Spontaneous Surface Charging and Janus Nature of the Hexagonal Boron Nitride-Water Interface

YK Wang and HJ Luo and XR Advincula and ZP Zhao and A Esfandiar and D Wu and KD Fong and L Gao and AS Hazrah and T Taniguchi and C Schran and Y Nagata and L Bocquet and ML Bocquet and Y Jiang and A Michaelides and M Bonn, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 147, 30107-30116 (2025).

DOI: 10.1021/jacs.5c07827

Boron, nitrogen, and carbon are neighbors in the periodic table and can form strikingly similar twin structures-hexagonal boron nitride (hBN) and graphene-yet nanofluidic experiments demonstrate drastically different water friction on them. We investigate this discrepancy by probing the interfacial water and atomic-scale properties of hBN using surface-specific vibrational spectroscopy, atomic-resolution atomic force microscopy (AFM), and machine learning-based molecular dynamics. Spectroscopy reveals that pristine hBN acquires significant negative charges upon contacting water at neutral pH, unlike hydrophobic graphene, leading to interfacial water alignment and stronger hydrogen bonding. AFM supports that this charging is not defect-induced. pH- dependent measurements suggest OH- chemisorption and physisorption, which simulations validate as two nearly equally stable states undergoing dynamic exchange. These findings challenge the notion of hBN as chemically inert and hydrophobic, revealing its spontaneous surface charging and Janus nature, and providing molecular insights into its higher water friction compared to carbon surfaces.

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