Exploring the reactivity of distinct electron transfer sites at CVD grown monolayer graphene through the selective electrodeposition of MoO2 nanowires

Abstract

The origin of electron transfer at Chemical Vapour Deposition (CVD) grown monolayer graphene using a polymer-free transfer methodology is explored through the selective electrodeposition of Molybdenum (di)oxide (MoO₂). The electrochemical decoration of CVD monolayer graphene with MoO₂ is shown to originate from the edge plane like- sites/defects. Edge plane decoration of MoO₂ nanowires upon monolayer graphene is observed via electrochemical deposition over short time periods only (ca. -0.6 V for 1 second (vs. Ag/AgCl)). At more electrochemically negative potentials (ca. -1.0 V) or longer deposition times, a large MoO₂ film is created/deposited on the graphene sheet, originating and expanding from the original nucleation points at edge plane like- sites/defects/wrinkles. Nanowire fabrication along the edge plane like- sites/defects of graphene is confirmed with Cyclic Voltammetry, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Raman Spectroscopy. Monitoring the electrochemical response towards [Ru(NH₃)₆]^3+/2+ and comparing the heterogeneous electron transfer (HET) kinetics at CVD grown monolayer graphene prior and post nanowire fabrication reveals key understandings into the fundamental electrochemical properties of carbon materials. The HET kinetics ([Formula: see text]) at MoO₂ nanowire decorated monolayer graphene sheets, when edge plane like- sites/defects have been coated/blocked with MoO₂, are significantly reduced in comparison to the unmodified graphene alternative. Interestingly, MoO₂ nucleation originates on the edge plane like- sites/defects of the graphene sheets, where the basal plane sites remain unaltered until the available edge plane like- sites/defects have been fully utilised; after which MoO₂ deposition propagates towards and onto the basal planes, eventually covering the entire surface of the monolayer graphene surface. In such instances, there is no longer an observable electrochemical response. This work demonstrates the distinct electron transfer properties of edge and basal plane sites on CVD grown monolayer graphene, inferring favourable electrochemical reactivity at edge plane like- sites/defects and clarifying the origin of graphene electro-activity.

Figure 1

SEM images of a monolayer graphene electrode (A), 1 second electrodeposition of MoO₂ at −0.6 V (vs. Ag/AgCl) (C) and 10 second electrodeposition of MoO₂ at −0.6 V (vs. Ag/AgCl) (E). Additionally presented is cyclic voltammetric analyses recorded using 1 mM [Ru(NH₃0₆]^3+/2+/0.1 M KCl, using a monolayer graphene sheet (B), 1 second electrodeposition of MoO₂ at −0.6 V (vs. Ag/AgCl) (D) and 10 second electrodeposition of MoO₂ at −0.6 V (vs. Ag/AgCl) (F) (Scan rate: 25 mV s⁻¹). Note that the monolayer graphene samples (A) might contain dust/air impurities due to the manufacturing process explained within the Experimental Section, observed as nanoparticulates.

Figure 2

Schematic highlighting the MoO₂ deposition process upon monolayer CVD graphene sheets, where MoO₂ nucleation starts upon the graphene edge plane like- sites/defects. Note that longer deposition times result in the MoO₂ growing from the edge plane like- sites/defects over onto the basal plane sites until there is a complete insulating layer covering the entire monolayer graphene sheet, resulting in substantially reduced electrochemical activity. Note, sizes are not to scale.