Synthesis and characterization of NiO nanoclusters via thermal decomposition

Article abstract of DOI:10.1016/j.poly.2009.01.026

Thermal decomposition process has been developed to synthesize nickel oxide (NiO) nanoclusters via the reaction between a new precursor, nickel oxalate [Ni(O₄C₂)(H₂O)₄] and oleylamine (C₁₈H₃₇

Full text of DOI:10.1016/j.poly.2009.01.026

Polyhedron 28 (2009) 1111–1114
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of NiO nanoclusters via thermal decomposition
b
a,b
Masoud Salavati-Niasari a,b, , Noshin Mir , Fatemeh Davar
*
a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Iran
b Department of Chemistry, Faculty of Science, University of Kashan, Kashan, P.O. Box 87317-51167, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Thermal decomposition process has been developed to synthesize nickel oxide (NiO) nanoclusters via the
reaction between a new precursor, nickel oxalate [Ni(O4C2)(H2O)4] and oleylamine (C18H37N). The com-
bination of triphenylphosphine (C18H15P) and C18H37N were added as surfactants to control the particle
size. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM),
transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, thermogravi-
metric analysis (TGA) and ultraviolet–visible (UV–Vis) spectroscopy. The synthesized NiO nanoclusters
have a cubic structure with average size 2–10 nm.
Received 1 September 2008
Accepted 27 January 2009
Available online 6 February 2009
Keywords:
Nanomaterials
Nickel oxide
Inorganic materials
Characterization methods
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
hydrothermal methods [18]. More recently, the NiO nanoparticles
were synthesized using two different microemulsion approaches
Recently, nanomaterials have attracted extensive interest be-
cause of their unique properties in various fields in comparison
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 materials
in many applications. Nanosized nickel(II) oxide has demonstrated
excellence properties such as catalytic [2], magnetic [3], electro-
chromic [4], optical and electrochemical properties [5]. Further-
by Palanisamy and Raichur. It was reported that Ni(OH)2 particles
were first formed which were then calcined at 600 °C to obtain
47 nm nickel oxide NiO particles [19]. Gao and co-workers synthe-
sized NiO nanorods consist of numerous NiO nanoparticles by ther-
mal decomposition of rod-like nickel oxide precursor achieved by
microwave-assisted method [20]. Rodríguez-Llamazares et al. syn-
thesized nickel/nickel oxides nanoparticles by a method based on
ligand displacement of bis(1,5-cyclooctadiene)-nickel(0), zerova-
lent organometallic precursor and simultaneous formation of a
thiourea inclusion compound [21]. In this study, it is tried to syn-
thesis relatively small NiO nanoclusters from a simple and non-
toxic precursor in a short time and with an easy, low temperature,
low cost method without using any special instrument.
more, 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].
In accomplishing the synthesis and manipulation of the nano-
structured nickel oxide, a variety of strategies have been employed,
such as evaporation [9], sputtering [10], electrodeposition [11],
thermal decomposition [12] and sol–gel techniques [13]. Thermal
decomposition method has some advantages such as simple pro-
cess, low cost and easiness to obtain high purity products hence
it is quite promising and facile rout for industrial applications.
Many reports have concerned the synthesis of NiO nanocrystals,
including NiO nanorings [14], nanosheets [15], nanoribbons [16]
and nanoclusters [17]. Recently, Shah reported synthesis of 50–
60 nm nickel oxide nanoparticles after 12 h using sonication and
Nanostructured metal clusters are isolable particles of sizes be-
tween 1 and 50 nm. In order to prevent agglomeration, these nano-
sized entities have to be stabilized by ligand molecules or a whole
plethora of ‘‘protecting shells’’ [22]. Nanoclusters are polycrystals
in which the size of the individual crystallites is of the order of sev-
eral nanometers. Therefore, a large fraction of the atoms in nano-
cluster particles is surface or interface atoms. Any structure
sensitive to physical properties is expected to differ between nano-
cluster particles and bulk materials.
However, an improvement in the thermal decomposition pro-
cess should be made in preparing NiO nanocrystals with uniform
size in order to extend the application areas and satisfy the needs
of fundamental research. Although the thermal decomposition of
nickel oxalate has been studied in the past, no previous studies
have investigated the synthesized nickel oxide nanoclusters. In this
paper, we report synthesis of NiO nanoclusters by thermal decom-
* Corresponding author. Address: Institute of Nano Science and Nano Technology,
University of Kashan, Kashan, P.O. Box 87317-51167, Iran. Tel.: +98 361 555333;
fax: +98 361 5552935.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.01.026

1112
M. Salavati-Niasari et al. / Polyhedron 28 (2009) 1111–1114
position of [Ni(O4C2)(H2O)4], without any additional reducing
agents. This precursor is a cheap and easy prepared complex with
low toxicity.
such as C6H12 (Scheme 1). The synthesized nanoclusters were char-
acterized by XRD, TEM, FT-IR and UV–Vis techniques.
3. Results and discussion
2. Experimental
In order to make sure of the precursor composition, elemental
analysis was done. The TGA curve of the as-prepared precursor is
shown in Fig. 1. It can be seen that there are two distinct weight
loss steps. The first weight loss occurs at 110–190ꢀC, which corre-
sponds to the evaporation of crystallized water, that is to say, nick-
el oxalate in the stage. The second weight loss step occurs at the
temperature range 210–370ꢀC. The weight loss at 210–370ꢀC may
be ascribed to the decomposition of the oxalate. The weight loss
of two steps is about 17.80% and 41.18%, which is close to the the-
oretical value.
The XRD diffraction patterns of the sample of nickel oxides were
recorded for the identification of the products (Fig. 2). The main
diffraction peaks were observed at 37.18ꢀ, 43.42ꢀ, 62.82ꢀ, 74.20ꢀand
79.2ꢀin the XRD pattern of the nickel oxide which confirm that
the formed material is nickel oxide. The diffraction peaks match
with the standard values and they can be indexed to pure cubic
phase of NiO (Fm3 m) with lattice parameter a = 4.1771 Å (JCPDS:
78-0429). The crystal size of the NiO nanoclusters was estimated
by Debye–Scherrer equation (d = 0.9k/bcosh) and it was about
2.7 nm, which is in agreement with the other observation results.
The diffraction peaks are clearly broadened, which can be the re-
sult of the reduced particle size.
All the chemical reagents used in our experiments were of ana-
lytical grade and were used as received without further purifica-
tion. XRD patterns were recorded by a Rigaku D-max C III, X-ray
diffractometer using Ni-filtered Cu Ka radiation. Transmission
electron microscopy (TEM) images were obtained on a Philips
EM208 transmission electron microscope with an accelerating
voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were
recorded on Shimadzu Varian 4300 spectrophotometer in KBr pel-
lets. The elemental analysis (carbon, hydrogen and nitrogen) of the
materials was obtained from Carlo ERBA Model EA 1108 analyzer.
Thermogravimetric-differential thermal analysis (TG-DTA) were
carried out using a thermal gravimetric analysis instrument (Shi-
madzu TGA-50H) with a flow rate of 20.0 mL minÀ1 and a heating
rate of 10 °C minÀ1. The electronic spectra of the complexes were
taken on a Shimadzu UV–visible scanning spectrometer (Model
2101 PC).
Nickel oxalate precursor was synthesized according to this pro-
cedure: the Ni(COOCH3)2 Á 4H2O was dissolved in 10 mL of distilled
water to form a homogeneous solution. A stoichiometric amount of
potassium oxalate dissolved in an equal volume of distilled water,
was dropwise added into the above solution under magnetic stir-
ring. The solution was stirred for about 15 min and a green precip-
itate was isolated, washed and dried at 50ꢀC. The obtained product
was characterized by FT-IR, elemental analyses and thermogravi-
metric analysis (TGA) [5]. Anal. Calc. for [Ni(O4C2)(H2O)4]: C,
10.98; H, 3.69; Ni, 26.83. Found: C, 10.89; H, 3.57; Ni, 26.74%.
The main synthetic procedure in order to approach to final
products is a modified version of the method developed by Hyeon
and others for the synthesis of metal nanocrystals that employ the
thermal decomposition of transition metal complexes [23,24]. In
current synthesis, NiO nanoclusters were prepared by the thermal
decomposition of nickel oxalate as precursor. First 0.6 g of
[Ni(O4C2)(H2O)4] and 5 mL of C18H37N were mixed to produce
[Ni(O4C2)(H2O)2]–oleylamin as suggested intermediate precursor.
The solution was placed in a 50 mL three-neck distillation flask
and heated up to 140ꢀC for 60 min. Then 5 g of C18H15P was in-
jected into the resulting metal-complex solution at 240ꢀC. The col-
or of the solution changed from green to black, indicating
generation of colloidal products. The black solution was aged at
240ꢀC for 45 min, and was then cooled to room temperature. The
black products were precipitated by adding excess C2H5OH to the
solution. The NiO nanoclusters were washed with C2H5OH. These
products could easily be re-dispersed in nonpolar organic solvents,
To investigate whether the surface of the nanoparticles was
capped with organic surface, the Fourier transformed infrared
(FT-IR) of the as-synthesized samples was performed. Fig. 3 shows
the FT-IR spectra of NiO nanoclusters and progressive formatted
Fig. 1. TGA of the nickel oxalate precursor.
OH2
OH2
P
H2N
H2N
H2O
H2O
O
O
Thermolysis
O
O
P
P
P
O
O
P
P
P
Oleylamine
O
O
Ni
Ni
Ni
P
Triphenylphosphine
OH2
OH2
Scheme 1. Schematic diagram illustrating the formation of NiO nanoclusters.

M. Salavati-Niasari et al. / Polyhedron 28 (2009) 1111–1114
1113
CH2 groups, terminal –CH3 and @CH of C18H37N. The H–OH bend-
ing vibration appears at 1635 cm–1. The sharp peak at 1430 cm–1
can be assigned to the phenyl stretching mode t(P–ph) (curve c).
Also a characteristic broad band due to the C–N stretching mode
was present at about 1150 cmÀ1. So the oleylamine and TPP serves
as the capping ligand that controls growth. Triphenylphosphine
(TPP) was widely used in the synthesis of phosphine-stabilized me-
tal nanoparticles [25]. Finally, the peak at 458 cmÀ1 in spectrum
(curve c), showing Ni–O bonds, gave clear evidence about the pres-
ence of the crystalline NiO. In curve a, the narrow band at 465 cmÀ1
is attributed to the bending mode of –COO group [26]. The spectra
of NiO nanoclusters shows two weak stretch vibrations at 2921
and 2865 cmÀ1 attributing to the C–H stretching modes of the
C18H37N carbon chain, indicating that C18H37N molecules have
been absorbed on the surface of NiO nanoclusters. So, the
C18H37N serves as the capping ligand that controls growth [24].
The surface morphological study of the nickel oxide nanoclus-
ters was carried out using SEM images. Fig. 4a shows the SEM
images of the nickel oxide samples. From the SEM analyses, one
can conclude the formation of nanoclusters flower-like structure
(Fig. 4a). The transmission electron microscopic (TEM) photo-
graphs and selected area electron diffraction (SAED) pattern of
the product have been given in Fig. 4b and c. The size of nanoclus-
ters obtained from the XRD diffraction patterns are in close agree-
ment with the TEM studies which show sizes of 2–10 nm (Fig. 4c).
The particle size distributions are shown as insets in Fig. 4b and c.
Most of the particles in Fig. 4b and c have diameters of 3 and 5 nm,
respectively, which compare well with the values calculated from
the XRD pattern. These sizes are nearly spherical. Some loose
spheres formed by aggregation of small particles can be observed
in TEM images. We may consider that when the reaction was car-
ried out at 240ꢀC, most organic molecules such as oleylamine and
Fig. 2. XRD pattern of the NiO nanoclusters.
triphenylphosphine were decomposed. Since only a few C18H37
N
molecules were adsorbed on NiO nanoclusters loose solid was fi-
nally produced. The SAED pattern in the inset in Fig. 4b reveals that
the NiO nanoclusters are crystalline.
Since the surface areas of metal nanoclusters are enormously
relative to their masses, they have an excess free energy surface
comparable to the lattice energy, making them thermodynamically
unstable. Protective agents are therefore essential in order to be
able to outweigh the attractive van der Waals forces by the repul-
sive steric forces. To achieve sufficient inter-particle separation, it
can be helpful to use sterically demanding substituents [22]. In this
work we have used C18H15P, (TPP) and as a ligand stabilizing agent
to make nanoclusters stable and prevent their agglomeration.
C18H15P is a high-boiling point surfactant with a patulous structure
providing greater steric hindrance. So, this agent might slow down
the rate of agglomeration of the nanoparticles during their growth,
Fig. 3. FT-IR spectra of (a) K2C2O4 Á H2O, (b) [Ni(O4C2)(H2O)4] and (c) NiO
nanoclusters.
materials within the procedure. The board peaks at ca. 3350–
3400 cmÀ1 in all three spectra are representative of H2O. Curve c
shows that the surface was hydroxylated because the sample
was exposed to the ambient after calcinations and some water
was adsorbed probably on the external surface of the samples dur-
ing handling to record the spectra. In our measurement, number of
resulting in much smaller nanoclusters. Also we have used C18H37
N
weak peaks between
attributable to the symmetric and asymmetric stretching of the –
m1000–1400 and m
2800–3000 cmÀ1 are
as an additional surfactant in order to reduce the particle size
much further and resulted in uniform size distribution (Fig. 4).
Fig. 4. (a) SEM; (b) and (c) TEM images of NiO nanoclusters (the inset shows the SAED pattern and the histograms of the particle size distributions).

1114
M. Salavati-Niasari et al. / Polyhedron 28 (2009) 1111–1114
[Ni(O4C2)(H2O)4] calcined at 450ꢀC for 2 h and resulted 9 nm nano-
particles (sample 1) [5], 0.5 g [Ni(O4C2)(H2O)4] and 5 mL of oleyl-
amine were mixed and heated in 240ꢀC for 45 min so led to NiO
agglomerations which show size of more than 1000 nm (sample
2) (Fig. 6). Also 0.5 g [Ni(O4C2)(H2O)4] and 5 g of TPP were heated
together in 240ꢀC for 45 min, after washing with ethanol no precip-
itations were obtained. To prepare NiO nanoclusters (sample 4),
the procedure mentioned in Section 2, including C18H37N and
TPP, was done. Considering different condition employed to pre-
pare these samples, it is obvious that using both C18H37 N and
TPP as capping agent is preferred for NiO nanoclusters synthesis.
4. Conclusion
NiO nanoclusters have been successfully synthesized through a
thermal decomposition of new precursor [Ni(O4C2)(H2O)4] in the
presence of TPP and C18H37N. The as-synthesized nickel oxide
nanoclusters show NiO with cubic structure without any other
impurities. From the results of XRD, FT-IR and TEM, the NiO
nanoclusters show good morphologies corresponding to nanosize
about 2–10 nm, relatively.
Fig. 5. UV–Vis absorption spectrum of the NiO nanoclusters dispersed in C2H5OH.
Table 1
Preparation of NiO under different conditions.
Sample
Capping agent
Temperature (ꢀC)
Time
Particles size
1
2
3
4
450
240
240
240
2 h
9 nm [5]
Acknowledgement
oleylamine
TPP
oleylamine + TPP
45 min
45 min
45 min
agglomeration
no product
2.7 nm
Authors are grateful to council of University of Kashan for pro-
viding financial support to undertake this work.
References
[1] C.C. Chen, A.B. Herhold, C.S. Johnson, A.P. Alivisatos, Science 276 (1997) 398.
[2] Y. Wang, J. Zhu, X. Yang, L. Lu, X. Wang, Thermochim. Acta 437 (2005) 106.
[3] Y.R. Uhma, J.H. Park, W.W. Kima, C.H. Chob, C.K. Rhee, Mater. Sci. Eng. B 106
(2004) 224.
[4] S.H. Lin, F.R. Chen, J.J. Kai, Appl. Surf. Sci. 254 (2008) 3357.
[5] X. Wang, J. Song, L. Gao, J. Jin, H. Zheng, Z. Zhang, Nanotechnology 16 (2005)
37.
[6] H. Sato, T. Minami, S. Takata, T. Yamada, Thin Solid Films 236 (1993) 27.
[7] V. Srinivasan, J. Weidner, J. Electrochem. Soc. 144 (1997) L210.
[8] J. He, H. Lindström, A. Hagfeldt, S.E. Lindquist, J. Phys. Chem. B 103 (1999)
8940.
[9] R. Cinnsealach, G. Boschloo, S.N. Rao, D. Fitzmaurice, Sol. Energy Mater. Sol.
Cells 57 (1999) 107.
[10] K. Yoshimura, T. Miki, S. Tanemura, Jpn. J. Appl. Phys. 34 (1995) 2440.
[11] C. Natarajan, H. Matsumoto, G. Nogami, J. Electrochem. Soc. 144 (1997) 121.
[12] C.S. Carney, C.J. Gump, A.W. Weimer, Mater. Sci. Eng. A 431 (2006) 1.
[13] A. Surca, B. Orel, B. Pihlar, P. Bukovec, J. Electroanal. Chem. 408 (1996) 83.
[14] J.H. Liang; Y.D. Li, Chem. Lett. 32 (2003) 1126.
[15] Z.H. Liang, Y.J. Zhu, X.L. Hu, J. Phys. Chem. B 108 (2004) 3488.
[16] D.N. Yang, R.M. Wang, J. Zhang, Z.F. Liu, J. Phys. Chem. B 108 (2004) 7531.
[17] R.H. Kodama, S.A. Makhlouf, A.E. Berkowitz, Phys. Rev. Lett. 79 (1997) 1393.
[18] M.H. Shah, Nanoscale Res. Lett. 3 (2008) 255.
[19] P. Palanisamy, A.M. Raichur, Mater. Sci. Eng. C 29 (2009) 199.
[20] X. Song, L. Gao, J. Am. Ceram. Soc. 91 (2008) 3465.
[21] S. Rodríguez-Llamazares, J. Merchán, I. Olmedo, H.P. Marambio, J.P.Muñoz, P.
Jara, J.C. Sturm, B. Chornik, O. Peña, N. Yutronic, M.J. Kogan, J. Nanosci.
Nanotech. 8 (2008) 3820.
[22] B. Corain, G. Schmid, N. Toshima (Eds.), Catalysis and Materials Science: The
Issue of Size Control, Elsevier Publishers, Oxford, 2008, p. 21.
[23] J. Park, E. Kang, S.U. Son, H.M. Park, M.K. Lee, J. Kim, et al., Adv. Mater. 17
(2005) 429.
[24] M. Salavati-Niasari, F. Davar, M. Mazaheri, M. Shaterian, J. Magn. Magn. Mater.
320 (2008) 575.
[25] H.T. Yang, Y.K. Su, C.M. Shen, T.Z. Yang, H.J. Gao, Surf. Interf. Anal. 36 (2004)
155.
[26] K. Nakamoto, Infrared and Raman spectra of Inorganic and Coordination
Compounds, 4th Ed., Wiley, VCH, 1986, p. 245.
[27] M. Salavati-Niasari, Z. Fereshteh, F. Davar, Polyhedron 28 (2009) 126.
[28] M. Salavati-Niasari, F. Davar, N. Mir, Polyhedron 27 (2008) 3514.
[29] G. Boschloo, A. Hagfeldt, J. Phys. Chem. B 105 (2001) 3039.
[30] T. Tsuboi, W. Kleeman, J. Phys.: Condens. Matter 6 (1994) 8625.
Fig. 6. NiO agglomerations (sample 2) synthesized in presence of oleylamine.
C18H37N has been known as a ligand that binds tightly to the metal
nanoparticles surface. The combined effects of TPP and C18H37
N
were much more profound than those of individual contributions.
Furthermore, as capping agents, TPP and oleylamine surround ini-
tial nuclei and result in decomposition of precursor in lower tem-
perature (240 ꢀC). However, the decomposition without these
capping agents (see Fig. 1) occurs in higher temperature (370ꢀC)
[27,28].
Fig. 5 is the UV–Vis spectrum of the as-synthesized NiO nanocl-
usters dispersed in ethanol. A strong absorption in the UV region is
observed at wavelengths about 365 nm, while a broad absorption
appears in the visible region. These absorptions can be attributed
to intra-3d transition of Ni2+ in the cubic structure of NiO [29,30].
In addition, the nickel oxide nanoparticles have been prepared
under different condition (Table 1). By controlling the quality
of nanoparticles, NiO with various sizes can be formed.