Abstract
Understanding the mechanisms of charge generation and their recombination in rutile TiO2 is of key importance in the design of optoelectronic and photocatalytic devices. In this study, we investigate the impact of both extrinsic and intrinsic defects on photoluminescence (PL) decay dynamics. The exploitation of two-photon fluorescence lifetime imaging microscopy (FLIM) enabled differentiation to be made between photoluminescence originating from dislocations and bulk in Nb-doped rutile TiO2. It was found that dislocations pinned at grain boundaries feature lower photoluminescence intensity and faster decay times (by 100 ps) than those in the bulk. This can be reversed upon reduction, whereby trap states are preferentially formed near to dislocation sites. We also evaluated the dependence of the extrinsic doping level on the charge carrier dynamics of rutile, and show that PL lifetimes are governed by predominant Auger processes that are insensitive to reduction, unlike dislocations. We believe that the oxygen deficiency suppression of charge carrier recombination at grain boundaries is the key factor in improving the photocatalytic activity of real TiO2-based materials.
Highlights
Titanium oxides present themselves as promising materials w ith respect to variouscatalytic applications, m ostly because o f the ease o f bandgap engineering v ia reduction, w hich is currently being w id ely investigated [1 -3 ]
The HAADF-STEM im age reveals a periodic contrast at the boundary, as shown in Fig. 1d, suggesting an array o f individual dislocations separated b y 36 nm, propagating in the (0 1 0 ) direction
The m onocrystalline rutile structure was found in both crystals, as demonstrated b y a selected-area electron diffraction (SAED) pattern ( Fig. 1d insets)
Summary
Titanium oxides present themselves as promising materials w ith respect to various (photo)catalytic applications, m ostly because o f the ease o f bandgap engineering v ia reduction, w hich is currently being w id ely investigated [1 -3 ]. Dislocations can be m echanically-induced in transition metal oxides, which allows for the tailoring materials with desired properties [1 8 ] This relationship between the properties o f oxides and the presence o f defects o f different dimensionalities is not always fu lly understood, and a m odel system w ith precisely controlled density defects is needed. W e em ploy two-photon FLIM microscopy equipped with time-correlated single-photon counting in order to achieve the best temporal sensitivity and spatial resolution b elow a single micrometer This enabled, for the first tim e in the case o f titanium oxides, the sepa ration o f signals from the bulk, dislocations, and area close to the grain boundary. W e prove that two-photon excitation photoluminescence microscopy enables differentiation to be drawn between the charge carrier dynamics in bulk and dislocations, as w e ll as the effects o f oxygen vacancy self-doping upon reduction and aliovalent Nb doping
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