phenomenon could be caused by the low doping concentration of Ti
3+ species. Zhu
et al. and Liu et al. regarded that the concentration of Ti
3+ should be high enough to
introduce a continuous impurity state below the conduction band of TiO 2–x [79, 84],
because low concentration of Ti
3+ doping leads to the formation of localized oxygen
defect states, which is harmful for the mobility of electron and photocatalytic
activities of the catalysts [79]. That is, Ti
3+ species and oxygen vacancies in a proper
concentration would promote the charge separation of the photogenerated carriers
[21]. With continuous formation of oxygen vacancies and Ti
3+ doping, the isolated
impurity states will enhance and mix with the edge of CBM, leading to the
narrowing of the bandgap of TiO 2–x photocatalysts [79, 85].
It is well known that Ti
3+ species on the surface or top few layers of TiO 2 are
usually not stable, and they are easy to be oxidized by oxygen in air or in water
[30, 62]. Surface oxygen vacancies could be healed by treating reduced TiO 2 with
oxygen exposure [86]. However, XPS characterization can only reach few layers of
TiO 2 surface, which limits the depth of its detection to ~10 nm. In order to test the
information of titanium chemical states in the bulk of TiO 2–x , an argon sputtering
treatment was introduced to remove the top few layer of TiO 2 before the XPS
measurement was carried out [53]. TiO 2–x shows a blue shift of the locations of Ti
2p 3/2 and Ti 2p 1/2 in XPS spectra, because of the introduction of Ti
3+ species [20, 40,
87]. The generation of oxygen vacancies could also be evidenced by O 1 s XPS tests
[51, 64, 80].
EPR is efficient for the detection of Ti oxidation states of TiO 2 , especially in the
bulk of TiO 2 . The g-factors, which are independent of the microwave frequency in
EPR characterization, are usually used for the identification of a compound. Generally speaking, the EPR absorption of a solid with an isotropic magnetic moment
shows a sharp peak, where the peak location in the absorption derivative spectra is
associated with the g tensor (g x ¼ g y ¼ g z ) [87], as shown in Fig. 4.9, left side. The
EPR absorption of a solid with an axial magnetic moment will show doublet peaks,
which can be fit into two peaks in the absorption derivative spectra corresponding to
g ⊥ (g x ¼ g y ) and g k (g z ) [87], Fig. 4.9, right side. EPR signals with g ⊥ ¼ 1.992 and
g k ¼ 1.962 were reported as the typical Ti
3+ centers in the lattice of anatase TiO 2
photocatalysts [88–91]. EPR signals with g ⊥ ¼ 1.976 and g k ¼ 1.945 have been
reported by massive works, which should be attributed to Ti
3+ ions in the bulk or
subsurface of TiO 2–x [15, 26, 70, 74, 87].
In addition, other characterization techniques are introduced for the identification
of structural and chemical changes in TiO 2–x catalysts. With the help of STM,
oxygen vacancies can be visually displayed [61, 92, 93]. And the oxidation state
of Ti and concentration of Ti
3+ species are studied by X-ray absorption near-edge
structure (XANES) [81, 94], extended X-ray absorption fine structures (EXAFS)
[94], and superconducting quantum interference device (SQUID) measurement [45],
respectively. The structural information for TiO 2–x can be further investigated by
Raman spectroscopy [16, 51, 95].
86
4 Preparation of Reduced TiO 2–x for Photocatalysis
3+ species. Zhu
et al. and Liu et al. regarded that the concentration of Ti
3+ should be high enough to
introduce a continuous impurity state below the conduction band of TiO 2–x [79, 84],
because low concentration of Ti
3+ doping leads to the formation of localized oxygen
defect states, which is harmful for the mobility of electron and photocatalytic
activities of the catalysts [79]. That is, Ti
3+ species and oxygen vacancies in a proper
concentration would promote the charge separation of the photogenerated carriers
[21]. With continuous formation of oxygen vacancies and Ti
3+ doping, the isolated
impurity states will enhance and mix with the edge of CBM, leading to the
narrowing of the bandgap of TiO 2–x photocatalysts [79, 85].
It is well known that Ti
3+ species on the surface or top few layers of TiO 2 are
usually not stable, and they are easy to be oxidized by oxygen in air or in water
[30, 62]. Surface oxygen vacancies could be healed by treating reduced TiO 2 with
oxygen exposure [86]. However, XPS characterization can only reach few layers of
TiO 2 surface, which limits the depth of its detection to ~10 nm. In order to test the
information of titanium chemical states in the bulk of TiO 2–x , an argon sputtering
treatment was introduced to remove the top few layer of TiO 2 before the XPS
measurement was carried out [53]. TiO 2–x shows a blue shift of the locations of Ti
2p 3/2 and Ti 2p 1/2 in XPS spectra, because of the introduction of Ti
3+ species [20, 40,
87]. The generation of oxygen vacancies could also be evidenced by O 1 s XPS tests
[51, 64, 80].
EPR is efficient for the detection of Ti oxidation states of TiO 2 , especially in the
bulk of TiO 2 . The g-factors, which are independent of the microwave frequency in
EPR characterization, are usually used for the identification of a compound. Generally speaking, the EPR absorption of a solid with an isotropic magnetic moment
shows a sharp peak, where the peak location in the absorption derivative spectra is
associated with the g tensor (g x ¼ g y ¼ g z ) [87], as shown in Fig. 4.9, left side. The
EPR absorption of a solid with an axial magnetic moment will show doublet peaks,
which can be fit into two peaks in the absorption derivative spectra corresponding to
g ⊥ (g x ¼ g y ) and g k (g z ) [87], Fig. 4.9, right side. EPR signals with g ⊥ ¼ 1.992 and
g k ¼ 1.962 were reported as the typical Ti
3+ centers in the lattice of anatase TiO 2
photocatalysts [88–91]. EPR signals with g ⊥ ¼ 1.976 and g k ¼ 1.945 have been
reported by massive works, which should be attributed to Ti
3+ ions in the bulk or
subsurface of TiO 2–x [15, 26, 70, 74, 87].
In addition, other characterization techniques are introduced for the identification
of structural and chemical changes in TiO 2–x catalysts. With the help of STM,
oxygen vacancies can be visually displayed [61, 92, 93]. And the oxidation state
of Ti and concentration of Ti
3+ species are studied by X-ray absorption near-edge
structure (XANES) [81, 94], extended X-ray absorption fine structures (EXAFS)
[94], and superconducting quantum interference device (SQUID) measurement [45],
respectively. The structural information for TiO 2–x can be further investigated by
Raman spectroscopy [16, 51, 95].
86
4 Preparation of Reduced TiO 2–x for Photocatalysis