Spectroscopic Studies of Hole Transfer to and Diffusion Along Semiconductor Surfaces for Water Oxidation

Artificial photosynthesis holds promise as a solution to the intermittency problem of solar photovoltaics. By storing the energy of sunlight in the chemical bonds of hydrogen, solar power can be generated around-the-clock and carbon-free. Current state-of-the-art devices for artificial photosynthesis, however, are not efficient, stable, and inexpensive enough to allow mainstream adoption of the technology. The efficiency problem stems from the rate-limiting water oxidation reaction at the anode of the device. While some of the molecular intermediates of the water oxidation reaction on the surface of the anode are known, a complete picture of the reaction requires knowledge of the behavior of the charge carriers from generation in the bulk of the device to reaction at the anode/electrolyte interface. In this thesis, two principal experiments are described. The first details a novel spectroscopic method to detect charge carrier motion within a model device as well as charge carrier arrival at a catalytic surface layer. The second experiment describes new applications of transient grating spectroscopy to track important changes in the in situ behavior of surface charge carriers at a catalyst surface./nA common photoanode geometry incorporates a catalytic overlayer with a light-absorbing substrate, connected by a buried junction. The magnitude and band bending of the electric field at the buried interface is crucial for device quantum efficiency, but no reliable metric exists to measure either quantity in real devices. Chapter 3 describes initial transient absorption spectroscopy (TAS) experiments designed to probe charge injection and electric field magnitude at a buried junction. A model photoanode device consisting of a np-GaAs junction and transition metal oxide (TMO) overlayer was designed to allow TAS with photo-excitation from either side of the device. Large coherent acoustic oscillations are observed only when the TMO overlayer is present and when probing with a laser wavelength that has a penetration depth on the order of the TMO film thickness. The oscillations are shown to possess two distinct frequencies that vary with probe wavelength and correspond to coherent longitudinal acoustic phonons (CLAPs) in the GaAs light absorber and TMO film, respectively. Additionally, large acoustic echoes are shown to originate from both the np-GaAs junction electric field and the p-GaAs/TMO interfacial electric field. The echoes traverse the length of the photoanode device with an amplitude that is proportional to the electric field strength at the location of the echo origin. The generation of the CLAP oscillations is attributed to charge carrier injection at the np-GaAs interface and the p-GaAs/TMO interface; the charge carriers screen the electric field at those locations, causing phonon emission via the inverse piezoelectric effect (IPE). The varied echo magnitude, experiments with varied pump fluence, and initial experiments in electrolyte environment strongly support the assignment of phonon generation to the IPE. With further development, the technique shows promise as a quantitative measure of charge carrier injection and electric field strength at buried interfaces in artificial photosynthesis devices./nChapter 4 describes the development of a novel application of transient grating spectroscopy (TGS) to connect the diffusivity of charge carriers at an n-GaN surface to the reactivity of that surface. GaN is widely studied both experimentally and theoretically as a model for applications in catalysis because of its interesting surface chemistry. TGS was used to measure the diffusivity of surface holes in the presence and absence of water oxidation intermediates on the surface. Results show that the lateral, interfacial hole diffusivity increases by a factor >2 from air at the n-GaN/aqueous electrolyte interface for both pH = 1.4 (0.1 M HBr) and pH = 7 (0.1 M Na2SO4). The increase in diffusivity is not reproduced on undoped GaN, where there is not a large density of surface intermediates, and is not sensitive to defect centers, since the surface recombination velocity, also obtained from the TGS kinetics, differs by almost an order of magnitude between the two electrolytes. The origin of this increase in diffusivity in n-GaN is attributed to a new current pathway for surface holes due to the presence of mobile intermediates of the water oxidation reaction on the surface of the n-GaN. In essence, surface holes move faster when they encounter a surface adsorbed with reaction intermediates. The result shows for the first time a connection between surface charge diffusivity and surface reactivity.

Advisor: Cuk, Tanja