Solvation dynamics at liquid/metal-IV oxide interfaces

This thesis presents a comparative study of the ultrafast solvation dynamics of liquids in bulk conditions, and at the interfaces of metal-IV oxides (specifically TiO2 and ZrO2), using ultrafast spectroscopy techniques. In the first part of the thesis we report on results of photon-echo peak-shift and pump super-continuum probe spectroscopy, that provided complementary ways for characterizing the solvation function of a dye, Eosin-Y, in aqueous solution. The solvation dynamics of the dye was studied both, dissolved in an aqueous buffer, and adsorbed to the surface of ZrO2 nanoparticles that were in turn, suspended in an aqueous medium. The results from both techniques indicate that only minor changes between the bulk and interfacial environments, are manifested in the solvation dynamics of Eosin-Y on the fastest timescales of the process (sub-picosecond). On the other hand, on the longest timescales ( >1 ps), we obtained consistent evidences for a slower solvation dynamics at the interface than in bulk water. From these results we concluded, that the presence of the ZrO2 surface affects the dynamics of librational motions and intermolecular vibrations of the hydrogen-bond network of water, only in a very narrow region of no more than 0.5 nm around the metal oxide. The long time behavior, on the other hand, was explained as due to hindered translational diffusion dynamics of the solvent molecules in the proximity of the interface. In order to overcome the limitations inherent to using dyes as probe targets of the solvation dynamics at interfaces, in the second part of this thesis we performed time-resolved Optical Kerr Effect experiments on liquids in the pores of nano-structured films of ZrO2. Three liquids were investigated: acetonitrile (an aprotic polar solvent), cyclohexane (apolar) and water (polar, strongly H-bond networked solvent). In all the cases, significantly slower dynamics were detected in the pores, as compared to the bulk behavior of each solvent. The most significant case was that of water, where the characteristic time constants of the dynamics in the bulk, were increased by a factor of 3 in the film. The results are discussed considering the possible physical models that determine the dynamics of solvation at the interface. In addition to this, using the same experimental setup, we carried out a detailed characterization of the non-resonant nonlinear optical response of nano-structured films of TiO2, by means of Transient Lensing, Cross Phase Modulation measurements, and Optical Kerr Effect spectroscopy. This investigation led us to define future experimental developments, that will allow the extension of our present investigations of solvent dynamics at the surface of ZrO2, to the interfaces of TiO2, of special relevance in several applications.