Luminescence of rare earth ions activated YNb0.5M0.5O4 (M = P, V) phosphors by chemical co-precipitation composing hybrid precursors

Article abstract of DOI:10.1016/j.jallcom.2006.10.088

A novel synthesis technology was applied to fabricate YP_0.5Nb_0.5O₄:RE³⁺ and YNb_0.5V_0.5O₄:RE³⁺ phosphors. In this method, the rare earth coordination polymers with salicylic acid were used as precursors and the polyethylene glycol (PEG) was used as dispersing media. By a modified co-precipitation process, the hybrid precursors have been prepared, and then were calcinated to obtain the samples. Both SEM indicated that the particles present cobble-like crystalline state, especially YNb_0.5V_0.5O₄:RE³⁺ phosphors. Besides this, crystalline grain sizes were in the range of around 0.5-1 μm. The optical properties of all the powder presented that the characteristic transitions of rare earth ions were detected.

Full text of DOI:10.1016/j.jallcom.2006.10.088

Journal of Alloys and Compounds 448 (2008) 298–302
Luminescence of rare earth ions activated YNb0.5M0.5O4 (M = P, V)
phosphors by chemical co-precipitation composing hybrid precursors
Xiu-Zhen Xiao, Bing Yan∗
Department of Chemistry, Tongji University, Shanghai 200092, China
Received 15 June 2006; received in revised form 12 October 2006; accepted 16 October 2006
Available online 1 December 2006
Abstract
A novel synthesis technology was applied to fabricate YP0.5Nb0.5O4:RE3+ and YNb0.5V0.5O4:RE3+ phosphors. In this method, the rare earth
coordination polymers with salicylic acid were used as precursors and the polyethylene glycol (PEG) was used as dispersing media. By a modified
co-precipitation process, the hybrid precursors have been prepared, and then were calcinated to obtain the samples. Both SEM indicated that the
particles present cobble-like crystalline state, especially YNb0.5V0.5O4:RE3+ phosphors. Besides this, crystalline grain sizes were in the range of
around 0.5–1 ␮m. The optical properties of all the powder presented that the characteristic transitions of rare earth ions were detected.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Phosphors; Chemical synthesis; Luminescence
1. Introduction
silica fibers have been widely applied in optical telecommuni-
cation field due to its emission at 1.53 ␮m [14]. Bulk phosphors
were prepared by high-temperature solid-state reactions provid-
ing agglomerated powders, which can permit to easily change
the structural characteristics of the powders [15–17]. Lately,
the development of so-called wet or soft chemical technology
for the preparation of micron or submicron or nanomaterials
may increase the possibilities for high effective rare earth phos-
phors and a new sol–gel technology based on the hydrolysis and
polycondensation process is a good example, which has been
verified to be very available for the synthesis of nonagglomer-
ated particles with surface accessibility for treatments and able
to be used in thin-film techniques related to the sol–gel process
[18–21].
The aim of this article is to introduce a novel synthesis tech-
nology employed in preparing YNb0.5V0.5O4 and YNb0.5P0.5O4
activated with rare earth ions and discuss the luminescent
properties of those phosphors in details. We use rare earth
coordination polymers with ortho hydroxylbenzoic acid as the
precursors of rare earth oxides components for they readily
form infinite chain-like polymeric structure similar to organic
polymer template [22]. Polyethylene glycols (PEG), Nb2O5,
NH4H2PO4 and NH4VO3 were composed to assemble inor-
ganic/organic hybrid precursor by a modified in situ chemical
co-precipitation process. Urea was used to modify the decom-
position behavior and ammonia for the pH value. Through an
Advanced technologies for the design of new devices such
as flat panel displays, integrated optics and biological labeling
provide new motivations for the research of luminescent materi-
als with optimized properties [1–3]. Tremendous improvement
in the field of luminescent materials has been made by the
introduction of rare earth ions as luminescent centers. The
important breakthrough was the use of Eu3+activated material
as the red component for color television screens. Presently,
rare earth activated powder phosphors are playing a commer-
cial role in the production of luminescent lamps and X-ray
intensifying screens [4,5]. For example, Eu3+ and Tb3+ were
widely used in color television and lamp phosphors [6–8]. Ce3+,
Tb3+ coactivated bulk LaPO4 is a highly efficient and com-
mercially applied green phosphors in fluorescent lamps and
Eu3+ activated YVO4 was an important commercial red-emitting
phosphors used in color television, the cathode ray tube, and
the high-pressure mercury lamp [9–12]. The Sm3+ and Er3+ are
also very important lanthanide ions for optical and other appli-
cations. For example, Y3Al5O12:Sm3+ films can be possibly
used as a pressure–temperature sensors [13], and Er3+-doped
∗
Corresponding author. Tel.: +86 21 65984663; fax: +86 21 65982287.
E-mail address: byan@tongji.edu.cn (B. Yan).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.10.088

X.-Z. Xiao, B. Yan / Journal of Alloys and Compounds 448 (2008) 298–302
299
in situ wet chemical co-precipitation process, multi-component
hybrid polymeric precursors were constructed to afford those
phosphors.
usual zircon-type tetrahedral xenotime structure [23–26]. By
considering the fact that the highest luminescence is not always
elicited from a single-phase line compound, a mixed com-
pound could be more suited to the host of rare earth ions
emission. The inclusion of YNbO4 may constitute an obsta-
cle to make single-phase solid solutions. It was observed that
the XRD pattern of Nb compounds has also the trace of the
other apex compounds. So from Fig. 1, it can be seen that the
mixed compounds are not a single-phase. In Fig. 1A and B,
there exists xenotime structure of YVO4 (YPO4) and fergu-
sonitestructureof YNbO4 in YNb0.5M0.5O4. Usingthescanning
electron microscope we obtain some cobble-like morphologies
of samples. Fig. 2 shows the representative morphologies of
YNb0.5V0.5O4:Sm3+ (A) and YNb0.5P0.5O4:Tb3+ (B). Com-
paring two SEM morphologies of YNb0.5M0.5O4, it can be
observed that YNb0.5V0.5O4 presents better crystalline than
that of YNb0.5P0.5O4 due to the similar chemical and phys-
ical properties of V and Nb. From Fig. 2, it can be found
that the typical crystalline grain of all products is estimated
to be around 0.5–1 ␮m in dimension. Besides this, some con-
glomeration among the crystalline grain are observed in both
of SEM figures because of the high temperature 1000 ◦C of
thermal decomposition, especially YNb0.5P0.5O4. The particle
size of YNb0.5P0.5O4 is smaller than that of YNb0.5V0.5O4
powders, which be easy to be conglomerated. In this method,
rare earth coordination polymers and the organic polymer were
used to form the interpenetrating polymeric network for they
show the similar infinite polymeric structure [27]. From the
co-precipitation composition, the hybrid precursors with micro-
porous fragment structure, which can control the morphology
of the final phosphors’ particle, can be assembled with rare
earth coordination polymers, PEG (organic polymer) and other
functional components (Nb2O5, etc.). After the thermal decom-
position, organic components lose and the particles of phosphors
present such microstructure. Therefore, this preparation technol-
ogy connecting the assembly of hybrid material with synthesis
of micrometer material can be expected to be a candidate for
the synthesis of other luminescent materials based on rare earth
oxides.
2. Experimental
YNb0.5P0.5O4:xTb3+ (x = 0.01, 0.02, 0.03, 0.05, 0.07) phosphor particles
were prepared by in situ co-precipitation method. The initiative materials Y2O3
and Tb4O7 were firstly dissolved into concentrated nitric acids. Then the prepa-
ration of the precursors was described in details in the following: superfluous
salicylic acid (HSal) (6.0 mmol) was dissolved into 95% ethanol and its pH
value was then adjusted to 7.0–8.0 with ammonia solution. Then the rare earth
nitrates solutions (Y(NO3)3 and Tb(NO3)3) were added and mixed homoge-
nously. After being stirred for half hours, the urea and PEG were added into the
above solutions with appropriate ratio to RE3+. After an hour, the solutions were
heated to 70 ◦C and the pH value was adjusted to about 7.5. At the same time,
Nb2O5 powders and NH4H2PO4 were added into the solutions. After this, the
solutions were stirred until they became homogenous systems. After filtered,
the precursors were achieved and heat-treated at the temperature 1000 ◦C for
about 4 h to obtain phosphors. The scheme of for the synthesis can be shown as
below:
Y(NO3)3 + Tb(NO3)3 + HSAl + NH3 · H2O
→ [RE (Sal)3]n coordination polymer + x Nb2O5 + (1 − x)NH4H2PO4
2
+ PEG + CO(NH2)2 + NH3 · H2O → YNb0.5P0.5O4 : Tb3+ + CO2
↑
+ H2O ↑ +NH3 ↑ +NO2
↑
The similar process was employed in YNb0.5P0.5O4:Er3+
,
Sm3+and
YNb0.5V0.5O4:Er3+, Sm3+ phosphors except for the Tb3+ replaced by Er3+
,
Sm3+ and NH4H2PO4 replaced by NH4VO3.
The particle sizes were characterized by means of X-ray diffraction
(XRD, Bruke, D8-Advance, 40 kV and 20 mA, Cu K␣). The morphology and
microstructrue were characterized by scanning electronic microscope (SEM,
Philps XL-30). Excitation and emission spectra at room temperature were deter-
mined with Perkin-Elmer LS-55 model fluorophotometer.
3. Results and discussion
The XRD pattern of YNb0.5V0.5O4:Sm3+ (A) and
YNb0.5P0.5O4:Tb3+ (B) was measured and they showed the
similar structure, which was shown in Fig. 1. Although the
YNbO4 has a different crystalline structure (fergusonite struc-
ture), YPO4 and YVO4 are all isomorphor belonging to the
Fig. 1. The XRD patterns of the YNb0.5V0.5O4:Sm3+ (A) and YNb0.5P0.5O4:Tb3+ (B) crystalline powders.

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X.-Z. Xiao, B. Yan / Journal of Alloys and Compounds 448 (2008) 298–302
Fig. 2. SEM micron morphologies of YNb0.5V P
0.5O4:Sm3+ (A) and YNb0.5 0.5O4:Tb3+ (B).
It was well known that among YPO4, YNbO4 and YVO4,
only the YPO4 host never undergoes the energy transfer from
the host to PO4 group en route because the 2p-and 3d-like
states of P are located far above the 4d-like states of Y.
Namely, the excitation energy is absorbed by the host lattice
and transferred directly from the host lattice to the activator
center. Otherwise, in YNbO4 and YVO4 systems, the excita-
tion energy is absorbed first by the host lattice, which involves
the transition between 4d-like states of Y and 2p-like states
of O. The absorbed energy may then be transferred to RO4
groups and last transferred directly from the host lattice to
the activator center. Therefore, when those compounds are
mixed, then the excitation and emission mechanism will be
different, which involves both of cases. All the excitation spec-
tra of YNb0.5P0.5O4:RE3+ and YNb0.5V0.5O4:RE3+ show the
similar features [26]. Figs. 3–5 show the representative excita-
tion spectra for YNb0.5V0.5O4:xSm3+, YNb0.5V0.5O4:xEr3+ and
YNb0.5P0.5O4:xTb3+ (x = 0.01, 0.02, 0.03, 0.05, 0.07) annealed
Fig. 4. Excitation spectra of YNb0.5V0.5O4:Er3+ (λem = 552 nm).
Fig. 3. Excitation spectra of YNb0.5V0.5O4:Sm3+ (λem = 605 nm).
Fig. 5. Excitation spectra of YNb0.5P0.5O4:Tb3+ (λem = 543 nm).

X.-Z. Xiao, B. Yan / Journal of Alloys and Compounds 448 (2008) 298–302
301
at 1000 ◦C, which is monitored at an emission wavelength of
605 nm (4G5/2 to H11/2 orange emission) for Sm3+ system,
6
552 nm (4S3/2 to 4I15/2 green emission) for Er3+ one and 543 nm
7
(5D4 to F5 green emission). For Fig. 3, the excitation spec-
trum consists of strong broad absorption band with maximum
peak at 243 nm in the short wavelength region from 200 to
280 nm (Fig. 3), which originates from the transitions toward
the charge transfer state (CTS) corresponding to Sm3+–O2−
interactions. Besides, the bands around 210 nm can be ascribed
3−
3−
to the host absorption band of NbO4
and VO4
(HAB)
[26,28–32]. In the longer wavelength region (320–450 nm), the
f–f transitions within the Sm3+ 4f5 electron configuration as
well can be checked to be a wide band. For Fig. 4, a strong
broad absorption band also exist in short ultraviolet region from
200 to 280 nm (Fig. 4) with three stronger peaks at 222, 243
and 280 nm, corresponding to the f–d transitions of Er3+. Sim-
ilar to Sm3+, the peaks around 210 nm should be attributed as
3−
the host absorption band of NbO4 (HAB). The long wave-
Fig. 6. Emission spectra of YNb0.5P0.5O4:Tb3+ (λex = 243 nm).
length at the range of 320–500 nm, come from the f–f transitions
of Er3+ ions, which is still weak. The excitation spectra of
YNb0.5P0.5O4:Tb3+activated powders in Fig. 5, presents three
strong bands centered at 210, 243, 300 nm and a weak band in
the region 350–400 nm. The band at 210 nm was ascribed to the
host absorption. The ground states (4f8) of Tb3+ are 7FJ config-
urations, when one electron is promoted to 5d shell, it can give
rise to two 4f75d1 excitation states; the high-spin states with 9DJ
configuratios or low-spin states with 7DJ configurations [33].
It is obvious that 9DJ states will be lower in energy accord-
ing to Hund’s rule. And the transitions between 7FJ and 7DJ
are spin-allowed, while 7FJ–9DJ transitions are spin-forbidden.
Therefore, the band at 243 nm would seem to be attributed to the
7D level of the 4f–5d transition of Tb3+ due to s spin-allowed
4f8–4f75d1 transitions, with higher energy; the band at 256 nm
could be assigned to the 9D level of the 4f75d1 configuration,
with lower energy. Besides this, there also appeared a weak band
in the region 300–400 nm resulting from the Tb3+ f–f transition.
It was well known that the crystalline environment has greater
effect on interconfigurational transitions of RE3+ than on intra-
configurational transitions within the shielded 4f because of the
excited states such as 5d are not shielded from the surrounding
electronic shell, and the 5d-electron has a strong interaction with
the neighboring anion ligand in the compound [34].
(J = 5/2, 7/2, 9/2) emission lines of Sm3+, which is located
6
6
at 562.5 nm (4G5/2 → H5/2), 601 nm (4G5/2 → H7/2), and
6
642 nm (4G5/2 → H9/2), respectively. Among the orange tran-
6
sition (4G5/2 → H7/2) exhibits the strongest emission. From
the emission spectra of YNb0.5V
0.5O4:Er3+ (Fig. 8), two
apparent luminescent bands in the green region are centered
4
at 524 (531.5) nm (2H11/2 → I15/2), and 544 nm (553.5 nm)
4
(4S3/2 → I15/2), respectively. This indicates that an efficient
energy transfer from host to the rare earth ions of Er3+ and
Sm3+ also occurs. The concentration quenching also appears as
for the systems of YNb0.5V0.5O4:Er3+ and YNb0.5V0.5O4:Sm3+
and we obtain the optimum dopant concentrations, which is 1,
and 3 mol% for Er3+ and Sm3+ ions, respectively. In contrast
to the concentration of Tb3+ (5 mol%) in YNb0.5P0.5O4 host,
their concentrations are much lower, which may be explained
that the cross-relaxation process in close Tb3+–Tb3+, Er3+–Er3+
and Sm3+–Sm3+ pairs play an important role in their lumines-
Fig. 6 wears the corresponding emission spectra of
YNb0.5P0.5O4:Tb3+. Upon excitation in the 243 nm band,
the emission spectra present the four feature lines at 487,
546, 588 and 621 nm, corresponding to the transition of
5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3, respectively. All
the emission spectra of YNb0.5V0.5O4:Tb3+ wears the sim-
ilar features. Besides this, the luminescence intensity varies
with increasing concentration from 1 to 8 mol% and reaches
the strongest emission at 5 mol%. As the doping concentration
of 8 mol%, the corresponding emission decrease dramatically,
so concentration effect takes place. Obviously the lumines-
cence intensities strongly depend on the doping concentration.
Fig. 7 shows the emission spectra of YNb0.5V0.5O4:Sm3+
with different doping concentrations under the excitation of
Fig. 7. Emission spectra of YNb0.5V0.5O4:Sm3+ (λex = 243 nm).
4
6
243 nm. The emission spectra are composed of G5/2 → HJ

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X.-Z. Xiao, B. Yan / Journal of Alloys and Compounds 448 (2008) 298–302
observed and the optimum concentrations for Tb3+, Er3+, Sm3+
are 5, 1, 3 mol%, respectively.
Acknowledgements
The work was supported by the Science Fund of Shanghai
University for Excellent Youth Scientists and National Natural
Science Foundation of China (20301013).
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4. Conclusions
In summary, YNb0.5V0.5O4:RE3+ and YNb0.5P0.5O4:RE3+
(RE = Tb, Er, Sm) phosphors have been achieved by a novel
in situ assembling hybrid precursor co-precipitation tech-
nology. Both XRD and SEM results indicate that these
phosphor particles exhibit novel crystal morphology, especially
YNb0.5V0.5O4:RE3+ due to the similar chemical and physical
surroundings of Nb and V. Upon UV excitation in the host,
the characteristic luminescence of the three rare earth ions is