Adsorption Of Carbon Atoms Research Articles

Effect of methane flow in inverse diffusion flame and surface morphology on synthesis of copper–carbon nanotubes composite

ABSTRACT In an inverse diffusion flame of fuel-rich methane–air mixture, a distinct separation between a yellow flame situated atop a high-temperature blue flame structure enabled the growth of carbon nanotubes on relatively low melting temperature copper substrates within a carbon-rich precursor environment. However, high-affinity passivation layers formation on copper prevented the diffusion of carbon precursors, rendering the growth of carbon nanotubes on copper challenging. Removal of passivation layers via sulphuric acid treatment facilitates the synthesis of carbon nanotubes within the flame. Nevertheless, the impact of sulphuric acid treatment on the surface morphology of copper, and its influence on the growth of carbon nanotubes in the presence of methane–air mixtures, remains relatively unexplored. This research investigates the effects of sulphuric acid treatment and methane–air mixtures towards the growth of carbon nanotubes on copper substrates. Application of sulphuric acid treatment yields a surface characterised by high surface area-to-volume nano-hills morphology, which enhances the adsorption of carbon atoms and leads to the growth of carbon nanotubes with lifted copper nanoparticles. The optimal growth of carbon nanotubes occurs at a specific methane flow rate, where the supply of carbon precursors aligns with the diffusion of carbon atoms and growth rate of carbon nanotubes.

Effect of Scanning Electron Beam Pretreatment on Gas Carburization of 22CrMoH Gear Steel

22CrMoH was selected for the gear steel material in this work, and the temperature field change in the scanning electron beam was analyzed to determine the optimal scanning parameters and explored the effect of scanning electron beam pretreatment (Abbreviated as: SEBP) on gas-carburizing (GC) efficiency and organizational properties of gear steel. The results showed that the scanning electron beam caused the material to form a thermally deformed layer 110 μm thick, and it promoted the adsorption of carbon atoms on the surface and their inward diffusion. Under the same gas-carburizing conditions, the carburizing efficiency was improved, and the thickness of the carburized layer increased from 0.78 to 1.09 mm. Furthermore, the hardness of the GC specimens with the SEBP increased from 615 to 638 HV0.05 at 0.1 mm of the sample surface, whereas the hardness of the cross-sectional region decreased gradually, indicating that the scanning electron beam enhanced the adhesion between the carburized layer and matrix zone. A comparative analysis of the microstructures of the GC specimens with and without the SEBP showed that the carbide particles in the surface layer of the samples become smaller and that of volume fraction of residual austenite reduced in size. In terms of the mechanical properties, the surface friction coefficient decreased from 0.87 to 0.46 μ and the GC specimen with the SEBP had a higher cross-sectional hardness gradient. Its friction coefficient was reduced from approximately 0.8 to almost 0.45 μ, and the wear amount of the specimens with SEBP was 47.7% of that of the matrix specimens.

Study of the Mechanical and Thermal Properties of Refractory Mortars from Kaolin and Bentonite

In this paper, Mortar was prepared from medium alumina refractory grog, bricks crashed as a mean material to a particular size, and Iraqi raw (kaolin or bentonite) as binding materials. Refractory bricks were crushed, milled, then sieved to three particle sizes: fine as (1.18 >fine> 0) mm, medium as (2.36 > medium > 1.18) mm, crushed as (400 > coarse > 2.36) mm. Then these particle sizes were mixed with Iraqi raw kaolin or bentonite with selected ratios (10,15,20,30 and 40) %. Specimens were formed by the wetting method, then drying it at laboratory temperature for one day, followed by firing it at 1200 ℃. Results showed that the porosity of specimens decreases when increasing the clay ratio from 3-4% (kaolin or bentonite), and the bond strength between grog and clay increases when increasing the clay ratio from 2-3% (kaolin or bentonite). Also, the diametrical strength increases when increasing the clay ratio from 4-7% (kaolin or bentonite). The thermal shock results showed that K-mortar is better than B-mortar, depending on the results we obtained through the effect of temperature and diametrical strength. The SEM results showed that mortar structure was produced by adding 40% bentonite with small irregularly shaped. The mortar was produced by adding 40% of kaolin which possesses regular mullite crystals. Finally, the results of the test EDS that K-mortar were revealed in showed that there is no adsorption of carbon while B-mortar showed that there is adsorption of carbon atoms.

Effect of rare-earth doping on adsorption of carbon atom on ferrum surface and in ferrum subsurface: A first-principles study

The adsorption of carbon atom on Fe surface and in Fe subsurface with and without rare earth (La and Ce) substitution in the surface layer and subsurface layer was studied by first-principles calculations. The carbon atom is predicted to adsorb at hollow and long bridge site on Fe(100) and Fe(110), respectively. However, the carbon atom shifts to occupy preferentially hollow site on both Fe(100) and Fe(110) with rare earth atom doping at surface layer. The lower adsorption energies involved with stronger adsorption abilities were obtained for carbon atoms on Fe surface with rare earth doping at surface layer, which was determined by the electronic structure of the surface atoms. The La atom was pulled out the surface after carbon adsorption due to strong interaction of La–C, which is consistent with the more charge transfer. In the subsurface region, the carbon atom prefers to occupy at octahedral site with rare earth doping at surface layer in Fe slab. These strong adsorption energies of the carbon atoms on Fe surface and in Fe subsurface with rare earth pose relevant insights into the interaction between carbon and rare earth, which helps to understanding the influence mechanism of rare earth in carburizing.

Crystallographic orientation dependence of work function: carbon adsorption on Au surfaces

We investigate the work function (WF) variation of different Au crystallographic surface orientations with carbon atom adsorption. Ab initio calculations within density-functional theory are performed on carbon deposited (100), (110), and (111) gold surfaces. The WF behaviour with carbon coverage for the different surface orientations is explained by the resultant electron charge density distributions. The dynamics of carbon adsorption at sub-to-one- monolayer (ML) coverage depends on the landscape of the potential energy surfaces. At higher ML coverage, because of adsorption saturation, the WF will have weak surface orientation dependence. This systematic study has consequential bearing on studies of electric-field noise emanating from polycrystalline gold ion-trap electrodes that have been largely employed in microfabricated electrodes.

Prediction of Adsorption Energies for Chemical Species on Metal Catalyst Surfaces Using Machine Learning

Computational catalyst screening has the potential to significantly accelerate heterogeneous catalyst discovery. Typically, this involves developing microkinetic reactor models that are based on parameters obtained from density functional theory and transition-state theory. To reduce the large computational cost involved in computing various adsorption and transition-state energies of all possible surface states on a large number of catalyst models, linear scaling relations for surface intermediates and transition states have been developed that only depend on a few, typically one or two descriptors, such as the carbon atom adsorption energy. As a result, only the descriptor values have to be computed for various active site models to generate volcano curves in activity or selectivity. Unfortunately, for more complex chemistries the predictability of linear scaling relations is unknown. Also, the selection of descriptors is essentially a trial and error process. Here, using a database of adsorption energi...

Characteristic of Low Temperature Carburized Austenitic Stainless Steel

Low temperature carburizing process has been carried out on austenitic stainless steel (ASS) type AISI 316L, that contain chromium in above 12 at%. Therefore, conventional heat treatment processes that are usually carried out at high temperatures are not applicable. The sensitization process due to chromium migration from the grain boundary will lead to stress corrosion crack and decrease the corrosion resistance of the steel. In this study, the carburizing process was carried out at low temperatures below 500 °C. Surface morphology and mechanical properties of carburized specimens were investigated using optical microscopy, non destructive profilometer, and Vicker microhardness. The surface roughness analysis show the carburising process improves the roughness of ASS surface. This improvement is due to the adsorption of carbon atoms on the surface of the specimen. Likewise, the hardness test results indicate the carburising process increases the hardness of ASS.

Intrasurface electron transition contribution to energy of adsorption of silicon at the SiC(0001) surface – A density functional theory (DFT) study

Adsorption of silicon and carbon atoms at polar SiC(0001) and SiC(0001) surfaces at low coverage was studied using ab initio calculations. The single Si atom is adsorbed in the H3 site at both surfaces, saturating broken bonds of the three neighboring atoms. The adsorption of carbon atom at carbon terminated SiC(0001) surface leads to sp2 graphene-like reconstruction and the release of silicon atom. The adsorption energy of all constituents is high, exceeding 6eV. The energy barriers for diffusion of all atoms were calculated showing that Si atom is highly mobile at SiC(0001) surface. C adatom at SiC(0001) and Si adatom at SiC(0001) mobility is several order of magnitude lower. C adatom destroys SiC(0001) lattice, thus excess of carbon is detrimental to SiC growth.

Ring structures formed inside voids in SiO2 layer on Si(100) during thermal decomposition

Ring structures inside voids in the SiO2 layer on a Si(100) substrate, which are concentrically formed by repeating thermal annealing in vacuum, have been investigated by scanning electron microscopy and atomic force microscopy. We demonstrate that slight exposure of the surface to volatile organic compounds during a cooling process significantly affects the formation of the ring structures. This result clearly shows that the key to ring-structure formation is surface adsorption of carbon atoms, which probably suppresses surface migration of silicon atoms. Our research provides a novel technique for the fabrication of nanostructured semiconductors for such applications as quantum effect devices.

Atomic carbon adsorption on Ni38/Co38 clusters and three low-index Ni/Co surfaces: a density functional theory study

An investigation of the adsorption of carbon atoms on Ni and Co models including Ni38/Co38 clusters and Ni/Co (100), (111) and (110) surfaces using the density functional theory is described. The carbon atoms adsorbed on Ni models and those on Co models were compared in terms of binding energy and bond length. Also, the density of electronic states and the electron density distributions in carbon atoms adsorbed on the Ni and Co surfaces were investigated. The preferred sites for carbon adsorption were obtained, while the reconstruction of Ni and Co during the process of carbon adsorption was observed. The results revealed that the (100) hollow sites in Ni and Co models were preferred for carbon adsorption, while the adsorption capacity of the Ni38 cluster was higher than that of the Co38 cluster.

Theoretical Investigation of Carbon Atom Adsorption and Diffusion on the Surfaces of Cu

The adsorption and diffusion of carbon atom on Cu (111) and (100) surfaces have been investigated based on first-principles density-functional theory. For Cu (111) surface, the hexagonal close-packed and face-centered cubic sites are the most stable sites with little energy difference in the adsorption energy. For Cu (100) surface, the hollow site is the most stable. There is charge transfer from Cu surface to the adsorbed carbon atom. Moreover, the diffusions of carbon atom on Cu surfaces have been investigated, and the results show that the diffusion of carbon atom prefers to happen on Cu (111) surface.

Computational study of graphene growth on copper by first-principles and kinetic Monte Carlo calculations.

In this work the growth of a graphene monolayer on copper substrate, as typically achieved via chemical vapor deposition of propene (C3H6), was investigated by first-principles and kinetic Monte Carlo calculations. A comparison between calculated C1s core-level binding energies and electron spectroscopy measurements showed that graphene nucleates from isolated carbon atoms adsorbed on surface defects or sub-superficial layers upon hydrocarbon fragmentation. In this respect, ab initio nudged elastic band simulations yield the energetic barriers characterizing the diffusion of elemental carbon on the Cu(111) surface and atomic carbon uptake by the growing graphene film. Our calculations highlight a strong interaction between the growing film edges and the copper substrate, indicative of the importance of the grain boundaries in the epitaxy process. Furthermore, we used activation energies to compute the reaction rates for the different mechanisms occurring at the carbon-copper interface via harmonic transition state theory. Finally, we simulated the long-time system growth evolution through a kinetic Monte Carlo approach for different temperatures and coverage. Our ab initio and Monte Carlo simulations of the out-of-equilibrium system point towards a growth model strikingly different from that of standard film growth. Graphene growth on copper turns out to be a catalytic, thermally-activated process that nucleates from carbon monomers, proceeds by adsorption of carbon atoms, and is not self-limiting. Furthermore, graphene growth seems to be more effective at carbon supersaturation of the surface-a clear fingerprint of a large activation barrier for C attachment. Our growth model and computational results are in good agreement with recent X-ray photoelectron spectroscopy experimental measurements.

Catalytically healing the Stone–Wales defects in graphene by carbon adatoms

Graphene with a perfect hexagonal network structure is desirable for various reasons, e.g., mechanical, thermal conductivity and transport properties. Yet, the embedded defects generated either in synthesis or usage stages have posed obstacles for graphene applications. Therefore, removal of the structural defects in graphene has remained an important task. Stone–Wales (SW) defects are one typical topological structure in the carbon nanomaterials. Unfortunately, the SW defects in graphene have to overcome a very high restoration barrier (ca. 6 eV). Very recent theoretical work has shown the promise to reduce the restoration barrier by the adsorbed transition metal atoms down to 2.86 eV (for W) (yet this is still too high). In the present density functional theory (DFT) study, we find that through a mechanically different process, the adsorption of carbon atoms can dramatically reduce the restoration barrier to hitherto the lowest value, i.e., 20.0 kcal mol−1 (0.87 eV), which could make the SW-healing experimentally accessible. Subsequently, the C-adatom can migrate very easily on the graphene surface. As a result, one carbon adatom could principally catalyze the healing of all the SW defects in a cascade mode if no termination steps exist. During the graphene growth, the presently proposed carbon-adatom catalytic mechanism could have played a role in healing the SW defect. Moreover, we propose that in the post-treatment of graphene, adsorption of the carbon adatom could be used as an effective catalyst for the SW-healing. The catalytic role of carbon atoms on the SW defect should be included in the modeling of graphene growth.

First-principles study of carbon atoms adsorbed on MgO(100) related to graphene growth

We have performed density functional theory calculations to understand the initial growth of graphene by studying the adsorption of carbon atoms on the oxide substrates such as magnesium oxide. For adsorption behaviors of carbon atoms on the MgO(100) surface, their adsorption geometries and binding energies are calculated. The binding of a carbon atom is the most stable at the on-top oxygen site on MgO(100). Such strong C–O binding is analyzed by examining the projected density of states. Then, we also increase the number of carbon atoms on MgO(100) to investigate their adsorption behaviors. Due to strong binding between carbon atoms, adsorbed carbon atoms form chain-like or graphene-like structures on the surface. Combined with relatively strong C–O binding, this result may explain the graphene growth on MgO(100) observed in available experiments.

Computational studies of graphene growth mechanisms

Density functional theory (DFT) and semiempirical tight-binding (TB) methods have been used to study the mechanism of graphene growth in the presence and absence of a catalytic surface. Both DFT and TB geometry optimized structures relevant to graphene growth show that the minimum energy growth mechanism is via the sequential addition of carbon hexagons at the edge of the graphene sheet. Monte Carlo (MC) simulations based on the TB model show that defect-free graphene sheets can be grown provided one has the proper combination of temperature, chemical potential, and addition rate. In this work, growth of perfect graphene structures has been simulated at the atomic level. Comparison of the growth mechanism in the absence and presence of a nickel catalyst surface shows that the catalyst (i) allows for adsorption of carbon atoms at surface and subsurface sites, (ii) enables formation of long, stable strings of carbon atoms, and (iii) stabilizes small flakes of graphene that can act as precursors to subsequent growth.

Effect of boron promotion on the stability of cobalt Fischer–Tropsch catalysts

Density functional theory (DFT) calculations indicate that boron atoms are thermodynamically stable at step, p4g clock, and subsurface sites of a Co catalyst under Fischer–Tropsch synthesis (FTS) conditions. Moreover, the presence of boron at step and clock sites is calculated to destabilize the adsorption of carbon atoms at neighboring sites by +160 and +108 kJ/mol, respectively. The calculations hence suggest that boron promotion can selectively block the deposition, nucleation, and growth of resilient carbon species. To experimentally evaluate this concept, the deactivation of a 20 wt.% Co/γ-Al 2O 3 catalyst promoted with 0.5 wt.% boron was studied for 200 h during FTS at 240 °C and 20 bar. Boron promotion was found to reduce the deactivation rate more than six-fold, without affecting the initial activity or selectivity. Characterization with X-ray photoelectron spectroscopy (XPS) and temperature-programmed hydrogenation (TPH) confirms that boron promotion reduces the deposition of resilient carbon species.

Carbon on Platinum Substrates: From Carbidic to Graphitic Phases on the (111) Surface and on Nanoparticles

The formation of carbonaceous deposits on Pt(111) surfaces and Pt nanoparticles has been studied using suitable models and density-functional calculations. The study addresses a broad range of processes, from the very first stage of carbon deposition up to a final building of graphene monolayers (ML) defined as a 1:1 ratio of the number of C atoms to surface Pt atoms. A carbidic phase is formed below a coverage of approximately 0.3 ML, when negatively charged carbon atoms are strongly adsorbed preferentially on fcc hollow sites. On Pt nanoparticles, the adsorption of carbon atoms seems to be enhanced near particle edges due to the special flexibility of defect sites. Above a coverage of approximately 0.3 ML, the formation of small C(n) aggregates becomes possible. Interestingly, thermodynamics favors the formation of C(3) trimers at a coverage of 0.33 ML, whereas the formation of C(2) dimers requires a higher coverage of 0.5 ML. The covalently bonded C(2) species is supposed to be the key fragment for the formation of benzene-like rings at coverages above 0.6 ML. These rings are expected to be the building blocks for the graphene monolayer. However, the typical electronic structure of graphene is not observed until a coverage above approximately 1.8 ML is reached. We corroborated the experimentally suggested carbon double-layer to be stable. It is proposed to consist of a monolayer of carbidic atoms C adsorbed on Pt with a graphene layer adsorbed on the carbidic layer. Some of the carbidic atoms serve as anchors for the graphene layer, with noticeably strong covalent bonds formed. This double-layer model would imply a much higher adhesion of the graphene layer than in the single-layer model.

Carbon atom adsorption on and diffusion into Fe(110) and Fe(100) from first principles

We employ spin-polarized periodic density functional theory (DFT) to examine carbon atom adsorption on, absorption in, and diffusion into Fe(110) and Fe(100). We find that carbon atoms bind strongly with Fe surfaces and prefer high coordination sites. The carbon atom is predicted to adsorb on the long-bridge site on Fe(110) and the fourfold hollow site on Fe(100). Due to the very short distance between the carbon atom and the subsurface Fe atom of Fe(100), the carbon atom binds more strongly with Fe(100) than with Fe(110). In the subsurface region, the carbon atom prefers the octahedral site, as in bulk Fe. We find that the carbon atom is more stable in the subsurface octahedral site of Fe(110) than that of Fe(100), since the strain caused by the interstitial carbon atom is released by pushing one surface Fe atom towards vacuum by 0.5 \AA{} in Fe(110), while the distortion in Fe(100) propagates far into the lattice. Diffusion of carbon atoms into Fe(110) and Fe(100) subsurfaces goes through transition states where the carbon atom is coordinated to four Fe atoms. The barriers to diffusion into Fe(110) and Fe(100) are 1.18 eV and 1.47 eV, respectively. The larger diffusion barrier into Fe(100) is mainly due to the stronger bonding between carbon and the Fe(100) surface. We predict that the rate-limiting step for $\mathrm{C}$ incorporation into bulk Fe is the initial diffusion to subsurface sites, while the rate-limiting step for absorbed carbon segregation to the surface is bulk diffusion, with no expected difference between rates to segregate to different surfaces. Lastly, we predict that graphite formation will be more favorable on $\mathrm{C}$-covered Fe(110) than $\mathrm{C}$-covered Fe(100).

Adsorption and Reaction of C2N2 on Si(100)-2 × 1: A Computational Study with Single- and Double-Dimer Cluster Models

The adsorption and decomposition of C2N2 on the Si(100)-2 × 1 surface have been calculated by the hybrid density functional B3LYP method with Si9H12 and Si15H16 as single- and double-dimer models, respectively. The result of our single-dimer surface model calculation shows that the surface reaction started from a single N atom of C2N2 molecularly adsorbed on one silicon atom of the dimer. From the result of bond distance changes and vibrational frequency analysis, this process occurs by direct interaction of the C⋮N group with a silicon dangling bond. Two different reaction paths follow; the first path occurs by the adjoined carbon atom adsorption producing four-member-ring product −Si−N−C−(CN)Si−, similar to the HCN molecule adsorbing sideway on this surface. The other path occurs by the adsorption of the second nitrogen atom with another Si atom producing a six-member-ring product, −Si−N−C−C−N−Si−. The mechanisms for the decomposition of these adsorbates have been elucidated. The result of our calculation with the double-dimer surface model reveals that the reaction barriers are somewhat lower than single-dimer system for either CC or CN bond-breaking processes but with a similar trend. The predicted stabilities of various surface species from physisorbed C2N2 to chemisorbed CN radicals and silicon nitride and silicon carbide are consistent with the results of our previous studies using HREELS, XPS, UPS, and TPD methods as well as with others' data for similar reactions of CN-containing species with the Si(100)-2 × 1 surface. The predicted result for adsorption of two C2N2 based on both surface models indicates a significant decrease in the adsorption energy for the second C2N2 molecule.

Two mechanisms of diamond phase formation during magnetron sputter deposition of amorphous carbon films

The diamond phase formation mechanisms are analysed by a new phenomenological model of deposition of diamond-like carbon (DLC) films on Si(001) substrate. Experimental observations that make base for this model are reviewed briefly. Only the main processes, such as adsorption of carbon atoms on the surface, formation of SiC, diamond, and graphite sites, are assumed to take place. Compressive stress, induced by ion subplantation, is regarded as a main factor causing the transformation of graphite to diamond. Relationship between stress and ion energy is given by Davis’ formula. According to this model, dependences of film deposition rate versus time and dependences of film composition versus film thickness are calculated that are in agreement with experimental ones. Also, the ratio of sp/sp volume fractions as a function of substrate bias voltage was calculated. Modelling of experimental curves show that under conditions of sputter deposition of DLC films there are at least two diamond phase formation mechanisms: (i) on the surface during carbon adsorption, and (ii) in the bulk of film by subplantation induced graphite transition to lonsdaleite (hexagonal diamond).