Dual Effect of Ag Doping and S Vacancy on H₂ Detection for SnS₂ Based Photo-Induced Gas Sensor at Room Temperature

1. Introduction

As a carbon-neutral energy carrier with high gravimetric energy density, hydrogen has emerged as a pivotal renewable energy source for applications ranging from chemical synthesis to fuel cell technologies and industrial combustion systems.[1] Yet, the widespread adoption of hydrogen energy necessitates urgent resolution of inherent safety concerns, particularly given its hazardous characteristics including a wide explosive concentration range (4.0-75.0 vol%), high diffusion coefficient (0.61 cm²/s), and substantial combustion enthalpy (285.5 kJ/mol).[2,3,4] thus, it is of great significance to selectively detect trace hydrogen[5]. These intrinsic properties underscore the critical requirement for developing reliable detection systems capable of selective trace-level hydrogen monitoring (particularly <1000 ppm) in operational environments. Recently, Current hydrogen detection methodologies primarily employ analytical techniques such as gas chromatography (GC), semiconductor gas-sensor, and optical spectroscopy approaches[1,5] In contrast, semiconductor-based chemiresistive sensors have garnered significant research interest due to their inherent advantages of low-cost fabrication, scalable manufacturing processes, and balanced performance metrics in sensitivity-stability-selectivity.

SnS₂ materials, as a 2D layered transition metal disulfide, has been gradually regarded as promising gas-sensing materials, due to its large surface and great electrical character[6]. A series of SnS₂ based materials exhibited the good gas-sensitive property for NO₂, NH₃, Xylene, C₂H₅OH and H₂S molecules[7,8,9,10,11]. W. Gao et al. proven that Pd/SnS₂/SnO₂ sensor exhibited high response and response time to 500 ppm H₂ at 300 ^oC[4]. However, for activating the gas-detection ability of semiconductor, a high operating temperature range of 100-450 ^oC was typically required with an additional heater[12,13]. Facing with its high resistance and weak adsorption to H₂ molecule at room temperature(RT), the especial structure designing of SnS₂ surface and external non-thermal field condition played two crucial roles for solving the above issues[13,14]. Due to the excellent light absorption capacity of SnS₂, light irradiation could be an effective alternative to thermal driving to activate the adsorption behavior of gas molecules [14]. Under illumination, abundant electron-holes pairs could promote the electron transfer, further improving the gas-sensing properties. While UV illumination (λ < 400 nm) demonstrated effective photoactivation, its practical application is limited by photon-induced material degradation and potential health hazards. Recent advances in LED technology offer a safer alternative through visible light irradiation, combining sufficient photon energy for electron excitation with low power consumption[12]. Therefore, the synergistic combination of defect-engineered SnS₂ nanostructures and LED photoactivation to develop an energy-efficient H₂ sensor operating at RT.

Besides, the strategic introduction of sulfur vacancies (Vs) in SnS₂ lattices enables selective oxygen chemisorption through isovalent orbital interactions, capitalizing on the chalcogen-group electronic affinity that facilitates superoxide radical(O₂^-) formation at defect sites. Simultaneously, noble metal dopants with high work functions demonstrated superior hydrogen spillover effects, their d-orbital electron configurations enabling optimized hydrogen binding energies through charge-transfer interactions[15,16]. In the intrinsic 2D SnS₂ lattice, the Sn atom was sandwiched between two layers of S atoms maintained by van der Waals forces[17]. Doped Ag(I) impurity not only brought out the adsorbed-activate site for hydrogen, but also facilitated the formation of S vacancy due to charge and size difference between Ag(I) and Sn(IV) in SnS₂[17]. Thus, it would be of great interest to investigate the synergistic effect of S vacancy and Ag impurity on the sensing performance of Ag doped SnS₂ sensors.

In this regard, a series of Ag doped SnS₂ (Ag-SnS₂) sheet was synthesized and used as the sensing material in sensing H₂ with the help of LED light illumination. The photo-activated Ag-SnS₂ sheet showed a significantly enhanced H₂ sensing performance with wide serviceable range from 5.0 ppm to 2500 ppm at RT. Such excellent gas-sensitivity was attributed to the effect of Ag doping and light illumination. The influence of Ag doping on the surface structure was investigated by ESR and TEM. Additionally, through O₂ absorption measurement and in-situ X-ray photoelectron spectroscopy(XPS), the possible mechanism was proven.

2. Materials and Methods

The SnS₂ based sample was synthesized through the one-step solvothermal method. Initially, 2.5 mL Triton X-100, 5.0 mmol citric acid and 5.0 mmol SnCl₄·5H₂O were orderly dissolved in 30 mL of deionized water and then added into 0.15 mmol of AgNO₃ under stronger stirring conduction. The 5 mL of thioacetamide solution (2.0 mmol/mL) was slowly added into the above colloidal solution under stirring. After the ultrasonic processing for 30 min, the resulting mixture was transferred to the Teflon-lined autoclave and heated at 150 ℃ for 12 h. The Ag-SnS₂ composite was finally obtained by centrifugation and washed by water/ethanol (three times) with subsequent drying at 70 ℃. This sample was named as xAg-SnS₂, in which x was on behalf of the ideal ratio (mol/mol %) between Ag and Sn element. The pristine SnS₂ sample was prepared using the above similar method without addition of AgNO₃. The SnS₂ based gas sensors were formed and the synthesized process was detailed in supplementary materials S1. The gas-sensitive property was measured using reconstructive W30-A gas-sensitive equipment (Figure S1)[18,19]. The structural characterization of the obtained SnS₂ samples were detailed in supplementary materials S2.

3. Results

As shown in Figure 1, five obvious peaks at 15.0^o, 28.2^o, 32.1^o, 50.0^o and 52.5^o were corresponding to the characteristic peaks of (0 0 1), (1 0 0), (1 0 1), (1 1 0) and (1 1 1) plane of hexagonal-2T SnS₂ materials. With Ag addition, the intensity of one broad peak at about 22.0 ^o gradually increased, while no new peak appeared. Based on the ICP-OES result (Table S1), Ag impurity was existed in the Ag-SnS₂ sample and its content increased with increasing Ag feedstock input. In the high resolution Sn 3d spectra (Figure 1b), two stronger peaks at the binding energies of 487.2 and 495.7 eV were associated with the Sn 3d_5/2 and 3d_3/2 of Sn(IV).[20,21] As displayed in Figure 1c, two peaks at 161.9 eV and 163.1 eV was corresponding to S 2p_3/2 and 2p_1/2, respectively (Figure 1c) [22,23]. In Figure 1d, two peaks at 368.0 and 374.1 eV were ascribed to 3d_5/2 and 3d_3/2 of Ag(I) [17,20]. The peak of both Sn and S shift negatively after Ag doping, indicating change of surroundings electronegativity around atom. In ESR spectra (Figure 1e), a clear sign of S vacancy (g=2.004) appeared in the sample of Ag doped SnS₂ and its intensity enhanced with increasing Ag input[17]. Combined with XRD result, the ordered arrangement of Sn-S was destroyed by doped Ag impurity, further forming the S vacancy. As shown in Figure S2, the sheet morphology appeared in SnS₂ and 3Ag-SnS₂ samples. In Figure 1f, the lattice fringes with spacing 0.278 and 0.182 nm were corresponding to the (1 0 1) and (1 1 0) crystal faces of hexagonal-2T SnS₂. Therefore, a series of Ag doped SnS₂ sheet with S vacancy were obtained through one-step hydrothermal process.

The gas-sensitive property for H₂ detection of SnS₂ based materials was investigated. In Figure 2a, all SnS₂ materials exhibited the sensitive property for H₂ gas at room temperature, and its response under illumination was apparently higher than that in dark. With increasing Ag addition, the response value of Ag-SnS₂ sensor was gradually promoted and 3Ag-SnS₂ sample showed the highest response value for H₂ detection. Beyond the optimal value, its sign exhibited a marked decline, which was caused by the excess defect in SnS₂ surface, further decreasing the concentration of active electron. For further exploring the liner between gas concentration and sensor sign, the results(Figure 2b) manifested that, although the fast response rate of 3Ag-SnS₂ photoinduced sensor appeared, its recovery time was gradually lengthened with increasing concentration of H₂ gas. Lucky, its response and recovery time in 2500 ppm H₂ condition still kept ~4 s and ~18 s, respectively. Besides, a linear correlation (y=0.01498x+2.428, R² 0.9923) could be determined between 5 ppm to 2500 ppm of H₂ concentration. In Figure 2c, facing with multiple different target gases, the 3Ag-SnS₂ sensor exhibited the good selectivity for H₂ gas. The cycling stably of gas sensor was not ignored. As shown in Figure 2d, the photoinduced gas sensor of 3Ag-SnS₂ exhibited the stable response value of H₂ detection at room temperature in 35 days. Compared with the previous SnS₂ or SnO₂ based gas sensor as summarized in Table S2, the 3Ag-SnS₂ photo-induced sensor exhibited the good sensitivity, selectivity and stability for H₂ detection with wide concentration range(5.0 to 2500 ppm) in operating temperature of room temperature[24,25,26,27,28,29,30,31,32,33].

For exploring the reason about enhanced photo-induced gas sensitivity of Ag-SnS₂, the band structure and surface state of the obtained samples were further investigated. Firstly, the influence of Ag impurity on the band structure of SnS₂ was studied. In Figure 3a, the light-response ranges of 3Ag-SnS₂ sample showed wider than that of pristine SnS₂ and their Eg value were calculated and summarized in Table S3. Then the VB band position could be measured through VB-XPS measurement. With Ag doping, the VB value modest decreased in Figure 3b. Finally, the CB position could be calculated and the information about their band structure was summarized in Table S2. Additionally, the photoinduced current curve was showed in Figure S3. Although the photoinduced current density of Ag-SnS₂ exhibited all higher than that of SnS₂, excess Ag impurity never bring out more abundance photoinduced electrons. It was the possible reason that S vacancy was easily formed around doped Ag site due to the effect of charge balance, but superfluous vacancy made it a recombination center of photoinduced electron-holes, leading to the weak current density. This also explained that 3Ag-SnS₂ sample exhibited the better gas-sensitive property, compared with other.

In Figure 4a, all obtained samples showed the adsorptive property of O₂ gas at room temperature. It was notably that the existence of Ag impurity enhanced the O₂ adsorbing capacity of SnS₂ and its variation tendency agreed with the change of S vacancy(Figure 1e). In Figure 4b, the variation trend of H₂ adsorption property was similar to that of O₂ adsorbing. With increasing the density of Ag site on surface, the H₂ adsorbing capacity of SnS₂ was increased, which was due to the superior hydrogen-binding capability of high work function metal element. Additionally, the in situ-XPS result (Figure 4c) showed that the peak position of Ag impurity in light shifted toward high energy direction and it reversely moved after light off. It indicated that the electron in Ag site lose in light and this change of Ag atom in light/dark condition was reversible[34]. Hence, doped Ag site may play a role for surface transfer of photo-induced hole. Based on the above results, as shown in Figure 4c, the S vacancy around Ag site of surface could easily adsorbed the oxygen molecule. And then, metal Ag site could capture hydrogen molecule in air, which formed two neighboring adsorbed O₂/H₂ molecules. Simultaneously, under illumination, the photogenerated electron and hole transferred from SnS₂ to adsorbed O₂ and H₂ molecules, respectively. The superoxide radical(O₂^-) and hydrogen ion(H⁺) were formed and reacted for each other, leading to electron consumption and the change resistance of SnS₂. The possible mechanism for H₂ detection of photoinduced gas sensor was showed in Figure 4d.

4. Conclusions

In conclusion, a series of sheet-like Ag doped SnS₂ materials were prepared through the one-step hydrothermal method. The S vacancy was generated along with the Ag impurity doping, leading to enhanced O₂ adsorbed property. Compared to the pristine SnS₂, which was hard to sense H₂ at RT, the Ag-SnS₂ sample exhibited an excellent gas-sensitive property for H₂ detection with wide concentration range from 5.0 ppm to 2500 ppm under blue LED illumination. Facing with the high concentration condition of H₂ gas (2500 ppm), its response and recovery time still kept ~4 s and ~18 s, respectively. Moreover, the sensor maintained the gas response after five weeks of relaxation. The synergistic effect of S vacancy and Ag impurity on enhanced O₂/H₂ adsorbed property has been proven. The Ag impurity site played a key role for photo-induced hole transfer to active adsorbed hydrogen on SnS₂ surface under illumination. On account of remarkable gas sensing performances, the formation of Ag doped SnS₂ sheet contributed to enhance the potential and fueled the exploitation of next-generation high-performance gas sensing light modulated devices at RT.