1. ELECTROCHROMIC AND STRUCTURAL
BEHAVIORS OF MOO3 AND WO3 THIN
FILMS USING LOWER
CONCENTRATIONS OF HCL
ELECTROLYTE.
By
Fadayomi Oladeji Taiwo
2. OUTLINE
• Purpose of work
• Introduction of electrochromic device
• Overview of WO3 and MoO3
• Device fabrication.
• Data analysis
• Conclusion
3. Purpose of work:
• To Fabricate an electrochromic devices using transition metal oxides and lower
concentration of HCl as electrolyte.
• To investigate the mechanism responsible for coloration.
• Raman study of thin film used.
• To study the optical behaviors of the devices during each cycle.
• To draw up conclusions.
4. Introduction of electrochromic device.
• An electrochromic material is characterized by its ability to sustain reversible and persistent changes of optical properties
when voltage is applied to it and used in smart windows to reduce radiation.
Reaction: 푊푂3 + 푥푒− + 푥푀+ ↔ 푀푥푊푂3 − 퐶표푙표푟푒푑 푀 = 퐻, 퐿푖, 푁푎, 퐾,
• Smart windows are capable of regulating the inflow of radiant energy in buildings or cars.
• Tungsten trioxide is a well know material that has been studied mostly. Electrochromic properties are mostly
exhibited by transition metal oxides.
• Electrical Conductors used are transparent, which is why we will be using FTO glass (12-14 Ω/sq).
• The optical changes observed in the device are due to ion/electron intercalation into the transition metal
crystal structure.
• The transferred ion is only used for charge balancing.
• The mechanism of electrochromic device is similar to that of Li-ion battery.
• They are usually fabricated using a stack of four to five layers of components.
5. (Cont’d) Introduction of electrochromic device.
Performance of EC windows depends strongly on the following:
• Microstructure
• Crystal sizes
• Porosity or compactness
• Thickness of sample
• Reversibility of cycles
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• Li+ intercalated ECD are most common yet they have a slower switching ability
compared to HCl.
• Hydrochloric acid is less popular because of its high dissolution rate and is not
encouraged when using porous thin films as the electrode.
6. Fabrication of electrochromic Device
• Electrochromic device consist of an oxide ‘active layer’ and an ion source sandwiched
between a pair of electrodes (4-layered).
• The ion source layer can be a liquid electrolyte (acid) or solid-state film, but must contain
small mobile ions, e.g. Univalent donating ions: H+ , Li+, Na+, K+.
• Potential is applied across the electrodes-a large electric field is generated due to the small
gap (microns) between the contacts.
• ‘Open-circuit memory’. The voltage is only required to change the color.
• Why is FTO glass used as the electrodes?
• Working electrode is the material deposited on a conducting glass.
• Counter electrode is the ionic storage/ electrolyte used.
• Thin films deposition techniques used were PLD and HWCVD.
• WO3 , MoO3 and hybrid samples were used as the oxide active layer.
Substrate
Electrical Conductor
Transition Metal Oxide for absorption
IONIC CONDUCTOR
Ion Storage (Counter Electrode)
Electrical Conductor
Substrate
V
Schematic diagram of electrochromic device
7. Why Tungsten Oxide?
Tungsten oxide is the most widely used transition metal in design of electrochromic devices
because it has many interesting optical, electrical and defect properties.
Coordinate geometry- Octahedra (WO6)
Temperature dependent structure-
Monoclinic (Stable form) at 17 oC to 330 oC.
Orthorhombic at 330 oC to 740 oC.
Tetragonal at >740 oC.
WO3 is a wide band gap and highly insulating semiconductor with
complex polymorphism and defect chemistry.
WO3 is a modification of the perovskite-type ABO3 lattice, in which
the A site remains unoccupied and the tungsten atoms occupy the
B site. This produces a three-dimensional network of corner-shared
octahedraWO6
During intercalation, the electron passes through the circuit into
the WO3, causing a net electron charge
Oxidation number of tungsten oxide reduces from W6+ to W5+
during coloration
Electrons transit from W5+ to the neighbouringW6+ within the
cubic structure
The metallic ions are not responsible for color change, but only for
ion balancing within the perovskite structure
8. Why Molybdenum trioxide?
Pros:
• It has a similar structure to that of WO3
• Orthorhombic α-MoO3 consists of double layers of MoO6
octahedral held together by covalent bonds and vander waal
forces in between the layers.
• In all three compounds, one oxygen atom, pointing in the
direction of the opposite layer, is terminal and strongly bound
with a very short Mo–O distance, ∼1.69 Å .
• The four oxygen atoms are bridging and form a skeleton of the
layer: they have intermediate Mo–O distances ranging from 1.73
to 2.25 Å.
• The sixth oxygen atom is located opposite to the first one at an
Mo–O distance of 2.29–2.34 Å. It corresponds to a terminal weakly
bound water molecule in hydrates, while it is a bridging atom for
the other two MoO6 octahedra in α-MoO3.
9. Cons?
• Inter layer distance of
~6 Å making it
highly porous and
easily dissolved in
HCl electrolyte.
The ReO3 structure, used as an approximation for
monoclinicWO3, is obtained by considering only the tungsten
and oxygen atoms. The lattice constant a, is defined, and the
octahedron, built up by oxygen atoms surrounding the tungsten
atom, is shown.
11. XRD characterization of as-deposited thin films
• Several thin films of WO3, MoO3 and
hybrid samples were made on silicon
and FTO glass, using HWCVD and PLD
technique.
• Initially we used a wide range of
temperature (Room temperature – 600
oC) to find a suitable parameter for
deposition.
12. XRD Plots of γ-monoclinic WO3 and α-MoO3
PLD WO3 at 400 oC
PLD WO3 at 500 oC
PLD MoO3 at 500 oC
19. RAMAN characterization of as-deposited thin films
Molybdenum trioxide Raman spectrum
MoO3 Bands Representation
• 158 cm-1 (Ag, B1g) Originates from
translation of rigid chains
• 285 cm-1 (B2g, B3g) Doublet peak from
wagging modes of terminal oxygen atoms
• 667 cm-1 (B2g, B3g) Asymmetric stretching
of Mo-O-Mo
• 820 cm-1 (Ag, B1g) Symmetric stretch of
terminal oxygen atoms
• 995 cm-1 (Ag, B1g) Asymmetric stretch of
terminal oxygen atoms
20. Tungsten trioxide Raman spectrum
monoclinic peaks at 267cm-1, 323cm-1
deformation vibration
710cm-1 and 801cm-1 represent the (W-O-W)
stretching vibration
The 950cm-1 band is near invisible
indicating nonexistence of nanostructure
in the film.
The monoclinic peak at 185cm-1 is only
slightly visible as a small
Both samples were made deposited at 450
oC temperature but differing number of
shots.
Raman spectrum FTO
21. • Comparing their two most prominent peaks at 710 cm-1
and 801 cm-1, both samples had similar full width half
maximum (FWHM) value- 43.46o and 27.1o for 140614WO3
(40,000 shots) and 50o and 28o for 211014-1-WO3 (10,000
shots).
• The value of the FWHM is useful in determining the crystal
size of the sample.
• For both samples of tungsten trioxide, the ratio of their
intensities
퐼 (40,000 푠ℎ표푡푠)
퐼 (10,000 푠ℎ표푡푠)
≈ 2.35.
• The thicker sample yielded more intensity in
counts/second.
24. Bleach
Dye
ECD experiment of ion/electron insertion and extraction
25. WO3
• Amorphous tungsten oxide recorded multiple cycling.
• The darkness of the color change fades as the cycle progressed.
• HWCVD WO3, MoO3 (40,000 shots) and hybrid samples had a few
challenges such as
Irreversibility from colored to bleached state
Porous thin film surface (Dissolved in HCl).
Hold time at vertex point: 2 minutes
Applied Bias Electrical Voltage: -2.5 V (reduction) to +3.0
V (Oxidation)
Scan Ewe dE/dt: 100.00mV/s
Irange: 100 mA
Bandwidth: 5- medium
Epa
Epc
26. Cycle 1 ion insertion and bleaching. Cycle 20 ion insertion and bleaching.
Vs
Vs
Transmittance in electrochromic device increases at as the
cycle progressed. The plot shows the difference between
the transmittance measured in the first cycle to that of the
20th cycle.
27. 10,000 PLD shots of
MoO3
0.02 N vs 0.05 N
HCl concentration
(-2V to +2V)
40,000 shots of PLD samples of molybdenum trioxide were not
suitable for device fabrication. The surface layer of thin film washed
away.
Reasons:
Weak interlayer bond and less density of 4.69 g/cm3
Tungsten trioxide = 7.16 g/cm3
8 cycles was recorded for 10,000 shots of MoO3 before coloration
began to fade.
28. Cycle 1 ion insertion and bleaching. Cycle 8 ion insertion and bleaching.
Vs
Vs
10,000 pulsed
laser shots of
MoO3 thin film
used as the
working
electrode.
(8 cycles were
recorded, 0.02 N
HCl
concentration
(-2V to +2V)
29. NON- BLEACHING HWCVD SAMPLES
HWCVD 220914A-WO3 transmittance progress
recorded using 0.05 N HCl and -2.5 V potential
As-deposited HWCVD WO3
Colored state- HWCVD WO3
Non-bleaching state HWCVD WO3
Idle HWCVD WO3- Post CV experiment
30. NON- BLEACHING Hybrid SAMPLES (MoO3/WO3 /MoO3/WO3)
Non-bleaching state Peeling off of the thin film surface
layer due to the weak interlayer bond
in MoO3
31. Effect of HCl concentration on optical properties of WO3
32. WO3
Transmittance of as-deposited thin films (PLD)
Comparison of optical properties and potential steps at different HCl conc.
34. Tungsten trioxide (HWCVD)
Disappearance of monoclinic stretching vibration bands
Blue shifts in the bands
35. PLD tungsten trioxide sample (Post CV)
Disappearance of monoclinic stretching vibration bands at 600 cm-1 range
Blue shifts in the bands also.
The 700 cm-1 peak broadens and disappears
Before CV- 10,000 PLD
Post CV- 10,000 PLD
36. Molybdenum trioxide
Forming new peaks and
disappearance of
prominent peaks at 656
cm-1 and 817 cm-1
Peak at 994.3 cm-1 shifts
to right
Before CV- 10,000 PLD
Post CV- 10,000 PLD
37. Hybrid sample (MoO3/WO3 /MoO3/WO3)- Total of 40,000 shots
o Broad peaks
representing the
combination of peaks
from both compounds.
---------------------------------
o MoO3 at 293.9 cm-1,
342.1 cm-1 and 823.8 cm-1
while 999.5 cm-1 is
exclusive to
molybdenum trioxide.
---------------------------------
o WO3 at 275.7 cm-1, 333.0
cm-1, 811.8 cm-1.
---------------------------------
o Red shift in peak band
could be identified as the
formation of hydrogen
bonding in the
structures.
---------------------------------
o It is also evidence of
tensile stress in the film.
38. Conclusions
• Electrochromism can be achieved at much lower concentration of 0.02 N and 0.05 N of
hydrochloric acid, as long as sufficient bias potential is passed across the device cause
ions and electrons to dissociate and become more mobile.
• Performance of the device is adversely affected by sample thickness especially for porous
materials having weak interlayer bonds such as MoO3.
• A better device can be built using HWCVD WO3 by baking the film after deposition to
create a more compact structure and possibly a transparent film.
• Transmittance of the device at its colored state is dependent on potential applied.
However its not dependent on the concentration of the HCl electrolyte used.
• Raman study shows the formation of hydrogen bonding in the structure of the thin film
causing tensile stress and also proves the stretching vibration in the case of WO3 .
• Number of cycles before fading could be enhanced by increasing the value of applied
voltage.
39. THANK YOU
• DR. Mitra (Research advisor)
• DR. Ghosh
• … and all my course mates
