Electrolytic gas evolution is a significant phenomenon in many electrochemical technologies from water splitting, chloralkali process to fuel cells. Although it is known that gas evolution may substantially affect the ohmic resistance and mass transfer, studies focusing on the electrochemistry of individual bubbles are critical but also challenging. Here, we report an approach using scanning electrochemical cell microscopy (SECCM) with a single channel pipet to quantitatively study individual gas bubble nucleation on different electrode substrates, including conventional polycrystalline Pt and Au films, as well as the most interesting two-dimensional semiconductor MoS 2 . Due to the confinement effect of the pipet, well-defined peak-shaped voltammetric features associated with single bubble nucleation and growth are consistently observed. From stochastic bubble nucleation measurement and finite element simulation, the surface H 2 concentration corresponding to bubble nucleation is estimated to be ∼218, 137, and 157 mM, with critical nuclei contact angles of ∼156°, ∼161°, and ∼160°at polycrystalline Pt, Au, and MoS 2 substrates, respectively. We further demonstrated the surface faceting at polycrystalline Pt is not specifically correlated with the bubble nucleation behavior.

2D transition metal dichalcogenides (TMDs) are characterized by the presence of multiple crystal structures or phases, even at the ultrathin limit. Controlling phase transformations, namely, phase engineering, of 2D TMDs is crucial for realizing high‐performance 2D devices by combining phases with distinct physical and chemical properties. As a powerful approach for large‐scale production of high‐quality 2D TMDs, chemical vapor deposition (CVD) offers unique advantages in phase engineering due to its highly controllable synthesis processes. Starting with an introduction of the crystal structures and phase transformations of 2D TMDs, this review summarizes the recent developments in CVD‐mediated phase engineering strategies of TMDs, including control of temperature, precursors, catalysis, atmosphere, composition, and strain during the deposition process. Moreover, the representative applications of CVD‐based phase‐engineered TMDs in the field of transistors, photodetectors, photovoltaic cells, and catalysis are overviewed. Finally, the challenges, expectations of CVD‐based phase engineering, and future development of this versatile technique are discussed.

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In this study, we demonstrated the chemical vapor deposition (CVD) of vertically standing molybdenum disulfide (MoS2) nanosheets, with an unconventional combination of molybdenum hexacarbonyl (Mo(CO)6) and 1,2-ethanedithiol (C2H6S2) as the novel kind of Mo and S precursors respectively. The effect of the distance between the precursor’s outlet and substrates (denoted as d) on the growth characteristics of MoS2, including surface morphology and nanosheet structure, was investigated. Meanwhile, the relationship between the structure characteristics of MoS2 nanosheets and their catalytic performance for hydrogen evolution reaction (HER) was elucidated. The formation of vertically standing nanosheets was analyzed and verified by means of an extrusion growth model. The crystallinity, average length, and average depth between peak and valley (Rz) of MoS2 nanosheets differed depending on the spatial location of the substrate. Good crystalized MoS2 nanosheets grown at d = 5.5 cm with the largest average length of 440 nm, and the highest Rz of 162 nm contributed to a better HER performance, with a respective Tafel slope and exchange current density of 138.9 mV/decade, and 22.6 μA/cm2 for raw data (127.8 mV/decade and 19.3 μA/cm2 for iR-corrected data).

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