Langmuir-Blodgett Deposition of Graphene Oxide â Identifying

Subscriber access provided by the Henry Madden Library | California State University, Fresno

Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Langmuir-Blodgett Deposition of Graphene Oxide — Identifying Marangoni Flow as a Process that Fundamentally Limits Deposition Control Alexander Holm, Cody Wrasman, Kun-Che Kao, Andrew R. Riscoe, Matteo Cargnello, and Curtis W. Frank Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00777 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Langmuir-Blodgett Deposition of Graphene Oxide — Identifying Marangoni Flow as a Process that Fundamentally Limits Deposition Control

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

†

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

#

Department of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis,

Stanford University, Stanford, CA 94305, USA

1 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

ABSTRACT Langmuir-Blodgett deposition is a popular route to produce thin films of graphene oxide for applications such as transparent conductors and biosensors. Unfortunately, film morphologies vary from sample to sample, often with undesirable characteristics such as folded sheets and patch-wise depositions. In conventional Langmuir-Blodgett deposition of graphene oxide, alcohol (typically methanol) is used to spread the graphene oxide sheets onto an air-water interface before deposition onto substrates. Here we show that methanol gives rise to Marangoni flow, which fundamentally limits control over Langmuir-Blodgett depositions of graphene oxide. We directly identified the presence of Marangoni flow by using photography, and we evaluated depositions with atomic force microscopy and scanning electron microscopy. The disruptive effect of Marangoni flow was demonstrated by comparing conventional Langmuir-Blodgett depositions to depositions where Marangoni flow was suppressed by a surfactant. Because methanol is the standard spreading solvent for conventional Langmuir-Blodgett deposition of graphene oxide, Marangoni flow is a general problem and may partly explain the wide variety of undesirable film morphologies reported in the literature. INTRODUCTION Because of its tunable physical and chemical properties, graphene oxide (GO) is interesting for a wide range of applications.1–3 Many of these applications require that morphologically welldefined, continuous thin monolayer films of GO single layer sheets can be deposited on largearea substrates.4–16 Unfortunately, it is a difficult task to control GO morphology in thin films, and methods such as spin-coating or drop-casting offer poor morphological control.7,17 Langmuir-Blodgett (LB) deposition is a reliable technique for depositing well-defined films of various nanoparticles,18–20 and the technique also offers better control over GO depositions.4 2 ACS Paragon Plus Environment

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

In conventional LB assembly and deposition of GO single layer sheets (henceforth called conventional LB), GO is dispersed in a mixture of alcohol (typically methanol) and water, and subsequently added drop-wise onto a water surface in an LB-trough (Figure 1a).4,5,8,10,11,13,21–35 Moving barriers are then used to change the area of the air-water interface, thereby controlling the surface density of GO sheets. Because GO single layer sheets are amphiphilic,10,26 sufficient compression of a GO film at the air-water interface reduces the surface tension and consequently increases the surface pressure (Π). Thus, the surface density of GO single layer sheets is probed by measuring the surface pressure as the GO-loaded air-water interface is compressed.10 When a suitable surface pressure and corresponding GO surface density is achieved, the GO film is collected onto a substrate by dip-coating (Figure 1a). By this method, Huang assembled continuous close-packed monolayer films of both overlapping and non-overlapping GO single layer sheets on various substrates.4 Unfortunately, film morphologies obtained by conventional LB vary widely among workers. As examples, Kim5 and Pasricha29 used different GO samples and in their conventional LB depositions, single layer sheets folded and overlapped well before continuous films were obtained. The results suggest that close-packed films of non-folded or non-overlapping single layers would be challenging to achieve in these cases. Thus, although conventional LB can be used in some cases to tune GO film morphology from dilute to closepacked to overlapping sheets, this is not usually the case. Instead, a wide variety of film morphologies with patch-wise depositions of folded and/or overlapping sheets is reported in the literature.5,10,11,21,22,26,27,29,30,32–34 The wide variability of GO film morphologies has typically been attributed to differences in intrinsic properties between GO samples, such as GO hydrophilicity and sheet sizes.10,22,26,34 Here we suggest, and experimentally support, that another — previously


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

overlooked — mechanism fundamentally limits control over GO film morphology in conventional LB.

Figure 1. Schematic structure of GO (1) and structure of stearic acid (2). GO contains oxidized (hydroxyl, carboxyl and epoxy groups) domains interspersed with non-oxidized polyaromatic domains.36 The oxidized domains are hydrophilic, while the non-oxidized domains are hydrophobic, which renders the single layer sheets amphiphilic.26 (a) Illustration of conventional LB assembly and deposition of GO. GO is dispersed in a mixture of water and alcohol (typically methanol) and then spread onto the air-water interface from a syringe. The air-water interface is


Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

then compressed by barriers until a GO film with desirable surface concentration is assembled at the air-water interface. Finally, the GO film is transferred to a substrate by dip-coating. (b) Illustration of stearic acid LB of GO single layer sheets. Stearic acid and GO are spread simultaneously from water-methanol mixtures onto the air-water interface. The interface is then compressed, and depositions are collected.

In conventional LB, after spreading GO (dispersed in an alcohol-water mixture), the liquid close to the surface is a mixture of alcohol and water. Such a mixture displays Marangoni flow37–40 as the meniscus spreads on a substrate that is being pulled through the air-water interface. A gradient in surface tension builds up because alcohol evaporates from the substrate quicker than water, making the surface tension of liquid on the substrate higher than of liquid in the subphase, which pulls liquid from the subphase onto the substrate, thus creating a flow (Figure 2).37–40 In this work, we show that Marangoni flow is present in conventional LB, and that it disrupts GO films as they deposit onto substrates. Using photography, we observed Marangoni flow and showed that it was present during depositions of both highly oxidized GO and less oxidized, partially reduced GO (rGO). We then used a surfactant (stearic acid LB, Figure 1b) which offsets surface-tension gradients,41 thereby suppressing the Marangoni flow which allowed better control over depositions. More importantly, comparison between conventional LB and stearic acid LB suggests that Marangoni flow is a fundamental and general problem inherently limiting control in conventional LB of GO. Our work thus suggests that disruptive Marangoni flow may have contributed to inhomogeneous film morphologies, such as patch-wise depositions and/or folded sheets displayed in other work.5,10,11,21,22,26,27,29,30,32–34


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

Figure 2. (a) Schematic illustration of Marangoni flow during conventional LB of GO single layer sheets. As a substrate is pulled through the liquid interface containing alcohol (typically methanol) and water, a thin liquid film spreads on the substrate. Methanol evaporates quicker than water from the film, which enriches the film in water and makes the surface tension of the film (γfilm) higher than the surface tension of the subphase (γsubphase). The surface tension gradient then pulls fresh liquid from the subphase onto the substrate, thus sustaining an upward flow. Liquid, enriched in water, accumulates at the top of the film where drops form and roll back into the subphase. These drops are called ‘tears of wine’ because they form in alcoholic beverages such as wine.37–40

EXPERIMENTAL SECTION Materials and cleaning procedures. Natural flake graphite (grade 230) was obtained from Asbury Carbons. SiO2/Si wafers with 500 nm thermally grown SiO2 was purchased from Pure Wafer (previously WRS materials). Anhydrous ethanol, mineral oil, L-ascorbic acid, stearic acid, sodium nitrate (NaNO3) and potassium permanganate (KMnO4) were purchased from Sigma Aldrich. Anhydrous methanol, ammonium hydroxide (NH4OH, 30 % in water), hydrogen peroxide (H2O2, 30 % in water), sulfuric acid (H2SO4) and sodium hydroxide (NaOH) were purchased from Fisher Scientific. Ultrapure MilliQ water was supplied by a Millipore system and used in all experiments. Glassware was first cleaned in a base bath containing 8 L isopropyl alcohol, 2 L deionized (DI) water and 500 g KOH, then rinsed copiously in DI water and finally rinsed in MilliQ water before drying in a clean oven at 120 oC.


Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

GO synthesis. GO was synthesized by a modification of the Hummers42 method. Graphite (2 g) was mixed with 92 mL sulfuric acid in a flask and stirred on an ice bath. 2 g NaNO3 was then added followed by 12 g of KMnO4. The mixture was then brought to 35 oC and stirred for 2 hours. After reaction, the mixture was cooled in an ice bath while 220 mL water was slowly added such that the temperature of the reaction mixture was kept below 10 oC. 60 mL of H2O2 (10% in water) was then slowly added until no more gas evolution was observed. Finally, the mixture was filtered under vacuum, the filter cake was re-dispersed in 500 mL water and the pH in suspension was set to pH 5 using 1 M NaOH (aq). The GO suspension was then dialyzed against pure water for 5 days while changing the water daily. To remove large GO sheets and unexfoliated graphitic oxide, the suspension was centrifuged at 2500 rpm (Sorvall, ST8, thermo scientific, HIGHConic III fixed angle rotor) for 20 min after which the sediment formed was discarded and the supernatant was collected. The centrifugation step was repeated two times. To remove small GO sheets, the suspension was centrifuged at 8000 rpm for 20 min after which the supernatant was discarded and the sediment re-dispersed in water. This step was also repeated two times. Finally, a stable GO stock dispersion with a concentration of 1.6 g/L was obtained. rGO synthesis. We synthesized rGO by modification of literature protocols.43,44 An aqueous GO suspension (2 mL, 1 g/L) was prepared in a glass vial. Ascorbic acid solution in water (0.11 mL, 1.5 g/L) was then added to the GO suspension. A protective layer of 0.5 mL mineral oil was added on top of the aqueous phase, and the vial was closed with a screw cap. The mineral oil layer eliminates the air-water interface, where rGO otherwise would aggregate, and allows rGO to remain well dispersed during and after reaction.45 The vial was then added to an oil bath (pre-heated to 85 oC) for 30 min. After reaction, 1.8 mL of the rGO sample was collected


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

and diluted with 9 mL methanol to form a stable rGO stock dispersion (0.17 g/L in 1:5 water:methanol). Conventional LB and stearic acid LB experiments. Substrates for LB deposition should be hydrophilic. Substrates used in this work were prepared by immersing SiO2/Si wafer pieces in freshly prepared solution consisting of 30 mL NH4OH (30 % in water) and 30 mL H2O2 (30 % in water) for 60 min, the wafers were then copiously rinsed in MilliQ water and dried under a N2(g) 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 150 x 580 mm was used (see Figure S1 for experimental setup). 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 syringe used for spreading GO or rGO dispersions and the (separate) syringe used for spreading stearic acid were also cleaned in this fashion. Before spreading the syringes were flushed with the respective spreading solutions. The trough was then filled with the water subphase. The temperature was set to 21 oC and the subphase allowed to equilibrate for 60 min before the air-water interface was cleaned by aspirating from the surface with a pipette during compression of the trough (75 cm2/min). The SiO2/Si wafer piece (typically 22 x 75 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. For conventional LB, GO (or rGO) spreading dispersions (0.025 g/L in 1:5 water:methanol) were prepared, sonicated for 15 minutes and then spread dropwise onto the water surface in the trough using a glass syringe and a syringe pump with a flow rate of 0.1 mL/min up to a volume of 13.6 mL (total amount of GO or rGO spread: 340 µg). For stearic acid


Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

LB, 6.8 mL of stearic acid solution (0.0026 g/L in 1:5 water:methanol) and 6.8 mL of GO dispersion (0.05 g/L or 0.1 g/L in 1:5 water:methanol) were simultaneously spread onto the airwater interface using two glass syringes and a syringe pump. The rate of spreading was 0.1 mL/min (0.05 mL/min from each syringe). The total amounts spread on the water surface in the trough were 340 µg or 680 µg GO, 18 µg stearic acid and 13.6 mL 1:5 water:methanol. After spreading, films at the air-water interface equilibrated for 20 min before surface pressure-area isothermal plots and depositions were collected. Isotherm plots were collected at a compression rate of 20 cm2/min. The air-water interface was equilibrated for 20 min before depositions were collected on pre-immersed substrates with a pull-up rate of 2 mm/min. For the pure stearic acid isotherm plot, 18 µg stearic acid in 13.6 mL 1:5 water:methanol was spread dropwise onto the air-water interface and the film was allowed to equilibrate for 20 min before the isotherm plot was collected at 20 cm2/min compression rate. Characterization. Scanning electron microscopy (SEM) images were collected on a field emission XL30 Sirion microscope operated at 5 kV. AFM micrographs were collected on a Veeco Multimode III system in tapping mode. Videos and photographs were acquired using a Canon EOS T5i camera equipped with an EFS 18-135 mm lens. UV-VIS spectra were collected on an Agilent Cary300 Bio spectrophotometer. Far-UV cuvettes with transmittance down to 170 nm were used. XPS spectra were collected using a ULVAC-PHI PHI 5000 Versaprobe spectrometer with an Al Kα source with incident photon energy of 1486.6 eV. Films of GO and rGO were dropcast onto SiO2/Si wafers for analysis. A 200 µm incident beam was used to collect C1s spectra at 5 points on each sample.

RESULTS AND DISCUSSION 9 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

Graphene oxide and reduced graphene oxide characterization. To test the influence of degree of GO oxidation on conventional LB depositions, we synthesized two differently oxidized GO samples. The more oxidized GO was produced by slight modification of Hummer’s method and the less oxidized sample was obtained by partial reduction of this GO to form reduced GO (rGO). The partial reduction was carried out using ascorbic acid following slight modification of published protocols.43,44 We used a small ratio of ascorbic acid to GO (0.08 g ascorbic acid / g GO) for partial reduction and note that for full reduction, ~ 0.9 g ascorbic acid / g GO is required.44 To contrast with the more oxidized GO, we label the less oxidized sample reduced GO (rGO), but emphasize that this partially reduced sample was still highly oxidized. GO and rGO were characterized using UV-VIS spectroscopy (Figure 3a) and rGO showed higher absorbance across the spectrum, suggesting that GO was more oxidized than rGO.43 We note that compared to other studies where more complete reduction was reported,43 the increase in light absorption of our rGO over GO was modest, suggesting that rGO was only slightly reduced. To further characterize the degree of oxidation in the GO and rGO, we used X-ray photoelectron spectroscopy (XPS) and present the results in Figure 3c,d. Deconvolution of the C 1s peak revealed three components at ~284.8 eV, ~ 287.0 eV and ~288.1 eV which have previously been attributed to (C-C), (C-O) and (C=O) structures, respectively.7,44 A small increase in the relative intensity of the (C-C, ~284.8 eV) peak in the rGO sample compared to the GO sample (Figure 3c,d) further indicates that the degree of oxidation was higher in GO than in rGO, as expected. Although the difference in relative intensity of the (C-C, ~284.8 eV) peak between the GO and rGO samples is small, the difference is significant which was confirmed by collecting spectra at five different locations on both the GO and rGO samples (Figure S2).


Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 3. (a) UV-VIS spectra of (1) GO and (2) rGO dispersions (0.03 g/L, 1 cm path length). (b) Photographs of (1) GO and (2) rGO dispersions (0.3 g/L). (c) Representative high-resolution (0.1 eV) core-level C 1s XPS spectrum of GO and of (d) rGO. Deconvolution revealed peaks at ~ 284.8 eV (C-C), ~ 287.0 eV (C-O) and ~ 288.1 eV (C=O). The percentage of non-oxidized carbon (C-C), which is indicated in the figure, was calculated from spectra taken at five randomly chosen locations (additional spectra in Supporting Information, Figure S2).

Conventional LB. Typical isothermal surface pressure-area plots from compressing GO at the air-water interface showed no increase in surface pressure (Figure 4a). Depositions were made at the end of compression, but typical SEM and AFM micrographs indicate that no GO sheets were 11 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

deposited (Figure 4b,c). By contrast, in conventional LB with rGO, the surface pressure was increased upon compression (Figure 4d), and films of rGO single layer sheets were successfully deposited (Figure 4 e–h). Further representative AFM and SEM micrographs of conventional LB depositions are presented in the supporting information (Figure S3 and S4). The absence of an increase in surface pressure during compression of GO at the air-water interface could indicate that the GO single layer sheets were sufficiently oxidized (and thus sufficiently ionized46) such that all sheets migrated into the subphase,10,26 and no sheets remained at the air-water interface. The rGO had a lower oxygen content than the GO (Figure 3), and the sheets were thus less ionized and less likely to migrate into the subphase.10,26 It is therefore not surprising that compression of rGO at the air-water interface gave rise to increased surface pressure and that rGO films were successfully deposited.


Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 4. Influence of degree of oxidation in conventional LB of GO and rGO. Before isotherm plots and depositions were collected, 340 µg of GO or rGO in 13.6 mL of 1:5 water:methanol was spread onto the air-water interface. (a) Typical isotherm plot (surface pressure (Π) versus trough area) collected while compressing GO at the air-water interface. (b) Typical SEM micrograph, and (c) typical AFM micrograph of deposition made at end of isothermal compression marked in (a). (d) Isotherm plot collected during compression of rGO. (e)-(h) Typical SEM and AFM micrographs of deposition made at the end of isothermal compression marked in (d). (e) SEM micrograph. (f) AFM micrograph with (g) enlarged section. (h) Height profile marked in (g). The height profile with incremental heights of ~ 1 nm confirms that the rGO sample consisted of single layers.47


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

As suggested previously,10,26 it is possible that a difference in affinity for the air-water interface between GO and rGO explains why rGO — but not GO — was successfully deposited (Figure 4). However, because methanol (the most common spreading solvent)4,5,8,10,11,13,21–35 was used to spread GO and rGO onto the air-water interface, another explanation is also possible. As described in the Introduction and in Figure 2, a water-methanol mixture will cause a flow on substrates that are pulled through the air-water interface,37–40 and it is plausible that such flow is disruptive of films as they deposit. In Figure 5, typical photographs are shown of wafer pieces as they were pulled through various air-water interfaces. As wafers were pulled through a pure air-water interface, the spreading water films dried, and no Marangoni flow was observed (Figure 5a and Supporting Movie S1). In contrast, when wafers were pulled through the air-water interface after spreading 13.6 mL 1:5 water:methanol, Marangoni flow was clearly observed as a climbing liquid film on the wafers (Figure 5b and Supporting Movie S2). At the top of the film, liquid accumulated and formed drops (called ‘tears of wine’37–40) that subsequently rolled back into the subphase. Note that the volume (13.6 mL) of 1:5 water:methanol spread onto the air-water interface is typical in conventional LB of GO single layer sheets.10,23–25 In addition, Marangoni flow was also present with smaller volumes, and in Figure S5 and Supporting Movie S3 we show that Marangoni flow was observed after spreading only 5 mL of 1:5 water:methanol onto the air-water interface. This volume is smaller than volumes used in most published works,4,5,8,10,21,23–25,28,29,32 and Marangoni flow is therefore relevant for most previous work on conventional LB of GO. In the present study, we investigate Marangoni flow resulting from methanol, but Marangoni flow is also present (Figure S6, Supporting Movie S4 and S5) with other alcohols such as isopropyl alcohol


Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

or ethanol that are sometimes used for conventional LB of GO.27,31,33 Thus, the present work is relevant also for those studies.

Figure 5. Representative photographs of SiO2/Si wafers pulled (2 mm/min) through (a) pure air-water interface, (b) after spreading 13.6 mL 1:5 water:methanol, (c) after spreading 340 µg GO in 13.6 mL 1:5 water:methanol and (d) after spreading 340 µg rGO in 13.6 mL 1:5 water:methanol. Photographs (c) and (d) were acquired during depositions marked in Figure 4 a and d, respectively. Note that an optical reflection of the LB trough is seen in the wafers in all images as an artifact. This reflection is outlined in the Supporting Information, Figure S7. Key to numbering: (1) Subphase; (2) Top of liquid film; (3) Tears of wine; (4) Deposited film.

We closely inspected the SiO2/Si wafers as they were withdrawn from the subphase during conventional LB of GO, and Marangoni flow was observed as a climbing liquid film with droplets (tears of wine) forming at the top of the film (Figure 5c and Supporting Movie S6). In conventional LB of rGO, Marangoni flow (climbing liquid film) was also observed (Figure 5d and Supporting Movie S7) but to a lesser extent than for GO, such that no visible tears of wine were observed. The fact that a climbing liquid film (Marangoni flow) was present (Figure 5d and Supporting Movie S7) during conventional LB of rGO is evident if one compares with 15 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

wafers pulled through pure air-water interfaces where no climbing liquid film was present (Figure 5a and Supporting Movie S1). Marangoni flow is driven by formation of a surface tension (γ) gradient between liquid on the substrate and liquid in the subphase (Figure 2).37–40 Thus, Marangoni flow can be offset by surfactants, because these quickly re-distribute in response to changes in surface tension and act to smooth surface tension-gradients.41 GO single layer sheets are amphiphilic, with their surface activity increasing with decreasing degree of ionization.10,26 Because degree of ionization decreases with decreasing degree of oxidation,46 rGO is expected to be more surface active than GO. The surface pressure increased during compression of rGO, but not during compression of GO, which suggests that rGO was more surface active than GO, as expected. Thus, rGO is itself a surfactant and may have partially offset the surface tension-gradient between the substrate and subphase, thus partially suppressing the Marangoni flow and allowing rGO films to deposit. In contrast, the absence of increased surface pressure during compression of GO (Figure 4a) shows that GO was only weakly surface active. Thus, we would not anticipate Marangoni flow to be suppressed by GO. We wanted to test if Marangoni flow was disruptive of GO films as they deposited and may have caused the difference in deposition characteristics between the GO and rGO samples. Thus, we hypothesized that Marangoni flow — not migration of GO into the subphase — caused the absence of GO sheets in conventional LB depositions. To test this hypothesis, we added surfactant (stearic acid) to suppress Marangoni flow and investigated if GO could be deposited in this case. Stearic acid assisted LB assembly. Stearic acid (pKa=10.15,48 sparingly ionized on neutral water subphase) was co-spread with GO before isotherm plots and depositions were collected 16 ACS Paragon Plus Environment

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(Figure 6). Stearic acid offsets41 the surface tension-gradient that would otherwise build up between the substrate and subphase. Thus, stearic acid suppressed the formation of Marangoni flow during depositions, and in contrast to conventional LB of GO (Figure 5c and Supporting Movie S6), no climbing liquid film was observed (Figure 7 and Supporting Movie S8). Consequently, GO that could not be deposited by conventional LB (Figure 4a–c) was now successfully deposited (Figure 6). In Figure 6a, isotherm plots are presented of pure stearic acid and of stearic acid coassembled with GO. For isotherm plot (1), 18 µg of stearic acid was spread onto the air-water interface. For isotherm plot (2) and (3), 340 µg and 680 µg of GO, respectively, were co-spread with 18 µg of stearic acid. For collection of all isotherm plots, the volume of spreading solvent was 13.6 mL of 1:5 water:methanol. Comparison of isotherm plots (1) and (2) shows that the isotherm was shifted to larger trough areas after GO addition. In isotherm (3) where twice the amount of GO was present compared to isotherm (2), the isotherm was further shifted. The shift between isotherms (2) and (3) was roughly the same as the shift between isotherms (1) and (2), suggesting that the shift was roughly proportional to the amount of GO added to the air-water interface. A model that rationalizes the shifts of isotherm plots (1)-(3) is to assume that GO and stearic acid did not interact, but instead occupied separate domains in the air-water interface. In this model, the addition of GO reduces the available area for stearic acid at the air-water interface, thus shifting isotherms to larger trough areas.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

Figure 6. Stearic acid LB. (a) Typical isotherm plots (surface pressure, Π, versus trough area) of (1) pure stearic acid (18 µg in 13.6 mL of 1:5 water:methanol), (2) Stearic acid (18 µg in 6.8 mL of 1:5 water:methanol) and GO (340 µg in 6.8 mL of 1:5 water:methanol) and (3) Stearic acid (18 µg in 6.8 mL 1:5 water:methanol) and GO (680 µg in 6.8 mL 1:5 water:methanol). (b-d) and (e-g) Typical SEM and AFM micrographs of depositions collected at LB trough positions marked in (a). (b, e) SEM micrographs. (c, f) AFM micrographs with (d,g) enlarged sections and associated height profiles. The height profiles with incremental heights of ~ 1 nm confirms that the GO sample consisted of single layer sheets.47 Key to numbering in enlarged sections: (I) GO, (II) Stearic acid, (III) Underlying SiO2/Si wafer. Additional representative AFM and SEM micrographs are presented in Figure S8.


Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 7. Photograph of SiO2/Si wafer pulled (2 mm/min) through the air-water interface after simultaneously spreading stearic acid (18 µg in 6.8 mL of 1:5 water:methanol) and GO (340 µg in 6.8 mL of 1:5 water:methanol). The photograph was acquired during the deposition marked (b–d) in Figure 6a. Note that an optical reflection of the LB trough is seen in the wafer as an artifact. This reflection is outlined in the Supporting Information, Figure S8. Key to numbering: (1) Subphase; (2) Top of liquid film.

The notion that GO and stearic acid did not interact but instead occupied separate domains in the air-water interface is further supported by AFM images of the depositions (Figure 6c,d and f,g). In the depositions, stearic acid is clearly seen (areas appearing porous) between, but not on, GO single layer sheets. In fact, the resistance to GO/stearic acid interaction was such that at higher compression of the GO/stearic acid film, instead of stearic acid being forced onto GO sheets, GO sheets started to fold and overlap (Figure 6f and g). To confirm that areas appearing porous in the micrographs consisted of stearic acid, the substrate was washed with ethanol. Representative AFM micrographs of the substrate before and after washing clearly


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

demonstrate that porous patches disappeared after washing, but that GO sheets remained (Figure 8a,c). Height profiles of the substrates before and after washing (Figure 8b,d) show that the substrate surface became substantially smoother after washing, while the height of the GO sheets remained the same, further demonstrating that stearic acid resided between, but not on GO sheets. Taken together, the shifts of the isotherm plots (Figure 6a) and the morphologies of the depositions (Figure 6b–g) conclusively show that GO and stearic acid did not interact, but instead occupied separate domains at the air-water interface.

Figure 8. (a) AFM micrograph of stearic acid LB deposition with (b) height profile. (c) AFM micrograph of stearic acid LB deposition after washing with ethanol and (d) height profile.

The fact that GO single layer sheets and stearic acid occupied separate domains in the airwater interface — without interacting — is significant, because it suggests that GO resided at the air-water interface also in conventional LB. The notion that GO resided at the air-water interface in stearic acid LB but not in conventional LB is not plausible, since there was no attractive interaction between GO and stearic acid, and it is unlikely that stearic acid would be involved in 20 ACS Paragon Plus Environment

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

pinning GO sheets to the air-water interface. The results thus support the hypothesis that GO sheets were present at the air-water interface during conventional LB, but were not deposited (Figure 4a –c) due to Marangoni flow (Figure 5c and Supporting Movie S6) that disrupted the GO films during conventional LB deposition. A plausible reason as to why no surface pressure increase was observed during conventional LB of GO (Figure 4a), even though sheets were present, is that highly oxidized GO may acquire a lubricating layer of water that allows the sheets to easily slide across each other.10 Finally, Marangoni flow — demonstrated here to be disruptive of GO films — was also present (only slightly suppressed) in conventional LB of rGO (Figure 5d and Supporting Movie S7). Thus, it is possible that the poor film characteristics such as wrinkled and folded sheets seen in the rGO deposition (Figure 4e–h) resulted from disruptive Marangoni flow. CONCLUSION In conventional LB deposition of graphene oxide (GO), the subphase surface consists of a mixture of methanol and water, which leads to formation of Marangoni flow. The Marangoni flow was disruptive and prevented GO films from depositing. The fact that GO was present at the air-water interface was supported by experiments using stearic acid, which suppressed Marangoni flow and allowed GO deposition. We also deposited less oxidized (and therefore more surface-active) rGO by conventional LB, and in this case, Marangoni flow was partially suppressed which allowed deposition of rGO films. However, disruptive Marangoni flow was still observed and may have contributed to poor control over film morphology. In summary, our results demonstrate that when alcohol is used as a spreading solvent in conventional LB of GO, Marangoni flow is inherent and has a disruptive effect on depositions.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

Thus, because alcohol is usually used to spread GO for conventional LB,4,5,8,10,11,13,21–35 we believe Marangoni flow may explain poor control over depositions also observed elsewhere in the literature. We suggest that researchers in the field carefully monitor their substrates as they are drawn through the air-water interface during conventional LB. If Marangoni flow is observed, it is likely that depositions will not be well controlled, and that other methods for deposition (such as cationic assisted LB assembly49–51) may provide better control. ASSOCIATED CONTENT Supporting Information Experimental setup of LB trough; Additional high-resolution core-level C 1s XPS spectra of GO and rGO; Additional SEM and AFM micrographs of conventional LB depositions of GO and rGO;

Photograph of SiO2/Si wafer pulled (2 mm/min) through air-water interface after

spreading 5 mL of 1:5 water:methanol; Photographs of SiO2/Si wafers pulled (2 mm/min) through air-water interfaces after spreading 13.6 mL 1:5 water:ethanol and 13.6 mL 1:5 water:isopropyl alcohol, respectively; Outlines of optical reflection artifacts in Figure 5 and Figure 7; Movies showing SiO2/Si wafer pulled (2 mm/min) through pure air-water interface (Movie S1, played at 5x speed), after spreading 13.6 mL 1:5 water:methanol (Movie S2, played at 5x speed), after spreading 5 mL 1:5 water:methanol (Movie S3, played at 5x speed), after spreading 13.6 mL 1:5 water:isopropyl alcohol (Movie S4, played at 5x speed), after spreading 13.6 mL 1:5 water:ethanol (Movie S5, played at 5x speed), during conventional LB of GO (Movie S6, played at 5x speed), during conventional LB of rGO (Movie S7, played at 5x speed), during stearic acid LB of GO (Movie S8, played at 5x speed). AUTHOR INFORMATION


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

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 as well as unrestricted funds. 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 ECCS1542152.

REFERENCES (1)

Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and

Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906–3924. 23 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(2)

Page 24 of 31

Loh, K. P.; Bao, Q. L.; Eda, G.; Chhowalla, M. Graphene Oxide as a Chemically Tunable

Platform for Optical Applications. Nat. Chem. 2010, 2, 1015–1024. (3)

Chung, C.; Kim, Y. K.; Shin, D.; Ryoo, S. R.; Hong, B. H.; Min, D. H. Biomedical

Applications of Graphene and Graphene Oxide. Acc. Chem. Res. 2013, 46, 2211–2224. (4)

Cote, L. J.; Kim, F.; Huang, J. Langmuir - Blodgett Assembly of Graphite Oxide Single

Layers. J. Am. Chem. Soc. 2009, 131, 1043–1049. (5)

Zheng, Q.; Ip, W. H.; Lin, X.; Yousefi, N.; Yeung, K. K.; Li, Z.; Kim, J.-K. Transparent

Conductive Films Consisting of Ultralarge Graphene Sheets Produced by Langmuir - Blodgett Assembly. ACS Nano 2011, 5, 6039–6051. (6)

Eda, G.; Fanchini, G.; Chhowalla, M. Large-Area Ultrathin Films of Reduced Graphene

Oxide as a Transparent and Flexible Electronic Material. Nat. Nanotechnol. 2008, 3, 270–274. (7)

Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. Evaluation of

Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano 2008, 2, 463–470. (8)

Zheng, Q.; Shi, L.; Yang, J. Langmuir-Blodgett Assembly of Ultra-Large Graphene

Oxide Films for Transparent Electrodes. Trans. Nonferrous Met. Soc. China 2012, 22, 2504– 2511. (9)

Lin, X.; Jia, J.; Yousefi, N.; Shen, X.; Kim, J.-K. Excellent Optoelectrical Properties of

Graphene Oxide Thin Films Deposited on a Flexible Substrate by Langmuir-Blodgett Assembly. J. Mater. Chem. C 2013, 1, 6869–6877.


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(10)

Cote, L. J.; Kim, J.; Zhang, Z.; Sun, C.; Huang, J. Tunable Assembly of Graphene Oxide

Surfactant Sheets: Wrinkles, Overlaps and Impacts on Thin Film Properties. Soft Matter 2010, 6, 6096-6101 (11)

Yin, Z.; He, Q.; Huang, X.; Zhang, J.; Wu, S.; Chen, P.; Lu, G.; Chen, P.; Zhang, Q.;

Yan, Q.; Zhang, H. Real-Time DNA Detection Using Pt Nanoparticle-Decorated Reduced Graphene Oxide Field-Effect Transistors. Nanoscale 2012, 4, 293–297. (12)

Liu, Y.; Yu, D.; Zeng, C.; Miao, Z.; Dai, L. Biocompatible Graphene Oxide-Based

Glucose Biosensors. Langmuir 2010, 26, 6158–6160. (13)

Mangadlao, J. D.; Santos, C. M.; Felipe, M. J. L.; de Leon, A. C. C.; Rodrigues, D. F.;

Advincula, R. C. On the Antibacterial Mechanism of Graphene Oxide (GO) Langmuir–Blodgett Films. Chem. Commun. 2015, 51, 2886–2889. (14)

Zhang, L.; Yu, J.; Yang, M.; Xie, Q.; Peng, H.; Liu, Z. Janus Graphene from Asymmetric

Two-Dimensional Chemistry. Nat. Commun. 2013, 4, 1443. (15)

Yu, D.; Nagelli, E.; Naik, R.; Dai, L. Asymmetrically Functionalized Graphene for

Photodependent Diode Rectifying Behavior. Angew. Chem., Int. Ed. 2011, 50, 6575–6578. (16)

Holm, A.; Park, J.; Goodman, E. D.; Zhang, J.; Sinclair, R.; Cargnello, M.; Frank, C.W.

Synthesis, Characterization, and Light-Induced Spatial Charge Separation in Janus Graphene Oxide. Chem. Mater. 2018, In press, DOI: 10.1021/acs.chemmater.8b00087 (17)

Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. A Chemical Route to Graphene

for Device Applications. Nano Lett. 2007, 7, 3394–3398.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(18)

Page 26 of 31

Tao, A. R.; Huang, J.; Yang, P. Langmuir - Blodgettry of Nanocrystals and Nanowires.

Acc. Chem. Res. 2008, 41, 1662–1673. (19)

Tao, A. R.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P.

Langmuir−Blodgett Silver Nanowire Monolayers for Molecular Sensing Using SurfaceEnhanced Raman Spectroscopy. Nano Lett. 2003, 3, 1229–1233. (20)

Acharya, S.; Panda, A. B.; Belman, N.; Efrima, S.; Golan, Y. A Semiconductor-

Nanowire Assembly of Ultrahigh Junction Density by the Langmuir-Blodgett Technique. Adv. Mater. 2006, 18, 210–213. (21)

Zheng, Q.; Shi, L.; Ma, P.-C.; Xue, Q.; Li, J.; Tang, Z.; Yang, J. Structure Control of

Ultra-Large Graphene Oxide Sheets by the Langmuir–Blodgett Method. RSC Adv. 2013, 3, 4680-4691. (22)

Hidalgo, R. S.; López-Díaz, D.; Velázquez, M. M. Graphene Oxide Thin Films: Influence

of Chemical Structure and Deposition Methodology. Langmuir 2015, 31, 2697–2705. (23)

Sutar, D. S.; Narayanam, P. K.; Singh, G.; Botcha, V. D.; Talwar, S. S.; Srinivasa, R. S.;

Major, S. S. Spectroscopic Studies of Large Sheets of Graphene Oxide and Reduced Graphene Oxide Monolayers Prepared by Langmuir-Blodgett Technique. Thin Solid Films 2012, 520, 5991–5996. (24)

Harrison, K. L.; Biedermann, L. B.; Zavadil, K. R. Mechanical Properties of Water-

Assembled Graphene Oxide Langmuir Monolayers: Guiding Controlled Transfer. Langmuir 2015, 31, 9825–9832.


Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(25)

Botcha, V. D.; Narayanam, P. K.; Singh, G.; Talwar, S. S.; Srinivasa, R. S.; Major, S. S.

Effect of Substrate and Subphase Conditions on the Surface Morphology of Graphene Oxide Sheets Prepared by Langmuir-Blodgett Technique. Colloids Surf., A 2014, 452, 65–72. (26)

Kim, J.; Cote, L. J.; Kim, F.; Yuan, W.; Shull, K. R., Huang, J. Graphene Oxide Sheets at

Interfaces. J. Am. Chem. Soc. 2010, 132, 8180–8186. (27)

Imperiali, L.; Liao, K. H.; Clasen, C.; Fransaer, J.; Macosko, C. W.; Vermant, J.

Interfacial Rheology and Structure of Tiled Graphene Oxide Sheets. Langmuir 2012, 28, 7990– 8000. (28)

Wang, X.; Bai, H.; Shi, G. Size Fractionation of Graphene Oxide Sheets by pH-Assisted

Selective Sedimentation. J. Am. Chem. Soc. 2011, 133, 6338–6342. (29)

Kumar, V.; Bahadur, N.; Sachdev, D.; Gupta, S.; Reddy, G. B.; Pasricha, R. Restructural

Confirmation and Photocatalytic Applications of Graphene Oxide-Gold Composites Synthesized by Langmuir-Blodgett Method. Carbon 2014, 80, 290–304. (30)

Zheng, Q.; Zhang, B.; Lin, X.; Shen, X.; Yousefi, N.; Huang, Z.-D.; Li, Z.; Kim, J.-K.

Highly Transparent and Conducting Ultralarge Graphene Oxide/Single-Walled Carbon Nanotube Hybrid Films Produced by Langmuir-Blodgett Assembly. J. Mater. Chem. 2012, 22, 2507225082 (31)

Silverberg, G. J.; Pearce, P.; Vecitis, C. D. Controlling Self-Assembly of Reduced

Graphene Oxide at the Air-Water Interface: Quantitative Evidence for Long-Range Attractive and Many-Body Interactions. ACS Appl. Mater. Interfaces 2015, 7, 3807–3815.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(32)

Page 28 of 31

Han, F.; Yang, S.; Jing, W.; Jiang, Z.; Liu, H.; Li, L. A Study on near-UV Blue

Photoluminescence in Graphene Oxide Prepared by Langmuir-Blodgett Method. Appl. Surf. Sci. 2015, 345, 18–23. (33)

Silverberg, G. J.; McClelland, A. A.; Griesse-Nascimento, S.; Girabawe, C.; Kadow, J.

P.; Mahadevan, L.; Vecitis, C. D. Controlling the Roughness of Langmuir-Blodgett Monolayers. J. Phys. Chem. B 2017, 121, 5078-5085 (34)

Lõpez-Díaz, D.; Velázquez, M. M.; de la Torre, S. B.; Pérez-Pisonero, A.; Trujillano, R.;

Fierro, J. L. G.; Claramunt, S.; Cirera, A. The Role of Oxidative Debris on Graphene Oxide Films. ChemPhysChem 2013, 14, 4002–4009. (35)

Zhang, X.; Sun, H.; Yang, S. Self-Limiting Assembly of Two-Dimensional Domains

from Graphene Oxide at the Air/water Interface. J. Phys. Chem. C 2012, 116, 19018–19024. (36)

Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene

Oxide. Chem. Soc. Rev. 2010, 39, 228–240. (37)

Fournier, J. B.; Cazabat, A. M. Tears of Wine. Europhys. Lett. 1992, 20, 517–522

(38)

Neogi, P. Tears-of-Wine and Related Phenomena. J. Colloid Interface Sci. 1985, 105,

94–101. (39)

Vuilleumier, R.; Ego, V.; Neltner, L.; Cazabat, A. M. Tears of Wine: The Stationary

State. Langmuir 1995, 11, 4117–4121. (40)

Gugliotti, M. Tears of Wine. J. Chem. Educ. 2004, 81, 67–68.


Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(41)

Ikura, Y. S.; Tenno, R.; Kitahata, H.; Suematsu, N. J.; Nakata, S. Suppression and

Regeneration of Camphor-Driven Marangoni Flow with the Addition of Sodium Dodecyl Sulfate. J. Phys. Chem. B 2012, 116, 992–996. (42)

Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc.

1958, 80, 1339 (43)

Zhang, J.; Yang, H.; Shen, G.; Cheng, P.; Zhang, J.; Guo, S. Reduction of Graphene

Oxide via L-Ascorbic Acid. Chem. Commun. 2010, 46, 1112–1114. (44)

Fernández-Merino, M. J.; Guardia, L.; Paredes, J. I.; Villar-Rodil, S.; Solís-Fernández,

P.; Martínez-Alonso, A.; Tascón, J. M. D. Vitamin C Is an Ideal Substitute for Hydrazine in the Reduction of Graphene Oxide Suspensions. J. Phys. Chem. C 2010, 114, 6426–6432. (45)

Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable Aqueous

Dispersions of Graphene Nanosheets. Nat. Nanotechnol. 2008, 3, 101–105. (46)

Krishnamoorthy, K.; Veerapandian, M.; Yun, K.; Kim, S. J. The Chemical and Structural

Analysis of Graphene Oxide with Different Degrees of Oxidation. Carbon 2013, 53, 38–49. (47)

Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.;

Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558–1565. (48)

Kanicky, J. R.; Shah, D. O. Effect of Degree, Type, and Position of Unsaturation on the

pKa of Long-Chain Fatty Acids. J. Colloid Interface Sci. 2002, 256, 201–207.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(49)

Page 30 of 31

Kim, H.; Jang, Y. R.; Yoo, J.; Seo, Y. S.; Kim, K. Y.; Lee, J. S.; Park, S. D.; Kim, C. J.;

Koo, J. Morphology Control of Surfactant-Assisted Graphene Oxide Films at the Liquid–Gas Interface. Langmuir 2014, 30, 2170–2177. (50)

Szabó, T.; Hornok, V.; Schoonheydt, R. A.; Dékány, I. Hybrid Langmuir–Blodgett

Monolayers of Graphite Oxide Nanosheets. Carbon 2010, 48, 1676–1680. (51)

Kim, T. H.; Kim, H.; Choi, K. I.; Yoo, J.; Seo, Y. S.; Lee, J. S.; Koo, J. Graphene Oxide

Monolayer as a Compatibilizer at the Polymer-Polymer Interface for Stabilizing Polymer Bilayer Films against Dewetting. Langmuir 2016, 32, 12741–12748.


Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table of contents graphic.


Langmuir-Blodgett Deposition of Graphene Oxide â Identifying

Recommend Documents

Langmuir-Blodgett Deposition of Graphene Oxide â Identifying