Intrinsic intermediate gap states of TiO2 materials and their roles in charge carrier kinetics

Abstract

Titanium dioxide (TiO 2) is regarded as an important prototype photocatalytic material for several decades. The charge carrier kinetics determines the photocatalytic properties of TiO 2 materials; this is found to be greatly dependent on electronic structures. It has been revealed that the intrinsic intermediate gap states (intrinsic GSs) play a significant role in charge carrier kinetics that drive the photocatalytic processes of TiO 2 materials, which are not well summarized until now. Motivated by this thought, the purpose of this review focuses on physiochemical science of the intrinsic GSs of TiO 2 materials and their important role in charge carrier kinetics. We first give a summary on the chemical resources of the intrinsic GSs in TiO 2 and their physiochemical nature. Their general energy distribution, charge carrier population, and the associated thermodynamic properties are also elaborated from an overall viewpoint. We further carefully summarize and compare the experimental studies on the energy and the density distribution of the intrinsic GSs and discuss the associated chemical resources and charge carrier localizations. Trapping is the dominant function of intrinsic GSs in the charge carrier kinetics of TiO 2 materials. The significant effect of trapping on the transport, recombination, and interfacial transfer of charge carriers are also comprehensive summarized. Furthermore, the effects of charge carrier kinetics on photocatalytic performances are also discussed to some extents. Because of the importance of intrinsic GSs in modulating charge carrier kinetics, it is expected to increase the photocatalytic activity by engineering the intrinsic GSs, not only for TiO 2 materials, but also for the other semiconductor photocatalysts.

The Effect of Photoinduced Surface Oxygen Vacancies on the Charge Carrier Dynamics in TiO2 Films

“…Besides, researchers have recently highlighted the important role polarons may play in the trapping of other charge carriers and charge carrier transport in TiO 2 . 53 For rutile TiO 2 , the self-trap energy for These measurements were performed initially in the dark and, subsequently, under a UV exposure followed by another period in the dark. The vacancy formation saturated within ∼90 min.…”

Recent Advancements in the Understanding of the Surface Chemistry in TiO2 Photocatalysis

“…A recent review by Fujishima et al [10] summarized the literature data on defects in bulky TiO2 surfaces such as oxygen vacancies, hydroxyl groups, interstitial Ti atoms, and Ti vacancies, as well Oxygen vacancies that are often present in the surfaces of TiO 2 and in the bulk give rise to states at approximately 1 eV below the Fermi level [9]. Theoretical methods resulted in a large scattering of values for the location of these states (from 0.1 to 1.8 eV below the conduction band), mostly reflecting differences in the applied computational methods.…”

“…A recent review by Fujishima et al [10] summarized the literature data on defects in bulky TiO 2 surfaces such as oxygen vacancies, hydroxyl groups, interstitial Ti atoms, and Ti vacancies, as well as on the surface electron and hole polarons. Thus, we do not consider these questions in the present review.…”

New Insight into the Role of Electron Transfer to O2 in Photocatalytic Oxidations of Acetone over TiO2 and the Effect of Au Cocatalyst

“…18 This implies that the electron transfer to O 2 might limit many photocatalytic organic oxidations. 8,13,19 Increasing the kinetics of electron transfer is therefore considered to be important in increasing photocatalytic activity.…”