Proceeding of ICNM - 2009
1st International Conference on Nanostructured Materials and
Nanocomposites (6 – 8 April 2009, Kottayam, India)
Published by : Applied Science Innovations Private Limited, India.
http://www.applied-science-innovations.com
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Sol-Gel synthesis and characterization of Sr2CeO4
blue nano phosphor
R. Seema and Nandakumar Kalarikkal*
School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam-683 560, Kerala,
India.
Abstract :A simple sol-gel route involving citric acid (CA) and ethylene glycol (EG) was
employed to prepare the Sr2CeO4 blue nano phosphor. The XRD study confirms the structure of
the system as orthorhombic. High resolution electron transmission microscopy revealed that the
Sr2CeO4 (0.5M CA & 0.5M EG sintered at 1000 C for 2hrs) sample composed of rod like
structures and has the length of ~135nm and width of ~26nm. Sr2CeO4 exhibits
photoluminescence due to the charge transfer (CT) mechanism. The sample displays a broad
excitation spectrum which peaks at ~ 292nm and the emission spectrum which peaks at ~467nm
which is attributed to the energy transfer between the molecular orbital of the ligand and charge
transfer state of the Ce4+ ion. The Commission International de l’Eclairage co-ordinates are x =
0.15 and y = 0.21 for the Sr2CeO4 sample.
Introduction :
In 1998, Danielson and co-workers reported unusual luminescence of the inorganic oxide
compound Sr2CeO4 using combinatorial technique [1]. Sr2CeO4 exhibits photoluminescence due
to the charge transfer (CT) mechanism [2, 3, 4, 5]. In CT transitions an electron is transferred
from a ligand to the 4fn shell of a rare earth (RE) ion. Some states that arise as a result of such a
transition are stable and can relax to the ground state with a photon emission (CT luminescence).
This phosphor exhibits blue-white luminescence efficiently under excitation with UV light,
cathode ray or X-ray. Sr2CeO4 also acts as a sensitizer to transfer the absorbed energy to the
dopants (activators) such as rare earth ions [2, 3]. Since Sr2CeO4 was found as a novel and
promising blue luminescent material by combinatorial chemistry method [1, 6], Sr2CeO4
phosphor has been widely studied because of its importance in the realization of a new
generation of optoelectronic and displaying devices [7]. The conventional solid state reaction
process has disadvantages in controlling the morphology and maintaining the uniformity in
composition of phosphor particles. Therefore new processes to overcome these disadvantages are
under investigation. The sol-gel method [8, 9] is a useful and attractive technique for preparation
of oxide – based materials especially complex metal oxides. We report the results of our
investigations on sol-gel synthesized Sr2CeO4 phosphors.
278
Experimental :
The Sr2CeO4 sample was prepared by the sol-gel reaction method using citric acid (CA) as the
chelating agent and ethylene glycol (EG) as the polymerizing agent. Stoichiometric amounts of
Strontium nitrate (Sr(NO)2), Cerium nitrate hexahydrate (Ce(NO)3.6H2O), Citric acid and
Ethylene Glycol were dissolved in distilled water. The solution of strontium nitrate and cerium
nitrate hexa hydrate (Sr2+: Ce4+, 2:1) was kept on a magnetic stirrer maintained at 50C for 30
minutes, followed by adding 0.5M CA and then 0.5M EG to the aqueous solution[8, 9, 10, 11].
The resultant solution was then constantly stirred for few hours with the temperature maintained
at 90 - 100C. Once the mixture is transformed into a yellowish gel, TG-DTA analysis has been
performed to finalize the sintering temperature. The sample was kept in an oven at 200 C in air
for 2hrs. The obtained powder was ground thoroughly and subsequently, the sample was sintered
in steps at 400C, 900C and at 1000C for 2hrs at a heating rate of 10C /min with intermediate
grinding. The thermogravimetric and differential thermal analysis (TG-DTA) curve of the gel
was recorded using a Shimadzu DTG60 thermal analyzer with a heating rate of 10C. The FTIR
spectrum of Sr2CeO4 (0.5M CA & 0.5M EG sintered at 1000C for 2hrs) was taken using the
Shimadzu 8400S FTIR spectrometer. The XRD spectrum was taken using Phillips X’Pert Pro XRay Diffractometer with Cu-K radiation. The Scanning electron micrograph was taken using
JEOL JSM - 6390. The Transmission electron microscopy has been performed using JEOL JEM
3010. Photoluminescence measurements were performed with spectrofluorophotometer
(SHIMADZU, RF-5301PC) using Xe lamp as excitation source.
Result and Discussion :
Fig. 1 shows the TG-DTA curves of the gel after heating at a rate of 10C min-1. The curve
reveal a first weight loss from room temperature to 130C, combined with three endothermic
peaks at 58, 90 and 120C, which result from the loss of water absorbed by the precursor and the
excess citric acid, respectively. The decomposition of the organic groups and nitrate salts result
in exothermic peaks at 228 and 274C, and in a second weight loss at temperatures between 130
and 330C. The third stage is the decomposition of SrCO3 and CeO2, with the release of CO2,
and formation of Sr2CeO4 between 330~1000C. The third weight lost may be due to SrCO3
reaction with CeO2 at temperature between 648 and 678C, forming the Sr2CeO4 particles. A
small weight loss occurs beyond 850C and the TG curve became almost a straight line from
900C.
279
8
100
TG
mg 7
DTA
uV
80
60
6
40
5
20
4
0
-20
3
-40
2
0
200
400
600
-60
1000
800
0
Temperature( C)
Figure (1) : The TG-DTA curve of the gel.
The FTIR spectrum of the Sr2CeO4 is shown in Fig. 2. The peaks at 3700 cm-1 are assigned to H-1
2O from any source. 3450 and 1110 cm is assigned to the hydrogen bonding in water and
impurities, usually present in KBr respectively. The absorption peaks at 1770, 1450, 1022, 856,
705 and 699 cm-1 were assigned to stretching characteristics of SrCO3 [12]. The absorption peak
at 600-300 cm-1 is assigned to the metal oxide frequency bands.
140
1022
856
703
699
458
2934
120
3427
110
100
1450
Transmittance ( % )
130
90
4000
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
Figure (2) : The FTIR spectrum of Sr2CeO4
280
500
The XRD pattern of the sample Sr2CeO4 is shown in Fig. 3. The pattern matches well with the
JCPDS file no 50-0115. The XRD study confirms the structure of the system as orthorhombic.
The XRDA3.1 software has been used to calculate the lattice parameters and found to be a =
6.07Å, b = 10.32Å and c = 3.62Å.
130
111
820
800
780
132
151
002
330
140
211
700
221
200
720
021
001
740
110
Intensity (a.u)
760
680
660
640
10
20
30
40
50
60
70
80
2
Fig. 3: The XRD spectrum of Sr2CeO4
The scanning electronic micrograph of Sr2CeO4 sample is shown in Fig. 4. The sample exhibits
grain like morphology with different sizes and shape. At low magnification the particles appear
agglomerated and at high enough magnification, the nature of the individual crystallites is clearly
evident.
Fig. 4: The SEM Micrographs of Sr2CeO4
281
The Transmission electron microscopy of the Sr2CeO4 is shown in Fig. 5 (a) and has the length
of ~135nm and width of ~26nm. Fig. 5 (b) is the lattice fringe image, displaying crystalline
planes in a single crystalline grain. The interplanar distance measured from HRTEM image in
Fig.5 (b) is 0.30 nm and is associated to (130) planes in Sr2CeO4. The corresponding FFT pattern
indicates Sr2CeO4 phosphor with good crystallinity.
(a)
(b)
Figure (5) : (a) TEM images of Sr2CeO4 (b) The lattice fringe image.
The inset is the Fast Fourier Transform (FFT) of the image; the d-spacing is 0.30nm.
1000
Intensity(arb. units)
800
600
400
200
0
200
250
300
350
400
450
Wavelength (nm)
Fig. 6: The excitation spectrum of Sr2CeO4 sample.
282
The excitation spectrum of Sr2CeO4 is shown in Fig. 6. The spectrum displays a broad band with
two strong peaks, one at ~ 292 nm and the other at ~345 nm. The two bands could be assigned to
the different Ce4+- O distances in the lattice [2]. The excitation band of the sample could be
attributed to the transition t1gf, where f is the lowest excited charge transfer state of the Ce4+
ion and t1g is the molecular orbital of the surrounding ligand in the six-fold oxygen co-ordination
[1].
1000
Intensity (arb. units)
800
600
400
200
0
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Figure (7) : The emission spectrum of Sr2CeO4 excited at 292 nm.
The emission spectrum of Sr2CeO4 excited at 292nm is shown in Fig. 7. The spectrum shows
broad band in the region 300 - 700 nm with a peak around 467 nm. The emission band can be
assigned to the ft1g transitions of Ce4+ ions. The emission spectrum of Sr2CeO4 observed with
345 nm excitation is similar to that observed with 292 nm. The only difference is the emission
intensity for excitation at 292 nm is higher than that for excitation with 345 nm.
From the literature, it is known that a typical Stoke’s shift for a CT transition on a RE ion ranges
from 4000 cm-1 upto 17,000 cm-1. Based on the difference between the first excitation maximum
(345 nm) and the emission maximum (467 nm), the Stokes shift is 7572 cm−1, which is within
the range of CT transitions on Ce4+ ions [12, 13].
283
The CIE co-ordinates were calculated by the spectrophotometric method using the spectral
energy distribution of the Sr2CeO4 sample and is shown in Fig. 8. The colour co-ordinates for the
Sr2CeO4 sample are x = 0.15 and y = 0.21. This data does not match with coordinates reported by
Danielson et al. (x = 0.20, y = 0.30 [1, 6]), Desong et al. (x=0.176, y=0.26 [14]) and those of
Jiang et al. (x = 0.19, y = 0.26 [15]). But these values are closer to the values of Serra et al. (x =
0.16, y = 0.21 [16]), Liu et al.(x=0.15 and y=0.21[17]) and Rahul et al. (x = 0.16, y = 0.25 [8]).
Conclusion :
Sr2CeO4 sample is prepared by a simple sol-gel route. The XRD study confirms the structure of
the system as orthorhombic. High resolution electron transmission microscopy revealed that the
Sr2CeO4 sample composed of rod like structures and has the length of ~135nm and width of
~26nm. The excitation spectrum recorded for Sr2CeO4 displays a broad band with two peaks, one
at ~ 292 nm and the other at ~345 nm, which is attributed to the two different Ce4+- O distances
in the lattice. The emission spectra of Sr2CeO4 at the corresponding peak excitation value
284
showed broad spectrum in the region 300 - 700 nm with a peak around 467 nm. The Commission
International de l’Eclairage co-ordinates are x = 0.15 and y = 0.21 for the Sr2CeO4 sample.
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