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We analyzed the surfaces of hydrogen-intercalated graphene on SiC(0001) by photoelectron spectroscopy and Raman spectroscopy after applying electric currents to the graphene for carrier mobility measurements while the sample was exposed to air. We found that Si at the interface between graphene and SiC oxidizes very gradually, whereas the graphene does not. The Si of this Si oxide is mainly divalent, and the oxide is attributed to SiO molecules. These molecules are volatile, and they have not been observed either on the oxidized surface of SiC(0001) or on oxygen-intercalated graphene/SiC(0001) formed by annealing in air or oxygen. This anomalous oxidation, which is triggered by the application of current to the graphene overlayer, could be observed because of the presence of the graphene overlayer and the oxidation at room temperature.

Recent studies revealing exceptionally rapid water flow across graphene oxide membranes have highlighted them for potential filtration and separation applications. The physical and chemical features in graphene oxide membranes are heterogeneous, and there remains a great deal of speculation as to what is responsible for the facile water percolation. One potential contributing feature is the variation of interlayer spacing, which can occur naturally or be artificially induced. Herein, water flow across pristine, oxidized, and mixed membranes with interlayer distances of 0.7, 0.9, and 1.2 nm, corresponding respectively to the formation of discrete mono-, bi-, and trilayer water structures, was studied via molecular dynamics simulations. The interlayer spacing of 0.7 nm results in the formation of square ice for the pristine graphene membrane, which leads to collective motion, inhibiting equilibrium transport but allowing for rapid nonequilibrium flow comparable to that in the membranes with larger interlaye...

Light-element-doped graphene is a promising support for polymer electrode fuel cell electrode catalysts. To understand the behavior of catalyst atoms on light-element-doped graphene, we investigated the adsorption states and diffusion behavior of a single Pt atom on pristine graphene and B-, N-, O-, Si-, P-, and S-doped graphene by first-principles calculations based on density functional theory. We found that localized orbitals, which can trap Pt atoms, are created in the vicinity of dopants. Thus, these doped graphene supports prevent Pt atom desorption. In particular, O- and P-doped graphene showed adsorption energies of −3.93 and −3.59 eV, respectively, while that of pristine graphene is −1.45 eV. We also calculated the diffusion coefficients of Pt atoms on these doped graphene at 100 °C, which is a typical operating temperature of polymer electrode fuel cells. The obtained diffusion coefficients of Pt atoms on doped graphene were less than that on pristine graphene. The diffusion coefficient on P-dop...

It is commonly accepted that the presence of nitrogen atoms in a graphene lattice improves many properties of carbon materials and particularly enhances their electrochemical capacity in Li ion batteries. Here, we present model experiments for revealing the difference in interaction of lithium with N-doped and N-free graphene samples by monitoring the changes in their electronic states after the deposition of Li vapors. Graphene and N-doped graphene films have been grown by chemical vapor deposition on copper substrates using methane and acetonitrile as precursors. The electronic structure of the films transferred onto SiO2/Si substrates was examined by X-ray photoelectron spectroscopy (XPS) before and after deposition of lithium from a Li evaporation source under vacuum conditions. A comparison between two graphene samples using in situ XPS measurements has detected a higher accumulation of lithium on the N-doped graphene, which implies its high prospects in energy storage applications. Analysis of the X...

Three-dimensional (3D) graphene has attracted increasing attention in electrochemical devices. However, the existing preparation technologies usually involve a solvent process, which introduces defects and functional groups into the 3D network. Here, we find the defects and functional groups influence the electrochemical stability of graphene. After an electrochemical process, the current decreases by more than 1 order of magnitude, indicating remarkable etching of graphene. To improve the electrochemical stability, we develop a solvent-free preparation process to produce 3D graphene for the first time. After growth on a 3D microporous copper by chemical vapor deposition (CVD), the copper template is removed by a high temperature evaporation process, resulting in 3D graphene network without any solvent process involved. The samples exhibit remarkably improved stability with durable time 2 times, compared with normal CVD samples, and 55 times, compared with reduced graphite oxide, and no obvious etching is...

Graphene oxide (GO) is an attractive option for large scale production of graphene. On the other hand, the graphene obtained by the reduction of GO has inevitable structural defects, and the vacant lattice sites will significantly restrict its conductivity. It has been demonstrated that thermal annealing in hydrogen is an efficient method to reduce defects and heal the lattice in GO samples. However, it is still not clear how the defects and/or disordering influence the photoelectric conversion efficiency and the carrier relaxation pathway in GO. Herein, time-domain terahertz (THz) spectroscopy is employed to characterize the properties of the multilayer GO films which were annealed in hydrogen at various temperatures. Upon photoexcitation, a transient increase of the conductivity was observed for the reduced graphene oxide (RGO) samples. The ultrafast carrier relaxation process can be well assigned to the carrier–carrier scattering and carrier–phonon coupling. Our results demonstrated that the RGO films ...

Functionalized graphene oxide papers are assembled materials with a wide range of potential applications; however, scientific studies about how the physical properties of functionalized graphene oxide papers are affected by the presence of new functional groups have not been addressed. Here, electrical resistance, optical band gap, and mechanical properties of flexible and free-standing functionalized graphene oxide papers with nitroxide moieties (NGOP’s) have been studied, and two of them (mechanicals and electricals) were correlated with the interlayer d spacing for the first time; herein, a new insight about the key role that an optimum amount of functionalization agent plays over the physical properties of NGOP’s is presented. In all cases, (functionalized or pristine) graphene oxide papers were prepared using anhydrous DMF instead water in order to study and highlight the pure effect of the functionalized groups over their physical properties. The functionalized nanomaterials exhibited significantly ...

We present a study of the process of reduction of thin graphene oxide (GO) films consisting of flakes with lateral size of up to 100 μm through soft ultraviolet irradiation in the argon atmosphere. It was found out that the reduction process leads to a significant decrease in the overall content of the basal-plane functional groups, namely, epoxides and hydroxyls, but with simultaneous increase in the total number of the edge-located carboxyl groups. Obtained transmission electron microscopy images showed that this effect is related to formation of nanoscale holes in the course of reduction. Based on the data obtained, we have proposed a mechanism of the observed GO structural modification in terms of photoinduced chemical reactions between the carbon network and functional groups. These reactions result in progressive growth of the initially existing and newly formed vacancies with formation of the nanoholes with size of up to 100 nm. Thus, reduced graphene oxide films with the restored conjugated networ...

Surface properties of olivine phosphate LiFePO4, as cathodes in lithium ion batteries, are of importance for overall performance as the nanoscale of particles has become indispensable. Using the first-principles total energy calculations, the effects of graphene or Mn dopant on the electrochemical properties of graphene/LiFePO4 have been comprehensively investigated. It is revealed that the interfacial binding between graphene and LiFePO4 in parallel orientation is stable and improved in the process of doping. The Li adsorption energy at different sites elucidates the core–shell model in Li extraction/insertion process and indicates the anomalous Li storage in the interface between graphene and LiFePO4. Mechanisms underlying influences of adsorption site, Mn dopant, and graphene modification on the Li adsorption energy are discussed through edge effect, doping stability, and interfacial binding strength, respectively. The surface conductivity is improved in the presence of graphene or Mn dopant with respe...

Introduction of graphene layer in-between a cobalt overlayer and an oxide support is proposed as a simple and efficient method to modify the morphology and redox properties of cobalt in reactive environments. Graphene-covered ZnO or SiO2 substrates were decorated by vapor-deposited Co nanoparticles and studied by spectroscopy and microscopy techniques accompanied by numerical simulations. It is shown that at low temperature graphene improves the dispersion of cobalt as compared to oxide substrates, while upon annealing it provokes particle agglomeration. In 7 mbar of oxygen ambient graphene has a minor effect on the Co oxidation process, however, it ameliorates the reducibility of cobalt oxides under hydrogen. Micro-Raman spectra show that in contact with cobalt single-layer graphene becomes defective, while areas with bilayer graphene or free of cobalt proved to be more stable under the gas treatment.

Single-crystal bilayer graphene is attractive for functional electronic and photonic devices because of its high carrier mobility as well as a tunable band gap induced by a perpendicular electric field. Chemical vapor deposition (CVD) has been increasingly reported for the production of single-crystal bilayer graphene, but it is limited with respect to grain size because of nonsynchronic growth between the second layer and the first layer of bilayer graphene. In this research, we established a nonisothermal atmosphere CVD (NI-APCVD) system to realize synchronization of nucleation and growth of single-crystal bilayer graphene that contains the surface adsorption for the decomposed carbons and the shallow bulk phase segregation for the dissolved carbons. Based on the above process engineering, we successfully produced large hexagonal single-crystal bilayer graphene (∼1.0 mm) on the polycrystalline Cu foils. This work paves a pathway to the synchronous growth of wafer-scale single-crystal bilayer graphene, w...

Most sorbents display poor capacity for elemental mercury at elevated temperatures. Graphene is the potential candidate among different high-temperature sorbents. We have studied the physical properties of mercury films on partially hydrogenated imperfect graphene, as well as their heating and bombardment with xenon clusters, by means of molecular dynamics. Hydrogenated edges of a graphene sheet containing Stone–Wales defects withstand heating to 1100 K. Formation of the droplet leads to a decrease in the blunt contact angle. The bombardment of a target with a Xe13 cluster beam at energies of 5–30 eV and incidence angles of 0–60° aiming to remove a mercury film from imperfect graphene has been performed. The graphene is completely cleaned of mercury at a cluster energy of EXe ≥ 15 eV. Mercury is removed from the graphene film via sputtering of single atoms and droplet detachment. A stress in graphene resulting from forces normal to the sheet plane is noticeably higher than that due to forces acting in its...

We demonstrate the deposition of crystalline palladium nanoparticles on graphene nanoplatelets (Pd/graphene) via atmospheric pressure atomic layer deposition (ALD) carried out in a fluidized bed reactor. The nucleation and growth of Pd nanoparticles on the inert graphene surface was enabled by applying an ozone pretreatment step, without significantly affecting the graphene crystalline structure. Uniform nucleation on both basal planes and edges of the graphene was obtained. The Pd loading and dispersion as well as the average particle size could be controlled by varying the number of ALD cycles. By analyzing the evolution of the particle size distribution and spatial density, we obtained insights into the nucleation and growth of Pd ALD on graphene. Furthermore, by shortening the pretreatment time, selective growth of Pd nanoparticles on the edges of the graphene was achieved. The Pd/graphene fabricated with our method showed a significantly lower level of impurities compared to the Pd/graphene synthesiz...

Free-floating, and copper-supported, few-layer graphene sheets were spontaneously modified from an aqueous solution containing nitrobenzenediazonium ions. The infrared spectra of the chemically modified (copper etched) free-floating graphene were measured in transmission mode by manipulating the sheets onto a KBr disc. The major advantage to this method is the ability to release the graphene sheets off the disc to refloat on a water bath, allowing the graphene to be further modified or deposited onto a new substrate suitable for other analysis. In this study, graphene sheets were then mounted onto highly ordered pyrolytic graphite (HOPG) for atomic force microscopy imaging and electrochemical measurements. The results show there are at least two reaction pathways for spontaneous film grafting to graphene: the commonly accepted aryl radical leading to films containing −C–C– linkages and a direct reaction of the diazonium cation with graphene to give films containing −N═N– linkages. The ability to manipulat...

In view of large-scale applications, electrochemical exfoliation of graphite for the production of graphene sheets must follow chemical processes that ensure high quality of the products—wide-size graphene foils, single- or few-layer thickness, and low level of defectivity—in order to guarantee high electrical transport and good mechanical properties. Understanding the exfoliation process of graphite at the atomic scale, that is, the intercalation of graphene layers in the electrolyte solution, is fundamental to really be able to control and optimize such processes. This can be obtained, for instance, by investigation of the exfoliated graphite—the surface of the original crystal left behind in the chemical solution—and by real-time monitoring of graphite surface morphological and structural modifications during the exfoliation process. Here, we monitor graphite surface changes as a function of the electrochemical potential by both electrochemical (EC) atomic force microscopy and EC scanning tunneling mic...

Recent advancements in chemical vapor deposition techniques for the growth of graphene have enabled access to large-area and high-quality graphene film for various applications. The key step in determining the quality of the final graphene films is the transfer process to desired substrates, with the goal of minimizing the amount of structural and chemical modification to the film. Multiple methods exist for this transfer process, with each method varying in the extent to which the graphene film is altered. Four fundamentally different transfer methods, including chemical etching and electrochemical delaminating, were employed to obtain graphene films from chemical vapor deposition graphene grown on Ni foil and the resulting films characterized using scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. These graphene films showed noticeable differences in their material properties in terms of amount of defects and elemental purities, with a dramatic i...

Tuning of the electrical properties of graphene and functionalized graphene is very important to its use in optoelectronic devices. In this work, we study the electrical properties of graphene and graphene with different π-stacking organic molecules. The different π-stacking organic molecules used in the present study were hemin, 1-amino 2-naphthol 4-sulfonic acid, and ferrocene. The noncovalently functionalized reduced graphene oxides were characterized by UV–visible spectroscopy, Fourier transformed infrared spectroscopy, atomic force microscopy, scanning electron microscopy, thermogravimetric analysis, and Raman spectroscopy. The noncovalently functionalized reduced graphene oxide show higher ac conductivity than graphene oxide (GO). The enhancement of conductivity shown can be attributed to higher mobility, and the density of π-electron and higher surface area of hybrid nanocomposites system. The reason is supported because that interaction of rGO with non-π-system like 18-crown-6 did not help in incr...

The functionalization of graphene oxide (GO) and graphene by TiO2 and other metal oxides has attracted considerable attention due to numerous promising applications in catalysis, energy conversion, and storage. Hydrogenation of this class of materials has been proposed as a promising way to tune catalytic properties by altering the structural and chemical transformations that occur upon H incorporation. Herein, we investigate the structural changes that occur during the hydrogenation process using in situ powder X-ray diffraction and pair distribution function analysis of GO–TiO2 and TiO2 under H2 reduction. Sequential Rietveld refinement was employed to gain insight into the evolution of crystal growth of TiO2 nanoparticles in the presence of two-dimensional (2D) GO nanosheets. GO sheets not only significantly retarded the nucleation and growth of rutile impurities, stabilizing the anatase structure, but was also partially reduced to hydrogenated graphene by the introduction of atomic hydrogen into the h...

We theoretically propose a surface-plasmon-based fiber optic biosensor in metal/graphene/MoS2 configuration with molybdenum disulfide (MoS2) as a bio recognition layer. The proposed configuration works in the visible region of the electromagnetic spectrum with a very high sensitivity. A comparative theoretical study of the sensors with different metallic layers of gold (Au), copper (Cu), and aluminum (Al) has been performed. The sensor has been found to be the most sensitive in both Cu/graphene/MoS2 and Al/graphene/MoS2 configurations with sensitivity of 6.2 μm/RIU. In both of the configurations the thicknesses of Cu and Al layers is 50 nm and the number of layers of graphene is 16 and 27, respectively, while only a single layer of MoS2 has been used. The sensitivity of the sensor in the Au/graphene/MoS2 configuration is 5.0 μm/RIU with comparatively high depth of resonance.

Graphene oxide is a promising solution-processable precursor for the mass production of graphene thin films. However, during the wet chemical oxidation and reduction process toward reduced graphene oxide (rGO) a large number of defects are created. Although it is possible to synthesize rGO with an average defect distance of 3–4 nm, the performance is still limited. Here we demonstrate the partial restoration of the graphene basal plane of rGO by annealing in Ar/H2/isopropanol flow. Detailed statistical Raman analysis over large areas corroborates that the mean defect distance increases from initially 2–3 to 10–12 nm after CVD annealing. Some areas even reach defect distances of up to 18 nm. However, residual manganese impurities from the oxidation process lead to undesired carbon nanotube growth on the substrate under these conditions and had to be removed before the deposition of graphene oxide flakes on the substrate. The observed defect reduction during CVD annealing indicates that the lattice defects ...

The quantum capacitance from large area, 6- to 7-layered graphene, and chemically modified graphene, was determined using electrochemical impedance spectroscopy measured at 100 Hz in a three-electrode electrochemical cell and aqueous acidic solution. Aryldiazonium chemistry was used to modify one side of the few-layered graphene sheets with methoxy- and iodo-phenyl groups. The graphene sheets were then mounted via an aqueous transfer method onto an epoxy substrate. It was found that both sides of the graphene sheets could be accessed by the electrolyte and thus the sheets were in a pseudo free-floating arrangement. The results show a complex quantum capacitance behavior with applied electrode potential, and that behavior was not stable with potential cycling. Chemically modified samples have similar quantum capacitance minimum to unmodified graphene, and all samples have a shallow U-shaped relationship with respect to applied electrode potential. The results show chemically modifying one side of few-layer...

Graphene has been heralded as a promising electrode material for high energy and power density electrochemical supercapacitors. This is in spite of recent work confirming the low double-layer capacitance (CDL) of the graphene/electrolyte interface limited by graphene’s low quantum capacitance (CQ), an effect known for the basal plane of graphite for over four decades. Consistent with this limit, much of the supercapacitor research implies the use of pristine graphene but, in contrast, uses a functionalized and defective graphene formed through the reduction of graphene oxide, without clarifying why reduced graphene oxide is needed to achieve high capacitance. Herein, we show that an optimal level of functionalization and lattice disorder in reduced graphene oxide yields a 4-fold increase in CDL over that of pristine graphene, suggesting graphene-based materials can indeed be tailored to engineer electrodes with significantly higher gravimetric capacitance limits exceeding 450 F/g than what has been achiev...

Hybrid nanostructures combining semiconductor quantum dots and graphene are attracting increasing attention because of their optoelectronic properties promising for photovoltaic applications. We present a hot-injection synthesis of a colloidal nanostructure which we define as quasi core/shell PbS/graphene quantum dots due to the incomplete passivation of PbS surfaces with an ultrathin layer of graphene. Simulation by density functional theory of a prototypical model of a nonstoichiometric Pb-rich core (400 atoms) coated by graphene (20 atoms for each graphene sheet) indicates the possibility of surface passivation of (111) planes of PbS with graphene resulting in a decrease in trap states and recombination sites. The graphene coating of the PbS quantum dots decreases the exciton lifetime up to 0.78 μs as compared to 1.2 μs for the oleic acid passivated PbS quantum dots due to the fast extraction of carriers. We have employed PbS/graphene as well as Cd-doped PbS/graphene quantum dots as active layers of bu...

The cathodic and anodic behavior of graphene oxide was investigated in aqueous suspensions on platinum. The anodic deposition relied on the polyanionic character of graphene oxide with increasing pH. It took place simultaneously with the oxidation of phosphate to peroxodiphosphate and the oxygen evolution. Scanning electrochemical microscopy showed an inhomogeneous anodic adsorption. The cathodic deposition is based on the reduction of graphene oxide to reduced graphene oxide, which showed reduced solubility. Anodic oxygen evolution led to the desorption of graphene oxide anions, whereas hydrogen evolution blocked further adsorption of reduced graphene oxide but caused no desorption.

The use of membranes for gas permeation and phase separation offers many distinct advantages over other more energy-dependent processes. The operational efficiencies of these membranes rely heavily on high gas permeability. Here, we report membranes with significantly increased permeability without a considerable decrease in mechanical strength or selectivity, synthesized from a polymer nanocomposite that incorporates graphene and polydimethylsiloxane (PDMS). These graphene–PDMS nanocomposite membranes were able to enhance the gas permeation of N2, CO2, Ar, and CH4 in reference to pristine PDMS membranes. This is achieved by creating interfacial voids between the graphene flakes and polymer chains, which increases the fractional free volume within the nanocomposites, giving rise to an increase in permeation. An optimal loading of graphene was found to be 0.25 wt%, while greater loading created agglomeration of the graphene flakes, hence reducing the effective surface area. We present the enhancements that...

One of the more common routes to fabricate graphene is by chemical vapor deposition (CVD). This is primarily because of its potential to scale up the process and produce large area graphene. For the synthesis of large area monolayer Cu is probably the most popular substrate since it has a low carbon solubility enabling homogeneous single-layer sheets of graphene to form. This process requires a very clean substrate. In this work we look at the efficiency of common pretreatments such as etching or wiping with solvents and compare them to an oxidation treatment at 1025 °C followed by a reducing process by annealing in H2. The oxidation/reduction process is shown to be far more efficient allowing large area homogeneous single layer graphene formation without the presence of additional graphene flakes which form from organic contamination on the Cu surface.

The mechanism of bilayer graphene nucleation and growth has been investigated by using quantum chemical molecular dynamics simulations. The results indicate that the presence of embedded nickel atoms in the upper-layer (first-layer) graphene has little impact on the evolution mechanism of the second-layer graphene precursor. The nucleation process occurs after the rapid precipitation of internal carbon atoms along with the degradation of nickel catalyst and the formation of discrete carbon polyyne chains. The second-layer graphene exhibits an attachment-limited growth on the rugged Ni(111) surface. The quality of the second-layer graphene can be reduced, and large structural holes are induced when the metal atoms are involved in the upper-layer graphene. On the contrary, high-quality upper-layer graphene can act as an excellent template for the growth of the second-layer graphene. These simulations, therefore, suggest that through carefully controlling the growth conditions, different kinds of bilayer gra...

The pre-H2O treatment and Al2O3 film growth under a two-temperature-regime mode in an oxygen-deficient atomic layer deposition (ALD) chamber can induce n-type doping of graphene, with the enhancement of electron mobility and no defect introduction to graphene. The main mechanism of n-type doping is surface charge transfer at graphene/redox interfaces during the ALD procedure. More interestingly, this n-type doping of graphene is reversible and can be recovered by thermal annealing, similar to hydrogenated graphene (graphane). This technique utilizing pre-H2O treatment and an encapsulated layer of Al2O3 achieved in an oxygen-deficient ALD chamber provides a simple and novel route to fabricate n-type doping of graphene.

Graphene oxide (GO) is composed of isolated graphene-like sheets containing various oxygen functionalities. The electrochemical reduction of GO enables the direct deposition of the graphene layer on electrode surfaces. Furthermore, a partial reduction of GO can be performed, enabling easy modification of the surface with oxygen-containing groups. In this contribution, the electrochemical reduction of GO is carried out on a gold electrode roughened by electrochemical oxidation–reduction cycles. Such a gold electrode contains nanostructures ranging in size from tens to 300 nm. The electrochemically reduced GO layer is subsequently used for horseradish peroxidase (HRP) immobilization. Two types of electrochemically reduced GOs, partially reduced graphene oxide (ERGOP) and fully reduced graphene oxide (ERGOF), are used in this study. Although HRP immobilized on both types of surfaces directly catalyzes electroreduction of the hydrogen peroxide, the enzyme immobilized on the ERGOP layer exhibits slightly highe...

We study long-term electrochemical sodium and lithium cycling, and tensile testing behavior of thermally reduced graphene oxide (rGO) paper electrodes. We find strong dependence of annealing temperature and gas environment on the electrical conductivity, electrochemical capacity, and rate capability of the electrodes. The effect, however, was opposing for the two cell types. Lithium charge capacity increased with increasing annealing temperatures reaching a stable value of ∼325 mAh·ganode–1 (∼100 mAh·cm–3anode at ∼48 μA·cm–2 with respect to total volume of the electrode) for specimen annealed at 900 °C, while a sharp decline in Na charge capacity was noted for rGO annealed above 500 °C. Maximum sodium charge capacity of ∼140 mAh·g–1anode at 100 mA·g–1anode (∼98 mAh·cmanode–3 at ∼70 μA·cm–2) was realized for specimen reduced at 500 °C. These values are the highest reported for GO paper electrodes. More important, annealing of GO in NH3 environment resulted in a complete shutdown of its Na-ion cyclability s...

The precursor material to graphene aerogels is a hydrogel formed from an aqueous solution of graphene oxide. We investigate the time evolution of the physical and chemical properties of a graphene oxide suspension as it transitions to a hydrogel. Fully formed hydrogels undergo densification during reaction, forming mechanically stable monoliths. We demonstrate that the gelation process removes oxygen functional groups, partially re-forms the sp2 network, and creates bonds between graphene oxide sheets. Furthermore, these changes to the physical and chemical properties occur on exactly the same time scale, suggesting that they have a common origin. This discovery lends greater understanding to the formation of graphene oxide-based hydrogels, which could allow more flexibility and tunability in synthetic methods for graphene-like materials.

We demonstrated a coffee-ring structure of graphene derivatives as a one-step solution fabrication route for graphene-based transistors. The key finding is that the coffee-ring structure formed after solvent evaporation of chemically modified graphene nanosheet solutions has an equivalent structure to all graphene-based transistors, the source/drain electrodes and semiconducting channel of which consisted monolithically of graphene nanosheets. These transistors have a thick stack of reduced graphene oxide at the edge of the coffee-ring structure, providing highly conductive thick graphite-like contact as the source and drain electrodes, and the thin film in the midsection of the coffee-ring pattern can simultaneously behave as the semiconducting channel. Here, we provide a holistic approach for achieving coffee-ring graphene-based transistors addressing the entire process flow starting with the coffee-ring structure formation from aqueous graphene oxide dispersion, proceeding through a rapid photoreductio...

Graphene-related materials have been of significant interest in the field of electrochemical sensing and biosensing. Four main methods of synthesizing large quantities of graphene from graphite via graphene oxide have been used to date, using either chlorate (Staudenmaier and Hofmann methods) or permanganate (Hummers and Tour methods) oxidants and strong mineral acids to generate graphite oxide with subsequent reduction. In electrochemical applications, electrochemical reduction is often used to prepare reduced graphenes so as to eliminate any electrochemically reducible groups on the surface of graphene oxides which could interfere with the analytical signals. Here, we show that electrochemical reduction of oxygen-containing groups at graphene oxide surfaces indeed results in materials without inherent electrochemistry for chlorate-based graphene oxides; however, permanganate-based electrochemically reduced materials exhibited significant limitation in the anodic region, starting from ∼+0.1 V (vs Ag/AgCl...

We report a new method for the codoping of boron and nitrogen in a monolayer graphene film. After the CVD synthesis of monolayer graphene, BN-doped graphene is prepared by performing power-controlled plasma treatment and thermal annealing with borazine. BN-doped graphene films with various doping levels, which were controlled by altering the plasma treatment power, were found with Raman and electrical measurements to investigate exhibit p-doping behavior. Transmission electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy were used to demonstrate that the synthesized BN-doped graphene films have a sp2 hybridized hexagonal structure. This approach to tuning the distribution and doping levels of boron and nitrogen in monolayer sp2 hybridized BN-doped graphene is expected to be very useful for applications requiring large-area graphene with an opened band gap.

The surface properties of the electrode materials play a crucial role in determining the performance and efficiency of energy storage devices. Graphene oxide and nanostructures of 3d transition metal oxides were synthesized for construction of electrodes in supercapacitors, and the electronic structure and oxidation states were probed using near-edge X-ray absorption fine structure. Understanding the chemistry of graphene oxide would provide valuable insight into its reactivity and properties as the graphene oxide transformation to reduced-graphene oxide is a key step in the synthesis of the electrode materials. Polarized behavior of the synchrotron X-rays and the angular dependency of the near-edge X-ray absorption fine structures (NEXAFS) have been utilized to study the orientation of the σ and π bonds of the graphene oxide and graphene oxide–metal oxide nanocomposites. The core-level transitions of individual metal oxides and that of the graphene oxide nanocomposite showed that the interaction of graphene oxide with the metal oxide nanostructures has not altered the electronic structure of either of them. As the restoration of the π network is important for good electrical conductivity, the C K edge NEXAFS spectra of reduced graphene oxide nanocomposites confirms the same through increased intensity of the sp2-derived unoccupied states π* band. A pronounced angular dependency of the reduced sample and the formation of excitonic peaks confirmed the formation of extended conjugated network.

The adsorption of 1-hexanol from cyclohexane-d12 at single-layer graphene/α-Al2O3 interfaces was probed at mole percent values as low as 0.05 in the C–H stretching region using vibrational sum frequency generation (SFG). The SFG spectra are indiscernible from those obtained in the absence of graphene, and from those obtained in the presence of graphene oxide films prepared via oxygen plasma treatment of pristine single-layer graphene. A Langmuir adsorption model yields observed free adsorption energies of −19.9(5) to −20.9(3) kJ/mol for the three interfaces. The results indicate that the molecular structure of the hexanol alkyl chain is subject to the same orientation distribution when graphene, oxidized or not, is present or absent at the α-Al2O3/cyclohexane-d12 interface. Moreover, it appears that the adsorption of 1-hexanol in this binary mixture is driven by hexanol interactions with the underlying oxide support, and that a single layer of graphene does not influence the extent of this interaction, ev...

Graphene, a lamellar structured material, easily shears at the contact interfaces and exhibits excellent mechanical strength and conductivity, which promises its potential for tribological applications. However, the dispersion of graphene in lube media is a big challenge. Herein, we developed a chemical approach for selective inclusion of long alkyl chains on the edges and defects sites of reduced graphene oxide sheets through the amide linkage, which facilitates their stable dispersion in the lube oil. Chemical and structural features of site-selective chemically functionalized reduced graphene oxide are monitored by FTIR, XPS, XRD, TG-DTA, FESEM, and HRTEM. Tribological test results showed that the chemically functionalized reduced graphene oxide, as an additive to 10W-40 engine oil, significantly reduced both the friction and the wear of steel balls. The stable dispersion of chemically functionalized reduced graphene oxide provides low resistance in a sheared contact owing to weak van der Waals interac...

Scanning positive column of atmospheric-pressure glow discharge (AGD) plasma is proposed as an eco-friendly and cost-effective strategy toward the instantaneous (∼2 s) reduction of graphene oxide (GO) paper, further leading to a reduced graphene oxide (rGO) paper with unimpeded liquid permeation for high performance supercapacitors. Unlike previous finding that AGD plasma has no effect on GO, a well-designed AGD plasma containing scanning positive column region is demonstrated to be capable of converting GO paper to rGO paper. With the synergy of energetic electrons (electron density 1.03 × 1016 cm–3) and surface touch heating (translational temperature ∼800 K), restoration of graphene’s excellent electrical properties is realized, evidenced by a substantially (5 orders of magnitude) improved electrical conductivity. Moreover, the ultrafast reduction of oxygen functionalities results in the creation of exposed open channels and the expansion of graphene layers, leading to a rGO paper supercapacitor electr...

We have investigated the UV-light-induced defect formation in single- and multilayer graphenes on a SiO2 (300 nm)/Si substrate. Remarkable selectivity for the number of layers was observed; only a single layer graphene is damaged while a multilayer graphene is stable against UV irradiation. We also performed UV irradiation on two kinds of single-layer graphene on Pt(111): the graphene grown on Pt(111) by chemical vapor deposition process and the graphene transferred onto Pt(111). UV irradiation of the graphene grown on Pt(111) did not lead to defect formation, while the graphene transferred onto Pt(111) was damaged by UV irradiation. The differences imply the importance of interfacial interaction between the graphene and the substrate. We have concluded that the π–π stacking in a multilayer graphene or the strong interaction with metal substrates in a single-layer graphene prevents the formation of defect via suppression of corrugation, which is likely to occur in a free-standing-like single-layer graphene.

The mechanism and kinetics of graphene formation from amorphous nickel carbides have been investigated employing quantum chemical molecular dynamics (QM/MD) simulations. Amorphous Ni3C, Ni2C, and NiC were employed to elucidate the role of the subsurface carbon density (ρC) on graphene formation. In each case, the nickel carbide phase underwent rapid carbon precipitation, resulting in a segregated nickel–carbon structure. The kinetics of graphene formation was most favorable for high carbon densities. At low ρC, i.e., Ni3C and Ni2C, there was a tendency for the formation of a number of small carbon fragments that failed to coalesce due to their inability to diffuse over the nickel surface. Graphene formation was only observed in the presence of high carbon densities that were relatively localized. These simulations, therefore, suggest that graphene nucleation is not immediately related to the presence of catalyst carbide phases.

In this study, a new core–shell heterostructure of multiwalled carbon nanotube@reduced graphene oxide nanoribbon (MWCNT@rGONR) was prepared by modified microwave-assisted synthesis step and a chemical reduction. The core–shell heterostructure of MWCNT@rGONR was used as the catalytic film of the counter electrode (CE) of a dye-sensitized solar cell (DSSC). The chemical state and the degree of defects on the surface of MWCNT@rGONR were investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectra, respectively. Transmission electron microscopy (TEM) image of the film of MWCNT@rGONR shows graphene sheet, covering on a MWCNT, indicating a core of the carbon nanotube and its shell of graphene. Photocurrent density–voltage characteristics of the DSSCs, using commercial graphene nanopowder (GNP), MWCNT, and MWCNT@rGONR as the CE materials were obtained at 100 mW cm–2. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to study the electrocatalytic abilities of the films...

Face-to-face restacking is one of the main reasons for low surface utilization of multilayered graphene. In this study, multilayered reduced graphene oxide/polymer architecture was fabricated by gas phase or chemical reduction of thin films composed of graphene oxide monolayers using poly(diallyldimethylammonium) as the cationic binder deposited by layer-by-layer self-assembly. The electrochemical behavior of the thin films in acidic and neutral electrolytes was investigated by using cyclic voltammetry and electrochemical impedance spectroscopy. A transmission line model was adopted to simulate the electrochemical impedance data. The electrochemical data were analyzed and deconvoluted into charge storage due to non-Faradaic electrical double layer capacitance and pseudocapacitance arising from Faradaic surface redox reactions. Pseudocapacitance observed in acidic electrolyte is proportional to the amount of surface functional groups. An overall volumetric capacitance as high as 364 F cm–3 was achieved for...

We investigated mechanism of the metal-catalyst-free direct growth of graphene films in chemical vapor deposition on sapphire surfaces using three kinds of substrates, randomly stepped (0001), phase-separated (0001) and randomly stepped (1120). The graphene growth process includes pit formation in the early stage by hydrogen and methane mixed gas flow, appearance of catalytic action inside the pit, nucleation of graphene on the pit surfaces, and expansion and fusion of graphene films. It was found that the quality of graphene films depends on the crystallographic plane. In addition, even on an identical plane, a domain-selective growth mode was observed between domains with different step-terrace structures. This study indicates that morphology of the sapphire surface is crucial in direct growth of graphene films because a certain density of atomic-steps on the surface is favorable to nucleation of graphene films.

We explore structural changes of the carbon in MnO2/reduced graphene oxide (RGO) hybrid materials prepared by the direct redox reaction between carbon and permanganate ions (MnO4–) to reach better understanding for the effects of carbon corrosion on carbon loss and its bonding nature during the hybrid material synthesis. In particular, we carried out near-edge X-ray absorption fine structure spectroscopy at the C K-edge (284.2 eV) to show the changes in the electronic structure of RGO. Significantly, the redox reaction between carbon and MnO4– causes both quantitative carbon loss and electronic structural changes upon MnO2 deposition. Such disruptions of carbon bonding have a detrimental effect on the initial electrical properties of the RGO and thus lead to a significant decrease in electrical conductivity. Electrochemical measurements of the MnO2/reduced graphene oxide hybrid materials using a cavity microelectrode revealed unfavorable electrochemical properties that were mainly due to the poor electric...

While almost all attempts for bulk synthesis of graphene result in agglomeration of flat graphene layers, a simple solvothermal route is reported for the preparation of a graphene powder consisting of very flexible sheets. Curvature of these graphene sheets avoids vital agglomeration, which normally blocks the accessible surface of graphene sheets. The thickness of the graphene sample varies from one to a few layers, but the spaces among the graphene layers are sufficiently large enough to provide internal accessibility. Thus, this graphene sample shows the general (bulk) properties of graphene single sheets. For instance, it has a high specific surface area of ca. 1168 m2 g–1 and starts to burn at 350 °C because of a large interlayer spacing of graphene sheets (i.e., 5.1 A). It should be considered that this specious interlayer is mostly due to the graphene curvature rather than to filling with functional groups (as in the case of graphite oxide). Owing to the high accessibility and active sites over a l...

A heating treatment is often used in graphene research to remove adsorbates and resist materials from graphene. Heating graphene followed by air exposure is also known to result in heavy hole doping in graphene, although the role of heating has been unclear. Here, we demonstrate that a practical graphene sample fabricated using the commonly used growth and transfer techniques is unstable against heating in a high vacuum. Structural disorder likely due to defect formation is induced by heating, and the disorder is accompanied by hole doping. Our analysis shows that the main cause of the defect formation is graphene reacting with O2 and H2O molecules inserted between graphene and the substrate. The hole doping caused by air exposure after heating is explained by gas adsorption at the defect sites.

We report on the electrical characterization and Raman spectroscopy of chemical vapor deposition copper-grown graphene transferred onto a Si/SiO2 substrate by high-speed (1 mm/s) electrochemical delamination. We determine graphene’s sheet resistance, carrier mobility, and concentration as well as its physical quality as a function of the electolyte concentration. Graphene’s electrical properties are investigated with standard Hall measurements in van der Pauw geometry and a contactless method that employs a single-post dielectric resonator operating at microwave frequencies. These properties are related to the widely used copper etching technique. The results prove that the high-speed electrochemical delamination provides good-quality graphene within a short time scale.

Graphene, grown by chemical vapor deposition, is transferred onto Nb-doped SrTiO3(001) surface and the interface properties are characterized by scanning tunneling microscopy and photoemission spec...

Integration of functional organic molecules with graphene is expected to promote the development of graphene-based flexible electronics with novel properties. Here, the self-assembled structure of dipole phthalocyanine molecules, chloro-aluminum phthalocyanine (ClAlPc), on single-layer graphene grown by chemical vapor deposition (CVD) over a Cu film was characterized by low-temperature scanning tunneling microscopy (LT-STM). The phthalocyanine molecules show highly ordered assembled structures on the CVD graphene, and these molecular layers extend continuously over the steps of the Cu film. We also observe specific boundaries in the self-assembled molecule arrays, which can be explained by the presence of domain boundaries in the graphene. The STM results suggest that CVD graphene is as a good molecular assembly template for surface functionalization and that these molecular arrays facilitate the study of domain structures in CVD graphene.

The reducibility of iron oxide nanoparticles (NPs) supported over few-layer thick graphite upon annealing in hydrogen is investigated by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), and high-resolution transmission electron microscopy (HR-TEM). It is found that the stability of the iron oxide NPs toward reduction is enhanced by the interaction with the graphene nanosheets as compared to the bulk iron oxide. Postannealing TEM micrographs reveal the existence of both core/shell and homogeneous iron oxide NPs with the latter forming irregular trenches into the graphene sheets. EELS analysis and TEM images clearly demonstrate that the reducibility of iron oxide particles depends on the specific graphene site on which they are attached. Furthermore, we show that graphene etching can be mediated by iron oxide NPs at relatively mild reduction conditions.