Environmental Engineering Reference
In-Depth Information
5nm
figure 10.5
High-resolution transmission electron microscopy image showing faceted TiO
2
nanocrystals in the [100] projection.
Reproduced with permission from Ref. [84]. © 2008, Academic Press.
10.6
coNtrolled crystal groWth
Growth of stable {101} facets is generally favored with respect to reactive {001} facets in anatase because it results in a
minimum energy configuration. While reducing the particle size increases the specific surface area, it also decreases the total
fraction of photoreactive {001} facets. for this reason, reducing the particle size alone may not be the most effective way of
improving photocatalytic activity. Nevertheless, anatase nanocrystals with well-defined, truncated bipyramidal shapes were
found to have superior photocatalytic properties than commercially produced nanoparticles [87]. This can be explained by high-
resolution transmission electron microscopy studies of commercial TiO
2
nanoparticles [88] that revealed a wide range of shapes
defined by other low index facets in addition to {001} and {101}. Some of the recent progress in crystal shape engineering by
selective growth of {001} facets using novel synthesis methods will be described next.
macroscopic (~1-µm-diameter) anatase crystals with a higher percentage area of reactive facets were synthesized by a
hydrothermal method using an aqueous solution of Tif
4
as the precursor and Hf as a crystallographic control agent [89]. Due
to its low bonding energy, the presence of f strongly bound to Ti lowers the surface energy and thereby promotes preferential
growth of {001} facets. The degree of truncation of the crystals could be controlled by varying the pH of the solution and B/A
ratios of up to 0.84, equivalent to a surface area consisting of 47% {001} facets, was achieved. An alternative solvothermal
method was subsequently developed [90], using a 2-propanol and Hf mixture as the capping agent, which allowed synthesis of
anatase TiO
2
nanosheets with 64% of {001} facets. measurements of the rate of hydroxyl radical formation under UV irradia-
tion indicated an increased photoreactivity per unit area compared with conventional TiO
2
; however, the practical photocatalytic
efficiency was restricted due to the large crystal size. Improved results were obtained with nanosized {001} faceted TiO
2
crystals
synthesized from Ti(SO
4
)
2
in Hf by means of a hydrothermal technique [91].
Anatase nanocrystals with a truncated bipyramidal form were prepared by dissolving TiO
2
fibers in an aqueous acetic acid
solution followed by a hydrothermal treatment [92]. Their shape was dependent on the acidity of the solution: 9.6% of the
overall surface area consisted of {001} facets at a pH of 1.6, while lower pH values tended to suppress their formation, resulting
in a sharp tetragonal shape. An ionic liquid-assisted hydrothermal synthesis method, using a Ticl
4
precursor in Hf or H
2
SO
4
,
was effective in controlling the crystal shape and inhibiting phase transformation [93]. The nanoparticles produced with Hf
varied from square plates to highly truncated bipyramids, while those prepared using H
2
SO
4
had a zigzag morphology, elongated
in the [001] direction, that consisted of alternating truncated pyramidal and parallelepiped sections connected by common {001}
interfaces. TiO
2
nanosheets with elevated percentages of {001} facets have also been produced by a different hydrothermal
method using tetrabutyl titanate in a 47% aqueous solution of Hf as the precursor [94].
While hydrothermal methods are the most commonly employed, various other synthesis methods for nanoparticles with
controlled shapes have been reported. Decahedral anatase nanocrystals with high photocatalytic activities were prepared by
gas-phase reaction of Ticl
4
with O
2
using a rapid heating and quenching technique [95, 96]. Nanocrystals with a large percentage
of exposed high-energy {001} and {010} facets, synthesized by a nonaqueous method, using titanium isopropoxide as the pre-
cursor [97], showed significantly increased photocatalytic activity in comparison to commercial TiO
2
nanopowder. A solvother-
mal technique has been developed to enable production of various nanocrystal morphologies, including rhombic, truncated
