Surface Science 603 (2009) 2087–2095

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Surface Science

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First principles investigation on the stabilization mechanismsof the polar copper terminated Cu2O(1 1 1) surface

Mazharul M. Islam *, Boubakar Diawara *, Vincent Maurice, Philippe MarcusLaboratoire 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, 75231 Paris Cedex 05, France

a r t i c l e i n f o

Article history:Received 13 January 2009Accepted for publication 5 April 2009Available online 15 April 2009

Keywords:Copper oxideDensity functional theoryMolecular dynamicsSimulated annealingPolar surfacesSurface reconstructionHydroxylation

0039-6028/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.susc.2009.04.005

* Corresponding authors. Tel.: +33 144276736; fax:E-mail addresses: rana-islam@enscp.fr (M.M. I

(B. Diawara).

a b s t r a c t

The stabilization of the unstable, polar copper terminated Cu2O(1 1 1) surface by reconstruction andhydroxylation was studied theoretically with static and molecular dynamics calculations at ab initio den-sity functional theory (DFT) level. Surface reconstruction was investigated using extensive finite temper-ature molecular dynamics (MD) combined with a simulated annealing technique. Both the globalminimum energy structure obtained during annealing the system at higher temperature (300 K) andthe final ‘quenched’ structure which was obtained after cooling the system to 0 K show the expectedreconstruction of the adsorbate-free surface. The copper atoms in the first layer and oxygen atoms inthe second and third layers are markedly displaced, and the atomic planes merge together to form a uni-form mixed layer, thereby minimizing the polarity of the surface. Surface hydroxylation by adsorption ofOH� or dissociated water was investigated using static optimization at 0 K. The results show that adsorp-tion is exothermic and that the reconstruction characterizing the annealed OH-free surface does notoccur in the presence of adsorbed OH. A surface coverage of 50% results in the surface structure that isthe closest to the unrelaxed bulk terminated surface.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Oxide surfaces are subject to complex stabilization processesthat ultimately determine their physical and chemical properties.Understanding the factors controlling the stability and adsorptionproperties at solid/gas and solid/liquid interfaces is a challengingissue and a key step to the design of appropriate surface propertiesfor numerous applications. Copper oxide is of considerable interestfor corrosion protection, energy storage and catalysis. Cu2O ultra-thin films (i.e. nano-objects) grown at the surface of copper providecorrosion protection to the metal substrate in aqueous environ-ments [1–5]. Cu2O submicroscopic spheres have potential applica-tions as negative electrode material for lithium ion batteries [6–8].Copper based catalysts are commercially used in the water–gasshift reaction to purify the hydrogen gas for fuel cells [9], in meth-anol synthesis from syngas (CO, CO2 and H2) [10,11], and in thepartial oxidation of methanol to formaldehyde [12]. Cu2O is alsoan effective photocatalyst able to decompose water into H2 andO2 under visible light [13–17].

Cu2O has a cubic structure (space group Pn3) with six atoms perunit cell and the oxygen and copper atoms forming bcc and fcc

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+33 146340753 (M.M. Islam).slam), bob-diawara@encp.fr

sub-lattices, respectively [18,19]. The oxygen atoms occupy twoof the eight tetrahedral sites defined by the Cu fcc sub-lattice.Along the h1 1 1i directions, atomic tri-layers consisting of oneCu plane embedded between two O planes are repeatedly piled-up (this can be seen in Fig. 2). The repeating unit is –O–Cu4–O–with the O planes forming a (2 � 2) superstructure with respectto the Cu plane, as described previously [20]. The Cu2O(1 1 1) sur-face belongs to class II of the Tasker’s classification of polar sur-faces with a non-polar O termination (O–Cu4–O–O–Cu4–O–. . .)and a polar Cu termination (Cu4–O–O–Cu4–O–O. . .) [21].

So far, experimental and theoretical investigations on the stabil-ity, structure and adsorption properties of the Cu2O(1 1 1) haveconcentrated on the O-terminated non-polar surface at the solid/gas interface. The Low Energy Electron Diffraction (LEED) studyby Schulz and Cox [22] showed that the (1 1 1) surface can be pre-pared in a nearly stoichiometric non reconstructed (1 � 1) form byion bombardment and annealing in vacuum. The absence of recon-struction was assigned to the non-polar property of the O-termi-nated surface. Adsorption of CO [23–28], NO [23,24,29,30], C3H6

[31], H2S [32,33], CH3OH [34], NH3 [35], H2O [33], benzotriazole[36], O2 [37], and N2O [38,39] was investigated on perfect anddefective surfaces. Metastable surface oxides for the O–Cu system,possibly catalytically relevant, were investigated by DFT modeling[40,41]. There it was shown that the polar Cu-terminatedCu2O(1 1 1) surface could be stabilized by the presence of stoichi-ometric defects (Cu vacancies).

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The possible routes of stabilization of the polar oxide surfacesconsist of various kinds of physical–chemical processes, eitherintrinsic or extrinsic [21]. They include the modification of the sur-face region by reconstruction, by hydroxylation or addition ofcharge provided by the adsorption of foreign chemical species orig-inating from the environment, or by the presence of stoichiometricdefects. Experimental investigations of the solid/liquid interfaceshowed that (1 1 1)-oriented Cu2O ultra-thin films are grown onthe Cu(1 1 1) surface by anodic oxidation in alkaline aqueous solu-tions [2,4]. Electrochemical Scanning Tunneling Microscopy (EC-STM) images revealed a (1 � 1) surface hexagonal lattice consistentwith the non-defective Cu sub-lattice, suggesting an unrecon-structed Cu termination. It was proposed that such a terminationis stabilized by surface hydroxylation of the Cu2O(1 1 1) surfaceat the interface with the aqueous solution. However, to the bestof our knowledge, there is no theoretical investigation of thestabilization of Cu-terminated Cu2O(1 1 1) surface by surfacehydroxylation.

Here we report a theoretical investigation of the stabilization ofthe polar Cu-terminated Cu2O(1 1 1) surface via complex surfacereconstruction and surface hydroxylation. In order to account forthe non defective surface observed at the solid/liquid interface[2,4], our study does not include mechanisms involving the intro-duction of vacancies and/or interstitial defects as reported previ-ously [40,41]. Static optimization at 0 K and finite temperaturemolecular dynamics (MD) calculations in combination with simu-lated annealing (SA) at room temperature were used at DFT level.The structural relaxation and the charge distribution were ana-lyzed for the MD simulated minimum energy and quenched struc-tures for the non-hydroxylated surface. The effect of surfacehydroxylation, canceling structural relaxations and stabilizing thesurface, was studied by static optimization at 0 K.

-142,5

-142,4

-142,3

-142,2

-142,1

0 100 200 300 400 500

Number of steps

Ener

gy (e

V)

Fig. 1. Potential energy curve for the annealing step at 300 K of the SA protocolapplied to a six tri-layer slab of Cu-terminated Cu2O(1 1 1). The global energyminimum structure is observed at step 94.

2. Computational method

Periodic calculations using the DFT method were performed.Procedural and computational details are described in details in aprevious 0 K static optimization study of Cu2O that gave excellentagreement between calculated and experimental bulk properties[20]. The Perdew–Burke–Ernzerhof (PBE) exchange-correlationfunctional [42] based on the GGA approximation was employed.This method was used as implemented in the plane-wave programVASP [43–45]. The projector-augmented wave (PAW) potentials[46,47] were used for the core electron representation. With planewave methods, the quality of the basis set is determined by a singleparameter, the energy cutoff Ecut. We used a converged value of Ecut

= 520 eV, as optimized in our previous study [20]. The valence elec-tron configurations for Cu and O are 3d104p1 and 2s22p4, respec-tively. The integration in reciprocal space was performed with aMonkhorst–Pack grid [48]. Based on the optimized structure ofbulk Cu2O, slabs of Cu2O(1 1 1) were modeled with an increasingnumber of tri-layers. A vacuum layer of 15 Å along the z directionperpendicular to the surface (the x and y directions being parallelto the surface) was employed to prevent spurious interactions be-tween the repeated slabs. A minimum of six tri-layers was foundnecessary to obtain converged values of the surface energy. Theconverged k-point grid for bulk Cu2O is (6 � 6 � 6) and that forthe six tri-layer slabs is (2 � 2 � 2) as optimized in the previousstudy [20]. The polarity was compensated by introducing dipolarcorrections along the axis perpendicular to the surface. Dipolarcorrections are required for fastening the energy convergence withrespect to supercell size as discussed in [49,50].

All MD simulations presented in this work were performed onthe exact Born-Oppenheimer surface in the microcanonical ensem-ble (NVE; N = number of the particles in the system, V = volume of

the system, E = energy of the system) and velocity rescaling with atime step of 2.5 fs. The simulated annealing (SA) protocol was thefollowing: heating the system up to the target temperature(300 K), then equilibrating the system at 300 K, and finally coolingthe system gradually down to 0 K, each process containing 500steps. A further geometry optimization was performed on the final‘quenched’ structure. Longer equilibration and cooling times werefound to have no effect on the final results.

Surface hydroxylation was simulated by adsorbing hydroxylgroups (OH) or dissociated water (OH + H) at the slab surface andvarying the surface coverage. Note that studying the mechanismsof dissociative adsorption of the water molecule on the oxide sur-face is not relevant in this case since the experimental studiesshowed that hydroxide ion adsorption on the metal surface occursbefore anodic oxidation [1–4], and thus that adsorbed hydroxylgroups can be expected at the oxide surface during the growth ofthe ultra-thin Cu2O(1 1 1) film. Geometry optimizations were per-formed at 0 K by relaxing all the atoms of the slab. All modelingwas performed by using the graphical interface software Model-View [51].

The charge distribution on the atoms was investigated usingBader’s topological analysis [52,53]. In this approach, atomiccharges are calculated using the decomposition of electroniccharge density into atomic contributions by dividing the space intoatomic regions with surfaces at a minimum in the charge density.We have used the ‘Bader’ code developed by Henkelman et al. [54],which assigns each point on a regular (x, y, z) grid to one of the re-gions by following a steepest ascent path on the grid. Here, theoptimized grid was 160 � 160 � 480.

3. Results and Discussion

3.1. Simulated annealing of the bare surface

The converged value of the surface energy Es for the adsorbate-free Cu2O(1 1 1) surface obtained by static optimization at 0 K was1.48 J/m2 [20]. This is an average value for a Cu-terminated and O-terminated surface because the slabs have two different chemicalterminations as discussed previously [20]. The stabilization energy(calculated as difference of the slab energy per surface area afterand before relaxation) was 0.44 J/m2. This is an indication of largerelaxation effect of the Cu-terminated surface that was confirmedby the structural analysis. Molecular dynamics in combinationwith simulated annealing was performed in order to determinethe energetically most stable structure of this surface. Fig. 1 shows

Fig. 2. Structure of the Cu-terminated Cu2O(1 1 1) surface (side and top views): (a) unrelaxed bulk termination, (b) global minimum energy structure annealed at 300 K, thebox in the middle shows the three tri-layers that are fixed, (c) final annealed structure quenched at 0 K. Small and large spheres represent Cu and O atoms, respectively. In thetop views, the Cu1 and Cu2 atoms are differentiated in yellow and blue, respectively, and O atoms of the second (OL2) and third layer (OL3) in red and green, respectively. (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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the variation of the energy vs. the number of steps during theannealing step at a constant temperature of 300 K. It is observedthat the global minimum energy structure (according to our simu-lation conditions as described above) is obtained at the 94th step.The annealed structure was then cooled down to 0 K to obtain thefinal quenched structure. Further optimization of this quenchedstructure produces Es = 1.23 J/m2, which gives a stabilization ofthe surface by 0.25 J/m2 compared to the statically optimizedstructure. In the following, we analyze the structural relaxationand the charge distribution in these two structures.

3.1.1. Surface reconstructionIn Fig. 2, the annealed structure of the global minimum energy

and the final quenched structure are compared with the bulk ter-minated surface. Three tri-layers in the middle of the six tri-layerslab were frozen to simulate the bulk and the top and bottomtri-layers were allowed to relax, as shown in Fig. 2b. It means thatat the top 1 complete tri-layer + 2 atomic layers (1 Ox + 1 Cu) arerelaxed and at the bottom 1 complete tri-layer + 1 Ox atomic layer

are relaxed for optimization. The middle three tri-layers are fixedas our previous investigation showed that these tri-layers remainunchanged during optimization [20]. The resulting slab is asym-metric as is the starting slab. It should be noted that there is noway to build a symmetric slab of the defect-free and adsorbate-freecopper terminated Cu2O(1 1 1) surface.

The Cu-terminated Cu2O(1 1 1) surface contains three differentsurface atoms, namely, the oxygen (O) atom of the second layer,the copper atom of the first layer (Cu1) laterally bonded to the sec-ond layer oxygen, and the copper atom of the first layer (Cu2) per-pendicularly bonded to the third layer oxygen. Structuralrelaxation was first analyzed by measuring the inter-atomic dis-tances. For the annealed structure of global minimum energy, theO–Cu1 and O–Cu2 bond lengths have increased from 1.87 Å to2.11 Å and 1.91 Å, respectively, and the Cu1–Cu2 bond lengthhas decreased from 3.05 Å to 2.58 Å. Similarly, the quenched struc-ture shows an increase of O–Cu1 and O–Cu2 bond lengths of 0.19 Åand 0.01 Å, respectively, and a decrease of the Cu1–Cu2 bondlength of 0.53 Å with respect to bulk terminated surface.

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Marked atomic displacements with respect to the positions inthe bulk terminated surface are also observed. For the global min-imum energy structure, the Cu1 atoms in the first layer havemoved differently. Average values of Dx = �2.43 Å, Dy = �0.98 Åand Dz = �0.91 Å were measured. The Cu2 atom in the first layerhas also moved along all directions (Dx = �1.81 Å, Dy = �2.30 Åand Dz = �1.04 Å). The oxygen atom of the second atomic layerhas moved along x (Dx = �2.23 Å), y (Dy = �0.46 Å) and z(Dz = �0.33 Å), and is positioned nearly into the same plane asthe copper atoms of the first atomic layer (Fig. 2b). The oxygenatom in the third layer also moved along the x, y and z directions(Dx = �2.26 Å, Dy = �0.43 Å and Dz = +0.96 Å). As a final result,the top Cu and O layers merge together to form one layer. A similarrelaxation is also observed in the case of the quenched structure(Fig. 2c).

This large extent of surface relaxation is confirmed by measur-ing the inter-planar distances along the surface normal betweenthe top three atomic layers. For the unrelaxed surface, the dis-tances between the first copper plane and the second (OL2) andthird (OL3) oxygen atom are 0.62 Å and 1.87 Å, respectively. Forthe annealed structure of global minimum energy, these distances

Fig. 3. Charge distribution according to Bader’s analysis on (a) bulk Cu2O, (b) unrelaxedand (d) final annealed structure quenched at 0 K. Small and large spheres represent Cu athe center of gravity for negative and positive charges, respectively. (For interpretationversion of this article.)

are reduced to 0.05 Å and 0.09 Å, respectively. Similarly, the Cu2–OL2 and Cu2–OL3 distances are reduced to 0.00 Å and 0.14 Å,respectively. This drastic surface relaxation is confirmed by thequenched structure.

Top views of the unrelaxed, annealed and quenched structuresare also presented in Fig. 2. Here the Cu2 atoms are marked in blue.In the unrelaxed structure (Fig. 2a), all the copper atoms are in thesame plane and they are organized in three straight rows equiva-lent to 120�, forming a hexagonal pattern. For both the annealed(Fig. 2b) and quenched (Fig. 2c) structures, this hexagonal patternis disrupted by the formation of ‘zig-zag’ rows in the copper layerdue to the atomic displacements (relative to the bulk position). Inthe unrelaxed structure, the three Cu1 atoms are bonded to theoxygen atoms of the second atomic layer in such a way that theyform an O–Cu3 symmetric pyramidal unit (as marked in Fig. 2a).Due to the atomic displacements along the x, y and z directions,the O–Cu3 units tend to become planar in the annealed andquenched structures (as marked in Fig. 2b and c). Both the ‘zig-zag’ rearrangement of the copper rows and the planarization ofthe O–Cu3 pyramidal units confirm the previous results obtainedfrom 0 K static optimization of this surface [20]. They are indicative

Cu-terminated Cu2O(1 1 1), (c) global minimum energy structure annealed at 300 K,nd O atoms, respectively. The light-blue and purple intermediate spheres representof the references to colour in this figure legend, the reader is referred to the web

M.M. Islam et al. / Surface Science 603 (2009) 2087–2095 2091

of a strong relaxation stabilizing the defect-free and adsorbate-freeCu-terminated Cu2O(1 1 1) surface. Combined with the local dis-ruption of the hexagonal symmetry of the (1 1 1)-oriented Cusub-lattice, this shows surface reconstruction.

From the structural point of view, the quenched structureresembles the global minimum energy structure which has beencorrectly trapped during the SA process. Upon further static opti-mization at 0 K, iso-energetic structures were obtained. It confirmsthat the quenched structure was not a local minimum trapped dur-ing cooling down to 0 K.

3.1.2. Charge distributionIn Fig. 3, the charge distribution of the Cu-terminated

Cu2O(1 1 1) surface is compared with that in the bulk Cu2O.According to our previous investigation [20], the Bader chargesare +0.53 e on Cu and –1.05 e on O in the bulk (Fig. 3a). Therefore,bulk Cu2O is not purely ionic. This is in agreement with a recenthigh-energy synchrotron experimental investigation on chargedensities of Cu2O [55], where the charges were found to range from+0.84 e to +0.57 e on Cu atoms and from –1.68 e to –1.14 e on Oatoms. Our calculated charges are also in line with the Badercharges obtained with the DFT method based on the GGA func-tional, where the charge on Cu and O atom is +0.53 e and –1.07 e, respectively [56].

For the unrelaxed Cu-terminated Cu2O(1 1 1) surface (Fig. 3b),the top-most layer consists of positively charged copper atomswith a sum of +0.96 e and the second layer contains negativelycharged oxygen atom with –1.02 e. The bottom layer of the slabconsists of a negatively charged oxygen atom with a partial chargeof�0.61 e. The positive and negative charges have different centersof gravity which are marked in Fig. 3b by light-blue and purplespheres, respectively. Therefore, the unrelaxed surface possessesa dipole moment along the surface normal. Both in the annealed(Fig. 3c) and quenched (Fig. 3d) structures, the distance betweenthe positive and negative centers of gravity is shorter as a resultof the structural relaxation. The distance has reduced from 1.12 Åfor the unrelaxed structure to 0.4 Å for both relaxed structures,thus evidencing that the decrease of polarity is the driving forcefor the surface reconstruction in the absence of stoichiometry de-fects or adsorbates.

The analysis of the charge distribution in the unrelaxed struc-ture (see Fig. 3b) also reveals that all the inner tri-layers, character-istic of an ideal non-polar Cu2O(1 1 1) orientation, possess a 0 echarge, but not the tri-layer formed by the 1st Cu layer, the 2ndO layer and the bottom O layer. The sum of charge of these threeatomic layers (two top-most and one bottom-most) is –0.67 e. Inthe annealed (see Fig. 3c) and quenched (see Fig. 3d) structures,there is a significant reduction of this sum to ��0.3 e. However,the sum of charge is not completely reduced to 0 e, which couldbe due to the fact that the electrostatic stability of a polar surfacecan not be completely achieved by surface reconstruction [21].

Mechanisms of stabilization of polar surfaces characterized by arelaxation-induced reduction of the center of gravity of the nega-tive and positive charges have been reported for other oxides. Itis the case for polar surfaces of MgO(1 1 1) [57] and Fe3O4(1 0 0)[58,59]. The present theoretical study predicts that it also holdsfor the Cu-terminated Cu2O(1 1 1) surface. However, to our knowl-edge, there is no experimental evidence for the existence of such areconstruction on a bare and defect-free Cu2O(1 1 1) surface at thesolid/gas interface. The disruption of the local hexagonal symmetryof the Cu sub-lattice characterizing the reconstructed surface al-lows us to exclude that it is this surface which is stabilized at thesolid/liquid interface for which an unreconstructed (1 � 1) Cusub-lattice is observed [2,3], and thus an alternative mechanismof stabilization must operate.

3.2. Surface hydroxylation

The stabilization of the unreconstructed polar oxide surfaces bysurface hydroxylation has been reported for (1 1 1)-oriented rocksalt structure (e.g. MgO [60,61], NiO [62], CoO [63]), for theZnO(0 0 0 1) surface [64,65] and for the stoichiometric Al-termi-nated a-Al2O3(0 0 0 1) surface [66–69]. In the present study, wehave investigated the stabilization of the Cu2O(1 1 1)-Cu surfaceby partial and complete surface hydroxylation. Partial surfacehydroxylation was obtained by the adsorption of hydroxyl groupsbonded via their oxygen atoms to the copper atoms of the first toplayer of the slab. The corresponding model may be negativelycharged on the exposed side. Calculations were performed withoutexplicit compensation of the charge. Since there are four copperatoms on this first layer, a maximum number of 4 OH groups canbe added by unit cell. The addition of 1 OH, 2 OH, 3 OH and 4OH by unit cell is termed in the following 25%, 50%, 75% and100% hydroxylation, respectively.

A complete surface hydroxylation by the classical dissociativeadsorption of water molecule, as performed on a-alumina(0 0 0 1) surface [66], is not possible on Cu-terminated Cu2O(1 1 1). Indeed, due to the inaccessibility of oxygen atoms of thesecond layer, the hydrogen atoms coming from the dissociationof water molecules cannot bind to neighbouring oxygen atoms.In the present study, a complete surface hydroxylation was ob-tained by combining the addition of OH groups as described abovewith the addition of H atoms on the unsaturated O atoms at thebottom surface of the slab. Thus there is charge compensationthrough the adsorption of hydrogen atoms onto the O-terminatedsurface of the opposite side. This is similar to the strategy used tostudy the hydroxylation of MgO(1 1 1) [61] and ZnO(0 0 0 1) [64]surfaces. Only three O atoms are unsaturated at the bottom sur-face, limiting this combined addition to three (OH + H) per unit cell(25%, 50% and 75% hydroxylation). In both partial and completehydroxylation processes, the hydroxyl groups were adsorbed onthe bridge sites between two neighboring Cu atoms. Preliminarytest calculations show that the adsorption of OH� on the top ofCu atom is thermodynamically less stable. This is in line with thehydroxylation of the polar O-terminated ZnO surface [70].

3.2.1. Hydroxylation energyIn order to evaluate the stability of the hydroxyl groups ad-

sorbed on the Cu2O(1 1 1) surface, the hydroxylation energy wasestimated according to the following equilibrium

Cu2Oð111Þsurf þ nH2Ofree $ Cu2Oð111Þsurf þ nOHads þ nHads ð1Þ

where n is for the number of water molecules. The energy ofhydroxylation (DE) associated with the Eq. (1) was calculated usingthe following equation,

DE ¼ ½EðCu2Oð111Þsurf þ nOHads þ nHadsÞ� fEðCu2Oð111ÞsurfÞ þ nEðH2OfreeÞg�=n; ð2Þ

The calculated DE values are compiled in Table 1. The simulatedsystem does not allow calculating the DE value for the 100%hydroxylated surface. Here, E(H2Ofree) = �14.74 eV, E(Cu2O(1 1 1)surf) = �157.23 eV and E(Cu2O(1 1 1)surf + nOHads + nHads) =�174.19 eV, �190.04 eV and –203.02 eV, for 25%, 50% and 75%hydroxylation, respectively. It is observed that in all other casesthe hydroxylation energy values are negative (i.e. exothermic reac-tion), which indicates that the adsorption of OHads and Hads speciesis thermodynamically favorable with respect to the non-adsorbedwater molecules. There is a gradual decrease of energy gain withthe increase of the hydroxylation coverage. This can be attributedto the increased repulsion between adsorbed hydroxyls on thesurface. However, the energy gain is not enough to overcome the

Table 1Hydroxylation energy DE (eV) for the optimized Cu-terminated Cu2O(1 1 1) surfacesafter 25%, 50% and 75% hydroxylation.

Structure (%) Hydroxylation H2O per unit cell DE

1-H2O (25%) 25 1 �2.222-H2O (50%) 50 2 �1.673-H2O (75%) 75 3 �0.53

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activation barrier for the dissociation of water (2.96 eV). This impliesthat the hydroxyl groups have to originate from the solution and/orthe dissociation of water can be promoted by the electrochemicalpotential in the experimental conditions of observation [2,4].

A similar approach could be developed for the adsorption of theOH groups alone. However, in this case, the free OH groupcorresponds to a radical and its high energy would necessarily re-sult in negative but non significant DE values. A more significantapproach to calculate the adsorption energy of the hydroxidegroups would be to simulate their solvation by water molecules.This requires the appropriate simulation of the water solvent atthe interface with the oxide surface which is beyond the scope ofthis paper.

Fig. 4. Cu2O(1 1 1) slab after 50% surface hydroxylation (side views). 2 OH per unit cerelaxation. Small and large spheres represent Cu and O atoms, respectively. The smalles

3.2.2. Structural relaxationStructural analysis of the surface structures obtained before and

after static optimization at 0 K revealed that the changes of the in-ter-atomic bond distances are smaller for the optimized hydroxyl-ated surfaces than for the adsorbate-free surface after simulatedannealing. For, 25% hydroxylation, the O–Cu1 bond length in-creases by +0.16 Å whereas the O–Cu2 bond length remains nearlyunchanged. The Cu1–Cu2 bond length decreases by �0.3 Å. For 50%hydroxylation, both O–Cu bond lengths are unchanged within±0.01 Å. The Cu–Cu bond length decreases by –0.11 Å which is alsoless than for 25% hydroxylation. For 75% hydroxylation, a largerrelaxation is observed as for 25% hydroxylation. The O–Cu1 bondlength increases by +0.05 Å and the O–Cu2 bond length remainsnearly unchanged, but the Cu1–Cu2 bond length decreases by�0.27 Å. For a 100% coverage, the Cu1–Cu2 bond length is nearlyunchanged (+0.01 Å) and the oxygen–copper distances have in-creased only by 0.06 Å. The changes of bond distances are thusminimized for 50% and 100% hydroxylation (shown in Figs. 4 and5, respectively), indicating that these surfaces are good modelsfor the unreconstructed Cu-terminated Cu2O(1 1 1) surface.

The analysis of the atomic displacements confirms that less sur-face relaxation of the first Cu and second and third O planes is

ll: (a) before and (b) after relaxation. 2 H2O per unit cell: (c) before and (d) aftert spheres are H atoms.

Fig. 5. Cu2O(1 1 1) slab after 100% surface hydroxylation (side views). 4 OH per unit cell: (a) before and (b) after relaxation. Small and large spheres represent Cu and O atoms,respectively. The smallest spheres are H atoms.

M.M. Islam et al. / Surface Science 603 (2009) 2087–2095 2093

obtained for the optimized structures for 50% and 100% hydroxyl-ation. For 25% hydroxylation, the average movements of the firstlayer Cu1 atoms along the x, y and z directions are Dx = �0.1 Å,Dy = +0.2 Å and Dz = �0.3 Å. The Cu2 atom moves along the y(Dy = +0.6 Å) and z (Dz = �0.2 Å) directions and the second layerO atom moves along the y (Dy = +0.40 Å) and z (Dz = +0.1 Å) direc-tions. The position of the third layer O atom is nearly unaffected.For 50% hydroxylation, the Cu1 atoms move only along y(Dy = +0.30 Å) and z (Dz = �0.1 Å), and the Cu2 atom moves alongz (Dz = +0.46 Å) direction. The movements of the second and thirdlayer O atoms are restricted along the z direction, Dz = +0.3 Å and+0.2 Å, respectively. For 75% hydroxylation, the copper atoms movealong all the x, y and z directions. In this case, the Cu1 atoms moveup to Dx = �0.3 Å, Dy = +0.2 Å and Dz = +0.1 Å and Cu2 atom

Fig. 6. Distribution of Bader’s charge on (a) the unrelaxed Cu-terminated Cu2O(1 1 1), (b)respectively. The light-blue and purple intermediate spheres represent the center ofreferences to colour in this figure legend, the reader is referred to the web version of th

moves up to Dx = �0.5 Å, Dy = �0.15 Å and Dz = �0.18 Å. The sec-ond layer O atom moves along x (Dx = �0.4 Å) and y (Dy = +0.2 Å)directions, while the third layer O atom remains nearly unchanged.For 100% hydroxylation, the Cu1 atoms move along x, y and z(Dx = +0.1 Å, Dy = +0.1 Å and Dz = +0.15 Å), the Cu2 atom movesonly along y (Dy = �0.16 Å) direction, and the second layer O atommoves along y and z (Dy = +0.23 Å and Dz = �0.1 Å).

The analysis of the interlayer copper–oxygen distances in theunrelaxed and optimized hydroxylated structures reveals that,for 25% hydroxylation, the average Cu–OL2 and Cu–OL3 distancesare 0.24 Å and 1.70 Å, respectively, showing a significant tendencyto merging of the first Cu plane and second O plane. For 50%hydroxylation, these distances are 0.58 Å and 1.81 Å, respectively,in close agreement to the bulk terminated values of 0.62 Å and

after 50% surface hydroxylation. Small and large spheres represent Cu and O atoms,gravity for negative and positive charges, respectively. (For interpretation of theis article.)

2094 M.M. Islam et al. / Surface Science 603 (2009) 2087–2095

1.87 Å, respectively. For 75% hydroxylation, the Cu1–OL2 and Cu2–OL2 interlayer distances markedly differ as well as the Cu1–OL3 andCu2–OL3 interlayer distances, which is indicative of a strong relax-ation. For 100% hydroxylation, these distances are again closer tothe bulk terminated values, but the average values, 0.86 Å and1.96 Å, indicate less good agreement than for 50% hydroxylation.

The charge distribution of the hydroxylated surface was inves-tigated for all the structures. For simplicity, only the charge distri-bution on the structure with 50% coverage is compared with that ofthe non-hydroxylated surface in Fig. 6. It is observed that due tohydroxylation, the centers of gravity of positive charge (light-bluesphere) and negative charge (purple sphere) are located in thesame plane along the z direction (see Fig. 6b). This allows a signif-icant reduction of polarity, and therefore, the resulting structure ismore stable than the non-hydroxylated surface. A similar mecha-nism of the polarity reduction was observed for the hydroxylatedsurface with 100% coverage. The reduction of polarity for the struc-tures with 25% and 75% coverage is less, in agreement with the lar-ger relaxation observed in both cases compared to the 50%coverage.

The present study of the Cu-terminated Cu2O(1 1 1) surfaceshows that among the four investigated hydroxylated structures,the 50% hydroxylated surface shows the lowest structural relaxa-tion with respect to the bulk terminated unrelaxed structure. Thepolarity reduction is highest for this structure. Therefore, thehydroxylated surface with 50% coverage appears as the best modelto describe the unreconstructed Cu sub-lattice observed at theCu2O(1 1 1)/liquid water interface.

4. Conclusions

The stabilization mechanisms of the unstable, polar Cu-termi-nated Cu2O(1 1 1) surface were investigated by means of periodicquantum chemical calculations based on the first principles DFTapproach.

The extensive finite temperature molecular dynamics approachat the DFT level combined with the simulated annealing techniqueallowed us to determine unambiguously the lowest energy struc-ture for the adsorbate-free and defect-free reconstructed surface,although there is no experimental account for such a structure.In this approach, a converged six tri-layer and Cu-terminatedCu2O(1 1 1) slab was heated from 0 K to 300 K, then annealed atthe constant temperature of 300 K and finally cooled down to0 K. The lowest energy structure was obtained during the constantheating at 300 K. The final structure obtained after cooling (0 K)was then compared with the lowest minimum energy structure.It is observed that, in both cases, there is a considerable reconstruc-tion of the surface. The Cu1 and Cu2 atoms in the first layer andoxygen atoms in the second and third layers move along the x, yand z directions, and the atomic planes merge together to form auniform mixed layer, thereby minimizing the polarity of thesurface. Due to the movement of Cu1 atoms of the first layer, a‘‘zig-zag” arrangement of the Cu rows appears, disrupting the localhexagonal symmetry in this layer. Due to the movement of theoxygen atom of the second layer towards the topmost copper layer,the O–Cu3 symmetric pyramidal units become planar.

The 0 K static optimization approach was applied to investigatethe alternative possibility of stabilizing the Cu-terminatedCu2O(1 1 1) surface by hydroxylation. Water dissociative adsorp-tion was found to be favorable. The energy gain decreases withincreasing OH coverage. The structural reconstruction characteriz-ing the annealed OH-free surface was diminished by OH adsorp-tion, with the first copper layer and second and third oxygenlayers showing minor relaxations. The coverage variation showedthat a surface hydroxylation of 50% obtained by adsorbing 2 OH

groups or by dissociating 2 H2O molecules per Cu2O(1 1 1) unit cellgave the surface structure that is the closest to the unrelaxed bulkterminated surface.

This study shows that the polar Cu-terminated Cu2O(1 1 1)surface can be stabilized by surface reconstruction or by hydroxyl-ation. Among the resulting surface structures, the nearly unrelaxedstructure obtained with 50% hydroxylation appears as the bestmodel of a stabilized surface consistent with the experimentalobservation of the unreconstructed Cu sub-lattice at the surfaceof Cu2O(1 1 1) thin films in contact with aqueous solutions.

Acknowledgement

A post-doctoral Grant of the French Ministry of Research for thesupport of M.M. Islam is acknowledged.

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