photocatalysis and photoelectric conversion efficiency [16–24], and expansion of the
optical response range [25–29]. However, traditional single-phase TiO 2 has defects
in the photocatalytic performance, which are mainly on account of two aspects: on
one hand, owing to the limits of its bandgap, the absorption of visible light is little,
causing the inefficient utilization of the sunlight [3–5]; on the other hand, the high
recombination rate of photo-generated electrons and holes greatly limits the
photocatalytic performance of TiO 2 [30, 31].
Researches have reported that modification of TiO 2 by organic dye photosensitization [32–36], noble metal deposition [37–41], or doping [42–45] or semiconductor compounding [45–49] can effectively enhance the photon utilization and reduce
the recombination rate of photo-generated electrons with holes, thereby improving
the photocatalytic efficiency of TiO 2 . However, those methods have many disadvantages such as low economic effect, poor reaction stability and controllability, and
complicated operation process, among others. In these respects, mixed-phase TiO 2
with relatively simple preparation process, low cost, and adjustable crystal type has
attracted much attention. In the mixed-phase photocatalyst, the effective separation
of the photo-generated electrons and holes, taken place on the two-phase interface,
results in the reduction of their recombination rate. Moreover, adding rutile TiO 2
with a narrower bandgap into pure anatase TiO 2 to form mixed-phase TiO 2 of rutile
and anatase can enhance the utilization of visible light to some extent, further
significantly improving the photocatalytic activity of TiO 2 [50–53]. These
abovementioned advantages of the mixed phase can efficiently enhance the comprehensive properties of TiO 2 photocatalyst. Apart from these researches, other
kinds of mixed-phase TiO 2 have also been studied in depth, such as anatase/brookite
[54, 55], anatase/brookite/rutile [56], and brookite/TiO 2 (B) [57]. The results of these
photocatalytic studies have demonstrated that the photocatalytic activity of mixedphase TiO 2 is higher than that of single-phase TiO 2 , which results from the existence
of different band positions and the promoted separation rate of photo-generated
electron–hole pairs and catalytic “hot spots” at the interface of different phases
[51, 56].
In this chapter, first, we briefly introduce three main kinds of TiO 2 phases
(anatase, rutile, and brookite), and then we focus on recent advances in the development of mixed-phase TiO 2 photocatalysts, including the synthesis for the mixedphase catalysts and their applications to various photocatalytic reaction systems such
as photocatalytic hydrogen production, photoreduction of CO 2 , and photocatalytic
degradation of organic pollutants. Following this we discuss the mechanism of
enhanced photocatalytic activity of the mixed-phase TiO 2 . Finally, the existing
problems of the mixed-phase TiO 2 are summarized, and the application prospects
of this kind of nanomaterials are outlooked.
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6 Phase Control of TiO 2 Photocatalyst
optical response range [25–29]. However, traditional single-phase TiO 2 has defects
in the photocatalytic performance, which are mainly on account of two aspects: on
one hand, owing to the limits of its bandgap, the absorption of visible light is little,
causing the inefficient utilization of the sunlight [3–5]; on the other hand, the high
recombination rate of photo-generated electrons and holes greatly limits the
photocatalytic performance of TiO 2 [30, 31].
Researches have reported that modification of TiO 2 by organic dye photosensitization [32–36], noble metal deposition [37–41], or doping [42–45] or semiconductor compounding [45–49] can effectively enhance the photon utilization and reduce
the recombination rate of photo-generated electrons with holes, thereby improving
the photocatalytic efficiency of TiO 2 . However, those methods have many disadvantages such as low economic effect, poor reaction stability and controllability, and
complicated operation process, among others. In these respects, mixed-phase TiO 2
with relatively simple preparation process, low cost, and adjustable crystal type has
attracted much attention. In the mixed-phase photocatalyst, the effective separation
of the photo-generated electrons and holes, taken place on the two-phase interface,
results in the reduction of their recombination rate. Moreover, adding rutile TiO 2
with a narrower bandgap into pure anatase TiO 2 to form mixed-phase TiO 2 of rutile
and anatase can enhance the utilization of visible light to some extent, further
significantly improving the photocatalytic activity of TiO 2 [50–53]. These
abovementioned advantages of the mixed phase can efficiently enhance the comprehensive properties of TiO 2 photocatalyst. Apart from these researches, other
kinds of mixed-phase TiO 2 have also been studied in depth, such as anatase/brookite
[54, 55], anatase/brookite/rutile [56], and brookite/TiO 2 (B) [57]. The results of these
photocatalytic studies have demonstrated that the photocatalytic activity of mixedphase TiO 2 is higher than that of single-phase TiO 2 , which results from the existence
of different band positions and the promoted separation rate of photo-generated
electron–hole pairs and catalytic “hot spots” at the interface of different phases
[51, 56].
In this chapter, first, we briefly introduce three main kinds of TiO 2 phases
(anatase, rutile, and brookite), and then we focus on recent advances in the development of mixed-phase TiO 2 photocatalysts, including the synthesis for the mixedphase catalysts and their applications to various photocatalytic reaction systems such
as photocatalytic hydrogen production, photoreduction of CO 2 , and photocatalytic
degradation of organic pollutants. Following this we discuss the mechanism of
enhanced photocatalytic activity of the mixed-phase TiO 2 . Finally, the existing
problems of the mixed-phase TiO 2 are summarized, and the application prospects
of this kind of nanomaterials are outlooked.
134
6 Phase Control of TiO 2 Photocatalyst