Sol-gel synthesis and the luminescent properties of CaNb2O6 phosphor powders

Article abstract of DOI:10.1016/j.jallcom.2008.07.142

Synthesis and luminescence properties of CaNb₂O₆ oxides by the sol-gel process were investigated. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence spectroscopy and absorption spectra. The PL spectra excited at 257 nm have a broad and strong blue emission band maximum at 457 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO₆]^7-. The optical absorption spectra of the 700 °C sample exhibited the band-gap energies of 3.53 eV.

Full text of DOI:10.1016/j.jallcom.2008.07.142

Journal of Alloys and Compounds 475 (2009) 698–701
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Sol–gel synthesis and the luminescent properties of
CaNb O phosphor powders
2
6
a
a
a
b,∗
Yu-Jen Hsiao , Chien-Wei Liu , Bau-Tong Dai , Yen-Hwei Chang
a
National Nano Device Laboratories, Tainan 741, Taiwan
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
2 6
Synthesis and luminescence properties of CaNb O oxides by the sol–gel process were investigated.
Received 23 April 2008
Accepted 29 July 2008
Available online 31 October 2008
The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-
resolution transmission electron microscopy (HRTEM), photoluminescence spectroscopy and absorption
spectra. The PL spectra excited at 257 nm have a broad and strong blue emission band maximum at 457 nm,
7−
corresponding to the self-activated luminescence of the niobate octahedra group [NbO6] . The optical
Keywords:
CaNb2O6
Luminescence
Sol–gel process
◦
absorption spectra of the 700 C sample exhibited the band-gap energies of 3.53 eV.
©
2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
The electro-optical and the luminescent properties of metal nio-
bates of LiNbO [1], KNbO [2], LnNbO (Ln = La, Gd, and Y) [3,4] and
The CaNb O6 powders were prepared by the sol–gel method
2
3
3
4
using calcium nitrate Ca(NO ) , niobium chloride (NbCl5), ethylene
3
2
ANb O6 (A = Ba, Sr and Ca) [5–7] compounds have been studied
2
glycol (EG) and citric acid anhydrous (CA). Their purities are over
9.9%. First, the stoichiometric amount of cadmium nitrate, and nio-
bium ethoxide were dissolved in distilled water. Niobium ethoxide,
extensively. The initial investigations of calcium niobate CaNb O6
2
9
was of interest because this particular crystal has shown to be a
strong source of coherent light that could be useful in holography
Nb(OC H5)5, was synthesized from niobium chloride and ethanol,
2
applications [8]. CaNb O6 exhibits very strong blue luminescence
2
C2H5OH, according to the general reaction as follows:
with excitation by ultraviolet radiation of 253 nm, even at room
temperature [9]. Ballman et al. [10], as well as Yariv and Gordon
NbCl5 + 5C H5OH → Nb(OC H5)5 + 5HCl.
(1)
2
2
[
11], reported that it was used both as a laser and a laser host mate-
rial. It was also proposed as a low cost lamp phosphor when doped
Sufficient amount of citric acid were added to the former solu-
tion as a chelating agent to form a solution. Citric acid to the total
metal ions in the molar ratio of 3:2 was used for this purpose. EG is
also added to the above solution as a stabilizing agent. The precur-
with Eu3+ and co-doped with Ti [12].
4+
The main purpose of this work is to explore a sol–gel syn-
thetic route for the preparation of single phase CaNb O6 oxides
2
◦
with a columbite structure (orthorhombic, space group Pbcn). The
major advantage of sol–gel processing is that there is a low-
temperature processing. Chemically synthesized ceramic powders
often posses better chemical homogeneity and a finer particle
together with better control of particle morphology than those
produced by the mixed oxide route [13]. Recently, works of the
niobate-based complex formed by the citric gel method are few.
Therefore, the luminescence and optical absorption properties of
sor containing Ca and Nb were dried in an oven at 120 C for 10 h
and then the CaNb2O6 powders were obtained after calcinations at
◦
600–700 C for 3 h in air.
The burnout behaviors of powders were analyzed by differen-
tial thermal analysis and thermogravimetry analysis (DTA–TGA,
PE–DMA 7). The phase identification was performed by X-
ray powder diffraction (Rigaku Dmax-33). The morphology
and microstructure were examined by transmission electron
microscopy (HR-TEM, HF-2000, Hitachi). The excitation and emis-
CaNb O6 nanocrystals have been investigated by a sol–gel for the
2
first time.
sion spectra were recorded on a Hitachi-4500 fluorescence
spectrophotometer equipped with xenon lamp. The absorption
spectra were measured using a Hitachi U-3010 UV–vis spectropho-
tometer. All of the above measurements were taken at room
temperature.
∗ Corresponding author. Tel.: +886 6 2757575x62968; fax: +886 6 2382800.
E-mail address: enphei@mail.ncku.edu.tw (Y.-H. Chang).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.07.142

Y.-J. Hsiao et al. / Journal of Alloys and Compounds 475 (2009) 698–701
699
Fig. 1. DTA and TG curves for CaNb2O6 precursor.
3. Results and discussion
The TG and DTA curves of the dry CaNb O precursor are shown
2
6
◦
in Fig. 1. The endothermic peak at about 110 C in DTA accounted
for 10% of the initial weight loss in TG, was assigned to the loss of
ethanol and free water. A fast weight loss stage of about 46% in the
Fig. 2. X-ray diffraction patterns of CaNb2O6 precursor powders annealed at (a) 500,
(b) 600 and (c) 700 C for 3 h.
◦
◦
range of 110–400 C that was accompanied by one exothermic peak
peak angle, and ˇ is half the peak width. The average grain sizes
◦
◦
at 370 C. The exothermic peak at 370 C was due to the burnout
of the low boiling organic species and the other exothermic peak
originated from the burnout of the organic groups in citric acid. In
◦
of powders calcined at 600 and 700 C were about 22.3 and
2
8.6 nm, respectively. The particle size increased as the sintering
temperature was increased. It is believed that a higher temper-
ature enhanced higher atomic mobility and caused faster grain
growth.
◦
addition, a manifest exothermic peak around 610 C, that was asso-
ciated with the decomposition the amorphous gel into main phase
CaNb O . In this experiment, the possible chemical reactions for
2
6
TEM analysis of the crystal provided further insight into the
the synthesis of CaNb O6 powders can be expressed as following:
◦
2
structural properties of as-synthesized CaNb O6 at 700 C. Fig. 3a
2
showed the low-magnification TEM image, and the morphology
was clearly observed. The big particles were condensed by assem-
bled nanograins. It was conjectured that the assemble effect arising
from nanocrystals, are responsible for the decreasing of surface
energy. There was only one orthorhombic crystalline phase exist-
ing in the ceramic matrix. The well-defined selected area electron
diffraction (SAED) pattern clearly shows the diffraction spots in
Fig. 3b, that was calculated the inter-planar spacing of the diffrac-
tion spots in patterns and the experimental d values well-fitted the
JCPDS card. The EDS results confirmed the composition with Ca and
Nb ion content and the molar ratio of Ca to Nb almost about 0.5 in
Fig. 3c.
CA
Ca(NO ) + 2Nb(OC H5) −→ CaNb O + NO ↑ +H O ↑ +CO2
↑
3
2
2
5
2
6
2
2
+C2H5OH ↑
(2)
◦
Therefore, the weight loss between 110 and 610 C in the TG
curve was caused by the generation of organic groups and many
kinds of gas from the precursor.
The amorphous metal-organic gel was heat-treated to pyrolyze
the organic components for crystallization. XRD patterns of the
precursor powders at heat-treatment temperatures of 500–700 C
for 3 h are shown in Fig. 2. Calcined temperatures at 500 C had
shown almost amorphous phases. The content of CaNb O phase
had a rapid product at 600 C that is due to decompose amor-
phous gel. When the precursor sintered at temperatures 700 C,
◦
◦
2
6
Fig. 4 presents the excitation spectra of the CaNb O sam-
2 6
◦
◦
ples at temperatures of 600 and 700 C. The photoluminescence
results reveal that the sample prepared at 700 C exhibits greater
◦
◦
the samples exhibited a single phase and all of the peaks were
absorption intensity at 257 nm than other samples by monitoring
fluorescence at a wavelength of 457 nm. Blasse [15] indicated that
identified to be the orthorhombic CaNb O6 phase (JCPDS file
2
7
−
the niobate complexes had two kinds of absorbing groups [NbO ]
6
No. 71-2406). Note that the intensity of the diffraction peaks
becomes sharper at higher temperatures, indicating that the
and [NbO4]3 , respectively. Only one peak was observed at wave-
−
◦
crystallinity of CaNb O6 increases with the increase of the cal-
lengths of 257 nm, as the calcining temperature at 600 and 700 C.
2
cination temperature. The average grain sizes were determined
from XRD powder pattern according to the Scherrer’s equation
Therefore, the peaks of excitation, at about 257 nm, were associated
with charge transfer bands of [NbO6]7 in the CaNb O system. The
−
2
6
[
14],
crystal structure of CaNb O was described as a layer structure built
2 6
up by edge-sharing of NbO6 trigonal prisms [16].
kꢀ
D =
(3)
The PL emission spectral wavelength distribution curves of
ˇ cos ꢁ
CaNb O6 phosphors under 257 nm excitation at room tempera-
2
where D is the average grain size, k is a constant equal to 0.89,
is the X-ray wavelength equal to 0.1542 nm, ꢁ is the (3 1 1)
ture are shown in Fig. 4. The PL spectra show a broad and strong
blue emission peaks at about 457 nm. Here, the edge-shared NbO6
ꢀ

700
Y.-J. Hsiao et al. / Journal of Alloys and Compounds 475 (2009) 698–701
◦
Fig. 3. (a) TEM images of as-synthesized nanocrystals at 700 C, (b) high-resolution TEM image of the nanocrystal and electron diffraction pattern and (c) EDX analysis of
CaNb2O6 nanocrystal.
groups are efficient luminescent centers for the blue emission,
which may be ascribed to self-trapped exciton recombination [17].
This luminescence effect depends on the Nb–O–Nb bonding that
5+
the conduction band is composed of Nb 4d orbitals, and the
valence band of O2 2p orbitals between the corner-sharing octa-
hedra [18]. In other word, this luminescence was originated from
the crystals of absorbing groups of the niobate octahedra group
−
[
NbO6]7
−
.
We have measured the UV–vis absorption spectra of the as-
prepared CaNb O6 nanocrystals and estimated the band gap from
2
the absorption inset in Fig. 5. For a direct band gap semiconductor,
the absorbance in the vicinity of the onset due to the electronic
transition is given by the following equation [19]:
1/2
C(hꢂ − Eg)
˛
=
(4)
hꢂ
where ˛ is the absorption coefficient, C is the constant, hꢂ is the
photon energy and Eg is the band gap. The inset of Fig. 5 shows the
relationship of (˛hꢂ)2 and hꢂ. Extrapolation of the linear region
gives a band gap of 3.53 eV. In particular, the oxygen vacancies
played an important role in the formation of these defect levels.
In our experiment, organic networks fiercely burnt out in a very
short time, and this process consumed a great amount of oxy-
gen, which induced the absence of oxygen sites and a great deal
of oxygen vacancies [20]. In this way, the small absorption peak
about at 360 nm may be caused by the absorption of defect levels
Fig. 4. The room-temperature excitation (ꢀem = 457 nm) spectra and emission
◦
in nanosized CaNb O .
(ꢀex = 257 nm) spectra of CaNb2O6 phosphors heat-treated at 600 and 700 C.
2 6

Y.-J. Hsiao et al. / Journal of Alloys and Compounds 475 (2009) 698–701
701
4
57 nm, were originated from the niobate octahedra group. The vis-
◦
ible light absorption edge of 700 C sample was at 352 nm, which
corresponded to band-gap energies of 3.53 eV.
Acknowledgment
The authors would like to thank the National Science Council of
Republic of China, Taiwan for financially supporting this research
under contract No. NSC 96-2811-E-006-014.
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Fig. 5. Absorption spectra of CaNb2O6 precursor powders annealed at 700 C for 3 h
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2
6
Ca(NO3)2 and NbCl5. The well-crystallized orthorhombic CaNb O
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◦
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