Luminescent properties of Bi3+-activated Ca2Sb2O7 nano-phosphor prepared by co-precipitation method

Article abstract of DOI:10.1016/j.jallcom.2015.09.047

A series of Ca₂Sb₂O₇:xBi³⁺ (x = 0, 0.5, 1, 1.5, 2, 4 and 8 mol%) blue phosphors were synthesized using a chemical co-precipitation technique with further calcination. The X-ray diffraction (XRD) and scanning electron microscope (SEM) results revealed that the obtained Ca₂Sb₂O₇:xBi samples are orthorhombic weberite structure with particle size ranging from 20 nm to 40 nm. X-ray photoelectron spectroscopy (XPS) confirmed that the introduction of bismuth into the Ca₂Sb₂O₇ crystal was in the form of Bi³⁺ and no other chemical states were found. The Bi-doped Ca₂Sb₂O₇ powders exhibit a broad emission band centered at 438 nm upon UV-irradiation and it is characterized by double-luminescence centers. The dependence of the luminescent intensity on the Bi-doping concentration in Ca₂Sb₂O₇ was studied in detail. As a result, the optimized concentration of Bi³⁺ is 1.5 mol% for the highest emission intensity peaking at 438 nm, and the concentration quenching occurs when the concentration of Bi³⁺ exceeds 1.5 mol%. The luminescence mechanism of Bi³⁺ in Ca₂Sb₂O₇ was also discussed. The favorable luminescent properties of Ca₂Sb₂O₇:Bi³⁺ make it promising for lighting and display applications.

Full text of DOI:10.1016/j.jallcom.2015.09.047

Journal of Alloys and Compounds 653 (2015) 345e350
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Luminescent properties of Bi3þ-activated Ca2Sb2O7 nano-phosphor
prepared by co-precipitation method
Shiyue Yao, Xuan Zhou, Yanling Huang, Zuoshan Wang, Yumei Long**, Weifeng Li*
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
a r t i c l e i n f o
a b s t r a c t
A series of Ca2Sb2O7:xBi3þ (x ¼ 0, 0.5, 1, 1.5, 2, 4 and 8 mol%) blue phosphors were synthesized using a
chemical co-precipitation technique with further calcination. The X-ray diffraction (XRD) and scanning
electron microscope (SEM) results revealed that the obtained Ca2Sb2O7:xBi samples are orthorhombic
weberite structure with particle size ranging from 20 nm to 40 nm. X-ray photoelectron spectroscopy
(XPS) confirmed that the introduction of bismuth into the Ca2Sb2O7 crystal was in the form of Bi3þ and
no other chemical states were found. The Bi-doped Ca2Sb2O7 powders exhibit a broad emission band
centered at 438 nm upon UV-irradiation and it is characterized by double-luminescence centers. The
dependence of the luminescent intensity on the Bi-doping concentration in Ca2Sb2O7 was studied in
detail. As a result, the optimized concentration of Bi3þ is 1.5 mol% for the highest emission intensity
peaking at 438 nm, and the concentration quenching occurs when the concentration of Bi3þ exceeds
1.5 mol%. The luminescence mechanism of Bi3þ in Ca2Sb2O7 was also discussed. The favorable lumi-
nescent properties of Ca2Sb2O7:Bi3þ make it promising for lighting and display applications.
© 2015 Elsevier B.V. All rights reserved.
Article history:
Received 15 December 2014
Accepted 6 September 2015
Available online 9 September 2015
Keywords:
Ca2Sb2O7:Bi3þ
Luminescence
Nanoparticle
Co-precipitation
1. Introduction
states are 3P0, 3P1, 3P2 and 1P1 in order of increasing energy [9,10].
Although electron transitions between 1S0 level and 3PJ levels are
In the past years, great efforts have been dedicated to search for
new materials and synthesis techniques to improve the properties
of phosphors including but not limited to high luminous efficiency,
energy saving, environment-friendliness, small volume, and long
persistence [1], which can provide improvements in lighting and
display systems. Most studies have focused on rare earth and
transition metal ions. As a matter of fact, some main group metal
theoretically spin forbidden, 1S0 / 3P1 transition become partly
allowed due to spineorbit coupling. Because the energy level of 3P1
excited state is lower than that of 1P1 excited state, the emission
spectrum of Bi3þ ion usually show a single broad band owing to the
3P1 / 1S0 transition [11]. Moreover, the emission band of Bi3þ ion
can appear in the ultraviolet, blue, or even red wavelength region,
which is strongly dependent on the composition and crystal
structure of the host lattice. It is therefore interesting that the
emission properties of Bi3þ ion can be modulated by choosing
appropriate matrixes.
Ca2Sb2O7, one of ternary antimonates, belongs to the large
family of p-block semiconductors. Ca2Sb2O7 adopt weberite-type
structure with Ca and Sb cations forming a distorted, face-
centered cubic array [12]. This structure is relatively compact.
Furthermore, the Ca2Sb2O7 compound is optical transparent (band
gap: 4.02 eV) [13] and chemically stable. Therefore, the Ca2Sb2O7
could be a favorable luminescence host material and the lumines-
cence property of Sm3þ-doped Ca2Sb2O7 has recently been re-
ported [14].
ions with ns2 electronic configuration including Bi3þ, Pb2þ, Sn2þ
,
Sb3þ and Tlþ are also excellent luminescence activators, and inor-
ganic materials containing such metal ions have been found wide
applications [2,3]. For example, Bi4Ge3O12 is a well-known scintil-
lation material used in medical X-ray imaging [4].
Trivalent bismuth ions with 6s2 outer electronic configuration
exhibit interesting optical properties in many host material [5e8],
and its luminescence is attributed to the electron transition be-
tween the 6s2 ground states and the 6s16p1 excited states. The
ground state of the free Bi3þ ion is 1S0 while the 6s16p1 excited
* Corresponding author.
** Corresponding author.
E-mail addresses: Yumeilong@suda.edu.cn (Y. Long), liweifeng@suda.edu.cn
(W. Li).
Most antimonites reported were synthesized by solidestate
reaction, which requires an elevated temperature and prolonged
calcination time. However, the luminescence quenching of Bi3þ at
http://dx.doi.org/10.1016/j.jallcom.2015.09.047
0925-8388/© 2015 Elsevier B.V. All rights reserved.

346
S. Yao et al. / Journal of Alloys and Compounds 653 (2015) 345e350
temperature room has been observed when the samples were
synthesized using solidestate reaction method, which is probably
ascribed to the volatility of Bi2O3 and the tendency of Bi3þ to be
oxidized to Bi5þ in the conventional sintering process [15e17]. In
this paper, Ca2Sb2O7:xBi3þ phosphors were prepared by a modified
chemical co-precipitation method, which is one of the most effec-
tive techniques for achieving ultrafine, pure and single-phase
powders and can proceed at moderate temperatures resulting
from enhanced reaction kinetics [18e20]. Finally, we systematically
investigated the photoluminescence properties of the
Ca2Sb2O7:xBi3þ samples and the luminescent mechanism was also
studied.
2. Experimental
2.1. Synthesis of Ca2Sb2O7:xBi nanoparticles
Bi2O3, Sb2O3, CaCl2ꢀ2H2O, NaOH and Na2C2O4 were obtained
from Sinopharm Chemical Reagent Co. Ltd. All the reagents were of
analytical grade and used without further purification. Deionized
water was used throughout the experiment.
Fig. 1. XRD patterns of Ca2Sb2O7:xBi3þ (x ¼ 0, 0.5, 1, 1.5, 2, 4 and 8 mol%) as well as
JCPDS card No. 26e0293 for Ca2Sb2O7. Inset is the amplification of XRD patterns.
The Ca2Sb2O7:xBi precursors were prepared by a chemical co-
precipitation method according to previous work [21]. In brief,
the required amounts of Bi2O3 and Sb2O3 were dissolved in dilute
hydrochloric acid to obtain BiCl3 and SbCl3 solutions, respectively.
Meanwhile, CaCl2 solution was prepared by dissolving CaCl2ꢀ2H2O
in deionized water. Then, BiCl3, SbCl3 and CaCl2 solutions were
mixed together with certain volume ratio according to atom ratio of
Bi:Ca:Sb ¼ x:(2ꢁ1.5x):2 (x ¼ 0, 0.005, 0.01, 0.015, 0.02, 0.04 and
0.08) and formed a clear and homogeneous solution. The mixture
solution was added dropwise into 120 mL of NaOHeNa2C2O4 so-
lution with continuous stirring, and the white precipitate was ob-
tained. The precipitate was washed with dilute ammonia and
anhydrous alcohol for several times, and dried at 80 ꢂC for 10 h.
Finally, the obtained precursor was calcined at 1000ꢂCin air for 4 h.
indexed by weberite structure (JCPDF No. 26e0293). The results
reveal that all the obtained samples are single phase and doping of
Bi3þ at the investigated concentrations does not change the crystal
structure or induce a new phase. However, the diffraction peaks
shift to smaller two theta angles by Bi-doping (inset of Fig.1), which
is in agreement with the qualitative analysis of Bragg's law:
nl ¼ 2dsinq
(1)
In which n,
l, d, and
q
are an integer, wavelength of incident X-
rays (here is 1.5406 Å), distance of lattice planes and Bragg angle,
respectively. When smaller radius of Ca2þ (1.12 nm) is replaced by
large radius of Bi3þ (1.17 nm), the inter-atomic spacing increases
and the lattice expands. As a result, the Bragg angle decreases. This
phenomenon also confirms that Bi3þ ions successfully enter the
crystal lattice of Ca2Sb2O7.
SEM imagines of pure Ca2Sb2O7 and Ca2Sb2O7:0.015Bi samples
are displayed in Fig. 2a and b, respectively. As can be seen, both of
them consist of nearly spherical particles with a particle size in the
range of 20e40 nm. There is little difference in the morphologies
and particle shape between pure Ca2Sb2O7 and Ca2Sb2O7:0.015Bi
samples, indicative of little effect of Bi-doping on the morphology
of Ca2Sb2O7. However, slight aggregation of these particles can be
observed, which might result from the ultrafine effect of the sam-
ples and/or limit of synthetic method.
Fig. 3 shows the XPS spectrum of the Ca2Sb2O7:0.015Bi. From the
whole XPS survey spectrum (Fig. 3a), the Ca2Sb2O7:0.015Bi sample
contains C, Ca, Sb, Bi and O elements, in which the carbon (C 1s:
285 eV) is ascribed to remaining carbonaceous species formed
during the sample preparation and is used for calibration as a
reference. The binding energies of Sb 3d core level in Ca2S-
b2O7:0.015Bi sample are illustrated in Fig. 3b. Two main peaks are
observed at 539.9 eV and 530.5 eV, corresponding to Sb(V) 3d3/2
and 3d5/2 levels, respectively, with a doublet spitting of 9.4 eV,
which matches the typical Sb(V) 3d spineorbit [22]. Furthermore,
the 530.5 eV peak can be fitted into two sub-peaks (Fig. 3c), one
peaking at 530.5 eV and the other centering at 532.1 eV. The latter is
attributed to O 1s binding energy due to the overlapping between
Sb(V) 3d5/2 and O 1s. One should note that no other antimony
species contribution has been found. Fig. 3d gives the high reso-
lution XPS spectra of the Bi 4f region. XPS signals of the Bi 4f can be
found at binding energies of 164.6 eV (4f5/2) and 159.3 eV (4f7/2),
2.2. Characterization
Crystal structure of the Ca2Sb2O7:xBi samples was characterized
by powder X-ray diffractometry on a X'Pert Pro MPD diffractometer
(Pananalytical, Holland) equipped with a Cu Ka radiation source
(l
¼ 1.5406 Å). Morphology of the products was investigated by
scanning electron microscope (SEM, Hitachi S-4600, Japan). The
chemical state identification of the constituent elements was
analyzed by X-ray photoelectron spectroscopy (XPS). XPS spectra
were recorded on an Axis Ultra HAS X-ray photoelectron spec-
trometer using Al Ka radiation with a resolution of 0.68 eV.
The photoluminescence (PL) spectra were obtained on an F-
2500 fluorescence spectrometer (Hitachi, Japan) at room temper-
ature. The diffuse reflectance spectra (DRS) of samples were ob-
tained on a UV-3150 spectrophotometer (Shimadzu, Japan) using
BaSO4 as
a reference standard in the wavelength range of
200e800 nm. The third harmonic (355 nm) of a pulsed Nd:YAG
laser (Spectron Laser Sys. SL802G) was employed to examine the
luminescence decay and decay profiles were recorded with a
LeCloy 9301 digital storage oscilloscope in which the signal was fed
from PMT.
3. Results and discussion
3.1. Characterization
Fig. 1 shows the XRD patterns of the pure and Bi3þ-doped
Ca2Sb2O7 at different concentrations. It can be clearly seen that all
samples have the similar patterns and all diffraction peaks are well

S. Yao et al. / Journal of Alloys and Compounds 653 (2015) 345e350
347
Fig. 4 gives the absorption spectra of samples derived from the
KubelkaeMunk method. The optical gap band of pure Ca2Sb2O7
was estimated to be 285 nm (i.e. 4.3 eV) and a broad absorption
band extending from 265 nm to 355 nm can also be observed from
the absorption spectra of the Ca2Sb2O7:0.015Bi sample.
Typical excitation spectra of Ca2Sb2O7:xBi3þ (0 ꢃ x ꢃ 0.08) by
monitoring the emission wavelength at 438 nm are presented in
Fig. 5. No excitation band can be found for the pure Ca2Sb2O7.
However, there exist two broad excitation bands for Bi3þ-doped
Ca2Sb2O7 samples, peaking at 337 nm and 274 nm, respectively.
Both the excitation bands have the same trends with increasing
Bi3þ concentration, in which the optimal Bi3þ concentration is
1.5 mol%. Obviously, these excitation bands should be attributed to
the Bi3þ-related absorption due to the absence of them in the pure
Ca2Sb2O7 sample, which is in agreement with the observed DRS
(Fig. 4). The excitation band at 274 nm is readily assigned to the host
lattice excitation because it is in good agreement with the band gap
of Ca2Sb2O7. The appearance of the excitation band of the host
lattice in the excitation spectrum of Bi3þ indicates the occurrence of
energy transfer from host lattice to Bi3þ ions. On the other hand, the
dominant excitation band peaking at 337 nm could be assigned to
1S0/3P1 transition of Bi3þ [3]. It is worth noting that the high
absorption of Ca2Sb2O7:Bi3þ in the spectral region of 290e390 nm
perfectly matches with the radiation light from the InGaN-based
LED. It is therefore expected that the Ca2Sb2O7:Bi3þ may be po-
tential for white LED application.
The photoluminescence properties of the Ca2Sb2O7:Bi3þ sam-
ples are investigated under the ultraviolet irradiation at 337 nm
and the results are shown in Fig. 6a. The pure Ca2Sb2O7 sample
shows no photoluminescence. After incorporating Bi3þ in Ca2Sb2O7
crystal, a strong photoluminescence band extending from 380 nm
to 520 nm can be clearly observed. The observed photo-
luminescence spectra are very similar to those reported for Bi-
doped materials [21,25,26]. Thus, the broad emissions of the
Ca2Sb2O7:xBi3þ are essentially assigned to 3P1/1S0 transition of
the Bi3þ ion. It is worthwhile to note that the emission band is not
symmetrical and it can be defined as a superposition of at least two
Gaussian components with approximate peaking wavelengths:
P1 ¼ 432 nm, P2 ¼ 456 nm, as shown in the inset of Fig. 6a. They
should correspond to different luminescence center. In addition,
both the emission bands have the same trends with various Bi3þ
concentrations, displayed in the Fig. 6b. As the Bi-doping concen-
tration increases from 0.5 mol% to 1.5 mol%, the emission band
notably enhances. However, the further increase of the Bi3þ con-
centration (2.0, 4.0 and 8.0 mol%) results in the decrease of emis-
sion intensity due to the concentration quenching effect.
Fig. 2. SEM images of (a) pure Ca2Sb2O7 and (b) Ca2Sb2O7:0.015Bi3þ samples.
with a doublet splitting of 5.3 eV, consistent with the Bi 4f
spineorbit [23]. The XPS results evidence the presence of only Bi3þ
ions [24], indicating the co-precipitation method can effectively
inhibit the oxidation of Bi3þ ions to higher chemical valence, such
as Bi5þ
.
3.2. Optical properties
The DRS of Ca2Sb2O7 and Ca2Sb2O7:0.015Bi3þ samples are dis-
played in Fig. 4. Both undoped and Bi3þ-doped samples exhibit
sharp drop in reflection spectra around 300 nm, corresponding to
the valence-to-conduction band transitions of the Ca2Sb2O7 host.
There is intense reflection in the visible spectral range, which
agrees with the observed white daylight color for the Ca2Sb2O7
matrix. It is thereby expected that Ca2Sb2O7:Bi could achieve high
luminescent efficiency owing to the high reflectance in emission
region. However, the introduction of Bi3þ ions causes an immediate
change in optical behavior. In order to better understand the
3.3. Luminescence mechanism
To understand the experimental data well, it is necessary to give
a short introduction of Ca2Sb2O7 crystal structure and to evaluate
the possible doping mechanism. It is believed that Bi3þ ions occupy
Ca2þ crystallographic sites considering the ionic radius and chem-
ical states (rcn¼8 ¼ 1.17 nm for Bi3þ and rcn¼8 ¼ 1.12 nm for Ca2þ).
However, there exist two types of Ca2þ ions occupying the Wyckoff
4a and 4d sites, respectively [12]. Fig. 7 shows the coordination
spheres for Ca2þ in the orthorhombic weberite structure. CaO8 (4d)
is a distorted cube coordination with two different Ca4deO bond
lengths and CaO8 (4a) group is a di-hexagonal pyramidal coordi-
nation with three different Ca4aꢁO bond lengths. The doped Bi3þ
ions are randomly distributed onto the Ca2þ (4d) and Ca2þ (4a)
sites. Since the luminescence of Bi3þ is sensitive to the coordination
polyhedra, the Ca2Sb2O7:xBi3þ exhibits two emission components
(the inset of Fig. 6a) and they have the same trends with various
Bi3þ concentrations, resulting from Bi3þ (4d) and Bi3þ (4a) centers.
thresholds for host lattice absorption and the absorption by Bi3þ
,
the absorption spectra of undoped and Bi3þ-doped Ca2Sb2O7 were
obtained using KubelkaeMunk method which is based on the
following equation:
FðRÞ ¼ ð1 ꢁ RÞ2=2R ¼ K=S
(2)
Where R is the reflectance of an extremely thick material and can be
substituted by the experimental reflectance here. K and S are the
absorption and the scattering coefficient, respectively. The inset of

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S. Yao et al. / Journal of Alloys and Compounds 653 (2015) 345e350
Fig. 3. (a) XPS survey scan of the Ca2Sb2O7:0.015Bi sample; (b) The high resolution XPS spectra of Sb 3d; (c) Deconvolution of 530.5 eV XPS peak; (d) The high resolution XPS spectra
of Bi 3d.
Fig. 4. UVevis DRS of Ca2Sb2O7 and Ca2Sb2O7:0.015Bi samples. Inset shows the ab-
Fig. 5. Excitation spectra of Ca2Sb2O7:xBi3þ (0 ꢃ x ꢃ 0.08) by monitoring the emission
sorption spectra derived from the KubelkaeMunk formula.
wavelength at 438 nm.
However, the emission spectra of the two Bi3þ centers at room
temperature overlap remarkably. The low-temperature optical
spectra would be a useful tool for the further explanation of
decay curve shows a double exponential decay and the two decay
times are estimated to be 291.4 0.9 ns and 85.6 0.5 ns (the
goodness of fit R ¼ 99.5%), respectively. This result coincides with
the overlap in the emission spectra of two bands, which can be
explained as being the consequence of having two different Ca2þ
luminescence centers in the Ca2Sb2O7:Bi3þ
.
Fig. 8 presents the room temperature luminescence decay pro-
file of the Ca2Sb2O7:0.015Bi3þ phosphor excited by 355 nm. The

S. Yao et al. / Journal of Alloys and Compounds 653 (2015) 345e350
349
Fig. 6. (a) Emission spectra of Ca2Sb2O7:xBi3þ (0 ꢃ x ꢃ 0.08) under UV excitation at 337 nm. Inset exhibits Gaussian decomposition. (b) Relative luminescence intensity excited by
337 nm light as a function of the Bi3þ-doping concentration.
Fig. 7. The oxygen atom coordinations of the two different Ca sites in Ca2Sb2O7.
Fig. 9. The CIE chromaticity diagram for Ca2Sb2O7:xBi3þ phosphors. Inset shows
photograph of Ca2Sb2O7:0.015Bi3þ excited by 365 nm.
and y ¼ 0.0446, which is close to pure blue tone (x ¼ 0.14, y ¼ 0.08).
The inset of Fig. 9 shows the photograph of Ca2Sb2O7:0.015Bi3þ
under 365 nm excitation. The blue emission can be clearly observed
by the naked eye, which is in good agreement with the photo-
luminescence properties of the phosphors.
4. Conclusion
In summary, we reported a novel blue-emitting nano-phosphor
of Bi3þ-doped Ca2Sb2O7 synthesized by a facile co-precipitation
method. All the as-synthesized samples exist in a pure ortho-
rhombic weberite structure having particle size of 20e40 nm and
their chemical states were examined using XPS. Under the excita-
tion of 337 nm, the Ca2Sb2O7:Bi3þ phosphor exhibited the intense
blue emission extending from 380 nm to 520 nm at room tem-
perature, which is ascribed to 3P1/1S0 transition of the Bi3þ ion.
The emission band of Ca2Sb2O7:Bi3þ phosphor consists of two
components, which is attributed to the two luminescence centers,
Bi3þ (4d) and Bi3þ (4a). The luminescence properties of
Ca2Sb2O7:xBi3þ are dependent on the Bi3þ concentration and con-
centration quenching occurs as the Bi3þ concentration exceeds
Fig. 8. The luminescence decay curves of Ca2Sb2O7:0.015Bi3þ under an excitation
wavelength of 355 nm at room temperature.
and thus also Bi3þ sites in the Ca2Sb2O7 crystal structure. According
to CIE (Commission International de L'Eclairage, France, 1931)
standard, the CIE chromaticity coordinates (x, y) of
Ca2Sb2O7:0.015Bi3þ phosphors were determined and shown in
Fig. 9. Its chromaticity coordinate was estimated to be x ¼ 0.1498

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S. Yao et al. / Journal of Alloys and Compounds 653 (2015) 345e350
1963.
1.5 mol%. In view of the desirable luminescence and suitable exci-
tation spectrum, Ca2Sb2O7:Bi3þ is potentially applicable as an
ultraviolet-convertible phosphor for lighting and display devices.
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