Does Interfacial Charge Transfer Compete with Charge Carrier Recombination? A Femtosecond Diffuse Reflectance Investigation of TiO₂ Nanoparticles

Abstract

The charge carrier dynamics of opaque, aqueous suspensions of Degussa P-25 TiO2 are probed with femtosecond time-resolved diffuse reflectance spectroscopy. Comparison of ultrafast pump−probe diffuse reflectance measurements of P-25 suspensions with dry P-25 powder and the transient absorption of transparent, aqueous Q-TiO2 solutions allows the observed kinetics to be assigned to charge carrier recombination. The electron−hole recombination kinetics are consistent with a second-order process as demonstrated by a laser fluence dependence study. Interfacial hole transfer dynamics of the P-25 TiO2/SCN- complex are probed as a function of thiocyanate ion concentration. A dramatic increase in the population of trapped charge carriers is observed within the first few picoseconds, demonstrating that interfacial charge transfer of an electron from the SCN- to a hole on the photoexcited TiO2 effectively competes with electron−hole recombination on an ultrafast time scale. The experimental dependence of the charge carrier dynamics are shown to be consistent with a kinetic model of competing second-order processes. The implications of the results on the use of nanoscale TiO2 for photocatalysis are discussed.

Review of the anatase to rutile phase transformation

“…Mixed-phase photocatalysts with rutile-anatase compositions have been reported to exhibit enhanced photoactivity relative to single-phase titania [23,34,54,55,68,73,80,110,112,125,130,131,[141][142][143][144][145][146]. It is considered widely that this is a result of improved charge carrier separation, possibly through the trapping of electrons in rutile and the consequent reduction in electron-hole recombination [78,117,147].…”

Titania Photocatalysis beyond Recombination: A Critical Review

“…Bowman and coworkers first reported pump-probe diffuse-reflectance spectroscopic measurements of heterogeneous photocatalytic reactions [4][5][6][7]. Decay kinetics of electrons trapped in levels located below the CB bottom (electron trap = ET) or electrons in the CB after photoexcitation by femtosecond ultraviolet pump pulses was observed in the picosecond time region and analyzed.…”

Synthesis, Characterization, and Photocatalytic Activities of Nanoparticulate N, S-Codoped TiO2 Having Different Surface-to-Volume Ratios

“…The TRDR technique [34][35][36][37] has also been used to study the decay of the electron in TiO 2 powders. 38,39 The absorption of the electron was followed at 600 nm ( 600 ) 1200 M -1 cm -1 ). 32,33 The transient absorption spectrum in Figure 8 for D-TKP 102-A powder undergoing a laser pulse with λ ) 450 nm showed the existence of a maximum around 550 nm after 5 s of laser pulse and extending to the limit of the oscilloscope of 2 ms due to electron trapping on TiO 2 dehydrated shallow traps.…”

“…38,39 The absorption of the electron was followed at 600 nm ( 600 ) 1200 M -1 cm -1 ). 32,33 The transient absorption spectrum in Figure 8 for D-TKP 102-A powder undergoing a laser pulse with λ ) 450 nm showed the existence of a maximum around 550 nm after 5 s of laser pulse and extending to the limit of the oscilloscope of 2 ms due to electron trapping on TiO 2 dehydrated shallow traps. 34 The signal of transient absorption upon visible laser pulse in Figure 9 shows that the undoped TKP 102 powder did not reveal any signal while D-TKP 102-A showed the signal for electrons on TiO 2 , with two components: a faster one due to the electron decay and a long-lived component with 1/4 of the initial amplitude due to the electron trapping on shallow TiO 2 traps.…”