SrTiO ₃ Sol–Gel Synthesis Enables Enhanced Photocatalytic Overall Water Splitting Activity by Al and/or Ga (co-)Doping and in-situ Formation of a SrTiO ₃ /SrCO ₃ Heterojunction

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. Herein, we address this phenomenon in the context of photocatalytic overall water splitting, and we introduce a sol–gel synthesis that 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 feasible with Al3+ and Ga3+ ions but incomplete at 850 °C and therefore contributing to the formation of a SrCO3 side phase. The highest overall water splitting activity was measured for an Al,Ga:SrTiO3/SrCO3 composite (3.5% Al, 3.5% Ga, and 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.