Atomic dopants and defects play a crucial role in creating new functionalities in 2D transition metal dichalcogenides (2D TMDs). Therefore, atomic‐scale identification and their quantification warrant precise engineering that widens their application to many fields, ranging from development of optoelectronic devices to magnetic semiconductors. Scanning transmission electron microscopy with a sub‐Å probe has provided a facile way to observe local dopants and defects in 2D TMDs. However, manual data analytics of experimental images is a time‐consuming task, and often requires subjective decisions to interpret observed signals. Therefore, an approach is required to automate the detection and classification of dopants and defects. In this study, based on a deep learning algorithm, fully convolutional neural network that shows a superior ability of image segmentation, an efficient and automated method for reliable quantification of dopants and defects in TMDs is proposed with single‐atom precision. The approach demonstrates that atomic dopants and defects are precisely mapped with a detection limit of ≈1 × 1012 cm−2, and with a measurement accuracy of ≈98% for most atomic sites. Furthermore, this methodology is applicable to large volume of image data to extract atomic site‐specific information, thus providing insights into the formation mechanisms of various defects under stimuli.
Among transition metal dichalcogenides (TMdCs) as alternatives for Pt‐based catalysts, metallic‐TMdCs catalysts have highly reactive basal‐plane but are unstable. Meanwhile, chemically stable semiconducting‐TMdCs show limiting catalytic activity due to their inactive basal‐plane. Here, metallic vanadium sulfide (VSn) nanodispersed in a semiconducting MoS2 film (V–MoS2) is proposed as an efficient catalyst. During synthesis, vanadium atoms are substituted into hexagonal monolayer MoS2 to form randomly distributed VSn units. The V–MoS2 film on a Cu electrode exhibits Pt‐scalable catalytic performance; current density of 1000 mA cm−2 at 0.6 V and overpotential of −0.08 V at a current density of 10 mA cm−2 with excellent cycle stability for hydrogen‐evolution‐reaction (HER). The high intrinsic HER performance of V–MoS2 is explained by the efficient electron transfer from the Cu electrode to chalcogen vacancies near vanadium sites with optimal Gibbs free energy (−0.02 eV). This study provides insight into ways to engineer TMdCs at the atomic‐level to boost intrinsic catalytic activity for hydrogen evolution.
Two-dimensional (2D) van der Waals (vdW) heterostructures exhibit novel physical and chemical properties, allowing the development of unprecedented electronic, optical, and electrochemical devices. However, the construction of wafer-scale vdW heterostructures for practical applications is still limited due to the lack of well-established growth and transfer techniques. Herein, we report a method for the fabrication of wafer-scale 2D vdW heterostructures with an ultraclean interface between layers via the aid of a freestanding viscoelastic polymer support layer (VEPSL). The low glass transition temperature (T g) and viscoelastic nature of the VEPSL ensure absolute conformal contact between 2D layers, enabling the easy pick-up of layers and attaching to other 2D layers. This eventually leads to the construction of random sequence 2D vdW heterostructures such as molybdenum disulfide/tungsten disulfide/molybdenum diselenide/tungsten diselenide/hexagonal boron nitride. Furthermore, the VEPSL allows the conformal transfer of 2D vdW heterostructures onto arbitrary substrates, irrespective of surface roughness. To demonstrate the significance of the ultraclean interface, the fabricated molybdenum disulfide/graphene heterostructure employed as an electrocatalyst yielded excellent results of 73.1 mV·dec–1 for the Tafel slope and 0.12 kΩ of charge transfer resistance, which are almost twice as low as that of the impurity-trapped heterostructure.
In recent years, one-dimensional (1D) transition-metal chalcogenide nanowires have been considered as potential candidates to replace noble-metal-based electrocatalyst in water electrolysis because they exhibit high surface area and have plenty of exposed chalcogen atoms to act as active sites. Herein, we report the fabrication of the noblemetal-free electrocatalyst of selenium-rich 1D single-chain niobium selenide (Nb 2 Se 9 ) for efficient hydrogen evolution reaction (HER). The Nb 2 Se 9 electrocatalyst is simply prepared by the filtration of dispersed Nb 2 Se 9 in isopropanol through porous carbon paper used as a filter and electrode, which enables the fabrication of a binder-free electrocatalyst. HER activity is gradually increased with reduction in the bundle size of nanowire due to the increment of active sites where the selenium atoms are more exposed, eventually reaching a low onset potential of −27 mV, Tafel slope of 63.7 mV dec −1 , and a large exchange current density of 0.25 mA cm −2 as well as a high hydrogen turnover frequency of (∼2 H 2 s −1 ) at −0.2 V. Furthermore, the remarkably stable structure of Nb 2 Se 9 demonstrates the considerable importance of the stability and cyclic durability of the catalyst in acidic medium for practical application. To probe into the catalytic active sites of Nb 2 Se 9 for HER, density functional calculations are performed, revealing that the selenium-rich site in Nb 2 Se 9 serves as the primary active site for HER.
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