Tuning the Properties of Pd Nanoclusters by Ligand Coatings

Apr 15, 2014 - Tuning the properties of palladium nanoparticles using different protecting ... The Journal of Physical Chemistry C 2015 119 (20), 1096...
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Tuning the Properties of Pd Nanoclusters by Ligand Coatings: Electronic Structure Computations on Phosphine, Thiol, and Mixed Phosphine−Thiol Ligand Shells Barbara Fresch and F. Remacle* Department of Chemistry, B6c, University of Liege, B4000 Liege, Belgium S Supporting Information *

ABSTRACT: Tuning the properties of palladium nanoparticles using different protecting ligand shells is an important step toward the application-orientated design of nanoparticles for nanoelectronics and catalysis. We present a density functional theoretical characterization of Pd13 and Pd55 metal cores protected by only thiol, only phosphine, and mixed phosphine−thiol ligand shells. We analyze the ligand contributions to the frontier orbitals and the charge redistribution between the ligand shell and the metal core and show that these properties control the values of the charging energy and the catalytic activity. The charge transfer character of the metal−ligand interaction is influenced by the presence of other ligands in the capping system indicating a cooperative effect in the ligand induced charge redistribution. Because of the interplay between the stabilization of the frontier orbital due to the contribution of the sulfur and the charge donation by the phosphine, the charging energy of the mixed phosphine−thiol protected cluster is larger than that of the only phosphine and the only thiol systems. The complementary point of view is adopted for rationalizing the catalytic properties of the clusters by analyzing the effect of the interaction with the metallic core on the properties of the ligand. The impact of solvation on the electronic structure of the ligand capped Pd13 cluster is investigated by including explicitly a layer of water molecules in the model system.

1. INTRODUCTION Experimental and theoretical efforts to exploit the potentialities of metal nanoparticles (NPs) have improved our understanding of their structure−property−performance relationships. In particular, the controlled synthesis of noble metal nanoparticles protected by an organic ligand shell led to wide applications in catalysis,1−5 nanoelectronics,6,7 and nanomedicine.8,9 NPs exhibit single-electron characteristics10−13 (such as quantized capacitance charging) and can be organized through selfassembly methods into well-ordered structures, with the nanoparticles at controlled locations.14,15 Because of these characteristics, metal NPs are among the most suitable candidates as simple building blocks for nanoelectronics, including junctions, heterojunctions, and single-electron transfer devices.6,16−18 More recently, size selective synthesis and characterization of sub-nano metal clusters (NPs smaller than 1 nm, composed of dozens of atoms) have been achieved and promising results on sensing and catalytic performance demonstrated.19−23 One of the most attractive properties of these materials is the ability to engineer ligand shells composed of different molecules.24 Each of the ligand shell molecules provides a different property,25,26 ideally enabling tuning the nanoparticle properties according to the needs of different applications. Sometimes properties are not simply additive, and cooperative effects emerge so that the presence of more than one ligand type in the ligand shell imparts novel properties to the nanoparticles.27,28 © 2014 American Chemical Society

We present a theoretical characterization of the electronic structure of ligated palladium NPs (Pd13 and Pd55) with the aim of gaining insights into the role of the ligand shell and the emergence of cooperative effects on their chemical behavior. Pd NPs have a complex electronic structure and a better understanding on how they are controllable by tuning the ligand shell impacts the engineering of their unique catalytic, sensing,1,4,29 and magnetic30 properties. Electronic structure features allow the characterization of different properties of the nanoparticles depending on the point of view of the analysis: by focusing on the effect of the ligand shell on the metal core, we provide understanding on the modulation of the nanoparticle properties through the control of their capping agents. On the other hand, by investigating how the molecular properties of the ligands are affected by the interaction with the metal core, we gain insights into the catalytic activity of the nanoparticle. Phosphine and thiols are common stabilizing ligands for noble metal nanoparticles.4,31−33 A possible route to synthesize thiol protected NPs is ligand exchange starting from phosphinestabilized precursors,34−36 which is also a mean to obtain mixed ligand shell phosphine/thiols species. In ref 37, we studied the interaction of methane-thiols and phosphines with the subnanometer sized Pd13 cluster, focusing on the effects of the Received: February 8, 2014 Revised: April 10, 2014 Published: April 15, 2014 9790

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interacting with 14 molecules of water. While not a complete solvation shell, this system allows us to study the effects of the water molecules that closely surround the ligand-protected nanoparticle. We discuss the consequences of these different interactions on the related spectroscopic observables and reactivity of the metallic core.

ligands on the geometry and on the high magnetic moment characterizing the neutral bare cluster.38,39 For catalysis of red-ox reactions and nanoelectronic purposes, it is important to characterize processes involving electron transfer from and to the metal clusters40 and to provide values for ionization potential (IP), electron affinity (EA) and charging energy (Uc) in different environments. Here we characterize the neutral and the charge states of the only phosphine (Pd13(PH3)12), only thiol (Pd13(SCH3)12), and mixed ligand shell (Pd13(PH3)6(SCH3)6) palladium clusters by means of the quantum chemistry implementation of density functional theory. Our computations indicate that the mixed ligand shell cluster exhibits a cooperative effect between the two kinds of ligands. Its charging energy is higher than the charging energy of both pure-phosphine and pure-thiol clusters. We rationalize this effect on the basis of the electronic properties and charge redistribution induced by the ligand molecules. The analysis of both the neutral and the charged species of the different ligated Pd13 clusters allows identifying the key properties of the neutral cluster related to the energetics of the charging process, that is, composition of the highest occupied molecular orbital (HOMO) and charge redistribution between metal core and ligand shell. The identification of the chemical properties of the neutral species governing the charging processes is important to make reasonable predictions on the charging energetics of larger clusters, for which reliable electronic structures can be obtained for the neutral close-shell species only. As an example, we consider the larger nanosized metal particles, Pd 5 5 (PH 3 ) 2 0 , Pd 5 5 (SCH 3 ) 2 0 , and Pd55(PH3)10(SCH3)10, whose metallic cores have a diameter of about 1.1 nm and characterize the main effects of the three different capping systems on the neutral species. On this basis, we argue that the cooperative effect on the charging energy that we observe explicitly for the Pd13 mixed coverage cluster scales to the larger size Pd55 one. While charging of the nanoparticle requires the addition/ removal of a whole electron, catalytic activity is often related to partial charge redistribution between the metal core and the adsorbate. Specifically, the reactivity and catalytic properties of transition metals are closely related to the population of their dband41 below the Fermi level. For example, the high reactivity of Pd compared to Au has been explained in terms of its highly populated d-band near the Fermi level. The closer the d-band center is to the Fermi level, the easier the charge transfer between the metal surface and the adsorbate, which makes palladium capable of breaking bond of adsorbates on its surface.42 Experimentally, the occupied portion of the d-band can be studied with photoemission and X-ray emission techniques,43 while the unoccupied densities of states of dcharacter can be investigated with the L3,2-edge X-rayabsorption near edge structure (XANES). We report on the electronic configurations of the Pd13 and Pd55 metal clusters induced by the different capping systems. Thiols withdraw charge from the metal and induce a decrease in its density of states (DOS) around the Fermi level, while phosphines have the opposite effect. Moreover, ligand-protected nanoparticles are often in solution especially when they are developed for catalytic applications. In this case, in addition to the effects of the ligand shell, the interaction with the molecules of the solvent may influence the reactivity of the system. In order to assess to what extent the electronic structure of the ligand− metal core complex is affected by solvation, we optimize and investigate the structure of the thiol protected Pd13 cluster

2. METHODS All the structures of the neutral and charged ligand protected Pd clusters were fully optimized using the B3LYP44,45 hybrid functional with relativistic LANL2DZ46,47 pseudopotentials and basis sets for the Pd atoms and the 6-31+G(d) Gaussian basis set for the ligands, as implemented in the GAUSSIAN0948 quantum chemistry suite of programs. To the best of our knowledge, this is the first study on palladium cluster as large as Pd55 that employs a hybrid, exchange corrected functional. The B3LYP hybrid functional is recognized to be more adequate than pure functionals in describing the interactions between the metal core and organic ligands,49−51 which is the focus of this paper. Electronic structures of open shell species were calculated within the unrestricted DFT formalism and they show negligible spin contamination (see Table SI-1 in the Supporting Information (SI)). The equilibrium geometries are fully relaxed since no symmetry constraint has been imposed. Frequency calculations on optimized geometries showed no imaginary frequencies and give the thermochemical characterization of the optimized structures. We fully optimized the cation and the anion of the three ligated Pd13 clusters in order to calculate ionization potentials (IP), electron affinities (EA), and reorganization energies (λ), together with hole extraction potential (HEP) and electron extraction potential (EEP).52 The IP is calculated as IP = E(N − 1) − E(N) where E(N) denotes the electronic energy of the neutral complex while E(N − 1) refers to the cation. The electron affinity is calculated from the energy of the neutral and of the anion as EA = E(N + 1) − E(N). IP and EA can be either for vertical (VIP and VEA; computed at the equilibrium geometry of the neutral) or adiabatic excitations (AIP and AEA; computed for the equilibrium geometries of the neutral and the charge species). In addition, HEP is the energy difference from M (neutral molecule) to M+ (cationic), computed at the M+ equilibrium geometry, and EEP is the energy difference from M to M− (anionic), computed at the M− equilibrium geometry. The reorganization energy (λ) quantifies the geometric and electronic relaxation accompanying the charge transfer, it is defined as λh = VIP − HEP for electron detachment and λe = VEA − EEP for electron attachment. The charge distribution within the ligand protected cluster has been calculated according to the natural bond orbital (NBO) analysis53,54 that defines electronic populations as well as natural electron configuration for each atom of the molecular system. We have shown in ref 37 that different schemes (NBO, Mulliken and charges fit to the electrostatic potential at points selected according to the Merz−Singh−Kollman scheme) lead to the same trend for ligand protected Pd cluster. The density of states (DOS), partial density of states (PDOS), and overlap population density of states (ODOS) have been generated with AOMix program55,56 using the default MPA (Mulliken Population Analysis) and a Gaussian line-shape with width at half-height equal to 0.2 eV. The Wiberg index for bond orders57 has been calculated from both the canonical molecular orbitals (MOs) in the AO basis and from the NBO basis, agreement in the highlighted trends has been verified. 9791

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Figure 1. Optimized geometries of the ligand protected Pd13 (top) and Pd55 (bottom) neutral clusters. The metal core of each complex is shown separately to emphasize the different structural distortions induced by the capping system. For the Pd55 nanoparticles an isocontour (isovalue 0.03 Å−3) of the HOMO orbital is shown and the HOMO energy reported.

3. RESULTS AND DISCUSSION 3.1. Structures of Ligand Protected Pd Clusters. The optimized geometries of the ligand protected Pd13 and Pd55 neutral clusters are shown in Figure 1. Starting geometries with icosahedral cores have been selected on the basis of previous studies. For the bare Pd13 core, while several low energy structural isomers have been identified,58,59 we showed that ligand capping significantly stabilizes the compact icosahedrallike geometry.37 The ground state geometry of the Pd55 core has been studied by molecular dynamics with thermal quenching procedure,60 with DFT in both local density approximation61 and with generalized gradient approximation.62 The results of all these studies pointed out that an icosahedral-like geometry is energetically favored over the other structural motifs for Pd55. More recently, the icosahedral structure of the anion of the Pd55 core has been experimentally verified by trapped-ion electron diffraction on size-selected cluster.63 While the structures obtained for the Pd55 are all diamagnetic (singlet spin state), we have shown in ref 37 that for the smaller clusters of Pd13 several magnetic states coexist. For the phosphine-covered cluster, the magnetic ground state of Pd13 at room temperature is a triplet state, the singlet being 0.19 eV higher in free energy. For the mixed ligand cluster, we found that singlet, triplet and quintet states are practically degenerate. Here we will consider the electronic structure of the singlet states for both Pd13(PH3)12 and Pd13(PH3)6(SCH3)6. The corresponding ionic species are doublets. The singlet state of the thiol capped Pd13 cluster is 1 eV higher in energy with respect to the magnetic ground state that is a quintet, so we will refer to the quintet state in the following analysis. Since the ionization of Pd13(SCH3)12 can produce ions in a quartet(q) or in a sextet(s) spin states, we will consider both the processes. For a more detailed analysis of the equilibrium geometry and the magnetic properties of the neutral Pd13 clusters protected by phosphines, thiols, and a

mixture of both, see ref 37. We briefly summarize here the points essential for the comparison with the charge species and with the neutral Pd55 clusters. The thiols are strongly bonded to the metal core with an adsorption energy per ligand equal to −1.61 eV in Pd13(SCH3)12 while the phosphines bind to the cluster through a relatively weak chemisorption interaction with an adsorption energy per ligand of −0.49 eV in Pd13(PH3)12. In ref 37, we studied the adsorption modes of one single ligand on the surface of Pd13 clusters: the energetically favored binding mode of a thiol molecule is a bridge/hollow configuration while phosphine is generally found in an on-top bonding. However, the on-top bonding of phosphine is computed to be degenerate with a bridge configuration for the icosahedral Pd13 core. Based on these results, we prepare the starting structures of the fully covered clusters by placing thiols initially in a bridge configuration between two Pd surface atoms and the phosphines in an on-top position. In Pd13(PH3)12 each Pd atom of the cluster surface interacts with a phosphine molecule that conserve the on-top binding mode upon optimization. In the bigger core Pd55, several phosphines change the initial coordination position to assume a bridge configuration around the Pd55 core, in order to maximize the number of Pd atoms interacting with the ligand shell. Overall, the interaction with the phosphine capping system does not significantly perturb the structure of the metal core, which maintains a distorted icosahedral-like geometry. On the contrary, because of the strength of the interaction and the tendency to bind on bridge sites, the optimized structures of the thiol-capped metal cores are extremely distorted (see Figure 1). Both the Pd13 and the Pd55 thiol-capped metal cores show structural disorder and a tendency toward amorphization of the cluster surface. The optimized geometries clearly show that thiol adsorption on Pd55 is able to extract Pd atoms, forming RS-Pd(adatom)-SR “staple motifs”, similar to those observed in thiol capped gold cluster.64,65 The distortion of the metal core and the extraction 9792

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Figure 2. (A) Ionization of the phosphine protected Pd13 cluster: the panel shows the HOMO and LUMO orbitals (isovalue 0.03 Å−3) of the neutral Pd13(PH3)12 cluster, the spin density (isovalue 0.0008 Å−3) of the ions resulting from the vertical ionization/electron attachment and the effects of the subsequent structural relaxation on the spin densities and HOMO orbitals of the ions. (B) Ionization of the thiol protected Pd13 cluster: the upper part of the panel shows the difference between the total electronic density of the ionic species and that of the neutral species for the ions resulting from different vertical ionization/electron attachment processes (isovalue = 0.0016 Å−3; dark blue = positive value; light blue = negative value); see the main text for discussion. For comparison, the lower part of panel (B) shows the frontier orbitals, the singly occupied orbitals (SOMOs), and the spin density of the neutral cluster Pd13(SCH3)12. (C) Ionization of the mixed ligand shell Pd13 cluster: the panel shows the HOMO, LUMO, and LUMO+1 orbitals (isovalue 0.03 Å−3) of the neutral Pd13(PH3)6(SCH3)6 cluster, the spin density (isovalue 0.0008 Å−3) of the ions resulting from the vertical ionization/electron attachment, and the effects of the subsequent structural relaxation on the spin densities and HOMO orbitals of the ions. 9793

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turns out to be the sextet spin state (ΔG(sextet-quartet) = −0.75 eV for the cation and −0.25 eV for the anion). Upon geometry relaxation, the Kohn−Sham orbitals of the quartet ions depart significantly from the neutral species while for the sextet spin states the electronic relaxation is minimal (as also indicated by the smaller reorganization energy of the sextet ions, see Table 1). The α-HOMO is the same for neutral and cation cluster and the β-HOMO of the neutral becomes the βLUMO of the cation in agreement with the removal of a β electron from the neutral cluster. Similarly the α-HOMO of the anion reflects the α-LUMO of the neutral (see Figure 2B). Ionization and electron addition in the mixed ligand shell cluster Pd13(PH3)6(SCH3)6 (see Figure 2C) induce a more pronounced reorganization than in the all-phosphine and allthiol protected clusters. The cation shows an increase in the average bond length of 0.04 Å with respect to the neutral species, and the reorganization energy amounts to 1 eV. The spin density of the vertical ionized species is consistent with the extraction of one electron from the HOMO of the neutral cluster but geometry relaxation changes the spin density considerably enhancing the spatial segregation of the α and β density (see Figure 2C). The addition of one electron induces the stabilization of the LUMO+1 orbital of the neutral cluster that is occupied instead of the LUMO as is demonstrated by the spin density distribution and the α-HOMO of the anion (see Figure 2C). The differences in the values of the VIP between the different ligand systems correlate almost quantitatively with the relative energies of the HOMO orbitals of the corresponding neutral species (see Table SI-1 in the SI for the energy of the frontier orbitals). Subsequent geometry relaxation of the cations lifts the quantitative correspondence between relative HOMO energies and relative IP but preserves a qualitative relation. The IP value of the thiol and mixed ligand shell clusters is about 1 eV higher than that of the phosphine protected cluster (5.71 and 5.85 eV for Pd13(SCH3)12 and Pd13(PH3)6(SCH3)6, respectively, against 4.76 eV for Pd13(PH3)12). Moreover, the AIP of the mixed ligand cluster is very similar to that of the thiol-capped cluster. This similarity can be explained by inspection of the composition of the HOMO orbitals. When thiols are part of the ligand shell the p-orbitals of the sulfur significantly contribute to the HOMO orbital of the cluster (>10%), which stabilizes it with respect to the HOMO of the only phosphine capped cluster. The contribution of the ligand atomic orbital to the HOMO orbital of the clusters can be easily visualized from the iso-contours of the orbitals (see Figure 2): while the HOMO is well delocalized on sulfur in the thiols and in the mixed ligand case, the HOMO of the phosphine capped cluster remains mainly localized on the metal core. In the case of the anionic clusters, the relative energy of the LUMO of the neutral species correlates only qualitatively with their relative electron affinity (see Table SI-2, SI). Consequently, the energy of the LUMO orbital correlates poorly with the energy associated with electron attachment and the HOMO−LUMO gap is not a good indicator for the charging energy. This is not surprising since electronic as well as geometric relaxations upon addition and removal of an extra electron are not negligible as discussed above. The electron attachment is energetically more favorable for the thiol-capped system than in the phosphine protected one (AEA = −2.65 and −2.03 eV, respectively). This is easily related to the well-known electron-attractor character of thiol ligands on palladium.37,42,66

of Pd adatoms are also observed in the mixed ligand case, though to less extent. 3.2. Anions and Cations: Charging Energies in Pd13. We now examine the ionic species deriving from attachment/ removal of one electron in the different Pd13 clusters. Figure 2 illustrates the electronic and geometric modifications upon charging while a summary of the energetics involved in ionization and electron attachment processes is given in Table 1. The addition or removal of one electron from the phosphineTable 1. Vertical and Adiabatic Ionization Potentials (VIP and AIP), Hole Extraction Potential (HEP) Reorganization Energy upon Ionization, λh, Vertical and Adiabatic Electron Affinity (VAE and AAE), Electron Extraction Potential, EEP, Reorganization Energy upon Electron Attachment, λe, and Charging Energy, Uc (calculated as AIP+AEA), of the Pd13 Clustersa VIP AIP HEP λh VEA AEA EEP λe Uc

Pd13(PH3)12

Pd13(PH3)6(SCH3)6

Pd13(SCH3)12

5.07 4.76 (4.98) 4.49 0.58 −1.76 −2.03/(−2.04) −2.24 0.48 2.74

5.41 5.85 (5.41) 4.38 1.03 −2.04 −2.11/(−2.36) −2.44 0.40 3.73

5.62(q), 5.85(s) 5.71 (5.68) (s) 5.43(q), 5.57(s) 0.42(q), 0.28(s) −2.54(q), −2.46(s) −2.65/(−2.73) (s) −3.13(q), −2.86(s) 0.59(q), 0.32(s) 3.06(s)

a

All the energies are in eV. The vertical energies and the charging energies are given as electronic energies differences at 0 K, while for the adiabatic IP and EA we report both the electronic energy difference at 0 K and the free energy difference at room temperature (values in parentheses). For the thiol-capped cluster, (s) means sextet and (q) means quintet.

protected cluster does not trigger drastic geometric or electronic reorganization (Figure 2A). The spin density plot of the vertical ionized cation reflects the HOMO orbital of the singlet neutral species while the spin density of the anion resulting from the vertical attachment of one electron reflects the LUMO orbital of the neutral cluster (see Figure 2A), the values of VIP and VEA are reported in Table 1. The subsequent geometry relaxation is accompanied by adjustments of the electron density as one can see from the spin density of the relaxed ions (Figure 2A); the process is characterized by a reorganization energy of about 0.5 eV for both the anion and the cation. The change in the values of the AIP and AEA is about 0.3 eV compared to the vertical value. For the thiol capped species different magnetic isomers of the ions have to be considered since the ground state of Pd13(SCH3)12 is a high spin state of multiplicity 5. In terms of vertical ionization and affinity, ions of spin multiplicity 4 are favored (VIP(4) = 5.62 eV against 5.85 for the sextet and VEA(4) = −2.54 eV against −2.46 for the sextet). The analysis of the difference between the total electron density of the neutral and the ionic species (see Figure 2B) reveals that the quartet species deriving from vertical ionization corresponds to the removal of the electron in the α-HOMO of the neutral species while the sextet results from the removal of the electron in β-HOMO. Similarly, for the anion, the quartet species derives from the addition of an electron in the β-LUMO of the neutral while the sextet derives from the extra electron being placed in the α-LUMO. Nonetheless, the magnetic ground state of the relaxed ions 9794

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Table 2. Partition of the Total Charge Amongst the Pd Metallic Core, Thiols, and Phosphines for Neutral and Ionic Pd13 Clusters, and For Neutral Pd55 Nanoparticlesa Pd total charge of the cluster Pd13(PH3)12 Pd13(SCH3)12 Pd13(PH3)6(SCH3)6 Pd55(PH3)20 Pd55(SCH3)20 Pd55(PH3)10(SCH3)10

0 −2.45 +0.39 −0.94

+1 −2.06 +0.77 −0.37 −4.46 +0.22 −2.34

SCH3 −1 −2.68 +0.17 −1.09

0 − −0.39 (−0.03) −0.64 (−0.11)

PH3

+1

−1

+0.23 (0.02) −0.27 (−0.05) − −0.22(−0.01) −0.23(−0.023)

−1.17 (−0.10) −0.98 (−0.16)

0 +2.45 (0.204) − +1.58 (0.26)

+1 3.06 (0.26)

−1 +1.68 (0.14)

+1.64 (0.27) +4.46 (0.223) − +2.57 (0.257)

1.08 (0.18)

a Atomic charges are calculated according to the Natural Population Analysis and are given in unit of the elementary electronic charge |e|. The number in brackets denotes the charge per ligand.

value of the charging energy for the mixed ligand cluster, which is calculated to be 3.73 eV, against 2.74 eV of the only phosphine and 3.06 eV of the only thiol capped clusters. For the larger size Pd55 clusters, a reliable determination of the electronic structure of the ligand-protected charged species remains prohibitive at the quantum chemistry level, because of high spin contamination and instability of the self-consistent field procedure. Nonetheless, charging properties and cooperative effects induced by the ligands can be characterized from the analysis of the HOMO frontier orbitals and of the charge distribution in the neutral species, which show features similar to the smaller Pd13 core clusters. There is a significant contribution from the p orbitals of the sulfur to the HOMO orbitals of both the only thiol capped cluster, Pd55(SCH3)20, and the mixed-ligand cluster, Pd55(PH3)10(SCH3)10, while the HOMO of the only phosphine cluster is localized on the metallic core only (see Figure 1). The energies of the HOMO orbital of the pure thiol and mixed clusters (−4.23 eV and −4.17 eV, respectively) are therefore similar and stabilized with respect to the only phosphine capped cluster (−3.97 eV). The distribution of charge follows the same trend as highlighted above for the Pd13 complexes. The Pd55 metal core acquires a partial negative charge when the phosphines are present in the ligand shell (−4.46|e| in Pd55(PH3)20 and −2.34|e| in Pd55(PH3)10(SCH3)10) and a partial positive charge (+0.22) in the only thiol capped system. The partial charge distributions in the three systems (see Table 2) reflects the same cooperative effect as in Pd13, namely an enhancement the electron acceptor/donor character of the ligands in the mixed ligand cluster. Therefore, we can reasonably predict that the energetics of the charging processes in the Pd55 nanoparticles with different capping environments will obey the same trend characterized in the smaller Pd 13 cluster. Due to the stabilization of the HOMO and the withdrawing of electronic density from the metal core, the thiol capping will induce an increase of the IP and of the EA relative to the phosphine capping. Moreover, the mixed ligand shell nanoparticle is expected to display a relatively high IP similar to the only thiol system and a relatively low electron attaching energy closer to the phosphine-protected system. 3.3. Influence of Ligands and Solvation on the Electronic Structure of the Metal Core. To understand how the electron density redistribution induced by the different capping systems affects the reactivity of the metal core the orbitals of the complexes near the frontier orbital were analyzed in details. Molecular orbitals (MO) just below the HOMO can be strongly affected by the chemical nature of the ligand shell. For palladium, in particular, the occupation of the d-states of the metal atoms near the Fermi level is an indicator of its

Phosphines have a clear tendency of donating electronic charge to the metal core while the more electronegative thiols tend to deplete charge from the metal core: in our previous study37 we found that a single phosphine binding in an on-top position donates about 0.2|e| to the Pd13 core while a single thiol withdraws around 0.02|e| from the metal core. The actual values of the charge distribution (calculated according to the NBO scheme) in the neutral and charged species considered here are reported in Table 2. The electron affinity (AEA) of the mixed ligand system is much closer to the values of the only phosphine clusters (notice that the energy of the HOMO of the anion [Pd13(PH3)6(SCH3)6]− is very close to that of the anion of the phosphine capped cluster). To intuitively understand why the mixed ligand cluster shows an electron affinity closer to the only phosphine ligand shell, we have to consider the different distribution of charge induced by the two capping systems. In the neutral Pd13(PH3)12, the palladium core acquires a net negative charge of −2.45|e|, which means that each phosphine of the ligand shell donates about 0.2|e| to the metal core, as in the case of single ligand interaction. In the only thiol cluster Pd13(SCH3)12, the palladium core is slightly positive (+0.39|e|), with each thiol withdrawing about −0.03|e|from the metal core. The distribution of charge in the mixed capping system Pd13(PH3)6(SCH3)6 reflects the stronger tendency of phosphines to donate charge: the six phosphine molecules accumulate a total positive charge of +1.58|e|, corresponding to a donation of 0.26|e| each. Correspondingly, the Pd13 metal core acquires a negative charge of −0.94|e| while the remaining −0.64|e| is transferred to the thiol ligands, each of them acquiring a negative charge of −0.11|e|. The presence of both ligands therefore induces a cooperative effect on the charge distribution: the electron-donor character of phosphines is slightly enhanced by the presence of thiols (each one donating 0.26 rather than 0.20) and the electron acceptor character of thiols is significantly enhanced by the presence of phosphines (each thiol withdrawing −0.11 rather than −0.03). Moreover, the charge donated by each phosphine in the neutral Pd13(PH3)6(SCH3)6 is the same as in the cationic cluster [Pd 13(PH 3 ) 12 ]+ and the acquired charge per thiol in Pd13(PH3)6(SCH3)6 is the same as in the anionic species [Pd13(SCH3)12]−; see Table 2. Because of this cooperative effect, in the mixed ligand cluster both the Pd13 metal core and the electron-acceptor thiols are already relatively electron-rich and the energetic associated with the accommodation of an extra-electron is thus similar to that observed in the only phosphine complex. The interplay between the delocalization of the HOMO on the ligand shell induced by thiols and the charge donation due to phosphines results in the maximum 9795

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Figure 3. Optimized geometry, spin density (isovalue = 0.0004 Å−3), and isosurface of the HOMO orbital (isovalue = 0.02 Å−3) of the solvated cluster Pd13(SCH3)12(H2O)14.

population of the d-orbital of the thiol protected Pd13 cluster. The depletion of the d-electronic density by electron attractor adsorbates can be experimentally detected by XANES spectroscopy (X-ray-absorption near-edge structure) which is extremely sensitive to the distribution and the filling of the d states in the vicinity of the Fermi level.67,71 The L2,3-edge in the XANES spectrum of Pd nanoparticle exhibits an intense spike (commonly known as a white line) and is associated with electronic transitions from the 2p state to an unoccupied 4d state.66,72 An increase of the white line intensity, corresponding to a depletion of d-electrons, in thiol-protected Pd nanoparticle has been indeed measured by this technique.66 Small metal clusters have discrete, molecular-like, density of states, instead of a continuous band. To get further insights into the effects of the different capping systems on the electronic structure of the clusters we examine the total and partial DOS near the frontier orbital. We report in Figures SI-2 and 3 the different contributions to the DOS for the Pd13 clusters and in Figure 4 the partial density of states for Pd55(PH3)20, Pd55(SCH3)20 clusters and for the mixed Pd55(PH3)10(SCH3)10. In the only phosphine capped system, the bonding interactions between the metal core and the ligand shell are confined to a rather narrow region between −9 and −10 eV in Pd13(PH3)12 (Figure SI-2) and between −10 and −11 eV in the Pd55(PH3)20 (Figure 4A). The contribution of the Pd atoms to this set of bonding orbitals (shown in Figure 4A) is mainly of s-type, therefore they are well separated from the d-band of the metal that starts at higher energies. The main difference in the DOS of the thiol-capped cluster (Figure 4B) is the larger mixing between the sulfur atomic orbital and the d-band of the cluster. In both Pd13(SCH3)12 and Pd55(SCH3)20, the bonding orbitals between Pd and ligand shell are located at the beginning of the d-band (see the insets of Figure 3B for an example) but a nonnegligible mixing of the ligand and the metal orbitals persists along all the d-band. This feature entails a broadening of the dband of the thiol-capped cluster and a marked reduction of the density of states near the Fermi level, evident in both the Pd13 and the Pd55 clusters. The PDOS of the mixed ligand cluster Pd55(PH3)10(SCH3)10, Figure 4C, presents features characteristic of both the ligands. The density of states near the Fermi level impacts the affinity of the metal cluster toward adsorbates and their catalytic properties.3,73 It is widely recognized that platinum and palladium nanoparticles are characterized by relatively high densities of states around the Fermi level because of the fractional d-band filling. The different shapes of the DOS near the HOMO energy for the different clusters of a given nuclearity (see Figures 4 and SI-2) suggest that the interaction with different ligands strongly affect the density of states near

catalytic efficiency, and it can vary due to chemical interactions with different ligand environments and solvents. We investigate the impact of solvation in the electronic structure of the ligand protected nanoparticle in the case of the Pd13(SCH3)12 that have been reoptimized in the presence of 14 water molecules. The magnetic ground state of Pd13(SCH3)12(H2O)14 does not change with respect to the unsolvated cluster. Figure 3 shows the structure, the spin density and the HOMO orbital of the ligand−cluster−water system. The water molecules establish a robust network of H-bonds between themselves, in some cases involving the hydrogen of the methyl group of the thiols. Water molecules close to accessible Pd atoms orient in some cases with the H pointing toward the metal and in some other cases by exposing to the metal the oxygen atom; the distance between Pd atoms and the atoms of the water is never smaller than 2.4 Å. The water shell influences the charge distribution on the cluster; specifically, the Pd13 core is slightly less positive than in the unsolvated cluster (+0.16|e| against +0.39|e|) while the thiols acquire more negative charge (−0.73|e| against −0.39| e|). Globally, the 14 water molecules donate 0.57|e| to the thiolcapped cluster. Spin density and the frontier orbital of the solvated system remain quite localized on the ligand-capped cluster, indicating that the electronic structure of the system is not strongly modified by the interaction with surrounding water molecules. From the natural bond orbital analysis, we can characterize the effect of the ligand shell on the occupancy of the d-states of the metal cores. In Pd single atoms, the d-orbitals are completely filled. In the bulk, the d-band is formed and, due to the s−p−d rehybridization, the valence d-band becomes partially unfilled. It is expected that going from the bulk to the single atom the d-electron count increases due to a less pronounced s-p-d hybridization.43,67,68 We report in Table SI-2 of the SI the natural electron configuration of the Pd13 and Pd55 metal cores in the different capping environments. The average occupation of the d orbitals is higher in the smaller core for all the complexes as expected.43,69,70 In the populations of the Pd55 cores, one can further recognize a difference in the degree of sp-d hybridization between the “internal” icosahedral core composed of 13 atoms (labeled Pd1−Pd13 in Table SI-2) and the other Pd atoms located at the cluster surface. In agreement with the model introduced in ref 43, the surface atoms contain a greater fraction of d-states than more delocalized s−p-states in comparison with the more “bulklike” internal Pd atoms. The thiol ligands induce an increase of d-holes (a decrease in the d natural population) with respect to the electron donor phosphine in both the Pd13 and Pd55 metal cores. The same trend has been measured in Au nanoparticles.68 Solvation by water does not change the natural 9796

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Figure 5. Number of states near the Fermi level for the only phosphine (red), only thiol (green), solvated thiol (cyan), and mixed ligand (blue) Pd13 and Pd55 clusters. The bars indicates the number of states located within a discrete interval of energy [E, E + ΔE] where ΔE is 0.3 and 0.18 eV for Pd13 and Pd55, respectively. At x = 0, only the HOMO is counted. The bars located at x = −ΔE denote the number of states between HOMO and HOMO − ΔE, the bars at x = −2ΔE denote the number of states between HOMO − ΔE and HOMO − 2ΔE and so on. In the case of Pd13(SCH3)12, the number of states is calculated as average between the counting of the alpha and the beta electron energies. Figure 4. DOS of the phosphine (A), thiol (B), and mixed (C) ligand capped Pd55 nanoparticles. The panels show the contributions of the ligand shell (red and green lines) and of the Pd metal core (blue line) to the total density of state (black dashed line) near the frontier orbitals. Vertical black and red lines indicate the energy of the HOMO and the LUMO orbital, respectively. The set of bonding orbitals for Pd55(PH3)20 and a representative bonding orbital of Pd55(SCH3)20 are shown in panel (A) and (B), respectively.

quenching of the catalytic activity is clearly connected to the withdrawing of charge from the metal and the decrease in the DOS around the Fermi level. The depletion of states around the frontier orbital is due to the broadening of the d-band of the Pd core upon the mixing with the p-orbitals of the sulfur. The mixed ligand cluster does show a decreasing of the DOS near the Fermi level relative to the only phosphine species but less marked than in the only thiol-protected cluster. Moreover, we report above that electron donor character of the phosphine effectively mitigates the depletion of charge on the metal core due to the thiols (see Table 2). These two properties of the mixed ligand cluster suggest that a possible route to mitigate the lost of catalytic efficiency of thiol-capped nanoparticles could be the addition of electron donor ligands as phosphines. On the other hand, the charge transfer from the solvation layer to the ligand−metal complex in Pd13(SCH3)12(H2O)14 is weak when compared to that of the phosphines and the number of states near the frontier orbital is not influenced significantly by solvation. To conclude our analysis we consider the effect of the Pd−S interaction on the thiol molecule. Analysis of X-ray photoelectron spectroscopy (XPS) data of thiol self-assembled monolayers on Pd surface and Pd NPs suggests the formation of a passivating sulfide layer before the adsorption of thiols and the formation of the ligand monolayer becomes possible.32,33,42 The incorporation of sulfur as sulfide on the NP surface requires the scission of the S−C bond that would be activated by electronic charge donation from the metal d-band to the antibonding orbitals of the thiol, with consequent weakening of the S−C bond. In Table SI-3 of the SI, we report the S−C

the frontier orbital. To better quantify the influence of the different capping systems to the DOS near the Fermi level, we discretized the energy region just below the HOMO orbital and we calculated the number of states located in each interval. The result is shown in Figure 5 for both Pd13 and Pd55 by using a discretization of the energy axis of 0.3 and 0.18 eV, respectively. For the Pd13 thiol-capped cluster, the number of states is calculated for both the alpha and the beta electron energies and the average value is taken. There is a clear reduction of the number of states near the frontier orbital in the case of the only thiol-capped cluster for both Pd13 and Pd55 metal particles and in the case of the thiols-protected Pd13 core the presence of the water layer does not affect significantly the number of states near the frontier orbital. The modifications induced by sulfur to the valence band of metals can lead to significant changes in the chemical and catalytic properties of these elements.74,75 In general, one expects a decrease in the activity of Pd, due to the thiol capping, for adsorption reactions that involve donation of electrons to the adsorbate (H2, CO, NO, olefins, etc.). For example, sulfur has been found to hinder the dissociation of H2 on Pd surfaces76,77 and sulfur poisoning of the automotive exhaust catalysts based on Pd is a well-documented process.75 The 9797

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the redistribution of charge and the electronic structure depends on the relative abundance of the two ligands will give valuable insights for an application-oriented optimization of the nanoparticle capping system.

bond length and the Wiberg index for the S−C bond order for all the thiols bounded to the Pd13 metal core in the gas phase and in the solvated cluster. We find that the S−C bond length on the Pd clusters is slightly elongated with respect to the S−C bond length of the isolated thiol radical (an average of 1.844 Å against 1.813 Å). Frequency calculations give access to the IR spectrum of the nanoparticle: the S−C bond stretching mode appears at 712.6 cm−1 in the IR spectrum of the isolated radical, while the same mode is found in the region 667−693 cm−1 for the thiols bound to the Pd13 core (for both the unsolvated and the solvated system) and between 654 and 689 cm−1 in the thiols bound to the Pd55 core. The shift of the S−C stretching toward lower frequency is a clear sign of the bond weakening. The bond orders also decrease going from the isolated thiol to the thiols coordinated to the metal core (see Table SI-3) without a significant effect due to the solvation layer. The activation of the S−C bond toward cleavage is compatible with the formation of a sulfide layer in thiol protected nanoparticle as suggested on the basis of EXAFS studies.33,66 However, the S−C bond cleavage is not a low energy process. The study of the reaction path and relative activation energies for the dissociation of the S−C bond on thiol adsorbed on Pd clusters is an interesting topic for future research.

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ASSOCIATED CONTENT

S Supporting Information *

Table of the spin states with calculated ⟨S2⟩ and energy of the frontier orbitals (eV) of the ligand protected clusters. Figures of the frontier orbitals of the neutral thiol capped Pd13 cluster and of the different contributions to the total density of states in the different Pd 13 clusters. Table of the natural electron configurations of the Pd core in the different Pd13 and Pd55 ligand-protected clusters. Table of the S−C bond length and bond order in the isolate thiol radical and in the solvated and unsolvated thiol-capped Pd13 cluster. Coordinates of the optimized structures discussed in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully thank the NANOFORCE ARC project of the University of Liege. B.F. acknowledges a postdoctoral fellowship from the University of Liège. F.R. acknowledges support from the Fonds National de la Recherche Scientifique (Belgium).

4. CONCLUSIONS We analyzed the electronic properties of Pd13 and Pd55 metal cores protected by only phosphine, only thiol, and mixed ligand shells. For the smaller Pd13 clusters, the ions resulting from the attachment or removal of one electron are completely characterized allowing computing the ionization potential, electron affinity, and charging energy. For the larger size Pd55 cluster, only the neutral species are investigated. The nature of the ligand shell strongly influences the charging properties. The sulfur is an electron-attractor, and its p-orbitals contribute to the frontier orbitals of the clusters, stabilizing the HOMO and enhancing the ionization potential with respect to the phosphine capped system. Phosphines have a strong tendency to donate electronic charge to the metal core reducing the electron affinity of the phosphine-capped system with respect to the thiol-protected cluster. When both molecules are present in the ligand shell the electron attractor character of the thiols and the electron donor character of the phosphine are enhanced. This cooperative effect is present both in the small Pd13 and the larger Pd55 clusters, and it has been recently highlighted in gold clusters.78 In the case of Pd13, the interplay of the orbital mixing and the charge redistribution in the mixed ligand system leads to a substantial increase of the charging energy with respect to the only phosphine and only thiol clusters. On the basis of the analysis of the neutral Pd55 species, we argue that the same effect is present in the larger cluster. The density of states near the Fermi level of the neutral cluster is related to the reactivity and catalytic activity of the nanoparticles. We showed that it is strongly influenced by the chemical nature of the ligand shell while the presence of a restricted water solvation layer does not affect it significantly. The depletion of d-electron density due to the interaction with the thiol ligand shell is mitigated by the presence of phosphine molecules but for both metal cores remains significant. The detailed analysis of the effects of the two capping systems and of the cooperative effect resulting from the interaction of phosphines and thiols is a step forward in the rational design of nanoparticle with specific properties required by nanoelectronics and catalytic applications. Further studies on how

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REFERENCES

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