Further Understanding of the Adsorption Mechanism of N719 Sensitizer on Anatase TiO2 Films for DSSC Applications Using Vibrational Spectroscopy and Confocal Raman Imaging

Vibrational spectroscopic studies of N719 dye-adsorbed TiO2 films have been carried out by using SERRS, ATR-FTIR, and confocal Raman imaging. The high wavenumber region (3000-4000 cm(-1)) of dye adsorbed TiO2 is analyzed via Raman and IR spectroscopy to investigate the role of surface hydroxyl groups in the anchoring mode. As a complementary technique, confocal Raman imaging is employed to study the distribution features of key dye groups (COO-, bipyridine, and C=O) on the anatase surface. Sensitized TiO2 films made from two different nanocrystalline anatase powders are investigated: a commercial one (Dyesol) and our synthetic variety produced through aqueous synthesis. It is proposed the binding of the N719 dye to TiO2 to occur through two neighboring carboxylic acid/carboxylate groups via a combination of bidentate-bridging and H-bonding involving a donating group from the N719 (and/or Ti-OH) units and acceptor from the Ti-OH (and/or N719) groups. The Raman imaging distribution of COOsym- on TiO2 was used to show the covalent bonding, while the distribution of C=O mode was applied to observe the electrostatically bonded groups.