Flower-shaped d-MnO2 was synthesized using the followinghydrothermal method. Potassium permanganate [KMnO4, BDH Chemicals; 4.5 mmol] was dissolved in 150 mL of deionized water (DI) and mixed with ammonium persulfate [APS; (NH4)2S2O8, Aldrich; 0.75 mmol] in a Teflon-lined stainless steel reactor. The solution was stirred for 20 min and subsequently heated in an oven at 90 C for 24 h. The reaction mixture was then furnace cooled to room temperature and a brownish product obtained was filtered, washedwithDIwateruntilapH$7,anddriedat60Cfor24hfor characterization.