ABSTRACT:As a new form of carbon, graphene is attracting intense interest as an electrode material with widespread applications. In the present study, the heterogeneous electron transfer (ET) activity of graphene is investigated using scanning electrochemical cell microscopy (SECCM), which allows electrochemical currents to be mapped at high spatial resolution across a surface for correlation with the corresponding structure and properties of the graphene surface. We establish that the rate of heterogeneous ET at graphene increases systematically with the number of graphene layers, and show that the stacking in multilayers also has a subtle influence on ET kinetics.Graphene-based materials are having a huge impact in electrochemistry and electrochemical technologies, with promising applications in areas such as supercapacitors, 1 batteries, 2 electrocatalytic supports, 3 sensors for electroanalysis 4 and transparent electrodes. 5 These important technologies typically use graphene produced by chemical vapor deposition (CVD) 6 and other scalable methods, yet important fundamentals questions concerning heterogeneous electron transfer (ET) at such materials -intrinsic to many of these applications-remain to be addressed. Electrical measurements have revealed that the electron mobility 7 and the electronic band structure 8 are sensitive to the number of graphene layers and their stacking order, with implications for electrochemistry. In this communication, we thus seek to elucidate how both the number of graphene layers and arrangement of the layers influence heterogeneous ET kinetics.Graphene grown by CVD on nickel substrates 9 (see Supporting Information section 1) was optimal for the present study because it presents a heterogeneous continuous layer of microsized multilayered flakes, which can be addressed with high resolution scanning electrochemical cell microscopy (SECCM). [10][11][12][13] Thus, on one sample it is possible to make thousands of individual electrochemical (EC) measurements at different locations and relate these to the corresponding graphene structure. This provides datasets on a scale that would be unfeasible with conventional photolithographic techniques of the type employed in recent EC studies of exfoliated graphene. [14][15][16] In order to study the unambiguous electrochemical response of graphene without any interference from a conductive substrate, CVD graphene layers were transferred to a silicon substrate with a 300 nm thermal grown oxide layer. This substrate allowed optical visualization and identification of the morphological film features characteristic of graphene, 17,18 for direct correlation with the local electrochemistry. Importantly, the approach described herein makes possible the study of graphene surfaces with minimal intrusion and avoids the need for any post-processing lithographic step, which may result in unavoidable damage and possible interference of residues. 19 Ferrocene-derivatives have proven particularly suitable for the study of the ET activity of sp 2 ca...