Materials Sciences and Applicatio ns, 2011, 2, 265-269
doi:10.4236/msa.2011.24034 Published Online April 2011 (http://www.scirp.org/journal/msa)
Low Temperature Gas Sensing Coatings Made
through Wet Chemical Deposition of Niobium
Doped Titanium Oxide Colloid
Naji Al Dahoudi
Physics Department, Al Azhar University, Gaza, Palestine.
Received January 6th, 2011; revised February 28th, 2011; accepted March 17th, 2011.
ABSTRACT
Niobium doped titanium oxide (TiO2) colloid was synthesized to fabricate a hydrog en gas sensor layer on oxidized sili-
con wafer substrate. The layers were obtained using spin coating technique and then heated in air at 500˚C for 30 min.
The doping of TiO2 led to a significant en hancement of the sens itivity of the layer especially at low op erating tempera-
ture. The effect of doping was found effective of operating the sen sor at relatively low tempera ture (150˚C). The layers
show a very smooth nanostructure with average roughness of less than 0.5 nm. The behavior of the sensing characteris-
tics of such layers was discussed related to their chemical compositions, morphology and their crystalline structure.
The morphological and structural characteristics of the layers were studied through X-ray diffraction (XRD) and
Atomic force microscopy (AFM).
Keywords: Colloid, Titanium Oxide , Niobium, Gas Sensor, Spin Coating
1. Introduction
Semiconducting oxide materials have been subjected to
much of developing researches for their use as essential
components in many industrial applications. Extensive
researches have been done to develop devices that can
detect inflammable and toxic gases using such materials.
Semiconducting metal oxides such as tin oxide (SnO2),
zinc oxide (ZnO), indium oxide (In2O3), and titanium
oxide (TiO2) have been widely used as active materials in
solid-state gas sensing devices [1-5]. Titanium oxide
(TiO2) is one of the most important materials that have a
wide multidisciplinary applications, such as photocata-
lytic applications (self cleaning and self sterilization),
inorganic membrane material for ultrafilteration, photo-
electric properties, and antireflection optical thin films
and as gas sensing materials [6-9]. The gas sensing
mechanism of such materials can be understood as a
function of the adsorption of gases on the semiconductor
surface and the corresponding change in the electrical
conductivity of the material. The adsorption of some
gases species leads to the formation of charge depletion
layer affecting the transfer of the charge carriers from
site to another. The adsorption of oxidizing gases such as
NO or O3 lead to an increase of the sensor resistance,
however reducing gases such as H2 or CH3OH causes a
reduction in the adsorb ed oxygen leading to a decrease of
the electrical resistance of the sensor material [7,10,11].
Nanostructured materials possessing very large specific
area which makes them very attractive to realize chemi-
cal or physical sensors, as the solid state sensing mecha-
nism is governed primarily by the available surface area.
Furthermore, the wet chemical sol-gel coating is becom-
ing a real candidate process compared with the estab-
lished vacuum processes, because of the low cost, the
higher flexibility regarding the geometry of substrates
and the high coating quality.
Detecting hydrogen gas is an important issue for in-
dustrial process control, combustion control, and as well
for some medical application where some bacterial infec-
tion can be detected using such sensor [12]. In this work,
a colloidal sol of niob ium doped titanium oxide was pre-
pared for obtaining sensor layers that can detect hydro-
gen gas. The layers were obtained using spin coating
process.
2. Experimental
2.1. Coating Colloid
The coating sols were prepared by the hydrolysis of Ti-

Low Temperature Gas Sensing Coatings Made through Wet Chemical Deposition of Niobium
266 Doped Titanium Oxide Colloids
tanium and Niobium alkoxides. For a pure TiO2 colloid,
a 0.0015 mole of Titanium isopropoxide was mixed in
0.105 mole of 2-methoxyethanol and stirred at RT for 10
min. A 0.1 g of bidistilled water in Metho xy ethanol was
added to the alkoxide solution under stirring for the hy-
drolysis at RT for 1 hr. The addition of water resulted in
a precipitation, where a hydrophobic colloidal was for-
med. The resulted solution was redipersed by adding a
0.5 g of nitric acid in Methoxy ethanol. A clear colloid
was formed, which is then filtered using a 0.2 µm filter
before the deposition procedure. For the Niobium doped
colloid, different concentrations between 2 mol% to 30
mol% of Niobium (V) n-Butoxide was added to the tita-
nium alkoxide solution, and the rest of the method as
before.
Film Formation: The layers were formed by depositing
the coating colloids onto a clean Si/SiO2 wafer substrate
using the spin coating technique. Three layers was ob-
tained and between each step the wet layer was subjected
to a drying process at 150˚C for 5 min., and then all lay-
ers were heated in an oven at 500˚C for 30 min. A struc-
tured gold film was sputtered onto the layers, and then a
cupper electrode contact was made for the gas sensing
measurements.
2.2. Characterizations
The gas sensing properties of the assembled layers were
carried out using a computer controlled set-up, including
a gas chamber, flow meters (Bronkhorst, Germany), gas
delivery pipes, a heating plate with a PID temperature
controller and a source meter (Keithly 2601). Air was
used as the carrier gas and a mixture of N2 and H2
(N2/H2 : 95/5) as doping gases. Interdigitated front elec-
trodes were deposited and contacted with copper wires
using silver paste. The gas was delivered in the required
concentration in form of short pulses of 40 s duration.
The sensitivity of the samples was defined as
a
S=R
g
Rwhere Ra and Rg are the measured electrical
resistance of the samples in air and gas, respectively. The
thickness of the layers as well as their refractive indices
were carried out using a high resolution Ellipsometer
(DR 155, Germany) operated at a wavelength of 632 nm.
The structural properties of the layers (determination of
the phase and crystal size) were carried out using the X
ray diffractometer. The topography of the layers has been
measured using an atomic force microscopy (AFM) (SIS,
Nanostation, Germany).
3 Results and Discussions
The thickness of the three layered sensor for pure and
doped TiO2 was measured after the ellipsometry to be
ranged between 160 - 170 nm. The same value was con-
firmed after a cross sectional image done by the SEM.
The microstructure of the obtained TiO2 and Nb doped
20 30 40 50 60
0
20
40
60
80
100
120
140
Intensity (a. u.)
2
(
de
g
.
)
TiO2-20Nb
TiO2-10Nb
Anatase
Si
101
112
Figure 1. The XRD pattern for spin coated 10% and 20 %
Nb doped TiO2 layers deposited on Si substrate and heated
at 500˚C.
TiO2 thin layers on Si/SiO2 substrates were examined via
XRD and those results for 10% and 20% Nb doping are
shown in Figure 1. The XRD pattern exhibited only
peaks related to the anatase structure and no rutile struc-
ture was observed and also no Niobium related phase
was found. The anatase phase was expected, as the heat-
ing temperature of the layers is relatively low (500˚C). It
is reported that the deposited amorphous TiO2 film crys-
tallizes into anatase structure at around 350˚C and the
anatase-rutile transformation occurs at high temperatures
of 600˚C - 900˚C [13]. The tendency of the formation of
anatase phase is enhanced by the doping of Niobium,
where the 112 peak for the 20% Nb is sharper than that
of the 10% one. The refractive index of the layers meas-
ured using the ellipsometry show a value of 2.5 of 10%
Nb doping TiO2, and 2.54 for 20% Nb doping TiO2,
which is closer to the theoretical anatase phase.
The surface morphology of the three TiO2 layers is
exhibited in Figures 2(a,b) using the AFM. The surface
of all layers shows a very fine and smooth structure with
an average roughness less than 0.5 nm. This smooth
structure of the layers surface refers to the high mono-
dispersion of the crystalline nanoparticles in the coating
sol and due to the low temperature annealing (500˚C) of
the wet layers. The SEM technique was used to test the
morphology of the surface of the layer, however because
of the smoothness of the surface, a blank surface was
only observed.
At the operating temperatures 150, 200 and 250˚C the
electrical resistance of the pure and doped TiO2 layers
were measured in air. Figure 3 shows the variation of the
electrical resistance of the sensor layers in air as a func-
tion of the Nb doping concentration from 0 to 30 mol%
at different operating temperature. Similar to all semi-
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Low Temperature Gas Sensing Coatings Made through Wet Chemical Deposition of Niobium
Doped Titanium Oxide Colloid
267
Figure 3. Variation of electrical resistance of pure and Nb
doped TiO2 layers at different operating temperatures.
(a)
tion, by which charge carriers are localized between the
two Nb ions [10].
The sensitivity of the hydrogen gas for pure and Nb
doped TiO2 layers measured at different operating tem-
peratu res 150, 200 and 250 ˚C is shown in Figure 4. It is
observed that the doping of niobium enhances the sensi-
tivity of the sensor effectively especially at low operating
temperature. At higher operating temperature (T = 250˚C)
the sensitivity of the layers decreases and the doping of
niobium add no further enhancement compared with the
values obtained for pure TiO2 layers. The smaller crystal-
lite size of nanocrystallite niobium-doped titanium oxide
than that of undoped-titanium oxide benefits the catalytic
activity for sensing gas [11]. It is found that 5 mol% of
Niobium doping operated at T = 150˚C gives the best
results, where at this doping concentration it pocesses a
sensitivity 8 times greater than pure TiO2. The doping of
niobium lead to a reduction of the operating temperature
of the sensor, where for example at 5% Nb doping the
sensitivity is 8.5 at T = 150˚C which decreases to 1.5 by
increasing the operating temperature to 250˚C. However,
for pure TiO2, the sensitivity is becoming higher by in-
creasing the operating temperature. The inset in Figure 4
shows the measured sensitivity in % in relation with the
Nb concentration at different operating temperatures.
These results show that such sensors are adequate to be
operated at low temperature with higher sensitivity by
the niobium doping.
(b)
Figure 2. (a) Atomic force microscopy (AFM) of the surface
(1 µm × 1 µm) of the TiO2 layers deposited on Si wafer; (b)
The scanning surface potential microscopy (SSPM) for the
sample in (a).
conducting oxides, the material shows a reduction of the
electrical resistance by raising the operating temperature,
which is understood as a result of the increase of charge
carrier density in the conduction band. The effect of Nb
doping leads to further gradual decrease of the electrical
resistance of the coatings. This indicates that the Nb in
corporation into TiO2 lattice leading to a reducing effect,
which means adding extra free charges to the lattice. It is
observed that the Niobium doping for concentrations
higher than 10% is not effective for further decrease in
the electrical resistance of the doped films, however in
some samples an increase of the electrical resistance is
observed beyond 10% of Nb doping. The increasing
electrical resistance or decreasing conductivity with in-
creasing Nb concentration is attributed to the pair forma-
The decrease of the electrical resistance which is be-
lieved due to the absorbance of the sensing gas is antici-
pated to a reduction of the surface oxygen species O–
such as . This behavior is reversi-
ble by flowing air into the gas chamber, where electron
recombination leading to O– regeneration. Figure 5
–
22
H+O HO+e–

Low Temperature Gas Sensing Coatings Made through Wet Chemical Deposition of Niobium 268 Doped Titanium Oxide Colloid
Figure 4. The sensitivity of the hydrogen gas for layers of pure and different Nb doped TiO2 layers at different operating
temperature. The inset shows the sensitivity percent as a function of the Nb doping concentration.
shows the response and the recovery states of 5% Nb
doped TiO2 layer for different sensing gas concentration.
The film exhibit a remarkable sensation at gas concentra-
tion of 300 ppm. The change in the electrical conductiv-
ity is increasing from 1.5 to 5 by increasing the concen-
tration of the gas from 300 to 1300 ppm. The film also
exhibits faster response time than the recovery time for
all gas concentrations. At gas concentration of 1300 ppm,
the response time is 42 s, where the recov ery time is 112
s. This means that the regeneration of the O– species is
slower than the reduction process.
4. Conclusions
The synthesis of nanocrystalline niobium doped titanium
oxide colloid was successful to be deposited on oxidized
Figure 5. The change of the electrical resistance of 5% Nb
doped TiO2 layer as a function of the reducing gas concen-
trations (pulse duration is 40 s, operating temperature @
150˚C).
Si wafer substrate for producing a hydrogen gas sensor
thin film. The coating was carried out using spin coating
technique, dried and then heated in air at 500˚C for 30
min. The obtained layers have a very smooth surface
with low av erag e rou ghne ss as observed by the AFM and
exhibited an anatase phase structure according to the
obtained XRD pattern. The niobium doping of TiO2 led
to a significant enhancement of the hydrogen gas sensi-
tivity of the layer especially at low operating temperature.
The effect of doping was found effective of operating the
sensor at relatively low temperature (150˚C). The behav-
ior of the sensing characteristics of such layers was dis-
cussed related to their chemical compositions, morphol-
ogy and the concentration of the hydrogen gas. An ob-
servable sensation of the gas was observed clearly at low
hydrogen gas concentration.
5. Acknowledgments
I would like to thank the German Academic Exchange
Service (DAAD) for their short visit scholarship to Ger-
many in 2007 and 2009. Furthermore I would like to
thank Prof. Mohammed Es-Souni from the IMST in Kiel
and Prof. Rolf Clasen from the Powder Technology for
Glass and Ceramics department at the University of Saar-
land for hosting me during my research study visit to
Germany.
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