Photoluminescence of tetragonal ZrO2 nanoparticles synthesized by microwave irradiation

Article abstract of DOI:10.1021/ic025532q

Polymer-stabilized tetragonal ZrO₂ nanopowders with average size of ca. 2.0 nm have been prepared by microwave heating in an aqueous solution containing Zr(NO₃)4·5H₂O, PVA, and NaOH. The photoluminescence of the synthesize

Full text of DOI:10.1021/ic025532q

Inorg. Chem. 2002, 41, 3602−3604
Photoluminescence of Tetragonal ZrO Nanoparticles Synthesized by
2
Microwave Irradiation
,†
Jiahe Liang,† Zhaoxiang Deng,† Xin Jiang,‡ Fuli Li,‡ and Yadong Li*
Department of Chemistry, Tsinghua UniVersity, Beijing 100084, P.R. China, and Department of
Physics, Capital Normal UniVersity, Beijing 100037, P.R. China
Received February 9, 2002
Polymer-stabilized tetragonal ZrO nanopowders with average size
of ca. 2.0 nm have been prepared by microwave heating in an
tation,13 surfactant templating method,14 and spray pyroly-
2
sis.15 However, it is very beneficial for us to find a fast,
simple, and energy efficient approach to produce fine t-ZrO2
powders.
aqueous solution containing Zr(NO ) ‚5H O, PVA, and NaOH. The
3 4
2
photoluminescence of the synthesized ZrO fine particles has been
2
Microwave-assisted synthesis is another way to produce
inorganic compounds since 1986. Compared with conven-
tional methods, microwave synthesis has the advantages of
very short reaction time and production of small particles
with narrow size distribution and high purity.16 Only a few
publications reported studies on microwave-assisted inorganic
synthesis, and some of them describe the fabrication of
zirconia particles by microwave heating.17 However, it is a
challenge to find an efficient way to prepare ZrO2 powders
with particle size about several nanometers by using the
microwave method, and there have been no reports on the
luminescent properties of nanoscale ZrO2. In the present
work, a microwave-assisted method for the preparation of
polymer-stabilized nanocrystalline t-ZrO2, without addition
of any mineralizer, is reported, and its luminescent property
is investigated. We think PVA has an important influence
on the stabilization of the as-prepared t-ZrO2 nanoparticles.
ZrO2 nanoparticles were successfully prepared by means
of hydrolysis of Zr(NO3)4‚5H2O (Tianjin chemical, AR)
under microwave irradiation in NaOH (Beijing chemical,
AR) aqueous solutions. In a typical synthesis, an aqueous
solution containing 0.10 mol L-1 Zr(NO3)4‚5H2O and 5 mol
L-1 NaOH was exposed to microwave radiation at a power
of 650 W. The microwave irradiation was operated in 30-
second cycles (on for 10 s, off for 20 s) for 6 min, and then,
investigated.
In recent years, there has been considerable interest in
photoluminescent properties of many metal oxides such as
TiO2,1 ZnO,2 WO3,3 and In2O3,4 because they could possibly
be used, for example, as nanoscopic optical storage elements
or as probes in living systems.5 However, comparatively few
studies,6 to the best of our knowledge, have been carried
out concerning the luminescence of nanoscale ZrO2 crystal-
lites.
Nanometric ZrO2 particles are a technologically important
class of materials with a wide range of applications.7 ZrO2
has three polymorphs: monoclinic(m), tetragonal(t), and
cubic(c) phases. The monoclinic phase is thermodynamically
stable up to 1100 °C, the tetragonal phase exists in the
temperature range 1100 -2370 °C, and the cubic phase is
found above 2370 °C.8 The existence of metastable t-ZrO2
at low temperature has been reported, and fine powders of
t-ZrO2 at mild temperature have been prepared by many
methods, including forced hydrolysis,8,9 sol-gel method,10
hydrothermal method,11 thermal decomposition,12 coprecipi-
* To whom correspondence should be addressed. E-mail: ydli@
tsinghua.edu.cn. Fax: +86-10-62788765. Phone: +86-10-62772350.
† Tsinghua University.
‡ Capital Normal University.
(1) Liu, Y. J.; Claus, R. O. J. Am. Chem. Soc. 1997, 119 (22), 5273.
(2) Vanheusden, K.; Warren, W. L.; Seaeger, C. H.; Jallant, D. R.; Voigt,
J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983.
(3) Villarica, R. M.; Nash, F.; Chaiken, J.; Osman, J.; Bussjager, R. Mater.
Res. Soc. Symp. Proc. 1996, 397, 347.
(4) Zhou, H. J.; Cai, W. P.; Zhang, L. D. Appl. Phys. Lett. 1999, 75, 495.
(5) Peyser, L. A.; Vinson, A. E.; Bartko, A. P.; Dickson, R. M. Science
2001, 291, 103.
(10) Nav`ıo, J. A.; Hidalgo, M. C.; Colo´n, G.; Botta, S. G.; Litter, M. I.
Langmuir 2001, 17 (1), 202.
(11) Soˆmiya, S.; Akiba, T. J. Eur. Ceram. Soc. 1999, 19, 81.
(12) (a) Wu, J. M.; Wu, C. M. J. Mater. Sci. 1988, 23, 3290. (b) Zhang,
Y. C.; Davison, S.; Brusasco, R.; Qian, Y. T.; Dwight, K.; Wold, A.
J. Less-Common Met. 1986, 116, 301.
(13) Gulino, A.; Delfa, S. L.; Fragala`, I.; Egdell, R. G. Chem. Mater. 1996,
8, 1287.
(6) Bonola, C.; Camagni, P.; Omenetto, N.; Samoggia, G. J. Lumin. 1991,
48-49, 797.
(14) Wang, Y. Q.; Yin, L. X.; Palhcik, O.; Hacohen, Y. R.; Koltypin, Y.;
Gedanken, A. Chem. Mater. 2001, 13, 1248.
(15) Murugave, P.; Kalaiselvam, M.; Raju, A. R.; Rao, C. N. R. J. Mater.
Chem. 1997, 7 (8), 1433.
(16) Liao, X. H.; Zhu, J. M.; Zhu, J. J.; Xu, J. Z.; Chen, H. Y. Chem.
Commun. 2001, 937.
(17) Bellon, K.; Chaumont, D.; Stuerga, D. J. Mater. Res. 2001, 16 (9),
2619.
(7) (a) Si, J.; Desu, S. B.; Tsai, C. Y. J. Mater. Res. 1994, 9 (7), 1721.
(b) Kao, A. S.; Gorman, G. L. J. Appl. Phys. 1990, 67, 3826.
(8) Bohe´, A. E.; Andrade-Gamboa, J.; Pasquevich, D. M.; Tolley, A. J.;
Pelegrina, J. L. J. Am. Ceram. Soc. 2000, 83(4), 755.
(9) Hu, M. Z. C.; Harris, M. T.; Byers, C. H. J. Colloid Interface Sci.
1998, 198, 87.
3602 Inorganic Chemistry, Vol. 41, No. 14, 2002
10.1021/ic025532q CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/12/2002

COMMUNICATION
Figure 1. XRD patterns of the microwave synthesized ZrO2 samples in
the absence (a) and presence (b) of PVA, and the ZrO2 products annealed
at 320 °C for 6 h (c). The noisy curves in a and b are experimental results,
and the smooth curves in a and b are simulated results.
Figure 2. TEM image of the as-synthesized ZrO2 product (sample b).
Inset shows an electron diffraction (ED) pattern taken on a selected area of
the sample.
the solutions were allowed to cool to room temperature
naturally. The resulting precipitates were collected, washed
with distilled water and absolute alcohol, and dried in air
for 2 h. The product obtained in this procedure is referred
to as sample a. For the synthesis of polymer-stabilized ZrO2
nanoparticles (referred to as sample b in the following text),
the procedure was the same except that PVA (Beijing
chemical, AR) was existent in 30 wt % along with other
reactants. Heating treatments of the synthesized ZrO2 nano-
particles were conducted at 320 °C in air.
X-ray diffraction (XRD) patterns of the obtained samples
were recorded on a Brucker D8-advance X-ray powder
diffractometer. Morphology analysis of the samples was
conducted with H-800 transmission electron microscopy
(TEM) operated at 200 kV. The photoluminescence (PL) of
as-prepared sample was observed under excitation by UV
light at 254 and 412 nm in air with a Perkin-Elmer LS-50B
fluorescence spectrophotometer. Optical diffuse reflectance
of the samples were recorded on Shimadzu UV-2100S
spectrophotometer based on the procedure described in the
literature.18
Figure 3. UV-vis absorbance of the microwave synthesized ZrO2 samples.
synthesis in the absence of PVA was shown in Figure 1c.
On the basis of the Debye-Scherrer equation, the particle
sizes calculated from the fwhm (full width at half-maximum)
of the four strong reflectance peaks are 18 ([101]), 11.6
([002], [110]), 9.7 ([112], [200]), and 10.1 nm ([103], [211]).
The smaller values derived from the latter three peaks were
in agreement with the fact that all of them are composed of
two reflectances, which is characteristic of tetragonal ZrO2.
Therefore, the real particle size of the annealed product is
about 18 nm calculated from the fwhm of [101] reflectance.
Similar results were also obtained when sample b was treated
in the same way. The good crystallinity of the tetragonal
ZrO2 product obtained only through heating at 320 °C for 6
h, along with the good fitting between the simulated and
experimental XRD results as shown in Figure 1, reveals that
the initial product through microwave synthesis is nanosized
tetragonal ZrO2.
XRD patterns of the samples obtained directly by micro-
wave synthesis in the absence or presence of PVA are given
in Figure 1 a,b, respectively. Both of them exhibit broad
diffraction peaks, implying the extremely small dimensions
of the prepared ZrO2 powders. Using PowderCell 2.3,19 the
two experimental XRD results (noisy curves in Figure 1)
were fitted by simulated XRD profiles (smooth curves in
Figure 1) for tetragonal ZrO2, and an average crystallite size
of ca. 2.0 nm for sample b could be derived from the
simulation results. On the basis of the XRD reflectances, it
could be found that sample b (Figure 1b) crystallizes better
than sample a (Figure 1a). These facts prove that PVA played
a significant role in the formation and stabilization of the
t-ZrO2 phase via the current microwave method. The XRD
pattern of the annealed sample obtained by microwave
TEM and ED pictures of sample b are shown in Figure 2.
Because the particle size is only about 2 nm as judged from
the XRD results, it is reasonable that the TEM does not show
individual particles under the magnification used. The ED
pattern shows well-defined quasicontinuous diffraction rings,
which is consistent with the fact that the particles are much
smaller than the radius of the electron beam employed for
the electron diffraction.
(18) Li, J.; Chen, Z.; Wang, X.-X.; Proserpio, D. M. J. Alloys Compd.
1997, 28, 262.
(19) Kraus, W.; Nolze, G. PowderCell 2.3 for Windows; Federal Institute
for Materials Research and Testing: Berlin, Germany, 1999.
Inorganic Chemistry, Vol. 41, No. 14, 2002 3603

COMMUNICATION
sions at 402 nm, 420 nm, and 459 nm, respectively, could
be observed. The PL spectrum in Figure 4b was obtained at
412 nm excitation, exhibiting a maximum at 608 nm, with
a weak satellite peak at 530 nm. Although the detailed PL
mechanism for the nano-ZrO2 is still under research, we
could ascribe the emissions that appear at short wavelength
excitation to the near band-edge transitions. In the case of
the emission in Figure 4b, it should be due to the involvement
of mid-gap trap states, such as surface defects and oxygen
vacancies.21 Large amounts of surface defects should exist
on the as-synthesized nano-ZrO2 particles because of their
high surface area.
In conclusion, a microwave-assisted method has been
successfully used for the preparation of ultrafine polymer-
stabilized nanocrystalline t-ZrO2 powders. Photolumines-
cence of the as-synthesized t-ZrO2 particles was observed
and investigated. The microwave method is rapid, simple,
and energy-saving, which can be easily extended to the
preparations of other oxide nanocrystals.
Figure 4. PL spectra of the ZrO2 samples excited at 254 nm and 412 nm.
The diffusive reflectance spectrum of the as-synthesized
ZrO2 nanoparticles is given in Figure 3. Because the
individual particle size is much less than the thickness of
the sample layer, an ideal diffuse reflectance with constant
scattering coefficient could be expected. The Kubelka-Munk
function,20 which is the ratio between the absorption and
scattering factors, is then used for the absorbance plotting,
which shows a clear absorption onset arising at about 3.5
eV. The photoluminescence (PL) spectra obtained at two
different excitations for the nano-ZrO2 samples are shown
in Figure 4. Figure 4a shows the PL spectrum with an
excitation wavelength of 254 nm; three fluorescence emis-
Acknowledgment. This work was supported by NSFC
through the National Outstanding Youth Science Fund (No.
20025102) and the State key project of Fundamental
Research for Nanomaterials and Nanostructures (No.
G19990645-03).
IC025532Q
(21) (a) Wu, X. C.; Wang, R. Y.; Zou, B. S.; Wang, L.; Liu, S. M.; Xu, J.
R. J. Mater. Res. 1998, 13 (3), 64. (b) Huang, M. H.; Wu, Y.; Feick,
H.; Tran, N.; Weber, E.; Yang, P. D. AdV. Mater. 2001, 13 (2), 113.
(20) Kortu¨m, G. Reflectance Spectroscopy: Principles, Methods, Applica-
tions; Springer-Verlag: Berlin-Heidelberg, 1969.
3604 Inorganic Chemistry, Vol. 41, No. 14, 2002