Impact of Photosensitizing Multilayered Structure on Ruthenium(II)-Dye-Sensitized TiO(2)-Nanoparticle Photocatalysts

[Image: see text] To improve the efficiency of photoinduced charge separation on the surface of dye-sensitized TiO(2) nanoparticles, we synthesized the Ru(II)-photosensitizer-immobilized, Pt-cocatalyst-loaded TiO(2) nanoparticles RuCP(2)@Pt–TiO(2), RuCP(2)–Zr–RuP(6)@Pt–TiO(2), and RuCP(2)–Zr–RuP(4)–Zr–RuP(6)@Pt–TiO(2) (RuCP(2) = [Ru(bpy)(2)(mpbpy)](2–), RuP(4) = [Ru(bpy)(pbpy)(2)](6–), RuP(6) = [Ru(pbpy)(3)](10–), H(4)mpbpy = 2,2′-bipyridine-4,4′-bis(methanephosphonic acid), and H(4)pbpy = 2,2′-bipyridine-4,4′-bis(phosphonic acid)) using phosphonate linkers with bridging Zr(4+) ions. X-ray fluorescence and ultraviolet–visible absorption spectra revealed that a layered molecular structure composed of Ru(II) photosensitizers and Zr(4+) ions (i.e., RuCP(2)–Zr–RuP(6) and RuCP(2)–Zr–RuP(4)–Zr–RuP(6)) was successfully formed on the surface of Pt–TiO(2) nanoparticles, which increased the surface coverage from 0.113 nmol/cm(2) for singly layered RuCP(2)@Pt–TiO(2) to 0.330 nmol/cm(2) for triply layered RuCP(2)–Zr–RuP(4)–Zr–RuP(6)@Pt–TiO(2). The photocatalytic H(2) evolution activity of the doubly layered RuCP(2)–Zr–RuP(6)@Pt–TiO(2) was three times higher than that of the singly layered RuCP(2)@Pt–TiO(2), whereas the activity of triply layered RuCP(2)–Zr–RuP(4)–Zr–RuP(6)@Pt–TiO(2) was less than half of that for RuCP(2)@Pt–TiO(2). The photosensitizing efficiencies of these Ru(II)-photosensitizer-immobilized nanoparticles for the O(2) evolution reaction catalyzed by the Co(II)-containing Prussian blue analogue [Co(II)(H(2)O)(2)](1.31)[{Co(III)(CN)(6)}(0.63){Pt(II)(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 H(2) and O(2) evolution reactions involve not only the Ru(II)-complex-TiO(2) interface but also the multilayered structure of the Ru(II)-photosensitizers on the Pt–TiO(2) surface.