SYNTHESIS OF POROUS NANOCRYSTALLINE TiO2 PASTES AND FILMS FROM
OCTYLPHENOL ETHOXYLATE AND THE SURFACE MODIFIER, ACETYL-ACETONE
D.S. Tsoukleris1, I.M. Arabatzis1, T. Maggos2, A.I. Kontos1, C. Vassilakos2, A.O. Ibhadon3*
and P. Falaras1
1
Institute of Physical Chemistry, NCSR "Demokritos", 153 10 Aghia Paraskevi Attikis,
Athens, Greece.
2
Environmental Research Lab/INT-RP, NCSR “Demokritos”, 153 10
Aghia Paraskevi Attikis, Greece
3
University of Hull, Faculty of Science and the Environment, Cottingham Road, Hull, HU6
7RX, England
Abstract
A direct reaction between a surface modifier,
Acetyl
acetone,
a
binder
molecule,
Octylphenol ethoxylate(Triton X-100) and a
semiconductor powder results in a paste from
which porous nanocrystalline TiO2 films are
made. X-ray diffraction analysis of the
titania films after sintering at 450˚C indicate
a well
organised structure of titania
nanoparticles. Analysis by scanning electron
microscopy (SEM) revealed that the surface
of the films possess a sponge like structure,
with extended roughness and complex
characteristics. Atomic Force Microscopy
show that the film particles are made up of
high mountains and deep valleys and their
height histogram shows a Gaussian-like
distribution in accord with
roughness
analysis. Films are made up of porous
network with extended surface area and are
ideal for heterogeneous energy conversion
processes.
The
synthesis,
characterisation
and
development of novel, efficient, well structured
porous, high surface area and complex forms of
titanium dioxide
for deployment in
environmental applications including sorption
media, filters
and as photocatalysts
in
films and pastes is reported as part of our research
work into nanocrystalline materials and catalysts.
Although a lot of published research in the area of
photocatalysis exists, there is lack of sufficient data
on the synthesis and characterisation of suitable
TiO2 based photocatalysts such as films, foams and
pastes for photocatalytic reactor applications. For
this reason we have developed a paste based on
titanium dioxide
from which nanocrystalline
photocatalyst films have been produced. It is
important in photocatalytic applications to control and
optimise the surface properties of these titania films.
Synthetic methods designed to prepare efficient
pastes and films of titanium dioxide are of great
interest. The composition of the precursor paste is
important for the homogeneity, adherence and
roughness of the final TiO2 films. In this work,
commercially available TiO2 powder, Degussa P25,
was used for the preparation of the titania pastes
due to its nanoparticle characteristics and
availability. Important parameters including paste
composition, addition of binder molecules and
dispersion temperature were taken into account in
the synthetic procedure. Prior to paste preparation,
the semiconductor powder was heated overnight at
200˚C in order to remove excess moisture. 0.6g of
TiO2 was added slowly to a surface modifier
consisting of a mixture of acetyl-acetone and distilled
water in ratio 1:10 to produce the paste.
advanced oxidation processes (AOP) for air
purification is an important area of an on-going
research[1-2]. In these applications, titania based
pastes and films must possess specific and
important surface properties and characteristics. In
a previous publication in Nanoletters[3] we reported
the synthesis of porous nanocrystalline foam. In
this work, the synthesis of porous nanocrystalline
*Corresponding author: email: a.o.ibhadon@hull.ac.uk Tel: +44-1723-357318
Table 1. Chemical composition of the titanium
dioxide paste
Component
Material
Paste
Semiconductor
TiO2 Degussa P25 (gr)
0.6
Binder
Triton X-100 (drop)
2
Surface
Modifier
Acetyl-acetone (ml)
2
Solvent
H2O (ml)
1.2
The addition of the semiconductor powder and water
is slow and the mixture is homogenized under
continuous stirring for 60 min. Then, two drops of
Triton X-100 binder were added while mechanical
stirring of the final paste continued for two hours. At
the end of this process, the paste is treated in an
ultrasonic bath for 1 hour to ensure the absence of
titanium dioxide aggregates.
Thin, nanocrystalline and porous TiO2 films were
produced by depositing the paste prepared unto
appropriate support such as glass spherules or
beads. The beads or spherules should be
ultrasonically cleaned in ethanol prior to deposition in
the paste for 5 minutes. The glass spherules coated
with titania paste are then dried at 120˚C for 15
minutes and annealed at 450˚C for exactly 90
minutes. This thermal treatment
ensured the
removal of any organic load and facilitated the
interconnection (sintering) of titanium dioxide
nanoparticles.
The crystallinity of the films was studied with a
Siemens D- 500 X-ray diffractometer, using CuKa
radiation while Raman spectroscopy was employed
to elucidate the vibrational modes of the films.
Raman measurements were carried out with a triple
Jobin-Yvon
spectrometer
equipped
with
a
microscope and a CCD detector and a 514.5 nm
Argon laser. Detailed surface images were obtained
by means of a scanning electron microscope (SEM)
with numerical image acquisition (LEICA S440).
Carbon deposition was performed to avoid problems
arising from surface charge effects. X-ray from the
SEM microscope probe (at horizontal incidence
beam) was used for non-destructive qualitative and
quantitative chemical analysis of the modified films.
Surface morphology, roughness and fractality of the
titania photocatalysts were examined with a Digital
Instruments Nanoscope III atomic force microscope
(AFM), operating in the tapping mode (TM) [4-5].
The total TiO2 surface developed on the glass
spherules as a thin film is 50cm2. The films are
opaque and extremely rough. Their thickness was
determined by an Ambios Technology (XP-2)
profilometer and found to be about 20 m. X-ray
diffraction results of the titania films sintered at
450˚C, and shown in Figure 1, indicate a well
organized crystal structure of titania nanoparticles.
The inset picture zooms at the A(101) anatase and
R(110) rutile peaks in the region of 24-28 degrees.
The ratio of the two peak intensities was
approximately the same for the films and Degussa
P25 powder, indicating similar weight percentages of
the anatase to rutile phases. The rutile content in the
film is 25%, while the anatase content is 75%. This
confirmed that the initial crystalline composition
remained in the films produced. In addition, the grain
size was determined from the width at half maximum
(w) of the A(101) anatase peak according to the
Scherrer formula [6]:
D=
0.9λ
w ∗ cos θ
(1)
and a value of D=20±1 nm was obtained for the
films compared to D=24±1 nm for Degussa P25).
Fig. 1. The XRD patterns of titania photocatalysts.
Raman spectroscopy is a flexible, non-destructive
technique for characterization of nanostructured
semiconductors. The technique is capable of
elucidating the titania structural complexity as peaks
from each crystalline phase are clearly separated in
frequency, and therefore the anatase and rutile
phases are easily distinguishable [7-9]. Moreover,
the technique is able to detect carbonic species and
evaluate the quality of thermal annealing. The
Raman spectra confirm that the films are well
crystallized, without overlapped peaks and low
number of imperfect sites. Vibration peaks at 142 ± 2
*Corresponding author: email: a.o.ibhadon@hull.ac.uk Tel: +44-1723-357318
cm-1 (Eg, vs), 194 ± 3 cm-1 (Eg, w), 393 ± 2 cm-1 (B1g,
s), 512 ± 1 cm-1 (A1g, s), 634 ± 2 cm-1 (Eg, s) are
present in the Raman spectra of the TiO2
nanocrystalline films, unambiguously attributed to the
anatase
modification.
Although
anatase
nanoparticles are the predominant species, rutile
phase is also observed as a broad peak at 446 cm-1.
Surface morphology is the most important factor for
an efficient thin film photocatalyst. Analysis
performed by scanning electron microscopy (SEM),
Figure 2, revealed that the surface of the titania films
possess a sponge like structure, with extended
roughness and complex characteristics.
From the top-view image (two-dimensional picture) it
can be seen that the films display a complex
configuration. In order to evaluate and compare the
geometric complexity of the film surfaces, qualitative
analysis including measurements of feature
frequency and fractal dimension Df
[10] (a
parameter which reflects the scaling behaviour and
is an intrinsic property of the material, 3≥Df≥2) was
performed.
Fig.2 Microscopic characterization of the titania
films (SEM)
The mean diameter of the nanocrystallites is
controlled
by
the
original
Degussa
P25
semiconductor material. Results show that
modification with an organic carrier does not induce
aggregation or additional growth of the TiO2
nanoparticles. In general, the appearance of the
films resembles a porous network with extended
surface area, ideal for heterogeneous energy
conversion processes, such as the photocatalytic
reactions.
In order to fully characterise the properties of the
films and express these in terms of surface
parameters, characterization by Atomic Force
Microscopy (AFM) was undertaken. Figures 3a and
3b show the top view and surface plot images (twodimensional and three dimensional representations
respectively) for the films. The films consist of
interconnected grain particles fused together to form
the semiconductor solid material. The average grain
diameter of the film is 20 nm, in agreement with Xray diffraction results.
The particles are made up of high mountains and
deep valleys and their height histogram shows a
Gaussian-like distribution with a max 145 nm for the
films. These results are in agreement with the
roughness analysis carried out. The Rms (Rms = the
standard deviation of the Z values, Z being the total
height range analysed) values show that the films
exhibit elevated values of roughness (Rms):21.24
nm.
Fig. 3.
AFM characterization of the titania
Films: (a) AFM top view (two
dimensional image), (b) AFM surface
plot (three dimensional image).
The fractal analysis yielded a Df value of 2.09
(±0.02) and showed that the films exhibit a relative
poor self-affine scaling character. The fractal
dimension (Df) influences the effective surface
extension and therefore fractal films show a higher
ability to efficiently capture photons, through complex
semiconducting network acting in a ‘‘sponge’’-like
way to achieve high photocatalytic efficiency.
However, the grain diameter of the TiO2
nanoparticles is also an essential parameter [11].
Taking
into
account
the
heterogeneous
photocatalytic mechanism of a thin film TiO2 catalyst
[12], the height and roughness of surface features
are also important. The films show a complex
surface structure and increased roughness resulting
from surface characteristics of important height.
*Corresponding author: email: a.o.ibhadon@hull.ac.uk Tel: +44-1723-357318
These films are endowed with a high real surface
extension, which
favours photodecomposition
processes. This type of surface not only permits the
adsorption of a greater number of pollutant
molecules, but also creates a rough environment
where multiple light reflection can occur, thus
considerably increasing the amount of adsorbed
photons.
The results obtained in this study represent a
simple, one step and low cost method for TiO2 film
preparation and opens up the possibility of
developing more efficient photocatalysts in the form
of porous and high surface area inorganic oxide
matrixes such as foams by using different precursor
materials.
Acknowledgements
The financial support from NATO (EST.CLG.979797)
and
GSRT/Ministry
of
Development-Greece
(Excellence
in
the
Research
Institutes1422/B1/3.3.1/362/2002 project and Greek – British
bilateral project is gratefully acknowledged. Thanks
are due Dr A.G. Kontos for Raman investigations
and helpful discussions as well as to Dr. M.C.
Bernard for SEM pictures and Dr. I. Raptis for
thickness measurements.
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*Corresponding author: email: a.o.ibhadon@hull.ac.uk Tel: +44-1723-357318