Despite decades of research into almost every known stable material, the hunt for an efficient solar photocatalyst for water splitting is still ongoing. The next frontier in this search can be found at the boundaries of thermodynamic stability, i.e., for metastable materials. Metastable materials can frequently exhibit superior properties; however, their syntheses are often challenging and empirical in nature. Results for the flux synthesis of a series of new metastable perovskite oxides, (Ba1-xSnX)(Zr1-yTiy)O3 (BSZT), will be presented together with an analysis of the relationship between its metastability and enhanced photocatalytic properties. A wide range of BSZT compositions could be prepared via low-temperature ion-exchange reactions using BZT precursors and a SnClF flux. The highest Sn(II) cation concentration reached is x = 0.6 (i.e., 60% Sn(II)) in (Ba0.4Sn0.6)(Zr0.5Ti0.5)O3 with a calculated metastability being a remarkably high ~0.25 eV atom-1. Reaction energies for decomposition show that the degree of metastability is a function of its composition, with increasing Sn(II) and Zr(IV) concentration leading to higher decomposition energies. Despite these thermodynamic considerations, kinetic stabilization of the highest Sn(II) compositions is achieved by the high cohesive energy of the perovskites containing Zr(IV) and the increased ion diffusion required for decomposition of the mixed Zr(IV)/Ti(IV) compositions. Finally, the underlying relationship between metastability and superior photocatalytic properties for water oxidation is revealed. The smallest bandgaps (e.g., ~1.95 eV for (Ba0.7Sn0.3)(Zr0.25Ti0.75)O3 and ~2.14 eV for (Ba0.4Sn0.6)(Zr0.5Ti0.5)O3) and greatest photocatalytic rates (e.g., ~216 μmol O2 h-1 g-1; AQY ~0.39% under visible light irradiation) are achieved for chemical compositions with the highest calculated metastability.

Metastable perovskite oxides for visible-light-driven water oxidation

ACS Spring Meeting 2021

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