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.