Gas Sensor and Sensitivity

Abstract

Gas sensors help to detect toxic and flammable gases in the atmosphere, and the use of these devices can reduce or prevent severe consequences for people and the environment. Metal oxides are one of the best materials used in the preparation of gas sensors, and they have proven in general that they have resistance to high temperatures Also, they are characterized by optical transparency at visible wavelengths, and they have a wide band gap. Whereas the interactive properties of metal oxides are the applications key chemical sensor. One of the characteristics of sensitivity is sensitivity, which is defined as the rate of change in the resistance of the thin film in the presence and absence of gas. Sensitivity is affected by several factors, including the relative humidity, the temperature of the sensors, the response time of the sensitivity, the time of exposure of the membranes to the gas, the background of the gas composition, and the thickness of the film.The chapter includes an explanation of the sensor parts and measurement sensitivity.

1. Introduction

According to the scientific and technological developments, researchers has witnessed big dealing in the study of the applications of sensor including gas sensors and for its fundamental applications in the various fields of life like industrial industries, power generation, food, and beverages, medical, and therapeutic as well as agricultural industries. As a result of current industries possesses that increasingly involve the use and manufacture of highly hazardous materials especially toxic gases and flammable create a potential hazard to industrial facilities and their employees even the people who live near them. Moreover, all events that take place in the world represent as reminder for this problem [1].

2. Classification of gas-sensitive materials

According to the base of gas sensing, gas-sensitive materials are divided into two kinds based on electrochemical components and other principles, as shown in Figure 1 [2].

Figure 1.

Classification of gas-sensitive materials [2].

There are three materials utilized as sensing components: metal oxide semiconductor, conductive polymer composites, and carbon nanomaterial [2].

Metal oxides are among the best materials used in the preparation of gas sensors as shown in Figure 2 and it proves generally that it resists high temperature. It is optically transparent at visible wavelengths and has a wide band gap [1, 3]. Both thin film and bulk semiconducting metal oxide materials have been widely used for the detection of a wide range of chemicals such as H₂, CO, NO₂, H₂S, ethanol, acetone, and human breath [4].

Figure 2.

Metal oxides are used in sensors [1].

The reactive properties of metal oxide are the key to chemical sensing applications and when exposed to oxidizing gases including (O2) and gas (NO2) where the common denominator in the reactions of these gases is that they tend to form oxygen ions (O⁻) or (O₂⁻) that electrically active in order for the oxygen ion to be stabilized, it needs to diffuse into the vacant levels formed as a result of crystal defects within the composition of the substance in the following equations [3].

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to {{\mathrm{O}}^{-}}_2$$E1

$${{\mathrm{O}}^{-}}_2+{\mathrm{e}}^{-}\to {\text{2O}}^{-}$$E2

Electrochemical cells contain individual electrodes. They are sites of chemical reactions that involve electron transfer with charged species (ions) that are transferred by electric current as it is showing in Figure 3 where the anode is the electrode where the oxidation reaction takes place (the loss of electrons to the electrical circuit) while the cathode takes the results of the reduction reaction (gaining electrons from electric circuit) where electrical circuits permit the balance of the charge by electron transfer. It also provides the necessary electrical voltage and where the increase and decrease in resistance begin when exposed to one of the gases; therefore, the chemical sensors depend on the change in the resistance. The semiconductors are from the type (n-type) give a change in the resistance from the highest value to the lowest value if there is gas while (p-type) the change in resistance is in opposite way (from the lowest value to the highest value) in addition to the impurities here play a vital role in improving the interactive properties of the sensor modifying the reaction pathway sensitization and selectivity and increasing the detection limits of gas. Adding some kind of impurities is a catalyst and activator for these properties in addition to particle size and surface porosity are all activating factors to improve the sensitivity of gas and it is one of the necessary principles that must be noticed at the beginning of choosing materials for the chemical sensors [4].

Figure 3.

Components of an electrochemical cell [4].

Recent studies showed that it could improve the properties of sensors including selectivity and others by using a mixture of oxides of nanomaterials. The properties of the sensor are influenced greatly by granular size and structure porosity of materials. The particle boundary can be increased by decreasing the particle size and that lead to increase in the sensitivity and reduce the operating temperature which saves energy as well as recent researchers have turned to materials with nano-crystalline structure since it increases the improvement of sensors properties like eclectic and response time and this is achieved by providing a massive increase in the surface area [5]. Figure 4 shows the gas component system.

Figure 4.

Gas sensing system [6].

3. Sensing properties

1. Sensitivity: Sensitivity can be defined as the percentage change in the resistance of the thin film in the existence of gas and lack thereof. Sensitivity is affected by factors including relative humidity, temperature of the sensors, the response time of the sensitivity, the time of exposure of the films to the gas composition, and the thickness of the film are symbolized by the symbol for (S) and can be shown in the following relation [1, 7, 8].

   $$\mathrm{S}=\frac{\left|∆\mathrm{R}\right|}{{\mathrm{R}}_{\mathrm{o}}}\times 100\%=\frac{{\mathrm{R}}_{\mathrm{gas}}-{\mathrm{R}}_{\mathrm{air}}}{{\mathrm{R}}_{\mathrm{air}}}\times 100\%$$E3

   Where:

   $\mathrm{S}$: Sensitivity.

   (${\mathrm{R}}_{\mathrm{gas}})$: Electrical resistance.

   (${\mathrm{R}}_{\mathrm{air}})$: Resistance in dry air.

   (${\mathrm{R}}_{\mathrm{o}})$: Resistance when entering the analytical gas

2. Selectivity: Selectivity can be defined as the ability of the sensor on identifying a specific gas inside a mixture of gases where the metal oxides have more sensitivity to the types of chemical gases in the air; therefore, studies underway to increase the selectivity of oxide semiconductor sensors.

3. Response time and recovery time: It is intended as the time the sensor takes for response in the case of changing the gas from zero to a specific value and reaches a stable value of about 90% from the final value. It is also defined as the time of recovery. It is time for the sensor signal to return to its initial value after removing the gas flow and reaching 10% [9].

4. Stability: Is the ability of the sensor to stabilize for a specific time while maintaining the selectivity, sensitivity, response time, and recovery time [10].

5. Detection limit:It is the minimum concentration of analysis that can be detected by the sensor.

6. Dynamic range:It is the range of concentration of examines between the maximum limit and the detection limit.

7. Resolution:It is the lowest focus that the sensor can detect.

8. Linearity:Is the relative deviation of the experimental graph on the ideal straight line in the ideal parameters.

9. Hysteresis:It is the large difference in the value of the output when the value converges with the decrease and increase in the degree of focus [11].

4. The effective factors on gas sensor properties

The following are the effective factors on gas sensor properties.

1. Porosity: The porous structure increases the reaction area, which increases the speed of response and improves the sensitivity and it helps to activate the molecules of gas on the surface of the semiconductor as the pores in the membranes act as channels to transport gas molecules which causes an increase in the response and retrieval speed [12]. The researcher Nayel et al. studied manufactured from a mixture of silver oxide and indium of the porous silicon slides, the results of the sensitivity tests showed that the sensor gave a high sensitivity at all temperatures to the base, where the sensitivity values ranged from single indium oxide, due to the effect of porous silicon [13].

2. Grain Size: The basic process that is related to the surface is adsorption and one of the effective factors in the adsorption on the sensor’s surface is the grain size for the material and surface area. When grain size decreases, that will increase from the surface area leading to improve gas sensor properties. This is where sensors show high sensitivity towards gases. So if the grain size for the sensor membrane has a nano scale size, as the ratio of the surface to volume increases. Thus most of the studies tend to manufacture sensors of semiconductors membrane (nano). The decrease in grain size leads to increase in grain boundaries to increase the adsorption surface area to increase interaction resulting in increasing response speed and sensitivity value [14]. The effect of particle size on the sensitivity of chemically resistant nano-gas sensors was studied by the researcher Rothschild et al. by means of the effective carrier concentration as a function of the surface-state density of the SnO₂ sensor material, with different particle sizes ranging from 5, 80 nm [15].

3. Dopant and Mixture: It can be defined as adding impurities from a certain material to the pure semiconductors. These impurities act on controlling of optical, electrical, and constructive properties of the semiconductors. When adding impurities from certain materials to semiconductors, it works on the appearance of one type of charge carrier, which is called the majority charge carrier, and the diminishing of the other type, which is called the minority charge carrier. This led to the increase in electricity supply. The increase of impurities ratio works on decrease in grain size and this leads to the increase of surface area and grain boundaries and this helps to increase the interaction whereas gas displaces the oxygen atoms which is related to the grain. This lowers the voltage barrier and thus lowers the resistance and conductivity increases. Since dopant increases the number of one type of carrier so the electrical conductivity depends on the number of atoms of the added material that is, focuses on free electrons generated from it [16]. The researcher Navaneethan M, et al., was able to study the improvement of sensitivity to ammonia gas sensor, as the sensitivity measurements for ammonia gas showed the highest response rate when Ag impregnation of ammonia gas [17].

4. Junction Effect (P-N): When mixing two or more oxides to make a gas sensor, differential-difference will be created between them, due to the different affinity and work function p-n or p–p, or n-n was the type of electronic conductivity for those oxides. Thus, the number of charge carriers increases across the junction which leads to increase or decrease the electrical resistance for the sensor which is due to the contact of oxide p-n [16].

5. Effect of Substrate: The base directly affects the sensitivity properties of the prepared films usually using different types of substrate on which films are deposited such as silicon, quartz, glass, and aluminum. The porous silicon to work on improving sensor properties. Thus, the use of silicon substrate to precipitate films on it gives special different properties to oxides of semi-conductivity materials due to its influences on surface electric charges. Then, its effects on gas interaction with the sensor makes it required in the industry of gas sensors. The silicon etching process is one of the used techniques in the improvement of sensor properties due to the increase of surface area where it determines the regulation of pore size. The glass substrate is known for its optical properties as it has an absorption approach to zero and high transmittance within the visible area so it is used as a substrate to study the optical properties, constructive, and the sensitivity to the prepared film. Also, the proposed silicon acts to improve the sensor [18]. The researcher Rahman et al. studied the sensitivity of the engrafted nano silver oxide prepared by the solution method. The substance was deposited on glass carbon electrodes to give sensitivity with a quick response to methanol in the liquid phase and it gave good and stable allergic results.

6. Active surface area and types of nanostructures: In many gas sensors, the conductivity response is decided by the efficiency of catalytic reactions with detected gas participation, taking place at the surface of the gas-sensing material. As a result, control of the catalytic activity of gas sensor material is one of the most generally used means to improve the performances of gas sensors. But, in practice, the widely used gas-sensing metal oxide materials such as TiO₂, ZnO, Ga₂O3, SnO₂, Cu₂O, and Fe₂O₃ are the least active with the catalytic point of view. The pure SnO₂ thin film without any catalyst shows a very poor sensitivity (∼3) confirming this statement [12]. Tian et al. presented a study of the gas sensing properties of Tio2 nanostructured materials and the effect of different forms of titanium dioxide on their sensitivity to gas. It was obtained that TiO₂ nanostructured materials with different dimensions show unique sensing properties for the detection of gases, in addition to that the ultra-small nanostructured materials with large and specific surface areas achieve rapid response for the detection of various gases [19].

5. Experimental gas sensor

5.1 Masks preparation and electrodes deposition

Preparation of the masks used for depositing the electrodes in the gas sensor, where these masks are made of steel material in the form of a single clip. The space between every two fingers of the clamp is ($400\mathit{\mu m})$ and the width of the fingers of the clamp is ($400\mathit{\mu m})$. Each piece includes five fingers its length is (3.3$\mathit{mm}$) and there is an active distance after the preparation of the masks, it is installed on the front part of the floors on which the material films are deposited to deposit the electrodes on top of it by thermal evaporation under vacuum conditions using a purity aluminum wire (99.9%) where the sedimentation takes place under pressure is shown in Figure 5.

Figure 5.

(a) Aluminum mask planner for sensor gas. (b) the sample after the deposition of aluminum electrodes for gas sensor.

5.2 Gas sensor preparation electrochemistry

The sensor can be prepared through the following steps:

1. Prepare the suitable floors (glass, Silicon)

2. Material deposition substances.

3. Deposition of the appropriate electrodes to make the sensor by thermal evaporation method in a vacuum.

4. Take the necessary measurements to determine the sensitivity of the material by using gas (NO₂, NH_{4,……..etc.}).

5.3 Description of the sensor system

The gas sensing chamber is a (cylinder or chamber) made of stainless steel, the cylinder is (30 cm) in diameter and (35 cm) high. The chamber contains several openings, including the gas pumping hole, the unloading hole, and the window opening to monitor what is going on inside the chamber, and a hole lead through is used for electrical connection between the parts inside the vacuum chamber and the measuring devices outside the vacuum chamber.

The sensing system contains a needle valve that controls the entry and exit of gas and is connected to a tube to a flask containing a source of gas, which supplies the gas to be tested in the chamber.

One of the important parts of the system is a laptop computer that is used to record the difference from sensors when exposed to a percentage of gas whose sensitivity is to be measured by the prepared membranes.

One of the parts of the sensing system is the hot plate, which is a base on which samples are placed inside the sensing chamber, the purpose of which is to raise the temperature of the membrane, as shown in Figure 6, and in order to control the operating temperature, the sensors are connected to a digital meter (thermometer).

Figure 6.

The system used to measure sensitive membranes of gases [20].

The resistance of the membrane in the air is measured first, then the samples are entered into a vacuum chamber, and the resistance is measured as a function of time. The gas is pumped in and the change in the resistance of the membrane with time is read for every second with constant temperature, and readings are taken for both the pure and tainted cobalt films.

The gas sensing characteristics of (Tio₂/ rGo) nanocomposite to NH₃ sensor application has been studied by Ref. 17, and we have reached that Ammonia sensing acts for (Tio₂/ rGo) layered film tackled . Sensing behavior of the (Tio₂/ rGo) nanocomposites tested under the 100 ppm of environment. The (Tio2/ rGo) nanocomposite explained a high reaction to NH3 gas (48%) at room temperature.

It is clear in Figures 7 and 8 that sensitivity (S) of NH3gas sensors based on (TiO₂/rGO) nanocomposite prepared by using pulsed laser ablation in Double distilled and deionized water (DDDW) at wavelength (532) nm to 100 ppm NH3gas in room-temperature. So that to obtain a measurement of sensitivity of the sample produced in this work, electrical resistance of nanocomposites was measured in the air and the presence of gas in room temperature. The resistive of gas sensors is called the relative change in resistance or conductivity for the nanocomposite. A known quantity of intended gas is introduced after the ohmic strength of the sensor matter gets stability. The recovery features (as the target gas is withdrawn) are also controlled as a function of time. Sensitivity (S) can be calculated from Eq.(3). The sensitivity of TiO2/rGO-1 and TiO2/r GO-2 nanocomposites are (48) (25), respectively. The sensitivity of the (TiO₂/rGO-1) nanocomposite for ammonia gas is higher than that of (TiO2/rGO-2) nanocomposite because the sensitivity is based on grain size and grain boundary. If the distance between the grains is small, rising the interaction between oxygen absorbed and gases is rising, also the grain boundary will increase interaction and increase sensitivity, response, and recovery time is due to the first definition under exposure to NH3 at room temperature. It is necessary that both responses time and remedy time relied on gas focus and the temperature at which the sensor perform in this work. The operating temperature is constant (RT) and gas concentration is also constant and is estimated at 100 ppm, When exposed nanocomposites (TiO₂/rGO) to NH3 gas, the resistance of the nanocomposite decreases and thus make the sensor performance at room temperature, where Ammonia is an electron supplier and might assist electrons to the (TiO₂rGO) sensing matters at the sensation actions. Semiconductor gas sensors are generally employed at the pressure in the atmosphere. Therefore, atmospheric oxygen on the surface adsorbs electrons from the conduction band of n-type (TiO₂rGO) nanocomposite film, forming O₂-and an electron-depleted layer at the surface of the film. NH3 gas was adsorbed, and electrons freed into the conduction band due to Equation below [4], decreasing resistance.

Figure 7.

Repetitive response curves of (TiO₂/rGO-1) nanocomposite exposed to 100 ppm NH3[21].

Figure 8.

Repetitive response curves of (TiO₂/rGO-2) nano composite exposed to 100 ppm NH3 [21].

$$4{\mathrm{NH}}_{3\left(\mathrm{g}\right)}+3{\mathrm{O}}_{2\left(\mathrm{ads}\right)}^{-}\to 2{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}$$E4

6. Conclusion

This chapter shows the importance of the study of gas sensors and their fundamental applications that are used in different aspects of life in addition to their importance in the dictation of toxic gases. The use of these devices can reduce the risks. To that, it is turn out the importance of classifying the materials that are used in gas sensitivity and components of gas sensors as it is explained the steps of sensitivity measurement with a description of the system used to sense the gases.