“…In particular, a remarkable attention has been focused on manganese dioxide, thanks to its low-cost, natural abundance, environmental friendliness, and versatile chemico-physical properties [ 12 , 13 , 14 , 15 ]. MnO 2 exhibits at least six different structurally related crystalline modifications ( α , β , γ , δ , ε , and λ ) [ 1 , 2 , 4 , 7 , 10 , 16 , 17 , 18 , 19 ]. All these polymorphs are semiconductors with low resistivity [ 2 , 20 , 21 ], and have emerged as attractive candidates for several end-uses, including electrodes in Li- and Na-ion batteries and supercapacitors, thermoelectric materials, chemical sensors, photo- and electrocatalysts for pollutant degradation and hydrogen production, and magnetic devices useful for information storage [ 5 , 8 , 11 , 12 , 13 , 14 , 15 , 18 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ].…”