Morphology and oxygen vacancy investigation of strontium titanate-based photo electrochemical cells

In this paper single band gap photo electrochemical cells (PECs) are presented, which consist of strontium titanate (SrTiO₃) photo anodes on nickel cathodes in potassium hydroxide electrolyte. SrTiO₃ powders are deposited on nickel substrates by electrophoresis before sintering with varying temperatures, times, cooling rates, gas types, and gas flow rates. The external quantum efficiency (EQE) of such PECs mainly depends on the morphology and the amount of oxygen vacancies in SrTiO₃ lattice. At first, the morphology is investigated, which can be adjusted by the particle size as well as the sinter temperature and time. Nanopowder-based PECs sintered above the starting sinter temperature indicate the best charge carrier transport and hence allow high EQEs. The sinter time influences the specific surface area, but not the EQE in this investigation. Secondly, the generation of oxygen vacancies is investigated, which depends on the oxygen partial pressure and the equilibration temperature. Low oxygen partial pressures and high equilibration temperatures increase the amount of oxygen vacancies, which can be set by the gas type and its flow rate or the cooling rate and an additional heating step, respectively. It can be shown that PECs have to possess a low amount of oxygen vacancies to reach high EQE values, but not too low to allow for sufficient conductivity. This point is shown through our finding that the samples with lower and higher concentrations exhibit very low photo activity. Oxygen vacancies can be considered as intrinsic donors and hence increase electrical conductivity which is necessary but also act as recombination centers. For SrTiO₃ nanopowder-based samples, which have been sintered at 1200 °C for 20 min with a cooling rate of 10 K/s in reducing gas (with 5 vol% H₂) and a low flow rate of 1.7 l/h, very high external quantum efficiencies of 64.2 % under 365 nm illumination can be achieved.