Layer-by-Layer Assembly of Thin Films Containing Exfoliated Pristine

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Layer-by-Layer Assembly of Thin Films Containing Exfoliated Pristine Graphene Nanosheets and Polyethyleneimine Alison Y. W. Sham† and Shannon M. Notley*,‡ †

Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia ‡ Faculty of Life and Social Sciences, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia S Supporting Information *

ABSTRACT: A method for the modification of surface properties through the deposition of stabilized graphene nanosheets is described. Here, the thickness of the film is controlled through the use of the layer-by-layer technique, where the sequential adsorption of the cationic polyethyleneimine (PEI) is followed by the adsorption of anionic graphene sheets modified with layers of polyethylene oxide− polypropylene oxide−polyethylene oxide (PEO−PPO−PEO) surfactants. The graphene particles were prepared using the surfactantassisted liquid-phase exfoliation technique, with the low residual negative charge arising from edge defects. The buildup of the multilayer assembly through electrostatic interactions was strongly influenced by the solution conditions, including pH, ionic strength, and ionic species. Thereby, not only could the thickness of the film be tailored through the choice of the number of bilayers deposited but the viscoelastic properties of the film could also be modified by changing solution conditions at which the different species were deposited. The quartz crystal microbalance was used to measure the mass of graphene and polyelectrolyte immobilized at the interface as well as to probe the energy dissipated in the adsorbed layer.

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cohesive energy between graphene sheets.10,20 In this manner, cationic, anionic, and non-ionic surfactants have been used to prepare stable graphene suspensions with various concentrations.11,21,22 It has been shown that the colloidal stability of these suspensions, which result from graphene surface interactions, is highly dependent upon background solution pH and ionic strength,23 in addition to the surfactant species and concentration.13 Moreover, the continuous addition of the surfactant throughout the sonication stage further increases the concentration of graphene suspensions by maintaining a reduced surface tension as the surfactant is depleted from solution because of adsorption to the exfoliated particle surface.13 These resultant concentrated pristine graphene suspensions are well-suited for use in thin-film applications. The ability to create thin films that incorporate graphene is highly desirable, because it provides a potential route for producing novel conducting films9 or hybrid composites with enhanced mechanical properties among other applications. An established method of creating thin films on substrates is through the layerby-layer (LbL) technique, which typically involves the buildup of consecutive layers of oppositely charged species.24−27 The

INTRODUCTION Graphene has received tremendous interest in a broad range of research areas because of its unique structure, superior properties, and remarkable application potential. Graphene is comprised of a single monolayer of sp2-bonded carbon atoms arranged in a planar, hexagonal lattice.1,2 It is this unique twodimensional structure that gives rise to its excellent mechanical,3 thermal,4 and electronic properties.5 There are many methods for the production of graphene,6 one of which is the ultrasonic exfoliation of graphite.7−11 Unlike traditional mechanical exfoliation methods, the ultrasonic exfoliation of graphite is a wet chemical technique particularly suited to large-scale solution processing of graphene particles. It distinguishes itself from other common wet chemical techniques, such as the Hummer’s method, by its ability to produce pristine graphene sheets that retain their sp2 character without oxidation and reduction steps, which may impact the electronic and mechanical properties.12 Ultrasonic exfoliation also has the benefits of being facile, inexpensive, and environmentally friendly, with water as the solvent of choice. With the aid of a surfactant, ultrasonic exfoliation has been used to form high-concentration, aqueous dispersions of twodimensional materials.13−19 The presence of a surfactant species produces stable graphene suspensions by improving particle stability and promoting the exfoliation of the graphite by reducing the surface tension of the solution to match the © 2014 American Chemical Society

Received: December 11, 2013 Revised: February 16, 2014 Published: February 16, 2014 2410

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resultant multilayer films possess a range of properties distinct from the underlying substrate, including altered adhesion,28 elasticity, biocompatibility, mechanical properties,29 and wettability.30,31 This is in addition to a high surface coverage of polymers or particles and controllable film thickness through the choice of a number of deposited bilayers. This technique has traditionally been applied to form multilayers with both polyelectrolyte24,32,33 and inorganic systems.34 Previous work has focused on incorporating graphene oxide or reduced graphene oxide into multilayered films.35 Recently, however, multilayers from surfactant-exfoliated graphene subsequently modified with the cationic polyelectrolyte polyethyleneimine (PEI) and the anionic polyelectrolyte poly(acrylic acid), were constructed using the LbL technique.36 It was shown that multilayer growth was governed by parameters, such as pH and polyelectrolyte, because of the weakly ionizable nature of PEI and poly(acrylic acid), which, hence, influences the overall surface charge of the graphene particles and counter polyelectrolyte. Furthermore, a lower charge on the modified graphene particle surface was found to increase the adsorbed amount and overall surface coverage.36 Consequently, there is a clear need to study multilayers formed with low-surface-charge graphene particles, such as those found in non-ionic surfactant-exfoliated dispersions, and polyelectrolytes. To realize the full potential of these thin films, however, it is also necessary to investigate and optimize the conditions under which these multilayers form. Here, we describe the preparation of graphene multilayer films by the LbL approach using pristine graphene particles stabilized with polymeric block copolymer surfactant coupled with a cationic polyelectrolyte. The small intrinsic charge on the graphene particles because of edge defects is sufficient to allow for the formation of multilayers. By modifying the effects of solution conditions under which graphene-containing multilayers form, the quality and thickness of the resultant multilayers can be controlled and optimized for a given application.

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pH 3.5 and 9.5. pH adjustment of the suspension was performed manually using the appropriate amount of NaOH or HCl. Raman spectroscopy was conducted on the graphene particles using a Horiba Jobin Yvon Raman system, with a 633 nm excitation laser. Samples were prepared by direct deposition of the undiluted graphene suspension onto silicon wafers and measured in the dry state. The UV−vis spectrum of the diluted graphene suspension was measured using a Cary 300 UV−vis spectrophotometer over the wavelength range of 200−800 nm. Particle sizing was performed using DLS with the Malvern Zetasizer Nano. Strictly speaking, DLS allows for the determination of the equivalent sphere particle size distribution and is, therefore, not typically applicable to platelet-like sheets. However, a recent study demonstrated a strong correlation in the particle size distribution peak position, aDLS, determined using DLS, and the mean lateral size of graphene sheets, ⟨L⟩, according to eq 1.37 1.5 ± 0.15 ⟨L⟩ = (0.07 ± 0.03)aDLS

(1)

Adsorption Measurements. Multilayers of graphene particles and PEI were prepared using the LbL deposition technique on silica surfaces. In a typical experiment, adsorption of PEI to the silica surface was followed by a rinsing step with electrolyte solution. A solution of block copolymer-stabilized graphene was then exposed to the surface, followed by another rinse step with electrolyte solution to produce a bilayer. The process of bilayer formation was then repeated until the appropriate number of bilayers was deposited. Both solutions used in the deposition process were adjusted to the appropriate pH. In this study, the formation of multilayers was monitored using a quartz crystal microbalance (QCM, KSV QCM-500). The QCM was used to monitor the change in resonant frequency (Δf) and overtones (Δf 3, Δf5, and Δf 7) as the multilayer was constructed on a QCM quartz resonator crystal. The third overtone (Δf 3) was selected for subsequent analysis in preference to the resonant frequency and other overtones because it exhibits enhanced stability and consistency over the fundamental resonant frequency, arising from better energy trapping.38 As layers are deposited and the adsorbed mass on the surface increases, the change in resonance frequency of the crystal decreases. The Sauerbrey relationship was used to convert this change in frequency because of adsorption into a sensed mass. The Sauerbrey relationship (eq 2) states that the total change in mass (ΔM) is directly proportional to Δf of the quartz resonator39 1 ΔM = − C Δfn n

EXPERIMENTAL SECTION

Materials. In this study, synthetic graphite powder with a nominal particle size of less than 20 μm was used as received from SigmaAldrich. The non-ionic triblock poloxamer Pluronic F108 [Mn ∼ 14.6 kDa; HO(C2H4O)141(C3H6O)44(C2H4O)141H] was also obtained from Sigma-Aldrich. The cationic polyelectrolyte PEI (30% aqueous; Mn ∼ 70 kDa) was obtained from PolySciences, Inc. NaCl and KNO3 were used as electrolytes. All solutions were prepared using Milli-Qgrade water and adjusted to the appropriate pH using NaOH and HCl. Methods. Preparation of Stock Graphene Suspensions. Stock graphene suspensions were prepared via the method of ultrasonic exfoliation of graphite, with continuous surfactant addition.13 In a typical experiment, a 10% (w/w) solution of Pluronic F108 (90 mL) was added at a rate of approximately 1 drop per second to a 2% (w/w) suspension of graphite (98 mL) powder in Milli-Q water under ultrasonication for 90 min. The suspension was then centrifuged at 2500 rpm for 20 min to sediment larger, non-exfoliated graphite particles. The resulting suspension was dialyzed against Milli-Q water for a minimum of 48 h to remove unadsorbed surfactant from the stock solution. Samples of the suspension were weighed, dried, and reweighed to determine the average graphitic content of the suspensions. Particle Characterization. The exfoliated graphene particles were characterized using ζ potential measurements, Raman spectroscopy, ultraviolet−visible (UV−vis) spectrophotometry, and a dynamic light scattering (DLS) technique. The ζ potential of the graphene particles was determined using a Malvern Zetasizer Nano. ζ potential measurements were performed at intervals of 0.5 pH units between

(2)

where C is a constant based on the physical characteristics of the quartz crystal (in this case, 17.7 ng cm−2 Hz−1) and n is the overtone number. The relationship is valid for uniform, rigidly adsorbed films, occurring when the Δf values of the different overtones are similar when scaled by n.40 This condition was satisfied for all solution conditions trialed, with the maximum average difference between overtones for each experiment being less than 11 Hz. The QCM was also used to measure dissipation values of the multilayer films indirectly via impedance analysis. Dissipation is a parameter that quantifies the sum of all energy losses in the QCM chamber per oscillation cycle and, thus, provides an indication of adsorbed multilayer film rigidity. Consequently, the dissipation values, ΔDn, were used to characterize the rigidity of the prepared films. Although film rigidity is a continuum, in this study, we define a threshold41 for ΔDn/(−Δf n/n) of 4 × 10−7 Hz−1 as a guide to distinguish between films that are more rigid and films that exhibit more viscoelastic behavior. Although most samples in this study were prepared using the QCM, a small number of multilayer films were prepared using a manual dipcoating process on silicon wafer surfaces. The samples were prepared using the same adsorption regimes used to prepare samples for QCM measurements. Multilayer Characterization. The resultant multilayers were characterized using both atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements in the dry state. XRD measurements were performed on multilayer samples, using a Bruker D8 ADVANCE 2411

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X-ray powder diffraction setup with a Cu Kα X-ray source operating at a voltage of 40 kV and current of 40 mA. The samples were continuously rotated throughout the measurements. The surfaces of the dip-coated multilayers were imaged using a MultiMode scanning probe microscope (SPM, Veeco, Inc., Santa Barbara, CA). Standard, non-contact Si3N4 cantilevers with a spring constant of 0.4 N/m and SiO2 tip chemistry (Veeco, Inc., Santa Barbara, CA) were used for all measurements in this study. The surfaces were imaged using the peak force mode. The scan rate was typically 0.5 Hz, with gains between 15 and 20.

confirming that the extended conjugation of the graphene sheet is maintained. Adsorption Measurements. Adsorption measurements were performed by preparing multilayer films from exfoliated graphene suspensions and PEI using the LbL technique in a variety of solution conditions at 23 °C. These measurements were performed to determine the effect of pH, graphene concentration, electrolyte, electrolyte concentration, and polyelectrolyte concentration on successful film formation, adsorbed mass, and film rigidity. Film Formation for Extended Films. Multilayers comprised of an extended number of graphene/PEI bilayers were assembled. Using the successive deposition of 100 ppm PEI and 0.5391 mg/mL Pluronic F108 exfoliated graphene suspension in 10−2 M KNO3 at pH 4, multilayers comprised of one, three, five, seven, and nine bilayers were prepared. Figure 2

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RESULTS AND DISCUSSION Graphene Characterization. The optical, vibrational, and physical properties of the exfoliated graphene particles were characterized prior to their use in subsequent experiments, using UV−vis spectroscopy, Raman spectroscopy, and ζ potential measurements. The ζ potential of the exfoliated graphene particles, in NaCl and KNO3, was measured as a function of pH to determine the effective surface charge of the graphene particles (Figure 1). The ζ potential measurements of

Figure 2. Change in frequency as a function of time for the nine bilayer system formed with 100 ppm PEI and 10% graphene suspension in 10−2 M KNO3 at pH 4. Figure 1. ζ potential measurement of (blue diamonds) graphene exfoliated in surfactant solution in 10−4 M NaCl and (red squares) graphene exfoliated in surfactant solution in 10−4 M KNO3.

shows the kinetics of this process for the construction of a multilayer comprised of nine bilayers. First, a baseline was recorded with the pH-adjusted background electrolyte in the QCM chamber. PEI was then adsorbed to the silica crystal surface for approximately 300 s, during which a corresponding decrease in signal was recorded, indicating adsorption of the cationic polymer. An equilibrium-adsorbed amount was attained during this step. Next, a rinsing step was performed with electrolyte for approximately 300 s, resulting in an increase in the signal, suggesting removal of loosely bound PEI from the surface. Graphene suspension was then added to the chamber for approximately 300 s, resulting in a decrease in frequency lower than that observed for the PEI, followed by a rinsing step with electrolyte, where the signal increased again. The process was then repeated to create the desired number of bilayers. The decrease in signal compared to the baseline upon rinsing, regardless of species introduced to the chamber prior to the rinse step, indicates the adsorption of successive layers and buildup of the multilayer. The results obtained for the Sauerbrey mass and dissipation of the nine bilayer film are presented in Figure 3. The film exhibited linear film growth with respect to the graphene additions, suggesting that it is possible to create multilayers with an extended number of bilayers. The only exception to this observation is the first bilayer, where a greater mass of PEI is adsorbed to compensate for the negative surface charge of silica. This behavior is consistent with the known adsorption kinetics of polyelectrolyte multilayers, which have been shown

the graphene particles were consistent with the literature for surfactant-exfoliated graphene,13 exhibiting a slight negative charge between −6.38 and −18.2 mV in 10−4 M NaCl and between −8.6 and −25.1 mV in 10−4 M KNO3, becoming more negative with increasing pH. This negative charge is acquired because of the oxygenated edge defect sites on the graphene sheets, introduced during the sonication procedure.23,36 Thus, as the pH decreases, protonation of the oxygen-containing surface groups occurs, increasing the ζ potential. Regardless of the pH, however, the ζ potential values still remain well within the region of colloidal instability (±25 mV), suggesting that the particles will tend toward reaggregation on the basis of insufficient electrostatic repulsion. However, previous studies indicate that agglomeration is prevented by the irreversible adsorption of block copolymer surfactant molecules on the surface of the graphene sheets.13 Because the stock suspension was stable and the surfactant is non-ionic, this suggests that the observed colloidal stability is imparted primarily through steric repulsion. Measurement of the light absorption of the graphene particles through UV−vis spectrophotometry supports that little to no basal defects are introduced in the exfoliated sheets because the position of the primary peak was at 269 nm. Furthermore, strong absorption across the spectrum, as shown in Figure S1 of the Supporting Information, was observed, 2412

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layers are adsorbed, layers closer to the substrate may collapse because of further interlayer interactions. This would cause the film to become more rigid as an increasing number of layers is adsorbed. Effect of pH and Electrolyte on Film Formation. Multilayer films were prepared at pH 4, 7, and 9 to determine the effect of pH on the formation and rigidity of the multilayers. The charge on the graphene particles increases with increasing pH, as shown in Figure 1; hence, the adsorbed amount may vary with pH. In addition, as the pH increases, the charge on the PEI macromolecules decreases. The multilayers, comprised of three bilayers each, were constructed using the consecutive adsorption of 100 ppm PEI and 0.5391 mg/mL Pluronic F108 exfoliated graphene suspension, in 10−2 M NaCl and 10−2 M KNO3. Two different salts were used to probe the influence of the ionic species on the film formation. The graphene is stabilized using a polyethylene oxide−polypropylene oxide− polyethylene oxide (PEO−PPO−PEO) block copolymer, which has some small conformational changes at elevated ionic strength, which is dependent upon the specific ions in solution. For both systems shown in Figures 4 and 5, it is clear that assembly proceeds to a greater extent at either acidic or basic

Figure 3. (a) Sauerbrey mass and (b) dissipation as a function of time for the nine bilayer system formed with 100 ppm PEI and 10% graphene suspension in 10−2 M KNO3 at pH 4.

to increase either linearly24 or exponentially.42 Linear film growth occurs as the result of the electrostatic attraction between the surface and the adsorbing molecule, with further deposition resulting in charge overcompensation at the interface. This suppresses further adsorption via electrostatic repulsion within the layer and gives rise to distinct layers with blended interfaces. Exponential film growth arises as the result of molecules adsorbing to the film via electrostatic interactions and diffusing throughout the film toward the substrate, forming a homogeneous film. The linear trend in observed film growth of the multilayers may be related to the size and planar nature of the exfoliated graphene nanoparticles. The continuous surfactant technique used to process the graphene particles in this study typically results in graphene particles with a lateral size of up to 100 nm,13 which is significantly larger than PEI molecules. Furthermore, the most kinetically favorable configuration of the sheets maximizes contact with the active surface. As a result, the size and orientation of the graphene sheets may hinder interdigitation of PEI molecules into the graphene layers. This could result in distinct layers where charge overcompensation is achieved for adjacent layers, resulting in a linear growth regime. The films also exhibited the predicted relationship between Sauerbrey mass and dissipation. As the mass adsorbed to the resonator increases, the dissipation increases upon the addition of both PEI and graphene. Interestingly, however, the change in dissipation and adsorbed mass decreases as successive PEI layers are added but remains relatively constant for graphene additions. This suggests that either less PEI is being adsorbed but the overall rate of increase in adsorbed mass remains the same or entrained water is removed from the multilayer upon further additions of PEI, which is supported by the trend in overall film rigidity. As the number of bilayers increases, dissipation as a function of frequency decreases, indicating the increased overall film rigidity. The presence of KNO3 promotes film rigidity through the removal of entrained water within the layers, regardless of the number of layers. Additionally, as more

Figure 4. Sauerbrey mass as a function of the layer number for multilayers fabricated using PEI at various pH values in 10−2 M NaCl. The maximum standard deviation of the measurements was 1.14 mg/ m2.

Figure 5. Sauerbrey mass as a function of the layer number for multilayers fabricated using PEI at various pH values in 10−2 M KNO3. The maximum standard deviation of the measurements was 0.62 mg/ m2.

conditions but not neutral pH conditions. This is attributed to orientation of the molecules, arising from the protonation and deprotonation of amine groups on PEI. At higher pH, deprotonation occurs, lowering charge density and causing 2413

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the polyamine molecules to extend away from the active surface, as previously demonstrated from surface force measurements.43,44 Typically, this results in a greater adsorbed amount, which is in agreement with theory and experiments of polyelectrolyte adsorption.45 Additionally, at high pH, the graphene particles acquire a more negative charge, as shown previously by ζ potential measurements. Consequently, a higher amount of PEI is required to be adsorbed to achieve charge compensation upon the second addition of PEI. As a result, the thickness and density of the molecules in the PEI layer increase, causing a higher mass to adsorb to the surface during each addition of PEI at high pH. Conversely, the PEI molecules adopt a flat conformation against the substrate at lower pH. The graphene particles have a very low negative charge at pH 4; hence, a higher adsorbed amount of particles should result. At neutral pH, both graphene particles and PEI have significant charge density, which results in relatively lower adsorbed amounts in comparison to solutions of high or low pH. Such behavior has previously been demonstrated for the assembly of multilayers using polyelectrolytes with weakly ionizable groups.31,46 Data from this series of experiments also suggested that the overall film rigidity of the multilayers was dependent upon pH. For example, multilayers formed in 10−2 M NaCl at pH 4 demonstrate more viscoelastic behavior, as evidenced by the higher energy dissipation of the film, which is typically greater than 10−6 per 10 Hz (Figure 6). In contrast, films formed at pH 7 and 9 in 10−2 M NaCl show an increase in film rigidity with increased pH. This can be attributed to the orientation of the PEI in addition the oscillation of the QCM crystal. As the QCM crystal oscillates, it generates a standing shear wave that results in a shear force parallel to the surface. At lower pH values, the PEI molecules deposit along the surface and may experience more slippage because of the shearing action of the oscillator, increasing the energy dissipated in the layer. Conversely, at higher pH values, the cationic polyelectrolyte extends away from the active surface. This encourages chain entanglement, increasing layer density and, thus, reducing the dissipation of the film. It was also shown that film rigidity and adsorbed mass were influenced by the electrolyte selected. At each pH level investigated, multilayers formed in the presence of NaCl exhibited a higher Sauerbrey mass than those of KNO3. However, films formed using KNO3 typically exhibited more rigid film behavior, as demonstrated by lower dissipation values. Here, the presence of K+ ions disturb the hydration layer surrounding PEO portions of the surfactant chain,47 dehydrating the adsorbed block copolymer. Consequently, a reduction in entrained water occurs, causing the films to become denser and, therefore, more rigid. Specific ion effects influence the solution properties of PEO, and hence, it is reasonable to assume that the solution conformation of block copolymers incorporating PEO are also similarly influenced. Usually, it would be expected that Na+ would be more hydrated and, therefore, result in a greater change in the conformation of the polymer; however, for K+, the dominant factor is the hydration of the polymer and not the hydration of the ionic species. The trend in adsorbed mass observed for the multilayers prepared at pH 4 and 7 in NaCl is also of particular interest. Here, the adsorbed mass and dissipation increase upon the addition of graphene and then decrease slightly upon adsorption of PEI. The adsorbed mass does not revert to the mass measured prior to adsorption of the graphene layer, as

Figure 6. Dissipation as a function of the normalized frequency for systems at (a) pH 4, (b) pH 7, and (c) pH 9 in 10−2 M NaCl. The red diagonal line illustrates a dissipation of 4 × 10−7 Hz−1, indicative of the threshold between rigid and viscous films defined previously. This threshold arises from the relationship between the dissipation per unit of frequency (or energy) applied to films deposited on a 5 MHz quartz crystal resonator.41 Data further below the line indicate more rigid multilayer films, whereas data above the line indicate increased viscosity.

may be expected when the layer is completely removed. Instead, the alternating increase and decrease in mass upon the addition of solution to the QCM chamber is characteristic of water entrained within the adsorbed layers. At pH 4 and 7, the exfoliated graphene is expected to undergo protonation of the oxygenated defect sites to a greater extent than at pH 9. The expulsion of entrained water upon the addition of PEI suggests that hydrogen bonding occurs between the protonated defect sites on the graphene and the trapped water molecules. Upon the addition of PEI, electrostatic interactions and interpenetration of the layers force the entrained water from the adsorbed graphene layer. Effect of the Graphene Concentration on Film Formation. The formation of multilayer films as a function of the graphene concentration was also investigated. Again, the multilayers were constructed of three bilayers, using the successive adsorption of 100 ppm PEI and non-ionic block copolymer-stabilized graphene suspension, in 10−2 M NaCl and 10−2 M KNO3. The measurements were performed for graphene concentrations of 2, 5, 10, and 20% of a 5.142 mg/ mL stock graphene solution at pH 4, where previous results showed enhanced multilayer formation. Regardless of the electrolyte used, a much higher effective mass was sensed for graphene additions in systems where the graphene concentration was greater than 2% (Figures 7 and 8). 2414

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the dip-coated multilayer films using AFM. Films of one, three, five, seven, and nine bilayers were prepared from 100 ppm PEI and Pluronic F108 exfoliated graphene suspension, in KNO3 at pH 4, using LbL deposition onto a silicon wafer. For each sample, height images of 10 representative areas were obtained and the corresponding Rq values were determined. Figure 9 shows representative peak force error (deflection) height images of the prepared samples, together with the corresponding average Rq value for each sample. The average Rq value typically increases with the number of adsorbed bilayers. This is consistent with common observations; as increasingly more material is added to the surface, the underlying observed surface roughness is intensified. This indicates that the films remain stable during the drying and dewetting process. Multilayer films constructed from surfactant-exfoliated graphene and PEI using the LbL approach show great potential in providing a means to creating thin-film coatings with tailored properties. In particular, the surface roughness, overall film thickness, and viscoelastic properties of the thin films may be altered by simply modifying the number of adsorbed layers, pH, and electrolyte used when constructing the films. This, in turn, could provide a method for producing thin films with controlled thickness, varied adhesion properties, and elasticity. Consequently, thin films prepared using this method could prove highly desirable in applications, such as conductive films and composites. One of the challenges in preparing composites from graphene is compatibility with matrix material. Polymer− graphene hybrid thin films can be conveniently prepared through the methods described here. The LbL approach has been used extensively over the past decade and has a sufficient degree of flexibility of the chemistries involved that a range of surface functionality can be modified. It is anticipated that multilayers incorporating graphene will have altered mechanical properties and possess other unique properties.

Figure 7. Sauerbrey mass as a function of the layer number for multilayers fabricated using PEI and various concentrations of graphene suspension in 10−2 M NaCl. The maximum standard deviation of the measurements was 0.68 mg/m2.

Figure 8. Sauerbrey mass as a function of the layer number for multilayers fabricated using PEI and various concentrations of graphene suspensions in 10−2 M KNO3. The maximum standard deviation of the measurements was 0.98 mg/m2.

For the 2% graphene systems, the adsorbed mass of consecutive PEI and consecutive graphene additions plateaus following the first addition of graphene. This suggests that a 2% graphene concentration is too low to enable self-limiting formation of a uniform graphene layer, suppressing further assembly. The effect is more pronounced when KNO3 is present because the salt acts to dehydrate the bilayers. Concentrations greater than 2% had little effect on the overall adsorption because the combination of conditions all approached similar values. This results in a maximum Sauerbrey mass of ∼4.06 mg/m2 observed for films formed in the presence of NaCl, with a graphene concentration of 20%. Characterization of Extended Numbers of Bilayers. XRD was used to further characterize multilayers adsorbed to silica-coated QCM resonator surfaces. Multilayer films with one, three, and five bilayers were prepared from 100 ppm PEI and 0.5142 mg/mL Pluronic F108 exfoliated graphene suspension, in KNO3 at pH 4. The XRD measurements of all three samples (see Figure S3 of the Supporting Information) demonstrate a peak at 26.5°, associated with the (002) diffraction line of graphite and, therefore, the 3.4 Å spacing between graphene sheets in graphite. 48 As a result, reaggregation of the graphene particles is likely to have occurred during the formation of the multilayers, although the low peak intensity shows this is a minor effect. Furthermore, none of the three XRD measurements indicate the presence of graphene oxide, which typically exhibits a peak at 11.8°. This indicates that the sp2 structure of the adsorbed graphene particles is conserved upon the formation of multilayers. The relationship between surface roughness and the number of adsorbed bilayers was investigated by imaging the surfaces of

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CONCLUSION There is a strong demand for surface coatings involving novel materials with the ability to improve a range of properties. Here, a silica surface was successfully modified through the LbL technique using pristine graphene nanoparticles. The graphene particles used in this study were prepared through aqueousphase exfoliation of graphite in the presence of a non-ionic triblock polymeric surfactant. This method for producing graphene is scalable in quantity and also flexible, allowing for the stabilization of particles with various surfactants and polymers. The graphene sheets were shown by Raman and UV−vis spectroscopy to have few to no defects aside from the edges, which gives rise to an inherent anionic charge, thereby allowing for the buildup of multilayers through electrostatic interactions. The solution conditions for the sequential adsorption of the polycation PEI and graphene sheets had a significant influence on the adsorbed amount of material. The ionic species, ionic strength, and pH of the solution all impacted the thickness and viscoelastic properties of each deposited bilayer. Deposition of an extended number of bilayers was achievable, with linear film growth observed for most of the conditions selected. This type of film growth suggests that graphene particles may hinder diffusion of the polyelectrolyte through the films because of their lateral size and shape, thereby preventing exponential growth of the film. It was also shown that films formed in the presence of KNO3 had a lower adsorbed mass but were more rigid than those prepared in NaCl solutions. This is likely due 2415

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Figure 9. AFM peak force error (deflection) images of (a) silicon wafer and (b) one, (c) three, (d) five, (e) seven, and (f) nine graphene−PEI bilayer films. The samples have average Rq surface roughness values of 0.191, 0.209, 0.268, 0.368, 0.256, and 0.627 nm, respectively.

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to disruption of the hydration layer surrounding portions of the triblock surfactant species used to stabilize the graphene particles in suspension. Importantly, the presence of the polymeric surfactant for stabilizing the graphene sheets did not prevent the formation of multilayer films; rather, it provides another variable that can be used to tune the film properties. Basic and acidic conditions were shown to favor the formation of multilayers based on the orientation and resultant packing of the PEI molecules because the charge on the graphene sheets hardly varies over the pH range studied. The growth rate of the multilayers was also dependent upon the graphene concentration, with a minimum graphene concentration required to ensure complete surface coverage and charge compensation of the adsorbed layer. By considering the effect of each of these parameters, the deposition and formation of multilayers containing graphene can be easily optimized for a given thin-film application. Thus, a convenient method for the modification of a surface with graphene was demonstrated, along with a description of some design rules for optimizing or tailoring the film properties.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +61-392148635. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported through a grant from the Australian Research Council under the Future Fellowships scheme. REFERENCES

(1) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6 (3), 183−191. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306 (5696), 666−669. (3) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321 (5887), 385−388. (4) Shahil, K. M. F.; Balandin, A. A. Thermal properties of graphene and multilayer graphene: Applications in thermal interface materials. Solid State Commun. 2012, 152 (15), 1331−1340. (5) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Ultrahigh electron mobility in suspended graphene. Solid State Commun. 2008, 146 (9−10), 351− 355. (6) Allen, M. J.; Tung, V. C.; Kaner, R. B. Honeycomb carbon: A review of graphene. Chem. Rev. 2009, 110 (1), 132−145. (7) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.;

ASSOCIATED CONTENT

S Supporting Information *

Characterization data of graphene particles, including UV−vis spectra, Raman spectra, and lateral size dimensions. This material is available free of charge via the Internet at http:// pubs.acs.org. 2416

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