Photoluminescence
(PL) spectroscopy was used to infer that oxygen
adsorption changes the band bending of the anatase phase of TiO2 within P25 nanopowder in different ways. On the one hand,
oxygen can adsorb through irreversible reaction with defects which
reduces the intrinsic upward band bending at the TiO2 surface
and results in increased PL emission. On the other hand, oxygen exposure
also leads to molecular chemisorption that yields an outermost negative
charge at the surface which increases the upward band bending of TiO2 and decreases the PL emission. Since band bending plays an
active role in directing charge carrier migration to the surface,
the finding that oxygen adsorption can have two different, and quite
opposite, effects on the band bending of TiO2 provides
a new perspective on how oxygen may influence photocatalytic reaction
efficiencies.
Photoluminescence spectroscopy was employed to study the photoinduced charge transfer between TiO 2 nanoparticles and Au nanoparticles under vacuum. We found that small coverages of 3 nm Au nanoparticles deposited on TiO 2 significantly diminish the 540 nm (2.3 eV) photoluminescence emission from TiO 2 because of the redistribution of the photoexcited charge to Au nanoparticles that are capable of accepting negative charges behind the Schottky barrier. The lack of development of the photoluminescence emission of Au/TiO 2 during continuous UV irradiation occurs because of a short circuit established through Au nanoparticles in which transferred electrons in Au recombine nonradiatively with holes in TiO 2 . The photoexcited electron transfer from TiO 2 to the Au nanoparticles occurs beyond the Au particle perimeter over a distance of at least 4 nm. The quenching of photoluminescence by resonance energy transfer from TiO 2 to Au nanoparticles is unimportant as Au plasmonic absorption is not observed.
Photoluminescence
(PL) spectroscopy was employed to study the effect of embedded single-walled
carbon nanotubes (SWNTs) on charge transport in powdered TiO2. It was found that 1–5 wt % SWNTs mixed with TiO2 accept electrons from photoexcited TiO2 and quench the
PL intensity from TiO2. The PL quenching efficiency by
SWNTs is proportional to the fractional occupancy of TiO2 particles which experience electrical contact with SWNTs. Ultraviolet
light was used to cause surface charging of the SWNT/TiO2 sample, and the charging/discharging rate was measured using PL.
The PL charging/discharging rate of all SWNT/TiO2 mixtures
is identical to that of pure TiO2, indicating that SWNTs
only accept photoexcited electron from TiO2 but do not
transport electrons under conditions of the experiment. This is due
to the effect of positively charged TiO2 particles which
immobilize the photoexcited electrons transferred to SWNTs at the
interface between first-layer TiO2 and SWNTs, inhibiting
charge transport through SWNT channels. The inhibition of charge transport
through SWNTs may be removed by applying an external voltage to overcome
the built-in electric field which prevents current flow in the SWNTs.
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