Abstract
Titanium dioxide, the most popular photocatalyst, is inactive under visible light, which limits the practical application of TiO<sub>2</sub> as a solar energy harvesting catalyst. This study investigated the photocatalytic hydrogen production on TiO<sub>2</sub> nanoparticles whose surface was modified in different ways. Dye-sensitized TiO<sub>2</sub>, nafion-coated TiO<sub>2</sub>, CdS/TiO<sub>2</sub> nanocomposites, and surface fluorinated/platinized TiO<sub>2</sub> were prepared and tested for the hydrogen generation under visible or UV irradiation. We synthesized six ruthenium sensitizers having different numbers of carboxylic (c-RuL<sub>3</sub>) or phosphonic (p-RuL<sub>3</sub>) linkage groups, anchored them onto TiO<sub>2</sub> surface, and tested their visible light reactivity for hydrogen production. p-RuL<sub>3</sub> with two phosphonate groups was the most efficient for hydrogen production. On the other hand, Ru(bpy)<sub>3</sub><sup>2+</sup> (as a cationic form) whose bipyridyl ligands were not functionalized with carboxylic acid groups was bound within the nafion layer on TiO<sub>2</sub> through electrostatic attraction. The visible light-sensitized H<sub>2</sub> production on Nf/TiO<sub>2</sub> using Ru(bpy)<sub>3</sub> <sup>2+</sup> was far more efficient than that on c-RuL<sub>3</sub>-TiO<sub>2</sub>. The roles of nafion layer on TiO<sub>2</sub> in the sensitized H<sub>2</sub> 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. TiO<sub>2</sub> nanoparticles sensitized with CdS quantum dots were also investigated for H<sub>2</sub> production. Finally, the simultaneously platinized and fluorinated TiO<sub>2</sub> (F-Pt-TiO<sub>2</sub>) was tested for the generation of hydrogen under UV illumination. The production of hydrogen was negligible with F-TiO<sub>2</sub> and Pt-TiO<sub>2</sub> but was significant with F-Pt-TiO<sub>2</sub> 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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