This study utilized first-principles calculations based on density functional theory (DFT) using the Vienna ab initio simulation package (VASP). The Perdew-Burke-Ernzerhof (PBE) functional in the form of generalized gradient approximation (GGA) was employed to calculate the exchange-correlation function. To ensure accuracy, convergence criteria were set as follows: a cut-off energy of 450 eV, force on each atom less than -0.01 eV/Å between two ionic steps, and electronic self-consist energy convergence less than 10-6 eV. Additionally, Grimme’s scheme (DFT-D2) was used for long-range dispersion correction to better describe van der Waals interaction. Monkhorst-pack scheme with a 5 × 5 × 1 k-mesh for geometry optimization and 9 × 9 × 1 k-mesh for electronic structure calculations was applied to sample the Brillouin zone. A vacuum space of 25 Å along the c-axis prevented any interaction between periodically repeating images. Finally, Bader charge analysis was conducted to examine the charge transfer mechanism between gas molecule and substrate.
