*3.2. Fabrication and Characterization of Ethylene Sensitive Films*

Four sensitive material systems were fabricated including rGO, rGO/WSe2, rGO/Pd, and rGO/WSe2/Pd to achieve the most promising candidates for room-temperature ethylene sensing. Firstly, the self-assembled films were characterized by XPS to investigate their chemical composition and surface states. Figure 2a,b shows the C 1s spectra of GO and rGO films, respectively. For GO films, C-C, C-O, and C=O are the three main types of carbon bonds, corresponding to the characteristic peaks at 284.8 eV, 287.0 eV, and 288.4 eV. After annealing, the peak of the C-O bond decreases greatly and the C-C bond becomes the main peak, indicating that most of the oxygen-containing functional groups in GO are removed and thus GO is reduced. After deposition of WSe2 nanosheets, the chemical composition of the rGO/WSe2 sensitive films was examined by XPS as well, shown in Figure 2c. Two obvious peaks appeared at 34.2 and 32.1 eV and were attributed to W4f7/2 and W4f5/2 of W4+, respectively, while the peaks at 54.2 eV and 55 eV correspond to the

Se3d3/2 and Se3d5/2 of Se2-. Then, the Raman spectrum was also recorded to exhibit the structural properties of the rGO/WSe2 sensitive films, as displayed in Figure 2d. The characteristic peak of WSe2 at 251 cm−<sup>1</sup> is clearly observed, which could be attributed to the overlapped peaks of the interlayer mode A1g (253 cm<sup>−</sup>1) and in-plane vibration mode E1 2g (250 cm−1). The peaks at 1349 cm−<sup>1</sup> and 1590 cm−<sup>1</sup> correspond to the D and G band of rGO, respectively, and its intensity ratio is almost 1.0, indicating the reduction of GO into rGO with some defects. Moreover, the broad 2D band at 2470–3000 cm−<sup>1</sup> shows that the fabricated rGO film is composed of some layers of the rGO sheets.

**Figure 2.** XPS C 1s spectrum of (**a**) GO and (**b**) rGO. (**c**) XPS spectra of W 4f and Se 3d of WSe2. (**d**) Raman spectrum of rGO/WSe2 sensitive films.

The surface morphology of rGO and its heterojunction films were observed through SEM. The pure rGO films show uniform morphology with a few wrinkles (Figure 3a). After deposition of lamellar WSe2 nanosheets, the rGO/WSe2 composite films remain uniform with excellent interlayer coupling between rGO and WSe2 nanosheets (Figure 3b,c). As shown in Figure S2, the EDS element mapping images of W, Se, C, and O confirm that these four elements are homogeneously present in the whole film region, which further proves formation of rGO/WSe2 heterojunction. Pd NPs are evenly distributed on rGO and rGO/WSe2 films (Figure 3c,d), which was beneficial for the sensing enhancement effects of Pd NPs. The corresponding high-resolution TEM (HRTEM) images exhibit a specific lattice spacing of the (102) facet for WSe2, (100) facet for WSe2, and (111) facet for Pd, which is measured to be 0.260 nm, 0.282 nm, and 0.225 nm, respectively, as shown in Figure 3d,e.

**Figure 3.** SEM images of the self-assembled (**a**) rGO, (**b**) rGO/WSe2, (**c**) rGO/Pd, and (**d**) rGO/WSe2/Pd, high-resolution TEM images of (**e**) rGO/WSe2 and (**f**) rGO/WSe2/Pd.
