# Titanium Dioxide-Based64∘ YX LiNbO3 Surface  Acoustic Wave Hydrogen Gas Sensors

Authors: A. Z. Sadek; D. Buso; A. Martucci; P. Mulvaney; W. Wlodarski; K. Kalantar-zadeh
Journal: Journal of Sensors
 (2008)

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## Abstract

Amorphous titanium dioxide (TiO2) and gold (Au) doped TiO2-based surface acoustic wave (SAW) sensors have been investigated as hydrogen gas detectors. The nanocrystal-doped TiO2 films were synthesized through a sol-gel route, mixing a Ti-butoxide-based solution with diluted colloidal gold nanoparticles. The films were deposited via spin coating onto 64∘ YX LiNbO3 SAW transducers in a helium atmosphere. The SAW gas sensors were operated at various temperatures between 150 and 310∘C. It was found that gold doping on TiO2 increased the device sensitivity and reduced the optimum operating temperature.


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## Body

## 1. Introduction

Gas sensing instruments are required to meet increasingly
stringent legal restrictions and industrial health and safety requirements as
well as for environmental monitoring, automotive applications, and for
manufacturing process  control. To meet these demands, the sensitivity,
selectivity, and stability of conventional devices need to be drastically
improved [1]. Recent advances in the development of nanostructured catalysts
such as metal oxide nanoparticles, nanowires, nanorods, and nanobelts [2–5] provide the
opportunity to greatly increase the response of these materials, as sensor
performance is directly related to granularity, porosity, and ratio of surface
area to volume in the sensing element. It has been established that the sensitivity
of semiconductor metal oxide gas sensors increases with decreasing grain size
[6], as the entire thickness of the sensitive layer can be affected by the
redox reactions of the gas species. Thus, low-dimensional nanostructured
materials, which have an increased surface to volume ratio when compared to
conventional polycrystalline structures, facilitate rapid diffusion of gases
into and out of the materials' nano- and microporosities, which in turn
increases the reaction rate, resulting in faster sensor response and recovery
time. Semiconductor metal oxide thin films also offer low cost, consistent
performance and easy fabrication [7]. In particular, titanium dioxide films
have been investigated as sensors for H2 [2, 8], CO and NO2 [3], O2 [9], and hydrocarbons [10]. The gas sensing capability is
due to changes in the film conductivity in the presence of oxidizing or
reducing gases.It has been reported that
addition of metal dopants to TiO2 further improves the gas sensing
performance [11], but there has been little attempt to date, to optimize, or to
understand the mechanism in these composite materials. In this paper, TiO2 and Au nanocrystal-doped TiO2 films are deposited onto the active
area of surface acoustic wave (SAW) devices. A SAW sensor coated with a gas
sensitive layer changes the film's physical properties such as mass, electrical
conductivity, elastic moduli, viscosity, and permittivity when exposed to a
target gas altering the surface acoustic wave's propagation parameters. A
sensor response may be due to single parameter or a combination of these
parameters. However, the main perturbation mechanisms such as mass-loading and
acoustoelectric changes (conductivity) are generally utilized for sensing
applications. A detailed explanation of the SAW sensing mechanisms can be found
elsewhere [12, 13].The sensing mechanism of author's developed layered SAW sensors is
predominantly conductometric sensitivity. In SAW devices, the change in
electrical conductivity of the sensing layer perturbs the velocity of the
propagating acoustic wave due to piezoelectric effects. The center frequency of
a SAW device is given by the equation,v=fp, where v is the surface wave velocity, f is the center frequency, and p is the interdigitated transducer
period. Therefore, the variation in velocity can be monitored by measuring the
changes in resonant frequency of the SAW device. For small gas concentrations,
this change in frequency is directly proportional to the gas concentration in
the environment. The strength of the SAW-based sensor is the direct frequency
output signal and hence higher accuracy than from the metal oxide (MOX)
sensors. Additionally, in SAW sensors, sheet conductivity of the thin films can
be tuned to get maximum sensitivity. Ricco and Martin [12] developed the basic
theory of a SAW sensor responding to a conductivity change. They proposed that
the change in velocity and attenuation due to the sensing layer conductivity
modulation can be approximated by the following relationships:(1)Δffo≅Δvvo=−K2211+(σs/σor)2, where fo is the
unperturbed oscillation frequency, and vo is the unperturbed
surface acoustic wave phase velocity, Δf is the
changes in oscillator centre frequency due to the changes in acoustic wave
phase velocity Δv, σs is the sheet
conductivity, and σor is the resonant
sheet conductivity of the sensing film. It is important to note that the
conductivity sensitivity is a function of the sheet resistance of the thin
layer and to achieve high sensitivity, the sheet conductivity of the layer must
be matched to the velocity-permittivity product of the SAW mode.The gas sensing efficiency is determined by the matrix porosity, which
determines the rate of gas percolation and mass transfer into the film, the
conductivity of the composite, which controls the redox reactions and
determines the sensitivity of the transduction process, and the metal
nanocrystal catalyst which can enhance adsorption and dissociation of the
substrate gas. Our goal in this paper is to try and keep the properties of the
matrix metal oxide film as constant while introducing a homogeneously dispersed
metal catalyst. Since ease of fabrication and scale-up are important issues for
these sensors, the TiO2 and TiO2-Au nanoparticles (NPs)
have been synthesized by conventional sol-gel processing. The SAW sensors based
on TiO2 and TiO2-Au NPs layered 64° YX LiNbO3 substrates have been investigated as online H2 gas sensors at
temperatures between 150°C
and 310°C.


## 2. Experimental Details

### 2.1. Fabrication of SAW Device

The sensors consisted of two-port resonators with 38-finger pairs
in each input and output interdigital transducers (IDT), 160 reflectors, 700μm
aperture width, and a periodicity of 40 μm. The IDTs were formed by patterning
20 nm layer of titanium (Ti) and an 80 nm gold layer. The titanium layer was
added to improve adhesion of the gold thin film.


### 2.2. Synthesis of Colloidal Gold

Colloidal gold was synthesized by reducing HAuCl4 by means of
tri-sodium citrate in water through the Turkevich method [14]. 1% tri-sodium
citrate solution was added to a boiling water solution of 4.5×10−4 M HAuCl4.


### 2.3. Synthesis of Sol-Gel Solution

Separately, a sol-gel solution containing the TiO2 precursor
was realized. Titanium (IV) butoxide, acetylacetone, and ethanol were mixed in
1 : 1.6 : 1.2 molar ratios and allowed to react under stirring for 25 minutes in
a closed vial. Acetylacetone complexes the titanium (IV) butoxide molecules
through a slight exothermic reaction and reduces the rate of hydrolysis, while
ethanol decreases the overall viscosity of solution.


### 2.4. Synthesis of Films

The gold suspension was first diluted in methanol (MeOH)
and then mixed with the mother sol-gel solution in the ratio gold suspension : MeOH : sol-gel = 1 : 3 : 1. In undoped films, pure water was added instead of the
gold sol. Further details on the synthesis of the doped films and their
characterization can be found elsewhere [15].Films were then deposited via spin coating onto the
active area of 64° YX LiNbO3 substrates at 3500 rpm for 25
seconds in a helium atmosphere. Films were also deposited on silica and silicon
substrates at room temperature for XRD and ellipsometric measurements. Samples
underwent thermal annealing in a tube furnace at temperatures from 100°C to 300°C, with temperature steps of 100°C for 30 minutes duration.


### 2.5. Experimental Setup

Extinction spectra of nanocrystal solutions and films were collected
using a Cary 5 UV-Vis-NIR spectrometer in the 200–800 nm wavelength range.
Transmission electron microscope (TEM) images were taken using a Philips CM10
TEM. Ellipsometry was carried out using a Jobin-Yvon UVISEL spectroscopic
ellipsometer and fitted to standard dispersion formulae. All the films were characterized
by X-ray diffraction (XRD) using a Philips diffractometer equipped with
glancing-incidence X-ray optics. The analysis was performed using CuKα Ni filtered radiation at 40 kV and 40 mA.The sensor operating principles involve
mainly two consecutive physical effects: the TiO2 or TiO2-Au
sensitive layer, which interacted with the gas media by changing conductivity,
and the SAW transducer which changed its operating frequency in response to
this conductivity change. Using the layered
SAW device as a positive feedback element in a closed loop circuit with an
amplifier, an oscillator was formed. A frequency counter was used to measure
the operational frequency of the transducer. The operational frequency of the
sensor was found to be approximately 102 MHz in dry synthetic air at a
temperature of 230°C.The sensors were mounted inside an enclosed environmental cell. A
computerized mass flow controller system was used to vary the concentration ofH2 gas in synthetic air. The gas mixture was delivered at a constant
flow rate of 0.2 litres per minute. The sensor responses were displayed in
real-time and saved for offline processing and analysis. Gas exposure time was
fixed for each pulse of H2 gas, and the cell was purged with
synthetic air between each pulse to allow the surface of the sensor to recover.
The sensors were exposed to a hydrogen gas pulse sequence of 0.06%, 0.12%,
0.25%, 0.50%, 1%, and 0.12% concentrations in synthetic air at various
temperatures between 150°C and 310°C. A Fluke high-resolution counter
(PM66860B) was used to measure the frequency changes of the SAW sensor.


## 2.1. Fabrication of SAW Device

The sensors consisted of two-port resonators with 38-finger pairs
in each input and output interdigital transducers (IDT), 160 reflectors, 700μm
aperture width, and a periodicity of 40 μm. The IDTs were formed by patterning
20 nm layer of titanium (Ti) and an 80 nm gold layer. The titanium layer was
added to improve adhesion of the gold thin film.


## 2.2. Synthesis of Colloidal Gold

Colloidal gold was synthesized by reducing HAuCl4 by means of
tri-sodium citrate in water through the Turkevich method [14]. 1% tri-sodium
citrate solution was added to a boiling water solution of 4.5×10−4 M HAuCl4.


## 2.3. Synthesis of Sol-Gel Solution

Separately, a sol-gel solution containing the TiO2 precursor
was realized. Titanium (IV) butoxide, acetylacetone, and ethanol were mixed in
1 : 1.6 : 1.2 molar ratios and allowed to react under stirring for 25 minutes in
a closed vial. Acetylacetone complexes the titanium (IV) butoxide molecules
through a slight exothermic reaction and reduces the rate of hydrolysis, while
ethanol decreases the overall viscosity of solution.


## 2.4. Synthesis of Films

The gold suspension was first diluted in methanol (MeOH)
and then mixed with the mother sol-gel solution in the ratio gold suspension : MeOH : sol-gel = 1 : 3 : 1. In undoped films, pure water was added instead of the
gold sol. Further details on the synthesis of the doped films and their
characterization can be found elsewhere [15].Films were then deposited via spin coating onto the
active area of 64° YX LiNbO3 substrates at 3500 rpm for 25
seconds in a helium atmosphere. Films were also deposited on silica and silicon
substrates at room temperature for XRD and ellipsometric measurements. Samples
underwent thermal annealing in a tube furnace at temperatures from 100°C to 300°C, with temperature steps of 100°C for 30 minutes duration.


## 2.5. Experimental Setup

Extinction spectra of nanocrystal solutions and films were collected
using a Cary 5 UV-Vis-NIR spectrometer in the 200–800 nm wavelength range.
Transmission electron microscope (TEM) images were taken using a Philips CM10
TEM. Ellipsometry was carried out using a Jobin-Yvon UVISEL spectroscopic
ellipsometer and fitted to standard dispersion formulae. All the films were characterized
by X-ray diffraction (XRD) using a Philips diffractometer equipped with
glancing-incidence X-ray optics. The analysis was performed using CuKα Ni filtered radiation at 40 kV and 40 mA.The sensor operating principles involve
mainly two consecutive physical effects: the TiO2 or TiO2-Au
sensitive layer, which interacted with the gas media by changing conductivity,
and the SAW transducer which changed its operating frequency in response to
this conductivity change. Using the layered
SAW device as a positive feedback element in a closed loop circuit with an
amplifier, an oscillator was formed. A frequency counter was used to measure
the operational frequency of the transducer. The operational frequency of the
sensor was found to be approximately 102 MHz in dry synthetic air at a
temperature of 230°C.The sensors were mounted inside an enclosed environmental cell. A
computerized mass flow controller system was used to vary the concentration ofH2 gas in synthetic air. The gas mixture was delivered at a constant
flow rate of 0.2 litres per minute. The sensor responses were displayed in
real-time and saved for offline processing and analysis. Gas exposure time was
fixed for each pulse of H2 gas, and the cell was purged with
synthetic air between each pulse to allow the surface of the sensor to recover.
The sensors were exposed to a hydrogen gas pulse sequence of 0.06%, 0.12%,
0.25%, 0.50%, 1%, and 0.12% concentrations in synthetic air at various
temperatures between 150°C and 310°C. A Fluke high-resolution counter
(PM66860B) was used to measure the frequency changes of the SAW sensor.


## 3. Results

### 3.1. Characterization

XRD spectra of deposited TiO2 and TiO2-Au thin films are
shown in Figure 1. The spectra show the
microstructural composition of TiO2 and TiO2-Au films annealed
up to 300°C. The peaks detectable in the (a) pattern arise from (111) and (200)
planes of the cubic lattice of crystalline metal gold (powder diffraction file
no. 04-0784, International Centre for Diffraction Data, Newton Square, Pa, USA).
The crystal size evaluated from the Scherrer equation using the FWHM values of
the diffraction peaks was 5 nm with standard deviation of 8%. On the other hand,
the spectra of the undoped TiO2 film ((b) pattern) exhibited no
discernible diffraction peaks.Figure 1
XRD patterns of TiO2 and TiO2-Au films
heated up to 300°C.Figure2
shows Au as synthesized gold nanocrystals in water (inset) with a mean diameter
9.72 nm with 0.9 nm standard deviation. The main TEM image (see Figure 2) shows
the same particles homogeneously dispersed inside the TiO2 matrix.
The image shows there is no aggregation of Au NPs inside the TiO2 matrix core. The mean size of the gold particles in the matrix was obtained by
counting over two hundred NPs and is 9.51±0.89 nm, in perfect agreement with
dimensions observed for clusters in water. This indicates there is no
dissolution, coalescence, or morphological changes to the nanocrystals during
the dispersion into the films, nor is there any effect of the annealing process
on their size distribution.Figure 2
TEM images of Au NPs as synthesized in water (inset) and
inside the TiO2 matrix (main) heated up to 300°C.Optical absorption spectra of TiO2 film, TiO2-Au film, and Au NPs
suspension in water are reported in Figure 3. The latter are characterized by
the strong absorption band associated with the surface plasmon resonance (SPR)
of spherical gold nanoclusters. The band is centered at 520 nm for NPs
suspension in water and at 595 nm for the same NPs embedded inside the TiO2 matrix. The difference in SPR band positions is related to the different
refractive indices of the media, water 1.34 [16], and TiO2 2.14,
respectively. The average refractive index of the doped, porous TiO2 film was determined by ellipsometry. The film porosity was estimated using the
refractive indices of the films (both measured and calculated) through the
correlation proposed by Zhao et al.  [17] and Ahn et al. [18], assuming a value of n=2.52
for the refractive index of bulk TiO2.Figure 3
Optical absorbance
spectra of Au NPs in solution, of TiO2-Au film and of TiO2 film.Ellipsometric measurements
performed on the samples gave thickness, refractive index, and relative
porosity values summarized in the following table. Porosity values have been
estimated by Lorentz-Lorentz relation using 2.52 as reference refractive index
for dense TiO2 at 589 nm.


### 3.2. Test Results

The dynamic responses of TiO2 and TiO2-Au sensors to a
sequence of different H2 concentrations in synthetic air are shown
in Figures 4 and 5, respectively. The introduction of H2 gas to the
sensor surface causes an increase in the device's resonant frequency for both
of the sensors. With hydrogen exposure, the conductivity of the TiO2 and TiO2-Au layers decreases resulting in an increase in the
acoustic wave velocity, thereby increasing the resonant frequency. The sensor
response is defined as the variation in operating frequency of oscillation due
to the interaction with the target gas. The measured sensor response was
approximately 6.5 and 7.4 kHz towards 1% of H2 at 230°C for the TiO2 and Au-TiO2 sensors, respectively. The frequency shift (sensitivity)
as a function of the operating temperature in the presence of 1%H2 for both of the sensors is shown in Figure 6. Although Au doping slightly
decreases the film porosity as shown in Table 1, it increases the device
sensitivity and reduces the optimal operating temperature. The optimum
operating temperatures for the TiO2-Au-based sensor are in the range
of 230–245°C and for TiO2-based
sensor are in the range of 245–260°C. Both sensors exhibited reproducible responses and consistent baseline
stability, as indicated when a second pulse of 0.12% hydrogen was introduced
into the sensor chamber. Repeatability was confirmed by testing the sensor
continuously over a 7-day period.Table 1
Values of thickness, refractive index, and porosity of TiO2 and TiO2-Au films measured by ellipsometry.
SamplesFilm thicknessRefractive indexPorosity(nm)at 589 nm(%)TiO2 at 300°C482.0937TiO2-Au at 300°C482.1433Figure 4
Dynamic response of a TiO2-based 64° YX LiNbO3 SAW
sensor towards H2 at 230°C.Figure 5
Dynamic response of a TiO2-Au-based 64° YX LiNbO3 SAW
sensor towards H2 at 230°C.Figure 6
Frequency shift versus operational temperature
for TiO2 and TiO2-Au-based 64° YX LiNbO3 SAW
sensors towards 1%H2 concentration.The gas sensing
mechanism is governed by the reaction switch that occurs at surface between the
TiO2 nanoparticles and gas molecules. It involves chemisorption of
oxygen on the particle surface followed by charge transfer during the reaction
of chemisorbed oxygen with target gas molecules [19]. In air, oxygen molecules
adsorb onto the surface of the TiO2 layer in several forms such as:
O2−, O−, and O2− depending on the
operational temperature and form a depletion region at the oxide surface [19, 20].
This reduces the film conductivity. The addition of a reducing gas such as H2 increases the film conductivity when the metal oxide is n-type and decreases
the film conductivity when the metal oxide is p-type. It is evident from test
results that (see Figure 6) gold nanoparticles addition to the TiO2 has
increased the sensor response towards H2 by 1 kHz and decreased
optimum operational temperature approximately 15°C. It is therefore supposed that the Au NPs have an enhancing effect on
the catalytic activity of TiO2 for H2 sensing in SAW
device.


## 3.1. Characterization

XRD spectra of deposited TiO2 and TiO2-Au thin films are
shown in Figure 1. The spectra show the
microstructural composition of TiO2 and TiO2-Au films annealed
up to 300°C. The peaks detectable in the (a) pattern arise from (111) and (200)
planes of the cubic lattice of crystalline metal gold (powder diffraction file
no. 04-0784, International Centre for Diffraction Data, Newton Square, Pa, USA).
The crystal size evaluated from the Scherrer equation using the FWHM values of
the diffraction peaks was 5 nm with standard deviation of 8%. On the other hand,
the spectra of the undoped TiO2 film ((b) pattern) exhibited no
discernible diffraction peaks.Figure 1
XRD patterns of TiO2 and TiO2-Au films
heated up to 300°C.Figure2
shows Au as synthesized gold nanocrystals in water (inset) with a mean diameter
9.72 nm with 0.9 nm standard deviation. The main TEM image (see Figure 2) shows
the same particles homogeneously dispersed inside the TiO2 matrix.
The image shows there is no aggregation of Au NPs inside the TiO2 matrix core. The mean size of the gold particles in the matrix was obtained by
counting over two hundred NPs and is 9.51±0.89 nm, in perfect agreement with
dimensions observed for clusters in water. This indicates there is no
dissolution, coalescence, or morphological changes to the nanocrystals during
the dispersion into the films, nor is there any effect of the annealing process
on their size distribution.Figure 2
TEM images of Au NPs as synthesized in water (inset) and
inside the TiO2 matrix (main) heated up to 300°C.Optical absorption spectra of TiO2 film, TiO2-Au film, and Au NPs
suspension in water are reported in Figure 3. The latter are characterized by
the strong absorption band associated with the surface plasmon resonance (SPR)
of spherical gold nanoclusters. The band is centered at 520 nm for NPs
suspension in water and at 595 nm for the same NPs embedded inside the TiO2 matrix. The difference in SPR band positions is related to the different
refractive indices of the media, water 1.34 [16], and TiO2 2.14,
respectively. The average refractive index of the doped, porous TiO2 film was determined by ellipsometry. The film porosity was estimated using the
refractive indices of the films (both measured and calculated) through the
correlation proposed by Zhao et al.  [17] and Ahn et al. [18], assuming a value of n=2.52
for the refractive index of bulk TiO2.Figure 3
Optical absorbance
spectra of Au NPs in solution, of TiO2-Au film and of TiO2 film.Ellipsometric measurements
performed on the samples gave thickness, refractive index, and relative
porosity values summarized in the following table. Porosity values have been
estimated by Lorentz-Lorentz relation using 2.52 as reference refractive index
for dense TiO2 at 589 nm.


## 3.2. Test Results

The dynamic responses of TiO2 and TiO2-Au sensors to a
sequence of different H2 concentrations in synthetic air are shown
in Figures 4 and 5, respectively. The introduction of H2 gas to the
sensor surface causes an increase in the device's resonant frequency for both
of the sensors. With hydrogen exposure, the conductivity of the TiO2 and TiO2-Au layers decreases resulting in an increase in the
acoustic wave velocity, thereby increasing the resonant frequency. The sensor
response is defined as the variation in operating frequency of oscillation due
to the interaction with the target gas. The measured sensor response was
approximately 6.5 and 7.4 kHz towards 1% of H2 at 230°C for the TiO2 and Au-TiO2 sensors, respectively. The frequency shift (sensitivity)
as a function of the operating temperature in the presence of 1%H2 for both of the sensors is shown in Figure 6. Although Au doping slightly
decreases the film porosity as shown in Table 1, it increases the device
sensitivity and reduces the optimal operating temperature. The optimum
operating temperatures for the TiO2-Au-based sensor are in the range
of 230–245°C and for TiO2-based
sensor are in the range of 245–260°C. Both sensors exhibited reproducible responses and consistent baseline
stability, as indicated when a second pulse of 0.12% hydrogen was introduced
into the sensor chamber. Repeatability was confirmed by testing the sensor
continuously over a 7-day period.Table 1
Values of thickness, refractive index, and porosity of TiO2 and TiO2-Au films measured by ellipsometry.
SamplesFilm thicknessRefractive indexPorosity(nm)at 589 nm(%)TiO2 at 300°C482.0937TiO2-Au at 300°C482.1433Figure 4
Dynamic response of a TiO2-based 64° YX LiNbO3 SAW
sensor towards H2 at 230°C.Figure 5
Dynamic response of a TiO2-Au-based 64° YX LiNbO3 SAW
sensor towards H2 at 230°C.Figure 6
Frequency shift versus operational temperature
for TiO2 and TiO2-Au-based 64° YX LiNbO3 SAW
sensors towards 1%H2 concentration.The gas sensing
mechanism is governed by the reaction switch that occurs at surface between the
TiO2 nanoparticles and gas molecules. It involves chemisorption of
oxygen on the particle surface followed by charge transfer during the reaction
of chemisorbed oxygen with target gas molecules [19]. In air, oxygen molecules
adsorb onto the surface of the TiO2 layer in several forms such as:
O2−, O−, and O2− depending on the
operational temperature and form a depletion region at the oxide surface [19, 20].
This reduces the film conductivity. The addition of a reducing gas such as H2 increases the film conductivity when the metal oxide is n-type and decreases
the film conductivity when the metal oxide is p-type. It is evident from test
results that (see Figure 6) gold nanoparticles addition to the TiO2 has
increased the sensor response towards H2 by 1 kHz and decreased
optimum operational temperature approximately 15°C. It is therefore supposed that the Au NPs have an enhancing effect on
the catalytic activity of TiO2 for H2 sensing in SAW
device.


## 4. Conclusions

SAW gas sensors have been fabricated using
titania and gold nanocrystal-doped titania films, synthesized by sol-gel
processing. The response toH2 gas at different operating
temperatures between 150 and 310°C has been measured. The sensors show a
repeatable and stable response towards H2. The results of this study
indicate that the optimum operating temperatures for the TiO2 sensor
are in the range of 230–245°C and for the TiO2-Au sensor are in the
range of 245–260°C. We have demonstrated that the nanocrystal doping process
can be carried out with no aggregation of the dopants and this enables a wide
variety of nanocrystal doping effects to be investigated including the effects
of doping concentration, particle shape, size, and composition. Furthermore,
these thin films can be characterized by optical methods, something not
possible with conventional catalyst development routes.



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*Source: 254283-2008-07-15.xml*