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Birnessite flower-like and α-type tubular MnO₂ nanostructures were selectively synthesized through simple decomposition of KMnO₄ under hydrochloric acid condition by controlling reaction temperature using a microwave-assisted hydrothermal method. The as-prepared samples were characterized in detail by various techniques including X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, Fourier transform-infrared spectroscopy, and Raman scattering spectroscopy. While the growth of flower-like birnessite-MnO₂ might follow a widely accepted Ostwald ripening process, we proposed a formation mechanism of the nanotubular α-MnO₂ based on our evidence, which was assembly of nanorods through an “oriented attachment” process.