Adsorption of Atoms on Cu Surfaces: A Density Functional Theory

The surface Brillouin zone was sampled using a 4 × 6 special k point for Cu(111) and Cu(100) and a 4 × 4 special k point for Cu(110) with the p(3 ×...
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Langmuir 2007, 23, 4910-4917

Adsorption of Atoms on Cu Surfaces: A Density Functional Theory Study Xian-Yong Pang,† Li-Qin Xue,† and Gui-Chang Wang*,‡ College of Chemistry and Chemical Engineering, Taiyuan UniVersity of Technology, Taiyuan 030024, and Department of Chemistry, Nankai UniVersity, Tianjin 300071, People’s Republic of China ReceiVed October 22, 2006. In Final Form: January 30, 2007 The chemisorption of atoms (H, N, S, O, and C) on Cu surfaces has been systematically studied by the density functional theory generalized gradient approximation method with the slab model. Our calculated results indicate that the orders of the adsorption energy are H < N < S < O < C on Cu(111) and H < N < O < S < C on Cu(110) and Cu(100). Furthermore, the adsorption energies of the given atoms on Cu(100) are larger than those on Cu(111) and Cu(110). The preferred adsorption sites are a 3-fold hollow site on Cu(111) and a 4-fold hollow site on Cu(100), but the preferred adsorption sites on Cu(110) are different for different adatoms. The energy, as well as the geometry, is in good agreement with the experimental and other theoretical data. In addition, this study focuses on the electronic and geometric properties of the metal-atom (M-A) bond to explain the difference in adsorption energies among adatoms. A detailed investigation of the density of states curves explains the nature of the most stable site. Finally, we test the effect of the coverage and find that the surface coverage has no influence on the preferred adsorption sites of the given adatoms on Cu(110) with the exception of hydrogen and oxygen, but has much influence on the value of the adsorption energy.

1. Introduction Copper is one of the best catalysts in the process of heterogeneous catalyst reactions in some chemical industries, such as WGS (water-gas shift), the potential usefulness of CO2 as a reactant in hydrocarbon synthesis, and steam re-forming of methanol, and it also plays an important role in the electrochemistry field.1-5 Furthermore, the chemisorption of adatoms (H, N, S, O, C) takes an important role in the reactions of the chemical industry, which are contained in the reactions of hydrocarbon production, ammonia synthesis, oxidation, corrosion, and petroleum re-forming.6-10 In addition, the adsorption behavior of adatoms on the metal surfaces is useful information to comprehend many electrochemically and heterogeneously catalyzed reactions.1 The intrinsically fundamental interest of the interaction of adatoms with metal surfaces is relevant to catalytic chemistry. The interaction of atomic species with copper single-crystal surfaces has been investigated using a wide variety of techniques and theoretical calculation methods in the past few years. The overlayer structure of hydrogen on Cu(111) has been studied using reflection-absorption infrared spectroscopy (RAIRS),11 low-energy electron diffraction (LEED),11 high-resolution elec* To whom correspondence should [email protected]. † Taiyuan University of Technology. ‡ Nankai University.

be

addressed.

E-mail:

(1) Rhodin, T. N.; Ertl, G. The Nature of the Surface Chemical Bond; NorthHolland: Amsterdam, 1979. (2) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (3) Ammon, Ch.; Bayer, A.; Held, G.; Richter, B.; Schmidt, Th.; Steinruck, H.-P. Surf. Sci. 2002, 507-510, 845. (4) Wschs, I. E.; Madix, R. J. J. Catal. 1978, 53, 208. (5) Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95, 190. (6) Besenbacher, F.; Nørskov, J. K. Prog. Surf. Sci. 1993, 44, 5. (7) Somorjai, G.A. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. (8) Gomer, R., Ed. Interactions on Metal Surface; Springer: Berlin, 1975. (9) Shustorovich, E. Surf. Sci. Rep. 1986, 6, 1. (10) Shustorovich, E.; Sellers, H. Surf. Sci. Rep. 1998, 31, 1. (11) Mccash, E. M.; Parker, S. F.; Pritchard, J.; Chesters, M. A. Surf. Sci. 1989, 215, 363.

tron energy loss spectroscopy (HREELS),12 and density functional theory (DFT) calculations.13,14 The interaction between a nitrogen atom and a Cu(111) surface has been observed with scanning tunneling microscopy (STM),15 DFT calculations,16 etc. Adsorbed sulfur atom on Cu(111) has been studied with LEED,17 STM,18-20 normal-incidence X-ray standing wavefield (NIXSW),21 and surface-extended X-ray absorption fine structure (SEXAFS)22 techniques. The chemisorption of oxygen on Cu(111) has been studied by UV photoelectron spectroscopy (UPS),23 STM,24 Auger electron spectroscopy (AES),23 and LEED.23,25,26 In addition, atomic hydrogen adsorption on Cu(100)1,27 and Cu(110)28,29 has been investigated by HREELS, LEED, STM, DFT calculations, etc. The chemisorption of atomic sulfur and oxygen on Cu(100)25-27,30-32 and Cu(110)25,33 has been studied (12) Lee, G.; Plummer, E. W. Surf. Sci. 2002, 498, 229. (13) Shustorovich, E. AdV. Catal. 1990, 37, 101. (14) Nobuhara, K.; Nakanishi, H.; Kasai, H.; Okiji, A. Surf. Sci. 2001, 493, 271. (15) Silva, S. L.; Leibsle, F. M. Surf. Sci. 1999, 441, L904. (16) Biemolt, W.; Jansen, A. P. J.; Neurock, M.; van de Kerkhof, G. J. C. S.; van Santen, R. A. Surf. Sci. 1993, 287-288, 183. (17) Saidy, M.; Mitchell, K. A. R. Surf. Sci. 1999, 441, 425. (18) Wang, D.; Xu, Q. M.; Wan, L.; Wang, C.; Bai, C. Surf. Sci. 2002, 499, L159. (19) Domange, J. L.; Ouder, J. Surf. Sci. 1968, 11, 124. (20) Driver, S. M.; Woodruff, D. P. Surf. Sci. 2001, 479, 1. (21) Jackson, G. J.; Driver, S. M.; Woodruff, D. P.; Cowie, B. C. C.; Jones, R, G. Surf. Sci. 2000, 453, 183. (22) Prince, N. P.; Seymour, d. L.; Ashwin, M. J.; McConville, C. F.; Woodruff, D. P. Surf. Sci. 1990, 230, 13. (23) Spitzer, A.; Lu¨th, H. Surf. Sci. 1982, 118, 136. (24) Matsumoto, T.; Bennett, R. A.; Stone, P.; Yamada, T.; Domen, K.; Bowker, M. Surf. Sci. 2001, 471, 225. (25) Simmons, G. W.; Mitchell, D. F.; Lawless, K. R. Surf. Sci. 1967, 8, 130. (26) McDonnell, L.; Woodruff, D. P. Surf. Sci. 1974, 46, 505. (27) Lee, R. N.; Farnsworth, H. E. Surf. Sci. 1965, 3, 461. (28) Hayden, B. E.; Lackey, D.; Schott, J. Surf. Sci. 1990, 239, 119. (29) Jacobsen, K. W.; Norskov, J. K. Phys. ReV. Lett. 1987, 59, 2764. (30) Kittel, M.; Polcik, M.; Terborg, R.; Hoeft, J.-T.; Baumga¨rtel, P.; Bradshaw, A. M.; Toomes, R. L.; Kang, J.-H.; Woodruff, D. P.; Pascal, M.; Lamont, C. L. A.; Rotenberg, E. Surf. Sci. 2001, 470, 311. (31) Habraken, F. H. P. M.; Mesters, C. M. A. M.; Bootsma, G. A. Surf. Sci. 1980, 97, 264. (32) Noonan, J. R.; Zehner, D. M.; Jenkins, L. H. Surf. Sci. 1977, 69, 731.

10.1021/la063097x CCC: $37.00 © 2007 American Chemical Society Published on Web 03/28/2007

Adsorption of Atoms on Cu Surfaces

with a variety of experiments and theoretical methods, such as thermal desorption electron spectroscopy (TDS), UPS, X-ray photoelectron spectroscopy (XPS), AES, LEED, HREELS, DFT, the linear muffin-tin orbital (LMTO) technique, and so on. For atomic nitrogen and carbon, the overlayer structures on Cu(100)27,34-37 and Cu(110)35,38 surfaces have been investigated by LEED, the extended electron energy loss fine structure (EXELFS) technique, STM, XPS, AES, TDS, and the ab initio cluster model. Rhodin and Ertl1 have reported that the adsorption site of an atom on a metal surface usually leans to the high-symmetry adsorption site. Koper et al.39 have also detected that O and OH radicals are first adsorbed on the 3-fold hollow site on some metal surfaces. Generally, an adatom occupies the highest coordination hollow sites on the low Miller index single-crystal metal surfaces, because adatoms there could touch more metal atoms to gain the largest adsorption energy. Mavrikakis et al.40-42 have reported atomic and molecular adsorption on Ir(111), Rh(111), and Pt(111). In our previous work,43 adatoms of C, H, O, and S on Cu(111) have been systematically investigated by DFT calculations with the cluster model, the results of which agreed well with the experimental data. However, questions remain: What are the preferred adsorption sites for the given atoms on Cu(111), Cu(100), and Cu(110)? What are the main factors to affect the preferred site? In the present work, the adsorption actions of adatoms (H, N, O, S, C) on different single-crystal surfaces of copper (Cu(111), Cu(100), Cu(110)) have been systematically investigated by the DFT method with the periodic slab model. 2. Computational Methods and Models All the calculations are implemented with a package “STATE” (simulation tool for atom technology) which has been successfully applied to the adsorption problem in the case of semiconductors and metal surfaces.44,45 Our calculations are based on a generalized gradient approximation46 in density functional theory (GGA-DFT) with the slab model. Also we used the Perdew, Burke, and Ernzerhof exchange-correlation functional47 as well as Vanderbilt’s ultrasoft pseudopotentials.48 The energy cutoffs of the plane wave basis sets are 25 and 400 Ry for the wave function and charge density, respectively. Different unit cells have been considered: p(3 × 2), p(2 × 2), p(2 × 1), and p(1 × 1) with corresponding coverages of 1/6, 1/4, 1/2, and 1 monolayer (ML), respectively. The vacuum region was defined at a 10 Å thickness between two successive slabs. The surface Brillouin zone was sampled using a 4 × 6 special k point for Cu(111) and Cu(100) and a 4 × 4 special k point for (33) Liem, S. Y.; Kresse, G.; Clarke, J. H. R. Surf. Sci. 1998. 415, 194. (34) Ricart, J. M.; Torras, J.; Rubio, J.; Illas, F. Surf. Sci. 1997, 374, 31. (35) Dai, Q.; Gellman, A. J. Surf. Sci. 1991, 248, 86. (36) Cohen, C.; Ellmer, H.; Guigner, J. M.; L’Hoir, A.; Pre´vot, G.; Schmaus, D.; Sotto, M. Surf. Sci. 2001, 490, 336. (37) Ellmer, H.; Repain, V.; Rousset, S.; Croset, B.; Sotto, M.; Zeppenfeld, P. Surf. Sci. 2001, 476, 95. (38) Baddorf, A. P.; Zehner, D. M. Surf. Sci. 1990, 238, 255. (39) Koper, M. T. M.; van Santen, R. A. J. Electroanal. Chem. 1999, 472, 126. (40) William, P.; Krekelberg, P.; Greeley, J.; Mavrikakis, M. J. Phys. Chem. B 2004, 108, 987. (41) Ford, D. C.; Xu, Y.; Mavrikakis, M. Surf. Sci. 2005, 587, 159. (42) Mavrikakis, M.; Rempel, J.; Greeley, J. J. Chem. Phys. 2002, 117, 6737. (43) Wang, G. C.; Jiang, L.; Cai, Z. S.; et al. J. Mol. Struct.: THEOCHEM 2002, 589-590, 371. (44) Morikawa, Y.; Iwata, K.; Nakamura, J.; Fujitani, T.; Terakura, K. Chem. Phys. Lett. 1999, 304, 91. (45) (a) Wang, G. C.; Zhou, Y. H.; Morikawa, Y.; Nakamura, J.; Cai, C. Z.; Zhao, X. Z. J. Phys. Chem. B 2005, 109, 65. (b) Wang, G. C.; Morikawa, Y.; Matsuoto, T.; Nakamura, J. J. Phys. Chem. B 2006, 110, 9. (c) Jun, L.; Li, R. F.; Wang, G. C. J. Phys. Chem. B 2006, 110, 14300. (46) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (47) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (48) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892.

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Figure 1. Constructions and adsorption sites of Cu(111), Cu(100), and Cu(110). t ) top site, br ) bridge site, 3h1 ) fcc, 3h2 ) hcp, 4h ) 4-fold hollow site, sb ) short bridge site, lb ) long bridge site, and 3h ) 3-fold hollow site. Red spheres denote first layers, yellow spheres denote second layers, and blue spheres denote third layers of copper surfaces. The hcp site resides above a subsurface atom in the second substrate layer, and the fcc site resides above a subsurface atom in the three substrate layer. Cu(110) with the p(3 × 2) unit cell. The structure was optimized until the force on the atom was smaller than 0.001 hartree/bohr. The adsorption energy or binding energy (Eads) is calculated according to the formula Eads ) Eclean + Eadsorbent - Eclean+adsorbent

(1)

where Eclean, Eadsorbent, and Eclean+adsorbent denote the calculated energy of a clean copper surface, the free adatom, and a copper slab with the adatom, respectively. The adsorption energies are verified to converge within 0.07 eV with respect to the k point sampling density and slab thickness.

3. Results and Discussion Generally, there are four different adsorption sites on Cu(111): the top site at which the adsorbent adsorbs above a surface atom, two 3-fold hollow sites (3h) including the “fcc site” (facecentered cubic) and the “hcp site” (hexagonal close-packed) (the hcp site resides above a subsurface atom in the second substrate layer, but the fcc site resides above a subsurface atom in the third substrate layer), and the bridge site (br), which lies halfway between the fcc and hcp sites. Furthermore, three different adsorption sites and five different adsorption sites exist on Cu(100) and Cu(110), respectively. These sites are schematically illustrated in Figure 1. In this work, it is hard to test the adsorption energies of adatoms on the top site of the Cu(110) surface, so we did not discuss the top site. This work is organized as follows. In section 3.1, we give the calculated results on the adsorption energy of atoms. In section 3.2, we discuss the reasons from the electronic effect and geometry effect points of view. We test the coverage effect on the adsorption energy in section 3.3. 3.1. Adsorption Energies of Atoms on Cu Surfaces. The calculation of adsorption energies is performed with the p(3 × 2) model with a corresponding coverage of 1/6 ML. The initial calculations are carried out with the adatoms relaxed and the three-layer metal atoms fixed in their bulk-truncated positions. These adsorption energy values on the preferred sites are almost repeated with the adatoms and the upper three layers relaxed and others fixed in the six-layer metal atom model. The calculated adsorption energies on the preferred adsorption sites are listed in Tables 1-3 and Figure 2, with the three-layer and the sixlayer models. The adsorption energies on all the adsorption sites with the three-layer model are listed in Tables 4 and 5 and Figures 3-5. 3.1.1. Adsorption of Atoms on Cu(111). The trend of the variation in the adsorption energies is listed in Figures 2a and 3-5 and the order of adsorption energies for all above adatoms on the Cu(111) surface is determined: top < br < hcp < fcc. One

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Table 1. Calculated Adsorption Energies and Structures of Atoms on the Preferred Adsorption Sites of the Cu(111) Surface Based on the Slab Model three-layer model a

six-layer model

adsorbate

Eads/eV

RA-Cu /Å

Eads/eV

RA-Cua/Å

H (fcc) N (fcc) S (fcc) O (fcc) C (fcc)

2.43 3.40 4.17 4.32 4.53

1.78 1.85 2.23 1.90 1.85

2.46 3.61 4.29 4.45 4.87

1.74 1.83 2.23 1.91 1.84

a RA-Cu is the nearest-neighbor bond between adatoms and the nearest copper atom.

Table 2. Calculated Adsorption Energies and Structures of Atoms on the Preferred Adsorption Sites of the Cu(100) Surface Based on the Slab Model three-layer model

six-layer model

adsorbate

Eads/eV

RA-Cu/Å

Eads/eV

RA-Cu/Å

H (4h) N (4h) O (4h) S (4h) C (4h)

2.45 4.57 4.86 4.90 6.15

1.87 1.89 1.87 2.29 1.88

2.43 4.59 4.88 4.89 6.13

1.95 1.92 1.89 2.28 1.91

Figure 2. Calculated results on the adsorption energies on the preferred adsorption sites. (a)-(c) show the adsorption energies of adsorbates on Cu(111), Cu(100), and Cu(110), respectively. Black lines denote the 3 × 2 cell with the three-layer model, and red lines denote the 3 × 2 cell with the six-layer model.

Table 3. Calculated Adsorption Energies and Structures of Atoms on the Preferred Adsorption Sites of the Cu(110) Surface Based on the Slab Model three-layer model

six-layer model

adsorbate

Eads/eV

RA-Cu/Å

Eads/eV

RA-Cu/Å

H (sb) N (lb) O (3h) S (4h) C (lb)

2.30 3.66 4.43 4.72 5.27

1.62 1.84 1.81 2.28 1.84

2.52 3.78 4.45 4.67 5.36

1.64 1.89 1.84 2.26 1.89

can see clearly that the preferred adsorption site is the fcc site. The order in adsorption energies on the preferred adsorption site is H < N < S < O < C. The least strong binding adatom is hydrogen, and the adsorption energy is 2.43 eV on the fcc site with the nearest-neighbor Cu-H distance of 1.78 Å (see Table 1). This adsorption energy agrees well with the experimental value of 2.43 eV.13 The corresponding perpendicular distance from the hydrogen to the first copper plane is 0.99 Å. Nobuhara et al.14 have also suggested that a H atom on Cu(111) is favored on the fcc site at 0.9 Å e z e 1.3 Å, especially at distance z ≈ 0.9 Å, which is based on the DFT calculation results and the model potential energy surfaces (PESs). For the most stable site of the fcc site, the adsorption energy with the six-layer model is determined to be 2.46 eV, with a nearestneighbor Cu-H distance of 1.74 Å, which have little difference from the calculated results from the three-layer model. For the nitrogen atom, the adsorption energy is 3.40 eV, with a nearest-neighbor Cu-N bond length of 1.85 Å. The calculated result for the nitrogen atom by the Amsterdam density functional (ADF) program is 3.51 eV by Biemolt et al.16,49 In addition, the adsorption energy calculated using the six-layer model is 3.61 eV, with a nearest-neighbor Cu-N bond length of 1.83 Å. The next strongest binding atom studied is sulfur. The adsorption energy on the most stable site is 4.17 eV, and the nearest-neighbor Cu-S bond length is 2.23 Å. According to STM, X-ray, LEED, and SEXAFS experiments,17-21 the preferred adsorption site for sulfur is the 3h site, which supported our (49) Neurock, M.; van Santen, R. A.; Biemolt, W.; Jansen, A. P. J. J. Am. Chem. Soc. 1994, 116, 6860. (50) Greeley, J.; Mavrikakis, M. J. Catal. 2002, 208, 291.

Figure 3. Adsorption energies of atomic adsorbates on Cu(111).

calculated results. Furthermore, the precise adsorption energy is 4.29 eV, with a nearest-neighbor Cu-S bond length of 2.23 Å. In the case of atomic sulfur adsorption on Cu(111), the predicted binding energy on the fcc site is 4.43 eV by Alfonso et al.51 Stronger adsorbing atomic oxygen is also displayed. The adsorption energy is 4.32 eV, with a nearest-neighbor Cu-O bond length of 1.90 Å on the perfect adsorption site, the fcc site. Mavrikakis et al.52,53 have reported that the best adsorption site for atomic oxygen on Cu(111) is the fcc site, and the adsorption energy is 4.30 eV, with a corresponding perpendicular distance of 1.19 Å. Experimental data54 from the SEXAFS method indicated that oxygen atoms are more closed to the relaxed Cu(111) surface on the 3h sites. The result for the preferred adsorption site with the six-layer model is 4.45 eV, with a nearest-neighbor Cu-O bond length of 1.91 Å. The strongest binding adatom investigated is carbon. The adsorption energy is 4.53 eV, with a nearest-neighbor Cu-C bond length of 1.85 Å. The adsorption energy on the 3h site of 4.53 eV is nontrivially greater than the calculated results of 3.89 eV55 from the cluster model. By using the six-layer model, the (51) Alfonso, D. R.; Cugini, A. V.; Sholl, D. S. Surf. Sci. 2003, 546, 12. (52) Xu, Y.; Mavrikakis, M. Surf. Sci. 2001, 494, 131. (53) Xu, Y.; Mavrikakis, M. Surf. Sci. 2003, 538, 219. (54) Haase, J.; Kuhr, H.-J. Surf. Sci. 1988, 203, L695.

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Table 4. Calculated Adsorption Energies of Atoms on All the Adsorption Sites of Cu(111) and Cu(100) with the p(3 × 2) Three-Layer Model adsorption site

Eads(H)/eV

Eads(N)/eV

Eads(O)/eV

Eads(S)/eV

Eads(C)/eV

top site of Cu(111) br site of Cu(111) hcp site of Cu(111) fcc site of Cu(111) top site of Cu(100) br site of Cu(100) 4h site of Cu(100)

2.01 2.29 2.42 2.43 1.83 2.36 2.45

1.36 2.93 3.30 3.40 1.37 2.87 4.57

2.70 3.87 4.27 4.32 2.78 3.97 4.86

3.00 3.95 4.15 4.17 3.04 3.91 4.90

2.68 4.06 4.44 4.53 2.77 4.08 6.15

Table 5. Distances of the Adsorbed Atom and Copper in Different Adsorption Cases on the Cu(110) Surfacea adatom and adsorption site H, 4h H, lb H, 3h H, sb N, sb N, 4h N, 3h N, lb O, sb O, 4h O, lb O, 3h S, sb S, lb S, 3h S, 4h C, sb C, 4h C, 3h C, lb

Eads/eV 2.03 2.24 2.24 2.30 2.73 3.33 3.54 3.66 4.07 4.17 4.26 4.43 3.82 4.29 4.67 4.72 4.01 4.69 4.80 5.27

RA-M-1/Å b

1.64 1.82 1.68 1.62b 1.77b 1.84b 1.89b 1.84b 1.81b 1.92b 1.90b 1.81b 2.14b 2.16b 2.28b 2.28b 1.80b 1.85b 1.89b 1.84b

n

RA-M-2/Å

n

RA-M-3/Å

n

RA-M-4/Å

n

1 2 2 2 2 1 2 2 2 1 2 2 2 2 1 1 2 1 1 2

2.26 1.86 1.88 2.92 3.09 2.30 1.93b 2.01b 3.14 2.32 2.23 2.05 3.51 2.77 2.43 2.44 3.13 2.30 1.94b 2.00b

4 2 1 2 2 4 1 2 2 4 2 1 2 2 2 4 2 4 2 2

3.06 3.16 2.97 3.79 4.00 3.17 2.77 3.17 4.06 3.21 3.20 2.92 4.23 3.36 2.46 3.44 4.05 3.17 2.69 3.17

2 4 2 2 2 2 2 4 2 2 4 2 4 4 2 2 4 2 2 4

3.68 3.20 3.19 3.89 4.03 3.84 3.22 3.36 4.07 3.91 3.60 3.29 4.35 4.58 3.44 4.30 4.06 4.09 3.19 4.16

4 2 2 4 4 4 2 2 4 4 2 2 4 6 2 2 4 2 2 4

a RA-M-x is the distance between the atom and copper metal, and n is the corresponding number. x ) 1 refers to the nearest-neighbor bond length, x ) 2 refers to the second-nearest-neighbor bond length, and so on. b The adatoms form bonds with the copper atoms.

adsorption energy is 4.87 eV and the corresponding nearestneighbor Cu-C bond length is 1.84 Å. Unfortunately, we did not get any experimental data for atomic carbon adsorption. The general binding trends for adatoms on Cu(111) are displayed in Figures 2a and 3 and Table 1. These results are in good agreement with the values from the bond order conservation (BOC) model,9 which describes the character of the adatoms. In addition, the values for atomic oxygen, hydrogen, and nitrogen adsorbed on the 3h sites are in good agreement with experimental11,12,15,23-26,54 and theoretical calculation results.20,22,49,50,52,53 In Figure 2a, one can see that the adsorption energies on the preferred adsorption site, the fcc site, between the three-layer and the six-layer models have little difference. Therefore, the size of the slab has little effect on the adsorption energies in the present study. 3.1.2. Adsorption of Atoms on Cu(100). Figures 2b and 4 and Table 2 exhibit the adsorption trend of adatoms (H, N, O, S, C) on Cu(100) and the calculated results of the adsorption energies and the optimized structures on the most stable adsorption sites. In Figure 4, the results show that the order of the adsorption energy for all the adatoms on Cu(100) is top < br < 4h. One can see clearly that the preferred adsorption site is the 4h site, and the order of adsorption energy here is H < N < O < S < C. The calculated adsorption energy of atomic hydrogen on the preferred site, the 4h site, is 2.45 eV, with a nearest-neighbor Cu-H distance in the chemisorbed structure of 1.87 Å. The (55) Au, C. T.; Ng, C. F.; Liao, M. S. J. Catal. 1999, 185, 12. (56) Mattsson, A.; Panas, I.; Siegbahn, P.; Wahlgren, U.; Akeby, H. Phys. ReV. B 1987, 36, 7389.

adsorption energy obtained with the six-layer model is 2.43 eV, and the nearest-neighbor Cu-H distance is 1.95 Å (see Table 2). For nitrogen, the adsorption energy on the best adsorption site, the 4h site, is 4.57 eV, with a nearest-neighbor Cu-N bond length of 1.89 Å. According to the EXELFS technique, STM, HREELS, and spot profile analyzing low-energy electron diffraction (SPA-LEED),27,35-37,57 the 4h site is the best adsorption site, which supports our calculated results. The adsorption energy obtained from the six-layer model is 4.59 eV, and the nearestneighbor Cu-N bond length is 1.92 Å. The next atom investigated is oxygen. In Table 2, the adsorption energy is 4.86 eV, with a nearest-neighbor Cu-O bond length of 1.87 Å on the 4h site, which is the preferred adsorption site for oxygen. The adsorption energy calculated from the six-layer model is 4.88 eV, and the nearest-neighbor Cu-O bond length is 1.89 Å. Many experimental results for the adsorption of oxygen on copper single-crystal surfaces studied by HREELS,57 LEED,32 and AES58 have concluded that the hollow site is the preferred adsorption site. The more strongly bound atom sulfur is studied. The preferred adsorption site is the 4h site, and the adsorption energy is 4.90 eV, with a nearest-neighbor Cu-S bond length of 2.29 Å. Noonan et al.32 have tested the preferred adsorption site by AES and LEED techniques, and their study supports our results. Using the large slab model with the six-layer model, the adsorption energy on the 4h site is 4.89 eV, with a nearest-neighbor Cu-S bond length of 2.28 Å. (57) Mohamed, M. H.; Kesmodel, L. L. Surf. Sci. Lett. 1987, 185, L467. (58) Spitzer, A.; Lu¨th, H. Surf. Sci. 1982, 118, 121.

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Figure 4. Adsorption energies of atomic adsorbates on Cu(100).

Figure 5. Adsorption energies of atomic adsorbates on Cu(110).

The most strongly bound atom investigated is carbon. On the most favorable adsorption site, the 4h site, the adsorption energy is 6.15 eV and the nearest-neighbor Cu-C bond length is 1.88 Å. In Table 2, the adsorption energy calculated from the sixlayer model is 6.13 eV, and the nearest-neighbor Cu-C bond length is 1.91 Å. Unluckily, there are few experimental and theoretical data for us to compare. The general binding trends for adatoms can be seen from Figures 2b and 4 and Table 2. In Figure 4, there is a distinct trend for the preferred adsorption sites and the order of adsorption energies. The adsorption energies of adatoms calculated for the preferred adsorption site by using the six-layer model on Cu(100) are larger than those on Cu(111) with the exception of hydrogen. We will discuss the reason in the following paragraph. 3.1.3. Adsorption of Atoms on Cu(110). The calculated results of atomic adsorptions on Cu(110) are listed in Figures 2c and 5 and Table 3. In Figure 5, one can see that the general trend in adsorption energies among the adatoms is not obvious. The following orders of adsorption energies for each adatom are different. The preferred adsorption site of carbon and nitrogen is the 1b site; the 3h site is the preferred adsorption site of oxygen; the 4h site is the preferred site of sulfur and the sb site is the best adsorption site for hydrogen. The order of adsorption energies on the preferred adsorption site is H < N < O < S < C. For hydrogen, the order of adsorption energies is 4h < lb ≈ 3h < sb. The preferred adsorption site is the sb site, and the adsorption energy is 2.30 eV, with a nearest-neighbor Cu-H

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bond length of 1.62 Å, while the calculated adsorption energy using the six-layer model is 2.52 eV, and the nearest-neighbor Cu-H bond length is 1.64 Å (see Table 3). The results from other DFT theory calculations by Bae et al.59also have suggested that the sb site of the Cu(110) surface is the preferred adsorption site, and the corresponding adsorption energy is 2.37 eV, which supports our calculated results. Our calculated results are also in good agreement with the data of experiments using HREELS, which reported that the sb site is the most stable adsorption site.60 The next least strongly bound atom is nitrogen. The preferred adsorption site was the lb site, with an adsorption energy of 3.66 eV and a nearest-neighbor Cu-N bond length of 1.84 Å. In Figure 5, the order of adsorption energies is sb < 4h < 3h < lb. In addition, the adsorption energy of the lb site from the six-layer model is 3.78 eV, and the shortest Cu-N bond length is 1.89 Å. The N-Cu bond length is 1.81 Å for nitrogen adsorbing on the Cu(110) surface determined by the EXELFS technique.35 For the oxygen adatom, the order of adsorption energies is sb < 4h < lb < 3h. The preferred adsorption site is the 3h site, and the adsorption energy is 4.43 eV, with a nearest-neighbor Cu-O bond length of 1.81 Å. The adsorption energy of the 3h site calculated from the six-layer model is 4.45 eV, and the nearestneighbor Cu-O bond length is 1.84 Å. The calculated results including the adsorption energy and structures are all in agreement in with the previous DFT calculation results by Liem et al.33 Spitzer et al.58 and Simmons et al.25 have also reported that the 3h site of the Cu(110) surface is the best adsorption site of oxygen by the LEED technique. Stronger binding to Cu(110) is displayed by sulfur. The most favorable adsorption site is the 4h site, with an adsorption energy of 4.72 eV and a nearest-neighbor Cu-S bond length of 2.28 Å. In Figure 5, the order of adsorption energies is sb < lb < 3h < 4h. The adsorption energy on the 4h site obtained from the six-layer model is 4.67 eV, and the nearest-neighbor Cu-S bond length is 2.26 Å. The most strongly bound atom investigated on Cu(110) is carbon. The preferred adsorption site is the lb site, with an adsorption energy of 5.27 eV and a nearest-neighbor Cu-C bond length of 1.84 Å. The order of adsorption energies is sb < 4h < 3h < lb, which is similar to the order for nitrogen. In addition, the adsorption energy on the lb site from the six-layer model is 5.36 eV, and the nearest-neighbor Cu-C bond length is 1.89 Å. The general binding trends for adatoms are exhibited in Figures 2c and 5. The order of adsorption energies on the best adsorption site is H < N < O < S < C. Atomic oxygen prefers the 3h site. The sulfur atom likes to be adsorbed on the 4h site. Other adatoms incline to the bridge site. Atomic carbon and nitrogen favor the lb site, and atomic hydrogen prefers the sb site. Furthermore, the adsorption energies of the preferred adsorption site on Cu(110) are between those on Cu(111) and those on Cu(100) except for atomic hydrogen. For hydrogen, the order of the adsorption energies from the six-layer model is Cu(100) < Cu(111) < Cu(110). 3.2. Discussion of the Influence of Electronic and Geometrical Factors on the Atomic Preferred Adsorption Site. In this section, we attempt to explore the reason for the preferred adsorption site of an atom from both the electronic and geometrical factors. First, the adsorption of atomic carbon is chosen as an example to analyze the electronic effect. For atomic carbon adsorbed on (59) Bae, C. S.; Freeman, D. L.; Doll, J. D.; Kresse, G.; Hafner, J. J. Chem. Phys. 2000, 113, 6926. (60) Astaldi, C.; Bianco, A.; Modesti, S.; Tosatti, E. Phys. ReV. Lett. 1992, 68, 90.

Adsorption of Atoms on Cu Surfaces

Figure 6. PDOS for the carbon atom (p bands) and Cu(110) firstlayer atoms (d bands) on the lb, 3h, 4h, and sb sites from top to bottom. The dotted lines denote the centers of the energy bands.

Cu(110), we calculate the projected density of states (PDOS) of atomic carbon adsorbed on the sb, 4h, 3h, and lb sites and compare the atomic carbon p bands with the copper d bands for various adsorption configurations. In Figure 6, the adsorption configuration of atomic carbon on the lb site of Cu(110) gives the best

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overlap between the carbon p band and copper d band centers because of the closest distance between these two band centers (i.e., -4.99 eV vs -6.42 eV), and the adsorption energy on the lb site is also the largest one, so one may conclude that, on the same single-crystal surface, the smaller the distance between these two band centers, the more stable the adsorption. Second, we attempt to explore the reason for the preferred adsorption site from the geometric effect. It may be assumed that if the atom can form more bonds with metal atoms at the given adsorption site, then its adsorption energy may be larger than that of other adsorbed sites. For the bond formation, it is assumed that the distance is not larger than r ) rm + ra (r denotes the radius). Here, we take the Cu(110) surface, for example. In Table 5, we list the data for the distance between adatoms and the copper atoms for the various cases. For the case of atomic hydrogen adsorption, hydrogen forms only one bond on the 4h site, and the corresponding nearest-neighbor bond length is 1.64 Å. Furthermore, it does not form any bonds on the 3h and the lb sites, because of the larger distances between atomic hydrogen and the nearest-neighbor copper of 1.68 and 1.82 Å, respectively. However, atomic hydrogen can form bonds with two metal atoms on the sb site, and the nearest-neighbor bond lengths are 1.62 Å. Since atomic hydrogen can form more bonds with metal atoms on the sb site, the preferred site for hydrogen is the sb site. Next, atomic nitrogen on the sb site forms bonds with two metal atoms, and the bond length is 1.77 Å, while the distance between nitrogen and the two next closest copper atoms is 3.09 Å. Meanwhile, nitrogen on the 4h site forms a bond with only one metal atom, and the bond length is 1.84 Å. However, there are four other copper atoms located nearby with a distance of 2.30 Å, which makes, overall, nitrogen adsorbed on the 4h site more stable than on the sb site. When nitrogen adsorbs on the 3h site, it forms bonds with three copper atoms, including two first-layer metal atoms and one second-layer metal atom. The bond lengths are 1.89 and 1.93 Å, respectively. The nitrogen atom on the lb site forms bonds with four copper atoms, including two first-layer metal atoms and two second-layer metal atoms, and the bond lengths are 1.84 and 2.01 Å, respectively, so the lb site is overall more stable than the other sites. In the case of atomic oxygen, the adsorption energy order is different from those of the other atoms. It forms bonds with two metal atoms on the sb site, and the bond length is 1.81 Å, but it hardly forms bonds with other metal atoms, because the distance between them is too great. When atomic oxygen adsorbs on the 4h site, it forms only one bond with the second-layer metal atom, and the corresponding bond length is 1.92 Å, but there are four other metal atoms nearby, 2.32 Å away, which makes oxygen adsorbed on the 4h site more stable than on the sb site. On the lb site, oxygen adsorbed forms bonds with two metal atoms, and the corresponding bond length is 1.90 Å. However, there are also two second nearest-neighbor metal atoms located 2.23 Å away, which makes this site more stable. In the case of oxygen adsorbed on the 3h site, the oxygen atom forms bonds with two first-layer metal atoms, and the corresponding bond length is 1.81 Å. There is also one copper atom nearby, 2.05 Å away, which is the shortest distance among the oxygen adsorption cases. Therefore, the 3h site for atomic oxygen is the preferred adsorption site. The sulfur atom forms bonds with two metal atoms on the sb site and the lb site with corresponding bond lengths of 2.14 and 2.16 Å, respectively, but the nearer-neighbor distance between atomic sulfur and metal atoms on the lb site is 2.77 Å, which is shorter than on the sb site. Therefore, the adsorption energy of sulfur on the lb site is larger than on the sb site. In addition, it forms only one bond on the 3h site with a corresponding bond

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Figure 7. Adsorption energies of the S atom adsorbed on a series of adsorption sites of Cu(110) at a variety of coverages.

length of 2.28 Å. However, the second nearest-neighbor distance is 2.43 Å, and there are two second nearest-neighbor metal atoms, which makes sulfur adsorbed on the 3h site more stable than on the sb site and the lb site. On the 4h site, the sulfur atom forms a bond with one metal atom, and the bond length is 2.28 Å. There are four second nearest-neighbor metal atoms, and the corresponding length is 2.44 Å. Therefore, the 4h site is the preferred site of the sulfur atom. The case of carbon is similar to that of atomic nitrogen. On the sb site, carbon forms bonds with two copper atoms, and the bond length is 1.80 Å. However, other metal atoms are too far away to form bonds. On the 4h site, atomic carbon forms only one bond, and the bond length is 1.85 Å. There are four second nearest-neighbor metal atoms located at a distance of 2.30 Å. When carbon adsorbs on the 3h site, it forms bonds with three

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metal atoms, and the bond lengths are 1.89, 1.94, and 1.94 Å, respectively. The two second nearest-neighbor metal atoms are 2.69 Å away, which form a bond with the carbon atom. On the lb site, the carbon atom forms bonds typically with four metal atoms, including two first-layer metal atoms and two secondlayer metal atoms, and the bond lengths are 1.84 and 2.00 Å, respectively. Therefore, the lb site is the preferred adsorption site. From the above analysis, if the adatoms can form bonds with more metal atoms and have more metal atoms nearby, the adsorption ability is greater. 3.3. Discussion about the Coverage Effect. In this section, we test the adsorption energy of adatoms on Cu(110) with a variety of coverages in the three-layer model, and the calculated results are listed in Figures 7 and 8. In Figure 7, the adsorption energies of atomic sulfur at a series of coverages from 1 to 1/6 ML are first listed. At the lower coverage (1/6 ML) the order of adsorption energies on different sites is sb < lb < 3h < 4h, and the adsorption energies are lower than those at the 1/4 ML coverage. At the 1/4 ML coverage, the order of adsorption energies on different sites is similar to that at the low coverage; however, the adsorption energies are largest among the coverages investigated here. At the 1/2 ML coverage, the order has no change, and the adsorption energies are larger than those at the high coverage. At the high coverage, the order is also sb < lb < 3h < 4h, while the adsorption energy is the lowest. Anyway, the order for sulfur atom on Cu(110) with a series of coverages is the same. The calculated results indicate that the coverage has no effect on the order of adsorption energies on different sites, but does have an effect on the values of the adsorption energy. In addition, the calculated results for other adatoms are listed in Figure 8. The order of adsorption energies on different sites for all adatoms has no change except for hydrogen and oxygen. Figure 8a indicates that different coverages have an effect on the

Figure 8. Adsorption energies of adatoms adsorbed on a series of adsorption sites of Cu(110) at two coverages: (a) hydrogen, (b) nitrogen, (c) oxygen, and (d) carbon.

Adsorption of Atoms on Cu Surfaces

order for hydrogen. At the 1/4 ML overage, the order of the adsorption energies on different sites is 3h > sb > lb > 4h. However, at all other coverages, the order charges and is the same, i.e., sb > 3h > lb > 4h. At the 1/4 ML coverage, the values of the adsorption energies are the largest. At the 1/6 ML coverage, the values of the adsorption energies are the smallest. At the high coverages of 1/2 and 1 ML, the values are in the middle. The adsorptions of nitrogen and carbon are similar (Figure 8b,d). The general trend of the binding strength is obvious. The coverage has no effect on the order of adsorption energies on different sites, but adsorption energies at the 1 ML coverage are the smallest, and those at the 1/2 ML coverage on the lb site are the largest, which is different from hydrogen adsorption. For the oxygen atom, in Figure 8c, the difference in the coverages does have an effect on the order. At the lower coverage, the preferred adsorption site is the 3h site, but at the higher coverage, the preferred adsorption site is the lb site, which is in good agreement with calculation results by Liem et al.33

4. Conclusions The chemisorption of atoms (H, N, O, S, C) on Cu surfaces has been systematically investigated. DFT-GGA and slab model calculations have been performed to decide the favorable adsorption sites, the chemisorbed structures, the adsorption energies, and the coverage effect. The order of adsorption energies is determined: H < N < S < O < C on Cu(111), H < N < O < S < C on Cu(100), and H < N < O < S < C on Cu(110). The adatoms show a preference for the fcc site on Cu(111). A

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preference for the 4h site for the adatoms is observed on Cu(100). The preference sites for adatoms on Cu(110) are complex: the preference for the lb site for atomic carbon and nitrogen is observed, the preferred adsorption site of the sulfur adatom is the 4h site, the preferred adsorption site for oxygen is the 3h site, and the favorable adsorption site for hydrogen is the sb site. All the calculated results compare well with the experimental and theoretical results. In addition, the order of adsorption energies and the preferred adsorption site can be well explained by the DOS and the geometric analysis. That is, the greater the overlap between the atomic p band and the copper d band centers, the stronger the adsorption. When the adatom forms more bonds with copper atoms and has more copper atoms nearby, the adsorption ability will be stronger. Finally, the coverage effect on the order of adsorption energies on different sites for all adatoms investigated has been examined. The results show that it has little effect on the order, except for hydrogen and oxygen atoms, whose preferred adsorption sites change from the 3h site at the 1/4 ML coverage to the sb site at other coverages for the hydrogen atom and from the 3h site at the low coverage to the lb site at the high coverage for the oxygen atom. The results overall agree well with other calculation results and experimental data. Acknowledgment. This work was supported by the National Science Foundation of China (Grant Nos. 20273034 and 20673063) and the NKStar HPC program. LA063097X