anatase. This is because when O 2 acts as an electron acceptor, it is quite sensitive to
properties of the catalyst materials in the process of the photocatalytic reactions. The
surface structure and the low-energy band of rutile may make it show low transfer
efficiency of electrons to O 2 , and thus rutile exhibits low catalytic activity when O 2 is
used as the electron acceptor. Therefore, rutile usually exhibits low photocatalytic
activity based on the fact that most researchers use O 2 for the studies of
photocatalytic reactions.
Because there have been few studies and experiments on brookite and the general
products are mixed-phase TiO 2 , including anatase/brookite [80], brookite/rutile [81],
or anatase/brookite/rutile instead of pure brookite, the photocatalytic activity of
brookite TiO 2 is quite controversial [44].
Recently, lots of studies have demonstrated that the mixed-phase TiO 2 crystals in
an appropriate composition ratio exhibit higher photocatalytic activity than that of
single-phase TiO 2 crystals. Bacsa et al. [82] found that the single-phase TiO 2 such as
pure anatase or pure rutile has relatively lower photocatalytic activity, but a mixture
of anatase and rutile in various ratios achieved enhanced photocatalytic activity than
100% anatase or 100% rutile, when the mixed phase in the ratio of 30% rutile and
70% anatase had the best photocatalytic activity. Thus the TiO 2 catalyzer with two
kinds of phases has a certain synergistic effect for improving photocatalytic activities. And the commercial photocatalyst P25 with considerable high photocatalytic
activity is also a kind of mixed-phase TiO 2 instead of single-phase TiO 2 . Therefore,
great efforts have been made to study in depth on the synthesis and photocatalysis
applications of mixed-phase TiO 2 .
6.3 Synthesis of Mixed-Phase TiO 2 Photocatalysts
It has been found that adjusting the experimental parameters of the synthesis can
change the structure and properties of the three types of crystals, making the
preparation of different mixed-phase TiO 2 come true. At present, there is a large
number of methods to fabricate mixed-phase TiO 2 , such as pulsed laser deposition
(PLD), hydrothermal method, hydrolysis method, solvothermal method,
microemulsion-mediated solvothermal method, solvent mixing and calcination
method (SMC method), high-temperature calcination method, high-temperature
vapor decomposition method, and so on [83]. In the synthesis process of mixedphase TiO 2 , with different preparation methods, the same factor has different effects
on the mixed-phase crystals. In all kinds of synthesis methods, the mixed-phase
ratio, morphology, and surface properties of photocatalysts are controllable via
optimizing the experimental conditions, such as calcination temperature, pressure,
concentration, and types of reagents. The ratio of different crystals in an excellent
mixed-phase photocatalyst should have the ability to be tuned according to the actual
demands, and the product should possess controllable shape as well as uniform
dispersion and should be difficult to agglomerate. The preparation conditions and
138
6 Phase Control of TiO 2 Photocatalyst
properties of the catalyst materials in the process of the photocatalytic reactions. The
surface structure and the low-energy band of rutile may make it show low transfer
efficiency of electrons to O 2 , and thus rutile exhibits low catalytic activity when O 2 is
used as the electron acceptor. Therefore, rutile usually exhibits low photocatalytic
activity based on the fact that most researchers use O 2 for the studies of
photocatalytic reactions.
Because there have been few studies and experiments on brookite and the general
products are mixed-phase TiO 2 , including anatase/brookite [80], brookite/rutile [81],
or anatase/brookite/rutile instead of pure brookite, the photocatalytic activity of
brookite TiO 2 is quite controversial [44].
Recently, lots of studies have demonstrated that the mixed-phase TiO 2 crystals in
an appropriate composition ratio exhibit higher photocatalytic activity than that of
single-phase TiO 2 crystals. Bacsa et al. [82] found that the single-phase TiO 2 such as
pure anatase or pure rutile has relatively lower photocatalytic activity, but a mixture
of anatase and rutile in various ratios achieved enhanced photocatalytic activity than
100% anatase or 100% rutile, when the mixed phase in the ratio of 30% rutile and
70% anatase had the best photocatalytic activity. Thus the TiO 2 catalyzer with two
kinds of phases has a certain synergistic effect for improving photocatalytic activities. And the commercial photocatalyst P25 with considerable high photocatalytic
activity is also a kind of mixed-phase TiO 2 instead of single-phase TiO 2 . Therefore,
great efforts have been made to study in depth on the synthesis and photocatalysis
applications of mixed-phase TiO 2 .
6.3 Synthesis of Mixed-Phase TiO 2 Photocatalysts
It has been found that adjusting the experimental parameters of the synthesis can
change the structure and properties of the three types of crystals, making the
preparation of different mixed-phase TiO 2 come true. At present, there is a large
number of methods to fabricate mixed-phase TiO 2 , such as pulsed laser deposition
(PLD), hydrothermal method, hydrolysis method, solvothermal method,
microemulsion-mediated solvothermal method, solvent mixing and calcination
method (SMC method), high-temperature calcination method, high-temperature
vapor decomposition method, and so on [83]. In the synthesis process of mixedphase TiO 2 , with different preparation methods, the same factor has different effects
on the mixed-phase crystals. In all kinds of synthesis methods, the mixed-phase
ratio, morphology, and surface properties of photocatalysts are controllable via
optimizing the experimental conditions, such as calcination temperature, pressure,
concentration, and types of reagents. The ratio of different crystals in an excellent
mixed-phase photocatalyst should have the ability to be tuned according to the actual
demands, and the product should possess controllable shape as well as uniform
dispersion and should be difficult to agglomerate. The preparation conditions and
138
6 Phase Control of TiO 2 Photocatalyst