A Comparison between Solution-Based Synthesis Methods of ZrO₂ Nanomaterials for Energy Storage Applications

The present study is focused on the synthesis of zirconium dioxide (ZrO₂) nanomaterials using the hydrothermal method assisted by microwave irradiation and solution combustion synthesis. Both synthesis techniques resulted in ZrO₂ powders with a mixture of tetragonal and monoclinic phases. For microwave synthesis, a further calcination treatment at 800 °C for 15 min was carried out to produce nanopowders with a dominant monoclinic ZrO₂ phase, as attested by X-ray diffraction (XRD) and Raman spectroscopy. The thermal behavior of the ZrO₂ nanopowder was investigated by in situ XRD measurements. From the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, the presence of near spherical nanoparticles was clear, and TEM confirmed the ZrO₂ phases that comprised the calcinated nanopowders, which include a residual tetragonal phase. The optical properties of these ZrO₂ nanopowders were assessed through photoluminescence (PL) and PL excitation (PLE) at room temperature (RT), revealing the presence of a broad emission band peaked in the visible spectral region, which suffers a redshift in its peak position, as well as intensity enhancement, after the calcination treatment. The powder resultant from the solution combustion synthesis was composed of plate-like structures with a micrometer size; however, ZrO₂ nanoparticles with different shapes were also observed. Thin films were also produced by solution combustion synthesis and deposited on silicon substrates to produce energy storage devices, i.e., ZrO₂ capacitors. The capacitors that were prepared from a 0.2 M zirconium nitrate-based precursor solution in 2-methoxyethanol and annealed at 350 °C exhibited an average dielectric constant (κ) of 11 ± 0.5 and low leakage current density of 3.9 ± 1.1 × 10⁻⁷ A/cm² at 1 MV/cm. This study demonstrates the simple and cost-effective aspects of both synthesis routes to produce ZrO₂ nanomaterials that can be applied to energy storage devices, such as capacitors.