Structure, electronic and optical properties of bilayer anatase nanoribbons

Abstract

The highest surface area anatase TiO2 containing two layers of titanium atoms and exposing facets (0 0 1) is investigated. The material has maximal theoretical surface area of 1080 m2/g, but its stability has been questionable so far. Nanoribbons of this material were considered with the lateral size of 0.4–4.6 nm with varied length of the hydroxylated and non-hydroxylated edges. All the nanoribbons were stable and had flat (0 0 1) surface according to both computational methods used PM6 and SCC-DFTB. The nanoribbons had electronic band gap of around 3.4–3.6 eV and optical band gap of 3.7 eV. They possessed XRD patterns different from those of anatase nanoparticles showing different peaks. Distribution of top orbitals of valence band and bottom orbitals of conduction band as representation for locations of photogenerated holes and electrons showed that orbitals are localized differently depending on the width of nanoribbons, but HOMO and LUMO always occupied different space. Hence, the bilayer nanoribbons are expected to be excellent photocatalysts and sensor materials with good separation of charges. The effect of different conformations of hydroxyl groups on properties of nanoribbons was studied. Interestingly, alignment of all hydroxyl groups in each edge to form hydrogen bonds changes the HOMO, LUMO positions and electronic band gap by up to 0.5 eV, while the optical band gap remained unchanged. It was demonstrated that nanoribbons with even number of layers of Ti atoms had flat structure, while odd-layer nanoribbons showed curvature radius equal to or larger than 3 nm and they can form nanotubes. Given the unique adsorptive and photocatalytic properties of the bilayer nanosheets they are expected to be of great importance in sensors, photocatalysts and adsorbents.