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2008, 112, 12606–12609 Published on Web 07/31/2008
Self-Assembly of Terephthalic Acid on Rutile TiO2(110): Toward Chemically Functionalized Metal Oxide Surfaces Antoni Tekiel,* Jakub S. Prauzner-Bechcicki, Szymon Godlewski, Janusz Budzioch, and Marek Szymonski Research Center for Nanometer-scale Science and AdVanced Materials (NANOSAM), Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian UniVersity, Reymonta 4, 30-059 Krakow, Poland ReceiVed: June 5, 2008; ReVised Manuscript ReceiVed: July 12, 2008
Self-organization of 1,4-benzenedicarboxylic acid molecules (terephthalic acid, TPA) on a rutile TiO2(110)(1×1) surface is studied by means of ultra-high vacuum scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM). When saturation coverage is achieved with the formation of one monolayer, STM images reveal two alternating contrast patterns: (i) a well-organized (2×1) structure and (ii) a mixed structure of molecular rows oriented along the 001 crystallographic direction. Complementary STM images recorded with two different tip terminations prove that the two contrasting patterns indicate the same stable surface structure. The nc-AFM imaging confirms the mixed molecular row structure. It is concluded that TPA molecules are adsorbed in an upright position. This occurs with one of the carboxyl group bound dissociatively in a bi-dentate fashion with the two 5-fold coordinated Ti atoms. The second carboxyl group is exposed to the vacuum interface. This carboxyl terminated surface is discussed in terms of surface chemical functionalization. Chemically funtionalized surfaces play an important role in many nanotechnogical applications. In particular, self-assembled monolayers (SAMs) have attracted a great interest in recent years.1 The most extensively studied class of SAMs is derived from the adsorption of alkanethiols on metals such as gold, silver, and copper. No attempt has been made so far to create a chemically functionalized metal oxide surface by adsorption of organic molecules, even though metal oxide surfaces provide anisotropy and specific adsorption sites that lead to well-ordered, monocrystalline monolayers. Here we report results of STM and nc-AFM measurements that address the adsorption of 1,4benzenedicarboxylic acid (terephthalic acid, TPA). This work has two motivations: First, TPA/TiO2(110)-(1 × 1) is a promising system to create an overlayer of upright oriented molecules terminated with carboxyl groups. This kind of chemical functionalization could have wide-ranging applications, similar to those of carboxyl-terminated SAMs.1 Second, terephthalic acid is a constituent of some metal-organic frameworks (MOFs) introduced by Yaghi et al.2,3 The MOFs are attracting great interest due to their gas-storage capacities and the possibility of forming metal clusters with special electronic and/or magnetic properties inside the framework pores. The so-called secondary building units (SBUs) of MOFs are bridged by organic ligands to form a porous and chemically robust framework. For example, in the case of the MOF-5, tetrahedral [Zn4O]6+ clusters and terephthalates act as SBUs and organic ligands, respectively. The possibility of fabricating a two-dimensional network formed by well-ordered and appropriately oriented organic ligands would facilitate the growth and anchoring of MOFs at surfaces, which is the focus of many industrial applications.4-7 * Corresponding author. E-mail:
[email protected].
10.1021/jp804979t CCC: $40.75
Monolayers of simple monocarboxylic acids, such as formic, acetic and trimethylacetic acids, form well-ordered structures of the (2 × 1) type on rutile TiO2(110)-(1 × 1) surfaces when adsorbed at room temperature.8,9 In general, the carboxyl group of the acid dissociates to give carboxylate and hydrogen. The negatively charged carboxylate adsorbs on a pair of 5-fold coordinated Ti atoms in a bridge form with the O-C-O plane aligned in the 001 crystallographic direction. The adsorption of larger, aromatic monocarboxylic acids (e.g., benzoic and isonicotinic) follows this trend.10-12 The adsorbate-adsorbate interactions, however, lead to linkage effects such as dimerization or trimerization that distort the (2 × 1) symmetry of the overlayer. All experiments reported here are performed in an ultra-high vacuum. A polished TiO2(110) wafer (MaTecK GmbH) is mounted on the sample holder with a Si wafer as a resistive heater. The temperature of the TiO2 sample is monitored by an infrared pyrometer. The TiO2(110)-(1 × 1) surface is prepared by cycles of Ar+ sputtering and subsequent annealing at ∼690 °C. Terephthalic acid (Fluka, g99% purity) is outgassed in vacuum and then evaporated from a Knudsen cell onto the TiO2 sample kept at room temperature. Scanning probe imaging is carried out with a commercial ultra-high vacuum Omicron VT-AFM/STM microscope. STM images of empty-states are acquired in the constant current mode at positive sample bias voltages with electrochemically etched tungsten tips. Atomicforce images are obtained in the non-contact mode with frequency modulation. Commercially available silicon cantilevers are used as probes. The TPA monolayer (1 ML ) 2.61 × 1014 molecules/cm2, i.e., two surface lattice unit cells accommodate one molecule) exhibits two different molecularly resolved STM patterns (Figure 2008 American Chemical Society
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Figure 1. Two typical molecularly resolved STM patterns for the TiO2(110)-(1 × 1) surface with 1 ML TPA. (a) Long range ordered monolayer of the dominant (2 × 1) symmetry. Image area 10 nm × 10 nm; sample bias 2 V; tunneling current 2 pA. (b) Structure of single and double molecular rows oriented along the 001 crystallographic direction. Image area 19 nm × 13 nm; sample bias 2 V; tunneling current 2 pA.
1, panels a and b). Figure 1a demonstrates the presence of a long-range ordered monolayer of the dominant (2 × 1) symmetry (one spot corresponds to one molecule). In contrast, Figure 1b shows a monolayer that is less regular, having a structure of single and double molecular rows aligned along the 001 crystallographic direction. Random, reversible switching between these two STM images are observed during the scans, which we attribute to a change of the STM tip termination, e.g. by an adsorbed molecule or its further reorientation. In Figure 2, panels a and b, we provide evidence that the observed STM patterns arise from the unchanged structure of the TPA overlayer. The STM images in Figure 2 are of a TPA covered surface with an elevated TiO2(110) island, recorded with a double STM tip. The two apexes of the STM tip are horizontally separated by ∼30 nm. The difference in their relative altitude is small in comparison to the TiO2(110) step height (3.24 Å). Each of the apexes records different STM pattern of the same TPA covered TiO2(110) island. When imaging the elevated surface island with the double tip, the feedback system of the STM microscope is locally dominated by the tunneling current flowing through one of the two apexes of the tip. In this way, a single STM frame comprises two different images of the same island shifted by ∼30 nm. This allows effectively independent measurements to be performed with two apexes of the STM tip. Moreover, they provide information that the observed effect is not due to any tip-induced surface modification: images recorded in multiple scans do not change. To further elucidate the structure of the TPA monolayer, we perform nc-AFM probing different properties than the STM. A high resolution nc-AFM image is presented in Figure 3, which shows structures strikingly similar to those resolved in the mixed STM pattern (compare Figures 1b and 2b). A high-resolution contrast in nc-AFM imaging results mostly from the chemical interaction between the outermost atom of the tip apex and atoms of the surface that are in immediate proximity. nc-AFM thereby provides information mostly from the outermost part of the surface, whereas the STM method could also be sensitive to deeper structures.
Figure 2. Two STM patterns of the same TiO2(110)-(1 × 1) island covered with 1 ML of TPA molecules. Both STM images are recorded with a double STM tip that provides independent measurements (see text). Image parameters: area 31.3 nm × 27.7 nm; sample bias 2 V; tunneling current 2 pA.
The observed patterns can be explained by considering the adsorption geometry of the TPA molecules on a TiO2(110)(1 × 1) surface. The additional carboxyl group in the TPA
12608 J. Phys. Chem. C, Vol. 112, No. 33, 2008
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Figure 3. Topographic nc-AFM image of a TiO2(110)-(1 × 1) surface with 1 ML TPA. There is a mixed structure of dimer and monomer rows oriented along 001 crystallographic direction. Image parameters: ∆f ) -109 Hz, f0 ) 237 kHz; area 29.1 nm × 23.2 nm; sample bias 0.3 V.
molecule makes it unique among other carboxylic acids on TiO2(110)-(1 × 1) because it can introduce significant complications to the adsorption process. For example, it would be possible that the TPA molecules adsorb as dicarboxylates, similarly to the bi-isonicotinic acid,13,14 giving rise to a flatlying geometry. This would make surface chemical functionalization impossible. Our observations, however, demonstrate that TPA molecules adsorb in a fashion similar to monoacids since the monolayer forms a (2 × 1) structure. In this case, one molecule occupies 38.31 Å2 of the surface area, which is not enough to accommodate a flat-lying TPA molecule (i.e., this geometry would prefer a (2 × 2) structure). We conclude that TPA molecules adsorb in an upright position. We also observe enhanced molecule diffusion along the 001 direction and domain boundaries separating two (2 × 1) structures shifted in the 001 direction by one surface lattice unit (2.95 Å) (Figure 1a). This behavior is typical for monocarboxylic acids adsorbed on the TiO2(110)-(1 × 1) surface.9-11,15 It is reasonable to assume that the TPA molecules follow the same dissociative adsorption path. In Figure 4 we present a schematic model of the TPA monolayer adsorbed on the TiO2(110)-(1 × 1) surface. One carboxyl group in each TPA molecule dissociates to a COOion and binds in a bi-dentate fashion to the two 5-fold coordinated Ti atoms. The remaining carboxyl group is oriented toward the vacuum interface and terminates the surface. The adsorbate-adsorbate interaction leads to linkage of neighboring molecules resulting in irregular, mixed dimer and monomer rows that can be seen by both nc-AFM (Figure 3) and STM (Figures 1b and 2b). As a consequence, the molecules within the dimer rows may not be oriented exactly perpendicular to the surface. When the STM tip is in a state where it is sensitive to deeper lying electronic structure, a regular overlayer of the (2 × 1) symmetry is recorded (Figure 1a). In this condition, the STM image is primarily influenced by electronic states related to the adsorption sites. These sites are determined by the TiO2(110)(1 × 1) surface structure causing a regular structure of the (2 × 1) symmetry to be recorded. Similar linkage effects on the TiO2(110)-(1 × 1) surface have been reported for benzoic and isonicotinic acids. An STM study showed that benzoic acid molecules form dimers and trimers at monolayer coverage.10 X-ray absorption spectroscopy was also used to study monolayers of isonicotinic acid suggesting dimer formation.11,12 In both cases the interaction responsible for molecular pairing led to the rotation of the aromatic ring around the molecular axis. In the case of benzoic acid, this
Figure 4. Schematic diagram of TPA molecules on a TiO2(110)(1 × 1) surface. Molecules adsorb in the upright position with one carboxyl group dissociatively bound to the 5-fold coordinated Ti atoms, while the other carboxyl group terminates the surface. Adsorbateadsorbate interactions lead to formation of dimer and monomer rows along the 001 direction that can be both seen by STM and nc-AFM methods. When the STM tip is unintentionally modified, a regular (2 × 1) lattice can be observed.
would facilitate the interaction between a hydrogen atom from the benzyl ring and the π-orbital of the molecule adsorbed on the neighboring Ti row. For isonicotinic acid, a geometry accommodating more than one ring orientation was proposed.11,12 In our work, the additional carboxyl group in the TPA/TiO2(110) system may lead to more complex situation than pairing induced by aromatic ring interaction. It remains to understand how much the terminating carboxyl group contributes to the molecular pairing process. Apart from the interaction between the benzyl cores, preferring a T-like structure, the carboxyl groups of the neighboring molecules can form cyclic and acyclic dimers. The scanning probe techniques used in the present study are not capable of definitively determining the nature of mutual molecular interaction. The TPA/TiO2(110)-(1 × 1) system can, however, be compared to the adsorption of TPA molecules on Cu(110),16 where saturation coverage of TPA molecules forms a p(2 × 1) structure of upright oriented molecules. Although TPA molecules are more densely packed on Cu(110) compared to TiO2(110)-(1 × 1) (adsorbate lattice unit cells are 5.11 Å × 3.62 Å and 5.91 Å × 6.49 Å, respectively), reflection adsorption infrared spectroscopy of the TPA/Cu(110) system indicated no significant hydrogen bonding between the terminating acid groups. The TPA overlayer on TiO2(110)-(1 × 1) has a much larger molecule-molecule distance in the direction perpendicular to the plane of the carboxylate
Letters (6.49 Å). Therefore, molecules can incline toward each other within the dimer row to form head-to-head -COOH · · · HOOCdimers. On the other hand, similar dimerization occurs also in the absence of the additional acid group, as can be concluded from an investigation of benzoic acid.10 Figure 1b shows that the mixed structure of dimer and monomer rows is frequently locally distorted: monomer rows gradually evolve into dimer rows and vice versa. This indicates that the structure of the TPA monolayer can be stabilized by interactions between several nearest and next-nearest neighboring molecules including longrange electrostatic forces. It has been reported that self-assembly of the (2 × 1) structure of trimethylacetic acid on TiO2(110)(1 × 1) can be accompanied by regular -OH group formation, presumably stabilizing the overlayer of the carboxylate on the substrate surface.17 In our study, the mixed dimer and monomer structure is likely determined by the detailed adsorbate-adsorbate interaction, which makes the molecule geometry very complicated. To summarize, our combined STM and nc-AFM study shows that monolayers of TPA molecules adsorb on TiO2(110)-(1 × 1) surfaces as monocarboxylates in an upright position. This forms a mixed structure of single and double molecular rows along the 001 crystallographic direction. As a consequence, a surface with carboxyl groups exposed at the vacuum interface is created. In the present studies, involving only microscopic techniques, it is impossible to verify if the terminating carboxyl groups are paired by hydroxyl bonds. Similar linkage effects such as dimerization are also present for aromatic monocarboxylic acids on the TiO2(110)-(1 × 1) surface, though the terminating acid group may remain intact and retain the chemical properties of the carboxyl group. Acknowledgment. This work was supported by the sixth Framework Program of the European Commission within the Specific Targeted Research Project “Anchoring of metal-organic frameworks, MOFs, to surfaces, SURMOF”, Contract No. NMP4-CT-2006-032109. A.T. gratefully acknowledges Prof.
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12609 U. Diebold and Dr. O. Dulub from Tulane University, New Orleans, LA, for their hospitality and helpful discussions on metal oxide surface preparation. Two of us (M.S. and S.G.) would like to acknowledge the support received from the Polish Foundation for Science (Contract for Subsidy No. 11/2007). References and Notes (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (2) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319–330. (3) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705–714. (4) Hermes, S.; Schro¨der, F.; Chelmowski, R.; Wo¨ll, C.; Fischer, R. A. J. Am. Chem. Soc. 2005, 127, 13744–13745. (5) Shekhah, O.; Wang, H.; Strunskus, T.; Cyganik, P.; Zacher, D.; Fischer, R.; Wo¨ll, C. Langmuir 2007, 23, 7440–7442. (6) Shekhah, O.; Wang, H.; Kowarik, S.; Schreiber, F.; Paulus, M.; Tolan, M.; Sternemann, C.; Evers, F.; Zacher, D.; Fischer, R. A.; Wo¨ll, C. J. Am. Chem. Soc. 2007, 129, 15118–15119. (7) Biemmi, E.; Scherb, C.; Bein, T. J. Am. Chem. Soc. 2007, 129, 8054–8055. (8) Diebold, U. Surf. Sci. Rep. 2003, 48, 53–229. (9) Henderson, M. A.; White, J. M.; Uetsuka, H; Onishi, H. J. Am. Chem. Soc. 2003, 125, 14974–14975. (10) Guo, Q.; Williams, E. M. Surf. Sci. 1999, 433-435, 322–326. (11) Schnadt, J.; Schiessling, J.; O’Shea, J. N.; Gray, S. M.; Patthey, L.; Johansson, M. K.-J.; Shi, M.; Krempaskyı`, J.; Åhlund, J.; Karlsson, P. G.; Persson, P.; Mårtensson, N.; Bru¨hwiler, P. A. Surf. Sci. 2003, 540, 39–54. (12) Schnadt, J.; O’Shea, J. N.; Patthey, L.; Schiessling, J.; Krempaskyı`, J.; Shi, M.; Mårtensson, N.; Bru¨hwiler, P. A. Surf. Sci. 2003, 544, 74–86. (13) Patthey, L.; Rensmo, H.; Persson, P.; Westermark, K.; Vayssieres, L.; Stashans, A.; Petersson, Å; Bru¨hwiler, P. A.; Siegbahn, H.; Lunnel, S.; Mårtensson, N. J. Chem. Phys. 1999, 110, 5913–5918. (14) Persson, P.; Lunell, S.; Bru¨hwiler, P. A.; Schnadt, J.; So¨dergren, S.; O’Shea, J. N.; Karis, O.; Siegbahn, H.; Mårtensson, N. J.; Ba¨sller, M; Patthey, L. J. Chem. Phys. 2000, 112, 3945–3948. (15) Onishi, H.; Iwasawa, Y. Langmuir 1994, 10, 4414–4416. (16) Martin, D. S.; Cole, R. J.; Haq, S. Phys. ReV. B 2002, 66, 1554271– 1554278. (17) Lyubinetsky, I; Yu, Z. Q.; Henderson, M. A. J. Phys. Chem. C 2007, 111, 4342–4346.
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