Ag Nanoparticles Decorated with ZnO Thin Films Synthesis via Spray Pyrolysis for Gas Sensing

Silver oxide, Ag₂O is effective in gas sensing due to; High surface reactivity—Ag atoms catalyze gas adsorption/desorption reactions. Ag₂O is p-type semiconductor behavior—enhances charge carrier modulation during gas exposure. In addition to good conductivity and redox properties, Ag₂O can exchange electrons rapidly with target gases, improving sensitivity and response time [1]. Several factors affect the gas sensing process in conductometric semiconducting metal oxide sensors. Such as surface reactions, chemical components, surface modification, microstructures of sensing layers, temperature, and humidity, as these factors significantly affect the sensitivity of Ag/ZnO gas sensing devices [2]. Due to industrialization and rapid economic growth, toxic, explosive, flammable, and hazardous gases have significantly increased [3]. Zinc oxide (ZnO) is a wide-bandgap semiconductor with excellent electrical and optical properties. It has been extensively investigated for gas sensing applications due to its high sensitivity, low cost, and stability at elevated temperatures [4]. ZnO sensors can detect gases such as NO₂, CO, NH₃, and H₂ [5]. Silver dioxide (Ag₂O) is another material considered for gas-sensing applications. Like ZnO, it is a semiconductor, and its properties can be tailored to enhance its gas-sensing capabilities [6]. As a result of their gas-sensing properties, ZnO and Ag₂O have been the subject of several investigations. In contrast, ZnO has a larger bandgap and is less vulnerable to environmental changes due to its high electrical conductivity [7]. When ZnO and Ag₂O are mixed, the resulting nanocomposite can exhibit improved gas-sensing performance over either material alone. This is due to the formation of a heterojunction between the two materials, which creates a region of high electric field sensitive to molecules [8]. CdO to Ag₂O can also improve the sensor selectivity, meaning it can be more specifically designed to detect a particular gas. For example, an Ag/ZnO sensor optimized for detecting O₂ may respond lower to other gases such as H₂S or NO₂ [9]. It is an essential application where it is necessary to distinguish between different gases, such as in environmental monitoring or industrial safety [10]. Ag/ZnO nanocomposites offer a promising platform for developing next-generation gas sensors [11]. The combination of high sensitivity and selectivity ZnO improves the stability and tunability of Ag₂O, making them well-suited for a wide range of applications [12]. The sensors can be worked at relatively low temperatures, saving energy, and extending the sensor's lifespan. The sensors are relatively inexpensive to fabricate and can be mass-produced [13]. Ag/ZnO gas sensors are still under development, but they have the potential to revolutionize the way that detect and monitor gases in our environment [14]. Ag/ZnO thin films were deposited via several techniques such as spray pyrolysis [15], Sputtering [16], electron-beam evaporation [17], PLD [18], ALD [19], Sol-Gel [20], spin coating [21], and others [22]. The synthesis method of Ag/ZnO nanocomposites using spray pyrolysis has gained much attention due to its simplicity, reproducibility, and scalability [23]. ZnO has a wide bandgap semiconductor and has been widely studied for gas sensing applications due to its high sensitivity, selectivity, and stability in detecting various gases [24]. However, to improve ZnO sensing properties, Ag doping has been proposed [25]. Thus, the Ag/ZnO nanocomposites synthesized using spray pyrolysis have immense potential in gas sensing applications due to their high sensitivity, selectivity, and stability [26]. Ag/ZnO nanocomposites have shown promising results in gas sensing applications due to their increased surface area, enhanced catalytic activity, and high electron mobility [27]. The present study aims to investigate the Ag/ZnO gas sensing for NO₂ with Ag varying.

The crystallinity of thin films was characterized by X-ray diffraction. The XRD patterns of thin films were analyzed to determine the crystal structure, phase composition, and orientation of the crystallites. The results revealed a high degree of crystalline phases, as presented in Figure 1. A polycrystalline hexagonal phase was observed in the deposited samples for ZnO and a cubic phase for Ag₂O. Figure 1 shows the data indicates that the reflection peaks of ZnO primarily occur along directions related to the crystallographic planes 100, 002, 101, 102, and 110. These planes correspond to reflection angles of 31.77°, 34.54°, 36.41°, 47.87°, and 56.96°, respectively. The XRD results agreed with Naqvi et al. [28] and Cai et al. [29]. XRD patterns were examined to match the crystallography card No. (96-210-7060) for ZnO [30]. Meanwhile, XRD analysis of the mixture 0.1 Ag showed 2$\theta$ = 36.41° and 55.30° at planes 200 and 220, respectively. While at mixture (0.2 Ag) showed 2$\theta$ = 32.67°, 37.76°, and 54.51°) at planes 111, 200, and 220, respectively. The CdO samples aligned with crystallographic diffraction data No. (96-101-0605) [31]. A decent similarity exists between the observed and reported values of d-spacing. The collected d-spacing values were observed with about 98% accuracy in calculations that agree with Ling et al. [7] and Yang et al. [9]. Crystallite size (D) was determined from the FWHM of the XRD spectrum using the Debye-Scherrer formula [32].

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Figure 1. XRD patterns for ZnO and (0.1, 0.2) Ag₂O deposited films

Regarding Figure 1, expression data is listed in Table 1. Data were recorded and plotted on the same intensity scale 20°-60° to highlight differences between planned patterns, along with full-width half maximum, d-spacing, crystallite size, and crystal planes (hkl).

$\mathrm{D}_{\mathrm{hkl}}=\frac{\mathrm{K} \lambda}{\beta \cos \left(\theta_{\mathrm{hkl}}\right)}$                        (1)

Table 1. XRD parameters of deposited Zno at different Ag/Zno

Assuming $\beta$ is the wavelength of the X-ray beam, Bragg's angle $\theta$, and $\beta$ is the FWHM of diffracted X-rays in radians.

Figure 2 shows the morphology of a ZnO thin layer that was examined via FESEM. Meanwhile, the average particle size of ZnO is around 68 nm. Figure 3 displays the morphology of the ZnO and Ag/ZnO. It is worth noting that a distinct nanostructure was detected at Ag (0.1).

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Figure 2. FESEM image of ZnO of the deposited thin films

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Figure 3. FESEM image of ZnO 0.1 Ag₂O and ZnO 0.2 Ag₂O thin films

The particle size distribution reveals that the particles fall within a range of 67 nm, with an average diameter of 70 nm, determined using ImageJ software.

The deposited films exhibited thicknesses ranging from 195 to 200 nm, measured using the Fizeau method with a green laser operating at a wavelength of 532 nm [33]. Absorption measurements were conducted to gather information on Ag/ZnO. Meanwhile, Figure 4 shows the optical absorption of the prepared sample. It was apparent that redshift was taking place as the amount of Ag₂O increased.

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Figure 4. Absorption lines of ZnO and Ag/ZnO

Absorption Lines of ZnO and Ag/ZnO; the UV-Vis absorption spectra of the synthesized thin films reveal distinct optical transitions for ZnO and Ag-doped ZnO (Ag/ZnO) nanocomposites. The absorption edge of pure ZnO appears around 309 nm, consistent with its wide direct bandgap (~3.21eV), characteristic of wurtzite ZnO semiconductors. Upon doping with Ag (0.1 and 0.2 molar ratios), the absorption spectra exhibit a redshift, with absorption edges shifting to longer wavelengths 319nm and 325 nm, respectively.

The reason for the shift in Transmittance Intensity with Increasing Ag₂O Content due to the concentration of Ag₂O in the ZnO thin films increases, a decrease in transmittance intensity is observed, particularly in the UV-visible range. This behavior can be attributed to the following factors: Increased light absorption due to plasmonic effects, bandgap narrowing and redshift, increased defect states and scattering centers, optical density increase [25].

Tauc formula [34] was employed to plot (αhν)² vs (hν) as bandgap calculations that presented in Figure 5. This shift indicates a narrowing of the optical bandgap, calculated via Tauc plots to be 3.12eV for 0.2 Ag/ZnO and 3.07eV for 0.1 Ag/ZnO, compared to 3.21eV for undoped ZnO. The redshift and bandgap reduction can be attributed to: Localized Surface Plasmon Resonance (LSPR) effects from Ag nanoparticles, Defect states and impurity levels introduced into the ZnO band structure, Enhanced charge carrier interactions at the Ag/ZnO heterointerface [35]. These modifications in the optical absorption behavior suggest improved light-matter interaction and potential enhancement in charge transfer processes favorable for gas sensing applications, especially in detecting oxidizing gases like NO₂.

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Figure 5. Energy bandgap of ZnO and Ag/ZnO

The FTIR spectrum is illustrated in Figure 6, showing that finding is consistent with the information provided by the study [36]. At 0.1 Ag, the bandgap decreased by 4.4%, which is promising for thin film sensing applications. The 3607,3391 cm^-1 bands show stretching vibrations of the hydroxyl groups as O-H is present in all samples [37]. The C-H bond was reported to be in the stretching mode with a frequency of 2880 cm^-1 [38]. A minor absorption peak at 2321 cm^-1 Indicates the occurrence of -COH stretching bond [39]. The bending vibration modes of H₂O observed at 1690 cm^-1 as mentioned by Carpenter et al. [40]. An investigation -products resulting from the chemical reactions emitted in the form of NO₂, which exhibited a vibrational frequency of 1602 cm^-1 [41]. Other by-products showed a C-O stretching vibration at a wavenumber of 1176 cm^-1, while C-O-O was seen at 903 cm^-1 [42]. The targeted metal oxides have appeared in the FTIR fingerprint region. Zn-O bond materialized at 453 cm^-1 as documented by Sowri Babu et al. [43]. The metal oxides of interest have been noticed within the fingerprint region of the FTIR. The formation of a Zn-O bond is seen at a wavenumber of 453 cm^-1 As reported by Sowri Babu et al. [43]. In contrast, a discernible fingerprint corresponding to the Ag-O bond was detected at a wavelength of 519 cm^-1 [44].

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Figure 6. FTIR spectroscopy of ZnO and Ag/ZnO

As part of the sensing process, gas detection is influenced by surface regulation. At the same time, chemisorbed oxygen contributes to surface resistance. However, the reaction mechanism involved is quite complex, involving multiple intermediate steps. As displayed in Figure 7, ZnO and Ag/ZnO samples react with NO₂. The measurements were carried out at three distinct operating temperatures 100, 200, and 300℃, in conjunction with their corresponding response and recovery durations. The gas sensing data were conducted with low detection limits ranging from 10 to 100 ppm. The results of the gas sensing examination are listed in Table 2, showing the response time and recovery time at the operating temperature. Furthermore, the sensitivity of the prepared samples was calculated using Eq. (2) [45].

$S \%=\frac{R_{{gas }}-R_{{air }}}{R_{ {air }}} \times 100 \%$                        (2)

According to the equation above, R_air and R_gasrefer to the deposited sample's resistance in air and an examining gas, respectively.

Table 2. Gas sensing examination, sensitivity, response, and recovery time

Figure 7 illustrates the variation in resistance during the gas examination. Based on the results in Figure 7, the 0.1 Ag/ZnO sample possesses the highest sensitivity among all the specimens prepared.

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Figure 7. Sensitivity of samples vs. temperature

The sensing explanation of limit of detection for NO₂ as the manuscript mentions that the gas sensors based on ZnO and Ag/ZnO thin films were tested with NO₂ concentrations ranging from 10 to 100 ppm, but it does not explicitly state the minimum concentration at which a reliable detection was achieved. Where The 0.1 Ag/ZnO composition showed the highest sensitivity (68% at 200℃), likely due to an optimal balance between catalytic activity and charge carrier mobility.