The monolayer MoS2 consists of a monatomic Mo-layer between two monatomic S-layers like a sandwich structure, in which Mo and S atoms are alternately located at the corners of a hexagon. In order to determine the favorable Stattic cost adsorption configuration, four adsorption sites are considered, namely, H site (on top of a hexagon), TM (on top of a Mo atom), TS (on top of a S atom), and B site (on top of a Mo-S bond). The gas

molecule is initially placed with its center of mass exactly located at these sites. For each site, configurations with different molecular orientations are then examined. Take NO as an example, three initial molecular orientations are involved, one with NO axis parallel check details to the monolayer and two with NO axis perpendicular to it, with O atom above N atom and O atom below N atom [see Additional file 1 for more detailed adsorption configurations]. The adsorption energy is calculated as , where is the total energy of MoS2 with an

absorbed molecule and and E molecule are the total energies of pristine MoS2 and isolated molecule, respectively. A negative value of E a indicates that the adsorption is exothermic. Table 1 summarizes the calculated values of equilibrium height, adsorption energy, and charge transfer for the adsorption of gas molecules on monolayer MoS2. The values for each adsorbate correspond to its favorable adsorption configurations obtained at different sites. The equilibrium height is defined as the vertical distance between the center of mass of the molecule and the top S-layer of the MoS2 sheet. Note that the adsorption energies are often overestimated at the LDA level, MDV3100 in vitro but this is not very essential here because we are primarily interested in the relative values of adsorption energies for different configurations and finding the most favorable one among them. From Table 1, we see that

for both H2 and O2, the TM site is found to be their most favorable site with the adsorption energies of -82 and -116 meV, respectively. The corresponding structures are shown in Figure 1a,b. Nevertheless, it seems that the two molecules adopt distinct orientations. While H2 has an axis perpendicular to the monolayer, that of O2 is nearly parallel mTOR inhibitor to the monolayer with its center of mass on top of the TM. H2O, NH3, and NO2 are preferably adsorbed at the H site, resulting in the adsorption energies of -234, -250, and -276 meV, respectively. Structures for the three systems are shown in Figure 1c,d,f. Contrary to the configuration for H2O where H-O bonds adopt tilted orientation with H atoms pointing at the monolayer, all the H atoms of NH3 point away from the monolayer. NO2 is bonded with O atoms close to MoS2. In our calculations, H2, O2, H2O, and NH3 fail to have stable configuration at the B site; this is because they tend to migrate to other sites during structural relaxations.