Deposition of Lath-Shaped -MoO3 Particles by Solution Precursor
Plasma Spray
Mehdi Golozar, Ken Chien, and Thomas W. Coyle
Centre for Advanced Coating Technologies, University of Toronto, Toronto, Canada
Abstract: Compared to electrical double-layer capacitors (EDLCs) using carbon-based electrodes, recent studies
have shown greater electrical energy storage capacity associated with electrodes of transition metal oxides and
nitrides. This new generation of super capacitors exhibits mixed double-layer and pseudo-capacitive properties.
The possibility of using Solution Precursor Plasma Spray (SPPS) to fabricate super-capacitor electrodes from
transition metal oxides/nitrides is under investigation in the current study.
SPPS uses precursor solutions as the feedstock injected into the DC-arc plasma instead of the powder used in
conventional thermal spray processes to deposit coatings of fine grain size and high surface area. Large specific
surface area is a determinant factor for the large electrical energy storage capability of super-capacitors. A wide
range of lath-like MoO3 structures has been observed, varying in size, alignment and configuration. Various spray
operating parameters affecting the thermal history of the precursor solution droplets, including the number of
spray passes and standoff distances, were investigated. The results of x-ray diffraction phase analyses and
scanning electron microscopy observations will be discussed and related to the deposition parameters.
Keywords: Molybdenum Oxide (-MoO3), Super-Capacitor, and Solution Precursor Plasma Spray (SPPS)
1. Introduction
2. Experimental Procedures
Plasma spray deposition technology is a costeffective and well-established industrial process.
Solution Precursor Plasma Spray (SPPS) uses
precursor solutions as the feedstock injected into the
DC-arc plasma, instead of the powder used in
conventional thermal spray processes, to deposit
oxide coatings, Fig. 1. The deposits can exhibit fine
grain sizes, high porosity levels, and high surface
area. Studies have revealed fractured hollow spheres
or crust to be the signature structure of these porous,
high surface area coatings. These hollow spheres are
produced due to droplet surface precipitation caused
by the low concentration of the precursor solution
[1, 2]. SPPS can produce high surface area -MoO3
deposits possessing continuous pore network ideal
for application as super-capacitor electrodes, Fig. 3.
Understanding the evolution of the precursor
solution and controlling the nucleation and growth
behavior of the high surface area -MoO3 features
are crucial in developing coatings of the desired
structure. This paper investigates the thermal history
of deposits and explains the formation of a wide
range of -MoO3 surface morphologies produced
using an SPPS coating process.
Solution Precursor Plasma Spray (SPPS)
A stream of the precursor solution was injected
radially using the EFD MicrosprayTM valve into the
plasma using a 200μm nozzle, where it broke up into
small droplets. The stream was injected into the hot
zone of the plasma jet to maximize in-flight solvent
evaporation, solute precipitation and precipitate
pyrolysis prior to the substrate impact. Coatings
were deposited on 1mm thick 304 stainless steel
coupons, 1.25cm x 7.5cm.
Solution
Precursor
Atomization Gas
Actuating Gas
Plasma Gas
Cathode
Nozzle
Control
Console
Pressure
Vessel
Substrate
Anode
Air
Air
Ar
CO2
CH4
+
Cooling
System
Substrate
Solution
Plasma
Standoff Distance
Flow Rate
Concentration
Flow Rate
Composition
Power
Substrate Standoff Distance
• Distance: 55-155mm
• Fly Time: 50-200μs
Power
Supply
55, 75, 95, 115, 135 and 155mm
23 ml/min
10 wt%
35 SLM
Ar, CO2 and CH4
35 kW
Figure 1. SPPS experimental setup and operating parameters.
Coating Characterization
The morphology of the deposits was examined using
secondary electron imaging in a scanning electron
microscope (SEM). The samples were sufficiently
conductive that coating of the samples was not
necessary. Phase analyses of the deposited and heattreated material were carried out by x-ray diffraction
(XRD) using copper K radiation ( = 1.5Å); the
amperage and voltage were 40mA and 40V,
respectively. The 2 diffraction angle range of 5° 45° was studied, where most high-intensity peaks
were previously observed. The scan rate was 1°/min.
Electrochemical study of the deposits was conducted
using cyclic voltammetry in the electrolyte solution
of 0.5M H2SO4. The sweep rate was 100mV/sec.
Figure 2. Increase in the surface charge storage capability of -MoO3
super-capacitor electrodes at shorter substrate standoff distances.
a
3. Results and Discussion
Porous nanocrystalline -MoO3 coatings were
produced by 16 passes of the torch at 6 substrate
standoff distances, Fig. 1. Fig. 2 shows a larger
capacitance for coatings deposited at shorter standoff
distances, which correlates with the presence of high
surface area, sub-micron lath structures, Fig. 3a.
These coatings were found by XRD analyses to be
predominantly -MoO3 with only a trace of
undecomposed ammonium molybdate at the larger
standoff distances, Fig. 4. It is worth noting that
shorter standoff distance deposits exhibited a higher
degree of crystallinity that may also contribute to
their larger capacitance, Fig. 4. A similar effect was
observed by comparing the charge storage capability
of crystalline vs. amorphous -MoO3 [3]. To further
increase the surface charge storage capacity of the
deposit, it is necessary to understand and control the
nucleation and growth behavior of these high surface
area -MoO3 crystallographic lath features.
Deposit Thermal History
The nucleation and growth behavior of the lathshaped features are controlled by the thermal history
of the deposit. This includes heating of the droplets
in-flight, and heating of previously deposited
material by the plasma tail and/or the substrate.
In-Flight Heating of the Droplets
Single-pass deposition tests were conducted to study
the effect of the in-flight droplet thermal history on
5μm
b
5μm
Figure 3. 75mm deposits, a, exhibited the presence of lath-shaped MoO3 features responsible for the high capacitance. No such surface
features were observed at a farther standoff distance of 135mm, b.
155mm
135mm
Ammonium Molybdate
-MoO3
10
115mm
15
20
95mm
75mm
-MoO3
55mm
0
5
10
15
20
25
30
35
40
Scattering Angle (2)
45
50
55
27.0
60
65
27.5
28.0
Figure 4. XRD patterns illustrated the formation of -MoO3 as the
major phase at all standoff distances with only the trace of ammonium
molybdate in the far coating.
the formation of the surface lath structures by
minimizing the effects originating from either direct
exposure of the deposits to the plasma tail or heating
of the substrates. The longer the in-flight droplet
residence time, the higher is the chance of complete
solvent evaporation, solute precipitation, and
precipitate decomposition; thereby forming the lathshaped crystallites prior to impact on the substrate.
XRD analyses of the single-pass spray trials showed
that undecomposed ammonium molybdate deposited
on all standoff distance substrates, including the
155mm deposits for which the in-flight droplet
exposure to the plasma heat is most prolonged.
However, ammonium molybdate partly transformed
into -MoO3 at shorter standoff distances due to the
high heat flux; small peaks arising from ammonium
molybdate remained at short standoff distances, Fig.
5. No sharp-edged features were found at any
standoff distance, Fig. 6. At 55mm it appears that
rapid solvent evaporation from the droplet occurred
at impact due to the high heat flux. The droplets
boiled and the deposits dried into the mixture of
ammonium molybdate and -MoO3. In contrast, at
155mm the heat flux was lower; the evaporation was
slower and more uniform resulting in a denser
coating with sporadic protrusions. The collapsed
protrusions were likely caused by the escape of
water vapor trapped during dehydration of the
precursor, Fig. 6b.
80
60
40
20
155mm
100
135mm
Intensity
50
150
100
50
115mm
200
95mm
Undecomposed Peak Range
(ammonium molybdate)
•
•
•
0
•
500
75mm
•
•
•
0
500
55mm
•
•
•
•
•
0
0
5
10
15
20
25
Scattering Angle (2)
•
30
•
35
40
45
Figure 5. XRD patterns of the single-pass coatings showing peaks of
undecomposed ammonium molybdate, unless stated otherwise: mixture
of ammonium molybdate/-MoO3 peaks (), and pure -MoO3 peaks (•).
Since the in-flight droplets did not completely
decompose to form the sharp-edged -MoO3
features prior to impact, the exposure of previously
deposited material to the plasma tail during
subsequent passes of the torch must play a major
role in the formation of -MoO3 lath structures.
a
20μm
b
50μm
Figure 6. SEM micrographs of the single-pass coatings at 55mm, a,
and 155mm, b, standoff distances.
Post Deposition Heating of the Deposits
To investigate the effects of heating previously
deposited material, a coating was produced with 2
passes of the torch at a 55mm standoff distance. The
deposit was subsequently heat treated by passing the
plasma jet over the surface with no solution injected
into the plasma, resulting in the formation of a wide
range of -MoO3 surface features, Fig. 7.
Far from the centre of the torch path, in zone 1, no
crystallographic lath features were observed in the
deposit. It resembled the single-pass deposits
obtained at the shortest standoff distance, Fig. 6a. In
zone 2, Fig. 7b, the formation and subsequent
growth of lath-shaped features superimposed on the
spherical particles is evident, similar to that shown
in Fig. 3a. The formation and growth of such lathshaped features can therefore be attributed to
exposure of the previously deposited material to the
plasma tail. Near the centerline, zone 5, where the
thermal flux from the torch was highest, the deposit
did not exhibit any of the spherical particles seen in
the other micrographs, which presumably formed
from the droplets. No crystallographic lath features
a
200μm
b (zone 2)
5μm
were observed. Rather, the coating was made up of
sub-micron, blocky or equi-axed grains. The deposit
was relatively flat at scales larger than the grain size,
suggesting that the MoO3 had melted to form a
uniform layer on the surface, then solidified after the
passage of the torch. A similar microstructure was
obtained by heating dried precursor solution droplets
to 500°C for 10min to ensure the formation of MoO3, followed by heat-treatment of the -MoO3 at
1000°C (above its melting point of 795°C) for
10min, then furnace cooled. Complete pyrolysis of
ammonium molybdate was found by DTA method to
occur at 360°C producing pure -MoO3 phase [4].
At intermediate distances from the centerline, zone 3
and 4, very large laths were formed, often organized
into flower-like shapes, on top of a uniform dense
layer similar to that found in zone 5, Fig. 7c and 7d.
Given the high vapor pressure of MoO3 at its
melting point, and the steep temperature gradient
between zone 5 and 3, it is likely that these features
formed by vapor deposition.
c (zone 3)
4. Conclusions
d (zone 4)
This paper showed that SPPS can inherently produce
porous, high surface area -MoO3 deposits ideal for
application as super-capacitor electrodes; however, it
was found that the surface charge storage capacity of
deposits can be further enhanced by increasing the
specific surface area without adding extra coating
material. This was achieved by growing -MoO3
lath features out of the previously deposited porous
material. The nature of the -MoO3 crystallographic
lath features was shown to be primarily determined
by the post-deposition exposure to the plasma jet.
10μm
References
[1] D. Chen, E. Jordan, and M. Gell, Surface and
Coatings Technology, 2007, vol. 202, pp. 2132-2138
[2] S. Basu and B.M. Cetegen, Intl. J. of Heat and
Mass Transfer, 2007, vol. 50 (17-18), pp. 3278-3290
50μm
Figure 7. SEM images illustrating the effect of plasma tail temperature
gradient on the formation of various -MoO3 morphologies.
[3] T. Brezesinski, J. Wang, S.H. Tolbert, and B.
Dunn, Nature MATL, 2010, vol. 9 (2), pp. 146-151
[4] M. Golozar, K. Chien, and T.W. Coyle, Intl.
Thermal Spray Conference, Germany 2011