influences of different NaOH concentrations and distinct reaction times on TiO 2
crystals are shown in Fig. 6.2a, b. As NaOH concentration increases, the content of
brookite increases, and when the reaction time is over 6 h, the crystalline formation
of the product does not change significantly. In addition, single-phase brookite TiO 2
can be obtained by replacing TiS 2 with sodium titanate. Since Na
+ can stabilize the
layered structure, when there is no Na
+ ion in the reaction system, a layered structure
is instable, and the hydrothermal process causes the destruction of the structure,
forming anatase TiO 2 ; when Na
+ is excessive, the stable layered structure does not
easily collapse and ultimately form titanate; when Na
+ concentration is proper, the
layered structure partially collapses, and part of the layered structure remains
Fig. 6.2 The composition of anatase and brookite in the mixed-phase TiO 2 prepared with (a)
various NaOH concentrations and (b) different reaction times under hydrothermal reaction [91]; (c)
The scheme of formation of anatase and brookite in mixed-phase TiO 2 [91]. (Reprinted with
permission from Ref. [91]. Copyright 2013, American Chemical Society)
140
6 Phase Control of TiO 2 Photocatalyst