pubs.acs.org/Langmuir © 2010 American Chemical Society
Facile One-Step Template-Free Synthesis of Uniform Hollow Microstructures of Cryptomelane-Type Manganese Oxide K-OMS-2 Hugo M. Galindo,† Yadira Carvajal,‡ Eric Njagi,† Roger A. Ristau,‡ and Steven L. Suib*,†,‡,§ †
Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060, ‡Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136, and §Chemical, Materials and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269-32222 Received June 12, 2010. Revised Manuscript Received July 8, 2010
Hollow microstructures of cryptomelane-type manganese oxide were produced in a template-free one-step process based on the fine-tuning of the oxidation rate of manganese species during the synthesis. The tuning of the reaction rate brought about by a mixture of the oxidants oxone and potassium nitrate becomes apparent from the gradual physical changes taking place in the reaction medium at early times of the synthesis. The successful synthesis of the hollow uniform structures could be performed in the ranges 120-160 °C and 8.2-10.7 for temperature and mass ratio oxone/ potassium nitrate, respectively. Independent of the conditions of the synthesis, all of the complex microstructures showed the same pattern for the array of very long nanofibers in which some of these elongated around the surface confining the cavity and the other fibers grew normal to the surface created by the previous arrangement. A mechanism based on the heterogeneous nucleation of the cryptomelane phase on the surface of an amorphous precursor and the growth of the nanoscale fibers by processes such as dissolution-crystallization and lateral attachment of primary nanocrystalline fibers is proposed to explain the formation of the hollow structures.
Introduction The self-assembly of nanoscale particles into complex structures has been intensively investigated by scientists to carry out the synthesis of materials displaying enhanced properties. Without doubt, hollow structures of metal oxides are some of these complex assemblies giving rise to enormous interest due to their high performance in applications regarding fields such as batteries,1-3 catalysis and photocatalysis,4-7 gas sensors,8 and drug delivery.9,10 Template-assisted processes based on organic and inorganic templates are by far the most used to perform the synthesis of hollow micro- and nanostructures of metal oxides.2,8,10-14 This approach has been usually carried out involving previous steps to the synthesis in which the template is prepared and its surface modified to create electrostatic or chemical interactions capable of driving the growing and selfassembly of the new material on the surface. Thermal or chemical *To whom correspondence should be addressed. E-mail: Steven.Suib@ uconn.edu. Fax: (860) 486-2981. (1) Yang, H. X.; Qian, J. F.; Chen, Z. X.; Ai, X. P.; Cao, Y. L. J. Phys. Chem. C 2007, 111, 14067–14071. (2) Du, N.; Zhang, H.; Chen, J.; Sun, J.; Chen, B.; Yang, D. J. Phys. Chem. B 2008, 112, 14836–14842. (3) Wang, Y.; Su, F.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2006, 18, 1347–1353. (4) Yang, Z.; Han, D.; Ma, D.; Liang, H.; Liu, L.; Yang, Y. Cryst. Growth Des. 2009, 10, 291–295. (5) Li, H.; Bian, Z.; Zhu, J.; Zhang, D.; Li, G.; Huo, Y.; Li, H.; Lu, Y. J. Am. Chem. Soc. 2007, 129, 8406–8407. (6) Song, X.; Gao, L. J. Phys. Chem. C 2007, 111, 8180–8187. (7) Yu, J.; Yu, X. Environ. Sci. Technol. 2008, 42, 4902–4907. (8) Li, X.-L.; Lou, T.-J.; Sun, X.-M.; Li, Y.-D. Inorg. Chem. 2004, 43, 5442– 5449. (9) Yang, J.; Lee, J.; Kang, J.; Lee, K.; Suh, J.-S.; Yoon, H.-G.; Huh, Y.-M.; Haam, S. Langmuir 2008, 24, 3417–3421. (10) Zhu, Y.; Kockrick, E.; Ikoma, T.; Hanagata, N.; Kaskel, S. Chem. Mater. 2009, 21, 2547–2553. (11) Titirici, M.-M.; Antonietti, M.; Thomas, A. Chem. Mater. 2006, 18, 3808– 3812. (12) Li, H.; Ha, C.-S.; Kim, I. Langmuir 2008, 24, 10552–10556. (13) Yu, J.; Liu, W.; Yu, H. Cryst. Growth Des. 2008, 8, 930–934. (14) Wang, X.; Hu, P.; Fangli, Y.; Yu, L. J. Phys. Chem. C 2007, 111, 6706–6712.
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treatments are sometimes required to remove the template. Spheres of materials such as silica,2,13 polystyrene,6,15-22 and colloidal carbon14 have been successfully used as a guide for arrangement of the targeted material on their surfaces. In addition to processes based on the sole use of templates, solvothermal and hydrothermal processes have also been used to carry out the in situ synthesis of the spherical templates through the thermal decomposition of a third component in the reaction medium into gas bubbles or carbon cores. Tetrabutylammonium hydroxide23 and hydrogen peroxide24 are examples of components leading to gas products, and carbohydrates7,11 to carbon cores. Besides the processes based on templates, spray precipitation has been included in the set of available methods to obtain hollow structures of metal oxides.25 Hollow spheres of silica,15,20 indium oxide,2 tin oxide,1-3 cerium oxide,4 titanium oxide,23,25,26 zirconium oxide,25,26 aluminum oxide,25 copper oxide,27,28 iron oxide,26 and cadmium hydroxides and oxides29 have been prepared following one of the techniques described earlier. (15) Wu, X.; Tian, Y.; Cui, Y.; Wei, L.; Wang, Q.; Chen, Y. J. Phys. Chem. C 2007, 111, 9704–9708. (16) Li, L.; Ding, J.; Xue, J. Chem. Mater. 2009, 21, 3629–3637. (17) Imhof, A. Langmuir 2001, 17, 3579–3585. (18) Cheng, X.; Chen, M.; Wu, L.; Gu, G. Langmuir 2006, 22, 3858–3863. (19) Yang, J.; Lind, J. U.; Trogler, W. C. Chem. Mater. 2008, 20, 2875–2877. (20) Deng, Z.; Chen, M.; Zhou, S.; You, B.; Wu, L. Langmuir 2006, 22, 6403– 6407. (21) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400–409. (22) Caruso, F.; Caruso, R. A.; Mohwald, H. Chem. Mater. 1999, 11, 3309– 3314. (23) Kim, Y. J.; Chai, S. Y.; Lee, W. I. Langmuir 2007, 23, 9567–9571. (24) Cheng, S.; Yan, D.; Chen, J. T.; Zhuo, R. F.; Feng, J. J.; Li, H. J.; Feng, H. T.; Yan, P. X. J. Phys. Chem. C 2009, 113, 13630–13635. (25) Chou, T.-C.; Ling, T.-R.; Yang, M.-C.; Liu, C.-C. Mater. Sci. Eng., A 2003, 359, 24–30. (26) Song, X.; Ding, X.; Li, P.; Ai, Z.; Zhang, L. J. Phys. Chem. C 2009, 113, 5455–5459. (27) Jun, L.; Dongfeng, X. Adv. Mater. 2008, 20, 2622–2627. (28) Zhu, H.; Wang, J.; Xu, G. Cryst. Growth Des. 2009, 9, 633–638. (29) Wang, W.-S.; Zhen, L.; Xu, C.-Y.; Shao, W.-Z. J. Phys. Chem. C 2008, 112, 14360–14366.
Published on Web 07/22/2010
DOI: 10.1021/la102404j
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Figure 1. Structure of cryptomelane-type manganese oxide K-OMS-2.
The unique physical and chemical properties of cryptomelanetype manganese oxide K-OMS-2 have given this material a wide range of potential applications in heterogeneous catalysis,30-37 adsorption of metal cations in aqueous systems,38 and control of gas emissions.39 K-OMS-2 displays a tunnel structure built up from the arrangement of four double-wide slabs of edge-shared octahedral [MnO6] units. The double-wide slabs, which are rotated 90° each other, are joined through corner oxygens producing the 1D tunnels parallel to the [001] direction. Because the double-wide slabs resemble the walls of the tunnels, this structure is called a 2 2 tunnel. A net negative electric charge is distributed all around the framework of K-OMS-2 due to the coexistence of manganese with both 3þ and 4þ oxidation states, which produces an average oxidation state lower than four. Potassium cations, which are incorporated inside the tunnels, balance this negative charge. Figure 1 shows the structures of cryptomelane K-OMS-2 based on the array of the octahedral units. In general, the synthesis of this type of manganese oxide is carried out through processes involving oxidation of Mn2þ cation,33,40 reduction of MnO4- species,30,41,42 and comproportionation of a mixture of Mn2þ and MnO4- ions.39,43-46 Hydrothermal,45 reflux,30,32,39 and microwave heating41,44 have been the thermal treatments usually performed to drive the reactions of (30) Villegas, J. C.; Garces, L. J.; Gomez, S.; Durand, J. P.; Suib, S. L. Chem. Mater. 2005, 17, 1910–1918. (31) Ding, Y.-s.; Shen, X.-f.; Sithambaram, S.; Gomez, S.; Kumar, R.; Crisostomo, V. M. B.; Suib, S. L.; Aindow, M. Chem. Mater. 2005, 17, 5382–5389. (32) Frias, D.; Nousir, S.; Barrio, I.; Montes, M.; Lopez, T.; Centeno, M. A.; Odriozola, J. A. Mater. Charact. 2007, 58, 776–781. (33) Fan, C.; Lu, A.; Li, Y.; Wang, C. J. Colloid Interface Sci. 2008, 327, 393– 402. (34) Suib, S. L. J. Mater. Chem. 2008, 18, 1623–1631. (35) Sriskandakumar, T.; Opembe, N.; Chen, C.-H.; Morey, A.; King’ondu, C.; Suib, S. L. J. Phys. Chem. A 2009, 113, 1523–1530. (36) Luo, J.; Zhang, Q.; Garcia-Martinez, J.; Suib, S. L. J. Am. Chem. Soc. 2008, 130, 3198–3207. (37) Ghosh, R.; Shen, X.; Villegas, J. C.; Ding, Y.; Malinger, K.; Suib, S. L. J. Phys. Chem. B 2006, 110, 7592–7599. (38) Dyer, A.; Pillinger, M.; Newton, J.; Harjula, R.; Moller, T.; Amin, S. Chem. Mater. 2000, 12, 3798–3804. (39) Li, L.; King, D. L. Ind. Eng. Chem. Res. 2005, 44, 7388–7397. (40) Yuan, J.; Li, W.-N.; Gomez, S.; Suib, S. L. J. Am. Chem. Soc. 2005, 127, 14184–14185. (41) Zhang, Q.; Luo, J.; Vileno, E.; Suib, S. L. Chem. Mater. 1997, 9, 2090–2095. (42) Ching, S.; Roark, J. L.; Duan, N.; Suib, S. L. Chem. Mater. 1997, 9, 750– 754. (43) Portehault, D.; Cassaignon, S.; Baudrin, E.; Jolivet, J. P. Chem. Mater. 2007, 19, 5410–5417. (44) Yang, L.-X.; Zhu, Y.-J.; Wang, W.-W.; Tong, H.; Ruan, M.-L. J. Phys. Chem. B 2006, 110, 6609–6614. (45) Gao, T.; Glerup, M.; Krumeich, F.; Nesper, R.; Fjellvag, H.; Norby, P. J. Phys. Chem. C 2008, 112, 13134–13140. (46) Cheney, M. A.; Birkner, N. R.; Ma, L.; Hartmann, T.; Bhowmik, P. K.; Hodge, V. F.; Steinberg, S. M. Colloids Surf., A 2006, 289, 185–192.
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synthesis from solutions of the molecular precursors to the oxide material. In this paper, a template-free one-step synthesis of uniform hollow microstructures of cryptomelane-type manganese oxide K-OMS-2 is reported. This facile one-step synthesis gives the process a major advantage regarding its scale up to commercial production. Concisely described, the novel process involves the production of K-OMS-2 through the oxidation of Mn2þ carried out in an aqueous medium. A mixture of two oxidizing agents was used to bring about the synthesis under hydrothermal treatment at temperatures 120, 160, and 200 °C. The hollow microstructures were composed of an arrangement of nanofibers displaying two levels of self-organization: the first level involved the array of nanofibers around the surface confining the empty domains of the microstructures, and the second one was characterized by an assembly of fibers normal to the surface built by the previous array. Some of these hollow structures also exhibited holes through whose diameters can be tuned according to the conditions of the synthesis.
Experimental Details Materials and Reagents. All of the syntheses were done with chemicals used as received. Potassium permanganate (KMnO4), certified ACS reagent, was from Fisher. Manganese(II) acetate tetrahydrate (MnAc2 3 4H2O) 99% was from ACROS ORGANICS. Oxone monopersulfate compound (triple salt 2KHSO5 3 KHSO4 3 K2SO4) and potassium nitrate (KNO3), ACS reagent, were bought from Aldrich. Synthesis of the Hollow Microstructures. The synthesis of the hollow microstructures of cryptomelane-type manganese oxide K-OMS-2 involved the oxidation process of Mn2þ based on the use of a mixture of two oxidizing agents. Potassium nitrate and oxone was the oxidizing system for the majority of the synthesis reported in this work, and the mixture potassium permanganate and potassium nitrate was also tried in one of the experiments. Manganese(II) acetate tetrahydrate was the inorganic precursor used to build the oxide network. In a typical synthesis, two aqueous solutions were prepared and mixed under intense stirring: the first solution involved the dissolution of 0.2 g of manganese(II) acetate tetrahydrate and 0.1653 g of potassium nitrate in 5 mL of double deionized water; the second solution was prepared by dissolving 1.5 g of oxone in 10 mL of double deionized water. After the mixing of the two solutions, the liquid reaction medium was placed in a Teflon lined stainless steel bomb and then heated up to 120 °C. This temperature was kept for 20 h. Hydrothermal alteration was the thermal treatment applied to drive both kinetically and thermodynamically the chemical reaction of synthesis toward the product. At the end of the reaction time, the reaction vessel was cooled down and opened, and the solids were separated from the mother liquid by centrifugation. Three cycles composed of washing with double deionized water and centrifugation were applied to the solids, which were finally dried at room temperature and characterized to establish their chemical identity, morphology, structure, chemical composition, and thermal stability. Table 1 reports the experimental conditions for the syntheses carried out in this work. These experiments did not constitute a formal experimental design to quantify the effect of variables such as temperature or manganese precursor/potassium nitrate ratio, but instead they were organized to establish a morphology-determining phenomenon for the synthesis of the hollow structures first and then a range of conditions for the mentioned variables in which hollow structures could be obtained. Finally, a synthesis without the use of potassium nitrate was also performed to set a baseline of the effect of the nitrate salt. Investigation of the Mechanism. The mechanism of formation of the complex hollow structures was also investigated performing further experiments based on the same conditions than the ones used for the synthesis of K-OMS-2 (1) but varying Langmuir 2010, 26(16), 13677–13683
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Article Table 1. Experimental Conditions for the Syntheses of the Hollow Microstructures of K-OMS-2
synthesis label
MnAc2 3 4H2O (g)
KNO3 (g)
oxidant
oxidant mass (g)
temperature (°C)
time (h)
K-OMS-2 (1) K-OMS-2 (2) K-OMS-2 (3) K-OMS-2 (4) K-OMS-2 (5) K-OMS-2 (6)
0.2 0.2 0.2 0.2 0.2 0.2
0.182 0.182 0.14 0.1653 0.1653 0.1653
oxone oxone oxone oxone KMnO4 oxone
1.5 1.5 1.5 1.5 0.13 1.5
120 160 120 120 120 200
20 20 20 24 24 20
the reaction times to 0.5, 1, 2, and 10 h. Morphological and structural data were acquired for the synthesized powders in order to establish the physical and chemical phenomena taking place during the steps of the synthesis. Characterization. Morphology and Structure. Field emission scanning electron microscopy (FESEM) studies were done to the prepared materials to acquire morphological data. SEM micrographs were collected on a LEO/Zeiss DSM 982 Gemini FESEM instrument with a Schottky emitter at an accelerating voltage of 2 kV and a beam current around 1 μA. FESEM data were also acquired on a FEI Strata 400S DualBeam scanning electron microscope equipped with a focused ion beam (FIB). The samples used to perform FESEM were prepared by dispersing the solid materials in ethanol and dropping small amounts of these dispersions on gold-coated silicon wafers. Phase identification and characterization of the structure of the synthesized materials were carried out by X-ray powder diffraction (XRD) and High resolution transmission electron microscopy (HRTEM). XRD patterns were collected on a Scintag model XDS 2000 diffractometer using Cu KR radiation (λ = 1.54 A˚). The beam voltage and the beam current were -45 kV and 40 mA, respectively. The analyses were performed in a 2θ range between 5 and 70° using a continuous scan rate of 0.75°/min. The identification of the phases was accomplished by comparison with standard patterns contained in a JCPDS database. HRTEM performed to collect further data about the crystalline structure of the solids was carried out on a JEOL 2010 FasTEM instrument with a 200 kV thermionic electron source. Four high-resolution CCD cameras allowed images to be acquired digitally. The samples for HRTEM analyses were prepared following a procedure similar to the one used in the FESEM characterization, but dropping the dispersions on carbon film coated copper grids instead of the silicon wafers. The d-spacings were obtained from selected area electron diffraction (SAED) patterns. Chemical Composition. Chemical composition data were collected for materials K-OMS-2 (1) to K-OMS-2 (4) using atomic absorption spectroscopy (AAS). The sample was prepared by carrying out an acid digestion at low temperature. A mixture of 1 mL of concentrated nitric acid and 3 mL of concentrated hydrochloric acid diluted to 50 mL with double deionized water was used for the digestion. When the solution turned clear, the digested sample was cooled to room temperature and diluted to a final volume of 50 mL. The concentrations of Kþ and Mn2þ were calculated from a calibration curve established for each one of the evaluated ions at concentration 5, 10, 15, and 20 ppm. A PerkinElmer 3100 atomic absorption spectrometer (air-acetylene flame) was used to perform these analyses. Thermal Stability. Thermal stability of the prepared materials was investigated by thermogravimetric analysis (TGA) performed on a TA Instruments TGA 2950 apparatus. A heating rate of 10 °C/min, a continuous flow of nitrogen, and a temperature range between 60 and 800 °C were the settings used to run the thermal analysis. Thermal stability was also analyzed by a combined temperature programmed desorption-mass spectrometry (TPD-MS) technique. Around 10 mg of the sample was packed in a quartz tube that was loaded into a tubular oven. A continuous flow of nitrogen of 40 sccm (standard cubic centimeter per minute) was used as an environment to perform the thermal treatment in the TPD analysis. Before beginning the heating, the tubular reactor and the sampling lines were purged with nitrogen Langmuir 2010, 26(16), 13677–13683
Figure 2. SEM micrographs of materials synthesized under the experimental conditions reported in Table 1. (a) K-OMS-2 (1), (b) K-OMS-2 (2), (c) K-OMS-2 (3), (d) K-OMS-2 (4), (e) K-OMS-2 (5), and (f) K-OMS-2 (6). The white bar for the insets is 2 μm. under room temperature for 1 h and then the tubular reactor was heated up to 800 °C keeping a heating rate of 10 °C/min. An Agilent Technologies 5975 C inert XL MSD with Triple-Axis Detector mass spectrometer was used to carry out the analysis.
Results Synthesis. Morphology and Structure. SEM micrographs portrayed in Figure 2a-d for materials K-OMS-2 (1) to K-OMS2 (4), respectively, as well as the corresponding insets on the same images show no major morphological differences between these synthesized manganese oxides. The morphology for the four solids is mainly characterized by uniform hollow microstructures produced by the complex arrangement of long nanoscale fibers. A more detailed view of the hollow structures of K-OMS-2 (1) depicted in Figure 3 shows two levels of self-organization of the nanofibers: The first level followed a pattern characterized by the growing of the nanofibers around the surface confining the cavity of the hollow structure. The second self-arrangement is characterized by the growing of the fibers normal to the surface created by the previous array. The fibers, which also display a pineneedle-like morphology, are long enough to get a close mechanical interaction with nearby microstructures. Micrographs of the materials K-OMS-2 (5) and K-OMS-2 (6) portrayed in Figure 2e and f, respectively, depict the usual needle-like DOI: 10.1021/la102404j
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Figure 3. Detailed SEM micrograph of the self-assembly of K-OMS-2 (1) nanofibers producing the complex hollow microstructures. The pine-needle-like morphology of the nanofibers can be observed in the enclosed area.
Figure 5. HRTEM images for K-OMS-2 (1). (a) TEM micrographs of the nanoscale fibers. Micrographs (b)-(d) show lattice fringes of the planes (110), (310), and (200). Micrographs (b) and (c) show line defects along the [001] direction. (e) SAED pattern of K-OMS-2 (1). (f) FFT for the nanocrystalline fibers.
Figure 4. X-ray powder diffraction patterns of (a) K-OMS-2 (1), (b) K-OMS-2 (2), (c) K-OMS-2 (3), (d) K-OMS-2 (4), (e) K-OMS2 (5), (f) K-OMS-2 (6), and (g) JCPDS card file 29-1020. (/) Sample holder.
structure of cryptomelane for these materials. No hollow arrays of the needles are observed for these two experiments, which were performed under conditions introducing higher oxidation rates when compared with the other four syntheses. The intrinsic low activation energy mechanism followed by the decay of potassium permanganate accounts for the higher rates in the synthesis of K-OMS-2 (5) and the high temperature (200 °C) for the experiment producing K-OMS-2 (6). The experiment performed without the use of potassium nitrate produced dispersed particles of cryptomelane, displaying the usual needle-like morphology. No characterization of the synthesis process is reported here. The XRD patterns of the synthesized solids, which are displayed in Figure 4, show a set of reflections matching the corresponding ones for the standard pattern of the pure tetragonal cryptomelane phase (JCPDS card file 29-1020). No other phases were observed for the six experiments reported in this work. Peak broadenings showed a remarkable difference for the crystal sizes between the K-OMS-2 synthesized at high temperature (200 °C) and the one based on potassium permanganate, with the first one having the largest crystal size and the last one the lowest. Materials synthesized with the mixture of oxone/potassium nitrate at temperatures 120 and 160 °C exhibited peak broadening in between the ones for K-OMS-1 (5) and K-OMS-2 (6). HRTEM images of materials K-OMS-2 (1) and K-OMS-2 (4) depicted in Figures 5 and 6, respectively, show the nanoscale size for the fibers self-assembled into the complex hollow structures. 13680 DOI: 10.1021/la102404j
Figure 6. HRTEM images for K-OMS-2 (4). (a) TEM micrographs of the nanoscale fibers. Micrographs (b)-(d) show lattice fringes of the planes (110) and (200). Micrograph (b) displays a set of line defects produced by the lateral attachment of aligned nanocrystalline fibers. (e) SAED pattern. (f) FFT confirms the nanocrystalline state of the nanofibers.
Both of these figures contain micrographs of particles showing clear and well-defined lattice fringes, which extend along the [001] direction of the crystallographic axes of cryptomelane. The lattice fringes shown in Figure 5a-c have d-spacings 0.69, 047, and 0.3 nm that correspond to the ones of the (110), (200), and (310) planes of the cryptomelane structure, respectively. Figure 5b and c also shows a collection of dislocations in the crystal structures composed by the planes (110) and (200) and along the [001] direction. Fast Fourier transform (FFT) analysis of the section enclosed in Figure 5d and displayed in Figure 5f shows a good crystal development for the nanocrystalline fibers composed mainly of planes (110). The SAED pattern for material K-MS-2 (1) depicted in Figure 5e shows around seven major reflections that are assigned to the planes (200), (310), (211), (301), (411), (521), and (541) of cryptomelane according to the calculated Langmuir 2010, 26(16), 13677–13683
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Article Table 2. Weight Loss in wt % of the Synthesized Materials temperature ranges (°C) material
60-300
300-500
500-650
650-730
K-OMS-2 (1) K-OMS-2 (2) K-OMS-2 (3) K-OMS-2 (4)
2.2 2.6 2.4 4.4
1.9 1.8 2.1 1.9
4.8 4.8 4.8 4.3
1.3 1.3 1.2 0.5
Table 3. Chemical Composition of the Synthesized Solids synthesis
Mn4þ (wt %)
Kþ (wt %)
molar ratio of Mn4þ/Kþ
KxMnO2
K-OMS-2 (1) K-OMS-2 (2) K-OMS-2 (3) K-MOS-2 (4)
47.5 42.4 43.5 49.1
3.8 3.2 3.9 3.7
8.9 9.3 7.9 9.5
x = 0.112 x = 0.108 x = 0.126 x = 0.106
Figure 7. (A) TGA profiles for (a) K-OMS-2 (1), (b) K-OMS-2 (2), (c) K-OMS-2 (3), and (d) K-OMS-S (4). (B) DTGA profiles for (a) K-OMS-2 (1), (b) K-OMS-2 (2), (c) K-OMS-2 (3), and (d) K-OMS-S (4).
d-spacings (not reported in this paper), which match the ones for the corresponding reflections in the XRD pattern of material K-OMS-2 (1). Figure 6b and c shows lattice fringes corresponding to the (110) and (200) planes of cryptomelane according to the calculated d-spacings 0.69 and 0.47 nm, respectively. FFT analysis of the enclosed area in Figure 6d and reported in Figure 6f shows a well ordered crystalline structure for the nanocrystalline fibers of material K-OMS-2 (4). The SAED pattern for material K-OMS-2 (4), which is depicted in Figure 6e, displays reflections indexed to the crystal planes (200), (310), (211), (301), (411), (521), and (541) of cryptomelane based on the calculated spacings, which are consistent with the ones for the same reflections in the XRD pattern of standard cryptomelane (JCPDS card 29-1020). Figure 6c also contains a cross section of one of the nanocrystals in which an array of tunnels for K-OMS-2 (4) can be observed. Just like the case of K-OMS-2 (1), cryptomelane K-OMS-2 (4) also displays a set of dislocations along the c crystallographic axis and around the boundary of well structured nanocrystals as is indicated by the white arrow in Figure 6b. The presence of these defects in the structures of both K-OMS-2 (1) and (4) can be attributed to the mechanism of particle growth based on the lateral attachment of well developed nanocrystalline fibers (as seen in the FFT analyses) that is brought about by the selforganization of close particles having a common crystallographic orientation.43,47,48 Chemical Composition. Chemical compositions of the synthesized powders reported in Table 3 are consistent with the molar ratio Kþ/Mn4þ calculated from the molecular formula of cryptomelane having half-occupancy of the tunnel sides KMn8O16. This table also shows differences for this molar ratio lower than 16% for the produced materials, indicating a good reproducibility of the synthesis under the investigated ranges of temperature and oxidant amounts. The chemical compositions for the synthesized K-OMS-2 (1) to (4), the XRD patterns, as well as the SEM micrographs for the same materials show that the synthesis of the hollow microstructures using this process based on a mixture of oxidants can be successfully performed in the ranges 120-160 °C and 8.2-10.7 for temperature and mass ratio of oxone/potassium nitrate, respectively. (47) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969–971.
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Figure 8. Oxygen TPD for manganese oxides (a) K-OMS-2 (1), (b) K-OMS-2 (2), (c) K-OMS-2 (3), and (d) K-OMS-S (4).
Thermal Stability. TGA profiles depicted in Figure 7A show similar patterns for the solids K-OMS-2 (1) to K-OMS-2 (4). The major feature of these profiles is the presence of four weight losses in the temperature ranges 60-300, 300-500, 500-650, and 650-730 °C. These ranges were determined with the thermal profiles provided by the derivative of the weight in terms of temperature (derivative thermogravimetric analysis, DTGA). DTGA profiles are provided in Figure 7B. As seen in Table 2, which summarizes the features of the thermal profiles, all of the materials had similar weight losses in the temperature ranges mentioned earlier, with the exception of material K-OMS-2 (4) that shows a higher weight loss for the temperature range 60300 °C. This difference accounted for the displacement toward lower wt % of the TGA profile of this cryptomelane when compared with the other ones. Desorption of physisorbed and chemisorbed water in the temperature ranges 60-300 and 300-500 °C, respectively, were the source of the first two weight losses.31,49 In addition to water, some release of chemisorbed oxygen also contributed to the loss of weight for the second temperature range according to the weak peak displayed in the O2 TPD profiles reported in Figure 8. Oxygen TPD profiles also allowed the weight changes at temperatures 500-650 and 650-730 °C to be correlated to the evolution of oxygen from the lattice structure of cryptomelane. The structural release of oxygen has been reported elsewhere30,31,36,37 in studies (48) Penn, R. L. J. Phys. Chem. B 2004, 108, 12707–12712. (49) Chen, X.; Shen, Y.-F.; Suib, S. L.; O’Young, C. L. Chem. Mater. 2002, 14, 940–948.
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Galindo et al.
After 1 h of reaction (Figure 10b), the nanofibers exhibit higher diameters and lengths, and some of the microstructures already show holes indicating an early development of the cavities. Insets in Figure 10b give a more detailed morphological view of the incipient hollow spaces. The upper and the lower-right insets show the second phase confined inside and around the growing cavity. These second phases also provide surfaces for the growth of the nanofibers. The lower-left inset shows a more developed hollow structure in which the cavity still has some remaining material of the second phase. The SEM micrograph in Figure 10c corresponding to the materials obtained after 2 h of reaction shows nanofibers having morphologies very close to the ones for full reaction times. Some small amount of the second phase inside the cavities is observed in this micrograph. The micrograph of materials synthesized for 10 h, which is depicted in Figure 10d, shows cavities completely developed without any presence of the second phase around the hollow space. This figure also shows a morphology of the fibers almost identical to the one seen in longterm experiments (Figure 3). Figure 9. X-ray diffraction patterns of K-OMS-2 (1) synthesized at different reaction times to follow the morphological and structural changes leading to the self-organization of the hollow structures: (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 10 h, and (e) cryptomelane reflections from JCPDS card 29-1020. (/) Sample holder.
Figure 10. SEM micrographs of K-OMS-2 (1) materials synthesized for (a) 0.5 h, (b) 1 h, (c) 2 h, and (d) 10 h.
involving synthesis of cryptomelane and its catalytic activity. The temperature at which the release of framework oxygen begins has been generally accepted as an indication of the thermal degradation of cryptomelane; however, this temperature is atmosphere sensitive, being lower for nitrogen or helium rich environments and higher for oxygen rich atmospheres.49 Mechanism. X-ray diffraction patterns and SEM micrographs of K-OMS-2 (1) powders synthesized at different reaction times are reported in Figures 9 and 10, respectively. Cryptomelane phase was produced since very early stages of the synthesis according to the XRD pattern of the material obtained for the shortest reaction time (0.5 h), which is reported in Figure 9a. Longer times for the synthesis brought about the growth of the crystals as is observed through the gradually decrease of the peak broadening displayed in the patterns of the materials synthesized for 1, 2, and 10 h (Figure 9b, c, and d respectively). The morphology after the synthesis performed for 0.5 h (Figure 10a) shows shorter and thinner nanofibers when compared with longer time experiments. These fibers also are elongated normal to the surface of particles composed mainly of amorphous material (called hereafter a second phase) according to the XRD pattern. 13682 DOI: 10.1021/la102404j
Discussion Although all of the experiments done in this work produced pure cryptomelane phases displaying a chemical composition corresponding to the one for cryptomelane having a half occupancy of the tunnel sites, there was a major morphological difference between the materials obtained under conditions producing high initial oxidation rates and the ones synthesized with modulated kinetics of oxidation. The strong relationship between the oxidation rate and the morphological features of the array of nanofibers becomes apparent from the observation of the physical changes displayed by the reaction media immediately after the mixing of the aqueous solutions containing the precursor and the oxidants. When oxone and potassium nitrate were used, a slow continuous change in the physical appearance of the liquid phase took place, going from one clear and colorless to one that was pink and transparent, which turned brownish and capable of scattering a laser beam. For the system based on potassium permanganate and the nitrate salt, the change to a dark medium characterized by big chunks of black particles was almost instantaneous after the mixing. Regarding the experiment involving high temperature (200 °C), there was an additional effect due to the increased solubility of the amorphous material, which led to the dissolution of the cores before the growing fibers could assemble into the hollow structures. The determining role of the kinetics of oxidation on the morphological features of the synthesized powders is even more evident when a comparison between the standard electrode potentials (E°) of potassium peroxymonosulfate (the active ingredient of oxone) and potassium permanganate (1.85 and 1.70 V, respectively) shows a higher E° value for oxone. The syntheses carried out for short times show the early development of cryptomelane under conditions of temperature and type of thermal treatment (hydrothermal) that have been reported elsewhere to require long reaction times to obtain cryptomelane when other types of oxidizing agents are used.50,51 This rapid evolution toward cryptomelane is attributed to the formation of the amorphous precursor at the initial steps of the synthesis whose surface provides a set of sites to start the heterogeneous nucleation of the new cryptomelane phase. This precursor is also a source of material to support the growth of the (50) Liu, J.; Makwana, V.; Cai, J.; Suib, S. L.; Aindow, M. J. Phys. Chem. B 2003, 107, 9185-9194. (51) Polverejan, M.; Villegas, J. C.; Suib, S. L. J. Am. Chem. Soc. 2004, 126, 7774–7775.
Langmuir 2010, 26(16), 13677–13683
Galindo et al.
Figure 11. Proposed mechanism of formation of the hollow structures of K-OMS-2. (a) Mixing of the molecular precursor with the oxidants, (b) early formation of particles of amorphous material, (c) heterogeneous nucleation and growth of cryptomelane nanofibers on the surface of the amorphous particles, (d) advanced stage of the synthesis showing well-defined and long fibers of cryptomelane organized on the surface of the amorphous core, and (e) micrograph showing a detailed view of the processes of nucleation and growth on the surface of the amorphous phase; the enclosed area shows a crystal displaying a growth normal and parallel to the surface.
nanocrystalline fibers by the dissolution-crystallization mechanism, which is improved at high temperatures and low pH such as the ones used in this work.43 The thermodynamic instability of the small amorphous particles is also a factor driving the dissolution of this precursor. The formation of lamellar phases of manganese oxides and disordered arrays of octahedral units [MnO6] at early advances of the synthesis and their evolution toward the cryptomelane phase has been previously documented.30,31,43,50 However, the rearrangement of the octahedral layers toward the tunnel structure of cryptomelane, which is claimed to happen in some of these papers,30,50 is not believed to take place in this process based on mixtures of oxone and potassium nitrate. Instead, a high energy surface is assumed to be created, allowing the fast development of the heterogeneous nucleation and growing as was pointed out earlier. Suggested Mechanism. The interaction between oxone and potassium nitrate brought about a fine-tuning of the oxidation rate of the manganese species which drove the synthesis through different stages, with one of them being the formation of particles of amorphous material whose surfaces supported the growth of nanofibers of cryptomelane by processes already described. The surface of the particles also restricted the growth of the nanoscale
Langmuir 2010, 26(16), 13677–13683
Article
fibers to hemispherical domains defined by the space above the surface and the surface itself. After the heterogeneous nucleation, the growth of the nanofibers was conducted by the mechanisms of lateral attachment of primary nanocrystalline fibers and dissolution-crystallization. The latter is responsible for the depletion of material of the cores on which the growth took place. The lateral attachment mechanism is believed to account for the pine-needle-like morphology observed for some of the nanofibers due to the small angle remaining between the oriented particles when the attachment occurred. HRTEM images show clearly the nonperfect alignment of the primary nanocrystalline fibers. Figure 11 gives a graphic description of the proposed mechanism for the formation of the hollow structures. The evolution of the reaction from the mixing of the precursor and oxidants to the growing of nanofibers on the surface of the amorphous cores is depicted in Figure 11a-c. A clear view of an advanced state of the synthesis showing an amorphous particle and the nanofibers growing on the surface of this particle is given in Figure 11d. A porous morphology for the core indicating the depletion of material due to its dissolution can be observed in this picture. Finally, Figure 11e shows a cross section of the surface supporting the nucleation and growth of the nanofibers of cryptomelane; a crystal growing parallel and normal to the surface is shown in the enclosed area in this figure.
Conclusions In summary, uniform hollow microstructures of pure cryptomelane-type manganese oxide K-OMS-2 having diameters of around 2 μm were synthesized using a template-free one-step process. These structures are the outcome of the self-assembly of nanocrystalline fibers following a complex pattern composed by two oriented arrangements of the nanoscale fibers, with one being around the surface confining the hollow space and the other being normal to this surface. The fine control of the oxidation rate of manganese species obtained by the use of oxone and potassium nitrate was the key factor for the successful synthesis of the hollow assemblies because the synthesis was allowed to go through intermediate states involving the formation of an amorphous core and the nucleation and growth of cryptomelane on the surface of these cores. The chemical composition of the synthesized structures showed molar ratios of Kþ/Mn4þ between 0.106 and 0.126, which are close to the one for cryptomelane with a half occupancy of the tunnel sides K0.125MnO2. Finally, temperatures between 120 and 160 °C and mass ratios of oxone/potassium nitrate between 8.2 and 10.7 were well established for the synthesis of the hollow microstructures. Acknowledgment. We acknowledge support by the Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.
DOI: 10.1021/la102404j
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