2
158
M.A. Ahmed, M.F. Abdel-Messih / Journal of Alloys and Compounds 509 (2011) 2154–2159
around 1600 cm 1 can be assigned to OH (bending modes) of
−
transformation, while some additives would retard the transforma-
tion by increasing the lattice defect concentration of Ti interstitials
in titania [34–39].
hydroxyl group and surface adsorbed water. In the low spectral
region (400–1000 cm ), the band of a Ti–O–Ti bond of a titanium
oxide network is detected at 640 cm [41].
−
1
−
1
It is clear from our results that nano alumina particles play a pri-
mordial role on the nature of phases and crystallites size of titania
in composite samples. It is most probably that during the sol–gel
process a homogenous mixture which contain Ti–O–Al bond is
formed. By thermal treatment the mentioned bonds are broken
and independent crystallization of the two oxides occurs. However,
only titania is present in the XRD pattern because its tendency to
crystallize is higher than that of alumina. Incorporation of some
proportion of alumina into titania crystallites in TA10 and TA20
sample can be a suitable cause for acceleration the anatase-to-rutile
The FTIR spectra of alumina show an intense band centered
−
1
−1
and a broad band at 1640 cm , which are
around 3447 cm
assigned to stretching and bending modes of adsorbed water
−
1
−1
[42]. A small band at 1073 cm and a shoulder at 1163 cm are
detected due to the symmetric and asymmetric bending modes of
Al–O–Al bonds, respectively [42]. The OH torsional mode observed
at 750 cm overlaps with Al–O stretching vibrations [42]. The band
observed at 620 cm is attributed to stretching mode of AlO [42].
The broad band in the region of 500–750 cm is assigned to ␣-
AlO , whereas the shoulder at 890 cm is assigned to ␣-AlO4 [42].
The FTIR spectra of the composite samples show a pronounced
bands at 3400 and 1620 cm
−
1
−
1
6
−
1
3+
−1
transformation. The reason is that Al ions with a valence lower
than Ti4+ can substitute it and create oxygen vacancies due to the
necessity of charge balance [21,40], thus the addition of this propor-
tion of alumina favors the nucleation of rutile phase on the surface
6
−
1
due to the hydroxyl group of the
oxides and surface adsorbed water [41]. A broad band centered at
−
1
and spread their growth into TiO particles. The composite samples
640 cm is detected which can assigned to Al–O–Al and Ti–O–Ti.
2
−
In addition, a new small bands are detected at 583 and 439 cm
1
containing high content of alumina (40–90%) are predominant pos-
sesses anatase phase of titania and ␣-phase of alumina. It is worth
noting that TA60 sample which contains 60% alumina are predom-
inant possesses anatase phase of titania and small peaks associated
with ␣-phase of alumina. We can suppose that part of alumina is
diffused on the surface grain boundary as a highly dispersed amor-
phous oxide coating the TiO2 nanocrystallites and suppresses the
grain growth of anatase and the other part segregates forming a
separate ␣-alumina. This result is in agreement with some authors
which can be assigned to hetero metal–oxygen bonds of –Ti–O–Al–,
these bands are more illustrated in the samples TA10 and TA20 [43]
and are probably implying the incorporation of alumina into the
framework of titania.
3.4. Scanning electron microscope
Fig. 4a–f depicts the microstructure of the pure and mixed
oxides. The micrograph of titania shows that the material consists of
two different types of pieces of partially crystallized gels of different
sizes; one can be attributed to the anatase crystal and the other cor-
responds to the rutile phase. The micrograph of alumina indicates
that a sample contains transparent gel particles of small size. The
micrograph of a sample TA20 indicates that a material possesses
both crystal of titania and transparent alumina particles but the
titania particles are predominant and a pronounced reduction in
particle size of the composite sample is detected, on the other hand
a sample TA80 possesses titania and alumina particles but alumina
particles are predominant. A characteristic rod is observed in Fig. 4e
for the composite sample TA20 on which a small spherical particle
is attached. On examining the micrograph of TA80, one can notice
that the transparent alumina particles covered completely the tita-
nia gel which confirm the formation of alumina layers between
titania particles that prevent the enlargement of titania crystal-
lites and stabilize the anatase phase as discussed previous in XRD
section.
[
37,39] who suggest that the presence of high content of alumina
would prevent the nucleation of rutile by interfering the mutual
contact of TiO2 particles, so by this way some proportion of alu-
mina prevents the small particles of titania from growing to a size
necessary to allow the commencement of the transition into the
stable rutile modification. In this way, a considerable proportion of
alumina species may have been excluded from participation in the
formation of alumina crystallites, thereby explaining the reduction
in the intensity of diffraction peaks associated with the alumina
modification.
3
.2. Surface characterization
The specific surface area measurements of all samples are shown
in Table 1. It is clear that increasing in the alumina content is accom-
panied by increase in the surface area of the composite samples as
the role of alumina is to prevent the small particles from agglomer-
ation and prevent the formation of large particles. This is confirmed
from X-ray measurements by the large reduction in grain size on
increasing in alumina content as indicated in Table 1. Al O3 acts as
a barrier against the advancement of grain boundaries of TiO2 and
effectively prevented the grain growth and loss in surface area.
2
4. Conclusions
Titania–alumina nanocomposites containing various composi-
tions of titania and alumina were prepared by controlled sol–gel
method using simple salts asTiCl4 and AlCl3 and all the composite
samples are in nanometric dimension. Alumina plays a primordial
role on the nature of crystalline phases, surface area and crystal-
lite size of titania. Materials with up to 20% alumina possess a
rutile phase while those with an excess of alumina exhibit only
anatase crystalline phase of titania in addition to ␣-phase of alu-
mina. The formation of tialite compound in the samples in which
alumina is present in high content is detected. The transforma-
tion of anatase–rutile phase in the samples TA10 and TA20 can be
explained by the incorporation of alumina in the crystal lattice of
titania. The stabilization of the anatase phase in the samples pos-
sesses high content of alumina can be attributed to the formation of
alumina layers between titania crystallites particles, alumina plays
the role of a surfactant in preventing the small titania particles from
agglomeration into large particle phase which is accompanied by
decrease in particle size and increase in surface area.
3
.3. FTIR spectra
FTIR is a very sensitive and well established tool for studying
the orientation, transformation and nature of hydroxyl bonds in
both alumina and titania hydrogel. The position and intensity of
the IR peaks are strongly influenced by the crystallization behav-
ior, degree of crystallinity, morphology and particle size. Therefore,
the FTIR results could well be correlated with the crystallization
behavior and phase evolution in titania–alumina composites.
Fig. 3 displays the FTIR spectra of titania–alumina nano com-
posites. The spectrum of pure titania shows a large broad band
−1
at 3600–3400 cm
stretching modes). This band is in the hydroxyl stretching region
and O–H vibration of the Ti–OH groups and surface adsorbed H O
that can be assigned to bridged OH modes
(
2
−1
molecules. The band around 3720 cm
minal O–H vibration of the Ti–OH groups [41]. The narrow band
can be assigned to ter-