Ultrastrong Freestanding Graphene Oxide Nanomembranes with

Ultrastrong Freestanding Graphene Oxide Nanomembranes with Surface-Enhanced Raman Scattering Functionality by SolventAssisted Single-Component Layer-by-Layer Assembly Rui Xiong,†,‡,§ Kesong Hu,‡,§ Shuaidi Zhang,‡ Canhui Lu,† and Vladimir V. Tsukruk*,‡ †

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States

‡

S Supporting Information *

ABSTRACT: We report single-component ultrathin reduced graphene oxide (rGO) nanomembranes fabricated via nonconventional layer-by-layer assembly (LbL) of graphene oxide flakes, using organic solvent instead of water to provide strong complementary interactions and to ensure the uniform layered growth. This unique approach does not require regular polymeric from the assembly process or intermediate surface chemical modification. The resulting ultrastrong freestanding graphene oxide (rGO) LbL nanomembranes with a very low thickness of 3 nm (three GO monolayers) can be transferred over a large surface area across tens of square centimeters by using a facile surface-tension-assisted release technique. These uniform and ultrasmooth nanomembranes with high transparency (up to 93% at 550 nm) and high electrical conductivity (up to 3000 S/m) also exhibit outstanding mechanical strength of 0.5 GPa and a Young’s modulus of 120 GPa, which are several times higher than that of other reported regular rGO films. Furthermore, up to 94 wt % of silver nanoplates can be sandwiched between 5 nm GO layers to construct a flexible freestanding protected noble metal monolayer with surface-enhanced Raman scattering properties. These flexible rGO/Ag/rGO nanomembranes can be transferred and conformally coat complex surfaces and show a cleaner Raman signature, enhanced wet stability, and lower oxidation compared to bare Ag nanostructures. KEYWORDS: flexible nanomembranes, reduced graphene oxide, mechanical performance, layer-by-layer assembly, surface-enhanced Raman scattering

F

materials with high electrical and opening new possibilities for flexible terials. Among various rGO-based aerogels, films, fibers, and quantum

lexible graphene oxide (GO) nanomembranes with high mechanical performance can enhance the durability of GO-based integrated devices under practical operating conditions (such as repeatedly bending and folding). A GO sheet as a graphene derivative is an attractive material due to its excellent mechanical properties, biocompatibility and optical transparence.1,2 GO sheets can be readily chemically reduced to reduced graphene oxide (rGO), yielding low-cost graphene-like © 2016 American Chemical Society

thermal conductivities,3 multifunctional nanomananomaterials such as dots, freestanding nano-

Received: March 23, 2016 Accepted: June 22, 2016 Published: June 22, 2016 6702

DOI: 10.1021/acsnano.6b02012 ACS Nano 2016, 10, 6702−6715

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Figure 1. (a) Fabrication of rGO nanomembranes from methanol GO dispersion: (1) SA-LbL assembly; (2) reduction of GO components using hydroiodic acid (HI) (bottom images are for the water contact angle of GO (left) and rGO (right) nanomembranes); (3) release of rGO nanomembranes at the air−water interface. (b) AFM image of GO sheets and its height profile (inset). (c) Thickness of the GO nanomembrane with an increasing number of deposition cycles. (d) AFM image of the first deposited GO monolayer. The 3 nm freestanding rGO nanomembrane on the water surface (e) and transferred on silicon wafer (f). (g) Transparent 3 nm nanomembrane transferred on a flexible poly(ethylene terephthalate) substrate.

removal (Figure 1a). However, previously reported LbL assembly of GO nanomembranes required additional components (such as polyelectrolytes and nanoparticles), which are oppositely charged to anionic graphene oxide nanosheets, to serve as interlayer binding and to achieve the uniform layered growth.15 Besides, combinations of these materials into the GO film often involve a complicated drying mechanism, resulting in excessive wrinkling and folding of flexible GO sheets. Additionally, the presence of polyelectrolytes and low molar weight binding components leads to deterioration of interfacial transport, resulting in a decrease in electrical and thermal conductivities.16,17 Recently, ultrathin all-rGO nanomembranes were fabricated by the LbL technique without any additional binders.16 However, all these LbL approaches are still based on the conventional LbL concept of requiring complementary components and involve an additional step for the chemical modification of GO sheets. Although hydrogen bonding and hydrophobic−hydrophobic interactions can be utilized for the LbL assembly of GO sheets, single-component LbL of GO sheets has been problematic without harsh (thermal or chemical) “curing” the deposited materials due to the redissolution of previously deposited layers, compromising the integrity of the interfacial morphology.18,19

membranes that consist of a limited number of monolayers of graphene-like sheets exhibit some interesting combinations of unique physical properties, such as high optical transmittance, conformability, mechanical strength, and highly anisotropic thermal and electrical conductivity. These unique properties arise from the ultrahigh aspect ratio of nanomembranes, which possess nanoscale thicknesses (around 1 nm) and microscopic (several microns) lateral dimensions.4,5 These rGO-based freestanding nanomembranes hold promising potential for a wide range of applications in ultrafiltration,6 transparent and flexible electronics,7 gas barriers,8 and protective coatings.9 However, there are still several urgent issues to be addressed in terms of the fabrication of rGO nanomembranes. Currently, GO-laminated nanocomposites can be fabricated on various substrates via dip-assisted self-assembly,10 spincoating,11 drop-casting,12 or vacuum-assisted filtration (VAF).13 Generally, drop-casting or vacuum filtration of a GO aqueous suspension involves a slow, two-stage gelation-and-solidification assembly, which results in random wrinkling and folding during the final drying processcompromising the GO film’s resultant physical properties.14 Compared with other assembly techniques, spin-assisted layer-by-layer assembly (SA-LbL) enables rapid tailoring of nanomembrane thickness, size, and morphology by applying centrifugal force to aid fast solvent 6703


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Figure 2. Chemical transformations of 20-layer nanomembranes: (a) Raman spectra of GO and rGO nanomembranes; (b) D band Raman map of rGO nanomembranes; (c,d) XPS spectra of GO and rGO nanomembranes.

the reported freestanding ultrathin membranes is greater than 50−100 nm, with much thinner films reported using complicated processing routines. For example, the bubbles produced in the chemical reduction process were used to release the rGO nanomembranes from the porous surface.5,28 Thick rGO films were exfoliated into ultrathin membranes with poorly controlled thicknesses at the liquid/air interface.29 Other researchers employ sacrificial polymer layers dissolved in orthogonal solvents to release the nanomembranes.30,31 However, all of these obtained nanomembranes are still greater than 10−20 nm due to the weak mechanical performance. In this study, we demonstrate that single-component ultrastrong freestanding rGO nanomembranes can be fabricated by SA-LbL assembly by balancing hydrophobic interactions with solvent replacement (Figure 1a). The highly uniform LbL growth of GO nanomembranes benefits from limited GO solubility in methanol due to the moderate interfacial interactions and the fast drying of the low-surface-energy methanol solvent. After chemical reduction, these as-prepared ultrathin nanomembranes can be readily released from the silicon substrate without employing the sacrificial supports due to the reduced interfacial adhesion between the GO nanomembrane and the Si substrate. The strength and the elastic modulus of these freestanding rGO nanomembranes reached 500 MPa and 120 GPa, respectivelyvalues that are superior to those of all other reported GO-based films. Due to the excellent mechanical performance, the transferrable nanomembranes with a thickness as low as 3 nm (three monolayers) were demonstrated, which is the thinnest reported freely standing rGO nano-

It is important to note that due to the poor interactions between GO layers, the outstanding mechanical properties of an individual GO sheet (elastic modulus around 250 GPa)20 frequently do not directly translate into the mechanical properties of assembled GO films.21 Moreover, when GO nanomembranes are exposed to moist air or a wet environment, water molecules diffuse into the GO stacks and spontaneously exfoliate the GO multilayers significantly, weakening the mechanical integrity of the nanostructures under ambient working conditions.22 In order to improve the mechanical properties of the GO nanomembranes, various binders, such as chitosan,23 poly(vinyl alcohol),24 silk fibroin,25 and cellulose nanocrystals,26 have been introduced in additional processing steps. However, these steps usually compromise other important nanomembrane properties such as optical transmittance and electrical conductivity.27 Therefore, it is critical to find a facile way to assemble robust GO nanomembranes without adding organic binders. Another common problem in the fabrication of ultrathin freestanding GO nanomembranes is the difficulty in releasing the film from the solid substrate with minimal film damage during the transfer to active devices. Although GO nanomembranes can be easily deposited on various substrates, complete release from the substrate with preservation of their integrity requires overcoming strong interfacial adhesion. Cracks or voids can be caused by the local stresses during the releasing processing, especially for these ultrathin nanomembranes with a thickness less than 100 nm, undermining overall physical properties of the freestanding structures. Such damage is the main reason why the majority of the thickness of 6704


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following chemical states: sp2 carbon (285.5 eV), epoxide/ether (287.5 eV), carbonyl (288.6 eV), and carboxyl (289.5 eV) groups.35 Analysis of the deconvoluted peaks and calculation of the area ratio of the sp2 carbon to total carbon content reveal that about 50% of carbon atoms on the initial GO surface were in oxidized chemical states. In contrast to initial specimens, after the reduction of the GO component, the C/O atomic ratio increases from 2.3 to 5.8 and the oxygen content decreases significantly to 25.7% (Figure 2d). This change of atomic ratio suggests that the fraction of defected benzene rings in the graphene oxide basal plane decreases from 87 to 34% [assuming that each benzene ring can accommodate, on average, one oxygen-containing functional group with one oxygen atom (C−OH)].35 The relatively low ratio in our study might be due to the milder experimental conditions compared to that in previously reported procedures.33 In our reduction method, HI acid was used to treat the GO nanomembranes for only 10 min at room temperature in contrast to higher temperature (80 °C) treatment for 5 h reported in the literature. Overall, these results show that the removal of oxygenated groups and partial recovery of the conjugated structures during the chemical reduction process result in significant change in the surface energy and interfacial interactions with dominating graphitic surface regions in reduced sheets that facilitate the release process without damage. For this study, it is critically important that the chemical reduction of the GO LbL nanomembrane increases the surface energy mismatch between the nanomembrane and the hydrophilic silicon oxide surface. This increase in surface energy mismatch and following decrease in interfacial adhesion are crucial for enabling a nondamaging release process without employment of the traditional sacrificial supports. Indeed, when we gradually dip the substrate carrying the rGO nanomembrane into water at a shallow angle, the rGO nanomembrane easily floats onto the water surface (Figure 1a). After being fully submerged, the full rGO nanomembrane is readily detached from the supporting substrate and floats freely on the water−air interface. Indeed, the thinnest three-monolayer freestanding rGO nanomembrane reported can freely float on the water−air interface and retain the original shape of the silicon substrate (Figure 1e).28 The thickness of this rGO nanomembrane after release remains close to 3.0 nm (Figure 1f).36 Although singleor double-monolayer rGO nanomembranes can be assembled by our procedure, they will disintegrate when released from the substrate. These three-monolayer rGO nanomembranes can further be transferred onto other flexible substrates, such as poly(ethylene terephthalate) (PET), demonstrating its high flexibility and high optical transparency (see below) (Figure 1g). It is worth noting that this release method can also be universally applied to the preparation of other freestanding polymer nanomembranes from hydrophilic or hydrophobic materials by adding the low surface tension coating on the substrate (see Figure S2). Single-Component Assembly of LbL Nanomembranes. In conventional LbL assemblies, complementary interactions (e.g., hydrogen bonding, ion pairing, or Coulombic attractions) between two or more components play a critical role in binding the stable assembly.36,37 As is well-known, LbL growth of a single component without any post-treatment is a really grand challenge38 because of the redissolution of the previously deposited layer by exposure to solvent in subsequent

membrane. Furthermore, they are highly transparent (>93% at 550 nm wavelength) and highly electrically conductive (>3000 S/m after chemical reduction), which makes them transparent and conductive flexible nanoelements with exceptional mechanical robustness. Finally, we demonstrated that up to 94 wt % of silver nanoplates can be integrated into 10 nm GO nanomembranes to construct a sandwiched freestanding rGO surface with surface-enhanced Raman scattering (SERS) activity of protected noble metal nanoplatelets. The as-prepared sandwiched nanostructures not only show enhanced antioxidation ability and wet stability but also show a less noisy enhanced Raman signature.

RESULTS AND DISCUSSION Fabrication, Chemical Reduction, and Release of Nanomembranes. The GO sheets employed here are prepared by Hummers’ method.32 The GO monolayers are around 1 nm thick with lateral dimensions ranging from 2 to 4 μm, and ζ-potential of −19.7 ± 0.9 mV in methanol that indicates a highly oxidized state (Figure 1b). The thickness of the GO nanomembrane grows linearly with the increase of deposition cycles, indicating consistent and uniform deposition of GO sheets (Figure 1c). To study the reproducibility of this LbL fabrication method, we fabricated four independent GO samples and measured the thickness growth of these GO samples. As shown in Figure S1a, the data points from different samples match very well with each other, and the average thickness per layer is determined to be 1.35 ± 0.01 nm with the variation below 1%, demonstrating good reproducibility. After chemical reduction, these samples can be released from the substrate and transferred to the copper grids with a 300 μm aperture for further mechanical testing (Figure S1b). These nanomembranes from different samples can be suspended on the grids uniformly without any wrinkles or pinholes, demonstrating their mechanical robustness. Atomic force microscopy (AFM) of a single spin-cycle GO nanomembrane shows that the surface is fully covered by mostly planar GO sheets with few wrinkles and low surface roughness (Rq) of 1.0 nm for a 5 × 5 μm2 area (Figure 1d). Concentrated hydroiodic acid (HI) is used here to reduce GO in order to enhance interlayer interactions and induce electrical conductivity (see the Experimental Section).33 After the chemical reduction, a significant fraction of oxygen-containing surface groups were removed and the π−π interactions of pristine graphene were partially restored, thus increasing the interlayer binding of GO nanosheets and reducing slightly the thickness.1,34 Chemical reduction of the GO sheets decreases the interfacial energy between the nanomembrane and the substrate, facilitating membrane release from the substrate. The Raman spectra of GO and rGO nanomembranes showed that the intensity ratio of Raman D to G bands (ID/IG) increased from 0.98 to 1.34 after the chemical reduction, indicating the efficient removal of oxygen-containing surface groups (Figure 2a).10 Finally, a highly uniform chemical composition of the rGO nanomembrane is observed in the D band Raman mapping, indicating the uniform reduction throughout the whole sample at a microscale (Figure 2b). The surface energy of the partially reduced nanomembranes is significantly reduced, as indicated by the increasing water contact angle from 21.4 to 56.1° (Figure 1a). The X-ray photoelectron spectroscopy (XPS) C 1s spectrum of an ultrathin GO nanomembrane shows four characteristic peaks (Figure 2c). These peaks are attributed to carbon in the 6705


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Figure 3. (a) XRD data and (b) d-spacing change of GO paper swollen in methanol for different time periods. (c) Fourier transform infrared data of the dried LbL GO nanomembranes fabricated from GO methanol solution.

correspond to two molecular layers of methanol assuming selective localization in mostly graphitic surface areas (Figure S4b). Similar treatment with a water droplet also results in swelling of the GO film initially with the interlayer spacing increasing much more to 1.46 nm (Figure S4). However, in contrast to methanol treatment, the (001) peak intensity drops during swelling and the width of the peak increases dramatically, indicating subsequent exfoliation and disintegration of the laminated GO stacks after water infusion (Figure S4a). When 6706


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Figure 4. Mechanical properties of rGO nanomembranes with different number of GO monolayers: (a) Young’s modulus, (b) ultimate stress, (c) toughness, and (d) ultimate strain.

both swollen GO films are air-dried, the (001) diffraction peak returns to the original position with d-spacing of 0.78−0.79 nm, indicating restoration of the initial stacking with some minor residual solvent presence (Figure S5). Furthermore, the XRD scans of a 200 nm LbL GO nanomembrane assembled from methanol solution was carried out to study its nanostructure (Figure S6). The (001) peak appeared at 11.54°, attributed to the d-spacing of 0.77 nm, which is only slightly smaller than that of VAF GO paper discussed above. The size of stacked monolayers in LbL nanomembranes was calculated to be 5.1 nm, indicating the extension of correlations to around 5−6 monolayers. Next, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was conducted on the LbL GO nanomembranes from methanol in order to monitor the intermolecular interactions within stacked rGO sheets (Figure 3c). The as-prepared GO nanomembrane shows a broad peak at 3402 cm−1 ascribed to −OH group stretching vibration.47 This absorption band is red-shifted compared to its typical position at 3500 cm−1, indicating the formation of a hydrogen bonding network between the adjacent GO sheets.48 It is worth noting that there are weak peaks at 2820 and 2950 cm−1 related to −CH2 group stretching vibration, confirming our aforementioned suggestion that a small amount of methanol is trapped in the dry GO nanomembranes. Such trapped molecules can interact with the hydrophobic graphitic regions to enhance the interfacial strength of dry stacked GO sheets via hydrophobic− hydrophobic interactions mediated by residual methanol.45

It is important to note that methanol as a dispersing medium shows additional advantage over water during the fabrication process due to its low surface tension (22.7 mN/m) in comparison to that of water (72.8 mN/m).43 Thus, the GO methanol solution wets the surface of the freshly cleaned silicon wafer quickly, facilitating fast, consistent, and uniform GO sheet deposition and high surface coverage. Also, the high volatility of methanol ensures the fast solvent removal during the spinning process and lower capillary forces, which are critically important for efficient LbL deposition and morphology control of the GO sheets with minimum folding and wrinkling. The GO methanol environment not only facilitates the uniform assembly of GO sheets without dissolving preceding layers but also enhances the overall efficiency of the LbL deposition, resulting in much higher surface coverage (close to 100%; see Figure 1d). This is much higher than the usual partial coverage of around 50% observed for assembly from water dispersion under similar conditions.25 As a result, we observed significant improvement of the overall mechanical properties and integrity of the LbL films. Mechanical Properties of Ultrathin Freestanding rGO Nanomembranes. The mechanical properties were determined from stress−strain data collected by a bulging test, which is a widely utilized technique in the mechanical characterizations of ultrathin films (Figure S7).25,49,50 Figure S8 demonstrates the 10 nm thick rGO nanomembrane suspended over a 300 μm copper aperture, which shows excellent mechanical robustness and can be deformed multiple times to generate reliable stress−strain curves. By using the relationships σ = Pr2/4hd and ε = 2d2/3r2, where r is the 6707


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Figure 5. (a) Representative typical stress−strain curves of rGO nanomembranes fabricated by SA-LbL assembly, drop-casting, and VAF of similar compositions and thicknesses (Table 1). The AFM images of rGO nanomembranes fabricated by (b) LbL, (c) drop-cast, and (d) VAF methods and corresponding GO assembly schematics. (e) Comparison of the ultimate stress and Young’s modulus values reported for pure GO or rGO films of different origins.27,34,42,51−55,57

and VAF films (Figure 5a). The strain−stress curves of these traditional rGO nanomembranes are very different from those measured here for SA-LbL specimens: the initial slopes and the ultimate stress are much lower and corresponding mechanical characteristics are significantly lower (Table 1).

radius of the opening (300 μm), d is the vertical deflection, and h is the film thickness, the pressure difference loading versus the nanomembrane deformation was used to generate stress−strain plots to calculate mechanical characteristics.25,49 Stress−strain data were obtained for rGO nanomembranes with different thicknesses from 5 to 80 nm or number of monolayers from 5 to 80. We found that the Young’s modulus of 120 ± 10 GPa of the rGO nanomembranes evaluated here is consistent for various nanomembranes (Figure 4). This value is well above all elastic modulus values reported to date for traditional rGO-based films (usually within 10−50 GPa) (Figure 4a).1 Surprisingly, very thin, 5 nm nanomembranes show the smallest variability in the Young’s modulus, which might result from the smallest surface roughness and minimum wrinkling (Figure S9). Other important mechanical characteristics, such as ultimate stress, ultimate strain, and toughness, were all found to increase gradually with the increasing number of GO monolayers (Figure 4). Increasing the number of rGO monolayers from 5 to 30 results in all mechanical characteristics increasing 2-fold, with the highest mechanical properties reached for the number of monolayers above 30. Overall, 40 monolayer thick nanomembranes were found to have the ultimate stress of 500 ± 57 MPa and the ultimate strain of 0.41 ± 0.02%, with overall toughness of 1.1 ± 0.1 MJ/m3. The correlation between monolayers and the mechanical properties primarily originates from the ability of defect tolerance in the thicker laminated materials with random stacked sheets. These high mechanical characteristics of the rGO nanomembranes could also be attributed to their uniform morphology without significant wrinkling of flexible sheets or voids (see below). To compare the mechanical performance of the rGO nanomembranes fabricated here with other traditional rGO membranes, we fabricated laminated rGO films of comparable thicknesses via other popular methods such as drop-cast films

Table 1. Mechanical Properties of rGO Nanomembranes Fabricated by SA-LbL Assembly, Drop-Cast, and VAF Procedure samples

thickness (nm)

ultimate stress (MPa)

Young’s modulus (GPa)

strain (%)

SA-LbL rGO drop-cast rGO VAF rGO

72 80 60

447 ± 22 257 ± 64 107 ± 9

120 ± 4 33 ± 6 8±1

0.37 ± 0.03 0.78 ± 0.05 2.68 ± 0.17

A high residual stress of 30−40 MPa has been revealed in all three specimens due to the tension during transfer and drying (Figure 5a). The ultimate stress of the SA-LbL rGO nanomembrane with similar thicknesses is 2 and 4 times that of drop-cast and VAF membranes, respectively (Table 1). The Young’s modulus of the SA-LbL nanomembrane is 3 and 14 times higher than that of drop-cast and VAF films, respectively. On the other hand, stretchability of these nanomembranes is higher, which can be related to different unfolding and shearing behavior facilitated by different morphologies. In order to visualize these differences, the surface morphology of all different types of rGO nanomembranes was studied with AFM (Figure 5b−d). The AFM images show that both drop-cast and VAF nanomembranes show relatively rough surfaces with a high concentration of microscopic wrinkles. High density of folded rGO sheets and excess heights of long wavy ridges were observed for VAF rGO films. The surface microroughness is 25.5 nm within 1 × 1 μm2 (here and below) for VAF rGO films. Similar wrinkled morphology but 6708


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Figure 6. (a) Optical transmittance (at 550 nm) and (b) electrical conductivity of rGO nanomembranes with different numbers of monolayers.

G band at 1580 cm−1 is of E2g symmetry, corresponding to the hexagonal network of carbon atoms.57 In comparison to GO films assembled from water, a methanol-dispersed film shows the increased ratio of ID/IG (Figure S10). Such change indicates that the sp2 domains in the GO basal plan are bound by the methanol to much greater extent than that deposited from the water dispersion.41 To compare the mechanical performance of rGO nanomembranes fabricated in this study with existing sets of data for GO or rGO films available in literature, we summarized the results on the ultimate stress and Young’s modulus of the reported pure GO or rGO films in the Ashby plot (Figure 5e).27,34,44,48,49,53−57 This comparison shows that SA-LbL rGO nanomembranes fabricated from methanol extend beyond the property space reported for conventional GO-based materials (Figure 5e). For the rGO films fabricated to date, the ultimate stress and Young’s modulus values are concentrated mainly within the regions of 100−300 MPa and 10−40 GPa, respectively. Virtually all published results are concentrated within a relatively narrow region that apparently indicates consistent and uniform mechanical behavior of disordered materials with wrinkled and folded flexible GO sheets. The higher strength of the reported rGO film (about 600 MPa) was achieved by long-time low-temperature controlled oxidation of pristine graphite with a well-stacked morphology.42 However, even in this case, the Young’s modulus is much lower than that of this study (indicated as a red star in Figure 5e). Another unique example worth mentioning is the rGO paper prepared by high-temperature thermal annealing of GO paper.58 Although both the strength and Young’s modulus reached 300 MPa and 40 GPa, respectively, they are significantly lower than the values reported here. Optical Properties and Electrical Conductivity of rGO Nanomembranes. Achieving a balance between mechanical robustness, optical transmittance, and electrical conductivity is a long-lasting challenge for graphene-based films if they are considered as prospective transparent flexible electrodes and sensors.1 Usually, the enhanced mechanical robustness due to the use of organic or polymeric binders is accompanied by reduced electrical and thermal conductivity. On the other hand, the chemical reduction usually results in compromised optical transmittance. In fact, relatively thick graphene-based films are nontransparent even if the monolayer graphene has been reported to transmit 97.7% of visible light at

with lower concentration of random folds and much lower microroughness of 5.8 nm was observed for drop-cast films. In comparison, SA-LbL nanomembranes with 2.7 nm microroughness are much smoother, uniform, and show no developed wrinkles and folds (Figure 5b). The differences in surface morphologies and uniformity of the rGO films observed here can be caused by differences in solvent removal processes during the GO assembly process. To compare rGO nanomembranes prepared by different methods, we analyzed the GO assembly before reduction (Figure 5b−d). We suggest that, during drop-casting, the evaporation of water will drive the GO nanosheets along the liquid−air interface, which causes local dewetting. The Brownian motion and capillary forces result in asymmetric GO membranes, and a locally nonuniform solvent removal process would lead to wrinkling and folding of the GO nanosheets in the drying liquid−air interfaces.42 This dewetting and receding of the liquid microdroplets drag flexible GO sheets further, facilitating bonding and aggregation of the nanomembrane during drying along the receding circular fronts, as discussed for drying of carbon nanotube dispersions (Figure 5c).56 On the other hand, in the VAF process, the microflux induced by the high-pressure difference across the filtered film causes GO nanosheets to be forcedly deposited onto the rough and nonuniform surface of the porous filter membrane. Basically, the original status of the GO nanosheets in an agitated dispersion (extremely flexible, high aspect ratio sheets with entanglements, rollings, wrinklings, and stackings) will be solidified in the final nanomembranes after drying (Figure 5d). In contrast, for the SA-LbL assembling process, the high spinning process produces strong centrifugal force, high shearing stresses, and fast solvent removal and evaporation, which all benefit the unfolding of GO nanosheets, prevent the formation of the GO wrinkles and jams, and result in smooth surface, uniform morphology, high surface coverage, and effective stacking of planar GO sheets (Figure 5b). Trace amounts of solvent trapped in the hydrophobic (methanol) and hydrophilic (water) surface regions of GO sheets can act as specific binder sites to anchor the adjacent GO sheets and strengthen the loading transfer. The strong interactions between entrapped methanol and graphene oxide sheets are confirmed by Raman spectra of GO components (Figure S10). As is known, the D band at 1330 cm−1 is related to the defect-induced breathing mode of A1g symmetry, and the 6709


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Figure 7. (a,b) Dark-field optical images of a freestanding rGO−AgNP−rGO nanomembrane suspended on a 300 μm aperture. (c,d) AFM images and TEM image (inset). (e) Scheme of silver nanoplates encapsulated between rGO sheets. (f) AFM profile of a silver nanoplate.

550 nm wavelength.59 Thus, a lower number of stacked monolayers in the nanomembrane is critical for ensuring high optical transmittance. For instance, stacking 100 rGO monolayers (common for mechanically robust graphenebased films reported in literature) would result in a maximum transmittance of only 9.7%. Reducing the thickness of the rGO films to a few monolayers typically results in poor mechanical stability. Conversely, significantly increasing the number of GO monolayers to ensure integrity of these films results in the dramatic reduction of the optical transmittance. On the other hand, most attempts to fabricate GO films with very few monolayers and high optical transmittance fail because of their poor integrity and mechanical strength. The optical transmittance at 550 nm was measured for rGO nanomembranes with different numbers of rGO monolayers (Figure 6a). The highest optical transmittance for the thinnest

transferrable three-monolayer rGO nanomembrane was measured to be close to 93%, which is very close to that expected for a simple trilayer graphene stack (93.2%), as can be evaluated from single-monolayer transmission.59 Moreover, even after 30 rGO monolayers are stacked into a very robust nanomembrane, the transmittance stays relatively high, greater than 60%, which is higher than the theoretical value of 30 layers of graphene. We suggest that the only partial reduction of GO nanomembranes is beneficial for the optical transmittance. These results demonstrate the unique advantage of the fabrication strategy reported here for the production of highly transparent, robust, and portable GO nanomembranes for optical applications. Generally, the ultrathin rGO films have an electron conductivity lower than that of thicker corresponding rGO films (Figure 6b). Higher electrical conductivity of thicker rGO films can also be achieved by intense chemical reduction. However, one notable point we have to mention is that the 6710


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Figure 8. Raman spectra of R6G on silicon, rGO, AgNP surfaces, and the rGO−AgNP−rGO nanomembrane. Numbers show the intensity scale.

been resolved completely. Generally, chemical adsorption induced vibrations, molecular deformation and distortion, charge transfer, and metal-catalyzed side reactions during the direct contact of the metal and molecules affect the scattering properties.65 Recently, it was reported that an ultrathin SiO2 shell introduced onto the metal nanoparticle surface as a spacer is an efficient approach to fabricate clean and stable SERSactive substrates.66 Uniform ultrathin coatings, such as layers of graphenes, were explored for wrapping a rough gold SERS substrate to collect a cleaner SERS signal.67 Due to the weak mechanical performance of the pristine graphene layers, these well-defined nanostructures require various substrates to support them in the further applications, resulting in signal deterioration.68 In a typical SERS substrate, a rough metal surface or well-defined metal nanostructures are required to provide the enhanced electromagnetic field, and direct adsorption of molecules on metal hot spots is required but can quickly deteriorate molecular structures. Thus, we suggest a sandwiched nanostructure of rGO− metal−rGO as SERS-active nanomembranes containing silver nanoplatelets (AgNPs) (Figure 7). The metal nanostructures used here are silver nanoplates with a rough surface produced by ultrasonic treatment (see Experimental Section). A freestanding rGO−AgNP−rGO nanomembrane was extremely stable and can be transferred and suspended across a 300 μm aperture without any pinhole or cracks (Figure 7a). An optical microscope image shows very uniform distribution of individual silver nanoplates with lateral dimensions of about 500 nm (Figure 7b). The thickness of the silver nanoplates was measured with AFM to be around 40 nm, and the total thickness of the rGO coating was determined to be 10 nm (Figure 7c,d and 0Figure S11). The AFM images and the profile of the silver nanoplate show a rough silver surface conformally covered by the 5 nm thick rGO layer (Figure 7f). The choice of the 5 nm rGO nanomembrane is based on a combination of mechanical performance and lowest loss of electromagnetic enhancements, with the 5 nm nanomembrane being robust enough to sandwich the AgNPs to obtain the freestanding rGO/Ag/ rGO nanomembranes. The increase of thickness would result in higher loss of electromagnetic enhancements due to increasing distance of absorbed molecules to Ag nanoplates. The volume fraction of silver nanoplates in the nanomembrane of 74% was

high optical transparency is also a valuable feature for rGO nanomembranes. Generally, these two properties are orthogonal and cannot be achieved simultaneously, and the enhancement of one property has to be traded with another. Therefore, the optical transparence should be taken into consideration to seek a balance of optical transparence and electrical conductivity, rather than aiming for large improvement in only one performance aspect at the expense of another, and optimal balance should be considered for different applications. Although ultrathin nanomembranes reported here cannot be directly compared with traditionally thicker films, the electrical conductivity of 30-layer rGO nanomembranes was found to be 3000 S/m (Figure 6b). This conductivity is very high and comparable to that of the previously reported rGO ultrathin films or any other conjugated polymers.60 Even for thinner, 10 nm LbL nanomembranes, the electrical conductivity still remains high, at 1000 S/m. The value obtained here is much higher than that reported for most transparent and flexible conductive polymers (10−2−10 −1 S/m), which cannot sustain even modest mechanical stresses for films with 10 nm thickness.61 Such a high level of electrical conductivity can meet these basic requirements of electrical properties for many applications including electrically conductive conformal coatings and portable nanofilms. Although the combination of properties for as-prepared samples shows a good balance in a wide thickness range from 10 to 30 nm, it still is hard to determine which thickness is the best choice in terms of overall mechanical property, optical transparency, and electrical conductivity at the same time, with particular balance to be tailored to a particular application. For example, if the mechanical property is the first priority, a 30−40 nm rGO nanomembrane should be favorable. If mechanical property, optical transparency, and electrical conductivity are equally important, rGO nanomembranes with 10−20 nm thickness might be more appropriate. If the highest transparency is the priority, the thinnest nanomembranes with 5−10 nm thickness should be considered. SERS Activity of rGO Nanomemebranes with Silver Nanoplates. SERS phenomena can enable nondestructive and even single-molecule detection.62 Although great efforts focus on the synthesis of metal nanostructures,63,64 the issue of high SERS fluctuations induced by metal−molecule contact has not 6711


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solidification of GO stacks. Moreover, we demonstrated that the ultrathin rGO nanomembranes can be used to encapsulate the silver nanoplates to fabricate freestanding SERS-active nanomembranes with a high-quality SERS signal of R6G. With the outstanding micromechanical properties, high conductivity, and high optical transmittance, these portable transferrable flexible rGO nanomembranes in their supported and freestanding states can be valuable for potential applications in flexible electronic devices, protective molecular coatings, optical sensors, SERS sensing elements, energy harvesting, and ion separators.

estimated from the large-area optical microscope images, and the weight percentage was calculated to be very high, 94% (Figure 7b). Conformally wrapping of AgNPs with 5 nm rGO coatings can be realized due to the excellent flexibility of rGO nanosheets and strong intercomponent interactions (Figure 7c,d). Some rGO wrinkles extended from silver nanoplates (Figure 7c, marked by arrows), further confirming that silver nanoplates were indeed conformally encapsulated between flexible rGO sheets to form densely packed sandwiched structures (Figure 7e). The high water contact angle of 57° was much higher than that for the bare silver nanoplates (19°), additionally confirming the encapsulation of wettable nanoplates into partially hydrophobic rGO sheets (Figure S11a). Finally, we tested the SERS performance of these nanomembranes by placing the known Raman marker Rhodamine 6G (R6G) on their surface (Figure 8).68 We collected a series of Raman spectra from 12 different spots for rGO/Ag/rGO and Ag, as shown in Figure S12. The Raman spectra shown in Figure 8 are the representative spectra from a single SERS acquisition cycle. The Raman scattering of R6G on a control silicon substrate and rGO surface is hardly observable due to the strong fluorescence (Figure 8). The SERS spectrum of R6G on AgNPs shows characteristic R6G peaks along with a high noisy background. SERS spectra for R6G on the rGO−AgNP surface are more reproducible and show sharper characteristic peaks at 611, 634, 661, 770, 1180, 1312, 1360, 1510, and 1647 cm−1 from R6G67 but with better signal-to-noise ratio, flat baseline, and minimized fluorescence background (Figure S12). This high-quality SERS spectrum with suppressed secondary peaks is in contrast with the corresponding spectrum on bare AgNPs, with additional peaks resulting from the secondary and mixed vibrational modes and potential locally damaged R6G molecules (see details in the Supporting Information). Limits of sensitivity of these and similar sandwiched structures will be tested in a forthcoming study. These as-prepared freestanding rGO−Ag−rGO SERS were ultrathin, mechanically robust and flexible, and optically transparent, which can be transferred on various topologically complex surfaces (paper and Al foil or glass) (Figure S13). They can serve as a robust sensing platform for chemical identification, compared with these intractable pure nanoparticle dispersions, which are difficult to recover.69 Additionally, the rGO/Ag/rGO demonstrate some other advantages over the bare AgNPs in terms of stability under wet conditions and reduced oxidation and impermeability to all liquids and aggressive chemicals (see Supporting Information Figures S14 and S15).

EXPERIMENTAL SECTION Materials and Fabrication. GO flakes were prepared using the Hummers’ method.32 Aqueous GO flakes were then redispersed in methanol (0.1 wt %) by solvent exchange using ultracentrifugation. Generally, the yellow GO aqueous solution was centrifuged at 10 000 rpm for 30 min, and the supernatant was decanted; the resulting slurry was washed three times with methanol to exchange and remove residual water. Finally, the concentration of the GO methanol solution was adjusted to 0.1 wt %. For SA-LbL assembly, GO nanomembranes were deposited onto piranha-treated silicon wafer supports (sizes on the range of tens of cm2) by spin-casting a GO methanol suspension at 3000 rmp for 30 s until the desired thickness was reached. The reduction of the GO component was conducted with HI solution (57 wt %) at room temperature for 10 min according to literature.33 The rGO nanomembranes were dried at room temperature before further characterization. Silver nanoplates were synthesized according to the literature procedure.70 As-prepared silver nanoplates in methanol (10 wt %) were ultrasonically treated for 3 h to roughen the metal surface. For the fabrication of rGO−SERS nanomembranes, five monolayers of GO (5 nm) were deposited on the substrate, then silver nanoplates were drop-cast on the GO surface, followed by spin-casting an additional five GO monolayers to form a sandwiched nanostructure. In this design, the reduction of GO monolayers was conducted with hydrazine vapor treatment according to literature.44 To prepare the AgNP coating for SERS experiments, we drop-cast the AgNP solution on the silicon wafer under the same condition use for rGO/AgNP/ rGO fabrication. The wavelength of the laser was chosen as 532 nm because the strongest SERS performance generally corresponded to the highest extinction near the localized surface plasmon resonance (LSPR) band. The as-prepared Ag nanoplates show a broad LSPR band from 400 to 550 nm, as shown in Figure S16. Characterization. The morphology of the nanomembranes was studied by AFM (Dimension 3000, Digital Instruments) with soft tapping mode at 0.7 Hz.71 The thickness of the nanomembrane was measured independently using a spectroscopic ellipsometer (M2000U, Woolam). Optical transmittance was determined by a Shimadzu 1601 UV−visible spectrometer. ATR-FTIR measurements were carried out on a Bruker FTIR spectrometer, Vertex 70, equipped with a narrowband mercury cadmium telluride detector. XRD data were collected with XPert Pro Alpha-1 diffractometer. XPS spectra were obtained with a Thermal Scientific K-alpha XPS instrument. Raman spectra and surface maps were measured with a WiTek Alpha 300R confocal Raman microscope. For the SERS measurements, rGO−AgNP−rGO nanomembranes were immersed in a 100 μM R6G ethanol solution for 1 min and washed with pure ethanol and dried in air. A contact angle was measured on a CAM 100 KSV instrument. Electrical conductivity was measured by a four-probe method with 1 mm probe distances. All of the bulging tests were conducted at 23 °C and 45% RH. All samples were air-dried overnight, and the bulging tests were done after 24 h of air-drying according to the established procedure with 150− 300 μm copper apertures according to the usual procedure.49 The copper aperture was mounted on an airtight holder connected to a linear pump that can supply negative hydraulic pressure to the

CONCLUSIONS In conclusion, we fabricated a single-component freestanding rGO nanomembrane as thin as 3 nm that exhibits high strength (500 MPa) and high elastic modulus (120 GPa) due to its uniform and smooth morphology with very low surface roughness and minimized defects and wrinkles (a common defect of assembled flexible GO flakes). This excellent mechanical performance is combined with high optical transmittance (up to 93% at 550 nm for thinner films) and high electrical conductivity (up to 3000 S/m for thicker films after chemical reduction). Replacement of water with methanol as the solvent for dispersing GO flakes is the key factor to enable the uniform LbL growth without any chemical modification or employing additional binding agents due to methanol’s selective solubility, fast spreading, and firm 6712


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ACS Nano suspended nanomembranes. The hydraulic pressure exerted on the deflected nanomembranes was recorded by a pressure gauge directly connected to the pressure application system. A homemade singlewavelength interferometer equipped with a He−Ne laser was used to monitor the vertical deflection of the apex of the film as the film was bulged. rGO nanomembranes can be easily assembled and released from the substrate onto the water surface. For the bulging test, these freestanding nanomembranes were transferred to the copper grid with 150−300 μm apertures. The 5 nm (and thicker) rGO nanomembranes are robust enough to be transferred onto a 150 μm aperture without any cracks or pinholes, and thus these nanomembranes were chosen for mechanical testing.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02012. Details of freestanding PS and PS/CA nanomembranes, dissolving tests, and characterization of graphene oxide paper in water and methanol, the water contact angle measurements, thickness measurements, XPS, and stability testing (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions §

R.X. and K.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of China (51473100), China Scholarship Council (201406240067), Air Force Office for Scientific Research Grant FA9550-14-1-0269, and the U.S. National Science Foundation CBET-1401720. The authors thank Ruilong Ma for helpful technical assistance. REFERENCES (1) Hu, K.; Kulkarni, D. D.; Choi, I.; Tsukruk, V. V. GraphenePolymer Nanocomposites for Structural and Functional Applications. Prog. Polym. Sci. 2014, 39, 1934−1972. (2) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906−3924. (3) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282−286. (4) Cheng, W.; Campolongo, M. J.; Tan, S. J.; Luo, D. Freestanding Ultrathin Nano-Membranes via Self-assembly. Nano Today 2009, 4, 482−493. (5) Liu, H.; Wang, H.; Zhang, X. Facile Fabrication of Freestanding Ultrathin Reduced Graphene Oxide Membranes for Water Purification. Adv. Mater. 2015, 27, 249−254. (6) Han, Y.; Xu, Z.; Gao, C. Ultrathin Graphene Nanofiltration Membrane for Water Purification. Adv. Funct. Mater. 2013, 23, 3693− 3700. (7) Yin, Z.; Sun, S.; Salim, T.; Wu, S.; Huang, X.; He, Q.; Lam, Y. M.; Zhang, H. Organic Photovoltaic Devices Using Highly Flexible Reduced Graphene Oxide Films as Transparent Electrodes. ACS Nano 2010, 4, 5263−5268. (8) Yang, Y. H.; Bolling, L.; Priolo, M. A.; Grunlan, J. C. Super Gas Barrier and Selectivity of Graphene Oxide-Polymer Multilayer Thin Films. Adv. Mater. 2013, 25, 503−508. 6713


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