and bulk structure of TiO 2–x were carefully studied. Therefore, it is necessary to
make a review of the current development of TiO 2–x .
In this chapter, we will first discuss the synthesis approaches of TiO 2–x
photocatalysts, including annealing under H 2 atmosphere, vacuum activation, in
situ reduction, metal reduction, electrochemical reduction, UV irradiation, plasma
treatment, and partial oxidation starting from Ti, Ti(II), or Ti(III) precursors, etc.
Then various characterization techniques are discussed to investigate the properties
of TiO 2–x , such as the formation of structural distortions inherent, Ti
3+ species,
oxygen vacancies, or surface hydroxyl bonds. We also summarize the applications
of reduced TiO 2 in the photocatalytic and photoelectric areas, point out further
modification approaches for TiO 2–x in improving their performance, and demonstrate the sustainability of photocatalysis as a solution to environment pollution and
energy crisis.
4.2 Synthesis of TiO 2–x Photocatalysts
Theoretically, reduced TiO 2 photocatalysts could be obtained either by the reduction
of TiO 2 solid materials or Ti(IV) precursors or the oxidation of low-oxidation-state
titanium compounds. Until now, a large number of methods have been developed for
the synthesis of TiO 2–x materials, including thermal treatment under H 2 atmosphere
or other reducing gases, in situ reduction, vacuum activation treatment, metal
reduction, hydrogen or water plasma treatment, electrochemical reduction, UV
light irradiation treatment, and partial oxidation starting from Ti, Ti(II), and Ti(III)
precursors [11]. In general, all these reported methods for the synthesis of TiO 2–x
could be classified by these two main aspects: under reducing or oxidizing
atmosphere.
4.2.1 Reduction Method
4.2.1.1 Thermal Treatment Under Reducing Gases
Thermal treatment under H 2 -gas atmosphere was early used for the synthesis of
TiO 2–x materials, developed by Chen et al. [12]. The pristine TiO 2 nanocrystals were
hydrogenated in a 20-bar H 2 atmosphere at 200
C for 5 days. The obtained TiO 2–x
nanoparticles became black colored (Fig. 4.1a) and disorder engineered (Fig. 4.1b).
Wang et al. investigated the hydrogenation of TiO 2 by H 2 –gas heating at various
temperatures [13]. The color of the obtained hydrogenated TiO 2 depends on the
hydrogen annealing temperature, and it becomes darker with temperature increasing
in the range of 200–500
C [13]. The color implies the extended solar light
absorption of TiO 2 as the result of hydrogenation. Besides, after thermal hydrogenation treatment, disorder surface shell is formed outside the crystalline TiO 2 core
76
4 Preparation of Reduced TiO 2–x for Photocatalysis
make a review of the current development of TiO 2–x .
In this chapter, we will first discuss the synthesis approaches of TiO 2–x
photocatalysts, including annealing under H 2 atmosphere, vacuum activation, in
situ reduction, metal reduction, electrochemical reduction, UV irradiation, plasma
treatment, and partial oxidation starting from Ti, Ti(II), or Ti(III) precursors, etc.
Then various characterization techniques are discussed to investigate the properties
of TiO 2–x , such as the formation of structural distortions inherent, Ti
3+ species,
oxygen vacancies, or surface hydroxyl bonds. We also summarize the applications
of reduced TiO 2 in the photocatalytic and photoelectric areas, point out further
modification approaches for TiO 2–x in improving their performance, and demonstrate the sustainability of photocatalysis as a solution to environment pollution and
energy crisis.
4.2 Synthesis of TiO 2–x Photocatalysts
Theoretically, reduced TiO 2 photocatalysts could be obtained either by the reduction
of TiO 2 solid materials or Ti(IV) precursors or the oxidation of low-oxidation-state
titanium compounds. Until now, a large number of methods have been developed for
the synthesis of TiO 2–x materials, including thermal treatment under H 2 atmosphere
or other reducing gases, in situ reduction, vacuum activation treatment, metal
reduction, hydrogen or water plasma treatment, electrochemical reduction, UV
light irradiation treatment, and partial oxidation starting from Ti, Ti(II), and Ti(III)
precursors [11]. In general, all these reported methods for the synthesis of TiO 2–x
could be classified by these two main aspects: under reducing or oxidizing
atmosphere.
4.2.1 Reduction Method
4.2.1.1 Thermal Treatment Under Reducing Gases
Thermal treatment under H 2 -gas atmosphere was early used for the synthesis of
TiO 2–x materials, developed by Chen et al. [12]. The pristine TiO 2 nanocrystals were
hydrogenated in a 20-bar H 2 atmosphere at 200
C for 5 days. The obtained TiO 2–x
nanoparticles became black colored (Fig. 4.1a) and disorder engineered (Fig. 4.1b).
Wang et al. investigated the hydrogenation of TiO 2 by H 2 –gas heating at various
temperatures [13]. The color of the obtained hydrogenated TiO 2 depends on the
hydrogen annealing temperature, and it becomes darker with temperature increasing
in the range of 200–500
C [13]. The color implies the extended solar light
absorption of TiO 2 as the result of hydrogenation. Besides, after thermal hydrogenation treatment, disorder surface shell is formed outside the crystalline TiO 2 core
76
4 Preparation of Reduced TiO 2–x for Photocatalysis