Capillary-Force-Assisted Self-Assembly (CAS) of Highly Ordered and

Article pubs.acs.org/JPCC

Capillary-Force-Assisted Self-Assembly (CAS) of Highly Ordered and Anisotropic Graphene-Based Thin Films Rachel Tkacz,† Rudolf Oldenbourg,‡ Alex Fulcher,§ Morteza Miansari,† and Mainak Majumder*,† †

Nanoscale Science and Engineering Laboratory (NSEL) and Mechanical and Aerospace Engineering Department, Monash University, Clayton, VIC 3800, Australia ‡ Marine Biological Laboratory, Woods Hole, Massachusetts 02543, United States, and Physics Department, Brown University, Providence, Rhode Island 02912, United States § Monash Micro Imaging (MMI), Monash University, Clayton, VIC 3800, Australia S Supporting Information *

ABSTRACT: We report capillary-force-assisted self-assembly (CAS) as a method for preparation of thin films of chemically reduced graphene oxide (rGO) with unidirectional organization of rGO sheets. The films were initiated at the contact line of the air−liquid−solid interface and form directly on solid substrates dipped in an isotropic colloidal suspension of rGO. Assisted by capillary forces at the contact line, the suspension undergoes an isotropic-toanisotropic phase transition and becomes aligned with the film growth direction as the contact line moves across the substrate surface. We determined the degree of order in rGO films and assemblies by birefringence and diattenuation imaging. The slow axis of the rGO platelets within the CAS films displayed a narrow angular distribution (±3°) within a film area of 1 mm2, resulting in the highest possible order parameter (S) of ∼1 with 8-fold enhancement of electrical conductivity compared to films formed by traditional techniques such as filtration. Our straightforward film fabrication technique is scalable to produce large areas of films, and by controlling the rates of convective to diffusive mass transport, films with varying degree of order can be produced. methods such as vacuum-assisted filtration,16,17 dip-coating (or assembly at the air−liquid−solid interface),4,18,19 and spincoating.20−22 Another approach is to use the natural tendency of GO sheets to self-assemble at interfaces with hydrophobic (air or liquid) phases, which arise from the hydrophobic domains of GO. Chen et al.,23 followed by Zhao et al.,24 prepared GO films at the liquid−air interface by heating an aqueous suspension of GO for a short time, while Chen et al.25 utilized the liquid−liquid interface of water and pentane with ethanol to reduce the solubility of GO and promote film formation. However, GO films are insulating due to sp3 hybridization of the oxidized carbons, and therefore need to be reduced, thermally4,20,26−28 or chemically,20,29,30 for partial restoration of the sp2 hybridization and conductivity. Preparation of films from reduced GO (rGO) aqueous suspensions poses several challenges because the hydrophobic rGO sheets tend to aggregate in water and destabilize the suspension;29 however, under controlled conditions of pH and concentration of rGO, the electrostatic repulsion of the carboxylic acid residues can stabilize the suspension.30 Once the suspension is stable, films can be prepared by flow-assisted methods, or directed-assembly processes, such as vacuum filtration30,31 and spin-coating.32 As with GO, an elegant approach is the use of interfacial properties of graphene

1. INTRODUCTION Thin films of graphene are promising materials as conductive and transparent electrodes for the growing market of photovoltaics, LCD’s, touch screens, and other technological applications. In addition to the combination of electrical conductivity1−3 and transparency, these films are endowed with other exciting properties such as high chemical and thermal stability and mechanical flexibility that make these materials multifunctional.4 Graphene, in the form of stable liquid dispersions, can be synthesized from graphite by mechanical exfoliation1,5 in various organic solvents6−8 or without mechanical means as graphite exfoliates spontaneously in superacids by a protonation mechanism enabling production of graphene-based thin films.9 Yet, processing of films from aqueous suspensions possesses a wider appeal and advantages in large-scale manufacturing from the reduced cost of chemicals and environmental friendliness. Graphite can be exfoliated in water in the presence of surfactant10,11 which stabilizes graphene suspensions; however, films made from these suspensions usually suffer from low conductivity due to the insulating properties of surfactants, which are difficult to remove postfabrication of thin films. A popular method to produce graphene in water is via oxidative exfoliation of graphite and mild sonication to generate graphene oxide (GO),12,13 which is easily dispersible in water due to the presence of hydrophilic domains.14,15 Therefore, it is not surprising that a large number of studies have reported film fabrication from aqueous suspensions of GO using various © 2013 American Chemical Society

Received: August 10, 2013 Revised: December 6, 2013 Published: December 9, 2013 259

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platelets to form self-assembled thin films. For example, Zhu et al. formed rGO films at the liquid−air interface,33 while Biswas et al. used the interface of chloroform and water to assemble graphene sheets at the liquid−liquid interface.34 Dip-coating is usually applicable for suspensions with relatively high viscosity; therefore, it has found frequent application with GO suspensions, but stable rGO suspensions are very dilute and do not have sufficient viscosity to enable film formation by dipcoating. Despite the published literature on self-assembly of GO18 at the air−liquid−solid triple phase, this route has been relatively unexplored for rGO whereby conductive films formation directly on a solid surface suitable for device applications is possible. Graphene films are made from small rGO platelets where the particle−particle junction and the degree of order will influence the electron cloud distribution and the electrical conductivity. The degree of anisotropy has a direct relationship with the degree of order and the electronic transport properties in assemblies of 1D nanomaterials such as CNTs35,36 and is likely to impact the performance of films made from assemblies of 2D materials such as graphene. So far there has been a lack of effort to develop metrologies for characterizing the degree of anisotropy of graphene thin films, especially by optical means, which are fast, simple, and nondestructive. Here we report capillary-force assisted self-assembly (CAS) of rGO sheets to form thin films at the air−liquid−solid triplephase interface of a concave meniscus (Figure 1a) from stable

sized graphene assemblies and the in-plane order parameter of the films.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Suspensions and Films. GO was prepared from graphite using the modified Hummers’s procedure12,13 and ultrasonicated in water to form a GO suspension, which was then chemically reduced with hydrazine and ammonia in a procedure similar to Li et al.’s30 (for more details see section S1 in the Supporting Information). The resultant suspension was centrifuged for 20 min at relative centrifugal force (RCF) of 1300, and the supernatant was collected. Without disposing of the settled particles, a stable suspension was re-extracted three more times with deionized water from the precipitate at the bottom and combined to make a larger stock of rGO suspension. This rGO suspension was stable at longer duration of centrifugation, indicating that the re-extraction step does not compromise the quality of the suspension. The concentration of the suspension was determined by measuring the absorbance at 660 nm, using an Ocean Optics USB4000 UV−vis spectrometer. A calibration curve was prepared gravimetrically (see section S2 in Supporting Information for details). The absorption coefficient, α, was calculated using the Lambert−Beer law, taking the slope of the absorbance versus the concentration, measured in a 1 cm length cuvette. We obtained α = 6240 mL mg−1 m−1, a similar value to other aqueous graphene suspensions.11 The average size of the colloids was determined using scanning electron microscope (SEM, JEOL Ltd.). Measurements were done on 90 sheets and a length of 0.9 ± 0.4 μm was determined (see section S3 in Supporting Information). Films were prepared by immersing glass slides vertically in the rGO suspension, which was left partially covered in a fume hood under ambient conditions. Films were formed on the glass slide at the contact line between the solid, liquid, and air (Figure 1), at a rate of ∼3 × 10−4 mm/min, due to water evaporation. 2.2. Characterization of Films. The thickness of the films was measured using a Veeco Dektak 150 surface profilometer, with stylus force of 1 mg and radius of 12.5 μm. The evaluated error for these measurements is approximately 5 nm. SEM images were taken on a silicon wafer coated with 100 nm of silicon oxide. The polarized light microscopy was done using a Leica DM IRB microscope with the LC-PolScope (LPS)−Abrio imaging system from CRI Inc. and a custom LC-PolScope for diattenuation measurements, as described in the next paragraph. Surface resistance was measured with Jandel Model RM3 four point probes, and the resistivity was calculated by multiplying the surface resistance with the average thickness measured by the profilometer. The measurements were done on at least three different areas of each film, at two perpendicular directions. Transmittance was measured using a UV−vis spectrometer at 550 nm. 2.3. Background on Quantitative Estimation of the Degree of Anisotropy in rGO Films by Polarized Light Microscopy. Graphite exhibits two optical phenomena: diattenuation, or dichroism, and birefringence37−40 which are a direct consequence of the in-plane and out-of-plane anisotropy of graphene. Diattenuation refers to the differential transmittance for light polarized parallel and perpendicular to the principal axes of the material and birefringence refers to the differential refractive index for light polarized in the same directions. Given that the building blocks of our films, i.e. graphene-based sheets, are optically anisotropic, polarized light

Figure 1. (a) Schematic representation of the formation of CAS films at the air−liquid−solid interface. In the bulk of the isotropic rGO colloidal suspension repulsive electric double layer forces, Uedl, are dominant, and the suspension is kinetically stable. At the top of the meniscus stretching by capillary forces yield an anisotropic phase of rGO. (b) CAS film formed on a glass slide (scale bar is 10 mm).

isotropic rGO suspensions. In addition, we demonstrate the use of the LC-PolScope-Abrio (LPS), a new polarized light microscopy technique, for characterizing the orientational order of graphene-based films and a quantitative measure for comparing the degree of anisotropy using the standard deviation of the azimuth distribution between micrometer260

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microscopy is a useful tool to characterize graphene assemblies, in particular to evaluate the degree of order and the alignment of the sheets. We note that this technique has been widely used in characterization of other ordered molecular or particle structures41,42 as well as graphene oxide liquid crystals.43−45 In contrast to previous research on graphene-based materials, we characterized the anisotropy of our films using the LPS imaging system.46−48 The main advantage of this system compared to traditional polarizing microscopes is that it measures the parameters at all points of the image simultaneously and with no need to mechanically rotate the specimen. This contributes significantly to the accuracy and the sensitivity of the measurement as well as the rapidity. We carried out our measurements primarily on the Birefringence LC-PolScope while the Diattenuation LC-PolScope was utilized to more closely examine the physical origin of the measured anisotropy. An explanation for these imaging modes and the physical relationship between diattenuation and birefringence are detailed in Mehta et al.49 Briefly, the polarization components of the Birefringence LC-PolScope are comprised of a linear polarizer, two variable electro-optical retarder plates instead of the traditional compensator, and an analyzer for circularly polarized light. The linear polarizer followed by two variable retarder plates is called a universal compensator and is used to generate light of circular and elliptical polarization. The polarization analyzer is used for analyzing the polarization of light transmitted through the specimen. A sensitive CCD camera and a digital image processing system provide fast measurement of the optical anisotropy, the retardance, and the orientation of the slow axis. As the birefringence measurement is sensitive to both diattenuation and birefringence, and since rGO is mainly dichroic, we will refer to the measured parameters as “apparent” retardance and azimuth. The diattenuation measurement, on the other hand, is only sensitive to differential transmittance. We have turned the birefringence setup into the diattenuation setup by simply removing the polarization analyzer from the imaging path of the microscope. The universal compensator was used to sequentially generate four linear polarizations of the transilluminating light. An image of the specimen was acquired for each polarization, and the four images were used to determine the average and differential transmittance and polarization orientation for maximum transmittance for each resolved specimen point.49 As mentioned previously, the optical properties of individual rGO sheets are expected to be highly anisotropic. On the basis of the atomic structure, we assume a uniaxial anisotropy with an optic axis perpendicular to the plane of carbon atoms (or parallel to the plane’s normal). To describe the orientation of an individual rGO sheet with respect to the light path of the microscope, we define two angles within a laboratory frame of reference x−y−z: (i) the inclination angle, which is the angle between the normal of an rGO sheet and the x−y plane, and (ii) the azimuth angle, which is the angle between the projection of the normal on the x−y plane and the x-axis, as shown in Figure 2. The x−y plane is the object plane imaged onto the camera by the microscope optics, and the z-axis is the microscope’s optical axis.

Figure 2. Relationship between the alignment of an rGO sheet within a film and its optical properties. The azimuth is the polarization angle in the x−y plane that gives the maximum transmittance, and the inclination angle is the angle between the normal of the rGO sheet and the x−y film plane. The two extreme cases of inclination angles are shown to the right. rGO is presented as a rigid plate for simplification only.

apparent retardance and azimuth orientation of two morphologies of rGO assemblies. For comparison, we show images of CAS films and rGO aggregates. In the CAS film images (Figure 3a) we see a film broken into four pieces each one made up of stripes of different film thickness as determined by the transmittance of the stripes seen in the top panel. A darker stripe indicates less transmittance and thicker film, while thicker film stripes have more apparent retardance and therefore appear brighter in the central panel. Despite the variability in film thickness and anisotropy, the azimuth angles of CAS films are constant, as shown by the uniform hue and vector direction in the center and right images of Figure 3a, in contrast to the random rGO aggregates in the respective images of Figure 3b. To quantify the consistency of alignment of rGO assemblies, we use a scalar order parameter S for the distribution of azimuth angles in the x−y plane. In analogy to liquid crystals,50 we define S = ⟨2 cos2 ϕ − 1⟩, where ϕ is the angle between the azimuth at each pixel and the mean azimuth. S = 1 represents a perfectly oriented system and S = 0 an isotropic one. We calculated S from the azimuth data of about 1000 pixels and obtained S ∼ 1 for the CAS films and S ∼ 0.3 for the aggregates, which may result from some alignment in the aggregation process or during transfer to the glass slide. The standard deviation of the azimuth for the CAS film is also narrow and ∼3°. We note that due to the circular nature of the distribution the standard deviation can only be compared between relatively ordered films (distribution 0.5 nm (scale bar is 10 μm). At the right polar histograms of the azimuth angles and the inplane order parameters (taken from ∼550 pixels).

vacuum filtration, with FOM of (15 ± 4) × 103 and (132 ± 68) × 103, for CAS films and films formed by vacuum filtration, respectively. A table showing the measurement on CAS and vacuum filtration films is shown in section S7 of the Supporting Information. We note that films prepared by vacuum filtration yields high ordering and high conductivity only when the films are thick (>1 μm),16,30 since at the initial stage of this process the sheets are randomly assembled and therefore the structure is not compact.16 The average values, as well as the standard deviations, indicate that the CAS films have a compact morphology that allows good contact between the layers and therefore high electrical conductivity. The images from the LPS and the azimuth distribution analysis in Figure 6b show that the films formed by vacuum filtration are not anisotropic within the film plane (S ∼ 0.3) and are not as uniform as the CAS films (Figure 5a). Spray-coating, another standard technique for preparation of thin films, is likely to yield less ordered structures than vacuum filtration since the films are formed by rapid evaporation of solvent resulting in rapid convection and hence was not discussed here.66 Annealing the CAS films at 400 °C under Ar/H2 flow overnight increased the conductivity from 1000 to 5000 S/m and decreased the FOM to 3 × 103. The CAS films are significantly superior to films made by slow vacuum filtration of GO suspensions and their subsequent reduction. For example, Eda et al.17 prepared thin films of GO by slow vacuum filtration, which after reduction with hydrazine vapors yielded FOM values larger than 2 × 106 (calculated from the data in the graphs, Figure 3b of ref 17), which is a few orders of magnitude larger than our CAS films. While it is difficult to compare the performance of CAS films made from rGO versus dip-coated films from GO with further reduction because of different processing conditions, the CAS films are slightly better than GO films prepared by dip-coating and subsequent chemical reduction with hydrazine for 20 h and thermal reduction at 700 °C, such as in Dickinson et al.67 that yielded FOM of 3.6 × 103. 264

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4. CONCLUSIONS In summary, we show that highly ordered and optically anisotropic rGO based films can be formed from isotropic suspensions by capillary-force-assisted self-assembly (CAS) process. The films have good electrical conductivity due to a compact layered structure. Contrary to conventional dipcoating processes in which convection and shear forces direct the particles to a uniform structure, in CAS films rGO colloids self-assembled into a highly ordered architecture through a diffusion controlled and capillary-force-assisted process. In addition, we demonstrate a method for quantitative measurement of anisotropy of films and suspensions using the LPS polarized light system, a technique that to the best of our knowledge has not been previously utilized in characterizing graphene-based materials. We demonstrate that diattenuation measurements provide a reliable axis of alignment of the rGO sheets, while distribution of azimuth angle and scalar order parameters, calculated from the birefringence or diattenuation mode, are good measures for the degree of anisotropy in the film plane. CAS films with dimensions of ∼25 × 18 mm2 were formed directly on glass substrate in the laboratory; the size is being limited only by the volume of the suspension. We believe that the capabilities of the LPS technique in characterizing graphene-based thin films and suspensions can connect fundamental metrology to manufacturing of graphene-based transparent conducting films.

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ASSOCIATED CONTENT

S Supporting Information *

GO and rGO preparation, films by filtration, absorption coefficient evaluation, SEM images, rGO sheets size evaluation, suspensions characterized by the LPS, Peclet number and diffusion coefficient calculations, LPS, and figure of merit. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +61399056255 (M.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge use of facilities within the Monash Center for Electron Microscopy, funding from the Australian Research Council (LP 110100612 and DP 110100082), and a grant from the US National Institute of Biomedical Imaging and Bioengineering (grant R01EB002045) awarded to R.O.

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REFERENCES

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dx.doi.org/10.1021/jp4080114 | J. Phys. Chem. C 2014, 118, 259−267

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2011 MRS Fall Meeting, Nov 28, 2011−Dec 2, 2011, Boston, MA; Materials Research Society: Boston, MA, 2012; pp 155−160.

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dx.doi.org/10.1021/jp4080114 | J. Phys. Chem. C 2014, 118, 259−267