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Energies 2022, 15, 6452 12 of 21 Energies 2022, 15, x FOR PEER REVIEW particles [90]. A similar phenomenon may be occurring in the present materials, explaining the redshift observed in the peak position of the band after calcination. In the case of Ashraf et al. [90], further annealing in air at higher temperatures (1500 ◦C) promoted additional diffusion of oxygen into the crystals, leading to a decrease in the broad band PL intensity, strengthening the assumption of the involvement of oxygen vacancy defects in the origin of this band. Their PLE spectra showed an excitation band between ~250 nm and 350 nm, which suffered a blueshift from ~300 nm (~4.13 eV) to ~280 nm (~4.43 eV) after annealing. In 2021, Lokesha et al. [88] also studied monoclinic ZrO2 samples, observing a broad band between 400 and 650 nm and peaked at 499 nm (~2.48 eV), which, in conjugation with other techniques, such as EPR, they attributed to the F2+ center related with aggregates of the singly occupied oxygen vacancies. These broad bands are frequently deconvoluted into different components (recombination channels), which can be associated with different charged states of the vacancy-related defect [88,90,97]. As a result, the existence of distinct charge states of the F-centers can account for the fact that three excitation maxima were observed in the present samples, subsequently leading to a shift in the band peak position depending on the excitation wavelength, particularly in the case of the calcinated material. Hence, the presence of oxygen vacancy-related defects seems to be a fair hypothesis for the nature of the defects involved in the PL emission identified in the present materials. 3.2. Structural Characterization of the ZrO2 Powder and ZrOx Thin Films Produced by Solution Combustion Synthesis 13 of 23 3.2.1. ZrO2 Powder X-ray Diffraction X-ray Diffraction Solution combustion synthesis is known to produce materials in powder form but also Solution combustion synthesis is known to produce materials in powder form but as thin films [58,60,98]. The ZrO2 powder was obtained by conventional solution combus- also as thin films [58,60,98]. The ZrO2 powder was obtained by conventional solution com- tion synthesis, with zirconium oxynitrate and urea as precursors and 2-methoxyethanol bustion synthesis, with zirconium oxynitrate and urea as precursors and 2-methoxy-◦ (2-ME) as a solvent. The prepared solution was further annealed in air at 350 C for 1 h to ethanol (2-ME) as a solvent. The prepared solution was further annealed in air at 350 °C obtain the ZrO2 powder. The XRD diffractogram of the ZrO2 powder is shown in Figure 7. for 1 h to obtain the ZrO2 powder. The XRD diffractogram of the ZrO2 powder is shown The ZrO2 tetragonal phase is clearly identified with the peak at 30.176◦, associated with the in Figure 7. The ZrO2 tetragonal phase is clearly identified with the peak at 30.176°, asso- (111) diffraction plane [99] (ICDD 00-050-1089), along with other diffraction maxima that ciated with the (111) diffraction plane [99] (ICDD 00-050-1089), along with other diffrac- were assigned to the monoclinic phase (ICDD 00-037-1484). No impurities were detected, tion maxima that were assigned to the monoclinic phase (ICDD 00-037-1484). No impuri- whichsuggeststhepresenceofahighlypureZrO powder. ties were detected, which suggests the presence of a highly2 pure ZrO2 powder. Figure 7. XRD diffractogram of ZrO2 powder produced by solution combustion synthesis and an- Figure 7. XRD diffractogram of ZrO2 powder produced by solution combustion synthesis and nealed in air at 350 °C for 1 h◦ (represented in red). For comparison, the simulated monoclinic, te- annealed in air at 350 C for 1 h (represented in red). For comparison, the simulated monoclinic, tragonal and cubic zirconia structures are also presented. tetragonal and cubic zirconia structures are also presented. 3.2.2. Electron Microscopy Figure 8 shows the SEM images of the ZrO2 powder resultant from the solution com- bustion synthesis. The formation of micro-sized plate-like structures is clear, as seen in Figure 8a. Nevertheless, Figure 8b indicates the presence of nano-sized grains composing the micro-sized structure, and Figure S2 suggests the stacking of several nanolayers. Indi-PDF Image | Comparison between Solution-Based Synthesis Methods of ZrO2
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