UC Berkeley

How charges transform chemical bonds at the solid-liquid interface is at the heart of energy storage technologies today. In these technologies, electrical charges transform into an energy-dense product. At this interface, the transformation process happens by localizing photogenerated charges in radical forms at ultrafast time scales. Later in the microsecond time scale, the generated radicals react with liquid to form new chemical bonds and complete the catalytic cycle. Our goal is to dynamically deconstruct the mechanism of this process. To do that, several areas of chemistry such as electrochemistry, inorganic chemistry and spectroscopy have to be combined.

In this dissertation, I focus on studying charge (hole) dynamics in a heterogenous water oxidation reaction, which is crucial in renewable energy applications, at the n-SrTiO3/aqueous interface. Several methods, including ultrafast optical spectroscopy, ultrafast infrared spectroscopy and microsecond optical spectroscopy, are applied to investigate how photogenerated holes transform water into oxygen gas.

By applying an ultrafast near infrared probe (800 nm) to isolate intra-band transitions, the dynamics of hole transfer from valance band to surface is revealed. By applying an ultrafast infrared probe to isolate interfacial vibrations and a sub band-gap probe (400 nm and white light) to isolate the mid-gap transitions, the initial formation dynamics of radicals are identified. Together with theoretical calculations, the initial formation dynamics, with a 1.3 ± 0.2 ps time constant, are assigned to two radical classes: the titanium oxyl (Ti-O•), which has been assigned through a sub-surface vibration at 800 cm-1 and the in-plane bridge radical (Ti-O•-Ti), which has been assigned through its in-plane optical transition dipole. Interestingly, the time constants of 1.3 ps is characteristic of hydroxyl stretch relaxation in an H-bonded water network, indicating stabilization of radicals by hydrogen bonding.

On microsecond time scales, by applying a sub-band gap probe (400 nm) to isolate the mid-gap transitions, the dynamics of radicals transforming into new chemical bond are found to involve two distinct processes: the fast process is mainly affected by solution ionic strength. The slower process also has an H/D isotope effect with D lengthening the reaction rate.

Together, these studies provide vital information towards understanding how charges transform chemical bonds at the solid-liquid interface.