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

Abstract

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.