8400
Langmuir 2002, 18, 8400-8406
Infrared and Atomic Force Microscopy Imaging Study of the Reorganization of Self-Assembled Monolayers of Carboxylic Acids on Silver Surface Y. T. Tao,*,† C. Y. Huang,‡ D. R. Chiou,§ and L. J. Chen§ Institute of Chemistry, Academia Sinica, Taipei, Taiwan, Republic of China, National Hsin-Chu Teachers College, Hsin-Chu, Taiwan, People’s Republic of China, and Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, Republic of China Received April 4, 2002. In Final Form: July 11, 2002 Self-assembled monolayers of carboxylic acids on a silver surface underwent a reversible reorganization process to form discrete clusters of hydrogen-bonded free acids upon exposing the monolayer assembly to H2S vapor. The clusters had a wide range of sizes, from nearly one micrometer to tens of nanometers or smaller. Restoration of the monolayer film from the clusters, driven by the reaction between the carboxylic acid molecules and the basic surface sites, proceeded on the surface through surface migration of molecules in the clusters. The process was real-time imaged by tapping mode atomic force microscopy.
Introduction n-Alkanoic acids and their aromatized derivatives can form highly ordered monomolecular assemblies on metal oxide surfaces.1-5 In particular, the monolayers formed on the silver surface exhibit nearly crystalline packing of the hydrocarbon chains.3 Although the ionic nature of the binding interaction in this system renders it a less stable film as compared to the n-alkanethiolate monolayer on coinage metals of Cu, Ag, and Au,6 this system offers a good opportunity to understand the interplay of binding interactions and intermolecular interactions in determining the ultimate structure of a monolayer assembly.3 We reported recently that a brief exposure of the carboxylate monolayer on Ag to HCl or H2S vapor would lead to a reorganization process such that the carboxylate adsorbates in the monolayer assembly, after being protonated, aggregated into discrete clusters of hydrogenbonded dimers of the carboxylic acid.7,8 The infrared spectroscopic results also suggested that the process was reversible in that a carboxylate monolayer was restored if the aggregated sample substrate was left in the ambient atmosphere for enough time. Yet there was no direct evidence of the reorganization process nor the size information of the particles. We hereby report a direct imaging of the reorganization process by atomic force microscopy (AFM), which is in corroboration with the spectroscopic results. Crystallites with sizes ranging from nearly one micrometer to tens of nanometers were obtained * Corresponding author e-mail:
[email protected]. † Academia Sinica. ‡ National Hsin-Chu Teachers College. § National Taiwan University. (1) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (2) Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986, 132, 93. (3) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350. (4) Chang, S. C.; Chao, I.; Tao, Y. T. J. Am. Chem. Soc. 1994, 116, 6792. (5) Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032. (6) Laibinis, P.; Whitesides, G. M.; Parikh, A. N.; Tao, Y. T.; Allara, D. L.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (7) Tao, Y. T.; Hietpas, G. D.; Allara, D. L. J. Am. Chem. Soc. 1996, 118, 6724. (8) Tao, Y. T.; Lin, W. L.; Hietpas, G. D.; Allara, D. L. J. Phys. Chem. B 1997, 101, 9732.
upon exposure to H2S. The crystallites slowly reassembled into a monolayer. Both the forward and the reverse processes were highly temperature dependent. Experimental Section The saturated n-alkanoic acids were obtained from Merck and were recrystallized from ethanol. The aromatized acids (4alkoxybenzoic acid and 4-alkoxybiphenyl-4′-carboxylic acid) were prepared as described before.8 The silver substrates were prepared by thermally evaporating (at a rate of ∼5 Å/s at 3 × 10-6 Torr) 2000 Å of silver on the surface of a chromium-primed 2-in. silicon wafer. The monolayer was prepared by selfassembling from the hexadecane or hexadecane-THF solution of the respective acids as described before.3,9 For H2S-induced reorganization experiments, the substrate with monolayer assembly was placed in a 1.0-L chamber prepurged with nitrogen and pre-equilibrated at the ambient or desired temperature. Ten milliliters of H2S gas (99%, Aldrich Chemical) was injected into the chamber through a needle and a syringe to make up ∼1% concentration. After 30 s (or a specified length of time), the substrate was taken out and subjected to infrared (IR) or AFM characterization. For restoration to a monolayer, the H2S-exposed substrate surface was kept in a sample holder in an ambient or oven-maintained atmosphere at a specified temperature. IR measurements were taken at different time intervals. The reflection-absorption IR spectra were recorded in a single reflection mode with a grazing incidence angle of 86°, using a Digilab FTS 60A Fourier transform infrared spectrometer (Bio-Rad, Cambridge, MA). A liquid nitrogen-cooled MCT detector was used. For fast kinetic IR measurements, the sample substrate was placed in the sample compartment of a custom-made external beam optic. H2S vapor was injected into the compartment to make up ∼1% concentration, and spectra were taken every 2 s at 4 cm-1 resolution. The AFM measurements were performed with a NanoScope IIIa AFM (Digital Instruments, Santa Barbara, CA) in an airconditioned room at around 25 °C. Commercial silicon cantilevers (Nanosensors, Germany) with typical spring constants of 21-78 N/m were used to operate the AFM in tapping mode. The sample heater has two thermocouples for temperature measurements at the sample and the heater base, usually in direct contact with a scanner cap. An embedded microheater was attached to the sample back to control the sample surface temperature. The temperatures were controlled at desired values to within (0.1 (9) Tao, Y. T.; Lee, M. T.; Chang, S. C. J. Am. Chem. Soc. 1993, 115, 9547.
10.1021/la025805u CCC: $22.00 © 2002 American Chemical Society Published on Web 09/24/2002
Self-Assembled Carboxylic Acid Monolayers on Silver
Langmuir, Vol. 18, No. 22, 2002 8401
Figure 1. Reflection-absorption IR spectra of the n-eicosanoic acid monolayer before and after H2S exposure. °C. Every sample was scanned for about 1 h. Images were taken continuously with a scan rate of 1 Hz. It took ∼4 min to finish one image.
Results and Discussion Figure 1 shows the reflection-absorption IR spectra of the n-eicosanoic (C20) acid monolayer before and after exposure to H2S vapor. Direct evidence for protonation of the carboxylate headgroups was provided by the emerging of a peak at 1701 cm-1 for the carbonyl stretching of the free acid at the expense of the peak at 1400 cm-1 for the carboxylate. The frequency 1701 cm-1 was indicative of (as well as in agreement with) the hydrogen-bonded (Hbonded) dimer of the carboxylic acid molecules.10 Accompanying the protonation process was a dramatic increase (∼15-fold) of the peak intensities for the symmetric and asymmetric CH2 stretching modes at 2916 cm-1 (νaCH2) and 2849 cm-1 (νsCH2), respectively. On the basis of the selection rule for reflection-absorption IR,11 the increase was attributed to the orientation change of the molecular chains: from the nearly perpendicular alignment of the molecular axis for the trans-extended chains to an alignment closely parallel to the substrate surface.7 The formation of a cyclic H-bonded dimer from two carboxyl groups was indicated by the out-of-plane bending of O-H- -O at 942 cm-1.12 The spectroscopic results suggested that the monolayer assembly had reorganized to discrete clusters of H-bonded carboxylic acids.7,8 To gain more insight into the process, fast kinetics IR measurements were carried out. When H2S vapor was injected into the IR sample chamber where the monolayer surface was loaded, spectra were taken every 2 s for the first 30 s, after which the spectra appeared the same as the (10) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Wiley: New York, 1975. (11) Allara, D. L.; Parikh, A. N. J. Chem. Phys. 1992, 96, 927. (12) Hayashi, S.; Umemura, J. J. Chem. Phys. 1975, 63, 1732.
Figure 2. Kinetic measurement of the n-hexadecaoic acid monolayer surface upon exposure to H2S. The consecutive traces are 2 s apart.
spectrum taken 15 min after the H2S exposure. Figure 2 shows the results. It was noted that over the first 30 s the carboxylate peak at 1400 cm-1 decreased continuously, whereas the carbonyl group for the -CO2H group at 1701 cm-1 grew concomitantly. In the high-frequency region, a broadening of the peak toward the higher frequency side of the methylene stretching vibration at 2918 cm-1 occurred initially (as shown by the arrows in the lower traces). Then the peak grew in intensity and eventually approached a more symmetrical and sharp band shape (indicated by the arrow in the top trace) at 2916 cm-1. This indicated an initial disordering of the hydrocarbon chains, followed by crystallization of the molecules at a later stage.13 As the peak at 1701 cm-1 was assigned to the carbonyl stretch for the H-bonded dimer, the presence of only this peak for the carbonyl group all through the process suggested direct reorganization to the H-bonded dimer form after protonation. This is in contrast with the situation where more than one type of carbonyl stretches was present with certain aromatized derivatives (see below). That the carboxylate absorption decreased continuously and the carboxyl absorption at 1701 cm-1 increased continuously implied that the protonation occurred progressively, as is the formation of the H-bonded dimer. The observation of one and only one type of carbonyl stretching vibration also ruled out the possibility that protonation of all acids occurred at the same time and formed a meta-stable assembly in which the carboxyl groups are H-bonded to the neighbors (see below) and reorganized afterward. This sulfidization-protonationreorganization process was completed in tens of seconds. (13) The disordered hydrocarbon gives CH2 absorption at higher frequency: Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta. 1963, 19, 85.
8402
Langmuir, Vol. 18, No. 22, 2002
Tao et al.
Figure 3. AFM micrographs of (a) a bare Ag surface, (b) n-eicosanoate monolayer on Ag before H2S exposure, and (c) after H2S exposure.
Figure 3 shows the AFM images of the effect of H2S treatment on the monolayer surface. A featureless surface was observed for both the bare silver surface and the surface with a monolayer of n-eicosanoic acid adsorbed. At higher resolution, one observed a similarly rough and grainy surface for both the bare silver and the monolayercovered silver. A large number of grain boundaries and defect regions are conceivable for the monolayer-covered surface. After the substrate was exposed to H2S for 30 s, clusters of various sizes appeared. The size distribution was wide. The section analysis on the representative sample showed that the largest one could reach a lateral dimension of 1 µm and a height of ∼200 nm. The smaller ones were in the order of tens of nanometers in dimension. On the basis of the volume of a single molecule (assuming
a rod of ∼20 Å2 × 27 Å),14 the largest cluster would contain ∼108 molecules! However, the total number of molecules present in the clusters was estimated to be less than the number of molecules present in the original monolayer.15 The wide distribution in sizes may relate to the heterogeneous nature of a deposited silver surface. This reorganization of monolayer assembly into clusters was (14) The cross sectional area for fatty acids: Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966; p 186. The length is estimated by calculation. (15) For an area of 10 µm × 10 µm, there would be at least 9 × 108 molecules based on a roughness factor of ∼1.8 for our deposited Ag surface. The total number of molecules present in the clusters in Figure 3c (by measuring the volume of visible clusters) was estimated to be ∼7 × 108. Discrepancies may be due to the detection limit of very small clusters by the AFM at this resolution.
Self-Assembled Carboxylic Acid Monolayers on Silver
Langmuir, Vol. 18, No. 22, 2002 8403
Figure 4. AFM micrographs of the restoration process of the monolayer from clusters of n-eicosanoic acids. Except for panel a (which was taken at 25 °C as the starting state) panels b-h were taken at 38 °C. Each micrograph is ∼8 min apart.
rationalized to be driven by the formation of Ag2S, a low Ksp salt. The Ag2S layer or the S adlayer blocked the carboxylic acid from interacting with the silver underlayer.16 Formation of H-bonded dimers, driven by an interaction energy of ∼12.5 kcal,17 prevailed. It was a surface version of precipitation-crystallization of the carboxylic acid by acidification of a two-dimensional carboxylate solution. Ambient storage of the clustered sample led to a gradual recovery of the original IR spectra. The intensities of the methylene stretching modes gradually diminished to nearly the original intensities, and the carboxylate absorption at 1400 cm-1 reappeared. A new peak at 1512 (16) Sulfur treatment has been shown to change the adsorption behavior of heteroaromatic compounds on Pt: Svetlicic, V.; Clavilier, J.; Zutic, V.; Chevalet, J. J. Electroanal. Chem. 1993, 344, 145. (17) Herzberg, G. Molecular Spectra and Molecular Structure II, 1st ed.; Van Nostrand Reinhold Inc.: New York, 1945; p 536.
cm-1 (assigned to the asymmetric stretch of carboxylate10) showed up, which suggested a slight change in the anchoring geometry of the headgroup.3 Instead of binding symmetrically to the surface, the carboxylate group now binds to the surface in a tilted fashion because of a change of the property of the surface (a sulfidized silver). The restoration process was temperature dependent. While at ambient temperatures (∼26 °C), 3-4 days were necessary to recover the original spectra; the recovery process completed in less than 1 h at 50 °C. Evaporation of the acid molecules can be excluded as a crystalline packing of the hydrocarbon chains within a monolayer was suggested from the IR results (2918 and 2849 cm-1 observed for νaCH2 and νsCH2, respectively, in recovered spectra, typical of crystalline packing13). The projected process was real-time imaged by AFM. Figure 4 shows the time-dependent micrographs of the clusters of the
8404
Langmuir, Vol. 18, No. 22, 2002
Tao et al.
Figure 5. AFM micrographs of the restoration of the monolayer from clusters of 4-hexadecyloxybenzoic acids at 60 °C.
acid molecules. The temperature was kept at 38 °C. Clearly a reassembling into monolayers was completed in ∼90 min. Re-exposure to H2S gave the clusters again, thus confirming that the featureless surface in Figure 4h was not due to evaporation of the film materials, as already shown in the IR observations. Similar observations were obtained for the n-hexadecanoate monolayer when monitored at 28 °C. The reassembling process was suggested to be driven by an acid-base reaction between the carboxyl groups and the basic sites (an interaction energy of ∼33.5 kcal18). The re-formation of basic silver oxide was indicated by XPS.19 (18) Canning, N. D. S.; Madix, R. J. J. Phys. Chem. 1984, 88, 2437.
Introduction of a phenyl ring into the long alkyl chain increases the intermolecular interaction. The clustering of 4-hexadecyloxybenzoic acid molecules still occurred when the carboxylate monolayer was exposed to H2S at ambient temperatures. A single carbonyl stretch at 1689 cm-1 resulted. The restoration of the monolayer was nevertheless much slower. The peak for carbonyl stretching at 1689 cm-1 decreased slowly but did not totally (19) An XPS study of the silver surface exposed to H2S showed sulfur S(2P1/2) and S(2P3/2) at 160.9 and 162.1 eV, respectively. After being kept in air for 3 days, the intensity of sulfur decreased by a factor of 2, whereas the O(1S) at 531.7 eV increased substantially. Regeneration of silver oxide was suggested. Whether the surface sulfur was lost to air (in the form of SO2 by oxidation) or migrated to the interior was not clear.
Self-Assembled Carboxylic Acid Monolayers on Silver
Langmuir, Vol. 18, No. 22, 2002 8405
Figure 6. Schematic representation of a structural change upon H2S exposure of a 4-hexadecyloxybiphenyl-4′-carboxylic acid monolayer.
disappear after 1.5 months storage of the substrate surface at ambient temperatures. Increasing the temperature of the substrate accelerated the restoration so that a complete recovery of the monolayer was achieved after ∼1 h at 60 °C. Figure 5 provides micrographs for the restoration of the monolayer of 4-hexadecyloxybenzoate from the clustered sample. The clusters appeared larger and fewer than those from the straight-chain acids. This may relate to the different aggregation properties of aromatic acids. A further increase in the intermolecular interactions, by introducing a biphenyl group to the long alkyl chain, caused some different observations. Upon exposure to H2S, a moderate increase (2-3-fold) in the intensities for methylene stretches was obtained, far less than the dramatic increase (∼15-fold) found for the saturated C16 and C20 acids and the n-hexadecyloxybenzoic acid monolayer. In the low-frequency region, the carboxylate absorption at 1401 cm-1 completely disappeared, indicating total protonation of the headgroups. Nevertheless, a major peak at 1710 cm-1 and a minor shoulder at 1686 cm-1 were observed in the carbonyl stretch region. Through comparison with the spectrum of the bulk acid sample, the minor shoulder was assigned to the H-bonded dimer of the aromatic acid molecules. The higher frequency at 1710 cm-1 suggested the presence of another form of carboxylic acid. It was assigned to acid molecules that are H-bonded with neighbors into a H-bonded network8 (Figure 6). The implication is that the strong intermolecular interaction combined from a π-π stacking interaction of the biphenyl groups and the packing interaction of the C16 hydrocarbon chains locked the -CO2H headgroups in place rather than forming H-bonded dimers. The moderate increase in the intensities for CH2 stretches may result from a mixture of the uniform free acid monolayer together with a small amount of reorganized molecules. When the H2S exposure was carried out at 80 °C in an oven, the IR spectrum showed different amounts of two carbonyl stretching peaks, depending on the exposure time (Figure 7). After being exposed to H2S at this temperature for 1 min, two carbonyl stretching peaks
Figure 7. Reflection absorption IR spectra of a monolayer of 4-hexadecyloxybiphenyl-4′-carboxylic acid upon exposure to H2S at 80 °C for 1 and 10 min.
of equal intensity were observed. This suggested a substantial amount of reorganization or clusterized acid molecules. For a sample that had been exposed to H2S for 10 min at 80 °C, only one carbonyl stretch at 1686 cm-1 was left, indicating the full formation of the H-bonded
8406
Langmuir, Vol. 18, No. 22, 2002
Tao et al.
Figure 8. AFM micrographs of (a) a monolayer of 4-hexadecyloxybiphenyl-4′-carboxylate on Ag, (b) after the monolayer surface was exposed to H2S at 80 °C for 1 min, and (c) after being exposed for 10 min.
dimeric form. The intensities for CH2 stretching vibrations also increased dramatically. The biphenyl moieties were lying nearly flat, as indicated by the strong out-of-plane bending for ring C-H bonds at 834 cm-1.20 Figure 8 provides supporting AFM evidence of the process. After 1 min of exposure to H2S, only a few small particles appeared. These may be due to dimer formation at particular sites such as grain boundaries and defect regions. For the sample that was exposed to H2S at 80 °C for 10 min, clusters of various sizes were found. It was noted that the clusters were more or less rod-shaped, some of which are several micrometers in length. It should be noted that this molecule is liquid-crystalline.8 The slow crystallization process at raised temperatures led to long rod crystals. Conclusion In summary, we showed definite evidence for the clustering of the molecules in the carboxylate monolayer (20) Varsanyi, G. Vibrational Spectra of Bezene Derivatives; Academic Press: New York, 1974.
on silver upon exposure to H2S as well as the reassembling process from clusters to the monolayer. The clustering process initiated as a result of protonation of the carboxylate groups at the interface. The free carboxylic acid obtained has much lower adhesion with a sulfurized silver surface. The process involves an activation barrier that is proportional to the intermolecular interaction. With higher intermolecular interaction, such as in the case of 4-hexadecyloxybiphenyl-4′-carboxylic acid, a quasi-stable free acid monolayer can be maintained. Raising the temperature can initiate the reorganization. The reverse process, the reassembling into the monolayer from the aggregated particles, was much slower. A much higher kinetic barrier due to intermolecular interaction and the H-bonding interaction in the carboxylic acid dimer is involved. Acknowledgment. We thank the National Science Council, Republic of China, and Academia Sinica for financial support of the work. LA025805U
