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

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 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{k}}}_{{\boldsymbol{obs}}}^{{\bf{0}}}$$\end{document}kobs0) 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.


Optimising the electrochemical deposition of MoO2 upon the monolayer graphene sheets
A monolayer graphene sheet covering half of the SiO2 wafer surface, was used to highlight the edge plane sites/defects of the monolayer graphene, creating a single step of one carbon atom as depicted in Figure S1. The monolayer graphene electrode was immersed in solutions of 0.5 and 1 mM Na2MoO4 (in 1 M NaCl and 1M NH4Cl adjusted to pH 8.5 with liquid NH3).
Linear sweep voltammetry was performed from 0.5 to -1.5 V (vs. Ag/AgCl) as depicted in Figure S4, where the electrochemical deposition of MoO2 onto the electrode surface is detected via the electrochemical reduction peak at -0.6 V (vs. Ag/AgCl). In this electrochemical process, the reduction of Mo 6+ to Mo 4+ occurs through the following reaction mechanism: MoO4 2-+ 2H2O + 2e - MoO2 + 4OH -, producing MoO2 deposited on the monolayer graphene.
Note that in the experiments performed herein, the potential at which the MoO2 electrodeposition occurs is shifted to a less negative potential compared to -1.0 V as reported previously when using HOPG 1,2 and graphitic SPEs 3 , which is likely due to the use of inert binders in the SPEs, and that we have exposed the edge plane of the monolayer graphene making it more readily available. Inspection of the voltammetric signals shown in Figure S4 reveal deposition potentials of -0.6 V (vs. Ag/AgCl), which corresponds to the onset of MoO2 deposition, and resultantly the required chronoamperometry was subsequently performed at this potential.

Raman characterisation
Raman characterisation of the MoO2 decorated monolayer graphene is recorded over the range: 20-3300 cm -1 . The schematic presented in Figure S5A depicts the electrodeposition process of MoO2 nucleating onto the monolayer graphene edge plane like-sites/defects, when the electrochemical decoration is held -0.6 V (vs. Ag/AgCl) for 1 second. Figure Figure S5. Schematic of selective MoO2 deposition process (chronoamperometry at -0.6 V (vs. Ag/AgCl) for 1 second), where the monolayer graphene sheet covers half of the SiO2 wafer (A). Raman spectra from the edge of the monolayer graphene (B) with a MoO2 peak at 308 cm -1 , the monolayer graphene where MoO2 has not been electrodeposited yet (C), and an area where there is only SiO2 wafer (D). Figure S6. Raman spectra from the edge of the monolayer graphene before (A) and after (B) its decoration with MoO2 (chronoamperometry at -0.6 V (vs. Ag/AgCl) for 1 second). Raman peak at 308 cm -1 corresponds to the MoO2 nanowires on the decorated graphene electrode.

AFM characterisation of the selective electrodeposition of MoO2 upon a monolayer graphene sheet
AFM images were collected in order to characterise the MoO2 nucleation upon the edge plane like-sites/defects as depicted in Figure S7, where the length and width of the wires is 1-2 µm and 30-75 nm respectively, which corroborates with the selective nucleation characterised by Rowley-Neale et al. 3 . Figure S7. AFM analysis of a monolayer graphene sheet following the electrodeposition of MoO2 at -0.6 V for 1 second (vs. Ag/AgCl). Figure