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

## TL;DR

This study uses 3D atomic force microscopy to reveal how water layers form on van der Waals materials like MoS2 and graphite under different pH conditions.

## Contribution

The study provides angstrom-scale insights into interfacial water layer structures on van der Waals materials across varying pH levels.

## Key findings

- MoS2 and graphite surfaces showed 2–3 hydrocarbon layers separated by ∼0.45 nm regardless of pH.
- Hydrocarbon layers remained stable on van der Waals materials even in acidic or alkaline solutions.
- Hydrophilic mica surfaces formed hydration layers of ∼0.28 nm spacing under both acidic and basic conditions.

## Abstract

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.