Morphology-sensitive trapping states of photogenerated charge carriers on SrTiO3 particles studied by time-resolved visible to Mid-IR absorption spectroscopy: The effects of molten salt flux treatments

Abstract

A B S T R A C TThe effects of the morphology-change of SrTiO 3 particles on the behavior of photogenerated charge carriers are studied by time-resolved absorption (TA) spectroscopy from the visible to mid-IR region. In the case of as-purchased defect-rich commercial SrTiO 3 particles, most of the charge carriers are deeply trapped, showing a transient absorption peak at 11,000 cm À1 . Scanning electron microscopy reveals that the irregular-shaped primary particles are heavily aggregated and that the photocatalytic activity for the water splitting reaction is negligibly small. However, when this powder is flux-treated by SrCl 2 , fine cubic SrTiO 3 crystals exposing well-defined surfaces are formed and the photocatalytic activity is greatly improved. TA measurements show that the spectral shape is changed dramatically: the number of deeply trapped electrons is reduced, and that of shallowly trapped electrons producing the absorption band at 2500 cm À1 is increased. The change in electron trap depth, observed upon flux treatment, is due to the decrease in the number of defects. We also found that further flux treatment in an Al 2 O 3 crucible (i) induces Al doping into SrTiO 3 , (ii) enhances the photocatalytic activity, (iii) changes the spectral shape, and (iv) prolongs the lifetime of shallowly trapped electrons. The increase in photocatalytic activity is presumably due to the change in lifetime.

Effect of Na‐Doping on Electron Decay Kinetics in SrTiO3 Photocatalyst

“…On the other hand, the absorption at 12,000 cm −1 (Figure c) becomes stronger both in O 2 and CH 3 OH than in vacuum. As reported in our previous paper, such a behavior can occur when both the photoexcited electrons and holes contributes to the absorption. Therefore, the transient absorption at 12,000 cm −1 is ascribed to a mixture of trapped electrons and holes.…”

“…To obtain deeper understanding of the mechanism, it is essential to know how the behaviors of photoexcited electrons and holes are changed by doping. The transient absorption spectroscopy is a powerful tool to observe the photocarrier dynamics . In this work, we investigated the Na‐doping effect on photocarrier dynamics in SrTiO 3 by measuring transient absorption from visible to mid‐IR region.…”

KCl flux-induced growth of isometric crystals of cadmium-containing early transition-metal (Ti 4+ , Nb 5+ , and Ta 5+ ) oxides and nitridability to form their (oxy)nitride derivatives under an NH 3 atmosphere for water splitting application

“…The broad absorption from 14000 -4000 cm -1 can be assigned to the transient absorption of deeply trapped electrons and/or holes at the defects. This absorption intensity is higher in Na-Ta 3 N 5 than in Cd-Ta 3 N 5 and Na-Cd-Ta 3 N 5 , indicating that the electron and/or hole is deeply trapped at defects in Na-Ta 3 N 5 [45,46]. The absorption below 4000 cm -1 is associated with shallowly trapped electrons and/or free electrons.…”

Oxygen‐Doped Ta3N5 Nanoparticles for Enhanced Z‐Scheme Carbon Dioxide Reduction with a Binuclear Ruthenium(II) Complex under Visible Light

“…The shape of the transient absorption spectra was dependent on the nitridation temperature, which suggests that trap states in the band gap of Ta 3 N 5 are altered by the nitridation temperature. The transient absorption in the mid‐IR region (3000–1000 cm −1 ) is assigned to free electrons in the conduction band or shallowly trapped electrons, while the absorption in the visible to near‐IR region (16000–3000 cm −1 ) is attributed to the deeply trapped electrons and/or holes in mid‐gap states . Nitrogen vacancies have been reported as the most probable trap states in Ta 3 N 5 ; therefore, the trap states in the band gap can be ascribed to nitrogen vacancies.…”

“…To investigate the difference in activity for CO 2 reduction using Ta 3 N 5 /SiO 2 prepared at various temperatures, the behavior of photogenerated electrons and holes in Ta 3 N 5 /SiO 2 was investigated by recording transient absorption spectra. Although quantitative comparison of the transient absorption signal remains a challenge, the technique gives us very useful information on carrier dynamics of semiconductor materials . Figure shows transient absorption spectra for Ta 3 N 5 /SiO 2 synthesized at various temperatures.…”