Microstructure Of Carbon Nanofibers Research Articles

Correlation between microstructure and surface chemistry of carbon nanofibers grown using different adhesive layers

Carbon nanofibers (CNFs) have applications in a wide range of technological and scientific fields. The connections between their micro- and macrostructure and observed performance are, however, currently lacking. This hinders the realization of their full potential. In this paper, we correlate the microstructure of CNFs grown on two types of substrates: (1) Si + 20 nm Ti + 20 nm Ni, and (2) Si + 80 nm Cr + 20 nm Ni, to their surface chemistry. We use transmission electron microscopy (TEM), supported by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis, to describe the morphology and structure of CNFs as well as the underlying interfacial layers. Then, we study the similarities and differences in chemistry of these two types of CNFs using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) and correlate them with the observed structural features of the fibers. Vertically aligned, tip-type fiber growth was observed on both substrates. TEM micrographs show that the CNFs grown on the Cr + Ni substrates have a slightly distorted herringbone-like structure, whereas fibers grown on the Ti + Ni substrates have relatively ill-defined structure with basal planes pointing outwards. Consequently, the latter possess a richer surface chemistry, which is apparent from the wider peaks and more spectral features observed during XAS and XPS measurements. This analysis provides us with some of the missing structure-chemistry connections, which can subsequently be expanded towards including correlations of these features with observed performance of the CNFs in different applications. Ultimately, this enables us to tailor features of the CNFs for specific target fields.

Catalytic graphitization of bacterial cellulose–derived carbon nanofibers for stable and enhanced anodic performance of lithium-ion batteries

Bacterial cellulose (BC) produced through microbial fermentation has emerged as a viable precursor for carbon nanofibers (CNF) anode used in lithium-ion batteries. However, the low capacity and fading behavior of BC-derived CNFs render their usage in its pure form. Tuning the microstructure of CNFs in such cases plays an essential role in overcoming these negative ramifications and improves battery performance. In this study, the fermentation media used for BC production is modified by the addition of an iron catalyst, which can induce graphitization in the derived CNFs. Pure BC and catalyst-incorporated BC are pyrolyzed at 900 °C and 1800 °C to obtain CNFs, and the properties of derived CNFs are compared for understanding the role of incorporated catalyst. The structural, morphological, and electrochemical properties of CNFs are analyzed through X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, impedance spectroscopy, galvanostatic charge-discharge studies, and cyclic voltammogram studies. By possessing a higher graphitic content, catalyst-incorporated BC–derived CNFs exhibit an enhanced rate performance with a reversible capacity of 529 mAh g−1 after 100 continuous charge/discharge cycles at a current density of 0.2C.

The formation of highly ordered graphitic interphase around embedded CNTs controls the mechanics of ultra-strong carbonized nanofibers

Templating graphitization, i.e., the transformation of certain polymers to highly-ordered graphitic (HOG) domains upon pyrolysis in the vicinity of graphitic nanomaterials, such as carbon nanotubes (CNTs), is known to be an effective approach to modify the microstructure of carbon nanofibers (CNFs). In this work, the microstructure of CNFs subjected to the templating effect of functionalized single-walled CNTs (f-SWNTs) and the effect of templating on mechanical properties of CNF/f-SWNTs hybrid nanofiber are studied. The CNF/f-SWNTs were fabricated via pyrolysis of electrospun polyacrylonitrile precursors with CNT inclusions. Prior to pyrolysis, the precursors were subjected to thermomechanical treatments, known as hot-drawing, to enhance chain and CNT alignment and packing. The study of the microstructure of the precursor and CNFs indicates the crucial role of precursor hot-drawing in enhancing the microstructure of the precursor and CNFs, leading to drastically enhanced templating effect, as evidenced from the thickness of the HOG that forms around CNTs. Mechanical tests on single nanofibers using custom-designed microdevices led to the realization that the templating effect of CNTs on CNFs, when properly implemented via precursor hot-drawing, can considerably increase the strength of CNFs. The average tensile strength and modulus of CNF/f-SWNTs in which HOG domains had clearly formed were measured to be 7.6 and 268 GPa, respectively, which are the highest value reported to date among similar types of materials. The existence and evolution of the HOG around CNTs inside CNFs and mechanical reinforcing of HOG were thoroughly discussed in conjunction with finite element models of building blocks of CNFs, alluding to the stress fields around HOG and CNTs in the CNF. The high-performance 1-D hybrid graphitic nanostructure developed here, CNF/f-SWNTs, can serve as an outstanding reinforcement material for weight sensitive applications.

Study of Microstructure Change of Carbon Nanofibers as Binder-Free Anode for High-Performance Lithium-Ion Batteries.

Flexible and binder-free film of N, O-doped carbon nanofibers (CNFs) is the ideal anode for high-energy-density batteries. Here, CNFs flexible films which the N, O dopant give defect in graphite structure results in high specific surface area more than 500 m2 g-1. A flexible film of CNF800 carbonized at 800 °C delivers initial capacities of 2000 and 755 mAh g-1 at the current densities of 5 and 10 A g-1, respectively. After 500 cycles, CNF800 remains the capacities of 1251, 865, 702, and 305 mAh g-1 at 0.5, 1, 5, and 10 A g-1, respectively. The microstructures of CNFs under various state of charge are studied by HRTEM, XPS, 13C NMR, and so forth. The lithiation/delithiation mainly happens to the interlayer of graphite domain of CNFs. The dopants of nitrogen and oxygen involve in lithiation, but much of Li-N is irreversible. The excellent performances of CNFs film can be attributed to the N, O doped structure of graphite domain that has increased the conductivity and lithium storage ability. Further development of N, O doped CNFs may enable practical applications as flexible anode in high-performance lithium-ion batteries.

Strong magnetic field-assisted growth of carbon nanofibers and its microstructural transformation mechanism.

It is well-known that electric and magnetic fields can control the growth direction, morphology and microstructure of one-dimensional carbon nanomaterials (1-DCNMs), which plays a key role for its potential applications in micro-nano-electrics and devices. In this paper, we introduce a novel process for controlling growth of carbon nanofibers (CNFs) with assistance of a strong magnetic field (up to 0.5 T in the center) in a chemical vapor deposition (CVD) system. The results reveal that: 1) The CNFs get bundled when grown in the presence of a strong magnetic field and slightly get aligned parallel to the direction of the magnetic field; 2) The CNFs diameter become narrowed and homogenized with increase of the magnetic field; 3) With the increase of the magnetic field, the microstructure of CNFs is gradually changed, i.e., the strong magnetic field makes the disordered “solid-cored” CNFs transform into a kind of bamboo-liked carbon nanotubes; 4) We propose a mechanism that the reason for these variations and transformation is due to diamagnetic property of carbon atoms, so that it has direction selectivity in the precipitation process.

Piezoresistive effect of individual electrospun carbon nanofibers for strain sensing

Piezoresistive behavior of individual electrospun carbon nanofibers (CNF) was studied for the first time via a microelectromechanical systems platform. The gage factor of CNFs was found to vary from 1.96 to 2.55, not correlating with nanofiber diameter. The measured strain sensitivity of electrical resistance of individual CNFs could not be solely explained based on strain induced dimensional changes of CNFs, pointing to piezoresistivity in nanofibers. The microstructure of CNFs was studied via TEM imaging and Raman spectroscopy, suggesting the presence of sp2 and sp3 hybridized carbon atoms in CNFs. The piezoresistivity of CNFs was explained in light of their hybrid structure. A one-dimensional model was adopted to relate CNFs piezoresistivity to their microstructure and electron tunneling between sp2 hybridized regions through sp3 hybridized regions. The calibrated model revealed tunneling distances of 0.15–0.3nm between sp2 hybridized atoms. Moreover, our study pointed to the degree of graphitization and elastic mismatch between differently hybridized carbon atom regions in CNFs as critical parameters controlling CNFs’ piezoresistivity. This study sets the stage for the utilization of CNFs, not just as load bearing elements, but also as multifunctional nanoscale components with strain sensing capabilities, for instance in Nanoelectro-mechanical systems applications.

Porosity and pore size distribution measurement of cement/carbon nanofiber composites by 1H low field nuclear magnetic resonance

The dispersion effect of carbon nanofibers (CNFs) in aqueous solution and the mechanical properties, porosity, pore size distribution and microstructure of CNFs reinforced cement-based composites were investigated in this paper. To achieve effective dispersion of CNFs, a method utilizing ultrasonic processing and a commercially surfactant were employed. CNFs were incorporated to cementitious materials with the addition of 0.1 wt% and 0.2 wt% of cement with a water/cement ratio of 0.35. The mechanical properties of CNFs/cement composites were analyzed, the porosity and pore size distribution were characterized by 1H low field nuclear magnetic resonance (NMR), and the microstructure was observed by scanning electron microscopy (SEM). The results indicate that the optimum concentration ratio of MC to CNFs is 2:1 for dispersing in aqueous solution. Moreover, in the field of mechanical properties, CNFs can improve the flexural strength and compressive strength. The increased mechanical properties and the decreased porosity of the matrices correspond to the increasing CNFs content and CNFs act as bridges and networks across cracks and voids.

Formation of Carbon Nanofibers from Supported Pt Catalysts through Catalytic Chemical Vapor Deposition from Acetylene

Carbon nanofibers (CNFs) were synthesized by catalytic chemical vapor deposition from acetylene from a series of supported Pt catalysts derived from Pt-containing Mg–Al layered double hydroxide precursors. The materials were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, temperature programmed reduction, low-temperature N2 adsorption–desorption experiments, X-ray photoelectron spectroscopy, and Raman spectroscopy. The effects of reaction temperature and Pt content on the morphologies and microstructures of CNFs were investigated. The results revealed that the reaction temperature of 600 °C was appropriate for the growth of uniform CNFs with regular shape. Furthermore, the structural defects and the diameters of CNFs were reduced with the increasing Pt content, which is attributable to the high dispersion of smaller Pt nanoparticles as well as the quick deposition rate of carbon atoms on active metal particles. The present work developed an additional approach to optimize the growth of CNFs.

Effect of growth of carbon nanofibers on the electrical conductivity of carbon fibers

Recently, research on various carbon materials such as carbon fibers (CFs), carbon black (CB), carbon nanofibers (CNFs), carbon nanotubes (CNTs), fullerenes, and graphene has received considerable attention. In particular, CFs that exhibit a graphite-layered structure have a strong structural anisotropy, and the alignment of CFs along the direction of an applied electrochemical application field (antistatic material, electromagnetic interference shielding material, sensing elements, heating elements, and packing of electronic components and equipment) has been attempted by utilizing their mechanically strong, durable and electrically conductive properties. Most recently, there have been many studies of the improvement of electrical conductivity of CFs. Its electrical conductivity strongly depends on the thermal treatment, microtexture, hybridization, surface treatments, content of heteroatoms, length. A number of studies have shown that the various methods for the enhancement of electrical conductivity of CFs such as plating with conductive metals electrochemical or plasma oxidation, fluorination, and the growth of carbon nanomaterials on its surfaces, etc. For example, Park et al. prepared the activated carbon fibers (ACFs) coated with CNFs for enhancement electrical conductivity of ACFs due to the CNFs have unique electrical properties. Jiang et al. reported that the use of Ag as a metallic film is coated onto the surface of chopped CFs or ACFs to increase the conductivity of conductive fillers. Zeng et al. synthesized the CNTs on CFs substrate by the floating catalyst method for field electron emission material. The catalytic chemical vapor deposition (CCVD) was a most common method for the synthesis of carbon nanomaterials on the different substrates. However, there are some problems in the synthesis of carbon nanomaterials (CNFs or CNTs) on CF surfaces by CCVD method: (a) the ununiformly dispersed-metal catalysts (such as Fe, Ni, and Co catalyst) on CF surfaces due to its low specific surface area; (b) the metal catalysts were easily diffused into the CFs; (c) the metal catalysts were easily aggregated into large particles and formed mixed phases of carbon materials (CNFs, CNTs, and amorphous carbon); (d) the products require the purity process. So, the uniform dispersion of catalysts is a critical process to get uniformly grown CNFs on CF surfaces. As few research reported, the mesoporous silica films are excellent catalyst supports and have been used in the growth of carbon nanomaterials. Zheng et al. reported that ordered meso silica films can direct the growth of CNTs from perpendicular to parallel to the substrate surface through control of the distribution of catalysts. In this work, we have synthesized CNFs on CF surfaces by using Ni-doped mesoporous silica film at different temperatures, and the density of CNFs on CF surfaces is uniform and densely. The Ni-doped mesoporous silica film was used to control the distribution of catalysts on CF surfaces. We examined the microstructure of CNFs coated CFs, physical properties, electrical conductivity, of the paper.

Effect of carbon nanofibers microstructure on electrocatalytic activities of Pd electrocatalysts for ethanol oxidation in alkaline medium

Pd electrocatalysts supported on three types of carbon nanofibers (CNFs), viz. platelet CNFs (p-CNFs), fish-bone CNFs (f-CNFs) and tubular CNFs (t-CNFs) are prepared and the effect of CNFs microstructure on the activities of the electrocatalysts for ethanol oxidation reaction (EOR) is investigated. The information on structural characteristics is obtained by high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. Electrochemical techniques are employed to characterize the microstructure effect of CNFs on the catalytic activities of catalysts. HRTEM images indicate the microstructure of CNFs has a powerful influence on the distribution of Pd particles. The results of the electrochemical characterization also indicate that the structure of CNFs significantly influences the catalytic activities of the electrocatalysts and p-CNFs supported Pd electrocatalyst has the best performance for ethanol oxidation in an alkaline medium because p-CNFs has the highest ratio of edge atoms to basal atoms and correspondingly the fastest electrode kinetics and strongest Pd–CNFs interaction.

Effect of nitrogen in the reaction atmosphere on the microstructure of carbon nanofibers grown by thermal chemical vapor deposition

Carbon nanofibers (CNFs) with different microstructures were synthesized by thermal chemical vapor deposition using different growth temperatures and methane/nitrogen gas mixtures. High-resolution transmission electron microscopy images revealed that bamboolike structure could be formed both by increasing the growth temperature and by increasing the nitrogen content in the reaction atmosphere at a lower growth temperature. Elemental analysis results indicated that no significant change in the nitrogen concentration was found regardless of the increase of nitrogen flow in the feed gas. The formation of bamboolike structure of CNFs and the effect of nitrogen gas on the microstructure change of CNFs were discussed.

Catalytic Vapor Decomposition of Methane over Nickle Catalyst: Growth Rate and the Corresponding Microstructures of Carbon Nanofibers

Time-resolved intrinsic reaction rate of catalytic vapor deposition of methane into carbon nanofibers (CNFs) and hydrogen on supported Ni catalysts was determined under different reaction temperatures (773–973 K) and feed compositions (PH2 = 0–0.29 atm, PCH4 = 0:28–0.71 atm) and the microstructure of the produced CNFs was characterized by Raman and TEM. The results showed that, increasing the reaction temperature, methane partial pressure and decreasing hydrogen partial pressure increased the initial reaction rate but generally led to fast catalyst deactivation. A high carbon yield per unit catalyst was obtained as a compromise between initial reaction rate and catalyst deactivation rate. The microstructures of the produced CNFs had a close relationship with the initial reaction rate. The CNFs produced with high initial rate tended to have a small angle between the graphene layers and the growth axis, thin diameter and a hollow core. This phenomenon can be used for monitoring and controlling the CNFs synthesis process.

CNFs-supported Pt catalyst for hydrogen evolution from decalin

Abstract Carbon nanofibers (CNFs) with different microstructures were synthesized as the support of Pt catalyst for decalin dehydrogenation to evolve pure hydrogen. The results show that in a liquid-film state at the temperature of 240 °C for reactor external heating, 5 wt% Pt catalyst supported on platelet CNFs gives a TOF of 732.4 mol H 2 /mol metal in 2 h. The microstructure of CNFs and the metal particle size have great impact on the performance of the catalyst for the reaction.

Oxygen reduction reaction properties of carbon nanofibers: Effect of metal purification

Carbon nanofibers (CNFs) are grown on metal catalysts and electrochemical treatment is used to remove the metal catalyst residuals from the as-grown CNFs. For comparison, the CNFs are also purified by a chemical method and a thermal method. The oxygen reduction reaction (ORR) properties of CNFs purified by these three methods are examined by cyclic voltammetry. CNFs treated by the electrochemical method have a more positive ORR onset reduction potential and peak potential compared with those treated by chemical and thermal methods, and this is because the microstructures of CNFs are less changed by electrochemical method. However, they have a lower electrochemical capacity and ORR peak current than those treated by the chemical method. Cyclic voltammetric measurements at different scan rates confirm that the oxygen reductions on CNFs treated by electrochemical and chemical methods are controlled by diffusion, while on CNFs treated by thermal method is partially influenced by diffusion.

Microstructure effect of carbon nanofiber on electrocatalytic oxygen reduction reaction

Carbon nanofibers (CNFs) with controlled microstructures, i.e. platelet CNF (p-CNF), fish-bone CNF (f-CNF) and tube CNF (t-CNF), are synthesized, and their behaviors in electrocatalytic oxygen reduction reaction (ORR) in acid media are investigated in this paper. The physico-chemical properties of the CNFs are characterized by high resolution transmission electron microscope (HRTEM), N 2 adsorption–desorption and Raman spectrum. Cyclic voltammetry experiments show that the CNFs have higher ORR activities than graphite. The p-CNF, which has the highest ratio of edge atoms to basal atoms, demonstrates the most positive ORR onset potential and ORR peak potential. The f-CNF, which has the largest amounts of ORR active sites, exhibits the highest ORR peak current. The t-CNF demonstrates the most negative ORR onset potential, negative ORR peak potential, and the least ORR peak current, which is a result of the fewest catalytic active sites. Furthermore, the microstructures of CNFs can impact the reaction process. The ORR on p-CNF or f-CNF is controlled by diffusion, while the ORR on t-CNF is jointly controlled by surface reaction and diffusion.

Catalytic Combustion Synthesis of Carbon Nanofibers

The catalytic combustion technique was used to synthesize carbon nanotubes and carbon nanofibers. In this paper, we report that carbon nanofibers were synthesized by ethanol catalytic combustion technique. The as-grown products were characterized by means of scanning electron microscopy, transmission electron microscopy, Raman spectroscopy. The results showed that the products have a mass of carbon nanofibers. However, morphology and microstructure of carbon nanofibers are affected by synthesis conditions, such as stability of flames and sampling time, sampling temperature etc. Different Influence factors were depicted in detail. Ethanol catalytic combustion technique offer a simple method to synthesize carbon nanotubes and carbon nanofibers, it also has some advantages, such as flexible synthesis conditions, simple setup, and environment-friendly.

Conformation and microstructure of carbon nanofibers deposited on foam Ni

Bundles of pure carbon nanofibers were prepared by catalytic decomposition of acetylene on foam Ni. The morphological and structural characteristics of the carbon nanostructures, in the as-prepared state, were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HTEM). A special conformation of carbon nanofibers composed of segmented structures was found among the products by both SEM and TEM observations. Further HTEM examination indicated that the segments were stacked with well ordered graphite platelets arranged perpendicular to the axis of the filaments.