Characterization of silica nanocomposites obtained by sol-gel process using positron annihilation spectroscopy

Article abstract of DOI:10.1016/S0022-3697(98)00263-7

SiO₂ matrix was prepared by a sol-gel method using tetraethoxysilane, TEOS, ethanol and water in a 1/3/10 mole ratio, with HCl and HF as catalysts. This silica gel was doped with copper with different precursors and different contents of dopant. The samples were prepared into monolithic shape, dried at 110 °C for 24 h and thermally treated for 2 h at 500, 900 and 1100 °C. The structural evolution was studied by positron annihilation lifetime spectroscopy (PALS) which has been shown to be a useful tool for pore size analysis in several materials, X-ray diffraction (XRD) and nitrogen gas adsorption. All the samples have shown, according to the positron spectroscope results, a stable pore structure up to 900 °C and a strong densification process at 1100 °C.

Full text of DOI:10.1016/S0022-3697(98)00263-7

JPCS 1547
Journal of Physics and Chemistry of Solids 60 (1999) 211–221
Characterization of silica nanocomposites obtained by sol–gel
process using positron annihilation spectroscopy
a,
b
b
*
´
Edesia Martins Barros de Sousa , Welington F. de Magalha˜es , Nelcy D.S. Mohallem
a
´
´
CDTN/CNEN, Centro de Desenvolvimento da Tecnologia Nuclear, CP: 941, Rua Prof. Mario Werneck s/n, Campus Universitario, Pampulha,
Belo Horizonte, MG, Brazil CEP 30123-970
b
´
´
´
Departamento de Quımica, ICEX, UFMG, Laboratorio de Espectroscopia de Aniquilac¸a˜o de Positrons, LEAP, C.P. 702,
31270-901 Belo Horizonte, MG, Brazil
Received 31 March 1998; accepted 19 July 1998
Abstract
SiO2 matrix was prepared by a sol–gel method using tetraethoxysilane, TEOS, ethanol and water in a 1/3/10 mole ratio, with
HCl and HF as catalysts. This silica gel was doped with copper with different precursors and different contents of dopant. The
samples were prepared into monolithic shape, dried at 110ЊC for 24 h and thermally treated for 2 h at 500, 900 and 1100ЊC. The
structural evolution was studied by positron annihilation lifetime spectroscopy (PALS) which has been shown to be a useful tool
for pore size analysis in several materials, X-ray diffraction (XRD) and nitrogen gas adsorption. All the samples have shown,
according to the positron spectroscope results, a stable pore structure up to 900ЊC and a strong densification process at 1100ЊC.
᭧ 1998 Elsevier Science Ltd. All rights reserved.
Keywords: B. Sol–gel growth; C. Positron annihilation spectroscopy
1. Introduction
by supported copper chloride has been much studied [6]. In
earlier work on copper catalysis, several factors promoting
alkane formation were the subjects of research. Nowadays,
alternatives for copper-chromite catalysis are the subject of
extensive research and copper–silica-based catalysts have
been described in the scientific literature [7, 8]. However,
the sol–gel process allows the dissolution of the salt into the
silica matrix, which results in a mechanically more stable
catalyst. This material can probably be used for longer,
when compared with silica-supported copper catalysts
prepared by precipitation of salt onto silica particles.
According to Duval et al. [9], silica gels doped with copper
become ferromagnetic after heating and baking up to 400–
800ЊC. This fact is probably a consequence of the cluster
shape and of the interation of Cu2ϩ ions with the silica. The
third-order optical nonlinearity of the Cu-doped silica is
becoming an important area of research, and hence attract-
ing renewed interest from the perspective of production of
the metal-doped glasses [10, 11]. The incorporation of
copper ions into silica glass prepared by the sol–gel method
allows the establishment of a coloring mechanism by the
change of oxidizing state. The structural characterization
of these materials is of fundamental importance for the
The sol–gel method [1, 2] to produce glasses, ceramics
and glass–ceramics has been utilized for a variety of
products ranging from optical fibers, lenses, and special
coatings to the production of ultra-pure powders. The sol–
gel method allows a nanostructural control of ceramics,
based on the performance upon the formation and evolution
of nanometric units through process variables. Materials
obtained by this process present high surface area and
chemical composition homogeneity [3, 4]. By this process
it is possible to obtain composite materials through incor-
poration of small amounts of different components in the
ceramic matrix, at low temperatures, because it deals with
liquid reagents and network development by chemical reac-
tion [5].
Several ceramic composites have been obtained for a
wide variety of applications. Silica matrix embedded with
copper particles exhibits important properties in catalysis,
sensors, electronics, etc. The catalytic oxyhydrochlorination
* Corresponding author. Fax: ϩ 31-499-33990; e-mail: sousaem
@urano.cdtn.br
0022-3697/99/$ - see front matter ᭧ 1998 Elsevier Science Ltd. All rights reserved.
PII: S0022-3697(98)00263-7

212
E. Martins Barros de Sousa et al. / Journal of Physics and Chemistry of Solids 60 (1999) 211–221
control of the processing and final properties of these
materials.
2. Experimental
Different techniques have been used for morphological
characterization, such as pore diameters. Positrons have
been used as a nano-probe by PALS, in order to inves-
tigate the free volume (Vf) in several materials [12, 13].
When a positron enters a condensed medium, it may
annihilate directly with an electron, or it may capture
an electron to form a hydrogen-like atom, called positro-
nium, Ps. Positronium can exist in two forms: para-
positronium, p-Ps, and ortho-positronium, o-Ps. The
p-Ps is the singlet state with total spin of zero. It has
a self-annihilation lifetime in vacuum of 0.125 ns and
decays via two gamma emissions. The o-Ps is the triplet
state with a total spin of one. Its free space lifetime is
much longer, 140 ns, and decays via three gamma
emissions [14]. In this way, o-Ps survives long enough
to interact with surrounding atomic and molecular elec-
trons. This interaction leads to the shortening of its life-
time and to an annihilation with two gamma emission
through the process called pick-off annihilation. Measure-
ment of this o-Ps shortened lifetime can thus yield infor-
mation about the physical and chemical properties of the
medium.
The SiO2 matrix was prepared by the sol–gel method
using TEOS, ethanol and water in a 1/3/10 molar ratio,
with HCl and HF as catalysts. These silica gels were
doped with copper adding CuCl or CuSO4, at different
copper content (1 and 5 mol%), in the starting solution,
during agitation. The samples were prepared in monolithic
shape, dried at 110ЊC for 24 h and thermally treated for 2 h
at 500, 900 and 1100ЊC.
The structural evolution was studied by XRD in a
RIGAKU Geigerflex 3034 apparatus using a curved crystal
monocromator of graphite. All measurements were recorded
with 40 kV voltage and 30 mA current at room temperature.
The pore characteristics of the samples were analyzed by
using a nitrogen gas adsorption machine type Autosorb
(Quantachrome Nova 1200), which is an automated physical
adsorption system that provides accurate equilibrium
adsorption and desorption. Nitrogen gas was used for
approx. 5 h with a 22-point adsorption–desorption cycle.
The adsorber or desorbed volume data points obtained at
various relative pressure are reduced to produce surface
area, pore size distribution, micropore volume and average
pore radius by BET. The samples were outgassed for 3 h at
200ЊC.
PALS measurements were carried out using a fast–fast
coincidence system (ORTEC), with a time resolution of
280 ps, from the 60Co prompt curve. The 22Na positron
source, with 4.0 × 105 Bq activity, was sandwiched between
two 7 mm thick foils of KAPTON. Lifetime spectra were
resolved into three components, by the POSITRONFIT-
EXTENDED [16] program. In the lifetimes, ti, and asso-
ciated intensities, Ii, the subscripts i ˆ 1, 2 and 3 refer to p-
Ps, the free positron and o-Ps, respectively, and the super-
script ‘0’ refers to parameters obtained for the pure matrix.
The errors of the PAL parameters are nearly in the range of
s(t3) ഡ 3–5 ns and s(I3) ഡ 0.5–1.5%. These errors are stan-
dard deviations estimated from three or more spectra of each
sample obtained in the POSITRONFIT-EXTENDED
analysis.
The annihilation rate of the different positron species (li),
is given by:
Z
li ˆ tϪi 1 ˆ const × rϪꢀr†·rϩꢀr†dr
ꢀ1†
where rϪꢀr† and rϩꢀr† are the density of the electron and
positron at position r, respectively, and the parameter
‘const’ is the normalization constant related to the number
of the electrons involved in the annihilation. According to
the free volume model for o-Ps pick-off annihilation, Ps is
confined to a rigid spherical potential well, its mean lifetime
depends on the radius, R, of this free volume following the
equation [13–15]:
ꢂ
ꢀ
ꢁꢃ
Ϫ1
1
R
1
2pR
ˆ t3 ˆ 0:5 ns 1 Ϫ
ϩ sin
ꢀ2†
l3
R0
2p R0
˚
where DR ˆ R0 Ϫ R ˆ 1:656 A corresponds to the width of
the electron layer of the atoms of the material that penetrates
inside the potential well of radius R0, in which one Ps acts
and annihilates by pick-off. As by hypothesis o-Ps is
confined to the spherical free volume, the relation between
R and Vf is given by:
3. Results and discussion
3.1. Sample preparation
After the drying stage, monolithic porous gels samples
were obtained without macroscopic defects. The samples
remained undamaged after the different thermal treatments.
The dried matrixes were transparent above 500ЊC. The
doped gels, initially green, changed from the original
color to blue at 500ЊC and then dark brown at 900ЊC. At
1100ЊC, the samples doped with CuCl presented a black
color, and that one doped with CuSO4, presented a dark
yellow color.
Vf ˆ 4pR3=3
ꢀ3†
In this work SiO2 matrixes were prepared by the sol–gel
process and doped with copper salts. This work was
intended to analyze the structural evolution of Cu-doped
porous silica gels through PALS, XRD and BET techniques
following specific thermal treatments.

E. Martins Barros de Sousa et al. / Journal of Physics and Chemistry of Solids 60 (1999) 211–221
213
Fig. 1. Variation of o-Ps lifetime of the pure silica matrix (A), doped with 1% (W) and 5% (K) CuCl, as a function of temperature.
3.2. Positron annihilation lifetime spectroscopy (PALS)
that a t3 decrease is expected with the increasing C accord-
ing to:
The PALS parameters for the compounds obtained are
shown in Table 1, determined from the spectra analyses
with three components (exponential decay) fixing the life-
time of p-Ps at 0.120 ns.
Ϫ1
ꢀt3† ˆ ꢀto3†Ϫ1 ϩ koxC
ꢀ4†
where to3 is the o-Ps lifetime in the pure matrix. A decrease
of t3 effectively observed in the dried samples at 110ЊC
confirms the chemical attribution of the quenching effect;
thus, the small values of t3 in these samples could not be
associated with the small size of the pores.
The high values of the t3 observed in the samples doped
with CuCl, after thermal treatment at 500 and 900ЊC, can be
understood by either or both of the following hypotheses:
Fig. 1 shows the variation of the o-Ps lifetime formed in
the matrix doped with 1 and 5% of CuCl as a function of
different thermal treatments. By analyzing the results of the
pure silica matrix, it is observed that the o-Ps lifetime, t3, in
this material, treated below 1100ЊC, is high and almost
constant. A significant decrease of t3 is observed at
1100ЊC. These results suggest that at this temperature an
intense decrease in the free volume size occurs, according
to values of the free volume radii and average free volume
size, obtained by Eq. (2) and Eq. (3), respectively, and are
shown in the Table 1.
For the binary systems containing 1 and 5% of the CuCl,
the general behavior is similar to that in silica matrix, except
for the composites dried at 110ЊC. In this case, t3 takes
values as small as those found in the composites densified
at 1100ЊC. This result is attributed to a chemical effect
promoted by the copper ions dissolved into the matrix
which reduce the o-Ps lifetime by an o-Ps oxidation reaction
• a change in the chemical state of the copper ions leading
it to chemically inactive form from the perspective of Ps
inhibition and quenching;
• a segregation of these ions to a second phase of copper
oxide, transforming a solid solution into a biphasic
system after thermal treatment. In this case, as the system
is practically pure silica phase, the t3 values are almost
that of the pure matrix.
Fig. 2 shows the variation of the o-Ps lifetime formed in
the pure matrix and doped with 1 and 5% of CuSO4 as a
function of different thermal treatments. As the relation
between the pore radius, R, and t3 is almost linear in the
range of right t3, the variation of R as a function of the
heating temperature has the same pattern, except for the
samples doped with 1% of CuSO4 at 110ЊC (and also 1
and 5% of CuCl at 110ЊC), as depicted in Fig. 2 (and also
Fig. 1). By analyzing the results of the sample doped with
1% of CuSO4, it could be verified that the general behavior
is the same as that observed for the samples doped with 1
and 5% of CuCl; that is, t3 presents short values at 110ЊC
which are related to the quenching discussed previously.
This reaction transforms the long lifetime species into
another of smaller lifetime. This process is known as Ps
quenching. The efficiency of this process increases with
the concentration, C, of the quencher and depends on the
reaction rate constant, kox. In these cases, it could be shown

214
E. Martins Barros de Sousa et al. / Journal of Physics and Chemistry of Solids 60 (1999) 211–221
Fig. 2. Variation of o-Ps lifetime of the pure silica matrix (A), doped with 1% (W) and 5% (K) of CuSO4, as a function of temperature.
However, for the system with 5% of CuSO4 content, this
behavior is not verified at 110ЊC. Here, t3 presents a high
value similar to those of pure silica in the same condition.
This fact could be related to the immiscibility of the salt in
this system during the sol preparation, gelation or drying
process. The segregation of the salt could have occurred
in a second phase leading to a silica network free of the
copper ions, known as Ps inhibitor and quencher. Once
the dopant content is low, the volume fraction of this salt
phase is very small when compared with that of the silica
phase. This results in a low probability of finding the
injected positron in the salt phase. In this salt phase, no Ps
formation is expected, as is the case in all ionic solids, and
all the Ps formed in the system comes from the silica phase.
When the density and the positron linear absorption coef-
ficient (or positron penetration profile) of both phases are
comparable, the o-Ps yield, I3, is a linear function of the
Table 2
Table 1
Structural characteristics of the samples studied at different
temperatures obtained by mercury porosimetry (measurements are
accurate to within 7%)
PALS parameters for the silica composites with different composi-
tion and thermal treatment temperatures and free volume radii and
volume
T
(ЊC)
rpic
rpor
Porosity Radio
(g/cm3) (g/cm3) (%)
˚
3
T (ЊC) t3 (ns) I3 (%) R (A) Vf × 10Ϫ4 (A )
Sample
Matrix
(A)
˚
˚
Sample
Matrix
110
500
900
1100
110
500
900
1100
110
500
900
1100
46.97 28.98 12.32 0.7834
52.35 33.48 12.84 0.8876
64.69 47.04 13.92 1.1304
53.72 2.38
11.23
55.24 27.14 13.11 0.9441
57.21 32.14 13.29 0.9827
42.37 1.70
2.14
48.93 16.28 12.52 0.8212
65.89 12.14 14.02 1.1542
48.26 1.78
11.52
53.00 18.92 12.91 0.9003
53.35 21.51 12.94 0.9071
110
500
900
0.63
0.58
1.89
0.92
0.90
1.94
—
1.25
1.01
1.29
—
1.19
1.17
1.32
—
0.94
0.91
1.34
—
38
52
42
4
54
53
50
—
62
65
68
—
64
61
58
—
66
67
61
—
63
59
62
—
1.54
5.21
0.0057
—
1100 2.14
Si–1% CuCl
Si–5% CuCl
—
Si–1% CuCl
Si–5% CuCl
110
500
900
0.70
0.68
0.83
46
51
46
5
1.21
1.82
0.0032
—
1100 1.89
—
110
500
900
0.87
0.74
1.10
45
52
32
7
1.08
2.60
0.0024
—
1100 1.17
Si–1% CuSO4 110
—
Si–1% CuSO4 110
0.74
0.69
0,95
45
47
43
9
500
900
500
900
1100 1.54
1100
1.10
67.19 1.81
0.0025
Si–5% CuSO4 110
47.36 14.88 12.36 0.7909
43.36 10.30 11.95 0.7140
50.49 22.37 12.67 0.8515
Si–5% CuSO4 110
0.72
0.70
0.74
1.13
0.79
1.14
—
30
48
28
11
500
900
500
900
1100 0.98
1100
1.07
44.84 1.77
0.0023

E. Martins Barros de Sousa et al. / Journal of Physics and Chemistry of Solids 60 (1999) 211–221
215
Fig. 3. Variation of o-Ps intensity of the pure silica matrix (A), doped with 1% of CuCl (W) and 1% of CuSO4 (K) as a function of temperature.
contents of the phase forming Ps, and t3 is independent of
the dopant content and equal to that of the phase forming Ps
[15]. In our case the salt phase has a higher density
(rꢀCuSO :5H O† ˆ 2.28 g/cm3) than the silica phase (Table 2)
0.52 (mol%)Ϫ1 at 900ЊC. Thus, between 900 and 1100ЊC
the average free volume size strongly decreases and the
number or concentration of free volumes increases with
the heat thermal treatment.
In the system doped with CuSO4, there is no clear beha-
vior for the inhibition effect as a function of salt concentra-
tion. This result is evidence for the complex behavior of the
SiO2/CuSO4 composite concerning its structure and reactiv-
ity towards positronium and their precursors.
4
2
and certainly a higher positron absorption coefficient. Under
this condition a lower I3 is expected than that predicted by
the linear relation, as was observed (Table 1).
In these materials a crescent intensity of the o-Ps forma-
tion I3 was observed, whose temperature reached a maxi-
mum value at 1100ЊC. Fig. 3 shows the variation of the
intensity of the o-Ps formation I3 in the pure matrix and
doped with 1% of CuCl and 1% of CuSO4 as function of
the different temperatures. The decrease of I3 with the
increase in CuCl concentration is consistent with a process
of total inhibition with a inhibition constant of nearly
2 (mol%)Ϫ1 at 110ЊC, 0.22 (mol%)Ϫ1 at 500ЊC and
3.3. XRD characterization
For the SiO2 matrix treated up to 1100ЊC, the XRD
patterns exhibit amorphous behavior. However, an increase
in the intensity and a narrowing in the diffraction band is
observed with increasing temperature. This fact indicates an
Fig. 4. X-ray diffraction patterns for the samples doped with 5% CuCl and heat at (a) 900ЊC; and (b) 1100ЊC.

216
E. Martins Barros de Sousa et al. / Journal of Physics and Chemistry of Solids 60 (1999) 211–221
Fig. 5. X-ray diffraction patterns for the samples doped with 5% CuSO4 and heat at (a) 900ЊC; and (b) 1100ЊC.
increase in the tetrahedron organizations of the silica
samples. For the binary systems, Fig. 4 and Fig. 5 show
the evolution of X-ray patterns of samples doped with 5%
of CuCl and CuSO4, respectively, treated at 900 and 1100ЊC.
The results show traces of the crystallization of silica gel by
diffraction peaks of quartz and cristobalite at 1100ЊC. It
could be observed that the introduction of the dopants in
silica gels results in the crystallization of the glass. SiO2
matrix with 5% content of CuCl exhibits amorphous beha-
vior up to 500ЊC. The very weak diffraction peaks present at
900ЊC were attributed to the crystal structure of tenorite,
CuO. However, a significant effect was verified as the
sample was treated at 1100ЊC. At this temperature, the
amorphous matrix converts to cristobalite and quartz
Table 3
Average values of the structural characteristics of the samples at different temperatures obtained by BET (measurements are accurate to within
5%)
T
(ЊC)
Area
Pore size
(A)
Vp × 103
Micro area
Meso Area
3
Sample
Matrix
(m2/g)
(cm /A)
(m2 gϪ1
)
(m2 gϪ1
)
˚
˚
110
500
900
1100
110
500
900
1100
110
500
900
1100
110
500
900
1100
110
500
900
1100
382
484
10.68
—
306
442
8.21
1.11
312
485
40
4.42
293
318
159
6.47
254
351
163
8.36
16.89
17.10
7.44
171
445
0.131
—
161
274
3.02
0.83
189
251
25
3.85
408
409
172
1.76
196
611
143
10.55
243
341
0.35
—
214
358
4.74
0.73
279
401
33
3.37
213
194
90.2
3.73
237
313
94
101
143
0.33
—
—
Si–1% CuCl
Si–5% CuCl
Si–1% CuSO4
Si–5% CuSO4
16.95
17.02
12.73
5.0
12.15
15.53
12.13
7.42
15.68
16.92
11.73
5.03
12.08
16.93
13.91
5.21
91.0
84.0
3.47
0.38
32.0
84.0
6.28
1.05
79.0
124
69.0
2.71
17.0
39.0
69.0
4.10
4.26

E. Martins Barros de Sousa et al. / Journal of Physics and Chemistry of Solids 60 (1999) 211–221
217
Fig. 6. Variation of density as a function of temperature for silica matrix (A), doped with 1 (W) and 5% (K) CuCl and 1 (S) and 5% (*) CuSO4.
beyond to favor the crystallization of the tenorite. The X-ray
patterns of samples containing 5% of CuSO4 show the
decrease in the content of amorphous silica with the increase
in temperature. At 110 and 500ЊC, the CuSO4.5H2O phase
was detected, which could be related to the immiscibility of
the salt at the sol preparation. This fact indicates the prob-
ability of the concentration being near or above the solubi-
lity of CuSO4 in this system. At 900ЊC, typical peaks of the
copper oxide, CuO can be observed. The X-ray patterns at
1100ЊC show a set of feature peaks of different silica phases,
cristobalite and quartz, of CuO, besides suggesting the
appearance of the second phase of the copper oxide,
Cu2O. This phase is not very distinct in the X-ray patterns
because their most intense peaks are near or over those of
the crystalline silica. This fact made the identification of the
Cu2O in this sample difficult. However, this was further
corroborated with EXAFS according to the study by Sousa
[17].
3.4. Structural characteristics measured by BET and
mercury porosimetry
The structure of the gel strongly depends on the experi-
mental conditions during the hydrolysis reaction, that is,
water/precursors ratio, type and concentration of cata-
lysts, temperature, and so on. When the gels are
produced by the complete hydrolysis in solution, the
dried gels can present a high concentration of hydroxyl
Fig. 7. Variation of surface area as a function of temperature for silica matrix (A), doped with 1 (W) and 5% (K) CuCl and 1 (S) and 5% (*)
CuSO4.

218
E. Martins Barros de Sousa et al. / Journal of Physics and Chemistry of Solids 60 (1999) 211–221
Fig. 8. Pore size distribution from the BET for the pure silica sample at (A) 110ЊC; (W) 500ЊC; and (K) 900ЊC.
groups chemically bonded, due to the re-esterification
reaction during the dry process. As the gels are heated,
the alkoxy groups and the hydroxyl groups are removed
by condensation reaction, which provokes substantial
weight loss in the material. These reactions produce cross-
linking and consequently gel network shrinkage or densifi-
cation occur.
Table 2 shows the values of the structural characteristics
obtained by mercury porosimetry and picnometry for dried
samples and treated at 500, 900 and 1100ЊC. In this table,
rpic and rpor represent volumetric density of picnometry and
porosimetry, respectively. The surface area, average pore
size, pore size distribution and total pore volume were
obtained through BET. These results are shown in Table
3, for the samples treated at different temperatures.
The organic retained and the dopants added to the xerogel
can exert effects on the structure of the material. The density
measured by picnometry and the surface area measured by
BET of different samples are shown in Fig. 6 and Fig. 7,
respectively as a function of the temperature. Analysis of the
values obtained, reveals that the density of the gels
decreases at 500ЊC. At this temperature, the surface area
and the porosity present maximum values. At 110ЊC, the
pure silica samples were yellowish, probably due to the
organic impregnation at the network. As the samples were
treated at 500ЊC, the organics were removed and the samples
Fig. 9. Pore size distribution from the BET for the silica sample doped with 1% CuCl at (A) 110ЊC; (W) 500ЊC; (K) 900ЊC; and (K) 1100ЊC.

E. Martins Barros de Sousa et al. / Journal of Physics and Chemistry of Solids 60 (1999) 211–221
219
Fig. 10. Pore size distribution for the silica by mercury porosimetry at (A) 110ЊC; (W) 500ЊC; and (K) 900ЊC.
lost weight. Despite this weight loss in the material, this
temperature range provokes little shrinkage. Consequently,
the density decreases, considering that the weight loss is
more significant than the shrinkage at this temperature. Once
the organics are eliminated in the network, the increase of the
free volume contributes to the increase of the porosity and the
solid–pore interface, which can be verified in Fig. 7. Beyond
500ЊC, the samples began densification as the pores gradually
collapsed with the increasing temperature, causing high
shrinkage in the samples. Values of the porosity obtained by
the mercury porosimetry confirm this behavior, that is, the
materials present maximum porosity at 500ЊC, which suggests
a large amount of the free volume in the gel networks.
The surface area of the samples dried at 110ЊC is approx.
350 m2/g. Although an increase of the surface area is
observed at 500ЊC, it decreases significantly at 900ЊC and
above. With the aid of the Autosorb program, it is possible
to separate the contributions of the micropore area and the
mesopore area to the total surface area. It is important to
˚
note that pores with openings not exceeding 20 A are called
˚
micropores, and sizes between 20 and 500 A are called
mesopores. Table 3 shows the contributions of these two
ranges of pore size for the different samples.
These results show that there is a significant contribution
of mesopores in the material dried at 110ЊC. As the firing
temperature increases from 500ЊC on, the surface area due
Fig. 11. Pore size distribution for the silica sample doped with 5% CuCl by mercury porosimetry at (A) 110ЊC; (W) 500ЊC; and (K) 900ЊC.

220
E. Martins Barros de Sousa et al. / Journal of Physics and Chemistry of Solids 60 (1999) 211–221
the presence of micro and mesopores decreases, but the contri-
bution of the mesopore area to the total surface becomes
progressively bigger. These results can be attributed to the
large number of micropores present in the samples treated at
low temperature. As the sample is heated from 900ЊC, a large
decrease of the micropore volume in the gel occurs, without
altering significantly the amount of the mesopore.
porosimetry provided suitable data for characterization of
the samples studied and proved to be complementary tech-
niques used to better understand the structural evolution of
these materials.
As the positron probe is able to investigate all the pores,
even though not connected to the surface of the crystals, and
particularly the narrow pores, the average pore radius
measured by PALS is the lowest among these three techni-
ques. The shrinkage of the pores observed by BET and
mercury porosimetry at 900ЊC and further at 1100ЊC trans-
The distribution of pore volume of the different materials
with respect to pore size obtained by BET (Figs. 8 and 9 )
presents some important features. The average pore size of
all samples treated up to 900ЊC is very small, ranging from
˚
forms the large pores into little free volumes of nearly 2 A or
˚
˚
12 to 17 A, and approximately 5 A for those treated at
less. This appears to be more efficient at inducing Ps forma-
tion when compared with the large pores, considering the
higher o-Ps intensity observed at the higher firing tempera-
ture. This conclusion implies that Ps is formed inside the
silica particles and then migrates toward the pores, where it
survives.
1100ЊC. Equally important, the distribution is very narrow
˚
with the maximum value not exceeding 80 A. That is, the
mesopores present in the material are only 5 times the size of
the average micropore. Mercury porosimetry confirms this
˚
fact. Samples containing pore sizes above 30 A can be
analyzed via this technique. The behavior was similar for
all the samples.
The XRD patterns indicate that for the samples prepared
from CuSO4, the formation of CuO occurs at 900ЊC and the
evolution to Cu2O at 1100ЊC. In the case of CuCl samples, it
occurs only after the formation of CuO at 900 and 1100ЊC.
For temperatures lower than 500ЊC, an amorphous material
is formed. The data showed the significant evolution for the
crystalline phases of the silica of cristobalite and quartz at
1100ЊC.
By analyzing the results obtained by mercury porosimetry
for the different materials, it is observed that for those for
pure silica, the variation in the pore size distribution occurs
in such a way that the total volume of pores decreases while
˚
the average pore size remains fairly constant, around 50 A,
as densification proceeds. At 900ЊC the samples suffer a
strong densification process, according to Fig. 10.
The samples doped with different salt content behave
similarly to the matrix. This presents an average pore size
of approx. 60 A, and a strong drop of the pore volume occurs
Acknowledgements
˚
at 900ЊC. These results can be observed in Fig. 11.
This behavior was similar for all the samples doped with
different copper contents. However, the values of the density
and surface area are different for each sample. The differ-
ence in the densification process can be attributed to differ-
ences in the degree of crosslinking that occurred as a result
of the combination of the silanol groups (Si-OH) on the pore
surface with the liberation of H2O and the formation of Si–
O–Si bonds.
Molecular water can be removed at relatively a low
temperature, but hydroxyl groups were only removed in
the range 800–1000ЊC. It was observed that the gel bloated
or cracked into fine pieces around 800ЊC when heating was
performed as initially proposed. This suggests that there is a
critical temperature for water removal at approx. 800ЊC.
The proposed procedure for the heat treatment from 900
to 1100ЊC (heating rate of 10ЊC/min from 150ЊC) leads to
closure of the pores with trap water, organic residues, and
carbon causing bloating. In this way, careful heat treatment
around this temperature was conducted to try to obtain bulk
glass.
This work has been supported by CNPq, FAPEMIG and
PADCT. The authors thank Walter de Brito of CDTN/
CNEN for the XRD measurements.
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