Direct Deposition of Graphene Nanomaterial Films on Polymer-Coated Glass by Ultrasonic Spraying

Modesto-López, L. B., Miettinen, M., Torvela, T., Lähde, A., & Jokiniemi, J. (2015). Direct deposition of graphene nanomaterial films on polymer-coated glass by ultrasonic spraying. Thin Solid Films, 578, 45-52.

Abstract- In this work, suspensions of two types of graphene nanomaterials — graphene nanoflowers (GNFs) and multilayer graphene (MLG) flakes — were ultrasonically sprayed into micrometer-sized droplets to directly form films on glass and methacrylic acid copolymer (MA)-modified glass substrates. Poly(vinylidene fluoride) (PVDF) was added to the suspensions to enhance the structural robustness of films. While PVDF enhanced nanomaterial binding, the MA coating boosted nanomaterial adhesion to the substrate. The films' morphology, adhesion, and electrical properties were investigated. MLG flakes produced highly irregular films, conversely GNFs formed thinner and more uniform layers. In the case of MLG flakes, films on MA-coated glass showed lower resistances compared to those on glass. GNF suspensions sprayed on MA-coated glass resulted in graphene nanostructures embedded in a predominantly dielectric polymer matrix. Thus, in the case of GNFs, films on glass exhibited lower resistances compared to those on MA-coated glass. The approach described herein can be readily extended to create films of other graphene-based materials on a variety of substrates.

Keywords: Graphene nanoflowers; Multi-layer graphene; Poly(vinylidene fluoride); Ultrasonic spray; Adhesion

Films of Graphene Nanomaterials Formed by Ultrasonic Spraying of Their Stable Suspensions

Modesto-López, L. B., Miettinen, M., Riikonen, J., Torvela, T., Pfüller, C., Lehto, V. P., ... & Jokiniemi, J. (2015). Films of graphene nanomaterials formed by ultrasonic spraying of their stable suspensions. Aerosol Science and Technology, 49(1), 45-56.

Abstract- Graphene, a two-dimensional carbon allotrope, exhibits excellent optoelectronic properties. The assembly of graphene into films provides a platform to deepen the study of its interaction with varying surfaces, to engineer devices, and to develop functional materials. A general approach to produce graphene films consists of preparing a dispersion and laying it on a substrate of choice, followed by solvent evaporation. Here, we report the preparation of stable suspensions of new types of graphene nanomaterials namely, graphene nanoflowers (GNFs) and multi-layer graphene (MLG) flakes, in ethanol,N,N-dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP). Sprayable suspensions of both GNFs and MLG were prepared in DMF/ethanol, which showed high stability, without addition of any surfactant. The stable suspensions were used to deposit micrometer-thick MLG/GNF films on glass substrates. Calculations of initial droplet size and of timescale of droplet evaporation are performed and possible thermophoretic effects on droplet deposition discussed as well. Coating glass substrates with a methacrylic acid–methyl methacrylate (MA) copolymer prior to the deposition significantly improved the adhesion of the nanomaterials to the substrate. With the MA coating, a substrate coverage of nearly 100% was achieved at 14-min spraying time for 0.05 wt% GNF and 0.1 wt% MLG suspensions. Raman spectra of the GNF and MLG films reveal that the films were made of MLG in which the individual graphene layers rotated from each other as in turbostratic graphene. This work provides a general approach to prepare graphene nanomaterial suspensions and to create films for a variety of applications. The spraying process applied in the current work is highly scalable and allows control of film characteristics through process parameters.

Photocatalytic Activities of Heterostructured TiO2-Graphene Porous Microspheres Prepared by Ultrasonic Spray Pyrolysis

Yang, J., Zhang, X., Li, B., Liu, H., Sun, P., Wang, C., ... & Liu, Y. (2014). Photocatalytic activities of heterostructured TiO 2-graphene porous microspheres prepared by ultrasonic spray pyrolysis. Journal of Alloys and Compounds, 584, 180-184.

Abstract- TiO2-graphene porous microspheres were prepared by ultrasonic spray pyrolysis (USP) of aqueous suspension of graphene oxide containing TiO2 nanoparticles (Degussa P25). The composite microspheres were characterized with SEM, XPS, photoluminescence, Raman and UV–Vis absorption spectra. TiO2-graphene porous microspheres displayed higher photocatalytic activity for the degradation of methylene blue solution than pristine TiO2 microspheres under the irradiation of Xe lamp, and the highest activity was obtained at a weight percentage of graphene around 1%. The effect of graphene on photocatalytic activity of porous microsphere was discussed in terms of the enhanced charge separation by TiO2-graphene heterojunction, increased absorption of the visible light, as well as the possible hindrance of mass transportation in microspheres.

Keywords: Porous microsphere; TiO2; Graphene; Photocatalysis; Ultrasonic spray pyrolysis

Temperature Dependent Photoluminescence Study on Zno/Graphene Nanocomposite Films

Lee, E., Kim, J. Y., Park, Y. I., & An, S. J. (2015). Temperature dependent photoluminescence study on ZnO/Graphene nanocomposite films. Current Applied Physics, 15(4), 563-566.

Abstract- We report observation of both free and defect-mediated excitonic emissions from temperature-dependent PL study on ZnO/graphene oxide (G-O) nanocomposite grown by ultrasonic assisted spray pyrolysis (UASP). From the temperature-dependent photoluminescence (PL) spectra of the ZnO/G-O nanocomposite, new graphene-related peak was observed at 372 nm along with the exciton transition bound to neutral acceptors or deep donors. The PL intensity of new graphene-related peaks (3.33 eV) become more prominent with increasing G-O concentration, and it was saturated or decreased with the addition of >7.0 wt% of G-O. This feature indicates that new graphene-related states were created below conduction band of ZnO, which supports the excitonic PL enhancement by graphene-embedding is contributed not by charge transfer, but by vacancy filling effect of G-O.

Keywords: Graphene; ZnO; Vapour synthesis; Photoluminescence

Time To Failure Modeling of Silver Nanowire Transparent Conducting Electrodes And Effects of a Reduced Graphene Oxide Over Layer

Kwan, Y. C. G., Le, Q. L., & Huan, C. H. A. (2016). Time to failure modeling of silver nanowire transparent conducting electrodes and effects of a reduced graphene oxide over layer. Solar Energy Materials and Solar Cells, 144, 102-108.

Abstract- Transparent conducting electrodes (TCE) with increased robustness and stability were produced from a composite of silver nanowires (AgNW) and reduced graphene oxide (GO) using ultrasonic spray coating. Ultrasonic spray coating is suited for large area depositions and thin films of AgNW and reduced GO TCEs with consistent optoelectronic properties were produced using this method and the reduction of GO was kept environmentally friendly by using ascorbic acid (AA) as the reducing agent. TCEs produced in this manner were highly conductive and had an average sheet resistance of 5.3 Ω/□ and an average transmittance of 64.9% ― the best TCE produced in this manner had a figure of merit of 190. Here, AgNWs were protected from humidity induced degradation and the effect of capillary instability at elevated temperatures on the nanowires was retarded by the introduction of a GO–AA over layer. A τ value of 1.5×10−9was found to signify the onset of capillary instability induced failure in the AgNW TCE and from experimentally supported calculations, it was predicted that the lifetimes of nanowire TCEs under thermal degradation could be extended significantly by keeping the gaseous environment oxygen free.

Keywords: Silver nanowire; Graphene oxide; Transparent conducting electrode; Spray coating; Green process; Time to failure modeling