Page 191 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

176 Principles and Methods

as the surface trapping state. The electron is trapped as Ti(III)OH (a
reduced Ti site on the surface) and the hole is trapped as surface-bound
.
.
hydroxyl radical, Ti(IV)OH . The trapped hole, Ti(IV)OH , is a power-
ful oxidizing species that is able to undergo either direct electron transfer
reactions or hydrogen abstraction reactions, depending on the chemical
nature of the electron donor. On the other hand, the trapped electron
[ Ti(III)OH] is a moderate to weak reductant, although it is still capa-
ble of transferring electrons to dioxygen (O ) adsorbed on the surface.
2
The DRIFT spectra of Figure 5.19 show a pronounced peak shift in the
2 2
presence of oxygen (i.e., O 1 e tr S O 2 ) versus the same system in a
2
vacuum. For example, the band that appears at 3716 cm 1 has been
identified as the trapped electron, Ti(III)OH, which results from the
localization of conduction band electrons in surface trapping sites. In the
presence of O , the trapped electron disappears and surface-bound super-
2
2.
oxide, O 2 , is formed.
tr ←⎯⎯
O e ⎯→⎯ O . (34)
2 2
As an alternative, Eq. 34 can be written as

⎯
O ⎯→ O ⋅ Ti(IV)OH (35)

Ti(III)OH ←⎯⎯
2 2

The trapped electron, Ti(III)OH , is completely removed by expo-
sure to Br in the gas phase. The trapped electron has an ESR (elec-
2
tron spin resonance) signal with g 5 1.957, and g 5 1.990.
i
'
However, after irradiation the 3716 cm 1 band persists under a
at 300 K, whether in the dark
1.0 atmosphere of dry (0 percent RH) O 2
or under illumination, whereas the TiO surface must be exposed to
2
water vapor before the trapped electron band at 3716 cm 1 disap-
pears. ab initio calculations of oxygen-deficient TiO in the rutile
2
form indicate that excess charge in the bulk remains spin-paired and
localized at vacant oxygen sites. Inter-bandgap states on reduced
1
TiO 2 surfaces are associated with spin-polarized Ti(III) 3d and Ti(II)
2
3d configurations.
The conduction band electrons in the trapped state electrons have
been observed using a variety of laser-based pump-probe photolysis
experiments. The so-called blue electron in either a deep or swallow
state trap—that is, an internal Ti(III) site or a surface Ti(III)OH
site—has a characteristic spectrum in the visible with a band peak at
600 nm. The appearance and disappearance of the “blue electron” can
be followed kinetically by rapid-scan spectroscopy in order to get an
estimate of the actual lifetimes of mobile electrons that are available
for electron transfer.