Infrared Spectroscopic Measurements of Excited Polaronic States of TiO ₂ (110) Rutile and TiO ₂ (101) Anatase Single Crystals

The long-standing differences in the activity of different phases of a given semiconductor for charge transfer reactions are well-known and have been the subject of many studies in particular for the anatase and rutile phases of TiO₂ (the two most studied oxide semiconductor photocatalysts). The many parameters affecting charge transfer in polycrystalline activity, such as the degree of crystallinity, lattice expansion/contraction, pore sizes, and crystallographic orientations, prevent extraction of fundamental properties that may help in designing a given material. Within this context, polarons of model single crystals of two different phases of TiO₂, anatase (101) and rutile (110), were monitored by diffuse reflectance infrared spectroscopy at atmospheric pressure in the temperature range between 300 and 623 K, under UV excitation. The linear signal increases with light fluency, and the sign of its response to excited electron and hole traps sheds light on their polaronic nature. The extracted activation energy from the change of the signal intensity with increasing temperature was found to be much smaller for anatase (101), 80-100 meV, when compared to that of rutile (110), 300-330 meV, which might be linked to the differences in their polaronic nature. Moreover, the signal increase and decay upon light excitation/de-excitation at different temperatures were monitored. It was found that the higher the temperature, the faster the signal decay was. The extracted rate constants for the anatase (101) were found to be on the order of 10^-2 s^-1 with activation energy close to <i>kT</i> values (15 meV). The fast kinetics observed in the single crystals, in sharp contrast to those routinely observed in their polycrystalline counterparts, may be linked to the absence of particle-to-particle charge hopping, as is the case for polycrystalline semiconductors. Based on the obtained results, the high activity of the anatase phase for hydrogen ion reduction when compared to that of the rutile phase can be linked to their different polaronic nature.