The dependence of persistent phosphorescence on annealing temperatures in CaTiO3:Pr3+ nanoparticles prepared by a coprecipitation technique

Article abstract of DOI:10.1016/j.jssc.2007.11.022

Red emitting phosphors of CaTiO₃:Pr³⁺ nanoparticles with size ranging from 6 to 95 nm have been prepared by a coprecipitation technique and structurally characterized by X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The fluorescence and phosphorescence of CaTiO₃:Pr³⁺ nanoparticles as a function of annealing temperature are investigated. It is found that fluorescence intensities monotonously increase with increasing temperature. However, a maximum in phosphorescence with the increase of annealing temperature occurs for the sample prepared at 700 °C. Based on the measurement of fluorescence emission, fluorescence excitation and reflectance spectra as well as time decay patterns of fluorescence and phosphorescence, it is demonstrated that the dependence of fluorescence and phosphorescence on annealing temperature originates from the decrease of surface defects with the increase of temperature.

Full text of DOI:10.1016/j.jssc.2007.11.022

ARTICLE IN PRESS
Journal of Solid State Chemistry 181 (2008) 393–398
www.elsevier.com/locate/jssc
The dependence of persistent phosphorescence on annealing
temperatures in CaTiO3:Pr3+ nanoparticles prepared by a
coprecipitation technique
Ã
Xianmin Zhanga,b, Jiahua Zhanga, , Xinguang Rena, Xiao-Jun Wanga,c
aKey Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,
16 Eastern South Lake Road, Changchun 130033, China
bGraduate School of Chinese Academy of Sciences, Beijing 100039, China
cDepartment of Physics, Georgia Southern University, Statesboro, GA 30460, USA
Received 9 October 2007; received in revised form 16 November 2007; accepted 18 November 2007
Available online 24 November 2007
Abstract
Red emitting phosphors of CaTiO3:Pr3+ nanoparticles with size ranging from 6 to 95 nm have been prepared by a coprecipitation
technique and structurally characterized by X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The
fluorescence and phosphorescence of CaTiO3:Pr3+ nanoparticles as a function of annealing temperature are investigated. It is found that
fluorescence intensities monotonously increase with increasing temperature. However, a maximum in phosphorescence with the increase
of annealing temperature occurs for the sample prepared at 700 1C. Based on the measurement of fluorescence emission, fluorescence
excitation and reflectance spectra as well as time decay patterns of fluorescence and phosphorescence, it is demonstrated that the
dependence of fluorescence and phosphorescence on annealing temperature originates from the decrease of surface defects with the
increase of temperature.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Photoluminescence; Phosphorescence; CaTiO3; Pr3+; Nanoparticle
1. Introduction
transitions [4–6], confinement on energy transfer [7,8],
and surface effects [9–12], have attracted great interest.
Among these properties, surface defects have attracted
intensive attention as a major factor affecting the
fluorescence efficiency of nanosized phosphors. Generally,
surface defects are regarded as a bad factor to the
nanophosphors because they quench the luminescence
efficiency of conventional phosphors [9]. Nevertheless, in
phosphorescence (light emission that persists after the
cessation of excitation) materials, defects acting as traps of
energy storage benefit phosphorescence emission [13–17].
To improve the phosphorescence intensity and persistent
time, the increase of density of traps in these materials is
needed [18]. The surface defects might act as traps that are
beneficial for the generation of phosphorescence when
they are adjusted in suitable depth. However, at present,
it is insufficient to study the effect of surface defects on
phosphorescence.
It is well known that reduction of the particle size in a
crystalline system can result in remarkable modifications of
properties differing from those of the bulk, which mainly
come from the high surface-to-volume ratio and the
quantum confinement effect of nanomaterials. Rare earth
(RE)-doped phosphors have been intensively applied in
luminescent and display devices [1,2]. In 1994, Bhargava
et al. [3] reported that the radiative transition rate of
ZnS:Mn nanocrystals increased 5-fold in comparison with
that of the bulk. Despite the fact that this result was
strongly criticized later, studies on the fluorescence (light
emission from the materials while it is irradiated by light)
properties of RE-doped nanophosphors, such as electronic
Ã
Corresponding author. Fax: +86 431 8617 6317.
E-mail address: zjiahua@public.cc.jl.cn (J. Zhang).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.11.022

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394
Pr3+-doped CaTiO3 phosphors have attracted intense
at 65 1C under vacuum. CaTiO3:Pr3+ nanoparticles were
obtained after annealing the as-prepared sample in air. The
bulk powder was synthesized by conventional solid-state
reaction at 1400 1C for 3 h in air.
The structural characterization was analyzed by X-ray
diffraction (XRD; Rigaku D/max-rA) spectra with the
interest to meet the challenge of achieving of intense red-
emitting phosphors in the field of display and lighting
technologies [19,20]. The single emission peaking at
1
610 nm originates from the intra—4f D2–3H4 transition
of Pr3+ with the chromaticity coordinates at x=0.680
and y=0.311, which is very close to the coordinates
of the ‘ideal red’ [21]. Moreover, as an oxide phosphor,
CaTiO3:Pr3+ has better physical and chemical stabilities
than that of the conventional sulfide phosphors [22].
To investigate the optical properties of nanosized
CaTiO3:Pr3+ phosphors or thin films, many preparation
methods have been used, such as sol–gel process [23–26],
radio frequency sputtering [27], and spray pyrolysis [28].
To the best of knowledge, no studies have been reported on
the preparation of CaTiO3:Pr3+ nanoparticles using a
coprecipitation technique. In previous studies, the empha-
sis of the investigation was on the fluorescence property. It
is found that the fluorescence intensity reduces in
CaTiO3:Pr3+ nanoparticles due to the increase of surface
defects as nonradiative transition centers [23–26]. However,
little work has been done to study the effect of surface
defects on phosphorescence, although it has been observed
[23]. Recently, we found that some surface defects acting as
traps of energy storage contribute to the phosphorescence
in CaTiO3:Pr3+ nanoparticles prepared by an improved sol
gel technique [29]. It is significant to study the effect of
surface defects on fluorescence and phosphorescence, and
further manipulate them to design novel luminescent
materials.
In this paper, CaTiO3:Pr3+ nanoparticles have been
prepared by a coprecipitation technique. The temperature
dependence of fluorescence and phosphorescence in
CaTiO3:Pr3+ nanoparticles is investigated. It is found that
fluorescence intensities monotonously increase with in-
creasing temperature. However, a maximum in phosphor-
escence with the increase of annealing temperature occurs
for the sample prepared at 700 1C. The dependence of
fluorescence and phosphorescence on the decrease of
surface defects with the increase of annealing temperature
is discussed by measuring fluorescence (photoluminescence,
photoluminescence excitation) and reflectance spectra as
well as time decay patterns of fluorescence and phosphor-
escence.
˚
CuKa line of 1.540 78 A. The morphology of products was
observed by field emission scanning electron microscopy
(FESEM, Hitachi S-4800). Energy-dispersive spectroscopy
(EDS) was performed on a GENESIS2000XMS 60S
(EDAX Inc). Fluorescence emission and excitation spectra
as well as phosphorescence decay curves were measured
using a Hitachi F-4500 fluorescence spectrophotometer.
Fluorescence and phosphorescence are measured after
irradiation by 365 nm ultraviolet (UV) light for 10 min.
For lifetime measurement, the third (355 nm) harmonic of
an Nd–YAG laser is used as the excitation source, and the
signal is detected with a Tektronic digital oscilloscope
mode TDS 3052.
3. Results and discussion
3.1. Microstructure character of the CaTiO3:Pr3+
nanoparticles: XRD, SEM and EDS studies
Fig. 1 shows the XRD patterns of CaTiO3:Pr3+
nanoparticles prepared at various annealing temperatures
and a bulk material. The as-prepared sample is amorphous,
as shown in Fig. 1a. XRD patterns of nanoparticles (for
b–f) are consistent with the phase of bulk, which is
orthorhombic CaTiO3 (JCPDS Card No. 82-0228). Several
small peaks from TiO2 (n) and CaO (,) are also observed,
respectively. The XRD peak of CaTiO3:Pr3+ nanoparticles
is wider and the position shifts to the smaller angle than
that of the bulk because of size effect [30]. By applying the
CaTiO3:Pr3+
Δ TiO2
CaO
bulk
g
f
Δ
Δ
Δ
Δ
Δ
900°C
e
d
c
800°C
700°C
2. Experimental section
600°C
500°C
Ca(NO3)2 ꢀ 4H2O (99.99%) 0.03 M and PrCl3 solution
(Pr6O11 was dissolved in HCl, the concentration of PrCl3 is
2.82 mg/ml) were first dissolved in the ethanol and NH3OH
solution (pH ¼ 12). Under constant magnetic stirring,
0.03 M Ti (OC4H9)4 was added in a dropwise manner. To
reduce the hydrolytic rate of Ti (OC4H9)4, the solution was
kept around 0 1C by an ice–water mixture all the time.
After stirring for 10 min, the solution was then precipitated
for 10 h. The precipitate was collected by centrifuge,
washed several times using distilled water and then dried
b
a
as-prepared
20
40
50
60 70
30
2 Theta (deg)
Fig. 1. XRD patterns of CaTiO3:Pr3+ nanoparticles obtained at different
annealing temperature (a) as-prepared, (b) 500 1C, (c) 600 1C, (d) 700 1C,
(e) 800 1C, (f) 900 1C and (g) bulk sample. Several small peaks from TiO2
(n) and CaO (,) are also observed.

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100 nm
500 nm
Ca
O
Ti
Ti
Ca
Ti
5
1 μm
0
1
2
3
4
6
Energy (Kev)
Fig. 2. The representative SEM images of CaTiO3:Pr3+ nanoparticles for sample b, d and f. (h) EDS analysis of sample d.
Scherrer formula to the full-width at half-maximum of the
EX
EM
diffraction peaks, one can calculate the mean particle sizes
of samples b, c, d, e and f to be 6, 11, 42, 63 and 95 nm,
respectively, which is consistent with that determined by
the representative SEM images of samples b, d and f, as
shown in Fig. 2. The analysis of chemical composition of
sample d using EDS (Fig. 2d) confirms the results of XRD
on the phase composition.
750
600
450
300
150
0
CaTiO3:0.4%Pr3+
A
B
C
g
x1/3
3.2. Temperature dependence of the fluorescence
f
Fluorescence emission (EM) and excitation (EX) spectra
of CaTiO3:Pr3+ nanoparticles prepared at various anneal-
ing temperatures are presented in Fig. 3, where the curve
for bulk powders is also plotted for comparison. The EM
spectra show a red emission line at 615 nm, originating
from the intra—4f 1D2–3H4 transition of Pr3+ [21]. The EX
spectra monitoring the red emission mainly consists of
three broad bands (A–C) in the UV region, which are located
at 320 nm (A), 265nm (B) and 375nm (C), respectively. In
comparison with the spectra of CaTiO3:Pr3+ bulk powders
(dashed line), band A shifts from 330 nm in the bulk to
320 nm in the nanoparticles. The position of band A is
experimentally observed to be consistent with that of the
band edge absorption of the CaTiO3 host [21], indicating
that the blue shift of band A in the nanoparticles is the result
of size confinement effect [27]. Band B exhibiting no obvious
shift in the nanoparticles is attributed to the 4f2-4f5d
absorption of Pr3+ ions [21]. Band C originate from a
low-lying intervalence charge transfer state (Pr3+–Ti4+) [31].
The blue shift of the band-edge absorption of CaTiO3
e
d
c
b
250
300
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 3. Fluorescence excitation (EX) and emission (EM) spectra of
CaTiO3: Pr3+ nanoparticles obtained at different annealing temperature
and bulk sample. EM is detected by exciting into the strong absorption
band of the host with 320 nm for the nanoparticles and 330 nm for the
bulk.
nanoparticles in comparison with that of the bulk is also
confirmed with the diffused reflectance spectra, as shown in
Fig. 4. A strong absorption edge of the host around 320 nm

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for nanoparticles (Fig. 4b–f) and 330 nm (Fig. 4g) for bulk
powder is clearly presented, being in good agreement with
the position of the corresponding band A observed in PLE
spectra of Fig. 3. It is found that the reflectance of
nanoparticles is higher than that of the bulk powders, as
shown in Fig. 4, which is due to the decrease of internal
bulk defects in CaTiO3:Pr3+ nanoparticles. In our previous
work [22], Ca and Ti vacancies are mainly crystal defects in
bulk CaTiO3:Pr3+ phosphors, which act as nonradiation
centers reducing the fluorescence emission efficiency. The
absorption in the whole visible range (Ti vacancies) and at
around 380 nm (Ca vacancies) reduces the reflectance
(around 0.6 at 600 nm) in bulk CaTiO3:Pr3+ phosphors.
The addition of RE oxides Ln2O3 (Ln ¼ Lu, La, Gd) in
CaTiO3:Pr3+ can reduce these defects because of the
substitution for Ca2+ or Ti4+ by Ln3+. The body color of
bulk CaTiO3:Pr3+ without Ln2O3 addition is brown and
becomes nearly white when Ln2O3 is added. In this report,
the body color of CaTiO3:Pr3+ nanoparticles is whitish.
This indicates that there are nearly no Ca and Ti vacancies
in nanparticles. Therefore, we deduce that the defects in
CaTiO3:Pr3+ nanoparticles mainly come from the surface
defects rather than from the internal defects.
CaTiO3:0.4%Pr3+
g
f
6
5
4
3
2
1
0
e
d
c
To further investigate the mechanism of fluorescence
intensity dependence on annealing temperature, the life-
1
b
times of the D2 level of Pr3+ in CaTiO3 nanoparticles are
measured in room temperature, as shown in Fig. 5(b, d–f).
The decay curve can be well fitted as a function of time t by
a double exponential equation
IðtÞ ¼ A1 exp ðꢁt=t1Þ þ A2 exp ðꢁt=t2Þ,
where I is the fluorescence intensity, A1 and A2 are
the amplitudes for each component, and t1 and t2 are
the decay constants for the two components, respectively.
(1)
300
400
500
600
Wavelength (nm)
Fig. 4. Diffused reflectance spectra of CaTiO3:Pr3+ nanoparticles for
samples b–g (bulk powder).
Fig. 5. Fluorescence lifetime of Pr3+ (1D2–3H4) in CaTiO3:Pr3+ nanoparticles for samples b, d, e, and f.

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350
300
250
200
150
100
50
PL intensity
lifetime
42
36
30
24
18
12
6
0
500
600
700
800
900
Sintering temperature (°C)
Fig. 6. Dependence of the lifetimes of the 1D2 level and the fluorescence
intensities of red emission (lex ¼ 320 nm) on annealing temperature.
Fig. 7. Phosphorescent decay curves of CaTiO3:Pr3+ nanoparticles for
samples b–f after irradiation by 365 nm UV light for 10 min.
The average lifetime for Pr3+ can be determined by the
formula [32]
200
CaTiO3:0.4%Pr3+
t ¼ ðA1t21 þ A2t22Þ=ðA1t1 þ A2t2Þ.
(2)
According to Eqs. (1) and (2), the dependence of lifetimes
(t) on annealing temperature is plotted in Fig. 6. It is found
that the lifetimes become longer with the increase of
annealing temperature, showing the enhancement of
1D2–3H4 emission efficiency of Pr3+. For comparison, the
dependence of fluorescence EM intensities on annealing
temperature is also presented, showing that fluorescence
intensities increase with the increase of annealing tempera-
ture in the same scale as the increase of lifetimes of
1D2–3H4 emission efficiency. This indicates that the
fluorescence enhancement is due to the increase of the
1D2–3H4 emission efficiency of Pr3+. In nanosized phos-
phors, the numerous defects on their surfaces due to large
surface-to-volume ratio acting as nonradiative centers
reduce luminescent efficiency [23–26]. With the increase
of preparing temperature, the nonradiative centers reduce
because of the decrease of surface defects. Thus, the
1D2–3H4 emission efficiency of Pr3+ in CaTiO3 nanopar-
ticles is enhanced with the increase of annealing tempera-
ture. In this report, it is assumed that all kinds of surface
defects reduce at the same rate with the increase of
annealing temperature because they all depend on the
surface-to-volume ratio. Therefore, surface defect proper-
ties are independent of annealing temperature.
160
120
80
40
0
500
600
700
800
900
Sintering temperature (°C)
Fig. 8. Dependence of phosphorescence intensities of CaTiO3:Pr3+ on
annealing temperature.
The existence of maximum phosphorescence for sample d
was demonstrated. In view of some surface defects that can
act as traps for phosphorescent emission [29], it is deduced
that the appearance of phosphorescent maximum origi-
nates from the decrease of surface defects. The phosphor-
escence intensity is proportional to the number of traps for
energy storage and the recombination efficiency of the
released carries with luminescent centers. With the increase
of annealing temperature, the increase of particle size
reduces the number of surface defects on the particles. On
the one hand, this reduces the number of traps for energy
storage. On the other hand, the transfer efficiency from
filled trap to luminescent centers enhances because of the
reduction of nonradiative recombination centers. As a
result, the maximal phosphorescence intensity with the
3.3. Temperature dependence of the phosphorescence
Fig. 7 illustrates the time decay curves of phosphores-
cence in CaTiO3:Pr3+ after irradiation with 365 nm UV
light for 10 min. The largest integrated area under the
decay curve is experimentally observed for the sample d
prepared at 700 1C. Fig. 8 shows the dependence of
phosphorescence intensities on preparing temperature.

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increase of annealing temperature occurs at a temperature,
which is 700 1C for CaTiO3:Pr3+ nanoparticles.
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coprecipitation technique. It is found that the fluorescence
intensities increase with the increase of annealing tempera-
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1D2–3H4 emission efficiency. This indicates that the
fluorescence enhancement is due to the increase of the
1D2–3H4 emission efficiency of Pr3+. With the increase of
annealing temperature, the reduction of surface to volume
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1
recombination centers and increases the D2–3H4 emission
efficiency. The phosphorescence of CaTiO3:Pr3+ nanopar-
ticles depends on the surface defects. With the increase of
preparation temperature, the reduction of trap numbers
and the enhancement of the recombination efficiency of
released carries lead to the appearance of the maximal
phosphorescence for the sample prepared at 700 1C. The
work promotes our knowledge about the effect of surface
defects on the fluorescence and phosphorescence of
nanophosphors.
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This work is financially supported by the MOST of China
(2006CB601104, 2006AA03A138) and the National Natural
Science Foundation of China (10574128, 10774141).
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