Morphology and atomic-scale structure of single-layer WS2 nanoclusters

Abstract

Two-dimensional sheets of transition metal (Mo and W) sulfides are attracting strong attention due to the unique electronic and optical properties associated with the material in its single-layer form. The single-layer MoS₂ and WS₂ are already in widespread commercial use in catalytic applications as both hydrotreating and hydrocracking catalysts. Consequently, characterization of the morphology and atomic structure of such particles is of utmost importance for the understanding of the catalytic active phase. However, in comparison with the related MoS₂ system only little is known about the fundamental properties of single-layer WS₂ (tungstenite). Here, we use an interplay of atom-resolved Scanning Tunneling Microscopy (STM) studies of Au(111)-supported WS₂ nanoparticles and calculated edge structures using Density Functional Theory (DFT) to reveal the equilibrium morphology and prevalent edge structures of single-layer WS₂. The STM results reveal that the single layer S–W–S sheets adopt a triangular equilibrium shape under the sulfiding conditions of the synthesis, with fully sulfided edges. The predominant edge structures are determined to be the (100) W-edge, but for the smallest nanoclusters also the (010) S-edges become important. DFT calculations are used to construct phase diagrams of the WS₂ edges, and describe their sulfur and hydrogen coordination under different conditions, and in this way shed light on the catalytic role of WS₂ edges.