Graphene-supported SnO2 nanoparticles prepared by a solvothermal approach for an enhanced electrochemical performance in lithium-ion batteries

SnO₂ nanoparticles were dispersed on graphene nanosheets through a solvothermal approach using ethylene glycol as the solvent. The uniform distribution of SnO₂ nanoparticles on graphene nanosheets has been confirmed by scanning electron microscopy and transmission electron microscopy. The particle size of SnO₂ was determined to be around 5 nm. The as-synthesized SnO₂/graphene nanocomposite exhibited an enhanced electrochemical performance in lithium-ion batteries, compared with bare graphene nanosheets and bare SnO₂ nanoparticles. The SnO₂/graphene nanocomposite electrode delivered a reversible lithium storage capacity of 830 mAh g⁻¹ and a stable cyclability up to 100 cycles. The excellent electrochemical properties of this graphene-supported nanocomposite could be attributed to the insertion of nanoparticles between graphene nanolayers and the optimized nanoparticles distribution on graphene nanosheets.     

Defect-curing function of self-limiting Al2O3 thin films in graphene materials

Impedance spectroscopy was applied to 2-dimensional graphene materials that were thermally grown on copper substrates to quantitatively monitor the quality of the as-grown graphene materials without the subsequent transfer process. The presence of the graphene layer prevents the dissolution of the metallic copper elements in the corrosive electrolyte and provides an interface between the ionic electrolyte and electronic graphene/copper materials. The highest impedance appears at the graphene/electrolyte to be associated with electrochemically robust graphene materials, i.e., the as-grown graphene materials subjected to atomic layer deposition of Al₂O₃. Such an effect is attributed to the anti-corrosive protection of graphene materials and the defect-curing function of Al₂O₃ in graphene materials. The impedance-based information can be exploited in-situ without the use of any destructive approaches to evaluate the electrical perfectness vulnerable to preparation environments.     

Raman enhancement by graphene-Ga2O3 2D bilayer film

2D β-Ga₂O₃ flakes on a continuous 2D graphene film were prepared by a one-step chemical vapor deposition on liquid gallium surface. The composite was characterized by optical microscopy, scanning electron microscopy, Raman spectroscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy (XPS). The experimental results indicate that Ga₂O₃ flakes grew on the surface of graphene film during the cooling process. In particular, tenfold enhancement of graphene Raman scattering signal was detected on Ga₂O₃ flakes, and XPS indicates the C-O bonding between graphene and Ga₂O₃. The mechanism of Raman enhancement was discussed. The 2D Ga₂O₃-2D graphene structure may possess potential applications.     

Process intensification of uniform loading of SnO2 nanoparticles on graphene oxide nanosheets using a novel ultrasound assisted in situ chemical precipitation method

In this paper, a novel ultrasound assisted, solution-based chemical synthesis method for the preparation of SnO₂–graphene nanocomposite is presented. Graphene oxide (GO) was prepared by the modified Hummers–Offeman method in presence of ultrasonic irradiation. Further loading of SnO₂ on GO was carried out with an ultrasound assisted solution-based synthesis route. The prepared GO and SnO₂–graphene nanocomposite were characterized by XRD, TEM, FTIR spectra, TGA and DTA analysis in order to confirm the formation of graphene–SnO₂ nanocomposite. TEM analysis of ultrasonically prepared graphene–SnO₂ composite shows the uniform and fine loading of SnO₂ particles (3–5 nm) on graphene nanosheets. However agglomerated morphology was observed in case of conventionally prepared graphene–SnO₂ composite. The cavitational effects generated due to the ultrasonic irradiations during the synthesis of graphene–SnO₂ composite improve the fine and uniform loading of SnO₂ on graphene nanosheets by oxidation–reduction reaction between GO and SnCl₂·2H₂O compared to conventional synthesis methods. The formed material was used for the preparation of anode in lithium ion batteries and its electrochemical performance was characterized by cyclic voltammetry and charge/discharge cycles. It is found that the capacity of SnO₂–graphene nanocomposite based Li-battery is stable for around 120 cycles, and the battery could repeat stable charge–discharge reaction.     

Synthesis and photocatalytic performance of PVA/TiO2/graphene‐MWCNT nanocomposites for dye removal

TiO₂/graphene‐MWCNT nanocomposite was prepared using solvothermal reaction for the effective distribution of TiO₂ nanoparticles on carbonaceous materials. TiO₂/graphene‐MWCNT nanocomposite was immobilized in poly(vinyl alcohol) (PVA) matrix for a convenient recovery after wastewater purification. MWCNT was incorporated in a nanocomposite not only to prevent the restacking of graphene but also to increase the electron transfer from TiO₂. The detailed characterization of the nanocomposite was performed using SEM, EDX, XRD, XPS, and FTIR. The photocatalytic performance of PVA/TiO₂/graphene‐MWCNT nanocomposite was investigated by UV spectroscopy on the basis of degradation of organic pollutants. PVA/TiO₂/graphene‐MWCNT nanocomposite showed improved photocatalytic decomposition of more than 70% of residual dye left in case of using PVA/TiO₂/graphene nanocomposite due to the improved electron transfer and the higher adsorption of organic pollutants. PVA/TiO₂/graphene‐MWCNT nanocomposite was suitable as a promising material for the recyclable photocatalytic wastewater purification system. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40715.     

A first-principles study of calcium-decorated, boron-doped graphene for high capacity hydrogen storage

Hydrogen adsorption and storage on calcium-decorated, boron-doped graphene was explored using density functional theory simulations based on local density approximation and generalized gradient approximation methods. The clustering problem for calcium-decorated graphene was investigated and it was shown that individual calcium atoms are not stable on pure graphene, and formation of aggregates is favorable. Substitutional boron doping can eliminate the clustering problem for Ca atoms on graphene. Up to four hydrogen molecules can stably bind to a Ca atom on a graphene plane with substitutional doping of a single boron atom. The average binding energy of ∼0.4 eV/H₂ is in the range that permits H₂ recycling at ambient conditions. Two binding mechanisms contribute to the adsorption of H₂ molecules: polarization of the H₂ molecule under the electric field produced by the Ca–graphene dipole, and hybridization of the 3d orbitals of Ca with the σ orbitals of H₂. Double-sided Ca-decorated graphene doped with individual boron atoms of 12 at.% can theoretically reach a gravimetric capacity of 8.38 wt.% hydrogen.     

Probing substrate influence on graphene by analyzing Raman lineshapes

We provide a new approach to identify the substrate influence on graphene surface. Distinguishing the substrate influences or the doping effects of charged impurities on graphene can be realized by optically probing the graphene surfaces, included the suspended and supported graphene. In this work, the line scan of Raman spectroscopy was performed across the graphene surface on the ordered square hole. Then, the bandwidths of G-band and 2D-band were fitted into the Voigt profile, a convolution of Gaussian and Lorentzian profiles. The bandwidths of Lorentzian parts were kept as constant whether it is the suspended and supported graphene. For the Gaussian part, the suspended graphene exhibits much greater Gaussian bandwidths than those of the supported graphene. It reveals that the doping effect on supported graphene is stronger than that of suspended graphene. Compared with the previous studies, we also used the peak positions of G bands, and I_2D/I_G ratios to confirm that our method really works. For the suspended graphene, the peak positions of G band are downshifted with respect to supported graphene, and the I_2D/I_G ratios of suspended graphene are larger than those of supported graphene. With data fitting into Voigt profile, one can find out the information behind the lineshapes.     

Layer-dependent morphologies of silver on n-layer graphene

The distributions of sizes of silver nanoparticles that were deposited on monolayer, bilayer, and trilayer graphene films were observed. Deposition was carried out by thermal evaporation and the graphene films, placed on SiO₂/Si substrates, were obtained by the mechanical splitting of graphite. Before the deposition, optical microscopy and Raman spectroscopy were utilized to identify the number of the graphene layers. After the deposition, scanning electron microscopy was used to observe the morphologies of the particles. Systematic analysis revealed that the average sizes of the nanoparticles increased with the number of graphene layers. The density of nanoparticles decreased as the number of graphene layers increased, revealing a large variation in the surface diffusion strength of nanoparticles on the different substrates. The mechanisms of formation of these layer-dependent morphologies of silver on n-layer graphene are related to the surface free energy and surface diffusion of the n-layer graphene. The effect of the substrate such as SiO₂/Si was investigated by fabricating suspended graphene, and the size and density were similar to those of supported graphene. Based on a comparison of the results, the different morphologies of the silver nanoparticles on different graphene layers were theorized to be caused only by the variation of the diffusion barriers with the number of layers of graphene.     

Effects of annealing on copper substrate surface morphology and graphene growth by chemical vapor deposition

Understanding the mechanism of graphene synthesis by chemical vapor deposition and the effect of process parameters is critical for production of high-quality graphene. In the present work, we investigated the effect of H₂ concentration during annealing on evolution of Cu surface morphology, and on deposited graphene characteristics. Our results revealed that H₂ had a smoothening effect on Cu surface as its surface roughness was reduced significantly at high H₂ concentration along with the formation of surface facets, dents and nanometer-sized particles. Furthermore, H₂ content influenced the graphene morphology and its quality. A low H₂ concentration (0% and 2.5%) during annealing promoted uniform and good quality bilayer graphene. In contrast, a high concentration of H₂ (20% and 50%) resulted in multilayer, non-uniform and defective graphene. Interestingly, the annealed Cu surface morphology differed considerably from that obtained after deposition of graphene, indicating that graphene deposition has its own impact on Cu surface.     

Essential roles of defects in pure graphene/Cu2O photocatalyst

The pure graphene flakes with different abundance of defects were synthesized by CVD method to successfully prepare the graphene/Cu₂O composites for the photocatalytic degradation of methyl orange (MO) under visible light irradiation. The results illustrate that catalytic activity was strongly dependent on the abundance of defects in graphene flakes. The highest activity was measured to be 80.10% in 30 min light irradiation. Furthermore, we demonstrate that the defects in graphene could facilitate to construct an efficient interface between graphene and Cu₂O for narrowing the band gap of original Cu₂O semiconductor and inhibiting the recombination of photo-generated electron–hole pairs.     

Synthesis and strengthened microwave absorption properties of three-dimensional porous Fe3O4/graphene composite foam

The three-dimensional porous Fe₃O₄/graphene composite foam as a new kind of absorbing composite with electrical loss and magnetic loss was successfully synthesized by a facile method. Fe₃O₄ was evenly attached on structure of graphene sheets which overlapped with each other to form three-dimensional porous graphene foam. The results revealed that when the mass ratio of graphene oxide (GO) and Fe₃O₄ was 1:1, the Fe₃O₄/graphene composite foam possessed the best absorption properties: the minimum reflection loss was up to ??45.08?dB when the thickness was 2.5?mm and the bandwidth below ??10?dB was 6.7?GHz when the content of the composite foam absorbents was just 8%. The micron-sized three-dimensional porous structure provided more propagation paths, enhancing the energy conversion of incident electromagnetic waves. The addition of Fe₃O₄ contributed to improving the impedance matching performance and magnetic loss. The three-dimensional porous Fe₃O₄/graphene composite foam was a kind of high-efficiency wave absorber, providing a new idea for the development of microwave absorbing materials.     

Transfer-free growth of graphene on SiO2 insulator substrate from sputtered carbon and nickel films

Here we demonstrate the growth of transfer-free graphene on SiO₂ insulator substrates from sputtered carbon and metal layers with rapid thermal processing in the same evacuation. It was found that graphene always grows atop the stack and in close contact with the Ni. Raman spectra typical of high quality exfoliated monolayer graphene were obtained for samples under optimised conditions with monolayer surface coverage of up to 40% and overall graphene surface coverage of over 90%. Transfer-free graphene is produced on SiO₂ substrates with the removal of Ni in acid when Ni thickness is below 100 nm, which effectively eliminates the need to transfer graphene from metal to insulator substrates and paves the way to mass production of graphene directly on insulator substrates. The characteristics of Raman spectrum depend on the size of Ni grains, which in turn depend on the thickness of Ni, layer deposition sequence of the stack and RTP temperature. The mechanism of the transfer-free growth process was studied by AFM in combination with Raman. A model is proposed to depict the graphene growth process. Results also suggest a monolayer self-limiting growth for graphene on individual Ni grains.     

A SnO2/graphene composite as a high stability electrode for lithium ion batteries

A simple solution-based synthesis route, based on an oxidation–reduction reaction between graphene oxide and SnCl₂•2H₂O, has been developed to produce a SnO₂/graphene composite. In the prepared composite, crystalline SnO₂ nanoparticles with sizes of 3–5 nm uniformly clung to the graphene matrix. When used as an electrode material for lithium ion batteries, the composite presented excellent rate performance and high cyclic stability. The effect of SnO₂/graphene ratio on electrochemical performance has been investigated. It was found that the optimum molar ratio of SnO₂/graphene was about 3.2:1, corresponding to 2.4 wt.% of graphene. The composite could deliver a charge capacity of 840 mAh/g (with capacity retention of 86%) after 30 charge/discharge cycles at a current density of 67 mA/g, and it could retain a charge capacity of about 590 and 270 mAh/g after 50 cycles at the current density of 400 and 1000 mA/g, respectively.     

Microstructure and properties of a graphene platelets toughened boron carbide composite ceramic by spark plasma sintering

A kind of B₄C/SiC composite ceramic toughened by graphene platelets and Al was fabricated by spark plasma sintering. The effects of graphene platelets and Al on densification, microstructure and mechanical properties were studied. The sintering temperature was decreased about 125–300?°C with the addition of 3–10?wt% Al. Al can also improve fracture toughness but decrease hardness. The B₄C/SiC composite ceramic with 3?wt%Al and 1.5?wt% graphene platelets sintered at 1825?°C for 5?min had the optimal performances. It was fully densified, and the Vickers hardness and fracture toughness were 30.09?±?0.39?GPa and 5.88?±?0.49?MPa?m^1/2, respectively. The fracture toughness was 25.6% higher than that of the composite without graphene platelets. The toughening mechanism of graphene platelets was also studied. Pulling-out of graphene platelets, crack deflection, bridging and branching contributed to the toughness enhancement of the B₄C-based ceramic.     

Nucleation and growth of graphene/Mo2C heterostructures on Cu through CVD

We investigated the chemical vapor deposition synthesis of Mo₂C/graphene heterostructures on a partially wetted liquid copper surface, studied the morphology of resulting phases using electron and optical microscopy, and determined the rate-limiting step for the growth of Mo₂C on graphene. The morphology of the Mo₂C crystals varied from the center to the edge of the copper substrate because of the change in the Mo diffusion pathways owing to the variation in the thickness of the Cu substrate. Thin, hexagonal-shaped crystals of Mo₂C were found in the central region, where Cu is the thickest. In addition, the growth pressure substantially affects the nucleation and growth kinetics of both Mo₂C and graphene. At high pressures (750 Torr), the graphene layer fully covered the Cu surface and Mo₂C crystals formed with a regular shape, while at low pressures (5 Torr), the nucleation of both domains was suppressed, leading to the evolution of Mo₂C crystals with irregular shapes. The activation energy for the growth of Mo₂C on graphene was calculated to be 3.76 ± 0.3 eV, and the diffusion of Mo to the Cu surface through uncovered Cu or graphene vacancies/defects was determined to be the rate-limiting step.     

TiO2 nanoparticles assembled on graphene oxide nanosheets with high photocatalytic activity for removal of pollutants

A novel method was developed to synthesize graphite oxide/TiO₂ composites as a highly efficient photocatalyst by in situ depositing TiO₂ nanoparticles on graphene oxide nano-sheets by a liquid phase deposition, followed by a calcination treatment at 200 °C. The two-dimensional porous graphene oxide/TiO₂ composites had specific surface area of 80 m² g⁻¹ being considerably larger than that of P25 and the similarly prepared neat TiO₂ particles without using graphene oxide. The composites exhibited excellent photocatalytic activity, being influenced by post-calcination temperature, graphene oxide content and solution pH. Under optimal conditions, the photo-oxidative degradation rate of methyl orange and the photo-reductive conversion rate of Cr(VI) over the composites were as high as 7.4 and 5.4 times that over P25, respectively. The excellent enhancing effect of graphene oxide nano-sheets on the photocatalytic properties of TiO₂ was attributed to a thin two-dimensional sheet support, a large surface area and much increased adsorption capacity, and the strong electron transfer ability of the thermally reduced graphene oxide in the composite.     

Grand canonical Monte Carlo simulations of nitrogen adsorption on graphene materials with varying layer number

Fabrication of monolayer graphene is a challenge and many processes yield few-layer or multi-layer graphene materials instead. The layer number is an important property of those materials and a quality control variable in graphene manufacture. We demonstrated that N₂ adsorption on graphene materials was used to distinguish its layer number. We performed grand canonical Monte Carlo simulation of N₂ adsorption on graphene materials with 1–10 layers to indicate the possibility of distinction of layer number by evaluating the dependence of N₂ adsorption characteristics on the layer number of graphene materials as well as the adsorption mechanism. The threshold relative pressures of monolayer adsorption of N₂ on monolayer and two-layer graphene were 1 × 10⁻³ and 2 × 10⁻⁴, respectively, while those of the others were 1 × 10⁻⁴. In contrast, the threshold pressures of second layer adsorption of N₂ were similar to each other. The difference of threshold pressures is attributed to stabilized energies induced by interactions with graphene materials. Therefore, the layer number of graphene materials could be evaluated from the threshold pressures of adsorption, providing a guide to aid fabrication of graphene materials.     

Electrochemical behaviors of graphene-ZnO and graphene-SnO2 composite films for supercapacitors

Graphene, graphene-ZnO and graphene-SnO₂ films were successfully synthesized and used as electrode materials for electrochemical supercapacitors, respectively. The screen-printing approach was employed to fabricate graphene film on graphite substrate while the ZnO and SnO₂ were deposited on graphene films by ultrasonic spray pyrolysis. The electrochemical performances of these electrodes were comparatively analyzed through electrochemical impedance spectrometry, cyclic voltammetry and chronopotentiometry tests. The results showed that the incorporation of ZnO or SnO₂ improved the capacitive performance of graphene electrode. Graphene-ZnO composite electrode exhibited higher capacitance value (61.7 F/g) and maximum power density (4.8 kW/kg) as compared with graphene-SnO₂ and pure graphene electrodes.     

Influence of the Reduction of Graphene Oxide with Hydroiodic Acid on the Structure and Photoactivity of CdS–rGO Hybrids

The temperature used in the chemical reduction of graphene oxide (GO) with hydroiodic acid has a significant influence on the removal of surface oxygenated functional groups, on the residual iodine species and on the rupture, stacking and graphitization of the graphene sheets in the reduced graphene oxides. The modification in the characteristics of the reduced graphene oxides induces changes in the surface area, the exposition of reduced graphene oxide entities and in the concentration of small CdS nanocrystals with strong confinement effect on the CdS-reduced graphene oxide hybrids. The hybridization of the reduced graphene oxide with CdS modifies in different way their photocatalytic behavior for hydrogen production from aqueous solutions of Na₂S and Na₂SO₃ under simulated sunlight irradiation. Only the hybrid formed between the CdS and the reduced graphene oxide treated at higher temperature showed improved hydrogen production rate respect to the bare CdS reference associated with the better conductivity of the reduced graphene oxide and with the increase in the concentration of small CdS nanocrystals sith strong confinement effect observed in the hybrid.     

Fabrication and properties of monolayer graphene reinforced Al2O3/TiN ceramic tool material

Al₂O₃/TiN/graphene ceramic tool materials were prepared by spark plasma sintering technology and the strengthening and toughening mechanisms were studied. The influence of monolayer graphene content on the mechanical properties and microstructure of the composite material were analyzed and the strengthening and toughening mechanisms were researched. The results showed that with an addition of .5 vol.% graphene the mechanical properties of the material reached the best. The bending strength, hardness, and fracture toughness were 624 MPa, 23.24 GPa, and 6.53 MPa·m^1/2, respectively. Graphene existed in the forms of few-layer and multilayer. The toughening mechanism of few-layer graphene was mainly graphene breaking, and that of multilayer graphene included graphene breaking and pulling-out. Graphene could contribute to the uniform growth of grains due to the excellent electrical conductivity and the high thermal conductivity. The addition of nano-TiN introduced many endocrystalline structures and graphene promoted this phenomenon. Micro-TiN grains made the crack extension show a combination of transgranular fracture, intergranular fracture, crack bridging, and crack deflection, while graphene introduced weak grain interfaces and made the crack appear more branches. The layered graphene made the material fracture change from two-dimension to three-dimension.