Comparing photoelectrochemical water oxidation, recombination kinetics and charge trapping in the three polymorphs of TiO2

Abstract

In this article we present the first comparative study of the transient decay dynamics of photo-generated charges for the three polymorphs of TiO2. To our knowledge, this is the first such study of the brookite phase of TiO2 over timescales relevant to the kinetics of water splitting. We find that the behavior of brookite, both in the dynamics of relaxation of photo-generated charges and in energetic distribution, is similar to the anatase phase of TiO2. Moreover, links between the rate of recombination of charge carriers, their energetic distribution and the mode of transport are made in light of our findings and used to account for the differences in water splitting efficiency observed across the three polymorphs.

Chemical Vapor Deposition of Photocatalytically Active Pure Brookite TiO2 Thin Films

“…We should make it clear that this was not because holes in anatase cannot be scavenged strongly by methanol, indeed they can, where nanopowders or mesoporous structures often show a strong hole scavenging effect. 62,64,65 However, in the case of a flat and dense sample, the quantum yield of this interaction is too low to see a hole scavenging effect (as bulk electron-hole recombination dominates). 65 Overall, our TAS results show that the innate electron-hole recombination dynamics are similar in brookite and anatase, akin to previous reports.…”

Tuning Charge Carrier Dynamics and Surface Passivation in Organolead Halide Perovskites with Capping Ligands and Metal Oxide Interfaces

“…The decays appear linear on a log–log plot (Figure S12A, Supporting Information), indicating power‐law decay kinetics. At low fluences, the decays become more linear on a lin–log plot (Figure S12B, Supporting Information), potentially an indication of recombination of deeply trapped charges by tunneling . To gain more insight into the recombination mechanism, we attempted to fit the TAS decay dynamics by a model that considers first‐order monomolecular, second‐order bimolecular, and third‐order Auger processes (see the Supporting Information).…”

Brookite: Nothing New under the Sun?

“…Notably, to the best of our knowledge, no theoretical study has been reported on electron trapping at the defects of brookite, even if similar results to anatase and rutile are expected. Some of the authors recently reported a study on trapped carriers in different TiO 2 polymorphs in the ms-s timescales, typical of photocatalytic reactions [58]. Also in these slow timescales, anatase and brookite exhibit dispersive power law recombination dynamics, in accordance with shallow charge trapping, while rutile exhibits logarithmic decay kinetics, indicative of deeper charge trapping.…”

“…The density of such occupied mid-gap trap states (DOTS) can be studied by time-resolved vis-IR absorption spectroscopy (TRAS) [57,58]. In a recent experimental study, the depth of electron-traps in brookite was estimated to be 0.4 eV, which is deeper than that of anatase (<0.1 eV), but shallower than that of rutile (∼0.9 eV) [57].…”