Anja Hofmann

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 relationship in photocatalytic hydrogen (H2) evolution. In Sr2TiO4, the 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, 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 limits beneficial fluorination effects. 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 symmetry and photocatalytic performance in RP-type titanates.

SrTiO3 is a well-known photocatalyst for overall water splitting due to its suitable band edge positions and perovskite structure, which allows for versatile doping strategies that alter electronic or optical properties. However, side phase formation in SrTiO3 syntheses is common and SrCO3 formation on SrTiO3 surfaces is known to be ubiquitous and independent of the synthesis method. The influence of SrTiO3/SrCO3 heterojunctions should therefore be considered for any photocatalytic application. Herein, we address this phenomenon in the context of photocatalytic overall water splitting, and we introduce a new sol-gel synthesis which allows for more control over SrCO3 formation and lower calcination temperatures than common SrTiO3 molten salt syntheses. We show how aliovalent (co-)doping at the Ti4+ lattice position is incomplete at 850 °C using higher amounts of dopant, therefore contributing to the formation of a SrCO3 side phase. However, doping with larger ions (Ga3+) as well as smaller ions (Al3+) is generally feasible at this temperature. The highest overall water splitting activity was measured for Al,Ga:SrTiO3/SrCO3 (3.5% Al, 3.5% Ga, 3% Sr excess), comparable to the activity of Al:SrTiO3/SrCO3 obtained from a common molten salt approach while saving 71% in energy consumption during calcination. This direct approach of doping and in-situ formation of a beneficial SrTiO3/SrCO3 heterojunction was transferred to a calcination temperature as low as 550 °C, allowing for the synthesis of porous composite materials.