Chemical exfoliation synthesis of boron nitride and molybdenum disulfide 2D sheets via modified Hummers’ method

Abstract

To realize long-cherished dreams of employing inorganic graphene cousins such as boron nitride (BN) and molybdenum disulfide (MoS₂) for large area coating applications, e.g., miniaturized electronic chips, radio wave devices, energy storage, LEDs, and solar cells, scalable synthesis pathways are presently being hunted with unprecedented curiosity to obtain large-scale 2D exfoliated sheets. Modified Hummers’ method provides an apt alternate and has already been established for graphene. However, it currently faces limitations in scalable production of BN and MoS₂ as the nature of binding in their crystallographic lattice as well as their interactions with the solvents are different in these materials’ vis-a-vis graphene. Therefore, intensely focused efforts are needed to optimize synthesis parameters to produce scalable quantities of these advanced materials at an economical cost. By hybridizing BN with MoS_2, for example, one can maneuver the material’s dielectric functionality, such as frequency bandwidth where it functions. In this report, we present a facile synthesis of BN and MoS₂ via modified Hummers’ synthesis. While the fixed BN:KMnO₄ precursor ratio of 1:6 was considered to obtain 2D BN, it (MoS₂:KMnO₄) was altered, i.e., 1:6, 1:5, and 1:4, for obtaining MoS₂ sheets. Interestingly, while precursor ratio 1:6 resulted in average lateral dimension ~100 nm, precursor ratio of 1:4 yielded lateral dimensions ~500 nm as evidenced from TEM measurements. Solvothermal (200 °C for 2 h) reduction was carried out for both BN and MoS₂ to remove the surface functionalities employing reducing solvent DMF. We obtained few-layer crystalline atomic sheets of BN and MoS₂. While inter-atomic distance in 2D BN was measured to be ~1.5 A° as evident from HRTEM imaging, E_2g vibration mode ~ 1360 cm⁻¹ was recorded in Raman spectroscopy. While HRTEM imaging reveals that average inter-atomic (Mo-S) distances in MoS₂ atomic sheets was obtained to be ~2.41 A°, two distinct MoS₂ Raman modes, viz., E_2g ~ 404 cm⁻¹ and A_1g ~385 cm⁻¹, were witnessed. Microwave blending of 2D MoS₂ with 2D BN resulted in significant modulation of dielectric behavior. While the modified Hummers’ synthesis for BN and MoS₂ will be a boon in disguise for several dream applications envisaged earlier, it may inspire future generations of devices and sensors with novel functionalities, it is believed.