Morphology Control of Surfactant-Assisted Graphene Oxide Films at

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Morphology Control of Surfactant-Assisted Graphene Oxide Films at the Liquid−Gas Interface Hyeri Kim,†,‡ Young Rae Jang,† Jeseung Yoo,§ Young-Soo Seo,*,§ Ki-Yeon Kim,† Jeong-Soo Lee,† Soon-Dong Park,† Chan-Joong Kim,† and Jaseung Koo*,† †

Division of Neutron Science, Korea Atomic Energy Research Institute (KAERI), 989-111 Daedeok-daero, Yuseong-gu, Daejeon, 305-353, South Korea ‡ Department of Chemical and Biological Engineering, Korea University, Seoul, 136-701, South Korea § Department of Nanoscience Technology, Sejong University, Seoul, 143-747, South Korea S Supporting Information *

ABSTRACT: Control of a two-dimensional (2D) structure of assembled graphene oxide (GO) sheets is highly desirable for fundamental research and potential applications of graphene devices. We show that an alkylamine surfactant, i.e., octadecylamine (ODA), Langmuir monolayer can be utilized as a template for adsorbing highly hydrophilic GO sheets in an aqueous subphase at the liquid−gas interface. The densely packed 2-D monolayer of such complex films was obtained on arbitrary substrates by applying Langmuir− Schaefer or Langmuir−Blodgett technique. Morphology control of GO sheets was also achieved upon compression by tuning the amount of spread ODA molecules. We found that ODA surfactant monolayers prevent GO sheets from sliding, resulting in formation of wrinkling rather than overlapping at the liquid−gas interface during the compression. The morphology structures did not change after a graphitization procedure of chemical hydrazine reduction and thermal annealing treatments. Since morphologies of graphene films are closely correlated to the performance of graphene-based materials, the technique employed in this study can provide a route for applications requiring wrinkled graphenes, ranging from nanoelectronic devices to energy storage materials, such as supercapacitors and fuel cell electrodes. manner.20 However, this technique, namely a direct spreading of GO on water surface, can not be applied for small-sized GO sheets since smaller sheets with the high edge-to-area ratio possess greater numbers of hydrophilic oxygenated functional groups, thereby exhibiting higher solubility in the water and sinking into aqueous subphases upon compression.23 In this work, we used an amine-terminated surfactant, octadecylamine (ODA) Langmuir monolayer, as a template for mediating the surface anchoring of GO sheets and inducing the formation of assembled GO monolayer at the liquid−gas interface (Figure 1a). This technique using surfactant Langmuir monolayers has been previously applied for clay monolayer formation.24−27 Recently, Gengler et al. also employed this technique for GO monolayer preparation on solid substrates. However, they obtained isolated GO sheets with only a low packing density even at the high pressure.28 In the current work, we achieved the full coverage of GO−ODA hybrid monolayer on the large substrate and successfully controlled coverage and morphologies of GO films by using various concentrations of GO and ODA upon compressing hybrid monolayers. The layer structure was measured by atomic force

1. INTRODUCTION Graphene exhibits intrinsic features of optically transparent two-dimensional (2-D) structure with remarkable electrical conductivity and excellent mechanical strength, thereby gathering substantial interest in recent years for a wide range of important technological applications, such as optoelectronics, photovoltaics, electrochemical energy storage, display devices, and sensors.1−7 A key to success in such application is to develop procedures to obtain isolated graphene single layers on different substrates for large scale. Various methods, such as mechanically cleaving Scotch tape method,8 epitaxial graphene growth,9,10 and chemical vapor deposition (CVD),11,12 have been explored for fabrication devices on the single layer of graphene sheets. Recently, the chemical approach for exfoliated graphene in solution phase has also drawn attention because this method allows researchers to transfer graphene monolayers from solutions to any arbitrary surfaces.13−19 Graphene oxide (GO) sheets, obtained from chemical exfoliation of graphite, are well-dispersed in the water. Their amphiphilic property, due to hydrophilic carboxylic group at the edges and hydrophobic unoxidized basal plane, facilitates close-packed assembly of GO monolayers at the air−water interface and deposition on solid substrates by a Langmuir−Blodgett (LB) technique.13−17,20−22 Originally, Li et al., for the first time, achieved high-quality graphene sheets by the LB technique in a layer-by-layer © 2014 American Chemical Society

Received: August 26, 2013 Revised: February 4, 2014 Published: February 5, 2014 2170

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stirred at 35 °C for 2 h. Subsequently, DI water (92 mL) was added below 50 °C. The reaction was terminated by adding a large amount of distilled water (280 mL) and 30% H2O2 solution (8 mL). The color of the mixture changed to bright yellow. After a day, the mixture was washed with 4 M HCl solution in order to remove metal ions, followed by centrifuging with DI water. The graphene oxide solution was obtained with sonication for 10 min. In order to prepare GO films, we used the aqueous GO suspension at the desired concentration, c, ranging from 2 to 100 ppm in the deionized (DI) water (pH 7.4) and applied Langmuir−Schaefer (LS) or Langmuir−Blodgett (LB) technique. The octadecylamine (ODA) (Aldrich, St. Louis, MO) solution (volume, v = 25 μL) at the concentration of 1 mg/mL in chloroform was then spread on the subphase of the prepared suspension (total area (A) = 367 cm2) at 20 °C and pH 7.4 in a LB trough (KSV 2000, KSV NIMA, Espoo, Finland). The pH values were checked using a pH meter (Thermo Scientific, Waltham, MA). After 15 min, the GO film was compressed at a speed of 5 cm2/min with monitoring the surface pressure−area (π−A) isotherm using a 25 mm wide platinum Wilhelmy plate suspended from a microbalance by a motor-controlled barrier with feedback. After the desired surface pressure was achieved, the films were then horizontally transferred on hydrophobic HF-etched silicon wafers, as shown in Figure 1b. After deposition, the substrates with attached GO sheets were gently washed several times with DI water and annealed under the vacuum at 80 °C for 2 h. For reduction of GO films, a hydrazine monohydrate 98% was purchased from Sigma-Aldrich (St. Louis, MO). We used a hydrazine vapor reduction method to produce reduced graphene oxide (rGO) films. (The detail procedures were described in the previous report.34) After hydrazine treatment, dry GO films on Si substrates were then heated to 400 °C in an Ar atmosphere for 3 h and were cooled down to room temperature. The surface structure of GO film on the substrate was characterized by an atomic force microscopy (AFM) equipped with a piezo scanner (Nanoscope IIIa; Veeco Instruments Inc., Plainview, NY). We applied the noncontact tapping mode using a silicon nitride tip. Unpolarized Raman spectra were recorded at room temperature for GO and reduced GO films deposited on silicon substrates. InVia Raman microscope (Renishaw Ltd., Gloucestershire, U.K.) with the excitation source of the 633 nm line from a He−Ne laser was used. Their elemental composition was also characterized by using X-ray photoelectron spectroscopy (XPS) (ULVAC-PHI, Quantera SXM, Osaka, Japan). Thermogravimetric analysis (TGA) (TA Instruments, SDT Q600, New Castle, DE) was performed for thermal characterization of the dry ODA powder, with heating at a rate of 5 °C/min under purified nitrogen gas flow. The specular neutron reflectivity (NR) experiments were conducted with the REF-V reflectometer at the Cold Neutron Laboratory Building of the HANARO at the Korea Atomic Energy Research Institute (Daejeon, Korea) with a wavelength (λ) of 4.75 Å and ΔQ/Q = 0.02−0.06.

Figure 1. (a) Schematic presentation of GO−ODA hybrid film formation at the interface between air and GO suspension in the water (ODA molecules and GO sheets are not to scale). The exfoliated GO sheets can be floated to the surface due to interaction between the positively charged amine groups of ODA surfactants and the negatively charged oxygenated groups of GO sheets. (b) Experimental setup for the LS deposition of GO−ODA hybrid films. The trough is filled with diluted GO dispersion. The ODA monolayer is spread and compressed while the surface pressure is monitored.

microscopy (AFM) and neutron reflectivity (NR) after being transferred by LB and Langmuir−Schaefer (LS) approaches. We proposed the mechanism of wrinkle formation of GO− ODA hybrid Langmuir films. The technique employed in this study can provide multiple depositions on different substrates upon compression with maintaining the pressure by feedback, representing a route for its incorporation into automated manufacturing system of the graphene film fabrication.

2. EXPERIMENTAL SECTION Si(100) wafers (LG Siltron Corp., South Korea) were treated with a modified Shiraki technique:29,30 the substrates were immersed in a mixed sulfuric acid solution, H2O:H2O2:H2SO4 (=1:1:1 vol) for 20 min at 80 °C, rinsed in deionized (DI) water, and immersed in very diluted hydrofluoric acid solution (H2O:HF = 10:1 vol) for 30 s, at 21 °C, to create a hydrophobic surface. The HF etched substrates were then rinsed with DI water again and dried in N2 gas before the film coating. Ultralarge GO was synthesized by modified Hummers and Offeman’s method13,31−33 from flake graphite (Aldrich, St. Louis, MO), while GO for a small size was purchased from Angstron Materials (Dayton, OH). For preparation of GO sheets, K2S2O8 (10 g) and P2O5 (10 g) were dissolved completely in concentrated H2SO4 (50 mL) at 90 °C. Graphite flake (12 g) was added to the solution and stirred at 80 °C for 5 h. Subsequently, DI water (2 L) was added and stored at room temperature overnight. The mixture was then carefully diluted with DI water and filtered through a filter paper with 5 μm pore until the pH became neutral. Dried product (2 g) and NaNO3 (1 g) were added to cold concentrated H2SO4 (46 mL), and then KMnO4 (6 g) was gradually added with stirring while keeping the solution temperature below 10 °C in an ice bath. The mixture was

3. RESULTS AND DISCUSSION Graphene oxide (GO) sheets are the product of chemical exfoliation of graphite, derivatized by carboxylic acid at the edges, and phenol hydroxyl and epoxide groups mainly at the basal plane.35,36 Therefore, GO sheets can be readily dispersed in water against flocculation or coagulation by electrostatic repulsion between layers.37 In order to induce the formation of the GO monolayers at the liquid−gas interface, we spread the amine-terminated alkyl surfactant monolayer from chloroform solution on the surface of GO suspensions. The positively charged amine groups on surfactants, i.e., octadecylamine (ODA), interact with the oxygenated groups on GO sheets, thereby inducing the exfoliated GO sheets to anchor at the interface between air and GO suspension, as illustrated in Figure 1a. We monitored the surface pressure−area (π−A) isotherms during the compression of the GO−ODA complex hybrid 2171

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monolayer. Figure 2 shows the isotherms of GO−ODA films prepared on the GO suspension at various concentrations.

Figure 2. Surface pressure−area (π−A) isotherms recorded for ODA molecules after spreading 25 μL of the solution (1 mg/mL) on the surface of aqueous GO suspensions with various concentrations, 0, 2, 4, 10, and 40 ppm at pH 7.4 and 20 °C. The dotted line represents the slope of isotherm for 40 ppm GO suspension. The slope was changed near π = 20 mN/m, indicating that the close-packed GO monolayer can be considered to be formed at this pressure.

Figure 3. AFM images of GO−ODA films transferred by LS deposition before compression at 66 Å2/cation of the area per ODA molecule after spreading 25 μL of ODA (1 mg/mL) on the surface of GO suspended solutions with various concentrations, (a) 2, (b) 10, (c) 40, and (d) 100 ppm. The inset is the cross section profile along the line in part a.

From the results, one can see that the lift-off area (Al, the area where the pressure starts to increase) for ODA on 2 ppm GO suspension was obtained as 23 Å2/cation which is larger than that (Al = 18 Å2/cation) for the ODA on the DI water (namely, without GO in the subphase), indicating that GO sheets in the subphase adhered to ODA monolayers at the liquid−gas interface. Values of Al were also shifted from 23 to 53 Å2/cation upon increasing the concentration of GO suspensions from 2 to 10 ppm. This reflects the presence of more GO sheets at the interface at the higher GO concentrations in subphases. However, relatively small shift occurred between 10 and 40 ppm of GO concentrations (the results were summarized in Table 1), which is consistent with AFM results in Figure 3. The

from the slope of the π−A isotherm. We determined π1 and π2 in the region with a linear increase (5 and 15 mN/m, respectively). The results were also listed in Table 1, where one can find that surface compressional moduli increase with decreasing concentration of GO subphases. This indicates that the ODA monolayer on the water subphase without GO exhibits a more elastic property, whereas the hybridized GO− ODA monolayer gets a lower modulus as a function of the GO concentration. This is because, as the concentration of GO suspension decreases, the overall contribution of GO sheets on the surface pressure becomes less, thereby reaching the modulus of the ODA Langmuir monolayer on the water surface. We transferred GO−ODA sheets at the interface to the HFetched silicon substrates by LS deposition (i.e., with the plane of the substrate’s surface parallel to the plane of the Langmuir film) before and after compression. From this technique, hydrophobic substrates interact with alkyl chains of ODA surfactants, while GO sheets adsorb on head groups of the surfactant monolayer on the other side. We also achieved the large-scale sample deposition (for example, 4 in. silicon wafer as shown in Figure 1b) with a tunable coverage of GO−ODA films at the different surface pressures or using various ODA and GO concentrations, by simply juxtaposing the HF-etched surface of the substrate with ODA monolayer at the air− suspension interface. Without spreading surfactants, however, no increase in the surface pressure occurred on 40 ppm GO suspension solution upon compression. Neither LB nor LS technique worked for the GO film deposition in the absence of the ODA monolayer. It is also worth noting that surfactants with shorter alkyl chains (i.e., hexylamine, octylamine, and dodecylamine) were not capable of increasing pressure upon

Table 1. Lift-Off Area and Surface Compressional Modulus of GO−ODA Langmuir Monolayers as a Function of GO Subphase Concentrations GO concentration (c, ppm)

lift-off area (Al, Å/ molecule)

surface compressional modulus (K)

0 2 4 10 40

18 23 47 53 80

153 109 80 67 47

results show that the coverage of GO sheets was considerably changed between 2 and 10 ppm of GO concentration, while there was no significant difference between 10 and 40 ppm in term of a packing density. In order to characterize physical properties of the GO−ODA monolayer from π−A isotherm results, we also investigated a surface compressional modulus, K = −a1(π2 − π1)/(a2 − a1), where a1 and a2 are the areas per molecule at π1 and π2.25,38−40 This value also equals the reciprocal quantity of the compressibility of monolayer, C, which can be simply obtained 2172

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compression on the 40 ppm GO suspension due to their lowere hydrophobicity. (The results were not shown here.) In order to investigate effects of the GO concentration on the Langmuir GO−ODA film formation, we measured the surface morphology using AFM after transferring GO−ODA films onto hydrophobic Si substrates before compression. Figure 3a shows the AFM image of the film transferred by LS deposition technique at 66 Å2/cation of the area per ODA molecule using 2 ppm of the GO suspended solution. From the result, one can see that GO sheets form a single layer without coagulation and flocculation by assistance of ODA surfactant monolayers. The thickness of the GO layer is 1.1 nm (Figure 3a), indicating a single layer. This is in good agreement with previous AFM results where the GO platelet is around 1 nm thick, deposited by LB13 and spin coating technique.41 Wang et al.42 and Gao et al.43 have functionalized the GO surface with ODA surfactants, and obtained around 1.8 nm thick GO−ODA films from AFM results where they found that the GO sheets were coated with ODA on both sides during functionalization. From our results of neutron reflectivity, however, the ODA surfactants were grafted on only one side of GO surface and also squeezed by drying. Thickness of squeezed ODA layers is very thin, compared to the GO sheets. (The details were discussed in neutron reflectivity results in Figure 5.) Therefore, GO−ODA hybrid monolayers deposited by LS technique may have a similar thickness with bare GO single layers. The density of GO sheets on Si substrates can be altered by the concentration of GO suspensions. As the GO concentration increased, more GO sheets attached to the ODA monolayer, hence forming closepacked GO monolayers from 40 ppm GO suspension at the pressure of 0.5 mN/m (Figure 3c). At 100 ppm of GO concentration, however, GO sheets start to partially overlap each other even before compression. In contrast to successful deposition of small GO fragments (below the sized of 1 μm2) with assistance of ODA molecules, it is difficult to achieve the Langmuir film formation of a small GO by direct spreading of the GO monolayer on the water subphase.23 The small GO fragments tend to sink down into the water subphase due to their high hydrophilicity, resulting in slightly darkening the color of the subphase as shown in Supporting Information Figure S1. On the other hand, surfactant-assisted GO film formation method provides a stable Langmuir monolayer at the liquid−gas interface, and therefore can be readily obtained regardless of the size of GO sheets in the suspension. We further controlled the density of GO sheets at the interface by isothermal compression and transferred onto substrates at various surface pressures. Figure 4a−d shows AFM images of GO−ODA films transferred by LS deposition using 40 ppm GO suspension at the surface pressure of 0.5, 20, 40, and 68 mN/m, respectively. We chose 40 ppm as the concentration of GO suspension because the full profile of π−A isotherm can be obtained at this concentration, as shown in Figure 2, and also the GO−ODA hybrid monolayer does not show overlapping before compression (Figure 3c), facilitating formation of densely packed single layer of GO sheets upon compression and further morphology control at various surface pressures at the liquid−gas interface. The result for this GO concentration also exhibited the single layer formation of GO sheets at π = 0.5 mN/m before compression (Figure 4a). At π = 20 mN/m, GO sheets became closer with each other, so that only a very small amount of free space remained between the sheets (Figure 4b) with little overlap. Note that the π−A

Figure 4. AFM images of GO−ODA films transferred on HF etched silicon substrates by LS deposition at the surface pressures of (a) 0.5 (before the compression), (b) 20, (c) 40, and (d) 68 mN/m after spreading 25 μL ODA solution (1 mg/mL) on GO suspension (40 ppm).

isotherm for 40 ppm GO suspension starts to increase linearly up to ∼20 mN/m (Figure 2), indicating that the film can be at the onset of a monolayer at this pressure. In Figure 4b, the bright regions pointed with white arrows are folding regions, occurring during the LS deposition, most likely due to the high water surface tension at the Si−water interface. The folding structure was not produced in the case of transferring by LB method (Supporting Information Figure S3). For further compression to 40 mN/m, GO sheets started to contact each other, forming wrinkles on the monolayer with random orientation (Figure 4c). The extensive wrinkles appeared at π = 68 mN/m (Figure 4d). The wrinkle formation of GO−ODA hybrid films also occurred for a lower concentration of GO suspension (i.e., 2 ppm) when the monolayer was compressed up to 46 mN/m (Supporting Information Figure S2), which indicates that wrinkles can be formed at the high pressure regardless of the initial GO surface density combined with the ODA monolayer at the air−water interface. These monolayer depositions of GO sheets can be achieved on various surfaces, ranging from polymeric film surfaces (polystyrene and PMMA), to inorganic surfaces (Si3N4 and hydrophilic SiO2) as shown in Supporting Information Figures S3 and S4. The film structure of GO monolayers on the polymer thin film has also been investigated by a neutron reflectivity measurement. Since the scattering length density (SLD) contrast between GO and ODA is high, one can obtain the distinct interface between the layers. (The SLD values for ODA and graphene are −0.3 × 10−6 and 7.3 × 10−6 Å−2, respectively.) In contrast to the AFM scanning, furthermore, this measurement uses the incident neutron beam which can average over the entire length of the sample. Hence, neutron reflectivity can be one of the best candidates to investigate the structure of GO−ODA hybrid monolayers. Figure 5a shows the neutron specular reflectivity profile for a deuterated polystyrene (dPS) thin film spun cast on Si substrates before and after GO−ODA hybrid monolayer deposition. We plotted the reflected scattering intensity as a function of the momentum transfer normal to the surface, qz = 4π sin θ/λ, where θ is the 2173

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Figure 5. (a) Neutron reflectivity profiles of dPS thin film spun cast on silicon wafers before and after LS deposition of the GO−ODA hybrid monolayer at 20 mN/m (spreading 25 μL of ODA (1 mg/mL) on 40 ppm GO suspension). The solid lines are the best fits to the data. The curves are shifted for clarity. (b) Corresponding scattering density profiles derived for GO on the polymer surface. The inset depicts the model of the GO− ODA on the solid substrate.

Figure 6. AFM images of LGO−ODA films transferred by LS deposition at various surface pressures: (a, b) 10 mN/m, (c, d) 20 mN/m, and (e, f) 56 mN/m, after spreading 25 μL ODA solution (1 mg/mL) on LGO suspension (40 ppm). Scan size: 20 μm × 20 μm for parts a, c, and e, and 50 μm × 50 μm for parts b, d, and f.

grazing angle of incidence and λ is the neutron wavelength, respectively. Before deposition, the reflectivity profile showed several distinct fringes with a periodicity corresponding to the thickness of the dPS layer. One can clearly see that, after deposition, the peak positions of Kiessig fringes shifted to smaller momentum transfer (qz) due to GO−ODA film thickness. According to the fitted scattering length density (SLD) profile in Figure 5b, the overall thickness of GO monolayer was obtained to be 1.3 nm, which is in a good agreement with AFM results. The ODA layer thickness was obtained to be 0.3 nm which is much thinner than the extended length of the ODA molecule (2.3 nm), indicating that the ODA layer is compressible once the water is removed. This is because the grafting density of the surfactant is not high and the hydrocarbon chains in between GO and substrate may not be oriented as illustrated in the inset of Figure 5b. Previously, we have also obtained similar findings in the case of organo-clay LB monolayers on solid substrates.44 Similar to small GO sheets, large GO (LGO) sheets also formed a monolayer of isolated sheets as shown in AFM images in Figure 6a,b. After compression up to 20 mN/m, however, they formed overlapping morphology as identified by a white arrow although an empty area unoccupied by GO sheets

(pointed by a black arrow) still existed (Figure 6c,d). This may be due to the low edge-to-area of large GO sheets with fewer surfactants, incapable of suppression of sliding between GO sheets. For further compression approaching a maximal pressure of 56 mN/m, the extensive amounts of wrinkles and overlaps exist as shown in Figure 6e,f. For the direct spreading of the GO suspension at the air− water interface, ultralarge GO sheets, which ranged from about 10 to 10 000 μm2, also produce wrinkle structures,14 whereas the small GO sheets with the size less than 10 μm2 tend to overlap without forming wrinkles due to their rigidity.13 In this work, however, small GO fragment can also form wrinkles by assistance of ODA monolayer. Interestingly, the ODA monolayer itself was not able to form wrinkles when the ODA molecules were spread and compressed to the maximum pressure at the air−water interface (Supporting Information Figure S5). In this case, the ODA layer transferred by the LS deposition was dewetted since hydrophilic amine head groups facing upward are unstable in the air. We can therefore conclude that the GO−ODA wrinkle formation is not due to the structure of surfactant layers. Herein, two possible mechanisms for the GO wrinkling phenomena are proposed as illustrated in Figure 7. (i) The 2174

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Figure 7. Schematic presentation of proposed mechanism of GO−ODA hybrid film formation. (a) If the ODA monolayer is sufficiently spread at the air−GO suspension interface, a number of ODA molecules (colored in green) will adsorb on GO sheets. Nonbound ODA molecules (colored in purple) are occupied on the surface of GO suspension. (b) After compression, GO sheets form a close-packed monolayer. Nonbound ODA molecules are also densely packed at the air−GO suspension interface. The GO−ODA hybrid monolayer hardly formed overlapping structure since adsorbed ODA molecules suppress sliding of GO sheets during the compression. (c) If a small amount of ODA is spread at the air−GO suspension interface; however, few ODA molecules (colored in green) will adsorb on GO sheets. (d) Upon compression, GO sheets are overlapped each other at the interface.

ODA monolayer may alter a flexibility of assembled GO sheets after hybrid. ODA surfactant layers which are occupied on GOfree space (colored in purple) play a crucial role in overall GO film flexibility. Upon compression, they are fully covered between isolated GO sheets, as illustrated in Figure 7b. In this case, the hybrid GO−ODA layer may act like one large sheet connected by ODA monolayers as evidenced by AFM results where the wrinkles were extended longer than the size of the GO even across the GO-free space as indicated with an arrow (Supporting Information Figure S6). Therefore, it become flexible enough to form wrinkles, compared to the individual flake of a rigid small GO sheet. This is consistent with the fact that larger GO sheets can easily wrinkle due to their flexibility. (ii) Furthermore, ODA surfactants on GO sheets (colored in green as shown in Figure 7b) can hinder GO sheets from sliding during the compression. A number of surfactants may tend to be attached at the edge of GO sheets by an interaction between positively charged surfactants and negatively charged edges of GO. Hence, surfactants on GO sheets function as obstacles to sliding GO sheets, resulting in formation of wrinkles, instead of overlaps. This was checked by changing the spread amount of ODA molecules. If a smaller amount of ODA (1 μg) was spread on GO suspension, GO sheets exhibited overlapping before wrinkling at the pressure of 25 mN/m. On the other hand, at the same pressure, the GO monolayer did not overlap in the case of spreading a greater amount of ODA molecules (25 μg) as shown in Figure 8. Note that, in other words, the GO coverage for 25 μg of ODA can also be lower than that for 1 μg of ODA at the same pressure, although before compression, as the more ODA was spread, the more GO sheets were attached to the ODA monolayer (see Figure 3c and Supporting Information Figure S7 for 25 and 1 μg of ODA, respectively). This is probably because 25 μg of spread ODA amount is more than enough to attract GO sheets; hence, the considerable amount of nonbound ODA may be also occupied in the gap between exfoliated GO platelets at the interface. The nonbound ODA can contribute on the isotherm increase, which eventually results in lower coverage of GO compared to that in the case of spreading a smaller amount of ODA at the same pressure. We also found that, in both cases, wrinkles start to form at 25 mN/m. When the GO−ODA hybrid layers compressed further up to 30 mN/m, the GO−ODA with 2 μg

Figure 8. AFM images of GO−ODA films transferred from GO suspended solution (40 ppm) by LS technique after spreading various amounts of ODA: (a) spreading 1 μg of ODA (v = 25 μL and c = 40 μg/mL) and compressing up to π = 25 mN/m, (b) spreading 25 μg of ODA (v = 25 μL and c = 1 mg/mL) and compressing up to π = 25 mN/m, and (c) spreading 2 μg of ODA (v = 25 μL and c = 80 μg/mL) and compressing up to π = 30 mN/m.

of ODA also forms more wrinkles as can be seen from the AFM result in Figure 8c. Reduction of GO films has also been performed by the chemical treatment, namely exposure to hydrazine vapor, followed by the thermal treatment at 400 °C in the Ar atmosphere. Raman spectra were measured before and after reduction treatment of GO sheets as shown in Figure 9a. From the results, one can see that, in both cases, Raman spectra exhibited typical peaks of GO sheets such as G, D, G + D, and 2D band. However, the intensity ratios of between G and D bands and between 2D and G + D bands, IG/ID and I2D/IG+D, respectively, were increased after the treatments, indicating that structural imperfection by the attachment of hydroxyl and epoxide groups on the carbon basal plane, were recovered to the graphitic structures by the reduction process.14,45 We also measured X-ray photoelectron spectroscopy (XPS) for the GO−ODA films before and after reduction, and checked the removal of oxygenated groups from GO sheets after reduction. Figure 9b shows the C1s XPS spectra of the GO and rGO sheets, where one can clearly see that the spectrum for GO− ODA monolayer exhibits a peak from the nonoxygenated ring carbon (CC, 284.2 eV) with the shoulder (CO, 286.1 eV and CO, 288.0 eV) due to considerable oxygen content. After reduction, however, the shoulder significantly disappears, indicating that most oxygen containing groups are effectively removed by reduction treatment. In addition, ODA molecules are removable during thermal treatment since the degradation 2175

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures including (1) photograph of Langmuir− Blodgett experimental setup after spreading GO suspension at the air−water interface, (2) AFM images of GO−ODA films transferred by LS deposition at 468 mN/m, (3) AFM image of GO−ODA films transferred by LB deposition, (4) AFM images of GO−ODA films on various surfaces, (5) AFM images of ODA films transferred by LS deposition at 62 mN/m, (6) AFM images of GO−ODA films transferred by LS deposition at 30 mN/m, (7) AFM images of GO−ODA films transferred by LS deposition at 0.5 mN/m after spreading 2 μg of ODA, and (8) TGA measurement for the ODA powder. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82 42 868 8436. *E-mail: [email protected]. Phone: +82 2 3408 3692.

Figure 9. (a) Raman spectra for GO and rGO sheets before and after reduction by the hydrazine vapor treatment and the thermal treatment at 400 °C in the Ar atmosphere. The GO film was transferred by LS deposition at 68 mN/m after spreading 25 μL of ODA solution (1 mg/mL) on GO suspension (40 ppm). (b) Their C1s XPS spectra before and after reduction. (c) The corresponding AFM image after reduction.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Tae Hwan Kang and Wang Sik Lee at the Hanbat National University for use of the furnace and AFM, respectively. This work was supported primarily from a grant from by the National Research Foundation of Korea under Contract NRF-2012M2A2A6004261.

temperature of ODA (226 °C) measured by a thermogravimetric analysis (TGA) is significantly lower than the reduction temperature (Supporting Information Figure S8). The AFM image of GO−ODA films deposited at 68 mN/m after reductive treatments were shown in Figure 9c where one can see that the wrinkling morphology was maintained even after the treatment and the rGO layers were stable without delamination due to the fairly strong interaction between films and substrates, which indicates that this process can be applied to enhance performance of graphene-based electrode.

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REFERENCES

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4. CONCLUSIONS Here, we demonstrated a method to form stable monolayers of highly hydrophilic GO sheets by assistance of positively charged surfactant, octadecylamine (ODA) monolayers on the surface of GO suspension. We proposed a model whereby the surfactant monolayer functions as a template for adsorbing GO sheets dispersed in an aqueous subphase and mediating the surface anchoring of GO sheets at the liquid−gas interface. Neutron reflectivity results showed that the GO layer was 1.3 nm thick, corresponding to a single layer thickness of GO. The single layer deposition of GO sheets with tunable surface coverage on solid substrates has been achieved at the different surface pressures by applying LS or LB technique. We also controlled the morphology of compressed GO sheets by spreading various amounts of the ODA surfactants. We found that ODA surfactant monolayers prevented GO sheets from sliding at the liquid−gas interface, resulting in formation of wrinkling rather than overlapping during the compression. These results can provide a route for applications requiring wrinkled graphenes, ranging from nanoelectronic devices46−48 to energy storage materials, such as supercapacitors49 and fuel cells.50 2176

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