Preparation and upconversion luminescence of Y2O3:Yb3+, Ho3+ nanocrystalline powders coated with SiO2

Article abstract of DOI:10.1016/j.jallcom.2008.03.042

Y₂O₃:Yb³⁺, Ho³⁺ nanocrystalline powders were prepared via a reverse-strike co-precipitation method, which were agglomerated shown from FESEM image. After dispersing and then coating with SiO₂ by St?be

Full text of DOI:10.1016/j.jallcom.2008.03.042

Journal of Alloys and Compounds 471 (2009) 201–203
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
3
+
Preparation and upconversion luminescence of Y O :Yb ,
2
3
3
+
Ho nanocrystalline powders coated with SiO
2
∗
Jian Zhang, Liqiong An, Shiwei Wang
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China
a r t i c l e i n f o
a b s t r a c t
Y O :Yb3+, Ho3+ nanocrystalline powders were prepared via a reverse-strike co-precipitation method,
Article history:
2
3
Received 30 January 2008
Received in revised form 10 March 2008
Accepted 12 March 2008
Available online 23 April 2008
which were agglomerated shown from FESEM image. After dispersing and then coating with SiO2 by
St o¨ ber method, spherical and uniform particles were obtained. The upconversion luminescent properties
of these samples were studied under the excitation of 980 nm. Enhanced upconversion luminescence was
observed in the sample coated with SiO2. Possible upconversion mechanism was proposed.
©
2008 Elsevier B.V. All rights reserved.
Keywords:
Y2O3
Coating
Upconversion luminescence
(∼430–550 cm−1) [6], which would reduce the possibility of non-
radiative process and then increase the upconversion efficiency.
Several studies reported the upconversion properties of nanocrys-
1
. Introduction
The ultrasensitive fluorescent labeling materials have been
widely studied for their analytical and biophysical applications
talline and bulk Y O3 synthesized by different procedures, such
2
[
1–3]. Traditionally, the commonly used fluorescent labels are
as sol–gel method, chemical vapor synthesis, combustion method,
and co-precipitation method [7–12]. Meanwhile, nanocrystalline
powders can be modified by surface treatment, for example, coating
with the same material or some other material. This modification
can reduce the surface unsatured bond density and defects and
thus enhance luminescence of the powders [13]. Further, selective
attachment to cell bonds of specific bacteria can be achieved after
the powders are properly coated, which is in favor of application in
fluorescent labeling. Therefore, it is necessary to modify the surface
organic dyes, such as rhodamine, fluorescein isothiocyanates (FITC),
and cyanine dyes (Cy3, Cy5 and Cy7). Recently, semiconductor
nanocrystals have been applied in biological detections [2,4,5].
They preserve high quantum yield, tunable emission wavelength
and high stability against photobleaching. However, the main prob-
lem of these materials is the autofluorescence (noise) from the
analytes under UV and visible light. Due to the unique fluores-
cent properties, up-conversion fluorescent labels have attracted
much attention for their potential application in bio-labeling. By
absorbing two or more photons of low near-infrared (NIR), the
upconversion materials can emit visible light or even UV light,
which show very low background light and minimal photodam-
age to biological tissues. Moreover, the commercially high-power
laser diodes used as the excitation sources are available now at
a relatively low cost for the rapid development of semiconductor.
Therefore, it will be extraordinarily convenient to employ upcon-
version materials for the purpose of bio-labeling.
of Y O3 nanocrystalline powders.
2
In this paper, we synthesized the Y O :Yb3+, Ho3+ nanocrys-
2
3
talline powders via a reverse-strike co-precipitation method and
the as-prepared powders were coated with SiO by St o¨ ber method.
2
The upconversion luminescent properties of these samples were
studied and possible upconversion mechanism was proposed.
2.
Experimental
3
+
Y2O3:Yb3+, Ho3+ nanocryatalline powders were prepared by a reverse-strike
Recently, Y O :RE has been chosen as upconversion lumin-
2
3
co-precipitation method. Details of the procedure were presented previously [12].
The as-prepared powders were carefully dispersed and then coated with SiO2 by
St o¨ ber method. Analytical grade tetraethyl orthosilicate (TEOS), ethanol and ammo-
nia water were used as the starting materials. TEOS, ethanol and ammonia water
scent material for its favorable physical and chemical properties
and ease of synthesis in nanometer region. Especially, the
host material yttria preserves relatively low phonon energy
3+
were mixed with a certain ratio and the mixture was kept stirring when Y2O3:Yb
,
3
+
Ho nanocrystalline powders were slowly added. The TEOS hydrolysis reaction can
be expressed as following:
∗ Corresponding author. Tel.: +86 21 52414320; fax: +86 21 52413903.
E-mail address: swwang51@mail.sic.ac.cn (S. Wang).
Si(OC2H5)4 + 4H2O → Si(OH)4 + 4C2H5OHSi(OH)4 → SiO2 + 2H2O
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.03.042

2
02
J. Zhang et al. / Journal of Alloys and Compounds 471 (2009) 201–203
Fig. 1. FESEM images for (a) Y2O3:Yb3+, Ho3+ calcined at 1000 C for 2 h, (b) SiO2–Y2O3:Yb3+, Ho3+ dried at 120 C for 5 h and (c) SiO2–Y2O3:Yb3+, Ho3+ calcined at 550 C for
◦
◦
◦
2
h.
After reaction, the slurry was dispersed ultrasonically, filtered, washed with
was stronger than that of the uncoated sample. Surface modifica-
tion may contribute to this phenomenon. SiO2 coating in our case,
to some extent, might eliminate the defect on the surface of the
nanocrystalline powders and accordingly reduce the probability of
multiphonon relaxation process. Therefore, the upconversion lumi-
nescent intensity of the powders was enhanced after coated with
SiO2.
To clarify the upconversion emission mechanism, we measured
the dependence of the emission intensity (Iup) on the excitation
power (Iex). The number of exciting photons (n) required in the
◦
3+
ethanol for several times and dried at 120 C to obtain the SiO2 coated Y2O3:Yb
,
3
+
Ho powders. The particle size of the coated powders can be controlled by the
amount of ethanol and ammonia water within a certain range. We employed a
volume ratio of ethanol and ammonia water to be 2:1 and a solid load of 40 wt.%
3+
3+
Y2O3:Yb , Ho nanocrystalline powders.
The morphology and particle size were detected by the field emission scanning
electron microscope (FESEM, JSM-6700F, JEOL, Japan). Upconversion luminescence
spectra of the samples were recorded at room temperature by a spectrofluorometer
Fluorolog-3, Jobin Yvon, France) equipped with Hamamatsu R928 photomultiplier
tube. A 980 nm continuous wave diode laser was used as the excitation source.
(
n
upconversion process was approximated by the formula Iup ∝ (Iex)
5
5
5
3
. Results and discussion
[14]. As shown in Fig. 3, n = 2.02, 1.80 and 1.67 for the F4, S2 → I8,
5 5 5 5 5
F5 → I and F , S → I7 transition, respectively. It implied that
8
4
2
Fig. 1 shows the FESEM pictures of the prepared Y O :Yb3+, Ho3+
two photons were involved in the upconversion process.
The upconversion mechanism in Yb–Ho system has been widely
studied in many host materials [15–17]. Because of the character-
2
3
3+
3+
powders and SiO -coated Y O :Yb , Ho powders. It can be seen
2
2
3
from Fig. 1(a) that the powders synthesized by reverse-strike co-
precipitation method were agglomerated with 60 nm in diameter
◦
when calcined at 1000 C for 2 h. Fig. 1(b) and (c) are typical FESEM
3
+
3+
◦
pictures of SiO -coated Y O :Yb , Ho powders dried at 120 C
2
2
3
◦
for 5 h and then calcined at 550 C for 2 h, respectively. The coated
powders were spherical in shape and very uniform in size with
an average size of about 200 nm (Fig. 1(b)). Even after calcined at
◦
5
50 C for 2 h, the coated powders were still uniform in size, with
no apparent grain growth.
Fig. 2 shows the upconversion luminescent spectra of the pre-
3
+
3+
pared Y O :Yb , Ho powders and the coated powders excited
2
3
by a 980 nm continuous wave laser diode. There were three dom-
inant emission bands. The green emission centered at 549 nm
dominates the whole spectrum corresponding to the multiplets
5
5
5
3+
F , S → I transition of the Ho ions. The red emission was
4
2
8
5
5
detected around 666 nm associated with the F5 → I transition.
8
The very weak infrared emission centered around 755 nm was also
Fig. 2. Upconversion spectra of the prepared powders excited by 980 nm LD, (a)
5
5
5
observed, which was assigned to the F , S → I7 transition. What
3+
3+
◦
3+
3+
◦
4
2
Y O :Yb , Ho calcined at 1000 C for 2 h, (b) SiO –Y O :Yb , Ho dried at 120 C
2
3
2
2
3
for 5 h and (c) SiO2–Y2O3:Yb , Ho3+ calcined at 550 C for 2 h.
◦
3
+
is more, the upconversion intensity of the sample coated with SiO2

J. Zhang et al. / Journal of Alloys and Compounds 471 (2009) 201–203
203
as follows:
2
2
2
:MROW>
(Yb ) + hꢀ980 → 2F
3+
:MROW>
3+
F
F
F
(Yb
)
(GSA)
<
MML
:MROW>
MML
<MML
(Yb ) + I8(Ho ) → 2F
3+
5
3+
:MROW>
(Yb ) + I6(Ho
3+
5
3+
)
(ET)
<
<MML
:MROW>
(Yb ) + I6(Ho ) → 2F
3+
5
3+
:MROW>
3+
5
5
3+
(Yb ) + ( S2, F4)(Ho
)
(ET)
<MML
<MML
(MPR)
3+
5
5
3+
5
3+
(
S2, F4)(Ho ) → F5(Ho
)
5
3+
3+
5
5
I6(Ho ) + hꢀ980 → ( S2, F4)(Ho
)
(ESA)
5
2
5
5
3+
)
I6(Ho ) → I7(Ho
(MPR)
:MROW>
(Yb ) + I7(Ho ) → 2F
3+
5
3+
:MROW>
(Yb3+) + 5F5(Ho3+
F
)
(ET)
<MML
<MML
I7(Ho ) + hꢀ980 → 5F5(Ho
3+
3+
)
(ESA)
5
5
3+
5
5
3+
(
S2, F4)(Ho ) → I8(Ho ) + hꢀgreen
(RT)
(RT)
5
3+
3+
F5(Ho ) → I8(Ho ) + hꢀred
(RT)
(5
S2, F4)(Ho3+) → I7(Ho ) + hꢀNIR
5
5
3+
4. Conclusion
Y O :Yb3+, Ho3+ nanocrystalline powders were prepared via a
2
3
reverse-strike co-precipitation method. The spherical and uniform
particles were obtained when coating the as-prepared powders
with SiO2 by St o¨ ber method. The dominant green, weak red and
infrared upconversion emissions were observed in these samples
under excitation of 980 nm. The intensity of upconversion emis-
sions became stronger after the powders were coated with SiO2,
which was ascribed to the elimination of the defect on the surface
Fig. 3. Upconversion emissions intensity as a function of the excitation power.
of nanocrystalline powders by SiO coating. The dependence of the
2
upconversion intensity upon the exciting power revealed that two
photons were involved and the emitting levels were populated via
energy transfer process.
Acknowledgments
This paper was financially supported by China National
863 Project (2006AA03Z535) and Shanghai Fundamental Project
(07DJ14001).
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+
[
[
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3
+
5
5
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5
5
5
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[
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5
5
3+
(
MPR) process from F , S2 levels. The other is that Ho ions in
4
[
5
5
the I level transfered to the I7 level by MPR and then was excited
to the F5 level by ET and ESA. Therefore we can observe the red
6
5
emission band. The above upconversion processes were expressed