Environmental Engineering Reference
In-Depth Information
11.2.2.1 Metal Doping
Selective doping of metal ions into the crystalline matrix of semiconductors has been
proven to be an effective route for improving visible light photoreactivity by creating
intraband levels in the forbidden band, which can serve as either a donor level above the
original VB or an acceptor level below the original CB. Thus far, metal-ion-doped TiO
2
,
SrTiO
3
, and ZnS have been reported. It is reported that the photoreactivity of the metal-
doped semiconductor exhibits a complex dependence on the dopant concentration, dopant
energy levels within the semiconductor lattice, distribution of dopants, d-electron conigu-
ration, and other physicochemical variables.
Several studies have analyzed the doping process by transition elements: irst row (V, Cr,
Mn, Fe), second row (Nd, Mo), third row (W), lanthanide (Nd), and others (Ge, Sn, Pb). For
the irst transition row, the 3
d
states of the dopants could progressively decrease the bot-
tom edge of the CB as the doping level increases. Both Mo and W can lower the bottom of
the CB, whereas Nd can modify both the CB and VB band edges.
9
The anatase TiO
2
band-
gap measurement as a function of the doping levels is shown in Figure 11.6. As described,
the electronic state can be modulated by the nature and content of the metal ion in order
to obtain optimum solar light absorption ability. Furthermore, doping with a transition
metal ion usually induces the formation of oxygen vacancies simultaneously, which facili-
tate the formation of
O
2
−
upon chemisorptions of oxygen.
In metal-doped semiconductors, 3
d
-transition element (V, Cr, Mn, Fe, Co, and Ni)-doped
TiO
2
attracts considerable attention.
10
Theoretical studies reveal that the localized 3
d
states
can split and mix with the CB and VB, insert an occupied level, and subsequently cause
the absorption band to redshift. The extent of the redshift depends on the amount and the
type of the doping metal ion. However, doping with 4 transition elements usually causes
bulk defects, which can act as recombination sites. Moreover, the discrete levels formed by
the localized
d
-states suppress the migration of carriers. To overcome these drawbacks, the
formation of new VB by orbitals without O2
p
is necessary. Orbitals of Pb6
s
in Pb
2+
, Bi6
s
in
Bi
3+
, Sn5
s
in Sn
2+
, and Ag4
d
in Ag
+
can form valence bands above the VB consisting of O2
p
orbitals in metal oxide photocatalysts, such as RbPb
2
Nb
3
O
10
and PbBi
2
Nb
2
O
9
.
11
Moreover,
the degree of the contribution of these cations to the VB formation depends on the crystal
3.00
2.75
TiO
2
Fe/TiO
2
Mo/TiO
2
W/TiO
2
Nd/TiO
2
2.50
Ca/TiO
2
V/TiO
2
Cr/TiO
2
2.25
2.00
0
1
2
3
4
5
(M/(M+Ti)) (at %)
FIGURE 11.6
Effect of doping level of various metal ions on anatase TiO
2
band gap. (Adapted with permission from Kubacka
A., Fernández-García M., Colón, G.,
Chem. Rev
., 112, 1555. Copyright 2012, American Chemical Society.)
