Sol-gel zirconia nanopowders with α-cyclodextrin as organic additive

Article abstract of DOI:10.1016/j.jallcom.2011.12.070

Nanomaterials present unique structural and physicochemical properties due to their ultra fine size of particles that make them very useful in many domains. The most spectacular applications of nanosized zirconia include ceramics, piezoelectrics, refractories, pigments, solid electrolytes, oxygen sensors, catalysts, ultrafiltration membranes, and chromatography packing materials. Nanostructured zirconia powders can be prepared using various methods, such as sol-gel process, coprecipitation, hydrothermal synthesis, and reverse micelle method. The aim of the present work was to prepare zirconia nanopowders through the sol-gel method, using α-cyclodextrin as organic additive and to establish its influence on the structural and textural properties of the obtained product. A white, amorphous ZrO₂ powder containing α-cyclodextrin was prepared, which became a crystalline, stable one, after removing the organic matter by thermal treatment. The resulted nanocrystalline powder contains both monoclinic and tetragonal zirconia phases and is very stable. It presents a relatively reduced tendency of agglomeration of particles and contains closed pores which are embedded in the zirconia matrix. The zirconia powders were characterized using the following methods: thermal analysis, IR spectroscopy, UV-vis spectroscopy, X-ray diffraction, and electron microscopy (SEM, TEM, and HRTEM coupled with SAED).

Full text of DOI:10.1016/j.jallcom.2011.12.070

Journal of Alloys and Compounds 517 (2012) 157–163
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Sol–gel zirconia nanopowders with ␣-cyclodextrin as organic additive
M. Ra˘ileanua,∗ , L. Todana,∗ , D. Cris¸ ana , N. Dra˘gana , M. Cris¸ ana , C. Stana , C. Andronescua , M. Voicescua ,
B.S. Vasileb, A. Ianculescub
a “Ilie Murgulescu” Institute of Physical Chemistry, Roumanian Academy, Splaiul Independent¸ei 202, 060021 Bucharest, Romania
b Department of Oxide Materials Science and Engineering, “Politehnica” University of Bucharest, 1-7 Gh. Polizu, P.O. Box 12-134, 011061 Bucharest, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanomaterials present unique structural and physicochemical properties due to their ultra fine size
of particles that make them very useful in many domains. The most spectacular applications of nano-
sized zirconia include ceramics, piezoelectrics, refractories, pigments, solid electrolytes, oxygen sensors,
catalysts, ultrafiltration membranes, and chromatography packing materials. Nanostructured zirconia
powders can be prepared using various methods, such as sol–gel process, coprecipitation, hydrothermal
synthesis, and reverse micelle method. The aim of the present work was to prepare zirconia nanopowders
through the sol–gel method, using ␣-cyclodextrin as organic additive and to establish its influence on the
structural and textural properties of the obtained product. A white, amorphous ZrO2 powder containing
␣-cyclodextrin was prepared, which became a crystalline, stable one, after removing the organic mat-
ter by thermal treatment. The resulted nanocrystalline powder contains both monoclinic and tetragonal
zirconia phases and is very stable. It presents a relatively reduced tendency of agglomeration of particles
and contains closed pores which are embedded in the zirconia matrix. The zirconia powders were char-
acterized using the following methods: thermal analysis, IR spectroscopy, UV–vis spectroscopy, X-ray
diffraction, and electron microscopy (SEM, TEM, and HRTEM coupled with SAED).
Received 26 July 2011
Received in revised form
13 December 2011
Accepted 15 December 2011
Available online 24 December 2011
Keywords:
Oxide materials
Sol–gel process
Thermal analysis
Scanning electron microscopy
Transmission electron microscopy
X-ray diffraction
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
by phase transformation reactions (of monoclinic polymorphic
modification into tetragonal, of tetragonal modification into cubic,
Zirconia represents one of the most studied oxide materi-
als due to its numerous and important physical and chemical
properties, such as high hardness, low thermal conductivity, high
melting point, resistance to high temperatures and corrosion,
chemical inertness, and amphoteric character [1–3]. Although it
has been extensively studied over few decades, the interest of the
researchers is still increasing due to the wide range of its appli-
cations. The most spectacular ones include ceramics, bioceramics,
refractories, piezoelectrics, pigments, cosmetics, solid electrolytes,
oxygen sensors, active phases and/or supports in catalysis, column-
packing materials for high-performance liquid chromatography
(HPLC), and photonics [1,4–13].
Pure zirconia exhibits three polymorphs: low-temperature
monoclinic (at T < 1170 ◦C), high-temperature tetragonal
(at temperatures between 1170 ◦C and 2370 ◦C), and high-
temperature cubic (for T > 2370 ◦C). The phase relationship and the
structure–property correlation have been very intensively stud-
ied [3,9,14–16]. The systems containing ZrO2 are characterized
and/or by the formation of some intermediate metastable phases
during these transformations). The high-temperature polymorphs
cannot simply be quenched to room temperature because of
their spontaneously martensitic transition to the monoclinic form
[3,9,13]. When zirconia is obtained by conventional methods, it
is often characterized by a mixture of monoclinic and tetragonal
phases which, by coexisting, affect part properties of the product
[4,17,18]. The transition between the phases, which implies
volume changes, can turn out be highly problematic [16]. In order
to avoid this kind of problems, many researchers recommend the
doping of ZrO2 with small quantities of different metal oxides, such
as MgO, CaO, Y2O3, Gd2O3, CeO2 and Sc2O3 [15,16,19–21]. Another
way by which the stabilization of the tetragonal and cubic zirconia
polymorphs could be obtained is by controlling the size of particles
[9]. It is very well known that materials with nanoscale grain sizes
present different properties in comparison with themselves in bulk
form. They are named “nanostructured materials” and because of
their special properties they are intensively studied.
The increasing interest in the nanotechnology of zirconia
materials in our days is really remarkable. Many researchers
[4,9,14,17,22–24] underlined that, compared to common zirco-
nia powders, the nanocrystalline ones exhibit better physical
and chemical properties which lead to wider applications. It is
∗
Corresponding authors. Tel.: +40 213 167 912; fax: +40 213 121 147.
E-mail addresses: malina raileanu@yahoo.com (M. Ra˘ileanu), ltodan@icf.ro,
l todan@yahoo.co.uk (L. Todan).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.12.070

158
M. Ra˘ileanu et al. / Journal of Alloys and Compounds 517 (2012) 157–163
considered that all the properties (mechanical, electrical, chemical
as well as catalytic) of ZrO2 can be improved by using nanopow-
ders instead of conventional micron-sized zirconia [23]. In the
last decade, the methods of obtaining nanoparticles have signifi-
cantly developed. They are numerous and include wire explosion
techniques, the polymerizable complex method, flame synthesis of
nanoparticles, the sonochemical method, solid state reactions, pre-
cipitation and co-precipitation from solutions, sol–gel synthesis,
forced hydrolysis, microemulsion techniques, microwave-assisted,
and hydro- and solvo thermal methods [7,23,25–29].
The selection of the preparation method of nondoped zirconia
powders is extremely important, the fact being recognized that
the used technique, together with its experimental conditions, sig-
nificantly influence the characteristics of the final product of the
synthesis [4,7,18,30,31]. The sol–gel method represents one of the
most popular among those previously mentioned in order to pre-
pare zirconia nanoparticles [10,32–38].
The present work deals with the preparation of a stable,
nanocrystalline zirconia powder using the sol–gel approach.
In order to confer special properties to the final product, ␣-
cyclodextrin has been used as organic additive. In the last years
there are some researchers which studied the synthesis of zirconia
powders in the presence of different template agents, especially
in order to obtain mesoporous materials. Some examples are: Yu
and Hu [22] which used cationic and anionic surfactants (hex-
adecyltrimethylammonium bromide and sodium dodecyl sulphate,
respectively), Suciu et al. [23] that used sucrose and pectin, Ortiz-
Landeros et al. [39] which have used polystyrene latex particles,
Duan et al. [10] with poly(methyl methacrylate) template, and
Heshmatpour and Aghakhanpour [16] with glucose and fructose.
The last mentioned researchers [16] suggested that the used
organic additives produced some desirable effects on the crystal-
lite size and on the crystal phase of the prepared zirconia powders.
Besides these effects, in our study a special powder morphology
consisting in embedded pores inside the nanocrystalline ZrO2 par-
ticles was obtained. Only very few researchers such as Chang et al.
[26] have reported till now the preparation of porous zirconia crys-
tals. They prepared the material using the combination of sol–gel
and hydrothermal methods and they refer to the obtained porous
structure as “unique”, considering it very important in applications
such as thermal barrier coatings (TBCs).
Fig. 1. Thermal behaviour of ␣-cyclodextrin.
were recorded with a Perkin Elmer Lambda 35 Spectrometer. As a reference, a cer-
tified reflectance standard has been used. During reflectance measurements, the
reflected component of the sample beam was collected by the integrating sphere
(Spectralon 50 mm Internal Diameter) and detected by the sphere detector. The
sphere detector was a silicon photodiode while the detector signal represented the
part of the sample beam that is not transmitted and not absorbed by the sample.
The sample holder was 8◦ wedge and the used parameters were data interval, 1 nm;
scan speed, 240 nm min−1 and slit, 1 nm; (4) X-ray diffraction with a Philips PW
1050 diffractometer with Bragg-Brentano geometry and Cu K␣ radiation; the pat-
terns have been scanned in steps of 0.02◦ (2ꢀ) from 8◦ to 80◦ with a constant counting
time of 30 s; (5) the scanning electron microscopy analysis were made by using a
Quanta Inspect F microscope (FEI, the Netherland’s) with field emission gun (FEG)
having a 1.2 nm resolution, equipped with an energy-dispersive X-ray spectrom-
eter (EDX) with a resolution at MnK of 133 eV. In what the sample preparation
was concerned in the SEM-FEG (Scanning Electron Microscopy with Field Emission
Gun) measurements, a small amount of powder was put onto a SEM aluminium
sample holder on top of a carbon conductive tape; (6) for the transmission elec-
tron microscopy measurements, the bright field and high resolution images were
obtained using a TecnaiTM G2 F30 S-TWIN transmission electron microscope (FEI, the
Nederland’s), equipped with a STEM/HAADF detector, EDS (energy dispersive X-ray
analysis) and EFTEM-EELS spectrometer (Electron Energy Loss Spectroscopy). The
microscope operates at an acceleration voltage of 300 kV (Shottky field emitter) with
˚
˚
a TEM point resolution of 2 A and a TEM line resolution of 1.02 A. The microscope
was operated at an extraction voltage of 4500 V. The sample preparation was accom-
plished as follows: a small amount of powder was diluted into pure ethylic alcohol
and left into an ultrasonic bath for approximately 15 min. After that, a small drop
of the diluted solution was put onto a 400 mesh, holey carbon coated film Cu grid;
(7) the specific surface area of the thermally treated zirconia powder has been mea-
sured by the Brunauer–Emmett–Teller (BET) method, using a Micromeritics ASAP
2020 apparatus. The sample has been outgassed in flowing N2 at 300 ◦C for 2 h.
2. Materials and methods
ZrO2 powder has been synthesized via hydrolysis of zirconium(IV) i-propoxide,
i-Zr(OC3H7)4 (70 wt.% solution in 1-propanol) from Fluka, in an alcoholic solution
of 1-propanol alcohol, C3H8O, also from Fluka. The necessary water has been added
as aqueous solution of ␣-cyclodextrin (␣-CD), C36H60O30, a cyclic oligosaccharide
composed of six glucose monomers, from Wacker-Chemie GMBH which has been
used as organic additive.
3. Results and discussion
The mixture of the mentioned alcohol and alcoxide in the molar ratio of 7/1 has
been maintained under agitation for 15 min (solution 1). An 8 wt.% aqueous solution
of ␣-CD (solution 2) was added drop by drop to solution 1, in a quantity correspond-
ing to the molar ratio H2O/alcoxide of 18/1, under vigorous stirring. The pH of the
reaction mixture was 7. After an induction time of few tens of seconds, a sudden
precipitation has been observed. The formation of ZrO2 particles was evident from
the white colour of the precipitate. A very slow stirring has been continued for
2 h, then the static conditions have been maintained for 24 h, in order to ensure
the completion of both hydrolysis and condensation reactions. The next step was
the drying at 80 ◦C until the complete removal of the solvent. The obtained material
has been crushed in order to obtain the powder which has been named “ZrCD-untt”,
representing the untreated (before the thermal treatment) zirconia powder contain-
ing ␣-CD. Based on the thermal behaviour of both ZrCD-untt, and ␣-CD materials,
the temperature of the thermal treatment has been chosen, in order to eliminate
the organic material from the zirconia powder. The resulted sample was named
“ZrCD-tt” (thermally treated).
3.1. Thermal analysis
Once prepared, the zirconia powder containing ␣-cyclodextrin
(ZrCD-untt sample) has been subjected to the characterization. In
order to establish the thermal treatment which must be applied to
remove the organic additive (for obtaining the ZrCD-tt sample) the
characterization has begun with the thermal analysis.
The derivatograms of ␣-cyclodextrin and of the zirconia pow-
ders before (ZrCD-untt) and after thermal treatment (ZrCD-tt) are
presented in Figs. 1–3.
The thermal behaviour of ␣-cyclodextrin (Fig. 1) has been fully
described in a previous paper [40]. It evidences the following ther-
mal effects: (a) two endothermic effects corresponding to 73 and
107 ◦C, respectively, due to water elimination; (b) one endothermic
effect at 274 ◦C which is assigned to cyclodextrin decomposition;
and finally (c) an exothermic effect at 499 ◦C, due to the cyclodextrin
combustion.
Both powders have been characterized using the following methods: (1) thermal
analysis up to 1000 ◦C, using a Mettler Toledo Star System TGA/SDTA851/LF 1600 ◦C,
with a heating rate of 10 K min−1, in dynamic air atmosphere, and with a flow of
50 mL min−1; (2) infrared spectroscopy analysis (IR), using a FT-IR NICOLET 6700
(400–4000 cm−1) spectrometer; (3) the absorption measurements (% Reflectance)

M. Ra˘ileanu et al. / Journal of Alloys and Compounds 517 (2012) 157–163
159
Zr CD - tt
Zr CD - unt
-CD
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4000 3500 3000 2500 2000 1500 1000
Wavenumbers (cm-1)
500
Fig. 2. Thermal behaviour of the untreated sample of ZrO2 containing ␣-
Fig. 4. IR spectra of both zirconia powders, before (ZrCD-untt) and after thermal
treatment (ZrCD-tt), and of ␣-cyclodextrin.
cyclodextrin (ZrCD-untt).
The thermal analysis of the sample ZrCD-untt (Fig. 2) which
represents the as-synthesized sol–gel zirconia powder containing
␣-CD puts in evidence the influence of the ZrO2 matrix on the
thermal behaviour of the organic component. In its presence the
temperature corresponding to the cyclodextrin combustion is sig-
nificantly modified. Thus, ␣-CD is eliminated from the sample at
much lower temperature (267 and 450 ◦C) compared to the tem-
perature of combustion of the pure cyclodextrin which is around
499 ◦C. This observation is also valid in the case of sol–gel silica
matrices [40] and could be considered as a proof of the fact that
the cyclodextrin was included in the zirconia matrix. Based on the
thermal behaviour study, the temperature of the thermal treatment
has been chosen, in order to eliminate the organic molecules from
the zirconia sample. After 20 h at 550 ◦C, the amorphous powder
ZrCD-untt became a nanocrystalline one (ZrCD-tt), as XRD, SEM,
and TEM analyses evidence. Its thermal behaviour is presented in
Fig. 3.
as a quite complex process which includes several mutually related
steps, such as dehydration. According to their results, the loosely
bound water is released at temperatures below 170 ◦C, whereas
the reticular one persists up to 450 ◦C. Such results show that the
synthesis of our powder led to the incorporation of a certain amount
of water into the framework of the gel, part of which (around 1%)
still persists in the sample even after the thermal treatment. On
the other hand, Rovira-Bru et al. [41] found on the TGA curve of the
studied zirconia a total mass loss of about 0.42% in the domain of
temperature from ∼100 to ∼960 ◦C. They assume that up to 350 ◦C
the weight loss is due to loss of strongly hydrogen bound water,
while surface hydroxyl condenses between 350 and 800 ◦C.
Comparing the TGA data of the amorphous zirconia powder and
of the crystalline one, respectively, the difference between their
mass losses can be observed. The first one is clearly larger than the
second one, due to the fact that the amorphous material looses not
only water, but also organic groups. This is in good agreement with
the literature data [4,42].
It could be assumed that the effect of the organic additive (␣-
cyclodextrin) in the reaction mixture was similar to that of the
different metal oxides (e.g. Y2O3, Ce2O3, MgO, CaO, etc.) generally
used as dopants in order to stabilize the zirconia against phase
transformation. This is in agreement with some literature data
which suggest that organic additives can produce some desirable
effects on the crystallite size and on the crystal phase of zirconia
[16].
It could be considered that a stable zirconia powder has been
obtained. The thermogravimetric analysis curve (TGA) shows mass
losses (of around 1% entirely) which are not coupled with thermal
effects and could be assigned to dehydration of physically adsorbed
water and desorption of chemically bound water (as OH− groups),
respectively [7,25]. Adamski et al. [18] consider the ZrO2 formation
3.2. IR spectroscopy
In order to check the presence of the organic additive in the
zirconia matrix, besides the thermal analysis, spectral methods (IR
and UV–vis spectroscopy) have been used to characterize sample
ZrCD-untt. Applying the mentioned methods to sample ZrCD-tt, the
obtaining of a zirconia powder has been evidenced.
The results of the infrared spectroscopy investigations are pre-
sented in Fig. 4 and in Tables 1 and 2.
The measured wavelength values for ␣-cyclodextrin and ZrCD-
tt sample have been presented in Tables 1 and 2 together with the
literature data, for comparison. As it can be seen, in both cases
the values overlap almost totally, which proves both the quality
of the used organic additive and its elimination from the thermally
treated zirconia powder. In the IR spectrum of the ZrCD-tt sample
from Fig. 4, beside the wavelengths typical for ZrO2 presented in
Fig. 3. Thermal behaviour of the thermally treated sample of ZrO2 containing ␣-
cyclodextrin (ZrCD-tt).

160
M. Ra˘ileanu et al. / Journal of Alloys and Compounds 517 (2012) 157–163
Table 1
Reference
ZrCD - untt
150
IR bands frequencies for ␣-cyclodextrin according to our measurements and to the
literature and the corresponding assignments.
ZrO
+
CD; 550 C, 20h
2
125
100
75
Wavenumber
in our sample
Wave number in
Assignment
literature (cm−1
)
(cm−1
)
3412
2925
2356
1646
3412 [43]
2925 [44]
2361 [5,45,46]
1646 [47,48]
OH stretching
CH stretching
CO2 asymmetric stretch
Deformation mode of
crystallization water
present in the cyclodextrin
cavity
CH deformation
CO stretching + OH bending
CO/CC stretching
50
640
216 nm
229 nm
1458, 1368 and 1336
1156
1087 and 1025
755, 711, 608 and 572
1458 and 1368 [49]
1156 [43]
1087 and 1025 [50]
755, 710 and 572
[51,52]
25
0
500
600
700
800
Pyranose ring vibration
nm
-25
200
300
400
500
600
700
800
900
Table 2, some peaks corresponding to water traces (1646 cm−1),
to CO2 absorbed from the atmosphere (2337 cm−1), and to CH
stretching (2921 and 2852 cm−1) which could belong to the resid-
ual ␣-cyclodextrin from the thermally treated sample are present.
The quantity of these traces seems to be extremely low, since they
are not found in the thermal analysis of the ZrCD-tt sample (Fig. 3).
In what the IR spectrum of the ZRCD-untt sample is con-
cerned, the values of the determined wavelengths were: 3412,
2925, 2848, 1630, 1564, 1356, 1152, 1029, and 478 cm−1, respec-
tively. As it can be seen, these represent a combination of IR
signals of both ␣-cyclodextrin and ZrO2. Some of them are slightly
shifted, proving the inclusion of the organic molecule in the ZrO2
matrix.
Wavelength nm
Fig. 5. UV–vis reflectance spectra of the prepared zirconia sample, before (ZrCD-
untt) and after thermal treatment (ZrCD-tt) in comparison with the reference.
of magnitude smaller by comparison to that before the thermal
treatment. Yu and Hu [22] reported the presence of the absorp-
tion peaks in the UV–vis spectra of zirconia prepared using organic
surfactants as hexadecyltrimethyl ammonia bromide (C16TMABr)
and sodium dodecyl sulphate (SDS), at 200 and 210 nm, respec-
tively.
In what the UV spectrum of ZrCD-tt sample is concerned, it evi-
dences a higher absorption than that of the ZrCD-untt sample and
it agrees well with those previously reported for zirconia. Chen
et al. [58] give for zirconia an absorption peak at 230 nm, which
So, it could be concluded that, as well as thermal analysis, the
presented IR experimental data evidenced that the organic additive
(␣-cyclodextrin) was included by synthesis in the zirconia powder,
from which it has been removed after the thermal treatment.
is assigned to Zr
O Zr linkage. For bulk phase ZrO2, a band max-
imum in the UV–vis spectrum is also signalled between 230 and
240 nm, by Zaitseva and Gushikem [59]. According to López et al.
[60] monoclinic phase of zirconia absorbs at lower energy (cor-
responding to 240 nm) than the tetragonal one (which absorbs
around 200–210 nm). In line with Aita and Kwok [61] the band at
216 nm may be also corresponding to the interband transition in the
monoclinic zirconium oxide. On the other hand, there are studies
that indicate almost the same value of the absorption wavelength
for both monoclinic and tetragonal zirconia phases that is 238, and
240 nm, respectively [62,63].
Apart from the main peaks of the zirconium presented spec-
tra, a broad and weak peak can be observed in Fig. 5 at around
640 nm which could arise from transitions involving defect states
or impurities [64].
It can be concluded that the UV–vis spectroscopy analysis has
evidenced, once again, besides the thermal behaviour and the IR
spectroscopy, the presence of the organic additive in the matrix of
zirconia as a result of the synthesis process as well as its elimination,
due to the proceeded thermal treatment.
3.3. UV–vis spectroscopy
Typical UV–vis reflectance spectra for zirconia sample, before
(ZrCD-untt) and after thermal treatment (ZrCD-tt) are presented in
Fig. 5.
As it can be seen, there is a difference between the absorption
peaks of the zirconia powder before the thermal treatment (sample
ZrCD-untt) which contains the organic compound (␣-CD), and the
thermally treated sample (ZrCD-tt) from which the cyclodextrin
has been eliminated. The presence of cyclodextrin in the sol–gel
prepared zirconia powder is responsible for the appearance of the
13 nm batochromic shift of the absorption band, from 229 nm (in
ZrCD-tt) to 216 nm (in ZrCD-untt). Also, it was observed that after
thermal treatment, the reflectance is approximately two orders
Table 2
IR bands frequencies for ZrO2 as it can be found in ZrCD-tt sample and in the litera-
ture and the corresponding assignments.
3.4. X-ray diffraction
Wavenumber in
sample named
Wavenumber for
ZrO2 in literature
Assignment
ZrCD-tt (cm−1
)
(cm−1
)
Being a typical structural characterization method, the XRD
analysis has been applied to both zirconia powders: before (sample
ZrCD-untt) and after thermal treatment (sample ZrCD-tt).
The results are presented in Fig. 6. It can be seen that the initial
synthesized powder unthermally treated (ZrCD-untt) which con-
tains ␣-cyclodextrin is amorphous. After the thermal treatment it
becomes crystalline (ZrCD-tt).
From the view point of its composition, the zirconia powder
resulted after the thermal treatment represents a mixture of two
crystalline phases, the monoclinic one (M) which is predominant
3428
–
–
3427 [53]
764 [53]
Zr OH
Zr O bending
ZrO2
800–200 [54]
710 (745–490)
[11,16]
734, 580 and 514
[55]
734, 580, 514 and
440 [39,54,56,57]
480 [56]
701 and 657
Zr O Zr bond
580 and 514
514 and 440
Zr O vibration
Monoclinic ZrO2
Tetragonal ZrO2

M. Ra˘ileanu et al. / Journal of Alloys and Compounds 517 (2012) 157–163
161
Table 3
Structural factors calculated from the computerized profile analysis of the XRD spectra of the sample “ZrCD-tt” and of the zirconia standard (reference) “Ref.” (according to
the ASTM files 37–1484 and 42–1164) and the corresponding errors.
Identified phases
Structural factorsa
a (Å)
b (Å)
c (Å)
ˇ (◦)
UCV (Å3)
ꢀDꢁ (Å)
ꢀSꢁ ×103
Monoclinic (M)
Tetragonal (T)
Ref.
ZrCD-tt
Ref.
5.3129
5.2585(42)
3.64
5.2125
5.1789(72)
–
–
5.1471
5.1323(32)
5.27
99.218
99.26(18)
–
–
140.70
137.95(46)
69.83
–
–
388(10)
–
214(15)
1.40(10)
–
2.19(20)
ZrCD-tt
3.5488(97)
5.2253(183)
65.81(59)
a
a, b, c, ˇ – lattice parameters; UCV – unit cell volume; ꢀDꢁ – average crystallite size; ꢀSꢁ – average internal strain.
(91.4%) and the tetragonal (T) which is secondary (8.6%), confirming
the literature data which assert a value of about 8% to the tetragonal
phase for pHs 7–10 [34]. The relative amount of the monoclinic
and tetragonal phases formed during calcination process has been
determined based on Eq. (1) from the literature data [9,65–70].
3.5. Electron microscopy
In order to complete the XRD characterization, electron
microscopy has been used and correlations between both methods
have been established, regarding the obtained crystalline zirconia
nanopowder (ZrCD-tt).
¯
Im(1 1 1) + Im(1 1 1)
xm
=
(1)
¯
Im(1 1 1) + Im(1 1 1) + It(1 1 1)
3.5.1. Scanning electron microscopy (SEM)
SEM results for the ZrCD-tt sample are presented in Fig. 7. They
confirm the literature data which assert that organic additives make
particles assume spherical shape and reach fairly uniform size as
well as prevent their agglomeration [16,22,69,72].
As it can be seen, equiaxial particles, uniform as shape and
size, with a relatively reduced tendency of agglomeration can be
observed. These observations are in good agreement with the lit-
erature data which indicate that a nanosized monoclinic zirconia
powder from Degussa AG, Hanau presents a typical image of parti-
cles having sizes around 50 nm [73].
where “I” represents the diffraction intensity of different lattice
planes.
Table 3 presents the experimental data which have been
obtained from computerized analysis of the XRD spectra with a
proper X-RAY5.0 program that represents the upgrade form of the
previous X-RAY3.0 program [71]. It also contains the corresponding
calculated errors.
As it can be seen from Table 3, the values of the structural factors
obtained from the crystalline powder ZrCD-tt are well correlated.
The monoclinic phase presents a higher value of the average crys-
tallite size than the tetragonal one, while the internal strains have
the opposite behaviour, being smaller for the monoclinic symmetry
than in the tetragonal case.
3.5.2. Transmission electron microscopy (TEM) and high
resolution transmission electron microscopy (HRTEM) coupled
with selected area electron diffraction (SAED)
Comparing the structural factors values of the prepared zir-
conia powder ZrCD-tt with those of the reference (Ref.), it can
be concluded that both elemental cells of the monoclinic phase
and of the tetragonal one contract. Connecting this contrac-
tion with the high value of the internal strain of the T phase,
important distortions in the lattice of our sample could be
suggested.
XRD results proved that a nanocrystalline ZrO2 powder has been
prepared with average crystallite sizes of 38.8 nm (for the mono-
clinic phase), and 21.4 nm (for the tetragonal phase), respectively.
In order to complete the SEM characterization of the thermally
treated zirconia nanopowder, TEM, HRTEM and SAED analyses have
been carried out. Fig. 8 presents the obtained results.
The TEM image of Fig. 8(a) allowed estimating an average parti-
cle size of 39.2 nm, in good agreement with the average crystallite
size of 38.8 nm calculated from the XRD data (Table 3) for the mon-
oclinic major phase and, thereby, proving the single crystal nature
of the ZrO2 particles.
The HRTEM image of Fig. 8(b) shows a particular porous mor-
phology of the ZrO2 particles obtained as a result of the organic
ZrCD- tt
ZrCD-uunnttt
6000
monoclinic ZrO2
tetragonal ZrO2
5000
4000
3000
2000
1000
0
10
20
30
40
50
60
70
80
2 (o)
Fig. 6. XRD patterns of zirconia powder before (ZrCD-untt) and after thermal treat-
ment (ZrCD-tt).
Fig. 7. SEM images of sample ZrCD-tt.

162
M. Ra˘ileanu et al. / Journal of Alloys and Compounds 517 (2012) 157–163
Fig. 8. TEM (a), HRTEM (b and c) and SAED (d) images of the ZrCD-tt sample.
compound (␣-cyclodextrin) release during the thermal treatment.
The light grey areas which are covered with consecutive lattice
fringes suggest that the pores are closed and embedded within the
grains. This hypothesis is also confirmed by the BET results which
indicated a value of 19.11 m2 g−1 for the specific surface area of the
thermally treated ZrO2 powder.
Particles between 32 and 50 nm respectively have been
detected. The mean size particle was of 39.18 nm, in a very good
agreement with the estimated value (39.20 nm) from the TEM
image of Fig. 8(a).
The higher magnification HRTEM image of Fig. 8(c) underlines
the crystallinity of the ZrO2 particles, showing well resolved and
˚
equidistant lattice fringes. These fringes are separated by 2.84 A
˚
and 1.84 A, respectively, which agrees well with the interplanar
spacing corresponding to the (1 1 1) and (0 2 2) planes of monoclinic
zirconia.
The high crystallinity degree of the ZrCD-tt nanopowder was
also confirmed by the well-marked, bright spots forming the con-
centric rings which correspond to the SAED patterns of monoclinic
ZrO2 (Fig. 8(d)).
The mean particle size of the ZrCD-tt powder was calculated
using the OriginPro 8.5 software (statistics on column) by tak-
ing into account approximately 500 non-agglomerated particles
(from images obtained from various microscopic fields), whose
diameter was accurately measured using the microscope soft-
ware DigitalMicrograph 1.8.0. The histogram describing the particle
size distribution based on these measurements is presented in
Fig. 9.
Fig. 9. Particle size distribution obtained by means of TEM measurements per-
formed on ∼500 non-agglomerated particles taken from various microscopic fields.

M. Ra˘ileanu et al. / Journal of Alloys and Compounds 517 (2012) 157–163
163
4. Conclusions
[24] V.G. Zavodinski, A.N. Chibisov, J. Phys. Conf. Ser. 29 (2006) 173–176.
[25] M. Rezaei, S.M. Alavi, S. Sahebdelfar, L. Xinmei, Z.F. Yan, J. Mater. Sci. 42 (2007)
7086–7092.
[26] Q. Chang, J.-E. Zhou, Y. Wang, G. Meng, Adv. Powder Technol. 20 (2009)
371–374.
[27] F. Prete, A. Rizzuti, L. Esposito, A. Tucci, C. Leonelli, J. Am. Ceram. Soc. 94 (2011)
3587–3590.
[28] C.J. Reidy, T.J. Fleming, S. Hampshire, M.R. Towler, Int. J. Appl. Ceram. Technol.
8 (2011) 1475–1485.
[29] R. Pazhani, H.P. Kumar, A. Varghese, A.M.E. Raj, S. Solomon, J.K. Thomas, J. Alloys
Compd. 509 (2011) 6819–6823.
[30] X. Bokhimi, A. Morales, O. Novaro, M. Portilla, T. López, F. Tzompantzi, R. Gómez,
J. Solid State Chem. 135 (1998) 28–35.
[31] S.-G. Chen, Y.-S. Yin, D.-P. Wang, J. Mol. Struct. 690 (2004) 181–187.
[32] O. Van Cantfort, B. Michaux, R. Pirard, J.P. Pirard, J. Sol–Gel Sci. Technol. 8 (1997)
207–211.
The sol–gel synthesis of a zirconia nanopowder has been per-
formed, in the presence of ␣-cyclodextrin as organic additive. The
inclusion of the oligosaccharide in the zirconia matrix has been
evidenced.
The crystalline nanopowder resulted after thermal treatment
(sample ZrCD-tt), consisted of a mixture of monoclinic (predom-
inant: 91.4%) and tetragonal (8.6%) zirconia phases. The organic
additive seems to produce some desirable effects on the crystal-
lite size and on the crystal phase of zirconia, ensuring its stability
against the phase transformation.
The ␣-cyclodextrin has influenced the powder properties by
making the particles to assume spherical shape and reach fairly
uniform size as well as preventing their agglomeration. Further-
more, the organic additive led to a certain porous morphology of
the zirconia particles that is the pores are embedded within the
grains.
[33] C. Stöcker, A. Baiker, J. Sol–Gel Sci. Technol. 10 (1997) 269–282.
[34] G. Fetter, P. Bosch, T. López, J. Sol–Gel Sci. Technol. 23 (2002) 199–203.
[35] L.F. Liotta, G. Pantaleo, A. Macaluso, G. Marcì, S. Gialanella, G. Deganello, J.
Sol–Gel Sci. Technol. 28 (2003) 119–132.
[36] T. Rivera, J. Azorín, M. Barrera, A.M. Soto, R. Sosa, C. Furetta, Radiat. Eff. Defect
Solid 162 (2007) 597–603.
[37] A. Taavoni-Gilan, E. Taheri-Nassaj, R. Naghizadeh, H. Akhondi, Ceram. Int. 36
(2010) 1147–1153.
In conclusion, using only the sol–gel method, in the experimen-
tal conditions mentioned before, at room temperature and in the
presence of ␣-cyclodextrin (a non-toxic and available organic com-
pound) it was possible to obtain a stable, nanocrystalline zirconia
powder with a special porous morphology which allows its appli-
cation in the thermal barrier coatings domain.
[38] S. Salehi, M.H. Fathi, Ceram. Int. 36 (2010) 1659–1667.
[39] J. Ortiz-Landeros, M.E. Contreras-Garciá, H. Pfeiffer, Adv. Technol. Mater. Mater.
Proc. J. 9 (2007) 119–124.
[40] M. Raileanu, L. Todan, M. Crisan, A. Braileanu, A. Rusu, C. Bradu, A. Carpov, M.
Zaharescu, J. Environ. Prot. 1 (2010) 302–313.
[41] M. Rovira-Bru, F. Giralt, Y. Cohen, J. Colloid Interface Sci. 235 (2001) 70–79.
[42] M.J. Bockmeyer, R. Krüger, J. Am. Ceram. Soc. 91 (2008) 1070–1076.
[43] D. Bongiorno, L. Ceraulo, M. Ferrugia, F. Filizzola, A. Ruggirello, V.T. Liveri,
ARKIVOC xiv (2005) 118–130.
[44] F.B. De Sousa, J.D.T. Guerreiro, M. Ma, D.G. Anderson, C.L. Drun, R.D. Sinisterra,
R. Langer, J. Mater. Chem. 20 (2010) 9910–9917.
Acknowledgements
[45] M. Avram, Gh.D. Mateescu, Spectroscopia în infraros¸ u. Aplicat¸ii în chimia
organic, Editura Tehnica˘ Bucures¸ ti, 1966, p. 55.
This work has been performed in the frame of 7.2 theme of the
Romanian Academy programme: oxide systems obtained by the
sol–gel method (2011).
[46] E. Schmidt, N. Aslesen, J. Phys. Chem. Lab. 9 (2005) 54–63.
[47] J.C. Netto-Ferreira, V. Wintgens, L.F. Vieira Ferreira, A.R. Garcia, L.M. Ilharco,
M.J. Lemos, J. Photochem. Photobiol. A: Chem. 132 (2000) 209–217.
[48] J.M. Gavira, A. Hernanz, I. Bratu, Vib. Spectrosc. 32 (2003) 137–146.
[49] G. Bals¸ , G.M. Simu, J. Eng. Ann. Fac. Eng. Hunedoara VII (2009) 107–109.
[50] T. Uyar, F. Besenbacher, Eur. Polym. J. 45 (2009) 1032–1037.
[51] S. Jaiswal, B. Duffy, A.K. Jaiswal, N. Stobie, P. McHale, Int. J. Antimicrob. Agents
36 (2010) 280–283.
[52] I. Bratu, S. Astilean, C. Ionesc, E. Indrea, J.P. Huvenne, P. Legrand, Spectrochim.
Acta Part A 54 (1998) 191–196.
[53] H. Wang, P. Xu, W. Zhong, L. Shen, Q. Du, Polym. Degrad. Stabil. 87 (2005)
319–327.
References
[1] V. Ramaswamy, M. Bhawat, D. Srinivas, A.V. Ramaswamy, Catal. Today 97
(2004) 63–70.
[2] P. Moravec, J. Smolík, H. Heskinen, J.M. Mäkelä, V.V. Levdansky, Aerosol Air
Qual. Res. 7 (2007) 563–577.
[3] A.P. Naumenko, N.I. Berezovska, M.M. Biliy, O.V. Shevchenko, Phys. Chem. Solid
State 9 (2008) 121–125.
[4] B.L. Kirsch, S.H. Tolbert, Adv. Funct. Mater. 13 (2003) 281–288.
[5] L.E. Davies, N.A. Bonini, S. Locatelli, E.E. Gonzo, Latin Am. Appl. Res. 35 (2005)
23–28.
[6] J. Widoniak, S. Eiden-Assmann, G. Maret, Eur. J. Inorg. Chem. (2005) 3149–3155.
[7] J.P. Markovic´, S.K. Milonjic´, J. Serb. Chem. Soc. 71 (2006) 613–619.
[8] I.-K. Jun, Y.-H. Koh, J.-H. Song, H.-E. Kim, J. Am. Ceram. Soc. 89 (2006) 2021–2026.
[9] A. Adamski, P. Jakubus, Z. Sojka, Mater. Sci. Poland 26 (2008) 373–380.
[10] G. Duan, C. Zhang, A. Li, X. Yang, L. Lu, X. Wang, Nanoscale Res. Lett. 3 (2008)
118–122.
[11] S. Nagarajan, N. Rajendran, J. Sol–Gel Sci. Technol. 52 (2009) 188–196.
[12] X.H. Jia, J.J. Yang, Y.M. Zuo, Chinese Chem. Lett. 12 (2001) 439–442.
[13] K. Joy, S. Lakshmy, P.B. Nair, G.P. Daniel, J. Alloys Compd. 512 (2012) 149–155.
[14] G. Dercz, K. Prusik, L. Pajak, JAMME 18 (2006) 259–262.
[15] H.A. Abbas, F.F. Hamad, A.K. Mohamad, Z.M. Hanafi, M. Kilo, Diffus. Fundam. 8
(2008) 7.1–7.8.
[16] F. Heshmatpour, R.B. Aghakhanpour, Powder Technol. 205 (2011) 193–200.
[17] J.-C. Yu, S.-W. Hu, Chinese J. Struct. Chem. 25 (2006) 1512–1516.
[18] A. Adamski, P. Jakubus, Z. Sojka, Nukleonika 51 (2006) S27–S33.
[19] Y. Kan, S. Li, P. Wang, G.-J. Zhang, O. Van der Biest, J. Vleugels, Solid State Ionics
179 (2008) 1531–1534.
[54] M. Zhang, E.K.H. Salje, A.H. Wang, X.J. Li, C.S. Xie, S.A.T. Redfern, R.X. Li, J. Phys.:
Condens. Matter 17 (2005) 6363–6376.
[55] A. Agraval, Effect of nickel concentration on stabilization of tetragonal zirco-
nia, Thesis for Bachelor of Technology in Ceramic Engineering, Department of
Ceramic Engineering National Institute of Technology Rourkela, 2009.
[56] F. del Monte, W. Larsen, J.D. Mackenzie, J. Am. Ceram. Soc. 83 (2000) 628–634.
[57] H.C. Zeng, S. Shi, J. Non-Cryst. Solids 185 (1995) 31–40.
[58] X.-R. Chen, Y.-H. Ju, C.-Y. Mou, J. Phys. Chem. C 111 (2007) 18731–18737.
[59] G. Zaitseva, Y. Gushikem, J. Brazil Chem. Soc. 13 (2002) 611–617.
[60] E.F. López, V.S. Escribano, M. Panizza, M.M. Carnasciali, G. Busca, J. Mater. Chem.
11 (2001) 1891–1897.
[61] C.R. Aita, H.K. Kwok, J. Am. Ceram. Soc. 73 (1990) 3209–3214.
[62] H.R. Sahu, G.R. Rao, Bull. Mater. Sci. 23 (2000) 349–354.
[63] G.R. Rao, H.R. Sahu, Proc. Indian Acad. Sci. (Chem. Sci.) 113 (2001) 651–658.
[64] L. Kumari, G.H. Du, W.Z. Li, R. Selva Vennila, S.K. Saxena, D.Z. Wang, Ceram. Int.
35 (2009) 2401–2408.
[65] J. Matta, J.-F. Lamonier, E. Abi-Aad, E.A. Zhilinskaya, A. Aboukais, Phys. Chem.
Chem. Phys. 1 (1999) 4975–4980.
[66] J. Moon, H. Choi, C. Lee, J. Ceram. Process. Res. 1 (2000) 69–73.
[67] J. Tang, F. Zhang, P. Zoogman, J. Fabbri, S.-W. Chan, Y. Zhu, L.E. Brus, M.L. Steiger-
wald, Adv. Funct. Mater. 15 (2005) 1595–1602.
[68] S. Shukla, S. Seal, Rev. Adv. Mater. Sci. 4 (2003) 123–126.
[69] G.-H. Li, Z.-L. Hong, H. Yang, Chinese J. Struct. Chem. 27 (2008) 498–502.
[70] W. Pabst, J. Havrda, E. Gregorová, B. Krcˇmová, Ceram. Silikáty 44 (2000) 41–47.
[71] N. Dra˘gan, D. Cris¸ an, C. Lepa˘datu, Rom. J. Mater. 33 (2003) 133–148.
[72] L. Zhou, J. Xu, X. Li, F. Wang, Mater. Chem. Phys. 97 (2006) 137–142.
[73] C. Oetzel, R. Clasen, J. Mater. Sci. 41 (2006) 8130–8137.
[20] Y.-W. Hsu, K.-H. Yang, K.-M. Chang, S.-W. Yeh, M.-C. Wang, J. Alloys Compd.
509 (2011) 6864–6870.
[21] P.M. Abdala, D.G. Lamas, M.C.A. Fantini, A.F. Craievich, J. Alloys Compd. 495
(2010) 561–564.
[22] J.-C. Yu, S.-W. Hu, Chinese J. Struct. Chem. 24 (2005) 1133–1139.
[23] C. Suciu, L. Gagea, A.C. Hoffmann, M. Mocean, Chem. Eng. Sci. 61 (2006)
7831–7835.