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Journal of Nanomaterials
Volume 2012 (2012), Article ID 607870, 7 pages
http://dx.doi.org/10.1155/2012/607870
Research Article

Controlled Synthesis of Manganese Dioxide Nanostructures via a Facile Hydrothermal Route

Department of Chemistry, Faculty of Resource Science and Technology, Universiti Malaysia Sarawak, Sarawak, 94300 Kota Samarahan, Malaysia

Received 4 October 2011; Accepted 5 December 2011

Academic Editor: Arava Leela Mohana Reddy

Copyright © 2012 Suh Cem Pang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Manganese dioxide nanostructures with controllable morphological structures and crystalline phases were synthesized via a facile hydrothermal route at low temperatures without using any templates or surfactants. Both the aging duration and aging temperatures were the main synthesis parameters used to influence and control the rate of morphological and structural evolution of MnO2 nanostructures. MnO2 nanostructures comprise of spherical nanoparticulate agglomerates and highly amorphous in nature were formed at lower temperature and/or short aging duration. In contrast, MnO2 nanostructures of sea-urchin-like and nanorods-like morphologies and nanocrystalline in nature were prepared at the combined higher aging temperatures and longer aging durations. These nanostructures underwent notable phase transformation from δ-MnO2 to α-MnO2 upon prolonged hydrothermal aging duration and exhibited accelerated rate of phase transformation at higher aging temperature.

1. Introduction

One-dimensional manganese dioxide (MnO2) nanostructures such as nanorods, nanowires, and nanofibers have generated intense research interests over the past recent years due to their superior optical, electrical, catalytic, magnetic and electrochemical properties [13]. Such manganese dioxide nanostructures are of considerable importance in technological applications and have been intensively investigated as promising electrode materials in primary/secondary batteries and electrochemical capacitors due to their excellent electrochemical properties, low-cost, environmentally benign, and ease of preparation [47]. Various approaches have been used to fabricate manganese dioxide, such as self-reacting microemulsion [8], precipitation [9], room-temperature solid reaction [10], sonochemical [11], and hydrothermal methods [12]. The hydrothermal method is a powerful synthesis approach for synthesizing various forms of manganese oxides and affords various advantageous features including the use of mild synthesis conditions such as pH and temperature, and a wide range of precursors that can be used.

Various types of inorganic nanowires and nanorods have been synthesized with the aid of templates or catalysts. Templates are being used to confine the growth of crystals, while catalysts may act as energetically favorable sites for the adsorption of reactant molecules [13]. However, the introduction of templates or catalysts to a reaction system is often accompanied by drawbacks such as the need to prepare or select appropriate templates or catalysts. Besides, impurities in the final product may be difficult to be removed, thereby making the overall synthesis process more complicated and costly. As such, any synthetic method without the need to use any catalyst or template is more favorable for the preparation of low-dimensional nanostructures. Recently, a hydrothermal or solvothermal method has been employed to prepare one-dimensional nanoscaled materials, for example, α-MnO2, without the use of templates or catalysts [14]. This method is superior to traditional methods since no specific and expensive equipment is required for synthesizing nanostructured materials at low temperatures. The hydrothermal preparation of manganese dioxides involved mainly redox reactions of M n O 4 and/or Mn2+ or the phase transformation of granular manganese dioxide precursors [15]. A common approach for the synthesis of single-crystalline α-MnO2 nanorods was based on the hydrothermal reaction of MnSO4 and KMnO4 [16]. DeGuzman et al. prepared fibrous α-MnO2 through redox reactions between KMnO4 and MnSO4 [17]. However, some minor differences in the morphology of final products have been observed as specific reaction conditions were being altered slightly. Parameters such as temperature, time and capping molecules can influence the growth of nanocrystals under nonequilibrium kinetic growth conditions in the solution-based approach [18]. Henceforth, the controlled synthesis of manganese dioxide nanostructures with favorable surface morphology, phase structure, crystallinity, and high reproducibility remains a considerable challenge.

This paper reports the controlled synthesis of various MnO2 nanostructures via a facile and mild hydrothermal route without using any physical template and addition of any surfactant. Both δ-MnO2 and α-MnO2 nanostructures were synthesized based on the hydrothermal reaction of MnSO4 and KMnO4 in aqueous medium and at mild temperatures. Effects of hydrothermal synthesis conditions on the evolution of structural morphology and phase transformation of MnO2 nanostructures were investigated.

2. Materials and Methods

2.1. Synthesis of MnO2 Nanostructures

The synthesis of MnO2 nanoparticles was based on the method reported by Xiao et al. with some modifications [19]. MnO2 nanoparticles were synthesized by mixing aqueous solutions of KMnO4 and MnSO4 at ambient temperature and pressure, and the pH of solution mixture was adjusted to ~1 with concentrated HNO3. The aging temperatures were fixed at 25°C and 80°C, whereas the aging duration varied between 1 hour and 24 hours. The reaction product was collected by filtration, washed, and then air-dried at room temperature.

2.2. Characterization of MnO2 Nanostructures

The surface and structural morphologies of MnO2 samples were studied by scanning electron microscope (SEM) (JEOL Model JSM 6390LA) and transmission electron microscope (TEM) (JEOL Model JEM-1230). For SEM imaging, all MnO2 samples were pre-coated with a thin platinum layer using an Ion Sputtering Device (JFC-1100 E) in order to reduce the inherent charging effect. As for TEM imaging, MnO2 samples were first being dispersed well in ultrapure water by ultrasonication. 1 μL of the resulting dispersions were then drop-coated onto Formvar-covered copper grids and air-dried. The specific surface area and pore size distribution of MnO2 samples were determined using the nitrogen adsorption-desorption (BET) analyzer (Micrometrics ASAP 2010) at 77 K. The phases of MnO2 samples synthesized at different aging durations and temperatures were studied using a X-ray diffractometer (XRD) (Scintag) with Cu Kα radiation source.

3. Results and Discussion

A facile and mild hydrothermal route was being used to synthesize MnO2 nanostructures without the use of any template or surfactant. Figures 1 and 2 show SEM micrographs of MnO2 samples after being aged for various durations at temperatures of 25°C and 80°C, respectively. Both the aging duration and aging temperature were observed to have substantial effect on the shape and morphology of MnO2 nanostructures formed. At an aging temperature of 25°C, spherical agglomerates of MnO2 nanoparticles were obtained at aging durations of between 0 hour and about 4 hours (Figure 1). In absence of surfactant, MnO2 nanoparticles showed high tendency to aggregate and formed spherical agglomerates of various sizes [20]. However, upon prolonged aging for 8 hours, nanorod-like structures began to develop on surfaces of individual MnO2 nanoparticles. Upon aging for more than 24 hours, well-defined nanorods had developed around these spherical MnO2 nanoparticles to form sea-urchin-like MnO2 nanostructures.

fig1
Figure 1: SEM micrographs of MnO2 samples synthesized at various aging durations at 25°C.
fig2
Figure 2: SEM micrographs of MnO2 samples synthesized at various aging durations at 80°C.

As shown in Figure 2, MnO2 samples synthesized initially at 80°C comprised of mainly spherical agglomerates. However, such spherical agglomerates were no longer discernible but large aggregates of nanorod-like structures were observed upon being aged for 4 hours at 80°C. Well-defined and fully developed MnO2 nanorods were formed after being aged for extended durations of 8 and 24 hours at 80°C, respectively. The aging temperature was found to play a crucial role in accelerating the rate of evolution of MnO2 nanostructures from spherical agglomerates to aggregates of well-defined nanorods. On the contrary, no MnO2 nanostructure was formed at the aging temperature of below 20°C even after being aged for a week. At the aging temperature of 25°C, the rate of structural evolution was observed to be rather slow, with MnO2 nanostructures of sea-urchin-like shape formed only after being aged for 24 hours (Figure 1). However, at the elevated aging temperature of 80°C, distinctive and well-defined MnO2 nanorod-like nanostructures were clearly discernable after being aged for 4 or more hours (Figure 2). A higher aging temperature appeared to favor the growth of one-dimensional (1D) MnO2 nanostructures which could be attributed to the accelerated rate of decomposition of MnSO4 to form MnO2 at elevated temperatures. These nanorod-like nanostructures continued to grow in length due to their anisotropic nature and eventually led to the formation of nanowires [21]. Henceforth, hydrothermal synthesis conditions could be controlled and optimized was for the synthesis of MnO2 nanostructures of desired morphology and crystalline phase.

Figure 3 shows TEM micrographs of MnO2 nanoparticles synthesized at various aging durations at aging temperatures of 25°C. The evolution of nanorod-like nanostructures was observed to have initiated from the surfaces of MnO2 nanoparticles after being aged for 4 hours at 25°C. More distinctive and defined nanorod-like nanostructures had evolved from spherical MnO2 nanoparticles after being aged for 8 hours. Long and well-defined nanorods were observed to have developed on MnO2 nanoparticles forming sea-urchin-like nanostructures after being aged for 24 hours at 25°C.

fig3
Figure 3: TEM micrographs of MnO2 samples synthesized at various aging durations at 25°C.

As shown in Figure 4, TEM micrographs depicted the rapid evolution of well-defined nanorods from individual spherical MnO2 nanoparticles at elevated aging temperature of 80°C. Agglomerates of MnO2 nanoparticles were observed to have transformed rapidly into sea-urchin-like nanostructures after being aged for 4 hours at 80°C. All MnO2 nanoparticles were observed to have transformed completely into well-developed nanorods after being aged for 8 hours. No notable morphological changes of nanorods were observed after prolonged aging duration for up to 24 hours at 80°C. The diameter and length of well-defined MnO2 nanorods ranged from 20 to 30 nm and 300 to 400 nm, respectively.

fig4
Figure 4: TEM micrographs of MnO2 samples synthesized at various aging durations at 80°C.

The hydrothermal synthesis route used in the present study had been shown to be a facile and mild synthesis approach for the synthesis of manganese dioxide nanostructures of desired morphology through judicious control of both the aging temperature and aging duration. In this synthesis approach, neither catalyst was needed to provide energetically favorable sites for the absorption of reactant molecules nor template was needed to direct the growth of nanorods. The driving force for the growth of MnO2 nanorods during the synthesis process could be derived from the inherent crystal structure of MnO2 material and its chemical potential in solution [22]. Based on our experimental observations, the formation mechanisms of MnO2 nanostructures could entail the following processes. MnO2 nanoparticles were initially produced by the redox reaction between MnSO4 and KMnO4.These MnO2 nanoparticles would subsequently aggregate to form spherical agglomerates due to their high surface energies. During prolonged aging duration, MnO2 nanoparticles would gradually transform into nanorods under the specific aging conditions. The gradual transformation of MnO2 nanoparticles into nanorods could be attributed to their one-dimensional growth and anisotropic nature. Such processes obey the well-known ‘‘Ostwald Ripening” process, in which larger nanorods grow at the expense smaller ones because of differences in their surface energies. Similar formation mechanism of MnO2 nanorods had been reported byTang et al.with single-crystalline α-MnO2 nanorods being successfully synthesized via a facile hydrothermal approach without any template and surfactant [23].

Figure 5 shows the effect of aging duration on the specific surface area and total pore volume of MnO2 nanostructures synthesized at different aging temperatures of 25°C and 80°C. For MnO2 samples synthesized at aging temperature of 25°C, both specific surface area and total pore volume were observed to increase with increasing aging durations. The specific surface area and total pore volume of MnO2 samples increased from 91.1 m2/g and 0.225 cm3/g as prepared (or without aging) to 130.5 m2/g and 0.410 cm3/g, respectively, after being aged for 24 hours. Such increases could be attributed to microstructural changes associated with the gradual transformation of tightly packed nanoparticles into nanorod-like structures during aging. MnO2 nanostructures of evolving nanorods should possess higher porosity as indicated by the increasing specific surface area and total pore volume with longer aging durations. In contrast, MnO2 samples synthesized at aging durations between 0 and 8 hours at 80°C showed substantially higher values of specific surface area and total pore volume which were comparable or even higher than MnO2 samples synthesized after being aged for 24 hours at 25°C (Figure 5). However, there was a notable decrease in both specific surface area and total pore volume for samples synthesized after being aged for 24 hours (Figure 5(b)) which could be due to aggregation and realignment of fully developed nanorods. These results showed that a substantially higher rate of transformation from spherical nanoparticles to nanorods occurred at elevated aging temperature, with complete transformation being achieved within 1 hour of aging duration at 80°C as compared to more than 24 hours at 25°C.

fig5
Figure 5: Effect of aging duration on the specific surface area and the total pore volume of MnO2 samples synthesized at (a) 25°C and (b) 80°C.

Figure 6 shows X-ray diffractographs of MnO2 samples as prepared and after being aged for 24 hours at 25°C and 80°C. Both types of as-prepared MnO2 samples without any post synthesis aging showed similar broad diffraction peaks which can be indexed to δ-MnO2 phase albeit with low degree of crystallinity. The absence of other manganese dioxide diffraction peaks indicated the high purity of these as-prepared δ-MnO2 samples. δ-MnO2 samples synthesized at 25°C were observed to have only partially converted to α-MnO2 phase as indicated by the 211 peak but have remained mostly amorphous in nature after being aged for 24 hours (Figure 6(a)). In contrast, samples of highly crystalline α-MnO2 phase were obtained after being aged for 24 hours at a higher aging temperature of 80°C. The presence of well-defined and sharp characteristic diffraction peaks of α-MnO2 phase showed a complete phase transformation of δ-MnO2 into α-MnO2 phase upon aging at 80°C for 24 hours (Figure 6(b)). Such phase transformation could be associated with the simultaneous morphological transformation from spherical nanoparticles to nanorods during prolonged aging at 80°C. These results indicated that the rates of phase and morphological transformation were temperature dependent, with accelerated rate occurred at elevated aging temperature. Our findings concurred with observations reported by other researchers that a higher aging temperature would promote more rapid phase transformation of MnO2 samples [24, 25]. The phase transformation from δ-MnO2 to α-MnO2 could also be attributed to the stability of the polymorphs of MnO2. Since δ-MnO2 is less stable than α-MnO2 and β-MnO2, it will be transformed into α-MnO2 or β-MnO2 with increasing temperature [26]. However, due to the mild reaction conditions used in the present study, the β-MnO2 phase could not be formed. These above results indicated that both the morphological structure and crystalline phase of MnO2 samples could be modulated by varying the hydrothermal aging duration and temperatures.

fig6
Figure 6: XRD diffractographs of MnO2 samples synthesized at (a) 25°C and (b) 80°C with different aging duration.

4. Conclusion

In conclusion, MnO2 nanostructures of spherical nanoparticulate agglomerates could be synthesized and transformed into nanostructures of sea-urchin-like or agglomerates of nanorods by a facile and mild hydrothermal route without the use of any templates or catalyst. MnO2 nanostructures underwent accelerated rate of transformation from δ-MnO2 to α-MnO2 phase and associated morphological changes from spherical nanoparticles to nanorods at elevated aging temperature. Both the morphological structure and crystalline phase of the MnO2 nanostructures could therefore be modulated by varying the hydrothermal aging duration and temperatures.

Acknowledgment

This work was supported in part by the Universiti Malaysia Sarawak under the special fundamental Research Grant 01(K03)/557/2005(56).

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