influence parameters of mixed-phase TiO 2 will be discussed in the following
subsection, combining concrete synthesis methods.
6.3.1 Hydrothermal Method and Solvothermal Method
The hydrothermal method is used most widely in the laboratory fabrication of TiO 2
nanomaterials [25, 84–90]. Using aqueous solution as the reaction system, through
heating or other means, the target photocatalysts are often synthesized in a sealed
reaction vessel (such as an autoclave), which creates an environment of high
temperature and high pressure during the hydrothermal process. In this condition,
with the increase of the water vapor pressure, the density and surface tension of the
materials decrease, and the ion product increases, leading to the dissolution and the
recrystallization of the substances which can be dissolved in water at room temperature. The hydrothermal method is one of the liquid-phase chemical synthesis
methods, and its main factors influencing the properties of the product are vessel
volume, reaction temperature, heating rate, and reaction time.
Chen and Sum et al. [91] fabricated highly crystalline single-phase TiO 2 brookite
and two-phase anatase/brookite TiO 2 nanostructures in NaOH solutions via a simple
hydrothermal method, using TiS 2 as the precursors. They successfully controlled the
phase composition via varying the solution concentration as well as reaction time. In
the reaction system, the precursors TiS 2 underwent a hydrolysis reaction as shown in
the following two equations:
TiS 2 þ 4 NaOH ! Ti
4þ
þ 4 OH
À
þ 2 Na 2 S
ð6:1Þ
Ti
4þ
þ x OH
ð Þ
À ! Ti OH
ð Þ
4Àx
x
ð6:2Þ
The reaction began with the hydrolysis of TiS 2 , forming Ti
4+ ion (Eq. 6.1). Then
Ti
4+ reacted with OH
À in the solution during the hydrothermal process (Eq. 6.2). The
formation of different crystal phases is related to concentration of OH
À . When the
concentration of OH
À is low and thus the hydrolysis rate of Ti
4+ is slow, the
hydrothermal reactions are more likely to form anatase TiO 2 via connecting Ti
4+
with OH
À
; when the OH
À concentration is high, the fast hydrolysis rate of Ti
4+ leads
to the formation of mixed-phase TiO 2 nanomaterials of brookite and anatase. The
mass fraction of brookite (B) and anatase (A) can be calculated by the following two
formulas [92]:
W B ¼ K B I B = K A I A þ K B I B
ð
Þ
ð 6:3Þ
W A ¼ K A I A = K A I A þ K B I B
ð
Þ
ð 6:4Þ
In the formula, K A ¼ 0.886, K B ¼ 2.721, and I A represent the integral intensity of
the strongest diffraction peaks in anatase, and I B represents that in brookite. The
6.3 Synthesis of Mixed-Phase TiO 2 Photocatalysts
139
subsection, combining concrete synthesis methods.
6.3.1 Hydrothermal Method and Solvothermal Method
The hydrothermal method is used most widely in the laboratory fabrication of TiO 2
nanomaterials [25, 84–90]. Using aqueous solution as the reaction system, through
heating or other means, the target photocatalysts are often synthesized in a sealed
reaction vessel (such as an autoclave), which creates an environment of high
temperature and high pressure during the hydrothermal process. In this condition,
with the increase of the water vapor pressure, the density and surface tension of the
materials decrease, and the ion product increases, leading to the dissolution and the
recrystallization of the substances which can be dissolved in water at room temperature. The hydrothermal method is one of the liquid-phase chemical synthesis
methods, and its main factors influencing the properties of the product are vessel
volume, reaction temperature, heating rate, and reaction time.
Chen and Sum et al. [91] fabricated highly crystalline single-phase TiO 2 brookite
and two-phase anatase/brookite TiO 2 nanostructures in NaOH solutions via a simple
hydrothermal method, using TiS 2 as the precursors. They successfully controlled the
phase composition via varying the solution concentration as well as reaction time. In
the reaction system, the precursors TiS 2 underwent a hydrolysis reaction as shown in
the following two equations:
TiS 2 þ 4 NaOH ! Ti
4þ
þ 4 OH
À
þ 2 Na 2 S
ð6:1Þ
Ti
4þ
þ x OH
ð Þ
À ! Ti OH
ð Þ
4Àx
x
ð6:2Þ
The reaction began with the hydrolysis of TiS 2 , forming Ti
4+ ion (Eq. 6.1). Then
Ti
4+ reacted with OH
À in the solution during the hydrothermal process (Eq. 6.2). The
formation of different crystal phases is related to concentration of OH
À . When the
concentration of OH
À is low and thus the hydrolysis rate of Ti
4+ is slow, the
hydrothermal reactions are more likely to form anatase TiO 2 via connecting Ti
4+
with OH
À
; when the OH
À concentration is high, the fast hydrolysis rate of Ti
4+ leads
to the formation of mixed-phase TiO 2 nanomaterials of brookite and anatase. The
mass fraction of brookite (B) and anatase (A) can be calculated by the following two
formulas [92]:
W B ¼ K B I B = K A I A þ K B I B
ð
Þ
ð 6:3Þ
W A ¼ K A I A = K A I A þ K B I B
ð
Þ
ð 6:4Þ
In the formula, K A ¼ 0.886, K B ¼ 2.721, and I A represent the integral intensity of
the strongest diffraction peaks in anatase, and I B represents that in brookite. The
6.3 Synthesis of Mixed-Phase TiO 2 Photocatalysts
139