Single-crystalline ultrathin 2D TiO2 nanosheets: A bridge towards superior photovoltaic devices

Ultrathin two-dimensional (2D) TiO2 nanostructures with atomic-level thickness are urgently demanded to achieve high-performance photocatalytic, photovoltaic, and electrochemical devices, by taking advantage of their unique characteristics of large surface-to-volume ratio, distinctive electronic properties, and intriguing chemical reactivity. Here, we present a molecular self-assembly of single-crystalline ultrathin 2D TiO2 nanosheets from a micelle-assisted solution. Via this synthesis strategy, highly-homogeneous ultrathin 2D TiO2 nanosheets with a thickness around 3 nm were obtained with relatively large production. The ultrathin 2D TiO2 nanosheet composite has a dominant anatase phase and a trace of rutile phase. It is interesting that highly reactive, non-equilibrium surfaces, such as the anatase {010} and rutile {001} facets, were observed in the fabricated nanosheets, which are very difficult to obtain by conventional synthesis methods but extremely useful for improving the performance of TiO2 nanostructures. Owing to the unique features of single-crystallinity, atomic-level thickness, highly reactive exposed surfaces, etc., the ultrathin 2D TiO2 nanosheets possess superior photochemical properties. With photoanodes made from ultrathin 2D TiO2 nanosheets, dye-sensitized solar cells showed significantly enhanced efficiency (8.28%) compared to ones with commercial Degussa P25 TiO2 nanoparticle anode (5.12%). This new family of graphene-like TiO2 nanosheets thus paves the way to further improving the performance of TiO2-based energy conversion and storage devices.