Effect of oxygen vacancies and crystal phases in defective Pt/ZrO2−x on its photocatalytic activity toward hydrogen production

Abstract

Among insulating transition-metal oxides, ZrO₂ has the potential to be a photocatalyst; however, its wide bandgap (∼5 eV) interferes with its ability to absorb sufficient light. Defect engineering—that is, the creation of oxygen vacancies (V_Os) and/or Zr³⁺ ions, is a promising strategy for adjusting the bandgap of ZrO₂ for photocatalytic applications. This study demonstrates that the included V_Os play an important role in boosting photocatalytic activity. In addition, the crystal phase of ZrO₂ (e.g., monoclinic or tetragonal) strongly influences both V_O formation and photocatalytic activity. After a Pt cocatalyst was deposited onto ZrO₂ with a single crystal phase (i.e., monoclinic (m-) or tetragonal (t-)), the specimens were reduced at 200, 400, and 600 °C to obtain photocatalysts with different amounts of V_Os. As-synthesized m-ZrO₂ possessed Zr³⁺ ions in the absence of V_Os, and the reduction treatment promoted V_O formation accompanied by the disappearance of Zr³⁺. The as-synthesized t-ZrO₂ originally included V_Os, and their amount in Pt/t-ZrO_2−x increased with increasing reduction temperature. Pt/t-ZrO_2−x possessed a greater amount of V_Os than Pt/m-ZrO_2−x reduced at the same temperature, suggesting that V_O formation occurs more easily in t-ZrO₂ than in m-ZrO₂. Photocatalytic activity increased with increasing reduction temperature, which is the same tendency observed for the V_O amount. In a comparison of the crystal phases, Pt/m-ZrO_2−x exhibited relatively higher activity than Pt/t-ZrO_2−x despite its fewer V_Os, suggesting that the V_O formed in the monoclinic ZrO₂ contributes to the enhancement of the photocatalytic activity more efficiently than in the tetragonal one.