Synthesis of uniform polycrystalline tin dioxide nanofibers and electrochemical application in lithium-ion batteries

Under optimized synthesis conditions, very large area uniform SnO₂ nanofibers consisting of orderly bonded nanoparticles have been obtained for the first time by thermal pyrolysis and oxidization of electrospun tin(II)2-ethylhexanoate/polyacrylonitrile (PAN) polymer nanofibers in air. The structure and morphology were elaborated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The SnO₂ nanofibers delivered a reversible capacity of 446 mAh g⁻¹ after 50 cycles at the 100 mA g⁻¹ rate and excellent rate capability of 477.7 mAh g⁻¹ at 10.0 C. Owing to the improved electrochemical performance, this electrospun SnO₂ nanofiber could be one of the most promising candidate anode materials for the lithium-ion battery.     

Structure and electrical conductivity of nitrogen-doped carbon nanofibers

The effect of nitrogen concentration in carbon nanofibers (CNFs) on the structural and electrical properties of the carbon material was studied. CNFs with nitrogen concentration varied from 0 to 8.2 wt.% (N-CNFs) with “herringbone” structure were prepared by decomposition of ethylene and ethylene mixture with ammonia over 65Ni-25Cu-10Al₂O₃ (wt.%) catalyst at 823 K. Detailed investigation of the CNFs and N-CNFs by XPS, FTIR and Raman spectroscopy showed that the nitrogen introduction in carbon material distorts the graphite-like lattice and increases the structure defectiveness. Both effects become more significant as the nitrogen concentration in N-CNFs grows.The electrical conductivity of N-CNFs with different nitrogen concentrations is caused by the competition of the nanofiber graphite-like structure disordering after introduction of nitrogen atoms and doping of an additional electron into the delocalized π-system of the graphite-like material. As a result, the maximum electrical conductivity among the samples studied was observed at nitrogen concentration in N-CNFs equal to 3.1 wt.%.     

Synthesis of nitrogen-containing carbon nanofibers by catalytic decomposition of ethylene/ammonia mixture

The formation of carbon nanofibers (CNFs) doped with nitrogen was investigated during decomposition of C₂H₄/NH₃ mixtures at 450-675 °C over metal catalysts: 90Ni-Al₂O₃, 82Ni-8Cu-Al₂O₃, 65Ni-25Cu-Al₂O₃, 45Ni-45Cu-Al₂O₃, 90Fe-Al₂O₃, 85Fe-5Co-Al₂O₃, 62Fe-8Co-Al₂O₃, 62Fe-8Ni-Al₂O₃. It was found that the yield of CNFs, their structural and textural properties, as well as nitrogen content in CNFs are strongly dependent on the synthesis conditions such as: catalyst used, feed composition, temperature and duration. The 65Ni-25Cu-Al₂O₃ was proved to be the most efficient catalyst for the production of nitrogen-containing carbon nanofibers (N-CNFs) with nitrogen content up to 7 wt.%. Ammonia concentration in the feed equal 75 vol.%, temperature 550 °C and duration 1 h were found to be the optimum reaction parameters to reach the maximum nitrogen content in N-CNFs. TEM studies revealed that the nanofibers have a helical morphology and a “herringbone” structure composed of graphite sheets. According to the XPS data, the nitrogen incorporation in the N-CNF structure leads to the formation of two types of nitrogen coordination: pyridinic and quaternary, and their abundance depends on the reaction conditions.     

Thermogravimetric studies of carbon nanofiber formation from methane at low temperature over Ni-based skeletal catalysts and the effect of substrate pre-carburization

Using thermogravimetry (TG) under conditions that minimize inhibition by the hydrogen produced, the intrinsic catalytic rates of skeletal Ni, pure and alloyed with solute metals Fe, Co, or Cu, were evaluated in methane decomposition to carbon nanofibers. In “standard” tests, i.e., after pre-reduction in H₂ and exposure to CH₄ directly at 450 °C, several catalysts reached stable activities exceeding 4 mg C/mg cat./h, comparable with literature values obtained at 500 °C or above. TG evidence is presented for partial bulk carburization of Ni in CH₄ below 350 °C, which leads to substantially increased coking rates. TEM evidence supports the view that carburization promotes catalyst particle disintegration, thereby inducing faster and more stable nanofiber growth. Irregularities in alloy response to carburization are interpreted in terms of the stability of the respective mixed-metal carbides. TEM also shows that alloying changes the metal nanocrystallite shape (habit), with consequences for the carbon nanofiber structure. Evidence for the easy dissociation of CH₄ is corroborated by direct catalyst activation in the absence of H₂. Reduction begins in pure hydrocarbon around 300 °C and leads to coking activities at 450 °C comparable to those for samples pre-reduced in H₂. Skeletal metal catalysts offer distinct advantages in low-temperature natural gas conversion.     

Electrocatalytic properties of Pt/carbon composite nanofibers

Pt/carbon composite nanofibers were prepared by electrodepositing Pt nanoparticles directly onto electrospun carbon nanofibers. The morphology and size of Pt nanoparticles were controlled by the electrodeposition time. The resulting Pt/carbon composite nanofibers were characterized by running cyclic voltammograms in 0.20 M H₂SO₄ and 5.0 mM K₄[Fe(CN)₆] + 0.10 M KCl solutions. The electrocatalytic activities of Pt/carbon composite nanofibers were measured by the oxidation of methanol. Results show that Pt/carbon composite nanofibers possess the properties of high active surface area and fast electron transfer rate, which lead to a good performance towards the electrocatalytic oxidation of methanol. It is also found that the Pt/carbon nanofiber electrode with a Pt loading of 0.170 mg cm⁻² has the highest activity.     

Structural changes in a nickel-copper catalyst during growth of nitrogen-containing carbon nanofibers by ethylene/ammonia decomposition

Structural changes in a 65Ni-25Cu-Al₂O₃ catalyst during the decomposition of a C₂H₄/NH₃ mixture at different stages of nitrogen-containing carbon nanofiber (N-CNF) synthesis were studied by X-ray diffraction (XRD) analysis, X-ray diffraction using anomalous scattering effect and extended X-ray absorption fine structure (EXAFS) spectroscopy. The N-CNF growth over a catalyst particle was supposed to proceed via the formation of an oversaturated solid solution of carbon and nitrogen in a nickel-enriched alloy “NiCu_xC_yN_z”. The latter resulted in an increase of the alloy lattice parameter to abnormally large values а = 3.616-3.706 Å without destruction of its cubic structure. The formation of the “NiCu_xC_yN_z” phase and its presence in the system coincide with the optimum time for the synthesis of N-CNFs with the highest nitrogen concentration and maximum values of specific surface area and total pore volume.     

Effects of precursor concentration on crystalline morphologies and particle sizes of electrospun WO3 nanofibers

Tungsten oxide (WO₃) nanofibers with different crystalline morphologies and various particle sizes were fabricated using an electrospinning technique. The nanofibers were prepared from mixtures of polyvinyl alcohol (PVA) and ammonium metatungstate hydrate (AMH) of various concentrations ranging from 4.2%w/v to 50.0%w/v. After calcination at 500 °C for 2 h, the nanofibers were observed to have a monoclinic crystal structure with diameters ranging from 30 to 250 nm. AMH concentration had a large influence on the resulting nanofiber morphology. Very low AMH concentration of 4.2%w/v led to the formation of WO₃ nanofibers having a very large area of monocrystalline structure. Higher AMH concentrations result in polycrystalline WO₃ nanofibers with joined nanoparticles along the fiber axis. The average particle size within the nanofibers increased from 29 to 66 nm as the AMH concentration increased from 8.3%w/v to 50%w/v. At these precursor concentration levels, primary particles were formed before PVA was completely burnt off, resulting in agglomeration of primary particles along the nanofiber axis.     

Plasma-assisted preparation of Fe-Cu bimetal catalyst for higher alcohols synthesis from carbon monoxide hydrogenation

Plasma-assisted Fe-Cu/SiO₂ catalysts were prepared by impregnation technique and characterized by X-ray diffraction (XRD), nitrogen adsorption and desorption isotherms, X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (H₂-TPR) techniques. Catalytic performances for carbon monoxide hydrogenation to higher alcohols were carried out in a fixed-bed reactor at the conditions of T = 300 °C, P = 5 MPa, H₂/CO = 2, GHSV = 6000 ml/g_cat h. Plasma-promoted Fe-Cu bimetal catalyst (FeCuSi-PC) possessed much better catalytic performances than those of conventional sample in the selective hydrogenation of carbon monoxide. XRD and XPS analysis suggested that the plasma assistance in the catalyst preparation remarkably diminished the particle size, improved the catalyst dispersion, and issued an exposure of more copper and iron species on the catalyst surface. The mechanism of plasma on catalyst crystallize size was also discussed.     

The characterization and application of p-type semiconducting mesoporous carbon nanofibers

The microstructures of mesoporous carbon nanofibers were characterized by scanning electron microscopy, transmission electron microscopy, nano-Raman, nitrogen adsorption-desorption and optical transmission. They possessed a high specific surface area 840 m² g⁻¹ and a 1.07 eV band gap. All mesoporous carbon nanofiber network can act as the channel material in p-type field-effect transistor devices with field-effect mobilities over 10 cm²/V s. Furthermore, mesoporous carbon nanofiber network exhibits better sensitivity and faster response to NO₂ gas than that of carbon nanotubes, which makes it a promising candidate as poisonous gas sensing nanodevices.     

Effects of various barium precursors and promoters on catalytic activity of Ba-Ti perovskite catalysts for oxidative coupling of methane

The influences of barium precursor and promoter type on the catalytic performance of perovskite catalysts in OCM reaction were studied. Catalysts (BaTiPO₃, P: promoter) were prepared by carbonate, hydroxide and propionate precursors of barium and SnCl₂ and CeO₂ as promoters by sol-gel method, tested in a fixed-bed microreactor and characterized by XRD, BET, CO₂-TPD, FT-IR and UV-Visible analysis. The experiment results showed that based on the extent of effect upon catalyst efficiency, the barium anions can be ranked as; propionate > carbonate > hydroxide, and the CeO₂ promoted catalysts were more active than the SnCl₂ promoted ones. The characterization results showed that the substitution of metal precursors caused formation of different phases with different particle sizes, influenced the basicity of the catalysts, resulted in the appearance of the peaks corresponding to different groups in IR spectroscopy, and shifted the absorption peaks in UV-Visible spectra. These results suggested that OCM reaction over perovskite catalysts is structure sensitive and depended on the type of used precursor and promoter.     

Preparation and electrochemical studies of electrospun TiO2 nanofibers and molten salt method nanoparticles

The TiO₂ nanofibers and nanoparticles are prepared by electrospinning and molten salt method, respectively. The materials are characterized by X-ray diffraction scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and a thermal analysis. The SEM and TEM studies showed that fibers were of average diameter ∼100 nm and composed of nanocrystallites of size 10-20 nm. Electrochemical properties of the materials are evaluated using cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy. Cyclic voltammetric studies show a hysteresis (ΔV) between the cathodic and the anodic peak potentials for TiO₂ nanofibers and nanoparticles (sizes ∼15-30 nm) are in the range, 0.23-0.30 V and a redox couple Ti^4+/3+ around ∼1.74/2.0 V. Electrochemical cycling results revealed that the TiO₂ nanofibers have lower capacity fading compared to that of the nanoparticles. The capacity fading for 2-50 cycles was ∼23% for nanofibers, which was nearly one-third of that of corresponding nanoparticles (∼63%). We discussed the effect of particle size on hysteresis and cycling performance of TiO₂ nanoparticles. Impedance analysis of TiO₂ nanofibers and nanoparticles during first discharge cycle is analyzed and interpreted.     

Control of catalyst particle crystallographic orientation in vertically aligned carbon nanofiber synthesis

Vertically aligned carbon nanofibers (VACNF) have been synthesized where the crystallographic orientation of the initial catalyst film was preserved in the nanoparticle that remained at the nanofiber tip after growth. A substantial percentage of catalyst particles (75%), amounting to approximately 200 million nanofibers over a 100 mm Si wafer substrate, exhibited a sixfold symmetry attributed to a cubic Ni(1 1 1)∥Si(0 0 1) orientation relationship which was verified by X-ray diffraction studies. The Ni catalyst films were prepared by rf-magnetron sputtering under substrate bias conditions to yield a single (1 1 1) film texture. The total energy of the Ni thin film was estimated by calculating the sum of the surface free energy and strain energy. The total film energy was minimized by the evolution of the plane of lowest surface free energy, the (1 1 1) texture. This result was in agreement with X-ray diffraction measurements. The preferred orientation present in the Ni catalyst film prior to nanofiber growth was preserved in the Ni catalyst particles throughout the VACNF growth process. The Ni catalyst particles at the nanofiber tips were not pure single crystals but rather consisted of a mosaic structure of Ni nanocrystallites embedded within Ni catalyst nanoparticles (200-400 nm). The tip-located nanoparticles exhibited a faceted, crystal morphology with the faceting transferred to the underlying carbon nanofiber during the growth process. The possibility of precisely and accurately controlling VACNF growth velocity over macroscopic wafer dimensions with uniformly aligned catalyst particles is discussed.     

Characterization and electrocatalytic properties of PtRu/C catalysts prepared by impregnation-reduction method using Nd2O3 as dispersing reagent

A simple impregnation-reduction method introducing Nd₂O₃ as dispersing reagent has been used to synthesize PtRu/C catalysts with uniform Pt-Ru spherical nanoparticles. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis have been used to characterize the composition, particle size and crystallinity of the catalysts. Well-dispersed catalysts with average particle size about 2 nm are achieved. The electrochemically active surface area of the different PtRu/C catalysts is determined by the CO_ad-stripping voltammetry experiment. The electrocatalytic activities of these catalysts towards methanol electrooxidation are investigated by cyclic voltammetry measurements and ac impedance spectroscopy. The in-house prepared PtRu/C catalyst (PtRu/C-03) in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 30 °C display a higher catalytic activity and lower charge-transfer resistance (R_t) than that of the standard PtRu/C catalyst (PtRu/C-C). It is mainly due to enhanced electrochemically active specific surface, higher alloying extent of Ru and the abundant Pt⁰ and Ru oxides on the surface of the PtRu/C catalyst.     

Effects of hydrogen on carbon nanotube formation in CH4/H2 plasmas

Carbon nanotubes (CNTs) were synthesized using CH₄/H₂ plasmas and plasmas simulated using a one-dimensional fluid model. The thinnest and longest CNTs with the highest number density were obtained using CH₄/H₂ = 27/3 sccm at 10 Torr. These conditions allowed CNTs to grow for 90 min without any meaningful loss of catalyst activity. However, an excess H₂ supply to the CH₄/H₂ mixture plasma made the diameter distribution of the CNTs wider and the yield lower. Hydrogen concentration is considered to affect catalyst particle size and activity during the time interval before starting CNT growth (=incubation period). With CH₄/H₂ = 27/3 sccm for a growth time of 10 min efficient CNT growth was achieved because the amount of carbon atoms in the CNTs and that calculated from simulation showed good agreement. The effect of hydrogen etching on CNTs was analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy by observing CNTs treated by H₂ plasma after CNT growth. It was confirmed that (a) multi-walled CNTs were not etched by the H₂ plasma, (b) the C 1s XPS spectra of the CNTs showed no chemical shift after the treatment, and (c) C-H bonds were produced in CNTs during their growth.     

Carbon nanofibers: Synthesis, characterization, and electrochemical properties

Carbon nanofibers (CNFs) have been synthesized by co-catalyst deoxidization process by a reaction between C₂H₅OC₂H₅, Zn and Fe powder at 650 °C for 10 h. These nanofibers exhibit diameters of ∼80 nm and lengths ranging from several micrometers to tens of micrometers. X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy indicate that as-prepared CNFs possess low graphitic crystallinity. The resultant CNFs as electrode shows capacity of ∼220 mAh/g and high reversibility with little hysteresis in the insertion/deintercalation reactions of lithium-ion. In addition, the possible growth of CNFs is discussed.     

On the origin of the high performance of MWNT-supported PtPd catalysts for the hydrogenation of aromatics

This work reports some surface and structural features of multi-wall carbon nanotubes (MWNT)-supported PtPd crystallites in order to explain their very good ability to hydrogenate aromatic rings in comparison with other catalyst systems consisting of the same metallic function but changing the support substrate (amorphous SiO₂-Al₂O₃ (ASA) and silica-delaminated zirconium phosphate (ZrPSi)). The bimetallic PtPd catalysts were prepared by simultaneous impregnation of the respective supports with metal salt precursors and characterized using chemical analysis, nitrogen adsorption-desorption isotherms at 77 K, temperature-programmed desorption-mass spectroscopy (TPD-MS), DRIFT of adsorbed NH₃ (DRIFT-NH₃), X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy measurements (XPS). The catalysts were tested in the vapor-phase simultaneous hydrogenation (HYD) of naphthalene and toluene in the presence of dibenzothiophene (DBT; 100 ppm of S) at 5 MPa of total pressure. Irrespectively of the reactant molecule, the PtPd/MWNT catalyst showed a higher initial turnover frequency (TOF) than the PtPd/ZrPSi and PtPd/ASA counterparts. The enhancement of activity observed with the PtPd/MWNT catalyst was related to the ensemble effect of Pt₄₈Pd₂₅ alloy located on the outer surface of the MWNT. Upon on-stream conditions the PtPd/MWNT showed the lowest thioresistence, sintering and coking among the catalysts studied. The improved resistance of this catalyst to metal sintering and coking is interpreted in terms of the reactivity changes of the coke precursors induced by adsorbed sulfur as well as the lowest acidity of this catalyst.     

Design of polymer nanofiber systems for the immobilization of homogeneous catalysts - Preparation and leaching studies

In this paper a novel general concept for the immobilization of catalysts is presented. It will be shown that catalysts covalently bound to low-molecular weight polystyrene (M_n > 4000 g/mol) can be immobilized into high molecular weight polystyrene nanofibers using the electrospinning process. The immobilized catalyst-oligostyrene conjugates are well dispersed within the fibers as shown by DSC and X-ray studies. In DMSO, the oligostyrene tails of the catalysts suppress the leaching of the catalysts out of the fibers into the solution for thermodynamic reasons. Leaching studies are conducted using naphthalene-conjugated oligostyrenes using fluorescence spectroscopy. The naphthalene-polystyrene conjugates with defined molecular weight are readily prepared using nitroxide-mediated radical polymerization (NMP). As a model catalyst system, proline-polystyrene conjugates are synthesized by NMP to study catalyst leaching out of the polystyrene nanofibers used as a catalyst matrix.     

Effects of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels in acidic chloride solutions

The role of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels was investigated in 0.5 M H₂SO₄, 3.5% NaCl and 0.5 M H₂SO₄ + 0.5 M NaCl solutions using potentiodynamic polarization, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy techniques. The passive film stability was enhanced in 0.5 M H₂SO₄ and the pitting resistance was improved in 3.5% NaCl solution by more nitrogen addition. The influence of nitrogen extended the whole anodic polarization region in 0.5 M H₂SO₄ + 0.5 M NaCl solution, as demonstrated by the enhanced dissolution resistance, promoted adsorption and passivation process, improved film protection and pitting resistance with increasing nitrogen content. Possible mechanisms relating to the role of nitrogen in different potential regions were discussed.     

Adsorption of H2S or SO2 on an activated carbon cloth modified by ammonia treatment

The aim of this research is to investigate how ammonia treatment of the surface can influence the activity of a viscose-based activated carbon cloth (ACC) for the oxidative retention of H₂S and SO₂ in humid air at 25 °C. Surface basic nitrogen groups were introduced either by treatment with ammonia/air at 300 °C or with ammonia/steam at 800 °C. The pore structure of the samples so prepared was examined by adsorption measurements. Changes in the surface chemistry were assessed by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and temperature programmed desorption (TPD). The change of ACC activity could not be merely attributed to surface nitrogen groups but to other changes in the support. Ammonia/steam treatment improved ACC performance the most, not only by introducing nitrogen surface groups, but also by extending the microporosity and by modifying the distribution of surface oxygen groups. Successive adsorption-regeneration cycles showed important differences between oxidative retention of H₂S and SO₂ and the subsequent catalyst/support regeneration process.     

Effect of 2-propanol and water contents on the crystallization and particle size of titanium dioxide synthesized at low-temperature

Two series of titanium dioxide, TiO₂, powder were prepared at a temperature of 50 °C without any catalyst. The effects of 2-propanol and water contents on the formation of crystalline powder mixture of anatase and brookite were systematically studied. The characteristics of produced powder were determined by employing X-ray diffraction, transmission electron microscopy, nitrogen adsorption test and Fourier transform infrared spectroscopy.