Electrochromic behavior of NiO film prepared by e beam evaporation 2017 Journal of Science Advanced Materials and Devices

Journal of Science: Advanced Materials and Devices 2 (2017) 225e232

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Original Article

Electrochromic behavior of NiO film prepared by e-beam evaporation
D.R. Sahu a, b, *, Tzu-Jung Wu b, Sheng-Chang Wang c, Jow-Lay Huang b, d, **
a

Department of Natural and Applied Sciences, Namibia University of Science and Technology, Private Bag 13388, Windhoek, Namibia
Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701, Taiwan, ROC
c
Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan 710, Taiwan, ROC
d
Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan, ROC
b

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 1 December 2016
Received in revised form
4 May 2017
Accepted 5 May 2017
Available online 12 May 2017

The NiO thin films were prepared by the electron beam evaporation method using synthesized sintered
targets. As-prepared films were characterized using X-ray diffraction, scanning electron microscopy,
UVeVIS spectroscopy and cyclic voltammetry. The thicker films were found to exhibit a well-defined
structure and a well-developed crystallite size with greater transmittance modulation and durability.
The as-deposited thinner films of 170 nm showed a faster response time during electrochromic cycles
with a coloration efficiency of 53.1 C/cm2 than the thicker ones. However, the thicker films showed no
enhanced electrochromic properties such as a larger intercalated charge than the thinner ones. The
electrochromic properties of the thinner films became deteriorated after 800 cycling tests.
© 2017 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />
Keywords:
E-beam evaporation
Nickel oxide
Electrochromic properties

1. Introduction
Nickel oxide (NiO) is used as an efficient electrochromic (EC)
material in EC devices. It is an anodic coloring material, which is
optically and electrochemically compatible with the well-known
tungsten trioxides [1e3]. The electrochromics is a special property of a material to change its color reversibly by the application of
a voltage. The electrochromic process is unique and it strongly
depends on the method of preparation of the materials [4e6]. To
ensure the sustainable development of its functional properties in
devices, high quality thin films can be produced as a coating on the
surface of the devices. The properties of the thin films are thickness
dependent [6e8] and this makes the film thickness a more
important parameter not only as a geometrical but also as a functional one in the design of materials and devices.
Efforts have been explored to understand the thickness dependence of optical properties in nanocrystalline oxide films [6e9] and
demonstrated that the optical constants of very thin oxide films may

deviate considerably from those of the bulk materials. This means

*Corresponding author. Department of Natural and Applied Sciences, Namibia
University of Science and Technology, Private Bag 13388, Windhoek, Namibia.
** Corresponding author. Department of Materials Science and Engineering,
National Cheng-Kung University, Tainan 701, Taiwan, ROC. Fax: þ886 6 276 3586.
E-mail addresses: (D.R. Sahu),
(S.-C. Wang), (J.-L. Huang).
Peer review under responsibility of Vietnam National University, Hanoi.

that the respective materials constant is sometimes no longer constant in thin films. As a matter of fact, systematic investigations of
thickness-dependent film properties are necessary for device
application. NiO films have been prepared by chemical, electrochemical techniques and by vacuum technologies, such as e-beam
evaporation and sputtering [1,4,5,10e12]. Among these methods,
those which employ vacuum technologies provide highly stable EC
NiO films with good electrochemical efficiency. The electrochromic
property is also affected by the structure, binding conditions, water
contents, stoichiometry and thickness of the films [13,14]. In this
work, nickel oxide films with different thickness were prepared by
e-beam evaporation methods and investigated. Their EC properties
have been compared with those of different films with respect to
structure, surface morphology and electrochemical behavior for
possible device applications.
2. Experimental
The NiO thin films were deposited on Indium tin oxide (ITO)
glass substrates in an e-beam evaporation system with electron
energy in the order of 4 keV using a NiO sintered target. The target
was prepared by pressing high purity NiO powder (OSAKA, purity
99.99%, <325 mesh) into a disk type pellet of 11 nm in diameter and
4 mm in thickness followed by 4 h sintering procedure at 1300  C.

For fabricating the NiO films, the electron beam was focused on the
sintered target mounted on a water-cooled holder while the substrate (of 3 cm  5 cm in size) was placed parallel to the target at a

/>2468-2179/© 2017 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license
( />

226

D.R. Sahu et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 225e232

distance of 130 mm. The e-beam chamber was pumped down to
7 Â 10À7 torr prior to the deposition, whereas deposition of the NiO
films was performed at a pressure of about 1 Â 10À5 torr in the
evaporation chamber. Specimens of 170 nm, 270 nm, 380 nm and
540 nm thickness, respectively were fabricated by adjusting the
appropriate depositing time.
The thickness of the films was measured using a surface profiler
(Alpha-step 500, TENCOR). Conventional qe2q XRD characterization of the as-prepared films was carried out in a Rigaku (D/MAX
2500) diffractometer using the CuKa radiation. Surface morphology
was observed on a field emission scanning electron microscope (FESEM, XL-40). The XPS analysis was carried out with a VG Scientific

(111)

3. Results and discussion

*ITO

Intensity (arb. unit)

(220)

540 nm *

*

(200)

*

*

380 nm
270 nm
170 nm
ITO

20

30

40

50

60

210, UK employing the AlKa line. AFM micrographs were taken
using an AFM of, Digital Instrument INC, NanoScope, California,
USA. Optical transmittance was measured in the range of
300e800 nm by a UVeVISeIR spectrophotometer (Hewlett Packard
8452A, Palo Alto CA). The electrochromic properties of the films

were studied by cyclic voltammetry (CV-Autolab Potentiostat 30)
using a three-electrode cell system with an electrolyte solution of
lithium perchlorate in propylene carbonate of 0.5 M strength (0.5 M
LiClO4-PC). The working electrode was made of NiO film on Indium
Tin Oxide (ITO) glass substrate. The reference and counter electrodes were a saturated calomel electrode (SCE) and a platinum
grid, respectively.

70

80

2θ (deg.)
Fig. 1. XRD patterns of various NiO films with different film thicknesses.

Fig. 1 presents the XRD pattern of the NiO films of various
thicknesses (170, 270, 380 and 540 nm) as prepared on the ITO glass
substrate. The XRD patterns reveal the polycrystalline structure for
all the films under investigation. As the thickness of the film increases, the diffraction intensity increases due to more crystallites
grown in the structures [15]. The diffraction pattern of each film is
featured by three peaks. The peaks located at 37, 43 and 63 are
indexed as (111), (200) and (220), respectively, using a cubic NiO
structure (Space group: Fm3m) similarly to the analysis in Ref. [16].
It has been reported that all films have (111) preferential planes
which is different than the preferred (200) growth [9,17] and
furthermore, that with the increase of the film, other peaks
appear [9,17,18]. In our study, however, we have observed all these
peaks in all films and this may be due to the fact that the different
conditions of the film growth in our investigations are in overall
also similar to those used by other authors [14,19]. The crystallite

Fig. 2. SEM morphology of NiO films with different thickness: (a) 170, (b) 270, (c) 380 and (d) 540 nm.


D.R. Sahu et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 225e232

sizes calculated using Scherrer formula [20] from the full width and
half maximum of (111) peak show that the crystallite sizes increase
with the increase of film thickness, which may be interpreted in
terms of a columnar grain growth. This is in good consistency with
the discussions made by other authors [18] that, the peak intensity
increase with the increasing film thickness also indicates an
improvement of crystallinity. This suggests that e-beam evaporation provides more favorite conditions than other methods for the
growth of NiO polycrystalline films with high texture planes
[21,22]. The SEM images of NiO film with different thickness are
shown in Fig. 2. Observations on the surface morphology of NiO
films reveal that the grain size increases in the film with greater

540nm
380nm
270nm
170nm

+2

Counts

Ni

+3

Ni

525

530

535

Binding Energy (eV)
Fig. 3. XPS spectra of NiO films with different film thicknesses.

540

227

film thickness which corroborates the XRD results. There is more
agglomeration of particles for thicker films. This also improves the
effect of ion insertion/extraction due to the change of surface
morphology [2,17].
The composition of the nickel oxide films was determined by
XPS. Fig. 3 shows the O1 spectra for three NiO films. Two binding
energy peaks at 529.7 and 531.5 eV are identified. The binding
energy of 529.7 eV corresponds to the O1s peak of NiO [23] and that
of 531.5 eV corresponds to the O1s peak of Ni2O3 [23,24]. This
confirms the formation of Ni2þ and Ni3þ in the films. There was no
change found in the chemical composition and in the orientation of
the films with different thickness. These XPS results are in good
agreement with XRD data and the data presented as potential cyclic
curves below as well. The AFM pictures of the films with different
thickness (170, 270, 380 and 540 nm) presented in Fig. 4 indicate a

roughness of about 2.5 nm for the films under investigation which
slightly increases in the film with greater thickness. However, no
pronounced (obvious) variation of the roughness with the film
thickness could be derived from these results.
The transmittance spectra of the as-deposited NiO films presented in Fig. 5(a) show that the transmittance decreases with increase of the thickness of the film. The transmittance spectra fall
rapidly at low wavelength region. The transmittance spectra show
different modulated pattern due to the interference of reflected
light from both faces of the layer. There occurs the change of phase
as the thickness of the film increased from 170 nm to 270 nm and
this could be related to the constructive or destructive interference
between rays coming from the top and bottom of the film [25].
There is also a crossover of maximum transmittance value from
170 nm thickness to 270 nm evaluated at 550 nm. In thicker films,
the onset of absorption edge became less sharp, this is due to the

Fig. 4. AFM micrograph of NiO films with different thickness: (a) 170, (b) 270, (c) 380 and (d) 540 nm.


228

D.R. Sahu et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 225e232

Fig. 5. (a) The transmittance spectra of as-deposited NiO films with different thicknesses. (b) The transmittance spectra of as-deposited, bleached and color state of NiO films with
different thicknesses. (c) The change of the optical density in NiO films with different thicknesses.


D.R. Sahu et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 225e232

229

Table 1
Comparison on electrochromic properties of NiO film having different thickness at 550 nm wavelength.
Thickness (nm)

As deposited
transmittance (T)

Bleach state
transmittance
(TBleach)

Color state
transmittance
(Tcolored)

Difference
(DT)

Response
time

CE
(C/cm2)

Durability

170
270
380
540

61.8
63.3
57.7
47.5

80.4
92.8
82
90.4

53.2
53.5
46.1
36.2

27.2
39.3
35.9
54.2

Fast
Fast
Slow
Slow

53.1
39.8
33.6
32.4

Bad
Bad
Good
Good

where Tb and Tc refer to the transmittance in the bleached and
colored states, respectively. It shows that the thick film has higher
optical density than the thin film. In the visible wavelength region
the change in the optical density is highest compared to the other
regions. This indicates that the electrochromic material changes its
optical density upon the inflow and outflow of the electron. Further,
there are few maximum in the curve of optical density vs wavelength in the visible wavelength region. An electrochromic material
shows an optical density rise when subjected to application of a
voltage to receive or discharge electron and shows an optical
density drop when electron moves in the direction opposite that of
density rise. However, the rapid changing in OD means that there is
smooth flow of electrons into the EC materials and smooth outflow
of electrons from the EC materials. Here in the thick film, the first
average OD is in a wavelength range 350e380 nm, 2nd average OD
in a wavelength range 420e450 nm, third in 450e500 nm, fourth in
500e540 nm and so on. The increased maximum in OD also indicates increasing coloration. The peak also indicates the change of
color of the film due to the degree of crystallinity of the film. As the
crystalline size increases with thickness, there is shift of the absorption band of OD curve towards larger wavelength giving rise to
different color. The changes in OD also indicate that coloration is a
result of the creation of induced defect center at these bands.
Maximum peak around 400 nm indicates brown color of the film.
The blue color of the film arises in the region of 450e500 nm and
gray color at higher wavelength region [32,33].
The electrochromic behavior of the nickel oxide was tested by

means of the standard electrochemical technique of cyclic voltammogram. Fig. 6 shows the voltammogram obtained for the films

0.0012

170nm
270nm
380nm
540nm

0.0009
0.0006
0.0003

Current (A)

DOD ¼ logðTb =Tc Þ;

with different thicknesses under investigation. From the figure, it is
observed that the applied potential position of the anodic and the
cathodic peak shifts towards positively with the positive current
and negatively with the negative current, respectively. These
changes of peak positions correlated with the change of the
injected charge. There is an increase of reaction charges with the
increased film thickness. So the polarization of charges increases
with film thickness. The positions of the anodic and cathodic peaks
shift to higher and lower potentials, respectively, with increasing
thickness with respect to a higher polarization [34]. The set of
peaks is the signature of the Ni3þ/Ni2þ redox process which corroborates the XPS results. Fig. 7 presents the charge intercalated
during the electrochromic cycling for the films under investigation.
It is seen that charge intercalation increases in the films with

increased film thickness. The difference in the charge intercalation

0.0000
-0.0003
-0.0006
-0.0009
-0.0012
-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Potential (V) (vs SCE)
Fig. 6. Cyclic voltammograms of the NiO films with different thicknesses.

0.0027

170 nm
270 nm
380 nm
540 nm

0.0024
0.0021

Q in (C)

presence of bigger crystallites sizes and increased scattering due to
surface roughness [26,27]. With the increasing film thickness, the
onset of the fundamental absorption is observed to shift towards
the shorter wavelength [28,29]. The optical absorption edge is
related to the defect electronic states, which are associated with the
nickel vacancy and interstitial oxygen atoms. The transmittance
spectra for the deposited, bleached and colored state of the films for
different thicknesses are presented in Fig. 5(b). It shows that all the
films have less transmittance in the color state than that of as
deposited and bleached state. The transmittance spectra for the
deposited, bleached and colored state at 550 nm wavelength also
show that the 270 nm thick film has greater transmittance at
bleached and color state than other films. Detailed results are
presented in Table 1. However, the DT (Tbleached À Tcolored) increases
in the films with increased thickness. The transmission modulation
results are comparable to those obtained on other NiO films which
generally depend on the growth techniques and the electrolytes
used [30,31]. Our results are similar to those other research groups
generally obtained on post deposition films [18,21,22].
The change in optical density (DD) of the films with wavelength
is presented in Fig. 5(c). The change in optical density [9] is
defined as

0.0018

0.0015
0.0012
0.0009
0.0006
0

200

400

600

800

Number of cycles
Fig. 7. Charge intercalated during electrochromic cycling for different films.


230

D.R. Sahu et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 225e232

100

Relative capacity (%)

first state during which the capacity increases may last from a few
to hundreds of cycles depending on the reference electrode concentration or on the scanned potential window. In a more important manner, the activation period, during which the film nature is
progressively modified, appears longer duration for the thinner and
less agglomerated films which is observed for the film with 170 nm

thickness. Whatever the film thickness will be, coulometric capacity reaches once its maximum in the steady state and then decreases in the degradation period. This degradation period is of
much larger intensity in case of the thicker agglomerated films.
After about 800 cycles, 78% of capacity was found left for the
540 nm thick films.
The response time of the films was tested using potential step
measurements. Fig. 9 shows the results of the potential step measurement for the films under investigation. It indicates that the
thicker film needs longer time to respond than the thinner film, so
the response time is fast in thin films. The coloration efficiency (CE)
is defined as

Intensity (arb. unit)

is due to the increase in the depth of diffusion (e.g. diffusion length)
with the film thickness. The amount of charge transferred back and
forth upon cycling within a certain potential range also depends on
the film thicknesses. The electrolytes as well as the surface
morphology plays a decisive role in the ionic intercalation/deintercalation processes [21,35]. During the electrochromic cycling, it
initially increases up to 150 cycles and then decreases with the
increase of cycles in the case of the thick films. However, in the case
of thin films the charge intercalation increases with the increase of
cycles. With the increase of film thickness, the charge intercalation
increases, so there is an increase of transmittance modulation. For
the comparison between the relative capacity of the maximum
amount of charge intercalation for the film with different thickness,
the relative capacity as a function of the number of cycles is presented in Fig. 8. It is observed that the 540 nm thick film has the
maximum amount of charge intercalation at 150 cycles. However, it
reaches to 650 and 300 cycles for the 170 nm and 270 nm thin films,
respectively. The relative capacity of the film decreases with the
increase film thickness and that is due to the increase in the amount
of grain boundaries and of the reaction surfaces [36]. So it is

obvious that thinner films needed to be cycled with more cycles to
reach the maximum amount of charges. The cycling life of the NiO
films is typically based on a three-step process, namely the activation period, the steady state and the degradation period [34]. The

90

80

cycle 800 times

as-deposited

170 nm
270 nm
380 nm
540 nm

70

60

20

30

40

50

60

70

80

2θ (deg.)

50
0

200

400

600

800

Number of cycles

Fig. 10. XRD patterns of 540 nm thick NiO films in as-deposited state and after 800
cyclic tests.

Fig. 8. Relative capacity as a function of the number of electrochromic cycles for
different films with different thicknesses.

540 nm
170 nm

Current (A)

0.008

0.006

0.004

intensity (arb. unit)

0.010

(111)

cycle 800 times

0.002

as-deposited
0.000

0

20

40

Elapsed time (s)
Fig. 9. Electrochromic response time during coloration of 170 nm and 540 nm thick
films.

20

30

40

50

60

70

80

2θ (deg.)
Fig. 11. XRD patterns of a 170 nm thick NiO in as-deposited state and after 800 cyclic
tests.


D.R. Sahu et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 225e232

CE ¼ DOD=Qin :
It is found to decrease with the increasing film thickness. It is
about 53.1 C/cm2 for the 170 nm and 32.4 C/cm2 for the 540 nm
film. There is an increase of the injected charge and a decrease in
the transmittance. As a result the coloration efficiency becomes
smaller for thicker films [37]. Our result is in contrast with those
reported by other authors that the CE increases with the thickness
of NiO films where the value obtained at 630 nm responses to our
results at 550 nm [9,38]. Here, the CE decreases rapidly for thinner

films after undergoing electrochromic cycles. The thicker films have
longer cycling life than that of the thinner film.
The XRD pattern of the 540 nm thick film after 800 cycles presented in Fig. 10 indicates that there is no change in the film structure after 800 cycles of electrochromic cycling. However, in the case
of the XRD pattern for the 170 nm thick film shown in Fig. 11, there
appears the (111) peak after 800 cycles of electrochromic cycling.
The film structure changes with the electrochromic cycles along
with the changes in the surface morphology. Films degrade with
cyclic tests. The cyclic property is related to film integrity [39].
Therefore, thin films appear to have short response time and high
coloration efficiency and so they are fast responding. The thick films
have greater transmittance modulation and durability. This suggests
that e-beam evaporation synthesized NiO films can well meet the
requirement of application in electrochromic devices.
4. Conclusion
NiO films with various thicknesses were successfully deposited
on ITO glass using the e-beam evaporation method. The relation
between the thickness of NiO films and their electrochromic
properties is investigated and discussed. In thicker films, welldeveloped crystal structure and crystallite size have been developed which enable greater transmittance modulation and durability. As-deposited thinner films of 170 nm thickness show fast
response time during electrochromic cycles with coloration efficiency in the order of 53.1 C/cm2. The thicker films are found to
have larger amounts of intercalated charge than the thinner ones
but the electrochromic properties do not increase proportionally.
The thickness variation of electrochromic properties NiO films depends on the film preparation conditions and methods along with
the electrolytes used. The e-beam evaporation deposition method
without using post deposition technique seems to be suitable for
the production of future electrochromic devices.
Acknowledgments
Authors are thankful for the financial grant received from National Science Council, Taiwan under Contract No: NSC 96-2218-E006-006.
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