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Energies 2022, 15, 6452 4 of 21 demonstrated the fabrication of ultrathin ZrOx films by deep ultraviolet irradiation, which revealed a leakage current density as low as 10−11 A cm−2 at 1 MV/cm, a capacitance of 260 nF cm−2 (at 1 MHz), high dielectric constant (22) and good breakdown voltage (around 6 MV/cm) [74]. In addition, Luo et al. prepared high-quality ZrO2 films using an oxygen-doped precursor solution (ODS). The ODS-ZrO2 films showed a low leakage current density of 10−7 A cm−2 (at 2 MV/cm), high breakdown electric field (7.0 MV/cm) and dielectric constant of 19.5 [75]. However, the challenge still relies on the fabrication of metal-oxide films with a high-quality surface (a smooth surface with a dense network) at a low temperature and through a simple approach to guarantee a low leakage current density and high breakdown field [75]. In this study, ZrO2 nanomaterials were produced in the form of powders or thin films through solution-based processes, i.e., a hydrothermal method assisted by microwave irradiation and solution combustion synthesis. The microwave synthesized powder was further calcinated at 800 ◦C for 15 min under atmospheric conditions. The ZrO2 nanomate- rials were characterized by XRD, Raman spectroscopy, SEM coupled with energy dispersive X-ray spectroscopy (EDS) and focused ion beam (FIB) and TEM. The thermal behavior of the nanopowder produced under microwave irradiation was investigated through in situ XRD, and these powders had their optical properties assessed through PL and PLE at RT. The ZrOx thin films produced by the solution combustion synthesis were further tested as capacitors. 2. Materials and Methods 2.1. Hydrothermal Synthesis of ZrO2 Nanoparticles Assisted by Microwave Irradiation The ZrO2 nanoparticle synthesis route was adapted from ref. [4]. In a typical synthesis, 50 mL of an aqueous (aq.) solution of 0.2 M of zirconium (IV) oxynitrate hydrate (Sigma- Aldrich, St. Louis, MO, USA, 99.9%, CAS: 14985-18-3, ZrO(NO3)2·xH2O) is mixed with 50 mL of an aq. solution of 0.4 M of sodium hydroxide (Labchem, CAS: 1310-73-2, NaOH). The reagents were used without any further purification. The 100 mL solution was left to stir for 30 min. The molar ratio of zirconium precursor and sodium hydroxide was kept at 1:2. Microwave synthesis was then carried out with a CEM microwave digestion system, Matthews, NC, USA (MARS one), and the applied microwave parameters were 1000 W, 230 ± 10 ◦C and 25 min. Afterwards, the previous solution was equally distributed into Teflon vessels of 75 mL (each vessel containing 20 mL of solution). Subsequently, the centrifugation of the resultant nanopowder was performed for 3 min at 4750 rpm and washed three times alternately with deionized water and isopropyl alcohol (IPA). Finally, the nanopowder was dried in a desiccator at 60 ◦C for 5 h. The yield was around 0.77 g of nanopowder/batch. After microwave synthesis, the dried ZrO2 nanopowder was further calcinated in an alumina ceramic crucible at 800 ◦C for 15 min using a Nabertherm furnace under atmospheric conditions. The calcination treatment aimed to guarantee the formation of the mostly thermodynamically stable ZrO2 phase, i.e., the monoclinic phase. 2.2. Solution Combustion Synthesis A solution with a concentration of 0.2 M of zirconium (IV) oxynitrate hydrate (Sigma- Aldrich, 99.9%, CAS: 14985-18-3) was prepared in 2-methoxyethanol (2-ME, ACROS Organ- ics, 99%, C3H8O2), and it was left to stir at room temperature for 2 h. Urea (Sigma-Aldrich, 98%, CO(NH2)2) was used as fuel for the combustion reaction and continued stirring for a minimum of 1 h. The molar proportion between urea and the zirconium oxide precursor was 5:3 to ensure the redox stoichiometry of the reaction. The powder was produced considering 10 mL of the combustion solution with the same concentration and transferred to an alumina ceramic crucible for further annealing in an air furnace at 350 ◦C for 1 h. As for the thin films, and prior to the ZrOx deposition, a 2.5 × 2.5 cm p-type single crystal 100-oriented silicon substrate (resistivity ∼= 1–2 Ω·cm) was cleaned with acetone for 10 min in an ultrasonic cleaning bath at 60 ◦C. This cleaning process was repeated withPDF Image | Comparison between Solution-Based Synthesis Methods of ZrO2
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