Bulk and surface properties of Cu 2 O: A first-principles investigation Mazharul M. Islam * , Boubakar Diawara * , Vincent Maurice, Philippe Marcus Laboratoire de Physico-Chimie des Surfaces, CNRS-ENSCP (UMR # 7045), Ecole Nationale Supérieure de Chimie de Paris, Université Pierre et Marie Curie, 11 rue Pierre et Marie Curie, 75005 Paris, France article info Article history: Received 1 July 2008 Accepted 19 November 2008 Available online 5 March 2009 Keywords: Copper oxide Density-functional theory Surface structure Reconstruction Electronic properties abstract The bulk and surface properties of Cu 2 O were studied theoretically at the density-functional level (DFT). The calculated structural parameters, binding energy per Cu 2 O molecule (E u ) and electronic properties were compared with available experimental bulk data. The convergence of the surface energy was inves- tigated for both non-polar (oxygen terminated) and polar (copper terminated) (1 1 1) surfaces. The elec- tronic properties of both surfaces showed that there are surface states both at the top of the valence band and the bottom of the conduction band, indicating strong surface excitons decreasing the band gap. The structural relaxation was investigated for both cases. The study shows that the stoichiometric O-termi- nated Cu 2 O(1 1 1) surface exhibits minor relaxation, whereas the Cu-terminated surface undergoes exten- sive relaxation minimizing the surface polarity and indicative of surface reconstruction for this termination. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Copper oxides are of considerable interest in catalysis since cop- per based catalysts are commercially used in numerous important chemical reactions. For fuel cells operating with hydrogen as fuel, Cu-based water–gas shift catalysts provide primary CO cleanup as well as secondary H 2 production in the fuel processing [1,2]. Cu/ZSM-5 catalysts show interesting performance properties for clean-burn NO x reduction in fully formulated simulated exhaust gases [3]. There it is observed that copper-alkyl species are formed by an oxidative activation and these intermediates add nitric oxide to form N-nitroso-N-alkylhydroxylamate species bound to cupric centers. The decomposition and reduction of N 2 O are performed using Cu/Carbon, Cu/Al 2 O 3 , Cu/SiO 2 and Cu/ZSM-5 catalysts [4]. Cu-based catalysts are also used for methanol synthesis from syn- gas (CO, CO 2 and H 2 ) [5,6], and for the partial oxidation of metha- nol to formaldehyde [7]. CuO/ZrO 2 catalysts are widely used for low temperature CO oxidation [8]. Cu 2 O is also an effective photo- catalyst able to decompose water into H 2 and O 2 under visible light [9–11]. In addition, recent investigations show that copper oxide nanoparticles produced by the chemical vapor deposition process appear to have a great relevance as catalysts to oxidize organic compounds at lower costs compared to noble metals [12,13]. Copper oxides are also important for their corrosion protection properties. Cu 2 O ultra-thin films grown at the surface of copper provide corrosion protection to the metal substrate in aqueous environments [14–17]. Energy storage is also a potential applica- tion since Cu 2 O submicroscopic spheres could be used as negative electrode material for lithium ion batteries [18]. In all these applications, performance is governed by the surface properties. Therefore, it is a challenge to understand the factors controlling not only the adsorption and reactivity of adsorbates but also the stability of the surface both in gaseous and liquid envi- ronments. In the present work, we focus on the Cu 2 O oxide and its (1 1 1) surface that can have an oxygen non-polar or a copper polar termination. The Cu 2 O lattice is highly symmetric, with a cubic structure (space group Pn3). It has six atoms per unit cell with the oxygen and copper atoms forming bcc and fcc sub-lattices, respectively, as shown in Fig. 1 [19,20]. The oxygen atoms occupy two of the eight tetrahedral sites defined by the Cu fcc sub-lattice. From a structural viewpoint, Cu 2 O is interesting because it pos- sesses unusual linear O–Cu–O bonding [21]. From an electronic viewpoint, bulk Cu 2 O is interesting because of its Wannier-exciton spectrum [22]. Both theoretical and experimental investigations of the structural properties [20,23–27], energetic properties [20,27] and electronic properties [22,24–31] of Cu 2 O have been performed. The experimental value of enthalpy of atomization (E u ) is 1093 kJ/mol [32] and the band gap (E g ) is 2–2.2 eV [22,28,29]. There are numerous experimental [33–36] and theoretical [24– 27, 37–39] investigations on the stability, structural and electronic properties of Cu 2 O surfaces. For the solid/gas interface, the low-en- ergy electron diffraction (LEED) study by Schulz and Cox [33] showed that the (1 1 1) surface can be prepared in a nearly stoichi- ometric non reconstructed (1  1) form by ion bombardment and annealing in vacuum. The absence of reconstruction was assigned to the non-polar property of the oxygen terminated surface. For 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.02.037 * Corresponding authors. E-mail addresses: rana-islam@enscp.fr (M.M. Islam), bob-diawara@enscp.fr (B. Diawara). Journal of Molecular Structure: THEOCHEM 903 (2009) 41–48 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem