Article pubs.acs.org/cm
Structural Characteristics and Eutaxy in the Photo-Deposited Amorphous Iron Oxide Oxygen Evolution Catalyst Joshua A. Kurzman,† Kevan E. Dettelbach,‡ Andrew J. Martinolich,† Curtis P. Berlinguette,‡ and James R. Neilson*,† †
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States Departments of Chemistry and Chemical & Biological Engineering, The University of British Columbia, 2036 Main Mall, Vancouver, Canada V6T 1Z1
‡
S Supporting Information *
ABSTRACT: A central challenge for hydrogen generation via electrolytic water splitting is the identification of efficient oxygen evolution reaction (OER) catalysts; a key aspect of the challenge hinges on an ability to relate atomic-scale structure to observed activities. Amorphous iron-based oxide(hydroxides) prepared by photochemical metal organic decomposition (PMOD) are proven OER catalysts, but their atomistic structures have been elusive. Here, a combination of powder diffraction and pair distribution function (PDF) analyses enables the formulation of a set of structural characteristics that capture the salient features of amorphous iron oxide(hydroxide) (a-FeOx), a model compound for this class of materials. a-FeOx contains only octahedrally coordinated iron atoms, which form clusters of both edge- and corner-sharing octahedra. A degree of “eutaxy” with predominantly ABC-type anion stacking persists at length scales beyond the dimensions of cluster domains−consistent with thermally induced crystallization into the defect spinel γ-Fe2O3. Evidence for considerable octahedral irregularities suggests the presence of a large number of bridging and terminal hydroxyl or water ligands, which would provide a high concentration of potential active sites. The structural features of a-FeOx are reminiscent of other first-row transition metal oxyhydroxide OER catalysts that comprise layers of edge-sharing octahedral ions capable of electron transfer and ligand association/dissociation. In keeping with ABC anion stacking, however, the title compound more closely resembles a highly defective spinel lattice rather than a layered hydroxide.
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INTRODUCTION Electrochemical water splitting offers the opportunity for clean production of hydrogen fuel. Practical implementation of this approach necessitates mastery of the four-electron oxygen evolution half-reaction (H2O →4H+ + O2 + 4e−), which is the kinetically limiting step in the efficient production of H2 at useful rates. Oxygen evolution reaction (OER) catalysts have employed a range of metal ions across a host of materials paradigms, including molecular complexes,1−5 bulk oxides,6−8 nanometric crystalline (oxy)hydroxides,9−11 and amorphous comounds.12−20 Amorphous metal-oxide(hydroxide) formulations offer advantages in OER catalysis by providing a higher concentration of defect sites, which can lead to smaller overpotentials and steeper (smaller) Tafel slopes as required for practical hydrogen generation.21 Biologically, oxygen evolution is catalyzed in photosystem II (PS II) by a protein-bound Mn4CaO5 complex that contains a distorted Mn3CaO4 cubane motif,22−25 illustrated in Figure 1a. Manganese octahedra in the PS II cluster are connected by sharing edges; the bridging ligands are frequently described as di-μ-oxo linkages, in reference to the sharing of two μ-oxo © XXXX American Chemical Society
species between cations (Figure 1b). This contrasts to mono-μoxo linkages for polyhedra that share corners (panel 1c), and tri-μ-oxo species for polyhedra that share a face. In PS II calcium sits above a tetrahedral hole formed by three of the Mn octahedra. Synthetic mimics of the PS II complex have been reported to function both in the presence and absence of Ca.26,27 Structural analogy to the core atomic connectivity in the active site of PS II is ripe among synthetic first-row transition metal compounds. Edge-sharing connectivities represent a fundamental building block in many of the oxide and (oxy)hydroxide materials studied for oxygen evolution catalysis. A series of representative structural depictions are shown in Figure 1d−h: (d) layered (oxide)hydroxide, (e) diaspore, (f) hollandite, (g) boehmite, and (h) λ-MnO2-type spinel. A wide range of compositions from these and other structural families have been examined in the electro- or photocatalytic evolution Received: March 6, 2015 Revised: April 13, 2015
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first-row transition metal catalysts prepared by electrodeposition is the presence of edge-sharing octahedra that assemble in layered hydroxide-like clusters or sheets, which are well-described by models deriving from crystalline compounds38−41 and perform comparably to layered hydroxide thin films.43 Electrodeposited Co and Ni catalysts have been shown to assemble as exclusively edge-sharing layered hydroxide-type materials.40,41 For Mn, a wider range of polymorphs can be accessed by electrochemical deposition, and the presence of corner-sharing connectivities or defects (i.e., mono-μ-oxo bridging) has been associated with smaller (more favorable) Tafel slopes.21 In the case of Fe, the oxyhydroxides produced by electrochemical deposition are diaspore-type α-FeOOH and boehmite-type γ-FeOOH, but only the γ-polymorph is stable under electrochemical OER conditions.35,44 In contrast, very little is currently known about the underlying structures of transition metal oxide-hydroxides prepared by photochemical metal organic decomposition (PMOD). Photochemically deposited amorphous iron oxyhydroxide is estimated to contain nearly twice as many hydroxyl ligands as oxo ligands,13 ≈ Fe9O7(OH)13, or a-FeOx for convenience. This difference is consistent both with the expected high concentration of surface-terminated hydroxyls for a material lacking any significant structural regularity, as well as with the emergence of porosity in annealed films.12 Photodeposited a-FeOx offers an advantage in overpotential of approximately 100 mV over electrodeposited γ-FeOOH at a current density of 10 mA cm−2.35,45 X-ray absorption measurements confirmed that films of a-FeOx contain only Fe3+, but Fe K-edge EXAFS provided very limited information beyond the first coordination shell. It was suggested that aFeOx contains only octahedral iron, but little could be ascertained beyond the immediate average environment of Fe in the material.46 In the present study, we apply high-energy X-ray scattering coupled with pair distribution function (PDF) analysis to elucidate the structural characteristics of PMOD-derived aFeOx. This material serves as a model compound for the class of photodeposited amorphous mixed-metal oxide(hydroxide) catalysts. An examination of the amorphous to crystalline phase transformation upon thermal annealing provides a foundation for assessing both the local chemical environments and aspects of medium-range topological order existing in a-FeOx. PDF analysis reveals the presence of heterogeneous domains comprising a mixture of edge- and corner-sharing octahedra. Simulations of the diffraction profiles for small cluster models consistent with the local connectivity, but differing in their anion packing sequence, enables the proposal of a structural description that bridges short- and medium-range length scales. a-FeOx is shown to primarily contain octahedral connectivities consistent with cubic ABC anion close-packing, rather than hexagonal ABAB anion close-packing.
Figure 1. (a) Distorted cubane-containing structure of the Mn4CaO5 oxygen evolution complex (OEC) found in photosystem II;25 for clarity the cubane motif is outlined in black. (b) Edge-sharing M4O4based cubane highlighting the bond geometry of di-μ3-oxo species. (c) Typical corner-sharing linkage highlighting the bond geometry of mono-μ3-oxo species. (d−h) Representative structure types among oxide(hydroxide) oxygen evolution catalysts.
of oxygen.21,28−35 All of these structures can be viewed as resulting from different connectivities between sheets of edgesharing octahedra. In the periodic limit: layered (oxide)hydroxides consist of unconnected sheets; diaspore and hollandite are built from two-octahedra-wide chains attached through corner-sharing linkages; boehmite is also built from two-octahedra-wide chains, connected to neighboring chains through edge-sharing, forming corrugated layers; and λ-MnO2, which adopts the spinel structure devoid of tetrahedra, is a three-dimensional network of edge-sharing sheets containing vacancies in the center of every 3 × 3 cluster of octahedra. At the nanoscale, of course, the periodic limit is no longer applicable and the fraction of terminal hydroxyl and water ligands is significantly increased, as are the number of coordinatively unsaturated bridging μ2-hydroxyl ligands. Despite the fundamental analogy between these representative structures, there is an important distinction with respect to oxo(hydroxo) ligand environments. In edge-sharing motifs the O(H) ligands are approximately tetrahedral, where for cornersharing linkages the O(H) coordination is nearly planar (Figure 1b,c). Which type of ligand environment offers greater advantage for OER catalysis remains an open question that is inextricable from the structure−composition interplay. Indeed, recent works point to iron as the principal active specie among the first-row transition-metal oxyhydroxides,11,35,36 and also to the important role of other metal ions (e.g., Co, Ni) in enhancing the electrical conductivity of mixed-metal catalysts.37 In all cases, elucidation of catalytically relevant motifs is contingent upon careful structural determination. Especially as it pertains to nanoscopic and amorphous oxide(hydroxide) OER catalysts, X-ray pair distribution function (PDF) and extended X-ray absorption fine structure (EXAFS) analyses are excellent, complementary probes that have been used extensively in the field.14,38−42 Both techniques have certain advantages, with EXAFS offering elemental specificity, and PDF providing sensitivity to medium-range structural correlations beyond the experimental limitations of EXAFS. The PDF also offers the convenience of being interpretable by direct inspection. A common feature among
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EXPERIMENTAL SECTION
a-FeOx Film Formation and Preparation for X-ray Scattering Measurements. Amorphous iron oxide(hydroxide) films were prepared by drop-casting a 0.3 M solution of iron(III) 2-ethylhexanoate in hexanes (50% w/w in mineral spirits, Strem Chemicals) onto glass substrates and decomposing the organometallic precursor under deep UV irradiation (λ = 185 and 254 nm). The thick film was removed by sonication in hexanes for 15 min and vacuum filtered through a medium porosity glass frit. For ex situ heating studies, aliquots were annealed in air for 1 h at a range of temperatures up to B
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Figure 2. Diffraction (left) and PDF (right) data of as-prepared a-FeOx and the ex situ annealing series of samples. Data are shown as colored crosses ( × ); Rietveld refinements, where possible, as solid dark traces. Expected reflection positions are denoted by ticks below the diffraction patterns: vacancy-disordered γ-Fe2O3 (γ), thick black lines; vacancy-ordered γ-Fe2O3 (γvo), thin gray lines; α-Fe2O3 (α), thick green lines. Approximate phase fractions for multiphase samples are listed adjacent to the respective annealing temperature. In the right panel, a dividing line at 8 Å demarcates different x-axis scales at short and longer r. 600 °C and loaded as powder into 1 mm diameter Kapton tubes. On the basis of previously reported XANES for a-FeOx,46 and the wellknown stability of Fe3+ in air within this temperature regime,47 no spectroscopic confirmation of oxidation state was obtained for the samples examined in this study. For in situ heating studies, the aliquot was pretreated by heating overnight under vacuum at 240 °C, and sealed under vacuum in a 0.8 mm diameter extruded quartz tube. X-ray Scattering Measurements. High-energy X-ray scattering experiments were conducted at the Advanced Photon Source of Argonne National Laboratory at beamlines 11-ID-B and 11-ID-C. Data were collected at X-ray energies of about 58 keV (λ = 0.2114 Å, 11-IDB) and 120 keV (λ = 0.11165 Å, 11-ID-C) with an amorphous silicon area detector. Total scattering measurements suitable for PDF analysis were performed at sample-to-detector distances of ∼16 cm (11-ID-B) and ∼35 cm (11-ID-C); medium resolution measurements suitable for Rietveld analysis were performed at a sample-to-detector distance of ∼90 cm (11-ID-B); calibration was performed by measurement of a CeO2 standard at each condition. The electron density pair distribution function, G(r), was obtained from the backgroundsubtracted scattering data by the ad-hoc approach applied in PDFgetX3.48 The reduced scattering structure function, S(q), was transformed to G(r) using a maximum momentum transfer of Qmax = 24 Å−1 for data collected at 11-ID-B, and Qmax = 20 Å−1 for data collected at 11-ID-C. A powdered nickel standard was used to determine the resolution truncation parameters Qdamp and Qbroad used in PDFgui simulations and refinements.49
the observed correlation distances, none of the structures offers a group of atom pairs that is consistent with only the observed correlations. Similar comparisons using molecular fragments extracted from the crystal structures did not provide improved descriptions of G(r). The absence of a suitable crystalline fragment to model the a-FeOx structure is indicative of extensive connective heterogeneity and motivates alternative strategies to elucidate the underlying atomic arrangement. An examination of phase behavior upon thermal annealing provides a handle with which to develop such an understanding. Thermal evolution of the amorphous to crystalline phase transformation for a-FeOx was examined ex situ by annealing in air, and in situ by annealing in vacuum. Diffraction patterns and corresponding PDFs of the as-prepared and ex situ annealed sample series are shown in Figure 2; PDFs from the in situ heating experiment are presented in Figure SI-2 in the Supporting Information. In both cases, the first crystalline phase observed is spinel: a-FeOx forms γ-Fe2O3 and then αFe2O3 when annealed in air, whereas under vacuum, magnetite Fe3O4 is the only phase observed up to the maximum temperature of the study, 600 °C. The formation of magnetite when heated in vacuum is possibly due to residual ethylhexanoate on the sample, which would serve as a carbothermal reducing agent under such conditions.47 We focus on analysis of the ex situ heating experiment because of its complementary diffraction data (collected at a longer sample-to-detector distance), which provides signatures of medium-range order (MRO) in a-FeOx beyond that captured by G(r). Synchrotron X-ray diffraction profiles of as-prepared a-FeOx and samples annealed at 100 and 200 °C present broad diffuse features rather than true Bragg reflections, but with maxima at the same angular positions and with some variation of the observed intensities as a function of annealing temperature (Figure 2, left panel). We note that these features are not
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RESULTS AND DISCUSSION Thermal Evolution and Local Coordination Environments. A preliminary survey of plausible motifs to model the as-prepared a-FeOx was conducted by comparing its observed G(r) with simulated PDFs of all the well-defined oxides and oxyhydroxides of Fe3+ (Figure SI-1 in the Supporting Information). The experimental data show pairwise correlations extending to about 7 Å (Figure 2), indicating a greater degree of short-range order than previously detected using EXAFS.46 Although many of the models−particularly ϵ-Fe2O3 and the oxyhydroxides−contain comparable features at approximately C
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Chemistry of Materials resolved without the use of a high flux/low-background diffractometer (i.e., synchrotron). The as-prepared material and samples annealed below 300 °C all display similar features in G(r) that terminate around 7 Å (Figure 2, right panel). However, as shown in Figure 3, the first sharp diffraction peak
Evolution of the UV−visible absorbance spectra across the amorphous to γ-Fe2O3 transition suggests an absence of tetrahedral Fe in the as-prepared sample, as proposed from EXAFS analysis.46 As shown in Figure 4a, the optical band edge
Figure 3. Room-temperature diffraction patterns of as-prepared aFeOx and samples annealed in air at 100, 200, and 300 °C. Scattering from the polyimide container has been subtracted. The region containing the first sharp diffraction peak (FSDP) is highlighted in the left panel; the position of this feature shifts moderately to lower Q (larger d-spacings) upon annealing. The entire wide-angle region is shown in the right panel and patterns are offset vertically for clarity.
(FSDP) of these samples appears at a d-spacing of more than 14 Å (Q ≈ 0.43 Å−1). On the basis of the full-width at halfmaximum (fwhm), the coherence length of the periodic spacing indicated by the FSDP is on the order of 36 Å [2π/ΔQ = 36.7(2) Å for as-prepared a-FeOx].50−57 Both the small and wide angle regions of the diffraction patterns are suggestive of MRO. Medium-range order in a-FeOx is a consideration we return to shortly. The diffraction pattern of the sample annealed at 300 °C contains similar broad features to the as-prepared sample and those heated at lower temperatures, with the exception of a drastic reduction in relative intensity at Q ≈ 1.5 Å−1. Although the 300 °C sample lacks the well-defined reflections needed for Rietveld analysis, it is consistent with poorly crystalline γ-Fe2O3 (maghemite); the comparable profiles observed after annealing at 325 and 350 °C support this assignment. Incipient crystallization is reflected in the PDFs by the appearance of pair correlations beyond 7 Å, which increase in intensity at higher annealing temperatures. Bragg reflections become defined after annealing at 350 °C and allow for Rietveld refinement of the data against a cubic, vacancy-disordered Fd3̅m model, which indicates preferential occupation of the tetrahedral site [site occupancy factor (SOF) = 0.90(1)] and a significant degree of vacancies on the octahedral site [SOF = 0.61(1)]. Real-space refinements of G(r) agree well with reciprocal-space observations and analyses. The material is markedly more crystalline after annealing at 375 °C, and adopts the vacancy-ordered P41212 structure indicated by the superstructure reflections of this phase.58−60 Additionally, a small fraction of α-Fe2O3 [10(2)mol %] is present. Annealing at 500 and 600 °C leads to a progressive increase in the α-Fe2O3 phase fraction at the expense of the γ-Fe2O3 component.
Figure 4. (a) Diffuse-reflectance UV−visible absorbance spectra of asprepared a-FeOx and samples annealed ex situ at 200, 300, and 400 °C (adapted from Smith et al.12). The spectrum of a sample annealed at 100 °C closely resembles the as-prepared material, and is omitted for clarity. (b) Evolution of the nearest-neighbor, and (c) next-nearestneighbor peaks in G(r) as a function of annealing temperatures shown in (a). The legend applies to panels a, b, and c. (d) Observed diffraction patterns up to the lowest examined annealing temperature at which γ-Fe2O3 is present, 300 °C. (e) DIFFaX simulated diffraction profiles for small (ca. 15 Å) spinel clusters containing variable fractional occupancy of the tetrahedral iron site, denoted as percentages. Cartoons illustrate the end members (small orange spheres are oxygen; large spheres iron: edge-sharing octahedral Fe are blue and black; corner-sharing tetrahedral Fe are pink).
undergoes a red-shift with increasing annealing temperature. This shift is consistent, although not diagnostic, with a progressive occupation of tetrahedral sites. However, in asprepared a-FeOx the sharp absorbance feature just above 350 nm is blue-shifted relative to the characteristic feature of tetrahedral Fe3+ that is present in samples annealed at and above 200 °C.61 The PDF data also suggest the presence of only octahedrally coordinated Fe in the as-prepared material. The nearestneighbor Fe−O peak at r ≈ 2 Å (Figure 4b) has a symmetric, Gaussian shape in the as-prepared sample, which broadens and shifts to shorter r as the annealing temperature is increased, indicating a contraction of the average Fe−O bond distance; this is consistent with both dehydration and an increasing fraction of tetrahedrally coordinated Fe. The intensity of the feature in G(r) at r ≈ 3.4 Å (Figure 4c) increases with D
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Figure 5. (a) DiFFaX-simulated diffraction patterns of layered hydroxide-based sheets; profiles were generated from 3-layer clusters with layer edge lengths of 12 Å (solid lines) and 24 Å (dashed lines) with anion stacking as listed. (b) Schematics of the slabs and their stacking arrangements. The series of models were built up from sheets containing nine edge-sharing octahedra (oxygen shown as small orange spheres, iron as large blue spheres), and variations were explored by removing or introducing cation sites. Removal of the central Fe produces a honeycomb layer (upper “top view” cartoon); as shown in the lower “top view” cartoon, a honeycomb layer can be capped over the central vacancy (white sphere, corner-sharing connectivity) and/or a tetrahedral hole (black spheres, edge-sharing connectivity). The application of different stacking operations yields a variety of distinct eutactic structures. Patterns shown here were calculated for honeycomb (AB and ABBCCA stacking) or capped-honeycomb (ABC and ABAC stacking) models containing only octahedral Fe. Simulated profiles for AB stacking enlist a (0.5,0.5,1) shift between layers; the profiles resulting from (0,0,1) layer shifts are shown in Figure SI-3 in the Supporting Information. (c) Observed diffraction of as-prepared a-FeOx, and profile simulated from a 240 atom fragment of a reverse Monte Carlo (RMC) fit to G(r). (d) RMC fit of the as-prepared a-FeOx PDF using an interconnected cluster model containing only octahedral Fe, derived from ϵ-Fe2O3. (e) A representative cluster from the RMC starting configuration; the respective capping sites retain the color scheme introduced in panel b. Although the RMC approach succeeds in describing the local structure reflected in G(r), the model does not capture the predominantly ABC anion-stacking at medium ranges as suggested by the simulated diffraction profiles.
annealing temperature, and the centroid shifts to longer r. In γFe2O3 these correlations derive predominantly from Fe−Fe distances involving tetrahedra: corner-sharing Feoct−Fetet pairs contribute at 3.46 Å, and nonoxygen-sharing (i.e., adjacent) Fetet−Fetet pairs occur at 3.62 Å. The grouping of next-nearestneighbor correlations at r ≈ 3 Å involve edge-sharing Fe−Fe and edge-connecting O−O pairs; O−O distances along tetrahedral edges are longer (3.07 Å) than along octahedral edges (2.95 Å). Shifting of the feature centered at r ≈ 3.0 Å first to longer r upon heating (as-prepared to 300 °C) then back to shorter r (300 to 400 °C) suggests that tetrahedral sites are preferentially occupied during crystallization, consistent with reciprocal- and real-space Rietveld results. Further evidence to support the absence of tetrahedra in the as-cast material is provided by considering the diffraction data across the amorphous to crystalline transformation (Figure 4, panels d and e). Successive tetrahedral population dramatically reduces the scattered intensities at approximately 1.5 Å−1 and 3.0 Å−1, due to destructive interference of what becomes, respectively, the {111} and {400} planes in γ-Fe2O3. The fwhm of the {111}-related feature in the as-prepared sample is ≈ 0.38 Å−1, which gives an estimated coherence length on the order of 16 Å. The simulations shown in Figure 4e were conducted with ≈ 12 × 12 × 17 Å3 clusters; the representative cartoons have half the layer diameter employed in the simulations, as defined by the blue spheres.
Given an absence of tetrahedra in as-prepared a-FeOx, PDF correlations in the vicinity of 3.4 Å can arise from Fe−Fe pairs between corner-sharing octahedra, or from Fe−Fe pairs between irregular (distorted) octahedra that share edges. Corner-sharing Fe in α-FeOOH occur at approximately this distance (3.44 Å), as do some of the edge-sharing octahedra in ϵ-Fe2O3 (3.32 Å). The as-cast material can thus be inferred to contain a combination of edge and corner-sharing octahedra. Medium-Range Eutaxy. Transformation of an amorphous iron (hydr)oxide to vacancy-ordered maghemite (γ-Fe2O3) has been observed previously,62 and is notable in that the only welldefined oxyhydroxide of iron that undergoes direct thermal conversion to maghemite is the γ-FeOOH polymorph, lepidocrocite.63−65 The distinct behavior of lepidocrocite has been rationalized on the basis of a topotactic relation between γ-FeOOH and γ-Fe2O3:66 γ-Fe2O3 and γ-FeOOH are the only iron(III) oxide(hydroxide) phases that possess ABC-based anion stacking arrangements (γ-FeOOH requiring an ABCA′B′C′ description to account for the shift of adjacent layers by a/2 along the a axis). This suggests a possible topological relation between a-FeOx and γ-Fe2O3. The most common stacking motif in crystalline iron oxide(hydroxide) compounds is based upon hexagonal ABAB anion stacking, appearing in αFe2O3 (hematite), α-FeOOH (goethite), δ-FeOOH (feroxyhyte), and the high-pressure rutile-like phase ϵ-FeOOH. ABAC stacking is not especially common, although ϵ-Fe2O3 and both models proposed for ferrihydrite contain this E
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Chemistry of Materials arrangement.67−69 As with the majority of iron oxide and oxyhydroxide phases, ferrihydrite and ϵ-Fe2O3, as well as the nonclose-packing hollandite structured β-FeOOH, undergo thermal conversion directly to α-Fe2O3 at ambient pressure.70−79 Despite the short-range of correlated chemical environments in a-FeOx, indicated by the PDF, the diffraction profile points to elements of topological correlation and coherence over significantly longer length scales. While a precise physical assignment of the FSDP spacing and coherence length remains uncertain, we note that an interpretation related to interparticle separation is one possibility. This is consistent with an observed shift of the FSDP to larger d-spacing (lower Q) upon annealing (see left panel of Figure 3), which suggests coarsening driven by dehydration. If this feature were related to an interlayer spacing then dehydration would be expected to shift it to higher Q. Another interpretation is that the FSDP arises from periodically spaced voids in the material, which would also be expected to increase in size upon dehydration. Given the tenuous assignment of the FSDP, a more compelling piece of evidence for the existence of MRO is provided by the smooth transition of the diffraction profile upon annealing (Figure 4d), supported by the aforementioned ∼16 Å coherence length of the diffracted intensity at ∼1.5 Å−1. We interpret these signatures as indicating a degree of eutaxy, that is, anion close packing with cations filling tetrahedral and octahedral voids.80 To reconcile the local connectivity and medium-range eutaxy in a-FeOx, we examine the relation between plausible anion stacking motifs and their resulting diffraction features (Figure 5a, b). A series of stacking models based on AB (CdI2 type), ABC (CdCl2 type), ABBCCA (hydrotalcite), and ABAC anion packing were constructed from 3 × 3 layered-hydroxide-type slabs of edge-sharing octahedra, and further variations were explored by removing or introducing cation sites. Removing the central Fe produces a honeycomb layer (see upper “top view” in Figure 5b), which can be capped over the central vacancy (light sphere in lower “top view” depiction) and/or over a tetrahedral hole (dark spheres in lower “top view” depiction) to yield a wide range of eutactic structures. For example, by capping both sides of the central vacancy and one of the tetrahedral holes, the application of ABC stacking to this layer cluster generates the spinel structure (bottom cartoon in Figure 4e). Fully occupied and honeycomb slabs are compatible with all of the considered stacking arrangements. The introduction of capping cations above vacant sites of the honeycomb layers constrains the possible arrangements to avoid unphysical short bond distances and the improbable trigonal prismatic coordination environment. To preserve the aforementioned criterion of solely octahedral cation environments: AB and ABAC stacking motifs accommodate capping over the central honeycomb vacancy, which introduces corner-sharing sites; the ABC motif is compatible with capping over tetrahedral holes, which produces edge-sharing cubane motifs. For ABBCCA stacking, neither of the capping sites results in octahedral coordination environments. Simulations of different stacking relationships in slabs consistent with the MRO length scales were conducted using the DiFFaX program.81 The generated diffraction profiles and schematics of the stacking motifs are presented in panels a and b of Figure 5, respectively. Here we have elected to show the profiles resulting from honeycomb sheets with AB and ABBCCA stacking (top row in panel b), and capped-
honeycomb sheets with ABC (cation over tetrahedral hole) and ABAC (cation over central vacancy) stacking. The solid traces in panel a were generated from layers with edge lengths of 12 Å and dashed traces from layers with edge lengths of 24 Å; all simulations shown here were conducted for models containing three layers, providing cluster heights comparable to the estimated length of coherence (∼16 Å). Although these models obviously contain a much greater degree of short-range chemical order than is observed experimentally, this does not obviate their utility because the objective is to assess the reciprocal-space influence of different stacking variations. The simulated profiles display subtle but significant differences for different stacking arrangements (Figure 5a). When the layer diameter is small (solid lines), all of the stacking motifs offer some agreement, albeit varying, with the scattering data observed for as-prepared a-FeOx (Figure 5c). In the region between 2 Å−1 and 3 Å−1, the reflection positions are notably shifted for AB and ABBCCA stacks, whereas ABC and ABAC motifs provide comparable agreement. For wider layers (dashed lines), only ABC stacking avoids the introduction of sharpened reflections that are inconsistent with the data. We note that although the boehmite-type polymorph γ-FeOOH contains an ABC-related stacking configuration, its diffraction profile is markedly different and provides poor agreement with the observed pattern. A comparison of fully occupied sheets (i.e., no central vacancy) vs honeycomb sheets is shown in Figure SI-3 in the Supporting Information. For all of the stacking motifs, the central vacancy leads to a significant increase of diffracted intensity in the vicinity of 1.5 Å−1, whereas higher angle features are relatively insensitive to the vacancy’s presence. Considering the feature observed experimentally at Q ≈ 1.5 Å−1 is quite prominent, the presence of vacancies within the layers is critical for reproducing the general features of the measured diffraction profile. Furthermore, the introduction of corner-sharing connectivity, as inferred from the PDF, is enabled by the presence of such vacancies. Octahedra that share more than one corner with edge-sharing clusters necessarily induce a degree of local ABA anion stacking. Such a model is consistent with the observed preference for occupying and ordering tetrahedral sites when a-FeOx is converted to poorly crystalline γ-Fe2O3: the local stacking changes from ABA to ABC stacking when corner-sharing octahedra undergo condensation and form tetrahedra. Given that AB hexagonal close-packing offers arguably the worst agreement with the experimentally observed diffraction data, a situation where some ABAC-type motifs exist appears more likely. To lend support for the inferred local prevalence of ABAC anion arrangements, a model was constructed from a 2 × 2 × 2 supercell of ϵ-Fe2O3 by removing all tetrahedra and select octahedra to yield an interconnected network of small clusters. A representative cluster is shown in Figure 5e. Because a-FeOx is sufficiently heterogeneous to preclude the use of any single cluster model, an ∼8000 atom supercell was modeled against the data by reverse Monte Carlo (RMC) simulation (Figure 5d).82 Within the constraints of purely octahedral connectivity, the PDF cannot be adequately modeled in the absence of significant octahedral irregularities: the prevalence of distorted octahedra is consistent with a large fraction of coordinatively unsaturated −OH and/or terminal −OH2 ligands, suggesting a high concentration of potential active sites. Despite the RMC approach yielding an ensemble of edgeand corner-sharing octahedra that capture the distribution of F
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irregular (i.e., distorted), consistent with the large degree of hydroxylation in the material; hydroxyl ligands are presumably present on both mono-μ-oxo and di-μ-oxo bridging sites.
local coordination environments, simulation of the Bragg profile83,84 generated from a fragment of the RMC box clearly shows that the medium-range eutaxy is not reproduced by this method (Figure 5c); specifically, the “real” material contains a much greater degree of ABC anion stacking than afforded by this local model. This apparent discrepancy can be reconciled by assuming a lesser degree of coherence is actually present between corner-sharing connectivities than imposed by the starting model. To a reasonable approximation, photochemically deposited a-FeOx can be regarded as a severely defective spinel that contains no tetrahedral sites, but does contain a non-negligible fraction of octahedral Fe in vacancy-capping sites (where tetrahedra are found in spinel). The ensemble of local structural motifs present in a-FeOx cannot be adequately captured by any single cluster model. An artistic representation of the structure appears in Figure 6, highlighting the primary features identified
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CONCLUSIONS The analyses presented here provide a general set of characteristics that describe the coordination and connectivity present in a-FeOx prepared by the photochemical metal organic decomposition method. The network: (1) consists strictly of octahedral iron; (2) contains both edge- and corner-sharing octahedral linkages; (3) displays medium-range order dictated by ABC-type anion packing; and (4) involves extensive octahedral irregularities that can be attributed to a large proportion of terminal and bridging hydroxyl ligands. Despite the considerable heterogeneity inherent to the material, these guidelines help to differentiate photochemically deposited aFeOx from electrochemically deposited γ-FeOOH, and provide a foundation for evaluating the structural evolution of the photodeposited catalyst under operating conditions.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Three supporting figures and additional information about DIFFaX simulations, including a representative control file. This material is available free of charge via the Internet at http://pubs.acs.org/.
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge Dr. Olaf J. Borkiewicz for assistance collecting the scattering data. C.P.B. thanks Canada Foundation for Innovation and Canada Research Chairs for financial support. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract DE-AC02-06CH11357.
Figure 6. Cartoon depiction highlighting the salient structural features of a-FeOx prepared by photochemical metal organic decomposition. The as-deposited material contains octahedrally coordinated iron, with a combination of edge- and corner-sharing linkages. Iron is shown as large blue spheres, and oxygen as small orange spheres. The representative network cluster (shown on top) contains primarily ABC-type anion packing, with terminal ABA-type packing to provide some corner-sharing linkages. The dimensions shown here are comparable to the 16 Å coherence length of the Q ≈ 1.5 Å−1 feature in the diffraction profile. Fe atoms are rendered as blurry and smeared to portray the limited extent of short-range chemical coherence in the material; O atoms conforming to ABC stacking are not blurred, signifying the presence of eutaxy at medium-range length scales. A magnified view of a small portion of the cluster is shown below (indicated by the dotted oval); the octahedral irregularities (i.e., distortions) present here are characteristic of other regions throughout the material, consistent with the degree of hydroxylation in a-FeOx. As deduced from the PDF data, Fe−Fe distances typical of edge- and corner-sharing connectivities are indicated.
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