Study of the Effect of Gamma (γ) Rays on ZnO Thin Films Nanoparticles as a Gas Sensor for NO2

INTRODUCTION
One of the many issues that facing humans in the twenty-first century is the monitoring and detection of dangerous and poisonous gases, such as CO, CH4, NO2, etc. Factory and thermal power plants generate nitrogen dioxide (NO2), a dangerous gas, during combustion. It plays a major role in the atmospheric reactions that produce ozone at ground-level, which is a major component of smog and the main cause of acid rain [1,2]. The environment contains NO2, a reddish-brown toxic gas with a pungent odor. The American Conference of Governmental Industrial Hygienists has set the threshold value for NO2 at 3 ppm. Long-term exposure to NO2 at concentrations higher than 40–100 μg/m3 can increase the risk of respiratory disease and may reduce lung function.[3] For the detection of hazardous gases, various metal oxide semiconductor-based sensors (Fe2O3 and TiO2, CuO, WO3, ZnO) have gained widespread use in recent years. Among these metal oxides, ZnO has gained widespread use as a gas sensing material due to its wide operating temperature range, stability, and flexibility, which make it a promising candidate for sensing applications for hazardous gas detection. Because of its wide energy bandgap (3.37 eV), high binding energy (60 MeV), and excellent mechanical and thermal stability at ambient temperature [4][5]. Several post-formation treatments are used to make metal oxide thin films better. These include annealing in a controlled environment and exposing them to high-energy ionizing radiation like gamma rays, protons, and charged particles. Radiation at different doses or fluxes leads to noticeable modifications in the compositional and morphological properties of metal oxides and creates a wide variety of defects [6]. The effect of radiation on metal oxides at high doses leads to two main effects: (1) temporary effects resulting from the generation of electron-hole pairs and (2) permanent effects resulting from the change in the crystal lattice. Therefore, we anticipate that exposing the ZnO sensor to gamma rays could improve its gas sensing capabilities [7]. Researchers Lontio Fomekong, R., & Saruhan, B. studied the gas sensing properties of a thin film of zinc oxide (ZnO) prepared by the (spray-CVD) method and the direction of the gases NO2, CO, and NO in the temperature range (400-500 ◦C). They demonstrated good selectivity, with the ZnO membrane’s selectivity ratio increasing for NO2 gas compared to CO (1 to 2.4) [8]. Researcher Waikar, Maqsood R., et al, studied the effect of gamma rays on the sensor properties of a thin ZnO membrane prepared chemically, found that after irradiating the membrane with 30 KGy of gamma rays, it became 7.29 times more sensitive to ammonia gas at a 400 ppm concentration, compared to its pre-irradiation value of 1.01[9]. The researcher kumar Anbalagan, Aswin, and his colleagues conducted a study in which they exposed thin films of zinc oxide to varying doses of gamma rays, aiming to create different defects that could impact the sensing of NO2 gas direction. They demonstrated that the irradiated sensors enhanced their sensitivity by 185%, 160%, and 120% at a 500 ppm concentration under radiation doses of (1.5, 4.5, and 6) KGy. The study confirmed that increasing oxygen vacancies, concentrations of defects, and grain boundaries resulting from radiation can achieve high sensitivity to NO2 gas [10]. This study aims to increase the sensitivity and effectiveness of sensors that use thin zinc oxide films prepared by the thermal chemical spray method and exposed to different doses of gamma rays to detect NO2 gas. 

MATERIALS AND METHODS
Chemicals like Thomas Baker aqueous zinc nitrate (NO3)2 6H2O, possessing a molecular weight of 297.5 g/mol and swiftly dissolving in distilled water, were utilized to produce thin zinc oxide films. Determined the solution concentration (0.2 M) by dissolving 2.975 mg of zinc nitrate in 50 ml of distilled water. prepared thin films using the thermal spray pyrolysis approach, which involved spraying the solution onto used glass substrates, which cleans the glass bases well with distilled water, ethanol, and acetone and then with distilled water to remove any stuck-on traces because the presence of impurities greatly affects the accuracy of the resulting measurements. Used a 55 sec spray duration and a 9 sec pause time between depositions to maintain the thermal stability of the glass substrate. After that, repeatedly spray until achieve the desired film thickness. The current investigation determined the ideal spray rate to be 0.35 ml/min, with an air pressure of 1.75 bar, and a nozzle distance of 35 cm from the substrate. We also set the substrate’s temperature at (380 ± 5 ◦C), an important parameter that significantly influences the membrane preparation. Fig. 1 illustrates the appearance of a homemade thermal spray pyrolysis system. 
The weighing method was adopted to measure the thickness of thin films after determining the mass of the film deposited on the surface of the substrate with a known area through the following relationship [11].

Where T the thickness of the thin film is measured in nanometers (nm), A The area of the substrate on which the thin film is deposited, ρ the density of zinc oxide is equal to 5.606 g/cm3 [12]. ∆m Change in mass after deposition. It was determined that the thin film produced for this study had a thickness of (250 ± 10) nm. Exposed the ZnO thin films produced by the thermal spray pyrolysis method to gamma rays using a 60CO source, which produces gamma rays with an energy of 1.25 MeV [13], at different times, the sample before irradiation was indicated as S0, while the samples exposed to radiation were indicated as (S15, S30, S45, and S60) at times of exposure (15, 30, 45, and 60) minutes, respectively. 

Gas sensor measurements
An apparatus that transforms chemical and physical signals into electrical impulses is known as a chemical sensor. This transformation is the result of the difference in the chemical or physical properties of sensitive materials when exposed to detection under the influence of a certain gas [14]. In general, the reducing gas is the electron donor while the oxidizing gas is the electron acceptor. Therefore, the interaction between the metal oxide surface and the gas molecules, which increases or decreases the number of majority carriers, causes the material’s resistance or electrical conductivity to change when exposed to gas. So, whether the resistance of the metal oxide goes down or up depends on the type of majority carriers in the semiconductor and the type of gas particles (reduced or oxidized) in the air around it. For p-type materials, gas reduction (donor) increases the resistance of thin films while oxidizing gases (acceptor) decrease it, and vice versa for n-type materials [15,16]. Fig. 2 shows a cross-sectional view of the gas sensor testing system and the homemade test chamber. The unit comprises a cylindrical stainless steel test chamber, hermetically sealed, measuring 27 cm in diameter and 10 cm in height, with a detachable bottom base. The effective volume of the chamber is 5298.5 cm³; it contains an inlet to allow the flow of the test gas and an air inlet valve to allow atmospheric air after evacuation. The chamber provides a third port for connecting to the vacuum gauge. Through spring-loaded screws, multi-terminal feeding through the chamber base makes it possible to connect the heater assembly and the sensor electrodes to the electrical grid. By opening the valve, we allow the target gas, which is NO2 with a known concentration, to flow into the test chamber during the test. The variation in the sensor’s resistance to the known concentration of the test gas is observed using a digital mustimeter connected to the computer ( NI-UT81B).
All samples (S0, S15, S30, S45, and S60) undergo testing before and after irradiation, with the working temperature set at 70 ◦C and the desired gas concentration set at 100 ppm, NO2 was produced by reacting copper Cu with concentrated nitric acid HNO3 to create nitrogen dioxide, which is subsequently converted to nitrogen dioxide upon contact with air [17].

Eq. 3 calculates the sensitivity (S) of zinc oxide thin film sensors [18].

Where Rgas represents the resistance after being exposed to the gas, Rair represents the sensor resistance in the air. When the sensor introduces the target gas, it takes the response time to achieve a 90% change in resistance. Similarly, when the sensor stops the target gas, it takes the recovery time to achieve a 90% change in resistance [19]. 

RESULTS AND DISCUSSIONS
Structural study
Fig. 3 shows an XRD analysis of ZnO thin films before and after exposure to gamma rays. Observed the highest peak intensity at (002), with lower intensity peaks at (100), (101), and (102). XRD analysis confirmed that ZnO has a hexagonal wurtzite crystalline structure [20]. The hexagonal structure of ZnO thin films has the lowest surface energy, which explains why ZnO exhibits a deterministic growth in the (002) level [21] These results are acceptable compared with JCPDS card no. 36-1451 [22]. The average grain size D values of the samples are calculated by Scherrer formula [23].

where θ is Bragg diffraction angle, λ is wavelength of X-rays (0.1540nm) and β is FWHM full width at half the peak’s maximum. Considering that the FWHM value depends on the length and width of the peak density, Table 1 reveals that the increase in the irradiation period led to a rise in the FWHM value. Conversely, a decrease in grain size Slightly at a rate of (15.631, 14.950 and 14.314) nm, resulted in an increase in lattice strain Ԑ; its value is calculated through equation (3) [24]. This could suggest the existence of gamma radiation-induced defects, leading to distortions in the crystal lattice [25]. This aligns with the researcher’s findings [26]. 

Gas NO2 Sensing Results
Due to the gas absorption, the resistance of the surface layer of the thin film in the oxidizing gas sensor changes. Examine the sensor properties of all ZnO thin films before and after irradiation as a function of time, at an operating temperature of 70 ◦C. Inject NO2 gas into a tightly controlled gas chamber with an amount of 100 ppm for varying times, then record the changes in resistance using an electrical measuring device, specifically a digital mustimeter, connected to the computer (NI-UT81B). To find out how sensitive the membranes were to absorbing the oxidizing gas, a (300±20) nm thick aluminum mask was put on top of the membrane in the shape of a mesh to act as a sensor for the gas that was deposited on the glass. Exposure to the gas NO2 in the presence of oxygen changes the surface properties of the thin film, altering its resistance, and when the gas stops, it returns to its initial value. Fig. 4 demonstrate an increase in resistance upon exposure of the ZnO thin film to NO2 gas, an oxidizing gas, both before and after irradiation of the samples (S0, S45, and S60). The ZnO films contain oxygen voids and zinc voids, as show n-type conductivity [27].
After the zinc oxide-sensitive thin film is exposed to nitrogen dioxide (NO₂), NO₂ is absorbed by the sensitive film, and NO₂ acts as an electron acceptor in the reaction, resulting in increased resistance of the sensor [28]. The surface of the sensor undergoes the following chemical reactions [29]:

The above reactions lead to a decrease in the concentration of electrons on the surface of zinc oxide (ZnO), resulting in an increase in the resistance of the material. In addition, the following interaction occurs between O-ads and NO2 (ads) -.

Thus, the periodic interactions continued: 

Fig. 5 shows that at a low radiation dose of gamma rays in samples (S15, S30), the excitation process leads to the creation of some charge centers and defects on the surface of the thin film. These generated electrons attract more oxygen, which increases the number of oxygen ions absorbed on the surface as the depletion zone decreases further, Additionally, the presence of nitrogen in the surface reactions of the ZnO membrane acts as a shallow surface acceptor, which can contribute to an increase in the concentration of gaps at the surface in ZnO membranes exposed to a low dose, reducing the resistance of the thin films compared to the thin film not exposed to radiation when exposed to the target gas. Defects from deformation (spaced vacancies or atomic overlap) in the ZnO films lead to an increase in the depletion region as the irradiation time for samples (S45, S60) increases. Although the saturation process at this dose reaches the maximum level of absorbed oxygen, the depletion zone does not shrink to the same degree as it did at lower doses. At high doses, the crystal structure deteriorates, which increases the resistance of the film at the surface, as in Fig. 4 above [30,31]. This is consistent with what the researcher indicated [32]. Several reports have shown that testing metal oxide-based gas sensors with nanoparticles alters their surface behavior in terms of resistance [33]. Rani et al. discovered that putting NO₂ gas on SO₂ membranes and then shining a low-dose electron beam on them lowered the surface resistance. When the dose was raised, the resistance went up in the same way it did before irradiation [34]. Majhi et al. also reported that zinc oxide films exposed to gamma rays showed behavior different from the surface resistance at low doses compared to high doses of gamma rays during exposure to NH3 gas [4].
Sensitivity (S%) was calculated using equation (2), and the response time when opening the gas, changing resistance, and recovery time were also calculated. The above-mentioned figures show different responses depending on the change in radiation dose. Table 2 shows the values of sensitivity, response time, and recovery time calculated before and after irradiation for the ZnO thin film sensor. 
The relative change in the response of the sensor after irradiation increased to 242.4% compared to the non-irradiated sensor, which equals 22.81%, as shown in Fig. 6-a. The increase in response when the film is exposed to gamma rays can be attributed to the significant changes in the surface morphology of the ZnO films, as revealed by XRD analysis, which indicated that the grain size decreased with increasing irradiation time, which increases the porosity of the films and leads to an increase in the surface area. This enhances the surface’s interaction area with the target gas [35]. In addition, the presence of oxygen and zinc vacancies in the irradiated ZnO films contributes to the increased sensitivity. The presence of interspersed zinc atoms results in a shallow donor level below the conduction band. The electrons at the shallow donor level will acquire sufficient thermal energy to enter the conduction band at operating temperature. Thus, these surface electrons are expected to enhance the oxygen-adsorbing chemical ions on the surface, this results in an enhancement of the sensor’s responsiveness [36]. This agrees with the researchers [9,10]. 
Fig. 6-b, also showed a decrease in the sensor’s recovery time (from 6.039 to 3.111 sec) as the irradiation time increased. Recovery time refers to the duration required for the sensor to revert to its initial state (base) following the removal of the target gas. The shorter the recovery time, the better the sensor’s performance, as it allows the device to operate faster and respond more frequently to continuous detections, thereby increasing its effectiveness in practical applications such as toxic gas detection [37].

CONCLUSION
This study examined the impact of gamma radiation on the ZnO thin films under NO2 gas. For all samples, the XRD test showed the sharpest peak at the (002) level. This level is for the hexagonal wartzite phase of ZnO. In addition, we observed that the grain size decreased at a rate of (15,631, 14,950, and 14,314)nm, with increasing irradiation time. This resulted in a high lattice strain Ԑ, which is a sign of defects caused by gamma radiation. Gamma rays greatly affect the relative response of the sensor. As the irradiation time increased, the response increased at a rate of (81.89%, 75.18%, 181.4% and 242.4%) compared to the samples before irradiation, which reached a value of (22.81%). Due to an increase in membrane surface area, the target gas interaction area increased. This was confirmed by XRD analysis. It also showed a decrease in sensor recovery time (from 6.039 to 3.111 sec) with increasing irradiation time. The shorter the recovery time, the better the performance of the sensor, because it allows the device to operate faster and respond more frequently to persistent detections.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.