Article Cite This: J. Phys. Chem. C 2018, 122, 3903−3914
pubs.acs.org/JPCC
Resolving Iron(II) Sorption and Oxidative Growth on Hematite (001) Using Atom Probe Tomography Sandra D. Taylor,* Jia Liu,† Bruce W. Arey, Daniel K. Schreiber, Daniel E. Perea, and Kevin M. Rosso* Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ABSTRACT: The distribution of Fe resulting from the autocatalytic interaction of aqueous Fe(II) with the hematite (α-Fe2O3) (001) surface was directly mapped in three dimensions (3D) for the first time, using Fe isotopic labeling and atom probe tomography (APT). Micrometer-sized hematite platelets were reacted with aqueous Fe(II) enriched in 57Fe and prepared for APT using conventional focused ion beam lift-out techniques. Mass spectrum analyses show that specific Fe-ionic species (i.e., Fe2+ and FeO+) accurately reproduce isotopic ratios within natural abundance in the hematite bulk, and thus were utilized to characterize the distribution of 57Fe and quantify Fe isotopic concentrations. 3D reconstructions of Fe isotopic positions along the surface normal direction showed a zone enriched in 57Fe, consistent with oxidative adsorption of Fe(II) and growth at the relict hematite surface reacted with 57Fe(II)aq. An average net adsorption of 3.2−4.3 57Fe atoms nm−2 was estimated using Gibbsian interfacial excess principles. Statistical, grid-based frequency distribution analyses show a heterogeneous, nonrandom distribution of 57Fe across the surface, consistent with Volmer−Weber-like island growth. The unique 3D nature of the APT data provides an unprecedented means to quantify the atomic-scale distribution of sorbed 57Fe atoms and the extent of atomic segregation on the hematite surface. This new ability to spatially map growth on specific crystal faces will potentially enable resolution of long-standing unanswered questions about underlying mechanisms for electron transfer and atom exchange involved in redox-catalyzed processes at this archetypal and broadly relevant interface.
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INTRODUCTION Fe(III) oxides, such as hematite and goethite, are abundant in natural systems and are being developed into cost-effective (photo)catalytic substrates for energy conversion devices.1,2 In contact with water, the redox potential plays a commanding role in determining the structure and chemical behavior at the Fe(II)−Fe(III) oxide interface. The interconversion between surface Fe(III) (which is stable in the oxide) to Fe(II) (relatively stable in solution), and vice versa, is fundamental. In particular, under anoxic conditions or when cathodically biased, the system redox potential is poised such that both oxidation states can be present. This can in turn facilitate electron transfer and atom exchange reactions (described below), making the Fe(II)−Fe(III) oxide interface uniquely dynamic. For example, enhanced redox reaction rates are observed for organic3−7 and inorganic8−10 contaminants in these systems. Formation of Fe(II)−Fe(III) mixed valence surface phases can occur or the Fe(II)(aq)-Fe(III)-oxide interface can remain “sharp”, yet still undergo relatively rapid Fe atom exchange.11,12 In soils and sediments, microbial respiration enables these dynamical interfacial conditions.13,14 In electrochemical systems, Fe(II)(aq) is created when the applied potential exceeds that needed for reductive dissolution of the Fe(III) oxide. Thus, a substantial amount of experimental and theoretical work is underway to © 2018 American Chemical Society
understand atom and electron exchange processes at Fe(II)(aq)−Fe(III) oxide interfaces in microscopic detail. When Fe(II)(aq) is in contact with a Fe(III) oxide, the most conspicuous but daunting experimental challenge has been quantifying the flux of Fe atoms across the interface, as Fe(II)/ Fe(III) can swap identities by electron transfer. That is, Fe(II) in the solution phase can adsorb onto a Fe(III)−oxide surface, such as that belonging to hematite (α-Fe2O3), and transfer its electron to an adjacent Fe(III) in the lattice. Electrons can potentially be conducted through the surface/bulk in specific crystallographic directions, ultimately leading to the reductive dissolution and release of Fe from elsewhere from the solid. Evidence for electron transfer at the surface has come from 57Fe Mössbauer spectroscopy analyses on Fe(III) oxides (comprised predominantly of 56Fe) reacted with 57Fe(II)aq.3,15 The measurements suggest that the amount of electron transfer at the interface depends on the average surface density of sorbed Fe(II), as Fe(II) oxidizes and forms a surface phase similar to the underlying oxide at coverages below a monolayer while a mixed Fe(II)−Fe(III) phase forms at higher coverages.15 Received: December 5, 2017 Revised: January 23, 2018 Published: January 24, 2018 3903
DOI: 10.1021/acs.jpcc.7b11989 J. Phys. Chem. C 2018, 122, 3903−3914
Article
The Journal of Physical Chemistry C
solids and at interfaces with subnanometer resolution.31−33 It has also been used in geochronology to reveal the existence of nanoclusters containing radiogenic elements like Pb in zircon and elemental isotopic ratios.34,35 Although its usefulness in characterizing relatively small mass-dependent fractionations at interfaces is limited by signal sensitivity, for enriched tracer studies where isotopic contrasts can, in principle, be made much larger, APT holds great promise.36,37 Here for the first time, we develop and apply APT as a microscopic probe of the elusive atom exchange front as implied from research on the 57Fe(II) interaction with 56Fe(III)-oxides.15,18,19,30 While APT as a mass spectrometric technique is not appropriate for resolving oxidation states, it is unique in its ability to differentiate and visualize the spatial distribution of Fe isotopes in atomic detail in 3D. In our experiments, we recreate aqueous 57Fe(II) tracer conditions for Fe atom exchange with hematite (at natural Fe isotopic abundance) and directly probe the resulting distribution of 57Fe at the reaction front in 3D using APT. To accomplish this goal, specimen preparation obstacles at the nanoscale first had to be overcome, and the performance of the APT technique for providing quantitative information had to be benchmarked. Therefore, the aims of this study were 2-fold: (1) assess the applicability of APT in analyzing isotopically enriched surfaces and (2) deepen insight into the interaction between sorbed Fe(II) on hematite, by characterizing the resulting 57Fe distribution in 3D at the interface for the first time. Hematite is an ideal candidate material because micrometer-sized euhedral crystallites are straightforward to synthesize, interpret in terms of crystallography, and nanoengineer into APT samples. The (001) surface was specifically analyzed because it is a key reactive interface with Fe(II) and its interaction with Fe(II) has been studied using the bespoken surface-sensitive spectroscopies and X-ray scattering methods. The results from this study provide new insight into the 3D nature of Fe redox interactions on the hematite basal surface, applicable to a broad range of research and development objectives focused on Fe oxide−water interfaces.
Analytical mass spectrometry complements these measurements and provides evidence of atom exchange occurring; i.e., experiments that contact 57Fe(II)aq or 55Fe(II)aq with Fe(III) oxides show that the Fe(II)aq becomes enriched in 56Fe in a matter of days while the solid becomes enriched in the tracer isotope, without substantial change in solid phase, mass, or physical characteristics.16−19 Rapid recrystallization of the solid is implied, nominally enabled by solid-state electron conduction through crystallites linking Fe(II) oxidative adsorption to its spatially distinct reductive release.11,12 The interaction of Fe(II) with individual oriented crystal faces available on large hematite single crystals has been probed using surface-sensitive spectroscopies, providing insight into Fe sorption and electron transfer mechanisms. Measurements using second harmonic generation,20 crystal truncation rod (CTR) diffractometry,21,22 and X-ray reflectivity (XR)23 all appear to support the notion that Fe(II) oxidatively adsorbs to the surface in a quasi-homoepitaxial manner, yielding hematite growth. This is consistent with related computational molecular simulations.24−28 Models fit to CTR21,22 and XR23 data sets also indicate that the (001) surface continues to be hematitelike but is modified following interaction with Fe(II). These techniques provide valuable information on the sorbed Fe structure based on indirect and averaged observations. Information on the local structure of the deposited surface film (e.g., the morphology and distribution at the surface) is however difficult to determine as these measurements are principally in the surface normal direction and require spatial averaging laterally over large regions of the interface. Microscopy can address this to some extent though it can suffer from ambiguous identification of the sorbed phases.17,29 Thus, direct visualization and explicit characterization of the sorbed Fe and its location on Fe surfaces would aid in determining adatom sorption and growth mechanisms, though this has been a persistent scientific challenge. Studies investigating the 57Fe(II) interaction with Fe(III) oxides macroscopically imply the existence of atom exchange fronts in crystallites (i.e., by monitoring the mass transfer of the 57 Fe tracer out of aqueous phase18,19 and the valence of the 57Fe reaction product15,30), though these fronts have yet to be identified. Conceptually the recrystallized regions would preserve a record of the solution’s Fe isotopic composition during the reaction, thus enabling identification of the exchange fronts in these experiments via isotopic contrast. That is, 57 Fe/56Fe ratios would be expected to be highest near the particle surface and decay inward to natural abundance (NA) ratios in the remnants of the original particle interior. Such a record could also contain information on crystallographic sites and directions where atom exchange occurs preferentially, providing insight into interfacial structure effects. Knowledge of these atom exchange fronts are critical to being able to follow the prospectively separate electron transfer and atom exchange pathways that underlie the dynamics at the Fe(II)−Fe(III) oxide interface. However, because batch tracer experiments have had to rely on high surface area micro- and nanoparticles, there are very few mass dependent probes capable of the high spatial resolution necessary to explore the front directly in individual crystallites and thus such a record of atom exchange remains elusive experimentally. Atom probe tomography (APT) is a powerful technique that can be used to directly probe these exchange fronts at the atomic scale in three-dimensions (3D). For example, 3D APT can map the elemental and isotopic distributions of dopants in
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METHODS Batch Experiments. Our study is based on reacting micrometer-sized hematite platelets with NA Fe isotope ratios with aqueous Fe(II) enriched in 57Fe. Euhedral, tabular hematite platelets were synthesized using a hydrothermal technique as described by Sapieszko and Matijevic.38 More specifically, the particles were precipitated from a solution initially 0.04 M ferric nitrate, 0.2 M triethanolamine, 1.0 M sodium hydroxide, and 1.2 M sodium acetate. All chemicals were of reagent grade quality and first prepared to the above concentrations in separate plastic containers. The solution was hydrothermally aged and mixed in Parr reaction bombs (Parr Instrument Company, Moline, IL) in a mechanical convection oven (Blue M Model OV-472A-2, retrofitted with a rotisserie system) preheated to 250 °C for 2.5 h. The reactors were then cooled in an ice bath and the precipitates were separated via centrifugation. The solids were also centrifuged and washed in ultrapure Milli-Q water (typically 18.2 MΩ cm at 25 °C) three times, and finally dried in an oven at 70 °C. This procedure yields hematite platelets that are generally ∼2−20 μm in diameter and ∼1−4 μm thick. The (001) surface is the dominant exposed face, truncated by (012) edge surfaces. Phase purity was confirmed with X-ray diffraction. The specific surface area (SSA) of the hematite particles was measured to be 3904
DOI: 10.1021/acs.jpcc.7b11989 J. Phys. Chem. C 2018, 122, 3903−3914
Article
The Journal of Physical Chemistry C
Figure 1. (a) Mass spectrum obtained within the bulk hematite region. Mass spectra for (b) Fe2+ and (c) FeO2+ states, highlighting the assignment of Fe isotopes.
0.2 m2 g−1 using the Brunauer−Emmett−Teller method with N2 adsorption and a degas temperature of 100 °C. Batch 57Fe(II) reaction experiments with loose powders of the platelets were conducted similar to that done by LareseCasanova and Scherer.15 The 0.8 M 57Fe(II) stock solution was prepared by first dissolving 57Fe metal (Cambridge Isotopes) in 5 M HCl, filtering it through a 0.22 μm membrane, and diluting it to 0.1 M HCl. The 57Fe metal had a reported isotopic abundance of 95.93% 57Fe (compared to a NA value of 2.12%), which was confirmed by inductively coupled mass spectrometry. A particle loading of 200 g L−1 hematite was used to obtain a surface loading of 40 m2 L−1. Hematite was reacted in high density polyethylene bottles at room temperature in a pH 7.5 solution (25 mM KBr, 25 mM HEPES buffer) in the absence (a control sample referred to as the “unreacted hematite” sample hereon) and presence (“reacted hematite”) of 2 mM 57Fe(II) for 24 h. All experiments were conducted in an anoxic glovebox (N2/H2 atmosphere) to avoid Fe(II) oxidation by ambient oxygen. All solutions used for the Fe(II)−hematite reaction were mixed using anoxic, degassed water. The water was prepared by boiling ultrapure Milli-Q water in Pyrex Corning glass bottles on a hot plate for ∼30 min under vacuum. Afterward the water was immediately transferred into the anoxic glovebox where purified N2 was blown onto and sparged into it overnight. Following reaction the aqueous and solid fractions were separated via centrifugation. The supernatant was removed and analyzed with the ferrozine method (using a wavelength of 562 nm on a UV−vis spectrophotometer) to determine the amount of Fe(II) remaining in solution, [Fe(II)aq]. Weakly bound Fe on the hematite surface was removed by first rinsing and centrifuging the powders with water once and then exposing them to a 0.4 M HCl (trace metal basis) solution for 10 min. The supernatant from this extraction was also analyzed to determine [Fe(II)extr], a quantity used to estimate the amount of 57Fe adsorbed to the powders according to eq 1:
ion beam sputter deposition system. These coatings protect the hematite surface, and provide a fiducial marker to identify the relative position of the particle surface, enabling this region to be placed at depth in the eventual APT tip. The Ni layer is used for imaging contrast during focused ion beam (FIB) milling while Cr was placed in direct contact with the hematite because it adheres well to Fe oxides and has a slightly lower but similar predicted evaporation field to Fe.39 Together these attributes facilitated a relatively smooth field evaporation from the protective capping layers into the Fe oxide during APT measurement. As discussed shortly, isobaric interferences at the interface exist from 54Cr and 54Fe species though these were systematically deconvoluted from one another. One of the advantages and reasons for working with the micrometer-size particles in this study was to utilize conventional FIB milling and lift-out techniques for APT specimen preparation. Following Cr−Ni deposition, individual particles with well-defined (001) basal surfaces were selected for APT analyses using scanning electron microscopy (SEM; FEI Helios Nanolab). A protective Pt capping layer was deposited on a rectangular section of the basal surface (e.g., ∼3 × ∼20 μm) by ion-beam induced deposition using the dual-beam FIB microscope, further protecting the Cr/Fe2O3 interface from damage and Ga contamination during ion milling. A lamellar wedge (∼20 μm long) was created by trench milling on both sides with the ion beam (Ga+ ion, 30 kV) and was extracted with the Omniprobe micromanipulator. The lamellar wedge lift-out contained enough material to make several APT tips (e.g., 2−3), each representing a different region on the basal surface of a single particle. Thus, sections of the wedge were mounted over several Si microposts and were annular milled to obtain needle-shaped APT specimens with
