Structural origins of photocatalytic properties in Ruddlesden-Popper Srn+1TinO3n+1 (n = 1, 2) and their topochemically fluorinated phases Srn+1TinO(3n+1)-xF2x (x ≈ n)

Abstract

We present a comparative study of n = 1 and n = 2 Ruddlesden-Popper (RP) titanates Sr2TiO4 and Sr3Ti2O7, as well as their oxyfluorides Sr2TiO3F2 and Sr3Ti2O5F4, synthesized via low-temperature topochemical fluorination to study structure–property relationships in photocatalytic hydrogen (H2) evolution. In Sr2TiO4, fluorination lowers the symmetry from I4/mmm to P4/nmm, placing Ti in a locally non-centrosymmetric site and producing ordered, asymmetric TiO5F units. By contrast, Sr3Ti2O7 retains its tetragonal I4/mmm symmetry upon fluorination with Ti remaining in a locally non-centrosymmetric environment. Photocatalytic H2 evolution shows a ∼6.3-fold enhancement for Sr2TiO3F2 compared to Sr2TiO4, which might be attributed to inversion symmetry breaking and the resulting internal local dipole fields that promote charge separation. In contrast, Sr3Ti2O7 exhibits much higher intrinsic activity than Sr2TiO4 (∼30-fold), likely governed by dimensionality, while its fluorinated phase Sr3Ti2O5F4 shows suppressed performance due to the absence of local polarity, which limits beneficial fluorination effects and diminishes charge-separation efficiency, as supported by transient absorption spectroscopy (TAS) results. TAS reveals distinct charge-carrier dynamics, with Sr3Ti2O7 showing the highest photogenerated electron population and Sr2TiO3F2 exhibiting enhanced long-lived electrons, consistent with improved charge separation. Density functional theory (DFT) calculations show that fluorination deepens the O 2p valence band, slightly raises the Ti 3d conduction edge, and reduces c-axis dispersion, consistent with the UV-Vis and Butler–Ginley analyses. The resulting increase in electron effective mass suppresses charge mobility along the c-axis, lowering conduction dimensionality. These findings establish anion sublattice engineering as a selective route to tune local symmetry, charge-carrier dynamics, and photocatalytic performance in RP-type titanates.