Template-free solvothermal synthesis of size-controlled yttria-stabilized-zirconia hollow spheres

Article abstract of DOI:10.1016/j.jallcom.2011.06.111

Tetragonal yttria-stabilized-zirconia (YSZ) hollow spheres were prepared through a template-free solvothermal method in a homogeneous solution of butanol/acetylacetone or ethanol/acetylacetone with ZrOCl₂· 8H₂O and Y(NO₃)₃·6H₂O as the reaction reagents. The hollow spheres were composed of nanometer-sized particles. The sizes of the hollow spheres could be tuned from 300 nm to 900 nm by changing the concentration of ZrOCl₂·8H₂O or altering the solvent. Time-dependent experiments showed that the hollow spheres transformed from amorphous solid spheres composed of nanoparticles. The Ostwald ripening process is proposed to explain the formation of the hollow structures. Er³⁺ doped zirconia (ZrO₂) hollow spheres were also synthesized using the same method. Characteristic emission patterns (515-565 nm) and (640-690 nm) were observed from Er³⁺ doped zirconia hollow spheres with 980 nm excitation.

Full text of DOI:10.1016/j.jallcom.2011.06.111

Journal of Alloys and Compounds 509 (2011) 9200–9206
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Template-free solvothermal synthesis of size-controlled
yttria-stabilized-zirconia hollow spheres
Zhanxia Shu, Xiuling Jiao, Dairong Chen∗
Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Tetragonal yttria-stabilized-zirconia (YSZ) hollow spheres were prepared through a template-free
solvothermal method in a homogeneous solution of butanol/acetylacetone or ethanol/acetylacetone
with ZrOCl2·8H2O and Y(NO3)3·6H2O as the reaction reagents. The hollow spheres were composed of
nanometer-sized particles. The sizes of the hollow spheres could be tuned from 300 nm to 900 nm by
changing the concentration of ZrOCl2·8H2O or altering the solvent. Time-dependent experiments showed
that the hollow spheres transformed from amorphous solid spheres composed of nanoparticles. The Ost-
wald ripening process is proposed to explain the formation of the hollow structures. Er3+ doped zirconia
(ZrO2) hollow spheres were also synthesized using the same method. Characteristic emission patterns
(515–565 nm) and (640–690 nm) were observed from Er3+ doped zirconia hollow spheres with 980 nm
excitation.
Received 15 March 2011
Received in revised form 28 June 2011
Accepted 28 June 2011
Available online 5 July 2011
Keywords:
Zirconia
Hollow spheres
Template-free
Solvothermal synthesis
Size-controlled
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
and they have potential applications in catalysis [20], drug-delivery
carriers [16,21], nanoreactors [22], drug release [23], etc. The routes
Zirconia (ZrO2), an important functional material in industry,
has been widely investigated due to its unique properties, such
as low thermal conduction, high melting point, high strength,
high ionic conductivity and chemical inertness [1]. At atmo-
spheric pressure, ZrO2 has three polymorphs-cubic, tetragonal, and
monoclinic-depending on the temperature [2]. If larger cations
of lower valence (e.g., Y3+) are substituted for Zr4+, the high-
temperature structures (cubic and tetragonal) can be stabilized
at room temperature because the substitution introduces oxy-
gen vacancies which compensate for the difference in charge
[3]. Yttria-stabilized-zirconia (YSZ) is widely used as a catalyst,
in oxygen sensors, and as the electrolyte in solid oxide fuel
cells, among other applications, because of its excellent proper-
ties of low thermal conductivity, high ionic conductivity, high
corrosion resistance and high refractive index [4–7]. As to zirco-
nia and doped zirconia materials, many kinds of morphologies
have been fabricated, such as nanoparticles [8], nanospheres [9],
nanobelts [10], nanowires [11,12], nanotubes [13], flake-like struc-
ture [14], mesoporous structures [15], hollow structures [16–19],
and etc.
to prepare hollow structures involve templates [21,24,25] and
template-free methods [26–29]. In previous reports, both tem-
plates [30–37] and template-free [38] methods have been applied
in the preparation of hollow zirconia spheres. However, most of
the synthetic processes are complicated and costly, for exam-
ple, polystyrene/ZrO2 [32], carbon/ZrO2 [30], SiO2/ZrO2 [31], and
Fe2O3/ZrO2 [17] spheres were firstly prepared, and calcining, alka-
line or acid etching was used to remove the hard templates. Also,
hollow ZrO2 microspheres were prepared using yeasts or pollen
as bio-template [35–37]. Alternatively, a precursor (Zr(OH)4 or
ZrOC2O4 hollow spheres) was prepared firstly, then ZrO2 hol-
low spheres were obtained by calcining [33,38]. Although Li and
co-workers [34] have prepared hollow ZrO2 microspheres with
a size in the range of 1.0–2.0 ␮m through an in situ source-
template-interface reaction route, the uniformity and size of
the hollow zirconia spheres could not be controlled. Therefore,
finding a simple method to prepare ZrO2 hollow spheres with
controllable size and excellent uniformity still remains great chal-
lenge.
In this paper, we produced uniform YSZ hollow spheres
with controlled size (300 nm–900 nm) through a template-free,
solvothermal method in an ethanol/butanol-acetylacetone sys-
tem using ZrOCl2·8H2O and Y(NO3)3·6H2O as the reagents. We
also investigated the mechanism of formation of the hollow
spheres. Er3+ doped zirconia hollow spheres were also pre-
pared, and their up-conversion luminescence properties were
studied.
Hollow inorganic spheres have special properties such as low
densities, high specific surface areas and excellent permeability,
∗
Corresponding author. Tel.: +86 531 8836 4280; fax: +86 531 8836 4281.
E-mail address: cdr@sdu.edu.cn (D. Chen).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.06.111

Z. Shu et al. / Journal of Alloys and Compounds 509 (2011) 9200–9206
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2. Experimental
2.1. Materials
Zirconium (IV) oxychloride octahydrate (ZrOCl2·8H2O, AR, Sinopharm Chemi-
cal Reagent Co. Ltd., ≥99.0%), yttrium (II) nitrate hexahydrate (Y(NO3)3·6H2O, AR,
Sinopharm Chemical Reagent Co. Ltd., ≥99.0%), n-butanol (CH3(CH2)2CH2OH, AR,
Tianjin Guangcheng Chemical Reagent Co. Ltd., ≥99.0%), acetylacetone (C5H8O2, AR,
Tianjin Kermel Chemical Reagent Co. Ltd., ≥99.5%) and absolute ethanol (C2H5OH,
AR, Tianjin Standard Co. Ltd., 99.7%) were used as raw materials without further
purification.
2.2. Synthesis and preparation of yttria-stabilized-zirconia hollow spheres
In a typical synthesis, 0.322 g of ZrOCl2·8H2O and 0.033 g of Y(NO3)3·6H2O
(molar ratio for Zr/Y = 92:8) were dissolved in a 20.0 mL mixture of n-butanol and
acetylacetone (volume ratio = 1:1) under magnetic stirring to form a clear solution.
Then the mixture was transferred to a 25 mL Teflon-lined stainless steel autoclave
and heated at 65 ◦C for 4 h, then reacted at 200 ◦C for 12 h. After that, the autoclave
was allowed to cool to room temperature naturally. The product was collected by
Fig. 1. XRD pattern (a) and Raman spectrum (b) of the YSZ hollow spheres.
peaks for monoclinic and cubic phase are observed [40,41]. The
XRD and Raman characterization revealed tetrahedral phase-pure
nature of the as-obtained YSZ. From the breadth of the (1 0 1) reflec-
tion, the crystallite size calculated using the Scherrer equation is
about 3.0 0.2 nm.
centrifugation, and washed with anhydrous ethanol 3 times and then dried at 60 ◦
for 6 h in vacuum.
C
2.3. Characterization
The X-ray diffraction (XRD) patterns were collected on
a Rigaku D/Max
Fig. 2a shows the TEM image of the as-synthesized product.
The obvious contrast difference between the margin and the cen-
ter of the particles confirms the existence of hollow structures in
the resulting spheres, and the uniformity of the hollow spheres
can be seen clearly from the image. The TEM image also demon-
strates that the average diameter of the hollow spheres is about
400 nm with shell thickness of ca. 60 nm. The SAED pattern (inset
in Fig. 2a) of a single hollow sphere (denoted by the arrow) shows
clear diffraction rings, revealing the polycrystalline nature of the
hollow sphere. The hollow structure was further investigated by
FE-SEM. Fig. 2b indicates that the product is composed of spheres
with an average diameter of 400 nm, and a sphere with an aperture
could also be observed in the image, confirming that the spheres
have hollow interiors. From the surface of the spheres, it can also be
observed that the spheres are composed of small particles. The HR-
TEM image (Fig. 2c) indicates that the hollow spheres were formed
by disoriented aggregation of nanoparticles, which is consistent
with the SAED pattern.
2200PC diffractometer with graphite monochromator and CuK␣ radiation
a
(ꢀ = 0.15418 nm). The morphology and microstructure of the products were deter-
mined using a field emission scanning electron microscope (FE-SEM, TOSHIBA
S4800), TEM (JEM-100CXII) with an accelerating voltage of 80 kV, and high-
resolution TEM (HR-TEM, JEOL-2010) with an accelerating voltage of 200 kV. TG
analysis was carried out on a Mettler Toledo SDTA851e thermal gravimetric analyzer
at a heating rate of 10 ◦C/min under air atmosphere. The infrared (IR) spectra were
measured on a Nicolet 5DX Fourier transform infrared (FT-IR) spectrometer using
KBr pellets for the solid samples and the liquid-membrane method for liquid sam-
ples. Elemental analysis (C, H contents) were conducted on a Vario EI III elemental
analyzer. Room-temperature fluorescence spectra were recorded on a Hitachi F-
4500 spectrophotometer equipped with a 150 W Xe-arc lamp at a fixed band-pass
of 0.2 nm with a 10 nm excitation slit, 8 nm emission slit and 700 V PMT voltage. The
up-conversion emission spectra were measured on a modified Hitachi F-500 spec-
trophotometer with an external tunable 2 W 980 nm laser as the excitation source in
place of the xenon lamp in the spectrometer. Raman spectra were recorded at room
temperature using LabRAM HR 800 (HORIBA Jobin Yvon) instrument equipped with
a 632.8 nm laser beam.
3. Results and discussion
Fig. 1a shows the XRD pattern of the product. All the diffraction
peaks can be indexed to the tetragonal structure of YSZ (JCPDS,
No. 48-0224), but the peak broadening, which might be due to the
small crystalline size of the product, makes it difficult to distin-
guish the tetrahedral and cubic phases. Raman technique is more
sensitive to phase identification of zirconia, so it is selected to fur-
ther elucidate the structure. In order to improve the Raman signal,
YSZ hollow spheres were annealed at 400 ◦C for 1 h to remove the
surface organics [39], and the XRD pattern of the annealed product
hardly changed. As shown in Fig. 1b, the peaks at 145, 261, 315,
460, 638 cm−1 can be attributed to the tetragonal zirconia, and no
The IR spectrum of the YSZ hollow spheres is shown in Fig. 3a.
According to the literature [42], the band at 3436 cm−1 can be
assigned to the asymmetric stretching vibration of OH. The absorp-
tions at 2960, 2929 and 2870 cm−1 are assigned to the out-of-plane
asymmetric CH3 stretching vibration, and the symmetric stretching
vibrations of C H in CH3 appear at 1457 and 1340 cm−1. The strong
absorption at 1563 cm−1 is ascribed to the stretching vibration of
C
O in acetylacetonate. The shift of the C O vibration compared
to pure acetylacetone (1621 cm−1) reveals that the acetylacetone
is complexed to Zr4+. The absorptions at 615 and 468 cm−1 are due
to Zr O vibrations and the peak at 1026 cm−1 to C O vibrations.
Fig. 2. TEM (a), FE-SEM (b) and HR-TEM (c) images of the YSZ hollow spheres. The inset in (a) is the SAED pattern of the YSZ hollow sphere denoted by the white arrow.

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Fig. 3. IR spectrum (a) and TG curve (b) of the YSZ hollow spheres.
(1621 cm−1, denoted by the square, SDBS No.1030). At the same
time, the characteristic absorption of butyl acetate (1737 cm−1
Fig. 3b gives the TG curve of the YSZ hollow spheres, and the
whole weight loss of ca. 21% can be observed from room tem-
perature to 500 ◦C. The first drop (up to ∼160 ◦C, 7.19%) is due
to desorption of absorbed water, and the second one (160–500 ◦C,
14.05%) is attributed to the decomposition of the organic materi-
als on the surface of the particles composing the hollow spheres
and the loss of water from the hydroxyls. Since the elimination
of n-butanol on the particle surfaces should occur from 120 ◦C
to 160 ◦C, it is concluded based on the IR and TG analyses that
the second weight loss step is mainly due to the decomposition
of acetylacetone. From the IR and TG analyses of the YSZ hollow
spheres, we conclude that there are water molecules, hydroxyls
and some coordinated acetylacetone species on the surface of the
hollow spheres.
,
denoted by the triangle, SDBS No.3292) appears. Extending the
reaction time from 70 min to 2 h causes the absorptions of acety-
lacetone to gradually decrease, accompanied by an enhancement
of the absorptions of butyl acetate. Therefore, a reaction occurs
between acetylacetone and n-butanol as shown in Eq. (1), which
is consistent with that proposed by Adkins et al. [43]. Upon further
prolonging the reaction time, no obvious changes can be detected.
The organic reaction is basically completed after reacting at 200 ◦C
for 2 h, and this reaction and the formation of the solid spheres
occurs at the same time.
O
O
O
O
⎯→
+
CH3COC4 H9 CH3CCH3
(1)
+
CH3CCH2CCH3
C4H9 OH
In order to investigate the formation of the hollow structures,
a detailed time-dependent experiment was conducted at 200 ◦C.
Fig. 4 gives the TEM images and XRD patterns of the products
synthesized at different times. When the solvothermal reaction
time is 60 min, there is no solid product, only a yellow solu-
tion. When the reaction time is 70 min, solid spheres with broad
size distribution from 150 nm to 300 nm are obtained (Fig. 4a).
After 2 h, the solid spheres become uniform and larger, ca. 400 nm
(Fig. 4b), but their surfaces become coarse as the reaction time
increases to 3 h (Fig. 4c). It also can be observed that some spheres
combined with each other. After reacting for 4 h, a thin shell is
formed outside the shrunken solid spheres (Fig. 4d). The FE-SEM
image (inset in Fig. 4d) shows a broken hollow sphere with a core
inside, which is consistent with the TEM observation. Further FE-
SEM observation (Fig. S1) indicates that some conjoint spheres are
detached from each other, which resulted in the aperture of the
spheres. At longer reaction times, the inside solid spheres grad-
ually diminish, and the shells of the spheres gradually thicken
(Fig. 4e–g). Finally, hollow spheres with a size of ca. 400 nm are
formed after a reaction time of 12 h (Fig. 4 g). The XRD patterns
(Fig. 4 h) revealed that the solid spheres are amorphous, and the
tetragonal-structured zirconia appears with the appearance of the
shell. As the reaction time increases from 4 h to 12 h, the crys-
tallinity of the product gradually increases. But when the reaction
time is increased beyond 12 h, no obvious changes of the product
can be observed.
Table 1 shows the contents of C, H in the products obtained after
reacting at 200 ◦C for 2 h (solid spheres) and 12 h (hollow spheres)
and the O contents in the decomposable fraction of the correspond-
ing products. The C contents in the hollow spheres and solid spheres
are almost the same, but the H and O contents in the solid spheres
are much higher than those in the hollow spheres, suggesting a
larger amount of absorbed water.
Comparing the TG analyses of the solid and hollow spheres
(Fig. 3b, Fig. 6a, respectively), the evidence also suggests that a
relatively larger amount of absorbed water is present in the solid
spheres. The TG curve of the solid spheres (Fig. 6a) shows more
weight loss than the hollow spheres before 160 ◦C, and the weight
losses after 160 ◦C are almost the same. The weight gain from 502 ◦C
to 666 ◦C may be related to the annihilation of surface defects by
oxygen [44], and the drop from 666 ◦C to 800 ◦C can be attributed
to the loss of hydroxyls. It is difficult to determine their precise
weight, as the two processes overlap.
Fig. 6b shows the IR spectra of the two samples described
in Table 1. The absorptions at 1631 (denoted by the arrow) and
3412 cm−1 in the spectrum of the solid spheres belong to the
stretching and bending modes of the hydroxyls in the absorbed
water, while in the spectrum of the hollow spheres the absorp-
tion at 1631 cm−1 almost disappears, indicating a decrease of the
absorbed water in the hollow spheres. From the results of IR, TG
and elemental analyses, it can be concluded that the organic con-
tents in the solid spheres and the hollow spheres are similar, while
the solid spheres have more absorbed water.
On the basis of the above experimental results, we believe that
Ostwald ripening should be the underlying mechanism for the for-
mation of YSZ hollow spheres. The formation process is proposed as
follows. At the beginning, zirconium is chelated with acetylacetone
to form zirconium acetylacetonate (Eq. (2)), which can dissolve in
To further clarify the formation mechanism of the YSZ hollow
spheres, IR spectra were taken of the solvents at different points
in the reaction process. According to Fig. 5, the IR spectra of the
freshly prepared solution, and those treated at 65 ◦C for 4 h, and at
200 ◦C for 1 h are almost the same, primarily composed of acetylace-
tone and n-butanol. After a reaction time of 70 min, the content of
acetylacetone significantly reduces, which can be concluded from
the decreasing of the characteristic absorption of acetylacetone

Z. Shu et al. / Journal of Alloys and Compounds 509 (2011) 9200–9206
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Fig. 5. IR spectra of the filtrate at different points in the procedure.
Then the reaction between the solvents occurs under the
solvothermal conditions, i.e. acetylacetone undergoes alcoholysis
in n-butanol to produce butyl acetate and acetone, which con-
sumes acetylacetone and makes the reaction shown in Eq. (2)
moving to the left, and release Zr4+ ions. If the concentration of Zr4+
reaches a critical value, Zr4+ hydrolyzed to form amorphous solid
nanospheres. With the release of Zr4+, the solid spheres gradually
grow and spheres as large as 400 nm are formed after almost all the
Zr4+ is released. As the amorphous solid spheres are surrounded
by polar solvents (water and alcohol), the primary particles in the
outer surfaces are relatively easy to crystallize. Therefore, less-
crystalline particles in the inner region are easy to dissolve [46], and
dissolution and recrystallization tend to happen. Then the hetero-
geneous nucleation of tetragonal YSZ on the solid spheres surface
occurs. With the dissolution of the interiors of the amorphous
solid spheres, the YSZ shell gradually thickened and the hollow
structures formed finally based on an inside-out Ostwald ripening
process.
Considering the crystal phase of the zirconia, both the stabilizing
effect by Y3+, Mg2+, etc. and the size effect are reported to pro-
mote the formation of the tetragonal polymorph [3,47]. Here, Y3+
was selected as the stabilizer, and HRTEM observation indicates
that the hollow spheres are composed of ca. 3 nm nanoparticles,
which are much smaller than the critical size of tetragonal zirco-
nia. To clarify the formation mechanism of the tetragonal zirconia,
ZrO2 hollow spheres were also synthesized under the same condi-
tions but without any stabilizer. As shown in Fig. 7a, all the XRD
reflections of the product can be indexed to monoclinic zirconia
(JCPDS, No.37-1484). The Raman spectrum (Fig. 7b) also reveals
the monoclinic phase-pure nature of the product [40]. The TEM
and SEM images (Fig. 8a and b) show the similar morphology and
size of the product to the YSZ hollow spheres in the typical syn-
thesis. The SAED pattern (inset in Fig. 8a) of a single hollow sphere
shows clear diffraction rings, revealing the polycrystalline nature of
the hollow sphere. Further HR-TEM observation (Fig. 8c) revealed
that the zirconia hollow spheres are also composed of disoriented
Fig. 4. TEM images (a–g) of samples prepared at different reaction times (200 ◦C)
and the corresponding XRD patterns (h). (a) 70 min, (b) 2 h, (c) 3 h, (d) 4 h, (e) 6 h,
(f) 8 h, (g) 12 h. The scale bars in all the images represent 300 nm. The insert in (d)
is the FE-SEM image of a broken sphere corresponding to TEM (d).
organic solvents, and there are a few dissociated Zr4+ ions in the
solution [45].
O
O
O
O
4+
+
4CH CCH CCH + Zr
Zr(CH CCHCCH ) + 4H
3
2
3
3 3 4
(2)
Table 1
Elemental analysis of the samples after reacting at 200 ◦C for 2 h and 12 h.
Samples
Decomposable fraction
(wt%, from TG)
C (wt%)
H (wt%)
O in the decomposable
fraction (wt%)
C/H/O (molar ratio)
Solid spheres
Hollow spheres
29.43
21.24
5.629
5.640
3.016
1.883
20.78
13.72
1/6.43/2.77
1/4.01/1.83

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Fig. 6. (a) TG curve of the solid spheres obtained by reacting at 200 ◦C for 2 h, (b) IR spectra of the products after reaction at 200 ◦C for different times (2 h and 12 h), the arrow
in (b) indicates the newly appeared absorption at 1631 cm−1
.
Fig. 7. XRD pattern (a) and Raman spectrum (b) of the ZrO2 hollow spheres.
aggregated nanoparticles with similar size to that of YSZ hollow
low structures can be clearly seen from the contrast difference on
the TEM images and the cracked spheres on the SEM images. If
the reaction takes place in a mixture of n-butanol and acetylace-
tone, hollow spheres with sizes of 300 nm and 400 nm are obtained
at ZrOCl2·8H2O concentrations of 0.0375 mol/L and 0.05 mol/L,
respectively (Fig. 9a and b). Using ethanol instead of n-butanol,
hollow spheres with sizes of 500 nm, 600 nm, 700 nm, 900 nm are
prepared by simply adjusting the concentration of ZrOCl2·8H2O to
0.0625 mol/L, 0.075 mol/L, 0.0875 mol/L or 0.10 mol/L (Fig. 9c–f).
The corresponding XRD patterns reveal that all the products are
phase-pure tetragonal YSZ hollow spheres. In the FE-SEM images
(Fig. 8b and Fig. 9), some of the hollow spheres with one aperture
spheres. According to the analysis, the crystal phases of the YSZ
hollow spheres are dependent on the Y3+ stabilizer rather than the
size effect. We speculate that the aggregation of the nanoparticles
results in a significant decrease of the surface energy, which means
the size effect does not control the crystal phase in our system.
By altering the solvent or changing the concentration of
ZrOCl2·8H2O and Y(NO3)3·6H2O (keeping the Zr/Y molar ratio at
92:8), the sizes of the hollow spheres can be adjusted from 300 nm
to 900 nm, and the shell thickness of the hollow spheres varies
between 60 and 150 nm. The SEM images shown in Fig. 9 indicate
that all the spheres with different sizes are uniform, and the hol-
Fig. 8. TEM (a), FE-SEM (b) and HR-TEM (c) images of ZrO2 hollow nanospheres. The inset in (a) is the SAED patterns of the hollow spheres.

Z. Shu et al. / Journal of Alloys and Compounds 509 (2011) 9200–9206
9205
Fig. 9. SEM images of YSZ hollow spheres prepared by altering the solvent or changing the concentration of ZrOCl2·8H2O. (a) n-butanol, 0.0375 mol/L; (b) n-butanol,
0.05 mol/L; (c) ethanol, 0.0625 mol/L; (d) ethanol, 0.075 mol/L; (e) ethanol, 0.0875 mol/L; (f) ethanol, 0.10 mol/L. The scale bars are 200 nm. Insets are the corresponding TEM
images.
were observed, which are caused by the departure of one sphere
from another.
rates with acetylacetone, and then show a slight influence on the
size of the spheres.
As described in the above section, the formation of the solid
spheres is related to the dissociation of the complex of Zr4+ and
acetylacetone. In the solution, most of the Zr4+ ions are coordinated
by acetylacetone. Under solvothermal conditions, Zr4+ is released
by the reactions between the acetylacetone and the alcohol, and the
solid spheres are formed through hydrolysis as soon as the concen-
tration of Zr4+ reaches the critical value, then grow larger with the
continuous release of Zr4+. With the increasing of the ZrOCl2·8H2O
concentration, more Zr4+ would be released during the growth pro-
cess, and then larger solid spheres are formed, which results in the
larger hollow spheres. Different alcohols exhibit different reaction
It is reported that Er3+ doped zirconia has an up-conversion
luminescence property [10]. Here, in addition to YSZ hollow
spheres, we synthesized tetragonal zirconia hollow spheres doped
with Er3+ instead of Y3+. The obtained hollow spheres were cal-
cined in a boat crucible at 800 ◦C in air for 1 h at a heating rate of
1 ◦C/min. Both the Y3+ and Er3+ doped zirconia hollow spheres show
almost the same down-conversion emissions (Fig. 10a, excited at
280 nm), which contain a broad band ranging from 380 nm to
480 nm with a maximum at 425 nm. Zr4+ itself has no lumines-
cence, so the observed luminescence from ZrO2 should be related
to defects and/or impurities in the system [9], such as the oxygen
vacancies induced by the doping of Er3+ and Y3+ into ZrO2 hol-
low spheres [48]. In the up-conversion spectrum of the Er3+-doped
hollow spheres (Fig. 10b, excited at 980 nm), characteristic emis-
sion patterns (515–565 nm) and (640–690 nm) can be observed,
which can be assigned to H11/2 + 4S3/2 → I15/2 and F9/2 → I15/2
2
4
4
4
transitions of Er3+, respectively.
4. Conclusions
Yttria-stabilized-zirconia hollow spheres with uniform and
adjustable sizes between 300 nm and 900 nm are synthesized by a
facile template-free solvothermal strategy. The hollow spheres are
produced by initial formation of amorphous solid spheres and then
an inside-out Ostwald ripening process. By varying the concentra-
tion of the Zr precursor and changing alcohols (ethanol or butanol),
YSZ hollow spheres with controllable and uniform size can be pre-
pared, which have potential industrial application. Hollow spheres
doped with Er3+ show down- and up-conversion luminescence
properties, which have value for basic functional optical research
and the development of optical devices.
Fig. 10. (a) Room-temperature down-conversion excitation (left) and emission
(right) spectra of the YSZ hollow spheres (dash line) and hollow spheres doped
with Er3+ (solid line). (b) The corresponding up-conversion emission spectrum of
the ZrO2/Er3+ hollow spheres. The sample was excited at 980 nm with a 2 W diode
laser.

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This work is supported by the 973 program of the Ministry of Sci-
ence and Technology of China (no. 2010CB933504) and the Science
Funds for Distinguished Young Scientists of Shandong Province
(JQ200903). The authors thank Dr. Pamela Holt for proof reading
the manuscript.
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