Opto-structural, electrical and electrochromic properties of crystalline nickel oxide thin films prepared by spray pyrolysis

Article abstract of DOI:10.1016/S0921-4526(01)01024-9

Polycrystalline nickel oxide films with preferential growth along (1 1 1) plane were deposited onto glass substrates, maintained at 350°C, by the spray pyrolysis technique using nickel chloride as starting solution. The effect of solution concentration on their structural, electrical, and optical properties was studied. Using X-ray diffraction, the structural characteristics have been studied and due to the high degree of preferred orientation, Voigt analysis of single reflection was used to determine the microstructural properties (crystallite size and microstrain). The refractive index n and the extinction coefficient k have been computed from the corrected transmittance and reflectance measurements over the spectral range 300-2400 nm. Analysis of the absorption versus photon energy curves revealed a direct transition with optical band gap, E_g, of 3.6 eV and indirect transition within the range 3.97-3.75 eV as solution molarity increases from 0.05 to 0.3 M. The electrochromic behaviour of polycrystalline nickel oxide film were investigated by means of cyclic voltametry in 1 M KOH aqueous solution. Cycling showed significant increase in solar optical modulation reaching a value of 0.23 after 150 cycles.

Full text of DOI:10.1016/S0921-4526(01)01024-9

Physica B 311 (2002) 366–375
Opto-structural, electrical and electrochromic properties of
crystalline nickel oxide thin films prepared by spray pyrolysis
S.A. Mahmouda,*, A.A. Akla, H. Kamalb, K. Abdel-Hadya
a Department of Physics, Faculty of Science, Minia University, Minia, Egypt
b Department of Physics, Faculty of Science, Ain Shams University, Cairo, Egypt
Received 27 April 2001; received in revised form 12 September 2001
Abstract
Polycrystalline nickel oxide films with preferential growth along (1 1 1) plane were deposited onto glass substrates,
maintained at 3501C, by the spray pyrolysis technique using nickel chloride as starting solution. The effect of solution
concentration on their structural, electrical, and optical properties was studied. Using X-ray diffraction, the structural
characteristics have been studied and due to the high degree ofpreferred orientation, Voigt analysis ofsingle reflection
was used to determine the microstructural properties (crystallite size and microstrain). The refractive index n and the
extinction coefficient k have been computed from the corrected transmittance and reflectance measurements over the
spectral range 300–2400 nm. Analysis ofthe absorption versus photon energy curves revealed a direct transition with
optical band gap, Eg; of3.6 eV and indirect transition within the range 3.97–3.75 eV as solution molarity increases from
0.05 to 0.3 M. The electrochromic behaviour ofpolycrystalline nickel oxide film were investigated by means ofcyclic
voltametry in 1 M KOH aqueous solution. Cycling showed significant increase in solar optical modulation reaching a
value of0.23 atfer 150 cycles. r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Electrochromic properties; Nickel oxide thin films; Spray pyrolysis
1. Introduction
has the advantage ofincreasing the optical density
variation ofthe device, since both electrode
colours and bleach simultaneously. Other impor-
tant applications ofnickel oxide include prepara-
tion ofalkaline batteries (as a cathode material),
antiferromagnetic layers and p-type transparent
conducting films [2–5]. Due to the wide band gap
(3.6–4.0 eV), it has a wide range ofapplications in
opto-electronics as well as the thermal applica-
tions. Nickel oxide films are a new optical
recording material ofNi-NiO heterogeneous sys-
tems that would realize portable medium by using
a laser diode beam for recording and reading. The
new medium needs no protection because ofits
Nickel oxide is an anodic electrochromic ma-
terial (EC), which colours upon reduction (ion
extraction). It exhibits large dynamic range and
high electrochromic efficiency [1]. The phenomen-
on ofanodic coloration ofnickel oxide allows
potential applications ofthis material as a counter
electrode in conjunction with tungsten oxide as
working electrode in assembling EC device. This
*Corresponding author. Tel.: +20-2-86346891; fax: +20-2-
68342601.
E-mail address: s.a.mahmoud@link.net (S.A. Mahmoud).
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 0 2 4 - 9

S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
367
stability against atmosphere corrosion agents,
unlike conventional materials like tellurium [6].
The increased interest in this material, due to its
widespread applications, has prompted the re-
searchers to associate measurements ofdeiffrent
physical properties.
With more nickel ions present on the sprayed
droplets, greater clusters were formed upon
deposition.
Different solution concentrations in the range
0.05–0.30 M were used to prepare these films. The
flow rate, substrate temperature, deposition time,
nozzle to substrate distance are kept unvaried
during the deposition process at 15 cm3/min,
3501C, 40 s, and 40 cm, respectively.
The film thickness ofthe prepared samples was
measured using multiple-beam Fizeau fringes at
reflection using either white light or monochro-
matic light (Hg, lg ¼ 546 nm). The coloured
interference fringes enabled the determination of
the order of magnitude of the fringe shift, while the
monochromatic fringe shift as a fraction of order
separation has been measured using an eyepiece
micrometer.
Various methods have been used to prepare
nickel oxide films, including thermal evaporation
[7], RF magnetron sputtering [8], electrochemically
deposited films [9] and chemical vapour deposition
[10]. Quite recently [2], thin films ofnickel oxide
have been deposited onto glass substrates from
chemical route containing Ni2+ ion and urea.
Some properties ofthe as-deposited and post-
heated films have been reported [11]. It is known
that EC variation is based on absorption modula-
tion. This may cause a rise in device temperature
with subsequent possible deterioration in the
device performance. This could be overcome by
using crystalline EC materials based on reflection
modulation.
To investigate the structure ofthe films, a JEOL
X-ray diffractometer (model JSDX-60PA) using
(
Ni-filtered Cu:Ka radiation (lK ¼ 1:5418 A) was
a
In this present work, polycrystalline films of
nickel oxide have been deposited onto glass
substrates using spray pyrolysis nickel chloride as
starting solution. Influence ofsolution concentra-
tion on the microstructural, electrical and optical
properties has been investigated and discussed.
Furthermore, the influences ofsolution molarity
on the electrochromic properties ofcrystalline nickel
oxide films have been investigated. Effect of cycling
on the optical modulation has been dealt with.
employed. Continuous scanning was applied with
a slow scanning speed and a small time constant. A
range of2 y from 61 to 601 was scanned, so that all
possible diffraction peaks could be detected. The
crystallite size as well as the microstrain were
determined using single-order Voigt profile analy-
sis [13,14]. The microstrain was calculated as the
fractional change, Dd=d0; in the interplanar spa-
cing, d; ofthe (1 1 1) plane, where d0 is that ofthe
standard powder (ICDD card no. 47-1049). On the
other hand, resistivity measurements were per-
formed on films using the four-point probe.
2. Experimental details
The spectral transmittance and reflectance ofthe
films were recorded in the wavelength range from
300 to 2400 nm using Shimadzu UV 3101 PC, UV-
VIS-NIR double-beam spectrophotometer with
reflection attachment ofV–N type (incident angle
51). A developed computer program [11] based on
solving the exact equations was used to calculate
the refractive index, n; and the extinction coeffi-
cient, k: The prepared nickel oxide films deposited
onto ITO substrates were subjected to electro-
chemical ion insertion/extraction in an electroche-
mical solution containing 1 M KOH aqueous
solution. A three-electrode cell has been used.
Besides the working electrodes (nickel oxide films),
a platinum sheet has been used as counter
Nickel oxide thin films were deposited from
aqueous solution ofnickel chloride (NiCl 2 ꢀ 6H2O),
by spray pyrolysis technique. A homemade spray
system described previously [12] has been used.
The layers have been deposited onto two kinds of
substrates: Indium-doped Tin Oxide (ITO)-coated
glass and microscopic slides. The substrates have
been chemically and ultrasonically cleaned. The
overall reaction process can be expressed as heat
decomposition ofnickel chloride to of rm clusters
ofnickel oxide in the presence ofwater as follows:
NiCl2 þ H2O H-eat 2HCl þ NiO:
ð1Þ

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S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
electrode. The reference electrode was a saturated
calomel electrode (SCE). The solar transmittance,
Ts; ofthe prepared samples, in the coloration and
bleached states was obtained by integrating the
measured transmittance TðlÞ; weighted by the
solar spectrum at air mass2 [15],
Each data point represents the mean value offive
measurements taken at different locations on the
film, the calculated error was found to be 76% as
represented by bars.
3.1. Structural properties
R
2400 TðlÞ GðlÞ
300
R
X-ray diffraction (XRD) patterns for layers
deposited at glass substrates (Tsub ¼ 3501C) are
presented in Fig. 2 as a function of solution
molarity (0.1–0.3 M). Films prepared from
0.05 M solution proved to be amorphous. By
calculating the interplanar spacing ofthe lines in
the investigated X-ray patterns and matching the
results with ICDD card no. (47-1049), it is
concluded that the lines in the patterns [1 1 1]
and [2 0 0] are ofthe NiO phase with a cubic
structure. The calculated lattice parameters were
found to be a ¼ b ¼ c ¼ 0:416 mm. The films show
preferred growth along the [1 1 1] direction, as the
solution molarity increases, while an enhancement
in the preferred orientation along both [1 1 1] and
Ts ¼
;
ð2Þ
2400 GðlÞ
300
where GðlÞ is the solar spectrum at AM2.
3. Results and discussion
Nickel oxide films were grown on a glass
substrate with different molarities of the starting
solution. The variation ofthe thickness ofthe
deposited films, as obtained interferometrically, as
a function of the solution molarity is shown in
Fig. 1. It is clear that, as the solution concentra-
tion increases, the amount ofmaterial that
participates in forming the deposited film increases
with subsequent increase in the film thickness.
Fig. 2. X-ray diffraction patterns for nickel oxide films onto
function of solution molarity
Fig. 1. The variation ofthe nickel oxide film thickness as a
function of the solution molarity.
glass substrate as
(Tsub ¼ 3501C).
a

S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
369
[2 0 0] plane is observed. The increase in the
intensity ofthe peaks may be attributed to either
the grain growth associated with largest thickness,
or the increase in the degree ofcrystallinity by
increasing the solution molarity or both.
The full width at half maximum (FWHM) was
found to decrease markedly with solution molar-
ity. Such a decrease reflects the decrease in the
concentration oflattice imperefctions due to a
decrease in the internal microstrain within the film
and/or an increase in the crystallite/domain size
(CS). Considering that the size broadening is the
sole effect [16], the CS, was determined using Voigt
method for single-line profile analysis. The para-
meters required for determining CS and the
magnitude ofmicrostrains are the Lorentzian
(bfL) and Gaussian (bfG) integral breadths of
the sample profile, bf : According to the apparent
CS, e is
1
Fig. 3. The dependence ofthe CS ofnickel oxide films on the
solution molarity.
e ¼
ð3Þ
bfL
and the microstrain jej is given by
Dd
d0
1
2
jej ¼
¼
cot Y Dð2YÞ:
ð4Þ
ofmagnitude, with increasing solution concentra-
tion and tends to be constant, acquiring a value of
1.2 Â 108 O/cm2 at higher values ofsolution con-
centrations. This is attributed to the variation of
film thickness associated with the increase in
molarity. Variation ofsheet resistance Rs with
the reciprocal ofthe film thickness (1 =d) yielded a
straight line as shown in Fig. 5. This linearity
implies that the solution concentration (film
thickness) has no influence on the resistivity, since
Rs ¼ r=d; the resistivity is calculated as 1.1 kO cm.
In order to estimate the value ofthe activation
energy Ea ofcrystalline nickel oxide films, the
variation ofthe film resistivity, which deposited at
Tsub ¼ 3501C and 0.1 M, was recorded versus the
temperature in the range from 300 to 440 K. This
range has intrinsic semiconducting transport
phenomenon which applies Arrhenius-like law
[17,18] between the resistivity r and temperature
TðKÞ in which the resistivity dependence on
temperature is approximated to
The microstrains are equivalent to a variation in d-
spacing within domains by the amount that
depends on the elastic constants ofthe material
and the nature ofinternal stresses.
Fig. 3 shows the variation ofthe CS calculated
from the (1 1 1) reflection as a function of the
solution molarity. The CS is by definition mea-
sured in the direction normal to the reflection
plane, i.e. in the /1 1 1S direction, and conse-
quently, perpendicular to the substrate. Therefore,
the observed increase in the CS may be interpreted
in terms ofa columnar grain growth.
3.2. Electrical properties
The electrical dark resistance ofthe nickel oxide
films was measured using the conventional four-
point probe assuming homogeneous conduction
throughout the film depth, while the change ofthe
sheet resistance with the starting solution molarity
is shown in Fig. 4. It is clear that the sheet
resistance decreases slightly, within the same order
r ¼ AeE =2kT
;
ð5Þ
a

370
S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
Fig. 6. The dependence ofln ðrÞ versus the reciprocal absolute
temperature.
Fig. 4. The relation between sheet resistance ofnickel oxide
films and solution molarity.
can be obtained from the slope of the plot between
logarithmically scaled resistivity and reciprocal
absolute temperature, Fig. 6, was found to be
0.78 eV. This value is in agreement with the value
estimated from the work of Pejova et al. [2]. In
Fig. 6, some differences between experimental data
and straight line fitting in the range of100–140 1C
were observed, which is attributed to water losses.
Thermogravitometry measurements on nickel
oxide films prepared by electrochemical technique
showed similar finding characterized by a small
mass decrease around 1001C [18]. Also, TGA
measurements during heating in N2 or O2 of
colloidally precipitated nickel oxide films, indi-
cated a mass decrease around 1001C that attrib-
uted to water loss [2].
3.3. Optical properties
Fig. 5. The dependence ofsheet resistance on the reciprocal of
the film thickness for films prepared from different solution
molarities.
Measurements ofspectral transmittance and
reflectance ofthe sample showed that the films
are transparent with transmittance exceeding 80%
all over the spectral range 300–2400 nm as shown
in Fig. 7. Films that were prepared from higher
where A is constant, k is Boltzmann constant, T is
the absolute temperature and Ea is the activation
energy. The estimation ofactivation energy, which

S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
371
Fig. 7. Transmission and reflection spectra ofnickel oxide films
for different solution molarities.
Fig. 8. The dependence of
ultraviolet range for nickel oxide films prepared at different
a
on the wavelength in the
solution concentrations.
solution concentration showed a relatively lower
transmittance. This is attributed to the increase in
the film thickness, with subsequent increase in
absorption. In the thicker films, the onset of
absorption edge became less sharp, this is due
to the fact that, as the concentration increases,
bigger clusters are deposited; and the scattered
radiation became remarkable due to the surface
roughness.
formula [19],
Aðhn À EgÞm
a ¼
;
ð7Þ
hn
where A is constant, hn is the incident photon
energy, and m depends on the nature ofband
transition; m ¼ 12 or 23 for direct allowed and direct
forbidden transitions, and m ¼ 2 or 3 for indirect
allowed and indirect forbidden transitions. It was
noticed from Fig. 9a that for m ¼ 12; the extrapola-
tion ofthe linear part gives the value ofthe indirect
energy gap. The values show concentration depen-
dence varying from Egind ¼ 3:97 to 3.75 eV for
solution molarity varying from 0.05 to 0.3 M. This
may be attributed to the variation offilm thickness
as well as the improved crystallinity. In addition,
the extrapolation ofthe linear part of ðahnÞ1=2
versus hn plot is shown in Fig. 9b, the direct energy
gap is deducted as Egd ¼ 3:6 eV. The values of
direct and indirect transition agree with the
reported values ofband gap [2,20,21]. A compara-
tive study with the same previous finding is given
in Table 1.
The absorption coefficient a was obtained
through formula (6), where d is the film thickness
in nm, RðlÞ and TðlÞ are the reflectance and
transmittance at the specified wavelength l:
ꢀ
ꢁ
104
f1 À RðlÞg2
aðlÞ ¼
log10
:
ð6Þ
d
TðlÞ
Fig. 8 shows the spectral variation ofthe absorp-
tion coefficient, a; in the range of300–400 nm,
with 5 nm step, at different solution molarities.
The results show an absorption band in the UV
region.
The optical band gaps, Eg; for the as-deposited
films were calculated on the basis ofthe optical
spectral absorption using the well-known

372
S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
Fig. 9. The dependence of(a) ðahnÞ2 and (b) ðahnÞ1=2 on the incident photon energy ðhnÞ for samples prepared at different solution
molarities.
Table 1
A review ofsome published data on the optical energy gap and its comparison with the present findings
Ref.
[2]
Eg (eV)
Transition type
Comment
Preparation technique
Solution growth
3.6
Direct energy gap
Annealed at 3501C
[3]
3.25
3.75
4.0
Annealed at 3801C
Annealed at 5501C
Bulk
Direct energy gap
Solution growth
[5]
4.0
Indirect energy gap
Indirect energy gap
Indirect energy gap
Indirect energy gap
Bulk
Thermal evaporation
Thermal evaporation
[22]
[23]
3.6–4.0
3.15–3.5
2.5–3.7
Different substrate temperature
Ts ¼ 30024801C
For different distance between nozzle and substrate
Spray pyrolysis
Spray pyrolysis
Present work
3.6
3.97–3.75
Direct energy gap
Indirect energy gap
Ts ¼ 3501C
Thickness dependent
The optical transitions observed by different
workers are mostly in agreement with one another
as far as the gap width in the range 3.6–4.0 eV is
concerned. However, regarding the character of
the transition, some authors found it to be direct
while others found it to be indirect with similar
and unsimilar values. Our results given in Table 1
confirm both the direct and indirect characters of
the optical transition.
The calculation ofthe rerfactive index ofthe
samples was calculated using our developed
programme based on solving the exact equation

S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
373
et al. [26] reported a value of D2.05 at l ¼ 576 nm
for NiOx films prepared at low pressure using the
reactive e-beam evaporation.
3.4. Electrochromic properties
Polycrystalline nickel oxide films were grown
by spray pyrolysis onto ITO substrate to investi-
gate the dependence ofthe electrochromic per-
formance on solution molarity. Typical samples,
deposited under the above conditions, were sub-
jected to electrochromic investigations in 1 M
KOH aqueous solution. In the first cycle,
the variation ofsolar optical modulation
(DTs ¼ Tsb À Tsc) with solution molarity is shown
in Fig. 11. With the increase ofsolution molarity,
DTs initially increases up to 0.1 M, then decreases
for a higher solution molarity. This can be
attributed to the transofrmation ofsome ofthe
nano-crystalline nickel oxide molecules to Ni
(OH)2 that act as the active species wherein ion
insertion transforms the phase from or to one of
Ni (III) phases [14,15] according to the Bode
reaction scheme [27].
Fig. 10. The dependence ofrreafctive index,
n; on the
wavelength for samples prepared from different solution
concentrations.
by iteration [24]. The method is basically a
bivariate search technique based on minimizing
ðDRÞ2 ¼ jRC À RXj2;
ðDTÞ2 ¼ jTC À TXj2
ð8Þ
simultaneously, where the suffices C and X
referred to calculated and experimental values,
respectively. In the present work, Tomlin functions
[25], namely ð1 þ RÞ=T and ð1 À RÞ=T; were used
as two simultaneous variables instead of T and R:
The spectral variation ofthe refractive index, n; of
crystalline nickel oxide films deposited at different
solution molarities is shown in Fig. 10. The solid
line represented the mean value indicating the
decrease of n from 2.6 at l ¼ 400 nm to 1.4 at
l ¼ 2400 nm. The n values decrease with l show-
ing normal dispersion according to Couchy’s
formula. The solid line shows that the averaged
values with probable error varies from 3% in the
visible region to 6%, as l increases. Such an error
is acceptable in experimental determination of n
from spectral measurements of R and T: Bange
Fig. 11. The variation ofsolar optical modulation ofpoly-
crystalline nickel oxide films with different solution molarities.
At the first cycle.

374
S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
Fig. 12. The effect of cyclic voltammetry on the solar modula-
tion ofpolycrystalline nickel oxide films.
Fig. 13. XRD patterns ofpolycrystalline nickel oxide films
onto ITO substrate after 150 cycle, (a) as-deposited film, (b)
bleached state and (c) coloured state.
For a subsequent investigation, nickel oxide film
has been prepared at the optimum condition of
solution molarity, namely at 0.1 M. The spectral
transmittance has been measured in the colora-
tion/bleaching states at a potential of 72 V.
Fig. 12 shows that the solar optical modulation
increases markedly with the number ofcycles
reaching a value of0.23 atfer 150 cycles. In this
stage the injected charge during bleaching is
equal to that extracted upon coloration reaching
stable electrochromic performance. It is to be
noted that such aging procedure (cycling) had no
effect on the electrochromic characterization of the
amorphous NiOx prepared by electrochemical
deposition [11].
Fig. 13 shows the XRD ofnickel oxide films
deposited onto ITO glass substrates in the as-
deposited (a), bleached (b), and coloured (c) states
after 150 cycles. The variation of the peak
characteristics is clear as shown in the enlarged
display. It is evident that cycling (as well as ion
insertion) increases the amorphousness ofthe film
as it appeared in the increase ofthe peak width
and in the decrease ofpeak intensity.
4. Conclusion
Polycrystalline nickel oxide films were prepared
onto heated glass and ITO substrate kept at 3501C
by spray pyrolysis technique. Physical and electro-
chromic properties ofthe prepared films at
different solution molarities were investigated.
The obtained results lead to the following conclu-
sion:
1. The films show a preferred growth along the
[1 1 1] direction. As increase the solution mo-
larity, enhance the peak intensity and crystallite
size increases. The observed increase in the CS
may be interpreted in terms ofa columnar grain
growth.
2. The sheet resistance decreases slightly with
increasing solution molarity and tends to be
constant, acquiring a value of1.2 Â 108 O/cm2
at higher values ofsolution molarity represent-
ing the bulk resistivity.

S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
375
3. The values ofoptical energy gap show varying
concentration dependence as indirect transition,
from Egind ¼ 3:9723:75 eV for solution molarity
0.05–0.3 M, respectively. The direct energy gap
is deducted as Egd ¼ 3:6 eV.
[8] Y. Ushio, A. Ishikawa, T. Niwa, Thin Solid Films 280
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4. The dispersion curves shows normal dispersion
following Couchy’s formula.
5. With the increase ofsolution molarity, solar
optical modulation, DTs initially increases up to
0.1 M, then decreases for higher solution
molarity.
[11] S.A. Mahmoud, S.A. Aly, M. Abdel-Rahman, K. Abdel-
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with number ofcycles reaching a value of0.23
after 150 cycles.
7. Both cycling and ion insertion increases the
amorphousness ofthe polycrystalline nickel
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