Hydrothermal Synthesis of Iridium-Substituted NaTaO(3) Perovskites

Iridium-containing NaTaO(3) is produced using a one-step hydrothermal crystallisation from Ta(2)O(5) and IrCl(3) in an aqueous solution of 10 M NaOH in 40 vol% H(2)O(2) 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 NaTaO(3), while XANES spectroscopy at the Ir L(III)-edge reveals that the highest Ir-content materials contain Ir(4+). The inclusion of Ir(4+) into the perovskite by replacement of Ta(5+) 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 H(2) from aqueous methanol. Compared to the parent NaTaO(3), the Ir-substituted material shows a more than ten-fold enhancement of hydrogen yield with a significant proportion ascribed to visible light absorption.