Single-layer MoS 2 is proving to be a versatile material for a wide variety of electronic, optical, and chemical applications. Sulfur depletion, without destabilization of the single layer, is considered a prudent way for making the basal plane of the layer catalytically active. Based on the results of our density-functional-theory examination of vacancy structures on one side of an MoS 2 layer, we show that the formation energy per sulfur vacancy is the lowest (energetically favorable) when the vacancies form a row and that the longer the row, the lower the formation energy. In addition, we find that the lowest energy barrier for the diffusion of sulfur vacancy at the row structures through the exchange of a vacancy with a nearby sulfur atom is 0.79 eV and that this barrier increases as the row elongates. We also evaluate the propensity for catalytic activity of an MoS 2 layer with two types of sulfur-vacancy structures (row and patch) and find the energetics for alcohol synthesis from syngas to be more favorable for the layer with a sulfur-vacancy patch.

We present results of ab initio density functional theory (DFT) based calculations of the geometry, electronic structure, and reactivity of subnanometer-sized (29-atom) transition metal nanoparticles (NPs) (Cu29, Ag29, and Au29) supported on single-layer MoS2. As compared to its pristine form, defect-laden MoS2 (with a S vacancy row) has relatively larger effect on the above properties of the NPs. The NPs bind more strongly on defect-laden than on pristine MoS2 (in the order Cu29 > Ag29 > Au29), confirming the important role of vacancies in stabilizing the NPs on the support. The presence of vacancies also leads to an increase in charge transfer from the NPs to MoS2 (with the same elemental trend as for their binding energy) and to a shift of the d-band center of the NPs further toward the Fermi level, in turn influencing their propensity toward chemical activity. We examine the adsorption and dissociation of O2 as the prototype reactions and find that there is no barrier for O2 to adsorb on top of an atom at the NP apex, where the frontier orbitals are localized, and that the dissociation channel proceeds through a chemisorbed state. The presence of the support leads to increase in the number of sites at which O2 can adsorb with similar binding energy (<0.1 eV difference). Interestingly, energy barriers for both dissociation and recombination of O2, when adsorbing at the NP apex, increase in the presence of the MoS2 support. However, since the increase in the barrier for recombination is much larger than for dissociation, the latter should be more favored. In particular, for defect-laden MoS2 supported Au29 the recombination faces a barrier of 1.36 eV whereas the dissociation does 0.5 eV, implying that the defect-laden support may significantly improve the catalytic performance of Au29 toward oxidation reaction.

Novel technological applications in catalysis and bactericidal formulation have emerged for zinc oxide (ZnO) nanoparticles owing to their ability to generate reactive oxygen species by fostering H2O dissociation. Rational improvement of those properties requires a mechanistic understanding of ZnO nanoparticle reactivity, which is currently lacking. Here, we determine the structural and electronic properties of nanometer-sized ZnO, determine the binding energetics of H2O adsorption, and compare to an extended macroscopic surface. We show that the electronic density of states of ZnO nanoparticles is size-dependent, exhibiting a decreasing bandgap with the increase of nanoparticle diameter. The electronic states near the Fermi energy dominantly arise from O 2p states, which are spatially localized on “reactive” surface O atoms on the nanoparticle edges that are doubly coordinated. The frontier electronic states localized at the low coordinated atoms induce a spontaneous dissociation of H2O at the nanoparticle edges. The surface Zn and O atoms have inhomogeneous electronic and geometrical/topological properties, thus providing nonequivalent sites for dissociative and molecular H2O adsorption. The free energy of H2O binding is dominated by the electronic DFT interaction energy, which is site-dependent and correlated with the Bader charge of surface Zn atom. Entropy is found to stabilize the bound form, because the increase in the vibrational contribution is greater than the decrease in the translational and rotational contribution, whereas solvation stabilizes the unbound state. The absence of rough edges on an extended, macroscopic ZnO surface prevents spontaneous dissociation of a single H2O. This study underlies the importance of coupling geometrical and electronic degrees of freedom in determining the reactivity of nanoparticles and provides a simple elucidation of the superior catalytic activity of ZnO nanoparticles compared to ZnO in macroscopic forms.

We have employed ab-initio density functional theory (DFT) to study the adsorption, dissociation, diffusion, and vibration of oxygen on Ag(110). We find that the four-fold hollow site is the preferred site 68.43.Jk, 68.43.Pq