Abstract
Since the first isolation of graphene monolayer via mechanical exfoliation in 2004, many other production methods have emerged in the following years. These methods include chemical vapour deposition, liquid phase graphite exfoliation, graphite oxide reduction and epitaxial growth, each having its pros and cons. The unique properties of graphene, such as high electron mobility, large specific surface area, mechanical strength and high transparency, ignited an enormous research interest to explore the electrochemical properties of graphene sheets, with potential uses in many immediate applications such as energy storage, solar cell technology and corrosion protection. Understanding how the electron transfer kinetics of a redox reaction between the graphene surface and a molecule compares to graphite or other carbon based materials is crucial to future use of graphene as a ‘supreme’ electrode material.1 Herein, the electrochemical response of mono-layer and multi-layer graphene electrodes, prepared using both chemical vapour deposition and mechanical exfoliation, is presented. Experiments were carried out using a microinjector, micromanipulator and optical microscopy, enabling precise deposition of size-controlled droplets on the flake surface.1 The insulating substrates used for flake preparation include inorganic materials and organic polymers. Graphene/graphite flakes were characterised using optical microscopy, Raman spectroscopy and atomic force microscopy.The electron transfer rate at the basal planes, edge planes and boundaries between graphene flakes was determined using voltammetric techniques. The redox couple, number of graphene layers, presence of defects and nature of the substrate are all shown to have a significant effect on the electrochemical activity.2-3 Furthermore, the stability of the droplets on the surface depends on various factors, such as the solvent choice, electrolyte content, number of graphene layers, underlying substrate and method of flake preparation.1. Valota, A. T.; Toth, P. S.; Kim, Y. J.; Hong, B. H.; Kinloch, I. A.; Novoselov, K. S.; Hill, E. W.; Dryfe, R. A. W., Electrochimica Acta, 2013, 110, 9-15.2. Valota, A. T.; Kinloch, I. A.; Novoselov, K. S.; Casiraghi, C.; Eckmann, A.; Hill, E. W.; Dryfe, R. A. W., ACS Nano, 2011, 5 (11), 8809-8815.3. P. S. Toth; A. Valota; M. Velicky; I. Kinloch; K. Novoselov; E. W. Hill; R. A. W. Dryfe, Chemical Science, 2013, DOI:10.1039/C3SC52026A.
Highlights
Graphene has attracted significant interest due to its unique properties, namely, record charge carrier mobility,[1] high thermal conductivity,[2] and mechanical strength,[3] discovered following its isolation in 2004.4 While applications of graphene in high-frequency transistors, flexible touch screens or photodetectors[5,6] will require high-quality material, other properties of this two-dimensional (2D) nanomaterial, such as high specific surface area, optical transparency,[7] and the ability to sustain large current densities,[8] can be exploited using medium-quality material
electron transfer (ET) measurements on individual graphene sheets were first reported by Li et al.,[16] using both mechanically exfoliated (ME) and chemical vapor deposition (CVD) grown flakes with ferrocenemethanol (FcMeOH) redox mediator, and Valota et al.,[17] using ME flakes and Fe(CN)63À: both reported accelerated kinetics on monolayer compared to bilayer samples (∼2-fold)[17] and graphite (∼10-fold),[16] respectively
The quasi-reversible nature of a reaction, where ET rate is comparable with diffusion rate, results in a peak-to-peak separation (ΔEp) increase with scan rate (ν) and allows the heterogeneous ET rate (k0) to be determined, using the analysis developed by Nicholson.[31] ψ where ψ is the dimensionless kinetic parameter determined from ΔEp, R is the transfer coefficient, n is the number of electrons transferred, F is the Faraday constant, and R and T have their usual meanings
Summary
Graphene has attracted significant interest due to its unique properties, namely, record charge carrier mobility,[1] high thermal conductivity,[2] and mechanical strength,[3] discovered following its isolation in 2004.4 While applications of graphene in high-frequency transistors, flexible touch screens or photodetectors[5,6] will require high-quality material, other properties of this two-dimensional (2D) nanomaterial, such as high specific surface area, optical transparency,[7] and the ability to sustain large current densities,[8] can be exploited using medium-quality material. ET measurements on individual graphene sheets were first reported by Li et al.,[16] using both mechanically exfoliated (ME) and chemical vapor deposition (CVD) grown flakes with ferrocenemethanol (FcMeOH) redox mediator, and Valota et al.,[17] using ME flakes and Fe(CN)63À: both reported accelerated kinetics on monolayer compared to bilayer samples (∼2-fold)[17] and graphite (∼10-fold),[16] respectively. Most reports of graphene electrochemistry use a mixture of graphene platelets of various thicknesses and lateral dimensions, usually prepared via liquid-phase exfoliation or reduction of graphene oxide, immobilized on a conducting substrate.20À22 While this method is convenient for characterization of graphene composites, sensing layers or paints, it does not provide insight into electrochemical activity of individual graphene flakes and the roles of the basal/edge plane and defects, due to the sample's polycrystalline nature and the presence of the underlying conductor. Ambrosi et al found that open graphene edges exhibit faster kinetics than folded edges (Fe(CN)63À),[25] while Goh and Pumera concluded that the ET rate is independent of the number of graphene layers (dopamine and ascorbic acid mediators).[21]
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