Abstract
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 2p3/2 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 10Dq 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.
Highlights
Iron oxides are highly abundant metal-oxide minerals on Earth[1] and play a prominent role in many environmental and technological processes,[2,3,4,5,6] relevant for instance in mineralogy and atmospheric science, including corrosion, catalysis, crystal growth and dissolution, as well as photo-electrochemical water splitting
In recent years several experimental developments have been demonstrated. These are (1) ambient pressure photoelectron (AP-PE) spectroscopy,[7,8,9,10,11] (2) photoelectron spectroscopy from liquid cells consisting of a few layers thick graphene membrane with large transmission for electrons in the
With respect to the second point, we show here that liquid-jet PE spectroscopy is capable to detect the electronic structure of the hematite NP–aqueous solution interface despite the small electron mean free path in solution.[62]
Summary
Iron oxides are highly abundant metal-oxide minerals on Earth[1] and play a prominent role in many environmental and technological processes,[2,3,4,5,6] relevant for instance in mineralogy and atmospheric science, including corrosion, catalysis, crystal growth and dissolution, as well as photo-electrochemical water splitting. The water-catalyzed dissociation is argued to result from the stabilization of the dissociated state due to the strong hydrogen bond between H2O and OH which lowers the kinetic barrier for water dissociation.[57] observed small oxygen-1s binding energy shi s of adsorbed OH as a function of water coverage are possible indications of the occurrence of different OH species or aFe2O3(0001) surface reconstruction.[57] We would like to point out an MD simulation of hematite NPs in water.[58] Smaller NPs (1.6 nm) were observed to exhibit larger disorder of the crystalline structure, and the immediate two water layers are less ordered than for the larger (2.7 nm) particles studied These results are in accord with a combined vibrational spectroscopy and MD simulations study.[59].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
