Formation and Decomposition of CO2 Intercalated Graphene Oxide

Article pubs.acs.org/cm

Formation and Decomposition of CO2 Intercalated Graphene Oxide Siegfried Eigler,*,† Christoph Dotzer,† Andreas Hirsch,*,† Michael Enzelberger,‡ and Paul Müller‡ †

Department of Chemistry and Pharmacy, University Erlangen-Nürnberg and Institute of Advanced Materials and Processes (ZMP), Henkestrasse 42, 91054 Erlangen and Dr.-Mack Strasse 81, 90762 Fürth, Germany ‡ Department of Physics, Universität Erlangen-Nürnberg, Erwin-Rommel-Strasse 1, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: The formation, stability, and decomposition of CO2 intercalated graphene oxide was analyzed by FTIR, TGAMS, TGA-IR, AFM, and SEM for the first time. We found that the formation starts at 50 °C and develops up to 120 °C. The formation process can be best observed by FTIR spectroscopy, and the product is stable at ambient conditions. At higher temperatures, the decomposition of CO2 intercalated graphene oxide occurs due to the release of water, CO2, and CO that can be monitored by TGA-MS and TGA-IR analysis. AFM and SEM images can visualize the formation of blisters in GO films that become instable at 210 °C. We further prepared graphene oxide with a low water-content and found that the formation of CO2 was significantly suppressed and CO became the major species responsible for the weight loss. In addition we prepared 18OH2 treated graphene oxide to elucidate the formation process of CO2 and found C16O18O by TGA-MS analysis that proves the crucial role of water during CO2 formation. From these experiments we propose that hydrate species are key-intermediates for the formation of CO2. Hence, it seems likely that rearrangement reactions that can proceed via hydrate intermediates, known from organic chemistry, are probably responsible for the formation of carboxylic acids at the edges of graphene oxide sheets after sonication of graphite oxide. Further, our investigations prove that graphene oxide is less stable than shown by TGA measurements. This has a high impact on the electronic properties of reduced graphene oxide, especially for all those using it for electronic applications. KEYWORDS: graphene, graphite oxide, graphene oxide, intercalation compound, mechanism, decomposition

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TEM imaging.20 Thermal treatment of GO films in the solid state causes decomposition accompanied by the release of water and CO2 as main products at a temperature of 150 °C, according to TGA analysis.21 The role of intercalated water in multilayered GO was studied by Acik et al., and they found that CO2 formation comes along with the generation of additional carbonyl groups and defects at a temperature of about 125 °C.22 We find that the generation of CO2 from GO occurs already at 50 °C and a compound that can be termed as CO2 intercalated GO is formed between 50 and 120 °C. This phase decomposes after the release of CO2, water, and CO at about 160 °C. Besides, it is important to know that graphene oxide already decomposes at 50 °C to some degree. This causes defects that limit the electron transport in reduced GO, e.g. if it is used in transistors. It is interesting to note that early observations by Charpy in 1909 describe the evolution of CO2 at 45 °C during the oxidation of graphite in sulfuric acid by potassium permanganate − a preparation method similar to that reported by Hummers.23

INTRODUCTION Graphene oxide (GO) is regarded to be one of the most promising materials for the bulk production of graphene-like sheet structures. It was not only used in composite1 and electronic materials2 but also as a conductive and transparent layer for the substitution of ITO electrodes.3,4 One of the most striking advantages of GO is its availability in large amounts in contrast to graphene prepared by mechanical exfoliation of graphite and by CVD methods.5 Recent reviews describe such advantages of GO in great detail.5,6 Moreover, GO was used to prepare supercapacitors,7,8 sensors,9 reduced GO transistors,10,11 liquid crystals,12 or used for DNA analysis.13 The structure of GO is inhomogeneous and depends on the preparation process.14,15 According to the Lerf-Klinowski model, the most important functional groups are epoxy and hydroxyl moieties bound above and below the carbon plain and carboxylic acids at the edges of the GO sheets.16,17 In order to use GO for electronic applications the π-system should be restored as much as possible by removing these groups. Next to various chemical reduction methods, like hydrazine reduction,18 annealing of GO is often used.19 Especially during thermal annealing defects are introduced that cannot be healed perfectly. This was revealed, for example, by high resolution © 2012 American Chemical Society

Received: October 27, 2011 Revised: March 9, 2012 Published: March 12, 2012 1276

dx.doi.org/10.1021/cm203223z | Chem. Mater. 2012, 24, 1276−1282

Chemistry of Materials

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GO with Reduced Sulfur Content. Freeze-dried graphite oxide (100 mg) was dispersed in water (100 mL) and sonicated using a cuphorn sonicator (30 W, pulsed 1 s on, 1 s off). After sonication the dispersion was centrifuged at (10000 g, 20 min) to remove minor amounts of few layered GO. The GO was further washed by water using a centrifuge (20000 g, 1 h) for five times. The centrifuged GO was dispersed in sodium hydroxide solution (1 M) and centrifuged (20000 g, 1 h). The contact time of base with GO was kept short to avoid decomposition reactions. The GO was further washed by water using a centrifuge (20000 g, 1 h) for five times. Finally, the GO solution was freeze-dried, and an orange colored fluffy felt was obtained. TGA-MS experiment did not detect significant amounts of SO2 in the temperature-range between 24 and 1000 °C to prove the reduced sulfur-content. Elemental Analysis: C 46.98, H 2.37, S close to detection limit (see the Supporting Information). 18 OH2 Exchanged Graphene Oxide. Freeze-dried GO (15 mg) was dispersed in 18O labeled water (1 mL) and sonicated in a closed vial for 10 min using a bath sonicator. The dispersion was stored in the refrigerator for 1 week and then the dispersion was freeze-dried and used for TGA-experiments. TGA-IR of Graphene Oxide. Freeze-dried GO (6.5 mg) was used for TGA experiments (rt-700 °C, 15 K/min, He 80 mL/min). The TGA was coupled with Bruker Tensor FTIR spectrometer, and spectra were recorded in the range of 4500 and 500 cm−1 with a resolution of 4 cm−1. Preparation of SEM and AFM Samples. The samples for microscopy were prepared by the drop casting methods. About 100 micro liters were dropped on a polished silicon wafer and dried for several hours. The samples with 2 to 4 layers have been produced by repeating this method. The samples were heated on a hot plate to the desired surface temperature and subsequently analyzed with both microscopic techniques. The procedure was repeated for all temperatures with the same samples.

EXPERIMENTAL PROCEDURES

General Methods. Natural flake graphite was obtained from Kropfmuehl AG, Germany. The quality used for the oxidation was SC20O. 18O labeled water, potassium permanganate, sodium nitrate, and sulfuric acid were obtained from Sigma-Aldrich. Thermogravimetric analysis (TGA) equipped with a mass spectrometer (MS) was accomplished on a Netzsch STA 409 CD instrument equipped with a Skimmer QMS 422 mass spectrometer (MS/EI) with the following programmed time dependent temperature profile: 24−700 °C (24− 800 °C or 24−1000 °C, TGA-MS) with 10 K/min gradient, and cooling to 24 °C. The initial sample weights were about 6 to 7 mg, and the whole experiment was accomplished under inert gas atmosphere with a He gas flow of 80 mL/min. m/z 32 was used to monitor the background during measurement. TGA analysis equipped with infrared spectrometer was accomplished on a Bruker Tensor FTIR spectrometer with Skimmer coupling. Bruker Tensor FTIR spectrometer equipped with ZnSe was used for the measurements of GO films. Freeze-drying was accomplished on an ALPHA 1-4 LDplus from Matrtin Christ, Germany. Sonication was done using a Sonoplus HD3200 with 13 mm sonotrode from Bandelin, Germany. For centrifugation a Sigma 4K15 centrifuge, Sigma Laborzentrifugen GmbH, Germany, was used. Optical microscope images are made with Axio M1m Imager, Carl Zeiss, Germany. Atomic force microscopy measurements were performed on an Asylum Research MfP 3d AFM in tapping mode or SolverPro from NT-MDT. The oscillation amplitude was usually chosen to be 5 nm; the set point was about 3 nm. SEM microscopy was performed on a Zeiss Leo 1530. The images were recorded at 10 kV acceleration voltage with the Inlens detector. Raman spectra were recorded using LabRAM Aramis Raman spectrometer from Horiba Scientific. The duoscan option was used measuring average spectra of 900 μm2. Elemental analysis was performed by combustion and gas chromatographic analysis with an EA 1110 CHNS analyzer from CEInstruments. Graphite Oxide. Graphite (1 g) and sodium nitrate (0.5 g) were suspended in sulfuric acid (24 mL) at room temperature. After stirring for 30 min the mixture was cooled to 5 °C in an ice-bath. Potassium permanganate (3 g) was added in small portions (500 mg, each) under stirring using a mechanical stirring unit (200 rpm). The temperature did not exceed 30 °C. After complete addition of the oxidation agent the dispersion was stirred for an additional hour at RT. During stirring the viscosity increased. The oxidation was quenched by adding ice-cold water (250 mL) under stirring (200 rpm). The temperature was raised to 80 °C. After 30 min ice-cubes were added to cool the dispersion, and hydrogen peroxide (10 mL, 3%) was added in small portions until the dispersion became yellow. After stirring for an additional 30 min the reaction mixture was centrifuged (4000 g, 10 min) and washed with water until the pH was neutral. Totally 4 L water were used. The graphite oxide dispersion was freeze-dried at a concentration of about 5 mg/mL and a brownish felt was obtained. Elemental Analysis: C 45.89, H 2.40, S 3.88. Graphene Oxide. Freeze-dried graphite oxide (100 mg) was dispersed in water (100 mL) and sonicated using a cup-horn sonicator (30 W, pulsed 1s on, 1s off). After sonication the dispersion was centrifuged at (10000 g, 20 min) to remove minor amounts of few layered GO. The GO was further washed by water using a centrifuge (20000 g, 1 h) for five times. Finally, the GO dispersion was freezedried and a gray fluffy felt was obtained. Elemental Analysis: C 47.14, H 2.75, S 3.32. GO Films on ZnSe. A diluted dispersion of GO was drop-casted on a ZnSe window, and the film was formed by evaporation of the solvent at ambient conditions. The thickness of the film was determined using the Z-indicator of the Zeiss microscope. Thereto, the film was scratched, and either the top of the film or the substrate was focused. Annealing of GO Films. The GO films were heated on a hot plate. The temperature of the film was measured on the substrate. The annealing was done in air. As a reference experiment, a glovebox (argon) was used during annealing to ensure that air has no effect on the material.

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RESULTS AND DISCUSSION GO was prepared according to Hummers’ method.24 As starting material natural graphite from Kropfmuehl AG was used. The graphite was oxidized by potassium permanganate in sulfuric acid with the aid of sodium nitrate. After one hour of oxidation the mixture was diluted with water. Subsequently hydrogen peroxide was used to oxidize manganese salts. The yielded graphite oxide was excessively washed with water and repeatedly centrifuged until the pH of the supernatant was neutral. After the sonication assisted exfoliation GO was isolated by freeze-drying. The obtained GO was characterized by FTIR using ZnSe windows. For this purpose, thin films (about 3 μm) of GO were prepared by drop casting from aqueous GO dispersions. The atmospheric background especially the CO2 signals were compensated, and the spectra were recorded iteratively after heating in steps of 10 °C. The results are shown in Figure 1. In a reference experiment a GO film on ZnSe was heated under argon instead of air, and the FTIR spectra were recorded after each heating step. We could prove that oxygen has no effect in this temperature range (Figure S1). The signals observed between 3100 and 3700 cm−1 are due to the O−H-vibrations of the C−OH-groups, H2O and carboxylic acids. These signals disappear upon annealing at more the 140 °C. Moreover, carbonyl-groups can be identified by the absorption at 1742 cm−1. They are stable during thermal treatment. The signal at 1628 cm−1 fades away upon heating between 50 and 100 °C and can be assigned to O−H of adsorbed water, as can be seen from TGA-MS measurements (Figure 3). By the elimination of water the signal at 1580 cm−1 that can be assigned to CC bonds22 becomes visible. Further, distinct vibrational modes for epoxides (C−O−C) at 1200−1300 and 1277



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the signal intensity at 2336 cm−1 increases up to 120 °C followed by a subsequent decrease. From the FTIR absorbance of CO2 we estimate the mean distance between two CO2 molecules to be about 2 nm (see the Supporting Information). This process plays a major roll in GO films due to the high degree of functionalization. The CO2 formation in GO is accompanied with carbon loss causing defects in the carbon sheets. According to TGA-MS analysis CO2 is barely detectable until 130 °C (m/z 44, Figure 3). This temperature is about 80 °C higher compared with the

Figure 3. TGA-MS spectra of freeze-dried GO between rt and 800 °C.

Figure 1. FTIR spectra of GO films on ZnSe after annealing between 25 and 150 °C.

FTIR results. At about 130 °C the intercalated CO2 is released accompanied with CO and water due to bursting and flaking of the GO film. As a consequence we find CO2 intercalated GO that was formed at 120 °C to be stable at ambient conditions according to the complementary FTIR and TGA-MS analysis. We further studied the decomposition of GO by TGA-MS analysis. The weight loss occurs in four steps. Between RT and 120 °C mainly water (m/z 18, 10%) is released, followed by CO2 and CO (m/z 44, 28, 18%) in the temperature range of 120 to 190 °C. A further weight loss of 17.5% that originates from water, CO, CO2, and SO2 (m/z 18, 28, 44, 64) occurs between 200 and 300 °C. The formation of water and SO2 originates from the treatment with H2SO4 that remains on GO, even when the pH of the supernatant of a GO dispersion is neutral. At higher temperatures the weight loss is dominated by CO formation. The species generated during thermal treatment were also analyzed by TGA-IR coupling, and it was possible to identify the SO2 species according to its FTIR absorption at 1360 cm−1 (Figures S2−S5). The thermal annealing process was further evaluated by using both, SEM imaging for large area analysis and AFM to gain deeper insights into the morphology of the GO films. The thermal annealing process of GO films was done in steps of 10 °C. The film consists of overlapping GO flakes and the flake structure is still visible and forms a closed network of overlapping GO flakes (Figure S6). At 160 °C the SEM micrographs reveal the decomposition of CO2 intercalated GO. At this temperature CO2 can diffuse through GO layers and starts to form blisters within the film. At 180 °C the amount of blisters increases, and at 210 °C it is possible to detect collapsed blisters forming a caldera (Figure 4). We further analyzed the formation process of CO2 by SEM analysis of GO films in dependence on the film thickness. For this purpose, GO films with a thickness of 1, 2, and 3 μm were investigated. We found that the amount of blisters increases with the film thickness (Figure S9). The size of the initially formed CO2 blisters was measured by AFM. Especially the

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about 800−900 cm can be identified. Of special significance is the signal at 2336 cm−1 that can be assigned to CO 2 intercalated between the GO planes. In Figure 2 GO with

Figure 2. Illustration of CO2 intercalated GO, based on Lerf-Klinowski model; GO layers are AM1 optimized, CO2 is placed manually between two layers, residual water is omitted for clearance; gray: carbon atoms, white: hydrogen atoms; red: oxygen atoms.26

CO2 intercalated between the layers is shown. The model is based on the Lerf-Klinowski model to illustrate a typical arrangement of such an architecture.25 Obviously, during thermal annealing CO2 is formed (compare Figure 1, 120 °C) from GO leading to a stage 1 intercalation compound, as graphite oxide is. Some water is still present according to FTIR spectra, which are omitted in Figure 2. The intercalation and adsorption of CO2 in carbon allotropes has been described in the literature, e.g. by Fastow et al. who observed a vibrational mode at 2341 cm−1 at low temperatures.27 Moreover, CO2 was also trapped in carbon nanotube bundles and identified by a signal at 2330 cm−1.28 Recently, trapped CO2 was detected after annealing multilayered GO to 125 °C during studying the role of water in graphite oxide.22 As can be seen from Figure 1 we find that the CO2 formation already starts at a much lower temperature of about 50 °C, and 1278



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be concluded from IR spectroscopy. By increasing the temperature, the intercalated CO2 can diffuse through the layered GO flakes and CO2 blisters start to form due to CO2 oversaturation. Upon further raising the temperature, the blisters increase in size until they finally burst. This in turn means that GO intercalated by CO2 is stable up to 120 °C. We analyzed the GO films on ZnSe by Raman spectroscopy after annealing at 60, 120, and 160 °C (532 nm laser) and did not find any significant changes in the spectra (Figure S10). In order to see changes, the distance between defects must be on the nm scale, and this is not the case here.29 Further TGA analysis of differently treated GO provided insights in the GO structure. In addition, we observed the combined formation of CO2 and water in the temperature region of 50−200 °C and at 250 °C CO2 is formed together with SO2 (Figure 3). Therefore, we studied the influence of water and its effect on the formation of CO2 more in detail. First, we reduced the residual sulfur content by washing GO once with 1 M NaOH followed by further washing steps to reach neutral pH again. TGA analysis shows 3% less weight loss up to 200 °C, because some elimination of functional groups might have occurred. In general the material is still highly functionalized, which can be concluded from the total weight loss of more than 40%. Further, sulfur species that decompose up to 1000 °C could be removed effectively, and the signal m/z 64 corresponding to SO2 was almost not detected any more (Figure S11). The corresponding TGA-MS spectra of freezedried GO with reduced sulfur content are shown in Figure 6.

Figure 4. SEM-images of GO-films at RT, 160, 180, and 210 °C (left to right, up to down); Formation of CO2 blisters (160, 180 °C), collapsed CO2 blisters (210 °C).

height of the blisters can be determined with this technique (Figure 5). Besides some larger ones, typical blisters have a height between 8 and 14 nm with an estimated diameter of about 20−30 nm. As can be seen from Figure 4, during the annealing process the size of CO2 blisters increases to typically 300 nm before bursting. Obviously, CO2 is formed at about 50 °C and remains intercalated between the densely packed GO flakes, which can

Figure 6. TGA-MS spectra of freeze-dried GO with reduced sulfur content between rt and 800 °C.

The main weight loss of 15% was found at about 160 °C due to the release of water, CO2, and CO. This temperature is almost the same for GO and GO with reduced sulfur content. Up to 300 °C the weight loss can be assigned to water, CO2 and CO release and at higher temperatures CO-formation is dominating. TGA-analysis reveals a water-content of about 10% in GO for adsorbed water. This can be concluded from TGA-MS spectra confirming that the main weight loss is caused by water (m/z 18). Now we analyzed the water content in freeze-dried GO and tried to reduce the water content by making thin films of GO on ZnSe, freeze-dried the films, and further dried the films over P2O5 up to two weeks, as it is described in the literature.30 We compared the FTIR spectra of the freeze-dried product, before drying and after the drying process over P2O5 and found no significant differences (Figure S12). So we can exclude any transformation of functional groups due to the

Figure 5. AFM-image of GO-film after heating to 180 °C and heightprofile of three blisters. 1279



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drying process and conclude that the residual water is strongly bound to GO, comparable to water of crystallization. Next, we decreased the water content in GO by exchanging the solvent from water to THF in a similar way as it was demonstrated with ethanol.31 This was done by iterative centrifugation and subsequent evaporation of the solvent. The material was analyzed by TGA-MS, and no significant THF was detectable (m/z 72). In the temperature range between rt and 120 °C, we found a significantly lower weight loss of less than 3% (Figure 7). In this sample, the removal of water leads to a

Figure 8. TGA-MS of GO; water exchanged with 18OH2; C16O18O evolution compared with GO as reference material.

suggest that hydrates can be formed in GO from carbonyl groups (Scheme 1a). Scheme 1. Proposed Mechanism for 18O Incorporation from 18 OH2 in GO with Hydrates As Key-Intermediatesa

Figure 7. TGA-MS spectra of dried THF treated GO between rt and 800 °C.

40 °C higher release-temperature for water, CO2, and CO (200 °C), compared to freeze-dried GO. Moreover, it is eye-catching that the amount of produced CO2 is much lower compared to freeze-dried GO. From Figure 7 one can conclude that the amount of produced CO2 is only about one-half compared to GO with higher water content (Figure 6). Simultaneously, the detected amount of water and CO are almost constant. At higher temperatures almost no additional water is formed from which we conclude that the weight loss is attributed to the formation of CO instead of CO2. These finding suggest that water molecules are involved in the formation of CO2. To get further evidence for the influence of water, freeze-dried GO was treated with 18O labeled water (followed by an additional freeze-drying step). This procedure should lead to an exchange of adsorbed water to yield a mixture of GO with normal 16O water molecules and 18O labeled water molecules next to the functional groups of GO. Indeed C16O18O is detected next to C16O2 by TGA-MS (Figure 8). We note, that we did not find a significant signal corresponding to C18O2 (Figure S13) and a shift of the C16O18O is not observed by FTIR, as it is known from other studies.32 The question is how can water exchange with functional groups present in GO to form C16O18O. There are several types of functional groups present in GO, mainly epoxides and hydroxyl groups. Further, carbonyl groups at the edges or at defective structures in the GO plane are present also. From the chemical point of view it is not possible to exchange OH groups in alcohols with water or O atoms in epoxides. On the other hand, carbonyl groups and water can react to form hydrates. Acids also present in GO can catalyze this reaction. The formation of hydrates can be further promoted due to additional electron withdrawing groups like epoxides or alcohols in α-position to carbonyl groups. Therefore, we

GO − abbreviation for inhomogeneous graphene oxide. a) Exchange of water and hydrate formation leading to CO and CO2, b) Exchange of water in carboxylic acids, and c) Rearrangement reaction of αepoxy ketones leading to carboxylic acids. a

Especially carboxylic acids that tend to decarboxylate at elevated temperatures can be formed by the influence of water. In particular this process might start either at defects or at highly oxidized carbon atoms. We annotate that the chemistry of GO is far away from being completely understood. One can assume that the formation of carboxylic acids occurs at defects or at the edges of GO sheets (Scheme 1b). On the other hand, rearrangement reactions with epoxides and hydrate species involved in the mechanism are well-known in organic chemistry, e.g. the base induced rearrangement of α-epoxy ketones that is related to the benzylic acid rearrangement (Scheme 1c).33 We want to annotate further that rearrangement reactions yielding carboxylic acids might be formed in the plane of GO and at its edges. If such a reaction is possible in GO, then carboxylic acids can be formed, and the exchange with water results in C16O18O upon heating. As a consequence, rearrangement reactions can also give an explanation why it is possible to form carboxylic acid groups at the edges of GO planes during the exfoliation 1280



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material for applications such as electronic devices, that require a material with a minimum of defects. If reduced GO should meet such demands in the future, it is necessary to avoid elevated temperatures, as we could prove by the formation of CO2 intercalated GO.

process of graphite oxide when no oxidation agent is present. With Hummers’ method graphite oxide is formed and purified during aqueous workup. Hydrogen peroxide is used to remove manganese salts and to break manganese esters formed during the oxidation process. So, the strong oxidation agents are probably responsible for the formation of carboxylic acids at the edges of graphite oxide sheets. Next, the formation of GO is done by sonication. During this sonication process the GO sheets can break and even if the graphite oxide flakes have a diameter of 20 or more μm finally GO flakes with a diameter of 1−5 μm are found, as it is observable by optical microscopy (Figure S15) and AFM (Figure S16). It seems likely that breaking GO sheets by sonication generates dangling bonds that are hydrated by the aqueous solvent but not oxidized. If only hydration occurs it is difficult to explain carboxylic acids at the newly formed edges. On the other hand, rearrangement reactions can certainly yield carboxylic acids in both graphite oxide and GO, respectively. Finally, all types of carboxylic acids present in GO can form hydrates and decarboxylate upon thermal treatment (Scheme 1b).

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra of GO films on ZnSe annealed under argon; details of TGA-IR analysis of GO between RT and 700 °C; AFM image of GO flake structure in GO films; SEM images of heated GO film at 180 °C and CO2 intercalated GO at 110 °C and CO2 formation in GO films with different film thickness; Raman spectra of GO film on ZnSe after annealing at 60, 120, and 160 °C; FTIR spectra of freeze-dried GO on ZnSe after 2 weeks dried over P2O5, TGA-MS analysis of 18OH2 exchanged GO, microscope images of graphite oxide and GO and AFM image of GO. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS We report on the first systematic generation of CO 2 intercalated GO. The phase is formed between 50 and 120 °C by decarboxalation, which is predominantly promoted by the water present in the starting materials. Carbon dioxide intercalated GO is stable at ambient conditions, and the intercalated CO2 cannot escape from the layered material at room temperature. At elevated temperatures intercalated CO2 can unambiguously be detected by TGA-MS. Moreover, the related CO2 release was also analyzed by SEM and AFM due to the appearance of blisters in between the layers formed at temperatures between 160 and 210 °C. First, smaller blisters with a size of about 10−20 nm are formed. Further increase of the temperature causes the amount of blisters to increase until they burst at about 210 °C. At this stage they approach typical diameters of 300 nm. The blister density also depends on the thickness of GO films as proved by SEM. TGA-MS and TGAIR analysis was found to be a very powerful tool to investigate the purity of GO. We discovered that SO2 elimination originating from previously introduced sulfuric acid takes place as well. This can be avoided by applying a preceding washing step in the presence of base. In this way it was possible to prepare GO with reduced sulfur content. Importantly we also discovered that water is responsible for the formation of CO2 from GO. As a consequence, the formation of CO2 is significantly suppressed after removing water from GO. Waterexhange experiments using 18O labeled water clearly demonstrated the critical role of water by the observation of C16O18O formation. A plausible mechanism of water incorporation is suggested with the formation of hydrate intermediates. Keto groups are one important source for formation of CO2. In addition also α-epoxy ketones are expected to rearrange via hydrate species to form carboxylic acids which represents a well-known reaction in organic chemistry. We also conclude that rearrangement reactions play an important role in forming carboxylic acids in GO during its preparation by sonication that lead to small GO flakes after breaking the sheets. These carboxylic acids can be used to further functionalize GO sheets. Finally, carboxylic acids can exchange water and incorporate 18 O to form C16O18O during annealing. Our results provide new insights into the chemical properties and stability of GO, which is of importance for the development of GO as new

AUTHOR INFORMATION

Corresponding Author

*A.H.: Phone: +49 (0)9131 8522537. Fax: +49 (0)9131 8526864. E-mail: [email protected]. Corresponding author address: Department of Chemistry and Pharmacy, University Erlangen-Nürnberg, Henkestrasse 42, 91054 Erlangen, Germany. S.E.: Phone: +49 (0)911 6507865005. Fax: +49 (0)911 6507865015. E-mail: siegfried. [email protected]. Corresponding author address: Department of Chemistry and Pharmacy, University Erlangen-Nürnberg and Institute of Advanced Materials and Processes (ZMP), Henkestrasse 42, 91054 Erlangen and Dr.Mack Strasse 81, 90762 Fürth, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Cluster of Excellence “Engineering of Advanced Materials” at the University of Erlangen-Nuremberg for financial support.

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