Impact of Ti Incorporation on Hydroxylation and Wetting of Fe₃O₄

Abstract

Understanding the interaction of water with compositionally tuned metal oxides is central to exploiting their unique catalytic and magnetic properties. However, processes such as hydroxylation, wetting, and resulting changes in electronic structure at ambient conditions are challenging to probe in situ. Here, we examine the hydroxylation and wetting of Fe (3−x) Ti x O 4 (001)-oriented epitaxial films directly using ambient pressure X-ray photoelectron spectroscopy under controlled relative humidity. Fe 2+ formation promoted by Ti 4+ substitution for Fe 3+ increases with hydroxylation, commensurate with a decrease in the surface work function or change in the surface dipole. The incorporation of small amounts of Ti (x = 0.25) as a bulk dopant dramatically impacts hydroxylation, in part due to surface segregation, leading to coverages closer to that of TiO 2 than Fe 3 O 4 . However, the Fe (3−x) Ti x O 4 compositional series shows a similar affinity for water physisorption, which begins at notably lower relative humidity than on TiO 2 . The findings suggest that relative humidity rather than surface hydroxyl density controls wettability. Studies of this kind directly relate to rational design of doped magnetite into more active catalysts for UV/Fenton degradation, the adsorption of contaminants, and the development of spin filters.

Surface Orientation Dependent Water Dissociation on Rutile Ruthenium Dioxide

“…This suggests a largely molecular adsorption of water at high partial pressures. Shifts in the gas phase peak relative to the lattice oxygen peak, used to probe changes in work function 82 or formation of a surface dipole due to the formation of surface adsorbates, such as -OH 83,84 (Figure S13, S14) also show that the offset is largest for the (101) facet, followed by the (110), (001) and (100) ( Figure S14). Figure 2: (A) Comparison of the O 1s spectra measured at a photon energy of 735 eV for the noted orientations.…”

Structure, Magnetism, and the Interaction of Water with Ti-Doped Fe3O4 Surfaces

“…The enrichment of Ti at the surface, particularly for the higher energy facets, persists before and after water exposure (Table S3). Because this solid solution has a high-temperature solvus, spinodal decomposition leaves the uppermost surface Ti-enriched with a composition similar to Fe 2 TiO 4 ulvöspinel in the top unit cell . Thus, small amounts of Ti doping can have dramatic consequence on the surface reactivity.…”

“…The stoichiometry of Fe (3– x ) Ti x O 4 is as expected in the bulk as probed by RBS (Table S1), but both x = 0.25 and x = 0.5 surfaces show comparable Ti enrichment in the topmost 5 nm, as measured by XPS. Similar to the low-energy (100) facet, the incorporation of Ti requires the reduction of Fe 3+ to Fe 2+ for charge balance (Table S4). A concurrent reduction in the hydroxylation behavior, as probed using a water isobar and AP-XPS, is expected based on that observed for the lower-energy (100) facet …”

“…Intensities of the Fe 3p (probed at an incident photon energy, IPE, of 490 eV, with a resultant kinetic energy, KE of ∼430 eV) and Ti 2p (IPE of 600 eV, KE ∼160 eV) were corrected by (1) tabulated cross sections, (2) an estimate of the different photon fluxes comparing the C 1s at both IPEs, (3) the estimated difference in spectrometer efficiencies at the two KEs obtained by comparing the O 1s (KE of ∼70 eV at an IPE of 600 eV) and O 2s (KE of ∼470 eV at an IPE of 490 eV) corrected by the flux and cross section, and (4) the inelastic mean free paths (IMFPs) calculated for Fe 3 O 4 . Note that the IMFP correction assumes that the film composition is uniform (taking into account the ratio of over which 95% of the signal was collected), although RBS and previous depth profiling for the (100) orientation in ref indicates Ti is enriched at the surface. This enrichment would result in the Fe/Ti ratio being lower than expected, as observed.…”

“…Moreover, none have yet simultaneously examined facet-dependent water reactivity with doping effects. Recent work by our group has examined the influence of Ti 4+ doping on the hydroxylation behavior of Fe 3 O 4 (100) surfaces, which showed that while increasing Ti content leads to an increase in the Fe 2+ /Fe 3+ ratio it also led to a disproportionately Ti-rich surface that exhibited a hydroxylation behavior less like iron oxides and more like TiO 2 . Beyond this initial work, compositional effects along this Fe (3– x ) Ti x O 4 magnetite-titanomagnetite binary on stable surface structures and their reactivity with water remain poorly explored.…”