Carbon Coverage Research Articles

Wettability of Nafion and Nafion/Vulcan Carbon Composite Films

The wettability of the Pt/carbon/Nafion catalyst layer in proton exchange membrane fuel cells is critical to their performance and durability, especially the cathode, as water is needed for the transport of protons to the active sites and is also involved in deleterious Pt nanoparticle dissolution and carbon corrosion. Therefore, the focus of this work has been on the first-time use of the water droplet impacting method to determine the wettability of 100% Nafion films, as a benchmark, and then of Vulcan carbon (VC)/Nafion composite films, both deposited by spin-coating in the Pt-free state. Pure Nafion films, shown by SEM analysis to have a nanochanneled structure, are initially hydrophobic but become hydrophilic as the water droplet spreads, likely due to reorientation of the sulfonic acid groups toward water. The wettability of VC/Nafion composite films depends significantly on the VC/Nafion mass ratios, even though Nafion is believed to be preferentially oriented (sulfonate groups toward VC) in all cases. At low VC contents, a significant water droplet contact angle hysteresis is seen, similar to pure Nafion films, while at higher VC contents (>30%), the films become hydrophobic, also exhibiting superhydrophobicity, with surface roughness playing a significant role. At >80% VC, the surfaces become wettable again as there is insufficient Nafion loading present to fully cover the carbon surface, allowing the calculation of the Nafion:carbon ratio required for a full coverage of carbon by Nafion.

Structural Patterns in Carbon Chemisorption on an Icosahedral 2 Iron Cluster

Carbon chemisorption on iron nanoparticles at small carbon coverage has been studied by using a Fe13 particle as a model because it possesses a nearly icosahedral geometry, and complications with additional effects associated with the surface inhomogeneity do not arise. The electronic and geometrical structures of Fe13Cn are computed for n = 0–20 using an all-electron density functional theory with generalized gradient approximation and a rather large basis set. It is found that the energetically preferred structures correspond to the formation of carbon dimers up to Fe13C12 and trimers up to Fe13C18 in octahedral configurations of the dimers and trimers with the Fe13 cluster being endohedral. The trend for the formation of carbon tetramers breaks at Fe13C20. We found that the dependence of the total energy on the total spin is nearly the same for Fe13 and Fe13C8. When the number of chemisorbed carbon atoms exceeds 6, chemisorption quenches the total magnetic moment to 36 μB from the value of 44 μB in the...

First-principles study of the mechanism of carbon deposition on Fe(111) surface and subsurface

A theoretical study of the carbon atoms adsorption and diffusion on the surface and into the subsurface of Fe (111) is performed using DFT calculations. Before the carbon coverages up to 1 ML, the adsorbed carbons tend to exist in an isolated atomic state and cause a reconstruction of Fe (111) surface. The configurations of “mC2+nC” are energetically favorable on the Fe (111) surface at 1 ML ≤ θC ≤ 2 ML. At a higher coverage, complicated adsorbed patterns such as chains and islands are found, and we predict that these carbon islands can function as the nucleation center of the precipitation of graphite or carbon nanotubes on the Fe(111) surface. In the subsurface region, the carbon atom prefers the octahedral site. The barriers for diffusion on and into the Fe (111) surface and subsurface are 0.45 eV and 0.73 eV, respectively. Actually, C2 formation is thermodynamically favored, whereas C migration into the subsurface region is kinetically feasible.

Direct graphene growth on Co3O4(111) by molecular beam epitaxy

Direct growth of graphene on Co3O4(111) at 1000 K was achieved by molecular beam epitaxy from a graphite source. Auger spectroscopy shows a characteristic sp2 carbon lineshape, at average carbon coverages from 0.4 to 3 ML. Low energy electron diffraction (LEED) indicates (111) ordering of the sp2 carbon film with a lattice constant of 2.5(±0.1) Å characteristic of graphene. Sixfold symmetry of the graphene diffraction spots is observed at 0.4, 1 and 3 ML. The LEED data also indicate an average domain size of ∼1800 Å, and show an incommensurate interface with the Co3O4(111) substrate, where the latter exhibits a lattice constant of 2.8(±0.1) Å. Core level photoemission shows a characteristically asymmetric C(1s) feature, with the expected π to π* satellite feature, but with a binding energy for the 3 ML film of 284.9(±0.1) eV, indicative of substantial graphene-to-oxide charge transfer. Spectroscopic ellipsometry data demonstrate broad similarity with graphene samples physically transferred to SiO2 or grown on SiC substrates, but with the π to π* absorption blue-shifted, consistent with charge transfer to the substrate. The ability to grow graphene directly on magnetically and electrically polarizable substrates opens new opportunities for industrial scale development of charge- and spin-based devices.

Energetics of Carbon deposition on Fe(100) and Fe(110) surfaces and subsurfaces

Spin-polarized density functional theory calculations have been performed to investigate carbon deposition and carbide formation on Fe(100) and Fe(110) at different carbon coverage. On Fe(100) with increasing carbon coverage, the most stable carbon adsorption configuration changes from surface carbon adsorption to surface carbon diffusion into subsurface, and surface carbon clustering is not favored thermodynamically. However, surface carbon clustering is more favored kinetically than carbon diffusion; and carbon diffusion into subsurface and surface carbon clustering become competitive. On Fe(110) with increasing surface carbon coverage, the most stable adsorption configuration changes from surface carbon adsorption to surface carbon diffusion into subsurface, and this process is favored both kinetically and thermodynamically, and surface carbon clustering is neither favored nor competitive. Surface carbon deposition might form on Fe(100), while carbide formation might be found on Fe(110).

Molecular behavior of water in TiO2 nano-slits with varying coverages of carbon: a molecular dynamics simulation study

It is well known that titanium dioxide (TiO(2)) is biocompatible and environmentally friendly. Consequently, TiO(2) is widely applied in many fields, such as implant materials, photocatalysis, pigments, cosmetic additives, etc. Mesoporous TiO(2) finds many industrial applications, because of its high surface area and stable structure. However, the strong interaction between TiO(2) and water molecules sometimes limits its application to solution environments. Our previous computational work showed that changes to the surface chemistry of TiO(2) can affect the hydrogen bond network of water molecules on the TiO(2) surface, and so influence the diffusion of water in the slits. Thus, a carbon-modified TiO(2) surface could be an alternative way to avoid this limitation. In this work, a slit pore model with a modified TiO(2) surface (pore widths 1.2 nm, 1.6 nm and 2.0 nm) with varying carbon coverages (0%, 7%, 47%, 53%, 93% and 100%) was presented. Molecular dynamics (MD) simulations were then performed to investigate the sorption and diffusion of water in these slits. Simulation results showed that the interfacial water molecules on bare TiO(2) regions were little affected by the neighboring carbon, and they have the same properties as those on bare TiO(2) surfaces. However, the diffusion of water molecules in the center of the slit was enhanced on increase of carbon coverage, because the carbon layer broke the hydrogen bond network between the interfacial water molecules and those on the bare TiO(2) surface. It was found that in the slits (>1.2 nm) fully covered by carbon the diffusion coefficients of water are larger than that of bulk water. Moreover, large pore sizes caused an increase in the mobility of water molecules in carbon-modified TiO(2), in agreement with previous experimental work.

Parametric studies on iron–carbon composite nanoparticles synthesized by laser pyrolysis for increased passivation and high iron content

Iron/iron carbide core and carbon shell nanoparticles with improved magnetic properties were successfully synthesized by laser pyrolysis. As iron and carbon precursors, iron pentacarbonyl and pure or argon-diluted acetylene/ethylene mixtures, respectively, were used. The aim of the present optimization is the improvement of the magnetic properties of the nanomaterials by the increase of the iron percent in powders simultaneously to the maintaining of the protective character of the carbon coverage of nanoparticles. The chemical content and the crystalline structure were monitored by EDX, XRD and TEM techniques. In the first study, the content of acetylene as carbon source was diminished from 75% to 0%. Consequently the percent iron increased from 10 at.% to 28 at.% while oxygen remained relatively constant (around 5 at.%). In the second step, only diluted ethylene was used (maximum 87.5 vol.% Ar). In this case, an increase of iron to 46 at.% is observed. An optimum 50% carbon source dilution was found. Above this value, the carbon content increases and below it, superficial oxidation increases through the diminishing of the carbon shell. The magnetic properties and the Fe phase composition of the Fe–C samples were analyzed by temperature dependent Mössbauer spectroscopy.

Quantitative investigation of the influence of carbon surfactant on Ge surface diffusion and island nucleation on Si(100)

We investigated the surface diffusion and island nucleation of Ge on Si(100) in presence of a submonolayer coverage of carbon as surfactant by using scanning Auger microscopy and atomic force microscopy. Ge stripes have been deposited and lithographically etched on a Si substrate and used as sources for the surface diffusion of Ge promoted by annealing at 600, 650, and $700\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The diffusion coefficient has been determined by fitting the postannealing coverage profiles measured by Auger microscopy with a one-dimensional continuous model. The carbon coverage has been spatially modulated on a single sample, allowing the measurement of the diffusion coefficient as a function of the C thickness at $600\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. We show that the reduction in the diffusion coefficient while increasing the surfactant coverage is described by a linear dependence of the diffusion activation energy on the C coverage. This dependence is discussed in terms of the chemical interactions among Si, C, and Ge, of the surface roughness and the local strain field induced by the C surfactant. Spontaneous nucleation of SiGe islands coexists with the continuous surface diffusion of Ge. The transition of the island nucleation as a function of the carbon coverage is observed to be continuous from the Stranski-Krastanov mode to the Volmer-Weber regime. We propose a consistent scenario correlating diffusion and nucleation parameters within a diffusion limited growth regime and show the existence of a threshold for C coverage below which no effect is observed.

Effect of Nanostructuring and Ex situ Amorphous Carbon Coverage on the Lithium Storage and Insertion Kinetics in Anatase Titania

Implications of nanostructuring and conductive carbon interface on lithium insertion/removal capacity and insertion kinetics in nanoparticles of anatase polymorph of titania is discussed here. Sol−gel synthesized nanoparticles of titania (particle size ∼6 nm) were hydrothermally coated ex situ with a thin layer of amorphous carbon (layer thickness: 2−5 nm) and calcined at a temperature much higher than the sol−gel synthesis temperature. The carbon-titania composite particles (resulting size ∼10 nm) displayed immensely superior cyclability and rate capability (higher current rates ∼4 A g−1) compared to unmodified calcined anatase titania. The conductive carbon interface around titania nanocrystal enhances the electronic conductivity and inhibits crystallite growth during electrochemical insertion/removal thus preventing detrimental kinetic effects observed in case of unmodified anatase titania. The carbon coating of the nanoparticles also stabilized the titania crystallographic structure via reduction in t...

Storage performance of LiFe1 − x Mn x PO4 nanoplates (x = 0, 0.5, and 1)

Although LiFePO4 (LFP) is considered to be a potential cathode material for the lithium-ion batteries, its rate performance is significantly restricted by sluggish kinetics of electrons and lithium ions. Several attempts have been made so far to improve the performance of LiFePO4 by reducing the grain size, doping with aliovalent atoms, and coating conductive materials such as carbon or RuO2. We report here synthesis of LFP nanoplates by solvothermal method, tailoring the thickness as well as carbon coverage at surfaces to explore their influence on the storage performance. Due to the fact that Li+ ion diffuses along the b-axis, solvothermal method was aimed to control the thickness of nanoplates across the b-axis. We synthesized several nanoplates with various plate thicknesses along b-axis; among those, nanoplates of LFP with ∼30-nm-thick b-axis having thin (2–5 nm) and uniform layer of carbon coating exhibits high storage capacity as well as high rate performances. Thus, a favorable morphology for LiFePO4 has been achieved via solvothermal method for fast insertion/extraction of Li+ as compared to spherical nanoparticles of carbon-coated LFP. Galvanostatic cycling shows a capacity of 164 ± 5 mAh g−1 at 0.1 C rate, 100 ± 5 mAh g−1 at 10 C rate, and 46 ± 5 mAh g−1 at 30 C rate, with excellent capacity retention of up to 50 cycles. Further attempts have been made to synthesize LiMnPO4 (LMP) as well as Li(Fe1 − x Mn x )PO4/C (x = 0.5) nanoplates using solvothermal method. Although LiMnPO4 does not exhibit high storage behavior comparable with that of LiFePO4, the mixed systems have shown an impressive storage performance.

Reconstruction of Metabolic Pathways, Protein Expression, and Homeostasis Machineries across Maize Bundle Sheath and Mesophyll Chloroplasts: Large-Scale Quantitative Proteomics Using the First Maize Genome Assembly

Chloroplasts in differentiated bundle sheath (BS) and mesophyll (M) cells of maize (Zea mays) leaves are specialized to accommodate C(4) photosynthesis. This study provides a reconstruction of how metabolic pathways, protein expression, and homeostasis functions are quantitatively distributed across BS and M chloroplasts. This yielded new insights into cellular specialization. The experimental analysis was based on high-accuracy mass spectrometry, protein quantification by spectral counting, and the first maize genome assembly. A bioinformatics workflow was developed to deal with gene models, protein families, and gene duplications related to the polyploidy of maize; this avoided overidentification of proteins and resulted in more accurate protein quantification. A total of 1,105 proteins were assigned as potential chloroplast proteins, annotated for function, and quantified. Nearly complete coverage of primary carbon, starch, and tetrapyrole metabolism, as well as excellent coverage for fatty acid synthesis, isoprenoid, sulfur, nitrogen, and amino acid metabolism, was obtained. This showed, for example, quantitative and qualitative cell type-specific specialization in starch biosynthesis, arginine synthesis, nitrogen assimilation, and initial steps in sulfur assimilation. An extensive overview of BS and M chloroplast protein expression and homeostasis machineries (more than 200 proteins) demonstrated qualitative and quantitative differences between M and BS chloroplasts and BS-enhanced levels of the specialized chaperones ClpB3 and HSP90 that suggest active remodeling of the BS proteome. The reconstructed pathways are presented as detailed flow diagrams including annotation, relative protein abundance, and cell-specific expression pattern. Protein annotation and identification data, and projection of matched peptides on the protein models, are available online through the Plant Proteome Database.

Origin of non-Faradayicity in electrochemical promotion of catalytic ethylene oxidation

The electrochemical promotion of catalytic C(2)H(4) oxidation has been investigated under low pressure conditions (p approximately 10(-6)-10(-4) mbar) with a Pt film on yttrium stabilized zirconia (YSZ) as catalyst. All measurements were conducted with a UHV system with a differentially pumped quadruple mass spectrometer (QMS) for rate measurements and a photoelectron emission microscope (PEEM) for spatially resolved measurements. A pronounced rate hysteresis upon cyclic variation of p(C(2)H(4)) was observed under open circuit conditions which is attributed to carbonaceous CH(x) adlayer inhibiting O(2) adsorption and hence poisoning the reaction. Application of a positive potential causes a partial removal of the inhibiting CH(x) adlayer by spillover oxygen thus triggering a transition from thus poisoned state of the surface to an active state with reduced carbon coverage. The ignition effect and therefore also the electrochemical promotion effect are linked to the presence of a carbon adlayer on the surface. The non-Faradayicity which is observed in this reaction system is explained as an ignition effect.

Morphology controlled synthesis of LiFePO4/C nanoplates for Li-ion batteries

Lithium iron phosphate, LiFePO4 (LFP), is considered to be a potential cathode material for lithium-ion batteries but its rate performance is significantly restricted by sluggish kinetics of electrons and lithium ions. A simple solvothermal method has been described in this article to synthesize carbon coated LFP (LFP/C) nanoplates with varying thickness from 20 to 500 nm by using different iron precursors. The influence of solvents on the morphology of the LFP in the solvothermal synthesis is also investigated. A uniform carbon coverage at the surfaces has been achieved by a selective chelating carbonising source, D-gluconic acid lactone. The smallest dimension of the nanoplates has been found to be the b-axis where the Li+ ion diffuses quickly. The overall capacity and rate performance have, in general, been found to increase with the decrease of thickness of the nanoplates. Hierarchical LFP/C with ∼30 nm thickness shows the best electrochemical performance of 167 mA h g−1, followed by spindle (<20 nm thickness but aggregated, 121 mA h g−1), plates (200–300 nm thickness, 110 mA h g−1) and diamond shaped LFP/C (300–500 nm thickness, 82 mA h g−1) at a current rate of 17mA g−1 (0.1C rate). The spindle shaped LFP/C shows unexpected electrochemical performance since the nanoplates are heavily agglomerated in the bulk which prevents access for the liquid electrolyte, as well as additive Super P carbon, between neighbouring nanoplates during the fabrication of the composite electrodes. Hence, only the peripheral plates of the spindle are actively involved in the insertion/extraction of Li+, while the core of the spindle shaped LFP/C is almost inactive, resulting in moderate storage behaviour.

Carbon Adsorption and Absorption in the (111) L12 Fe3Al Surface

Density functional theory is used to investigate the adsorption of carbon on the (111) surface of the L1 2 Fe 3 Al structure under various carbon coverages and the absorption in interstitial sites of the subsurface. Surface reconstruction is observed after carbon is adsorbed on the surface, and as carbon coverage increases carbon structures are formed in most cases with enhanced adsorption energies. C structures evolve, becoming sp3-like at intermediate coverage, and at much higher coverage sp2 networks form a graphene sheet that separates from the metal surface. The surface Fe magnetic moments are decreased in the presence of C, and the change is dependent on the position of the adsorbed C atom. At low C coverage, C interacting with Fe has an induced magnetic moment on the order of 0.15 μB that becomes reduced as C-C structures are formed. The interstitial octahedral sites of the first interlayer below the surface are the most stable sites for carbon absorption, where carbon is more stable than when adsorbed on the surface at the same total coverage. Carbon diffusion from the surface to the subsurface at low coverage has a barrier of 0.99 eV, but the barrier is significantly reduced to 0.33 eV at higher C surface coverage. Strong lattice distortions occur due to the incorporation of carbon on the surface and subsurface, giving some insights into possible carburization pathways of the L1 2 F 3 Al phase.

The influence of Mg on the C adsorption on Ni(1 0 0): A DFT study

In this work, we performed density functional theory (DFT) calculations to study carbon adsorption on the Ni(1 0 0) surface and absorption in the bulk of nickel-based catalysts. The ideal catalyst surface was modeled as a five-layer Ni(1 0 0) slab. We also studied the influence of magnesium, considered as an atomic substitutional impurity, on carbon adsorption and migration into the bulk. To study the effect of Mg on the structure of Ni-based catalysts, we replaced a Ni atom by a Mg atom, on the surface and also in the bulk. Then, we relaxed the Mg and Ni positions. Our results showed that Ni atoms were pushed away from Mg. We also performed energy calculations to evaluate carbon interactions on different preferential sites of the Ni-slab, and also the effect of Mg atom on these deposition processes. The calculations showed that carbon adsorption on the bare Ni(1 0 0) surface was more favorable than deposition on this surface doped with magnesium. Our results also indicated that Mg could improve the interaction of surface-adsorbed carbon with gas phase oxygen, leading to the formation of a CO precursor species. In this way, magnesium promotes Ni-based catalysts hindering the surface from attaining a high carbon coverage, and thus making difficult the formation of carbon agglomerates and carbon whiskers. We also considered the effect of Ca in the C–O interaction on the metal surface.

Carbon-stimulated self-organization of Ge nanoislands during molecular beam epitaxy of Ge/CaF2/Si(111) heterostructures

The self-organization of germanium nanoislands on the surface of calcium fluoride was studied using atomic-force microscopy and reflection electron diffraction. A Ge/CaF2/Si(111) structure was grown by molecular beam epitaxy. The surface of the calcium fluoride film was modified by submonolayer carbon coverage to stimulate the formation of germanium nanoislands. The parameters of an array of nanoislands were found to depend on the coverage.

Atomic O and H exposure of C-covered and oxidized d-metal surfaces

Carbon coverage, oxidation and reduction of Au, Pt, Pd, Rh, Cu, Ru, Ni and Co layers of 1.5 nm thickness on Mo have been characterized with ARPES and desorption spectroscopy upon exposure to thermal H and O radicals. We observe that only part of the carbon species is chemically eroded by atomic H exposure, yielding hydrocarbon desorption. Exposure to atomic O yields complete carbon erosion and CO 2 and H 2O desorption. A dramatic increase in metallic and non-metallic oxide is observed for especially Ni and Co surfaces, while for Au and Cu, the sub-surface Mo layer is much more oxidized. Although volatile oxides exist for some of the d-metals, there is no indication of d-metal erosion. Subsequent atomic H exposure reduces the clean oxides to a metallic state under desorption of H 2O. Due to its adequacy, we propose the atomic oxygen and subsequent atomic hydrogen sequence as a candidate for contamination removal in practical applications like photolithography at 13.5 nm radiation.

Monte Carlo simulation of the oscillatory behavior in partial oxidation of methane on nickel catalyst under nonisothermal conditions

The Monte Carlo method has been used to simulate the kinetic oscillations during partial oxidation of methane under nonisothermal conditions. The oscillatory behavior can be found with the selected parameters by using oxide formation and removal model. From the simulation, the temperature variation during the reaction synchronizes well with the oscillations of product formation rates, and also with the rates of oxide formation and reduction processes. Compared with the isothermal simulation results, the oscillations under the nonisothermal conditions are observed to have a slightly shorter period, lower maximum carbon coverage and higher nickel oxide coverage.

Diamond nucleation on iridium: Local variations of structure and density within the BEN layer

The diamond nuclei generated by the bias enhanced nucleation (BEN) on iridium are gathered in well defined areas (“domains”). In atomic force microscopy (AFM) measurements they become manifest in a 1 nm downward step. The fine structure of the carbon layer inside and outside these domains has been studied by small spot Auger electron spectroscopy (AES), high resolution transmission electron microscopy (HRTEM), AFM and lateral force microscopy (LFM). The Auger spectra of the carbon KLL peak taken in an ultra high vacuum setup revealed diamond features inside and more graphitic features outside the domains. The comparison with the intensity of the Auger signal originating from the underlying Ir film indicates a carbon coverage inside the domains which is only by about 20% lower than outside. Cross section HRTEM images after BEN and after a short growth step, as well as AFM measurements after softly etching off the carbon layer, give no indication that the Ir had experienced a major modification by the domain formation process. Combining the information deduced from these experiments we conclude a significantly higher density of the carbon matrix within the domains. The lower friction forces measured in these regions by LFM confirm the interpretation of a material with higher elastic modulus. The present results allow to refine our understanding of structure and formation of the BEN layer on Ir.

Benzene and Its Dissociation Products on Ir{100}

Benzene forms a disorded overlayer when adsorbed on unreconstructed Ir{100} at low temperatures, but is known to dissociate yielding an ordered c(2 × 4) benzyne overlayer at moderate temperatures. Predosing the surface with a low coverage of atomic carbon, however, causes the benzyne overlayer to adopt a c(4 × 4) pattern instead. Here, we investigate the structure and energetics of these overlayers by means of density functional theory calculations, rationalizing the thermodynamic imperatives that drive the transitions between these various phases.