Templated epitaxy of TiO₂(B) on a perovskite

The bronze-phase TiO2(B) has an open atomic framework that makes it a good candidate for applications in photochemical and electrochemical conversion of energy. However, the synthesis of bronze-phase TiO2(B) thin films on perovskite substrates, such as SrTiO3 (STO), which is one of the most conventional versatile substrates for oxide epitaxy, has been extremely challenging owing to the preferential formation of the anatase TiO2 over TiO2(B). The main reason is that the anatase TiO2 has not only a smaller lattice mismatch than TiO2(B), but also a better structural symmetry match when grown on STO. Here, we demonstrate a way to circumventing this problem by using a VO2(B) buffer layer, yielding the growth of a high-quality single crystalline TiO2(B) film on a (001)-oriented STO substrate. From the resulting TiO2(B) film, we found that this film has a large optical bandgap of ∼3.6 eV, which is close to the known theoretical value, the largest among TiO2 polymorphs, and useful for developing high-power energy devices.