Simple and rapid synthesis of NiO/PPy thin films with improved electrochromic performance

Article abstract of DOI:10.1016/j.electacta.2009.11.087

Nickel oxide/polypyrrole (NiO/PPy) thin films were deposited by a two step process in which the NiO layer was electrodeposited potentiostatically from an aqueous solution of NiCl₂·6H₂O at pH 7.5 on fluorine doped tin oxide (FTO) coated conducting glass substrates, followed by the deposition of polypyrrole (PPy) thin films by chemical bath deposition (CBD) from pyrrole mixed with ammonium persulfate (APS). The NiO/PPy films were further characterized for their structural, optical, morphological and electrochromic properties. X-ray diffraction study indicates that the films composed of polycrystalline NiO and amorphous PPy. Infrared transmission spectrum reveals chemical bonding between NiO and PPy. Rectangular faceted grains were observed from scanning electron microscopy results. The electrochromic (EC) property of the film was studied using cyclic voltammogram (CV), chronoamperometry (CA) and optical modulation. The NiO/PPy presents superior EC properties than their individual counterparts. The coloration/bleaching kinetics (response time of few ms) and coloration efficiency (358 cm²/C) were found to be improved appreciably. The dramatic improvement in electrochemical stability (from about 500 c/b cycles for PPy to 10,000 c/b cycles for NiO/PPy) was observed. This work therefore demonstrates a cost-effective and simple way of depositing highly efficient, faster and stable NiO/PPy electrodes for EC devices.

Full text of DOI:10.1016/j.electacta.2009.11.087

Electrochimica Acta 55 (2010) 2344–2351
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Simple and rapid synthesis of NiO/PPy thin films with improved
electrochromic performance
A.C. Sonavanea, A.I. Inamdara,b, D.S. Dalavia, H.P. Deshmukhc, P.S. Patila,∗
a Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India
b Department of Semiconductor Science, Dongguk University, Seoul 100-715, Republic of Korea
c Department of Physics, Y.M. College, Bharati Vidyapeeth, Erandwane, Pune, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel oxide/polypyrrole (NiO/PPy) thin films were deposited by a two step process in which the NiO
layer was electrodeposited potentiostatically from an aqueous solution of NiCl2·6H2O at pH 7.5 on fluorine
doped tin oxide (FTO) coated conducting glass substrates, followed by the deposition of polypyrrole (PPy)
thin films by chemical bath deposition (CBD) from pyrrole mixed with ammonium persulfate (APS). The
NiO/PPy films were further characterized for their structural, optical, morphological and electrochromic
properties. X-ray diffraction study indicates that the films composed of polycrystalline NiO and amor-
phous PPy. Infrared transmission spectrum reveals chemical bonding between NiO and PPy. Rectangular
faceted grains were observed from scanning electron microscopy results. The electrochromic (EC) prop-
erty of the film was studied using cyclic voltammogram (CV), chronoamperometry (CA) and optical
modulation. The NiO/PPy presents superior EC properties than their individual counterparts. The col-
oration/bleaching kinetics (response time of few ms) and coloration efficiency (358 cm2/C) were found
to be improved appreciably. The dramatic improvement in electrochemical stability (from about 500
c/b cycles for PPy to 10,000 c/b cycles for NiO/PPy) was observed. This work therefore demonstrates a
cost-effective and simple way of depositing highly efficient, faster and stable NiO/PPy electrodes for EC
devices.
Received 22 August 2009
Received in revised form
24 November 2009
Accepted 26 November 2009
Available online 2 December 2009
Keywords:
Nickel oxide thin films
Polypyrrole (PPy)
Electrodeposition
Chemical bath deposition (CBD)
EC properties
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Conjugated polymers by virtue of their high electrical conduc-
tivity and easy processibility have attracted a great attention and
Organic–inorganic hybrid materials represent a new class of
materials as they can combine the advantages of both the com-
ponents and offer special properties through modification of each
other [1]. It is therefore anticipated that the improved elec-
trochromic device based on hybrid material would deploy a
material with high coloration efficiency, faster response time and
an outstanding operation lifetime. The organic polymers have
good mechanical flexibility, toughness, and formability, while inor-
ganic ceramics have excellent durability to chemical attacks, high
thermal stability, and surface hardness. Inorganic–organic com-
posite materials thus have combined interesting properties and
hence gained much attention [2]. However, further studies are
necessary to unveil the still unexplored properties of hybrid mate-
rials. The successful implementation of these materials in to the
devices would further require facile synthetic strategies. At present
organic polymer/inorganic nanocomposites have been synthesized
by adopting several chemical routes [3–5].
consequently have been extensively explored as an alternative
to metals or inorganic semiconductors in fabrication of optoelec-
tronic, microelectronic and microelectrochemical devices [6–8].
Amongst the important conducting polymers, polypyrrole (PPy)
has potential applications in drug delivery [9,10], sensors [11],
biomolecules [12], adhesives [13], actuators [14], immunosen-
sors [15] and corrosion protection [16]. Due to the intrinsic
semiconducting properties, PPy has been used as an electronic com-
ponent in electronic devices, e.g., photoelectrochemical devices
[17], organic light-emitting diodes [18], and rectifying devices [19].
Some organic/inorganic composites have already been reported
in the literatures, such as PPy/TiO2 [20], PPy/Ti [21], PPy/Au [22],
PPy/Ag [23], PPy/Pt [24], PPy/Pd [25], Fe3O4/PPy [26], PPy/Ta2O5
[27], PPy/Cu1.4Mn1.6O4 [28], RuO2-PPy [29], FeO-PPy [30], and
PPy/ZnO [31] for various applications. Interestingly, PPy has been
effectively used to enhance/improve several properties of the inor-
ganic oxides, hence it offers promising organic counterpart in the
electronic devices.
Five kinds of uniform metal oxide particles (a-Fe2O3, CeO2, CuO,
NiO, and SiO2) were coated with polypyrrole by reacting the dis-
persed solids with pyrrole in a water/ethanol medium without
∗
Corresponding author. Tel.: +91 231 2609230; fax: +91 231 2691533.
E-mail address: psp phy@unishivaji.ac.in (P.S. Patil).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.11.087

A.C. Sonavane et al. / Electrochimica Acta 55 (2010) 2344–2351
2345
the use of a soluble oxidant. Fe2O3 and SiO2 were found to be
inactive in the polymerization, while CeO2 and CuO react with
the adsorbed pyrrole molecules through a reductive-dissolution
process, in which the monomers are oxidized, causing a release
of reduced metal ions [32]. An EC window based on PPy/nickel
oxide combination in an aqueous electrolyte has been reported by
Scrosati and co-workers [33] as a light-modulation window with
high cycling efficiency (93%), long life (1000 cycles) and fast switch-
ing time (of the order of few seconds). This paper demonstrates
a promising way to improve the EC property of inorganic oxide
with an organic counterpart. It is quite surprising to note that there
are no further reports on this system, to the best of the author’s
knowledge. Hence we revisited this NiO/PPy system for further
exploration, with a slight change in synthetic strategy.
In this paper NiO/PPy samples have been prepared by using
simple electrodeposition and CBD techniques. Electrodeposition
is of particular interest due to low-cost, environmental friendly
process, and feasibility of room temperature growth on large area
for the deposition of NiO films. On the other hand, the advantage of
chemical synthesis is that it offers mass production at a reasonable
cost for polypyrrole films, without compromising on quality. The
EC performance of NiO/PPy film was tested in LiClO4-PC. In-situ
transmittance with time was used to determine the response
time of the coloration-bleaching process of the device. The elec-
trochemical stability was examined, by repeating color/bleach
cycles in an electrolyte for prolonged period. The NiO/PPy presents
superior EC properties than their individual counterparts. The
coloration/bleaching kinetics, coloration efficiency and electro-
chemical stability were found to be improved appreciably. This
work therefore demonstrates a cost-effective and simple way of
depositing highly efficient, faster and stable NiO/PPy electrodes
for EC devices.
2. Experimental
NiO/PPy films were prepared sequentially by adopting elec-
trodeposition and CBD techniques. In the first part nickel oxide
thin films were electrodeposited from an aqueous nickel chlo-
ride bath containing 0.5 M nickel chloride (LOBA CHEMIE Extra
Pure 97%) and 0.1 M KCl onto FTO (15–20 ꢀ/ꢀ) coated conduct-
ing glass substrates. The bath solution was complexed using EDTA
(A.R. 99%) and pH was adjusted at 7.5 by addition of KOH. The three-
electrode electrochemical cell was used to deposit NiO thin films
in potentiostatic mode in which graphite was counter electrode,
SCE was reference electrode. The electrochemical quartz crystal
(EQCM) measurements were taken using an electrochemical ana-
lyzer (model—CHI-400A) made by CH Instrument, USA. The EQCM
measurements were performed on a platinum electrode of 1 cm2
area in a specially designed electrochemical cell.
The cyclic voltammogram was recorded in the range from +0.3
to −0.8 V vs SCE in a precursor solution at 10 mV/s. Fig. 1(a) and
(b) is associated with the reduction-oxidation behaviour of nickel
species on the Pt electrode (area 1 cm2) accompanied by a concur-
rent shift in the frequency of the quartz crystal electrode with the
applied voltage. The arrows indicate the scan direction. The scans
were initiated at +0.3 V vs SCE, reversed at −0.8 V (vs SCE) and ter-
minated at +0.3 V (SCE). During the cathodic scan, the increment in
current density beyond −0.2 V (vs SCE) is due to reduction of Ni+2
ions, where slight increase in mass takes place (Fig. 1(b)). The peak
at −0.4 V is due to oxygen reduction [34–36]. This causes local pH
to increase and a dip at −0.5 V on the I–V curve. The exponential
increase in current density beyond −0.5 V is due to Ni (OH)2 reduc-
tion (Fig. 1(a)) [37]. Subsequently, NiO growth takes place via inter-
mediate steps of hydroxide formation as given in Eqs. (1) and (2).
Fig. 1. (a) The reduction–oxidation behaviour of the electrolyte on the Pt electrode
(area 1 cm2) accompanied by (b) a concurrent shift in the delta mass of the quartz
crystal electrode with the applied voltage.
Ni(OH)2 → NiO + H2O
(2)
Upon reversal of the plot scan, the mass deposited onto the elec-
trode continues to increase up to −0.5 V and leveled off till 0 V (vs
AgCl) (Fig. 1(b)). The nickel oxide thin films were deposited at room
temperature under potentiostatic condition at −0.75 V [38].
In the second part, polypyrrole (PPy) was synthesized using
monomer Pyrrole (0.03 M in 100 ml distilled water) chemically
polymerized with ammonium persulfate ([(NH4)2S2O8] APS 0.06 M
in 50 ml distilled water). This reaction mixture was stirred continu-
ously at a constant low temperature (5 ◦C) to obtain PPy. As soon as
the pyrrole is mixed with the APS, the mixture turned to a charac-
teristic black color with change in pH from 5 to 2, indicating that the
organic polymerization reaction began immediately. The reaction
was allowed to proceed for 10 min. The pre-deposited NiO films
were dipped vertically in PPy solution for 10 min, lifted and rinsed
with double distilled water. This dip-lift cycle was repeated twice
to obtain PPy film of adequate thickness.
The NiO, PPy and NiO/PPy films were prepared for EC inves-
tigations with a standard three electrode electrochemical cell of
configuration,
NiCl2 · 6H2O + H2O → Ni(OH)2 + 2HCl + 5H2O
(1)
Glass/FTO/NiO/PPy/1 M LiClO4 + PC/SCE

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A.C. Sonavane et al. / Electrochimica Acta 55 (2010) 2344–2351
Fig. 2. X-ray diffraction patterns for (a) NiO, (b) PPy and (c) NiO/PPy samples where
asterisk specifies the peaks of FTO of JCPDS data (77-0447). The standard JCPDS data
(4-0835) for NiO is shown as (1 1 1) and (2 0 0) peaks.
Fig. 3. IR transmittance spectra for (a) NiO, (b) PPy and (c) NiO/PPy sample recorded
in the wavenumber of 400–4000 cm−1
.
(FTIR) measurements (PerkinElmer IR spectrometer model-783) in
the range 400–4000 cm−1. The cyclic voltammetry (CV), chronoam-
perometry (CA), chronocoulometry (CC) measurements of the
NiO/PPy films were carried out in a three compartment cell contain-
ing 1 M LiClO4 + 1 mM propylene carbonate (PC) as the electrolyte,
a saturated calomel electrode (SCE) as the reference electrode and
graphite as the counter electrode on a CHI400 electrochemical
workstation. The CV measurements were carried out in the poten-
tial range −1.5 to +1.5 V (vs SCE) with a scanning rate of 20 mV/s
A sharp step was created onto the sample and their thicknesses
were measured using Ambios surface profiler (XP-1) (NiO = 104 nm,
PPy = 210 nm and NiO/PPy = 290 nm). The structural and morpho-
logical characterization of the samples were carried out using
Philips, PW 3710 X-Ray diffractometer (XRD) with CuK␣ radia-
tion (wavelength 1.5432 A◦) and JEOL-JSM-6360 scanning electron
microscope (SEM) respectively. The powders collected from the
deposited films were characterized by Fourier-transform infrared
Fig. 4. Scanning electron microscopy for (a) NiO, (b) PPy and (c) NiO/PPy samples deposited onto FTO coated conducting glass substrates. All the films are recorded at ×10,000
magnifications.

A.C. Sonavane et al. / Electrochimica Acta 55 (2010) 2344–2351
2347
at room temperature. Optical absorption and transmittance spec-
tra of the samples in the colored and bleached states were recorded
using UV–vis-NIR spectrophotometer, (Systronics-model 119) over
350–1000 nm. The values of response times were also deduced
from the in-situ transmittance studies at 630 nm. The variation of
the transmittance with time has been recorded with X-Y recorder
(EG&G Princeton Applied Research model RE0091).
3. Results and discussion
3.1. Structural characterization
The XRD patterns of NiO, PPy and NiO/PPy films deposited on
FTO substrates are presented in Fig. 2. Nickel oxide thin film exhibits
cubic NiO phase (JCPDS file 4-0835) whereas pure PPy films were
found to be amorphous in nature. For the NiO/PPy films, the diffrac-
tion peaks of NiO were observed whereas no diffraction peaks of
PPy films were found, which indicates, formation of cubic poly-
crystalline NiO/amorphous PPy samples. The peaks observed at
37◦ and 43◦ are of (1 1 1) and (2 0 0) reflections of cubic NiO. The
FTO substrate peaks are denoted by asterisk (*) (JCPDS data file
77-0447).
Fig. 5. Cyclic voltammogram recorded in 1 M LiClO4 + PC electrolyte for (a) NiO (also
shown in inset), (b) PPy and (c) NiO/PPy sample. The potential is swept from +2 to
−1.2 V (vs SCE) for NiO and +1.5 to −1.5 V (vs SCE) for PPy and NiO/PPy at the scan
rate of 20 mV/s.
The samples in powder form were collected from NiO, PPy and
NiO/PPy with the help of doctor blade. The IR spectra of NiO, PPy
Fig. 6. Photographs of a sample (a) NiO, (b) PPy and (c) NiO/PPy under different applied potentials.

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A.C. Sonavane et al. / Electrochimica Acta 55 (2010) 2344–2351
and NiO/PPy powders in its palletized form with KBr had been
recorded in the range of 400–4000 cm−1. Fig. 3 shows the FTIR spec-
tra for NiO, PPy and NiO/PPy samples. The FTIR spectrum of NiO
shows the presence of 6 intensive bands centered at 3438, 1594,
1398, 1114, 668 and 453 cm−1 due to OH stretching band of hydro-
gen bonded water, water bending vibration, in-plane C–H bending
mode, incorporation of chloride group and the nickel–oxygen inter-
action [39,40]. The PPy spectrum shows the broad strong band
at 3420 cm−1 corresponds to the absorption of N–H stretching
of polypyrrole [41]. The frequency at 2923 and 2853 cm−1 refers
to the stretching vibration of C–H bond. The bands at 1565 (2,5-
substituted pyrrole) and 1462 cm−1 may be assigned to typical PPy
ring vibrations. The bands at 1288, 1041 cm−1 may correspond −to1
in-plane vibration of C–H band. Transmittance peaks 1171 cm
band was assigned to N–C stretching band and band at 923 cm−1
was for the presence of polymerized pyrrole. The IR peak observed
at 889 cm−1 may be assigned to the out of plane vibration of C–H
indicating polymerization of pyrrole [42]. Whereas in NiO/PPy sam-
ple mostly all the stretching vibrations were same as that of NiO
and PPy, only a slight shift of IR absorption to lower frequencies in
NiO/PPy spectra is observed as compared with NiO and PPy which
suggests that an interaction between the polymer and NiO occurs.
For NiO films a strong band centered at 453 cm−1 corresponds to
the stretching vibration of Ni–O. However the shift in NiO stretch-
ing band at 490 cm−1 was observed in NiO/PPy films. This observed
Fig. 7. Variation of transmittance for NiO, PPy and NiO/PPy as a function of time for
the first ten switching cycles between −1.2 and 2.2 V for NiO and −1.5 and +1.5 V
vs SCE for PPy and NiO/PPy in 1 M LiClO4/PC. The inset shows the variation for one
complete switching cycle, for all the samples.
Fig. 8. Optical transmittance c/b at various potentials vs SCE in 1 M LiClO4 + PC electrolyte for 60 s of (a) NiO recorded in the wavelength range 350–800 nm, (b) PPy within
first five c/b cycles recorded in the wavelength range 350–1000 nm and (c) NiO/PPy film in the wavelength range 350–1000 nm where dots represent the transmittance after
10,000th c/b cycle.

A.C. Sonavane et al. / Electrochimica Acta 55 (2010) 2344–2351
2349
shift in stretching vibration of NiO is due to binding of organic
compound to the NiO [43].
The transition from NiOx to LiyNiOx after intercalation and dein-
tercalation of Li+ ions causes charge transfer from Ni+3 to Ni+2. Due
to charge transfer from Ni+2 (transparent Fig. 6(a)) to Ni+3, films get
colored (brownish gray Fig. 6(a).
For PPy films, the potential scans were started from +1.5 V (vs
SCE) reversed at −1.5 V (vs SCE) and terminated at +1.5 V (vs SCE)
(Fig. 5 curve (b)). A well-defined cathodic peak at −0.75 V (vs SCE)
during cathodic scan and anodic peak at 0.5 V (vs SCE) during anodic
scan were observed. Under the above mentioned circumstances,
the reaction (4) for PPy films can be described by the scheme [44]:
The SEM images of NiO, PPy and NiO/PPy films are shown in
Fig. 4. Surface morphology exhibited by the PPy film deposited onto
the amorphous glass substrate is compact, dense and amorphous-
like. Nevertheless few grains can be seen. However, when PPy is
deposited onto the granular NiO film, a large number of faceted
grains elongated and rectangular are observed. The underneath
NiO grains might have acted as seeds for the growth of these PPy
grains. Although randomly grown, their population density is quite
uniform.
p−doping
−
(C4H5N)n
Insulating(Orange−yellow)
+ (ny)ClO4 ←→ [(C4H5Ny+)(ClO4−)y] + ny(e−
)
(4)
conducting(Black−violet)
n
3.2. Electrochromic and electrochemical properties
PPy films show electrochromism with color changes from
orange–yellow in the reduced state (Fig. 6(b)) to black–violet in
the oxidized state (Fig. 6(b)). Films also exhibit an intermediate
state changing from yellowish green (−0.8 V vs SCE) to bluish green
(−0.62 V vs SCE).
Fig. 5(a)–(c) shows the CVs recorded for NiO, PPy and NiO/PPy
films in 1 M LiClO4 + 1 mM PC solution at 20 mV/s. The potential
scans for NiO thin films (inset of Fig. 5—curve (a)) were started
from +2.2 V (vs SCE) reversed at −1.2 V (vs SCE) and terminated at
+2.2 V (vs SCE). A well-defined cathodic peak at 0.1 V (vs SCE) during
cathodic scan and anodic peak at 0.9 V (vs SCE) during anodic scans
were observed. A simplified reduction scheme for representing
the gradual optical change that takes place under ion intercala-
tion/deintercalation in EC NiO films is represented in Eq. (3),
The shape of the CV of NiO/PPy film (Fig. 5 curve (c)) is
very similar to the CV of PPy films having cathodic peak at
−0.32 V (vs SCE) and anodic peak at +0.65 V (vs SCE) but with
less current density, which indicates the presence of NiO. NiO/PPy
shows electrochromism with color changes from brown–yellow to
black–violet (Fig. 6(c)) which represents that PPy plays leading role
in EC of NiO/PPy.
NiOx + yLi+ + e−
⇔
LiyNiOx
Coloured(Brownish gray)
(3)
The switching characteristics of the NiO, PPy and NiO/PPy films
were studied from in-situ transmittance at 630 nm. The studies
Transparent
Fig. 9. Overlay of CVs for (a) NiO sample after 5th and 10,000th c/b cycle, (b) PPy sample after 5th and 500th c/b cycle and (c) NiO/PPy sample after 5th and 10,000th c/b
cycles in 1 M LiClO4 + PC, demonstrating the electrochemical stability of the films at a scan rate of 20 mV/s.

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were performed by switching the samples from an oxidized state
to a reduced state by applying alternating square potentials (2.2
and −1.2 V for NiO and 1.5 and −1.5 V for PPy and NiO/PPy). Fig. 7
shows the resultant transmittance–time response for all the sam-
ples up to first ten cycles. The inset shows the response for one
cycle. In these experiments we have chosen the switching times as
the times required to switch 95% of the maximum optical contrast
at the wavelength of 630 nm.
NiO exhibits slower response speed with about 2.75 s for col-
oration (oxidation) and 1.84 s for bleaching (reduction) kinetics.
The response speed is faster in case of PPy samples (1.61 s for
coloration and 1.15 s for bleaching). The NiO/PPy samples exhibit
much faster response speed (0.601 s for coloration and 0.395 s for
bleaching) as compared to pure NiO and pure PPy. The much faster
response of NiO/PPy samples stem from the penetration of con-
ducting PPy in to NiO pores [38], which facilitates easy and fast
diffusion of ions. Similar results pertaining to the improvement in
response time in case of NiO/PEDOT has been reported by Xia et al.
[45].
4. Conclusions
An electrochromic NiO/PPy film was assembled with a configu-
ration of FTO/NiO/PPy and characterized for its electrochromic and
optical performance in LiClO4 + PC. XRD and FTIR indicate that the
film composed of NiO in cubic crystalline form and PPy is amor-
phous in nature. Scanning electron micrographs reveals a large
number of faceted rectangular grains. The CE of the NiO/PPy film
was observed to be 358 cm2/C. These results were suitable to oper-
ate the device with perceptible color changes from brown–yellow
in the reduced state to black–violet in the oxidized state in the
potential range from −1.5 to +1.5 V (vs SCE). The response time
of the film for coloring and bleaching process were found to be
0.601 and 0.395 s, respectively. This indicates the faster insertion
and deinsertion kinetics. The stability of NiO/PPy film is about 104
c/b cycles. The NiO/PPy film thus exhibits enhanced EC property
owing to the chemical bonding between them.
Acknowledgement
Fig. 8(a)–(c) shows the transmission spectra of the NiO, PPy and
NiO/PPy films under its colored and bleached states. The trans-
mittance of NiO film is c/b at 0.7 V vs SCE is shown in Fig. 8(a).
The coloration efficiency (CE), an important parameter to probe
the potential of the material as an EC material, was calculated at
633 nm by using following relations:
The authors wish to acknowledge the U.G.C., New Delhi for the
financial support through the DST-FIST (2002–2007) program and
UGC-ASIST (2005–2010) program.
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ꢂOD
(
)
630 nm
CEꢁ=630 nm
=
(5)
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