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.     

Self-assembly of silver–graphene hybrid on electrospun polyurethane nanofibers as flexible transparent conductive thin films

A method of integrating hybrid thin films of graphene nanosheets (GNSs) and silver nanoparticles (AgNps) by in situ chemical reduction to prepare transparent conductive films (TCFs) is studied. The surface functional groups of graphite oxide (GO) serve as nucleation sites of silver ions for adsorption of AgNps. To fabricate conductive films with high transmittance, polyurethane (PU) nanofibers are introduced to help construct two-dimensional conductive networks consisting of AgNps and GNSs (AgNps–GNSs). This method requires only a low percentage of conducting AgNps–GNSs covering the transparent substrate, thereby improving the transmittance. The flexible GNSs serve as nanoscale bridges between conductive AgNps and PU nanofibers, resulting in a highly flexible TCF. The optical transmittance can be further increased after melting the PU nanofibers at 100 °C. A fused film obtained after electrospinning (ES) a PU solution for 120 s and immersion in 0.05 wt.% AgNp–GNS (5:1) solution has a surface resistance of 150 Ω/sq and 85% light transmittance. Mechanical testing shows that AgNps–GNSs on flexible substrates yield excellent robustness. Thus, TCFs with a 3:1 ratio of AgNps:GNSs have high conductivity, good mechanical durability, and barely one order of magnitude increase of surface resistance when bent to an angle of 90°.     

Internal stress in multilayer silver–bismuth coatings

An investigation was carried out on the influence of electrodeposition conditions on internal stress of cyclically modulated Ag–Bi alloys. Pulse currents of different duration and height were used, leading to different composition and thickness of the deposited sublayers, and the resulting changes in the internal stress were measured in situ. The investigated multilayer coatings with Bi contents up to 20 wt.% reveal a positive (tensile) internal stress, and at Bi contents higher than 20 wt.% the internal stress is negative (compressive). The possibility of deposition of stress-free coatings is shown.     

Easy synthesis of nitrogen-doped graphene–silver nanoparticle hybrids by thermal treatment of graphite oxide with glycine and silver nitrate

Nitrogen-doped graphene–silver nanoparticle hybrids were prepared by thermal treatment of graphite oxide (GO) with glycine and silver nitrate at 500 °C. Glycine was used to reduce the nitrate ions, resulting in the decomposition of a glycine–nitrate mixture near 200 °C. The products of decomposition act as sources for nitrogen doping. The thermal treatment of a mixture of GO, glycine and silver nitrate results in the formation of silver nanoparticles at 100 °C, promotes the reduction of GO near 200 °C, and generates pyrrolic and pyridinic type nitrogen doping in graphene at 300 and 500 °C, respectively. The atomic percentage of nitrogen in as-prepared sample is about 13.5%. This approach opens up a new possibility for the synthesis of nitrogen-doped graphene decorated with various metallic nanoparticles, which could find important applications in the fields of energy storage and conversion devices.     

Synthesis of reduced graphene oxide–Fe3O4 multifunctional freestanding membranes and their temperature dependent electronic transport properties

A simple and scalable method for the synthesis of reduced graphene oxide (RGO) based conductive and magnetic multifunctional films (membranes) is reported. A RGO–iron oxide (Fe₃O₄) freestanding film is fabricated using a versatile chemical route followed by vacuum infiltration. Temperature dependent electronic transport properties of the magnetic GO and RGO films were measured using a four probe technique from room temperature to 15 K. A conduction mechanism based on variable range hopping is suggested for explaining of the electronic conductivity variations. Possible applications of this multifunctional membrane are also discussed.     

Bio-inspired composite films with integrative properties based on the self-assembly of gellan gum–graphene oxide crosslinked nanohybrid building blocks

Mimicking natural structures to synthesize novel structural materials is attracting considerable attention, but progress in practical applications remains slow. Natural composites achieve excellent balance between strength and toughness from the “brick-and-mortar” arrangement of organic and inorganic layers, accompanied with various toughening mechanisms. We emulate the structural features of natural nacre by combining graphene oxide (GO) and a gellan gum (GG) biopolymer. We also reveal the mechanism of integrative mechanical performance. Several GO nanosheets with GG coating and crosslinking are used as optimal building blocks with intrinsic hard/soft features. These materials are induced to rapidly self-assemble into aligned nacre-like films by vacuum filtration to produce strong and tough bio-inspired composite films with fracture strength of 88.7 MPa, fracture stain of 0.84%, tensile modulus of 25.4 GPa and good biocompatibility. This study has merit of unrestricted fabrication of a homogeneous colloidal suspension of crosslinked nanohybrid building blocks of GO. Composite films constructed using these building blocks are innovative because of combined interactions, including coordination bonding, ionic bonding, and hydrogen bonding among their constituents. These films differ from other reported bio-inspired GO composite films with single adhesion interaction and thus provide good integrative mechanical performance through a multiple energy dissipation mechanism.     

Process-dependent conductivity and film homogeneity of slot-die-coated PEDOT:PSS–PVA composite films

In this study, we investigate the impact of process parameters on homogeneity and electrical conductivity of slot-die-coated PEDOT:PSS–PVA composite films that are doped with DMSO. Due to a strong correlation between conductivity and morphology of PEDOT:PSS films and the latter’s dependency on the processing step itself, we apply slot die coating for maximized process control and systematically evaluate the impact of coating gap, speed, and film thickness. Since the entire coating and drying process is run in batch mode, the setup is optimized regarding steady-state conditions and high homogeneity of the films. Overall, for the films manufactured in batch mode, we obtain a reproducibility film thickness of 99% and a low deviation from the set film thickness (below 8%). In order to analyze the impact of the coating parameters, stable operating points derived from the viscocapillary model are chosen and either the dimensionless gap or the capillary number is varied. Coating gap and film thickness emerged as dominating parameters, leading to an increase in conductivity of 40% and 70%, respectively, or, when changing both simultaneously, of 157%. Only a minor impact of shear forces (increase of 10%) was found.     

Catalyst-free,self-assembly,and controllable synthesis of graphene flake–carbon nanotube composites for high-performance field emission

Catalyst-free and self-assembled growth of graphene flakes (GFs) on carbon nanotube (CNT) arrays have been realized by using microwave plasma enhanced chemical vapor deposition. The shape of GFs was highly manipulated by adjusting the growth time, C concentration, and microwave power. We qualitatively discussed the nucleation and growth mechanism of GFs based on the growth parameter–GF shape studies. The field emission (FE) properties of graphene flake–carbon nanotube (GF–CNT) composites for different GF shapes were measured and found to be strongly influenced by the GF distribution. The optimal shape of GFs for FE had small scales, sharp edges, and sparse distribution on CNTs. The best FE properties with the optimal shape were observed with a low turn-on electric field of 0.73 V/μm and excellent stability, which are superior to those of the as-grown CNT arrays and GF–CNT composites covered by densely distributed GFs. We consider that the large aspect ratio of CNTs and the unique FE stability of GFs play a synergetic effect on the improved FE properties.     

Thermal conductivity of a graphene oxide–carbon nanotube hybrid/epoxy composite

Thermally conductive graphene oxide (GO)–multi-wall carbon nanotube (MWCNT)/epoxy composite materials were fabricated by epoxy wetting. The polar functionality on the GO surface allowed the permeation of the epoxy resin due to a secondary interaction between them, which allowed the fabrication of a composite containing a high concentration of this hybrid filler. The thermal transport properties of the composites were maximized at 50 wt.% of filler due to fixed pore volume fraction in filtrated GO cake. When the total amount of filler was fixed 50 wt.% while changing the amount of MWCNTs, a maximum thermal conductivity was obtained with the addition of about 0.36 wt.% of MWCNTs in the filler. Measured thermal conductivity was higher than the predicted value based on the by Maxwell–Garnett (M–G) approximation and decreased for MWCNT concentrations above 0.4%. The increased thermal conductivity was due to the formation of 3-D heat conduction paths by the addition of MWCNTs. Too high a MWCNT concentration led to increased phonon scattering, which in turn led to decreased thermal conductivity. The measured storage modulus was higher than that of the solvent mixed composite because of the insufficient interface between the large amount of filler and the epoxy.     

Novel synthesis of CuO nanofiber balls and films and their UV–visible light filteration property

Self-assembled cupric oxide (CuO) nanofiber balls and films were synthesized via a facile solvothermal route directly from cupric acetate monohydrate (Cu(CH₃COO)₂·H₂O) in water and ethanol without any chemical additions or high temperature treatment. The CuO balls with size of 150 nm–1.5 µm had rough surfaces which consisted of lots of about 10 nm nanofibers in diameter. The sizes of CuO balls were controllable by changing reaction time and volume ratio of water to ethanol. CuO nanofiber films were prepared with the aid of the in situ hydrolysis of Cu(CH₃COO)₂·H₂O coating layer on a substrate at 60 °C. CuO films showed excellent UV–visible light filteration property and could be used as a potential candidate of UV–visible light filter. Compared with traditional method to fabricate CuO films, neither precursor nor Cu substrate was needed in this study. This technique could be used to produce CuO films without being confined to our template and to produce CuO powders in large scale with low cost.     

Effect of number of graphene layers on mechanical and dielectric properties of graphene–epoxy nanocomposites

In this study, mechanical and dielectric properties of epoxy nanocomposites with two types of graphene, <?10 layer stacks (GEC10) and <?30 layer stacks (GEC30) were investigated. Results showed that the number of graphene layers remarkably affected the dielectric properties of epoxy nanocomposites. The real and imaginary parts of relative permittivity and loss tangent of GEC10 samples were noticeably enhanced and reached to 1.29, 20 and 15.6 times respectively for 1?wt-% graphene sample compared to GEC30 samples. Meanwhile, tensile tests showed a peak for tensile strength of GEC10 and GEC30 samples with 0.1?wt-% graphene, which improved by 13 and 7.9% with respect to pure epoxy respectively. In addition, flexural properties did not change significantly compared to the pure epoxy.     

The synthesis of novel perylene-like dyes and their aggregation properties in Langmuir and Langmuir–Blodgett films

The synthesis of novel perylene-like dyes with different length and molecular structure of terminal chains substituted to the main perylene core is described. The dyes are able to form compressible and stable monolayers at the air–water interface (Langmuir films), which can be easily transferred onto solid substrates (Langmuir–Blodgett films). In the Langmuir and Langmuir–Blodgett films, the dye molecules show tendency to creation of self-aggregates, both in the ground and excited electronic states. The influence of the molecular structure of the substituents on the aggregation properties is observed and discussed.     

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.     

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.     

A molecular dynamics study of the thermal conductivity of graphene nanoribbons containing dispersed Stone–Thrower–Wales defects

Classical molecular dynamics with the AIREBO potential is used to investigate the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing different densities of Stone–Thrower–Wales (STW) defects. Our results indicate that the presence of the defects can decrease thermal conductivity by more than 50%. The larger the defect density, the lower the conductivity, with the decrease significantly higher in zigzag than in armchair nanoribbons for all defect densities. The effect of STW defects in the temperature range 100–600 K was also determined. Our results showed the same trends in thermal conductivity decreases at all temperatures. However, for higher defect densities there was less variation in thermal conductivity at different temperatures.     

The anodic behavior of silver in chloride solutions—I. The formation and reduction of thin silver chloride films

The initial stages of growth and the subsequent reduction of AgCl films at a silver electrode in aqueous chloride solutions has been investigated using potentiodynamic and potentiostatic methods. The first step in the growth process involves the two-dimentional deposition of either a partial monolayer (50–75 μC cm⁻²) of AgCl film or the adsorption of chloride ions involving a partial charge transfer. Following this, at an overpotential of typically 20–40 mV, the nucleation and growth of three-dimensional AgCl films commences. Reduction of this type of AgCl film is characterized by two cathodic peaks, possibly depicting the reduction of AgCl nuclei formed by two nucleation mechanisms. Continuous potential cycling results in significant increases in the nucleation currents and charges (not due to conventional electrode roughening), indicating that active silver nuclei are being left on the electrode surface upon AgCl film reduction. However, by maintaining the potential at a value significantly negative of AgCl film growth for a period of time, the currents due to the three-dimensional nucleation and growth process diminish in magnitude. This is evidence for an active reconstruction of the silver surface leading to the loss of these silver nuclei.     

Facile preparation of three-dimensional porous Pd–Au films and their electrocatalytic activity for methanol oxidation

Pd–Au porous foam films with three-dimensional hierarchical pores consisting of interconnected dendrite walls are obtained by using the hydrogen bubble dynamic template. The films are characterized by scanning electron microscope, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, as well as electrochemical techniques. The CV curves have confirmed that the Pd–Au catalysts have high catalytic activity toward methanol oxidation. The catalytic activities of these Pd–Au catalysts are strongly dependent on the atomic ratio of Pd/Au. Among all the Pd–Au catalysts, the Pd₁Au₁ catalyst is found to possess superior catalytic activity and stability toward methanol oxidation.     

Synthesis of graphene oxide-intercalated α-hydroxides by metathesis and their decomposition to graphene/metal oxide composites

Graphene oxide-intercalated α-metal hydroxides were prepared using layers from the delaminated colloidal dispersions of cetyltrimethylammonium-intercalated graphene oxide and dodecylsulfate-intercalated α-hydroxide of nickel/cobalt as precursors. The reaction of the two dispersions leads to de-intercalation of the interlayer ions from both the layered solids and the intercalation of the negatively charged graphene oxide sheets between the positively charged layers of the α-hydroxide. Thermal decomposition of the intercalated solids yields graphene/nanocrystalline metal oxide composites. Electron microscopy analysis of the composites indicates that the nanoparticles are intercalated between graphene layers.