Water‐Driven Stacking Structure Transformation of Ultrathin Ru Nanosheets for Efficient Hydrogen Evolution Reaction

Abstract Ultrathin crystalline materials are a class of popular materials that can potentially exhibit fascinating physical and chemical properties dictated by their unique stacking freedom. However, it is challenging to achieve the controllable synthesis over their stacking structure for ultrathin crystalline materials. Herein, water is employed as a key regulatory factor to realize phase engineering in ultrathin nanosheets (NSs), thereby altering stacking faults to achieve distinct stacking arrangements. Ruthenium (Ru) NSs with consistent specific surface areas but different stacking manners are fabricated through the systematic regulation of water. Based on this, it is demonstrated that the hydrogen evolution reaction (HER) performance can be significantly influenced by their stacking structures. Further in‐depth investigations reveal that the distinct stacking structures of Ru NSs, featuring a limited area of side facets, will influence the energy barrier of sluggish Volmer step in HER. Ru NSs with ABC stacking exhibit an accelerated Volmer process with outstanding catalytic activity, demonstrating a remarkably low overpotential (25 mV at 10 mA cm ⁻² ) and Tafel slope (29 mV dec ⁻¹ ) than most of the reported HER catalysts. The work will advance the understanding of controllable synthesis methods and illuminate the structure‐activity relationships in ultrathin crystalline nanomaterials.