Letter pubs.acs.org/JPCL
Spontaneous Oxidation of Ni Nanoclusters on MgO Monolayers Induced by Segregation of Interfacial Oxygen M. Smerieri,†,⊥ J. Pal,†,‡,⊥ L. Savio,*,† L. Vattuone,†,‡ R. Ferrando,†,§ S. Tosoni,∥ L. Giordano,∥ G. Pacchioni,∥ and M. Rocca†,‡ †
IMEM-CNR, U.O.S. Genova, Via Dodecaneso 33, 16146 Genova, Italy Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy § Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31, 16146 Genova, Italy ∥ Dipartimento di Scienza dei Materiali, Università Milano Bicocca, via R. Cozzi 55, 20125 Milano, Italy ‡
S Supporting Information *
ABSTRACT: We report the study of Ni nanoclusters deposited on MgO/Ag(100) ultrathin films (one monolayer) at T = 200 K. We show by STM analysis and DFT calculations that in the limit of low Ni coverage the formation of nanoclusters of four to six atoms occurs and that these aggregates are flat rather than 3D, as expected for Ni tetramers, pentamers, or hexamers. Both the shape of the clusters and the interatomic distance between neighboring Ni atoms are indicative that the nanoparticles do not consist of pure metal atoms but that a NiyOx structure has formed thanks to the availability of atomic oxygen accumulated at the MgO/Ag interface, with Ni clusters acting as oxygen pumps. Besides being of relevance in view of the use of metal nanoclusters in catalysis and other applications, this finding gives a further proof of the peculiar behavior of ultrathin oxide films.
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related to the accumulation of oxygen atoms at the MgO/Ag interface, which reduces the stress of the oxide layer favoring the formation of extended terraces.21 A large, flat MgO monolayer is an ideal substrate for NP deposition, such as Ni clusters. We investigated this system with respect to cluster size and geometry by low-temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT) calculations, focusing our attention on the smallest NPs, up to six Ni atoms in size. We find flat structures completely different from those predicted for Ni4, Ni5, and Ni6 clusters, which are expected to be 3D already for the tetrameric configuration,17,22 and we demonstrate that NiyOx rather than Ni clusters form and are stabilized on the surface. This result is due to the high interface oxygen density available immediately below the MgO layer, which segregates to the surface and binds to the reactive Ni atoms. The latter ones thus act as pumps to extract oxygen from the substrate. Besides being of interest in view of the different catalytic properties of Ni23−27 and NiO,28−30 our results demonstrate once more the peculiarity of ultrathin oxide films with respect to the corresponding bulk materials and suggest that the physics of these systems is still far from being completely understood.
he growing interest in metal nanoparticles (NPs) is related to their increasing importance for a series of applications ranging from nanoelectronics to magnetism, pharmaceutics (e.g., for biological labeling), and catalysis. It is now well established that the NPs’ properties depend critically on their size and shape1−4 and that some peculiar geometries may appear when reducing the cluster size.5−7 Despite the vast literature on the topic, most of the attention has concentrated so far on metal NPs made of a few tens of atoms or more.2,3,8 For metal clusters of just a few atoms supported on insulating surfaces,9−14 the catalytic activity is size-dependent and it is directly related to the intrinsic electronic and geometric properties of the NPs and to the size-dependent NP−substrate interaction. This was proved experimentally for the interaction of CO with small Au and Pt NPs9 and for that of acetylene with Pd NPs,10 while the initial nucleation of a metal cluster has been described theoretically for several systems,15 as Pd on MgO16 and Co and Ni on MgO.17,18 Considering the technological relevance of these nano-objects, a clear description of their properties also in the very low size limit is important. This is particularly true in the case of Ni, which combines its magnetic nature to a strong catalytic activity. MgO is a very common substrate for the deposition of metal NPs, mainly because of its simple atomic and electronic structure and its wide band gap.19 Despite that, the optimal experimental conditions for growing a flat, extended monolayer film on Ag(100)20,21 were found only recently. The quality of the film is © 2015 American Chemical Society
Received: June 26, 2015 Accepted: July 22, 2015 Published: July 22, 2015 3104
DOI: 10.1021/acs.jpclett.5b01362 J. Phys. Chem. Lett. 2015, 6, 3104−3109
Letter
The Journal of Physical Chemistry Letters Figure 1a shows a typical overview of the 0.7 ML MgO film covered with Ni nanoclusters of different size and shape,
randomly distributed on the MgO terraces. Atomic resolution is achieved for clusters consisting of a single layer of Ni atoms (estimated from the apparent height of the cluster; see height profiles in Figures 2 and 3), as those reported in the side panels c−f. These structures consist of two, four, five, or six bright lobes, respectively. Although a detailed analysis is reported in the following, we anticipate that from the average size of each lobe and from its round shape, we can safely assign each of them to a single Ni atom. Therefore, we identify the different features present on the surface as dimers, tetramers (T), pentamers (P), and hexamers (H). These nanostructures are stable on the surface at low Ni coverage and for 100 ≤ T ≤ 250 K. Increasing the amount of Ni and the surface temperature, aggregation into larger, multilayer clusters occurs (images not shown). Under the optimal conditions (T = 200 K, ΘNi ≈ 0.2 ML), the tetramers are the most common monolayer feature (35% of over 200 clusters analyzed), followed by pentamers (10−15%). Hexamers and dimers are only a few, while the rest consists of larger aggregates formed by at least two Ni layers. Although monolayer clusters are ∼50% of the NPs, being the smallest features, they represent only a minor fraction of the overall Ni coverage. As evident at a first glance, the nature of these clusters is quite different from what is expected from the literature,17 in which calculations predict the formation of 3D NPs already for Ni4 clusters adsorbed on MgO bulk. As reported in the Supporting Information, we have simulated two Ni4 isomers adsorbed on a supported 1 ML thick MgO film, giving rise to a tetrahedral 3D structure and to a flat 2D structure (Figure S1), respectively. Similarly to what found for the MgO(100) surface, the 3D isomer results in being more stable by 0.72 eV. The same outcome was found for adsorption at an oxygen vacancy. In the 3D structure, the Ni−Ni interatomic distances span in the range 2.3 to 2.5 Å. In the flat 2D isomer, Ni−Ni distances are 2.3 Å along the sides of the square aggregate and 3.2 Å on the diagonals. None of the
Figure 1. STM images of Ni nanoclusters deposited at 200 K on 0.7 ML MgO/Ag(100). (a) Overview. Tetramer (T) and pentamer (P) structures are imaged with atomic resolution. Image size: (10 × 10) nm2, V = +1.0 V, I = 0.14 nA. (b) Atomically resolved image of the clean Ag(100) substrate, from which the high symmetry directions (marked by arrows) and the calibration parameters are deduced. Image size: (2.4 × 2.3) nm2, V = 0.1 V, I = 0.2 nA. (c−f) Close-up views of a dimer, tetramer, pentamer, and hexamer cluster, respectively. For all images, image size: (1.5 × 1.5) nm2, V = +1.0 V, I = 0.14 to 0.15 nA.
Figure 2. (a) Atomically resolved STM images of a tetramer structure recorded at V = +1.0 V and −1.0 V, respectively (15 × 15 Å2, I = 0.14 A). The highsymmetry directions of the substrate are marked in panel a1. (b) Line scans cut along the lines drawn in panels a1 and a2, showing the characteristic dimensions of the cluster. Continuous lines correspond to V = +1.0 V; dashed lines correspond to V = −1.0 V. (c) Tersoff−Hamann images of a Ni4O5 aggregate with bias V = +1.0 and −1.0 V. The ISO level of charge density is set to 10−5 |e|/Å3. To make an easier comparison, in this and in the following Figures the experimental images have been rotated to orient the axis coherently with the simulated one. (d,e) Most stable Ni4O5 aggregate on MgO/ Ag(100) as deduced from DFT calculations (side and top views). This configuration was used as input for the simulated image. 3105
DOI: 10.1021/acs.jpclett.5b01362 J. Phys. Chem. Lett. 2015, 6, 3104−3109
Letter
The Journal of Physical Chemistry Letters
Figure 3. (a) Atomically resolved STM image of a pentamer structure (18 × 18 Å2, V = 1.0 V, I = 0.14 nA). The high-symmetry directions of the substrate are marked aside. (b) Line scans cut along the lines drawn in panel a and showing the characteristic dimensions of the cluster. (c) Tersoff− Haman image of a Ni5O8 aggregate with bias V = +1.0 eV. The ISO level of charge density is set to 10−5 |e|/Å3. (d,e) Most stable Ni5O8 aggregate on MgO/Ag(100) as deduced from DFT calculations (side and top views). This configuration was used as input for the simulated image.
geometries fit with the 4-lobes experimental image reported in Figure 1d, which shows a flat arrangement with a distance of ∼2.7 Å between adjacent lobes (see Figure 2b); the question on the structure of the detected small clusters remains thus open. As next step, we repeated the calculations of the 3D and 2D structures and relative stability in the presence of different concentrations of oxygen at the MgO/Ag interface (ΘOint), a parameter neglected so far, to determine how it affects the morphology of the Ni/MgO/Ag system. We found that only at a very high loading of Oint (>50%) the 2D isomer is stable (see Table S1). We can thus draw the first conclusion that the presence of oxygen in the region immediately below the MgO layer affects the geometry of the Ni clusters; however, Oint concentrations much higher than the experimentally estimated ones (∼0.3 ML under the present experimental conditions20,21) would be required to justify the STM observation. Moreover, the dimensions of the 2D Ni4 aggregates are remarkably underestimated compared with those deduced by STM (see Table S2). This result indicates that nonoxidized Ni clusters do not fit with the reported STM images and that other effects should be considered. Because of the high affinity of metallic nickel for oxygen one can conceive that some oxygen could be incorporated in the clusters, producing NiyOx particles. This oxygen is unlikely to come from the residual atmosphere during and after the deposition, in view of the good background pressure (P < 2 × 10−9 mbar) in the UHV chamber and of the absence of O2 signal in the corresponding mass spectra. Alternatively, oxygen atoms may be extracted from the oxygen-rich interface. This latter mechanism is supported by the easy passage of atomic oxygen through the highly flexible MgO monolayer. Indeed, while on the MgO surface or thick film, O atoms adsorb at the surface on the oxygen site, forming a peroxo group, on the MgO monolayer the
adsorbed oxygen pushes the oxygen atom of the MgO lattice at the interface with a large energy gain (∼2 eV) and virtually no barrier (
