*3.1. Theoretical Design of the Ethylene Sensitive Material System*

A model of graphene consisting of 55 atoms was constructed. The length of the C-C bond in basal plane was 1.42540 Å, which was close to the C-C bond length of the graphite planar structure [27]. The geometric structure of the 16 monolayer WSe2 was also discussed [28,29] and the length of the W-Se bond varied from 2.51006 to 2.51244 Å. The 4 Pd atoms with each volume of 62.01 Å<sup>3</sup> were adopted. Lattice constants of graphene and WSe2 were calculated as 12.34 and 13.26, respectively. After compounding, the lattice constant of graphene/WSe2 was 12.64, indicating graphene was stretched while WSe2 was compressed. The length between ethylene and graphene, WSe2, and Pd, were 2.86 Å, 2.88 Å, and 2.98 Å, respectively, forming no chemical bonds and declaring van der Waals adsorption, as shown in Figure 1. While in ternary structure, there was a chemical bond (1.74 Å) linked between Pd and WSe2, and the distance between ethylene and WSe2 was shortened to 2.2 Å, indicating that the adsorption had been changed.

Energies of the above structures and the ethylene adsorbed on the structures were obtained (Table 1). After assemble with WSe2, the energy of graphene almost doubled. While after metal Pd modification, the energy of the graphene became slightly more powerful. Adsorption energies were calculated and are shown in Table 2. The adsorption of graphene to ethylene was only −0.14; a negative value indicated the adsorption occurred energetically and a weak value declared the van der Waals adsorption. Compounded Pd or WSe2 helped to enhance the adsorption capacity of graphene to ethylene. It was noticed that capacity drastically increased to 5.5 times through ternary compounding.

**Figure 1.** Optimized geometric models of (**a**) graphene and (**b**) graphene/WSe2, and adsorption structures of ethylene molecules on (**c**) graphene, (**d**) graphene/Pd, (**e**) graphene/WSe2, and (**f**) graphene/WSe2/Pd.



**Table 2.** Adsorption energies of the ethylene adsorption system.

