Light to Electrons to Bonds: Imaging Water Splitting and Collecting Photoexcited Electrons

Abstract

Photoelectrochemical devices can store solar energy as chemical bonds in fuels, but more control over the materials involved is needed for economic feasibility. Both efficient capture of photon energy into electron energy and subsequent electron transfer and bond formation are necessary, and this thesis explores various steps of the process. To look at the electrochemical fuel formation step, the spatially-resolved reaction rate on a water-splitting electrode was imaged during operation at a few-micron scale using optical microscopy. One method involved localized excitation of a semiconductor photoanode and recording the growth rate of bubbles to determine the local reaction rate. A second method imaged the reactant profile with a pH-sensitive fluorophore in the electrolyte to determine the local three-dimensional pH profile at patterned electrocatalysts in a confocal microscope. These methods provide insight on surface features optimal for efficient electron transfer into fuel products. A second set of studies examined the initial process of photoexcited electron transport and collection. An independent method to measure the minority carrier diffusion length in semiconductor photoelectrodes was developed, in which a wedge geometry is back illuminated with a small scanned spot. The diffusion length can be determined from the exponential decrease of photocurrent with thickness, and the method was demonstrated on solid-state silicon wedge diodes, as well as tungsten oxide thin-film wedge photoanodes. Finally, the possibility of absorbing and collecting sub-bandgap illumination via plasmon-enhanced hot carrier internal photoemission was modeled to predict the energy conversion efficiency. The effect of photon polarization on emission yield was experimentally tested using gold nanoantennas buried in silicon, and the correlation was found to be small.