Coexistence of Two Types of Liquid Structures at Platinum-Water Interfaces

Platinum-water interfaces underpin many electrochemical energy conversion processes. However, despite decades of research, the real-space liquid structure of these interfaces remains elusive. Using three-dimensional atomic force microscopy (3D-AFM), we mapped Pt-water interface in real space with angstrom-level resolution. Topographic imaging revealed atomically flat (type I) and stripe-like (type II) surface nanodomains. Force-distance profiles above type I domains exhibited oscillatory decay patterns with periodicity of ~0.33 nm, consistent with water. In contrast, type II domains showed stronger oscillations with larger periodicity of ~0.45 nm and extended decay lengths, indicative of a different liquid structure with stronger correlation and ordering. Wide-angle X-ray scattering (WAXS) measurements of pure water and a series of liquid n-alkanes revealed peaks at ~0.31 nm and ~0.46 nm, in agreement with 3D-AFM observations of type I and type II structures, respectively. Our findings uncover the coexistence of two types of liquid structures at Pt-water interfaces modulated by surface heterogeneity, enabling new understandings and design principles for energy conversion applications.