Modified Carbon Nanofibers Research Articles

Enhanced Pt performance with H2O plasma modified carbon nanofiber support

The insufficient durability and catalytic activity in low loading of platinum (Pt) are main obstacles to the development of low-temperature fuel cells. Our study demonstrated an efficient way to simultaneously improve the durability and electro-catalytic activity of Pt catalysts on carbon supports by water vapor (H2O) plasma functionalization. We report the finding that H2O plasma modification can introduce hydroxyl groups on carbon nanofiber (CNF) surface, and at the same time, highly preserve the microstructure of carbon support. Pt/CNF-H2O electrode possesses ultra-low Pt loading and superior electro-catalytic activity, poisoning-resistance ability and stability, suggesting a good candidate for fuel cell applications.

Hollow carbon nanofibers as an effective electrode for brackish water desalination using the capacitive deionization process

Capacitive deionization (CDI) is strongly recommended as an environmentally friendly and economical technique for removing salt ions from saline water. In this study, highly efficient hollow carbon nanofiber electrodes for capacitive deionization were prepared using co-axial electrospinning of poly(methyl methacrylate) (core) and poly(acrylonitrile) (shell) polymer solutions, followed by oxidative stabilization and then carbonization. The morphology, pore structure and electrochemical performance were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), nitrogen adsorption–desorption isotherms, and cyclic voltammetry, respectively. The synthesized hollow carbon nanofibers had a specific capacitance of 222.3 F g−1, which is almost 4 times higher than the corresponding value for solid carbon nanofibers (63 F g−1). Moreover, the surface area of the hollow nanofibers (186 m2 g−1) was 10 times greater compared to the surface area of solid carbon nanofibers (17.7 m2 g−1). Accordingly, the salt ion electrosorption capacity of the modified carbon nanofibers was greatly enhanced; the hollow nanofibers exhibited an excellent desalination performance (∼86%) and a better cycling ability. These properties are attributed to the hollow structure. Overall, the proposed modification to carbon nanofibers makes them adequate not only for use as promising electrodes for the CDI process but also for any application requiring carbonaceous materials with a high specific surface area.

Effective Modified Carbon Nanofibers as Electrodes for Capacitive Deionization Process

Carbon materials have the advantages of good electrical conductivity and excellent chemical stability, so many carbon materials have been introduced as electrodes for the capacitive deionization (CDI) process. Due to the low surface area compared to the other nanocarbonaceous materials, CNFs performance as electrode in the CDI units is comparatively low. This problem has been overcome by preparing high surface area carbon nanofibers and by creating numerous long pores on the nanofibers surface. The modified CNFs have been synthesized using low cost, high yield and facile method; electrospinning technique. Stabilization and graphitization of electrospun nanofiber mats composed of polyacrylonitrile (PAN) and poly (methyl methacrylate) (PMMA) leads form longitudinal pores CNFs. The utilized characterizations indicated that the CNFs obtained from electrospun solution having 50% PMMA have surface area of 181 m2/g which are more than the conventional CNFs. Accordingly, these nanofibers revealed salt removal efficiency of ~90% and specific capacitance of 237 F/g.

Improving toughness of ultra-high molecular weight polyethylene with ionic liquid modified carbon nanofiber

Ionic liquids (ILs) with long alkyl substituted groups, including 1-docosanyl-3-methylimidazolium bromide (IL-1) and 1-docosanyl-3-methylimidazolium hexafluorophosphate (IL-2), were synthesized and used to modify the surface of carbon nanofibers (CNF). The nanocomposite film prepared by solution-blending of ionic liquid modified CNF (i-CNF) and ultrahigh molecular weight polyethylene (UHMWPE) displayed better toughness when compared with pure UHMWPE even at very low concentrations (e.g. 0.4 wt%). The effect of ionic liquids on the elongation-to-break ratio of this nanocomposite system was investigated. The ionic liquid with hexafluorophosphate as the anion was more efficient to increase the toughness of UHMWPE due to the improved compatibility of IL with UHMWPE in the polymer matrix than that of the bromide. The rheological behavior of molten nanocomposites revealed that the storage modulus and the complex viscosity decreased with increasing ionic liquid content in the high frequency region. However, a reverse trend was observed when the frequency was less than 0.05 s−1. In-situ monitoring in the change of crystallinity of the nanocomposite during tensile deformation suggested a mechanism of sliding between UHMWPE crystal regions and the surface of carbon nanofibers.

Hydrogenation of unsaturated compounds in the presence of palladium-containing modified carbon nanofibers

Palladium-containing carboxylated carbon nanofibers were studied as catalysts for hydrogenation of double bond >C=C< in olefins, unsaturated alcohols, and acids, as well as for hydrogenation of nitroarenes. The developed catalyst is 7 times more efficient than the industrial analog (Pd/C).

Improvement of the hydrophilicity of electrospun porous carbon nanofibers by grafting phenylsulfonic acid groups

Electrospun porous carbon nanofibers fabricated by electrospinning and steam activation were functionalized by spontaneous grafting of phenylsulfonic acid (PSA) groups. SEM and Raman observation suggested that modified electrospun nanofibers did not exhibit distinct changes of surface morphology and intrinsic graphitic structure. Nitrogen adsorption showed decrease in surface area and pore volume of modified carbon nanofibers. XPS and elementary analysis were conducted to characterize the surface chemistry of carbon nanofibers. The hydrophilic character was measured by the contact angles and static adsorption isotherms of water. The adsorption isotherms of benzene and butanone showed that the increasing hydrophilicity led to enhancement of adsorption performance for butanone on carbon nanofibers, indicating that the adsorption selectivity to hydrophilic organic compounds was improved.

Hydrogenation catalyst based on modified carbon nanofibers

The aim of this work was to study the palladium-carboxylated carbon nanofibers (CNF) as a catalyst for the hydrogenation of nitrobenzene model reaction. It is shown that the efficiency of the catalyst obtained more than 6 times higher than that of the industrial counterpart (Pd/C).

2-Methyl oxazoline-grafted carbon nanofibers: preparation, characterization and their role in elastomeric actuators

Convex surface modification of carbon nanofibers (CNFs) through covalent functionalization plays a pivotal role in the world of polymeric nanocomposites. The free radical ‘grafting from’ technique was successfully applied for the first time for synthesis of 2-methyl oxazoline-grafted carbon nanofibers (Oxa-g-CNF). The resulting Oxa-g-CNF was characterized by infrared spectroscopy, thermogravimetric analysis and wide angle X-ray studies. Moreover, the synthesised Oxa-g-CNF was utilized for the preparation of acrylic elastomer-based nanocomposite targeted for model elastomeric actuators by solution casting technique. The nanocomposite showed excellent increase in relative permittivity proportional to actuation pressure, when compared with pristine acrylic elastomer and commercial materials. The improvement of properties was ascribed to excellent interaction between modified carbon nanofibers and polar group of acrylic elastomer. This technique adopted promises to open up a new avenue for facile preparation of acrylic elastomeric actuators.

Study of the Surface Chemistry of Modified Carbon Nanofibers by Oxidation Treatments in Liquid Phase

Carbon nanofibers (CNF) were grown by thermocatalytic decomposition of methane. Their texture and surface chemistry were modified by different oxidation treatments with HNO3 at different concentrations or a mixture of HNO3-H2SO4 to optimise their ability of dispersing active metal particles, because this material will be used as electrocatalytic support for polymeric electrolyte fuel cells. The effect of liquid phase oxidation on the surface chemistry and the textural properties of the CNF was studied by temperature programmed desorption (TPD), scanning electron microscopy (SEM) and N2-physisorption. Moreover, their thermal stability was studied by temperature programmed oxidation (TPO). During oxidation treatments functional groups were created and their number was function of the oxidation treatment conditions. Results indicated that an increase in severity of the oxidation treatment produces an increase in the number of surface oxygen groups and in the thermal stability. However, a very severe treatment can destroy partially the structure of carbon nanofibers.

Organic functionalization of carbon nanofibers for composite applications

AbstractThe present work reports the study of the effect of different chemical functionalization methods on the interfacial characteristics of the polymer composites formed. The chemically modified carbon nanofibers were blended with polypropylene (PP) and PP modified with maleic anhydride on a mini twin‐screw extruder. The functionalization methods were designed for minimum fiber damage and enhanced fiber/matrix interactions by chemical reaction of the functional groups introduced at the nanofibers surface with the maleic anhydride grafted at the PP. The degree of nanofiber functionalization was studied using thermogravimetric analysis. The composites were analyzed for nanofiber dispersion, mechanical properties, and electrical resistivity. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers

Deposition of Au colloids on plasmachemically modified carbon nanofibers

The surface properties of fishbone (FB) carbon nanofibers (CNFs) were altered. This was achieved by H 2O-plasma treatment (FB-plasma) and oxidation in HNO 3 (FB-HNO 3). By zeta-potential measurements, the surfaces of as-grown and acid treated CNFs were found to be negatively charged at all pHs. Meanwhile, basic entities were introduced by H 2O-plasma treatment, indicated by an isoelectric point of pH 4.6. Ultraviolet–visible measurements indicated the successful deposition of negatively charged Au colloids on FB-plasma, and incomplete deposition on FB-HNO 3. The nature of the basic sites accommodating colloid deposition is discussed based on X-ray photoelectron spectroscopy (XPS) results. The XPS study indicated significant differences in the type and amount of surface groups. Also, a variation in the fraction of defect and/or edge sites was found. The possible sites responsible for colloid immobilization are discussed based on the XPS results. Plasma treatment is demonstrated as a simple and effective procedure for altering the surface characteristics of CNFs.

Use of a microcolumn packed with modified carbon nanofibers coupled with inductively coupled plasma mass spectrometry for simultaneous on‐line preconcentration and determination of trace rare earth elements in biological samples

In this work, a new method was developed for the determination of trace rare earth elements (REEs) in biological samples by inductively coupled plasma mass spectrometry (ICP-MS) after preconcentration on a microcolumn packed with modified carbon nanofibers (CNFs). CNFs oxidized with nitric acid have been proved to possess an exceptional adsorption capability for REEs due to their surface functionalization. The effects of the experimental parameters, including pH, sample flow rate and volume, elution solution and interfering ions, on the recoveries of the analytes have been investigated systematically. A 100-fold enrichment factor was obtained. The adsorption capacity of CNFs was found to be 18.1, 19.3, 23.6, 17.6, 22.3 and 19.5 mg/g for La, Ce, Sm, Eu, Dy and Y, respectively. Under the optimum conditions, the detection limits of this method ranged from 0.2 pg/mL (Dy) to 1.2 pg/mL (Ce) with an enrichment factor of 15-fold, and the relative standard deviations (RSDs) for the determination of REEs at the 1.0 ng/mL level were less than 4% (n = 9). This method was applied to the analysis of trace REEs in a real sample of human hair with recoveries of 95-115%. In order to validate the proposed method, a certified reference material of human hair (GBW 07601) was analyzed with satisfactory results.

Relationship between structure and dynamic mechanical properties of a carbon nanofiber reinforced elastomeric nanocomposite

The tensile and dynamic mechanical properties of a nanocomposite, containing modified carbon nanofibers (MCNFs) homogenously dispersed in an elastomeric ethylene/propylene (EP) copolymer semicrystalline matrix (84.3 wt% P), have been correlated with the structure development. These properties were characterized by in situ synchrotron X-ray diffraction during stretching, dynamic mechanical analysis and X-ray analysis techniques over a wide temperature range. Upon sequential drawing, the tensile strength of the nanocomposite film was notably higher than that of the unfilled polymer even though both samples exhibited a similar amount of crystal fraction and the same degree of crystal orientation, revealing the effect of nanofiller reinforcement in the semicrystalline matrix. The mechanical spectra of the 10 wt% MCNF filled samples in both stretched and non-stretched states showed broadening of the elastic modulus at high temperatures, where the corresponding crystallinity index also decreased. It is conceivable that a significant fraction of chain orientation is induced in the vicinity of the nanofillers during stretching, and these stretched chains with reduced mobility significantly enhance the thermal mechanical properties.

Improving the Mechanical Properties of Polycarbonate Nanocomposites with Plasma‐Modified Carbon Nanofibers

The mechanical properties of polycarbonate film embedded with carbon nanofibers were studied based on plasma surface modification of carbon nanofibers by the use of polystyrene. The nanofiber surfaces were modified by various processing conditions including plasma polymerization power, nanofiber concentration, and ultrasonication time. The tensile strength and Young's modulus of the carbon nanofiber‐polycarbonate composites were then measured. The mechanical behavior of the composite was found to be affected by dispersion of the nanofibers. Higher plasma power resulted in improved mechanical strength. A maximum strength (10% increase) was achieved at a low concentration (1 wt.%) of nanofibers. The optimization of ultrasonication time indicated that the maximum strength occurred at different times for the composites with different concentrations of the modified carbon nanofibers.

Shear‐Induced Orientation and Structure Development in Isotactic Polypropylene Melt Containing Modified Carbon Nanofibers

The shear‐induced crystallization behavior of isotactic polypropylene (iPP) nanocomposite melt containing modified carbon nanofibers (MCNFs) was investigated by rheo‐SAXS (small‐angle X‐ray scattering) and rheo‐WAXD (wide‐angle X‐ray diffraction) techniques using synchrotron radiation. Under quiescent conditions, the nucleating effect of MCNFs on crystallization of iPP was pronounced and the system exhibited a remarkably low saturation point (ca. 0.05 wt% of MCNF). In‐situ SAXS and WAXD results showed the development of shear‐induced crystalline structures and lamellar morphology in nanocomposite melts. Under the same shear conditions, the filled system exhibited notably faster kinetics compared with the unfilled system. The oriented crystalline fraction was found to decrease with the MCNF loading, indicating the competition between oriented crystals (induced by shear) and unoriented crystals (due to the nucleating effect of MCNF). At the early stages of crystallization, the amount of the oriented crystals increased with the MCNF concentration, suggesting that the nanofiller hindered the motion of polymer chains after the cessation of flow resulting in the delayed relaxation of stretched polymer segments. Dedicated to Prof. Phillip H. Geil's seventy‐fifth birthday.

Rheological study of carbon nanofiber induced physical gelation in polyolefin nanocomposite melt

The rheological behavior of nanocomposites based on elastomeric ethylene–propylene (EP) random copolymer (84.3 wt% P) and well-dispersed modified carbon nanofibers (MCNFs) with concentrations from 0.5 to 20 wt% were studied by oscillatory-shear rheometry at varying temperatures. At relatively low temperatures, the entanglement density of the polymer chains appeared to increase with filler concentration, ensuring strong MCNF–polymer interactions. At elevated temperatures, pronounced deviations from the ideal melt behavior in the low frequency domain (i.e. positive slopes of the tan δ( ω) curves) were observed, indicating the formation of a three-dimensional percolated network. Following the Winter–Chambon criterion, the transition from pseudo-solid-like to liquid-like behavior (i.e. the critical gel point) upon cooling was determined. The physical gelation induced by MCNFs is a thermo-reversible phenomenon and its origin can be traced to the interactions of nanofillers, where the percolation temperature decreases with filler concentration. Abrupt changes in the critical gelation temperature, the stiffness of the gel and the relaxation exponent were observed in nanocomposites with MCNF loading above 10 wt%. This behavior indicates a change of the mechanism of physical gelation at high MCNF loadings that can be explained by the concept of bridge formation of polymer segments between two adjacent nanofillers in concentrated nanocomposites.

Deposition Precipitation for the Preparation of Carbon Nanofiber Supported Nickel Catalysts

Deposition precipitation of nickel hydroxide onto modified carbon nanofibers has been studied and compared to deposition onto silica. The carbon nanofiber support materials consisted of graphite-like material of the fishbone-type with a diameter of 20-50 nm and a specific surface area of 150 m2/g. Modification involved surface oxidation (CNF-O) optionally followed by partial reduction (CNF-OR) or thermal treatment (CNF-OT). Titration of the support materials showed the presence of 0.17 and 0.03 mmol/g carboxylic acid groups for CNF-O and CNF-OR, respectively. For the CNF-OT only basic groups were present. The deposition precipitation of 20 wt % nickel onto these supports has been studied by time dependent pH and nickel loading studies. With silica, nickel ion adsorption did not occur prior to nucleation of the nickel hydroxide phase at pH = 5.6. With CNF-O, nickel ion adsorption took place right from the start of the deposition process at pH = 3.5, and at pH = 5.6 already 4 wt % nickel was adsorbed. Nucleation of nickel hydroxide onto adsorbed nickel ion clusters proceeded subsequently. Characterization of the dried Ni/CNF-O samples with TEM and XRD showed well dispersed and thin (5 nm) platelets of nickel hydroxide adhering to the carbon nanofibers. After reduction at 773 K in hydrogen the Ni/CNF-O contained metallic nickel particles of 8 nm homogeneously distributed over the fibers. With CNF-OR and CNF-OT, precipitation of large platelets (> 500 nm) separate from the support took place. Clearly, the presence of carboxylic acid groups is essential to successfully deposit nickel hydroxide onto modified carbon nanofibers.

Uniaxial deformation of an elastomer nanocomposite containing modified carbon nanofibers by in situ synchrotron X-ray diffraction

Structure and property of a nanocomposite consisting of modified carbon nanofibers (MCNFs), homogenously dispersed in an elastomeric ethylene/propylene (EP) random copolymer (84.3 wt% P) matrix, were studied by in situ synchrotron X-ray diffraction during uniaxial deformation. The MCNF acted as a nucleating agent for crystallization of the α-form of isotactic polypropylene (iPP) in the matrix. During deformation at room temperature, strain-induced crystallization took place, while the transformation from the γ phase to α phase also occurred for both unfilled and 10 wt% MCNF-filled samples. The tensile strength of the filled material was consistently higher than that of pure copolymer. However, when compared with pure copolymer, the highly stretched nanocomposite exhibited a higher amount of unoriented crystals, a lower degree of crystal orientation and a higher amount of γ crystals. This behavior indicated that polymer crystals in the filled nanocomposite experienced a reduced load, suggesting an effective load transfer from the matrix to MCNFs. At elevated temperatures, the presence of MCNFs resulted in a thermally stable physically cross-linked network, which facilitated strain-induced crystallization and led to a remarkable improvement in the mechanical properties. For example, the toughness of the 10 wt% nanocomposite was found to increase by a factor of 150 times at 55 °C.

In-Situ X-ray Scattering Studies of a Unique Toughening Mechanism in Surface-Modified Carbon Nanofiber/UHMWPE Nanocomposite Films

The toughening mechanism of nanocomposite films comprising ultrahigh molecular weight polyethylene (UHMWPE) and modified carbon nanofiber (MCNF) was investigated by in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques during uniaxial stretching. Surface modification of carbon nanofibers included oxidation and subsequent chemical reaction with octadecylamine. At room temperature, the toughness of melt-pressed nanocomposite films was found to increase over 10 times by addition of 0.2 and 5 wt % of MCNF compared to that of pure UHMWPE. WAXD and SAXS results indicated that MCNF acted as a solvent carrier in the stiff UHMWPE matrix, whereby the grafted short hydrocarbon chains (n = 18) plasticized the surrounding UHMWPE chains in the nanoscale vicinity (10−20 nm) and induced interfacial flow under stretching, resulting in a large elongation-to-break ratio (>500%). A martensitic crystal transformation in UHMWPE was detected in all samples during deformation,...

In situ synchrotron SAXS/WAXD studies during melt spinning of modified carbon nanofiber and isotactic polypropylene nanocomposite

The structural development of a nanocomposite, containing 95 wt% isotactic polypropylene (iPP) and 5 wt% modified carbon nanofiber (MCNF), during fiber spinning was investigated by in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. The modification of carbon nanofibers (CNFs) was accomplished by a chemical surface treatment using in situ polymerization of olefin segments to enhance its compatibility with iPP, where the iPP/MCNF nanocomposite was prepared by twostep blending to ensure the dispersion of MCNF. X-ray results showed that at low spin-draw ratios, the iPP/MCNF nanocomposite fiber exhibited much higher iPP crystalline orientation than the control iPP fiber. At higher spin-draw ratios, the crystalline orientation of the nanocomposite fiber and that of the pure iPP fiber was about the same. The crystallinity of the composite fiber was higher than that of the control iPP fiber, indicating the nucleating effect of the modified carbon nanofibers. The nanocomposite fiber also showed larger long periods at low spin-draw ratios. Measurements of mechanical properties indicated that the nanocomposite fiber with 5 wt% MCNF had much higher tensile strength, modulus and longer elongation to break. The mechanical enhancement can be attributed to the dispersion of MCNF in the matrix, which was confirmed by SEM results.