Ab initio calculations of the reaction pathways for methane decomposition over the Cu (111) surface

Optimizing Hydrogen Production: Influence of Promoters in Methane Decomposition on Titania-Modified-Zirconia Supported Fe Catalyst

This study addresses the pivotal challenge of hydrogen production through methane decomposition, offering a pathway to achieving clean energy goals. Investigating the utilization of titania-modified zirconia dual redox supports (10TiZr) in iron or doped iron-based catalysts for the CH4 decomposition reaction, our research involves a thorough characterization process. This includes analyses of the surface area porosity, X-ray diffraction, Raman-infrared spectroscopy, and temperature-programmed reduction/oxidation. The observed sustained enhancement in catalytic activity over extended durations suggests the in situ formation of catalytically active sites. The introduction of Co or Ni into the 30Fe/10TiZr catalyst leads to the generation of a higher density of reducible species. Furthermore, the Ni-promoted 30Fe/10TiZr catalyst exhibits a lower crystallinity, indicating superior dispersion. Notably, the cobalt-promoted 30Fe/10TiZr catalyst achieves over 80% CH4 conversion and H2 yield within 3 h. Additionally, the Ni-promoted 30Fe/10TiZr catalyst attains a remarkable 87% CH4 conversion and 82% H2 yield after 3 h of the continuous process.

Mechanisms for Graphene Growth on SiO2 Using Plasma-Enhanced Chemical Vapor Deposition: A Density Functional Theory Study

Plasma-enhanced chemical vapor deposition (PE-CVD) of graphene layers on dielectric substrates is one of the most important processes for the incorporation of graphene in semiconductor devices. Graphene is moving rapidly from the laboratory to practical implementation; therefore, devices may take advantage of the unique properties of such nanomaterial. Conventional approaches rely on pattern transfers after growing graphene on transition metals, which can cause nonuniformities, poor adherence, or other defects. Direct growth of graphene layers on the substrates of interest, mostly dielectrics, is the most logical approach, although it is not free from challenges and obstacles such as obtaining a specific yield of graphene layers with desired properties or accurate control of the growing number of layers. In this work, we use density-functional theory (DFT) coupled with ab initio molecular dynamics (AIMD) to investigate the initial stages of graphene growth on silicon oxide. We select C₂H₂ as the PE-CVD precursor due to its large carbon contribution. On the basis of our simulation results for various surface models and precursor doses, we accurately describe the early stages of graphene growth, from the formation of carbon dimer rows to the critical length required to undergo dynamical folding that results in the formation of low-order polygonal shapes. The differences in bonding with the functionalization of the silicon oxide also mark the nature of the growing carbon layers as well as shed light of potential flaws in the adherence to the substrate. Finally, our dynamical matrix calculations and the obtained infrared (IR) spectra and vibrational characteristics provide accurate recipes to trace experimentally the growth mechanisms described and the corresponding identification of possible stacking faults or defects in the emerging graphene layers.

Predicting Graphene Growth on Cu: Universal Kinetic Growth Model and Its Experimental Verification

The kinetics of the chemical vapor deposition (CVD) of graphene on Cu in CH₄ + H₂ were investigated by monitoring the graphene flake size as a function of CVD growth time. A growth model was set up which relates the CVD parameters to the mass action constant Q_exp of the methane decomposition reaction toward graphene at a given temperature T. Graphene growth was shown to proceed from pre-equilibrated adsorbed carbon (C_ad) within a wide CVD parameter range. The model not only leads to the correct scaling relation of the growth kinetics but quantitatively determines how far the CVD parameters deviate from thermal equilibrium and correctly predicts the absolute flake size increase per time. Fitting experimental data delivers the energy barrier for carbon detachment from the graphene island edge (E_det = 4.7 ± 0.3 eV) and the methane decomposition entropy toward C_ad on Cu (Δ_decS° = 260 ± 20 J mol^-1 K^-1). The latter value is used to estimate the vanishingly small C_ad equilibrium concentration of 3 × 10^-10 monolayers at 1045 °C. The universal validity of the model is proven by comparison with literature data providing the correct order of magnitude growth velocities up to 1000 μm/h. The performed reactor experiments deliver data that match the predicted flake growth velocity with a precision of about 50%. The obtained results can be used to calibrate any hot wall CVD reactor setup for the methane decomposition reaction toward graphene on Cu. The description can be directly applied for any hydrocarbon in the gas feed, and the technique can be easily applied for other catalytic support surfaces.

Bonding of C1 fragments on metal nanoclusters: a search for methane conversion catalysts with swarm intelligence

There is a need for a catalyst that can directly convert methane into useful substances. The use of Ni as a catalyst for the steam reforming of methane has led us to look at Ni nanoclusters as potential candidates for the direct conversion of methane. Fe, Co, Cu, and Zn nanoclusters are also focused on. How the type of C1 fragments (CH4, CH3, CH2, CH, and C) stabilized by the metal nanoclusters as adsorbed species varies with metal species is theoretically investigated. The particle swarm optimization algorithm, which is based on swarm intelligence, as well as density functional theory, is used for this calculation. The Ni nanoclusters are found to preferentially adsorb C as a stable species, the Fe and the Co nanoclusters both CH and CH3, and the Cu nanoclusters CH3; the Zn nanoclusters are found not to chemisorb any C1 fragment. The methane activation capacity can be ranked in the order of Ni > Fe > Co > Cu > Zn. The highest methane activation capacity of Ni is due to the strongest covalent nature of the interaction between Ni and the adsorbed species. The ionicity of the bond between Fe and the adsorbed species is higher than that between Co and the adsorbed species, while the covalent nature of the bonds is comparable for both. The weak methane activation ability of Cu compared to Fe, Co, and Ni is found to be due to the fact that both the covalent and ionic bond strengths between Cu and the adsorbed species are weak. Zn and the adsorbed species form neither ionic nor covalent bonds. These results indicate that the Fe and the Co nanoclusters as well as the Ni may lead to the over-oxidation of methane, whereas the Zn nanoclusters cannot activate methane in the first place; therefore, their application to direct methane conversion catalysts is unlikely. Since the Cu nanoclusters do not adsorb C and CH as stable species, but CH3 stably, the Cu nanoclusters are expected to work as a catalyst for the direct conversion of methane.

Chemical vapour deposition of graphene on copper-nickel alloys: the simulation of a thermodynamic and kinetic approach

Chemical vapour deposition (CVD) of graphene on transition metals is generally believed to be the fabrication route best suited for the production of high-quality large-area graphene sheets. The mechanism of CVD graphene growth is governed by interactions in both the gas phase and at the surface. Here we present a simulation of the CVD graphene growth mechanism which includes thermodynamics, gas phase kinetics and the surface reaction in a sequential manner. The thermodynamic simulation shows that the deposition driving force is the greatest for high carbon to hydrogen ratios and reaches a maximum at around 850 °C. No graphene growth is observed below this temperature. The surface kinetic model also shows that below this temperature, the carbon surface concentration is less than the solubility limit, thus no film can grow. The effect of the reaction chamber geometry on the product concentrations was clear from the gas phase decomposition reactions. The gas residence times studied here (around 0.07 s) show that the optimum gas phase composition is far from that expected at thermodynamic equilibrium. The surface kinetics of CH₄ reactions on Ni, Cu and Cu-Ni surfaces shows good agreement with the experimental results for different growth pressures (0.1 to 0.7 mbar), temperatures (600 to 1200 °C) and different Ni thicknesses (25-500 μm). Also, the model works well when substrates with various C solubilities are used. The thermodynamic and kinetic models described here can be used for the design of improved reactors to optimise the production of graphene with differing qualities, either single or multi-layer and sizes. More importantly, the transfer to a continuous process with a moving substrate should also be possible using the model if it is extended from 2D to 3D.

Review of fabrication methods of large-area transparent graphene electrodes for industry

Graphene is a two-dimensional material showing excellent properties for utilization in transparent electrodes; it has low sheet resistance, high optical transmission and is flexible. Whereas the most common transparent electrode material, tin-doped indium-oxide (ITO) is brittle, less transparent and expensive, which limit its compatibility in flexible electronics as well as in low-cost devices. Here we review two large-area fabrication methods for graphene based transparent electrodes for industry: liquid exfoliation and low-pressure chemical vapor deposition (CVD). We discuss the basic methodologies behind the technologies with an emphasis on optical and electrical properties of recent results. State-of-the-art methods for liquid exfoliation have as a figure of merit an electrical and optical conductivity ratio of 43.5, slightly over the minimum required for industry of 35, while CVD reaches as high as 419.

Controlled Growth of Single-Crystal Graphene Films

Grain boundaries produced during material synthesis affect both the intrinsic properties of materials and their potential for high-end applications. This effect is commonly observed in graphene film grown using chemical vapor deposition and therefore caused intense interest in controlled growth of grain-boundary-free graphene single crystals in the past ten years. The main methods for enlarging graphene domain size and reducing graphene grain boundary density are classified into single-seed and multiseed approaches, wherein reduction of nucleation density and alignment of nucleation orientation are respectively realized in the nucleation stage. On this basis, detailed synthesis strategies, corresponding mechanisms, and key parameters in the representative methods of these two approaches are separately reviewed, with the aim of providing comprehensive knowledge and a snapshot of the latest status of controlled growth of single-crystal graphene films. Finally, perspectives on opportunities and challenges in synthesizing large-area single-crystal graphene films are discussed.

Synthesis of Diverse Green Carbon Nanomaterials through Fully Utilizing Biomass Carbon Source Assisted by KOH

With multiple properties, green carbon nanomaterials with high specific surface area have become extensively attractive as energy storage devices with environmental-friendly features. The primary synthesis attempts were based on alkalis activation, which, however, faced the dilemma of low utilization rate of carbon sources. Herein, the green carbon with ultrahigh surface area (up to 3560 m²/g) was prepared by the KOH-assisted biomass carbonization. Moreover, the redundant K₂O steam and C_xH_y flow were further utilized; as a result, the carbon materials with a wide range of morphological diversity were collected on the Cu foam. Accordingly, we carried out density functional theory simulations to reveal the mechanism of O-adatom-promoted CH₄ dissociation over the Cu surface for carbon formation. The electrodes of electrochemical capacitor fabricated by carbon synthesis possess a 170% higher specific capacitance compared with commercial carbon electrodes. As such, this strategy might be promising in developing hierarchical carbons along with sufficient carbon sources for broadening their potential applications.

Graphene growth through a recrystallization process in plasma enhanced chemical vapor deposition

Thermal chemical vapor deposition (TCVD) is the current method of choice to fabricate high quality, large area graphene films on catalytic copper substrates. In order to obtain sufficiently high growth rates at reduced growth temperatures an efficient dissociation of the precursor molecules already in the gas phase is required. We used plasma enhanced chemical vapor deposition (PECVD) to fabricate high quality graphene films at various temperatures. The efficient, plasma-induced dissociation of the precursor molecules results in an activation energy of 2.2 eV for the growth rate in PECVD, which is reduced by almost a factor of 2 compared to TCVD growth in the same reactor. By varying the growth time, we demonstrate that crystalline graphene grains surrounded by amorphous carbon formed during the early stage of growth merge into an almost defect-free graphene film with growth time via a recrystallization process. Almost defect-free graphene is prepared with negligible (I _D/I _G < 0.1) contributions of the D peak in Raman spectroscopy and with a sheet resistance down to 470 Ω/sq.

Highly transparent light emitting diodes on graphene encapsulated Cu nanowires network

The internal quantum efficiency of blue LEDs is almost close to the limit, therefore, advanced transparent electrode has been long explored for gaining high external quantum efficiency. However, work function mismatch at electrode-semiconductor interface remains the fundamental difficulty in obtaining low resistance ohmic contact. Here, we demonstrate the gas phase encapsulation of graphene layer on superfine Cu nanowires network by chemical vapor deposition for highly transparent LEDs. The fast encapsulation of graphene shell layer on Cu nanowires achieves high optoelectronic performance (33 Ω/sq @ 95% T), broad transparency range (200~3000 nm) and strong antioxidant stability. A novel phenomenon of scattered-point contact is revealed at the Cu nanowires/GaN interface. Point discharge effect is found to produce locally high injection current through contact points, which can effectively overcome Schottky barrier and form ohmic contact. The transparent LED on Cu@graphene nanowire network is successfully lighted with bright blue emission.

Intergrain Diffusion of Carbon Radical for Wafer-Scale, Direct Growth of Graphene on Silicon-Based Dielectrics

Graphene intrinsically hosts charge-carriers with ultrahigh mobility and possesses a high quantum capacitance, which are attractive attributes for nanoelectronic applications requiring graphene-on-substrate base architecture. Most of the current techniques for graphene production rely on the growth on metal catalyst surfaces, followed by a contamination-prone transfer process to put graphene on a desired dielectric substrate. Therefore, a direct graphene deposition process on dielectric surfaces is crucial to avoid polymer-adsorption-related contamination from the transfer process. Here, we present a chemical-diffusion mechanism of a process for transfer-free growth of graphene on silicon-based gate-dielectric substrates via low-pressure chemical vapor deposition. The process relies on the diffusion of catalytically produced carbon radicals through polycrystalline copper (Cu) grain boundaries and their crystallization at the interface of Cu and underneath silicon-based gate-dielectric substrates. The graphene produced exhibits low-defect multilayer domains ( L_a ∼ 140 nm) with turbostratic orientations as revealed by selected area electron diffraction. Further, graphene growth between Cu and the substrate was 2-fold faster on SiO₂/Si(111) substrate than on SiO₂/Si(100). The process parameters such as growth temperature and gas compositions (hydrogen (H₂)/methane (CH₄) flow rate ratio) play critical roles in the formation of high-quality graphene films. The low-temperature back-gating charge transport measurements of the interfacial graphene show density-independent mobility for holes and electrons. Consequently, the analysis of electronic transport at various temperatures reveals a dominant Coulombic scattering, a thermal activation energy (2.0 ± 0.2 meV), and two-dimensional hopping conduction in the graphene field-effect transistor. A band overlapping energy of 2.3 ± 0.4 meV is estimated by employing the simple two-band model.

Surface-assisted dehydrogenative homocoupling and cyclodehydrogenation of mesityl groups on a copper surface

We present for the first time an adjacent dehydrogenative homocoupling and cyclodehydrogenation reaction of mesityl groups to form a synthetic nanoribbon on Cu(110). Submolecular resolution scanning tunneling microscopy (STM) observations and density functional theory (DFT) calculations confirm the formation of an ethylene (-CH[double bond, length as m-dash]CH-) unit or an aromatic six-membered ring group.

A review of theoretical study of graphene chemical vapor deposition synthesis on metals: nucleation, growth, and the role of hydrogen and oxygen

Graphene has attracted intense research interest due to its extraordinary properties and great application potential. Various methods have been proposed for the synthesis of graphene, among which chemical vapor deposition has drawn a great deal of attention for synthesizing large-area and high-quality graphene. Theoretical understanding of the synthesis mechanism is crucial for optimizing the experimental design for desired graphene production. In this review, we discuss the three fundamental steps of graphene synthesis in details, i.e. (1) decomposition of carbon feedstocks and formation of various active carbon species, (2) nucleation, and (3) attachment and extension. We provide a complete scenario of graphene synthesis on metal surfaces at atomistic level by means of density functional theory, molecular dynamics (MD), Monte Carlo (MC) and their combination and interface with other simulation methods such as quantum mechanical molecular dynamics, density functional tight binding molecular dynamics, and combination of MD and MC. We also address the latest investigation of the influences of the hydrogen and oxygen on the synthesis and the quality of the synthesized graphene.

An ab initio study of the nickel-catalyzed transformation of amorphous carbon into graphene in rapid thermal processing

Ab initio molecular dynamics (AIMD) simulations are employed to investigate the chemical mechanism underlying the Ni-catalyzed transformation of amorphous carbon (a-C) into graphene in the rapid thermal processing (RTP) experiment to directly grow graphene on various dielectric surfaces via the evaporation of surplus Ni and C at 1100 °C (below the melting point of bulk Ni). It is found that the a-C-to-graphene transformation entails the metal-induced crystallization and layer exchange mechanism, rather than the conventional dissolution/precipitation mechanism typically involved in Ni-catalyzed chemical vapor deposition (CVD) growth of graphene. The multi-layer graphene can be tuned by changing the relative thicknesses of deposited a-C and Ni thin films. Our AIMD simulations suggest that the easy evaporation of surplus Ni with excess C is likely attributed to the formation of a viscous-liquid-like Ni-C solution within the temperature range of 900-1800 K and to the faster diffusion of C atoms than that of Ni atoms above 600 K. Even at room temperature, sp(3)-C atoms in a-C are quickly converted to sp(2)-C atoms in the course of the simulation, and the graphitic C formation can occur at low temperature. When the temperature is as high as 1200 K, the grown graphitic structures reversely dissolve into Ni. Because the rate of temperature increase is considerably faster in the AIMD simulations than in realistic experiments, defects in the grown graphitic structures are kinetically trapped. In this kinetic growth stage, the carbon structures grown from sp(3)-carbon or from sp(2)-carbon exhibit marked differences.

Insights into the synergistic role of metal–lattice oxygen site pairs in four-centered C–H bond activation of methane: the case of CuO

Of the three mechanisms for activation of methane on copper and copper oxide surfaces, the under-coordinated Cu–O site pair mediated mechanism on CuO surfaces has the lowest activation energy barriers.

Ethylene decomposition on Ir(111): initial path to graphene formation

The complete mechanism behind the thermal decomposition of ethylene (C₂H₄) on Ir(111), which is the first step of graphene growth, is established for the first time employing a combination of experimental and theoretical methods.

Cation doping size effect for methane activation on alkaline earth metal doping of the CeO2 (111) surface

Alkaline earth metal doping of CeO₂ improves the dissociative adsorption of methane with the behaviour showing a correlation to dopant ionic radius.

Molecular understandings on the activation of light hydrocarbons over heterogeneous catalysts

Due to the depletion of petroleum and the recent shale gas revolution, the dropping of the price for light alkanes makes alkanes an attractive feedstock for the production of light alkenes and other valuable chemicals. Understanding the mechanism for the activation of C-H bonds in hydrocarbons provides fundamental insights into this process and a guideline for the optimization of catalysts used for the processing of light alkanes. In the last two decades, density functional theory (DFT) has become a powerful tool to explore elementary steps and mechanisms of many heterogeneously catalyzed processes at the atomic scale. This review describes recent progress on computational understanding of heterogeneous catalytic dehydrogenation reactions of light alkanes. We start with a short description on basic concepts and principles of DFT as well as its application in heterogeneous catalysis. The activation of C-H bonds over transition metal and alloy surfaces are then discussed in detail, followed by C-H activation over oxides, zeolites and catalysts with single atoms as active sites. The origins of coking formation are also discussed followed by a perspective on directions of future research.

Cooperative island growth of large-area single-crystal graphene on copper using chemical vapor deposition

In this work we explore the kinetics of single-crystal graphene growth as a function of nucleation density. In addition to the standard methods for suppressing nucleation of graphene by pretreatment of Cu foils using oxidation, annealing, and reduction of the Cu foils prior to growth, we introduce a new method that further reduces the graphene nucleation density by interacting directly with the growth process at the onset of nucleation. The successive application of these two methods results in roughly 3 orders of magnitude reduction in graphene nucleation density. We use a kinetic model to show that at vanishingly low nucleation densities carbon incorporation occurs by a cooperative island growth mechanism that favors the formation of substrate-size single-crystal graphene. The model reveals that the cooperative growth of millimeter-size single-crystal graphene grains occurs by roughly 3 orders of magnitude increase in the reactive sticking probability of methane compared to that in random island nucleation.

Mechanisms of graphene growth on metal surfaces: theoretical perspectives

Graphene is an important material with unique electronic properties. Aiming to obtain high quality samples at a large scale, graphene growth on metal surfaces has been widely studied. An important topic in these studies is the atomic scale growth mechanism, which is the precondition for a rational optimization of growth conditions. Theoretical studies have provided useful insights for understanding graphene growth mechanisms, which are reviewed in this article. On the mostly used Cu substrate, graphene growth is found to be more complicated than a simple adsorption-dehydrogenation-growth model. Growth on Ni surface is precipitation dominated. On surfaces with a large lattice mismatch to graphene, epitaxial geometry determin a robust nonlinear growth behavior. Further progresses in understanding graphene growth mechanisms is expected with intense theoretical studies using advanced simulation techniques, which will make a guided design of growth protocols practical.

Atomic-scale engineering of the electrostatic landscape of semiconductor surfaces

A low-temperature scanning tunneling microscope was used in conjunction with density functional theory calculations to determine the binding sites and charge states of adsorbed Ga and Mn atoms on GaAs(110). To quantify the adatom charge states (both +1e), the Coulomb interaction with an individual Mn acceptor is measured via tunneling spectroscopy and compared with theoretical predictions. Several methods for positioning these charged adatoms are demonstrated, allowing us to engineer the electrostatic landscape of the surface with atomic precision.

Activation energy paths for graphene nucleation and growth on Cu

The synthesis of wafer-scale single crystal graphene remains a challenge toward the utilization of its intrinsic properties in electronics. Until now, the large-area chemical vapor deposition of graphene has yielded a polycrystalline material, where grain boundaries are detrimental to its electrical properties. Here, we study the physicochemical mechanisms underlying the nucleation and growth kinetics of graphene on copper, providing new insights necessary for the engineering synthesis of wafer-scale single crystals. Graphene arises from the crystallization of a supersaturated fraction of carbon-adatom species, and its nucleation density is the result of competition between the mobility of the carbon-adatom species and their desorption rate. As the energetics of these phenomena varies with temperature, the nucleation activation energies can span over a wide range (1-3 eV) leading to a rational prediction of the individual nuclei size and density distribution. The growth-limiting step was found to be the attachment of carbon-adatom species to the graphene edges, which was independent of the Cu crystalline orientation.