Self-Diffusion of Individual Adsorbed Water Molecules at Rutile (110) and Anatase (101) TiO2 Interfaces from Molecular Dynamics

Stephanie J Boyd,Run Long,Dáire O’Carroll,Niall J English,Yogeshwaran Krishnan

doi:10.3390/cryst12030398

Abstract

The distribution of individual water molecules’ self-diffusivities in adsorbed layers at TiO2 surfaces anatase (101) and rutile (110) have been determined at 300 K for inner and outer adsorbed layers, via classical molecular-dynamics methods. The layered-water structure has been identified and classified in layers making use of local order parameters, which proved to be an equally valid method of “self-ordering” molecules in layers. Significant distinctness was observed between anatase and rutile in disturbing these molecular distributions, more specifically in the adsorbed outer layer. Anatase (101) presented significantly higher values of self-diffusivity, presumably due to its “corrugated” structure that allows more hydrogen bonding interaction with adsorbed molecules beyond the first hydration layer. On the contrary, rutile (110) has adsorbed water molecules more securely “trapped” in the region between Ob atoms, resulting in less mobile adsorbed layers.

Highlights

Since Fujishima and Honda stated the possibility of producing hydrogen and oxygen gas from water splitting when exposing titanium dioxide (TiO2, known popularly as titania) to light irradiation [1], and, attributed to its non-toxicity and natural abundance, TiO2 has been widely investigated for various promising renewable energy applications using photoelectrochemical dissociation [2–4]
We provide the distribution of self-diffusivities of individual water molecules in the inner and outer layers at TiO2 surfaces to highlight the complex nature of distinct molecular behaviour from immobile adsorbed water molecules to more mobile and flimsy bounded water
The present study elucidates water dynamics in a singlemolecule point of view, and it gives a better understanding of the structural and dynamical properties changes of individual water molecules at interfaces rather important in solarenergy conversion applications applying a distinct layering ordering method based in a combined hydrogen-bonding analysis and local order parameters (LOPs)

Summary

Introduction

Since Fujishima and Honda stated the possibility of producing hydrogen and oxygen gas from water splitting when exposing titanium dioxide (TiO2 , known popularly as titania) to light irradiation [1], and, attributed to its non-toxicity and natural abundance, TiO2 has been widely investigated for various promising renewable energy applications using photoelectrochemical dissociation [2–4]. A vital point for the analysis of structural and dynamical properties of interfacial water, such as self-diffusivity, is to assure simulation times will have a long and adequate length, as well as favorable statistics when utilizing well-tested empirical potentials [25] This is, one of the main concerns of the authors here, as suitable simulation length will allow properly convergence for both surfaces and the achievement of meaningful insights with regard to layered water molecules diffusivity. The present study elucidates water dynamics in a singlemolecule point of view, and it gives a better understanding of the structural and dynamical properties changes of individual water molecules at interfaces rather important in solarenergy conversion applications applying a distinct layering ordering method based in a combined hydrogen-bonding analysis and local order parameters (LOPs). Crystals 2022, 12, 398 energy conversion applications applying a distinct layering ordering method based in a combined hydrogen‐bonding analysis and local order parameters (LOPs)

Computational Details

Structure

Dynamic Interfacial Structure and Layers Identification

Density

Diffusivity and Mobility of Adsorbed Water Layers

Conclusions