ARTICLE pubs.acs.org/Langmuir
Single-Layered Graphene Oxide Nanosheet/Polyaniline Hybrids Fabricated Through Direct Molecular Exfoliation Guan-Liang Chen,† Shi-Min Shau,† Tzong-Yuan Juang,*,‡ Rong-Ho Lee,*,† Chih-Ping Chen,§ Shing-Yi Suen,† and Ru-Jong Jeng*,|| †
Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan § Materials and Chemical Laboratories, Industrial Technology Research Institute, Hsinchu, 310, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
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bS Supporting Information ABSTRACT: In this study, we used direct molecular exfoliation for the rapid, facile, large-scale fabrication of single-layered graphene oxide nanosheets (GOSs). Using macromolecular polyaniline (PANI) as a layered space enlarger, we readily and rapidly synthesized individual GOSs at room temperature through the in situ polymerization of aniline on the 2D GOS platform. The chemically modified GOS platelets formed unique 2D-layered GOS/PANI hybrids, with the PANI nanorods embedded between the GO interlayers and extended over the GO surface. X-ray diffraction revealed that intergallery expansion occurred in the GO basal spacing after the PANI nanorods had anchored and grown onto the surface of the GO layer. Transparent folding GOSs were, therefore, observed in transmission electron microscopy images. GOS/PANI nanohybrids possessing high conductivities and large work functions have the potential for application as electrode materials in optoelectronic devices. Our dispersion/exfoliation methodology is a facile means of preparing individual GOS platelets with high throughput, potentially expanding the applicability of nanographene oxide materials.
’ INTRODUCTION Much effort has been exerted recently in the high-yield production of individual graphene nanosheets (GSs)—2D atomthick platelets of sp2-hybridized carbon atoms—and in understanding the exfoliation mechanism from 3D stacked graphite organized in multilayer honeycomb carbon networks.14 Several techniques have been used for the production of single- or fewlayered GSs, including the micromechanical cleavage of graphite,5 chemical vapor deposition, epitaxial growth, super acid dissolution,6 arc discharging, thermal exfoliation, the chemical exfoliation of graphite oxide (GO),79 and hydrogen exfoliation under mild conditions.10 Nevertheless, a simple method for the largescale direct exfoliation of layered graphite into individual 2D single-layered platelets has remained a challenge.3,1113 The most promising methods for the rapid, economical, and efficient large-scale production of high-quality layered GS are based on the reduction of GO with simple deionized water.14 Chemical methods for the exfoliation and stabilization of individual layered GS in solution are bottom-up approaches with the potential for high throughput and high yields. Generally, layered GO derived from oxidized graphite can be considered to be stacks of 2D sheets. In the layered GO sheets, the oxygen atoms are bound to the carbon atoms mainly in the form of hydroxyl and epoxy functional groups but also with some carbonyl functionalities. Because of the attractive interactions that organize the layered GO sheets into well-structure r 2011 American Chemical Society
multilayer stacks, with many polar intermolecular interactions, it can be difficult to cleave the sheets. In earlier studies, we enlarged the basal spacings between the 2D layered silicate materials over a wide range through incorporation of globular macromolecules of various sizes and shapes.1519 We manipulated the interlayer distances of intercalated organic/inorganic nanohybrids in the pursuit of exfoliated single platelets, thereby developing an approach to constructing molecular conformations upon layered templates. The intercalation of several polymers within layered graphite has been developed previously.20,21 Considerable progress has also been made in the preparation of GO/polymer composites, in some cases with a significant enhancement of the mechanical and thermal properties of GO. The insertion of electrically conductive polyaniline (PANI) into GO is of interest because of the possibility of the resulting hybrids possessing synergistic properties (e.g., enhanced mechanical strength and increased electrical conductivity). Matsuo et al. developed a PANI-intercalated GO through an exchange reaction of n-hexadecylamine-intercalated GO with PANI.21 By changing the amine content in the starting material, the interlayer spacing of the PANI-intercalated GO varied over a wide range (1.141.68 nm). Moreover, Wang et al. reported the synthesis Received: August 19, 2011 Revised: October 17, 2011 Published: October 19, 2011 14563
dx.doi.org/10.1021/la203253m | Langmuir 2011, 27, 14563–14569
Langmuir of poly(aniline-co-o-anisidine)-intercalated GO composites exhibiting electrical conductivity that was 3 orders of magnitude higher than that of GO.22 Such GO/PANI derivative composites with excellent electrochemical capacitance have the potential for applications as supercapacitors or other power source systems.23,24 From an examination of the influence of the raw material GO content on the electrochemical behavior of PANI-intercalated GO composites,25 Wang et al. found that the GO/PANI composites exhibited excellent electrochemical capacitance, as high as twice that of pure PANI.25 Xu et al. reported the preparation of reduced GO/PANI composites through the in situ oxidative polymerization of aniline;9 these nanocomposites exhibited good specific capacitance during galvanostatic charging and discharging when used as capacitor electrodes. Zhang et al. also revealed that reduced graphene/PANI nanocomposites with high conductivities through the hydrazine process can serve as supercapacitor electrodes.26 Furthermore, the feasibility of using electropolymerized nanographite/polyaniline composite films as counter electrodes in dye-sensitized solar cells (DSSCs) was demonstrated.27 The considerable progress described above confirmed that the GO/PANI composites exhibited a significant enhancement of the electrical conductivity in electro-optical applications. We were interested, however, in understanding some of the critical issues behind the layered GO exfoliation and overcoming the strong GO platelet attraction. Indeed, the in situ growth of PANI molecules on the GO interface by oxidative polymerization has been reported in the literature.9,23,25,26,2830 However, the exfoliated mechanism of in situ PANI growth with galleryexpanded GO depending on the reaction time for single-layered GO nanosheet (GOS)/PANI nanohybrids has not been reported before. In this article, we report the preparation and characterization of GOS/PANI nanohybrids. We prepared GO nanosheets from raw graphite flakes using a modified version of Hummers’ method.31 We then formed single-layered GOS/ PANI nanohybrids through the intercalation and exfoliation of PANI within the GO sheets. Varying the GO/aniline ratio and the reaction time allowed us to determine the mechanism of the intercalation and exfoliation of multilayer GO from morphological observations made using scanning electron microscopy (SEM), transmission electronic microscopy (TEM), and X-ray diffraction (XRD). Next, we compared the conductivity of the single-layered GOS/PANI nanohybrid with those of unmodified GO and pure PANI. Moreover, we also investigated the work function of a thin film incorporating a single-layered GOS/PANI nanohybrid.
’ EXPERIMENTAL SECTION Materials. Graphite (particle size
