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Novel Synthesis of Layered Graphite Oxide-Birnessite Manganese Oxide Nanocomposite Xiaojing Yang,* Yoji Makita, Zong-huai Liu, and Kenta Ooi* National Institute of Advanced Industrial Science and Technology, 2217-14 Hayashi, Takamatsu 761-0395, Japan Received September 24, 2002 Revised Manuscript Received November 26, 2002 Exfoliation/restacking of a two-dimensional host material is obviously an important and effective approach to preparing a nanocomposite.1 Graphite oxide (GO) is a pseudo-two-dimensional solid in bulk form,2,3 and its powders can be exfoliated in a dilute (alkaline) aqueous solution4 or an aqueous saline solution.5 Nanocomposite materials of GO and some organic component guests are obtained, via the restacking of exfoliated GO layers4,6-8 or, alternatively, restacking of layers on the guest (the layer-by-layer assembly method)5,9 processes. Layered birnessite-type manganese oxide (BirMO) exhibits excellent properties as a molecular sieve, lithium ion secondary battery or catalyst material, and precursor for preparing many other manganese oxides.10-13 GOBirMO nanocomposite is attractive as a candidate for the electrode of a high-capacity battery or capacitor. However, there are no reports to date of a manganese oxide as the guest intercalated into GO layers. The weak dispersion ability of manganese oxide in (alkaline) aqueous solution makes it difficult to apply directly the exfoliation/restacking of GO layers approach. Alkali metal-birnessite can be synthesized by the oxidation of a Mn2+ salt by air14 or hydrogen peroxide15 in concentrated alkali metal aqueous solution; therefore, we propose an in situ precipitation of Li-BirMO in suspension with exfoliated GO layers, which is dispersed stably in LiOH solution. A stable suspension of a GOBirMO nanocolloid could be obtained by this method. Subsequent air-drying resulted in the smooth restacking * To whom correspondence should be addressed. Fax: +81-87-8693551. E-mail:
[email protected];
[email protected]. (1) Leroux, F.; Besse, J. P. Chem. Mater. 2001, 13, 3507. (2) Nakajima, T.; Matsuo, Y. Carbon 1994, 32, 469. (3) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477. (4) Liu, P.; Gong, K.; Carbon 1999, 37, 706. (5) Cassagneau, T.; Fendler, J. H. Adv. Mater. 1998, 10, 877. (6) Matsuo, Y.; Fukutsuka, T.; Sugie, Y. Carbon 2002, 40, 955. (7) Liu, P.; Gong, K.; Xiao, P.; Xiao, M. J. Mater. Chem. 2000, 10, 933. (8) Liu, Z-h.; Wang, Z.-M.; Yang, X.; Ooi, K. Langmuir 2002, 18, 4926. (9) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771. (10) Cao, H.; Suib, S. L. J. Am. Chem. Soc. 1994, 116, 5334. (11) Duncan, M. J.; Leroux, F.; Corbett, J. M.; Nazar, L. F. J. Electrochem. Soc. 1998, 145, 3746. (12) Brock, S. L.; Duan, N.; Tian, Z.-R.; Giraldo, O.; Zhou, H.; Suib, S. L. Chem. Mater. 1998, 10, 2619. (13) Feng, Q.; Kanoh, H.; Ooi, K. J. Mater. Chem. 1999, 9, 319. (14) Li, L.; Pistoia, G. Solid State Ionics 1991, 47, 231. (15) Feng, Q.; Yanagisawa, K.; Yamasaki, N. J. Ceram. Soc. Jpn. 1996, 104, 897.
Figure 1. Wet-state XRD patterns of (a) GO treated with LiOH solution, (b) GO-birnessite nanocomposite, and (c) Libirnessite. d-values in Å.
of a nanocolloid having a unique structure with GO and BirMO sheets stacked alternatively, aligning their c-axes in the same direction. Graphite oxide was obtained by the oxidation of the graphite powder (brand name CMX-40, Nippon Graphite Industries, Ltd., average size 38 µm, apparent density 0.05 g/cm3), with KMnO4 in concentrated H2SO4 by a modified Hummers method.16,7 The GO obtained had 50.0 wt % carbon, 37.6 wt % oxygen, and 2.0 wt % hydrogen and the repeat distance along the c-axis, Ic, 0.770 nm. The lithium-ion exchange capacity was 5.2 mmol/g of GO, as determined by alkalimetry in a mixed solution of LiOH and LiCl according to the literature.17 The obtained GO (1 g) was immersed in a 0.15 M LiOH aqueous solution (40 mL), sonicated for 30 min, and then stirred overnight. The amount (6 mmol) of LiOH added was slightly more than the ion exchange capacity of GO. A stable colloidal suspension was obtained (abbreviated as Li-GO) as shown in Figure 1a. After 2 mol of H2O2 (2 M) was added, the suspension was dropped into a 0.3 M Mn(NO3)2 aqueous solution (38 mL) with stirring. Finally, a mixed solution of 0.5 M LiOH (64 mL) and 6 mL of H2O2 (2 M) was poured into the suspension and stirred for 2 h. A stable colloidal suspension with a brownish color was obtained. The product was centrifuged and washed with distilled water several times until pH ≈ 7. The product is abbreviated as GBH. For comparison, Li-birnessite was synthesized using the same procedure. (16) Hummers, W. S. Jr.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (17) Ooi, K.; Miyai, Y.; Sakakihara, J. Langmuir 1991, 7, 1167.
10.1021/cm025697x CCC: $25.00 © 2003 American Chemical Society Published on Web 02/20/2003
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Chem. Mater., Vol. 15, No. 6, 2003 1229 Table 1. Compositions of the GO-Birnessite Nanocomposite and Li-Birnessite
sample
Li (mmol g-1)
Mn (mmol g-1)
GO (wt %)
H2Oa (wt %)
GBH Li-birnessite
3.25 2.61
5.08 9.48
35.2
12 15
ZMnb
Li/Mn
formula
3.68
0.64 0.27
Li3.86Mn14O27.7
a
Estimated from the weight loss in the TG-DTA curves from the starting temperature to 110 °C for GBH and to 400 °C for Libirnessite. b The mean oxidation state of manganese, by the standard oxalic acid method, Japan Industrial Standard M8233, 1969.
Figure 3. TEM images (left) and SAD patterns (right) of GObirnessite nanocomposite. (a) and (b): Surface. (c) and (d): Cross section.
Figure 2. XRD patterns of (a) GO treated with LiOH solution, (b) GO-birnessite hybrid, and (c) Li-birnessite, all of which were put on glass slides, dried sufficiently at room temperature, and then further dried at 70 °C for 1 day. (b′) The powdered sample for GO-birnessite nanocomposite conventionally dried in air. d-values in Å, (hkl) cf. JCPDS 43-1456.
The centrifuged sediments were subjected to X-ray diffraction (XRD) measurement in the wet state, on a Rigaku RINT 2100 diffractometer using Cu KR radiation, a scanning speed of 2°/min, and sampling 0.01°. The XRD patterns of Li-GO and GBH gave no clear peaks but only a broad diffraction halo in a 2θ range of 20-40°, whereas the Li-birnessite sample had clear peaks with Ic ) 0.71 nm, although the weak hump existed (Figure 1). The broad halo for Li-GO is most likely related to scattering from dispersed single sheets of exfoliated GO and water as a solvent, similar to the layered titanic acid.18 The absence of diffraction peaks for GBH suggests that stable nanocolloids of GOBirMO composite are formed, but not the simple mix(18) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 129, 4682.
ture of GO and BirMO solids. Scanning electron microscope observations of the freeze-dried samples showed that the lamellar GBH particles had the same regular boundaries as the GO particles but rough surfaces similar to those of the Li-birnessite particles. Energydispersive X-ray microanalysis showed the uniform distribution of Mn, C, and O over GBH particles, indicating that GO and BirMO were well-mixed. When the samples were dried normally, the layered structure appeared in XRD patterns as shown in Figure 2. The exfoliated Li-GO sheets restacked to a layered structure with Ic ) 0.85 nm; the Ic value has been known to vary between 6.1 and 11 Å depending upon the amount of water adsorbed.19 Very sharp, strong peaks are observed in the pattern of the GBH film dried on a glass substrate. The intensity of (001) interference is 20 times higher than that obtained by conventional airdrying (Figure 2b′). It has Ic ) 0.697 nm, smaller than that of Li-GO, and even a little smaller than that of Libirnessite under the same drying conditions. The existence of birnessite in the GBH sample is confirmed from the XRD pattern of the powdered sample (Figure 2b′), where the peaks of (hkl) interferences of birnessite, not parallel to (001), appear to be the same as those of Libirnessite in Figure 2c. The GBH dried on a glass substrate, however, shows only the peaks of (00l) interferences (Figure 2b). The difference in XRD patterns between (b) and (b′) of Figure 2 indicates that the (19) Dekany, I.; Kruger-Grasser, R.; Weiss, A. Colloid Polym. Sci. 1998, 276, 570.
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Figure 4. Schematic representation of GO-birnessite nanocomposite.
birnessite sheet tends to orientate its c-axis parallel to the c-axis of GO, resulting in a highly oriented composite of GO and BirMO. The presence of GO sheets is essential for this high orientation because such orientation was barely observed in the Li-birnessite sample (Figure 2c). The manganese and lithium contents in the samples were determined by atomic absorption spectrophotometry after being dissolved in a mixed solution of HCl and H2O2. The residual solid GO was dried at 50 °C and weighed. As shown in Table 1, Li-birnessite was yielded at Li/Mn ) 3.3 in the solutions, as has been reported in the literature.15 The lithium content 3.25 mmol/g in GBH can be portioned into 1.4 mmol/g at the acidic sites of birnessite according to the Li/Mn ratio of the Libirnessite sample, and 1.85 mmol/g. The latter is equal to the lithium-ion exchange capacity (0.352 × 5.2 ) 1.83 mmol/g) of GO. This result suggests that most of the ion-exchange sites of GO are charge-compensated for by lithium cations, not by manganese ions. On the basis of the composition and high-orientation property, we hypothesize that a lithium cation layer exists between the birnessite and GO layers to compensate for the negative charge of birnessite and GO. The TEM (high-resolution transmission electron microscope) and SAD (selected area electron diffraction) observation was performed using a JEOL, JEM-3010 transmission electron microscope at an accelerating voltage of 300 kV. For the cross-section observation, the sample was embedded in epoxy resin and sliced by ultra microtomy. The samples were supported on microgrids. Figure 3 shows the TEM images and SAD patterns of
the GBH sample dried normally at room temperature. Interestingly, an elliptic SAD pattern (Figure 3b) is observed for a GBH particle. Such an elliptic pattern was not observed for Li-birnessite or Li-GO and shows the unique structure of GBH. This pattern can be explained as a “textured polycrystalline structure”,20 where polycrystals have a particular crystal axis aligned along a certain direction (the fiber axis) with a random arrangement of azimuthal orientations about this axis. The semiminor axis of the smallest ellipse in Figure 3b shows a d value of 2.2 Å corresponding to the birnessite (201) plane (Figure 2c,b′). Thus, it is most probable that the elliptic pattern arises due to each birnessite crystal having the same crystal plane (00l) parallel to the plane of GO layer. The high-magnification observation from the cross section shows a lattice image of layered structure in which the d spacing of the composite is 7 Å, which coincides with the XRD result (Figure 2b). TEM/SAD observation of the Li-GO sample did not show any lattice image because of the instability of Li-GO under the same observation conditions. In accordance with the above discussion, a schematic representation of the composite formation is given in Figure 4. The process can be divided into two steps: (1) formation of stable nanocolloid composites by in situ precipitation of birnessite crystals; (2) highly regulated stacking of nanocolloids by air-drying. The nanocolloid (20) Hirsch, P. B.; Howie, A.; Nicholson, R. B.; Pashley, D. W.; Whelan, M. J. Electron Microscopy of Thin Crystals, Reprinted and revised; Butterworth: London, 1967; p 116.
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obtained by step 1 is stable enough to tolerate dispersion in water for a long time. The GO and BirMO sheets may form a nanocomposite since Li-birnessite alone tends to stack as shown in Figure 1c. Since the surface charge density of BirMO is about half that of GO, heterocoagulation of GO and BirMO may take place to overcome the electric repulsive force between negatively charged GO and BirMO sheets. The specific area was estimated as follows: the framework density of GO is supposed to be the same as graphite, that is, 2.3 g/cm3; the carbon framework content was evaluated as 50% of GO from the TG-DTA analysis. The thickness of GO is known to be 0.61 nm.19 Hence, the surface area of GO was 1 g (GO) × 50%/(2.3 × 10-6)/(0.61 × 10-9) ) 0.7 × 103 m2/g of GO. From the area 0.147 nm2 for each
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manganese atom in the manganese oxide layer and the moles of manganese atoms in the experiment, 0.3 mmol/ mL × 38 mL ) 11.4 mmol, the surface area of BirMO is evaluated as 1 × 103 m2/g of GO. Finally, the specific surface areas of GO and BirMO in GBH were found to be nearly the same. Therefore, it is possible for birnessite to form a single layer on the GO surface In conclusion, birnessite manganese oxide precipitates in situ on exfoliated GO layers with its layers aligned parallel to the GO layers; the charge between them is compensated for by lithium ions. The restacking of GO layers encapsulated with Li-birnessite layers produces the GO-birnessite nanocomposite. CM025697X
