Graphene Cover Research Articles

Utilization of graphene as an alternative sustainable amendment in improving soil health through accelerated decomposition of oil palm mulch and enhanced nutrient availability

Graphene has unique properties for improving soil health properties such as nutrient availability, soil physical and chemical properties, and controlled release of essential elements. This research aimed at determining the impact of graphene amendment on the decomposition of oil palm frond mulching and on soil health status. The study was conducted using a factorial experiment in completely randomized design with two main factors: (i) covering conditions: cover with plastic sheet and no cover, and (ii) graphene application that included T1 (control): oil palm frond mulching (OFM), T2: OFM + graphene (G), T3: OFM + G + chemical fertilizer, and T4: OFM + G + goat manure. The results indicated that there were significant differences among graphene applications, between cover conditions, and in interactions between graphene applications and cover conditions for all soil characteristics in the most observed month. In the third month of soil analysis, the treatment of graphene applications showed higher electrical conductivity (T2: 151.7 ± 6.8 µS cm−1), available phosphorus (T3: 9.0 ± 6.7 mg kg−1), exchangeable potassium (T2: 67.1 ± 24.9 mg kg−1), and exchangeable calcium (T3: 95.4 ± 5.1 mg kg−1), compared to control. The cover condition showed suitable soil pH (5.0 ± 0.2), higher soil available phosphorus (7.1 ± 5.0 mg kg−1), and exchangeable calcium (599.1 ± 235.2 mg kg−1), but the no-cover condition presented higher soil organic matter (0.7% ± 0.2%), exchangeable potassium (60.3 ± 19.1 mg kg−1), and exchangeable magnesium (96.7 ± 11.4 mg kg−1). Correlation results indicated that most soil characteristics were correlated under graphene applications. Principal component analysis showed that the treatments of graphene application dominated most soil characteristics. The results suggest that graphene has potential for improving soil health properties and can be applied as an alternative sustainable amendment to accelerate the decomposition of oil palm frond mulch and enhance nutrient availability for oil palm. In addition, the authors suggest that further investigations should consider more soil health parameters in long-term field studies for a better understanding and to provide recommendations to farmers.

Adsorption of water, hydrogen and oxygen on graphene/Pt(111) surfaces: 2D graphene cover effect studied by temperature programmed desorption

Understanding the interaction between reactive gas and graphene overlayers on metal substrates is prerequisite to study the surface reactions confined at the interfaces between graphene cover and metal surfaces. Here, we report H2O, H2 and O2 adsorption on graphene overlayer covered Pt(111) surfaces by using temperature programmed desorption (TPD). H2O has weak interaction with graphene overlayers and does not intercalate under graphene overlayers under UHV adsorption conditions. Submonolayer H2O desorption on graphene is about 20 K lower directly compared to that on Pt(111) on the submonolayer graphene/Pt(111) surfaces with well-defined metal-graphene interfaces. Both H2 and O2 can intercalate on submonolayer graphene/Pt(111) but not on the graphene fully covered Pt(111) surfaces under UHV adsorption conditions. Graphene cover weakens H2 adsorption at the graphene-Pt(111) interfaces with the H2 desorption temperature lowered by about 30 K and desorption energy lowered by 8.4 ± 0.6 kJ/mol . The intercalated oxygen under graphene reacts with the graphene overlayers above 600 K with CO2 and CO as the oxidation products.

Near-field thermal rectification via an InSb/graphene/3C–SiC-nanowire heterostructure

Thermal rectifiers allow asymmetric heat flux when the temperature bias between two terminals is reversed. Based on the near-field radiative heat transfer, the performance of thermal rectifiers can be significantly enhanced. Here, we propose a near-field thermal rectifier based on the InSb/graphene/3C–SiC-nanowire heterostructure. The rectifier consists of two terminals, one terminal is an InSb slab with a graphene-coated bottom, and the other terminal is a 3C–SiC nanowire array with a graphene cover on its top. By using fluctuational electrodynamics, we calculate the radiation heat flux and the corresponding thermal rectification efficiency (TRE). The results show that the strong asymmetric surface plasmon polaritons (SPPs) and hyperbolic phonon polaritons (HPPs) in the forward and reverse heat transfer scenarios guarantee a robust TRE. Besides, the TRE of the proposed heterostructure can be dynamically controlled and optimized via the chemical potential of graphene, the volumetric filling ratio of the nanowire array, and the terminal temperature. Specifically, the thermal rectification efficiency (1 – min (Qf, Qr)/max (Qf, Qr) with Qf and Qr denoting the forward and reverse heat flux, respectively) can exceed 80% at a wide-ranged terminal gap and reaches a maximum of 92.96%.

Porous 2D Catalyst Covers Improve Photoelectrochemical Water-Oxidation Performance.

Confined catalysis under the cover of 2D materials has emerged as a promising approach for achieving highly effective catalysts in various essential reactions. In this work, a porous cover structure is designed to boost the interfacial charge and mass transfer kinetics of 2D-covered catalysts. The improvement in catalytic performance is confirmed by the photoelectrochemical oxidation evolution reaction (OER) on a photoanode based on an n-Si substrate modified with a NiOx thin-film model electrocatalyst covered with a porous graphene (pGr) monolayer. Experimental results demonstrate that the pGr cover enhances the OER kinetics by balancing the charge and mass transfer at the photoanode and electrolyte interface compared to the intrinsic graphene cover and cover-free control samples. Theoretical investigations further corroborate that the pore edges of the pGr cover boost the intrinsic catalytic activity of active sites on NiOx by reducing the reaction overpotential. Furthermore, the optimized pores, which can be easily controlled by plasma bombardment, allow oxygen molecules produced in the OER to pass through without peeling off the pGr cover, thus ensuring the structural stability of the catalyst. This study highlights the significant role of the porous cover structure in 2D-covered catalysts and provides new insight into the design of high-performance catalysts.

Effect of temperature on CO oxidation over Pt(111) in two-dimensional confinement.

Confined catalysis between a two-dimensional (2D) cover and metal surfaces has provided a unique environment with enhanced activity compared to uncovered metal surfaces. Within this 2D confinement, weakened adsorption and lowered activation energies were observed using surface science experiments and density functional theory (DFT) calculations. Computationally, the role of electronic and mechanical factors responsible for the improved activity was deduced only from static DFT calculations. This demands a detailed investigation on the dynamics of reactions under 2D confinement, including temperature effects. In this work, we study CO oxidation on a 2D graphene covered Pt(111) surface at 90 and 593K using DFT-based ab initio molecular dynamics simulations starting from the transition state configuration. We show that CO oxidation in the presence of a graphene cover is substantially enhanced (2.3 times) at 90K. Our findings suggest that 2D confined spaces can be used to enhance the activity of chemical reactions, especially at low temperatures.

Threshold conditions for transversal modes of tunable plasmonic nanolasers shaped as single and twin graphene-covered circular quantum wires

We implement the lasing eigenvalue problem (LEP) approach to study the electromagnetic field in the presence of a circular quantum wire (QW) made of a gain material and wrapped in graphene cover and a dimer of two identical graphene-covered QWs, at the threshold of stationary emission. LEP delivers the mode-specific eigenvalue pairs, namely the frequencies and the threshold values of the QW gain index for the plasmon and the wire modes of such nanolasers. In our analysis, we use quantum Kubo formalism for the graphene conductivity and classical Maxwell boundary-value problem for the field functions. The technique involves the resistive boundary conditions, the separation of variables in the local coordinates, and, for the dimer, the addition theorem for the cylindrical functions. For single-wire plasmonic laser, we derive approximate engineering expressions for the lasing frequencies and threshold values of the gain index that complement the full-wave computations. For the dimer, we derive separate determinantal equations for four different classes of symmetry of the lasing supermodes and solve them numerically. Our investigation of the mode frequencies and thresholds versus the graphene and QW parameters shows that plasmon modes or, for the dimer, plasmon supermodes have lower frequencies and thresholds than the wire modes provided that the QW radius is smaller than 10 μm, however in thicker wires they are comparable. Only the plasmon-mode characteristics are well-tunable using the graphene chemical potential. In the dimer, all lasing supermodes form closely located quartets, however, they quickly approach the single-wire case if the inter-wire separation becomes comparable to the radius. These results open a way for building essentially single-mode plasmonic nanolasers and their arrays and suggest certain engineering rules for their design.

Oxygen-Induced Dissociation of a Single Water Molecule in Confined 2-D Layers: A Semiempirical Study.

Semiempirical quantum mechanical methods provide a middle ground to molecule-surface interactions between computationally demanding full ab initio quantum chemistry calculations and force-field calculations. In the present study, the PM7 semiempirical method is used to evaluate the adsorption energy values of X@h-BN monolayer [X=O, OH, and H2 O], followed by a mechanistic study of oxygen-induced water dissociation on a free-standing h-BN monolayer. Based on oxygen adsorption configurations, two reaction pathways for water dissociation are studied that yield two distinct configurations of double OH-functionalized h-BN monolayer. The effect of a graphene cover layer on these proposed mechanistic pathways is then investigated by placing the graphene cover layer on the top of the h-BN monolayer and continuously tuning the separation (dGr/h-BN ) between these two layers.

Boudouard reaction under graphene cover on Ni(1 1 1)

We investigated the interaction of CO with graphene/Ni(111) and the Boudouard reaction at 3.7 mbar by Near Ambient Pressure X-Ray Photoemission Spectroscopy (NAPXPS), i.e. at one order of magnitude higher pressure than previously explored in-operando conditions. In this regime, CO intercalates under the graphene layer causing its partial detachment from the Ni substrate. The so-obtained high local CO coverage opens the way to CO2 formation via the Boudouard reaction. Its onset is witnessed by observing physisorbed CO2 accumulating below the graphene cover. The so-generated additional carbon atoms transform carbide into graphene, causing the expansion of the graphene islands. In addition, CO adsorption occurs on the strongly interacting areas of the graphene layer, confirming previous results obtained by some of us at low temperatures and in ultra-high vacuum conditions.

Water Chemistry beneath Graphene: Condensation of a Dense OH-H2O Phase under Graphene.

Room temperature oxygen hydrogenation below graphene flakes supported by Ir(111) is investigated through a combination of X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations using an evolutionary search algorithm. We demonstrate how the graphene cover and its doping level can be used to trap and characterize dense mixed O–OH–H2O phases that otherwise would not exist. Our study of these graphene-stabilized phases and their response to oxygen or hydrogen exposure reveals that additional oxygen can be dissolved into them at room temperature creating mixed O–OH–H2O phases with an increased areal coverage underneath graphene. In contrast, additional hydrogen exposure converts the mixed O–OH–H2O phases back to pure OH–H2O with a reduced areal coverage underneath graphene.

Numerical Study of Graphene/Au/SiC Waveguide-Based Surface Plasmon Resonance Sensor.

A new waveguide-based surface plasmon resonance (SPR) sensor was proposed and investigated by numerical simulation. The sensor consists of a graphene cover layer, a gold (Au) thin film, and a silicon carbide (SiC) waveguide layer on a silicon dioxide/silicon (SiO2/Si) substrate. The large bandgap energy of SiC allows the sensor to operate in the visible and near-infrared wavelength ranges, which effectively reduces the light absorption in water to improve the sensitivity. The sensor was characterized by comparing the shift of the resonance wavelength peak with change of the refractive index (RI), which mimics the change of analyte concentration in the sensing medium. The study showed that in the RI range of 1.33~1.36, the sensitivity was improved when the graphene layers were increased. With 10 graphene layers, a sensitivity of 2810 nm/RIU (refractive index unit) was achieved, corresponding to a 39.1% improvement in sensitivity compared to the Au/SiC sensor without graphene. These results demonstrate that the graphene/Au/SiC waveguide SPR sensor has a promising use in portable biosensors for chemical and biological sensing applications, such as detection of water contaminations (RI = 1.33~1.34), hepatitis B virus (HBV), and glucose (RI = 1.34~1.35), and plasma and white blood cells (RI = 1.35~1.36) for human health and disease diagnosis.

Observation of a topological defect lattice in the charge density wave of 1T-TaS2

Charge density wave (CDW) phases native to the transition metal dichalcogenide 1T-TaS2 are probed through a graphene layer covering its surface by using local scanning tunneling microscopy/spectroscopy and transport measurements. We show that over a wide range of temperatures, the graphene cover, while protecting the air-sensitive 1T-TaS2 from oxidation, does not obstruct access to the electronic properties of the CDW. At room temperature, where the system is in the nearly commensurate CDW phase, we observe contiguous domains of the CDW phase separated by a network of topological defects. These topological defects, dubbed CDW vortices, form a lattice with a quasi-long range translational order, which resembles the Bragg glass phase in Abrikosov vortex lattices in type-II superconductors. While the quasi-long range order in the CDW itself is not uncommon, its existence in the CDW vortex lattice is observed here for the first time. Finally, when reducing the temperature to 78 K into the commensurate CDW phase, we find that the graphene cover layer acquires properties consistent with a proximity-induced CDW.

Temperature induced dynamics of water confined between graphene and MoS2.

Water trapped between MoS2 and graphene assumes a form of ice composed of two planar hexagonal layers with a non-tetrahedral geometry. Additional water does not wet these ice layers but forms three-dimensional droplets. Here, we have investigated the temperature induced dewetting dynamics of the confined ice and water droplets. The ice crystals gradually decrease in size with increasing substrate temperature and completely vanish at about 80 °C. Further heating to 100 °C induces changes in water droplet density, size, and shape through droplet coalescence and dissolution. However, even prolonged annealing at 100 °C does not completely dry the interface. The dewetting dynamics are controlled by the graphene cover. Thicker graphene flakes allow faster water diffusion as a consequence of the reduction of graphene's conformity along the ice crystal's edges, which leaves enough space for water molecules to diffuse along the ice edges and evaporate to the environment through defects in the graphene cover.

Graphene growth on Ni (1 1 1) by CO exposure at near ambient pressure

We investigated the interaction of CO with a Ni(111) sample partially covered with carbide by Near Ambient Pressure (PCO ~ 2 mbar) X-Ray Photoemission Spectroscopy. Above 500 K we observe the formation of strongly and weakly interacting graphene in comparable amount and CO intercalation underneath the latter moiety. Above 600 K, physisorbed CO2 forms, a process which we ascribe to the onset of the Boudouard reaction under the graphene cover.

Preparation of large area graphene on SiC(0 0 0 −1) by moderate vacuum technology

Owing to its compatibility with existing Si technology, epitaxial graphene on SiC is expected to replace III-V materials in future. This paper proposes an available approach to overcome the low quality and the high cost properties of traditional epitaxial growth access. The epitaxial graphene grows in low vacuum environment, using Ar as the filling gas. The epitaxial graphene was investigated with optical microscope, atomic force microscopy and Raman spectroscopy. It is found that large area graphene cover on SiC C-face conformally. The Raman results show that the materials obtained in lower vacuum environment are ordered and put free standing monolayer graphene.

Integral Equation Analysis of Terahertz Backscattering From Circular Dielectric Rod With Partial Graphene Cover

We study how the backward scattering cross-section (BSCS) of a microsize dielectric rod illuminated by an H-polarized plane wave can be efficiently manipulated using a partial graphene cover and associated plasmon resonances. Our treatment is based on the hyper-singular boundary integral equation, discretized with a Nystrom-type algorithm, which has a mathematically grounded convergence. We compute the BSCS and the extinction cross-section (ECS) in the sub-THz and THz ranges with controlled accuracy and demonstrate that BSCS can be both enhanced and reduced due to the graphene strip.

Self-regenerable carbon nanofiber supported Fe – Mo2C catalyst for CH4-CO2 assisted reforming of biomass to hydrogen rich syngas

In-situ self-regenerable Fe / Fe3C – Mo2C – CNF catalyst was developed and evaluated extensively over 15 cycles of CH4 – CO2 assisted biomass reforming. In-situ regeneration was observed through nucleation on carbon saturated Fe3Cx sites due to abundance of free-C from reaction and undercoordinated carbon in support. Molybdenum existed as stable β-Mo2C whereas Fe composition changed from Fe° sites to Fe3Cx. Due to highly dispersed small nanoclusters (< 5 nm) in lower Fe loading catalysts, graphene nanosheets (GNS) synthesis was observed whereas on 5 % Fe catalysts, large agglomerated nanoparticles (∼10 to 20 nm) facilitated CNF / CNT synthesis. Regeneration on 0.5 % Fe occurred through CNF growth while graphene cover regenerated 5 % Fe catalysts. Fe – Mo2C – CNF catalyzed reactions produce hydrogen rich syngas over 15 cycles with H2 concentration > 60 % and H2:CO ratio of 2–4. Hydrodeoxygenation reforming of biomass was typically between 40–60 % of the amount of oxygenates.

Strong ferromagnetic coupling and tunable easy magnetization directions of FexCo1−x layer(s) intercalated under graphene

We report the magnetic response of FexCo1−x layer(s) intercalated under graphene, compared with the bulk equiatomic FeCo(1 0 0) specimen. Graphene grown on Ir(1 1 1) can act as a protective membrane, allowing confined epitaxial growth of homogeneous FexCo1−x layer(s). Few intercalated FexCo1−x layers exhibit a significant enhancement of the spin (Seff) and orbital (L) moments with respect to both the single elements and the bulk equiatomic FeCo(1 0 0) configuration, as demonstrated by means of magnetic circular dichroism experiments. Reducing the intercalant thickness down to a single layer of FexCo1−x, the easy magnetization direction switches from parallel to perpendicular to the surface plane, with a magnetic response being influenced by the underlying metal. Element-sensitive hysteresis loops disclose identical magnetization response at the Fe and Co sites, thus constituting a single magnetic unit with enhanced ferromagnetic response, with Fe acting as a strong ferromagnet, differently from its bulk bcc configuration. This is a significant step towards the control at the atomic scale of the magnetic response in low dimensional 3d transition metal alloys, with tunable easy-magnetization direction, enhanced spin moments and magnetic anisotropy, in a inert and protected configuration thanks to the 2D graphene cover.

Water-induced hydrogenation of graphene/metal interfaces at room temperature: Insights on water intercalation and identification of sites for water splitting

Though it is well recognized that the space between graphene cover and the metal substrate can act as a two-dimensional (2D) nanoreactor, several issues are still unresolved, including the role of the metal substrate, the mechanisms ruling water intercalation and the identification of sites at which water is decomposed. Here, we solve these issues by means of density functional theory and high-resolution electron energy loss spectroscopy experiments carried out on graphene grown on (111)-oriented Cu foils. Specifically, we observe decomposition of H2O at room temperature with only H atoms forming bonds with graphene and with buried OH groups underneath the graphene cover. Our theoretical model discloses physicochemical mechanisms ruling the migration and decomposition of water on graphene/Cu. We discover that the edge of graphene can be easily saturated by H through decomposition of H2O, which allows H2O to migrate in the subsurface region from the decoupled edge, where H2O decomposes at room temperature. Hydrogen atoms produced by the decomposition of H2O initially form a chemical bond with graphene for the lower energy barrier compared with other routes. These findings are essential to exploit graphene/Cu interfaces in catalysis and in energy-related applications.

New method allows for improved ion images of neurons and skin cells

Using a graphene cover and air plasma treatment, researchers greatly improved ion images in time-of-flight secondary ion mass spectrometry.

Oxygen Adsorption on Graphene-Encapsulated Palladium Nanoparticles Imaged by Kelvin Probe Force Microscopy

Graphene-encapsulated metal nanoparticles (G@NPs) offer a possibility to observe confined reactions in the nanocontainer formed by the NP's facets and graphene. However, direct experimental detection of adsorbed atomic and molecular species under the graphene cover is still challenging, and the mechanisms of intercalation and adsorption are not well understood. Here, we show that Kelvin probe force microscopy can largely contribute to the understanding of adsorption and desorption at the single NP level, which we exemplify by comparing oxygen adsorption experiments obtained at as-prepared PdNPs and G@PdNPs, both supported on highly oriented pyrolytic graphite and studied under ultrahigh vacuum (UHV) conditions. We show that oxygen adsorption at room temperature occurs at a much higher partial oxygen pressure on G@PdNPs compared to as-prepared PdNPs. Similarly, the removal of oxygen via a reaction with the residual gas of the UHV is slower on the G@PdNPs compared to as-prepared PdNPs. The differences can be explained by a limited facility for reactant and product molecules to enter and desorb from the nanocontainer via the defects of the graphene. Experimental observations are supported by assisting density functional theory calculations.