Structures of Aniline and Polyaniline Molecules Adsorbed on Au(111

Jul 9, 2009 - Marco Di Giovannantonio , Massimo Tomellini , Josh Lipton-Duffin , Gianluca Galeotti , Maryam Ebrahimi , Albano Cossaro , Alberto Verdin...
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Structures of Aniline and Polyaniline Molecules Adsorbed on Au(111) Electrode: as Probed by in Situ STM, ex Situ XPS, and NEXAFS Shuehlin Yau,* YiHui Lee, and ChinZen Chang Department of Chemistry, National Central UniVersity, Jhongli, Taiwan 320, ROC

LiangJen Fan* and YawWen Yang National Synchrotron Radiation Research Center, Hsinchu, Taiwan 300, ROC

Wei-Ping Dow* Department of Chemical Engineering, National Chung Hsing UniVersity, Taichung 40227, Taiwan, ROC ReceiVed: March 26, 2009; ReVised Manuscript ReceiVed: June 17, 2009

In situ scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) were used to examine the electrified interface of Au(111) in 0.1 M H2SO4 containing 0.030 M aniline. In agreement with cyclic voltammogram (CV), which revealed two pairs of peaks at 0.48 and 0.62 V, in situ STM imaging yielded two highly ordered aniline adlattices, (19 × 5) at 0.55 V and (3 × 23)rect at 0.85 V [vs reversible hydrogen electrode, RHE]. According to XPS results obtained with Au(111) emersed at 0.85 V from 0.1 M H2SO4 + 0.030 M aniline, bisulfate anions were coadsorbed in an amount equal to that of aniline. The (3 × 23)rect-aniline structure was examined carefully by STM using different imaging conditions. Results revealed that imaging with a tunneling current of 10 nA at a -300 mV bias voltage allowed molecular resolution of both aniline admolecules and bisulfate anions. These species could form acid-base pairs and mingled uniformly on the Au(111) electrode. NEXAFS results were also obtained at 0.85 V, showing that the phenyl rings of aniline admolecules on average was tilted away from the Au(111) substrate by 47°. At E > 0.95 V, aniline molecules were oxidized to cation radicals, which initiated intermolecular coupling between aniline molecules to form polyaniline (PAN). The as-formed PAN assuming the form of emeraldine salt exhibited distinct linear conformations, which is proposed to derive from a unique head-to-tail arrangement of aniline monomers in the (3 × 23)rect structure. The coadsorbed bisulfate anions played an important role in the production of surface-bound PAN emeraldine salts, whose high conductivities facilitated molecular resolution STM imaging up to a thickness of four PAN layers. Introduction In situ STM has been used extensively to study the structures of electrified interfaces, revealing structures of pristine substrate or atomic or molecular adsorbates on single-crystal electrodes.1-4 In addition to acting as a structural probe, STM has evolved as an important tool in the modern era of nano science and technology, providing insights into dynamics of surface reactions, fabricating nanometer features, and measuring the conductivities of nanometer features or even single molecules.5-9 Scanning probes have also been used to study polymeric materials, and a number of studies reported thus far are able to provide details of polymers at a molecular level.6,7,10,11 For example, STM was used to create local polymerization of 10,12nonacosadiynoic acid on a graphite surface.10 More recently, in situ STM yields structural details and electronic properties of polythiophene on iodine-modified Au(111) electrode.6,7 Although organic conducting polymers such as PAN have many applications, their structures are frequently poorly defined. Recently, we used STM to illustrate that PAN produced on an ordered Au(111) electrode can have well-defined structures and dimensions, as linear PAN molecules sprawling up to 50 nm in * Corresponding authors. E-mail: [email protected] (S.Y.); dowwp@ dragon.nchu.edu.tw (W.-P.D).

length were observed.12 Because the same procedure led to crooked PAN on a rough gold electrode, it seems that aniline polymerize preferentially on electrode surfaces, and thus the lateral arrangements of aniline admolecules could determine the final conformations of PAN. The observation of a highly ordered Au(111)-(3 × 23)rect structure at the onset potential for polymerization attests this view.12 Furthermore, we gathered in situ STM and ex situ XPS results to address the importance of potential on the conformational changes of PAN.13 Nonetheless, as we reflected upon the intriguing results observed with aniline and PAN on Au(111), we recognize the importance of the organization of electrified interfaces, which influences not only the structure of monomers but also the conformation of polymers.13 In this report, we address the transformation of aniline structures with the modulation of potential. We also used XPS techniques to characterize the make-ups of the aniline adlayer. It is fair to state that the study of well-defined electrified interface in the past decade has not established a congruent picture concerning the adsorption of amine molecules on gold electrode.14-23 Researchers have focused attention on the adsorption configurations of admolecules, and the role of anions such as bisulfate and halide has been mostly defined. Anions such as bisulfate and halide have

10.1021/jp9027194 CCC: $40.75  2009 American Chemical Society Published on Web 07/09/2009

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proven to play important roles in structuring electrified interfaces, as noted for aniline and 4-aminophenol on Au(111).13,16 In this study, we combined in situ STM and ex situ XPS and NEXAFS to characterize the structures and chemical compositions of the Au(111) interface in 0.1 M H2SO4 containing 0.030 M aniline. Results obtained here are consistent, showing that the Au(111)-(3 × 23)rect structure comprised equal amounts of aniline molecules and bisulfate anions. We are able to delineate precisely how these species were adsorbed on Au(111) electrode and how this ordered array could facilitate intermolecular coupling to yield linear PAN upon anodization at E > 1 V. In addition to addressing the linearity of PAN, we outline how defects in the (3 × 23)rect structure could guide the propagation of molecular coupling, leading to imperfections in PAN molecules. Experimental Section Electrode Preparation and Electrochemical Measurements. The Au(111) single crystal electrode used in this study was homemade by melting a gold wire (φ ) 0.8 mm) by using a hydrogen torch. Details for electrode preparation were already reported.2,24 All electrochemical experiments were performed with the conventional hanging meniscus method in a threeelectrode cell. A reversible hydrogen electrode (RHE) and Pt wire acted as the reference and the counter electrodes, respectively. The potentiostat used was a CHI 703. The supporting electrolyte was typically 0.1 M H2SO4 containing 0.030 M aniline. Ultrapure H2SO4 was purchased from Merck (Darmstadt, DFG), while aniline was obtained from Aldrich (Saint Louis, MO). Millipore triple-distilled water (resistivity 18 MΩ) was used to prepare all solutions. Scanning Tunneling Microscope (STM). The STM use in this study was a Nanoscope E (Digital Instruments, Santa Barbara, CA) with a single tube scanner (high-resolution A-head, maximal scan area 500-600 nm). Tungsten tips (diameter 0.3 mm) prepared by electrochemical etching in 2 M KOH were used throughout this study. The tip was water-rinsed, dried by acetone washing, and finally painted with nail polish for insulation. XPS and NEXAFS. The substrates used for ex situ XPS and NEXAFS experiments were borosilica slides coated with 2000 Å thick gold (Arrandee, DFG). The substrate was annealed with a butane flame, followed by quenching in Milli-Q water. This process could yield large (111) grains of hundreds of nanometer in breath. The as-prepared gold substrate was then submerged into solution containing 0.03 M aniline and 1 M H2SO4 under potential control. The Au(111) electrode was then emersed from the solution under potential control and transferred onto the STM stage. STM imaging of Au(111) in a dry nitrogen environment started after it was air-dried. Finally, the sample was transferred into a UHV chamber where XPS and NEXAFS experiments were performed. STM molecular resolution images indicated well-order (3 × 23) structure, as observed also in situ. This result suggests that the ex situ XPS and NEXAFS results obtained in this study reflected the genuine structure of aniline adsorbed on Au(111). A typical ex situ STM image is shown in the Supporting Information. XPS and NEXAFS experiments were performed at a UHV end station hooked up to BL24A wide-range spherical grating beamline at NSRRC. The end station housed a differentially pumped sputter ion gun for sample cleaning, a SPEC PHOIBOS150 energy analyzer for XPS, a multi channel plate (MCP) detector for NEXAFS, and a load-lock sample transfer mechanism. The XPS binding energy scale was referenced to

Figure 1. Cyclic voltammogram recorded at 50 mV/s with Au(111) immersed in 0.1 M H2SO4 and 0.03 M aniline.

the bulk Au 4f7/2 core level located at 84.00 eV relative to Fermi level. The XPS spectra were acquired at a photon energy of 630 eV for the survey scan and a photon energy of 500 eV for high resolution N 1s and S 2p scans. The combined-instrument energy resolution for high-resolution scan is estimated to be better than 0.3 eV. Polarization-dependent carbon k-edge NEXAFS measurements were performed based on the total electron yield (TEY) method with a current amplifier measuring the drain current. The photon energy scale was calibrated against an intense 1s f π* transition of the HOPG graphite located at 285.38 eV. The spectra were first normalized to the photon flux, forming a so-called Io-normalized spectrum, with the flux obtained by measuring the ionization current of argon gas in a specially designed gas ionization chamber situated in front of the sample.25 The X-ray absorption features of the substrate were eliminated by dividing the Io-normalized TEY spectrum of a aniline-covered Au(111) surface with the Io-normalized TEY spectrum of a clean Au(111) surface with the spectra measured separately but under the same photon incident conditions. All of the spectra presented were treated with the same mathematic method. Results and Discussion Cyclic Voltammetry. Shown in Figure 1 is the CV recorded at 50 mV/s with a Au(111) electrode immersed in 0.1 M H2SO4 containing 0.030 M aniline.12 The potential program used here was 0.8 f 0.05 f 1.0 V, which yielded two pairs of peaks, A/A′ and B/B′, at 0.48 and 0.62 V. Both peaks are very sharp and highly reversible, suggesting that they are associated with surface processes, such as the adsorption and desorption of aniline monomers and (bi)sulfate anions. This view is supported by in situ STM results (see below), revealing two highly ordered molecular arrays at the positive ends of peak A and B, respectively. Aniline molecules were oxidized at E > 0.95 V, yielding cation radicals and later to PAN molecules. Without aniline in the solution, the CV reveals two pairs of broad peaks due to the phase transition between the reconstructed and (1 × 1) structures of Au(111). Adsorbed bisulfate anions undergo a disorder-to-order phase transition at 1.1 V, where a pair of current peaks emerges.26 In Situ STM of Au(111)-(3 × 23)rect: Aniline Monomer at 0.85 V. STM imaging experiments were performed under the conditions used for the CV measurements (Figure 1). The potential of the Au(111) electrode was initially set at 0.85 V in 0.1 M H2SO4 containing 0.030 M aniline. After

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Figure 2. In situ STM images showing the formation of a long-range ordered aniline adlayer (a) and patches of rotational domains (b) on Au(111) electrode at 0.85 V in 0.1 M H2SO4 and 0.03 M aniline. These images were obtained in two separated experiments.

satisfactory results were collected, the potential was switched negatively to 0.55 V to unravel the effect of potential on the arrangement of aniline monomers. Shown in Figure 2 are typical STM results observed at 0.85 V, which indicate unambiguously the presence of a long-range ordered aniline adlattice with a single ordered domain spanning more than 50 × 50 nm (Figure 2a). The image in Figure 2b reveals multiple ordered domains on a terrace with molecular resolution of the internal molecular arrangements. These images were collected with typical imaging conditions of -300 mV in bias voltage and 1 nA in set point current. It is thought that the extent of ordering in the aniline adlayer hinged on the surface quality of the Au(111) electrode and the cleanliness of the electrochemical environment. Each domain consisted of welldefined zigzag chains aligned in parallel to the main axis of the Au(111) substrate, as marked by arrows. Two neighboring domains frequently have their molecular rows intersected at a 120° angle, suggesting that they were rotational domains of an identical structure. Two of the three possible rotational domains were observed in Figure 2b. The molecular-resolution STM images shown in Figure 3 provide details of this aniline structure. These STM results are also intended to show how the appearance of the molecular STM image varied with tunneling conditions. The typical imaging conditions of -300 mV and 1 nA yielded a rectangular array defined by two unit vectors of 0.90 and 1.03 nm in length. The fact that these unit vectors are aligned in the 〈110〉 and 〈121〉 directions indicates an Au(111)-(3 × 23)rect structure containing two protrusions in each unit cell. Tentatively, we attribute these features to aniline admolecules, which results in a coverage of 0.17 (2/12). We then varied the tunneling conditions to see their effects on the appearance of STM. Because the tungsten tip could have behaved as a microelectrode leading to redox processes and faradaic current that interfered the tunneling current, the more common practice in electrochemistry STM is then to change the feedback current with its potential fixed at an optimal value (∼0.5 V). This act of changing the tunneling current effectively modulated the tip-and-sample distance. Assuming a tunneling barrier of 1 eV as reported by others,27 increasing the feedback current from 1 to 10 nA lowered the tip by ∼0.23 nm. This act resulted in marked differences in the STM appearance, as Figure 3b shows. In addition to aniline admolecules, the STM revealed another set of spots arranged also in (3 × 23)rect denoted by the rectangle in solid lines. These new features result in a coverage of 2/12 (0.17), the same as that of aniline admolecules. It is likely that they stemmed from bisulfate anions, which exhibited corrugation heights 0.03 nm lower than those of aniline molecules. This selective imaging mechanism could be repro-

Figure 3. Molecular-resolution STM images acquired with Au(111) electrode at 0.85 V in 0.1 M H2SO4 and 0.03 M aniline. The tunneling current was set at 1 nA (a) and 10 nA (b) with a bias voltage of -0.3 V, which rendered molecular resolution of aniline admolecules marked by the dotted rectangle (a) and of aniline + bisulfate anions (b). The image in (c) was obtained with the tunneling current switched abruptly from 1 to 10 nA as the tip scanned to one-half of the frame, which is marked by the dotted line in (c). This ordered structure is explained by the ball model depicted in (d).

duced with different tips, although the quality of STM resolution varied. The adsorption of bisulfate anions on metal electrodes of Cu, Ag, Au, Pt, Ru, Rh, and Pd has been extensively examined by STM.26,28-34 It is shown that water molecules, which could act as bases, are coadsorbed with bisulfate anions in a highly ordered (3 × 7) structure. This idea of acid-base coadsorption on electrode surface could also hold for the aniline + bisulfate system. To further illustrate the effect of imaging condition on the appearance of STM images, we switched the feedback current abruptly from 1 to 10 nA when the tip was scanning upward to the middle of a frame. This yielded a composite STM image shown in Figure 3c, where the structures of aniline and aniline + bisulfate were seen respectively in the lower and upper halves of the frame. These two domains are clearly resolved, as the dotted line indicates. This selective imaging process was reversible; it did not cause any change and damage to the adlayer. This result shows clearly the importance of the tipand-sample distance in effecting STM imaging of two different chemisorbates. These information lead to a model for this (3 × 23)rectaniline structure shown in Figure 3d. First, we note that several theoretical studies show that the on-top site provides the most favorable coordination between the amino group and Au(111) substrate.35,36 Thus, all aniline admolecules are assigned to ontop sites of Au(111). Second, according to our NEXAFS results obtained in this study (see below), the phenyl rings of aniline molecules were tilted by 47°. Close inspection of this model reveals that aniline admolecules are arranged in zigzags in the main axis of the Au(111) surface with two nearest ad-particles separated by ∼0.52 nm, a value nearly equal to that between two phenyl rings in a PAN molecule. Because PAN molecules

Structures of Aniline and Polyaniline Molecules

Figure 4. High resolution N 1s (a) and S 2p (b) XPS spectra obtained with Au(111) electrode dosed with aniline at 0.85 V in 0.1 M H2SO4 + 0.03 M aniline.

produced by anodization at E > 0.95 V took on well-defined linear conformations, it is tempting to propose that aniline admolecules in the zigzag chains were arranged in a head-totail fashion, because this readily facilitated coupling between neighboring aniline molecules. Bisulfate anions were also incorporated in this structure with half of them lying closer to aniline admolecules and the other half positioned in more isolated locations. This model is supportive to the XPS results showing two types of aniline molecules, protonated and unprotonated ones. Compared to the mixed adlayer of bisulfate and water molecules, which each occupies 20% of the electrode surface of Au(111), aniline and water molecules were less populated on Au(111). Whether the amounts of basic molecules, aniline or water molecules, depended solely on their physical dimension is not clear now. Furthermore, as bisulfate + water molecules are coadsorbed in the (3 × 7) structure on most fcc(111) oriented electrodes,29,32-34 it would be interesting to investigate if the (3 × 23)rect - aniline + bisulfate structure is formed at other electrode such as Ag(111), Cu(111), etc. XPS and NEXAFS of Au(111)-(3 × 23)rect-Aniline + Bisulfate. Ex situ XPS measurements were performed on a Au(111) electrode after its emersion from 0.1 M H2SO4 + 0.03 M aniline under potential control at 0.85 V, where in situ STM revealed a highly ordered (3 × 23)rect structure. First we mention that ex situ STM imaging of an emersed Au(111) in nitrogen disclosed the same (3 × 23)rect structure. (See the Supporting Information for details.) Thus, it is fair to state that the electrified interface of Au(111) was stable against the emersion and transfer processes and the (3 × 23)rect structure should remained intact in the ultrahigh vacuum chamber. The XPS survey scan taken over a wide binding energy region indicates the presence of carbon, nitrogen, sulfur, oxygen and gold species with an elemental ratio of 1/6/1/4 (carbon/nitrogen/ sulfur/oxygen). These results attest the cleanliness of the

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Figure 5. Polarization dependent carbon k-edge NEXAFS spectra for Au(111) emersed at 0.85 V in 0.1 M H2SO4 and 0.03 M aniline. The variation of integrated peak intensity for the two π* transitions together with two theoretical curves for incline angles of 45° and 50° are shown in b.

molecular film. Shown in Figure 4 are high-resolution N 1s (a) and S 2p (b) XPS spectra. Two distinct peaks ascribable to surface bound aniline (Ph-NH2) and protonated aniline (PhNH3+) species were found at 399.9 and 401.5 eV (Figure 4a).37-40 In Figure 4b, the broad spin double peak at 168.34 eV is attributed to S 2p3/2 arising from adsorbed (bi)sulfate.40 The molecular ratio of aniline and (bi)sulfate, estimated from the intensity of N 1s and S 2p peaks, is close to one. The Au(111)-(3 × 23) aniline structure found at 0.85 V in sulfuric acid therefore consisted of equal amounts of aniline and (bi)sulfate anions. Half of the adsorbed aniline molecules were protonated; whereas the other half were not. The broadness of the S 2p peak suggests that not all adsorbed bisulfate anions in the (3 × 23) structure resided at the same environment. These results support our ball model for the structure shown in Figure 3d, where bisulfate anions could be adsorbed at two different sites. NEXAFS experiments employing a wide range of incident angles were performed to probe the adsorption configurations of aniline admolecules in the Au(111)-(3 × 23) structure. In NEXAFS spectroscopy,41 tunable and linearly polarized X-rays impinging on the samples are absorbed via resonant excitations of core electrons to unoccupied molecular orbitals of π* or σ* character. Thus, features in the NEXAFS spectra reflect the electronic structure of admolecules. Moreover, this type of dipole transition, or the extent of X-ray absorption varies with the dot product between the vector of transition dipole moment and the electrical vector of impingent X-ray. The socalled polarization-dependent measurement lies in analyzing how a given X-ray transition varies with the incident angle of X-ray. This sort of measurements can yield the orientation of a particular molecular constituent responsible for the X-ray photoemission. Figure 5a shows the carbon K-edge NEXAFS spectra for aniline adsorbed on Au(111) at 0.8 V. A number of peaks appear

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Figure 6. In situ STM images, (a) and (b), showing the Au(111)-(19 × 5)-aniline + bisulfate structure at 0.55 V in 0.1 M H2SO4 and 0.03 M aniline. Brighter and dimer spots are ascribed to aniline admolecules and bisulfate anions, respectively. The ball model in (c) with ovals and triangles denoting aniline and bisulfate species is proposed to account for this structure.

between 283 and 310 eV, in agreement with the results for aniline adsorbed on various surface.39 The peaks at 285.5 and 286.3 eV are assigned to C 1s f π*1 and C 1s f π*2 transitions of the phenyl ring, respectively.42-44 A feature due to C 1s f σ*C-H is observed at 288.7 eV. At even higher energy two broad * states were fount at 294 and peaks due to transitions to the σCC 302 eV. The first two π* peaks are broader than those observed with gas phase aniline, which suggests that aniline admolecules interacted strongly with the Au(111) substrate. As shown in Figure 5a, intensity of the two π* resonances is highest at the grazing incident angle of 20°, indicating that the phenyl ring of aniline admolecule was inclined toward the Au(111) surface than the magic angle of 55°. The inclination angle can be readily estimated from the relationship between absorbance and incident angles.41,45 The variation of integrated peak intensity for the two π* transitions together with two theoretical curves for incline angles of 45° and 50° are shown in Figure 5b. The aromatic plane of adsorbed aniline molecule is determined to be tilt away from the Au(111) substrate by 47°. Combining all available information from ex situ XPS, NEXAFS, and in situ STM leads to the conclusion that the unit cell of the Au(111)-(3 × 23) structure comprised two aniline molecules and two bisulfate anions.29 In Situ STM Imaging of Au(111)-(19 × 5)-Aniline Structure at 0.55 V. Once the aniline adlayer was disclosed by the STM at 0.85 V, we lowered the potential to 0.55 V, the negative end of B′ in the CV, in order to reveal restructuring of the aniline adlayer. STM images shown Figure 6a indicate that the aniline adlayer was still highly ordered. The applied potential of 0.55 V was not negative enough to transform the Au(111) surface from (1 × 1) into the reconstructed structure. Figure 6b shows a close-up STM scan, which reveals the internal molecular arrangement of this superlattice. This square-like unit cell is defined by two unit vectors measured 1.3 and 1.5 nm in length. The former (latter) is rotated by 23° (0°) from the main axis of the Au(111) surface, which implies that this is a (19 × 5) structure. It consisted of paired rows of protrusions prescribed to aniline admolecules in the 19 direction. Meanwhile, bleary features ascribed to coadsorbed bisulfate anions were discerned at the gaps between paired molecules rows. The aniline coverage in this structure is calculated as 4/25 (0.16), which is slightly less than that (0.166) of the (3 × 23) structure observed at 0.85 V. It appears that this difference in aniline coverage could be too little to account for the appearance of the sharp peak at 0.62 V. Instead, we propose that the change of bisulfate coverage could be more relevant to the CV feature of B/B′. More specifically, we propose that there were two bisulfate anions in each (19 × 5) unit cell, yielding a bisulfate coverage of 2/25 or 0.08. This is only one-half of that determined for the (3 × 23) structure. Thus, the increase of bisulfate coverage in response

Figure 7. In situ STM image acquired with Au(111) at 0.95 V in 0.1 M H2SO4 and 0.03 M aniline. Long linear features running preferentially along the 〈110〉 direction are attributed to PAN molecules. Protruded lines with different corrugation height are PAN molecules at different layer. The ordered pattern seen in the background is due to aniline monomers. The model in (b) shows how defects of 120°-bent and 180°fold could be produced.

to the shift of potential from 0.55 to 0.85 V ultimately drove the restructuring of the interface of Au(111) in 0.1 M H2SO4 + 0.03 M aniline. Close examination of the STM images shown in Figure 6b reveals that the corrugation heights of protrusions varied little ((0.03 nm), but not all protrusions have the same shape. Tentatively, these differences stemmed from unlike adsorption configurations of aniline admolecules, although we are not sure how to extrapolate information from these STM images, concerning the coordination of aniline molecules on Au(111). If the (19 × 5) structure is stable against emersion, it is possible to use NEXAFS techniques to probe the adsorption configurations of aniline admolecules. At this moment we can only speculate that aniline molecules were adsorbed at the ontop sites of Au(111) mainly via their nitrogen ends, as suggested by others.36 The tilt angles of the phenyl rings of aniline molecules are not known, although a theoretical study suggests that the phenyl ring of aniline is tilted by 18° on Au(111).36 A tentative model is proposed to account for the (19 × 5) structure. In Situ STM Imaging of PAN on Au(111). According to the CV shown in Figure 1, increasing the potential to 0.95 V resulted in oxidation and polymerization of aniline. Expectedly, real-time in situ STM imaging revealed drastic and rapid changes in the aniline adlayer. A typical STM image obtained at 0.98 V is shown in Figure 7. Distinct linear features, ascribed to the as-produced PAN molecules, began to sprout. The prominent linear conformation of PAN is thought to result from the unique zigzag arrangement of aniline admolecules illustrated by Figure 3d. PAN molecules grew longer and the film grew thicker with time. These results indicate show unambiguously that electrochemical poly-

Structures of Aniline and Polyaniline Molecules merization proceeded laterally, rather than vertically, on Au(111) electrode, at least in the initial stage. The dimension of PAN molecules varied with the extent of ordering in the (3 × 23) aniline adlayer seen at the onset potential for polymerization. The longest linear PAN chain observed thus far is 50 nm, comprising roughly 100 aniline molecules, as seen in Figure 7a. More frequently, in situ STM showed that PAN molecules grew linearly to several tens of nanometer before they bent by 120° or folded by 180°. Without these sorts of defects, PAN could grow to 100 nm or longer. PAN molecules were found to align themselves in only three directions, namely the 〈110〉 azimuth of the Au(111) electrode. These directions are exactly how zigzag aniline admolecules are aligned in the model shown in Figure 3d, which underlines the important correlation between the structure of monomers and the conformation of polymers. Although the structure of Au(111)-(3 × 23)-aniline + bisulfate anions could be optimal for the polymerization process, internal molecular defects such as bent and U-turn were always observed. These internal molecular defects could result from coupling of aniline molecules sitting at irregular sites in the course of polymerization, as illustrated by the model shown in Figure 7b. Polymerization of aniline could propagated initially in the direction of arrow, but at some points one of the adsorbed aniline molecules or an aniline molecule diffusing from solution could land at an irregular surface site, which then guided the polymerization reaction in directions bent by 120° or 180° from the original direction. Also, close inspection of the linear pan molecules seen in Figure 7a discloses that not all linear chains gave rise to the same corrugation height, suggesting that bilayer polyaniline molecules grew simultaneously before the first aniline adlayer was completed. Evidently, aniline molecules needed to produce the second layer of PAN had to come from the solution. Intriguingly, even PAN molecules in the second layer exhibited well-defined linear conformation, and they appear to perch directly atop the first PAN layer. These STM results indicate an epitaxial molecular film deriving from the strong intermolecular interaction, such as the π-π interaction noted in many organic systems.46 The conductivity of PAN film could decrease with its thickness and became so low that eventually disrupted the imaging process. High quality STM image was observed for a PAN film of 4 molecular layers. Conclusions Molecular-resolution STM imaging reveals two ordered aniline structures, Au(111)-(19 × 5) and (3 × 23)rect, at 0.55 and 0.85 V in 0.1 M H2SO4 and 0.03 M aniline. In situ STM results show that aniline coverages in these two structures are nearly the same at 0.16, whereas those of bisulfate anion could double (from 0.08 to 0.16) as the potential is increased from 0.55 to 0.85 V. STM imaging at 10 nA in feedback current and -300 mV in bias voltage renders molecular resolutions of aniline admolecules and bisulfate anions in the Au(111)-(3 × 23)rect structure simultaneously. XPS results are supportive to the 1:1 molar ratio of aniline: bisulfate in the (3 × 23)rect structure. Furthermore, half of the aniline admolecules are protonated by the coadsorbed bisulfate anion, whereas the other half are adsorbed as unprotonated aniline molecules. Aniline admolecules are adsorbed with their nitrogen ends bound to the Au(111) substrate and their phenyl rings are tilt by ∼47° away from the substrate plane. In the Au(111)-(3 × 23)rect structure aniline admolecules are arranged in a zigzag, headto-tail configuration along the main axis of Au(111) substrate.

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13763 This unique molecular arrangement eventually leads to welldefined linear PAN chains at E > 0.95 V. According to STM results, linear PAN chains grow in epitaxial fashion up to four molecular layers on a Au(111) electrode within minutes at 1 V in 0.1 M H2SO4 + 0.03 M aniline. Acknowledgment. The authors thank technical help from Prof. C. C. Su (Institute of Organic and Polymeric Materials, National Taipei University of Technology). The financial support is provided by the National Science Council of Taiwan (NSC 98-2113-M-008-001). Supporting Information Available: The structure of aniline molecules adsorbed on Au(111) was imaged by STM without potential control but under nitrogen. A typical STM image is depicted. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Yau, S. L.; Vitus, C. M.; Schardt, B. C. J. Am. Chem. Soc. 1990, 112, 3677. (2) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (3) Gewirth, A. A.; Niece, B. K. Chem. ReV. 1997, 97, 1129. (4) Magnussen, O. M. Chem. ReV. 2002, 102, 679. (5) Kolb, D. M.; Ullmann, R.; Will, T. Science 1997, 275, 1097. (6) Sakaguchi, H.; Matsumura, H.; Gong, H.; Abouelwafa, A. M. Science 2005, 310, 1002. (7) Sakaguchi, H.; Matsumura, H.; Gong, H. Nat. Mater. 2004, 3, 551. (8) Zhou, X.-S.; Wei, Y.-M.; Liu, L.; Chen, Z.-B.; Tang, J.; Mao, B.W. J. Am. Chem. Soc. 2008, 130, 13228. (9) Xu, B.; Tao, N. J. Science 2003, 301, 1221. (10) Okawa, Y.; Aono, M. Nature 2001, 409, 683. (11) Mena-Osteritz, E.; Meyer, A.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2000, 39, 2679. (12) Yang, L. Y. O.; Chang, C.; Liu, S.; Wu, C.; Yau, S. L. J. Am. Chem. Soc. 2007, 129, 8076. (13) Lee, Y.; Chang, C.; Yau, S.; Fan, L.; Yang, Y.; Yang, L. O.; Itaya, K. J. Am. Chem. Soc. 2009, 131, 6468. (14) Wandlowski, T.; Lampner, D.; Lindsay, S. M. J. Electroanal. Chem. 1996, 404, 215. (15) Hoon-Khosla, M.; Fawcett, W. R.; Chen, A.; Lipkowski, J.; Pettinger, B. Electrochim. Acta 1999, 45, 611. (16) Batz, V.; Schneeweiss, M. A.; Kramer, D.; Hagenstro¨m, H.; Kolb, D. M.; Mandler, D. J. Electroanal. Chem. 2000, 491, 55. (17) Ikezawa, Y.; Koda, Y.; Shibuya, M.; Terashima, H. Electrochim. Acta 2000, 45, 2075. (18) Kong, D.-S.; Wan, L.-J.; Han, M.-J.; Pan, G.-B.; Lei, S.-B.; Bai, C.-L.; Chen, S.-H. Electrochim. Acta 2002, 48, 303. (19) Mayer, D.; Dretschkow, T.; Ataka, K.; Wandlowski, T. J. Electroanal. Chem. 2002, 20, 524–525. (20) Dretschkow, T.; Wandlowski, T. Electrochim. Acta 1999, 45, 731. (21) He¨bert, P.; Le Rille, A.; Zheng, W. Q.; Tadjeddine, A. J. Electroanal. Chem. 1998, 447, 5. (22) Cai, W.-B.; Wan, L.-J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992. (23) Cunha, F.; Tao, N. J. Phys. ReV. Lett. 1995, 75, 2376. (24) Chang, C.-C.; Yau, S.-L.; Tu, J.-W.; Yang, J.-S. Surf. Sci. 2003, 523, 59. (25) Fan, L.-J. F.; Yang, Y.-W.; Lee, K. AIP Conf. Proc. 2007, 882, 920. (26) Edens, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357. (27) Nagy, G.; Wandlowski, T. Langmuir 2003, 19, 10271. (28) Magnussen, O. M.; Hotlos, J. H.; Behm, R. J. Faraday Discuss. 1992, 94, 329. (29) Wan, L.-J.; Yau, S.-L.; Itaya, K. J. Phys. Chem. 1995, 99, 9507. (30) Cuesta, A.; Kleinert, M.; Kolb, D. M. Phys. Chem. Chem. Phys. 2000, 2, 5684. (31) Schweizer, M.; Kolb, D. M. Surf. Sci. 2003, 544, 93. (32) Kim, Y.-G.; Soriaga, J. B.; Vigh, G.; Soriaga, M. P. J. Colloid Interface Sci. 2000, 227, 505. (33) Lu, P.-C.; Yang, C.-H.; Yau, S.-L.; Zei, M.-S. Langmuir 2002, 18, 754. (34) Broekmann, P.; Wilms, M.; Kruft, M.; Stuhlmann, C.; Wandelt, K. J. Electroanal. Chem. 1999, 467, 307. (35) Bilic´, A.; Reimers, J. R.; Hush, N. S.; Hafner, J. J. Chem. Phys. 2002, 116, 8981.

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