Article pubs.acs.org/JPCC
Atomic Structure of Cr2O3/Ag(111) and Pd/Cr2O3/Ag(111) Surfaces: A Photoelectron Diffraction Investigation Alex S. Kilian,† Fabiano Bernardi,† Alexandre Pancotti,‡ Richard Landers,§ Abner de Siervo,§ and Jonder Morais*,† †
Instituto de Física, Universidade Federal do Rio Grande do Sul, Caixa Postal 15051, 91501-970 Porto Alegre, RS, Brazil Universidade Federal de Goiás, Campus Jataı ́, Rodovia BR364 3800, 75801-615 Jataí, GO, Brazil § Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, Rua Sérgio Buarque de Holanda 777, 13083-859 Campinas, SP, Brazil ‡
S Supporting Information *
ABSTRACT: A detailed investigation concerning the atomic structure of Cr2O3 and Pd/Cr2O3 ultrathin films deposited on a Ag(111) single crystal is presented. The films were prepared by MBE (molecular beam epitaxy) and characterized in situ by LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy), and XPD (X-ray photoelectron diffraction). Evidences of rotated domains and an oxygen-terminated Cr2O3/Ag(111) surface were observed, along with significant contractions of the oxide’s outermost interlayer distances. The deposition of Pd atoms on the Cr2O3 surface formed a four-monolayer film, fcc packed and oriented in the [111] direction, which presented changes in monolayer spacing and lateral atomic distance compared to the expected values for bulk Pd. The observed surface structure may shed light on new physical properties such as the induced magnetic ordering in Pd atoms.
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structure determination of (√3 × √3)R30° α-Cr2O3 films, Cr-terminated surfaces were observed in several cases. Some authors report a single Cr layer17,19 and others a double Cr layer18,20 as the outermost layers of the Cr2O3. In all cases, a contraction of the outermost interlayer distances compared to the bulk values was observed. Nevertheless, theoretical papers21,22 have also predicted the possibility of Cr2O3 films with an oxygen-terminated surface, depending on different growth parameters (environment and surface defects). Considering the transition metals, palladium stands out due to its important role in catalysis,23 for instance, due to its high activity and selectivity toward several reactions,24−26 such as Heck, Suzuki, Sonogashira, and olefin metathesis.27−30 Pd is also applied as an automotive catalyst in the three-way process.23 Several theoretical studies have been performed to estimate the atomic interlayer distances of the bulk Pd surface.31,32 For example, Wan et al.33 employed the MEAM (modified embedded atom method) to study the relaxation of the outermost interlayer distances of several metals, including Pd, considering a slab of eight layers for energy minimization in the relaxation procedure. Their findings indicated that the Pd(111) surface would present an interlayer distance between the first and second monolayers (d12) with a contraction of
INTRODUCTION A full understanding of the short-range order crystalline structure, especially for ultrathin films1,2 (below 10 nm thickness), is a key issue since a two-dimensional material displays distinct electronic and structural properties compared to those observed in its bulk form.3−5 Modifications of the physical properties induced by the atomic distribution of the surface atoms induce very particular properties6 and hence allow a variety of technological applications, for instance, in microelectronics and catalysis.7 The determination of the packing and interatomic and interlayer distances of the outermost layers is essential for the development and improvement of advanced materials and devices.8 However, it is quite challenging to obtain such parameters in a straightforward way. The surface atomic structures of several oxide thin films have been investigated.9,10 Among those, Cr2O3 has attracted much attention due to its wide applications, ranging from its use as a catalyst,11−13 as a support for Pd islands in a model catalyst,14 or in the form of magnetic films.15,16 It has been reported that Cr2O3 films grown on different metallic substrates and having thickness below 5 Å present a p(2 × 2) surface structure.17,18 The crystallographic structure and stoichiometry of such thin films are still objects of strong debate in the literature. However, for films thicker than 10 Å the typical surface structure is the (√3 × √3)R30°, in the α-Cr2O3 phase.18 Among the experimental results obtained in the surface © 2014 American Chemical Society
Received: June 30, 2014 Revised: August 7, 2014 Published: August 12, 2014 20452
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in the chamber was kept at 2 × 10−6 mbar. In this way, the simultaneous deposition of Cr and O atoms onto the ordered surface of Ag(111) single crystal formed a CrxOy film. The sample was then annealed up to approximately 900 K for 2 min under 2.4 × 10−6 mbar oxygen pressure in order to get a sharp LEED pattern. The Cr2O3 film thickness was determined by angle-resolved X-ray photoelectron spectroscopy (ARXPS). Thereafter, Pd atoms were deposited by the MBE technique onto the chromium oxide film for 30 min while the sample was kept at room temperature (300 K) in UHV. The sample was then annealed up to approximately 773 K for 2 min, and a wellordered surface structure was verified by LEED. After each step of the film preparation described above, the sample was analyzed by XPS and XPD using an Al Kα X-ray source (1486.6 eV). The overall resolution for the measurements was about 0.8 eV. The energy calibration of the analyzer was performed by measuring the Ag 3d5/2 peak at 368.3 eV of the Ag(111) single crystal. Concerning the Cr2O3/Ag(111) system, the XPD data were acquired using the Cr 2p3/2 (kinetic energy of 905 eV) and O 1s (kinetic energy of 955 eV) photoemission peaks. Only Cr 2p3/2 was considered for the XPD pattern due to the significant Cr 2p3/2 satellite peak contribution in the Cr 2p1/2 region. Analogously, for the Pd/Cr2O3/Ag(111) system, XPD was performed, acquiring core-level Cr 2p3/2 (kinetic energy of 905 eV) and Pd 3d5/2 (kinetic energy of 1146 eV) photoemission peaks. The base pressure was maintained below 7 × 10−10 mbar in the analysis chamber during the XPS and XPD measurements. The XPD measurements were performed in the angular mode, that is, the sample polar angle θ (the angle between the analyzer axis and the surface normal) and the azimuthal angle ϕ were varied, while the photon energy was fixed. Since the LEED patterns for all films showed C6 or C3 symmetries, the ϕ angle was varied in 3° steps over a range of only 120°, with the data replicated for the remaining azimuthal angles. The polar angle (θ) was varied in steps of 3° from 18° to 72° for the Cr2O3/ Ag(111) system and from 30° to 72° for the Pd/Cr2O3/ Ag(111) system. A full spectrum of each photoemission peak was acquired and fitted at every angular setting, and the component area was used as the XPD intensity. The experimental XPD patterns represent the atom-specific photoemission peak intensity variations as a function of the angles θ and ϕ. Theoretical XPD patterns, one for each photoemission peak (Cr 2p3/2, O 1s, Pd 3d5/2), were calculated using the MSCD package, taking into account multiple scattering in the Rehr− Albers approximation.42,43 A cluster of atoms with a paraboloid shape, top radius of 9 Å, and depth ranging from 18 to 25 Å, which consisted of approximately 330 atoms, was used during simulations. All calculations were performed considering multiple scattering up to the sixth order, and the fourth order for the Rehr−Albers approximation. The phase shifts for the scattered electrons were obtained using a muf f in-tin potential for Pd and Cr atoms in their respective bulk phases. A routine based on a genetic algorithm (GA) implemented by Viana et al.44 was used to optimize the relaxation of the interlayer distances of Cr2O3 (d12, d23, d34, and d45) and Pd (d12, d23, and d34), where d23 is the interlayer distance between the second and third planes of atoms and so forth. The relaxation of the atomic lateral distance in the outermost Pd monolayer and the Debye temperature (TD) was also performed using the same algorithm. In order to compare experimental XPD results with
0.32% in comparison with the bulk value. The next interlayer distances remain almost unchanged. Few investigations of the Pd/Cr2O3 system have been reported. An interesting study applied FTIR (Fourier transform infrared) and HREELS (highresolution electron energy loss spectroscopy) techniques to characterize a Pd-based model catalyst, where Pd depositions on Cr2O3 were performed at different temperatures. In particular, at 300 K, three-dimensional Pd islands with a very low aspect ratio were formed.14 Particularly, the lateral Pd−Pd distance expansion could play an important role in inducing a magnetic ordering in Pd atoms, as predicted theoretically.34−36 A possible way to induce ferromagnetism in Pd is by modifying the atomic arrangement of nonmagnetic atoms.37 The existence of ferromagnetism in Pd films with fcc structure, with an expansion of between 10 and 32% in the lattice parameter,36 where it would reach the value of 0.36 μB, has been predicted. In the present work, we aim to prepare a Cr2O3 film on Ag(111) and Pd ultrathin films on Cr2O3/Ag (111), followed by in situ structural characterization by X-ray photoelectron diffraction (XPD) measurements. XPD has been successfully used for structural characterization of several surfaces and thin films (semiconductors, metals, and oxides). Depending on the studied system, XPD can provide valuable information about the atomic arrangements at the surface, adsorbed molecular orientation, and bond symmetry and distances, with the advantage of being element and chemical state specific.38−41 Here, XPD and X-ray photoelectron spectroscopy (XPS) enabled us to determine that the Cr2O3/Ag(111) film was oriented along the [0001] direction with an oxygen surface termination under the preparation conditions used in the present work. The four-monolayer Pd film deposited on the oxide presented an orientation along the [111] direction, and the relaxation of the outermost interlayer distances was investigated.
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EXPERIMENTAL SECTION The experiments were performed using a dedicated surface science workstation equipped with evaporators, LEED optics, and a fixed-geometry high-resolution hemispherical electron analyzer (Omicron HA-125HR). A sample manipulator with five degrees of freedom (x, y, z, θ, and ϕ) was also available, with the possibility of heating the sample up to 1300 K. The substrate used was a high-purity (99.999%) Ag(111) single crystal with a diameter of 10 mm. The single crystal was cleaned by repeated cycles of argon ion bombardment (sputtering) and annealing. The sputtering was performed with 1.3 kV argon ions impinging at a grazing incidence of 30° with respect to the sample’s surface at a pressure of 2 × 10−8 mbar in the chamber. The sample was heated on the back side of the crystal by electron bombardment up to 873 K during a few minutes. The process was repeated until no impurities were observed by the XPS measurements. In the final cycle, the sputtering was performed with 1.0 kV ions followed by a final sample heating up to 873 K for 2 min in order to produce a sharp LEED pattern, characteristic of the Ag(111) ordered surface. The chromium oxide film was deposited onto the clean Ag(111) single crystal by the MBE (molecular beam epitaxy) technique associated with the in situ oxidation of the Cr atoms. The Cr atoms were evaporated from an Al2O3 crucible, which was previously degassed by bombardment with an energetic electron beam. The deposition time was 90 min. The substrate was kept at 600 K during film growth, and the oxygen pressure 20453
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the simulated patterns, the agreement was quantified by the reliability factor Ra45 of the normalized XPD intensities. On the basis of the statistical analysis of the Ra factor, the uncertainty assumed in the resulting structural parameters is 0.05 Å.46
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RESULTS AND DISCUSSION Cr 2O 3/Ag(111). The LEED patterns (using 62 eV electrons) of the p(1 × 1) Ag(111) single crystal surface and the (√3 × √3)R30° surface structure of the Cr2O3 deposited on the Ag(111) substrate are displayed in Figure 1. The LEED pattern of the Cr2O3 indicates growth in the [0001] direction.
Figure 2. XPS spectra showing the Cr 2p region of the Cr2O3/ Ag(111) system.
different θ angles (corrected by the relative sensitivity factors and analyzer transmission). The Cr/O ratio obtained was 0.62, which is consistent with the Cr2O3 stoichiometry.18,50 The Cr2O3 thickness obtained by ARXPS was 15.0 Å, which corresponds to slightly more than one complete unit cell of the corundum structure of Cr2O3. As discussed in a previous work,18 the alpha phase (α-Cr2O3) is expected for Cr2O3 films thicker than 10 Å. The O 1s XPS peak of the Cr2O3/Ag(111) system was observed at 530.9 eV binding energy (Supporting Information), in agreement with the peak position reported for Cr2O3.48,51 After the preparation and precharacterization of the ordered Cr2O3 film, the XPD measurements were performed for the Cr 2p3/2 and O 1s regions. Figure 3a and b displays the experimental Cr 2p3/2 and O 1s XPD patterns, corresponding to photoelectrons emitted with kinetic energies of 905 and 955
Figure 1. LEED patterns (62 eV) for the (a) clean p(1 × 1) surface of Ag(111) and (b) the (√3 ×√3)R30° surface of the Cr2O3/Ag(111) film.
Figure 2 presents the XPS spectrum of the Cr 2p region for the Cr2O3/Ag(111) sample. The binding energy observed at 576.9 eV (green line) corresponds to the Cr 2p3/2 peak position reported for Cr2O3.47,48 As previously reported, nonmonochromatized X-ray sources provide a Cr 2p3/2 XPS spectrum with only one component,49 which was the case of the XPS spectrum shown in this work for any θ angle in the range of 18° to 72°. The binding energies for the Cr 2p3/2 and Cr 2p1/2 satellites (violet) and the X-ray satellite (red) agree with the values reported in the literature.49 The peaks observed at 573.5 and 604.2 eV (orange curve) refer to Ag 3p3/2 and Ag 3p1/2 components from the substrate.47 The surface stoichiometry may be checked by the peak intensity ratio between the Cr 2p and O 1s XPS regions considering the azimuthal average of both peak intensities for
Figure 3. Experimental XPD patterns of the Cr2O3/Ag(111) system obtained by measuring the (a) Cr 2p3/2 photoelectrons (905 eV kinetic energy) and (b) O 1s photoelectrons (955 eV kinetic energy). The simulated XPD results for the (c) Cr 2p3/2 and (d) O 1s patterns, which resulted in the lowest Ra factors. 20454
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eV, respectively. These kinetic energy values ensured that the XPD measurements occurred in the forward scattering regime. The brighter spots represent a higher number of photoelectrons emitted from the sample surface and detected for a given (θ, ϕ) angle. The experimental XPD patterns indicated a 6-fold symmetry. MSCD calculations were performed in order to investigate the influence of the rotation of the surface domains on the Ra factor value. The lowest Ra was achieved when two domains rotated by 60° were used. This case will be considered in the subsequent steps. The Cr2O3 structure belongs to the R3c space group, which consists of hexagonal close-packed layers of O atoms, where two-thirds of the octahedral interstices between them are filled with Cr atoms. The oxygen atoms follow hcp packing (ababab···), while Cr atoms follow fcc packing (ABCABC···). A representation of this structure, oriented in the [0001] direction, is shown in Figure 4.
Figure 5. Ra factor values obtained from Cr 2p3/2 XPD simulations for the different surface terminations of Cr2O3 considering bulk interlayer distance values (dashed orange line) and after the interlayer distance relaxation (solid blue line).
the different structural models and provided high Ra factor values. Therefore, the interlayer distances obtained from the Cr 2p XPD pattern simulations were used in the next steps. The results obtained after the relaxations are displayed in Table 1. Table 1. Interlayer Distances for Cr2O3 with aBCbA and bABaB Surface Terminations with the Lowest Ra Factors (Figure 5)a surface termination aBCbA
Figure 4. Schematic representation of the α-Cr2O3 unit cell (corundum structure) grown in the [0001] direction with AaBCb surface termination.
bABaB
Therefore, in the following step, structural models for the Cr2O3 surface were proposed with different surface terminations: AaBCb, aBCbA, bABaB, BCbAB, and CbABa. The XPD simulations were performed considering the TD values for Cr and O atoms as 630 and 600 K,52 respectively. The MSCD calculations considering Cr atoms as emitters and bulk values for interlayer distances (d12, d23, d34, and d45) achieved the lowest Ra factor for aBCbA and bABaB (Ra = 0.269 and Ra = 0.261), which have an oxygen-terminated surface top layer. Relaxations of the interlayer distances of all structural models were allowed. The interlayer distances were varied during the search process for the lowest Ra factor. Again, the lowest Ra factors found for the Cr 2p3/2 XPD pattern were obtained for aBCbA (Ra = 0.156) and bABaB (Ra = 0.157). Figure 5 shows a comparison between the Ra factors obtained before and after the structural relaxation for all structural models proposed for the Cr 2p3/2 XPD simulations. Some previous investigations have shown the possibility of O-terminated surfaces in Cr2O3.20−22,53 Our earlier XPD simulations indicated the presence of two domains rotated by 60°; therefore, it was not possible to discriminate packing aBCbA and bABaB since they only differentiate by a 60° rotation. It is noteworthy that this difficulty in distinguishing the two O-termination models has been reported.54 MSCD simulations were also carried out considering the O atoms as photoelectron emitters, but they were nonsensitive to
aBCbA (proposed)
a
interlayer distances (Å) (±0.05 Å) d12 d23 d34 d45 d12 d23 d34 d45 d12 d23 d34 d45
(O−Cr) = 1.04 (Cr−Cr) = 0.00 (O−Cr) = 0.49 (Cr−O) = 0.59 (O−Cr) = 0.84 (Cr−Cr) = 0.18 (O−Cr) = 0.49 (Cr−O) = 0.62 (O−Cr) = 0.79 (Cr−Cr) = 0.11 (O−Cr) = 0.31 (Cr−O) = 0.59
[−] contraction/[+] expansion (%) relative to the bulk values +10.4 −100 −48 −37.3 −10.4 −54 −48 −34.4 −15 −69 −63 −37.3
The last row corresponds to the proposed aBCbA structure.
Hence, considering the two O-terminated possibilities as the best candidates, the next step was to emphasize the surface contribution and verify the contradictory d12 values (contraction and expansion) obtained for both aBCbA and bABaB, as observed in Table 1. Again, the influence of the d12, d23, and d34 interlayer distances on the Ra factor values was calculated, but this time only the grazing polar angles, from 54° to 72°, were considered in the simulations and compared to the respective experimental points. The structural relaxation was performed for the Cr 2p3/2 XPD simulation by varying d12 from 0 Å (−100%) up to 1.73 Å (+84%), d23 from 0 Å (−100%) to 0.8 Å (+105%), and d34 from 0 Å (−100%) up to 1.73 Å (+84%). All variations were made in 0.05 Å steps and one distance at a time. As result, very similar equivalent interlayer distances and Ra values were achieved for both oxygen terminations. For the aBCbA case, the d34 relaxation resulted in a minimum of the Ra factor for d34 = 0.31 ± 0.05 Å (−63%). Figure 6 shows a 20455
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Figure 6. Contour map of the interlayer distances d12 and d23 in the aBCbA structure versus the corresponding Ra factor values. d34 was kept constant at its best value of 0.31 Å.
contour map of the interlayer distances d12 and d23 versus the corresponding Ra factor values for d34 = 0.31 ± 0.05 Å. The global minimum for the aBCbA structure is observed at d12 = 0.79 ± 0.05 Å (−15%) and d23 = 0.11 Å (−69%) with Ra = 0.118. For instance, similar procedure for the bABaB model lead to Ra = 0.126. Therefore, taking into account only the grazing polar angles, the contraction of d12 is confirmed, while for d23 and d34 behavior similar to that achieved previously was observed. On the basis of these results, our final model considered oxygen terminated surface structure with the interlayer distances d12, d23, and d34 obtained in the simulations for grazing polar angles and the d45 value obtained considering all polar angles (Table 1). The simulated XPD patterns for the aBCbA model using these final interlayer distances are plotted in Figure 3c and d for the Cr 2p and O 1s, respectively. Their corresponding Ra factors of 0.161 and 0.268 corroborated our conclusions. Additionally, Figure 7 shows comparisons between exper-
Figure 8. (a) The p(1 × 1) LEED pattern (62 eV) and (b) the Pd 3d XPS region of the Pd/Cr2O3/Ag(111) surface.
surface with the same symmetry as that observed for the oxide, showing that the Pd film was oriented along the [111] direction. The (√3 × √3)R30° surface structure was not noticed in this pattern, probably indicating that the Pd film covered most of the Cr2O3 surface. The Pd 3d XPS spectrum for the Pd/Cr2O3/Ag(111) (Figure 8b) displays the Pd 3d5/2 peak at 335 eV and the Pd 3d3/2 peak at 340.3 eV, which is in very good agreement with the expected values for metallic palladium (Pd0).55−57 Also observed in Figure 8b is the Pd 3d shakeup satellite peak (orange line), typical of Pd in the metallic state.58 The XPS spectrum of the Cr 2p region kept the same chemical component observed before the deposition of Pd. As expected, its intensity decreased since it is buried under the Pd layers. XPD patterns of the Cr 2p3/2 and Pd 3d5/2 XPS regions were collected. The kinetic energy of these photoelectrons is higher than 500 eV, and therefore, both XPD patterns were obtained in the forward scattering regime. Figure 9 presents the experimental XPD patterns obtained for (a) Pd 3d5/2 and (b) Cr 2p3/2 regions. Since XPD was not able to distinguish between two possible oxygen terminated Cr2O3 surface structures (aBCbA and bABaB), both were considered for Pd 3d5/2 and Cr 2p3/2 XPD patterns simulations. No preferable Pd/Cr2O3 interface was observed in all simulations, which allowed us to choose one termination (aBCbA) for the following steps. The proposed Pd structure is displayed in Figure 10a. Several papers concerning the atomic structure of Pd films prepared under different deposition conditions have been published. There are experimental and theoretical investigations reporting the possibility of Pd films formed with both fcc and hcp structures, depending on the substrate used as well as the growing conditions. An unconventional hcp Pd film with the lateral lattice parameter expanded is also predicted by DFT calculations to hold spontaneous ferromagnetic ordering.37 Therefore, different packing possibilities and crystalline structures (fcc and hcp) for the [111] direction were tested,
Figure 7. Experimental (open points) and simulated (gray lines) azimuthal scans (for θ = 24°, 48°, 54°, 60°, and 72°) for (a) Cr 2p3/2 (open green points) and (b) O 1s (red open points) photoelectrons emitted from the Cr2O3/Ag(111) surface.
imental and simulated azimuthal scans of the Cr2O3/Ag(111) surface (at selected polar angles), for both Cr 2p3/2 and O 1s photoelectrons. It shows excellent agreement for curves measured at high polar angles (more surface sensitive) as for those corresponding to low polar angles (bulk sensitive), confirmed by the corresponding low Ra factors. Pd/Cr2O3/Ag(111) System. A predominant p(1 × 1) LEED pattern was observed after the deposition of Pd on the Cr2O3/Ag(111) film (Figure 8a). It indicated a well ordered 20456
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Table 2. Structural Models Used to Simulate the XPD Patterns and Their Respective Ra Factor Values model 2 2 3 3 3 3 4 5 6
ML ML ML ML ML ML ML ML ML
structure
packing
fcc hcp hcp fcc hcp fcc fcc fcc fcc
AB AC ABA ABC ACA ACB ABCA ABCAB ABCABC
Ra factor Pd emitter Ra factor Cr emitter 0.353 0.348 0.473 0.331 0.473 0.325 0.304 0.377 0.387
0.446 0.459 0.540 0.489 0.566 0.496 0.420 0.439 0.416
compared to those with a different number of monolayers. The best Ra factor value for this system was found considering two domains rotated by 60°, as in the Cr2O3/Ag(111) case. Relaxation of the Debye temperature (TD), a nonstructural parameter, was allowed to improve accuracy of the simulations. As result, TD = 274 K was found, which is nearly the same as the TD of bulk Pd (111) (275 K). The TD used for Cr was 630 K.52 The high kinetic energy of the photoelectrons in our case provided the forward scattering in the crystallographic directions, and then the relaxation of TD was not effective in lowering the Ra factor value. Subsequently, during the simulation of the Pd 3d5/2 XPD pattern, the relaxation in the Pd interlayer spacing was allowed. The interlayer distances d12 and d23 were varied at the same time in the search procedure for the lower Ra factor. Figure 11 shows the contour map
Figure 9. Experimental XPD patterns for (a) Pd 3d5/2 (1146 eV) and (b) Cr 2p3/2 (905 eV) photoelectrons from the Pd/Cr2O3/Ag(111) system. The corresponding simulated (c) Pd 3d5/2 and (d) Cr 2p3/2 XPD patterns, and respective Ra factors.
Figure 10. (a) The proposed model for Pd/Cr2O3 and (b) unit cell of the fcc Pd with ABC packing in the [111] growth direction.
Figure 11. Results for Pd XPD interlayer relaxation of the d12 and d23 distances, considering the four monolayers of Pd, ABCA packing, and bulk values for the lateral distances.
where the bulk lattice parameter (a = 3.89 Å), interlayer distance (2.25 Å), and lateral distance between Pd atoms (2.75 Å) were fixed. Figure 10b shows the Pd film with the fcc structure, ABCA packing, and bulk distances proposed for the simulations. Table 2 shows the resulting Ra factors obtained for the proposed models considering Pd and Cr atoms as photoelectron emitters. On the basis of the Ra values presented, one may discard the hcp structure for the Pd atoms. The fcc structure with ABCA packing was selected for the next steps of simulations since the Ra values, for both Cr and Pd, were the lowest ones. Thereafter, we started a search for the correct number of Pd monolayers. The proposed model of four monolayers presented a significant drop in the Ra factor
produced in the search for the best Ra factor of the d12 and d23 distances. A minimum value was found for d12 = 2.23 ± 0.05 Å and d23 = 2.51 ± 0.05 Å corresponding to Ra = 0.239. These values represent a contraction of 0.9% for d12 and an expansion of 11.5% for d23, both compared to the bulk values. The d12 and d23 values determined in the search procedure were used for the simulation of the Cr 2p3/2 XPD pattern, considering the four monolayers of Pd in an ABCA packing. The simulation led to a drop in the Ra factor compared to the initial value obtained with bulk values (from 0.420 to 0.381). The same procedure was performed for the three-monolayer fcc Pd with packing ABC (Ra = 0.273) and ACB (Ra = 0.274), and four-monolayer hcp Pd with ABAB (Ra = 0.597) and ACAC (Ra 20457
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= 0.579). The resulting Ra factors were always higher than in the four-monolayer ABCA packing case. Structural relaxation of the d34 interlayer distance was also simulated, but the results did not show significant variations in the interlayer distances, so the bulk value was considered. The interlayer distances of the bulk Pd surface have been studied and reported,59,60 mostly from the theoretical point of view. For instance, Table 3 shows some reported values for the Pd(111) surface31,61,62 compared to the results presented here. Table 3. Reported Interlayer Distance Values for the Pd[111] Surfacea interlayer distance [−] contraction/[+] expansion (%) relative to the bulk values (%)
a
ref
d12
Methfessel et al.31 Rodriguez et al.61 Trinble32 Ning et al.62 Wuan et al.33 this work
−0.1 −2.4 −3.2 −3.17 −0.32 −0.9
d23
Figure 12. Ra factor versus the four-monolayer-thick Pd coverage on Cr2O3. During these simulations, the best values (relaxed) found for the Pd interlayer and lateral distances were used.
1.3 0.27 −0.02 11.5
experimental XPD patterns. The concordance is evidenced by Ra = 0.237 for Pd, while Ra = 0.348 was achieved for the Cr atom. The latter suggests a medium agreement between experimental and simulated patterns that may be induced by the low signal intensity due to attenuation of the Cr 2p3/2 photoelectrons caused by the top Pd film. As shown above, an Ra factor of around 0.2 was obtained without Pd coverage in previous results for the Cr2O3/Ag(111) system. Others studies using HREELS and FTIR and aiming at the elucidation of the Pd morphology on Cr2O3 showed that one monolayer of Pd deposited on the substrate at 90 K produces two-dimensional islands covering the entire oxide. When the Pd was deposited at 300 K, the islands were three-dimensional but with a low aspect ratio.14 There are also reports on island formation for Pd deposited on oxide surfaces with the corundum structure.14,63−66 Future experiments are planned to extend this investigation for different Pd thicknesses in order to shed light on the Pd growth mechanism on Cr2O3.
The percentages were calculated with respect to the bulk values.
The contraction of d12 observed in the present work has been previously reported.32 The interlayer distance d23 indicates an expansion of 11.5%, which is considerably larger than the values reported in the literature,33,61 but those cases did not refer to a few monolayers of Pd on a metal oxide. In the following step, relaxation of the lateral distances of the Pd atoms deposited on Cr2O3 was allowed during simulations, again for four monolayers of Pd and ABCA packing. The structural relaxation was performed for Pd atoms located at the first, second, and third outermost monolayers. The best Ra factor (0.237) was achieved for a distance of 2.83 ± 0.05 Å in the first monolayer, 2.84 ± 0.05 Å for the second monolayer, and 2.85 ± 0.05 Å for the third monolayer. The bulk value for the lateral distance of Pd atoms at the (111) surface is 2.75 Å, and these results suggest expansions of around 3.0%, 3.3%, and 3.6% for the first, second, and third monolayers, respectively. The results reported here show an expansion of 3.3% in the lattice parameter. The expansion of the lateral distance is caused by the epitaxial Pd deposition on Cr2O3. The lateral distance O−O in Cr2O3 (0001) is 2.86 Å, and the atoms of the Pd film at the interface assume nearly the same value. Finally, we performed a series of Cr 2p3/2 XPD pattern simulations aiming to estimate the coverage extent of the 4 ML Pd on the Cr2O3 surface. For this purpose, a linear combination of a Pd/Cr2O3/Ag system (considering 100% coverage of a 4 ML Pd film) and a Cr2O3/Ag system (0% Pd coverage) was used. In this case XPD patterns for the Cr 2p3/2 photoelectrons were calculated. Figure 12 shows the Ra factor as a function of the 4 ML Pd coverage. A minimum of the Ra factor is achieved only with 100% coverage. This result indicates the formation of a continuous 4 ML Pd film over the Cr2O3 surface. The possibility of nonuniform Pd thickness cannot be discarded; however, the XPD simulations of the model proposed here provided a surface structure in a satisfactory and simplified way. Figure 9c and d shows the best simulated XPD patterns for the Pd 3d5/2 and Cr 2p3/2 regions, respectively, considering the optimized structural parameters obtained in this work. The arrangement of diffraction spots shows similarity with the
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CONCLUSIONS A detailed in situ XPD investigation of the surface structure of Cr2O3/Ag(111) and Pd/Cr2O3/Ag(111) systems was presented. The XPD simulations presented here lead to Ra factor values as low as 0.161 for the Cr 2p3/2 pattern of the Cr2O3/ Ag(111) surface and 0.237 for the Pd 3d5/2 pattern obtained from Pd/Cr2O3/Ag(111). Those values imply a good agreement between simulated and experimental results. XPD simulations for the Cr2O3/Ag(111) system pointed to a structural model with oxygen termination (aBCbA and/or bABaB) and domains rotated by 60°. The interlayer distances of the outermost layers present significant contraction in the [0001] direction. The surface structural model proposed in this work is corroborated by theoretical predictions.67 To the best of our knowledge, there is no direct experimental observation and structural characterization of oxygen terminated Cr2O3 surface. The experimental exploration of such termination has been the subject of intense research in the past years due to their promising applications.67−72 For instance, DFT calculations showed that the oxygen terminated surface structure of Cr2O3 is particularly protective in a hydrochloric acid solution, which is of interest in corrosion studies. Cline et al. suggested that the investigation of oxygen terminated Cr2O3 surfaces 20458
dx.doi.org/10.1021/jp506507e | J. Phys. Chem. C 2014, 118, 20452−20460
The Journal of Physical Chemistry C
Article
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could provide a path for insight into novel magnetochemical effects and their implications for catalysis.73 The Pd/Cr2O3/Ag(111) system indicated that four monolayers of Pd with an fcc structure, and ABCA packing was epitaxially deposited on Cr2O3 as a continuous film. The Pd interlayer distances d12 and d23, as well as the lateral Pd−Pd distance on the first, second, and third monolayers, displayed significant relaxations compared to the bulk values. The ability to control the surface structure raises the chances of tailoring the physical properties of materials. The results reported here show an expansion of 3.3% in the lattice parameter of the Pd film, and further investigations could lead to even finer control of the film structure, which makes this system a model case to study the theoretical predictions on magnetic ordering of Pd.
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ASSOCIATED CONTENT
S Supporting Information *
XPS survey spectra of the Ag(111) substrate, Cr2O3/Ag(111), and Pd/Cr2O3/Ag(111) systems; the O 1s XPS spectrum of the Cr2O3/Ag(111) system; linear fit for the Cr2O3/Ag(111) ARXPS analysis; Ra values for the Pd XPD pattern simulations as a function of the number of Pd monolayers; simulated XPD patterns, for both Pd 3d5/2 and Cr 2p3/2 photoelectrons using the bulk values for the interlayer distances, and considering different structural models of the Pd film; and results obtained during relaxation of the lateral distances of the Pd atoms in the first, second, and third monolayers, considering four monolayers and ABCA packing. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the LNLS staff for their support. A.S. Kilian thanks CNPq for his Ph.D. fellowship. This work was funded by CNPq, CAPES, FAPERGS, and FAPESP.
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REFERENCES
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