Advances in Ma terials Physics and Che mist ry, 2012, 2, 16-20
doi:10.4236/ampc.2012.24B005 Published Online December 2012 (htt p://
Copyright © 2012 SciRes. AMPC
Sol-Gel Synthesis of TiO2 Thin Films from In-House
Nano-TiO2 Powder
Mohd Zainizan Sahdan1, Nafarizal Nayan1, Samsul Haimi Dahlan1, Mahdi Ezwan Mahmoud2, Uda Hashim3
1Microelectroni c and Nanotechnology-Shamsuddi n Research Cen tre (MiNT-SRC), Faculty of Electrical and El ectronic Engi ne e r ing ,
Universiti Tun Hussein Onn Malaysia, Batu Pahat, Johor, Malaysia
2Material Technology Group, Nuclear Agency of Malaysia, 43000 Kajang, Selangor, Malaysia
3Institute of Nano Electronic Engineering (INEE), Universiti Malaysia Perlis, Arau, Perlis, Malaysia
Received 2012
This paper presents the optimization process in sol-gel technique to synthesize Titanium dioxide (TiO2) thin films using in-house
Nano-TiO2 powder. Nano-TiO2 powder was previously synthesized in our lab from ilmenite which is a tin mining byproduct using a
modified hydrothermal method. By varying the mass of Nano-TiO2 powder and acetic acid (catalyst) concentration in the sol-gel
process, highly transparent TiO2 thin films were obtained . The thin films were character ized by field ef fect scanning elect ron micro-
scope (FESEM), atomic force microscopy (AFM), thickness profiler, ultra-violet-visible sp ectrometer (UV-Vis) and curr ent-voltage
(I-V) measure ment syste m. This p aper also d emonstrates th e TiO2 thin films are sensitive towards isopropanol (IPA) solution where
the I-V response of the thin films changed sharply as IPA was dropped onto the thin film’s surface. The electrical property shows the
thin film has potential applications for chemical sen s ors and solar cells .
Keywords: Itanium Dioxide; Ilmenite; Sol-gel; Tin mining
1. Introduction
Tianium dioxide (TiO2) or known as titania has been reported
widely for its numerous applications from optoelectronics to
cosmetics [1-3]. TiO2 has excellent photocatalytic oxidative
properties that depend on the crystallinity and crystal form [4].
Due to the photocatalytic activity, TiO2 has been used in water
and air pollution treatments [5]. It also exhibits unique electrical
and chemical properties that can be utilized in various techno-
logical and engineering applications such as humidity sensor,
gas sensor and membrane [6,7] . In addition, TiO2 is also pro-
posed for solar cells and laser diodes for its high refractive index
and stability [8]. Although the starting material of TiO2 powder
can be obtained easily in the market, the price is quite expen-
sive especial ly for resear ch purposes i n Malaysia. Th erefore, an
alternative way of using in-house nano-TiO2 powder (anatase)
synthes ized from Ilmenite powder (from Malaysian Tin mining
waste), is propo sed. Using this in-house nano-TiO2 powder, the
cost o f the starti ng material can be reduced up to 80%.
The problem of using nano-TiO2 powder is the low solubility
in organic solvent such as ethanol and isopropanol. Therefore,
optimization on the mass of the starting material and catalyst is
required. Sol-gel process is proposed since it is a convenient
and versatile method for preparing transparent thin film at low
temperature [9]. The sol-gel process involved many complex
processes for both chemical and structural nature. Before gel
formation (polymerization), two stages are indentified: i) hy-
drolysis of the organometallic group precursor, and ii) poly-
condensation. The physical, chemical and mechanical proper-
ties are much dependant on the properties of the precursor solu-
tion [10]. Therefore, optimizing the precursor solution may
produce great results of TiO2 thin film. Sol-gel process is very
convenient to deposit transparent materials in combination with
spin coating technique. The resulting coatings are of high purity
and structural homogeneity depending on the parameters opti-
2. Experimental
Indium Tin Oxide (ITO) was used as the substrate which has
dimension of 1.5 cm x 1.5 cm. The su bstrat e was cleaned usin g
acetone in ultrasonic bath for 5 minutes at 50ºC. Then, it was
blown dry with nitrogen gas.
Different TiO2 solution was prepared using different mass of
nano-TiO2 powder which is 1g, 0.4 g, 0.1 g and 0.05 g. Each
powder will be stirred in 30 ml of ethanol mixed with 6 ml of
acetic acid. After underwent ageing process for 20 hours, the
solution was spin coated onto the ITO substrate for 10 layers.
The deposition was using 2-steps spin coating (1000 r.p.m. for
30 s and 3000 r.p.m. for 60 s). Every layer was preheated at
100ºC for 3 minutes. The thin films were annealed at 500ºC for
1 hour to improve the structural property. Again, after
underwent slow cooling at room temperature, the thin films
were charact er ized to find the optim um Nano-TiO2 mass.
The acetic acid concentration was optimized using different
acetic acid volumes which are 0 ml, 6 ml, 10 ml and 30 ml. It
was mixed with nano-TiO 2 powder using the optimum mass in
the previous experiment. It was stirred in 30 ml of ethanol for
20 hours. Using the same spin coater step, the TiO2 thin films
were deposited onto the ITO substrates. After annealing at
500ºC for 1 hour, the thin films were undergoing slow cooling
at room temperature.
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The thickness of each sample was characterized using KL
Tenko surface pro filer. The surface topology and roughness were
characterized by an XE-100 Park system atomic force micro-
scope (AFM). The optical property was characterized with a
Lambda-750 Perkin Elmer ultra violet-visible spectrometer
(UV-Vis) . The structural property was charact erized b y an Ad-
vance Bruker X-ray diffractometer (XRD) and the electrical
property of the sample was measured by a 2400 Keithley current -
voltage (I-V) measurement system.
3. Results and Discussion
3.1. Nano-TiO2 Mass Optimization
Figure 1 shows the AFM topography of the sample deposited
using different mass of nano-TiO2 powder. Generally, the film’s
roughness changes as the mass of the nano-TiO2 powder changed.
All films exhibit particles-packed morphology rather than sheet-
packed. Lo wering the mass o f nano-TiO2 powder contribute to
the reduction of the surface roughness of the films. The surface
roughness for film deposited using 1g, 0.4g, 0.1g and 0.05g of
nano-TiO2 powder is 55.6, 18.6, 23.8 and 26.6, respectively. It
is found that the optimum mass of nano-TiO2 powder for optimum
roughness is 0.4g. The thicknesses of the sample deposited
using 1g, 0.4g, 0.1g and 0.05g of nano-TiO2 powder is 230, 140,
110 and 98 nm, respectively.
Figure 2 shows the transmittance of the TiO2 thin films us-
ing different mass of nano-TiO 2 powder. As shown in the figure,
it is clearly observed that the transmittance increases as the
mass of the nano-TiO2 powder decreases. This may due to the
reduction of the thin film’s thickness as th e mass of nano-TiO2
powder reduced. The TiO2 thin film absorbed light which has
energy greater than 3.4 eV (~365 nm). However for 0.4g sample,
it absorbed photon energy greater than 3.83 eV (~324 nm) or in
other word, extends the transparency in which applicable for
photovoltaic application. Figure 3 shows the XRD spectra of
the TiO2 thin films. It is proven that all TiO2 films exhibit ana-
tase form. The intensity of the XRD spectra differs due to the
mass difference of n ano-Ti O2 powder.
Figu re 1. The AFM topograph y of TiO2 thin films using different mass of nano-TiO2 powder; (a) 1g; (b) 0.4g; (c) 0.1g; (d) 0.05g.
Figure 2. The UV-Vis spectra of TiO2 thin films deposited using
different mass of nano-TiO2 powder.
Figure 3. The XRD spectra of TiO2 thin films using different mass
of nano-TiO2 powder; (a) 1g; (b) 0.4g; (c) 0.1g; (d) 0.05g.
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3.2. Acetic Acid Concentration Optimization
Figure 4(a) shows the AFM topography of the sample deposited
using 0.4g of nano-TiO2 powder without the presence of acetic
acid catalyst. The surface roughness obtained from AFM is 32.
Fig ure 4( b) shows the topography of TiO2 thin film when 3 ml
of acetic acid was added in the solution. The surface roughness
is reduced to 25.9. However, Figure 4(c) show different mor-
phology of TiO2 thin film when 10 ml of acetic acid was used.
The particle morphology is obviously seen and the surface
roughness is reduced to 3.8 when the acetic acid volume was
increased to 10 ml. On the other hand, Figure 4(d) shows al-
most similar morphology with that of Figure 4(c). The surface
roughness increased slightly to 4.9 when the acetic acid volume
was 30 ml. It is found that the optimum acetic acid volume is 10
ml which results a uniform TiO2 thin films as shown in Figure
4(c). All samples exhibit almost similar thickness which is ap-
proximately 130 nm.
Figure 5 sh ows the trans mitt ance sp ectra of the s ample depo-
sited using different acetic acid concen tration. Generall y, as the
acetic acid volume increases, the transmittance of the TiO2 thin
film also increased. However for 10 ml sample, the
trans-mi ttance for wavelength from 419 to 547 nm decreased
below the transmittance of 3 ml sample. The effect of adding
acetic acid on the band gap i s evaluated u sing Tauc’s plot from
the eq uations;
( )
[ ]
1ln 1tT
= ×
E hc
where α, t and T are the absorption coefficient, film’s thickness
and transmittance, respectively. While Eg, h, c and λ are the
energy gap, plank constant (4.136 × 10-15 eV), speed of light (3
× 108 m.s-1) and wavelength, respectively. It has been found
that the band gap of the TiO2 thin films for 0, 3 and 30 ml sam-
ples is around 3.2 eV. However, the 10 ml sample has different
band gap value which is around 2.2 eV. Figure 6 shows the
XRD sp ectra of the samples whi ch ind icates all TiO2 thin films
are still in anatase form although the intensity is low. This low
intensity of the film is due to the low thickness of TiO2 thin
3.3. Sensing Properties of TiO2 Thin Film
In order to test current-voltage (I-V) char acteristic of the sample,
Platinum (Pt) electrodes were deposited on the TiO2 thin film
using a d.c. sputter coater. With Pt thickness around 15 nm, I-V
probes were contacted and supplied with voltages from -2 V to
+7 V using Keithley 2400 source meter. Figure 7 shows the I-V
characteristic of the optimized TiO2 films (nano-TiO2 powder:
0.4g, acetic acid: 10 ml) when dropped with IPA. As shown in
the figure, the TiO2 thin film exhibits Schottky response with Pt
due to large difference of work function. The threshold voltage
is around 6.7V. The threshold voltage in creased to 2 .4V as IP A
was dropped on the thin film. The current value was gradually
decreased by time and obviously seen after 30 second. The I-V
response returned back to origin after 5 minutes. This phenomena
is due to the chemical reaction between TiO2 particles and the
IPA. The sensitivity of structural stability, porousity and surfa ce-
to-volume ratio. TiO2 thin films prepared by sol-gel process
provide a backbone that can be use as a microporous support in
which analyte-sensitive species are trapped and into which
analyte molecules may effecti vely dif fu s e and interact [11].
Figu re 4. The AFM topograph y of TiO2 thin films using diffe re nt acetic aci d conc entration; (a) 0 ml; (b) 3 ml; (c) 10 ml; (d) 30 ml.
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Figure 5. The transmittance spectra of TiO2 thin films using differ-
ent acetic acid concentration; (a) 0 ml; (b) 3 ml; (c) 10 ml; (d) 30
ml .
Figure 6. The XRD spectra of TiO2 thin films using diff erent acetic
acid concentration; (a) 0 ml; (b) 3 ml; (c) 10 ml; (d) 30 ml.
4. Conclusion
This paper presents the results of the optimization process to
produce uniform and transparent TiO2 thin films using sol-gel
technique. Two types of optimizations were performed. First
was the mass of nano-TiO2 powder and second was the acetic
acid co ncentr ation.
The results from the AFM analysis confirmed that 0.4 g
sample has the least TiO2 thin film roughness. Then by adding
10 ml of acetic acid has resulted optimum uniformity and
roughness of the TiO2 thin film. The transmittance for the op-
timum film is around 80% which is sufficient for optoelectronic
application especially for solar cell. The XRD result indicates
that all films are in anatase form. Finally, it has been demon-
strated in this paper th at the prepared TiO2 thin film is sensitive
towards o rganic so lvent whi ch could i ncrease th e current val ue.
Therefore, it is applicab le for chemical s ensing appli cation.
Figure 7. The sensin g property of TiO2 thin film toward IPA solvent.
5. Acknowledgements
The authors would like to thank Universiti Tun Hussein Onn
Malaysia for providing the technical supports and Ministry of
Higher Education Malaysia (MOHE) for the financial support
through fundamental research grant scheme (FRGS) vote No
1059 and MTUN COE res ear ch grant vote No C020.
[1] S. Angkaew and P. Limsuwan, "Preparation of silver-titanium
dioxide core-shell (Ag@TiO2) nanoparticles: Effect of Ti-Ag
mole ratio," Proce di a Engine er i ng, vol. 32, pp. 649-655 , 2012.
[2] V. Brezová, et al., "Photoactivity of mechanochemically pre-
pared nanoparticulate titanium dioxide investigated by EPR
spectroscopy," Journal of Photochemistry and Photobiology A:
Chemistry, vol. 206, pp. 177-187, 2009.
[3] R. K. Keswani, et al., "Room temperature synthesis of titanium
dioxide nanoparticles of different phases in water in oil micro-
emulsion," Colloids and Surfaces A: Physicochemical and Engi-
ne ering Aspects , vol. 369 , pp. 75-81, 2010.
[4] A. Kiselev, et al., "Solar light decomposition of DFP on the
surface of anatase and rutile TiO2 prepared by hydrothermal
treatment of microemulsions," Surface Science, vol. 584, pp.
98-105, 2005.
[5] J. Taranto, et al., "Photocatalytic air purification: Comparative
efficac y and pressu re drop of a TiO2-c oated thin mesh and a ho-
neycomb monolith at high air velocities using a 0.4 m3
close-loop rea ctor," Separation and Purification Technology, vol.
67, pp. 187-193, 2009.
[6] J. Moon, et al., "Pd-doped TiO2 nanofiber networks for gas
sensor applications," Sensors and Actuators B: Chemical, vol.
149, pp. 301-305, 2010.
[7] A. L. Ahmad, et al., "Synthesis and characterization of TiO2
membrane with palladium impregnation for hydrogen separa-
tion," Journal of Membrane Science, vol. 366, pp. 166-175,
[8] S. Nad, et al., "Anomalous nanostructured titanium dioxide,"
Journal of Colloid and Interface Science, vol. 264, pp. 89-94,
[9] R . Gup t a, et al., "E ffect of et hanol vari ation on the int ernal en vi-
ronment of solgel bulk and thin films with aging," Biosensors
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and Bi o e le ctron ic s, vol. 21, pp. 549-556, 2005.
[10] J. Calabria A, et al., "Synthesis of solgel titania bactericide
coatings on adobe brick," Construction and Building Materials,
vol. 24, pp. 384-389, 2010.
[11] S. H. Si, et al., "Improvement of piezoelectric crystal sensor for
the detection of organic vapors using nanocrystalline TiO2
films," Sensors and Actuators B: Chemical, vol. 108, pp.
165-171, 2005.