The effect of calcination on photocatalytic activity of TiO2 particles: femtosecond study

The transient absorption properties of TiO2 nanocrystalline anatase powder samples calcinated at different temperatures were studied by femtosecond diffuse reflectance spectroscopy. The measurements were performed in TiO2 aqueous suspensions in the presence and absence of thiocyanate ions, SCN−. Transient absorption decay behaviour of the TiO2 particles was independent of calcination temperature. Both the shapes of the transient absorption spectra and the decay kinetics were practically the same for all the samples. The multiexponential global fitting analysis applied to the transient absorption data of the TiO2/SCN− system revealed the spectra and the lifetimes of the transient species. After the hole transfer from TiO2 to SCN− the consecutive formation of a weekly coupled dimeric thiocyanate radical anion, (SCN⋯SCN)−, and its structural stabilization to (SCN)2− were observed with time constants of a few picoseconds and few tens to hundreds of picoseconds, respectively. The time constants were dependent on the calcination temperature of the TiO2 particles being the faster the lower was the calcination temperature. Also the relative amounts of the formed (SCN⋯SCN)− and (SCN)2− were dependent on the calcination temperature of the TiO2 samples. The extent of the initial fast hole transfer reaction correlated with the calcination temperature being the more intensive the lower was the calcination temperature. On the other hand, relatively more (SCN)2− were formed at longer timescales the higher was the calcination temperature.