J Nanopart Res (2009) 11:1107–1115 DOI 10.1007/s11051-008-9517-6
RESEARCH PAPER
Controlled synthesis and characterization of layered manganese oxide nanostructures with different morphologies Naicai Xu Æ Zong-Huai Liu Æ Xiangrong Ma Æ Shanfeng Qiao Æ Jiaqi Yuan
Received: 14 August 2008 / Accepted: 11 September 2008 / Published online: 28 September 2008 Ó Springer Science+Business Media B.V. 2008
Abstract Layered manganese oxide nanostructures with different morphologies, such as nanowire bundles, cotton agglomerates, and platelikes were successfully fabricated by a simple and template-free hydrothermal method based on a reaction of KMnO4 and KOH solutions with different concentrations. The obtained nanowire bundles were assembled by nanowires with diameters of 10 to 200 nm and lengths up to 5–10 lm. The cotton agglomerates were composed of manganese oxide layers with a thickness of about 10 nm. Both the concentration of KOH solutions and the reaction temperature played an important role in the formation of layered manganese oxide nanostructures with different morphologies. XRD, SEM, TEM, HRTEM, SAED, TG-DTA, and chemical analysis were employed to characterize these materials. On the basis of the experimental results, a possible formation mechanism of layered manganese oxide
Electronic supplementary material The online version of this article (doi:10.1007/s11051-008-9517-6) contains supplementary material, which is available to authorized users. N. Xu Z.-H. Liu X. Ma S. Qiao J. Yuan Key Laboratory of Aplied Surface and Colloid Chemistry, Shaanxi Normal University, Ministry of Education, Xi’an 710062, People’s Republic of China N. Xu Z.-H. Liu (&) X. Ma S. Qiao J. Yuan School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an, Shaanxi 710062, People’s Republic of China e-mail:
[email protected]
nanostructures with different morphologies was presented. Keywords Layered manganese oxide Nanocomposites Morphology Nanowire bundles Hydrothermal synthesis
Introduction In recent years, 1-D nanostructures with different morphologies have been demonstrated to exhibit excellent optical, electrical, mechanical, and thermal properties, and can be used in many fields, such as microelectronic and optoelectronic devices (Shi et al. 2001; Xia et al. 2003; Pan et al. 2001; Peng et al. 2000). Just for this reason, more and more studies have been focused on synthesizing various nanostructures with novel morphologies. Manganese oxide, as an important functional metal oxide, has been widely studied because of its distinctive properties and wide applications in catalysis, ion exchange, molecular adsorption, energy storage in secondary batteries, and soft magnetic materials (Feng et al. 1999). Recently, lots of efforts have also been devoted to the preparation of low-dimensional manganese oxide nanostructures with various polymorphs; because many research results show that both the electronic and optical properties of the manganese oxide nanostructures have highly relied on their morphologies, dimensionality, size, and
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crystallographic forms (Wang et al. 2004; Bach et al. 1990). Up to now, a variety of the prepared methods such as reflux, coprecipitation, sol–gel, solid-state chemical reaction, template-assisted, as well as hydrothermal treatment have been developed to fabricate 1-D manganese oxides nanorods (Ferreira et al. 2006), nanowires (Zhang et al. 2007), nanobelts (Ma et al. 2004a, b), nanofibers (Shen et al. 2005), nanoneedles (Yang et al. 2006), and nanotubes (Ma et al. 2004a, b). Meanwhile, 0-D and 2-D manganese oxide nanostructures with different morphologies have also been synthesized by other research groups. Birnessite and birnessite-related manganese oxides have a 2-D layer structure, which consist of edgeshared MnO6 octahedra with hydrated cations in the interlayer space to compensate for the layer charge ˚ (Feng deficit and a basal spacing of about 7.10 A et al. 1999). Since manganese oxides with a layered structure show easy ion-exchange properties and can be used as precursors for the synthesis of manganese oxides with a tunnel structure, a large number of studies have been carried out by many researchers. For example, birnessite-related 1-D hierarchical nanostructures of ultralong layered KxMnO2 (x \ 0.3) bundles with diameters of 50 to 100 nm and lengths up to 50–100 lm have been prepared by a PEG-assisted hydrothermal method (Ge et al. 2006). By a hydrothermal treatment of KMnO4 with nitric acid, KxMnO2 yH2O with a layered structure and small spheres morphology has also been fabricated by Chen et al. (1996). Although the layered manganese oxides with different morphologies have been successfully synthesized by using many methods, the synthesis process in the solution system consists of many chemical reactions. Meanwhile, controlled optimization of the reaction parameters such as temperature, concentration, and pH is needed in order to obtain high purity materials. In addition, many reactions introduce various templates and substrates, which will cause heterogeneous impurities and increase the synthetic cost. Thus, how to develop facile, easily controlled, and template-free synthesis technology to synthesize layered manganese oxide nanostructures with novel morphologies and crystallographic forms is of great importance. Herein, we report a facile and template-free route to manipulate the synthesis of layered manganese oxide nanostructures with various morphologies by a direct and mild hydrothermal
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method based on the reaction of KMnO4 and KOH solutions with different concentrations. The structure, morphology, and particle size of the as-prepared manganese oxide nanostructures were characterized by using X-ray powder diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM), and a possible formation mechanism of layered manganese oxide nanostructures with various morphologies was proposed.
Experimental section Materials All chemicals were of analytical grade and were used without further purification. KMnO4 and KOH were purchased from Xi’an Chemical Reagent Company and Tianjin Chemical Reagent Company, respectively. Deionized water was used throughout the experiment. Synthesis of layered manganese oxide nanostructures with different morphologies A typical synthesis procedure for the layered manganese oxide nanostructure with nanowire bundles morphology was conducted as follows: 3.16 g (0.02 mol) KMnO4 was dispersed in an aqueous solution of KOH (1 M, 38 mL) to obtain an amaranth solution after stirring for 10 min. The amaranth solution was then transferred into a Teflon (PTFE)lined autoclave of 50 mL capacity, sealed and maintained at 240 °C for 48 h. In order to synthesize layered manganese oxide nanostructure with cotton agglomerate morphology, the concentration of KOH solution was increased to 15 M with other parameters unchanged. In company with the increase in the KOH concentration to 20 M, the obtained layered manganese oxide nanostructure had platelike morphology. All obtained brownish-black products were filtrated off, washed with deionized water several times, and finally dried at 50 °C for 12 h in air. In order to investigate the effect of the reaction conditions on the morphology and particle size of the obtained products, a series of parallel contrastive experiments were carried out by altering the reaction parameters such as hydrothermal treatment temperature and reaction time.
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Chemical analysis The average oxidation number (Z*Mn) of manganese was evaluated from the value of available oxygen, which was determined by a standard oxalic acid method (JIS 1969). The alkali metal and manganese contents were determined by atomic absorption spectrometry after the samples were dissolved in a mixed solution of HCl and H2O2. Characterization The powder X-ray diffraction pattern was recorded on a D/Max-3c X-ray diffractometer with Cu-Ka ˚ ), using an operation voltage radiation (k = 1.5406 A and current of 40 kV and 40 mA, respectively. SEM images were taken using Quanta 200 environmental scanning electron microscopy operated at an accelerating voltage of 20 kV. TEM, the corresponding selected area electron diffraction (SAED) pattern and high-resolution TEM (HRTEM) were taken on a JEM-3010 transmission electron microscope (CCD, Gatan894). Specimens for TEM observation were prepared by dispersing the obtained manganese oxide nanostructures with different morphologies into alcohol by ultrasonic treatment. Thermal analysis was determined on DSC-TGA analysis (Q1000DSC? LNCS?FACSQ600SDT) with a heating rate of 10 °C/min in flowing nitrogen gas.
Results and discussion Structure and morphology Layered manganese oxide nanostructures with various morphologies could be obtained by hydrothermal treating of KMnO4 in KOH solutions with different concentrations at 240 °C for 2 days. XRD patterns of the as-prepared materials in KOH solutions with different concentrations are similar to each other, showing a typical layered structure with a basal ˚ spacing of about 7.10 A (Fig. 1). The structure corresponds to one molecular layer of water between the manganese oxide sheets, and K? ions may be randomly exchanged with the monolayer water molecules (Feng et al. 1999). When a purple solution of KMnO4 was hydrothermally treated in a solution of 1 M KOH at 240 °C for 2 days, manganese oxide
Fig. 1 XRD patterns of layered manganese oxide nanostructures obtained in KOH solutions with different concentrations at 240 °C for 48 h. a 1 M. b 5 M. c 10 M. d 15 M. e 20 M. f 25 M
brownish-black powders were obtained, which had a ˚ layered structure with a basal spacing of 7.06 A (Fig. 1a). All diffraction peaks could be readily indexed to a K?-birnessite phase of manganese oxides with a monoclinic structure, and the lattice ˚, constants were calculated to be a = 5.15 A ˚ ˚ b = 2.844 A, and c = 7.159 A, respectively. In company with the increase in the KOH concentrations, the obtained materials still maintained a layered structure, only their basal spacing and the crystallinity little changed (Fig. 1b–f). These results indicate that a K?-type birnessite manganese oxide ˚ can be obtained with a basal spacing of about 7.10 A in an alkali solution at 240 °C for 2 days, but the concentration of the alkali solution had small influence on the formation of K?-type birnessite manganese oxides. Although the concentration of the alkali solution had small influence on the formation of K?-type birnessite manganese oxides, the morphologies of the
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Fig. 2 SEM images of layered manganese oxide nanaostructures obtained in KOH solutions with different concentrations at 240 °C for 48 h. a 1 M. b 5 M. c 10 M. d 15 M. e 20 M. f 25 M
obtained materials showed an obvious difference (Fig. 2). K?-type birnessite manganese oxides with nanowire bundle morphology was obtained in 1 M KOH solution at 240 °C for 2 days (Fig. 2a). In company with the increase in KOH concentrations, the morphologies of the obtained materials change to cotton agglomerates (Fig. 2d), and finally to platelikes (Fig. 2f) from nanowire bundle morphology. These results indicate that the morphology of the obtained materials could be controlled by adjusting the concentration of KOH solution. The obtained materials with nanowire bundles and cotton agglomerates morphologies were further characterized by TEM and HRTEM. Figure 3a shows the overview of manganese oxide nanowire bundles, it can be seen obviously that the bundles are self-assembled by lots of nanowires with diameters of 10 to 200 nm and lengths up to 5–10 lm. An individual nanowire bundle is clearly presented in Fig. 3b, and the corresponding HRTEM is shown in Fig. 3c. The axis of the wire is found to be parallel to the (001) direction, indicating that the nanowire bundle grows along the ˚ from the (001) direction. The width of 7.10 A
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neighboring fringes corresponds to the d001-spacing observed in the XRD patterns of layered manganese oxide nanostructure. On the other hand, the TEM image of the layered manganese oxide nanostructure with a cotton agglomerate morphology notices clearly that the cotton agglomerates are composed of manganese oxide layers with a thickness of about 10 nm, which may be due to the electrostatic attraction between the manganese oxides layers and the K? ions in the solution (Fig. 3d). The SAED patterns corresponding to the appointed cotton agglomerate show ˚ hexagonal arrangements with d100 = 2.42 A (Fig. 3e), indicating that the obtained material has a single crystalline nature. Figure 3f is a high-resolution TEM image corresponding to the SAED pattern, and a clearly resolved lattice image of manganese oxide layers is shown viewed along the (001) direction. There is a big chemical potential of manganese oxide layers in concentrated KOH solutions (Zhao et al. 2007), which may drive the manganese oxide layers to self-assemble to form the layered manganese oxide nanostructure with cotton agglomerate morphology.
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Fig. 3 a TEM of overview manganese oxide nanowire bundles. b A typical manganese oxide nanowire bundle. c HRTEM image of corresponding signal nanowire. d TEM image. e SAED pattern. f HRTEM image of manganese oxide cotton agglomerate
On the basis of the element analyses, the compositions of the materials obtained in KOH solutions with different concentrations indicate clearly that the contents of K ions increase, while the contents of Mn gradually decrease in company with the increase in the KOH concentrations (Table 1). The average oxidation number of manganese slightly increases with the increase in KOH concentrations. The K?-birnessite with nanowire bundle morphology had a chemical formula of K0.30MnO1.91 0.33H2O and an average manganese oxidation number of 3.59, while the K?birnessite with cotton agglomerate morphology had a chemical formula of K0.34MnO2.04 0.43H2O and an
average manganese oxidation number of 3.73. Although the crystal phase of the materials obtained in different KOH concentrations is K?-birnessite layered manganese oxides; their morphologies are various when the reaction parameters are different. The average manganese oxidation number of the synthesized samples is very similar to that of the K?type birnessite manganese oxides reported by references (Feng et al. 1999), which is between 3.60 and 3.80. The subtle change in the average manganese oxidation number may be ascribed to the disproportionation reaction of MnIII to MnII and MnIV under alkali conditions.
Table 1 Chemical analysis of obtained layered manganese oxide nanostructures with various morphologies in KOH solutions with different concentrations at 240 °C for 48 h Samples
K content (mmol/g)
Mn content (mmol/g)
Total O content (mmol/g)
ZMna
H2O (mmol/g)
Formula
1M
3.03
10.34
19.53
3.59
3.42
K0.30MnO1.91 0.33H2O
5M
3.06
10.31
20.10
3.60
3.40
K0.30 MnO1.95 0.33H2O
10 M
3.03
10.37
19.80
3.63
4.35
K0.29MnO1.91 0.42H2O
15 M
3.28
9.51
19.40
3.73
4.06
K0.34MnO2.04 0.43H2O
20 M
3.65
9.19
19.45
3.73
3.82
K0.39MnO2.12 0.42H2O
25 M
3.60
9.53
19.62
3.74
3.81
K0.38MnO2.06 0.40H2O
a
The average oxidation state of manganese
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Effects of time and temperature In order to investigate the effects of reaction time and temperature on the structure and morphology of the obtained materials, a purple solution of KMnO4 was hydrothermally treated in a solution of 15 M KOH at different reaction times and temperatures. The XRD patterns of the materials obtained in 15 M KOH at 240 °C for different hydrothermal times show that the obtained materials maintain a layered structure characteristic, only the crystallinity displays a little change (Support information 1, Figure S1). The morphology evolution of the materials obtained in 15 M KOH solution at 240 °C for different reaction times is given in Fig. 4. It can be seen clearly that when a purple solution of KMnO4 was hydrothermally treated for 0.5 h, the obtained material showed stacked nanoparticle morphology (Fig. 4a). On extending the reaction time to 1 h, partial cotton agglomerate morphology was observed (Fig. 4b). The obtained material nearly shows the cotton agglomerate morphology for the reaction time of 2 h. Further prolonging the reaction time to 5, 12, and
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24 h, respectively, only the particle size of the obtained materials increases slightly, but the cotton agglomerate morphology does not change any more (Fig. 4c–f). These results indicate that the structure of the layered manganese oxide nanostructure mainly depends on the hydrothermal treatment temperature, while their morphology and particle size connect with the hydrothermal treatment time to some extent. In general, the hydrothermal treatment temperature plays a key role on the structure and morphology of the obtained materials. The XRD patterns of the obtained materials at different treatment temperatures show that the layered manganese oxide nanostructure can be formed at 180 °C, when the reaction is performed in 15 M KOH solution for 2 days. By increasing the hydrothermal temperature, the obtained materials still showed the layered structure character, only the basal spacing decreased slightly (Support information 2, Figure S2). The morphology evolution of the materials synthesized at different hydrothermal temperatures shows the material obtained at 180 °C displays aggregated nanoparticle morphology (Fig. 5a). The particle sizes of layered
Fig. 4 SEM image evolution of the materials obtained in 15 M KOH solution at 240 °C for different hydrothermal treatment times. a 0.5 h. b 1 h. c 2 h. d 5 h. e 12 h. f 24 h
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Fig. 5 SEM images of the materials obtained in 15 M KOH solution for 48 h at different hydrothermal temperatures. a 180 °C. b 200 °C. c 220 °C. d 240 °C
manganese oxide nanostructure gradually increase in company with the increase in the hydrothermal treatment temperature, and the layered manganese oxide nanostructure with cotton agglomerate morphology was finally formed. (Fig. 5b–d). These results indicate that the layered manganese oxide nanostructure is already formed at 180 °C, while their morphology and size depend on the hydrothermal treatment temperature.
Formation mechanism It is of importance to study the formation mechanism of the layered manganese oxide nanostructures with different morphologies, because through this, we can realize the controllable synthesis of the manganese oxide nanostructures with various morphologies. Up to present, many formation mechanisms have been reported for the formation of the manganese oxide nanostructures with different structures and morphologies. For example, Li and co-workers (Wang and Li 2003) have reported a rolling and phase transformation
mechanism of 1-D MnO2 nanostructure under hydrothermal conditions. Song et al. (2007) have presented a nucleation-dissolution-anisotropic growth-recrystallization mechanism. Furthermore, some other formation mechanisms such as compression and collapse mechanism (Shen et al. 2006), oriented attachment mechanism (Penn and Banfield 1998), and solutionliquid-solid mechanism (Trentler et al. 1997) have also been proposed by other researchers. The present experimental results indicate that the structure and crystallization of the layered manganese oxide nanostructures obtained at 180–240 °C are hardly affected by the concentration of KOH solutions, but their morphology obviously depends on the concentration of KOH solutions. The layered manganese oxide nanostructure with nanowire bundle morphology is formed in 1 M KOH solution. In company with the increase in KOH concentrations, the morphology of the obtained materials gradually changes to irregular flakes in 10 M KOH solution, to cotton agglomerates in 15 M KOH solution, and finally to an obvious platelike morphology of the size of 10 nm in 25 M KOH solution. It is well known
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Fig. 6 Schematic illustrain of the formation of layered manganese oxide nanostructures with nanowire bundles and cotton agglomerate morphologies obtained at 240 °C for 48 h
that in order to have a lower energy, single-crystal particles always have especial and regular morphologies, because a single-crystal particle has to be enclosed by crystallographic facets (Wu et al. 2005; Wang and Feng 2003). In general, the anisotropy of crystal structure or the crystal surface reactivity is regarded as the primary driving force for the growth of the anisotropic structure. Peng and Peng (2001, 2002) reported that the chemical potential had some influence on the morphology evolution of the obtained materials, and it is favorable for the formation of crystalline particles to have a high chemical potential in the solution. A possible formation mechanism for the layered manganese oxide nanostructures with different morphologies is presented in Fig. 6. We think that KOH acts as a surfactant in the hydrothermal process, and a different chemical potential surrounding for the formation of layered manganese oxide nanostructures with different morphologies is probably obtained in KOH solutions with different concentrations. In order to start with, an amaranth solution of KMnO4 is obtained by dissolving it in KOH solution with appropriate concentrations. Then, the KMnO4 solution is hydrothermally treated at different reaction times and temperatures, causing the permanganate to decompose and produce a great deal of amorphous manganese oxide nanoparticles. The amorphous manganese oxide nanoparticles nucleate and grow bigger during the hydrothermal process, and at the same time, an Ostwald ripening process is carried out under hydrothermal environment, that is the smaller
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nanoparticles redissolve and the bigger ones grow into platelike particles with a lamellar structure. Finally, the platelike particles with a lamellar structure exhibit the tendency to curl, break into wirelike particles, and then assemble to form layered manganese oxide nanostructure with wire bundle morphology in KOH solutions with lower concentrations. This intriguing conversion process is very similar to the rolling mechanism (Wang and Li 2003) proposed by Li and his co-workers. However, in concentrated KOH solutions, enough high K? concentrations probably restrain the curling of the platelike particles with a lamellar structure, and cause a high chemical potential for the growth of platelike particles in size. These influence factors finally make the reaction system to form the layered manganese oxides nanostructure with cotton agglomerates instead of wire bundle morphology. Therefore, an appropriate alkali concentration is needed for the formation of the layered manganese oxide nanostructures with various morphologies.
Conclusions In summary, layered manganese oxide nanostructures with various morphologies were fabricated by a facile and template-free hydrothermal procedure by decomposing KMnO4 in KOH solutions with different concentrations. The structure and crystallinity of the obtained layered manganese oxide nanostructures had a little connection with the KOH concentrations,
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while their morphology obviously depended on the reaction alkali concentrations. A formation mechanism for the layered manganese oxide nanostructures with various morphologies was proposed on the basis of the experimental results. Acknowledgments We thank the National High Technology Research and Development Program of China (2007AA03Z248) for financially supporting this research.
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