Precursor-Regulated Hydrothermal Reconstruction of α-MnO2 Thin Films for Electrochromic Modulation

Abstract Manganese dioxide (MnO2) is a promising electrochromic material owing to the natural abundance of manganese, its multivalent redox chemistry, and its tunable optical response. However, achieving sufficient electrochemically active material loading while maintaining efficient ion and electron transport remains challenging. Herein, an electrodeposition-assisted hydrothermal strategy is developed to construct α-MnO2 electrochromic thin films on fluorine-doped tin oxide (FTO) substrates. Potentiostatic electrodeposition is first employed to regulate the initial deposition state of a MnOOH precursor, followed by KMnO4-assisted hydrothermal conversion and structural reconstruction into crystalline α-MnO2. By varying the electrodeposition duration, we examined the relationships among precursor deposition, structural evolution, electrochemical response, and electrochromic behavior. Among the resulting films, MnO2-100 prepared by potentiostatic electrodeposition at 1.0 V (vs. SCE) for 100 s in an aqueous solution containing 0.1 M MnSO4·H2O and 0.1 M Na2SO4, followed by hydrothermal treatment in 0.1 M KMnO4 at 110 °C for 12 h, exhibits the highest electrochromic response, achieving an optical modulation (ΔT) of 57.1% at 460 nm, coloring and bleaching times of 13 and 11 s, respectively, and a coloration efficiency of 159 cm2 C−1. The superior response of MnO2-100 is associated with an intermediate precursor deposition state that favors the formation of a relatively open nanoplate-like architecture and more favorable electrochemical kinetics. In contrast, insufficient precursor deposition limits the amount of electrochromically active MnO2, whereas excessive deposition leads to film thickening and nanosheet aggregation, increasing transport limitations and reducing effective material utilization. These results reveal a nonmonotonic structure−property relationship governed by precursor-regulated hydrothermal reconstruction and provide a simple strategy for tailoring MnO2 architectures toward efficient electrochromic modulation.