the high-resolution transmission electron microscopy (HRTEM), the phase interface
could be seen clearly, indicating a close interaction existing between the two phases.
Bojinova et al. [104] synthesized an anatase/rutile mixed-phase TiO 2 by the SMC
method using ethanol as the solvent. In the product, the grain size of anatase was
42 nm and that of rutile was 56 nm which are calculated according to the Scherrer
formula. Liu et al. [105] used a layer-by-layer self-assembly technique to make the
anatase small particles adsorbed onto rutile nanorods with assist of polystyrene
sulfonate (PSS) as a medium, and finally the PSS was removed by calcination.
The anatase content in the mixed-phase TiO 2 crystal could be controlled by adjusting
the number of loading cycles.
The phase content of the mixed-phase TiO 2 can be regulated easily by using the
SMC synthesis method. However, because the final calcination process requires high
temperature, the TiO 2 is easy to agglomerate. In addition, it is usually difficult to
obtain the mixed-phase TiO 2 with anatase and rutile mixing uniformly, causing
products with many pure rutile aggregates as well as pure anatase aggregates.
Therefore, the photocatalytic performance of the mixed-phase TiO 2 crystals produced by this method is limited.
6.3.5 High-Temperature Calcination Method
The high-temperature calcination method, which is mostly used to research the
phase transition of TiO 2 , is one of the earliest methods for the synthesis of mixedphase TiO 2 [25].
Through the high-temperature calcination method, Nair et al. [106] succeeded in
preparing the mixed-phase TiO 2 nanomaterials with different anatase and rutile
contents. They firstly prepared small anatase particles by a sol–gel method followed
by calcinating the anatase particles at elevated temperatures. It was found that the
samples which were calcined at low temperatures (T < 600
C) remained pure
anatase phase. The phase transformation began at the temperature of 650
C.
When the calcination temperature was raised to 850
C, the product converted to
pure rutile. As calculated by the Scherrer formula, the grain sizes of anatase and
rutile in the mixed-phase products were 33.66~51.48 nm and 45.2~60.6 nm, respectively. Figure 6.9 showed that the particle size of anatase was about 100 nm and that
of rutile was about 200 nm, and the mixed-phase particle size was between the above
two and increased with the increase of calcination temperature. The phase transformation includes nucleation and growth. However, there is some dispute on the
concrete process. Gouma et al. [107] considered that the rutile firstly formed nuclear
on the surface of anatase and then expanded to the bulk, while Zhang et al. [108]
verified the phase transformation of anatase–rutile occurred in the bulk at first and
then spread to the surface with the increase of calcination temperature via UV
Raman spectroscopy. Figure 6.10 shows the phase’s transformation.
The simple high-temperature calcination method can be used to obtain mixedphase TiO 2 nanomaterials with perfect polymorphs and tunable phase content.
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6 Phase Control of TiO 2 Photocatalyst
could be seen clearly, indicating a close interaction existing between the two phases.
Bojinova et al. [104] synthesized an anatase/rutile mixed-phase TiO 2 by the SMC
method using ethanol as the solvent. In the product, the grain size of anatase was
42 nm and that of rutile was 56 nm which are calculated according to the Scherrer
formula. Liu et al. [105] used a layer-by-layer self-assembly technique to make the
anatase small particles adsorbed onto rutile nanorods with assist of polystyrene
sulfonate (PSS) as a medium, and finally the PSS was removed by calcination.
The anatase content in the mixed-phase TiO 2 crystal could be controlled by adjusting
the number of loading cycles.
The phase content of the mixed-phase TiO 2 can be regulated easily by using the
SMC synthesis method. However, because the final calcination process requires high
temperature, the TiO 2 is easy to agglomerate. In addition, it is usually difficult to
obtain the mixed-phase TiO 2 with anatase and rutile mixing uniformly, causing
products with many pure rutile aggregates as well as pure anatase aggregates.
Therefore, the photocatalytic performance of the mixed-phase TiO 2 crystals produced by this method is limited.
6.3.5 High-Temperature Calcination Method
The high-temperature calcination method, which is mostly used to research the
phase transition of TiO 2 , is one of the earliest methods for the synthesis of mixedphase TiO 2 [25].
Through the high-temperature calcination method, Nair et al. [106] succeeded in
preparing the mixed-phase TiO 2 nanomaterials with different anatase and rutile
contents. They firstly prepared small anatase particles by a sol–gel method followed
by calcinating the anatase particles at elevated temperatures. It was found that the
samples which were calcined at low temperatures (T < 600
C) remained pure
anatase phase. The phase transformation began at the temperature of 650
C.
When the calcination temperature was raised to 850
C, the product converted to
pure rutile. As calculated by the Scherrer formula, the grain sizes of anatase and
rutile in the mixed-phase products were 33.66~51.48 nm and 45.2~60.6 nm, respectively. Figure 6.9 showed that the particle size of anatase was about 100 nm and that
of rutile was about 200 nm, and the mixed-phase particle size was between the above
two and increased with the increase of calcination temperature. The phase transformation includes nucleation and growth. However, there is some dispute on the
concrete process. Gouma et al. [107] considered that the rutile firstly formed nuclear
on the surface of anatase and then expanded to the bulk, while Zhang et al. [108]
verified the phase transformation of anatase–rutile occurred in the bulk at first and
then spread to the surface with the increase of calcination temperature via UV
Raman spectroscopy. Figure 6.10 shows the phase’s transformation.
The simple high-temperature calcination method can be used to obtain mixedphase TiO 2 nanomaterials with perfect polymorphs and tunable phase content.
148
6 Phase Control of TiO 2 Photocatalyst