Growth of nickel nanoparticles on NiO/Ni(0 0 1): Evidence of adsorbed oxygen on metal particles by metastable induced electron spectroscopy (MIES) Andrea Berlich * , Yan-Chun Liu, Harald Morgner Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Fakultät für Chemie und Mineralogie, Universität Leipzig, Linnestr. 2, D-04103 Leipzig, Germany article info Article history: Received 22 May 2008 Accepted for publication 14 October 2008 Available online 21 October 2008 Keywords: Metastable induced electron spectroscopy (MIES) X-ray photoelectron spectroscopy (XPS) Ultraviolet photoelectron spectroscopy (UPS) Work function measurements Surface electronic phenomena Nickel Nickel oxides Adatoms abstract We present results – obtained with X-ray as well as ultraviolet photoelectron spectroscopy (XPS and UPS) and metastable induced electron spectroscopy (MIES) – for the deposition of nickel on a NiO film of about 10 monolayers. It was found that up to a coverage of 0.5 monolayers single atoms or non-metallic clusters with up to 7 nickel atoms are formed. A MIES reference spectrum for this situation has been recon- structed from the experimental spectral series and the surface density of states has been obtained by deconvolution of this spectrum. Above 0.5 monolayer nominal nickel coverage a metallic nickel overlayer is formed as indicated by UPS. However, only MIES with its perfect surface sensitivity provides direct experimental evidence that on top of the metallic nickel a small amount of oxygen is adsorbed which originates from the underlying oxide. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The study of metal nanoparticles on oxide surfaces is a very ac- tive field of surface science with respect to such applications like heterogeneous catalysis or electronic devices [1–4]. However, the understanding of the elemental processes occurring at the hetero- geneous surface still remains an open question in some cases. Especially, the role of oxygen and oxides for the catalytic activity either at the surface or at the interface between metal and oxide is controversially discussed [5–7]. Schalow et al. used a molecular beam CO titration experiment to determine the amount of chemisorbed oxygen on Pd particles on Fe 3 O 4 [8] as well as of interface oxides by recording the CO 2 for- mation. Their kinetic measurements allowed to distinguish be- tween chemisorbed oxygen and interface oxides because of their different reactivity. In contrast to this indirect method the amount of adsorbed oxygen and surface oxides on metal particles can be measured directly using MIES (metastable induced electron spec- troscopy). This method provides perfect surface sensitivity over- coming the problem of a certain escape depth of photoelectrons in UPS or XPS that makes a quantitative evaluation more difficult. Although MIES has been applied to a number of solid and liquid surfaces including defect structures and magnetic surfaces [9–13] there have been difficulties to evaluate MIE spectra from heteroge- neous surfaces quantitatively. During the last years efforts have been made in our group to overcome this problem, a prerequisite to analyse inhomogeneous surfaces such as model systems for oxide supported metal catalysts. Our quantitative analysis is based on the assumption that the measured spectrum can be described by a linear combination of a limited number of reference spectra which can either be mea- sured directly or are reconstructed from a series of spectra. This concept has been successfully applied to a number of heteroge- neous solid surfaces such as the oxidation of transition metals [14] or to self assembled monolayers on gold and silver [15,16]. An overview is given in [17]. In a previous communication [18], we have successfully applied this algorithm to spectra obtained during nickel deposition on a NiO layer grown on Ni(0 0 1). Not only that we could reconstruct different reference spectra for the oxide and deposited nickel atoms. We also found hydrocarbons and we could even conclude that the hydrocarbons were exclusively adsorbed on Ni particles. It was assumed that the source of carbon was the graphite crucible used to evaporate nickel. In this work, the presence of these hydrocarbons is excluded by evaporating the Ni from a rod instead from a graphite crucible. It turns out that on top of the surface of metallic nickel chemisorbed oxygen acts as a surfactant for the growth of the metallic Ni film. Such a behaviour has been observed before for the growth of Ni 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.10.014 * Corresponding author. Tel.: +49 (341) 97 36 426; fax: +49 (341) 97 36 090. E-mail address: aschau@rz.uni-leipzig.de (A. Berlich). Surface Science 602 (2008) 3737–3744 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc