Detection of the electronic structure of iron-(iii)-oxo oligomers forming in aqueous solutions

Abstract

The nature of the small iron-oxo oligomers in iron-(iii) aqueous solutions has a determining effect on the chemical processes that govern the formation of nanoparticles in aqueous phase. Here we report on a liquid-jet photoelectron-spectroscopy experiment for the investigation of the electronic structure of the occurring iron-oxo oligomers in FeCl aqueous solutions. The only iron species in the as-prepared 0.75 M solution are Fe monomers. Addition of NaOH initiates Fe hydrolysis which is followed by the formation of iron-oxo oligomers. At small enough NaOH concentrations, corresponding to approximately [OH]/[Fe] = 0.2-0.25 ratio, the iron oligomers can be stabilized for several hours without engaging in further aggregation. Here, we apply a combination of non-resonant as well as iron 2p and oxygen 1s resonant photoelectron spectroscopy from a liquid microjet to detect the electronic structure of the occurring species. Specifically, the oxygen 1s partial electron yield X-ray absorption (PEY-XA) spectra are found to exhibit a peak well below the onset of liquid water and OH (aq) absorption. The iron 2p absorption gives rise to signal centered between the main absorption bands typical for aqueous Fe. Absorption bands in both PEY-XA spectra are found to correlate with an enhanced photoelectron peak near 20 eV binding energy, which demonstrates the sensitivity of resonant photoelectron (RPE) spectroscopy to mixing between iron and ligand orbitals. These various signals from the iron-oxo oligomers exhibit maximum intensity at [OH]/[Fe] = 0.25 ratio. For the same ratio, we observe changes in the pH as well as in complementary Raman spectra, which can be assigned to the transition from monomeric to oligomeric species. At approximately [OH]/[Fe] = 0.3 we begin to observe particles larger than 1 nm in radius, detected by small-angle X-ray scattering.

Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces: Vibrational Sum Frequency and X-ray

“…Although the [FeCl 2 (H 2 O) 4 ] +1 complex is most abundant (next to free Cl – ) in the bulk for 1.0 m FeCl 3 and higher concentration solutions, we cannot expect that speciation at the surface will be at all similar to bulk speciation due to the asymmetric hydration environment of the interface. Considering the enrichment of Fe­(III) in the surface region, one might expect polynuclear Fe­(III) species at the surface; however, Winter and co-workers recently found that iron-oxo oligomers are formed only when there is excess OH – …”

“…Considering the enrichment of Fe(III) in the surface region, one might expect polynuclear Fe(III) species at the surface; however, Winter and co-workers recently found that iron-oxo oligomers are formed only when there is excess OH − . 122 Depicted in Figure 6, we show possible scenarios for the low and high concentration aqueous surfaces. For the high concentration, a centrosymmetric Fe(III) complex and the SFG-active hydrating water molecules as proposed from SFG spectral analysis (spectral fit, Supporting Information, Table S2) are depicted.…”

Low-energy constraints on photoelectron spectra measured from liquid water and aqueous solutions

“…4 Furthermore, core-level binding energies (BEs) are sensitive to covalent bonding interactions [5][6][7] and solute charge states, 8,9 relative peak intensities reveal stoichiometry 10,11 as well as surface propensity (via the so-called depth-probing technique), 12,13 and resonant signal enhancements can be used, e.g., to increase detection sensitivity. 5,14,15 These applications rely on extracting photoelectron (PE) peak areas and/or kinetic energies by taking into account experimental factors such as ionization and electron collection geometries, detection efficiency, and/or ionization cross sections (CSs), although the latter are typically unknown in the aqueous phase.…”

Characterization of Fe2+ Aqueous Solutions with Liquid Jet X-ray Photoelectron Spectroscopy: Chloride Depletion at the Liquid/Vapor Interface Due to Complexation with Fe2+

“…As the pH increases, the aqua ligands around the Fe 2+ center will deprotonate to form hydroxido ligand coordination before eventually collapsing to insoluble Fe 2+ -hydroxido nanoparticles. ,, Fe 3+ complexes have more complicated aqueous solution chemistry as Fe 3+ -hydroxyl complexes are stable as monomers in solution but can also form dimers, trimers, and polymers . The presence of small inorganic ions in solutions, especially Cl – ions, has been shown to generate large networks of polymeric species in solution by the use of resonant X-ray absorption spectroscopy on a liquid jet solution . Small inorganic ions are also known to have a propensity for the air–water interface and significantly affect the chemistry near the surface. − …”