Microwave-assisted rapid synthesis of graphene-analogue hexagonal boron nitride (h-BN) nanosheets and their application for the ultrafast and selective adsorption of cationic dyes from aqueous solutions

Abstract

Herein, graphene-analogue nanoscale hexagonal boron nitride (h-BN) with a sheet-like morphology is successfully synthesized by reacting borax (Na₂B₄O₇·10H₂O) and NH₄Cl powder as inexpensive starting materials in air via a facile microwave-assisted method with a short reaction time (10 min). The as-prepared product is structurally characterized via X-ray powder diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), Raman spectroscopy, energy dispersive spectroscopy (EDS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and BET surface area measurements. X-ray diffraction indicates the formation of hexagonal boron nitride (h-BN) with lattice constants of a = 2.50 and c = 6.67 Å. The purity of the BN nanosheets is confirmed via X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. TEM and FE-SEM indicate that the as-synthesized product is composed of fine loosely aggregated plate-like particles with lengths in the range of 0.5 to 3.5 μm and thicknesses of 20–30 nm. EDX gives an average B/N atomic ratio of about 1 : 1. FT-IR displays strong B–N absorptions at 1396 and 805 cm⁻¹. In addition, ultraviolet-visible (UV-vis) spectroscopy reveals that the as-prepared h-BN has an obvious band gap (5.92 eV). The as-prepared BN nanosheets also have a relatively large specific surface area of 177.07 m² g⁻¹. Due to these characteristics, the as-prepared BN nanosheets exhibit a very quick adsorption rate for cationic methylene blue (MB) and Rhodamine B (RhB) dye and their adsorption capacity is as high as 472.4 mg g⁻¹. Furthermore, these BN nanosheets can also selectively adsorb MB from binary and ternary mixed dye solutions. Therefore, these BN nanosheets have wide promising applications in wastewater treatment and the recovery of valuable organic compounds from wastewater.