(Invited) Time Resolved Spectroscopy of Charge Carriers in TiO2 Anatase and Rutile Single Crystals. Effect of Oxygen Defects and Electrons/Holes Scavengers

Abstract

In an effort to probe into the fundamentals of charge carriers dynamics relevant to photo-catalytic materials, we have performed femtosecond time resolved spectroscopy (TRS) on anatase and rutile single crystals in the gas phase as well as in aqueous environment. By studying the effect of pump fluence, differences in charge carrier recombination rates and nature between both crystals were observed. The time constants for carrier recombination are two orders of magnitude slower for anatase (101) when compared to those of rutile (110) single crystal. Bulk defects introduced by sample reduction via annealing in ultra-high vacuum resulted in faster recombination rates for both polymorphs. Both surfaces (fresh and reduced) probed by pump fluence dependence measurements revealed that the major recombination channel in fresh and reduced anatase and reduced rutile is the first-order Shockley–Reed mediated. The effect of a hole trapping agent (ethanol) and of electron trapping agents (Ag+ and Ce4+ cations) is investigated over TiO2(110) rutile single crystal. Ethanol adsorption resulted in increasing the transient absorption signal in the 700-950nm with a slight decrease of their lifetime. No change was seen in the 500nm region commonly attributed to the presence of holes. The decay of the 700-950 nm signal mainly composed of two components is considerably perturbed by the presence of Ag+ or Ce4+ cations, which may indicate its surface origin. Work in progress to extract kinetic information from the study of the concentration effect of these metal cations in both the “nominally” hole and electron transient absorption regions.