Environmental Engineering Reference
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structure and the doping level. A typical example is the Bi-based photocatalyst BiVO 4 ,
which mainly exists in a tetragonal phase or monoclinic phase crystalline structure. With
the CB composed of V3 d orbitals in both cases, the top of the VB in the tetragonal BiVO 4
consists of only O2 p orbitals, while Bi6 s orbitals occupy the VB in the monoclinic case,
resulting in a narrowed band gap of 2.34 eV for the monoclinic structure compared with
3.11 eV for the tetragonal structure. 12 It is noteworthy that the VB composed of Bi6 s orbitals
in monoclinic BiVO 4 can induce water oxidation to O 2 evolution by four-electron oxidation.
The doped metal ion can occupy two different positions in a semiconductor lattice: substitu-
tional and interstitial, which is dependent on the ionic radius of the dopant compared with the
matrix cation. The different positions of dopants may cause distinct effects on the electronic
structure of the semiconductor. For example, in doped TiO 2 nanoparticles, Nd 3+ and Pd 2+ act
as interstitial dopants and induce a large distribution of the potential energy, while Fe 3+ and
Pt 2+ are in substitutional positions and have very little or no potential energy disturbance. 13
Codoping with two suitable heteroatoms can also achieve substantial synergistic effect.
For instance, codoping of Cr 3+ /Ta 5+ , Cr 3+ /Sb 5+ , Ni 2+ /Ta 5+ , and doping of Rh cations is effec-
tive for the sensitization of SrTiO 3 to visible light. Cr- or Fe-doped La 2 Ti 2 O 7 and Rh-doped
SrTiO 3 can also function as effective visible-light-driven photocatalysts. TiO 2 codoped with
Cr 3+ /Sb 5+ , Rh 3+ /Sb 5+ , and Ni 2+ /Nb 5+ is active for O 2 evolution. 14 In these photocatalysts,
codoped metal cations can compensate the charge imbalance, and thus suppress the for-
mation of recombination sites and maintain the visible light absorption ability.
Furthermore, metal doping can greatly change the charge carrier lifetime and then affect
photoreactivity. For example, Grätzel and Howe reported that Fe 3+ - and V 4+ -doped TiO 2
exhibited a drastically extended lifetime of the photogenerated electron-hole pairs compared
with pristine TiO 2 . 15 On the other hand, doped metal ions can also act as recombination sites,
especially at a high doping level. Indeed, no visible light photoreactivity is observed in Fe 3+ -
doped TiO 2 at an Fe loading of 10 wt%, while optical absorption toward visible light is con-
siderable. Several other effects, such as surface hydrophilicity, adsorption behavior toward
reactant molecules, and crystal structure transitions, would be also affected by metal doping.
It is therefore necessary to control the doping process for speciic photocatalytic applications.
11.2.2.2 Nonmetal Doping
Nonmetal doping is another approach used to narrow band gap and enhance the visible-
light-driven photoreactivity. Compared with metal doping that is likely to form a donor
level in the forbidden band, nonmetal doping usually induces the VB edge upward. In
principle, there are two requirements for nonmetal doping to elevate the VB maximum of
the metal oxides: (i) the nonmetal dopant should have a lower electronegativity than that
of oxygen, and thus favors the newly formed VB locating at its top; (ii) the radius of the
nonmetal dopant should be comparable to that of the lattice O atom to facilitate the uni-
form distribution of the dopant atoms within the whole matrix.
The best studied case involves doping anatase TiO 2 with N. 16 The experimental results and
theoretical calculation document the presence of oxygen vacancies in both substitutional and
interstitial N-doping samples without the concomitant existence of Ti 3+ species. Essentially,
N doping produces occupied states with an energy of approximately 0.0-0.2 and 0.5-0.6 eV
above the VB for substitutional and interstitial doping, respectively. 17 In addition to N, C, S, B, F,
I, P and N/F, N/B, N/C have all been tested as dopants of the anatase TiO 2 system. Figure 11.7
summarizes the experimentally observed electronic states for N, F, C, S and N, F codoped
anatase TiO 2 . For instance, C-doping at an interstitial position can decrease the TiO 2 band gap
owing to the interactions present between carbon impurities and the corresponding anion
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