CH₃COCH₃ belongs to the family of volatile organic compounds (VOCs) that are employed as solvents in various industries. CH₃COCH₃ at higher levels may have a negative impact on the central nervous system and may be harmful to the eyes and nose. Therefore, the occupational threshold limit value for CH₃COCH₃ was set at 250 ppm considering a time-weighted average of 8 h [47]. Accordingly, the development of sensitive CH₃COCH₃-sensing devices is required from the perspective of safety. In this regard, some attempts have been made to establish selective CH₃COCH₃ sensors by adding Ag into SMOs. For instance, Xu et al. [48] described the synthesis of Ag-decorated SnO₂ hollow nanofibers (NFs) with large surface areas and their CH₃COCH₃ sensing properties. Typically, noble metals have higher electrical conductivities that enable the rapid transfer of electrons and catalysis of the oxidation of reducing gas molecules such as VOCs [45,46,47,48]. Thus, noble metal-incorporated sensing materials can be suitable candidates for detecting VOCs. The developed Ag-decorated SnO₂ sensor showed an excellent response and higher selectivity for CH₃COCH₃ at 160 °C. A schematic of the sensing interactions between the CH₃COCH₃ gas molecules and pure and Ag-decorated SnO₂ is shown in Figure 1. Owing to the presence of p-type Ag₂O crystals, p-n heterojunction interfaces formed between p-type Ag₂O crystals and n-type SnO₂, generating a broader depletion layer on the SnO₂ side, thereby increasing the initial resistance of the sensing material. Upon exposure to CH₃COCH₃, significant modulation of the heterojunctions occurred and contributed to the sensor response. The Ag NPs produced Ag₂O in air, which has facilitated the development of highly electron-depleted layers while removing electrons from SnO₂. This process has increased the total resistance and the area adjacent to the Ag–SnO₂ interfaces, thus increasing the sensitivity of SnO₂ to CH₃COCH₃. Furthermore, one-dimensional (1D) SnO₂ hollow nanostructures fabricated via electrospinning considerably facilitated the diffusion and transport of electrons in CH₃COCH₃ molecules, thereby rapidly and substantially changing the sensor resistance. Additionally, SnO₂ hollow NFs with both outer and inner surfaces exhibited high surface areas and eventually contributed to high CH₃COCH₃ responses. In summary, the significantly high CH₃COCH₃ sensing performances of Ag/SnO₂ composites are attributed to the Ag dopant and 1D NF structures of SnO₂. Ag with high electrical conductivity enabled the rapid transfer of electrons and catalyzed the oxidation of the reducing CH₃COCH₃ gas molecules. Moreover, the SnO₂ NFs with higher surface areas led to an effective dispersion of catalyst particles and possessed a porous tube-like structure that promoted rapid gas flow and a superior ability to store and release oxygen ions.

In another study, Kilic et al. [49] reported the CH₃COCH₃ sensing characteristics of Ag-loaded TiO₂ nanorods (NRs). They used a seed-mediated hydrothermal approach to grow TiO₂ NRs and subsequently loaded Ag onto TiO₂ NRs by thermally evaporating metallic Ag at different times of 30, 45, and 90 s. The authors demonstrated that the enhancement in the CH₃COCH₃ detection performance of the Ag-loaded TiO₂ (45 s) sensor was due to the catalytic activity of Ag. Actually, Ag loading increased the number of adsorption sites on the surface of TiO₂ and accelerated the rate of electron exchange between the TiO₂ surface and CH₃COCH₃ molecules. Under a CH₃COCH₃ atmosphere, the bonds between CH₃COCH₃ molecules were easily dissociated by Ag, allowing these molecules to quickly interact with the chemisorbed oxygen species. Moreover, the decoration of Ag NPs onto TiO₂ NRs pinned the Fermi level of TiO₂ because of the transfer of electrons from TiO₂ to Ag. This led to surface band bending and induced a more pronounced electron–hole separation effect, thereby enhancing the sensitivity of the sensor to CH₃COCH₃. The lower CH₃COCH₃ response of the Ag-loaded TiO₂ (90 s) sensor was associated with the accumulation of Ag clusters on the TiO₂ surface, which impeded O₂ diffusion within TiO₂ and reduced the catalytic activity of Ag.

Cl₂ gas is very hazardous to the human respiratory mucous membrane at concentrations in the range 0.2–3.5 ppm. It causes psychological disorders, skin infections, and even liver damage. Hence, the early detection and successive monitoring of hazardous Cl₂ by reliable gas sensors are highly important. Few studies have been reported on the Cl₂ detection properties of Ag-loaded sensors. Li et al. [50] illustrated the effect of Ag loading on the Cl₂ response of bismuth ferrite (BiFeO₃, BFO) nanospheres (BFO NSs); they synthesized BFO NSs using a sol–gel route and successively loaded Ag NP onto BFO NSs via a photodeposition technique. Typically, BFO has a distinct surface reactivity, O₂ adsorption ability, and a narrow band gap, which are beneficial for improving the gas-sensing characteristics. Therefore, BFO NSs are important candidates for the detection of Cl₂. The observed response of the optimum 4 mg AgNO₃-modified BFO sensor to 10 ppm Cl₂ was 72.62 at a working temperature of 240 °C. This response was 2.5 times that of the pure BFO sensor. A schematic of the energy band structure and hole transfer after the loading of Ag NPs is shown in Figure 2. Upon the loading of Ag NPs onto BFO, the positive charges on BFO reduced, and downward band bending occurred because the holes on the BFO NSs transferred to the Ag NPs. Along with the separation of electrons and holes, the electrons probably combined with O₂ and Cl₂ on the surface of the gas sensor. No barriers were formed because of the reduction in the number of conduction band electrons, thus supporting the progress of the reaction. Moreover, the Fermi levels of Ag and BFO correspond to one another at the same level, following contact because BFO (Φm) has a lower work function than that of Ag (Φs). Here, Ag NPs might also be used as distinct adsorption sites for O₂ and Cl₂. Furthermore, at the Ag/BFO interface, a Schottky junction was created that decreased the intergranular barriers and enhanced the interfacial effect [50].

In contrast, the number of electrons at the reaction sites increased, and the reaction quickly saturated because of the rapid transfer of charge carriers from BFO to Ag. Consequently, the gas response significantly improved, and the response time shortened. However, the amount of adsorbed O ions decreased when high amounts of Ag were loaded onto the BFO NSs, which reduced the reactions of O₂ and Cl₂, thereby decreasing the sensor response. Thus, this study demonstrates that the addition of noble metals presents a new way of enhancing the sensitivity of p-type semiconductors toward Cl₂ gas.

C₂H₂ is a flammable and colorless gas with a peculiar odor and is commonly utilized as a fuel in oxyacetylene welding and metal cutting and as a raw material in various industries. C₂H₂ poses serious threats due to its inherent instability, primarily in the cases of liquefaction, pressurization, heating, or mixing with air. This means that C₂H₂ can cause massive explosions if it leaks. Accordingly, from environmental and safety perspectives, the development of highly efficient C₂H₂ sensors is becoming increasingly important [31,51,52]. In this regard, Uddin et al. [51] described the C₂H₂ sensing properties of chemically synthesized 0–5 wt.% Ag-loaded ZnO-reduced graphene oxide (ZG-Ag) ternary hybrids. Generally, herein, the use of rGO was preferred over that of graphene because of difficulties in the large-scale production of graphene and lack of functional groups and band gaps in graphene [53]. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images (Figure 3) support the Ag/ZnO/Gr configuration of these heterostructures.

The Ag-loaded sensor based on ternary hybrids showed a higher response than that of the sensor without Ag. The sensor without Ag could detect 16–100 ppm C₂H₂ at 250 °C. The response of the 3 wt.% Ag-loaded ZnO–Gr (ZG-Ag3) sensor was 22 at 150 °C, indicating that 3 wt.% was the optimum loading amount of Ag. The loading of 3 wt.% Ag onto the ZnO–Gr sensor not only enhanced the sensor response but also reduced the optimum sensing temperature. Ag has a superior capacity to dissociate O₂ than ZnO, and it catalyzes the dissociation of molecular O₂ on the sensor surface, which generates higher C₂H₂-sensing active sites and thereby a higher sensor response. The development of depletion layers around the ZnO NPs due to the presence of Ag was mostly correlated with the modulation of nano-Schottky barriers, thereby improving the low-temperature surface reactivity. Upon the loading of Ag onto the ZnO–Gr sensor, further dynamic sites generated at the sensing layer/Ag interfaces because of the spillover effect, and a larger number of C₂H₂ molecules adsorbed on the sensor surface. Therefore, a high sensor response was obtained when the loading amount of Ag was optimal. Nevertheless, when an excess amount of Ag was loaded, the O₂ molecules dissociated on the surface and surpassed the percolation threshold, which caused an overlapping spillover zone, ultimately affecting the efficient transfer of O₂ and decreasing the chances of C₂H₂ adsorption; this reduced the sensor response.

In another study associated with the detection of C₂H₂, Uddin et al. [54] designed a C₂H₂ sensor using Ag-decorated ZnO NRs supported by a flexible polyimide (PI)/polytetrafluoroethylene (PTFE) substrate (Figure 4).

Flexible gas sensors should have the following characteristics: (i) the ability to detect gases at low concentrations and (ii) low- or room-temperature operation [55]. In this study, radio frequency magnetron sputtering (125 W, 7 mTorr) was applied to load Ag NPs onto the sensing layer of ZnO NRs in an Ar environment at different loading times of 6, 8, and 10 s. Gas-sensing analysis demonstrated that the flexible sensor developed using 8 s Ag-loaded ZnO NRs showed a higher response of 13.8 toward 100 ppm C₂H₂ at 200 °C than those of the pristine (3.21), 6 s (9.33), and 10 s (9.57) Ag-loaded sensors. Furthermore, upon repeated bending and relaxation for up to 5 × 10⁴ cycles, the sensor response only slightly decreased, which demonstrated the outstanding sustainability and mechanical strength of the designed flexible sensor. This performance improvement was attributed to the integration of the Ag–ZnO structure with the flexible PI substrate, which improved the physical binding and adherence of the Ag–ZnO structure to the supporting substrate. ZnO NRs with small diameters ensured an improved surface area and a larger aspect ratio of the resulting sensor. This caused inadequate surface atomic coordination and higher surface energy, leading to better adsorption of O ions and hence improved sensor response. Furthermore, because of catalytic interactions, the Ag NP/ZnO NR interface might produce further charge carriers or O vacancies on the sensor surface. In the case of Ag-loaded ZnO NRs, the ZnO surface transformed from an electron-depletion state to an almost flat band state along with a redox change at Ag. Additionally, the improved C₂H₂ sensing activity observed at an optimum temperature of 200 °C was ascribed to the height of the Schottky barrier between the ZnO grains and Ag in the Ag-loaded ZnO NRs. The improvement in the performance of the Ag-loaded ZnO NRs may also be controlled by changing the Ag concentration for the sensitive variation of the Ag oxidation state during exposure to C₂H₂. When the Ag loading amount was high, a thick Ag layer developed on the ZnO NR surface, which decreased the efficacy of the open surface porosity of ZnO, thereby reducing the gas sensor response. Herein, the observed higher selectivity of the Ag/ZnO NR sensor toward C₂H₂ was attributed to the chemical sensitization effect. Since Ag has excellent catalytic activity, it acts as a certain adsorption site for the dissociation of O₂ and separation of H₂ molecules owing to the spillover effect.

TEA, a clear and flammable liquid with a strong odor of ammonia, is widely used in chemical industries [56]. However, TEA can cause many severe health issues, such as pulmonary edema, gastroenteritis, headaches, and even death, because of its toxicity [57]. Additionally, dead fish and other decaying marine products may release TEA, and the concentration of TEA is expected to progressively increase over time [58]. Consequently, TEA may serve as a chemical indicator to assess the quality of marine food [59]. Therefore, designing an advanced sensor for the detection and monitoring of TEA is required. In contrast, recently, three-dimensional (3D) structural nanomaterials have received considerable attention in gas-sensing applications. In fact, 3D structures may offer highly efficient specific surface areas that improve the response of the corresponding gas sensor. Accordingly, Shen et al. [60] reported the TEA sensing characteristics of Ag-loaded 3D porous ZnO microspheres. Their morphological studies indicated that the ZnO microspheres (3–5 μm in size) were assembled by numerous thin and porous nanosheets with sizes of ≈20 nm. The developed sensor exhibited better cross-selectivity toward TEA, which was significantly attributed to the differences between the reactivity of the target gases caused by their different bond energies and chemical molecular structures. The reported C-N, C-O, C-H, C=C, C=O, and O-H bond energies are 307, 326, 414, 610.3, 798.9, and 458.8 kJ/mol, respectively. As a result of the lower C-N bond energy, the TEA molecules were more easily reduced by Ag–ZnO.

In this case, the baseline resistance of the Ag-loaded sensor (2737.8 MΩ) was higher than that of the pristine sensor (28.5 MΩ). Furthermore, O species may more readily adsorb on the Ag NPs via a known spillover effect. Thus, highly active negative O ions spilled on the ZnO surface, which extracted the electrons from the conduction band of ZnO, thereby increasing the thickness of the electron-depletion layer and consequently the sensor resistance. Furthermore, the active O species accelerated the reaction of the sensor with TEA. Overall, the widely dispersed Ag NPs were highly favorable to the spillover effect and might catalyze the sensing reaction to increase the response of the sensor to TEA (Figure 5a,b). Finally, the special hierarchical 3D porous nanostructure comprised hundreds of porous nanosheets was an additional meaningful component for the excellent TEA sensing properties of the Ag-loaded gas sensor. This unique structure with a high surface area and large pores (Figure 5c,d) increased the regions of the sensing reaction and promoted the TEA sensing performance of the sensor.

HCHO is extensively utilized in several industrial applications. Typically, HCHO is a colorless gas with a pungent odor, and it easily evaporates from the products into the indoor air. Many health-related issues, including headaches, nausea, cancer, and mucosal and respiratory irritation, are associated with exposure to HCHO [61,62,63,64]. Therefore, the development of highly sensitive HCHO gas sensors is urgently required to ensure the safety of people. In this regard, Xing et al. [65] prepared highly porous 1–5 at % Ag-loaded ZnO for application in HCHO sensors. In this study, the sensor fabricated using 1 at % Ag-loaded ZnO exhibited the maximum response to HCHO (170.42) at an optimal temperature of 240 °C. Unique hierarchically structured porous Ag-loaded ZnO provided an adequate surface area for interaction between the HCHO molecules and the sensing material. The high response of the optimal gas sensor to HCHO was ascribed to the production of heterojunctions between Ag and ZnO in addition to the catalytic activity of Ag. Schematics of the energy bands of Ag/ZnO in the presence of air and HCHO gas are shown in Figure 6a,b, respectively. Herein, the electrons transferred from Ag to ZnO because the work function of Au (Wm = 4.4 eV) is smaller than that of ZnO (Ws = 5.2 eV), generating an electron-depletion layer because of the increase in the concentration of electrons at the Ag/ZnO interface. When the Ag/ZnO sensor was exposed to air, the electrons released from ZnO were captured by the adsorbed O ions to form O⁻_x, which showed strong oxidation activity. This process reduced the sensor resistance. Upon exposure to HCHO, the previously adsorbed O⁻_x strongly reacted with the HCHO molecules and oxidized HCHO into CO₂ and H₂O, thereby decreasing the sensor resistance. This study reveals that the HCHO sensing performance of the sensor is directly influenced by the Ag/ZnO heterojunction and the developed O⁻_x.

Furthermore, Wang et al. [66] reported the HCHO sensing activity of a two-step solution-processed Ag-loaded sunflower-like hierarchical In₂O₃ nanoarchitecture. The observed HCHO sensing performance was attributed to the highly porous and hierarchically designed sunflower-like nanostructure and the catalytic activity of Ag. The exclusive hierarchical 3D sunflower-like nanostructures possess numerous radial nanobranches with irregular surfaces, which offer a larger exposed surface area and additional paths for the exchange of electrons during the entire gas diffusion and surface reaction processes. An appropriate Ag loading amount plays an important role in chemical and electronic sensitization during the detection of HCHO by the Ag-loaded In₂O₃ sensor. Chemical sensitization favored gas reactions by dissociating HCHO via a spillover effect, and electronic sensitization substantially enhanced the direct exchange of electrons between In₂O₃ and Ag. However, higher Ag loading amount resulted in the conduction of electrons along the metallic Ag NPs irrespective of chemoresistive variations, thereby deteriorating the sensor response.

CO is among the most hazardous gases as it is invisible, tasteless, odorless, and colorless and thus cannot be detected by human sensory organs [67]. Consequently, numerous efforts have been made to detect CO gas by various materials and strategies. Molybdenum disulfide (MoS₂) is a distinct graphene-like two-dimensional (2D) layered transition metal dichalcogenide. In addition to a conventional band gap of 1.8 eV, it has a large surface area-to-volume ratio with excellent physical/mechanical properties [68]. Nevertheless, the gradual degradation of MoS₂ nanosheets occurs under ambient conditions owing to atmospheric oxidation and surface contamination, which may eventually decrease their sensing performances. Thus, to achieve an appropriate CO sensing performance of pure MoS₂ nanosheets, sensing should be performed under an inert N₂ atmosphere, significantly restricting their commercial applications. To overcome this limitation, Zhang et al. [69] fabricated a novel ternary Ag-loaded ZnO/MoS₂ nanocomposite via layer-by-layer self-assembly for the detection of CO gas.

The presence of ZnO NRs, Ag NPs, and MoS₂ nanosheets was verified by morphological investigations (the scanning electron microscopy (SEM) image is shown in Figure 7a). The developed nanocomposite sensor demonstrated excellent room temperature CO sensing performance. This confirmed that the existence of MoS₂ might reduce the operating temperature and energy consumption probably because of the higher specific surface area and better conductivity of MoS₂ along with the synergistic effect between ZnO and MoS₂. Ag effectively improved the catalytic activity of the sensor and mobilities of the carriers for the interaction of gas with the surface of MoS₂ or ZnO by modifying the energy band structure and surface morphology of the sensing material. Furthermore, substantial dynamic sites for CO adsorption might be generated upon Ag loading. After Ag modification, the interactions of CO molecules with O ions dramatically reduced the resistance of the Ag–ZnO/MoS₂ nanocomposites as compared to that of the ZnO/MoS₂ film, resulting in a high CO response of the sensor. A schematic of the interactions between the CO molecules and the Ag–ZnO/MoS₂ sensor is depicted in Figure 7b. Herein, Ag and Pt were employed to improve the CO sensing performance of the ZnO/MoS₂ composite sensor. The Ag-loaded sensor exhibited the maximum response as compared to that of the Pt-loaded sensor under similar experimental conditions. The higher response of the Ag-loaded sensor was explained with respect to the work function. Upon the introduction of a noble metal into ZnO/MoS₂, the electrical characteristics of ZnO/MoS₂ may be influenced by the existence of a Schottky barrier. Therefore, noble metals with lower work functions are preferred to reduce the Schottky barrier and thus improve the response of the corresponding ZnO/MoS₂ sensor. As Ag has a lower work function (4.26 eV) than that of Pt (5.65 eV), the Ag–ZnO/MoS₂-based sensor demonstrated higher CO sensing performance.

In another study, Ag-modified flower-like ZnO microspheres were prepared using a combination of solvothermal strategy and an impregnation approach [70]. Dual selective sensing of methane (CH₄) and CO using 1.5 at % Ag-loaded ZnO by monitoring the working temperature was reported in this study. Ag exhibits high catalytic activity for low-temperature oxidation of CO. However, the low-temperature oxidation of CH₄ is very difficult to achieve, as CH₄ is a thermally stable sp³-hybridized non-polar molecule. Accordingly, no considerable low-temperature response of the sensor to CH₄ was observed. In contrast, at higher temperatures, CH₄ was sufficiently activated by Ag because there was adequate thermal energy to overcome the barrier, thereby increasing the sensor response. Consequently, the sensor fabricated using 1.5 at % Ag-loaded ZnO exhibited temperature-modulated dual selectivity toward CO and CH₄ at 130 and 200 °C, respectively.

C₂H₅OH is one of the most commonly used VOCs. Exposure to C₂H₅OH vapors may cause headache, drowsiness, eye irritation, liver damage, and breathing difficulties. Furthermore, the consumption of C₂H₅OH is the leading cause of motor vehicle accidents worldwide, and alcohol-impaired driving fatalities account for 31% of the total number of traffic-related deaths in the United States [71]. Thus, the design of low-temperature operating and selective sensors for the detection and monitoring of C₂H₅OH is required from the viewpoint of safety. Graphitic carbon nitride (gC₃N₄, g-CN) is a non-metallic polymer semiconductor with a 2D layered structure. As a result of its unique physicochemical characteristics, suitable band gap, and outstanding thermal and chemical stabilities, g-CN can be utilized as an active sensing element in gas-sensing applications. In this regard, Tomer et al. [72] fabricated well-ordered mesoporous Ag-decorated meso-CNs using a hard template by nanocasting to detect C₂H₅OH at trace levels. The response of cubic meso-CN to C₂H₅OH gas was approximately two times that of conventional g-CN. This improvement was caused by the higher meso-CN surface area along with uniform pore channels through which the target gas penetrated the deep portions of the sensor. The response of the 3 wt.% Ag-decorated meso-CNs to C₂H₅OH gas was two times that of meso-CN. A schematic of the C₂H₅OH sensing mechanism of the Ag/g-CN sensor is shown in Figure 8. In this case, the electrons transferred from g-CN to Ag due to the lower work function of g-CN (4.3 eV) than that of Ag (4.7 eV), thereby creating a potential barrier. This barrier prevented the transfer of electrons through the mesoporous sensor, and the electrons existing on the sensor surface developed dynamic sites for the adsorption of O₂, thereby increasing the sensor response. Moreover, the loading of Ag NPs strongly accelerated catalytic oxidation and chemical sensitization, which ultimately enhanced the number of dynamic O species on the surface of meso-CN. In fact, the chemical sensitization effect (namely, spillover effect) of Ag NPs improved the rate of adsorption–desorption of molecular O₂ on the sensor surface and transformed molecular O₂ into O ions. Furthermore, the excellent catalytic activity of Ag NPs accelerated the decomposition of C₂H₅OH gaseous molecules into active radicals, thus enhancing the interactions between the chemisorbed O ions and C₂H₅OH molecules. Owing to the transfer of electrons from meso-CN to Ag NPs, a negatively charged layer was produced around the Ag/meso-CN interface. Under a C₂H₅OH atmosphere, the electrons returned to the sensing layer via the Ag₂O/Ag redox reaction, and thus, the areas along the Ag/meso-CN interface became very sensitive to C₂H₅OH.

NO₂ is a highly poisonous and polluting gas that produces acid rain and photochemical smog [73]. As a result of its highly toxic nature, detection and monitoring systems for NO₂ are needed. Accordingly, Wang et al. conducted NO₂ gas-sensing studies employing Ag-loaded mesoporous WO₃ designed using 3D cubic KIT-6 as a hard template [74]. Herein, Ag at different molar ratios of 0.2, 0.5, and 1.0% was loaded onto mesoporous WO₃. The sensor developed using 0.5% Ag exhibited a higher response of 44 to 1 ppm NO₂ at 75 °C because of its larger surface area and diffusion paths. NO₂ is a polar molecule having a positive charge localized on the N atom and a negative charge over the O atom. The interaction of electrons with Ag repels the O atom and attracts the positively charged N atom. Ag was used as a dynamic catalyst to create more active sites for the detection of NO₂. Although the addition of Ag may enhance the sensing properties of the material, the loading of Ag at excessively high concentrations can decrease the catalytic performance of the material. When the Ag concentration exceeded 0.5%, the total surface area reduced, and interconnected Ag NPs changed the electron path to be from Ag instead of WO₃.

The advantages of UV-activated sensors are their high stabilities, abilities to detect flammable gases, and low energy consumptions. In a typical UV-based gas-sensing device, the UV wavelength and power intensity determine the energy of the photons arriving at the exposed surface. This energy influences the steady-state surface reactions and plays a significant role in the response curve. Espid et al. [75] explained the NO₂ gas detection characteristics of Ag-loaded ZnO nanoellipsoids, which were synthesized by a simple coprecipitation method, under UV illumination. When the surface of the gas sensor was irradiated with the photons released from a UV light-emitting diode, the electrons of the sensor hopped from the valence band to the conduction band, leaving holes in the valence band. A schematic of electron mobilization and the related reactions of the Ag-loaded ZnO sensor with NO₂ gas under UV illumination is depicted in Figure 9. Photogenerated electron/hole pairs supported the direct adsorption of O₂ molecules. Under an air atmosphere, O₂ interacted with the excited electrons and adsorbed in the ionic form on the sensor surface. This slightly increased the sensor resistance. Under a NO₂ atmosphere, the gas molecules reacted with the chemisorbed oxygen ions or directly adsorbed on the sensor surface by accepting electrons. A change in the electron flow stimulated by these reactions modulated the electrical resistivity. Under continuous UV irradiation, when the NO₂ flow was stopped, the accelerated photons eliminated the NO₂ species from the sensor surface. The observed enhancement in the NO₂ detection performance of ZnO nanoellipsoids after Ag loading was associated with the synergistic effects of the semiconductor composite and Ag. Additionally, the improvement of the sensing response in the presence of Ag was linked to the increased electron utilization ratio and the decreased recombination rate of photogenerated electrons and holes due to the trapping of electrons in the Ag NPs during excitation. Similarly, the adsorption ability was directly correlated with the number of unoccupied oxygen sites. When Ag was embedded in the ZnO lattice, O-vacancies formed because of the differences between the charges of Ag⁺ and Zn²⁺ ions, which indicated that additional adsorption sites were created, resulting in a higher sensor response.

Furthermore, Zhang et al. [76] designed a room-temperature light-assisted NO₂ gas sensor using Ag-decorated ZnO NPs. The sensor developed using 3 mol.% Ag exhibited the highest NO₂ response. A schematic of the NO₂ detection mechanism of pure and Ag-loaded ZnO NPs under various light conditions is shown in Figure 10. Herein, characterization studies revealed the development of a heterojunction between Ag NPs and ZnO NPs. Nevertheless, the improved NO₂ detection performance of the sensor after Ag loading was ascribed to prominent interactions between Ag NPs and the O vacancies. In this study, Ag NPs and surface oxygen vacancies acted as electron sinks to boost charge separation and catalytic activities of Ag NPs for promoting the adsorption of gas molecules. The effective separation of photogenerated electron–hole pairs due to the interfacial charge transfer between ZnO and Ag and charge-carrier trapping through the surface oxygen vacancies enabled further accumulation of unpaired electrons on the surfaces of ZnO NPs. Moreover, Ag NPs and oxygen vacancies on the surface enhanced the adsorption of gas molecules, and thus, more oxygen molecules adsorbed on the sensor surface. An excess of Ag NPs reversibly acted as charge-carrier recombination centers, leading to negatively charged electrostatic attraction between Ag and the positively charged holes, thereby reducing the photoquantum efficiency. Additionally, sometimes, it is difficult to completely desorb the molecular or atomic oxygen species chemisorbed on the Ag surface through light irradiation at room temperature. In brief, O₂ molecules play an essential role in the absorption/desorption of NO₂ molecules on the surfaces of ZnO NPs. Specifically, the exposure degree of O⁻_2(ads) (hν) directly stimulates the sensing characteristics, such as response and response/recovery speeds, of light-assisted NO₂ sensors.

CH₃SH is a gas that is commonly used as an additive to other gases including propane and natural gas. The rotten egg odor of CH₃SH facilitates the detection of CH₃SH leakage. The recommended airborne exposure limit for CH₃SH is 0.5 ppm during an 8 h work shift. Exposure to higher levels of CH₃SH results in eye and throat irritation, drowsiness, and even bronchitis [77]. Consequently, designing a sensor that can detect CH₃SH at low levels is highly desirable. Additionally, CH₃SH gas-sensing properties of SMOs have rarely been reported. In this regard, Garcia et al. [77] constructed a sensor using mesoporous Ag-loaded hematite (α-Fe₂O₃) to detect CH₃SH at room temperature. Herein, the loading of 3 wt.% Ag onto α-Fe₂O₃ significantly enhanced the sensor response owing to the deeper electron-depletion layer. Ag loaded onto the surface of α-Fe₂O₃ served as a catalyst to accelerate the rate of conversion of O₂ into ionic O. Therefore, more electrons were trapped, which generated a thicker depletion layer. When the Ag-loaded α-Fe₂O₃ sensor was exposed to CH₃SH, the CH₃SH molecules favorably chemisorbed due to the strong affinities of thiol groups for metallic Ag. Ag loading accelerated the reaction of CH₃SH with oxygen ions via a spillover effect (Figure 11). In this case, the deep electron-depletion layer converted into a flat layer, which resulted in a better sensor response. However, when the concentration of Ag exceeded 3 wt.%, the Ag NPs generated a connected network on the surface of α-Fe₂O₃, and consequently, the resistance of the sensor reduced. Thus, the adsorption of O₂ and the gas–O₂ interaction substantially reduced, causing an inferior response of the sensor to the target gas.