Orthorhombic distortion-induced anatase-like optoelectronic properties of rutile TiO2

Titanium dioxide (TiO2) is an excellent photocatalyst and transparent conducting oxide. It has two major crystal structures: rutile and anatase. Anatase TiO2 is valuable from an industrial point of view because it typically displays better photocatalytic and electronic transport properties than rutile TiO2. To further extract the functional properties of TiO2, understanding the correlation between the electronic structure and the crystal structure is essential. Because the electronic structure strongly depends on the crystal structure, introducing lattice distortion to rutile TiO2 should effectively modulate its electronic structure. Here, we show that Nb-doped rutile TiO2 epitaxial films on (11¯00) α-Al2O3 substrates exhibit anatase-like optoelectronic properties due to orthorhombic lattice distortions and experimentally observe the change in the electronic structure. Reducing the film thickness increases the orthorhombic distortion ratio (b/a) up to 3.4%. As b/a increases, the carrier effective mass decreases from 35 to 3 m0 (m0: electron mass) and the optical bandgap significantly increases. The present observations provide insight into regulating the TiO2 physical properties and should be beneficial for designing TiO2-based photocatalysts and transparent conducting electrodes.