Insights into dye-sensitization of planar TiO₂: Evidence for involvement of a protonated surface state

The pH dependence of sensitized photocurrent for porphyrin-derivatized planar TiO₂ films in regenerative photoelectrochemical cells is reported. The porphyrin sensitizers 5-(4-carboxyphenyl)-10,15,20-trimesitylporphinatozinc(II) (1), 5-(4-carboxyphenyl)-10,15,20-trimesitylporphine (2), and 5-(4-carboxyphenyl)-10,15,20-trimesitylporphinatoplatinum(II) (3) were studied. The TiO₂ conduction band edge potential (E_CB) shifted from -0.43 V to +0.16 V vs Ag/AgCl_(aq) from pH 12 to pH 2, a shift of 59 ± 2 mV per pH unit. Excited-state potentials (E_1/2^+/*) of 1-3 ranged from -1.57 to -0.91 V vs Ag/AgCl_(aq), well negative of E_CB. Nonetheless, for all three porphyrins a 10-fold increase in the magnitude of sensitized photocurrent was observed upon acidification of the electrolyte from pH 12 to pH 2. Photocurrent vs pH data did not depend on the sensitizer excited-state potential. A model is proposed wherein protonation of a surface state is required for charge compensation and photocurrent production. Therefore, the magnitude of sensitized photocurrent is determined by surface protonation/deprotonation equilibria.