Synthesis of boron nitride nanosheets powders using a plasma based bottom-up approach

Abstract

Boron nitride nanosheets (BNNS) are graphene-like materials with large bandgap and excellent thermal/chemical stability. Current BNNS synthesis methods show low yield and/or purity preventing effective implementation in real-life applications. This work reports two catalyst-free bottom-up approaches for BNNS synthesis using induction thermal plasma. High enthalpy and cooling rates of this plasma allow BNNS to form homogeneously when using solid ammonia borane (AB) as a precursor. In this case, clusters of B x N y H z nucleate to form particles of critical sizes on which BNNS propagate while releasing H2. Using boron powders instead of AB produces BNNS through a heterogeneous route. In this case, boron undergoes spheroidization while active nitrogen species diffuse on the liquid surface to form boron nitride nanowalls which propagate into BNNS. The operating pressure and nitrogen loading are shown to control BNNS nucleation and growth and minimize by-products. An optimum operating pressure of 62 kPa provides axial velocities necessary for decomposing AB and melting boron. The synthesis process is well controlled yielding homogeneously grown BNNS of 30 × 30 nm sheet lateral sizes and ∼8 atomic layers, while the heterogeneously grown BNNS are of 100 × 100 nm and ∼20 layers. Both homogeneous and heterogeneous routes lead to BNNS structures showing exceptional crystallinity with defect-free stacking of the BN planes.