performance, because the enhancement of solar light harvesting could increase the
temperature localized at the water–air interface rather than heating the whole bulk of
water [165].
4.5.6 TiO 2–x with Special Facets Exposed
The formation of high percentage Ti
3+ in TiO 2 is a great challenge due to its
instability in air, and the obtained Ti
3+ species may also act as electron and hole
recombination sites, which are harmful for the photocatalytic performance of the
catalysts [68]. In order to solve these problems, Fang et al. successfully fabricated
deep-blue TiO 2–x photocatalysts with dual {001} and {101} facets exposed, starting
from the TiCl 3 precursor [78]. The spin intensity in EPR spectra reaches as high as
24.6 Â 10
22 spins per g. Besides, the partial oxidation of Ti
3+ was controlled and
stabilized by the adding of aqueous HF solution. In a dual {101}-{001} facets
system, the holes are like to gather at the {001} facets, while the electrons prefer
to migrate to the {101} facets, which contributes to the separation of photogenerated
electrons and holes [78, 168, 169].
During the preparation of {001} facet-dominated TiO 2 nanomaterials, HF is
usually injected as the structure-directing agent [78, 87, 131, 170]. The introduction
of HF may promote the formation of oxygen vacancies. However, this effect is not
strong enough to introduce high concentrated Ti
3+ in the bulk of TiO 2 [171]. The
subsequent hydrogenation of F-modified TiO 2 with exposed {001} facets by thermal
treatment under high-pressure H 2 atmosphere could produce a large number of Ti
3+
species and oxygen vacancies, resulting in enhanced solar light absorption and
improved photocatalytic activities [172]. Chen et al. compared the effect of hydrogenation treatments on TiO 2 with exposed {001}, {110}, and {101} facets, respectively, and demonstrated that the electric field formed between the reduced
subsurface and stoichiometric surface is the key for the high concentrated Ti
3+
species and F
1+ color centers of hydrogenated TiO 2–x with exposed {001}
facets [173].
Li et al. synthesized TiO 2 nanosheets with exposed {101}-{001} facets first and
then employed low-temperature plasma treatment to introduced Ti
3+ and unique
core-shell structure on the obtained TiO 2 [131]. By adjusting the working gases, N
co-doping could be realized by choosing NH 3 as the plasma source [131]. In
addition, other approaches for {001} facet-dominated TiO 2-x were also reported:
metallic Zn [170] or Ti [68] powders were added in the HF-containing solution,
acting as the reductants for the synthesis of TiO 2–x .
4.5 Modification on TiO 2–x Photocatalysts
95
temperature localized at the water–air interface rather than heating the whole bulk of
water [165].
4.5.6 TiO 2–x with Special Facets Exposed
The formation of high percentage Ti
3+ in TiO 2 is a great challenge due to its
instability in air, and the obtained Ti
3+ species may also act as electron and hole
recombination sites, which are harmful for the photocatalytic performance of the
catalysts [68]. In order to solve these problems, Fang et al. successfully fabricated
deep-blue TiO 2–x photocatalysts with dual {001} and {101} facets exposed, starting
from the TiCl 3 precursor [78]. The spin intensity in EPR spectra reaches as high as
24.6 Â 10
22 spins per g. Besides, the partial oxidation of Ti
3+ was controlled and
stabilized by the adding of aqueous HF solution. In a dual {101}-{001} facets
system, the holes are like to gather at the {001} facets, while the electrons prefer
to migrate to the {101} facets, which contributes to the separation of photogenerated
electrons and holes [78, 168, 169].
During the preparation of {001} facet-dominated TiO 2 nanomaterials, HF is
usually injected as the structure-directing agent [78, 87, 131, 170]. The introduction
of HF may promote the formation of oxygen vacancies. However, this effect is not
strong enough to introduce high concentrated Ti
3+ in the bulk of TiO 2 [171]. The
subsequent hydrogenation of F-modified TiO 2 with exposed {001} facets by thermal
treatment under high-pressure H 2 atmosphere could produce a large number of Ti
3+
species and oxygen vacancies, resulting in enhanced solar light absorption and
improved photocatalytic activities [172]. Chen et al. compared the effect of hydrogenation treatments on TiO 2 with exposed {001}, {110}, and {101} facets, respectively, and demonstrated that the electric field formed between the reduced
subsurface and stoichiometric surface is the key for the high concentrated Ti
3+
species and F
1+ color centers of hydrogenated TiO 2–x with exposed {001}
facets [173].
Li et al. synthesized TiO 2 nanosheets with exposed {101}-{001} facets first and
then employed low-temperature plasma treatment to introduced Ti
3+ and unique
core-shell structure on the obtained TiO 2 [131]. By adjusting the working gases, N
co-doping could be realized by choosing NH 3 as the plasma source [131]. In
addition, other approaches for {001} facet-dominated TiO 2-x were also reported:
metallic Zn [170] or Ti [68] powders were added in the HF-containing solution,
acting as the reductants for the synthesis of TiO 2–x .
4.5 Modification on TiO 2–x Photocatalysts
95