Mechanical Properties of Water-Assembled Graphene Oxide Langmuir Monolayers: Guiding Controlled Transfer

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1 Supporting Information Mechanical Properties of Water-Assembled Graphene Oxide Langmuir Monolayers: Guiding Controlled Transfer Katharine L. Harrison, Laura B. Biedermann, and Kevin R. Zavadil This document provides supporting information for the manuscript entitled above. It is structured with the same headings as appear in the manuscript with additional subheadings for clarity. Information is presented in the order it was discussed in the manuscript. Experimental Preparation of GO and RGO spreading solutions As-prepared dried GO powder was dispersed in a 1 mg/ml aqueous suspension by sonicating for 35 minutes using a Cole-Parmer 8890 ultasonicator and stirring overnight. The 1 mg/ml dispersion was then diluted to 0.1 mg/ml and stirred for at least one hour, followed by centrifugation at 10,000 rpm (relative centrifugal force ~8160 x g) for 10 min in an Eppendorf 5415C centrifuge to precipitate the largest flakes and any agglomerates. In a typical experiment, 9 ml of the 0.1 mg/ml suspension was centrifuged. The precipitate was then collected and the water was replaced with 22.5 ml of a 5:1 methanol:water mixture following the method described by Cote et al. 1 The methanol and water GO suspension was then stirred for 1 h and was centrifuged in a Fisher Scientific Marathon 8k centrifuge at 500 rpm (relative centrifugal force ~39 x g) for 7 min to remove any agglomerates. Partially reduced graphene oxide (RGO) was prepared from a GO suspension using the ascorbic acid reduction proposed in Fernandez-Merino et al. 2 Ammonium hydroxide was used to adjust the ph of the GO suspension to ~10 and L-ascorbic acid was added such that the concentration was 0.73 mm. The ph was readjusted to ~10. The GO suspension was held at ~80 o C in a water bath for 1 hour. The suspension was placed in an ice bath for about 10 min to stop the reduction reaction and then centrifuged in a Fisher Scientific Marathon 8k centrifuge at 2500 rpm (relative centrifugal force ~978 x g) three times for min and washed in between. S1

2 After the third centrifugation, the partially reduced RGO was resuspended in ph ~10.5 Milli-Q water (with ammonium hydroxide) and sonicated in a Cole-Parmer 8890 ultasonicator for 2 min. The amount of water used for resuspension was the same as the initial amount of water decanted such that the concentration was not intentionally changed, though some RGO was lost during washing. Preparation of Hydrophilic and Hydrophobic Gold Gold substrates were prepared by depositing 20 nm Ti and 200 nm Au onto 650 m thick Si wafers, where the Ti layer serves as an adhesion layer. The wafers were then diced into 15x15 mm squares. The gold substrates were cleaned first by a warm soak (~45 o C) in acetone for 15 min followed by thorough isopropanol and deionized (DI) water rinses. Then the substrates were soaked in piranha solution (5:1 H 2 SO 4 :H 2 O 2 ) for 15 min. Caution should be used when working with piranha solution, as it is very corrosive. The substrates were then rinsed well with DI water, dried with nitrogen, and finally annealed with a butane torch. To further enhance the hydrophilicity of the gold, the substrates were treated for 5 min in a UV ozone cleaner (Jelight Company Inc, Model No. 42 UVO Cleaner). 3 To obtain hydrophobic surfaces, the substrates were soaked in ethanol for 10 min. 4, 5 The contact angle was measured by a First Ten Ångstroms Dynamic Contact Angle Analyzer to be 25 o for the hydrophilic Au and 80 o for the hydrophobic Au. Although hydrophobic materials are generally defined with a contact angle > 90 o, our Au samples are nearly hydrophobic. More importantly, they are hydrophobic enough that typical LB transfer (both upstroke and downstroke) is unsuccessful, so these samples serve our purpose as hydrophobic substrates which can be compared to the hydrophilic Au substrates. Moreover, substrates with contact angles close to 90 o should be the most difficult to transfer by LB. If the contact angle is near zero, upstroke deposition is aided by the strong upward curvature of the meniscus. Conversely, if the contact angle is superhydrophobic with a contact angle much greater than 90 o, downstroke deposition is aided by the strong downward curvature of the meniscus. S2

3 Characterization of RGO The RGO data described in the manuscript was partially reduced. It was very difficult to prepare a stable spreading solution with highly reduced RGO due to problems with agglomeration, despite keeping the solution basic. By only partially reducing the RGO, it was much easier to prepare a stable spreading solution and to obtain a stable surface phase. However, we were able to obtain similar results with more highly reduced RGO. The moduli for the partially and highly-reduced RGO were similar and we were unable to transfer either monolayer. We chose to present the partially-reduced RGO in the manuscript because it was easier to reliably prepare a stable spreading solution for the purpose of reproducibility and to deposit a stable surface phase that did not agglomerate. The degree of reduction is characterized below by UV-Vis and XPS data. Comparisons of UV-Vis data (Figure S1a) to X-ray diffraction (XRD) data in the literature suggests that when the UV-Vis absorption peak is ~255 nm, the XRD still shows a greatly reduced d spacing (2 = 22.9, d = 0.39 nm) compared to GO (typically 2 ~ 10, d ~ 0.85 nm) and is similar to graphite (typically 2 ~ 26.4, d ~ 0.34 nm). 6 Therefore, a significant fraction of the oxygen groups on the basal plane is removed even when the RGO is only partially reduced, consistent with the trends observed for C(1s) spectra shown in Figure S1b. The decrease in oxygen groups on the basal plane is important because it shows that even our partially reduced RGO should have a small d spacing, indicating that the sheets will likely slide over one another easier as we have discussed in the manuscript. S3

4 Figure S1. (a) UV-Vis and (b) X-ray photoelectron spectroscopy data for the GO and partially reduced RGO that are presented in the manuscript. The sample labeled RGO produced similar moduli and transfer results as the partially reduced RGO, but it was more difficult to reproducibly obtain a Langmuir monolayer on the trough. Brewster Angle Microscope Characterization The KSV Nima MicroBAM focuses a collimated beam of 659-nm, p-polarized light at 53 o to the air-water interface. At this angle, the Brewster angle, Snell s law states that there will be no reflection at the air-water interface. A monolayer with a different index of refraction than water reflects this polarized light and appears bright in the BAM images. This BAM has a field of view 3.6 mm x 4.0 mm and an image resolution of 12 μm; thus only aggregates of GO/RGO flakes are observable. S4

5 Results and Discussion Oscillatory Barrier Measurements Mechanical Properties Calculations Elastic moduli are calculated from the surface pressure and area oscillations shown in Figures 2, S6, and S7. The initial target area (A 0 ) was subtracted from the area data and a quadratic offset was subtracted from the surface pressure data to obtain area and surface pressure values that oscillated around zero. The area and surface pressure data with the Wilhelmy plate in both orientations (perpendicular and parallel to the barriers) were than fit in Matlab to cosine functions, as described in equations S1-S4. Fit_A (t) = ΔΑ cos(t ω + φ A, ) (S1) Fit_Π (t) = ΔΠ cos(t ω + φ Π, ) (S2) Fit_A (t) = ΔΑ cos(t ω + φ A, ) (S3) Fit_Π (t) = ΔΠ cos(t ω + φ Π, ) (S4) ΔΑ, ΔΠ, ΔΑ, and ΔΠ are the amplitudes of the oscillations in area (cm 2 ) and surface pressure as measured in the parallel ( ) and perpendicular ( ) Wilhelmy plate orientations. Similarly, φ A,, φ Π,, φ A,, and φ Π, are the phases (rad) of the area and surface pressure oscillations as measured in the parallel and perpendicular Wilhelmy plate orientations. Finally, t is time (sec) and is the frequency of the oscillations ( rad/sec). The phase shifts, θ and θ, between the surface pressure and area oscillations with the Wilhelmy plate in the parallel and perpendicular orientations are defined by equations S5 and S6, in which was subtracted to account for the natural radian phase difference between area and surface pressure oscillations. θ = φ Π, + π φ A, θ = φ Π, + π φ A, (S5) (S6) S5

6 Tables S1-S3 present oscillatory barrier results for GO on basic and intrinsic ph subphases and RGO on a basic subphase, as calculated from cosine fits to the raw oscillatory ΔΠ barrier data. A 0 and A ΔΠ 0 are relevant parameters for calculating the complex moduli as described in Eqs. 4 and 5 in the main manuscript. 7 and are the storage and dissipative components of the compression moduli as defined by Eq. 2 in the main manuscript, respectively. G and G are the storage and dissipative components of the compression modulus as defined by Eq. 3 in the main manuscript, respectively. Table S1: Intrinsic ph subphase oscillatory barrier measurement results as calculated from the data shown in Figure 2 for a GO monolayer. regime A 0 ΔΠ A 0 ΔΠ θ (rad) θ (rad) G G percolation 8.4± ± ± ± ± ± ± ±0.02 high stiffness 55±4 33± ± ± ±3-3.7±0.3 11±2 0.0±0.2 overlapping 35±1 11±1 0.08± ± ± ±0.6 12±1 1.9±0.2 Table S2: Basic ph subphase oscillatory barrier measurement results as calculated from the data shown in Figures S7 for a GO monolayer. regime A 0 ΔΠ A 0 ΔΠ θ (rad) θ (rad) G G percolation 8.5±0.3 8± ± ± ± ± ± ±0.1 high stiffness 51±2 27±2-0.08± ± ±2 2.1±0.6 12±1 2.1±0.3 overlapping 58±2 12±1 0.11± ±0.1 34±2 5.1±0.8 23±1 1±1 Table S3: Basic ph subphase oscillatory barrier measurement results as calculated from the data shown in Figures S6 for an RGO monolayer. regime A 0 ΔΠ A 0 ΔΠ θ (rad) θ (rad) G G percolation 9 ± ± ± ± ± ± ± ± 0.3 high stiffness 40 ± 2 40 ± ± ± ± ± ± ± 0.2 overlapping 5.1 ± ± ± ± ± ± ± ± 0.05 S6

7 The phase shifts shown in Tables S1-S3 are generally minimal, indicating RGO and GO behave primarily as elastic monolayers. The slight (positive) phase shifts seen for the GO monolayers at higher surface pressures indicate that the GO monolayers transition to become slightly viscoelastic under heavy compression. In this regime, viscoelasticity may result from sheets sliding onto one another irreversibly relative to the time scale of the measurements. The phase shifts are dependent on frequency and applied strain because any energy loss factors such as friction are exacerbated by high frequencies or large strains. Indeed, we found that the phase shifts were larger (more positive) with larger strains or shorter frequencies. Negative dissipation values ( and G ) are unphysical and, in all cases, arise from negative θ and θ values. These negative phase shifts, in turn, indicate the area oscillations lagging the surface pressure oscillations. This result is not physical and represents a limitation in the measurement instrumentation. We attribute this measurement error to a lag between the barrier oscillation motor drive signal, from which A is calculated, and the barrier motion. As discussions with the instrument designer have not resolved this timing issue, we chose to treat this lag as a determinate error in an effort to explore the potential energy dissipating nature of the GO/RGO monolayers. The high degree of precision (generally less than 0.1 rad) in our measurements indicates that the phase offset is a systematic error rather than a random error. We would expect a monolayer with little dissipation to have zero phase shift, especially in the percolation regime. The negative values that we show in Tables S1-S3 are unphysical and should be greater than or equal to zero. Despite our inability to define the exact phase values, we can be sure that the RGO phase values are consistently more negative than the GO phase values. If we were to offset the values such that they were all positive, the GO phase shifts would be more positive, indicating more dissipation than the RGO monolayer. The for RGO is large and negative only as a result of the erroneously negative phase shift. We expect that correcting for the systematic error would lead to very close to zero for RGO. At higher surface pressures, we do observe some finite dissipation for the GO monolayers. This represents loss of energy which could manifest itself as friction between the GO sheets as they wrinkle or slide upon one another. Because the frictional force is much lower for RGO than for GO, there is energy loss associated with compressing the GO monolayers. S7

8 We note that the negative phase values have less than a 10% effect on the storage terms. If the angles are small (close to zero), as in our case, the cosines of the angles are close to unity, ΔΠ so the dominant terms affecting the storage components of the moduli are A 0 and A ΔΠ 0 rather than the phase shifts. Therefore, we are confident that the storage components of the shear and compression moduli are reasonably correct, but the dissipation terms cannot be further studied without accurate synchronizing of the surface pressure and area signals. The storage components are calculated with cos θ and cos θ so the storage terms are positive even if the angles are slightly negative. Small, negative G values in Tables S1-S3 result from A 0 ΔΠ ΔΠ values slightly larger than A 0. These small, negative values are always consistent with positive numbers within the measurement uncertainty. Mechanical Measurements Guiding Transfer to Substrates Area Coverage Analysis GO area coverage was automatically calculated from SEM image contrast using a custom Matlab code built upon MathWorks Curve Fitting and Image Processing Toolboxes, as shown in Figure S2. After automatically adjusting the image contrast, Gaussian fits to the image intensity histogram identified regions of uncovered Au, monolayer GO, and thicker GO. Figure S2. Example of GO area coverage analysis result. The cyan lines outline the border between uncovered Au and GO (92% covered with GO); red lines encircle regions of bilayer and thicker GO (28% multiple layer, 64% monolayer). S8

9 Uniformity and Morphology Analysis Associated with Figure 4 Figure S3 shows additional images of some of the samples introduced in Figure 4. The hydrophobic samples dipped by upstroke deposition at 30 o and 90 o angles are excluded for brevity, but show similar trends. Representative images were taken near the top, middle, and bottom of the Au substrates. Most samples show very uniform coverage, with a slight coverage increase in the bottom of the sample. The hydrophobic Au sample dipped at 90 o shown below exhibited significant non-uniformity. Figure S3. Additional SEM images corresponding to those presented in Figure 4 for some of the dipping procedures. Images are shown near the top, the middle, and bottom of the dipped portions of the substrates and complement Figures 4c, 4f, 4d, and 4h, respectively, from left to right. Figure S4 shows magnified images for some of the samples introduced in Figure 4 for easier visualization of the sheet interactions and morphology of the monolayer. The hydrophilic Au sample shows slight overlap of sheets at the edges with some slight edge wrinkling. The hydrophobic sample transferred by upstroke deposition shows a few gaps in the monolayer. These gaps are generally associated with sheets curling up at the edges and pulling away from S9

10 the hydrophobic substrate. This result indicates that there was a continuous monolayer on the trough, but the non-ideal dipping conditions caused the sheets to wrinkle on the edges in some locations. The hydrophobic sample transferred by downstroke deposition shows highly overlapping sheets with little evidence of wrinkling or buckling of the monolayer. The morphology of this sample contrasts sharply with that of the hydrophilic sample for which the sheets tile together to fill gaps but exhibit minimal overlap. The dipping geometry and surface interactions with the sheets modify the morphology of the transferred monolayer. Figure S4. Additional SEM images corresponding to those presented in Figure 4f, 4g, and 4h. Magnified images show the sheet morphology. Impact of Dipping Angle on GO Transfer The impact of mechanical strain on GO film stability is further highlighted by examining the transferred film quality at intermediate dipping angles. Results of GO film transfer to hydrophobic Au are shown in Figure S5 for dipping angles at intermediate values of 45 o and 60 o for both upstroke and downstroke directions. Films deposited by upstroke dips show monotonic decreases in GO coverage with an increase in dipping angle (Figures 4d, 4g, S5a, and S5b), consistent with the idea of increased mechanical strain applied to the GO film during transfer at increased angle. The lack of large area film uniformity observed at 60 o signals a threshold for S10

11 mechanical failure of the film. Films produced by downstroke deposition (Figures 4e, 4h, S5c, and S5d) also exhibit a monotonic decrease in GO coverage with increasing dipping angle. At 45 o and 60 o, we observe a new failure mechanism; the GO film unevenly covers a trapezoidal region of the Au substrate, as shown in Fig. S5c and S5d. We believe that the sharp edges of the Au-coated Si coupons create local meniscus perturbations that tear the GO film at the coupon edge, producing the characteristic trapezoid shape. GO film coverage s dependence on dipping angle indicates the importance of minimizing strain during LB deposition of 2D nanosheets. Figure S5. Photographs and SEM images of GO monolayers transferred at a surface pressure of 15 mn/m (high stiffness regime) by 45 o upstroke (a), 60 o upstroke (b), 45 o downstroke (c), and 60 o upstroke (d) deposition. The SEM images in each column show regions of the sample near the top, middle, and bottom of the dipped substrates. S11

12 Shear Modulus as a Predictor of Transfer Success Oscillations for RGO and GO Monolayers Figures S6 and S7 show the orientation-dependent surface pressure response to an oscillating barrier at 5% strain for RGO and GO, respectively, both on basic subphases. The RGO surface pressure oscillations show no Wilhelmy plate orientation dependence. Contrastingly, the GO monolayer on the basic subphase shows Wilhelmy plate orientation dependence in the high stiffness regime, in agreement with the results for the GO monolayer on the intrinsic ph subphase (Figure 2). Figure S6. Basic subphase RGO monolayer area (black lines) and surface pressure (symbols) oscillations with the Wilhelmy plate in the perpendicular ( ) and parallel ( ) orientations in the percolation (a), high stiffness (b), and overlapping (c) regimes. A subset of the oscillations is shown here. S12

13 Figure S7. Basic subphase GO monolayer area (black lines) and surface pressure (symbols) oscillations with the Wilhelmy plate in the perpendicular ( ) and parallel ( ) orientations in the percolation (a), high stiffness (b), and overlapping (c) regimes. A subset of the oscillations is shown here. Transfer of RGO to Hydrophilic and Hydrophobic Au We were unable to transfer RGO monolayers from a basic water subphase (in the absence of any surfactants).figure S8 shows an example of an attempt to transfer RGO to hydrophilic Si with upstroke deposition at a 90 o angle relative to the air-water interface. Similar results were seen for hydrophilic Si at a 30 o angle as well as hydrophobic gold substrates with upstroke and downstroke deposition at a 30 o angle. Most of the area was void of RGO sheets. A few areas could be found with slight deposition and some agglomerated sheets as shown below, but the coverage was very low. S13

14 Figure S8. Representative SEM images of RGO sheets on hydrophilic and hydrophobic gold substrates with upstroke and downstroke deposition techniques at 30 and 90 degree dipping angles. All dips were performed at a surface pressure of 15 mn/m. Transfer of GO in Basic ph In the high stiffness regime, 90 o dip transfers of GO monolayers in basic and intrinsic ph subphases result in similar coverage on hydrophilic substrates (Figure S9), as to be expected due to similar G. Furthermore, dipping at a 30 o improves transfer to hydrophobic substrates for the basic ph subphase GO monolayer, similarly to the intrinsic case shown in Figure 4d,g. Thus, if a monolayer exhibits a significant G, minimizing strain on the monolayer will improve transfer. S14

15 Figure S9. Representative SEM images of transferred GO monolayers with varied subphase ph, dipping angle, and substrate hydrophilicity as indicated in each image. All dips were performed at a surface pressure of 15 mn/m. Interpretation of BAM Movies Brewster angle microscopy (BAM) movies show the compression and relaxation of the GO and RGO monolayers, highlighting the differences in the long-range interactions. In BAM_GO_Intrinsic_pH.gif and BAM_GO_Basic_pH.gif, the formation of large-scale GO networks is observed in the percolation regime. Under further compression to the maximum stiffness regime, these structures coalesce into a dense network, in-between which flows a lower density of GO flakes. These networks clearly resist moving around one another. In BAM_RGO_Basic_pH.gif, the formation of small-scale RGO agglomerates occurs in the percolation regime. Even after the transition to the overlapping regime, these structures easily and randomly move around one another. Unlike GO, no large-scale networks are observed during the RGO isotherms. S15

16 Note that the overlapping regime for RGO was easily accessible with both the BAM and the Wilhelmy plate in the center of the trough, so the isotherm data in the BAM movie corresponds to the BAM images exactly. The surface pressure is a little lower for the RGO isotherm shown in the BAM movie than is shown in the manuscript. That is because more material was added to the subphase for the BAM movie such that the percolation regime is not present even with the barriers fully expanded; the whole isotherm is shifted to lower surface pressures relative to the zero surface pressure. For GO, it was difficult to access the overlapping regime with both the Wilhelmy plate and the BAM present because the barriers had to be very highly compressed. Therefore, the isotherms in the BAM movies for GO were run just before the BAM images were recorded. Barrier compression to maximum and minimum areas was run while recording BAM movies as for the isotherms; space constraints required removing the Wilhelmy plate during the BAM acquisition. References 1. Cote, L. J.; Kim, F.; Huang, J. Langmuir Blodgett assembly of graphite oxide single layers. J. Am. Chem. Soc. 2008, 131 (3), Fernandez-Merino, M.; Guardia, L.; Paredes, J.; Villar-Rodil, S.; Solis-Fernandez, P.; Martinez-Alonso, A.; Tascon, J. Vitamin C is an Ideal Substitute for Hydrazine in the Reduction of Graphene Oxide Suspensions. J. Phys. Chem. C 2010, 114 (14), King, D. E. Oxidation of Gold by Ultraviolet Light and Ozone at 25 C. J. Vac. Sci. Technol., A 1995, 13 (3), Ron, H.; Matlis, S.; Rubinstein, I. Self-Assembled Monolayers on Oxidized Metals. 2. Gold Surface Oxidative Pretreatment, Monolayer Properties, and Depression Formation. Langmuir 1998, 14 (5), Tabor, R. F.; Morfa, A. J.; Grieser, F.; Chan, D. Y.; Dagastine, R. R. Effect of Gold Oxide in Measurements of Colloidal Force. Langmuir 2011, 27 (10), Wang, Y.; Zhang, P.; Liu, C. F.; Zhan, L.; Li, Y. F.; Huang, C. Z. Green and Easy Synthesis of Biocompatible Graphene for Use As an Anticoagulant. RSC Adv. 2012, 2 (6), Cicuta, P.; Terentjev, E. Viscoelasticity of a Protein Monolayer from Anisotropic Surface Pressure Measurements. Eur. Phys. J. E 2005, 16 (2), S16