Novel SrTiO3 Sol-gel Synthesis Enables Enhanced Overall Water Splitting Activity by Al and/or Ga (co-)doping and in-situ Formation of a SrTiO3/SrCO3 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. 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.