Spontaneous reduction of Eu3q ion in Al co ... - Davidson Physics

May 1999
Ž .
Materials Letters 39 1999 227–231
www.elsevier.comrlocatermatlet
Spontaneous reduction of Eu 3q ion in Al co-doped sol–gel silica
matrix during densification
A. Biswas ) , C.S. Friend, P.N. Prasad
Photonics Research Laboratory, State UniÕersity of New York at Buffalo, Buffalo, NY 14260, USA
Received 7 December 1998; accepted 11 December 1998
Abstract
Eu and Eu-Al co-doped silica glasses have been prepared by impregnating the pores of a base catalysed tetraethylorthosilicate
Ž TEOS.
gel with the nitrate salt of Eu and Al and subsequent densification around 1125 to 11508C. Absorption,
emission and excitation spectra of these glasses indicate that Eu 3q ions are spontaneously reduced to Eu 2q in the presence
of Al 3q during sintering of the glasses above 10008C. q 1999 Published by Elsevier Science B.V. All rights reserved.
Keywords: Sol–gel; Silica glass; Post-doping; Divalent europium; Optical properties
1. Introduction
Glasses incorporated with luminescent species are
of considerable interest for applications as solid state
lasers wx 1 , optical data storage units w2–4 x, chemical
sensors w5,6x and waveguides w7,8 x. The silica glass
Ž SiO .
2 is one of the most technologically important
materials in optical Ž e.g., fibers. and electrical Že.g.,
insulator.
applications due to its many attractive
properties w9,10 x. These properties include high UV
transparency, high mechanical strength, high T g,ex-
tremely low thermal expansion, and low refractive
index. These features make the glass a strong candidate
as a host for rare earth dopants.
Of particular interest among the rare earth ions
are Eu 2q and Sm 2q since the electronic transitions,
4f ny1 5d4f n , of these divalent ions are dipole
) Corresponding author. Tel.: q1-716-6800-x2098; Fax: q1-
716-645-6915
allowed. Therefore, the emission of divalent rare
earth ions should be approximately 10 6 times more
intense than the f–f transitions of the corresponding
trivalent rare earth ions. The divalent rare earth ion
may be incorporated in a glass through a melt method,
but this requires high temperature, a strong reducing
atmosphere and usually results in contamination from
the substrate w11 x. The sol–gel process has been
shown to be a better choice due to its moderate
conditions of temperature and atmosphere w11–13 x.
w x
2q
Nogami et al. 11 have prepared Sm doped
porous aluminosilicate glasses by the sol–gel method
using H 2–N2
gas at 8008C. They also demonstrated
persistent spectral hole burning at room temperature
w x
2q
14 . The same group has also prepared Eu doped
glasses by reducing Eu 3q ions with a H 2–N2
gas
mixture at 8008C w12,15 x. An enhanced emission of
Eu 2q at 440 nm was observed in Al 2O3
co-doped
silica glasses.
w x
3q
Zaitoun et al. 16 prepared Eu doped silica
gels and found the presence of Eu 2y . They argued
00167-577Xr99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved.
Ž .
PII: S0167-577X 99 00011-7

228
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A. Biswas et al.rMaterials Letters 39 1999 227–231
that the electron hole carrier was responsible for the
surface-assisted reduction of Eu 3q .
Recently, Cordoncillo et al. w17x
have incorporated
Eu 2q in an organic inorganic hybrid matrix at
room temperature. They used zirconium tetrapropoxide
to liberate the H 2 gas, which reduced Eu 3q to
Eu 2q in situ.
In this letter, we report for the first time, to our
knowledge, spontaneous reduction of Eu 3q to Eu 2q
in an Al co-doped sol–gel silica matrix.
2. Experimental
Ž .
Tetraethylorthosilicate TEOS was hydrolyzed
with a 0.1 N NH OH solution. The molar ratio of
4
TEOS:H 2O was 1:10. The mixture was magnetically
stirred to obtain a clear homogeneous solution Ž sol ..
The sol was cast into a polystyrene petri-dish and
covered with a polyethylene film. The sol transformed
into a gel within 24 h at room temperature.
The gels were aged and dried at 458C for 2 weeks
and finally heat treated at 10008C to form a rigid
porous silica matrix.
The porous silica gels were soaked with an
ethanolic solution of EuŽ NO .
3 3P5H 2O for 12 h. For
aluminum co-doping, AlŽ NO .
3 3P9H 2O was added
to the solution. The concentration of EuŽ NO .
3 3P
5H 2O was varied from 0.01 to 0.1 M and that for
AlŽ NO . P9H O from 0.1 to 0.5 M. After removing
3 3 2
from the solution, the gels were dried at 508C for 12
h and then densified at around 1125 to 11508C to
form dense, pore free silica glasses.
The pore volume and size were measured by
nitrogen adsorption on a Micromeritics AFAP-2010.
The UV–Vis absorption spectra were measured on a
Shimadzu UV-3101 spectrophotometer. Emission and
excitation spectra were recorded on a Shimadzu
RF5000U spectrofluorophotometer.
they became clear and pore free. Fig. 1 shows the
pictures of porous gels heated to 10008C and sintered
at 11408C. The concentrations of Eu 2O3 and Al 2O3
in the final glasses were calculated to be 1.1 and
0.325 wt.%, respectively, for the samples soaked in a
0.1 M solution of their nitrate salts.
Fig. 2 shows the room temperature absorption
spectra of Eu and Eu–Al impregnated densified
glasses. The Eu-doped glass Ž Fig. 2a.
shows two
absorption peaks at 396 and 465 nm corresponding
to the transition from the fundamental 7 F0
to the
excited 5 L and 5 6 D2
levels of Eu 3q ions, respectively,
and similar to those observed by others w13,17 x.
The Eu–Al doped glass, on the other hand, shows an
intense peak at 288 nm Ž Fig. 2b.
which corresponds
to the dipole allowed f–d transition of the Eu 2q ions
w15,16 x. Thus, in the Al co-doped glass Eu is present
in its divalent state.
Fig. 3 shows the fluorescence spectra of the Eu
and Eu–Al co-doped gels fired at 8008C. The samples
show similar spectra with peaks at 578, 590 and
614 nm for both samples, except with higher intensity
in the Eu–Al co-doped gel. The emission peaks
are due to the 5 D 7 0 FjŽ js0, 1, 2. transitions of Eu 3q
ions and are in good agreement with those reported
earlier w12,18,19 x. The increase in emission intensity
of Eu 3q with Al co-doping in a sol–gel glass has
also been reported previously w12 x. In the Al co-doped
silica matrix, the rare-earth ions are preferentially
surrounded by the Al 3q ions forming Al–O–RE
bonds in order to share oxygen atoms w20 x. Thus, a
3. Results and discussion
The 10008C fired undoped gels have a highly
porous interconnecting network. The adsorption
measurement shows the average pore size to be 8–10
nm with 60% porosity. The gels have a faint milky
appearance. However, when fired at 1125–11508C,
Fig. 1. Photograph of Eu–Al impregnated gel fired at Ž. a 10008C
and Ž. b 11408C.

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A. Biswas et al.rMaterials Letters 39 1999 227–231 229
Fig. 2. Room temperature absorption spectra of Ž. a Eu-doped glass
Ž. b Eu–Al co-doped glass, densified at 11408C. Inset shows
magnified spectrum of Ž. a .
Fig. 4. Fluorescence spectra of the glasses densified above and
Ž. Ž.
doped with a Eu and b Eu–Al.
larger separation among the rare earth ions is expected
in alumina co-doped silica.
The fluorescence spectra of the Eu and Eu–Al
co-doped glasses densified at 11408C are shown in
Fig. 4. The Eu-doped sample shows emission similar
to that heated at 8008C Ž Fig. 4a .. The Eu–Al doped
sample, however, does not exhibit any emission
related to Eu 3q ions. Instead, it shows a broad
intense peak at 430 nm Ž Fig. 4b .. The emission at
430 nm is due to the 4f 6 5d 1 4f 7 5d 0 transition of
Eu 2q ions and similar to that observed by Nogami et
al. w12x
in a porous Eu-doped glass reduced by
flowing H 2. The spectrum also matches with that
2q
due to the Eu ions in silica xerogel w16 x.
The reduction of Eu 3q ions in Al co-doped glasses
is further supported by the excitation spectra. Fig. 5
shows the excitation spectra of Eu and Eu–Al codoped
samples fired at 8008C, monitoring the emission
at 614 nm. Both spectra are identical with peaks
at 368, 387, 396 and 463 nm. The peaks represent
the typical transition of Eu 3q ions in a silicate matrix
as reported earlier w18,21 x.
The excitation spectrum of a Eu-doped glass fired
at 11408C Ž Fig. 6a.
is similar to its 8008C fired
counterpart Ž Fig. 5a ., but no excitation peak is observed
for the Eu–Al co-doped glass corresponding
to the 614 nm emission. When monitored at 430 nm,
the Eu–Al co-doped dense glass shows a broad
Ž. Ž.
Fig. 3. Fluorescence spectra of a Eu-doped gel and b Eu–Al
doped gel fired at 8008C.
Fig. 5. Excitation spectra of the gels fired at 8008C containing Ž. a
Eu and Ž. b Eu–Al.

230
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A. Biswas et al.rMaterials Letters 39 1999 227–231
excitation peak at 378 nm corresponding to the Eu 2q
ions Ž Fig. 6b ..
The emission spectra, excitation spectra and UV–
Vis absorption spectra all point to the fact that the
Eu 3q ions have been reduced to Eu 2y in the presence
of Al 3q during the densification of the gel
around 10008C. The Eu-doped gel heat treated at
10008C shows a broad emission with a peak at 354
nm Ž Fig. 7 .. This emission is associated with oxygen
Ž
y
associated hole centers O states.
and was reported
earlier w16 x. Since Eu ions are in the trivalent state,
there must be charge compensation within the silica
matrix from the nonbridging oxygen atoms. The
absence of a sufficient number of nonbridging oxygen
atoms to coordinate the isolated rare earth ions
in a rigid silica network may cause oxygen associated
hole centers at high temperature. The electron
emitted from the defect centers may react with Eu 3q
in the silica matrix to form Eu 2q in the presence of
alumina. Defect center induced reduction of Eu 3q
ions has already been reported in xerogels at room
temperature w16 x.
The spontaneous reduction of Eu 3q ions in Al
co-doped glasses can be explained by the observaw
x
3q
tion of Nogami et al. 11 who found that the Sm
ions can only be reduced by flowing H 2–N2
gas
mixture in the presence of Al 2O 3. They attributed
this to a phenomenon classified as microscopic basicity
in which the electron donating ability of oxygen
surrounding the rare earth ions is affected by the
3q
Al ions as proposed by Duffy and Ingram w22 x.
Fig. 7. Emission spectra of Eu 3q -doped gel heated to 10008C.
The lowered basicity in Al 3q co-doped gel together
with the low reduction potential of Eu 3q Eu 2q
Ž y0.32 E8, V. w23x
may account for the spontaneous
reduction of Eu 3q by the electron ejected from the
defect center at higher temperatures w17 x.
4. Conclusion
Eu-doped silica glasses with and without alumina
have been prepared by impregnating porous silica
monoliths with ethanolic solutions of EuŽ NO .
3 3P
5H O and AlŽ NO .
2 3 3P9H 2O and subsequent densification.
In the silica matrix, Eu maintains its trivalent
state throughout the densification process. In the
Eu–Al co-doped silica, the Eu ion is in a trivalent
state only up to around 10008C. In the presence of
Al 2O 3, it spontaneously reduces to its divalent state
during densification above 11008C.
Acknowledgements
Fig. 6. Excitation spectra of the glasses densified above and doped
Ž. Ž.
with a Eu and b Eu–Al.
This work was supported in part by the Directorate
of Chemistry and Life Sciences of the Air
Force Office of Scientific Research through contract
number F496209610124 and in part by the MURI
program through the University of Southern California
primary contract number 000507.

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A. Biswas et al.rMaterials Letters 39 1999 227–231 231
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