Temperature dependent photoluminescence of photocatalytically active titania nanopowders

Temperature photoluminescence (PL) in the sub-ambient range has been developed as a tool for characterizing photocatalytic materials. The use of well-characterized TiO2 nanoparticles with calibrated particle sizes allowed to face the temperature dependent PL results with the photocatalytic activity and several important physico-chemical parameters. In the relaxation of the photoexcited electron/hole pairs, the transfer towards surface sites is in competition with radiative and non-radiative recombinations. Temperature dependent PL appears thus to be a very sensitive technique to study the efficiency of the electron or the hole trapping at the surface of titania nanoparticles where they take part to the oxidation/reduction reactions. The PL shows a peak around 610 nm with a width of about 130 nm. The quenching of this emission is measured as a function of the temperature, from 9.9 up to 230 K. The kinetics of this phenomenon can be described by fitting the temperature dependent PL results with a double activation law while a simple activation model fails. It suggests that two ways are involved in the trapping process of the carriers, with activation energies in the range of a few meV and a few tenths of meV, respectively. The relationship between the PL parameters linked to the two different energetic processes associated to two energy barriers (activation energy and transition rate), the powder characteristics and the photoactivity is discussed.