increasing by the abundant WN bond. To the best of our knowledge, the experimental synthesis of 2D nitrogen-rich tungsten nitrides is not reported in literature because of the harsh synthesis condition, stemming from the sluggish reaction thermodynamics of penetrating nitrogen into tungsten lattice. [9] Overall, high pressure and temperature (P-T) synthetic method (5 GPa and 880-2570 K) was adopted to prepare various nitrogen-rich tungsten nitrides such as W 2 N 3 , W 3 N 4 . [10] However, this harsh synthesis condition is laborious to control the morphology of tungsten nitrides, especially for the 2D structure. [11][12][13][14] As an alternative strategy, ammoniating tungsten metals or compounds are extensively studied to produce tungsten nitrides. [9,15] Yet, the reaction is always incomplete, leading to low nitrogen ratio in final products with unreacted tungsten precursor (tungsten, tungsten oxides, etc.). [16] Consequently, developing a facile and real-world synthetic strategy, with favorable reaction thermodynamics and facile morphology control for 2D nitrogen-rich tungsten nitrides, is highly desired but still thought provoking.Herein, we synthesize atomically thin 2D nitrogen-rich hexagonal W 2 N 3 (h-W 2 N 3 ) nanosheets via salt-templated method at atmospheric pressure for the first time. In this strategy, h-W 2 N 3 2D transition metal nitrides, especially nitrogen-rich tungsten nitrides (W x N y , y > x), such as W 3 N 4 and W 2 N 3 , have a great potential for the hydrogen evolution reaction (HER) since the catalytic activity is largely enhanced by the abundant WN bonding. However, the rational synthesis of 2D nitrogen-rich tungsten nitrides is challenging due to the large formation energy of WN bonding. Herein, ultrathin 2D hexagonal-W 2 N 3 (h-W 2 N 3 ) flakes are synthesized at atmospheric pressure via a salt-templated method. The formation energy of h-W 2 N 3 can be dramatically decreased owing to the strong interaction and domain matching epitaxy between KCl and h-W 2 N 3 . 2D h-W 2 N 3 demonstrates an excellent catalytic activity for cathodic HER with an onset potential of −30.8 mV as well as an overpotential of −98.2 mV for 10 mA cm −2 .
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