Synthesis of MnO2 Nanostructures with Sea Urchin Shapes by a Sodium Dodecyl Sulfate-Assisted Hydrothermal Process Xu Chun
Song,*,†
Yang
Zhao,‡
and Yi Fan
Zheng#
Department of Chemistry, Fujian Normal UniVersity, Fuzhou 350007, P. R. China, Department of Chemistry, Henan Normal UniVersity, Xinxiang 453007, P. R. China, College of Chemical Engineering & Materials Science, Zhejiang UniVersity of Technology, Hangzhou, Zhejiang 310014, P. R. China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 159-162
ReceiVed August 10, 2006; ReVised Manuscript ReceiVed October 13, 2006
ABSTRACT: MnO2 nanostructures with sea urchin shapes have been prepared by an sodium dodecyl sulfate (SDS)-assisted hydrothermal treatment method. The products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). XRD patterns indicated that the sea urchin shaped MnO2 nanostructures were tetragonal phase. Furthermore, SEM and TEM revealed that MnO2 nanostructures with sea urchin shapes consisted of MnO2 nanorods 150-200 nm wide and several micrometers long. A growth mechanism of MnO2 nanostructures with sea urchin shapes was suggested and explained in detail. 1. Introduction In the past few years, controlling the shape of nanostructures at the microscopic level has been one of the challenging issues presently faced by material scientists. One-dimensional (1D) nanoscale building blocks, such as nanotubes, nanobelts, nanowires, and nanorods, have attracted intensive interest due to their importance in fundamental research and potential wideranging applications.1-6 Many efforts have been focused on the integration of 1D nanostructure building blocks into two- and three-dimensional (2D and 3D) ordered superstructures or complex functional architectures, which is essential for the success of bottom-up approaches toward future nanodevices and would offer opportunities to explore their novel collective optical, magnetic, and electronic properties.7-11 Generally, routes toward these ordered structures include two accesses. One is the use of various template precursors, such as droplets, silica, and block copolymers.12-14 Another is the control of some factors to organize the components into multidimensional morphologies.15,16 Recently, multiarmed and radially aligned semiconductor nanorods were fabricated by solution-growth methods.17 Manganese dioxide is one of the most attractive inorganic materials because of its physical and chemical properties and wide applications in catalysis, ion exchange, molecular adsorption, biosensors, and particularly, energy storage.18-22 MnO2 exists in many polymorphic forms (such as R, β, γ, and δ), which are different because the basic unit [MnO6] octahedra are linked in different ways.23,24 Recently, R-, β-, γ-MnO2 was prepared in different morphologies, such as rods, wires, tubes, etc., using hydrothermal techniques.25-28 However, few reports have been focused on the formation of MnO2 hierarchical architecture.29,30 The development of facile, mild, and effective methods for creating novel architectures based on nanowires remains a key scientific challenge. Herein, we report on the facile synthesis of sea urchin shapes of MnO2 nanorods directed by a sodium dodecyl sulfate (SDS)-assisted hydrothermal * Corresponding author. Tel: +86-591-87441126; fax: +86-59183465376; e-mail:
[email protected]. † Fujian Normal University. ‡ Henan Normal University. # Zhejiang University of Technology.
Figure 1. XRD pattern of the MnO2 nanostructures with sea urchin shapes.
process, which provides a novel method for direct hydrothermal growth of hierarchical nanostructure based on inorganic nanorods. 2. Experimental Procedures 2.1. Sample Preparation. All the chemicals were analytic grade reagents used without further purification. MnO2 nanostructures with sea urchin shapes were synthesized under hydrothermal conditions. Experimental details were as follows: MnSO4 (1.5 g) was dissolved in 10 mL of distilled water and SDS (2 g) was added slowly to it with vigorous stirring. When the solution was clarified, 10 mL of aqueous solution containing 2.5 g of KClO3 was added to the above solution under continuous stirring. The resulting transparent solution was then transferred into a Teflon-lined stainless steel autoclave (50 mL) of 80% capacity of the total volume. The autoclave was sealed and maintained at 150 °C for 12 h. After the reaction was completed, the autoclave was allowed to cool to room temperature naturally. The solid black precipitate was filtered, washed several times with distilled water and absolute ethanol to remove impurities, and then dried at 60 °C in air. The obtained black powders were collected for the following characterization. 2.2. Characterization. The morphologies were characterized using scanning electron microscopy (SEM, Hitachi S-4700 II, 25 kV) and transmission electron microscopy (TEM, JEM200CX, 120 kV). The composition of the product was analyzed by an energy dispersive X-ray detector (EDX, Thermo Noran VANTAG-ESI, 120 kV). X-ray diffraction (XRD, Thermo ARL SCINTAG X’TRA with Cu KR irradiation, λ ) 0.154056 nm.) was used to analyze the crystallinity.
3. Results and Discussion The phase and purity of the products were examined by XRD. The XRD pattern of the MnO2 nanostructure with sea urchin
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Figure 2. (a) SEM and (b) TEM images of the MnO2 nanostructures with sea urchin shapes.
Figure 3. Low-magnification SEM image (a) and high-magnification SEM image (b) of typical MnO2 nanostructures with sea urchin shapes.
shapes prepared by the SDS-assisted hydrothermal method is shown in Figure 1. All the diffraction peaks can be indexed as the tetragonal phase [space group: I4/m (No. 87)] of R-MnO2 with lattice constants of a ) 9.7845 and c ) 2.8630 Å, which are in agreement with the reported values (JCPDS 44-0141). No other phase was detected in Figure 1 indicating the high purity of the final products. Figure 2a shows the low-magnification SEM image of the MnO2 nanostructures with sea urchin shapes. It is found that the MnO2 nanostructures with sea urchin shapes are composed of many MnO2 nanorods. The MnO2 nanorods radiate from the center of the crystals forming a spherical sea urchin shaped structure with a diameter of about 2-10 µm. For the firm MnO2 nanostructures that could not be destroyed after a long period of ultrasonic treatment, Figure 2b shows the TEM image of the final product after long-time ultrasonic treatment. It reveals the product with sea urchin shapes, which is the same as the SEM results. The selected area electron diffraction (SAED) pattern reveals the single-crystalline nature of the R-MnO2 nanorods. The diffraction pots can be indexed as the body-centered tetragonal phase. These results are in good agreement with the results of XRD. Figure 3a,b shows the low- and highmagnification SEM images of the typical MnO2 nanostructure with sea urchin shapes. The typical MnO2 nanostructure with sea urchin shapes consisted of MnO2 nanorods that are 150200 nm in width and several micrometers in length. Energy dispersive spectrometry (EDS) analysis was employed to determine the composition of the MnO2 nanostructures. As shown in Figure 4, only oxygen and manganese elements existed in the nanostructures with a molar ratio of about 2 (O/Mn). They should therefore be attributed to MnO2. The influence of the reaction time on the growth of the nanowires and nanorods was investigated. The corresponding samples were tested by SEM. Figure 5 shows SEM images of the as-obtained samples measured (a) after 1 h, (b) after 4 h, (c) after 8 h, (d) after 12 h, and other conditions kept constant at the same time. As can be seen, the reaction lasted for 1 h; the products were composed of aggregated particles (see Figure 5a). When the reaction time was prolonged to 4 h, products
Figure 4. EDS patterns of the MnO2 nanostructures with sea urchin shapes.
were imperfect sea urchin shaped MnO2 crystals (see Figure 5b). This process continued, and more MnO2 nanostructures with sea urchin shapes formed after 8 h (see Figure 5c). When the reaction time was extended to 12 h, most of the products were MnO2 nanostructure with sea urchin shapes. Generally, temperature is believed to have a great impact on the crystal forms of final products. We have carried out analogous experiments at different temperatures for comparison. The results revealed that there was no MnO2 precipitation formed at 110 °C; when the temperature was at 130, 150, and 170 °C, the products were all MnO2 nanostructures with sea urchin shapes. It was found that temperature did not have much effect on the crystal shapes of products during this range. Therefore, this method was very effective for the large-scale synthesis of MnO2 with sea urchin shapes. But the products were mainly MnO2 nanorods at 190 °C. It was possible that at this temperature the SDS capsules were destroyed under hydrothermal conditions. To substantially understand the effect of SDS on MnO2 nanostructures with sea urchin shapes, the experiments of the hydrothermal process with different concentrations of SDS were carried out. Figure 6a is the SEM image of the sample obtained without SDS. The result showed that the products synthesized were only MnO2 nanorods with no sea urchin shaped MnO2 crystals observed. With the addition of 1 g of SDS, the
MnO2 Nanostructures with Sea Urchin Shapes
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Figure 5. The SEM images of products obtained for various reaction times: (a) 1 h, (b) 4 h, (c) 8 h.
Figure 6. SEM images of MnO2 synthesized at 150 °C for 12 h with different amounts of SDS: (a) 0 g, (b) 1 g (low magnification), (c) 1 g (high magnification), (d) 1 g (TEM), (e) 1.5 g, and (f) 1.5 g (loose structure).
morphologies of synthesized MnO2 shown in Figure 6b is approximately spheres assembled with MnO2 nanocrystals. Further research with high-magnification SEM images showed that the approximate sphere shapes were the disorder aggregation of MnO2 nanorods (see in Figure 6c), and after ultrasonic treatment, disperse MnO2 nanorods were formed, as shown in Figure 6d. As the content of SDS reaches 1.5 g, Figure 6e shows the SEM image of the solid sample, where the products are a mixture of nanorods and nanostructures with sea urchin shapes. Figure 6f shows that the MnO2 nanostructures with sea urchin shapes has a loose center, and it is predicted that such a loose structure could be decentralized into nanorods by ultrasonic treatment. The formation of this structure probably relates to the low concentration of SDS. With the addition of 2 g of SDS, MnO2 nanostructures with sea urchin shapes are the major product. The growth mechanism of the individual MnO2 nanorods can be explained with the different growing rates of various MnO2 crystal facets. Synthesis of sea urchin shaped MnO2 crystals is
Figure 7. Schematic illustration of assembled process of the MnO2 nanostructures with sea urchin shapes.
greatly affected by adjusting the SDS concentration in the solution. Therefore, the process of assembling MnO2 nanorods into sea urchin shaped forms can be illustrated as shown in Figure 7. The higher concentration SDS solution will generate microspherical capsules (step 1). In aqueous solution, the hydrophilic group of the SDS, namely, the sulfonic group, points
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than the cmc, the formed spherical micelle in solution are favored for the production of sea urchin shaped MnO2 from nanorod-like MnO2. The method may be used in the assembly of other nanomaterials with structure and properties similar to MnO2. References
Figure 8. SEM image of the imperfect sea urchin shaped MnO2 crystals.
to the outer surface of the capsules, while the hydrophobic end points to the inner. Because of the electrostatic interaction of the sulfonic group and the Mn2+, the outer surface of the capsules is occupied by a lot of Mn2+ cations (step 2). Then the absorbed Mn2+ cations are converted into MnO2 nuclei during the hydrothermal process (step 3). Figure 8 shows imperfect sea urchin shaped MnO2 crystals. From the SEM image, it is found that incomplete rodlike MnO2 particles assemble in the center of the crystals. This result supports the above growing steps well. When the hydrothermal treatment time is prolonged, longer MnO2 nanorods will be generated resulting in the formation of entire sea urchin shaped MnO2 crystals (step 4). According to the above analysis of the growing mechanism for the sea urchin shaped MnO2 crystals, the experimental result of our work could be explained as follows. First, when the concentration of SDS is lower than the critical micelle concentration (cmc), the sphere micelle could not be formed, and we only get the rodlike MnO2 crystals. Second, when the concentration of SDS reaches the cmc, the same situation occurs as when 1.5 g of SDS was added into the solution in our experiment: mixtures of nanorods and sea urchin shapes MnO2 crystals were produced. When the concentration is not high enough, imperfect sphere micelle were formed and resulted in the production of sea urchin shaped MnO2 with loose centers. Third, only when the concentration of SDS in the solution reaches a suitable value that is higher than the cmc, can perfect sea urchin shaped MnO2 be produced. 4. Conclusion In summary, sea urchin shaped MnO2 nanostructures, which consisted of MnO2 nanorods, have been prepared by an SDSassisted hydrothermal process at a low temperature (150 °C). The sea urchin shaped MnO2 nanostructures are tetragonal phase. When the concentration of SDS in the solution is higher
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