Highly Selective and Fast Response/Recovery Cataluminescence Sensor Based on SnO2 for H2S Detection

In the present work, three kinds of nanosized SnO2 samples were successfully synthesized via a hydrothermal method with subsequent calcination at temperatures of 500 °C, 600 °C, and 700 °C. The morphology and structure of the as-prepared samples were characterized using X-ray diffraction, transmission electron microscopy, selected area electron diffraction, Brunauer–Emmett–Teller analysis, and X-ray photoelectron spectroscopy. The results clearly indicated that the SnO2 sample calcined at 600 °C had a higher amount of chemisorbed oxygen than the SnO2 samples calcined at 500 °C and 700 °C. Gas sensing investigations revealed that the cataluminescence (CTL) sensors based on the three SnO2 samples all exhibited high selectivity toward H2S, but the sensor based on SnO2−600 °C exhibited the highest response under the same conditions. At an operating temperature of 210 °C, the SnO2−600 °C sensor showed a good linear response to H2S in the concentration range of 20–420 ppm, with a detection limit of 8 ppm. The response and recovery times were 3.5 s/1.5 s for H2S gas within the linear range. The study on the sensing mechanism indicated that H2S was oxidized into excited states of SO2 by chemisorbed oxygen on the SnO2 surface, which was mainly responsible for CTL emission. The chemisorbed oxygen played an important role in the oxidation of H2S, and, as such, the reason for the SnO2−600 °C sensor showing the highest response could be ascribed to the highest amount of chemisorbed oxygen on its surface. The proposed SnO2-based gas sensor has great potential for the rapid monitoring of H2S.


Introduction
Hydrogen sulfide (H 2 S) is a noxious gas that smells like rotten eggs [1][2][3].Even exposure to a low concentration of H 2 S can cause serious harmful effects, including damage to the respiratory and central nervous systems [4,5].Moreover, death occurs immediately upon exposure to a high concentration of H 2 S. The threshold limit for an 8 h exposure to H 2 S is 10 ppm, which was established by the Health and Safety Executive [6].In addition, H 2 S is also a flammable compound that can cause fires and explosions when exposed to a flame or high temperatures.However, what is more worrying is that H 2 S is found both in the natural environment and in a variety of production processes, such as oil extraction and metal smelting.In other words, H 2 S is present in the human living environment.Therefore, in order to prevent harm from H 2 S, there is an urgent need to develop high-performance gas sensors for the timely detection of H 2 S.
Cataluminescence (CTL) a kind of in situ chemiluminescence, which occurs through catalytic oxidation reactions at gas-solid interfaces [7,8].On account of its numerous advantages, such as a rapid response, extraordinary sensitivity, high selectivity, good repeatability, and convenience, CTL-based sensors show promising application in environmental monitoring [9,10], material characterization [11,12], food analysis [13,14], catalyst evaluation [15,16], and clinical diagnosis [17][18][19].CTL-based sensors are considered one of the most attractive and effective tools for gas sensing.In 2004, Zhang et al. pioneered the development of a CTL-based sensor for H 2 S using Fe 2 O 3 as the sensing material [20].Since then, the development of CTL-based sensors for H 2 S has attracted extensive attention.In particular, the Lv group has conducted a series of innovative studies on the development of a high-performance CTL-based sensor for H 2 S. Examples include a H 2 S sensor based on a hierarchical hollow microsphere and flower-like In 2 O 3 [21], a H 2 S sensor using an α-Fe 2 O 3 /g-C 3 N 4 composite [22], and an F-doped cage-like SiC as a metal-free sensing material for H 2 S [23].Although significant progress has been made, there are still some drawbacks, such as the high operating temperature, poor selectivity, and complicated procedures for synthesis of the sensing material [24].Therefore, persistent efforts should be made to overcome these drawbacks.
The sensing material directly determines the sensing performances, including the sensitivity, selectivity, reproducibility, etc. Metal oxide semiconductors have been widely developed as sensing materials owing to their high sensitivity, low cost, good reliability, and fast response [25,26].Tin dioxide (SnO 2 ) is a stable n-type wide-bandgap semiconducting metal oxide with stable chemical transduction properties [27].SnO 2 easily adsorbs oxygen on its surface because of its natural non-stoichiometry, providing it with high reactivity toward reducing gases [28].Nowadays, SnO 2 is widely used as a base material to design electrical gas sensors for the detection of harmful gas, including H 2 S [29][30][31][32].However, electrical gas sensors based on nanostructured SnO 2 usually have poor selectivity.It was reported that CTL produced based on a solid catalyst must meet three conditions [33].First, the reaction must release sufficient energy, but not all of the reactions can release enough energy.Second, the reaction pathway must favor a change in the energy to form an electronically excited molecule.Third, the electronically excited molecule must release its energies via radiative transition but not nonradiative transition.Unfortunately, radiative transition may appear in low proportions.Therefore, CTL sensors show good selectivity due to the three essential conditions described above.However, a CTL sensor using SnO 2 for H 2 S has not been reported, which inspired us to employ SnO 2 to design a CTL sensor for H 2 S to address some of the issues in H 2 S sensing.
Herein, three kinds of SnO 2 were synthesized by a simple hydrothermal method at three different calcination temperatures.Among them, the SnO 2 samples annealed at 600 • C (SnO 2 −600 • C) showed the highest CTL response toward H 2 S and exhibited sufficient selectivity.Subsequently, the influencing parameters and sensing performance of the SnO 2 −600 • C sensor were investigated in detail.The sensing mechanism was also explored.Compared with most reported H 2 S sensors, the outstanding advantage of the present sensor is that it shows high selectivity at a relatively low operating temperature.).An increase in the calcination temperature resulted in a decrease in the full width at half maxima, which mainly results from increased crystallinity.This result agrees with those of previous reports [34,35].In addition, no impurity peak is presented in the XRD patterns, which clearly proves that highly purified SnO 2 was successfully synthesized.

Structural and Morphological Characterizations
The morphology and size of the as-prepared SnO 2 samples were investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM).Figure 2a-c reveal that the three SnO 2 samples are monodispersed nanoparticles with an average particle diameter of about 20 nm.The HRTEM images in Figure 2d,e show that both the SnO 2 −500 • C and SnO 2 −600 • C have a (101) plane with a lattice space of 0.264 nm, a (110) plane with a lattice space of 0.335 nm, and a (211) plane with a lattice space of 0.176 nm. Figure 2f shows that SnO 2 −700 • C has a (211) plane with a lattice space of 0.176 nm.The corresponding selected area electron diffraction (SAED) images in Figure 2g-i display the observed diffraction patterns consisting of concentric rings, which implies that the SnO 2 samples have a polycrystalline structure.Meanwhile, the SAED fringe patterns are consistent with the peaks observed in the XRD spectra.The morphology and size of the as-prepared SnO transmission electron microscopy (TEM) and high-reso croscopy (HRTEM).Figure 2a-c reveal that the three S nanoparticles with an average particle diameter of abou Figure 2d,e show that both the SnO2−500 °C and SnO2− lattice space of 0.264 nm, a (110) plane with a lattice spa with a lattice space of 0.176 nm. Figure 2f shows that Sn a lattice space of 0.176 nm.The corresponding selected images in Figure 2g-i   The specific surface areas and pore size distribution of the as-prepared samples were measured by nitrogen adsorption−desorption and obtained using Barrett−Joyner−Halenda (BJH) calculations of the adsorption branches.Figure 3a-c show that all the SnO 2 samples produced typical type IV curves and have mesoporous features.As shown in Table 1,     The elemental and valence state analyses were performed based on the XPS spectra.Figure 4 shows the XPS spectra of SnO 2 −500 • C, SnO 2 −600 • C, and SnO 2 −700 • C, which were composed of tin and oxygen core levels.The Sn 3d peak was deconvoluted into two peaks: Sn 5/2 and Sn 3/2 .For SnO 2 −500 • C, the Sn 3d peaks centered at the binding energies of 486.88 eV and 495.28 eV, respectively, yielding a peak-to-peak separation of 8.4 eV (Figure 4a).Similarly, SnO 2 −600 • C exhibited corresponding peaks at 486.74 eV and 495.14 eV, with a separation of 8.4 eV (Figure 4b), and SnO 2 −700 • C exhibited corresponding peaks at 486.39 eV and 494.79 eV, with the same separation of 8.4 eV (Figure 4c).The above results indicates that all the Sn 3d peaks can be assigned to the highest oxidation state of Sn 4+ for SnO 2 [36,37].The elemental and valence state analyses were performed based on the XPS spectra.Figure 4 shows the XPS spectra of SnO2−500 °C, SnO2−600 °C, and SnO2−700 °C, which were composed of tin and oxygen core levels.The Sn 3d peak was deconvoluted into two peaks: Sn5/2 and Sn3/2.For SnO2−500 °C, the Sn 3d peaks centered at the binding energies of 486.88 eV and 495.28 eV, respectively, yielding a peak-to-peak separation of 8.4 eV (Figure 4a).Similarly, SnO2−600 °C exhibited corresponding peaks at 486.74 eV and 495.14 eV, with a separation of 8.4 eV (Figure 4b), and SnO2−700 °C exhibited corresponding peaks at 486.39 eV and 494.79 eV, with the same separation of 8.4 eV (Figure 4c).The above results indicates that all the Sn 3d peaks can be assigned to the highest oxidation state of Sn 4+ for SnO2 [36,37].

2023, 28, x FOR PEER REVIEW
The measured O 1s core level spectra of the SnO2 samples are presented in Figure 4df.The asymmetric peak of O 1s was fitted by two deconvoluted peaks for all samples.The main components of the O 1s signal, centered at 530.32-530.89eV, are attributed to the lattice oxygen (OL) as O 2− in SnO2.The higher binding energy peaks, centered at 532.11-532.504eV, can be ascribed to the chemisorbed oxygen (Oc) on the SnO2 surface [36,38].The binding energy value and the peak area ratio of oxygen species in each sample are listed in Table 2.The percentages of OC were 34.12%, 45.30%, and 26.97% for SnO2−500 °C, SnO2−600 °C, and SnO2−700 °C, respectively.The SnO2−600 °C sample had the highest amount of chemisorbed oxygen.Many studies have demonstrated that abundant chemisorbed oxygen can improve the sensing sensitivity.

Evaluation of the Sensing Materials
The three as-prepared SnO2 samples were used to fabricate gas sensors to sense H2S. Figure 5a shows the seven parallel determinations of H2S at 50 ppm by the three SnO2 sensors.The relative standard deviations (RSDs) of the response signals of the three sensors were 2.6%, 2.5%, and 3.6%, respectively, indicating that all the sensors have good reproducibility.However, the response signal of SnO2−600 °C was significantly higher than that of SnO2−500 °C and SnO2−700 °C.Then, the selectivity of the sensors toward H2S was studied by three parallel determinations of different gases at 2000 ppm, including ammonium sulfide, ethanethiol, dimethyl sulfide, trimethylamine, dimethylamine, triethylamine, ethylenediamine, ammonia, benzene, toluene, o-xylene, m-xylene, p-xylene, styrene, trichloroethylene, methanol, ethanol, acetone, formaldehyde, acetaldehyde, propionaldehyde, carbon monoxide, carbon dioxide, and sulfur dioxide.As Figure 5b shows, 2000 ppm of ammonium sulfide produced weak responses, but its concentration was The measured O 1s core level spectra of the SnO 2 samples are presented in Figure 4d-f.The asymmetric peak of O 1s was fitted by two deconvoluted peaks for all samples.The main components of the O 1s signal, centered at 530.32-530.89eV, are attributed to the lattice oxygen (O L ) as O 2− in SnO 2 .The higher binding energy peaks, centered at 532.11-532.504eV, can be ascribed to the chemisorbed oxygen (O c ) on the SnO 2 surface [36,38].The binding energy value and the peak area ratio of oxygen species in each sample are listed in

Evaluation of the Sensing Materials
The three as-prepared SnO 2 samples were used to fabricate gas sensors to sense H 2 S. Figure 5a shows the seven parallel determinations of H 2 S at 50 ppm by the three SnO 2 sensors.The relative standard deviations (RSDs) of the response signals of the three sensors were 2.6%, 2.5%, and 3.6%, respectively, indicating that all the sensors have good reproducibility.However, the response signal of SnO 2 −600 • C was significantly higher than that of SnO 2 −500 • C and SnO 2 −700 • C.Then, the selectivity of the sensors toward H 2 S was studied by three parallel determinations of different gases at 2000 ppm, including ammonium sulfide, ethanethiol, dimethyl sulfide, trimethylamine, dimethylamine, triethylamine, ethylenediamine, ammonia, benzene, toluene, o-xylene, m-xylene, p-xylene, styrene, trichloroethylene, methanol, ethanol, acetone, formaldehyde, acetaldehyde, propionaldehyde, carbon monoxide, carbon dioxide, and sulfur dioxide.As Figure 5b shows, 2000 ppm of ammonium sulfide produced weak responses, but its concentration was much higher than that of H 2 S. It was found that ammonium sulfide could not produce responses from all the sensors when its concentration was reduced to 1000 ppm.It is worth mentioning that methanol, ethanol, acetone, and carbon monoxide are common interferents for H 2 S sensors [29][30][31][32]39].However, the present sensors did not produce responses toward high concentrations of these gases, indicating that the as-prepared SnO 2 samples are qualified for the selective sensing of H 2 S. Because SnO 2 −600 • C had the most sensitive response and possessed satisfactory selectivity, the sensing performance of a sensor based on SnO 2 −600 • C was studied in detail in the subsequent work.

Optimization of Conditions
The detecting wavelength, operating temperature, and flow rate have sizable effects on CTL sensing.First, the effect of the detecting wavelength on the CTL response signal and signal-to-noise ratio (S/N) was investigated by changing the optical filters in a range of 230-520 nm.As shown in Figure 6a, both the CTL response signal and the S/N value reached their maximums at 350 nm.Consequently, 350 nm was chosen as the optimal

Optimization of Conditions
The detecting wavelength, operating temperature, and flow rate have sizable effects on CTL sensing.First, the effect of the detecting wavelength on the CTL response signal and signal-to-noise ratio (S/N) was investigated by changing the optical filters in a range of 230-520 nm.As shown in Figure 6a, both the CTL response signal and the S/N value reached their maximums at 350 nm.Consequently, 350 nm was chosen as the optimal wavelength for the CTL sensing of H 2 S.

Optimization of Conditions
The detecting wavelength, operating temperature, and flow rate have sizable effects on CTL sensing.First, the effect of the detecting wavelength on the CTL response signal and signal-to-noise ratio (S/N) was investigated by changing the optical filters in a range of 230-520 nm.As shown in Figure 6a, both the CTL response signal and the S/N value reached their maximums at 350 nm.Consequently, 350 nm was chosen as the optimal wavelength for the CTL sensing of H2S.For wavelength optimization, the operating temperature was 210 °C, and the flow rate was 800 mL/min.For temperature optimization, the detecting wavelength was 350 nm, and the flow rate was 800 mL/min.For flow rate optimization, the detecting wavelength was 350 nm, and the operating temperature was 210 °C.
Next, the effect of the operating temperature on the CTL response signal and S/N was investigated.Figure 6b shows the change trends of the response signal and S/N versus operating temperature ranging from 133 to 288 °C.The CTL response signal increased monotonically with the operating temperature.For the S/N value, it increased with operating temperature before 210 °C and then deceased with increasing temperature, which can be mainly attributed to the background noise emitted by the thermal radiation that increases with the increasing temperature.Therefore, an operating temperature of 210 °C was chosen as the optimal working temperature, as the maximum S/N was observed at this temperature.Next, the effect of the operating temperature on the CTL response signal and S/N was investigated.Figure 6b shows the change trends of the response signal and S/N versus operating temperature ranging from 133 to 288 • C. The CTL response signal increased monotonically with the operating temperature.For the S/N value, it increased with operating temperature before 210 • C and then deceased with increasing temperature, which can be mainly attributed to the background noise emitted by the thermal radiation that increases with the increasing temperature.Therefore, an operating temperature of 210 • C was chosen as the optimal working temperature, as the maximum S/N was observed at this temperature.
Finally, the effect of the flow rate on the CTL sensing of H 2 S was investigated.As Figure 6c shows, both the CTL response signal and the S/N value increased gradually with the increasing flow rate.Even when the flow rate reached the maximum rate (1000 mL/min) of the mass control flowmeter, the CTL response signal and the S/N value did not show decreasing trends.This means that the rapid oxidation rate of H 2 S on the SnO 2 −600 • C surface and the total reaction rate is controlled by the transfer rate of H 2 S from the gas phase to the SnO 2 −600 • C surface.Thus, an increase in the flow rate accelerates the total reaction rate, causing the increase in the CTL signal and S/N.Because the change in the CTL response signal and S/N tended to be stable when the flow rate exceeded 800 mL/min, a flow rate of 800 mL/min was chosen as the optimal flow rate.

Analytical Characteristics
Figure 7a shows the response signal of the sensor based on SnO 2 −600 • C toward different concentrations of H 2 S under the optimal conditions.A good linear relationship between the CTL response signal and H 2 S concentration was found in the range of 20-420 ppm with a correlation coefficient of 0.9926.The linear equation was I = 68.39c− 1153.1,where the I is the average CTL response signal of three parallel determinations, and c represents the H 2 S concentration (ppm).The limit of detection (LOD) was 8 ppm at an S/N ratio of 3. The LOD of the sensor is lower than the threshold limit value of 10 ppm for H 2 S in a workplace (8 h time weighted average), indicating its promising application to monitor H 2 S in the workplace for safety management.ceeded 800 mL/min, a flow rate of 800 mL/min was chosen as the optimal flow rate.

Analytical Characteristics
Figure 7a shows the response signal of the sensor based on SnO2−600 °C toward different concentrations of H2S under the optimal conditions.A good linear relationship between the CTL response signal and H2S concentration was found in the range of 20-420 ppm with a correlation coefficient of 0.9926.The linear equation was I = 68.39c− 1153.1,where the I is the average CTL response signal of three parallel determinations, and c represents the H2S concentration (ppm).The limit of detection (LOD) was 8 ppm at an S/N ratio of 3. The LOD of the sensor is lower than the threshold limit value of 10 ppm for H2S in a workplace (8 h time weighted average), indicating its promising application to monitor H2S in the workplace for safety management.A comparison of the sensing performances between the present sensor and other H2S sensors is summarized in Table 3.The present sensor has a relatively lower operating temperature and a faster response and recovery speed than most electrical sensors.Although the electrical sensors are more sensitive than the present sensor, our sensor exhibits higher selectivity toward H2S.Gas sensors that can operate at low or room temperatures are highly desired for reducing the risk of gas explosions, minimizing energy consumption, and increasing security.Further endeavors could be made to improve the sensitivity and reduce the operating temperature of CTL sensors based on SnO2 materials via doping noble metals, loading porous materials, or engineering the oxygen vacancy.A comparison of the sensing performances between the present sensor and other H 2 S sensors is summarized in Table 3.The present sensor has a relatively lower operating temperature and a faster response and recovery speed than most electrical sensors.Although the electrical sensors are more sensitive than the present sensor, our sensor exhibits higher selectivity toward H 2 S. Gas sensors that can operate at low or room temperatures are highly desired for reducing the risk of gas explosions, minimizing energy consumption, and increasing security.Further endeavors could be made to improve the sensitivity and reduce the operating temperature of CTL sensors based on SnO 2 materials via doping noble metals, loading porous materials, or engineering the oxygen vacancy.Figure 7b displays the CTL response profiles of the sensor toward different concentrations of H 2 S under the optimal conditions.The injection time was 20 s, and it can be seen that the response signal can reach the maximum value within 3.5 s, and the maximum response signal can decay back to baseline within 1.5 s for all concentrations of H 2 S, indicating a fast response/recovery behavior of the sensor and highlighting its ability to rapidly monitor H 2 S.

Mechanism Study
SnO 2 is a typical n-type semiconductor material; when a SnO 2 sensor is exposed to the air atmosphere, the O 2 molecules in the air react with SnO 2 to produce chemisorbed oxygen via receiving electrons, such as O 2− , O 2− , and O − [31,45].It was reported that O − is the predominant chemisorbed oxygen species on metal oxide surfaces between 100 and 300 • C [46], and thereby the O − species is the preferential chemisorbed oxygen because the optimal working temperature of the sensor is 210 • C. We found that the CTL signal could not be detected on a heated ceramic heater without sintering nanosized SnO 2 , indicating that the catalyst is essential to the CTL emission.In addition, no CTL signal was detected when the air carrier was replaced by nitrogen, helium, or argon, implying that the surface chemisorbed oxygen but not the lattice oxygen takes part in the reaction.The influence of the oxygen content on CTL intensity was also investigated.As shown in Figure 8, the CTL intensity increased gradually with increasing oxygen content, and then remained stable when the oxygen content exceeded 5%.Oxygen is the second most abundant gas in the air at about 21%, indicating that the use of air as a carrier gas can provide sufficient oxygen for the oxidation of H 2 S on SnO 2 surfaces.
not be detected on a heated ceramic heater without sinteri that the catalyst is essential to the CTL emission.In additio when the air carrier was replaced by nitrogen, helium, or ar chemisorbed oxygen but not the lattice oxygen takes part in the oxygen content on CTL intensity was also investigated.intensity increased gradually with increasing oxygen cont when the oxygen content exceeded 5%.Oxygen is the sec air at about 21%, indicating that the use of air as a carrier gas for the oxidation of H2S on SnO2 surfaces.In order to further explore the sensing mechanism, gas chromatography-mass spectrometer was used to identify the reaction product from the oxidation of H 2 S on the SnO 2 surface.SO 2 and H 2 O were detected in the exhaust gas from the oxidation of H 2 S on the SnO 2 surface.According to the accepted theory, the generation of excited state intermediates is necessary for the generation of CTL during the reaction process [47].It was reported that SO 2 is a strong chemiluminescence continuum band between 250 and 500 nm with a peak wavelength of around 350 nm [48,49].The CTL emission spectrum shown in Figure 6a also exhibits similar characteristics.Therefore, the excited state of SO 2 (SO 2 *) is the possible luminophore, which is responsible for CTL emission from the oxidation of H 2 S on the SnO 2 surface.According to the above discussion, the possible oxidation reaction for H 2 S on the SnO 2 surface in air can be described as follows: The above reactions display the important role of chemisorbed oxygen in CTL emission.Abundant chemisorbed oxygen can promote the oxidation of H 2 S, enhancing the CTL intensity.The XPS results show that the order of the amount of chemisorbed oxygen is SnO 2 −600 • C > SnO 2 −500 • C > SnO 2 −700 • C, which is in accordance with the order of the sensitivity.Therefore, the amount of chemisorbed oxygen can explain the different sensitivities of the sensors based on the different SnO 2 samples.However, extensive work is still needed to explain the underlying sensing mechanism.For example, the adsorption behavior of H 2 S and the evolution of oxygen species on heated SnO 2 surfaces should be studies by in situ diffuse reflectance infrared Fourier transform spectroscopy or in situ Raman spectroscopy.

Synthesis of SnO 2
Approximately 7.012 g (0.02 mol) of SnCL 4 •5H 2 O was dissolved in 80 mL deionized water and stirred for 15 min for complete dispersion; the final concentration of SnCL 4 •5H 2 O was 0.25 mol/L.Then, 10 mL of 14.8 mol/L ammonium hydroxide (28% NH 3 in H 2 O) was added dropwise into the solution, and the mixture was stirred continuously for 15 min.After that, the obtained solution was transferred into a Teflon-lined stainless steel autoclave and kept at 100 • C for 12 h.After naturally cooling down to room temperature, the assynthesized precursors were washed several times with deionized water and ethanol and dried at 60 • C for 12 h in an oven.Then, the as-prepared powders were annealed at 500 • C, 600 • C, and 700 • C for 3 h, and the resulting annealed SnO 2 samples were labeled as SnO 2 −500 • C, SnO 2 −600 • C, and SnO 2 −700 • C, respectively.

Characterization and Apparatus
The phase structures of the SnO 2 samples were investigated using powder X-ray diffraction (XRD, Rigaku, Ultima IV) with Cu Kα radiation (λ = 1.5406Å).The morphology and exposed surfaces of the SnO 2 samples were studied by transmission electron microscopy (TEM; Titan G260-300 FEI, Stanford, CA, USA).The specific surface areas and pore size distribution of the SnO 2 samples were investigated using a surface area analyzer (ASAP2460, Micromeritics, Norcross, GA, USA).The chemical status and the composition of elements of the SnO 2 samples were investigated by K-Alpha X-ray photoelectron spectroscopy (XPS; Thermo Scientific, Waltham, MA, USA) with monochromatic Al Kα X-ray radiation.

Fabrication of the Gas Sensor
The detailed fabrication methods of the SnO 2 sensors were similar to those reported in previous works [8,10], and a schematic diagram of the sensing setup is shown in Figure 9.Briefly, the as-prepared SnO 2 samples were mixed with deionized water to obtain a suspension.The suspension was then brush-coated on the surface of a ceramic heater (35 W, diameter is 0.5 cm) to obtain a thin layer (about 1 mm in thickness).The ceramic heater was placed in a homemade quartz tube (with a length of 9 cm and an inner diameter of 1.2 cm) with a gas inlet and outlet.The quartz tube was placed into the sample chamber of a commercial BPCL ultra-weak luminescence analyzer (Guangzhou Microphotonics Technologies Co., Ltd., Guangzhou, China) equipped with a photomultiplier (tube type: GDB-52) at the bottom of sample chamber.The sensors were aged at 400 • C for 20 min in a stable airflow at 800 mL/min.An air pump was used to carry and transport the sample, and a mass flow controller was used to control the flow rate.A voltage regulator was used to adjust the operating temperature.The detecting wavelength was changed using different interference filters.The sample was injected into the injection port, and then the CTL signal was recorded and processed by the BPCL ultra-weak luminescence analyzer.and a mass flow controller was used to control the flow rate.A volta to adjust the operating temperature.The detecting wavelength was ent interference filters.The sample was injected into the injection p signal was recorded and processed by the BPCL ultra-weak lumine

Conclusions
In conclusion, three kinds of nanosized SnO2 samples were s hydrothermal method.Structural characterizations indicated that th of chemisorbed oxygen of the three nanosized SnO2 samples was Sn °C > SnO2−700 °C.The gas sensing results indicated that the senso nosized SnO2 samples all exhibited high selectivity toward H2S, bu sor exhibited the highest sensitivity, which can be attributed to th resulting in the highest amount of chemisorbed oxygen.The mec that H2S was oxidized into SO2 and H2O by chemisorbed oxygen on excited state of the SO2 produced by the oxidation reaction was m CTL emission.The designed CTL sensor for H2S has good prospec tion of H2S in the fields of environmental monitoring, food saf health, owing to its advantages of sufficient sensitivity, high sele sponse and recovery capacity.

Figure 1
Figure 1 shows the XRD patterns of the three samples annealed at different temperatures.Diffraction peaks at 2θ = 26.58• , 33.88 • , 37.94 • , 51.78 • , and 54.76 • correspond to the (110), (101), (200), (211), and (220) planes of tetragonal SnO 2 with standard lattice constants of a = b = 4.7382 Å and c = 3.1871 Å (JCPDS card no.41-1445).An increase in the calcination temperature resulted in a decrease in the full width at half maxima, which mainly results from increased crystallinity.This result agrees with those of previous reports[34,35].In addition, no impurity peak is presented in the XRD patterns, which clearly proves that highly purified SnO 2 was successfully synthesized.The morphology and size of the as-prepared SnO 2 samples were investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM).Figure2a-c reveal that the three SnO 2 samples are monodispersed nanoparticles with an average particle diameter of about 20 nm.The HRTEM images in Figure2d,e show that both the SnO 2 −500 • C and SnO 2 −600 • C have a (101) plane with a lattice space of 0.264 nm, a (110) plane with a lattice space of 0.335 nm, and a (211) plane with a lattice space of 0.176 nm.Figure2fshows that SnO 2 −700 • C has a (211) plane with a lattice

Figure 1 .
Figure 1.XRD patterns of the as-prepared samples.
display the observed diffraction rings, which implies that the SnO2 samples have a poly the SAED fringe patterns are consistent with the peaks o The specific surface areas and pore size distribution measured by nitrogen adsorption−desorption rett−Joyner−Halenda (BJH) calculations of the adsorption

Figure 1 .
Figure 1.XRD patterns of the as-prepared samples.

Molecules 2023 , 15 Figure 5 .
Figure 5. (a) The response curves of the three sensors toward H2S.(b) The responses of the three sensors toward 50 ppm of H2S and 2000 ppm of potentially interfering gases.

Figure 5 .
Figure 5. (a) The response curves of the three sensors toward H 2 S. (b) The responses of the three sensors toward 50 ppm of H 2 S and 2000 ppm of potentially interfering gases.

Figure 5 .
Figure 5. (a) The response curves of the three sensors toward H2S.(b) The responses of the three sensors toward 50 ppm of H2S and 2000 ppm of potentially interfering gases.

Figure 6 .
Figure 6.The effects of the detecting wavelength (a), operating temperature (b), and flow rate (c) on the CTL response signal and S/N.The concentration of H2S used was 50 ppm.For wavelength optimization, the operating temperature was 210 °C, and the flow rate was 800 mL/min.For temperature optimization, the detecting wavelength was 350 nm, and the flow rate was 800 mL/min.For flow rate optimization, the detecting wavelength was 350 nm, and the operating temperature was 210 °C.

Figure 6 .
Figure 6.The effects of the detecting wavelength (a), operating temperature (b), and flow rate (c) on the CTL response signal and S/N.The concentration of H 2 S used was 50 ppm.For wavelength optimization, the operating temperature was 210 • C, and the flow rate was 800 mL/min.For temperature optimization, the detecting wavelength was 350 nm, and the flow rate was 800 mL/min.For flow rate optimization, the detecting wavelength was 350 nm, and the operating temperature was 210 • C.

Figure 7 .
Figure 7. (a) Linear relationship between the CTL response signal and H2S concentration.(b) CTL response profiles of the sensor toward different concentrations of H2S.Inset: enlarged view of the CTL response profiles.

Figure 7 .
Figure 7. (a) Linear relationship between the CTL response signal and H 2 S concentration.(b) CTL response profiles of the sensor toward different concentrations of H 2 S. Inset: enlarged view of the CTL response profiles.

Figure 8 .
Figure 8.The influence of the oxygen content on the CTL intensit SnO2−600 °C surface.

Figure 8 .
Figure 8.The influence of the oxygen content on the CTL intensity of the H 2 S response on the SnO 2 −600 • C surface.

Figure 9 .
Figure 9.The schematic diagram of the sensing setup.

Table 1 .
The BET, Langmuir, and BJH results for the three samples.

Table 2 .
XPS results of different chemical states of O elements at the surface of the as-prepared SnO2 nanomaterials.

Table 2 .
The percentages of O C were 34.12%, 45.30%, and 26.97% for SnO 2 −500 • C, SnO 2 −600 • C, and SnO 2 −700 • C, respectively.The SnO 2 −600 • C sample had the highest amount of chemisorbed oxygen.Many studies have demonstrated that abundant chemisorbed oxygen can improve the sensing sensitivity.

Table 2 .
XPS results of different chemical states of O elements at the surface of the as-prepared SnO 2 nanomaterials.

Table 3 .
Comparison of the sensing performances of H 2 S sensors.