Crystal structure, morphology and formation mechanism of a novel polymorph of lead dioxide, γ-PbO₂

Nanosecond-pulsed spark discharge between two lead electrodes in liquid nitro­gen has been used to synthesize hexagonal lead nanosheets. These original nanostructures are collected on a substrate located under the electrodes. After the full evaporation of the liquid nitro­gen, the nanosheets are oxidized in air and transform into a lead dioxide. The resulting hexagonal sheets have typical widths of around 1 µm and typical thicknesses of around 10 nm. Investigations by energy dispersive spectroscopy microanalysis, transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy and electron microdiffraction were performed in order to identify the crystal structure in which these hexagonal nanosheets crystallize. An analysis of the chemical composition pointed to a stoichiometric lead dioxide, PbO₂. This PbO₂ lead dioxide crystallizes in the hexagonal system (a = 0.912 nm and c = 1.265 nm) and belongs to the space group P6/m2/m2/m. On the basis of group theory (symmetry analysis), the nanosheets develop a hexagonal-prismatic shape in liquid nitro­gen, assumed to be an isotropic medium. From the energetic point of view, this shape, dictated by the 6/m2/m2/m point group, corresponds to an absolute extremum, an indicator of the stability of this lead dioxide. A mechanism similar to that of the ledge mechanism explaining the formation of thin plates in a metallic matrix has been adapted and proposed for the formation of the lead nanosheets in the liquid nitro­gen. When the liquid nitro­gen is removed, the lead nanosheet is oxidized, leading to a lead dioxide, inheriting the nanosheet morphology. As far as the authors are aware, this is the first time that such a lead dioxide has been synthesized by spark discharge in liquid nitro­gen followed by oxidation in air. The crystallographic structure is determined and the morphology is explained. A mechanism for the development of the lead nanosheets and their oxidation is proposed. This hexagonal phase, designated γ-PbO₂, is thought to be the third polymorph after the α-PbO₂ and β-PbO₂ phases of lead dioxide, the former being orthorhombic and the latter being tetragonal.