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found that the optimal Fe
3+
dopant concentration for enhancing catalytic
activity strongly depends on the particle size of the TiO
2
nanocatalyst. h e
optimal Fe
3+
concentration was found to decrease with increasing particle
size. Yao
et al.
[143] reported that La (Fe)-doped bismuth titanate shows
good photocatalytic activity. Feng [144] showed that nanosized Fe-doped
TiO
2
synthesized by sol-gel pillaring technique was excellent photocatalyst
for discoloration and mineralization of Orange II. For the nanosized pho-
tocatalysts, high specii c surface areas and small crystal sizes are benei cial
for ei cient photocatalytic reactions. By acting as both hole and electron
traps, Fe can enhance the photocatalytic activity of TiO
2
[145].
h e general pathway of photochemical reaction is in the presence of
UV light; Fe(III) in TiO
2
matrix generates Fe
2+
, Fe
4+
and charge-pair (Eq.
15.1), and at the same time Fe
3+
is also regenerated through electron or
hole trapping by reversible reaction. h e photocatalytic reaction involves
the migration (Eq. 15.2) and recombination (Eqs. 15.3, 15.4, 15.5) of the
charge between the iron and titanium ions. h e dye is discolored within
the time of reaction through the mutual contact between the dye and the
catalyst. h e discoloration of the dye occurs because the dye is sensitized
and the dye cation is formed. h e dye cation is unstable and decomposes,
injecting an electron in the conduction band of Fe
2+
. h is band-gap elec-
tron starts the production of highly oxidative radical species through the
formation of super oxide radical anion O
2
• −
on the surface of the catalyst
as long as the dye is present. In later steps of the reaction, when long-lived
colorless intermediates are present, the Fe
2+
absorbs the light and produces
the conduction-band electron and valence-band holes. h e mechanism
of Fe
3+
-doped TiO
2
samples [41, 145-148] is illustrated in the following
equations.
hv
3
+
2
+
4
+
−
+
2Fe
Fe
+
Fe
+
e
+
h
(15.1)
CB
NB
2
+
4
+
3
+
3
+
Fe
+
Ti
Fe
+
Ti
(15.2)
4
+
−
3
+
Fe
+
OH
Fe
+
OH
(15.3)
2
+
3
+
−
Fe
+
OH
Fe
+
OH
(15.4)
4
+
3
+
3
+
4
+
Fe
Ti
Fe
+
Ti
(15.5)
Dye
+
hv
Dye
∗
(15.6)
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