Ali, H.; Winter, B.; Seidel, R.: The Metal-Oxide Nanoparticle-Aqueous Solution Interface Studied by Liquid-Microjet Photoemission. Accounts of Chemical Research 56 (2023), p. 1687-1697
10.1021/acs.accounts.2c00789
Open Access Version
Abstract:
The liquid-microjet technique combined with soft X-ray photoelectron spectroscopy (PES) has become an exceptionally powerful experimental tool to investigate the electronic structure of liquid water and non-aqueous solvents, and solutes, including nanoparticle suspensions since its first implementation at the BESSY II synchrotron radiation facility 20 years ago. This Account focuses on nanoparticles (NPs) dispersed in water, offering a unique opportunity to access the solid–electrolyte interface for identifying interfacial species by their characteristic photoelectron spectral fingerprints. Generally, the applicability of PES to a solid–water interface is hampered due to the small mean free path of the photoelectrons in solution. Several approaches have been developed for the electrode–water system and will be reviewed briefly. The situation is different for the NP–water system. Our experiments imply that the TMO NPs used in our studies reside close-enough at the solution–vacuum interface such that electrons emitted from the NP–solution interface (and from the NP interior) can be detected. We were specifically exploring aqueous-phase transition-metal oxide nanoparticles (TMO) that have a high potential for (photo-) electrocatalytic applications, e.g., for solar fuel generation. The central question we address here is how H2O molecules interact with the respective TMO NP surface. Liquidmicrojet PES experiments, performed from hematite (α-Fe2O3, iron(III) oxide) and anatase (TiO2, titanium(IV) oxide) NPs dispersed in aqueous solutions, exhibit sufficient sensitity to distinguish between free bulk-solution water molecules and those adsorbed at the NP surface. Moreover, hydroxyl species, resulting from dissociative water adsorption can be identified in the photoemission spectra. An important aspect is that in the NP (aq) system the TMO surface is in contact with a true extended bulk electrolyte solution, rather than with a few monolayers of water, as is the case in experiments using single-crystal samples. This has a decisive effect on the interfacial processes that can occur since NP–water interactions can be uniquely investigated as a function of pH and providing an environment allowing for unhindered proton migration. Our studies confirm that water is dissociatively adsorbed at the hematite surface and molecularly adsorbed at the TiO2 NP surface at low pH. In contrast, at near-basic pH the water interaction is dissociative at the TiO2 NP surface. The liquid-microjet measurements presented here also highlight the multiple aspects of photoemission necessary for a full characterization of TMO nanoparticle surfaces in aqueous environments. For instance, we exploit the ability to increase species-specific electron signal via resonant photoemission, so-called partial-electron-yield X-ray absorption (PEY-XA) spectra and from valence photoelectron and resonant Auger-electron spectra. We also address the potential of these resonance processes, and the associated ultrafast electronic relaxations, for determining charge transfer or electron delocalization time, e.g., from Fe3+ located at the hematite nanoparticle interface into the aqueous-solution environment.