Vitreous Carbon Surface Research Articles

Reactive and inelastic scattering dynamics of hyperthermal O and O2 from a carbon fiber network

Pulsed beams containing O and O2 with incident velocities of ∼7900 m s−1 were directed at a carbon fiber preform network (FiberForm) at temperatures in the range, 1023–1823 K, and the products that scattered from the network were detected with a rotatable mass spectrometer as a function of their velocities and scattering angles. A beam containing pure Ar atoms was also directed at the network, allowing multiple-bounce effects in the absence of reaction to be characterized. Scattered O, O2, and Ar exhibited the dynamical characteristics of impulsive (non-thermal) scattering and thermal desorption (TD). On the other hand, CO and CO2 exhibited only TD dynamics. The fluxes of all products were quantified as a function of sample temperature and, for two focus temperatures, as a function of scattering angle. CO was the dominant reactive product. A temperature dependent hysteresis in the CO flux was quantified, and a comparison was made between the hysteresis on the carbon fiber network and on a planar vitreous carbon surface. The new results may be used to increase the fidelity of oxygen-carbon ablation models for hypersonic flight.

Oxidation and nitridation of vitreous carbon at high temperatures

Molecular beam-surface scattering experiments were used to obtain fundamental data on gas-surface interactions that are central to the ablation of carbon during hypersonic flight through air. Continuous beams containing O or N atoms with incident velocities of ∼2000 m s−1 were directed at a vitreous carbon surface at temperatures in the range, 800–1873 K, and the products that desorbed from the surface were detected with a rotatable mass spectrometer detector as a function of their velocity and scattering angle. All products exhibited the dynamical characteristics of thermal desorption. The efficiencies of the gas-surface interactions, both reactive and non-reactive, were quantified as a function of surface temperature. In addition to reacting with carbon to produce CO2 (minor product) and CO (major product), oxygen atoms may recombine on the surface to produce O2 with an efficiency that is somewhat lower than that to produce CO. Nitrogen atoms may recombine on the surface to produce N2 or react to produce CN. The recombination efficiency of N atoms is generally more than an order of magnitude higher than the reaction efficiency to produce CN. The quantitative reaction efficiencies reported here are useful for the development of air-carbon models for hypersonic ablation.

Gas-surface interactions of atomic nitrogen with vitreous carbon

We have conducted beam-surface scattering experiments with the intent of identifying the reaction mechanisms that are relevant to the ablation of carbon by N atoms at surface temperatures relevant to hypersonic flight. A pulsed molecular beam containing N and N2 with translational energies of 460 and 808 kJ mol-1, respectively, was directed at a vitreous carbon surface held at temperatures in the range 1023–1923 K. Time-of-flight distributions were collected for scattered products with fixed incidence and final angles of θi = θf = 45°. Inelastically scattered N and N2 and reactively scattered CN were the only products observed. The scattering dynamics of N and N2 were independent of surface temperature and were indicative of purely impulsive scattering. The scattering dynamics of CN suggested that this reaction product is formed by a mechanism that occurs in thermal equilibrium with the surface – i.e., a Langmuir-Hinshelwood mechanism. The reaction probability to produce CN has an Arrhenius temperature dependence with an activation energy of 207 kJ mol-1. The relevance of the current results to the hypersonic ablation of carbon in the presence of dissociated air is discussed.

Construction of finite rate surface chemistry models from pulsed hyperthermal beam experimental data

A general approach for constructing finite rate surface chemistry models using time-of-flight (TOF) distribution data acquired from pulsed hyperthermal beam experiments is presented. First, a detailed study is performed with direct simulation Monte Carlo (DSMC) to analyze the TOF distributions corresponding to several types of reaction mechanisms occurring over a wide temperature range. This information is used to identify and isolate the products formed through different reaction mechanisms from TOF and angular distributions. Next, a procedure to accurately calculate the product fluxes from the TOF and angular distributions is outlined. Finally, in order to derive the rate constant of the reactions within the system, the inherent transient characteristic of the experimental pulsed beam set up must be considered. An analysis of the steady-state approximation commonly used for deriving the rate constants reveals significant differences in terms of the total product composition. To overcome this issue, we present a general methodology to derive the reaction rate constants, which takes into account the pulsed setup of the beam. Within this methodology, a systematic search is performed through the rate constant parameter space to obtain the values that provide the best agreement with experimentally observed product compositions. This procedure also quantifies the surface coverage that corresponds to the rates of product formation. This approach is applied to a sample system: oxidation reaction on vitreous carbon surfaces to develop a finite-rate surface chemistry model. Excellent agreement is observed between the developed model and the experimental data, thus showcasing the validity of the proposed methodologies.

Development and validation of a finite-rate model for carbon oxidation by atomic oxygen

Molecular beam experiments of hyperthermal oxygen atoms scattering from a vitreous carbon surface were simulated and analyzed using direct simulation Monte Carlo (DSMC). The purpose of this study was to construct and validate a surface chemistry model in DSMC by matching observed features in experimental time-of-flight (TOF) and angular distribution data. A variety of reaction mechanisms including adsorption, desorption, and several types of Langmuir-Hinshelwood (LH) reactions are used to characterize the surface processes in the DSMC surface chemistry framework. This framework is used to numerically simulate the beam-surface scattering experiments of oxygen on vitreous carbon with surface temperatures ranging from 600 to 2000 K. Detailed analysis of the experimental data including TOF and angular distributions, and product fluxes derived from these data, is used to propose modifications to the carbon surface oxidation model developed by Poovathingal et al. This revised model successfully captures the important features in the experimental TOF distributions, including: (i) the impulsively scattered and thermally desorbed components in the oxygen (O) TOF distributions, and (ii) the relatively slow long-tail components in the carbon monoxide (CO) TOF distributions. Comparisons between the simulated (DSMC) and experimental TOF and angular distributions, as well as product fluxes, show excellent agreement.

Electro-polymerisation of 3,4-ethylenedioxythiophene on reticulated vitreous carbon in imidazolium-based chloroaluminate ionic liquid as energy storage material

This work shows the electro-polymerisation of thin film poly(3,4-ethylenedioxythiophene) on three-dimensional reticulated vitreous carbon substrates by cyclic voltammetry and pulsed polymerisation methods from a Lewis neutral chloroaluminate ionic liquid containing 3,4-ethylenedioxythiophene monomer. The polymer composite is attractive as an energy storage electrode for sustainable and high-performance technologies due to its unique properties of battery and capacitor in one system, i.e., the redox reaction occurring simultaneously with the anion doping/de-doping of the conductive polymer with AlCl4− ionic species contained in the ionic liquid. The structure of the polymer films, their doping/de-doping mechanism and the stability in the ionic liquid were characterised by scanning electron microscopy and cyclic voltammetry and compared with films electro-polymerised on planar vitreous carbon. The typical granular and nano/micro-porous polymer structure observed on planar vitreous carbon was successfully replicated on the macro-porous reticulated vitreous carbon surface. The polymer films show approximately 45% higher capacity than films on planar substrates and similar efficient redox behaviour, proving that the material has hybrid battery-capacitor properties enhanced by the higher area per unit volume of reticulated vitreous carbon.

Influence of graphitic materials microstructure in the hydrogen evolution in aqueous solution of tetra-alkylammonium-sulfonic acid ionic liquid

Vitreous carbon (VC), pyrolytic carbon (PC) and moulded graphite (MG) were tested as cathodic materials in hydrogen production by water electrolysis in the presence of the ionic liquid tetrafluoroborate of 3-triethylammonium-propane sulfonic acid (TEA-PS.BF4). The physical characterization of the carbon materials indicated large differences in the microstructure of VC, PC, and MG and this significantly affected their electrochemical response. The mechanism presented for all the materials studied in the hydrogen evolution reaction (HER) was Volmer–Heyrovsky, where the H2 desorption at the catalytic surface is the determining step. The MG electrode presented an unfavourable performance due to the formation of nanobubbles that coalesced without H2 desorption and led to the deactivation of the catalytic sites. This behaviour is attributed to the presence of large and ordered crystallites at the material surface, with a greater number of hydrophobic domains in comparison with VC and PC material surfaces. The VC and PC electrodes presented a higher performance compared to the Pt cathode, showing lower activation energy, higher cathodic exchange current and lower charge transfer resistance. The set of results indicates VC and PC as promising alternative materials to constitute cathodes for the electrolysis of water using TEA-PS.BF4 aqueous solution as electrolyte.

Inelastic and Reactive Scattering Dynamics of Hyperthermal O and O2 on Hot Vitreous Carbon Surfaces

We have undertaken a series of experiments to learn the mechanisms of carbon oxidation over a wide range of temperatures that extend to the conditions encountered during atmospheric re-entry, with a particular interest in understanding how these mechanisms change with temperature. We report here the hyperthermal scattering dynamics of ground-state atomic oxygen, O(3P), and molecular oxygen, O2(3Σg–), on vitreous carbon surfaces at temperatures from 600 to 2100 K. A molecular beam containing neutral O and O2 in a mole ratio of 0.93:0.07 was prepared with a nominal velocity of 7760 m s–1, corresponding to a translational energy of 481 kJ mol–1 for atomic oxygen. This beam was directed at a vitreous carbon surface, and angular and translational energy distributions were obtained for inelastically and reactively scattered products with the use of a rotatable mass spectrometer detector. Unreacted oxygen atoms exited the surface through both impulsive scattering and thermal desorption. The preferred scattering ...

Formation of Au/Pd-Sn trimetallic particles onto vitreous carbon

The sequential deposition process of Pd and Sn on Au microcrystals, previously electrodeposited onto vitreous carbon (VC) was investigated in sulphuric acid solution by cyclic voltammetry and Auger electron spectroscopy (AES). Scanning electron microscopy was used to characterize Au deposits. The results reveal that the deposition of Pd and Sn is selective on the Au microcrystals, uniformly distributed on the VC surface, due to the strong interaction energy between the three metals. From the cyclic voltammetric and AES measurements in the VC/Au/Pd/Sn system, the existence of a Sn core–shell structure with Pd can be inferred. Similar voltammetric results were obtained for the VC/Au/Sn/Pd system, where in this case, the Pd adsorbed layer screened the Sn deposits on the VC/Au interface. However, the AES spectrum shows that Sn is still present on the surface, indicating that probably, the Sn deposits have diffused to the surface, or they are not completely covered by Pd. The VC/Au/Pd/Sn modified substrate appears as an interesting electrocatalyst material for the reduction of nitrate ions. Copyright © 2013 John Wiley & Sons, Ltd.

Study of reticulated vitreous carbon surface treated by plasma immersion ion implantation for electrodes production

RVC samples were treated by nitrogen plasma immersion ion implantation (N-PIII) for electrodes production. High-voltage pulses with amplitudes of −3.0 kV or −10.0 kV were applied to the RVC samples while the treatment time was 10, 20 and 30 min. The samples were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The SEM images present an apparent enhancement of the surface roughness after the treatment probably due to the surface sputtering during the PIII process. This observation is in agreement with the specific electrochemical surface area (SESA) of RVC electrodes. An increase was observed of the SESA values for the PIII-treated samples compared to the untreated specimen. Some oxygen and nitrogen containing groups were introduced on the RVC surface after the PIII treatment. Both plasma-induced process: the surface roughening and the introduction of the polar species on the RVC surface are beneficial for the RVC electrodes application.

Formation of Cu/Pd bimetallic crystals by electrochemical deposition

The early stages of the palladium electrodeposition process onto a vitreous carbon (VC) substrate as well as the deposition of Cu on such Pd/VC modified surface were investigated using classical electrochemical techniques, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Within the potential range considered the kinetics of the Pd electrodeposition from a PdCl 2 acid solution can be described by a model involving progressive nucleation on active sites and diffusion-controlled 3D growth. The nucleation rate constant, A 0, and the number of active sites of the substrate, N 0, were determined from the analysis of potentiostatic current transients on the basis of an existing theoretical model. The AFM images corroborated the progressive nucleation mechanism showing irregular palladium crystals randomly distributed over the VC surface, with different sizes and 3D morphological characteristics. The electrodeposition of Cu was carried out onto the characterized Pd/VC modified surface from a Cu 2+ containing solution using a well defined polarization routine. The SEM/EDX images confirmed the formation of Cu/Pd bimetallic crystals uniformly distributed on the VC surface and the in situ AFM images obtained during this process corroborated that Cu formed a core–shell structure with the Pd crystals. Nevertheless, the subsequent anodic stripping produced only a partial dissolution of the Cu deposits, and therefore, the formation of a Cu/Pd alloy could be inferred.

Electrochemical behaviour of uranium (IV) in DMF at vitreous carbon

The electrochemical behaviour of UCl 4 (0.01 mol L −1 up to 0.05 mol L −1) in 0.1 mol L −1 TBAPF 6/DMF solution at vitreous carbon was studied, at room temperature, by cyclic voltammetry and potentiostatic techniques. The electrolytic solutions were analyzed by UV spectroscopy (UV), and the electrodeposited films were characterized by Rutherford Backscattering Spectroscopy (RBS) and X-ray diffraction (XRD). The cyclic voltammetric results, at low UCl 4 concentrations (0.01 mol L −1), point that the reduction of U(IV) to U(0) occurs in two steps involving mainly U(IV) and U(III) species. The first electron transfer reaction is quasi-reversible and the second irreversible. The diffusion coefficient of U(IV) in DMF and the charge rate constant were determined to be 4.78 × 10 −7 cm 2 s −1 and 1.93 × 10 −3 cm s −1 (at 0.02 V s −1), respectively. RBS data obtained from samples prepared at constant potential (−3.10 V) during 3 h at room temperature, indicated the presence of uranium particles deposited all over the vitreous carbon surface with aggregates in some places, confirming that the second reduction step corresponds to uranium electrodeposition. No crystallographic ordering could be seen by XRD, pointing to an amorphous character of the uranium films.

Mechanism and kinetics of the electrochemical formation of polypyrrole under forced convection conditions

This work studies the process of electrochemical formation of polypyrrole onto a vitreous carbon rotating disk electrode, from a pyrrole aqueous solution containing sodium nitrate, applying different potentials to the electrode for both conditions: stagnant and forced convection. It was found that the analyses of the experimental current transients, in both cases, could be done using the same theoretical model. The latter involves three different contributions: a first one due to pyrrole oxidation on the vitreous carbon surface, to form different oligomers in solution, a second one related with bidimensional nucleation of the oligormeric form of pyrrole (formed during the first stage) that saturated the solution near the electrode surface and a final contribution due to oxidation of pyrrole and/or its oligomers onto the surface of the polypyrrole film deposited during the 2D nucleation. The last contribution can be related to the polypyrrole three dimensional growth through direct incorporation of the monomer to the polymeric film or to a similar processes as described on the first two stages. For the stagnant solution the contribution related with the third stage was the highest observed, for a given applied potential, however, when hydrodynamic conditions were imposed, at a constant potential, the contribution due to this stage diminishes with the angular speed of the RDE electrode, because the more reactive oligomeric forms of pyrrole are forced away from the electrode surface, to practically reach a steady value. From AFM images it is shown that toroidal structures became apparent when deposition was done potentiodynamically and that the whole electrode surface was covered by small nuclei which possess an average height that is quite similar to that theoretically predicted by the deposition model here proposed to explain the potentiostatic Ppy electrodeposition.

Application of vitreous and graphitic large-area carbon surfaces as field-emission cathodes

Numerous carbon bulk or thin-film materials have been used as field-emission cathodes. Most of these can be made into large-area and high-current field-emission cathodes without the use of complex IC fabrication techniques. Some of these exhibit low-extraction field, low work-function, high ruggedness, chemical stability, uniform emission, and low-cost manufacturability. A comparison of all of these materials is presented. Two viable cathode materials, reticulated vitreous carbon (RVC) and graphite paste are examined here and compared.

Effect of Potential on the Early Stages of Nucleation and Growth during Silver Electrocrystallization in Ammonium Medium on Vitreous Carbon

In this work, a systemic study of silver electrocrystallization process was performed on a vitreous carbon (VC) electrode in a medium of at pH 11. The analysis of current transients, using different theoretical models, shows that the systematic evolution of electrocrystallization process is observable through the gradual modification of energetic conditions of the interface by either changing the concentration of electroactive species to or the potential imposed to the electrode. At low concentrations of electroactive species and not so negative potentials, 2D-2D growth of silver may be observed forming deposits less than the monolayer coverage. At more negative potentials 2D-3D growths are obtained. In this type of transition, two kinds of silver growth sites are identifiable, first on the VC and afterward, on the recently deposited silver. At concentrations and less negative potentials, 3D growth of nuclei can be seen, forming deposits far beyond a monolayer. Their growth is influenced by either diffusion of or the incorporation of silver adatoms into the substrate lattice, depending on the conditions of imposed potential. At concentrations and very negative potentials, where the deposited silver was much greater than a monolayer, 2D growths were found associated with the growth of Ag° on the previously deposited silver. It is worth noting that this is the first time, that in the same chemical system, the systematic evolution of electrocrystallization of 2D and 3D stages of nuclei growth on heterogeneous VC surfaces can be observed, through the gradual modification of energetic conditions at the interface by either changing the concentration of electroactive species or the potential imposed to the electrode. © 2004 The Electrochemical Society. All rights reserved.

Considerações sobre a eletrogeração de peróxido de hidrogênio

This paper presents some results that may be used as previous considerations to a hydrogen peroxide electrogeneration process design. A kinetic study of oxygen dissolution in aqueous solution is carried out and rate constants for oxygen dissolution are calculated. Voltammetric experiments on vitreous carbon cathode shown that the low saturation concentration drives the oxygen reduction process to a mass transfer controlled process which exhibits low values of limiting currents. Results have shown that the hydrogen peroxide formation and its decomposition to water are separated by 400 mV on the vitreous carbon surface. Diffusion coefficients for oxygen and hydrogen peroxide are calculated using data taken from Levich and Tafel plots. In a series of bulk electrolysis experiments hydrogen peroxide was electrogenerated at several potential values, and concentration profiles as a function of the electrical charged passed were obtained. Data shown that, since limiting current plateaus are poorly defined onto reticulated vitreous carbon, cathodic efficiency may be a good criterion for choosing the potential value in which hydrogen peroxide electrogeneration should be carried out.

Electrodeposition of Zn and In onto vitreous carbon

The deposition of indium and zinc on vitreous carbon was studied by voltammetric, galvanostatic and single potentiostatic pulse techniques. The morphology and composition of deposits were analysed by SEM/EDX. The codeposition process occurs without the formation of alloys or intermetallic compounds. On the one hand under stagnant conditions or at low electrode rotation speeds, localized alkalization produced by the hydrogen evolution reaction (HER) favours deposition through an indium hydroxide layer, and deposits with the same atomic percentage of In and Zn are attained. On the other hand, under electrode rotation, preferred deposition of Zn takes place. In this case, the reduction of H+ by the In+ species, intermediate in the In3+ reduction process, diminishes the electrochemical HER on the substrate thus favouring Zn deposition. The higher nucleation rate on metallic deposits previously formed on the vitreous carbon surface is also likely.

Reactive Electrophoretic Activation of a Microelectrode for Enzyme-Amplified Recognition and for Melting-Temperature Determination of 105 Copies of a Simple Oligonucleotide

The hybridization of 105 copies of 25−30-base single-stranded poly(deoxythymidine)-5‘-phosphate [ss-pd(T)25-30] was detected amperometrically with a 10 μm-diameter microelectrode. The melting of 105 copies of the hybrid with horseradish peroxidase (HRP)-labeled poly(deoxyadenine)-5‘-phosphate [ss-pd(A)25-30-HRP] was also tracked by amperometry. The microelectrode was coated with its hybridization-sensing layer in a two-step process involving electrophoretic deposition, which yielded reproducible electrode coatings. In the first step, a thin film of an electron-conducting redox polymer was deposited electrophoretically at constant current on the vitreous carbon surface of the microelectrode. In the second step, carbodiimide-activated 5‘-phosporylated ss-pd(T)25-30 was reactively electrophoretically deposited and covalently attached to the redox polymer film. Subsequent hybridization led to electrical contact between the HRP label of ss-pd(A)25-30 and the conducting redox polymer. This contact resulted in catalysis of H2O2 electroreduction to water at 0.0 V vs Ag/AgCl. The 20 ± 2 pA current produced by 105 copies of hybridized pd(A)25-30-HRP was at least 8 times greater than the 2.5 ± 2.5 pA current measured with noncomplementary HRP-labeled poly(deoxyguanidine)-5‘-phosphate and 40 times greater than the 0.5 pA electrical background noise.

Identification of different silver nucleation processes on vitreous carbon surfaces from an ammonia electrolytic bath

We performed an electrochemical study of silver electrodeposition from an electrolytic bath containing 1 M NH4OH and 1 M KNO3 (pH=11), over a Ag(I) concentration range of 10−4 to 0.3 M, and identified silver nucleation processes on the vitreous carbon surface. Depending upon the silver concentration in the deposition bath, growth occurred either two-dimensionally (2D), controlled by adatom incorporation, or three-dimensionally (3D), controlled by diffusion or lattice incorporation. Qualitative and quantitative characterization of the observed nucleation processes were based on the results of cyclic voltammetry and chronoamperometry analysis. For quantitative characterization, different theoretical models related to electrocrystallization processes were used. Atomic force microscopy (AFM) was employed to probe the surface morphology of the silver deposit. AFM images revealed that silver deposits formed from ammonia baths with different concentrations of Ag(I) ion possess different morphological characteristics. Indeed, many of the surface characteristics clearly corroborated the mechanism of silver deposition proposed by electrochemical analysis.

Recovery of mercury using an electrochemical flow-by reactor. Part I Reaction rate expression for mercury deposition in the presence ofa side reaction

This paper analyses the deposition of the mercuric complexes from dilute solutions ofHgCl(2−n)n and high concentrations of NaCl, taking into account the reduction of O2 as a simultaneous electrochemical reaction. Studies with a vitreous carbon rotating disk electrode were performed to determine the potential regions where the reactions take place and also to calculate the kinetic parameters under experimental conditions. Reproducible results were obtained only when a mercury film was deposited on the vitreous carbon surface prior to each experiment. Potentiodynamic polarizations were obtained from 5×10−4 M Hg(II) solutions and different NaCl concentrations ranging from 1 M to 4 M, in the presence and absence of dissolved oxygen. The mercuric complex deposition and the oxygen reduction proceeded via mixed control within the first 50 mV overpotential, followed by a diffusion control. After −0.450 V vs sce the H2 evolution started. This reaction occurred at free vitreous carbon left between the liquid mercury droplets where the hydrogen overpotential is smaller. Kinetic parameters as well as the reaction rate expression for mercury deposition from its chloride complexes were obtained.