New process for synthesis of nickel oxide thin films and their characterization

Article abstract of DOI:10.1016/j.jallcom.2011.06.029

Sol-gel spin coating method has been successfully employed for the deposition of nanocrystalline nickel oxide (NiO) thin films. The films were annealed at 400-700 °C for 1 h in an air and changes in the structural, morphological, electrical and optical pr

Full text of DOI:10.1016/j.jallcom.2011.06.029

Journal of Alloys and Compounds 509 (2011) 9065–9070
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
New process for synthesis of nickel oxide thin films and their characterization
B.T. Rauta, S.G. Pawara, M.A. Chougulea, Shashwati Senb, V.B. Patila,∗
a Materials Research Laboratory, School of Physical Sciences, Solapur University, Solapur 413255, M.S., India
b Crystal Technology Section, Bhabha atomic Research Centre, Mumbai, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Sol–gel spin coating method has been successfully employed for the deposition of nanocrystalline nickel
oxide (NiO) thin films. The films were annealed at 400–700 ◦C for 1 h in an air and changes in the struc-
tural, morphological, electrical and optical properties were studied. The structural properties of nickel
oxide films were studied by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM).
XRD analysis shows that all the films are crystallized in the cubic phase and present a random orientation.
Surface morphology of the nickel oxide film consists of nanocrystalline grains with uniform coverage of
the substrate surface with randomly oriented morphology. The electrical conductivity showed the semi-
conducting nature with room temperature electrical conductivity increased from 10−4 to 10−2 (ꢀ cm)−1
after annealing. The decrease in the band gap energy from 3.86 to 3.47 eV was observed after annealing
NiO films from 400 to 700 ◦C. These mean that the optical quality of NiO films is improved by annealing.
© 2011 Elsevier B.V. All rights reserved.
Received 28 May 2011
Received in revised form 6 June 2011
Accepted 7 June 2011
Available online 8 July 2011
Keywords:
Nickel oxide
Sol–gel method
Structural properties
Optoelectronic properties
1. Introduction
ture range of 330–350 K. Pramanik and Bhattacharya [12] prepared
nickel oxide thin films from an aqueous solution composed of nickel
Metal oxides can adopt a large variety of structural geometries
with an electronic structure that may exhibit metallic, semicon-
ductor, or insulator characteristics, endowing them with diverse
chemical and physical properties. Therefore, metal oxides are the
most important functional materials used for chemical and biolog-
ical sensing and transduction. Moreover, their unique and tunable
physical properties have made themselves excellent candidates for
electronic and optoelectronic applications. Nanostructured metal
oxides have been actively studied due to both scientific interests
and potential applications [1,2].
NiO is an important antifromagnetic p-type semiconductor with
excellent properties such as gas-sensing, catalytic and electro-
chemical properties, and has been studied widely for applications
in solid-state sensors, electrochromic devices and heterogeneous
catalysts as well as lithium batteries. The nickel oxide thin films
have been prepared using various techniques including thermal
evaporation [3], spray pyrolysis [4], chemical vapor deposition
[5], electrochemical deposition [6], sol–gel [7,8], sputtering [9–11],
chemical solution deposition [12–16], etc. Among these, chemical
solution deposition, also called as a chemical bath deposition, is
an advantageous technique due to its low cost, low-temperature
operating condition and freedom to deposit materials on a vari-
ety of substances. Varkey and Fort [14] deposited nickel oxide thin
films using nickel sulfate and ammonia solution over the tempera-
sulfate, potassium persulfate, and ammonia at room temperature.
Han et al. [15] studied growth mechanism of nickel oxide thin
films following Pramanik’s chemistry. Banerjee et al. [16] obtained
hexagonal mesoporous nickel oxide using dodecyl sulfate as a sur-
factant and urea as a hydrolyzing agent.
To the best of our knowledge, few works are available in the lit-
erature on the sol–gel synthesis and characterization of NiO-based
nanosystems [7,8].
In the present study, we report new method of synthesis and
characterization of nanocrystalline NiO thin films by simple and
inexpensive sol–gel spin coating technique and effect of anneal-
ing on their structure, morphology, electric transport and optical
properties.
2. Experimental details
Nanocrystalline NiO thin films have been synthesized by a sol–gel method using
nickel acetate Ni(CH3COO)2·4H2O as a source of Ni. In a typical experiment; 3.322 g
of nickel acetate was added to 40 ml of methanol and stirred vigorously at 60 ◦C for
1 h, leading to the formation of light green colored powder. The as prepared pow-
der was sintered at various temperatures ranging from 400 to 700 ◦C with a fixed
annealing time of 1 h in an ambient air to obtain NiO films with different crystal-
lite sizes. The nanocrystalline NiO powder was further dissolved in m-cresol and
solution was continuously stirred for 11 h at room temperature and filtered. The
filtered solution was deposited on to a glass substrate by a single wafer spin pro-
cessor (APEX Instruments, Kolkata, Model SCU 2007). After setting the substrate on
the substrate holder of the spin coater, the coating solution (approximately 0.2 ml)
was dropped and spin-casted at 3000 rpm for 40 s in an air and dried on a hot plate
at 100 ◦C for 10 min. Fig. 1 shows the flow chart for the sol–gel synthesis of the NiO
films prepared by using the spin-coating technique. The structural properties of the
films were investigated by means of X-ray diffraction (XRD) (Philips PW-3710, Hol-
∗
Corresponding author. Tel.: +91 2172744771; fax: +91 217274770.
E-mail address: drvbpatil@gmail.com (V.B. Patil).
˚
land) using Cu K␣ radiation (ꢁ = 1.5406 A). The surface morphology of the films was
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.06.029

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B.T. Raut et al. / Journal of Alloys and Compounds 509 (2011) 9065–9070
Fig. 1. Flow diagram for NiO films prepared from the sol–gel process using the spin-coating method.
examined by scanning electron microscopy (SEM) (Model Japan), operated at 20 kV.
The room temperature dc electrical conductivity measurements were performed
using four probe techniques. The optical absorption spectra of the NiO thin films
were measured using a double-beam spectrophotometer Shimadzu UV-140 over
200–1000 nm wavelength range. The thickness of the film was measured by using
weight difference method and Dektak profilometer.
where t is film thickness of the film; m is actual mass deposited
onto substrate; A is area of the film and ꢂ is the density of nickel
oxide (6.67 g/cc2).
It was observed that increasing the annealing temperature
resulted in a decrease in film thickness from 0.9061 ␮m (400 ◦C
annealing) to 0.4997 ␮m (700 ◦C annealing). The NiO thin film
thickness is also confirmed by using Dektak profilometer and is
presented in Table 1.
3. Results and discussion
3.1. NiO film formation mechanism and thickness measurement
3.2. Structural analysis
The mechanism of NiO film formation by the sol–gel spin coating
method can be enlightened as follows:
Structural analysis of the NiO films annealed at 400–700 ◦C
was carried out by using CuK␣ radiation source of wavelength
Ni(CH3COO)2·4H2O + 2CH3–OH → Ni(OH)2 + 2CH3COOCH3 + 4H2O
˚
(ꢁ = 1.54056 A) and the diffraction patterns of films were recorded
Since to improve crystallinity and remove hydroxide phase,
films were annealed for 1 h pure NiO film is formed after air anneal-
ing by following mechanism:
by varying diffraction angle (2ꢃ) in the range of 20–80◦. Fig. 2
shows XRD pattern for the NiO films annealed at 400–700 ◦C. The
observed ‘d’ values are in good agreement with standard ‘d’ values
and the diffraction peaks are indexed to the cubic phase of NiO with
Ni(OH)2 ↓ +Carbonaceous compoundsAir a−nn→ealingNiO + H2O ↑
˚
a = b = c = 4.1678 A [Joint Committee on Powder Diffraction Stan-
Oxidation
dards (JCPDS) No. 73-1519]. It shows well-defined peaks having
orientations in the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes.
The absence of impurity peaks suggests the high purity of the nickel
oxide. Compared with those of the bulk counterpart, the peaks are
Thickness was calculated by weight difference method using;
m
t =
(1)
Aꢂ

B.T. Raut et al. / Journal of Alloys and Compounds 509 (2011) 9065–9070
9067
Table 1
Effect of annealing on NiO thin film properties.
Sr. No.
Annealing
Crystallite size (nm)
(from SEM)
Crystallite size (nm)
(from XRD)
Thickness (␮m)
Energy gap, Eg (eV)
Activation energy,
Ea␴ (eV)
temperature (◦C)
HT
LT
1.
2.
3.
4.
400
500
600
700
40
42
47
52
41.55
43.20
46.80
50.67
0.9161
0.7414
0.6425
0.4997
3.86
3.69
3.60
3.47
0. 11
0. 24
0.34
0.48
0.08
0.08
0.09
0.14
relatively broadened, which further indicates that the deposited
material has a very small crystallite size [17]. The crystallite size
(D) is calculated using equation as follows [18]:
reciprocal temperature (1000/T) is depicted in Fig. 4. After anneal-
ing, room temperature electrical conductivity was increased from
10−4 to 10−2 (ꢀ cm)−1, due to increase in the crystallite size and
reduced scattering at the grain boundary. Similar type of increase in
electrical conductivity has been observed by Patil et al. [22]. From
Fig. 4 it is observed that the conductivity of film is increases with
increase in annealing temperature, further it is observed that con-
ductivity obeys Arrhenius behavior indicating a semiconducting
transport behavior. The activation energies were calculated using
the relation:
0.94ꢁ
ˇ cos ꢃ
D =
(2)
where, ˇ is the half width of diffraction peak measured in radi-
ans. The calculation of crystallite size from XRD is a quantitative
approach which is widely accepted and used in scientific com-
munity [19–22]. The average crystallite size is increased with
increasing annealing temperature revealing a fine nanocrystalline
grain structure (Table 1).
ꢀ
ꢁ
Ea
ꢄ = ꢄ0exp
(3)
kT
where, ꢄ is the conductivity at temperature T, ꢄo is a constant, k
is the Boltzmann constant, T is the absolute temperature and Ea is
the activation energy. The activation energy represents the location
of trap levels below the conduction band. From Fig. 4, activation
energy (HT) was increased from 0.110 eV, to 0.481 eV, when film
annealed from 400 to 700 ◦C indicating no significant change.
3.3. Surface morphological studies
The two-dimensional high magnification surface morphologies
of NiO thin films annealed at 400–700 ◦C were carried out using
SEM images are shown in Fig. 3(a–d). From the micrographs, it is
seen that the film consists of nanocrystalline grains with uniform
coverage of the substrate surface with randomly oriented mor-
phology and the crystallite size is increased from 40 to 52 nm as
annealing temperature increases from 400 to 700 ◦C. The crystal-
lite size calculated from SEM analysis is quite in good agreement
with that of crystallite size calculated from XRD analysis. Similar
results are also observed by Patil et al. [22] for sol–gel derived TiO2
films.
3.4.2. Thermo-emf measurement
The dependence of thermo-emf on temperature is depicted in
Fig. 5. The thermo-emf was measured as a function of temperature
in the temperature range of 300–500 K. The polarity of the thermo
emf was negative at the hot end with respect to the cold end which
confirmed that nickel oxide thin films are of p-type semiconducting
similar to earlier report [23]. The plot shows increase in thermo-
emf with increase in temperature when film annealed from 400 to
700 ◦C. This is attributed to the increase in hole concentration as
the annealing temperature increases and also due to the increase
in crystallite size as discussed in Section 3.3. The thermoelectric
power was found to be of the order of 10−3 V/K when film annealed
from 400 to 700 ◦C.
3.4. Electrical transport properties
3.4.1. Electrical conductivity measurement
The four-probe technique of dark electrical conductivity mea-
surement was used to study the variation of electrical conductivity
of film with annealing temperature. The variation of log ꢄ with
3.5. Optical studies
The optical absorption spectra in the range of 200–1000 nm for
NiO thin films annealed at 400 to 700 ◦C were carried out at room
temperature without taking in account of scattering and reflec-
tion. Fig. 6 shows the optical absorption spectra of NiO thin films
annealed at 400–700 ◦C, it is observed that the absorption coeffi-
cient is very low for photon energy in the IR and visible region while
the sudden increase in the absorption coefficient occurs in the near
UV region. It was found that, the absorption coefficient of films is
increases with increase in annealing temperature. This could be
because of increase in the density of states of holes with increase
in annealing temperature, similar results are reported by Varkey
et al. [14] and Pejova [13]. The optical band gap (Eg) of NiO thin
films annealed at 400–700 ◦C is calculated on the basis of optical
absorption spectra using the following equation:
(a) 400
(b) 500
(c) 600
(d) 700
(d)
(c)
(b)
(a)
A(Eg − hv)
20
30
40
50
60
70
80
˛ =
(4)
hv
2 q (
)
degree
where ‘A’ is a constant, ‘Eg’ is the semiconductor band gap and ‘n’ is a
number equal to 1/2 for direct gap and 2 for indirect gap compound.
Fig. 2. X ray diffraction patterns of NiO film at different annealing temperatures.

9068
B.T. Raut et al. / Journal of Alloys and Compounds 509 (2011) 9065–9070
Fig. 3. SEM of NiO thin films anneled at (a) 400 ◦C, (b) 500 ◦C, (c) 600 ◦C and (d) 700 ◦C.
12
(a) 400oC
(d)
(c)
(b) 500oC
-2
-3
-4
-5
-6
10
(c) 600oC
(b)
(d) 700oC
(a)
8
6
4
2
0
0
400C
0
500C
0
600C
0
700C
300
350
400
450
500
Temperature ( 0C)
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
-1
1000/T (K )
Fig. 5. The variation of thermo-emf with temperature for of NiO thin film annealed
at different temperatures.
Fig. 4. Arrhenius plot of log conductivity vs. 1000/T of NiO thin film annealed at
different temperatures.

B.T. Raut et al. / Journal of Alloys and Compounds 509 (2011) 9065–9070
9069
increase of the ZnO grain size. Chaparro et al. [26] ascribed this ‘red
shift’ in the energy gap, Eg, to an increase in crystallite size for the
annealed ZnSe films. Bao and Yao [27] also reported a decrease in
Eg with increasing annealing temperature for SrTiO3 thin films, and
suggested that a shift of the energy gap was mainly due to both the
quantum-size effect and the existence of an amorphous phase in
thin films. In present case, the mean crystallite size increases from
40 to 60 nm (from XRD & SEM) after annealing from 400 to 700 ◦C.
Moreover, it is understood that the amorphous phase is reduced
with increasing annealing temperature, since more energy is sup-
plied for crystallite growth, thus resulting in an improvement in
crystallinity of NiO films. Therefore, it is believed that both the
increase in crystallite size and the reduction in amorphous phase
cause are decreasing in band gap of annealed NiO films. The change
in optical band gap energy, Eg, reveals the impact of annealing on
optical properties of the NiO films.
4.0
3.6
3.2
2.8
2.4
2.0
1.6
o
(a ) 400C
o
(b) 500C
o
(c) 600C
o
(d) 700C
(a)
(b)
(c)
(d)
400
600
800
1000
Wavelenght(nm)
Fig. 6. Variation of absorbance (˛t) with wavelength (ꢁ) of NiO thin film annealed
at different temperatures.
4. Conclusions
Nanocrystalline nickel oxide thin films were prepared by low-
cost sol–gel spin coating technique. The NiO films were annealed
for various temperatures between 400 and 700 ◦C. The XRD results
revealed that the NiO thin film has a good nanocrystalline cubic
structure. The SEM results depict that a uniform surface morphol-
ogy and the nanoparticles are fine with an average grain size of
about 40–52 nm. The dc electrical conductivity is increased from
10−4 to 10−2 (ꢀ cm)−1 for films annealed at 400–700 ◦C. Optical
absorption studies show low-absorbance in IR and visible region
with band gap 3.86 eV (at 400 ◦C) which was decreased to 3.47 eV
(at 700 ◦C). This has been attributed to the decrease in defect levels.
The P-type electrical conductivity is confirmed from thermo-emf
measurement with no appreciable change in thermoelectric power
after annealing.
The plots of (˛hꢅ)2 versus hꢅ of films annealed at 400–700 ◦C are
shown in Fig. 7. Since the plots are almost linear, the direct nature
of the optical transition in ␤-Ni(OH)2 and NiO is confirmed. Extrap-
olation of these curves to photon energy axis reveals the band gaps.
The band gap was found to be decreased from 3.86 to 3.47 eV for
films annealed at 400–700 ◦C. Varkey and Fort [14] reported the
slightly lower band gaps 3.75 and 3.25 eV for as-prepared NiOOH
and annealed NiO thin films [13]. The decrease in Eg with anneal-
ing temperature could be due to increase in crystalline size and
reduction of defect sites. This is in good agreement with the exper-
imental results of XRD analysis. According to XRD results, the mean
grain size has increased with increased annealing temperature. As
the grain size has increased, the grain boundary density of a film
decreased, subsequently, the scattering of carriers at grain bound-
aries has decreased [24]. A continuous increase of optical constants
and also the shift in absorption edge to a higher wavelength with
increasing annealing temperature may be attributed to increase in
the particle size of the crystallites along with reduction in porosity.
The decrease in optical band gap energy is generally observed
in the annealed direct-transition-type semiconductor films. Hong
et al. [25] observed a shift in optical band gap of ZnO thin films
from 3.31–3.26 eV after annealing, and attributed this shift to the
Acknowledgment
Authors (VBP) are grateful to DAE-BRNS, for financial support
through the scheme no.2010/37P/45/BRNS/1442.
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