Effect of Oxidation Level on the Interfacial Water at the Graphene Oxide-Water Interface: From Spectroscopic Signatures to Hydrogen-Bonding Environment.

Aashish Tuladhar,Christopher Arges,Le Zhang,Revati Kumar,Rolf David

doi:10.1021/acs.jpcb.0c05282

Abstract

The interfacial region of the graphene oxide (GO)-water system is nonhomogenous due to the presence of two distinct domains: an oxygen-rich surface and a graphene-like region. The experimental vibrational sum-frequency generation (vSFG) spectra are distinctly different for the fully oxidized GO-water interface as compared to the reduced GO-water case. Computational investigations using ab initio molecular dynamics were performed to determine the molecular origins of the different spectroscopic features. The simulations were first validated by comparing the simulated vSFG spectra to those from the experiment, and the contributions to the spectra from different hydrogen bonding environments and interfacial water orientations were determined as a function of the oxidation level of the GO sheet. The ab initio simulations also revealed the reactive nature of the GO-water interface.

Highlights

Several surface-specific techniques can be used such as environmental scanning electron microscopy (ESEM), secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES), etc.[42−46] One method, vibrational sum-frequency generation,[47,48] has received a lot of attention for characterizing interfaces experimentally[49−57] and in conjunction with simulations.[51,52,58−62] The synergy between vSFG experiments and molecular simulations allows for an in-depth probing of the interface, permitting a finer molecular interpretation of the underlying interfacial region
To characterize the interface between the Graphene oxide (GO) sheet and the water, the WillardChandler instantaneous interface[90] was employed, as it provides a robust definition of the interfacial region
This paper demonstrates that the orientation and the hydrogen-bonding class of water molecules plays a major role in the vSFG spectra and sheds light on the interactions specific to this interface

Summary

Introduction

Graphene oxide (GO), whether a single layer or a few layers of exfoliated sheets from graphite oxide, has recently received a lot of attention in the literature due to a range of potential applications.[1−35] GO consists of graphene sheets with oxygenated groups, and a number of studies have revealed a wide range of oxygen functional groups, such as hydroxyls and epoxides,[11] carboxylic acids, or sulfonates groups,[6,36] on these sheets as well as how these groups are arranged on the surface.[37−41] A key question that arises is how, depending on their number and partitioning, these oxygen functional groups can favor or prohibit reactions at the GO-liquid interface in aqueous media. An analysis of the graphene-oxide-water interface by ab initio molecular dynamics (AIMD) at different levels of oxidation was performed to provide insight on the effect of the different structural domains of graphene-oxide (organic, aromatic rich regions vs oxygenrich hydrophilic regions) on the interfacial water structure. The effect of the oxidation level of the GO sheet on the water structure was studied. These results are put into perspective with the experimental vSFG spectra of these systems as a function of oxidation level, thereby not just confirming the accuracy of said ab-initio methods and providing insight into the molecular origins of the spectral signatures in the experimental vSFG spectra.

Methods

Results

Conclusion