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 degrees C for 24 h within a low-melting SnCl2/SnF2 salt, the new (Ba1-xSnx)Nb4O15 (x = 0-0.5; P (3) over bar m1; a = 5.79650(5) angstrom, c = 11.79288(8) angstrom; Z = 2) has been prepared in high purity with up to similar 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, similar to 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, similar to 77 mu mol O-2 h(-1) g(-1) (visible irradiation) and, similar to 159 mu mol O-2 h(-1)g(-1) (UV-vis irradiation), with apparent quantum yields of similar to 0.35 and 0.52%, respectively. By comparison, pure Ba5Nb4O is 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 5s(2) valence orbitals. Thus, visible-light bandgap excitation occurs from electronic transitions predominantly involving the Sn(II) (5s(2)) to Nb(V) (4d(0)) 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.

Automated summary

A new strategy has been developed to incorporate Sn(II) cations into the structure of a photocatalytic oxide, Ba5Nb4O15, by leveraging the asymmetric coordination environments. This strategy enables the reduction of the band gap and the enhancement of photocatalytic activity under visible light by introducing Sn(II) cations. The study demonstrates the powerful utility of low-temperature Sn(II)-exchange reactions to sensitize layer-type oxide photocatalysts to the visible region of the solar spectrum.