ARTICLE pubs.acs.org/JPCC
Formation of Phenol Groups in Hydrated Graphite Oxide D. W. Lee*,†,‡ and J. W. Seo†,‡ † ‡
Division of Physics and Applied Physics, Nanyang Technological University, 637371 Singapore Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom ABSTRACT: We investigated the reaction of graphite oxide with water molecule in the air with X-ray diffraction, solid-state nuclear magnetic resonance, and Fourier transform infrared spectroscopy. X-ray diffraction data show that the layer distance of graphite oxide increases when it becomes hydrated in the air. Solid-state nuclear magnetic resonance and Fourier transform infrared spectra reveal that hydrated graphite oxide in the air has phenol groups on its layer edges. The overall results indicate that graphite oxide reacts with water molecules in the air and phenol groups are formed in graphite oxide.
1. INTRODUCTION When graphite is oxidized into graphite oxide (GO), it loses electrical conductivity and becomes amorphous with short-range atomic order. It preserves the layered structure and is very sensitive to humidity.13 Although its structure is under debate, experiments with various techniques410 have revealed that GO has chemical groups: O groups, OH groups, CdO groups, and so on. Moreover, XPS measurements show that GO has two kinds of carbon: sp2 and sp3.57 In addition to the abovementioned chemical groups, the presence of phenol has been proposed11,12 because GO shows a peak at 110 ppm in solid-state nuclear magnetic resonance (NMR) spectra, which corresponds to phenol and furan. GO is easily hydrated when it is exposed to humid air because it is hydrophilic.13,14 Although the basal spacing of GO is 6.71 Å, it is proportional to the degree of oxidation1517 and it increases by ∼3 Å as a layer of water is inserted between GO layers. The basal spacing of fully hydrated GO increases up to ∼12 Å.2,3 Two neighboring hydroxyl groups in GO try to be away because of repulsive force, shown in Figure 1a. However, oxygen atoms in water molecules are attracted to hydroxyl groups in GO due to hydrogen bonding. Hydrogen atoms in water molecules are also attracted to epoxy groups in GO. The electronegativity of carbon is 2.55 and that of oxygen is 3.44,22 causing charge transfer from carbon atoms to oxygen atoms. As a result, GO layers become electrically positive and water molecules are prone to adsorb on GO layers, which makes it almost impossible to remove all water molecules from GO even at low humidity.8,14 Besides, water molecules are present in noninterlamellar voids as well as interlayer space in GO (Figure 1b) and it is not possible to distinguish between confined water in the interlayer space and in noninterlamellar voids since they have similar local environments. According to Buchsteiner et al.,3 bulklike water molecules are present in GO with highest water contents and each layer in r 2011 American Chemical Society
GO has different filling; in other words, some layers are fully filled while others are partially filled. The hydration of GO has been studied in two ways: hydration in the air2,3,13,18,19 and hydration of wet GO at high pressure.20,21 Studies on the hydration of GO in the air have focused on the dynamics, kinetics, and mobility of water molecules in GO,2,3,13,18,19 while the research on the hydration of wet GO at high pressure has revealed that the layer distance gradually increases by 30% above 1 GPa at ambient temperature with the insertion of an additional water layer.20,21 GO does not become wet if it is hydrated in the air, while GO becomes wet when it is hydrated in water at high pressure. Furthermore, the reaction of GO with water molecules in the air has not been considered. Thus, we report here that GO reacts with H2O in the air and phenol groups are formed on its surface.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. GO samples were prepared by the Brodie process.23 The Brodie process is as follows. 5.0 g of graphite was added into 62.5 mL of fuming nitric acid. After the mixture was cooled in an ice bath, we slowly added 25.0 g of potassium chlorate (KClO3) into the mixture. After it reached room temperature, it was placed in a water bath and was slowly heated up to 45 °C. It was kept at this temperature for 20 h. Subsequently, the mixture was poured into 125 mL of cold distilled water to be warmed to 70 °C. It was centrifuged, decanted, and dried overnight at 70 °C. The whole process was repeated. It took 3 weeks to prepare GO samples by the Brodie method. To insert a layer of H2O inside GO, GO was placed for 24 h in a desiccator with ca. 78% relative humidity, as Received: February 14, 2011 Revised: May 26, 2011 Published: May 26, 2011 12483
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Figure 3. XRD spectra of graphite, GO, and hydrated GO (H-GO).
Figure 1. (a) Water molecules in GO layers. Carbon grids are presented as layers. Water molecules are attracted to hydroxyl groups and epoxy groups in GO due to hydrogen bonding. (b) Schematic diagram of GO interlamellar spaces. There are voids among GO particles.
Figure 4. (a) 13C MAS NMR spectrum of GO acquired with HPDEC. (b) 13C CP/MAS NMR spectrum of GO acquired with 3050 μs contact time. Figure 2. Hydration of GO (H-GO). GO was placed in a desiccator with NaCl solution for 1 day.
shown in Figure 2. The correct humidity was achieved by placing saturated NaCl solution in a desiccator for 1 day.24 2.2. Sample Characterization. The samples were characterized by powder X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FT-IR). XRD patterns were recorded on a Bruker D8 Avance powder diffractometer with Cu KR radiation with a step size of 0.02°. The 13C solid-state NMR spectra were obtained at 9.4 T by use of a Bruker Avance 400 MHz spectrometer and 4 mL zirconia magic-angle spinning (MAS) rotors spun in air at 6 kHz. 13C MAS spectra with high-power decoupling (HPDEC) were acquired at 100 MHz with 100 kHz 90° pulses of 1.25 μs duration. 1H13C cross-polarization (CP) MAS spectrum was recorded at 3050 μs. To characterize the structure of the samples, we also used a Perkin-Elmer FT-IR spectrophotometer.
3. RESULTS AND DISCUSSION Figure 3 shows XRD patterns of graphite, GO, and hydrated GO (H-GO). The XRD pattern of graphite shows its typical
peaks at 26.54° and 54.64°, which denote the reflections (002) and (004), respectively. The basal spacing of graphite is 6.71 Å. The principal peak of GO appears at 12.61°, which refers to the basal spacing of 7.02 Å. H-GO has a main peak at 9.45° and its basal spacing is 9.36 Å, implying that a layer of water is inserted inside it.2,3 Figure 4 compares the experimental 13C MAS NMR spectrum of GO acquired with HPDEC and 13C CP/MAS NMR spectrum of GO recorded at 3050 μs. There are four peaks at 60, 70, 130, and 190 ppm from tetramethylsilane (TMS) in common. The peak at 60 ppm, which is not cross-polarized with nearby hydrogen atoms, must come from carbons linked to the epoxy groups.3,25,26 The peak at 70 ppm must be from carbons linked to the hydroxyl groups27 since it is cross-polarized with nearby hydrogen atoms. The peaks at 130 ppm are from the double bonds, which are relatively stable in comparison with the epoxy and hydroxyl groups.3,25,28 The peak at 190 ppm is not crosspolarized and we assigned the peak at 190 ppm as ketone groups in our previous paper.4 Figure 5 shows the 13C MAS NMR spectrum of H-GO acquired with HPDEC. It is different from the spectra in Figure 4. A very sharp peak at 110 ppm appears. In contrast, the spectra in Figure 4 do not show such a peak around 110 ppm. The peaks 12484
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Figure 5. 13C MAS NMR spectrum of H-GO acquired with HPDEC. (Inset) NMR spectra of GO and H-GO in the region of 110 ppm. CP/ MAS and HPDEC spectra of GO (HD-GO and CP-GO) do not show any peak near 110 ppm, while H-GO shows peaks near 110 ppm.
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Figure 7. FT-IR spectra of GO and H-GO.
Table 1. FT-IR Peak Assignments for GO and H-GO frequency (cm1)
assignment GO
1070
O, epoxy group
1640
dO, vibration mode, ketone group
3430
OH, stretching mode, hydroxyl group stretching mode
1080
O, epoxy group
1246
CO, stretching mode, phenol group
H-GO
Figure 6. (a) 13C NMR chemical shift of phenol. (b) 13C NMR chemical shift of furan. (c) Possible structure at the edge region of H-GO.
above 130 ppm and the peak at 110 ppm in Figure 5 are due to the formation of phenol and/or aromatic diol groups.28,29 Their formation requires that OH groups attached to the carbon ring are converted to phenols/aromatic diols or one double bond is present in the ring. The inset compares the spectra of GO and H-GO in the region of 110125 ppm. Phenol and furan have two similar peaks in common. One appears at 110 ppm and the other at 150 ppm (Figure 6a,b). However, there is no peak near 150 ppm in Figure 5. The structure of furan is different from that of GO, while the structure of phenol is similar to that of graphite oxide. Although two double bonds are also difficult to create, it is possible for hydroxyl groups to be bonded within the plane shown in Figure 6c. For this reason, the peak might appear not at 150 ppm but at 110 ppm in Figure 5. In Figure 7, GO shows three main peaks centered at 1070, 1640, and 3430 cm1. The peak at 1070 cm1 arises from epoxy groups. The peak 3430 cm1 denotes COH stretching. The peak at 1640 cm1 corresponds to the vibrational mode of the ketone groups, which can be located only on the edge, and it is influenced by the edge conditions such as the shape and the dangling bonds. H-GO has peaks at 1080, 1246, 1400, 1620, and
1400
CO, bending mode, hydroxyl and phenol group
1620
dO, vibration mode, ketone group
3140
OH, stretching mode, hydroxyl and phenol group
3140 cm1. The peaks are assigned in Table 1. Unlike GO, the peak at 1400 cm1 from H-GO becomes very sharp. The peak at 1246 cm1 grows. The peaks at 1245 and 1400 cm1 are strong evidence that phenol groups is formed in hydrated GO.
4. CONCLUSIONS Structural properties of GO samples have been investigated. GO swells when it becomes hydrated and the layer distance increases to 9.7 Å. After a layer of water molecules is inserted inside GO, it shows new NMR peak at 110 ppm. It also exhibits strong peaks at 1246 and 1400 cm1 in the FT-IR spectrum. We suggest that phenol groups are formed by reaction of GO with water. ’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected]; phone þ65 6513 8459; fax þ65 6795 7981.
’ ACKNOWLEDGMENT J.W.S. acknowledges financial support by the Korean Research Foundation (Grant KRF-2005-215-C00040). ’ REFERENCES (1) Hofmann, U.; Frenzel, A.; Csalan, E. Liebigs Ann. Chem. 1934, 510, 1. 12485
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