Elucidation of the Deposition Processes and Spatial Structures of

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Elucidation of the Deposition Processes and Spatial Structures of Alkanethiol and Arylthiol Molecules Adsorbed on Pt(111) Electrodes with in Situ Scanning Tunneling Microscopy Yaw-Chia Yang,† Ya-Pei Yen,† Liang-Yueh Ou Yang,†,‡ Shueh-Lin Yau,*,†,§ and Kingo Itaya*,‡,§ Department of Chemistry, National Central University, Chungli, Taiwan 320, Republic of China, Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 04, Sendai 980-8579, Japan, and CREST, JST, 4-1-8 Kawaguchi, Saitama 332-0012, Japan Received May 9, 2003. In Final Form: July 12, 2004 In situ scanning tunneling microscopy (STM) was used to examine the spatial structures of n-alkane thiols (1-hexanethiol, 1-nonanethiol, and 1-octahexanethiol) and arylthiols (benzenethiol and 4-hydroxybenzenethiol) adsorbed on well-ordered Pt(111) electrodes in 0.1 M HClO4. The electrochemical potential and molecular flux were found to be the dominant factors in determining the growth mechanisms, final coverages, and spatial structures of these organic adlayers. Depending on the concentrations of the thiols, deposition of self-assembled monolayers (SAMs) followed either the nucleation-and-growth mechanism or the random fill-in mechanism. Low and high thiol concentrations respectively produced two ordered structures, (2 × 2) and (x3 × x3)R30°, between 0.05 and 0.3 V. On average, an ordered domain spanned 500 Å when the SAMs were made at 0.15 V, but this dimension shrank substantially once the potential was raised above 0.3 V. This potential-induced order-to-disorder phase transition resulted from a continuous deposition of thiols, preferentially at domain boundaries of (x3 × x3)R30° arrays. All molecular adlayers were completely disordered by 0.6 V, and this restructuring event was irreversible with potential modulation. Since all thiols were arranged in a manner similar to that adopted by sulfur adatoms (Sung et al. J. Am. Chem. Soc. 1997, 119, 194), it is likely that they were adsorbed mainly through their sulfur headgroups in a tilted configuration, irrespective of the coverage. Both the sulfur and phenyl groups of benzenethiol admolecules gave rise to features with different corrugation heights in the molecular-resolution STM images. All thiols were adsorbed strongly enough that they remained intact at a potential as negative as -1.0 V in 0.1 M KOH.

Introduction Aided by modern surface characterization tools and the use of single-crystal electrodes, the study of electrochemical interfaces in the past decade has reached an atomic or molecular-level sophistication.1-3 Many studies have shown that the environment can dominate the adsorption and reactions of adsorbates at interfaces. For example, an early study on the adsorption of methanol molecules on Pt(111) revealed the formation of methoxy and Cbonded species in a vacuum and an electrochemical environment, respectively.4 A marked contrast is also noted for the adsorption of organoiodide compounds at Pt electrodes.5 While it is shown that iodobenzene molecules readily decompose to produce benzene and iodine adatoms on Pt(111) in a vacuum,6 they could be adsorbed molecularly on a Pt(111) electrode at room temperature under * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 81-22-2145380. Fax: 81-222145380. † National Central University. ‡ Tohoku University. § CREST, JST. (1) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (2) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (3) Soriaga, M. P. Chem. Rev. 1990, 90, 771. (4) Franaszczuk, K.; Herror, E.; Zelenay, P.; Wieckowski, A.; Wang, J.; Masel, R. I. J. Phys. Chem. 1992, 96, 8509. (5) Chen, J.-H.; Yau, S.-L.; Chang, S.-C. J. Phys. Chem. B 2002, 106, 9079. (6) Cabibil, H.; Ihm, H.; White, J. M. Surf. Sci. 2000, 447, 91.

proper potential control.5 Among all possible factors, the electrochemical potential, which governs the charge density at metal electrodes, frequently determines the configuration and strength of adsorption. There are a wide variety of techniques used for interface characterization, and scanning tunneling microscopy (STM) is arguably the most direct means to probe surface structures. Numerous STM studies have illustrated its use in probing the realspace structures and dynamics of electrode processes in real time.1,5,7 In terms of organic adsorbates, self-assembled monolayers (SAMs) of organosulfur compounds on Au(111) have attracted the most attention in the past decade, as they can be used to design specific sensors, to study surfactants in electrodeposition, for corrosion prevention, and to mimic electron transfer.8-17 Au(111) in the form of a single crystal (7) Wu, H.-C.; Yau, S.-L. J. Phys. Chem. B 2001, 105, 6965. (8) Ulman, A. Chem. Rev. 1996, 96, 1533. (9) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219. (10) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (11) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444. (12) Chidsey, C. E. D.; Bertozzi, C. R.; Putrinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (13) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. J. Electroanal. Chem. 1998, 441, 109. (14) Schneeweiss, M. A.; Kolb, D. M. Phys. Status Solidi A 1999, 173, 51. (15) Poirier, G. E. Langmuir 1997, 13, 2019. (16) McDermott, C. A.; McDermott, M. T.; Green, J. B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257.

10.1021/la030198b CCC: $27.50 © 2004 American Chemical Society Published on Web 10/08/2004

Spatial Structures of Alkanethiols and Arylthiols

or film has been the most popular substrate to support SAMs, because gold is the easiest metal to work with in the ambient environment. The production of ordered SAMs on Au(111) derives from the strong S-Au interaction and the van der Waals intermolecular interaction between the organic functional groups.8 It has been shown that on Au(111) alkanethiol molecules are adsorbed generally with their sulfur headgroups bonded at the thermodynamically most favorable face-centered cubic (fcc) 3-fold hollow sites and the carbon backbones are tilted by ∼27° from the surface normal to optimize intermolecular interactions.8 On the other hand, it was reported that n-alkyl thiols can dimerize to form disulfides on Au(111).18 The bonding schemes of alkanethiol molecules on other metal surfaces, such as silver and copper,19,20 were also examined, showing substantial differences in molecular orientation and spatial structure. We reported recently SAMs on Ru(0001) electrodes, showing that thiols are adsorbed strongly enough that they are not desorbed at the onset of water reduction in alkaline solutions.21 Reported studies of SAMs on Pt surfaces are reviewed here. Stern et al. used rigorous vacuum techniques to study arylthiols adsorbed on Pt(111) and found that these molecules are oriented upright with the sulfur headgroups bonded to Pt(111).22 Low-energy electron diffraction (LEED) shows that benzenethiol adlayers are disordered, a result presumably due to the insufficient molecular coverages produced by dosing in aqueous solutions.22 The study of alkanethiols on Pt(111) is limited, and methanethiol seems to be the only alkanethiol molecule studied in a vacuum thus far.23,24 LEED, X-ray photoelectron spectroscopy (XPS), normal incidence X-ray standing wave spectroscopy, and time-of-flight scattering and recoiling spectroscopy (TOF-SARS) were used to examine methanethiol/Pt(111). Raising the temperature from 250 to 373 K induces dehydrogenation reactions to produce a series of organic fragments, including methanethiolate, thioformaldehyde, and hydrogen-free S-C species.23 A diffuse (x3 × x3)R30° LEED pattern, ascribable to the adsorption of methanethiolate, has been identified.23,24 Results from TOF-SARS suggest that the methanethiol adspecies are adsorbed through their sulfur heads at fcc 3-fold hollow sites and the S-C axis is tilted by 45° from the surface normal.24 In comparison, benzenethiol admolecules are adsorbed upright on Au(111), forming a disordered adlayer in air but an ordered (x13 × x13)R14° structure in electrolytes.25-27 To our knowledge, there has been no report of organosulfur compounds adsorbed on Pt single-crystal electrodes in the ambient environment. In this study, we show (17) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409. (18) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (19) Nemetz, A.; Fischer, T.; Ulman, A.; Knoll, W. J. Chem. Phys. 1993, 98, 5912. (20) Kondoh, H.; Saito, N.; Matsui, F.; Yokoyama, T.; Ohta, T.; Kuroda, H. J. Phys. Chem. B 2001, 105, 12870. (21) Ou Yang, L.-Y.; Yau, S.-L.; Itaya, K. Langmuir 2004, 20, 4596. (22) Stern, D. A.; Wellner, E.; Salaita, G. N.; Laguren-Davidson, L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. Soc. 1988, 110, 4885. (23) Lee, J. J.; Fisher, C. J.; Bittencourt, C.; Woodruff, D. P.; Chan, A. S. Y.; Jones, R. G. Surf. Sci. 2002, 516, 1. (24) Kim, S. S.; Kim, Y.; Kim, H. I.; Lee, S. H.; Lee, T. R.; Perry, S. S.; Rabalais, J. W. J. Chem. Phys. 1998, 109, 9574. (25) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C.-H. Langmuir 1997, 13, 4018. (26) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Surf. Sci. 1999, 425, 101. (27) Wan, L.-J.; Terashima, M.; Noda, H.; Osawa, M. J. Phys. Chem. B 2000, 104, 3563.

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that the electrochemical potential plays a decisive role in guiding the spatial structures of SAMs on Pt(111). Only between 0.05 and 0.3 V did in situ STM reveal long range ordered adlattices of (2 × 2) and (x3 × x3)R30° for all organosulfur molecules examined in this study. These structures, the same as those of sulfur adatoms on Pt(111),28,29 suggest the prominent role of the S-Pt interaction in guiding the adsorption of these organosulfur molecules. Experimental Section Pt single-crystal bead electrodes were prepared by melting the end of a Pt wire, and their pretreatment used the annealing and quenching procedure.30 The surface state of the as-prepared Pt electrode could be diagnosed easily with cyclic voltammetry or high-resolution STM imaging in 0.1 M HClO4. Typically, the thermal oxide layer produced by the annealing procedure was reduced electrochemically by applying a potential of 0.05 V prior to the introduction of the organic compound of interest. Ultrapure perchloric acid, 1-hexanethiol, 1-dodecanethiol, and 1-octadecanethiol were purchased from Merck Inc. (Darmstadt, Germany). 1-nonanethiol was obtained from Lancaster (Lancashire, U.K.), and ethanol was purchased from Aldrich (Saint Louis, MO). They were used as received without further purification. Millipore triple-distilled water was used to prepare all the needed solutions. The concentrations of benzenethiol, 1-hexanethiol, 1-nonanethiol, 1-dodecanethiol, and 1-octadecanethiol in water are 7.58, 1.50, 0.041, 0.0011, and 0.000 83 mM at 25 °C.31 A concentration of 0.1 mM in solution is equivalent to 1 Torr of pressure in the gas phase, which is roughly 6-7 orders of magnitude higher than the typical dosing level in an ultrahigh vacuum (UHV). The drastically different dosing level might influence the orientations of molecular adsorbates. The scanning tunneling microscope was a Nanoscope-E (Santa Barbara, CA), and the tip was made of tungsten (diameter, 0.3 mm) prepared by electrochemical etching in 2 M KOH. The details of the STM imaging experiments were described previously. We always used the constant-current mode in the in situ STM imaging experiments. This operation mode was able to give highquality STM images. Reversible hydrogen electrodes (RHEs) were used in the electrochemical and STM measurements, and all potentials refer to a RHE scale.

Results and Discussion Linear Sweep Voltammetry. Figure 1a shows the steady-state cyclic voltammograms (CVs) obtained with an as-prepared Pt(111) electrode without (solid line) and with (dotted line) a monolayer of benzenethiol (BT) molecules in 0.1 M HClO4. The characteristics contained in the solid trace confirm the well-defined surface state of the Pt(111) electrode.30 Adsorption of BT on the Pt(111) electrode resulted in the elimination of all features, as the dotted line in Figure 1a shows. These CV results indicate that BT interacted with Pt(111) so strongly that it inhibited the adsorption of ionic species, such as protons and perchlorate anions, and water molecules. Meanwhile, the double-layer charging current is subsequently reduced to one-half of that of a bare Pt(111). All organosulfur admolecules (1-hexanethiol, 1-nonanethiol, 1-dodecanethiol, and 1-octadecanethiol) on Pt(111) produced similar CV results (not shown here). We conducted extensive potential cycling between 0.05 and 0.9 V to examine the stability of these organosulfur adlayers on Pt(111), and results show that all thiol monolayers were stable within (28) Billy, J.; Abon, M. Surf. Sci. 1984, 146, L525. (29) Sung, Y.-E.; Chrzanowski, W.; Zolfaghari, A.; Jerkiewicz, G.; Wieckowski, A. J. Am. Chem. Soc. 1997, 119, 194. (30) Clavilier, J.; Rodes, A.; El Achi, K.; Zamakhchari, M. A. J. Chem. Phys. 1991, 88, 1291. (31) Howard, P. H., Meylan, W. M., Eds. Handbook of Physical Properties of Organic Chemicals; CRC Press: New York, 1997.

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Figure 1. (a) Cyclic voltammograms (CVs) obtained with clean (solid trace) and benzenethiol-coated (dotted trace) Pt(111) electrodes in 0.1 M HClO4. (b) The 41st CV scan between 0.05 and 1.15 V for a Pt(111) electrode modified with a monolayer of 1-hexanethiol. The scan rate was 50 mV/s.

this potential range. Opening the potential window to 1.15 V, however, yielded marked increases of current density at potentials more positive than 1.0 V for all admolecules. These anodic reactions are ascribed to the irreversible oxidation or desorption of the admolecules, since extensive potential cycling between 0.05 and 1.15 V partially restored the Pt(111) surface. For example, Figure 1b shows the CV profile of the 41st potential cycle between 0.05 and 1.15 V at 50 mV/s for a Pt(111) electrode coated with a monolayer of 1-hexanethiol. The morphology of this CV profile resembles that of a well-ordered Pt(111) electrode.30 Apparently, the thiol molecules were gradually but eventually oxidized to CO2, leaving a bare Pt surface. Since multiple electrons were involved in this electrode process, extensive potential cycling was necessary to facilitate these changes. As demonstrated previously with SAMs on Au(111), thiol admolecules can be reductively desorbed at potentials more negative than -0.85 V (vs saturated calomel electrode (SCE)) in 0.1 M NaOH.32-34 The potential at which an admolecule desorbs reflects the strength of intermolecular interaction. For example, propanethiol is desorbed from Au(111) at -0.85 V. This is 150 and 300 mV more positive than that of hexanethiol and dodecanethiol, respectively.34 We adapted this idea and performed similar experiments with Pt(111) coated with organosulfur adsorbates. However, potential excursion to the onset of water reduction in 0.1 M KOH could not detach any of the molecules studied here. Thus, organosulfur admolecules may be adsorbed more strongly on Pt(111) than on Au(111). The situation of Ru(0001) electrodes resembles those of Pt(111) in this regard.21 Also, to investigate how an organic adlyer affects electron transfer across an interface, we performed voltammetric experiments with Pt(111) electrodes modified with sulfur adatoms and organosulfur molecules in 0.1 M HClO4 containing 1 mM hydroquinone. These experiments could be helpful to elucidate the nature of thiols on Pt(111). Sulfur atoms and organosulfur molecules (32) Yang, D.-F.; Morin, M. J. Electroanal. Chem. 1997, 429, 1. (33) Wong, S.-S.; Porter, M. D. J. Electroanal. Chem. 2000, 485, 135. (34) Vela, M. E.; Martin, H.; Vericat, C.; Andreasen, G.; Herna´ndez Creus, A.; Salvarezza, R. C. J. Phys. Chem. B 2000, 104, 11878.

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Figure 2. CVs obtained with Pt(111) modified with a monolayer of (a) sulfur atoms and (b) benzenethiol in 1 mM hydroquinone + 0.1 M HClO4. The multiple CV traces in part b were obtained with different scan rates ranging from 15 to 500 mV/s. The insets show the linear relationship between the peak currents and the square roots of the scan rates.

were adsorbed under potential control at 0.2 V from aqueous dosing solutions containing Na2S and benzenethiol, respectively. The results obtained with Smodified Pt(111) electrodes shown in Figure 2a reveal the presence of a well-developed redox couple near 0.7 V for all scan rates ranging from 15 to 100 mV/s. This reversible redox feature with a peak separation (∆Ep) of 60 mV is ascribed to the 2e-/2H+ redox process of hydroquinone and benzoquinone. A redox couple was also observed at BT-modified Pt(111) (Figure 2b), but with a much larger peak separation (∆Ep ) 300 mV). The insets in Figure 2 reveal the relationship between peak current (ip) and xscan rate (xv), and the linear correlation indicates that this redox feature is diffusion-controlled.35 Also, these results strongly suggest that the BT admolecules did not decompose to produce sulfur adatoms and organic fragments upon their adsorption at 0.2 V. SAMs of all the other thiols examined in this study yielded results similar to those of BT. In Situ STM Imaging of Organosulfur Molecules on Pt(111). We performed in situ STM experiments on SAMs/Pt(111) under potentiostatic conditions in 0.1 M HClO4. Normally, we would achieve atomic resolution of the Pt(111) substrate prior to the addition of a dosing solution. Benzenethiol, hexanethiol, and 1-nonanethiol, but not alkanethiols with backbones longer than C10, are sufficiently soluble in 0.1 M HClO4 to yield full monolayers. An organic solvent such as hexane was used to carry the less soluble thiol molecules to perform dosing, but in situ STM results show that 1-dodecanethiol and 1-octadecanthiol were mostly disordered. Ethanol, which works well for gold electrodes, could not be used for Pt because it oxidizes readily.36 STM Imaging of the Adsorption Process of Benzenethiol on Au(111). We first describe the time-dependent in situ STM results obtained at the beginning of benzenethiol adsorption. It will be made clear later that the electrochemical potential was the dominant factor in determining (35) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; J. Wiley: New York, 1994. (36) Iwasita, T.; Pastor, E. Electrochim. Acta 1994, 39 (4), 531.

Spatial Structures of Alkanethiols and Arylthiols

Figure 3. Time-sequenced constant-current (height mode) STM images showing the nucleation and growth of benzenethiol molecules at Pt(111) potentiostated at 0.15 V in 0.1 M HClO4. The imaging parameters were 300 mV in bias voltage and 5 nA in setpoint current. The time marks for these images are indicated. The dotted circles indicate molecular patches containing ordered BT molecular arrays. The image in part d acquired in a separate experiment shows the ordered BT adlattice and Pt(111) substrate in T1 and T2, respectively. The insets highlight the internal arrangements within these two domains. Scan area: (a-c) 200 × 200 Å2; (d) 300 × 300 Å2.

the spatial structures of organosulfur molecules on Pt(111). Only between 0.05 and 0.3 V did STM reveal ordered molecular adlattices. Typically, the potential of Pt(111) was held at 0.15 V as an admolecule of interest was introduced. All organosulfurs were adsorbed instantaneously, but the time needed to form a monolayer varied with their concentrations. Time-sequenced STM images shown in Figure 3 were acquired to show the adsorption process of BT on Pt(111) within the initial 8.5 min of dosing with 10 µM BT. It was difficult to completely avoid thermal drifts in the imaging process, so that distinct surface features, such as the step ledge at the low right of the image in Figure 3, were used as a marker to ensure that the same area is compared here. Imaging was conducted at 300 mV in bias voltage and 5 nA in setpoint current. The first image (Figure 3a) acquired shortly after BT was added reveals nanometer-scale pits distributed randomly on terraces and at steps. The appearance of pits, rather than protruded islands, of molecular aggregates could be an imaging artifact, associated with the existence of chemical species within the tunneling gap, as noted previously by others.37 These features, unseen prior to the addition of benzenethiol, were aggregations of BT, produced by random collisions of admolecules. Larger aggregates circled in dotted lines clearly contained short range ordered arrays with many defects. Small BT aggregates were mobile on the surface, which produced transient trenchlike features (indicated by arrows) in the STM images in Figure 3b. These lateral motions of molecular clusters were likely to be spontaneous, rather (37) Bohringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R. Surf. Sci. 2000, 457, 37.

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than induced by the tip. Similar results are also noted by others.38,39 It seems that molecular clusters drifted randomly before they collided with others to produce larger clusters or were trapped by an immobile island. A molecular island with an area of approximately 200 × 100 Å2 found at the lower end of a step ledge (Figure 3c) appeared to be stable toward STM imaging 7.5 min after dosing. This feature expanded laterally with time, as more and more molecules were trapped at its perimeter. This random addition of admolecules led to a molecular array containing a high density of defects, imaged as dark spots. However, possibly driven by intermolecular van der Waals interaction, BT admolecules were relocated to form a closepacked hexagonal lattice. These in situ STM results clearly reveal that the SAM of BT on Pt(111) was formed via a nucleation-and-growth mechanism, at least at the beginning of adsorption. In a separate experiment, we obtained the high-quality STM image shown in Figure 3d which reveals the formation of patches of ordered BT along with the atomic lattice of the Pt(111) electrode. The ordered hexagonal adlattices of BT and the Pt(111) substrate appear on the upper (T1) and lower (T2) terrace, respectively. The insets in Figure 3d show a portion (30 × 30 Å2) of the ordered adlattice and the Pt(111) substrate, respectively. Evidently, the protrusions within these two hexagonal patterns are aligned parallel to each other and the nearest neighbor spacing of the BT adlattice (5.6 Å) is 2 times larger than that of Pt(111). These results clearly indicate a (2 × 2) BT adlattice with one molecule per unit cell or a coverage of 0.25 (one BT molecule on every four Pt atoms). The same corrugation heights of the protrusions in the (2 × 2) BT adlattice suggest that all BT admolecules could reside at identical lattice sites, for example, the most stable fcc 3-fold hollow sites on Pt(111).40 It is noteworthy that BT admolecules were arranged this way even at the initial stage of adsorption, as seen in Figure 3. This finding suggests that BT admolecules were adsorbed upright and interacted with attraction at submonolayer coverages. Effect of Concentration on the Structure of Benzenethiol. To see how the molecular flux of BT affected its coverage, growth mode, and spatial structures, we raised the concentration of BT from 10 to 100 µM. In situ STM imaging revealed that the adsorption process of BT became faster. Within 1 min or so, the whole Pt(111) surface was covered with BT admolecules. A long range ordered monolayer might have been formed through a random fill-in as the initial step followed by a disorder-to-order phase transition, rather than the nucleation-and-growth mechanism, when the Au(111) electrode was dosed with solutions containing 100 µM (or higher) BT. This is illustrated by the time-sequenced STM images shown in Figure 4 which recorded the surface evolution for 5 min after the introduction of BT from a 0.1 M HClO4 solution containing 100 µM BT. The images were consecutively collected at time intervals of ∼1 min. The STM image in Figure 4a reveals that a 500 × 500 Å2 wide terrace was fully covered with BT admolecules with nearly one-half of them arranged in an ordered array (D1 domain) and the other half adsorbed randomly (D2 domain). The 30 × 30 Å2 high-resolution STM scan in Figure 4a reveals the internal arrangement of the ordered array in D1. This ordered structure was (2 × 2), as seen in Figure 3d. These (38) Briner, B. G.; Doering, M.; Rust, H.-P.; Bradshaw, A. M. Science 1997, 278, 257. (39) Frenken, J. W. M.; Kuipers, L.; Hoggeman, M. S. Ber. BunsenGes. Phys. Chem. 1994, 98, 307. (40) Hayek, A.; Glassl, H.; Gutmann, A.; Leonhard, H. Surf. Sci. 1985, 152/153, 419.

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Figure 4. Time-sequenced constant-current STM images (500 × 500 Å2) showing the disorde-to-order phase transition observed with benzenethiol on Pt(111). The time differences of these images are 1 min. The dotted lines in these images denote the domain boundary that separates the ordered (D1) and disordered (D2) domains. The high-resolution STM scan (30 × 30 Å2) shown as the inset in part a reveals the internal arrangement of D1. The arrows and dotted circles in part b denote the sites where the phase transition was initiated. The potential of Pt(111) was at 0.15 V, and the imaging conditions were 100-200 mV and 1-5 nA.

Figure 5. Time-sequenced constant-current STM images (500 × 500 Å2) showing the phase transition from (2 × 2) to (x3 × x3)R30° for the benzenethiol adlayer on Pt(111). The time differences are 1 min. The dotted lines and circle in part b highlight the initial sites of restructuring. The high-resolution STM scans (insets in part c, 30 × 30 Å2) reveal the internal arrangement of the adlattices seen before and after the transition. The potential of Pt(111) was 0.15 V, and the imaging conditions were 200 mV and 3 nA.

two domains had changed and disappeared in Figure 4c. The disorder-to-order transformation through both growths of the D1 domain (indicated by arrows) and nucleation of ordered domains (circled in dotted lines) was nearly completed within 2 min. It is not clear whether this disorder-to-order phase transition involved relocation of BT admolecules or only realignments of their phenyl groups. The completion of this well-ordered adlayer eliminated the original domain boundary, producing an ordered domain spanning a distance of 500 Å (Figure 4c). However, since two neighboring ordered domains might not be aligned with each other, a domain boundary was found at the interface between them. The BT adlayer continued to evolve with prolonged STM imaging. The time-dependent STM images in Figure 5 were obtained after the whole terrace was occupied by the ordered (2 × 2) structure. The potential of Pt(111) was still at 0.15 V, and parameters of 200 mV and 3 nA were used to acquire these images in time differences of 1 min. The first image in Figure 5a shows that the terrace was mostly covered with the (2 × 2) structure, but 1 min later, another ordered array appeared in nearly the whole left end and a patch in the middle/upper part of the scan area (Figure 5b). Meanwhile, the degree of ordering of the (2 × 2) domain deteriorated. Figure 5c acquired 1 min after Figure 5b shows nearly the whole scan area was occupied by the new ordered structure. Thus, it appears that this phase transition of the ordered adlattice was completed within 3 min. The insets in Figure 5c highlight the (2 × 2) structure (upper) and the new structure (lower) seen in Figure 5c. Apparently, the close-packed molecular rows of these two lattices are misaligned by 30°. The intermolecular spacing of this new structure is 4.8 Å, as

Figure 6. (a and b) In situ STM images showing the long range ordered adlattice of Pt(111), (x3 × x3)R30°, benzenethiol. The image in part c reveals the internal molecular structure. The models in parts d and e reveal the spatial arrangements of sulfur headgroups and phenyl groups, respectively. The ovals and x in part e represent the π-electrons and hydrogen atoms of the phenyl groups. The images in parts a and b were obtained at 0.2 V with a bias voltage of 330 mV and a setpoint current of 3 nA, while 200 mV and 10 nA were used to acquire that in part c.

compared to 5.6 Å of the (2 × 2) structure. These results indicate that this new structure is (x3 × x3)R30°. Figure 6a presents a 1500 × 1500 Å2 scan to show the degree of ordering of the (x3 × x3)R30° structure, while

Spatial Structures of Alkanethiols and Arylthiols

Figure 6b reveals the molecular arrangements of BT on the terraces. The former reveals two atomically flat terraces separated by a 70 Å wide, 2.3 Å deep depressive gap. The winding protruded lines (indicated by arrows) are attributed to domain boundaries delineating ordered domains of the (x3 × x3)R30° structure. It is impressive to see that an ordered domain could span as wide as 1000 Å. The high-resolution STM scan shown in Figure 6b reveals that the BT adlayer is indeed remarkably ordered with only a few vacancy defects in a scan area of 500 × 500 Å2. A further high-resolution STM scan shown in Figure 6c provides a close-up view of this ordered array, which appears to be hexagonal with a nearest neighbor spacing of 4.8 Å. Close examination of this STM image reveals that this array contains two sets of spots with apparently different corrugation heights, as outlined by the two rhombuses drawn on the image. If all features are associated with separate BT admolecules, the adsorbates would be unrealistically close. Hence, these two sets of spots should have different origins. For example, BT admolecules could be adsorbed tilted with their sulfur headgroups bound to the Pt(111) surface. The brighter spots should be associated with the sulfur headgroups, whereas the dimmer ones could arise from the phenyl groups which only interacted weakly with the Pt(111) substrate. The same corrugation heights exhibited by the sulfur headgroups indicate that all BT molecules were adsorbed at an identical type of sites,40 possibly the favorable fcc 3-fold hollow sites, as revealed by the model depicted in Figure 6d. To account for the arrangements of the phenyl groups (the weaker spots in Figure 6c), a model in Figure 6e illustrates the parallel alignment of the phenyl groups along the [11h 0] direction, which is needed to minimize steric hindrance between neighboring BT admolecules. This arrangement allows a lateral intermolecular spacing of 8.3 Å, which is larger than the van der Waals dimension of a phenyl group. One can envisage that this arrangement enables each molecule to interact with four of its nearest neighbors through the π-π stacking attractive force, as indicated by the rectangle in the model.41,42 The ovals and x in the model of Figure 6e denote π-electrons and hydrogen atoms of phenyl rings, respectively. Overall, (x3 × x3)R30° and (2 × 2) were the only two ordered structures found for benzenethiol; raising or lowering [BT] by 10 times did not produce other ordered structures. BT admolecules are aligned upright in these two ordered structures even at the very early stages of adsorption (Figure 3), which is a surprising finding, given the fact that benzene molecules strongly interacts with Pt(111).43 In addition to the S-Pt interaction, the phenyl group could also interact with the Pt substrate to enhance the adsorption strength of BT. Indeed, BT molecules are shown to lie parallel on Au(111) and Rh(111) at low coverage in a vacuum.44,45 This difference in molecular orientation might arise from the chemical nature of substrates, the environment of interfaces, and the molecular fluxes toward a substrate. Further efforts are needed to resolve this issue. Our present finding of two well-ordered BT adlattices on Pt(111) contrasts markedly with a previous study of BT on Au(111), as the latter gave (41) Waters, M. L. Curr. Opin. Chem. Biol. 2002, 6, 736. (42) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525. (43) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (44) Whelan, C. M.; Barnes, C. J.; Walker, C. G. H.; Brown, N. M. D. Surf. Sci. 1999, 425, 195. (45) Bol, C. W. J.; Friend, C. M.; Xu, X. Langmuir 1996, 12, 6083.

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Figure 7. In situ STM images showing the surface morphology of Pt(111) in benzenethiol-containing 0.1 M HClO4 at 0.45 V. The bias voltage and setpoint current are 100 mV and 5 nA, respectively. The high-resolution STM scan (50 × 50 Å2) in the inset of part b highlights the ordered structure on the terrace.

no ordered structure when the BT adlayer was prepared by soaking in 1 mM BT + ethanol.25 It was proposed that the S-Au bond for BT/Au(111) is more sp3-like, as compared to sp hybridization for benzyl mercaptans/Au(111). This difference in bonding was thought to render rigid and flexible molecular orientations, thus unlike (disordered vs ordered) molecular packing on Au(111).25 Our results are partially in agreement with the conclusions drawn from Auger electron spectroscopy (AES) and LEED measurements where BT admolecules were shown to align upright.22 The packing density of BT determined from AES reaches a maximum of ∼0.356 nmol/cm2, or an equivalent coverage of 0.14 (BT/per Pt atom).22 This is less than one-half of the value 0.33 determined by the present in situ STM study. Stern et al. proposed that this low coverage was mostly due to the virtual insolubility of BT in aqueous solutions. However, according to our present in situ STM results, their AES measurements could have underestimated the coverage of BT. Meanwhile, the adsorption of 4-hydroxybenzenethiol (HBT) on Pt(111) was also examined with in situ STM to elucidate how molecular structure would affect the spatial arrangement of arylthiol molecules. Results (not shown here) indicate that these two molecules behaved similarly on Pt(111). Effect of Potential on the Benzenethiol Adlayer. To explore the effect of potential on the orientation and the spatial structure of BT admolecules, the potential of Pt(111) was stepped from 0.15 V to more positive values in 0.1 M HClO4 containing ∼10 µM benzenethiol molecules. It was found that the (2 × 2) adlattice observed at 0.15 V could be converted into a more closely packed structure of (x3 × x3)R30° at the potential of 0.2 V. This potentialinduced restructuring event was irreversible to potential; stepping the potential back to 0.15 V or more negative values did not result in the (2 × 2) structure again. Upon raising the potential to an even more positive value, in situ STM imaging revealed obvious changes of

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Figure 8. In situ STM imaging of (a and b) 1-hexanethiol and (c) 1-nonanethiol at Pt(111) in 0.1 M HClO4. The imaging parameters were 100-200 mV bias voltage and 3-10 nA, and the potential of Pt(111) was 0.2 V. The image in part a was obtained 30 s after the dosing solution of 1-hexanethiol was added.

the surface morphology. For example, Figure 7a obtained 5 min after the potential was stepped from 0.15 to 0.45 V reveals protruded ridges with poorly defined thicknesses and heights. Figure 7b reveals a close-up view of a terrace site, revealing locally ordered (x3 × x3)R30° and corrugated ridges and blotches. The degree of ordering of the BT adlayer deteriorated substantially at more positive potentials, and the situation simply worsened as the potential was raised even more positively. These structural changes were irreversible to the modulation of potential, suggesting that incorporation of ions or water molecules in the organic films was not responsible for these changes. Also, since the BT adlayer was not oxidized at 0.45 V, these structural changes were not due to the decomposition of BT admolecules. Rather, it seems more likely that these changes stemmed from additional deposition of molecules at more positive potentials. Because the adsorption of organosulfur molecules is an oxidative process,46 involving cleavage of the S-H bond, we expect positive potentials would favor an increase of coverage. This contention stems from the results observed for the sulfur adlayer on Pt(111),29 where the coverage of sulfur adatoms increases with positive potentials. Further deposition of molecules likely occurred at sites where adsorbates had more freedom to move and to accommodate the incoming molecules. Domain boundaries seemed to be the most likely locations for these events. Finally, the voltammetry was not sensitive enough to identify these changes, as evidenced by the featureless CV profile in the dotted trace in Figure 1a. STM Atomic Resolution of 1-Hexanethiol and 1-Nonathiol on Pt(111). In situ STM imaging of the spatial structures of SAMs consisting of 1-hexanethiol (HT) and 1-nonanethiol (NT) on Pt(111) was conducted similarly. Dosing with dilute 10 µM solutions of each molecule resulted in ordered adlattices at 0.2 V in 0.1 M HClO4. The STM images in Figure 8a and b show the adsorption of HT, and Figure 8c reveals the NT adlayer. Figure 8a obtained 30 s after the introduction of HT is intended to show the initial stage of adsorption, where molecular aggregates resulted in protruded patches seen at terraces and steps. This result indicates that the formation of SAMs for HT also followed the nucleationand-growth mechanism observed earlier for benzenethiol. Monolayers of HT and NT were produced within a few minutes, depending on the concentrations of these molecules. High-resolution STM imaging of HT and NT revealed hexagonal arrays with nearest neighbor spacings of 5.4 Å, and the protrusions are aligned along the major substrate rows of Pt(111). Thus, these arrays are (2 × 2) (46) Ron, H.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13444.

with coverages of 0.25. However, dosing with solutions containing saturated 1-hexanethiol and 1-nonanethiol produced different adlattices, identified as (x3 × x3)R30° for both cases (not shown). The effect of potential on the spatial structures of these molecules was also examined, and results show that they became mostly disordered once the potential was made more positive than 0.3 V. These results are consistent with those observed for benzenethiol on Pt(111). It seems that the S-Pt interaction the adsorption of thiols on Pt(111). It would be of interest to examine the adsorption of organosulfur molecules with larger organic groups to see how these molecules are adsorbed on Pt(111). Finally, we comment on the imaging mechanism of these thiol molecules. It was shown in previous vacuum STM studies that the tunneling resistance needs to be as high as 50 GΩ to prevent tip-and-sample interaction and avoid physical interruption of the organic thin films.47,48 Throughout this study, the typical STM tunneling resistance was ∼20 MΩ, which amounts to only 1/2500 of those used in UHV.44,45 According to what was observed in UHV, the tip should have been already in physical contact with the substrate at this low tunneling resistance. Nevertheless, as revealed by the present results, STM imaging was stable and high-quality molecular resolution was possible with this low tunneling resistance. Hence, either the tip was not contacted with the film during imaging or these organic films were rigid enough to resist physical rubbing by the tip. It seems likely that this contrast in STM imaging in UHV and in solution can arise from the much lower tunneling barrier observed in the electrochemical environment.49 High-quality molecular-resolution STM images of other upright organic molecules, such as iodobenzene and iodoheptane, were obtained with similar imaging parameters in electrolytes.5 Conclusions High-quality in situ STM results indicate that, depending on the concentrations of thiols, two well-ordered adlattices, (2 × 2) and (x3 × x3)R30°, were observed on Pt(111) electrodes between 0.05 and 0.3 V. Organosulfur molecules are oxidatively adsorbed on Pt(111), involving the cleavage of S-H bonds and the formation of S-Pt bonds. Because sulfur adatoms are arranged similarly on Pt(111), it is likely that the S-Pt interaction controls the adsorption of thiols on Pt(111). This view is supported by (47) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259. (48) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (49) Nagatani, Y.; Hayashi, T.; Yamada, T.; Itaya, K. Jpn. J. Appl. Phys. 1996, 35, 720.

Spatial Structures of Alkanethiols and Arylthiols

STM molecular resolution, which reveals the spatial arrangements of the sulfur headgroups and phenyl groups of BT admolecules. The electrochemical potential and molecular flux control the growth mechanisms and spatial structures of thiols on Pt(111). Dosing with 10 µM benzenethiol renders a well-ordered (2 × 2) adlattice via the nucleation-and-growth mechanism. In contrast, raising the concentration to 100 µM enables a random fill-in growth process, followed by a disorder-to-order phase transition to produce a metastable (2 × 2) structure before (x3 × x3)R30° prevails at the end. The as-formed organic monolayers can span as wide as 1000 Å without any domain boundary. Raising the potential to 0.3 V or

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more positive values yields deposition of thiols at domain boundaries first, then at terraces. This continuous deposition of thiols greatly decreases the degree of ordering within the SAMs. This order-to-disorder phase transition is not reversible to the alternation of potential. Acknowledgment. This work is supported by the National Science Council of the Republic of China under Contract No. NSC 93-2119-M-008-002. This work was partially supported by the Ministry of Education, Culture, Sport, Science and Technology, a Grant-in-Aid for the COE project, Giant Molecules and Complex Systems, 2004. LA030198B