Morphology of WS 2 nanoclusters in WS 2/C hydrodesulfurization catalysts revealed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging

Abstract

Tungsten disulfide (WS 2) nanoclusters supported on high surface area graphite are studied by use of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging. It is found that HAADF-STEM, in contrast to high-resolution transmission electron microscopy (HRTEM), enables ready observation of all the WS 2 nanoclusters and gives direct insight into their morphology. Such measurements may thus provide a more detailed and quantitative basis for establishing activity correlations. The S W S layers of the WS 2 nanocluster structure are found to be oriented parallel to the graphite surface and with an apparent topotactic orientation relationship. Whereas many WS 2 nanoclusters are observed to be irregular in shape, hexagonal and more often triangular or truncated triangular shapes are also encountered. The shape variations within a particular sample are larger than the shape variations between different samples each prepared by a particular sulfiding procedure. The vast majority of nanoclusters contains only a single S W S layer even after sulfiding at 1073 K.