Formation Mechanism of Boron-Based Nanosheet through the Reaction of MgB₂ with Water

A recent experiment demonstrated that ultrasonication of MgB₂ in water yields Mg-deficient hydroxyl-functionalized boron nanosheets at room temperature. Herein, we examined the mechanism of nanosheet formation. Analysis of the reaction products and temporal variation in pH and H₂ production shows that the reaction between MgB₂ and water comprises two steps: (i) an ion-exchange process between protons and a part of Mg cations of MgB₂ with its exfoliation and (ii) the hydrolysis reaction between Mg-deficient boron hydride and water to produce H₂ and Mg-deficient hydroxyl-functionalized boron sheets. The sheets with a stacking periodicity of 0.70 nm were obtained as the supernatant of the reaction product of water with MgB₂. The stacking sheets can be further exfoliated if the reaction is conducted under ultrasonication. The derived nanosheets are composed of sp²-bonded boron framework and possess a disordered structure containing hydroxyl species and oxidized magnesium.