Fuel generation by photoelectrochemical (PEC) water splitting is a promising renewable energy technology and also a complex and fascinating mesoscale science problem. Efficient photocatalytic water splitting requires both a high quantum yield for transport of charge carriers to the reactive interface (occurring on ps timescales) and for charge transfer (occurring on ns and longer time scales). Using in situ ultrafast optical methods, we have developed an experimental framework to separately measure recombination losses during charge separation and interfacial charge transfer in nanostructured semiconductor photocatalysts. This approach allows us to quantify and understand the origins of differences in performance between different materials through concurrent electrical characterization and in situ optical studies carried out in a customized PEC cell. Using a transient emission technique with sub-ps resolution, we preferentially probe photogenerated charge carriers and directly monitor parasitic recombination processes that occur on timescales shorter than interfacial charge transfer. Performing these measurements at various applied potential, we tune the depletion region to direct the flow of photogenerated minority carriers and to correlate the measured carrier dynamics to the electrical properties of the photoelectrochemical device. Losses occurring on picosecond timescales are found to represent a major loss channel in heavily doped nanostructured photocatalytic materials, even when the size of the nanostructure is below its carrier diffusion length. Measurements of parasitic losses be combined with complementary transient absorption studies of surface trapped species. Together, these measurements allow us to evaluate the role of ultrathin (~ 1 nm) surface layers in enhancing chemical activity in addition to reducing corrosion.

In situ transient optical studies of charge transport in nanostructured photocatalytic materials

254th National Meeting (2017)

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