2D SnSe2 micro-flower decorated with 0D In2O3 nanoparticles for low-temperature low-concentration TEA detection

Abstract

Triethylamine, a crucial industrial raw material, poses significant threats to both the ecosystem and human health. However, detecting lower concentrations of TEA remains an arduous task. In this study, we report the facile hydrothermal and ultrasonic treatment synthesis of 2D SnSez micro-flower modified with OD In203 nanoparticles to form SnSe,/In203 heterojunctions for the first time. The SnSe2/In203 sensor has a response value of 4.86 for 10 ppm TEA gas at 120 °C, with response and recovery times of 18 s and 79 s respectively, and detection limits as low as 100 ppb. In addition, the SnSe2/In203 sensor is essentially unaffected by humidity in the 30% RH to 60% RH range, and the SnSe2/In2O3 sensor response value decreases slightly in the 70% RH to 97% RH range, demonstrating excellent humidity tolerance. More importantly, the sensor maintained excellent cyclicstability performance during a four-month cyclic stability test. The improved gas-sensitive performance can be attributed to the large number of n-n heterojunctions in the SnSez/In203 material, which enhances the interfacial charge transfer, as well as the active-sites on the material surface. This work serves as a valuable complement to the TEA gas sensor and holds significant potential for detecting low concentrations of TEA at low temperatures in environmental sensing applications.

Headnote

Keywords:

SnSe>

In2O2

Ultra-sonication composite

Triethylamine sensor

n-n heterojunctions

Low-power consumption

ABSTRACT

Triethylamine, a crucial industrial raw material, poses significant threats to both the ecosystem and human health. However, detecting lower concentrations of TEA remains an arduous task. In this study, we report the facile hydrothermal and ultrasonic treatment synthesis of 2D SnSez micro-flower modified with OD In203 nanoparticles to form SnSe,/In203 heterojunctions for the first time. The SnSe2/In203 sensor has a response value of 4.86 for 10 ppm TEA gas at 120 °C, with response and recovery times of 18 s and 79 s respectively, and detection limits as low as 100 ppb. In addition, the SnSe2/In203 sensor is essentially unaffected by humidity in the 30% RH to 60% RH range, and the SnSe2/In2O3 sensor response value decreases slightly in the 70% RH to 97% RH range, demonstrating excellent humidity tolerance. More importantly, the sensor maintained excellent cyclicstability performance during a four-month cyclic stability test. The improved gas-sensitive performance can be attributed to the large number of n-n heterojunctions in the SnSez/In203 material, which enhances the interfacial charge transfer, as well as the active-sites on the material surface. This work serves as a valuable complement to the TEA gas sensor and holds significant potential for detecting low concentrations of TEA at low temperatures in environmental sensing applications.

ARTICLE INFO

(ProQuest: ... denotes formulae omitted.)

1. Introduction

Triethylamine (TEA) has been widely used as catalysts, preservatives, surfactants, organic solvents, etc. in fisheries, agriculture, medicine and health, aviation and many other fields [1-4]. Moreover, because decaying fish and shellfish release TEA, it is often used to test the freshness of marine fish [5]. However, human inhalation or dermal exposure to TEA can cause severe tissue damage [6]. TEA is also a potent carcinogen, which can cause pulmonary edema, laryngitis, asthma and other diseases [7]. When the TEA content in the air is too high, there may be a potential explosion hazard [8]. According to the Occupational Safety and Health Administration (OSHA), the permitted TEA' s concentration in the air should not outstrip 10 ppm [9]. Both the American Conference of Governmental Industrial Hygienists and the European Commission have established limits of 1 ppm (8 h TWA (Time-weighted average)) for TEA [8,10,11]. Commercial TEA detection methods mainly include gas tube detection and gel chromatography [12]. However, the cumbersome testing process and high testing costs limit their further development in practical applications [13]. Therefore, the development of a simpler and cheaper miniature TEA sensor for real-time TEA monitoring in complex environments is essential.

In recent decades, resistive gas sensors based on metal-oxidessemiconductor (MOS) have made significant progress due to their advantages of simple preparation, high sensitivity, and simple back-end electrical signal processing [5,14-18]. Park et al. reported that Bi»O3-decorated InyO3 nanorods showed an extremely high response to 200 ppm ethanol at 200 °C, but it also showed a significant response to gases such as methanol [19]. Shi et al. reported that Co304/In203 (2.5 wt%) HMs showed excellent gas sensing performance for TEA, but still required high temperature operation at 250 °C [20]. In a word, the main problems faced by these materials are the potential risks posed by high operating temperatures and the low selectivity caused by various gases surface adsorption. In recent years, unique atomic layered structure of two-dimensional (2D) transition metal dihalide compounds (TMDs) have receiving much attention, because of their high specific surface areas close to theoretical extremes, excellent semiconductor properties, superior gas adsorption capacity and lower operating temperature [21,22]. Typically, SnSez layers are stacked in the form of Se-Sn-Se with a CdIy-type hexagonal structure, which is a typical anisotropic binary layered material [23,24]. Atoms within the same layer are bound by covalent bonding, while layers are bonded by van der Waals weak interactions [25-27]. Due to its huge van der Waals surface and high surface electron mobility, SnSe> has unique advantages in the field of gas sensors [28-32].

In recent years, to further improve the sensing performance of gas sensors, researches have been focusing on developing new advanced materials by combining the characteristics of various types of nanomaterials, making MOS and TMD each other through morphology adjustment, doping, noble-metal modification, construction of heterogeneous structures, etc. [33]. Taking TMDs SnSe; as an example, a large number of studies mainly focus on detection of relatively small harmful gas molecules and improvement of gas sensitivity. Guo et al. prepared an In-doped SnSez sensor with excellent sensitivity for detecting SO, gas at room temperature, but it has a disadvantage of long response time [34]. Pan et al. used an Au modified SnSez sensor prepared by a hydrothermal method, to detect №3 repeatedly and stably at room temperature, but the sensor has the disadvantages of a long response time and poor selectivity [35]. Nonetheless, SnSez has shown excellent gas-sensitive performance in the detection of small molecule gases (e.g., NO; [24], SO, [36], Ho [37], NH3 [35], CH4 [38], etc.) at low temperatures. However, current research seems to be focused only on detection of the above gases, and the applications of gas sensors with SnSez as a sensitive material to detect VOC gas at low temperatures is almost blank.

In this work, a novel n-n heterojunction of layered SnSe> micro-flower decorated by In203 nanoparticles is designed, to increase the number of active sites and accelerate charge transport. The synthesis of 2D SnSe, micro-flower modified with zero-dimensional (OD) In203 nanoparticles by hydrothermal and ultrasonic treatments and the formation of SnSez/ In203 heterojunctions are reported for the first time. Low-temperature TEA gas detection performance of the SnSe»/In203 composite sensor was systematically investigated. The SnSe>/In>03 sensor has a response value of 4.86 for 10 ppm TEA gas at 120 "C, with response and recovery times of 18 s and 79 s respectively, and detection limits as low as 100 ppb. Compared with original SnSez and In2O3 sensors, the SnSe2/In203 sensors showed significant improvements in response value, response/ recovery time, repeatability, detection sensitivity, humidity tolerance and stability. The formation of n-n heterojunctions between SnSez and In203 nanomaterials was confirmed using TEM and XPS results. The mechanism of the enhancement in sensing TEA by the SnSe»/In203 gas sensor at low temperatures is elucidated based on energy band theory.

2. Experimental section

2.1. Synthesis of SnSez 2D micro-flower

Firstly, 455 mg SnCla-2H20 was added to a solution consisting of a 1:1 solution of deionized water and ethylene glycol with magnetic stirring until complete dissolution. Secondly, 445 mg SeO, was vigorously stirred into the solution obtained in the first step until it reached an apricot color. Finally, the above solution was slowly titrated with 3 ml of hydrazine hydrate until a brick red color developed. The brick-red solution was subjected to a hydrothermal reaction at 180 °C for 20 h in a hot oven, resulting in the formation of SnSez sample.

2.2. Synthesis of In203 nanoparticles

For prepare In203 nanoparticles, 1 g In(NO)3-4.5H,0 was added to a mixture solution of isopropanol and deionized water in a volume ratio of 3:5 under vigorous magnetic stirring until the solution became clear again. The above solution was supplemented with 1 g of CTAB (Hexadecyl Trimethyl Ammonium Bromide) and stirred until the additive had fully dissolved. Finally, NaOH was added to adjust PH to 8. The solution underwent a hydrothermal reaction at 200 °C for 20 h in an oven. The resultant precipitate was subjected to centrifugation, drying, and annealing processes in order to obtain In203 nanoparticles.

2.3. Synthesis of 2D/OD SnSe2/In203 n-n heterostructures

For the synthesis of SnSe2/In203 composites, the following procedure was employed: Firstly, Firstly, 50 mg of pre-prepared SnSez powder introduced into a beaker containing 40 mL of deionized water. Subsequently, the dispersion was subjected to ultrasonication at a power of 150 W for 25 min to achieve a homogeneous dispersion with a concentration of 1.5 mg mL·. Then, different masses of InyO3 nanoparticle powders (5 mg, 10 mg, 15 mg, 20 mg) were added to the above dispersion solution and ultrasonicated for 1 h to form composites with SnSe>. Finally, the dispersion was thoroughly washed, centrifuged and subjected to vacuum drying in order to obtain a high-quality SnSez/In203 composite powder.

2.4. Material characterization

The morphology, structure and composition of the samples were analyzed using field emission scanning electron microscope (FESEM, AMBER) and field emission transmission electron microscope (FETEM, JEOL JEM-F200), as well as energy dispersive X-ray spectroscopy (EDS) equipped with TEM instruments. The synthesized sample's phase forms were characterized using a SmartLab-3kW X-ray powder diffractometer (XRD) with Cu Ka radiation. X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi) was utilized for characterizing the elemental species and chemical states present, with binding energy being adjusted to the C 1s energy of 284.8 eV as a reference point. The specific surface area and pore size distribution of SnSe> and SnSe2/In203 were determined by a specific surface area analyser (JW-BK122W, BET). The direct band gaps of SnSe> and SnSe2/In2O3 samples were obtained by UV-visible spectrophotometer (Hitachi U-4100) measurements. The work functions of the SnSe> and In203 samples were measured using ultraviolet photoelectron spectroscopy (UPS, Thermo Fisher Escalab Xi+).

2.5. Device fabrication and sensing performance measurement

The sensing devices are prepared and tested as shown in step 3 of Fig. 1. The sensors devices, utilizing In203, SnSe> and SnSez/In203 materials, were fabricated using a simple coating strategy as outlined below. In brief, 10 mg of prepared powders were placed into an agate mortar and a dropwise addition of ethanol solution was made while grinding the mixture in a clockwise direction until it became a homogeneous slurry. The thickness of the SnSe>/In203 composite sensing-material coated on commercial ceramic-tubes is about 10 pm. Next, delicately apply the sample onto a commercial ceramic tube (purchased from Winsen Electronics) using a hook pen. Subsequently, weld the ceramic tube to the base of six terminal posts and utilize it as a gas sensing cell. Finally, the fabricated sensor was subjected to vacuum drying at 60 °C for 24 h prior to subsequent sensing tests.

Gas sensing characteristics were evaluated using a static system (WS30B, Winsen Electronic Technology Co. Ltd., China). The sensor is situated within the 18 L testing chamber that has been filled with air, and once the resistance stabilizes at a value of Ra, the introduction of the target gas into the testing chamber may cause a change in sensor resistance to Rg. The sensing response is defined as R-RA,/Rç.

3. Results and discussion

3.1. Microstructure characterization of 2D/0D SnSe2/In203 n-n heterojunction

The phase structure of the prepared crystals was analyzed using X-ray diffraction (XRD). Fig. 2 displays the XRD spectra of In,O3 (red), SnSe; (blue), and SnSe2>/In203 (black) composites. The simultaneous presence of diffraction peaks for In203 and SnSe, in the SnSey/In,O3 composite indicates successful preparation of the composite sample via ultrasonic method.

The surface morphology of In203, SnSe> and SnSe2/In2O3 samples was analyzed by FESEM. The SEM images in Fig. 3 (a) display the SEM images of pure SnSe>. SnSe> is uniformly distributed into flower-like structures throughout the field of view. Fig. 3 (b) depicts In203 with a standard OD nanoparticle structure, exhibiting diameters ranging from 20 to 50 nm. The SEM images of the SnSez/In203 composite are shown in Fig. 3 (c). The overall structure of the composite material is loose, which is conducive for the diffusion of gases molecules inside the sensing material. Fig. 3 (а) shows the magnified details of the SnSe2/In203 composite. The results show that the OD In>03 nanoparticles are closely grown on the surface of the 2D SnSe> micro-flower, which is conducive to the formation of heterojunctions [39].

TEM, HRTEM, SAED and EDS were used to further investigate the structural characteristics and elemental distribution of the synthesized materials and the detailed results are shown in Fig. 4. As shown in Fig. 4 (a), the TEM image of the SnSe2/In2O3 composite exhibits a thin sheetlike structure, which coincides with the observation by SEM. In addition, it is obvious that the In203 nanoparticles are distributed on the surface of the SnSe> micro-flower. This provides more reactive sites for oxygen adsorption reactions. In Fig. 4 (b), the high magnification TEM image shows clearly that the In,O3 nanoparticles are tightly suspended on the surface of the SnSe> micro-flower, thus it is reasonable to speculate that there exists n-n heterojunctions interface between the In,O3 папоparticles and SnSe; micro-flower. The HRTEM images of SnSe2/In2O3 composites are shown in Fig. 4 (с). The red stripes in the figure correspond to crystal plane spacing d of 0.292, 0.184 and 0.128 nm for (222), (521) and (111) crystal planes of In>03, respectively. The blue stripes in the figure correspond to crystal plane spacings d of 0.191 and 0.291 nm corresponding to the (110) and (101) crystal planes of SnSe>, respectively. The above results, in agreement with the XRD results, indicate that heterogeneous junction surfaces do exist between SnSe> and In203. SAED in Fig. 4 (а) shows that the SnSe> micro-flower match the hexagonal crystal system, while the In203 nanoparticles correspond to the cubic crystal system. Both show a polycrystalline trend. This can also corroborate the formation of n-n heterogeneous junction surfaces. The EDS mapping image of SnSez/In,03 are shown in Fig. 4 (e). It is clearly observed that the distribution of In and O elements is showing granularity. And the distribution of Sn and Se elements is almost distributed in the lower left part of the picture. The Sn element shows enhanced signal at the In203 particles probably due to the fact that Sn and In elements are adjacent elements and EDS is not sufficient to distinguish them clearly. Fig. 4 (f) can explain this point well.

The XPS analysis is employed to verify the elemental composition and chemical state of the prepared material. The high-resolution Sn 3d spectra of SnSe> and SnSe2/In203 composite nanomaterials exhibit two distinct peaks, as illustrated in Fig. 5 (a). The peaks observed at 486.3 and 494.8 eV of the SnSe> samples can be attributed to photoelectron emissions from Sn 3ds,» and Sn 3d3, orbitals, respectively. The results indicate that the predominant chemical state of Sn is Sn··, which is in agreement with SnSe, [40]. Compared to the Sn3d spectrum of pristine SnSey, the Sn 3d3,2 and 3d5/2 spin orbitals of SnSe>/In203 exhibit a slight shift towards higher binding energies at 486.8 and 495.3 eV respectively, indicative of the change of chemical environments of Sn atoms [41]. Most probably due to the formation of SnSe,/In;03 heterojunction. Fig. 5 (b) shows the high-resolution spectra Se 3d of SnSez and SnSe»/In203 composite nanomaterials. The binding energies located at 54.4 and 55.5 eV belong to Se 3d5/2 and Se 3d3/2, respectively, indicating the presence of the oxidation state of Se" in the SnSe> sample [42]. The high-resolution In 3d spectrum (Fig. 5 (c)) can be divided into In 3d3/2 (451.6 eV) and In 3d5/2 (444.1 eV), matching the chemical composition of Iny03. The За peaks of In in the SnSey/In,O3 composites were 451.3 eV and 443.8 eV, which were shifted by 0.3 eV toward the lower binding energy compared with the pristine In203 nanomaterials, probably due to the electron gain of In203 from SnSe> [43,44]. The high-resolution O1s spectra of the In203 and SnSe2/In2O3 samples are shown in Fig. 5 (d). The peaks at 530.2, 532.2 and 533.3 eV can be attributed to lattice oxygen (Oy), defective oxygen (Oy) and adsorbed oxygen (Oc) on the sample surface. Compared with the In;O; nanomaterials, the Op in the SnSe>/In203 composite nanomaterials slightly decreased, while the percentage of Oy and Oc showed an obvious increment. This indicates the presence of more adsorbed and defective oxygen in SnSe>/In>O3z compared to In203.

3.2. Gas-sensing properties of SnSe>/In203 n-n heterojunction

The gas-sensing performance of resistive gas sensors depends on the adsorption/desorption of gases on the surface of the sensitive material. The adsorption/desorption of gases is highly dependent on the surface state of the material, the activation energy barrier that must be overcome for a chemical reaction to occur between the surface and the gas. To explore the effect of the activation level of the surface reaction on the In203, SnSe> and SnSe,/In203 sensors, the response of the three sensors to 10 ppm TEA was examined over a range of operating temperatures. As shown in Fig. 6 (a), the response of all three sensors increases with increasing the temperature until 200 °C. At temperatures above 200 °C, pure In203 and SnSe2/In2O3 rapidly increase, while the response of the pure SnSe; sensor decreases. Firstly, at room temperature, the response values of all three sensors are not high. This is due to the lower activity of the gas at lower temperatures. When the temperature increases, the activity of the gas reaction on the surface of the gas-sensitive material gradually increases. For the two-dimensional gas-sensitive material (SnSey), the main active site comes from the dangling bonds at the edges [45]. The number of such active sites is very limited, as a result, SnSe> has a low response value to TEA gas at any temperature. The decrease in SnSe; response may be caused by the high temperature, where the rate of resolution of TEA gas in the SnSe> material is greater than the rate of adsorption. For InyO3 sensing materials, which are metal oxides, the operating temperature is usually in the range of (200 to 500 °C) [46]. As the working temperature increases, the active sites in In>O3 are gradually activated. the adsorption rate of TEA gas on the surface of In>O3 is greater than the desorption rate, and the response is gradually strengthened. The response value reaches the maximum when the dynamic equilibrium is reached. After that, the desorption rate was larger than the adsorption rate, and the response showed a decreasing trend. The reason for the increased response values of SnSe,/In203 sensitive materials is in addition to the above-mentioned-advantages of SnSe> and In203 sensitive materials. n-n heterojunction resulting in charge transfer also plays an important role. Fig. 6 (b) shows the resistance of all sensors in air at a temperature range from 20 to 300 °C. It can be seen that the resistance of all sensors in air gradually decreases as the operating temperature increases. It indicates that the carrier concentration in the sensing material gradually increases and the reactive activity of the material surface gradually increases. Notably, when comparing the Ra of the three sensors at identical temperatures, there is a tendency that In203 to exhibit higher resistance than SnSez/In203 and SnSe». Both SnSez and In Og are n-type semiconductors, and the work function of InyOs is larger than that of SnSe». Therefore, the electron carriers transfer to from SnSe> to In,03 due to the formation of SnSez/In203 heterojunctions, such that the overall resistance (Ra) of the SnSe,/In,03 sensor was increased because of less carrier centration. The modification of In203 nanoparticles increases the specific surface area of SnSez microscale flowers, providing more catalytically active sites for gas diffusion and adsorption. Oxygen molecules will preferentially adsorb on In,03 nanoparticles and then dissociate into O5. The large amount of Oz adsorption increases the base resistance of the In;O3-modified SnSez micelles. When triethylamine gas molecules enter, they can react with more adsorbed O3 and release electrons, which leads to a rapid change in resistance and a significant improvement in gas-sensitive performance. Fig. 6 (с) shows the response time graph of the SnSe>/In>0O3 sensor at different operating temperatures. It is evident that as the operating temperature increases to 120 °C, the response and recovery time exhibit a rapid reduction while the response value experiences a significant enhancement. Considering the power consumption issue and response time of the sensor, subsequent measurements will be conducted at 120 °C to evaluate sensing performance. The effect of the ratio of the two substances on the gas-sensitive properties was also discussed (Supporting Information).

Important parameters to evaluate the gas sensitivity performance of gas sensors also include the selectivity for different gases. The gassensitive response of all samples to different gases is shown in Fig. 7 (a). It is easily found that all sensors, especially the SnSe>/In203 sensor, exhibit a great selectivity for TEA in comparison to 10 ppm toluene, ammonia, formaldehyde, acetone, ethanol, ethylene glycol, and nitrogen dioxide. In addition, the response of SnSe2/In2O3 to different gases is significantly higher than that of In203 and SnSe», further indicating that SnSe>/In>O3 is more suitable as a TEA gas sensing material. The good TEA selectivity may be due to its low C-N bond energy [47,48]. The response characteristics of the three sensors at 120 °C for different con.

. . centrations of TEA are shown in Fig. 7 (b). The response values of the SnSe /In,03 sensor for TEA at 0.1, 0.5, 1, 10, 25, 50, 100, 200 and 300 ppm are 1.76, 2.11, 3.65, 4.81, 7.09, 7.39, 8.16, 9.13 and 9.98, respectively. In comparison, the response values of SnSe> were 1.32, 1.33, 1.51, 2.36, 3.72, 4.54, 5.02, 5.41 and 5.93, respectively, and the response of SnSe> micro-flower modified with In203 nanoparticles is improved by a factor of 1.83 on average. Correspondingly, the response values of In203 ? are 1.01, 1.09, 1.65, 1.68, 3.00, 3.84, 3.85, 6.12, and 7.52, respectively, and the response of SnSez micro-flower modified with In203 nanoparticles is improved by a factor of 1.95 on average. Fig. 7 (c) illustrates the dynamic resistance curve of all sensors exposed to varying concentrations (100 ppb-300 ppm) of TEA gas. It is evident that the SnSe>/In>203 sensor exhibits a somewhat suppressed baseline drift in comparison to the SnSe> and In203 sensors. Fig. 7 (?), (e) and (f) show the experimental results of the recovery characteristics of the sensor response at 120 °C for SnSe2/In203, In203 and SnSe> sensors at 10 ppm TEA, respectively. The response/recovery times of SnSe2/In203, In203, and SnSe> sensors were 18 5/79 5, 32 s/133 5, and 10 s/169 s respectively. the recovery time of 79 s seems to be relatively metal-oxide-semiconductors longer because of the low working temperature of 120 °C [5,18]. As can be seen from Table 1, the prepared SnSe2/In2O3 sensor has obvious advantages in terms of operating temperature, gas response, and detection concentration compared with the previously TEA sensors based on 2D materials [3, 40-44].

It is important to note that the relative humidity of the environment also affects the gas-sensitive performance of the sensor. Therefore, it is essential to investigate the response recovery characteristics of the sensor at different humidity levels. The dynamic humidity response curves of the SnSe2/In2O3 sensor in a TEA gas environment were investigated at 120 °C, with changes of relative humidity (RH) from 30% to 97%. Results are presented in Fig. 8 (a) and (b). It can be verified that the SnSe2/In203 sensor works properly at different values of relative humidity. Interestingly, the response of the SnSe>/In203 sensor exhibits an upward trend in the range of 30-60% RH when exposed to TEA, followed by a decline from 60% RH to 97% RH and reaching its peak at 60% RH. The response change trend observed in Fig. 8 (с) is consistent with the findings reported by Guo et al. [56]. Unlike previous reports, the base resistance of our prepared sensors shows a tendency to increase and then decrease. In the range of 30-60% RH, H,0 and O; molecules are competitively adsorbed on the surface of the material, confining the electrons on the surface and leaving the material in a high resistance. This is expressed as a decrease in the concentration of electrons involved in the conductivity of the sensor, resulting a higher base resistance. In the range of 60-97% RH, a Grotthuss transport mechanism is established on the surface of the material due to the high relative humidity. As the humidity increases, the concentration of hydrogen ions increases, causing the sensor resistance to decrease. As shown in Fig. 8(d), the SnSe>/In203 sensor response recovery time becomes longer as the relative humidity increases. The longer response recovery time may be due to the fact that gas molecules diffuse at a lower rate than in dry air environments when the relative humidity increases [57].

Reproducibility and long-term stability are important parameters for gas sensors. Eight consecutive cycles of the SnSe2/In203, In203 and SnSe> sensors at 10 ppm TEA are shown in Fig. 9 (a). All three sensors have a stable response to TEA, however, the SnSe»/In,03 sensor has an enhanced response value compared to the other two sensors. In addition, the introduction of SnSe> micro-flower can effectively suppress the baseline drift of conventional In203 sensors at low temperatures. This provides a reliable new idea for the detection of VOCs gases in conventional MOS at low temperatures or even room temperature. Thereafter, the stability performance of the three sensors against 10 ppm TEA gas was tested at 120 °C for 8 cycles at an elapsed time of 1, 2, 3, 4 months. The results are presented in Fig. 9 (b), (с) and (а), respectively. It can be seen that SnSe2/In2O3 sensor shows a good cycling performance with a slight decrease in the response value after 4 months. The response and recovery times remain essentially unchanged. More detailed statistics of the response values for the SnSe>/In203 composite sensor are shown in Fig. 10. Compared to the first month, the response only decreased by 2.13% in the second month, 3.38% in the third month, and 15.2% in the fourth month. This indicates that the prepared SnSe>/In203 composite sensor possesses excellent reliability in long-term cycling tests. The enhanced stability of SnSe2/In2O3 sensors in air may be related to the depletion layer formed at the interface of the n-n heterojunction [58].

3.3. Sensing mechanism of SnSez/In203 n-n heterojunction

In order to get a comprehensive understanding of the sensing mechanism of the prepared sensor, a schematic diagram of the TEA adsorption model is shown in Fig. 11. According to previous reports, SnSe> is a typical n-type gas-sensitive material, and its main carriers are electrons [59]. When a pure SnSe> sensor is exposed to air at 120 °C, O2 (gas) molecules are adsorbed onto the surface of the SnSez sensing material to form a large amount of O, (ads) molecules. O; (ads) molecules can be converted into chemisorbed oxygen anions (05) by trapping free electrons from the conduction band of SnSe> sensing materials as shown in Egs. (1) and (2) [35,60].

... (1)

... (2)

When injected with TEA gas, TEA molecules can be adsorbed on the SnSe> surface and react with the highly reactive Oz adsorbed on the SnSe> surface, as illustrated in equations (3) to (6) [61]. This process releases the captured electrons back into the conduction band, and as a result, a sharp drop in the resistance of the sensor can be observed [61].

... (3)

... (4)

... (5)

... (6)

For the SnSe>/In203 sensor, the selectivity measurements demonstrated that in all tested gases due to both SnSe; and In203 showed comparatively strong adsorption capability to TEA gases. It is believed that the combination of SnSes and In203 contributes to the increased TEA gas sensing performance. The enhanced gas sensing performance of the obtained SnSe2z/In,03 sensor may be attributed to the following factors.

(1) The unique 2D layered structure of SnSez micro-flowers provides a large surface area (Supporting Information Fig. S2), while In203 nanoparticles on the SnSe, surface increases the number of surface-active-sites. XPS results demonstrated that the introduction of In203 nanoparticles to the surface of SnSe> flowers led to obvious increment in the concentration of chemisorbed oxygen molecules and oxygen vacancies in In203 nanoparticles, which can provide more active sites for the SnSey/ In203 sensor. Hence, larger contact areas and more reaction sites are introduced on the surfaces of SnSe>/In>0O3 for target gas molecules, which enhance the oxidation reaction and results in a significantly higher gas response [62].

(2) The electron migration through the formed SnSe;/In;O3 n-n heterojunction plays a crucial role in modulating the gas-sensitive properties of SnSey/InyO3 composites. As shown in Fig. 11 (a), the work functions of SnSe> and In,03 are approximately 3.7 eV and 5.0 eV (Supporting Information Fig. S3). The bandgaps of SnSe; and InyO3 are about 1.72 eV and 3.6 eV. Thus, when the SnSe> micro-flowers and InyO3 nanoparticles are in contact each other, electrons can easily immigrate from the conduction band of SnSe; to the conduction band of InyO3 until their Fermi energy levels are balanced. In addition, the direct band gaps of SnSez and SnSe,/In,03 were calculated based on UV-visible diffuse tance spectra Information Fig. S4). The results show that the bandgap of SnSe2/In2O3 composites increases by 0.59 eV compared with that of SnSe> micro-flower material, which may be attributed to the migration of electrons from the SnSe> side to In203 after the formation of the n-n heterojunction, resulting in the shift of conduction bands towards the Fermi energy level. Accordingly, a schematic diagram for a SnSe; micro-flower decorated with InyO3 nanoparticles and the corresponding energy band structure of SnSe>/In>03 when exposed to air are drawn in Fig. 11 (b). It's well known that an electron depletion layer with certain thickness in n-n heterojunction can be formed due to the contact between InyO3 nanoparticles and SnSez nanosheet. In air, because of the electron capture on the chemisorption O2 molecules, more electrons immigrate through the n-n heterojunction from the SnSe; nanosheets to In203 nanoparticles, which further thicken the electron depletion layer. Thus, the main conductive moiety of the SnSez/In203 composite, i. e. the petals of SnSe> micro-flowers, is in a highly resistive state due to decrement in the density of electron carries of SnSey. Exposed to TEA gas, as shown in Fig. 11 (с), TEA molecules react with adsorbed O; while releasing electron trapped by Oj as illustrated in Egs. (3) to (6). These electrons are returned to the SnSe> petal of the microflowers through the n-n heterojunction until the Fermi level of SnSe> and InyO3 are balance, providing plenty of carries and leaving the SnSe>/In203 sensor in a lower resistance state. Therefore, the presence of the n-n heterogeneous interface enhances the electron migration between the SnSe> micro-flowers and In>03 nanoparticles as well as the charge transfer between the SnSe>/In203 sensing-material and the gas molecules, which strengthens the gas-sensitive performance of the SnSez/In203 Sensor.

(3 The high selectivity of SnSe>/In203 composites for TEA is mainly attributed to the properties of TEA gas molecules and the strong effect between SnSe2/In2O3 composites and the target gases. The bond dissociation energy of TEA (C-N, 307 kJ/mol) is lower than that of №3 (N-H, 386 kJ/mol), ethanol/ethylene glycol (O-H, 458.8 kJ/mol) and toluene (C=C bond, 798.9 kJ/mol), suggesting that the reactivity of TEA molecules with chemisorbed oxygen is higher than that of other gas molecules [63]. The nitrogen atoms in TEA have a lone pair of electrons, which can be donated to form a bond, and thus TEA molecules are more inclined to be adsorbed on the cationic site. In addition, the increase in the number of oxygen vacancies as active reactive sites can improve the gas-sensitive performance of the SnSe»/In203 sensor.

Generally, the construction of n-n heterojunction between SnSe> and In203 provides large specific surface area and abundant active sites, which can be maximized the overall sensing performance of TEA gas Sensors.

4. Conclusion

In summary, SnSe2/In203 composites were synthesized via a hydrothermal-ultrasonic composite method. Meanwhile, the gassensitive properties of SnSej, In,O3 and SnSe,/In203 sensors were systematically investigated. The SnSe>/In203 sensor has a response value of 4.86 for 10 ppm triethanolamine gas at 120 °C. Response and recovery times are 18 and 79 5, respectively. The detection limit is as low as 100 ppb. In addition, the SnSey/In,O3 sensor is essentially unaffected by humidity in the range of 30% RH to 60% RH, whereas in the range of 70% RH to 97% RH, the response of the SnSe>/In203 sensor decreases slightly. Thus, the SnSez/In203 sensor is shown to have excellent humidity resistance. More importantly, the sensor maintained excellent cyclic stability performance during a four-month cyclic stability test. The TEM and XPS results have confirmed that the enhanced gas-sensitive performance of SnSe>/In203 is primarily attributed to the formation of an n-n heterojunction between the SnSe> micro-flower and InyO3 папоparticles. The TEA sensing mechanism was elucidated through the application of energy band theory. The results showed that an n-n heterogeneous interface is formed between SnSe; and In203, and the carrier motion under the action of the built-in electric field increases the electron concentration on the surface of InyO3, leading to an enhanced oxygen adsorption reaction and an increase in the charge depletion region on the surface of the sensing layer. Therefore, SnSe2/In2O3 sensors prepared via simple methods exhibit great potential as efficient detectors for lowconcentration TEA detection at low temperatures.

CRediT authorship contribution statement

Li Wang: Conceptualization, Investigation, Methodology, Writingoriginal draft. Jianpeng Li: Writing-review & editing, Validation, Formal analysis. Cheng Xu: Writing-review & editing, Project administration, Supervision. Ziqin Yang: Resources, Validation, Formal analysis. Xiangyun Tan: Validation, Formal analysis, Visualization. Zhihu Dong: Validation, Formal analysis, Visualization. Li Xu: Validation, Formal analysis, Visualization. Dongwei Zhang: Validation, Formal analysis, Visualization. Chunging He: Writing - review & editing, Funding acquisition, Project administration, Supervision.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgements

of China (NSFC) (Nos. 12075172 and 12375288), National Key R&D

Program of China (Grant No. 2019YFA0210003). We thank the Core

Facility of Wuhan University for Field Emission Transmission Electron

Microscope (FETEM, JEOL JEM-F200).

Y. Luo, Y. Zheng, Z. Luo, S. Hao, C. Du, Q. Liang, Z. Li, K.A. Khor, K. Hippalgaonkar, J. Xu, ©. Yan, С. Wolverton, М.С. Kanatzidis, n-Type SnSe, oriented-nanoplatebased pellets for high thermoelectric performance, Adv. Energy Mater. 8 (2017).

X. Wang, F. Sun, Y. Duan, Z. Yin, W. Luo, Y. Huang, J. Chen, Highly sensitive, temperature-dependent gas sensor based on hierarchical ZnO nanorod arrays, J. Mater. Chem. C 3 (2015) 11397-11405.

Sidebar

Received 2 November 2023; Accepted 19 April 2024 Available online 7 March 2024

2589-9651/© 2024 Chongqing University. Publishing services by Elsevier В.М. on behalf of KeAi Communications Co. Ltd. This 15 an open access article under the CC

BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Footnote

* Corresponding author. E-mail address: [email protected] (C. He).

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This work was supported by the National Natural Science Foundation

Appendix A. Supplementary data

Supplementary data to this article can be found online at https ://doi.org/10.1016/j.nanoms.2024.02.010.