Influence of Water on the Electronic and Molecular Surface Structures of Ru-Dyes at Nanostructured TiO₂

The influence of water on the surface electronic and molecular properties of three Ru-dyes adsorbed at nanostructured TiO 2 was investigated using photoelectron spectroscopy (PES). The dyes investigated were the Ru(dcbpy) 2 (NCS) 2 in its acid (N3) and in its 2-fold deprotonated form (N719) as well as a similar dye (Z-907) containing the hydrophobic ligand 4,4′-dinonyl-2,2′-bipyridine. Trends in surface structures depending on water exposure were followed for the three dyes. The results showed that the hydrophobic chains of the Z-907 dye effectively inhibit surface reorganization while large changes in surface electronic and molecular structure were observed for the N3 and N719 molecular layers. Specifically, large effects involving the thiocyanate ligands were detected, and the S2p and N1s core level spectra indicate that the changes involve mixing of only two dominating surface configurations. Moreover, the PES results also showed water-induced changes in the energy level matching between the dye and the TiO 2, and water induced desorption of the TBA + counterion. Basic photoelectrochemical trends depending on water exposure to dye-sensitized solar cell systems were also verified.