We report on electronic structure measurements of the interface between hematite nanoparticles (6 nm diameter) and aqueous solutions. Using soft X-ray photoelectron spectroscopy from a liquid microjet we detect valence and core-level photoelectrons as well as Auger electrons from liquid water, from the nanoparticle–water interface, and from the interior of the aqueous-phase nanoparticles. Most noteworthy, the method is shown to be sufficiently sensitive for the detection of adsorbed hydroxyl species, resulting from H₂O dissociation at the nanoparticle surface in aqueous solution. We obtain signal from surface OH from resonant, non-resonant, and from so-called partial-electron-yield X-ray absorption (PEY-XA) spectra. In addition, we report resonant photoelectron measurements at the iron 2p excitation. The respective Fe iron 2p_3/2 edge (L₃-edge) PEY-XA spectra exhibit two main absorption peaks with their energies being sensitive to the chemical environment of the Fe³⁺ ions at the nanoparticle–solution interface. This manifests in the 10D_q value which is a measure of the ligand-field strength. Furthermore, an observed intensity variation of the pre-peak, when comparing the PEY-XA spectra for different iron Auger-decay channels, can be assigned to different extents of electron delocalization. From the experimental fraction of local versus non-local autoionization signals we then find a very fast, approximately 1 fs, charge transfer time from interfacial Fe³⁺ into the environment. The present study, which is complementary to ambient-pressure photoemission studies on solid-electrolyte systems, also highlights the multiple aspects of photoemission that need to be explored for a full characterization of the transition-metal-oxide nanoparticle surface in aqueous phase.

中文翻译：

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α-Fe2O3 纳米颗粒-水溶液界面处形成的分子种类†

我们报告了赤铁矿纳米颗粒（直径 6 nm）和水溶液之间界面的电子结构测量。使用来自液体微射流的软X射线光电子能谱，我们检测来自液态水、纳米颗粒-水界面以及水相纳米颗粒内部的价态和核心级光电子以及俄歇电子。最值得注意的是，该方法对于检测水溶液中纳米颗粒表面H ₂ O 解离产生的吸附羟基物质具有足够的灵敏度。我们从共振、非共振和所谓的部分电子产额 X 射线吸收 (PEY-XA) 光谱中获得表面 OH 信号。此外，我们报告了铁 2p 激发下的共振光电子测量。相应的 Fe 铁 2p _3/2边缘（L ₃ -边缘）PEY-XA 光谱显示出两个主要吸收峰，其能量对纳米颗粒-溶液界面处Fe ³⁺离子的化学环境敏感。这体现在 10 D _q值中，该值是配体场强度的量度。此外，当比较不同铁俄歇衰变通道的 PEY-XA 光谱时，观察到的前峰强度变化可以归因于不同程度的电子离域。从局部与非局部自电离信号的实验分数中，我们发现从界面 Fe ³⁺到环境的电荷转移时间非常快，约为 1 fs。本研究是对固体电解质系统的常压光电子发射研究的补充，还强调了光电子发射的多个方面，需要探索这些方面来全面表征水相中的过渡金属氧化物纳米粒子表面。