Transmission electron microscopy (TEM) is used to study growth of MoS2 nanocrystals in situ. The nanocrystals are formed from a submonolayer molybdenum oxide dispersed on an oxide support by sulfidation in an H2S/H2 atmosphere. From series of time-resolved TEM images, it is revealed that single-layer MoS2 nanocrystals form preferentially and that multi-layer nanocrystals form late in the sulfidation process. The TEM images pinpoint that step sites in the support can act as nucleation centers for single-layer nanocrystals and that single-layer nanocrystals grow along the support surface. Moreover, the TEM images reveal that multi-layer MoS2 nanocrystals form in a layer-by-layer mode by the homogeneous nucleation of additional MoS2 layers onto already formed single-layer MoS2 nanocrystals. Hereby, the atomic-scale observations suggest that the formation of multi-layer MoS2 nanocrystals is an energetically more activated process than growth of single-layers. These findings explain why process parameters, such as temperature, can tune the relative fraction of single- to multi-layer MoS2 nanocrystals, which is important for their use in, e.g., hydrotreating catalysis.
The observation of the multi-layer MoS2 nanocrystal formation at the atomic scale and noted that it is a more dynamic process in comparison to that of single-layers. The researchers concluded that the difference in characteristics can explain why process parameters can modify the relative fraction of single- to multi-layer MoS2 nanocrystals.