Kmno4

δ-MnO₂ was prepared following Gaillot et al.^{38 ,39} KMnO₄ (Wako Chemical) crystalline powder was thermally decomposed at 700 °C in air. In order to remove soluble by-products, the powder was washed with distilled water several times until a colorless solution was obtained. The remaining insoluble black powder was filtered and dried in vacuum at 80 °C. ICP optical emission spectrometry confirmed that the composition of the resulting powder was K_0.33MnO₂.
X-ray powder diffraction (XRD) analysis was performed using a Rigaku SmartLab diffractometer with MoKα radiation (λ = 0.70926 Å). In situ·XRD analysis at controlled temperatures was carried out at a heating rate of 5 °C min⁻¹ with periods of constant temperature for 30 min to collect diffraction patterns. The samples were placed in ambient air atmosphere (60%RH at 24 °C) or dry N₂ atmosphere during measurement. Crystal structures of MnO₂ polymorphs were visualized with the VESTA program⁴⁸ .
For reference, transmission electron microscopy (TEM) observation was also conducted by JEM-2000EXII (JEOL) for as-prepared sample (initially in a sufficiently hydrated state), the results of which are given in Supplementary Fig. 8.

Benzene (C₆H₆, >99.5% purity, Kanto Chemical Co., Inc., Japan) and potassium permanganate (KMnO₄, >99.3% purity, Wako Pure Chemical Industries Ltd., Japan) were selected as the carbon and manganese oxide precursors, respectively. DCH18C6 (C₂₀H₃₆O₆, >98.0%, Tokyo Chemical Industry Co., Ltd., Japan) was used as the main chemical agent for preparing purple benzene. Manganese(IV) oxide powder (85% purity, Kanto Chemical Co., Inc., Japan) was used as a commercial MnO₂ sample for comparing the dye removal performance with the as-prepared adsorbents. Ethanol (C₂H₅OH, >99.5%), hydrochloric acid (HCl, 35–37%), and a sodium hydroxide solution (NaOH, 48–50%) were purchased from Kanto Chemical Co., Inc., Japan. MB (C₁₆H₁₈ClN₃S·3H₂O, 98.5% purity, Kishida Chemical Co., Ltd., Japan), Rh B (C₂₈H₃₁ClN₂O₃, Guaranteed reagent, Wako Pure Chemical Industries Ltd., Japan) and MO (C₁₄H₁₄N₃NaO₃S, ACS reagent, dye content 85%, Sigma-Aldrich, Japan) were selected as typical cationic and anionic organic dyes. Distilled water was obtained from an Aquarius water distillation apparatus (RFD250NB, Advantec, Japan) with a resistivity of 18.2 MΩ cm at 25 °C. All chemicals were used without further purification.