Valence Photoelectron and Auger-Emission Spectroscopy from the Solid-Aqueous Solution Interface under Operando Photochemical Conditions

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Solar energy is an inexhaustible natural source of energy, and hence plays an exceptional role in our search for future solutions to the global energy needs due to an increasing world population and increasing demand in clean energy. Here, the main challenge is an efficient collection and storage of solar energy in chemical bonds, in so-called solar fuels. This is analogous to the ability of plants' ability to convert sunlight, water and CO2 into usable forms of energy through photosynthesis. One of the most promising processes for technological application, which eventually replaces the technologies based on traditional non-renewable energy sources, is the splitting of (abundant) water at a semiconductor surface acting as a catalyst, in the presence of sunlight. The device to realize such processes is a photoelectrochemical cell (PEC), consisting of an aqueous electrolyte, a semiconductor anode exposed to light, and a metal cathode. Two principle reactions characterize the water decomposition, the evolution reaction of molecular oxygen, O2, (at the photoanode) and the evolution reaction of hydrogen gas, H2, at the cathode. The work proposed here focuses largely on the former half-reaction. From the technologic point of view the crucial issues are the development of stable, inexpensive and abundant materials for high-performance PEC electrodes, especially the improvement of the photoanode properties for efficient water splitting. Such a material optimization must however go hand in hand with an in-depth understanding of the underlying atomic-level interactions at the solid-water interface, both with and without illumination. It is undisputed that one of the most powerful experimental tools to access the desired electronic-structure information is photoelectron (PE) spectroscopy which however only now has been developed for application to the solid-water interface. This field of spectroscopy is still very young, and perhaps a dozen scientific works have been reported, and among the most recent accomplishments is the application of PE spectroscopy from an actual PEC. My aim is to investigate recently discovered aspects of PE spectroscopy from a liquid microjet (based on the detection of Auger and valence electrons) from neat aqueous solutions to advance our understanding of the atomic-level processes at the solid-solution interface. This is an important step, and complementary to previous PE spectroscopy studies, in uncovering mechanisms of photocatalytic water splitting towards developing improved solar-converted energy concepts.