Hydrothermal Synthesis of Iridium-Substituted NaTaO₃ Perovskites

Iridium-containing NaTaO₃ is produced using a one-step hydrothermal crystallisation from Ta₂O₅ and IrCl₃ in an aqueous solution of 10 M NaOH in 40 vol% H₂O₂ heated at 240 °C. Although a nominal replacement of 50% of Ta by Ir was attempted, the amount of Ir included in the perovskite oxide was only up to 15 mol%. The materials are formed as crystalline powders comprising cube-shaped crystallites around 100 nm in edge length, as seen by scanning transmission electron microscopy. Energy dispersive X-ray mapping shows an even dispersion of Ir through the crystallites. Profile fitting of powder X-ray diffraction (XRD) shows expanded unit cell volumes (orthorhombic space group Pbnm) compared to the parent NaTaO₃, while XANES spectroscopy at the Ir LIII-edge reveals that the highest Ir-content materials contain Ir⁴⁺. The inclusion of Ir⁴⁺ into the perovskite by replacement of Ta⁵⁺ implies the presence of charge-balancing defects and upon heat treatment the iridium is extruded from the perovskite at around 600 °C in air, with the presence of metallic iridium seen by in situ powder XRD. The highest Ir-content material was loaded with Pt and examined for photocatalytic evolution of H₂ from aqueous methanol. Compared to the parent NaTaO₃, the Ir-substituted material shows a more than ten-fold enhancement of hydrogen yield with a significant proportion ascribed to visible light absorption.