ConspectusThe 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 nonaqueous
solvents and solutes, including nanoparticle (NP) suspensions, since
its first implementation at the BESSY II synchrotron radiation facility
20 years ago. This Account focuses on 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 transition-metal
oxide (TMO) NPs used in our studies reside close enough to the solution–vacuum
interface that electrons emitted from the NP–solution interface
(and from the NP interior) can be detected.We were specifically
exploring aqueous-phase TMO NPs 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.
Liquid-microjet 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
provides an environment allowing for unhindered proton migration.
Our studies confirm that water is dissociatively adsorbed at the hematite
surface and molecularly adsorbed at the TiO2NP surface
at low pH. In contrast, at near-basic pH the water interaction is
dissociative at the TiO2NP 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 signals
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 times, e.g.,
from Fe3+located at the hematite nanoparticle interface
into the aqueous-solution environment.