The introduction of oxygen vacancies into zirconia is an effective strategy for enhancing its light absorption ability and photocatalytic performance. However, the cost-efficient preparation of oxygen-deficient zirconia (ZrO_2−x) remains challenging, which severely limits its broad application. In this study, flash sintering treatment was used to fabricate ZrO_2−x bulk in very short time of 90 s. Oxygen vacancies were introduced into ZrO₂ bulk through electrochemical reduction reactions. The as-prepared black ZrO_2−x exhibited excellent optical absorption capability, a small band gap (2.09 eV for direct and 1.67 eV for indirect), and a reduced conduction band energy, which is ascribed to the generation of oxygen vacancies and reduction of Zr cations. The as-prepared ZrO_2−x showed remarkable photocatalytic activity due to excellent solar light absorption and low recombination rate of electron‒hole pairs. Flash sintering treatment provides a cost-efficient approach for rapidly fabricating ZrO_2−x bulk materials with high concentrations of oxygen vacancies, which can also be applied to other materials.

The challenge in synthesizing high-entropy ceramic (HEC) nanopowders is to suppress severe grain coarsening and particle agglomeration, which occur at elevated temperatures. This challenge could be addressed by the polyacrylamide gel method. In this work, single-phase high-entropy (La_0.2Nd_0.2Sm_0.2Gd_0.2Yb_0.2)₂Zr₂O₇ and (La_0.2Nd_0.2Y_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ nanopowders without agglomeration were successfully synthesized using the polyacrylamide gel method for the first time. The results showed that phase composition, particle size, and agglomeration degree of the nanopowders were greatly influenced by the molar ratio of acrylamide (AM)/Zr and calcination temperature. These as-synthesized high-entropy zirconate (HEZ) nanopowders could be sintered into fully dense ceramics at 1500 ℃ for 2 h. These HEZ nanopowders showed a phase transformation from a defect-fluorite phase to a pyrochlore phase with the increase of sintering temperature. Additionally, two-step sintering of these nanopowders was conducted, and the HEZ ceramics with fine grains were prepared. The polyacrylamide gel method is simple and easily operated, which is a facile approach of producing the HEC nanopowders with excellent sinterability.