Low Temperature Gas Sensing Coatings Made Through Wet Chemical Deposition of Niobium Doped Titanium Oxide Colloid

doi:10.4236/msa.2011.24034

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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.

Email: [email protected]

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

S=R

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

100

120

140

Intensity (a. u.)

2

(

)

TiO2-20Nb

TiO2-10Nb

Anatase

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-

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

–

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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