Brookite TiO₂ Thin Film Epitaxially Grown on (110) YSZ Substrate by Atomic Layer Deposition

Epitaxial brookite TiO₂ (B-TiO₂) film was deposited on (110) yttria-stabilized zirconia (YSZ) substrate using plasma-enhanced atomic layer deposition, and its structural, optical, and gas sensing properties were investigated. As-deposited TiO₂ film was a pure brookite and (120) oriented. The determined in-plane orientation relationships were [21̅0]_B–TiO2//[1̅10]_YSZ and [001]_B–TiO2 //[001]_YSZ. The B-TiO₂ film showed ∼70% transmittance and the optical band gap energy was 3.29 eV. The B-TiO₂ film-based gas sensor responded to H₂ gas even at room temperature and the highest magnitude of the gas response was determined to be ∼150 toward 1000 ppm of H₂/air at 150 °C. In addition, B-TiO₂ sensor showed a high selectivity for H₂ against CO, EtOH, and NH₃.