Titanium dioxide, the most popular photocatalyst, is inactive under visible light, which limits the practical application of
TiO2 as a solar energy harvesting catalyst. This study investigated the photocatalytic hydrogen production on TiO2
nanoparticles whose surface was modified in different ways. Dye-sensitized TiO2, nafion-coated TiO2, CdS/TiO2
nanocomposites, and surface fluorinated/platinized TiO2 were prepared and tested for the hydrogen generation under
visible or UV irradiation. We synthesized six ruthenium sensitizers having different numbers of carboxylic (c-RuL3) or
phosphonic (p-RuL3) linkage groups, anchored them onto TiO2 surface, and tested their visible light reactivity for
hydrogen production. p-RuL3 with two phosphonate groups was the most efficient for hydrogen production. On the other
hand, Ru(bpy)32+ (as a cationic form) whose bipyridyl ligands were not functionalized with carboxylic acid groups was
bound within the nafion layer on TiO2 through electrostatic attraction. The visible light-sensitized H2 production on
Nf/TiO2 using Ru(bpy)32+ was far more efficient than that on c-RuL3-TiO2. The roles of nafion layer on TiO2 in the
sensitized H2 production are proposed to be two fold: to provide binding sites for cationic sensitizers and to enhance the
local activity of protons in the surface region. TiO2 nanoparticles sensitized with CdS quantum dots were also
investigated for H2 production. Finally, the simultaneously platinized and fluorinated TiO2 (F-Pt-TiO2) was tested for the
generation of hydrogen under UV illumination. The production of hydrogen was negligible with F-TiO2 and Pt-TiO2 but
was significant with F-Pt-TiO2 even in the absence of organic electron donors. The hydrogen production was highly
enhanced in the presence of 4-chlorophenol, which realized the simultaneous degradation of organic substrates and the
production of hydrogen.
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