Anion‐Directed Horizontal Growth Toward Atomically Thin Metal Phosphorus Chalcogenides in Low‐Temperature Molten Salts

Two‐dimensional metal phosphorus chalcogenides (MPY 3 ) represent an emerging class of layered materials in which strong structural anisotropy and unusual electronic properties give rise to pronounced light‐matter interactions and thickness‐dependent functionalities. However, accessing these intrinsic properties at the atomic thickness limit has been fundamentally impeded by the lack of synthetic strategies capable of precise thickness control. Herein, we develop an anion‐directed thickness regulation strategy in low‐temperature molten salts to induce the horizontal growth of FePS 3 , enabling the synthesis of atomically thin nanosheets with a well‐defined three/four‐layer structure and selectively exposed (001) planes. It is revealed that selective adsorption of anions (Br − and I − ) on the (001) surface effectively modulates surface free energy, suppresses vertical stacking, and continuously tunes the nanosheet thickness from ≈14 nm to ≈2 nm. This strategy is further extended to a broad family of MPY 3 materials, such as NiPS 3 , CdPS 3 , and FePSe 3 . The resulting atomically thin MPY 3 nanosheets exhibit excellent photothermal conversion performance and enable efficient photothermal‐enhanced electrocatalytic hydrogen evolution, thereby offering mechanistic insights into thickness‐dependent structure‐property relationships and guiding the rational design of light‐thermal and light‐thermal‐electrical synergistic energy conversion systems.