A mixed salt-assisted ambient-pressure rapid chemical vapor deposition (CVD) approach was developed for synthesis of large-size, ultrathin SnS₂ nanosheets based on rapid CVD methods developed in our reports [26,27]. The typical experimental procedure to grow SnS₂ nanostructures with single salt is as follows: 0.020 g SnO₂ (Shanghai Aladdin Biochemical Technology Co., Ltd.) powder was uniformly mixed with 0.002 g KI (Shanghai Macklin Biochemical Technology Co., Ltd., 99.9% purity) to serve as the tin source. A freshly cleaved mica sheet (approximately 1 × 2 cm²) was placed above the tin source. Sulfur powder (Shanghai Aladdin Biochemical Technology Co., Ltd.) was placed in a separate quartz boat upstream of the tin source at a certain distance. The quartz tube was evacuated and purged with argon to remove air, then backfilled to atmospheric pressure. A constant argon flow of 50 sccm was maintained, and the outlet was sealed with a water seal. When the furnace reached 540°C, the quartz tube was rapidly moved to position the tin source at the heating center and bring the sulfur powder near the heating zone. After the reaction for 10 min, the furnace was turned off and allowed to cool to room temperature before the sample was removed. The growth process of SnS₂ nanostructures using different salts remains the same, with a constant mass ratio of SnO₂ to salt fixed at 10:1. All salts are of analytical grade and used as received without further purification (purchased from Shanghai Macklin Biochemical Technology Co., Ltd. or Shanghai Aladdin Biochemical Technology Co., Ltd.). The procedure for growing SnS₂ nanosheets with mixed salts follows the same protocol, except that a mixed salt (KI and CsCl) is used instead of a single salt. The mass ratio of SnO₂ to the mixed salt is kept constant at 10:1, while the ratio of KI to CsCl can be adjusted according to the experimental design.

SnS₂ nanosheets were transferred from mica to 300 nm SiO₂/Si substrates or Cu grids by a water-assisted ultrasonic transfer as described in our previous reported [27]. The as-grown SnS₂ nanosheets were transferred onto mica to characterize polarized optical image using a mechanical transfer method. Briefly, the mica sheet carrying the as-grown SnS₂ nanosheets was brought into face-to-face contact with a freshly cleaved mica. The two substrates were gently pressed together and then slowly separated. Consequently, some SnS₂ nanosheets were transferred onto the target substrate.

The morphology and spatial distribution of SnS₂ nanosheets were examined by reflection-mode optical microscopy (Olympus BX51M) and atomic force microscopy (AFM, Bruker Dimension Icon). Crystal structure, lattice resolution, and compositional uniformity were assessed by X-ray diffraction (XRD, Rigaku Ultima IV, Cu Kα, λ = 0.15418 nm), transmission electron microscopy (TEM, JEOL JEM-F200, 200 kV) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB Xi, Al Kα, hν = 1486.6 eV). The polarized optical imaging was characterized using an OM equipmented with crossed polarizer and analyzer plates. Raman spectra were acquired at room temperature with a HORIBA Jobin-Yvon Evolution spectrometer using a 532 nm excitation laser kept below 5 mW to avoid sample heating.

An ambient-pressure rapid CVD method combined with a salt-assisted growth strategy was used to investigate the effects of various salt additives on the size, morphology, and thickness of SnS₂ nanosheets. Fig. 1a shows the schematic of the mixed salt-assisted rapid CVD process [26,27]. It offers several advantages, including rapid heating-up, negligible precursor loss during the heating process, and precise control over growth temperature and duration. In the absence of salt additives, only a few small and thick SnS₂ crystal are observed by optical microscopy (OM) as shown in Fig. 1b. In recent years, salts, particularly alkali metal halide salts, have been widely employed as promoters in the CVD growth of 2D materials [28,29]. While certain salts, such as NaCl and KI, have also been utilized in the CVD synthesis of SnS₂, however, the existing studies on salt-assisted SnS₂ growth remain sporadic and lack a systematic investigation. In this work, we comprehensively investigate the influence of 36 distinct salts, including alkali metal halide salts and other common salt types, on the growth of SnS₂ nanosheets (Fig. 1c). Based on careful screening and comparative analysis of these salts, an efficient mixed salt system was successfully identified and established. This optimized system enables the controllable growth of ultrathin, large-area SnS₂ nanosheets with excellent crystalline quality, providing a reliable approach for the synthesis of high-quality 2D materials.

When 12 salts such as AlCl₃, Cr(NO₃)₃·H₂O, Mn(CH₃COO)₂·4H₂O, Fe(NO₃)₃, Co(NO₃)₂·6H₂O, Ni(NO₃)₂, CuSO₄, Zn(CH₃COO)₂·2H₂O, Rb₂CO₃, SrCl₂·4H₂O, WCl₆ and AgNO₃ were introduced into the growth system, the nucleation and subsequent growth of SnS₂ were significantly suppressed. As shown in Fig. 2, no observable SnS₂ crystal were detected on the sample surface under these conditions, indicating a complete inhibition of crystal formation. This inhibitory effect may be attributed to the strong interaction between the metal cations (such as Al³⁺, Fe³⁺, Rb⁺ and Ag⁺) or their corresponding anions with the precursors or the growth substrate, which likely hinders the initial nucleation process. These results demonstrate that certain salt additives can act as growth inhibitors rather than promoters, highlighting the dual role of salts in the CVD growth of 2D materials.

The addition of 12 different salts, including LiCl, MgSO₄, CaSO₄, MoCl₅, Ba(CH₃COO)₂, SbCl₃, BiCl₃, InCl₃·4H₂O, SnCl₂·2H₂O, HfCl₄, CdCl₂·2.5H₂O, and PbI₂, led to distinct morphological outcomes, as shown in Fig. 3. When salts such as LiCl, MgSO₄, CaSO₄, MoCl₅, Ba(CH₃COO)₂, SbCl₃, BiCl₃ and InCl₃·4H₂O were introduced into the precursor, only a few small SnS₂ crystals with varying brightness were observed in the OM images (Fig. 3a–h). In contrast, the addition of SnCl₂·2H₂O and HfCl₄ produced significantly larger SnS₂ crystals (Fig. 3i,j), suggesting that these salts may promote precursor conversion or facilitate crystal growth under identical conditions. Furthermore, the addition of CdCl₂·2.5H₂O led to the formation of multilayer SnS₂ nanosheets (Fig. 3k), indicating that CdCl₂·2.5H₂O plays a regulatory role in modulating interlayer van der Waals interactions. These weak interactions govern the vertical assembly of 2D layers, and CdCl₂·2.5H₂O appears to enable controlled layer-by-layer stacking, offering a promising route for multilayer SnS₂ synthesis. Notably, the introduction of PbI₂ induced the formation of one-dimensional (1D) nanowires (Fig. 3l). Further analysis overturns the conventional view of PbI₂ as merely an auxiliary agent. As demonstrated by the structural and compositional analysis in Fig. S1, PbI₂ instead acts as a critical reactant, deeply participating in the lattice formation process. Consequently, the synthesized product is identified as the ternary lead-tin-sulfur compound PbSnS₃, not the binary tin disulfide SnS₂. These contrasting observations highlight the specificity of salt-assisted growth and underscore the importance of carefully selecting salt additives to achieve desired structural outcomes.

The alkali metal halide salts constitute a distinct category of additives. Within this group (NaF, NaCl, NaBr, NaI, KF, KCl, KBr, KI, CsF, CsCl, CsBr, and CsI), only CsCl, CsBr, and KI demonstrated significant regulatory capabilities in the SnS₂ growth process (Fig. 4). Specifically, both cesium salts effectively promoted lateral expansion while simultaneously suppressing vertical growth, leading to larger but thinner crystals. CsCl was more potent in this regard than CsBr; however, both yielded products with irregular morphologies. By contrast, KI primarily enhanced the crystalline quality and shape regularity of the nanosheets, although the products remained relatively thick with notable thickness variation. The remaining alkali halides showed little to no effect, with NaF, KF, CsF acting as a strong inhibitor of normal crystal development.

To better understand the effects of salts on the growth of SnS₂ nanostructures, a periodic table of the metal salts with different color scheme were draw as shown in Fig. 1c. These salts can be divided into three categories: 15 salts marked in light blue (such as AlCl₃, AgNO₃, NaF, et al.) represent suppressed growth, 16 salts marked in blue (such as LiCl, CaSO₄, BiCl₃, NaCl, et al.) represent no significant effect on growth, and 5 salts marked in dark blue (CdCl₂·2.5H₂O, PbI₂, KI, CsCl and CsBr) represent enhanced growth. Most of salts had a weak influence or even suppression on growth, and only a few salts enhanced growth. Among them, CsCl significantly enhance lateral size and reduce thickness of SnS₂, while KI optimizes its crystallinity and morphology. This classification strategy enables intuitive screening of auxiliary growth agents with potential application value, providing new insights for the controllable preparation of 2D materials.

To elucidate the synergistic mechanism of CsCl and KI and the effects of their mixing ratio on the growth morphology, size, thickness, and crystalline quality of SnS₂, a series of comparative experiments was designed based on single salt-assisted growth shown above. The CsCl/KI ratio was systematically varied while keeping the total amount of SnO₂ and mixed salts constant. Five experimental groups were set up, namely CsCl:KI = 0:2, 1:2, 2:2, 2:1, and 2:0, while the total amount of SnO₂ and the mixed salts was strictly maintained at a constant. The growth morphology of each group was characterized by OM firstly, and the results are shown in Fig. 5. The results show that the growth characteristics of SnS₂, such a morphology, lateral size, and thickness, are strongly depend on the CsCl/KI ratio. Increasing the CsCl proportion promotes lateral growth, leading to larger lateral sizes and reduced thickness, consistent with the effect of CsCl alone. In contrast, a higher KI content (CsCl:KI = 0:2 or 1:2) results in smaller lateral sizes but smoother edges, clearer contours, and a pronounced triangular morphology, aligning with the high crystallinity and regularity achieved with KI alone. To address this, the atomic force microscopy (AFM) of SnS₂ growth with assistant of the KI, CsCl and KI/CsCl (KI:CsCl = 1:1) were performed and the results are shown in Fig. S2. For SnS₂ nanosheets grown with KI assistance, the average edge length is 34.4 μm, the average thickness is 26.0 nm, and the edges are smooth. In contrast, SnS₂ nanosheets grown with CsCl assistance exhibit a significantly larger average edge length of 155.1 μm, a considerably smaller average thickness of 3.7 nm, but with rough edges. When a mixed salt (KI/CsCl) is used, the resulting SnS₂ nanosheets show an average edge length of 66.3 μm, an average thickness of 10.6 nm, and smooth edges. Thus, CsCl promotes lateral expansion and thinning, while KI enhances morphological regularity and crystalline quality. In summary, the CsCl/KI ratio synergistically regulates the lateral and vertical growth rates of SnS₂, jointly determining its overall morphology and crystalline properties.

To gain mechanistic insight into how CsCl promotes lateral growth and KI improves morphology, theoretical calculations were performed as shown in Fig. S3. First-principles calculations reveal that the formation energies of SnS₂ on the (100), (010), and (001) planes are 0.0339 eV, 0.0339 eV and 0.0457 eV, respectively. The equal formation energies of the (100) and (010) planes are consistent with the sixfold symmetric structure of SnS₂. Notably, the formation energies of the in-plane (100) and (010) planes are lower than that of the out-of-plane (001) plane, indicating that 2D growth is energetically favorable. When Cs and K ions are introduced onto the (100), (010), and (001) surfaces of SnS₂, the binding energies on the (001) surface are the lowest for both ions, suggesting a preferential adsorption on this basal plane. This promotes lateral growth, which is in good agreement with experimental observations. Furthermore, the binding energy of Cs⁺ on the (001) surface is lower than that of K⁺, indicating that Cs⁺ is more effective than K⁺ in promoting lateral growth and thus facilitating the formation of thinner SnS₂ nanosheets. Additionally, the binding energies of Cs⁺ on the (100) and (010) surfaces are lower than those of K⁺, implying that K⁺ exhibits weaker binding on these edge planes. This weaker interaction favors the formation of well-defined, sharp edges, consistent with the experimental observation that KI-assisted growth yields more regular morphologies. These theoretical results are in strong agreement with the experimental findings shown above. These findings provide important guidance for optimizing SnS₂ growth processes and preparing high quality samples.

To systematically characterize the morphology, structure, and chemical composition of the ultrathin large-size SnS₂ nanosheets, atomic force microscopy (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed. As shown in Fig. 6a, the lateral size of SnS₂ nanosheets can reach 248 μm when the salt mixing ratio (CsCl:KI = 2:2) is fixed and growth conditions are further optimized. AFM imaging (Fig. 6b) and cross-sectional analysis reveal a thickness of approximately 1.87 nm, confirming a bilayer structure [30]. XRD (Fig. 6c) exhibits distinct peaks at 14.9°, 30.2°, 46.0°, and 62.9°, corresponding to the (001), (002), (003), and (004) planes of SnS₂ (PDF#23-0677), respectively. No secondary phases are detected, confirming the single-phase nature of the as-prepared samples. The survey spectrum on as-grown mica substrate (Fig. 6d) reveals the expected Sn and S as, in addition to the weak C 1s peak always present at 284.8 eV for samples handled in air. Notably, Cs, Cl, K and I are clearly detected, and no other impurities are observed. While only Cs remains detectable, while K, Cl, and I are no longer observed after transfer to SiO₂/Si (Fig. S4). High-resolution XPS analysis (Fig. 6e–j) provides detailed chemical state information. The Sn 3d core level splits into two peaks at 495.1 eV and 486.6 eV due to spin-orbit coupling, corresponding to Sn 3d_3/2 and Sn 3d_5/2, respectively, indicating Sn in the Sn⁴⁺ state. The S 2p spectrum shows two peaks at 161.6 eV and 162.8 eV, assigned to S 2p_3/2 and S 2p_1/2, respectively, confirming sulfur in the S²⁻ state, these values are consistent with the previous reports [31]. The Cs 3d core level splits into two peaks at 738.6 eV and 724.6 eV, corresponding to Cs 3d_3/2 and Cs 3d_5/2, respectively, indicating Cs in the Cs⁺ state; the Cl 2p spectrum shows two peaks at 199.0 eV and 197.6 eV, assigned to Cl 2p_1/2 and Cl 2p_3/2, confirming Cl in the Cl⁻ state [32]. The K 2p spectrum exhibits two peaks at 296.0 eV and 293.2 eV, corresponding to K 2p_1/2 and K 2p_3/2, confirming K in the K⁺ state; the I 3d spectrum shows two peaks at 630.5 eV and 618.9 eV, assigned to I 3d_3/2 and I 3d_5/2, confirming I in the I⁻ state [33]. The clearly detection of Cs, Cl, K, and I further demonstrates that the mixed salt (CsCl/KI) plays an important role in regulating both the lateral and vertical growth rates of SnS₂. Collectively, these results confirm that the as-grown SnS₂ nanosheets are of high quality.

To investigate the atomic-scale microstructure, growth orientation, and chemical composition distribution of SnS₂ nanosheets, transmission electron microscopy (TEM) was employed. Fig. 7a shows a TEM image of a transferred SnS₂ nanosheet, revealing its overall morphology. The high-resolution TEM (HR-TEM) image (Fig. 7b) reveals a hexagonal lattice structure free of atomic vacancies or lattice distortions. The measured lattice spacing is approximately 0.32 nm, matching the (100) interplanar spacing of standard SnS₂ (0.317 nm) and confirming the high crystal orientation. The selected area electron diffraction (SAED) pattern (Fig. 7c) exhibits regular hexagonal spots, confirming the hexagonal phase and excellent single-crystal quality of the as-prepared SnS₂ nanosheets. Moreover, energy-dispersive X-ray spectroscopy (EDS) elemental maps (Fig. 7d–f) show uniform distribution of Sn and S across the nanosheet, with no evidence of elemental segregation or enrichment.