illustrates the size and morphological
development in the TiO2 sol particles prepared at
different acidities. It can be seen from Fig. 2 (a)
that spherical particles with an approximate
diameter of 4 nm, which is in agreement with the
average crystal size (3.67 nm) determined by XRD,
were uniformly dispersed in the whole aqueous
solution when the pH was 1.0. As the pH was
increased to 2.0, a few irregular and larger TiO2
particles of about 10–15 nm in size started to form
in comparison with Fig. 2 (a). In addition, the
conglomeration of TiO2 particles can be also
observed in Fig. 2 (b). As shown in Fig. 2
(c)–(e), some TiO2 particles of about 20 nm in size
appeared and tended to form more compact
clusters with the increase of pH value. Aging of
the hydrolyzed precipitates in abundant acidic
aqueous solution is a growth process of crystallites,
which is promoted by electrostatic repulsion
between protonated hydrolyzed species. Electrostatic
repulsion counteracts the particle agglomeration
induced by natural attractive Van der
Waals forces. A TiO2 particle was just a single
crystallite as the acidity of the solution was high
enough (pH ¼ 1.0). However, TiO2 particles were
composed of more crystallites and prone to
agglomerate as the acidity of the solution was
decreased. The fact suggests that the higher acidic
condition favors to the formation of smaller and
well-dispersed TiO2 particles.
illustrates the size and morphologicaldevelopment in the TiO2 sol particles prepared atdifferent acidities. It can be seen from Fig. 2 (a)that spherical particles with an approximatediameter of 4 nm, which is in agreement with theaverage crystal size (3.67 nm) determined by XRD,were uniformly dispersed in the whole aqueoussolution when the pH was 1.0. As the pH wasincreased to 2.0, a few irregular and larger TiO2particles of about 10–15 nm in size started to formin comparison with Fig. 2 (a). In addition, theconglomeration of TiO2 particles can be alsoobserved in Fig. 2 (b). As shown in Fig. 2(c)–(e), some TiO2 particles of about 20 nm in sizeappeared and tended to form more compactclusters with the increase of pH value. Aging ofthe hydrolyzed precipitates in abundant acidicaqueous solution is a growth process of crystallites,which is promoted by electrostatic repulsionbetween protonated hydrolyzed species. Electrostaticrepulsion counteracts the particle agglomerationinduced by natural attractive Van derWaals forces. A TiO2 particle was just a singlecrystallite as the acidity of the solution was highenough (pH ¼ 1.0). However, TiO2 particles werecomposed of more crystallites and prone toagglomerate as the acidity of the solution wasdecreased. The fact suggests that the higher acidiccondition favors to the formation of smaller andwell-dispersed TiO2 particles.
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