Article pubs.acs.org/JPCC
Impact of a Mixed Oxide’s Surface Composition and Structure on Its Adsorptive Properties: Case of the (Fe,Cr)3O4(111) Termination of the α‑(Fe,Cr)2O3(0001) Surface M.A. Henderson*,† and M.H. Engelhard‡ †
Fundamental and Computational Sciences Directorate, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ABSTRACT: Characterization of an α-(Fe0.75,Cr0.25)2O3(0001) mixed oxide single crystal surface was conducted using X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD). After sputter/anneal cleaning in ultrahigh vacuum (UHV), the mixed oxide surface became terminated with a magnetite-like (111) structure based on the presence of (2 × 2) spots in LEED and Fe2+ in XPS. The composition of the surface was close to that of M3O4 based on XPS, with the metal (M) content ratio of Fe2+/3+ and Cr3+ being close to 1.4:1, despite the film’s bulk being prepared with a 3:1 ratio of these cations. Enrichment of the surface with Cr was not altered by high temperature oxidation in UHV but could be partially reversed by exposure to air. Adsorption of various probe molecules (NO, O2, CO2, and H2O) was used in an effort to identify the active cation sites present on the (Fe,Cr)3O4(111)-terminated surface. Although both XPS and SIMS indicated that the near-surface region was enriched in Cr3+, little adsorption behavior typically associated with Cr3+ sites on αCr2O3 single crystal surfaces was detected. Instead, the TPD behaviors of O2 and CO2 pointed toward the main active sites being Fe2+ and Fe3+, with O2 preferentially adsorbing at the former and CO2 at the latter. NO was observed to bind at both Fe2+ and Fe3+ sites, and H2O TPD looked nearly identical to that for H2O on the Fe3O4(111) surface. Competition for adsorption sites between coadsorbed combinations of CO2, O2, H2O, and NO corroborated these assignments. These results indicate that the surface composition of a mixed oxide can vary significantly from its bulk composition, depending on the treatment conditions. Even then, the surface composition does not necessarily provide direct insights into the active adsorption sites. In the case of the (Fe,Cr)3O4(111) termination of the α-(Fe0.75,Cr0.25)2O3(0001) surface, Cr3+ cations in the near-surface region appears to be fully coordinated and mostly unavailable for adsorbing molecules.
1. INTRODUCTION Iron oxides are attractive catalytic and photocatalytic materials because of their natural abundances, their varying oxidation states, and the overlap of their optical absorption spectra with the solar spectrum. Many groups have shown that iron oxide materials can function adequately as water splitting photocatalysts if the materials are suitably engineered or chemically modified.1−6 Combinations of iron with other transition metals in mixed or composite oxides also show considerable promise for promoting visible-light photocatalysis,7,8 with mixed chromium−iron oxides as a specific example.9−13 This latter class of mixed oxides is attractive because Cr inclusion into hematite (α-Fe2O3) can form a solid solution with lower optical absorption onset and improved charge carrier dynamics.10,11 Fundamental understanding of the interfacial properties of complex (mixed) oxide photocatalysts often lags the efforts aimed at bandgap and band edge engineering. A candidate material that adequately absorbs solar radiation may fail as a solar photocatalyst because its interfaces are ill-suited for promoting catalysis and charge transfer. The identification of surface structures, sites, and compositions is a significant challenge in understanding the interfacial properties of a mixed oxide. While considerable progress has been made in achieving © XXXX American Chemical Society
fundamental understanding of binary oxide surfaces [such as rutile TiO2(110)14], the more conceptually complex problems associated with a ternary oxide surface have been largely unaddressed. In this study, we examine the surface composition, structure, and simple adsorptive properties of an α-(Fe0.75,Cr0.25)2O3(0001) mixed oxide surface using UHV approaches. We find that vacuum treatment (sputter/ annealing) alters not only the mixed oxide’s surface structure, transforming it from the corundum (0001) to a magnetite-like (111) phase, but also the cation composition ratio in the near surface region, with significant enhancement occurring in the Cr3+ concentration. The impacts of these changes on surface chemistry are explored using a series of typical probe molecules.
2. EXPERIMENTAL SECTION The α-(Fe,Cr)2O3(0001) film employed in this study was prepared using oxygen plasma-assisted molecular beam Special Issue: John C. Hemminger Festschrift Received: April 21, 2014 Revised: June 9, 2014
A
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the sample in a line-of-sight configuration positioned ∼1 cm from the apertured entrance to the QMS at a heating rate of ∼2 K/s. In the XPS system, the sample was transferred to a UHVcompatible processing chamber in which annealing and exposure to gases were accomplished.
epitaxy15 with a nominal 75% Fe and 25% Cr composition by Chamberlin et al.11 as a 500 Å thin film on an α-Al2O3(0001) crystal. The crystal was mounted in a UHV chamber (base pressure below 2 × 10−10 Torr) possessing an apertured quadrupole mass spectrometer (QMS) for TPD and SIMS measurements, and a LEED apparatus. The sample was pressed against a Au-coated Ta plate, which could be cooled to ∼40 K with a He cryostat and heated resistively to >1000 K. A consequence of using the He cryostat for cooling was that background gases condensed on the Cu blocks on which the sample leads were mounted. The Cu blocks were at a base temperature of ∼20 K but rose in temperature (to ∼40 K) during TPD causing desorption of background gases (e.g., CO, CO2, O2) as the sample temperature exceeded 500 K. This effect was not present for strongly condensing molecules (e.g., NO, H2O). Temperature was monitored with chromel−alumel thermocouples attached to various surfaces (e.g., the Ta plate, the crystal, and the Cu block holder). Because the film was grown on an insulating substrate (sapphire), charging prevented the use of charged-particle surface characterization techniques at temperatures below 450 K. Transfer of the film through air from the growth chamber to the UHV chambers used in this study resulted in surface contamination that could only be removed by sputtering. The crystal surface was cleaned by 1 kV Ne+ ion sputtering (above 450 K to alleviate charging) and annealing at ∼950 K, with cleanliness monitored by SIMS. After initial cleaning, surface cleanliness was maintained by brief heating to 950 K, resulting in consistently repeatable TPD spectra. After completion of the TPD measurements, the sample was transferred through air to the XPS system. XPS measurements were performed with a Physical Electronics Quantera Scanning X-ray Microprobe, which employed a focused monochromatic Al Kα X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 32 element multichannel detection system. A 100 W X-ray beam focused to 100 μm diameter was rastered over a 1.3 × 0.1 mm rectangle on the sample. The X-ray beam was incident normal to the sample and the photoelectron detector was typically at 45° off-normal. High-energy resolution spectra were collected using a passenergy of 69.0 eV with a step size of 0.125 eV and a ±20° analyzer acceptance angle. For angle-resolved XPS, the analyzer angular acceptance was decreased to ±4°. XPS measurements were acquired at different spots on the crystal face with multiple measurements at each spot. The largest signal variations (approximately ±3%) were seen at the shallow takeoff angles, with variations decreasing (to approximately ±1%) at surface normal. For the Ag 3d5/2 line, these conditions produced a fwhm of 0.91 eV. The sample experienced charging (see above), which could be alleviated using low-energy (450 K using a 90 eV beam energy.
sputter/anneal treatment. The film extensively charged in the LEED beam below 450 K because it was grown on a sapphire substrate. The image in Figure 2 was therefore obtained >450 K, which unfortunately resulted in a high background and slightly diffuse spots. As the LEED image revealed, the sputter/ anneal treatment generated a magnetite-like (111) surface phase reflected by the (2 × 2) spots.17,18,22,30−32 There was no B
dx.doi.org/10.1021/jp5038975 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 2. Ball and stick model of the Fe3O4(111) surface (from the side), showing 4-coordinate Fe cations (Fe4c, dark blue balls), 6coordinate Fe cations (Fe6c, light blue balls), and O anions (gray balls), along with three possible terminations of the surface.
indication in the image for biphase-ordering,19 indicating the absence of FeO(111)-like surface domains. However, the LEED image also possessed weak (√3x√3)R30° spots, which possibly reflect a surface phase composed of a metal halfoccupied corundum (0001) surface structure33−35 or a partial ordering of Cr cations within the reconstructed magnetite-like (111) surface. This LEED pattern was not altered by oxidation at temperatures up to 1000 K with O2 pressures up to 2 × 10−6 Torr, indicating its resistance to transforming back to the corundum (0001) structure in UHV. The surface structure of the termination of Fe3O4(111) remains controversial.30,34−37 Figure 2 shows a ball and stick model of the possible terminations of Fe3O4(111). In the bulk, the inverse spinel structure for Fe3O4(111) has octahedral (Fe6c) and tetrahedral (Fe4c) cations, with all of the Fe4c sites occupied by Fe3+ and the Fe6c sites occupied half and half by Fe2+ and Fe3+. That, of course, is in the bulk. The situation of valency and site occupancy in the surface is not clear. The Fe3O4(111) surface can, in principle, be terminated at three cation sheets (not counting O2− sheets): with both Fe6c and Fe4c cations (T1), with only Fe4c cations (T2), or with only Fe6c cations (T3). The most commonly accepted model of the Fe3O4(111) surface termination involves the mixed Fe6c and Fe4c cation sheet in which either both Fe6c and Fe4c cation sites are present (T1) or with only the Fe4c cation sites (T2). The impact of Cr3+ inclusion on these terminations is not known. The surface was also characterized using static SIMS (1 kV Ne+ and 90%) as a result of the 900 K anneal. The more prominent increase in the Cr composition (relative to that of Fe) after the anneal suggests an enrichment in the surface with Cr as a result of the anneal. With a focus only on the Fe, Cr, and O in Table 1, the stoichiometry of the annealed surface was approximately Fe1.9Cr1.2O4, or M3.1O4 (if both cation compositions are combined), consistent with a magnetite-like surface composition. To further address the compositional variations occurring in the magnetite-like (111) termination of the α(Fe,Cr)2O3(0001) surface, Figure 5 presents angle-resolved C
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Figure 4. XPS spectra in the (a) Fe 2p and (b) Cr 2p spectral regions for the “as-received” film transferred through air and after annealing in UHV at 900 K for 1 h.
Table 1. Elemental Compositions (%) in the (Fe,Cr)3O4(111) Termination of the α-(Fe,Cr)2O3(0001) Surface After Through-Air Transfer (“As-Received”) and After Annealing in UHV at 900 K for 1 ha as-received annealed
Fe
Cr
O
C
misc.
23.8 26.5
8.5 16.2
50.9 55.4
15.7 0.7
1.1 1.2
Analyzer acceptance angle was ±20°. See text for details of the quantifications and description of “misc.” impurity elements.
a
XPS measurements of the 900 K annealed surface. For these measurements, the analyzer acceptance angle was narrowed from ±20° (as used in Figure 4) to ±4° in order to get more accurate angular dependence and great depth specificity. The elemental compositions for O (●), Fe (▲), and Cr (■) and miscellaneous impurities (see above; ◆) show different trends with changes in the electron takeoff angle from surface normal detection (90°), which favors bulk detection, to glancing angle detection (20°), which favors surface detection. As one might expect, the impurities (e.g., Al, P, etc.) were concentrated at the surface. The Cr composition increased as the electron takeoff angle decreased from 90 to 20°, suggesting that the surface was enriched in Cr. In contrast, the Fe composition decreased as the electron takeoff angle was varied from bulk to surface sensitivity. Table 2 presents the composition variations of Figure 5. The data suggest that the composition of the film was slightly more oxidized at the surface than in the near-surface region (i.e., greater O:M ratio, where “M” is the sum of the Fe and Cr compositions), although all of the ratios are suggestive of a magnetite-like composition (O:M of 1.33:1). This is consistent with the surface being reduced as a result of annealing in UHV. The data also indicate that the reduced surface was enriched in Cr (Fe:Cr ratio of ∼1.4:1) relative to the as-grown composition (Fe:Cr ratio of 3:1), consistent with the Fe+ and Cr+ yields measured by static SIMS (Figure 3). The
Figure 5. Compositional variations for O, Fe, and Cr and miscellaneous impurity species for (Fe,Cr)3O4(111) as a function of the electron takeoff angle.
angle-resolved XPS measurements confirm the conclusion from LEED, SIMS, and normal emission XPS that the surface has a magnetite-like (111) structure enriched with Cr. The structural implications of alloying Cr into the Fe3O4(111) surface structure are not known. As mentioned above, magnetite has an inverse spinel structure with the octahedral metal cations occupied half with Fe2+ and half with D
dx.doi.org/10.1021/jp5038975 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Table 2. Elemental Compositions (%) and O:M Ratio in the (Fe,Cr)3O4(111) Termination of the α-(Fe,Cr)2O3(0001) Surface after Annealing at 900 K as a Function of Electron Takeoff Angle (90° = Surface Normal)a electron takeoff angle
Fe
Cr
O
misc.
O:M ratio
20° 45° 90°
24.4 26.6 28.8
17.6 17.0 16.8
55.6 54.7 53.2
2.5 1.6 1.3
1.32:1 1.25:1 1.17:1
a Analyzer acceptance angle was ±4°. See text for details of the quantifications and description of misc. impurity elements.
Fe3+ and all of the tetrahedral cations being Fe3+. In contrast, chromite (FeCr2O4) is spinel with tetrahedral sites occupied by Fe2+ and octahedral sites occupied by Cr3+.42 In bulk studies of Cr-doped Fe3O4(111), Liang et al.43 found that the inverse spinel (magnetite) structure was retained for Cr compositions up to 30%, with Cr3+ in octahedral sites and Fe2+/3+ distributed between octahedral and tetrahedral sites. Similarly, Magalhães et al.44 found that at low Cr concentrations (below ∼25%), Cr3+ was present in the lattice only at the octahedral sites; however, Cr3+ also began occupying tetrahedral sites as the concentration was increased above 25%. From these bulk studies, one might expect that incorporation of high percentages of Cr into a Fe3O4(111) surface should alter the coordination environments of Fe2+ and Fe3+. The Cr concentration in the magnetite-like (111) surface in this study (from angular-resolved XPS) was ∼40%. Therefore, a wide distribution of coordination environments could exist for all three cations (i.e., Cr3+, Fe3+, and Fe2+), not to mention the potential of Cr cations possessing some slightly reduced character. Determining the termination, composition, and site occupancy of a mixed (Fe,Cr)3O4(111) termination will be considerably more difficult to arrive at than that of pure Fe3O4(111), which remains controversial despite considerable attention. Insights into the surface character of the mixed (Fe,Cr)3O4(111) termination of the α-(Fe,Cr)2O3(0001) film were obtained in the next section from TPD of various probe molecules. 3.2. Surface Chemistry of Probe Molecules. This section describes the adsorption/desorption behaviors of selected probe molecules (NO, O2, CO2, and H2O) on the mixed (Fe,Cr)3O4(111) termination of the α(Fe,Cr)2O3(0001) film using TPD as a diagnostic technique. Selection of probe molecules was based on their known chemical sensitivities to either the α-Fe2O3(0001), αCr2O3(0001), and/or Fe3O4(111) surfaces.17,26,27,45−57 Figure 6 presents a survey of the TPD spectra for exposures near saturation of the surface’s first layer (as gauged by the onset of multilayer desorption). From top-to-bottom in Figure 6, O2 yielded two prominent TPD features that could be associated with “chemisorbed” species at 105 and 230 K, along with a sharp feature at ∼55 K due to physisorbed O2.58 The ratio of O2 desorbing in the 105 and 230 K TPD states was ∼1:1.7 at saturation. The two peaks grew in simultaneously with increasing O2 exposure (data not shown). Available data in the literature will be relied on in assigning the O2 TPD features in Figure 6. Dillmann et al.52 found that O2 desorbed in TPD from the Cr-terminated α-Cr2O3(0001) surface at ∼300 K. The Cr-terminated α-Cr2O3(0001) surface possesses 3-coordinate Cr3+ sites in nominal octahedral geometry. In contrast, York et al.56 found that O2 desorbed from the α-Cr2O3(0112̅ ) surface, which possesses only 5-
Figure 6. TPD spectra for exposures of O2, CO2, H2O, and NO that approximately saturate the first layer on (Fe,Cr)3O4(111). Spectra are displaced vertically for clarity.
coordinate Cr3+ sites, at 220 K. However, both Dillmann et al. and York et al. found the considerable O2 dissociative adsorption accompanied molecular O2 desorption to the extent that the surfaces became predominately O-terminated after repetitive exposures of O2, with the Cr-terminated surface regenerated by heating to high temperature. The capping of Cr3+ sites with O atoms appears to be consistent with a property of α-Cr2O3 surfaces,24,27,28,33,46,52,53,56,57,59 with O termination removed by annealing. In contrast, this author51 showed that O2 desorbed from the reduced α-Fe2O3(011̅2) surface in a peak at 235 K that resulted from O2 molecules bound at surface Fe2+ sites, with no indication of O 2 dissociation. No O2 adsorption was detected on the oxidized α-Fe2O3(011̅2) surface, which is terminated with 5-coordinated Fe3+ sites. Although not conclusive, the presence or absence of dissociative O2 adsorption may be an indicator of the type of site(s) present on the (Fe,Cr)3O4(111) surface. As it turned out, the same O2 exposure used in Figure 6 could be repeated in consecutive TPD experiments up to 600 K with only a small (
