Article pubs.acs.org/Langmuir
Nematic Order Drives Macroscopic Patterns of Graphene Oxide in Drying Drops Yanqi Luo, Gregory A. Braggin, Grant T. Olson, Alexandra R. Stevenson, Wanda L. Ruan, and Shanju Zhang* Department of Chemistry and Biochemistry, California Polytechnic State University, San Luis Obispo, California 93407, United States S Supporting Information *
ABSTRACT: We report on a series of experiments on large-area ordered patterns of graphene oxide on solid substrates deposited from aqueous dispersions by directed drop evaporation. The aqueous dispersion of graphene oxide exhibits phase transitions from isotropic to liquid crystalline nematic phases via a biphasic region with increasing concentration. In the single nematic phase, schlieren textures accompanied by oriented bands are frequent. Drying of drops in each phase results in deposition covering the whole drop base. The dynamic process of drop drying is analyzed based on the weight loss, radius change, and texture change over time. It is found that the radial bands develop in the nematic drops in the vicinity of the receding of the contact line and subsequently transform into birefringent stripes after drying. Study into the structure and morphology of the stripes reveals anisotropic wrinkling of graphene oxide sheets. The nature of stripe orientation is strongly dependent on the local nematic order at the dewetting water front. Various macroscopic patterns with different stripe orientations including radial spokes, spider webs, and parallel stripes have been generated by tuning the nematic order of drops.
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INTRODUCTION Graphene is a two-dimensional (2D) monatomic layer of sp2hybridized carbon atoms in a honeycomb lattice and possesses the unique combination of intriguing Dirac-like electronic properties, exceptional mechanical properties, and high thermal conductivities.1−3 Translating the excellent properties of individual graphene sheets as a building block into macroscopic material forms may pave the way for the ultimate utilization of this nanostructured carbon allotrope for various emerging applications including optoelectronics,4 supercapacitors,5 photocatalysis,6 actuators,7 biomedicine,8 sensors,9 water purification,10 and so on. In particular, the fine control of spatial ordering of graphene is highly needed for the full exploitation of its excellent features. Currently, it is challenging to achieve macroscopically ordered assemblies of graphene. Graphene oxide (GO) has been received great attention as an intermediate in the graphene synthesis.11−13 Reduction of GO to graphene has been considered as a cost-effective route for the large-scale production of graphene. From the geometric perspective, graphene and GO can be viewed as highly anisometric plate-like nanoparticles. Onsager’s steric theory states that entropy-driven liquid crystalline (LC) nematic order occurs spontaneously in solutions (or dispersions) of anisotropic moieties above the critical volume fraction as a consequence of simple excluded volume interactions.14 The system undergoes a phase transition from an isotropic to a nematic phase passing through a biphasic region in which © XXXX American Chemical Society
isotropic and nematic phases coexist. Experimentally, the LC behavior of GO has been recently reported in aqueous and organic dispersions.15−20 Interestingly, both nematic and smectic LC phases have been reported in aqueous disperions of GO.21−23 Remarkably, GO sheets in the LC phases are readily aligned to form ordered structures under weak external forces such as shear flow, electric fields, and magnetic fields.16,24,25 In this regard, GO sheets in the LC phase demonstrate the large anisotropy of polarizability and giant optical sensitivity to the external fields.25 Thus, LC properties of GO provide unique opportunities toward fabricating macroscopic ordered assemblies of graphene. Evaporation of drops of colloidal dispersions on substrates is a facile and powerful approach to organize colloids into wellordered structures.26−28 A well-known phenomenon during evaporation is a coffee-ring stain in which the colloids form a ringlike deposit along the perimeter of the original drop.29 It is believed that drop edges become pinned to the substrate during evaporation, and capillary flow outward from the drop center brings colloids to the edge. Drop drying has been utilized to produce well-ordered structures of DNA,30 virus particles,28 conjugated polymers,26 semiconducting nanorods,31 carbon nanotubes,32 etc. However, the flow instability in the drying Received: September 15, 2014 Revised: November 4, 2014
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drop often yields irregular structures.26 To obtain large-scale well-ordered patterns with controllable alignment, location, and interspacing, controlled evaporation is required.26,33 To this end, LC ordering has been employed to direct the ordered deposition of anisotropic particles from solutions onto substrates.34 In this work, we report on macroscopic patterns of GO in drying drops driven by LC nematic order. GO solutions undergo an isotropic-to-LC transition via a biphasic region with increasing concentration. Surprisingly, the coffee-ring effect is suppressed after complete evaporation of GO drops. To this end, various large-scale ordered patterns of GO including radial spokes, spider webs, and parallel stripes have been obtained by tuning the local LC nematic order.
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series of GO dispersions with different concentrations were prepared to examine optical birefringence under crossed polarizers. As expected, a phase transition from an isotropic to a LC phase via a biphasic region was found with increasing concentration. Figure 1 shows the phase diagram and
EXPERIMENTAL SECTION
Preparation of Graphene Oxide. Graphene oxide was prepared from natural graphite flakes using a modified Hummers method.35 In a typical synthesis procedure, 1.0 g of graphite flakes was treated with a mixture of concentrated sulfuric acid (30 mL) and nitric acid (10 mL) at room temperature under ultrasonication for 3 h. The acid-treated graphite flakes were rinsed with deionized (DI) water five times to collect the paste-like solid. The dried solid sample was then added into 200 mL of concentrated sulfuric acid at 0 °C, in which 10 g of potassium permanganate was subsequently added under 20 °C. The mixture was then heated to 35 °C and stirred at this temperature until the mixture became light brown in color with a significant increase in viscosity. To terminate the oxidization reaction, 200 mL of DI water was added to the mixture under stirring for 30 min in an ice bath, followed by dropwise addition of 3 mL of 30% hydrogen peroxide. To collect the product, the dispersion was centrifuged to produce a bright yellow precipitate, which was then washed with 3 mL of 1.0 M hydrochloric acid. The precipitate was further washed with DI water until the pH reached ∼5. After centrifugation, the gel-like aqueous dispersion of graphene oxide was obtained. Characterization Methods. Fourier transform infrared (FTIR) spectra were collected on a Nicolet iS10 FT-IR spectrometer in an attenuated total reflection (ATR) mode. UV−visible (UV−vis) absorption spectra were obtained in aqueous solution on a Jasco V550 spectrophotometer. X-ray diffraction (XRD) analysis was taken on a Siemens D5000 diffractometer in a reflection mode with a 1.54 Å Cu Kα radiation source. Scanning electron microscopic (SEM) images were collected on a FEI Quanta 200 microscope operated at an acceleration voltage of 20 kV. Atomic force microscopic (AFM) images were taken on a Park Xe-70 microscope. The liquid crystalline textures were determined on a Leica DM2500P optical microscope under crossed polarizers. The Leica ICC50 HD video camera was used to record optical images during drop drying at regular time intervals and the drop radius was evaluated using ImageJ. The drop mass of GO dispersions during evaporation was measured using Sartorius analytical balance with 0.01 mg resolution. To study the effect of the shear flow on the pattern formation, the thin layer of the LC dispersion of GO sheets on the clean glass slide was formed by casting and shearing at a high coating speed.
Figure 1. Phase diagram of GO sheets in aqueous dispersions as a function of concentration. Typical optical micrographs in isotropic, biphasic, and nematic liquid crystalline phases are demonstrated at the top. The scale bar represents 500 μm.
representative optical micrographs in each phase. The biphasic range was estimated to be between 0.015 and 0.35 wt %. Such low critical concentration values of phase transitions are attributed to the large aspect ratio of GO sheets.17 In a dilute dispersion below 0.015 wt %, GO sheets as individual entities move randomly and produce an isotropic phase that appears dark under crossed polarizers. With increasing concentration beyond 0.015 wt %, GO sheets start to align themselves due to steric interactions to develop birefringent nematic domains in the isotropic phase, forming the biphasic regime. Further increasing concentration beyond ∼0.35 wt %, nematic domains grow and join together to form a single LC phase, showing typical schlieren textures with black brushes.36 Interestingly, schlieren textures with both two brushes and four brushes are frequent (Figures 1 and 2c,d), which correspond to disclinations with strengths of s = ±1/2 and ±1, respectively.37 This observation is consistent with the literature report on aqueous GO LCs.16,22 In general, integer disclination defects are thermodynamically unstable and rarely observed in LCs.37 It is believed that the giant GO sheets may kinetically trap integer disclinations. Indeed, we occasionally observed that the single integer disclination decomposed into two half disclinations during imaging. Upon rotation of the crossed polarizers, the black brushes rotate in the same direction as the polarizers for positive disclinations or in the opposite direction for negative disclinations. At 45 degree rotation, the black and bright regions change alternatively (SIFigure 3). Other than disclination defects, inversion wall defects are frequently observed, which appear as black lines under crossed polarizers (Figure 2a). Typically, isolated inversion walls have mirror symmetry, and the LC director possesses a rotation of π from one side of the wall to the other38 (Figure 2b). As the LC director of GO is normal to the GO sheets,17 the splay LC director corresponds to bending of the GO sheets, and vice versa. Remarkably, the walls are often aligned to form banded textures. In particular, both radial and circular bands around the integer disclinations are often observed (Figure 2c,d and SI-
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RESULTS AND DISCUSSION Liquid Crystallinity. In this work, GO sheets were prepared using the modified Hummers method.35 It is evident that GO sheets are single-layered with a thickness of ∼1.1 nm and a mean size of ∼5.0 μm (SI-Figure 1). The average aspect (width/thickness) ratio of GO sheets could reach ∼4.5 × 103. The chemical structure of GO was confirmed by means of FTIR, XRD, and UV−vis (SI-Figure 2). As-prepared GO sheets are highly soluble in water, and the concentration of GO dispersions could reach ∼15 mg/mL (1.5 wt %) in which gelation occurs spontaneously. To study the LC behavior, a B
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Figure 3. Drop drying of GO sheets in the aqueous dispersion: (a) side view of changes in the drop during evaporation; (b) top view of patterns during drying, and arrows show compression forces; (c, d) spoke patterns in the dried drops from 0.30 and 1.0 wt % dispersions, respectively. The scale bar represents 1 mm.
Figure 2. (a) Optical micrograph of an isolated inversion wall defect. (b) illustration of bend and splay patterns of LC director fields around the isolated inversion wall defects. Solid lines represent GO sheets, and dashed lines represent inversion walls. (c) Optical micrograph of radial bands around an s = +1 disclination. (d) Optical micrograph of circular bands around an s = +1 disclination. The scale bar represents 500 μm.
Figure 3). It is well-known that the polymeric LCs form banded textures under shear flow, and the bands are associated with a periodic variation in LC director orientation about the flow axis.39−41 The GO sheets can be viewed as giant oblate macromolecules, and they are highly sensitive to external forces due to the large aspect ratio and excessive mechanical flexibility.21 When casting the LC dispersion of GO on substrate, local compression stresses due to the osmotic compressibility during drop evaporation may cause GO sheet buckling to generate the periodic bands. Recently, the similar circular banded texture has also been reported in the literature on aqueous GO LCs.21 As the elastic distortion of the LC director is determined by a balance of splay and bend elastic constants,38 our observation on both radial and circular bands indicates that both splay and bend elastic constants of GO sheets are of the same order of magnitude. Drop Drying. To obtain macroscopic assemblies of GO sheets, a drop drying approach was employed. Typically, 10 μL of drops was placed on the glass slide to permit evaporation of water (Figure 3a,b). Surprisingly, deposition of radial spoke patterns (Figure 3c,d) rather than coffee-ring patterns forms in drops with a wide range of concentrations from an isotropic to a biphasic to a LC nematic phase after complete evaporation. Suppression of coffee-ring patterns is indicative of depinning of the contact line during evaporation42 (Figure 3a,b). Interestingly, the radial spoke patterns show optical birefringence under the crossed polarizers (Figure 3c,d), indicating changes in local orientation of GO sheets (Figure 3b). The oriented stripes in the spoke pattern are normal to the drop edge (Figure 3c). With increasing concentrations of starting drops, the spoke patterns become more birefringent with larger periodicities of stripes while the center of the spoke patterns becomes less ordered (Figure 3d). To understand the physical origin of macroscopic patterns, the dynamic process of drop drying has been investigated. The drop mass of GO suspensions of isotropic, biphasic, and LC phases has been analyzed during evaporation as shown in Figure 4a. It is evident that the drop mass decreases linearly with time. The evaporation rates of drops from the isotropic, biphasic, and LC nematic phases are similar to that of pure
Figure 4. Dynamic process of drop drying from GO dispersions with different concentrations: (a) the drop mass as a function of drying time; (b) the drop radius ratio rc/r0 as a function of normalized drying time tc/tf. The inset in each figure shows the concentration of starting drops. rc and r0 stand for the drop radius evaporation continues and the original one, respectively. tc and tf stand for the time evaporation continues and the time evaporation finishes, respectively.
water, which was determined to be ∼2.5 μg/s. However, the evaporation rate of LC gels from 1.5 wt % dispersion is slightly smaller due to the higher viscosity. To this end, video microscopy was used to record drops from isotropic, biphasic and LC nematic phases during evaporation. The diameter of drops in each phase was calculated at three different angles, and the averaged value was then analyzed. Figure 4b shows the drop radius as a function of evaporation time in each phase. At the beginning of evaporation, the drop radius remains constant, and the contact line is therefore pinned. When the evaporation time reaches the critical value tcr, the drops start to shrink until water is completely evaporated. With increasing concentration, the value of tcr decreases accordingly, indicating fast depinning.43 We found that depinning occurred at tcr ∼ 0.60 tc/tf from 0.010 wt % isotropic drops while it did at tcr ∼ 0.15 tc/tf from 1.5 wt % LC gels (Figure 4b), where tc and tf represent the time evaporation continues and the time evaporation finishes, respectively. This depinning phenomenon is in contrast with drop drying of spherical colloids. The latter keeps a fixed base radius during evaporation and thus produces coffee-ring deposition.29 Figure 5 shows a series of optical images under cross polarizers of drops from a biphasic region during evaporation. The optical birefringent ring forms at tc/tf ∼ 0.25, while the contact line is still pinned at the initial location. The birefringent rim displays a biphasic-to-LC phase transition. During this process, the convective flow associated with the solvent evaporation brings the GO sheets from the drop C
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Figure 5. A series of optical micrographs of a drop from 0.30 wt % solution at different time tc/tf during the evaporation. (a) tc/tf = 0, (b) tc/tf = 0.3, (c) tc/tf = 0.4, (d) tc/tf = 0.5, (e) tc/tf = 0.8, and (f) tc/tf = 1.0. The scale bar represents 1 mm.
interior to the periphery and makes the local concentration sufficiently high to initiate the transition from the biphasic to the LC nematic state.32 The resulting LC-like rim then serves as a nucleation site for the formation of fingerlike structures when the contact line is depinned (Figure 5c−e). As the stripes propagate inward, the radial spoke pattern will develop when the drop is completely dried (Figure 5f). While the formation procedures of radial spoke patterns in isotropic and LC phases are similar to that in the biphasic region, there is one significant difference in the LC phase. The banded textures in the LC phase are apparent at the dewetting front (Figure 6). Upon rotation of the sample in the presence
Figure 7. SEM images of the radial spoke pattern in the dried GO film: (a) the whole deposition, (b) the edge, (c) the region away from the edge, and (d) the center. The scale bar represents 500 μm in (a), 100 μm in (b), 200 μm in (c), and 100 μm in (d).
literature.44,45 It is believed that a LC gel-like thin skin develops on the top free surface of the drop in the vicinity of the retreating water front during evaporation.44,46,47 When the drop starts to shrink, the LC gel-like skin tends to buckle under compression to generate wrinkles (Figure 3b).46 In this regard, the inversion walls of banded textures in the local LC region act as nucleation sites for promoting anisotropic wrinkled structures that may store elastic energies. To this end, the resulting radial spoke pattern in the dried GO film is closely correlated to local LC nematic order at the dewetting front. This observation suggests a possible approach to produce various macroscopic patterns in drop-dried GO films by simply tuning the LC nematic order at the dewetting front. Directed Assembly. Directed assembly refers to a process by which nanoparticles organize into ordered structures with aids of the directing fields.48−50 Such directing fields include shear flow, surface tension, molecular forces, templates, electric, and magnetic fields, or combinations thereof. The directing fields can modulate the thermodynamic forces that drive nanoparticle assembly with controllable orientation and spacing. To obtain macroscopic patterns of GO sheets, combination of surface fields, shear flow, and LC order was utilized during drop drying. Figure 8 shows a series of optical images under crossed polarizers during evaporation of the LC drop containing radial bands around a single integer disclination in the center. It shows that the fingerlike structure gradually appears during evaporation (Figure 8a,b). Before water is completely evaporated, the disclination texture with radial bands in the drop remains undisrupted (Figure 8b,c), and the radial bands promote radial stripes to eventually form the radial spoke pattern in the dried deposition of the drop (Figure 8d). Figure 9 shows a series of optical images during evaporation of the LC drop containing circular bands around a single integer disclination in the center. At the beginning of evaporation, the contact line is pinned and radial capillary flow carries GO sheets to the edge. During this process, GO sheets reorient to adopt homeotropic anchoring at the edge.44 In the vicinity of the receding of the contact line, the radial bands form and subsequently transform into radial stripes when the dewetting occurs (Figure 9a). As the dewetting front moves
Figure 6. Optical micrographs of the drop from 1.5 wt % dispersion during evaporation. Banded textures (a) without and (b) with retardation plate and (c) schematic of the banded texture. The scale bar in (a) and (b) represents 1 mm. Solid lines represent GO sheets, and dashed lines represent inversion walls of bands.
of a retardation plate under the crossed polarizers, the continuous changes of alternating blue and orange/red bands are demonstrated. It has been recognized that at the liquid/ substrate and liquid/air interfaces GO sheets adopt homeotropic (face-on) anchoring to minimize the excluded volume entropy and interfacial adsorption energy.44 When the drop shrinks due to depinning, large orthoradial compression stresses will be generated in the LC phase. As a result, a periodic distortion of LC director forms (Figure 6c).30 Therefore, the physical origin of banded textures lies in the coupling between the orthoradial compression stress in the vicinity of the receding of the contact line and the LC elasticity of the GO dispersion. Subsequently, the inversion walls in the banded textures serve as nuclei to promote the radial stripes of the spoke pattern when the drop is completely dried. Figure 7 shows typical SEM images of radial spoke patterns in the dried GO film. The surface wrinkles of GO sheets in the spoke patterns are apparent, and they correspond to birefringent fingerlike features under crossed polarizers. The anisotropic wrinkling behavior in this work is remarkably different from the random wrinkling of GO sheets in the D
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Figure 8. Optical micrographs of the formation of the radial spoke patterns from the drop containing radial bands around a +1 disclination in the center during evaporation: (a) nematic texture with the center focus, (b) same as (a) with the edge focus, (c) the pattern formation at the later stage of evaporation, and (d) radial spoke pattern in the dried deposition. The scale bar represents 1 mm.
Figure 10. Optical micrographs of the formation of the parallel stripe patterns from the sheared LC dispersion of GO upon evaporation: (a) banded texture under simple shear flow, (b) schematic of the banded pattern, and (c, d) parallel stripe patterns in the dried deposition. The arrow in (a) shows the direction of shear flow. Solid lines in (b) represent GO sheets, and dashed lines represent inversion walls. The scale bar represents 500 μm in (a, c) and 200 μm in (d).
45° to the polarizer but becomes zero when the bands are parallel to the polarizer. After water evaporation, the bands transform into parallel stripes on the deposition (Figure 10c,d). Interestingly, the orientation of the stripes is least affected by the substrate boundaries, and therefore, the stripes are parallel to the band direction at the edge as well (Figure 10c). It seems that the dewetting process is perturbed by defects on the substrate, leaving behind twisted or branched stripes (Figure 10d).
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CONCLUSIONS We have demonstrated a facile bottom-up approach for the fabrication of large-area ordered assemblies of graphene oxide on a substrate by directed drop evaporation. Aqueous dispersions of graphene oxide undergo isotropic to biphasic to liquid crystalline nematic transitions with increasing concentration. Contrary to well-known coffee-ring stains, depinning of graphene oxide during drop drying in each phase occurs. After evaporation, various macroscopic patterns such as radial spokes, spider webs, and parallel stripes are formed by controlling the local liquid crystalline nematic order at the receding water front. As graphene oxide is readily reduced to generate graphene using thermal treatments or strong reducing reagents, our work may provide opportunities for the large-scale production of ordered assemblies of graphene by utilizing liquid crystallinity for various emerging applications.
Figure 9. Optical micrographs of the formation of the spider-web patterns from the LC drop containing circular bands around a +1 disclination in the center during evaporation: (a) formation of primary radial stripes, (b) formation of secondary lateral stripes, (c) formation of spider-web patterns, and (d) the dried deposition. The scale bar represents 1 mm.
toward the drop center, the secondary lateral stripes develop between the primary radial stripes (Figure 9b,c), resulting in a spider-web pattern in the dried deposition of the drop (Figure 9d). In this regard, the width and periodicity of the secondary lateral stripes are much smaller than those of the primary radial stripes. It is evident that the formation of the secondary lateral stripes is closely associated with the initial circular bands around the single integer disclination. In the drop center, LC polydomain structures transform into disordered stripes when water is completely evaporated (Figure 9d). To develop well-aligned bands for the production of parallel stripe patterns, a simple shear flow was applied to the LC dispersion of GO sheets. The resulting optical bands are well aligned along the direction of shear flow (Figure 10a). It is believed that high shear flow can induce GO sheets to reorient along the shear direction.44 During the relaxation, GO sheets will form zigzag structures that display optical banded textures under the crossed polarizers (Figure 10b).41 The periodicity of the bands is about tens of micrometers, and it is dependent on the GO concentration and the flow rate. Light transmission under the crossed polarizers is maximized when the bands lie
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ASSOCIATED CONTENT
S Supporting Information *
Data of AFM, SEM, FTIR, XRD, UV−vis, and POM. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel +1 805 756 2591; Fax +1 805 756 5500; e-mail
[email protected] (S.Z.). Notes
The authors declare no competing financial interest. E
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ACKNOWLEDGMENTS We thank Dr. Gregory Scott for AFM analysis and Dr. Satish Kumar for SEM aids. This work is primarily supported by the Extramural Funding Initiative of Cal Poly. Y.L. and A.R.S. acknowledge summer research support from the Frost Research Fellowship of Cal Poly. S.Z. acknowledges financial support from the National Science Foundation (CMMI-1345138) and American Chemical Society-Petroleum Research Fund (53970UR7).
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