The Metal-Oxide Nanoparticle-Aqueous Solution Interface Studied by Liquid-Microjet Photoemission

Conspectus The liquid-microjet technique combined withsoft X-ray photoelectronspectroscopy (PES) has become an exceptionally powerful experimentaltool to investigate the electronic structure of liquid water and nonaqueoussolvents and solutes, including nanoparticle (NP) suspensions, sinceits first implementation at the BESSY II synchrotron radiation facility20 years ago. This Account focuses on NPs dispersed in water, offeringa unique opportunity to access the solid-electrolyte interfacefor identifying interfacial species by their characteristic photoelectronspectral fingerprints. Generally, the applicability of PES to a solid-waterinterface is hampered due to the small mean free path of the photoelectronsin solution. Several approaches have been developed for the electrode-watersystem and will be reviewed briefly. The situation is different forthe NP-water system. Our experiments imply that the transition-metaloxide (TMO) NPs used in our studies reside close enough to the solution-vacuuminterface that electrons emitted from the NP-solution interface(and from the NP interior) can be detected. We were specificallyexploring aqueous-phase TMO NPs that havea high potential for (photo)electrocatalytic applications, e.g., forsolar fuel generation. The central question we address here is howH(2)O molecules interact with the respective TMO NP surface.Liquid-microjet PES experiments, performed from hematite (alpha-Fe2O3, iron(III) oxide) and anatase (TiO2, titanium(IV) oxide) NPs dispersed in aqueous solutions, exhibitsufficient sensitity to distinguish between free bulk-solution watermolecules and those adsorbed at the NP surface. Moreover, hydroxylspecies resulting from dissociative water adsorption can be identifiedin the photoemission spectra. An important aspect is that in the NP(aq)system the TMO surface is in contact with a true extended bulk electrolytesolution rather than with a few monolayers of water, as is the casein experiments using single-crystal samples. This has a decisive effecton the interfacial processes that can occur since NP-waterinteractions can be uniquely investigated as a function of pH andprovides an environment allowing for unhindered proton migration.Our studies confirm that water is dissociatively adsorbed at the hematitesurface and molecularly adsorbed at the TiO2 NP surfaceat low pH. In contrast, at near-basic pH the water interaction isdissociative at the TiO2 NP surface. The liquid-microjetmeasurements presented here also highlightthe multiple aspects of photoemission necessary for a full characterizationof TMO nanoparticle surfaces in aqueous environments. For instance,we exploit the ability to increase species-specific electron signalsvia resonant photoemission, so-called partial electron yield X-rayabsorption (PEY-XA) spectra, and from valence photoelectron and resonantAuger-electron spectra. We also address the potential of these resonanceprocesses and the associated ultrafast electronic relaxations fordetermining charge transfer or electron delocalization times, e.g.,from Fe3+ located at the hematite nanoparticle interfaceinto the aqueous-solution environment.

Automated summary

The liquid-microjet technique combined with soft X-ray photoelectron spectroscopy is a powerful tool for investigating the electronic structure of liquid water and nonaqueous solvents and solutes, including nanoparticle suspensions. This Account focuses on NPs dispersed in water, exploring the interaction between water molecules and the surface of transition-metal oxide NPs.