© Copyright 2001 by the American Chemical Society
VOLUME 105, NUMBER 10, MARCH 15, 2001
LETTERS Hydrogen-Bond Induced Surface Core-Level Shift in Isonicotinic Acid James N. O’Shea,* Joachim Schnadt, Paul A. Bru1 hwiler,* Hendrik Hillesheimer, and Nils Mårtensson† Department of Physics, Uppsala UniVersity, Box 530, S-751 21 Uppsala, Sweden
Luc Patthey and Juraj Krempasky Swiss Light Source, Paul-Scherrer-Institut, CH-5232 Villigen, Switzerland
ChuanKui Wang, Yi Luo, and Hans A° gren Theoretical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden ReceiVed: October 6, 2000; In Final Form: December 5, 2000
Intermolecular hydrogen-bonding in thick films of isonicotinic acid evaporated onto rutile TiO2(110) has been investigated with X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The rate of deposition is found to be a key factor in overcoming the kinetic barriers to extensive hydrogenbond formation, which, when present, gives rise to large energy shifts between bulk and surface in both the N1s XPS and XAS. The origin of the surface core-level shift is attributed to the presence of non-hydrogenbonded nitrogen atoms in the surface layer.
Introduction Noncovalent interactions, and in particular hydrogen-bonding, are becoming increasingly exploited in the fabrication of selfassembled nanoscale structures.1-7 Pyridinecarboxylic acids are an interesting class of molecules in this respect due to their ability to form intermolecular hydrogen bonds in the solid state between the carboxylic group and the pyridine nitrogen atom.8 The high directionality of hydrogen bonds9 makes it possible to form highly oriented structures from units capable of forming head-to-tail interactions1-3 for which isonicotinic acid is a good candidate. In this letter we present the results of our investigation of hydrogen bonding in isonicotinic acid thick films using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spec* To whom correspondence should be addressed.E-mail: jamesna@ fysik.uu.se and
[email protected]. † Also Max-lab, Box 118, S-221 00 Lund, Sweden.
troscopy (XAS). The use of core-level spectroscopy to directly probe the effects of hydrogen-bonding in the condensed phase has not often been exploited,10 and to our knowledge has so far yielded little direct information. However, we show that these techniques exhibit strong hydrogen-bond-induced effects, which can be a powerful tool for the investigation of such systems. Experimental and Theoretical Section The experiments were performed at beamlines I51111 and D101112,13 of the MAX II synchrotron storage ring at MAXLab in Lund, Sweden. The substrate used for this investigation is a rutile TiO2(110) single crystal annealed at high temperatures in order make the crystal conducting via the creation of bulk defects, which we then eliminate from the surface region by treatment with oxygen.17 Isonicotinic acid (4-pyridinecarboxylic acid) was deposited under UHV conditions using a home-built thermal evaporator equipped with temperature and pressure
10.1021/jp003675x CCC: $20.00 © 2001 American Chemical Society Published on Web 02/20/2001
1918 J. Phys. Chem. B, Vol. 105, No. 10, 2001
Figure 1. N1s XPS (measured at 160 K) for an isonicotinic acid thick film adsorbed on rutile TiO2 at 160 K measured at the indicated emission geometries with respect to the surface normal. The highbinding energy feature is enhanced at normal emission angle and therefore localized mainly in the bulk of the film. The inset shows N1s XPS spectra measured at normal emission for slow and rapid evaporations, where a single feature is observed for the rapid film. All spectra are normalized to the intensity of the low binding energy feature.
monitors. Where comparison is made between two different preparation conditions, referred to in this letter as “slow” and “rapid”, this corresponds to approximately equal exposures (film thickness) carried out at different adsorbate pressures and evaporation times. The corresponding pressures as measured in the preparation chamber for the slow and rapid evaporations are ∼1 × 10-8 Torr and ∼1 × 10-6 Torr, respectively. X-ray absorption spectra were measured using an electron yield detector. The XAS photon energy scale was calibrated using first- and second-order light. X-ray photoelectron spectra were measured using Scienta hemispherical electron analyzers. Since no photoelectron signal originating from the substrate could be measured through the thick organic films, XPS binding energies were instead calibrated using the C1s signal of the ring carbon atoms (287.4 eV; arbitrary). This procedure is adequate for the purpose of this discussion; however, we stress that binding energies should not be considered as absolute values. Molecular geometries used for calculations of the H-bonding distances and N1s ionization potentials were optimized at the hybrid density functional theory (DFT) method B3LYP with the standard 6-31G basis set using the Gaussian 98 program.14 The ionization potentials were calculated using the DFT code deMon15 with triple-ζ plus polarization basis sets. Results and Discussion N1s XPS for a thick film of isonicotinic acid evaporated onto TiO2(110) at 160 K is shown in Figure 1. The limited mean free path of the photoelectrons is exploited to give a higher degree of surface sensitivity at more grazing emission angles.16 Isonicotinic acid contains one nitrogen atom per molecule and in the absence of interaction-induced chemical shifts is thus expected to exhibit a single N1s feature in the XPS. However, it is shown in Figure 1 that we observe two N1s peaks separated by 1.7 eV binding energy. Furthermore, the angular dependence of the spectra shows that the high-binding energy feature is localized in the bulk of the thick film. A similar effect is also observed in X-ray absorption, shown in Figure 2. It appears that the lowest unoccupied molecular orbital (LUMO) assigned to a π* antibonding state is split into two components compared to the single feature expected based on data for bi-isonicotinic acid17,18 and pyridine.19,20 By increasing the retardation voltage
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Figure 2. N1s XAS (measured at 160 K) of thick-film isonicotinic acid adsorbed at 160 K on TiO2. Comparison of spectra measured at total (bulk sensitive) and partial (surface sensitive) yield modes (0 and -340 V retardation, respectively) shows that the first peak is emphasized with enhanced surface sensitivity. The inset shows spectra measured in total yield mode for slow and rapid evaporations of isonicotinic acid, illustrating that the splitting is quenched for the rapid evaporation.
of the detected electrons (partial yield mode), we filter out with increasing efficiency the low energy scattered electrons originating from the bulk. The lowest energy feature is enhanced in this more surface-sensitive partial yield mode, indicating that the higher-energy resonance is associated with molecules situated beneath the surface layer. Our results show that we have a surface core-level shift (SCLS), where a different electronic structure is observed for the surface molecules compared to those in the bulk of the film. Contributions to the SCLS can generally be divided into several contributions.21 An altered dielectric core-hole screening response near the surface as compared to within the bulk is one candidate. This process is known to result in a surface binding energy shift for nonpolar organic molecules in the condensed phase such as anthracene22 and C60.23 Such shifts are typically in the range of 0.1 to 0.4 eV24 and positiVe due to less efficient screening at the surface. The dominant contribution to the large negatiVe SCLS observed for isonicotinic acid is therefore attributed to an effect of the chemical environment, which for molecules in the surface must be different from those in the bulk. We attribute this difference to intermolecular hydrogen bonding between the carboxylic group and the nitrogen of the pyridine ring. Such head-to-tail hydrogen bonding is already known from X-ray crystallography studies to connect the isonicotinic acid molecules in infinite chains in the solid state.8 Support for the assignment of the photoelectron shifts to a hydrogen bond interaction is provided by our calculations. Results from the DFT calculations carried out for H-bonded isonicotinic dimer and trimer systems are shown in Figure 3. The strength of the hydrogen-bond interaction increases with cluster size due to a process known as cooperative hydrogen bonding25,26 exhibited by systems such as water, amides, and peptides.27 The calculated shift of the H-bonded peak (N2) with respect to the free nitrogen (N1) is 1.24 eV for the dimer, and a total shift of 1.65 eV is obtained for the trimer (N3-N1). These calculated shifts are in good agreement with the experimental shift of 1.7 eV shown in Figure 1, strongly supporting the assignment of this effect to a hydrogen bond interaction. The trimer calculation also illustrates the different ionization potentials of the H-bonded nitrogen atoms in different environ-
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J. Phys. Chem. B, Vol. 105, No. 10, 2001 1919
Figure 4. Schematic representation of an isonicotinic acid thick-film assembled via hydrogen-bond interactions. The molecular plane is found experimentally to lie tilted with respect to the surface normal, not shown here.
Figure 3. Optimized hydrogen-bonding geometries of isonicotinic acid dimer and trimer molecules. The decreased H-bonding distance (indicated on the figure) for the trimer is the result of cooperative hydrogen bonding. Calculated N1s ionization potentials are given for each nitrogen atom, which give rise to the calculated total energy shifts shown.
ments (N2 and N3). These energy differences are relatively large in the case of the trimer; however, it is feasible that for the case of polymeric H-bonded chains (known for the solid-state structure of isonicotinic acid) that these different energies might tend to converge, resulting simply in a broadening of the N1s XPS feature. Such a complex calculation is beyond the scope of this letter; nevertheless, a Gaussian-Lorentzian curve fit of the XPS data of Figure 1 does indeed show that the fwhm of the H-bonded feature is greater (1.53 eV) than that of the terminating nitrogen (1.36 eV). Experimental18 and theoretical28 investigations of monolayers of the isonicotinic acid dimer, bi-isonicotinic acid (2,2′bipyridine-4,4′-dicarboxylic acid), show that the molecule covalently bonds to the rutile TiO2(110) surface through the deprotonated carboxylic groups, with an upright geometry. We have found via X-ray absorption and photoelectron spectroscopy experiments that the isonicotinic acid monolayer also binds to the TiO2(110) surface in this way, adopting a maximum tilt angle of 30° with respect to the surface normal.29 We therefore propose a possible structure for the thick film similar to that shown schematically in Figure 4, combining aspects of the known bulk and interface structures. Subsequent layers of isonicotinic acid
are hydrogen bonded to the previously adsorbed layers to form polymeric chains.2 This model also satisfies the observation of a surface core-level shift because the terminating layer must contain non-hydrogen-bonded nitrogen atoms. However, such chains, if they are indeed formed at the surface, must be tilted with respect to the surface normal (more so than the monolayer) because we still observe significant H-bond induced intensity in the XPS at grazing emission angle. Angle resolved XAS confirms that the aromatic ring adopts a tilted geometry. The formation of hydrogen-bonded polymers on the surface requires a highly ordered molecular arrangement. We have investigated the bonding kinetics by comparing XPS and XAS data measured for two different preparations of isonicotinic acid as described in the Experimental Section. We refer to these preparation conditions as “slow” and “rapid”, for which the total exposure and substrate temperature remains constant, but the adsorbate pressure is 100 times greater for the rapid evaporation. It can be seen from the inset of Figure 1 that the high-binding energy N1s feature in the XPS is strongly suppressed for the rapid evaporation. Similarly, virtually all H-bonding-associated π* structure is absent in the XAS of the rapid evaporation, as shown in the inset of Figure 2. These results suggest that the rate at which subsequent layers impinge on the surface plays an important role, such that the rate-determining step is the molecular reorientation that allows further interaction of the free nitrogen. We have shown that hydrogen-bond interactions give rise to very large XPS and XAS shifts and that these techniques can be successfully applied to the investigation of hydrogen bond interactions at interfaces of organic molecules. XAS in particular is flexible enough to be applied to buried interfaces or buried films, with a clear energetic signature for H-bonding. In addition, the magnitudes of the shifts observed in the photoelectron spectra are large enough to allow the use of laboratory-based X-ray sources for such analysis. These techniques are therefore powerful analysis tools applicable to the new generation of self-
1920 J. Phys. Chem. B, Vol. 105, No. 10, 2001 assembled nanoscale devices that exploit noncovalent interactions such as hydrogen bonding for their fabrication. Acknowledgment. We thank R. Nyholm, S. Wiklund, K. Hansen, and A. Sandell for help with beamline D1011, M. Nagasono and D. Nordlund for help with beamline I511, and M. Lundwall for useful discussions. Also, L. Bolkegård and J.-O. Forsell for help with the design and construction of the evaporation equipment. We are grateful to the Consortium on Clusters and Ultrafine Particles, which is funded by Stiftelsen fo¨r Strategisk Forskning, and to Go¨ran Gustafsons Sfiftelsen and Tekrikvetenskapliga Forskningsra˚det for financial support. References and Notes (1) Cai, C.; Bo¨sch, M.; Mu¨ller, B.; Tao, Y.; Ku¨ndig, A.; Bosshard, C.; Gan, Z.; Biaggio, I.; Liakatas, I.; Ja¨ger, M.; Schwer, H.; Gu¨nter, P. AdV. Mater. 1999, 11, 745. (2) Cai, C.; Mu¨ller, B.; Weckesser, J.; Barth, J. V.; Tao, Y.; Bo¨sch, M.; Ku¨ndig, A.; Bosshard, C.; Biaggio I.; Gu¨nter, P. AdV. Mater. 1999, 11, 750. (3) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. 2000, 39, 1230. (4) Fredericks, J. R.; Hamilton A. D. ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Lehn, J.-M., Eds.; Pergamon: New York, 1996; Vol. 9, p 565. (5) Witesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (6) Philip, D.; Stoddart, J. F. Angew. Chem. 1996, 108, 1242. (7) Philip, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154. (8) Takusagawa, F.; Shimada, A. Acta Crystallogr. 1976, B32, 1925. (9) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (10) Siegbahn, K.; Nordling, C.; Johansson, G.; Hedman, J.; Heden, P. F.; Hamrin, K.; Gelius, U.; Bergmark, T.; Werme, L. O.; Manne, R.; Baer, Y. ESCA Applied to free molecules, North-Holland 1971, 127. (11) Denecke, R. et al., J. Electron Spectrosc. Relat. Phenom. 1999, 101-103, 971. (12) Nyholm, R.; Svensson, S.; Nordgren J.; Flodstro¨m, A. Nucl. Instrum. Meth. A 1986, 246, 267.
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