Boron Nitride was purchased from Kojundo Chemical Lab. Co., Ltd. (purity ∼99%, particle size ∼10 μm) without any additional treatment. We confirmed that the sample powders show Bragg X-ray diffraction peaks attributed only to the hBN phase. Sulfuric acid (95 %) was obtained from Wako Pure Chemical Industries, Ltd. and was used as purchased. We prepared h-BN/H2SO4 intercalation compounds according to the scheme shown schematically in Fig. . A 30 mg amount of h-BN was added to 0.5 mL of sulfuric acid in a 50 mL Teflon-lined stainless steel autoclave. The autoclave was heated at 200 °C for 24 h, and then naturally cooled to room temperature, producing a slightly brownish slurry. The slurry was transferred into a test tube and was neutralized with an aqueous solution of NaHCO3, resulting in an opaque solution. During neutralization, sodium sulfate salt (Na2SO4) is expected to be formed in between the hBN layers, leading to spontaneous exfoliation of hBN layers The solution was centrifuged at 15000 rpm for 45 min to remove insoluble materials, and then the supernatant was transferred to a new test tube. The thus obtained supernatant is an aqueous solution containing Na2SO4 and exfoliated hBN nanosheets. A solvent extraction method was utilized to isolate the exfoliated hBN nanosheets from the supernatant solution. That is, the solution was mixed with an equal volume of organic solvent in a separatory funnel, leading to migration of hBN nanosheets into the organic solvent. We found that 1-pentanol is the most suitable solvent for the present purpose. Although the extracted 1-pentanol solvent was transparent, a colloidal dispersion of the exfoliated hBN nanosheets was confirmed by the Tyndall effect (Fig. ).