Basic photocatalytic activity of ZrO 2 thin films fabricated by a sol-gel method under UV-C irradiation

Highlights: • ${\text{ZrO}}_2$ thin films were fabricated by a sol gel method. • Annealing temperature dependence of their photocatalytic activity was investigated. • ${\text{ZrO}}_2$ thin films have photocatalytic activity under ultraviolet C irradiation. • Formation of interstitial O^- states decreases their photocatalytic activity. • Photocatalytic activity of ZrO₂ was approximately ten times as high as that of ${\text{TiO}}_2$. The photocatalytic activity of zirconia (${\mathrm{ZrO}}_2$) thin film fabricated by a sol-gel method was investigated using methyl red decomposition under ultraviolet C (UV-C) irradiation. The dependence of ${\mathrm{ZrO}}_2$ photocatalytic properties on annealing temperature was also investigated. The photocatalytic activity of ${\mathrm{ZrO}}_2$ was compared with that of ${\mathrm{TiO}}_2$, a standard photocatalyst material. Zirconia annealed at 500–700 °C has a combined tetragonal and monoclinic crystal structure, while that annealed at 800–980 °C has an almost exclusively monoclinic structure. The photocatalytic activity of ${\mathrm{ZrO}}_2$ annealed at 500–700 °C was found to be higher than that of ${\mathrm{ZrO}}_2$ annealed above 800 °C. The low activity of ${\mathrm{ZrO}}_2$ annealed above 800 °C is considered to result from the increase in interstitial $O^{-}$ states formed by phase transitions, which lead to increased absorption at $\sim$5.4 eV. The photocatalytic activity of ${\mathrm{ZrO}}_2$ annealed at 500–700 °C was approximately ten times as high as that of ${\mathrm{TiO}}_2$, which suggests that pure ${\mathrm{ZrO}}_2$ with wide bandgap energy is a promising candidate for photocatalysis under UV-C irradiation.