Possibility of NCS Group Anchor for Ru Dye Adsorption to Anatase TiO₂(101) Surface: A Density Functional Theory Investigation

We examined the possibility of the presence of isothiocyanate (NCS) group anchor for Ru dye adsorption to anatase TiO₂(101) surface in typical dye-sensitized solar cells (DSCs), motivated by the recent X-ray photoelectron spectroscopy (XPS) experimental observations of the NCS anchors. A variety of adsorption configurations were examined with model molecules of CH₃NCS by density functional theory (DFT) calculations. To compare with the experimental XPS spectra, we also calculated core–electron binding energies using (TiO₂)₃₈ cluster model. We demonstrated that 0.6–0.7 eV of the observed chemical shift in the S 2p XPS spectra can be assigned to the S interactions with the surface Ti atom and O vacancy, while 8.8 eV shift in the S 1s level is connected to the S–Ti vacancy interaction. On the other hand, it is also confirmed that the adsorption energies of COOH group and acetonitrile solvent molecule are usually larger than that for the NCS group. Therefore, the Ru dye adsorption through the NCS anchor is less probable even on the surface in vacuum and further decreases in the solution environment. With these results, we conclude that the NCS group anchoring is possible in the vacuum environment as shown by the XPS studies, while it will be negligible on the working electrode of the solar cell