Facile Synthesis of Novel Nanostructured MnO2Thin Films and Their Application in Supercapacitors
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Nanostructured α-MnO2thin films with different morphologies are grown on the platinum substrates by a facile solution method without any assistance of template or surfactant. Microstructural characterization reveals that morphology evolution from dandelion-like spheres to nanoflakes of the as-grown MnO2is controlled by synthesis temperature. The capacitive behavior of the MnO2thin films with different morphologies are studied by cyclic voltammetry. The α-MnO2thin films composed of dandelion-like spheres exhibit high specific capacitance, good rate capability, and excellent long-term cycling stability.
KeywordsSupercapacitor MnO2 Nanostructure Thin film Cyclic voltammetry
In recent years, manganese oxides (MnO2) have attracted considerable interests due to their distinctive physical and chemical properties and wide applications in catalysis, ion exchange, molecular adsorption, biosensor, and energy storage [1, 2, 3, 4, 5]. Specifically, manganese oxides have been extensively evaluated as electrode materials for supercapacitors due to their low cost and environmental benignity compared to noble metal oxides such as RuO2 [6, 7, 8]. In the development of supercapacitors, nanostructured electrode materials have received great interests as they exhibit higher specific capacitance and rate capability compared to traditional bulk materials. Over the years, various nanostructured manganese oxides, including one-dimensional (1-D) (nanorodes, nanowires, nanobelts, nanoneedles, and nanotubes), two-dimensional (2-D) (nanosheets, nanoflakes), and three-dimentional (3-D) (nanospheres, nanoflowers, hollow urchins) nanostructures, have been synthesized [9, 10, 11, 12, 13, 14, 15, 16]. 3-D hierarchical porous structures often produce more active sites and exhibit more favorable electrochemical properties than 2-D and 1-D structures. However, facile synthesis and mass production of complex 3-D nanostructures are still a challenge in the areas of materials science [17, 18, 19, 20]. It has been reported that a core-shell structure with spherically aligned nanorods of α-MnO2 can be prepared through a simple room temperature reaction between MnSO4 and (NH4)2S2O8 with a catalyst of Ag+ in an acid solution . A similar method used by Gong et al. is able to synthesize MnO2 hollow urchins with a reactive template of carbon spheres . Wang et al. also reported the synthesis of hierarchical α-MnO2 spheres by the reaction between MnSO4 and K2S2O8 with the addition of CuSO4 in an acidic solution . However, the preparation of 3-D nanostructured MnO2 in the thin film form has never been reported. Since the use of composite electrodes introduces additional undesirable interfaces in the electrode material with the risk of negating the benefits of electrochemistry using nanostructures, thin film electrodes enable us to investigate the electrochemical properties of the active material itself without the influence of binders and conductive additives as required for composite electrodes.
In this paper, we propose the stratagem to synthesize 3-D α-MnO2dandelion-like spheres and 2-D α-MnO2nanoflakes by a reaction between MnSO4and (NH4)2S2O8in a Na2SO4solution at low temperatures. With a platinum (Pt) substrate submerged into the reaction solution, the nanostructured MnO2can be directly deposited on the Pt substrate in the thin film form. The effects of the synthesis temperature on the morphology of the films are investigated, and the capacitive behaviors of nanostructured MnO2thin films with different morphologies are studied and compared.
Synthesis of Nanostructured MnO2
Analytical grade MnSO4, (NH4)2S2O8, and Na2SO4from Sigma–Aldrich were used. A typical synthesis of nanostructured MnO2was performed by dissolving MnSO4, (NH4)2S2O8, and Na2SO4with a molar ratio of 1:1:1 in 30 mL deionized water at room temperature. The concentrations of Mn2+, S2O82−, and SO42−in the solution are the same as 0.1 mol L−1. A Pt substrate was submerged into the solution, while the solution was magnetically stirred in a beaker at room temperature (RT) for 12 h or at 80 °C for 2 h. One side of the Pt substrate was covered with Parafilm, so that MnO2can only be deposited on one side. After the reaction, the Pt substrate was washed using distilled water and then dried in the vacuum at 60 °C overnight.
Structure and crystallinity of thin films were characterized using a Shimadzu XRD-6000 X-ray diffractometer with Cu Kα radiation at a scanning rate of 1°min−1. Surface morphology of the as-deposited thin films was characterized using a Hitachi S-4100 field emission scanning electron microscope (FESEM). Weights of the MnO2thin films were measured using a microbalance with an accuracy of 0.01 mg.
All electrochemical measurements were conducted using a Solartron 1287 electrochemical interface combined with a Solatron 1260 frequency response analyzer. For the electrochemical measurements, a three-electrode cell system composed of a MnO2thin film electrode as the working electrode, a high surface carbon rod as the counter electrode, and an Ag/AgCl reference electrode was employed. The capacitive behaviors of the as-deposited MnO2thin films were characterized by cyclic voltammetry (CV) in 1 M Na2SO4electrolyte at room temperature. CV measurements were performed on the three-electrode cells in the voltage window between 0 and 0.9 V at different scan rates from 20 to 200 mV s−1. Electrochemical impedance spectra (EIS) of different thin film electrodes were measured at the open-circuit potential in the frequency range from 100 kHz to 10 mHz.
Results and Discussion
α-MnO2 has usually been found to be the product of the oxidation of Mn2+ by S2O82− either through a hydrothermal reaction  or through a mild solution reaction [21–23]. It has been observed in this study that the formation of nanostructured MnO2 is preferred to deposit on the Pt substrate rather than in the solution. Therefore, the preparation of MnO2 thin films (as shown in Fig. 1c) in this study is quite simple and convenient compared with electrochemical deposition, which is usually employed to prepare MnO2 thin films.
The possible formation mechanism for the hierarchical MnO2 spheres is schematically illustrated in Fig. 2c. Generally speaking, the crystal growth process always includes two steps: the initial nucleation stage and following crystal growth stage [25, 26]. Initially, MnO2 colloids are slowly formed and attached to the Pt substrate. After which, the absorbed MnO2 colloids on the Pt substrate tend to aggregate loosely to form spherical appearance due to their high surface energies. Because the reaction temperature is at RT, Gibbs energy for nucleation of new MnO2 sites is low. As a consequence, MnO2 colloids tend to attach on the habit planes of existing MnO2 sites, leading to the formation of 1-D nanowhiskers from the initial colloidal microspheres. With increase in the processing duration, finally dandelion-like 3-D microspheres of MnO2 on the Pt substrate appear. However, on the contrary to Wang’s finding , the increase in reaction temperature in this study is unable to improve the formation of 3-D hierarchical microspheres of MnO2 but leads to the formation of 2-D nanoflakes. This phenomenon can be explained by the change in growth mechanisms as shown in Fig. 2f. When the reaction temperature is increased to 80 °C, the reaction rate is greatly enhanced along with the high rate of adsorption of MnO2 colloids to the Pt substrate. Under such circumstances, in addition to the one-dimentional growth of the nuclei along the low energy direction , the growth of the nuclei along other directions can also happen due to the fast nucleation and adsorption rates of MnO2 at an elevated temperature. Therefore, nanoflakes instead of nanowhiskers form resulting in the morphology evolution.
MnO2thin films with nanostructures have been prepared on Pt substrates by a facile and mild solution method. The MnO2film prepared at RT with a long reaction time is composed of dandelion-like microspheres, which consists of nanowhiskers with very small size. The reaction temperature plays an important role in controlling the surface morphology of the film. As the reaction temperature was increased to 80 °C, a film composed of nanoflakes can be prepared in a very short time. The CV measurements indicate that MnO2thin films prepared by this method are promising as electrodes for supercapacitors. The film composed of dandelion-like microspheres exhibited a higher specific capacitance and better rate capability than the film composed of nanoflakes, which is probably due to the high surface area and smaller feature of the microspheres. The excellent cycling stability and good rate capability of the film composed of dandelion-like microspheres coupled with the simple and low cost synthesis method make this material attractive for large applications.
This research is supported by National University of Singapore and Agency for Science, Technology and Research through the research grant R-265-000-292-305 (072 134 0051).