Physico-Chemical Interaction of MgB2 with Water during Ultrasonication: An Avenue to Realize Boron-Rich Quantum Dots, Nanodiscs, and Nanoplatelets

Abstract

Layered magnesium diboride (MgB2), with magnesium atoms sandwiched in between boron honeycomb planes, present a rich opportunity to access graphenic forms of boron. It has been recently demonstrated by us that MgB2 crystals can be exfoliated by ultrasonication in water to yield boron-rich nanosheets. However, the knowledge on the fate of layered metal borides during ultrasonication in an aqueous phase is in its incipient stages. This thesis presents a detailed investigation of the physico-chemical interaction of MgB2 with water during ultrasonication. We observe, that in addition to undergoing exfoliation to yield nanosheets, MgB2 crystals also undergo a chemical reaction with water which results in their degradation to form gaseous boron hydrides and quantum dots rich in boron. These quantum dots are found to be short-lived and subsequently ripen to form boron rich nanodiscs and nanoplatelets. While the quantum dots are observed to be <10 nm in lateral dimensions, the nanodiscs and nanoplatelets have a lateral dimensional range of ~50-400 nm. These nanostructures are few-layer thick exhibiting a transverse dimension of ~6-8 nm and are found to be decorated with hydrides and oxy-functional groups. The chemical modification and nano scaling of MgB2 results in transparent aqueous dispersions which absorb only in UV regime, making the nanostructure prospective candidates for fabricating UV absorbing materials. This study presents an entirely new perspective on the bottom-up formation of planar boron nanostructures. The ability to form versatile morphologies of boron-rich nanostructures from a single starting material significantly adds to the current state of knowledge on boron-based nano structures.