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J. Phys. Chem. C 2007, 111, 14694-14697
Shape Evolution of Single-Crystalline Mn2O3 Using a Solvothermal Approach Wei-Na Li,† Lichun Zhang,† Shanthakumar Sithambaram,‡ Jikang Yuan,† Xiong-Fei Shen,† Mark Aindow,† and Steven L. Suib*,†,‡ Department of Chemical, Materials, and Biomolecular Engineering, and Department of Chemistry, Unit 3060, 55 North EagleVille Road, UniVersity of Connecticut, Storrs, Connecticut 06269-3060 ReceiVed: June 12, 2007; In Final Form: August 9, 2007
A new and facile route was developed to manipulate the growth of hierarchically ordered Mn2O3 architectures via a solvothermal approach. Various solvents are employed to control the product morphologies and structures. Mn2O3 with unique cuboctahedral, truncated-octahedral, and octahedral shapes are obtained, and a possible formation mechanism is proposed.
There has been extensive recent research into fabrication and control of micro- and nanostructured materials with novel morphologies because of their shape-dependent properties that these materials exhibit and the novel ways in which they can be applied.1-6 Various techniques, including capping agents and templates, have been employed to accomplish this morphology control. However, these techniques are complex and often require further purification to remove the controlling agents, which may introduce impurities into the synthesis system and increase the production cost.1-6 Therefore, developing facile and template-free methods for shape- and structure-controlled synthesis is of particular interest. Solvothermal methods have been used to generate a wide range of materials with specific shapes, such as semiconductor nanowires and nanorods, transition metal oxide hollow spheres, multipods, and so on.7 However, the way in which the choice of solvents affects the growth in solvothermal synthesis has not been studied systematically. Manganese oxide materials are of considerable importance becauseoftheirdistinctivepropertiesandextensiveapplications.4-6,8-11 Polymorphs of Mn2O3 have been employed as environmentally friendly catalysts to remove carbon monoxide and nitrogen oxide from waste gases.11,12 Studies of NO and N2O decomposition showed that Mn2O3 is the most stable catalyst among different manganese oxides including MnO, Mn2O3, Mn3O4 and MnO2.10-12 Mn2O3 can also serve as an inexpensive precursor for the preparation of soft magnetic materials such as manganese zinc ferrite, and for electrode materials of rechargeable lithium batteries via lithiation. Mn2O3 has been synthesized in a variety of different morphologies including rods, wires, or cubes. However, there is little evidence of morphological control in these syntheses with the shapes of final products being dictated to a great extent by those of the precursors.10-12 Herein, we report a new and facile route to manipulate the growth of hierarchically ordered Mn2O3 architectures via a solvothermal approach. Various solvents are employed to control the morphologies and structures of the products, and a possible mechanism for formation of Mn2O3 octahedra will be proposed. The obtained products exhibit great potential for use in catalysis. * Corresponding author. Email:
[email protected]. † Department of Chemical, Materials, and Biomolecular Engineering. ‡ Department of Chemistry.
A typical synthesis was as follows: 4 mmol of Mn(NO3)2 was dissolved in an organic solvent followed by a vigorous stirring at room temperature for half an hour in a Teflon liner. Then the Teflon liner was transferred and sealed in an autoclave for solvothermal treatment at 120 °C for 20 h. A variety of different solvents was used to investigate the effect of solvents on the morphology of the resultant Mn2O3. In order to investigate the development of Mn2O3 crystals, the reactions were also conducted at different temperatures from 100 °C to 180 °C for 20 h and at times from 1.5 h to 20 h at 120 °C, all using ethanol as the solvent. The phase purities of the samples obtained using different solvents were investigated by X-ray diffraction (XRD) as shown in Figure 1. The spectra labeled a-d in Figure 1 were obtained using ethanol, 1-butanol, 2-ethanol, and acetone as the solvent, respectively, and all of the peaks in these spectra can be indexed to a pure cubic phase of Mn2O3 (bixbyite-c, JCPDS 41-1442). The sharp and intense peaks in the XRD spectra labeled indicate good crystallinity of pure Mn2O3, while the crystallinity of the samples prepared in acetone (spectrum d) is relatively low. The samples synthesized in cyclohexane and benzene, however, give XRD peaks corresponding to pure γ-MnO2 (JCPDS 17-510, spectra 1e,f). Conventionally, synthesized Mn2O3 materials have either wirelike or rodlike shapes.10-12 In comparison, the Mn2O3 materials obtained in this work show unique octahedral morphologies when ethanol, 1-butanol, and 2-butanol were used as solvents in this system. Field emission scanning electron microscopy (FESEM) image, shown in Figure 2a, indicates that most of the Mn2O3 octahedra obtained in ethanol have a side length of 200 nm to 2 µm, although there are some aggregates of smaller octahedra. An example of one large octahedron is shown in the inset to Figure 2a (enlarged FESEM image is available in Supporting Information), and this crystal has sides 1.5 µm in length. A similar morphology is observed in FESEM pictures of samples prepared in 1-butanol (Figure 2b). When the solvent was changed to 2-butanol, however, truncated octahedra were observed with a side length from 300 nm to 3 µm and small square facets at the vertices (Figure 2c): a typical example of such a truncated octahedron is shown in the inset FESEM image (enlarged FESEM image is available in Supporting Information). Small amounts of rodlike materials are also found in the samples obtained using 2-butanol; however,
10.1021/jp0745539 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007
Shape Evolution of Single-Crystalline Mn2O3
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14695
Figure 1. XRD patterns for the samples prepared in different solvents: (a) ethanol, (b) 1-butanol, (c) 2-butanol, (d) acetone, (e) cyclohexane, and (f) benzene.
Figure 2. FESEM images of products synthesized in different solvents: (a) ethanol, (b) 1-butanol, (c) 2-butanol, and (d) acetone.
Figure 3. (1) FESEM images of products synthesized under different reaction periods (a) 1.5 h, (b) 2 h, and (c) 3 h. (2a) TEM image and the corresponding MDP in the [100]* axis (insert), and (2b) the [100]* high-resolution TEM (HRTEM) image showing the details in the right edge of the nanocube I of Mn2O3 prepared in ethanol for 1.5 h, (c) TEM image and the corresponding MDP in the [11h0]* axis (insert) and (d) the [11h0]* HRTEM image showing the details in the top edge of the nano-octahedron II of the Mn2O3 prepared in ethanol for 3 h.
XRD data show that they are the same phase of Mn2O3. Urchinlike aggregates of Mn2O3 nanorods are produced in acetone (Figure 2d), while rodlike materials with lengths up to several micrometers are observed in cyclohexane and benzene solvents. FESEM pictures of the samples prepared in benzene and cyclohexane are given in Supporting Information. For the materials produced using ethanol as solvent, the Mn2O3 octahedra were formed over a range of temperatures from 100 °C to 160 °Cwithout any significant change in morphology and crystallinity. However, when the temperature was increased to 180 °C, a mixture of bixbyite-c and γ-Mn2O3 was produced with a rough surface of octahedra. Bixbyite-c octahedra thus are not stable up to 180 °C (see Supporting Information). The reactions in ethanol were also conducted for different periods to understand the formation mechanism of these novel Mn2O3 microstructures. FESEM pictures from the samples obtained after 1.5 h reaction in ethanol show nanocubes with a side length of less than 100 nm (Figure 3(1a)). When the reaction time was lengthened to 2 h, cuboctahedral products are produced (Figure 3(1b)), and the cuboctahedra evolved into octahedra after 3 h of reaction (Figure 3(1c)). TEM imaging was employed to further investigate the morphologies and crystallographic features of the various Mn2O3 products. The bright-field TEM image shown in Figure 3(2a) was obtained from the sample after 1.5 h reaction in ethanol: this clearly shows cube-shape of the products less than 100 nm in size. The nanocube labeled “I” in Figure 3(2a) was tilted so that the side faces were exactly edge on. The corresponding microdiffraction pattern (MDP) is shown in the inset of Figure 3(2a): this confirms not only that the Mn2O3 adopts cubic crystal structure, but also that the faces of the cube lie exactly parallel
to the {001} planes. Further evidence for this is shown in Figure 3(2b), which is a high-resolution lattice image obtained from the same nanocube showing the structure of these {001}-type facets. The Mn2O3 materials synthesized in ethanol for 3 h were also studied by TEM methods (Figure 3(2c,d)). In the bright-field TEM image (Figure 3(2c)), the nano-octahedron labeled “II” was tilted until the side faces were exactly edge-on, whereupon it exhibited a characteristic diamond shape in projection. The inset in Figure 3(2c) is the corresponding MDP, which can be indexed as the [11h0]* pattern of the Mn2O3 cubic phase. Figure 3(2d) is a high-resolution lattice image from one corner of this nano-octahedron. A series of such TEM images was used to determine that the Mn2O3 octahedra grow with well-defined {111}-type faces. Very recently, multipod- and dumbbell-shaped MnO nanocrystals have been prepared in organic solutions.4,5 A two-step mechanism of orientation and recrystallization is proposed to explain the formation of MnO nanomaterials. Crystal growth theory from rough surfaces was used to explain the preparation of cross-shaped MnO nanocrystals.6 Another commonly used technique to achieve nanostructures with tunable shapes is to kinetically control the growth rates of various facets of a seed via the use of controlling agents, such as capping agents or surfactants.2,14,15 Although no controlling reactants were employed in our case, the synthesis of Mn2O3 octahedral particles more likely follows this mechanism. On the basis of our FESEM and TEM results, the possible formation mechanism is proposed. First, Mn(NO3)2 precursor was dissolved in the solvents, and this then started to decompose very slowly and smoothly under solvothermal conditions so that the nuclei grew homogeneously. Aggregation of the manganese
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Li et al. TABLE 1: Preparation of Quinoxaline Catalyzed by Mn2O3 Catalysts
Figure 4. Scheme of the possible growth mechanism of Mn2O3 octahedra.
catalyst
Mn2O3 obtained in ethanol
Mn2O3 obtained in 1-butanol
Mn2O3 obtained in 2-butanol
commercial Mn2O3
conversion selectivity
74% 100%
71% 100%
68% 100%
51% 100%
generation of Mn2O3, and MnO2 may be the original products produced in situ at the beginning as described in the following reactions:
Mn(NO3)2 f MnO2 + 2NO2 Figure 5. Preparation of quinoxaline catalyzed by Mn2O3 catalysts.
Figure 6. Mechanism of preparation of quinoxaline catalyzed by Mn2O3.
oxide nuclei then formed small particles, which grew into nanocubes with {001} faces as demonstrated in FESEM (Figure 3(1a)) and TEM images (Figure 3(2a,b)). With a lengthened reaction period (2 h), the nanocubes coarsen via a process akin to Ostwald ripening, with a concomitant transition to a cuboctahedral morphology (Figure 3(1b)) due to the different growth rates of {001} and {111} faces.13 After 3 h reaction, {001} faces disappeared and only the {111} faces were identified as shown in Figure 3(1c,d). A simple scheme of the octahedral particle evolution is demonstrated in Figure 4. This shape evolution is very similar to the diamond transformation from octahedra to cubes reported in the literature, in which intermediate cuboctahedra were observed.14 These researchers proposed that the growth rate difference between the two directions [111] and [100] plays a key role in shape control which is affected by reaction conditions. As a result, the shapes of the samples can be tuned by adjusting the experimental factors. Certain concentrations of capping agents were also found to help control the growth of {001} and {111} planes for preparations of PbS and Cu2O octahedra. Concentrations of capping agents may favor or block the growth of {001} or {111} planes, which in turn affects the final shapes of the products.1-3 In the current system, the selective interaction between solvents and various crystallographic planes of Mn2O3 could favor the growth of {111} planes when the reaction continues, which contributes to the formation of Mn2O3 with different morphologies. The different properties of the used organic solvents, such as polarity and viscosity, may play important roles in the shape control of the Mn2O3 materials. In addition, on the basis of the FESEM and XRD patterns, rodlike γ-MnO2 was observed in cyclohexane and benzene instead of the Mn2O3 octahedra formed in other organic solvents. Therefore, -OH groups are believed to be critical for the
2MnO2 + CH3CH2OH f Mn2O3 + CH3CHO + H2O
(1) (2)
Since no reducing agents exist, such as -OH, in cyclohexane and benzene, no Mn2O3 was obtained. Catalytic reactions were carried out to prepare quinoxaline at 110 °C for 1 h using 50 mg of Mn2O3 as catalyst as shown in Figure 5. Compared to 51% conversion for commercial Mn2O3, the Mn2O3 catalysts obtained in our system give increased conversions around 70% for generation of quinoxaline from pyridoin under the same reaction conditions (Table 1). The reaction mechanism involves an oxidation and a condensation step as shown in Figure 6. This type of transformation has been done with active manganese oxide (as a stoichimetric reagent) and molecular sieves.16 Manganese oxide and molecular sieves in combination have been used in producing quinoxalines. This process requires excessive amounts of manganese oxide (∼15 equiv) and is done in chlorinated solvent. Moreover, the time of reaction was 20 h. Transformation with Mn2O3 required only a small amount of catalyst for a time of only 1 h. In conclusion, we have developed a selective solvothermal method to synthesize Mn2O3 materials with unique and tunable morphologies. The hierarchically ordered Mn2O3 octahedra evolve from nanocubes, then cuboctahedra, and finally octahedra due to the different growth rate of {001} and {111} faces. Such control over the morphology suggests great potential for shapecontrolled syntheses of other transitional metal and metal oxides from their nitrates such as zinc oxides, iron oxides, nickel oxides and silver metal, which are under investigation in our lab. Acknowledgment. We acknowledge support by the Division of Chemical, Geosciences and Biosciences, Office of Basic Energy Sciences, Office of Sciences, U.S. Department of Energy. We would like to thank Dr. Jim Romanow for access to their FESEM facilities in the Physiology and Neurobiology Department, UConn. Supporting Information Available: Experiment details on temperature effects on the morphologies and crystal structures. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Zhang, Z.; Sui, J.; Zhang, L.; Wan, M.; Wei, Y.; Yu, L. AdV. Mater. 2005, 17, 2854. (2) Lee, S. M.; Jun, Y. W.; Cho, S. N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244. (3) Zhang, X.; Xie, Y.; Xu, F.; Liu, X.; Xu, D. Inorg. Chem. Commun. 2003, 6, 1390. (4) Zitoun, D.; Pinna, N.; Frolet, N.; Belin, C. J. Am. Chem. Soc. 2005, 127, 15034. (5) Zhong, X.; Xie, R.; Sun, L.; Lieberwirth, I.; Knoll, W. J. Phys. Chem. B 2006, 110, 2.
Shape Evolution of Single-Crystalline Mn2O3 (6) (a) Ould-Ely, T.; Prieto-Centurion, P.; Kumar, A.; Guo, W.; Knowles, W. V.; Asokan, S.; Wong, M. S.; Rusakova, I.; Lu¨ttge, A.; Whitmire, K. H. Chem. Mater. 2006, 18, 1821. (b) Rusakova, I.; Ould-Ely, T.; Hofmann, C.; Prieto-Centurio´n, D.; Levin, C. S.; Halas, N. J.; Lu¨ttge, A.; Whitmire, K. H. Chem. Mater. 2007, 19, 1369-1375. (7) Zou, G.; Li, H.; Zhang, Y.; Xiong, K.; Qian, Y. Nanotechnology 2006, 17, S313. (8) (a) Tian, Z. R.; Tong, W.; Wang, J. Y.; Duan, N. G.; Krishnan, V. V.; Suib, S. L. Science 1997, 276, 926. (b) Yuan, J.; Li, W. N.; Gomez, S.; Suib, S. L. J. Am. Chem. Soc. 2005, 127, 14184. (9) Son, Y. C.; Makwana, V. D.; Howell, A. R.; Suib, S. L. Angew. Chem., Int. Ed. 2001, 40, 4280.
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14697 (10) Yamashita, T.; Vannice, A. J. Catal. 1996, 163, 158. (11) Koyanaka, H.; Takeuchi, K.; Loong, C. K. Sep. Purif. Technol. 2005, 43, 9. (12) He, W.; Zhang, Y.; Zhang, X.; Wang, H.; Yan, H. J. Cryst. Growth 2003, 252, 285. (13) Marqusee, J. A.; Ross, J. J. Chem. Phys. 1984, 80, 536. (14) Kobashi, K.; Nishimura, K.; Kawate, Y.; Horiuchi, T. Phys. ReV. B 1988, 38 (6), 4067 (15) Xia, Y.;Yang, P.; Sun, Y.; Wu, Y.; Maybers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15 (5), 353. (16) Raw, S. A.; Wilfred, C. D.; Taylor, R. J. K. Chem. Commun. 2003, 2286-2287.
