Anatase TiO2 single crystals with a large percentage of
reactive facets
Author
Yang, Hua Gui, Sun, Cheng Hua, Qiao, Shi Zhang, Zou, Jin, Liu, Gang, Smith, Sean Campbell,
Cheng, Hui Ming, Lu, Gao Qing
Published
2008
Journal Title
Nature
DOI
https://doi.org/10.1038/nature06964
Copyright Statement
© 2008 Nature Publishing Group. This is the author-manuscript version of this paper.
Reproduced in accordance with the copyright policy of the publisher. Please refer to the journal
website for access to the definitive, published version.
Downloaded from
http://hdl.handle.net/10072/60737
Griffith Research Online
https://research-repository.griffith.edu.au
Anatase TiO
2
single crystals with a large percentage
of reactive {001} facets
Hua Gui Yang
1*
, Cheng Hua Sun
1,2*
, Shi Zhang Qiao
1
, Jin Zou
3
, Gang Liu
1,4
, Sean
Campbell Smith
1,2
, Hui Ming Cheng
4
& Gao Qing Lu
1
1
ARC Centre of Excellence for Functional Nanomaterials, School of Engineering and
Australian Institute for Bioengineering and Nanotechnology, The University of
Queensland, QLD 4072, Australia
2
Centre for Computational Molecular Science, Australian Institute for
Bioengineering and Nanotechnology, The University of Queensland, QLD 4072,
Australia
3
Centre for Microscopy and Microanalysis and School of Engineering, The
University of Queensland, QLD 4072, Australia
4
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China
*These authors contributed equally to this work.
1
Due to their scientific and technological importance, inorganic single crystals
with highly reactive surfaces have long been targeted
1-13
. Unfortunately, surfaces
with high reactivity usually diminish rapidly during the crystal growth process
as a result of the minimization of surface energy. A typical example is titanium
dioxide (TiO
2
), which has promising energy and environmental applications
14-17
.
Most available anatase TiO
2
crystals are mainly dominated by the
thermodynamically stable {101} facets (more than 94% according to Wulff
construction
10
),
as opposed to the much more reactive {001} facets.
8-13, 18-20
. Here,
we demonstrate that for fluorine-terminated surfaces this relative stability is
reversed - {001} is energetically preferable to {101}. We explored this effect
systematically for a range of non-metallic atoms (H, B, C, N, O, F, Si, P, S, Cl, Br,
I) by first-principle quantum chemical calculations. Based on theoretical
predictions, we have synthesized uniform anatase TiO
2
single crystals with a
high percentage (47%) of {001} facets by using hydrofluoric acid (HF) as a
morphology controlling agent. Moreover, the fluorated surface of anatase single
crystals can be easily cleaned to render a fluorine-free surface by heat treatment
without altering the crystal structure and morphology.
The surface stability and reactivity of inorganic single crystals (SCs) have long been
thought to be dominated by their surface chemistry, whose effect on the equilibrium
morphology is critical for the synthesis of SCs with high reactivity
1-13, 21
. For anatase
TiO
2
, both theoretical and experimental studies found that the minority {001} facets
in the equilibrium state are especially reactive
8
. However, large high-quality anatase
SCs with a high percentage of {001} facets have not been realized
22-24
. An early
study
23
showed that the hydrothermal treatment of hydrous titanium (IV) oxide in the
presence of HF resulted in irregular aggregates of polymorphic TiO
2
with anhedral
2
morphology. Recently, anatase SCs were synthesized by chemical transport reactions,
but such process suffered from long reaction time, low-purity and no {001} facets
24
.
Therefore, preparation of uniform and high-purity anatase SCs with controllable
crystallographic facets still remains a great challenge.
To this end, various adsorbate atoms have been attempted to change the relative
stabilities of different crystal facets
19-23
. For anatase TiO
2
, among oxygenated surfaces,
{100} is the most stable, rather than {101}, in clean and hydrogenated conditions
5,10,21
.
However, both H- and O-terminated anatase surfaces present high surface energies (γ),
which restrict the formation of large single-crystal anatase. High values of γ are
mainly attributed to the high bonding energies (D
0
) of H-H (436.0 kJ/mol) and O-O
(498.4 kJ/mol)
25
. Therefore, to find a low D
0
element with high bonding to Ti could
be an effective solution for stabilizing the faceted surfaces. Interestingly, F is such an
element as D
0
F-F
= 158.8 kJ/mol
25
and D
0
F-Ti
= 569.0 kJ/mol
26
. To further explore the
effects of various adsorbate atoms, we carried out a systematic investigation of 12
non-metallic atoms X (X = H, B, C, N, O, F, Si, P, S, Cl, Br, I) using first-principle
calculations. Fig. 1a-d depicts the models of clean and X-terminated surfaces of (001)
and (101). The calculated γ values for different adsorbates are illustrated in Fig. 1e,
from which two conclusions can be drawn: (i) among the 12 non-metal-terminated
surfaces and the clean surfaces, F-terminated anatase surfaces have the lowest γ for
both (001) and (101); and (ii) for F-terminated anatase surfaces, (001) is more stable
than (101). These results indicate that a high percentage of anatase {001} facets may
be achievable if their surface is surrounded by F. Furthermore, based on the shape
dependent thermodynamic model proposed by Barnard et al
27
, the optimized ratio of
B/A (as denoted in the inset of Fig. 1) and the percentage of {001} facets can be
predicted if γ is known. As shown in Fig. 1f, the F-terminated surfaces have the
3
highest degree of truncation (approximately B/A→1) and, in turn, the F-terminated
surfaces of anatase TiO
2
should be dominated by {001} facets.
To verify these theoretical predictions, titanium tetrafluoride (TiF
4
) aqueous solution
and HF were used as the anatase SCs precursor and crystallographic controlling agent,
respectively, to generate the truncated anatase bipyramids. Representative scanning
electron microscopy (SEM) images of the products synthesized with different
concentrations of TiF
4
and reaction times are shown in Fig. 2a and 2d. Based on the
symmetries of anatase TiO
2
, the two flat squared surfaces must be {001} facets while
the other eight isosceles trapezoidal surfaces are {101} facets of anatase SCs (further
proved in Fig. 3 and Fig. S7). The yield of anatase TiO
2
SCs is around 90%, even
though some agglomerates and/or irregular particles (see Fig. S4) could be
occasionally observed. To examine the uniformity of the synthesized anatase crystals,
the length of A and the ratio of B/A were statistically analyzed and their results are
presented in Figs. 2b, 2c, 2e, 2f, respectively. The average lengths are 1.66 μm and
1.64 μm with relative standard deviations (RSDs) of 8.4% and 15.8% (Figs. 2b and 2e)
for the cases shown in Figs. 2a and 2d. Their degrees of truncation (assigned as B/A)
are 0.77 and 0.84 with RSDs of 4.3% and 5.1% (Figs. 2c and 2f), respectively. The
percentages of {001} facets can be estimated as 35% and 47%, respectively. The fact
that anatase SCs exhibit a high degree of truncation generated under low
concentration of TiF
4
may be attributed to the higher fluorine density on the surface
thus making the isotropic growth more obvious. This is remarkably consistent with
our theoretical predictions, and can be well understood from the viewpoints of shape-
control chemistry
20, 21
. Previously, Lazzeri and Selloni found that the surface free
energy of (001) can be reduced to 0.51 J/m
2
due to the (1×4) reconstruction
13
,
suggesting that such reconstruction can also stabilize (001). However, based on their
4