Formation Mechanisms of Hexagonal Boron Nitride Nanosheets in Solvothermal Exfoliation

Abstract

Solvothermal techniques are widely used to exfoliate many two-dimensional materials, but the formation mechanisms of these nanomaterials have not been clearly revealed. Herein, we discovered the dissociation of hexagonal boron nitride (h-BN) in solvothermal exfoliation evidenced by the formation of B(OH) 3 , NH 4 B 5 O 8 •4H 2 O, and (NH 4 ) 2 B 10 O 16 •8H 2 O. In the selected solvents, the lateral sizes of the formed boron nitride nanosheets (BNNSs) are increased in the order of N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), acetonitrile (MeCN), and isopropanol (IPA), suggesting the decreased dissolving abilities of these solvents to h-BN in turn. The dissociation behaviors are the properties of solvents themselves, but the inclusion of lithium chloride (LiCl) and cetyltrimethylammonium bromide (CTAB) can elevate the dissociation degree and yield BNNSs with smaller lateral sizes due to the intercalating effects. The cation−π interactions make CTAB more effective in obtaining uniform BNNSs than using the neutral halogenated hydrocarbons as assistant reagents. The dissociation abilities of the solvents have strong relationships with the surface tension, Hansen solubility parameters distances (R a ), and polarities, whereas there is little relevance with the pressures. Meanwhile, we also observed the cracking of CTAB and the polymerization of MeCN in these reactions. Our findings indicate that the impurities are prone to be attached to the BNNSs exfoliated by the solvothermal route.

Recent advances in two-dimensional materials as catalysts for the electrochemical reduction of carbon dioxide

“…Reproduced with permission from Songfeng et al. 45 Copyright © 2023, American Chemical Society. (C) Schematic illustration of the CVD growth route.…”

“…Songfeng et al. 45 revealed the formation mechanisms of these nanomaterials by solvothermal techniques. As shown in Figure 1 B, it was revealed that hexagonal boron nitride (h-BN) dissociates in solvothermal exfoliation.…”

Hexagonal boron nitride exfoliation and dispersion

“…154 Finally, in 2023, E and coworkers studied the mechanism of solvothermal exfoliation. 155 They first pre-treated their hBN by ball In 2018, Rizvi and coworkers described a different approach using expansion of high-pressure gas to exfoliate several 2D nanomaterials, which was named compressible flow exfoliation (CFE). This strategy presented several advantages including being a faster, continuous process and producing fewer defects on the materials.…”

“…154 Finally, in 2023, E and coworkers studied the mechanism of solvothermal exfoliation. 155 They first pre-treated their hBN by ball milling, then sonicated the powder in either NMP, DMF, acetonitrile, or IPA. They found that the addition of LiCl and cetyltrimethyl ammonium bromide (CTAB) during the sonication process improved the dissociation of hBN, where the Li + ions and CTAB were suggested to intercalate the hBN layers through cation–π interactions.…”

“…sonicated the powder in either NMP, DMF, acetonitrile, or IPA. They found that the addition of LiCl and cetyltrimethyl ammonium bromide (CTAB) during the sonication process improved the dissociation of hBN, where the Li + ions and CTAB were suggested to intercalate the hBN layers through cation-π interactions 155. Furthermore, they determined that the solvothermal process causes dissociation of hBN, forming the byproducts B(OH) 3 , NH 4 B 5 O 8 •4H 2 O, and (NH 4 ) 2 B 10 O 16 • 8H 2 O as demonstrated by XRD, XPS, and FTIR.…”