*3.1. Characterization of MoS2 Nanosheets*

### 3.1.1. Microstructures and Morphology

To study the variation of the crystal structure during the MoS2-NS preparation, the precursor ((NH4)2MoS4), bulk MoS2 and MoS2-NS were analyzed by XRD and the results were illustrated in Figure 1. As shown in Figure 1, after calcination under N2, the characteristic peaks of (NH4)2MoS4, located at 2θ = 17.2◦, 18.44◦, and 29.08◦, fully vanished, suggesting the evident change of crystal structure. Instead, the peaks at 2θ = 14.6◦, 33.48◦, 39.82◦, and 58.94◦ were assigned to the (100), (103), (105), and (110) plane of hexagonal MoS2 phase (JCPDS card No. 65-0160), indicating that after calcination, the obtained bulk MoS2 was hexagonal phase. The results were similar to Zhang et al.'s findings [24,25]. In addition, after exfoliation by sonication, the resulting MoS2-NS still kept the same peaks with bulk MoS2, manifesting that the 2H-MoS2-NS was successfully obtained. However, compared to the bulk MoS2, the (002) plane peak of MoS2-NS became broadened and lower, suggesting an increase of the *d* spacing between MoS2 layers [26]. Based on the full width at half maximum (FHWM) of the (002) plane, the layer number of the prepared MoS2-NS could be calculated to be about ~2 layers through Scherrer's equation. To further confirm the structure of the exfoliated MoS2-NS, Raman spectroscopy (Figure 1b) was employed to characterize the bulk MoS2 and MoS2-NS. Both the bulk MoS2 and MoS2-NS exhibited two dominant peaks ranging from 340 to 450 cm<sup>−</sup>1, corresponding to the E<sup>1</sup> 2g and A1g mode of the hexagonal MoS2, respectively [26,27], which convincingly proved the successful exfoliation of MoS2-NS. Among which, the E1 2g mode peak at 381.6 cm−<sup>1</sup> involved the in-layer displacements of Mo and S atoms, whereas the A1g mode peak at 407.9 cm−<sup>1</sup> represented the out-of-layer symmetric displacements of S atoms along the *c*-axis [28]. Noticeably, compared to the bulk MoS2, the E<sup>1</sup> 2g (377.8 cm−1) and A1g (402.2 cm−1) mode peaks displayed evident blue shift, and the interval (Δ = 24.4 cm<sup>−</sup>1) between E1 2g and A1g peaks was lower than that of bulk MoS2 (Δ = 26.3 cm<sup>−</sup>1), which was ascribed to the decrease of the MoS2 thickness. According to the previous literature [4,25,29], it was found that both E<sup>1</sup> 2g and A1g peaks were the characteristic peaks of MoS2 and their frequencies would vary with the layer number. When the layer number increases, the interlayer van der Waals force in MoS2 suppressed atom vibration, resulting in higher force constants [30]. On the contrast, the force constants between the layers would weaken with the layer number decreases. Thus, both E<sup>1</sup> 2g and A1g modes were supposed to stiffen (blue-shift) along with the reduction of MoS2 layers.

Figure 2a–d presented the FESEM images of the bulk MoS2 and MoS2-NS. As seen in Figure 2a,b, the bulk MoS2 displayed varisized particle-like morphology but clearly thick layer-structure. After exfoliation, the MoS2-NS exhibited evident thin layer-structure with inconspicuous wrinkle (Figure 2c,d), which suggested the successful exfoliation of MoS2-NS. Similar to the bulk MoS2, the size of prepared MoS2-NS still differed widely. In addition, the thickness of the prepared MoS2-NS was analyzed by atomic force microscopy (AFM) (Figure 2). As seen in the AFM images (Figure 2e,f), the height profile of the two selected regions displayed a height of ~1.20 nm (±0.03 nm) for MoS2-NS, which was about 2 times as thick as the theoretical thickness of monolayer MoS2 (~0.65 nm) [27]. The evident platform of the height curves of the selected MoS2-NS revealed the smooth surface for MoS2-NS. Low-resolution TEM image (Figure 2g) also clearly depicted well-stacked layered structures (~2 layers) of the MoS2-NS, which strongly confirmed the results of XRD and AFM. In the high-resolution TEM (HRTEM) images (Figure 2h), the lattice spacing of 0.27 and 0.16 nm between two adjacent lattice planes could be resolved, which were assigned to the (100) and (110) plane of MoS2. In addition, the HRTEM image (Figure 2i) and associated fast Fourier transforms (FFT) (Figure 2j) from the center of the MoS2-NS evidently exhibited hexagonally symmetric structure, which was consistent with the results of XRD analysis. All the above results manifested that the obtained MoS2-NS still retained hexagonal single crystalline nature during pre-expansion and sonication treatments, which agreed with the previous findings [25,31,32]. However, the BET analysis (Figure 3) showed that the specific

surface areas of bulk MoS2 and MoS2-NS were 5.6 and 26.6 m<sup>2</sup> g−1, respectively, indicating that the exfoliation greatly changed the specific surface area of the MoS2 materials. With the decreasing of the layers, more and more MoS2 was exposed, resulting in a promotion of the BET surface.

**Figure 1.** (**a**) XRD patterns of (NH4)2MoS2, bulk MoS2, and MoS2-NS and (**b**) Raman spectra of bulk MoS2 and MoS2 nanosheets.

The chemical composition and element valence on the surface of MoS2 NS were analyzed with XPS (Figure 4). As depicted in Figure 4a, the survey spectra clearly confirmed the presentence of C, O, NS, and Mo elements. The weak C1s (~284 eV) peak was attributed into the calibration of binding energy with carbon, while the N1s (~400 eV) peak was ascribed into the adsorbed hydrazine hydrate during the sonication. In Mo3d core-level spectra (Figure 4b), the appearance of Mo3d5/2 (233.5 eV) and Mo3d3/2 (232.6 eV) peaks for Mo3d doublet indicated the characteristic +4 oxidation state [1]. Besides, two weak Mo6<sup>+</sup> 3d peaks (3d5/2 peak at 233.5 eV and 3d3/2 peak at 235.9 eV) were ascribed to

the slight oxidation of MoS2 NS edge during the MoS2 transfer under high temperature [25]. In the high-resolution scans of S2p (Figure 4c), two feature peaks (S2p1/2 and S2p3/2) were observed at 162.0 and 163.3 eV, respectively, which greatly matched the binding energy of S2<sup>−</sup> ions in 2H-MoS2 [2]. In addition, the appearance of O1s also confirmed the oxidation of MoS2. In the high-resolution spectra of O1s (Figure 4d), the peak of O2<sup>−</sup> species located at 532.0 eV and the peak at 533.5 eV was attributed into the absorbed oxygen-containing material like H2O [3].

**Figure 2.** Field emission scanning electron microscope (FESEM) images (**a**–**d**) of the bulk MoS2 and MoS2-NS, atomic force microscope (AFM) images (**e**,**f**), height profiles (inset), (transmission electron microscope) TEM and high-resolution transmission electron microscope (HRTEM) images of MoS2 nanosheets (**g**–**i**), and the fast Fourier transforms (FFT) pattern of MoS2 NS (**j**).

**Figure 3.** Brunauer–Emmett–Teller (BET) N2 isotherms of the bulk MoS2 and MoS2 NS.

**Figure 4.** XPS survey spectra of MoS2 (**a**) and high-resolution scans of Mo3d (**b**), S2p (**c**), O1s (**d**).
