UV induced local heating effects in TiO2nanocrystals

Abstract

When isolated TiO₂ nanocrystals are subjected to UV light at 77 K and pressures below 10⁻⁶ mbar, trapping of photogenerated hole centers occurs on the surface of the nanocrystals and can be tracked by time-resolved electron paramagnetic resonance spectroscopy. Irrespective of the selected UV irradiance used, the maximum concentration of trapped charges was found to be constant for a given number of nanocrystals (∼10¹⁵) and corresponds to one electron–hole pair per particle. On a time scale of seconds to minutes the dynamics for the trapping process depend on the number of photons with supra band gap energy. A local temperature rise of the TiO₂ nanocrystals was observed for irradiances above 1.55 mW cm⁻² (10¹⁵ photons cm⁻² s⁻¹). This is attributed to enhanced nonradiative recombination of photogenerated charge carriers via heat production and points to a substantial contribution of thermal chemistry in photocatalytic reaction cycles.