Influence of TiO₂ surface defects on the adsorption of N719 dye molecules

Surface defects influence the dye adsorption on TiO₂ used as a substrate in dye-sensitized solar cells (DSSCs). In this study, we have used different Ar⁺ sputtering doses to create a controlled density of defects on a TiO₂ surface exposed to different pre-heating temperatures in order to analyse the influence of defects on the N719 dye adsorption. TiO₂ was pre-treated using two different treatments. The first treatment involved heating to 200 °C with subsequent sputtering at different doses. The second treatment included heating only, but at four different temperatures starting at 200 °C. After the pre-treatments, the TiO₂ samples were immersed into an N719 dye solution for 24 hours at room temperature to dye the TiO₂ substrates. The amount of Ti³⁺ surface defects introduced by the different pre-treatments and their influence on dye adsorption onto the TiO₂ surface were examined by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES). Neutral impact collision ion scattering spectroscopy (NICISS) was used to determine the coverage of the TiO₂ surface by adsorbed dye molecules. It was found that Ti³⁺ surface defects were formed by Ar⁺ sputtering but not by pre-treatment through heating alone. MIES analysis of the outer-most layer and density of states calculations show that the thiocyanate ligand of the N719 dye becomes directed away from the TiO₂ surface. Both XPS and NICISS results indicate that the amount of adsorbed N719 dye decreases with increasing density of Ti³⁺ surface defects. Thus, the generation of surface defects reduces the ability of the TiO₂ surface to adsorb the dye molecules. Heating alone as pre-treatment of the TiO₂ substrates instead increases the dye adsorption, without causing detectable defects on the TiO₂ surface.