COLLOIDAL TiO2 NANORODS FOR PHOTOCATALYSIS: A FEMTOSECOND TRANSIENT ABSORPTION STUDY

Concept: The excitation-dependent relaxation dynamics of charge carriers in anatase TiO2 nanorods (NRs) was investigated by femtosecond transient absorption spectroscopy. This technique is a widely known effective tool with unique capabilities for elucidating the electron-hole (e--h+) recombination dynamics of nanocrystals. Colloidal dispersions of such anisotropic nanocrystals were excited in the UV−vis range using three different pump wavelengths, i.e. above, close to, and below the direct band gap of anatase. We show that the ultrafast dynamics strongly depends on the excitation wavelength, and influences most of the processes contributing to the relaxation dynamics. Motivations and objectives: TiO2 nanocrystals are successfully exploited in applications related to energy conversion, such as photocatalysis and photovoltaics, thanks to their ability to generate e--h+ pairs under proper lighting. In addition, the rod-like nanocrystal shape magnifies this effect, due to a larger surface/volume ratio and a higher number of active sites of interaction with the environment. The conversion efficiency is correlated to the lifetimes of e- and h+ before their recombination, which in turn are influenced by several factors. Excitation energy plays a major role, as it defines the initial potential of the carriers. An excitation-dependent investigation of the charge carrier dynamics is thus expected to provide major information crucial for understanding their reactivity and for further rationalizing their behavior in photocatalytic applications. Results and discussion: TiO2 NRs were synthesized by a colloidal chemistry route and dispersed in an organic solvent, thanks to oleate ions coordinating the nanocrystal surfaces. We performed pump-probe experiments in a weak-excitation regime by pumping at 300, 350, and 430 nm and probing in a broadband spectral range extending from 450 to 750 nm. The temporal evolution of photoinduced absorption changes was found to be strongly dependent on the excitation conditions, both at short and long time delays. Nonetheless, the initial charge carriers trapping in surface defect states occurs very rapidly after the photogeneration in all investigated cases. The two distinct TA bands at 500 and 700 nm, typically attributed to trapped h+ and e- in anatase, are accessible only when TiO2 nanorods are photoexcited well above the band gap, while there is no evidence of such bands when excitation occurs close to or below the band gap. In such cases the observed dynamics are attributed to excitonic states. This evidence is also supported by the persistence of a long-lasting TA contribution ascribable to such bound states.