Graphene Nanoflakes Research Articles

Graphene-Coated PVDF/PAni Fiber Mats and Their Applications in Sensing and Nanogeneration.

Forcespinning is a powerful technique to produce fiber systems with suitable properties for a vast array of applications. This study investigates the sensing and energy generation performance of PVDF/PAni fiber mat systems made by the forcespinning method with and without graphene coating. The developed fiber mats were coated with graphene nanoflakes by drop-casting. The graphene-coated nanocomposites show an average output voltage of 75 mV (peak-to-peak), which is 300% higher compared to bare fiber mats, and an output current of 24 mA (peak-to-peak) by gentle finger pressing. Moreover, graphene-coated PVDF/PAni showed a volume conductivity of 1.2 × 10-7 S/cm and was investigated as a promising system for temperature (5 times better sensitivity than normal fiber mat), vibration (2 times better voltage generation), and airflow sensing. The graphene-coated composite has been further investigated as a water tide energy harvesting piezoelectric nanogenerator, with the system generating ∼40 mV for a synthetic ocean wave with a flow rate of 30 mL/min. In the future, graphene-coated nanofiber mats can be a solution for low-powered sensors and to harvest blue energy and vibration energy.

Conformational change-modulated spin transport at single-molecule level in carbon systems

Controlling the spin transport at the single-molecule level, especially without the use of ferromagnetic contacts, becomes a focus of research in spintronics. Inspired by the progress on atomic-level molecular synthesis, through first-principles calculations, we investigate the spin-dependent electronic transport of graphene nanoflakes with side-bonded functional groups, contacted by atomic carbon chain electrodes. It is found that, by rotating the functional group, the spin polarization of the transmission at the Fermi level could be switched between completely polarized and unpolarized states. Moreover, the transition between spin-up and spin-down polarized states can also be achieved, operating as a dual-spin filter. Further analysis shows that, it is the spin-dependent shift of density of states, caused by the rotation, that triggers the shift of transmission peaks, and then results in the variation of spin polarization. Such a feature is found to be robust to the length of the nanoflake and the electrode material, showing great application potential. Those findings may throw light on the development of spintronic devices.

Effect of the Surface Functionalization of Graphene and MWCNT on the Thermodynamic, Mechanical and Electrical Properties of the Graphene/MWCNT-PVDF Nanocomposites.

The nanocomposites of poly(vinylidene fluoride) (PVDF) with pristine graphene nanoflakes (GNF) and a multi-wall carbon nanotube (MWCNT) were prepared by the solution casting method. Additionally, the GNF and MWCNT were functionalized by acid treatment, and nanocomposites of the acid-treated MWCNT/GNF and PVDF were prepared in the same method. The effect of the acid treatment of MWCNT and GNF on the mechanical, thermal and thermo-oxidative stability and the thermal conductivity of the MWCNT/GNF-PVDF nanocomposites was evaluated, and the results were compared with the untreated MWCNT/GNF-PVDF nanocomposites. In both cases, the amount of GNF and MWCNT was varied to observe and compare their thermal and mechanical properties. The functionalization of the GNF or MWCNT resulted in the change in the crystallization and melting behavior of the nanocomposites, as confirmed by the differential scanning calorimetry analysis. The addition of the functionalized GNF/MWCNT led to the improved thermal stability of the PVDF nanocomposites compared to that of the non-functionalized GNF/MWCNT-PVDF nanocomposites. The thermal and electrical conductivity of the functionalized and non-functionalized GNF/MWCNT-PVDF composites were also measured and compared. The functional groups, crystal structure, microstructure and morphology of the nanocomposites were characterized by Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively.

Facile Transformation of Waste Biomass into Nitrogen-Doped Graphene for Highly Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Reaction

Key words: Metal-free catalyst, Oxygen reduction reactions, Pyridinic nitrogen, Stability It is of significant interest to produce metal-free electrocatalysts for oxygen reduction reactions (ORR) derived from low-cost and environmentally friendly biomass. This work presents a facile thermal annealing approach for large-scale synthesis of the nitrogen-doped graphene (N-GO) nanoflakes using melamine as the nitrogen source and chestnut shells-based biomass as the carbon source. This approach allows us to achieve a high nitrogen doping level in graphene. Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Fourier Transform Infrared spectroscopy (FTIR) were used to characterize the N-GO. XPS analysis revealed the presence of pyridinic, graphitic, and pyrrolic nitrogen doping in graphene sheets, with pyridinic nitrogen being the most abundant. Electrochemical investigations showed that a higher concentration of pyridinic nitrogen configurations contribute significantly to the superior electrocatalytic activity toward oxygen reduction reactions (ORR) in alkaline electrolytes. The as-synthesized N-GO exhibits high ORR activity with the onset and half-wave potential of 0.90 V and 0.79 V in alkaline medium, respectively; which is among the best reported to the date for metal-free catalysts. Moreover, compared with the commercial Pt/C, optimized N-GO electrocatalyst exhibits more positive onset potential and better long-term stability, making it a promising metal-free catalyst for practical fuel cells. Acknowledgements: The authors are thankful to Campus France and the National Research Foundation of Korea for financing the PHC Star mobility project (45823YC). Figure 1

Effect of Fabrication Method on the Thermo Mechanical and Electrical Properties of Graphene Doped PVDF Nanocomposites.

Nanocomposites of poly (vinylidene fluoride) PVDF with graphene nanoflakes (GNF) were prepared using two different routes. Initially, a mix-melting method was used to prepare composites, and their thermal and mechanical properties were evaluated to choose the better method for future experiment and properties investigation. Then, nanocomposite films were prepared by a simple solution-casting technique using a PVDF/graphene solution. In both cases, the amount of graphene was varied to observe and to compare their thermal and mechanical properties. The addition of graphene to the PVDF matrix resulted in changes in the crystallization and melting behaviors as confirmed by DSC analyses. Increasing the graphene content led to improved thermal stability of the PVDF nanocomposites prepared using both methods. Improvements in mechanical properties by the addition of graphene were also observed. Better performance was observed by the nanocomposites prepared by a mix-melting technique suggesting better dispersion and strong interface bonding between PVDF and graphene particles. Thermal and electrical conductivity were measured and compared. Microstructure and morphology were characterized using FTIR, XRD, and SEM analyses.

Hydrogen Storages Based on Graphene Nano-Flakes: Density Functional Theory Approach

Carbon materials such as graphene, carbon nanotubes, fullerene, and graphene nanoflakes (GNFs) are used for hydrogen storage. The doping of alkali metals to these materials generally increases the accumulation density of molecular hydrogen (H2). However, the reason why the doping enhances the ability of the H2 storage of GNF is not clearly known, although there are some explanations. In addition, the information on the storage capacity of GNF is ambiguous. In the present review article, we introduce our recent theoretical studies on the interaction of GNF with H2 molecules carried out to elucidate the mechanism of hydrogen storage in alkali-doped GNFs. As alkali metals, lithium (Li), sodium (Na), and potassium (K) were examined, and the abilities of hydrogen storage were discussed. Next, the mechanism of Li-diffusion on GNF, which plays a crucial role in Li-battery, was presented. There are several unanswered questions. In particular, does lithium diffuse randomly on GNF? Or is there a specific diffusion path? We present our study, which elucidates the factors governing lithium diffusion on GNF. If the dominant factor is known, it is possible to arbitrarily control the diffusion path of lithium. This will lead to the development of highly functional battery materials. Finally, the molecular design of H adsorption–desorption reversible storage devices based on GNF will be introduced. Elucidating the mechanism of hydrogen storage, Li-diffusion on GNF, and molecular design of storage device is important in understanding the current molecular devices and provide a deeper insight into materials chemistry.

Edge‐Rich Graphene Nanospheres with Ultra‐High Nitrogen Loading Metal‐Free Electrocatalysts for Boosted Oxygen Reduction

Nitrogen (N) doping in graphene-based materials has been demonstrated as an effective strategy in constructing active sites of metal-free catalysts for oxygen reduction reaction (ORR). The practical applications of metal-free electrocatalysts in metal-air batteries or fuel cells, nevertheless, have been hampered by their unsatisfactory catalytic performance due to insufficient catalytic active sites. In this work, a novel N-rich graphene nanospheres (NGNs) have been achieved by adopting an edge engineering strategy through annealing the mixture of graphitic carbon nitride (g-C3N4) and edge-rich graphene nanospheres (GNs) composed of graphene nanoflakes. Benefiting from the exposure of edge defects in the GNs, the N loading surprisingly achieved as high as 14.01 at%. The half-wave potential and limiting current density of the synthesized NGNs-900 catalyst can reach 0.872 V and 4.25 mA cm−2, respectively, which are superior to that of the commercial Pt/C. Based on the experimental and theoretical results, the synergistic effect of graphitic-N and pyridinic-N in NGNs catalysts has been distinguished as the origin of the boosted ORR performance. This work proposes a facile synthesis strategy to optimize the N-doped carbon-based catalysts for ORR, which have great potential to replace noble-metal catalysts.

Edge magnetization and thermally induced spin current in nanostructured graphene

In this work, the magnetic states and thermally induced spin currents in graphene nanoflake sizes with different sizes and shapes have been investigated using Hubbard model combined with non-equilibrium Green’s function method. In addition to the antiferromagnetic (AFM) state governed by the sizes, shapes, armchair bond densities, and Coulomb energy, our calculations have also pointed out the emergence of ferromagnetic (FM) and complex magnetic states when the gate voltage is invoked in the graphene nanoflakes. More prominently, by exploiting the geometric symmetry of the nanoflakes without external fields, a pure spin current and zero charge current are generated in spin caloritronic device when the graphene nanoflakes are both in the AFM and FM states. The formation of pure spin currents driven by temperature difference depends on the graphene nanoflakes’ size, shape, temperature and gate voltage as well. The study also shows the outstanding advantages of diamond-shaped graphene nanoflakes in both magnetic properties and spin currents. This result paves the way for the possibility of practical applications of graphene materials in spintronics and spin caloritronics.

A theoretical simulation of the magnetic properties of nanocomposites based on graphene nanoflakes

AbstractUsing exact diagonalization, the density matrix renormalization group approach and the quantum Monte‐Carlo method, we studied the peculiarities of the magnetization profiles and the possibility of spin switching for the Heisenberg spin models of low‐dimensional composite magnets formed by graphene nanoclusters and complex transition‐metal compounds. For these models, we found the presence of intermediate plateaus in the low‐temperature magnetization profiles, which is in agreement with known experimental and theoretical studies of the magnetization profiles of nanostructured magnets with the ferrimagnetic spin ordering in the ground state. For spin‐chain model of composite magnets proposed, we also found a possibility of single and double spin switching with the macroscopic jump of the ground state spin results from frustrated spin interactions.

Revising quantum optical phenomena in adatoms coupled to graphene nanoantennas.

Graphene flakes acting as photonic nanoantennas may sustain strong electromagnetic field localization and enhancement. To exploit the field enhancement, quantum emitters such as atoms or molecules should be positioned in such close proximity to the flake that electron tunneling might influence the optical and electronic properties of the system. However, tunneling is usually not considered if the optical coupling mechanism between quantum emitters and nanoantennas is at focus. This work presents a framework for describing the electron dynamics in hybrid systems consisting of graphene nanoflakes coupled both electronically and optically to adatoms and subject to external illumination. Our framework combines the single-particle tight-binding approach with a nonlinear master equation formalism that captures both optical and electronic interactions. We apply the framework to demonstrate the impact of electron tunneling between the adatom and the flake on emblematic quantum optical phenomena: degradation of coherent Rabi oscillations and quenching of Purcell spontaneous emission enhancement in two-level adatoms in proximity of triangular graphene nanoflakes.

Quantum Revivals in Curved Graphene Nanoflakes.

Graphene nanostructures have attracted a lot of attention in recent years due to their unconventional properties. We have employed Density Functional Theory to study the mechanical and electronic properties of curved graphene nanoflakes. We explore hexagonal flakes relaxed with different boundary conditions: (i) all atoms on a perfect spherical sector, (ii) only border atoms forced to be on the spherical sector, and (iii) only vertex atoms forced to be on the spherical sector. For each case, we have analysed the behaviour of curvature energy and of quantum regeneration times (classical and revival) as the spherical sector radius changes. Revival time presents in one case a divergence usually associated with a phase transition, probably caused by the pseudomagnetic field created by the curvature. This could be the first case of a phase transition in graphene nanostructures without the presence of external electric or magnetic fields.

Graphene nanoflakes and fullerenes doped with aluminum: features of Al-C interaction and adsorption characteristics of carbon shell

The aluminium-carbon interaction in two core-shell systems (Al22 nanoparticle coated with graphene nanoflake and Al@C60 metallofullerene) is investigated within the density functional theory. A set of non-equilibrium configurations of the coated Al22 nanoparticle is obtained from the ab initio molecular dynamics simulation. The Morse parameters describing the Al-C interaction are fitted based on density functional calculations performed at the B3LYP / 6-31G* level of theory. The Grimme’s D3 dispersion corrections are added to accurately account for the non-covalent interactions. It is shown that the concave carbon surface interacts much weaker with the nanoparticle and is located further away from it compared to the usually considered convex surfaces. Negligible charge transfer from aluminum core to carbon shell confirms that Al22 nanoparticle do not change the shell reactivity. In contrast, a single Al atom endohedrally doped C60 fullerene strongly interacts with the carbon cage and distorts the frontier molecular orbitals and reactivity of the compound. We also compare the hydrogen F..H-O bonds formed between the fluorinated C60F2 or Al@C60F2 cages and niacin drug molecule. We found that Al leads to drastic weakening of this hydrogen bond. We conclude that Al22 nanoparticle do not change reactivity of the carbon shell, whereas a single Al atom reduces reactivity of the outer fullerene.

Effect of carbonaceous reinforcements on anticorrosive and magnetic properties of Ni-Cu based composite coatings prepared by pulsed electrodeposition

Magnetic and anticorrosive properties of Ni-Cu alloy-based composite coatings prepared by pulsed electrodeposition reinforced with CNT (Ni-Cu-CNT), graphene (Ni-Cu-G), and diamond (Ni-Cu-D) have been assessed in the current work. The corrosion behavior of the composite coatings was evaluated in a 3.5% NaCl corrosive medium. Linear polarization test performed within ±10 mV with respect to open circuit potential (OCP) unveiled that graphene added coatings exhibited maximum polarization resistance (~13.1 kΩcm2) followed by the Ni-Cu-D (~9.1 kΩcm2), the Ni-Cu-CNT (~8.3 kΩcm2), and the Ni-Cu with a minimum resistance of ~4.2 kΩcm2. Electrochemical impedance spectroscopy tests performed with a signal perturbation of ±10 mV with respect to OCP and frequency sweep from 105 Hz to 10−2 Hz elicited maximum film resistance of ~32 kΩ cm2 for the Ni-Cu-G coating, which could be attributed to the high area to volume ratio, α ~134/m, of graphene nanoflakes inducing strong particle-matrix interaction and to the formation of micro galvanic cell between the graphene particles and the Ni-Cu matrix where the surface was covered predominantly with graphene, eventually lowering the corrosion rate to 5.91 MPY. For Ni-Cu-D coating, the uniform dispersion of diamond particles prohibits localized corrosion. In contrast, agglomeration of CNTs in Ni-Cu-CNT causes ~50% reduction in corrosion resistance compared to diamond reinforced coating. However, corrosion resistance of CNT grafted coating was around 2.5 times higher than Ni-Cu (~4.2 kΩcm2), which can be imputed to the filling of the pores, cracks, and crevices by CNT fibers. The hysteresis loops of Ni-Cu composite coatings traced by a vibrating sample magnetometer indicated that CNT incorporation enhanced the magnetic saturation to ~1.89 Am2/kg from ~0.84 Am2/kg (Ni-Cu) and maximum coercivity of ~110 Oe (~78.91 Oe for Ni-Cu). However, graphene and diamond addition has negligible effect on the magnetic properties of the coated systems. The enhanced anticorrosive and magnetic properties of proposed coating designs consisting of Ni-Cu matrix with carbonaceous reinforcements impart multifunctionality to the system. The marine industry can successfully adopt the proposed coating systems to protect propellers shafts, ship hulls, and specialty sensors.

A C 54 B 2 Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity

A C ₅₄ B ₂ Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity

Reactions of Graphene Nano-Flakes in Materials Chemistry and Astrophysics

The elucidation of the mechanism of the chemical evolution of the universe is one of the most important themes in astrophysics. Polycyclic aromatic hydrocarbons (PAHs) provide a two-dimensional reaction field in a three-dimensional interstellar space. Additionally, PAHs play an important role as a model of graphene nanoflake (GNF) in materials chemistry. In the present review, we introduce our recent theoretical studies on the reactions of PAH and GNF with several molecules (or radicals). Furthermore, a hydrogen storage mechanism for alkali-doped GNFs and the molecular design of a reversible hydrogen storage device based on GNF will be introduced. Elucidating these reactions is important in understanding the chemical evolution of the universe and gives deeper insight into materials chemistry.

Near Room-Temperature Synthesis of Vertical Graphene Nanowalls on Dielectrics.

Vertical graphene nanowalls (VGNs) with excellent heat-transfer properties are promising to be applied in the thermal management of electronic devices. However, high growth temperature makes VGNs unable to be directly prepared on semiconductors and polymers, which limits the practical application of VGNs. In this work, the near room-temperature growth of VGNs was realized by utilizing the hot filament chemical vapor deposition method. Catalytic tantalum (Ta) filaments promote the decomposition of acetylene at ∼1600 °C. Density functional theory calculations proved that C2H* was the main active carbon cluster during VGN growth. The restricted diffusion of C2H* clusters induced the vertical growth of graphene nanoflakes on various substrates below 150 °C. The direct growth of VGNs successfully realized the excellent interfacial contact, and the thermal contact resistance could reach 3.39 × 10-9 m2·K·W-1. The temperature of electronic chips had a 6.7 °C reduction by utilizing directly prepared VGNs instead of thermal conductive tape as thermal-interface materials, indicating the great potential of VGNs to be directly prepared on electronic devices for thermal management.

Spatial Dissipative Solitons in Graphene-Based Active Random Metamaterials

We investigate dissipative nonlinear dynamics in graphene-based active metamaterials composed of randomly dispersed graphene nanoflakes embedded within an externally pumped gain medium. We observe that graphene saturable nonlinearity produces a subcritical bifurcation of nonlinear modes, enabling self-organization of the emitted radiation into several dissipative soliton structures with distinct topological charges. We systematically investigate the existence domains of such nonlinear waves and their spatial dynamics, finding that soliton vortices are unstable, thus enabling self-organization into single dissipative structures with vanishing topological charge, independently of the shape of the graphene nanoflakes. Our results shed light on self-organization of coherent radiation structures in disordered systems and are relevant for future cavity-free lasers and amplifier designs.

Boronic Acid Decorated Graphene Nano Flakes for Glucose Sensing in Diabetes: A DFT Prediction

The requirement of high-sensitivity, accurate and rapid determination of glucose in human blood is major challenge from clinical perspective. In this work, boronic acids (BAs) and derivatives were successfully functionalized over graphene nano flakes (GNF) surface as a sensing platform to spot Glucose molecule. The density functional theory-based investigation has been carried out by considering geometrical optimization, density of states, and molecular energy spectrum. Further, molecular properties like molecular orbital investigations, energy gap, chemical hardness, chemical softness, have been evaluated in order to distinguish the rate of reaction and binding energy between GNF-BA and Glucose. Results show that BAs derivatives were physisorbed/chemisorbed over GNF surface. Moreover, the analyte formed the covalent interactions with its diol group towards the trigonal structured B atom of BAs and provided the sensing signal by modifying the DOS peaks of sensing platform. Eventually, the sensitivity of different BA’s derivatives @GNF to Glucose sensing were compared. These findings show the variation at molecular and atomic level behaviour of GNF-BA sensing platform in presence/absences of Glucose. Therefore, our DFT predictions have application in designing sensor in area of biomolecules and bioelectronics for healthcare sector.

Effect of Graphene Addition on the Thermal and Persistent Luminescence Properties of Gd2.994Ce0.006Ga3Al2O12 and Gd2.964Ce0.006Dy0.03Ga3Al2O12 Ceramics.

The gadolinium, gallium, aluminum garnet doped with cerium and co-doped with dysprosium ions were prepared using sol gel method. The SEM images show that after synthesis, the grains are below 100 nm. The powders were ultrasonically mixed with graphene nanoflakes and ceramics were prepared using the high pressure low temperature sintering technique. A series of the ceramics was prepared using different graphene content. The structure of the samples was examined using X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman techniques. The spectroscopic properties were checked using conventional and persistent luminescence spectra measurements. The thermoluminescence glow curves and fading time of persistent luminescence measurements were performed to check how the graphene presence affects the electron traps number and depth. It was found that the addition of graphene improved the thermal conductivity of co-doped samples. This resulted in faster release of deeper traps and an increase in fading of persistent luminescence. The possibility of releasing energy from deep traps without additional stimulation may allow the use in different applications, the matrices and luminescent ions, which so far did not show persistent luminescence at room temperature.

Silicon-doped graphene nanoflakes with tunable structure: Flexible pyrolytic synthesis and application for lithium-ion batteries

Incorporation of heteroatoms into graphene layers is a powerful way of tuning their surface and structure. We developed for the first time a tunable approach for the synthesis of Si-doped jellyfish-like graphene nanoflakes (Si-GNFs) via template pyrolysis. The synthesis parameters were shown to affect the doping level, silicon bonding type, heteroatom localization in Si-GNFs, and the defectiveness of the synthesized material. The silicon incorporation into GNF layers creates acidic and basic Lewis sites as it was tested in the butanol-2 conversion over Si-GNF. Doping of GNFs with Si significantly boosted the specific capacity of GNF-based electrode of the lithium-ion battery up to 600 mAh*g−1. The capacity enhancement was ascribed to the redistribution of the electron density due to the difference in electronegativity between C and Si, corrugation of the graphene layers by heteroatoms, and reduction of silicon species. The synthesis conditions were shown to strongly affect the capacity and cycle stability of the electrode.