Spherical NiO nanoparticles (SNPs): Synthesis, characterization, and optical properties

Article abstract of DOI:10.1080/15533174.2011.609213

Spherical NiO nanoparticles have been synthesized by a simple and modified solvothermal method using NiCl₂·6H₂O and urea. After the precursor was calcined in the air, NiO could be obtained. Structural, optical, and morphological characterizations were done by x-ray powder diffraction, ultraviolet-visible (UV-vis) spectroscopy, and scanning electronmicroscopy. Fouriertransform infrared (FTIR) studies showa broad absorption around 422-425 cm^-1 corresponding to a surface-active mode, indicating the nanocrystalline nature of the oxide. The values of some important physical parameters of material are determined, such as refractive index (n), dielectric constant (e), ratio of molar volume (V_m) and molar refraction (R_m), and metallization criterion (M_(n)). Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15533174.2011.609213

This article was downloaded by: [University of Otago]
On: 15 March 2015, At: 04:27
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer
House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthesis and Reactivity in Inorganic, Metal-Organic,
and Nano-Metal Chemistry
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/lsrt20
Spherical NiO Nanoparticles (SNPs): Synthesis,
Characterization, and Optical Properties
A. Kalam a , Ayed S. Al-Shihri a , M. Shakir a , A. A. El-Bindary b , El Sayed S. Yousef a c
Gaohui Du d
&
a Department of Chemistry, Faculty of Science , King Khalid University , Abha, Kingdom
of Saudi Arabia
b Department of Chemistry, Faculty of Science (Demiatta) , Mansoura University ,
Demiatta, Egypt
c Department of Physics, Faculty of Science , Al Azhar University , Assuit, Egypt
d Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical
Chemistry , Zhejiang Normal University , Jinhua, China
Published online: 02 Dec 2011.
To cite this article: A. Kalam , Ayed S. Al-Shihri , M. Shakir , A. A. El-Bindary , El Sayed S. Yousef & Gaohui Du (2011)
Spherical NiO Nanoparticles (SNPs): Synthesis, Characterization, and Optical Properties, Synthesis and Reactivity in
Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:10, 1324-1330, DOI: 10.1080/15533174.2011.609213
To link to this article: http://dx.doi.org/10.1080/15533174.2011.609213
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained
in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of
the Content. Any opinions and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied
upon and should be independently verified with primary sources of information. Taylor and Francis shall
not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other
liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or
arising out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any
form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://
www.tandfonline.com/page/terms-and-conditions

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:1324–1330, 2011
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.609213
Spherical NiO Nanoparticles (SNPs):
Synthesis, Characterization, and Optical Properties
A. Kalam,1 Ayed S. Al-Shihri,1 M. Shakir,1 A. A. El-Bindary,2
El Sayed S. Yousef,1,3 and Gaohui Du4
1Department of Chemistry, Faculty of Science, King Khalid University, Abha, Kingdom of Saudi Arabia
2Department of Chemistry, Faculty of Science (Demiatta), Mansoura University, Demiatta, Egypt
3Department of Physics, Faculty of Science, Al Azhar University, Assuit, Egypt
4Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry,
Zhejiang Normal University, Jinhua, China
the nanocrystalline form, it shows a superparamagnetic property
due to surface effects.[5]
Spherical NiO nanoparticles have been synthesized by a sim-
ple and modified solvothermal method using NiCl2·6H2O and
urea. After the precursor was calcined in the air, NiO could
be obtained. Structural, optical, and morphological characteri-
zations were done by x-ray powder diffraction, ultraviolet–visible
(UV–vis) spectroscopy, and scanning electron microscopy. Fourier-
transform infrared (FTIR) studies show a broad absorption around
422–425 cm−1 corresponding to a surface-active mode, indicating
the nanocrystalline nature of the oxide. The values of some im-
portant physical parameters of material are determined, such as
refractive index (n), dielectric constant (ε), ratio of molar volume
(Vm) and molar refraction (Rm), and metallization criterion (M(n)).
Various researchers have synthesized NiO nanomaterials
which make them an ideal candidate for applications in alka-
line batteries as cathodes,[6] and as electrochemical capacitors,[7]
smart windows,[8] magnetic materials,[9] and gas sensors.[10] NiO
nanoparticles have been produced by calcinations,[11] thermal
decomposition of nickel carbonate,[12] and flame spray pyroly-
sis reactors,[13] and by using gelatin as a precursor along with
nickel chloride.[14] Palanisamy and co-workers[15] have synthe-
sized NiO nanoparticles using a microemulsion technique with
rhamnolipid as the surfactant. Wang[16] reported the fabrication
of hollow spheres of nickel oxide with hierarchical structures us-
ing styrene–acrylic acid copolymer (PSA) latex particles as the
template. Zhu et al.[17] obtained nickel oxide nanocolumns using
hydrazine and nickel chloride at low temperature. Le et al.[18]
obtained flower-shaped NiO architectures successfully synthe-
sized through a hydrothermal route. A number of reports ap-
peared on the synthesis of NiO nanomaterials involving urea by
adopting different methods.[19–21]
The ideal method of synthesis should yield essentially
naked particles so that different surface functionalities can be
readily introduced for nanoscale devices. There is a need for
the development of new methodologies to produce spherical
NiO nanoparticles. Keeping in view these requirements
we have developed a method for successful preparation of
spherical NiO nanoparticles by employing a modified and
simple solvothermal method in which a simple refluxing
glass assembly was employed instead of the sophisticated
and expensive solvothermal reaction setup. This simplified
Keywords chemical synthesis, FTIR and optical properties, scanning
electron microscopy, x-ray diffraction
INTRODUCTION
Synthesis of nanostructures with controllable sizes and shape
is stimulating worldwide interest in areas of materials sci-
ence.[1,2] One of the most commonly used metal oxides for a
wide range of applications is NiO. Nanostructured NiO is intro-
duced as a prototypical p-type semiconductor with stable wide
band gap (3.6–4.0 eV),[3] although, bulk NiO is an antiferro-
magnetic insulator with a Ne´el temperature (TN) of 523 K.[4] In
Received 28 April 2011; accepted 4 June 2011.
The authors are thankful to the Chairman Department of Chemistry,
King Khalid University, Abha, KSA, for providing necessary research
and laboratory facilities. The authors are thankful to the Chairman De- solvothermal route generated spherical NiO nanoparticles with
partment of Physics, King Khalid University, Abha, KSA, for extending
very low optical bandgap. Spherical NiO nanoparticles have
XRD and TGA facilities. The authors are also thankful to Dr. Refaat
A. Eid, College of Medicine, Electron Microscopy Unit, Pathology
Department, King Khalid University, Abha, KSA, for SEM analysis.
been obtained through calcination of as-synthesized precursor
formed by reacting NiCl2·6H2O and urea. The particle size,
lattice parameters, morphology and optical properties, bonding,
Address correspondence to Abul Kalam, Department of Chemistry,
Faculty of Science, King Khalid University, PO Box 9004, Abha 61413, and thermal behavior of spherical NiO nanoparticles have
Kingdom of Saudi Arabia. E-mail: abul k33@yahoo.com
been established by x-ray powder diffraction (XRD), scanning
1324

SPHERICAL NiO NANOPARTICLES
1325
electron microscopy (SEM), ultraviolet–visible (UV-vis)
spectroscopy, Fourier-transform infrared (FTIR) spectroscopy,
and thermogravimetric analysis (TGA). In addition, the optical
constants (n, ε, Rm/Vm) and the metallization criterion M(n)
have also been determined from the transmittance.
EXPERIMENTAL
Materials
The following chemicals are commercially available and
were used without further purification: NiCl2·6H2O (99.9%,
Sigma-Aldrich, USA), urea (99.5%, Sigma-Aldrich, USA),
2-propanol (99%, Sigma-Aldrich, USA), and absolute ethanol
(99.5%, Sigma-Aldrich, USA).
Preparation of NiO Nanoparticles
In a typical experiment, 2 g (0.0085 M) of NiCl2·6H2O was
dissolved in mixed solvents of 12 mL deionized water and 8 mL
2-propanol in a round-bottom flask to which 0.8 g (0.013 M)
urea was added; 50 mL ethanol was also added to the reaction
mixture to form the low-boiling-point azeotrope with water.
The round-bottom flask was fixed with a condenser, which was
airtight so that solvent vapors do not escape and get condensed
instantaneously, so as to keep the volume of the system constant
during the reaction. The reaction mixture was stirred on a
magnetic stirrer accompanied by refluxing for 60 h at 80◦C.
In this process, light green sediment (i.e., precursor) was
formed. The light green precipitate thus formed was separated
by centrifugation, washed with deionized water and acetone
three times, and dried in an oven at 90◦C overnight. Finally, the
nickel precursor was calcined in muffle furnace at 400◦C and
500◦C, respectively, for 2 h, resulting in a black solid mass.
FIG. 1. XRD patterns of NiO spherical nanoparticles calcined at (a) 400◦C
and (b) 500◦C.
RESULTS AND DISCUSSION
The NiO nanoparticles were obtained by calcining the pre-
cursor at different temperatures. Figures 1a and 1b show the
XRD patterns of the as-calcined NiO nanoparticles at two dif-
ferent temperatures. All diffraction peaks in the x-ray spectra
for NiO nanostructures can be indexed to a single face-centered
cubic phase (space group Fm3m, space group number 225)
NiO (JCPDS- ICDD card no. 001-1239), which indicates that
pure NiO is obtained. The lattice parameter, calculated at higher
2θ values, was found to be 4.1710, which is consistent with
monophasic nature of pure NiO crystal structure. XRD peaks
are markedly broader than the bulk cubic NiO powder and are lo-
cated at 37.2735◦, 43.3018◦, 62.8300◦, 75.2557◦, and 79.3738◦,
corresponding to (111), (200), (220), (311), and (222) crystal
planes. The broadening of diffraction peaks indicates that the
sample is on the nanometer scale. The crystallite size of NiO was
calculated from the major diffraction peak of the corresponding
NiO (200) using the Debye–Scherrer formula,[22]
Instrumental Techniques
The calcined NiO nanoparticles were characterized using
the x-ray powder diffraction (XRD) on a Shimadzu x-ray
diffractometer XRD-6000 with CuKα (λ = 1.54178 Å)
radiation for phase identification. XRD patterns were collected
at room temperature using a continuous scan over an angular
range of 2θ = 10–90◦ with step size of 0.02◦ and scan rate of
2◦ min−1. Thermogravimetric analysis (TGA) was carried out
using a Shimadzu TGA-50H with a heating rate of 10◦C/min.
On the basis of the TGA studies, we calcined the precursor
obtained by the modified solvothermal route at 400◦C and
500◦C for 2 h to ensure the complete conversion to nickel
oxide nanoparticles. The specimens were sputtered by a
BOC Edwards gold coating machine. The morphology of NiO
nanoparticles was analyzed using JEOL JSM-6390 LV scanning
electron microscopy (SEM). Infrared absorption spectroscopy
(IR) spectra were recorded at room temperature on an FTIR
spectrometer (JASCO 460 plus) using the KBr pellet technique.
Ultraviolet-visible absorption spectra were recorded using a
Shimadzu 1601PC UV-vis double-beam spectrophotometer
with a wavelength range of 250–800 nm.
D = 0.89λ/(β cos θ)
where λ is the wavelength of the x-ray (1.5418 Å) for CuKα
radiation, β the full width at half maximum (FWHM) of the
prominent intensity peak (using the 100% relative intensity
peak), and θ the peak position. The estimated grain sizes us-
ing the Debye–Scherrer formula were found to be about 13 and

1326
A. KALAM ET AL.
FIG. 2. Thermograph of the as-synthesized precursor (color figure available online).
14 nm (Figures 1a and 1b). It is interesting to note that the par- the typical morphology with spherical stacked layer structure.
ticle size of as-calcined NiO increases slightly with increasing After heating, spherical morphology (with stacked layered struc-
the calcination temperature, due to low agglomeration.
ture) changed to subunits of nanoparticles of NiO coalesced in
The TGA curve of the as-prepared precursor is shown in Fig- micrometer-size spherical particles (Figures 3c and 3d). Figure
ure 2. It can be seen that there are two distinct weight loss steps. 3c shows the SEM images of NiO nanoparticles at high magni-
The first weight loss occurs between 40 and 100◦C, correspond- fication, which suggest that the NiO nanoparticles are spherical
ing to the evaporation of crystallized water. The second weight in shape with smooth surface and the grain size of 5 µm. It is
loss occurs in the temperature range 325–540◦C, which may interesting that as the calcined temperature is increased from
be ascribed to the decomposition of the hydroxide. The weight 400 to 500◦C, the particle size remain unaltered. On the other
loss of the two steps is about 3.483% and 17.485%, which are hand, NiO at 500◦C consists of particles with relatively coarse
close to the theoretical values. Based on the results of TGA, surfaces.
the temperature of 400◦C was chosen to ensure the complete
Figure 4 exhibits FTIR spectra of as-synthesized and cal-
decomposition of precursor to form NiO. After the calcination cined products obtained at 400 and 500◦C temperatures. The
at 400◦C for 2 h in air, the XRD pattern resulted in all the peaks narrow band at 3640 cm−1 is attributed to the free hydroxyl
corresponding to NiO with a cubic structure (Figure 1). The group and the broad absorption band in the region 3600–
sample turned black in color after thermal treatment, indicating 3400 cm−1 represented the hydroxyl groups and water
complete hydroxide removal.
molecules, which were extensively hydrogen bonded,[23–28] and
SEM analysis was performed to observe the morphological the band at 1624 cm−1 is attributed to bending mode (H–O–H),
changes in the calcined NiO particles. The SEM micrograph indicating the as-synthesized precursor contains moisture. How-
of the as-synthesized precursor at ×2500 and ×6000 magni- ever, the absorption bands in the region 1530–1398 cm−1 in
fications are shown in Figures 3a and 3b, which demonstrate the observed precursor (Figure 4a) may be due to the bending

SPHERICAL NiO NANOPARTICLES
1327
FIG. 3. SEM images of (a, b) as-synthesized and NiO nanoparticles at (c) 400◦C and (d) 500◦C.
vibration of ionic CO32−, which gradually disappears after heat-
treated. The two interesting bands appearing at ∼2217 and ∼643
cm−1 confirm the presence of the intercalated NCO species in
the precursor similar to that reported in literature,[29] and it dis-
appears upon thermal treatment as shown in Figures 4b and 4c.
From the bands for the –NCO– and CO32− functional groups in
FT-IR spectrum, it is inferred that NCO and CO32− anions have
been intercalated in the precursor. The absorption band at ∼420
cm−1 is characteristics of Ni–O stretching vibrational mode,
which indicates the formation of nanoparticles as compared
to the bulk form NiO, the bands which lies between 390 and
Figure 5 shows the optical absorption characteristics of the
calcined NiO nanospheres in the UV–vis spectra, which were
ultrasonically dispersed in absolute ethanol. It is well known
that optical bandgap absorption in the UV region (Eg) can be
calculated on the basis of the optical absorption spectrum by the
following equation:
(Ahυ)n = B(hυ − Eg)
where hυ is the photon energy, A is absorbance, B is a constant
relative to the material, and n is either 2 for direct transition or
1/2 for an indirect transition.[33] Hence, the optical bandgap for
the absorption peak can be obtained by extrapolating the linear
portion of the (Ahυ)n−hυ curve to zero. As can be seen, the NiO
nanospheres have an absorption peak at 390 nm (or 1.515 eV)
and at 480 nm (or 1.182 eV) (Figure 6), for calcined samples at
400◦C and 500◦C, respectively. The strong absorption in the UV
region is attributed to bandgap absorption in the NiO and the
403 cm−1 [30]
.
As discussed in the case of as-prepared sample calcined at
400◦C and 500◦C, the band at 525.57 cm−1[31] suggested that
the formation of this NiO structure is due to the special mecha-
nism of the decomposition of high-temperature (HT)-like struc-
ture, i.e. thermal expansion and possible different ions arrange-
ments.[32]

1328
A. KALAM ET AL.
FIG. 6. Plots of (αhυ)n as a function of photon energy (hυ) for NiO nanopar-
ticles at (a) 400◦C and (b) 500◦C (color figure available online).
band gap values are lesser than those of bulk NiO (∼4 eV).[34]
The band gap energies of the NiO have been found to be 1.515
eV (at 400◦C) and 1.182 eV (at 500◦C). It is interesting to notice
that the values of the band gap energies are lower than the en-
ergy reported by Boschloo,[35] Z. Zhang,[36] and Chatterjee.[37]
No linear relation was found for n = 1/2, suggesting that the
calcined NiO nanoparticles are semiconducting with direct tran-
sition at this energy.[38] Furthermore, it is well known that the
energy gap of the semiconductor nanoparticles increases with
the decrease of the grain size. Normally, the nanomaterials show
a blue shift in the bandgap energy, but the synthesized material
show a red shift. The effect may be attributed to the chemical
defects or vacancies present in the crystals which results in the
electronic state in the bandgap to reduce the bandgap energy.
The optical absorption of the NiO nanosphere does not show
a blue shift upon size reduction (bulk value 4.0 eV).[34] Hence
there is no quantum confinement effect due to their small Bohr
radii. Duffy[39] has obtained an empirical formula that relates
energy gap Eg to molar refraction Rm for a large number of
simple oxides:
FIG. 4. FTIR of (a) as-synthesized and NiO nanoparticles at (b) 400◦C and
(c) 500◦C.
Eg = 20(1 − Rm/Vm)2
[1]
This equation gives the explicit expression for Rm:
ꢀ
Rm = Vm(1 − Eg/20)
[2]
The most familiar and widely used relationship that relates molar
refraction Rm to refractive index n and molar volume Vm is the
Lorentz–Lorenz equation:
FIG. 5. UV-vis absorption spectra NiO nanoparticles at (a) 400◦C and (b)
500◦C (color figure available online).
Rm = [(n2 − 1)/(n2 + 2)]Vm
[3]

SPHERICAL NiO NANOPARTICLES
1329
TABLE 1
tical absorption bandgap of the nickel oxide nanoparticles was
estimated to be 1.515 eV and 1.182 eV at temperatures of 400
and 500◦C, respectively. The synthetic method presented herein
is novel in the sense that it does not involve any templates or sur-
factants, so it may be suitable for bulk production of spherical
NiO nanoparticles.
Optical energy gap Eg (eV), refractive index n, dielectric
constant ε, molar volume Vm (cm3), molar refraction Rm, (cm3
mol−1), and metallization criterion M(n)
Sample number
Eopt
n
ε
Rm/Vm
M(n)
a
b
1.515 2.98
1.182 3.216 10.34
8.88
0.725
0.757
0.275
0.243
REFERENCES
1. Mishra, Y.K.; Mohapatra, S.; Avasthi, D.K.; Lalla, N.P.; Gupta, A. Adv.
Mat. Lett. 2010, 1, 151.
2. Rai, R.; Sharma, S. Adv. Mat. Lett. 2010, 1, 269.
3. Yang, H.; Tao, Q.; Zhang, X.; Tang, A.; Ouyang, J. J. Alloys Compd. 2008,
459, 98.
4. Guerra, C.D.; Remo´n, A.; Garc´ıa, J.A.; Piqueras, J. Phys. Stat. Sol. 1997,
163, 497.
5. Richardson, T.; Yiagas, D.I.; Turk, B.; Forster, K.; Twigg, M.A. J. Appl.
Phys. 1991, 70, 6977.
The Lorentz–Lorenz relationship, Eq. (3), becomes the
Clausisus–Mossotti expression on replacing n2 by the (high fre-
quency) dielectric constant, ε (strictly, this is for n at infinite
wavelength). If, next, molar refractivity, Rm, replaces polaris-
ability, αm, Eq. (3) becomes
6. Bahadur, J.; Sen, D.; Mazumder, S.; Ramanathan, S. J. Solid State Chem.
2008, 181, 1227.
Rm = Vmε − 1/ε + 2
[4]
7. Nathan, T.; Aziz, A.; Noor, A.F.; Prabaharan, S.R.S. J. Solid State Elec-
trochem. 2008, 12, 1003.
8. Granqvist, C.G. (Ed.), Handbook of Inorganic Electrochromic Materials;
Elsevier: Amsterdam, 1995.
(with Rm in cm3mol−1 and Rm = 2.5 αm). Equation (4) has very
important implications for the transition from the nonmetallic
state to the metallic state.[40]
9. Felic, A.C.; Lama, F.; Piacentini, M.J. J. Appl. Phys. 1997, 80, 3678.
10. Yang, H.X.; Dong, Q.F.; Hu, X.H. J. Power Sources 1998, 74, 46.
11. Xiang, L.; Deng, X.Y.; Jin, Y. Scr. Mater. 2002, 47, 219.
12. Turky, A.M. Appl. Catal., A Gen. 2003, 247, 83.
13. Seo, D.J.; Park, S.B.; Kang, Y.C.; Choy, K.L. J. Nanopart. Res. 2003, 5,
199.
14. Meneses, C.T.; Flores, W.H.; Garcia, F.; Sasaki, J.M. J. Nanopart. Res.
2007, 9, 501.
15. Palanisamy, P.; Raicher, A.M. Mater. Sci. Eng. C 2009, 29, 199.
16. Wang, D.B.; Song, C.X.; Hu, Z.S.; Fu, X. J. Phys. Chem. B 2005, 109,
1125.
According to the theory on metallization of the condensed
matter proposed by Herzfeld,[41] for the condition Rm/Vm = 1
in the Lorentz–Lorenz equation the refractive index becomes
infinite, which corresponds to the metallization of covalent solid
materials. In other words, the electron becomes itinerant and the
system acquires metallic status. The necessary and sufficient
conditions for predicting the nonmetallic or metallic nature of
solids are Rm/Vm < 1 (nonmetal) and Rm/Vm > 1 (metal). The
difference from 1 is the so-called metallization criterion[42]
17. Zhu, J.X.; Gui, Z.; Ding, Y.Y.; Wang, Z.Z.; Hu, Y.; Zou, M.Q. J. Phys.
Chem. C 2007, 111, 5622.
18. Zhou, Q.; Gu, F.; Li, C. J. Alloys Compounds 2009, 474, 358.
19. Bai, L.; Yuan, F.; Hu, P.; Yan, S.; Wang, X.; Li, S. Mater. Lett. 2007, 61,
1698.
20. Liu, J.; Du, S.; Wei, L.; Liu, H.; Tian, Y.; Chen, Y. Mater. Lett. 2006, 60,
3601.
21. Wu, L.; Wu, Y.; Wei, H.; Shi, Y.; Hu, C. Mater. Lett. 2004, 58, 2700.
22. Klug, H.; Alexander, L. (Eds.), X-Ray Diffraction Procedures; Wiley: New
York, 1962, p. 125.
M = 1 − Rm/Vm
[5]
Equation (5) indicates that when (1 – Rm/Vm) becomes zero the
transition to the metal state takes place. The physical properties
of the as-synthesized and calcined materials are provided in
Table 1.
CONCLUSION
23. Oliva, P.; Leonardi, J.; Laurent, J.F.; Delmas, C.; Braconnier, J.F.;
Figlarz, M.; Fievet, F.; De Guibert, A. J. Power Sources 1982, 8,
229.
24. Genin, P.; Delahaye-Vidal, A.; Portemer, F.; Tekaia-Elhsissen, K.; Iglarz,
M.F. Eur. J. Solid State Inorg. Chem. 1991, 28, 505.
25. Ho, K.C.; Jorne, J. J. Electrochem. Soc. 1990, 137, 149.
26. Soler-Illia, G.J.; Regazzoni, A.E.; Blesa, M.A. Chem. Mater. 1999, 11,
3140.
27. Jayalakshmi, M.; Venugopal, N.; Reddy, B.R.; Mohan, R.M. J. Power
Sources 2005, 150, 272.
28. Jeevanandam, P.; Koltypin, Y.; Gedanken, A. Nano. Lett. 2001, 1, 263.
29. Pejova, B.; Kocareva, T.; Najdoski, M.; Grozdanov, I. Appl. Surf. Sci. 2000,
165, 271.
Highly crystalline and spherical nanoparticles (SNPs) of NiO
have been prepared successfully by a simple, modified, and in-
expensive solvothermal method using NiCl2·6H2O and urea as
the starting materials and ethanol as the refluxing solvent. It is
found that the crystallite size of the nickel oxide varies with the
calcination temperature, such that the crystallite size increases
linearly with increase in temperature. The morphological dif-
ference induced by the final calcination temperature also affects
the optical properties of the nickel oxide. Evaluation of the
optical properties in combination with observation of the mor-
phology of nickel oxide by SEM analysis suggests that there
is an optimum calcination temperature for nickel oxide. FTIR
studies give broad absorption maxima in the frequency range
421–423 cm−1, suggestive of nanocrystalline particles. The op-
30. Biju, V.; Abdul Khaddar, M. Spectrochim. Acta A Mol. Biomol. Spectrosc.
2003, 59, 121.
31. Preudhomme, J.; Tarte, P. Spectrochim. Acta 1967, 23A, 2127.
32. Bellotto, M.; Rebours, B.; Clause, O.; Lynch, J.; Bazin, D.; Elkain, E. Phys.
Chem. 1996, 100, 8535.

1330
A. KALAM ET AL.
38. Li, X.; Zhang, X.; Li, Z.; Qian, Y. Solid State Commun. 2006, 137,
33. Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49.
34. Varkey, A.J.; Fort, A.F. Thin Solid Films 1993, 235, 47.
35. Boschloo, G.; Hagfeldt, A. J. Phys. Chem. B 2001, 105, 3039.
581.
39. Duffy, J.A. J. Solid State Chem. 1986, 62, 145.
36. Wang, X.; Song, J.; Gao, L.; Jin, J.; Zheng, H.; Zhang, Z. Nanotechnology 40. Edwards; Sienko. 1982.
2005, 16, 39.
41. Herzfeld, K. Phys. Rev. 1927, 29, 701.
42. Dimitrov, V.; Sakka, S. J. Appl. Phys. 1996, 79, 1741.
37. Chakrabarty, S.; Chatterjee, K. J. Phys Sci. 2009, 13, 245.