A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky prospect 31, 119991 Moscow, Russia.
Langmuir. 2012 Jan 17;28(2):1206-16. doi: 10.1021/la204429z. Epub 2011 Dec 20.
Superhydrophobic surfaces are attractive as self-cleaning protective coatings in harsh environments with extreme temperatures and pH levels. Hexagonal phase boron nitride (h-BN) films are promising protective coatings due to their extraordinary chemical and thermal stability. However, their high surface energy makes them hydrophilic and thus not applicable as water repelling coatings. Our recent discovery on the superhydrophobicity of boron nitride nanotubes (BNNTs) is thus contradicting with the fact that BN materials would not be hydrophobic. To resolve this contradiction, we have investigated BNNT coatings by time-dependent contact angle measurement, thermogravimetry, IR spectroscopy, and electron microscopy. We found that the wettability of BNNTs is determined by the packing density, orientation, length of nanotubes, and the environmental condition. The origins of superhydrophobicity of these BNNT coatings are identified as (1) surface morphology and (2) hydrocarbon adsorbates on BNNTs. Hydrocarbon molecules adsorb spontaneously on the curved surfaces of nanotubes more intensively than on flat surfaces of BN films. This means the surface energy of BNNTs was enhanced by their large curvatures and thus increased the affinity of BNNTs to adsorb airborne molecules, which in turn would reduce the surface energy of BNNTs and make them hydrophobic. Our study revealed that both high-temperature and UV-ozone treatments can remove these adsorbates and lead to restitution of hydrophilic BN surface. However, nanotubes have a unique capability in building a hydrophobic layer of adsorbates after a few hours of exposure to ambient air.
超疏水表面作为自清洁防护涂层在极端温度和 pH 值的恶劣环境中很有吸引力。六方相氮化硼（h-BN）薄膜因其非凡的化学和热稳定性而成为有前途的防护涂层。然而，它们的高表面能使它们亲水，因此不适用于防水涂层。我们最近发现氮化硼纳米管（BNNTs）具有超疏水性，这与 BN 材料不会疏水的事实相矛盾。为了解决这个矛盾，我们通过时变接触角测量、热重分析、红外光谱和电子显微镜研究了 BNNT 涂层。我们发现 BNNTs 的润湿性取决于纳米管的堆积密度、取向、长度和环境条件。这些 BNNT 涂层具有超疏水性的原因有两个：（1）表面形态和（2）BNNTs 上的碳氢化合物吸附物。碳氢化合物分子在纳米管的弯曲表面上比 BN 薄膜的平坦表面上更强烈地自发吸附。这意味着 BNNTs 的表面能由于其大曲率而增强，从而增加了 BNNTs 吸附空气中分子的亲和力，这反过来又降低了 BNNTs 的表面能，使它们疏水。我们的研究表明，高温和 UV-臭氧处理都可以去除这些吸附物，并使亲水 BN 表面恢复原状。然而，纳米管在暴露于环境空气数小时后具有构建疏水吸附物层的独特能力。
