Pure cubic ZrO2 nanoparticles by thermolysis of a new precursor

Article abstract of DOI:10.1016/j.poly.2009.06.032

Zirconia (ZrO₂) nanoparticles have been synthesized through the thermolysis of bis-aqua, tris-2-hydroxyacetophenato zirconium (IV) nitrate, [Zr(HAP)₃(H₂O)₂](NO₃), as a precursor in oleylamine (C₁

Full text of DOI:10.1016/j.poly.2009.06.032

Polyhedron 28 (2009) 3005–3009
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Pure cubic ZrO2 nanoparticles by thermolysis of a new precursor
b
a
Masoud Salavati-Niasari a,b, , Mahnaz Dadkhah , Fatemeh Davar
*
a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
b Department of Chemistry, Faculty of Science, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Zirconia (ZrO2) nanoparticles have been synthesized through the thermolysis of bis-aqua, tris-2-hydroxy-
acetophenato zirconium (IV) nitrate, [Zr(HAP)3(H2O)2](NO3), as a precursor in oleylamine (C18H37N) and
triphenylphosphine (C18H15P). The combination of C18H37N and C18H15P was added to act as surfactants
to control the particle size. C18H37N and C18H15P play an important role in preventing aggregation of ZrO2
nanoparticles. The products were characterized by X-ray diffraction (XRD), transmission electron micros-
copy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL) and Fourier
transform infrared (FT-IR) spectroscopy to depict the phase and morphology. The synthesized ZrO2 nano-
particles have a cubic structure. The FT-IR spectrum showed the purity of obtained cubic phase ZrO2
nanocrystals.
Received 28 April 2009
Accepted 7 June 2009
Available online 23 June 2009
Keywords:
Nanomaterials
Zirconium oxide
Thermolysis
Inorganic materials
Characterization methods
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
ing nanomaterials whilst controlling the nanocrystal size, shape
and distribution size.
Zirconia (ZrO2), because of its surface characteristics of possess-
ing both acid–basic and redox functions, has been widely used as a
catalyst in numerous chemical reactions. ZrO2 has three different
crystal phases, monoclinic, tetragonal and cubic, that strongly
influence the catalyst’s activities and selectivities [1,2].
Generally, the tetragonal phase of zirconia is preferentially
formed relative to the monoclinic phase during the crystallization
of amorphous hydrous zirconia. So, upon increasing the calcination
temperature, firstly most amorphous zirconia precursors convert
to the tetragonal phase and then transform to the monoclinic
phase at higher temperatures (ꢀ600 °C). The transformation is
completed at temperatures more than about 800 °C [3–5].
A variety of zirconium complexes have been prepared success-
fully and modified [21–24]. These precursors can be good candi-
dates for the synthesis of ZrO2 nanomaterials. Herein, we report
on the facile synthesis of ZrO2 nanocrystals via thermolysis of the
new precursor bis-aqua, tris-2-hydroxyacetophenato zirconium
(IV) nitrate; [Zr(HAP)3(H2O)2](NO3); in oleylamine and triphenyl-
phosphine (TPP). To the best of our knowledge, this is the first re-
port on the synthesis of ZrO2 nanocrystallites with this precursor.
In this process, oleylamine was used as both the medium and the
stabilizing reagent.
2. Experimental
ZrO2 has been synthesized via different routes, for example by
the microwave plasma method [6], chemical vapor synthesis [7],
sol–gel method [8–10], precipitation [11], hydrothermal synthesis
[12], etc. [13–15]. Several year ago we initiated a research work
aimed at the synthesis of metallic [16,17] and metal oxide nano-
particles [18–20] by thermal decomposition of inorganic com-
plexes as precursors. With this thermolysis method, the
utilization of simple, facile, cheap, air-stable and less-toxic molec-
ular precursors is expected to greatly facilitate the large-scale pro-
duction of oxide nanocrystals for the above applications. A major
interest at the moment is in the development of organometallic
or inorganic compounds for preparation of nanocrystals. Using no-
vel compounds can be useful and can open a new way for prepar-
2.1. Materials
Zirconyl nitrate pentahydrate (ZrO(NO3)2.5H2O), 2-hydroxyace-
tophenone (HAP), oleylamine, triphenylphosphine (TPP), toluene,
hexane, and ethanol were purchased from Aldrich and used as re-
ceived. The precursor complex [Zr(HAP)3(H2O)2](NO3) was pre-
pared according to the procedure described previously [25]:
0.01 mmol ZrO(NO3)2ꢁ5H2O, 0.06 mmol of 2-hydroxyacetophenone
and 10 ml of 2-methoxy ethanol were added to a 250 ml Erlen-
meyer flask to give a turbid solution. To this suspension 10 ml of
acetonitrile was added and the solution was refluxed for 12 h to
yield white crystals. The crystalline product was purified by wash-
ing with dry diethylether to yield 80% of the product (m.p. 99 °C).
Anal. Calc. for ZrO11C24H25 N: C, 48.47; H, 4.24; N, 2.36; Zr, 15.34.
Found: C, 48.29; H, 4.80; N, 2.41; Zr, 15.32%.
* Corresponding author. Tel.: +98 361 5555333; 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.2009.06.032

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M. Salavati-Niasari et al. / Polyhedron 28 (2009) 3005–3009
2.2. Characterization
XRD patterns were recorded by a Rigaku D-max C III, X-ray dif-
fractometer using Ni-filtered Cu K radiation. Elemental analyses
a
were obtained from Carlo ERBA Model EA 1108 analyzer. Scanning
electron microscopy (SEM) images were obtained on Philips XL-
30ESEM equipped with an energy dispersive X-ray spectroscope.
Transmission electron microscopy (TEM) images were obtained
on a Philips EM208 transmission electron microscope with an
accelerating voltage of 100 kV. X-ray photoelectron spectroscopy
(XPS) of the as-prepared products was measured on an ESCA-
3000 electron spectrometer with non-monochromatized Mg K
a
X-ray radiation as the excitation source. Room temperature photo-
luminescence (PL) was studied on an F-4500 fluorescence spectro-
photometer. Fourier transform infrared (FT-IR) spectra were
recorded on Shimadzu Varian 4300 spectrophotometer in KBr pel-
lets. The electronic spectra of the complexes were taken on a Shi-
madzu UV–Vis scanning spectrometer (Model 2101 PC).
Fig. 1. TG curve of the [Zr(HAP)3(H2O)2](NO3) precursor in air.
2.3. Synthesis of ZrO2 nanoparticles
the complex shows a gradual loss of weight of about 70% around
460 °C to give a residual mass consisting of about 30%, close to that
expected for a complete conversion to ZrO2. The weight loss from
250 to 460 °C is linked with burning of 2-hydroxyacetophenato
(exothermic) to acetone, CO2 and H2O [27]. There was no organic
residue left after 450 °C, as confirmed by the FT-IR spectrum of
the residual mass.
The current synthetic procedure is a modified version of the
method developed by the Hyeon group for the synthesis of various
nanoparticles of metals and oxides, which employs the thermal
decomposition of metal–surfactant complexes in hot surfactant
solution [26]. A stock solution of [Zr(HAP)3]–surfactant complex,
prepared by reacting 0.6 g of bis-aqua, tris-2-hydroxyacetophenato
zirconium (IV) nitrate, [Zr(HAP)3(H2O)2](NO3), with 5 ml of oleyl-
amine at 150 °C, was slowly injected into 5 g of triphenylphos-
phine (TPP) under vigorous stirring at 250 °C. The mixture was
heated at 250 °C and the resulting solution was stirred at this tem-
perature for 90 min. The initial white color of the solution slowly
turned yellow, indicating that nanoparticles were generated. The
reaction mixture was then cooled to room temperature and the
white nanoparticles were retrieved by adding 30 ml of ethanol, fol-
lowed by centrifugation. The final products were washed with eth-
anol at least three times to remove impurities, if any, and dried at
100 °C. The retrieved powder form of the nanoparticles could be
easily redispersed in non-polar organic solvents such as hexane
or toluene (Scheme 1).
3.2. XRD analysis of the products
The XRD diffraction pattern of the ZrO2 was recorded for the
identification of the product (Fig. 2). From the XRD data, the crys-
tallite size (Dc) of the as-prepared ZrO2 particles was calculated to
be 40 nm using the Debey–Scherrer equation [28],
3. Results and discussion
3.1. Thermogravimetric analysis of the [Zr(HAP)3(H2O)2](NO3)
precursor
The TGA curve for the decomposition of [Zr(HAP)3(H2O)2](NO3)
was recorded in air atmosphere and is shown in Fig. 1. As shown in
Fig. 1, a total of two weight losses were observed. A weight loss of
9% occurred between 100 and 180 °C, and this weight loss is indic-
ative of the loss of one coordinated H2O molecule. The TG curve of
Fig. 2. XRD patterns of ZrO2 nanoparticles.
Scheme 1. Synthetic procedure for ZrO2 nanoparticles.

M. Salavati-Niasari et al. / Polyhedron 28 (2009) 3005–3009
3007
Kk
b cos h
3.5. FT-IR spectra of the products
Dc ¼
The FT-IR spectra for the free TPP, oleylamine, [Zr(HA-
P)3(H2O)2](NO3) and the as-prepared ZrO2 nanoparticles are shown
in Fig. 5a–d. As shown in Fig. 5a and b, peaks corresponding to P–
Ph stretching at 1710 cmꢂ1, rocking and bending mode of the
where b is the breadth of the observed diffraction line at its half-
intensity maximum, K is the so-called shape factor, which usually
takes a value of about 0.9, and k is the wavelength of X-ray source
used in the XRD. All of the reflections of the XRD pattern in Fig. 2
can be indexed to the standard pattern of the pure cubic phase of
ZrO2, which is in good agreement with the reported data (JCPDS
No. 27–0997). This revealed that the ZrO2 sample produced a sin-
gle-phase cubic structure. Three observed peaks with 2h values of
30.46°, 50.45° and 60.37° correspond to the (1 1 1), (2 2 0) and
(3 1 1) diffraction peaks of crystalline zirconia, respectively. Zirco-
nia nanocrystallite with an average size of 35 nm was estimated
by using the Debye–Scherrer formula.
methylene group
phenyl group
x
(CH2) at 1270 cmꢂ1, the bending mode of the
(@CH) at 742 cmꢂ1and
m (benzene ring) at
c
697 cmꢂ1can be seen. The only difference among these character-
istic peaks is either the peak intensity or a slight shift in the peak
position. For example, the peak position of the longitudinal modes
of oleylamine and TPP shifts to lower wavenumbers after TPP and
oleylamine are adsorbed on the surface of the ZrO2 nanocrystals. In
the case of [Zr(HAP)3(H2O)2](NO3) (see Fig. 5c), the presence of
bands at 1526 cmꢂ1 and 1590 cmꢂ1 are due to metal bonded C–O
and bands at 1451 cmꢂ1 and 1382 cmꢂ1 are due to the C–C bond
of the 2-hydroxyacetophenone group [34,35]. In free 2-hydroxy-
acetophenone two C–O bonds give a band pattern at 1600 cmꢂ1
and 1500 cmꢂ1 whereas two C–C bonds give a band pattern at
1450 cmꢂ1 and 1260 cmꢂ1. The lowering of the absorption fre-
quency of these two bonds indicates chelation of the 2-hydroxy-
acetophenone group to the metal center. A very broad peak at
3440–3100 cmꢂ1 is assigned to a water molecule present in the
complex. Absorption peaks at 500–650 cmꢂ1 are due to a M–O
bond. However, there are no absorption bands around this range
in 2-hydroxyacetophenone. Therefore, the added 2-hydroxyaceto-
phenone was thought to be fully chelated to the zirconium ion,
and no free 2-hydroxyacetophenone exists in the solution. It is well
known that FT-IR spectra are very useful for the determination of
the crystal phase for ZrO2 [36]. The extended spectrum in the
400–4000 cmꢂ1 region signified a structure with the surface of
the sample having chemisorbed H2O molecules. The spectra of
ZrO2, (Fig. 5d), shows two very weak stretch vibrations at 2925
and 2865 cmꢂ1, attributed to the C–H stretching models of the car-
3.3. TEM studies
The TEM image of the sample is shown in Fig. 3, from which it
can be seen that the ZrO2 is composed of some agglomerated par-
ticles with an average size of about 30–40 nm. The TEM image
shows quasi-spherical nanoparticles. The size of the nanoparticles
obtained from the XRD diffraction patterns are in close agreement
with the TEM studies, which show sizes of 30–40 nm (Fig. 3).
3.4. Photoluminescence study
Fluorescence spectra were measured with several excitation
wavelengths between 230 and 290 nm. While the fluorescence
intensity changes to some extent with the excitation wave-
length, the fluorescence band position and the band shape re-
mained about the same for excitation wavelengths below
290 nm. This indicates that the fluorescence involves the same
initial and final states even when the excitation wavelength is
varied between 230 and 290 nm. This result can be explained
by fast relaxation from the final state reached by photo-excita-
tion to those states from which the fluorescence originates.
Fig. 4 shows one representative fluorescence spectrum with an
excitation wavelength of 265 nm. The spectrum features a broad
fluorescence band centered at 371 nm [29,30]. The broad band
and the substantial red shift of the band maximum compared
to the band gap (5.6 eV) of the bulk material [31,32] strongly
indicate that the fluorescence involves extrinsic states. Because
the particle size distribution is very narrow, the broad fluores-
cence band seems to be mostly caused by the small particle size
leading to an inhomogeneous broadening from a distribution of
surface or defect states [33].
Fig. 4. Photoluminescence spectra of ZrO2 nanocrystals (excited at 265 nm (blue
color), the excitation spectrum of the emission recorded at 371 nm (red color)). (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
Fig. 3. TEM image of ZrO2 nanoparticles.

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M. Salavati-Niasari et al. / Polyhedron 28 (2009) 3005–3009
bon chain of oleylamine that were adsorbed on the surface of the
ZrO2 nanocrystals. The spectrum in Fig. 5d showed broad bands
at 451, 540 and 705 cmꢂ1 related to the c-ZrO2 phase (Fig. 5)
[37]. The trend is consistent with the fact that for cubic ZrO2 only
one fundamental mode is IR active and the active modes increase
markedly upon lowering the symmetry of the structure by going
from cubic to tetragonal and then to monoclinic ZrO2. Two bands
of c-ZrO2 nanocrystallites were obtained at 865 and 1500 cmꢂ1
.
By comparing the infrared spectra of the nanoparticles with free
surfactants we can see that the organic molecules have indeed be-
come a small part of the nanoparticles.
3.6. X-ray photoelectron spectroscopy (XPS) analysis
The chemical state of the zirconia nanoparticles was analyzed
by XPS measurements. In an X-ray photoelectron spectroscopy
(XPS) experiment, the samples are exposed to monochromic X-
radiation and the properties of the inner-shell electrons are
probed. Fig. 6 displays the XPS spectra of the Zr 3d and O 1s core
levels. The peaks located at 181.3 and 183.8 eV are attributed to
the spin–orbit splitting of the Zr 3d components, Zr 3d5/2, Zr
3d3/2. The binding energy of O 1s in zirconia is located at
530.1 eV. The peak positions and their relative intensities are basi-
cally consistent with those of a standard cubic structure of zirconia
[31]. It is noted in Fig. 6 that the peaks of Zr 3d and O 1s shift to
higher binding energies with increasing particle diameter. The re-
sult is in good agreement with the results obtained by XRD.
3.7. Synthesis of ZrO2 under different condition
In addition, the ZrO2 nanoparticles have been prepared under
different conditions (Table 1). By controlling the quality of the
nanoparticles, ZrO2 with various sizes can be formed. [Zr(HA-
P)3(H2O)2](NO3) calcined at 600 °C for 4 h (sample 1) resulted in
ZrO2 spherical shapes with sizes of 40–50 nm (Fig. 7), 0.6 g of
[Zr(HAP)3(H2O)2](NO3) and 5 ml of oleylamine mixed and heated
Fig. 5. IR spectra for (a) TPP, (b) oleylamine, (c) [Zr(HAP)3(H2O)2](NO3) and (d) ZrO2
nanoparticles.
Fig. 6. XPS spectra of ZrO2 nanoparticles, Zr 3d and O 1s.
Table 1
Preparation of ZrO2 under different conditions.
Sample
Capping agent
Temperature (°C)
Time
Particles size
1
2
3
4
600
250
250
250
4 h
40–50 nm
30 nm (according to XRD)
no product
oleylamine
TPP
oleylamine + TPP
90 min
90 min
90 min
30–40 nm

M. Salavati-Niasari et al. / Polyhedron 28 (2009) 3005–3009
3009
using both oleylamine and triphenylphosphine as capping agents
are preferred for the synthesis of small size, pure ZrO2
nanoparticles.
4. Conclusion
ZrO2 nanoparticles powders were prepared via thermolysis of a
new precursor, [Zr(HAP)3(H2O)2](NO3). ZrO2 nanoparticles were
synthesized under different conditions. The TEM image shows that
the size of the quasi-spherical nanoparticles synthesized in the
presence of oleylamine and TPP are about 30–40 nm. According
to the XRD pattern, to prepare cubic ZrO2 nanospherical particles,
oleylamine and TPP must be used together. Thermolysis is a simple
method to prepare ZrO2 nanoparticles and is one that can be used
for large-scale syntheses.
Acknowledgment
Authors are grateful to the Council of the University of Kashan
for providing financial support to undertake this work.
Fig. 7. TEM image of ZrO2 nanoparticles ((sample 1); [Zr(HAP)3(H2O)2](NO3) was
calcined at 600 °C for 4 h).
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Fig. 8. SEM image of the ZrO2 nanoparticles (sample 2).
to 245 °C for 90 min led to ZrO2 nanoparticles with a quasi-spher-
ical shape and an average size of about 30 nm, but the products are
agglomerated (sample 2) (Fig. 8). Also 0.6 g [Zr(HAP)3(H2O)2](NO3)
and 5 g of TPP (sample 3) were heated together at 250 °C for
90 min, but after washing with ethanol no precipitation was ob-
tained. To prepare ZrO2 spherical nanocrystals (sample 4), the pro-
cedure mentioned in Section 2, including C18H37N and TPP, was
carried out. Considering the different conditions employed to pre-
pare these samples, it is obvious that using both C18H37N and TPP
as capping agents are preferred in ZrO2 nanoparticle synthesis.
In summary, pure zirconium oxide nanoparticles arrays were
successfully prepared by the thermolysis method. We have used
SEM, TEM, XRD, XPS, FT-IR and PL techniques to characterize the
structure of the zirconia nanoparticles. XRD analysis indicated that
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(NO3). The morphology and particle size of the obtained products
were compared with each other. Considering the different condi-
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