First-Principles Study of the Role of O2 and H2O in the Decoupling of Graphene on Cu(111)

An overview of contemporary developments and the application of graphene-based materials in anticorrosive coatings

Although graphene and graphene-based materials (GBMs) offer a wide range of possible applications, interest in their use as barrier layers or as reinforcements in coatings for the mitigation of corrosion has grown during the past decade. Because of its unique two-dimensional nanostructure and exceptional physicochemical characteristics, graphene has gotten a lot of attention as an anti-corrosion material. This enthusiasm is largely driven by the requirement to integrate more features, improve anti-corrosion effectiveness, and eventually prolong the service duration of metallic components. As barriers against metal corrosion, graphene nanosheets can be applied singly or in combination to create thin films, layered frameworks, or composites. Concurrently, over the past few years, significant advancements have been made in the establishment of scalable production methods for graphene and materials based on graphene. Since there is currently a wide variety of graphene material with various morphologies and characteristics, it is even more important that the production approach and the intended application be properly matched. This review gathers the most recent data and aims to give the reader a comprehensive overview of the most recent developments in the use of graphene and GBMs in various anti-corrosion strategies. The structure-property correlation and anticorrosion techniques in these systems are given special consideration. The current article offers a critical examination of this topic as well, stressing the areas that require more research.

Direct Electrochemical Functionalization of Graphene Grown on Cu Including the Reaction Rate Dependence on the Cu Facet Type

We present an electrochemical method to functionalize single-crystal graphene grown on copper foils with a (111) surface orientation by chemical vapor deposition (CVD). Graphene on Cu(111) is functionalized with 4-iodoaniline by applying a constant negative potential, and the degree of functionalization depends on the applied potential and reaction time. Our approach stands out from previous methods due to its transfer-free method, which enables more precise and efficient functionalization of single-crystal graphene. We report the suggested effects of the Cu substrate facet by comparing the reactivity of graphene on Cu(111) and Cu(115). The electrochemical reaction rate changes dramatically at the potential threshold for each facet. Kelvin probe force microscopy was used to measure the work function, and the difference in onset potentials of the electrochemical reaction on these two different facets are explained in terms of the difference in work function values. Density functional theory and Monte Carlo calculations were used to calculate the work function of graphene and the thermodynamic stability of the aniline functionalized graphene on these two facets. This study provides a deeper understanding of the electrochemical behavior of graphene (including single-crystal graphene) on Cu(111) and Cu(115). It also serves as a basis for further study of a broad range of reagents and thus functional groups and of the role of metal substrate beneath graphene.

Nanoscale characterization of the heterogeneous interfacial oxidation layer of graphene/Cu based on a SEM electron beam induced reduction effect

Characterization of the interfacial oxidation layer of graphene/metal is a challenging task using conventional spectroscopy techniques because interfacial oxidation is heterogeneous at the nanoscale underneath the graphene. Here we developed a feasible method for nanoscale characterization of the interfacial oxidation layer of graphene/Cu (Gr/Cu) based on scanning electron microscopy (SEM) electron beam irradiation (EBI) induced reduction of interfacial oxides (SEM EBI-RIO method) at room temperature. The change in the thickness and coverage of the interfacial Cu oxide layer induced by EBI is responsible for the observed contrast reversal or change in SEM images of a targeted area with a width down to 200 nm in the EBI time scale of seconds to minutes. This method offers the capability of mapping heterogeneous interfacial oxidation of Gr/Cu with sub-100 nm spatial resolution and determining the range of thickness (1-5 nm) of the interfacial oxide layer. The SEM EBI-RIO method will be a powerful method to complement X-ray photoelectron spectroscopy (XPS), Raman microscopy, and high resolution transmission electron microscopy (HRTEM) for characterization of the interfacial oxidation layer of 2D materials and devices.

Unexpectedly Spontaneous Water Dissociation on Graphene Oxide Supported by Copper Substrate

Water dissociation is of fundamental importance in scientific fields and has drawn considerable interest in diverse technological applications. However, the high activation barrier of breaking the OH bond within the water molecule has been identified as the bottleneck, even for the water adsorbed on the graphene oxide (GO). Herein, using the density functional theory calculations, we demonstrate that the water molecule can be spontaneously dissociated on GO supported by the (111) surface of the copper substrate (Copper-GO). This process involves a proton transferring from water to the interfacial oxygen group, and a hydroxide covalently bonding to GO. Compared to that on GO, the water dissociation barrier on Copper-GO is significantly decreased to be less than or comparable to thermal fluctuations. This is ascribed to the orbital-hybridizing interaction between copper substrate and GO, which enhances the reaction activity of interfacial oxygen groups along the basal plane of GO for water dissociation. Our work provides a novel strategy to access water dissociation via the substrate-enhanced reaction activity of interfacial oxygen groups on GO and indicates that the substrate can serve as an essential key to tuning the catalytic performance of various two-dimensional material devices.

Toward Epitaxial Growth of Misorientation-Free Graphene on Cu(111) Foils

The epitaxial growth of single-crystal thin films relies on the availability of a single-crystal substrate and a strong interaction between epilayer and substrate. Previous studies have reported the roles of the substrate (e.g., symmetry and lattice constant) in determining the orientations of chemical vapor deposition (CVD)-grown graphene, and Cu(111) is considered as the most promising substrate for epitaxial growth of graphene single crystals. However, the roles of gas-phase reactants and graphene-substrate interaction in determining the graphene orientation are still unclear. Here, we find that trace amounts of oxygen is capable of enhancing the interaction between graphene edges and Cu(111) substrate and, therefore, eliminating the misoriented graphene domains in the nucleation stage. A modified anomalous grain growth method is developed to improve the size of the as-obtained Cu(111) single crystal, relying on strongly textured polycrystalline Cu foils. The batch-to-batch production of A3-size (∼0.42 × 0.3 m²) single-crystal graphene films is achieved on Cu(111) foils relying on a self-designed pilot-scale CVD system. The as-grown graphene exhibits ultrahigh carrier mobilities of 68 000 cm² V^-1 s^-1 at room temperature and 210 000 cm² V^-1 s^-1 at 2.2 K. The findings and strategies provided in our work would accelerate the mass production of high-quality misorientation-free graphene films.

Scalable synthesis of nanoporous atomically thin graphene membranes for dialysis and molecular separations via facile isopropanol-assisted hot lamination

Scalable graphene synthesis and facile large-area membrane fabrication are imperative to advance nanoporous atomically thin membranes (NATMs) for molecular separations. Although chemical vapor deposition (CVD) allows for roll-to-roll high-quality monolayer graphene synthesis, facile transfer with atomically clean interfaces to porous supports for large-area NATM fabrication remains extremely challenging. Sacrificial polymer scaffolds commonly used for graphene transfer typically leave polymer residues detrimental to membrane performance and transfers without polymer scaffolds suffer from low yield resulting in high non-selective leakage through NATMs. Here, we systematically study the factors influencing graphene NATM fabrication and report on a novel roll-to-roll manufacturing compatible isopropanol-assisted hot lamination (IHL) process that enables scalable, facile and clean transfer of CVD graphene on to polycarbonate track etched (PCTE) supports with coverage ≥99.2%, while preserving support integrity/porosity. We demonstrate fully functional centimeter-scale graphene NATMs that show record high permeances (∼2-3 orders of magnitude higher) and better selectivity than commercially available state-of-the-art polymeric dialysis membranes, specifically in the 0-1000 Da range. Our work highlights a scalable approach to fabricate graphene NATMs for practical applications and is fully compatible with roll-to-roll manufacturing processes.

Spatial Confinement as an Effective Strategy for Improving the Catalytic Selectivity in Acetylene Hydrogenation

While control over chemical reactions is largely achieved by altering the intrinsic properties of catalysts, novel strategies are constantly being proposed to improve the catalytic performance in an extrinsic way. Since the fundamental chemical behavior of molecules can remarkably change when their molecular scale is comparable to the size of the space where they are located, creating spatially confined environments around the active sites offers new means of regulating the catalytic processes. We demonstrate through first-principles calculations that acetylene hydrogenation can exhibit significantly improved selectivity within the confined sub-nanospace between two-dimensional (2D) monolayers and the Pd(111) substrate. Upon intercalation of molecules, the lifting and undulation of a 2D monolayer on Pd(111) influence the adsorption energies of intermediates to varying extents, which, in turn, changes the energy profiles of the hydrogenation reactions. Within the confined sub-nanospace, the formation of ethane is always unfavorable, demonstrating effective suppression of the unwanted overhydrogenation. Moreover, the catalytic properties can be further tuned by altering the coverage of the adsorbates as well as strains within the 2D monolayer. Our results also indicate that for improving the selectivity, the strategy of spatial confinement could not be combined with that of single-atom catalysis, since the reactant molecules cannot enter the sub-nanospace due to the too weak adsorbate-substrate interaction. This work sheds new light on designing novel catalysts with extraordinary performance for the selective hydrogenation of acetylene.

Interfacial co-existence of oxygen and titanium vacancies in nanostructured TiO2 for enhancement of carrier transport

The interfacial co-existence of oxygen and metal vacancies in metal oxide semiconductors and their highly efficient carrier transport have rarely been reported. This work reports on the co-existence of oxygen and titanium vacancies at the interface between TiO₂ and rGO via a simple two-step calcination treatment. Experimental measurements show that the oxygen and titanium vacancies are formed under 550 °C/Ar and 350 °C/air calcination conditions, respectively. These oxygen and titanium vacancies significantly enhance the transport of interfacial carriers, and thus greatly improve the photocurrent performances, the apparent quantum yield, and photocatalysis such as photocatalytic H₂ production from water-splitting, photocatalytic CO₂ reduction and photo-electrochemical anticorrosion of metals. A new "interfacial co-existence of oxygen and titanium vacancies" phenomenon, and its characteristics and mechanism are proposed at the atomic-/nanoscale to clarify the generation of oxygen and titanium vacancies as well as the interfacial carrier transport.

Intercalation, decomposition, entrapment - a new route to graphene nanobubbles

Graphene nanobubbles (GNBs) have become the subject of recent research due to their novel physical properties. However, present methods to create them involve either extreme conditions or complex sample fabrication. We present a novel approach which relies on the intercalation of small molecules (NH3), their surface-mediated decomposition and the formation of larger molecules (N2) which are then entrapped beneath the graphene in bubbles. Our hypothesised reaction mechanism requires the copper substrate, on which our graphene is grown via chemical vapour deposition (CVD), to be oxidised before the reaction can occur. This was confirmed through X-ray photoelectron spectroscopy (XPS) data of both oxidised and reduced Cu substrate samples. The GNBs have been analysed through atomic force microscopy (AFM, after NH3 treatment) and XPS, which reveals the formation of five distinct N 1s peaks, attributed to N2 entrapment, N doping species and atomic nitrogen bonded with the Cu within the substrate. This method is simple, occurs at low temperatures (520 K) and integrates very easily with conventional CVD graphene growth, so presents an opportunity to open up this field of research further.

High-accuracy large-scale DFT calculations using localized orbitals in complex electronic systems: the case of graphene-metal interfaces

Over many years, computational simulations based on density functional theory (DFT) have been used extensively to study many different materials at the atomic scale. However, its application is restricted by system size, leaving a number of interesting systems without a high-accuracy quantum description. In this work, we calculate the electronic and structural properties of a graphene-metal system significantly larger than in previous plane-wave calculations with the same accuracy. For this task we use a localised basis set with the Conquest code, both in their primitive, pseudo-atomic orbital form, and using a recent multi-site approach. This multi-site scheme allows us to maintain accuracy while saving computational time and memory requirements, even in our exemplar complex system of graphene grown on Rh(1 1 1) with and without intercalated atomic oxygen. This system offers a rich scenario that will serve as a benchmark, demonstrating that highly accurate simulations in cells with over 3000 atoms are feasible with modest computational resources.

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.

Controlling Fundamental Fluctuations for Reproducible Growth of Large Single-Crystal Graphene

The controlled growth of graphene by the chemical vapor deposition method is vital for its various applications; however, the reproducibility remains a great challenge. Here, using single-crystal graphene growth on a Cu surface as a model system, we demonstrate that a trace amount of H₂O and O₂ impurity gases in the reaction chamber is key for the large fluctuation of graphene growth. By precisely controlling their parts per million level concentrations, centimeter-sized single-crystal graphene is obtained in a reliable manner with a maximum growth rate up to 190 μm min^-1. The roles of oxidants are elucidated as an effective modulator for both graphene nucleation density and growth rate. This control is more fundamental for reliable growth of graphene beyond previous findings and is expected to be useful for the growth of various 2D materials that are also sensitive to trace oxidant impurities.

Effect of hydrogen passivation on the decoupling of graphene on SiC(0001) substrate: First-principles calculations

Intercalation of hydrogen is important for understanding the decoupling of graphene from SiC(0001) substrate. Employing first-principles calculations, we have systematically studied the decoupling of graphene from SiC surface by H atoms intercalation from graphene boundary. It is found the passivation of H atoms on both graphene edge and SiC substrate is the key factor of the decoupling process. Passivation of graphene edge can weaken the interaction between graphene boundary and the substrate, which reduced the energy barrier significantly for H diffusion into the graphene-SiC interface. As more and more H atoms diffuse into the interface and saturate the Si dangling bonds around the boundary, graphene will detach from substrate. Furthermore, the energy barriers in these processes are relatively low, indicating that these processes can occur under the experimental temperature.

Surface chemistry and catalysis confined under two-dimensional materials

Interfaces between 2D material overlayers and solid surfaces provide confined spaces for chemical processes, which have stimulated new chemistry under a 2D cover.

Oxidation-assisted graphene heteroepitaxy on copper foil

We propose an innovative, easy-to-implement approach to synthesize aligned large-area single-crystalline graphene flakes by chemical vapor deposition on copper foil. This method doubly takes advantage of residual oxygen present in the gas phase. First, by slightly oxidizing the copper surface, we induce grain boundary pinning in copper and, in consequence, the freezing of the thermal recrystallization process. Subsequent reduction of copper under hydrogen suddenly unlocks the delayed reconstruction, favoring the growth of centimeter-sized copper (111) grains through the mechanism of abnormal grain growth. Second, the oxidation of the copper surface also drastically reduces the nucleation density of graphene. This oxidation/reduction sequence leads to the synthesis of aligned millimeter-sized monolayer graphene domains in epitaxial registry with copper (111). The as-grown graphene flakes are demonstrated to be both single-crystalline and of high quality.