Pure Graphene Research Articles

The effect of cesium dopant on APCVD graphene coating on copper

This study reports in-situ cesium-doped graphene (G/Cs) coating obtained by introducing Cs2CO3 into the atmospheric pressure chemical vapor deposition (APCVD) furnace during graphene deposition on copper. The successful Cs-doping of the graphene coating was confirmed via X-ray photoelectron spectroscopy (XPS). As compared to the spectra of pure graphene coating, the XPS spectra of the G/Cs coating revealed a shift of the C1s and Cs3d5/2 peaks to higher and lower binding energies, respectively; thus, implying the n-type character of the doping and indicating a charge transfer between Cs and graphene. Raman results show that a pure graphene coating is composed of fewer layers, fewer defects, and larger domain size than the G/Cs coating. Ultraviolet photoelectron spectroscopy was utilized to study the work function of graphene and the G/Cs and revealed that doping graphene with Cs dopants reduced the work function of graphene by 1.2 eV. Electrochemical testing during 15-day immersion in 0.1 M NaCl indicated the destructive effect of the G/Cs coating on the Cu substrate. The results showed that the G/Cs coating exhibits a higher corrosion rate and lower corrosion resistance than even the bare metal itself.

Graphene/ β‐ MnO 2 composites—synthesis and its electroanalytical properties study in the Mg storage battery

Graphene/β-MnO2 composites were synthesized via ultrasonication method, followed by calcination at 400°C for 4 hours. The structural properties of synthesized graphene and graphene/β-MnO2 composites were systematically studied using different instruments, such as XRD, FESEM, TEM, EDX, XPS and FT-IR spectroscopy. Furthermore, magnesium storage performance was scientifically studied using various electroanalytical characterizations like galvanostatic charge/discharge, electrochemical impedance spectra and cyclic voltammetry. The GNS/β-MnO2-20% composite displays the discharge capacity of 110 mAh/g within 60 cycles at a current density of 25 mA/g, which is an excellent rate capability, and magnesium storage performance around 100 cycles. There is a significant improvement in electrochemical properties of GNS/β-MnO2-20% composite materials as cathode for magnesium storage system due to the synergetic interaction between a pure β-MnO2 particle and graphene sheet.

An extensive case study on the dispersion parameters of HI-assisted reduced graphene oxide and its graphene oxide precursor

The formation of highly concentrated and stable graphene derivatives dispersions remains a challenge towards their exploitation in various applications, including flexible optoelectronics, photovoltaics, 3D-printing, and biomedicine. Here, we demonstrate our extensive investigation on the dispersibility of graphene oxide (GO) and reduced graphene oxide (RGO) in 25 different solvents, without the use of any surfactant or stabilizer. Although there is a significant amount of work covering the general field, this is the first report on the dispersibility of: a) RGO prepared by a HI/AcOH assisted reduction process, the method which yields RGO of higher graphitization degree than the other well-known reductants met in the literature, b) both GO and RGO, explored in such a great range of solvents, with some of them not previously reported. In addition, through calculation of their Hansen Solubility Parameters (HSP), we confirmed their dispersibility behavior in each solvent, while we indirectly validated the most advanced graphitization degree of the studied RGO compared to other reported RGOs, since its HSPs exhibit the highest similarity with the respective ones of pure graphene. Finally, high concentrations of up to 189 μg mL−1 for GO and ~ 87.5 μg mL−1 for RGO were achieved, in deionized water and o-Dichlorobenzene respectively, followed by flakes size distribution and polydispersity indices estimation, through dynamic light scattering as a quality control of the effect of a solvent’s nature on the dispersion behavior of these graphene-based materials.

Interconnected Graphene Hollow Shells for High-Performance Capacitive Deionization.

Electrochemical capacitive deionization (CDI) is a promising technology for distributed and energy-efficient water desalination. The development of high-performance capacitive electrodes is critical for enhancing CDI properties and scaling up its applications. Herein, a three-dimensional graphene porous architecture with high CDI performance is successfully constructed by assembling intentionally designed incomplete graphene-based spherical hollow shells. Small graphene oxide (GO) sheets are purposely adopted to prepare sphere shells by wrapping the surface of polystyrene sphere templates. Because the small-sized GO sheets cannot enwrap the spherical templates seamlessly, a unique graphene hollow shell structure with integrally interconnected feature forms upon removal of the templates. Compared to control samples with typical isolated pore structure (3DGA-C) prepared with commonly used large-sized GO sheets, such open and interconnected porous architectures (3DGA-OP) greatly increase their accessibility of specific surface area and pore volume, enabling superior electrochemical performance. The optimized CDI capacities of 3DGA-OP electrodes reach up to 14.4 mg·g-1 in NaCl aqueous of 500 mg·L-1 at 1.2 V, which is about 2 times the 3DGA-C ones (6.7 mg·g-1) and exceeds the CDI values of most reported pure graphene electrodes under the same experimental conditions. This strategy of improving the open interconnectivity between pores illuminates new avenues for developing high performance CDI porous electrodes assembled from two-dimensional materials.

A novel graphene oxide-based ceramic composite as an efficient electrode for capacitive deionization

In this study, N-doped and TiO2-decorated graphene oxides were developed as efficient nonprecious electrodes for capacitive deionization. The activity of this new material was evaluated in situ and in vivo. The performance of the synthesized material was measured in different saline solutions (0.1, 0.5 and 1.0 M NaCl) as an electrolyte. The results showed that the new material exhibits very good performance (157 F/g at 5 mV/s and 1.0 M NaCl compared to 19.5 F/g for pure graphene oxide). In the desalination test, which was performed in batch mode, the salt adsorption capacity and the efficiency of salt removal were 9.2 mg/g and 98%, respectively. To check the stability, the desalination test was repeated several times, and no change in the performance was observed. The results provide evidence that the newly synthesized material is a potential electrode material for CDI water desalination with satisfactory salt removal ability.

First-Principles Study of Molecular Adsorption of Hydrogen/s on Co-Adatom Graphene

The study of graphene and its allotropes help to understand fundamental science and their role in the industry. The adsorption of transition metal adatom on mono-layer graphene can tune the geometrical, electronic, and magnetic properties of the material according to the requirement for the practical applications. In the present work, the geometrical stability, electronic and magnetic properties, and also the redistribution of electronic charge of single cobalt atom (Co) adsorbed graphene with reference to pure graphene have been investigated to develop a model system for the effective storage of hydrogen. The density functional theory (DFT) based first-principles calculations by incorporating van der Waals (VDW) interactions within DFT-D2 levels of approximation implemented in the quantum ESPRESSO package was used. The band structure and density of states of cobalt-adatom graphene show that the material is metallic and magnetic with a total magnetic moment of 1.55 μB. The change in the electronic distribution of Co-adatom graphene has been found favorable for adsorbing molecular hydrogen/s with greater strength. The increasing number of adsorbed molecular hydrogen/s (n=1 to 7) onto the substrate shows varying binding energy per hydrogen molecule, high enough at low concentration (n=1, 2, and 3), and then decreases slowly on increasing the value of n. The nature of adsorption and binding energy per hydrogen molecule (with a range of 0.116 - 0.731 eV/ H2) are found useful to meet a standard target for hydrogen storage in such materials.

Nitrogen-Doped Graphene via In-situ Alternating Voltage Electrochemical Exfoliation for Supercapacitor Application.

Doping heteroatom, an effective way to enhance the electrochemical performances of graphene, has received wide attention, especially related to nitrogen. Alternating voltage electrochemical exfoliation, as a low cost and green electrochemical approach, has been developed to construct in-situ N-doped graphene (N-Gh) material. The N-Gh presents a much higher capacity than that of pure graphene prepared via the same method, which might be attributed to the introduction of nitrogen, which has much more effects and a disordered structure. As-prepared N-Gh exhibits a low O/C ratio that is helpful in maintaining high electrical conductivity. And the effects and disorder structure are also conductive to reduce the overlaps of graphene layers. A symmetric supercapacitor assembled with N-Gh electrodes displays a satisfactory rate behavior and long cycling stability (92.3% retention after 5,000 cycles).

Solvent-Solvent Correlations across Graphene: The Effect of Image Charges.

Wetting experiments show pure graphene to be weakly hydrophilic, but its contact angle (CA) also reflects the character of the supporting material. Measurements and molecular dynamics simulations on suspended and supported graphene often reveal a CA reduction due to the presence of the supporting substrate. A similar reduction is consistently observed when graphene is wetted from both sides. The effect has been attributed to transparency to molecular interactions across the graphene sheet; however, the possibility of substrate-induced graphene polarization has also been considered. Computer simulations of CA on graphene have so far been determined by ignoring the material's conducting properties. We improve the graphene model by incorporating its conductivity according to the constant applied potential molecular dynamics. Using this method, we compare the wettabilities of suspended graphene and graphene supported by water by measuring the CA of cylindrical water drops on the sheets. The inclusion of graphene conductivity and concomitant polarization effects leads to a lower CA on suspended graphene, but the CA reduction is significantly bigger when the sheets are also wetted from the opposite side. The stronger adhesion is accompanied by a profound change in the correlations among water molecules across the sheet. While partial charges on water molecules interacting across an insulator sheet attract charges of the opposite sign, apparent attraction among like charges is manifested across the conducting graphene. The change is associated with graphene polarization, as the image charges inside the conductor attract equally signed partial charges of water molecules on both sides of the sheet. Additionally, using a nonpolar liquid (diiodomethane), we affirm a detectable wetting translucency when liquid-liquid forces are dominated by dispersive interactions. Our findings are important for predictive modeling toward a variety of applications including sensors, fuel cell membranes, water filtration, and graphene-based electrode materials in high-performance supercapacitors.

Molecular and dissociative adsorption of tetrachlorodibenzodioxin on M-doped graphenes (M = B, Al, N, P): pure DFT and DFT + VdW calculations.

Tetrachlorodibenzodioxin (TCDD) is one of the most famous dioxin families that is hazardous to humans and the environment. Designing cheap and novel catalysts for its detecting and removing is an essential need for the environment. In this work, DFT + VdW is used to investigate the potentiality of proposed catalysts in adsorbing and dissociating TCDD. P-type and N-type charge carrier effects on the adsorption process are modeled by doping of B/Al and N/P atoms in the graphene. Al-doped graphene, with - 1.27eV adsorption energy, has the highest possibility to adsorb TCDD. P-type dopants have higher interactions with TCDD in comparing with N-type dopants. Introducing positive and negative charges on the studied complexes shows that in all complexes, the driving force of complexation is π-π stacking except for the Al-doped graphene. Dissociative adsorption studies show that unlike literature data, chlorine atoms on the surface of studied catalysts are not dissociated from TCDD, and instead, C-O bonds in TCDD are dissociated symmetrically and asymmetrically. Data show that Al-doped graphene is the best catalyst for symmetrical dissociation, and pure graphene is the best one for asymmetrical dissociation of TCDD.

Molecular dynamics simulations on chitosan/graphene nanocomposites as anticancer drug delivery using systems

Several chitosan (CS) drug delivery systems (DDSs) were designed by molecular dynamics (MD) simulations. The systems were composed of pure graphene (G), P-doped (GP) or N-doped (GN) graphene nanosheets for the anticancer drug cyclophosphamide (CP) with the aim of discovering the most appropriate drug carrier. Furthermore, the temperature influence was investigated on the systems characteristics through performing the simulations at four different temperatures including 25, 35, 45 and 55 °C. The diffusion coefficients and mean square displacements (MSDs) were boosted with the temperature increase. At 55 °C, the CS-G-CP exhibited the greatest diffusion coefficient (0.0658 × 10–5 cm2/s) but the CS-GN-CP had the smallest diffusion coefficient of 0.0553 × 10–5 cm2/s and the CS-GP-CP illustrated a medium amount (0.0595 × 10–5 cm2/s) indicating the most controlled/sustained drug diffusion could occur in the CS-GN-CP which could allow the most efficient drug delivery. Moreover, to evaluate the effects of PEG chains and VC molecule on the drug delivery capacity of the CS-GN-CP cell, they were added to the this system and the CS-GN-CP-PEG2-VC was nominated as the best DDS working at 35 °C (close to the human body temperature) due to it had a relatively high drug loading capacity as well as suitable CP diffusion coefficient and FV, FFV values.

Hydrodynamic slip can align thin nanoplatelets in shear flow

The large-scale processing of nanomaterials such as graphene and MoS2 relies on understanding the flow behaviour of nanometrically-thin platelets suspended in liquids. Here we show, by combining non-equilibrium molecular dynamics and continuum simulations, that rigid nanoplatelets can attain a stable orientation for sufficiently strong flows. Such a stable orientation is in contradiction with the rotational motion predicted by classical colloidal hydrodynamics. This surprising effect is due to hydrodynamic slip at the liquid-solid interface and occurs when the slip length is larger than the platelet thickness; a slip length of a few nanometers may be sufficient to observe alignment. The predictions we developed by examining pure and surface-modified graphene is applicable to different solvent/2D material combinations. The emergence of a fixed orientation in a direction nearly parallel to the flow implies a slip-dependent change in several macroscopic transport properties, with potential impact on applications ranging from functional inks to nanocomposites.

Ionic Liquid Concentration Dependent Hysteresis in Suspended Bilayer Graphene Transistor

Traditionally, graphene-based field effect transistors comprise graphene flakes lying on SiO2 substrate. On the other hand, suspended graphene structure can help investigate the intrinsic properties of graphene and even show improved sample quality. There have been several publications relating to the researches about transport hysteresis in graphene devices [1-3], which are important for the development of graphene-based sensor. However, there are seldom paper talking about graphene with one-side liquid structure, which means the effects of the substrate can be eliminated, leaving the structure of pure graphene and liquid. Therefore, we can further explore the intrinsic characteristics of graphene, such as hydrophilic or hydrophobic surface properties.In this work, we used the device structure as shown in Figure 1 to present an investigation compared with the previous device. The device of Figure 1 contains two gold contacts for drain/source electrodes, and the etched area which includes two larger regions for liquid injection and the main trench with some pillar for suspended graphene. We began by measuring the transport hysteresis in the device without trench, and then do the same measurement but using the device for suspended graphene. The solution gate voltage is swept from -1V to 1V in order to avoid any reaction, and both devices are in ambient environment.Figure 2 shows the different hysteresis directions observed in this work, which means that there are some differences between the carrier transport mechanism of these two devices. As shown in Figure 3, the magnitude of the hysteresis in gate voltage is quite different in the two devices, which it’s larger in suspended-graphene device than in normal device. Besides, the drain currents in two devices also show a large difference, which may result from the fabrication deviation, such as the contact resistance or the sheet resistance of bilayer graphene. On the other hand, the difference between the Dirac point (VDirac) in the potassium chloride (KCl) solution of various concentration shows an almost linear dependent relationship if we plot the concentrations on the logarithmic scale, as shown in Figure 4.It is believed that the more charge trapping will cause larger hysteresis; however, in this work we found that the magnitude of hysteresis on suspended graphene device is anomalously larger than on-substrate graphene. We suppose that this result is related to the air/graphene/ion liquid structure associated with the intrinsic properties of graphene surface. The second observation of the reduction hysteresis by increasing the solution concentration is also an interesting phenomenon, which may relates to the process of the competition between charge trapping effect and capacitive gating effect. [1]In conclusion, we compared the electrical characteristics shown by two different structures of graphene-based field effect transistor in this work, and found some interesting results of the transport hysteresis relating to device structure and ion concentration. This development has potentials for the chemical or biological sensing application. Furthermore, it can contribute to the fundamental researches about the interaction on the interface between graphene and solution.[1] Wang, H.; Wu, Y.; Cong, C.; Shang, J.; Yu, T., Hysteresis of Electronic Transport in Graphene Transistors. ACS Nano 2010,4(12), 7221-8.[2] Uysal, A.; Zhou, H.; Feng, G.; Lee, S. S.; Li, S.; Fenter, P.; Cummings, P. T.; Fulvio, P.F.; Dai, S.; McDonough, J. K.; Gogotsi, Y. Structural origins of potential dependent hysteresis at the electrified graphene/ionic liquid interface. J. Phys. Chem. C2013,118, 569-574.[3] Shih, C.-J., Paulus, G. L. C., Wang, Q. H., Jin, Z., Blankschtein, D., Strano, M. S., Understanding Surfactant/Graphene Interactions Using a Graphene Field Effect Transistor: Relating Molecular Structure to Hysteresis and Carrier Mobility. Langmuir, 28(22) Figure 1

A high-capacity graphene/mesocarbon microbead composite anode for lithium-ion batteries

The graphene/mesocarbon microbead (MCMB) composite is assessed as an anode material with a high capacity for lithium-ion batteries. The composite electrode exhibits improved cycling stability and rate capability, delivering a high initial charge/discharge capacity of 421.4 mA·h/g/494.8 mA·h/g as well as an excellent capacity retention over 500 cycles at a current density of 40 mA/g. At a higher current density of 800 mA/g, the electrode still retains 35% of its initial capacity which exceeds the capacity retention of pure graphene or MCMB reference electrodes. Cyclic voltammetry and electrochemical impedance spectroscopy reveal that the composite electrode favors electrochemical kinetics as compared with graphene and MCMB separately. Superior electrochemical properties suggest a strong synergetic effect between highly conductive graphene and MCMB.

Roles of structural and chemical defects in graphene on quenching of nearby fluorophores

Oxidation scanning probe lithography was used to systematically fabricate graphene/defective graphene ribbon arrays on a graphene sheet. The tribological, structural, and chemical natures of the defects created by o-SPL were characterized using lateral force microscopy (LFM), micro-Raman (μ-RS) and micro-X-ray spectroscopy (μ-XPS), respectively. While LFM revealed all fabricated patterns exhibited increase in friction, μ-RS and μ-XPS analysis showed that a transition of defect type from structural to chemical defects occurred in the defective graphene ribbons at a threshold voltage (8.5 V). Subsequently, micro-photoluminescence (μ-PL) quenching spectroscopy was measured for a thin layer of polystyrene containing isolated fluorophores (MEH-PPV) spin cast on the ribbon patterns. Whilst the quenching efficiency was suppressed more significantly as defect concentration increased, for pure structural defects, the μ-PL lifetime varied linearly with defect concentration and then saturated to a constant as chemical defects dominated. Among all oxygen related functional groups, the presence of CO bonds led to greatly reduced quenching efficiency while simultaneously reducing the strong blue spectral shift seen for pure graphene with the spectrum at higher CO bond concentration approaching to pristine MEH-PPV. This work demonstrated that local chemical and structural composition in 2D quencher strongly affects the quenching efficiency and dynamics.

The topology impact on hydrogen storage capacity of Sc-decorated ever-increasing porous graphene.

Hydrogen storage capacity of different scandium (Sc)-decorated topological porous graphene (PG) was examined through density functional theory calculations. PGs were selected considering odd and even topological symmetries. Our calculations demonstrate that the most preferable sites for adsorption of Sc are located on the center of carbon rings on the perimeter of pores of all sizes. This results in stronger polarization and hybridization perpendicular to the surface leading to enhanced binding. Thus, all PGs are suitable for hydrogen storage under surrounded settings. Furthermore, results showed that the adsorption energies of H2 molecules increased gradually with the size of pores. Analysis of charge density difference showed that the presence of Sc could play an efficient role for stronger adsorption of hydrogen molecules rather than increasing pore sizes. Furthermore, projected densities of states indicate that favorable systems for hydrogen storage are those that have higher overlap of individual states at Fermi level. Compared with H2 adsorption on pure graphene, injecting topological defect such as hexagon porous and decoration with a transition metal atom such as Sc can effectively create much more conductive states at Fermi energy. Eventually, Sc decoration leads to n-type doping of PGs that help in much easier transportation of charge carriers and desirable storage of H2 molecules.

First-Principles Study of 3<i>D</i> Transition Metal Doped Single-Layer Graphene

The electronic structure and magnetic properties of C atoms in Co, Ni-substituted graphene single-layers were studied by first-principles calculation method based on density functional theory. The study found that the pure graphene single-layer is an insulator, does not have magnetism, and we found that the doping of Co and Ni atoms alone does not make the system magnetic. Both Co and Ni atoms are capable of generating impurity levels in the graphene single-layer system. The impurity level of Co atom doping is 0.75 eV below the Fermi level, and the impurity level of Ni atom doping is 0.4 eV above the Fermi level. Studies on the coupling doping of Co and Ni atoms show that two different distance Co atoms or Ni atoms in the graphene single-layer are not always ferromagnetically coupled, and a stable magnetic ground state cannot be obtained. It can produce different magnetic ground states by controlling different doping distances, thus we provide one new way to control the spin properties.

Enhancement mechanism of the saturable absorption effect in reduced graphene oxide decorated with silver nanoparticles

Reduced graphene oxide (rGO) decorated with silver nanoparticles exhibits an enhanced nonlinear absorption effect compared with pure rGO. Using femtosecond time-resolved transient absorption spectroscopy, the enhancement mechanism and carrier dynamics of the composites are experimentally demonstrated. When the material is excited by laser pulses, the excited carriers in the conduction band of graphene will transfer to the d-band of silver before returning to the valence band. As the decay process (∼210 ps) is much longer than that of the relaxation time in pure graphene (∼fs), the bleaching effect of valence band is prolonged, resulting in enhanced saturable absorption effect.

Suitability of graphene monolayer as sensor for carcinogenic heavy metals in water: A DFT investigation

The present article discusses the sensing ability of graphene monolayer for the possible detection of carcinogenic heavy metals like Arsenic (As), Cadmium (Cd), Chromium (Cr), Mercury (Hg) and Lead (Pb)) present in water as pollutants and responsible for various health diseases. Density functional theory based first principle approach has been used to understand the sensing behavior of graphene monolayer by analyzing it’s electronic and transport properties both in vacuum and aqueous environment. The investigations have revealed that the pure graphene has no sensitivity towards water molecule, however interacts well with the carcinogenic heavy metal Cr with significant charge transfer, calculated through Bader Analysis. The significant variations in the band structure, density of states (DOS) profile and current–voltage characteristics of graphene monolayer due to presence of Cr, confirms that the graphene monolayer is relatively better candidate for the detection of Cr at small applied voltage of 400 mV in water, with a response of 82%, in comparison to the other heavy metals, reported in the present work.

Recent advances of 3D graphene-based adsorbents for sample preparation of water pollutants: A review

• Synthesis methods of 3D graphene were introduced. • Types of 3D graphene-based adsorbents were summarized. • 3D graphene-based adsorbents for removal of water pollutants were discussed. • Trends of 3D graphene-based adsorbents in sample preparation were outlooked. Recently, the researches about graphene-materials as efficient adsorbents have been promoted from two-dimension (2D) to three-dimension (3D). Compared with 2D graphene-materials, 3D graphene-based macrostructures not only preserve the original properties of constituent monomers, but also possess new collective physiochemical properties, making them more attractive as versatile adsorbents in sample preparation. In this review, we will innovatively summarize the recently published various kinds of 3D graphene-based adsorbents from the angles of material type (including pure 3D graphene and decorated with different materials: atom/molecule, inorganic-nanomaterials, polymers, carbon-nanomaterials and hybrid-composite-materials). Furthermore, we will discuss their applications for removing of various water pollutants (oils and organic solvents, dyes, metal ions, drugs, endocrine-disrupting chemicals and others). At last, the challenges and future outlook of 3D graphene-based adsorbents will also be discussed.

Electronic Structure of Nitrogen- and Phosphorus-Doped Graphenes Grown by Chemical Vapor Deposition Method.

Heteroatom doping is a widely used method for the modification of the electronic and chemical properties of graphene. A low-pressure chemical vapor deposition technique (CVD) is used here to grow pure, nitrogen-doped and phosphorous-doped few-layer graphene films from methane, acetonitrile and methane-phosphine mixture, respectively. The electronic structure of the films transferred onto SiO2/Si wafers by wet etching of copper substrates is studied by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy using a synchrotron radiation source. Annealing in an ultra-high vacuum at ca. 773 K allows for the removal of impurities formed on the surface of films during the synthesis and transfer procedure and changes the chemical state of nitrogen in nitrogen-doped graphene. Core level XPS spectra detect a low n-type doping of graphene film when nitrogen or phosphorous atoms are incorporated in the lattice. The electrical sheet resistance increases in the order: graphene < P-graphene < N-graphene. This tendency is related to the density of defects evaluated from the ratio of intensities of Raman peaks, valence band XPS and NEXAFS spectroscopy data.