Trends in Adhesion Energies of Gold on MgO(100), Rutile TiO2(110

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Trends in Adhesion Energies of Gold on MgO(100), Rutile TiO(110) and CeO(111) Surfaces: A Comparative DFT Study 2

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Sergio Tosoni, and Gianfranco Pacchioni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09429 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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The Journal of Physical Chemistry

Trends in Adhesion Energies of Gold on MgO(100), Rutile TiO2(110) and CeO2(111) Surfaces: A Comparative DFT Study Sergio Tosoni, Gianfranco Pacchioni* *

Dipartimento di Scienza dei Materiali, Università Milano – Bicocca, Via R. Cozzi 55, 20125 Milan, Italy

*

Corresponding author: [email protected]

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Abstract The adhesion of gold on three oxide surfaces (magnesia, titania and ceria) is studied by means of dispersion-corrected DFT+U calculations, considering in a systematic approach an isolated Au atom, Au20 clusters, and periodic extended interfaces to model large nanoparticles. The results show that for a Au1 monomer the adhesion energy on the three oxides is similar: 0.83 eV (TiO2), 0.99 eV (CeO2) and 1.09 eV (MgO). The picture is more complex for nanoclusters and extended interfaces, where morphological factors largely determine the binding capability of these oxides toward Au. For Au20, the adhesion on rutile TiO2 is smaller and dominated by long-range dispersion contributions, while the better match between Au atoms and surface oxygen anions lead to a larger binding on magnesia and ceria. Overall, a CeO2 > MgO > TiO2 trend is observed. For the extended interfaces, the trend is CeO2 ≈ MgO > TiO2. Notice that the adhesion energies of a 20 atoms cluster are 2-3 times larger than those of the extended interfaces because of (a) the structural flexibility of nanoclusters, and (b) the presence of several under-coordinated Au atoms at the cluster border in contact with the oxide surface. While for monomers dispersion contributions are of the order of 20% of the total adsorption energy, for clusters and nanoparticles they represent an important and sometimes dominant contribution to the adhesion energy. The picture is radically altered if the oxide supports are not perfectly stoichiometric. The presence of oxygen vacancies enhances the gold adhesion energy, an effect that may render a direct comparison of DFT results with experimental estimates of adhesion energies more complex due to the difficulty to fully control defects concentration at the oxide surfaces.


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1. Introduction The deposition of dispersed metal nanoclusters on oxide surfaces is at the base of many applications in heterogeneous catalysis.1-4 The interaction between metal particles and oxide surfaces influences the charge state and morphology of a supported nanocluster, affecting also its activity. This fact permits, in principle, to tailor new catalysts by tuning the metal-oxide interactions.5-6 Several metals are commonly employed in heterogeneous catalysis in form of supported clusters. Gold nanoparticles, however, are of particular interest, because they have shown high catalytic effect on important reactions such as the CO to CO2 oxidation,7-9 the water-gas shift reaction,10 and many others.11 It has been shown that several factors may influence the Au particles catalytic activity. In the case of Au particles supported on MgO, for instance, both the size of the cluster and the presence of surface defects in the oxide were proven to have a relevant influence for the activity in the CO oxidation reaction.12 A factor of general relevance in determining the catalytic activity of gold nanoparticles is their dispersion; namely, small and highly dispersed nanoparticles are usually more active than their bulk counterparts or large particles.13 In particular, it has been shown how the catalytic activity scales with the inverse of the particle diameter.14-18 At the normal operating conditions in heterogeneous catalysis, however, the dispersed supported nanoparticles tend to aggregate into larger and less active adducts. This process is known as thermal deactivation, or sintering. In particular, sintering proceeds via two mechanisms, i) particle migration and aggregation, and ii) the so-called Ostwald ripening process, which consists in the loss of single atoms from smaller aggregates toward larger ones.19,20 Many strategies have been applied to contrast thermal deactivation of gold catalysts, spanning from alloying of the Au nanoparticles with non-noble transition metals,21,22 to the usage of organic ligands to promote redispersion on the surface.23 However, it is clear that the sintering of oxide-supported gold nanoparticles primarily depends on the gold-oxide adhesion energy. Indeed, oxides that are capable to strongly bind gold aggregates are better candidates as supports for Au-based catalysts, because the sintering process will be slower. Understanding the trends in Au adhesion energies on metal oxide surfaces is thus crucial in catalysts design. In spite of the large amount of experimental and theoretical works published in the literature, the deposition of gold particles on oxide surfaces reveals complicated and partially unclear trends in the gold-oxide adhesion energy. A detailed discussion on this topic can be found in a recent study by Hemmingson and Campbell, where a crosscheck between calorimetric and morphological highvacuum measurements of metal nanoparticles over oxide supports is performed, spanning on various metals and oxide surfaces.24 Formerly, the adhesion energy of a given metal on a clean, 3 ACS Paragon Plus Environment


pristine oxide surface was suggested to correlate to the oxophilicity of the metal.25 DFT calculations of single transition-metal atom adsorption on oxides provide physical insight to this model, showing that the main contribution to the binding energy is normally due to the metal-oxygen interaction.19,26-30 Indeed, for several oxides, a good correlation is found between the adhesion energies of a metal M on a given oxide surface and its oxophilicity.24 On the contrary, the difference in adhesion energy of a given metal on various oxide surfaces is more difficult to rationalize. In the specific case of gold, the following trend in adhesion energy has been reported, based on a combination of experimental techniques, and in particular single-crystal adsorption calorimetry (SCAC), particle shape measurements by TEM, or X-ray scattering (GISXAS):24 MgO(100) ≈ TiO2 (110) < α- Al2O3 (0001) < CeO2 (111) ≈ Fe3O4 (111) One may expect that the metal-oxide adhesion energy increases with the increasing chemical potential of the oxygen ion in the oxide lattice: i.e., the more an oxygen ion is bound to the lattice, the less it is prone to coordinate adsorbed adatoms. However, this simple model conflicts with the evidence that gold nanoparticles display analogous adhesion energy on the surface of a rather reducible oxide semiconductor such as rutile TiO2 and on a strongly ionic, insulating and nonreducible oxide such as MgO.24 One might therefore speculate that the Au adhesion energy on an oxide surface depends on the interplay between electronic and structural factors, where not only the oxygen chemical potential in the oxide plays a role, but also the Au-O mismatch at the interface or the capability of the oxide surface to relax in contact to the Au nanoparticle are relevant. In order to shed some light on these aspects, we performed density functional theory (DFT) simulations (see section 2 for details) of gold deposition on MgO(100), TiO2 rutile (110) and CeO2 (111) surfaces with a systematic approach. We first treated the case of the adsorption of an isolated Au atom on the aforementioned oxide surfaces (section 3). Then, we deposited a small (compared to the dimension of the Au particles detected in the experiment) 3D-cluster, namely Au20, on the surfaces of the three oxides (Section 4). In order to simulate the interaction of large particles of several nm in diameter, we studied the case of the gold-oxide extended interfaces (Section 5). All these calculations have been done at the same theoretical level, considering perfect, stoichiometric surfaces. This is an advantage of theory compared to experiment: defectivity, often unavoidable in real oxide samples, can be introduced in a desired way. This is what has been done in the final part of the work where the effect of surface oxygen vacancies on the adhesion of Au is assessed, Section 6.


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2. Computational Details All reported calculations have been performed at the DFT level using a plane wave basis set using the code VASP 5.31 The core electrons are treated with the Projector Augmented Wave approach.32,33 O(2s, 2p), Mg(2p, 3s), Ti(4s, 3p, 3d), Ce (5s, 6s, 5p, 4f) and Au(5d, 6s) electrons are considered explicitly. The exchange and correlation energy is calculated with the generalized gradient approximation (GGA) of the density functional theory, using the PBE functional.34 To partially circumvent the well-known self-interaction error, which affects the electronic structure of semiconducting oxides calculated with GGA, in the case of rutile and ceria we adopt the so called GGA+U approach.35,36 An effective Hubbard’s parameter of 3 eV has been applied to the Ti(3d) orbitals and of 4.0 eV to the Ce (4f) orbitals.37,38 The adopted value for Ti(3d), though empirically set, represents a good compromise between the desirable increase in the oxide band gap with respect to bare GGA (2.1 eV versus 1.9 eV, respectively) and the risk of introducing artefacts in the surface reactivity, as reported for too large Hubbard’s parameters.39 In the case of ceria, we base our choice on a systematic investigation on the effect of U on the properties of bulk CeO2 and Ce2O3.40 For the ionic oxide MgO, we rely on the bare GGA approach. A recent study has shown that dispersion interactions are important also for metal clusters on oxide surfaces.41 Here, the long-range dispersion effects are treated with the DFT+D2’ approach. This approach is based on the DFT+D2 scheme originally proposed by Grimme,42 with a slight modification of the parameters, aiming at taking into accounts the ionic nature of Mg2+, Ti4+ , Ce4+, and O2-,39,43 as well as the metallic nature of Au.44-45 The D2’ method corrects partly the deficiencies of the standard D2 method, and has been checked with respect to other approaches, such as the non local correlation vdW functional proposed by Langreth, Lundqvist et al.46,47,48,49 showing that the two approaches perform similarly for oxide surfaces.39,50 Still, the exact extent of the dispersion forces at metal/oxide interfaces is difficult to assess. In this respect, the calculated adhesion energies have the great advantage to be obtained on the same footing for three different oxide surfaces, MgO, TiO2, and CeO2, thus allowing a direct comparison. The absolute values, on the other hand, must be taken with some care due to the well-known deficiencies of the DFT method in general, and of the classical approaches to treat dispersion. Lattice relaxations are performed with an increased kinetic energy cutoff of 600 eV. Ionic relaxations are subsequently performed with the standard cutoff of 400 eV. The increase by a factor 1.5 (i.e. 600 eV) is necessary when the lattice is relaxed, in order to keep the Pulay forces on the cell stress tensor reasonably small.



The following supercells are adopted in order to study the deposition of single Au atoms and Au20 clusters, simulating high-vacuum and high dilution conditions: 5x5 for MgO(100), 5x2 for rutile TiO2(110), and 4x4 for CeO2(111). All slabs are 4-layers thick, where the two bottom layers are kept frozen to the bulk lattice positions, while the two topmost layers are free to relax together with the adsorbate. For Au20 adsorption, given the larger size of the adopted supercells (vide infra), the sampling of the reciprocal space is reduced to the Γ-point. Convergence criteria of 10-5 eV and 10-2 eV/Å are adopted for electronic and ionic relaxations, respectively. For the extended interfaces the following Γ-centred high-density grid of k-points are adopted: 12x12x1 for Au/MgO, 12x1x1 for Au/TiO2, and 2x2x1 for Au/CeO2. The different number of k-points adopted for magnesia, titania and ceria is justified by the different dimensions of the respective supercells (see Section 5). All systems are simulated with 5ML slab models (for both gold and the oxide), including also a 12 Å empty space in the supercell to avoid spurious interactions between replicas of the slab. During the relaxation, the two bottom-layers of the oxide are frozen into the bulk lattice positions, while the three layers close to the metal/oxide interface are relaxed. All five metal layers are relaxed. The adsorption energy of an Aux adduct on a metal oxide MO is defined as Eb = E(Aux) + E(MO) - E(Aux+MO). We refer to Db as the contribution to Eb due to the long-range van der Waals forces. For Au20 clusters and extended interfaces, we define an adhesion energy (Ead), which is the adsorption energy normalized per gold surface unit. Please note that positive values in adsorption or adhesion energy imply that the Au/MO interface is stable. It is worth noting that the Zero Point Energy effects are not included in the present calculations. While this may affect a direct comparison to the experimental adhesion energy, the trends over the three considered oxides are maintained.

3. Au monomer on MgO(100), TiO2(110), and CeO2(111) We start the discussion by considering the interaction of a single Au atom with the three surfaces, Table 1. This case has been already widely studied in the past for MgO (100),29,51 TiO2 (110)52 and CeO2 (111)53. Nevertheless, it is considered here for comparative purposes as this represents the smallest gold unit that can be adsorbed on an oxide surface. Furthermore, our calculations include dispersion, at variance with most of the data reported in the literature. In the case of MgO(100), Figure 1a, previous reports clearly indicate the preference of the Au atom for oxygen-top (O-top) sites.49,54 For rutile TiO2(110), Figure 1b, the three-fold oxygen 6 ACS Paragon Plus Environment

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hollow site has been previously identified as the global minimum.50 On ceria, Figure 1c, there is a clear indication that the Au adatom is preferentially coordinated on top of a three-coordinated oxygen ion.51 Thus, we restricted our analysis to these sites, Figure 1 and Table 1.

Table 1. Deposition of a Au atom on MgO(100), TiO2 rutile(110) and CeO2 (111). Adsorption energy (Eb, eV), dispersion contribution to the adsorption energy (Db, eV), oxygen-gold bond distance (RO, Å), Bader charge of the Au atom (q, |e|), and surface dipole moment (µ, |e|/Å). Surface

Site

Eb (eV) Db (eV) RO (Å) qAu (|e|) µ (|e|/Å)

MgO(100)

O-top

1.09

0.19

2.26

-0.29

+0.90

TiO2(110)

O-hollow

0.83

0.25

2.30

+0.03

-0.39

CeO2(111)

O-top

0.99

0.24

2.03

+0.10

-0.18

Figure 1. Au atom adsorption on a) MgO(100), b) TiO2 rutile (110) and c) CeO2(111).

The adsorption energies are overall similar for the three considered oxides, being 1.09 eV for MgO, 0.83 eV for TiO2 and 0.99 eV for CeO2. It is worth noting that the contribution to the 7 ACS Paragon Plus Environment


adhesion energy due to the semiempirical treatment of the long-range dispersion forces is quite similar in the three cases, about 0.2 eV (see Dad in Table 1). The shortest Au-O bond distance is 2.3 Å on magnesia and titania and 2.03 Å on ceria, in close agreement to the previous reports.29,49-52 A special discussion is required for the nature of the Au-oxide bond. In general, the bonding is due to an hybridization of the gold 5d and 6s orbitals with the 2p levels of the surface oxygen anions.52 The Au monomer maintains a singly occupied 6s level, with the spin density strongly localized on Au. This has been proved by electron paramagnetic resonance experiments for the case of Au1/MgO(100).55 This indicates the absence of a net charter transfer in either direction. This is consistent with the Bader charges, Table 1, that are relatively small. The largest value is found for Au on MgO, -0.29 |e|, indicating a delocalization of electronic charge from the fully reduced O2ions of the MgO surface towards the highly electronegative gold atom. This gives rise to a positive surface dipole. Much smaller, and of opposite sign, are the Bader charges for an Au atom adsorbed on TiO2 and CeO2, Table 1. Consequently, also the surface dipole is smaller and of opposite sign compared to Au1/MgO, Table 1. It is worth mentioning that the nature of the interaction of an Au atom on CeO2 is a delicate issue, and that the bonding nature is highly method-dependent. Some theoretical studies have shown that Au adsorbed on the regular CeO2 (111) surface transforms into Au+ with corresponding reduction of a Ce4+ ion of the lattice to Ce3+;56 other studies57 have found an opposite result, i.e. that Au remains neutral. The discrepancy has been explained with the subtle interplay between geometric and electronic structure, and in particular with the different lattice constants predicted at the GGA+U or GGA levels.51 It should be mentioned that two electronic states with neutral and oxidized Au are also found using an hybrid functional (HSE06), but the energy is lower for the solution with neutral Au.51 This is also the situation found in our study. In conclusion, the order of stabilities for a Au atom adsorbed on the three oxides is TiO2 < CeO2 < MgO, but the differences are of the order of 0.1 eV, and one can consider the three adsorption energies as rather similar. When the adsorption energies obtained in the present work are compared to those previously reported in the literature, we notice that the main differences are due to the treatment of the longrange dispersion. For MgO, for instance, we obtained an adsorption energy of 1.09 eV, where 0.19 eV are due to the dispersion contribution, while in Ref. 29 (relying on the PBE functional) Au was reported to adsorb with an energy of 0.89 eV. Similarly, on titania, an adsorption energy of 0.60 eV is reported in Ref. 50 using the PW91 functional, which is very close to our value of 0.83 eV once the dispersion contribution (0.25 eV) is subtracted. As previously discussed, the case of gold on ceria is extremely delicate due to the pronounced dependency on the adopted computational 8 ACS Paragon Plus Environment

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scheme. As elucidated in ref. 55, the adsorption energy strongly depends on the value of U adopted for the Ce 4f orbitals, passing from 0.60 eV for U= 3 eV to 0.96 eV for U = 5 eV. In this work (Table 1), we obtain 0.99 eV where 0.25 eV are due to long-range dispersion. . Our value is thus in line with previous DFT studies reported in the literature. Things are less straightforward when the DFT results are compared to experiment. A value of 0.23 eV and of 2.18 eV, respectively, has been reported for the initial heat of adsorption of Au monomers on MgO(100) 24,58 and CeO2(111),59 with a factor 10 difference for the two surfaces. The adsorption of Au monomers on CeO2 is probably overestimated because of an important contribution of steps.24 The very low value found for MgO,56 however, can be hardly reconciled with the DFT results. The fact that DFT results obtained for ideal MgO and CeO2 surfaces (at 0 K) predict very similar binding of Au monomers, while the measured heat of adsorption at 100 K indicate a large difference between the two surfaces, represent an important discrepancy between theory and experiment that needs to be clarified with further work. Having considered the behaviour of the gold monomers, we can now move to the case of Au20 adsorbates.

4. Adsorption of Au20 cluster The adsorption of a gold cluster represents an intermediate case between the isolated atom and a large gold particle. Compared to an extended slab, metal clusters exhibit a pronounced structure flexibility and can thus better adapt to the surface morphology. Among gold nano-clusters, Au20 represents an interesting test case, because its magic number of electrons, filling exactly one shell in the jellium model, results in enhanced structural stability and large HOMO-LUMO gap when a pyramidal structure is considered.60 This is indeed the most stable arrangement, exposing unreconstructed Au(111) facets on all cluster sides (Figure 2). However, many other stable arrangements of Au20 adsorbed on MgO(100) have been found, with little differences in terms of charge transfer and cluster activity with respect to the pyramidal global minimum.61 In this work, beside the pyramidal Au20 isomer (Pyr), we have also considered a structure that contains 7 atoms in the bottom layer, 8 in the central one, and 5 on the top layer, Fig. 2. It may be considered as derived from an fcc structure (Fcc), and it will be considered to test the adhesion on the oxide surfaces of the two chosen isomers. Notice that we are not interested in the absolute minima of the Au20 isomer on each oxide surface, but in the adhesion energy. In this respect, we reduce the exploration of all possible isomers of Au20 cluster adsorbed on the oxide surfaces to two structures only. No claim is made to have fully explored all possible configurations. 9 ACS Paragon Plus Environment


Therefore, we have deposited the two clusters on the three oxide surfaces, and we have reoptimized the corresponding structures. The two clusters have a different number of Au atoms in contact to the oxide, 10 for Au20 Pyr and 7 for Au20 Fcc. The Pyr isomer exhibits regular triangular facets, whose area is 33.3 Å2. The surface area of the Fcc isomer cannot be exactly calculated due to its more irregular shape, and it is therefore estimated by rescaling the Pyr area by a factor 0.7, according to the different number of Au atoms in contact with the oxide surface (10 for Pyr and 7 for Fcc). The corresponding adhesion energies are computed with respect to the optimal Pyr and Fcc gas-phase isomers. In this way we obtain a direct measure of the strength of the bond of each isomer to the three considered oxide surfaces.

Figure 2. Pyramidal (left) and Fcc (right) gas-phase Au20 isomers

The relaxed structures for the Au20 cluster deposited on magnesia, titania and ceria are sketched in Figure 3a, 3b, and 3c, respectively. On MgO (100), the Au20 Pyr cluster is bound with an adsorption energy of 3.51 eV, while the Fcc isomer has an adsorption energy of 3.21 eV, Table 2. The contribution of the long-range dispersion term (Dad) to the total adhesion energy is close to 50% for both isomers. The interfacial cluster-surface distance (RI), calculated with respect to the average of the atoms in the topmost MgO layer and the Au atoms belonging to the bottom layer of the cluster, is, in the case of the pyramidal isomer, 2.68 Å, and a bit shorter for the Fcc isomer, 2.35 Å. The Pyr cluster bends slightly on the surface, in order to maximise the stabilizing O-Au interactions, Fig. 3a. The average shortest Au-O distance for the ten gold atoms in contact to the surface is 2.90 Å. The Fcc isomer maintains a flat position, but it rearranges its structure, leaving only six Au atoms in contact to the surface, with an average Au-O distance of 2.36 Å. For the Pyr isomer, a significant negative Bader charge is reported (-1.01 |e|), indicating a charge transfer from the oxide to the gold cluster. As expected, a positive interface dipole moment is associated to the reported charge transfer, +1.97 |e|/Å. Similar results are found for the less stable Fcc isomer, Table 2.


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Figure 3. Pyramidal (left) and Fcc (right) Au20 isomers adsorbed on: a) MgO (100); b) TiO2 (110); c) CeO2 (111).

Table 2. Au20 deposition on MgO(100) and rutile TiO2(110). Total adsorption energy (Eb, eV), dispersion contribution (Db, eV), adhesion energy per gold surface area (Ead, J/m2), mean interface distance (RI, Å), mean oxygen-gold bond distance (RO, Å), Bader charge of the Au cluster (q, |e|) and dipole moment (µ, |e|/Å). Surface MgO(100)

TiO2(110)

CeO2(111)

Au20 Eb (eV) Db (eV) Ead (J/m2) RI (Å) RO (Å) qAu (|e|) µ (|e|/Å) Pyr

3.51

1.62

1.70

2.68

2.90

-1.01

+1.97

Fcc

3.21

1.65

2.60

2.35

2.36

-1.36

+2.30

Pyr

2.33

2.02

1.13

2.08

2.89

+0.32

-0.03

Fcc

2.82

2.14

1.71

1.71

2.59

-0.30

-0.27

Pyr

4.52

2.64

2.19

2.18

2.58

-0.19

+0.90

Fcc

3.24

2.60

2.24

2.11

2.27

-0.08

+0.82

As reported in Table 2, the Au20 Pyr cluster adsorbed on TiO2 has an adsorption energy of 2.33 eV while the Fcc one is bound by 2.82 eV. It is important to note that the adsorption energy is 11 ACS Paragon Plus Environment


substantially due to dispersion, more than for MgO, Table 2. The surface morphology may play an important role in this case: MgO(100) is atomically flat, while rutile TiO2(110) displays rows of protruding O2c-ions which are in close contact to some Au atoms. Other Au atoms of the Au20 Pyr, however, are in a less favourable position on top of O3c or Ti ions (see Figure 3b). The interfacial basal distance is 2.08 Å, while the mean oxygen-gold bond distance is comparable to MgO (2.89 Å). For this reason the other Fcc isomer better adapts to the surface morphology and results in a more stable configuration, Fig. 3b. For both Au20 isomers the adsorption energy on TiO2 (110) is smaller compared to MgO (100), Table 2, confirming the trend observed for the isolated atom. This may be somewhat surprising given the rather unreactive nature of the MgO surface compare to TiO2, but here what plays a really important role is the close contact between the metal atoms and the surface oxide anions. On rutile TiO2 the Au20 Bader charge is negligible, and of opposite sign for the two isomers, which is consistent with an interaction dominated by dispersion with modest chemical bonding contributions. On ceria, Au20 Pyr has a very good match with respect to the distribution of surface oxygen ions. Differently from what observed for magnesia and titania, the cluster bottom layer remains flat with all the Au atoms in contact to a surface oxygen ion with an average distance of 2.6 Å (Figure 3c and Table 2). This reflects in a rather strong adsorption energy of 4.52 eV, much larger than for MgO and TiO2. Also in this case, the dispersion contribution is relevant, and in between that of TiO2 and MgO. The Fcc isomer, on the other hand, has a much less favourable match with the surface oxide anions, resulting in a significant distortion of the Au20 structure. The cost to deform the cluster results in an overall smaller binding energy, 3.24 eV. Only 7 gold atoms are interfaced to the surface oxygen ions, with a mean Au-O distance of 2.3 Å. The interface distance is as large as 2.1 Å. The adsorbate-support charge transfer, as depicted with the Bader formalism, is rather small and similar to what observed for TiO2. In summary, the results for the Au20 cluster show a clear trend in the adsorption energies which is CeO2 > MgO > TiO2, with differences for the different oxides of about 0.7-1.0 eV.

5. Gold-oxide extended interfaces We now extend the analysis to the gold-oxide extended interfaces. When modelling metal/oxide extended interfaces, it is essential to minimize the strain related to the lattice mismatch between metal and oxide to avoid artefacts in the calculated electronic structure and adhesion energies. We therefore begin with the comparison between experimental and calculated lattice parameters of 12 ACS Paragon Plus Environment

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MgO (100), rutile TiO2 (110), CeO2 (111), and Au(100) and Au(111) surfaces, Table 3. A general good agreement between X-ray diffraction data and DFT relaxed lattice parameters is found. In order to optimize the gold/oxide match, we considered the Au(100) surface for MgO and TiO2, and the Au(111) surface for CeO2 (111).

Table 3. Experimental and computed surface lattice parameters (Å) Experimental Calculated System

a

b

Au(100)a

4.08

4.08

4.12 4.12

Au(111)a

2.88

2.88

2.91 2.91

TiO2(110)b

2.96

6.49

3.01 6.56

MgO(100)c

4.21

4.21

4.23 4.23

d

3.83

3.83

3.88 3.88

CeO2(111)

a

b

a) X ray diffraction data from: ref. 62; b) ref. 63; c) ref. 64; d) ref. 65

In Table 4, the mismatch of the various interfaces is analysed. For Au(100)/MgO(100), there is a reasonable match between the smallest available cells, namely √2/2, √2/2. If the residual strain is transferred on Au, we have an elongation of 2.8% of the Au lattice parameters. Vice versa, if the strain is transferred to MgO, the oxide lattice constants are contracted by 2.8%. In the calculations both cases will be considered, strain on gold and strain on the oxide.

Table 4. Au(100)/MgO(100), Au(100)/TiO2(110), and Au(111)/CeO2(111) interfaces. Lattice mismatch based on calculated data and supercell composition. Positive strain indicates elongation of the lattice parameters, negative strain indicates contraction. Supercell Interface Au(100)/MgO(100)

Composition Au5Mg5O5

Lattice Au

√2/2 √2/2

Oxide

√2/2 √2/2


Strain (%) Strain on Au MgO

a

b

+2.8 +2.8 -2.8

-2.8


Au(100)/TiO2(110)

Au45Ti40O80

Au(111)/CeO2(111) Au80Ce45O90

√2/2 √2 9( ) 2 4×4

1×4 3×3

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Au

+3.3 +0.1

TiO2

-3.2

-0.1

Au

-0.1

-0.1

CeO2

+0.1 +0.1

The case of Au(100)/TiO2(110) interface is more complicated. We have considered various possible supercells. Finally we have chosen an interface constructed with a (1×9) Au(100) and (1×4) TiO2 (110) cells. Here the strain is about 3% along the a lattice vector, and 0.1% for the b direction, Table 4. Also in this case, both interfaces with the strain on Au and on TiO2 will be considered. Finally, for the case of Au(111)/CeO2(111), the strain becomes as small as 0.1% if a 4x4 Au(111) supercell is superimposed on a 3x3 CeO2 lattice, with a very small lattice compression or elongation if the strain is released on the gold slab, or on the oxide, respectively.

5.1

Au(100)/MgO(100) interface

In the case of the Au(100)/MgO(100) interface, three different registries have been studied: 1) Au atoms on top of O ions (O-top), 2) in hollow positions (Hollow), and 3) on top of Mg ions (Mgtop), Figure 4.


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Figure 4. Top and side views of the Au(100)/MgO(100) interface structures: O-top (left, Au on top of O positions), Hollow (middle, Au on hollow sites) and Mg-top (right, Au on top of Mg positions)

As reported in Table 5, when the strain is released on the Au slab and for the O-top registry, an adhesion energy of 0.79 J/m2 is obtained. The contribution due to the long-range dispersion is important (0.38 J/m2, about 50% of the total adhesion energy). The MgO-Au basal distance is 2.59 Å. The hollow registry is less stable (0.58 J/m2), the basal distance increases to 2.83 Å, and the dispersion contribution is now more than 50%. For the Mg-top registry, not surprisingly the adhesion energy further decreases to 0.39 J/m2 (predominantly due to long-range dispersion) while the basal distance increases up to 3.03 Å. In all cases, the gold slab displays a very small and negative Bader charge and the interface dipole is negligible, Table 5. As expected, the registry where the Au interface atoms sit on top of O anions is strongly favourable. Interestingly, when the strain is released on the MgO side of the interface the results do not change qualitatively, and also the quantitative differences are very small (0.05 J/m2), Table 2. This is an important finding, which evidences the robustness of the adopted structural model. The available experimental measurements of the adhesion energy strongly depends on the adopted methodology, spanning from Ead = 0.67 J/m2, as derived from microscopic HRTEM 15 ACS Paragon Plus Environment


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measurements,66 and Ead = 0.31 J/m2, as directly measured in calorimetric SCAC experiments.24 As we mentioned before, it is difficult to assess the accuracy of our DFT+D adhesion energies; however, the estimate of 0.75-0.79 J/m2, depending on where the strain is released, is closer to the value obtained from HRTEM measurements, 0.67 J/m2. Considered that our method probably overestimates the dispersion interactions, the two values are consistent. On the contrary, an adsorption energy of 0.31 J/m2 appears to be much too low to be reconciled with the present calculations.

Table 5. Au(100)(5ML)/Mg(100)/(5ML) interface. Total adhesion energy (Ead, J/m2), dispersion contribution to the adhesion energy (Dad, J/m2), interface basal distance (RI, Å), average Au-O distance (RO, Å), Bader charge of the Au slab normalized per interface Au atoms (q, |e|) and dipole moment (µ, |e|/Å) . Strain on Registry Ead (J/m2) Dad (J/m2) RI (Å) RO(Å)a qAu (|e|) µ (|e|/Å) Au

MgO

HRTEM SCAC a

O-top

0.79

0.38

2.59

2.59

-0.12

0.11

Hollow

0.58

0.33

2.83

3.20

-0.10

0.07

Mg-top

0.39

0.29

3.03

3.69

-0.08

0.06

O-top

0.74

0.40

2.66

2.66

-0.16

0.10

Hollow

0.57

0.36

2.87

3.22

-0.11

0.07

Mg-top

0.40

0.31

3.08

3.70

-0.09

0.06

b

c

0.67 0.31

Shortest Au-O distance at the interface, averaged over all non-equivalent Au atoms in the

supercell. b

Measurements from ref. 64

c


5.2

Au(100)/TiO2(110) interface

As shown in Figure 5, the Au(100)/TiO2(110) interface is modelled according to two possible registries. In one case (Figure 5, left), the Au atoms belonging to the interface layer are placed on top of the rows of O2c ions topping the rutile surface. In the other case (Figure 5, right panel), the gold atoms are placed along the rows of the underlying O3c ions. It is important to notice that, due to


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the rutile/gold structure mismatch, some Au atoms are placed on top of O, while others sit on less favorable Ti-top positions. This is a substantial difference with respect to the Au/MgO interface.

Figure 5. Top and side views of Au(100)/TiO2(110) interface structures: O2c-top registry (left), and O3c-top registry (right).

Table 6. Au(100)(5ML)/TiO2(110)(5ML) interface. Total adhesion energy (Ead, J/m2), dispersion contribution to the adhesion energy (Dad, J/m2), interface basal distance (RI, Å), average Au-O distance (RO, Å), Bader charge of the Au slab normalized per interface Au atoms (q, |e|), and dipole moment (µ, |e|/Å). Strain on

Registry Ead (J/m2)

Dad (J/m2) RI (Å) RO(Å)a qAu (|e|) µ (|e|/Å)

Au

O2c-top

0.56

0.47

1.97

2.70

+0.07

-0.41

O3c-top

0.41

0.41

2.18

3.07

+0.06

-0.42

O2c-top

0.54

0.47

2.02

2.61

+0.07

-0.36

O3c-top

0.41

0.42

2.19

3.05

+0.06

-0.37

TiO2

a

GISAXSb

0.54±0.10

HRSEMc

0.71±0.11


supercell. b


c




The O2c-top registry displays a larger adhesion energy compared to O3c-top one, 0.56 J/m2 versus 0.41 J/m2, Table 6. Compared to the Au/MgO case, the long-range dispersion contribution is now predominant. In the less favourable O3c-top case, in particular, the DFT result without dispersion correction would lead to a flat potential energy surface. The two registries, however, display interface basal distances (1.97 Å and 2.18 Å, respectively) shorter than in the case of Au-MgO. The average shortest Au-O bond distance is similar to the MgO case (2.70 Å and 3.07 Å for O2c-top and O3c-top, respectively). Interestingly, the Bader charge of the Au slab evidences, in this case, a small electron donation from Au to rutile, 0.07 |e| per interface Au atom, opposite to what found for MgO. The metal-oxide electron transfer leads also to a negative interface dipole moment, Table 6. As in the case of MgO, structural and electronic properties do not change significantly if the stress is released on the oxide or on the metal, Table 6. Thus, we can conclude that the adhesion energy of gold nanoparticles on TiO2(110) is of 0.55±0.01 J/m2. In the case of Au nanoparticles deposited on rutile TiO2(110), there are experimental data obtained only with morphological techniques: GISAXS measurements report an adhesion energy of 0.54±0.10 J/m2,67 while HRSEM gives 0.71±0.11 J/m2.68 The calculated adhesion energy for the most stable case, O2c-top, seems to be in good, although probably fortuitous, agreement with the GISAXS data, but not far from the HRSEM estimate.

5.3

Au(111)/CeO2(111) interface

We finally analyse the case of the Au(111)/CeO2(111) extended interface. Three registries have been considered, by putting the Au atoms of the bottom layers in three positions: O3c-top, O4c-top, and Ce-top, see Figure 6. As one can see from Table 7, however, the registry does not show a remarkable influence on the gold-ceria interface properties, in agreement with previous reports.69 In particular, the adhesion energy varies between 0.75 and 0.74 J/m2, with a substantial contribution arising from the long-range dispersion, Table 7. Accordingly, the charge transfer from gold to ceria and the resulting interface dipole moment are also almost unremarkable. The average interface distance is 2.41 Å, and the average Au-O distance varies between 2.8 and 2.9 Å, Table 7. As in the previous cases, the results are reproduced if the strain is transferred on the oxide, Table 7, Ead = 0.76 J/m2 (only the O3c-top case has been considered). As one can see, differently from the previous cases, the calculated adsorption energy (0.76 J/m2 in the most favourable case) is very similar to that obtained for MgO (0.77±0.03 J/m2). The adhesion energy of gold on CeO2 has been recently estimated experimentally based on calorimetric SCAC measurements,24 with values between 2.53 and 2.83 J/m2, depending on the concentration of oxygen vacancies. The difference with respect to the computed Au/CeO2 stoichiometric interface is very large, since the measured 18 ACS Paragon Plus Environment

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adhesion energy is 3-4 times larger than the computed one. We suggest that this difference is may be due to the presence of a relatively high number of oxygen vacancies on the not-fully stoichiometric ceria samples used for the measurements.24 In order to investigate on this aspect, in the next section we discuss results obtained for gold adhesion on reduced MgO, TiO2, and CeO2 surfaces. Table 7. Au(111)(5ML)/CeO2(111)(5ML) interface. Total adhesion energy (Ead, J/m2), dispersion contribution to the adhesion energy (Dad, J/m2), interface basal distance (RI, Å), average Au-O distance (RO, Å), Bader charge of the Au slab normalized per interface Au atoms (q, |e|), and dipole moment (µ, |e|/Å). Strain on Registry Ead (J/m2) Dad (J/m2) RI (Å) RO(Å)a qAu (|e|) µ (|e|/Å) Au

CeO2 SCAC a

b

O3c-top

0.75

0.62

2.41

2.79

+0.03

-0.03

O4c-top

0.74

0.62

2.41

2.92

+0.03

-0.03

Ce-top

0.74

0.61

2.44

2.82

+0.03

-0.03

O3c-top

0.76

0.62

2.40

2.78

+0.03

-0.04

2.53, 2.83


supercell. b

Measurements from ref. 24 and 57

Figure 6. Top and side views of Au(111)/CeO2(111) extended interfaces. O3c-top registry (left), O4c-top (middle) and Ce-top (right). 19 ACS Paragon Plus Environment


We can now compare the adhesion energies obtained with the Au20 Pyr cluster and the extended interfaces. For Au20 Pyr a contact area of 33.03 Å2 (i.e. the area of the basis of the Au20 pyramid) is used to calculated the adhesion energy in J/m2 for the three oxides, Table 2. We notice that the adhesion energies calculated for the extended surfaces are systematically smaller compared to those obtained for Au20 Pyr. In the case of MgO, for Au20 we obtain 1.70 J/m2 compared to 0.79 J/m2 for the Au/MgO extended interface. Similarly, for TiO2 the adhesion energy decreases from 1.13 J/m2 for Au20 to 0.54 J/m2 for the extended interface; finally, for CeO2 we observe a decrease from 2.19 J/m2 to 0.75 J/m2. Thus, there is reduction by a factor 2-3 going from the small cluster to the model of a large particle. This trend can be rationalized as the consequence of two contributions: (1) a cluster of finite size is more flexible and can adapt to the surface morphology and maximize the interaction with the oxide surface; (2) while a periodic interface has no lowcoordinated Au atoms, in the case of Au20 ten gold atoms are in contact to the oxide surface, but nine of them are located along the cluster boundaries. The undercoordination of these atoms makes them much more reactive toward oxide surfaces. This is an important conclusion, which shows that the adhesion energy of a nanoparticle is not only a function of the size, but in particular of the number of atoms at the boundary of the particle at the metal/oxide interface.

6. Effect of surface oxygen vacancies on Au adhesion energies In the previous sections, we have discussed results concerning stoichiometric oxide surfaces. We now focus on surfaces where an oxygen vacancy has been created. For the calculations we adopted the same supercells that were used in Section 4 and recalculated the adsorption energy of the Au20 tetrahedral (Pyr) clusters located on top of an oxygen vacancy (VO). The vacancy concentration is as follows: MgO: one VO every 25 surface O ions; TiO2: one VO every 10 bridging O ions; CeO2: one VO every 16 surface O ions. The three considered oxides, i.e. magnesia, titania, and ceria, display significant differences in terms of reducibility. When the stability of VO is calculated with respect to the pristine material and ½ O2, we find that creating a neutral oxygen vacancy costs 6.14 eV on MgO, 3.82 eV on TiO2 rutile (an O2c is removed) and 4.00 eV on CeO2. It must thus be considered that oxygen vacancies, while very rare on pristine MgO crystals,70 are likely present in titania and ceria samples. In the case of ceria in particular, calorimetric data have been reported as a function of the oxide stoichiometry, showing that the adhesion energy increases from 2.53 J/m2 to 2.83 J/m2 if the O:Ce ratio decreases from 1.95 to 1.80, suggesting thus that the oxide’s binding to gold is reinforced by a higher concentration of vacancies.57 20 ACS Paragon Plus Environment

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Table 8. Au20 (Pyr) deposition on MgO(100) (4ML), rutile TiO2(110) (4ML) and CeO2(111) (4 ML). Total adsorption energy (Eb, eV), dispersion contribution to the adsorption energy (Db, eV), adhesion energy normalized per gold surface area (Ead, J/m2), mean interface distance (RI, Å), mean oxygen-gold bond distance (RO, Å), Bader charge of the Au cluster (q, |e|), and dipole moment (µ, |e|/Å). Eb (eV) Db (eV) Ead (J/m2) RI (Å) RO (Å) qAu (|e|) µ (|e|/Å)

Surface

MgO(100) Stoic 3.51

1.62

1.70

2.68

2.90

-1.01

+1.97

5.53

1.74

2.68

2.60

2.90

-2.37

+2.35

Stoic 2.33

2.02

1.13

2.08

2.89

+0.32

-0.03

VO

2.20

2.00

1.07

2.06

3.02

+0.12

-0.23

CeO2(111) Stoic 4.52

2.64

2.19

2.18

2.58

-0.19

+0.90

2.70

2.70

2.07

2.66

-0.39

+0.71

VO TiO2(110)

VO

5.56

As reported in Table 8, the adhesion energy of Au20 on MgO strongly increases from 3.51 eV to 5.53 eV in presence of a surface oxygen vacancy, with a substantial charge transfer from the reduced surface to the gold particle, whose Bader charge goes from -1.01 |e| (pristine MgO) to -2.37 |e| (reduced MgO). In the case of rutile TiO2, differently from MgO, the adsorption energy of Au20 does not change significantly in presence of an oxygen vacancy, and actually a small decrease from 2.33 eV to 2.20 eV is reported, Table 8. This fits with the fact that the contribution of the Van der Waals forces to the adsorption energy is predominant for both stoichiometric and reduced titania, indicating a noncovalent nature of the Au20-titania interaction, which is confirmed also by the negligible charge transfer depicted by the Bader formalism. In a previous report on the Au20-rutile(110) interface, 71it was shown by means of molecular dynamics simulations that a strong binding of the Au20 in presence of an oxygen vacancy on the rutile surface (accompanied by a substantial charge transfer from the reduced surface to the cluster) is possible only by inducing a melting and structural reorganization of the pyramidal cluster, which can take place at temperatures above 700 K. In our study, we aim at comparing the Au binding capability of different oxide surfaces, which can be done consistently only if the cluster maintains comparable morphology and surface area, thus excluding radical rearrangement phenomena. We therefore restrict the present study to the Pyr and Fcc Au20 isomers. One should be thus aware that severe structural changes upon annealing at high temperature may induce strong cluster-support interactions. It is, finally, worth reporting that the 21 ACS Paragon Plus Environment


adhesion energy of the Au(100) extended surface on TiO2 increases from 0.56 J/m2 to 0.71 J/m2 upon support’s reduction. This significant increase in the adhesion energy, however, was reached at a concentration of surface oxygen vacancies of 25% of the O2c bridge lattice positions. In the case of CeO2, finally, the adhesion energy increases by more than 1 eV if the cluster is adsorbed on top of a surface oxygen vacancy compared to the stoichiometric case, Table 8. As in the case of TiO2, no major charge transfer occurs at the interface. Also in this case, however, if structural reorganization of the cluster at high temperature is taken into account, a charge flow from Au to the reduced ceria was reported.72 We can thus conclude that the trend in gold adhesion reported for perfectly stoichiometric oxides, i.e. TiO2 < MgO < CeO2, does not hold true if the presence of oxygen vacancies is taken into account. On the reduced ceria surface the Au20 cluster is bound more strongly than on the stoichiometric one. The same would hold true also for MgO, but it is well known that the concentration of oxygen surfaces on magnesia is negligible. On titania, finally, a stronger binding capability in presence of oxygen vacancies is not found for Au20, while is found for the case for the extended interfaces. However, also Au20 displays a higher affinity for reduced titania if a strong structural reorganization is allowed.69 This confirms the general notion of increased interaction between metal nanoparticles and oxide surfaces when defects are present on the surface. The details of this contribution, however, is difficult to estimate as it is proportional to the number of defects (O vacancies) and it also depends on the size and shape of the nanoparticle deposited on the surface.

7. Conclusions We have performed DFT calculations on the adhesion energy of gold nanoparticles on magnesia, titania, and ceria. To this end, we have considered three forms of gold deposits, an isolated atom, two Au20 nano-clusters, and a periodic extended interface that better represents a large gold particle. The advantage of a theoretical approach is that we can consider ideal, non-defective, stoichiometric surfaces and analyse the adhesion energy simply as a function of the two components, gold on one side and the oxide surface on the other. When looking at the adhesion energy of the gold atom, this is similar for the three oxides, close to 1 eV. Dispersion contributes 0.2-0.25 eV to the monomer binding energy. When this contribution is taken into account, our results are close to other DFT estimates reported in the literature. The trend in adsorption energy is TiO2 < CeO2 < MgO, with small differences of about 0.1 eV for the three surfaces. In all cases the Au atom is bound to oxygen, and retains the atomic configuration of atomic gold, 5d106s1. The fact that the bonding is 22 ACS Paragon Plus Environment

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similar, despite the very different nature of the oxide ion in the three oxides, is related to the nature of the bonding, with some hybridization and little charge transfer character. For Au20 the trend in adsorption energy is CeO2 > MgO > TiO2. Here the absolute differences are larger, of about 0.7-1.0 eV. Also in this case the bonding has a large contribution from dispersion, and depends in an important way on the morphology of the surface. For the extended interfaces, the trend found in adhesion is CeO2 ≈ MgO > TiO2. Once more, two completely different oxides, the reducible ceria and the non-reducible magnesia, bind gold with the same strength. Notice that these results are independent of the strain present when two materials are interfaced. The fact that titania is less binding than ceria and magnesia is simply due to the different morphology of the surface (more corrugated). The driving force for the gold-oxide adhesion is thus the morphological match between lattice oxygen anions and gold atoms, which is indeed definitely better in the cases of the atomically flat MgO surface or the oxygen-terminated CeO2 surface, while the zig-zag profile of rutile TiO2(110) allows only for a limited number of Au-O interactions. Recently, an analysis of existing experimental data has been performed by Hemmingson and Campbell.24 According to this study, the trend in stability of gold deposits on oxide surfaces follows the trend MgO (from 0.31 to 0.67 J/m2) ≈ TiO2 (from 0.54±0.10 to 0.71±0.11 J/m2)

Trends in Adhesion Energies of Gold on MgO(100), Rutile TiO2(110

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