Synthesis and red luminescence of Pr3+-doped CaTiO3 nanophosphor from polymer precursor

Article abstract of DOI:10.1016/S0022-4596(03)00175-0

Uniform and sphere-like nanoparticles of crystalline Pr³⁺-doped CaTiO₃ have been prepared from complex polymer precursor at 600°C, in which, metal atoms are previously dispersed by citric acid in ethylene glycol solvent. The decomposition process of the precursor, crystallization, and particle sizes of CaTiO₃ have been investigated by using thermal analysis, powder X-ray diffraction and transmission electron microscopy. Diffuse reflectance spectra, photoluminescence and decay curve indicate that a strong red emission located at the nearly NTCS ideal red site is deduced from the energy transfer from the band gap absorption to doping Pr³⁺ ions. The thermoluminescence curves exhibit that a potential long phosphorescent material based on Pr³⁺-doped CaTiO₃ will be explored in future.

Full text of DOI:10.1016/S0022-4596(03)00175-0

ARTICLE IN PRESS
Journal of Solid State Chemistry 174 (2003) 69–73
Synthesis and red luminescence of Pr3+-doped CaTiO3 nano-
phosphor from polymer precursor
Yuexiao Pan,a Qiang Su,a Huifang Xu,b Tianhu Chen,b Weikun Ge,c Chunlei Yang,c
and Mingmei Wua,b,ꢀ
a State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering,
Sun Yat-Sen (Zhongshan) University, 135 Xingang Rd. W, Guangzhou 510275, People’s Republic of China
b Department of Earth and Planetary Science, University of New Mexico, 200 Yale Blvd., Albuquerque, NM 87131, USA
c Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
Received 11 December 2002; received in revised form 14 March 2003; accepted 15 March 2003
Abstract
Uniform and sphere-like nanoparticles of crystalline Pr3+-doped CaTiO3 have been prepared from complex polymer precursor at
600ꢁC, in which, metal atoms are previously dispersed by citric acid in ethylene glycol solvent. The decomposition process of the
precursor, crystallization, and particle sizes of CaTiO3 have been investigated by using thermal analysis, powder X-ray diffraction
and transmission electron microscopy. Diffuse reflectance spectra, photoluminescence and decay curve indicate that a strong red
emission located at the nearly NTCS ‘‘ideal red’’ site is deduced from the energy transfer from the band gap absorption to doping
Pr3+ ions. The thermoluminescence curves exhibit that a potential long phosphorescent material based on Pr3+-doped CaTiO3 will
be explored in future.
r 2003 Elsevier Science (USA). All rights reserved.
Keywords: Nanophosphor; CaTiO3; Sol–gel chemistry; Red luminescence
1. Introduction
tonic applications. Lanthanide-doped nanocrystalline
oxides with particle diameters of 100 nm or less have
been drawing particular interest recently [6]. Addition-
ally, monodispersed, small and spherical particles
are required for their application in FED [7] while
coarse particles normally produce much more
light scattering and increase the consumption of
phosphors [8].
To obtain desired nanosized oxides and/or rare earth
ions doped phosphors, wet chemistry such as copreci-
pitation [9], hydrothermal synthesis [10], colloidal
chemistry [11–13] at relatively low temperature, has
been used extensively, as starting materials in these
methods can be mixed at molecular-level and the
temperature for forming nanostructured products is
relatively low. Citric acid can act as chelate ligands to
metal ions to yield a rigid polyester network in which
metal atoms are dispersed uniformly. Thus, the hydro-
lysis of the metal atoms can be inhibited and chemical
processing can be controlled [14]. Using citric acid as a
chelating agent has distinct advantages to preparing
pure complex oxides of uniform nanoparticles at a
relatively low temperature [2,15].
Recently, oxide phosphors have been found to be
optimal candidate in field emission display (FED) and
plasma display panel (PDP) devices as they are
sufficiently conductive to release electric charges stored
on the phosphor particle surfaces. Because oxides are
resistant to high-density electron irradiation, the lumi-
nescent efficiency and thermal stability can be main-
tained under prolonged Coulomb loading. Much work
has been concentrated on the novel phosphors based on
Pr3+-doped perovskite titanate [1–5], which are gener-
ally characterized with one strong and sharp emission
peak at about 610 nm. In general, the doping of Pr3+
will increase the n-type conductivity of a titanate.
Nanoparticles, on the other hand, have been recognized
to hold tremendous potential in the area of pho-
ꢀCorresponding author. State Key Laboratory of Optoelectronic
Materials and Technologies, School of Chemistry and Chemical
Engineering, Sun Yat-Sen (Zhongshan) University, 135 Xingang Rd.
W, Guangzhou 510275, People’s Republic of China. Fax: +86-20-
84111038.
E-mail address: ceswmm@zsu.edu.cn (M. Wu).
0022-4596/03/$ - see front matter r 2003 Elsevier Science (USA). All rights reserved.
doi:10.1016/S0022-4596(03)00175-0

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70
CaTiO3 is a chemically and thermally stable complex
obtained after calcinations at a temperature of/or over
600ꢁC for 5 h or a longer period in air.
oxide, and has been widely used in electronic devices as a
dielectric material. Pr3+-doped CaTiO3, as a relatively
new luminescence material, exhibits red cathodlumines-
cence located at x ¼ 0:680 and y ¼ 0:311 in the CIE
coordinate [4]. The CIE coordinate is very close to that
of the NTSC ‘‘ideal red’’ [3]. In addition, it is a potential
red long persistent phosphor, which will probably
substitute conventional sulfides in applications. New
and excellent red long decay phosphors are becoming
more and more important in science and technology,
especially for white light, since blue and green long
decay phosphors have been more developed than the red
one. Up to the present, much work has been focused on
this novel red phosphor, which has potential application
in optoelectronic industry [4,5].
Besides lower temperature processing to prepare
dispersed and uniform nanocrystalline particles, it is
necessary to protect both Pr3+ from oxidation and
Ti(IV) from reduction spontaneously during calcina-
tions, which can be achieved by the combustion of
organic components in air flow that provide atmosphere
with suitable strength of reduction–oxidation. In this
paper, we present the preparation and luminescence of
ultrafine CaTiO3:Pr, a red nanophosphor formed from a
citric acid chelating precursor.
2.2. Characterization
Thermogravimetric (TG) analysis of the precursor
was carried out in air flow on a NETZSCH TG 209
TGA instrument from ambient temperature to 930ꢁC
with a heating rate of 10ꢁC/min. Products were
characterized by powder X-ray diffraction (XRD)
collected on a Philips Model PW1830 diffractometer
with a graphite monochromator and CuKa radiation
(l ¼ 0:1541 nm). Particle sizes and shapes were observed
by transmission electron microscopy (TEM) on JEM-
1010 electron microscope under an accelerated voltage
of 100 kV. A sample for TEM examination was
prepared by depositing an ultrasonically dispersed
suspension of powder from a mixed solution of alcohol
and water on a carbon-coated copper grid. Diffuse
reflectance spectra were taken on a Cary 100 spectro-
photometer. Photoluminescence spectra were recorded
using a SPEX Fluorolog-2 spectrofluorometer, while
long afterglow emissions were recorded on a photo-
multiplier of SPEX Fluorolog-Spectrofluorometer after
irradiation by a UVP Standard mercury lamp peaking at
362 nm with a power density of 3300 lux for 10 min. For
the measurement of decay time, the photoluminescence
signal detected by the photomultiplier tube was analyzed
on a sr400 photon counter. The xeon lamp dispersed by
a spex1681 spectrometer was used as the excitation light
source. The resolution of the system was about 5 ns.
Thermoluminescence (TL) glow curves at a temperature
range from 20ꢁC to 500ꢁC at a heating rate of 2ꢁC/s
were detected on FJ-427A TL-meter of Beijing Nuclear
Instrument Factory. All measurements except TL
spectra were carried out at room temperature.
2. Experimental
2.1. Preparation
Pr6O11 was dissolved in HNO3 and an apple green
Pr(NO3)3 solution was formed. Excess HNO3 was
removed by evaporation in fume cupboard. n-Titanium
butoxide Ti(OC4H9)4 was added to a mixed solvent of
citric acid and ethylene glycol (EG) under a constantly
magnetic stirring, until the solvent became transparent.
Ca(NO3)2 solution was prepared by dissolving CaCO3 in
HNO3 and excess HNO3 was removed by evaporation in
fume cupboard. The mixed solution of Ca(NO3)2 and
Pr(NO3)3 with a molar ratio of 1000 CaO:2.0 PrO2, was
added dropwise to the above titanium organic solution
slowly to avoid titanium butoxide hydrolyzing, until the
molar ratio of calcium to titanium was up to unity. By
heating at 200ꢁC for 5 h, the solution color changed
from pale yellow to bright yellow, and then brown.
When the excess solvent was removed, the solution
became more viscous but without any visible precipita-
tion or turbidity. At last, dark-brown glassy resin-like
substance was obtained. After further firing at 400ꢁC for
2 h, a dry dark solid mass was obtained. After it was
cooled to room temperature, it was ground slightly into
powder with agate mortar. The black powder was called
as precursor in this work. Ultrafine white powders were
3. Results and discussion
3.1. Synthesis of CaTiO3 nanophosphor
The weight loss of the black precursor can be divided
into three stages according to the TG-DTG curves as
shown in Fig. 1. The weight loss up to 100ꢁC is mainly
attributed to the adsorbed moisture from air. The last
two stages with major weight loss up to 600ꢁC result
from the decomposition and evaporation of complex
network and organic molecules. Although the organic
components can be decomposed completely only at
600ꢁC, CaTiO3 have crystallized at 500ꢁC (Fig. 2a). As
illustrated in Fig. 2, phase-pure and crystalline CaTiO3
particles are prepared at 600ꢁC, which are further
characterized by TEM. Crystalline sizes calculated
from full-width at half-maximum (FWHM) by using
Scherrer’s equation are around 10 nm, which is in

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100
90
80
70
60
50
DTG
TG
0
200
400
600
800
1000
Temperature ( oC)
Fig. 1. Thermogravimeric (TG) curve (solid line) and differential
thermogravimeric (DTG) curve (dotted line) of the polymeric
precursor powder.
Fig. 3. TEM images of products prepared at 600ꢁC for 3 h (a), 5 h (b)
and 8 h (c); at 700ꢁC (d), 800ꢁC (e), and 900ꢁC (f) for 5 h. The insert on
the top-right of the photo (a) is a high-resolution TEM of a CaTiO3
nanograin with clear lattice fringes indicating its good crystallinity.
600 oC 5 h
100
600 oC 3 h
500 oC 5 h
undoped CaTiO3
CaTiO3:Pr
80
60
40
20
20
30
40
50
60
70
(degrees)
2 Theta
Fig. 2. Powder XRD patterns of CaTiO3 obtained at 500ꢁC for 5 h (a),
600ꢁC for 3 h (b), and 600ꢁC for 5 h (c).
400
500
600
700
Wavelength (nm)
0
200
300
400
500
600
700
800
accordance with the results supplied by TEM observa-
tion (Fig. 3a). TEM photos in Fig. 3 indicate that much
larger CaTiO3 nanograins are produced at higher
temperature. For example, CaTiO3 particles of about
60 nm are obtained at 900ꢁC for 5 h. At 600ꢁC for 8 h,
the particle size remains smaller than 15 nm, which is a
higher reaction temperature significant for promoting
the growth of CaTiO3 particles (Fig. 3).
Wavelength (nm)
Fig. 4. Diffuse reflectance spectra of undoped and Pr3+-doped
CaTiO3; Inset: enlarged difference of the two curves.
and its luminescent properties are further described as
follows.
Here, phase-pure crystalline CaTiO3 nanograins can
be produced at 600ꢁC, a much lower temperature than
normal solid-state reaction to produce phase-pure
CaTiO3. This is because the citric acid can chelate metal
atoms and these metal atoms disperse at molecular level,
showing high activity as the bonds between metal atoms
and ligands are broken. Moreover, due to the metal
atoms separated by organic molecules being present in
the network, uniform and dispersed nanocrystalline
CaTiO3 can be obtained through calcinations. Based on
this concept, red nanophosphor of Pr3+-doped CaTiO3
is successfully prepared. The presence of Pr3+ in CaTiO3
3.2. Luminescence
As shown in Fig. 4, the first derivative of the
reflectance curve gives a peak at around 350 nm, which
indicates the band gap of CaTiO3 is around 3.55 eV. A
trace doping of Pr3+ demonstrates a weak absorption
shoulder maximum at about 390 nm, which is attributed
to 4f 25d transitions of Pr3+ ions. Without doubt, the
three weak absorption peaks in the range 450–500 nm
are due to the transitions of Pr3+ from ground state to
3
excited state PJ (J ¼ 0; 1, 2), while the weak peak at

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72
X6
(a)
1000
(b)
(c)
0
50
100
150
200
250
Decay time (µs)
200
300
400
500
600
700
Fig. 7. Decay curve of the phosphorescence at 612 nm in CaTiO3:
Pr3+(lem ¼ 362 nm).
Wavelength (nm)
Fig. 5. Photoluminescence spectra of Pr3+-doped CaTiO3: excitation
spectrum (a) and its enlarged one (b) (lem ¼ 612 nm), and emission
spectrum (c) (lex ¼ 340 nm).
Sintering temperature (oC)
which the emission is the most intense [16]. However, it is
difficult for us to prepare nearly white Pr3+-doped
CaTiO3 powders with microsized grains while maintain-
ing the control of valence states of both Pr and Ti at a
much higher temperature than 900ꢁC, especially for a
longer reaction time. This is because Pr3+ will be readily
oxidized to Pr4+ by sintering the sample in air at high
temperature for a long time, whereas Ti4+ will be reduced
to Ti3+ in reductive atmosphere and the powder will
become gray. The suitable reductive atmosphere in our
work is produced by combustion of citrate at tempera-
tures lower than 1000ꢁC.
500
600
700
800
900
(a)
160
5 h
120
80
(b)
40
600 ˚C
The CIE coordinate (x ¼ 0:6897; y ¼ 0:3013), which is
calculated from the emission spectra of our sample, is in
good agreement with NTSC ideal red [3]. The decay of
the emission 3H4 to 1D2 at 612 nm has two different
speeds as shown in Fig. 7. The decay curve can be well
fitted by double exponential equation: IðtÞ ¼
A expðÀt=t1Þ þ B expðÀt=t2Þ; where I is the lumines-
cence intensity, A and B are constants, t is the time, t1
and t2 are the decay time for the exponential compo-
nents, respectively. The t1 and t2 extracted from the fit
are 24 and 230 ms, respectively. The rise time in the origin
of the decay curve is attributed to the energy transfer.
In addition, long lasting phosphorescence is observed
in our sample. In order to elucidate the doping of Pr3+
in CaTiO3, TL glow curve of the phosphor above room
temperature is supplied in Fig. 8. The lonely TL peak at
around 70ꢁC offers us a hint that the traps induced by
Pr3+ ions are all the same in a suitable depth. That is a
Pr3+ ion substitutes a Ca2+ ion to form a [PrCda] defect,
improving n-typed conductivity. The TL peak at around
70ꢁC also shows probably long lasting phosphorescence.
For measurement of long lasting property, a UVP
standard mercury lamp peaking at 362 nm with a power
density of 3300 lux is used to irradiate the products for
10 min and then the UVP is switched off. After 10 s, the
2.0
8.0
10.0
4.0 Reacti6o.n0 time (h)
Fig. 6. Photoluminescence intensity varied with sintering temperature
(a) and reaction time (b).
about 600 nm is assigned to the transition of Pr3+ from
3H4 to D2 (Fig. 4 inset).
1
Due to the band gap of CaTiO3 host, a strong
excitation band in the range of 250 and 350 nm is
observed in Pr3+-doped CaTiO3 (Fig. 5a). These much
weaker peaks at 457, 472 and 497 nm in the enlarged
curve (Fig. 5b) are assigned to the transition of Pr3+
3
3
3
3
from H4 to P2, P1 and P0, respectively. Significantly,
1
the typical D2–3H4 transition of Pr3+ in CaTiO3 host
provides us strong and sharp emission at 612 nm. The
unique red emission in the visible region from CaTiO3
nanocrystals may offer us an opportunity to exploit
novel PDP, FED and nanodevices.
The increasing intensity of nanocrystalline Pr3+-doped
CaTiO3 with either reaction temperature or time in Fig. 6
implies that larger nanocrystallites can enhance the red
emission. It has been reported that there is a critical
nanocrystal size of Eu3+-doped YVO4 phosphor, at

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Acknowledgments
This work was financially supported by the Provincial
Natural Science foundation of Guangdong, NNSFC,
and NSF of USA (NER02-10820 and CTS98-71292).
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