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Photoelectron spectroscopy of palladium-doped gold cluster anions; AunPd− (n=1–4)
31 May 2002
Chemical Physics Letters 358 (2002) 224–230 www.elsevier.com/locate/cplett
Photoelectron spectroscopy of palladium-doped gold cluster anions; AunPd ðn ¼ 1–4Þ Kiichirou Koyasu a, Masaaki Mitsui a, Atsushi Nakajima a
a,*
, Koji Kaya
b
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan b Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan Received 15 March 2002
Abstract Palladium-doped gold clusters, Aun Pd ðn ¼ 1–4Þ, were investigated using anion photoelectron spectroscopy at 4.66 eV photon energy. Electron affinities (EAs) and vertical detachment energies (VDEs) are determined, and the electronic structures of Pd-doped Aun clusters are compared to those of pure Aun clusters. A peak shape analysis reveals electronic and geometric similarity between Au n and Aun1 Pd clusters and it is found that (1) an electron promotion occurs from 4d to 5s orbital in the Pd atom, and that (2) the bond of Au–Pd is formed through r orbital between 6s of Au and 5s of Pd. Ó 2002 Published by Elsevier Science B.V.
1. Introduction Recently bimetallic clusters have become a matter of increasing interest in cluster research [1– 4]. The reason for this development is evident: when doping pure metals or metal surfaces with metallic heteroatoms, the new system often exhibits more tailored properties for applications than the non-doped pure metals [5,6]. One of the industrial applications is heterogeneous catalysis: enhanced catalytic activity and selectivity is often gained by using doped bimetallic catalysts [7]. The investigation of bimetallic or alloy clusters provides a fundamental tool to gain insights into
*
Corresponding author. Fax: +81-45-566-1697. E-mail address: [email protected] (A. Nakajima).
chemical and physical properties of bimetallic systems on geometric and electronic structures as a function of size and composition. These electronic/ geometric structures can be compared and sometimes correlated with size-dependent chemical reaction rates and other important properties. Small clusters of the coinage metals (Cu, Ag, and Au) have been studied by several experimental and calculation works [8–13] and these experimental studies have included chemical reactivity [14–16], photoelectron spectroscopy of negative ions [17– 19], and absorption spectroscopy of embedded clusters [20]. Especially, extreme size sensitivity of catalytic activity of supported Au clusters is worthy of particular attention [21]. Then, bimetallic clusters containing the coinage metals can be intriguing because the additive effect of the second metallic component plays an important role to control the activity and the stability.
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 5 6 2 - 6
K. Koyasu et al. / Chemical Physics Letters 358 (2002) 224–230
Anion photoelectron spectroscopy is a powerful experimental method to investigate the electronic structure of clusters as it affords the preparation of mass selected ions, thus avoiding the inherent problem of separating the cluster of interest from the other species. The electronic properties of the neutral clusters are readily observed from the photoelectron spectra, because the photodetachment corresponds to a transition from the ground state of anions into the ground state or electronic excited states of the corresponding neutrals. Information on the bonding of the clusters and thus the geometry has been generally obtained from the analysis of photoelectron spectra. In this Letter, we apply this approach to investigate the electronic states of small Aun Pd cluster anions ðn ¼ 1–4Þ in order to study the bonding feature between Au and Pd atoms. A Pd atom has a unique closed-shell electronic ground 10 0 state, 1 S0 ð½Krð4dÞ ð5sÞ Þ, and a Pd ð1 S0 Þ atom only forms a weak bonding through a van der Waals force. To increase bonding, at least one 4d electron must be excited to a 5s orbital, although the promotion energy of 0.8 eV is required. In the Au–Pd binary cluster, therefore, the hybridization of atomic s and d orbitals seems to be indispensable to making the heterogeneous cluster. The sizedependent additive effect is discussed on comparison between the photoelectron spectra of Au n and Aun Pd .
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of 40 mm length. The clusters are mass-analyzed by their m=z ratio with an in-line TOF-MS having a resolution of m=Dm ¼ 200. To decrease the amount of energy broadening of the photoelectron spectra caused by the Doppler effect, the mass-selected cluster anions are decelerated with a special deceleration technique originally reported by Handschuh et al. [23]. In the center of the magnetic bottle-type time-of-flight photoelectron spectrometer (TOF-PES) the clusters are irradiated with the fourth harmonics output (4.66 eV) of a Q-switched Nd3þ :YAG laser (laser fluence: 1–3 mJ=cm2 ). The kinetic energies of the detached electrons are measured by their TOF, and electron kinetic energy spectra are converted to the electron binding energy spectra by subtracting the kinetic energy from the photon energy. The TOF-PES is calibrated using the strong line of the ground state transition ð1 S0 ! 2 S1=2 Þ of the gold atomic anion [24,25]. The resolution of the spectrometer is better than 50 meV at 1 eV electron kinetic energy (eKE), and decreases according to ðeKEÞ3=2 at higher kinetic energies. In the current experiment, photoelectron (PE) spectra are measured by accumulating 10 000–30 000 experimental runs at 10 Hz repetition rate. No power-dependent processes for the spectrum shape were observed.
3. Results and discussion 3.1. Mass spectrometry of Aun Pdm
2. Experiments The apparatus used in this work consists of a cluster anion source, a time-of-flight mass spectrometer (TOF-MS), and a magnetic-bottle type electron TOF spectrometer, and most of them have been described in detail previously [22]. Therefore, only a brief description will be given. Generation of bimetallic Aun Pd cluster anions is performed as follows: two independently operating Nd3þ :YAG lasers (532 nm) are focused onto a rotating and translating Au rod (upstream location) and a Pd rod (downstream location), respectively. Then, clusters are formed by cooling the plasma with a high-pressure helium pulse (6 atm stagnation pressure) in a 3 mm diam. channel
Fig. 1 shows the mass spectrum of Aun Pd m cluster anions. In the spectrum, relatively prominent peaks are labeled with the notation of (n, m), denoting the numbers of Au atoms (n) and Pd atoms (m), respectively. It can be found that intensity distribution for pure Au n exhibits two local maxima at n ¼ 3 and 7. As discussed below, the rich abundance of Au 3 and Au7 is ascribed to relatively high electron affinity (EA) of Au compared to neighbors, which has been examined experimentally [17] and theoretically [9,13]. Especially, the enhanced abundance of Au 7 is attributed to the electron shell closing of 1p shell, because total eight valence electrons including a charging electron complete the 1p shell.
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Fig. 1. A typical TOF mass spectrum of Aun Pd m clusters. Bimetallic Aun Pd m clusters are observed less abundantly that the pure Au n clusters. Peaks are labeled with the notation of (n, m), denoting the numbers of Au atoms (n) and Pd atoms (m), respectively.
Between the Au n peaks, the peaks of bimetallic Au–Pd clusters are found with very low intensity. Surprisingly it was very hard to mix Au and Pd elements efficiently, even when the vaporization rate of the Pd rod was extremely enhanced by high laser fluence. The mass spectrum shown in Fig. 1 is one of the best spectra exhibiting the mixed Au–Pd cluster anions. The peak shape of Pd clusters becomes broad with increasing the number of Pd atoms (m) because of several isotopes of 102 Pd (1%), 104 Pd (11%), 105 Pd (22%), 106 Pd (27%), 108 Pd (26%), 110 Pd (12%), and then the height of the peaks tends to be low compared to those of Au clusters having single isotope of 197 Au. However, the distribution evaluated even by peak areas is apparently far from a binomial distribution as shown in Fig. 1. When the interaction energies between two elements are more or less similar, the distribution should approximately obey a binomial distribution. The binominal distribution was obtained between similar elements; lithium–sodium [26], cobalt–vanadium [27], and so on. For Au–Pd, however, the binding energies of their dimers of Au2 , AuPd, and Pd2 are very different; those of Au2 , AuPd, and Pd2 are 2.2, 1.4, and 0.7 eV, respectively [25]. Since the ground state of Pd atoms have ½Krð4dÞ10 ð5sÞ0 configuration ð1 S0 Þ, the interaction between two
Pd ð1 S0 Þ atoms should be weak. Indeed, the ground state of Pd2 is theoretically predicted to be 2 4 4 4 a triplet state having ð4drg Þ ð4dpu Þ ð4ddg Þ ð4ddu Þ 4 1 1 ð4dpg Þ ð4dru Þ ð5srg Þ , which is correlated to the dissociation limit of Pd ð1 SÞ þ Pd ð3 DÞ instead of Pd ð1 SÞ þ Pd ð1 SÞ [28]. It should be interestingly noted that the co-laser vaporization of a Pd rod could effectively enhance the formation of pure Au n clusters themselves. This is probably because the laser-plasma from Pd vaporization plays a role of an electron source toward Aun clusters without any severe mixing between Au and Pd. To our knowledge, laser vaporization of a magnesium (Mg) target also works as the electron source to generate cluster anions, because Mg atoms have closed electronic config2 uration of ½Neð3sÞ . In order to make cluster anions abundantly, the co-laser vaporization of Pd and Mg seems very useful. 3.2. Photoelectron spectroscopy of Aun Pdm Fig. 2 shows photoelectron spectra of Au n ðn ¼ 1–5Þ and Aun Pd ðn ¼ 1–4Þ at 266 nm photon energy. In the spectra, the horizontal axis corresponds to the electron binding energy, Eb , defined as Eb ¼ hm Ek , where Ek is the kinetic energy of the photoelectron, and arrows indicate the threshold energy ðET Þ, which corresponds to the upper limit of EA. The EA ðET Þ values are tabulated in Table 1, together with vertical detachment energy (VDE). In the figure prominent 0 peak(s) is labeled by An , Bn , Cn for Au n and by An , 0 0 Bn , Cn for Aun Pd . The photoelectron spectra of the pure Au n clusters have already been reported, which were detached by 157 nm (7.9 eV) for n ¼ 1–233 [17], by 193 nm (6.42 eV) for n ¼ 1–4 [19], and by 351 nm (3.53 eV) for n ¼ 1–5 [18]. Our photoelectron spectra are almost consistent with theirs. Namely, the obtained EAs in this work are almost the same with theirs within the experimental uncertainties. As for the pure Au n clusters, the ET values exhibit an even/odd alternation; ET at odd n is larger than that at even n. The even/odd alternation has been reported, and can be explained by pairing energy of 6s electrons including one charging electron [17]. Similarly to the pure Au n clusters, an even/
K. Koyasu et al. / Chemical Physics Letters 358 (2002) 224–230
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more, common spectral features are recognized between Au n and Aun1 Pd ; both the peak intensity and the peak splitting are very similar when the peaks labeled by B0n are excluded in Aun1 Pd . The striking similarity between Au and n Aun1 Pd cannot be explained by the fact that a Pd atom takes the configuration of ½Krð4dÞ10 ð5sÞ0 in the ground state. Since localized 4d electrons much less contribute to form bonding orbitals, Au n should have the same number of total valence electrons not with Aun1 Pd but with Aun Pd . Then, the plausible explanation for the similarity is that one of 4d electrons is promoted to 5s orbital, and the 5s electron contributes to form bonding between Au atom(s). In fact, the configuration of ½Krð4dÞ9 ð5sÞ1 is theoretically predicted to largely contribute to form AuPd [28] as well as Pd2 [29]. These results reasonably indicate that the Aun Pd cluster have n þ 2 valence electrons at small n.
Fig. 2. Photoelectron spectra of pure Au n clusters (left column: n ¼ 1–5 at 4.66 eV) and bimetallic Aun1 Pd clusters (right column: n ¼ 2–5 at 4.66 eV). Threshold energies ET ; EA) are indicated by downward arrows (see text). Between the photo electron spectra of Au n and Aun1 Pd , apparent similarity is found in ET , peak shape, and peak intervals. Table 1 Experimental threshold energies (ET ; EA) and VDEs of Aun Pd ðn ¼ 1–4Þ in eV Size (Aun Pd 1)
ET ; EA
VDE
1–1 2–1 3–1 4–1
1.88 3.80 2.51 2.69
2.00 3.89 2.71 2.82
odd alternation is found also in the ET of Aun Pd . Interestingly, the absolute values of ET are almost the same between Au n and Aun1 Pd . Further-
3.2.1. AuPd As discussed above, the configuration of Pd ð½Krð4dÞ9 ð5sÞ1 Þ should largely contribute to form the bond between Au and Pd atoms in the Aun Pd clusters. If d orbital bonding in neutral AuPd dimer is appreciable, one can expect the d orbitals to be split into dr, dp, dd, dd , dp , and dr atom and anti-bonding orbitals, giving a ground elec9 10 tronic state of ð4dPd Þ ð5dAu Þ r2 , 2 Rþ with the 4d
hole located a dr molecular orbital. If the bonding consists of sr2 bond with a weakly interacting 4d core, the 4d hole are preferentially placed in d orbitals, resulting in 2 D ground state. Namely, the bonding contribution of d orbitals results in the molecular state of 2 Rþ or 2 D for AuPd. The spectral similarity between Au and n Aun1 Pd indicates that the molecular states are governed particularly by r orbitals consisting of 5sPd and 6sAu , because the molecular states of Aun clusters are described by the contribution of 6s orbitals. Therefore, the ground state of AuPd is reasonably presumed to be 2 D, which can be rationalized by analogy of CuNi [30]. Then, peak A01 is assigned to the transition from the electronic ground state of the anion 3 D (or 1 D) (ð4dÞ9 r2 r ) to the ground state of the neutral 2 D ðð4dÞ9 r2 Þ. The
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threshold energy of 1.88 eV indicated by an arrow corresponds to the EA of AuPd dimer, as listed in Table 1. Compared with the reported EAs of Pd2 (1.68 eV) [31] and Au2 (1.94 eV) [18], the replacement of Pd atom by Au atom causes the monotonous increase of EA due to the larger EA of Au atoms (2.308 eV) than that of Pd atom (0.56 eV) [26]. Using our experimentally determined values for the EA of AuPd, the dissociation energy, D0 , was determined via the following thermochemical cycle: D0 ðAuPd Þ ¼ D0 ðAuPdÞ þ EAðAuPdÞ EAðAuÞ:
ð1Þ
Using the reported values for D0 ðAuPdÞ, 1.40 eV, and the EA of Au atom, 2.308 eV, a value of 0.97 eV is evaluated for D0 ðAuPd Þ (X3 D ðor 1 DÞ). The dissociation energy of AuPd is much smaller than that of the neutral AuPd. This is because the excess electron occupies the anti-bonding r orbital. The large difference between D0 ðAuPd Þ and D0 ðAuPdÞ should result in difference of bond length between AuPd and AuPd, which gives a reasonable explanation for the relatively broad peak A01 in Fig. 2. There have been little experimental data and no calculated results for the electronic excited states of AuPd dimer. Between the spectra of AuPd and Au 2 in the BE range up to 4.5 eV, however, the similarity can be recognized. Since the peak B2 in the Au 2 spectrum has been assigned as the transition from the electronic ground state of the anion 2 þ R ðr2 r Þ to the first excited state of the neutral a 1 þ R ðrr Þ, the analogy enables us to assign the peak C01 of AuPd to the transition from the electronic ground state of the anion X 3 D (or 1 D) to the excited state of the neutral anion 4 D (or 2 D) (ð4dÞ9 rr ). Furthermore, the peak B01 in the AuPd spectrum is assigned as the transition into 8 the first excited state having (ð4dÞ r2 r Þ configuration, where one of d electrons in 4d is ejected from the anion. By the analogy of NiCu [30], the 8 (ð4dÞ r2 r ) configuration is probably expressed as 8 3 ð4dÞ ð FÞr2 r , although theoretical calculations are necessary for the accurate assignment of the peaks.
3.2.2. Au2 Pd The negatively charged trimers of pure alkali and group-11 (conventionally group 1B; Cu, Ag, Au) clusters are linear [8–10,12,13] with a 1 Rþ g ground state. In the electron detachment for most of these species (Na 3 , K3 , Rb3 , Ag3 , and Au3 ) [9,18,32] the transition into the neutral ground state 2 Rþ u of the neutral trimer occurs [18]. For Au2 Pd 1 , the spectrum exhibits only a sharp peak at 266 nm detachment, and the spectral feature is essentially the same with that of the linear Au also 3 , implying that Au2 Pd 2 takes a linear structure having a D ground state with a weakly interacting 4d core, where the Pd atom configuration can be described as ½Krð4dÞ9 ð5sÞ1 . The sharp peak can be plausibly attributed to the transitions for linear geometries between the anion and the neutral, because the transitions for lower dimensional structure result in sharper peaks and short progressions. This has actually been reported in the C n spectra [33]. For the linear structure of Au2 Pd , two kinds of isomer are conceivable; Au–Au–Pd and Au–Pd– Au. According to our recent experiment [34] and the calculations [35] on Au2 Ag , the most stable structure is linear Au–Ag–Au , where the excess electron is preferably localized on the terminal Au atoms. Since the EA of Au (2.31 eV) is larger than that of Ag (1.30 eV), the structure of Au–Ag–Au is much more stable than that of Au–Au–Ag . When a Pd atom takes the ½Krð4dÞ9 ð5sÞ1 config10 1 uration, an Ag atom ð½Krð4dÞ ð5sÞ Þ can be regarded as an isoelectronic atom except for no vacancy in 4d orbitals. By analogy of Au2 Ag , namely, it seems reasonable to deduce that Au2 Pd takes the linear structure of Au–Pd–Au , because the EA of a Pd atom (0.56 eV) is very small [25]. Indeed, the ET s of both anions are almost the same; Au2 Pd (3.80 eV) and Au2 Ag (3.81 eV). A successive peak B02 is observed next to the peak A02 in the spectrum of Au2 Pd . This is a new peak compared to the spectrum of Au 3 . The peak B02 is seemingly assigned as the transition with 4d electron detachment from the Pd atom, which is a common origin for the peak B01 in AuPd .
K. Koyasu et al. / Chemical Physics Letters 358 (2002) 224–230
3.2.3. Aun Pd (n ¼ 3 and 4) In the PES spectra of Au3 Pd and Au4 Pd , the number of observed peaks is increased. Spectral similarity for number of peaks and for intensity ratio is still perceivable between Au 4 and Au3 Pd , and also between Au5 and Au4 Pd , although peaks are congested in higher binding energy in the spectra of Aun Pd . Recent theoretical calculations predict that Au 4 has two essentially degenerate structures: a C2v ‘T-shaped’ structure and a ‘zigzag’ chain structure [13]. Furthermore, the optimized structure of Au 5 is calculated to be a planar trapezoid [13]. Judging from the spectral similarity, the geometric structure is likely maintained by the substitution of an Au atom by a Pd atom, although it is hard to predict the preferable position of Pd atom in the substitution. Congested peaks in higher binding energy indicate the electronic contribution from 4d orbitals.
4. Conclusions We observed the photoelectron spectra of the small Au–Pd mixed clusters produced by the twolaser vaporization of Au and Pd rods. The spectral similarity between Au indicates n and Aun1 Pd that the Pd atom is mixed in the clusters with the promoted electronic configuration of ½Krð4dÞ9 ð5sÞ1 , and that the geometries are almost maintained in the substitution. The present work can provide an important base to understand not only the electronic but also the geometric structures of the small Au–Pd clusters.
Acknowledgements We are grateful to Mr. M. Ge and Prof. K.S. Kim for sending their results prior to publication. We are also grateful to Dr. A. Pramann and Mr. K. Miyajima for their technical help. This work is supported by a program entitled ‘Research for the Future (RFTF)’ of the Japan Society for the Promotion of Science (98P01203) and by a grantin-aid for scientific research (C) (No. 13640582) from the Ministry of Education, Science, Sports and Culture.
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References [1] G.M. Koretsky, K.P. Kerns, G.C. Nieman, M.B. Knickelbein, S.J. Riley, J. Phys. Chem. A 103 (1999) 1997. [2] T.G. Taylor, K.F. Willey, M.B. Bishop, M.A. Duncan, J. Phys. Chem. 94 (1990) 8016. [3] U. Heiz, A. Vayloyan, E. Schumacher, C. Yeretzian, M. Stener, P. Gisdakis, N. Roesch, J. Chem. Phys. 105 (1996) 5574. [4] O.C. Thomas, W.-J. Zheng, T.P. Lippa, S.-J. Xu, S.A. Lyapustina, K.H. Bowen Jr., J. Chem. Phys. 114 (2001) 9895. [5] D.M. Zehner, D.M. Goodman, Physical and Chemical Properties of Thin Metal Overlayers and Alloy Surfaces, vol. 83, Material Research Society, 1987. [6] J.A. Rodriguez, Surf. Sci. Rep. 24 (1996) 223. [7] J.H. Sinfelt, Bimetallic Catalysis: Discoveries Concepts and Applications, Wiley, New York, 1983. [8] K. Balasubramanian, J. Chem. Phys. 86 (1987) 5587. [9] V. Bonacic-Koutecky, L. Cespiva, P. Fantucci, J. Pittner, J. Koutecky, J. Chem. Phys. 100 (1994) 490. [10] R. Mitric, M. Hartmann, B. Stanca, V. Bonacic-Koutecky, P. Fantucci, J. Phys. Chem. A 105 (2001) 8892. [11] V. Kello, A.J. Sadlej, J. Phys. Chem. 103 (1995) 2991. [12] C.W. Bauschlicher Jr., S.R. Langhoff, H. Partridge, J. Chem. Phys. 91 (1989) 2412. [13] H. Hakkinen, U. Landman, Phys. Rev. B 62 (2000) R2287. [14] T.H. Lee, K.M. Ervin, J. Phys. Chem. 98 (1994) 10023. [15] M.P. Irion, P. Schnabel, A. Selinger, Ber. Bunsenges. Phys. Chem. 94 (1990) 1291. [16] L. Lian, S.A. Mitchell, P.A. Hackett, D.M. Rayner, J. Chem. Phys. 104 (1996) 5338. [17] K.J. Taylor, C.L. Pettiette-Hall, O. Cheshnovsky, R.E. Smalley, J. Chem. Phys. 96 (1992) 2483. [18] J. Ho, K.M. Ervin, W.C. Lineberger, J. Chem. Phys. 93 (1990) 6987. [19] H. Handschuh, G. Gantef€ or, P.S. Bechthold, W. Eberhart, J. Chem. Phys. 100 (1994) 7093. [20] E. Cottancin, J. Lerme, M. Gaudry, M. Pellarin, J.-L. Vialle, M. Broyer, B. Prevel, M. Treilleux, M. Melinon, Phys. Rev. B 62 (2000) 5179. [21] A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Hakkinen, R.N. Barnett, U. Landman, J. Phys. Chem. A 103 (1999) 9573. [22] A. Nakajima, T. Taguwa, K. Hoshino, T. Sugioka, T. Naganuma, F. Ono, K. Watanabe, K. Nakao, Y. Konishi, R. Kishi, K. Kaya, Chem. Phys. Lett. 214 (1999) 22. [23] H. Handschuh, G. Gantefoer, W. Eberhardt, Rev. Sci. Instrum. 66 (1995) 3838. [24] H. Hotop, W.C. Lineberger, J. Phys. Chem. Rev. Data 4 (1975) 539. [25] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 79th ed., CRC Press, Boca Raton, FL, 1998. [26] M.M. Kappes, P. Radi, M. Schar, C. Yeretzian, E. Schumacher, Z. Phys. D 3 (1986) 115.
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Chemical Physics Letters 358 (2002) 224–230 www.elsevier.com/locate/cplett
Photoelectron spectroscopy of palladium-doped gold cluster anions; AunPd ðn ¼ 1–4Þ Kiichirou Koyasu a, Masaaki Mitsui a, Atsushi Nakajima a
a,*
, Koji Kaya
b
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan b Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan Received 15 March 2002
Abstract Palladium-doped gold clusters, Aun Pd ðn ¼ 1–4Þ, were investigated using anion photoelectron spectroscopy at 4.66 eV photon energy. Electron affinities (EAs) and vertical detachment energies (VDEs) are determined, and the electronic structures of Pd-doped Aun clusters are compared to those of pure Aun clusters. A peak shape analysis reveals electronic and geometric similarity between Au n and Aun1 Pd clusters and it is found that (1) an electron promotion occurs from 4d to 5s orbital in the Pd atom, and that (2) the bond of Au–Pd is formed through r orbital between 6s of Au and 5s of Pd. Ó 2002 Published by Elsevier Science B.V.
1. Introduction Recently bimetallic clusters have become a matter of increasing interest in cluster research [1– 4]. The reason for this development is evident: when doping pure metals or metal surfaces with metallic heteroatoms, the new system often exhibits more tailored properties for applications than the non-doped pure metals [5,6]. One of the industrial applications is heterogeneous catalysis: enhanced catalytic activity and selectivity is often gained by using doped bimetallic catalysts [7]. The investigation of bimetallic or alloy clusters provides a fundamental tool to gain insights into
*
Corresponding author. Fax: +81-45-566-1697. E-mail address: [email protected] (A. Nakajima).
chemical and physical properties of bimetallic systems on geometric and electronic structures as a function of size and composition. These electronic/ geometric structures can be compared and sometimes correlated with size-dependent chemical reaction rates and other important properties. Small clusters of the coinage metals (Cu, Ag, and Au) have been studied by several experimental and calculation works [8–13] and these experimental studies have included chemical reactivity [14–16], photoelectron spectroscopy of negative ions [17– 19], and absorption spectroscopy of embedded clusters [20]. Especially, extreme size sensitivity of catalytic activity of supported Au clusters is worthy of particular attention [21]. Then, bimetallic clusters containing the coinage metals can be intriguing because the additive effect of the second metallic component plays an important role to control the activity and the stability.
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 5 6 2 - 6
K. Koyasu et al. / Chemical Physics Letters 358 (2002) 224–230
Anion photoelectron spectroscopy is a powerful experimental method to investigate the electronic structure of clusters as it affords the preparation of mass selected ions, thus avoiding the inherent problem of separating the cluster of interest from the other species. The electronic properties of the neutral clusters are readily observed from the photoelectron spectra, because the photodetachment corresponds to a transition from the ground state of anions into the ground state or electronic excited states of the corresponding neutrals. Information on the bonding of the clusters and thus the geometry has been generally obtained from the analysis of photoelectron spectra. In this Letter, we apply this approach to investigate the electronic states of small Aun Pd cluster anions ðn ¼ 1–4Þ in order to study the bonding feature between Au and Pd atoms. A Pd atom has a unique closed-shell electronic ground 10 0 state, 1 S0 ð½Krð4dÞ ð5sÞ Þ, and a Pd ð1 S0 Þ atom only forms a weak bonding through a van der Waals force. To increase bonding, at least one 4d electron must be excited to a 5s orbital, although the promotion energy of 0.8 eV is required. In the Au–Pd binary cluster, therefore, the hybridization of atomic s and d orbitals seems to be indispensable to making the heterogeneous cluster. The sizedependent additive effect is discussed on comparison between the photoelectron spectra of Au n and Aun Pd .
225
of 40 mm length. The clusters are mass-analyzed by their m=z ratio with an in-line TOF-MS having a resolution of m=Dm ¼ 200. To decrease the amount of energy broadening of the photoelectron spectra caused by the Doppler effect, the mass-selected cluster anions are decelerated with a special deceleration technique originally reported by Handschuh et al. [23]. In the center of the magnetic bottle-type time-of-flight photoelectron spectrometer (TOF-PES) the clusters are irradiated with the fourth harmonics output (4.66 eV) of a Q-switched Nd3þ :YAG laser (laser fluence: 1–3 mJ=cm2 ). The kinetic energies of the detached electrons are measured by their TOF, and electron kinetic energy spectra are converted to the electron binding energy spectra by subtracting the kinetic energy from the photon energy. The TOF-PES is calibrated using the strong line of the ground state transition ð1 S0 ! 2 S1=2 Þ of the gold atomic anion [24,25]. The resolution of the spectrometer is better than 50 meV at 1 eV electron kinetic energy (eKE), and decreases according to ðeKEÞ3=2 at higher kinetic energies. In the current experiment, photoelectron (PE) spectra are measured by accumulating 10 000–30 000 experimental runs at 10 Hz repetition rate. No power-dependent processes for the spectrum shape were observed.
3. Results and discussion 3.1. Mass spectrometry of Aun Pdm
2. Experiments The apparatus used in this work consists of a cluster anion source, a time-of-flight mass spectrometer (TOF-MS), and a magnetic-bottle type electron TOF spectrometer, and most of them have been described in detail previously [22]. Therefore, only a brief description will be given. Generation of bimetallic Aun Pd cluster anions is performed as follows: two independently operating Nd3þ :YAG lasers (532 nm) are focused onto a rotating and translating Au rod (upstream location) and a Pd rod (downstream location), respectively. Then, clusters are formed by cooling the plasma with a high-pressure helium pulse (6 atm stagnation pressure) in a 3 mm diam. channel
Fig. 1 shows the mass spectrum of Aun Pd m cluster anions. In the spectrum, relatively prominent peaks are labeled with the notation of (n, m), denoting the numbers of Au atoms (n) and Pd atoms (m), respectively. It can be found that intensity distribution for pure Au n exhibits two local maxima at n ¼ 3 and 7. As discussed below, the rich abundance of Au 3 and Au7 is ascribed to relatively high electron affinity (EA) of Au compared to neighbors, which has been examined experimentally [17] and theoretically [9,13]. Especially, the enhanced abundance of Au 7 is attributed to the electron shell closing of 1p shell, because total eight valence electrons including a charging electron complete the 1p shell.
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Fig. 1. A typical TOF mass spectrum of Aun Pd m clusters. Bimetallic Aun Pd m clusters are observed less abundantly that the pure Au n clusters. Peaks are labeled with the notation of (n, m), denoting the numbers of Au atoms (n) and Pd atoms (m), respectively.
Between the Au n peaks, the peaks of bimetallic Au–Pd clusters are found with very low intensity. Surprisingly it was very hard to mix Au and Pd elements efficiently, even when the vaporization rate of the Pd rod was extremely enhanced by high laser fluence. The mass spectrum shown in Fig. 1 is one of the best spectra exhibiting the mixed Au–Pd cluster anions. The peak shape of Pd clusters becomes broad with increasing the number of Pd atoms (m) because of several isotopes of 102 Pd (1%), 104 Pd (11%), 105 Pd (22%), 106 Pd (27%), 108 Pd (26%), 110 Pd (12%), and then the height of the peaks tends to be low compared to those of Au clusters having single isotope of 197 Au. However, the distribution evaluated even by peak areas is apparently far from a binomial distribution as shown in Fig. 1. When the interaction energies between two elements are more or less similar, the distribution should approximately obey a binomial distribution. The binominal distribution was obtained between similar elements; lithium–sodium [26], cobalt–vanadium [27], and so on. For Au–Pd, however, the binding energies of their dimers of Au2 , AuPd, and Pd2 are very different; those of Au2 , AuPd, and Pd2 are 2.2, 1.4, and 0.7 eV, respectively [25]. Since the ground state of Pd atoms have ½Krð4dÞ10 ð5sÞ0 configuration ð1 S0 Þ, the interaction between two
Pd ð1 S0 Þ atoms should be weak. Indeed, the ground state of Pd2 is theoretically predicted to be 2 4 4 4 a triplet state having ð4drg Þ ð4dpu Þ ð4ddg Þ ð4ddu Þ 4 1 1 ð4dpg Þ ð4dru Þ ð5srg Þ , which is correlated to the dissociation limit of Pd ð1 SÞ þ Pd ð3 DÞ instead of Pd ð1 SÞ þ Pd ð1 SÞ [28]. It should be interestingly noted that the co-laser vaporization of a Pd rod could effectively enhance the formation of pure Au n clusters themselves. This is probably because the laser-plasma from Pd vaporization plays a role of an electron source toward Aun clusters without any severe mixing between Au and Pd. To our knowledge, laser vaporization of a magnesium (Mg) target also works as the electron source to generate cluster anions, because Mg atoms have closed electronic config2 uration of ½Neð3sÞ . In order to make cluster anions abundantly, the co-laser vaporization of Pd and Mg seems very useful. 3.2. Photoelectron spectroscopy of Aun Pdm Fig. 2 shows photoelectron spectra of Au n ðn ¼ 1–5Þ and Aun Pd ðn ¼ 1–4Þ at 266 nm photon energy. In the spectra, the horizontal axis corresponds to the electron binding energy, Eb , defined as Eb ¼ hm Ek , where Ek is the kinetic energy of the photoelectron, and arrows indicate the threshold energy ðET Þ, which corresponds to the upper limit of EA. The EA ðET Þ values are tabulated in Table 1, together with vertical detachment energy (VDE). In the figure prominent 0 peak(s) is labeled by An , Bn , Cn for Au n and by An , 0 0 Bn , Cn for Aun Pd . The photoelectron spectra of the pure Au n clusters have already been reported, which were detached by 157 nm (7.9 eV) for n ¼ 1–233 [17], by 193 nm (6.42 eV) for n ¼ 1–4 [19], and by 351 nm (3.53 eV) for n ¼ 1–5 [18]. Our photoelectron spectra are almost consistent with theirs. Namely, the obtained EAs in this work are almost the same with theirs within the experimental uncertainties. As for the pure Au n clusters, the ET values exhibit an even/odd alternation; ET at odd n is larger than that at even n. The even/odd alternation has been reported, and can be explained by pairing energy of 6s electrons including one charging electron [17]. Similarly to the pure Au n clusters, an even/
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more, common spectral features are recognized between Au n and Aun1 Pd ; both the peak intensity and the peak splitting are very similar when the peaks labeled by B0n are excluded in Aun1 Pd . The striking similarity between Au and n Aun1 Pd cannot be explained by the fact that a Pd atom takes the configuration of ½Krð4dÞ10 ð5sÞ0 in the ground state. Since localized 4d electrons much less contribute to form bonding orbitals, Au n should have the same number of total valence electrons not with Aun1 Pd but with Aun Pd . Then, the plausible explanation for the similarity is that one of 4d electrons is promoted to 5s orbital, and the 5s electron contributes to form bonding between Au atom(s). In fact, the configuration of ½Krð4dÞ9 ð5sÞ1 is theoretically predicted to largely contribute to form AuPd [28] as well as Pd2 [29]. These results reasonably indicate that the Aun Pd cluster have n þ 2 valence electrons at small n.
Fig. 2. Photoelectron spectra of pure Au n clusters (left column: n ¼ 1–5 at 4.66 eV) and bimetallic Aun1 Pd clusters (right column: n ¼ 2–5 at 4.66 eV). Threshold energies ET ; EA) are indicated by downward arrows (see text). Between the photo electron spectra of Au n and Aun1 Pd , apparent similarity is found in ET , peak shape, and peak intervals. Table 1 Experimental threshold energies (ET ; EA) and VDEs of Aun Pd ðn ¼ 1–4Þ in eV Size (Aun Pd 1)
ET ; EA
VDE
1–1 2–1 3–1 4–1
1.88 3.80 2.51 2.69
2.00 3.89 2.71 2.82
odd alternation is found also in the ET of Aun Pd . Interestingly, the absolute values of ET are almost the same between Au n and Aun1 Pd . Further-
3.2.1. AuPd As discussed above, the configuration of Pd ð½Krð4dÞ9 ð5sÞ1 Þ should largely contribute to form the bond between Au and Pd atoms in the Aun Pd clusters. If d orbital bonding in neutral AuPd dimer is appreciable, one can expect the d orbitals to be split into dr, dp, dd, dd , dp , and dr atom and anti-bonding orbitals, giving a ground elec9 10 tronic state of ð4dPd Þ ð5dAu Þ r2 , 2 Rþ with the 4d
hole located a dr molecular orbital. If the bonding consists of sr2 bond with a weakly interacting 4d core, the 4d hole are preferentially placed in d orbitals, resulting in 2 D ground state. Namely, the bonding contribution of d orbitals results in the molecular state of 2 Rþ or 2 D for AuPd. The spectral similarity between Au and n Aun1 Pd indicates that the molecular states are governed particularly by r orbitals consisting of 5sPd and 6sAu , because the molecular states of Aun clusters are described by the contribution of 6s orbitals. Therefore, the ground state of AuPd is reasonably presumed to be 2 D, which can be rationalized by analogy of CuNi [30]. Then, peak A01 is assigned to the transition from the electronic ground state of the anion 3 D (or 1 D) (ð4dÞ9 r2 r ) to the ground state of the neutral 2 D ðð4dÞ9 r2 Þ. The
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threshold energy of 1.88 eV indicated by an arrow corresponds to the EA of AuPd dimer, as listed in Table 1. Compared with the reported EAs of Pd2 (1.68 eV) [31] and Au2 (1.94 eV) [18], the replacement of Pd atom by Au atom causes the monotonous increase of EA due to the larger EA of Au atoms (2.308 eV) than that of Pd atom (0.56 eV) [26]. Using our experimentally determined values for the EA of AuPd, the dissociation energy, D0 , was determined via the following thermochemical cycle: D0 ðAuPd Þ ¼ D0 ðAuPdÞ þ EAðAuPdÞ EAðAuÞ:
ð1Þ
Using the reported values for D0 ðAuPdÞ, 1.40 eV, and the EA of Au atom, 2.308 eV, a value of 0.97 eV is evaluated for D0 ðAuPd Þ (X3 D ðor 1 DÞ). The dissociation energy of AuPd is much smaller than that of the neutral AuPd. This is because the excess electron occupies the anti-bonding r orbital. The large difference between D0 ðAuPd Þ and D0 ðAuPdÞ should result in difference of bond length between AuPd and AuPd, which gives a reasonable explanation for the relatively broad peak A01 in Fig. 2. There have been little experimental data and no calculated results for the electronic excited states of AuPd dimer. Between the spectra of AuPd and Au 2 in the BE range up to 4.5 eV, however, the similarity can be recognized. Since the peak B2 in the Au 2 spectrum has been assigned as the transition from the electronic ground state of the anion 2 þ R ðr2 r Þ to the first excited state of the neutral a 1 þ R ðrr Þ, the analogy enables us to assign the peak C01 of AuPd to the transition from the electronic ground state of the anion X 3 D (or 1 D) to the excited state of the neutral anion 4 D (or 2 D) (ð4dÞ9 rr ). Furthermore, the peak B01 in the AuPd spectrum is assigned as the transition into 8 the first excited state having (ð4dÞ r2 r Þ configuration, where one of d electrons in 4d is ejected from the anion. By the analogy of NiCu [30], the 8 (ð4dÞ r2 r ) configuration is probably expressed as 8 3 ð4dÞ ð FÞr2 r , although theoretical calculations are necessary for the accurate assignment of the peaks.
3.2.2. Au2 Pd The negatively charged trimers of pure alkali and group-11 (conventionally group 1B; Cu, Ag, Au) clusters are linear [8–10,12,13] with a 1 Rþ g ground state. In the electron detachment for most of these species (Na 3 , K3 , Rb3 , Ag3 , and Au3 ) [9,18,32] the transition into the neutral ground state 2 Rþ u of the neutral trimer occurs [18]. For Au2 Pd 1 , the spectrum exhibits only a sharp peak at 266 nm detachment, and the spectral feature is essentially the same with that of the linear Au also 3 , implying that Au2 Pd 2 takes a linear structure having a D ground state with a weakly interacting 4d core, where the Pd atom configuration can be described as ½Krð4dÞ9 ð5sÞ1 . The sharp peak can be plausibly attributed to the transitions for linear geometries between the anion and the neutral, because the transitions for lower dimensional structure result in sharper peaks and short progressions. This has actually been reported in the C n spectra [33]. For the linear structure of Au2 Pd , two kinds of isomer are conceivable; Au–Au–Pd and Au–Pd– Au. According to our recent experiment [34] and the calculations [35] on Au2 Ag , the most stable structure is linear Au–Ag–Au , where the excess electron is preferably localized on the terminal Au atoms. Since the EA of Au (2.31 eV) is larger than that of Ag (1.30 eV), the structure of Au–Ag–Au is much more stable than that of Au–Au–Ag . When a Pd atom takes the ½Krð4dÞ9 ð5sÞ1 config10 1 uration, an Ag atom ð½Krð4dÞ ð5sÞ Þ can be regarded as an isoelectronic atom except for no vacancy in 4d orbitals. By analogy of Au2 Ag , namely, it seems reasonable to deduce that Au2 Pd takes the linear structure of Au–Pd–Au , because the EA of a Pd atom (0.56 eV) is very small [25]. Indeed, the ET s of both anions are almost the same; Au2 Pd (3.80 eV) and Au2 Ag (3.81 eV). A successive peak B02 is observed next to the peak A02 in the spectrum of Au2 Pd . This is a new peak compared to the spectrum of Au 3 . The peak B02 is seemingly assigned as the transition with 4d electron detachment from the Pd atom, which is a common origin for the peak B01 in AuPd .
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3.2.3. Aun Pd (n ¼ 3 and 4) In the PES spectra of Au3 Pd and Au4 Pd , the number of observed peaks is increased. Spectral similarity for number of peaks and for intensity ratio is still perceivable between Au 4 and Au3 Pd , and also between Au5 and Au4 Pd , although peaks are congested in higher binding energy in the spectra of Aun Pd . Recent theoretical calculations predict that Au 4 has two essentially degenerate structures: a C2v ‘T-shaped’ structure and a ‘zigzag’ chain structure [13]. Furthermore, the optimized structure of Au 5 is calculated to be a planar trapezoid [13]. Judging from the spectral similarity, the geometric structure is likely maintained by the substitution of an Au atom by a Pd atom, although it is hard to predict the preferable position of Pd atom in the substitution. Congested peaks in higher binding energy indicate the electronic contribution from 4d orbitals.
4. Conclusions We observed the photoelectron spectra of the small Au–Pd mixed clusters produced by the twolaser vaporization of Au and Pd rods. The spectral similarity between Au indicates n and Aun1 Pd that the Pd atom is mixed in the clusters with the promoted electronic configuration of ½Krð4dÞ9 ð5sÞ1 , and that the geometries are almost maintained in the substitution. The present work can provide an important base to understand not only the electronic but also the geometric structures of the small Au–Pd clusters.
Acknowledgements We are grateful to Mr. M. Ge and Prof. K.S. Kim for sending their results prior to publication. We are also grateful to Dr. A. Pramann and Mr. K. Miyajima for their technical help. This work is supported by a program entitled ‘Research for the Future (RFTF)’ of the Japan Society for the Promotion of Science (98P01203) and by a grantin-aid for scientific research (C) (No. 13640582) from the Ministry of Education, Science, Sports and Culture.
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