(III) precursors. The oxidation process should be well controlled, and complete
oxidation of these low-oxidation-state titanium compounds should be avoided.
4.2.2.1 Starting from Metallic Ti Powder
According to the best of our knowledge, Zuo et al. firstly reported rutile TiO 2–x
photocatalysts with active {111} and {110} facets via a simple hydrothermal method
starting from the mixture of titanium powder and hydrochloric acid [66]. The Ti
3+
species in the final TiO 2–x samples resulted from the reduction of Ti
4+ by H 2 gas,
which was induced by the reaction between Ti and HCl at the high vapor pressure
and high-temperature atmosphere. The desired dual {111} and {110} were reported
to collect holes and electrons, respectively, which resulted into the separation of
photogenerated electron and hole pairs. Liu et al. reported similar experiment
starting with Ti and HCl, however, with additional injection of HF solution
[67]. Because of the adding of HF solution, the obtained Ti
3+ species were further
stabilized by F-termination of the TiO 2–x samples. In addition, the adding of HF
facilitated to the formation of {001} facets of TiO 2–x catalysts [67, 68], owing to the
low energy of (001) surface decreased by high Ti-F bonding energy [69].
4.2.2.2 Starting Form Ti(II) Precursors
Using TiH 2 as the starting material for the synthesis of TiO 2–x photocatalysts was
investigated by Huang and co-workers [70–73]. Hydrothermal treatment was
employed on the mixture of TiH 2 and H 2 O 2 aqueous solution [70]. The using of
TiH 2 can guarantee a high doping concentration of Ti
3+ in the final TiO 2–x samples.
The choosing of H 2 O 2 as the oxidation agent is the key to the innovation because the
oxidation process could be controlled and new chemical contaminations could be
avoided [74]. During the hydrothermal process, TiH 2 was oxidized by H 2 O 2 , and a
“solid interface diffusion–redox” reaction was proposed by Liu et al. for the generation of Ti
3+ species [70]. Apart from hydrothermal treatment, heating at high
temperature under argon atmosphere was also employed on the gel dried from the
mixture of TiH 2 and H 2 O 2 solution [71, 74], as shown in Fig. 4.7. Ar–gas protection
during the thermal treatment process is necessary for the maintaining of Ti
3+ species.
Otherwise, the obtained Ti
3+ species will be oxidized to Ti
4+ by oxygen while
annealing in air [74]. Xin et al. combined the hydrothermal and post-annealing
processes together for the synthesis of TiO 2-x catalyst, and they demonstrated that
the distribution of surface/subsurface defects could be controlled by the further
annealing of the as-prepared TiO 2–x photocatalysts [75]. Zhu et al. prepared TiO 2–
x via thermal treatment on the mixture of commercial Evonik P25 and TiH 2 under
vacuum [76]. In this case, active hydrogen atoms were proposed to release from the
decomposition of TiH 2 while heating over 400
C during this process, resulting into
the corporation of H on the final obtained TiO 2–x samples, rather than the generation
of Ti
3+ species [76].
4.2 Synthesis of TiO 2–x Photocatalysts
83
oxidation of these low-oxidation-state titanium compounds should be avoided.
4.2.2.1 Starting from Metallic Ti Powder
According to the best of our knowledge, Zuo et al. firstly reported rutile TiO 2–x
photocatalysts with active {111} and {110} facets via a simple hydrothermal method
starting from the mixture of titanium powder and hydrochloric acid [66]. The Ti
3+
species in the final TiO 2–x samples resulted from the reduction of Ti
4+ by H 2 gas,
which was induced by the reaction between Ti and HCl at the high vapor pressure
and high-temperature atmosphere. The desired dual {111} and {110} were reported
to collect holes and electrons, respectively, which resulted into the separation of
photogenerated electron and hole pairs. Liu et al. reported similar experiment
starting with Ti and HCl, however, with additional injection of HF solution
[67]. Because of the adding of HF solution, the obtained Ti
3+ species were further
stabilized by F-termination of the TiO 2–x samples. In addition, the adding of HF
facilitated to the formation of {001} facets of TiO 2–x catalysts [67, 68], owing to the
low energy of (001) surface decreased by high Ti-F bonding energy [69].
4.2.2.2 Starting Form Ti(II) Precursors
Using TiH 2 as the starting material for the synthesis of TiO 2–x photocatalysts was
investigated by Huang and co-workers [70–73]. Hydrothermal treatment was
employed on the mixture of TiH 2 and H 2 O 2 aqueous solution [70]. The using of
TiH 2 can guarantee a high doping concentration of Ti
3+ in the final TiO 2–x samples.
The choosing of H 2 O 2 as the oxidation agent is the key to the innovation because the
oxidation process could be controlled and new chemical contaminations could be
avoided [74]. During the hydrothermal process, TiH 2 was oxidized by H 2 O 2 , and a
“solid interface diffusion–redox” reaction was proposed by Liu et al. for the generation of Ti
3+ species [70]. Apart from hydrothermal treatment, heating at high
temperature under argon atmosphere was also employed on the gel dried from the
mixture of TiH 2 and H 2 O 2 solution [71, 74], as shown in Fig. 4.7. Ar–gas protection
during the thermal treatment process is necessary for the maintaining of Ti
3+ species.
Otherwise, the obtained Ti
3+ species will be oxidized to Ti
4+ by oxygen while
annealing in air [74]. Xin et al. combined the hydrothermal and post-annealing
processes together for the synthesis of TiO 2-x catalyst, and they demonstrated that
the distribution of surface/subsurface defects could be controlled by the further
annealing of the as-prepared TiO 2–x photocatalysts [75]. Zhu et al. prepared TiO 2–
x via thermal treatment on the mixture of commercial Evonik P25 and TiH 2 under
vacuum [76]. In this case, active hydrogen atoms were proposed to release from the
decomposition of TiH 2 while heating over 400
C during this process, resulting into
the corporation of H on the final obtained TiO 2–x samples, rather than the generation
of Ti
3+ species [76].
4.2 Synthesis of TiO 2–x Photocatalysts
83