The synthesis and photoluminescence of nanocrystalline Zr 0.80Zn0.20O1.80+δ powders via glycine nitrate process

Article abstract of DOI:10.1016/j.materresbull.2005.09.027

Nanocrystalline Zr_1-xZn_xO_2-x+δ (x = 0, 0.20, 1.00) powders were synthesized by glycine nitrate process route and the Zr_0.80Zn_0.20O_1.80+δ powders were calcined in the temperature range between 500 and 800 °C. An intense UV emission band centered at 382 nm with excitation at 292 nm has been observed in Zr_0.80Zn_0.20O_1.80+δ powders calcined at 600 °C, and X-ray diffraction analysis indicates that the powders exhibit a single phase with cubic ZrO₂ structure with the average grain size is about 7 nm. According to the results of photoluminescence and annealing experiments in different atmospheres, it can be proposed that the intense UV emission band is related to the defect states involving oxygen vacancies. Compared with pure ZrO₂, the incorporation of Zn²⁺ ions enhances UV emission intensity. Our experimental results also show that photoluminescence intensity depends on the concentration of defects and the peak position is related to the crystal phase structure. The novel strong UV emission properties of this material may be very interesting for further application.

Full text of DOI:10.1016/j.materresbull.2005.09.027

Materials Research Bulletin 41 (2006) 873–878
www.elsevier.com/locate/matresbu
The synthesis and photoluminescence of nanocrystalline
Zr Zn O
powders via glycine nitrate process
0
.80 0.20 1.80+d
a
Zhen Wang , Beifang Yang , Zhengping Fu , WeiWei Dong ,
a,
a
a
*
b
Jiachun Xie , Wenqi Liu
c
a
Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China
b
Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR China
c
Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, PR China
Received 13 June 2005; received in revised form 23 September 2005; accepted 30 September 2005
Available online 21 October 2005
Abstract
Nanocrystalline Zr1 Zn O
(x = 0, 0.20, 1.00) powders were synthesized by glycine nitrate process route and the
ꢀx
x
2ꢀx+d
Zr0.80Zn0.20O1.80+d powders were calcined in the temperature range between 500 and 800 8C. An intense UV emission band
centered at 382 nm with excitation at 292 nm has been observed in Zr0.80Zn0.20 1.80+d powders calcined at 600 8C, and X-ray
diffraction analysis indicates that the powders exhibit a single phase with cubic ZrO structure with the average grain size is about
nm. According to the results of photoluminescence and annealing experiments in different atmospheres, it can be proposed that
the intense UV emission band is related to the defect states involving oxygen vacancies. Compared with pure ZrO , the
O
2
7
2
2
+
incorporation of Zn ions enhances UV emission intensity. Our experimental results also show that photoluminescence intensity
depends on the concentration of defects and the peak position is related to the crystal phase structure. The novel strong UVemission
properties of this material may be very interesting for further application.
#
2005 Elsevier Ltd. All rights reserved.
Keywords: A. Composites; B. Chemical synthesis; C. X-ray diffraction; D. Luminescence
1
. Introduction
Zirconia-based ceramics have received considerable attention owing to their excellent properties such as high
thermomechanical resistance, low thermal conductivity, high ionic conductivity and dielectric constant [1,2]. These
properties give them great potential for applications such as fuel cell electrolytes [3], catalysts [4], oxygen sensors [5],
damage resistant optical coatings and gate dielectric [6,7]. It is well known that zirconia has three crystalline structures,
¯
namely, monoclinic (20–1170 8C, P2 /c), tetragonal (1170–2370 8C, P4 /nmc) and cubic (2370–2680 8C, Fm3m)
1
2
phases. Thetwohightemperaturephases (tetragonalandcubic) are unstableinthe bulkform atroom temperature, though
they are more valuable than the monoclinic phase in the technological applications mentioned above.
Special efforts have been devoted to stabilizing the tetragonal and cubic zirconia phases at room temperature by
doping with different valence metal oxides (such as CaO, MgO, CeO , Al O , Y O , etc.) [1,8–10], and the studies on
new doping agents to exploit ZrO -based ceramics with improved physical and chemical properties are still of great
2
2
3
2 3
2
*
Corresponding author.
E-mail address: bfyang@ustc.edu.cn (B. Yang).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.09.027

874
Z. Wang et al. / Materials Research Bulletin 41 (2006) 873–878
interest. In recent years, there has been considerable interest in the PL properties of ZrO and ZrO -based powders, for
2
2
it is an important method of acquiring the information of the surface states and electronic states [11–13]. Moreover,
these studies of the PL properties are also expected to develop new photoactive and fluorescence materials.
ZnO is a wide band-gap semiconductor material with a direct band-gap of 3.37 eVat room temperature. It has been
considered as an excellent optoelectronic material. Several papers have been reported on ZnO-stabilized zirconia.
Qadri et al. predicted that ZnO–ZrO film had potential applications such as coating for hot corrosion resistance,
2
thermal barrier protection and transparent conducting oxide [15,16]. La et al. reported that doping with ZnO in ZrO2
increased its thermoluminescence intensity [17]. However, the photoluminescence (PL) phenomenon of nanosized
ZnO–ZrO solution powders has not been reported.
2
ZrO -based nanocrystalline powders have been synthesized by several methods such as sol–gel [18], coprecipitation
2
[
19], hydrothermal [20], solid state reaction and glycine nitrate process (GNP), etc. [21,22]. Compared with other
methods, the GNP route is a simple and inexpensive method. It has been widely applied to synthesize ultrafine
homogeneous single component or multi-component oxide nanocrystalline powders. In the reaction process, metal
nitrate is the oxidant, and glycine plays two roles. One role is to prevent the individual component from precipitation by
ꢀ
coordinating the transition metal ions with the amine group (NH2 ) in glycine, the other is to sere as fuel for the
combustion reaction. ZnO powders prepared by the GNP route have been reported [23]. To the best of our knowledge,
GNP route has notbeen used to synthesize ZnO–ZrO powders. In this work, nanosized Zr0.80Zn0.20O1.80+d powderswere
2
prepared by GNP route and an intense UVemission band was observed. Phase transformation as a function of calcination
temperature (T ) and characterization behaviors of the PL band were discussed.
c
2
. Experimental
Zirconium nitrate (Zr(NO ) ꢁ5H O), zinc nitrate (Zn(NO ) ꢁ6H O) and glycine (NH –CH –COOH) were used as
3
4
2
3 2
2
2
2
reactants (all the reagents purchased from Shanghai Chemical Co., AR). Zinc nitrate aqueous solution (0.39 M) and
glycine aqueous solution (0.50 M) were mixed in a beaker, and then solid zirconium nitrate was added into the beaker.
ꢀ
The molar ratio of NO3 to glycine was kept as 1:1. The mixture was vigorously stirred for 20 min and a transparent
precursor solution was obtained. Then the beaker was heated on an electric heater. As the distilled water evaporated,
the solution became viscous and finally turned to black ‘‘ash’’. The four as-obtained powders with the same molar ratio
of Zn/Zr equal to 1:4 were calcined in air-flowing tube furnace for 2 h at 500, 600, 700 and 800 8C, respectively. For
comparison, pure ZrO and ZnO powders were also prepared by the same method. The pure ZrO powders were
2
2
calcined in air-flowing tube furnace for 2 h at 600 8C and the pure ZnO powders at 500 8C. The X-ray diffraction
˚
XRD) patterns were measured by Philips X’pert prosuper diffractometer (l = 1.5418 A, Cu Ka). PL spectra were
(
obtained using a Hitachi-850 fluorescence spectrophotometer with a 300 W Xe lamp as the light source.
3
. Results and discussion
3
.1. XRD characterization
The initial uncalcined sample is the amorphous phase (not shown). XRD patterns of Zr0.80Zn0.20O1.80+d powders
calcined at 500, 600, 700 and 800 8C are shown in Fig. 1. When the T is equal to 500 and 600 8C, the XRD patterns
show a set of broadened peaks of cubic zirconia and the diffraction peaks of ZnO under our minimum detection limit,
c
2
indicating that Zn ions are incorporated into zirconia. The results are in good agreement with Qadri’s work, which
+
suggested that ZnO and ZrO could form solid solution of cubic zirconia in a large compositional range in firing film
2
forms [16]. When the T is above 700 8C, the diffraction peak of the hexagonal ZnO is observed at about 368 (2u),
c
indicating that ZnO is soluble enough in cubic phase up to 600 8C in the Zr0.80Zn0.20O1.80+d system. At the T of
c
7
of ZrO (a = b
00 8C, the distinct splittings of the peaks are observed at about 35 and 608 (2u). Normally, the tetragonal polymorphs
6¼ c) can be considered as the distortion of the cubic ones of ZrO (a = b = c) and the slight variation in
2
2
the lattice parameters (a = b
6¼ c) of tetragonal zirconia gives rise to the characteristic tetragonal peak splitting in XRD
pattern [24]. Therefore, the sample calcined at 700 8C was transformed into the tetragonal phase, though the presence
of small amounts of cubic zirconia phase was not ruled out. For the XRD pattern of powders calcined at 800 8C, the
diffraction peaks marked with M should be assigned to monoclinic phase. However, the tetragonal phase is the primary
phase. The fraction of monoclinic phase determined by the polymorph method [25] is about 15%.

Z. Wang et al. / Materials Research Bulletin 41 (2006) 873–878
875
Fig. 1. XRD patterns of Zr0.80Zn0.20O1.80+d powders calcined for 2 h at different temperatures. Z notations refer to ZnO.
In order to correlate the phase transformation with the grain size, the grain sizes of Zr0.80Zn0.20O1.80+d powders
calcined at each temperature were estimated by the Scherrer formula from the diffraction peak at 30.58 (2u) (cubic
phase (1 1 1) or tetragonal phase (1 0 1)) and summarized in Table 1. It is obvious that the higher T results in the
c
narrowing of the diffraction peaks (Fig. 1) and, hence, the greater grain growth (Table 1). As the T is elevated from 500
c
to 700 8C, Zr0.80Zn0.20O1.80+d nanocrystals gradually grow from 5.9 to 14.7 nm. Such a grain growth is a common
phenomenon in the calcination behavior of ZrO -based powders. When the T is 800 8C, an abrupt grain growth can be
observed with tetragonal phase partly transformed into the monoclinic phase. Gulino et al. have observed the similar
2
c
behavior in Bi O -doped zirconia system [24].
2
3
3
.2. PL characterization
As shown by curve a in Fig. 2, an intense UVemission band centered at about 382 nm with a narrow full widths at
half maximum (FWHM) of about 80 nm was observed for Zr0.80Zn0.20O1.80+d powders with the T of 600 8C. The UV
c
emission band appears to be asymmetric because of the 350 nm cutoff filter used to block off scattered incident light at
2
2
92 nm. In order to investigate the effects of Zn ions on the PL of Zr0.80Zn0.20O1.80+d powders, the emission spectra
+
of ZrO powders with the T of 600 8C and ZnO powders with the T of 500 8C were also measured, and are shown as
2
c
c
curves b and c in Fig. 2. The PL emission of ZrO powders is in a broad band from 350 to 700 nm, which consists of a
2
dominant visible emission band centered at about 447 nm with an UVemission shoulder at about 382 nm (Fig. 2b). It is
noteworthy that with the unique excitation light (292 nm) the peak position of the UVemission of Zr0.80Zn0.20O1.80+d
powders corresponds exactly to that of ZrO powders though the intensity is about two times higher than that of ZrO2
2
powders. This fact implies that the UVemission in Zr0.80Zn0.20O1.80+d powders originates from the same luminescence
centers as in ZrO2 powders, which is further confirmed by the distinct emission band differences between
Zr0.80Zn0.20O1.80+d and ZnO powders (Fig. 2c). Furthermore, the emission energy in the present work is far lower than
the band-gap of bulk ZrO (5.0–5.5 eV) [13], indicating that the UV emission involves defect states of zirconia [12].
2
Table 1
Average crystalline sizes of Zr0.80Zn0.20O1.80+d powders calcined at different temperatures
T
c
(8C)
Average crystalline size (nm)
5
6
7
8
00
00
00
00
5.9
7.1
14.0
41.2

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Z. Wang et al. / Materials Research Bulletin 41 (2006) 873–878
2
Fig. 2. Emission spectra of the samples (a) Zr0.80Zn0.20O1.80+d (lex = 292 nm) calcined at 600 8C (b) ZrO (lex = 292 nm) calcined at 600 8C and (c)
ZnO (lex = 373 nm) calcined at 500 8C.
Generally, the PL emissions of ZrO and ZrO -based powders are ascribed to the defects involving oxygen vacancies
2
2
+
such as F, F and F [13] In our previous work the emission band centered at about 385 nm for pure ZrO samples is
A
.
2
ascribed to oxygen-vacancy defects based on annealing experiments in different atmospheres [14]. Large amounts of
such surface defects should also form in the as-synthesized nanosized Zr0.80Zn0.20O1.80+d particles because of their
high surface area and contribute to the PL emission. Moreover, from the emission spectra of Zr0.80Zn0.20O1.80+d
powders annealed in different atmospheres we found a behavior similar to pure ZrO , viz the UV emission of the
2
sample annealed in a nitrogen atmosphere is the most intense and the UVemission of the sample annealed in an oxygen
2
+
atmosphere is the weakest (emission spectra of the samples were not shown). The incorporation of Zn ions will
further increase the amount of oxygen vacancies in Zr0.80Zn0.20O1.8+d powders to maintain the charge compensation,
and then enhance their PL emissions [17].
c
Fig. 3. Emission spectra of Zr0.80Zn0.20O1.80+d powders at different T . (a) 600 8C (lex = 292 nm), (b) 700 8C (lex = 292 nm) and (c) 800 8C
(l
ex = 290 nm).

Z. Wang et al. / Materials Research Bulletin 41 (2006) 873–878
877
The effects of T on the PL of Zr0.80Zn0.20O1.80+d powders was investigated and is shown in Fig. 3. When the T is
c
c
5
00 8C, the sample is of non-fluorescent materials. With T increasing from 600 to 700 8C, the emission intensity
c
decreases abruptly while the peak position remains approximately invariable. While T increases to 800 8C, the PL
c
intensity decreases slightly and the dominant peak position shifts to 450 nm with a shoulder peak at about 382 nm. The
substantial decrease of the PL intensity of Zr0.80Zn0.20O1.80+d powders with the T of 700 8C is tentatively ascribed to
c
the following factors. With T increasing to 700 8C, ZnO precipitates and modifies the surface of nanosized
c
Zr0.80Zn0.20O1.80+d particles, and will decrease the amount of oxygen vacancies dramatically. This is considered as the
primary factor for suppressing the PL intensity. In addition, the calcination will enlarge the particle size (Table 1) and
reduce the surface area. Meanwhile, the diffusion of oxygen into Zr0.80Zn0.20O1.80+d particles will also reduce the
concentration of oxygen vacancies. In the case of T increasing from 700 to 800 8C, the slight decrease of the PL
c
intensity is mainly due to the last two factors mentioned above. When T increases to 800 8C, the dominant peak
c
position of the PL emission exhibits a red-shift of about 170 nm. This can be explained as follows. The electronic
structures of the three phases of ZrO are different [26], which may result in discrepancies in the defect state energies.
2
This may contribute to the red-shift of the dominant peak position of the sample with partly monoclinic phase calcined
at 800 8C (Fig. 1). Su et al. observed that the monoclinic ZrO with impurity of ZnO had a PL band centered at about
2
480 nm [27]. The variance of peak position compared with our results is due to the fact that the crystal phase of
Zr0.80Zn0.20O1.80+d powders in our experiments is that of a mixed phase of tetragonal and monoclinic phase.
4
. Conclusion
Nanocrystalline Zr0.80Zn0.20O1.80+d powders were prepared by the GNP route. The powders exhibit a single phase
with cubic ZrO structure after calcination at temperature below 700 8C. When T is 700 8C, XRD analysis indicates
2
c
that both tetragonal zirconia phase and hexagonal ZnO phase appear, while T increases to 800 8C, tetragonal ZrO is
c
2
partly transformed into monoclinic ZrO . The results of the PL measurement of Zr0.80Zn0.20O1.80+d powders show that
2
2
the PL emission is related to the defect states involving oxygen vacancies. The incorporation of Zn ions increases the
+
concentration of oxygen vacancies, resulting in the enhancement of emission intensity. With T increasing, the
c
concentration of oxygen vacancies decreases, leading to decrease of emission intensity. The red-shift of peak position
of the powders after calcination at 800 8C is ascribed to the transformation from the tetragonal phase to monoclinic
phase. The novel intense UVemission of this new material may be very interesting for further study and applications.
Acknowledgement
This research was supported by the National Natural Science Foundation of the People’s Republic of China.
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