Molecular‐Level Understanding of the Photocatalytic Activity Difference between Anatase and Rutile Nanoparticles

Abstract

The generation of oxidants on illuminated photocatalysts and their participation in subsequent reactions are the main basis of the widely investigated photocatalytic processes for environmental remediation and selective oxidation. Here, the generation and the subsequent diffusion of .OH from the illuminated TiO2 surface to the solution bulk were directly observed using a single‐molecule detection method and this molecular phenomenon could explain the different macroscopic behavior of anatase and rutile in photocatalysis. The mobile .OH is generated on anatase but not on rutile. Therefore, the photocatalytic oxidation on rutile is limited to adsorbed substrates whereas that on anatase is more facile and versatile owing to the presence of mobile .OH. The ability of anatase to generate mobile .OH is proposed as a previously unrecognized key factor that explains the common observations that anatase has higher activity than rutile for many photooxidative reactions.

Photocatalytic Efficiency Tuning by the Surface Roughness of TiO2 Coatings on Glass Prepared by the Doctor Blade Method

“…The photocatalytic generation of HO˙and subsequent substrate oxidation mechanisms have been extensively discussed (15,38,(40)(41)(42)(43), and recently reviewed by the Nosaka´s group (7). It is now accepted that a fraction of the photogenerated HO˙at the TiO 2 surface (HO˙b ound ) can diffuse to the bulk solution as free radical species (HO˙f ree ) (41,42).…”

“…The photocatalytic generation of HO˙and subsequent substrate oxidation mechanisms have been extensively discussed (15,38,(40)(41)(42)(43), and recently reviewed by the Nosaka´s group (7). It is now accepted that a fraction of the photogenerated HO˙at the TiO 2 surface (HO˙b ound ) can diffuse to the bulk solution as free radical species (HO˙f ree ) (41,42). The relative amount of both types of HO˙species depends on the crystalline composition of TiO 2 , being anatase more prone to release HO˙while rutile does not, limiting in the latter case only the oxidation reaction with adsorbed substrates (15,38,41).…”

Why is Anatase a Better Photocatalyst than Rutile? The Importance of Free Hydroxyl Radicals

“…In a recent communication, Choi and co‐workers detected, for the first time, single molecules of OH . f in water, using a technique known as total internal reflectance fluoresence microscopy (TIRFM) 15. This technique has arisen as a relatively simple but powerful way to image photogenerated surface species16 (Figure 3).…”

“… Schematic of the TIRFM setup showing the arrangement of the TiO 2 catalyst surface and fluorescein‐glass substrate, and the oxidative cleavage of the quenching group. Figure reproduced with permission from Reference 15, copyright Wiley‐VCH 2014. …”

“… Schematic representation of hydroxyl radical formation and reaction with an organic substrate (S) upon rutile and anatase. Figure reproduced with permission from Reference 15, copyright Wiley‐VCH 2014. …”