Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, 35-41
doi:10.4236/jsemat.2011.12006 Published Online July 2011 (
Copyright © 2011 SciRes. JSEMAT
Effect of Annealing on Structural, Morphological,
Electrical and Optical Studies of Nickel Oxide
Thin Films
Vik as P at il1*, Shailesh P awar1, Manik Chougule1, Prasad Godse1, Ratnakar Sakhare1, Sh as hwat i Se n2,
Pradeep Joshi1
1Materials Research Laboratory, School of Physical Sciences, Solapur University, Solapur, India; 2Crystal Technology Section,
Bhabha atomic Research Centre, Mumbai, I ndia.
Received March 28th, 2011; revised May 3rd, 2011; accepted May 13th, 2011.
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˚C - 700˚C for 1 h in an air and changes in the structural, 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 crystal-
lized in the cubic phase and present a random orientation. Surface morphology of the nickel oxide film consists of na-
nocrystalline grains with uniform coverage of the substrate surface with randomly oriented morphology. The electrical
conductivity showed the semiconducting nature with room temperature electrical conductivity increased from 10-4 to
102 (Ωcm)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˚C - 700˚C. These mean that the optica l qu ality of NiO films is impro v e d by annealing.
Keywords: Nickel Oxide, Sol Gel Method, Structural Properties, Optoelectronic Prop erties
1. Introduction
Metal oxides can adopt a large variety of structural geo-
metries with an electronic structure that may exhibit me-
tallic, semiconductor, or insulator characteristics, en-
do wing t hem with d iverse chemical and physical proper-
ties. Therefore, metal oxides are the most important
functional materials used for chemical and biological
sensing and transduction. Moreover, their unique and
tunable physical properties have made themselves excel-
lent candidates for electronic and optoelectronic applica-
tions. Nanostructured metal oxides have been actively
studied due to both scientific interests and potential ap-
plications [1,2].
NiO is an important antifromagnetic p-type semicon-
ducto r with exc ellent p roper ties suc h as gas-se nsing, c at-
alytic and electrochemical properties, and has been stu-
died widely for applications in solid-state sensors, elec-
trochromic devices and heterogeneous catalysts as well
as lithium batter ies. The nickel oxide thin films have
been prepared using various techniques including ther-
mal 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 advanta-
geous technique due to its low cost, low-temperature
operating condition and freedom to deposit materials on
a variety of substances. Verkey and Fort [14] deposited
nicke l oxide t hin fil ms using ni ckel sul fate and ammonia
soluti on over the temp erature range 3 30 - 350 K. Prama-
nik and Bhattacharya [12] prepared nickel oxide thin
films from an aqueous solution composed of nickel sul-
fate, potassium persulfate, and ammonia atroom temper-
ature. Han et al. [15 ] studi ed gro wth mec hani sm of ni ck-
el oxide thin films following Pramanik’s chemistry. Ba-
nerjee et al. [16] obtained hexagonal mesoporous nickel
oxide using dodecyl sulfate as a surfactant and urea as a
hydrolyzing agent.
To the best of our knowledge, few works are available
in the literature o n the sol-gel synthesis and characteriza-
tion of NiO-based nanosystems [ 7,8].
Ef fect of Annealing on Structural, Morphological, Electrical and Optical Studies of Nickel Oxide Thin Fil ms
Copyright © 2011 SciRes. JSEMAT
In the present study, we report new method of synthe-
sis and characterization of nanocrystalline NiO thin films
by si mple and inexpe nsive sol-ge l spin c oatin g techniq ue
and effect of annealing on their structure, morphology,
electric transport and optic a l proper tie s.
2. Experimental Details
Nanocrystalline NiO t hin films have been synthesized by a
sol-gel method using Nickel a cetate Ni(CH3COO)2·4H2O
as a source of Ni. In a typical experiment; 3.322 gm of
nickel acetate was added to 40 ml of methanol and stirred
vigorously at 60˚C for 1 hr, leading to the formation of
light green colored po wder. The as prepared powder was
sintered at various temperatures ranging from 400 -
700˚C with a fixed annealing time of 1hr in an ambient
air to obtain NiO films with different crystallite sizes.
The nanoc rysta lli ne NiO powde r was furt her di ssolved in
m-cresol and s ol ution was c ontinuously stirred for 11 h at
room temperature and filtered. The filtered solution was
deposited on to a glass substrate by a single wafer spin
processor (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.
Figure 1 shows the f low char t fo r t he so l -gel s ynthe s is o f
the NiO films prepared by using the spin-coating tech-
nique. The structural properties of the films were inves-
tigated by means of X-ray diffraction (XRD) (Philips
PW-3710, Holland) using Cu Kα radiation (
= 1.5406
Å). The surface morphology of the films was 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
3. Results and Discussion
3.1. NiO Film Formation Mechanism and
Thickness Measurement
The mechanism of NiO film formation by the sol gel spin
coating method can be enlightened as follows:
( )
( )
3 23
3 32
⋅ + −→
Since to improve crystallinity and remove hydroxide
phase, films were annealed for 1 h pure NiO film is
formed after air annealing by follo wing me c hanism:
Figure 1. Flow diagram for NiO films prepared from the
sol-g el pro cess using the spin-coating method.
( )
Air annealing2
NiOHCarbonaceous compounds
→ +↑
Thickness was calculated by weight difference method
using formula:
t mA
= (1)
where t is film thickness of the film; m is actual mass
deposited onto substrate; A is area of the film and is the
densi ty of nickel oxide (6.67 g/cc2).
It was observed that increasing the annealing temper-
ature resulted in a decrease in film thickness from 0.906 1
μm (400˚C annealing) to 0.4997μm (700˚C annealing).
The NiO thin film thickness is also confirmed by using
Dektak profilometer and is presented in Table 1.
3.2. Structural Analysis
Structural analysis of the NiO films annealed at 400˚C -
700˚C was carri ed out b y usi ng CuK α radiation source of
wavelength (λ = 1.54056A˚) and the diffraction patterns
of films were recorded by varying diffraction angle (2θ)
in the range 20˚ - 80˚. Fig ur e 2 shows XRD pattern for
Ef fect of Annealing on Structural, Morphological, Electrical and Optical Studies of Nickel Oxide Thin Fil ms
Copyright © 2011 SciRes. JSEMAT
Table 1. Effect of annealing on NiO thin film properties.
Sr. No. Annealing temperature oC Crystallite size nm Thicknessμm Ene rg y gap Eg, eV Activ at i on en er g y, EeV
1 400 41.55 0.9061 3.86 0. 1 10 0.082
2 500 43.20 0.7414 3.69 0. 2 36 0.086
3 600 46.80 0.6425 3.60 0.344 0.096
4 700 50.67 0.4997 3.47 0.481 0.143
Figure 2. X-ray diffraction patterns of NiO film at different
annealing te mp eratures.
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 a = b = c = 4.1678 A˚ [Joint Committee on
Powder Diffraction Standards (JCPDS) No. 73-1519]. It
shows well-defined peaks having orie ntations in the (1 1
1), (2 0 0), (2 2 0), (3 1 1) and (222) planes. The absence
of impurity peaks suggests the high purity of the nickel
oxide. Compared with those of the bulk counterpart, the
peaks are relatively broadened, which further indicates
that the deposited material has a very small crystallite
size [17]. The crystallite size (D) is calculated using equ-
ation as follows [18]:
0.9 cosD
λβ θ
where, β is the half width of diffraction peak measured in
radians. The calculation of crystallite size from XRD is a
quantitative approach which is widely accepted and used
in scientific community [19-22]. The average crystallite
size is increased with increasing annealing temperature
revealing a fine nanocrystalline grain structure (Table 1).
3.3. Surface Morphological Studies
The two-dimensional high magnification surface mor-
phologies of NiO thin films annealed at 400˚C -700˚C
were carried out using SEM images are s hown in F igure
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 cr ystallite size is increased from 40 - 52
nm as annealing temperature increases from 400˚C -
700˚C. The crystallite 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
fil ms .
3.4. Electrical Transport Properties
3.4.1. Electrical Conductivity Measurement
The four-probe technique of dark electrical conductivity
measurement was used to study the variation of electrical
conductivity of film with annealing temperature. The
variation of log σ with reciprocal temperature (1000/T) is
depicted in Figure 4. After annealing, room temperature
electrical conductivity was increased from 104 to 102
·cm)1, due to increase in the crystallite size and re-
duced scattering at the grain boundary. Similar type of
increase in electrical conductivity has been observed by
Patil et al. [22]. From Figure 4 it is observed that the
conductivity of film is increases with increase in anneal-
ing temperature, further it is observed that conductivity
obeys Arrhenius behavior indicating a semiconducting
transport behavior. The activation energies were calcu-
late d using the r elation:
( )
E kT
= −
where, σ is the conductivity at temperature T, σo is a con-
stant, k is the Boltzmann constant, T is the absolute tem-
perature and Ea is the activation energy. The activation
energy represents the location of trap levels below the
conduction band. From Figure 4, activatio n energy (HT )
was increases from 0.110 eV, to 0.481 eV, when film
annealed from 400˚C - 700˚C indicating no significant
3.4.2. Thermo-emf Measurement
The dependence of thermo-emf on temperature is de-
Ef fect of Annealing on Structural, Morphological, Electrical and Optical Studies of Nickel Oxide Thin Fil ms
Copyright © 2011 SciRes. JSEMAT
Figure 3. S EM of N iO thin films annel ed at (a) 400˚C; (b) 500˚C; (c) 600˚C; and (d) 700˚C.
1.6 1.8 2.0 2.2 2.4 2.6 2.83.0 3.2 3.4
Log σ
1000/T (K
(a) 400
(b) 500
(c) 600
(d) 700
Figure 4. Arrhenius plot of log conductivity vs. 1000/T of
NiO thin film anneal ed at different temperatures.
picted in Figure 5. The thermo-emf was measured as a
function of temperature in the temperature range 300
500 K. The polarity of the thermo-emf was negative at
the hot end wit h respe ct to t he co ld end whic h confirmed
that nickel oxid e thin films are of p-t ype s em iconducting
similar to earlier report [23]. The plot shows increase in
th ermo-emf with increase in temperature when film an-
nealed from 400˚C -700˚C. This is attributed to the in-
crease 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 103 V/K when film annealed
from 400˚C - 700˚C
3.5. Optical Studies
The optical absorption spectra in the range of 200 - 1000
nm for NiO thin films annealed at 400 - 700˚C were car-
ried out at room temperature witho ut t a kin g i n a cc o u nt of
scattering and reflection. Figure 6 shows the optical ab-
sorption spectra of NiO thin films annealed at 400˚C -
700˚C, it is observed that the absorption coefficient is
very low for photon energy in the IR and visible region
while the sudden increase in the absorption coefficient
Ef fect of Annealing on Structural, Morphological, Electrical and Optical Studies of Nickel Oxide Thin Fil ms
Copyright © 2011 SciRes. JSEMAT
Figure 5. The variation of thermo-emf with tempera-
ture for of NiO thin film annealed at different tem-
400600800 1000
1. 6
2. 0
2. 4
2. 8
3. 2
3. 6
4. 0
( d)
( b)
Absorbanc e(a.u)
(a ) 400
(b) 500
(c) 600
(d) 700
Figure 6. Variation of absorbance (αt) with wavelength (λ)
of NiO thin f ilm annealed at different temperatures.
occurs in the near UV region. It was found that, the ab-
sorption coefficient of films is increases with increase in
annealing temperature. This could be because of increase
in the density of states of holes with increa s e in annealing
temperature, similar results are reported by Varkey et al.
[24] and Pejova [25]. The optical band gap (Eg) of NiO
thin fil ms annea led at 4 00˚C - 700˚C is calculated on the
basis of optical absorption spectra using the following
( )
A Egh
where ‘A’ is a constant, ‘Eg’ is the semiconductor band
gap a nd ‘n’ is a number equal to 1/2 for direct gap and 2
for indirect gap compound.
The plots of (α·)2 versus of films annealed at
400˚C - 700˚C are s hown in Figure 7.
Figure 7 Plot of (α·)2 versus (hv) of NiO thin film
for different annealing temperatures.
Since the plots are almost linear, the direct nature of
the optical transition in β-Ni(OH)2 and NiO is co nfir med.
Extrapolation of these curves to photon energy axis re-
veals the band gaps. The band gap was found to be de-
creased from 3.86 to 3.47 eV for films annealed at 400˚C -
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 [24]. The decrease in Eg with
annealing temperature could be due to increase in crys-
talline size and reduction of defect sites. This is in good
agreement with the experimental results of XRD analy-
sis. 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 a t
grain boundaries has decreased [25] .A continuous in-
crease 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 crystal lite s along with red uction in por os i ty.
The decrease in optical band gap energy is generally
observed in the annealed direct-transition-type semicon-
ductor films. Hong et al. [26] observed a shift in optical
band gap of ZnO thin films from 331 - 326 eV after
annealing, and attributed this shift to the increase of the
ZnO grain size. Chaparro et al. [27] ascribed this ‘red
shift’ in the energy gap, Eg, to an increase in crystallite
size for the annealed ZnSe films. Bao and Yao [28] also
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
12 (d)
( eV
Photon Energy (hυ)
(a) 400
(b) 500
(c) 600
(d) 700
Figure 7. P lot of (α· h ν)2 versus (h v) of NiO t hin fi lm for dif-
ferent ann eal ing temperatures.
Ef fect of Annealing on Structural, Morphological, Electrical and Optical Studies of Nickel Oxide Thin Fil ms
Copyright © 2011 SciRes. JSEMAT
reported a decrease in Eg with increasing annealing tem-
perature 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 in-
creases from 40 to 70 nm after annealing from 400˚C -
700˚C. Moreover, it is understood that the amorphous
phase is reduced with increasing annealing temperature,
since more energ y is supplied for cr ystallite growth, thus
resul ti n g i n an i mpr o ve me nt i n crysta llinit y o f N iO fi l ms .
Therefore, it is believed that both the increase in crystal-
lite 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. 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 to
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 morphology and the
nanopar ticles are fine with an average grain size of about
40 - 60 nm. The dc electrical conductivity is increased
fro m 104 to 102 ·cm) 1 for films annealed at 400˚C -
700˚C. Optical absorption studies show low-absorbance
in IR and visible region with band gap 3.86 eV (at
400oC) which was decreased to 3.47 eV (at 700oC). T his
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 ther-
moelectric power after a nnealing
5. Acknowledgments
Authors (VBP) are grateful to DAE-BRNS, for financial
support through the scheme
no.2010/37P/45/BRNS/1442. Thanks are also extended
to Dr.P.S.Patil, Department of Physics, Shivaji Universi-
ty, Kolhapur fo r providi ng S EM fa cility.
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