Stable Wettability Control of Nanoporous Microstructures by iCVD Coating of Carbon Nanotubes

Scalable
manufacturing of structured materials with engineered nanoporosity
is critical for applications in energy storage devices (i.e., batteries
and supercapacitors) and in the wettability control of surfaces (i.e.,
superhydrophobic and superomniphobic surfaces). Patterns formed in
arrays of vertically aligned carbon nanotubes (VA-CNTs) have been
extensively studied for these applications. However, the as-deposited
features are often undesirably altered upon liquid infiltration and
evaporation because of capillarity-driven aggregation of low density
CNT forests. Here, it is shown that an ultrathin, conformal, and low-surface-energy
layer of poly perfluorodecyl acrylate, poly­(1H,1H,2H,2H-perfluorodecyl
acrylate) (pPFDA), makes the VA-CNTs robust against surface-tension-driven
aggregation and densification. This single vapor-deposition step allows
the fidelity of the as-deposited VA-CNT patterns to be retained during
wet processing, such as inking, and subsequent drying. It is demonstrated
how to establish omniphobicity or liquid infiltration by controlling
the surface morphology. Retaining a crust of entangled CNTs and pPFDA
aggregates on top of the patterned VA-CNTs produces micropillars with
re-entrant features that prevent the infiltration of low-surface-tension
liquids and thus gives rise to stable omniphobicity. Plasma treatments
before and after polymer deposition remove the crust of entangled
CNTs and pPFDA aggregates and attach hydroxyl groups to the CNT tips,
enabling liquid infiltration yet preventing densification of the highly
porous CNTs. The latter observation demonstrates the protective character
of the pPFDA coating with the potential application of these surfaces
for direct contact printing of microelectronic features.

具有工程化纳米孔隙结构的结构化材料的规模化制备，对于储能器件（即电池与超级电容器）以及表面润湿性调控（即超疏水与超全疏表面）应用至关重要。垂直排列碳纳米管（vertically aligned carbon nanotubes, VA-CNTs）阵列中形成的图案，已针对此类应用得到广泛研究。然而，由于低密度碳纳米管阵列受毛细作用驱动发生团聚，沉积态结构在液体浸润与蒸发过程中常会发生非预期的形貌改变。
本研究发现，一层超薄、共形且低表面能的聚(1H,1H,2H,2H-全氟癸基丙烯酸酯)（pPFDA）涂层，可使VA-CNTs抵御表面张力驱动的团聚与致密化。仅需一步气相沉积工艺，即可在油墨涂布等湿法加工及后续干燥过程中，保留沉积态VA-CNT图案的保真度。
研究证实，可通过调控表面形貌实现超全疏性或液体浸润性。在图案化VA-CNT顶部保留缠结碳纳米管与pPFDA团聚体形成的表层，可制备出带有重入特征的微柱阵列，该结构可阻挡低表面张力液体浸润，从而实现稳定的超全疏性。在聚合物沉积前后进行等离子体处理，可去除缠结碳纳米管与pPFDA团聚体形成的表层，并在碳纳米管尖端接枝羟基，从而实现液体浸润同时避免高孔隙率碳纳米管发生致密化。上述结果证实了pPFDA涂层的防护特性，此类表面可应用于微电子结构的直接接触印刷。