Carbon Papers and Aerogels Based on Graphene Layers and

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Carbon papers and aerogels based on graphene layers and chitosan: direct preparation from high surface area graphite Vincenzina Barbera, Silvia Guerra, Luigi Brambilla, Mario Maggio, Andrea Serafini, Lucia Conzatti, Alessandra Vitale, and Maurizio Galimberti Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b01026 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Carbon papers and aerogels based on graphene layers and chitosan: direct preparation from high surface area graphite Vincenzina Barbera,*,† Silvia Guerra,† Luigi Brambilla,† Mario Maggio,‡ Andrea Serafini,† Lucia Conzatti,⊥ Alessandra Vitale,§ and Maurizio Galimberti*,† †

Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “G.

Natta”, Via Mancinelli 7, 20131 Milano, Italy ‡

Università degli Studi di Salerno, Department of Chemistry and Biology, Via Giovanni Paolo II

132, 84084 Fisciano (SA), Italy ⊥

Institute for the Study of Macromolecules, National Council of Research, Via De Marini 6,

16149 Genova, Italy §

Politecnico di Torino, Department of Applied Science and Technology, Corso Duca degli

Abruzzi 24, 10129 Torino, Italy

KEYWORDS: chitosan, graphene, carbon paper, monolithic aerogel.

ACS Paragon Plus Environment

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In this work, carbon papers and aerogels based on graphene layers and chitosan were prepared. They were obtained by mixing chitosan (CS) and a high surface area nanosized graphite (HSAG) in water in the presence of acetic acid. HSAG/CS water dispersions were stable for months. High resolution transmission electron microscopy revealed the presence of few graphene layers in water suspensions. Casting or lyophilization of such suspensions led to the preparation of carbon paper and aerogel respectively. In X-ray spectra of both aerogels and carbon paper, peaks due to regular stacks of graphene layers were not detected: graphene with unaltered sp2 structure was obtained directly from graphite without the use of any chemical reaction. The composites were demonstrated to be electrically conductive, thanks to the graphene. Chitosan thus makes it possible to obtain monolithic carbon aerogels and flexible and free standing graphene papers directly from a nanosized graphite, avoiding oxidation to graphite oxide and successive reduction. Strong interaction between polycationic chitosan and the aromatic substrate appears to be at the origin of the stability of HSAG/CS adducts. Cation-π interaction is hypothesized, also on the basis of XPS findings. This work paves the way for the easy large-scale preparation of carbon papers through a method that has a low environmental impact and is based on a biosourced polymer, graphene and water.


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1. INTRODUCTION Graphene1 is the thinnest material on earth,2 with very high aspect ratio and specific surface area,3 and is endowed with outstanding chemical, electronic, mechanical and thermal properties.4 Research has been particularly aimed at applications in nanoelectronics5 and at the preparation of high-speed transistors,6 sensors and devices for energy storage and conversion.7 Over the last years, increasing attention has been focused on graphene composites, as they can bring the exceptional graphene properties to the macroscopic scale, allowing the preparation of high performance materials. Graphene papers can find a wide variety of applications, such as electrochemical energy storage devices,8 catalyst supports and fuel cells,9 sensors and actuators,10 chemical filters and membranes,11 structural composites.12 Objectives for graphene papers are lightness, flexibility, robustness, conductivity as well as a preparation method characterized by a low environmental impact. Carbon aerogels are high surface area materials with great versatility.13-19 At the nanoscale, the pore structure can be controlled, building hierarchiral porosities. At the macroscale, they can be shaped as powders, thin-film composites, microspheres, monoliths. They have been studied as electrodes for double layer capacitors, pseudocapacitors, and capacitive deionization units,19 as adsorbents for the desulfurization of liquid hydrocarbon fuels for fuel cell applications,18 for catalysis applications,17 for hydrogen and electrical energy storage,13,16 to exploit their electrical conductivity,14 for the absorption of different organic liquids,15 applying then the absorption–squeezing process to collect oil. Graphene and graphene oxide are actively studied

13,14,16

for the preparation of carbon aerogels. Typical

methods for the preparation of graphene composites involve the oxidation of graphite or graphitic nanofillers, made by few layers of graphene, to graphene oxide (GO).20 Graphene papers and aerogels can then be obtained by performing the reduction of GO papers21 or starting


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from chemically reduced graphene oxide (known as CRGO).22 Both graphene and GO23 papers are finding increasing interest, mainly for their antibacterial properties and as parts of Li ion batteries.24 The preparation of GO involves harsh reaction conditions and, often, toxic reagents. The precise structure of GO is still unknown.25 It was reported26 that hydroxyl and epoxide groups are on the surface of basal planes and carbonyl and carboxyl groups are on the edges. However, it is widely acknowledged that oxidation leads to extensive modification of the graphene layers: sp2 hybridization of graphene is heavily disrupted and, consequently, its electrical properties are drastically damaged. In order to obtain graphene with target properties, the reduction step is thus mandatory. The aim of this work was the preparation of carbon papers and aerogels based on graphene layers with unaltered bulk structure. A particular objective was also to use biosourced materials such as chitosan for their preparation. A nanosized graphite with very high surface area (HSAG), higher than 300 m2 g-1, was used. HSAG was reported27 to have a low number of layers (about 35) stacked in the direction orthogonal to graphene planes and a high shape anisotropy, defined as the ratio between the crystallites size in directions parallel and orthogonal to the layers. Such HSAG has been so far successfully used for the preparation of nanocomposites with polymers from renewable sources, such as natural rubber28 and poly-L-lactide (PLLA).29 In this work, chitosan (CS) was selected as the biosourced material. Because of its good biocompatibility, biodegradability, and multiple functional groups, chitosan has attracted significant interest in a broad range of applications such as water treatment, separation membrane, food package, tissue engineering, and drug delivery.30 Nanocomposites based on chitosan and nanosized carbon allotropes have been reported in the scientific literature. In


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particular, carbon nanotubes (CNTs) were combined with chitosan to improve its mechanical properties31 and for applications such as electrochemical sensing,32,33 in particular DNA34 and enzyme35 biosensing, and scaffolds for bone tissue engineering.36-42 Reviews on this topic are present in the literature.43,44 A strong interaction between chitosan and CNTs was reported; it could be achieved either by carboxylic and/or hydroxyl groups on CNTs, or by relying on the morphology of the chitosan chains, wrapped around the tubes. Nanocomposites based on chitosan and graphene related materials were also prepared, for their application in ultrasensitive detection of human epidermal growth,45 bone tissue engineering,46 and large-scale preparation of highly compatible membranes.47 In these cases, graphene oxide or reduced graphene oxide were used. A challenging objective of this work was to achieve a strong interaction between chitosan and graphene layers without performing any chemical modification, either on HSAG or on chitosan, thus avoiding the oxidation reaction of the carbon allotrope (preparation and successive reduction of GO), adopting a simple and environmentally friendly preparation method. HSAG adducts with chitosan were prepared, exploring different HSAG/CS ratios, by simply mixing HSAG and chitosan in a mortar-pestle, preparing then a water suspension with the help of acetic acid. Water suspensions were studied by UV spectroscopy and their stability was investigated by means of centrifugation. HSAG/CS paper and aerogel were then prepared by casting water suspensions on a glass support or by lyophilization, respectively. Wide angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to characterize the HSAG/CS adducts.


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Thermal stability of the composites was studied by means of thermogravimetric analysis (TGA) and their direct electrical conductivity with the four-point probe method.

2. EXPERIMENTAL SECTION 2.1 Materials Reagents and solvents commercially available were purchased and used without further purification. Chitosan (high purity, Mw 110.000-150.000; degree of acetylation: ≤40 mol. %), acetic acid, n-hexane and dimethylformamide (DMF) were from Aldrich. High surface area graphite (Synthetic Graphite 8427®) was purchased from Asbury Graphite Mills Inc., with a minimum carbon mass % of 99.5 and a surface area of 330 m2 g-1. 2.2.

Structural characterization of the graphitic starting material

The selection of pristine graphite was performed after WAXD characterization of many disordered graphitic samples. HSAG was selected as it was shown to have high crystalline order inside the structural layers and a turbostratic structure, with a relatively low number of stacked layers (about 35) in crystalline domains and a high shape anisotropy (3.1).27 The lateral size of the graphitic layers was shown in 2D TEM images to be about 300 nm.48 In the technical data sheet of the compound, the carbon content and the surface area are reported to be at least 99 wt% and 330 m2 g-1, respectively. Elemental analysis was performed with a Thermo FlashEA 1112 Series CHNS-O analyzer, after pretreating samples in an oven at 100 °C for 12 h. Chemical composition, determined by elemental analysis, was: carbon 99.5 wt%, hydrogen 0.4 wt%, nitrogen 0.1 wt%, oxygen 0.0 wt%. BET surface area was determined by applying ASTM D6556 method and was found to be 330.3 m2 g-1.


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2.3.

Preparation of chitosan based nanocomposites

2.3.1. Water suspensions of HSAG/CS HSAG (0.4 g) and chitosan (0.4 g) were mixed for 5 minutes in a mortar with the help of a pestle. The mixture was dispersed in water (8 mL) and 4 drops of an aqueous solution of acetic acid 99.7% (0.010 g, 9.9x10-3 mol) were added, obtaining a homogenous suspension. Acetic acid was used in such an amount to lead to the protonation of about 7% of chitosan amino groups. Stable suspensions were also obtained using different amount of water (20, 30, 50 and 70 mL) and different ratios of chitosan and graphite (1:1, 1:2, 1:4, 1:6). 2.3.2 Hydrogel HSAG (0.4 g) and chitosan (0.4 g) were mixed for 5 minutes in a mortar with the help of a pestle. 4 drops of an aqueous solution of acetic acid 99.7% (0.010 g) and 10 mL of water were added to the mixture. The homogenous suspension obtained was poured into a glass vial. Hydrogel was formed leaving the mixture at 25°C for 1 hour. Hydrogels were also obtained using different amount of water: 20, 30, 50 and 70 mL. The equilibrium swelling was determined by a gravimetric method. The sample was immersed into water in a Petri dish at room temperature for a predetermined time (1 h), and then poured into a pre-weighed wet tea bag (200 mm × 100 mm). Thanks to gravity, the excess of water was allowed to drip off the sample, which was then weighed. 2.3.3 Chitosan carbon aerogel The hydrogel obtained as reported in 2.3.2 was cooled at -30°C and then lyophilized (EDWARDS MODULYO EF4-1596) using the following conditions: T=-50°C, P=5 mbar, lyophilization time t=12 h. Nominal composition of HSAG/CS aerogels was 1:1 by mass. Aerogel showed a bulk density (i.e., volumetric mass density of the substance, expressed in mass


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per unit volume) of about 0.026 g cm-3. The total volume of a pre-weighed aerogel sample was determined by placing the aerogel in a cylinder with a known volume, and then adding small beads to fill the volume. The beads total mass and packing density was also known. By subtracting the volume of the beads, the aerogel volume was ascertained. 2.3.4 Carbon paper based on HSAG and CS Casting of HSAG/CS suspensions, obtained as reported in 2.3.1, was performed on a glass plate in which an adhesive tape was used to delimit the area. Sheets were formed after water evaporation, at room temperature and at atmospheric pressure (24 hours). 2.4.

Characterization of HSAG/CS composites

2.4.1 UV-vis spectroscopy HSAG/CS suspensions (3 mL) were placed by pipette Pasteur in quartz cuvettes of 1 cm optical path (volume 1 or 3 mL) and analyzed by using a Hewlett Packard 8452A Diode Array Spectrophotometer. Pure water was used as blank. In the UV-vis spectrum, the absorption intensity was reported as a function of the wavelength of the radiation between 200 and 850 nm. 2.4.2. Thermogravimetric analysis TGA tests were performed under flowing N2 (60 mL/min) with a Mettler TGA SDTA/851 instrument according to the standard method ISO9924-1. Samples (10 mg) were heated from 30°C to 300°C at 10°C/min, kept at 300°C for 10 min, and then heated up to 550°C at 20°C/min. After being maintained at 550°C for 15 min, they were further heated up to 900°C and kept at 900°C for 30 min under flowing air (60 mL/min). Measurements were also performed under flowing air, for temperatures higher than 800°C. Before every TGA analysis, the sample was stored for 16 hours at 80°C. 2.4.3. Elemental analysis


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Elemental analysis was performed with a Thermo FlashEA 1112 Series CHNS-O analyzer, after pretreating samples in an oven at 100 °C for 12 h. 2.4.4. Infrared spectroscopy FT-IR absorption spectra were recorded in transmission mode using a diamond anvil cell (DAC) coupled with a ThermoElectron FT-IR Continuµm IR microscope (resolution: 4 cm-1; scans: 128). 2.4.5. Raman spectroscopy Raman spectra of powder samples were recorded with a Horiba JobinYvon Labram HR800 dispersive Raman spectrometer equipped with an Olympus BX41 microscope and a 50X objective (resolution: 2 cm-1; acquisition time: 30 seconds and 4 accumulation). The excitation line at 514.5 nm of an Ar+ laser was kept at 0.5 mW in order to prevent possible photo induced thermal degradation of the samples. Each Raman spectrum reported was obtained as average of four spectra recorded in different points of the sample. 2.4.6. Wide angle X-ray diffraction WAXD patterns were obtained in reflection, with an automatic Bruker D8 Advance diffractometer, with nickel filtered Cu–Kα radiation. Patterns were recorded in 4° – 80° as the 2θ range, being 2θ the peak diffraction angle. Distance between crystallographic planes of HSAG was calculated from the Bragg law. The Dhkℓ correlation length, in the direction perpendicular to the hkl crystal graphitic planes, was determined applying the Scherrer equation: Dhkℓ = K λ / (βhkℓ cosθhkℓ)

(1)

where K is the Scherrer constant, λ is the wavelength of the irradiating beam (1.5419 Å, Cu-Kα), βhkℓ is the width at half height, and θhkℓ is the diffraction angle. The instrumental broadening, b, was determined by obtaining a WAXD pattern of a standard silicon powder 325 mesh (99%),


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under the same experimental conditions. The width at half height βhkℓ = (Bhkℓ – b) was corrected, for each observed reflection with βhkℓ< 1°, by subtracting the instrumental broadening of the closest silicon reflection from the experimental width at half height, Bhkℓ. 2.4.7. Conductivity measurements: four-point probe method Direct current electrical conductivity (σ) was measured by the four-point probe (FPP) method49 by using a hand applied FPP device (Jandel Engineering Ltd.) with a probe head with line arrayed tungsten carbide needles (tip radii 300µm, needles spacing 635µm, loads 60 g) coupled with a Keithley 2601 electrometer. Data were acquired and analyzed by CSM/Win Semiconductor Analysis Program software (MDC). 2.4.8 High-resolution transmission electron microscopy HRTEM investigations on HSAG/CS samples were carried out with a Philips CM 200 field emission gun microscope operating at an accelerating voltage of 200 kV. Few drops of the water suspensions were deposited on 200 mesh lacey carbon-coated copper grid and air-dried for several hours before analysis. During acquisition of HRTEM images, the samples did not undergo structural transformation. Low beam current densities and short acquisition times were adopted. To estimate the number of stacked graphene layers and the dimensions of the stacks visible in HRTEM micrographs, Gatan Digital Micrograph software was used. 2.4.9. Scanning electron microscopy The thickness and the morphology of the external lateral surface of carbon papers (before and after folding) were characterized by means of a SEM instrument (Cambridge Stereoscan 360) operating at 20 kV. Before imaging, all the specimens were gold coated (approximately 10 nm thick gold coating) using a sputtering system. The coating procedure was necessary in order to prevent surface charging during the measurement and to increase the image resolution.


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2.4.10. Bending experiments Bend testing for HSAG/CS paper was carried out using a universal caliper until failure took place. A typical bending experiment on HSAG/CS paper was performed as follows: a specimen of carbon paper (2 cm2) was first cut using a cutter, folded up through a caliper, and then different pressures (from 1 to 5 bar) were applied until the paper was completely folded on itself. 2.4.11. Solvent resistance Solvent resistance test were performed pouring HSAG/CS adducts (1 cm2, thickness=50 nm for the paper samples; 1 cm3 for the aerogel samples) in 3 vials containing 2 mL of water, hexane and dimethyl formamide respectively. Vials were left for 2 months to test the solvent stability. Swelling of specimens was not observed by weighting the sample before and after the treatment. 2.4.12. pH resistance pH resistance tests were performed pouring HSAG/CS adducts (1 cm2, thickness=50 nm for the paper samples; 1 cm3 for the aerogel samples) in 5 vials containing 2 mL of water solutions at different pH. Vials were left for 2 months to test the solvent stability. Acids and bases used were: HCl (38%), CH3COOH (99.85%), NaHCO3, KOH. Swelling of specimens was observed only at pH from 1 to 4 by weighting the sample before and after the treatment. The equilibrium swelling was determined as reported for hydrogel. 2.4.13 High resolution X-ray Photoelectron Spectroscopy PHI 5000 Versa Probe instrument (Physical Electronics) was utilized for survey scan and high resolution XPS analyses. The samples were placed in a pre-chamber overnight, in order to avoid anomalous outgassing during the XPS characterization, performed in UHV condition (10-8 Pa). A monochromatic Al K-alpha X-ray source (1486.6 eV, 15 kV voltage and 1 mA anode current) and a power of 25.2 W were used for analysis. Different pass energy values were exploited:


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187.85 eV for survey spectra and 23.5 eV for high resolution peaks. Analyses were carried out with a take-off angle of 45° and with a 100 µm diameter X-ray spot size on a square area of 1400×1400 µm2, with the aim to have a good average and better statistics of sample behavior. A double beam (electron and argon ion gun) neutralization system, dedicated to reduce the charging effect on samples, was also employed during data acquisition. All binding energies (B.E.) were referenced to the C1s line at 284.8 eV. Multipak 9.6 software was used for spectra analysis and peak deconvolution.

3. RESULTS AND DISCUSSION Adducts of HSAG with chitosan were prepared as described in the experimental part and summarized in Figure 1. In brief, HSAG and chitosan were first premixed in a mortar with the help of a pestle. Suspensions were prepared by introducing the HSAG/CS mixture in a water solution of acetic acid. Acetic acid was selected in this work, as in many papers reporting the interaction of chitosan with CNTs,31,33-42 acetic acid was used to promote the solubility of chitosan in water, because it is known to be more efficient for this purpose than HCl.33

Figure 1. Block diagram for the preparation of HSAG/CS adducts. Different HSAG/CS mass ratios were used: 1:1, 2:1, 4:1 and 6:1. HSAG and chitosan are easily available and the experimental procedure followed is very simple, allowing the facile preparation of large amounts of suspensions. Contrary to HSAG water dispersions, the water dispersions of HSAG/CS mixture were observed to be stable for months (Figure 2). The suspension stability


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was tested for all the HSAG/CS ratios prepared, and for concentrations up to 5 mg/mL of HSAG/CS. Figure 2 shows water dispersions of HSAG (Figure 2a) and HSAG/CS with 1:1 as mass ratio and 1 mg/mL as HSAG/CS content, after 1 month storage (Figure 2b) and after 30 min centrifugation at 9000 rpm (Figure 2c).

Figure 2. Water dispersions of HSAG (a), HSAG/CS after 1 month storage (b), and after 30 min centrifugation at 9000 rpm (c). HSAG/CS were in 1:1 ratio and the concentration was 1 mg/mL.

Water dispersions of HSAG/CS 1:1 were analyzed by UV-vis spectroscopy. Figure 3 reports the UV-vis absorbance, as a function of wavelength, for dispersions with HSAG/CS concentration between 0.55 and 2.50 mg/mL. Spectra in Figure 3 show that, with the increasing of the HSAG/CS concentration, the absorbance monotonically raises and the absorption peak of the HSAG/CS dispersions red-shifts from 296 nm to 312 nm. In the scientific literature,50 UV-vis spectroscopy has been reported for stable aqueous colloids formed with chemically converted graphene sheets. Graphite was oxidized to graphite oxide and colloids were obtained through controlled deoxygenation by hydrazine reduction. Hydrazine


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reduction led to the gradual redshifts of the absorption peak of GO dispersion, from 270 nm to 231 nm. Red shift was attributed to the restored electronic conjugation within the graphene sheets. In the present work, UV-vis spectroscopy was performed on HSAG aqueous suspensions, in the absence and in the presence of a cationic species, as described in detail in the Supporting Information (Table S1): acid and dimethyl ditallow ammonium chloride (2HTCl) were used as the cationic species. The addition of each of them led to blueshift of the absorption peak. The available experiments suggest that the electronic perturbation of graphene sheets, extensive as in the case of GO, or confined to the surface as in the case of the interaction with cations, lead, to different extent, to blue shift of UV absorption peak. Redshift observed by diluting the suspension of chitosan/HSAG adduct could be thus interpreted with the reduction of interaction of chitosan with HSAG.

Figure 3. UV-vis absorbance of HSAG/CS dispersions in water/acetic acid solutions as a function of wavelength. HSAG/CS concentration (mg/mL) was: 2.50 (a), 1.60 (b), 1.25 (c), 0.71 (d), and 0.55 (e).


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To analyze the morphology of the HSAG/CS adducts, HRTEM was performed (Figure 4) on HSAG/CS mixtures (Figure 4a and 4b), HSAG/CS water suspensions (Figure 4c and 4d) and HSAG/CS supernatant suspensions after centrifugation for 30 minutes at 9000 rpm (Figure 4e and 4f). The lateral size of all HSAG/CS adducts is similar, indicating that the mixing step does not cause an appreciable breaking of the graphitic layers. As it can be seen in Figure 4e, the largest size of the graphitic layers obtained is about 300 nm, in agreement with the values reported in the literature.30 As this micrograph was taken on a HSAG/CS supernatant suspension, one could conclude that the graphitic layers with the largest size are able to remain in the supernatant suspension also after centrifugation. In Figure 4c and 4e, it can be seen that chitosan is adhered to the HSAG surface and even to the carbon grid (chitosan layer is indicated by an arrow in Figure 4c). Micrographs at high magnification (Figures 4b, 4d and 4f) allow to analyze the stacks of graphene layers (indicated in the boxes), when they are disposed perpendicularly to the beam, and to estimate the number of layers forming these nanosized stacks. In Figure 4b it can be seen that the stacks are mostly made by about 25-30 graphene layers, in Figure 4d the sample reveals stacks made by about 10-12 layers, while the stacks in Figure 4f (sample isolated after centrifugation) are made by 4-6 graphene layers. These results, in line with the findings obtained by WAXD analysis (discussed below in the text), indicate the prevailing of a disordered stacking of graphene layers and suggest that the combination of chitosan and nanosized graphite will lead to the preparation of carbon papers made by few layer graphene.


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Figure. 4 Micrographs of HSAG/CS samples taken from the mortar (a, b), from the water suspensions (c, d) and isolated after centrifugation at 9000 rpm for 30 min (e, f). Micrographs were taken with low magnification bright field TEM (a, c, e) and with HRTEM (b, d, f).

Stable water suspensions of few graphene layers were then used to obtain carbon papers and aerogels. For the preparation of carbon papers, HSAG/CS suspensions were poured on a glass plate and water evaporation was allowed, as reported in the experimental part. Whereas, monolithic aerogels of HSAG with chitosan were prepared as described in the experimental part and summarized in Figure 5. In brief, a stable HSAG/CS hydrogel was prepared adding a water solution of acetic acid to the graphitic mixture, then the hydrogel was subsequently frozen and lyophilized.

Figure 5. Block diagram for the preparation of HSAG/CS aerogels.

Structural characterization of HSAG/CS papers and aerogels was performed through TGA, FTIR, XPS, Raman spectroscopy, WAXD and SEM. TGA was carried out on HSAG/CS papers (1:1, 2:1, 4:1 as mass ratio) as well as on HSAG and chitosan. Thermographs of the analyses performed under nitrogen are shown in Figure 6. HSAG appears to be stable until 800°C and mass losses due to water removal cannot be detected. chitosan shows three main decomposition steps: the first occurs in the range from 50°C to 100°C, reasonably due to water release, with a mass loss of about 5%, while the decompositions


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due to oxygen and nitrogen containing functional groups are in the range from 250°C to 450°C.51 The thermographs of HSAG/CS papers have a trend similar to that of chitosan, with mass losses from 50°C to 100°C and from 250°C to 450°C. The TGA trace of HSAG/CS (1:1) paper, after soaking in DMF (reported in Figure 6B), do not show any indication of DMF absorption.

Figure 6. (A) TGA curves (under nitrogen, until 900°C) of HSAG (a), HSAG/CS papers with 4:1 (b), 2:1 (c) and 1:1 (d) mass ratio, and chitosan (e); (B) TGA trace of HSAG/CS paper 1:1 after soaking in DMF as solvent.

The FT-IR spectra of HSAG, chitosan, HSAG/CS 1:1 paper, HSAG/CS 1:1 aerogel and HSAG/CS 1:1 physical mixture are reported in Figure 7. In particular, Figure 7a shows the spectra recorded in the region 4000 cm-1 - 700 cm-1, while in Figure 7b are displayed the spectra in the fingerprint region, after baseline correction, to allow an easier comparison. The spectrum of HSAG (i in Figure 7a) is characterized by the feature near 1590 cm-1 that can be assigned to the absorption of E1u IR active mode of collective C=C stretching vibration of


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graphene layers. The increasing background toward high wavenumbers is due to diffusion/reflection phenomena of the IR beam passing through HSAG micro-particles. The spectrum of pure chitosan (v in Figure 7a) shows signals characteristic of chitin and deacetylated chitin units, as expected being chitosan a partially deacetylated chitin. In fact, in the region 3500 cm-1 – 3000 cm-1 it is possible to identify four peaks located at 3477 cm-1, 3444 cm1

, 3268 cm-1 and 3107 cm-1, which are assigned to the stretching vibrations of -OH, -NH-R and -

NH2 groups of chitosan. Bands in the region 3000 cm-1 - 2800 cm-1 are assigned to CH stretching modes. The strong doublet with maxima at 1659 cm-1 and 1625 cm-1 has been assigned to the C=O stretching vibrations (amide I) of the amide group -C=ONHCH3 for crystalline α-chitin.52 The strong and structured band at 1558 cm-1, with a shoulder at 1575 cm-1, could be assigned to the overlap of the -CN- stretching (amide II) of the C=ONHCH3 group (chitin units) and of the NH2 bending vibration of the primary amine (deacetylated chitin units). The sharp peak at 1378 cm-1 is assigned to the symmetric bending of methyl groups (“umbrella” motion of chitin groups). The four strong and sharp peaks in the region 1200 cm-1 - 1000 cm-1 could be assigned to CO stretching modes of –COH, -COC- and –CH2OH groups of the glycosidic ring.53 The occurrence of sharp absorption features and of the doublet at 1659 cm-1 and 1625 cm-1 suggests the presence of crystalline domains in the sample, as confirmed by WAXD analysis experiment (see below in the text). The spectrum of the HSAG/CS 1:1 physical mixture (iv in Figure 7b) substantially shows the same features observed in the spectrum of chitosan. However, bands are broader, more overlapped and some peaks are slightly shifted. Indeed in the region 3600 cm-1 – 3000 cm-1, the OH and NH stretching vibrations originate a very broad band and the doublet at 1659 cm-1 and 1625 cm-1 (assigned to the crystalline α-chitin) merges in a single broad band, suggesting that the


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physical mixing with HSAG introduces structural disorder into chitosan. The spectra of HSAG/CS 1:1 paper (iii in Figure 7b) and HSAG/CS 1:1 aerogel (ii in Figure 7a) are very similar to that of the physical mixture except for the strong intensity of the band at 1558 cm-1 in the spectrum of HSAG/CS 1:1 aerogel, which can be assigned to a higher contribution of the HSAG absorption at 1590 cm-1 with respect to HSAG/CS 1:1 paper and physical mixture.


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Figure 7. FT-IR spectra of HSAG (i), HSAG/CS 1:1 aerogel (ii), HSAG/CS 1:1 paper (iii), HSAG/CS 1:1 physical mixture (iv) and chitosan (v). (a): spectra in the 4000 cm-1 - 700 cm-1 region; (b): spectra after baseline correction in the 1800 cm-1 – 800 cm-1 fingerprint region.

XPS analyses were performed on HSAG and on HSAG/CS 1:1 paper. Spectra are shown in Figure 8. Wide scan spectrum of HSAG/CS 1:1 paper (Figure 8a) shows three main signals: C1s


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(B.E.=284.8 eV), O1s (B.E.=533.3 eV) and N1s (B.E.=398.1 eV). As reported in previous publications,54 for pristine HSAG only two main signals are present, due to C1s and O1s. Table 1 details the relative atomic ratios among the different elements calculated for HSAG/CS 1:1 paper from XPS analysis. The relative amount of atoms in pure chitosan can be calculated taking into consideration the chemical formula: [C6O4H9(NH2)x (NHCOCH3)y]r, where x and y are fractions of repeating units with free amine and acetylated amine, respectively. In the examined chitosan sample x=0.3 and y=0.7. On the basis of this data, the atomic concentration ratios for chitosan were evaluated: they are listed in Table 1. It appears that the atomic ratios measured for HSAG/CS carbon paper are very similar to the ones calculated for pure chitosan.

Table 1. Atomic concentration ratios and C1s components of HSAG/CS 1:1 paper deducted from XPS spectra and of pure chitosan, estimated considering a deacetylation degree of 30%.

C1s

HSAG/CS 1:1 paper

Chitosan

Experimental

Theoretical

N1s/C1s

0.10

0.14

N1s/O1s

0.21

0.21

O1s/C1s

0.49

0.63

Component 1

32.8

35.1

Component 2

50.7

44.8

Component 3

13.4

14.9

Component 4

3.1

5.2

The deconvolution of the N1s signal of HSAG/CS paper from a high resolution XPS spectrum (Figure 8b) shows only one component, which is attributed to free amine or amide groups of the chitosan chains,55 i.e. non protonated nitrogen functionalities.


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The deconvolution of the C1s signal of the carbon paper is in Figure 8c. It looks very similar to the deconvoluted spectrum of chitosan, reported in the literature56 and is completely different from the C1s envelope of HSAG (Figure 8d). In Figure 8c there are four main components indicating four different environments. The carbon atoms from the glucosamine and Nacetylglucosamine repetitive units are usually grouped as follows (see Figure S5 in the Supporting Information for carbon atoms labeling): atoms C2 and C6 from the glucosamine units, together with carbon atoms C2, C6 and C8 from the N-acetylglucosamine rings, are included into a single environment (component 1); a second carbon environment is produced by C3, C4 and C5 in the glucosamine segment and in the N-acetylglucosamine segment (component 2); a third environment is given by contributions of C1 atoms in both segments (component 3); a fourth signal arises from C7 in the acetylglucosamine unit (component 4). Taking into account this classification, the relative amount of the C1s components are very close to the theoretical values estimated for the chitosan used in the work, whose deacetylation degree is 30% (Table 1). The spectrum of carbon paper thus shows the features of pure chitosan. However, the binding energies of the individual bands of C1s reported in the literature for pure chitosan are at 285.1, 286.6, 288.1 and 288.8 eV. In the carbon paper spectrum (Figure 8c), the B.E.s are shifted to lower values. This experimental finding can lead to assume that C atoms charges are different and lower and the charging effects on the surface of chitosan can hence be due to HSAG. In other words, the results suggest that chitosan is at the surface of the carbon paper and covers HSAG, and that there are strong interactions between the graphite layers and the biopolymer. The absence of protonated nitrogen confirms the exclusive interaction of chitosan with HSAG.


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Figure 8. XPS analysis: wide scan spectrum of HSAG/CS 1:1 paper (a), high resolution N1s spectrum and its deconvolution for HSAG/CS 1:1 paper (b), high resolution C1s spectrum and its deconvolution for HSAG/CS 1:1 paper (c) and HSAG (d).

The structural characterization of HSAG/CS 1:1 paper and HSAG/CS 1:1 aerogel was performed by means of Raman spectroscopy, a tool widely employed for the study of carbonaceous materials.57-65 In Figure 9 the average Raman spectra of HSAG/CS 1:1 paper and HSAG/CS 1:1 aerogel are reported and compared to HSAG. We focused the Raman analysis on the two strong scattering lines, named D and G, located near 1350 cm-1 and at 1582 cm-1 respectively. The G peak is due to E2g collective C=C stretching vibration, correspondent to the only Raman active phonon of bulk crystalline graphite or extended graphene layers, whereas the D peak, whose


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frequency depends on the excitation laser wavelength, occurs in the presence of either chemical/structural disorder or confinement (e.g. by the edges) of the graphitic layers.61-67 Structural defects can be holes, sp3 or sp carbon atoms, free radicals, distortions from planarity, etc. Moreover, real graphitic layers have finite sizes with irregular boundaries containing dangling bonds or grafted functional groups. The HSAG used in this work was produced through ball milling. As reported in the literature,64 the reduction of the average crystallite size of the graphene layers and the presence of electronically perturbed region close to the edges, affected by confinement effects, can account for the presence of a strong D band at 1330 cm-1 in the Raman spectrum. The G peak at 1582 cm1

suggests the existence of an extended sp2 system with vibrational properties very similar to

those of infinite and ideal graphene layers. The spectra of HSAG/CS 1:1 paper and HSAG/CS 1:1 aerogel show slightly different ID/IG ratios with respect to the spectrum of HSAG, and no sizeable increase of the broad Raman signal located between the D and the G peak, usually associated to amorphous sp3 carbon structures.69 Hence the increase of ID/IG could be ascribed to a further structural disorder induced by the preparation of the sample.

Figure 9. Raman spectra with normalized intensities of HSAG (red), HSAG/CS paper 1:1 (blue) and HSAG/CS aerogel 1:1 (green).


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Figure 10 shows WAXD patterns of pristine HSAG, chitosan powder, chitosan treated with acetic acid, HSAG/CS 1:1 aerogel and paper. The crystallinity of chitosan (Figure 10f) is demonstrated by two reflections at 10.2° and 19.9° as 2θ angles. The diffractogram of HSAG (Figure 10a and the inset in Figure 10) shows (00ℓ) reflections, which indicate the order in the direction orthogonal to the structural layers: 002 at 26.6°, corresponding to an interlayer distance of 0.338 nm, and 004 at 54.3o. The in-plane order is revealed by 100 and 110 reflections, at 42.5° and 77.6° respectively. The out-of-plane (D┴) and the in-plane correlation length (D║) were calculated by using (002) and (110) reflections, respectively, applying the Scherrer equation (Equation 1). The calculated values were 9.8 nm for (D┴) and 30.2 nm for (D║). Hence, the inplane correlation length is larger than the out-of-plane correlation length and the number of stacked layers is quite low. From the values of out-of-plane correlation length and interlayer distance, the number of stacked layers was estimated to be about 35. Moreover, the interlayer distance (0.338 nm) is slightly larger than that reported in the literature for ordered graphite (0.335 nm).70 These experimental findings indicate that HSAG has a disordered structure, known as turbostratic,71 and appears thus a suitable nanosized graphite for giving rise to further exfoliation. In the WAXD pattern of the aerogel based on HSAG/CS (treated with acetic acid), shown in Figure 10b, the main peak is the (002) reflection: even if this peak corresponds to HSAG, it is however broader compared to that of pure graphite. By applying the Scherrer equation, the number of stacked layers was calculated as 13, while it was 35 for HSAG. No signals of chitosan crystals appear in the diffractogram. In the aerogel there is thus a profound modification of the solid state organization of both HSAG and chitosan.


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In order to understand the changes of crystallinity observed for the HSAG/CS aerogel, chitosan was treated with acetic acid, in the absence of HSAG, and the isolated film was analyzed by WAXD. The obtained pattern is shown in Figure 10e and reveals broad crystalline peaks at 20.5° and 11.8° as 2θ value. Compared with the chitosan powder peaks, they are less resolved and also slightly displaced. It can be supposed that the process of casting a chitosan film from an acetic acid solution allows some crystallization upon drying, while the presence of HSAG could inhibit the crystallization. Intriguing results are shown in the WAXD patterns of HSAG/CS paper (Figure 10c and 10d): compared to the peak of the aerogel, the (002) reflection (2θ=26.6°) decreases in the carbon paper prepared using 10 ml of water, and is absent in the case of the paper prepared using 50 ml of water. This finding indicates that graphite was exfoliated into graphene sheets by chitosan. To the best of our knowledge, this is the first case of graphene obtained directly from graphite, maintaining unaltered its sp2 structure, without the use of any strong reaction.


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Figure 10. WAXD patterns of pristine HSAG (a), HSAG/CS 1:1 aerogel (b), HSAG/CS 1:1 paper prepared using 10 ml of water (c), HSAG/CS 1:1 paper prepared using 50 ml of water (d), CS film in acetic acid (e) and chitosan powder (f). Samples of HSAG/chitosan adducts with chitosan/graphite ratios other than 1:1 have been characterized, via IR and WAXD analyses. Spectra of the HSAG/CS papers and aerogels 2:1, 4:1 and 6:1 as mass ratio (Figures S1 and S2 in the Supporting Information) show the same features observed in the IR spectrum of HSAG/CS paper and aerogel with 1:1 as the mass ratio. As expected, the intensity of peaks due to HSAG consistently increases with its relative amount. WAXD Patterns reveal the increase of 002 peak with HSAG content. HSAG exfoliation was not observed in samples when HSAG exceeded the mass content of chitosan (ratios larger than 1:1). The morphological properties of the aerogels were studied using SEM (Figure 11). Aerogels exhibited a well-developed highly porous structure, which is in line with the evaluated low density, i.e. 0.026 g cm-3. SEM micrographs show a honeycomb structure: on the walls of the cavity, an architecture made by chitosan in the presence or absence of aggregates of HSAG is identified. Both HSAG agglomerates and continuous HSAG networks can in fact be detected. Figure 11 allows to confirm that the aerogel formation does not disrupt the connectivity of the nanofiller inside the composite.


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Figure 11. SEM image of HSAG/CS aerogel.

Physical properties such as bending behavior, solvent and pH resistance, and electrical conductivity of HSAG/CS papers and aerogels were investigated. Figure 12 shows that free-standing HSAG/CS 1:1 paper, with a thickness of 0.15 mm, were easily obtained. The paper was very flexible and perfectly foldable, reaching a curvature radius close to 180° (Figure 12c) without the appearance of cracks. The density of the carbon paper was as low as 0.81 g cm-3. Preparation of free-standing papers became difficult whit a HSAG/CS ratio close to 4:1.

Figure 12. HSAG/CS 1:1 free standing paper (a) with 0.15 mm thickness (b) and high curvature radius (c).


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The morphological properties of the carbon papers, before (Figure 13a) and after bending to 180° (Figure 13b), were studied by using SEM: the two systems are very similar, showing a typical wrinkled morphology (Figure 13d), and no crack can be identified. The SEM image of HSAG/CS paper after soaking in a water solution with pH=14 (Figure 13c) reveals no surface damage.

Figure 13. SEM images of HSAG/CS paper 1:1 before (A) and after (B) bending to zero degree, after treatment at pH 14 (C). (D) shows the border of the carbon paper.

The resistance of HSAG/CS papers to solvents was preliminarily investigated: Figure 14 shows vials containing specimens of the paper kept for two months in H2O, hexane and DMF. After


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being recovered from the vials, the samples showed negligible swelling and TGA analysis did not reveal any mass loss that could be attributed to adsorbed solvent.

Figure 14. HSAG/CS paper, 1:1 ratio, in H2O (a), hexane (b) and DMF (c) after 2 months storage.

Moreover, the pH resistance of HSAG/CS paper in water solutions with different pH values was investigated (Figure 15). HCl, CH3COOH, NaHCO3 and KOH were used to modulate the pH of the solutions. Samples were found to be stable at pH = 7, 9, 14: specimens recovered from the vials did not show mass loss in TGA. Swelling was instead clearly detected at pH values ranging from 1 to 4.


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Figure 15. pH resistance of HSAG/CS paper 1:1. The resistance to solvents and to pH of aerogels was absolutely similar to the one observed for carbon paper. In particular, swelling was observed only when the aerogels were immersed in solutions with a pH ranging from 1 to 4. To account for the swelling behavior of carbon papers and aerogels, the fact that chitosan swells in water at pH not higher than 4 and becomes soluble at even lower values of pH72 should be considered. A competition could be hypothesized between water and graphene layers for cationic chitosan. This pH-sensitive swelling may find application in the pharmaceutical field for drug release.73

The results reported in this manuscript indeed indicate that HSAG/CS 1:1 adducts, both carbon papers and aerogels, have an outstanding stability, suggesting a strong interaction between HSAG and chitosan. As reported in the introduction, stable adducts of chitosan with CNTs were mainly obtained introducing hydroxyl or carboxy functional groups on CNTs,31,34-37,39-43 and the adducts stability was promoted by hydrogen bonds and by the interaction between the oxygenated functional groups on CNTs and the amino group on chitosan, which could also lead to the formation of amido groups,38 hence to the formation of covalent bonds. When CNTs were


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not functionalized,33,41 the interaction between nanotubes and chitosan was noncovalent in nature, with chitosan wrapping itself around the tubes during the sonication process. In the case of adducts based on graphene related materials, functional groups present on GO and, possibly, on RGO could be responsible for the graphene-chitosan interaction. The stability of chitosan adducts with graphene layers without functional groups, shown in the present work, is thus unprecedented and deserves indeed to be commented. The careful inspection of FT-IR spectra of HSAG/CS adducts did not reveal the presence of peaks that could be attributed to the formation of covalent bonds between HSAG and CS. Although the absence of an evidence is not the evidence of an absence, it can be said that the formation of covalent bonds is not supported by FT-IR results. The hypothesis here proposed explains the stability of HSAG/CS adducts with the occurring of a strong interaction between the cationic chitosan (after the reaction with acetic acid) and the aromatic rings of graphene. Such strong interaction is well documented with CNTs and graphene layers as the π systems. For instance, with carbon nanotubes and imidazolium compounds, ionic liquids gels75 and buckypaper76 were formed. An imidazolium compound was also confined in the narrow interior channel of CNTs.77 Interaction of ionic liquids with graphene layers was exploited for the electrochemical synthesis, directly from graphite, of graphene sheets functionalized with ionic liquids78 and for preparing dispersions of graphene layers, at high concentration, in ionic liquids.79 Such systems could be applied in the fields of gas sensing80 and electrochemical biosensing.81 The ionic liquid interaction with sp2 carbon allotropes is prevailingly interpreted as cation-π interaction,79,82 which is well known in organic chemistry.83 Indeed, molecular dynamics simulations led to predict a preferential adsorption and a significant enrichment of ionic liquid cations at the graphene surface.84 However, on the basis of spectroscopic investigations, it has been also reported that ionic liquids interact with carbon


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nanotubes through weak van der Waals interactions other than the generally assumed cation interaction.85 Although the debate on the nature of the interaction of cationic molecules with aromatic substrates appears thus still open, ionic liquids were used to promote the interaction of chitosan with CNTs33 and with graphene layers.86 Electrical conductivity of HSAG/CS papers and aerogels was measured by the four-point probe method.49 Table 2 reports the measured values of room temperature DC conductivity for composites with different HSAG/CS ratio. For carbon papers, a critical value of HSAG/CS ratio has to be exceeded in order to obtain values of electrical conductivity in line with those reported in the literature. Using a proper HSAG/CS ratio, sufficient values of conductivity for many electrical applications87-88 are reached. As an example, Figure S6 shows that HSAG/CS papers have sufficient electrical conductivity to light a light-emitting diode (LED). In addition, the carbon papers produced do not suffer from instability issues often associated with conductive polymer-based papers. HSAG/CS aerogels show an electrical conductivity even higher than the carbon papers with the same HSAG/CS ratio. This finding could appear surprising, as the electrical conductivity is known to increase with the material density. The following hypothesis can be formulated to explain these results: stacks of nanosized graphite, confined in the walls of the aerogel, could find it easier to establish a conductive pathway, both through direct contact or tunneling effect. As commented above, SEM micrographs revealed in fact that the aerogel formation did not disrupt the graphitic layers connectivity. Moreover, it has to be considered that also the method used to remove water from the samples (which is more efficient in the case of the freeze-dry approach to prepare aerogels) could affect the electrical conductivity.


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The results reported in Table 2 demonstrate, as a proof of principle, the suitability of HSAG/CS papers and aerogels for their application in the field of energy devices.

Table 2: DC conductivity of HSAG/CS composites with different graphite content.

a

HSAG/CS ratio a

σ (S/m)

Chitosan

1 E-8

1:1 b

8.27 E-5

2:1 b

1.05E-4

4:1 b

1.56E-3

6:1 b

1.9E-2

1:1 c

3.3E1

content respect to 100 mg of chitosan b paper c aerogel

In the introduction, applications of carbon papers and aerogels from graphene and graphene related materials have been reported. In the light of the above results, it is worth focusing the attention on the use of graphene-based materials for the fabrication of energy storage devices such as the supercapacitors.89-104 Indeed, the scientific and technological implications of capacitance performance of graphene-based supercapacitors are widely acknowledged.89 Moreover, supercapacitors have been prepared with polymer/graphene composites as electrodes,90-97 and can be configured into paperlike materials.98-99 Undoubtedly interesting appear the results obtained relying on cation-π interactions and hence avoiding chemical modification of the graphene layers: a high energy density has been achieved at room


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temperature by using an ionic liquid as electrolyte.89 In fact, it is known that functionalized graphene sheets can be used for the preparation supercapacitors: 1-atom thick sheets of functionalized carbon was used.100 However, larger capacitance values were obtained by restoring, as much as possible, the sp2 nature of carbon atoms. Reduction of graphene oxide was performed with gaseous hydrazine reduction,101 microwave irradiation102 or directly heating its suspension in propylene carbonate.103 Three-dimensional hierarchical porous composites have been obtained by using chitosan.104 Steps of the synthesis were: preparation of chitosan / graphene oxide mixture, gelling process through freeze-drying, carbonization and KOH activation. On the basis of the results shown in the present paper, an easier preparation of the electrode can be hypothesized. Challenging appears the achievement of a very high level of capacitance. Work is in progress in this direction, selecting the nanosized graphite endowed with the largest electrical conductivity and tuning the graphite/chitosan ratio. Doping is also considered, as already reported for the aerogels.74

4. CONCLUSIONS In this work, bio-nanocomposites based on graphene layers and chitosan were prepared. High surface area graphite and chitosan, from 1:1 to 6:1 as the mass ratio, were mixed in a mortar with the help of a pestle, and water suspensions were then prepared by adding acetic acid. Such suspensions were observed to be stable for months and graphene papers and carbon aerogels were obtained simply by casting or lyophilization. WAXD analysis of aerogel revealed the presence of HSAG stacks made by only few layers of graphene whereas the (002) reflection typical of stacked graphene layers was not observed in the WAXD pattern of carbon papers. 150 µm thick carbon papers were free standing, with curvature radius close to zero and were stable to


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different solvents and pH, maintaining unaltered properties. Conductive aerogel was prepared with 1:1 HSAG:CS ratio. To account for these results, multiple cation-π interaction is hypothesized. This work demonstrates that chemical reactions on graphitic materials, such as the oxidation to GO and the successive reduction, are not necessarily required to prepare carbon composites based on graphene and few layers graphene. An eco-friendly, inexpensive, simple process for producing high-quality graphene papers and aerogels could play a positive role for developing applications as electrodes in supercapacitors.

ASSOCIATED CONTENT Supporting Information. S1. Preparation of water suspensions of HSAG Table S1. UV-vis absorbance of HSAG water dispersions in hydrochloric acid and 2HTCl as a function of wavelength (concentration 1 mg/mL). Figure S1. FT-IR spectra of HSAG/CS aerogels 1:1 (i), 1:2 (ii), 1:4 (iii) and 1:6 (iv) in the 3700 cm-1 - 750 cm-1 region. Figure S2. FT-IR spectra of HSAG/CS papers 1:1 (i), 1:2 (ii), 1:4 (iii) and 1:6 (iv) in the 3700 cm-1 - 750 cm-1 region. Figure S3. WAXD patterns of HSAG/CS aerogels with chitosan/graphite ratios 1:2 (a), 1:4 (b) and 1:6 (c) Figure S4. WAXD patterns of HSAG/CS papers with chitosan/graphite ratios 1:2 (a), 1:4 (b) and 1:6 (c) Figure S5. Chemical structure of partially deacetylated chitosan and atom labelling.


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Figure S6. Circuit closed using HSAG/CS paper (1:1) and a light-emitting diode (LED).

AUTHOR INFORMATION Corresponding Authors * [email protected] (orcid.org/0000-0002-4503-4250) * [email protected] (orcid.org/0000-0001-5770-7208) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT “PRIN Research Project 2010-2011” and Fondazione Silvio Tronchetti Provera are acknowledged for the financial support.

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Novoselov, A. K.; Geim, S. V.; Morozov, D.; Jiang, Y.; Zhang, S. V.; Dubonos, I. V.; Grigorieva, A.; Firsov, A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666–669.

(2)

Geim, A. K.; MacDonald, A. H. Graphene: exploring carbon flatland. Phys. Today 2007, 60, 35-41.

(3)

Geim, A. K.; Novoselov, A. K. The rise of graphene. Nat. Mater. 2007, 6, 183-191.


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BRIEFS (Word Style “BH_Briefs”). If you are submitting your paper to a journal that requires a brief, provide a one-sentence synopsis for inclusion in the Table of Contents.


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TABLE OF CONTENTS GRAPHIC (TOC)


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Figure 1. Block diagram for the preparation of HSAG/CS adducts 236x44mm (300 x 300 DPI)


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Figure 2. Water dispersions of HSAG (a), HSAG/CS after 1 month storage (b), and after 30 min centrifugation at 9000 rpm (c). HSAG/CS were in 1:1 ratio and the concentration was 1 mg/mL. 187x126mm (300 x 300 DPI)


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Figure 3. UV-vis absorbance of HSAG/CS dispersions in water/acetic acid solutions as a function of wavelength. HSAG/CS concentration (mg/mL) was: 2.50 (a), 1.60 (b), 1.25 (c), 0.71 (d), and 0.55 (e). 184x124mm (300 x 300 DPI)


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Figure. 4 Micrographs of HSAG/CS samples taken from the mortar (a, b), from the water suspensions (c, d) and isolated after centrifugation at 9000 rpm for 30 min (e, f). Micrographs were taken with low magnification bright field TEM (a, c, e) and with HRTEM (b, d, f). 134x199mm (300 x 300 DPI)


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Figure 5. Block diagram for the preparation of HSAG/CS aerogels. 210x40mm (300 x 300 DPI)


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Biomacromolecules

Figure 6. (A) TGA curves (under nitrogen, until 900°C) of HSAG (a), HSAG/CS papers with 4:1 (b), 2:1 (c) and 1:1 (d) mass ratio, and chitosan (e); (B) TGA trace of HSAG/CS paper 1:1 after soaking in DMF as solvent. 328x146mm (300 x 300 DPI)


Biomacromolecules

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Figure 7. FT-IR spectra of HSAG (i), HSAG/CS 1:1 aerogel (ii), HSAG/CS 1:1 paper (iii), HSAG/CS 1:1 physical mixture (iv) and chitosan (v). (a): spectra in the 4000 cm-1 - 700 cm-1 region; (b): spectra after baseline correction in the 1800 cm-1 – 800 cm-1 fingerprint region. 109x166mm (300 x 300 DPI)


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Biomacromolecules

Figure 8. XPS analysis: wide scan spectrum of HSAG/CS 1:1 paper (a), high resolution N1s spectrum and its deconvolution for HSAG/CS 1:1 paper (b), high resolution C1s spectrum and its deconvolution for HSAG/CS 1:1 paper (c) and HSAG (d).

165x120mm (300 x 300 DPI)


Biomacromolecules

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Figure 9. Raman spectra with normalized intensities of HSAG (red), HSAG/CS paper 1:1 (blue) and HSAG/CS aerogel 1:1 (green). 89x62mm (300 x 300 DPI)


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Biomacromolecules

Figure 10. WAXD patterns of pristine HSAG (a), HSAG/CS 1:1 aerogel (b), HSAG/CS 1:1 paper prepared using 10 ml of water (c), HSAG/CS 1:1 paper prepared using 50 ml of water (d), CS film in acetic acid (e) and chitosan powder (f). 90x101mm (300 x 300 DPI)


Biomacromolecules

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Figure 11. SEM image of HSAG/CS aerogel. 188x150mm (300 x 300 DPI)


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Biomacromolecules

Figure 12. HSAG/CS 1:1 free standing paper (a) with 0.15 mm thickness (b) and high curvature radius (c). 133x37mm (300 x 300 DPI)


Biomacromolecules

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Figure 13. SEM images of HSAG/CS paper 1:1 before (A) and after (B) bending to zero degree, after treatment at pH 14 (C). (D) shows the border of the carbon paper. 223x141mm (300 x 300 DPI)


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Biomacromolecules

Figure 14. HSAG/CS paper, 1:1 ratio, in H2O (a), hexane (b) and DMF (c) after 2 months storage. 154x128mm (300 x 300 DPI)


Biomacromolecules

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Figure 15. pH resistance of HSAG/CS paper 1:1. 135x71mm (300 x 300 DPI)


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Biomacromolecules

TOC 93x88mm (300 x 300 DPI)


Carbon Papers and Aerogels Based on Graphene Layers and

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