Transparent Ultramicroelectrodes for Studying Interfacial Charge Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

Abstract

Scanning electrochemical microscopy (SECM) has been extensively applied to high-throughput screening and quantification of the surface and interfaces of a photoelectrochemical (PEC) system. It is highly desirable to have a semiconductor photocatalytic electrode with well-defined geometry and active area comparable to the SECM’s tip electrode for accurately quantifying interfacial charge transfer activities and photoelectrochemically generated redox species of a photoelectrode. This is because the broadening effects due to mass transfer gradient and non-local electron transfer at a planar semiconductor surface can be minimized by utilizing the well-defined semiconductor substrate. Here, we present a newly developed platform as SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 µm recessed disk shape is fully characterized with SECM for quantitatively understanding the interfacial electron transfer kinetics of IrCl6 2-/IrCl6 3- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO2 nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent interfacial photogenerated charge transfer kinetics and quantitative analysis of photocurrent contributions from water oxidation and SECM tip-generated redox mediators. The transparent UME-SECM method can be potentially expanded to other SECM operation modes for understanding the dynamics of the surfaces and interfaces of a PEC catalyst.