Synthesis of three-dimensional hierarchically porous reduced graphene oxide–TiO2 nanocomposite for enhanced hydrogen storage

Three-dimensional hierarchically porous graphene-TiO₂ (3D-HPGT) nanocomposites were synthesized through electrostatic assembly method. The obtained 3D-HPGT nanocomposites exhibited hierarchically porous structure with multi-level pores (macro-, meso- and micropores), high specific surface area (705?m²/g), large pore volume (0.41?cm³/g) and higher hydrogen storage capacity. At the pressure of 5?bar, 3D-HPGT nanocomposite showed a maximum hydrogen capacity of 4.11 and 1.48?wt% at 77 and 298?K, respectively, which were much higher than those of previously reported graphene-based materials. The enhanced hydrogen storage capacities were attributed to the three-dimensional hierarchically porous structure, evenly distributed TiO₂ nanoparticles on the graphene nanosheets, strong attachment of TiO₂ nanoparticles to the underlying graphene nanosheets, and hydrogen spillover effect originated from TiO₂ nanoparticles.     

Molecule–substrate interaction in functionalized graphene

The study of carbon-based hybrid nanostructures is an emerging field of current research. In particular, photo-active molecules have been shown to considerably influence optical properties of carbon nanotubes suggesting realization of molecular switches. Here, we focus on the qualitative nature of molecule–substrate coupling within carbon-based hybrid nanostructures including nanoribbons and graphene. Our theoretical approach is based on density-matrix formalism and predicts a molecule-induced splitting of the pristine spectral resonances combined with a considerable spectral shift. Both effects strongly depend on the electronic bandstructure of the substrate. Furthermore, we investigate the impact of the substrate dimension on the coupling by increasing the width of nanoribbons from the very narrow up to graphene. Our calculations reveal a clear increase of the optical absorption of graphene in the vicinity of the Dirac point and a peak broadening at the saddle point due to the appearance of a high-energy shoulder. Our results give new insights into the molecule–substrate coupling and can guide future experiments towards the realization of tailored hybrid materials with desired optical properties.     

Direct multipulse laser processing of titanium oxide–graphene oxide nanocomposite thin films

Nanocomposite thin films consisting of titanium oxide (TiO₂) nanoparticles (NPs) and graphene oxide (GO) platelets were deposited by a spin-coating technique. The obtained films were submitted to direct laser irradiation using a frequency quadrupled Nd:YAG (λ=266 nm, τ_FWHM≅3 ns, ν=10 Hz) laser source. The effect of the laser processing conditions, as laser fluence value and number of subsequent laser pulses incident onto the same target location, on the surface morphology, crystalline structure, and chemical composition of the TiO₂/GO nanocomposite thin films was systematically investigated. The laser fluence values were maintained below the vaporization threshold of the irradiated composite material. With the increase of the laser fluence and number of incident laser pulses melting and coalescence of the TiO₂ NPs into inter-connected aggregates as well as rippling of the GO platelets take place. The gradual reduction of GO platelets and the onset of anatase to rutile phase transition were observed at high laser fluence values.     

TEM characterization of spark plasma sintered ZrB2–SiC–graphene nanocomposite

The interfacial behavior of spark plasma sintered ZrB₂–SiC nanocomposite doped with graphene nano-platelets was investigated by transmission electron microscopy (TEM). A powder mixture including ZrB₂ matrix, 20?vol% SiC and 10?vol% graphene was used as the starting material. X-ray diffraction analysis did not exhibit any in situ phase formation in the prepared nanocomposite. TEM observations verified the diffusion-controlled sintering. This study clarifies that graphene nano-platelets additive in the prepared nanocomposite did not engage in reactive sintering process, unlike many previous research studies addressing reactive sintering role for carbon additives.     

Influence of hydrogen functionalization on the fracture strength of graphene and the interfacial properties of graphene–polymer nanocomposite

Using molecular dynamics and classical continuum concepts, we investigated the effects of hydrogen functionalization on the fracture strength of graphene and also on the interfacial properties of graphene–polymer nanocomposite. Moreover, we developed an atomistic model to assess the temperature and strain rate dependent fracture strength of functionalized graphene along various chiral directions. Results indicate that hydrogen functionalization at elevated temperatures highly degrade the fracture strength of graphene. The functionalization also deteriorates the interfacial strength of graphene–polymer nanocomposite. Near-crack-tip stress distribution depicted by continuum mechanics can be successfully used to investigate the impact of hydrogen passivation of dangling carbon bonds on the strength of graphene. We further derived a continuum-based model to characterize the non-bonded interaction of graphene–polymer nanocomposite. These results indicate that classical continuum concepts are accurate even at a scale of several nanometers. Our work provides a remarkable insight into the fracture strength of graphene and graphene–polymer nanocomposites, which are critical in designing experimental and instrumental applications.     

Thermal annealing induced cave in and formation of nanoscale pits in Ag–TiO2 plasmonic nanocomposite thin film

Ag–TiO₂ nanocomposite thin films on silica glass were prepared through thermal evaporation in combination with RF magnetron sputtering. Thermal annealing induced changes in the optical, morphological and structural properties of Ag–TiO₂ nanocomposites were examined using optical absorption, photoluminescence spectroscopy, Raman spectroscopy, FESEM, AFM and XRD. FESEM and AFM studies revealed cave in of the Ag–TiO₂ thin film at various places leading to the formation nanoscale pits upon thermal annealing at 600 °C. The computed average size of pits was found to be 54 nm. Raman studies indicated 600 °C annealing induced transformation of anatase phase of TiO₂ into anatase/rutile mixed phase TiO₂. Optical absorption spectra showed systematic changes due to the effects of mixed phase formation and variation in the plasmonic behavior upon annealing. PL results of the as deposited Ag–TiO₂ thin film showed peaks at 377, 402, 432 and 486 nm. PL studies of Ag–TiO₂ nanocomposites treated at different annealing temperatures revealed changes in defect concentration in TiO₂. The tentative mechanism for the creation of nanoscale pits in Ag–TiO₂ thin film through thermal annealing was proposed.     

Reinforcement in melt-processed polymer–graphene composites at extremely low graphene loading level

We have prepared polymer nanocomposites reinforced with exfoliated graphene layers solely via melt blending. For this study polyethylene terephthalate (PET) was chosen as the polymer matrix due to its myriad of current and potential applications. PET and PET/graphene nanocomposites were melt compounded on an internal mixer and the resulting materials were compression molded into films. Transmission electron microscopy and scanning electron microscopy revealed that the graphene flakes were randomly orientated and well dispersed inside the polymer matrix. The PET/graphene nanocomposites were found to be characterized by superior mechanical properties as opposed to the neat PET. Thus, at a nanofiller load as low as 0.07 wt%, the novel materials presented an increase in the elastic modulus higher than 10% and an enhancement in the tensile strength of more than 40% compared to pristine PET. The improvements in the tensile strength were directly correlated to changes in elongation at break and indirectly correlated to the fracture initiation area. The enhancements observed in the mechanical properties of polymer/graphene nanocomposites achieved at low exfoliated graphene loadings and manufactured exclusively via melt mixing may open the door to industrial manufacturing of economical novel materials with superior stiffness, strength and ductility.     

Enhanced properties of poly(styrene–b–ethylene–co–butylene–b–styrene) nanocomposites with in situ construction of interconnected graphene network

In this work, such elastomeric nanocomposites were fabricated with graphene (GE) sheets selectively distributing between polymer matrices and forming three-dimensional networks. The solvent evaporation process was first introduced to produce poly(styrene–ethylene–co–butadiene–b–styrene) (SEBS) microspheres and then reduced GE oxide attached to the surface of SEBS microspheres via electrostatic interaction and sonication-assisted reduction. The microstructure of nanocomposites, prepared by compression molding using SEBS/GE microspheres, was investigated using scanning electron microscopy and transmission electron microscopy. The results showed that interconnected GE networks formed in heat-pressing composite and was destroyed after twin-roll mixing. The SEBS/GE nanocomposites showed enhanced electrical, thermal, and mechanical properties. The electrical resistivity of nanocomposites obtained via heat-pressing reached to 1.1 × 10³ Ω m at a 2.5 wt % (1.07 vol %) content of GE. The thermal and mechanical properties were also characterized. It was found that the initial degradation temperature increased by nearly 40 °C and the mechanical properties continued to rise with GE content below 0.5 wt %. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47118.     

Decoration of graphene network with metal–organic frameworks for enhanced electrochemical capacitive behavior

Well-intergrown nanocrystals of zeolitic imidazolate frameworks (ZIF-8) supported on three-dimensional (3D) graphene were prepared by a counter diffusion technique. The incorporation of ZIF-8 crystals greatly improves the surface areas of the graphene composites. The carbonized graphene–ZIF composites with hierarchical pore structures showed high electrochemical capacitance and good stability. This work provides an efficient method to synthesize porous carbon materials with high capacitance.     

In situ sonochemical synthesis of Fe3O4–graphene nanocomposite for lithium rechargeable batteries

In this paper, we have presented experimental results for preparation of Fe₃O₄–graphene nanocomposite that uses an ultrasound assisted method. The graphene oxide (GO) was prepared from graphite powder using modified Hummers–Offeman method. Subsequently, the synthesis of graphene-Fe₃O₄ nanocomposite was carried out by ultrasound assisted co-precipitation of iron (II) and (III) chlorides in the presence of GO. The formation of GO and graphene-Fe₃O₄ nanocomposite was confirmed by X-ray diffraction (XRD), Energy dispersive X-ray (EDX) analysis and Fourier transform-infrared (FTIR) analysis. The particle size of Fe₃O₄ nanoparticles loaded on graphene nanosheets (observed from TEM image) was found to be smaller than 20 nm. The use of ultrasonic irradiations during synthesis of graphene-Fe₃O₄ nanocomposite resulted in uniform loading of Fe₃O₄ nanoparticles on graphene nanosheets. The prepared graphene-Fe₃O₄ nanocomposite material was used for the preparation of anode for lithium ion batteries. The electrochemical performance of the material was tested by cyclic voltammetry (CV) and charge/discharge cycles. It was observed that the capacity of Li-battery when the anode material was made using graphene-Fe₃O₄ nanocomposite showed stable electrochemical performance for around 120 cycles and the battery could repeat stable charge–discharge reaction.     

The formation of graphene–titania hybrid films and their resistance change under ultraviolet irradiation

We report the fabrication of hybrid films of graphene and monolayer titania using a simple electrostatic self-assembly method. Ultraviolet (UV) responses of the hybrid films based on the graphene–titania structure were investigated. We observed that the resistance of the graphene–titania hybrid increased exponentially with UV irradiation time and decreased exponentially when UV was turned off. Time constants of the order of hundreds of seconds were identified and found to be sensitive to the gas environment of graphene. The UV response as well as the time constant is tunable by varying the number of titania layers. Our results confirmed that UV irradiation played a significant role in the resistance modulation of graphene as well as graphene–titania hybrid films.     

Platinum–polymer–clay nanocomposite hydrogels via exfoliated clay-mediated in situ reduction

Pt nanoparticles (Pt NPs) are currently used in many areas of nanoscience and technology. Numerous studies have been reported on the design of Pt and Pt-based nanomaterials with different sizes, shapes, and compositions. Here, we report the synthesis, structure, and properties of a novel hydrogel-based nanostructured Pt material, Pt-NC gel, consisting of ultrafine Pt NPs strongly immobilized within a unique polymer−clay network. Pt-NC gels were synthesized through exfoliated clay-mediated in situ reduction of Pt ions in the NC gel at ambient temperature. Pt NPs were trapped on the clay surface, probably at the edges of the clay nanoplatelets. Ultrafine Pt NPs were also obtained as a stable suspension from the NC gel, without any stabilizing agents. The combination of ultrafine Pt NPs and mechanically tough NC gel may open up new possibilities for designing functional Pt-gel materials.     

Voltammetric determination of alkannin using an Au nanoparticles–poly(diallyldimethylammonium chloride)-functionalized graphene nanocomposite film

An electrochemical sensor based on Au nanoparticles (AuNPs)–poly(diallyldimethylammonium chloride) (PDDA)-functionalized graphene (AuNPs–PDDA-G) nanocomposite was fabricated for the sensitive detection of alkannin. The nanocomposite was characterized by X-ray diffraction, ultraviolet/visible spectra, scanning electron microscopy, and transmission electron microscopy. Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical behaviors of alkannin on the AuNPs–PDDA-G nanocomposite film-modified glassy carbon electrode. This electrochemical sensor displayed satisfactory analytical performance for alkannin detection over a range from 5.0 nmol L^?1 to 3.0 μmol L^?1 with a detection limit of 1.4 nmol L^?1 (S/N = 3). Moreover, the sensor also exhibited good reproducibility and stability, and could be used for the detection of alkannin in real samples with satisfactory results.     

Synthesis of exfoliated polyaniline–clay nanocomposite in supercritical CO2

This paper presents the works done to synthesize fully exfoliated polyaniline–clay nanocomposites (PCNs) with high purity via in situ polymerization of aniline in Cloisite 30B nano-clay suspension in supercritical CO₂ (ScCO₂) medium. The Cloisite 30B was first delaminated with ScCO₂ treatment in the presence of aniline monomers. Ammonium peroxydisulfate (APS) solution was added rapidly into the mixture of delaminated Cloisite 30B and aniline monomers to produce PCNs. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), UV–vis spectroscopy and XRD analysis have been used to characterize the morphology and structure of the as-synthesized product. SEM results reveal that nano-clays are fully exfoliated in the final nanocomposite which is synthesized in ScCO₂. FTIR and UV–vis analysis showed that the resulted polyaniline (PANI) had been in highly conductive emeraldine salt state and ScCO₂ does not have any effect on chemical structures of the PANI.     

Observation of the semiconductor–metal transition behavior in monolayer graphene

We have observed that during temperature-dependent four-terminal resistance measurement of monolayer graphene, the resistance exhibits anomalous rising and falling behavior at different temperature regions. At lower temperature region (2–200 K) the resistance decreases gradually, but when the temperature rise further it turn to a sudden increase, and after 280 K it resumes gradual decrease. The rising and falling resistance behavior is characteristic of semiconductor or metal property. Consequently, the resistance transition follows a phase of semiconductor–metal–semiconductor. However, when a perpendicular magnetic field is applied, the resistance shows reverse transition behavior which follows a sequence of metal–semiconductor–metal. The novel transition property is attributed to the competition between the disorder of lattice defects as a short-range scattering in monolayer graphene and the Landau levels interaction. Magneto-transport measurement reveals that the excitonic gap induced by magnetic field in the monolayer graphene show an anomalous thermally activated property.     

Strong spin–orbit splitting in graphene with adsorbed Au atoms

Spin–orbit (SO) splitting in graphene with adsorbed Au atoms is investigated by using a first-principles method. Considerable (～200 meV) Rashba-type SO splitting can be achieved in the graphene π bands. When a Au atom is adsorbed above a C–C bond, Dresselhaus-type SO splitting is found to be present due to the absence of inversion symmetry and the substantial contribution of Au 5d_xz components. The influence of strains in graphene on SO splitting is also explored. A slight strain with the strength of ?5% to 5% usually does not change much the SO splitting. The variation of SO splitting versus strain strength is rationalized through structural relaxation and effective hybridization between C 2p_z and certain Au 5d states. Our study predicts a new way to increase the SO splitting in graphene and provides useful understanding of the mechanism.     

Ignition and phase formation in the Zr–Al–C system

The ignition mechanism and the dependence of the composition of the products of combustion or thermal explosion in a mixture of 2Zr + Al + C on the initiation temperature and heat transfer conditions were studied. Heat transfer conditions were changed by varying the size of the samples and the gaseous medium in which the experiments were performed. Two contusion regimes were found: a low-temperature regime, in which zirconium aluminides formed and carbon and part of the zirconium remained unreacted, and a high-temperature regime in which the reaction products were zirconium carbide and aluminide. Upon re-initiation, the low-temperature combustion products reacted in the high-temperature combustion regime. The observed dependences are due to parallel reactions in the three-component system.     

UV light exposure of aqueous graphene oxide suspensions to promote their direct reduction,formation of graphene–metal nanoparticle hybrids and dye degradation

The use of UV light to trigger different processes involving graphene oxide sheets suspended in aqueous medium at room temperature has been investigated. These processes include (1) deoxygenation of the sheets in the absence of photocatalysts, reducing agents and stabilizers, (2) selective nucleation and growth of metal nanoparticles on the sheets to yield graphene-based hybrids and (3) decomposition of the dye molecule rhodamine B in the presence of only graphene oxide. Photoinduced heating of the suspended graphene oxide sheets by intense UV irradiation (～1 W cm^?2 delivered at the surface of the dispersion) was interpreted to generate at high temperature and reactive environment strictly localized at the sheets and their immediate aqueous medium, which in turn brings about the mentioned processes. In addition to providing a simple route toward reduction of graphene oxide dispersions, the present results suggest that intense UV light can be used to promote reactions at ambient conditions with this material that would otherwise require high temperatures, chemical reactants and/or catalysts.