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Cluster Nucleation and Growth from a Highly Supersaturated Adatom Subscriber access provided by Washington University | Libraries Article Cluster Nucleation and Growth from a Highly Supersaturated Adatom Phase: Silver on Magnetite Roland Bliem, Rukan Kosak, Lukas Perneczky, Zbynek Novotny, Oscar Gamba, David Fobes, Zhiqiang Mao, Michael Schmid, Prof. Dr. Peter Blaha, Ulrike Diebold, and Gareth S. Parkinson ACS Nano, Just Accepted Manuscript • Publication Date (Web): 19 Jun 2014 Downloaded from http://pubs.acs.org on June 20, 2014 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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Page 1 of 20 ACS Nano 1 2 3 4 5 6 7 8 Cluster Nucleation and Growth from a Highly 9 10 11 12 Supersaturated Adatom Phase: Silver on 13 14 15 Magnetite 16 17 18 y z y y 19 Roland Bliem, Rukan Kosak, Lukas Perneczky, Zbynek Novotny, 20 21 y { { y 22 Oscar Gamba, David Fobes, Zhiqiang Mao, Michael Schmid, Peter 23 24 Blaha,z Ulrike Diebold,y and Gareth S. Parkinson∗,y 25 26 27 Institute of Applied Physics, Vienna University of Technology, Vienna, Austria, Institute for 28 29 Materials Chemistry, Vienna University of Technology, Vienna, Austria, and Tulane University, 30 31 New Orleans, Louisiana 70118, USA 32 33 34 E-mail: [email protected] 35 36 37 38 39 Abstract 40 p p 41 × ◦ 42 The atomic-scale mechanisms underlying the growth of Ag on the ( 2 2)R45 -Fe3O4(001) 43 surface were studied using scanning tunneling microscopy and density functional theory based 44 45 calculations. For coverages up to 0.5 ML, Ag adatoms populate the surface exclusively; ag- 46 47 glomeration into nanoparticles only occurs with the lifting of the reconstruction at 720 K. 48 49 Above 0.5 ML, Ag clusters nucleate spontaneously, and grow at the expense of the surround- 50 51 ing material with mild annealing. This unusual behaviour results from a kinetic barrier asso- 52 p p 53 ciated with the ( 2 × 2)R45◦ reconstruction, which prevents adatoms from transitioning 54 55 ∗To whom correspondence should be addressed 56 yInstitute of Applied Physics, Vienna University of Technology, Vienna, Austria 57 zInstitute for Materials Chemistry, Vienna University of Technology, Vienna, Austria 58 {Tulane University, New Orleans, Louisiana 70118, USA 59 60 1 ACS Paragon Plus Environment ACS Nano Page 2 of 20 1 2 3 4 to the thermodynamically favourable 3D phase. The barrier is identified as the large separa- 5 tion between stable adsorption sites, which prevents homogeneous cluster nucleation, and the 6 7 instability of the Ag dimer against decay to two adatoms. Since the system is dominated by 8 p p 9 kinetics so long as the ( 2 × 2)R45◦ exists, the growth is not well described by the tradi- 10 11 tional growth modes. It can be understood, however, as the result of supersaturation within an 12 13 adsorption template system. 14 15 16 17 18 Keywords 19 20 21 metal adatoms, nanoparticle growth, adsorption template, metal oxide surfaces, Fe3O4, magnetite 22 23 Adsorption templates are increasingly utilized to investigate fundamental processes at surfaces 24 25 because they offer a stable, well-defined initial state from which the evolution of a system can 26 27 be studied, and the influence of external stimuli determined. To date, moire´ lattices,1–3 surface 28 29 reconstructions,4 and ordered point defects5–7 have been found to provide the necessary combina- 30 31 tion of nucleation sites and corrugated surface potential to stabilize monodisperse metal clusters 32 33 at elevated temperatures. Such systems are promising for studies of heterogeneous catalysis, for 34 35 example, since the ability to tune the size of the clusters is ideal for studies of the size-effect. 36 p p 37 The adsorption template provided by the ( 2 × 2)R45◦-Fe O (001) surface is unique in 38 3 4 39 that the reconstruction stabilizes single Au adatoms to unprecedented temperatures (as high as 40 41 673 K).8 As such, this model system is potentially ideal to probe the mechanisms underlying single 42 43 9–13 44 atom catalysis under reaction conditions. However, while Au adatoms are stable thermally, the 45 46 coverage of Au adatoms is limited to 0.17 ML by the onset of cluster nucleation. In this study, we 47 48 turn our attention to the adsorption of Ag on the Fe3O4(001) surface. This choice was motivated 49 50 by our desire to further establish the universality of the adatom template for the transition metals, 51 52 to discover whether adatom coverages in excess of 0.17 ML could be achieved, and by reports of 53 14 54 highly active sub-nano Ag clusters in the literature. 55 56 In addition to studies of catalytic activity, the adatom template system also offers an ideal basis 57 58 for fundamental studies of cluster nucleation and growth; a long standing topic of fundamental 59 60 2 ACS Paragon Plus Environment Page 3 of 20 ACS Nano 1 2 3 4 research that underpins vital technologies such as microelectronics and heterogeneous catalysis. 5 6 Volmer-Weber growth (V-W, 3D crystallites) dominates in such systems since the surface energies 7 15,16 8 of metal oxides are often lower than those of metals, but Stranski-Krastanov (S-K) growth, in 9 10 which 3D crystallites coexist with a stable wetting layer, occurs in systems with strong adatom- 11 12 substrate interactions. In this paper, we use scanning tunneling microscopy and density functional 13 14 theory based calculations to study the atomistic details of Ag growth at the Fe3O4(001) surface. 15 17 16 While the thermodynamic preference is for 3D crystallites, as shown previously, our results show 17 p p ◦ 18 that a ( 2 × 2)R45 reconstruction strongly influences the growth kinetics, and indeed a near 19 20 complete layer of adatoms to be stabilized at room temperature. This unusual behavior stems 21 22 from the large distance between stable adsorption sites and the relative instability of the Ag dimer, 23 24 which decays into two adatoms on formation. Since kinetics dominate over thermodynamics below 25 26 720 K, the system is not well described by the traditional growth modes. We propose an alternative 27 28 description in which the surface is treated as an adsorption template, and cluster nucleation results 29 30 from supersaturation effects. 31 32 33 34 35 Results 36 37 38 The STM images presented in Figure 1 provide an overview of the Fe3O4(001) surface following 39 40 the deposition of Ag at room temperature. The Fe3O4(001) substrate is characterised by rows of 41 42 protrusions running in the h110i directions, which correspond to the rows of surface Fe atoms in 43 44 Figure 2(a).18–21 The Fe row direction, indicated by the cyan lines in Figure 1(a), rotates by 90◦ 45 46 from one terrace to to the next as a consequence of the inverse spinel structure. At the lowest 47 48 Ag coverage (Figure 1(a,d); 0.33 ML) Ag adatoms (yellow arrow) are observed at equivalent sites 49 p p 50 ◦ within the ( 2 × 2)R45 unit cell. Note that 1 ML is defined as one adatom per Fe3O4(001)- 51 p p 52 ( 2× 2)R45◦ unit cell (1.42×1018 atoms/m2. The adatoms are statistically distributed and show 53 54 no preference for step edges or other defect sites. No clusters are observed. DFT calculations 55 56 reveal that the preferred adsorption site is identical to that observed previously for Au8 and Pd22 57 58 59 60 3 ACS Paragon Plus Environment ACS Nano Page 4 of 20 1 2 3 ≈ 4 adatoms, i.e. twofold coordinated to surface oxygen atoms, protruding 0.2 nm above the surface 5 6 plane. The Ag adatoms are strongly bound (1.44 eV) and possess a charge state of +1. Figure 2(a) 7 8 shows a perspective view of the optimized structure. Calculations of a Ag dimer find two similarly 9 10 stable configurations, flat and upright (see Figure 2(b,c) and Table 1). In both configurations the 11 12 bond length to the surface oxygen atom is larger than for a Ag adatom.
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