Adsorption of water on fluorinated graphene

doi:10.1016/j.jpcs.2018.08.030

Journal of Physics and Chemistry of Solids

Volume 124, January 2019, Pages 54-59

https://doi.org/10.1016/j.jpcs.2018.08.030 Get rights and content

Highlights

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The adsorption of water on fluorinated graphene is studied using state-of-the-art first-principles methods.
•
The water-carbon interactions are enhanced due to F decoration.
•
The dipole moment of water molecule aligns spontaneously along the surface normal.

Abstract

In this paper, we investigate the adsorption of water monomer on fluorinated graphene using state-of-the-art first-principles methods within the framework of density functional theory (DFT). Four different types of methods are employed to describe the van der Waals (vdW) interactions between water molecule and the carbon surface: The traditional DFT calculations within the generalized gradient approximation (GGA), and three additional types of calculations using respectively the semi-empirical DFT-D2 method, the original van der Waals density functional (vdW-DF) method, and one of its variants. Two situations and totally four adsorption configurations are considered. Compared with the adsorption on pristine graphene, the adsorption energies of water on fluorinated graphene are significantly increased, and the orientations of water diploe moment are notably changed. The most stable configuration is found to stay right above the top site of the C atom which is bonded with F, and the dipole moment of water molecule aligns spontaneously along the surface normal.

Introduction

The interaction between water and carbon-based materials such as carbon nanotube, graphite and graphene plays a critical role in the daily life phenomena ranging from wetting, lubrication, heterogeneous ice nucleation and growth, surface catalysis, to the function of carbon nanostructures in biological environment, etc. Investigations on this topic have attracted a lot of interests of research in the past decades. For instance, water can exhibit novel phases and anomalous properties when it is confined in carbon nanotube [1,2]. In the simulation of water in confinement [1] or the super-lubricity of water transport in carbon nanotube [3], the water-carbon interactions play a decisive role, which are usually described by empirical force filed. The parameters of empirical force filed are obtained either by fitting the interatomic potentials from quantum mechanical calculations on small molecules or by reproducing the bulk properties measured by experiments. It remains open about the validity of such empirical force filed in describing the properties of interfacial water at atomic scale. Traditional first-principles calculations based on density functional theory (DFT) are shown to be insufficient to describe the weak water-carbon interactions where the van der Waals (vdW) interactions count [4,5]. Indeed, the adsorption of water molecules on carbon-based systems serves as a good playground for testing the theory of vdW interactions [[4], [5], [6]].

Upon chemical modifications, graphene and the other recently discovered two-dimensional (2D) materials (e.g., phosphorene, borophene) can exhibit unique properties with comparison to the pristine ones [[7], [8], [9], [10], [11]]. Specially, the adsorption of water can have nontrivial effects on the structural and electronic properties of the underlying 2D material, which in return affects the adsorption structure of water molecules [[12], [13], [14]]. In this work, we study the adsorption of water on fluorinated graphene (abbreviated as F-Gr hereafter), i.e., graphene whose surface decorated with F atoms, to explore the effects of chemical modification on the strength of water-carbon interactions. Fluorination is chosen here by considering the fact that F is the most electronegative element and is expected to induce larger charge perturbations on the graphene substrate with comparison to hydrogenation and oxidation. It is found that the adsorption of water on the fluorinated graphene is enhanced with comparison to the case of water adsorption on pristine graphene [4,5]. Due to the dipole-diploe interactions between water molecule and the F-Gr substrate, the adsorption geometries are also significantly modified.

Section snippets

Computational and modeling methods

The first-principles calculations are performed using the VASP code [15,16], which is based on density functional theory (DFT). A plane wave basis set and the projector-augmented-wave (PAW) potentials [17,18] are employed to describe the electron wave function and the electron-ion interactions, respectively. The exchange-correlation interactions of electrons are described by the PBE type functional [19]. The energy cutoff for plane waves is 600 eV. For structural relaxation and total energy

Results and discussion

We begin with studying the adsorption of water monomer on pristine graphene. It is found by previous calculations using different types of functionals for the vdW interactions [4,5], that the configurations with one or two OH bonds of the water molecule pointing downward to the graphene sheet are energetically favored, which have similar adsorption energies and are therefore of similar stability. In this context, either of the two configurations is enough to model the water-graphene

Summary

In conclusion, we have examined the adsorption of water monomer on fluorinated graphene using first-principles methods, including traditional DFT-GGA calculations, the semi-empirical DFT-D2 method and the van der Waals density functional and one of its variants to treat the van der Waals interactions between water and carbon surface. The water-carbon interactions are found to be significantly enhanced and the dipole moments of the adsorbed water molecules align spontaneously with the surface

Acknowledgments

This work is jointly supported by the National Natural Science Foundation of China (No. 11664003, 11474285), Natural Science Foundation of Guangxi Province (No. 2015GXNSFAA139015), and the Scientific Research and Technology Development Program of Guilin (No.2016012002). We gratefully acknowledge the crew of Center for Computational Materials Science of the Institute for Materials Research, Tohoku University for their continuous support of the SR16000 supercomputing facilities. We also thank the

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Adsorption of water on fluorinated graphene

Highlights

Abstract

Introduction

Section snippets

Computational and modeling methods

Results and discussion

Summary

Acknowledgments

Prog. Mater. Sci.

Comput. Mater. Sci.

Comput. Mater. Sci.

Nature

Phys. Rev. Lett.

Nano Lett.

Phys. Rev. B

Phys. Rev. B

J. Chem. Phys.

Phys. Chem. Chem. Phys.

Phys. Rev. B

Small

Nanoscale