Carboxylic Anchoring Dye p‑Ethyl Red Does Not Adsorb Directly onto TiO2 Particles in Protic Solvents

Adsorption of the carboxylic anchoring dye, p-ethyl red (p-ER), onto TiO2 nanoparticles in protic vs aprotic solvents was studied in situ using the surface-specific technique, second harmonic light scattering (SHS). Two different adsorption schemes were proposed to account for p-ER interactions with TiO2 under different solvent environments. In aprotic solvents, p-ER adsorbs directly onto TiO2. Conversely, in protic solvents, in which solvent molecules bind stronger than p-ER with TiO2, the dye molecules adsorb onto the solvent shell around the particle but not directly to the TiO2 surface. In addition, a portion of the p-ER molecules form hydrogen bonds with the protic solvent molecules. The two different adsorption models reproduce the measured adsorption isotherms detected by SHS. Specifically, the p-ER molecules adsorb with a smaller free energy change and a larger density in protic solvents than in aprotic solvents. Our results indicate that protic solvents are undesirable for administering adsorption of carboxylic dyes in dye-sensitized solar cell applications as the dye molecules do not directly adsorb onto the TiO2 particle.