α-MnO₂ has usually been found to be the product of the oxidation of Mn²⁺ by S₂O₈ ²⁻ either through a hydrothermal reaction 24 or through a mild solution reaction 21 22 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 morphologies of the MnO₂ thin films synthesized at different temperatures are shown in Fig. 2. It can be seen from Fig. 2a that the film synthesized at RT is composed of uniform microscopic spheres with diameters ranging from 0.5 to 1 μm. The magnified FESEM image (Fig. 2b) shows that these microscopic spheres are composed of nanowhiskers, resulting in a dandelion-like morphology. Li et al. 22 reported that a core-shell structure microspheres (2–3 μm) of α-MnO₂ can be obtained by the reaction between MnSO₄ and (NH₄)₂S₂O₈ with the addition of AgNO₃ catalyst at RT for 1–2 days. The thin shell of this structure is composed of nanorods, and this morphology can only be obtained with the existence of Ag⁺. Wang et al. 23 reported that sea urchin-shaped microspheres (~1 μm) of α-MnO₂ can be obtained by the reaction between MnSO₄ and K₂S₂O₈ with the addition of CuSO₄ at 70 °C for 3 days. In the present work, the 3-D hierarchical structure of α-MnO₂ can be obtained at RT in a relatively short synthesis time (12 h) without using any AgNO₃ or CuSO₄ since the addition of catalyst of AgNO₃ or CuSO₄ may induce Ag⁺ or Cu²⁺ impurities in the final MnO₂ products 23 . It is assumed that reaction between Mn²⁺ and S₂O₈ ²⁻ in the Na₂SO₄ solution with a Pt substrate is relatively faster compared to the previous studies 22 23. It is interesting to observe a morphology evolution of the film, as the synthesis temperature increases. As shown in Fig. 2d, the film synthesized at 80 °C exhibits another type of porous structure with nanoflakes almost vertically aligned on the Pt substrate. The magnified FESEM image (Fig. 2e) shows that the average size of the nanoflakes is about 500 nm, and the thickness of the nanoflakes is less than 50 nm.

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 23 , 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 27 , 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.

The 3-D and 2-D nanostructured MnO₂ are promising as electrode materials for supercapacitors due to their porous structure, large surface area, and short diffusion length for protons or alkali cations. In order to evaluate the electrochemical properties of the as-deposited thin films, the thin film samples with different nanostructures were characterized in aqueous 0.1 M Na₂SO₄ electrolyte with CV measurements. The CV curves at various scan rates from 20 to 200 mV s⁻¹ for the sample A and B are shown in Fig. 3a, b (sample A represents MnO₂ with dandelion-like sphere morphology, and sample B represents MnO₂ nanoflakes). It can be seen that the curves at the low scan rate of 20 mV s⁻¹ for both samples exhibit ideal symmetrical rectangle-like shape indicating ideal capacitive behavior. As the scan rate increases, slight distortion from the ideal symmetrical rectangle shape can be observed from the CV curves for both samples. However, it is clear to see that the distortion from the rectangularity of the CV curves for the sample A is much less than that for the sample B, indicating much better rate performance of the sample A due to its finer nano-architecture. The specific capacitance of the nanostructured MnO₂ film can be obtained by the following equation:

where Q is the voltammetric charge, ΔE is the voltage window (0.9 V), and m is the mass of the active material of the electrode. The specific capacitances at different scan rates for both samples are shown in Fig. 3c. At the scan rate 20 mV s⁻¹, the sample A exhibits a much higher specific capacitance of about 230 F/g than 180 F/g of the sample B. As the scan rate increases, the specific capacitance for both samples decreases, which is typical for electrochemically active MnO₂ materials. At the highest scan rate of 200 mV s⁻¹, the sample A can maintain 66% of its full capacitance (we set the specific capacitance at 20 mV s⁻¹ as the full capacitance) while the sample B can only maintain 55% of its full capacitance, indicating that the sample A has much better rate capability than that of the sample B. Figure 3d compares the Nyquist plots for the MnO₂ thin films with different nanostructures. A depressed semicircle in the high-frequency range corresponding to the charge-transfer resistance, and a straight sloping line in the low-frequency range corresponding to the diffusive resistance can be observed for both samples. As shown in Fig. 3d, it is clear that the sample A has lower charge-transfer resistance and diffusive resistance compared with those of the sample B, confirming the superior capacitive behavior of the sample A. Based on the previous paper by Chou et al. 28 , surface area of thin film can be derived from the double layer capacitance by fitting EIS using equivalent circuit. The calculated surface areas of the sample A and the sample B are about 414 and 125 m² g⁻¹ respectively. Pseudocapacitance of MnO₂ mainly originates from the adsorption of cations in the electrolyte (M⁺ = Li⁺, Na⁺, and K⁺) on the surface of MnO₂ and also possible intercalation/deintercalation of H⁺ and alkaline metal cations in the bulk of MnO₂ 29 . Since the 3-D dandelion-like microspheres of the sample A are composed of much finer nanowhiskers with very small size, they provide a much larger surface area per gram and shorter diffusion length for cations, comparing to the relatively large 2-D nanoflakes of the sample B. Therefore, the morphology advantage of sample A explains why it can exhibit a higher specific capacitance and better rate capability than those of the sample B.

The sample A, showing better capacitive behavior in rate capability test, was further investigated for long-term cycling stability. The CV curves at different cycling stages and variation of specific capacitance over 2,000 cycles are shown in Fig. 4a, b. As shown in Fig. 4a, the CV curves for the 1st, 500th, 1000th, and 2000th cycles almost overlapped with each other, indicating excellent cycling stability. After 2,000 cycles, there is no degradation of the capacitive behavior, indicating no significant structural or microstructural changes in the MnO₂thin film electrodes. The CV curve after 2,000 cycles is noted to become more symmetrical with the rectangular shape compared with the first cycle, indicating improved capacitive behavior after long time cycling. The specific capacitance of the sample A (as shown in Fig. 4b) slightly decreases for the first 30–40 cycles then starts to increase very slowly with the cycling. As shown in the XRD results, there is a small amount of Mn₃O₄exist in the film. Mn with a lower oxidation state in the Mn₃O₄is probably oxidized to Mn⁴⁺during the long time CV cycling, resulting in improved capacitive behavior and an small increase of specific capacitance.