Structure, Stability, and Photocatalytic Activity of a Layered Perovskite Niobate after Flux-Mediated Sn(II) Exchange

A new strategy to incorporate the Sn(II) cation and its stereoactive lone pair into the structure of a photocatalytic oxide has been achieved by leveraging the asymmetric coordination environments within the (111)-oriented perovskite-type layers of Ba₅Nb₄O₁₅. This layered perovskite represents one of the few known photocatalysts capable of efficiently splitting water, but its activity is restricted to ultraviolet radiation owing to its large band gap. By reacting this layered niobate at 350 °C for 24 h within a low-melting SnCl₂/SnF₂ salt, the new (Ba_1-Sn)Nb₄O₁₅ (x = 0-0.5; P3̅m1; a = 5.79650(5) Å, c = 11.79288(8) Å; Z = 2) has been prepared in high purity with up to ∼50% Sn(II) cations. Statistical disordering of the Sn(II) cations was probed by neutron diffraction Rietveld refinements and found to occur predominantly over the asymmetric cation sites, Ba2 and Ba3, for the 40% Sn(II) composition of x = 0.4. An increasing Sn(II) amount significantly red-shifts the band gap (E_g) from 0% Sn for x = 0 (3.78 eV; ultraviolet, indirect) to 40% Sn for x = 0.4 (E_g = 2.35 eV; visible, indirect), as found by UV-vis diffuse reflectance. Density functional theory calculations show an increasing metastability, i.e., a thermodynamic instability toward decomposition to the simpler oxides SnO, Nb₂O₅, and SnNb₂O₆. A synthetic limit of ∼50% Sn(II) cations can be kinetically stabilized under these reaction conditions. For the highest Sn(II) amounts, photocatalytic rates are observed for the production of molecular oxygen from water of up to ∼77 μmol O₂ h^-1 g^-1 (visible irradiation) and ∼159 μmol O₂ h^-1 g^-1 (UV-vis irradiation), with apparent quantum yields of ∼0.35 and 0.52%, respectively. By comparison, pure Ba₅Nb₄O₁₅ exhibits no measurable photocatalytic activity under visible-light irradiation. Electronic structure calculations show that the decreased band gap stems from the introduction of the Sn(II) cations and the formation of a higher-energy valence band arising from the filled 5s² valence orbitals. Thus, visible-light bandgap excitation occurs from electronic transitions predominantly involving the Sn(II) (5s²) to Nb(V) (4d⁰) cations. This study demonstrates the new and powerful utility of low-temperature Sn(II)-exchange reactions to sensitize layer-type oxide photocatalysts to the visible region of the solar spectrum, which is facilitated by exploiting their asymmetric cation environments.