Thermally Evaporated Tungsten Oxide (WO 3 ) Thin Films for Gas Sensing Applications Submitted in fulfilment of the requirements of Doctor of Philosophy Mohammed Ahsan School of Engineering Systems Faculty of Built Environment and Engineering Queensland University of Technology Australia 2012
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Thermally Evaporated Tungsten Oxide (WO3) Thin Films for Gas Sensing
Applications
Submitted in fulfilment of the requirements of Doctor of Philosophy
Mohammed Ahsan
School of Engineering Systems Faculty of Built Environment and Engineering
Queensland University of Technology Australia
2012
ii
Statement of Original Authorship
The present thesis reports the results of the work done during the years of my PhD
project. The work has been carried out mostly at Queensland University of
Technology although some experiments were performed also at University of
Queensland, Australian National University, Australian Nuclear Science and
Technology Organization and RMIT University.
I hereby declare that:
All the experiments have been performed during the PhD project.
All the results presented come directly from the experimental activity done.
All the interpretations and observations are based on the results of the
experiments and have been often referred, with the reported reference, to previous
literature results.
For these reasons, I can declare, at the best of my knowledge, that this work is
original and never submitted before in any other academic institution for higher
degree qualification purposes.
Mohammed Ahsan
________________________
28th April 2012
iii
Acknowledgements This work has been done with the invaluable help of many people. I wish to thank
them all.
First of all I wish to thank immensely to Dr. Tuquabo Tesfamichael for
providing me the opportunity to conduct my PhD and for his advice, support and
constructive feedback throughout my PhD. He has been a great support on all fronts
and made my PhD journey a memorable experience. Special thanks to my associate
supervisor Prof. John Bell for encouraging me to join QUT and his advice and
support throughout my PhD project. I wish to thank to my associate supervisor Prof.
Prasad Yarlagadda, for his positive feedback and advices.
I deeply acknowledge the funding provided by Prof. Nunzio Motta through
NIRAP Project “Solar powered Nano Sensors” for purchases and visits during the
project work.
I wish to thank Prof. Barry Wood from UQ, Prof. Wojtek Wlodarski from
RMIT, Dr. Mihail Ionescu from ANSTO and Nina De Caritat from ANU for their
immense help and support during experimental work at these facilities. I extend my
thanks to Peter Hynes, Cristina Theodoropoulos, Lambert Bekessy, Thor Bostrom
and Tony Raftery from AEMF, QUT for their continuous support and guidance
during my PhD. I would also like to extend my thanks to the technical staff of Built
Environment and Engineering Faculty for their support. I acknowledge all the lovely
people from Research Portfolio Office, for their support and guidance during my
PhD. I extend my sincere thanks to all my friends and colleagues in O401 for their
support and encouragement during my entire stay.
iv
Words are not sufficient to thank the most special person in my life, my wife,
Tehmeena whose sacrifice, love, encouragement and immense support made this
PhD an easy journey for me. I owe special thanks and immense love to my beautiful
children Ayman, Akmal and Hamnah who have been away from their dear father for
more than 2 years. This thesis could not have been accomplished with the immense
support of my father Amrullah Sharief, my mother Rabia Begum, my brother Akbar
and my sister Najma. I take this opportunity to thank all my relatives including my
father-in-law Prof. Sofi Ali, brother-in-laws Mansoor, Masood, Moudood and Late
Maqsood for their encouragement and support.
Mohammed Ahsan
v
Abstract
In this thesis, the author proposed and developed gas sensors made of
nanostructured WO3 thin film by a thermal evaporation technique. This technique
gives control over film thickness, grain size and purity. The device fabrication,
nanostructured material synthesis, characterization and gas sensing performance have
been undertaken. Three different types of nanostructured thin films, namely, pure
WO3 thin films, iron-doped WO3 thin films by co-evaporation and Fe-implanted
WO3 thin films have been synthesized. All the thin films have a film thickness of 300
nm. The physical, chemical and electronic properties of these films have been
optimized by annealing heat treatment at 300ºC and 400ºC for 2 hours in air.
Various analytical techniques were employed to characterize these films. Atomic
Force Microscopy and Transmission Electron Microscopy revealed a very small
grain size of the order 5-10 nm in as-deposited WO3 films, and annealing at 300ºC or
400ºC did not result in any significant change in grain size. X-ray diffraction (XRD)
analysis revealed a highly amorphous structure of as-deposited films. Annealing at
300ºC for 2 hours in air did not improve crystallinity in these films. However,
annealing at 400ºC for 2 hours in air significantly improved the crystallinity in pure
and iron-doped WO3 thin films, whereas it only slightly improved the crystallinity of
iron-implanted WO3 thin film as a result of implantation. Rutherford backscattered
spectroscopy revealed an iron content of 0.5 at.% and 5.5 at.% in iron-doped and
iron-implanted WO3 thin films, respectively. The RBS results have been confirmed
using energy dispersive x-ray spectroscopy (EDX) during analysis of the films using
transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS)
revealed significant lowering of W 4f7/2 binding energy in all films annealed at 400ºC
vi
as compared with the as-deposited and 300ºC annealed films. Lowering of W 4f7/2 is
due to increase in number of oxygen vacancies in the films and is considered highly
beneficial for gas sensing. Raman analysis revealed that 400ºC annealed films except
the iron-implanted film are highly crystalline with significant number of O-W-O
bonds, which was consistent with the XRD results. Additionally, XRD, XPS and
Raman analyses showed no evidence of secondary peaks corresponding to
compounds of iron due to iron doping or implantation. This provided an
understanding that iron was incorporated in the host WO3 matrix rather than as a
separate dispersed compound or as catalyst on the surface.
WO3 thin film based gas sensors are known to operate efficiently in the
temperature range 200ºC-500 ºC. In the present study, by optimizing the physical,
chemical and electronic properties through heat treatment and doping, an optimum
response to H2, ethanol and CO has been achieved at a low operating temperature of
150ºC. Pure WO3 thin film annealed at 400ºC showed the highest sensitivity towards
H2 at 150ºC due to its very small grain size and porosity, coupled with high number
of oxygen vacancies, whereas Fe-doped WO3 film annealed at 400ºC showed the
highest sensitivity to ethanol at an operating temperature of 150ºC due to its
crystallinity, increased number of oxygen vacancies and higher degree of crystal
distortions attributed to Fe addition. Pure WO3 films are known to be insensitive to
CO, but iron-doped WO3 thin film annealed at 300ºC and 400ºC showed an optimum
response to CO at an operating temperature of 150ºC. This result is attributed to
lattice distortions produced in WO3 host matrix as a result of iron incorporation as
substitutional impurity. However, iron-implanted WO3 thin films did not show any
promising response towards the tested gases as the film structure has been damaged
vii
due to implantation, and annealing at 300ºC or 400ºC was not sufficient to induce
crystallinity in these films.
This study has demonstrated enhanced sensing properties of WO3 thin film
sensors towards CO at lower operating temperature, which was achieved by
optimizing the physical, chemical and electronic properties of the WO3 film through
Fe doping and annealing. This study can be further extended to systematically
investigate the effects of different Fe concentrations (0.5 at.% to 10 at.%) on the
sensing performance of WO3 thin film gas sensors towards CO.
viii
Table of Contents
Abstract ........................................................................................................................ v
Table of Contents ...................................................................................................... viii
List of Figures ............................................................................................................. xi
List of Tables .............................................................................................................. xv
List of Publication – Research Activities ................................................................. xvii
Backscattered Spectroscopy and Raman Spectroscopy. This is valuable in
assessing the microstructural and electronic properties of the films.
25
5. To characterize the sensing properties (sensitivity, selectivity, response and
recovery times) of these films to H2, ethanol and CO in the temperature range
100ºC to 300ºC.
6. Investigate the effect of microstructure, doping and post deposition heat
treatment on the sensing performance and operating temperature of these
films.
1.4 Thesis Organization
This thesis consists of six chapters and is presented as follows:
Chapter 1 gives an overview of author’s motivation for performing this
research and specific objectives.
Chapter 2 presents the literature review on various aspects of gas sensing. It
includes the construction principle and basic characteristics of gas sensors,
gas sensing mechanisms, role of additives, importance of dimensions in gas
sensing and limitations of current metal oxide based gas sensors. The
structural properties of tungsten oxide and brief overview of various
deposition techniques with a particular focus on thermal evaporation are also
described in this chapter.
Chapter 3 outlines the processes and procedures involved in the synthesis and
characterization of tungsten oxide thin films investigated in this research.
This chapter is broadly divided into three sections which describe the thin
film deposition, thin film characterization and gas sensor characterization. In
the first section of this chapter, thermal evaporation of nanostructured
tungsten oxide thin films is described. It is followed by the description of
various analytical techniques such as Transmission Electron Microscopy,
26
Atomic Force Microscopy, Rutherford Backscattered Spectroscopy, X-ray
Diffraction, X-ray Photoelectron Spectroscopy and Raman spectroscopy
which were employed to characterize these films. The last section describes
the details of the conductometric gas sensing setup used in this study.
Chapter 4 focuses on the physical, chemical and electronic characterization of
nanostructured thin films. The characterization outcomes using various
analytical techniques are presented and the effect of heat treatment and
doping are discussed.
Chapter 5 presents the experimental results obtained from the gas sensor
characterization of nanostructured tungsten oxide thin film based gas sensors.
The results are linked to the various physical, chemical and electronic
properties of these films investigated in Chapter 4 and the influence of
various factors such as surface morphology, grain size, crystallinity and
stoichiometry on gas sensing performance are discussed.
Chapter 6 presents the conclusions of this thesis and suggestions for possible
future work.
27
CHAPTER 2 : LITERATURE REVIEW
2.1 Introduction
In Chapter 1, conductometric gas sensors were described and the parameters
expressing the sensor performance have been discussed. In this chapter, the
fundamental characteristics of conductometric metal oxide gas sensors are discussed.
Then, a critical review of the state of the technology of metal oxide gas sensors is
presented with a focus on tungsten oxide.
2.2 Construction principles of conductometric metal oxide gas
sensors
In principle, conductometric metal oxide gas sensors are constructed by two key
functions: Receptor function and Transduction function.
2.2.1 Receptor function
Receptor function transforms chemical information into a form of energy which
can be measured by the transducer [21]. It is determined through various interactions
between the surface and target gas such as adsorption, ion exchange or
electrochemical reaction. In air, oxygen is adsorbed on the oxide grains as negatively
charged ions, inducing a surface space charge layer depleted of electrons (space
charge layer) which leads to a band bending [22]. The band bending and thickness of
space charge layer created due to oxygen species are further decreased when the
metal oxide is exposed to a reducing gas as reducing gases inject electrons to the
conduction band. For oxidizing target gasses such as NO2 the conductivity decreases
as oxidising gases extract electrons. Variation of the height of the potential barrier is
28
believed to be the origin of the conductance response to gases [23]. The receptor
function of metal oxides is largely affected by their intrinsic electronic properties and
any deviation from their stoichiometric chemical properties. Defects such as oxygen
vacancies are inherent in metal oxides. This creates the space charge layer depleted
of electrons and negatively charged oxygen ions on the surface. The mobility of main
carriers is of primary importance for semiconductors, because it provides the
proportionality constant of the change of electrical conductivity when the number of
main carriers changes as a result of gas–solid interactions [24]. As the metal oxides
approach stoichiometry, the conductivity becomes extremely low (high resistance).
WO3 and TiO2 films have very small electron mobility (high resistivity) which
ranges between 0.03 and 0.2 cm2 V−1 s−1 [25, 26]. Dopants and defects such as
interstitial cations or anion vacancies can play an important role in enhancing the
conductivity [14, 15]. Dopants are important to the increased formation of oxygen
vacancies and modifying of the electronic structure and band gap energy of metal
oxides. It has been reported that doping of TiO2 with Fe increases the oxidation
activity of the oxide and this has been related to a higher density of oxygen vacancies
[27].
2.2.2 Transduction function
Transduction function is related to the ability to transport electrons through grain
boundaries. In polycrystalline metal oxide, gas sensing reaction takes place at the
surface of the individual particles and at grain boundaries and it becomes easy for
electrons to conduct through different grains. Experiments on various metal oxide
films exposed to specific gases suggest that the electronic band structure and hence
conductivity are also dependent on film microstructure (i.e. particle size and film
porosity) [16-19, 28]. This shows that the magnitude of the conductivity change
29
depends mainly on the ratio between particle size and Debye length (distance over
which charge separation occurs in a semiconductor). If the grain size is large (>>
Debye length) the depletion of the space charge region between the grain boundaries
controls the conductivity variation. If the radius of the grain is extremely small (less
than the Debye length, e.g. for WO3, is 10 nm), the entire particle is depleted and no
band bending occurs which results in high conductivity-change (or high sensitivity)
when exposed to target gas. In practice nanomaterials of particle size less than 10 nm
favours large gas-active surface area and provide high sensor sensitivity.
2.3 Limitations of existing metal oxide gas sensors
The functions discussed above in Section 2.2 strongly influence the sensitivity of
semiconducting metal oxides. Although most of the semiconductor metal oxide
sensors that have been investigated to date are promising, they do not show response
to gases at lower operating temperatures (100ºC-200ºC) and must be thermally
activated at higher temperatures (200ºC-500ºC). These high operating temperatures
cause grain growth and changes in material properties that lead to long term stability
problems of the sensors. The higher optimum operating temperatures [29, 30] also
demand higher power consumption, which makes them unsuitable for battery
operated sensor devices that would be advantageous in some in situ applications.
Lower operating temperature metal oxide gas sensors with an acceptable sensitivity
would overcome these stability and high power consumption problems. By analogy
with the current high temperature metal oxide gas sensors, a method for enhancing
sensitivity to gas at lower temperatures is modification of the electronic structure of
the metal oxides by using mixed metal oxides such as SnO2-ZnO, Fe2O3-ZnO and
ZnO-CuO [28, 31-33]. Composites of SnO2-ZnO and SnO2-In2O3 have shown
enhanced sensitivity when compared with single oxide sensors when exposed to
30
ethanol [34]. Sensitivity is improved by selective catalytic activity of one component
of the mixed oxide to a particular gas. However, this approach has not shown any
significant reduction in optimum working temperature of the sensors [33].
The catalytic activity of the gas sensor material can also be enhanced by doping
with noble metals such as Pt, Au, Pd and Ag [35-39]. The noble metals chemically
sensitize the metal oxide surface i.e., they activate target gases by enhancing their
spill-over (more surface coverage), so that they react with oxygen adsorbates more
easily. The oxygen supply can also be improved by metal additives, at the surface of
which oxygen molecules from the ambient can be dissociated and migrate to the
surface of metal oxide. In this way, the additive enhances the sensing properties of
the metal oxide.
Metal oxides can also be electronically sensitized to improve the gas response
and lower the operating temperature [40]. Addition of fine particles of some metals
results in a rise of the base resistance in air. The electron concentration in the oxide
surface layer is low, which corresponds to an increase in the space-charge depth as a
result of the transfer of electron from the metal oxide to the metal loaded onto its
surface. When the metal surface is covered with oxygen adsorbates at elevated
temperatures in air, its oxidation state changes (the metal is oxidized).The oxygen
adsorbates extract electrons from this metal, which in turn extracts electrons from the
metal oxide, leading to a further increase in the space charge depth. Consumption of
oxygen adsorbates on the metal, in addition to those on the metal oxide surface, by
reaction with gas, enhances the sensitivity.
Another limitation of current metal oxide sensors is that they are non-selective
i.e. they are sensitive simultaneously to a wide range of gases. Selectivity can be
improved by exploiting the influence of operating temperature on the sensitivity of
31
the sensor [41, 42]. A change in sensor resistance is expected with change in
temperature, as the reactions occurring at the surface of the sensor
(chemisorption/redox reaction) are functions of temperature [9]. The temperature
dependence of the sensitivity of different gases can therefore be exploited to obtain
the selectivity to a particular gas.The selectivity can also be improved by depositing
a diffusion filter layer, such as SiO2 on top of the metal oxide [43]. By doing so, only
small molecules such as H2 are able to reach the surface of sensing material.
However, this has the undesirable effect of reducing the total number of gas
molecules that reach the sensor, hence reducing sensitivity.
One of the recent developments in improving selectivity of metal oxide gas
sensors is the use of a neural network. The idea is inspired from the biological
olfactory system. In human and mammalian noses, there are thousands of receptors
with bad selectivity to different odours, but the brain is able to derive specific odour
identification by processing the signals from all the receptors. The technological
analogue is called ‘electronic nose’ and mimics the natural olfactory process [5, 6].
In this system, an array of sensors with different functionality is employed and their
data are processed by neural networks to determine the gas concentrations.
2.4 Basic characteristics of a metal oxide conductometric sensors
In general, the electrical resistance of a conductometric metal oxide gas sensor
changes upon exposure to the molecules of the target gas. The nature of sensor
material (n-type or p-type semiconductor metal oxide) and the target gas (oxidizing
or reducing) governs the increase or decrease in electrical resistance. For an n-type
semiconductor exposed to reducing gas, the resistance decreases, whereas, upon
exposure to oxidizing gas, the resistance increases. The variation of resistance of
32
sensor with time on exposure and withdrawal of target gas is depicted by a typical
response curve as shown in Fig. 2-1.
Figure 2-1: A typical response curve of a conductometric gas sensor.
The performance of a gas sensor is characterized by the following five parameters
[8]:
1. Sensitivity or Response Amplitude
2. Response time
3. Recovery time
4. Selectivity
5. Long term stability
A brief description of these parameters follows below:
1. Sensitivity(S):
It is defined as the ratio of resistance change of a sensor upon exposure to target
gas to the resistance in target gas for n-type materials [8]. For p-type materials, it is
the ratio of resistance change of sensor upon exposure to target gas to the resistance
in air.
Response time
Recovery time
Gas on
Gas off
Time
Res
ista
nce
33
)1.2()( materialstypenforR
RS
gas
)2.2()( materialstypepforR
RS
air
where ΔR is the change in sensor resistance upon exposure to target gas. Sensors of
high S value are desirable in order to sense low concentration of gases.
As pointed out in the Section 1.3, the term ‘sensitivity’ is often used to indicate
response amplitude. However, in a true sense, sensitivity of a gas sensor is defined as
the derivative of the response to the gas concentration [11]. For the purpose of
simplicity, the term ‘sensitivity’ would be used to indicate the response amplitude of
the sensors in this study.
2. Response time:
This is the time interval over which resistance of the sensor material attains a
fixed percentage (usually 90%) of final value when the sensor is exposed to full scale
concentration of the gas. It is usually expressed as T90, T80, etc. A T80 of 50s
means that the sensor exhibits 80% of saturation value of resistance in 50s.
3. Recovery time:
It is the time interval over which sensor resistance reduces to 10% of the
saturation value when the sensor is exposed to full scale concentration of the gas and
then placed in the clean air. A sensor should have a small recovery time so that it can
be used over and over again.
4. Selectivity:
Most of the chemiresistive sensors exhibit high value of sensitivity to many
gases under similar operating conditions. Thus, selectivity of a sensor towards target
gas is expressed in terms of dimension that compares the concentration of the
corresponding interfering gas that produces the same sensor signal. It is expressed as
34
(2.3) d gasthe desirey towards Sensitivit
ering gasfor interf the sensorof y SensitivitySelectivit
5. Long term stability:
The ability of a sensor to maintain its properties when operated continuously for
long durations is called its stability. Good sensors have long term stability that last up
to several years without showing a drift in sensor performance.
All the above sensing parameters depend on several factors including the following:
Sensing material i.e. intrinsic properties of the metal oxide.
Sensing mechanisms i.e. interaction between the gas and sensor surface
(details in Section 2.5).
Operating conditions i.e. temperature, type of target gas.
Film properties such as microstructural features, film type (thick or thin film),
stoichiometry, etc.
In order to control these parameters, scientific understanding of gas - sensor
interaction (sensing mechanism) needs to be addressed, which follows.
2.5 Basic mechanisms of gas sensing in semiconductor metal oxide
sensors
Semiconductor metal oxides are used for two different types of gas sensing
applications. Broadly speaking, these applications can be categorized as
Determination of partial pressure of oxygen.
Determination of concentration of a minor constituent (oxygen partial
pressure remains constant).
The materials that have been used commercially for determining partial pressure
of oxygen are TiO2, Cr2O3 and Ga2O3 [44]. If the sensor operates at a high
35
temperature (700ºC and above), the mechanism responsible for the detection is bulk
conduction. The oxygen partial pressure and electrical conductivity are related as
[44]
(2.4)1/m2
B
A OPTk
Eexpσσ
where, is the electrical conductivity,
* is a constant,
EA is activation energy for conduction,
KB is Boltzman’s constant,
P[O2] is oxygen partial pressure,
m is the oxygen vacancy constant dependent on dominant type of bulk defect
involved in the reaction between sensor and oxygen and
T is absolute temperature.
The second application of semiconductor metal oxide gas sensors involves
situations where the oxygen partial pressure is constant and concentration of minor
constituent gases such as H2, CO, CH4 and H2S are to be determined.
For an n-type metal oxide semiconductor, possible gas sensing mechanisms are
[45]:
Surface reactions with adsorbed gases.
Ion exchange.
Direct gas adsorption.
For semiconductor materials, the observed sensor effects are dominated by direct
gas adsorption and surface reactions with preadsorbed molecules. The sensor
characteristics will vary depending on whether the sensor material is n-type or p-type
and whether the interacting species are reducing or oxidizing gases.
36
For n-type sensor material and reducing gas, there will be preadsorbed oxygen
ions on the surface of the sensor. The gas will then react with oxygen ions to form
neutral molecules, leading to electron transfer to the sensor material and
consequently decrease the resistance. Following is the detailed description:
In air environment, oxygen molecules adsorb onto the surface of metal oxide
layer to form O2-, O- and O2- species by extracting electrons from the conduction
band depending on the temperature [46] and type of metal oxide (n-type or p-type).
These oxygen adsorbates play an important role in detecting gaseous species.
It has been experimentally confirmed by TPD, FTIR and ESR techniques that
dominant oxygen species are [47]:
Molecular (O2-) below 150ºC.
Atomic (O-) between 150ºC and 300ºC.
Atomic (O2-) above 300ºC.
The oxygen adsorption can be described by the following rate equations:
)6.2()()( 221 adsOgO k
)7.2()()( 222 adsOadsOe k
)8.2()(2)(2 32 adsOadsOe k
)9.2()()( 24 adsOadsOe k
The reaction of the target gas X to be detected can be represented by:
)10.2()()( 225 egXOadsOX k
)11.2()()( 6 egXOadsOX k
)12.2(2)()( 22 7 egXOadsOX k
In an n-type semiconductor metal oxide, electrons are transferred to the surface
and then ionize the oxygen adsorbates to form O2- and O- which results in a negative
37
charge being developed on the surface. The surface layer is therefore depleted of
electrons, and so called depletion layer is created [48].
Fig. 2-2 shows the schematic of the surface layer and the corresponding electron
band structure. The conduction band energy levels of the bulk and the surface are
represented by Ecb and Ecs, respectively. The valence band energy levels for the bulk
and the surface are represented by Evb and Evs, respectively, and Ef denotes the Fermi
energy. Et is trap energy level of electrons at surface states due to adsorbed oxygen,
eVs is the height in energy of the band bending at the surface, d is the distance from
the surface and dd is the depth of depletion. The difference between Ecb and Evb is the
band gap energy Eg.
Figure 2-2: Schematic diagram showing the surface layer and corresponding electron band structure,
adapted from [48].
If the surface belongs to an ideal bulk material (large d) without grain
boundaries, the influence of the depletion layer is of little or no importance for the
conduction along the surface. However, in case of polycrystalline film, each interface
between grains gives rise to a band bending as depicted in Fig. 2-3. At the
intergranular contact, the conduction is restricted by this Schottky potential barrier
Evs Evb
EF
Ecb
Ecs eVs
d
Surface
O2-
O-
Et
Depletion layer (for n-type
semiconductor)
dd
38
due to depletion layer and the electrons have to overcome the energy barrier, eVs.
The change of the barrier height makes the electrical resistance of the material
dependent on the gaseous atmosphere [49]. The resistance and hence, gas sensitivity
in this case are not dependant on particle size.
Figure 2-3: Schematic view of physical and electron band structure model for a polycrystalline
material [50].
Fig. 2-4 shows three situations with different influence on the depletion layer
[50]. Fig. 2-4a demonstrates the situation where the area of the depletion zone at the
contact is less than the contact area. The depletion layer extends into the grain to a
depth marked by the dashed line. The resistivity is about that of the undepleted
region in the center of the neck. Fig. 2-4b illustrates a closed neck, where, the
depletion layers from the surfaces overlap resulting in a higher resistance in the
center of the constriction. The above two situations imply that the porous structure
would respond to the gas in the same way as a thin film sensor. Fig. 2-4c illustrates a
Barrier
eVs
O-
O2-
O-
O- O2
-
O- O2-
O- O-
O2- O-
O- O2- O
- O- O2
- O-
O-
O-
O2-
O- O- O- O2
- O- O- O-
Electronic current
Conduction band electrons
Adsorbed oxygen
Depletion layer
Physical Model
Band Model
39
situation where conduction is limited by the point of contact. This type of situation is
applicable to a porous material.
Figure 2-4: Schematic illustration of intergrain constriction situations: (a) Open neck (b) Closed neck and (c) Conduction limited by point of contact.
Wang [51] investigated the transition from open neck, neck controlled to point of
contact limited conduction of metal oxide gas sensors. He showed that the sensitivity
to adsorbed gas increased rapidly as the grain size became smaller than 40 nm, at
which point the conduction mechanisms for neck controlled and point of contact
limited conduction exist together.
2.6 Role of additives in gas sensing
Appropriate amounts of metal additives such as Au, Pt, Pd, etc. when added to
metal oxide sensors have shown improvement for various kinds of gases.
Enhancement in sensor response and a decrease in operating temperature for
maximum sensor response have also been achieved, in addition to decrease in
response time and better selectivity. The main idea of using metal additives is to
enhance the reaction rate of the gases when they come in contact with the sensor
surface.
40
Shimizu et al [52] identified that metal additives can lead to two different
sensitization mechanisms: Electrical sensitization and Chemical sensitization. These
mechanisms are shown in Fig. 2-5.
Figure 2-5: Schematic of additive role in gas sensing [40].
In chemical sensitization, the metal cluster catalyzes the reaction and reaction
products are subsequently spill over the semiconducting metal oxide surface. These
reaction products then cause the gas sensing response [40].
Electronic sensitization occurs due to the alignment of Fermi energies of the
metal oxide and the additive. This is similar to the Schottky barrier influencing the
surface charge region in the semiconductor material. Oxidation or reduction of metal
additive controls the band bending of the metal oxide thereby controlling the sensing
mechanism [53].
2.7 Importance of the dimension in gas sensing
Nanomaterials are defined as those that have at least one of their dimensions
≤100nm. Thus we may visualize them as structures produced by reducing one, two,
or three dimensions of a bulk material, thereby resulting in 2D nanolayers, 1D
nanowires or 0D nanoclusters [54, 55]. At such small length scales, most of the
Electrical sensitization Chemical sensitization
O-
e-
O2
O- 2R+O2 2RO
O-
O- O- O-
R RO
Noble Metal Cluster
41
atoms are surface atoms, thus significantly increasing the effective number of sites
available for reactions. Increase in surface area to volume ratio with decrease in grain
size is very important in the context of gas sensing. Thus reducing the grain size
plays an important role in applications that involve surface reactions such as
catalysis, chemical gas sensing, etc.
Fig. 2-6 demonstrates the grain size dependence of sensitivity of SnO2 films
exposed to CO and H2 [56]. It can be observed that grain size reduction below 10 nm
results in a drastic increase in sensitivity.
Figure 2-6: The effect of particle size on gas sensitivity of SnO2 sensor exposed to CO and H2, adapted from [56].
Fig. 2-7 shows the response of electron beam evaporated In2O3 film to NO2 as a
function of grain size [57]. A remarkable increase in sensitivity is observed upon
grain size reduction from 20 nm to 5 nm. The depletion layer depth (Debye length)
plays an important role if the grain size is very small. For most nanostructures,
Debye length is of the order of the grain diameter (considering spherical particles,
nanowires or nanotubes) or their width in case of nanobelts and other flat
nanostructures. Under such conditions the surface chemical processes strongly
influence the electronic properties.
42
Figure 2-7: Influence of grain size on gas response of In2O3 film to NO2, adapted from [57].
A 2 to 3 order increase in sensitivity was observed in the case of In2O3 films
when the grain size was decreased from 60-80 nm to 10-50 nm [58].
For grains large enough to have a bulk region unaffected by the surface
phenomena, i.e. when the grain diameter d >> λD (Debye length), the surface charge
carrier density ns, is given by [9]
)13.2(exp
Tk
qVnn
B
sbs
where nb is the concentration of free charge carriers (electrons), qVs is activation
energy, kB is Boltzman's constant and T is temperature in Kelvin.
When the grain size d ≤ λD is comparable to depth of depletion layer (Debye length),
the activation energy ΔE is related to Debye length as [9]
)14.2(4
DB
DTkE
where D is the grain diameter.
43
If ΔE is comparable to the thermal energy then a homogeneous electron
concentration is attained in the grain and leads to the flat band case. For grain sizes
lower than 10 nm, complete depletion of charge carriers occurs inside the grain and a
flat band condition results in a wide range of temperatures. Also, as the gas sensing
mechanisms involve adsorption processes, the physical properties and the shape of
the material determine the response of the nanosensor. Higher area/volume ratio
favors gas adsorption (and change in conductivity), decrease the response time and
increase the sensitivity of the device. Additionally, the time taken for gas molecules
to diffuse into and out of the volume of nanostructures is minimized [59].
Because of the advantages nanostructured material based sensors have over the
same sensor built with bulk material, nanoparticles and thin films of metal oxides
(less than 1000 nm thick) have been used to detect a wide variety of gases. As a
result, nanotechnology offers sensor devices with improved sensing properties.
2.8 Structural properties of tungsten oxide
Amorphous tungsten oxide film has large open pores and constitutes clusters
which are built from 3-8 WO6 octahedra [60]. These octahedra are linked together at
corners or edges by W-O-W bonds or water bridges [61, 62]. Random packing of the
clusters results in open structure or voids which are usually filled with molecular
water taken from the air [61]. The ionic conduction of amorphous WO3 film is
carried out by proton transport through water bridges in pores. On the other hand, the
electronic conduction is done by clusters linked through W-O-W bonds.
Tungsten oxide exhibits a cubic perovskite-like structure based on the corner
sharing of WO6 octahedra, with the O atoms at the corner of each octahedron [63]. A
schematic view of WO3 crystal structure is shown in Fig. 2-8. The symmetry of
tungsten oxide is lowered by two distortions: tilting of WO6 octahedra and
44
displacement of tungsten from the center of its octahedron [64]. These distortions
result in a number of temperature dependant phases of WO3, which are listed below
[64-68]:
Monoclinic ε – WO3 phase below -50ºC.
Triclinic δ – WO3 phase (from -50 to 17ºC).
Monoclinic γ – WO3 phase (from 17 to 330ºC) stable at room temperature.
Orthorhombic β – WO3 phase (from 330-740ºC).
Tetragonal α – WO3 phase above 740ºC.
In addition to the above phases, a metastable hexagonal WO3 phase has also been
*The sensitivity values presented in this table are absolute values as reported in the literature. Their magnitude cannot be compared as different authors have used different formula to calculate
sensitivity. The main intent of this table is to highlight the published results on WO3 based gas sensors.
Kawasaki et al [160] investigated NOx sensing properties of WO3 films
synthesized by Pulsed Laser Deposition (PLD) method. Their results demonstrated
that PLD technique is an efficient method to produce crystalline WO3 thin films
which are sensitive to NOx gases. It was also observed that substrate temperature (Ts)
49
is an important parameter in producing crystallinity in films and it increases with
increasing Ts. A sensor produced by using nanocrystalline WO3 and thin film
microfabrication technology showed a high degree of sensitivity to low NO2
concentration in the range from 50 to 550 ppb with relatively fast response and
recovery time [161]. The sensitivity was found to depend on surface structure, grain
size and geometrical heterogeneity of the films which were controlled by the
calcination temperature. The sensitivity was also found to depend on NO2 adsorbed
form on the surface which was affected by operating temperature. The optimal
sensing condition was found when the films were calcined at 550ºC for 1 hour and
operating the sensor at 300ºC. Ponzoni et al [139] obtained nanostructured WO3 gas
sensors by modified thermal evaporation technique which consisted of sublimation
from a metallic tungsten wire followed by oxidation in low vacuum conditions and
reactive atmosphere (PO2 = 0.22 mbar) with substrate heated at high temperature
(600ºC). The films were composed of agglomerates with nanometric size and present
high surface roughness and large effective area suitable for gas sensing applications.
Sensing measurements indicated high performance at a working temperature of
100ºC, high response towards sub-ppm concentrations of NO2 compared to lower
ones for NH3 and CO.
Iron-doped nanostructured WO3 thin films prepared by Electron Beam
Evaporation (EBE) technique were investigated towards acetaldehyde by
Tesfamichael et al [162]. Addition of 10 at.% Fe slightly decreased the band gap
energy and subsequent annealing at 300ºC for 1 hour in air further decreased the
band gap energy. The annealed Fe-doped WO3 sensor produced gas selectivity but a
reduced gas sensitivity towards acetaldehyde as compared to WO3 sensor. The NO2
sensing performance of pure and iron-doped WO3 thin films prepared by EBE
50
technique was investigated by Ahsan et al [163]. The pure WO3 films were found to
be highly sensitive to 5 ppm NO2 at lower temperature (150ºC). Doping with Fe was
found to decrease the film resistance significantly but also a reduced sensitivity. The
high sensitivity towards NO2 was attributed to the improved nanostructure obtained
through e-beam evaporation and subsequent annealing at 300ºC for 1 hour in air.
The H2S, N2O and CO sensing performance of Al-doped WO3 nanoparticle films
prepared by advanced gas deposition was investigated by Hoel et al [164]. A
maximum sensitivity towards H2S, N2O and CO was observed at temperatures
130ºC, 250ºC and 430ºC, respectively.
Iron addition lower than 10 at.% to WO3 films prepared by reactive RF
sputtering produced an enhancement in sensor response when exposed to NO2 [165].
Additionally, iron addition was found to be advantageous in sensing ozone, CO and
ethanol. NO2 and humidity sensing characteristics of WO3 thin films prepared by
vacuum thermal deposition and subsequent annealing in the temperature range of
300ºC-600ºC were investigated by Xie et al [166]. It was found that NO2 sensing was
strongly dependant on annealing and working temperature. Different WO3
mesoporous structures obtained by hard template route were used by Rossinyl et al
[167] to investigate their sensing response towards NO2. It was found that WO3 was
sensitive to NO2 even at low concentrations, although differences attributable to
different structures were observed. Introduction of copper as catalytic additive
improved both sensor response and response time [167].
Khatko et al [168] investigated the NO2, NH3 and ethanol sensing performance
of WO3 thin films deposited by reactive rf sputtering with interruptions during the
deposition process. Sensitivity was found to increase with increase in number of
interruptions and interruption time, which was attributed to observed grain size
51
reduction during interruption. In another study, the authors observed that the
response of these sensors to ozone is up to four times higher than that of the sensors
prepared using rf sputtering [169]. A high sensitivity to NO2 at a temperature of 50ºC
for a sensor made of WO3 particles of size ~36 nm was reported by Meng et al [170].
In this study, WO3 nanoparticles were prepared by evaporating tungsten filament
under a low pressure of oxygen gas, namely, by gas evaporation method. The
deposition was carried out under various oxygen pressures and samples were
annealed at different temperatures. The sensitivity was found to increase with
decreasing particle size, irrespective of oxygen partial pressure during deposition and
annealing temperature.
Solis et al [171] investigated the H2S response of nanocrystalline WO3 thick
films prepared by evaporation of tungsten metal by an electric arc discharge in
reactive atmosphere. The structure was found to consist of monoclinic and tetragonal
phases with a mean grain size of 40 nm. The influence of sintering temperature on
H2S sensitivity was studied. These films showed excellent sensing properties upon
exposure to low concentrations of H2S in air at room temperature. The conductance
of films sintered at 300ºC was found to increase by a factor of about 104 when
exposed to 10 ppm of H2S. A further rise in sintering temperature resulted in
decrease in sensor response. This effect was attributed to disappearance of the
tetragonal phase which may point at a specific crystal structure being responsible for
unique gas sensing properties of WO3. Nanoparticles and nanoplatelets of WO3 and
nanowires of WO2.72 were investigated for their H2S sensing characteristics by Rout
et al [172]. The WO2.72 nanowires emerged as good candidate for H2S sensors with
little effect of humidity (upto 60% relative humidity) as well as improved response
and recovery times.
52
The electrical response of WO3 based sensors for ozone detection was reported
by Boulmani et al [173]. Thin films (40 nm thick) of WO3 were deposited by rf
reactive magnetron sputtering on SiO2/Si substrate with Pt interdigitated micro
electrodes. The response towards ozone was found to strongly depend on film
morphology which depends on the oxygen concentration during the deposition
process. The sensor response was also affected by bias voltage, sputtering time and
oxygen concentration during deposition.
WO3 thin films with different effective surface area were deposited under
various discharge gas pressures at room temperature by using reactive magnetron
sputtering and their response towards H2 was investigated by Shen et al [174]. It was
observed that effective surface area and pore volume of WO3 thin films increased
with increasing discharge gas pressure. The peak sensitivity for H2 gas was observed
at 300ºC. The results indicate the importance of achieving high effective surface area
on improving the gas sensing performance. The hydrogen response of WO3
nanotextured thin films coated with a 2.5 nm Pt layer was investigated by Yaacob et
al [175]. The films exhibited gasochromic characteristics when tested in visible-NIR
(400-900 nm) range. The total absorbance in this range increased by 15% upon
exposure to 600 ppm H2 in synthetic air and 60% upon exposure to 10,000 ppm H2 in
synthetic air. The films were found to be highly sensitive with stable and repeatable
responses towards low concentrations of H2 at 100ºC. However, the recovery time
was found to be slow at room temperature.
The effect of cerium oxide additive on WO3 nanoparticles prepared by solgel
method towards Volatile Organic Compound (VOC) gases was investigated by Luo
et al [176]. The highest gas response of Ce-added WO3 samples was found to shift to
lower temperatures compared to pure WO3 samples. Grain boundaries were pinned
53
due to CeO2 which resulted in reduction in grain size and increase in surface area.
Complex impedence spectroscopy analysis indicated that grain boundary resistance
increased and grain boundary capacitance decreased with increasing concentration of
CeO2 which indicates that Ce ions mainly exist at WO3 grain boundaries and help to
improve the microstructure.
Tungsten oxide films have also shown a good sensing performance towards
ethanol [177-181]. The sensitivity towards ethanol has been attributed to the
desorption of oxygen at the surface of grains [181].
Carbon monoxide (CO) sensing characteristics of CoOOH-WO3 doped with Au
and SWCNT was investigated by Wu et al [158]. It was found that mixture with a
CoOOH-WO3 ratio of 2:1 had the highest sensor response at room temperature.
Doping with 1 wt% SWCNT and 0.1 wt% Au in CoOOH-WO3 was found to boost
the CO response by 3.6 times. Azad et al [182] investigated the sensing performance
of WO3 towards 100 ppm CO. The authors achieved sensitivity towards CO by
modulating ambient oxygen partial pressure to create oxygen deprivation on the
metal oxide surface. However, WO3 responded to CO only at 450ºC.
2.11 Deposition techniques of nanostructured metal oxide films
For the purpose of gas sensing, coatings or films of nanostructured metal oxides
are deposited on substrate. This section will briefly highlight the different processing
routes for deposition of metal oxide films. The methods can broadly be classified
into two main categories:
1. Thin film processes: Processes such as solgel, spray pyrolysis, physical
vapour deposition and chemical vapour deposition are classified under this
category.
54
2. Thick film processes: Processes such as screen printing and spin coating are
classified under this category.
Fig. 2-9 shows a schematic of the classification of different processing routes for
nanostructured materials. It is to be noted that there is a huge number of deposition
processes and only a few processes are discussed in the following sections.
Figure 2-9: A schematic of the classification of processes used for deposition of nanostructured films.
2.11.1 Thin Film Processes
2.11.1.1 Solgel
The process involves the hydrolysis of a metal organic compound such as a
metal alkoxide or inorganic salts such as chlorides to produce a colloidal solution
[183]. The hydrolysis can take place with the help of alcohol, acid or base. The sol is
then allowed to age and settle. This is called the gelation step. The sol can then be
coated on the substrate by either spin/dip coating to form a 'xerogel' film.
Alternatively, the solvent from the sol can be evaporated to precipitate particles of
uniform size and then these can be screen printed.
General Processing Routes for Nanostructured materials
Thin films Thick films
Wet process Vapour phase deposition
Solgel
Spray pyrolysis
Spin casting
Screen printing CVD PVD
RGTO
55
2.11.1.2 Spray pyrolysis
It involves atomization of a liquid precursor through a series of reactors, where
the aerosol droplets undergo evaporation, solution condensation within the droplet,
drying, thermolysis of the precipitate particle at higher temperature to form a
microporous particle which then gets sintered to give a dense film [184].
2.11.1.3 Chemical vapour deposition (CVD)
Chemical vapour deposition involves exposing a substrate of choice to a mixture
of volatile precursors that react or decompose on the substrate to give the desired
product. A wide variety of CVD techniques that are currently being in use are
at 300ºC for 2 hours in air and (c) annealed at 400ºC for 2 hours in air.
86
4.3 Characterization of chemical and electrical properties
4.3.1 Raman spectroscopy
The Raman spectra of as-deposited and annealed WO3 films are shown in Fig. 4-
14. Two characteristic bands are associated with WO3. The first band lies between
200-500 cm-1 and is associated with O-W-O bending vibration modes. The second
band lies in the range 600-1000 cm-1 and is associated with W-O stretching vibration
modes. The as-deposited WO3 film exhibited week and broad Raman band centred at
315 cm-1 and 799 cm-1. These features are characteristic of amorphous materials and
are usually assigned to O-W-O deformation modes and O-W-O stretching vibration
modes of monoclinic WO3 phase, respectively [201]. The amorphous nature of as-
deposited WO3 thin films is consistent to the results obtained by TEM and GIXRD
observations.
The film annealed at 300ºC also appears to be amorphous with a slight
broadening of peak at 315 cm-1. However, the crystallinity of this film increased after
annealing at 400ºC, as shown by the sharp peaks at 707 cm-1 and 799 cm-1 which are
characteristic of O-W-O stretching vibration modes [70], corresponding well with the
results obtained by GIXRD and TEM analysis.
The Raman spectra of as-deposited and annealed Fe-doped WO3 films are
shown in Fig. 4-15. The as-deposited Fe-doped WO3 thin film exhibited very weak
and broad Raman bands centered at 320 cm-1 and 804.4 cm-1, which are associated to
O-W-O deformation vibration modes and O-W-O stretching vibration modes,
respectively [201]. These weak and broad bands are indicative of the amorphous
nature of as-deposited Fe-doped WO3 thin film which is also confirmed by TEM and
GIXRD observations.
87
2000
2500
3000
3500
4000
4500
5000
5500
6000
200 400 600 800 1000 1200
WO3 annealed @ 300oC
WO3 annealed @ 400oC
as-deposited WO3
Inte
ns
ity
(AU
)
Raman shift (cm-1)
Si
Si
O-W-Odeformationmodes
O-W-Ostretchingmodes
799
707
315
Figure 4-14: Raman spectra of nanostructured WO3 films.
2400
2600
2800
3000
3200
3400
3600
200 400 600 800 1000 1200
as-deposited Fe-doped WO3
Fe-doped WO3 annealed @ 300oC
Fe-doped WO3 annealed @ 400oC
Inte
ns
ity
(AU
)
Raman shift (cm-1)
804.
4
712.
6320
394
O-W-Odeformationmodes
O-W-Ostretchingmodes
Si
Si
Figure 4-15: Raman spectra of nanostructured Fe-doped WO3 films.
Upon annealing at 300ºC for 2 hours in air, the intensity of these bands increased
slightly, indicating the onset of crystallinity growth of the film. Upon annealing at
400ºC for 2 hours in air, the peak intensity of O-W-O stretching modes at 712.6 cm-1
and 804.4 cm-1 increased significantly, indicating that the film is highly crystalline,
which corresponds well with the TEM and XRD observations. The O-W-O
stretching vibration mode peak positions of WO3 and Fe-doped WO3 films annealed
88
at 400ºC are compared in Table 4.1. A slight blue shift of about 5.5 cm-1 is observed
for both the peaks after doping with Fe. Such shifts are caused by shortening of O-
W-O bonds [202], which corresponds to slightly smaller cell parameters of Fe-doped
WO3 film as compared with WO3 film. The GIXRD analysis has shown that the
lattice parameters of Fe-doped WO3 film are slightly smaller than WO3 film.
However, the octahedral orientation of WO3 has been retained after doping with Fe,
indicating that the preferred oxidation state of Fe is Fe3+. This is evident from similar
XRD patterns of WO3 and Fe-doped WO3 films annealed at 400ºC and absence of
any Raman peaks associated with Fe in 400ºC annealed Fe-doped WO3 film.
Table 4-1: Comparison of the positions of O-W-O stretching vibration mode peaks observed for nanostructured WO3 and Fe-doped WO3 films annealed at 400ºC for 2 hours in air.
Raman Peak position (cm-1)
WO3 annealed at 400ºC 707 799 Fe-doped WO3 annealed at 400ºC 712.6 804.4
Blue Shift (cm-1) 5.5 5.5
Figure 4-16 shows the Raman spectra of Fe-implanted WO3 films. The as-
deposited film appears to be highly amorphous as no characteristic Raman peaks are
observed. Annealing the Fe-implanted film at 300ºC did not induce any crystallinity
in the film. However, after annealing at 400ºC, characteristic Raman peaks at 792
cm-1 and 706 cm-1 are observed. However, the intensity of these peaks is smaller than
those observed for WO3 and Fe-doped WO3 thin films annealed at 400ºC. This
indicates that the film is essentially amorphous even after annealing at 400ºC as
confirmed by GIXRD analysis.
89
2000
2500
3000
3500
4000
4500
5000
5500
6000
200 400 600 800 1000 1200
Fe-Implanted WO3
Fe-Implanted WO3 annealed @ 300oC
Fe-Implanted WO3 annealed @ 400oC
Inte
ns
ity
(AU
)
Raman shift (cm-1)
792
706
Si
Si
Figure 4-16: Raman spectra of nanostructured Fe-implanted WO3 films.
4.3.2 XPS analysis
Fig. 4-17 shows the XPS spectra obtained by wide survey scans on the surface of as-
deposited and annealed (300ºC and 400ºC) WO3 films between binding energies 0
and 1200 eV. Survey scan information is useful in identification of elements present
on the film surface. Peaks of O, N, C and W are observed in all the films. Presence of
carbon and nitrogen on the surface is attributed to atmospheric contamination. The C
peak measured at binding energy of 284.80 eV coincides with C 1s binding energy
reported in literature [203] and is, thus, used as a point for binding-energy reference.
Fig. 4-18 shows the XPS spectra of as-deposited and annealed (at 300ºC and 400ºC)
Fe-doped WO3 films between binding energies 0 and 1200 eV. Characteristic peaks
of O, N, C and W are observed as in the case of the pure WO3 film. However, no
characteristic peak of Fe was observed on the surface of all the films. The sensitivity
of XPS is only about 0.01 at.% over a depth of 10 nm. From RBS analysis, which is
discussed in the next section, the amount of Fe is only about 0.016 at.% over 10 nm
90
depth, which is much less than the critical amount that XPS can detect. This might
indicate the non-uniform deposition of Fe in the WO3. Hence, no Fe was observed on
the surface of the Fe-doped film as analysed using XPS.
0
5x104
1x105
1.5x105
2x105
2.5x105
3x105
020040060080010001200
as-deposited WO3
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1s
N 1s
W 4f
O 1s (a)
0
5x104
1x105
1.5x105
2x105
2.5x105
3x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1s
N 1s
W 4f
O 1sWO3 annealed @ 300oC (b)
0
5x104
1x105
2x105
2x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding Energy (eV)
O 1s
N 1sC 1s
W 4d
W 4f
WO3 annealed @ 400oC (c)
Figure 4-17: XPS wide spectra of nanostructured WO3 films, (a) as-deposited, (b) annealed at 300ºC for
2 hours in air and (c) annealed at 400ºC for 2 hours in air.
91
0
5x104
1x105
2x105
2x105
3x105
3x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1sN 1s
W 4f
O 1sas-deposited Fe-doped WO3 (a)
0
5x104
1x105
2x105
2x105
3x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding energy (eV)
W 4dC 1s
N 1s
W 4fO 1s
Fe-doped WO3 annealed @ 300oC (b)
0
5x104
1x105
2x105
2x105
3x105
020040060080010001200
Co
un
ts/s
eco
nd
(A
U)
Binding Energy (eV)
W 4fO 1s
N 1s
W 4d
C1s
Fe-doped WO3 annealed @ 400oC (c)
Figure 4-18: XPS wide spectra of nanostructured Fe-doped WO3 films, (a) as-deposited, (b) annealed at
300ºC for 2 hours in air and (c) annealed at 400ºC for 2 hours in air.
The XPS spectra of Fe-implanted WO3 films are shown in Fig. 4-19. Characteristic
peaks of O, N, C, W and Fe are observed in all the films.
0.2 CO 1000 150 Fe-implanted WO3 Imp annealed at 400ºC
5 Amorphous 5.5 35.70 37 0.05 H2 10,000 100
136
However, the Fe-implanted film shows poor gas sensing response as it was highly
damaged and annealing at 300ºC or 400ºC is not sufficient to crystallize this film.
The W 4f 7/2 binding energies of 400ºC annealed films has the maximum downshift
as compared to as-deposited and 300ºC annealed WO3 films, indicating high number
of oxygen vacancies in this film. However, no significant change is observed in W
4f7/2 binding energy of Fe-implanted film, thus, its stoichiometry is essentially
similar to that of pure WO3.This indicates that 400ºC annealed WO3 and Fe-doped
WO3 films have higher number of oxygen vacancies as compared to as-deposited
WO3 film and showed better sensing performance towards the tested gases (H2,
ethanol and CO).
Highest sensitivity to H2 is demonstrated by the 400ºC annealed pure WO3 film
at an operating temperature of 150ºC, which is due to the very small grain size (5
nm) and high porosity of the film, combined with high number of oxygen vacancies.
The highest sensitivity to ethanol is shown by the 400ºC annealed Fe-doped WO3
film at operating temperature of 150oC. Response to CO is also observed by the
400ºC annealed Fe-doped WO3 thin film at an operating temperature of 150ºC.
137
CHAPTER 6: CONCLUSIONS AND FUTURE WORK
This thesis presents the evolution of the author’s PhD research. The research
program commenced with the aim of optimizing the physical, chemical and
electronic properties of WO3 thin film for improved gas sensing performance. The
specific aim of this project was to optimize the operating temperature of the sensor
device and improve its sensitivity towards H2, ethanol and CO. Although WO3 thin
film gas sensors have shown excellent performance in detecting various gases, the
operating temperature of these films is still very high (300ºC-500ºC) and thus low
operating temperature is desirable. In addition, very little evidence is available in
literature on the CO sensing performance of WO3 thin film gas sensors which is
worthy of investigation through modification of the pure film by doping.
Thermal evaporation technique was used to develop pure nanostructured WO3
thin films. Iron doping of the thin films was performed through two methods: by co-
evaporation during thermal evaporation and by ion implantation. The films were
annealed at 300ºC and 400ºC in air for 2 hours to improve their film properties. The
properties of the films were characterized using AFM, TEM, XRD, RBS, XPS and
Raman to understand and evaluate their suitability for gas sensing. A number of
factors such as the target gas, operating temperature, crystallinity, stoichiometry and
presence of oxygen vacancies on the film surface influenced the sensing performance
The developed sensors were tested towards various gas concentrations within the
TLV range of H2 (600-10,000 ppm), ethanol (12-185 ppm) and CO (50-1000 ppm) in
the temperature range of 100ºC-300ºC and relative humidity of 0%. The gas sensing
properties, namely, sensitivity, response and recovery times, and baseline resistance
were evaluated. The results were thoroughly discussed by understanding the gas
138
sensing mechanism of the films at various temperatures. The sensing mechanism is
largely dictated by the target gas and the dominant oxygen species at the specific
operating temperature, which can lead to an opposite sensor response, as observed in
the present study. The concluding major findings of this research and potential for
future work are summarised in the following sections.
6.1 Conclusions
The major findings of this research program are summarized below:
Highly amorphous nanostructured WO3, Fe-doped WO3 and Fe-implanted
WO3 thin films (300 nm thick) with a grain size less than 15 nm have been
synthesized using thermal evaporation technique. To the best of author’s
knowledge, this was the first attempt to dope WO3 film with Fe using thermal
evaporation technique.
The as-deposited WO3 films showed a highly unstable response towards H2
and ethanol, owing to their amorphous nature. These films did not show any
response towards CO.
Doping with small amount (0.5 at.%) of iron increased the film resistance and
considerably improved the sensing performance. The film characterization
revealed that Fe was incorporated as a substitutional impurity in the WO3
matrix, rather than as a catalyst on the film surface.
Upon implantation with 5.5 at.% of Fe, the film became highly amorphous,
however, no additional compounds were revealed from characterization,
indicating that implantation did not induce any chemical changes in the film,
however, the morphology and grain structure were highly distorted.
139
Annealing at 300ºC for 2 hours in air showed an onset of the crystalline
properties of pure and Fe-doped WO3 films and induced sub-stoichiometry in
these films. These films showed a response towards H2, ethanol and CO,
however, the response was characterized by noise and a drifting baseline.
Annealing at 400ºC for 2 hours significantly improved the crystalline
properties and altered the stoichiometry in the WO3 and Fe-doped WO3 films,
which increased the number of oxygen vacancies in the films. An increase in
number of oxygen vacancies is considered to be highly beneficial for gas
sensing.
The nanostructured WO3 film annealed at 400ºC showed maximum response
to H2 and ethanol at an optimum operating temperature of 150ºC and no
response to CO.
The Fe-doped WO3 film annealed at 400ºC showed maximum response to
H2 at an optimum operating temperature of 200ºC. A response to both ethanol
and CO was observed at an optimum operating temperature of 150ºC.
The Fe-implanted WO3 film annealed at 400ºC showed maximum response to
H2 at 100ºC, but the sensitivity is very low compared to other films and
sensor was characterized by a drifting baseline. This film did not show any
response to ethanol and CO in the temperature range 100ºC-300ºC.
Upon comparison the sensing performance of all the films towards various
gases, firstly, it is observed that the author has been able to achieve a lower
operating temperature of 150ºC towards various gases by depositing
nanostructured thin films and doping of the films with Fe and subsequent heat
treatment. Secondly, doping the WO3 film with Fe has demonstrated a
140
response towards CO, which otherwise is non-sensitive to CO as per
available literature reports and author’s knowledge.
6.2 Recommendations for Future Work
This thesis has presented advances in the field of nanostructured WO3 thin film
based gas sensors. Throughout the course of this research, several areas of interest,
which have tremendous research potential, have been identified. In this section, some
proposals will be made for possible future development of the current research.
These proposals for future work are listed below:
In the present study, the films were annealed at 300ºC and 400ºC for 2 hours
in air. There was no increase in grain size after annealing the film at 400ºC,
however, the films were completely crystalline. This project can be extended
to investigate the effect of annealing temperature between 300-600ºC on
change in grain size and gas sensing behaviour. This will allow to achieve an
optimum annealing temperature of the WO3 based films for improved gas
sensing properties.
The Fe-doped WO3 thin film gas sensor has demonstrated to be a potential
candidate for low temperature CO gas sensing. However, in the present study,
only a fixed Fe concentration (0.5 at.%) of co-evaporated tungsten oxide
films have been investigated. The study can be extended to vary the iron
concentrations (0.5 at.% - 10 at.%) and systematically investigate the
optimum effect of Fe doping on the sensing performance and operating
temperature of the tungsten oxide thin films towards CO.
141
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