Research Article www.acsami.org
Facile and Scalable Synthesis Method for High-Quality Few-Layer Graphene through Solution-Based Exfoliation of Graphite Boon-Hong Wee, Tong-Fei Wu, and Jong-Dal Hong* Department of Chemistry, Research Institute of Natural Sciences, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon, 21022, Republic of Korea S Supporting Information *
ABSTRACT: Here we describe a facile and scalable method for preparing defect-free graphene sheets exfoliated from graphite using the positively charged polyelectrolyte precursor poly(p-phenylenevinylene) (PPV-pre) as a stabilizer in an aqueous solution. The graphene exfoliated by PPV-pre was apparently stabilized in the solution as a form of graphene/ PPV-pre (denoted to GPPV-pre), which remains in a homogeneous dispersion over a year. The thickness values of 300 selected 76% GPPV-pre flakes ranged from 1 to 10 nm, corresponding to between one and a few layers of graphene in the lateral dimensions of 1 to 2 μm. Furthermore, this approach was expected to yield a marked decrease in the density of defects in the electronic conjugation of graphene compared to that of graphene oxide (GO) obtained by Hummers’ method. The positively charged GPPV-pre was employed to fabricate a poly(ethylene terephthalate) (PET) electrode layer-bylayer with negatively charged GO, yielding (GPPV-pre/GO)n film electrode. The PPV-pre and GO in the (GPPV-pre/GO)n films were simultaneously converted using hydroiodic acid vapor to fully conjugated PPV and reduced graphene oxide (RGO), respectively. The electrical conductivity of (GPPV/RGO)23 multilayer films was 483 S/cm, about three times greater than that of the (PPV/RGO)23 multilayer films (166 S/cm) comprising RGO (prepared by Hummers method). Furthermore, the superior electrical properties of GPPV were made evident, when comparing the capacitive performances of two supercapacitor systems; (polyaniline PANi/RGO)30/(GPPV/RGO)23/PET (volumetric capacitance = 216 F/cm3; energy density = 19 mWh/cm3; maximum power density = 498 W/cm3) and (PANi/RGO)30/(PPV/RGO)23/PET (152 F/cm3; 9 mWh/cm3; 80 W/cm3). KEYWORDS: liquid exfoliated graphene, graphene oxide, poly(p-phenylenevinylene), flexible electrochemical capacitor, supercapacitor transported in RGO primarily by hopping from one sp2 cluster to the next due to the presence of residual oxygen moieties, which act as strong electron-scattering centers that tend to disrupt the flow of charge carriers.9,11 Therefore, it is desirable to establish a process for producing pristine nonfunctionalized graphene homogeneously dispersed in the liquid phase through the exfoliation of pristine graphite.12−24 Such a process would allow the straightforward fabrication of high-performance graphene thin films on sophisticated devices in a film thickness ranging from nanometers to tens of micrometers using various established solution-processable techniques including dip-, drop-, spin-, and spray-coatings, as well as vacuum filtration, Langmuir−Blodgett, layer-by-layer (LbL), electrophoretic deposition, and 3D printing techniques.3,4,25−34 One of the difficulties in producing homogeneous pristine and nonfunctionalized graphene dispersions has been their poor stability in the solution phase: Exfoliated graphene sheets tend to reaggregate owing to the strong π−π interactions and
1. INTRODUCTION Graphene is a two-dimensional allotrope of carbon with extraordinary electrical, mechanical, and optical properties, and has emerged as an attractive material in a broad range of applications including chemical sensors, field-effect transistors, optoelectronics, and energy storage devices.1−4 Many of these potential applications require defect-free graphene, similar in quality to that of pristine graphene, and would benefit by having this graphene produced on an industrial scale. Production of graphene on such a scale is typically accomplished by employing Hummers’ method,5 which produces single-layer graphene oxide (GO) dispersed in an aqueous solution. The GO is electrically insulating due to the presence of functional groups including epoxide, hydroxyl, and carboxylic groups, as well as other oxygen-containing moieties including carbonyl, phenol, lactone, and quinone.6−8 GO can be converted to reduced GO (RGO), which is a conductive form of GO, by either chemical or thermal treatment.9 However, the RGO produced by this conversion process does not have the same structure and electrical properties as pristine graphene, due to the disruptive impacts of the oxidation technique on the sp2 conjugated lattice.10 Charge is © 2017 American Chemical Society
Received: September 16, 2016 Accepted: January 17, 2017 Published: January 17, 2017 4548
DOI: 10.1021/acsami.6b11771 ACS Appl. Mater. Interfaces 2017, 9, 4548−4557
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic illustration for the exfoliation and stabilization of graphene by PPV-precursor in aqueous solution.
van der Waals attraction.9 Two approaches to prevent the aggregation of the exfoliated graphene sheets have been reported. The investigators using the first approach exfoliated graphite in organic solvents including dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, and N-dimethylformamide, each of which has a surface energy of ∼70 mJ·m−2, to yield stable dispersions of a single layer or a few layers of high-quality graphene.35−37 A disadvantage of this method was the use of solvents with high boiling points (>150 °C), and the removal of these solvents was difficult, and their presence tended to disrupt the electron tunneling channels between graphene sheets, resulting in conductivity levels undesirably low for electronics applications. When using the second approach, pristine graphene dispersions were obtained by sonicating graphite in an aqueous medium containing effective stabilizers such as surfactants,38,39 polymers,33,40−42 and aromatic “π−π stacking” molecules,43 which interact noncovalently with pristine graphene sheets through surface adsorption, micelle formation, or π−π interaction, and sterically or electrostatically prevent the restacking of graphene sheets.44 The preparation of aqueous pristine graphene dispersions in a high concentration (∼1 mg/mL) usually requires an excess amount of polymer or surfactant as the stabilizer.12,41 Since a complete removal of these residual stabilizers is impossible in the application stage,39 their presence in the graphene composites could adversely affect the electrical and mechanical properties of the final graphene-loaded products.44−46 Therefore, it is important to discover new stabilizers that would allow the preparation of homogeneous defect-free graphene dispersions, and that would display synergistic effects in various practical applications when forming a graphene/stabilizer composite. In this article, we describe our preparation of high-quality few-layer graphene sheets in the form of an aqueously dispersed composite of graphene and poly(p-phenylenevinylene) precursor (with the precursor denoted as PPV-pre and the entire composite as GPPV-pre), in which the positively charged PPVpre was employed as a stabilizer, as illustrated in Figure 1. PPVpre displays high solubility in water, and a high absorption free energy (−2.46 eV) toward the graphene surface due to the strong π−π interaction between 2D graphene and PPV-pre, leading to the formation of stable and aggregation-resistant graphene dispersions in aqueous solutions along with a low density of defects in the electronic conjugation of graphene. In addition, the cationic PPV-pre polyelectrolyte, when adsorbed onto the graphene surface, imparted positive charge (+22 mV)
to the GPPV-pre composite, and this composite was employed together with negatively charged GO in an alternating sequential layer-by-layer (LbL) assembly to fabricate a (GPPV-pre/GO)n multilayer film on a plastic substrate. The PPV-pre and GO components in (GPPV-pre/GO)n multilayer film were simultaneously converted to fully conjugated PPV and RGO by using a chemical treatment involving hydroiodic acid vapor (HI/H2O).28 Thus, all layer components in the resultant (GPPV/RGO)n multilayer film displayed fully conjugated π-electron rich structures. Also, the conjugated PPV is an excellent electron-conducting polymer that can be applied as an efficient organic current collector. This work is a series of the reports about development of nanostructured multilayer film electrodes including graphene and precursor phenylenevinylene.28,29 The superior electronic properties of the GPPV sheets (versus those of RGO obtained by Hummers’ method) were evaluated by comparing the electrical conductivities of (GPPV/RGO)23 and (PPV/RGO)23 multilayer films, and also the capacitive performances of two electrochemical supercapacitor systems: (1) (PANi/RGO)30/(GPPV/ RGO)23/PET (denoted as PG) (in which the (GPPV/RGO)23 stratum works as an organic current collector) and (2) (PANi/ RGO)30/(PPV/RGO)23/PET (denoted as RG) (in which the (PPV/RGO)23 stratum works as an organic current collector). It is noteworthy that RGO components (obtained by Hummers’ method) tend to form graphene sp2 conjugated lattice, which is partially broken up by the disruptive impacts of the oxidation techniques.10
2. EXPERIMENTAL SECTION 2.1. Materials. α,α′-Dichloro-p-xylene, tetrahydrothiophene, natural graphite powder (particle size >150 μm and 150 μm) was used for synthesizing graphene oxide (GO) for LbL-assembly of (GPPV-pre/GO) multilayer film. Natural graphite powder (particle size
