the electron delocalization and clustering can be estimated using magnetic resonance methods. Evidence for delocalization in a range of carbon-based materials such as diamond-like carbon thin films produced by chemical vapour deposition, nanostructured carbon produced by pulsed laser ablation and ultrananocrystalline diamond is presented.     

Recent developments in understanding the toxicity of PTFE thermal decomposition products

Fluropolymers, especially polytetrafluoroethylene (PTFE), have good fire-resistance properties, but their application is limited by concerns over the toxicity of their thermal decomposition products. In experiments using a tube furnace system similar to the DIN 53 436 method, the 30-minute (+ 14 days observation) LC₅₀ in mass loss terms was found to be 2.9 mgI^?1 (Standard Error 0.40) under non-flaming conditions, approximately ten times as toxic as wood and most other materials. Toxicity was due to upper respiratory tract and airway irritation, and was consistent with the known effects of carbonyl fluoride and hydrogen fluoride. When decomposed in the NBS cup furnace test under-non-flaming conditions, PTFE evolved extreme-toxicity products with an LC₅₀ of approximately 0.05 mgI^?1 (mass loss), approximately 1000 times as toxic as wood and most other materials. Toxicity was due to deep lung irritation and oedema. Investigations of the range of conditions under which the extreme toxicity of PTFE products occurs in both small-scale (200-litre) and intermediate-scale (6 m³) experiments have shown that the highest toxicity occurs when PTFE is decomposed under non-flaming conditions over a temperature range of 400–650°C, and when the primary decomposition products are subjected to continuous secondary heating. At higher or lower temperatures, when the sample is flaming, when decomposition products from wood are also present in the chamber, when secondary heating is curtailed, or when the molecule contains hydrogen as well as fluorine, the toxicity of the products is greatly reduced, tending towards the region of ten times the potency of most other materials. Extreme toxicity is associated with a particulate, but the particulate atmosphere is not always extremely toxic, the potency decreasing as the fumes age.     

Diamond-like carbon (DLC) films as electrochemical electrodes

Amorphous carbon can be any mixture of carbon bonds of sp³, sp², and even sp¹, with the possible presence of hydrogen. The group of mixture, of which there is a high fraction of diamond-like (sp³) bonds, is named diamond-like carbon (DLC). Unlike the crystalline carbon materials: diamond, graphite, carbon nanotube, fullerene and graphene, DLC can be deposited at room temperature without catalyst or surface pretreatment. Furthermore, its properties can be tuned by changing the sp³ content, the organization of sp² sites and hydrogen content, and also by doping. This paper firstly reviewed the electrochemical properties of DLC films and their applications.     

Oxidative dehydrogenation of ethylbenzene to styrene over ultra-dispersed diamond and onion-like carbon

The catalytic properties of sp³-hybridized ultra-dispersed diamond and sp²-hybridized onion-like carbon in the oxidative dehydrogenation of ethylbenzene to styrene were investigated, highlighting the structure sensitivity of the reaction. The sp³-carbon led initially to C-C cleavage and benzene formation, while a switchover of the main reaction pathway into the styrene formation occurred with time on stream due to the formation of surface sp² carbon, required for the selective styrene formation. This was confirmed by the behavior and the high stable styrene selectivity shown by onion-like carbons. High temperature oxygen pre-treatment created catalytically active species at the sp² carbon surface, confirming that a high thermal stability carbon-oxygen complex was the active surface site for forming styrene.     

Plasma-chemical bromination of graphitic materials and its use for subsequent functionalization and grafting of organic molecules

Plasma-chemical bromination has been shown to be well-suited for highly selective and efficient C-Br-functionalization of polyolefin surfaces and has now been applied to graphitic materials, including highly oriented pyrolytic graphite (HOPG), natural graphite, carbon nanotubes, and graphitized carbon fibres. In contrast to the radical substitution of hydrogen by bromine at hydrogen-terminated sp³-hybridized carbon, bromination of all-carbon bound sp²-hybridized carbon in graphene requires addition to aromatic double bonds. The related change of sp² to sp³ hybridization of the carbon atoms involved helps to explain experimental results of a loss in surface planarity on brominated HOPG. Bromine concentrations of up to 50 bromine atoms per 100 C-atoms were achieved in elemental bromine vapour under low-pressure plasma conditions using low-energetic inductively coupled radio-frequency plasma excitation. The time scale of the bromination progress was found to increase with the specific surface area of the material. The results of plasma-chemical bromination are compared to that of wet-chemical electrophilic addition reactions, which achieved 28 Br per 100 C for MWCNT. The resulting C-Br bonds from plasma-chemical processing of graphitic materials were shown to be well-suited for hydroxyl functionalization and for grafting of organic molecules by nucleophilic substitution with 1,6-diaminohexane and (3-aminopropyl)triethoxysilane.