Surface Functionalization of Ni(111) with Acrylate Monolayers

Boon-Siang Yeo, Zhi-Hua Chen, and Wee-Sun Sim*. Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260, Singapore...
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Langmuir 2003, 19, 2787-2794

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Surface Functionalization of Ni(111) with Acrylate Monolayers Boon-Siang Yeo, Zhi-Hua Chen, and Wee-Sun Sim* Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260, Singapore Received October 14, 2002. In Final Form: December 30, 2002 The adsorption and reactivity of acrylic acid on clean and preoxidized Ni(111) have been examined under ultrahigh vacuum (UHV) conditions using reflection absorption infrared spectroscopy (RAIRS). On clean Ni(111), acrylic acid molecules that adsorb as dimers at 140 K subsequently decompose between 240 and 310 K into surface-bound acrylate species and η1(O)-acrylic acid monomers attached to the surface through the lone pair electrons of the carbonyl O atom. On Ni(111)-p(2 × 2)-O, facile deprotonation of acrylic acid occurs at 240 K to yield upright symmetrically bound acrylate species. Increasing the coverage results in tilting of the acrylate species as a consequence of steric hindrance and repulsive dipolar interactions. An alternative synthetic route to stable surface-bound acrylate monolayers has also been achieved through O-induced nucleophilic substitution of acryloyl chloride on Ni(111)-p(2 × 2)-O.

1. Introduction Functionalization of metal surfaces via the modification of both their physical and chemical properties can be effected through the chemisorption of a wide range of organic molecules. Such hybridized structures have attracted great attention lately because of their promising utility in the development of optoelectronic devices, molecular sensors, and thin film polymer coated surfaces that are useful in areas that require protection against corrosion. Bifunctional molecules are particularly valuable in the pursuit of such an endeavor as one functionality can be used to form the chemisorption bond while the other can be selectively modified through functional group interconversion or further covalent bond formation with another different molecule.1,2 An excellent example of such an adsorbate is acrylic acid (CH2dCHCOOH), which undergoes reactions typical for both olefins and aliphatic carboxylic acids.3 Through conjugative and negative inductive effects of the carbonyl group, the β-C atom can act as an electrophile to aid in the addition of a large variety of nucleophiles to the vinyl group. The CdC bond can also participate in free radicalinitiated addition, Diels-Alder, as well as polymerization reactions. The carboxyl group is subject to reactions characteristic of carboxylic acids such as nucleophilic acyl substitution, reduction, and deprotonation. Compared to its simpler counterparts such as formic and acetic acids, there have been relatively few adsorption studies of acrylic acid on metal and metal oxide singlecrystal surfaces under ultrahigh vacuum (UHV) conditions. Acrylate (CH2dCHCOO) species have been identified on TiO2(100),4 Cu2O(100),5 Pd(111),6 and polycrystal* Corresponding author. E-mail: [email protected]. (1) Richardson, N. V.; Frederick, B. G.; Unertl, W. N.; El Farrash, A. Surf. Sci. 1994, 307-309, 124. (2) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (3) Ohara T.; Sato, T.; Shimizu, N.; Prescher, G.; Schwind, H.; Weiberg, O.; Marten, K. In Ullman’s Encyclopedia of Industrial Chemistry, 5th ed.; Gerhertz, W., Ed.; VCH: Weinheim, 1985; p 161. (4) Titheridge, D. J.; Barteau, M. A.; Idriss, H. Langmuir 2001, 17, 2120. (5) Schulz, K. H.; Cox, D. F. J. Phys. Chem. 1992, 96, 7394. (6) Davis, J. L.; Barteau, M. A. J. Mol. Catal. 1992, 77, 109.

line Al.7 Molecular acrylic acid is π-bonded via the CdO bond on Pt(111),8,9 while on evaporated Ag films, dative bonding to the surface through the lone pair electrons of the carbonyl O atom (hereafter referred to as η1(O)-acrylic acid) occurs initially, followed by the formation of flatlying H-bonded dimers at higher coverages.10 Transmission infrared spectroscopy, multiple reflection absorption infrared spectroscopy (MRAIRS), and inelastic electron tunneling spectroscopy (IETS) have also been employed to study the interaction of acrylic acid with oxide catalyst surfaces such as V2O5,11 SnO2,12 oxidized Al,13 and plasmagrown AlxOy,14 where acrylate species were the main adsorbate present. The purpose of this study is to use reflection absorption infrared spectroscopy (RAIRS) to investigate the adsorption behavior of acrylic acid on clean and preoxidized Ni(111) and to synthesize stable surface-bound acrylate species for detailed spectroscopic characterization using adsorbed O atoms as a deprotonating agent. Another possible reaction pathway for generating acrylate species has also been explored by using adsorbed O atoms to effect the nucleophilic substitution reaction of acryloyl chloride (CH2dCHCOCl). It can be envisaged that the free vinyl group of an upright-standing acrylate species produced by the above methodologies can be further reacted with other molecules to create surfaces with tailor-made functionalities. 2. Experimental Section The experiments were performed in a three-level UHV chamber with a base pressure of 450 K immediately prior to the start of each experiment to desorb any volatile impurities. To reliably assign the RAIR spectra, density functional theory (DFT) calculations have been performed to predict the vibrational frequencies of a series of model Ni complexes and molecules shown in Figure 1. The Becke-Perdew density functional model was used in conjunction with the DN** spin polarized numerical basis set in the Spartan molecular modeling program,16 and the Ni (15) Xu, Z.; Surnev, L.; Uram, K. J.; Yates, J. T. Surf. Sci. 1993, 292, 235. (16) Spartan, version 5.1; Wavefunction, Inc.: Irvine, California, 1998.

Figure 2. RAIR spectra of Ni(111)-p(2 × 2)-O exposed to increasing doses of acrylic acid at 240 K. atoms have been capped with CO ligands to simulate closed shell configurations that enable the structures to attain stable energyminimized molecular geometries. The raw-calculated frequencies agree sufficiently well with experimentally observed values and are as such presented without applying any scaling factors.

3. Results 3.1. Adsorption of Acrylic Acid on Ni(111)-p(2 × 2)-O at 240 K. The facile synthesis of acrylate monolayers on Ni(111)-p(2 × 2)-O is demonstrated in Figure 2, a and b, which shows the representative RAIR spectra of the surface dosed with 0.0025 and 0.01 L of acrylic acid, respectively, at 240 K. The complete absence of the normally strongly absorbing CdO stretch v(CdO) at ∼1760 cm-1 and bands characteristic of monomeric or dimeric acrylic acid17-19 provides strong evidence of the complete dissociative adsorption of acrylic acid. The resulting spectrum is instead fully consistent with that of a surface-bound acrylate species, with the strongest band at 1437-1442 cm-1 attributed to the coupled symmetric OCO stretch and CH2 deformation vs(OCO) + δ(CH2). The band at 1635 cm-1 is assigned to the CdC stretch v(CdC) and shows clearly that the π-electron system of the vinyl group remains intact and does not bind to the surface as any such interaction would result in a lowering of this frequency. The bands at 3105, 1371, 1277, and 1067 cm-1 are ascribed to the CH2 asymmetric stretch vas(CH2), another component of vs(OCO) + δ(CH2), (17) Feairheller, W. R., Jr.; Katon, J. E. Spectrochim. Acta, Part A 1967, 23, 2225. (18) Umemura, J.; Hayashi, S. Bull. Inst. Chem. Res., Kyoto University 1974, 52, 585. (19) Kulbida, A.; Ramos, M. N.; Rasanen, M.; Nieminen, J.; Schrems, O.; Fausto, R. J. Chem. Soc., Faraday Trans. 1995, 91, 1571.

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Table 1. Vibrational Frequencies and Mode Assignments for Acrylate assignment

CH2CHCOO Na salta (cm-1)

CH2CHCOO SnO2b (cm-1)

CH2CHCOO Pd(111)c (cm-1)

vas(CH2) v(CH) vs(CH2) v(CdC) vas(OCO) vs(OCO) + δ(CH2)

3085 3043 3012 1640 1540-1570 1450

1630 1510-1520 1425-1430

1430

vs(OCO) + δ(CH2) δ(CH) F(CH2) γ(CH) γ(CH2) v(CC) + δ(OCO) γ(OCO)

1368 1285 1055 991 952 902 837

1355-1365 1275 1050-1065 980 890 820

-

a

CH2CHCOO Ni(111) (cm-1) 3105 1635 1534 1437-1442 1462 1371 1277 1067 -

CH2CHCOONi2 calculated (cm-1) Cs(per) Cs(par) 3169 3088 3075 1640 1575 1424

3182 3124 3086 1637 1558 1399

1351 1269 1058 959 860 881 735

1331 1256 1049 992 918 886 800

Reference17. b Reference 12. c Reference 6.

C-H deformation δ(CH), and CH2 rock F(CH2), respectively. Further dosing of acrylic acid (>0.02 L) on the same surface yields several interesting features, with the band at 1442 cm-1 losing intensity while two new bands at 1462 and 1534 cm-1 appear (Figure 2c). The new bands are ascribed, respectively, to vs(OCO) + δ(CH2) and the asymmetric OCO stretch vas(OCO) of a second acrylate species, which is believed to be the consequence of steric and electronic effects that manifest themselves at high coverages. All the observed frequencies agree well with the vibrational spectra of the acrylate anion and acrylate species reported on metal and oxide surfaces, as well as the frequencies predicted by the DFT calculations as summarized in Table 1. The conversion of acrylic acid to acrylate should be accompanied by the stoichiometric transfer of protons to the O adatoms to yield surface OH groups. Our observed RAIR spectra, however, do not show any peaks that may be assigned to the O-H stretch v(OH) which is expected at ∼3600 cm-1. Previous RAIRS studies of formic acid adsorption on NiO(111)-p(2 × 2)/Ni(111)20 and H2O on NiO/Ni(110)21 indicate that v(OH) of OH groups typically appear at or below the noise level. It is also possible that the OH groups can further deprotonate more acrylic acid molecules and consequently desorb as H2O as has been observed for formic and acetic acid adsorption on a variety of preoxidized metal surfaces.22-30 3.2. Adsorption of Acryloyl Chloride on Ni(111)p(2 × 2)-O at 240 K. To verify the identity of the acrylate species on Ni(111), an alternative synthetic route using acryloyl chloride as the precursor molecule has been explored. Figure 3 shows the RAIR spectra obtained by exposing the Ni(111)-p(2 × 2)-O surface to acryloyl chloride at 240 K. It is immediately apparent that they are identical to the RAIR spectra obtained from acrylic acid adsorption. For an exposure of 0.0025 L (Figure 3a), the absorption (20) Bandara, A.; Kubota, J.; Wada, A.; Domen, K.; Hirose, C. J. Phys. Chem. 1996, 100, 14962. (21) Sanders, H. E.; Gardner, P.; King, D. A.; Morris, M. A. Surf. Sci. 1994, 304, 159. (22) Hung, W. H.; Bernasek, S. L. Surf. Sci. 1996, 346, 165. (23) Houtman, C. J.; Brown, N. F.; Barteau, M. A. J. Catal. 1994, 145, 37. (24) Davis, J. L.; Barteau, M. A. Surf. Sci. 1991, 256, 50. (25) Avery, N. R. J. Vac. Sci. Technol. 1982, 20, 592. (26) Sexton, B. A.; Madix, R. J. Surf. Sci. 1981, 105, 177. (27) Barteau, M. A.; Bowker, M.; Madix, R. J. J. Catal. 1981, 67, 118. (28) Xu, C.; Goodman, D. W. J. Phys. Chem. 1996, 100, 1753. (29) Sim, W. S.; Gardner, P.; King, D. A. J. Phys. Chem. 1996, 100, 12509. (30) Sim, W. S.; Gardner, P.; King, D. A. J. Am. Chem. Soc. 1996, 118, 9953.

Figure 3. RAIR spectra of Ni(111)-p(2 × 2)-O exposed to increasing doses of acryloyl chloride at 240 K.

bands in the RAIR spectrum can be readily assigned to the same acrylate species observed earlier (with vas(CH2) at 3105 cm-1, v(CdC) at 1635 cm-1, vs(OCO) + δ(CH2) at 1437 cm-1 and 1371 cm-1, δ(CH) at 1277 cm-1 and F(CH2) at 1067 cm-1). Increasing the exposure to >0.02 L (Figure 3b) causes the bands to increase in magnitude and attain saturation intensities that are identical in size to that observed for acrylic acid dissociation before the second acrylate species appears. There is no evidence of the second acrylate species being formed in this case, in contrast to the behavior for acrylic acid adsorption as described in the previous section. 3.3. Adsorption of Acrylic Acid on Clean Ni(111) between 140 and 310 K. The RAIR spectra of clean Ni(111) dosed with increasing quantities of acrylic acid at 140 K are shown in Figure 4. At an exposure of 0.0025 L

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Table 2. Vibrational Frequencies and Mode Assignments for Acrylic Acid Dimers

assignment

crystal (Raman)a (cm-1)

crystal (IR)b (cm-1)

2500-3200 1719, 1705 1679, 1676 v(CdC) 1657, 1637 1634, 1631, 1615 δ(COH) 1444 1441, 1435 δ(CH2) 1414 1395, 1382 vas(CCO) 1305, 1301, 1292, 1283 1323, 1301 δ(CH) 1252 1260, 1256 γ(COH) 1182, 1149 F(CH2) 1073 1078 γ(CH) 993 991 γ(CH2) 973 973, 895 v(CC) + δ(OCO) 867 875 γ(OCO) 818 813

vas(CH2) v(CH) vs(CH2) v(OH) v(CdO)

a

3108 3041 2996 1721

Reference 19. b Reference 18. c Reference 10.

d

dimer multilayer Ag multilayer monolayer multilayer calculated filmc (cm-1) Pt(111)d (cm-1) Ni(111) (cm-1) Ni(111) (cm-1) (cm-1) 1709

1704

1705

1635,1612 1443 1396,1385 1304 1259 1078, 1047 999 978, 916 817

1635 1445 1401 1310 1254 999 950, 917 -

1640, 1614 1411 1283 1207 1185 988 822

2500-3200 1708 1680 1638, 1614 1442 1396 1302 1255 1078 997 976, 911 818

3206, 3204 3156, 3132 3105, 3101 2778, 2604 1689 1615 1655, 1642 1479, 1450 1402, 1401 1310, 1303 1252, 1241 1076, 1034 1062, 1057 994, 991 969, 967 866, 854 810, 808

Reference 8.

Figure 4. RAIR spectra of clean Ni(111) exposed to increasing doses of acrylic acid at 140 K.

(Figure 4a), by comparison with the infrared spectrum of matrix-isolated acrylic acid,19 it can be concluded that the observed vibrational frequencies at 1163 and 1151 cm-1 originate from the coupled C-O stretching and COH deformation modes v(CO) + δ(COH) of the monomeric molecular species. Between 0.005 and 0.015 L exposure (Figure 4b and c), the presence of acrylic acid dimers is inferred from its characteristic vibrational frequencies at 1705, 1640, 1614, 1411, 1283, 1207, 1185, 988, and 822 cm-1as assigned in Table 2. With increasing coverage, the features due to acrylic acid dimers grow in size while absorption bands assigned to the monomers are gradually

attenuated. The simultaneous formation of surface-bound acrylate species throughout this exposure regime is also indicated by the observation of vs(OCO) + δ(CH2) at 14201432 and 1365 cm-1. As the exposure is increased to 0.065 L (Figure 4d), physisorbed acrylic acid multilayers are formed as demonstrated by the continuous increase in size of absorption bands characteristic of molecular acrylic acid dimers. These frequencies are virtually identical to those of the bulk crystalline dimeric species, which exist in the cis form,18,19 and to the multilayer vibrational spectra reported for acrylic acid dimers physisorbed on Pt(111)8 and evaporated Ag films.10 All the vibrational modes observed are listed and assigned in Table 2. Heating the multilayer-covered surface to 200 K results in the disappearance of features due to the physisorbed species (presumably caused by desorption) and leaves a RAIR spectrum essentially identical to that of a monolayer of surface-bound acrylate and chemisorbed acrylic acid dimers (Figure 5a). From 240 to 310 K, the RAIR spectra (Figure 5b and c) are characterized by the attenuation of the acrylic acid dimer bands and emergence of a new set of features at 1662, 1385, 1312, 1255, and 1145 cm-1. There is also a concurrent decrease in the sizes of the acrylate vs(OCO) + δ(CH2) bands at 1435 and 1367 cm-1 as the band at 1662 cm-1 grows to become the dominant feature in the RAIR spectrum. These new bands match perfectly with the infrared spectrum of η1(O)-acrylic acid on Ag.10 This is further corroborated by the vibrational frequencies predicted using DFT calculations for the model Ni complex with an η1(O)-acrylic acid ligand. Coordination of the lone pair electrons of the carbonyl O atom with surface Ni atoms weakens the CdO bond, and this is reflected in the lowering of v(CdO) from ∼1760 cm-1 in the free acrylic acid monomer to 1662 cm-1. The observed vibrational features are assigned and presented in Table 3. At first sight, it may also appear reasonable to ascribe these new bands to an acrylate species bonded in a different configuration, specifically, a monodentate moiety bound to the surface through only one O atom. This results in nonequivalence of the C-O bonds with one having much more double-bond character than the other. The reduction in surface symmetry of the fragment will also cause the vas(OCO) mode to have a component of its dynamic dipole moment perpendicular to the surface and it will thus become observable. However, the frequency of the band at 1662 cm-1 is on the high side compared to that usually

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Table 3. Vibrational Frequencies and Mode Assignments for η1(O)-Acrylic Acid

assignment v(OH) vas(CH2) v(CH) vs(CH2) v(CdO) v(CdC) δ(CH2) v(CO) + δ(COH) δ(CH) v(CO) + δ(COH) F(CH2) γ(CH) γ(CH2) v(CC) + δ(OCO) γ(OCO) a

CH2CHCOOH matrix-isolateda (cm-1) cis trans 3567, 3564, 3561 1764, 1763, 1746 1663, 1635, 1634 1412, 1411, 1408 1323 1252 1194, 1190, 1132, 1131 1061 994, 987 973, 971 830, 829 812

3576, 3572 1760, 1757, 1756 1651, 1626, 1624 1416 1330 1187, 1185 1019, 1017 999, 991 971, 968, 967 826 816

η1(O)-CH2CHCOOH Ag filmb (cm-1) 1678 1637, 1606 1421, 1389 1292 1234 977, 938 816 812

η1(O)-CH2CHCOOH Ni(111) (cm-1) 1662 1385 1312 1255 1145 -

η1(O)-CH2CHCOOHNi calculated (cm-1) cis trans 3680 3203 3108 3100 1699 1626 1405 1313 1268 1143 1054 988 959 838 773

3667 3185 3149 3089 1678 1632 1426 1309 1275 1171 1014 986 942 837 788

Reference 19. b Reference 10.

stable than their monodentate counterparts, presumably because of the enhanced surface-adsorbate interaction.22,26,31 There is thus no a priori reason monodentate acrylate should be formed at this higher temperature. High-resolution electron energy loss spectroscopy (HREELS) of acrylic acid adsorption on Pd(111) at 280 K yields a weak band at 1675 cm-1 that has been attributed to acrylate fragments bound almost parallel to the surface in an η5-(CCOCO) configuration.6 Such an assignment is unlikely as the in-plane vibration which gives rise to that band has a dynamic dipole moment parallel to the surfacesclearly not consistent with the large observed intensity of the 1662 cm-1 band in this case. On the basis of the above considerations, the likelihood of the new features being due to a second acrylate species is ruled out.

Figure 5. RAIR spectra of clean Ni(111) exposed to 0.065 L of acrylic acid at 140 K and then annealed to and maintained at the temperatures indicated.

observed for vas(OCO) of acrylate species (∼1500-1600 cm-1).7,11-14,17 Moreover, if there were such a large increase in the bond order of one C-O bond, it is reasonable to expect a corresponding decrease in the bond order of the other C-O bond. However, the RAIR spectra obtained show no other bands below 1400 cm-1 that can be reasonably assigned to vs(OCO) + δ(CH2) of monodentate acrylate. The new bands observed between 1100 and 1400 cm-1 are not consistent with the reported vibrational spectra of other acrylate species.7,11-14,17 Finally, a general trend observed with the absorption of organic acids such as formic and acetic acids on a variety of clean metal surfaces is that bridging carboxylates tend to be more

4. Discussion 4.1. Bonding Configurations of Surface-Bound Acrylate on Ni(111). It is important to first establish the bonding configuration of the acrylate species found in this work. To give an accurate description of the two forms of surface-bound acrylate identified on Ni(111)-p(2 × 2)-O at 240 K, an analysis of their infrared activity has been carried out. According to the metal-surface selection rule, only vibrational modes which belong to the totally symmetric representations, that is, A1, A′, or A, of the corresponding point group are infrared active.29,30,32 This implies that vibrational dipole excitations are limited to only dipole moment changes that possess a component normal to the surface. Rigorous application of the metalsurface selection rule usually involves a complete symmetry analysis and gives unambiguous bonding geometries for high-symmetry systems (Cnv), which often correspond to only one adsorbate orientation relative to the surface. However, for low-symmetry systems (Cs and C1), each symmetry group can correspond to several possible adsorbate configurations, making symmetry analysis alone inadequate, as is the case for surface-bound acrylate. In these situations, it is often more instructive to consider the directions of the dynamic dipole moments of the normal vibrations of the free adsorbate molecule relative to the surface plane under various bonding modes. If there is no significant coupling between the internal modes of the adsorbate and either the lattice or surface(31) Sexton, B. A. Surf. Sci. 1979, 88, 319. (32) Richardson N. V.; Sheppard, N. In Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T., Madey, T. E., Eds.; Plenum: New York, 1987; p 1.

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Table 4. Polarization States of the Dynamic Dipole Moments of the Fundamental Internal Modes of Acrylate Conformers (Reference Axes: z along the C-C Bond, y Perpendicular to the O-C-O Plane, x Parallel to the O-C-O Plane) polarization of dynamic dipole moment mode

Cs(par)

vas(CH2) v(CH) vs(CH2) v(CdC) vas(OCO) vs(OCO) δ(CH2) δ(CH) F(CH2) γ(CH) γ(CH2) δ(OCO) v(CC) γ(OCO)

x+z x+z x+z x+z x z x+z x+z x+z y y z z y

Cs(per) y+z y+z y+z y+z x z y+z y+z y+z x x z z y

C1 x+y+z x+y+z x+y+z x+y+z x z x+y+z x+y+z x+y+z x+y x+y z z y

adsorbate vibrations, then these predictions will reflect the true dipole moment changes for the surface-adsorbate complex. The free acrylate anion has one plane of symmetry that lies either parallel (Cs(par)) or perpendicular (Cs(per)) to the OCO plane, depending on the relative orientation of the vinyl group. Both energy-minimized conformations are essentially energetically equivalent according to our DFT calculations, although anecdotal evidence from the literature favors the parallel conformation. Rotation of the vinyl group such that the dihedral angle between the CdC bond and the OCO plane is between 0 and 90° will then reduce the symmetry to C1. Table 4 lists the directions of the dynamic dipole moments of the normal modes of each conformation of the acrylate species with respect to its three orthogonal axes. For an acrylate species symmetrically bound to a metal surface in an upright configuration, only modes whose dynamic dipole moments are polarized along the direction of the C-C bond (z-axis) would be infrared active. Any tilting along or normal to the OCO molecular plane would then cause additional modes (with dynamic dipole moments polarized along the x- and y-axes, respectively) to become infrared active. On the basis of these considerations, the first form of acrylate responsible for vs(OCO) + δ(CH2) at 1437-1442 cm-1 exhibits only absorption bands attributable to modes characteristic of an upright symmetrically bound species. In particular, no absorption band attributable to vas(OCO) is present between ∼1500-1600 cm-1. As the coverage increases, presumably because of steric crowding and repulsive dipolar interactions between the more densely packed adsorbates, some of the surface-bound acrylate species are forced to tilt along the OCO molecular plane. This implies that vas(OCO) will now have a component of its dynamic dipole moment orthogonal to the surface and become infrared active, and a band that may be assigned to this mode is indeed observed at 1534 cm-1. This is accompanied by the appearance of another band at 1462 cm-1 because of vs(OCO) + δ(CH2) of this tilted acrylate species, which gains intensity at the expense of its upright counterpart with increasing coverage. Such a phenomenon has been previously reported for formate on preoxidized Ag(111) where increasing the coverage forces the surface species to adopt as many as four different bonding configurations in an attempt to minimize unfavorable intermolecular repulsive interactions.29 The relative ease in doing so for formate can be attributed to the small H

Figure 6. Reaction pathways in surface-bound acrylate formation on Ni(111)-p(2 × 2)-O.

substituent, which is less bulky than the vinyl group of surface-bound acrylate. An empirical method has been previously proposed to deduce the nature of the bonding of carboxylates to metal centers by correlating the structure and vibrational spectra of a series of metal carboxylate complexes.33 The separation between vas(OCO) and vs(OCO) (∆v) of the chemisorbed carboxylate when compared with that of the carboxylate salt may be used to infer its bonding configuration, that is, ∆v(chelating species) < ∆v(carboxylate salt) ≈ ∆v(bridging species) < ∆v(monodentate species). On the basis of these criteria, the tilted acrylate species, with ∆v ) 72 cm-1 (compared to ∆v(sodium acrylate) ) 90-120 cm-1) probably still has both O atoms coordinated to surface Ni atoms, albeit in an asymmetrical fashion. Upright and tilted carboxylate species such as formate, acetate, and propanoate are widely known and have been identified on various surfaces including Mo(110),28 Fe(100),22 Ru(001),34,35 Rh(111),23 NiO(111),36 Ni(110),37 Pd(111),24 Pt(111),25 Cu(100),38 Ag(110),26 Ag(111),29,30 and Al(111).39 4.2. Surface Reaction Mechanisms for Acrylate Formation on Ni(111). The surface reaction mechanisms leading to acrylate formation on preoxidized Ni(111) are depicted in Figure 6. Adsorbed O plays the role of a Bro¨nsted base in the dissociation of acrylic acid on Ni(111)-p(2 × 2)-O, where it strips off the OH proton and in (33) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. (34) Garcia, A. R.; Da Silva, J. L.; Ilharco, L. M. Surf. Sci. 1998, 415, 183. (35) Weisel, M. D.; Chen, J. G.; Hoffmann, F. M.; Sun, Y. K.; Weinberg, W. H. J. Chem. Phys. 1992, 97, 9396. (36) Langell, M. A.; Berrie, C. L.; Nassir, M. H.; Wulser, K. W. Surf. Sci. 1994, 320, 25. (37) Haq, S.; Love, J. G.; Sanders, H. E.; King, D. A. Surf. Sci. 1995, 325, 230. (38) Sexton, B. A. Chem. Phys. Lett. 1979, 65, 469.

Surface Functionalization of Ni(111)

doing so suppresses the adsorption of intact acrylic acid molecules. This abstraction reaction involves the direct transfer of H from the adsorbed acrylic acid monomer to the O adlayer to produce surface-bound acrylate and OH groups. The OH groups may further deprotonate more acrylic acid molecules and desorb as H2O, freeing surface Ni atoms for acrylate binding. We have observed analogous H transfer processes for acetic and formic acids on preoxidized Ni(111) yielding acetate and formate, respectively. The facile and complete deprotonation of all acrylic acid molecules within the first monolayer yielding only chemisorbed acrylate species (as verified by the RAIR spectra) also testifies to the efficacy of preoxidized metal surfaces in generating stable surface-bound carboxylates. Their synthesis on surfaces via an O-mediated route has already been demonstrated in numerous studies. An excellent example would be that of formic acid, which adsorbs molecularly on an inert surface such as clean Ag(111) but decomposes to give multiple formate species on the preoxidized surface.29 From the organic chemistry literature, acryloyl chloride is known to have a good leaving group (Cl) attached to the acyl C atom. Thus, in the presence of nucleophiles such as H2O or an amine, it rapidly undergoes a substitution reaction to yield the corresponding acid or amide. This concept can also be extended to preoxidized Ni(111) where the O adatoms too can behave as nucleophiles, leading to the following mechanistic pathway for surface acrylate formation. The acryloyl chloride molecules adsorbed on Ni(111)-p(2 × 2)-O are subjected to nucleophilic attack at the carbonyl C by adsorbed O to form acrylate species and coadsorbed Cl atoms. The nucleophilic nature of surface O has been demonstrated previously for formaldehyde, acetaldehyde, and methyl formate adsorption on preoxidized metal surfaces, where following nucleophilic attack on the carbonyl C, the corresponding carboxylates are formed.29,40-42 The absence of any tilted acrylate species being formed in acryloyl chloride adsorption can be ascribed to the fact that the nucleophilic substitution reaction occurring on Ni(111)-p(2 × 2)-O is a 1:1 surface titration in which an O atom is consumed by an acryloyl chloride molecule so as to produce a surface-bound acrylate species and a coadsorbed Cl adatom, which may block potential acrylate binding sites. On the other hand, in acrylic acid adsorption on Ni(111)-p(2 × 2)-O, an O adatom is capable of deprotonating two acid molecules to yield a pair of acrylate species and a H2O molecule that desorbs during the dose. It is thus clear that the most likely reason that tilting is only observed in the latter system would be the larger saturation population of acrylate species achievable, which leads to increased intermolecular repulsive interactions. 4.3. Association and Dissociation of Acrylic Acid Species on Ni(111). The propensity for acrylic acid molecules in the physisorbed multilayers to undergo H-bonding is borne out in this work by the large perturbations in the v(CdO) and v(OH) frequencies, which are observed at 1708 and 2500-3200 cm-1 (this appears as a broad absorption envelope not shown in the RAIR spectra presented), respectively, as compared to that for monomeric acrylic acid, where the corresponding bands (39) Crowell, J. E.; Chen, J. G.; Yates, J. T., Jr. J. Chem. Phys. 1986, 85, 3111. (40) Barteau, M. A.; Bowker, M.; Madix, R. J. Surf. Sci. 1980, 94, 303. (41) Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985, 155, 366. (42) Davis, J. L.; Barteau, M. A. Surf. Sci. 1992, 268, 11.

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Figure 7. Adsorption, association, and dissociation of acrylic acid on clean Ni(111).

are expected at 1760 and 3560 cm-1 respectively.19 It has been shown from a statistical analysis of known crystal structures that with few exceptions, most monocarboxylic acid molecules, including acrylic acid, form dimers as opposed to catemeric chains through linking of the carboxyl groups of the monomers through H-bonding.43,44 The preference for dimer over catemer formation has been attributed to steric repulsion caused by the bulky substituents of the carboxylic acid molecules that obstruct chain formation in the crystal structure.45 The RAIR spectra of acrylic acid multilayers on Ni(111) does in fact agree fully with infrared spectrum of crystalline acrylic acid dimers. The sequence of events that occurs upon acrylic acid adsorption on the clean Ni(111) surface can thus be constructed and is depicted in Figure 7. At 140 K, acrylic acid adsorbs both molecularly and as dissociated acrylate moieties. Evidence for the initial presence of acrylic acid monomers is supported by the observation of v(CO) + δ(COH) at 1163 and 1151 cm-1. Association of the monomers at higher coverages then results in the formation of dimers, (43) Allen, F. H.; Samuel Motherwell, W. D.; Raithby, P. R.; Shields, G. P.; Taylor, R. New J. Chem. 1999, 23, 25. (44) Higgs, M. A.; Sass, D. L. Acta Crystallogr. 1963, 16, 657. (45) Leiserowitz, L. Acta Crystallogr. B 1976, 32, 775.

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with consequential attenuation of the monomer absorption bands. The weak relative intensity of v(CdO) at 1705 cm-1 even after v(CO) + δ(COH) of monomeric acrylic acid is almost completely attenuated implies that the dimers are lying essentially flat on the surface. This is consistent with earlier findings that monolayer acrylic acid dimers are oriented flat on Pt(111) at 95 K.8,9 The dimers are also responsible for the vibrational features of the multilayers which build up at higher exposures and result in the screening of the acrylate absorption bands. For the multilayers, a strong v(CdO) band is clearly observed at 1708 cm-1, suggesting either tilted or random dimer orientations relative to the surface in the condensed ice. The aggregation of acrylic acid monomers to give dimers on surfaces such as Pt(111)8 and polycrystalline Ag10 has been previously reported. The driving force is probably the strong intermolecular H-bonding within the molecular aggregates that outweighs the metalmonomer interaction. Heating the surface to 200 K causes the multilayers to desorb and leave behind surface-bound acrylic acid dimers and acrylate species. At 240 K, symmetrically bound upright acrylate is the major species observed in the RAIR spectra. There is also strong evidence of acrylic acid bound to Ni(111) through dative bonding of the lone pair electrons of the carbonyl O atom. This is believed to originate from the presence of both the dimers and acrylate species on the surface, which reduces the number of sites available for dissociative chemisorption of acrylic acid. The bulky vinyl groups present are also expected to add to the steric hindrance. Because of the lack of available surface binding sites, the complete dissociation of the acrylic acid dimers into acrylate is inhibited. Instead, the monomeric intermediate is formed, bound to the surface in an η1(O) configuration, which requires fewer surface binding sites. The coadsorbed acrylate and η1(O)-acrylic acid species are likely to mutually stabilize each other through H-bonding interactions as shown in Figure 7. As the surface is heated from 240 to 310 K, variation in the relative intensities of the bands in the RAIR spectra indicates that either conformational changes or reprotonation of acrylate species into η1(O)-acrylic acid may be occurring as the dimers break up to maximize the H-bonding within the overlayer. It has been reported that formic acid and formate can coexist on Cu(100), where at full surface coverage, all the adsorbed species are held together in large H-bonded arrays that stabilize the overlayer structure.46 A strong band observed at 1650 cm-1 was assigned to v(CdO) of the adsorbed formic acid, which is consistent with our assignment of the band at 1662 cm-1 to v(CdO) of η1(O)-acrylic acid on Ni(111). η1(O)-acrylic acid has in fact been observed previously on evaporated Ag films by vibrational spectroscopy.10 Carbonyl compounds such as formic acid, acetone, acetaldehyde, methyl formate, methyl acetate, and ethyl formate have also chemisorbed in the η1(O)-configuration on surfaces such as Ni(111),47 Pt(111),48 Ru(001),49 and Ag films,50 showing in each case a lowering of v(CdO) by up to 100 cm-1, a value consistent with that observed in

the present case for η1(O)-acrylic acid on Ni(111). The redshift of v(CdO) can be attributed to back-donation of d-electrons from the metal substrate into the antibonding orbital of the carbonyl group, which lowers its bond order. Such an adsorption mode is further corroborated by the fact that the highest occupied molecular orbital (HOMO) of acrylic acid belongs predominantly to the nonbonding electrons of the carbonyl O atom, which according to the Frontier Molecular Orbital theory, is the most reactive orbital in an acid-base (i.e., metal-adsorbate) reaction.51,52 The coordination of carboxylic acids to metal centers is well documented. A large variety of compounds containing acetic acid coordinated via the carbonyl O atom to divalent metal cations such as Mg2+, Mn2+, Co2+, Cu2+, Ni2+, and Zn2+, as well as in complexes with SbCl5, ReCl3, and SnCl4, have been synthesized and spectroscopically characterized.53 The [Ni(η1(O)-acetic acid)6](BF4)2 complex has been prepared and from X-ray crystallography, it was found that the Ni-O bond length in the crystal structure is similar to that found for Ni acetate.54 This has led to the conclusion that there is actually little difference in the strengths of the Ni-carboxylic acid and Ni-carboxylate bonds, which implies that carboxylic acid adducts should be more common in coordination chemistry than generally believed.

(46) Dubois, L. H.; Ellis, T. H.; Zegarski, B. R.; Kevan, S. D. Surf. Sci. 1986, 172, 385. (47) Zahidi, E.; Castonguay, M.; McBreen, P. J. Am. Chem. Soc. 1994, 116, 5847. (48) Avery, N. R. Surf. Sci. 1983, 125, 771. (49) Anton, A. B.; Avery, N. R.; Toby, B. H.; Weinberg, W. H. J. Am. Chem. Soc. 1986, 108, 684. (50) Munro, S.; Raval, R. In Catalysis and Surface Characterization; Dines, T. J., Rochester, T. H., Thomson, J., Eds.; Royal Society of Chemistry: Cambridge, U.K., 1992; p 118.

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5. Conclusions We have examined the formation of stable, surfacebound acrylate monolayers by reaction of either acrylic acid or acryloyl chloride with surface atomic O on Ni(111). Acrylic acid undergoes an acid-base reaction with O to yield an upright symmetrically bound acrylate species that tilts at higher coverages as a consequence of steric crowding and repulsive dipolar interactions. Acryloyl chloride is subject to nucleophilic substitution by O to give adsorbed acrylate and Cl, but no tilting is observed in this case because of the lower saturation coverages achievable. We have also studied the coverage- and temperature-dependent behavior of acrylic acid on clean Ni(111). Acrylic acid intially adsorbs at 140 K as a combination of intact monomers and dissociated acrylate fragments at low coverages. The monomers then associate into H-bonded dimers, which become the dominant species present both in the first monolayer and the condensed multilayers. Heating the surface to between 240 and 310 K results in the desorption of the multilayers and conversion of the adsorbed dimeric acrylic acid monolayer into a mixture of acrylate and η1(O)-acrylic acid monomers attached to the surface through the lone pair electrons of the carbonyl O atom. Acknowledgment. We acknowledge financial support for this work from the National University of Singapore (Grant No. R-143-000-074-112).

(51) Traven, V. F. Frontier Orbitals and Properties of Organic Molecules; Ellis Horwood: New York, 1992; p 67. (52) Katrib, A.; Rabalais, J. W. J. Phys. Chem. 1973, 77, 2358. (53) Van Leeuwen, P. W. N. M.; Groeneveld, W. L. Recl. Trav. Chim. Pays-Bas 1968, 87, 86. (54) Cramer, R. E.; Van Doorne, W.; Dubois, R. Inorg. Chem. 1975, 14, 2462.