Small Copper Clusters in Ar Shells - American Chemical Society

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Small Copper Clusters in Ar Shells: A Study of Local Structure V. L. Mazalova* and A. V. Soldatov Faculty of Physics, RostoV State UniVersity, Sorge 5, RostoV-on-Don, 344090, Russia

S. Adam HASYLAB at DESY, Notkestrasse 85, D-22603 Hamburg, Germany, and TransilVania UniVersity, B-dul Eroilor 29, BrasoV, 500036, Romania

A. Yakovlev Scientific Computing and Modelling NV, Vrije UniVersiteit, De Boelelaan 1083, Amsterdam, NL-1081HV The Netherlands

T. Mo¨ller Technische UniVersitaet Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany

R. L. Johnston School of Chemistry, UniVersity of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom ReceiVed: October 23, 2008; ReVised Manuscript ReceiVed: March 20, 2009

Small copper clusters embedded in argon shells were obtained using the pick-up technique. The clusters were analyzed using X-ray absorption near-edge spectroscopy (XANES) measured at the Cu L23-edge. Theoretical interpretation of the experimental spectra has been performed using full-potential finite difference method simulations and density functional theory (DFT) to optimize the geometry of the clusters. As a result, it was found that the icosahedral Cu13 nanocluster with the geometry optimized by DFT produces theoretical Cu L23-XANES similar to the experimental one, whereas the theoretical XANES of the cuboctahedral cluster differs from that of the experimental. The bonding energy for the icosahedral cluster obtained by the present DFT study has a higher absolute value than that for the cuboctahedral cluster, thus also supporting the higher stability of the icosahedral Cu13 nanocluster. I. Introduction The study of nanoclusters of metallic atoms is a multidisciplinary field ranging from physics1 to chemistry2 and even to environmental problems3 and biological sciences.4 It has grown tremendously during the past decade. Indeed, clusters are of fundamental interest due to their own intrinsic properties but also because of their intermediate position between molecular and condensed matter sciences;5 thus, experimental and theoretical investigations must concern both gas-phase and solid-state approaches.6 The geometry of free clusters is of particular importance because it impacts not only on surface properties, such as nucleation, growth morphology, and catalysis, but also on electronic properties, such as insulator-to-metal transitions.7 A detailed understanding of how the transitions in observed properties take place when going from the single atom or molecule to the bulk solid is connected to the knowledge of the cluster atomic structure and its evolution with cluster size. The most significant questions regarding the structures of these nanoclusters are related to the interatomic distances8-10 and to the modification of the atomic arrangement (symmetry) as a function of the cluster size,11-17 especially when the cluster size is reduced to the critical nucleation size. * Corresponding author. E-mail: [email protected].

We have chosen Cu13 clusters for the present study because of their high symmetry and because relatively high fluxes of these particles can be generated, though Christensen and Jacobsen have suggested that Cu13 clusters are not particularly stable in comparison with closed jellium shell clusters, such as Cu8 and Cu18.15 In some studies, a cuboctahedral geometry has been used to describe the Cu13 cluster,15,18 whereas others have found the icosahedral structure to be more stable.19,20 Theory predicts a crossover between icosahedral and cuboctahedral (fcclike) structures at a size of about 2000 atoms.8,20,21 Small Cu clusters have been successfully prepared using various techniques: radiolysis and electrolysis from aqueous solutions,22,23 gas aggregation,6 laser vaporization,24 etc. Depending on the production method, the clusters are studied in the gas phase, on the surface of a support, or embedded in raregas matrices or polymers. To date, only a few experimental studies have given direct insight into the geometric and electronic structures of small copper nanoclusters. X-ray absorption spectroscopy has demonstrated the unique ability to selectively probe the local surroundings of individual atomic sites.25,26 Experimental efforts alone are, however, insufficient to extract full information on the dependence of cluster structure on size. Indeed, recent investigations27,28 have clearly shown that high level, ab initio

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Figure 1. Schematic representation of the experimental setup. The pick-up process parameters are the stagnation pressure p0, the nozzle temperature T0, and the oven temperature Toven. The oven crucible is filled with high-purity bulk Cu. The size of pure Ar or embedded Cu clusters is measured with the time-of-flight mass spectrometer. The clusters are deposited on a cold Au substrate.

Figure 2. Cuboctahedral (a), icosahedral (b), and amorphous (c) models of the Cu13 nanocluster.

TABLE 1: Interatomic Distances (in Å) for the Three Types of Cu13 Nanoclusters Presented in Figure 2 Obtained by Different Methods of Calculationa DFTc structure Oh Ih amorphous

LCAO-MO

b

Gupta potential

d ) 2.43 d1 ) 2.40

d ) 2.47 d1 ) 2.42

d2 ) 2.52 -

d2 ) 2.54 ) 2.52

c

parametrized TB-LMTO

d

LDA

d2 ) 2.51 -

d ) 2.41 d1 ) 2.35 d2 ) 2.47 ) 2.39

GGA d ) 2.45 d1 ) 2.38 d2 ) 2.51 ) 2.44

a

b

The d1 is the interatomic distance between the central Cu and an outer Cu, and d2 is the distance between neighboring outer Cu atoms. Reference 49. c Present study. d Reference 13.

TABLE 2: Bonding Energy Eb (eV/atom) between the Atoms of the Cu13 Clusters DFTa

a

structure

LDA

GGA OPBE

parametrized TB-LMTOb

MO-LCAOc

Gupta potentiala

Oh Ih amorphous

2.91 2.94 2.90

1.96 1.99 1.93

2.25 2.46 -

2.64 -

2.52 2.60 2.48

Present study. b Reference 13. c Reference 49.

theory is required to extract full structural information from experimental results. Improvements in theoretical methods have been shown to lead to better understanding of the electronic structures of copper nanoclusters.16,29 Thus, in the present study, we apply both modern ab initio theory and the very powerful XANES experimental technique to gain deeper insight into the relationship between the local geometry and electronic structure of Cu13 nanoclusters. II. Method of Calculation The calculations of the Cu L23-edge XANES spectra were performed using the full-potential FDMNES_2007 code,30 which

runs within the real space cluster approach and uses the finite difference method (FDM) to solve the Schro¨dinger equation. Its main advantage is the possibility to have a totally model free potential shape, thus getting rid of the muffin-tin limitations. This is a key point because, as shown very recently,31 the classical multiple scattering scheme,32,33 which uses the muffintin approximation,34 is not able to give reasonable agreement with the experimental XANES spectra for atoms at the surface of clusters. For the Cu13 nanocluster, the surface/inside atom ratio is 12 to 1; thus, the surface atom contribution dominates the total XANES spectrum of the cluster. In the second step, the multielectronic processes are taken into account through an energy-dependent Lorentzian convolution of the spectrum. At

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Figure 5. Pictorial view of (a) the icosahedral core-shell Cu13Ar42 cluster and (b) the cuboctahedral core-shell Cu13Ar42 cluster (Cu13 forms the core, which is surrounded by Ar atoms).

Figure 3. Cu L3-edge XANES spectra of free Cu13 nanoclusters. From top to bottom: (1) experiment, (2) theory for free Cu13 nanocluster with icosahedral structure, (3) theory for free Cu13 nanocluster with cuboctahedral structure, and (4) theory for free Cu13 nanocluster with amorphous structure. Theoretical spectra are results of calculations for all nonequivalent positions of absorption atom in the cluster.

the BCGA to search for low-energy Cu clusters, modeled using the Gupta potential, have previously been presented.37 XANES simulations were performed using optimized geometries calculated by the all-electron density functional theory (DFT) approach implemented38 in the ADF2007 code. The electronic configuration of the cluster was described by an uncontracted double-ζ basis set of Slater-type orbitals. Energies were calculated using different types of exchange-correlation potentials: Vosko, Wilk, and Nusair’s local exchange-correlation potential39 (LDA) and the generalized gradient approximation (GGA) with OPBE exchange-correlation potential.40 The OPBE exchange-correlation correction is equivalent to OPTX’s exchange41 + PBE’s correlation.42 III. Experimental Section

Figure 4. Cu L3 XANES spectra. From top to bottom: (1) experiment; (2) theoretical curve for a Cu13 cluster in an argon matrix (icosahedral core-shell Cu13Ar42 cluster; see Figure 5a); and (3) theoretical curve for a Cu13 cluster in an argon matrix (cuboctahedral core-shell Cu13Ar30 cluster; see Figure 5b). Both models were obtained by geometry optimization using DFT theory. Theoretical spectra are results of calculation for all nonequivalent positions of the absorption atom in the cluster.

low energy, the convolution width includes only the core hole width and the experimental resolution; at higher energy, the broadening increases with the many-body processes induced by the photoelectron (e.g., inelastic losses). Small Cu clusters (N ) 13 atoms) were modeled using the Gupta atomistic potential, which comprises a pairwise additive repulsive term and a many-body attractive (cohesive) term.35 The parameters for Cu were taken from the work of Cleri and Rosato, who fitted the Gupta parameters to bulk properties of fcc Cu.35 Low-energy structures for these Cu clusters were found by using the Birmingham Cluster Genetic Algorithm (BCGA) code, which has been described previously.5 The genetic algorithm searches for optimal and near-optimal solutions using a combination of evolutionary operators, such as crossover, mutation, and natural selection, which are analogues of natural processes in biological evolution.36 Some results of applying

Experiments have been performed using a cluster source (the experimental setup was similar to that described elsewhere43) installed at the BW3 beamline of HASYLAB (Hamburg). The scheme of the experiment is represented in Figure 1. Previously, embedded clusters have been synthesized by means of the pickup technique mainly in free cluster (gas-phase) studies.44-48 Deposited clusters that are isolated inside rare-gas matrices are less represented in the literature partially due to technical challenges (very low temperature for substrates, soft landing conditions, etc.). Using a pick-up source, Ar embedded Cu clusters are produced with average sizes depending on the cluster source parameters (pressure and temperature) and oven temperature. The primary Ar cluster beam passes through a skimmer into the oven chamber. The general purpose of the skimmers is to maintain a differential pumping stage. Our experimental setup has four skimmers, and the result is different pressure in each section of the experimental setup during the cluster experiment. In the case of the first skimmer, it is possible to “select” mainly the condensed, unscattered part of the beam, that is, the Ar clusters. The crucible is filled with high-purity Cu, which is vaporized and then captured by the rare-gas clusters. First, the Cu atoms are stored on the Ar cluster surface. The kinetic energy of the guest atom and the bonding energy (EArCu) are transferred to the host rare-gas cluster. The excess energy is then dissipated by evaporation of an appropriate number of Ar atoms. Using a time-of-flight mass spectrometer, the distribution of Cu clusters is measured for oven temperatures in the range of 1190-1310 °C. The mass spectra clearly indicated the presence of small Cu clusters consisting of 5-15 atoms/cluster, but the distribution of the cluster sizes shows that most of the clusters have a size between 10 and 13. For this temperature regime, due to the presence of Ar clusters, one might conclude that Cu clusters are protected by an Ar shell. If the oven temperature is lower than 1190 °C, only pure Ar clusters are detected. For temperatures slightly higher than 1310 °C, only Cu clusters are measured, whereas for higher temper-

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Figure 6. Most stable geometries of the Cu3, Cu7, Cu13, Cu30, and Cu135 clusters.

TABLE 3: Mulliken Population of Atomic Orbitals As a Function of Copper Cluster Sizea cluster size Cu1 Cu3 Cu7 Cu13 Cu30 Cu135 Cubulk

electronic configuration [Ar]3d104s1 [Ar]3d9.94s0.94p0.194f0.005 top atoms [Ar]3d9.84s0.684p0.574f0.011 plane atoms [Ar]3d4s4p4f central atom [Ar]3d9.6934s0.614p1.0734f0.024 surface atoms [Ar]3d4s4p4f [Ar]3d4s4p4f all atoms [Ar]3d4s4p4f central atom [Ar]3d9.5884s1.0174p0.6754f0.017 surface atoms [Ar]3d4s4p4f [Ar]3d9.6414s0.6164p0.7294f0.015

a Because there are several nonequivalent atom positions in some clusters, we presented the averaged values for these cases in brackets.

atures, the Ar clusters are completely destroyed due to the high vapor pressure of Cu atoms inside the oven. This means that, by simply tuning the cluster source parameters (conical nozzle, temperature T0, and a stagnation pressure p0) and the oven temperature Toven, one can prepare different sizes of pure raregas clusters, Cu/Ar core/shell structures, or free Cu clusters. In the present study, we have used the following set of parameters: p0 ) 99 mbar, T0 ) 77 K, and Toven ) 1310 °C. The embedded Cu clusters are deposited on a high-purity gold foil. The substrate is kept cold (T ≈ 10 K) with the help of a liquid He cryostat. The XPS and XANES investigations reveal that, in the case of pure Ar cluster deposition, a solid rare-gas matrix with fcc structure is obtained on the cold substrate (T < 25 K). The rare-gas shell prevents the destruction of Cu clusters on contact with the substrate or coagulation on the substrate surface. Thin films (few tens of an angstrom) of Cu clusters isolated inside a rare-gas matrix were deposited on the substrate and investigated by means of spectroscopic techniques. The stability and homogeneity of the cold deposited clusters were monitored with the help of the photoelectron spectroscopy technique. XPS measurements show that the contamination (C or O atoms) of the deposited samples is low, with a stable chemical composition, and Cu/Ar ratios are independent of the film thickness.

The acquisition of the X-ray absorption spectra at the Cu L23edges is performed simultaneously in two different ways: total electron yield (TEY) and Auger electron yield. As the shape of obtained Auger and TEY spectra coincides and TEY shows better statistics, we show below only TEY spectra. With the experimental setup used in the present work, one can measure XANES spectra of clusters with a fixed size. The method consists of the following: One fixes the cluster size by using a time-of-flight mass spectrometer and choosing the peak of the mass spectrum corresponding to (for example) the Cu13 cluster size. Then, one starts changing the energy of incident X-rays and measuring the current of a mass spectrometer as a function of the energy of incident X-rays. IV. Results and Discussion There are two possible ways to study the structures of small free (or hosted in a rare-gas matrix) nanoclusters: theoretical predictions and experimental studies. A potential drawback of theoretical approaches is that the calculated bond lengths can be sensitive to the choice of the method and parameters (or functionals) used, as shown, for example, in Table 1, where the structural parameters for clusters having Oh and Ih symmetry and for a nonsymmetrical isomer are presented. As one can see, the differences between the three models are smaller than the differences in the same parameter (interatomic distance) obtained using different approaches, though it agrees that the interatomic spacing in the cuboctahedral and amorphous clusters is intermediate between the short (radial) and long (surface) bonds in the icosahedral structure. However, it is the combination of highlevel theory and experimental analysis that offers the best opportunity for extracting reliable structural information from the experimental data. To select the most probable structure of the Cu13 nanocluster, we also calculated the bonding energy between the atoms in the models in the study. In our DFT approach, the bonding energy is calculated as the energy of the cluster minus the energy of the constituent atoms. In Table 2, we present the bonding energy Eb for cuboctahedral, icosahedral, and amorphous Cu13 nanoclusters obtained using two types of exchange-correlation potentials: Vosko, Wilk, and Nusair in the LDA framework and the OPBE-type of potential.

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TABLE 4: HOMO and LUMO Energies EHOMO and ELUMO (eV) and Energy Gap Values ∆EHOMO-LUMO for Clusters of Different Sizes Obtained by DFT Using the LB9453 Model for the Potential cluster

symmetry

Cu1 Cu3 Cu6 Cu7 Cu13

C2V Oh D5h Ih

interatomic distance, Å

EHOMO, eV

ELUMO, eV

2.31 2.36 2.38 2.41

-11.624 -11.048 -11.506 -9.851 -11.046

-3.873 -5.586 -6.480 -7.116 -8.584

Our DFT calculations predict the icosahedral structure to be more stable than the Oh and nonsymmetrical isomer by around 0.03 and 0.06 eV/atom, respectively. The higher stability of the icosahedral Cu13 cluster is in agreement with the finding of Lammers and Borstel,13 Demuynck et al.,49 Seifert,50 and Valkealahti and Manninen11 but disagrees with some previous studies.18,15 Our results also did not show any evidence (at least under the conditions of the present study) for the amorphous nature of Cu13 clusters, which has been proposed.51 This is in agreement with most results obtained so far for small condensed nanoclusters for temperatures below the melting point. To study the problem of the most probable structure of the Cu13 nanocluster from the experimental point of view, we performed an analysis of the experimental Cu L3-edge XANES. In Figure 3, one can see a comparison of the low-energy part of the experimental Cu L3-edge XANES for Cu13 nanoclusters with theoretical spectra calculated using full potential (FDM) for three models of the structure: icosahedral (curve 2), cuboctahedral (curve 3), and amorphous (curve 4). The calculations were done using the real Hedin, Lundqvist, and Von Barth potential. All models were obtained as a result of cluster geometry optimization in the framework of DFT GGA theory. As a model for the cuboctahedral cluster, we took a fragment of bulk fcc copper (see Figure 2a); for the icosahedral structure, we used the cluster obtained on the basis of the Gupta empirical atomistic potential (see Figure 2b), and the amorphous cluster is one of the local minima for the Cu13 structure. A comparison of the Cu L3-edge XANES spectra for the three structural models (Figure 3) with the experimental curve clearly shows that, according to the position and relative intensity of peak B, the icosahedral structure gives much better agreement with the experimentally observed spectrum. Thus, a Cu13 cluster with Ih symmetry looks more probable than a cuboctahedral and amorphous one. Therefore, a comparison of theoretical XANES for Oh, Ih, and nonsymmetrical models with both experimental spectra and DFT bonding energy calculations shows the Ih structure to be more stable than the cuboctahedral and amorphous one for Cu13 nanoclusters. In Figure 4, we present a comparison of the experimental Cu L3-edge XANES of Cu13 nanoclusters with the FDM (nonmuffin-tin) simulations. There are three features (A, B, and C) on the experimental XANES spectrum (curve 1) of copper nanoclusters in the argon matrix (at a temperature of about 8 K). Curve 2 represents the theoretical spectrum of a core-shell Cu13Ar42 nanocluster with icosahedral geometry, optimized using DFT theory for both Cu13 and Ar42 subshells (Figure 5a shows the Cu13 cluster surrounded by a 42 Ar atom shell). Curve 3 represents the theoretical spectrum of a core-shell Cu13Ar42 nanocluster with cuboctahedral geometry, optimized using DFT theory for both Cu13 and Ar42 subshells (Figure 5b).The influence of the argon matrix results in the appearance of an additional shoulder C in the high-energy region of the experimental XANES spectrum at a low relative concentration of copper

|∆E

HOMO-LUMO|,

eV

7.750 5.463 5.026 2.735 2.462

atoms. Peaks B and C are mostly related to the icosahedral geometry of the copper atoms in the Cu13Ar42 nanocluster. The electronic structure of atomic copper is described by the configuration, [Ar]3d104s1, but a copper atom in the bulk crystal is thought to have the configuration, [Ar]3d10-x4s1+x, where x ) 0.4.52 This is a signature of chemical bonding changes, and thus, the study of changes in electronic configuration of copper atoms in clusters of different sizes is interesting. Within the DFT framework, we obtained the electronic configurations for the clusters containing 3, 7, 13, 30, and 135 copper atoms (see Figure 6) and for bulk copper. In Table 3, we show the Mulliken populations of atomic orbitals as a function of copper cluster size. As one can see, there are changes in atomic orbital populations during the transition from a single atom to the bulk. With increasing cluster size, the occupation of the 4p orbitals increases, whereas the number of electrons in 4s orbitals decreases. OurDFTcalculationsalsoshowanincreaseintheHOMO-LUMO energy gap with decreasing cluster size (Table 4), which is consistent with a size-induced transition from the metallic to the nonmetallic state. The results obtained for small copper nanoclusters agree with the assumption54 of the nonmetallic nature of small clusters constructed from metal atoms. V. Conclusion Cu L3-edge XANES spectra are rather sensitive to variations of nanocluster geometry (i.e., to bonding angles), in addition to the well-known sensitivity of XAFS spectra to small variation in interatomic distances. For small copper nanoclusters, the icosahedral arrangement is more favorable compared with the amorphous and cuboctahedral (observed in large clusters and bulk copper) ones. The stability of the icosahedral structure for Cu13 nanoclusters is supported both by experimental XANES analysis as well as by DFT calculations. Acknowledgment. Part of this research (for V.M. and A.S.) has been supported by Russian Foundation for Basic Research, Grant No. RFBR 09-02-12257-ofi_m. References and Notes (1) Ferretti, N.; Balkaya, B.; Vollmer, A.; Neeb, M.; Eberhardt, W. J. Electron Spectrosc. Relat. Phenom. 2007, 156-158, 124. (2) Tamura, K.; Oyanagi, H.; Kondo, T.; Koinuma, M.; Uosaki, K. J. Phys. Chem. B 2000, 104, 9017. (3) Chen, C. Y.; Wang, H. P.; Wei, Y. L.; Jou, C. J. G.; Huang, Y. C. Radiat. Phys. Chem. 2006, 75, 1913. (4) Alivisatos, P. Nat. Biotechnol. 2004, 22, 47. (5) Johnston, R. L. Dalton Trans. 2003, 4193. (6) de Heer, W. A. ReV. Mod. Phys. 1993, 65, 611. (7) von Issendorff, B.; Cheshnovsky, O. Annu. ReV. Phys. Chem. 2005, 56, 549. (8) Apai, G.; Hamilton, J. F.; Stohr, J.; Thompson, A. Phys. ReV. Lett. 1979, 43, 165. (9) Montano, P. A.; Shenoy, G. K.; Alp, E. E. Phys. ReV. Lett. 1986, 56, 2076.

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