A facile strategy to fabricate high-quality single crystalline brookite TiO₂ nanoarrays and their photoelectrochemical properties.

Vertically aligned high-quality single crystalline brookite TiO2 nanoarrays were synthesized for the first time using an environmentally benign one-step hydrothermal reaction. They have a unique bullet-shaped structure which has a length of 700-1000 nm and a width of 150-250 nm with a sharpened tip structure. By adjusting the concentration of NaOH in hydrothermal reaction, we could also synthesize other types of TiO2 nanostructures including anatase TiO2 nanotubes/nanowires. The morphologies and crystal structures of the products were confirmed by scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. Their vertically aligned structures facilitate their application as photoanodes in photoelectrochemical cells, and the photoelectrochemical properties such as photocurrent density and open circuit voltage were measured in a three-electrode electrochemical cell with TiO2 nanoarrays, Ag/AgCl and a Pt flag as the working, reference and counter electrodes, respectively, incorporating a 0.1 M NaOH electrolyte solution. The fabricated brookite TiO2 nanoarrays exhibited a highly enhanced photocurrent density and a longer electron lifetime compared with anatase TiO2 nanoarrays with similar lengths.