small enough (<13 nm), the small surface free energy will play a decisive role in the
phase transformation [74]. For TiO 2 nanocrystals with the size smaller than 11 nm,
anatase is the most stable phase, and the rutile phase with nanocrystals size bigger
than 35 nm performs the thermodynamic stability. As for brookite phase, its stability
lies between the anatase and rutile. In fact, as a metastable phase, the major physical
parameters of brookite are between those of anatase and rutile. For example, the
bandwidths of anatase and rutile are 3.19 and 3.0 eV, respectively, while that of
brookite is 3.11 [75].
6.2.3 Photocatalytic Activity of Rutile, Anatase, and Brookite
Among the three types of phases, anatase exhibits the highest photocatalytic activity,
which is on account of the following aspects:
1. The bandgap of anatase is 3.19 eV, while that of rutile and brookite are 3.0 eV and
3.11 eV, respectively [62]. Therefore, the electron–hole pair of anatase has more
positive or more negative potential, improving the oxidation ability [76].
2. The surface of anatase has a stronger adsorption ability for H 2 O, O 2 , and OH,
which conduces to high photocatalytic activity because the adsorption capacity of
the surface has a dramatic influence on the photocatalytic activity during the
photocatalytic reaction, and strong adsorption capacity benefits to high activity.
3. Compared to rutile and brookite, anatase usually exhibits smaller grain size and
larger specific surface area in the crystallization process, enhancing the
photocatalytic activity.
However, due to the crystallization process greatly influencing the photocatalytic
activity, the above rules do not suit for all situations. When the amorphous TiO 2
crystallize, rutile usually forms large grains with poor surface properties and thus
exhibits low photocatalytic activity. Under the same conditions, if the rutile can have
the same grain size and adsorption ability as anatase, it can also exhibit high
photocatalytic activity. For example, Lee et al. [77] found that through the treatment
of laser exposure, anatase phase of TiO 2 can transfer to rutile phase without the
change of the specific surface area and grain size. This resultant rutile TiO 2 exhibited
considerable high photocatalytic activity. Tsai et al. [78] fabricated anatase and rutile
TiO 2 via different methods and investigated their photocatalytic activity for the
degradation of phenol. It is found that the preparation methods as well as the
calcination temperature have a remarkable influence on the photocatalytic activity
of TiO 2 catalyzers. Under certain conditions, rutile TiO 2 exhibited very high catalytic activity. Therefore, whether anatase or rutile, the photocatalytic activity of
materials greatly depends on the grain size and the surface properties. Apart from
that, Ohno et al. [79] demonstrated that the photocatalytic activity of different phase
TiO 2 relates to the electron acceptors in the system. When the electron acceptor is
O 2 , the photocatalytic activity of anatase is relatively higher than that of rutile. And
when Fe
3+ is the electron acceptor, rutile exhibits higher catalytic activity than
6.2 Phases of TiO 2
137
phase transformation [74]. For TiO 2 nanocrystals with the size smaller than 11 nm,
anatase is the most stable phase, and the rutile phase with nanocrystals size bigger
than 35 nm performs the thermodynamic stability. As for brookite phase, its stability
lies between the anatase and rutile. In fact, as a metastable phase, the major physical
parameters of brookite are between those of anatase and rutile. For example, the
bandwidths of anatase and rutile are 3.19 and 3.0 eV, respectively, while that of
brookite is 3.11 [75].
6.2.3 Photocatalytic Activity of Rutile, Anatase, and Brookite
Among the three types of phases, anatase exhibits the highest photocatalytic activity,
which is on account of the following aspects:
1. The bandgap of anatase is 3.19 eV, while that of rutile and brookite are 3.0 eV and
3.11 eV, respectively [62]. Therefore, the electron–hole pair of anatase has more
positive or more negative potential, improving the oxidation ability [76].
2. The surface of anatase has a stronger adsorption ability for H 2 O, O 2 , and OH,
which conduces to high photocatalytic activity because the adsorption capacity of
the surface has a dramatic influence on the photocatalytic activity during the
photocatalytic reaction, and strong adsorption capacity benefits to high activity.
3. Compared to rutile and brookite, anatase usually exhibits smaller grain size and
larger specific surface area in the crystallization process, enhancing the
photocatalytic activity.
However, due to the crystallization process greatly influencing the photocatalytic
activity, the above rules do not suit for all situations. When the amorphous TiO 2
crystallize, rutile usually forms large grains with poor surface properties and thus
exhibits low photocatalytic activity. Under the same conditions, if the rutile can have
the same grain size and adsorption ability as anatase, it can also exhibit high
photocatalytic activity. For example, Lee et al. [77] found that through the treatment
of laser exposure, anatase phase of TiO 2 can transfer to rutile phase without the
change of the specific surface area and grain size. This resultant rutile TiO 2 exhibited
considerable high photocatalytic activity. Tsai et al. [78] fabricated anatase and rutile
TiO 2 via different methods and investigated their photocatalytic activity for the
degradation of phenol. It is found that the preparation methods as well as the
calcination temperature have a remarkable influence on the photocatalytic activity
of TiO 2 catalyzers. Under certain conditions, rutile TiO 2 exhibited very high catalytic activity. Therefore, whether anatase or rutile, the photocatalytic activity of
materials greatly depends on the grain size and the surface properties. Apart from
that, Ohno et al. [79] demonstrated that the photocatalytic activity of different phase
TiO 2 relates to the electron acceptors in the system. When the electron acceptor is
O 2 , the photocatalytic activity of anatase is relatively higher than that of rutile. And
when Fe
3+ is the electron acceptor, rutile exhibits higher catalytic activity than
6.2 Phases of TiO 2
137