in the hydrothermal reaction system could successfully control the contents of
brookite and anatase in the TiO 2 particles (Fig. 6.3b). According to the ligand field
theory, they explained the mechanism of the phase evolution between brookite and
anatase in which C 4 H 6 O 6 could chelate with Ti to form a stable titanium complex
[98]. When the C 4 H 6 O 6 /TiCl 3 molar ratio was below 0.75 in the reaction system,
there were two forms of Ti species. They were the insoluble Ti-contained species Ti
(OH) 4 and the soluble Ti-contained complex [Ti(OH)x(C 4 H 6 O 6 )y]z-. Under hydrothermal treatment, the amorphous Ti(OH) 4 could first transform to layered titanate.
Given a proper concentration of Na
+ and OH
À , the layered titanate would be
transformed to brookite [94]. Due to the large steric hindrance of carboxylic acidic
ligands, the [Ti(OH)x(C 4 H 6 O 6 )y]z- complexes were inclined to combine together by
sharing equatorial or apical edges and being arranged in zigzag chains, which was
beneficial to form anatase crystallites, thus resulting in an mixed-phase TiO 2 of
anatase and brookite. They also pointed out that the brookite as the pure phase or the
main phase could not be obtained on the conditions that the pH value of the reaction
system was lower than 9 or the hydrothermal temperature was below 180
C. In
other words, it was hard to control the contents of anatase and brookite in samples
just by changing the C 4 H 6 O 6 /TiCl 3 molar ratio while pH < 9 or temperature < 180
C
in the hydrothermal reaction system.
A variation of the hydrothermal method is the solvothermal method wherein
many kinds of organic solvents such as ethanol, glycol, and toluene can be used to
replace an aqueous system. Similar to the hydrothermal method, both the crystal
types and morphology of the TiO 2 nanomaterials can be well controlled by regulating parameters in the solvothermal reaction system, including temperature and
pressure inside the system, the reaction time, and the titanium source.
Through the solvothermal method, Li et al. [99] synthesized anatase/rutile mixedphase TiO 2 crystals with different rutile content by hydrolysis of
tetraisopropyltitanate in an acid alcoholic solution and studied influence of
hydrochloric acid on the rutile content in the mixed-phase crystal. The results
showed that high H 2 O/Ti mole ratio favored the formation of brookite/anatase
mixed-phase TiO 2 , and low H 2 O/Ti mole ratio benefited to form anatase/rutile
mixed-phase TiO 2 . Lei et al. [100] prepared anatase/rutile mixed-phase TiO 2
crystal using a low-temperature (80
C) solvothermal method by pre-oxidizing TiCl 3
into Ti
4+ with HNO 3 followed by diluting with urea, water, and ethanol. They found
that the anatase content in the mixed-phase crystal increased by increasing ethanol
content in the solution (Fig. 6.4). The average particle size of anatase and rutile in the
mixed-phase crystal is both below 10 nm calculated by the Scherrer formula.
The mixed-phase TiO 2 nanomaterials synthesized by hydrothermal and
solvothermal methods are usually well crystallized and are not required to be
calcinated at a certain high temperature. The size and phase type of TiO 2 can be
regulated by simply adjusting the experimental parameters such as the type of base
or acid in the reaction system, the reaction temperature, the heating rate, the reaction
time, and the autoclave pressure. Besides these advantages, there still remain some
drawbacks such as the requirement for the equipment to withstand high pressure as
well as high temperature and experimental safety concerns. Moreover, it is difficult
142
6 Phase Control of TiO 2 Photocatalyst
brookite and anatase in the TiO 2 particles (Fig. 6.3b). According to the ligand field
theory, they explained the mechanism of the phase evolution between brookite and
anatase in which C 4 H 6 O 6 could chelate with Ti to form a stable titanium complex
[98]. When the C 4 H 6 O 6 /TiCl 3 molar ratio was below 0.75 in the reaction system,
there were two forms of Ti species. They were the insoluble Ti-contained species Ti
(OH) 4 and the soluble Ti-contained complex [Ti(OH)x(C 4 H 6 O 6 )y]z-. Under hydrothermal treatment, the amorphous Ti(OH) 4 could first transform to layered titanate.
Given a proper concentration of Na
+ and OH
À , the layered titanate would be
transformed to brookite [94]. Due to the large steric hindrance of carboxylic acidic
ligands, the [Ti(OH)x(C 4 H 6 O 6 )y]z- complexes were inclined to combine together by
sharing equatorial or apical edges and being arranged in zigzag chains, which was
beneficial to form anatase crystallites, thus resulting in an mixed-phase TiO 2 of
anatase and brookite. They also pointed out that the brookite as the pure phase or the
main phase could not be obtained on the conditions that the pH value of the reaction
system was lower than 9 or the hydrothermal temperature was below 180
C. In
other words, it was hard to control the contents of anatase and brookite in samples
just by changing the C 4 H 6 O 6 /TiCl 3 molar ratio while pH < 9 or temperature < 180
C
in the hydrothermal reaction system.
A variation of the hydrothermal method is the solvothermal method wherein
many kinds of organic solvents such as ethanol, glycol, and toluene can be used to
replace an aqueous system. Similar to the hydrothermal method, both the crystal
types and morphology of the TiO 2 nanomaterials can be well controlled by regulating parameters in the solvothermal reaction system, including temperature and
pressure inside the system, the reaction time, and the titanium source.
Through the solvothermal method, Li et al. [99] synthesized anatase/rutile mixedphase TiO 2 crystals with different rutile content by hydrolysis of
tetraisopropyltitanate in an acid alcoholic solution and studied influence of
hydrochloric acid on the rutile content in the mixed-phase crystal. The results
showed that high H 2 O/Ti mole ratio favored the formation of brookite/anatase
mixed-phase TiO 2 , and low H 2 O/Ti mole ratio benefited to form anatase/rutile
mixed-phase TiO 2 . Lei et al. [100] prepared anatase/rutile mixed-phase TiO 2
crystal using a low-temperature (80
C) solvothermal method by pre-oxidizing TiCl 3
into Ti
4+ with HNO 3 followed by diluting with urea, water, and ethanol. They found
that the anatase content in the mixed-phase crystal increased by increasing ethanol
content in the solution (Fig. 6.4). The average particle size of anatase and rutile in the
mixed-phase crystal is both below 10 nm calculated by the Scherrer formula.
The mixed-phase TiO 2 nanomaterials synthesized by hydrothermal and
solvothermal methods are usually well crystallized and are not required to be
calcinated at a certain high temperature. The size and phase type of TiO 2 can be
regulated by simply adjusting the experimental parameters such as the type of base
or acid in the reaction system, the reaction temperature, the heating rate, the reaction
time, and the autoclave pressure. Besides these advantages, there still remain some
drawbacks such as the requirement for the equipment to withstand high pressure as
well as high temperature and experimental safety concerns. Moreover, it is difficult
142
6 Phase Control of TiO 2 Photocatalyst