Information on the morphology and edge structures of single-layer MoS2 and promoted CoMoS nanoparticles is essential in order to understand and accurately model the activity in hydrodesulfurization catalysis. Only the edge sites in single-layer MoS2 nanoparticles are known to be active in catalysis, but since different edge structures are possible, the activity and selectivity may depend critically on the exact morphology. From atom-resolved scanning tunneling microscopy (STM) experiments, we provide here a general overview of the atomic-scale effect of sulfo-reductive atmospheres, sulfiding agents, promoter atoms, and support type on the equilibrium morphology of single-layer MoS2. Overall strong morphology variations are seen. These variations are interpreted in the framework of the Wulff construction as differences in edge energies induced by the conditions of the experiment. The results highlight the STM as an increasingly. important experimental technique to investigate the atomic-scale structure of catalytically active nanoparticles in general and as one of the only tools, which can address single atom defects and single adsorption events on the atomic-scale. (C) 2013 Elsevier Inc. All rights reserved.