**5.2 Gas sensor preparation electrochemistry**

The sensor can be prepared through the following steps:


#### **Figure 5.**

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

#### **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).

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 roomtemperature. 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

**Figure 6.** *The system used to measure sensitive membranes of gases [20].*

**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].*

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.

$$\text{4NH}\_{3(g)} + \text{3O}^{-}\_{2(\text{ads})} \rightarrow 2\text{N}\_{2} + \text{6H}\_{2}\text{O} + \text{6e}^{-}\tag{4}$$
