DFT study of the adsorption of Ni on Anatase (0 0 1) surface E. Escamilla-Roa a , V. Timón b , A. Hernández-Laguna b,⇑ a Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain b Instituto Andaluz de Ciencias de la Tierra, CSIC-UGR, Avda Las Palmeras 4, 18100 Armilla, Granada, Spain article info Article history: Received 2 March 2011 Received in revised form 25 November 2011 Accepted 25 November 2011 Available online 3 December 2011 Keywords: DFT Anatase TiO 2 (0 0 1) Surface Ni Adsorption Spin density abstract Potential energy curves and different adsorption sites of Ni on Anatase (0 0 1) surface, at increasing cov- erage, were computed at singlet, triplet, and quintuplet multiplicity states. Coverage was increased until a complete adlayer is reached. A density functional theory (DFT) based method (periodic D Mol 3 included in MS package) was used. At the lowest coverage of our system (one Ni per 2  2  1 unit cells) different adsorption sites were found at the holes on the surface. At the triplet multiplicity state, two deep minima were found at 0.55 and 0.11 Å from the bridging oxygen plane (BOP) on the surface with adsorption ener- gies of 3.5 and 3.74 eV, respectively. Both of these minima showed spin transfer from the Ni to the Ti atoms at the relaxed slab surfaces. With singlet multiplicity a BOP minimum was found at 0.33 Å. For high coverages, spin multiplicity of the system turned out to be the singlet. This shows similar trends alike in the clustering of Ni on Anatase (0 0 1) surface. Calculated surface energy decreased until full cov- erage, and the surface energy per Ni also decreased up to saturation value at full coverage. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction TiO 2 is one of the most widely used metal oxide, with a wide range of technological applications. It is one of the most studied transition metal oxides within surface science, semiconductors, adsorptions, catalysis in general (especially photo-catalysis), solar cells, and many other fields (see for example Ref. [1] and references therein). Particularly in the oil industry, TiO 2 acts as potential sup- ports for hydrodesulphurisation processes and in methane-reform- ing catalysis [2–7]. It is an important substrate for metal support, such as Au, Cu, and Pb. Metals adsorbed onto the surface show important catalytic properties [8–10]. In nature, TiO 2 exits in three polymorphs, namely Rutile, Ana- tase, and Brookite [11,12]. The first and second ones are tetragonal and the third polymorph is orthorhombic. Rutile is the most com- mon phase, while Anatase is considered as a metastable phase and less dense than Rutile [13]. Up to crystal sizes of approximately 14 nm, TiO 2 seems to prefer the Anatase phase rather than the Ru- tile phase [14]. Anatase exhibits high photo activity for catalysis [15]. This can be understood by considering that Anatase shows the lowest average surface energy of the three polymorphs, whose equilibrium crystal shape is dominated largely by the highly stable (1 0 1) surface. It is well known that the Anatase (0 0 1) surface is the minority surface at the equilibrium shape of Anatase nano- crystals [14,16–19] and it was found to be highly reactive [17,18,20–23] with metals and organic compounds. Several theo- retical studies have predicted the special reactivity of Anatase (0 0 1) surface [20,24]. Furthermore, from experiments and DFT cal- culations, it is evidenced that Anatase (0 0 1) surface is especially stabilized as fluorate surface [25], in front of other adatoms such as H, from Boron to Oxygen and from Silicon to Chlorine, Br and I, letting this facet being the dominant in the fluorate adsorbed crystal form. Rutile and Anatase were extensively investigated as supports for gold nano-particles in catalytic CO oxidation [1,26–29]. Ni deposited onto the (1 1 0) surface of Rutile has been studied by dif- ferent spectroscopic techniques [30], and a charge transfer from the surface to the Ni at low coverage was found depending on the previous preparation of the surface. This charge transfer plays an important role in the chemisorption of Ni and its catalytic prop- erties. Furthermore, with the use of similar techniques quoted in Ref. [30], CO adsorbed onto Ni–TiO 2 (1 1 0) and clean surfaces, re- veals a low charge transfer from the Ni to the surface [31,32]. Re- cently, using X-ray Photoelectron Spectroscopy, annealing and Ar + ion bombardment on Ni nano-clusters adsorbed onto the TiO 2 (0 0 1) surface [33], a 3 eV satellite structure appeared in the Ni 2p 3/2 photoemission spectra. This was attributed to 3 F states of Ni, discarding a 3d 10 final-state configuration with a low charge transfer without transport of mass between Ni and the surface. However, this line at 3 eV shortened when the coverage was in- creased, suggesting a larger population of the 3d orbitals in larger clusters on the surfaces. Furthermore, in Ref. [33] authors suggest theoretical calculations in order to clear up this phenomenon. The 2210-271X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.comptc.2011.11.046 ⇑ Corresponding author. E-mail address: ahlaguna@ugr.es (A. Hernández-Laguna). Computational and Theoretical Chemistry 981 (2012) 59–67 Contents lists available at SciVerse ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc