Characterization by resonance Raman spectroscopy of sol–gel TiO2 films sensitized by the Ru(PPh3)2(dcbipy)Cl2 complex for solar cells application

The development of TiO₂ photoelectrochemical cells is largely dependent on the elaboration of efficient dyes molecules to sensitize the nanocrystalline titania surfaces. In the present work, a new system functioning on this basis is proposed: an Ru(II) complex bearing two triphenylphosphine (PPh₃) and one 2,2′-bipyridyl-4,4′-dicarboxylate (dcbipy) chromophoric ligands, deposited on TiO₂ prepared through a simple sol–gel process is tested in a wet regenerative photoelectrochemical cell. The grafting of the complex on TiO₂ surface is characterized using Raman spectroscopy. The measurement of the full-width at half-maximum (FWHM) of the TiO₂ lower frequency E_g Raman line allowed to compare the crystallite sizes of the sol–gel films (13–14 nm) with those of nanocrystalline anatase films (7–8 nm). Very intense Raman bands were observed (ex situ and in situ) for the Ru(PPh₃)₂(dcbipy)Cl₂ complex chemisorbed on TiO₂. The most important vibrations were unambiguously assigned to the PPh₃ or to dcbipy ligands. Altering the potential applied to the electrode for a given laser excitation energy can selectively enhance the vibrational modes of PPh₃. A reversible shift of the dcbipy Raman lines was also observed. The electronic absorption spectrum and the electrochemical data are taken into account in order to explain the obtained results for the chemisorbed dye on both nanocrystalline and sol–gel films.