Transparent and highly conductive liquid-phase exfoliated graphite films treated with low-temperature air-annealing

https://doi.org/10.1016/j.matchemphys.2013.08.017Get rights and content

Highlights

  • Highly stable graphene dispersion was prepared by exfoliation with the aid of Nafion.

  • Low-temperature annealing was employed to treat graphene films for the first time.

  • Graphene films possess sheet resistance of 2.86 KΩ sq−1 and light transmittance over 84%.

Abstract

This article presents a novel and simple method of liquid-phase exfoliation to fabricate graphene films that possess high conductivity and good light transparency. Graphite was exfoliated in water–ethanol mixture, with the aid of Nafion, to give highly stable graphene dispersion. Transparent graphene thin films were easily deposited by vacuum filtration from the Nafion-stabilized graphene dispersion. More important, low-temperature air-annealing (at 250 °C for 2 h) was employed to treat freshly-prepared graphene films for the first time. It demonstrates that the technique is advantageous and quite efficient for the fabrication of exfoliated graphite films with defect-free structure and high purity, confirmed by TEM, SEM, FTIR, XPS, and Raman spectra. The resulting graphene films possess a sheet resistance lower than 2.86 kΩ sq−1 and optical transmittance over 84% at a typical wavelength of 550 nm.

Introduction

Transparent conductive films (TCFs),1 with their unique combination of electronic conductivity and transparency in the visible region of the spectrum [1], play a critical role in many current and emerging optoelectronic devices, such as solar cells [2], [3], light-emitting diodes [4], flat-panel displays [5], antistatic and antiglare coatings [6], and low-emissivity windows [6], [7]. The traditional conducting and transparent metal oxides, such as indium tin oxide (ITO) and fluorine tin oxide (FTO), still remain some challenges in practical use due to their disadvantages in high manufacture cost, chemical instability, brittle nature and limited near-infrared transparency [8], [9].

Recently graphene has attracted tremendous attention as a novel optoelectronic material to replace the traditional transparent conductors. Graphene is the first example of truly two-dimensional materials. The researches yield graphene's opacity of only 2.3 ± 0.1% and negligible reflectance (<0.1%) due to its one-atom-thick structure, implying its good optically transparent properties [10], [11]. Furthermore, charge carriers in an individual graphene sheet delocalize over the entire sheet and can travel thousands of interatomic distances without scattering. As a zero-gap semiconductor, an individual graphene sheet exhibits very high in-plane conductivity with carrier mobility up to 200,000 cm2 V−1 s−1 [12]. Its high transparency and low resistivity make this two dimensional crystal ideally suitable for the fabrication of transparent conductor. Together with its high chemical stability and mechanical strength, graphene should improve the durability of the optoelectronic devices.

Thus far, two traditional types of TCF fabrication methods based on graphene have enjoyed reliable success: (i) Fabrication from graphene oxide (GO) followed by chemical reduction. It is well known that GO can be easily exfoliated to give monolayer with high yield and efficiency [13]. Therefore most of the recent efforts on graphene film investigation were based on GO. Through a liquid-phase reduction, the conductivity of the insulating GO films can be partially recovered by reducing agents, as typical as hydrazine. However, the reduction is not sufficient, for the resulting films often display resistivity higher than 104 Ω sq−1 at ca. 80% transmittance [14], [15], [16], [17]. High-temperature thermal treatment is able to further restore the sp2 structure of graphene and the resistivity can be decreased to ca. 103 Ω sq−1 [18], [19], but it is inapplicable with flexible substrate such as PET due to the high temperature; and (ii) Fabrication from chemical vapor deposition (CVD) graphene followed by transference. At present, CVD is the most promising and almost impeccable method for the preparation of graphene. The typical CVD process includes hydrocarbon pyrolysis of carbon species and graphene forms on the surface of the catalyst from the dissociated carbon species [20]. By using CVD method, some researchers have successfully synthesized graphene on nickel foil [21] or copper foil [22], which opens an avenue to the controllable synthesis of large area, high conductivity and uniform graphene. However, the preparation of graphene transparent conductive films from CVD is restricted to equipments and size, and its nondestructive transfer technology also has certain difficulties, thus it cannot simultaneously realize mass production and low cost manufacture.

Very recently, a novel and facile route for the fabrication of graphene TCFs from liquid-phase exfoliated graphite is becoming an increasing interest. Exfoliation of graphite in liquid-phase was firstly found to give oxide-free graphene monolayer with high quality and yield by Coleman and his co-workers [23], [24]. The approach is direct, simple and benign with no need of complicated oxidation–reduction process. Normally the oxidation–reduction process is instinctively necessary for the production of chemically reduced graphenes (CRGs), in which the oxidation causes extensive damage of π–π conjugated bonds in the carbon skeleton and the distortion of the structure is very difficult to be restored during reduction. The exfoliated graphite with low oxygen content can prevent electrode failing caused by the adsorption of the oxide thus have potential applications in electro-analytical NADH sensors, facilitating the exceptionally stable and sensitive detection [25]. Furthermore, the well protected sp2 structure during liquid-phase exfoliation guarantees high conductivity of the resulting graphene sheets, which attract the attentions of researchers to fabricate high-performance graphene TCFs from liquid-phase exfoliated graphite [26], [27], [28]. Kang et al. reported graphene TCFs fabricated by the top-down method with sheet resistance of 0.3 kΩ sq−1 and 73% transmittance at 550 nm, which is the lowest sheet resistance for graphene TCFs [28].

Herein we report a new method to prepare highly conductive and transparent graphene films from Nafion-assisted liquid-phase exfoliated graphite. In particular, graphite powder was exfoliated in water–ethanol mixture with the aid of Nafion to give a graphene dispersion. From the dispersion graphene films were deposited by vacuum filtration. Instead of the traditional high-temperature thermal annealing, a low-temperature air-annealing technique for graphene TCFs was employed in our approach. The annealing purpose here is only to get rid of impurities between the adjacent graphene sheets rather than reconstruct the conjugated system thanks to the low defect content for liquid-phase exfoliated graphite in comparison with that of CRGs. After low-temperature annealing treatment, films with a sheet resistance of 2.68 kΩ sq−1 and light transparency of 84% were obtained. Our method has obvious advantages of being eco-friendly by using acid-free water dispersion and energy-cheap by using low-temperature air-annealing.

Section snippets

Graphene dispersion preparation

Graphite powders (particle size of 1–44 μm, Sinopharm Chemical Reagent Co., Ltd.) were dispersed in a water–ethanol mixture solution (volume ratio 1:1) of perfluorosulfonated cation-exchange polymer (Nafion, Jiangsu Huayuan Hydrogen Technology Development Co., Ltd) by sonication for 1 h, using a tip sonication instrument (Scientz-IID, Ningbo Scientz Biotechnology Co., Ltd.). The resulting dispersion was left to stand overnight and then centrifuged at 5000 rpm for 30 min (TGL-16G, Shanghai

Graphene dispersion

Graphite powder was exfoliated into flakes in a water–ethanol solution, using Nafion as the stabilizing agent. The cartoon in Scheme 1 schematically illustrates the proposed process.

The graphene dispersion before centrifugation is dark-black. After centrifugation, a gray dispersion is obtained with concentration as high as 0.067 mg mL−1 which can remain stable for a month as shown in Fig. 1A. The high stability of the dispersion is from the efficient dispersing capacity of Nafion. Nafion is a

Conclusions

Graphite was exfoliated in water–ethanol mixture, with the aid of Nafion, to give a stable graphene dispersion with concentration up to 0.067 mg mL−1. The characterization of FTIR, XPS and Raman spectrum confirmed that liquid-phase exfoliation method avoids complicated oxidation–reduction process and can be applied to obtain graphene films with high-quality and defect-free structure. From Nafion-stabilized graphene dispersion, TCFs were easily deposited by vacuum filtration. For the first time,

Acknowledgments

This work was supported by Specialized Research Fund for the Doctoral Program of Higher Education of China (20123219110010), Natural Science Foundation of Jiangsu Province of China (Grant No. BK2012845) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

References (36)

  • J.K. Wassei et al.

    Mater. Today

    (2010)
  • K.I. Bolotin et al.

    Solid State Commun.

    (2008)
  • G. Venugopal et al.

    Mater. Chem. Phys.

    (2012)
  • S.J. Wang et al.

    Carbon

    (2010)
  • S. Obata et al.

    Carbon

    (2013)
  • Y. Ouyang et al.

    Phys. E

    (2008)
  • M. Kevin et al.

    Energy Environ. Sci.

    (2012)
  • R. Ishikawa et al.

    Jpn. J. Appl. Phys.

    (2012)
  • L.L. Jiang et al.

    Inorg. Mater.

    (2012)
  • G. Jo et al.

    Nanotechnology

    (2012)
  • E. Klaus

    Nat. Photonics

    (2012)
  • Y.Q. Sun et al.

    J. Polym. Sci. Polym. Phys.

    (2013)
  • R.S. Edwards et al.

    Acc. Chem. Res.

    (2013)
  • J.O. Hwang et al.

    ACS Nano

    (2012)
  • R.R. Nair et al.

    Science

    (2008)
  • J.J. Tang et al.

    Prog. Chem.

    (2012)
  • S. Stankovich et al.

    J. Mater. Chem.

    (2006)
  • G. Eda et al.

    Nat. Nanotechnol.

    (2008)
  • Cited by (10)

    • Macro copper-graphene composites with enhanced electrical conductivity

      2022, Journal of Alloys and Compounds
      Citation Excerpt :

      The Raman spectra for HD-GNS and LD-GNS are presented in Fig. 1. The defect density of the HD-GNS materials was 7.8 ± 0.8 × 1010 cm−2, while that of LD-GNS materials was determined to be 2.1 ± 0.1 × 1010 cm−2, which is similar to that of high purity graphene found in literature [39–41]. The D-peak of the LD-GNS material demonstrated an intensity of around 9 arbitrary units (a.u.) compared to the annealed HD-GNS D-peak intensity of approximately 60 a.u.

    • Which plasticizer is suitable for films based on babassu starch isolated by different methods?

      2019, Food Hydrocolloids
      Citation Excerpt :

      Thus, AS and KS starch films were less opaque when glycerol was used as plasticizer, whereas WS starch film was less opaque when glucose or sorbitol was employed as plasticizer. These plasticizers effectively increased polymer chain mobility and intermolecular spacing in the starch matrix (Saberi et al., 2017; Tong, Xie, Si, Che, & Xiao, 2013), which could facilitate light permeability through the film, thereby giving a low opacity value. Comparing the different babassu starch films (AS, WS, and KS), KS starch film was the least luminous and the most colored and opaque independent of plasticizer type.

    • Graphene and MXene-based transparent conductive electrodes and supercapacitors

      2019, Energy Storage Materials
      Citation Excerpt :

      Graphene can also be exfoliated with the aid of Nafion, forming a stable water/ethanol solution [148]. After a low annealing temperature (250 °C for 2 h) under vacuum, the resulting graphene films demonstrate a Rs of 2.86 kΩ sq-1 and a T of 84% at 550 nm [148]. Jo et al. exfoliated large size (> 1 µm) graphene nanosheets assisted by PEG.

    • Fast and simultaneous growth of graphene, intermetallic compounds, and silicate on Cu-Ni alloy foils

      2014, Materials Chemistry and Physics
      Citation Excerpt :

      All data clearly indicate that the monolayer graphene was formed over the area of 30 μm × 30 μm without fractions of bilayers and multilayers [17,23,27]. Fig. 2(d) confirms that the 2D peak position did not shift, indicating that the number of associated graphene layers did not increase but hold on monolayer [26,34]. Furthermore, the D-bands were not associated with the defects and/or disordered carbon atoms near 1350 cm−1 in the Raman spectra.

    View all citing articles on Scopus
    View full text