Room Temperature Sub-ppm NO₂ Gas Sensor Based on Ag/SnS₂ Heterojunction Driven by Visible Light

Abstract

As industrial waste gas, nitrogen dioxide (NO₂) is a serious hazard to air pollution and human health, and there is a pressing demand for developing high-performance NO₂ gas sensors. Tin disulfide (SnS₂), a representative two-dimensional metal sulfide characterized by a significant specific surface area, a suitable electron band gap, and an easily tunable layered structure, shows a broad application prospect in gas sensing applications. Nevertheless, SnS₂-based gas sensors suffer from poor sensitivity, which seriously hinders their application in room temperature gas sensing. In this study, Ag/SnS₂ heterojunction nanomaterials were synthesized by an in situ reduction approach. The findings reveals that the gas-sensitive response of the Ag/SnS₂ nanocomposites at room temperature under visible light irradiation can achieve 10.5 to 1 ppm NO₂, with a detection limit as low as 200 ppb, which realizes the room-temperature detection of Sub-ppm NO₂. Meanwhile, the sensor exhibits good selectivity, reproducibility (cyclic stability > 95%). The improved gas sensitivity of the Ag/SnS₂ sensor can be due to the synergistic effect of the carrier separation at the Ag/SnS₂ Schottky junction and the localized surface plasmon resonance (LSPR) of Ag nanoparticles. The LSPR effect significantly enhances light absorption and surface-active site density, facilitating trace NO₂ detection at room temperature. This study provides the foundation for the subsequent development of room temperature layered metal sulfide gas sensors.

1. Introduction

Nitrogen dioxide (NO₂), a noxious and hazardous gas, is prevalent in the environment, mainly stemming from the burning of fossil fuels, like oil, natural gas, and coal, as well as from industrial emissions [1,2]. The 2018 World Health Organization report identified NO₂ pollution as a significant threat to human health. When the human body inhales NO₂, the gas chemically reacts with tracheal tissue fluid, resulting in damage and corrosion of surface cells [3]. According to the American Industrial Hygiene Association, the safety threshold for long-term human exposure (LTEV) to NO₂ is 3 ppm and the short-term exposure threshold (STEV) is 5 ppm [4]. Prolonged exposure to NO₂ can strongly irritate the eyes, nose, throat and other parts of the human body, with symptoms such as headaches and pulmonary edema [5,6]. In the environment, excess NO₂ can undergo chemical reactions with atmospheric water and oxygen to generate nitric acid, forming nitric acid rain. This acidic precipitation not only erodes soil and damages plant growth but also causes irreversible damage to buildings [7]. Furthermore, the concentration of NO₂ in human respiratory gases can be analyzed to assist in the of intestinal and pulmonary diseases. Therefore, the accurate detection and monitoring of NO₂ concentrations is of critical importance for public health and environmental preservation.

The predominant NO₂ sensors commercially are traditional resistive gas sensors utilizing metal oxide semiconductors as the gas sensitive material. Metal oxide semiconductors typically require operation at temperature exceeding 200 °C [8,9,10]. Prolonged exposure to such high temperature may induce thermal growth of oxide grains, compromising the sensor’s durability. In addition, operating at high temperatures will elevate the sensor’s energy consumption, challenging its integration into portable devices like smart bracelets and mobile devices [11]. Consequently, developing novel gas-sensitive materials capable of detecting NO₂ at low or even room temperature has become a critical research focus in advancing NO₂ sensor technology. Compared with metal oxide semiconductors, layered metal sulfides typically exhibit effective gas sensing capabilities at lower temperatures, making them promising candidates for NO₂ detection at low temperature [8,11,12]. Tin disulfide (SnS₂), a typical metal sulfide, exhibits a distinctive two-dimensional layered structure that confers a substantial specific surface area, providing more active sites for the adsorption of NO₂ gas molecules. Compare with metal oxides, SnS₂ has a larger lattice spacing of 0.59 nm, allowing gas molecules to adsorb and desorb on the surface and inside active lattice sites [13,14]. Furthermore, SnS₂ exhibits higher carrier mobility, which can effectively mitigate charge accumulation during the gas-sensing process, enhancing the response of the sensor [15]. Previous studies have shown the potential of SnS₂ in low-temperature NO₂ detection. But pristine two-dimensional SnS₂-based gas sensors still face certain challenges, including high baseline resistance, and inadequate sensitivity at low temperatures, potentially constraining their practical utility in gas sensing applications [16]. Numerous strategies have been explored to improve the NO₂ sensing capabilities of SnS₂ gas sensors, including morphological tuning, heterojunction construction, and heteroatom doping [17,18,19,20,21]. Most of the approaches focus on enhancing the performance of SnS₂-based gas sensors by augmenting active sites through surface regulation.

Research demonstrates that noble metal sensitization has emerged an effective method to enhance gas sensing capabilities [22,23]. Noble metals exhibit superior catalytic properties and conductivity, which markedly improve the conductivity of gas-sensitive materials and reduce sensors’ baseline resistance. Furthermore, noble metals can induce a localized surface plasmon resonance (LSPR) effect under light excitation, and boost light absorption of sensitive materials. This effect facilitates redox reactions between gas molecules and sensitive materials, lowers the working temperature of sensors, and improves gas sensitive performance [24]. In our previous study, Au/SnS₂ nanoflowers enhanced the gas sensitivity to 8 ppm NO₂ at 100 °C by 1.5 times compared to the unmodified SnS₂ [25]. Wang et al. achieved a 1.8-fole increase in gas sensitivity to 2 ppm NO₂ at 100 °C by depositing Pt nanoparticles on SnS₂ microsphere via an in situ reduction method [26]. Similarly, Guo et al. synthesized Pt/SnS₂ nanoflowers using a hydrothermal method, which lowered the optimal operating temperature by 40 °C and improved the response to 30 ppm n-butanol by 3.4 times compared to pure SnS₂ [27]. The results indicate that while noble metal modification can enhance the response of gas -sensitive materials, it still relies on high operating temperature, limiting its applicability for room temperature detection.

Recently, photoexcitation, as an alternative to thermal excitation, has become a feasible way to improve the operating characteristics of gas-sensitive materials at room temperature [16,28]. By irradiating the sensitive materials with light of a specific wavelength, photo-generated carriers alter the semiconductor surface’s electronic properties, thereby optimizing the gas sensitivity [29,30]. Hu et al. demonstrated that compared with pure SnS₂ nanosheets, the prepared Au/SnS₂ nanosheets improved the selectivity of sensors by 130% and exhibited good repeatability under 420 nm light-assisted conditions [31]. Compared to Pt, Au, and Pd, Ag offers the benefits of lower cost and notable improvement of photoelectric performance, making it a common choice for modifying sensitive materials [32,33,34,35]. In photocatalysis, the surface plasmon resonance effect of Ag is more closely aligned with the optical absorption characteristics of SnS₂ within a specific wavelength range, effectively expanding the optical response range and promoting the efficient separation and movement of photoinduced charge carriers [24]. Therefore, the synergistic effect of Ag nanoparticle modification and photoexcitation is expected to be a feasible strategy for the NO₂ detection of SnS₂ nanomaterials at room temperature.

In this work, Ag/SnS₂ nanosheets were successfully synthesized via the in situ reduction method. A room-temperature, visible light-assisted Ag/SnS₂ gas sensor was developed by combining the electron and chemical properties of Ag with LSPR effect of metal nanoparticles. The fabricated sensor exhibits remarkable sensitivity down to 1 ppm NO₂, with a substantially lower baseline resistance compared to pure SnS₂. It achieves a response of 10.5 to 1 ppm NO₂ and has a detection limit of 200 ppb. The sensor also demonstrates excellent selectivity and long-term stability. Furthermore, the mechanisms of electronic and chemical sensitization, and the LSPR effects of Ag nanoparticles under visible light illumination were discussed in detail.

2. Materials and Methods

2.1. Materials

All chemical reagents were analytical grade without further purification. Thiourea, tin tetrachloride, silver nitrate, trisodium lemon and acetone were purchased from Sinopharm Chemical Reagent Co., Shanghai, China. Ammonia, formaldehyde and benzene were obtained from Aladdin Reagent Co., Shanghai, China. Xylene was provided by Shanghai Lingfeng Chemical Reagent Co., Shanghai, China. NO₂ gas was acquired from Changzhou Huayang Gas Co., Ltd., Changzhou, China.

2.1.1. Synthesis

SnS₂ nanosheets were produced using a hydrothermal route, as detailed in our previous work [36]. A precursor solution was prepared by dissolving SnCl₄·5H₂O and thiourea in a 1:3 molar ratio in 30 mL of deionized water under vigorous stirring. The precursor solution was then subjected to a 16 h hydrothermal treatment at 180 °C in a 50 mL reactor. The resulting product was subsequently washed with deionized water and ethanol to eliminate contaminants, followed by vacuum drying at 60 °C for 12 h to yield yellow SnS₂ powder.

The synthesis of Ag/SnS₂ composite material was carried out as follows. 200 mg of pre-synthesized SnS₂ was dispersed in 80 mL of deionized water and sonicated to achieve a homogeneous yellow suspension. To the suspension, 6 mL of 0.1 mol/L AgNO₂ solution was gradually introduced under continuous stirring for 30 min, forming a pale-yellow solution. Subsequently, 2 mL of 0.01 mol/L trisodium citrate solution was introduced in a dropwise manner, followed by stirring for an additional 10 min. The mixture was then transferred to a 60 °C water bath and stirred for 20 min. Upon completion, the composite material was washed alternately with deionized water and ethanol, and vacuum-dried at 60 °C for 12 h to obtain the Ag/SnS₂ powder. The samples were named Ag/SnS₂-1, Ag/SnS₂-2, Ag/SnS₂-3, and Ag/SnS₂-4, corresponding to the addition of 2 mL, 6 mL, 12 mL, and 18 mL of AgNO₂ solution, respectively. Figure 1 illustrates the synthetic procedure.

2.1.2. Material Characteristics

The crystallinity and phase formation of the samples were examined using Cu Kα radiation in the 10–70° range with a Philips Xpert X-ray diffractometer (XRD) (Malvern Panalytical, Almelo, The Netherlands). For microstructural analysis, Feisirion 200 field emission scanning electron microscope (FSEM) (Thermo Fisher Scientific, Eindhoven, The Netherlands) and FEI Tecnai G2 F20 transmission electron microscope (TEM) (Thermo Fisher Scientific) were utilized to investigate the sample morphologies and elemental distributions. Surface electronic states and chemical compositions were investigated via X-ray photoelectron spectroscopy (XPS, AXIS-ULTRA spectrometer, Al Kα radiation). The binding energies of all elements were calibrated using the C1s peak at approximately 284.8 eV. Furthermore, the absorption spectrum was recorded with a Shimadzu UV-2600 spectrophotometer (Shimadzu Corporation, Kyoto, Japa).

2.2. Sensor Fabrication

The sensor featured an Al₂O₃ substrate with an effective area of 2.0 cm × 1.0 cm, provided by Changzhou Xintu Technology Co., Ltd. (Changzhou, China). The interdigital electrodes (IDEs) consisted of 8 pairs of Au electrodes with an 10 μm interspacing gap. To activate the sensor, 10 mg of the as-prepared material was uniformly dispersed in 0.5 mL of ethanol to achieve a homogeneous mixture. Subsequently, 20 μL of the dispersion solution was drop-coated onto the sensor’s center, creating a sensitive film on the interdigital electrodes. Lastly, the fabricated sensor was transferred to a drying oven for overnight drying at 60 °C to solidify the sensitive material.

2.3. Sensor Measurement and Testing Methodology

Gas sensing performances were assessed using a self-assembled dynamic test system. A digital multimeter (Keysight 3445A, Keysight Technologies, Santa Rosa, CA, USA) in a two-wire configuration measured the sensor’s dynamic resistance. Throughout the testing procedure, the sensor operated under photo-excitation at room temperature, maintaining a constant LED lamp distance of 0.5 cm above the sensor. The LED light sources emitted wavelengths of red (620 nm), yellow (590 nm), green (530 nm) and blue (460 nm), with light intensity measurements conducted using a photometer. The response of a gas sensor is defined in the following manner:

$$\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{e}={\displaystyle \frac{R_g}{R_a}} (\mathrm{F}\mathrm{o}\mathrm{r} \mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{z}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{g}\mathrm{a}\mathrm{s}\mathrm{e}\mathrm{s},\mathrm{e}.\mathrm{g}.,{\mathrm{N}\mathrm{O}}_2)$$

(1)

$$\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{e}={\displaystyle \frac{R_a}{R_g}}(\mathrm{F}\mathrm{o}\mathrm{r} \mathrm{r}\mathrm{e}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{g}\mathrm{a}\mathrm{s}\mathrm{e}\mathrm{s},\mathrm{e}.\mathrm{g}.,{\mathrm{N}\mathrm{H}}_3,\mathrm{a}\mathrm{c}\mathrm{e}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{e})$$

(2)

where $R_a$ represents the baseline resistance in air, and $R_g$ denotes the steady-state resistance in the target gas, respectively.

The response time and recovery time were calculated as the duration needed for the sensor’s resistance to achieve 90% of the total change in resistance.

Throughout the long-term stability evaluation, the same sensor was used and stored in ambient air to closely simulate real-world operating conditions. No pre-treatment or regeneration was performed prior to each measurement cycle. The gas-sensing performance was measured periodically to systematically assess the sensor’s stability and reliability over extended use.

3. Results

3.1. Structural and Morphological Characterization

X-ray diffraction (XRD) analysis, depicted in Figure 2, was employed to investigate the crystal structure and composition of the sensitive material. The XRD spectrum reveals distinct diffraction peaks at 15.1°, 28.2°, 32.2°, and 49.9°, which correspond to the (001), (100), (101), and (110) crystal planes of SnS₂ (JCPDS: 23-0677), respectively. The spectra of Ag/SnS₂ samples reveal no discernible Ag-related peaks apart from the diffraction peak corresponding to pure SnS₂. It could be ascribed to the low Ag concentration and the limited diffraction intensity present in the samples under investigation. Figure 2b shows a slight shift in the diffraction peak at 15.1° in Ag/SnS₂ samples compared with pure SnS₂. As the Ag doping level increases, the peak gradually shifts toward lower angles, indicating lattice expansion of the SnS₂ crystal. This expansion may result from local tensile stress induced by Ag doping or from interstitial incorporation of Ag atoms into the SnS₂ lattice [33]. It suggests the possible formation of a metal–semiconductor Schottky heterojunction between Ag and SnS₂. Notably, the XRD spectrum does not exhibit any discernible impurity peaks, confirming the absence of extraneous impurities in the synthesized sample.

The surface morphologies of the as-prepared SnS₂ and Ag/SnS₂-2 were characterized by SEM and TEM measurement in Figure 3 and Figure 4. Figure 3a presents the SEM image of the synthesized SnS₂, revealing a nanosheet structure with a characteristic size of approximately 1 μm. Figure 3b shows the morphology of SnS₂ after surface modification with Ag nanoparticles. The micrograph indicates that the overall morphology remains largely unchanged following the surface functionalization, retaining its nanoflake structure. TEM was employed to characterize the internal electronic structures of as-synthesized SnS₂ and Ag/SnS₂ composites. As depicted in Figure 4a,b, the SnS₂ material displays a distinct layered morphology, aligning with SEM findings. High-resolution TEM images of SnS₂ (Figure 4c) reveals lattice fringes of 0.18 nm and 0.32 nm, which correspond, respectively, to (110) and (100) planes of SnS₂ (JCPDS No.22-0951). Figure 4d–f are TEM and HRTEM images of Ag/SnS₂ composite. Figure 4d illustrates that the modification of Ag does not alter the morphology of SnS₂, with the Ag/SnS₂ composite remains the nanosheets nanostructure. In Figure 4e, it is evident reveals that Ag nanoparticles preferentially nucleate and grow at the edges of SnS₂ nanosheets. It may be ascribed to the presence of high-energy defects along the edges of the SnS₂ nanosheets, which serve as preferential nucleation sites for the Ag nanoparticles. Figure 4f further illustrates that the Ag nanoparticles (highlighted by yellow circles) have an average size of around 3 nm and a lattice spacing of 0.238 nm, corresponding to the (111) crystal plane of Ag. The EDS spectrum of the Ag/SnS₂ heterostructure (Figure 4g,h) confirms the presence of Sn, S, and Ag elements, with the Ag weight percentage estimated to be around 2.79%. Additionally, the atomic ratio of S to Sn is determined to be 1.60:1.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition and elemental states of the SnS₂ and Ag/SnS₂-2 heterostructure. The C1s peak at 284.8 eV served as a reference for peak calibration. The high-resolution spectrum of Sn 3d in Figure 5a exhibits distinct peaks at 486.5 and 495 eV, corresponding to Sn 3d_3/2 and Sn 3d_5/2 states of Sn⁴⁺, respectively [25,37]. Notably, with the modification of Ag, the two characteristic peaks undergo a noticeable shift towards the high binding energies. This shift is indicative of changes in the chemical environment of tin ions, possibly attributed to the integration of Ag into the SnS₂ lattice. Furthermore, the high-resolution S 2p spectrum in Figure 5b reveals peaks at 163.14 eV (S 2p_1/2) and 161.91 eV (S 2p_3/2), which also experience a shift towards the lower energies upon Ag doping [38]. This phenomenon suggests a modification in the electronic structure of sulfur ions, likely influenced by the presence of Ag within the SnS₂ [39]. The XPS peaks corresponding to tin (Sn) and sulfur (S) suggest that the modification of Ag nanoparticles changes the binding energies of Sn and S. This modification induces surface detects, thereby promoting the formation of sulfur vacancies. Figure 5c presents the high-resolution Ag 3d spectrum, revealing prominent peaks at 368.1 eV and 374.1 eV, which are attributed to the binding energies of Ag 3d_3/2 and Ag 3d_5/2, respectively [33,40]. The observed 6.0 eV energy splitting between these peaks is characteristic of metallic Ag, indicating the successful incorporation of Ag species within the Ag/SnS₂-2 composites [41].

The optical properties of SnS₂ and Ag/SnS₂ were investigated through UV-vis spectroscopy. For comparative analysis, the absorption spectra of both SnS₂ and Ag/SnS₂ were normalized, as depicted in Figure 6a. As shown in the set of Figure 6a, the pure SnS₂ sample exhibits a broad absorption range from 400 nm to 700 nm in the visible light spectrum. Benefiting from the local surface plasmon resonance (LSPR) of the noble metal Ag, the Ag/SnS₂ sample exhibits enhanced light absorption across nearly the entire visible spectrum, with peak enhancement around 530 nm. The band gaps (E_g) of the samples were determined using the Tauc method, and the corresponding Tauc plots are presented in Figure 6b. As shown, the band gaps of SnS₂ and Ag/SnS₂ were 2.38 eV and 2.28 eV, respectively. The band gap of SnS₂ decreased after surface modification with the noble metal Ag, which can be attributed to the strong electronic interaction between Ag nanoparticles and SnS₂ nanosheets (known as strong metal–support interaction) [42]. The narrower bandgap of the Ag/SnS₂ composite facilitates the generation of a greater number of photo-induced charge carriers under illumination [43]. Efficient light absorption is essential for enhancing the gas-sensing capabilities of sensor devices, suggesting that the gas response of the Ag/SnS₂ composite can be effectively modulated under photoexcitation.

3.2. Gas Sensing Performances

The influence of Ag nanoparticle doping concentration on the NO₂ gas-sensing performance of Ag/SnS₂ sensors was investigated. Figure 7a,b present the gas response curves and dynamic resistance curves of Ag-doped Ag/SnS₂ gas sensors exposed to 1 ppm NO₂ under green light illumination at ambient temperature. Figure 7a demonstrates that Ag/SnS₂-x gas sensors exhibit enhanced NO₂ sensing responses compared to pure SnS₂ sensors. The gas-sensing response initially increased with the AgNO₃ content, reaching a maximum response at an AgNO₃ addition of 6 mL, which identifies Ag/SnS₂-2 as the optimal sensor. The sensor exhibits an response of 10.5 to 1 ppm NO₂, with its response time and recovery time being 135 s and 267 s, respectively. However, further increases in AgNO₃ content led to a decline in gas-sensing performance. The gas response of Ag/SnS₂-4 is lower than that of the pristine SnS₂ sensor. This reduction can be attributed to the fact that noble metal Ag itself lacks intrinsic gas-sensing activity. With increasing Ag loading, excessive Ag nanoparticles tend to agglomerate on the SnS₂ surface, occupying and reducing the number of active adsorption sites available for gas molecules. Furthermore, an excessive amount of Ag may modify the band structure and surface charge distribution of the composite, thereby weakening the synergistic contributions from electronic sensitization and the LSPR effect. The gas-sensing response of the Ag/SnS₂-2 sensor to 1 ppm NO₂ was 2.3 times greater than that of the pure SnS₂ sensor. Subsequent gas-sensing tests were conducted using the Ag/SnS₂-2 gas sensor. As shown in Figure 7b, the baseline resistance of the Ag/SnS₂ sensor decreased with increasing AgNO₃ content. Compared to the pure SnS₂ sensor, the baseline resistance of the Ag/SnS₂-2 sensor decreased by an order of magnitude. The work function difference between Ag and SnS₂ facilitates significant electron transfer, altering the electronic properties of SnS₂ and lowering the sensor’s initial resistance. The resistance curves reveal that both pure SnS₂ and Ag-doped SnS₂-X sensing materials exhibit a marked increase in resistance upon exposure to 1 ppm NO₂, followed by a decrease when exposed to air. This behavior confirms that the samples retain n-type semiconductor properties even after doping with Ag nanoparticles.

UV-vis analysis indicates that SnS₂ and Ag/SnS₂ effectively absorb visible light. The gas-sensing performance of the Ag/SnS₂-2 sensor for 1 ppm NO₂ was investigated under various visible light conditions, as shown in Figure 7c. The Ag/SnS₂ gas sensor exhibited the highest response to 1 ppm NO₂ under 530 nm light illumination, which may be attributed to its strongest light absorption at this wavelength. In contrast, under dark conditions, the sensor showed a significantly higher initial resistance and baseline drift. Its response to 1 ppm NO₂ was only 3.5, markedly lower than that under visible light excitation, and the recovery process was substantially prolonged, preventing complete recovery (Figure S1). These results indicate that visible light illumination can effectively enhance the room-temperature gas-sensing performance of the Ag/SnS₂ sensor toward low-concentration NO₂. Light intensity significantly influences the gas-sensing performance of sensors. To determine the optimal optical power, the response of the Ag/SnS₂ gas sensor to 1 ppm NO₂ was examined at varying optical powers of 530 nm, as illustrated in Figure 7d. It reveals that the sensor’s response initially increases and subsequently decreases with rising optical power. The maximum response at an optical power of 10 mW/cm² is attributed to the synergistic contribution of photoexcited nonequilibrium carriers and localized surface plasmon resonance (LSPR) on Ag nanoparticles, which facilitate adsorption and desorption of gas molecules (O₂, NO₂) in different ways. At low light intensities, gas adsorption onto the sensitive material dominates the gas-sensing reaction, thereby enhancing the sensor’s response. As the light intensity increases, the desorption process becomes the predominant factor in the gas-sensing reaction, leading to a decrease in the sensor’s response. In subsequent experiments, a light source with a wavelength of 530 nm and an intensity of 10 mW/cm² was identified as optimal light source for the Ag/SnS₂ sensor.

Figure 8a,b present the gas sensing properties of the Ag/SnS₂-2 sensor to NO₂ concentrations ranging from 200 ppb to 1 ppm under 530 nm light excitation at room temperature. Upon exposure to 0.2 ppm NO₂, the sensor exhibits an obvious change in resistance, achieving a response value of approximately 2.0, suggesting that the sensor exhibits a practical detection limit of 200 ppb (Figure S2). Figure 8b shows the sensor response increases proportionally with NO₂ concentration in the range of 0.2–1 ppm, with values of 2.1, 3.1, 4.5, 6.7, 5.4, 6.7, 7.8, 8.7, 8.9, and 10.2. This demonstrates a linear correlation between the Ag/SnS₂ sensor’s response and NO₂ concentration, described by the equation:

$$y=0.23+9.3x$$

(3)

Here, y denotes the device response, and x represents the NO₂ concentration (ppm).

Figure 8. (a) Dynamic sensing response curve and (b) Linear fit of the response of the Ag/SnS₂-2 sensor exposed to 0.2–1 ppm NO₂ under 530 nm light excitation at room temperature, (c) Response curve of the Ag/SnS₂-2 sensor for 10 cycles to 1 ppm NO₂ under 530 nm light excitation at RT, (d) Selectivity of Ag/SnS₂-2 sensor to 1 ppm NO₂ and other gas at 10 ppm, and (e) Long-term stability of Ag/SnS₂-2 sensor toward 1 ppm NO₂ at RT for 90 days.

Figure 8. (a) Dynamic sensing response curve and (b) Linear fit of the response of the Ag/SnS₂-2 sensor exposed to 0.2–1 ppm NO₂ under 530 nm light excitation at room temperature, (c) Response curve of the Ag/SnS₂-2 sensor for 10 cycles to 1 ppm NO₂ under 530 nm light excitation at RT, (d) Selectivity of Ag/SnS₂-2 sensor to 1 ppm NO₂ and other gas at 10 ppm, and (e) Long-term stability of Ag/SnS₂-2 sensor toward 1 ppm NO₂ at RT for 90 days.

With the correlation coefficient R² of 0.98832 (Inset of Figure 8b), the sensor indicates a better linearity at low NO₂ concentration. The theoretical detection limit is estimated to be 23 ppb by the least-squares method, indicating it has great application potential in trace gas detection [44].

Figure 8c shows the sensor’s gas sensing properties to 1 ppm NO₂ over 10-cycles gas test under 530 nm light excitation at room temperature. The consistent sensor response across all 10 cycles indicates the Ag/SnS₂ sensor possesses outstanding repeatability for NO₂ detection at ambient conditions. Selectivity, a crucial performance indicator for gas sensor performance, was evaluated for the Ag/SnS₂-2 gas sensor under 530 nm light excitation with various gases, as shown in Figure 8d. The potential interfering gases included Cl₂, O₃, ethanol (C₂H₅OH), acetone (CH₃COCH₃), ammonia (NH₃), benzene (C₆H₆), formaldehyde (CH₂O) and methanol (CH₃OH). It can be observed that the sensor’s response to NO₂ was markedly higher than to other gases at room temperature, which is due to SnS₂’s unique surface affinity for NO₂ molecules. The findings indicate the sensor’s potential for efficient room-temperature detection of NO₂. To assess the long-term stability of the Ag/SnS₂-2 sensor, a 3-month gas sensitivity test was conducted, as depicted in Figure 8e. The results indicated an 18% decrease in response value over the testing period, likely attributed to the gradual oxidation of Ag particles on the SnS₂ surface forming Ag₂O, which diminished the sensor’s gas sensitivity performance. Nevertheless, the Ag/SnS₂ composite remains an attractive gas-sensitive material for the development of room-temperature NO₂ sensors, exhibiting favorable characteristics such as high sensitivity, repeatability, and selectivity. A comparison of the gas-sensing performance of SnS₂-based sensors reported in the recent literature is presented in

Table 1. Companion NO₂ sensing properties of the Ag/SnS₂ sensor with other SnS₂-based gas sensors in the literature.

Gas Sensing Material	Operation Condition	NO2 (ppm)	Response (Rg/Ra)	Ref.
SnS2/graphene	RT	1	1.75	[45]
SnS2/SnSe2 heterostructures	RT	2	6.99	[46]
Mo-SnS2	150 °C	100	1.74	[47]
Ag-SnS2 nanoflowers	100 °C	5	8.37	[33]
Au/SnS2 heterostructures	RT 420 nm	1	3.10	[31]
Ag/SnS2 nanosheet	RT 530 nm	1 0.2	10.5 1.8	This work

RT = Room temperature.

. This comparison suggests that Ag/SnS₂-based sensors can effectively detect NO₂ gas at room temperature under the influence of visible light illumination.

3.3. Sensing Mechanism

As a typical n-type semiconductor metal sulfide, the sensing mechanism of the SnS₂-based gas sensor is intricately linked to the electron interaction between the target gas and the material surface during the sensing process [19,48,49]. When exposed to air, oxygen molecules adsorb on the surface of the material’s surface, capturing electrons in its conduction band to create chemisorbed oxygen (${\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}$) (≤100 °C). It results in a reduction in the free electron concentration on the material’s surface, leading to a substantial increase in sensor resistance. The specific redox reaction process can be represented as follows [50]:

$${\mathrm{O}}_{2\left(\mathrm{g}\mathrm{a}\mathrm{s}\right)}\to {\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}$$

(4)

$${\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}+{\mathrm{e}}^{-}\to {\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}(\le 100 °\mathrm{C})$$

(5)

When exposed to the oxidizing gas NO₂, as illustrated in Equation (6), NO₂ gas molecules capture electrons from the SnS₂ conduction band. It results in a reduction in the majority carrier (electron) concentration of SnS₂, a decrease in conductivity, and an increase in resistance [25].

$${\mathrm{N}\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}+{\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}$$

(6)

Even introducing the Ag modification, the sensor retains an n-type response to NO₂, indicating that the gas sensitivity is still attributed to SnS₂. Due to the disparity in work function between Ag and SnS₂, electron transfer occurs between the metal Ag and the SnS₂ sensitive material. As the lower work function of SnS₂ (4.4 eV) compared to that of Ag (4.74 eV), electrons move from SnS₂ to Ag until their Fermi levels align, establishing a Schottky barrier at the interface (Figure 9a,b) [34]. When exposed to NO₂ gas, the gas-sensitive material adsorbs NO₂ gas molecules, resulting in an increased potential barrier width at the interface, and amplifying the resistance change in the sensor [13,51]. Additionally, due to the spillover effect and catalysis sensitization of noble metals, the incorporation of Ag nanoparticles effectively increases the concentration of surface-active sites [22]. This enhancement promotes the adsorption of gas molecules, thereby further improving the gas-sensing performance of the device [33,34,39].

The NO₂ sensing mechanism of the Ag/SnS₂ heterojunction in both dark and under light excitation is illustrated in Figure 10. In the dark, oxygen molecules extract electrons from SnS₂, adsorbing on its surface as oxygen ions (${\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}$). These oxygen ions occupy numerous active sites on the gas materials’ surface (Equation (5)). Upon excitation with visible light at 530 nm wavelengths (photoelectron energy ≈ 2.34 eV > E_g = 2.28 eV), a large number of photogenerated electron-hole pairs are generated within the Ag/SnS₂ (Equation (7)) [28]. Due to the surface cleaning effect of the photogenerated holes ($h_{hv}^{+}$), $h_{hv}^{+}$ interact with the adsorbed oxygen ions (${\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}$), facilitating the desorption of the oxygen (Equation (8)). This process decreases the amount of adsorbed oxygen on the surface, increasing the number of active sites and enhancing the sensor’s response to NO₂ [16]. On the other hand, the localized surface plasmon resonance (LSPR) effect of Ag nanoparticles enhances the visible light absorption of SnS₂, increasing photogenerated carrier concentration. Simultaneously, hot electrons ($e_{LSPR}^{-}$) produced by LSPR excitation are transferred to the surface of SnS₂, further elevating electron concentration (Figure 9c) [52]. Both hot electrons ($e_{LSPR}^{-}$) and photogenerated electrons ($e_{hv}^{-}$) directly interact with NO₂ gas molecules, as depicted in Equations (3) and (4), thereby enhancing the Ag/SnS₂ gas-sensing performance [24].

$$hv\to h_{hv}^{+}+e_{hv}^{-}$$

(7)

$$h_{hv}^{+}+{\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}\to {\mathrm{O}}_{2\left(\mathrm{g}\mathrm{a}\mathrm{s}\right)}$$

(8)

$${\mathrm{N}\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}+e_{LSPR}^{-}\to {\mathrm{N}\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}$$

(9)

$${\mathrm{N}\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}+e_{hv}^{-}\to {\mathrm{N}\mathrm{O}}_{2\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)}^{-}$$

(10)

4. Conclusions

The present study reports the successful synthesis of Ag-doped SnS₂ nanomaterials via an in situ reduction method. The gas-sensing performance of Ag/SnS₂-based sensors towards NO₂ was systematically investigated under the combined influence of visible light excitation and the incorporation of noble metal doping. The gas-sensitive response of Ag/SnS₂ composites was significantly influenced by the Ag doping concentration and the optical wavelength. At a Ag content of 2.79% by weight, the Ag/SnS₂ nanocomposite exhibited the highest gas-sensitive response to 1 ppm NO₂ under 530 nm illumination at room temperature, with a response 2.3 times greater than that of the pure material. Furthermore, the sensor demonstrated a detection limit as low as 200 ppb, indicating its ability to detect sub-ppm levels of NO₂ at room temperature. The Ag/SnS₂ sensor exhibited an 18% decrease in response after a three-month stability test, likely due to the oxidation of Ag nanoparticles in air. Despite this decline, the sensor maintains excellent repeatability, linearity, and high selectivity towards various interfering gases, suggesting its suitability for detecting low concentrations of NO₂ at room temperature. The Ag/SnS₂ sensor’s superior sensing performance at room temperature is ascribed to the synergistic effects of visible light excitation, noble metal sensitization, and the localized surface plasmon resonance (LSPR) effect. The visible light-assisted Ag/SnS₂ gas sensor presented in this study provides a novel technological approach and material solution for the efficient detection of sub-ppm NO₂ gas at room temperature.