A microelectrochemical actinometer for scanning electrochemical microscopy studies of photochemical processes

A new in situ electrochemical actinometry method has been developed and used to determine the light flux through a quartz fibre, employed in a scanning electrochemical microscopy (SECM) system developed to study the kinetics of interfacial photochemical processes. In this system an ultramicroelectrode (UME) probe is positioned with high precision at a known distance close to the end of the fibre, through which light is guided, and used to detect reactants or products of the ongoing photochemical process. The microelectrochemical actinometer was developed using the well-known liquid phase potassium ferrioxalate actinometer. The approach involved recording the steady-state current for Fe(III) reduction at a 25 microm diameter disc-shaped ultramicroelectrode (UME) positioned close to the fibre. A step function in the light flux through the fibre (off-on) was then applied which led to a depletion in the local Fe(III) concentration. The resulting chronoamperometric behaviour at the UME, as a consequence of the solution photochemical process, was measured. A theoretical model has been developed to simulate experimental current-time profiles, which enabled measurements of the light flux initiating the photoprocess.