Angstrom-Scale Water Layer Structure on van der Waals Materials Probed by 3D Atomic Force Microscopy: From Acidic to Alkaline Aqueous Solutions

The structure of interfacial water governs the reactivity and surface properties of solid surfaces. Yet a molecular-scale understanding of how the concentration of H⁺ or OH⁻ species affects the interfacial hydration structure remains elusive. Here, we use three-dimensional atomic force microscopy (3D-AFM) to map with angstrom-scale resolution the interfacial liquid layer structure on mildly hydrophobic van der Waals (vdW) materials as a function of the pH. The liquid layer structure of MoS₂ and graphite surfaces revealed the presence of 2-3 hydrocarbon layers separated by ∼0.45 nm. The pH value neither prevented the presence of hydrocarbon layers nor facilitated the removal of hydrocarbon layers from graphite or MoS₂ surfaces. In contrast, the interfacial layer structure on a hydrophilic surface (mica) revealed the formation of 2-3 hydration layers separated by ∼0.28 nm for both acidic and basic conditions. A theoretical model predicted the formation of hydrocarbon layers on van der Waals materials in the presence of trace amounts of hydrocarbons (∼15 μg/m³). We propose that hydrocarbon layers are indispensable to explain the properties of van der Waals material-water interfaces.