1. February 1999
Ž .Materials Letters 38 1999 161–166
Phase transformation in sol–gel titania containing silica
S. Rajesh Kumar, C. Suresh, Asha K. Vasudevan, N.R. Suja, P. Mukundan,
K.G.K. Warrier )
( )Structural Ceramics Unit, Regional Research Laboratory, CSIR , ThiruÕananthapuram-695019, India
Received 19 March 1998; revised 14 July 1998; accepted 14 July 1998
Abstract
Titanium dioxide containing 5–10% silica has been prepared by the co-hydrolysis of titanium isopropoxide at pH 4 in
presence of TEOS in different solvents such as methanol, ethanol and water. The resultant gels have been analyzed by BET
surface area measurements and thermal analysis. Addition of 5% silica to TiO increases initial surface area of 89 to 1882
m2
rg. As the amount of silica is further increased to 10%, the surface area is further increased to 203 m2
rg. The
composition containing TiO –10% SiO in ethanol medium achieved the highest specific surface area 232 m2
rg while the2 2
corresponding sample in methanol had 217 m2
rg. Addition of silica has further resulted in considerable increase of
anatase–rutile transformation temperature )8008C. Addition of methanol and ethanol to titania–silica systems enhances the
formation of rutile phase. This phase transformation has been followed by X-ray powder diffraction data and further, through
impedance measurements on the precursor gel pellets heated to various temperatures and the latter has been presented as a
very simple and convenient tool to study phase transformation in titania ceramics. q 1999 Elsevier Science B.V. All rights
reserved.
Keywords: Sol–gel; Titania; Anatase–rutile phase transformation; Solvent effect; Impedance spectroscopy
1. Introduction
Titanium dioxide prepared by hydrolysis of alkox-
ides by sol–gel methods has been used in many
applications such as membranes, porous substrates,
photocatalytic oxides and also in electronic devices
where ultrafineness and homogeneity are the criteria
w x1–4 . Titania is also often modified by addition of
other metal oxides in order to obtain properties to
w xsuit specific applications 5,6 . Some of the additives
commonly developed for catalytic titania are WO ,3
w xMnO , CuO and V O 6,10 . Titania–silica system2 2 5
)
Corresponding author. Fax: q91-471-491712; E-mail:
warrier@csrrltrd.ren.nic.in
is considered a potential candidate as photocatalyst
w x6 . Further, the catalytic activity and anatase phase
w xstability are closely related 7 and silica addition has
been reported in general to shift the anatase–rutile
w xtransformation to higher temperature range 6 in
order to aid such catalysts to work at elevated tem-
peratures. However, no detailed study has been re-
ported on the effect of synthesis conditions on char-
acteristics of the composite powder with particular
reference to specific surface area as well as
anatase–rutile transformation temperature. Further
variation of solvent medium produces considerable
change in rate of gelation and nature of resultant gels
w x8,13 . This is also important since the medium plays
an important role in achieving better sol–gel coat-
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž .PII: S0167-577X 98 00152-9

2. ( )S. Rajesh Kumar et al.rMaterials Letters 38 1999 161–166162
ings on various surfaces. In the present work, an
effort has therefore been made to study the effect of
addition of silica as TEOS to titanium isopropoxide
in presence of solvents such as ethanol and methanol,
prepared by co-hydrolysis keeping the medium at pH
4. Further, the anatase–rutile transformation in the
precursor gels have been identified by XRD and
impedance measurements in line with earlier studies
w xon anatase based sensor materials 9–12 .
2. Experimental
Titania was prepared by hydrolysing Titanium
Ž .isopropoxide Aldrich Chemicals, USA and silica
was prepared by hydrolysing Tetraethyl orthosilicate
Ž .TEOS, Alfa Chemicals, USA . In all the experi-
ments we used a constant solventralkoxide volume
ratio of 10. In a typical experiment for the prepara-
tion of TiO –5% SiO , required amount of TEOS2 2
Ž .1.25 ml was measured and transferred to a beaker
Žcontaining 250 ml water at pH 4 acidified with
. Ž .HNO . Titanium isopropoxide 25 ml was added3
slowly to the above solution of TEOS kept under
stirring and the pH was further brought to 4.0 by
adding dil.HNO . After stirring for 3 h the resultant3
gel was dried at 708C. The titania containing 10%
SiO was also prepared by following an identical2
procedure. The method was repeated for other sol-
vent media such as methanol and ethanol. TiO and2
SiO gels were also made separately for comparative2
studies.
Precursor gel samples calcined at 4508C were first
degassed at 2008C and then measured for specific
surface area at liquid nitrogen temperature using a
ŽBET Surface Area Analyzer Gemini 2360, Mi-
.cromeritics, USA . The precursor gels calcined at
2008C were characterised by dilatometric measure-
Ž .ments Shimadzu TMA 50H . Differential Thermal
Analysis was done on the samples using a Shimadzu
DTA 50H. Thermogravimetry was done using Shi-
madzu TGA 50H. Impedance spectral analysis of the
samples were done using an HP 4192 LF Impedance
Analyzer over a range of frequencies 5 Hz to 13
MHz on discs of size 1 mm thick and 10 mm
diameter prepared by uniaxial compaction of precur-
sor gel after calcining at 2008C. Similarly pellets
preheated to 200, 400, 600, 800 and 10008C were
measured as above. In all cases, the pellets were
coated by silver on both sides and connected with
copper wires before measurements. The X-ray
Diffraction pattern of heated samples were taken
using a Philips Diffractometer PW 1710 in the range
2u, 20–608.
3. Results and discussion
The titanium isopropoxide is known to follow fast
hydrolysis and is pH dependent. Earlier studies have
indicated that change of pH in the acidic range
w xinfluences the high temperature phase formation 11 .
Ž .Silica in acidic sols pH 2–7 or in presence of
flocculating salts aggregates into three-dimensional
network and forms gels. The hydroxyl groups of the
solvent, forms hydrogen bonds with the hydronium
ions of the acid and controls the rate of hydrolysis of
w x Ž .the alkoxides 13 . Hence an acidic pH pH 4 was
maintained in the experiments.
The surface area values of selected precursor gels
calcined at 4508C are given in Table 1. Sample
derived from pure titania gel has the lowest surface
area of 89 m2
rg. Addition of 5% silica to TiO2
increases the surface area to 188 m2
rg and further to
203 m2
rg for 10% silica addition. Precursor gels
containing 10% silica prepared in ethanol medium
showed specific surface area of 232 and 217 m2
rg
for methanol medium. As is already known, the
gelation of metal alkoxide condensed polymers is a
result of complex sequence of hydrolysis and con-
Table 1
Surface area values of samples calcined at 4508C
System TiO TiO –5% SiO TiO –10% SiO TiO –10% SiO TiO –10% SiO2 2 2 2 2 2 2 2 2
Ž . Ž .methanol ethanol
2Ž .Surface area m rg 89 188 203 217 232

3. ( )S. Rajesh Kumar et al.rMaterials Letters 38 1999 161–166 163
densation reactions which are catalysed in presence
w xof acid 13 , such as
The condensation reactions lead to highly net-
worked systems under acid catalysed conditions and
are characterised by large specific surface area and
w xmicroporosities 8,13 . Further, addition of silica en-
hances the strengthening of gel structure and thus
increases resistance to drying stress resulting in still
w xhigher surface area 14 . The gel structure is modi-
fied in presence of solvents through substitution of
hydroxyls. Ethanol and methanol being less polar
solvents than water form weak hydrogen bonds with
surface hydroxyl groups which also reduce strain on
the gel structure during drying. The surface tension
of ethanol during gelation is preferentially less, and
w xresults in high specific surface area 14 .
Thermogravimetric analysis curves are presented
in Fig. 1. The weight loss towards dehydroxylation
of pure titania gel observed between 350 and 5008C
which is shifted to higher temperature in the case of
TiO –SiO samples. This shows that addition of2 2
silica influences the gel structure which is reflected
in the decomposition behaviour.
Differential thermal analysis curves of pure titania
and that containing 10% silica are presented in
Ž . Ž .Fig. 1. TGA curves of a titania b titania containing 10% silica
Ž .aqueous medium .
Ž . Ž .Fig. 2. DTA curves of a titania b titania containing 10% silica
Ž .aqueous medium .
Fig. 2. Desorption of physically bonded water and
alcohol from the precursor gel takes place at around
1008C for all the samples. The exothermic peak at
3858C for pure TiO sample shifts to about 4008C2
for samples containing silica. In TiO sample con-2
taining 10% silica, there is yet another exothermic
peak at 5508C which is not observed in the case of
pure TiO . This could be due to the breaking of2
Si–O–Ti linkages. Owing to the density difference
between anatase and rutile, a volume change is
known to be associated with the anatase–rutile trans-
formation. The dilatometric curves of selected sam-
ples are presented in Fig. 3. In pure TiO sample2
shrinkage starts at 3508C and almost reaches comple-
Ž . Ž .Fig. 3. Dilatometric curves of a titania b titania containing 10%
Ž .silica aqueous medium .

4. ( )S. Rajesh Kumar et al.rMaterials Letters 38 1999 161–166164
Ž . Ž . Ž . Ž . ŽFig. 4. A XRD patterns of titania samples calcined at 8008C consisting of a 10% silica ethanol medium b 5% silica methanol
. Ž . Ž . Ž . Ž . Ž . Ž . Ž . Žmedium c 10% silica aqueous medium d 5% silica ethanol medium e 5% silica aqueous medium f 10% silica methanol
. Ž . ŽŽ . Ž . . Ž . Ž . Žmedium g 0% silica o rutile; x anatase . B XRD curves of titania samples calcined at 10008C consisting of a 10% silica ethanol
. Ž . Ž . Ž . Ž . Ž . Ž . Ž . Žmedium b 5% silica methanol medium c 10% silica methanol medium d 5% silica ethanol medium e 5% silica aqueous
. Ž . Ž . ŽŽ . Ž . .medium f 10% silica aqueous medium o rutile; x anatase .
tion by about 7808C. This is due to the formation of
anatase phase from the gel and onset of rutile forma-
tion. The corresponding change in TiO –10% SiO2 2
sample is at 534 and 9998C, respectively, indicating
shift in the transformation due to added silica. These
results also emphasize the influence of SiO on the2
high temperature phase formation of titania.
X-ray diffraction patterns of the heated samples
are presented in Fig. 4A and B. XRD pattern of TiO2
gel heated at 8008C shows only rutile phase. XRD
Table 2
Distribution of phases determined by X-ray powder diffraction in titania–silica precursor gels heated to 800 and 10008C
Ž .Temp 8C TiO TiO q5% SiO TiO q10% SiO TiO q10% SiO TiO q10% SiO2 2 2 2 2 2 2 2 2
Ž . Ž .methanol ethanol
800 R A A A A
1000 R AqR A AqR AqR
Asanatase.
Rsrutile.

5. ( )S. Rajesh Kumar et al.rMaterials Letters 38 1999 161–166 165
Ž .Fig. 5. A Impedance spectra of TiO –SiO samples calcined at2 2
Ž . Ž . Ž .4008C a TiO b TiO q10% SiO . B Impedance spectra of2 2 2
Ž . Ž .TiO –SiO samples calcined at 8008C c TiO d TiO q10%2 2 2 2
Ž .SiO . C Impedance spectra of TiO –SiO samples calcined at2 2 2
Ž . Ž .10008C e TiO f TiO q10% SiO .2 2 2
Žpatterns of TiO –SiO samples heated at 8008C Fig.2 2
.4A show anatase phase, indicating that the
anatase–rutile transformation is shifted to higher
temperatures in presence of silica. The interaction of
SiO with TiO on dehydroxylation of the gel results2 2
in two different types of phases, one containing
Ž .SiO –TiO part and the other, the unreacted excess2 2
TiO . These unreacted Ti atoms could be distributed2
at longer distances from one another due to the
presence of silica and therefore require larger amount
of energy and longer time to form the regular array
w xof anatase or rutile arrangement 5 . All the composi-
tions investigated except TiO –10% SiO in water2 2
medium indicate a mixture of anatase and rutile on
Ž .heating at 10008C Fig. 4B, Table 2 .
Impedance spectral measurements are presented
in Fig. 5. Silica gel dehydroxylates slower than
w xtitania gel 13 , the dehydroxylation being incom-
plete at 4008C, residual ions are present which con-
tribute to the total conductivity and hence the
impedance curve of silica–titania system indicates
Ž .greater conductivity Fig. 5a,b . Once dehydroxyla-
tion is complete at 10008C, however, the contribu-
tion of silica towards conduction becomes very less
Ž .Fig. 5e,f . Once anatase has been formed silica
enhances the transformation temperature to high
temperatures which is supported by the XRD data.
Since the resistances for these samples are in the
high frequency range, measurement of resistance by
Fig. 6. Resistivity vs. calcination temperature for TiO –SiO2 2
Ž . Ž . Ž .samples. ` TiO I TiO q5% SiO ^ TiO q10% SiO2 2 2 2 2
Ž . Ž . Ž . Ž .) TiO q10% SiO methanol v TiO q10% SiO ethanol .2 2 2 2

6. ( )S. Rajesh Kumar et al.rMaterials Letters 38 1999 161–166166
usual methods becomes difficult. The resistances
were therefore calculated by fitting the impedance
curves to semicircles. The points where the semi
circles cut the X-axis were taken as resistance values
for each of these samples and are plotted against the
Ž .temperature for each of these samples Fig. 6 . At
2008C all the samples show low resistivity, due to
presence of several factors that contribute to conduc-
Ž .tion ions, hydroxyl groups, etc. . With further in-
crease in temperature, dehydroxylation reactions get
completed with attendant increase in resistivity. A
maximum in the resistivity value is noted at ;4008C
which first decreases at almost 6008C, and further
increases with increase in temperature. Anatase starts
forming at about 4008C and a corresponding de-
crease in resistivity is shown. On further increase in
temperature rutile phase starts forming accompanied
by increase in resistivity.
4. Conclusion
1. The present investigation has indicated that the
presence of SiO in titania mixed xerogels has2
been effective in raising the anatase to rutile
transformation temperature.
2. Addition of solvents like ethanol and methanol
results in considerable increase in the surface area
probably due to modifications in gel structure.
3. In titania–silica precursors without solvents
anatase phase is fully retained at 10008C while in
presence of ethanol and methanol medium pres-
ence of rutile is also indicated.
4. Impedance spectroscopy has been found to be
effective to detect the anatase–rutile transforma-
tion in TiO and TiO –SiO bulk samples. This2 2 2
makes the characterisation of the phases very
simple.
Acknowledgements
Dr. Asha. K. Vasudevan acknowledges the sup-
port given by CSIR, Government of India for a
research associateship. S. Rajesh Kumar thanks
CSIR, Government of India and C. Suresh thanks
STEC, Government of Kerala for financial support
as research fellows.
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