Graphene free-standing film-like or paper-like materials have attracted great attention due to their intriguing electronic, optical and mechanical properties and potential application in chemical filters, molecular storage and supercapacitors. Although significant progress has been made in fabricating graphene films or paper, there is still no effective method targeting ultrathin free-standing graphene films (UFGFs). Here, we present a modified filtration assembly method to prepare these ultrathin films. With this approach, we have fabricated a series of ultrathin free-standing graphene oxide films and UFGFs, up to 40 mm in diameter, with controllable thickness from micrometre to nanoscale (approx. 40 nm) dimensions. This method can be easily scaled up and the films display excellent optical, electrical and electrochemical properties. The ability to produce UFGFs from graphene oxide with a scalable, low-cost approach should take us a step closer to real-world applications of graphene.
Graphene is a fascinating material with marvellous mechanical, physical and chemical properties [1,2]. Since the first samples were produced in 2004 by mechanical cleavage of graphite , many methods to date have been developed to prepare graphene. These include mechanical exfoliation , epitaxial growth , chemical vapour deposition [5,6] and solution-phase dispersion [7,8]. Compared with other methods, the reduction in graphene oxide (GO) brings forth superiorities in many respects [7,9]. GO can display homogeneous dispersion after introducing oxygen functional groups on basal planes and edges of graphite via oxidation, thus the processability of GO sheets awards them widespread applications in many fields, such as chemical filters, energy storage, composites and components of electrical batteries [2,10–12]. To realize such applications, several routes have been developed to prepare graphene films and paper based on GO dispersion, which involve Langmuir–Blodgett assembly [13,14], vacuum filtration , molecular templates  and spin-casting . Currently, flow-directed assembly [18–20] is one of the important methods in preparing GO paper because of its controllable film thickness. Dikin et al.  successfully produced free-standing GO paper with a thickness ranging from 1 to 30 μm by this method, but great difficulties appeared with further reduction in thickness. Although the preparation of free-standing GO films with a thickness of 0.5 μm has been realized by a liquid/air interface assembly recently , there are still no effective methods for the fabrication of ultrathin free-standing graphene films (UFGFs). Here, we develop a facile modified filtration method to prepare these nanoscale graphene films. The method is easy to scale up, and we have extended the controllable thickness of UFGFs from micrometre to nanoscale dimensions. These films show excellent optical, electrical and electrochemical properties that could extend the applications of graphene.
2. Experimental set-up
(a) Preparation of graphene oxide and chemically reduced graphene oxide
GO sheets were synthesized by a modified Hummers method . A small amount of graphite powder (50 mg) was mixed with 30% H2O2 (4 ml), and concentrated H2SO4 (16 ml) was added while stirring. After 8 h reaction at room temperature, the mixture was added to excess deionized water, followed by washing with deionized water and drying in a vacuum. Next, the exfoliated graphite was dispersed into concentrated H2SO4 (10 ml), then 20% KMnO4 solution of concentrated H2SO4 (20 ml) was dropped into the dispersion. The reaction was maintained at room temperature for 3 h in order to gain sufficient oxidation of graphite. The suspension was then diluted with deionized water (200 ml) and treated with 30% H2O2 until the purple faded. Finally, the remaining sediment was washed carefully and dewatered by vacuum drying. One hundred milligrams of GO solid was dispersed in deionized water (20 ml), and then 80 ml of 80% hydrazine hydrate was added while stirring. After reaction at room temperature for 3 days, a homogeneous black dispersion of graphene was obtained without any stabilizers.
(b) Fabrication of ultrathin free-standing graphene oxide films and ultrathin free-standing graphene films
HA homogeneous suspension of GO sheets was prepared by dispersing 15 mg of GO powder into 100 ml of deionized water and then sonicated for 30 min. A certain amount of GO or chemically reduced GO dispersion was vacuum-filtrated though a nitrocellulose membrane (NM) with 0.22 μm pores, forming a superimposed membrane with controllable thickness on the surface of the NM. The membrane was then leant against a piece of filter paper with the graphene side in contact with the filter paper, and immersed in n-methyl pyrrolidone to remove the NM. Finally, the free-floating film was transferred into ethyl ether and fished out using a piece of filter paper. Accompanied by the volatilization of ethyl ether, the ultrathin free-standing graphene oxide films (UFGOFs) or UFGFs were thus easily separated from the filter paper if it was handled.
The samples were investigated using a wide range of characterization techniques. Scanning electron microscopy (SEM) was performed with a Hitachi S-4300 at 15 KV accelerating voltage. X-ray diffraction (XRD) patterns were recorded with a Rigaku Dmax 2500 diffractometer using Cu Kα radiation. Atomic force microscopy (AFM) imaging was obtained using an Innova microscope in the tapping mode. X-ray photoelectron spectroscopy (XPS) was carried out on a ESCA Lab220I-XL. Fourier transform infrared (FTIR) spectra were carried out with an RT-DLaTGS 27 spectrometer. Transmission ultraviolet-visible (UV-vis) spectra were recorded using a JASCO V-570 spectrophotometer.
(d) Device and electrochemical measurements
Supercapacitor test cells were fabricated with the two-electrode configuration. To obtain membrane electrodes, the UFGFs were annealed at 400°C for 3 h in a flow of Ar (200 sccm). Two symmetric electrodes were separated by a thin polymer in 30 wt% KOH aqueous electrolyte solution, and then sandwiched in a stainless steel test cell using Ni foil as current electrodes. The electrochemical properties of the electrodes were studied using a two-electrode system. Cyclic voltammetry and galvanostatic charge–discharge were carried out on an electrochemical workstation CHI600B.
3. Results and discussion
Figure 1a shows the schematic drawing illustrating the processing steps for fabrication of UFGOFs or UFGFs. The procedure consists of the preparation of graphene film on NM (step I) , followed by a distinctive separation (steps II, III and IV). According to previously reported results [15,19], the filtration of GO dispersions through a piece of the NM can yield a membrane with a controllable GO film on the surface. However, the mechanical strength of GO film and the interaction between the GO film and the NM limits the minimum thickness of free-standing graphene films. To obtain UFGOFs or UFGFs, an efficient approach is to dissolve the supporting NM. We found that the introduction of a piece of filter paper as an interim support can prevent GO or chemically reduced GO films from curling during dissolution of the NM; then the van der Waals attraction and hydrogen bonds between neighbouring sheets allow the free-standing films to float freely in organic solvents. After completely eliminating the NM, a piece of filter paper as supporting substrate is used to remove the free-standing films from organic solvent. Owing to the weak interactions between these graphene films and the filter paper, the GO or chemically reduced films separated easily from the filter paper if it was handled. Figure 1b–e and the electronic supplementary material, figure S1, show the optical images of our UFGOFs and UFGFs with different thicknesses. These films are uniform and transparent under the transmitted white light. Readable words seen through the ultrathin free-standing films (figure 1d) indicate that these films have good optical properties.
Figure 2a shows the SEM image of the UFGFs prepared with the aid of filter paper. A transparent membrane with smooth surface and uniform thickness is observed. The transparency reveals that the films composed of graphene sheets have relatively few layers. However, the SEM images with higher magnification reveal different morphologies of different sides of these ultrathin free-standing films (figure 2b,c). At an early stage of vacuum filtration, the favourable negative pressure produced by a vacuum system offers enough force for contact stacking of graphene sheets (figure 2b), while the lack of current and essential pressure in the later stage of vacuum filtration may cause a loose structure on one side of the UFGFs (figure 2c). These ultrathin films are flexible and capable of being folded (electronic supplementary material, figure S2). Wave-like nanostructure covers the whole surface, and the individual graphene sheets can be clearly identified as blocks for building the UFGFs.
The thickness of UFGOFs or UFGFs can be controlled from micrometre to nanoscale dimensions by the volume of colloidal dispersion of GO or chemically reduced GO sheets. The annealed UFGOFs are about 260 nm thick for the deposition at a filtration volume of GO dispersion (30 ml). With a smaller filtration volume of 7.5 ml, the thickness decreases to about 60 nm. The fractured edges of UFGFs when imaged via SEM revealed well-packed layers through almost the entire cross section. From the higher resolution SEM images (figure 2d,e), we can estimate the thickness of UFGFs. Figure 2f shows a typical tapping mode AFM image of a piece of chemically reduced graphene film. The sample was prepared by transferring it to quartz substrate. The cross-sectional contour illustrates that the step height between the upper surface and the substrate is about 40 nm. The result is well in accord with the SEM observation.
The UFGOFs and UFGFs were prepared by using GO or chemically reduced GO sheets. XRD patterns of our GO powder reveal a single sharp peak corresponding to a layer at a distance of approximately 0.89 nm (electronic supplementary material, figure S3), indicating the complete exfoliation of graphite layers using our modified Hummers method. Large graphene sheets are important for fabrication of graphene films and devices. As seen from the SEM images (electronic supplementary material, figure S4), our GO sheets are predominantly single-layer GO sheets with large lateral size. The electronic supplementary material, figure S5, shows a typical AFM image of a GO sheet deposited on SiO2/Si substrate. The line scans illustrate that the step height between the upper surface of single-layer GO sheets and the substrate is found to be about 1.1 nm. The height is in accord with one-atom-thick GO sheets on SiO2/Si substrates .
Figure 3a shows the FTIR spectrum of the UFGFs compared with the spectrum of NM. The peak at 1565 cm−1 is assigned to the stretching of C=C bonds, which becomes more intense after reduction because of increasing aromatization . Although the reduction has minimized the oxygen content, the residual epoxide groups and hydroxyl groups induce a broad absorption band at 1205 cm−1 corresponding to the high oscillator strengths of the C−O stretching mode [24,25]. The NM spectrum indicates the NO2 groups by two intense asymmetric and symmetric stretching bands at 1651 and 1281 cm−1. However, at the UFGF spectrum these bands basically disappear, illustrating that the majority of the NM has been removed after repeated immersion cleaning. We also present the XPS spectra of the two substances in figure 3b; the N1s photoelectron peak centred at about 400 eV ascribes to the non-covalent bonding between hydrazine and the reduced GO sheets [24,25]. Analysing the spectra of the UFGFs and the NM, a similar conclusion is also obtained by the severely weakened signal at 408 eV. These results are in accordance with the observations using SEM.
We have measured the electrical and optical properties of these UFGFs. The average sheet resistance of UFGFs is 6.1×104 Ω−2 and the average conductivity is about 400 S m−1. Moreover, the average sheet resistance can be further decreased to approximately 2.0×104 Ω−2 and the conductivity can be improved to approximately 1200 S m−1 by annealing. The optical transmittance of UFGOFs and UFGFs is shown in figure 3c. The UFGOFs permits high transmittance in the visible and near-infrared region, and at a wavelength of 1000 nm the transmittance is above 78%. This value is obviously higher than that of free-standing GO films made by other methods [19–21]. The optical properties of graphene sheets reduce with the restoration of the C−C conjugated system (figure 3c,d). Nevertheless, The UFGFs still permit high transmittance (approx. 60% at a wavelength of 1000 nm) after a mild hydrazine hydrate reduction, which further indicates the ultrathin structure of our free-standing graphene films.
In normal routes, the colloidal dispersion of GO sheets is filtrated through an Anodisc membrane filter, and free-standing GO paper is produced by directly exfoliating them from the supporting membrane [18–20]. However, the minimum membrane thickness of graphene paper is limited by the stronger interaction between the GO film and the Anodisc membrane filter. Comparatively, the modified filtration assembly is controllable and easy to scale up for producing UFGOFs or UFGOFs. First, the thickness can be adjusted well by the volume and the concentration of the GO dispersion, because this method is developed from vacuum filtration [15,19]. Second, a piece of general filter paper is introduced in the fabrication of UFGOFs or UFGOFs, which changes tight coupling between the graphene film and the NM into loose contact between the film and the filter paper; the change facilitates the separation of graphene films. Third, the method is easily implemented and there are no other complicated equipment and virulent reagents involved in the whole process, which makes it a good tool for scalable production of controllable UFGOFs and UFGFs.
To explore the electrochemical properties of these film-like materials, we fabricated them into supercapacitors. Figure 4a shows the typical CVs of the devices in the voltage window from 0 to 1 V. As we know, the integrated area of CVs represents the power density, and the shape of CV loops reflects the magnitude of contact resistance [26–29]. The CV loop of a supercapacitor should be rectangular, provided that there are low contact resistances. Our devices display rectangle-like CV loops with various scan rates, even at a high scan rate of 800 mV s−1, indicating excellent capacitance behavior and lower contact resistances in the supercapacitors. The magnitude of equivalent series resistance (0.7 Ω) can be obtained from the x-intercept of the Nyquist plot (figure 4b), which determines the charge–discharge rate and also is an important factor in determining the power density of the supercapacitors. Figure 4c illustrates the cycle performance of the devices under a constant current (0.5 mA). The discharge curves are linear in the total range of potential with nearly constant slopes, and the maximum specific capacitance (192 F g−1) can be calculated from the slope of the discharge curves according to the equation C=2(i)/(s×m), where i is the constant applied current, s is the slope of the discharge curve and m is the mass of electrode [27–29]. Figure 4d shows the variation in specific capacitance with cycle number at a constant current of 0.5 mA. The specific capacity decay is about 9% of the initial discharge capacity after 1000 cycles, which indicates excellent electrochemical stability, cycling perdurability and charge–discharge reversibility.
Stable dispersion of macroscopic graphene sheets is used to prepare UFGOFs and UFGFs by a modified filtration assembly strategy. In the process, GO or chemically reduced sheets are fabricated into films on the NM, followed by a distinctive separation. The process is controllable and the minimum thickness of UFGFs can reach several tens of nanometres. Such ultrathin free-standing films show higher optical, electrical and electrochemical properties. We believe that the preparation of UFGOFs and UFGFs would open up further widespread applications of graphene.
This work was supported by the National Natural Science Foundation of China (61101051, 51233006, 60911130231 and 21021091), the Major State Basic Research Development Program (2011CB808403, 2011CB932303 and 2011CB932700) and Chinese Academy of Sciences.
One contribution of 8 to a Theo Murphy Meeting Issue ‘Theo Murphy International Scientific Meeting between the UK and China on the chemistry and physics of functional materials’.
- © 2014 The Author(s) Published by the Royal Society. All rights reserved.