Photocatalytic degradation of 2,4-dichlorophenol on ZrO₂–TiO₂: influence of crystal size, surface area, and energetic states

ZrO₂–TiO₂ heterostructure with 5 mol% of ZrO₂ was synthesized by the sol–gel method and calcined at different temperatures (300–600 °C). The photocatalysts were characterized by thermal analysis, X-ray diffraction, physisorption of N₂, diffuse reflectance spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity was tested for the removal of 2,4-dichlorophenol under ultraviolet irradiation, being the materials exhibiting the best performance those calcined at 400 °C and 500 °C with 99% and 98% of degradation, respectively, after 150 min under irradiation. This behavior was related to a smaller crystallite size, higher surface area, and significant hydroxyl radicals produced. The (photo)electrochemical study showed that temperatures of 400 °C and 500 °C also generated an optimum amount of energetic states that act as electron traps and decrease the electron–hole pair recombination, favoring the oxidation of 2,4-dichlorophenol. However, at 300 °C and 600 °C, these energetic states act as an energy barrier that reduces the effective charge transfer and therefore decreases the photocatalytic activity of the materials.