## 3 Result and Discussion  
The atomic arrangements of the three metallic transition metal ditellurides (_TM_Te\({}_{2}\)), VTe\({}_{2}\), WTe\({}_{2}\), and MoTe\({}_{2}\), are shown in Figure 1a. One transition metal atom is surrounded by six tellurium atoms with the form of a distorted octahedron: one center _TM_ atom and six corner Te atoms. These layered metallic TMDs have exotic buckled layer structures due to the lattice distortion originating from strong correlations between spin, charge, and lattice in 2D materials [28, 29]. The buckled monolayer of _TM_Te\({}_{2}\) has non-uniform distribution of surface Te atoms, which is categorized as \(\alpha\) (orange circles) and \(\beta\) (yellow circles) according to the vertical positions at the basal plane as seen in Figure 1a. Different from the zig-zag chains (black lines) of Mo or W atoms in MoTe\({}_{2}\) and WTe\({}_{2}\), VTe\({}_{2}\) has mirrored double zig-zag chains (black lines) of V atoms, leading to the different surface densities of site \(\alpha\) and \(\beta\) for hydrogen evolution as described in Table 1.  
Different Te atoms in the buckled layer of the three _TM_Te\({}_{2}\) have different Gibbs free energies for hydrogen adsorption. We classified two Te atomic sites as \(\alpha\) sites and \(\beta\) sites to analyze the HER performances of metallic _TM_Te\({}_{2}\). We assume that \(\alpha\) site Te atoms are better catalytically active than \(\beta\) site Te atoms in the three _TM_Te\({}_{2}\), which is consistent with recently reported HER works on MoTe\({}_{2}\) and WTe\({}_{2}\)[22, 23, 27]. During the HER process, approaching proton atoms (H\({}^{+}\)) are captured at \(\alpha\) sites rather than \(\beta\) sites due to the differences in Gibbs free energies, which creates the intrinsic catalytic activity for hydrogen evolution [22, 23, 27]. The number densities of active \(\alpha\) site in three _TM_Te\({}_{2}\) are calculated with unit surface area, which contains one active \(\alpha\) site (squares with dashed lines in Figure 1a) in the basal plane (see Table 1). Figure 1: (a) Schematics of monolayer _TM_Te\({}_{2}\) (TM = V, W, and Mo) with side and top view. Protons (H+) in acidic electrolyte (0.5 M H\({}_{2}\)SO\({}_{4}\)) are just passing without reactions at catalytic inactive Te atoms (\(\beta\) site, yellow circles), but are adsorbed at catalytic active Te atoms (\(\alpha\) site, orange circles) and subsequently are converted to hydrogen gas (H\({}_{2}\)). (WTe\({}_{2}\); a = 3.498 Å and b = 6.338 Å, MoTe\({}_{2}\); a = 3.487 Å and b = 6.366 Å, VTe\({}_{2}\); b = 3.605 Å, a+2c = 9.529 Å). (b) X-ray diffraction of three _TM_Te\({}_{2}\) single crystals. To study the inherent catalytic activity of the basal plane in _TM_Te\({}_{2}\), _TM_Te\({}_{2}\) single crystals were used for this study. We synthesized single crystalline _TM_Te\({}_{2}\) using the NaCl-Flux method and the solid state reaction described in our previous studies [28; 29]. We confirmed that _TM_Te\({}_{2}\) single crystals have the orthorhombic (a = 3.498 A, b = 6.338 A, c = 15.432 A, \(\beta\) = 90\({}^{\circ}\)), monoclinic (a = b = 3.559 A, c = 40.693 A, \(\beta\) = 90\({}^{\circ}\)), and monoclinic (a = 14.826 A, b = 3.605 A, c = 9.366 A, \(\beta\) = 112.4\({}^{\circ}\)) structure for WTe\({}_{2}\), MoTe\({}_{2}\), and VTe\({}_{2}\), respectively, and the _TM_Te\({}_{2}\) was well grown with a growth axis in the (00n) direction using x-ray diffraction as seen in Figure 1b. All three _TM_Te\({}_{2}\) show metallic behaviors in which the resistivity decreases with a decreasing temperature (see Figure S2). The three _TM_Te\({}_{2}\) are known as semimetals (or bad metals) with few carriers due to the quasi gap at the Fermi surface [33; 34]. Thus, they have good charge transfer and also less of a screening effect. Figure 2a shows the experimental setup for electrochemical measurements of the three metallic _TM_Te\({}_{2}\).