Comment on “Hydrothermal Synthesis and Pseudocapacitance Properties of α-MnO2 Hollow Spheres and Hollow Urchins”
Résumé
We would like to comment about the recent work described by Xu et al. published in J. Phys. Chem. C2007, 111, 19141−19147. The paper deals with synthesis of α-MnO2 hollow spheres and urchins by a hydrothermal method. We believe that there is a misinterpretation on the explanation of the XRD pattern for the material synthesized via the hydrothermal reaction for 3 h. The pattern shows four distinct peaks around 12.3, 24.6, 36.6, and 65.7° in 2θ values. Although the authors have indexed the observed XRD pattern as a material crystallizing in tunnel structure of the α-MnO2 type (JCPDS 29−1020), the entire pattern can be accounted for by δ-MnO2 crystallizing in a layered-type structure composed of sheets of edge-shared MnO6 octahedra with bilayers of water stabilized between the MnO2 layers. 1 This is also in agreement with previous reports 2-4 and the standard pattern of the pure hexagonal birnessite phase (lattice parameter a = 2.85 and c = 7.1 Å) with a basal spacing of 7.1 Å (JCPDS 23−1046). The peak shape at 36.6° derives from a turbostratic structure, another characteristic of the layered structure, demonstrating that the XRD pattern of the sample synthesized under hydrothermal condition for 3 h is not α-MnO2 type with tunnel structure. 5 Also, it is known that δ-MnO2 is synthesized in milder synthetic conditions compared to α-MnO2 which needs severer conditions such as a longer reaction time and a higher reaction temperature, suggesting that there is a phase transition from δ-MnO2 to α-MnO2 between 3 and 6 h of hydrothermal reaction in the paper of Xu et al.