The ability to probe chemical and physical processes occurring in solution at small scales is developing rapidly. A new approach is to enclose a solution system by a very thin upper membrane which is transparent to radiation and/or electrons, enabling high resolution surface measurements of processes occurring at the interfacial liquid region. Graphene is an excellent choice of such membrane, being very thin yet mechanically robust. This concept has been applied recently in several studies. Nonetheless, interpretation of such measurements must account for graphene doping, width of double layer in liquid, influence of the graphene electromechanical response, and artifacts due to graphene contamination during preparation of the cell. In this work, we apply Kelvin probe force microscopy to study cells with 500 nm regions capped by graphene. The contact potential difference (CPD) was measured for systems at 2 different alkaline conditions, as a function of voltage applied between the single layer graphene membrane and a Pt counter electrode located remotely in the solution. We found that the changes in CPD can be explained by the electric double layer thickness and changes in the graphene doping due to the gating of the liquid cell. At higher voltages, CPD and topographical changes provided evidence of reversible graphene functionalization and electro-wetting, which will also be discussed. The graphene membrane is robust to scanning and allows direct measurement of phenomena occurring near the lower liquid/graphene interface. The insights gained here provide a sound basis for further studies on more complex systems such as electrocatalysts. A few examples with Cu nanoparticles will also be presented to show the potential of this approach to probe functional materials.

Operando Kelvin probe force microscopy measurements with graphene-capped electrochemical reactor cells

ACS Spring 2022

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