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 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.