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A selective process to extract high-quality Reduced Graphene Oxide (RGO) leaflets Ahmad Al Shboul, Mohamed Siaj, and Jerome P. Claverie ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01580 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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ACS Applied Nano Materials

A Selective Process to Extract High-Quality Reduced Graphene Oxide (RGO) Leaflets Ahmad M. Al Shboul,† Mohamed Siaj†* and Jerome P. Claverie§* † Université du Québec à Montréal, Department of Chemistry, Succ Centre-Ville, CP8888, Montréal (Québec) CANADA H3C 3P8 § Université de Sherbrooke, Department of Chemistry, 2500, boul. de l'Université, Sherbrooke (Québec) CANADA J1K 2R1

KEYWORDS: Reduced Graphene Oxide (RGO), selective process, Double Liquid Phase Extraction (DLPE), dispersion, cholesterol-based copolymer.

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ABSTRACT

One popular approach to prepare graphene on a large scale consists in converting Graphene Oxide (GO) into Reduced Graphene Oxide (RGO). However, this procedure yields graphene flakes with various amounts of oxygenated defects. Using a Double Liquid-Phase Extraction technique (DLPE) assisted by cholesterol-based polymers, we demonstrate that the RGO flakes of the highest quality, i.e. those with the highest π-conjugated network and with the lowest number of oxygenated defects can be selectively extracted in isooctane, while lower quality flakes remain in water. Thus, it is possible to collect single-layer graphene sheets of high-quality, as characterized by Raman spectroscopy (ID/IG below 0.2) starting from a RGO containing a heterogeneous mixture of leaflets (ID/IG ~ 1.3). The high-quality of the RGO leaflets extracted by DLPE was also confirmed by X-ray photoelectron spectroscopy, photoluminescence, Fourier transform infrared spectroscopy, X-ray diffraction, and atomic force microscopy. The conductivity of the films prepared with DLPE RGO flakes is an order of magnitude higher than the one of the films prepared with as-prepared RGO. The thermal stability of the extracted leaflets, as measured by thermal gravimetric analysis, is also greatly enhanced. Thus, sorting RGO by DLPE is a valuable process for the large-scale production of high-quality graphene.


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INTRODUCTION

Due to their astonishing electronic, chemical, thermal and mechanical properties, graphene-based materials show great promise for a myriad of applications.1–6 In order to massively exploit them, one must be able to mass-produce defect-free graphene flakes in suspension in a liquid using an economically-viable route.7,8 One popular route to access dispersions of graphene flakes in a solvent consists in the formation of Reduced Graphene Oxide (RGO).9,10 In short, graphite is exfoliated under highly oxidizing conditions to yield Graphene Oxide (GO) which is soluble in water. Then, using a variety of reducing agents,11–17 GO is reduced into Reduced Graphene Oxide (RGO), which consists of leaflets of graphene containing various levels of oxygenated defects which disrupt the π-conjugated network of graphene. The presence of these oxygenated defects is revealed by a relatively high D band in Raman spectroscopy,18,19 and by the presence of peaks characteristic of C-O, C=O and O-C=O by X-ray Photoelectron Spectroscopy (XPS).17,20,21 The optimization of the RGO synthesis to minimize the concentration of these defects has attracted significant attention in recent years.11–17 However, to our knowledge, no attempt has so far been made to develop a process whereby a given RGO sample would be sorted and separated according to the concentration of oxygenated defects in each leaflet. Our work hypothesis to achieve such a separation process is that flakes containing a high concentration of oxygenated moieties should exhibit a greater affinity for polar solvents due to hydrogen bonding,21,22 whereas flakes with a low amount of defects (low amount of oxygenated moieties) should be less polar and should exhibit a greater affinity for non-polar solvents, thus following the qualitative rule ʺlike dissolves likeʺ. Herein, we present a simple extraction process, coined Double Liquid-Phase Extraction process (DLPE), whereby flakes with the lowest concentration of oxygenated defects (referred below as high quality RGO) are separated from RGO using a


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biphasic system constituted of a polar (water, bottom layer) and non-polar (isooctane, top layer) solvent (Figure 1). In order to increase the affinity of RGO for isooctane, an organo-soluble polymer which contains pendant cholesterol groups (see Figure 1) has been added in the isooctane layer. It was shown by us that polymers bearing such pendant cholesterol groups form strong supramolecular interactions with graphene23 or with carbon nanotubes.24 The RGO flakes separated from the isooctane layer are nearly devoid of surface defects, as shown by a combination of a Raman spectroscopy, Photoluminescence (PL) measurements, XPS, X-ray diffraction (XRD), Thermogravimetric Analysis (TGA), and Fourier-transform infrared (FTIR) spectroscopy. Thus, we envision that this separation process offers a promising route toward the mass-production of high-quality graphene via an economically viable process.


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Figure 1: (A) Schematic representation of the DLPE process. RGO is layered with water and isooctane

containing

poly(CEM11-b-EHA7)

(CEM

=

cholesteryloxycarbonyl-2-

hydroxymethacrylate, EHA = 2-ethylhexyl acrylate). (B) Structure of the diblock copolymer poly(CEM11-b-EHA7). (C) Photograph of the biphasic DLPE medium, showing the aqueous phase at the bottom (pH = 6) and the isooctane phase (polymer = 0.25 mg.mL-1) after 24 hours at room temperature. (D) Evolution of the isooctane layer vs time in hour (time indicated on the cap) for RGO extracted by DLPE at pH=6, RT, and 0.25 mg.mL-1 polymer.


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EXPERIMENTAL SECTION Materials. Graphite powder (+100 mesh, 99.9995%), concentrated sulfuric acid (H2SO4, 98%), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), hydrazine monohydrate (N2H4.H2O, 65%), ammonium hydroxide (NH4OH), 2-hydroxyethyl methacrylate, 2-ethylhexyl acrylate (EHA), hydrochloric acid (HCl), 1,4-dioxane and isooctane were used as received from Sigma–Aldrich. Cholesteryl chloroformate was purchased from Alfa Aesar. Synthesis of poly(CEM11-b-EHA7). Block copolymer poly(CEM11-b-EHA7) was prepared using the

procedure

described

in

reference

24.24

In

short,

RAFT

agent

2-

{[(butylsulfanyl)carbonothioyl]sulfanyl}propanoic acid, (0.051g, 0.21 mmol, 1eq), 4.4′-azobis (4-cyanovaleric acid) (6.0 mg, 0.021mmol, 0.1eq), 2-ethylhexyl acrylate (EHA, 0.20 g, 1.08 mmol, 5eq) and 1,4-dioxane (4.0 g) were added to a 50 mL round-bottomed flask and degassed with nitrogen for 20 minutes. The flask was then heated in an oil bath at 70 ° C for 5 h under constant magnetic stirring. The obtained polymer was separated from the unreacted monomer by precipitation in methanol and was dried under vacuum. To this polymer, cholesteryloxycarbonyl2-hydroxymethacrylate (CEM, 1.7 g. 3.2 mmol, 15eq) and 1,4 dioxane (6.0 g) were added to a 50 mL round-bottomed flask and degassed with nitrogen for 20 minutes. The flask was then heated in an oil bath at 70 ° C for 6 h under constant magnetic stirring. The resulting polymer was recovered by precipitation in methanol and was left to dry for 2 days under vacuum. Yield: 1.2g (60%). Analysis by 1H NMR, using the procedure highlighted in reference 24, indicated that the resulting block copolymer was constituted of 7 units of EHA and 11 units of CEM. This product is later referred as poly(CEM11-b-EHA7)


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Synthesis of GO and of rGO. GO was synthesized from natural graphite by using the modified Hummers method.25,26 Briefly, 2 g of natural graphite flakes was mixed with 50 ml concentrated sulfuric acid (H2SO4) and 1 g NaNO3 for 1 hour and then cooled in an ice water bath. Then, 6 g of potassium permanganate (KMnO4) was gradually added into the mixture to produce a thick green mixture. The mixture was stirred for 2 hours while still cooled in the ice bath. Afterwards, the mixture was removed from the ice bath and allowed to stir at 35 ⁰C for 1 hour. Then, the prepared mixture was diluted slowly with 100 mL of distilled water and refluxed for 1 hour at 100 ⁰C to produce a brown pasty mixture. Subsequently, the mixture was diluted again with another 300 mL of distilled water followed with the slowly addition of hydrogen peroxide (H2O2) to reduce residual permanganate corresponding to no further gas evolution from the mixture. The resulting solid was filtered and washed several times with HCl solution (5%) to remove metal ions followed by distilled water to remove the acid. Finally, the obtained solid was dried at 60 °C in vacuum oven for overnight. RGO was prepared by reduction of GO aqueous solution with hydrazine monohydrate.27 First. 100 mg GO was dispersed in 100 mL distilled water for 2 hours followed by centrifugation of the suspension at 3500 rpm for 2 hours to precipitate the accumulated, partially oxidized and undispersed GO flakes. This step prevents unexfoliated graphite particles from contaminating GO and RGO. Afterwards the supernatant was adjusted at pH ≈ 9-10 and refluxed at 100 ⁰C with 1 mL (0.02 mol) hydrazine monohydrate for 2 hours. Finally, the obtained dark solution was filtered and washed extensively with distill water. The filtrated dark solid was dried at 60 °C in vacuum oven for overnight. DLPE Process for RGO. All experiments were performed by stirring the RGO powder (25 mg) in a system contains two layers. The lower layer was nanopure water (50 mL) and the upper layer was isooctane (50 mL) containing poly(CEM11-b-EHA7) (c = 0.25 mg·mL-1). The system


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was stirred continuously using a magnetic stir bar at ~ 1000 rpm. A sample of the organic layer (2 mL) was taken every hour for the first 10 hours in order to assess the quality and concentration of the extracted RGO, and a last one after 24 hours. In total, only 5.2% of the RGO was extracted during the sampling process, thus the sampling procedure did not significantly disturb the RGO partitioning between the aqueous and the organic layer. For each withdrawn sample, an aliquot of 2 mL of isooctane containing the polymer (c = 0.25 mg·mL-1) was added to maintain volumes of the DLPE phase. After 24 hours, the homogeneous isooctane layer was separated from the aqueous layer and from RGO precipitates which were floating at the interface between GO and RGO. Characterization. Absorbance was recorded on a UV-visible spectrophotometer (Cary 100 Bio, Agilent Technologies, USA) using a quartz cell with a path length of 1.00 cm The absorption of the dispersions in isooctane were measured at λ=600 nm. The concentration was calculated using an extinction coefficient of 24.60 mL mg-1 cm-1 at this wavelength.19 The morphology of the RGO flakes was characterized by AFM (Veeco/Bruker Multimode AFM, USA). The AFM specimens were deposited on mica (Ted Pella, inc.) using a Langmuir-Blodgett (LB) technique.28,29 Briefly, a drop of RGO dispersed in isooctane was gently layered on a flat surface of nanopure water. Since isooctane and water are immiscible, RGO flakes spread out on the water surface and the RGO flakes were collected by immersing and withdrawing a mica plate slowly and perpendicularly to the aqueous layer. The sample was dried in vacuum oven before analysis. The advantage of this procedure is that the graphene folding is minimized and that most, if not all, polymer is removed from the sample.28,29 Raman spectroscopy was performed on a confocal Raman microscope (CRM, Alpha300 R, Witec, Germany) using a 532 nm class 3B laser. Samples for Raman spectroscopy were prepared


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by depositing a few drops of RGO on a glass slide. Once dried, the excess polymer (that is to say the polymer which was not adsorbed on the surface of the graphene) was removed by immersing the slides in pure hexane for a few minutes. Typically, images of 20 per 20 m were acquired with a pixel size of 250 per 250 nm. Then, the raw data were processed using the Witec software. Background and cosmic ray subtraction were performed to create an identical baseline for each spectrum and to remove spikes. For each flake, the D and G modes, which are found at 1350 and 1575 cm-1 respectively were analyzed by averaging their intensity measured at each pixel over the entire flake. Photoluminescence (PL, Spectrofluorometer PerkinElmer LS-45, USA) was measured on liquid dispersions using λex= 325 nm. X-ray Photoelectron Spectroscopy (XPS, PHI 5600-ci (Physical Electronics, Eden Prairie, MN, USA) was measured on Si wafers which were washed with hexane in order to remove excess polymer. X-ray diffraction (XRD, Bruker D8, Germany) was recorded at a scanning rate of 0.5 o/min with a scan step of 0.02o in a 2θ range of 5-55° using a Cu Kα radiation at λ = 1.54 Å. Thermogravimetric analysis coupled mass spectroscopy (TGAMS) was performed on a TGA (Q500)/Discovery MS from TA Instruments. Samples were heated at 10 °C/min from 30 to 900 °C under air flow. Sheet resistance of raw RGO and DLPE RGO films was measured at room temperature using a four-point probe resistivity measurement system (EveBeing PE-4X probe station equiped with a Keithley 6400 analyzer). The raw RGO film was prepared by filtering a RGO dispersion in DMF on a PTFE membrane (0.22 um). The resulting film was washed with THF to remove excess DMF and dried at 40 °C overnight under vacuum. The DLPE RGO film was prepared in a similar manner but using a dispersion in isooctane rather than in DMF.


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RESULTS AND DISCUSSION Our strategy to sort RGO is schematized in Figure 1A. Our starting material is RGO which has been prepared by hydrazine reduction of GO. Characterization of GO by AFM (Figure S5), XPS, Raman spectroscopy and PL (see below) all indicate that graphite was fully exfoliated into GO during Hummer’s treatment. Reduction of GO by hydrazine led to a black RGO powder which is insoluble in isooctane. However, in the presence of poly(CEM11-b-EHA7) (structure shown in Figure 1B), RGO becomes dispersible in isooctane as supramolecular interactions are formed between graphene leaflets and cholesterol units (adsorption energy = 62 kJ/mol).24 In the DLPE experiment, RGO powder is submerged by first a layer of water and then by a layer of isooctane containing poly(CEM11-b-EHA7). After gently shaking this biphasic mixture on an orbital shaker, a portion of the solid RGO is dispersed in the isooctane layer (DLPE RGO) while RGO aggregates (PPT RGO) remain in the aqueous phase and at the water/isooctane interface (Figure 1C). The DLPE RGO dispersion is stable in the colloidal sense in isooctane (as shown in Figure 1D), and its concentration depends on experimental conditions such as shaking time, temperature, aqueous pH and polymer concentration. At near neutral pH (~ 6), the concentration of DLPE RGO decreases when temperature increases (Figure 2A). Such behavior can be explained by the fact that the driving force for transferring RGO from water to isooctane is the supramolecular adsorption of the polymer on the graphene surface. Higher temperatures promote polymer desorption rather than adsorption (due to entropic considerations), leading to low concentrations of RGO in isooctane. The pH of the aqueous phase (Figure 2B) also influences the outcome of the DLPE, with near neutral pH


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leading to a greater amount of RGO transferred to the organic phase. Highly acidic and basic pHs lead to the formation of positively and negatively charged groups at the surface of RGO (via protonation or deprotonation), which tend to render RGO more hydrophilic and less prone to be dispersed in an organic solvent. The polymer concentration in isooctane is also a key parameter of the DLPE process (Figure 2C). The amount of RGO dispersed in the organic phase is maximal when the polymer concentration reaches 0.25 mg.mL-1. Surprisingly, higher polymer concentrations lead to lower RGO amounts. A possible explanation is that at high polymer concentration (> 0.25 mg.mL-1), depletion flocculation occurs.23 Depletion flocculation can be observed when a polymer is dissolved in a colloidal suspension (here graphene leaflets suspended in isooctane). Under such conditions, the aggregation or flocculation of graphene releases solvent molecules which can be used to dilute the polymer chains, resulting in a favorable entropy gain.30,31 Thus, it is necessary to limit the amount of polymer in solution in order to maintain the colloidal stabilization of RGO. This preliminary study has established that neutral pH, ambient temperature and a polymer concentration of 0.25 mg.mL-1 in isooctane constitute the optimal operational parameters for the DLPE of RGO when starting from a raw RGO concentration of 0.5 mg.mL-1. The concentration of RGO dispersed in isooctane can be evaluated by measuring the optical absorption at 600 nm and using an extinction coefficient of 24.60 mL mg-1 cm-1.19,23 Thus, under optimal DLPE conditions which are the only ones used for the rest of this study, the concentration of DLPE RGO in isooctane reaches ~ 0.11 mg.mL-1 after 24 hours, indicating that approximately 20% of the RGO has been extracted (DLPE RGO) and 80% remains in water under the form of PPT RGO. The DLPE RGO remains in isooctane solution for days (Figure 1D), as it is stabilized by the polymer. Once dried, excess polymer, that is to say polymer which is not adsorbed at the surface of the flakes, can be partially removed


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by rinsing with hexane. As the supernatant remains clear, this rinsing step does not remove graphene.


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Figure 2: Concentration of RGO found in isooctane after 24 hours of the DLPE process (A) vs extraction temperature (pH = 6 and c = 0.25 mg.mL-1). (B) vs pH (room temperature and c = 0.25 mg.mL-1) (C) vs poly(CEM11-b-EHA7) concentration (room temperature and pH = 6). Red dots indicate experimental conditions used for the rest of this work. Raman spectroscopy is an effective technique to evaluate the electronic structure of RGO flakes. In a typical sample, the Raman spectrum shows a G peak at 1580 cm-1, corresponding to the E2g vibration mode of extended sp2 conjugated systems, and a D peak at 1350 cm-1, corresponding to the A1g vibration mode of sp2 carbons adjacent to defects. The ratio of the intensities of the D and G bands, ID/IG, is commonly taken as an indicator of the quality or graphitization degree of graphene.32–34 As shown in Figure 3A, the GO and as-obtained RGO had an ID/IG value of 1.15 and 1.3 respectively, which are characteristic values for GO and for RGO prepared by the Hummer process followed by reduction with hydrazine.35 Remarkably, after the DLPE experiment, the RGO dispersed in isooctane has a ID/IG value of 0.2, whereas PPT RGO had an ID/IG value of 1.2 which is similar to the value for the as-obtained RGO. Thus, during the DLPE experiment, the RGO leaflets with the lowest number of defects (low ID/IG value) are extracted in isooctane whereas all other leaflets (PPT RGO) remain in the aqueous phase. Further information about the quality of the RGO leaflets can be obtained by scrutinizing the 2D and 2D’ bands in the Raman spectra (Figure 3A). The 2D’ band is indicative of oxygenated defects in the RGO sample.36,37 As observed in Figure 3A, the DLPE RGO is devoid of 2D’ band whereas GO, asprepared RGO and PPT RGO all exhibit such a band. For the DLPE sample, the 2D band at 2700 cm-1 can be deconvoluted into four Lorentzian components referred as 2D1A, 2D2A, 2D1B, 2D2B, which can be used to assess the number of graphene layers in the RGO bunddle.32,38 The


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prominence of the 2D1B Lorentzian is indicative of bundles containing one or two graphene leaflets.

Figure 3: (A) Raman spectra and ID/IG for GO, raw RGO, DLPE RGO and PPT RGO samples. (B) Deconvolution the 2D band for DLPE RGO.

The high quality of the DLPE RGO was also confirmed by XPS, as shown in Figure 4A. The C 1s signals of the GO can be deconvoluted into five peaks at 284.5, 285.3, 286.4, 287.4 and 289.0 eV which are respectively attributed to C=C, C-C, C-O, C=O and O-C=O bonds.39,40 In the


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DPLE RGO sample, peaks corresponding to C-O, C=O and O-C=O bonds has near completely disappeared and only the C-C and C=C peaks remain in 1:1 proportions, which is coherent with a pure graphene structure. Figure 4B, shows the PL spectra for GO, raw RGO and DLPE RGO. These spectra were deconvoluted into two Gaussian-like peaks, labeled as IP1 and IP2, centered at 525 nm and 490 nm which are characteristic of leaflets respectively rich and poor in oxygenated moieties.41 The PL spectrum of GO is constituted of a main peak, Ip1, indicating that the presence of oxygenated groups is predominant. By contrast, the PL spectrum of raw RGO shows both Ip1 and Ip2 peaks, in a 1.9:1 intensity ratio. Thus, RGO contains a mixture of domains which are rich and poor in oxygenated groups, and, the PL intensity of the oxygen rich domains (Ip1) is more intense. For the DLPE RGO, the Ip2 peak (Ip1/Ip2 = 0.5) which indicates that the leaflets with the lowest amounts of oxygenated defects have been sorted in this process. Fourier transform infrared spectroscopy (FTIR, Figure S3) and X-ray powder diffraction (XRD, Figure S4) also confirm that high-quality RGO leaflets are selected in the DLPE process. To conclude on this section, Raman spectroscopy, XPS, PL, FTIR and XRD all indicate that high quality graphene is separated from RGO during the DLPE process.


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Figure 4: (A) C 1s XPS, and (B) PL spectra of GO, raw RGO and DLPE RGO

The results presented in Figure 3 and 4 are for DLPE RGO extracted after 24 hours. As shown in Figure 1D, the amount of DLPE RGO increases with time, to reach ~ 0.11 mg.mL-1 after 24 hours, corresponding to 20% extraction. The quality of the DLPE RGO extracted at various times has been assessed by Raman spectroscopy (Figure S1) and by PL (Figure S2). The quality of DLPE RGO increases when the duration of the DLPE experiment is increased. For example, after 2 hours, the ID/IG ratio of DLPE is 0.8, whereas it is 0.2 after 24 hours (after 24 hours, prolonging the experiment does not result in further improvement). When the DLPE experiment is started, RGO is under the form of an aggregated powder. The poly(CEM11-bEHA7) dispersant is not water soluble.23 RGO leaflets, either under the form a single layer or of


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bundles, are transferred to the organic solvent due to the favorable supramolecular interactions with the cholesterol containing polymer.23 The first leaflets which are dispersed in water are those which contain more oxygenated groups, as they are intrinsically more polar. Thus, at early times, these leaflets are also found in the organic phase, explaining the poor quality of the DLPE RGO after a two-hour experiment. Over time, more hydrophobic RGO leaflets are extracted from the powder. These leaflets have a lower concentration of oxygenated defects, the affinity of the poly(CEM11-b-EHA7) for these leaflets is greater, explaining why these leaflets are eventually found in isooctane after several hours. Since low-quality leaflets are found initially in the isooctane layer, but not found at later times, it indicates that they have migrated back to the aqueous layer. Therefore, the 24-hour duration corresponds to the time necessary to reach a partitioning equilibrium between both layers.

The morphology of the DLPE RGO deposited on mica was also analyzed by AFM (Figure 5 A), indicating that the size of the flakes is ~ 500 nm and their thickness is ~1.1 nm. The thickness of single-layer graphene on mica has been reported to be within 0.7 to 1 nm.42,43 As reported by us during the study of the direct exfoliation of graphene in isooctane,23 the 1.1 nm thickness corresponds well to a single layer of graphene covered by a monolayer of

the

poly(CEM11-b-EHA7) polymer. Zooming on the leaflets (Figure 5B) shows the presence of dips which can be explained by the presence of drops of solvent which were trapped under the RGO surface and which have burst during sample drying.42,43


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Figure 5: (A) AFM image of DLPE RGO flakes transferred on mica via Langmuir-Blodgett technique (B) zoom on one DLPE RGO flake (C) z-profile indicated by the white line in (B)

As the DLPE RGO contains a lower proportion of oxygenated defects, its physical properties also differ from those of raw RGO. For example, its thermal stability, as measured by TGA (Figure 6), is significantly enhanced. The TGA curve of DLPE RGO shows a 25% weight loss at 250 -370 ºC, which corresponds to the decomposition of the adsorbed cholesterol polymer, as shown by comparison with the TGA curve of the polymer alone. The rest of the DLPE RGO (~75%) remains stable to temperatures up to 655 ºC. By contrast, the raw RGO and PPT RGO decompose at temperatures of 460 ºC. Such weight loss is ascribed to the removal of oxygencontaining groups e.g. OH, epoxide and COOH etc., evaporation of H-bonded water molecules and the decomposition of some unstable or small structures formed through the oxidation and reduction process.69–72 At 600 oC, more than 80% of the as-prepared RGO is decomposed, whereas more than 50% of the DLPE RGO remains intact (the exact quantification being


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complicated by the presence of adsorbed polymer). Thus, the thermal stability of the DLPE RGO is greatly enhanced.

100 250 °C

460 °C

80 Weight loss (%)

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


60 40 20 0

RGO DLPE RGO PPT DLPE Polymer 150

300

585 °C 655 °C

450 600 Temp. (°C)

750

900

Figure 6: TGA curves (10 oC/min under air) of GO, of poly(CEM11-b-EHA7), of RGO, of DLPE RGO and of PPT RGO

As the DLPE RGO is of higher quality, i.e. the extent of its π-conjugation is greater, it also exhibits a greater electrical conductivity. Thus, the sheet resistance, Rs, of films of raw RGO and of DLPE RGO was measured. Both films had the same thickness, 6 µm, as measured by profilometry. The value of Rs decreased from 27 kΩ/□ for raw RGO to 1.2 kΩ/□ for DLPE RGO, which translates into conductivities of respectively 6 and 140 S/m. Overall, DLPE RGO shows s > 10 times enhancement on electrical performance. This improvement is attributed to the presence of an extended sp2 network free of oxygenated defects which act as traps for the circulating holes and electrons and which also prevent inter-leaflet electronic transfer. Despite its low value, the resistance of the DLPE RGO film may be overestimated because the sample contains some poly(CEM11-b-EHA7), and one could expect an even lower value for a sample free of polymer.


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CONCLUSIONS In this work, a DLPE process was used to extract the highest quality leaflets from an untreated sample of RGO. Approximately 20 wt% of the RGO was extracted, and the extracted portion was found to be constituted of monolayers of graphene of ~ 500 nm size which have a remarkably low ID/IG of 0.2, as measured by Raman spectroscopy. These leaflets exhibited a conductivity which was >10 greater than the conductivity of the as-prepared RGO before extraction. The very high quality of the graphene obtained by this process was further confirmed by XPS, PL, TGA and XRD measurements. This work first outlines the extraordinary heterogeneity of RGO and the need for discovering novel purification procedures. Furthermore, it points out that DLPE is an efficient purification procedure, as the higher quality RGO flakes are more hydrophobic and therefore show a greater affinity for hydrophobic solvents such as alkanes, leaving the lower quality RGO in water. Because the DLPE technique is simple and easily scalable, our results are promising for the mass-scale formulation of RGO-based inks with applications in printable electronics.

ASSOCIATED CONTENT SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website


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Raman and PL spectra of DLPE RGO extracted every 2 hours, FTIR spectra and XRD diffractograms for graphite, GO, raw RGO and DLPE RGO, AFM images of GO prepared by Hummer’s method.

This material is available free of charge via the Internet at

http://pubs.acs.org

AUTHOR INFORMATION Corresponding Authors * Jerome P. Claverie: E-mail: [email protected] * Mohamed Siaj: E-mail: [email protected] ORCID Mohamed Siaj: 0000-0003-0499-4260 Jerome Claverie: 0000-0001-7363-1186 Author Contributions A.A.S. performed all the experiments. A.A.S., M.S. and J.P.C. interpreted the data and cowrote the manuscript Notes The authors declare no competing financial interest ACKNOWLEDGMENTS


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We thank Dr G. Chamoulaud, J. Nguendia, W. Zhong, Pr. S. G. Cloutier and C.Trudeau for assistance and technical help. M.S and J.P.C. acknowledge funding from the Canada Research Chair, the Canadian Foundation for Innovation, NanoQuébec (major infrastructure program), and the National Science and Engineering Research Council (discovery program) for support.

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