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Chlorination of the Cu(110) Surface and Copper Nanoparticles: A Density Functional Theory Study Ibrahim A. Suleiman,† Marian W. Radny,*,‡ Michael J. Gladys,‡ Phillip V. Smith,‡ John C. Mackie,†,§ Eric M. Kennedy,† and Bogdan Z. Dlugogorski† †
School of Engineering and ‡School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, New South Wales 2308, Australia § School of Chemistry, The University of Sydney, Sydney, Australia ABSTRACT: The interaction of atomic chlorine with the Cu(110) surface is studied using density functional theory and ab initio atomistic thermodynamics. The calculated surface free energies of different Cl/Cu(110) structures as a function of chlorine chemical potential show that under ultrahigh-vacuum conditions, the c(2 2)-Cl structure is the most stable for coverages up to and including 1/2 ML, whereas the (2 3)-Cl and (1 4)-Cl configurations are the most stable at 2/3 and 3/4 ML coverages, respectively. It is also shown that although there are thermodynamically stable geometries for high (1 ML) coverage of Cl, these structures are only kinetically accessible. The morphology of a copper nanostructure terminated by low-index Cu surfaces in a chlorine environment has been predicted using a Wulff construction. It is found that the (111) facets dominate at low chlorine concentration, but as the chlorine concentration is increased, the (100) planes become predominant, resulting in a cubical crystal shape.
1. INTRODUCTION The chemistry of chlorine on metal surfaces is involved in many technological processes, such as corrosion and catalysis.14 In heterogeneous catalysis, the interaction of chlorine with metals usually poisons the surface layer, inhibiting various catalytic reactions.5,6 However, it has also been reported that chlorine can promote some catalytic reactions by increasing the selectivity of certain partial oxidation reactions.7 A large number of studies have been carried out to investigate chlorine chemisorption on the low-index (100), (110), and (111) copper surfaces.822 Chlorine adsorption on Cu(110) has been studied experimentally by several researchers,8,14,2325 but to our knowledge, there is no theoretical study devoted to the adsorption of Cl on this surface. The (110) plane has a more open geometry than the more densely packed (100) and (111) surfaces and, hence, is more reactive.26,27 It also exhibits a greater ability to reconstruct,28 thereby allowing the possible formation of more reactive structures. Stickney et al.14 have investigated the adsorption of gaseous HCl at room temperature under ultrahigh vacuum conditions on the low-index planes of copper using the techniques of angulardependent photoemission, low-energy electron diffraction, and X-ray absorption spectroscopy. They have reported that the initial exposure of HCl on Cu(110) forms a c(2 2) structure, and further exposure leads to a (3 2) phase. They propose that for the c(2 2) structure, the Cl atoms occupy the hollow sites, while the (3 2) structure is more densely packed with the Cl atoms occupying either the hollow or long-bridge sites. Carley et al.8,25 in a more recent study using X-ray photoemission spectroscopy and scanning tunnelling microscopy (STM) have r 2011 American Chemical Society
confirmed that the adsorption of HCl on Cu(110) at coverages up to 1/2 ML produces a c(2 2) structure at room temperature. Some studies have also been carried out to investigate the interaction of chlorine with the Cu(110) surface in aqueous environments. Stickney et al.14 studied the immersion of Cu(110) in an aqueous 1 mM HCl solution. Although they have not identified a particular structure, they have reported that the Auger spectrum suggests a coverage of close to 1/2 ML. An in situ room temperature STM experiment was carried out by Wan and Itaya23 to investigate the structure of the Cu(110) surface in various solutions containing halide ions, including Cl. They observed that Cl ions adsorbed on Cu(110) to form a (4 1)-Cl reconstructed configuration at 3/4 ML coverage. Li et al.24 in their STM study of chloride adsorption on Cu(110) in a hydrochloric acid aqueous solution have also observed a (4 1) structure. Despite all of this experimental work devoted to understanding Cl adsorption on Cu(110), however, some of the very basic properties, such as the actual surface structures and their thermodynamical stabilities, still need to be reliably determined. In this paper we report the results of ab initio atomistic thermodynamics and density functional theory calculations that have been performed to determine the thermodynamic stability of Cl-induced structures on Cu(110) under various Cl exposure (pressure and temperature) conditions. We have found that under ultrahigh vacuum (UHV) conditions, the c(2 2)-Cl configuration is the most stable structure for coverages up to and Received: April 19, 2011 Revised: June 2, 2011 Published: June 17, 2011 13412
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including 1/2 ML. This agrees with experiment, although the details of the underlying atomic configuration differ from that proposed previously. At higher pressures, the (2 3) structure at 2/3 ML coverage becomes more stable, followed by the (1 4) configuration at 3/4 ML. The 1 ML geometries, which form the most thermodynamically stable structures at high chlorine coverage, were found to be only kinetically accessible. The surface free energies of the Cl/Cu(110) adsorption system have also been compared with those of the Cl/Cu(100)21,22 and Cl/Cu(111)20 systems. Using the Wulff construction, this has enabled the shape of copper nanostructures in different chlorine environments to be determined.
Table 1. Energetics and Geometries Obtained for Different Locations and Coverages of Cl on the Unreconstructed Cu(110) Surfacea system
Eb (eV)
Δ12 (%)
Δ23 (%)
CuCl (Å)
8.9
þ3.4
1/9 ML, SB
2.31
6.6
þ2.0
2.299
1/9 ML, LB
2.16
6.7
þ1.9
2.376
1/4 ML, SB
2.28
4.1
0.0
2.284
1/4 ML, LB 1/4 ML, T
2.14 1.86
4.1 5.1
þ0.2 þ0.3
2.366 2.160
1/2 ML, SB
2.24
0.0
3.0
2.260
1/2 ML, LB
2.11
þ0.2
2.8
2.350
A. Computational Details. All electron calculations have been
2/3 ML, (2 3)
2.06
þ2.0
3.3
carried out using density functional theory (DFT) in the generalized gradient approximation (GGA) of Perdew and Wang (PW91)29,30 as implemented in the DMol3 software.31,32 The calculations have employed a double numeric basis set with polarization functions (DNP). A (5 5 1) MonkhorstPack33 k-point sampling set was employed for the p(2 2), p(2 3), and p(3 3) unit cells, and an (8 5 1) set was used for the p(1 4) surface unit cells. The calculated lattice constant of 3.67 Å, which is 1.7% larger than the experimental value34 of 3.61 Å, was used to build an eleven layer symmetric slab with the Cl atoms adsorbed on both surfaces. All atoms were allowed to relax except for the three layers in the center of the slab, which were held fixed in their bulk positions. The Fermi smearing, the real space cutoff, and the energy, gradient, and displacement convergence tolerances were set to 0.136 eV, 4.4 Å, 2 106 eV, 5 104 eV/Å, and 5 103 Å, respectively. The average binding energies, Eb, of the adsorbed Cl atoms have been calculated using the formula 1 NCl ECl2 ECl=slab Eslab þ ð1Þ Eb ¼ NCl 2
3/4 ML, (1 4)
1.96
þ2.5
3.0
1 ML, SB
1.07
þ2.0
4.1
2.382
1 ML, LB
1.13
þ4.0
4.8
2.430
1 ML, T 1 ML, H þ SB
1.06 1.69
1.6 þ6.4
3.4 3.9
2.248
2. METHODOLOGY
where Eslab, ECl2 and ECl/slab are the total energies of a clean slab, an isolated chlorine molecule, and the Cl/Cu (110) adsorption system, respectively, and NCl is the number of adsorbed chlorine atoms within each slab unit cell. B. Ab Initio Atomistic Thermodynamics. Ab initio atomistic thermodynamics calculations have been performed to determine the relative stability of the Cu(110) surface in contact with a molecular chlorine (Cl2) gaseous environment. The chlorine concentration in the gas phase is represented implicitly by the chlorine chemical potential, μCl, which is a function of the gas temperature, T, and partial pressure, p. At equilibrium, the thermodynamically preferred structure is that with the lowest surface free energy. This can be calculated from the expression γads ¼
1 ΔG T, p, NCu , NCl ΔNCu μCu ðT, pÞ þ NCl μCl ðT, pÞ A
ð2Þ where A is the cell surface area; ΔNCu is the difference between the number of Cu atoms of the Cl/Cu system and the clean ideal (unreconstructed) surface slab; μCu and μCl are the chemical potentials of Cu and Cl, respectively; and ΔG is the Gibbs free energy difference between the Cl/Cu(110) system and the ideal (unreconstructed), clean Cu(110) slab, ð3Þ ΔG ¼ GCl=slab Gslabideal Following the standard approach,20,35,36 we approximate the Gibbs free energy difference, ΔG, by the difference between the
clean surface
a Eb is the average binding energy calculated using eq 1. Δij is the average relaxation (as a percentage) between layers i and j, and CuCl is the bond length between a Cl atom and the closest Cu atom at the surface.
ground state DFT total energies of the adsorption Cl/Cu(110) system and the clean, ideal slab. Equation 2 can then be written as γads ¼
1 NCl ECl2 ΔNCu Ebulk N Δμ ECl=slab Eslabideal Cl Cl Cu 2 A
ð4Þ where ΔμCl = μCl (1/2)ECI2, and is the total energy of a Cu atom in bulk copper. Assuming that the Cl2 molecules in the gas reservoir behave ideally, the T and p dependence of the ΔμCl is given by " !# 0 1 pCl2 ~ T, p þ kB T ln ΔμCl ¼ μ ð5Þ 2 Cl2 p0 Ebulk Cu
In this equation, p0 is the atmospheric pressure, and the values of μ ~Cl2(T, p0), which include the contributions from the rotations and vibrations of a Cl2 molecule as well as the ideal-gas entropy at 1 atm, can be obtained from thermodynamics tables.37
3. RESULTS AND DISCUSSION A. The Clean Unreconstructed Cu(110) Surface. The calculated interlayer relaxation values for the clean Cu(110) surface are shown in Table 1. Our calculated value for the interlayer separation between the first and second layer, Δ12, of 8.9% is in good agreement with the experimental value of 10.0 ( 2.5%.38 By contrast, our predicted interlayer separation between the second and third layers, Δ23, of þ3.4% is somewhat higher than the experimental value of 0.0 ( 2.5%.38 Both of our calculated values are, however, in good agreement with previous DFTGGA calculations (Δ12 = 10.0%, Δ23 = þ3.0%).39 Our calculated surface energy per unit cell of 0.95 eV also correlates well with the previously calculated value of 0.97 eV.39 B. Chlorine on an Unreconstructed Cu(110) Surface. Four different adsorption sites on the unreconstructed Cu(110) surface— the hollow (H), top (T), long-bridge (LB), and short-bridge (SB) sites (see Figure 1)—were considered for atomic chlorine 13413
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Figure 1. Top views of the on-surface Cl atoms at different coverages on the hollow, long-bridge, short-bridge, and top sites. The red and black spheres represent the Cu atoms of the first layer of the Cu(110) surface, and the chlorine atoms, respectively.
adsorption at coverages of 1/4, 1/2, and 1 ML. For 1/9 ML coverage, only the LB and SB sites were examined using a (3 3) unit cell. The results obtained from minimizing the total energies of the various structures are summarized in Table 1. We have found that for Cl adsorption at coverage e1/2 ML, the most stable position for a Cl adatom on the unreconstructed Cu(110) surface is the SB site, followed by the LB position. It was also found that for a coverage of 1/2 ML, Cl atoms initially adsorbed at H and T sites are not stable and move to SB sites. Chlorine atoms adsorbed at T sites for 1/4 and 1 ML retain their initial geometry, but yield significantly lower binding energies than when adsorbed at the other sites (see Table 1). It should be noted that although our prediction of a c(2 2) structure as the most stable configuration for e1/2 ML agrees with the experimentally found Cl-induced surface reconstruction, our calculated preference for the SB sites does not support the atomic model proposed by Stickney et al.,14 who have suggested the H site for Cl atoms adsorbed on Cu(110) at coverages e1/2 ML. Our results are, however, consistent with the results of the cluster calculations of Alves et al.40 and the periodic slab calculations of Fu et al.41 for Cl adsorption on the Ag(110) surface, which predict the SB site as the most stable site. Erley in his extensive experimental study42 of Cl adsorption on the (110) faces of Ni, Pd, and Pt also proposed a bridge site for the Cl atom, although he has suggested the LB rather than the SB site. At 2/3 ML coverage, the adsorbed chlorine atoms form the compact (2 3)-Cl structure shown in Figure 2. The calculated average binding energy of 2.06 eV/Cl atom for this configuration is slightly lower in magnitude than those generally obtained for coverages e1/2 ML (see Table 1). A binding energy of 1.96 eV was also obtained for the Cl adatoms forming a 3/4 ML structure within the (1 4) unit cell (see Figure 2). This suggests that there is a slight overall decrease in the magnitude of the average binding energies of the Cl adatoms with increasing coverage from 1/9 ML to 3/4 ML (see Table 1). The (2 3)-Cl 2/3 ML structure has been observed by Stickney et al.14 at coverages beyond 1/2 ML. Erley has also reported similar compact structures for Cl adsorption on Ni, Pd, and Pt (110) surfaces at coverages higher than 1/2 ML.42 For stable structures at 1 ML coverage, the calculated Cl adatom average binding energies are observed to have decreased significantly in magnitude. This is believed to be due to the
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Figure 2. Top and side views of the final configurations of the (2 3) 2/3 ML and (1 4) 3/4 ML structures. The black balls represent the Cl atoms, and the red spheres show the first layer of the Cu(110) surface.
Figure 3. Average interlayer spacing between layers i and j, Δij, as a function of coverage. The horizontal solid and dotteddashed black lines indicate the calculated relaxation of the clean surface between the first and second, and second and third layers, respectively.
increased Coulombic repulsion between the adsorbed Cl atoms, which become closer together at higher coverages. Chlorine atoms defining 1 ML configurations formed solely from adsorption at either SB, LB, or T sites were found to remain at their initial sites. For the 1 ML H configuration, however, the Cl atoms changed their initial locations to form a packed structure with the Cl atoms occupying either H or SB sites (hereafter referred to as the 1 ML, H þ SB configuration). This transition reduces the repulsive forces between the Cl atoms, resulting in a higher average binding energy than when the Cl atoms occupy only SB, LB, or T sites. Table 1 shows the interlayer spacings and CuCl nearestneighbor distances for the different stable adsorption geometries. These data are also displayed in Figures 3 and 4. Figure 3 shows that for the most stable adsorption sites, SB and LB, the interlayer spacing between the first and second layers (Δ12) increases with increasing coverage and that this relaxation goes from being significantly inward at 1/9 ML to being outward at 1 ML for both the LB and SB adsorption sites. The opposite trend is observed for the relaxation between the second and third layers (Δ23), which is found to go from being outward at 1/9 ML to being inward at 1 ML. Figure 4 presents the CuCl nearest-neighbor distances for the stable SB and LB configurations. For both sites, the CuCl 13414
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Figure 4. The bond length between a Cl atom and its closest Cu atom at the surface as a function of coverage. The horizontal black solid and dotted lines represent the CuCl bond length in the Cu(I) and Cu(II) chlorides, respectively. Lines are drawn to guide the eye.
distances decrease with coverage up to and including 1/2 ML and then increase for 1 ML. The CuCl bond lengths of the SB configurations are observed to be consistently lower than those for the corresponding LB configurations. This behavior mirrors the calculated binding energies, with the SB structures, which are characterized by shorter CuCl bonds, being more stable than the LB configurations, which have longer CuCl bonds. The data in Figure 4 also show that the CuCl bond length in the 1/2 ML SB c(2 2) structure is equivalent to the CuCl bond length of the most common CuCl2 salt, talbochite.10 Carley et al.8 have found that HCl adsorption on a partially oxidized Cu(110) surface leads to the direct formation of a CuCl2 phase, without prior formation of the CuCl phase. The above result thus suggests that adsorption at the energetically favored SB site may play a dominant role in the formation of the CuCl2 phase, even without mediation from the oxygen atoms. C. Chlorine on the Reconstructed Cu(110) Surface. The ideal (unreconstructed) Cu(110) surface is thermodynamically more stable than the reconstructed surface. However, the energy required to reconstruct the Cu(110) surface is much lower than for the (100) and (111) planes.28 In our calculations for the unreconstructed Cu(110) surface, a reduction of ∼0.2 Å has been observed in the distance between the immediately adjacent Cu atoms when Cl is absorbed at the LB positions at 1/9, 1/4, and 1/2 ML coverages. This change in the distance between the Cu atoms bridging the LB site affects the bonding energy of the Cu atoms at the surface and, as such, may represent a possible pathway for reconstruction of the Cu(110) surface. As a result, we have also examined Cl adsorption on different reconstructed (110) surfaces. Total energy calculations were performed for the (1 2) missing row (MR) (Figure 5) and the (2 1)-MR (Figure 6) structures for coverages of 1/4, 1/2, and 1 ML, and the (1 4) added row (AR) configuration (Figure 7) for 3/4 ML coverage. For the (2 1)-MR and (1 2)-MR surfaces, the Cl atoms were assumed to adsorb at the SB and LB sites, respectively, of the original unreconstructed surface. This is because for the (2 1)-MR structure, the original LB sites are no longer available, and for the (1 2)-MR structure, the original SB sites no longer exist. The experimental observations of Wan and Itaya23 and Li et al.24 from the electrochemical interaction of Cl ions with Cu(110) guided our choice for the (1 4)-AR structure. The results of our total energy calculations for the adsorption of Cl on reconstructed Cu(110) are summarized in Table 2.
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Figure 5. Top and side views of the unreconstructed and (1 2) missing row structures for Cl at 1/4 ML coverage, showing only the first and second layers. The black balls represent the Cl atoms, the purple balls show the first layer of Cu atoms, and the red spheres are the underlying Cu atoms. The black rectangles indicate the unit cell.
Figure 6. Top and side views of the (2 1) missing row structure for Cl at 1/4 ML coverage, showing only the first and second layers. The black balls represent the Cl atoms, the purple balls show the first layer of Cu atoms, and the red spheres are the underlying Cu atoms. The black rectangle indicates the unit cell.
We have found that the average binding energies of Cl adatoms on reconstructed Cu(110) decrease in magnitude with increasing coverage, similar to the behavior on the unreconstructed Cu(110) surface. However, in contrast to unreconstructed Cu(110), the LB site is the preferred adsorption site (rather than SB) for Cl adatoms on reconstructed Cu(110). We also observe that both the Δ12 and Δ23 interlayer spacings increase with increasing coverage. This is in contrast to the behavior observed for the unreconstructed Cu(110) surface, where the Δ23 decreases with increasing coverage. This difference is most likely due to more effective interaction 13415
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Figure 7. Top and side views of the (a) initial and (b) final configurations of the (1 4) added row structure, showing only the first and second layers. The black balls represent the Cl atoms, the purple balls indicate the added rows of Cu atoms, and the red spheres denote the Cu atoms. The Cl concentration in this case corresponds to 3/4 ML coverage. The black rectangles indicate the unit cell.
Table 2. Energetics and Geometries Obtained for Different Locations and Concentrations of Cl on Reconstructed Cu(110) Surfacesa system
adsorption site
Eb (eV)
Δ12 (%)
Δ23 (%)
(2 1)-MR 1/4 ML
SB
2.36
2.1
4.4
(1 2)-MR 1/4 ML (2 1)-MR 1/2 ML
LB SB
2.49 2.11
4.3 1.6
4.7 1.1
(1 2)-MR 1/2 ML
LB
2.25
2.7
2.7
(2 1)-MR 1 ML
SB
1.11
þ1.8
þ0.4
(1 2)-MR 1 ML
LB
1.77
þ24.6
þ77.1
a Eb is the average binding energy calculated using eq 1, and Δij is the average relaxation (as a percentage) between layers i and j.
of the adatoms with the second layer of the reconstructed Cu(110) substrate. A contraction of the long CuCu bond was also observed when a Cl atom adsorbed at an LB site of the (1 2)-MR structures. Although this contraction is small for 1/4 ML (∼0.07 Å), it is significant at 1/2 ML (∼0.2 Å). For 1 ML coverage, our calculations predict that there is a substantial expansion of the (1 2)-MR reconstructed surface (see Table 2) and that some of the adsorbed Cl atoms are now located below the surface layer. Because of the similarity between a step edge and the (1 2)-MR reconstruction, this subsurface adsorption may represent a possible pathway for the Cl atoms to diffuse into the Cu bulk to form a CuCl phase. As discussed earlier, the (1 4)-AR structure with 3/4 ML coverage has been observed experimentally by Wan and Itaya23 and Li et al.24 The proposed geometry for this configuration involves pairs of atomic rows being added to the Cu(110) surface in the [100] direction, with every two adjacent rows being absent (see Figure 7). Three Cl atoms per unit cell were adsorbed on this (1 4)-AR structure, resulting in a 3/4 ML coverage. In the proposed structure, one of the three Cl atoms occupies a H site in the added Cu atomic rows, another sits at a SB site in the top layer of the Cu(110) substrate, and the third Cl atom occupies a triangular position defined by two copper atoms in the top layer of the substrate and one Cu atom of an added row (see Figure 7a). We have found that all three Cl atoms move from these initial locations and end up occupying positions close to SB sites (see Figure 7b).
D. Surface Energetics of Cl/Cu(110). To compare the thermodynamic stability of all of the above-mentioned structures, we have constructed the surface phase diagram as a function of chlorine chemical potential. This phase diagram is presented in Figure 8. We observe that when the Cu(110) surface is in a very dilute chlorine environment (ΔμCl < 2.35 eV), the clean unreconstructed Cu(110) surface is the most stable structure. With increasing concentration of chlorine, the Cl chemical potential increases, and over the narrow range 2.35 < ΔμCl < 2.15 eV, the (2 1)-MR 1/4 ML (Figure 6) and 1/4 ML SB (Figure 1) structures become more stable than the clean surface. Within the accuracy of our calculations, the surface energies of these two 1/4 ML configurations overlap, and hence, the phase diagram indicates that either or both of these phases could be formed under UHV conditions (p ≈ 1010 atm) at temperatures around 850 K. We would anticipate, however, that at such temperatures, kinetic processes (such as etching) are likely to occur, prohibiting these structures from being experimentally observed. In the range 2.15 < ΔμCl < 1.50 eV, the 1/2 ML SB c(2 2) structure (see Figure 1) becomes most stable. This agrees with the experimental observations of Stickney et al.,14 although the predicted SB adsorption site is different from the H adsorption site proposed by experiment. For the value of ΔμCl = 1.50 eV, the (2 3) 2/3 ML structure (see Figure 2) becomes more stable. This is also consistent with the work of Stickney et al.,14 who observed the (2 3) phase on Cu(110) when the coverage exceeded 1/2 ML. This (2 3) 2/3 ML structure remains stable up to ΔμCl = 1.22 eV, where the (1 4) 3/4 ML (Figure 2) and (1 4)-AR 3/4 ML (Figure 7) configurations become more stable. These two 3/4 ML phases were found to have virtually identical free surface energies. It is interesting to note that the (1 4)-AR structure has been observed experimentally23,24 when Cu(110) is immersed in HCl electrolyte solution, but to the best of our knowledge, only the c(2 2)-Cl 1/2 ML and (2 3)Cl 2/3 ML phases were detected for gaseous HCl.14 The most likely reason for this is that once the 2/3 ML configuration is obtained, to produce a further increase in the Cl concentration at the surface, the pressure must be increased, and more importantly, there must be room for an adsorbing molecule to dissociate at the surface. However, at 2/3 ML coverage, the majority of adsorption sites are now unavailable (see Figure 2) for diatomic molecules such as Cl2 or HCl to dissociate at the surface. As predicted by our calculations, however, if chlorine ions were adsorbed at the surface, then it would be expected that the (1 4) 3/4 ML structure would be observed. The 1 ML, H þ SB structure becomes the most stable structure above ΔμCl = 0.84 eV. It is worth noting that the (1 2)-MR subsurface Cl configuration at 1 ML coverage has a stability very similar to the 1 ML, H þ SB on-surface structure. However, because the stability curves of both configurations lie in the bulk CuCl stability area (ΔμCl > 1.15 eV), they can only be kinetically accessible. Following the work by Fishlock et al.43 on the adsorption of Br on the Cu(100) surface, in which a stable substitutional Br/Cu(100) configuration was found, we have also investigated the stability of structures that result from the substitutional adsorption of Cl atoms on Cu(110) at coverages of 1/4, 1/2, and 3/4 ML. We found, however, that none of these substitutional adsorption configurations form more stable structures than the adsorption configurations discussed above. This is consistent with DFT studies of Cl on the Cu(100)22 and Cu(111)20 surfaces, which found that substitutional adsorption yields less stable structures than on-surface configurations. 13416
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Figure 8. Surface free energy per unit surface area for the different Cl/Cu(110) structures as a function of the chlorine chemical potential. The horizontal solid black line represents the clean Cu(110) surface, and the vertical dotted brown line represents the chemical potential of CuCl bulk. The vertical dashed gray lines show the stability regions for [1] the clean surface, [2] the 1/4 ML (2 1)-MR and 1/4 ML SB structures, [3] the 1/2 ML SB structure, [4] the (2 3) 2/3 ML structure, and [5] the (1 4) and (1 4)-AR 3/4 ML structures.
Figure 9. Wulff construction of copper nanostructures in a chlorine environment. (a) Surface free energy per unit surface area of the low-index Cu(100), Cu(110), and Cu(111) surfaces as a function of chlorine chemical potential. (be) Predicted Cu nanostructure morphologies at different values of ΔμCl.
E. The Wulff Construction. The relative surface energetics of
different chlorinated Cu(hkl) surfaces were investigated using the Wulff construction. Such a construction allows the optimal shape of Cu nanostructures formed in a gaseous chlorine environment to be predicted. The experimental data suggests that the observed shapes of nanoparticles in a Cl environment are due mainly to the interaction of Cl with the low index Cu(111) and
Cu(100) surfaces.44 To verify this prediction, we have combined the minimum surface free energies for the chlorinated (110) surface discussed in this paper with previously calculated free energy curves for the chlorinated Cu(100)22 and Cu(111)20 surfaces. To incorporate the (111) surface, we have adopted the two lowest energy structures obtained by Peljhan and Kokalj,20 and recalculated the energies of these structures using DMol3. 13417
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The Journal of Physical Chemistry C √ √ A (5 5 1) k-point sampling set and a 3 3-R30° unit cell were used to perform these calculations on the (111) surface. The remaining parameters are similar to those described in section 2A. We have used the WINXMORPH program45,46 to produce the Wulff constructions47 for Cu nanostructures in a gaseous Cl environment. The obtained Wulf constructions as a function of Cl chemical potential are shown in Figure 9be. We have found that under very dilute conditions and, hence, large negative values of ΔμCl, such as 2.40 eV (see Figure 9b), the shape of a Cu nanoparticle is determined by the free energies of the clean Cu (100), (110), and (111) surfaces. At a higher chemical potential value (such as 2.00 eV), the three surfaces are chlorinated, and the (111) facets were found to dominate the shape of the formed particle (see Figure 9c). Beyond ΔμCl ≈ 1.60 eV, where the surface free energy for the (100) plane becomes the lowest (see Figure 9d), the (100) facets dominate in the crystallite structure. At ΔμCl = 1.17 eV and just prior to the copper chloride (CuCl) bulk phase, the (100) facet is clearly the predominant plane in the crystallite structure. This results in a cubical shape of the crystallite, as shown in Figure 9e. This agrees with the findings of Pileni and co-workers,44,48 who reported the formation of cubic nanostructures of copper when these crystals were grown in a Clh environment. It is also worth noting that a cubical nanostructure has also been predicted by Soon et al. for copper crystals in a high concentration of gaseous nitrogen.49 Beyond ΔμCl = 1.15 eV, bulk CuCl should be formed. We would thus anticipate that under these conditions, the Cu nanostructures would consist of bulk CuCl surfaces. Interestingly, our earlier results for Cl on Cu(100) showed that for ΔμCl g 1.15 eV, the 2 ML configuration with a structure resembling a CuCl bulklike phase22 is the most stable Cl/Cu(100) configuration.
’ CONCLUSIONS We have investigated the interaction of atomic chlorine with the Cu(110) surface by performing first-principles density functional and ab initio atomistic thermodynamics calculations. The c(2 2) structure was found to be the most stable configuration for coverages of e1/2 ML, in agreement with the experimental observations. However, we have found that the SB site is the most stable site for Cl adsorption, not the H site that has been previously proposed. We have also found, in agreement with experiment, that the (2 3) 2/3 ML and (1 4) 3/4 ML configurations represent stable structures for coverages higher than 1/2 ML. Finally, by comparing the free energies of the chlorinated (100), (110), and (111) surfaces as a function of Cl chemical potential and employing the Wulff construction, we have predicted the shape of copper nanostructures surrounded by a gaseous chlorine environment. We have found that with increasing Cl concentration, the (100) planes become predominant, resulting in a cubical shape for each nanoparticle. These results are also consistent with the experimental data. ’ AUTHOR INFORMATION Corresponding Author
*Phone: (þ61 2) 4921 5447. Fax: (þ61 2) 4921 6907. E-mail:
[email protected] ’ ACKNOWLEDGMENT I.A.S. acknowledges the award of a UNIPRS and UNRSC by the University of Newcastle and financial support from the Ellari Trading Establishment-Jordan. M.W.R., J.C.M., E.M.
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K., and B.Z.D. acknowledge the Australian Research Council (ARC) for support (Project no. DP0988907). The calculations were undertaken on the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government. Computational resources used in this work were also provided by Intersect Australia Ltd.
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