Nonlinear and Anisotropic Ion Transport in Black Phosphorus Nanochannels

Two-dimensional material nanochannelswith molecular-scale confinementcan be constructed by Van der Waals assembly and show unexpected fluidtransport phenomena. The crystal structure of the channel surfaceplays a key role in controlling fluid transportation, and many strangeproperties are explored in these confined channels. Here, we use blackphosphorus as the channel surface to enable ion transport along aspecific crystal orientation. We observed a significant nonlinearand anisotropic ion transport phenomenon in the black phosphorus nanochannels.Theoretical results revealed an anisotropy of ion transport energybarrier on the black phosphorus surface, with the minimum energy barrieralong the armchair direction approximately ten times larger than thatalong the zigzag direction. This difference in energy barrier affectsthe electrophoretic and electroosmotic transport of ions in the channel.This anisotropic transport, which depends on the orientation of thecrystal, may provide new approaches to controlling the transport offluids.