Characterization of Ultra-thin Films of Pd Deposited on Au(111) and Au Deposited on Pd(111)

Transcription

1 Characterization of Ultra-thin Films of Pd Deposited on Au(111) and Au Deposited on Pd(111) Alexandre Pancotti 1, Marcelo F. Carazzolle 1, Luis H. de Lima 1, Denise A. Tallarico 2, Abner de Siervo 1,3, Pedro A. P. Nascente 2, Richard Landers 1,3, George G. Kleiman 1 1 UNICAMP IFGW - DFA, Campinas, SP, Brazil 2 UFSCar DEMa, São Carlos, SP, Brazil 3 LNLS, Campinas, SP, Brazil

2 Summary - Introduction - Experimental Methods - Theoretical Methods - Results and Discussion - Conclusions

3 Acronyms XPS: X-ray photoelectron spectroscopy AES: Auger electron spectroscopy XAES: X-ray excited Auger electron spectroscopy LEED: low-energy electron diffraction XPD: X-ray photoelectron diffraction UPS: ultraviolet photoelectron spectroscopy LEISS: low-energy ion-scattering spectroscopy

4 Introduction Bimetallic surfaces present interesting catalytic, electronic, electrochemical, and magnetic properties [1-3]. The deposition of an ultra-thin metal film on a single crystal metal substrate can produce a bimetallic surface. The films constituted by transition and noble metals present particular interest in heterogeneous catalysis, since bimetallic surfaces have shown an enhanced activity for catalytic reaction as compared to pure metals [2]. [1] J.A. Rodrigues, Surf. Sci. Rep. 23, 223 (1996). [2] H. Dreyssé, C. Demangeat, Surf. Sci. Rep. 28, 65 (1997). [3] J.G. Chen, C.A. Menning, M.B. Zellner, Surf. Sci. Rep. 63, 201 (2008).

5 Gold and palladium are completely miscible in all proportions and there is only a slight lattice mismatch (less than 5% ) between the Pd(111) and Au(111) surfaces [4]. Pseudomorphic overlayers of Pd on Au(111) and Au on Pd(111) can be prepared [5]. No ordered LEED structures other than the substrate (1 x 1) pattern have been observed for Au overlayers deposited on Pd(111), and the lattice spacing was found to vary linearly with alloy composition, implying the formation of surface alloys [6]. [4] B.E. Koel, A. Sellidj, M.T. Paffett, Phys. Rev. B 46, 7846 (1992). [5] C.-W. Yi, D.W. Goodman, J. Phys. Chem. B 109, (2005). [6] Z. Li, F. Gao, Y. Wang, F. Calaza, L. Burkholder, W.T. Tysoe, Surf. Sci. 601, 1898 (2007).

6 Shen et al. [7] studied the growth mode and electronic structure of Pd overlayers on Au(111) by AES, LEED, and UPS, and proposed a layer-by-layer growth at room temperature. Koel and co-authors [4] studied the growth mode and surface chemistry of Pd ultra-thin films deposited on Au(111), characterizing the films by XAES, LEISS, XPS, and LEED. They described the growth mode at 150 K by an epitaxial layerby-layer mechanism for the first few layers. For substrate temperatures of 300 K and above, they observed surface alloy formation. [7] X.Y. Shen, D.J. Frankel, J.C. Hermanson, G.J. Lapeyre, R.J. Smith, Phys. Rev. B 32, 2120 (1985).

7 Lambert and co-authors [8] employed Automated Tensor LEED to determine the crystallographic structures formed during Pd deposition on Au(111). At 200 K, deposition of 0.2 monolayer Pd caused the lifting of the clean surface Au(111) reconstruction. Deposition of one monolayer at this temperature formed a disordered structure, which ordered on heating at 300 K to a (1 x 1) structure having a predominantly Pd top layer. Further deposition of Pd produced a disordered overlayer, which ordered on annealing at temperatures above 500 K to a 3x 3 R30 structure consisting of at least 2 layers of Pd 2 Au alloy. ( ) o [8] C.J. Baddeley, C.J. Barnes, A. Wander, R.M. Ormerod, D.A. King, R.M. Lambert, Surf. Sci. 314, 1 (1994).

8 Recently, there has been a considerable interest in Au-Pd systems. Goodman and co-authors [5] formed Au/Pd(111) alloys by depositing various amounts of Au and Pd onto a Mo(110) surface. The alloy formed a (111) structure on the Mo template upon heating above 800 K. Tysoe and co-authors [9] evaporated gold on Pd (111) and then heated to various temperatures in order to obtain a wide range of alloy compositions. [5] C.-W. Yi, D.W. Goodman, J. Phys. Chem. B 109, (2005). [9] Z. Li, O. Furlong, F. Calaza, L. Burkholder, H. C. Poon, D. Saldin, W. T. Tysoe, Surf. Sci. 602, 1084 (2008).

9 Tysoe and collaborators have reported on the adsorption and reaction of ethylene [10], acetic acid [11], and vinyl acetate [12] on Au/Pd(111) alloy surfaces. The formation of Au-Pd alloys had a considerable effect on the surface chemistry of the adsorbed molecules. [10] F. Calaza, F. Gao, Z. Li, W.T. Tysoe, Surf. Sci. 601, 714 (2007). [11] Z. Li, F. Calaza, F. Gao, W.T. Tysoe, Surf. Sci. 601, 1351 (2007). [12] F. Calaza, Z. Li, F. Gao, J. Boscoboinik, W.T. Tysoe, Surf. Sci. 602, 3523 (2008).

10 In this work, we employed XPS, LEED, and XPD generated by synchrotron radiation in order to investigate the growth, composition, and structure of two ultra-thin films of Pd deposited on Au(111), having thicknesses of the equivalent of one monolayer and approximately 3 monolayers, and an ultrathin film of Au deposited on Pd(111), having approximately 4 monolayers. We report on the surface structure of the Au(111) and Pd(111) substrates, the electronic structure of the thinner Pd film, and the atomic structure of the thicker Pd and Au films.

11 Experimental Methods The experiments were performed at the Brazilian Synchrotron Light Laboratory (LNLS) using the soft X-ray spectroscopy (SXS ev) and the spherical grating monochromator (SGM ev) beam lines, for Pd/Au(111) and Au/Pd(111), respectively, using a surface analysis system equipped with LEED optics, a high resolution electron analyzer (Omicron HA125HR with multi-detection) mounted in the plane of the storage ring, a differentially pumped argon ion sputter gun, a two axis sample manipulator, and a conventional Al Kα X-ray source [13]. The base pressure was kept at 2x10-10 Torr. [13] A. de Siervo, E.A. Soares, R. Landers, T.A. Fazan, J. Morais, G.G. Kleiman, Surf. Sci. 504, 215 (2002).

12 The X-ray photoelectron diffraction (XPD) experiment consists in varying the θ and φ angles in relation to the analyzer, and keeping the photon energy fixed.

13 We performed the angular XPD measurements by varying the azimuthal angle, in steps of 3 o over a range of 129 o, and generated the completed 360 o azimuth curve by using three-fold symmetry. The polar angle varied from 20 o to 60 o in steps of 5 o. For the Pd films deposited on Au(111), Pd 3p 1/2 (KE = 1280 ev) and Au 4p 3/2 (KE = 1293eV) peaks were used for the XPD measurements. Photon energy: 1840 ev. For the Au films deposited on Pd(111), Pd 3d 5/2 (KE = 165 ev) and Au 4f 7/2 (KE = 416 ev) peaks were used. Photon energy: 500 ev.

14 Theoretical Methods The theoretical simulations for the XPD patterns were performed by using a genetic algorithm (GA) modified multiple scattering calculation diffraction (MSCD) code, including an average T-matrix approximation (ATA) subroutine [14-16]. We used a linear combination of the intensities from surfaces completely covered with the adsorbate and surfaces free of the adsorbate. The comparison between the theoretical simulation and the experimental data was done by R-factor analysis. [14] Y. Chen, M.A. Van Hove, MSCD Multiple Scattering Calculation Diffraction Package, [15] E.A. Soares, A. de Siervo, R. Landers, G.G. Kleiman, Surf. Sci. 497, 205 (2002). [16] M.L. Viana, R. Diez Muino, E.A. Soares, M.A. Van Hove, V.E. de Carvalho, J. Phys. Condens. Matter 19, (2007).

15 Spectrometer installed at the LNLS SXS line

16 Clean and ordered Au(111) surface Several cycles of sputtering and annealing. Argon ion sputtering: 800 ev, 25 ma, 15 mpa, 10 minutes. Annealing: 1000 V, 20 ma, 1,94 A, 3 minutes, 750 C. Cleanliness was verified by XPS and ordering, by LEED. Intensity (a.u.) Au(111) clean h" = 1840 ev Au 4f 5/2 HW 0.134! = 0,009 Instrumental + foton = 1,15 ev Au 4f 7/2 HW 0.149! = 0,009 4p 3/2 4d 3/2 4d 5/2 4f s 4p 1/2 5p Binding Energy (ev)

17 (a) Experimental and simulated XPD azimuthal curves taken at different polar angles; (b) experimental and (c) simulated XPD patterns.

18 For simulating the Au(111) surface, we used a parabolic cluster with a radius of 7.5 Å and a depth of 23 Å having 163 atoms organized in FCC packing (ABCABC ) with a lattice parameter of 4.08 Å, and the interlayer distances obtained by the genetic algorithm were: d 12 = 2.4 ± 0.1 Å d 23 = 2.5 ± 0.1 Å d 34 = 2.7 ± 0.1 Å d 45 = 2.4 ± 0.1 Å Even tough the Au(111) surface had a reconstruction, the employed 1840 ev photon beam was not sensitive enough to detect this kind of reconstruction. ( 22x 3)

19 One monolayer of Pd on Au(111) Pd 3d 3/2 and Au 4p 3/2 peaks were used for monitoring the film growth. An equivalent of one monolayer was formed after evaporating Pd for 30 minutes on Au(111) at room temperature. 1/ln(1+I Pd 3d /IAu 4p ) ML of Pd Thin Film Linear Regression for polarscanau_f: Y = A + B * X Parameter Value Error 3 A B Cos(!)(rad)

20 Clean Au(111) and 1 ML Pd film on Au(111) h! = 1840 ev 4f Intensity(a.u.) Au(111) clean Pd on Au(111) Pd on Au(111) Au(111) clean Pd 4p 1/2 4d 3/2 4p 3/2 4d 5/2 5p Binding Energy (ev)

21 LEED patterns for the clean and ordered Au(111) substrate and 1 ML Pd film on Au(111) 1 ML Pd film deposited on Au(111) presented a not so well ordered (1 x 1) LEED pattern. Annealing at 450 C for 20 minutes enhanced the pattern.

22 Au 4d and Pd 3d photoemission spectra Annealing at 450 C for 20 minutes caused diffusion of Pd into the Au bulk.

23 Au 4d and Pd 3d (left) and Au 4p and Pd 3p (right) photoemission spectra Intensity Normalized h" = 1840 ev Au 4d 3/2 Pd 3d 5/2 Au 4d 5/2!E = 0,2 ev # = 30 o 450 o C Evap Clean Intensity Normalized h# = 1840 ev Pd 3p 1/2 "E = 0,2 ev Au 4p 3/2! = 30 o Pd 3p 3/2 450 o C Evap Clean Binding Energy (ev) Binding Energy (ev) Annealing at 450 o C caused a slight chemical shift in the Au 4d 5/2 peak, but not in the Pd 3d 3/2 peak, suggesting charge redistribution in the Au atoms. Negligible charge transfer was reported for the Au-Pd system [17]. [17] P.A.P. Nascente, S.G.C. de Castro, R. Landers, G.G. Kleiman, Phys. Rev. B 43, 4659 (1991).

24 Pd MVV Auger spectra for Pd(111) and Pd films on Au(111) For the 1 ML Pd film, the shape of the Auger spectrum indicates that part of the deposited Pd atoms is interacting with the Au atoms, suggesting a diffusion of Pd atoms into the Au substrate. The shoulder in evidence at the spectrum for the 3 ML Pd film, which also appears at the spectrum of pure Pd, indicates lesser Pd diffusion into the Au substrate for the deposition of the thicker film.

25 Three monolayers of Pd on Au(111) Pd 3d 3/2 and Au 4p 3/2 peaks were used for monitoring the film growth. 1/ln(1+I Pd 3d /IAu 4p ) ML of Pd Thick Film Au 4p 3/2 Pd 3p 1/2 Pd 3p 3/2 Y = A + B * X Parameter Value Error A E B Cos(!)(rad)

26 Due to the overlap of the Pd 3d 5/2 and Au 4d 5/2 peaks (KE = 1505 ev), we used the Pd 3p 1/2 (KE = 1280 ev) and Au 4p 3/2 (KE = 1293 ev) peaks for the XPD measurements. Au 4p 3/2 Shirley background Pd 3p 1/2 Pd 3p 3/2

27 XPD patterns excited with hν = 1840 ev. It is possible to obtain the substrate pattern since the mean free path of the Au 4p 3/2 photoelectrons (16.9 Å) is higher than the Pd film thickness. Experimental E kin = 1280 ev Experimental E kin = 1293 ev Bare Au (111) Experimental E kin = 1293 ev Pd 3p 1/2 Au 4p 3/2 Au 4p 3/2

28 We initiated the simulation with a bulk-like Pd structure on Au(111), employing a parabolic cluster with a radius of 10 Å and a depth of 20 Å, having 12 Au(111) layers packed in a FCC fashion with up to an additional 4 Pd monolayers on top of the Au slab. Initially it was used the lattice parameter of the Au(111) substrate without relaxation, 4.07 Å, and the same lattice parameter for the Pd film.

29 The principal models are displayed bellow. Special attention is given to models 3, 6, and 12.

30 R-factors obtained for the 12 models, without considering relaxation of the atomic planes. Model 1 (Pd overlayer) 2 (Pd overlayer) 3 (Pd overlayer) 4 (Pd overlayer) 5 (Pd multilayer) 6 (Pd multilayer) 7 (Pd multilayer) 8 (Pd multilayer) 9 (Au segregation) 10 (Au segregation) 11 (Au segregation) 12 (Au segregation) R-factor (no relaxation)

31 Pd 3p 1/2 and Au 4p 3/2 XPD patterns excited with hν = 1840 ev for the 3 ML Pd film on Au(111). Experimental Theory Pd 3p 1/2 R a = 0.41 Au 4p 3/2 R a = 0.42

32 The exact number of Pd layers included in all XPD simulations was determined by varying the number of Pd multilayers on Au(111) and performing a R-factor analysis. The best result for the Pd 3p 1/2 emitter was obtained using the model number 3, which has three Pd layers. However, the minimum R-factor for the Au 4p 3/2 emitter was obtained for the model number 1, which indicates the presence of Au surface regions uncovered by Pd. After determining that the number of Pd multilayers was three, the principal interlayer distances were relaxed for all models.

33 R-factors obtained for the 12 models, considering relaxation of the atomic planes. Model 1 (Pd overlayer) 2 (Pd overlayer) 3 (Pd overlayer) 4 (Pd overlayer) 5 (Pd multilayer) 6 (Pd multilayer) 7 (Pd multilayer) 8 (Pd multilayer) 9 (Au Segregation) 10 (Au Segregation) 11 (Au Segregation) 12 (Au Segregation) R-factor (with relaxation)

34 Interlayer distances obtained for the 12 models, considering relaxation of the atomic planes. The distances for the bulk Pd and Au crystals are 2.25 and 2.36 Å, respectively. Interlayer distance (Å) d 12 Δd 12 = % % % % % % % % % d 23 Δd 23 = % % % % % % % % % d 34 Δd 34 = % % % % % % % % d 45 Δd 45 = % % % % % % % d 56 Δd 56 = % % % % %

35 In XPD, sometimes the R-factor analysis involving the theoretical simulations and experimental data alone in not sufficient for deciding what the best proposed structure is, because it can yield very similar (or equal) values. In order to evaluate what model better fits the experimental data, we analyzed the variations of the mean azimuthal R-factor for each polar angle for the Pd 3p 1/2 emission.

36 Variation of the mean azimuthal R-factor values as a funciton of the polar angle, for the Pd 3p 1/2 emission. Small θ values carry information from the bulk, large θ values, information from the surface.

37 Models 3, 6, 10, and 12 presented similar R-factor values, but models 10 and 12 produced more R-factor values bellow 0.4. These two models suggest that gold segregated to the surface. A similar behavior was observed by Koel and co-authors [4] by using LEISS to characterize the surface of an ultra-thin Pd film deposited on Au(111) at room temperature. [4] B.E. Koel, A. Sellidj, M.T. Paffett, Phys. Rev. B 46, 7846 (1992).

38 Comparison among the experimental (left) and simulated (right) Pd 3p1/2 and Au 4p3/2 XPD patterns considering a segregated Au layer on top of 2 ML Pd film on bulk Au(111).

39 Considering the possibility of having surface regions covered by one and two monolayers of gold, we performed a linear combination of the theoretical intensities of the Pd 3p 1/2 emission through portions of the surface covered by 1 and 2 ML Au layers, which yielded a minimum for R-factor at approximately 0.3. This indicates that approximately 30% of the surface was formed by 2 ML Au layers, and 70% of the surface, by 1 ML Au layers.

40 The comparison between theoretical and experimental Pd 3p 1/2 XPD curves as a function of the azimuthal angle, for three polar angles, showed good agreement.! = 30 o Chi exp Chi MSCD o Chi exp! = 50 Chi MSCD Chi exp Chi MSCD! = 60 o Azimuthal angle (degrees)

41 Ultra-thin film of Au deposited on Pd(111) Photoemission spectra exited by synchrotron radiation (1 x 1) LEED pattern (1x1)

42 The film thickness was evaluated by ARXPS measurements

43 The comparison between experimental and theoretical XPD results yielded a lattice parameter for the Au film of 4.08 Å (bulk value: 4.07 Å) The interlayer distances were obtained by the genetic algorithm, yielding: d 12 = Å (-6.8%) d 23 = Å (-7.0%) d 34 = Å (+12.2%) d 45 = Å (-9.0%) d bulk, Au = Å

44 Pd 3dp 5/2 and Au 4f 7/2 XPD patterns excited with hν = 500 ev for the 4 ML Au film on Pd(111).

45 Conclusions 1 ML Pd film on Au(111): annealing at 450 o C enhanced the (1 x 1) LEED pattern, but caused the diffusion of Pd into the Au substrate, and also caused a slight chemical shift in the Au 4d 5/2 photoelectron peak, but not in the Pd 3d 3/2 peak, suggesting charge redistribution in the Au atoms. 3 ML Pd film on Au(111): the comparison between experimental and theoretical XPD results indicated that approximately 30% of the surface was formed by 2 ML Au layers, and 70% of the surface, by 1 ML Au layers.

46 4 ML Au film on Pd(111): the full analysis is still under development. Preliminary results indicate that the Au film grew in a Volmer-Weber mode at room temperature, i.e. the deposition of Au on Pd(111) formed three dimensional islands, and that the Au lattice parameter increased to 4.08 Å.

47 Acknowledgements The authors would like to thank Ana C. F. Felippi for her assistance in part of the experiments. Work supported by FAPESP, CAPES, CNPq, (Brazilian funding agencies) and LNLS (SXS projects #5759 and 7024).