Quantification of Dangling Bonds Formed via Plasma Treatment as Reactive Sites for Functionalization of Hexagonal Boron Nitride Nanoparticles

To elucidate the mechanism involved in the surface modification of hexagonal boron nitride (h-BN) via plasma treatment, we evaluated the number of reactive sites available for functionalization by quantifying the dangling bonds formed on h-BN particles during plasma treatment in solution. Electron spin resonance detected the boron dangling bonds formed on h-BN particles; these dangling bonds gradually decreased upon exposure to an atmospheric environment for up to 650 days as a result of oxygen termination, such as hydroxyl functionalization. These results indicated that plasma treatment increased the number of dangling bonds, up to 14-fold per unit mass, on h-BN surfaces as reactive sites for functionalization. The surface number density of boron dangling bonds as a function of air-exposure days could be expressed as Langmuir adsorption kinetics; the surface reactive-site density was estimated to be of 1015 m–2 on the plasma-treated h-BN. Polymer composite materials incorporating such functionalized h-BN after plasma treatment and oxygen termination showed markedly improved dispersion and flexibility. Overall, plasma treatment effectively introduced reactive sites even on chemically stable materials, while subsequent oxygen termination further promoted surface functionalization.