ABSTRACT
A gas sensor is a selective device used in monitoring the presence or concentration level of a particular gas in the ambient atmosphere. Gas sensors operate on the principle which is anchored on any of the following three classifications, that is, spectroscopic, optical, and solid- state gas sensing methods. In spectroscopic techniques, the gas sensor is based on basic gas properties such as molecular mass or vibration spectrum, while for optical gas sensors; measurements of the absorption spectra are involved. Solid-state gas sensors apply the fact that there is a change in the electrical properties of a sensing material whenever there is exposure to gas.
Data collected on hydrogen sensors indicate that all the sensors have a low response time and are less sensitive to respond to even very low leakages of hydrogen. This work was prompted by the fact that there is continued research and study of new gas sensing materials, and therefore a likelihood in improvement in terms of response to the gas sensing properties as well as widen the choice and variety of hydrogen gas sensors fabricated using different types of materials.
Thin films of Nb:TiO2 for gas sensing applications have been deposited using radio frequency (RF) magnetron sputtering. The samples were deposited at different partial pressures and sputtering power. The objectives were to analyse the optical, electrical and gas sensing properties of the thin films.
The general results on optical and electrical properties of pure TiO2 and doped 2%wt Nb: TiO2 and 4%wt Nb: TiO2 have shown the different amount of thin-film transmittance depending on deposition conditions. The increase in partial pressure has been observed to cause a decrease in transmittance in doped TiO2, which has been attributed to competition for oxygen molecules between TiO2 and NbO phase. The deposition power has also been observed to give similar results in terms of transmittance, this is because at lower power a thinner film forms while at a higher deposition power a thicker film is formed thus resulting in to decrease in transmittance. The amount of doping influences the number of free electrons and thus influencing the optical and electrical properties of thin films. The band gaps for the three types of thin films were observed to vary depending on the deposition conditions. The drop in bandgap after the post- deposition annealing was observed and is fact attributed to improved crystallinity due to an increase in electrically activated charge carriers. Finally, the sensing capability of the thin film device has been observed to improve with annealing, a factor that has been attributed to the crystallinity and charge carriers
TABLE OF CONTENTS
DECLARATION i
ABSTRACT ii
TABLE OF CONTENTS v
LIST OF TABLES viii
LIST OF FIGURES ix
LIST ABBREVIATIONS AND SYMBOLS x
FORMULAE OF CHEMICAL COMPOUNDS xi
CHAPTER ONE: INTRODUCTION
1.1 Background of study 1
1.2 Statement of the problem. 4
1.3 Objectives 4
1.3.1 Goal 4
1.3.2 Specific objectives 5
1.4. The significance of the study 5
1.5 Justification 5
CHAPTER TWO: LITERATURE REVIEW
2.1. Overview of hydrogen gas sensors. 7
2.2 Types of gas sensors 8
a) Catalytic bead sensors 8
b) Electrochemical sensors 8
c) The resistive palladium alloy sensor 8
d) Metal oxide semiconductor (SMO) gas sensors 9
2.3 The properties of TiO2 semiconductor 15
2.4 Phases of titanium dioxide 17
2.5 Effects of doping of titanium dioxide 17
2.6 Thin films 20
2.7 Thin film deposition techniques 21
(a) The chemical deposition method 21
(b) Chemical vapour deposition (CVD) 21
(c) Plasma enhanced chemical vapour deposition (PEVCD) 21
(d) The atomic layer deposition (ALD) method 22
(e) The sol- gel method 22
(f) Physical vapour deposition 22
(g) Molecular beam epitaxy (MBE) 22
(h) The pulsed laser deposition ( PLD) 22
(i) Thermal evaporation method 23
(j) Sputtering method 23
CHAPTER THREE: THEORY
3.0 Theory for thin film characterization and analysis 24
3.1 Optical properties 24
3.1.1 Optical reflectance 24
3.1.2 Optical transmittance 24
3.1.3 Tauc Optical band gap 25
3.1.4 Extinction coefficient, refractive index and absorption coefficient. 25
3.2 Electrical properties 26
3.2.2 The hot -probe technique 26
3.2.3 The Hall Effect measurement 26
3.2.4 The 4- point probe technique 26
CHAPTER FOUR: METHODOLOGY
4.1. Sample preparation 29
4.1.1. Substrate preparation 29
4.1.2. Varied Parameters 29
4.2. Thin film deposition 29
4.3 Properties of the thin films 30
4.3.1 Optical measurements of the thin film 30
4.3.1 Electrical properties measurements of the thin film 30
4.4 Gas sensing properties measurement 31
CHAPTER FIVE: RESULTS AND DISCUSSIONS
5.1 Introduction 33
5.2 Optical properties of Nb doped TiO2 thin films 33
5.2.1 Influence of variations in deposition pressure on transmittance 33
5.2.2. Influence of variations in deposition power in transmittance 35
5.2.3. Influence of niobium doping on transmittance 38
5.2.4. Influence of annealing on optical transmittance 38
5.3. Optical band gap of the thin films 41
5.3.1 Optical band gap for as-deposited thin films 41
5.3.2. Optical band gaps for the post annealed (450oc) for the thin films 42
5.4. Electrical properties of pure and doped TiO2 thin films 43
5.4.1. Electrical properties of the thin films as-deposited 43
5.4.2. Electrical properties of pure and TiO2 thin films as annealed 44
5.4.3. Comparison of electrical resistivity of the films as deposited and as annealed 44
5.5 Gas sensing properties 45
5.5.1 Influence of Deposition pressure on the gas sensing properties 45
5.5.2 Influence of Deposition power on the gas sensing properties 47
5.5.3 Influence of Niobium doping on the gas sensing properties of TiO2 thin films 49
5.5.4 Influence of annealing temperatures on the gas sensing properties 50
CHAPTER 6: CONCLUSION AND RECOMMENDATIONS
6.1 Conclusion 52
6.2 Recommendations 53
REFERENCES 54
LIST OF TABLES
Table 4.1: Sputter deposition parameters 29
Table 5.1: Comparison of band gaps as annealed and as-deposited 42
Table 5.2: Summary of the electrical properties as- deposited 43
Table 5.4: Summary of electrical properties for samples annealed at 450 0C 44
Table 5.5: The summary of the sheet resistance for the as-deposited and as annealed 44
LIST OF FIGURES
Figure 2.1: Acoustic gas sensor schematic 10
Figure 2.2: Schematic diagram of a Schottky diode sensor 11
Figure 2.3: A schematic of MOS capacitor sensor 12
Figure 2.4: schematic of a MOSFET sensor 13
Figure 3.1: Schematic of 4- point probe configuration 27
Figure 4.1: The lab assembled gas sensing unit 31
Figure 5.1: Variations in the transmittance spectra for the TiO2 thin films at different deposition pressure 34
Figure 5.2: The transmittance spectra for the thin films deposited at various RF power. 37
Figure 5.3: The transmittance spectra for pure and Nb-doped TiO2 thin films deposited. 38
Figure 5.4: The transmittance spectra for the thin films as-deposited and annealed 40
Figure 5.5: The Band gap curves for as-deposited thin films as deposited 41
Figure 5.6: Band gap curves for the post annealed films 42
LIST ABBREVIATIONS AND SYMBOLS
Α Absorption coefficient
Cr Chromium
Eg Optical band gap
eV Electron volts
HCl Hydrochloric acid
Nb Niobium
NIR Near-infrared
O3 Ozone
Pb Lead
PID Proportional Integral Derivative
PKT Packets
Ppm Parts per million
Pt Platinum
RF Radio Frequency
RRAM Resistive Random Access Memory
TCO Transparent Conducting Oxide
UV Ultraviolet
VIS Visible
FORMULAE OF CHEMICAL COMPOUNDS
CH3OH Methanol
CO2 Carbon dioxide
Fe2O3 Iron (iii) oxide
H2O Water
O2 Oxygen
NO Nitrogen monoxide
SnO2 Tin (IV) oxide
SrTiO3 Strontium titanate
V2O3 Vanadium trioxide
WO3 Tungsten trioxide
ZnO Zinc oxide
CHAPTER ONE
INTRODUCTION
1.1 Background of the study
The devices used in the detections of the existence of gas in domestic, laboratories, and industrial environment, for the simple purpose of monitoring toxic and combustible gases as a preventive measure, are called gas sensors (Wei- Cheng et al., 2013). These gadgets have been given a great attention in research activities in the last few years for diverse usage in various areas, such as alcohol level breath-tests/ breathalyser (Yang et al., 2009; Bihar et al., 2016; Barnett et al., 2017), environmental monitoring (Rossi and Brunelli, 2012; Novikov et al., 2016; Yang and Deng, 2019), indoor/ outdoor air quality (Prajapati et al., 2017; Arroyo et al., 2020), workplace health and safety (Kanaparthi and Singh, 2020; Thomas et al., 2018), and homeland security (Rout and Roy, 2016; Sathish et al., 2017). The gas sensors form essential components in contemporary electronic systems as a crossing point with the environment for monitoring gas molecules, identification, and bringing together. These days, data from environmental detection devices is vital in many scientific disciplines, and technology particularly for safety reasons, such as those deployed in environmental monitoring, industrial process monitoring, and the automobile industry (Claudio et al., 2012; Ghosh et al., 2019; Nazemi et al., 2019; Poloju et al., 2018).
The research work being reported in this thesis was devoted to hydrogen gas detection being among the many gases that are of interest in research in the recent years (Zhu and Zeng, 2017; Poloju et al., 2018). Hydrogen is highly inflammable in the air at a wide range (4-75%) by volume, but if properly packaged, it can act as a very important alternative source of clean energy (Chomkitich et al., 2012; Hames et al., 2018). The global concern on climate change due to environmental pollution by fossil fuels considers hydrogen gas energy source as one of the best alternative clean energy for the novel transportation scenario and hydrogen derived power- sources based on fuel cells. As a fact, hydrogen is an explosive gas that can explode in ambient air at a concentration as low as 4% vol. (Arndt and Simon, 2001). In recent years, Hydrogen has steadily started to emerge as a possible alternative fuel source to supplement mineral fossil fuels. It is important to note at this juncture, that hydrogen-based fuel cells, founded on various technologies, is the technique by which energy is obtained from the reaction of hydrogen and oxygen gases within the fuel cell with by product being water as exhaust material which is a non-pollutant (Arndt and Simon, 2001).
The consumer industry for hydrogen fuel cells is based on three large market segments namely: the portable fuel cells (Lalchand et al., 2014), the stationary or residential fuel cells (Ghenai et al., 2020), and the automotive fuel cells (Jacobson et al., 2005; Wiebe, et al., 2020). Two types of sensors are required for these types of fuel cells depending on purpose of usage; first to monitor the quality of the hydrogen feed gas, and then secondly and more important sensor systems for leak detection. Hydrogen fuel cells are an emerging technology and consequently the codes and standards for the fuel cells and the sensors supporting the fuel cells are still in their nascent stages. Sensor standards and specifications for these applications are still being written, with no specific standard yet passed for use. Hydrogen gas is odourless, colourless, and tasteless, it cannot be distinguished by t he human sense of smell (Xu et al., 2020). The hydrogen gas has a low ignition temperature as well as a wide flammable range making it easily inflammable and explosive. It is in this regard that, rapid and accurate detection system is necessary during the manufacturing, stowage, and use of hydrogen (Lalchand et al., 2014). When it is compared with domestic natural gas, sulphur-containing mercaptan gas are introduced to alert the consumer of possible gas leakage (Sun et al., 2020), but this approach is not advisable in case of hydrogen, given that such add-ons can poison the fuel cell catalyst (platinum) and hydrogen sensors are needed to monitor the environment around the fuel cell for hydrogen leaks. The human nose is commonly used for natural gas detection, as it is a remarkably reliable, superb low-level detection sensor. Hydrogen leak-detection sensors differ in that these sensors must detect over the general level of ambient hydrogen levels available in the detection environment. For example, automobile lead-acid batteries evolve hydrogen routinely and for a garage-based sensor, a sensor must be able to discriminate from certain level in parts per million in ambient sources of hydrogen and those which will be generated by a hydrogen leak. Once leakage has taken place, suppression of hydrogen gas is difficult since it diffuses readily through most materials due to its intrinsic property of being the lightest gas. As a result, there will always be hydrogen gas presence in the ambient environment, if hydrogen is present in a container or if the hydrogen has evolved (Oleg et al., 2008).
In order to fabricate a commercially viable hydrogen gas leak detector the following factors need to be put into consideration: redundant arrays of multiplexed sensors, selectivity to avoid false alarms; autonomous integrated fuel cell shutoff/venting measures; dependable sensing, calibration standards, and self-testing. It is a standard practise to have in place two type of sensors for leak detection, which serve two purposes: one to detect the presence of the gas and a second for alarm sensor which should be triggered when the 50% of the Lower Explosive Limit (LEL) of 4 % hydrogen in air is reached. In pour modern day society, hydrogen gas consumption is becoming common place and it is critical to have leak detection system in place. Places where large quantities of such gas are found maybe in suburban centres, fuelling stations, welding garages, and automotive repair shops (Adamyan et al., 2008).
Three-dimensional detection of hydrogen gas leak requires a detector element on a probe analogous to comparable leak-detection methods. The sensor element itself must be a short response time to avert operator fatigue, be able to detect down to 1000 ppm range, and larger upper detection limit as a necessity, depending on the usage. In addition, there is a necessity to have a leakage detector that is discriminatory and ambient autonomous (Seham et al., 2019), for example, a leakage sensor enclosed argon that can function accurately, this may be essential technique to prevent ignition of a compressed gas leak.
In this work, Niobium (Nb) doped Titanium dioxide (TiO2) thin films were used for fabricating hydrogen gas sensor. In the last few years, a number of materials have been used to fabricate solid-state gas sensors, among these materials, semiconductor metal oxides have dominated the materials used for the fabrication of the gas sensors (Dey, 2018). The semiconductor metal oxide gas sensors may also be known as semiconductor gas sensors. Some of the materials previously studied to fabricate the semiconductor gas sensors include SnO2 (Oyabo, 1982; Wen et.al., 2009), ZnO (Boccuzzi et al, 1992), WO3 (Lin et al, 1994), SrTiO3 (Hu et al., 2004), V2O5 (Schillini et al., 1994), Fe2O3 and TiO2 (Schilliniet al, 1994; Dey, 2018). For these semiconductor gas sensors, the detection indicator is founded on the alteration of the material’s resistivity after gas contact. The sensitivities of the sensor devices are usually high at temperatures ranging between 200oC to 800oC as this temperature range is in the same spectrum to the optimum conductivity of semiconductors.
The material proposed in this research work was selected because prior researchers had shown that Nb doping in TiO2 modifies the microstructure of TiO2 by influencing the grain size growth and therefore resulting in a modification in the conductivity of the resultant material (Sukon et al., 2011). In the prior literature work, a number of techniques have been described for the deposition of TiO2 thin films, these includes and not limited to sputtering (Alexandrov, et al, 1996), chemical vapour deposition (Rausch and Burte, 1993; Zhangand Griffin, 1995), screen printing (Bach et al., 1998; Marcos, et al, 2008), evaporation (Grahn et al,1998; Rao and Mohan, 1990), sol-gel method (Kim et al, 2006), tape-casting (Chao and Dogan, 2011), laser chemical vapour deposition (LCVD) (Gao et al, 2012; Guo et al., 2013), and spray pyrolysis (Oja et al., 2006).
In this research, Nb doped TiO2 thin films were prepared by vacuum radio frequency (RF) magnetron sputtering deposition method. The deposition method chosen has the following advantages over other coating techniques; low substrate temperatures are possible during deposition, good substrates adhesion of the thin film. Other factors are formation of a uniform thin film with homogeneous thickness and the ability of formation of compact thin film, as well as ease of co-deposition of different materials to form alloys, and compounds materials with possibility of varying the vapour pressures (Angelats-Silva, 2006).
1.2 Statement of the problem.
Fossil fuel currently is the main source of energy worldwide despite the environmental consequences associated with them. The scientist has estimated that one time in the future they will get depleted and therefore it is important to search for an alternative source of energy during this grace period while they are available and the climatic conditions are not yet severe. Several alternative sources of energy are being mulled as possible candidates that can occupy the gap which will be left by the fossil fuel once they get depleted. Hydrogen fuel cells are a possibility and seem to be having an emerging market niche, and therefore standards and codes for these cells are still being developed, similarly, sensor specifications are still being researched. The hydrogen sensors are already in use in the market, but optimisation of a hydrogen sensor or sensor suitable to respond quickly with short reaction time to sensing requirements for fuel cells is still ongoing due to the numerous challenges being faced. Data collected on hydrogen sensors indicate that all the sensors have a low response time (8-30 seconds), which is not the expected duty cycle needed for most applications. Also noted is the lack of a more sensitive gas sensor that can respond to even very low leakage of hydrogen. It is also a fact that the maturity and specialisation of these sensors will also necessitate considerable yet realistic cost reductions, as well as a better description from the emerging fuel cell market for the probable duty cycle loads, hydrogen fuel stream pressures, flow rates, and configuration.
1.3 Objectives
1.3.1 Goal
The main objective of the work was to study the properties of Niobium (Nb) doped Titanium dioxide (TiO2) thin films deposited by RF sputtering technique to be used as hydrogen gas sensor.
1.3.2 Specific objectives
1. To analyse the optical properties of niobium (Nb) doped Titanium dioxide (TiO2) thin films deposited at different partial pressure and sputtering power.
2. To investigate the electrical properties of Nb doped TiO2 thin films deposited at different partial pressure and sputtering power.
3. To determine the gas sensing properties of Nb doped TiO2 thin films deposited at different partial pressure and sputtering power.
1.4. The significance of the study.
It obvious from data collected on hydrogen sensors indicate that all the sensors have a low response time (8-30 seconds), which is not the expected duty cycle for most application. Also noted is the lack of a more sensitive gas sensor that can respond to even very low leakage of hydrogen. It is also a fact that the as this model of the sensor develops and gets highly optimised, it will be necessary to achieve some considerable and realistic cost reduction, as well as a better description from the emerging fuel cell market for the probable duty cycle loads, flow rates, hydrogen fuel stream pressures, and composition.
The continued research and study of new gas detection materials, as greater milestones are achieved there is a likelihood of improvement in terms of response to the gas sensing properties as well as widen the choice and variety of hydrogen gas sensors fabricated using a different type of materials.
The significance of materials design in innovating gas detection device is demonstrated by taking a semiconductor gas sensor as an analogy. By some innovations, the existing devices can be made more intelligent and more quantitative, which is an essential step for the further advancement of gas sensor technology. Therefore, new gas sensors for future markets can arise from the moulding of the existing gas sensor performances in realistic operational conditions utilizing a combination of spectroscopic and phenomenological techniques. Intrinsic properties of selectivity, sensitivity, and stability, which are limiting factors to the wide application of the current gas sensors still need to be addressed further.
1.5 Justification
The most important characteristic of TiO2 in this research work is that being a semiconducting oxide, it has the ability upon gas adsorption to change its electrical conductivity, and this change is utilised as a signature of detection when fabricating a gas sensor. The Titanium dioxide thin films have distinct characteristics at two different temperatures: At elevated temperature, Titanium dioxide thin films can be used as a thermodynamically controlled bulk defects sensor to determine gas over a large range of partial pressures, while secondly, at low temperature, as in our case, the addition of dopants like Nb and Pt. leads to high gas sensitivity and then tested widely for the sensitivity of a variety of gas.
Doping TiO2 with Niobium results in modification of the pristine material and leads to a new structurally different material with different characteristics from the pure TiO2 material. The same way, the levels of doping vary the characteristics of the material. Different deposition condition results in a material with different characteristics to be investigated. Since TiO2 sensor materials are already in use and it is a fact that sensitivity among other qualities of sensors remains a challenge in the market, this research will dwell on studying doped materials at different percentage, and changing the deposition conditions of sensor material to come up with a more sensitive sensor.
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