Nanoscopic diffusion of water on a topological insulator

Anton Tamtögl,Irene Calvo-Almazán,Martin Bremholm,Peter S M Townsend,Philip Hofmann,Marco Sacchi,William Allison,Nadav Avidor,John Ellis

doi:10.1038/s41467-019-14064-7

Abstract

The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water in fields such as catalysis, biophysics and nanotribology is challenging due to its ultrafast motion, and the complex interplay of inter-molecular and molecule-surface interactions. Here we present an experimental and computational study of the nanoscale-nanosecond motion of water at the surface of a topological insulator (TI), Bi{}_{2}Te{}_{3}. Understanding the chemistry and motion of molecules on TI surfaces, while considered a key to design and manufacturing for future applications, has hitherto been hardly addressed experimentally. By combining helium spin-echo spectroscopy and density functional theory calculations, we are able to obtain a general insight into the diffusion of water on Bi{}_{2}Te{}_{3}. Instead of Brownian motion, we find an activated jump diffusion mechanism. Signatures of correlated motion suggest unusual repulsive interactions between the water molecules. From the lineshape broadening we determine the diffusion coefficient, the diffusion energy and the pre-exponential factor.

Highlights

The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water in fields such as catalysis, biophysics and nanotribology is challenging due to its ultrafast motion, and the complex interplay of intermolecular and molecule-surface interactions
These processes are accessible with ultrafast optical spectroscopy[11,12,13], whereas the interfacial diffusion of molecules typically occurs in the pico- to nanosecond regime and is monitored either in real space using microscopic techniques or in reciprocal space using scattering techniques
We have studied the adsorption of H2O on Bi2Te3 for a number of different adsorption geometries and initial water configurations using van der Waals corrected density functional theory (DFT) calculations

Summary

Introduction

The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water in fields such as catalysis, biophysics and nanotribology is challenging due to its ultrafast motion, and the complex interplay of intermolecular and molecule-surface interactions. The motion of protons, the vibrational dynamics and electronic transitions of water at surfaces usually happens at ultrafast time scales (in the order of femtoseconds)[10] These processes are accessible with ultrafast optical spectroscopy[11,12,13], whereas the interfacial diffusion of molecules typically occurs in the pico- to nanosecond regime and is monitored either in real space using microscopic techniques or in reciprocal space using scattering techniques. To make these fast diffusive motions accessible to microscopy studies, the process typically needs to be considerably slowed down. It has been suggested that the TSSs can be used as an additional parameter to adjust the catalyst–adsorbate interactions, with the TSS acting as a tunable electron bath[22]

Methods

Results

Conclusion