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

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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.