Controlled Synthesis of Manganese Dioxide Nanostructures via a Facile Hydrothermal Route

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.


Introduction
One-dimensional manganese dioxide (MnO 2 ) 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 [1][2][3].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 [4][5][6][7].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-MnO 2 , 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 MnO 4 − and/or Mn 2+ or the phase transformation of granular manganese dioxide precursors [15].A common approach for the synthesis of single-crystalline α-MnO 2 nanorods was based on the hydrothermal reaction of MnSO 4 and KMnO 4 [16].DeGuzman et al. prepared fibrous α-MnO 2 through redox reactions between KMnO 4 and MnSO 4 [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 MnO 2 nanostructures via a facile and mild hydrothermal route without using any physical template and addition of any surfactant.Both δ-MnO 2 and α-MnO 2 nanostructures were synthesized based on the hydrothermal reaction of MnSO 4 and KMnO 4 in aqueous medium and at mild temperatures.Effects of hydrothermal synthesis conditions on the evolution of structural morphology and phase transformation of MnO 2 nanostructures were investigated.

Synthesis of MnO 2
Nanostructures.The synthesis of MnO 2 nanoparticles was based on the method reported by Xiao et al. with some modifications [19].MnO 2 nanoparticles were synthesized by mixing aqueous solutions of KMnO 4 and MnSO 4 at ambient temperature and pressure, and the pH of solution mixture was adjusted to ∼1 with concentrated HNO 3 .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.

Characterization of MnO 2
Nanostructures.The surface and structural morphologies of MnO 2 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 MnO 2 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, MnO 2 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 MnO 2 samples were determined using the nitrogen adsorption-desorption (BET) analyzer (Micrometrics ASAP 2010) at 77 K.The phases of MnO 2 samples synthesized at different aging durations and temperatures were studied using a X-ray diffractometer (XRD) (Scintag) with Cu K α radiation source.

Results and Discussion
A facile and mild hydrothermal route was being used to synthesize MnO 2 nanostructures without the use of any template or surfactant.Figures 1 and 2 show SEM micrographs of MnO 2 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 MnO 2 nanostructures formed.At an aging temperature of 25 • C, spherical agglomerates of MnO 2 nanoparticles were obtained at aging durations of between 0 hour and about 4 hours (Figure 1).In absence of surfactant, MnO 2 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 MnO 2 nanoparticles.Upon aging for more than 24 hours, well-defined nanorods had developed around these spherical MnO 2 nanoparticles to form sea-urchin-like MnO 2 nanostructures.
As shown in Figure 2, MnO 2 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 MnO 2 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 MnO 2 nanostructures from spherical agglomerates to aggregates of well-defined nanorods.On the contrary, no MnO 2 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 MnO 2 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 MnO 2 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) MnO 2 nanostructures which could be attributed to the accelerated rate of decomposition of MnSO 4 to form MnO 2 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 MnO 2 nanostructures of desired morphology and crystalline phase.
Figure 3 shows TEM micrographs of MnO 2 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 MnO 2 nanoparticles after being aged for 4 hours at 25 • C.More distinctive and defined nanorod-like nanostructures had evolved from spherical MnO 2 nanoparticles after being aged for 8 hours.Long and well-defined nanorods were observed to have developed on MnO 2 nanoparticles forming seaurchin-like nanostructures after being aged for 24 hours at 25 • C.
As shown in Figure 4, TEM micrographs depicted the rapid evolution of well-defined nanorods from individual spherical MnO 2 nanoparticles at elevated aging temperature of 80 • C. Agglomerates of MnO 2 nanoparticles were observed to have transformed rapidly into sea-urchin-like       nanostructures after being aged for 4 hours at 80 • C. All MnO 2 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 MnO 2 nanorods ranged from 20 to 30 nm and 300 to 400 nm, respectively.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 MnO 2 nanorods during the synthesis process could be derived from the inherent crystal structure of MnO 2 material and its chemical potential in solution [22].Based on our experimental observations, the formation mechanisms of MnO 2 nanostructures could entail the following processes.MnO 2 nanoparticles were initially produced by the redox reaction between MnSO 4 and KMnO 4 .These MnO 2 nanoparticles would subsequently aggregate to form spherical agglomerates due to their high surface energies.During prolonged aging duration, MnO 2 nanoparticles would gradually transform into nanorods under the specific aging conditions.The gradual transformation of MnO 2 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 MnO 2 nanorods had been reported byTang et al. with single-crystalline α-MnO 2 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 MnO 2 nanostructures synthesized at different aging temperatures of 25 • C and 80 • C. For MnO 2 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 MnO 2 samples increased from 91.1 m 2 /g and 0.225 cm 3 /g as prepared (or without aging) to 130.5 m 2 /g and 0.410 cm 3 /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.MnO 2 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, MnO 2 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 MnO 2 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.
Figure 6 shows X-ray diffractographs of MnO 2 samples as prepared and after being aged for 24 hours at 25 • C and 80 • C. Both types of as-prepared MnO 2 samples without any post synthesis aging showed similar broad diffraction peaks which can be indexed to δ-MnO 2 phase albeit with low degree of crystallinity.The absence of other manganese dioxide diffraction peaks indicated the high purity of these as-prepared δ-MnO 2 samples.δ-MnO 2 samples synthesized at 25 • C were observed to have only partially converted to α-MnO 2 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-MnO 2 phase were obtained after being aged for 24 .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.

Journal of Nanomaterials
Our findings concurred with observations reported by other researchers that a higher aging temperature would promote more rapid phase transformation of MnO 2 samples [24,25].
The phase transformation from δ-MnO 2 to α-MnO 2 could also be attributed to the stability of the polymorphs of MnO 2 .Since δ-MnO 2 is less stable than α-MnO 2 and β-MnO 2 , it will be transformed into α-MnO 2 or β-MnO 2 with increasing temperature [26].However, due to the mild reaction conditions used in the present study, the β-MnO 2 phase could not be formed.These above results indicated that both the morphological structure and crystalline phase of MnO 2 samples could be modulated by varying the hydrothermal aging duration and temperatures.

Conclusion
In conclusion, MnO 2 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.MnO 2 nanostructures underwent accelerated rate of transformation from δ-MnO 2 to α-MnO 2 phase and associated morphological changes from spherical nanoparticles to nanorods at elevated aging temperature.Both the morphological structure and crystalline phase of the MnO 2 nanostructures could therefore be modulated by varying the hydrothermal aging duration and temperatures.

Figure 1 :
Figure 1: SEM micrographs of MnO 2 samples synthesized at various aging durations at 25 • C.

Figure 2 :
Figure 2: SEM micrographs of MnO 2 samples synthesized at various aging durations at 80 • C.

Figure 3 :
Figure 3: TEM micrographs of MnO 2 samples synthesized at various aging durations at 25 • C.