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Pseudomorphic growth of Pd monolayer on Au(111) electrode surface
Surface Science 461 (2000) 213–218 www.elsevier.nl/locate/susc
Pseudomorphic growth of Pd monolayer on Au(111) electrode surface M. Takahasi a, *, Y. Hayashi a, J. Mizuki a, K. Tamura b, T. Kondo b, H. Naohara b, K. Uosaki b a Department of Synchrotron Radiation Research, Japan Atomic Energy Research Institute, Mikazuki-cho, Hyogo 679-5148, Japan b Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan Received 24 January 2000; accepted for publication 12 May 2000
Abstract The surface structure of a Pd layer electrochemically deposited onto Au(111) from PdCl2− solution has been 4 studied by surface X-ray diffraction. By combining the measurements of the specular and non-specular rod profiles, both in-plane and out-of-plane structures have been determined. On the Au(111) electrode surface, Pd occupies the cubic closest packing site and forms a pseudomorphic smooth monolayer. The formation of the stable Pd overlayer is explained by the lifting of the surface reconstruction of the Au substrate. The PdMAu bond length at the ˚ , which is close to the sum of the atomic radii of Pd and Au in Pd/Au(111) interface has been found to be 2.82 A each bulk. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical methods; Gold; Growth; Metal–electrolyte interfaces; Palladium; X-ray scattering, diffraction, and reflection
1. Introduction Palladium is one of the most important metals for industrial applications because of its high catalytic activities for various chemical reactions in vacuum and in solution. It has been shown that ultrathin Pd films grown on foreign metals have altered catalytic properties, compared with thick films or bulk [1,2]. For understanding the origin of these altered properties, studies on Pd thin films with well-defined structure are important. Palladium films deposited onto Au substrates are suitable for this purpose, because uniform and epitaxial growth is expected on account of only 5% lattice mismatch between Pd and Au. * Corresponding author. Fax: +81-791-58-2740. E-mail address: [email protected] (M. Takahasi)
One technique for the fabrication of such thin films is the vapor deposition in ultrahigh vacuum ( UHV ). On the basis of results given by a combination of Auger electron spectroscopy (AES), lowenergy ion scattering (LEIS ), X-ray photoemission spectroscopy ( XPS) and low-energy electron diffraction (LEED), Koel et al. [3] have reported that the in-vacuum growth process of Pd on Au(111) depends strongly on the substrate temperature. At a substrate temperature of 150 K, a pure Pd overlayer is formed on the Au(111) surface, as suggested by the evolution of AES signals. When the growth temperature is raised to 500 K, LEIS data indicative of intermixing of Pd and Au are obtained. The Pd–Au alloy formation has been detected even at the room temperature (RT ). The intermixed layer is composed of a Pd-rich alloy
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whose surface shows well-ordered (E3×E3)R30° periodicity [4]. The initial growth of Pd on Au(111) below the alloying temperature has been revealed by a recent scanning tunneling microscopy (STM ) study [5]. In the submonolayer region Pd ˚ rather than a forms islands as large as 30 A uniform monolayer. In subsequent deposition the growth of the second Pd layer commences before completion of the first layer, resulting in a Pd layer with a rough surface. Another approach to the construction of thin Pd films is the electrochemical deposition from a solution containing PdCl2−. Current–potential 4 curve studies on Cu underpotential deposition ( UPD) on Pd–Au [2,6 ] and a conventional X-ray diffraction study [7] have shown that the Pd layers electrochemically deposited on Au(111), Au(001) and Au(110) grow with the surface crystallographic orientation matching that of the substrate. Furthermore, recent STM studies have revealed that Pd on Au(111) and Au(001) grows in layerby-layer mode for several layers [6–8]. However, STM is sensitive only to the outermost surface and is incapable of identifying the atomic species, so that the crystallographic relationship between the Pd overlayer and the substrate has remained unresolved. In the present work we applied surface X-ray diffraction (SXD) to the structure analysis of Pd monolayers electrochemically deposited onto an Au(111) surface. Recently, SXD has proved to be well suited for the structural study of solid–liquid interfaces [9,10]. Our method is based on the measurements of diffracted intensities along the reciprocal rods normal to the surface. The intensity profiles of the reciprocal rods are modulated by the interference effect between the diffraction from the substrate regarded as an ideally terminated bulk crystal and the diffraction from the overlayer including foreign atomic species or atoms deviating from the ideal bulk position. Thus the positions of the deposited atoms with respect to the substrate can be determined from the analysis of the rod profiles. The specular rod measured in such a geometry that the scattering vector is aligned along the surface normal is the most sensitive to the structure in the out-of-plane direction, such as the
interlayer spacing between the surface monolayer and the first layer of the substrate. The information on in-plane adsorption sites is obtained from measurements along the non-specular rods that have lateral components. The combination of the measurements of the specular and non-specular rods has allowed us to confirm the pseudomorphic and two-dimensional growth of Pd on the Au(111) electrode.
2. Experimental A single Au crystal, which was shaped in a disk (10 mm in diameter and 5 mm in thickness), was used as the substrate. It was mechanically polished using 0.03 mm diamond slurry and electrochemically etched in a solution containing 0.1 M HClO and 5 mM NaCl [11,12]. After annealing 4 in an H flame and cooling slowly in air, the 2 crystal was transferred to an electrochemical cell specially designed for in situ SXD measurements. The body of the cell is made of polychlorotrifluoroethylene (PCTFE ). The cell is equipped with an inlet and an outlet for electrolyte and with electric feedthroughs for the Pt counter electrode and the Ag–AgCl reference electrode. It was sealed with a 6.0 mm thick Mylar film (Chemplex, Mylar 250), which served as an X-ray window. The electrochemical deposition of Pd was carried out in a solution containing 1 mM PdCl2− and 0.1 M 4 H SO , which was deaerated with N gas of 2 4 2 99.999% purity prior to each measurement. During the injection of the electrolyte solution, the potential applied to the sample was kept at +0.90 V (versus Ag/AgCl ), which is more positive than the UPD potential of Pd on the Au electrode. An outer chamber filled with flowing N gas was 2 attached to the topside of the cell to prevent oxygen from diffusing into the solution through the Mylar window. Electrochemical deposition of the Pd monolayer was carried out in this cell in such a way that the potential was first swept to the negative direction from +0.90 to +0.58 V, turned back to the positive direction and kept at +0.65 V. At this final potential, neither cathodic nor anodic current was observed. The amount of Pd deposited was deter-
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mined by the cathodic charge that flowed during a cycle of potential scan. In this paper, one monolayer (ML) is defined as 1.39×1015 cm−2 for the Au(111) surface. The solution between the electrode and the Mylar film was thicker than 5 mm during the deposition. After the deposition of Pd, the window was deflated so that an about 30 mm thick electrolyte layer remained on the sample surface. The SXD experiments were performed with the vertical-axis four-circle diffractometer installed on beamline 4C at the Photon Factory, Tsukuba, Japan. The X-rays were monochromatized by the Si(111) double crystal system and focused by a Pt-coated cylindrical bent mirror. The incident slit was set to 0.1 mm (vertical )×0.5 mm (horizontal ). The angular acceptance of the receiving slit was 3 mrad for the 2h direction and 30 mrad for the x ˚ was selected to direction. A wavelength of 1.283 A avoid the fluorescence from the Au substrate. All the measurements were carried out in such a mode that the incoming and outgoing angles with respect to the sample surface were equal. At each measuring point, the rocking scan of the v axis was performed for background subtraction and for the integration over the mosaic spread of the sample.
3. Results Fig. 1 shows the (a) specular and (b) nonspecular rod profiles for the Au(111) electrode onto which 1 ML Pd was deposited. For convenience, a measuring point in the reciprocal space Q is described in terms of the Miller indices (H, K, L) with the reciprocal basis a1=(2p/a ) 0 (2 , 2 , −4) , b1=(2p/a ) (−2 , 4 , −2) , c1= 0 3 3 3 cubic 3 3 3 cubic ], which corresponds to the hex(2p/a ) (1 , 1 , 1) 0 3 3 3 cubic agonal unit cell defined by a=a (1 , 0, −1) , 0 2 2 cubic , c=a (1, 1, 1) . b=a (−1 , 1 , 0) cubic 0 cubic 0 2 2 An important feature in the specular rod profile is the remarkable dips that are observed between two adjacent bulk Bragg peaks. Compared with the profile calculated for the ideally terminated Au(111) surface, which is represented by a dotted line in Fig. 1, the reflected intensity decreases by a factor of 10−2 around l=1.5 and l=4.5. These
Fig. 1. The specular (a) and the non-specular (b) rod profiles for 1 ML Pd/Au(111). The solid lines are the best fit curves based on a pseudomorphic growth model. The dotted line in the specular rod profile represents the profile for the ideally terminated Au(111) surface. The dashed and dot–dashed lines in the non-specular rod profile were calculated for the Pd adsorption to the hcp threefold hollow site and the atop site, respectively, with the same surface normal structure as the best fit model.
dips can be explained qualitatively by the formation of a uniform Pd layer on the Au(111) substrate. The scattering amplitude from the substrate is given by 1 f exactly at the middle of the two 2 Au Bragg points. When the topmost Au layer is replaced by a Pd monolayer, the scattering amplitude becomes 1 f −f +f =−1 f +f . 2 Au Au Pd 2 Au Pd Considering that the atomic form factor is approximately equal to the atomic number and that the
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atomic number of Pd is nearly half that of Au, it is easily known that the total scattering amplitude is depressed at the anti-Bragg condition. Hence these dips are strong evidence for the formation of an ordered Pd layer with respect to the surface normal direction. The quantitative surface normal structure was obtained from the least squares fitting with a kinematical calculation [13,14]. The calculation is based on a structure model consisting of the ideal Au substrate, a top Au layer, a Pd layer and a PdCl2− layer. For the latter three surface layers, 4 the surface normal positions z , the coverages r m m and the root mean square (RMS) of atomic displacements s , which includes the effects of static m corrugations as well as of thermal vibrations, can be the structural parameters to be fitted (m= 1, 2, 3). For simplicity, however, the coverages of the Au and Pd layers were fixed to unity and the RMS of the Au layer was assumed to be the ideal bulk value in the actual fitting procedure. As far as the specular rod is concerned, the in-plane positions (x , y ) (m=1, 2, 3) have no influence m m on the diffracted intensity. The best-fit structural parameters are listed in Table 1. A better fitting result was obtained by the model with a small amount of PdCl2− in the topmost layer than by 4 the model with a single layer of Pd. However, the coverage of the adsorbed ions is considerably smaller than that expected from the ordered PdCl2− layer that has been suggested by STM 4 observations [8,15]. Thus this fractional electron density might be caused by the excess of Pd or by
another kind of ion adsorbed onto the surface. Besides the structural parameters, the absorption correction originating from the solution, exp(−m/L), was also included in the fitting parameters. The resultant value was m=0.11, corresponding to a water layer of about 30 mm thickness. Provided that Au and Pd atoms have the same atomic radii as in each bulk, the evaluated interlayer spacing between the Au and Pd layers, ˚ , is consistent with the Pd adsorpz −z =2.27 A 2 1 tion onto the threefold hollow site. As shown in Fig. 2, however, there are two kinds of threefold hollow site with respect to the topmost Au atoms located at (2|a|, 1d|b|): one is the hexagonal closest 3 3 packing (hcp) site, i.e. (x , y )=(|a|, |b|), and the 2 2 other is the cubic closest packing (ccp) site, i.e. (x , y )=(1|a|, 2|b|). It is impossible to distinguish 2 2 3 3 between these two adsorption sites only by the specular rod profile analysis. In order to ascertain the in-plane structure, we carried out the measurement of the non-specular rod profile. In Fig. 1b, the observed profile of the (01) rod is compared with the profile calculated for the adsorption to the hcp, ccp and atop sites. It should be noted that no refinements of the
Table 1 Structural parameters obtained from the analysis of the specular rod profile of 1 ML Pd/Au(111). The atomic form factors f , Au f ,f and f are of Au, Pd, Pd2+ and Cl− respectively. Pd Pd2+ Cl− The third layer in the surface region is assumed to be composed of PdCl2− ions. The positions of the atomic layers are repre4 sented by the distance from the second Au layer, which is assumed to keep the ideal bulk position Atomic layer m
Atomic form factor f m
Position ˚) z (A m
Coverage r (ML) m
RMS ˚) s (A m
1 2 3
f Au f Pd f +4f Pd2+ Cl−
2.35 4.62 6.55
1.0a 1.0a 0.036
0.084a 0.080 0.2
a Values were fixed in the fitting procedure.
Fig. 2. Possible adsorption sites of Pd atoms on Au(111). Gray circles represent the Au atoms in the first layer and open circles represent the Au atoms in the second layer. The dashed parallelogram indicates the unit cell defined by the fundamental , b=a (−1 , 1 , 0) and c=a vectors a=a (1 , 0, −1) 0 2 2 cubic 0 0 2 2˚ cubic (1, 1, 1) (a =4.08 A ). Note that the hexagonal coordinate cubic 0 system is employed.
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structural parameters have been added to those obtained from the analysis of the specular rod, except for the small fraction of PdCl2− in the 4 topmost layer. Neglecting this contribution brought about a minor improvement in the fitting of the non-specular rod profile. This suggests that the additional topmost layer does not form an ordered structure with respect to the lateral direction. Adsorption onto the ccp hollow site gives the best fit, as shown by the solid line in Fig. 1. Other adsorption sites, such as the hcp hollow site (the dashed line) and the atop site (the dot–dashed line), fail to reproduce the observed profile. Hence it can be concluded that the Pd layer keeps the fcc stacking sequence of the Au substrate. Actually, this is intuitively apparent from the decrease in the intensity around l=3.5. For the same reason as in the case of the specular rod profile, the remarkable dip at the middle of the two Bragg peaks is caused by the destructive interference between the scatterings from the Au substrate and the ordered Pd layer. In the non-specular case, the ordering for the lateral direction is involved in the rod profile. Thus, the pseudomorphic growth of Pd is readily deduced from the combination of the specular and non-specular rod profiles.
4. Discussion The present SXD study has provided confirmation that the electrochemical deposition enables the construction of a pseudomorphic smooth Pd layer on Au(111). In our study, no signs of intermixing between Pd and Au has been observed; the destructive interference effect at the anti-Bragg condition would be indistinct if the topmost surface layer were composed of the mixture of Pd and Au. This is in contrast to the case in UHV deposition, where nucleation followed by threedimensional growth is observed at a low temperature and alloy formation takes place at the temperature higher than RT [3]. The difference in the growth modes between the electrochemical and UHV conditions is ascribable to the surface structure when Pd is deposited. In UHV, the Au(111) surface shows a reconstruction composed of linear discommensurations that sepa-
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rate regions with the normal ccp stacking sequence from regions with a faulted hcp stacking sequence. Owing to the symmetry of this surface, there are three equivalent directions along which the discommensurations are running. In the most stable phase, an ordered array of two out of the three equivalent domains forms the zig-zag pattern reported in the literature [16,17]. According to the STM observation [5], Pd islands seem to nucleate at the kinks between the discommensurations. Although the reconstruction is lifted by heating the substrate up to 900 K [17], Pd–Au alloy formation would be inevitable at this temperature. Therefore, a uniform pseudomorphic Pd layer cannot be obtained by the methods based on vacuum evaporation. Under electrochemical conditions, on the other hand, the surface reconstruction of Au(111) [18] is lifted at a potential of +0.35 V (versus Ag/AgCl ) in sulfuric acid, which was used as a supporting electrolyte in the present experiment. Since the Pd deposition occurs at +0.58 V, which is positive enough of the transition potential, Pd is allowed to be deposited onto the ideal 1×1 surface. The lattice distortion in the pseudomorphic Pd monolayer gives rise to a change in its electronic structure. It is likely that the altered electronic structure accounts for the unusual chemical properties of the Pd monolayer on Au(111). Although charge transfer, which is sometimes observed at the interface of dissimilar metals [19], may affect the chemical properties, structural deformation associated with the charge transfer has not been observed in the present system. From a calculation using the fitting parameters in Table 1, the bond length between Pd and Au atoms is evaluated as ˚ . This value is close to the average of the 2.82 A atomic radii of Au and Pd in each bulk. Furthermore, the interlayer spacing between the first and the second Au layers is not significantly changed from the ideal bulk value. Thus, there is no electron transfer between Pd and the Au substrate.
5. Conclusions The structure of a Pd monolayer electrochemically deposited onto Au(111) was analyzed on the
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basis of in situ SXD data. The Pd monolayer forms a flat pseudomorphic film, following the stacking sequence of the Au(111) substrate. Such a Pd film is available only under electrochemical conditions, where the surface reconstruction is lifted at the deposition potential. The chemical properties of the Pd monolayer on Au(111) are ascribed to the altered electronic structure induced by lattice distortion rather than to charge transfer.
Acknowledgements The authors would like to thank Dr Murakami and Mr Wakabayashi for their assistance at the Photon Factory. The present work was partly supported by a Grant-in-aid for Scientific Research on Priority Area of ‘Electrochemistry of Ordered Interfaces’ (no. 09237101) from the Ministry of Education, Science, Sports and Culture, Japan. The synchrotron radiation experiments were performed as projects approved by the Photon Factory Program Advisory Committee (proposal no. 98G091).
References [1] H. Naohora, S. Ye, K. Uosaki, in preparation. [2] M. Baldauf, D.M. Kolb, J. Phys. Chem. 100 (1996) 11 375.
[3] B.E. Koel, A. Sellidj, M.T. Paffet, Phys. Rev. B 46 (1992) 7846. [4] C.J. Baddeley, C.J. Barnes, A. Wander, R.M. Ormerod, D.A. King, R.M. Lambert, Surf. Sci. 314 (1994) 1. [5] C.J. Baddeley, R.M. Ormerod, A.W. Stephenson, R.M. Lambert, J. Phys. Chem. 99 (1995) 5146. [6 ] M. Baldauf, D.M. Kolb, Electrochim. Acta 38 (1993) 2145. [7] H. Naohara, S. Ye, K. Uosaki, J. Phys. Chem. B 102 (1998) 4366. [8] H. Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem. 473 (1999) 2. [9] A. Davenport, J.G. Gordon ( Eds.), Proceedings of the Symposium on X-ray Methods in Corrosion and Interfacial Electrochemistry, The Electrochemical Society, Pennington, 1992. [10] C.A. Melendres, A. Tadjeddine ( Eds.), Synchrotron Techniques in Interfacial Electrochemistry, Kluwer Academic, Dordrecht, 1994. [11] S. Ye, C. Ishibashi, K. Shimazu, K. Uosaki, J. Electrochem. Soc. 145 (1998) 1614. [12] S. Ye, C. Ishibashi, K. Uosaki, J. Electrochem. Soc. 145 (1998) 1614. [13] D. Gibbs, B.M. Ocko, D.M. Zehner, S.G.J. Mochrie, Phys. Rev. B 38 (1988) 7303. [14] R. Feidenhans’l, Surf. Sci. Rep. 10 (1989) 105. [15] L.A. Kibler, M. Klenert, R. Randler, D.M. Kolb, Surf. Sci. 443 (1999) 19. [16 ] J.V Barth, H. Brune, G. Ertl, R.J. Behm, Phys. Rev. B 42 (1990) 9307. [17] A.R. Sandy, S.G.J. Mochrie, D.M. Zehner, K.G. Huang, D. Gibbs, Phys. Rev. B 43 (1991) 4667. [18] J. Wang, B.M. Ocko, A.J. Davenport, H.S. Isaacs, Phys. Rev. B 46 (1992) 10 321. [19] J.X. Wang, N.S. Marinkovic´, R.R. Adzˇic´, B.M. Ocko, Surf. Sci. 398 (1998) L291.
Pseudomorphic growth of Pd monolayer on Au(111) electrode surface M. Takahasi a, *, Y. Hayashi a, J. Mizuki a, K. Tamura b, T. Kondo b, H. Naohara b, K. Uosaki b a Department of Synchrotron Radiation Research, Japan Atomic Energy Research Institute, Mikazuki-cho, Hyogo 679-5148, Japan b Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan Received 24 January 2000; accepted for publication 12 May 2000
Abstract The surface structure of a Pd layer electrochemically deposited onto Au(111) from PdCl2− solution has been 4 studied by surface X-ray diffraction. By combining the measurements of the specular and non-specular rod profiles, both in-plane and out-of-plane structures have been determined. On the Au(111) electrode surface, Pd occupies the cubic closest packing site and forms a pseudomorphic smooth monolayer. The formation of the stable Pd overlayer is explained by the lifting of the surface reconstruction of the Au substrate. The PdMAu bond length at the ˚ , which is close to the sum of the atomic radii of Pd and Au in Pd/Au(111) interface has been found to be 2.82 A each bulk. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical methods; Gold; Growth; Metal–electrolyte interfaces; Palladium; X-ray scattering, diffraction, and reflection
1. Introduction Palladium is one of the most important metals for industrial applications because of its high catalytic activities for various chemical reactions in vacuum and in solution. It has been shown that ultrathin Pd films grown on foreign metals have altered catalytic properties, compared with thick films or bulk [1,2]. For understanding the origin of these altered properties, studies on Pd thin films with well-defined structure are important. Palladium films deposited onto Au substrates are suitable for this purpose, because uniform and epitaxial growth is expected on account of only 5% lattice mismatch between Pd and Au. * Corresponding author. Fax: +81-791-58-2740. E-mail address: [email protected] (M. Takahasi)
One technique for the fabrication of such thin films is the vapor deposition in ultrahigh vacuum ( UHV ). On the basis of results given by a combination of Auger electron spectroscopy (AES), lowenergy ion scattering (LEIS ), X-ray photoemission spectroscopy ( XPS) and low-energy electron diffraction (LEED), Koel et al. [3] have reported that the in-vacuum growth process of Pd on Au(111) depends strongly on the substrate temperature. At a substrate temperature of 150 K, a pure Pd overlayer is formed on the Au(111) surface, as suggested by the evolution of AES signals. When the growth temperature is raised to 500 K, LEIS data indicative of intermixing of Pd and Au are obtained. The Pd–Au alloy formation has been detected even at the room temperature (RT ). The intermixed layer is composed of a Pd-rich alloy
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whose surface shows well-ordered (E3×E3)R30° periodicity [4]. The initial growth of Pd on Au(111) below the alloying temperature has been revealed by a recent scanning tunneling microscopy (STM ) study [5]. In the submonolayer region Pd ˚ rather than a forms islands as large as 30 A uniform monolayer. In subsequent deposition the growth of the second Pd layer commences before completion of the first layer, resulting in a Pd layer with a rough surface. Another approach to the construction of thin Pd films is the electrochemical deposition from a solution containing PdCl2−. Current–potential 4 curve studies on Cu underpotential deposition ( UPD) on Pd–Au [2,6 ] and a conventional X-ray diffraction study [7] have shown that the Pd layers electrochemically deposited on Au(111), Au(001) and Au(110) grow with the surface crystallographic orientation matching that of the substrate. Furthermore, recent STM studies have revealed that Pd on Au(111) and Au(001) grows in layerby-layer mode for several layers [6–8]. However, STM is sensitive only to the outermost surface and is incapable of identifying the atomic species, so that the crystallographic relationship between the Pd overlayer and the substrate has remained unresolved. In the present work we applied surface X-ray diffraction (SXD) to the structure analysis of Pd monolayers electrochemically deposited onto an Au(111) surface. Recently, SXD has proved to be well suited for the structural study of solid–liquid interfaces [9,10]. Our method is based on the measurements of diffracted intensities along the reciprocal rods normal to the surface. The intensity profiles of the reciprocal rods are modulated by the interference effect between the diffraction from the substrate regarded as an ideally terminated bulk crystal and the diffraction from the overlayer including foreign atomic species or atoms deviating from the ideal bulk position. Thus the positions of the deposited atoms with respect to the substrate can be determined from the analysis of the rod profiles. The specular rod measured in such a geometry that the scattering vector is aligned along the surface normal is the most sensitive to the structure in the out-of-plane direction, such as the
interlayer spacing between the surface monolayer and the first layer of the substrate. The information on in-plane adsorption sites is obtained from measurements along the non-specular rods that have lateral components. The combination of the measurements of the specular and non-specular rods has allowed us to confirm the pseudomorphic and two-dimensional growth of Pd on the Au(111) electrode.
2. Experimental A single Au crystal, which was shaped in a disk (10 mm in diameter and 5 mm in thickness), was used as the substrate. It was mechanically polished using 0.03 mm diamond slurry and electrochemically etched in a solution containing 0.1 M HClO and 5 mM NaCl [11,12]. After annealing 4 in an H flame and cooling slowly in air, the 2 crystal was transferred to an electrochemical cell specially designed for in situ SXD measurements. The body of the cell is made of polychlorotrifluoroethylene (PCTFE ). The cell is equipped with an inlet and an outlet for electrolyte and with electric feedthroughs for the Pt counter electrode and the Ag–AgCl reference electrode. It was sealed with a 6.0 mm thick Mylar film (Chemplex, Mylar 250), which served as an X-ray window. The electrochemical deposition of Pd was carried out in a solution containing 1 mM PdCl2− and 0.1 M 4 H SO , which was deaerated with N gas of 2 4 2 99.999% purity prior to each measurement. During the injection of the electrolyte solution, the potential applied to the sample was kept at +0.90 V (versus Ag/AgCl ), which is more positive than the UPD potential of Pd on the Au electrode. An outer chamber filled with flowing N gas was 2 attached to the topside of the cell to prevent oxygen from diffusing into the solution through the Mylar window. Electrochemical deposition of the Pd monolayer was carried out in this cell in such a way that the potential was first swept to the negative direction from +0.90 to +0.58 V, turned back to the positive direction and kept at +0.65 V. At this final potential, neither cathodic nor anodic current was observed. The amount of Pd deposited was deter-
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mined by the cathodic charge that flowed during a cycle of potential scan. In this paper, one monolayer (ML) is defined as 1.39×1015 cm−2 for the Au(111) surface. The solution between the electrode and the Mylar film was thicker than 5 mm during the deposition. After the deposition of Pd, the window was deflated so that an about 30 mm thick electrolyte layer remained on the sample surface. The SXD experiments were performed with the vertical-axis four-circle diffractometer installed on beamline 4C at the Photon Factory, Tsukuba, Japan. The X-rays were monochromatized by the Si(111) double crystal system and focused by a Pt-coated cylindrical bent mirror. The incident slit was set to 0.1 mm (vertical )×0.5 mm (horizontal ). The angular acceptance of the receiving slit was 3 mrad for the 2h direction and 30 mrad for the x ˚ was selected to direction. A wavelength of 1.283 A avoid the fluorescence from the Au substrate. All the measurements were carried out in such a mode that the incoming and outgoing angles with respect to the sample surface were equal. At each measuring point, the rocking scan of the v axis was performed for background subtraction and for the integration over the mosaic spread of the sample.
3. Results Fig. 1 shows the (a) specular and (b) nonspecular rod profiles for the Au(111) electrode onto which 1 ML Pd was deposited. For convenience, a measuring point in the reciprocal space Q is described in terms of the Miller indices (H, K, L) with the reciprocal basis a1=(2p/a ) 0 (2 , 2 , −4) , b1=(2p/a ) (−2 , 4 , −2) , c1= 0 3 3 3 cubic 3 3 3 cubic ], which corresponds to the hex(2p/a ) (1 , 1 , 1) 0 3 3 3 cubic agonal unit cell defined by a=a (1 , 0, −1) , 0 2 2 cubic , c=a (1, 1, 1) . b=a (−1 , 1 , 0) cubic 0 cubic 0 2 2 An important feature in the specular rod profile is the remarkable dips that are observed between two adjacent bulk Bragg peaks. Compared with the profile calculated for the ideally terminated Au(111) surface, which is represented by a dotted line in Fig. 1, the reflected intensity decreases by a factor of 10−2 around l=1.5 and l=4.5. These
Fig. 1. The specular (a) and the non-specular (b) rod profiles for 1 ML Pd/Au(111). The solid lines are the best fit curves based on a pseudomorphic growth model. The dotted line in the specular rod profile represents the profile for the ideally terminated Au(111) surface. The dashed and dot–dashed lines in the non-specular rod profile were calculated for the Pd adsorption to the hcp threefold hollow site and the atop site, respectively, with the same surface normal structure as the best fit model.
dips can be explained qualitatively by the formation of a uniform Pd layer on the Au(111) substrate. The scattering amplitude from the substrate is given by 1 f exactly at the middle of the two 2 Au Bragg points. When the topmost Au layer is replaced by a Pd monolayer, the scattering amplitude becomes 1 f −f +f =−1 f +f . 2 Au Au Pd 2 Au Pd Considering that the atomic form factor is approximately equal to the atomic number and that the
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atomic number of Pd is nearly half that of Au, it is easily known that the total scattering amplitude is depressed at the anti-Bragg condition. Hence these dips are strong evidence for the formation of an ordered Pd layer with respect to the surface normal direction. The quantitative surface normal structure was obtained from the least squares fitting with a kinematical calculation [13,14]. The calculation is based on a structure model consisting of the ideal Au substrate, a top Au layer, a Pd layer and a PdCl2− layer. For the latter three surface layers, 4 the surface normal positions z , the coverages r m m and the root mean square (RMS) of atomic displacements s , which includes the effects of static m corrugations as well as of thermal vibrations, can be the structural parameters to be fitted (m= 1, 2, 3). For simplicity, however, the coverages of the Au and Pd layers were fixed to unity and the RMS of the Au layer was assumed to be the ideal bulk value in the actual fitting procedure. As far as the specular rod is concerned, the in-plane positions (x , y ) (m=1, 2, 3) have no influence m m on the diffracted intensity. The best-fit structural parameters are listed in Table 1. A better fitting result was obtained by the model with a small amount of PdCl2− in the topmost layer than by 4 the model with a single layer of Pd. However, the coverage of the adsorbed ions is considerably smaller than that expected from the ordered PdCl2− layer that has been suggested by STM 4 observations [8,15]. Thus this fractional electron density might be caused by the excess of Pd or by
another kind of ion adsorbed onto the surface. Besides the structural parameters, the absorption correction originating from the solution, exp(−m/L), was also included in the fitting parameters. The resultant value was m=0.11, corresponding to a water layer of about 30 mm thickness. Provided that Au and Pd atoms have the same atomic radii as in each bulk, the evaluated interlayer spacing between the Au and Pd layers, ˚ , is consistent with the Pd adsorpz −z =2.27 A 2 1 tion onto the threefold hollow site. As shown in Fig. 2, however, there are two kinds of threefold hollow site with respect to the topmost Au atoms located at (2|a|, 1d|b|): one is the hexagonal closest 3 3 packing (hcp) site, i.e. (x , y )=(|a|, |b|), and the 2 2 other is the cubic closest packing (ccp) site, i.e. (x , y )=(1|a|, 2|b|). It is impossible to distinguish 2 2 3 3 between these two adsorption sites only by the specular rod profile analysis. In order to ascertain the in-plane structure, we carried out the measurement of the non-specular rod profile. In Fig. 1b, the observed profile of the (01) rod is compared with the profile calculated for the adsorption to the hcp, ccp and atop sites. It should be noted that no refinements of the
Table 1 Structural parameters obtained from the analysis of the specular rod profile of 1 ML Pd/Au(111). The atomic form factors f , Au f ,f and f are of Au, Pd, Pd2+ and Cl− respectively. Pd Pd2+ Cl− The third layer in the surface region is assumed to be composed of PdCl2− ions. The positions of the atomic layers are repre4 sented by the distance from the second Au layer, which is assumed to keep the ideal bulk position Atomic layer m
Atomic form factor f m
Position ˚) z (A m
Coverage r (ML) m
RMS ˚) s (A m
1 2 3
f Au f Pd f +4f Pd2+ Cl−
2.35 4.62 6.55
1.0a 1.0a 0.036
0.084a 0.080 0.2
a Values were fixed in the fitting procedure.
Fig. 2. Possible adsorption sites of Pd atoms on Au(111). Gray circles represent the Au atoms in the first layer and open circles represent the Au atoms in the second layer. The dashed parallelogram indicates the unit cell defined by the fundamental , b=a (−1 , 1 , 0) and c=a vectors a=a (1 , 0, −1) 0 2 2 cubic 0 0 2 2˚ cubic (1, 1, 1) (a =4.08 A ). Note that the hexagonal coordinate cubic 0 system is employed.
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structural parameters have been added to those obtained from the analysis of the specular rod, except for the small fraction of PdCl2− in the 4 topmost layer. Neglecting this contribution brought about a minor improvement in the fitting of the non-specular rod profile. This suggests that the additional topmost layer does not form an ordered structure with respect to the lateral direction. Adsorption onto the ccp hollow site gives the best fit, as shown by the solid line in Fig. 1. Other adsorption sites, such as the hcp hollow site (the dashed line) and the atop site (the dot–dashed line), fail to reproduce the observed profile. Hence it can be concluded that the Pd layer keeps the fcc stacking sequence of the Au substrate. Actually, this is intuitively apparent from the decrease in the intensity around l=3.5. For the same reason as in the case of the specular rod profile, the remarkable dip at the middle of the two Bragg peaks is caused by the destructive interference between the scatterings from the Au substrate and the ordered Pd layer. In the non-specular case, the ordering for the lateral direction is involved in the rod profile. Thus, the pseudomorphic growth of Pd is readily deduced from the combination of the specular and non-specular rod profiles.
4. Discussion The present SXD study has provided confirmation that the electrochemical deposition enables the construction of a pseudomorphic smooth Pd layer on Au(111). In our study, no signs of intermixing between Pd and Au has been observed; the destructive interference effect at the anti-Bragg condition would be indistinct if the topmost surface layer were composed of the mixture of Pd and Au. This is in contrast to the case in UHV deposition, where nucleation followed by threedimensional growth is observed at a low temperature and alloy formation takes place at the temperature higher than RT [3]. The difference in the growth modes between the electrochemical and UHV conditions is ascribable to the surface structure when Pd is deposited. In UHV, the Au(111) surface shows a reconstruction composed of linear discommensurations that sepa-
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rate regions with the normal ccp stacking sequence from regions with a faulted hcp stacking sequence. Owing to the symmetry of this surface, there are three equivalent directions along which the discommensurations are running. In the most stable phase, an ordered array of two out of the three equivalent domains forms the zig-zag pattern reported in the literature [16,17]. According to the STM observation [5], Pd islands seem to nucleate at the kinks between the discommensurations. Although the reconstruction is lifted by heating the substrate up to 900 K [17], Pd–Au alloy formation would be inevitable at this temperature. Therefore, a uniform pseudomorphic Pd layer cannot be obtained by the methods based on vacuum evaporation. Under electrochemical conditions, on the other hand, the surface reconstruction of Au(111) [18] is lifted at a potential of +0.35 V (versus Ag/AgCl ) in sulfuric acid, which was used as a supporting electrolyte in the present experiment. Since the Pd deposition occurs at +0.58 V, which is positive enough of the transition potential, Pd is allowed to be deposited onto the ideal 1×1 surface. The lattice distortion in the pseudomorphic Pd monolayer gives rise to a change in its electronic structure. It is likely that the altered electronic structure accounts for the unusual chemical properties of the Pd monolayer on Au(111). Although charge transfer, which is sometimes observed at the interface of dissimilar metals [19], may affect the chemical properties, structural deformation associated with the charge transfer has not been observed in the present system. From a calculation using the fitting parameters in Table 1, the bond length between Pd and Au atoms is evaluated as ˚ . This value is close to the average of the 2.82 A atomic radii of Au and Pd in each bulk. Furthermore, the interlayer spacing between the first and the second Au layers is not significantly changed from the ideal bulk value. Thus, there is no electron transfer between Pd and the Au substrate.
5. Conclusions The structure of a Pd monolayer electrochemically deposited onto Au(111) was analyzed on the
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basis of in situ SXD data. The Pd monolayer forms a flat pseudomorphic film, following the stacking sequence of the Au(111) substrate. Such a Pd film is available only under electrochemical conditions, where the surface reconstruction is lifted at the deposition potential. The chemical properties of the Pd monolayer on Au(111) are ascribed to the altered electronic structure induced by lattice distortion rather than to charge transfer.
Acknowledgements The authors would like to thank Dr Murakami and Mr Wakabayashi for their assistance at the Photon Factory. The present work was partly supported by a Grant-in-aid for Scientific Research on Priority Area of ‘Electrochemistry of Ordered Interfaces’ (no. 09237101) from the Ministry of Education, Science, Sports and Culture, Japan. The synchrotron radiation experiments were performed as projects approved by the Photon Factory Program Advisory Committee (proposal no. 98G091).
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