New Pyrochlore-like Oxyfluorides Na2-2xSnxM2O5F2 (M = Nb5+ or Ta5+ and 1 ≤ x ≤ 0) as Potential Candidates for Overall Water Splitting Photocatalysis

Lone pair containing oxyfluorides have recently emerged as efficient and stable photocatalysts for overall water splitting (OWS). In this work we have obtained several new potential OWS photocatalysts with pyrochlore structure (i.e., Na2-2xSnxM2O5F2 with M = Nb5+ or Ta5+ and 1 ≤ x ≤ 0) through kinetically controlled ion-exchange reaction from Na2M2O5F2. Rietveld refinement of the structures as well as DFT indicate the Sn2+ lone pair stereochemical activity. The partial density of states reveals contributions at the top of the valence band that are mainly composed of a Sn 5s-Sn 5p hybridization through the (O,F) 2p orbitals. This leads to a significant narrowing of the band gap and an improvement of the photoconduction response by a factor ca. 50 with respect to the x = 0 compound, Na2Nb2O5F2. The Mott-Schottky measurements show that all materials possess band edge positions encompassing the thermodynamic redox potential of H+/H2 and O2/H2O. However, the valence band of SnNb2O5F2 may not be oxidative enough to overcome the overpotential associated with the O2- oxidation and hence could be unsuitable for OWS photocatalysis in contrast with Na1.5Sn0.25Nb2O5F2.