Y2O3:Eu3+ (5 mol%) with Ag nanoparticles prepared by citrate precursor

Article abstract of DOI:10.1016/j.jssc.2010.07.010

Y₂O₃:Eu³ (5 mol% Eu³) and Y₂O₃:Eu³ (5 mol% Eu³) containing 1 mol% of Ag nanoparticles were prepared by heat treatment of a viscous resin obtained via citrate precursor. TEM and EDS analyses showed that Y ₂O₃:Eu³ (5 mol% Eu³) is formed by nanoparticles with an average size of 12 nm, which increases to 30 nm when Ag is present because the effect of metal induced crystallization occurs. Ag nanoparticles with a size of 9 nm dispersed in Y₂O ₃:Eu³ (5 mol% Eu³) were obtained and the surface plasmon effect on Ag nanoparticles was observed. The emission around 612 nm assigned to the Eu³ (⁵D₀→ ⁷F₂) transition enhanced when the Ag nanoparticles were present in the Y₂O₃:Eu³ luminescent material.

Full text of DOI:10.1016/j.jssc.2010.07.010

Journal of Solid State Chemistry 183 (2010) 2110–2115
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Y2O3:Eu3+ (5 mol%) with Ag nanoparticles prepared by citrate precursor
J.L. Ferrari a,n, M.A. Cebim a, A.M. Pires b, M.A. Couto dos Santos c, M.R. Davolos a
a Instituto de Qu´ımica, UNESP Univ. Estadual Paulista, 14800-970, Araraquara–Sa¨o Paulo, Brazil
b Depto. de F´ısica, Qu´ımica e Biologia, Faculdade de Ciˆencias e Tecnologia, UNESP Univ Estadual Paulista, P.O. Box 467, 19060-900, Presidente Prudente, Sa¨o Paulo, Brazil
c
~
~
Departamento de F´ısica/UFS, 49100-000 Sao Cristo´vao/SE/Brazil
a r t i c l e i n f o
a b s t r a c t
Y2O3:Eu3+ (5 mol% Eu3+) and Y2O3:Eu3+ (5 mol% Eu3+) containing 1 mol% of Ag nanoparticles were
prepared by heat treatment of a viscous resin obtained via citrate precursor. TEM and EDS analyses
showed that Y2O3:Eu3+ (5 mol% Eu3+) is formed by nanoparticles with an average size of 12 nm, which
increases to 30 nm when Ag is present because the effect of metal induced crystallization occurs. Ag
nanoparticles with a size of 9 nm dispersed in Y2O3:Eu3+ (5 mol% Eu3+) were obtained and the surface
plasmon effect on Ag nanoparticles was observed. The emission around 612 nm assigned to the Eu3+
(5D0-7F2) transition enhanced when the Ag nanoparticles were present in the Y2O3:Eu3+ luminescent
material.
Article history:
Received 11 February 2010
Received in revised form
5 June 2010
Accepted 12 July 2010
Available online 15 July 2010
Keywords:
Yttrium oxide
Europium ion
& 2010 Elsevier Inc. All rights reserved.
Ag nanoparticles
Luminescence spectroscopy
1. Introduction
citric acid (alpha-hydroxycarboxilic acid) contributes to form the
stable chelates through carboxylic groups with different metals and
provides precursors with a high level of homogeneity.
Europium-activated Y2O3 (Y2O3:Eu3+) has attracted much
attention as a red-emitting phosphor for commercial use in
fluorescent lighting and screen due to its high luminescence
emission around 612 nm. Because of the fast development of
nanotechnology, the optical properties of nanocrystalline
Y2O3:Eu3+ have also been extensively investigated for its
potential application in high resolution imaging for flat plasma
displays and in fundamental research [1,2]. Therefore, many
researchers have already performed investigations related to this
nanophosphor, such as the study of the influence of particle size
on the intense luminescence of nanocrystalline Y2O3:Eu3+ [3–12].
In order to achieve improved luminescent properties of
Y2O3:Eu3+ [13], the precursors as well as the synthetic route are
very important factors. In the literature, many papers have reported
different routes used to prepare this material, for example EDTA
complex sol–gel process [14], combustion synthesis [15,16], flame
spray pyrolysis [17,18], low temperature hydrothermal route [19],
firing precursors prepared by urea thermal decomposition [20],
microemulsion microwave heating [21], reverse micelles [22], sol–
gel process [23–25], homogeneous precipitation technique [26], Au
nanoparticles dispersed in Y2O3:Eu3+ films [27], and the citrate
method [30]. The use of citrate precursors has been reported as a
way to achieve materials with excellent features [31]. The use of
Besides, nanostructured metallic surfaces appear as an alter-
native to enhance the luminescence of Y2O3:Eu3+. Liu et al. [32]
have assigned the luminescence enhancement of Y2O3:Eu3+ to an
energy transfer from Ag to this material. Metallic nanostructures
have caused a drastic increase in the intensity of Raman scattering
or luminescence ions in close proximity with the metallic surface
[33]. Y2O3:Eu3+ containing Au nanoparticles has also been
reported with an increase of luminescence compared with the
same materials without Au, which may have potential applica-
tions in biological detection and nanoreactors [34].
This notable enhancement phenomenon stems from a reso-
nant excitation of surface Plasmon polarizations confined within a
metallic nanostructure leading to a consequent enhancement in
the induced dipole moment of the electronic transition respon-
sible for the emission. Based on some studies, research on the
fabrication of plasmonic nanomaterials and their high potential
for various applications in photonics and surface-enhanced
spectroscopy resulted in an increasing interest about the funda-
mental physics of the metal nanoparticles [35–39]. Moreover,
many papers have been published regarding the effect of Ag
metallic nanoparticles on the structure of annealed silica obtained
by sol–gel method.
In this work, we have focused on an easy method to prepare
the luminescent materials Y2O3 and Y2O3:Eu3+ (5 mol% Eu3+
)
using citrate precursors. Also, the effect of Ag nanoparticles on the
spectroscopic and structural properties of the doped material was
investigated.
n Corresponding author. Fax: +55 16 3633 2660.
E-mail addresses: jeffersonlferrari@gmail.com, ferrari@pg.ffclrp.usp.br
(J.L. Ferrari).
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.07.010

J.L. Ferrari et al. / Journal of Solid State Chemistry 183 (2010) 2110–2115
2111
2. Experimental procedure
As reported by Ferrari et al. [30] in previous work based on
spectroscopic properties, the best concentration of Eu3+ in the Y2O3
host matrix prepared by citrate precursor is 5 mol%. Y2O3,
Y2O3:Eu3+ (5 mol% Eu3+) and Y2O3:Eu3+ (5 mol% Eu3+) containing
1 mol% of Ag were prepared by citrate method [27–29]. Yttrium
oxide (Aldrich 99.999%), europium oxide (Aldrich 99.999%) and
silver nitrate (Acros Organic 99%) were used to obtain the
respective nitrate solutions with concentration of metals of
9.95ꢀ 10ꢁ3, 1.3ꢀ 10ꢁ2 and 0.01 mol Lꢁ1, respectively, to obtain
the stock solution. The volume of nitrate metal solutions and the
amount of citric acid (Vetec, P.A.) (the number of moles of citric
acid was 20 times that of the number of moles of metals) to
prepare each material were added in the beaker and deionized
water was added to complete 50 mL of final solution. Stoichio-
metric amounts of the reagents were used to obtain 1 g of Y2O3,
Y2O3:Eu3+ (5 mol% Eu3+) and Y2O3:Eu3+ (5 mol% Eu3+) containing
1 mol% of Ag of final product. The amount of citric acid changes for
each samples, because the amount of metals also changes. Then,
ammonium hydroxide solution (0.01 mol Lꢁ1) was added drop by
drop to adjust the pH to
4 to facilitate the formation of
homogeneous solution and to avoid possible metal hydrolysis
reaction, and the solution was slowly evaporated upon 80 1C until
the formation of a viscous resin. For the obtainment of the resin,
the solutions were prepared in dark environment to avoid the
reduction of silver by light. After that, the resins were fired in an
oven at 800 1C for 4 h to give the three materials. The powder
Fig. 1. X-ray diffraction patterns of Y2O3, Y2O3:Eu3+ (5 mol% Eu3+) and Y2O3:Eu3+
(5 mol% Eu3+) containing 1 mol% of Ag samples obtained by heat treating the resin
prepared by citrate precursor at 800 1C for 4 h in static air atmosphere. The
assignments of the indexed phases are also indicated (inset).
The Eu3+ ion might be replacing the Y3+ ions in the Y2O3 host
lattice because no diffraction pattern characteristic of Eu2O3
phase is detected. This is an expected effect since the difference
samples were characterized by X-ray diffractometry (XRD) using a
˚
Rigaku RINT 2000 (copper target,
0.031/2 s (15–701, 2y range). High resolution transmission electron
l¼1.5406 A) at a scanning rate of
between the Y3+ and Eu3+ atomic radii is quite small (0.047 A)
˚
[30,45]. Through careful examination of the powder XRD patterns
of phosphors with Ag content, it can be observe the extra
reflection peaks that are assigned to the metallic Ag phase, which
are consistent with the JCPDS no. 04-0783 card [46]. The peak of
microscopy (HRTEM) and energy-dispersive X-ray spectrometry
(EDS) analyses were acquired using a Philips CM 200 transmission
electron microscope in order to study the morphology, grain size
and composition of the particles. HRTEM images were also used to
determine an average particle size as well as the size distribution
and to build the histograms. The nanoparticles were suspended in
isopropyl alcohol and added onto copper grade to support the
samples the during analyses. Photoluminescence (PL) spectroscopy
measurements were carried out using a Fluorolog SPEX F212/Jobin-
Yvon fluorescence spectrometer equipped with an R928 Hama-
matsu photomultiplier. A 450 W ozone-free Xenon lamp was used
as the excitation source equipped with a cooled Hamamatsu R928
photomultiplier with excitation and emission slits fixed at 0.3 and
0.5 mm, respectively. Emission spectra in the range between 575
and 700 nm of Y2O3:Eu3+ (5 mol% Eu3+) and Y2O3:Eu3+ (5 mol%
Eu3+) containing 1 mol% of Ag were obtained at room temperature
with excitation at 261 nm, in the region of the charge-transfer band
related to the excitation of an electron from the oxygen 2p state to
Eu3+ 4f state [32,40–42].
Ag is localized around 2
y¼38.251 and assigned to the (1 1 1)
plane. In addition, a comparison among the half-band widths of
the diffraction peaks from the different samples (Fig. 1) suggests
that the Ag nanoparticles lead to sharper peaks referent to the
Y2O3 in the diffractograms, and this effect may be related to an
increase of the system crystallinity. In accordance to Herd et al.
[47] the addition of small amount of metals to some materials, the
metal-induced crystallization effect occurs, contributing to in-
creasing the crystallinity, or then, the formation of phases that are
normally obtained at higher temperature. Using an XRD diffracto-
gram, it was possible to estimate of the average nanocrystallite
size by Scherrer’s formula. The equation is described by the
relation t¼0.89
l/B cos y, where t is an average nanocrystallite
size, lA˚ the X-ray wavelength, B the full width half-maximum
(FWHM) and yrad the position of the peak used in calculus. The
estimations were realized based on the reflection peaks localized
near 2
Y¼29.281 assigned to the (2 2 2) main plane present in the
3. Results and discussion
diffractograms and the average nanocrystallite sizes for Y2O3,
Y2O3:Eu3+ (5 mol% Eu3+) and for Y2O3:Eu3+ (5 mol% Eu3+) in the
sample containing 1 mol% of Ag, were 9.6, 9.6 and 16.9 nm,
respectively. According to Taxak et al. [22], the Y2O3:Eu3+
nanoparticles obtained via tartaric acid sol–gel method, depicted
nanocrystallite with size around 20 nm for materials annealed at
500 1C and 45 nm for material annealed at 800 1C/3 h. Recently,
Wang et al. [48] reported the obtaining of Y2O3:Eu3+ by
homogeneous precipitation, and observed the formation of
nanocrystallite size around 41 and 30 nm dependeing on the
amount of europium and also the pH chosen in the reaction. As we
can see, the method reported in this work contributes to obtaining
Y2O3:Eu3+ material with smaller nanocrystallites size in compar-
ison with other works reported in the literature.
Fig. 1 shows the XRD patterns of Y2O3, Y2O3:Eu3+ (5 mol%
Eu3+) and Y2O3:Eu3+ (5 mol% Eu3+) containing 1 mol% of Ag
prepared from citrate precursors. The cubic structure of Y2O3
appears as the main phase and is present in all the formed
materials. Jung et al. [12] observed that the pure cubic phase of
Y2O3 is obtained from spray pyrolysis in the temperature range
from 800 to 1300 1C without transition to the monoclinic phase.
This crystalline system corresponds to a body-centered cubic
structure with Ia3 space group consistent with the JCPDS no. 43-
1036 card [43]. It is well known that the Y3+ ions are
accommodated in two different symmetries, i.e., the C2 without
inversion center and the S6 with inversion center [26,44].

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J.L. Ferrari et al. / Journal of Solid State Chemistry 183 (2010) 2110–2115
TEM images of the materials annealed at 800 1C for 4 h in air
are depicted in Fig. 2. Fig. 2(a) shows Y2O3:Eu3+ (5 mol% Eu3+
with an average particle size of 12 nm. EDS analysis evidences the
presence of a pure material composed only by yttrium, europium
and oxygen. The assigned energy peaks are in accordance with
those reported by Yongqing et al. [14].
Dhanaraj et al. [23] prepared Y2O3:Eu3+ by the gel–polymers
pyrolysis process and obtained materials with nanoparticles
between 10 and 20 m. Schmechel et al. [6] also reported the
formation of Y2O3:Eu3+ nanoparticles around 10 nm obtained
from chemical vapour synthesis with great optical properties.
The diffuse reflectances were recorded for samples with and
without Ag nanoparticles and the spectra obtained are depicted
(Fig. 4(a)). By analysing the reflectance spectra profile, it is
possible to verify that the investigated materials exhibit consi-
derable differences. Firstly, Y2O3:Eu3+ (5 mol% Eu3+) containing
1 mol% of Ag shows a reflectance edge in a region with higher
energy in comparison with the sample without Ag nanoparticle.
Secondly, in the spectrum of the sample containing Ag
nanoparticles, an absorption band around 400 nm is observed
and can be assigned to the plasmon surface effect, which is
usually displayed by metallic nanoparticles [49,50]. On the other
hand, in the Y2O3:Eu3+ (5 mol% Eu3+) spectrum this band is
absent and a band edge is observed in lower energy. The band
assigned to the plasmon effect, helps us verify the Ag
nanoparticles that are present, and the results obtained by XRD,
HRTEM and EDS also contribute to confirm with this observation.
The position of the band localized in higher energy observed in
the reflectance spectrum may be associated to the presence of Ag
nanoparticles, which are responsible for eliminating the possible
defects present on the surface of Y2O3:Eu3+ material, due to the
metal-induced crystallization effect. Consequently, the elimi-
nation of defects can lead to a more transparent Y2O3:Eu3+
material in the visible range containing Ag nanoparticles in
comparison to the material without Ag nanoparticles. The
bandgap values of the compounds were determined by using
)
TEM and EDS analyses of the sample containing 1 mol% of Ag
present in two different areas with different particle sizes and are
indentified as areas A and B (Fig. 2b). Area A is formed by
nanoparticles composed basically of Y2O3:Eu3+ with an average
size of 25 nm and area B by nanoparticle with size of about 9 nm.
The HRTEM images and EDS spectra of the nanoparticles depicted
in area B of Fig. 2(b) were recorded and are shown in Fig. 2(c). It is
possible to observe that the nanoparticles are composed by Ag
nanoparticles with an average size of about 9 nm. The copper
lines detected in all the EDS spectra are related to the copper
grade used to support samples for the HRTEM analysis. The
statistical histograms of average nanoparticle size are shown in
Fig. 3. Fig. 3(a) shows the histogram of the Y2O3:Eu3+ (5 mol%
Eu3+) nanoparticles in the absence of Ag while Fig. 3(b) shows the
histogram of the Y2O3:Eu3+ (5 mol% Eu3+) nanoparticles (area A)
and the Ag nanoparticles (area B). The histograms show that the
Ag nanoparticles promote an increase of the Y2O3:Eu3+ (5 mol%
Eu3+) particle size compared with the material obtained in the
absence of Ag nanoparticles. In general, the particle size of the
Y2O3:Eu3+ obtained by other researchers is very similar.
Camenzind et al. [17] observed that the particle size of
Y2O3:Eu3+ also containing 5 mol% of Eu3+obtained by one-step
flame spray pyrolysis depicts the size distribution between 10 and
25 nm that is dependent directly on the flame conditions.
Fig. 2. TEM images and EDS analysis of (a) Y2O3:Eu3+ (5 mol% Eu3+), (b) Y2O3:Eu3+ (5 mol% Eu3+) containing 1 mol% of Ag and (c) HRTEM image and EDS analysis of the Ag
nanoparticles.

J.L. Ferrari et al. / Journal of Solid State Chemistry 183 (2010) 2110–2115
2113
Fig. 4. (a) Diffuse reflectance spectra of Y2O3:Eu3+ (5 mol% Eu3+) and Y2O3:Eu3+
(5 mol% Eu3+) containing 1 mol% of Ag nanoparticles and (b) calculated bandgap of
Y2O3:Eu3+ (5 mol% Eu3+) and Y2O3:Eu3+ (5 mol% Eu3+) containing 1 mol% of Ag
nanoparticles obtained by heat treating the resin prepared by citrate precursor at
800 1C for 4 h in static air atmosphere.
Fig. 3. Representative particle size histograms of (a) Y2O3:Eu3+ (5 mol% Eu3+) and
(b) Y2O3:Eu3+ (5 mol% Eu3+) containing 1 mol% of Ag (area A¼Y2O3:Eu3+ (5 mol%
Eu3+) and area B¼Ag nanoparticles) obtained by heat treating the resin prepared
by citrate precursor at 800 1C for 4 h in static air atmosphere.
neighbourhood around the Eu3+ ions. This effect can be verified
by the shift observed in the emission spectra assigned
to 5D0-7F0 (C2), 5D0-7F1 (S6) and 5D0-7F1 (Fig. 5(b)). On the
basis of the Judd–Ofelt theory [40,51,52], the 5D0-7F0 and
5D0-7F1 transitions originate from electric and magnetic dipoles,
respectively. The electric dipole transition is allowed only when
Eu3+ occupies a site without an inversion center and is sensitive to
the local symmetry. Thus the shift observed in the spectrum of the
sample containing Ag is due to a microstructural change in the host
lattice, formed by the presence of Ag nanoparticles.
Luminescence efficiency for the materials reported in the
current work is unknown and still in progress. However, the
phosphor containing Ag nanoparticles shows higher intensity
luminescence, and displays around 34% more relative intensity
than the sample without Ag nanoparticles. This observation is
very important to certify that the Ag nanoparticles contribute to
the optical properties of the material obtained from citrate
precursors. The plasmon effect observed on the Ag nanoparticles
may contribute to an increase of the relative luminescence
intensity of Eu3+-doped Y2O3 host lattice (5 mol% Eu3+) as well
the Kubelka–Munk model with the Tauc, considering an indirect
bandgap for Y2O3 host matrix [50]. The determined bandgap
depicted in Fig. 4(b) leads to the same conclusion, that is, if the
compound containing 1% of Ag nanoparticles possess a higher
crystallinity than Y2O3:Eu3+ (5 mol% Eu3+) then one would expect
a higher bandgap for it.
Fig. 5 shows the emission spectra. The emission observed in
the 575–700 nm range is ascribed to the well-known Eu3+
5D0-7FJ (J¼0, 1, 2, 3 and 4) transitions. The maximum intensity
at 612 nm is assigned to the hypersensitive 5D0-7F2 transition
due to the forced electric dipole mechanism. All the other
emission peaks at ꢂ580 (5D0-7F0–C2), 582 (5D0-7F1–S6) and
at 587, 593 and 599 nm (5D0-7F1) were assigned and are shown
in Fig. 5(a). In the cubic Y2O3 there are two crystallographic sites,
one with C2 symmetry and the other with S6 symmetry. Therefore,
it can be concluded that the Eu3+ ions replace the Y3+ ions and
occupy both the C2 and S6 symmetries because the emissions
assigned to such symmetries were observed (Fig. 5(b)). As
discussed in the XRD results, the Ag nanoparticles may be
affecting the crystallinity of the system and consequently the

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J.L. Ferrari et al. / Journal of Solid State Chemistry 183 (2010) 2110–2115
feasible for the synthesis of crystalline materials. The plasmon
surface effect was also observed for Y2O3:Eu3+ (5 mol% Eu3+
)
containing Ag nanoparticles, thus confirming the presence of
metal nanoparticles together with the XRD and HRTEM analyses.
The presence of the Ag nanoparticles in the Y2O3 host lattice
might promote a decrease of the defects possibly present on the
particle surface, which can affect the Eu3+ neighbourhood. The
metal-induced crystallization effect was observed in this work,
and can be the main effect to contribute for the luminescence
enhancement of Y2O3:Eu3+ (5 mol% Eu3+). However, a systematic
study about the plasmon effect or host crystallinity on the
enhancement of luminescence as a function of different amounts
of Ag is still in progress.
Acknowledgments
The authors acknowledge the funding agencies FAPESP and
CNPq for the financial support. J.L. Ferrari thanks FAPESP for the
scholarship.
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The citrate method used in this work demonstrated to be a
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