Core-shell defective TiO2 nanoparticles by femtosecond laser irradiation with enhanced photocatalytic performance

Engineering defects in titanium dioxide (TiO₂) is becoming increasingly important to enhance its photocatalytic performance by increasing active sites and lowering its band gap from the UV region to the visible light region. Herein, we demonstrate a simple method to create stable surface defects by exposing TiO₂ dispersions to femtosecond laser irradiation. When using an 800 nm wavelength source, little change to the TiO₂ dispersion is observed. However, including the second harmonic of this source (400 nm wavelength) creates stable, dark colored, defective TiO₂, also known as black TiO₂. This occurs without the formation of a rutile phase, previously thought to be necessary for black TiO₂ formation. We demonstrate that the surface oxygen vacancy concentration can be tuned to a maximum of 8% with irradiation time. Beyond this, defect annihilation is observed. Unlike irradiation using longer pulse width lasers (e.g. nanosecond and picosecond lasers), which are known to create more bulk defects, modification using femtosecond pulse widths only modifies a thin shell at the surface, leaving the bulk of the particles unaffected. The most defective TiO₂ exhibits enhanced photocatalytic performance under UV light owing to the formation of defects which act as trapping states for photogenerated electron/hole pairs.