With the development of society, the detection of various gases is becoming more and more important in industrial production and environmental protection^[1,2]. At present, semiconductor gas sensors based on oxide gas sensing materials are most widely used due to their high sensitivity, ease of fabrication, simple structure and low cost^[3]. For example, WO₃, ZnO, and SnO₂ are used to make ethanol, acetone, and nitrogen dioxide sensors, respectively^[4,5,6]. However, most oxide semiconductor gas sensors tend to exhibit high operating temperatures, poor sensing selectivity and high limits of detection (LODs)^[7,8,9].

In recent years, two-dimensional (2D) transition metal dichalcogenides (TMDs) became a research hotspot in the field of nanomaterials^[10,11]. TMDs are promising gas sensing materials due to their planar crystal structure, large surface-to-volume ratio and low electronic noise^[12,13], which have not received deserved attention. Perkins, et al^[14] prepared a gas sensor using MoS₂ single- layer nanosheets obtained by the Scotch tape method. It was found that the gas sensor based on this MoS₂ single- layer nanosheet had a fast response to triethylamine and acetone. The response time of triethylamine gas is less than 15 s, and this sensor can detect triethylamine gas at concentrations as low as 10^-8. Studies by Perkins, et al^[14] have also shown that MoS₂-based sensors have better selectivity than carbon nanotube sensors. In particular, this MoS₂ nanosheet sensor has not strength response to water vapor, which is very important for the practical application of the sensor. Li, et al^[15] found that Metal- Oxide-Semiconductor Field-Effect Transistors (MOSFETs) based on MoS₂ nanosheets prepared by mechanical stripping can detect NO gas at concentrations as low as 0.8×10^-6. The synthesis of TMDs nanoplates (NPs) with controlled size has been of scientific and technological interest. Although tin is not belong to transition metal, SnS₂ shares the similar layered structure with the TMDs. SnS₂ has a layered hexagonal structure (a=0.3645 nm, c=0.5891 nm, space group: P3m1). It is composed of tin atoms sandwiched between two layers of hexagonally disposed close packed sulfur atoms, the adjacent sulfur layers are connected by the weak van der Waals bonding. Bulk SnS₂ is a semiconductor with a band gap of 2.35 eV^[16], which offers interesting applications in the fields of gas sensing. In the past few years, a few preparation methods have been developed for preparing ultrathin SnS₂ NPs. Huang, et al^[17] reported a FET based on monolayer SnS₂ flake by exfoliating SnS₂ crystals and explored its potential application in photodetection. Su, et al^[18] obtained the thin crystal arrays of SnS₂ at predefined locations, introducing a new idea for large-scale production by chemical vapor deposition. Unfortunately, due to obstacles such as high cost, difficulty of control, small output, the exfoliating and CVD methods are difficult to be used for large-scale production. Therefore, a facile, inexpensive and productive synthesis approach for SnS₂ NPs with good morphology and reduced thickness is still an important step to explore its future actual application. So simply synthesis route of wet chemistry method have attracted the attention of researchers.

However, the raw materials used in the reported hydrothermal approaches, especially the organometallic precursors and sulphureous sources (e.g. carbon disulfide, thioacetamide, thiourea), are toxic and expensive, thus limiting their practical applications^[16,19-20]. Recently, Kumar, et al^[21] prepared SnS₂ hexagonal nanoplates (NPs) by hydrothermal synthesis, with SnCl₄·5H₂O and sulfur used as precursors, which are cheap and environmental friendly. But the lateral size of these SnS₂ NPs is less than 60 nm, and the standard diameter of 2D materials is 100 nm^[22]. In addition, Du, et al^[23] demonstrated a one-pot chemical solution-phase approach to synthesize SnS₂ nanoplates using SnCl₄·5H₂O and sulfur powder as precursors. The size of samples is uniform, but their thickness is (35±10) nm, which may mask some unique properties of few-layer-structure SnS₂ NPs.

In this work, we report a facile chemical bath method to synthesize uniform 2D SnS₂ NPs in the presence of oleylamine (OAm), and oleic acid (OAc). The gas sensing results of the typical 2D SnS₂ NPs show a high response, selectivity, as well as low operating temperatures toward NO₂.

Tin (IV) chloride (SnCl₄·5H₂O, >99.9%, Aladdin) and sulfur powder (99.999%, Aladdin) were used as tin and sulfide precursor, respectively; oleic acid (OAc, >90.0%, Aladdin) and oleylamine (OAm, >90.0%, Aladdin), were used as surfactant; octadecene (ODe, >90.0%, Aladdin) was solvent. All the chemical reagents were used as received without further purification.

Fig. 1 is an experimental steps diagram. A simplified Schlenk line was used to protect the reaction from oxygen and moisture, and the whole synthesis process should be under a persistent flow of high-purity N₂. In a typical experiment, SnCl₄·5H₂O (1 mmol) was added to a mixture of OAc (10 mL) and ODe (20 mL) in a 100 mL three-neck flask. The mixed solution was degassed at 130 ℃ for 0.5 h to remove the moisture and the oxygen. Subsequently, the solution was heated to 280 ℃ within 15 min with a vigorous stir (700 r/min). The OAm-S solution (sulfide powder 2 mmol was dispersed into OAm 10 mL) was injected into the reaction system and the reaction was maintained at 280 ℃ for 30 min. After cooling the solution to room temperature naturally, the yellow-green product was collected and separated from the solution by centrifugation. The product was further washed twice by ethanol and cyclohexane (1/1, V/V) and finally dispersed in cyclohexane. Besides, different morphologies of products were checked by various amount of OAm/OAc while keep other parameters unchanged. The products are stable in air without further protection for further characterizations.

The shape and phase of the resulting SnS₂ NPs were examined by scanning electron microscopy (SEM) JEOL S4800 equipped with an energy-dispersive X-ray spectroscope (EDS) and transmission electron microscope (TEM) using a JEM-2100F (JEOL) at an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) patterns were taken on χ’Pert Pro MPD (Philips PANanalytical ) with Cu-Kα radiation at 45 kV and 40 mA. The XRD data was collected with the θ-2θ scanning scope started from 2θ=10º to 80º with a step of 6 (°)/s by using Cu-Kα radiation (0.15418 nm). The composition of the reaction liquid is characterized by Fourier Transform Infrared spectrometer (FT-IR, 8400 Shimadzu, Japan).

To make the gas sensing device, 0.1 g of as-prepared typical SnS₂ NPs and 5 mL of cyclohexane were added into a centrifuge tube with 5 mL capacity. This mixture was ultrasonicated for 10 min to form a concentrated suspension. There are two gold signal electrodes on the front of the flat gas sensing device (shown in Fig. 2(a)) with a platinum wire connecting to each electrode. The SnS₂-cyclohexane solution was dropped on the front surface of the device and then dried in ambient air naturally. Repeating the above processes 10 times, a gas sensing tablet whose front surface was completely and evenly covered by SnS₂ NPs was prepared. On the back of the device there is heating coating of rhodium, and the operating temperature of the gas sensor is controlled by the voltage across the heating coating.

Gas sensing tests were carried out on a self-built measurement system (shown in Fig. 2(b)) using ambient air as the dilute and reference gas. The operating temperature is controlled by the heating voltage applied to the back of the test device. A hygrometer showed a humidity of 60% at room temperature (25 ℃). The sensing time (t_sen) or recovery time (t_rec) is expressed as the time for the sensor output reaching 90% of its saturation after the test gas is applied or deactivated in a function step. The response S was defined as:

where R_a and R_g are the steady-state resistances in air and gas.

XRD analysis was applied to identify the crystal structure of typical SnS₂ NPs. Fig. 3(a) shows the powder XRD pattern of as-prepared SnS₂ NPs. All the diffraction peaks and relative intensity of samples are in accord with those of bulk SnS₂ and can be indexed as the 2H hexagonal structure, which is consistent with the standard data file (ICDD 23-0677). The 2H SnS₂ belongs to the space group (P-3m1) and has three atoms in the unit cell, which extend over only one monolayer. As described in Fig. 3(b), in this SnS₂ crystal, each Sn atom is six-fold coordinated, hexagonally packed between two three-fold coordinated S atoms, and S-Sn-S quintuple-layers are weakly bonded to other layers by van der Waals force. The primary diffraction peaks of as-prepared SnS₂ NPs at 2θ=15.04°, 28.27°, 32.17°, 41.95°, 50.05°, and 52.41° are respectively ascribed to the (001), (100), (101), (102), (110) and (111) planes, which is in accordance with pure 2H SnS₂ phase.

The obtained typical 2D nanoplates demonstrate unified hexagonal shape with lateral dimensions mainly ranging from 120 to 160 nm according to the SEM and TEM images shown in Fig. 4(a, b), respectively. As shown in Fig. 4(c), the cross-sectional high-resolution TEM (HRTEM) image of 2D SnS₂ flakes and its inset statistics obviously describe their layer structure and confirms that these 2D SnS₂ flakes consist of an average of (9±2) layers with an interlayer distance of 0.59 nm (profile intensity image along the line in Fig. 4(c). Fig. 4(d) shows the HRTEM image of the region circled by yellow rectangle in Fig. 4(c), in which a lattice fringe spacing of 0.318 nm is identified and corresponds to both the ($\bar{1}100$) and ($01\bar{1}0$) lattice planes of hexagonal SnS₂, while 0.587 nm is the distance between two adjacent (0001) molecular layers. To further confirm the growth crystalline of SnS₂ NPs, we performed EDS spectrum for the sample shown in Fig. 4(e). As shown in it, strong signals from Sn (L) and S (K) were detected by EDS. The peaks related to Al, O, C came from the Al₂O₃ substrate used in the SEM analysis. The estimated composition was 1 : 2, exactly showing the theoretical value of SnS₂. Combined with the observed hexagonal structure and lattice images, the NPs grown by our method are concluded to be single-crystalline SnS₂.

Here, we discuss the reaction course of SnS₂ NPs formation in current approach. In the process of heating at 130 ℃, SnCl₄·5H₂O was dissolved in ODe with the help of oleic acid. It is supposed to be a replacement reaction between SnCl₄ and oleic acid at high temperature showing in the following reaction equation:

We passed the effluent gas through a flask with deionized water, and after reaction the water became acidic. This proves the formation of hydrochloric acid.

After completion of the reaction, the suspension was allowed to stand for 6 h with 5 mL of ethanol, and the FT-IR spectrum of supernatant is shown in Fig. 5. The presence of strong acyclic C-H stretching at 2924 and 2855 cm^-1 demonstrates the coexistence of free oleic acid and oleylamine. The discernible peak at 1412 cm^-1 is assigned to the carboxylate (COO^-) stretch, while the obvious peaks at 1560 and 1647 cm^-1 are attributed to the C-N stretch of amide. Other peaks at 1051, 1083 and 3431 cm^-1 could be ascribed to vibrations of bonds in ethanol. These FT-IR data confirm that OAm and OAc make amidation reaction and produce H₂O, which is consistent with the experimental phenomenon that some low-boiling substances condenses on the upper part of the flask during synthesis reaction. This indicated that when we added the S-OAm solution into the three- necked flask, oleylamine molecules, which had been combined with S, was rapidly amidated with the original oleic acid to form water and rapidly released reactive S.

The 2D nano plate morphology arises from the anisotropic bonding nature of SnS₂, since the intra-layer covalent bond is stronger than the interlayer van der Waals force. Due to the large amount dangling bonds exist on the edge of the growing nucleation, the growth of SnS₂ crystals continues with a high anisotropic nature, and Sn and S atoms would selectively bind to these dangling bond sites. Thus, the lateral dimension grows much faster than the vertical thickness dimension, which results in the thin plate morphology of SnS₂.

In the typical synthetic method, the molar ratio between oleylamine and oleic acid was about 1 : 1, which resulted in a moderate S generation rate and nucleation rate, and subsequently further reaction prepared typical SnS₂ NPs with uniform hexagonal shape.

We also investigated the effect of surfactants on the morphology of SnS₂ NPs. Fig. 6 shows TEM images of SnS₂ NPs with different molar ratio between OAc and OAm. From Fig. 6(a) to 6(i), the amount of OAm decreases, while that of OAc increases. As shown in Fig. 6(a), with little oleic acid participating in the reaction, the morphology of product is quite different from the typical one. Its lateral size is up to 10 μm. When the mixed solution containing S and oil amine was injected into the flask, some of them made amidation reaction with OAc as above and generated SnS₂ nucleus, while the remaining S-OAm solution released H₂S^[24]. The latter process is much slower than the former, resulting in a low nucleation rate and less number of nucleation. During the whole reaction process, more S and Sn participate in the growth of the NPs; and some of oleylamine molecules tend to interact with S atom exposed on the outer (001) crystal face of SnS₂ NPs so that the laterial size of this sample is great. As shown in Fig. 6(i), when oleic acid is excessive, almost all OAm moleculars instantaneous amide with OAc moleculars and release a lot of S precursors, which produce much more nucles than those of typical synthesis. In addition, excessive OAc moleculars bonded to Sn atoms exposed on the edge of SnS₂ NPs, and this interaction hinders growing along the lateral dimension. Therefore the morphology of this sample is circle instead of hexagon, and its lateral size is small. The whole reaction and growth process is shown in Fig. 7.

A large number of studies has demonstrated that the operating temperature of a gas sensor is the most influential factor which affects the gas response of semiconductor sensors^[25]. It is known that sulfides are unstable in the air at high temperatures, so we examined the stability of the as-prepared SnS₂ NPs. After heating the NPs to 150 ℃ in air for 8 d, XRD result showed that there was no transition from SnS₂ to SnO_x. And our previous research also shows that SnS₂ phase tends to fade for the sample treated at 250 ℃^[26]. Therefore, we tested the gas sensing response of SnS₂ NPs at operating temperatures from room temperature to 150 ℃. At room temperature (20 ℃), there is no acceptable signal because the resistance of the sensing device is too large for the test limit of the ammeter. The response curves of sensors made of typical SnS₂ NPs in the presence of 10^-5 of NO₂ gas at 50-150 ℃ are shown in Fig. 8. The current in the test circuit in the air increased with the growing of temperature. This is because more electron- absorbing thermal energy transitions from the valence band to the conduction band, causing the resistance of the material to decrease. SnS₂ is an n-type semiconductor and NO₂ molecule is an electron acceptor. When we injected NO₂ gas into the test chamber, NO₂ molecules were adsorbed on the surface of SnS₂ NPs, and electrons in the conduction band of SnS₂ NPs transferred to NO₂ molecules, causing the number of conduction band electrons reducing, which made the resistance of SnS₂ NPs increase^[27]. When the temperature below 90 ℃, there were less conduction band electrons. So in NO₂ atmosphere, almost all electrons were transferred to NO₂ molecules, leading to the fact that resistance was extremely large and the current value was close to 0. At this time, S is infinite and the response of SnS₂ NPs to NO₂ gas is obvious. At the same time, however, the gas desorption rate at low temperatures was very slow, and it was clear that when the test device was placed back in the air, the resistance could not return to the pre-adsorption level in an acceptable time range. When the operating temperature reached 130 ℃, the sensing time (t_sen) and recover time (t_rec) was 98 s and 680 s, respectively, and S was 4.14. Then, when the temperature continued to rise to 150 ℃, t_sen increased to 168 s and S also decreased to 3.13. So 130 ℃ is the optimum temperature for the detection of NO₂ gas by SnS₂ NPs prepared in this study.

Fig. 9(a) shows the dynamic sensing-recovery curves of the sensors to NO₂ gas with concentrations ranging from 20×10^-6 to 0.5×10^-6 at 130 ℃. With the decrease of NO₂ concentration, S was getting weaker and weaker. This is due to less ethanol molecules adsorbed on the surface of the sensing material, and the resistance of SnS₂ NPs with less change. When the concentration of NO₂ gas decreased to 0.5×10^-6, the gas sensor still performed an obvious response, revealing the resistance of the prepared SnS₂ NPs is very sensitive to NO₂ gas. Furthermore, we believe that the lowest detection concentration should be less than 10^-7, but due to limitations of experimental conditions, we can’t accurately measure the response signal at lower concentrations.

In order to study their reversibility characteristics, sensing devices were exposed to 10^-5 of NO₂ gas and air alternately for 3 or more cycles. In Fig. 9(b), we can see the resistance of sensor in 10^-5 NO₂ gas was steady and can return to no-load level after each desorption. This result confirms that there is a minuscule amount of gas molecules during desorption process, and this SnS₂ NPs gas sensors have a good performance in reversible cycle tests. We also tested the responses of this SnS₂ NPs based sensors to different gases at 130 ℃. Experiments have shown that SnS₂ NPs was not sensitive to ammonia (100×10^-6), carbon dioxide (3%) and hydrogen (20×10^-6). This indicates the SnS₂ NPs based sensors exhibit superior selectivity to NO₂ gas.

Uniform hexagonal shape SnS₂ NPs with a thickness of 4-6 nm and lateral dimensions of about 150 nm were obtained by high temperature chemical bath method. The ratio between OAm and OAc is the key to control the morphology of SnS₂ NPs. During the process of nucleus growth, OAm will attach to exposed (0001) crystal surface and prevent NPs vertical growth. To add amount of OAm, we got a series of SnS₂ nanoplates with huge lateral size. The gas sensing properties of 2D SnS₂ NPs were investigated at different operating temperatures. 2D SnS₂ NPs based sensors exhibited excellent sensing properties to NO₂ gas with a response S of 4.14 at 130 ℃, a sensing time of 98 s and a recovery time of 680 s. This study demonstrates the potential of 2D SnS₂ NPs for NO₂ gas sensing applications.