ARTICLE pubs.acs.org/JPCC
Interplay between Layer-Resolved Chemical Composition and Electronic Structure in a Sn/Pt(110) Surface Alloy Stefano Agnoli,*,† Giovanni Barcaro,‡ Andrea Barolo,† Alessandro Fortunelli,‡ Mauro Sambi,† Francesco Sedona,† Marco Di Marino,† Tomas Skala,§ and Gaetano Granozzi† †
Dipartimento di Scienze Chimiche and INSTM Unit, University of Padova, 35131 Padova, Italy CNR-IPCF, Istituto per i Processi Chimico-Fisici, 56124 Pisa, Italy § Sincrotrone Trieste, Strada Statale 14, km 163.5, I-34149 Basovizza-Trieste, Italy ‡
ABSTRACT: PtSn alloys play an important role as oxygen reduction reaction (ORR) catalysts in fuel cell technology both for their role in rationalizing the mechanism responsible for the better performances of bimetallic catalysts and for their practical applications. Here we present a complete experimental and theoretical study on the geometric and electronic structure of a (41) termination over a Pt3Sn near-surface ordered alloy obtained by depositing Sn on the Pt(110) and subsequent annealing. LEED and STM measurements combined with DFT simulations allow us to determine the atomistic structure of this phase, and to rationalize the peculiar dependence of its STM pattern on the applied bias and the compositional order in deep layers. Both photoemission experiments and density functional calculations indicate that in this phase the SnPt alloying process determines important changes in the electronic properties, and in particular a relevant shift of the Pt 5d band centroid with respect to clean Pt.
’ INTRODUCTION Within the present energy landscape, fuel cells (FCs) are playing a very important role, which is supposed to greatly expand in the near future together with the development of the new hydrogen economy. However, FC technology still suffers of important limitations, e.g., fast degradation of active materials over time, scarce efficiency, and high costs, to which basic research is trying to give an answer.1 Pt is the most used and widespread material used in FC as catalyst, either at the cathode for the oxygen reduction reaction (ORR)2 or at the anode for the oxidation of hydrogen or alcohols; however, its steadily increasing cost and limitations (durability, poisoning, slow kinetics etc) are pushing toward the development of alternative solutions.35 In this respect, one among the several possible approaches consists in alloying Pt with other metals to reduce the costs and to induce new functionalities; actually, Pt based catalysts are still the ones with the best performances so far.68 Among these, SnPt alloys are playing an important role not only because they have practical applications9 but also because they allowed researchers to unravel the mechanism at the origin of the higher performances of bimetallic catalysts.6,7,1012 In the present paper, we report the preparation and full characterization of a novel (41) SnPt surface alloy obtained on the Pt(110) surface, a termination scarcely investigated so far, where quite high reactivity toward CO and ethanol oxidation13 and ORR14 have been documented. Using scanning tunneling microscopy (STM), high resolution photoemission spectroscopy from core and valence levels (XPS), low energy electron diffraction (LEED), and density functional theory (DFT) calculations, r 2011 American Chemical Society
we were able to demonstrate the formation of a new surface alloy characterized by a very peculiar atomic structure and quite interesting electronic properties. Moreover, we investigated the stability of these Sn overlayers, evidencing surface segregation and diffusion phenomena, which are the key factors for the design of a stable and durable real catalyst.
’ EXPERIMENTAL AND THEORETICAL METHODS The STM experiments were performed by using an Omicron variable temperature (VT) STM system custom-equipped with a monochromatized Gammadata Scienta SAX-100/XM-780 Al KR X-ray source and a SES-100 electron spectrometer for high resolution XPS. The instrument consists of an UHV preparation chamber with a base pressure of 2 1010 mbar containing equipment for sample sputtering, thermal annealing, Sn evaporation, precise gas dosing, XPS, and LEED, connected to a chamber equipped with the STM stage. The photoemission data were taken at the Materials Science beamline at Elettra synchrotron, whose experimental chamber is equipped with a high luminosity electron energy analyzer (Specs Phoibos, 150 mm mean radius, with 9 channels), an angle resolving electron energy analyzer (50 mm mean radius, single channel), a fast entry, LEED, sputter gun, gas inlet, mass spectrometer, and sample heating and cooling. In the conditions typically used for acquiring the photoemission data (hν = 125 eV), the photon flux on the Received: April 12, 2011 Revised: June 20, 2011 Published: June 20, 2011 14264
dx.doi.org/10.1021/jp2034278 | J. Phys. Chem. C 2011, 115, 14264–14269
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Photoemission spectra of the Sn(2MLE)/Pt(110) deposit taken with a photon of 125 eV and normal emission (if not differently indicated): (a) Pt 4d, (b) valence band (VB), and (c) Pt 4f7/2 regions. The (41)-Sn/Pt(110) surface alloy is formed starting from 350 °C (see text).
sample was 3 1010 photons/s at a resolving power of 4000. For all the experiments, the (12)-Pt(110) surface termination was obtained by cycles of Ar+ sputtering and annealing at 700 °C, followed by a short flash in oxygen to eliminate carbon contamination, as determined by LEED, and by STM measurements, showing the characteristic fish scale morphology.22 The Sn (Mateck 99.99%) overlayers were deposited by means of a Knudsen cell working at a temperature of 900 °C in UHV conditions. STM pictures were acquired in constant current mode at room temperature (RT) using PtIr chemically etched tips. For the photoemission experiments the same procedure was followed except for the usage of an e-beam evaporator for Sn deposition. The amount of Sn evaporated was determined either directly by STM measurements or by using angle resolved photoemission; the Sn monolayer equivalent (MLE) is defined as 1 Sn atom per Pt(110) surface unit cell, i.e., 1.8 1015 atoms/ cm2. The photoemission data (i.e., Pt 4f, Sn 4d, and VB spectra) were acquired at RT either in normal or in grazing emission (30° form surface) using photon energy values of 120 and 60 eV. Spin polarized DF calculations have been carried out using the QuantumEspresso computational code using ultrasoft pseudopotentials15 and the PW91 exchangecorrelation functional.16
Cutoff energies of 40 and 400 Ry were used as the energy cutoffs for the selection of the plane waves basis set for the description of the wave function and electron density, respectively. At least 10 Å of empty space between replicated cells along z was left. A (1,3,1) k-point grid, sufficiently accurate for this unit cell, has been employed for the sampling of the first Brillouin zone in the reciprocal space. A Gaussian smearing technique with a broadening of the one-particle levels of 0.03 eV was applied. A rectangular unit cell containing 46 atoms and measuring 11.3 Å (four times the DF-equilibrium first-neighbor distance between Pt atoms in the bulk, 2.82 Å) along the x direction and 4.00 Å (corresponding to the first-neighbor distance multiplied by square root of two) along the y direction has been used to model the p(41) superstructure. Along the z direction, the slab was made by 13 (110) layers. The central 5 layers were fixed at the lattice sites of an fcc bulk structure; the other 8 (4 on each side) were left free to relax according to the symmetry constraint of a mirror plane of symmetry at z = 0. The symmetry of the unit cell perpendicularly to the surface ensures the cancellation of unphysical dipole moments interacting along the z direction. STM images were computed at a distance between 2.0 and 2.5 Å above the topmost atomic layer using the TersoffHamann approach;17 14265
dx.doi.org/10.1021/jp2034278 |J. Phys. Chem. C 2011, 115, 14264–14269
The Journal of Physical Chemistry C
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Figure 2. LEED pattern (Ek = 103 eV) of (a) the (12)-Pt(110) clean surface and (b) the Sn(2MLE)/Pt(110) deposit after annealing at 350 °C. The substrate ((41) superstructure) unit cell is outlined in green (red).
values of 104 to 105 au for the isosurface contours were used in all cases.
’ RESULTS AND DISCUSSION The deposition of Sn on Pt(110) has been the object of a previous investigation:18 the authors reported that in the submonolayer regime (i.e., up to 0.7 monolayer, ML), the disruption of the missing row reconstruction of the (12)-Pt(110) surface occurs and, concurrently, either Sn ad-islands or, locally, a Pt3Sn(110) (22) superstructure are formed. Moreover, they documented that after annealing at 600 °C, most of the Sn atoms diffuse into the bulk and the morphology of the surface becomes extremely rough and defective. In the present work, 2 MLE of Sn were deposited on (12)Pt(110) and the evolution of the film as a function of temperature was followed by means of XPS, LEED, and STM. Figure 1 summarizes the XPS data taken with a photon of 125 eV at the Elettra synchrotron radiation facility (Trieste, Italy): the complex shape of the Sn 4d photoemission line demonstrates the presence of Sn atoms with different chemical states and undergoes remarkable changes as a function of temperature. A fitting procedure based on Voigt doublets (spin orbit splitting of 1.04 eV14) was used to identify individual chemically shifted components. The RT spectrum can be interpreted as the overlap of two distinct components: the higher binding energy (BE) doublet can be ascribed to Sn atoms at the interface with Pt (b component in Figure 1, with the 4d5/2 maximum centered at 24.30 eV), while the lower BE one is connected to the formation of 3D Sn islands (a component, 4d5/2 maximum at 23.96 eV). This assignment derives from the comparison of the spectra acquired at different angles: in normal emission the b peak has a much higher relative intensity with respect to the same spectrum taken in a grazing geometry (see Figure 1). As a consequence of annealing, the Sn 4d spectrum shape is modified, forming three different components. It can be seen that these changes take place between 200 and 350 °C, while no further difference can be observed for annealing at higher temperature, meaning that the surface has reached a stable structural configuration. In the same temperature range, we observed the formation of a sharp (41) LEED pattern (Figure 2). After annealing at 350 °C, the Sn 4d photoemission
line becomes very complex but nonetheless can be still interpreted as the sum of three different peaks. The most intense feature is the doublet labeled c in Figure 1 (4d5/2 maximum centered at 24.8 eV), while the b component, which was the most intense at RT, is now the weakest, and a rather intense peak a0 is formed at lower BE (4d5/2 maximum centered at 23.76 eV). The comparison between the spectra taken at different photon energies (i.e., with different probing depth; see Figure 1a where 400 °C data taken at different photon energies are compared) indicates that the c component comes from the near-surface region of the sample, while the a0 and b components are pure surface components. Therefore, we assign peak c to Sn atoms alloyed with Pt and forming a near-surface alloy, while the a0 and b components can be ascribed to two types of Sn atoms in a surface alloy, the difference in the BE being due to a different Pt coordination or structural environment. It is worth mentioning that, while the intensity of the c component depends on the amount of Sn deposited on the surface, the intensity ratio between the a0 and b components is always 2. Figure 1b reports the valence band (VB) data of the bare Pt substrate and of the Sn(2 MLE)/Pt(110) deposit prior to and after the annealing at 350 °C, taken at normal emission with a photon energy of 125 eV. After the deposition of Sn metal at RT there is a relevant shift of the spectral intensity toward higher BE with respect to the Pt spectrum. The complex structure of the Pt 5d band is replaced by two broad peaks centered at 4.5 and 3.1 eV, likely Sn 5p orbitals, while the intensity is strongly decreased in the region just below the Fermi level. After the annealing, the VB data radically change and the previously observed peaks shift toward opposite directions, originating two well separated maxima at 5.1 and 2.4 eV, respectively. The overall effect of the annealing is to push the centroid of the d band back to higher BE, but lower than bare Pt(110). According to a recently proposed procedure,19 the same information can be obtained by examining the Pt 4f spectra (shown in Figure 1c): as a consequence of Sn deposition, the Pt 4f7/2 bulk component shifts from 71.0 to 71.6 eV and, after the annealing at 350 °C, it moves to 71.2 eV. Sn deposition has been monitored also by STM (Figure 3).20 In the low coverage regime (
