Pyrene Coating Transition Metal Disulfides as Protection from Photooxidation and Environmental Aging.

Ruben Canton-Vitoria,Christopher Ewels,Raul Arenal,Yuman Sayed-Ahmad-Baraza,Bernard Humbert,Nikos Tagmatarchis

doi:10.3390/nano10020363

Abstract

Environmental degradation of transition metal disulfides (TMDs) is a key stumbling block in a range of applications. We show that a simple one-pot non-covalent pyrene coating process protects TMDs from both photoinduced oxidation and environmental aging. Pyrene is immobilized non-covalently on the basal plane of exfoliated MoS2 and WS2. The optical properties of TMD/pyrene are assessed via electronic absorption and fluorescence emission spectroscopy. High-resolution scanning transmission electron microscopy coupled with electron energy loss spectroscopy confirms extensive pyrene surface coverage, with density functional theory calculations suggesting a strongly bound stable parallel-stacked pyrene coverage of ~2–3 layers on the TMD surfaces. Raman spectroscopy of exfoliated TMDs while irradiating at 0.9 mW/4 μm2 under ambient conditions shows new and strong Raman bands due to oxidized states of Mo and W. Yet remarkably, under the same exposure conditions TMD/pyrene remain unperturbed. The current findings demonstrate that pyrene physisorbed on MoS2 and WS2 acts as an environmental barrier, preventing oxidative surface reactions in the TMDs catalyzed by moisture, air, and assisted by laser irradiation. Raman spectroscopy confirms that the hybrid materials stored under ambient conditions for two years remained structurally unaltered, corroborating the beneficial role of pyrene for not only hindering oxidation but also inhibiting aging.

Highlights

Transition metal disulfides (TMDs) consist of covalently bound chalcogen–metal–chalcogen layers interacting weakly via van der Waals forces
The MoS2/pyrene and WS2/pyrene nanoensembles, referred to hereafter as 1a and 1b, respectively, were obtained free of non-immobilized pyrene by filtering the reaction mixture over a PTFE filter (100 nm pore size) and subsequently washing the solid residue obtained onto the filter with dichloromethane
Pyrene has been demonstrated to bind to the basal plane of MoS2 and WS2 via multiple π-S interactions, yielding a close-packed continuous surface coverage with significant ground-state charge-transfer

Summary

Introduction

Transition metal disulfides (TMDs) consist of covalently bound chalcogen–metal–chalcogen layers interacting weakly via van der Waals forces. A handicap of the unique structure of monolayer TMDs is the direct exposure of all atoms to the environment, which can induce significant structural modifications, and affect the novel electronic, optical, and mechanical properties [5,6]. Edges and defect sites are susceptible to oxidation, under atmospheric conditions and/or moisture when exposed to light illumination. This forms oxides which can significantly degrade the performance in energy and tribology related applications [7]. The presence of oxygen on the surface of TMDs is a critical issue in micro- and opto-electronic applications, where high electrical conductivity and carrier mobility is required. Even brief ambient exposure (

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