22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Continuous synthesis of lithium titanates nanoparticles and nanowires in
inductive thermal plasma
F. Quesnel1, G. Soucy1, J. Veilleux1, P. Hovington2, W. Zhu2 and K. Zaghib2
1
Department of Chemical and Biotechnological Engineering, Université de Sherbrooke, Québec, Canada
2
IREQ, Varennes, Québec, Canada
Abstract: Nanoparticles and nanowires of lithium titanates were synthesized using a
thermal plasma torch. The influence of the ratio and type of the reagent as well as the
presence of seed material were investigated. The nanomorphologies were observed by
Scanning Electron Microscopy (SEM), while X-ray diffraction (XRD) combined with a
Rietveld refinement was used to quantify the composition.
Keywords: thermal plasma, lithium titanate, nanowires, nanoparticles
1. Introduction
Plasma synthesis of nanoparticles of titanates have a
long and successful history. Our work aims to further
investigate ways to emulate such success with lithium
titanates, a class of material being considered for anodes
in lithium-ion batteries, superconductors and fusion
reactor blanket [1-3].
Although lithium titanates nanowires and nanotubes
were produced before, their productions pathways mostly
relied on either an alkali catalysis to achieve directional
growth or the addition of lithium to TiO 2 morphologies
produced by hydrothermal or electrochemical processes
[4, 5]. Our work describes some of the first reports of
nanotructures of lithium titanates that were synthesized in
a reproducible manner with a plasma without catalyst.
2. Methods and materials
The experimental setup used a Tekna PL-50 inductive
thermal plasma torch operating at 3 MHz, 66 kPa and
40 kW (see Fig. 1). The sheath gas was a mixture of
10 L min-1 of H 2 and 75 L min-1 of Ar, while the central
gas was Ar at 30 L min-1. These gases flowed in a
water-cooled steel reactor and associated off-gas filtering
system.
The Li 2 O-TiO 2 system was explored with two mixtures
of reagents, in both cases containing Li 2 CO 3 , with the
TiO 2 source being either microsized agglomerates of
nanosized anatase (Mixture A) or nanosized rutile
(mixture B). Mixture A was ball milled and sieved to
80µm, while Mixture B was simply sieved to 80 µm. The
experiments involved gradients of stoichiometry (Li:Ti
ratio) and nanosized Li 4 Ti 5 O 12 seed material, as
described in Table 1.
The XRD patterns were acquired with an X’pert Pro
MRD device using a Cu Kα radiation source and the
scanning electronic microscope pictures were captured on
a Hitachi S-4700 device. A Rietveld refinement using the
Jade 10 software was developed and validated for these
applications in previous work [6].
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Fig. 1. Experimental setup.
Table 1. Definition of experiments.
Test
Powder
Feed rate
(g min-1)
Li:Ti
ratio
a
b
c
d
e
f
A
A
A
B
B
B
6
6
6
3.7
3.7
3.7
0.8
0.9
1
0.8
0.8
0.8
Li 4 Ti 5 O 12
Seed
(wt %)
0.0
0.0
0.0
0.0
0.3
3.0
1
3. Results
Nanopowders were collected in the off-gas filters.
These samples appeared to have different bimodal particle
size distributions, as illustrated in Figs. 2 and 3. Both
partial vaporization and particle agglomeration are likely
pathways for the origin of the larger droplets, with the
former being the most likely in tests a, b, c, while the
latter is the most likely for tests d, e, f.
oriented shapes, the overarching composition of the
samples is consistent with these containing a significant
fraction of Li 4 Ti 5 O 12 . This conclusion is supported by
the compositions shown in Table 2.
Fig. 4. Example of platelets (A) and short wires (B&C)
from Test f.
Fig. 2. Bimodal distribution from Test a.
Fig. 5. Magnified view of the irregularities on a droplet
from Test f.
Fig. 3. Bimodal distribution from Test d.
Some structures were also displayed in the shape of
nanoplatelets and nanowires, present both individually or
joined to larger particles. This can be observed in Figs. 4
and 5.
There appears to be an increase in the number and length
of non-spheroidal morphologies relative to the amount of
Li 4 Ti 5 O 12 seed material. It was also noted that among
the tests conducted with Mixture A, the Test b with a
Li:Ti ratio of 0.9 yields the highest frequency of oriented
shapes. While the characterization studies did not allow
for indisputable identification of the phases contained in
the
2
Table 2.
Compositions of collected samples as
determined by Rietveld refinement.
Phase
(wt %)
a
b
c
d
e
f
Li 4 Ti 5 O 12
23.8
23.9
18.3
31
42.7
48.1
Li 2 Ti 3 O 7
17.8
15.4
10.8
34.7
18.5
16.3
β-Li 2 TiO 3
16.1
23.9
28.3
8.9
15.2
10.1
γ-Li 2 TiO 3
TiO 2
(anatase)
TiO 2
(rutile)
21.7
25.2
35.7
4.3
17.6
12
13.6
7.3
3.2
2
0
0.8
0.4
0
0
8.2
3.7
7.8
Li 2 CO 3
6.5
4.3
3.7
10.8
2.4
4.9
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4. Discussion
Both TiO 2 and Li 2 CO 3 are difficult to vaporize and are
identified in the synthetized material. The pertinent TiO 2
allotropes possess a high density and vaporization point.
Li 2 CO 3 is soft and subject to agglomeration in grinding,
while it decomposes to Li 2 O and CO 2 at a comparatively
low temperature in a way that may induce its discharge
from the plasma plume. As a consequence, a shorter
residence time and lower vaporization would occur. A
gas recirculation pattern may also induce some level of
low-temperature effluent condensation on the cold shell.
Both of these mechanisms may have led to a higher
concentration on the reactor shell, which forms a gradient
that is highest near the torch. Furthermore, ultra-fast
quenching should facilitate the formation of anatase,
while the reducing gas should promote rutile [7].
However, it is reported that the TiO 2 phase(s) observed
mainly correspond to the injected phase.
The strong correlation between the TiO 2 phases may be
partly attributed to a seeding effect. It was illustrated
with the Li 4 Ti 5 O 12 seeds that significant effects (much
higher yield) can be achieved with marginal seeding
material. This would argue against the influence from
irregularities in the mixing or injection as the main source
of the residual reagents.
6. Acknowledgments
This research was enabled by the financial support of
the Natural Sciences and Engineering Research Council
of Canada (NSERC) and the Institut de Recherche en
Électricité du Québec (IREQ). We are especially grateful
to André Bilodeau for his contribution in the operation of
the plasma reactor.
7. References
[1] T. Hoshino, K. Kato, Y. Natori, M. Nakamura,
K. Sasaki, K. Hayashi, T. Terai and K. Tatenuma.
Fusion Engng. Design, 2-6, 84 (2009)
[2] D.C. Johnston, H. Prakash, W.H. Zachariasen and
R. Viswanathan. Mat. Res. Bull., 7, 8 (1973)
[3] T. Ohzuku. J. Electrochem. Soc., 5, 142 (1995)
[4] V. Kumar, J.H. Kim, J.B. Jasinski, E.L. Clark and
M.K. Sunkara. Crystal Growth Design, 7, 11
(2011)
[5] S.C. Lee, S.M. Lee, J.W. Lee, J.B. Lee, S.M. Lee,
S.S. Han, H.C. Lee and H.J. Kim. J. Phys. Chem.
C, 42, 113 (2009)
[6] F. Quesnel, G. Soucy, J. Veilleux, P. Hovington, W.
Zhu and K. Zaghib. TMS: Characterization of
Minerals, Metals and Materials. (2015)
[7] J.-G. Li, M. Ikeda, R. Ye, Y. Moriyoshi and
T. Ishigaki. J. Phys. D: Appl. Phys., 8, 40 (2007)
5. Conclusion
This study demonstrated that the plasma synthesis of
lithium titanates is sensitive to seeding effects in the
formation of both Li 4 Ti 5 O 12 and TiO 2 phases. It was
also shown that the specified plasma conditions allowed
for the production of low-dimension nanostructures in the
collected powders.
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