Investigation of structural, morphological and optical properties of nickel zinc oxide films prepared by sol-gel method

Article abstract of DOI:10.1016/j.jallcom.2010.11.054

NiZnO films have been prepared on glass substrates by sol-gel spin coating method. Nickel (II) nitrate hexahyrate and zinc acetate dihydrate were used as precursors. Surface morphology and crystalline structure of the films were investigated by field emis

Full text of DOI:10.1016/j.jallcom.2010.11.054

Journal of Alloys and Compounds 509 (2011) 2461–2465
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Investigation of structural, morphological and optical properties of nickel zinc
oxide films prepared by sol–gel method
∗
D. Duygu Dogan, Yasemin Caglar , Saliha Ilican, Mujdat Caglar
Department of Physics, Anadolu University, Eskisehir 26470, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
NiZnO films have been prepared on glass substrates by sol–gel spin coating method. Nickel (II) nitrate
hexahyrate and zinc acetate dihydrate were used as precursors. Surface morphology and crystalline
structure of the films were investigated by field emission scanning electron microscopy (FESEM) and X-
ray diffractometer, respectively. X-ray diffraction (XRD) patterns showed that the films are polycrystalline
nature. It was observed that crystal structure changed from wurtzite (ZnO) to cubic (NiO) structure. The
lattice constants crystallite size and preferred orientation were calculated from XRD data. The surface
morphology of the films changed with Ni concentration. The average transmittance of the ZnO film
decreased from 84% to 58% in the visible region. The absorption edge varied from 3.26 to 3.89 eV with
increasing Ni concentration.
Received 1 October 2010
Accepted 7 November 2010
Available online 18 November 2010
Keywords:
NiZnO
Sol–gel process
Crystalline structure
Absorption edge
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
Although pure ZnO and NiO films have been studied by many
research groups, the NiZnO films have been rarely studied. Also in
these works about NiZnO, in general, small amounts of Ni were
Zinc oxide (ZnO) films have been widely studied because of their
specific electrical, optical and mechanical properties, low material
cost and low deposition temperature. They can be used in optoelec-
tronic devices and it is useful material for the optoelectronic devices
as transparent conducting oxide (TCO) [1]. Also, recently there is
growing interest in nickel oxide (NiO), which is a metal oxide mate-
rial, because of its applications in many areas. Catalytic, magnetic,
electrochromotic, optical and electrochemical properties are that
some great feature of NiO [2]. In the literature, the optical band
gaps of bulk NiO and bulk ZnO are 4.00 eV and 3.37 eV, respectively
incorporated to ZnO in the NixZn1 O structure and the physical
properties of these films were investigated depending on the small
−x
x values [22,23]. In the NixZn1 O structure, while x value varies
−x
from 0 to 1, it is observed ZnO, NiO and related complex struc-
tures depending on the x value. The crystal structure changes from
wurtzite (ZnO) to cubic (NiO) structure. So, the determination of
the physical properties of the structural phase transition between
two different structures can be very useful. In our previous works,
the some physical properties of similar structures such as MgZnO
and CdZnO were reported [24–26].
[
3–5]. Furthermore NiO films are suitable for magnetoresistance
sensors, chemical sensors [6,7], electrochromic devices [8] and
transparent p-type semiconducting layer, smart windows [9] and
dye sensitized photocathodes [10–12]. Some methods can be used
for the preparation of NiZnO films such as sputtering [13], evapora-
tion [14], electrodeposition [15], thermal decomposition [16], pulse
laser deposition (PLD) [17–19], spray pyrolysis [20], and sol–gel
techniques [21]. Among these methods, the sol–gel method is an
attractive one due to its simplicity, safety, non-vacuum system of
deposition, and inexpensive. Other advantages of this method are
that it can be adapted easily for production of large-area films, and
to get varying band gap materials during the deposition process.
Although ZnO and NiO films produced by the sol–gel spin coat-
ing method have been reported by many research groups, NiZnO
films have seldom been studied. According to our knowledge,
there are a little detailed works on NiZnO films obtained by the
sol–gel spin coating method [15,24]. In this work, NiZnO films were
prepared by sol–gel spin coating method. The structural, morpho-
logical and optical properties of these films prepared at different
solution ratios to get the transition from ZnO to NiO structure were
studied.
2
.
Experimental
A set of NiZnO films was prepared by sol–gel process using a spin coating tech-
nique onto glass substrates. The glass substrates were first cleaned by detergent,
and then in methanol and acetone each for 10 min. At last, the substrates were
rinsed with deionized water and dried with nitrogen. To obtain the sol, the precursor
zinc acetate dihydrate (Zn(CH3COO)2·H2O, ZnAc) and nickel (II) nitrate hexahydrate
∗ Corresponding author. Tel.: +90 222 3350580; fax: +90 222 3204910.
E-mail addresses: yasemincaglar@anadolu.edu.tr,
ycaglar@semiconductorslab.com (Y. Caglar).
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.11.054

2
462
D.D. Dogan et al. / Journal of Alloys and Compounds 509 (2011) 2461–2465
NZO1
NZO2
30
40
50
60
70
30
40
50
60
70
2
θ
2θ
NZO3
NZO4
30
40
50
60
70
30
40
50
60
70
2
θ
2θ
NZO5
30
40
50
60
70
2
θ
Fig. 1. XRD patterns of the NiZnO films.
(
Ni(NO3)2·6H2O, NiN) were first dissolved into 2-methoxyethanol (C3H8O2) as a
3. Results and discussions
solvent and by adding monoethanolamine (C2H7NO,·MEA), which acts as the sta-
bilizer. Molar ratio of MEA to ZnAc or NiN was maintained at 1:1. ZnAc and NiN of
The XRD patterns of the NiZnO films have been realized in
order to evaluate crystalline phase and crystallite orientation. Fig. 1
shows the XRD patterns of these films. The observed indexed peaks
in these XRD patterns are fully matched with the corresponding
hexagonal wurtzite structure ZnO (zincite, PDF number: 036-1451
[27]) and cubic structure NiO (bunsenite, PDF number: 047-1049
[28]). The XRD results indicate that all the films have the poly-
crystalline structure. As seen in Fig. 1, the NZO1 film has highly
oriented (0 0 2) peak. The other peaks in this film are corresponds
to the (1 0 0) and (1 0 1) peaks of the hexagonal wurtzite structure.
As seen from Fig. 1, 2ꢀ values of (0 0 2) peaks of NZO1, NZO2 and
NZO3 films increased with increasing nickel concentration. With
the increasing nickel concentration, it is clearly observed that the
peak intensities of ZnO structure decrease and new peaks which
belong to NiO structure begin to appear. In the NZO3 film, the
0
.5 M were mixed together in different nominal solution volume ratios to obtain
ZnO, Ni(0.25)Zn(0.75)O, Ni(0.50)Zn(0.50)O, Ni(0.75)Zn(0.25)O, NiO films (named as
NZO1, NZO2, NZO3, NZO4, NZO5, respectively). The solutions were stirred at 60 C.
The substrates were placed on the sample holder and were rotated at a speed of
3
was done at 300 C for 10 min. This process was repeated ten times. The films were
finally annealed at 500 C for 1 h in air for crystallization and phase formation. The
obtained films have different colours, varying between colourless (transparent) for
the ZnO to brown for the NiO.
◦
000 revolutions per min (rpm) for 30 s. Immediate drying after successive coating
◦
◦
XRD experiments were performed in air with X-ray powder diffractometer
(
BRUKER D8 Advance). The diffractometer reflection of all the films was taken at
room temperature. The films were mounted on rotating sample holders. A sealed
X-ray tube operated at 40 kV and 40 mA with CuK␣ radiation was used. All measure-
◦
◦
ments were performed in reflection geometry as coupled ꢀ–2 ꢀ scans (30 ≤ 2ꢀ ≤ 70 )
at a divergent slit of 0.5 mm width. Surface morphology was studied using Zeiss
Ultraplus FESEM. Optical transmittance measurements of the films were carried out
by double beam Shimadzu UV 2450 spectrometer with an integrating sphere in the
wavelength range 190–900 nm.

D.D. Dogan et al. / Journal of Alloys and Compounds 509 (2011) 2461–2465
2463
Fig. 2. FESEM images of the NiZnO films.
peaks belong to both ZnO and NiO structures are shown. That is,
this complex structure includes both hexagonal and cubic struc-
tures. It is very interesting to note that the peaks belong to ZnO are
not observed in the NZO4 film although NiO peaks are little seen
in the NZO2 film. NZO5 film has an oriented (2 0 0) peak. The other
peaks in this film correspond to the (1 1 1) and (2 2 0) peaks of the
cubic structure. The miller indices (hkl), 2ꢀ, d values of the all films
are given in Table 1.
The crystallite size of the NiZnO films was calculated by using
the well-known Scherrer’s equation [29]
kꢁ
D =
(1)
ˇ cos ꢀ
where k is a constant (0.9), ꢁ is the wavelength of X-rays, ˇ is the
full width-at-half-maximum (FWHM) and ꢀ is the angle of diffrac-
tion. The crystallite size was calculated by resolving the highest
intensity peak belonging to the single phase. There are mixture
phases in some films. Therefore, the crystallite size values were
evaluated according to the highest peaks belonging to the ZnO and
the NiO phases in the NZO2 and NZO3 films. The crystallite sizes
and ˇ values of the films are given in Table 1. As seen in this table,
6
Fig. 3. FESEM image of NZO5 film at 1 × 10 magnifications.

2
464
D.D. Dogan et al. / Journal of Alloys and Compounds 509 (2011) 2461–2465
Table 1
100
Structural parameters of the NiZnO films.
Film
hkl
2ꢀ
d ( A˚ )
ˇ
D (nm)
8
6
4
0
0
0
NZO1
ZnO (1 0 0)
ZnO (0 0 2)
ZnO (1 0 1)
ZnO (1 1 0)
ZnO (1 0 3)
ZnO (2 0 0)
ZnO (1 0 0)
ZnO (0 0 2)
ZnO (1 0 1)
NiO (2 0 0)
ZnO (1 0 2)
ZnO (1 1 0)
NiO (2 2 0)
ZnO (1 1 2)
ZnO (1 0 0)
ZnO (0 0 2)
ZnO (1 0 1)
NiO (1 1 1)
NiO (2 0 0)
ZnO (1 0 2)
ZnO (1 1 0)
NiO (2 2 0)
NiO (1 1 1)
NiO (2 0 0)
NiO (2 2 0)
NiO (1 1 1)
NiO (2 0 0)
NiO (2 2 0)
31.731
34.395
36.214
56.546
62.856
67.934
31.781
34.440
36.266
43.075
47.618
56.618
62.874
67.927
31.782
34.460
36.241
37.106
43.111
47.501
56.763
62.854
37.084
43.077
62.569
37.261
43.308
62.868
2.81771
2.60530
2.47853
1.62623
1.47729
1.37870
2.81341
2.60201
2.47510
2.09827
1.35029
1.62434
1.47690
1.37882
2.81329
2.60053
2.47675
2.42097
2.09662
1.35342
1.62052
1.47734
2.42231
2.09817
1.48339
2.41123
2.08754
1.47705
–
–
0.222 37
–
–
–
–
–
–
–
–
–
–
@400-800nm; Tave
%
NZO2
NZO3
NZO1 84%
NZO2 74%
NZO3 64%
NZO4 62%
NZO5 58%
0.288 29
–
0.302 31
–
–
–
–
–
0.305 27
–
0.305 27
–
–
–
–
–
20
0
–
–
–
–
–
2
00
400
600
800
Wavelength (nm)
–
Fig. 4. Optical transmittance spectra of the NiZnO films.
–
–
–
–
crystallite size values calculated from the XRD results. It is observed
that the surface morphology of the films changes depending on the
increase in Ni concentration. As the Ni content increases, together
with the decrease in the crystallite size was also observed the aggre-
gations on the surface. Although not seen in FESEM images, as the Ni
content increases, some fractures were easily seen with the naked
eye on the surface. Also, Fig. 3 shows the image of the NZO5 film
taken at very high magnification. In this image, cubic structures can
be easily seen.
Fig. 4 shows the optical transmittance spectra of the NiZnO films
in the range 200–800 nm. The average transmittance values of the
films are given in Fig. 4. It was observed that NZO1 film has an aver-
age transmittance about 84% in the visible region and the highest
average transmittance value belongs to this film. The colour of the
films changes from transparent to dark brown with the increasing
Ni concentration. So, the average transmittance of the NiZnO film
decreases from 84% to 58%. The absorption edge of the films varied
from 380 nm to 318 nm.
NZO4
NZO5
0.327 25
–
–
–
–
–
–
0.405 21
–
–
the crystallite sizes decrease with increasing nickel molar concen-
tration and the largest crystallite size is observed in NZO1 film.
The ionic radius of Ni2+ (0.69 A˚ ) is smaller than Zn (0.74 A˚ ) [30],
2+
therefore, the substitution of Ni by Zn may reduce the crystallite
size.
The lattice parameters (ahex and chex) for hexagonal structure
were calculated using Eq. (2) for the NZO1, NZO2, and NZO3 films
[
29]:
ꢀ
ꢁ
2
2
l2
2
hex
1
d2
4
3
h + hk + k
=
+
(2)
2
a
c
hex
The calculated ahex and chex values are given in Table 2. Lattice
parameter (acub) for cubic structure calculated using Eq. (3) for
NZO2, NZO3, NZO4 and NZO5 films [29]:
3
.89eV
acub
NZO5
d = ꢂ
(3)
2
2
2
h + k + l
11.5
8.5
3.77eV
.29eV
NZO4
NZO3
The calculated a
parameters of the ZnO film were slightly decreased with increas-
ing Ni concentration. This decrease in the lattice parameters can
be expected when Zn ions are replaced by Ni ions because of
the smaller radius of Ni2+ ions than Zn ions. Similar results were
reported by Park and Kim [21] and Thota et al. [23].
The surface morphology of NiZnO films has been studied by
FESEM. Fig. 2 shows the FESEM micrographs of all the films. As
seen from these micrographs, the crystallite size decreases with
increasing Ni concentration. This result is in good agreement with
values are also given in Table 2. The lattice
cub
3
2
+
2+
3
.28eV
2+
λ
Τ
5
2
.5
.5
NZO2
NZO1
3
.26eV
Table 2
Lattice parameters of the NiZnO films.
Film
ZnO (hexagonal)
NiO (cubic)
ahex ( A˚ )
chex ( A˚ )
acub ( A˚ )
NZO1
NZO2
NZO3
NZO4
NZO5
3.25361
3.24864
3.24851
–
5.21060
5.20402
5.20106
–
–
-0.5
4.19654
4.19324
4.19556
4.17508
1
2
3
4
5
6
Photon energy (eV)
–
–
Fig. 5. The plot of dT/dꢁ versus photon energy of the NiZnO films.

D.D. Dogan et al. / Journal of Alloys and Compounds 509 (2011) 2461–2465
2465
To determine the absorption band edge of the NiZnO films, we
[3] Z.M. Jarzebski, Oxide Semiconductors, Pergamon Press, Poland, 1973.
[
[
[
[
4] P. Puspharajah, S. Radhakrishna, A.K. Arof, J. Mater. Sci. 32 (1997) 3001–
computed the first derivative of optical transmittance and are pre-
sented in Fig. 5. The plots of dT/dꢁ vs. photon energy give the
maximum peak position corresponding to the absorption band
edge. As seen in Fig. 5, the peak position of the curves shifts to
higher energies with Ni concentration. The absorption band values
are given in Fig. 5. This suggests that the absorption band edge shifts
from 3.26 eV to 3.89 eV with Nickel concentration.
3006.
5] A. Gupta, H.S. Bhatti, D. Kumar, N.K. Verma, R.P. Tandon, Digest J. Nanomater.
Biostruct. 1 (2006) 1–9.
6] U.S. Joshia, R. Takahashia, Y. Matsumotoa, H. Koinuma, Thin Solid Films 486
(
2005) 214–217.
7] H. Sato, T. Minami, S. Tanaka, T. Yamada, Thin Solid Films 236 (1993) 27.
[8] S.H. Lin, F.R. Chen, J.J. Kai, Appl. Surf. Sci. 254 (2008) 3357.
[9] Z. Zhu, N. Wei, H. Liu, Z. He, Adv. Powder Technol., doi:10.1016/j.apt.2010.06.
0
08.
10] X. Chen, Z. Zhang, C. Shi, X. Li, Mater. Lett. 62 (2008) 346–351.
[11] X. Ni, Q. Zhao, F. Zhou, H. Zheng, J. Cheng, B. Li, J. Cryst. Growth 289 (2006)
99–302.
12] K.-T. Kim, G.-H. Kim, J.-C. Woo, C. Kim, Surf. Coat. Technol. 202 (2008)
650–5653.
[
4
. Conclusions
2
[
The structural, morphological and optical properties of NiZnO
5
films deposited by sol–gel spin coating were investigated. X-ray
diffraction spectra indicated that the films are polycrystalline
nature. It was also shown from these spectra that the crystallite
structure changed from wurtzite to cubic. The structural parame-
ters of the films were determined by using XRD data. The surface
morphology was investigated by FESEM. The crystallite size values
of the NiZnO films varied from 37 to 21 nm with increasing Ni
concentration in solution. This decrease in the grain size was also
observed in the FESEM images. The absorption band edge of the
films shifted from 3.26 to 3.89 eV with increasing Ni concentration
in solution.
[13] A.A. Al-Ghamdi, E. Mahmoud Waleed, S.J. Yaghmour, F.M. Al-Marzouki, J. Alloy
Compd. 486 (2009) 9–13.
[
[
14] A. Berlich, Y.C. Liu, H. Morgner, Radiat. Phys. Chem. 74 (2005) 201–209.
15] X.H. Huang, J.P. Tu, X.H. Xi, X.L. Wang, J.Y. Xiang, L. Zhang, Y. Zhou, J. Power
Sources 188 (2009) 588–591.
[16] M.S. Niasari, N. Mir, F. Davar, Polyhedron 28 (2009) 1111–1114.
[
[
17] C.S. Carney, C.J. Gump, A.W. Weimer, Mater. Sci. Eng. A 431 (2006) 1.
18] B. Pandey, S. Ghosh, P. Srivastava, D.K. Avasthi, D. Kabiraj, J.C. Pivin, J. Magn.
Mater. 320 (2008) 3347–3351.
[19] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988.
[
20] R. Romero, F. Martin, J.R. Ramos-Barrado, D. Leinen, Thin Solid Films 518 (2010)
499–4502.
21] Y.R. Park, K.J. Kim, J. Cryst. Growth 258 (2003) 380–384.
4
[
[22] Y. Zuo, S. Ge, Z. Chen, L. Zhang, X. Zhou, S. Yan, J. Alloy Compd. 470 (2009)
7–50.
23] S. Thota, L.M. Kukreja, J. Kumar, Thin Solid Films 517 (2008) 750–754.
[24] M. Caglar, J. Wub, K. Li, Y. Caglar, S. Ilican, D. Xue, Mater. Res. Bull. 45 (2010)
84–287.
25] S. Ilican, Y. Caglar, M. Caglar, M. Kundakci, A. Ates, Int. J. Hydrogen Energy 34
2009) 5201–5207.
[26] Y. Caglar, M. Caglar, S. Ilican, A. Ates, J. Phys. D: Appl. Phys. 42 (2009) 065421.
4
[
Acknowledgements
2
[
This work was supported by Anadolu University Commission of
Scientific Research Projects under grant no. 061039 and 081029.
(
[
27] Joint Committee on Powder Diffraction Standards (JCPDS), Powder Diffraction
File, card no: 36-1451.
References
[
28] Joint Committee on Powder Diffraction Standards (JCPDS), Powder Diffraction
File, card no: 47-1049.
[
[
1] S. Ghosh, P. Srivastava, B. Pandey, M. Saurav, P. Bharadwaj, D.K. Avasthi, D.
Kabiraj, S.M. Shivaprasad, Appl. Phys. A 90 (2008) 765–769.
2] Q. Yang, J. Sha, X. Ma, D. Yang, Mater. Lett. 59 (2005) 1967–1970.
[29] B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, 3rd ed., Prentice Hall,
2001.
[30] R.D. Shannon, Acta Cryst. A32 (1976) 751.