Deciphering Adsorbate–Adsorbate Interactions from Scanning Tunneling Microscopy Images: Integrated Topological, Statistical, and First-Principles Analysis of Methoxy Adsorbates on Pt/TiO₂(110)

Adsorbate–adsorbate interactions and their distributions are fundamentally important to attaining an atomic-level understanding of heterogeneous catalysis. Scanning tunneling microscopy (STM) is one of the most powerful evaluation techniques because it can image individual atoms or molecules and give their distribution on a solid surface. Once the adsorbate locations are determined, a two-dimensional (2D) radial distribution function, which provides the azimuth and distance of one adsorbate relative to another, can be obtained to evaluate the lateral interactions between the two adsorbates. However, when adsorbates are distributed on different terraces and other adsorbates are present on the surface, automatically determining the adsorbate locations is challenging. Here, we applied topological data analysis using persistent homology to STM images of methoxy adsorbates on a Pt/TiO2(110) surface, and the locations of each methoxy adsorbate were selectively identified. We then calculated the 2D radial distribution function and found that occupation of the adjacent methoxy adsorbate at the nearest and second-nearest adsorption sites was significantly suppressed, indicating a repulsive interaction between them. Density functional theory calculations attributed this to steric hindrance between neighboring methyl groups and electrostatic repulsion between the methoxy adsorbates, resulting in destabilization in the adsorption energy. The distribution of methoxy adsorbates was well reproduced by Monte Carlo simulations using a one-dimensional random walk model, assuming that it followed the Boltzmann distribution depending on the adsorption energy and temperature. Our integrated topological, statistical, and first-principles approach is effective for visualizing and understanding adsorbate–adsorbate interactions using STM images and can be applied to various adsorbate systems.