Adsorption and Reaction of Terephthaloyl Chloride on Ag (111): X-ray

Jun 20, 2011 - Photoelectron Spectroscopy and Density Functional Theory. Investigations. Martin Schmid,. †. Wolfgang Hieringer,. ‡. Christoph H. S...
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Adsorption and Reaction of Terephthaloyl Chloride on Ag(111): X-ray Photoelectron Spectroscopy and Density Functional Theory Investigations Martin Schmid,† Wolfgang Hieringer,‡ Christoph H. Schmitz,§ Hans-Peter Steinr€uck,† Moritz Sokolowski,§ and J. Michael Gottfried*,†,^ †

Lehrstuhl f€ur Physikalische Chemie II and ‡Lehrstuhl f€ur Theoretische Chemie, Universit€at Erlangen-N€urnberg, 91058 Erlangen, Egerlandstrasse 3, Germany § Institut f€ur Physikalische und Theoretische Chemie, Universit€at Bonn, 53115 Bonn, Wegelerstrasse 12, Germany ^ Fachbereich Chemie, Universit€at Marburg, 35032 Marburg, Hans-Meerwein-Strasse, Germany

bS Supporting Information ABSTRACT: The adsorption and reaction of terephthaloyl chloride (TPC) on a Ag(111) surface was investigated with X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Below 120 K, TPC forms multilayers without reacting with the Ag(111) substrate. Multilayer desorption starts above 120 K and is accompanied by a chemical reaction between the TPC molecules and the Ag surface. This reaction involves scission of the carbon chlorine bond, resulting in the formation of adsorbed chlorine atoms and a p-phenylene dicarbonyl (pPDC) species. This molecular fragment is stable at room temperature and does not undergo further decomposition, presumably due to stabilization by adsorbate substrate interactions and/or oligomerization. The DFT calculations confirm that pPDC is a possible intermediate or final reaction product and show that the two carbonyl C atoms form covalent bonds to the Ag substrate.

1. INTRODUCTION The adsorption of thin organic films is a versatile method for surface functionalization because even ultrathin films with a thickness of one monolayer can profoundly change the physical and chemical properties of a substrate.1 Those modifications have been utilized in the fields of organic electronics,2,3 sensor technology,4 and catalysis.5 A general feature of organic monolayers is their ability to form ordered structures. This selfassembly, which results in well-defined surface morphologies, is often driven by relatively weak van der Waals dispersion interactions between the individual molecules and is present for both chemisorbed and physisorbed species.6,7 A consequence of the predominantly weak intermolecular interactions is a limited thermal and/or mechanical robustness of the ordered structures. A possible approach to overcome this limitation and to obtain well-ordered organic films with a high mechanical and thermal stability is covalent linking, that is, a polymerization reaction between the adsorbed molecules, subsequent to the initial ordering process. The formation of covalent bonds can be initiated, for example, by irradiation with electrons or photons.8,9 Alternatively, a recent approach uses thermally initiated polymerization between halogen-substituted molecules on coinage metal surfaces, resulting in well-ordered two-dimensional r 2011 American Chemical Society

networks.10 13 The cleavage of the carbon halogen bond and the formation of a new carbon carbon bond represent the critical steps in this reaction. Similar reactions are also possible with surface-assisted C H bond scission.14 Surface-confined polymerization has recently been achieved by a condensation reaction between terephthaloyl chloride Ia (TPC, 1,4-benzenedicarbonyl dichloride, cf. Figure 1) and p-phenylenediamine on the Ag(111) surface. This reaction leads to the formation of ordered domains of polyamide chains at room temperature.15 However, it was not clear whether the acid chloride is stable on Ag(111) under these conditions and can thus react directly with the amine or whether the acid chloride reacts with the substrate first, which would lead to a more complex polymerization reaction. To address this topic, we examined the adsorption of terephthaloyl chloride Ia on Ag(111) using photoelectron spectroscopy (XPS) at variable temperature in combination with density functional theory (DFT) calculations. Our study provides valuable insight into surface reactions of halogen-substituted adsorbates and thus tackles one of the key questions for a Received: April 8, 2011 Revised: June 20, 2011 Published: June 20, 2011 14869

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Figure 1. Molecular structures of terephthaloyl chloride (TPC, 1,4-benzenedicarbonyl dichloride) and p-phenylene dicarbonyl (pPDC, 1,4-dicarbonyl benzene).

detailed understanding of thermally activated surface-confined polymerization reactions and their control within a fabrication process.

2. METHODS 2.1. Experimental Methods. The XPS experiments were performed with a Scienta ESCA-200 photoelectron spectrometer with a monochromatized Al KR X-ray source (hν = 1486.6 eV), which was described in detail elsewhere.16 All binding energies were referenced to the Fermi edge of the Ag(111) surface. Photoelectrons were collected at an angle of 70 relative to the surface normal to increase surface sensitivity. The base pressure of the UHV system was in the range of 2  10 10 mbar. The substrate, a Ag(111) single crystal with surface alignment better than 120 K).

Figure 6. Shifts of the C 1s, Cl 2p, and O 1s XPS signals as a function of the sample temperature; the values are given relative to the initial peak positions at 110 K.

with increasing temperature starting at 120 K. Figure 5 also shows the intensity changes of the two chlorine species (“pristine” for unreacted TPC and “reacted” for the reaction product, chemisorbed Cl atoms) with increasing temperature. The relative intensity of the reacted species was calculated from the intensity of the respective signal (at 197.5 eV), including

correction for the signal attenuation due to the TPC multilayer. Irrespective of the changes in the Cl 2p region, the intensity ratio between the C 1s components related to the phenylene ring and the carbonyl C atoms remains at a value of ∼3:1 throughout the temperature range (Figure 4b). In the course of this temperaturedriven desorption/reaction process, the C 1s and O 1s signals and the Cl 2p signal of unreacted TPC shift uniformly toward lower binding energy. In contrast, the Cl 2p contribution at 197.5 eV (assigned to chemisorbed Cl from reacted TPC) shows a significantly smaller shift (Figure 6). These effects will be discussed in section 4. Between 250 and 400 K, the photoelectron spectra show less pronounced changes; Figure 4c illustrates the calculated layer thickness, based on the C/Ag intensity ratio. In this temperature range, no further desorption was observed. Furthermore, all signals show only small binding energy shifts, compared to the large shifts between 110 and 250 K (Figure 6). However, changes in shape and intensity of the peaks occur above 250 K. The Cl 2p spectra at 250 and 300 K show only a minor signal contribution at 200.8 eV, corresponding to pristine TPC; at 400 K, this signal has completely disappeared, and the Cl 2p spectrum consists solely of a single spin orbit doublet at 197.5 eV (position of the 2p3/2 component; Figure 5). This indicates that the C Cl bond scission is complete by 400 K. At this temperature, the O 1s signal consists of two overlapping components at 531.7 and 533.1 eV with an intensity ratio of 3:2 (Figure 7). The C 1s spectrum for this temperature shows an additional contribution at around 286.4 eV. The ratio of the carbonyl C 1s signal to this additional signal is also 3:2. Therefore, it can be speculated that the new O 1s and C 1s signals have a common origin and are possibly related to carbonyl groups in an oligomerization product of the pPDC species (see below). If the sum of the C 1s signals at 286.4 and 288.5 eV, which in this interpretation, both belong to heterocarbon, is compared to the intensity of phenylene carbon (284.5 eV), the Chetero/Cphenylene ratio is 1:2.9, very close to the ideal stoichiometric ratio of 1:3. Other possible interpretations of the split O 1s and hetero-C 1s signals will be discussed below in section 4. 3.2. Density Functional Theory (DFT). The XPS data presented in section 3.1 show that TPC Ia reacts with the Ag(111) surface above 120 K. Considering the substantial shift of the Cl 2p signal, we propose that the C Cl bond is cleaved and that pPDC (1,4-dicarbonyl benzene, Ib, see Figure 1) is formed as a possible product or intermediate of this reaction. Further arguments that support this interpretation will be discussed in section 4. For the neutral, isolated pPDC in the gas phase, the calculations predict a singlet ground state in which both carbonyl groups 14872

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Table 1. Salient Interatomic Distances [Å], Angles [degrees], and Adsorption Energies ΔEads [kJ/mol]a Ib d(O C) in Å

1.18/1.18

d(C Ag) in Å — (O C Ci) in  ΔEads in kJ/mol

179.8/180.0

IIa

IIb

1.18/1.18

1.18/1.18

1.21/1.21

3.50b/3.49b

3.51b/3.45b

2.31/2.29

178.6/178.6

177.6/178.5

76

76

III

121.5/122.4c 91

IV 1.25/1.17 2.20 122.3/179.6 70

a

C, Ci, and Ag denote the carbonyl carbon atoms, the ipso carbon atoms of the phenylene ring, and the closest silver atom, respectively. b Distance in the direction of the surface normal with respect to the average of all Ag atoms in the first surface layer in structures IIa/IIb. c Dihedral angles ϑ(O C C C) in III: 115.2/96.2.

Figure 8. DFT results for pPDC on Ag(111); illustrations of the optimized geometries of the adsorbed species IIa, IIb, III, and IV.

adopt a linear geometry (Cipso CdO angle close to 180, cf. Table 1). We furthermore consider three different adsorption modes of pPDC on the ideal Ag(111) surface, all of which proved to represent local minima in the DFT calculations. Optimized geometries of these adsorption modes are shown in Figure 8. In adsorption mode II, the molecule lies flat on the surface at a molecule surface distance of approximately 3.4 Å (average over all atoms, see Table S1 in the Supporting Information (SI) for the vertical atomic positions). The carbonyl oxygen atoms are farther away from the surface (3.6 Å) than the C atoms of the phenylene ring (average height ≈ 3.4 Å). Other than that, the geometry of the molecule is virtually the same as that in the gas phase; see Table 1 for selected structural data. For this coordination mode, we have considered two different orientations IIa and IIb of the molecule on the surface. In IIb, the molecule is rotated by 30 around the surface normal with respect to IIa; compare Figure 8. The molecular geometry is similar in both orientations, indicating an only weak interaction with the surface atoms. The adsorption energy is calculated to be 76 kJ/mol at the present dispersion-corrected density functional level. As an alternative to the physisorption mode II, we consider the possibility of a directed, covalent interaction of the carbonyl C atom with individual Ag atoms in the surface. Here, the CO groups adopt a bent, trigonal-planar geometry that resembles those found in organic carbonyl compounds (as, for example, in TPC). In coordination mode III, each CO group binds to a Ag atom of the surface. As can be seen from Table 1, this leads to a drastic decrease of the Ag Ccarbonyl distance by 1.2 Å and a slight increase in the C O distance by 0.03 Å; the C C O angle is close to 120, as is typical for a sp2 hybridization of the carbonyl C atom. In order to enable the coordination of both CO groups to surface atoms, the molecule adopts a slightly bent geometry, as

shown in Figure 8. As a result, the carbonyl C atoms are closer to the surface plane (2.44 2.48 Å) than the phenylene ring (2.67 2.81 Å, cf. Table S1 in the SI). Due to the sp2 hybridization of the carbonyl C atoms, the O atoms are farthest away from the surface (3.28 3.38 Å). The Ag atoms engaged in the C Ag bonds of III are lifted above the surface plane by 0.34 and 0.36 Å (see Figure 8), which helps to relieve the strain exerted on the molecule due to the coordination of both CO groups to the surface. The total adsorption energy for adsorption mode III is calculated to be 91 kJ/mol, 15 kJ/mol higher than that for the physisorbed mode II. In view of the strain present in III, we also considered a situation (IV, cf. Figure 8) where only one CO group is bound to a surface atom while the rest of the molecule is not in contact with the surface. Some structural parameters of the optimized geometry can be found in Table 1. As expected, the C Ag bond is shorter by ∼0.1 Å compared to the strained situation III. Nevertheless, the binding energy of this coordination mode is calculated to be only 70 kJ/mol.

4. DISCUSSION The experimental results presented in section 3.1 show that TPC forms multilayers on but does not react with Ag(111) below 120 K. This is concluded from the single Cl 2p contribution at 200.3 eV (assigned to intact TPC) and the constant intensities and binding energy positions for all examined core-level signals. Above 200 K, the situation is fundamentally different; the dominant Cl 2p feature is now located at 197.5 eV. This binding energy has previously been found for chemisorbed Cl on Ag29 and thus indicates scission of the C Cl bond. The formation of chemisorbed Cl atoms is further supported by the fact that the Cl 2p signal at 197.5 eV is only slightly shifted during the temperature variation, that is, the peak position is almost independent of the work function. This indicates a constant binding energy relative to the Fermi energy, which is typical for strongly chemisorbed species. In contrast, the Cl 2p signal of intact, unreacted TPC displays a larger shift (see Figure 6) and is therefore attributed to chlorine in a physisorbed species, for which the binding energies are typically constant relative to the vacuum level. Therefore, the binding energies change with the work function. The work function change induced by the chemisorbed Cl atoms is presumably also responsible for the uniform shifts of the C 1s and O 1s signals of intact physisorbed TPC (Figure 6). The scission of the C Cl bond leads to the formation of pPDC, a species with unsaturated carbonyl C atoms, which can bind to the substrate or to neighboring fragments. The XPS data suggest that pPDC does not undergo further decomposition, as indicated by the constant stoichiometric ratios for high- and 14873

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The Journal of Physical Chemistry C low-temperature spectra and the uniform, only work-functionrelated peak shifts. In particular, the persistence of the C 1s signal of the carbonyl C atom, combined with the lack of further reduction of the total amount of adsorbed carbon, indicates the presence of stable molecular fragments after the cleavage of the C Cl bond. It should be noted that the binding energy of the carbonylrelated C 1s signal is only weakly affected by the loss of the Cl atom and the concurrent formation of the Ag Ccarbonyl bond; from 110 to 400 K, the separation between the carbonyl and phenylene-related C 1s peaks increases by only 0.25 eV; compare Figure 6. This may seem counterintuitive considering the very different electronegativities30 of the bonding partners, 3.16 for Cl and 1.93 for Ag. However, Cl is not only a σ acceptor (negative inductive effect, I); it can also donate π electrons into a conjugated system (positive mesomeric effect, +M).31 In contrast, the Ag atom can only act as a weak σ donor. For the Cl atom, the π donor effect apparently compensates for the σ acceptor effect on the carbonyl C atom, such that the difference in electronegativity between Cl and Ag does not lead to a significant change in the C 1s binding energy of the carbonyl C atom. The fact that the pDCP species does not desorb at room temperature and above indicates chemisorptive interactions with the substrate, most likely via the unsatured carbonyl C atoms. Alternatively, the carbonyl C atoms may bind to each other via 1,2-dicarbonyl bridges and thus form oligomers, which are too large to desorb. Both products, monomeric pPDC species and the oligomer, may be formed simultaneously, which would explain why the C 1s and O 1s signals show additional components above 200 K (Figure 7). As an alternative explanation, it cannot be excluded that the splitting of the C 1s and O 1s signals is due to the simultaneous presence of pPDC monomer species in different bonding situations. The relatively small adsorption energy differences found in the DFT calculations could be further reduced by cooperative effects in the densely packed monolayer or by adsorption on defect sites such as steps, where another binding situation than on the terrace might be favored. Alternatively, it cannot be excluded that the additional contribution in the C 1s spectrum at 286.4 eV is a charge-transfer satellite of the main signal at 284.5 eV, that is, it may result from substrate-to-photoion charge transfer following the core excitation. A similar interpretation can be given for the additional O 1s signal at 533.1 eV. Similar satellites have been proposed to explain the structure of core-level signals of naphthalene tetracarboxylic acid dianhydride (NTCDA) on Ag(111).32 Another observation that is in line with the existence of a stable adsorbed dicarbonyl species is the formation of polyamide chains by reaction between coadsorbed TCP and p-phenylene diamine (1,4-diamino benzene).15,33 This is only possible if the carbonyl C atoms are accessible for a nucleophilic attack by the amine nitrogen atom. In addition, this study33 provides another unambiguous proof for the cleavage of the C Cl bond; after formation of the polymer, the released Cl remains on the surface, as revealed by the Cl 2p3/2 XPS peak at 197.5 eV. This is exactly the same binding energy as we reported above for reacted TPC on Ag(111). The DFT calculations also confirm that adsorbed pPDC is a possible intermediate or final product of the surface reaction. In the most stable adsorption mode III, the two unsaturated carbonyl C atoms form direct covalent bonds to the Ag substrate, while the oxygen atoms point away from the surface. This geometry is stabilized by 15 kJ/mol or more relative to the other

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adsorption modes IIa/IIb and IV considered here. The inherent strain of structure III is overcompensated by the covalent Ag Ccarbonyl bonds in combination with van der Waals dispersion interactions. This is shown by comparison with geometry IV, in which the pPDC fragment is unstrained but binds with only one carbonyl C atom to the substrate. The adsorption energy of this geometry is lower than that for geometry III. In addition, the Ag atoms engaged in the Ag Ccarbonyl bonds of III are lifted above the surface plane, which helps to relieve strain in the molecular fragment. Finally, we discuss our results in relation to previous studies of halogen-containing organic compounds on Ag(111). The methyl halides CH3Cl and CH3Br are clearly less reactive than TPC as they were found to desorb intact around 125 and 140 K, respectively, without reacting with the substrate. In contrast, CH3I dissociates into methyl and iodide between 130 and 190 K (a similar range as that found here for the C Cl scission in TPC), simultaneous with multilayer desorption above 130 K.34 The methyl groups desorb associatively as ethylene C2H6 at around 190 K, while iodine remains on the surface. The dissociation of adsorbed CH3I shifts the I 3d5/2 signal toward lower binding energies ( 1.75 eV), consistent with the even larger shift of the Cl 2p signal ( 2.8 eV) observed in this work. It noteworthy that no thermally induced C Cl bond scission was observed for CH3Cl, although the monolayer desorption temperature (around 125 K) is in the range of C Cl bond scission for TPC. Presumably, the unsaturated carbonyl C in the pPDC fragment is stabilized by the neighboring phenyl ring and carbonyl O atom, leading to a lower transition state for C Cl cleavage in TPC as compared to CH3Cl.34 The adsorption of iodobenzene on Ag(111) results in the formation of biphenyl due to a similar mechanism as that described above for CH3I.35 This reaction is prototypical for related cases of reactions of larger organic molecules,10 13 which include, for example, the formation of 2D covalent networks on Ag(111), initiated by the cleavage of carbon iodide bonds of a hexaiodo-substituted cyclohexa-m-phenylene macrocycle.10 The network formation always follows the same reaction sequence as described above: first the selective cleavage of the C I bond and a surface-induced stabilization of the remaining reactive fragments, followed by a polymerization reaction that can be thermally activated.10,34 According to these findings, the possibility of an initial stabilization and further, thermally activated oligomerization of the adsorbed TPC fragments needs to be further investigated. Especially STM could elucidate the existence and morphology of a surface oligomer (or polymer) in the examined temperature range.

5. CONCLUSIONS The adsorption and reaction of terephthaloyl chloride (TPC) on Ag(111) was examined using X-ray photoelectron spectroscopy and density functional theory. The results reveal a strong influence of temperature on the reactivity, especially with respect to the cleavage of the chlorine carbonyl bond. Below 120 K, a stable multilayer of TPC with no sign of a surface reaction was found, even for molecules in direct contact with the Ag(111) substrate. Multilayer desorption starts between 130 and 140 K, as indicated by a parallel decrease of the C 1s, O 1s, and Cl 2p intensities. Simultaneously, a selective scission of the C Cl bond for the molecules in the first layer was observed, resulting in the 14874

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The Journal of Physical Chemistry C formation of chemisorbed Cl atoms. The residual part of the molecule, a p-phenylene dicarbonyl species with two coordinatively unsaturated carbonyl C atoms, does not decompose or desorb at room temperature. In analogy to previous findings for reactive halogen-substituted hydrocarbons adsorbed on Ag(111), this behavior is interpreted as a surface-induced stabilization of the reactive organic species. DFT calculations confirm that p-phenylene dicarbonyl is a possible intermediate or final product of the reaction of TPC with Ag(111). In the most stable adsorption geometry, the two carbonyl C atoms form covalent bonds to the Ag substrate. The Ag atoms engaging in the C Ag bond are lifted above the surface plane.

’ ASSOCIATED CONTENT

bS

Supporting Information. Ag 3d XPS data and vertical atomic positions of p-phenylene dicarbonyl (pPDC) in different adsorption geometries as calculated by DFT. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Address: Philipps-Universit€at Marburg Fachbereich Chemie Hans-Meerwein-Str. 35032 Marburg, Germany. Tel.: +49 64212822-541. E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 583, Sonderforschungsbereich 624, and the Cluster of Excellence “Engineering of Advanced Materials” is gratefully acknowledged. We thank W. G. Schmidt for sharing with us his van der Waals DFT module for the VASP program. ’ REFERENCES (1) Prakash, S.; Karacor, M. B.; Banerjee, S. Surf. Sci. Rep. 2009, 64, 233–254. (2) Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D. Chem. Mater. 2004, 16, 4413– 4422. (3) Lee, S.; Koo, B; Shin, J.; Lee, E.; Park, H.; Kim, H. Appl. Phys. Lett. 2006, 88, 162109. (4) Takulapalli, B. R.; Laws, G. M.; Liddell, P. A.; Andreasson, J.; Erno, Z.; Gust, D.; Thornton, T. J. J. Am. Chem. Soc. 2008, 130, 2226– 2233. (5) Mochida, I.; Suetsugu, K.; Fujitsu, H.; Takeshita, K. J. Catal. 1982, 77, 519–526. (6) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201–341. (7) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (8) Eck, W.; K€uller, A.; Grunze, M.; V€olkel, B.; G€olzh€auser, A. Adv. Mater. 2005, 17, 2583–2587. (9) Zhou, X.-L.; Castro, M. E.; White, J. M. Surf. Sci. 1990, 238, 215– 225. (10) Bieri, M.; Nguyen, M.-T.; Gr€oning, O.; Cai, J.; Treier, M.; Ait-Mansour, K.; Ruffieux, P.; Pignedoli, C. A.; Passerone, D.; Kastler, M.; M€ullen, K.; Fasel, R. J. Am. Chem. Soc. 2010, 132, 16669–16676. (11) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nat. Nanotechnol. 2007, 2, 687–691. (12) Lipton-Duffin, J. A.; Ivasenko, O.; Perepichka, D. F.; Rosei, F. Small 2009, 5, 592–597.

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