Formation mechanism of TiO 2 nanoparticles

Figure 3.9. XRD patterns of products synthesized by hydrothermal treatment of TTIP-TMA solution with (A) R=0.7, (B) R=1.2, (C) R=1.8, (D) R=2.5, (E) R=4.0 and (F) R=8.0 at 160 ^oC for (a) 6, (b) 9, (c) 12, (d)15, (e) 18, (f) 21, (g) 24, and (h) 36h, respectively.

Table 3.2. Contents of anatase, brookite, and rutile TiO2 phases in products obtained by hydrothermal treatment of TTIP-TMA solutions with different R values at 160 ^oC for different reaction times.

The hydrothermal reaction time dependences of the brookite contents in the products with the different R values are presented in Figure 3.10. It is interesting that the changes of brookite contents are dependent on the R value. When the R value is 0.7, no brookite phase is formed in the reaction system. The brookite contents diminish with increasing the hydrothermal reaction time when the R values are 1.2 and 1.8, indicating the brookite phase is unstable and gradually transformed to the anatase phase under these hydrothermal conditions. When the R value is 2.5, the brookite content is almost constant during the hydrothermal reaction up to 36 h. However, when the R value is 4.0, the brookite content increases with increasing the hydrothermal reaction time, that is, the anatase phase is unstable and gradually transformed to brookite phase under the hydrothermal conditions. The single brookite phase is formed when the R value is 8.0, namely the large R value is beneficial to the formation of brookite phase under the

hydrothermal conditions.

Figure 3.10. Dependences of brookite contents in products synthesized by hydrothermal treatments of TTIP-TMAOH solutions with different R values at 160 ^oC on hydrothermal reaction time.

The above results reveal that the pH value and the concentration of TMA⁺ ions play the important roles in formations of the different TiO2 phases in the hydrothermal reaction system. The rutile, anatase, and brookite phases are easy to be formed in the lower pH range, middle pH range, and higher pH range, respectively. Cheng et al.^[43] havereported that under the lower pH conditions, there are less OH^- ligands in the solution, which facilitates the formation of the corner-shared bonding of TiO6 octahedra and results yielding the rutile phase, while under the higher pH conditions, there are more OH -ligands in the solution, which facilitates the formation of edge-shared bonding of TiO6

octahedra and results yielding the anatase phase.

For formation of the brookite phase, except the high pH, the high TMA⁺

concentration maybe is also necessary. Zhao et al.^[15,16] have reported that the cations, such as Na⁺ and NH4+, in the interlayers of a layered titanate play an important role in the phase transformation from the layered titanate to the TiO2 phases because the formation reactions of the TiO2 phases accompany the deintercalation of cations from the interlayer spaces. When the concentration of the cations is low, the deintercalation reaction occurs easily, which causes formation of relatively stable anatase phase. However, when the concentration of the cations is high, the deintercalation is difficult, which causes formation of relatively unstable brookite phase, where an unstable lattice shear can be formed in the phase transformation process from layered titanate to the TiO2 phase, resulting the formation of the unstable brookite phase.

A nanostructural study is carried out on the hydrothermally synthesized products by using FE-SEM and TEM. The fibril particles are formed for all samples prepared by hydrothermal treatments of the TTIP-TMAOH solutions for 1h, seen in Figure 3.11, where only the layered phase is formed. The fibril particle size increases with increasing the R value, due to the large R value and high pH are beneficial to the formation of layered phase under the hydrothermal conditions. The particle morphology changes dramatically with changing the R value when hydrothermal reaction time prolongs to over 3 h. At R=0.7, the uniform six-pointed star-like particles are formed during 3 to 18 h, seen in Figures 3.11A. The six-pointed star-like particles correspond to the anatase phase, in which the anatase lattice fringes with d-values of 0.233 nm and 0.237 nm with an angle of 60^o, corresponding to the (112) and (004) planes, are observed in the HR-TEM image (Figure 3.12(a)). The six-pointed star-like particles are transformed completely to anatase phase after reaction for over 6 h. Ban et al. ^[44] have proposed a mechanism for the formation of the six-pointed star-like anatase particles, namely, the nanosized layered

titanate nanosheets firstly stack, sequentially are transformed to anatase phase by a topotactic structural transformation reaction following self-assembly of the nanosheets.

Figure 3.11. FE-SEM images of products synthesized by hydrothermal treatment of TTIP-TMA solution with (A) R=0.7, (B) R=1.2, (C) R=1.8, (D) R=2.5, (E) R=4.0 and (F) R=8.0 at 160 ^oC for (a) 1h, (b) 3h, (c) 6h, (d) 9h, (e, h) 12h, (f) 15h, and (g) 18h, respectively.

Figure 3.12. TEM and HR-TEM images of products synthetized by hydrothermal treatment of TTIP-TMAOH solutions with R values of (a, b) 0.7, (c, d) 1.8, (e, f) 4.0 and (g, h) 8.0 at 160 ^oC for 3h, respectively.

In the R value range of 1.2 to 2.5, except the six-pointed star-like particles, the almond-like particles are formed also and fraction of the almond-like particles increases with increasing the R value, seen in Figures 3.11B-D. the six-pointed star-like particles are formed by the mechanism of the topotactic structural transformation reaction following self-assembly of the small layered titanate nanosheets similar to the case of R=0.7. The almond-like particles are formed by a self-assembly process of stacking large

layered titanate nanosheets. The increasing fraction of the almond-like particle can be ascribed to that the size increasing of layered titanate nanosheets with increasing the R value. Some bamboo-shoot-like nanocrystals are formed on the surface of the almond-like particles.

The lattice fringes of anatase and brookite phases are observed simultaneously in the HR-TEM image of one almond-like particle prepared at R=1.8 for 3h (Figure 3.12(c, d)).

The brookite phase in the almond-like particles is formed by a topotactic structural transformation reaction from the layered phase under the hydrothermal conditions, which retains the almond-like particle morphology in the reaction process. The bamboo-shoot-like nanocrystals on the almond-bamboo-shoot-like particle surface are formed by stacking small layered titanate nanosheets, sequentially are transformed to anatase phase by the topotactic structural transformation reaction similar to the case of the formation of the six-pointed star-like particle. The bamboo-shoot-like anatase nanocrystals loss their morphology and change to spherical anatase nanocrystals located on the surface of almond-like brookite particles by dissolving-precipitating reaction following Ostwald’s ripening rule during the hydrothermal process, resulting brookite-anatase almond-like nanocomposites.

At R=4, only the almond-like particles are formed due to the formation of large-sized layered titanate nanosheets under the large R value conditions, seen in Figures 3.11E.

Some bamboo-shoot-like nanocrystals are observed also on the almond-like particle surface, which are formed by stacking the small layered titanate nanosheets which are formed by broken the large nanosheets. The lattice fringes of anatase and brookite phases are observed simultaneously also in the HR-TEM image of one almond-like particle prepared at R=4 for 3 h (Figure 3.12(e, f)), namely, formation of the almond-like brookite-anatase nanocomposites. The formation mechanism of the almond-like brookite-brookite-anatase

nanocomposites is the same as the case of R=1.8 described above. At R=8, only the almond-like particles with a smooth surface without the bamboo-shoot-like nanocrystals are formed, seen in Figures 3.11F. The almond-like particles are transformed completely to single brookite phase after hydrothermal reaction for 6 h, seen in Figures 3.11F(c). The TEM result of R=8 at 3h suggests that the brookite phase is formed by the topotactic structural transformation reaction from the layered phase (Figure 3.12(g, h)), which retains the almond-like morphology.

Figure 3.13. (A) TG-DTA curves of layered phase sample synthesized by hydrothermal treatment of TTIP-TMA solution with R value of 1.6 at 160 ^oC for 1h and (B) XRD patterns of samples obtained by calcination of layered phase at (a) 65, (b) 280, (c) 350 and (d) 450 ^oC for 2h.

Based on the above results, a formation mechanism of the TiO2 crystals in the TMAOH hydrothermal reaction system is proposed as shown in Scheme 3.1. In the TTIP-TMAOH reaction system, firstly the lepidocrocite-type layered titanate phase is formed by an acid-base reaction. The TG-DTA and XRD results suggest that the lepidocrocite-type layered phase has a basal spacing of 1.696 nm with a monolayer of water molecule and a monolayer of TMA⁺ ions in the interlayer, seen in Figure 3.13. The formed layered

titanate with TMA⁺ ions in the interlayer is easily to be exfoliated to its nanosheets in the solution similar to the exfoliations of other layered titanates by the TMA⁺ intercalation in solution, ^[9,10] which results the layered titanate nanosheet solution. The size of the layered titanate nanosheets increases with increasing the R value. When R value is small, such as at R=0.7, the small-sized titanate nanosheets are formed, and self-assemble to the six-pointed star-like particles and then transformed into anatase phase by topotactic reaction to give the six-pointed star-like anatase particles under the hydrothermal conditions.

When R value is large, such as at R=8, the large-sized titanate nanosheets are formed, and self-assemble to the almond-like particles and then transformed into brookite phase by topotactic reaction to give the almond-like brookite particles under the hydrothermal conditions. When R value is middle, such as in R=1.2-2.5, both small- and large-sized titanate nanosheets can be formed. The small-sized nanosheets self-assemble to the six-pointed star-like particles and then transformed into the six-six-pointed star-like anatase particles. While the large-sized nanosheets self-assemble to the almond-like particles and then transformed into the almond-like brookite particles. The bamboo-shoot-like nanocrystals on the almond-like particle surface can be formed by stacking small-sized layered phase nanosheets which are formed by broken the large-sized layered phase nanosheets. The bamboo-shoot-like nanoparticles are transformed into anatase nanocrystals by the topotactic reaction, and then change to spherical anatase nanocrystals located on the surface of almond-like brookite particles by dissolving-precipitating reaction following Ostwald’s ripening rule with increasing the hydrothermal reaction time.

This reaction causes the brookite-anatase nanocomposite.

The formed brookite phase can be transformed to anatase phase under low pH conditions, such as at small R values of 1.2 and 1.8, by the dissolving-precipitating

reaction following Ostwald’s ripening rule. On the other hand, the formed anatase phase can be transformed to the brookite phase under high pH conditions, such as at large R value of 4.0, by the dissolving-precipitating reaction following Ostwald’s ripening rule.

The brookite fractions change with increasing reaction time can be described to the dissolving-precipitating reaction. The dissolving-precipitating reaction can change the particle morphology especially at high temperature, such as 200 ^oC.

Scheme 3.1. Formation reaction mechanism of TiO2 polymorphs in TTIP-TMAOH hydrothermal reaction system.