Role of sub-surface oxygen in Cu(100) oxidation Minyoung Lee, Alan J.H. McGaughey ⁎ Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, United States abstract article info Article history: Received 18 March 2010 Accepted 3 May 2010 Available online 17 May 2010 Keywords: Copper (100) oxidation Sub-surface oxygen Missing-row reconstruction Oxygen-induced restructuring Density functional theory Density functional theory (DFT) calculations are used to investigate the role of sub-surface oxygen in Cu(100) oxidation. We ﬁnd that the presence of sub-surface oxygen atoms causes the top copper layer of the missing-row reconstructed surface to rise by 1.7 Å compared to the bare surface. This prediction compares well to an earlier scanning tunneling microscopy measurement of 1.8 Å [Lampimaki et al. Journal of Chemical Physics 126 (2007) 034703]. When the missing-row reconstructed surface is exposed to an additional oxygen molecule, surface restructuring that leads to oxide-like structures is only observed when sub-surface oxygen is present. The oxide- like nature of these structures is conﬁrmed through structural, Bader, and electron density of states analyses. These ﬁndings, combined with our previous DFT results that predicted low energy barriers for the embedment of oxygen atoms into the sub-surface [Lee and McGaughey, Surface Science 603 (2009) 3404], demonstrate the key role played by sub-surface oxygen in Cu(100) oxidation. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Copper and its oxides (CuO and Cu 2 O) are relevant in many scientiﬁc and engineering applications. Cu 2 O, which is a direct band gap semiconductor, is a candidate material for next-generation photovoltaic cells [1,2]. During the electrochemical reaction in a solid state cell containing Cu + conducting electrolytes, a Cu/Cu 2 O interface is formed and detrimentally affects the cell performance [3]. To prevent CO poisoning, copper, CuO, and Cu 2 O powders and nanoparticles are being considered as substitutes for the platinum catalyst in fuel cells [4,5]. It is thus important to understand the atomistic structure and energetics of copper–oxide interfaces and how the associated free surfaces interact with their environments. Based on the existing experimental and theoretical data, the oxidation of a Cu(100) surface is a four-step process that proceeds as more oxygen molecules arrive at the surface: (i) Oxygen molecules dissociate and the oxygen atoms adsorb on the face-centered cubic (fcc) hollow sites up to 0.5 monolayer (ML) coverage [c(2×2) phase] [6–17]. (ii) The 2 ﬃﬃﬃ 2 p × ﬃﬃﬃ 2 p   R45- missing-row reconstruction is formed by the release of every fourth row of copper atoms from the top copper layer [18–29]. (iii) Merged missing-row reconstructed domains are formed and Cu 2 O islands nucleate [30,31]. (iv) Islands grow and merge with other islands to form a continuous oxide layer [32–36]. This behavior is different from typical metal-oxides (e.g., aluminium, iron, and barium), which grow as a uniform layer on a clean metal surface [37]. The shapes of the Cu 2 O islands in steps (iii) and (iv) vary with the oxidation temperature. They are triangles at 350 °C, squares at 500 °C, high aspect-ratio nanorods at 600 °C, and pyramids above 700 °C [34]. To use (and potentially control) the varied nanostructures of the Cu 2 O islands, an understanding of the atomic-level mechanisms of copper oxidation is needed. There have been theoretical and experimental investigations of the atomic oxygen adsorption [step (i)] [6–17] and the oxygen-induced surface reconstruction [step (ii)] [18–29]. A limited number of investigations, however, have studied the transition from the missing- row reconstruction to Cu 2 O island formation [step (iii)] [30,31,38–40]. Using scanning tunneling microscopy (STM), Lampimaki et al. observed that individual missing-row reconstructions merge to form an ordered domain that is 1.8 Å higher than the clean surface. They found that Cu 2 O islands form on these domains [30]. They suggest that the copper atoms released during the missing-row reconstruction form an adlayer that subsequently reconstructs due to further oxygen adsorption. This process elevates the surface and creates sub-surface oxygen. As we previously reported, however, oxygen can also directly penetrate into the sub-surface region through the missing row. Lahtonen et al. found, using STM and X-ray photoelectron spectroscopy, that oxygen penetra- tion into the sub-surface region plays a key role in the subsequent oxidation and the growth of Cu 2 O islands [31]. Using density functional theory (DFT) calculations, Kangas et al. found that a missing-row reconstructed Cu(100) surface that includes on- and sub-surface oxygen atoms is more energetically favorable than a missing-row reconstructed surface including only on-surface oxygen atoms [38]. Using DFT calculations, Kangas and Laasonen later found that oxygen embeds into the sub-surface more easily when there are adsorbed oxygen atoms both on the surface and in the sub-surface [39]. They also found signiﬁcant structural changes on the missing-row reconstructed Cu (100) surface when the sub-surface oxygen coverage increases from 0.25 to 1.0 ML. The unit cell p ﬃﬃﬃ 2 p × ﬃﬃﬃ 2 p   h i used in that study, however, was too small for the authors to identify possible Cu 2 O-like structures and the transition mechanism from the missing-row reconstructed Surface Science 604 (2010) 1425–1431 ⁎ Corresponding author. E-mail address: mcgaughey@cmu.edu (A.J.H. McGaughey). 0039-6028/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2010.05.004 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/ locate/susc