performed further research on this topic, including the study on brookite-containing
mixed-phase TiO 2 . They prepared bicrystalline anatase/brookite TiO 2 photocatalysts
via a hydrothermal method [118]. The as-prepared anatase/brookite TiO 2 was also
applied for CO 2 photoreduction with water vapor for the production of CO and CH 4 .
Compared with those of pure anatase, pure brookite, and a commercial anatase/rutile
TiO 2 (P25), the photocatalytic activities of bicrystalline anatase/brookite TiO 2 were
better. Wherein, the bicrystalline mixture with a composition of 75% anatase and
25% brookite showed the best photocatalytic performance, whose photocatalytic
activity was nearly twice as high as that of 100% anatase (A100) and three times as
high as that of pure brookite (B100) TiO 2 . Because pure anatase A100 possessed the
largest specific surface area and the smallest bandgap among three types of TiO 2
phases, the higher photocatalytic activity of bicrystalline anatase/brookite was very
likely ascribed to the interactions between the anatase and brookite nanocrystals. In
addition, the result that the anatase-rich bicrystalline anatase/brookite mixtures were
superior to anatase/rutile mixture P25 which indicated the interaction between
anatase and brookite of as-prepared mixed-phase TiO 2 seems to be more effective
than that of P25 in the photoreduction of CO 2 .
6.4.3 Photocatalytic Degradation of Organic Pollutants
on Mixed-Phase TiO 2
In 1977, S. N. Frank and A. J. Bard reported that TiO 2 could effectively decompose
the cyanide in aqueous medium under sunlight for the first time [119]. From then on,
the application of TiO 2 for photocatalytic degradation of organic pollutants such as
organic dyes has raised wide attention due to its effectiveness in degrading and
mineralizing the toxic, bio-refractory, and highly concentrated organic compounds
as well as the possibility of utilizing the solar ultraviolet and visible light spectrum.
Owing to the superiority that mixed-phase TiO 2 usually exhibit better photocatalytic
activity than single-phase TiO 2 , TiO 2 nanomaterials consisting of different phases
have also been widely used to degrade organic pollutants.
There have been many studies on the photocatalytic degradation of organic dyes
using mixed-phase TiO 2 as photocatalyst. For example, under simulated solar light
irradiation, the degradation of methyl blue (MB) was carried out in the aqueous
solution by anatase/rutile TiO 2 heterojunction nanoflowers [120]. It was found that
72% of MB could be degraded in 120 min in the presence of the mixed-phase
photocatalyst prepared in optimized experimental conditions, while only 30% of MB
could be degraded in the solution with 100% anatase. Besides, the TiO 2 nanoflower
photocatalysts showed excellent stability after nine cycles under the same conditions, suggesting that the mixed-phase anatase/rutile TiO 2 heterojunction nanoflower
materials have great potential for the future photodegradation of practical dye
wastewater. Since high surface area favors high photocatalytic activity, mesoporous
mixed-phase anatase/rutile TiO 2 was also fabricated [121] (Fig. 6.15a) to apply for
156
6 Phase Control of TiO 2 Photocatalyst
mixed-phase TiO 2 . They prepared bicrystalline anatase/brookite TiO 2 photocatalysts
via a hydrothermal method [118]. The as-prepared anatase/brookite TiO 2 was also
applied for CO 2 photoreduction with water vapor for the production of CO and CH 4 .
Compared with those of pure anatase, pure brookite, and a commercial anatase/rutile
TiO 2 (P25), the photocatalytic activities of bicrystalline anatase/brookite TiO 2 were
better. Wherein, the bicrystalline mixture with a composition of 75% anatase and
25% brookite showed the best photocatalytic performance, whose photocatalytic
activity was nearly twice as high as that of 100% anatase (A100) and three times as
high as that of pure brookite (B100) TiO 2 . Because pure anatase A100 possessed the
largest specific surface area and the smallest bandgap among three types of TiO 2
phases, the higher photocatalytic activity of bicrystalline anatase/brookite was very
likely ascribed to the interactions between the anatase and brookite nanocrystals. In
addition, the result that the anatase-rich bicrystalline anatase/brookite mixtures were
superior to anatase/rutile mixture P25 which indicated the interaction between
anatase and brookite of as-prepared mixed-phase TiO 2 seems to be more effective
than that of P25 in the photoreduction of CO 2 .
6.4.3 Photocatalytic Degradation of Organic Pollutants
on Mixed-Phase TiO 2
In 1977, S. N. Frank and A. J. Bard reported that TiO 2 could effectively decompose
the cyanide in aqueous medium under sunlight for the first time [119]. From then on,
the application of TiO 2 for photocatalytic degradation of organic pollutants such as
organic dyes has raised wide attention due to its effectiveness in degrading and
mineralizing the toxic, bio-refractory, and highly concentrated organic compounds
as well as the possibility of utilizing the solar ultraviolet and visible light spectrum.
Owing to the superiority that mixed-phase TiO 2 usually exhibit better photocatalytic
activity than single-phase TiO 2 , TiO 2 nanomaterials consisting of different phases
have also been widely used to degrade organic pollutants.
There have been many studies on the photocatalytic degradation of organic dyes
using mixed-phase TiO 2 as photocatalyst. For example, under simulated solar light
irradiation, the degradation of methyl blue (MB) was carried out in the aqueous
solution by anatase/rutile TiO 2 heterojunction nanoflowers [120]. It was found that
72% of MB could be degraded in 120 min in the presence of the mixed-phase
photocatalyst prepared in optimized experimental conditions, while only 30% of MB
could be degraded in the solution with 100% anatase. Besides, the TiO 2 nanoflower
photocatalysts showed excellent stability after nine cycles under the same conditions, suggesting that the mixed-phase anatase/rutile TiO 2 heterojunction nanoflower
materials have great potential for the future photodegradation of practical dye
wastewater. Since high surface area favors high photocatalytic activity, mesoporous
mixed-phase anatase/rutile TiO 2 was also fabricated [121] (Fig. 6.15a) to apply for
156
6 Phase Control of TiO 2 Photocatalyst