Synthesis and characterization of Y2O2S:Eu 3+, Mg2+, Ti4+ hollow nanospheres via a template-free route

Article abstract of DOI:10.1016/j.jallcom.2012.07.060

A red long-lasting phosphorescent material, monodisperse Y ₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ hollow nanospheres have been prepared successfully by a thiourea-based homogeneous precipitation combining with a postcalcining process. Self-assembly and Ostwald ripening process occured to form hollow structure of amorphous precursors. The crystalstructure and particle morphology were investigated by the X-ray diffraction (XRD) patterns, Fourier Transform Infrared (FT-IR) spectra, scanning and transmission electron microscopy (SEM and TEM) pictures, respectively. A possible growth mechanism was proposed to reveal the formation process. Luminescence properties of the Y₂O₂S:Eu ³⁺, Mg²⁺, Ti⁴⁺ hollow nanosphere compared with bulk material were analyzed by measuring the excitation spectra, emission spectra, afterglow decay curve and thermoluminescence curve. It was considered that this synthetic route may have potential applications for fabricating other lanthanide oxysulfides.

Full text of DOI:10.1016/j.jallcom.2012.07.060

Journal of Alloys and Compounds 542 (2012) 207–212
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Synthesis and characterization of Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanospheres
via a template-free route
b,
b
b
Suqing Deng a, Zhiping Xue a, Yingliang Liu b, , Bingfu Lei , Yong Xiao , Mingtao Zheng
⇑
⇑
a Department of Chemistry, Jinan University, Guangzhou 510632, People’s Republic of China
b College of Science, South China Agricultural University, Guangzhou 510642, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
A red long-lasting phosphorescent material, monodisperse Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanospheres
have been prepared successfully by a thiourea-based homogeneous precipitation combining with a post-
calcining process. Self-assembly and Ostwald ripening process occured to form hollow structure of amor-
phous precursors. The crystalstructure and particle morphology were investigated by the X-ray
diffraction (XRD) patterns, Fourier Transform Infrared (FT-IR) spectra, scanning and transmission electron
microscopy (SEM and TEM) pictures, respectively. A possible growth mechanism was proposed to reveal
the formation process. Luminescence properties of the Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanosphere com-
pared with bulk material were analyzed by measuring the excitation spectra, emission spectra, afterglow
decay curve and thermoluminescence curve. It was considered that this synthetic route may have poten-
tial applications for fabricating other lanthanide oxysulfides.
Article history:
Received 18 May 2012
Received in revised form 9 July 2012
Accepted 11 July 2012
Available online 20 July 2012
Keywords:
Yttrium oxysulfide
Hollow nanosphere
Luminescence
Rare earths
Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction
Due to the structural advantages, hollow materials have many
potential applications in the fields such as photonic device, drug
Afterglow phosphor is a kind of important optical functional
materials which can emit long-lasting phosphorescence (LLP) in
darkness after excited by the sunlight, UV (ultraviolet) lamp or
fluorescence lamp. These non-radioactive long afterglow phos-
phors are used increasingly in a range of fields such as emergency
light sources, luminous paint, road signs, billboards, graphic arts
and interior decoration. In 1996, Matsuzawa et al. firstly reported
the green and blue emitting LLP phenomenon from Eu2+-doped
alkaline earth aluminates [1]. At present, the performance of these
aluminates-based afterglow phosphors, such as SrAl2O4:Eu2+, Dy3+
(green), CaAl2O4:Eu2+, Nd3+ (blue), already meet the requirement
for practical applications [2,3]. However, the red afterglow phos-
phor with better luminescent property is difficult to achieve. From
the viewpoint of practical applications, the progress in red long-
lifetime phosphors is still very limited compared with those of
other colors. Therefore, researchers in this field are currently focus-
ing on the preparation and properties of red and orange emitting
afterglow phosphors. Yttrium oxysulfide has been known for a long
time as an excellent red phosphor host material. While doped with
Eu, Mg, Ti, a red long-lasting phosphor with the afterglow time of
above 3 h has been synthesized [4]. However, the progress on the
systemic research of Y2O2S:Eu3+, Mg2+, Ti4+ was very slow and their
luminescent mechanism was not well understood.
delivery, active-materials protection, and catalysis [5–8]. There-
fore, considerable researches have been focused on the fabrication
of these materials. Liu et al. prepared Y2O2S:Eu3+, Mg2+, Ti4+ hollow
microspheres by homogeneous precipitation using monodispersed
carbon spheres as hard templates [9]. Such hollow spheres of rare-
earth-doped phosphors can reduce the consumption of expensive
rare-earth metals in the synthesis. Additionally, owing to the low
density of hollow spherical materials, such phosphors can be well
dispersed, thereby enhancing the uniformity and packing density
of a phosphor coating [10]. Although template directing is the most
straightforward method, this strategy suffers from many disadvan-
tages including high cost and complicated procedures. Thus, one-
pot and template-free methods are more attractive and feasible
to produce hollow-structured materials [11–13].
In this paper, Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanospheres have
been prepared via a template-free route combining with a postcalc-
ining process. A possible growth mechanism was proposed to reveal
the formation process. The Y2O2S:Eu3+, Mg2+, Ti4+ hollow nano-
spheres havepromising long-lasting phosphorescencewith potential
scale-dependent applications in photonic devices. This synthetic
route is also suitable for fabricating other lanthanide oxysulfide.
2. Experimental
2.1. Preparation
⇑
Corresponding authors. Tel.: +86 20 85280323; fax: +86 20 85221813.
E-mail addresses: tliuyl@163.com (Y. Liu), tleibf@scau.edu.cn (B. Lei).
RE2O3 (RE = Y, Eu) (99.99%) were purchased from Hunan Institute of Rare Earth
and other chemicals were purchased from Guangzhou Chemical Reagent Factory.
0925-8388/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.07.060

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S. Deng et al. / Journal of Alloys and Compounds 542 (2012) 207–212
analysis demonstrates that Y2O2S:Eu3+, Mg2+, Ti4+ has hexagonal
crystal structure with the unit cell dimensions: a = 0.378 nm,
c = 0.659 nm, which are very close to the standard lattice parame-
ters. Obviously, the substitution of Y3+ with trace Eu3+, Mg2+ and
Ti4+ does not remarkably change the crystal structure and lattice
parameters of Y2O2S. No obvious diffraction peak appears in
Fig. 1e, which indicates that the precursor compound is amorphous
without calcination.
All chemicals are analytical grade reagents and used directly without further puri-
fication. Rare-earth nitrate stock solutions were prepared by dissolving the corre-
sponding metal oxide in nitric acid at elevated temperatures. In
a typical
procedure, 7.4 mL of Y(NO3)3 (0.5 M), 2 mL of Eu(NO3)3 (0.05 M), 2 mL of Mg(NO3)2
(0.05 M) and 2 mL of Ti(SO4)2 (0.05 M) were added into 50 mL of ethylene glycol
(EG), then 4.0 g of poly(vinyl pyrrolidone) (PVP K30, M = 40,000) was added into
the above solution. After vigorous stirring for 15 min, the PVP was dissolved thor-
oughly. Then 20 mL of ethanol solution containing 0.22 g of thiourea was added
dropwise into the above solution. The as-obtained solution was adjusted pH 7–8
by ammonia with the concentration of 25% (wt.%), and stirred for another 1 h. Then
the transparent feedstock was transferred to three 50 mL. Teflonlined stainless
autoclaves and heated at 200 °C for different time. After naturally cooling to
room-temperature, the precursors were separated by filtration, washed with etha-
nol and deionized water, and dried in atmosphere at 60 °C overnight.
For further insight into the chemical composition of the sam-
ples, Fig. 2 displays the FT-IR spectra of the as-prepared precursors
and the same samples calcined at 1000 °C and 1100 °C for 4 h. The
as-prepared precursors (Fig. 2a) show the absorption band at 1322
and 1621 cmꢀ1 can be attributed to the stretching mode of the ter-
tiary amine group and C@O, respectively [14]. This result indicated
the existence of PVP in the precursors. The band at 1414 cmꢀ1 is
attributed to the CAN stretching vibration in thiourea. The C@S
stretching vibration at 1084 cmꢀ1 in pure thiourea appears as a
doublet in the precursor at 725 and 609 cmꢀ1. Red-shifted from
729 cmꢀ1 in pure thiourea to 725 cmꢀ1 can be attributed to the re-
duced double-bond character of the C@S bond owing to the sulfur
bonding with the metal [15]. The absorption peak at 609 cmꢀ1 may
signify the presence of d(Y–S) mode, confirming the existence of Y–
S bond [16]. In Fig. 2b and c, the band at 1621 cmꢀ1 is attributed to
the absorption of residual CO23ꢀ after calcining [17], the peak at
1322 cmꢀ1 disappears and a new peak at 455 cmꢀ1 is assigned to
the vibration of the Y–O bond [17], indicating that PVP has been re-
moved and Y2O2S has formed after calcination, which is in good
accordance with the XRD result above. However, the precise struc-
ture of the samples cannot be obtained and needs to be probed
further.
Y2O2S:Eu3+, Mg2+, Ti4+ phosphors were prepared by a gas aided sulfur treatment
through calcining precursor using CS2 as a sulfurization agent. Sulfur powder was
put in a sealed graphite crucible, and pre-fired at 800 °C for 4 h. During the heat
treatment, sulfur reacted with graphite to form CS2, which was absorbed in the
vacancies within the graphite layer. Then, the dried precursors were placed in the
graphite crucible and calcined at desired temperatures (800–1100 °C) for 4 h.
2.2. Characterization
Powder XRD patterns were recorded on a MSAL-XD2 X-ray diffractometer with
Cu Ka radiation (36 kV, 20 mA, k = 1.54051 Å). FT-IR spectra were measured by an
Equinox 55 (Bruker) spectrometer with the KBr pellet technique ranging from
400 to 4000 cmꢀ1. The size and morphology of the samples were inspected using
a field emission scanning electron microscope (FE-SEM, JEOL JEM-7600F) equipped
with an energy dispersive X-ray (EDX) spectroscopy, transmission electron micros-
copy (TEM, Philips TECNAI 10) and JEOL-2010 transmission electron microscope at
the accelerating voltage of 200 kV. Photoluminescence (PL) excitation and emission
spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a
150 W xenon lamp as the excitation source. A long lasting phosphorescence spec-
trometer with a 1000LX standard Xe lamp was used to measure the afterglow prop-
erties, the measurements started after 15 min irradiation for each sample.
Thermoluminescence (TL) curves were recorded using a FJ-427A1 instrument (Bei-
jing Nuclear Instrument Factory) in the temperature range 30–300 °C at a heating
rate of 2 °C/s. All measurements were carried out at room temperature except for
the TL measurements.
3.2. Morphology
The microstructures of precursor collected at different intervals
are studied by the TEM images and presented in Fig. 3. It can be
seen from the micrograph that only aggregated particles with
diameter about twenty nanometers could be detected (Fig. 3a)
when the reaction was carried out for 30 min. If the reaction was
prolonged to 3 h (Fig. 3b), nanospheres with the diameter of about
130–200 nm were obtained and the spheres actually further con-
sist of small grains. The grains aggregate noncompactly and are
somewhat hollow as can be seen. This may be attributed to the re-
lease of gaseous NH3/CO2 during the decomposition process of
thiourea. As shown in Fig. 3c, hollow spheres with outer diameter
around 200 nm were formed and the shell thickness was measured
to be about 40–50 nm when the reaction was further elevated to
6 h. The growth of the spheres may be inhibited by PVP, because
precursor synthesized without PVP exhibits non-uniform distribu-
tion, and agglomerate to some extent (Fig. 3d).
3. Results and discussion
3.1. Structure of the products
Fig. 1 shows the XRD patterns of the precursor generated by
solvothermal process and Y2O2S:Eu3+, Mg2+, Ti4+ powders after
being calcined at different temperatures. The phosphor samples
calcined at either 800 or 900 °C exhibit peaks corresponding to
the Y2O3 phase with the presence of the Y2O2S phase, as illustrated
in Fig. 1a and b. The XRD lines observed in samples calcined at
1000 °C (Fig. 1c) and 1100 °C (Fig. 1d) matched those of Y2O2S
structure given in standard JCPDS files, No. 24-1424. The phase
Fig. 4 show a typical FE-SEM image and TEM images of the
phosphors annealed at 1000 °C with its EDX spectroscopy (inset
in Fig. 4a), revealing that the products consist of separated hollow
spheres with diameter of 90–150 nm. The nano-sized hollow
spheres are composed of nanoparticles with diameter of about
30 nm (Fig. 4b). Fig. 4c depicts the detailed structure of an individ-
ual hollow sphere and the inner diameter of the hollow sphere is
measured to be about 50 nm. The high-resolution transmission
electron microscopy (HRTEM) image (Fig. 4d) of the nanocrystal
composing the Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanospheres clearly
shows a lattice fringe with interplanar spacing of 0.364 nm that
corresponds to the (101) plane of Y2O2S. According to the results
of EDX and element analysis measurement, the product is com-
posed of Y, Eu, C, O, Ti and S elements. C element probably came
from conductive adhesive tape for supporting the sample. This re-
sult further confirmed the chemical composition of the final prod-
ucts to a certain degree.
Fig. 1. XRD patterns for the precursors and Y2O2S:Eu3+, Mg2+, Ti4+ nanospheres
annealed at 800 °C, 900 °C,1000 °C and 1100 °C.

S. Deng et al. / Journal of Alloys and Compounds 542 (2012) 207–212
209
Fig. 2. FT-IR spectra of the precursors and the products calcined at 1000 °C and 1100 °C.
Fig. 3. TEM images of the solvothermal precursors prepared with reaction time: (a) 30 min; (b) 3 h; (c) 6 h; (d) 6 h without PVP.
A possible growth mechanism can be schematically illustrated
in Fig. 5. Most probably, the bubbles of NH3/CO2 gas generated
by the decomposition of thiourea and PVP played as crucial roles
in the forming of the hollow spheres process. Under weak alkaline
conditions, thiourea decomposed in the reaction system and pro-
duces S2ꢀ and NH3/CO2. PVP were hydrolyzed to produce anions
which acted with Y3+, Eu3+, Mg2+ and Ti4+ ions. When the concen-
tration reached to its critical micelle concentration (CMC) in the
solution, the surfactant concentration was high enough to adsorb
on the faces of Y3+, Eu3+, Mg2+ and Ti4+ ions in all directions to en-
tail isotropic growth. At the same time, the NH3/CO2 bubbles gen-
erated from the decomposed thiourea may serve as the template to
the formation of the hollow structure [18]. Then the self-assembly
and Ostwald ripening process [19]occurs around the interface of
NH3/CO2. The precursor nanoparticles were covered with PVP,
and these molecules adsorbed on the surface of the particles
formed a protective layer to hold back the growth rate of precursor
nanoparticles, the surfactant adsorb on the faces of the particles in
all directions to entail isotropic growth, consequently stable spher-
ical precursors were obtained [16]. Finally, the precursors were an-
nealed to generate the final Y2O2S:Eu3+, Mg2+, Ti4+ hollow spheres.
The chemical reactions can be described as follows (Eqs. (1)–(6)):
NH3 ꢁ H2O ꢀ NHþ þ OHꢀ
ð1Þ
ð2Þ
ð3Þ
4
CSðNH2Þ þ 2OHꢀ ꢀ CO2 þ 2NH3 þ S2ꢀ
2
CO2 þ 2OHꢀ ꢀ CO2ꢀ þ H2O
3

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S. Deng et al. / Journal of Alloys and Compounds 542 (2012) 207–212
Fig. 4. FE-SEM image (a) and TEM images (b and c) of the phosphors annealed at 1000 °C with its EDX spectroscopy (inset in a), as well as HRTEM image (d) of the nanocrystal
composing the Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanospheres.
Fig. 5. Schematic illustration of growth process.
the excitation spectrum is a wide band with two peaks attributing
to the Eu3+–O2- CTB (charge transfer band) and Eu3+–S2ꢀ CTB. While
some weak and narrow peaks are attributed to the f–f transition of
ð4Þ
Eu3+ ions. The emission spectra of Y2O2S:Eu3+, Mg2+, Ti4+ hollow
spheres were shown in Fig. 6b. When the phosphors are excited
by 325 nm, they produce visible light with different wavelengths.
The strong red-emission lines at 615 and 625 nm are due to transi-
tion from 5D0 to 7F2 level of Eu3+ ion. Both the excitation and emis-
sion spectra are similar to those of the bulk Y2O2S:Eu3+, Mg2+, Ti4+
phosphors. Neither the Mg2+ ion nor the Ti4+ ion significantly affects
the shape of the excitation and emission spectra.
2Y3þ þ 2OHꢀ þ S2ꢀ þ CO32ꢀ ! Y2SðOHÞ CO3
ð5Þ
ð6Þ
2
Y2SðOHÞ2CO3 þ CS2 þ 2O2 ! Y2O2S þ CO2 þ SO2 þ H2O
3.3. Luminescence properties
The afterglow property of Y2O2S:Eu3+, Mg2+, Ti4+ hollow nano-
sphere is not as good as bulk material as shown in Fig. 7. For all
the samples, the decay curves cannot be fitted to a single exponen-
tial. According to Fig. 7, the luminous intensity decay’s equation
can be written as:
Fig.
6 illustrates the excitation and emission spectra of
Y2O2S:Eu3+, Mg2+, Ti4+ hollow spheres compared with bulk phos-
phors prepared by solid-state reaction. It is clear from Fig. 6a that

S. Deng et al. / Journal of Alloys and Compounds 542 (2012) 207–212
211
Fig. 6. Excitation spectrums (kem = 625 nm) and emission spectrums (kex = 325 nm) of Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanosphere and bulk material.
Table 1
Parameters of afterglow decay curves of Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanospheres
and bulk phosphors.
Samples
Nanospheres
Bulk
A1
s1 (s)
A2
2.1014
2.1055
0.4090
33.2700
0.4515
5.5406
0.2500
57.7878
s2 (s)
pears three peaks located at 70 °C, 145 °C and 186 °C, indicating
the presence of various traps with different depths in bulk
Y2O2S:Eu3+, Mg2+, Ti4+ phosphor. The depth of the trap energy level
can be calculated by the half peak width method [22]. Formula for
calculation is as follows:
2
E ¼ 2kðTmÞ =ðT2 ꢀ TmÞ
where k is the Boltzmann’s constant, Tm the temperature value cor-
responding to the peak of the thermoluminescent curve, T2 the
temperature value corresponding to the point on the right side of
the thermoluminescent curve, where the peak intensity is half of
the peak value. After calculating, the traps energy level of hollow
Fig. 7. Decay curves of Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanosphere and bulk material.
I ¼ A1 expðꢀt=s1Þ þ A2 expðꢀt=s2
Þ
Fit the above empirical equation, the values of A1,
s
1, A2 and s2 can
be obtained, as listed in Table 1. According to this equation, the
phosphor behaviors in term of a two exponential decay model, a ra-
pid decay at the beginning and the long-lasting phosphorescence
further. The afterglow mechanisms can be explained by the contri-
bution from the electron traps formed by the co-doped Mg2+ and
Ti4+ ions. When the Y2O2S:Eu3+ phosphor is co-doped with both
Mg2+ and Ti4+, the Mg2+ and Ti4+ ions occupy the same lattice sites
as Y3+ ions do [20]. The introduction of the Mg2+ and Ti4+ ions to the
Y2O2S compound causes the formation of new electronic donating
and accepting levels between the host lattice band gap. The trap-
ping of excited electrons and thermally released processes results
in the afterglow. At first it acts on the first exponential, when the
decay time is longer, the decay will act on the 2nd exponential.
The decay ratio speed-down when the decay time increases.
From the report [21], the glow peaks between 50 and 110 °C in
the thermoluminescent curve benefit the appearance of afterglow.
It is obvious that there is a peak around 58 °C, which is related to
the afterglow of Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanospheres, as is
shown in Fig. 8. The thermoluminescence curve of hollow nano-
sphere is a little different from the bulk sample synthesized by a
solid-state reaction. The shape of the thermoluminescent curve ap-
Fig. 8. Thermoluminescence curves of Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanosphere
and bulk material.

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S. Deng et al. / Journal of Alloys and Compounds 542 (2012) 207–212
nanosphere is about 0.86 eV and bulk material is about 0.93 eV,
which reveal suitable trap depth and therefore produce long lasting
afterglow for the Y2O2S:Eu3+, Mg2+, Ti4+ phosphors. This result is
good consistent with the conclusion of Fig. 7, and can be explained
by the action of additional, surface-related defects in nanosized
phosphors.
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4. Conclusions
In summary, Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanospheres have
been prepared via a template-free route combining with a post-
calcining process. Thiourea and PVP played as crucial roles in the
forming of the hollow spheres process, and the formation mecha-
nism has been proposed. While co-doped Mg2+ and Ti4+ ions, the
phosphorescence could be seen clearly with the naked eye in the
dark clearly for more than 1.5 h even after the irradiation source
had been removed. This synthetic route may be suitable for prepar-
ing other hollow-structured long-lasting phosphor. Furthermore,
the Y2O2S:Eu3+, Mg2+, Ti4+ hollow nanospheres have promising
long-lasting phosphorescence with potential scale-dependent
applications in photonic devices.
Acknowledgment
This present work was financially supported by the National
Natural Science Foundations of China
21171071).
(Nos. 21071063 and
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