Hydrothermal Synthesis of Iridium-Substituted NaTaO3 Perovskites.

David L Burnett,Reza J Kashtiban,Richard I Walton,Christopher D Vincent,Jasmine A Clayton

doi:10.3390/nano11061537

Abstract

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.

Highlights

The hydrothermal synthesis of ABO3 perovskite oxides has attracted a large amount of interest in the past decade [1]
The refined lattice parameters are in agreement with the literature values, Table 1, and the mixed-phase nature of NaTaO3 has been previously seen, with samples prepared by solid-state synthesis showing ~45% of the Cmcm polymorph [38]
We have presented a hydrothermal synthesis route to introduce iridium into a prototypical perovskite structure NaTaO3 that uses mild reaction conditions in a single-step process

Summary

Introduction

The hydrothermal synthesis of ABO3 perovskite oxides has attracted a large amount of interest in the past decade [1] This includes families of materials with important properties such as titanates (B = Ti) with dielectric properties [2], piezoelectric zirconate-titanates (B = Zr, Ti) [3], multiferroic chromites (B = Cr) [4], and ferrites (B = Fe) with applications in redox catalysis [5].

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