Abstract:
Two dimensional (2D) nanomaterials came into limelight with the discovery of graphene[1] in 2004. Graphene is a single atom thick layer of carbon having a hexagonal honeycomb lattice structure. It’s exceptional optoelectronic and mechanical properties[2] motivated the researchers to acquire analogs of graphene by delaminating them from their three dimensional (3D) parent crystals. Significant work has already been done to get 2D allotropes of conventional compounds. Boron, being immediate neighbor of carbon in periodic table, has always fascinated the scientific community on account of its rich properties. Thus specifically, researchers are investigating if boron could be arranged in form of nanosheets. Theoretical studies suggest that access to it’s quasi 2D planar structure could be feasible if
precursor possess a layered structure. Magnesium diboride (MgB ), which holds a layered structure with magnesium atoms sandwiched between the hexagonal layers of boron, presents a prospective material and can be exfoliated to acquire 2D nanostructures rich in boron.
We recently demonstrated that ultrasonication of MgB2 crystals in water can yield chemically modified MgB2 (CMMB) nanosheets decorated mostly with hydroxyl groups[3]. However, the yield and stability has been observed to be very low. In order to acquire the quasi 2D sheets in an appreciable amount, ionic liquid (IL) can prove to be a good solvent for exfoliation process. It is well known that ionic liquid (IL ) shows a double displacement reaction, thus it could displace Mg2+ ions to give boron based nanosheets with probable less degree of functionalization. In this work, we show that stable dispersion of few-layer-thick boron based nanosheets can be synthesized by tip ultrasonication mediated exfoliation of parent MgB crystals in ionic liquid ([BMIM][BF4]). We have been able to synthesize lower dimensional nanostructures, in powdered form as well as in colloidal dispersion. The nanosheets exhibit lateral dimensions in a range of 3-6 µm with an average thickness of ~2.5 nm. Whereas, lower dimensional boron rich nanostructures exhibit diameters in the range of 5-25 nm as estimated by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). We show that these nanostructures display a wavelength dependent photoluminescence over a broad spectrum of UV-visible wavelengths. The energy-dispersive X-ray (EDX) spectroscopy suggests that MgB2 is chemically modified after exfoliation. Selected Area Electron Diffraction (SAED) revealed that most of the nanosheets are polycrystalline along with few possess amorphous structure. The data obtained from UV-vis spectroscopy facilitated us to get the insight about the colloidal dispersion acquired after centrifugation of the sonicated sample. We also showed that these nanostructures exhibits optical band gap of ~3.3eV. Therefore these photoluminescent nanostructures with tuned band gap value have diverse application in optoelectronic and energy storage field. This one step approach of synthesizing boron-based nanosheets using IL not only presents a simple way to form lower-dimensional boron rich nanostructures, but also renders a previously unexplored interface of planar forms of boron with IL.