In situ picosecond transient diffuse reflectance spectroscopy of opaque TiO2 systems under microwave irradiation and influence of oxygen vacancies on the UV-driven/microwave-assisted TiO2 photocatalysis

Abstract

We report a picosecond transient diffuse reflectance study of commercially available pristine Ishihara ST01 titania, which upon treatment with hydrogen gas yields an oxygen-vacancy rich V_O-ST01 system. For comparison, a nitrogen-doped N-ST01 sample was also prepared using urea as the nitrogen donor. These were characterized by XRD and by diffuse reflectance spectroscopy. Transient decay kinetics at 550 nm for all three samples were determined in situ using a 150 ps Nd-YAG pulsed laser system (10 Hz) and a Xe flash lamp (2 μs pulses) probe while samples were being microwave-irradiated (2.45 GHz). The transient(s) absorbing at the probe wavelength displayed double exponential decay kinetics: a fast decay that occurred within ca. 5–12 ns ascribed to recombination of photogenerated shallow-trapped or free conduction band electrons with valence band holes, and a slower decay that occurred from hundreds of nanoseconds to several microseconds attributable to recombination of electrons trapped in deep traps (e.g., either as Ti³⁺ or as F color centers) with free holes. Significant differences were observed for the wet pristine ST01/H₂O and V_O-ST01/H₂O systems when subjected to microwave irradiation; results concurred with those from the degradation of the 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide in aqueous TiO₂ dispersions at 100 °C under UV/microwave irradiation (UV/MW) and UV irradiation with conventional heating (UV/CH).