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

太阳能是一种取之不尽、用之不竭的自然能源，因此，由于世界人口的增加和对清洁能源的需求不断增加，太阳能在我们寻求解决全球能源需求的未来解决方案方面发挥着特殊的作用。在这里，主要的挑战是如何有效地收集和储存化学键形式的太阳能，即所谓的太阳能燃料。这类似于植物通过光合作用将阳光、水和二氧化碳转化为可用能量的能力。最有希望的技术应用过程之一，最终取代了基于传统不可再生能源的技术，是在半导体表面作为催化剂的（丰富）水的分裂，在阳光的存在下。实现这一过程的装置是光电化学电池（PEC），由水电解质、暴露于光下的半导体阳极和金属阴极组成。两个主要反应表征了水的分解，即分子氧O2（在光阳极）的析出反应和氢气H2（在阴极）的析出反应。这里提出的工作主要集中在前半反应。从技术的角度来看，关键问题是开发稳定、廉价和丰富的高性能PEC电极材料，特别是提高高效水分解的光阳极性能。然而，这种材料的优化必须与对固体-水界面上潜在的原子级相互作用的深入理解携手并进，无论是否有照明。毫无疑问，光电子能谱是获得所需电子结构信息的最有力的实验工具之一，但直到现在才发展到应用于固水界面。这一领域的光谱学仍然很年轻，可能有十几项科学工作已经被报道，最近的成就之一是PE光谱在实际PEC中的应用。我的目的是研究最近发现的来自纯水溶液的液体微射流（基于对俄歇和价电子的检测）的PE光谱方面，以促进我们对固溶界面原子水平过程的理解。这是一个重要的步骤，并补充了以前的PE光谱研究，在揭示光催化水分解的机制，以发展改进的太阳能转换能源的概念。

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.