Few-layer Graphene Surface Research Articles

Adsorption of Rhodamine G from Aqueous Solutions onto Particles of Few-Layer Graphene Prepared by Self-Propagating High-Temperature Synthesis

The ability of few-layer graphene prepared by self-propagating high-temperature synthesis to adsorb Rhodamine G was studied. The dependences of the sorption capacity of few-layer graphene particles on the Rhodamine G concentration in aqueous solution, solution temperature, and adsorption time were determined. The adsorption of Rhodamine G on the surface of few-layer graphene is satisfactorily described by the Langmuir and Freundlich models. The process occurs under mixed diffusion control and can be described by a pseudo-second-order rate equation.

Towards Understanding the Impact of Electrochemical Double Layer on the Performance of Graphene Devices

The effect of electrochemical double layer (EDL) on the performance of graphene-based sensing platforms has been an area of controversy over the last two decades 1. The hydrophobic nature of bare graphene tends to repel the water and minimizes the solution/solid surface interactions 2. However, the presence of oxygen-based functional groups on graphene introduces negatively charged sites and causes a negative surface zeta potential. Hence, the instant formation of an EDL on a graphene surface in an aqueous solution is inevitable, affecting the graphene surface's chemical/physical interactions 3. Thus far, multiple theories and techniques have been developed to investigate the impact of EDL on graphene sensing performance; however, they either suffer from complexity in design or have ignored the co-existence of other solution parameters such as pH and oxidation-reduction potential 4.In this work, we propose the use of alkaline chloride salts to understand the impact of the ionic strength and EDL of the solution on the few-layer graphene (FLG) based chemiresistive sensors. Considering the full ionization of NaCl and the low redox potential of the generated ions, alkaline chlorides are considered good candidates to modulate the electronic band structure of FLG without altering its surface chemistry 5. By use of Helmholtz theory of EDL, it is postulated the Inner Helmholtz Layer (IHL) to be formed by Na+ ions due to the negative surface charge of FLG. Whereas the hydrated counter ions, Cl-, form the Outer Helmholtz Layer (OHL). Upon the formation of a positively charged immobile layer of Na+ ions on the surface (IHL), the electrostatic gating effect of EDL dopes the graphene with electrons. This charge separation can be modelled as two parallel plates of a capacitor with infinitely thin dielectric 6.Since the presence of oxygen-based functional groups on FLG is inevitable during the synthesis process, it is considered inherently p-doped. Accordingly, upon the addition of NaCl concentrations, the FLG surface current decreases. However, at higher concentrations (around 2000 ppm NaCl), the sensor response is inversed, and surface current increases by the addition of NaCl (Figure-left). In fact, high ionic strength increases the EDL compactness and decreases the charge screening length (Debye length). As a result of this sharper difference in solid/solution potentials (Figure-right), a more n-doped surface having electron as majority charge carriers is obtained, changing the nature of the response.We have also demonstrated that the formation of sodium-oxygen metal complexes on FLG is not likely in an aqueous environment. According to the Raman spectroscopy results of FLG, an increase in the intensity ratio of D (defect) to G bands is observed upon exposure to water and NaCl. Moreover, the noticeable right shift in the 2D band position demonstrates the p-doping of FLG 7. Furthermore, the reversible response of the sensor during multiple exposures to NaCl demonstrates the sensor response originates from EDL, not the formation of sodium-oxygen metal complexes. In fact, the full or half hydrated Na+ ions in IHL electrostatically interact with surrounding oxygen. However, since the oxygen atom in water is more negatively charged compared to the oxygen on the graphene surface, Na+ is unlikely to bound to the surface after hydration. Therefore, using NaCl could be a suitable medium to study the impact of EDL on the performance of graphene devices.Reference Jurado et al., Scientific Reports, 7(1), pp.1-12 (2017).Angizi, S et al., Langmuir, 37(41), pp.12163-12178 (2021).Jung et al., Nano Letters, 21(1), pp.34-42 (2020).Pak et al., The Journal of Physical Chemistry C, 118(38), pp.21770-21777 (2014).Lee et al., ACS Applied Materials & Interfaces, 11(45), pp.42520-42527 (2019).Kwon et al., The Journal of Physical Chemistry C, 116(50), pp.26586-26591 (2012).Bruna et al., ACS Nano, 8(7), pp.7432-7441 (2014). Figure 1

Realizing Long-life Zn Anode by Few-layer Graphene Ion-oriented Interface

Inspired by the nanostructured graphene-based membranes for ionic sieving, an ion-oriented interface of few-layer graphene (FLG) was rationally constructed to achieve long-life zinc (Zn) metal anode. The ion-oriented interface increased the nucleation sites and inhibited localized dendrite growth, which aided the uniform transmission of Zn ions. Moreover, the fewer oxygen-containing functional groups on the FLG surface demonstrated a lower energy barrier of ion transport and a stronger Zn ion-regulating function. The FLG@Zn anode exhibited a much larger capacity with significant stability in the cycling behavior against MnO2 cathode compared to the pristine Zn anode. Specifically, a nearly 97% capacity retention was achieved for FLG@Zn/MnO2 battery after 5000 cycles at 10 A·g–1. Our work provides a novel approach to the fabrication of high-performance Zn anode for ultra-stable aqueous batteries.

ROLE OF LOW SURFACE AREA FEW LAYER GRAPHENE IN ENHANCING MECHANICAL PROPERTIES OF POLY (1,4-CIS-ISOPRENE) RUBBER NANOCOMPOSITES

ABSTRACTNanofillers of type few layer graphene (FLG) characterized with “low” surface area (<40 m2/g) were studied as reinforcing agents in poly (1,4-cis-isoprene) (IR matrix), and their roles in enhancing mechanical properties of rubber nanocomposites were demonstrated. The adsorption isotherms show that FLG surface areas were 13.8 m2/g for SFG6, 23.8 m2/g for KS4, and 39.5 m2/g for EXG 9840. All FLG had remarkable in-plane crystallinity and were characterized by a lower number of layers stacked (45–48) in the crystalline domain. Rheometric curves were studied to estimate the scorch time (t′05) and curing time (t′90). Tensile and dynamic–mechanical analyses show that FLG forms filler networks at low filler content. Tensile properties show that EXG 9840/IR has a lower reinforcing factor and elongation at break than KS4 FLG nanofiller. A series of experiments shows the dependency surface area of FLG in enhancing mechanical properties of nanocomposites. The work suggests that lower surface area should be selected in order to have lower dissipation of energy and efficient filler networking.

Few-Layer Graphene Functionalized by Carbon Nanotubes and MnO2 Nanoparticles for High-Performance Supercapacitors

Few-layer graphene (FLG) exfoliated from graphite is an attractive electrode material due to its high conductivity and high chemical stability. But usually limited by their specific surface area (SSA), its capacitance needs to be improved when used in supercapacitors. In this study, FLG has been prepared by combining electrochemical exfoliation and thermal expansion. It has a SSA of 148 m2/g and delivers a capacitance of 55 F/g at a discharge current density of 1 A/g. To increase its capacitance, carbon nanotubes (CNTs) and MnO2 nanoparticles are functionalized onto FLG surface separately. Both modification could achieve a composite electrode with enhanced capacitance. The CNT/FLG and MnO2/FLG electrodes show a specific capacitance of 106 and 336 F/g at the current density of 1 A/g, respectively, while the former exhibits a much better rate performance and cycling stability than the latter. This work demonstrates an optional way to improve the performance of FLG-based supercapacitors.

Synthesis of macroporous ZnO-graphene hybrid monoliths with potential for functional electrodes

The favorable optoelectronic and catalytic properties of zinc oxide, combined with the large specific surface area of graphene molded in 3D network, are able to offer a promising pathway towards the development of hybrid structures with improved electrical, optical, and catalytic properties, that are well suited as active elements for a wide range of applications, especially those related to energy storage and conversion. Using a template-assisted method we demonstrate the synthesis of 3D, porous network monoliths of hybrid ZnO-graphene thin-films in various controlled configurations. The thin-film composites are realized by thermal CVD growth of few-layers graphene on macroporous nickel foam, followed by an ALD coating process of the ZnO film. Notably, the graphene shell fully preserved its network structure after performing complete chemical etching of the nickel scaffold. The challenge of ZnO layer deposition directly on the graphene surface was solved by hydrophobicity adjustment using UV-ozone treatment prior to the atomic layer deposition process. Structural and morphological characterization revealed the formation of hexagonal crystalline zinc oxide films, conformally deposited on the few-layer graphene surface, while the electrical conduction analysis indicates excellent conductivity and interlayer electrical contact for these hybrids. Our results demonstrate that this controlled fabrication process is able to provide highly functional materials with great potential for various electrode applications.

Surface structure of few layer graphene

Understanding surface structure of graphene is important for its integration into composite materials. Here, we have used synchrotron X-ray reflectivity (XRR) to study the structure of commercially available graphene samples (prepared via chemical vapor deposition, and marketed as graphene monolayers) on SiO2/Si at different temperatures. X-ray photoelectron spectroscopy, photoemission electron microscopy and atomic force microscopy (AFM) were employed to evaluate the composition and morphology of the graphene layer. Our results indicate that the samples we characterized consisted of 3–4 layers of graphene, which should thus be more accurately described as few layer graphene (FLG). Furthermore, a “contaminant” layer, comprising polymethylmethacrylate and graphene multilayers, was found present atop FLG. We also report tentative results on the effect of temperature on the graphene sample thickness. At 25 °C, the FLG thickness from XRR measurements was 13.0 ± 1.0 Å, in agreement with that obtained from AFM (13.9 ± 0.7 Å). Upon heating to 60 °C, the FLG thickness expanded to 13.8 Å, which further increased to 14.3 Å upon cooling to 25 °C. We attribute this temperature dependent thickness to the out-of-plane rippling of graphene as previously reported. These unprecedented results on the FLG surface structure are valuable to its potential bioanalytical applications.

Formation of Stable Solid–Electrolyte Interphase Layer on Few-Layer Graphene-Coated Silicon Nanoparticles for High-Capacity Li-Ion Battery Anodes

Silicon-based anode materials exhibit higher specific and volumetric capacities than other materials and have therefore received much attention for potential use in lithium-ion batteries. However, the continuous growth of a solid–electrolyte interphase at the surface of silicon is a primary cause of chronic capacity fading of silicon electrodes. In this paper, we report the formation of an electrochemically stable solid–electrolyte interphase layer on the surfaces of the few-layer graphene-coated silicon nanoparticles. During the first lithiation, electrolyte molecules were electrochemically decomposed and deposited on the surface of few-layer graphene, thus forming a stable protective layer. When combined with an ionic liquid electrolyte based on pyrrolidinium and bis(fluorosulfonyl)imide, an anode containing 75% few-layer graphene-coated silicon delivered a reversible capacity of 1770 mAh g–1 (1430 mAh/ccelectrode) at a current density of 400 mAh g–1 (2 mAh cm–2) after 200 cycles. Averaged over the firs...

Facile Doping and Work‐Function Modification of Few‐Layer Graphene Using Molecular Oxidants and Reductants

Doping of graphene is a viable route toward enhancing its electrical conductivity and modulating its work function for a wide range of technological applications. In this work, the authors demonstrate facile, solution‐based, noncovalent surface doping of few‐layer graphene (FLG) using a series of molecular metal‐organic and organic species of varying n‐ and p‐type doping strengths. In doing so, the authors tune the electronic, optical, and transport properties of FLG. The authors modulate the work function of graphene over a range of 2.4 eV (from 2.9 to 5.3 eV)—unprecedented for solution‐based doping—via surface electron transfer. A substantial improvement of the conductivity of FLG is attributed to increasing carrier density, slightly offset by a minor reduction of mobility via Coulomb scattering. The mobility of single layer graphene has been reported to decrease significantly more via similar surface doping than FLG, which has the ability to screen buried layers. The dopant dosage influences the properties of FLG and reveals an optimal window of dopant coverage for the best transport properties, wherein dopant molecules aggregate into small and isolated clusters on the surface of FLG. This study shows how soluble molecular dopants can easily and effectively tune the work function and improve the optoelectronic properties of graphene.

電界放出電子線を用いた低速電子回折装置の開発

Low-energy electron diffraction (LEED) apparatuses with field-emission (FE) tips were developed. The FE tips were fabricated by field-assisted gas etching to obtain an atomically sharp tip apex. These tips emit single-spot electron beams. Using the FE beams in conjunction with a lensless system, we observed backscattered electrons at the surface of few-layer graphene grown on SiC(0001). The obtained hexagonal patterns at sample biases of 93 V, 54 V, and 28 V were interpreted as diffraction spots derived from the graphene, the SiC(0001) substrate, and a buffer layer, respectively. On the basis of the opening angle of FE, the irradiated areas were estimated to be 350 nmφ when the sample bias was 100 V. Since the FE beams were easily focused by a magnetic lens, a LEED apparatus with the focused FE beams was also examined. This approach might be useful for improving the sharpness of LEED patterns.

Ultrafine nickel oxide quantum dots enbedded with few-layer exfoliative graphene for an asymmetric supercapacitor: Enhanced capacitances by alternating voltage

A green and one-step method of electrochemical alternating voltage has been utilized to form NiO quantum dots/graphene flakes (NiO-dots/Gh) for supercapacitor applications. NiO quantum dots (∼3 nm) are uniformly deposited on few-layer graphene surfaces by oxygen functional groups on graphene surface that is naturally utilized to bridge NiO and graphene through Ni–O–C bands, which exhibits outstanding specific capacitance 1181.1 F g−1 at a current density of 2.1 A g−1 and rate behavior 66.2% at 42 A g−1 as NiO dots can be fleetly wired up to current collector through the underlying graphene two-dimensional layers. The NiO-dots/Gh composite is further undertaken in asymmetric supercapacitors with high energy density (27.3 Wh kg−1 at 1562.6 W kg−1).

Dynamics of Gold Nanoparticles on Carbon Nanostructures Driven by van der Waals and Electrostatic Interactions.

Transmission electron microscopy studies on the assembly and growth of gold nanoparticles on carbon nanotubes supported on few-layer graphene and amorphous carbon reveal a competition between van der Waals forces and electrostatic interactions, enabling controlled positioning and sizing of adsorbed nanoparticles at the nanochannels formed between the carbon nanotube and the few-layer graph-ene surface.

Tuning between crystallographically aligned carbon nanotube growth and graphene etching

The catalytic activity of nanoparticles to form either crystallographically oriented etch tracks or carbon nanotubes on top of few-layer graphene is tuned through the application of a methane feedstock gas. The catalytic activity for these two processes is found to vary at different rates as a function of the amount of applied feedstock gas. These differences provide a window of growth parameters in which nanotubes can be grown on the surface of few-layer graphene without significant formation of etch tracks. Since this surface growth results in nanotubes which can be crystallographically aligned to the underlying graphene layers, this development could lead to improved electrical interfaces between nanotubes and graphene without the deleterious consequences of catalytic etching. Such improved interfaces could prove to be useful in applications benefiting from low interfacial electrical resistances between one-dimensional and two-dimensional materials, such as in supercapacitor applications.

Strain Engineering of Kapitza Resistance in Few-Layer Graphene

We demonstrate through molecular dynamics simulations that the Kapitza resistance in few-layer graphene (FLG) can be controlled by applying mechanical strain. For unstrained FLG, the Kapitza resistance decreases with the increase of thickness and reaches an asymptotic value of 6 × 10(-10) m(2)K/W at a thickness about 16 nm. Uniaxial cross-plane strain is found to increase the Kapitza resistance in FLG monotonically, when the applied strain varies from compressive to tensile. Moreover, uniaxial strain couples the in-plane and out-of-plane strain/stress when the surface of FLG is buckled. We find that with a compressive cross-plane stress of 2 GPa, the Kapitza resistance is reduced by about 50%. On the other hand it is almost tripled with a tensile cross-plane stress of 1 GPa. Remarkably, compressive in-plane strain can either increase or reduce the Kapitza resistance, depending on the specific way it is applied. Our study suggests that graphene can be exploited for both heat dissipation and insulation through strain engineering.

Electrical Transport in “Few-Layer Graphene” Film Prepared by the Hot-Spray Technique: The Effect of Thermal Treatment

We investigate charge transport of a randomly stacked few-layer graphene (FLG) film supported on quartz plate, as-prepared and after annealing treatment. The film is prepared by a simple, cheap and rapid hot spray technique from a FLG suspension, while FLG was obtained in our previous report by high yield mechanical exfoliation of pencil lead. According to Hall Effect measurements, the annealing process has strong impact on the conductivity of the FLG film, despite its low oxygen/defect content as confirmed by X-ray photoelectron spectroscopy (XPS), Raman, and photoluminescense spectroscopy. The nonohmic behavior along with a sheet resistance (R□) decreases from a few tens to a few kilo-ohms is observed for the FLG film, whereas after thermal treatment, a constant R□ of 0.7 kΩ is recorded. The improvement of conductivity in the FLG film is attributed mainly to desorption of toluene from the FLG surface according to the TPD-MS analysis and not only to the oxygen decrease as generally reported.

Hydrogen spillover at sub-2 nm Pt nanoparticles by electrochemical hydrogen loading

Hydrogen generation and storage is an essential component in the increasingly important field of energy storage. Electrochemical generation of hydrogen atoms at the surface of Pt like metals at select potentials is a widely accepted phenomenon. However, moving these adsorbed hydrogen atoms to high surface area support systems for storage is an issue. We show spillover of these adsorbed hydrogen atoms to the supporting structure for sub-2 nm Pt nanoparticles sputtered on Fluorine Doped Tin Oxide (FTO) and on few layer graphene (FLG) supports. Evidence of size-dependent hydrogen spillover was observed for Pt nanoparticles deposited using tilted target sputtering and a correlation between nanoparticle size, crystallinity, support characteristics, and hydrogen spillover is also reported. Evidence of C–H bonds formed on the FLG surface due to H spillover from 0.9 nm Pt nanoparticles was also confirmed through XPS analysis.

Few layer graphene decorated with homogeneous magnetic Fe3O4 nanoparticles with tunable covering densities

Magnetic iron oxide nanoparticles (NPs) with narrow size distribution (8 ± 2 nm), well defined chemical composition and crystalline structure are synthesized and homogeneously dispersed onto the surface of few-layer graphene (FLG) via a solvothermal decomposition method. The iron oxide NPs are strongly anchored to the graphene surface and confer a magnetic character to the final composite. The metal oxide/support interaction is high enough to avoid the NPs coalescence and/or agglomeration and thus to preserve the NPs size and dispersion after thermal treatment up to 400 °C. The introduced iron oxide NPs on FLG also play a role of nano-spacers to prevent the re-stacking of the graphene sheets upon the drying process. It is expected that such a composite could find use in several application fields such as catalyst support for liquid-phase reactions with easy magnetic separation, in electrochemical energy storage and in waste water treatment. The ability of the synthesized iron oxide NP/FLG composite to adsorb foreign elements (organic pollutants) is demonstrated in the methylene blue (MB) adsorption and its catalytic properties are evaluated in the selective oxidation of H2S.

High peroxidase catalytic activity of exfoliated few-layer graphene

Few-layer graphene prepared from graphite exfoliated by chitosan has a preserved structure without oxidation or destruction of the sp2 character of the carbon plane and exhibits a higher peroxidase catalytic activity than that of graphene oxide (GO) and its reduced form. The peroxidase catalytic activity of as-obtained few-layer graphene is 45 times higher than that of GO and 4 times higher than that of reduced GO with the same concentration of 30μgmL−1 and the detection limit of hydrogen peroxide is 10nM. The excellently catalytic performance can be attributed to the fast electron transfer on the surface of few-layer graphene, which is further confirmed by electrochemical characterization. The as-prepared few-layer graphene has been used to determine hydrogen peroxide in three real water samples with satisfactory results.

Evolution of surface morphology and electronic structure of few layer graphene after low energy Ar+ ion irradiation

We report on co-existing dual anisotropy ripple formation, sp bonding transformation, and variation in the delocalized π electron system in 1 keV Ar+ ion irradiated few-layer graphene surfaces. Ripples in directions, perpendicular and parallel to the ion beam were found. The irradiation effect and the transition from the sp2-bonding to sp3-hybridized state were analyzed from the deconvolution of the C (1s) peak and from the shape of the derivative of the Auger transition spectra. The results suggest a plausible mechanism for tailoring of few-layer graphene electronic band structure with interlayer coupling tuned by the ion irradiation.

Unusual surface and edge morphologies, sp2 to sp3 hybridized transformation and electronic damage after Ar+ ion irradiation of few-layer graphene surfaces

Roughness and defects induced on few-layer graphene (FLG) irradiated by Ar+ ions at different energies were investigated using X-ray photoemission spectroscopy (XPS) and atomic force microscopy techniques. The results provide direct experimental evidence of ripple formation, sp2 to sp3 hybridized carbon transformation, electronic damage, Ar+ implantation, unusual defects and edge reconstructions in FLG, which depend on the irradiation energy. In addition, shadowing effects similar to those found in oblique-angle growth of thin films were seen. Reliable quantification of the transition from the sp2-bonding to sp3-hybridized state as a result of Ar+ ion irradiation is achieved from the deconvolution of the XPS C (1s) peak. Although the ion irradiation effect is demonstrated through the shape of the derivative of the Auger transition C KVV spectra, we show that the D parameter values obtained from these spectra which are normally used in the literature fail to account for the sp2 to sp3 hybridization transition. In contrast to what is known, it is revealed that using ion irradiation at large FLG sample tilt angles can lead to edge reconstructions. Furthermore, FLG irradiation by low energy of 0.25 keV can be a plausible way of peeling graphene layers without the need of Joule heating reported previously.