Monitoring Hole Trapping in Photoexcited TiO₂(110) Using a Surface Photoreaction

Abstract

The hole-induced photodesorption of chemisorbed O2 from a TiO2(110) single crystal has been employed to monitor the kinetics of electron-hole pair (e-h) formation and hole trapping. Excitation is produced by 3.4 +/- 0.05 eV photons at 110 K. Two separate O2 desorption processes have been found which are characteristic of low photon fluxes and high photon fluxes. At a critical photon flux, Fhnu(crit), the slow O2 photodesorption process suddenly converts to a fast process, signaling the saturation of hole traps in the TiO2 crystal. Consequently, this allows photogenerated holes to more efficiently reach the surface, causing more rapid O2 photodesorption. The estimated bulk concentration of hole traps is approximately 2.5 x 10(18) cm(-3), involving a fraction of about 3 x 10(-5) of the atomic sites in the bulk. Both the slow and fast O2 photodesorption processes are described by a rate law that is proportional to Fhnu(1/2), indicating that the steady-state concentration of holes, [h], is governed by second-order e-h pair recombination kinetics. Effective use is made of a hole scavenger molecule, adsorbed methanol (CH3OH), to probe the role of added hole traps on the rate of the photodesorption of adsorbed O2 molecules and on the magnitude of Fhnu(crit).

WO3/TiO2 Nanotubes with Strongly Enhanced Photocatalytic Activity

“…From these results one may ascribe the strongly beneficial effect of WO 3 to the photocatalytic activity mainly to an enhanced hole-transfer kinetics to the electrolyte, while at the same time the detrimental effect of a too high WO 3 content may originate from charge trapping effects in the bulk. This is in line with literature, [36,39] which suggests the presence of WO 3 in TiO 2 to influence the re- combination rate of the photoproduced electron-hole (e À -h + ) pairs that may be either due to localized heterojunction formation [38] (due to mismatch of the TiO 2 and WO 3 band energies) or to the formation of activating surface species such as W VI states. The present work clearly favors the formation of specific surface features such as W VI states that act as mediators for charge transfer to the electrolyte.…”

“…[23,24,27,28] Investigations of their photocatalytic properties have shown that these tubular layers can be more efficient than classical nanoparticulate layers of a comparable thickness. [29,[39][40][41] Self-ordered oxide nanotubes cannot only be grown on pure Ti, but also on other transition metals such as Mo, W, Ta, Nb, and so forth, and a full range of Ti alloys including TiW, TiNb, TiAl, TiMo, TiTa. [30][31][32][33][34] In the present work, we demonstrate a very strong effect of tungsten addition to the TiO 2 nanotubes in terms of their photocatalytic activity.…”

A surface science perspective on TiO2 photocatalysis

“…They found that replacement of H in surface OH groups with D did not change the observed O − lineshape, but enrichment of the surface with 17 O did, leading them to conclude that holes did not trap at surface oxygen anion sites that were protonated. Additionally, some groups have also found evidence for subsurface hole traps [386,399,503,569,573]. Kerisit and coworkers [477] performed electrostatic calculations on hole (and electron) trapping at the unrelaxed R TiO 2 (110) surface.…”

Gas‐Phase Photooxidation of Alkenes by V‐Doped TiO2‐MCM‐41: Mechanistic Insights of Ethylene Photooxidation and Understanding the Structure–Activity Correlation

“…[11][12][13][14][15][16][17][18][19] In addition to large surfaces, zeolites and other ordered mesoporous materials help control the size of the confined guest species during the process of deposition, therefore they become an obvious choice. Doping TiO 2 with metal ions for effective modification of the bandgap or separation of the photon-induced electronhole pair [20,21] is one of the most efficient methods reported for enhancement of catalytic activity. Transition-metalloaded silica-titania have also been studied extensively for their physicochemical properties and photocatalytic activity.…”