The smallest Silver oxide
Surprisingly bulk-like structural motifs appear in Silver-oxides already in the sub-nanoscale regime. Thanks to the new size-selected cluster source developed in the Lab we measured at the SuperESCA beamline the unusual process of Ag oxidation at T=20 K.F. Loi et al., J. Mater. Chem. A 27, 14594 (2022)
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Atomic clusters formed by a very small number of atoms constitute a formidable research topic, since they exhibit unique and often unexpected properties. The most fascinating aspect is that when the number of atoms forming the aggregates is very small (less than 100 atoms) the discretization of the electron density of states alters the atomic interactions leading to unique geometric and electronic structures. While for nanoparticles of few nanometers these properties are still dominated by the outer crystal plane characteristics, clusters in the sub-nanoscale often display non-crystalline icosahedral, decahedral, or even more exotic structures, which depend on the exact number of atoms in the clusters A proper description at this scale is even more challenging in the case of oxides and it is still unclear and debated whether the structural and electronic properties of oxidized atomic clusters follow the non-monotonic behavior typical of their metallic counterparts or if it is possible to obtain bulk-like features already for low-nuclearity aggregates. Such issue is particularly relevant as it would give crucial information to achieve tailor-made growth of nano-oxides, which have strong potentialities in heterogeneous catalysis reactions. In this context, we investigated the oxidation of an Ag atomic cluster made of 11 atoms combining high-resolution x-ray photoelectron spectroscopy and ab initio calculations based on density functional theory (DFT). In this experiment, performed at the SuperESCA beamline, we generated mass selected Ag11clusters with ENAC (Exact Number of Atoms in each Cluster), the cluster source designed and built at the Nanoscale Materials Laboratory which allows to produce small clusters with a precision on the very single atom. ENAC was specifically designed to be connected to the SuperESCA end-station, allowing for the in situ characterization of the clusters deposited on suitable substrates. For this experiment, we chose to deposit the cluster on a graphene layer epitaxially grown on Ru(0001). This substrate is a highly corrugated heterostructure and the low interaction of Ag with graphene leaves the property of the clusters nearly unperturbed, but its corrugation strongly decreases the mobility of the deposited clusters. Dealing with the mobility of the clusters was a crucial part of the experiment, as their migration on the surface of the sample would lead to their coalescence, compromising the mass selection. To further prevent this from happening, we worked with an extremely low coverage of Ag11 clusters (about 9 x 10-3 clusters/nm2, with a statistical occupation of one cluster every 9 moiré cells) and the sample was kept at a temperature of 20 K throughout the experiment. This extremely low temperature raised a new issue that we had to face: the energetic barrier for O2 dissociation on Ag cannot be overcome at T = 20 K. In fact, the O2 dissociation barrier on Ag atomic agglomerates is even higher than for Ag single crystal surfaces, hence making the oxidation of Ag clusters particularly challenging. To overcome these difficulties, we adopted a strategy (schematized in Fig. 1a) which allowed us to provide atomic O to foster the oxidation the Ag11 clusters without compromising the system. The clusters were exposed to a very low pressure of O2 at 20 K and irradiated with soft x-rays (500 eV), resulting in an efficient dissociation of the O2molecules physisorbed on the clusters caused by the secondary electrons produced by the photoelectric process. Despite the extremely low coverage of clusters on the sample, we were able to follow their oxidation by measuring the Ag 3d5/2 core level thanks to the high photon flux of the SuperESCA. The Ag 3d5/2 core level shift towards lower binding energy upon oxidation of he Ag11 clusters is presented in Fig. 1b. This negative core level shift is a known anomaly of Ag related to final-state effects and represents a clear indication of the successful oxidation of the clusters. |
To interpret the experimental results, we performed DFT calculations on the structure of oxidized Ag11O2m clusters for different O coverages and we calculated the Ag 3d5/2 core levels energies for each of them. We focused our study on clusters with an even number of O atoms, as the latter are obtained from the dissociation of O2 molecules. The DFT calculations were used for the spectral analysis performed on the data in Fig. 1b, where we demonstrate that almost 40% of the total spectral weight can be attributed to Ag11O12 clusters. These clusters have an Ag:O ratio close to 1 and a deeper analysis based on DFT calculations showed structural and electronic properties that strongly resemble those of AgO bulk oxide. We investigated the average Ag – O bond lengths, the local morphology and the Bader charge distribution in the cluster to conclude that Ag11O12 has, like the AgO bulk oxide, an hybrid structure formed by Ag(I) and Ag(III) ions (Fig. 1c). More points of contact between the two systems were found by comparing the calculated projected density of states (PDOS) of the 4d-band of Ag ions in both systems (Fig. 1d), allowing us to conclude that Ag11O12 can be labelled as the smallest silver oxide.
Retrieve article Oxidation at the sub-nanoscale: oxygen adsorption on graphene-supported size selected Ag clusters Federico Loi, Monica Pozzo, Luca Sbuelz, Luca Bignardi, Paolo Lacovig, Ezequie Tosi, Silvano Lizzit, Aras Kartouzian, Ueli Heiz, Dario Alfè and Alessandro Baraldi J. Mater. Chem. A 27, 14594 (2022) |
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