Impact of Photosensitizing Multilayered Structure on Ruthenium(II)-Dye-Sensitized TiO2‑Nanoparticle Photocatalysts

To improve the efficiency of photoinduced charge separation on the surface of dye-sensitized TiO2 nanoparticles, we synthesized the Ru­(II)-photosensitizer-immobilized, Pt-cocatalyst-loaded TiO2 nanoparticles RuCP2@Pt–TiO2, RuCP2–Zr–RuP6@Pt–TiO2, and RuCP2–Zr–RuP4–Zr–RuP6@Pt–TiO2 (RuCP2 = [Ru­(bpy)2(mpbpy)]2–, RuP4 = [Ru­(bpy)­(pbpy)2]6–, RuP6 = [Ru­(pbpy)3]10–, H4mpbpy = 2,2′-bipyridine-4,4′-bis­(methanephosphonic acid), and H4pbpy = 2,2′-bipyridine-4,4′-bis­(phosphonic acid)) using phosphonate linkers with bridging Zr4+ ions. X-ray fluorescence and ultraviolet–visible absorption spectra revealed that a layered molecular structure composed of Ru­(II) photosensitizers and Zr4+ ions (i.e., RuCP2–Zr–RuP6 and RuCP2–Zr–RuP4–Zr–RuP6) was successfully formed on the surface of Pt–TiO2 nanoparticles, which increased the surface coverage from 0.113 nmol/cm2 for singly layered RuCP2@Pt–TiO2 to 0.330 nmol/cm2 for triply layered RuCP2–Zr–RuP4–Zr–RuP6@Pt–TiO2. The photocatalytic H2 evolution activity of the doubly layered RuCP2–Zr–RuP6@Pt–TiO2 was three times higher than that of the singly layered RuCP2@Pt–TiO2, whereas the activity of triply layered RuCP2–Zr–RuP4–Zr–RuP6@Pt–TiO2 was less than half of that for RuCP2@Pt–TiO2. The photosensitizing efficiencies of these Ru­(II)-photosensitizer-immobilized nanoparticles for the O2 evolution reaction catalyzed by the Co­(II)-containing Prussian blue analogue [CoII(H2O)2]1.31[{CoIII(CN)6}0.63{PtII(CN)4}0.37] decreased as the number of Ru­(II)-photosensitizing layers increased. Thus, crucial aspects of the energy- and electron-transfer mechanism for the photocatalytic H2 and O2 evolution reactions involve not only the Ru­(II)-complex-TiO2 interface but also the multilayered structure of the Ru­(II)-photosensitizers on the Pt–TiO2 surface.