Thermal and microchemical characterisation of sol-gel SiO2, TiO2 and xSiO2-(1-x)TiO2 ceramic materials

Article abstract of DOI:10.1023/A:1012471112566

Amorphous SiO₂, TiO₂ and xSiO₂-(1-x)TiO₂ ceramic materials with selected values of x 0.5, 0.7 and 0.9, have been prepared via sol-gel process using silicon tetraethoxysilane (TEOS) and titanium tetraisopropoxide Ti(OPri)₄. By means of the combined use of differential thermal analysis (DTA), thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photo-electron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES), the surface microchemical structure and morphology of the sol-gel materials have been studied as a function of thermal treatments carried out in air up to 1200°C. In the range of temperature from 50 to 450°C, DTA-TG results evidence a remarkable mass loss due to the evaporation of organic solvents entrapped in the sol-gel materials and of the remnant organic components of the precursor metal alkoxides. In the range of temperature from 400 to about 1000°C, by means of the combined use of DTA, XRD, XPS and XAES techniques as a function of temperature and of chemical composition, it is possible to evidence the formation of crystalline phases such as quartz, anatase and rutile. Furthermore, lineshape analysis of O 1s XPS peak allows to distinguish between single O-Ti and O-Si bonds and also to disclose the presence of cross linking Si-O-Ti bonds, that act as bridges between SiO₂ and TiO₂ moieties. As a function of temperature, Si-O-Ti bonds are broken and the formation of new Ti-O and Si-O bonds as in TiO₂ and SiO₂ takes place as well as a silica segregation phenomenon.

Full text of DOI:10.1023/A:1012471112566

Journal of Thermal Analysis and Calorimetry, Vol. 66 (2001) 37–46
THERMAL AND MICROCHEMICAL
CHARACTERISATION OF SOL-GEL SiO2, TiO2
AND xSiO2–(1–x)TiO2 CERAMIC MATERIALS
G. M. Ingo1, C. Riccucci1, G. Bultrini2, S. Dirè 3 and G. Chiozzini1
1CNR, Istituto di Chimica dei Materiali, CP 10, 00016 Monterotondo Stazione, Roma, Italy
2Dipartimento di Chimica, Università di Catania, v.le A. Doria 6, 95125 Catania, Italy
3Dipartimento Ingegneria dei Materiali, Università di Trento, 38050 Mesiano-Trento, Italy
Abstract
Amorphous SiO2, TiO2 and xSiO2–(1–x)TiO2 ceramic materials with selected values of x 0.5, 0.7 and
0.9, have been prepared via sol-gel process using silicon tetraethoxysilane (TEOS) and titanium
tetraisopropoxide Ti(OPri)4. By means of the combined use of differential thermal analysis (DTA),
thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photo-
electron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES), the surface
microchemical structure and morphology of the sol-gel materials have been studied as a function of
thermal treatments carried out in air up to 1200°C. In the range of temperature from 50 to 450°C,
DTA-TG results evidence a remarkable mass loss due to the evaporation of organic solvents entrapped
in the sol-gel materials and of the remnant organic components of the precursor metal alkoxides. In the
range of temperature from 400 to about 1000°C, by means of the combined use of DTA, XRD, XPS
and XAES techniques as a function of temperature and of chemical composition, it is possible to evi-
dence the formation of crystalline phases such as quartz, anatase and rutile. Furthermore, lineshape
analysis of O 1s XPS peak allows to distinguish between single O–Ti and O–Si bonds and also to dis-
close the presence of cross linking Si–O–Ti bonds, that act as bridges between SiO2 and TiO2 moieties.
As a function of temperature, Si–O–Ti bonds are broken and the formation of new Ti–O and Si–O
bonds as in TiO2 and SiO2 takes place as well as a silica segregation phenomenon.
Keywords: DTA-TG, microchemical structure, oxide ceramic materials, SEM, silica, sol-gel pro-
cessing, surface, titania, XPS
Introduction
The low-temperature sol-gel synthesis of glasses and ceramic materials received con-
siderable scientific and technological interest during the last years [1–8]. Indeed,
starting from liquid molecular precursor, it is possible to produce via hydroly-
sis-condensation reactions, a great variety of unusual multication ceramic oxide ma-
terials, the chemical composition and structure of which can be tailored by the chemi-
cal design of molecular precursors and synthesis parameters. Furthermore, the sol-gel
synthesis offers opportunities for producing also fine ceramic powders with a high
1418–2874/2001/ $ 5.00
Akadémiai Kiadó, Budapest
© 2001 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

38
INGO et al.: SOL-GEL SiO2, TiO2 AND xSiO2–(1–x)TiO2 CERAMIC MATERIALS
surface area and high purity as well as films and coatings with tailored chemical and
physical features.
By means of the combined use of differential thermal analysis (DTA), thermo-
gravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM),
X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectros-
copy (XAES), SiO2, TiO2 and xSiO2–(1–x)TiO2 sol-gel ceramic materials, with se-
lected values of x 0.5, 0.7 and 0.9, have been studied for identifying the variations of
the microchemical and microstructural structure as a function of temperature includ-
ing temperature dependent changes of the surface chemical composition, bonding
and structure.
Experimental
SiO2–TiO2 ceramic materials were prepared at room temperature (r.t.) according to the
experimental procedure reported in reference [3–7]. The method is based on the use of a
solution of tetraethoxysilane (TEOS) with absolute ethanol (EtOH) and water in the mo-
lar ratio TEOS:EtOH:H2O=1:2:1 adjusting the pH of water at 1 by adding hydrochloric
acid. After stirring the solution for 6 h, titanium tetraisopropoxide (TIPR) was added. The
sol was poured in an open vessel and the sol-gel transition took place in three days. The
gel was then dried at r.t. for seven days. SiO2 was obtained from TEOS:EtOH:H2O=1:2:1
solution according to the above mentioned conditions. TiO2 was prepared by adding wa-
ter to TIPR under stirring for 2 h. The TiO2 precipitate was filtered, washed with distilled
water and dried at 80°C.
DTA-TG measurements were carried out in air from room temperature to
1200°C using an automated thermal analyser (Stanton Redcroft STA-781) at a heat-
ing rate of 10°C min–1.
The samples for XPS and XRD analysis were prepared first drying the sol-gel
materials at room temperature for seven days and then air heating them to the desired
temperature with a heating rate of 10°C min–1 and maintaining the samples at the se-
lected temperature for 2 h.
XPS study was carried out on an Escalab Mk II spectrometer using MgKα1,2 radi-
ation as excitation source (hν=1253.6 eV). The electron hemispherical analyser was
operated in fixed analyser transmission mode (FAT) by selecting a constant pass en-
ergy of 20 eV. Under these conditions, the full width at half maximum (FWHM) of
argon ion cleaned Ag 3d5/2 line was 1.0 eV. Binding energies (BEs) were referenced
to the Fermi level of the analyser and confidence in the linearity of BE scale was
based upon setting the position of Au 4f7/2, Cu L3M45M45 and Cu 2p3/2 peaks at 84.0,
567.8 and 932.5 eV, respectively. Corrections for the energy shift, due to the steady
state charging effect, were accomplished by assuming the C 1s line, resulting from
the adsorbed hydrocarbons, as lying at 285.0 eV. Furthermore, on pure sol-gel SiO2
and TiO2 powders, 20 nm thick gold film has been deposited to provide an Au 4f7/2 BE
reference. As previously observed by other authors [9], two different charging cor-
rections procedures result in essentially identical BE data. Reproducibility in the re-
sults was ±0.15 eV. XRD patterns were recorded directly on the powders by multiple
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39
scanning using an automated Seifert XRD-3000 diffractometer. The identification of
the species was carried out by using a Seifert XDAL 3000 Software Index I. SEM and
EDS characterisation was carried out by using a Cambridge 360 scanning electron
microscope equipped with an EDS apparatus and a LaB6 filament.
Results and discussion
Figure 1 shows the micromorphology of the 50SiO2–50TiO2 and 90SiO2–10TiO2 sol-gel
materials in as prepared conditions after grinding (hereafter a.p.) and after an air thermal
treatment (t.t.) carried out at 1000°C for 2 h in air, micrographs A, C, B and D, respec-
tively. Under these experimental conditions, the back scattered electrons (BSE) images
do not disclose evidence of an appreciable variations of the morphology in the as pre-
pared sol-gels as a function of the chemical composition. Furthermore, BSE images show
that the air thermal treatment at 1000°C induces only the formation of cracks and of small
particles scattered on the surface of the particles.
Fig. 1 Back scattered electrons (BSE) images that show the micromorphology of the
50SiO2–50TiO2 sol-gel ceramic in as prepared condition, of the 90SiO2–10TiO2
sol-gel ceramic in as prepared conditions, of the 50SiO2–50TiO2 sol-gel ceramic
after an air thermal treatment at 1000°C and of the 90SiO2–10TiO2 sol-gel ce-
ramic after an air thermal treatment at 1000°C, micrographs A, C, B and D, re-
spectively
DTA-TG curves for SiO2, TiO2 and 50SiO2–50TiO2 materials are reported in
Fig. 2 for showing the typical air thermal behaviour of the sol-gel material. Further-
more, in order to show the occurrence of crystallisation processes during the air ther-
mal treatment, in Fig. 3 are reported some selected XRD patterns for TiO2 after an air
thermal treatment carried out for 2 h at 400, 800 and 1200°C.
In Table 1 are listed the binding energies (BEs, expressed in eV) of Si 2p, Ti
2p3/2, O 1s XPS peaks and XAES α’ parameters [10,11] for as prepared and air ther-
mal treated SiO2, TiO2 and SiO2–TiO2 sol-gels as well as for some reference com-
pounds.
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It is worth noting that XAES α’ parameter is obtained by means of the combined
use of both the photoelectron and X-ray induced or Bremstrahlung Auger line accord-
ing to:
α’=KE(Auger peak)–KE(photoelectron peak)+hν
So conceived α’ has the advantages of being independent of photon energy, of
always having positive values and is independent of the charge referencing operation.
Therefore, α’ is found with higher accuracy than the determination of either photo-
emission line energy alone.
Fig. 2 DTA-TG curves for SiO2, TiO2 and 50SiO2–50TiO2 sol-gel materials (air, heat-
ing rate 10°C min–1)
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41
The surface chemical composition has been obtained also via XPS analysis and
the presence of O, Si and Ti was monitored as well as a small amount of carbon only
on the as prepared materials. The presence of this latter element indeed disappears af-
ter the first thermal treatment carried out at 400°C in air.
This phenomenon can be related to the TG curves reported in Fig. 2 that show an
appreciable mass loss occurring in the temperature range from room temperature to
450°C as well as to the endothermic minima in the DTA curves. These results provide
information on the evaporation of physically absorbed water and other solvents and
on the cracking of organic compounds coming from the remnant contamination of the
precursor solution. Indeed, the presence of these latter compounds in the as prepared
samples is indicated by C 1s signals as well as from the lineshape analysis of the Ti
2p3/2 and Si 2p photoemission peaks (not shown). These latter signals are peaked at
459.2±0.1 and 103.3±0.1 eV, respectively and are slightly broadened on the low
binding energy side. This asymmetry is due to the presence of Ti–O–CH(CH3)2 and
Si–O–CH2CH3 bonds from the precursor solution and disappears when the sol-gel
materials are subjected to air thermal treatments at 400°C.
At higher temperature, the line shape of XPS peaks Si 2p and Ti 2p3/2 peaksdoes
not change appreciably thus indicating that the chemical bonds of silicon and tita-
nium can be described as Ti4+ and Si4+ oxides [3]. Furthermore, we note that BEs for
Ti 2p3/2 and Si 2p peaks, in some cases, slightly increase with increasing temperature,
but this variation is within the experimental uncertainty.
It is worth noting that the organic compounds are removed with increasing tem-
perature from the network of the sol-gel ceramic without inducing an appreciable
variation of the surface micromorphology i.e. without forming pores observable via
scanning electron microscopy (SEM).
With increasing temperature (higher than 400°C), other microchemical or mic-
rostructural changes could take place such as surface chemical composition variation
and crystallisation phenomena of the sol-gel ceramic materials. Both are important
phenomena associated with thermal treatment of sol-gel materials and these events
have been followed by means of the combined use of DTA-TG, XRD and
XPS-XAES techniques in the temperature region from about 400 to 1000°C.
DTA-TG curves shown in Fig. 2 for SiO2, TiO2 and 50SiO2–50TiO2 powders,
evidence thermal induced modifications disclosing exothermic peaks at different
temperatures likely due to pyrolysis phenomena while TG curves do not show appre-
ciable variations.
By combining XRD patterns, DTA curves and XPS-XAES results reported in
Table 1, it is possible to establish that a main variation suffered by SiO2, TiO2 and
xSiO2–(1–x)TiO2 sol-gel ceramics with increasing temperature is due to the variation
of structure. In particular, XPS-XAES results indicate that the silicon and titanium α’
Auger parameters for both SiO2, TiO2 and SiO2–TiO2 powders, vary as a function of
the thermal treatment temperature from about 1711.5 to about 1712.0 eV, and from
871.9 to 873.2±0.2 eV, respectively. These latter changes are due to the crystallisa-
tion of amorphous SiO2 and TiO2 to quartz, anatase and rutile. Indeed, the comparison
between XPS-XAES data for the sol-gel materials and the titanium α’ Auger parame-
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Table 1 Binding energies of Si 2p, Ti 2p3/2, O 1s photoemission signals and α’ Auger parameters
(expressed in eV) for as prepared (a.p.) and air thermal treated at 1000°C (t.t.) SiO2,
TiO2 and xSiO2–(1–x)TiO2 sol-gel materials as well as for some reference compounds
Sample
O 1s
Si 2p
–
Ti 2p3/2
459.3
α’(Si)
α’(Ti)
530.6
532.0
–
871.9
TiO2 a.p.
TiO2 t.t. (1000°C)
SiO2 a.p.
530.6
–
459.3
–
–
873.1
–
531.7
532.7
103.4
1711.4
SiO2 t.t. (1000°C)
532.7
103.6
103.6
–
1712.0
1711.4
–
530.6
531.4
532.7
459.2
871.9
50SiO2–50TiO2 a.p.
530.6
531.5
532.7
103.6
103.3
103.7
103.2
103.4
459.3
459.1
459.3
459.3
459.2
1712.0
1711.6
1711.9
1711.6
1711.9
872.8
871.8
873.1
871.5
873.4
50SiO2–50TiO2 (1000°C)
70SiO2–30TiO2 a.p.
530.7
531.6
532.7
530.7
531.6
532.6
70SiO2–30TiO2 (1000°C)
90SiO2–10TiO2 a.p.
530.6
531.6
532.6
530.6
531.7
532.7
90SiO2–10TiO2 (1000°C)
SiO2 (amorphous)
SiO2 (quartz)
532.7
532.7
530.6
530.6
530.5
103.4
–
1711.3
–
103.5
–
1712.0
–
TiO2 (rutile)
–
–
–
459.3
459.3
459.3
–
–
–
873.2
873.6
872.9
TiO2 (brookite)
TiO2 (anatase)
ter values for reference anatase, brookite and rutile crystals reported in Table 1, com-
bined to DTA and XRD results, indicate that the amorphous TiO2 changes its struc-
ture to the anatase form at about 420°C and to rutile at about 800°C. On the contrary,
the titanium α’ Auger parameter for xSiO2–(1–x)TiO2 powders as well as DTA
curves and XRD patterns indicate that anatase is formed at about 600°C and main-
tains this crystalline structure up to 1000°C. These results reveal that the variation of
the crystalline structure for xSiO2–(1–x)TiO2 sol-gel materials, takes place at differ-
ent temperature and is related to the chemical composition of the sol-gel ceramics. It
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43
is worth noting that structural information obtained via XPS-XAES results are in
good agreement with XRD results thus demonstrating that the surface structural mod-
ification occurs also in the bulk of the sol-gel materials. Furthermore, it is worth
remarking that the α‘ titanium Auger parameter for as prepared 90SiO2–TiO2 sol-gel
material is 871.3 eV. This result could be related to a possible fourfold co-ordination
of titanium in SiO2–TiO2 glasses with a TiO2 content lower than 10 mol% [12]. In-
deed, the calculations for α’ using the simple electrostatic model [10, 11], indicate
that a titanium fourfold co-ordinated is characterised by an α’ value lower than the
amorphous phase and the octahedral structure.
The other important variation induced by the thermal treatment could be the
change of the chemical bonds for silicon and titanium that can be studied via XPS as a
function of temperature.
The most interesting features are given by O 1s signal, the components of which
are characterised by different BEs values. These latter data are reported in Table 1
and show large differences for SiO2, TiO2 and SiO2–TiO2 materials. Indeed, a ready
distinction between SiO2 and TiO2 is achieved from O 1s lineshape analysis having
these latter compounds largely different BE values and being clearly indicative of a
mainly single chemical environment surrounding the photo-emitting oxygen. By way
of contrast, xSiO2–(1–x)TiO2 sol-gel ceramics are characterised by complex profiles
that are indicative of the presence of different oxygen chemical bonds. Curve fitting
process has been then performed on O 1s spectra in order to quantify the presence of
each chemical bonding in the composite bands that are indicative of the coexistence
of different oxygen chemical bondings peaked at 531.4±0.1 eV, 530.6±0.1 eV and
532.7±0.1 eV. On the basis of the measurements carried out on pure TiO2 and SiO2
powders we assign the two latter components to Ti–O–Ti and Si–O–Si bonds. The
first O 1s component centred at 531.4 eV, that has an intermediate BE value lying be-
tween the two extreme BEs, is assigned to Ti–O–Si bond. This assignment is pro-
posed on the basis of previously reported considerations [13–15] on the system
Fe2O3, SiO2 and Fe2SiO4 systems where O 1s component pertinent to Fe–O–Si bond,
has a BE value intermediate between Fe–O–Fe and Si–O–Si BEs.
As a function of temperature, O 1s spectra undergo striking modifications as the
Ti–O–Si component decreases at temperature higher than 600°C and new O–Ti and
O–Si bonds are formed, as long as O 1s spectrum for xSiO2–(1–x)TiO2 treated at
1000°C is characterised by a twofold line shape, consisting of a component centred at
530.6 along with a component at 532.7 eV. These latter values are exactly at the same
BE as disclosed by all the one-component spectra for pure sol-gel titanium and sili-
con oxides. It is worth noting that these surface chemical bonding variations do not
give rise to sharp exothermic or endothermic peaks in the DTA curves thus demon-
strating that the chemical reactions take place only on the surface of the sol-gel ce-
ramics or give rise to a low intensity and very broad signal along the whole tempera-
ture range from 400 to 1000°C.
The variation of the chemical bonding in xSiO2–(1–x)TiO2 materials as a func-
tion of temperature is associated also to the variation of the surface chemical compo-
sition. This change has been evidenced by the variation of Si/Ti atomic ratios ob-
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INGO et al.: SOL-GEL SiO2, TiO2 AND xSiO2–(1–x)TiO2 CERAMIC MATERIALS
Fig. 3 Selected X-ray diffraction (XRD) patterns for TiO2 sol-gel ceramic materials af-
ter an air thermal treatment carried out for 2 h at 400, 800 and 1200°C. The XR
diffractrograms disclose the presence of anatase in the TiO2 material after an air
thermal treatment carried out for 2 h at 400°C, of rutile with a small amount of
anatase for the TiO2 sol-gel ceramic after an air thermal treatment carried out for
2 h at 800°C and of rutile for TiO2 sol-gel material treated at higher temperature
tained via quantitative XPS analysis reported in Table 2. These latter Si/Ti atomic ra-
tios are based on Ti 2p3/2 and Si 2p peaks and are compared in Table 2 with Si/Ti
atomic ratios calculated from the intensity ratio measurements of O 1s single compo-
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45
nents. The data are in good agreement also with the chemical analyses and show that
SiO2 surface amount increases as a function of temperature. As already shown by
other authors [16] in 60 mol% SiO2–40TiO2 glasses subjected to air thermal treatment
at 1000°C, by means of transmission electron microscopy (TEM), a possible explana-
tion of the change of the surface chemical composition could include a SiO2 segrega-
tion and precipitation- agglomeration of TiO2.
Table 2 Si/Ti surface atomic ratios for xSiO2–(1–x)TiO2 in as prepared conditions (a.p.) and after
air thermal treatment (t.t.) carried out at 400, 600, 800 and 1000°C. The atomic ratios
have been obtained from XPS quantitative analyses based on Ti 2p3/2 and Si 2p peaks
(marked with an asterisk), and Si/Ti atomic ratios calculated from the intensity ratio
measurements of the O 1s single components
Sample
Temperature/°C
Si/Ti*
1.5
Si/Ti
1.6
50SiO2–50TiO2
50SiO2–50TiO2 t.t.
50SiO2–50TiO2 t.t.
50SiO2–50TiO2 t.t.
50SiO2–50TiO2 t.t.
70SiO2–30TiO2
70SiO2–30TiO2 t.t.
90SiO2–10TiO2
90SiO2–10TiO2 t.t.
–
400
600
800
1000
–
1.6
1.7
1.8
1.9
2.1
2.2
2.4
2.7
3.8
4.0
1000
–
4.0
4.2
7.2
6.7
1000
15.2
15.6
Conclusions
By means of the combined use of XPS-XAES, DTA-TG, XRD and SEM techniques,
the microchemical structure of sol-gel SiO2, TiO2 and xSiO2–(1–x)TiO2 ceramics
have been studied as a function of thermal treatments carried out up to 1200°C in air.
The proposed structure of the materials in as prepared conditions can be de-
scribed as a composite between TiO2 and SiO2 moieties with a not negligible number
of Si–O–Ti bonds that act also as crosslinking bonds between these units.
In the temperature range from 50 to 450°C, DTA-TG results evidence a remarkable
mass loss due to the evaporation of organic solvents entrapped in the sol-gel materials
and of the remnant organic components of the precursor metal alkoxides. Furthermore,
with increasing temperature the arrangements between the units Ti–O–Ti–O–Ti and
Si–O–Si–O–Si made by Si–O–Ti crosslinking bonds are broken thus inducing the forma-
tion of new Si–O and Ti–O bonds as well as a SiO2 segregation phenomena.
The structure of these newly formed materials can be described as composed by
a pure SiO2 matrix and TiO2 based particles crystallised in the quartz, anatase and
rutile forms as a function of the chemical composition of the sol-gel materials and of
temperature.
* * *
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The authors thank C. Veroli for XRD analysis and the SEM laboratory of Roma II University where
the characterisations have been performed by the authors.
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