Article pubs.acs.org/Langmuir
Selective Insertion of Sulfur Dioxide Reduction Intermediates on Graphene Oxide Eduardo Humeres,*,† Nito A. Debacher,† Alessandra Smaniotto,† Karen M. de Castro,† Luís O. B. Benetoli,† Eduardo P. de Souza,† Regina de F. P. M. Moreira,‡ Cristiane N. Lopes,‡ Wido H. Schreiner,§ Moisés Canle,∥ and J. Arturo Santaballa∥ †
Departamento de Química and ‡Departamento de Engenharia Química e de Alimentos, Universidade Federal de Santa Catarina, Florianópolis, SC, Brazil § Departamento de Física, Universidade Federal do Paraná, Curitiba, PR, Brazil ∥ Grupo de Reactividade Química e Fotorreactividade, Departamento de Química Física e Enxeñaría Química, Universidade da Coruña, Rúa da Fraga 10, E-15008 A Coruña, Spain ABSTRACT: Graphite microparticles (d50 6.20 μm) were oxidized by strong acids, and the resultant graphite oxide was thermally exfoliated to graphene oxide sheets (MPGO, C/O 1.53). Graphene oxide was treated with nonthermal plasma under a SO2 atmosphere at room temperature. The XPS spectrum showed that SO2 was inserted only as the oxidized intermediate at 168.7 eV in the S 2p region. Short thermal shocks at 600 and 400 °C, under an Ar atmosphere, produced reduced sulfur and carbon dioxide as shown by the XPS spectrum and TGA analysis coupled to FTIR. MPGO was also submitted to thermal reaction with SO2 at 630 °C, and the XPS spectrum in the S 2p region at 164.0 eV showed that this time only the nonoxidized episulfide intermediate was inserted. Plasma and thermal treatment produced a partial reduction of MPGO. The sequence of thermal reaction followed by plasma treatment inserted both sulfur intermediates. Because oxidized and nonoxidized intermediates have different reactivities, this selective insertion would allow the addition of selective types of organic fragments to the surface of graphene oxide. industrial plants to avoid acid rain17 and to activate carbon particles for further functionalization.18 The reaction follows the same stoichiometry, considering the sulfur to be S2 (eq 1), for a number of different carbon matrices, and the reactivity increases in the order graphite ≈ cokes < charcoal < activated carbon.19
1. INTRODUCTION The surface chemistry of Si, Ge, and C has been intensely studied recently because these elements are semiconductors1 and their functionalization may have important consequences for their use in microelectronics.2,3 Different forms of carbon, such as graphite, nanotubes, and fullerenes, have also been widely investigated as adsorbents, composites, and catalytic supports and for medical and pharmaceutical uses.4,5 The studies carried out so far have furnished a useful database and have identified important trends in the reactivity and selectivity of functionalized carbon matrices. Particular attention has been given to the properties and functionalization of graphene and graphite oxide,6,7 which has led to a number of other novel materials because of their outstanding mechanical,8,9 thermal,10,11 electronic,12 and catalytic13 properties. Sulfur can be incorporated in large amounts into the edges of graphite, and these graphite−sulfur composites present superconductive properties.14,15 First-principles calculations on graphene sheets functionalized with episulfide and thiol groups found that it has a very low reactivity against the thiol group. The results confirmed the experimental evidence, which indicated that the sulfur-containing groups present in sulfur− graphite nanocomposites are attached to the edges of graphite and that vacancy defect sites must be considered.16 The reduction of SO2 on carbons to elemental sulfur is an important reaction in eliminating SO2 in flue gases from © 2014 American Chemical Society
SO2 + C → CO2 +
1 S2 2
(1)
The reaction proceeds on a graphite surface at 900 °C, but the energy demand of the reaction decreases sharply by oxidation and the decreasing size and crystallinity of the carbon matrix.19,20 During the reaction of activated carbon with SO2, sulfur was rapidly incorporated into the carbon matrix up to a constant content when the system reached steady-state conditions. The XPS spectra in the S 2p region showed a band at 163.9 eV (nonoxidized sulfur) and a band at 168.2 eV (oxidized sulfur). The reaction of this carbon with CO2 produced only SO2, demonstrating that the reaction proceeds through these sulfur intermediates and that eq 1 consequently is reversible.20 It also showed that CO2 is indeed the primary product of the direct reaction. Received: January 11, 2014 Revised: February 19, 2014 Published: March 7, 2014 4301
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episulfide groups. The results showed that the thiolysis and photolysis in t-butanol occurred on the oxidized intermediates, whereas in the aminolysis only the nonoxidized intermediate was involved. These important differences would allow a selective functionalization of carbon particles. The inert nature of carbon particles, especially carbon nanotubes and graphenes, and their strong tendency to agglomerate limit their applications. Covalent and noncovalent functionalization of graphene oxide30,31 and epitaxial graphene32 modifies their properties with relevant potential applications.33,34 Advances to modify the chemical and physical properties of particles by changing their surface chemical composition, without affecting the bulk properties, have been achieved by using a nonthermal plasma.35−37 A nonthermal plasma is, in a general sense, any plasma not in equilibrium (thermodynamic, thermal, mechanical, or radioactive) because the ion temperature is different from the electron temperature or the velocity distribution of one of the species does not follow a Maxwell− Boltzmann distribution.35 Nonthermal plasma treatment is one of the most versatile surface treatment techniques because it is nonpolluting and has short reaction times.36−38 It is a simple technique for inserting a variety of atomic and molecular radicals on the surface of materials38 and for creating a specific environment for oxidative, reductive, or inactive reactions by altering the feed gas, thus it is possible to insert or interlock certain functional groups on the surface.36,39 Nonthermal dielectric barrier discharge (DBD) plasma technology can operate with different gases at atmospheric pressure, with reasonably high power levels and without sophisticated equipment.35 Low-pressure plasma treatment of Laponite clay,39 polymers,40 and highly oriented pyrolitic graphite41 in the presence of SO2 has been used to incorporate several sulfur species into the solid matrix. In this article, we report the selective insertion of SO2 into graphene oxide by excitation with nonthermal plasma or/and thermal treatment, as an oxidized or nonoxidized sulfur intermediate, and the subsequent formation of the expected final products of the reduction of SO2 on carbons. The samples were characterized by X-ray photoelectron spectroscopy (XPS) and TGA/DTA coupled with FTIR.
A theoretical study of the chemisorption process of SO2 on the graphite surface was carried out using a pyrene structure and two dehydrogenated derivatives as models of graphite, one containing an armchair edge with benzyne-like structure and the other containing a zigzag edge that corresponds to a triplet biradical structure.21 Models of graphene sheets also consider the existence of free edge sites of the carbyne type on the armchair sites and the carbene type on the zigzag sites.22 The results from the pyrene model showed that at 900 °C the sites of the zigzag edge are completely occupied through [3 + 3] cycloaddition to form a six-membered ring, 1,3,2-dioxathiolane (1), and/or [2 + 3] cycloaddition to form a five-membered ring, 1,2-oxathiolane 2-oxide or γ-sultine (2) (Figure 1). Both
Figure 1. Mechanism of the primary reaction.
species of oxidized sulfur intermediates are in equilibrium (at 900 °C, ΔGo = 1 to 2 kcal·mol−1).21 It has also been observed that the covalent functionalization of graphite and exfoliated graphene sheets occurs preferentially at the edges rather than in the domains of the basal plane.23−25 These results are summarized in the postulated primary mechanism involved in the SO2 reduction on carbons shown in Figure 1, where the zigzag diradical fragment represents the carbon matrix.26 Oxidized intermediate 1 decomposes to produce episulfide 3 (nonoxidized sulfur intermediate) that initiates a transport mechanism of sulfur out of the carbon matrix.26 The sulfur transfer from the dioxathiolane forming the episulfide must generate an oxidized moiety C(CO2) that will produce CO2 in a later step. The mechanism of CO2 formation has not yet been studied, and for this reason the fragment of the residual carbon matrix in Figure 1 is not considered. This mechanism has been found to be consistent for different carbons, such as charcoal, graphite, and activated carbon.19,20,27,28 The fact that the stoichiometry, entropy of activation, intermediates, and specific reactivity of the intermediates are essentially the same for graphite, graphite oxide, graphene oxide, and activated carbon strongly suggests that the reaction site of SO2 reduction on carbon matrixes is the same, independent of the oxidation state of the matrix.27,28 As mentioned above, the oxidation of the matrix produced a large acceleration of the reduction reaction. However, at the moment we do not know the mechanism of this activation. The characterization of the intermediates of the reduction of SO2 on carbons was done through the study of their reactivity with organic reagents. The reactivity was studied with respect to the basic hydrolysis, thiolysis, aminolysis, reaction with alkyl halides, and photolysis in t-butanol.18,28 The insertion of the organic moiety on the carbon matrix was observed by ssC13 NMR. The XPS spectra after the reaction was calculated by the atom inventory method,29 assuming reactions already described in the literature that involve dioxathiolanes, sultines, and
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Graphite microparticles (MPG) were obtained from Cia, Nacional de Grafite Ltd., Brazil, with a composition of 99.92% carbon, 0.08% ash, and 0.018% moisture and a d50 value of 6.20 μm. Sulfur dioxide supplied by White & Martins, Brazil, was 99.9% pure and diluted in nitrogen. MPG was characterized by elemental analysis (PerkinElmer-240, LECO-SC 132). The X-ray diffraction analysis was carried out on a Philips Xpert diffractometer operating at 40 kV and 30 mA with a 2q scanning speed of 0.05°/s and with a path width of 0 to 120°. SEM images were obtained with an LEO1525 field-emission gun scanning electron microscope from Carl Zeiss SMT AG. The specific surface area determination was performed with an Autosorb-1 Quantachrome Nova-2200e. A mass of 100 mg of each sample was outgassed under vacuum at 200 °C for 6 h prior to analysis. Measurements were performed at the temperature of liquid N2. The average specific area was determined by the BET method. TGA/DTA coupled with FTIR (model Vector 22, Bruker) was carried out under a flow of nitrogen (50 mL·min−1) using a TGA (model SDT2960, TA Instruments). The temperature was calibrated using an aluminum standard. Mass measurements were calibrated with appropriate alumina weights. Samples, ranging in size from 1 to 4 mg, were heated from room temperature to 800 °C at 5 °C/min. 4302
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Figure 2. (A) Experimental setup used in this study. (B) View of the reactor cross section.
Figure 3. X-ray diffractogram of graphite microparticles (a) before and (b) after oxidation. The XPS spectra were obtained in a VG Microtech ESCA 3000 spectrometer with an MG (Kα) source. The pressure of the system was 10−10 mbar, and the operational pressure was maintained at less than 10−8 mbar during the measurements. The calibration was carried out with respect to the main peak of C 1s at 284.5 eV. The concentrations of elements were calculated from the system databank, and the deconvolution of the peaks was obtained using the spectral data processor (SDP) v 4.5.42 2.2. Oxidation of Graphite Microparticles. The method was adapted from Bissessur et al.43 as described elsewhere28 using a 500 mL double-walled cylindrical glass vessel with mechanical stirring. Concentrated sulfuric acid (230 mL) was transferred to the reactor, and 10 g of MPG and 30 g of KMnO4 were slowly added. The mixture was cooled and stirred for 30 min. Distilled water (230 mL) was slowly added to the reactor, and the mixture was transferred to a beaker and diluted with 1.4 L of water and 100 mL of 30% hydrogen peroxide. Because filtering is an extremely slow method of washing the particles, the material was exhaustively centrifuged instead by decanting the supernatant and adding distilled water until the particles tested negative for sulfate. The particles were exfoliated in a tubular quartz reactor fitted with a temperature controller and heated in an electric oven. The sample was positioned at the bottom of the reactor and heated for 10 min under a 100 mL·min−1 flow of argon until the system reached 300 °C and maintained this temperature for 15 min.44 The FTIR spectrum of the exfoliated oxidized MPG, MPGO, showed the following bands: 3400 cm−1, broad, (O−H); 1700 cm−1 (CO, carbonyl and carboxylic groups); 1640 and 1576 cm−1 (C C, carbon rings); and 1110 and 1038 cm−1 (C−O−C).45,46 The presence of COOH, OH, and CO groups at the edges of the sheets has been previously described by NMR, IR spectroscopy, and electron
diffraction, whereas epoxide (1,2-ether) and OH functionalities have been found in the basal planes.47−51 2.3. Thermal Reaction of MPGO with SO2. The thermal reaction was carried out in a tubular quartz reactor (diameter 20 mm; length 250 mm) fitted with a temperature controller and heated in an electric oven. The sample was placed on a piece of steel sieve (80 μm) supported by a glass wool plug positioned in the middle of the reactor. The particles were pretreated at 700 °C for 3 h under a 40 mL·min−1 flow of argon controlled by a mass flow controller. The temperature was then adjusted to 630 °C, and a 100 mL·min−1 flow of SO2 (95% mol/mol in nitrogen) was passed through the sample for 60 min. 2.4. Reaction of MPGO with SO2 Using Nonthermal Plasma. The experimental system used in this study is shown in Figure 2. An ac 220/17 kV high-voltage neon transformer (60 Hz, Neonena, Brazil) was used to generate the plasma channels between the electrodes. The nonthermal dielectric barrier discharge (DBD) plasma bolt-type reactor was built using a cylindrical quartz tube that was 230 mm in length, 18 mm in diameter, and 1 mm in thickness with Teflon caps. The reactor had a quartz insulating barrier between the electrodes to prevent the discharge of sparks. The high-voltage electrode was made of stainless steel, 7 mm diameter, and it was concentrically positioned in the reactor. The ground electrode was made of aluminum foil, 38 mm wide, surrounding the outer region of the quartz tube. Sulfur dioxide diluted in nitrogen was used as the plasma feed gas. The current and voltage measurements were performed using a highvoltage probe (Tektronix P6015A) and a current probe (Agilent N2781A) to monitor the signals with an oscilloscope (Tektronix TDS5034B). The MPGO sample was placed in the plasma discharge zone between the inner surface of the quartz tube and the high-voltage electrode. The carbon particles were kept in the plasma discharge zone 4303
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Figure 4. SEM images (8000×) of microparticles of (a) MPG, graphite sheets; (b) MPGO, exfoliated graphite oxide; (c) plmMPGO3, MPGO after modification with SO2 using plasma for 3 h; and (d) plmMPGO3 after thermal shock at 400 °C for 10 min. Specific surface areas are in parentheses. using two glass wool plugs at the ends of the reactor chamber. The DBD reactor was purged with SO2 for a few minutes to eliminate contaminants, and the plasma system was then turned on with a SO2 flow of 100 mL·min−1 (95% mol/mol in nitrogen). Finally, the plasmamodified particles (plmMPGO) were removed from the DBD reactor and stored in an Eppendorf tube.
surface area changed very little after modification with SO2 using plasma and after thermal shock. Graphene oxide can be obtained by exfoliating graphite oxide but the precise chemical structure of graphene oxide has been the subject of considerable debate, and even to this day no unambiguous model exists. Chemically, graphene oxide is very similar to graphite oxide, but instead of a stacked structure, graphene oxide is exfoliated into monolayers or few-layer stacks.55 The exfoliation of graphite oxide can be obtained by different methods. Thermal exfoliation usually produces a partial or total reduction of graphene oxide, depending on the conditions.55,56 The degree of reduction of graphene oxide with temperature can be characterized through the increase in the atomic ratio of carbon to oxygen (C/O).52,53,55 Exfoliated graphite oxide, MPGO, presented a C/O ratio of 1.53, as determined by XPS, showing that the reduction upon exfoliation was negligible and that it can be considered to be graphene oxide. Similar graphite oxide exfoliated at 300 °C for 1 h showed an increase in C/O from 2.3 to 5.3.52 There is a characteristic distribution of oxidized functionalities related to carbon in exfoliated graphite oxide that can be determined from XPS spectra. These characteristics depend on the graphite,57 the method of oxidation,55,58 and the exfoliation conditions (atmosphere, temperature, time, and pressure).44,52,53 The C 1s XPS spectra of the graphene oxide shows the peak at 284.50 eV assigned to arylic carbons with important contributions of a variety of different carbon bonding configurations related to oxygen functionalities (in eV, C−O, 286.2; CO, 287.8; and C(O)O, 289.0).54 Upon heating, there is a decomposition of the oxygen-containing groups indicated by the decrease in the oxygen-related peaks and the
3. RESULTS AND DISCUSSION 3.1. Thermal Exfoliation of Graphite Oxide. The X-ray diffractogram of graphite microparticles before oxidation is presented in Figure 3a, showing the crystalline structure of the graphitic carbon in the 2θ range of 5 to 120°. The main peak displacement from 26.27 to 11.28° after oxidation indicates a change in the crystalline microstructure (Figure 3b) and that the interplanar separation distance of the graphene layers increased from 3.38 to 7.84 Å when graphite was transformed into graphite oxide.34,45,47,52 The 4.46 Å increase in height correlates well with the increase in the separation distance of the oxidized graphene layers containing functional groups on the C−C basal planes as well as the adsorption of intercalated water molecules.34,53 The SEM images of the microparticles of pristine graphite sheets and exfoliated graphite oxide are shown in Figure 4. Surface area measurements with nitrogen gas yielded a BET value of 204 m2·g−1 for the exfoliated graphite oxide as compared to 11 m2·g−1 for the pristine graphite. The ca. 20-fold surface area expansion is a good indicator of a successful exfoliation process. However, it is lower than the reported specific surface area for completely exfoliated and isolated graphene sheets,34,54 which is attributed to the agglomeration of the graphene oxide sheets upon oxidation. However, the values of nitrogen surface areas are, in general, lower than the corresponding carbon dioxide surface areas.45 The specific 4304
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Table 1. Binding Energies and Composition Obtained from XPS Spectra of MPGO Modified with SO2 by Nonthermal Plasma and after Thermal Shocka sample
after plasma,b plmMPGO60
initial, MPGO
element
eV (wt %)
S 2p oxi C 1s
284.5 (59.3) 285.6 (26.3) 287.6 (14.5) total
O 1s
531.9 (35.2) 533.8 (44.8) 535.3 (20.0) total
C/O
atom % 35.84 15.91 8.75 60.50 13.89 17.71 7.90 39.50 1.53
eV (wt %) 168.7 284.5 285.3 286.4
calcdc
plmMPGO60 after thermal shockd
atom %
atom %
eV (wt %)
0.77 58.96 12.40 13.15 84.51 2.41 12.32
0.79
(100.0) (69.8) (14.7) (15.6)
531.3 (16.4) 533.2 (83.6)
14.73 5.74
284.5 (71.7) 285.9 (14.0) 286.8 (14.3) 84.50 531.2 (45.4) 533.1 (30.6) 534.0 (24.0) 14.71
calcde atom %
atom %
0.00 66.23 12.93 13.21 92.37 3.47 2.34 1.83 7.63 12.11
0.00
92.74
7.26
Spectrum calibrated with reference to C 1s (284.5 eV). bTreatment with plasma, 100 mL·min−1 flow of 95% SO2 for 60 min. cCalculated from reactions 1 and 2. d15 min at 600 °C in an Ar atmosphere. eCalculated from reactions 3 and 4. (1) MPGO + SO2 → MPGO(SO2); (2) MPGO(SO2) → rMPGO(SO2) + CO2; (3) rMPGO(SO2) → rrMPGO(SO2) + CO2; (4) rrMPGO(SO2) → rrMPGO + S. r represents the reduction of graphene oxide. a
appearance of a broad tail toward higher binding energies53,54,59 The exfoliated graphite oxide, MPGO, presented 41 atom % of C−O bond-related peaks (Table 1). Therefore, MPGO essentially had the characteristics of graphene oxide, which, after certain reactions conditions described below, was partially reduced. 3.2. Graphene Oxide Modified Using Nonthermal Plasma. When MPGO was treated with plasma under a SO2 atmosphere at room temperature, the XPS spectrum showed that SO2 was inserted only as an oxidized intermediates because just one peak was observed at 168.7 eV in the S 2p region (Table 1). The sulfur content increased almost linearly with the reaction time (Table 2). The calculated XPS spectrum
Graphene oxide showed a linear increase with time of sulfur content induced by plasma (Table 2). Considering that initially the sample had no sulfur, the rate of SO2 insertion was 1.18 × 10−2 atom %/min (r = 0.997). Besides, a decrease of SO2 in nitrogen from 95 to 15% mol/mol produced a small decrease in the rate of insertion and the same specificity for the oxidized intermediates. The modified MPGO treated with SO2 excited with plasma for 60 min was submitted to a short thermal shock (15 min) at 600 °C under an Ar atmosphere with the elimination of sulfur and carbon dioxide (Table 1, reactions 3 and 4). Similarly, a sample of modified graphene oxide with SO2 using plasma for 180 min showed only the oxidized intermediate at 168.9 eV, and also sulfur and CO2 were eliminated from the solid after a thermal shock of 10 min at 400 °C. The XPS spectra after thermal shock are not compatible with SO2 desorption reaction 2
Table 2. Effect of Reaction Time on the Insertion of SO2 into Graphene Oxide Induced by Nonthermal Plasmaa reaction time, min S 2p, atom %, eV(wt %)
60b 0.77 168.7 (100.0)
120b 1.35 168.7 (100.0)
180c 1.86 168.9 (100.0)
SO2 MPGO → SO2 + MPGO
(2)
nor with a concerted reaction leading to sulfur and CO2 extrusion (reaction 3)
At room temperature; flow 100 mL·min−1; SO2 in nitrogen, % mol/ mol. b95% mol/mol. c15% mol/mol.
a
SO2 MPGO → S + CO2 + MPGO
(3)
They are consistent with the sequence of reactions according to the stoichiometry of the reaction (eq 1) and the primary mechanism (Figure 1). After the insertion of SO2 as an oxidized intermediate, sulfur and CO2 were produced as final products through the nonoxidized intermediate that, in this case, without a constant flow of SO2 to reach the steady state, was not detected There was an increase in the C/O ratio upon plasma treatment (5.74) and thermal shock (12.11) that indicated the partial reduction of graphene oxide.52,55 It is considered that the loss of oxygen in graphene oxide with temperature is due to the extrusion of carbon dioxide.34 Therefore, the loss of CO2 detected by the atom inventory is due, in part, to this reaction, as was shown by TGA/DTA analysis coupled with FTIR (see below). TGA/DTA analysis coupled with FTIR was performed on a sample of graphene oxide modified with SO2 using plasma for 120 min (mplMPGO120), and the results are presented in Figure 5. Up to 150 °C, the weight loss is ascribed to water (8.13 μmol).52 It is reasonable to assume that in the range of
composition, using the atom inventory technique,29 suggests that the insertion of SO2 occurs along with the partial reduction of MPGO and the formation of CO2 (Table 1, reactions 1 and 2). When plasma treatments with SO2 were performed on highly oriented pyrolytic graphite and glassy carbon, the insertion of two chemical states of S 2p was clearly favored.41 One peak had a binding energy of 164.0 eV and corresponds to low oxidized sulfur species, and the second peak of a highly oxidized species was found at 168.5 eV. The maximal incorporation on both carbons always remained under 5 atom %. The oxidation state of the adsorbed sulfur species depended on the bias potential applied to the sample. The higher oxidation state was preferred with a lower bias. The insertion should reach a steady-state concentration, as was measured for activated carbon at 630 °C. A constant concentration of sulfur of 7.19 atom % (from XPS) was obtained after 120 min of reaction.20 At the same temperature, after 180 min, graphite and graphite oxide reached concentrations of 0.74 and 1.99 atom %, respectively.28 4305
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peak in the XPS spectrum in the S 2p region at 164.0 eV (Table 3). The calculated XPS spectrum suggested that the insertion of SO2 occurred with reduction of graphene oxide, the formation of the nonoxidized episulfide intermediate, and the elimination of CO2 and sulfur (Table 3, reactions 1−4). Reactions 2, 3a, and 3b for the extrusion of CO2 are indistinguishable, as are reactions 4a and 4b, but they explain the insertion of sulfur as a nonoxidized intermediate and the severe reduction of MPGO shown by the increase in the C/O ratio to 10.02. The TGA/DTA/FTIR analysis of the sample obtained at 630 °C (Figure 7) showed a weight loss up to 400 °C that would correspond to 0.60 μmol of sulfur. In this case, water was previously eliminated during the thermal treatment. In the range of 400−625 °C, there was a 1.39 μmol loss of CO2. The FTIR spectrum at 422 °C showed a doublet at 2362−2338 cm−1 corresponding to CO2.60 The loss of CO2 occurred in two steps. In the second step in the range of 625−817 °C, there was a loss of 1.75 μmoles that can be ascribed to the reduction of graphene oxide.52 We note that according to the TGA analysis, samples obtained from both plasma treatment and thermal modification, upon increasing temperature, produced sulfur and CO2 in sequence, suggesting that the desulfurization and carbooxidation reactions are separate pathways, the first having a lower energy barrier than the second. 3.4. Thermal and Plasma Consecutive Treatment of MPGO with SO2. Partially reduced graphene oxide obtained by thermal reaction with SO2 at 630 °C was consecutively treated by nonthermal plasma under a SO2 atmosphere. The XPS spectrum in the S 2p region of the final product of modified graphene oxide showed bands at 163.5 and 168.1 eV corresponding to nonoxidized and oxidized sulfur intermediates, respectively (Table 3, Figure 8). The change in concentration of the XPS spectrum after the plasma treatment corresponded to reactions 1 and 2 in Tables 1 and 3. This sequence of treatment allows the selective insertion of two different functionalities of sulfur in the same graphene oxide sheet. As mentioned above, the incorporation of sulfur into the edges of graphite introduced important superconductive properties.16,17 Surface chemistry on carbon materials is an area in which information on the molecular level is difficult to obtain, and all techniques relating the structure and reactivity of carbon materials are of primary importance in predicting their applications. The reactivity of graphene oxide with respect to SO2 is mechanistically similar to that of other forms of carbon found in our previous studies.20,22 Carbon surface oxides play an important role of increasing the reactivity of the carbon matrix, and graphene oxide is decorated with a wide spectrum of organic functionalities from carboxylic acids to ketones and ethers. Information of the chemical properties of graphene oxide and its microstructure is imperative in many different fields.10−15 Therefore, the selective functionalization of graphene oxide and its reduced forms is an important result, especially because the different reactivities of the SO2 reduction reaction intermediates would allow the insertion of a variety of organic fragments on the carbon matrix.
Figure 5. TGA/DTA coupled with FTIR of graphene oxide modified with SO2 using plasma for 120 min. The FTIR spectrum was taken at 472 °C.
150−300 °C the loss is due to the extrusion of 6.0 μmol of sulfur from the maximum of 13.1 μmol of sulfur content on the sample surface (Figure 4c, specific surface, 199 m2/g; Table 2, 1.35 atom % sulfur). The loss of CO2 occurred in two steps. In the range of 300−750 °C, the sample lost 13.8 μmol of CO2 near the maximum expected from the sulfur content. The FTIR spectrum, taken at 472 °C, showed a doublet at 2361−2343 cm−1corresponding to CO2.60 Above 750 °C, the loss of 2.6 μmol of CO2 would correspond to the reduction of graphene oxide. The C/O ratio increased from 1.53 to 5.74 (Table 1). The results from the TGA experiment are consistent with the reactions used for the calculated XPS spectrum (Table 1) (i.e., after the SO2 insertion as oxidized intermediates, sulfur and carbon dioxide are thermally formed as final products of the reaction with the partial reduction of MPGO (Figure, 6)). 3.3. Thermal Modification of Graphene Oxide, mMPGO. The modification of graphene oxide with SO2 at 630 °C resulted exclusively in the insertion of nonoxidized intermediate episulfide, as shown by the observation of just one
4. CONCLUSIONS The functionalization of graphene oxide with sulfur can be obtained from the reaction with SO2 using nonthermal plasma and/or thermal treatment. Exfoliated graphite oxide produced graphene oxide with negligible reduction. The modification of
Figure 6. Reactions of graphene oxide submitted to modification by SO2 and plasma and short thermal shock. The loss of CO2 due to the partial reduction of MPGO is not included. 4306
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Table 3. Binding Energies and Composition from the XPS Spectrum of MPGO after Thermal Modification with SO2 at 630 °C, Followed by Plasma Treatmenta sample
initial MPGO
element
eV (wt %)
after thermal modificationb atom %
S 2p nonoxi
calcdc
284.5 (59.3) 285.6 (26.3) 287.6 (14.5) total
O 1s
532.0 (35.2) 533.8 (44.8) 535.3 (20.0) total
C/O
35.84 15.91 8.75 60.50 13.89 17.71 7.90 39.50 1.53
calcde
eV (wt %)
atom %
atom %
eV (wt %)
atom %
164.0(100.00)
2.03
2.03
163.5 (63.8) 168.1 (36.2)
284.5 (64.3) 285.5 (19.7) 286.2 (16.0)
57.28 17.58 14.23 89.09
2.57 1.46 4.03 31.13 24.58 29.34 85.05 5.97 4.95
oxi total C 1s
after plasma treatmentd
534.0(100.00)
284.5 (36.6) 285.4 (28.9) 288.1 (34.5) 88.84 532.0 (54.7) 533.9 (45.3)
8.89 8.89 10.02
9.13
10.92 7.79
atom %
4.15
84.94
10.92
a Spectrum calibrated by reference to C 1s (284.5 eV). bHeated to 630 °C for 60 min, flow of 100 mL·min−1, 95% SO2. cFrom reactions 1, 2, 3, and 4. dTreatment with plasma for 60 min, flow of 100 mL·min−1, 15% SO2. eFrom reactions 1 and 2. (1) MPGO + SO2 → MPGO(SO2); (2) MPGO(SO2) → rMPGO(SO2) + CO2; (3a) MPGO(SO2) → MPGO(S) + CO2; (3b) rMPGO(SO2) → rMPGO(S) + CO2; (4a) MPGO(S) → MPGO + S; (4b) rMPGO(S) → rMPGO + S. r represents the reduction of graphene oxide.
Figure 8. XPS spectrum in the S 2p region for graphene oxide after reaction with SO2: (a) thermal treatment, (b) nonthermal plasma treatment, and (c) thermal followed by nonthermal plasma treatment. The deconvoluted curves are shown.
Figure 7. TGA/DTA coupled with FTIR of graphene oxide modified with SO2 at 630 °C for 60 min. The FTIR spectrum was taken at 422 °C.
630 °C, only the nonoxidized episulfide intermediate is inserted. The sequence of thermal reaction followed by plasma treatment results in the insertion of both intermediates, oxidized and nonoxidized. The use of the reaction of SO2 for the functionalization of graphene oxide is efficient, and high levels of insertion are obtained. Because oxidized and nonoxidized intermediates have different reactivities with respect to thiolysis and aminolysis, sequential reactions can be conducted on the graphene surface, and various types of organic moieties can be selectively added to the surface.
graphene oxide with SO2 excited with plasma produced a partial reduction, and the thermal insertion of SO2 occurred with the severe reduction of graphene oxide. The treatment of graphene oxide with nonthermal plasma under a SO2 atmosphere results in the insertion of only the oxidized intermediates of the SO2 reduction. Short thermal shock eliminates sulfur and CO2 from the carbon matrix as expected from the primary mechanism of reduction. When graphene oxide is submitted to thermal reaction with SO2 at 4307
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(16) Denis, P. A. Density functional investigation of thioepoxidated and thiolated graphene. J. Phys. Chem. C 2009, 113, 5612−5619. (17) Kohl, A. L.; Nielsen, R. Gas Purification, 5th ed.; Gulf Publishing Company: Houston, TX, 2003. (18) Humeres, E.; Castro, K. M.; Moreira, R. F. P. M.; Peruch, M. G. B.; Schreiner, W. H.; Aliev, A. E.; Canle, M.; Santaballa, J. A.; Fernandez, I. Reactivity of the thermally stable intermediates of the reduction of SO2 on carbons and mechanisms of insertion of organic moieties in the carbon matrix. J. Phys. Chem. C 2008, 112, 581−589. (19) Humeres, E.; Moreira, R. F. P. M.; Peruch, M. G. B. Reduction of SO2 on different carbons. Carbon 2002, 40, 751−760. (20) Humeres, E.; Peruch, M. G. B.; Moreira, R. F. P. M.; Schreiner, W. Reactive intermediates of the reduction of SO2 on activated carbon. J. Phys. Org. Chem. 2003, 16, 824−830. (21) Pliego, J. R.; Resende, S. M.; Humeres, E. Chemisorption of SO2 on graphite surface: a theoretical ab initio and ideal lattice gas model study. Chem. Phys. 2005, 314, 127−133. (22) Radovic, L. R.; Bockrath, B. On the chemical nature of graphene edges: origin of stability and potential for magnetism in carbon materials. J. Am. Chem. Soc. 2005, 127, 5917−5927. (23) Sun, Z.; Kohama, S.; Zhang, Z.; Lomeda, J. R.; Tour, J. M. Soluble Graphene Through Ede-Selective Functionalization. Nano Res. 2010, 3, 117−125. (24) Lim, H.; Lee, J. S.; Shin, H. J.; Shin, H. S.; Choi, H. C. Spatially resolved spontaneous reactivity of diazonium salt on edge and basal plane of graphene without surfactant and its doping effect. Langmuir 2010, 26, 12278−12284. (25) Boukhvalov, D. W.; Katsnelson, M. I. Chemical functionalization of graphene with defects. Nano Lett. 2008, 8, 4373−4379. (26) Humeres, E.; Peruch, M. G. B.; Moreira, R. F. P. M.; Schreiner, W. H. Reduction of sulfur dioxide on carbons catalyzed by salts. Int. J. Mol. Sci. 2005, 6, 130−142. (27) Humeres, E.; Moreira, R. F. P. M. Kinetics and mechanisms in flow systems: reduction of SO2 on carbons. J. Phys. Org. Chem. 2012, 25, 1012−1026. (28) Humeres, E.; Castro, K. M.; Smaniotto, A.; Lopes, C. N.; Debacher, N. A.; Moreira, R. F. P. M.; Schreiner, W. H.; Aliev, A. E. Reactivity of the intermediates of the reduction of SO2. Functionalization of graphite, graphite oxide and graphene oxide, J. Phys. Org. Chem. 2014, DOI: 10.1002/poc.3267. (29) Humeres, E.; Castro, K. M.; Moreira, R. F. P. M.; Schreiner, W. H.; Aliev, A. E.; Canle, M.; Santaballa, J. A.; Fernandez, I. The use of XPS spectra for the study of reaction mechanisms: the atom inventory method. J. Phys. Org. Chem. 2008, 21, 1035−1042. (30) Grimme, S.; Muck-Lichtenfeld, C.; Antony, J. Noncovalent interactions between graphene sheets and in multishell (hyper)fullerenes. J. Phys. Chem. C 2007, 111, 11199−11207. (31) Liu, Z. B.; Xu, Y. F.; Zhang, X. Y.; Zhang, X. L.; Chen, Y. S.; Tian, J. G. Porphyrin and fullerene covalently functionalized graphene hybrid materials with large nonlinear optical properties. J. Phys. Chem. B 2009, 113, 9681−9686. (32) Geng, J.; Jung, H. Porphyrin functionalized graphene sheets in aqueous suspensions: from the preparation of graphene sheets to highly conductive graphene films. J. Phys. Chem. C 2010, 114, 8227− 8234. (33) Stankovich, S.; Piner, R. D.; Chen, X. Q.; Wu, N. Q.; Nguyen, S. T.; Ruoff, R. S. Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate). J. Mater. Chem. 2006, 16, 155−158. (34) Schniepp, H. C.; Li, J.-L.; McAllister, M. J.; Sai, H.; HerreraAlonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. Functionalized single graphene sheets derived from splitting graphite oxide. J. Phys. Chem. B 2006, 110, 8535−8539. (35) Fridman, A. Plasma Chemistry; Cambridge University Press: New York, 2008. (36) Xu, T.; Yang, J.; Liu, J.; Fu, Q. Surface modification of multiwalled carbon nanotubes by O2 plasma. Appl. Surf. Sci. 2007, 253, 8945−895.
AUTHOR INFORMATION
Corresponding Author
*Tel: 55 48 3282 1078. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Brazilian Government Agency Coordenaçaõ de ́ Aperfeiçoamento de Pessoal de Nivel Superior (CAPES), projeto CAPES/DGU 219/2010, and the Spanish Ministerio de Educación, Cultura y Deporte, DGU, project PHB2009-0057PC, for financial support. We also thank the Conselho Nacional ́ de Desenvolvimento Cientifico e Tecnológico (CNPq) and Programa de Bolsas do Fundo de Apoio à Manutençaõ e ao Desenvolvimento da Educaçaõ Superior do Estado de Santa Catarina (FUMDES) for research scholarships (A.S. and K.M.d.C., respectively).
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REFERENCES
(1) Ma, Z.; Zaera, F. Organic chemistry on solid surfaces. Surf. Sci. Rep. 2006, 61, 229−281. (2) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N., Jr. Cycloaddition chemistry of organic molecules with semiconductor surfaces. Acc. Chem. Res. 2000, 33, 617−624. (3) Bent, S. F. Organic functionalization of group IV semiconductor surfaces: principles, examples, applications, and prospects. Surf. Sci. 2002, 500, 879−903. (4) Fischer, J. E. Carbon Nanotubes: Structure and Properties. In Carbon Nanomaterials; Gogotsi, Y., Ed.; Taylor and Francis: Boca Raton, FL, 2006; pp 41−76. (5) Takami, A.; Xin, W.; Tomofumi, U.; Masanori, B.; Mitsuru, A. Protein direct delivery to dendritic cells using nanoparticles based on amphiphilic poly(amino acid) derivatives. Biomaterials 2007, 28, 3427−3436. (6) Rao, C. N. R.; Sood, A. K.; Voggu, R.; Subrahmanyam, K. S. Some novel attributes of graphene. J. Phys. Chem. Lett. 2010, 1, 572− 580. (7) Liu, Z.; Robinson, J. T.; Sun, X.; Dai, H. PEGylated nanographene oxide for delivery of water-insoluble cancer drugs. J. Am. Chem. Soc. 2008, 130, 10876−10877. (8) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321, 385−388. (9) Gomez-Navarro, C.; Burghard, M.; Kern, K. Elastic properties of chemically derived single graphene sheets. Nano Lett. 2008, 8, 2045− 2049. (10) Balandin, A. A.; Suchismita, G.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior thermal conductivity of single-layer graphene. Nano Lett. 2008, 8, 902−907. (11) Au, A.; Ramesh, P.; Itkis, M. E.; Bekyarova, E.; Haddon, R. C. Graphite nanoplatelet-epoxy composite thermal interface materials. J. Phys. Chem. C 2007, 111, 7565−7569. (12) Williams, G.; Seger, B.; Kamat, P. V. TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide. ACS Nano 2008, 2, 1487−1491. (13) Muszynski, R.; Seger, B.; Kamat, P. V. Decorating graphene sheets with gold nanoparticles. J. Phys. Chem. C 2008, 112, 5263− 5266. (14) Kurmaev, E. Z.; Galakhov, A. V.; Moewes, A.; Moehlecke, S.; Kopelevich, Y. Interlayer conduction band states in graphite-sulfur composites. Phys. Rev. B 2002, 66, 193402−193405. (15) Silva, R. R.; Torres, J. H. S.; Kopelevich, Y. Indication of superconductivity at 35 K in graphite-sulfur composites. Phys. Rev. Lett. 2001, 87, 147001−147005. 4308
dx.doi.org/10.1021/la500140m | Langmuir 2014, 30, 4301−4309
Langmuir
Article
(37) Kusano, Y.; Mortensen, H.; Stenum, B.; Goutianos, S.; Mitra, S.; Ghanbari-Siahkali, A.; Kingshott, P.; Sørensen, B. F.; Bindslev, H. I. Atmospheric pressure plasma treatment of glassy carbon for adhesion improvement. Int. J. Adhes. Adhes. 2007, 27, 402−408. (38) Khare, B. N.; Wilhite, P.; Quinn, R. C.; Chen, B.; Schingler, R. H.; Tran, B.; Imanaka, H.; So, C. R.; Bauschlicher, C. W.; Meyyappan, M. Functionalization of carbon nanotubes by ammonia glowdischarge: experiments and modeling. J. Phys. Chem. B 2004, 108, 8166−8172. (39) Fatyeyeva, K.; Poncin-Epaillard, F. Sulfur dioxide plasma treatment of the clay (laponite) particles. Plasma Chem. Plasma Process. 2011, 31, 449−464. (40) Holländer, A.; Kröpke, S. Polymer surface treatment with SO2containing plasmas. Plasma Process. Polym. 2010, 7, 390−402. (41) Coen, C. M.; Keller, B.; Groening, P.; Schlapbach, L. Functionalization of graphite, glassy carbon, and polymer surfaces with highly oxidized sulfur species by plasma treatments. J. Appl. Phys. 2002, 92, 5077−5083. (42) XPS International, Mountain View, CA, USA. (43) Bissessur, R.; Scully, S. F. Intercalation of solid polymer electrolytes into graphite oxide. Solid State Ionics 2007, 178, 877−882. (44) Talyzin, A. V.; Szabo, T.; Dekany, I.; Langenhorst, F.; Sokolov, P. S.; Solozhenko, V. L. Nanocarbons by high-temperature decomposition of graphite oxide at various pressures. J. Phys. Chem. C 2009, 113, 11279−11284. (45) Hontoria-Lucas, C.; López-Peinado, A. J.; López-González, J. D.; Rojas-Cervantes, M. L.; Martín-Aranda, R. M. Study of oxygencontaining groups in a series of graphite oxides: physical and chemical characterization. Carbon 1995, 33, 1585−1592. (46) Fuente, E.; Menendez, J. A.; Dıez, M. A.; Suarez, D.; MontesMoran, M. A. Infrared spectroscopy of carbon materials: a quantum chemical study of model compounds. J. Phys. Chem. B 2003, 107, 6350−6359. (47) Szabo, T.; Berkesi, O.; Forgo, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dekany, I. Evolution of surface functional groups in a series of progressively oxidized graphite oxides. Chem. Mater. 2006, 18, 2740−2749. (48) He, H.; Klinowski, J.; Forster, M.; Lerf, A. A new structural model for graphite oxide. Chem. Phys. Lett. 1998, 287, 53−56. (49) Szabo, T.; Berkesi, O.; Dekany, I. Drift study of deuteriumexchanged graphite oxide. Carbon 2005, 43, 3186−3189. (50) Aragon de la Cruz, F.; Cowley, J. M. Structure of graphitic oxide. Nature 1962, 196, 468−469. (51) Paci, J. T.; Belytschko, T.; Schatz, G. C. Computational studies of the structure, behavior upon heating, and mechanical properties of graphite oxide. J. Phys. Chem. C 2007, 111, 18099−18111. (52) Botas, C.; Á lvarez, P.; Blanco, C.; Santamaría, R.; Granda, M.; Gutiérrez, M. D.; Rodríguez-Reinoso, F.; Menéndez, R. Critical temperatures in the synthesis of graphene-like materials by thermal exfoliation−reduction of graphite oxide. Carbon 2013, 52, 476−485. (53) Cao, J.; Qi, G.-Q.; Ke, K.; Luo, Y.; Yang, W.; Xie, B.-H.; Yang, M. B. Effect of temperature and time on the exfoliation and deoxygenation of graphite oxide by thermal reduction. J. Mater. Sci. 2012, 47, 5097−5105. (54) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007, 45, 1558−1565. (55) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39, 228−240. (56) McAllister, M. J.; Li, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem. Mater. 2007, 19, 4396−4404. (57) Chee, S. Y.; Poh, H. L.; Chua, C. K.; Sanek, F.; Sofer, Z.; Pumera, M. Influence of parent graphite particle size on the electrochemistry of thermally reduced graphene oxide. Phys. Chem. Chem. Phys. 2012, 14, 12794−12799.
(58) You, S.; Luzan, S. M.; Szabó, T. A.; Talyzin, A. V. Effect of synthesis method on solvation and exfoliation of graphite oxide. Carbon 2013, 52, 171−180. (59) Bonanni, A.; Ambrosi, A.; Pumera, M. On oxygen-containing groups in chemically modified graphenes. Chem.Eur. J. 2012, 18, 4541−4548. (60) Cataldo, F.; Ursini, O. The role of carbon nanostructures in the ozonization of different carbon black grades, together with graphite and rubber crumb in an IR gas cell . Fullerenes, Nanotubes, Carbon Nanostruct. 2007, 15, 1−20.
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