Electrochromic properties of organic-inorganic composite materials

Article abstract of DOI:10.1016/j.jallcom.2017.05.222

A new type of electroactive composite material based on tungsten trioxide (WO₃) and poly (tri(4-(2′-thienyl)) phenylamine) (P3TPA) was prepared. Using the WO₃/P3TPA as electroactive material, the electrochromic device was fabricated,

Full text of DOI:10.1016/j.jallcom.2017.05.222

Journal of Alloys and Compounds 718 (2017) 379e385
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Electrochromic properties of organic-inorganic composite materials
a
a
a
a
a
a, *
Weishi Liu , Xiaoyuan Zhang , Jianqiang Liu , Xiaodan Ma , Jinming Zeng , Ping Liu
Tiangui Xu
a
,
b
State Key Laboratory of Luminescent Materials and Devices, Research Institute of Materials Science, South China University of Technology, Guangzhou,
5
10640, China
b
Shenzhen Academy of Inspection and Quarantine, China
a r t i c l e i n f o
a b s t r a c t
0
3
A new type of electroactive composite material based on tungsten trioxide (WO ) and poly (tri(4-(2 -
Article history:
Received 20 January 2017
Received in revised form
thienyl)) phenylamine) (P3TPA) was prepared. Using the WO
electrochromic device was fabricated, and the electrochromic properties of WO
gated. Compared with WO , the coloring switching time and the bleaching switching time of WO
was decreased. The response speed became faster. The open-circuit memory of WO /P3TPA became
better. The coloration efficiency of WO /P3TPA increased.
3
/P3TPA as electroactive material, the
/P3TPA were investi-
/P3TPA
3
7
May 2017
3
3
Accepted 20 May 2017
Available online 22 May 2017
3
3
©
2017 Elsevier B.V. All rights reserved.
Keywords:
Tungsten trioxide
Triphenylamine derivative
Composite
Electroactive material
Electrochromic device
Electrochromic property
1. Introduction
Subsequently, many other researchers expanded on the subject
14]. ECDs based on WO show outstanding electrochromic prop-
[
3
Electrochromic materials have attracted attention because of
their potential applications in energy conservation [1]. Electro-
chromic devices (ECDs) are used in smart windows, automotive
rear-view mirrors, displays, sunglasses, etc. [2]. The most important
prerequisites for ideal EC materials include high initial trans-
parency, large optical contrast between colored and bleach states,
short switching time, and long-term cyclic stability [3]. EC mate-
rials can be divided into two large kinds on the basis of the
component, organic and inorganic materials. Traditional metal
erties and have been used on smart windows [15], automotive rear-
view mirrors [16], and electrochromic displays [17]. Nevertheless,
despite the plentiful amount of work that has been done on the
coloration phenomena in thin films of various morphological WO ,
3
many contradictions still exist. Further commercialization is limited
by the long switching times and low coloration efficiency.
Among all the electrochromic materials, the conjugated poly-
mers have attracted much attention because of their low cost for
easy processing, multicolor adjustability and mechanical flexibility
[18,19]. Organic conjugated polymers such as polythiophene, pol-
yaniline (PANI), polypyrrole (PPy), Poly(3,4-ethylene-dioxythio-
phene) (PEDOT) and their derivatives, have been extensively
investigated [20] and many of them exhibit good electrochromic
properties. As a typical organic electrochromic material, PANI and
its derivatives are important organic conjugated polymers to ECDs
because of short coloring and bleaching switching times, high op-
tical contrast, and high coloration efficiency [21]. PEDOT and its
derivatives are also a class of outstanding EC conjugated polymers
due to their high coloration efficiency and ability of diverse colors
3 2 3 2 5
oxide semiconductors, such as WO [4], TiO [5], MoO [6], Nb O
[7,8], NiO [9] are some of the inorganic EC materials. At present,
some other researchers also investigate organic EC materials due to
their variety of visual coloration and versatile structural morphol-
ogies [10e12].
Among all the materials studied thus far, WO has emerged as
3
the most extensively studied material, not only for electro-
chromism, but also for a variety of other device applications. Deb in
3
1963 first reported on the electrochromic properties of WO [13].
[22,23]. However, the low electrochemical and thermal stability of
conjugated polymers results in a relatively short lifetime. These
disadvantages have hindered the further development of organic
*
Corresponding author.
E-mail address: mcpliu@scut.edu.cn (P. Liu).
http://dx.doi.org/10.1016/j.jallcom.2017.05.222
925-8388/© 2017 Elsevier B.V. All rights reserved.
0

380
W. Liu et al. / Journal of Alloys and Compounds 718 (2017) 379e385
ECDs applications [24].
measured on the CHI750A electrochemical workstation.
Usually, three methods are used to improve the performance of
organic ECDs [25]. Firstly, designing and synthesizing new organic
conjugated compounds; Secondly, developing new inorganic and
organic composite materials, or inorganic heterojunction; Thirdly,
electrolytes play an important role in determining the long-term
stability of the devices, so finding appropriate electroplates is
another way to improve the performance of ECDs [26]. The main
characteristics of inorganiceorganic composite electrochromic
materials are the advantages of each component and the improved
ECD performance. Recently, electrochromic inorganic-organic
0
2
.3. Synthesis of tri (4-(2 -thienyl)) phenylamine (3TPA)
The synthesis of 3TPA is described in Scheme 1. 3TPA was pre-
pared by Suzuki coupling reactions according to the procedure as
followed: 1.45 g tris(4-bromophenyl)amine,1.92 g 2-thienylboronic
2 3 2 2 3
acid, 90 mg Pdcl (PPh ) , 150 ml THF and 100 ml K CO (2 M) were
added into a dry and clean 500 ml three-neck flask. The mixture
was heated to reflux for 48 h, and then poured into a saturated
solution of ammonium chloride and extracted with dichloro-
methane for three times. The organic phase was washed with brine
and then dried over anhydrous sodium sulfate, filtered and
removed the solvent. The solid residue was purified by silica-gel
column chromatography to give a pale yellow crystalline solid.
Yield: 59%.
2 2
complexes such as TiO /polythiophene, IrO /polyaniline and
WO /polypyrrole have been studied [27e31].
3
In our previous work, we have investigated the electrochromic
0
properties of the P3TPA denotes poly(tri(4-(2 -thienyl)) phenyl-
amine) with triphenylamine and thiophene units (Scheme 1). It
was found that the P3TPA exhibited reversible, clear color change
from orange-yellow to blue on electrochemical doping and
dedoping. However its electrochromic stability is poor. In this
2
.4. Preparation of the WO
3
/P3TPA composite film
study, we prepared the inorganic-organic complex WO
Using the WO /P3TPA as electrochromic material, the electro-
chromic devices were prepared, and the electrochromic properties
of WO /P3TPA were investigated.
3
/P3TPA.
3
A 100e150 nm thick WO
glass substrate by vacuum deposition (vacuum pressure
3
film was deposited on a ITO-coated
3
ꢁ3
4
ꢀ 10 Pa).
At room temperature, the electropolymerization of 3TPA was
ꢁ
2
2
. Experimental
carried out in an ACN solution of 5.0 ꢀ 10 M 3TPA and 0.1 M TBAP
1
ꢁ
by repetitive cycling at scan rate of 100 mV s . The reference
2þ
2.1. Materials
electrode was the Hg/Hg electrode; the counter electrode was the
platinum wire, and the ITO-coated WO was the working electrode.
The P3TPA was directly coated onto WO . Fig. 1 shows the cyclic
voltammogram of the electropolymerization of 3TPA on the ITO-
coated WO electrode. The current density increased during the
3
Tungsten trioxide (WO
3
) and Pdcl
2
(PPh
3
)
2
were purchased from
3
Beijing HWRK Chem Co., Ltd. 2-Thienylboronic acid, tetrabuty-
lammonium perchlorate (TBAP) and tris(4-bromophenyl) amine
3
were purchased from Alfa Aesar. Potassium carbonate (K
2
CO
3
),
repeated potential scanning. The results suggest that the electro-
tetrahydrofuran (THF), acetonitrile (ACN), dichlormethane (DCM),
and sodium chloride (NaCl) were purchased from Tianjin Hongyan
polymerization of 3TPA was achieved and P3TPA finally formed.
Chem Co., Ltd. Lithium perchlorate (LiClO
Aladdin Industrial Corporation. Sodium sulfate (Na
4
) was purchased from
SO ) was pur-
2
4
chased from Beijing Liudian Chem Co., Ltd. Petroleum ether was
purchased from Guangdong JHD Co., Ltd.
2.2. Instruments
NMR spectra were obtained on a Bruker AVANCE-500 FT-NMR
using tetramethylsilane as internal standard. IR spectra were
recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer. Mass
spectra were recorded on a GCMS-QP 2010 mass spectrometer. The
cyclic voltammogram (CV) and electropolymerization were carried
out by the CHI750A electrochemical workstation. AFM images were
obtained on a DI/MultiMode from Veeco.
The parameters of the ECD including optical contrast and open-
circuit memory were measured on a Helios-
tometer. The coloration efficiency and switching time were
g UVevis spectropho-
Fig. 1. The electropolymerization of 3TPA (5.0 ꢀ 10ꢁ M) at 100 mV/s for ten times in
2
the ACN solution with 0.1 M TBAP.
Scheme 1. The Synthesis route of 3TPA.

W. Liu et al. / Journal of Alloys and Compounds 718 (2017) 379e385
381
4 3 3
Fig. 2. The coloring time and bleaching time of the electroactive film in the ACN solution with 0.05 M TBAP and 0.05 M LiClO . a: WO film and b: WO /P3TPA film.
2
.5. Preparation of the electrochromic device
and bleaching time. To evaluate the switching time of the ECD
based on the WO and the WO /P3TPA films, the chro-
noamperometry was used in typical three-electrode configuration.
Fig. 2 shows that the coloring time and bleaching time of WO film
is 2.80 s and 3.66 s, respectively, and the coloring time and
bleaching time of WO /P3TPA film is 2.24 s and 2.15 s, respectively.
Compared with the WO film, the coloring time and bleaching time
/P3TPA film are decreased by 20% and 41.3%,
3
3
3 3
Using the ITO and Pt wire as electrodes, the WO film or WO /
P3TPA composite film as active material, and the ACN solution
containing 0.05 M TBAP and 0.05 M LiClO as electrolyte, the ECD
was prepared. The device structure is ITO glass/active materials/
electrolyte solution/Pt.
3
4
3
3
of the WO
respectively.
3
3
. Results and discussion
Fig. 3 shows AFM images of the WO
3
and WO
3
/P3TPA films. The
Electrochromic properties of the WO
3
/P3TPA composite film
surface roughness (Rms) of WO and WO
3
3
/P3TPA film is 5.164 nm
and 31.817 nm, respectively. The rough surface morphology facili-
tates the injection and extraction of ions and reduces the switching
time of electrochromic materials [32,33]. Therefore, the coloring
3
The WO film exhibited a reversible, clear color change between
colorless and deep blue for applied voltages of þ2.5 V and ꢁ2.5 V
between the ITO and the platinum wire electrodes in the electrolyte
3
and bleaching times of the WO /P3TPA film are shorter than that of
solution. The WO
blue for applied voltage of þ2.5 V and from blue to yellow for
applied voltage of ꢁ2.5 V. The results suggest that the WO /P3TPA
composite film possess reversible electrochromic properties.
3
/P3TPA composite film switched from yellow to
the WO film.
3
3
3.2. Open-circuit memory
The open-circuit memory of an electrochromic material is
defined as the time that the material retains its color without
voltage application [34]. The open-circuit memory is an important
parameter of electrochromic materials. To evaluate the open-circuit
3
.1. Switching time
The switching or response time is one of the most important
parameters for ECDs. The switching time includes the coloring time
memory of electrochromic materials, the transmittance of the WO
3
3 3
Fig. 3. The AFM images of the (a) WO film and (b) WO /P3TPA film.

382
W. Liu et al. / Journal of Alloys and Compounds 718 (2017) 379e385
4 3 3
Fig. 4. The open-circuit memory of electroactive materials in the ACN solution containing 0.05 M TBAP and 0.05 M LiClO . a: WO film and b: WO /P3TPA film.
Fig. 5. The optical contrast of the electroactive materials at the coloring and bleaching state in ACN solution containing 0.05 M TBAP and 0.05 M LiClO
P3TPA film.
4 3 3
. a: WO film and b: WO /
Fig. 6. The CV of the electroactive materials in the CAN solution containing 0.05 M TBAPe0.05 M LiClO
4
. Scan rate ¼ 100 mV/s. a: WO
3 3
film; b: WO /P3TPA film.
and WO
photometer, respectively. Fig. 4a shows that when the applied
film at
the blue coloring state is 44.8%, whereas it is 89.1% at the bleaching
state. When the applied voltage of ꢁ2.5 V is removed, the trans-
mittance at 700 nm changes from 44.8% to 77.1% after nine days,
which represents a change of 72.9%. The open-circuit memory of
3
/P3TPA films were measured using a UVevis spectro-
change is 30.0%. The results suggest that the open-circuit memory
of the WO /P3TPA film is better than that of the WO film.
3
3
voltages is ꢁ2.5 V, the transmittance at 700 nm of the WO
3
3.3. Optical contrast
The optical contrast (
D
T) is a parameter of the color change of
the electrochromic material. It is defined as the transmittance gap
between two coloring states with different applied potentials [35].
the WO
applied voltage is þ2.5 V, the transmittance at 700 nm of the WO
P3TPA film at the blue coloring state is 55.7%. The transmittance at
00 nm of the WO /P3TPA film at the yellow state (the bleaching
3
/P3TPA composite film is shown in Fig. 4b. When the
3
/
Fig. 5 shows the transmittance spectra of the WO
films at the coloring and bleaching states. In Fig. 5a, the
00 nm of the WO film is 44.2% between the deep blue state and
the colorless transparent state, whereas the T at 700 nm of the
WO /P3TPA film is 26.9% between the blue and yellow state.
and WO
3
/P3TPA
3
DT at
7
3
7
3
state) is 82.8%. When the applied voltage of þ2.5 V is removed, the
D
transmittance is 72.4% after nine days and the transmittance
3

W. Liu et al. / Journal of Alloys and Compounds 718 (2017) 379e385
383
Fig. 7. The CV of active materials for the 1st and 500th cycle in the ACN solution containing 0.05 M TBAP-0.0 5 M LiClO
, Scan rate ¼ 100 mV/s. a: WO
film and b: WO
4
3
3
/P3TPA film.
Compared with WO
has about 17.3% decrease. This is because the P3TPA film is yellow
when it is in the bleaching state but WO
3
film, the optical contrast of WO
3
/P3TPA film
3.4. Coloration efficiency
The coloration efficiency (h) characterizes the charge utilization
3
film presents colorless.
ratio of electrochromic materials. The high coloration efficiency
indicates that the ECD exhibits large optical modulation for small
inserted or extracted charge. The coloration efficiency
h
is calcu-
Table 1
Coloration efficiency of WO
lated with the following equation [36].
3
and WO
3
/P3TPA.
WO
Parameters
3
WO
3
/P3TPA
h
¼
D
OD=Q ¼ log½T =T
b
c
ꢂ=QA
(1)
Optical density variation (
Injected charge (Q
D
OD)
)
0.298
7.06
0.173
3.93
2
ꢁ
2
where the
length,
injected charge, and T
transmittance, respectively.
Fig. 6 shows the CV curves of the electroactive materials. Fig. 7
shows the CV curves of the WO and WO /P3TPA films in the first
and 500 cycle. Compared with the WO film, there is no
h
(cm /C) is the coloration efficiency at a given wave-
A
, mC cm
2
(coloring charge)
DOD is the variable optical density, Q
A
(mC/cm ) is the
Extracted charge (Q
R
, mC cmꢁ2)
7.86
3.10
and T represent the bleaching and coloring
b
c
(coloring charge)
Charge loss ratio (%)
Coloration efficiency (
Improvement of (%)
11.4%
42.2
e
21.1%
55.8
24.4%
2
h
, cm /C)
h
3
3
th
3
3 3
Fig. 8. The AC impedance diagrams of (a)WO film, (b) P3TPA film, (c) WO /P3TPA film.

384
W. Liu et al. / Journal of Alloys and Compounds 718 (2017) 379e385
3 3
Fig. 9. The equivalent circuit used for fitting the experimental impedance data (a) WO film, (b) P3TPA film and WO /P3TPA film.
Table 2
Parameters of EIS plots.
Appendix A. Supplementary data
R
e
/
U
C
1
/F
Rct/
U
C
2
/F
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jallcom.2017.05.222.
6.19 ꢀ 10ꢁ
5
6
6
5
5
4
ꢁ5
WO
P3TPA film
WO /P3TPA film
3
film
27.75
82.78
53.24
1.906 ꢀ 10
2.254 ꢀ 10
1.281 ꢀ 10
4.726 ꢀ 10
ꢁ
2.11 ꢀ 10
e
e
3.82 ꢀ 10ꢁ
3
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