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)-photosensitizerimmobilized, 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-), 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 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/cm(2) for singly layered RuCP2@Pt-TiO2 to 0.330 nmol/cm(2) for triply layered RuCP2-Zr-RuP4-Zr-RuP6@Pt-TiO2. The photocatalytic H-2 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 f or the O-2 evolution reaction catalyzed by the Co(II)-containing Prussian blue analogue [Co-II(H2O)(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-TiO2 interface but also the multilayered structure of the Ru(II)-photosensitizers on the Pt-TiO2 surface.