Synthesis and the luminescent properties of CdNb2O6 oxides by sol-gel process

Article abstract of DOI:10.1016/j.jallcom.2008.03.081

Synthesis and luminescence properties of CdNb₂O₆ 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 272 nm have a broad and strong blue emission band maximum at 460 nm, corresponding to the self-activated luminescence center of CdNb₂O₆. The optical absorption spectra of the 700 °C sample exhibited the band-gap energies of 3.35 eV. Furthermore, the microstructure and luminescence spectra of the products were investigated.

Full text of DOI:10.1016/j.jallcom.2008.03.081

Journal of Alloys and Compounds 471 (2009) 259–262
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis and the luminescent properties of CdNb2O6 oxides by sol–gel process
Yu-Jen Hsiaoa, Yee-Shin Changb, Guo-Ju Chenc, Yen-Hwei Changa,∗
a Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
b Department of Electronic Engineering, National Formosa University, Yunlin 632, Taiwan
c Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis and luminescence properties of CdNb2O6 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 272 nm have a broad and strong blue emission band maximum at
460 nm, corresponding to the self-activated luminescence center of CdNb2O6. The optical absorption spec-
tra of the 700 ◦C sample exhibited the band-gap energies of 3.35 eV. Furthermore, the microstructure and
luminescence spectra of the products were investigated.
Received 11 December 2007
Received in revised form 19 March 2008
Accepted 19 March 2008
Available online 1 May 2008
Keywords:
CdNb2O6
Luminescence
Sol–gel process
© 2008 Published by Elsevier B.V.
1. Introduction
2. Experiments
The CdNb2O6 powders were prepared by the sol–gel method using cadmium
The synthesis and electro-optical properties of metal niobates
have been intensively investigated during the past half century
[1–3]. Additionally, niobates are known as interesting photoac-
tive host materials, and the luminescent properties of LiNbO3 [4],
KNbO3 [5], LnNbO4 (Ln = La, Gd, and Y) [6,7] and KNb3O8 [8] have
been studied extensively. However, to the best of our knowledge,
there are few investigations into the metal niobates (e.g., CdNb2O6).
CdNb2O6 is an important intermediate phase for the prepara-
tion of Cd2Nb2O7 ferroelectric ceramics [9], which is formed due
to distortions on octahedral NbO units, and, as a consequence, the
formation of short Nb O bonds takes place [10]. CdNb2O6 is also a
suitable reference material for dielectric ceramics [11].
Therefore, the main purpose of this work is to explore a sol–gel
synthetic route for the preparation of single phase CdNb2O6 oxides
with a columbite structure. The major advantage of sol–gel pro-
cess is that it is a low-temperature process. 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, and these fea-
tures often result in improved sinterability [12]. However, there are
few studies of works of the niobate-based complexes formed by
the sol–gel method, and the luminescence properties of CdNb2O6
oxides have been investigated by a sol–gel for the first time in this
paper.
nitrate [Cd(NO3)2·6H2O], niobium chloride (NbCl5), ethylene glycol (EG) and citric
acid anhydrous (CA), with purities of over 99.9%. First, the stoichiometric amount
of cadmium nitrate, and niobium ethoxide were dissolved in distilled water. Nio-
bium ethoxide, Nb(OC2H5)5, was synthesized from niobium chloride and ethanol,
C2H5OH, according to the general reaction [1]:
NbCl5 + 5C2H5OH → Nb(OC2H5)5 + 5HCl.
(1)
A sufficient amount of citric acid was added 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
and EG is also added as a stabilizing agent. The precursor containing Cd and Nb was
dried in an oven at 120 ◦C for 10 h, and then the CdNb2O6 powders were obtained
after calcinations at 400–700 ◦C for 3 h in air.
The burnout behaviors of powders were analyzed by differential thermal analy-
sis and thermogravimetry analysis (DTA–TGA, PE–DMA 7). The phase identification
was performed by X-ray powder diffraction (Rigaku Dmax-33). The surface mor-
phology and microstructure were examined by scanning electron microscopy (SEM,
S4200, Hitachi) and transmission electron microscopy (HR-TEM, HF-2000, Hitachi),
respectively. The excitation and emission spectra were recorded on a Hitachi-4500
fluorescence spectrophotometer equipped with a xenon lamp. The absorption spec-
tra were measured using a Hitachi U-3010 UV–vis spectrophotometer. All of the
above measurements were taken at room temperature.
3. Results and discussion
The amorphous metal–organic gel was heat-treated to pyrolyze
the organic components for crystallization. XRD patterns of the pre-
cursor powders at heat-treatment temperatures of 400–700 ◦C for
3 h are shown in Fig. 1. When calcined at temperatures of 400 ◦C
the powders showed some interphases such as Cd(OH)2 (JCPDS file
No., 18-0258) and H4Nb6O17 ·nH2O [13]. The content of the CdNb2O6
phase had a rapid product at 500 ◦C due to decompose interphases.
∗
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 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2008.03.081

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(e.g., Cd(OH)2 and H4Nb6O17 ·nH2O), and that around 480 ◦C corre-
sponded to the formation of the CdNb2O6 phase from XRD (Fig. 1).
In addition, an exothermic peak appeared around 575 ◦C, which was
associated with the decomposition of the second phase into main
phase CdNb2O6. In this experiment, the possible chemical reac-
tions for the synthesis of CdNb2O6 powders can be expressed as
follows:
Cd(NO3)2 + 2Nb(OC2H5)5
CA
−→ (1 − x)CdNb2O6 + xCd(OH)2
+
x H4Nb6O17 · nH2O
3
+2HNO3 + 10C2H5OH ↑→ CdNb2O6 + NO2 ↑ +H2O ↑ +CO2
↑
(2)
Fig. 1. X-ray diffraction patterns of CdNb2O6 precursor powders annealed at (a) 400,
(b) 500, (c) 600 and (d) 700 ◦C for 3 h.
When the precursor sintered at temperatures above 600 ◦C, the
samples exhibited a single phase and all of the peaks were found
to be the orthorhombic CdNb2O6 phase (JCPDS file No., 38-1428).
The variation of the relative amount of each phase as a function of
calcining temperature could be explained by the homogenization
of the composition with the enhancement of the diffusion process.
The TG and DTA curves of the dry CdNb2O6 precursor are shown
in Fig. 2. The endothermic peak at about 105 ◦C and small peak at
about 155 ◦C in DTA accounted for 11% of the initial weight loss
in TG, and this was assigned to the loss of ethanol and free water.
A fast weight loss stage of about 48% in the range of 160–440 ◦C
was accompanied by two exothermic peaks at 320 and 440 ◦C.
The exothermic peak at 320 ◦C was due to the burnout of the
low boiling organic species, and the other exothermic peak orig-
inated from the burnout of the organic groups in citric acid. The
exothermic peak around 440 ◦C was associated with the decom-
position into the oxide from the amorphous gel to second phase
Fig. 3. Scanning electron micrographs of CdNb2O6 precursor powders annealed at
(a) 500 ◦C, (b) 600 ◦C and (c) 700 ◦C for 3 h.
Fig. 2. DTA and TG curves for CdNb2O6 percursor.

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261
enhanced higher atomic mobility and caused faster grain growth.
Therefore, the particle size increased along with the sintering tem-
perature.
TEM analysis of the crystal provided further insight into the
structural properties of as-synthesized CdNb2O6 at 600 ◦C. Fig. 4
shows the low-magnification TEM image, and the morphology is
clearly observed. The diameters of the crystals are in the range
of 80–200 nm. However, the big particles were condensed by
assembled nanograins, as found in the previous SEM results. It is
conjectured that the assembly effect arising from the nanocrystals
is responsible for the decrease in surface energy. The associated
selected area electron diffraction (SAED) pattern in Fig. 4, recorded
perpendicular to the axis, can be indexed for the [2 0 0] zone axis of
crystalline CdNb2O6. The well-defined SAED pattern clearly shows
the diffraction spots representing {0 2 0} and {1 3 0} lattice planes
of CdNb2O6, and the interplanar spacing of the diffraction spots
in patterns and the experimental d values are well-fitted with
the JCPDS card. The results confirm that the crystalline phase was
CdNb2O6.
Fig. 5a presents the excitation spectra of the CdNb2O6 samples at
temperatures of 500–700 ◦C. The photoluminescence results reveal
that the sample prepared at 700 ◦C exhibits greater absorption
intensity at 272 nm than other samples by monitoring fluores-
cence at a wavelength of 460 nm. Both peaks were observed at
wavelengths of 272 and 330 nm, respectively, with the calcining
temperature at 600 ◦C. Blasse [14] indicated that the niobate com-
Fig. 4. TEM images of as-synthesized CdNb2O6 powders at 600 ◦C. The inset is the
electron diffraction pattern of the selected area corresponding along the CdNb2O6
[2 0 0] zone axis.
Therefore, the weight loss between 155 and 650 ◦C in the TG curve
was caused by the generation of organic groups and many kinds of
gas from the precursor.
plexes had two kinds of absorbing groups, [NbO6]7− and [NbO4]3−
,
The SEM pictures of the material sintered at 500, 600 and 700 ◦C
are shown in Fig. 3a–c. The rod-like particles seem to be distributed
homogeneously in Fig. 3c. It is believed that a higher temperature
respectively. This result corresponds to the excitation wavelengths
in Fig. 4 at calcining temperature 600 ◦C. Therefore, both peaks of
excitation, at about 272 and 330 nm, were associated with charge
Fig. 5. The room-temperature (a) excitation (ꢀem = 460 nm) spectra, (b) emission (ꢀex = 272 nm) and (c) emission (ꢀex = 330 nm) spectra of CdNb2O6 phosphors heat-treated
at various temperatures.

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tant 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 oxygen, which induced the
absence of oxygen sites and a great deal of oxygen vacancies [20].
In this way, the shifted absorption peak at 300nm may be caused by
the absorption of defect levels in nanosized CdNb2O6. The absorp-
tion edges were calculated according to the method reported by
Khan et al. [21]. Band-gap energy both was similar between 600
and 700 ◦C. The visible light absorption edge of 700 ◦C sample was
at 370 nm, which corresponded to the band-gap energy of 3.35 eV.
4. Conclusions
CdNb2O6 crystal was prepared by a sol–gel synthesis using
[Cd(NO3)2·6H2O] and NbCl5. The well-crystallized orthorhombic
CdNb2O6 can be obtained by heat-treatment at 600 ◦C from XRD.
The excitation wavelengths at a calcining temperature of 600 ◦C,
at about 272 and 330 nm, were associated with charge transfer
bands of [NbO6]7− and oxygen deficient niobate groups [NbO4]3−
.
Fig. 6. Absorption spectra of CdNb2O6 precursor powders annealed at 600 and
The PL spectra under 272 nm excitation show a broad and strong
blue emission peak at about 460 nm, originating from the niobate
octahedra group [NbO6]7−. The visible light absorption edge of the
700 ◦C sample was at 370 nm, which corresponded to the band-gap
energy of 3.35 eV.
700 ◦C for 3 h measured at room temperature.
transfer bands of [NbO6]7− and [NbO4]3− in the CdNb2O6 sys-
tem. The CdNb2O6 has an ordered columbite structure where the
pentavalent cations form pairs by face sharing of their coordina-
tion octahedral. Furthermore, in our experiment, rapid calcinations,
incomplete crystallization, and the departure of the Cd/Nb stoi-
chiometric ratio may generate various structural defects, such as
oxygen vacancies and Cd vacancies or interstitials. Therefore, the
concentration of extrinsic niobate groups (i.e., groups with an oxy-
gen deficiency or excess positive charge) [15,16] cannot be avoided.
Thus, the excitation bands at 330 nm, may be attributed to the
defects and impurities with different densities generated from
the extrinsic niobate groups (i.e., oxygen deficient niobate groups
[NbO4]3−).
The PL emission spectral wavelength distribution curves of
CdNb2O6 phosphors under 272 nm excitation at room temperature
are shown in Fig. 5b. The PL spectra show a broad and strong blue
emission peak at about 460 nm. Here, the edge-shared NbO6 groups
are efficient luminescent centers for the blue emission, which
may be ascribed to self-trapped exciton recombination [17]. This
Acknowledgement
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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