Thermally stable, solvent resistant and flexible graphene oxide paper

Abstract

The ability of graphene oxide (GO) aqueous suspensions to form robust GO paper is largely improved by basification of the suspension before processing. In particular, casting procedures, which are generally unsuitable for production of robust GO paper, become suitable for basified GO (b-GO) suspensions, leading to dense and free-standing papers, which are also highly flexible. Thermal or microwave treatments of paper from b-GO suspensions (b-GO paper) easily produce loss in stacking order of graphene oxide layers, with maintenance of a high degree of parallelism (0.6 < f ≡ orientation function < 0.7) with respect to the paper surface. Differently from usual GO papers, b-GO papers maintain their dimensional integrity when thermally treated or when dispersed in organic solvents or in aqueous solutions. Many relevant b-GO features (improved film ability by casting, maintenance of film integrity and reduced deoxygenation by heating and improved solvent resistance) can be rationalized by formation of covalent bridges between GO layers. Infrared spectra and simple chemical arguments suggest that these covalent bridges between GO layers could be mainly constituted by ether bonds.

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1 Introduction

Graphite oxide is a layered material, which can be obtained by chemical oxidation of graphite by treatment with strong mineral acids and oxidizing agents.¹ This material exhibits oxygen containing functional groups (epoxide and hydroxyl groups at the surface of the basal planes and carbonyl and carboxyl groups at the edges)^2,3 and can be easily exfoliated to graphene oxide (GO) layers. GO layers present, of course, polar surface properties, a strong hydrophilic character and catalytic activity toward many reactions.^4–8

Nowadays, it is widely acknowledged in the scientific community that large scale production of single or few layers of graphene can be achieved by top-down processes of exfoliation and reduction of GO, through chemical or thermal treatments.^9–12 It is also well recognized that the development of three-dimensional structures with graphene oxide and graphene layers, e.g. aerogels^13–19 or papers,^20–28 is expected to improve their manufacturability as well as to expand their applications.

In particular, free-standing GO paper can be obtained by vacuum filtration of colloidal dispersions of graphene oxide sheets.^20–28 However GO papers, due to the high content of polar functional groups, generally exhibit a strong tendency to go back into suspension in the presence of solvents (mainly if polar or highly polar, like water), which is a major inconvenience for many applications.

In this contribution, we present GO papers as prepared after basification of the colloidal GO aqueous suspension (thereafter defined as basified GO paper and shortly b-GO paper) that exhibit thermal stability and solvent resistance much higher than usual GO papers. The article also shows that basification of GO suspensions also facilitates processing, mainly solvent casting procedures that lead to definitely more flexible papers. Spectroscopic analyses of the obtained papers have been conducted aiming to a possible rationalization of the observed behavior.

2 Experimental

2.1 Materials and graphite oxide preparation

High surface area graphite, with Synthetic Graphite 8427® as trademark, was purchased from Asbury Graphite Mills Inc., with a minimum carbon wt% of 99.8 and a surface area of 330 m² g⁻¹. Nitric acid, sulfuric acid, sodium nitrate and sodium hydroxide were purchased from Sigma-Aldrich. All reagents were used as received without purification.

Graphite oxide samples were prepared by Hummers' method, from graphite samples. 120 mL of sulfuric acid and 2.5 g of sodium nitrate were introduced into a 2000 mL three-neck round bottomed flask immersed into an ice bath and 5 g of graphite were added, under nitrogen, with a magnetic stirring. After obtaining a uniform dispersion of graphite powders, 15 g of potassium permanganate were added very slowly to minimize the risk of explosion. The reaction mixture was thus heated to 35 °C and stirred for 24 h. Deionized water (700 mL) was added in small amounts into the resulting dark green slurry under stirring and, finally, gradually adding 5 mL of H₂O₂ (30%). The obtained sample was poured into 7 L of deionized water, and then centrifugated at 10 000 rpm for 15 min with a Hermle Z 323 K centrifuge. The isolated graphite oxide powder was first washed twice with 100 mL of a 5 wt% HCl aqueous solution and subsequently washed with 500 mL of deionized water. Finally, it was dried at 60 °C for 12 h. About 10 g of graphite oxide powders were obtained. The cation-exchange-capacity of the obtained graphite oxide was determined as CEC_GO = 5.8 mmol g⁻¹, by the procedure reported by Matsuo et al.²⁹

2.2 GO paper preparation

GO and basified GO^30–34 aqueous dispersions were prepared from graphite oxide powders (150 mg) in 20 mL of water and of 0.05 M NaOH aqueous solution, respectively. Complete dispersions were obtained with the aid of ultrasonication³⁵ for 4 h at room temperature.

Both kinds of dispersions were processed by vacuum filtration²⁰ as well as by casting in Petri dishes at 60 °C. By using these procedures GO and b-GO papers with thickness in the range 10–70 μm were obtained. All the results reported for GO paper refer to samples obtained by vacuum filtration, because the cast samples are too brittle. Most of the results reported for b-GO paper refer instead to cast samples, which present on the contrary improved flexibility (see Section 4.5).

b-GO papers were washed by water and, to remove Na⁺ ions, also by a 0.05 M HCl solution.

Robust b-GO papers have been obtained by using aqueous solutions with a suitable range of NaOH concentration (0.01–0.08 M).

3 Characterizations

Wide-angle X-ray diffraction (WAXD) patterns were obtained by an automatic Bruker D8 Advance diffractometer, in reflection, at 35 kV and 40 mA, using the nickel filtered Cu-Kα radiation (0.15418 nm). Details relative to evaluation methods for correlation lengths (D) and orientation functions (f) are reported as ESI.†

Elemental analysis was performed with a Thermo FlashEA 1112 Series CHNS-O analyzer, after pretreatment of the samples in an oven at 100 °C for 12 h.

Microwave treatments were carried out in a house-hold microwave oven (Delonghi, model MWJ62) in ambient conditions at 400 W and 900 W, at different times of exposure.

Most measurements have been conducted on films fully equilibrated with water at the relative humidity of 45% at 20 °C, as obtained by saturated aqueous potassium carbonate salt solutions.³⁶

Density measurements were obtained by flotation, at room temperature, in carbon tetrachloride and 1,2-dibromoethane solutions.

Nitrogen adsorption at liquid nitrogen temperature (77 K) was used to measure surface areas of carbon powders and papers with a Nova Quantachrome 4200e instrument. Before the adsorption measurement, samples were degassed at 60 °C under vacuum for 24 h. The surface area values were determined by using 11-point BET analysis.³⁷

Radius of curvature of the obtained papers has been evaluated by the minimum opening of a gauge, before sample breaking.

Thermogravimetric (TG) analyses were carried out on a TG 209 F1, manufactured by Netzsch Geraetebau, at a heating rate of 10 °C min⁻¹, under N₂ flow. Water content of the samples was determined by the weight loss below 100 °C.

Differential scanning calorimetry (DSC) was carried out under nitrogen from 30 °C to 300 °C at a heating rate of 10 °C min⁻¹ on a TA instruments (DSC Q2000).

IR-DRIFT spectra were obtained using a diffuse reflectance accessory ‘Collector’ from Thermo SCIENTIFIC, with a BRUKER Vertex70 spectrometer equipped with deuterated triglycine sulfate (DTGS) detector and a KBr beam splitter. The frequency scale was internally calibrated to 0.01 cm⁻¹ using a He–Ne laser. 64 scans were signal averaged to reduce the noise. Samples were analyzed at room temperature, by pouring them loosely into a sample cup of 8 mm depth and 5 mm diameter, without any dilution in KBr.

4 Results and discussion

4.1 Structural characterizations

The X-ray diffraction pattern (CuKα) of graphite oxide powder, obtained by oxidation of graphite samples exhibiting high shape anisotropy¹² and used as starting material of the present study, is shown in Fig. 1A. The pattern shows the (001) peak corresponding to an interlayer distance of 0.82 nm, with a correlation length of 4.5 nm as well as the (100) and (110) peaks (at d = 0.21 nm and d = 0.12 nm, respectively) corresponding to a long-range order in the graphitic planes (e.g., correlation length perpendicular to the (100) planes higher than 30 nm).

Fig. 1 X-ray diffraction patterns (CuKα) as collected by an automatic powder diffractometer (A–E) or by a photographic camera with the X-ray beam parallel (B′, D′ and E′ for EDGE diffraction geometry) or perpendicular to the sample surface (B′′, D′′ and E′′ for THROUGH diffraction geometry) for: (A) the starting graphite oxide powder; (B, B′ and B′′) GO paper obtained by vacuum filtration of GO suspension; (C) b-GO paper, by casting of a basified GO suspension, including 13 wt% of sodium ions; (D, D′ and D′′) b-GO cast paper, after sodium removal by washing with HCl solution; (E, E′ and E′′) b-GO paper treated at 100 °C for 15 s. The hk[small script l] Miller indexes of the main reflections are indicated.

The GO paper, obtained by vacuum filtration of a GO suspension,²⁰ has a thickness of nearly 30 μm, a C/O ratio equal to 1.4 (first line of Table 1) and a density of 1.79 ± 0.01 g cm⁻³, similar to those reported in the literature for analogous GO papers.²⁰

Table 1 Elemental analysis of different papers based on GO^a

Paper	C (wt%)	H (wt%)	O (wt%)	C/O
a Elemental composition of anhydrous samples: the error in C and O content is close to 1 wt% while the error in H content is close to 0.1 wt%; water contents (as evaluated by TGA, after equilibration at room temperature in air with a relative humidity of 45%) was in the range 13 ± 2 wt%, for all the examined samples.
GO	57.2	1.4	41.3	1.4
GO (300 °C)	81.8	0.6	17.6	4.6
b-GO	52.1	0.9	47.0	1.1
b-GO (300 °C)	62.4	0.8	37.6	1.7
b-GO (400 °C)	64.9	1.1	33.8	1.9
b-GO (500 °C)	71.4	1.2	27.4	2.6
b-GO (microwave)	52.4	1.0	46.6	1.1

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The X-ray diffraction pattern as taken by an automatic diffractometer of this GO paper (Fig. 1B) is similar to that one reported in Fig. 1 of ref. 20 and shows only the (001) diffraction peak. This clearly indicates that, irrespective of the full exfoliation in the aqueous dispersion, GO aggregates again to graphite oxide, coming back to the starting interlayer distance (0.82 nm), even with a slightly increased correlation length (from 4.5 up to 4.8 nm). The comparison between the patterns of Fig. 1A and B also shows an increased intensity of the (001) peak with respect to the amorphous halo as well as the disappearance of (100) and (110) peaks for the GO paper. These features are due to the occurrence in the GO paper of uniplanar orientation of the graphitic planes parallel to the paper surface. This uniplanar orientation is clearly shown by the photographic patterns of Fig. 1B′ and B′′, as taken with EDGE and THROUGH geometries, i.e. by placing the X-ray beam parallel and perpendicular to the paper surface, respectively (as sketched in the lower part of Fig. 1). In fact, as already observed for EDGE patterns of other graphite and GO papers and films,^32,33 the (001) reflection is present as arcs centered on the equator (corresponding to a degree of parallelism of the (001) graphite oxide planes with respect to the paper surface equal to f_00l ≈ 0.83) while the (100) reflection is present as broad arcs centered on the meridian (Fig. 1B′). Of course, the corresponding pattern as taken by an automatic powder diffractometer (Fig. 1B), which mainly collects information relative to the equator of the EDGE pattern, completely looses information relative to the hk0 reflections.

The b-GO paper, as obtained by casting of a b-GO suspension, has a thickness of nearly 30 μm, a C/O ratio equal to 1.1 (third line of Table 1). Hence, the adopted basification procedure leads to an increase of the oxygen content with respect to the GO paper, possibly mainly due to opening of epoxide groups on the GO layers as promoted by hydroxyl ions. Irrespective of its higher oxygen content (1^st and 3^rd line of Table 1) the density of the b-GO paper (1.82 ± 0.01 g cm⁻³) is even slightly higher than for the GO paper (1.79 ± 0.01 g cm⁻³).

The WAXD patterns of the cast b-GO paper, as taken by an automatic powder diffractometer, before and after sodium removal (≈13 wt%, corresponding to a C/Na ratio not far from 6), by washing with a 10 wt% aqueous HCl solution, are shown in Fig. 1C and D, respectively. A shift to a lower diffraction angle of the (001) reflection indicates a definitely increased interlayer spacing (d₀₀₁ = 1.0 nm, Fig. 1C), which disappears after sodium removal (d₀₀₁ = 0.8 nm, Fig. 1D). This clearly indicates the formation of an intercalate crystalline structure of graphite oxide^32,34 with sodium ions,³⁸ which goes back to crystalline graphite oxide, after sodium removal by hydrochloric acid treatment.

For both patterns of Fig. 1C and D, a relevant increase of the intensity of the amorphous halo (roughly centered at d ≈ 0.37 nm) is observed. Due to the increase of oxygen content (elemental analysis of Table 1), these halos although not far from the graphitic interlayer spacing (d = 0.34 nm) cannot be attributed to graphite oxide reduction. Their high intensity instead indicates a strong loss (of nearly 70%) of graphite oxide crystallinity.

X-ray diffraction patterns of the cast and washed b-GO paper as taken by a photographic camera with EDGE and THROUGH geometries, are shown in Fig. 1D′ and D′′, respectively. From the EDGE photographic pattern of Fig. 1D′, it is apparent that not only the (001) graphite oxide peak but also the broad amorphous halo are centered on the equator. Rather surprisingly, the degree of parallelism with respect to the paper surface of the amorphous halo (f_am ≈ 0.69) is even higher than for the narrower peak at d = 0.8 nm (Fig. 1D), corresponding to the (001) peak of the graphite oxide component (f_00l ≈ 0.60).

WAXD patterns similar to those of Fig. 1D, D′ and D′′ are obtained for different sonication conditions, with intensity of the amorphous halo gradually increasing with sonication time and temperature. For instance, after sonication of the GO dispersion at 80 °C for 12 h, the WAXD pattern of the obtained paper (not shown) exhibits an amorphous halo again centered at d ≈ 0.37 nm, whose intensity is nearly 10 times higher than the intensity of the (001) graphite oxide peak still located at d = 0.8 nm.

A complete loss of the (001) peak, i.e. of the crystalline order perpendicular to the graphite oxide planes can be reached by simple annealing of the b-GO paper. For instance, the WAXD pattern of the cast paper, after 15 s of treatment at 100 °C (Fig. 1E) shows only a broad halo centered at d ≈ 0.37 nm (Fig. 1F), which is very similar to the diffraction halo of amorphous carbon³⁹ as well as of largely oxidized amorphous carbon.⁴⁰ The two-dimensional patterns of Fig. 1E′ and 1E′′ show that the broad amorphous halo of the annealed b-GO paper is only present in the EDGE pattern (E′) and centered on the equator. This again indicates a rather high degree of parallelism of graphene oxide layers, with respect to paper surface (f ≈ 0.67).

The loss of crystalline order perpendicular to the GO planes, which characterize the b-GO paper of Fig. 1E, leaves essentially unaltered the in plane hk0 reflections, as shown by a comparison between (100) rings in the THROUGH patterns (Fig. 1B′′, D′′ and E′′).

Hence the present results clearly indicate that the basification procedure leads to macroscopic papers with loss of crystalline order perpendicular to the GO planes, without reducing the in-plane crystalline order of the GO planes. Moreover, the degree of parallelism with respect to the paper plane of the GO layers (f ≈ 0.67) is still comparable with that one of the graphite oxide crystallites being present in the GO paper of Fig. 1B (f ≈ 0.83).

4.2 Structural changes as a consequence of thermal and microwave treatments

GO and b-GO papers are largely different as for their structural changes as a consequence of thermal treatments. For instance, the changes of WAXD patterns, as collected by an automatic powder diffractometer, for GO and b-GO papers, as a consequence of thermal treatments in the range 200–400 °C are compared in Fig. 2A and B, respectively.

Fig. 2 X-ray diffraction patterns (CuKα) as collected by an automatic powder diffractometer for GO paper (A) and b-GO paper (B), as a consequence of different thermal treatments: (a) unannealed; (b) 200 °C for 30 s; (c) 200 °C for 3 min; (d) 200 °C for 30 min; (e) 200 °C for 300 min; (f) 400 °C for 3 min. The d spacing, expressed in nm, of the observed peaks is indicated. The insets in the top of the figures show photographs of GO (A) and b-GO (B) papers after treatment at 400 °C for 3 min.

As for GO paper treatments at 200 °C, the (001) peak progressively moves to higher diffraction angles and correspondingly the interlayer spacing reduces from 0.82 nm down to 0.40 nm. A weak but narrow graphitic peak at d = 0.34 nm, for all the considered annealing conditions, also appears (Fig. 2Ab–f) and clearly indicates the formation of a minor amount of ordered graphitic structure. Even after the thermal treatment at 400 °C (Fig. 2Af), the GO paper still contains minor amounts of reduced graphite oxide crystallites (peak at d = 0.40 nm) as well as of graphite crystallites (peak at d = 0.34 nm). Moreover, the GO paper after the thermal treatment at 400 °C is fully disintegrated (see inset in the upper part of Fig. 2A).

Largely different are the WAXD patterns of the b-GO paper after thermal treatments at 200 °C and 400 °C (Fig. 2Bb–f), which all present only a broad amorphous halo, as for the b-GO paper annealed at 100 °C (Fig. 1E, E′ and E′′).

Particularly relevant is the maintenance of integrity of the b-GO paper, even after the thermal treatment at 400 °C, which is instead fully lost for GO paper (compare insets in the upper part of Fig. 2A and B, respectively).

Structural changes for GO and b-GO papers, as a consequence of room temperature microwave treatments have also been studied by WAXD patterns (as collected by powder diffractometer), as shown in Fig. 3. The (001) graphite oxide peak of the GO paper is gradually shifted toward higher 2θ values, and after 40 min of treatment the d spacing decreases from 0.85 nm down to 0.69 nm, with a reduction of the correlation length perpendicular to the graphitic layers from 4.4 nm to 3.5 nm (Fig. 3A–C).

Fig. 3 X-ray diffraction patterns (CuKα), as collected by an automatic powder diffractometer, for: GO paper untreated (A), microwave treated at 900 W for 2 min (B) and for 40 min (C); b-GO paper untreated (D) and microwave treated at 400 W for 15 s (E).

The (001) graphite oxide peak of the b-GO paper (Fig. 3D), on the contrary, after only 2 min of microwave treatment completely disappears leading to a amorphous halo (Fig. 3E), which is again indistinguishable from that one observed for the thermally treated b-GO paper (Fig. 1E). Elemental analysis of the microwave treated b-GO paper (last line of Table 1) indicates a negligible reduction of oxygen content with respect to the starting b-GO paper (3^rd line of Table 1), clearly showing that the amorphous halo centered at d ≈ 0.37 nm is due to loss of three dimensional order of graphene oxide layers rather than to their reduction.

Hence, the minor graphite oxide component of the b-GO paper, differently from the large graphite oxide component of GO paper, can easily loose (both by thermal or microwave treatments) its crystalline order perpendicular to the graphene oxide layers, always maintaining the GO paper integrity.

Relevant information on structural organization of b-GO paper also comes from N₂ sorption experiments. In fact, as prepared b-GO papers that maintain graphite oxide crystalline order (Fig. 1D) exhibit negligible BET surface areas (<1 m² g⁻¹). b-GO paper samples, which have lost crystalline order perpendicular to the GO planes both by annealing at 100 °C (Fig. 1E) or by microwave treatments (Fig. 3E), present definitely higher BET values (4.6 ± 0.1 m² g⁻¹). BET surface area values further increase as a consequence of b-GO paper reduction processes, which maintain the complete loss of crystalline order perpendicular to the GO planes. For instance, for b-GO papers after 1 min of annealing at 400 °C and 500 °C, where the C/O weight ratio increases up to 1.9 and 2.6 (Table 1), the BET values increase up to 20 m² g⁻¹ and 75 m² g⁻¹, respectively.

4.3 Thermal behavior

TGA and DSC measurements of GO and b-GO papers are compared in Fig. 4.

Fig. 4 Scans at heating rate of 10 K min⁻¹ of GO paper (black line) and b-GO paper (red line): (A) TGA; (B) DTGA; (C) DSC.

TGA measurements (Fig. 4A) and the corresponding differential plot (DTGA, Fig. 4B) show the well known thermal degradation phenomenon, corresponding to deoxygenation mainly by CO₂ evolution,⁴¹ which for both GO and b-GO papers occurs at nearly 200 °C. However, the weight loss for the b-GO paper (11%) is close to one half of the weight loss of the GO paper (21%).

This nearly halved degradation phenomenon, for b-GO paper, is clearly confirmed by DSC measurements (Fig. 4C), which show that the corresponding exothermic peak⁴² (centered in the temperature range 195–198 °C) is also nearly halved. In fact, the enthalpy change is equal to 1.1 and 0.53 kJ g⁻¹, for GO and b-GO paper, respectively.

Both TGA and DSC measurements indicate that thermal deoxygenation phenomena, are largely depressed for b-GO paper with respect to GO paper. These data are also clearly confirmed by elemental analyses, as collected in Table 1. In fact, as consequence of treatment at 300 °C, the C/O weight ratio of GO paper increases from 1.4 up to 4.6 (1^st and 2^nd lines of Table 1) while for the b-GO paper only increases from 1.1 up to 1.7 (3^rd and 4^th lines of Table 1). Elemental analyses of b-GO paper treated at 400 °C and 500 °C (5^th and 6^th lines of Table 1) show that the C/O weight ratio increases only up to 2.6.

Hence, after treatment at 300 °C, the GO paper has an oxygen content of 17.8 wt% while the b-GO paper has a nearly double oxygen content (37 wt%). A possible molecular origin of the definitely more difficult thermal deoxygenation of b-GO papers, with respect to GO papers, is discussed in Section 4.6.

4.4 Dimensional stability in solvents

Although b-GO paper exhibits an oxygen content definitely higher than for usual GO paper (see, elemental analysis in Table 1), they present a much higher dimensional stability when dispersed in both polar and apolar solvents, mainly in the presence of sonication.

The dimensional stability in the presence of some solvents, after vigorous stirring and three months of storage, for GO paper (by vacuum filtration) and for b-GO papers (both by vacuum filtration and casting procedures), is shown by photographs of vials in Fig. 5.

Fig. 5 Photographs of vials containing three different solvents (dimethylformamide, water and NaOH 1 M aqueous solution) and three different kinds of papers: GO; b-GO by vacuum filtration (b-GO(VF)) and b-GO by casting (b-GO(C)). The photographs have been taken after vigorous stirring and three months of storage.

Both b-GO papers maintain their dimensional integrity after sonication in organic solvents like acetone, ethanol and hexane while the GO paper breaks in many pieces.

Both b-GO papers are also completely stable to sonication in polar organic solvents, like dimethylformamide (DMF, lower part of Fig. 5) and N-methyl-pyrrolidone (NMP), with solvents remaining fully transparent, while the GO paper in part goes back into suspension and the solvents become heavily turbid.

In water (intermediate part of Fig. 5), the GO paper comes back to GO suspension, the b-GO(VF) paper breaks in few pieces also losing some fine particles (less than 1 wt%) that generate water turbidity, while b-GO(C) paper remains intact in fully transparent water.

b-GO(C) paper also exhibits a remarkable dimensional stability in acidic and alkaline aqueous solutions, also for high concentrations. This is shown, for instance, in the upper part of Fig. 5, for a 1 M aqueous solution of NaOH, which fully destroys GO paper and becomes slightly turbid in the presence of b-GO(VF). An analogous behavior is observed for the same GO papers in 1 M aqueous solutions of HCl.

It is worth adding that b-GO papers exhibit the above-described high solvent resistance both before and after the acidic treatment leading to ion removal. Hence, the high solvent resistance of b-GO papers is not due to interactions involving the added metal cations or possible multivalent cationic metal contaminants.⁴³

4.5 Paper flexibility

Mechanical properties of b-GO paper are comparable or even better than those of the usual GO paper. This is shown, for instance, for bending tests evaluating radius of curvature (as one half of the minimum opening of a gauge, before sample breaking, see Fig. 6A) of papers of different thickness.

Fig. 6 (A) Bending tests evaluating the minimum opening of a gauge, before paper breaking. (B) The radius of curvature, calculated as one half of this minimum opening, reported versus the papers thickness, for b-GO papers (, ) and GO papers (, ). In particular, triangles (, ) indicate data for papers by vacuum filtration, squares () data for papers by casting while circles () indicate literature data taken from Ruoff et al.²⁰ An arrow indicates the cast b-GO paper that is fully foldable, i.e. does not break in our bending test. (C and D) Photographs of a 12 μm b-GO paper: (C) crumbled (D) and then laid out in a plane.

The plot of Fig. 6B clearly shows that papers by vacuum filtration present similar radius of curvature, irrespective of the preparation from GO or b-GO (upper curve). However, cast papers that were obtained only from b-GO suspensions exhibit a definitely higher flexibility (lower curve with red squares). In particular, the cast paper with lowest thickness (12 μm) is fully foldable, i.e., does not break in our bending test, even for radius of curvature equal to zero. It is worth adding that this 12 μm b-GO paper survived a repeated folding test (1000 folds) with a curvature radius of 200 μm. Moreover, after the repeated folding test the thin paper was crumpled (Fig. 6C) and then laid out in a plane (Fig. 6D), without any fracture.

4.6 FTIR characterization and possible rationalization of b-GO properties

Many favorable properties of b-GO paper with respect to GO paper could be rationalized by formation of covalent bonds between GO layers: (i) largely improved film ability, which is generally associated with higher molecular masses of the dispersed molecules; (ii) largely improved solvent resistance, which is generally associated with crosslink formation; (iii) paper integrity after complete loss of stacking order of GO as induced by thermal or microwave treatments, which again is favored by covalent bonds between graphene layers; (iv) higher paper density, irrespective of lower crystallinity, of higher oxygen content and of lower degree of planar orientation.

This section reports an infrared characterization of functional group on the GO layers, aiming to understand the chemical nature of the possible covalent links between GO layers. The FTIR reflection–absorption spectra of the prepared GO papers have been studied for the range 4000–400 cm⁻¹ and are compared for the spectral range 2000–800 cm⁻¹, in Fig. 7.

Fig. 7 FTIR reflection–absorption spectra of GO paper (A) and b-GO paper (B) for the spectral range 2000–800 cm⁻¹.

The FTIR spectra of the GO paper (Fig. 7A) present peaks at 825 cm⁻¹ and 1275 cm⁻¹, due to the bending and asymmetric stretching modes of the epoxy (C–O–C) group,^44,45 and a peak at 1740 cm⁻¹, which is generally attributed to C=O stretching vibration of carboxylic functionalities.⁴³ The intense peak at 1640 cm⁻¹, can be possibly attributed to an overlap of different vibrational modes as C=C stretching vibration⁴⁶ (which is activated in GO due to oxygen functional groups and defects) and bending mode of intercalated water.⁴⁷

Largely different are the spectra of the obtained b-GO papers that are mainly characterized by a very broad band roughly centered at 1335 cm⁻¹ (Fig. 7B), which does not help to establish the chemical nature of the functional groups on the layers. However, the b-GO spectra show, beside a clear reduction of the carboxylic 1740 cm⁻¹ peak (indicating a partial decarboxylation, usually associated with alkaline induced reduction),^30,41 the complete disappearance of the epoxy vibrations at 825 cm⁻¹ and 1275 cm⁻¹. The disappearance of the epoxy groups from the GO layers is also confirmed by ¹³C NMR spectra, as shown in the ESI.†

The disappearance of the epoxy groups shows that the NaOH concentration (0.05 M) used for b-GO preparation is effective to promote epoxy ring opening reaction (Fig. 8A), which produces alkoxy ions.⁴⁸ Some of these ions could attack the epoxy ring of a close layer (Fig. 8B), thus producing interlayer ether bonds (Fig. 8C).

Fig. 8 Proposed mechanism for interlayer covalent bond formation for b-GO paper.

The chemical mechanism for interlayer covalent bond formation, proposed in Fig. 8, is also able to rationalize the decreased thermal deoxygenation, as observed for b-GO papers (Fig. 4), irrespective of their higher oxygen content [(C/O)_b-GO = 1.1 vs. (C/O)_GO = 1.4]. In fact, the replacement of most hydroxyl groups by ether bonds in the paper preparation procedure would reduce the thermal decarboxylation reaction, which would be mainly based on disproportionation of hydroxyl groups of graphite oxide.⁴¹

5 Conclusions

The ability of GO aqueous suspensions to form robust GO paper is largely improved by basification of the suspension prior of vacuum filtration or casting procedures. In particular, casting procedures, which generally lead to brittle GO paper, for the case of basified GO suspensions lead to tough papers of macroscopic size, for a broad range of thickness (at least in the range 5–50 μm).

Flexibility of b-GO papers, when obtained by casting procedures, is definitely improved with respect to usual GO papers. In particular, cast b-GO papers with low thickness (≈10 μm) do not break in our bending test, even for radius of curvature equal to zero.

Differently from usual GO papers, b-GO papers exhibit a remarkable dimensional stability, after thermal or solvent treatments. Dimensional stability in solvents is observed even after sonication and differences with respect to GO papers are particularly impressive in the presence highly polar solvents, like water or DMF or NMP.

X-ray diffraction analyses indicate that b-GO papers have a much larger amorphous component. Thermal treatments of b-GO paper easily produce complete loss in stacking order of graphene oxide layers while thermal treatments of GO paper lead to their disintegration but only to partial loss in stacking order. Also microwave treatments at room temperature of b-GO paper easily lead to loss in stacking order of graphene oxide layers.

TGA and DSC measurements show that, for b-GO papers, thermal deoxygenation phenomena are less than halved with respect to those observed for GO paper.

Physical and chemical properties of b-GO paper suggest the formation of interlayer covalent bonds. Infrared characterizations indicate that the main change observed moving from GO to b-GO papers is the disappearance of the epoxy peaks. This indicates that relevant properties of b-GO are induced by reactions of the hydroxyl ions with the epoxy groups on the GO layers. A possible mechanism leading to formation of interlayer ether bonds, for b-GO paper, is proposed. Additional work is needed to confirm the presence of these interlayer ether links.

Acknowledgements

We thank Dr Luca Giannini of the Pirelli Tyre Research Center, Prof. Maurizio Galimberti of Politecnico of Milan, Prof. Pasquale Longo of University of Salerno for useful discussions and Ms Luisa Annunziata for experimental support. Financial support of Pirelli Tyre and of “Ministero dell' Istruzione, dell' Università e della Ricerca” is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09476g

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