Rapid Gas-Sensing Detection of Carbon Disulfide by a CdS/SnS Nanocomposite-Based Cataluminescence Sensor

Abstract

A CdS/SnS nanocomposite was prepared using a simple hydrothermal method and used as a sensitive material for the detection of carbon disulfide (CS₂) based on cataluminescence (CTL). The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the CdS/SnS nanocomposite sensor has a high sensitivity to CS₂ at a relatively low operating temperature (162 °C); the response time is about 3 s, and the recovery time is about 16 s. The modification of CdS effectively enhances the sensitivity of SnS sensors. The CTL intensity shows a good linear relationship at gas concentrations ranging from 6.75 to 168.75 ppm (R² = 09974), and the limit of detection (LOD) of CS₂ reached 0.96 ppm. In addition, the CdS/SnS sensor has excellent selectivity and good stability towards CS₂. The mechanism of the sensor is discussed in detail. This research shows that CdS/SnS has great potential for the detection of CS₂.

1. Introduction

Carbon disulfide (CS₂) is a typical organosulfur compound, which is a colorless and transparent fat-soluble liquid at room temperature and pressure with a spoiled rotten egg smell, and has strong volatility, flammability and explosiveness. The main sources are natural gas, coke oven gas and viscose fiber production exhaust gas [1,2,3]. The emission of CS₂ is harmful to human health. It is absorbed by the human body in the form of steam or through the mucous membranes of the eyes and skin and enters the blood. Therefore, long-term human exposure to low concentrations of CS₂ may lead to peripheral neuropathy [4,5,6]. CS₂ emissions are also extremely harmful to the environment, and once CS₂ enters the stratosphere, it is rapidly oxidized into carbon sulfide and sulfur dioxide [7], leading to acid rain and the formation of secondary organic aerosols. Therefore, in order to control the concentration of CS₂ below the threshold value, a sensitive, reliable and specific method is needed to monitor the content of CS₂ in the environment.

At present, the methods for the determination of CS₂ include flame photometric gas chromatography [8], the fluorescence sensor method [9], wavelength modulation spectroscopy [10], the quantum cascade laser method [11] and ultraviolet absorption spectroscopy [12]. Although these methods can accurately detect CS₂ gas, there are disadvantages, such as bulky detection instruments, cumbersome processes and the need for professional operation. Gas sensors are characterized by small sizes, simplicity, low cost and easy operation, and they play a vital role in environmental monitoring [13,14,15]. In recent years, CTL sensors have been widely developed and used in the field of VOC monitoring due to advantages such as low cost, small and convenient detection equipment, high sensitivity and excellent stability [16]. At the same time, with the introduction of nanomaterials into the design of CTL sensors, the performance of CTL sensors has been significantly improved due to characteristics such as a large surface area, high activity and the good adsorption of nanomaterials [17]. Another great advantage of CTL sensors is that only the target gas and oxygen are consumed during the sensing process, and the sensing element is not consumed as a catalyst. Because of the many advantages of CTL sensors, they have the ability to monitor analytes for a long time [18,19]. Therefore, CTL sensors are designed for CS₂ gas detection.

As a carbon-group element (IVA) (MX; M = Sn, Ge; X = S, Se), SnS has a large absorption coefficient (≈105 cm⁻¹), is low-cost and non-toxic and has an appropriate band gap (1.0–1.35 eV) [20,21,22,23,24,25,26]. SnS nanomaterials have inherent disadvantages, such as a small modulation depth. In recent years, element doping has been an effective method for enhancing the gas sensitivity of gas sensors, as it can control the grain size, band structure and surface state of the sensing material. Previous studies have shown that heterogeneous structures exhibit unique electrical and optical properties [27,28,29]. According to previous studies, the CdS/SnS heterostructure shows remarkable performance in all aspects [30]. For example, Chang et al. used the excellent properties of the CdS/SnS heterojunction to develop detectors with excellent performance [28]. Considering this, the application of CdS/SnS nanocomposites to CTL sensors may have a good performance. At present, no relevant reports on the application of CdS/SnS nanocomposites to CTL sensors could be found.

Herein, SnS and CdS/SnS composites were prepared by using a hydrothermal method. The CTL characteristics of SnS and CdS/SnS composites were compared, and the possible mechanism of the enhanced CTL performance of CdS/SnS composite sensors was explored based on influencing factors, namely, the temperature, concentration and carrier gas velocity. The results show that the material and CS₂ can produce a strong CTL phenomenon, and this work shows that the CdS/SnS nanojunction may be a new CTL-sensitive material, which provides a promising method for the development of high-performance CTL CS₂ sensors.

2. Experimental Procedures

2.1. Test Reagents

Stannous chloride (SnCl₂·2H₂O, 98.0%, Shanghai Sinopharm Chemical Reagent Co., Ltd., Shanghai, China); Absolute ethanol (C₂H₆O, 99.5%, Zhengzhou Yibang Industrial Co., Ltd., Zhengzhou, China); Thiourea (CH₄N₂S, 99.0%, Shanghai Rinen Technology Development Co., Ltd., Shanghai, China); Cadmium acetate (C₄H₆CdO₄·2H₂O, 99.9%, Tianjin Photofusion Chemical Reagent Co., Ltd., Tianjin, China); and Anhydrous glycol (C₂H₆O₂, 99.99%, Shanghai Sinopharm Chemical Reagent Co., Ltd., Shanghai, China).

2.2. Main Analytical Instruments

BPCL faint luminescence measuring instrument (Guangzhou Microlight Technology Co., Ltd., Guangzhou, China); X-ray diffractometer (Panalytical, Almelo, The Netherlands); scanning electron microscope (ZEISS, Jena, German); energy-dispersive spectrometer (ZEISS, German company); and X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).

2.3. Preparation of Nanocomposites

CdS/SnS nanocomposites were prepared using a hydrothermal method. First, 0.36 g of thiourea was added to a solution obtained by mixing 0.54 g of SnCl₂ with 60 ml of absolute ethanol. The reaction mixture was transferred to a 100 mL Teflon-lined stainless-steel reactor, which was placed in a constant-temperature heating box and heated at 180 °C for 12 h. Then, the samples were collected, washed alternately with deionized water and absolute ethanol and dried in a constant-temperature heating box at 70 °C for 8 h. Next, 0.46 g of cadmium acetate and 0.16 g of thiourea were added to a solution obtained by mixing 0.3 g of SnS powder with 50 ml of deionized water and stirred at 70 °C for 30 min. The sample was centrifuged and washed. Finally, it was dried in a constant-temperature heating box and ground to obtain black CdS/SnS powder.

2.4. CTL Device and Detection Method

The system diagram of the ultra-weak chemiluminescent instrument (BPCL-1, Guangzhou Micro Light Technology Co., Ltd., Guangzhou, China) is shown in Figure 1. The system is divided into three parts: (I) reaction chamber: the chamber is composed of a ceramic heating rod coated with CdS/SnS nanocomposites and a quartz tube with a gas inlet and outlet, and the gas to be measured enters the quartz tube from the air inlet and reacts when in contact with the material; (II) temperature and flow rate control device: the temperature control system increases the surface temperature of the heating rod, and the air pump controls the carrier air velocity; (III) photoelectric detection and data processing system: the system consists of the BPCL-1 ultra-weak luminescence analyzer and the computer used to catalyze the luminescent signal. The concentration of the detection gas is calculated by the following formula [31]:

$$\mathrm{C}=\frac{{\mathrm{V}}_{\mathrm{i}}\times {\mathrm{p}}_0}{{\mathrm{V}}_{\mathrm{c}}\times {\mathrm{p}}_{\mathrm{a}}}$$

(1)

where C, V_i, V_c, P₀ and P_a are the concentration of the target gas, the volume of the injected gas, the volume of the chamber, the equilibrium vapor pressure at room temperature and standard atmospheric pressure, respectively.

3. Results and Discussion

3.1. Material Characterization

The SEM images of SnS nanoflowers are shown in Figure 2a,b. SnS has a microstructure similar to a nanoflower, a regular morphology and a uniform distribution, and a large number of nanoparticles are stacked into nanospheres. Figure 2b shows its enlarged image, and the flower-like leaves are composed of ultra-thin nanosheet structures with clear and smooth surfaces. This composite has a large particle size. The size is large, and the synapses of the flower-like leaf structure on the surface are obvious, which can increase the contact area between the material and the gas, which is conducive to the adsorption and diffusion of gas. Figure 2c,d show images of CdS/SnS. Compared with the SEM of SnS, it is found that there are more agglomerated particles on the surfaces of the SnS nanoflowers. The particle size of the composite becomes smaller, the structure becomes denser, and the number of external nanosheets increases. In addition, the coated structure of the new composite helps to increase the specific surface area of the material that contacts the gas, provides more active sites for the adsorption and diffusion of the gas in the reaction, and also helps to improve the response and recovery times of the sensor.

EDS elemental mapping, EDS spectra (Figure 3) and the elemental content table (

Table 1. The elemental content of CdS/SnS.

Element	Weight %	Atomic %
C	0.34	16.95
O	0.74	28.73
S	0.70	29.75
Cd	0.67	7.23
Sn	2.57	17.34
Total	—	100

) show that S, Sn and Cd are characteristic elements with relatively uniform distributions, which once again shows the successful synthesis of CdS/SnS composites.

The X-ray diffraction (XRD) pattern for CdS/SnS is shown in Figure 4. According to the standard cards (PDF# 39-0354 and PDF# 42-1411), all diffraction peaks can be attributed to SnS and CdS, indicating the successful synthesis of CdS/SnS composites. The diffraction peaks at 2θ = 26.0°, 31.5°, 39.0°, 45.5° and 51.3° are attributed to the (120), (111), (131), (002) and (151) crystal faces of SnS, respectively, and the diffraction peaks at 2θ = 30.8°, 43.9° and 64.0° are attributed to the (200), (220) and (400) crystal faces of CdS, respectively. No diffraction peaks of other impurities were observed, which further indicates that CdS/SnS has a high purity.

The elemental composition and electronic states of CdS/SnS nanocomposites were analyzed by XPS. As shown in Figure 5, the complete spectrum of XPS clearly shows the presence of Sn, Cd and S in the composite, indicating significant changes in the electronic states of these three elements, as well as minor peaks due to O and C changes due to adsorption on the surface of the material. Figure 5b–d show the high-resolution XPS spectra of Cd 3d, Sn 3d and S 2p of CdS/SnS nanocomposite structures, respectively. The characteristic peaks of the Cd 3d spectrum are observed at 404.99 eV and 411.76 eV, corresponding to Cd 3d_5/2 and Cd 3d_3/2, respectively. The two peaks at 485.7 eV and 494.2 eV are attributed to Sn 3d_5/2 and Sn 3d_3/2, respectively, and the two characteristic peaks shown in the Cd 3d and Sn 3d spectra are located at two different orbitals, 5/2 and 3/2, respectively; the proportions are different, indicating that they are associated with the Cd²⁺ and Sn²⁺ states in CdS and SnS [32,33]. According to the periodic law of Cd and Sn, both of them have highly volatile electrons that form the divalent states Cd²⁺ and Sn²⁺. However, because the cationic oxidation of Sn is stronger than that of Cd, SnS is mainly used as a catalyst in the whole CTL process, and the incorporation of Cd metal elements promotes the absorption of O₂ on the surface of the material, which greatly improves the CTL performance of the composite material; the spectral characteristic peaks of the two elements are similar, and the peak crack at 8.5 eV can also confirm the formation of SnS [34]. Figure 5d shows the high-resolution spectrum of S 2p, which is divided into three asymmetric peaks; the peaks at 161.08 eV and 162.18 eV are S 2p_3/2 and S 2p_1/2, respectively, with an energy interval of 1 eV and a peak-intensity ratio of 1.2:1, indicating a change in the valence state of S.

3.2. Effect of CdS Modification on SnS CTL Performance

The CTL performance is shown in Figure 6. Both sensitive materials have the highest peaks of luminous intensity within 3 seconds of CS₂ introduction, indicating that the two materials have a fast response to CS₂. The signal intensity of CTL on CS₂ when CdS/SnS was used as a sensitive material was significantly higher than that when using SnS as a material. The composite exhibits a strong CTL reaction to CS₂. It is shown that compared with pure SnS materials, CdS/SnS nanocomposites exhibit excellent CTL properties. The introduction of cadmium ions increases the cation content on the surface of the material, and the oxygen vacancy gradually increases in order to balance the charge to offset the electrical deviation of the material due to the increase in cadmium ion content. Studies have shown that the higher the content of oxygen molecules adsorbed on the surface of the material, the more oxygen vacancies on the surface of the material [35]. The addition of CdS can effectively enhance the signal intensity of the CTL method of pure SnS materials to detect CS₂, and subsequent research on the sensitive characteristics of CS₂ was carried out with CdS/SnS nanocomposites as sensitive materials.

3.3. Effect of Working Temperature on CTL Intensity

Figure 7 was obtained under the following conditions: the flow rate was 280 mL/min, the gas concentration was 33.75 ppm, and the CTL intensity of CS₂ increased with increasing temperature; the reason for choosing these conditions is that CS₂ has a greater reaction conversion on CdS/SnS at higher temperatures. In the BPCL-1 experimental device, a photomultiplier tube was used to convert optical signals into electrical signals as output. Increasing the operating environment temperature of the photomultiplier tube will lead to an increase in hot-electron emission and the dark current, which will increase the noise of the sensor and directly affect the signal-to-noise ratio (S/N). When studying temperature-influenced factors, we took the S/N as an important research parameter. Considering the influence of the signal-to-noise ratio, the sensor does not achieve the best performance with a continuous increase in temperature [36]. When the temperature exceeds 162 °C, the S/N value decreases significantly with an increase in noise, and the signal strength of the sensor also decreases. Atomic ions have much higher activity at elevated temperatures, and the response of nanocomposites is very weak when the temperature is below the peak of the temperature polyline. However, when the temperature rises and is higher than the peak value, the signal strength of the sensor will also decrease, which may be due to the low operating temperature, the insufficient atomic ion activity of the reactant, and the relatively low response signal of the sensor. On the other hand, the higher the desorption rate of the target gas on the surface of the material, the greater the surface of the material that has not yet fully reacted, resulting in a decrease in the reaction performance [37]. The experiment finally selected a test temperature of 162 °C for further study. Reaction temperatures have always been an important factor limiting the range of use of CTL; many CTL sensors need to work at high temperatures, and researchers have been exploring sensitive materials that can enable CTL detection at lower temperatures. Compared with previous studies, the CS₂ sensor based on the CdS/SnS nanocomposite has a lower operating temperature, so it is more conducive to expanding the potential of the sensor for practical applications.

3.4. Effect of Carrier Air Velocity on CTL Intensity

As an important parameter, the flow rate also has an important impact on the CTL process. Figure 8 displays the effect of different carrier gas flow velocities in the range of 100–550 mL/min on the CTL performance, and it can be observed that the CTL reaction signal and S/N increase with the increase in the flow rate in a certain range (100–280 mL/min), and the CTL signal reaches a maximum value at 280 mL/min. When the carrier gas velocity is faster than 280 mL/min, due to the insufficient reaction time between the CS₂ vapor and the sensor, the CS₂ cannot be fully oxidized by oxygen. The CTL signal and S/N are reduced. Therefore, we selected a carrier air velocity of 280 mL/min for further study.

3.5. The Correspondence between the CTL Intensity and the Concentration of the Analyte

Under optimal conditions, the correspondence between the signal strength and the CS₂ concentration of the CTL sensor based on CdS/SnS composites in the CS₂ concentration range of 6.75~168.75 ppm was studied. Different concentrations of the compound vapor react with nanocomposites so that the sensor obtains different responses, because the equilibrium process of the CTL reaction is different at different concentrations. There is a dynamic equilibrium between the adsorption and desorption of gases on the surface of the sensing material. When the concentration of the compound vapor is low, the equilibrium process of the reaction takes a long time. Therefore, this stage of adsorption plays a leading role in the entire gas detection process. Conversely, when the concentration reaches a higher level, the reaction quickly reaches equilibrium, and then gas desorption dominates the sensor’s detection process [38]. Figure 9a shows the CTL signal produced by different concentrations of CS₂. The intensity of the CTL signal increases significantly as the concentration increases. Figure 9b shows the correspondence between the CTL intensity and the concentration of CS₂. It can be seen that the CTL intensity and the concentration of CS₂ show a good linear relationship; the linear regression equation is y = 187.95x − 903.65 (R² = 0.9974), where y is the average CTL intensity with CS₂ on the surface of the two-dimensional nanocomposite after four parallel experiments, x is the CS₂ concentration, and R is the correlation coefficient. The LOD is calculated according to the following formula, and the value is 0.96 ppm (S/N = 3).

$$\mathrm{D}=3\mathrm{N}\times \frac{\mathrm{Q}}{\mathrm{I}}$$

(2)

where Q is the minimum intake air concentration within the detection range; N is the noise corresponding to the minimum intake concentration within the detection range; and I is the signal response value corresponding to the minimum intake air concentration within the detection range.

3.6. Selectivity and Lifetime of the Sensor

In addition to high stability and good reproducibility, excellent selectivity is also critical for sensors, especially when applied to actual inspections. Therefore, under optimal operating conditions, the selectivity of CdS/SnS sensing for CS₂ can be studied using a series of possible interfering substances. The selectivity test was carried out under the same experimental conditions, as shown in Figure 10a. It can be observed that toluene, methylformate, trichloroethylene, propylene oxide, methanol, n-butyl ether, isobutyl alcohol and acetonitrile do not interfere with the sensor. The acetone signal strength is only 1% of that of CS₂. Based on the above analysis, it is shown that the sensor detects CS₂ with high selectivity.

In addition to excellent selectivity, the service life is also critical to the sensor. In order to test the life of the CdS/SnS sensor, the sensor was allowed to work for 8 h a day for two weeks. CS₂ vapor was continuously introduced into the reaction chamber, and the CTL intensity was measured every 2 h. The CTL intensity remained almost unchanged for two weeks under high-intensity long-term operation; 10 sets of representative data were taken, and the results are shown in Figure 10b. The CTL signal intensity change is small, the repeatability is better, the relative standard deviation (RSD = 4.82%) of the 10 measurements is less than 5%, and the results show that the CTL sensor has good stability and a long service life in detecting CS₂.

3.7. Mechanism Discussion

According to the commonly accepted mechanism of the CTL reaction [39], oxygen flows through the surface of the catalyst along with the carrier gas; excited intermediates are formed during the catalytic oxidation of CS₂ and the surface of the sensitive material, and luminescence can be detected when these excited intermediates fall to the ground state. Based on a comprehensive study of multiple gas sensitivity tests, the possible mechanism of CS₂ CTL is speculated.

For CTL gas-sensitive sensors, the CTL reaction that occurs on the surface of the solid catalytic sensing material can be divided into three steps: (1) the adsorption process, (2) the reaction process and (3) the desorption process. The first is the adsorption stage, in which ${\mathrm{O}}_2$ molecules enter the reaction chamber with the carrier gas, adsorb on the surface of the sensing material and extract free electrons from the conduction band of the material to form chemically adsorbed oxygen species (${\mathrm{O}}_2^{-}$, ${\mathrm{O}}^{-}$ and ${\mathrm{O}}^{2-}$), where ${\mathrm{O}}^{-}$ is considered to be the main reaction that is produced and plays a key role. As a representative n-type semiconductor gas-sensing material, SnS more readily exchanges lattice oxygen with oxygen in the air, improving the conversion efficiency of oxygen ions. The conversion process of oxygen ions is as follows [40,41]:

$${\mathrm{O}}_2 \left(\mathrm{gas}\right) \to {\mathrm{O}}_2 \left(\mathrm{ads}\right)$$

(3)

$${\mathrm{O}}_2 \left(\mathrm{ads}\right) + {\mathrm{e}}^{-} \to {\mathrm{O}}_2^{-} \left(\mathrm{ads}\right)$$

(4)

$${\mathrm{O}}_2^{-} \left(\mathrm{ads}\right) + {\mathrm{e}}^{-} \to 2{\mathrm{O}}^{-} \left(\mathrm{ads}\right)$$

(5)

$${\mathrm{O}}^{-} \left(\mathrm{ads}\right) + {\mathrm{e}}^{-} \to {\mathrm{O}}_2^{-} \left(\mathrm{ads}\right)$$

(6)

The more oxygen vacancies on the surface of the material, the higher the adsorbed oxygen content on the surface of the material. Oxygen vacancies are conducive to the working effect of CTL sensors, which can improve the type and content of oxygen adsorbed on the surface. When Cd²⁺ ions are present in the material, oxygen vacancies are bound to occur in order to maintain their electrical neutrality. Oxygen vacancies are an excellent adsorption site, which can increase the adsorbed oxygen content on the surface and accelerate the migration rate of oxygen. Oxygen atoms are important participants in the electron exchange between sensing materials and gas molecules, so the increase in the type and content of adsorbed oxygen plays a crucial role in improving the performance of CTL sensors [42].

Element doping is now considered to be a method that can effectively enhance the CTL gas-sensing performance. With the increase in the Cd element doping ratio, more ${\mathrm{O}}_2$ molecules will be adsorbed on the surface of the composite, more electrons (${\mathrm{e}}^{-}$) will be captured from the conduction band (EC) of the sensing material, and the conversion efficiency of electrons will increase. At the same time, ${\mathrm{O}}_2$ molecules are also used to convert the target gas into ${\mathrm{SO}}_2$ later, and the CTL phenomenon is formed by the state change of the product [43].

The most direct principle of CTL is the luminescence phenomenon produced when the excited state products produced during the catalytic oxidation reaction return to the ground state. In the catalytic oxidation of CS₂, CS₂ gas molecules are adsorbed on the surface of CdS/SnS composites and further react with the adsorbed ${\mathrm{O}}^{-}$ ions to form excited CS₂ (${\mathrm{SO}}_2$*), and the excited ${\mathrm{SO}}_2$* molecules then return to the basic state and emit light. Therefore, in this experiment, the possible reaction process of CS₂ is:

$$\mathrm{C}{\mathrm{S}}_2 \left(\mathrm{gds}\right) \to \mathrm{C}{\mathrm{S}}_2 \left(\mathrm{ads}\right)$$

(7)

$$\mathrm{C}{\mathrm{S}}_2 \left(\mathrm{ads}\right) \to \mathrm{C}{\mathrm{S}}^{4+}+{\mathrm{S}}^{4-}$$

(8)

$$\mathrm{C}{\mathrm{S}}^{4+}+4{\mathrm{O}}^{-} \to \mathrm{C}{\mathrm{O}}_2+\mathrm{S}{\mathrm{O}}_2$$

(9)

$$\mathrm{S}{\mathrm{O}}_2* \to \mathrm{S}{\mathrm{O}}_2+\mathrm{hv}$$

(10)

4. Conclusions

A CdS/SnS composite was prepared, and the cataluminescence properties were investigated. The optimal reaction conditions are determined to be 162 °C and 280 mL/min. Under the optimized experimental conditions, different concentrations of carbon disulfide gas are introduced into the reaction chamber, and in the range of 6.75–168.75 ppm, the cataluminescence intensity is proportional to the concentration of the gas to be measured, with a linear regression equation of y = 187.95x − 903.65 (R² = 0.9974, n = 6), along with a low detection limit of 0.96 ppm. The selectivity and stability of the sensor were studied, and the selectivity study shows that the sensor has high selectivity for carbon disulfide and can be used for the detection of carbon disulfide. The relative standard deviation (RSD = 4.82%) measured continuously in the stability experiment is less than 5%. The as-prepared nanocomposites working with carbon disulfide could produce a strong cataluminescence phenomenon, which shows that CdS/SnS nanojunctions may be a new cataluminescence-sensitive material, which provides a promising method for the development of high-performance low-temperature cataluminescence carbon disulfide detectors.