The hydration of graphene oxide (GO) membranes is the key to understand their remarkable selectivity in permeation of water molecules and humidity-dependent gas separation. We investigated the hydration of single GO layers as a function of humidity using scanning force microscopy, and we determined the single interlayer distance from the step height of a single GO layer on top of one or two GO layers. This interlayer distance grows gradually by approximately 1 Å upon a relative humidity (RH) increase in the range of 2 to ∼80%, and the immersion into liquid water increases the interlayer distance further by another 3 Å. The gradual expansion of the single interlayer distance is in good agreement with the averaged distance measured by X-ray diffraction on multilayered graphite oxides, which is commonly explained with an interstratification model. However, our experimental design excludes effects connected to interstratification. Instead we determine directly if insertion of water into GO occurs strictly by monolayers or the thickness of GO layers changes gradually. We find that hydration with up to 80% RH is a continuous process of incorporation of water molecules into single GO layers, while liquid water inserts as monolayers. The similarity of hydration for our bilayer and previously reported multilayered materials implies GO few and even bilayers to be suitable for selective water transport.

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Charge transfer at solid interfaces and ensuing interfacial electric fields on the order of 109 V/m are ubiquitous in nanostructures and hybrid materials. Here, we address how intrinsic interfacial electric fields alter the structural properties of intercalated molecules considering the optically transparent and atomically flat graphene–mica interface and confined rhodamine 6G dyes as a model system. Using a combination of Raman spectroscopy and atomistic simulations based on density-functional theory and classical molecular dynamics, we show that the observed softening of Raman-active modes of the confined molecules is due to mechanical deformations within the latter and to the action of interfacial electric fields exceeding 109 V/m. Our findings contribute to the general understanding of the role of interfacial electric fields in molecule/solid interfaces, thereby opening new perspectives for controlling catalytic activities of such complex systems.