Unveiling Stability Factors in Sn(II)-Containing Oxides: Discovery of a Polar Tin Titanate Perovskite and Photocatalytic Activity for Overall Water Splitting

The discovery of multinary Sn(II)-containing oxides has been severely limited by a lack of understanding of the factors leading to their thermodynamic stability, e.g., chemical compositions and structure types, as well as by the absence of productive synthetic routes. The relatively few reported Sn(II)-O-M (M = early transition metal cation) solids frequently decompose at moderate to low temperatures. Herein, a large-scale predictive modeling approach was used to assess the structural factors yielding their enhanced thermodynamic stability. This has resulted in ten predicted new Sn(II)-containing oxides that are proposed to fall within reasonable synthetic limits. Increasing stability was found for structures possessing smaller amounts of Sn(II) with local asymmetric coordination environments allowing expression of its stereoactive lone pair. As a test of these results, synthetic efforts to prepare one of the proposed compounds starting from BaLa4Ti4O15 yielded the predicted noncentrosymmetric layered perovskite SnLa4Ti4O15 (SLTO). The new SLTO crystallizes with hexagonal plate-shaped morphologies in the polar P3c1 space group (No. 158), as confirmed by Rietveld refinements of powder X-ray diffraction data and second harmonic generation activity. Full Sn(II) substitution was confirmed by 119Sn Mössbauer spectroscopy, SEM-EDS, and X-ray photoelectron spectroscopy. UV-vis diffuse reflectance data confirmed that SLTO has a visible-light bandgap of ~2.4 eV and is thus predicted to be promising photocatalyst for solar energy conversion. After loading its surfaces with a Rh/Cr2O3-CoOx dual-cocatalyst, SLTO is the first Sn(II)-containing oxide to show activity for overall water splitting into H2 and O2 with an apparent quantum yield of ~21.7%. Thus, these results highlight the synergistic combination of chemical intuition, predictive modeling, and synthetic design in the synthesis of new Sn(II)-containing oxides for promising optical properties and photocatalytic activities for water splitting.