Structure and optical properties of nanocrystalline NiO thin film synthesized by sol-gel spin-coating method

Article abstract of DOI:10.1016/j.jallcom.2009.06.139

NiO thin film was prepared by sol-gel spin-coating method. This thin film annealed at T = 600 °C. The structure of NiO thin film was investigated by means of X-ray diffraction (XRD) technique and scanning electron microscopy (SEM). The optical properties

Full text of DOI:10.1016/j.jallcom.2009.06.139

Journal of Alloys and Compounds 486 (2009) 9–13
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Structure and optical properties of nanocrystalline NiO thin film synthesized by
sol–gel spin-coating method
A.A. Al-Ghamdia, Waleed E. Mahmouda,b,∗, S.J. Yaghmoura, F.M. Al-Marzoukia
a King Abdulaziz University, Faculty of Science, Physics Department, Jeddah, Saudi Arabia
b Suez Canal University, Faculty of Science, Physics Department, Ismailia, Egypt
a r t i c l e i n f o
a b s t r a c t
NiO thin film was prepared by sol–gel spin-coating method. This thin film annealed at T = 600 ◦C. The
structure of NiO thin film was investigated by means of X-ray diffraction (XRD) technique and scanning
electron microscopy (SEM). The optical properties of the deposited film were characterized from the
analysis of the experimentally recorded transmittance and reflectance data in the spectral wavelength
range of 300–800 nm. The values of some important parameters of the studied films are determined,
such as refractive index (n), extinction coefficient (k), optical absorption coefficient (˛) and band energy
gap (Eg). According to the analysis of dispersion curves, it has been found that the dispersion data obeyed
the single oscillator of the Wemple–DiDomenico model, from which the dispersion parameters and high-
frequency dielectric constant were determined. In such work, from the transmission spectra, the dielectric
Article history:
Received 21 May 2009
Received in revised form 21 June 2009
Accepted 22 June 2009
Available online 30 June 2009
Keywords:
NiO
Sol–gel
Spin coating
Structure
Optical properties
constant (ε ), the third-order optical nonlinear susceptibility ꢀ(3), volume energy loss function (VELF)
∞
and surface energy loss function (SELF) were determined.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
sol–gel techniques [13]. Thermal decomposition method has some
advantages such as simple process, low cost and easiness to obtain
Recently, nanomaterials have attracted extensive interest
because of their unique properties in various fields in compari-
son with their bulk counterparts [1]. A reduction in particle size to
nanometer scale results in various interesting properties compared
with their bulk properties. Having a large surface area, metal oxide
nanomaterials show great advantages over conventional materi-
als in many applications. Transparent semiconducting materials,
which can be grown efficiently as thin films with low cost, are used
extensively for a variety of applications, including architectural
windows, solar cells, heat reflectors, light transparent electrodes,
and thin-film photovoltaic and many other opto-electronic devices.
Nanosized nickel oxide has demonstrated excellent properties
such as catalytic [2], magnetic [3], electrochromic [4], optical and
electrochemical properties [5]. Furthermore, nickel oxides can be
used as a transparent p-type semiconducting layer [6] and are
being studied for applications in smart windows, electrochemical
supercapacitors [7] and dye sensitized photocathodes [8]. A variety
of strategies have been employed for synthesis and manipulation
of nanostructured nickel oxide, such as evaporation [9], sputter-
ing [10], electrodeposition [11], thermal decomposition [12] and
high purity products hence it is quite promising and facile rout for
industrial applications.
In the present work, the structure properties of NiO thin film
prepared by sol–gel spin coating have been studied. The absorption
spectra are also measured in the wavelength range 300–800 nm.
In addition, the optical constants (n and k) are determined from
the transmittance T and the reflectance R of the film in the spectral
range from 300 to 800 nm. The transport and the onset energy gap
are also estimated.
2. Experimental
NiO film was deposited onto glass substrates by the sol–gel method using a spin-
coating technique [13]. Nickel acetate [Ni(CH3COO)2·4H2O] was used as a starting
material. 2-Methoxethanol (C3H8O2) and monoethanolamine (C2H7NO, MEA) were
used as a solvent and stabilizer, respectively. 2 mmol of nickel acetate was dissolved
into 20 ml of 2-methoxethanol then MEA added to them. The molar ratio of MEA
to nickel acetate was maintained at 1:1. The obtained mixture was stirred at 60 ◦C
for 2 h to yield a clear and homogeneous solution and then it was served as the
coating source after cooling down to room temperature. The glass substrates were
first cleaned by detergent, and then in methanol and acetone each for 10 min by
using ultrasonic cleaner. At last, the substrates were rinsed with deionized water and
dried with nitrogen. The coating solution was dropped into glass substrate, which
was rotated at 3000 rpm for 30 s using spin coater. After the spin coating, the film
was dried at 200 ◦C for 15 min in a furnace to evaporate the solvent and to remove
organic residuals, and then annealed at 600 ◦C in air for 30 min.
∗
Corresponding author at: Suez Canal University, Faculty of Science, Physics
Department, Ismailia, Egypt. Tel.: +20 16 2277227.
The structural characteristics of NiO film were investigated by X-ray diffraction
patterns. Philips X-ray diffractometer (model X’ Pert) was used for the measurement
of utilized monochromatic CuK␣ radiation operated at 40 kV and 25 mA. The diffrac-
E-mail address: w e mahmoud@yahoo.com (W.E. Mahmoud).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.06.139

10
A.A. Al-Ghamdi et al. / Journal of Alloys and Compounds 486 (2009) 9–13
Fig. 1. XRD of NiO thin film annealed at 600 ◦C.
Fig. 2. SEM photography of NiO thin film annealed at 600 ◦C.
tion patterns were recorded automatically with a scanning speed of 2◦ min−1. UV–vis
absorption spectra of NiO film, deposited onto optically flat fused quartz substrates,
were recorded on spectrophotometer. In addition, T and R of the annealed film were
measured at normal incidence in the spectral ranging from 300 to 800 nm. This is by
using a double-beam spectrophotometer (JASCO model V-570 UV–vis–NIR) attached
with constant angle specular reflection attachment (5◦).
tronic transition. The band-gap value could be obtained from the
optical absorption spectra by using Tauc’s relation [16];
A
˛ =
(hꢃ − Eg)n
(2)
hꢃ
where ˛ is the absorption coefficient (hꢄ) is the photon energy, A is
a constant and n assumes the values 1/2, 2, 3/2 and 3 for allowed
direct, allowed indirect, forbidden direct and forbidden indirect
transitions, respectively. Now for allowed direct type of transitions.
3. Results and discussions
3.1. Structural properties
˛hꢃ = A(hꢃ − Eg)1/2
(3)
Fig. 1 shows the X-ray diffraction (XRD) pattern of NiO thin
film annealed at 600 ◦C for 30 min. The observed indexed peaks
in this XRD pattern are fully matched with the corresponding pure
cubic-structured crystalline NiO. Several peaks are observed in the
pattern at 2Â = 37.1◦, 43.3◦, 62.8◦, 75.2◦, and 79.3◦ assigned to the
(1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystal planes, respectively.
The indexed peaks are fully consistent with the cubic-structured
crystalline NiO (JCPDS 47-1049) [14]. In addition to this, no peaks
for any impurities such as ␣-Ni(OH)2, ␤-Ni(OH)2, or other phases
were observed in the pattern which further confirm the crystalline
and pure phase of the cubic NiO. Moreover, the observed peaks
are sharper and higher in intensity which confirmed the well-
crystallization of the obtained NiO structures. The average diameter
was calculated by using Scherrer formula [15]
Fig. 4 shows the variation of (˛hꢄ)2 versus (hꢄ) for the NiO thin
film. The straight line nature of the plots over a wide range of photon
energy indicates the direct type of transition. The optical gap has
then been determined by the extrapolation of the linear region on
the energy axis as shown in Fig. 4. The optical gap for NiO film was
found to be 3.44 eV.
The average radius of NiO nanoparticles can be estimated in the
effective mass approximation [17]:
ꢀ
ꢁ
h¯ 2ꢂ2
2R2
1
me∗
1
mh∗
Eg
=
+
(4)
where Eg is the difference between the NiO nanoparticles Eg value
and the energy position of the long-wave edge of the absorption
band of NiO film and m∗e (=0.11 mo) and m∗ (=0.32 mo) [18] the effec-
tive masses of the conduction band electrhons and the valence band
holes of NiO, respectively. Using the above equation, the particle
kꢁ
d =
(1)
ˇ cos Â
where k is a constant which is taken to be 0.9, ꢁ the wavelength
of X-rays used (0.1789 nm), ˇ the full width at half maximum
(FWHM) and  is the angle of diffraction. The average diameter of
NiO nanoparticles was found to be about 25 nm.
The surface morphology of the as-prepared NiO nanoparticle
film, annealed at 600 ◦C for 30 min, has been studied by scanning
electron microscopy (SEM), Fig. 2. It is clearly observed from the
micrography that the nanoparticles are extra-fine, and uniformly
distributed on the glass substrate.
3.2. Optical properties
The analysis of optical absorption spectra is one of the most pro-
ductive tools for understanding and developing the band structure
and energy band gap, Eg, of crystalline structure. Fig. 3 shows the
dependence of absorption on the wavelength ꢁ in the spectral range
300–800 nm for NiO thin film. From such figure it is clear that,
there is absorption peak at 320 nm due to interband ꢂ–ꢂ* elec-
Fig. 3. Absorption spectrum of NiO thin film.

A.A. Al-Ghamdi et al. / Journal of Alloys and Compounds 486 (2009) 9–13
11
Fig. 6. The refractive index (n) and extinction coefficient (k) of NiO film as a function
of wavelength.
Fig. 4. The variation of (˛hꢄ)2 versus (hꢄ) for NiO thin film.
of 10−2 in visible region, it is a qualitative indication of excellent
surface smoothness of the NiO thin film [21].
In the normal dispersion region above (ꢁ > 320 nm), the refrac-
tive index dispersion is analyzed by using Wemple and DiDomenico
single oscillator model [22]. In terms of this model, the refractive
index as a function of photon energy can be expressed in the form:
size of NiO nanoparticles has been estimated which found to be
24.5 nm. This value is in good agreement with that obtained from
XRD data.
The transmittance and reflectance spectra of the NiO film were
measured to investigate the optical properties of the films and
are presented in Fig. 5. This figure depicts that the NiO film is
highly transparent and has an average transmittance within region
95–98% in the visible region. As seen in Fig. 5, in the visible region,
the reflectance is limited only by the surface reflectance of about
2–5%.
ꢂ
ꢃ
ꢂ
ꢃ
1
Eo
Ed
1
EoEd
=
−
(hꢄ)2
(6)
n2 − 1
where h is the Planck constant, ꢄ is the frequency, Eo is the oscilla-
tor energy and Ed is the dispersion energy, which is the measure of
the average strength of interband optical transition or the oscillator
strength. The single oscillator energy Eo and the dispersion energy
Ed can be obtained from Eq. (6) by plotting (n2 − 1)−1 against (hꢄ)2
as shown in Fig. 7. The determination of Eo and Ed can be made
from the plot, using the linear fit parameters. The values of Ed and
Eo are obtained from the intercepts and the slopes of the curves.
The oscillator energy, Eo, is related to the optical band gap, Eg is
in close approximation by Eo = 2Eg. Using the curve above the esti-
mated values of the oscillator parameters Eo, Ed and Eg were found
to be 6.84, 8.89, and 3.42 eV, respectively. The values of refractive
The refractive index of the films was calculated by the following
relation [19],
ꢄ
ꢂ
ꢃ
1 + R
1 − R
4R
(1 − R)2
n =
−
− k2
(5)
where R is the reflectance and k (k = ˛ꢁ/4ꢂ) is the extinction coeffi-
cient. The absorption coefficient ˛ [20] has been determined from
the spectra using the formula (˛ = ln(1/T)/d), where T is the trans-
mittance and d is the thickness of the film (=100 nm).
Fig. 6 shows the refractive index, n, and extinction coefficient,
k, of NiO film as a function of wavelength. The dispersion curve of
refractive index is fairly flat in the long wavelength region and rises
rapidly towards shorter wavelengths, showing the typical shape of
dispersion curve near an electronic interband transition. This sug-
gests that the film shows normal dispersion. The refractive index
is between 2.3 and 1.95 in the visible region. The extinction coef-
ficient decreases gradually with the shortening of wavelength. The
calculated k value included the total optical losses caused by both
absorption and scattering waves, this factor average is 1.2 × 10−2 at
visible range. The low value of extinction coefficient is in the order
index, n(0), and the constant ε = n(0)2 (for hꢄ → 0) extrapolated
∞
from the Wemple–DiDomenico model fit, and the high-frequency
dielectric [23] are found equal n(0) = 2.01 and ε = 4.04.
∞
The relation between the lattice dielectric constant (εL = n2),
wavelength (ꢁ) and refractive index (n) is given by [24].
ꢀ
ꢁ
e2N
4ꢂC2m∗
n2 = εL −
ꢁ2
(7)
where εL is the lattice dielectric constant and N/m* is ratio between
the number of free carriers to the effective mass. The relation
between n2 and ꢁ2 is shown in Fig. 8 which verifies the linearity of
Fig. 5. The spectral distribution of transmittance (T%) and reflectance (R%) of NiO
thin film.
Fig. 7. The relation between (n2 − 1)−1 against (hꢄ)2 for NiO thin film.

12
A.A. Al-Ghamdi et al. / Journal of Alloys and Compounds 486 (2009) 9–13
Fig. 10. The variation of ε1 and ε2 with the photon energy for NiO thin film.
Fig. 8. The relation between n2 and ꢁ2 for NiO thin film.
Eq. (7). The values of εL and (N/m*) are determined from the extrap-
olation of these plots to ꢁ2 = 0 and from the slopes of the graphs
and was found equal 4.3 and 2.3 × 1035 kg−1 m−3, respectively. It is
clear from the results that (ε < ε ), this is due to free charge carri-
∞
L
ers contribution in the polarization process that is occur inside the
material when the light incident on it.
The third order of nonlinear polarizability parameter, ꢀ(3), so-
called nonlinear optical susceptibility, is an important parameter,
because it gives an indication about the strength of the chemical
bonds between molecules of the prepared nanocrystalline thin film.
The third-order nonlinear susceptibility of NiO film is calculated by
using Eq. (8) [25], the spectrum is shown in Fig. 9.
ꢀ
ꢁ
Fig. 11. The variation of VELF and SELF with the photon energy for NiO thin film.
4
A
EoEd
4ꢂ[Eo2 − (hꢄ)2]
ꢀ(3) = A(ꢀ(1)
)
=
= A
(8)
(4ꢂ)4(n2 − 1)4
wave in the medium [26]. Fig. 10 represents the variation of ε1 and
ε2 with the photon energy for NiO thin film. As observed, the real
and imaginary parts show maxima related to the absorption value.
It is also possible to calculate the volume and surface energy loss
functions (VELF and SELF) by using Eqs. (9) and (10) [27] and plotted
in Fig. 11.
where A is a constant, A = 1.7 × 10−10 esu and ꢀ(1) is the linear opti-
cal susceptibility. Since the covalence and ionic chemical bonds
strongly influence the magnitude of the nonlinearity. One can see
that ꢀ(3) begins from 0.2 × 10−13 esu and get increase as the photon
energy hꢄ increases up to 1.7 eV, after which ꢀ(3) has a limit value
of 1.62 × 10−13 esu.
ε2
ε21 + ε22
VELF =
SELF =
(9)
The obtained absolute values of transmittance and reflectance
are used to determine the complex refractive index (n* = n − ik)
and the complex dielectric constant (ε* = ε1 − iε2) (where the real
dielectric constant ε1 = (n2 − k2) and the imaginary dielectric con-
stant ε2 = 2nk). The real part generally relates to dispersion, while
the imaginary part provides a measure to the dissipative rate of the
ε2
(10)
[(ε1 + 1)2 + ε22]
From such figure, it is clear that the volume energy loss is greater
than surface energy loss at all incident photon energies. It is also
clear that the maximum of SELF and minimum of VELF corresponds
to the absorption energy due to interband transition that occurs at
320 nm (3.9 eV).
4. Conclusion
NiO thin film has been deposited on a glass substrate by sol–gel
process and spin-coating method. X-ray diffraction showed that it
is a nanocrystalline structure. SEM revealed that the nanoparticles
are extra-fine, with an average particle size of about 22 3 nm. The
optical band energy gap of the film is estimated at 3.44 eV. The
dispersion of the refractive index in the film follows the single elec-
tronic oscillator mode. Using this model, the values obtained for
the oscillator strength (Ed) and the oscillator energy (Eo) are 8.89
and 6.84 eV, respectively. The dielectric constant ε is evaluated
∞
to 4.04, the refractive index is equal at 2.01. The third-order non-
linear susceptibility ꢀ(3) is calculated using Frumer model, and is
estimated to be 1.62 × 10−13 esu. The values of volume energy loss
Fig. 9. The nonlinear optical susceptibility (ꢀ(3)) as a function of photon energy (hꢄ).

A.A. Al-Ghamdi et al. / Journal of Alloys and Compounds 486 (2009) 9–13
13
of NiO thin film was found greater than surface energy loss at all
incident photon energies. It is also clear that the maximum of SELF
and minimum of VELF corresponds to the absorption energy due to
interband transition that occurs at 320 nm (3.9 eV).
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