Supplemental Information
One-Step Fast Synthesis of Li4Ti5O12 Particles Using an Atmospheric
Pressure Plasma Jet
Shih-Min Chang, Erwin Franco Rodríguez Tolava, Yao-Jhen Yang, Hsin-Chieh Li,
Rung-Chuan Lee, Nae-Lih Wu*, and Cheng-Che Hsu*
Department of Chemical Engineering
National Taiwan University, Taipei 10617 Taiwan
Corresponding Authors’ E-mail: Cheng-Che Hsu (chsu@ntu.edu.tw), Nae-Lih Wu
(nlw001@ntu.edu.tw; for battery characterization)
*
Figure S1 shows the microstructure of the particles fabricated under various
precursor concentrations and carrier gas flow rates when a preheater is not used. In
this set of experiment, a great number of particles with shell-like or porous structures
are observed. With a 0.05 M precursor concentration, particles with sizes from ca. 200
nm to 3 m are observed. The rather broad size distribution is the nature of the
nebulizing process, by which droplets with a broad range distribution are formed. An
increase in the precursor concentration results in the increase in the fraction of
number of porous particles with diameters greater than 1 m. To further examine the
interior microstrcture of spheric particles, the inner structure of a selected particle is
examined using FIB-SEM, as shown in the inset under the condition of 0.4 M and 1.5
slm in Fig. S1. It shows that the spherical particles are shell-structured. The formation
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of shell-like or porous particles can be well explained as following: when the droplets
are in contact with the plasma jet, fast solvent evaporation from the droplet surface
occurs. This results in the case where the solute concentration at the droplet surface
reaches critical supersaturation, while the solute concentration at the center of the
droplet is still lower than the saturation concentration. Under this circumstance, the
solute near the droplet surface precipitates, forming a crust, and shell-like or hollow
structure are therefore observed 1. During the heating process, the burst of solid crust
occurs occasionally when the inner pressure of the droplets is high and the crust is
dense.2, 3 For particles obtained under 0.4 M precursor concentration and 1.5 carrier
gas flow rate without the use of a pre-heater, the Ti/O ratio obtained by EDS and XPS
are 0.29 and 0.34, respectively. The BET specific surface area (SSA) for these
powders is 41.8 m2/g.
When a preheater is used, the precursor droplets are heated to 450 °C in a humid
atmosphere prior to entering the jet downstream. With the same carrier gas flow rate
used above (0.7 and 1.5 slm), the droplets are heated up with a lower rate than the
case when the preheater is not used. A relatively lower solvent evaporation rate at the
droplet surface is expected. Such a low evaporation rate allows the solute at the
droplet surface to diffuse toward the droplet center before precipitation at the surface
2
occurs. In this case, a spherical particle is formed, as shown in the cases with 0.7 and
1.5 slm carrier gas flow rate in Figure S2. Under the condition of 0.4 M and 1.5 slm,
the FIB-SEM image (inset, 0.4 M and 1.5 slm in Fig. S2) shows a porous structure
inside the spherical particles. Under the condition of 0.05 M and 0.7 slm, a higher
magnification SEM image of a seleced broken particle shown in the inset
demonstrates that the spherical particle is rather dense. An increase in the carrier gas
flow rate to 3 slm leads to a more rapid heating rate of the droplet because the
droplets take less time to travel in the preheater, and then enters the plasma jet. Such a
rapid heating rate results in a more rapid solvent evaporation at the droplet surface,
resulting in the increase in the solute concentration, followed by solute precipitation.
Sell-like and porous particles are obtained, as shown in the case with 3 slm carrier gas
flow rate in Figure S2. For particles obtained under 0.2 M precursor concentration and
1.5 carrier gas flow rate with the use of a pre-heater, the Ti/O ratio obtained by EDS
and XPS are 0.36 and 0.33, respectively. The SSA for these powders is 40.9 m2/g.
Such a high SSA is a result of the porous structure inside the spherical particles, as
described above.
At a given carrier gas flow rate, the particle size increases with the precursor
concentration. For example, with 0.7 slm carrier gas flow rate, the particle size
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increases from approximately 100-300 to 400-2200 nm when the precursor
concentration increases from 0.05 to 0.4 M. Such an observation can be well
explained by the mass balance of the droplet to particle conversion. Under this
conversion, the droplet undergoes solvent evaporation and the reaction of the solute to
the product. If the particle is dense, its size can be estimated by the following
equation4:
d p ,v  d d ,v (
MC
p
)1 / 3
(S1)
where dp, v and dd,v are the product particle and precursor droplet diameters, M is the
molecular weight of precursor solute, C is the precursor concentration, and p is the
density of product material. Due to the fact that the precursor solution is prepared using
the stoichiometric amount of Li and Ti, the molecular weight of LTO is used for M.
Given the precursor droplet size, the molecular weight, and density of LTO, the particle
sizes are estimated to be 120-940, 190-1520, and 235-1880 nm when the precursor
concentrations are 0.05 M, 0.2 M, and 0.4 M, respectively. With a carrier gas flow rate
of 0.7 slm, the observed particle size shown in Figure S2 reasonably agrees with the
estimated size shown above. At a higher carrier gas flow rate, particles with larger sizes
are observed. This is primarily due to the fact that the droplets are heated at a higher rate
and less dense particles are formed.
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A. Gurav, T. Kodas, T. Pluym and Y. Xiong,"Aerosol Processing of Materials,"
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(2008).
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Figure S1: SEM images of Li4Ti5O12 particles prepared using different precursor
concentration and carrier gas flow rates by using the APPJ without the use of a
pre-heater. The plasma is operated under the condition of 30 slm N2 flow, 275 V
applied voltage, and a 7 s /33 s on/off duty cycle.
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Figure S2: SEM images of Li4Ti5O12 particles prepared using different precursor
concentration and carrier gas flow rates by using the APPJ with pre-heater(450℃).
The plasma is operated under the condition of 30 slm N2 flow, 275 V applied voltage,
and a 7 s /33 s on/off duty cycle.
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