Decorating reduced graphene oxide with Co3O4 hollow spheres and their application in supercapacitor materials

In this paper, a composite of reduced graphene oxide decorated by Co₃O₄ hollow spheres (Co₃O₄/RGO composite) has been synthesized by a one-pot solvothermal method. The samples are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR), Raman spectra and so on. The results demonstrate that the Co₃O₄ hollow spheres with good purity and homogenous size are absorbed onto the reduced graphene oxide sheets as spacers to prevent the aggregation of the graphene oxide sheets. Furthermore, the well electrochemical properties demonstrate that the Co₃O₄/RGO composite might have potential applications as electrode materials for supercapacitors.     

Preparation and characterization of graphene/CdS nanocomposites

Graphene-based nanocomposites are emerging as a new class of materials that hold promise for many applications. In this paper, we present a facile approach for the preparation of graphene/CdS nanocomposites through simple reflux processes, in which thiourea (CS(NH₂)₂) and thioacetamide (C₂H₅NS) act as a sulphide source, respectively. The samples were characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectrum (FT-IR), ultraviolet-visible (UV-vis) spectroscopy and thermogravimetry analysis. It was shown that in the nanocomposites, the CdS nanoparticles were densely and uniformly deposited on the graphene sheets, and the sulphide source used has a great influence on the morphology, structure and property of the graphene/CdS nanocomposites. The good distribution of CdS nanoparticles on graphene sheets guarantees the efficient optoelectronic properties of graphene/CdS and would be promising for practical applications in future nanotechnology.     

Green synthesis of biocompatiable chitosan–graphene oxide hybrid nanosheet by ultrasonication method

Ultrasound-induced synthesis of chitosan-modified nano-scale graphene oxide (CS-NGO) hybrid nanosheets, which has great potential pharmaceutical applications, in supercritical CO₂ without catalyst was presented for the first time. The preparation process does not require organic solvent and post-processing, and CO₂ easily escapes from the product. The morphology and structure of the CS-NGO, characterized using scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis, confirms that it was combined via the amide linkage, and had excellent dispersibility and stability toward acidic and physiological aqueous solution, which implies that it could be used as a drug-carrier. The sonication power played a crucial role in inducing forming amidation, and the conversion rate increased with the sonication time. The mechanism of this reaction was explained.     

Decorating graphene sheets with Pt nanoparticles using sodium citrate as reductant

In this paper, we proposed a novel and green approach for the synthesis of graphene nanosheets (GNS) and Pt nanoparticles-graphene nanosheets (Pt/GNS) hybrid materials, employing graphene oxide (GO) as precursor and sodium citrate as environmentally friendly reducing and stabilizing agent. The microstructures of GO and Pt/GNS were characterized by high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Raman spectroscopy, atomic force microscopy (AFM), X-ray diffraction (XRD) and electrochemical measurements. The results confirmed that the uniform size distribution of Pt nanoparticles on the surface of GNS without agglomerates could be easily obtained via using sodium citrate as reductant, moreover the Pt/GNS hybrids exhibited high electrochemical activity.     

Surface adhesion properties of graphene and graphene oxide studied by colloid-probe atomic force microscopy

Surface adhesion properties are important to various applications of graphene-based materials. Atomic force microscopy is powerful to study the adhesion properties of samples by measuring the forces on the colloidal sphere tip as it approaches and retracts from the surface. In this paper we have measured the adhesion force between the colloid probe and the surface of graphene (graphene oxide) nanosheet. The results revealed that the adhesion force on graphene and graphene oxide surface were 66.3 and 170.6 nN, respectively. It was found the adhesion force was mainly determined by the water meniscus, which was related to the surface contact angle of samples.     

Impact of synthesis routes on the chemical,optical, and electrical properties of graphene oxides and its derivatives

Solution-processed graphene oxides in their reduced forms are prominent prospective functional materials for organic optoelectronics. For graphene oxide synthesis, several methods have been developed, which induce varying properties in their products. However, the dependence of the graphene oxide properties on their synthesis methods has rarely been studied, hindering the selection of the optimum synthesis route for a target application. In this study, we report our study results on the properties of synthesized graphene oxides and their reduced forms created using several synthesis methods, such as the modified Hummers' method, the improved method, and the Staudenmaier's method as well as from two commercial sources, Angstron Material, Inc. and Graphos, Inc. Focusing on the transparent electrode application, the properties of thin films were investigated using UV–visible spectroscopy, Hall measurements, atomic force microscopy, Raman spectroscopy, work function measurements, and X-ray photoelectron spectroscopy. Our results reveal significant morphological, elemental, structural, and optoelectrical property variations among the as-prepared and reduced thin films of graphene oxides by their synthesis methods. In addition, the results show that the graphene oxides synthesized using the modified Hummers' method and the product from Angstron Material, Inc. are the most suitable materials for the transparent electrode application.     

NMR of localized magnetic states in graphene

The NMR relaxation rate is studied on the magnetic states of an impurity in bilayer graphene within a tight-binding scenario. The dependencies of the relaxation rate on temperature, interlayer interaction and also the chemical potential have been considered. Although for low temperatures we observe the usual Korringa relation, a characteristic of the conventional fermions, the rate increases with the increase in temperature and tends to saturate for high temperatures. For small interlayer interactions (t_⊥) the system can be either magnetic or non-magnetic. However for higher t_⊥ we observe the existence of only a pure magnetic state. In graphene this transition is also observed with two cusps related to the magnetic to non-magnetic transition, which modifies to a single hump for higher t_⊥, where the system is purely magnetic for any value of chemical potential.     

Structures of Pt clusters on graphene by first-principles calculations

The interactions between Pt_n clusters (n?13) and a graphene sheet have been investigated by first-principles calculations based on density functional theory. For single Pt-atom and Pt₂-dimer adsorptions, the stable adsorption sites are bridge sites between neighboring carbon atoms. When the number of Pt atoms in a cluster increases, the Pt-C interaction energy per contacting Pt atom becomes smaller. For smaller clusters (3?n?7), the adsorption as a vertical planar cluster is more stable than that as parallel planar or three-dimensional (3D) clusters, due to the stability of a planar configuration itself and the stronger planar-edge/graphene interaction, while the adsorption as a parallel planer cluster becomes stable for larger cluster (n?7) via the deformation of the planar configuration so as to attain the planar-edge/graphene contact. For much larger clusters (n?10), the adsorption as a 3D cluster becomes the most stable due to the stability of the 3D configuration itself as well as substantial Pt-C interactions of edge or corner Pt atoms. The interfacial interaction between a Pt cluster and graphene seriously depends on the shape and size of a cluster and the manner of contact on a graphene sheet.     

Topological vortices in chiral gauge theory of graphene

Generation mechanism of energy gaps between conductance and valence bands is at the centre of the study of graphene material. Recently, Chamon, Jackiw et al. proposed a mechanism of using a Kekulé distortion background field φ and its induced gauge potential A_i to generate energy gaps. In this paper, various vortex structures inhering in this model are studied. Regarding φ as a generic background field rather than a fixed Nielson-Oleson type distribution, we have found two new types of vortices on the graphene surface—the velocity field vortices and the monopole-motion induced vortices—from the inner structure of the potential A_i. These vortex structures naturally arise from the motion of the Dirac fermions instead of from the background distortion field.     

A green synthetic approach to graphene nanosheets for hydrogen adsorption

A green and facile strategy of preparing graphene by reducing exfoliated graphite oxide (GO) with glucose was developed in this study. The as-prepared samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Atomic force microscopy (AFM). The characterization results indicated that the graphene sheets (GS) were of high quality with smooth surface, rich pore structure and few layer graphene. The samples have a BET specific surface area of 1205.8 m² g⁻¹ measured by N₂ adsorption at 77 K. The hydrogen storage capacity of 2.7 wt.% at 298 K and 25 bar demonstrated that the as-prepared graphene employing glucose as reductant is supposed to be a promising material with outstanding property for hydrogen storage.     

Adsorption-induced magnetism properties in graphene

Using first-principles calculations, we investigate magnetic properties and electronic structures of graphene with H, N and P adsorptions. With a change in adsorption density from 1/50 to 1/162 a band gap changing from ∼1.2 to 0.1 eV emerges in a H-absorbed graphene, leading to the semiconducting graphene and showing ferromagnetism with the magnetic moment of the system changing from 0.76 to 0.42μ_B. The unpaired electrons in the absorbed N/P atoms are polarized and thus it exhibits magnetic moment per N/P atom changing from 0.38/0.20 to 0.60/0.14μ_B and metallic and half-metallic magnetism, respectively. The spin-polarized graphene system has a great application prospect in spintronics.     

Density functional theory prediction for diffusion of lithium on boron-doped graphene surface

The density functional theory (DFT) investigation shows that graphene has changed from semimetal to semiconductor with the increasing number of doped boron atoms. Lithium and boron atoms acted as charge contributors and recipients, which attracted to each other. Further investigations show that, the potential barrier for lithium diffusion on boron-doped graphene is higher than that of intrinsic graphene. The potential barrier is up to 0.22 eV when six boron atoms doped (B₆C₂₆), which is the lowest potential barrier in all the doped graphene. The potential barrier is dramatically affected by the surface structure of graphene.     

A DFT study of formaldehyde adsorption on functionalized graphene nanoribbons

Density functional theory (DFT) based ab initio calculations were done to monitor the formaldehyde (CHOH) adsorptive behavior on pristine and Ni-decorated graphene sheet. Structural optimization indicates that the formaldehyde molecule is physisorbed on the pristine sheet via partly weak van der Waals attraction having the adsorption energy of about −15.7 kcal/mol. Metal decorated sheet is able to interact with the CHOH molecule, so that single Ni atoms prefer to bind strongly at the bridge site of graphene and each metal atom bound on sheet may adsorb up to four CHOH. The findings also show that the Ni decoration on graphene surface results in some changes in electronic properties of the sheet and its E_g is remained unchanged after adsorption of CHOH molecules. It is noteworthy to say that no bond cleavage was observed for the adsorption of CHOH on Ni-decorated graphene.     

Developing accelerometer based on graphene nanoribbon resonators

We investigated an ultrahigh sensitive accelerometer based on graphene nanoribbon resonators. Sensing acceleration can be made by their resonance frequency shift and/or their capacitance change. Schematics and the static properties were introduced and the dynamic properties were investigated via classical molecular dynamics simulation. As the acceleration increased, the oscillations of the deflections were going dramatically faster and the mean deflections increased, then the capacitance continually varied with large amplitudes and the resonance frequencies linearly increased in a log–log scale by power regression. The energy loss decreased with increasing time, and the average quality factors were dramatically reduced with increasing acceleration.     

How water makes graphene metallic

A simple explanation is given on the tendency of graphene to become metallic when the amount of doped water is increased, an effect which was previously obtained from ab initio band calculations. It is clarified how the effect is mainly determined by oriented water electric dipoles, which create a step like potential at the separation distance between graphene and water planes. By using perturbation theory and an effective potential coupled with a image-charge tail potential, we showed that under increasing the water doping, the lowest energy free band in graphene starts lowering their energies by approaching to the Fermi level. Moreover, we demonstrated that this crossing induces a huge increase of states at the Fermi level, an effect akin to the magic-angle flat-band appearance in bilayer graphene.     

Electromigration with enhanced green emission in the titanium dioxide nanotube/graphene composite

One of the most studied photoluminescence emission peaks of anatase titanium dioxide (TiO₂) is green, located at about 520 nm, which is assigned to the radiative recombination between a mobile electron in the conduction band and oxygen vacancy defect as a trapped hole in the bandgap. Composite materials of TiO₂ with graphene are normally shown by the gradual quenching of photoluminescence intensity as a result of carrier lifetime extension, which is important to enhance photocatalytic activity. Herein we report an observation of the intensity enhancement of the green PL emission in a composite TiO₂ nanotube (TNT) and graphene produced through facile hydrothermal synthesis. The heterojunction formation of graphene and TNT makes the excited photoelectrons easy to diffuse from TNT to graphene. Hence, the recombination rate of mobile electrons in graphene and trapped holes located on the nanotube surface is enhanced due to the high mobility of electrons in graphene.     

Plasmon modes of double-layer graphene at finite temperature

We calculate the dynamical dielectric function of doped double-layer graphene (DLG), made of two parallel graphene monolayers with carrier densities n₁ and n₂, and an interlayer separation of d at finite temperature. The results are used to find the dispersion of plasmon modes and loss functions of DLG for several interlayer separations and layer densities. We show that in the case of n₂=0, the finite-temperature plasmon modes are dramatically different from the zero-temperature ones.     

Scanning probe microscopy investigations of the electrical properties of chemical vapor deposited graphene grown on a 6H-SiC substrate

Sublimated graphene grown on SiC is an attractive material for scientific investigations. Nevertheless the self limiting process on the Si face and its sensitivity to the surface quality of the SiC substrates may be unfavourable for later microelectronic processes. On the other hand, chemical vapor deposited (CVD) graphene does not posses such disadvantages, so further experimental investigation is needed. In this paper CVD grown graphene on 6H-SiC (0 0 0 1) substrate was investigated using scanning probe microscopy (SPM). Electrical properties of graphene were characterized with the use of: scanning tunnelling microscopy, conductive atomic force microscopy (C-AFM) with locally performed C-AFM current–voltage measurements and Kelvin probe force microscopy (KPFM). Based on the contact potential difference data from the KPFM measurements, the work function of graphene was estimated. We observed conductance variations not only on structural edges, existing surface corrugations or accidental bilayers, but also on a flat graphene surface.     

Study of multi-faceted CoS2 introduced graphene aerogel hybrids via chemical approach for an effective electrocatalytic water splitting

The present article reports the synthesis of hybrid structure along with non-precious cobalt-disulfide. A simple hydrothermal method was used to fabricate multi-faceted CoS₂ introduced graphene aerogels. Studies on electrocatalytic activity showed that the presence of CoS₂ facets along with graphene aerogel played a prominent role in the enhancement of proton reduction to hydrogen gas. The CoS₂/graphene aerogel hybrid sample exhibits extremely low overpotential (160 mV vs. RHE), and high current density for HER in acidic solution. The activity enhancement can be attributed to increasing the active electrochemical surface area of graphene aerogel and faceted particles inside the 3D matrix of graphene. Furthermore, the CoS₂/graphene hybrid retained its high activity even after 1000 cycles of cyclic voltammetry scans, signifying longer stability under acidic condition. The results suggest that CoS₂/graphene aerogel hybrids show their potential application to hydrogen evolution reaction.     

Temperature as a key parameter for graphene sono-exfoliation in water

Graphene dispersions in water are highly desirable for a range of applications such as biomedicines, separation membranes, coatings, inkjet printing and more. Recent novel research has been focussed on developing a green approach for scalable production of graphene. However, one important parameter, which is often neglected is the bulk temperature of the processing liquid. This paper follows our earlier work where optimal sono-exfoliation parameters of graphite in aqueous solutions were determined based on the measured acoustic pressure fields at various temperatures and input powers. Here, we take the next step forward and demonstrate using systematic characterisation techniques and acoustic pressure measurements that sonication-assisted liquid phase exfoliation (LPE) of graphite powder can indeed produce high quality few layer graphene flakes in pure water at a specific temperature, i.e. 40 °C, and at an optimised input generator power of 50%, within 2-h of processing. UV–vis analysis also revealed that the exfoliation, stability and uniformity of dispersions were improved with increasing temperature. We further confirmed the successful exfoliation of graphene sheets with minimal level of defects in the optimized sample with the help of Raman microscopy and transmission electron microscopy. This study demonstrated that understanding and controlling processing temperature is one of the key parameters for graphene exfoliation in water which offers a potential pathway for its large-scale production.