# Insights into the Structure and Dynamics of Water at Co$_3$O$_4$(001) Using a High-Dimensional Neural Network Potential

## TL;DR

This study employs a high-dimensional neural network potential trained on DFT data to simulate and analyze the atomistic structure, dynamics, and reactivity of Co$_3$O$_4$(001)-water interfaces over extended time scales, revealing key surface behaviors.

## Contribution

The paper introduces a neural network potential that extends ab initio molecular dynamics simulations to larger scales, enabling detailed investigation of Co$_3$O$_4$-water interfaces.

## Key findings

- Distinct hydration structures for A and B terminations.
- B-termination stabilizes a well-organized hydration layer.
- Proton transfer pathways and surface hydroxylation dynamics.

## Abstract

Co$_3$O$_4$ is an important catalyst for the oxidation of organic molecules in the liquid phase. Still, understanding the atomistic details of Co$_3$O$_4$-water interfaces under operando conditions remains extremely challenging. While ab initio molecular dynamics have become an essential tool for investigating these dynamic interfaces in silico, they are limited to only a few picoseconds and a few hundred atoms. In this work, we overcome these limitations by training a high-dimensional neural network potential (HDNNP) on density functional theory data, which allows us to significantly extend the accessible time and length scales. Employing this HDNNP, we perform simulations to unravel the structure, dynamics, and reactivity of Co$_3$O$_4$(001)-water interfaces in detail. Our simulations reveal distinct characteristics of the two possible A and B terminations. The B-terminated surface stabilizes a compact, quasi-epitaxial hydration layer with strong templating effects, enhanced hydroxylation, and a well-organized hydrogen-bond network. In contrast, the A-termination forms a more diffuse contact layer with weaker templating, lower hydroxylation, and less ordered interfacial water. Extended simulations further uncover proton transfer pathways, including intermittent protonation of surface hydroxyls, migration of water molecules into the epitaxial layer, and rare hydronium-like configurations.