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Synthesis of Single Crystal Manganese Oxide Octahedral Molecular Sieve (OMS) Nanostructures with Tunable Tunnels and Shapes Wei-Na Li,† Jikang Yuan,† Sinue Gomez-Mower,† Shantakumar Sithambaram,‡ and Steven L. Suib*,†,‡ Institute of Materials Science and Department of Chemistry, UniVersity of Connecticut, Storrs, Connecticut 06269 ReceiVed: September 20, 2005; In Final Form: December 28, 2005
A new and facile route is reported to manipulate the self-assembly synthesis of hierarchically ordered RbOMS-2 and pyrolusite with an interesting flowerlike morphology by a direct and mild reaction between rubidium chromateand manganese sulfate without any organic templates. The crystal forms, morphologies, and tunnel sizes of the obtained OMS materials can be controlled. A mechanism for the growth of manganese dioxides with flowerlike architectures was proposed. The obtained products exhibit potential for use in catalysis and other applications.
1. Introduction Control of the shapes and structures of nanomaterials has been one of the most challenging issues faced by materials scientists.1-9 The intrinsic properties of nanostructures can be tailored by controlling shape, crystallinity, size, and composition, which makes it possible for the design and fabrication of new functional nanomaterials. Assembly of nanoparticles into twodimensional (2D) and three-dimensional (3D) well-ordered structures represents some of the key issues in this area.5-9 However, solid templates which are normally used can introduce impurities into the synthesis system and have to be removed after the reaction.8,9 Therefore, developing facile and templatefree approaches to build novel self-generated patterns with controllable morphologies and structures is of great interest. Manganese oxide materials are a family of microporous transition metal oxides and can form mixed-valent semiconducting octahedral molecular sieves (OMS) with 1D tunnel structures of various sizes. These OMS materials have been extensively used as ion-sieves, chemical sensing and energy storage materials, and catalysts. For example, selective oxidation of alcohols with a conversion up to 100% can be realized.10-13 Various types of manganese oxides, such as cryptomelane (designated OMS-2), pyrolusite, and γ-MnO2, have been synthesized by adjusting reaction parameters, such as temperature, pH, concentration, anions, and cations.10-13 However, a transformation from layered precursors, such as birnessite, to tunnel-structured OMS materials is generally necessary in the traditional syntheses.10-13 The OMS nanostructures demonstrate fibrous or flaky morphologies. Kijima et al. recently produced manganese oxides (R-MnO2) with flower shapes using ozone as oxidants.14a However, according to their report, the obtained manganese oxide nanoparticles were “ragged” needlelike single crystals covered with amorphous components and further aging processes were required to prepare pure manganese dioxide nanorods.14a To our best knowledge, flowerlike pyrolusite samples have not been reported. * Address correspondence to this author. Phone: (860) 486-2797. Fax: (860) 486-2981. E-mail:
[email protected]. † Institute of Materials Science. ‡ Department of Chemistry.
Inorganic cations are believed to play critical roles in directing the tunnel dimensions of OMS materials because they can balance charge from mixed-valent framework manganese and stabilize different tunnel sizes. For instance, Mg2+ tends to stabilize 3 × 3 tunnels, while K+ and NH4+ are believed to be suitable cations for 2 × 2 tunnels of the OMS materials. The cations in the tunnels or framework sites of OMS materials have effects on their electronic and catalytic performance.10d,12 The preparation of MnO2 with Rb+ cations (designated Rb-OMS2) in the 2 × 2 tunnels has been studied before.13 However, a layered precursor, an extensive reaction time, and/or high pressure (0.2 GPa) were required in multistep reactions. There is no report of single-crystal Rb-OMS-2 from a single step reaction without layered precursors. Although a great deal of effort has been devoted to synthesize OMS materials with similar tunnel sizes, few studies involve direct control of crystal forms, shapes, and tunnel dimensions of OMS structures. More recently, we have synthesized K-OMS-2 nanoflowers.14b However, in this work the structures and tunnels of the synthesized nanostructures cannot be controlled.14b Therefore, fabrication of OMS architectures with tailored shapes and tunnel sizes under mild conditions is highly desirable. Herein, we report a new and facile route to manipulate the self-assembly synthesis of hierarchically ordered Rb-OMS-2 (2 × 2 tunnel) and pyrolusite (1 × 1 tunnel) three-dimensional flowerlike architectures by a direct and mild reaction. The obtained products exhibit potential for use in catalysis and other applications. 2. Experimental Section 2.1. Synthesis of OMS Nanomaterials. Diluted MnSO4‚H2O was reacted with rubidium chromate under acidic conditions to synthesize OMS nanomaterials with different structures and morphologies based on the following chemical equation
2Rb2CrO4 + 2H2SO4 + 3MnSO4 f 3MnO2 + 2Rb2SO4 + Cr2(SO4) + 2H2O (1) which consists of the following three half reactions (eqs 2, 3, and 4):
10.1021/jp0553380 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/26/2006
MnO2 Octahedral Molecular Sieve Nanostructures
2H+(aq) + 2CrO42-(aq) / Cr2O72-(aq) + H2O
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(2)
MnO2(s) + 4H+(aq) + 2e-1 f Mn2+(aq) + 2H2O (Eo ) 1.23 V)15 (3) Cr2O72-(aq) + 14H+(aq) + 6e- f 2Cr3+(aq) + 7H2O (Eo ) 1.36 V)15 (4) Rb-OMS-2 was prepared in the following way: 6 mmol of MnSO4‚H2O and 4 mmol of Rb2CrO4 were added to a Teflonliner with a volume of 23 mL. Condensed H2SO4 (0.984 mL) was then added dropwise into 14 mL of deionized water (DI) and this solution was poured into the mixture in the Teflonliner under vigorous stirring at room temperature. The obtained clear solution was then transferred into an autoclave and heated at 120 °C for 20 h. After the autoclave was cooled to room temperature, the product was washed and centrifuged several times with DI water to remove all soluble metal ions. To synthesize pyrolusite nanoflowers, hydrothermal temperature was increased from 120 to 180 °C with other parameters unchanged. The processes for the preparations of OMS materials are summarized in Figure 1. To investigate how the OMS nanoflowers formed, the reactions were performed under a varying reaction time from 2 to 6 h at 120 °C. 2.2. Characterization. 2.2.1. X-ray Pattern (XRD). Powder X-ray diffraction data were collected with a Scintag PDS 2000 diffractometer with Cu KR X-ray radiation. A 45 kV beam voltage, and a 40 mA beam current were used. Aqueous slurries of manganese oxide were spread onto glass slides and allowed to dry at room temperature. 2.2.2. FESEM. The morphologies of the obtained manganese oxides were investigated with use of a Zeiss DSM 982 Gemini field-emission scanning electron microscope (FESEM) at an accelerating voltage of 2 kV with a Schottky emitter. The sample suspension in water was dispersed on AuPd-coated silicon chips that had been mounted onto the stainless steel sample holders with two-sided carbon tape. There was no evidence of electron beam induced radiation damage. Magnifications up to 200 000× were used (vide infra). 2.2.3. TEM. High-resolution electron microscope (HRTEM) studies were carried out on a JEOL 2010 at accelerating voltages of 200 kV with an EDS analyzer. The samples were prepared by dispersing the material in 2-propanol. Then a drop of the dispersion was placed on a carbon coated copper grid and allowed to dry. Chemical compositions of samples were determined by energy-dispersive X-ray analysis (EDAX, model EDAX Phoenix with an ultrathin window). 3. Results 3.1. XRD. X-ray diffraction (XRD) was used to study the phase purity of the obtained OMS products as shown in Figure 2a,b. All the reflections of the XRD pattern in Figure 2a can be indexed to a pure tetragonal phase of cryptomelane-type MnO2 (JCPDS 29-1020) for the sample formed at 120 °C, while the XRD pattern of pyrolusite phase (JCPDS 24-735) was observed for the samples prepared at 180 °C (Figure 2b). The XRD patterns with sharp and intense peaks indicate that pure RbOMS-2 and pyrolusite with good crystallinity were obtained. The relative intensities of the (hk0) planes for the Rb-OMS-2 XRD pattern are higher than those in the standard JCPDS data, which can be caused by preferred orientation of the nanofibrous structure. For pyrolusite, the relative intensity of the (101) reflection is higher than the one reported in the JCPDS file.
Figure 1. Schematic illustration of the synthesis for Rb-OMS-2 (2 × 2 tunnel) and pyrolusite (1 × 1 tunnel).
Figure 2. XRD patterns of (a) Rb-OMS-2 obtained at 120 °C and (b) pyrolusite obtained at 180 °C.
3.2. FESEM. Conventionally, the OMS-2 materials have either platelike or rodlike shapes.10-13 Very recently, flowerlike OMS-2 materials have been synthesized.14 However, the crystal forms and tunnel sizes of the products cannot be controlled in these syntheses. A closer observation of a typical highmagnification field emission scanning electron microscopy (FESEM) image of the obtained Rb-OMS-2 (Figure 3b,c,d) indicates that the nanoflowers have a diameter of 3-4 µm, and are formed of uniform nanotetragonal prisms. The cross-section of the nanotetragonal prisms is square and seems fairly smooth (Figure 3c,d). Nanowires with novel rectangular cross section were recently obtained via chemical vapor deposition (CVD) methods under high temperature and low pressure.3 However, more work needs to be done for precisely controlling lengths and particle sizes at high temperatures. Pyrolusite (rutile structure) is composed of single chains of edge-sharing MnO6 octahedra which share corners with neighboring chains to form 1 × 1 tunnels (Figure 1b). Due to the chain structure, pyrolusite typically has a needlelike morphology,16 which has been observed by various researchers.11,17 A facile method was developed here to produce pyrolusite nanoflowers composed of tetragonal nanorods. Compared with Rb-OMS-2, pyrolusite exhibits flowers with a similar diameter of 3-4 µm (Figure 3e). The building blocks of the flowers, however, are different. Although tetragonal prisms are clearly shown in Figure 3f,g, a rough top of the prisms covered by extended nanowires was observed instead of flat tops for the prisms of Rb-OMS-2. Further information on the OMS nanomaterials synthesized under different temperatures was investigated by TEM as shown in Figure 4. The morphologies of the products prepared after a reaction period from 2 to 6 h are shown in Figure 5. When the reaction was run for 2 h, the nanoflowers with an average diameter of 2 µm are observed (Figure 5a,b), and the nanorods composed of the nanoflowers are covered by a lot of small dendritic particles (Figure 5c,d). Prolonged reaction time results in the
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Figure 3. FESEM images: (a) overall product morphology for RbOMS-2 materials; (b) detailed view of an average-sized Rb-OMS-2 single nanoflower; (c) detailed view of the nanoprisms for the RbOMS-2 nanoflower; (d) detailed view of a single nanoprism for the Rb-OMS-2 nanoflower (200 000×); (e) detailed view of an averagesized pyrolusite single nanoflower; and (f) side-view and (g) top-view of the prisms of the pyrolusite nanoflower (100 000×).
Figure 4. (a) Low-magnification TEM image of Rb-OMS-2 single nanoprisms and an SAED pattern, (b) HRTEM image of Rb-OMS-2 single nanoprisms, (c) low-magnification TEM image of pyrolusite single nanoprisms and an SAED pattern, and (d) HRTEM image of pyrolusite single nanoprisms.
smoother surface of the nanorods as indicated in Figure 5e-g, and some dendrites were only found on the top ends of the nanoprisms after a 6 h of reaction (Figure 5g).
Li et al.
Figure 5. FESEM images of samples prepared at different reaction times: (a) overall product morphology at 2 h; (b) detailed view of a single nanoflower at 2 h; (c) detailed view of the nanoprisms at 2 h (100 000×); (d) detailed side-view of the nanoprisms at 2 h (100 000×); (e) detailed view of the single nanoflowers at 3 h; (f) detailed view of the nanoprisms at 3 h (100 000×); (g) detailed view of the nanoprisms at 6 h (100 000×).
3.3. TEM. Transmission electron microscopy (TEM) images of Rb-OMS-2 and pyrolusite as well as selected area electron diffraction (SAED) patterns are shown in Figure 4a-d. Figure 4a shows a typical TEM image for an individual tetragonal prism, the building blocks for the Rb-OMS-2 nanoflowers. The inset SAED demonstrates that the nanoprisms are single crystals. From high-resolution transmission electron microscopy (HRTEM) (Figure 4b), the distances between the lattice fringes are around 4.9 Å, which correspond to the d-spacings of the (200) plane of Rb-OMS-2. Combining the TEM data and the XRD patterns for Rb-OMS-2, the nanoprisms tend to lie on the (100) planes. Figure 4c, combined with the FESEM images, indicates that the prisms of pyrolusite have a larger average width than those of Rb-OMS-2, which may be due to a faster crystal growth rate at a higher temperature of 180 °C. The extended nanowires around the nanotetragonal prisms are clearly observed from the TEM pictures for pyrolusite, which confirmed the results from FESEM. The inset SAED pattern shows that the obtained pyrolusite was composed of single crystals. The HRTEM image of pyrolusite (Figure 4d) indicated that the distance between the lattice fringes is 2.3 Å, which is close to the d-spacings (2.40 Å) of the (101) planes. The TEM results are consistent with XRD data confirming the pure phase and good crystallinity of the as-prepared OMS nanostructures. Figure 6 shows the TEM images for the sample obtained after 2 h, which are in good agreement with the results from FESEM. Several nanorods grow radially from a single core, which is believed to be the nucleus of the nanoflowers (Figure 6a). The nanorods composed of the nanoflowers are covered by many dendrites as indicated in Figure 6b. The dendrites around the
MnO2 Octahedral Molecular Sieve Nanostructures
Figure 6. TEM images of OMS nanoflowers prepared after 2 h: (a) overall morphology of the nanoflower; (b) a detailed view of the building blocks; and (c) the HRTEM image of the building blocks.
nanorods show short-range order, while the nanorods are well ordered (Figure 6c). The distance between the neighboring lines of the nanorods shown in the HRTEM picture (Figure 6c) is about 4.9 Å, which corresponds to (200) planes of Rb-OMS-2. 4. Discussion 4.1. Temperature Effects on the Growth of OMS Nanoflowers. Temperature is critical in controlling the crystal structure and morphology of the obtained nanostructures. At 120 °C Rb-OMS-2 nanomaterials were produced, while formation of pyrolusite was observed with the temperature increased to 180 °C. This occurs because enough thermal energy is available at higher temperatures (180 °C) to overcome the activation energy required for the conversion from Rb-OMS-2 to pyrolusite as proposed.17 Temperature is also related to selfgenerated pressure. Increasing temperatures in the autoclave led to higher pressures, which seem to be beneficial for preparing pyrolusite. Temperature also plays a role in controlling the morphologies of the OMS samples. At 180 °C, the reaction proceeds more vigorously than that at 120 °C, resulting in a faster nucleation and growth of the materials. Therefore, the nanoprisms of pyrolusite nanoflowers have bigger diameters than those of RbOMS-2. The appearance of extended nanowires around the prisms constructed of pyrolusite nanoflowers may also arise from the faster growth at a higher temperature of 180 °C. Several experiments were done with various cations. For example, Li+, Na+, and Cs+ ions also yielded flower morphologies. However, the building blocks of the nanoflowers are nanorods instead of tetragonal prisms for Rb+ and K+. Chromium ions must be present for the formation of the flowerlike OMS materials. Li+ and Na+ are beneficial for the preparation of pure nsutite (an intergrowth of 1 × 1 and 1 × 2 tunnels), while a mixture of pyrolusite (1 × 1 tunnel) and nsutite was prepared with Cs+. OMS-2 nanostructures were produced with Rb+ and K+. However, how the cations affect the formation of the nanoflowers and the structures of the final products is not known and is still under investigation. 4.2. Possible Growth Mechanism of the OMS Nanoflowers. To understand the formation of the novel OMS nanostructures, the reactions under different reaction periods were carried out. Crystallization is an essential step in 1D nanostructure formation.5 Two basic steps, nucleation and growth, are involved in the evolution of a solid from a vapor, liquid, or solid phase. First, MnO2 units were produced from the redox reaction between Cr2O72- and Mn2+ in the solution. These units then tended to aggregate to form small particles, which may serve as nuclei after the concentrations of MnO2 solids in the solution grow sufficiently high. Because the difference of the redox potentials between Cr2O72-/Cr3+ and MnO2/Mn2+ in the redox reaction is very small, around 0.13 eV, the redox reaction can continue very slowly and smoothly, which allows nuclei to grow homogeneously. All these small nuclei may grow larger into
J. Phys. Chem. B, Vol. 110, No. 7, 2006 3069 dendrites, which fuse and evolve into single nanotetragonal prisms under the mild reaction as shown in Figure 5. The FESEM images clearly indicate that the building blocks of the OMS nanoflowers prepared after 2 h have a fluffy surface resulting from the dendritic particles around the tetragonal prisms (Figure 5a-d). The existence of dendrites was confirmed by TEM results, in which short-ordered dendrites were observed to cover the surface of the well-ordered nanoprisms as shown in Figure 6a-c. A radial arrangement of nanoprisms around one core can be clearly observed from Figure 6a, which confirms that the nanorods grow from one nucleus. After a 3 h reaction, some dendrites can still be observed around the nanoprisms (Figure 5,f). However, the surfaces of the nanoprisms are smoother than those of samples obtained after 2 h. The beginning stage is similar to the transportation of nanoparticles to nanowires as reported.2 With a longer reaction time to 6 h (Figure 5g), the surface of the single nanoprism becomes smoother, and dendrites only appear on the top face, which may contribute to the dots on the end of the Rb-OMS-2 samples prepared after 20 h (Figure 3c,d). At 180 °C, extended nanowires were clearly observed around the nanoprisms (Figure 3e-g), which may further confirm that the single tetragonal prism is evolved from dendritic particles. With a continuous supply of the building blocks, the nanoprisms self-organized into nanoflowers due to interfacial tension and hydrophilic properties of OMS nanoparticles. The proposed mechanism is in contrast to the one suggested by Dick et al., who observed growth of nanotrees from different levels branching from the main nanotrunks.1 The branching structures make it possible for the nanotrees to convert photons into electrical current. 5. Conclusions In conclusion, we have successfully synthesized OMS threedimensional architectures with tunable tunnel structures via a soft chemical method. The hierarchically ordered manganese oxides exhibit a primary structure of pores with tunnels constructed of MnO6 octahedral blocks; a secondary structure of tetragonal prisms, which are made up of the primary structures; and a tertiary structure composed of self-assembly of nanotetragonal prisms to form complex nanostructures. Such control over the crystal structure, morphology, and tunnels suggests great potential for these semiconducting OMS materials to generate novel properties which can be utilized in nanoscale devices and systems. Specific catalytic applications include selective oxidations of alcohols10d and generation of imines. RbOMS-2 is active in these reactions. Acknowledgment. We would like to thank Dr. Mark Aindow for access to the JEOL 2010 TEM in the Institute of Materials Sciences, UConn, and Dr. Jim Romanow for providing access to their FESEM facilities in the Physiology and Neurobiology Department, UConn. We acknowledge Dr. Francis S. Galasso, Dr. Lichun Zhang, Dr. Luis-Javier Garces, and Xiongfei Shen for helpful discussions and Geosciences and Biosciences Division of the Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Office of Basic Energy Science for support of this research. References and Notes (1) Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380. (2) Tang, Z.; Wang, Y.; Sun, K.; Kotov, N. A. AdV. Mater. 2005, 17, 358. (3) Guiton, B. S.; Gu, Q.; Prieto, A. L.; Gudiksen, M. S.; Park, H. J. Am. Chem. Soc. 2005, 127, 498.
3070 J. Phys. Chem. B, Vol. 110, No. 7, 2006 (4) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (5) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (6) Cao, M.; Liu, T.; Gao, S.; Sun, G.; Wu, X.; Hu, C.; Wang, Z. L. Angew. Chem., Int. Ed. 2005, 44, 2. (7) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (8) (a) Reuter, T.; Vidoni, O.; Torma, V.; Schmid, G.; Nan, L.; Gleiche, M.; Chi, L.; Fuchs, H. Nano Lett. 2002, 2, 709. (b) Lopes, W. A. Phys. ReV. E 2002, 65, 031606. (9) Giraldo, O.; Brock, S. L.; Marquez, M.; Suib, S. L.; Hillhouse, H.; Tsapatsis, M. Nature 2000, 405, 6782. (10) (a) Shen, Y. F.; Zerger, R. P.; DeGuzman, R. N.; Suib, S. L.; McCurdy, L.; Potter, D. I.; O’Young, C. L. Science 1993, 260, 51l. (b) Tian, Z. R.; Tong, W.; Wang, J. Y.; Duan, N. G.; Krishnan, V. V.; Suib, S. L. Science 1997, 276, 926. (c) Yuan, J.; Laubernds, K.; Villegas, J.; Gomez, S.; Suib, S. L. AdV. Mater. 2004, 16, 1729. (d) Son, Y. C.;
Li et al. Makwana, V. D.; Howell, A. R.; Suib, S. L. Angew. Chem., Int. Ed. 2001, 40, 4280. (11) Shen, X. F.; Ding, Y. S.; Liu, J.; Cai, J.; Laubernds, K.; Zerger, R. P.; Vasiliev, A.; Aindow, M.; Suib, S. L. AdV. Mater. 2005, 17, 805. (12) Suib, S. L. Curr. Opin. Solid State Mater. Sci. 1998, 3, 63. (13) (a) Liu, J.; Makwana, V.; Cai, J.; Suib, S. L.; Aindow, M. J. Phys. Chem. B 2003, 107, 9185. (b) Yamamoto, N.; Oka, Y.; Tamada, O. Mineral. J. 1990, 15, 41. (c) Ohzuku, T.; Hirai, T. Proc. Electrochem. Soc. 1985, 85, 262. (14) (a) Kijima, N.; Yasuda, H.; Sato, T.; Yoshimura, Y. J. Solid State Chem. 2001, 159, 918. (b) Yuan, J.; Li, W. N.; Gomez, S.; Suib, S. L. J. Am. Chem. Soc. 2005, 127, 14184. (15) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and ReactiVity; HarperCollins College Publisher: New York, 1993; pp A-35-A-37. (16) Post, J. E.; Proc, E. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3447. (17) Walanda, D. K.; Laurance, G. A.; Donne, S. W. J. Power Sources 2005, 139, 325.
