Air Pollutant Gases And Gas Sensors

Introduction

A sensor is a device that converts physical or chemical stimuli to electric signals. Similarly, a gas sensor is defined as a device that detects the presence or measures the concentration of a particular atom, molecule or ion in an atmosphere. Gas sensors is a type of chemical sensors have found extensive applications in process control industries and environmental monitoring. Growing industrialization and increasing pollutants from vehicular exhaust has resulted into increased air pollution. Some of the air pollutant gases released from vehicles and their effects are mentioned below:

Carbon monoxide (CO) is produced when carbon-based fuel is burnt incompletely and is poisonous at higher concentrations.

Carbon dioxide (CO2) is produced when burning carbon containing fuel and contributes to global warming.

Hydrocarbons (HC), also referred to as Volatile Organic Compounds (VOC), come from unburnt fuel and contribute to ozone and photochemical smog. VOC can contribute to respiratory illness.

Oxides of nitrogen (NOx) are produced in the combustion chamber of engines at high temperatures. NOx reacts with hydrocarbons in sunlight and produce ozone and photochemical smog. NOx can contribute to respiratory illnesses and acid rain.

Particulate matters (PM) are small particles consisting mainly of unburnt carbon, but they also contain heavy metals and toxic substances. PM contributes to respiratory illness and more serious cardiopulmonary diseases.

Depending on the technology applied in the development of gas sensor, it is classified into three major groups: (i) Optic (ii) Spectroscopic and (iii) Solid sensors.

Optical sensors measure absorption spectra after the target gas has been stimulated by light. To analyse the absorbed spectra, complex measurement system with a monochromatic excitation source and an optical sensor is required and also employs expensive analytical techniques (such as infrared spectroscopy, ultraviolet fluorescence, chromatography, etc.).

In spectroscopic systems direct analysis of the molecular mass or vibrational spectrum of the target gas is made. The composition of the different gases is measured and employs Gas chromatography and mass spectrometry. Spectroscopic and optic systems are too expensive for domestic use and sometimes difficult to implement in reduced spaces such as car engines. However a solid state sensor has advantages due to their fast sensing response, simple implementation and low prices. They are portable, low-power consuming, inexpensive because of the considerable production of semiconductor materials.

Semiconductor oxide gas sensor

The solid state gas sensors are widely used in several domestic, commercial, and industrial gas sensing systems during past few decades. Three different types of solid state gas sensors are based on electrochemical behaviour, catalytic combustion, or resistance modulation of semiconductor oxide. In the solid state gas sensing methods, the semiconductor oxide gas sensor devices are commonly used due to their several advantages such as low cost, small size, measurement simplicity, durability, ease of fabrication, low detection limits (< ppm levels), long-life and resistant to poisoning. Semiconducting metal oxide sensors are chemiresistive gas sensors which react with target gas and undergo reduction and oxidation process. This process causes exchange of electrons from sensor with target gas at certain characteristic rate, thereby sensor’s resistance changes. Several materials are synthesized and developed to enhance the sensing characteristics of the sensor. Semiconductor oxides based gas sensor can be designed with variations in the composition of the semiconductor oxide materials and film deposition methods. These deposition methods include either physical vapour deposition (PVD) or chemical vapour deposition (CVD) techniques.

Sensing mechanism of semiconductor oxide Gas Sensors

The sensing mechanism is due to the features of adsorption ability, electro-physical and chemical properties, catalytic activity, thermodynamic stability and adsorption/desorption properties of the surface. The gas sensing of sensors involve two major functions such as receptor and transducer functions. The receptor and transducer function describes electronic change of the oxide surface and electrical resistance change of the sensor respectively, when target gas is passed. The optimization involved in the mentioned features of gas sensor has resulted in major advances in sensor technology where material’s responses are improved.

Key characteristics to gas sensor performance

In chemiresistive sensor, change (increase or decrease) in resistance depends on the nature of sensor material (n-type or p-type) and the gas (reducing or oxidizing) when exposed to the molecules of analysing gas. A typical response curve with variation of resistance of sensor with time on exposure and withdrawal of analysing gas is shown in Figure 1.3. The response curve of a sensor is characterised by following five parameters:

• Sensitivity
• Selectivity
• Stability
• Response time
• Recovery time.

The parameters are discussed in detail:

1. Sensitivity: It is defined as the resistance of the sensor in air to the resistance of the sensor upon exposure to the target gas.
2. Selectivity: It is defined as the ratio of the sensitivity of one analyte gas relative to another analyte gas under same conditions.
3. Stability: It is a characteristic that specifies repeatability of device measurements after a long use.
4. Response Time: It is defined as the time interval over which resistance attains a fixed percentage (usually 90%) of final value when the sensor is exposed to full scale concentration of the gas.
5. Recovery Time: It is defined as time interval over which the sensor resistance reduces to10% of the saturation value when the sensor is exposed to full scale concentration of the gas and then placed in clean air. A good sensor should have a small recovery time so that sensor can be used again. There are several factors leading to gas sensor’s instability, such as design errors, structural changes, phase shifts, poisoning during chemical reactions and influence of the surrounding environment. In order to make the sensor more stable and reduce the response time, the following methods should be considered such as identifying metals with chemical and thermal stability, optimizing elemental composition and grain size of sensing materials and utilizing specific technology during surface pre-treatment of sensors.

Tin dioxide (SnO2)

In this research work, the SnO2 is identified as a suitable metal oxide gas sensor with suitable dopant to increase sensor’s performance for sensing CO2 gas. SnO2 is an n-type semiconductor that has a wide band gap of 3.6 eV at 300K. The working principle of SnO2 as gas sensor comes from changes in the electrical conductivity of SnO2 grains, which results from the reaction between oxygen and gas reduction. SnO2 in thin film is commonly used for gas sensing applications due to advantages such as high sensitivity, low cost, fast response and recovery speed. Various additives are used to improve the material sensitivity and selectivity and decrease the response time and the operating temperature of the sensitive layer. Dopant such as iron, manganese, cobalt, etc., can be added with SnO2 to improve sensor response.

Reasons for detecting CO2 gas

In this research work, using thin film SnO2, CO2 gas which is one of the air pollutant is sensed.CO2 is safe for human’s up to 5000 ppm and is dangerous when it reaches concentrations of 40,000 ppm. A short term exposure of 30,000 ppm is bearable. The level of carbon dioxide in the atmosphere is changing every year. The present level of carbon dioxide in the atmosphere is over 400 ppm. Hence there is a need for developing sensor to sense carbon dioxide in the Earth’s atmosphere as a means of prevention.

Literature review

Over the last decades, there has been a significant work on the synthesis of SnO2 thin film using various deposition techniques, characterization, fabrication for gas sensing applications. Keeping this in view, the studies carried out by various researchers related to the synthesis of SnO2 nano particles, characterization, fabrication, gas sensing performances of sensors are reviewed and presented in this section.

Korotecenkov et al. (2009) reported SnO2 films of differing thickness prepared using spray pyrolysis technology. The response time is shorter for the reduced thickness of the film. For sensor design, if rate of sensor response is not a critical parameter, use of thicker films are preferred due to which provides better sensitivity at lower temperature. The influence of thickness on the parameters of film morphology, sensor response, rate of response and working temperature are analysed.

Rezvani et al. (2010) reported Cu-doped SnO2 thin films with various concentrations deposited on glass substrate by spray pyrolysis method as O2-gas sensors. The films were prepared using SnCl4, 5H2O and CuCl2, 2H2O. The SEM and XRD results confirm nano-structure of films in thin film sensor. Measurement of the electrical resistivity of films shows that with increasing of Cu doping in films upto 12.5%, the electrical resistivity increase sharply. By Cu-doping in SnO2 films, O2-sensing properties of sensor is enhanced and the best sensing of O2-gas is obtained in 10% cu-doping and substrate temperature=470ᵒC.

Quang Trung Khuc et al. (2010) synthesized SnO2 nanorods by hydrothermal method using tin chloride, liquid ammonia, sodium hydroxide and cetyltrimethyl ammonium bromide. SnO2 nano rods with diameters of 100-300 nm and lengths of several micro meters have been synthesized. The effect of hydrothermal temperature on the morphology and diameters of products were investigated. In particular, when the hydrothermal temperature was increased to 230◦C, the bended foils and plates of SnO2 were obtained. The structural properties and surface morphologies of SnO2 is characterized by XRD and SEM. XRD patterns reveals that obtained product exhibit tetragonal rutile structure of SnO2.

Vijayalakshmi et al. (2012) reported the comparison between TiO2 nano particles prepared via two different routes: via sol gel route and by hydrothermal method. It is found that preparing under the same ambient conditions viz temperature pressure etc. and keeping all the parameters same viz precursors, mole ratio, solvent etc., the nano particles prepared via sol-gel route are found to be crystalline and having higher crystallite size as compare to the one prepared by hydrothermal method. The crystallinity and the crystallite size are examined by XRD and TEM.

Faith C Bancolo et al. (2012) reported SnO2 nanomaterial fabricated using Horizontal Vapour Phase Growth technique (HVPG) for sensing CO2 gas. It is noticed that sensor exhibited greatest response at the least dwell times which was grown at 4 hours. The best result sensitivity responses has the average valu of S=1.142. The surface topography, morphology and elemental composition of the nanomaterials are investigated using SEM and EDX.

Manal Madhat Abdullah et al. (2012) presented the synthesis of Tin oxide thin film using spray pyrolysis method from SnCl2.2H2O isopropyl mixing with water solution on the glass substrate for sensing butane gas. The maximum sensitivity was obtained at an operating temperature of 470◦ C for the exposure of 5% of butane gas. The films are characterized by XRD. The crystallite size was evaluated to be 4.413 nm by using Scherrer’s equation. The surface morphology is studied using AFM.

Kavitha et al. (2013) studied the growth, physical and chemical characterization of TiO2 nanostructures prepared by sol-gel and hydro thermal method. The structural, morphological and photocatalytic activity of the prepared nanostructures are analysed. The prepared samples are calcined at different temperature and analysed.

Tetsuya Lida et al. (2013) reported SnO2 based films with different particle and pore sizes synthesized by hydrothermal method. A seed mediated growth approach using 4 nm SnO2 nano particles ranging from 9.5 to 17 nm in average diameter under hydrothermal conditions at 250◦C are developed. The gas sensor response is greatly influenced by these factors. The sensor responses are analysed for H2, CO and H2S gases.

Sharmila Devi et al. (2014) synthesized the nano sized titanium dioxide (TiO2) powder via sol-gel method using titanium tetraisopropoxide (TTIP) as the precursor. The phase transformation is investigated by an X-ray diffractometer (XRD). The anatase structure of titanium dioxide is obtained after calcination. The microstructure is characterized by a Scanning Electron Microscope (SEM).Ramesh et al (2014) synthesized nanocrystalline as-prepared SnO2 thin film by sol-gel dip coating technique for sensing H2 gas with precursors as tin chloride dehydrate, ethanol and glycerin. The nanocrystalline SnO2 thin films gives maximum gas response (S=360) at 75 ◦C. The crystal structure, surface morphology and microstructure property was characterized by XRD, FE-SEM and TEM. The structural and microstructural properties confirm that as-prepared tin oxide thin films are polycrystalline and nanocrystalline in nature. The nanocrystalline thin film exhibits rapid response recovery time.

Zheng-Dong Lin et al (2015) developed flexible CNT-based gas sensor on a low cost polyimide substrate that respond immediately to CO2 at room temperature. Deposition is done by sputtering technique. CNT grown on SiO2 substrate is transferred on to the acrylic adhesive coated flexible substrate. The surface morphology of the obtained sample is characterized by FESEM and HRTEM. The sensitivity, speed and stability of the response of the flexible gas sensor are analysed. The sensor exhibited a high sensitivity of 2.23% at room temperature when the concentration of CO2 gas was 800 ppm.

Brian Yuliarto et al. (2015) studied the nanostructure SnO2 as a pollutant gas sensor. For most SnO2 gases, the working temperatures are above 250ᵒC. However addition of other substances like metal oxide to SnO2 could lower its working temperature. The development of fabrication of SnO2 nanostructures synthesis and functionalization of SnO2 are explained in detail. Zaki et al. (2016) reported fabrication and characterization of metal oxides (TiO2, SnO2, ZnO) based sensor for detection of formaldehyde gas. These metal oxides are deposited on top of aluminium/ glass substrate using sol-gel technique. From the obtained result, SnO2 is found to be having excellent capability due to good uniformity and high surface to volume ratio compared to TiO2 and ZnO. The surface morphology is characterized by using AFM.

Korotcenkov et al. (2016) reported SnO2 and In2O3 thin films as materials for the design of solid-state conductometric ozone sensors. The analysis of the fundamentals of SnO2 and In2O3 based conductometric ozone sensor operation are explained in detail. The main focus is on the description of mechanisms of ozone interaction with metal oxides (SnO2 and In2O3), the influence of air humidity on sensor response, processes that control the kinetics of sensor response to ozone.

Yadav et al. (2017) reported CO2 sensing properties of La2O3 thin film synthesized by spray pyrolysis deposition method. The structural, morphological and optical properties are studied. La2O3 thin film shows hexagonal, crystalline nature with porous morphology. The porous surface morphology is analysed and found to be sensitive for CO2 by using SEM. The maximum response of 15% at 623 K is observed on exposure to 400 ppm of CO2 gas.

Christoph Willa et al. (2017) developed P/Al2O3 composite material based light and flexible substrate. Synthesis of composite material is done by E-beam evaporation and lift-off technique. The strong humidity dependence of impedance indicates that variation in proton conduction is contributing to the sensitivity. The parameters such as relative humidity and responses are studied. Karthik et al. (2017) reported CO2 sensing properties of tin oxide (SnO2) and zinc oxide (ZnO) thin films deposited onto the macro porous silicon (PS) substrates. PS was synthesized by electrochemical etching of p-type monocrystalline silicon along with the metal oxide (MO) thin film deposition using precipitation method. X-ray diffraction (XRD) confirms the presence of tetragonal and wurtzite phases of SnO2 and ZnO respectively. Scanning electron microscopy (SEM) reveals the formation of substrate porosity dependent porous metal oxide nanostructures. CO2 sensing properties of SnO2/PS and ZnO/PS were studied as a function of deposition time, gas concentration and operation temperature.