Polycrystalline TiO
2
Anatase with a Large Proportion of
Crystal Facets (001): Lithium Insertion Electrochemistry
Milan Bousa,
a,c
Barbora Laskova,
a,c
Marketa Zukalova,
a
Jan Prochazka,
a,
*
Alison Chou,
b
and Ladislav Kavan
a,c,
*
,z
a
J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic,
CZ-18223 Prague 8, Czech Republic
b
Australian Research Council Centre of Excellence for Functional Nanomaterials, The University of
Queensland, Queensland 4072, Australia
c
Department of Inorganic Chemistry, Faculty of Science, Charles University, CZ-12840 Prague 2,
Czech Republic
The electrochemical behavior of TiO
2
anatase with a predominant 001 face ANA001 was studied by cyclic voltammetry of Li
insertion and chronoamperometry. Both voltammetric and chronoamperometric diffusion coefficients and rate constants proved the
higher activity of ANA001 toward Li insertion compared to that of a reference anatase material C240 with dominating 101
facets. The enhanced activity of the anatase 001 face for Li insertion stems from synergic contributions of a faster interfacial
charge transfer at this surface and a facile Li transport within a more open structure of the anatase lattice in the direction parallel
to the c-axis. Despite the larger particle size of ANA001, the values of integral charge capacity and Li-insertion coefficient further
confirmed its improved Li-insertion properties. The results of this study further complete the analogous data published on
single-crystal anatase electrodes and evidence their validity for nanocrystalline materials too.
© 2010 The Electrochemical Society. DOI: 10.1149/1.3479192 All rights reserved.
Manuscript submitted June 10, 2010; revised manuscript received July 16, 2010. Published August 25, 2010.
The insertion of lithium into TiO
2
anatase has been a subject of
intense research in the past see, e.g., Ref. 1-6, for review Ref. 7.
Anatase accommodates lithium electrochemically, which can be
schematically described by the equation
TiO
2
+ xLi
+
+e
-
→ Li
x
TiO
2
; x 0.5 1
Fundamental studies have been carried out on anatase single-crystal
electrodes.
5,6
For the fabrication of electrodes, macroscopic
millimeter-sized single crystals were grown in two orientations
with the exposed faces 101 or 001, respectively.
5,6
The 001 face
had more negative flatband potential and was more active for Li
insertion than the 101 face.
5
The conclusion about the flatband
potential shift has been recently confirmed also on polycrystalline
electrodes.
8
A different activity for Li insertion was attributed to a
more open structure of the anatase lattice in the direction parallel to
the c-axis.
5
The actual argument was that the anisotropy of Li
+
transport normal to the 101 and 001 faces was a consequence of
different numbers the Li
+
hopping events between pseudo-
octahedral positions in the anatase lattice.
5
Due to its thermodynamic stability, the 101 face is dominating
in the usual TiO
2
materials 94% of the total surface area of
ordinary crystals.
9
Hence, many earlier studies of polycrystalline
anatase electrodes addressed the effects occurring virtually on this
face only. Obviously, the studies of polycrystalline electrodes, made
from industrial powders
10
or from laboratory-made materials,
11,12
should rather be regarded as the investigation of anatase 101 be-
cause the proportion of other crystal faces is, in general, very low.
The remaining face on anatase crystal is 001, which is consistent
with the conclusion that a truncated bipyramid is the corresponding
crystal morphology.
13
Only rarely, the rhombic crystals are found,
exposing the 010 face.
14
Both 001 and 010 facets are called
“high energy” or “reactive” ones. They show interesting activity in
catalysis and photocatalysis.
14-16
Recently, Yang et al.
9
have discovered that micrometer-sized
platelets of anatase with 47% of 001 facets can be grown hydro-
thermally from a TiF
4
aqueous solution with HF; the latter acts as
the morphology directing agent. The unusual crystal shape of very
flat truncated bipyramids was stabilized by fluorine termination of
the surface, but the surface can be cleaned from fluorine by heat-
treatment without affecting the crystal shape. This work was an-
nounced as the first demonstration of large anatase crystals with
high percentage of 001 facets.
9
Although there were also earlier
works on the pure 001 face,
5
the facile synthesis of microcrystal-
line anatase enriched with 001 faces
9
opened a pathway to a
deeper investigation of orientation-dependent effects in polycrystal-
line anatase materials. The follow-up studies
15-18
have reported on
materials enriched up to 89% with the 001 face.
To our knowledge, there is only one paper reporting on Li inser-
tion into a F-terminated anatase 001 nanosheet.
17
It concluded that
the nanosheet showed improved charge capacity and cycle life.
However, it is unclear whether the effect is caused by the F-atom
modified surface doping or by the crystal morphology itself.
17
The
investigation of F-free materials remains to be a challenge, which
was the central motivation of our study. Also, we report here on a
detailed investigation of Li-insertion kinetics, which upgrades the
earlier works on single-crystal electrodes
5
and polycrystalline 001
nanosheets.
17
Experimental
Preparation of electrodes.— Anatase was prepared as follows:
1.2 mL of HF 50%; Ajax Finechem was added to 10 mL of tita-
niumIV butoxide. The mixture was sealed in a Teflon cell encased
in a stainless steel autoclave and heated at 200°C for 24 h. The
sample was collected after 24 h of heating and washed with copious
amounts of Milli-Q water. The solid was dried at 100°C. The as-
received material contained 8 wt % of F, as determined by energy-
dispersive X-ray EDX analysis see below for details. After cal-
cination 450°C, 30 min the F content dropped practically to zero.
X-ray diffraction XRD patterns and scanning electron microscopy
SEM images data not shown confirmed that the material was
phase-pure anatase with the plateletlike morphology,
9,17
whereas
both the phase composition and crystal morphology remained intact
during this thermal treatment. The material is further abbreviated as
ANA001. For comparison was used a material coded as C240.
10
Briefly, C240 is a nanocrystalline anatase with the Brunauer–
Emmett–Teller BET surface area, S
BET
= 89 m
2
/g. It was pre-
pared by hydrolysis of titanium tetraisopropoxide, which was fol-
lowed by hydrothermal recrystallization at 240°C in autoclave.
19
The crystal morphology of C240 is characterized by particles ca.
10–20 nm in size exposing mainly the 101 facets.
20
A second
reference material was titanium dioxide P90 from Degussa AG,
Germany powder, with a S
BET
of 100 m
2
/g rutile/anatase mixture
with 90% anatase with trace amount of TiO
2
B.
21
For the prepa-
ration of electrodes, the powder samples were sonicated in pure
* Electrochemical Society Active Member.
z
E-mail: kavan@jh-inst.cas.cz
Journal of The Electrochemical Society, 157 10 A1108-A1112 2010
0013-4651/2010/15710/A1108/5/$28.00 © The Electrochemical Society
A1108
Downloaded 13 Sep 2010 to 147.231.29.120. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp