High yield and concentration exfoliation of defect-free 2D nanosheets via gentle water freezing-thawing approach and stabilization with PVP

Abstract

The scalable exfoliation of bulk hexagonal boron nitride (h-BN) into high quality mono- or few-layer nanosheets still remains an enormous challenge and becomes the bottleneck to essential research and applications of h-BN. Here, a gentle water freeze-thaw technique, with a particular focus on the use and efficacy of polyvinylpyrrolidone (PVP) as a dispersant, is put forth as a novel and efficient approach for exfoliation of BNNSs in a large yield (32%) and high concentration (1.64 mg ml−1). Through the investigation into the exfoliation mechanism, it’s found that the essence of peeling driving forces comes from the high adsorption ability of PVP, which presents strong π-π interaction with few-layered h-BN nanosheets (BNNS) and the expansion of the volume of water upon freezing. The results of systematic characterization for thin flakes show that dispersions with PVP to initial h-BN concentration ratio as high as 27%, should be favorable for the preparation of samples and devices with minimal interference from PVP. Furthermore, the synthesized BNNSs after 30 repetitive freeze−thaw cycles (BNNSs-30) with original lateral dimensions consists primarily of fewer than five atomic layers, as evidenced by electron microscopy. XRD, XPS and Raman results confirm that as prepared materials have pure phase, and no defects are observed. This simple and novel strategy can synthesize large quantities of defect-free BNNSs. Above all, the PVP assisted freeze-thaw method not only paves the way for wide applications, but also can be versatile, having been used to produce dozens of different 2D nanosheets, such as MoS2 and MoO3 nanoflakes, in large yields.