ARTICLE pubs.acs.org/Langmuir
Ethanol-Assisted Graphene Oxide-Based Thin Film Formation at Pentane Water Interface Fuming Chen, Shaobin Liu, Jianmin Shen, Li Wei, Andong Liu, Mary B. Chan-Park, and Yuan Chen* School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore
bS Supporting Information ABSTRACT: Graphene oxide (GO) can be viewed as an amphiphilic soft material, which form thin films at organic solvent water interfaces. However, organic solvent evaporation provides little driving force, which results in slow GO transfer in aqueous phase, thus dawdling GO film formation processes for various potential applications. We present an ethanol-assisted self-assembly method for the quick formation of GO or GO-based composite thin films with tunable composition, transmittance, and surface resistivity at pentane water interface. The thickness of pure GO and reduced GO (rGO) films ranging from ∼1 nm to more than 10 nm can be controlled by the concentration of GO in bulk solution. The transmittance of rGO films can be tuned from 72% to 97% at 550 nm while the surface resistivity changes from 8.3 to 464.6 kΩ sq 1. Ethanol is essential for achieving quick formation of GO thin films. When ethanol is injected into GO aqueous dispersion, it serves as a nonsolvent, compromising the stability of GO and providing driving force to allow GO sheets aggregate at the water pentane interface. On the other hand, neither the evaporation of pentane nor the mixing between ethanol and water provides sufficient driving forces to allow noteworthy amount of GO sheets to migrate from the bulk aqueous phase to the interface. This method can also be extended to prepare GO-based composites thin films with tunable composition, such as GO/ single walled carbon nanotube (SWCNT) composite thin films investigated in this work. Reduced GO/SWCNT composite films show much lower surface resistivity compared to pure rGO thin films. This ethanol-assisted self-assembly method opens opportunities to design and fabricate new functional GO-based hybrid materials for various potential applications.
’ INTRODUCTION Graphene is a two-dimensional sheet of sp2-hydridized carbon,1 which has attracted great interests because of its exceptional properties, such as large specific surface area, high intrinsic mobility,2,3 and high optical transmittance (∼97.7% for a single layer).4,5 Potential applications of graphene require the production of pure graphene or graphene-based composite films.6 8 Direct synthesis of graphene by chemical vapor deposition9 13 have produced films with 200 Ω/S sheet resistance at 85% optical transmittance,10 and the size of graphene film can reach 30 in.14 An alternative approach for forming graphene films is using chemically derived graphene oxide (GO).15 GO films are first produced using water-soluble GO dispersions, and then they are reduced to produce reduced GO (rGO) films.16 18 The GO thin-film preparation methods include drop-casting, spincoating,19,20 dip-coating,21 spraying,22,23 vacuum filtration,24,25 spin-assisted assembly, Langmuir Blodgett (LB) assembly,26,27 and other self-assembly processes at liquid air28 31 or liquid liquid interfaces.27,32,33 The most exciting advantages of these GO-based graphene thin-film formation methods are their low cost and massive scalability.34 Among these GO thin-film preparation methods, some, such as drop-casting, spin-coating, and dip-coating, would result in multilayer aggregates and crumpled sheets because of the uncontrolled flow and dewetting during solvent evaporation, which force the soft GO sheets to fold and wrinkle.27,34 Developing and understanding the r 2011 American Chemical Society
GO assembly method with improved controllability is necessary for their potential applications. Because the GO sheet can be viewed as an amphiphilic soft material, Huang et al. demonstrated that GO films can form at the chloroform water interface.27,35 The strong electrostatic repulsion prevented GO sheets from overlapping when compressed at interfaces, which is favorable for fabricating large-area, flat GO films.33 However, chloroform evaporation provides little driving force, which results in slow GO transfer in water phase, thus a dawdling GO film formation process. To speed up GO migration, gas bubbles (nitrogen or solvated CO2) were used to catch GO sheets and lift them up to water surface.27,31 From the perspective of developing large-scale assembly methods, creating controllable upward convection flows is essential to achieve fast formation of GO films. Several previous studies have shown some solvents could serve as nonsolvent in aqueous media, which helps to form 2D films comprised of carbon nanotubes and nanoparticles.36 38 Here, we present an ethanol-assisted self-assembly method, which allow the fast formation of GO or GO-based composite thin films with tunable composition, transmittance, and surface resistivity at the pentane water interface. Process parameters, Received: April 4, 2011 Revised: June 1, 2011 Published: June 29, 2011 9174
dx.doi.org/10.1021/la201230k | Langmuir 2011, 27, 9174–9181
Langmuir
ARTICLE
Figure 1. Schematic illustration of the standard GO thin film preparation procedure used in this work.
such as GO concentration in bulk solution and volume of pentane and ethanol, which could influence the thin film formation, were examined. The role of ethanol as nonsolvent is identified. The method was also extended to produce GO/singlewalled carbon nanotube (SWCNT) composite films. The transmittance and surface resistivity of these films were compared.
’ EXPERIMENTAL SECTION GO powder was synthesized by a modified Hummers method from graphite powder (Aldrich, synthetic,
