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The facile synthesis of MnO 2 crystals with a controllable structure was achieved through a simple hydrothermal reaction. The MnO 2 grew to form a quasicolumnar shape with a structure that can be controlled through the simple variation of the pH during the synthesis. This control strategy could be applied to other solvents to obtain materials with unique structures. Furthermore, composites of the as-synthesized hollow MnO 2 with N-doped graphene (rGN) are electrocatalysts for the [a] 1315 Figure 2. (A) XRD patterns of MnO 2 -rGN-10, MnO 2 -rGN-6, MnO 2 -rGN-3, and MnO 2 -10. (B) Raman spectra of MnO 2 -rGN-10, MnO 2 -rGN-6, MnO 2 -rGN-3, rGN, MnO 2 -rGO-10, and MnO 2 -10.