In situ TEM study of edge reconstruction and evolution in monolayer black phosphorus.

As symmetry-breaking interfaces, edges inevitably influence material properties, particularly for low-dimensional materials such as two-dimensional (2D) graphene and black phosphorus (BP). Hence, exploiting pristine edge structures and the associated edge reconstruction is important. In this study, we revealed edge reconstruction and evolution in monolayer BP (ML-BP) via in situ high-resolution transmission electron microscopy. Under our typical experimental conditions, spontaneous edge reconstruction occurred in all types of as-prepared edges that include zigzag, Klein zigzag, diagonal, and Klein diagonal edges. Reconstruction induces a periodic variation of the bond length and bond angles of edge atoms: an out-of-plane bending for zigzag and diagonal edge atoms and a dimerization for two neighboring edge atoms on the Klein edge, respectively. Surface atom diffusion can also induce edge structural evolution as evidenced by the atomic scale dynamics captured for the zigzag edge. Experimentally resolved edge configurations and reconstruction were further corroborated by ab initio first-principles calculations. This study explores the understanding of the edge stability in 2D BP materials and may provide routes for precisely controlled edge structure engineering.

Composition and Interface Engineering of Alloyed MoS2 x Se2(1- x ) Nanotubes for Enhanced Hydrogen Evolution Reaction Activity.

Hierarchical MoS2 x Se2(1- x ) nanotubes assembled from several-layered nanosheets featuring tunable chalcogen compositions, expanded interlayer spacing and carbon modification, are synthesized for enhanced electrocatalytic hydrogen evolution reaction (HER). The chalcogen compositions of the MoS2 x Se2(1- x ) nanotubes are controllable by adjusting the selenization temperature and duration while the expanded (002) interlayer spacing varies from 0.98 to 0.68 nm. It is found that the MoS2 x Se2(1- x ) (x = 0.54) nanotubes with expanded interlayer spacing of 0.98 nm exhibit the highest electrocatalytic HER activity with a low onset potential of 101 mV and a Tafel slope of 55 mV dec(-1) . The improved electrocatalytic performance is attributed to the chalcogen composition tuning and the interlayer distance expansion to achieve benefitting hydrogen adsorption energy. The present work suggests a potential way to design advanced HER electrocatalysts through modulating their compositions and interlayer distances.

Stable Metallic 1T-WS2 Nanoribbons Intercalated with Ammonia Ions: The Correlation between Structure and Electrical/Optical Properties.

Stable metallic 1T-WS2 nanoribbons with zigzag chain superlattices, highly stabilized by ammonia-ion intercalation, are produced using a facile bottom-up process. The atomic structure of the nanoribbons, including W-W reconstruction and W-S distorted octahedral coordination, results in distinctive electrical transport and optical Raman scattering properties that are very different from semiconducting 2H-WS2 . The correlations between structure and properties are further confirmed by theory calculations.