General Self-Assembly Method for Deposition of Graphene Oxide into

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General Self-Assembly Method for Deposition of Graphene Oxide into Uniform Close-Packed Monolayer Films Alexander Holm, Larissa Kunz, Andrew R. Riscoe, Kun-Che Kao, Matteo Cargnello, and Curtis W. Frank Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03994 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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General Self-Assembly Method for Deposition of Graphene Oxide into Uniform Close-Packed Monolayer Films

Alexander Holm,†,* Larissa Kunz, †,# Andrew R. Riscoe, †,# Kun-Che Kao,† Matteo Cargnello†,# and Curtis W. Frank†,*

†Department

#

of Chemical Engineering, Stanford University, Stanford, CA 94305, USA

SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, CA 94305,

USA

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ABSTRACT Depositing a morphologically uniform monolayer film of graphene oxide (GO) single-layer sheets is an important step in the processing of many composites and devices. Conventional LangmuirBlodgett (LB) deposition is often considered to give the highest degree of morphology control, but film microstructures still vary widely between GO samples. The main challenge is in the sensitive self-assembly of GO samples with different sheet sizes and degrees of oxidation. To overcome this drawback, here we identify a general method that relies on robust assembly between GO and a cationic surfactant (cationic surfactant-assisted LB). We systematically compared conventional LB and cationic surfactant-assisted LB for three common GO samples of widely different sheet sizes and degrees of oxidation. While conventional LB may occasionally provide satisfactory film morphology, cationic surfactant-assisted LB is general and allows deposition of films with tunable and uniform morphologies — ranging from close-packed to overlapping single layers — from all three types of GO samples investigated. Because cationic surfactant-assisted LB is robust and general, we expect this method to broaden and facilitate the use of GO in many applications where precise control over film morphology is crucial.


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INTRODUCTION Graphene oxide (GO) single-layer sheets in the form of thin films are useful precursors in the fabrication of a large range of materials, composites and devices. Examples include transparent conductors,1–10 sensors,11–14 photovoltaics,15–17 actuators,18,19 molecular sieving membranes,20–26 antibacterial surfaces,27–31 electron spectroscopy windows32 and Janus graphene oxides.33–35 GO is suitable for this large range of applications because its properties, such as conductivity and reactivity, are widely tunable by modifying sheet size, degree of oxidation, and surface chemistry.36,37 However, in addition to tuning GO properties to fit the intended application, GO films should ideally be deposited with control over film microstructure, which determines macroscopic properties.38 Regardless of the morphology needed, it is also critical that films can be deposited uniformly over large substrate areas.38 Many methods have been assessed for the production of GO thin films,3,4,8,18,23,39–42 and the subject has been reviewed.38 However, a general and robust method that can control film morphology at the single sheet level and that can be used with different GO samples of different sheet sizes and degrees of oxidation has not been identified. Methods such as spray coating and drop-casting typically offer poor morphological control and lead to non-uniform films.38,39 Vacuum filtration and spin-coating offer better control, but close-packed monolayer films of single-layer sheets have not been achieved.3,4,18,23,38,40–42 Langmuir-Blodgett (LB) deposition has resulted in a high degree of morphological control in some cases but suffers from poor reproducibility among GO samples because the method is very sensitive to the characteristics of the GO used.1,2,7,11,43–55 As an example, Cote et al.1 obtained monolayer films with uniform morphologies of close-packed, non-overlapping or close-packed, overlapping GO single-layer sheets. However, using the same conventional protocol for LB deposition, but a different GO 3 ACS Paragon Plus Environment

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sample, Zheng et al.50 instead obtained folded and overlapping sheets before a close-packed film could be achieved, which means that close-packed, non-overlapping films would be difficult to obtain. Imperiali et al.43 used conventional LB of yet another GO sample, but only obtained patchwise deposition. Other examples of variability in film morphologies obtained by using conventional LB with different GO samples are numerous in the literature.1,2,7,11,43–57 Thus, while conventional LB can sometimes be used to obtain satisfactory film morphologies, the process is rarely straightforward, and a general approach for uniform morphologies would be highly valuable. Szabó et al. first used cationic surfactant-assisted LB assembly and deposition (henceforth called cationic LB) to deposit monolayer films of GO,58 but the method has, to the best of our knowledge, only been used once since its introduction.59 In the previous work, degrees of GO oxidation or GO sheet size distributions were not quantified, although these parameters are known to strongly influence GO self-assembly.7,55,57 Therefore, robustness of cationic LB with respect to these crucial GO properties has not been evaluated. Moreover, because conventional LB is considered the state-of-the-art for control over GO film morphology,1,38 it is important to benchmark cationic LB with respect to this technique. The purpose of the present work is to both evaluate the robustness of cationic LB with respect to GO samples of different chemical and physical properties, and also to benchmark the technique to conventional LB. In conventional LB, sheets self-assemble due to a complex combination of charge and vander Waals interactions1 and due to hydrophobic effects and specific hydrogen bonding.7 These forces are highly dependent on sheet size and type of functional groups in different GO samples, leading to large variability in self-assembly.1,2,7,11,43–57 In addition, it has been shown that the common use of alcohol in conventional LB causes Marangoni flow, which substantially reduces 4 ACS Paragon Plus Environment

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control over depositions.57 We propose that cationic LB makes use of a different mechanism for self-assembly — interfacial exchange of counterions — that overcomes problems with both variable GO self-assembly and Marangoni flow. We show that cationic LB, in contrast to conventional LB, is robust and can be used to obtain highly ordered, morphologically uniform monolayer films from different GO samples with a wide range of both sheet sizes and degrees of oxidation.

RESULTS AND DISCUSSION Graphene Oxide Characterization. To compare the robustness of cationic LB and conventional LB, we deposited GO monolayer films by both methods and with three different and commonly used GO samples:1–3,7,9,16,27,32,33,39,43,46,50,52,55–57,60–63 (1) GO made by Hummers’ method from Asbury Carbons (grade 230, natural flake graphite); (2) GO made by Hummers’ method from Bay Carbon (grade SP-1); and (3) Graphenea GO (commercial source). All GO samples consist predominantly of single-layer sheets as demonstrated by typical AFM micrographs of depositions obtained with the cationic LB method that will be described below (Figure 1a-c). Sheet size distributions of the different GO samples were estimated from SEM images acquired at random positions in the films (Figure 1d-f). Asbury and Bay GO had similar and large average lateral sizes of 23 µm and 19 µm, respectively, while Graphenea GO sheets were substantially smaller with an average size of 4 µm. The degrees and types of oxidation in the different GO samples were quantified by X-ray photoelectron spectroscopy (XPS) (Figure 1g-i and Figure S1). Deconvolution of the C1s peak


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revealed three components at ~285 eV, ~ 287 eV and ~288 eV, which have previously been attributed to (C-C), (C-O) and (C=O) species, respectively.4,64 The XPS characterization reveals that Asbury and Bay GO were similarly oxidized with, respectively, 54% and 52% oxidized carbon ((C-O) and (C=O) species combined). However, the type of oxygen groups varied significantly between the two samples with Bay GO having significantly more (C=O) species and less (C-O) species than Asbury GO. Graphenea GO was more oxidized (59% oxidized carbon) than either Asbury or Bay GO. Although the difference in degrees of oxidation between Graphenea GO and Asbury or Bay GO may not seem large, similar differences have previously been shown to substantially change deposition morphology in conventional LB.57 Therefore, the characterization shows that the samples vary significantly in properties important for their self-assembly: sheet size, degree of oxidation, and types of oxygen groups.


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Figure 1. (a)-(c) Typical AFM micrographs with associated height sections of Asbury, Bay and Graphenea GO deposited with cationic LB. The height profiles with heights of ~1 nm confirm that samples consisted of single layers.65 (d)–(f) Distributions of lateral sheet sizes for Asbury, Bay and Graphenea GO. More than 1000 sheets were used to estimate each distribution. Details of data treatment are given in the experimental section and in Figure S24. (g)-(i) Representative highresolution core-level C 1s XPS spectra of Asbury, Bay and Graphenea GO. Deconvolution revealed contributions at ~285 eV (C-C), ~287 eV (C-O) and ~288 eV (C=O). The percentages of (C-C), (C-O) and (C=O) carbon species are indicated in the figure and were calculated from spectra taken at five randomly chosen locations (additional spectra in the Supporting Information, Figure S1).

Conventional vs. Cationic LB deposition. In conventional LB deposition of GO, the single-layer sheets are dispersed in a water/alcohol (typically methanol) mixture and spread onto the air-water interface.1,2,7,11,43–57 Moving barriers are used to decrease the area of the interface, thereby increasing the surface concentration of GO single-layer sheets. During compression of the air-water interface, the surface concentration of GO sheets can be monitored by measuring the


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surface pressure (Π). When the air-water interface has been compressed to yield a desirable GO surface concentration, the GO film can be transferred to a suitable substrate by dip-coating or other methods (Figure 2a). In cationic LB, GO is instead dispersed directly in the subphase, and a cationic surfactant is spread on the air-water interface.58,59 After LB trough compression and equilibration, negatively charged GO sheets diffuse and assemble at the positively charged surface (Figure 2b). We propose that this process is analogous to layer-by-layer assembly of polyelectrolytes, where deposition of polyelectrolyte on a charged surface liberates low molecular weight counterions which increases the entropy.66 Thus, we propose that the process is driven by entropic gain associated with replacement of low molecular weight anions at the cationic surfactant loaded air-water interface with negatively charged GO sheets. The process is self-limiting, because as negatively charged sheets attach to the positively charged air-water interface, the net charge is reduced such that eventually no more sheets attach. It has been shown that polyanions show strong thermodynamic drive towards assembly by counterion exchange with cationic surfactants at the air-water interface.67 GO can be regarded as a two-dimensional and cross-linked anionic polymer,1 and it is therefore plausible that GO also shows strong and robust affinity for cationic surfactant at the air-water interface. We therefore hypothesized that, with respect to different GO samples, selfassembly in cationic LB would be more robust than in conventional LB so that greater control over film microstructure could be exercised. To test this hypothesis, we attempted to deposit monolayer films with uniform morphologies (close-packed, non-overlapping and close-packed, overlapping single-layer sheets, respectively) from each GO sample and using each of the LB methods. To facilitate direct comparison of the films obtained by conventional and cationic LB, AFM images of morphologies obtained by both methods are summarized in Figure 3. It is clear that conventional LB could only be used to deposit overlapping films (figure 3 d-f), while close-packed non-


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overlapping films could not be deposited (Figure 3 a-c). In addition, only multilayer films could be obtained (Figure 3 d-f). In contrast, cationic LB affords monolayer deposition of close-packed, non-overlapping single layer sheets (Figure 3 g-i) and close-packed, overlapping single layer sheets (Figure 3 j-l) from all samples used in this study. A detailed description and discussion of results from both conventional and cationic LB is provided in the following sections

Figure 2. Structures and schematic illustrations. (1) Model structure of a small sheet of GO. (2) Structure of cationic surfactant, 1,2-di-(9Z-octadecenoyl)-3-trimethylammonium-propane 9 ACS Paragon Plus Environment

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(DOTAP). (3) Structure of cationic surfactant oleylamine (OLAM). (a) Schematic illustration of conventional LB (graphic modified from reference 57) and (b) cationic LB.


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Figure 3. Typical AFM micrographs showing attempts to deposit monolayer films of single-layer sheets from three different GO samples (Asbury, Bay and Graphenea GO) by two different LB techniques (conventional and cationic LB). (a-c) Conventional LB attempts to deposit closepacked, non-overlapping sheets. (d-f) Conventional LB attempts to deposit close-packed, overlapping sheets. (g-i) Cationic LB attempts to deposit close-packed, non-overlapping sheets. (j-l) Cationic LB attempts to deposit close-packed, overlapping sheets.

Conventional LB deposition results. Conventional LB was carried out by dispersing GO in 1:5 v/v water/methanol (0.17 mg mL-1, 10 mL) that was spread onto an air-water interface. The interface was then compressed and isothermal surface pressure–area plots and depositions were collected (Figure 4 and Figure S2- Figure S10). For Asbury GO, no surface pressure increase was observed during compression (Figure 4a), and for Graphenea GO, only a small surface pressure increase was observed (Figure 4c), which indicates that the sheets were not very surface-active (i.e. not prone to accumulate at interfaces and increase surface pressure). For both Asbury and Graphenea GO, few or no sheets were deposited at low or medium compression (regions (1) and (2) in Figure 4a,c), but multilayer films with overlapping and folded sheets were deposited at high trough compression (region (3) in Figure 4a,c). The fact that multi-layer films were deposited at high trough compressions shows that sheets were present at the surface also at low and medium compression, but did not deposit. In contrast, for Bay GO there was significant surface pressure increase upon compression (Figure 4b), suggesting that Bay GO was more surface-active than either Asbury or Graphenea GO. In depositions at low or medium compression (regions (1) and (2) in Figure 4b), single-layer sheets were deposited, but sheets were folded and overlapping before a close-packed monolayer film could be achieved. Depositions made at high compression yielded multi-layer films with overlapping and folded sheets (region (3) in Figure 4b), just as for Asbury and Graphenea GO. The GO preparation and LB deposition method used here for Bay GO was 11 ACS Paragon Plus Environment

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essentially the same as described by Cote et al. in their seminal paper.1 Thus, our difficulties to precisely replicate the highly ordered film morphologies described by Cote et al. exemplify how conventional LB depositions are very sensitive to small differences in the preparation and deposition of GO sheets.

Figure 4. Conventional LB deposition process characterized (from left to right columns) by representative isotherm plots (surface pressure, Π, vs. trough area) and AFM micrographs of depositions. The AFM micrographs were taken of depositions made at different regions marked in the isotherms: (1) low compression, (2) medium compression and (3) high compression regions. (a) Asbury GO (b) Bay GO (c) Graphenea GO.

Recently, we demonstrated that the alcohol used in conventional LB leads to Marangoni flow, which disrupts films as they deposit on the substrate during LB deposition.57 Because a 1:5 12 ACS Paragon Plus Environment

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v/v water/methanol mixture was used here to replicate previous work,1,2,11,44,48,51,52,57 it is likely that Marangoni flow contributed strongly to the poor reproducibility in the deposition of different GO samples. Marangoni flow is strongly decreased or eliminated by using surfactants, and more surface-active amphiphiles are more effective at suppressing the flow.57 Bay GO was more surface-active than either Asbury or Graphenea GO, as demonstrated by the higher surface pressures achieved during the isotherms (Figure 4), and is therefore expected to be more effective in suppressing Marangoni flow.57 We therefore propose that it is the higher surface-activity of Bay GO compared to Asbury and Graphenea GO that allows Bay GO to deposit at low and medium compression. Interestingly, Bay GO appears substantially more surface-active than Asbury GO even though the samples have similar degrees of oxidation and sheet sizes (Figure 1d,e,g,h). We therefore suggest that the ratio of (C=O) species to (C-O) species, which is substantially higher in Bay GO than in Asbury GO (Figure 1g,h), strongly influences surface-activity. Finally, although Bay GO could be deposited onto substrates, Marangoni flow was likely still present, thus contributing to undesirable film characteristics (such as folded sheets and patch-wise deposition). The fact that the same conventional LB protocol was applied to the three different GO samples and gave rise to substantially different film morphologies (Figure 4 and Figure S2- Figure S10) clearly demonstrates the poor sample-to-sample reproducibility in conventional LB deposition. Cationic LB deposition results. Cationic LB deposition was carried out by dispersing GO in a water subphase (0.05 g L-1) with pH adjusted to 10, followed by spreading of a cationic surfactant (DOTAP, Figure 2) in chloroform (7.2 mM, 15 µL) on the air-water interface. DOTAP was used for cationic LB because it was hypothesized that the ester groups in DOTAP (Figure 2) would bind to GO through hydrogen bonding with GO hydroxyl and carboxyl groups, thereby facilitating ion exchange. However, other cationic surfactants could also be used, such as oleylamine (OLAM, 13 ACS Paragon Plus Environment

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Figure 2), and OLAM-assisted cationic LB depositions show the same high quality as obtained with DOTAP (Supporting Information, Figure S11). After spreading DOTAP, the trough area was compressed, and isotherm plots and depositions were collected for Asbury, Bay and Graphenea GO (Figure 5 and Figure S12 - Figure S20). Under these conditions, it was straightforward to obtain monolayer films of both close-packed, non-overlapping and close-packed, overlapping GO single-layer sheets. It is worth noting that when no cationic surfactant was spread on the air-water interface, no GO sheets could be deposited, indicating that cationic surfactant is necessary for assembly. Deposited films covered whole SiO2/Si wafers uniformly, as demonstrated by AFM and SEM micrographs acquired at multiple regions on the whole wafer (Figure S12– Figure S20). These excellent deposition characteristics, obtainable for all GO samples, are in stark contrast with depositions achievable by the conventional LB described above.


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Figure 5. Cationic LB characterized (from left to right columns) by representative isotherm plots (surface pressure, Π, vs. trough area) and AFM micrographs of depositions. The AFM micrographs were taken of depositions made at different regions marked in the isotherms: (1) low compression, (2) medium compression, and (3) high compression regions. (a) Asbury GO (b) Bay GO (c) Graphenea GO.

By cationic LB, any desired morphology, ranging from dilute to close-packed to overlapping single-layer sheets, could be deposited from all GO samples simply by choosing the surface pressure at which films were collected. Thus, dilute films of single-layer sheets (Region (1) in Figure 5a-c) were obtained by depositing films in regions where the surface pressure was 00.25 mN m-1 (Asbury GO) or 0-1 mN m-1 (Graphenea or Bay GO). Monolayer films of closepacked, non-overlapping single-layer sheets (Region (2) in Figure 5a-c) were deposited in regions where the surface pressure was 0.25-0.8 mN m-1 (Asbury GO) or 1-2.4 mN m-1 (Graphenea or Bay 15 ACS Paragon Plus Environment

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GO). Monolayer films of close-packed, overlapping single-layer sheets (Region (3) in Figure 5ac) were deposited in regions where the surface pressure was higher than 0.8 mN m-1 (Asbury GO) or higher than 2.4 mN m-1 (Graphenea or Bay GO). Therefore, surface pressure represents a very valuable metric to allow highly reproducible depositions of uniform GO monolayer films. The fact that surface pressure is a useful parameter for controlling film morphology of different GO samples can be rationalized by considering a plausible mechanism for GO assembly. The likely driving force for assembly is entropic gain from exchanging low molar mass anionic counterions at the positively charged air-water interface with polyanionic graphene oxide (analogous to polyelectrolyte adsorption to a charged solid surface66). In layer-by-layer deposition of polyelectrolytes, three factors primarily determine self-assembly: charge density of the surface onto which the polyelectrolytes adsorb, charge density of the polyelectrolyte, and molecular weight of the polyelectrolyte.66,68,69 By analogy to this process, we propose that three similar factors also primarily determine self-assembly of GO in cationic LB: positive charge density at the surfactant-loaded air-water interface, negative charge density on GO, and GO sheet size. Therefore, surface pressure — correlated with positive charge density at the air-water interface — is expected to modulate the driving force for attaching differently charged and differently sized GO sheets to the air-water interface. Surface pressure controls GO deposition in cationic LB. To further demonstrate that surface pressure is a useful metric to control the deposition of GO with different degrees of oxidation (and therefore ionization), we chemically reduced Asbury GO using ascorbic acid,57,64,70 thus reducing the percentage of oxidized carbon ((C-O) and (C=O)) from 54% in Asbury GO to 49% in reduced Asbury GO (rGO, Figure 6a,b). Again, although this change in degree of oxidation may not seem substantial, it has a significant impact on conventional LB depositions, as shown previously.57 16 ACS Paragon Plus Environment

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Cationic LB deposition of Asbury rGO was then performed (Figure 6c (1)-(3) and Figure S21Figure S23). The same morphologies (close-packed, non-overlapping and close-packed, overlapping) as for the more oxidized Asbury GO sample were readily prepared, but higher surface pressures were required (Figure 6c). These results confirm that differently oxidized sheets of the same size can readily be deposited with the same final film morphologies simply by choosing the appropriate surface pressures. If, as we hypothesize, ion exchange drives self-assembly, then films with similar morphologies are assembled by similar levels of ion exchange. Therefore, it is not surprising that a higher surface pressure (and corresponding higher surface charge density) is needed to attract the less oxidized (and therefore less ionized) Asbury rGO as compared to Asbury GO.

Figure 6. Representative high-resolution core-level C 1s XPS spectra of (a) Asbury GO (reproduced from Figure 1g) and (b) Asbury rGO. The percentages of (C-C), (C-O) and (C=O) carbon species are indicated in the figure and were calculated from spectra taken at five randomly chosen locations (additional spectra in the Supporting Information, Figure S1). (c) Isotherm plot 17 ACS Paragon Plus Environment

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from cationic LB of Asbury rGO (black curve). The isotherm from cationic LB of Asbury GO (grey curve, reproduced from Figure 5a) is added for comparison. AFM micrographs were taken of depositions made at different regions marked in the Asbury rGO isotherm: (1) low compression, (2) medium compression, and (3) high compression regions.

Uniformity of the depositions. To assess the uniformity of film coverage at the wafer scale, large wafer pieces were used for depositions (22x88 mm). Low-magnification SEM micrographs were collected at several locations, and representative micrographs of cationic LB depositions for all GO samples are presented in Figure 7a-d (close-packed, non-overlapping films) and Figure 7e-h (close-packed, overlapping films). Further representative SEM micrographs are presented in the Supporting Information (Figure S12 – Figure S23). From this characterization, it is clear that film morphologies are highly uniform across entire wafers.


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Figure 7. SEM micrographs of cationic LB depositions and fractions of wafer area in depositions covered with zero, one and two or more layers of GO sheets. (a)-(d) Medium compression deposition (close-packed, non-overlapping sheets) of Asbury GO, Asbury rGO, Bay GO and Graphenea GO. (e)-(h) High compression deposition (close-packed, overlapping sheets) of Asbury GO, Asbury rGO, Bay GO and Graphenea GO. In the SEM micrographs, the brightest, darker and darkest areas correspond to zero, one and two or more GO layers respectively.62 (i)-(l) Fractions of wafer area covered with zero, one and two or more layers of GO sheets in the depositions of Asbury GO, Asbury rGO, Bay GO and Graphenea GO. The width of the error bars represents two standard deviations from the mean. Key for bar charts: medium compression, blue vertical stripes; high compression, orange horizontal stripes.


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Low resolution SEM also offers the opportunity for more quantitative characterization of the film morphologies investigated in this work. In the SEM micrographs, the brightest areas correspond to the plain wafer surface with no sheets deposited; darker areas correspond to surfaces covered with a single layer of graphene oxide; and the darkest areas correspond to surfaces covered with two or more layers of graphene oxide (signifying overlaps, folds or unexfoliated material).62 The image contrast in SEM images was used to quantify the fraction of area covered with zero, one and two or more layers of graphene oxide, respectively, and the results are summarized in Figure 7i-l. For all samples, the fraction of area covered with a single layer is exceptionally high at >70%, and reached 86% for Graphenea GO. Moreover, the fractions of area covered with zero and two or more layers clearly reveals the difference between close-packed, non-overlapping and close-packed, overlapping morphology. In close-packed, non-overlapping depositions (made at medium trough compressions), the area fraction with zero layers is larger than the fraction with two or more layers, while in close-packed, overlapping depositions (made at high trough compressions), the relationship is reversed with the area fraction of two or more layers being larger than the fraction of zero layers. In close-packed, non-overlapping depositions, the fraction of area covered with two or more layers is extremely low, below 5% for all GO samples. Such depositions are ideal if the target is asymmetric functionalization, such as in the production of Janus graphene oxide.33–35 In close-packed, overlapping depositions, the fraction of area with two or more layers is in the range 5%-25% (while the fraction of surface with zero layers is below ~ 5%). Such depositions are ideal for applications where a conductive film is required (although films have to be chemically reduced), such as for preparing transparent electrodes1–10 or sensors.11–14 The fact that uniform films can be easily obtained from all GO samples shows that cationic LB deposition is substantially more robust compared to conventional LB. Given that no special 20 ACS Paragon Plus Environment

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requirements/actions are needed to perform cationic LB deposition as compared to the conventional process besides the addition of a surfactant, this method represents a powerful way to obtain GO films with desired morphology for both fundamental studies and advanced applications. CONCLUSIONS We have shown that cationic LB deposition is a general and robust method for obtaining monolayer films of single-layer graphene oxide sheets over large areas. Two uniform GO film morphologies — close-packed, non-overlapping and close-packed, overlapping single-layer sheets — were conveniently obtained from a wide range of different, commonly used GO samples. It was demonstrated that surface pressure can be used as a control parameter to modulate the driving force for GO assembly at the air-water interface so that GO samples from different sources and with different sizes and degrees of oxidation could be deposited with the same final desired morphologies. In contrast, conventional LB of the same samples could only be used to obtain a subset of desirable morphologies and only from some samples. Thus, even though conventional LB sometimes offer high control over film morphology, the method is not robust with respect to different GO samples. In stark contrast, cationic LB deposition is a robust and convenient alternative to produce a uniform film morphology from a given GO sample. We therefore believe cationic surfactant-assisted LB will significantly facilitate and expand the use of GO in thin film applications.

EXPERIMENTAL SECTION


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Chemicals and cleaning procedures. Graphite powder used to synthesize Asbury GO was supplied by Asbury Carbons (grade 230, natural flake graphite). Graphite powder used to synthesize Bay GO was supplied by Bay Carbon (grade SP-1). Graphenea GO was supplied as a 4 g L-1 dispersion in water from Graphenea. SiO2/Si wafers with 500 nm thermally grown SiO2 were obtained from Pure Wafer (previously WRS materials). Anhydrous ethanol, mineral oil, Lascorbic acid, sodium nitrate (NaNO3), potassium permanganate (KMnO4) and oleylamine (OLAM) were purchased from Sigma Aldrich. Methanol, ammonium hydroxide (NH4OH, 30 wt. % in water), hydrogen peroxide (H2O2, 30 vol. % in water), sulfuric acid (H2SO4), sodium hydroxide (NaOH), potassium hydroxide (KOH), chloroform, acetone and isopropyl alcohol were purchased from Fisher Scientific. 1,2-di-(9Z-octadecenoyl)-3-trimethylammonium-propane (DOTAP, chloride salt in chloroform) was obtained from Avanti Polar Lipids. MilliQ water (electrical resistance >18.2 MΩ cm) was supplied by a Millipore system and used in all experiments. Glassware was cleaned in a base bath containing 8 L isopropyl alcohol, 2 L deionized (DI) water and 500 g KOH and then rinsed copiously in DI water, and finally in MilliQ water, and dried in a clean oven at 120 oC before being used for any experiment. Graphene oxide synthesis from Asbury and Bay GO. Graphene oxide was synthesized by the modification of Hummers’ method reported by Cote et al.1,71 In a 500 mL round-bottom flask, 2 g of graphite was stirred with 92 mL sulfuric acid and 2 g NaNO3 for 20 min while being cooled in an ice bath. Then, 12 g of KMnO4 was added in small portions. The temperature was kept below 10 oC during the additions. The mixture was then increased to 35 ± 5 oC and stirred for 60 minutes. 160 mL MilliQ water was then added, raising the temperature to 90 ± 5 oC. The mixture was stirred for another 30 minutes. Finally, 230 mL of MilliQ water was added followed by dropwise addition of 30 vol. % hydrogen peroxide (12 mL), and the mixture was stirred until no gas evolution could 22 ACS Paragon Plus Environment

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be observed from the reaction mixture. While the reaction mixture was still warm, it was vacuum filtered. The filter cake was washed with 100 mL MilliQ water. The filter cake was then recovered with a spatula and suspended in 250 mL MilliQ water by stirring. To remove reaction by-products, un-exfoliated graphitic oxide and small sheets, the sample was placed in centrifuge tubes and purified by repeated centrifugation (Sorvall ST8, thermo scientific, HIGHConic III fixed angle rotor) using the following procedure: -

4 x 8000 rpm (8500g) for 20 min. Supernatant discarded, sediment re-dispersed (vortexing) in MilliQ water.

-

1 x 8000 rpm (8500g)

for 20 min. Supernatant discarded, sediment re-dispersed

(vortexing) in MilliQ water. The sample was then sonicated (Branson M1800) for 15 min and subjected to further purification by repeated centrifugation: -

2 x 1000 rpm (135g) for 3 min. Supernatant collected, sediment discarded.

-

2 x 8000 rpm (8500g) for 20 min. Supernatant discarded, sediment re-dispersed (vortexing) in MilliQ water.

-

2 x 2500 rpm (840g) for 10 min. Supernatant collected, sediment discarded.

Reduction of Asbury GO by Ascorbic Acid. Reduction of Asbury GO was carried out by modification of our previously reported protocol.57 16.5 mL of an ascorbic acid solution in water (1.5 g L-1) was added to an Asbury GO dispersion in water (300 mL, 0.3 g L-1) and stirred in a 500 mL round-bottom flask. 50 mL of mineral oil was added on top of the reaction solution, and the flask was capped. The mineral oil layer prevents the formation of an air-water interface, where rGO would otherwise aggregate, and allows rGO to remain well dispersed during and after reaction.63 The mixture was then immersed in a pre-heated oil bath (85 oC) for 30 min, and was 23 ACS Paragon Plus Environment

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gently stirred with a magnetic Teflon stir bar. After reaction, the aqueous phase was collected with a glass syringe. General Procedure Common to all LB Experiments. Substrates for LB deposition used in this work were prepared by immersing SiO2/Si wafer pieces in a freshly prepared solution consisting of 30 mL NH4OH (30 wt. % in water) and 30 mL H2O2 (30 vol. % in water) for 60 min to make them hydrophilic. The wafers were then copiously rinsed with MilliQ water and dried under nitrogen flow. This treatment results in wafer pieces that are completely wetted by water. For LB assembly and depositions, a KSV 5000 Nima trough (Biolin Scientific) with dimensions 150x580 mm was used. The trough and barriers were carefully washed with a soft brush and then rinsed, first with DI water, then with ethanol, and finally with DI water again. The trough was then filled with the aqueous subphase (pure MilliQ water for conventional LB or GO dispersion for cationic LB). The temperature was set to 21 oC and the subphase allowed to equilibrate for at least 60 min before the air-water interface was cleaned by aspirating liquid from the surface with a pipette during compression of the trough (75 cm2 min-1). A SiO2/Si wafer piece (typically 22x88 mm) was then immersed into the subphase. A paper Wilhelmy plate (supplied by Biolin Scientific) attached to a tensiometer was used to record the surface pressure in the trough. Before immersing the Wilhelmy plate into the subphase, it was immersed for 2 hours in DI water and then rinsed in DI water. LB films were deposited onto SiO2/Si wafer pieces by pulling the pre-immersed substrate through the air-water interface at a speed of 2 mm min-1. Conventional LB Assembly and Deposition. Conventional LB was carried out as previously reported by Cote et al.1 To spread GO on the air-water interface, GO dispersions (0.17 g L-1) were prepared in 1:5 v/v water/methanol. The GO dispersion was then spread dropwise on top of the water surface in the trough using a glass syringe and a syringe pump with a flow rate of 0.1 mL 24 ACS Paragon Plus Environment

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min-1 up to a volume of 10 mL. After the GO dispersion was spread, the film at the air-water interface was allowed to equilibrate for at least 45 minutes before the first isothermal compression was started. Isotherm plots were acquired at a compression speed of 20 cm2 min-1. Before each deposition was collected, the air-water interface was equilibrated for 25 min. Cationic surfactant-assisted LB Assembly and deposition. We slightly modified our recently reported protocol for cationic LB.33 Asbury, Bay or Graphenea GO or Asbury rGO were dispersed in water to yield a concentration of 0.05 g L-1. The dispersion was then set to pH ~10 using 1 M NaOH(aq) and pH indicator strips, and the dispersion was added to the LB trough as the subphase. A microsyringe (Hamilton) was rinsed with chloroform and used to spread cationic surfactant (15 μL, 7.2 mM DOTAP or OLAM in chloroform) on top of the subphase. The LB trough was then allowed to rest for at least 45 minutes before the interface was compressed (20 cm2 min-1) until reaching a surface pressure (Π) corresponding to a desired film morphology. The trough was then allowed to rest for 25 minutes to allow for equilibration before a deposition was performed. The surfactant was removed from the deposited films by washing the films with acidified acetone (500 mL acetone and 1 mL 0.1 M aqueous HCl) and acidified ethanol (500 mL ethanol and 1 mL 0.1 M aqueous HCl). Note that the subphase of GO can be recycled, so that the same GO stock subphase can be used for multiple depositions. In practice, we suggest that when working with a new GO sample, it is helpful to perform a series of depositions at different trough compressions (and Π), empirically finding the optimal Π for deposition of close-packed, non-overlapping films or close-packed, overlapping films. Characterization. Atomic Force Microscopy (AFM) height mode measurements were performed using the non-contact mode on a Park NX-10 instrument. SEM micrographs were collected on a FEI Sirion Scanning Electron Microscope. LB isotherms and depositions were collected using a 25 ACS Paragon Plus Environment

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Nima technology trough, model KSV5000 with dimensions 150x580 mm from Biolin Scientific. X-ray photoelectron spectroscopy (XPS) was performed using a ULVAC-PHI 5000 Versaprobe spectrometer with an Al Kα source with incident photon energy of 1486.6 eV. Films of GO and rGO were drop-casted onto SiO2/Si wafers for analysis. A 200 μm incident beam was used to collect C1s spectra at five points on each sample. Data Analysis. To perform sheet size analysis in depositions of Asbury, Bay and Graphenea GO, area normalized sheet size distributions were estimated from randomly acquired scanning electron microscopy (SEM) images of cationic depositions. More than 1000 sheets were measured for each distribution using ImageJ software. Further details of raw data treatment for generation of sheet size distributions are given in Figure S24. To estimate the fraction of wafer area in cationic LB depositions covered with zero, one and two or more layers of GO sheets, image contrast in randomly acquired SEM images of depositions was adjusted to isolate zero, one and two or more sheets respectively using ImageJ software, which allows quantification of percent surface coverage. Further details are given in Figure S25. ASSOCIATED CONTENT Supporting Information Additional high-resolution core-level C 1s XPS spectra; Additional AFM and SEM micrographs of conventional and cationic surfactant assisted LB depositions of Asbury, Bay and Graphenea GO and Asbury rGO; Treatment of raw data (SEM micrographs) for generation of area normalized sheet size distributions; Treatment of raw data (SEM micrographs) for quantification of fraction of area covered with zero, one and two or more layers in the deposited GO films. AUTHOR INFORMATION 26 ACS Paragon Plus Environment

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Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Alexander Holm: 0000-0002-3660-4389 Matteo Cargnello: 0000-0002-7344-9031 Curtis W. Frank: 0000-0002-0708-1048 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS A.H. acknowledges support from the Sweden-America Foundation and the Blanceflor Boncompagni Ludovisi, née Bildt Foundation, unrestricted funds as well as a TomKat seed grant from Stanford University. M.C. acknowledges support from the School of Engineering at Stanford University and from a Terman Faculty Fellowship. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF, Stanford University), supported by the National Science Foundation under award ECCS-1542152.

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General Self-Assembly Method for Deposition of Graphene Oxide into

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