Search for: All records

The adsorption and dissociation of water molecules on two-dimensional transition metal dichalcogenides (TMDs) is expected to be dominated by point defects, such as vacancies, and edges. At the same time, the role of grain boundaries, and particularly, mirror twinboundaries (MTBs), whose concentration in TMDs can be quite high, is not fully understood. Using density functional theory calculations, we investigate the interaction of water, hydroxyl groups, as well as oxygen and hydrogen molecules with MoSe 2 monolayers when MTBs of various types are present. We show that the adsorption of all species on MTBs is energetically favorable as compared to that on the basal plane of pristine MoSe 2 , but the interaction with Se vacancies is stronger. We further assess the energetics of various surface chemical reactions involving oxygen and hydrogen atoms. Our results indicate that water dissociation on the basal plane should be dominated by vacancies even when MTBs are present, but they facilitate water clustering through hydroxyl groups at MTBs, which can anchor water molecules and give rise to the decoration of MTBs with water clusters. Also, the presence of MTBs affects oxygen reduction reaction (ORR) on the MoSe 2 monolayer. Unlike Se vacancies which inhibit ORR due to a high overpotential, it is found that the ORR process on MTBs is more efficient, indicating their important role in the catalytic activity of MoSe 2 monolayer and likely other TMDs. 

Water adsorption on transition metal dichalcogenides and other 2D materials is generally governed by weak van der Waals interactions. This results in a hydrophobic character of the basal planes, and defects may play a significant role in water adsorption and water cluster nucleation. However, there is a lack of detailed experimental investigations on water adsorption on defective 2D materials. Here, by combining low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we study in that context the well-defined mirror twin boundary (MTB) networks separating mirror-grains in 2D MoSe 2 . These MTBs are dangling bond-free extended crystal modifications with metallic electronic states embedded in the 2D semiconducting matrix of MoSe 2 . Our DFT calculations indicate that molecular water also interacts similarly weak with these MTBs as with the defect-free basal plane of MoSe 2 . However, in low temperature STM experiments, nanoscopic water structures are observed that selectively decorate the MTB network. This localized adsorption of water is facilitated by functionalization of the MTBs by hydroxyls formed by dissociated water. Hydroxyls may form by dissociating of water at undercoordinated defects or adsorbing of radicals from the gas phase in the UHV chamber. Our DFT analysis indicates that the metallic MTBs adsorb these radicals much stronger than on the basal plane due to charge transfer from the metallic states into the molecular orbitals of the OH groups. Once the MTBs are functionalized with hydroxyls, molecular water can attach to them, forming water channels along the MTBs. This study demonstrates the role metallic defect states play in the adsorption of water even in the absence of unsaturated bonds that have been so far considered to be crucial for adsorption of hydroxyls or water.