Self‐Limited on‐Site Conversion of MoO₃ Nanodots into Vertically Aligned Ultrasmall Monolayer MoS₂ for Efficient Hydrogen Evolution

MoS 2 has emerged as a promising alternative electrocatalyst for the hydrogen evolution reaction (HER) due to high intrinsic per‐site activity on its edge sites and S‐vacancies. However, a significant challenge is the limited density of such sites. Reducing the size and layer number of MoS 2 and vertically aligning them would be an effective way to enrich and expose such sites for HER. Herein, a facile self‐limited on‐site conversion strategy for synthesizing monolayer MoS 2 in a couple of nanometers which are highly dispersed and vertically aligned on 3D porous carbon sheets is reported. It is discovered that the preformation of well‐dispersed MoO 3 nanodots in 1–2 nm as limited source is the key for the fabrication of such an ultrasmall MoS 2 monolayer. As indicated by X‐ray photoelectron spectroscopy and electron spin resonance data, these ultrasmall MoS 2 monolayers are rich in accessible S‐edge sites and vacancies and the smaller MoS 2 monolayers the more such sites they have, leading to enhanced electrocatalytic activity with a low overpotential of 126 mV at 10 mA cm −2 and 140 mV at 100 mA mg −1 for HER. This state‐of‐the‐art performance for MoS 2 electrocatalysts enables the present strategy as a new avenue for exploring well‐dispersed ultrasmall nanomaterials as efficient catalysts.