Unique Effects of Iron(III) Ions on Photocatalytic and Photoelectrochemical Properties of Titanium Dioxide

Abstract

Photocatalytic oxidation of water on TiO2 (rutile) powder proceeded with a fairly high efficiency (about 9%) when iron(III) ions were used as the electron acceptor. The reaction continued until all iron(III) ions added to the solution were reduced into iron(II) ions. This behavior was in marked contrast to other reversible photocatalytic reactions, whose reaction rates decelerate as the result of the back reactions of the products on the photocatalysts. The efficient oxidation of water in the presence of iron(III) ions was attributed to preferential adsorption of iron(III) ions on TiO2 over iron(II) ions, which enabled efficient oxidation of water, although this reaction was thermodynamically less favorable than oxidation of iron(II) ions. Furthermore, from the measurements of photocurrents at crystalline TiO2 electrodes, iron(III) ions were concluded to have a catalytic function for the oxidation of water on photoirradiated TiO2.

Review of the anatase to rutile phase transformation

“…These studies involved highsurface-area rutile of acicular morphology [25], rutile containing residual anatase [78], and iron-doped rutile [81]. It is possible that electron transfer between rutile and a residual quantity of anatase [78] may facilitate improved photo-oxidative reactions, as in mixed-phase titania catalysts.…”

“…Quantification of phase proportions usually is carried out by X-ray diffraction (XRD) [1,78,81,105,[120][121][122][123][124][125]. Such analyses often are done using the method of Spurr and Myers [126], which utilises the ratio of the rutile (110) peak at 27.355°2h to the anatase (101) peak at 25.176°2h.…”

Overall Water Splitting under Visible Light through a Two‐Step Photoexcitation between TaON and WO3 in the Presence of an Iodate–Iodide Shuttle Redox Mediator

“…Thus, to construct a two-step water splitting system, it is necessary to develop a photocatalytic system with a high selectivity for the forward reactions (solid arrows in Scheme 2). [13] Using this strategy, we have developed TiO 2 -rutile and Ptloaded WO 3 photocatalysts that possess high selectivity for the oxidation of water, allowing selective O 2 evolution even in the presence of both IO 3 À and I À anions. [7,8] We have demonstrated two-step water splitting under visible light by combining the Pt-WO 3 photocatalyst with a SrTiO 3 (Cr, Ta-doped) photocatalyst [14] in the presence of an iodate-iodide (IO 3 À /I À ) shuttle redox mediator.…”

Development of a New System for Photocatalytic Water Splitting into H2 and O2 under Visible Light Irradiation

“…These results clearly indicate that anatase and rutile have a different reactivity in the competitive oxidation of I ¹ and H 2 O. Ohno and co-workers have also reported the preferential oxidation of H 2 O over rutile TiO 2 photocatalysts in the presence of an Fe 3+ /Fe 2+ redox couple. 167 Therefore, rutile itself surely possesses an active site for the effective oxidation of H 2 O, as suggested by some research. For example, Nakato and coworkers have suggested that the (100) face of rutile TiO 2 possesses favorable properties for water oxidation.…”