Photocatalytic hydrogen production using surface-modified titania nanoparticles

Abstract

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)₃ ²⁺ 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.