Adsorption of Chlorine on Cu(111): A Density-Functional Theory Study

J. Phys. Chem. C 2009, 113, 14363–14376

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Adsorption of Chlorine on Cu(111): A Density-Functional Theory Study Sebastijan Peljhan and Anton Kokalj* Department of Physical and Organic Chemistry, Jozˇef Stefan Institute, JamoVa 39, SI-1000 Ljubljana, SloVenia ReceiVed: March 13, 2009; ReVised Manuscript ReceiVed: April 30, 2009

The dissociative adsorption of chlorine on a perfect Cu(111) surface has been studied and characterized by means of extensive density functional theory calculations. A few properties of the bulk chlorides CuCl and CuCl2 are also reported, because they may be viewed as a limiting case for Cl adsorption. Calculations predict that the chemisorption energy of Cl at low coverage is about -1.9 eV and remains so up to the coverage of 1/3 ML due to a good screening of metal electrons. Upon further increase of coverage, its magnitude decreases. It is further found that the substitutional adsorption mode is unstable, except at very large coverage (3/4 ML), where the mixed on-surface + substitutional structure is the most stable. The diagram of the adsorption free energy as a function of chlorine chemical potential reveals that the on-surface (3 × 3)R30° adsorption phase is thermodynamically the most stable over a very broad range of Cl chemical potentials. The analysis of electronic structure points out that although the Cl adatoms are negatively charged, which results in an increase of the work function, the Cl-Cu interaction is not purely ionic but is to some extend also covalent, as witnessed by the formation of bonding and antibonding states. Results reveal that several Cl adsorption properties are almost unchanged up to the coverage of 1/3 ML, and at larger coverage, several new characteristics appear, such as occupation of nonoptimal surface sites, reduction of adatom net charge, and more covalent nature of the adsorbate-substrate interaction. 1. Introduction Due to its good mechanical properties and excellent electrical and thermal conductivity, copper is one of the most useful materials and plays a prominent role in many fields. Some among them are heating and cooling piping, heat exchanging, electrical appliances, architecture, and catalysis. Hence, copper ware is frequently exposed to various harsh environments, chloride media representing one of the most frequent. Adsorption of chlorine on copper surfaces therefore represents a key issue for understanding the basic surface processes connected with corrosion, electrodeposition, and catalysis. Because of the importance of the applications mentioned above, the chlorine-copper interaction has attracted great experimental1-12 interest over the past years, whereas theoretical studies are less frequent.13-16 Structural parameters were obtained using low-energy electron diffraction (LEED),1,2 normal-incident X-ray standing wave (NIXSW),3,4 surfaceextended X-ray adsorption fine structure (SEXAFS),5-7 shadowcone-enhanced secondary-ion mass spectrometry (SIMS),8 and scanning-tunnelling microscopy (STM)9-12 methods, showing that there is a strong interaction between Cl and Cu atoms. Chlorine adsorbs dissociatively on the Cu(111) surface, leading to formation of well-ordered phases, with the (3 × 3)R30° adlattice at the coverage of 1/3 ML as the most stable structure at room temperature.1,9,11 Coincidental superstructure lattices result from progressive compression of hexagonal layers of Cl atoms.1 In order of increasing coverage, there is a continuous transition between them as follows: (3 × 3)R30°, (123 × 123)R30°, (47 × 47)R19.2°, and (63 × 63)R30°.1,11 Cl atoms prefers to adsorb onto face-centered cubic (fcc) hollow positions over the hexagonal close-packed (hcp) hollow, bridge, and top positions.2 At higher coverages, adsorption of halogens * Corresponding author. Tel: +386-1-477-35-23. Fax: +386-1-477-3822. E-mail: [email protected]. URL: http://www-k3.ijs.si/kokalj/.

on metal surfaces yield halides in the form of islands.17,18 Some theoretical investigations have been performed using various approaches based either on the cluster13 or the periodic slab model.14,16 To the best of our knowledge, only the (3 × 3)R30° (refs 14, 16) and (3 × 3) (ref 16) structures of the Cl/Cu(111) system have been considered in the existing density functional theory (DFT) periodic slab model studies. For this reason, we present in this paper a systematic study of Cl adsorption on Cu(111) utilizing extensive DFT calculations. On-surface and substitutional adsorption over a broad range of Cl coverages, ranging from 1/16 to 1 ML, is considered. The aim of this study is to understand why the (3 × 3)R30° structure is thermodynamically most stable and to ascertain the main factors that govern the adsorbate-substrate interaction. A number of energetic, structural, and electronic properties, such as chemisorption energies, Cl-Cu bond distances and their interlayer spacings, adatom net charges, adsorption-induced work function, and Cl-Cu bonding characteristics, are calculated. This paper is organized as follows. In section 2 we present the computational framework and define several quantities that are used throughout the paper. Section 3 contains the results: in subsection 3.1 we deal with the basic constituents [Cu bulk, Cu(111), gas-phase Cl2, and bulk CuCl and CuCl2 chlorides] and in subsection 3.3 we deal with energetic and structural properties of Cl chemisorption on Cu(111). In section 4 we discuss the results in terms of chemisorption energies (subsection 4.1), electronic structure (subsection 4.3) and adsorption free energy (subsection 4.4). Section 5 finally contains our conclusions. 2. Computational Details and Definitions Calculations were performed in the framework of DFT using the generalized gradient approximation (GGA) of Perdew-Burke -Ernzerhof (PBE).19 We used the pseudopotential method with

10.1021/jp902273k CCC: $40.75  2009 American Chemical Society Published on Web 06/19/2009

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ultrasoft pseudopotentials.20,21 Kohn-Sham orbitals were expanded in a plane-wave basis set up to a kinetic energy cutoff of 30 Ry (240 Ry for the charge-density cutoff). Brillouin-zone (BZ) integrations have been performed with the Gaussiansmearing22 special-point technique,23 using a smearing parameter of 0.03 Ry. All calculations have been done using the PWscf code from the Quantum ESPRESSO distribution,24 while molecular graphics were produced by the XCRYSDEN25 graphical package. The Cu(111) was modeled by periodic slabs. Properties of clean Cu(111) were evaluated with slabs consisting of 7, 9, 11, and 13 layers and a (1 × 1) unit cell, whereas adsorption calculations were performed with four-layer slabs. Cl atoms were adsorbed on one side of the slab, and a dipole correction26 has been applied to cancel out the fictitious dipole created along the surface normal direction. The thickness of the vacuum regionsthe distance between adjacent slabsswas set to about 20 Å. BZ integrations were performed using a (8 × 8 × 1), (6 × 6 × 1), (4 × 4 × 1), (3 × 3 × 1), and (2 × 2 × 1) uniformly shifted k-mesh27 for the (1 × 1), (3 × 3)R30°, (2 × 2), (3 × 3), and (4 × 4) surface supercells, respectively. 2.1. Energy Equations. The estimation of surface energy, σ, is based on the relation

Eslab(N) ) 2σ + NEbulk

(1)

where Eslab(N) and Ebulk are total energies of the N-layer slab and bulk atom, respectively. The actual values of σ are then obtained by evaluating this equation in three different ways: (i) used directly, (ii) by fitting the equation for large enough values of N and extrapolating to N ) 0, and (iii) by approximating Ebulk with

1 Ebulk ) [Eslab(N + 2) - Eslab(N)] 2

(2)

The work function, Φ, is calculated as the difference between the average electrostatic potential in the vacuum and the Fermi level:

Φ ) Evacuum - EF

(3)

The average binding energy of Cl onto Cu(111) surface is calculated as

1 Eb ) [ECl/slab - (Eslab + nECl)] n

(4)

where n is the number of adsorbed Cl atoms per supercell, and ECl and ECl/slab are the total energy of isolated Cl atom (nonspherical spin-polarized solution; see note 28) and Cl/Cu(111) adsorption system, respectively. The average chemisorption energy for dissociative adsorption of chlorine is calculated as

Echem )

1 n E - Eslab + ECl2 n Cl/slab 2

[

(

)]

tot εchem )

1 n E - Eslab + ECl2 A Cl/slab 2 n ) Echem A

[

(

(6)

where A is the surface area spanned by the supercell. The average substitutional chemisorption energy, per Cl atom, of n Cl atoms adsorbed into n vacancy on Cu(111) is given by subst Echem )

1 n E + nEbulk - Eslab - ECl2 n Cl/vac 2

[

]

(7)

where ECl/vac is the total energy of the substitutional adsorption system (note that Eslab is the total energy of the slab without subst takes into account the the vacancy). This definition of Echem vacancy formation energy, which is given by

Evf )

1 [E + mEbulk - Eslab] m vac

(8)

where Evac is the total energy of the Cu slab with m vacancies. In the case of mixed on-surface + substitutional adsorption, the net average chemisorption energy is calculated as subst Echem )

1 n ECl/vac + mEbulk - Eslab - ECl2 n 2

[

]

(9)

where n is the total number of Cl atoms, m the number of vacancies and substitutionally adsorbed Cl atoms, and n g m. The average gross chemisorption energy of n Cl atoms into n preformed vacancies is defined as vac Echem )

1 n E - Evac - ECl2 n Cl/vac 2

[

]

(10)

To make a link between the Echem and the effect of temperature and chlorine partial pressure on various adsorption structures containing different amount of Cl, the adsorption free energy per unit area, γads, as a function of chlorine chemical potential, µCl, is considered. The γads can be written as

γads )

∆G - [mµCu + nµCl] A

(11)

where the Gibbs free energy difference, ∆G ) GCl/slab - Gslab, is approximated by the difference between the energies of the adsorption system and clean slab, ECl/slab - Eslab. The n is the number of adsorbed Cl atoms and m is a difference between the number of Cu atoms in the chlorinated and clean surface (i.e., as in eq 9) and is different from zero only for substitutional adsorption. The µCu is the chemical potential of copper, and its reference state is chosen to be the total energy of Cu atom in the bulk, µCu ) Ebulk. The zero reference state of µCl is set to the total energy of Cl in isolated chlorine molecule, µCl ) 1/2ECl2 ) 0. With this choice, the Echem and the γads are related by

(5)

where ECl2 is the total energy of isolated Cl2 molecule. The total chemisorption energy per unit area is calculated as

)]

γads )

n(Echem - µCl) A

For more details about such treatment, see refs 29, 30.

(12)

Adsorption of Chlorine on Cu(111)

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2.2. Definitions Related to the Analysis of the AdsorbateSubstrate Bonding. The charge transfer between the adsorbate and the substratesas well as the formation of a chemical bonds between themscan be characterized in terms of the charge density difference, ∆F(r), defined as

where ψV,k(r) is the Vth occupied Bloch orbital at wave vector k, and V,k is the corresponding energy eigenvalue. φ˜ (r) is defined as

∑ Sµν-1/2φν(r)

φ˜ µ(r) )

(19)

ν

n

∆F(r) ) FCl/slab(r) - Fslab(r) -

∑ FCl (r) i)1

i

(13)

where the subscripts Cl/slab, slab, and Cli stand for adsorption system, bare surface, and the ith adsorbed Cl atom (note that the geometry of the latter two is kept the same as in Cl/slab). We have utilized a planar averaged charge density difference, ∆Fj(z), defined as

∫A ∆F(x, y, z) dx dy

1 n

∆F¯ (z) )

1 n

∫-az z∆F¯ (z) dz

1 n

∫-aC-a z∆F¯ (z) dz

where C is size of the supercell along the z-direction (i.e., direction perpendicular to the surface), and -a and C - a lie in the center of the vacuum region. Note that the symbol µ is used to designate the chemical potential (eq 11) and the dipole moment (eq 16). We believe it will be clear from the context when each of the two is used. The adsorption-induced dipole moment µ and the change of work function ∆Φ are related by the Helmholtz equation, which in atomic units reads

∆Φ ) 4πθµ

∑ ∑ |〈ψV,k|φ˜ 〉|2δ( - V,k) V

k

∑ ∑ |〈ψV,k|φ˜ µ〉|2 V

(21)

k

This is the so-called Lo¨wdin population analysis. The total electronic charge of a given atom, QA, is calculated by summing Qµ values of all its atomic orbitals, QA ) ∑µ∈AQµ. The net charge of an atom is defined as the difference between the total electronic and nuclear charge

qA ) QA - ZA

(22)

where ZA is the atomic number of atom A or its nuclear charge (note that in the current case, due to the use of pseudopotentials, ZA actually corresponds to the valence charge). By this definition qA is in units of electron charge, i.e., positive values represent negatively charged atoms. In the projection of plane-wave eigenstates onto atomic orbital basis set, some fraction of electrons is lost. Hence the sum of all atomic orbital charges, Qtot ) ∑µQµ, does not equal the number of electrons Nel in the system. If the fraction of lost electrons is designated by a spilling parameter s,31 then

s)1-

Qtot Nel

(23)

A typical value of spilling parameter in current Cl/Cu(111) calculations is about 1%. To account for this electron loss, the Lo¨wdin atomic charges can be renormalized as

(17)

where ∆Φ is in hartrees, µ is a dipole moment per Cl adatom in units of e bohr, and θ is an absolute coverage in units of bohr-2 (i.e., θ ) Θ/A0, where Θ is a fractional coverage and A0 is the area of the (1 × 1) surface unit cell). A positive value of µ stands for an inward-pointing dipole, that is, from negatiVe adsorbate to the positiVe surface. The adsorbate-substrate bonding can be further analyzed in terms of density of states (DOS) and some of its variants. The DOS projected onto individual orthonormalized atomic orbitals (PDOS), say φ˜ (r), is defined as

nφ() )

occ

(15)

(16)

(20)

The charge associated with the orthonormalized atomic orbital µ is then

Qµ )

The first moment function µ(z) is related to adsorption-induced dipole moment µ per adsorbed Cl adatom, because for the slab models utilized in the current studyswith adatoms adsorbed only on one side of the slabsthe latter can be calculated as

µ)

Sµν ) 〈φµ |φν〉

(14)

where z is the direction perpendicular to the surface, n is the number of adsorbed chlorine atoms per supercell, ∆F(x,y,z) ≡ ∆F(r) and is defined by eq 13, and the integration is performed over the area A spanned by the surface supercell. We also define its first moment function, µ(z), as

µ(z) )

where Sµν-1/2 is the element of the square root inverse of the overlap matrix S of the atomic orbitals φi(r), whose elements are

(18)

Ã ) Q

QA 1-s

(24)

The renormalized net charge of an atom is then calculated accordingly as

˜ A - ZA qÃ ) Q

(25)

In the present study, the local DOS (LDOS) is also utilized. It is defined as

n(, r) )

∑ ∑ |ψV,k(r)|2δ( - V,k) V

k

(26)

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The bonding properties belonging to some given energy range, [min, max], can be analyzed using the integrated local density of states (ILDOS), defined as

∫ n(,r) d ≡ ∑ ∑ ∫ |ψV,k(r)|2 × δ(-V,k) d

N(min, max, r) )

max

min

max

V

k

min

(27)

In the current casesdue to the use of ultrasoft pseudopotentialsseqs 18, 21, 26, and 27 contain also an augmentation term, and the corresponding expressions are written in ref 32. 2.3. Structural Parameters. To facilitate the characterization of Cl/Cu structures, the following structural parameters are defined: d12 is the interlayer distance between the surface and subsurface Cu layers. It is calculated as the difference between the average height of surface Cu atoms and the average height of subsurface Cu atoms. ∆d12 is the relaxation of the surface layer, i.e., the difference between the d12 and the ideal bulk interlayer spacing. ∆d23 is the relaxation of the subsurface layer, calculated analogously to ∆d12. surf is the approximate measure of the adsorption-induced hCu corrugation of the surface. It is defined as the difference in height between the highest and lowest Cu atom in the surface layer. dCl-Cu is the average distance between on-surface Cl adatom and nearest-neighbor Cu atoms. ∆zjCl-Cu is the distance between the height of the Cl adatom and the average height of surface Cu atoms. ∆zCl-Cu is the distance between the height of the Cl adatom and the height of the nearest neighbor Cu atoms. Other more obvious parameters will be explained when used. 3. Results 3.1. Basic Constituents. A few properties of individual basic constituentssCu bulk, Cu(111) surface, isolated Cl2 molecule, and bulk copper chloridessare described first. 3.1.1. Cu Bulk and Cu(111). The experimental lattice parameter of copper bulk is 3.615 Å,35 whereas our GGA calculations yield 3.67 Å, an overestimation of 1%. Our result is in excellent agreement with the value from previous studies performed by the pseudopotential plane-wave (PPPW) method, 3.67 Å,33 while the full-potential linearized augmented-planewave (FP-LAPW) method gives a value of 3.63 Å.34 The bulk modulus, B0, was calculated by fitting the total energy vs volume data points with the Murnaghan equation of state, and a value of 127 GPa is obtained (the experimental value is 138 GPa).35 The convergence of a few properties of the clean Cu(111) surface with respect to the slab thickness is presented in Table 1, whereas in Table 2 we compare our results for Cu bulk and Cu(111) surface to those available in the literature. 3.1.2. Cl2 Molecule. As for the isolated Cl2 molecule, we obtain a bond length of 2.00 Å and a bond strength of 2.85 eV, in fair agreement with previous GGA results,39 where bond length is 2.05 Å and the bond strength is 2.69 eV. Experimentally measured bond length and strength is 1.99 Å and 2.48 eV, respectively.35 3.1.3. Bulk Chlorides: CuCl and CuCl2. A few basic properties of bulk copper chlorides are presented, because they may represent the limiting case of Cl adsorption on Cu surfaces in the presence of harsh chloride media. Bulk copper chlorides

TABLE 1: Convergence of Properties of Clean Cu(111) with Respect to Slab Thicknessa no. of layers

Φ (eV)

σb (meV/Å2)

σc (meV/Å2)

∆d12 (%)

∆d23 (%)

7 9 11 13

4.69 4.69 4.68 4.67

80 80 79

78 77 77 76

-1.1 -1.0 -1.0 -1.0

-1.0 -0.2 -0.6 -0.5

a

Φ, work function; σ, surface energy; ∆d12, relaxation of surface layer, and; ∆d23, relaxation of subsurface layer. b Calculated with Eslab(N) ) 2σ + N/2[Eslab(N + 2) - Eslab(N)]. c Calculated with Eslab(N) ) 2σ + NEbulk.

TABLE 2: Comparison of Our Calculated Properties of Cu Bulk and Cu(111) Surface to Those Available in the Literaturea GGA this work

PPPWb

FP-LAPWc

expt

a0 (Å) B0 (GPa)

3.67 127

Cu bulk 3.67d 134d

3.63e 142e

3.62f 138f

Φ (eV) σ (meV/Å2) σ (J/m2) ∆d12 (%) ∆d23 (%)

4.68 81i 1.30i -1.0 -0.5

Cu(111) 4.98g

4.78e

4.98h

1.11g -1.9g -0.4g

1.41e -1.2e -0.7e

1.79j -0.7k

property

a a0, lattice parameter; B0, bulk modulus. Other labels have the same meaning as in Table 1. b PPPW, plane-wave pseudopotential method. c FP-LAPW, full-potential linearized-agumented plane wave. d Reference 33. e Reference 34. f Reference 35. g Reference 16. h Reference 36. i Calculated by fitting the eq 1. j Reference 37. k Reference 38.

Figure 1. Crystal structure of copper(I) chloride, CuCl, and copper(II) chloride, CuCl2. Cl atoms are bright green and Cu atoms are colored copper-like. The choice (direction) of lattice vectors is indicated for CuCl2.

exist in two forms: as copper(I) chloride, CuCl, and as copper(II) chloride, CuCl2. Crystal structure of CuCl is sphalerite under ambient conditions and may be described by two interpenetrating fcc lattices, e.g,. with Cu atom located at the origin and Cl at (1/4, 1/4, 1/4). Hence each Cl (Cu) atom has four Cu (Cl) nearest neighbors in a tetrahedral environment; see Figure 1a. At ambient conditions the CuCl2 forms a base-centered monoclinic Bravais lattice.40 The Cu atom is in an axially distorted octahedral environment with four equatorial nearestneighbors Cl atomsslocated in the plane spanned by the b and c lattice vectorssand two axial Cl atoms; see Figure 1b. The Cu-Cl distance for the latter is substantially elongated. The calculated properties of bulk CuCl and CuCl2 are compared to experimental data in Table 3. As for the CuCl2, the calculations were performed spin-polarized,45 because the magnetic solution is more stable than the nonmagnetic one by 0.19 eV per CuCl2 formula unit. The formation energies, Ef, of



TABLE 3: A Few Calculated Properties of Bulk CuCl and CuCl2 Compared to Available Experimental and Computational Dataa β B0 a (Å) b (Å) c (Å) (deg) (GPa)

Efj (eV)

Ecohk (eV)

CuCl this work FP-LAPWb PPPWc expt

5.47 5.46 5.44 5.42d

this work expt

7.52 6.90i

3.35 3.30i

CuCl2 7.29 119 6.82i 122i

47 48

-0.98

38e

-1.42f

-5.72 -7.46 -6.38 -6.18g -6.32h

-1.82 -7.99 -2.28f -8.29g

a a, b, c, β, lattice parameters; B0, bulk modulus; Ef, formation energy; and Ecoh, cohesive energy (calculated with respect to spin-polarized solutions28 of isolated Cu and Cl atoms). b Reference 41. c Reference 42. d Reference 35. e Reference 43. f Reference 35. The value is the standard enthalphy of formation. g Reference 35. The value is calculated from standard enthalphies of formation of Cl(g), Cu(g), and bulk CuCln, n ) 1, 2. h Reference 44. i Reference 40. j Ef ) ECuCln(bulk) - ECu(bulk) - n/2ECl2(g). k Ecoh ) ECuCln(bulk) ECu(g) - nECl(g).

Figure 2. Reaction energy profile for dissociation of Cl2 molecule on Cu(111).

CuCl and CuCl2 as obtained in this work are -0.98 and -1.82 eV, respectively. This quantity is calculated with respect to a Cu atom in the bulk Cu and Cl2 molecule in the gas phase. For CuCl, it represents the energy gained per Cl atom to dissociate Cl2 and to form CuCl from Cu bulk, whereas for CuCl2 the equivalent energy gain per Cl atom equals half the formation energy, that is, -1.82/2 ) -0.91 eV/Cl atom. Calculated formation energies strikingly underestimate experimental standard heats of formation, which are -1.42 and -2.28 eV for CuCl and CuCl2, respectively.35 The calculations also substantially overshoot the a and c lattice parameters of CuCl2. Note that the structure of the bulk CuCl2 may be seen as to consist of CuCl2 polymers extending along the b lattice vector direction. Namely, the interatomic distances within the polymers are much smaller than those between the polymers. This indicates that dispersion interactions may contribute significantly to the attraction between the polymers. We therefore attribute the substantially overshot a and c lattice parameters to the inability of the current density functional to properly describe the van der Waals interaction. 3.2. Dissociation of Cl2. The adsorption of molecular chlorine, Cl2, is dissociative on Cu(111). Calculations predict no energy barrier for the dissociation of Cl2 during the adsorption. This has been established by climbing image nudged elastic band calculations,46,47 and the corresponding result is displayed in Figure 2. For this reason, merely atomic Cl is considered from now on. 3.3. Chemisorption of Chlorine on Cu(111). In this subsection the results about different Cl adsorption geometries on

Figure 3. Relative stability of on-surface high-symmetry sites for Cl adsorption on Cu(111) at low coverage (Θ ) 1/4 ML).

Cu(111) surface, as a function of Cl coverage and adsorption mode (on-surface and substitutional), are presented. We build on the work of a related Cl/Ag(111) study48 coauthored by one of us. Only a subset among the analogue structures presented in ref 48 in particular, those that were found the most stable, are considered here. 3.3.1. RelatiWe Stability of High-Symmetry Sites. As for the on-surface Cl adsorption, the relative stability of high-symmetry sites at low coverage follows the order fcc ≈ hcp > bridge > top (see Figure 3). A tiny preference for fcc over the hcp site is in accord with experimental observations.3,4 At 1/4 ML, the calculated energy differences with respect to the fcc site, ∆E ) E - Efcc, are 0.01, 0.08, and 0.42 eV for hcp, bridge, and top sites, respectively, in good agreement with the previous GGA results of Doll et al.14 These values are also almost identical to those calculated for Cl adsorption on Ag(111).48 The bridge and top sites are not local minima at low coverage, and the corresponding energies have been obtained from symmetry constrained structural optimizations. The difference between the bridge and hollow sites is below 0.1 eV, indicating a very small diffusion barrier (see note 49) and an eventual occupation of nonhollow sites at larger coverage due to repulsive electrostatic interaction between neighboring adatoms, which prevails the energy differences between the various sites (see below). 3.3.2. On-Surface Adsorption. In Figure 4, optimized onsurface Cl configurations at coverages ranging from 1/16 to 1 ML are displayed and the corresponding results are reported in Table 4. These structures contain Cl adsorbed exclusively into fcc sites, except those at 1/2 and 3/4 ML are combinations of fcc + hcp and fcc + hcp + top sites, respectively. Note that the relaxed Cl configurations display a honeycomb pattern with either void or centered hexagons. They do so to maximally avoid each other due to a repulsive lateral electrostatic interaction between negatively charged adatoms. This implies that at high enough coverage, Θ J 1/2 ML, not all adatoms are adsorbed into an optimal fcc site, which is indeed the case: at 1/2 ML coverage the Cl adatoms are equally distributed among fcc and hcp sites (Figure 4e), whereas at 3/4 ML they are equally distributed among fcc, hcp, and top sites (Figure 4g). An estimate of the repulsive lateral Cl-Cl interaction can be inferred from the comparison between two structures at 1/2 ML, i.e., the most stable fcc + hcp structure and the constrained configuration with Cl adatoms located exclusively in the fcc sites. The latter structure is 0.33 eV/supercell less stable because its Cl-Cl lateral distance is 0.4 Å shorter. As for the comparison with the data available in the literature, the majority of data correspond to the (3 × 3)R30° structure. Our result for the Cl binding energy, Eb, at 1/3 ML (-3.28 eV) is in good agreement to the value reported by Migani et al.,16 -3.24 eV, calculated by the plane-wave PAW method, whereas Doll et al.14 reported a more exothermic value of -3.70 eV, calculated by utilizing a localized Gaussian-type orbital basis set. The experimentally estimated Cl binding energy at low coverage is -2.80 eV and was derived from the temperatureprogrammed desorption data.2

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Figure 4. On-surface Cl adsorption structures in the coverage range from 1/16 to 1 ML with corresponding average chemisorption energies, Echem. Corresponding supercells used in the calculations are also stated. Unless explicitly labeled, the Cl atoms are adsorbed into fcc sites. Cl atoms are bright green, Cu are colored copper-like.

TABLE 4: Calculated Structural Parameters and Chemisorption Energies, Echem, in eV/Cl Atom for On-Surface Cl Configurations as a Function of Coverage (Θ)a

c

Θ (ML)

structure

1/16 1/9 1/4 1/3 1/2 2/3 3/4 1

fcc fcc fcc fcc fcc/hcp fcc/fcc fcc/hcp/top fcc

Echem (eV) Figure 4a Figure 4b Figure 4c Figure 4d Figure 4e Figure 4f Figure 4g Figure 4h

-1.85 -1.89 -1.80 -1.86 -1.18 -0.28 -0.68 +0.50

Eb (eV)

∆zjCl-Cu (Å)

∆zCl-Cu (Å)

dCl-Cu (Å)

surf hCu (Å)

∆d12 (%)

-3.28 -3.31 -3.23 -3.28 -2.60 -1.70 -2.10 -0.92

1.88 1.89 1.90 1.86 1.85/1.89b 1.92 1.67/1.73/2.48c 2.30

1.85 1.84 1.87 1.86 1.82/1.86b 1.92 1.74/1.79/2.28c 2.30

2.41 2.41 2.41 2.40 2.36/2.39b 2.44 2.31/2.33/2.28c 2.74

0.08 0.09 0.11 0.00 0.11 0.00 0.27 0.00

-1.4 -1.7 -1.8 -1.5 -1.5 -1.2 -0.3 -1.6

a Other labels are defined in section 2. For pure Cu(111), ∆d12 ) -1.0%. Corresponds to Cl at fcc, hcp, and top sites, respectively.

The calculated structural parameterssthe Cl-Cu nearest neighbor distance (dCl-Cu ) 2.40 Å) and the Cl-Cu interlayer spacing (∆zjCl-Cu ) 1.86 Å)sfor the (3 × 3)R30° adsorption phase are in excellent agreement with experimentally determined values: Crapper et al.5 reported a distance of 2.39 ( 0.02 Å, while Way et al.8 reported an almost identical value of 2.38 ( 0.04 Å. For the corresponding interlayer spacing, the values of 1.81 ( 0.05 Å3,6,7 and 1.87 ( 0.04 Å8 have been reported. Our dCl-Cu is also in good agreement with the previously reported GGA value of 2.39 Å calculated by Migani et al.16 The same authors reported a value of 1.92 Å for the Cl-Cu interlayer spacing, whereas Doll et al.14 reported a value of 1.89 Å. Way et al.8 reported that at low coverages, i.e. Θ ≈ 0.1 ML or lower, the Cl-Cu bond length is elongated by 0.1 Å and attributed this to the increased ionicity of the Cl-Cu bond at lower coverage. This is in disagreement with our calculations, according to which the Cl-Cu bond length is pretty constant for coverages up to 1/3 ML, with dCl-Cu ) 2.41 ( 0.01 Å; an identical difference of 0.01 Å in dCl-Cu on going from 1/9 to 1/3 ML has been also calculated by Migani et al.16 It can be seen, however, from Table 4 that the Cl-Cu interlayer spacing, ∆zjCl-Cu, is increased by about 0.03 Å at low coverages compared to Θ ) 1/3 ML. This is due to adsorption-induced buckling of the neighboring Cu atoms: the vertical buckling is about 0.1 Å, surf values. Moreover, as will be presented as witnessed by the hCu

b

Corresponds to Cl at fcc and hcp sites, respectively.

in section 4.3, the analysis of electronic structure does not discern any significant change in the ionicity of Cl-Cu bond in the coverage range from 1/16 to 1/3 ML. It is found, however, that the ionicity of the Cl-Cu bond is reduced at larger coverages, Θ > 1/3 ML, and also the dCl-Cu distance is shortened (except for the metastable 2/3 and 1 ML structures). 3.3.3. Substitutional Adsorption. A sample initial substitutional configuration is shown in Figure 5a, whereas Figure 5b displays the resulting structures for the substitutional adsorption mode. Corresponding results are summarized in Table 5, whereas vacancy formation energies are reported in Table 6. Remarkably, during structural relaxation the substitutional Cl atoms are substantially up-shifted (see side views), and also displaced laterally to an off-bridge, hollow, and bridge site for 1/9, 1/4 and 1/3 ML structures, respectively. These structures will be named fake-substitutional, to remind us that they were obtained from Cl atoms located initially in the substitutional sites. According to the calculations, substitutional sites are therefore unstable. The calculations further show that the potential energy surface around the vacancy is extremely flat, i.e., there is a very flat and wide plateau above the vacancy. For comparison purposes, we performed a calculation at 1/3 ML with Cl atom constrained laterally at the vacancy. In the so-optimized structure, the Cl atom ends up 1.15 Å directly above the vacancy with the Echem of -1.01 eV. Hence, this



Figure 5. Substitutional Cl/Cu(111) configurations. (a) Sample initial configuration with Cl located in the vacancy at the surface plane and (b) optimized configurations at 1/9, 1/4, and 1/3 ML. Note that after relaxation Cl atoms are not adsorbed into the vacancy but are substantially shifted upward and also displaced laterally to either (off-)bridge or hollow sites. (c) Mixed on-surface + substitutional optimized configuration at 3/4 ML with Cl adsorbed into fcc, hcp, and vacancy sites. Cl adsorbed in the latter is darkened.

TABLE 5: Calculated Structural Parameters and Chemisorption Energies, Echem, in eV/Cl Atom for Fake-Substitutional and Mixed On-Surface + Substitutional Cl Configurationsa Θ (ML)

structure

1/9 1/4 1/3 3/4

sub sub sub sub/fcc/hcp

a

Figure 5b Figure 5b Figure 5b Figure 5c

Echem (eV)

vac Echem (eV)

∆zjCl-Cu (Å)

∆zCl-Cu (Å)

dCl-Cu (Å)

surf hCu (Å)

-1.32 -1.47 -1.45 -0.82

-2.05 -2.24 -2.23 -1.08

1.86 1.81 1.82 0.03/1.84/1.82b

1.79 1.81 1.82 0.03/1.84/1.82b

2.29 2.36 2.24 2.60/2.37/2.36b

0.10 0.01 0.00 0.00

∆d12 (%) -2.8 -4.2 -4.6 +2.1

Labels are defined in section 2. For pure Cu(111), ∆d12 ) -1.0%. b Corresponds to Cl at substitutional, fcc, and hcp sites, respectively.

TABLE 6: Vacancy Formation Energy, Evf, As a Function of Vacancy Coverage, Θvaca Θvac (ML)

Evf (eV/vacancy)

surf hCu (Å)

∆d12 (%)

1/9 1/4 1/3

0.73 0.77 0.78

0.07 0.00 0.00

- 2.8 - 4.9 - 5.7

a Other labels are defined in section 2. For pure Cu(111), ∆d12 ) -1.0%.

structure is 0.44 eV less stable than the fully relaxed structure shown in Figure 5b. The only structure with the true substitutional Cl is the mixed on-surface + substitutional configuration at 3/4 ML shown in Figure 5c, where Cl atoms are equally distributed among fcc, hcp, and substitutional sites (Cl atoms adsorbed in the latter are darkened in the figure). 4. Discussion 4.1. Trend of Cl Chemisorption on Cu(111). In Figure 6 the average chemisorption energy as a function of coverage is displayed. The following trend can be observed: 1 For Θ e 1/3 ML, the Echem of on-surface adsorbed Cl (red circles) is independent of the coverage (actually, there is some small scatter, which is on the order of the estimated computational accuracy). 2 For Θ > 1/3 ML, the magnitude of Echem decreases with the coverage. 3 At high coverage, Θ ) 3/4 ML, the mixed on-surface + substitutional adsorption (back square) is preferred over the pure on-surface adsorption. 4 Fake-substitutional adsorption (blue stars) is less stable compared to on-surface adsorption. This can be attributed to the energy cost required for the vacancy formation (vacancy formation energies are reported in Table 6). 5 The figure suggests that the highest possible coverage of on-surface Cl is about 3/4 ML. Note that the structure at

Figure 6. Average chemisorption energy, Echem, of Cl adsorbed on Cu(111) as a function of coverage. Lines are drawn to guide the eye.

1 ML coverage (Figure 4h) is actually not stable and was obtained due to the symmetry constraint imposed by the (1 × 1) unit cell (also the corresponding Echem is endothermic). By breaking the symmetry, e.g., using a larger (2 × 2) supercell and making the Cl atoms symmetryinequivalent, 50% of Cl will desorb and recombine into Cl2 molecule. The same is true also for 2/3 ML structure (Figure 4f). For this reason, these two structures are designated as metastable. To further elaborate on the issue of highest possible coverage of on-surface Cl, a total chemisorption energy normalized to tot defined by eq 6sas a function of the surface unit areasεchem tot data are coverage is plotted in Figure 7. In this plot, the εchem fitted to a third-order polynomial (red curve). Let us first focus tot increases on the calculated points: the exothermicity of εchem up to 1/3 ML coverage and then decreases. This plot suggests that the coverage of 1/3 ML is thermodynamically the most stable among the calculated configurations (this issue will be discussed in terms of adsorption free energy in section 4.4). At

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Peljhan and Kokalj TABLE 7: Electronic Properties of Cl/Cu(111) System as a Function of Coveragea

tot Figure 7. Total chemisorption energy per surface unit area, εchem (in eV/Å2), of on-surface Cl as a function of coverage. Open pentagons designate two nonoptimal overlayer structures at 1/2 and 2/3 ML, whereas the metastable 1 ML structure is designated by an open circle. The red curve represents the third-order polynomial fit (the two “pentagon” points were excluded). The vertical blue-dotted line at Θ ) 0.45 ML corresponds to the Cl coverage of (63 × 63)R30° superstructure.

tot 1/2 and 3/4 ML, the εchem is slightly less exothermic than at 1/3 ML, whereas for the metastable 2/3 and 1 ML structures the tot εchem values are substantially larger (more endothermic), thus witnessing the instability of the corresponding structures. The highest experimentally observed coverage of on-surface Cl is Θ ) 0.45 ML,1 which corresponds to (63 × 63)R30° superstructure. This coverage is marked by a vertical blue-dotted line in Figure 7. Interestingly, this coverage is relatively close tot fitted curve, which is at Θ ≈ 1/2 to the minimum of the εchem ML. As evident from Figure 7, the εtot chem of 1/2 ML and in particular the 2/3 ML structure (void pentagons) is more endothermic than predicted by the third-order polynomial fit (red curve). The reason for the low stability of the two structures can be attributed to the lateral repulsion between the Cl adatoms in the overlayer configuration: all considered structures but the 1/2 ML@(2 × 2) (Figure 4e) and 2/3 ML@(3 × 3)R30° (Figure 4f) display the optimal centered-honeycomb structure with the maximal lateral nearest-neighbor (nn) Cl-Cl distance, whereas the two mentioned structures display a nonoptimal void-honeycomb structure. This is visualized in Figure 8, which shows the comparison between the actual adatom nn distance (points) and the maximal lateral adatom nn distance (curve) as a function of coverage. For an fcc(111) surface the maximal lateral adatom nn distance is

dmax nn (Θ)

)

dCu

√Θ

(28)

where dCu is the bulk Cu-Cu nearest neighbor distance and Θ the adsorbate coverage in ML. Note that the two nonoptimal overlayer configurations are an artifact of the finite size effects, i.e., small surface supercells do not allow for a better lateral adsorbate arrangement for specific coverages. 4.2. Why Chlorine Desorbs as Atomic Cl. Goddard et al.1 reported that chlorine desorbs exclusively as atoms from Cu(111) and explained it by the fact that the activation energy of Cl desorption is smaller than the activation energy of Cl2 desorption. By simple arguments they attributed this to two effects: (i) nonactivated adsorption of Cl2 and (ii) small Cl2 bond

Θ (ML)

qLo¨wdin (electrons)

q˜Lo¨wdin (electrons)

∆Φ (eV)

µ (e bohr)

1/16 1/9 1/4 1/3 1/2 2/3 3/4 1

0.50 0.49 0.48 0.47 0.44/0.45b 0.31 0.32/0.33/0.41c 0.13

0.58 0.58 0.56 0.56 0.52/0.54b 0.40 0.40/0.41/0.49c 0.21

0.14 0.23 0.59 0.63 1.23 0.74 2.06 0.62

0.14 (0.36) 0.13 (0.33) 0.14 (0.36) 0.12 (0.29) 0.15 (0.38) 0.06 (0.17) 0.17 (0.43) 0.04 (0.10)

a qLo¨wdin, Lo¨wdin net charge of adsorbed Cl (eq 22); q˜Lo¨wdin, renormalized qLo¨wdin (eq 25); ∆Φ, adsorption induced change of work function; µ, adsorption induced dipole moment (values in parentheses are in debye). b Corresponds to Cl at fcc and hcp sites, respectively. c Corresponds to Cl at fcc, hcp, and top sites, respectively.

energy (i.e., breaking the second Cl-metal bond is not compensated for by making the Cl-Cl bond). Our calculations are completely consistent with this reasoning. First, they predict a vanishing energy barrier for the dissociation of Cl2 on Cu(111), similar to that on Ag(111).48 Hence the desorption energy for Cl2 desorption is

Edes(Cl2) ) 2DCl-Cu(111) - DCl-Cl

(29)

where DCl-Cu(111) is the magnitude of binding energy of Cl onto Cu(111), i.e. |Eb|, and DCl-Cl is the Cl-Cl bond energy. On the other hand, the desorption energy of atomic Cl is simply given by

Edes(Cl) ) DCl-Cu(111)

(30)

Up to the coverage of 1/3 ML, the DCl-Cu(111) is ≈3.3 eV (Table 4) and is roughly independent of the coverage. The highest experimentally observed coverage is Θ ) 0.45 ML, and on the basis of the interpolation curve of Figure 7, the corresponding DCl-Cu(111) of about 3.0 eV can be estimated. As for the Cl-Cl bond energy, our calculations give a value of 2.85 eV. Hence, it can be written as

Edes(Cl) - Edes(Cl2) ) DCl-Cl - DCl-Cu(111) ) -0.45 eV (low coverage) ) -0.15 eV (high coverage)

(31)

This demonstrates that, according to calculations, the desorption of atomic chlorine is more exothermic than the desorption of molecular chlorine over the whole experimentally achievable coverage range, in agreement with finding of Goddard et al.1 4.3. Electronic Properties. To better understand the bonding of Cl onto Cu surface and the origin of the repulsive lateral adatoms interactions, the electronic structures of the various systems studied in this work are examined in some detail now. Several electronic properties discussed in this subsectionse.g., adatom Lo¨wdin net charges, adsorption induced dipole moments, and work function changessare reported in Table 7. 4.3.1. Lo¨wdin Population Analysis. Due to the large difference in electronegativity between Cl and Cusin Pauling scale the values are 3.16 and 1.90, respectively35sit is reasonable to expect that the Cl atoms are negatively charged. For this reason, the Lo¨wdin net charge of Cl adatom as a function of coverage


J. Phys. Chem. C, Vol. 113, No. 32, 2009 14371 Figure 9a), yet it has larger charge than the corresponding ideal overlayer structure with the same Cl-Cl nn distance (e.g., the nonideal 2/3 and ideal 1 ML overlayer structures have the same dnn, yet the charge on the former is larger). Hence the effective Cl-Cl nn distance at a given coverage is defined as a geometric mean between the actual and ideal Cl-Cl nn distance at that coverage: max deff nn ) √dnndnn

Figure 8. Comparison between the actual nearest-neighbor Cl adatoms distances, dnn (blue points), and the maximal possible lateral nn distance as a function of coverage (red curve). Note that the 1/2 ML@(2 × 2) and 2/3 ML@(3 × 3)R30° overlayer structures (open pentagons) display the nonoptimal dnn.

is plotted in Figure 9a. In this figure, the two nonoptimal overlayer structures discussed above [i.e., 1/2 ML@(2 × 2) and 2/3 ML@(3 × 3)R30°] are marked with open pentagons. At low coverage, the net charge on Cl is about half of an electronsin agreement with Bader analysis of ref16sand it remains so up to the coverage of 1/3 ML. Then with the further increase of coverage a depolarization occurs. Note that the charge of 2/3 ML@(3 × 3)R30° metastable structure is smaller than would be expected on the basis of interpolation (red curve). This is so because its nn adatom distance, dnn, is smaller than the corresponding optimal nn distance (see Figure 8). In particular, the dnn of 2/3 ML structure is even smaller than the dnn of the 3/4 ML structure. Hence, in the former structure the Cl-Cl repulsive interaction is stronger and the charge on Cl flows back to the substrate to stabilize the system. Therefore, the Cl charge of the 2/3 ML structure is smaller than that of the 3/4 ML one. To account for this nonoptimal overlayer configuration, an effective Cl-Cl nn distance is defined. For the optimal centeredhoneycomb structures, the effective nn distance equals the actual nn distance, whereas for nonoptimal structures it should be in between the actual and ideal nn distance. This is so, because the actual structure has smaller adatom charge, as would have the corresponding ideal structure at the same coverage (see

(32)

In Figure 9b, the Lo¨wdin net charge of Cl adatom as a eff , is plotted. function of just defined effective nn distance, dnn This plot reveals that the adatom charge remains constant up eff of about 4 Å, and upon further reduction of distance, to the dnn a depolarization occurs. This plot also explains why the Cl charge of nonoptimal 1/2 ML@(2 × 2) overlayer structure lies close to the interpolation curve in Figure 9a: for this structure the effective nn distance is large enough and is located just outside the region of very steep slope of the charge vs distance curve. Figure 9b also indicates by a vertical blue-dotted line eff of the (63 × 63)R30° what would be the GGA dnn superstructureswhich is the structure with the highest Cl eff ) 63/7 coverage that was observed experimentally1si.e., dnn × a0/2 ) 3.86 Å, where a0 is the calculated Cu bulk lattice parameter. 4.3.2. Work Function Change. The negatively charged Cl adatoms indicate the work function increase on adsorption, because in a simple picture a negatively charged adatom should induce an inward pointing dipole (the adsorption of N on W(100)50 and Cl on Rh(100), Pd(100), and Pt(100)15 are notable exceptions). The adsorption-induced dipole moment and the corresponding change of work function are related by the Helmholtz equation, eq 17. Figure 10 displays the change of the work function and the corresponding induced dipole moment as a function of coverage. It can be seen that the work function increases linearly with the coverage, with the exception of the two metastable structures at 2/3 and 1 ML (open circles) for which the ∆Φ is substantially smaller. The induced dipole moment is approximately 0.15 e bohr/Cl adatom (disregarding the two metastable structures) and shows a weak increasing dependence on coverage (Figure 10b). A similar linear increase of the work function without a saturation at large coverage has been also reported for oxygen adsorption on Ag(111) by Li et

Figure 9. (a) Lo¨wdin net charge (in units of electrons) of Cl adatom adsorbed on Cu(111) as a function of coverage. The two nonoptimal overlayer configurations are marked with open pentagons and were not included in the fit (curve). (b) Lo¨wdin net charge on Cl adatom as a function of eff effective Cl-Cl nearest-neighbor distance, deff nn (see the text). The vertical blue-dotted line indicates what would be the GGA dnn of the high coverage (63 × 63)R30° superstructure.

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Figure 10. (a) Adsorption-induced work function change and (b) dipole moments for Cl on Cu(111) as a function of coverage (in atomic units, e bohr/Cl atom; 1 e bohr ) 2.54 D). The data for the metastable 2/3 and 1 ML structures are shown as open circles. Lines are drawn to guide the eye. In part a, the vertical blue-dotted line at Θ ) 0.45 ML corresponds to the Cl coverage of (63 × 63)R30° superstructure.

Figure 11. Planar averaged charge density difference ∆Fj(z) (red curve), defined by eq 14, and its first moment function, µ(z) (blue curve), defined by eq 15, for adsorbate coverages Θ ) 1/4, 1/3, 1/2, 2/3, and 3/4 ML. The positions (heights) of the Cu layers and Cl adatoms are indicated by brown and green vertical lines, respectively. The adsorption-induced dipole moment, µ, equals the value of µ(z) on the right side of the plot.

al.51 Another similarity with the O@Ag(111) system is the deviation of the work function at 1/3 ML from the linear behavior: in the current case, the deviation is negative, whereas for O@Ag(111) it is positive. This results in a noticeable decrease (increase) of induced dipole moment for Cl@Cu(111) [O@Ag(111)] (see Figure 10b and ref 51).

Our results are in agreement with those of Migani et al.,16 who calculated ∆Φ of 0.24 eV at 1/9 ML, compared to our 0.23 eV at the same coverage. An experimentally reported maximum of ∆Φ for Cl/Cu(111) is 1.2 eV, as reported by Goddard et al.1 The calculated ∆Φ is therefore in good agreement with this experiment, provided that the highest experimental coverage of 0.45 ML is considered. This coverage is indicated by a vertical blue-dotted line in Figure 10a, and a value of ∆Φ ) 1.16 eV can be extracted. Goddard et al., however, reported that significant depolarization occurs at high coverages, yet our calculations do not show any ∆Φ saturation up to the coverage of 3/4 ML, which is significantly higher than the maximal reported experimental coverage. The only exception may be the already mentioned negative deviation at 1/3 ML. The induced dipole moment should be roughly proportional to the adatom charge times its height above the metal surface (more precisely above the image plane).52,53 However, Figure 9 shows that for coverages larger than 1/4 ML the Cl adatom charge decreases with increasing coverage, hence it is not obvious why the induced dipole moment slightly increases with the coverage (disregarding the deviation at 1/3 ML and the two metastable structures at 2/3 and 1 ML). To answer this question, the adsorbate-substrate interaction should be considered in more details. We start by considering the planar averaged charge density difference, ∆Fj(z), defined by eq 14, and its first moment function, µ(z), defined by eq 15, which are shown in Figure 11 for adsorbate coverages ranging from 1/4 to 3/4 ML. The positions (heights) of the Cu layers and Cl adatoms are also indicated. In this figure, the ∆Fj(z) (red curve) is plotted in units of electron charge; therefore, positive values represent electron excess regions. To facilitate the discussion and to track the changes of ∆Fj(z) and µ(z) in Figure 11 with increasing adsorbate coverage, the ∆Fj(z) peaks and depressions are labeled from 1 to 7. It is first noted that the adsorptioninduced perturbation of the metal substrate is located mainly around the topmost Cu layer and extends up to the first subsurface layer (peaks 1-3 and depression 4). Just above the position of surface Cu layer there is an electron deficit region (depression 4), whereas around the position of Cl adatom there is an electron excess region (peaks 5 and 6), in accord with the Lo¨wdin population analysis presented above. Peaks 5 and 6 are followed by a tiny depression, labeled 7, which grows with the coverage. The reduced dipole moment of 1/3 ML structure can be attributed almost exclusively to the latter feature: note how drastically the depression 7 reduces the µ(z) (blue curve). Due to this effect, one would expect that at larger 1/2 ML coverage the µ would reduce further. Indeed, the reductive effect of depression 7 on µ(z) is big (see how much it

Adsorption of Chlorine on Cu(111) reduces over the position of depression 7), yet due to other counteracting effects, the dipole actually increases. Peak 3, located just above the position of surface Cu layer, almost vanishes; hence, the center of gravity of the electron deficit region shifts toward the substrate, and in addition, peaks 5 and 6 become larger, resulting in an overall increase of µ. The reason for the small µ of the 2/3 ML structure is that depression 4 and peaks 5 and 6 are reduced. Moreover, the relative heights of peaks 5 and 6 are reversed, thus shifting the center of gravity of the electron excess region toward the substrate. These features together with the big depression 7 result in a small µ. As for the 3/4 ML structure, its largest adsorption-induced dipole moment can be attributed to Cl adatom adsorbed on the top site, which is located 0.79 Å above the two hollow Cl atoms (see Table 4) and contributes about 65% to the overall dipole moment. Due to this, depression 7 almost entirely vanishes. 4.3.3. Charge Density Difference. To understand the changes of the profile of ∆Fj(z) with coverage and to understand the nature of the adsorbate-substrate bonding, Figure 12 displays twodimensional contour plots of the charge density difference, ∆F(r), defined by eq 13. In the following text, a label CuCl will designate surface Cu atoms that are bonded to Cl adatoms. These plots distinctively show the electron excess region around the Cl adatoms (red regions) and the electron deficit region at the metal surface (blue regions). The ∆F(r) remains similar in shape up to the coverage of about 1/3 ML, whereas at larger coverage new features appear. At low coverage, the maxima of the charge accumulation are located below the adatoms (see sideview) in the direction of the Cl-Cu bonds (side view and top view). Also the CuCl atoms are polarized in the direction toward the Cl adatoms, i.e., a closer inspection reveals a rotated dz2-like blue shape and a rotated dxzlike red shape. The latter is responsible for a little peak 3 in Figure 11. At coverages larger than 1/3 ML, a charge deficit region starts to accumulate above the Cl adatom, so that together with the charge

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14373 depletion located below the Cl adatom resembles the pz orbital. This feature is responsible for the depletion 7 and the small depletion between the peaks 5 and 6 in Figure 11. Moreover, the surface CuCl atoms are not polarized anymore in the direction toward the Cl adatoms, but rather in a direction perpendicular to the surface, because each CuCl is bonded to two Cl adatoms. For this reason, a small charge accumulation region located around the CuCl atoms does not point outside the surface Cu layer, as it did at lower coverage. This may be the reason for the reduction (disappearance) of peak 3 in Figure 11. At 1 ML, the shape of ∆F(r) changes completely. First, the Cu-Cl interaction is weakened due to a reduced charge transfer, as witnessed by the smaller magnitude of charge excess and deficit regions. Also the Cl adatoms are not polarized in the direction of Cl-Cu bonds. In addition, the top view plot reveals an accumulation of charge in between the Cl neighbors, indicating the formation of the lateral Cl-Cl bonds. The nearestneighbor Cl-Cl bond formation is further visualized by an additional side view contour plot displayed along the Cl-Cl bond direction (shown on the right side of the corresponding top view plot). An approximate strength of such covalent lateral interaction can be estimated from the corresponding isolated Cl layer, and calculations predict a value of about -0.7 eV/Cl atom. Note that when such a layer is adsorbed on the surface, in addition to this lateral covalent attraction, there is also an electrostatic repulsion due to negatively charged Cl adatoms. 4.3.4. Screening by Metal Electrons. The top view plots of Figure 12 show another perspective of the charge accumulation regions around the Cl adatoms, whose sizes are given by a zerolevel (nodal) contour/isosurface. With increasing coverage these regions comes closer together and at a coverage of 1/2 ML they already overlap. It is interesting to notice that the average chemisorption energy, Echem, correlates with the observation just mentioned, because Echem remains approximately independent

Figure 12. Side view and top view contour plots of the charge density difference ∆F(r), defined by eq 13, for Cl adsorbed on Cu(111) at adsorbate coverages ranging from 1/4 to 1 ML. ∆F(r) presented on the left side were calculated with a (2 × 2) supercell, whereas those on the right with a (3 × 3)R30° supercell. Contours are drawn in linear scale from -0.006 to 0.006 e/a03, with the increment of 0.001 e/a03. The blue color represents the electron-deficit regions, while the electron-excess regions are colored red (i.e., charge flows from blue to red regions). The positions of the side view contour planes are also indicated, whereas the top view contour plane is located 0.42 Å below the fcc Cl adatoms.

14374


Figure 13. (Top) The density of states projected (PDOS) to s- (purple) and d-states (brown) of surface Cu atom. The remaining panels show the PDOS of Cl adatoms (green) and surface Cu atoms (brown) that are bonded with the adatoms for coverages from 1/4 to 1 ML.

of the coverage as long as the charge accumulation regions remain separated, which for the currently considered coverages is 1/3 ML. At larger coverage, the magnitude of Echem decreases, and charge accumulation regions overlap. The independence of the Echem on the coverage for Θ e 1/3 ML is therefore attributed to the good screening ability of metal electrons, which are able to screen the negatively charged Cl adatoms efficiently as long as the charge accumulation regions remain separated. 4.3.5. PDOS and ILDOS. To further analyze the electronic structure and the adsorbate-substrate bonding, we start considering the densities of states projected (PDOS) onto Cl adatoms and the CuCl atoms (i.e., Cu atoms that are bonded with Cl adatoms). The corresponding PDOSs are shown in Figure 13 for an adsorbate coverage ranging from 1/4 to 1 ML. In the following all energies are stated with respect to the Fermi level. It is first noted that the Cu d-band of bare surface is located from about 4.5 to 1.5 eV below the Fermi level, whereas the s-band is spread throughout the whole plotting region. The DOS projected onto the Cl adatom is distributed over the whole d-band region, indicating that Cl-Cu interaction is not purely ionic, but is to some extent also covalent. One of the first observations from Figure 13 is that the Cl PDOS broadens with

Peljhan and Kokalj the coverage, indicating that the covalent character of adsorbate-substrate interaction increases with the coverage. At low adsorbate coverage, Θ e 1/3 ML, the main contribution to the Cl PDOS is located at the bottom of the Cu d band, in particular from -6 to -3.5 eV. There is also a narrow peak located at the end of the Cu d band at -1.5 eV. An analysis in terms of integrated local density of states (ILDOS), defined by eq 27, reveals that these correspond to bonding and antibonding Cu-Cl states (see Figure 14). The first part of the bonding peak corresponds to the interaction between Cl pz states with Cu s and d states, whereas the second part corresponds to the interaction between Cl px,y states (where the symbol px,y stands for a linear combination of px and py orbitals) with Cu s and d states. The antibonding peak is due to a hybridization between Cl px,y with Cu d states. All these features are visible in the plots presented in Figure 14. At larger 1/2 ML coverage, the Cl PDOS broadens, in particular, the antibonding states become more pronounced, indicating that the interaction becomes more covalent. The ILDOS analysis of antibonding states shows the nodal plane not only between the Cl and Cu but also between the Cl neighbors. Another difference with the lower coverage is that the relative order of pz and px,y bonding states is reversed, the px,y state being the lowest lying now. The Cl PDOSs of metastable 2/3 and 1 ML structures are broadened even to a much larger extent. The reason is the short Cl-Cl nearest neighbor distance; hence, the Cl sp states hybridize laterally (i.e., between the Cl-Cl neighbors) and the shape of the Cl PDOS resembles the shape of the PDOS of a delocalized metallic sp-band. It is worth mention that recently Baker et al.54 argued on the basis of charge density difference, PDOS, and ILDOS analysis that the bonding of Cl onto Au(111) surface is mainly covalent in nature, and these authors attributed this finding to the small electronegativity difference between chlorine and gold, being merely 0.76. The analysis discussed above (i.e., Figures 12-14) shows very similar features to those presented by Baker et al. for Cl/Au(111), despite the fact that the electronegativity difference between Cl and Cu is larger, 1.26. Moreover, the Bader analysis gives the net charge of Cl on Cu(111) and Au(111) of 0.5 and 0.4 electrons,16 respectively; hence, the difference between Cu and Au is not that remarkable. 4.4. Adsorption Free Energy. To facilitate comparison between Cl/Cu structures at various adsorbate coverage and to take into account the effect of temperature and chlorine partial pressure, the adsorption free energy, γads, as a function of Cl chemical potential, µCl, is displayed in Figure 15. In this plot, the horizontal line at zero adsorption free energy, γads, represents the clean surface. Thermodynamically the most stable structure at given µCl is the one with the smallest γads. At very low chemical potential, µCl < -1.9 eV, the clean surface is thermodynamically the most stable (black horizontal line), then at µCl ≈ 1.9 eV, the low coverage lines intersect the horizontal clean surface line, indicating that at this tiny range of µCl low coverage 1/16 and 1/9 ML chemisorption structures are degenerate (within the estimated computational accuracy) and the most stable. From this point on and up to µCl ) 0.0 eV, the 1/3 ML@(3 × 3)R30° phase is the most stable (red thick line). This is in nice agreement with experimental observations.1 Therefore, the highcoverage structures (Θ g 1/2 ML) are never thermodynamically the most stable. On the other hand, the mixed on-surface + substitutional structure at Θ ) 3/4 ML (dark gray line) becomes the most stable at µCl ≈ 0.0 eV, hence, at a value of the Cl chemical potential that is substantially larger than that needed to form bulk



Figure 14. Integrated local density of states (ILDOS), defined by eq 27, and PDOS projected onto individual atomic orbitals of Cl adatoms and of their Cu nearest-neighbors at coverages of 1/4 ML (top panels) and 1/2 ML (bottom panels). The energy range associated with given ILDOS is indicated by blue stripes in the PDOS plots. The magnitude of the plotted increases from red to violet in rainbow fashion. Six contours are drawn in linear scale from 0.0 to 0.028ne/a03, where n stands for the number of electrons per Cl adatom obtained by integrating the PDOS of Cl within the corresponding energy range.

formation of high-coverage adsorption phases (Θ g 1/2 ML), if they would ever occur, cannot be due to thermodynamic equilibrium but can only result from kinetic effects. 5. Conclusions

Figure 15. Adsorption free energy, γads, for several low-energy Cl/ Cu(111) structures as a function of Cl chemical potential, µCl. The horizontal black line at γads ) 0.0 meV/Å2 corresponds to clean Cu(111). The vertical blue line at µCl ) -0.98 eV corresponds to bulk CuCl. The label on+subst stands for mixed on-surface + substitutional adsorption, respectively.

CuCl. According to the GGA results, the latter would form at µCl ≈ -1 eV (blue vertical line). This finding indicates that the

In this paper, the adsorption of atomic Cl on Cu(111) has been characterized in detail by means of extensive DFT-GGA calculations. On-surface and substitutional adsorption have been considered over a wide range of Cl coverages, ranging from 1/16 to 1 ML. It is found that the substitutional adsorption mode is unstablesCl adatom does not adsorb in the vacancy but at its edgesexcept at very large coverage (3/4 ML), where the mixed on-surface + substitutional structure is the most stable. It is observed that a number of adsorption properties is almost independent of the coverage up to 1/3 ML, and upon further increase of coverage new features appear: the magnitude of chemisorption energy reduces, Cl adatoms start to adsorb into nonoptimal sites, net adatom charge reduces, and the Cl-Cu interaction becomes more covalent. For this reason, the adsorption is classified into low-coverage regime (Θ e 1/3 ML) and high-coverage regime (Θ > 1/3 ML).

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At low coverage, the dissociative chemisorption energy is about -1.9 eV/Cl adatom (with respect to Cl2 molecule in the gas phase), and the Cl adatoms are negatively charged by about half an electron. This results in an increase of the work function. The analysis of electronic structure further reveals that the Cl-metal interaction is not purely ionic but also covalent to some extent, as witnessed by the formation of adsorbate-substrate bonding and antibonding states. The analysis further indicates that the independence of the Echem on the coverage for Θ e 1/3 ML is due to a good screening ability of metal electrons, which are able to screen the negatively charged Cl adatoms efficiently as long as the electron-charge accumulation regions remain separated, which is true for Θ e 1/3 ML. At larger coverage, the negatively charged adatoms will not be screened and a lateral electrostatic repulsion between them will push some fraction of Cl adatoms from the optimal fcc site so as to maximize their interatomic distance. This happens because the difference in energetic stability between various sites is relatively small (i.e., the stability of high-symmetry sites follow the order fcc ≈ hcp > bridge > top). Therefore, the formation of various observed Cl overlayer superstructures at Θ ∈ (1/3, 1/2) MLse.g., (123 × 123)R30°, (47 × 47)R19.2°, and (63 × 63)R30°sis attributed to the interplay between a small site preference and lateral electrostatic repulsion between adatoms. The analysis of adsorption free energy as a function of chlorine chemical potential reveals that the (3 × 3)R30° adsorption phase is thermodynamically the most stable over a very broad range of Cl chemical potential. Only at a very tiny range of very low Cl chemical potential, around -1.9 eV, are the low-coverage configurations [i.e., (3 × 3) and (4 × 4)] the most stable structures. At even lower chemical potential, clean Cu(111) surface is the most stable. Acknowledgment. This work has been supported by the Slovenian Research Agency (Grant No. J1-9516 and P2-0148). The authors thank Prof. Anton Meden for providing crystal structure data of bulk copper chlorides. References and Notes (1) Goddard, P. J.; Lambert, R. M. Surf. Sci. 1977, 67, 180–194. (2) Walter, W. K.; Manolopoulos, D. E.; Jones, R. G. Surf. Sci. 1996, 348, 115–132. (3) Kadodwala, M. F.; Davis, A. A.; Scragg, G.; Cowie, B. C. C.; Kerkar, M.; Woodruff, D. P.; Jones, R. G. Surf. Sci. 1995, 324, 122–132. (4) Shard, A. G.; Ton-That, C.; Campbell, P. A.; Dhanak, V. R. Phys. ReV. B 2004, 70, 155409. (5) Crapper, M. D.; Riley, C. E.; Sweeney, P. J. J.; McConville, C. F.; Woodruff, D. P. Europhys. Lett. 1986, 2, 857–861. (6) Woodruff, D. P.; Seymour, D. L.; McConville, C. F.; Riley, C. E.; Crapper, M. D.; Prince, N. P. Phys. ReV. Lett. 1987, 58, 1460–1462. (7) Woodruff, A. P.; Seymour, D. L.; McConville, C. F.; Riley, C. E.; Crapper, M. D.; Prince, N. P. Surf. Sci. 1988, 195, 237–254. (8) Way, W. K.; Pike, A. C.; Rosencrance, S. W.; Braun, R. M.; Winograd, N. Surf. Interface Anal. 1996, 24, 137–141. (9) Motai, K.; Hashizume, T.; Jeon, D. R.; Lu, H.; Tanaka, K.; Pickering, H. W.; Sakurai, T. Jpn. J. Appl. Phys. 1992, 31, L874–L876. (10) Kruft, M.; Wohlmann, B.; Stuhlmann, C.; Wandelt, K. Surf. Sci. 1997, 377-379, 601–604. (11) Sakurai, T.; Hashizume, T. Nanotechnology 1992, 3, 126–132. (12) Suggs, D. W.; Bard, A. J. J. Am. Chem. Soc. 1994, 116, 10725– 10733. (13) Ignaczak, A.; Gomes, J. A. N. F. Chem. Phys. Lett. 1996, 257, 609–615. (14) Doll, K.; Harrison, N. M. Chem. Phys. Lett. 2000, 317, 282–289. (15) Migani, A.; Sousa, C.; Illas, F. Surf. Sci. 2005, 574, 297–305. (16) Migani, A.; Illas, F. J. Phys. Chem. B 2006, 110, 11894–11906. (17) Andryushechkin, B. V.; Eltsov, K. N. Surf. Sci. 1992, 265, L245– L247.

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Adsorption of Chlorine on Cu(111): A Density-Functional Theory Study

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