Single-Molecule Interfacial Electron Transfer

Abstract

Interfacial electron transfer (ET) plays an important role in many chemical and biological processes. Specifically, interfacial ET in TiO 2 -based systems is important to solar energy technology, catalysis, and environmental remediation technology. However, the microscopic mechanism of interfacial ET is not well understood with regard to atomic surface structure, molecular structure, bonding, orientation, and motion. In this project, we used two complementary methodologies; single-molecule fluorescence spectroscopy, and scanning-tunneling microscopy and spectroscopy (STM and STS) to address this scientific need. The goal of this project was to integrate these techniques and measure the molecular dependence of ET between adsorbed molecules and TiO 2 semiconductor surfaces and the ET induced reactions such as the splitting of water. The scanning probe techniques, STM and STS, are capable of providing the highest spatial resolution but not easily time-resolved data. Single-molecule fluorescence spectroscopy is capable of good time resolution but requires further development to match the spatial resolution of the STM. The integrated approach involving Peter Lu at Bowling Green State University (BGSU) and Wilson Ho at the University of California, Irvine (UC Irvine) produced methods for time and spatially resolved chemical imaging of interfacial electron transfer dynamics and photocatalytic reactions. An integral aspect of the joint research was a significant exchange of graduate students to work at the two institutions. This project bridged complementary approaches to investigate a set of common problems by working with the same molecules on a variety of solid surfaces, but using appropriate techniques to probe under ambient (BGSU) and ultrahigh vacuum (UCI) conditions. The molecular level understanding of the fundamental interfacial electron transfer processes obtained in this joint project will be important for developing efficient light harvesting, solar energy conversion, and broadly applicable to problems in interface chemistry and surface physics.