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In Situ STM Revelation of the Adsorption and Polymerization of Aniline on Au(111) Electrode in Perchloric Acid and Benzenesulfonic Acid Yihui Lee, Sihzih Chen, Hsinling Tu, and Shuehlin Yau* Department of Chemistry, National Central University, Jhongli, Taiwan 320, ROC
Liangjen Fan and Yawwen Yang National Synchrotron Radiation Research Center 101 Hsin Ann Road, Hsinchu Science Park, Hsinchu, Taiwan 30076, ROC
Wei-Ping Dow* Department of Chemical Engineering, National Chung Hsing University, Taichung 40227, Taiwan, ROC Received October 11, 2009. Revised Manuscript Received November 25, 2009 In situ scanning tunneling microscopy (STM) was used to study the adsorption and polymerization of aniline on Au(111) single-crystal electrode in 0.1 M perchloric acid and 0.1 M benzenesulfonic acids (BSA) containing 30 mM aniline, respectively. At the onset potential of aniline’s oxidation, ∼0.8 √ V [vs reversible √ hydrogen electrode], aniline molecules were adsorbed in highly ordered arrays, designated as (3 2 3) and (4 2 3) in perchloric acid and BSA, respectively. These structures consisted of intermingled aniline molecules and perchlorate or BSA- anions zigzagging in the Æ110æ directions in HClO4 and in the Æ121æ directions in BSA. The coverage of aniline admolecule on Au(111) was lower in BSA than in HClO4. Raising the potential to 0.9 V or more positive values triggered the oxidation and polymerization of aniline. With aniline molecules arranging in a way similar to the backbone of PAN in HClO4, they readily coupled with each other to produce linear polymeric chains aligned predominantly in the Æ110æ directions of the Au(111). Compared with the results observed in H2SO4 (Lee et al. et al. J. Am. Chem. Soc. 2009, 131, 6468), the rate of polymerization was slower in HClO4 and the produced PAN molecules tended to aggregate on the Au(111) electrode. PAN molecules generated in HClO4 were anomalously shorter than those formed in H2SO4. In 0.1 M BSA, PAN molecules produced by small overpotential (η < 100 mV) could assume linear chains or 3D aggregates, depending on [aniline]. These results revealed molecular level details in electropolymerization of aniline, highlighting the important role of anion in controlling the conformation of PAN molecules and the texture of PAN film.
Introduction In the past several decades electropolymerization of aniline has been studied extensively to show that anions are needed to compensate the charges of PAN molecules. They affect not only the rate of polymerization but also the physical characteristics, such as conductivity and texture of PAN.1-6 The rate of aniline polymerization in inorganic acids is found to descend in the sequence of H2SO4 >HCl ≈ HNO3 >HClO4. Anions are important because they tend to form ion-pairs with aniline and affect the subsequent oxidation and reaction of aniline on metal electrode as well as on PAN molecules.2,7 A few studies have addressed the adsorption of ion-pairs by employing isotope-labeling and reflection absorption spectroscopy.7 Limited by sensitivity of the techniques used in the past, most previous reports could not provide details concerning the amount and adsorption config*Corresponding authors. E-mail: (S.Y.)
[email protected]; (W.-P.D.)
[email protected].
(1) Desilvestro, J.; Scheifele, W. J. Mater. Chem. 1993, 3, 263. (2) Duic, L.; Mandic, Z.; Kovacicek, F. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 105. (3) Choi, S.-J.; Park, S.-M. J. Electrochem. Soc. 2002, 149, E26. (4) Nunziante, P.; Pistoia, G. Electrochim. Acta 1989, 34, 223. (5) Tang, H.; Kitani, A.; Shiotani, M. Electrochim. Acta 1996, 41, 1561. (6) Cordova, R.; del Valle, M. A.; Arratia, A.; Gomez, H.; Schrebler, R. J. Electroanal. Chem. 1994, 377, 75. (7) Kazarinov, V. E.; Andreev, V. N.; Spytsin, M. A.; Shlepakov, A. V. Electrochim. Acta 1990, 35, 899.
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uration of adspecies. Thanks to the modern surface sensitive techniques of STM and X-ray photoelectron spectroscopy (XPS), we are able to address the coadsorption of aniline and bisulfate anions at a molecular level. For example, aniline √ and bisulfate are coadsorbed in an ordered Au(111) - (3 2 3) structure prior to aniline’s oxidation.8,9 In addition, STM imaging enables a direct and real-space vision of PAN molecules produced by anodization of Au(111) in aniline-containing sulfuric acid.8-10 The spatial arrangement of aniline monomers on electrode plays a decisive role in determining the shape of PAN molecules, as aniline molecules zigzagging in the Æ110æ directions of Au(111) eventually yielded parallel, elongate PAN chains spanning 40 nm or more. It is equally important to note that bisulfate anions intermingled uniformly with aniline molecules, which readily facilitated the production of emeraldine salt;the most conductive form of PAN.8,9 In this study in situ STM was used to examine the adsorption and polymerization of aniline on Au(111) electrolytes other than sulfuric acid. In particular, perchloric acid and BSA were used as the supporting electrolytes. Both acids are strong enough to (8) Ou Yang, L. Y.; Chang, C.; Liu, S.; Wu, C.; Yau, S. L. J. Am. Chem. Soc. 2007, 129, 8076. (9) Lee, Y.; Chang, C.; Yau, S.; Fan, L.; Yang, Y.; Yang, L. O.; Itaya, K. J. Am. Chem. Soc. 2009, 131, 6468. (10) Yau, S.; Lee, Y.; Chang, C.; Fan, L.; Yang, Y.; Dow, W.-P. J. Phys. Chem. C 2009, 113, 13758.
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induce protonation of PAN and impart its electronic conductivity.11 Although perchlorate and bisulfate are both tetrahedral in shape, they are rather different in lyophilicity.3,12 This characteristics, as defined by Hofmeister series, determines how strongly anion interacts with anilinum ions and how fast PAN molecules can be produced on the electrode.3 BSA differs vastly from bisulfate and perchlorate in size and the phenyl group expectedly imparts unlike chemical nature, which would influence polymerization of aniline. Indeed, high-quality STM results presented in this study show unambiguously that the physical and chemical nature of anions influence greatly the shape of PAN molecules and the texture of PAN films. It is worthwhile noting that anions are parts of the most conductive form of PAN;emeraldine salts. They can affect not only the conductivity but also the structures and electroactivity of PAN. For example, it is possible to produce PAN molecules with optical isomers by polymerization in media containing (þ)/(-)camphorsulfonic acid (CSA).13-15 The idea of secondary doping introduced in 1995 utilizes CSA as the dopant to uncoil PAN to harness more conductive PAN.16 Copolymerization of aniline and substituted aniline, which is difficult to implement in sulfuric acid, becomes feasible in BSA.17 Finally, β-naphthalenesulfonic acid when incorporated in PAN can be used as sensing materials for ascorbic acid.18
Experimental Section Au(111) single-crystal electrodes used in voltammetic and STM experiments were made by melting the end of a gold wire (φ 0.8 mm).19,20 One of the (111) facets was exposed by mechanical polishing after alignment by laser reflection. This electrode allowed voltammetric measurements. Pretreatment of the gold electrode involved annealing by a hydrogen torch, followed by quenching in H2-saturated Millipore triple distilled water (resistivity 18.3 MΩ 3 cm). The conventional hanging meniscus method was used to perform voltammetry in a three-electrode cell, equipped with a reversible hydrogen electrode (RHE) and a Pt wire acting as the reference and the counter electrodes, respectively. The potentiostat used was a CHI 703. The supporting electrolyte used in this study was typically 0.1 M HClO4 or BSA containing 30 or 0.3 mM aniline. Ultrapure HClO4 was purchased from Merck (Darmstadt, DFG), and BSA was obtained from Fluka chemicals (Tokyo, Japan). Aniline was purchased from Showa chemicals (Tokyo, Japan) and was distilled before uses. Triple-distilled Millipore water (resistivity 18.3 MΩ cm-1) was used to prepare all solutions. The STM used in this study was a Nanoscope E (Digital Instruments, Santa Barbara, CA) with a single tube scanner (high-resolution A-head, maximal scan area ∼600 nm). Tungsten tips (o.d. 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. The use of STM in studying electrified interface is reviewed.21,22 (11) Trivedi, D. C. J. Solid State Electrochem. 1998, 2, 85. (12) Zhang, Y.; Cremer, P. S. Curr. Opin. Chem. Biol. 2006, 10, 658. (13) Strounina, E. V.; Kane-Maguire, L. A. P.; Wallace, G. G. Synth. Met. 1999, 106, 129. (14) Majidi, M. R.; Ashraf, S. A.; Kane-Maguire, L. A. P.; Noms, I. D.; Wallace, G. G. Synth. Met. 1997, 84, 115. (15) Majidi, M. R.; Kane-Maguire, L. A. P.; Wallace, G. G. Polymer 1994, 35, 3113. (16) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1995, 69, 85. (17) Vijayan, M.; Trivedi, D. C. Synth. Met. 1999, 107, 57. (18) Zhang, L. Electrochim. Acta 2007, 52, 6969. (19) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1. (20) Chang, C.-C.; Yau, S.-L.; Tu, J.-W.; Yang, J.-S. Surf. Sci. 2003, 523, 59. (21) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (22) Magnussen, O. M. Chem. Rev. 2002, 102, 679.
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Figure 1. Cyclic voltammograms obtained at 50 mV/s with Au(111) in 0.1 M HClO4 (a) and 0.1 M BSA (b) without (dotted traces) and with 30 mM aniline (solid lines).
Results and Discussion Cyclic Voltammetry. Shown in Figure 1a and b were CVs recorded at 50 mV/s with Au(111) electrodes immersed in 0.1 M HClO4 (a) and BSA (b) without (dotted lines) and with (solid traces) 30 mM aniline, respectively. The potential scans used in Figure 1 started from the open-circuit potentials (∼0.7 V), first to 0.05 V, followed by a positive-going sweep to 1.0 V, giving rise to a few sharp peaks ascribable to the adsorption and restructuring of aniline adlattices, as observed also in sulfuric acid.9,10 The broad waves observed near 0.45 V in HClO4 are due to restructuring of the Au(111) substrate.19,23 Similar to the conclusion drawn for aniline in sulfuric acid, the adlayer on Au(111) included aniline molecules as well as anions (HSO4-, ClO4-, or BSA-) and acid molecules. The coverages of these species vary with potential.10 The CV profiles shown in Figure 2 were obtained by cycling the potential of Au(111) at 50 mV/s between 0.05 and 1.2 V in 0.1 M BSA þ 0.3 mM aniline. The precipitous increases of anodic current at E > 1.05 V is ascribed to the oxidation and polymerization of aniline, leading to a number of features evolving with cycle numbers. A pair of weak feature appeared near 0.4 V, followed by a more pronounced peak at 0.7 V. The anodic feature seen at 1.1 V diminished while the reduction feature at 0.8 V gained intensity with potential cycles. These characteristics resemble those observed with polymerization of aniline in sulfuric acid.3,9 The continuous increase of current indicates the gradual build-up of PAN film with potential cycles, while changes in the oxidation state of PAN were responsible for the redox features seen in the CV profiles. The redox couple found at 0.4 V is ascribable to the transformation from the fully reduced to the half oxidized state of PAN. It was not as developed as that seen in 0.1 M H2SO4, possibly because less PAN was produced in perchloric acid.3,9 The most pronounced feature seen at 0.7 V, previously attributed to the redox processes associated with side products formed at E > 1.1 V, could result from some different species. This feature persisted even after the electrode was rinsed with Millipore water, suggesting that it is associated with species strongly held at the gold electrode. The transformation from the half- to the fully oxidized state of PAN which produces a peak at 0.9 V in H2SO4 did not produce any well-defined CV feature. (23) Hamelin, A.; Martins, A. M. J. Electroanal. Chem. 1996, 407, 13.
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Figure 2. Cyclic voltammograms recorded at 50 mV/s with Au(111) in 0.1 M HClO4 (a) and 0.1 M BSA (b) containing 30 mM aniline. The precipitous increases of current seen in both cases are attributed to the oxidation and polymerization of aniline, leading to the pronounced features near 0.42 and 0.7 V, ascribable to redox processes of the as-formed polyaniline molecules on Au(111) electrodes.
The rate of polymerization of aniline in sulfuric acid and BSA should be proportional to the magnitude of peak current (ip). It appears that aniline polymerized faster in BSA than in HClO4, although the difference is only marginal. Their peak currents amount to only one-third of that observed in 0.1 M H2SO4, which agrees with reported results.24 This anionic effect on the rate of polymerization is thought to be associated with the lyophilicity of anions.6 In addition, because anions would have to diffuse into or out of the polymeric film in response to the modulation of potential, their size could also influence the rate of reaction. Lowering [aniline] from 30 to 0.3 mM slowed down the rate of polymerization by an order of magnitude. In situ STM Imaging of Aniline Adlattices on Au(111) in 0.1 M HClO4. Holding the potential at 0.9 V in 0.1 M HClO4 containing 0.03 M aniline, in situ STM imaging revealed typical terrace-and-step features and molecular structures on Au(111), as revealed by Figure 3a-d. Prior to its oxidation, aniline monomers were adsorbed in highly ordered arrays with one single ordered domain spanning 50 nm or more. Since ClO4- by itself did not yield this ordered structure, the highly ordered array seen in Figure 3b would have to involve aniline molecules. The optimal imaging conditions were -300 mV in bias voltage and 1 nA in feedback current. We identified a rectangular unit cell with two edges aligned in the Æ110æ and Æ121æ directions√ and measured 0.9 and 1.0 nm in length, which yielded a (3 2 3) structure, the same as that observed previously in 0.1 M H2SO4.9,10 In addition, as found in sulfuric acid, anions such as perchlo√ rate could be coadsorbed with aniline molecules in the (3 2 3) adlayer. Presuming each protrusion is associated with an aniline admolecule, the coverage of aniline is calculated as 2/12 or 0.167. Intriguingly, the spot inside the cell was lopsided toward the Æ110æ aligned edges, rather than sit in the center of the cell. These characteristics were observed also in sulfuric acid.9,10 The coadsorbed perchlorate anions could be seen more clearly under different imaging conditions,10 but we did not pursue this issue further. Switching the potential from 0.9 to 1.05 V resulted in drastic changes of the Au(111) surface, revealed by the STM image in Figure 3, parts c and d. The former, obtained shortly after the (24) Shin-Jung, C.; Su-Moon, P. J. Electrochem. Soc. 2002, 149, E26.
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Figure 3. In situ STM √ images reveal the well ordered adlattice of Au(111) - (3 2 3) - aniline þ perchlorate anion (a and b), denoted by rectangle, at 0.9 V in 0.1 M HClO4 þ 30 mM aniline. Those in parts c and d were acquired 1 and 10 min after the potential was made to 1.05 V, showing the prominent linear shapes of PAN molecules aligned preferentially with rows of aniline molecules on the Au(111) electrode. A few cases of misalignment between PAN and adsorbed aniline monomer are highlighted. Arrows marked in the images indicate the main axes of the Au(111) surface.
switch of potential, reveals protruded linear segments sitting on parallel linear patterns. Those protruded linear features were associated with PAN molecules produced by the shift of potential. The time allowed only a fraction of the surface to have PAN chains, and most Au(111) surface was still occupied by the (3 √ 2 3) structure. Most PAN molecules seen in Figure 3c bulged by 0.34 nm on the aniline monomers. Occasionally, PAN molecules, marked in Figure 3c, were aligned unparallel to the underlying aniline structure. This sort of imperfection was rare in H2SO4.9 According to our previous study, there is a close correlation between the spatial arrangement of aniline admolecules and the formation of PAN.10 Thus, the stability or rigidity of the adlattices made of aniline and bisulfate or perchlorate anions could have strong bearing with the formation of defects in the PAN chains. The length of linear PAN chains is thought to relate closely to the stability of the aniline adlattices. On average linear portion of PAN chains seen in HClO4 was 15 nm, as compared to 35 nm observed in H2SO4.9 Prolong STM imaging at 1.05 V revealed continuous deposition of PAN molecules on Au(111), as revealed by the STM image shown in Figure 3d, collected 20 min at 1.05 V. The tier-like topography indicates multilayered PAN with typical linear molecular shape, but with a lot more defects characterized as 120-bending and 180-folding. On average the linear portion of the PAN chains was 20 nm, the same as those found initially. PAN molecules lying in the upper layers appeared to run parallel to those in the lower planes. This packing habit can be likened as stacking bricks, which stemmed from attractive π-π interactions between two PAN layers. Typically PAN molecules grew independently; they rarely intersected with another PAN chain. STM imaging of these multilayered PAN was stable without any trace of the tip-and-sample contact, which implies that the as-produced PAN film was good electronic conductor. These results are mostly consistent with those found in sulfuric acid, although PAN produced in sulfuric acid could have a longer linearity.9 In situ STM Imaging of Aniline Adlattices on Au(111) in 0.1 M BSA. Figure 4a is a typical STM topo image obtained at 0.5 V in 0.1 M BSA, which reveals a largely smooth Au(111) surface with corrugated stripes spanning more than 100 nm in the Æ121æ directions. The reconstructed features of Au(111) were Langmuir 2010, 26(8), 5576–5582
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Figure 4. In situ STM imaging showing a homogeneous Au(111) surface onto which highly ordered BSA were adsorbed at 0.5 V in 0.1 M √ BSA (a). A few herringbone structures remained. The high resolution STM scan shown in part b yielded a (5 2 3) structure, which is accounted for by the ball model presented in part c. BSA, accompanied by water molecules, is likely to adsorb via its sulfonate group. Arrows marked in part b indicate the main axes of the Au(111) surface.
Figure 5. In situ STM images showing changes in the BSA adlayer as the potential was switched from 0.55 (a) to 0.75 (b), then to 0.85 V (c), the positive end of the sharp peak seen in the voltammogram of Figure 1b. In addition to the loss of the reconstructed features of Au(111), the ordered BSA adlayer was disrupted after the potential was held at 0.8 V for 20 min. Arrows marked in part b indicate the main axes of the Au(111) surface.
largely removed, possibly by the adsorption of BSA anions. The molecular-resolution STM image shown in Figure 4b reveals the ordered BSA adlattice with its unit cell marked in rectangle. This rectangle has a 1.5 1.0 nm dimension and its edges were√ aligned in the Æ110æ and Æ121æ directions, which yields a (5 2 3)rect structure. There were four BSA adspecies in each cell, resulting in a coverage of 2/20 √ or 0.1, √ which one-half of that determined for the Au(111)-( 3 7)-bisulfate structure. BSA could be adsorbed either at different sites or with different configurations, giving rise to unlike STM corrugation heights, as illustrated by the ball model depicted in Figure 4c. Although it is not known how BSA interacted with Au(111), we contend that it was adsorbed via its SO3- functional group while leaving its phenyl ring pending in solution. Water molecules, known √ to coadsorb with bisulfate, could be a part of this (5 2 3)rect - BSA adlattice also, as indicated by the ball model. To explore the nature of the anodic peak at 0.81 V, the potential was shifted first from 0.55 to 0.75 V, the negative end of this feature. PArts a and b of Figure 5 show the structure √ seen before and after the change of potential. The (5 2 3)rect-BSA pattern sitting on √ the herringbone features were imaged at 0.5 V, but the (5 2 3)rect structure residing on the fcc domains of Au(111) were apparently eliminated. The ordered BSA arrays sitting on the herringbone features were more resistive to the change of potential, and they were the last ones to comply with the potential. At 0.85 V, nearly all of the BSA structure and the herringbone structures were removed (Figure 5c). These results indicate that the sharp peak at 0.78 V is ascribed to an order-todisorder phase transition of the BSA adlayer. Meanwhile, a few pits appeared as the ordered structure of BSA disappeared at 0.82 V, which could be defects in a largely disordered molecular adlayer. In Situ STM Imaging of Au(111) in 0.1 M BSA þ 0.03 M Aniline. Shown in Figure 6a were STM images obtained at 0.7 V in 0.1 M BSA þ 0.03 M aniline, revealing a homogeneous adlayer cloaking a smooth Au(111) surface. The distinct, prolong linear Langmuir 2010, 26(8), 5576–5582
patterns were due to the ordered adlayer, which remarkably had a span of 136 nm. Figure 6b reveals the internal molecular arrangement of the ordered array, which is represented by a 1.2 1.0 nm rectangle with two edges aligned in the Æ110æ √ and Æ121æ directions. These characteristics indicate a (4 2 3)rect structure. By associating each protrusion with an aniline admolecule, STM resulted in a coverage of 2/16 or 0.125, which is 32% √ less than that (0.167) determined for the Au(111) - (3 2 3)rect aniline structure observed in sulfuric and in perchloric acids. √ As found with the Au(111)-(3 2 3)rect-aniline structure, √ two set of spots were imaged in the present (4 2 3)rect structure. We contend that the brighter and weaker ones are due to aniline admolecules and the coadsorbed BSA- anions, respectively. Thus, on Au(111) aniline admolecules formed zigzagged chains punctuated by strings of anions, such as bisulfate, perchlorate, and BSA-. Since BSA is physically larger than perchlorate and bisulfate anions, it is not surprising that two neighboring aniline chains were more separated to produce a larger unit cell. Each unit cell thus has two aniline molecules and two BSA anions, resulting in a coverage of 0.125. Thus, less aniline molecules were adsorbed on Au(111) in BSA than in H2SO4 and in HClO4. Theoretical calculations were performed to scrutinize the adsorption configuration of organic molecules bearing amino group on gold surface, which yields the on-top site as the most favorable registries on Au(111).25 These results lead to a tentative ball model shown in Figure 6c, where aniline molecules are arranged in zigzags in the Æ121æ direction of Au(111). It is likely that BSA- was adsorbed via its -SO3group. Not all aniline admolecules exhibited the same corrugation heights in the STM image (Figure 6b). In particular, the aniline admolecule located at (1/2, 2/3) is 0.02 nm lower than those at the corners. We attribute this feature to an aniline admolecule adsorbed in an orientation different from those of corner ones. (25) Hoft, R. C.; Ford, M. J.; McDonagh, A. M.; Cortie, M. B. J. Phys. Chem. C 2007, 111, 13886.
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Figure 6. In situ STM images revealing the morphology of Au(111) electrode (a) covered by a highly ordered adlayer of (4 2 3) seen in the
high resolution image in part b. The Au(111) electrode was fixed at 0.7 V and immersed in 0.1 M BSA þ 30 mM aniline. A ball model is proposed in part c, entailing a mixed adlattice of aniline and BSA and the adsorption configuration of these adsorbates. The bias voltage and feedback current were typically set at -200 mV and 1 nA, respectively. Arrows marked in the images indicate the main axes of the Au(111) surface.
Figure 7. Potential-dependent STM images showing the production of polyaniline molecules on Au(111) electrode immersed in 0.1 M BSA þ 30 mM aniline. The first two images (a and b) were obtained at 0.9 V; whereas the third (c) was obtained at 1.0 V. The sudden increases of blotches imaged by the STM were associated with PAN aggregates, although a few linear PAN molecules were seen initially.
For example, this aniline molecule could be rotated by 60 from the corner ones. This dependence of the STM appearance and adsorbed configuration of adsorbate is reported for many molecules, perhaps the adsorption of organosulfur compounds on Au(111) is the most well-known case.26 The aniline’s arrangement in this structure is particularly relevant to the shape of polymer produced at E > 1.0 V. In Situ STM Imaging of Electropolymerization of Aniline in 0.1 M BSA. Raising the potential of Au(111) to 0.9 V in 0.1 M BSA þ 0.03 M aniline triggered the oxidation and polymerization of aniline, as revealed by the STM images shown Figure 7a-c. The potential of Au(111) was set initially at 0.9 (a and b) but later switched to 1.0 V (c). The bias voltage and feedback current were -0.5 √ V and 1 nA, respectively. At the beginning the (4 2 3)rect-aniline þ BSA structure predominated with a few PAN chains imaged as protruded linear segments. As the potential of Au(111) was held at 0.9 V, in situ STM revealed a substantial increase of polyaniline molecules on the Au(111) electrode in 20 minutes, mostly in the form of poorly defined aggregates and some minor linear chains (Figure 7b). Raising the potential from 0.9 to 1.0 V increased the rate polymerization, adding more molecules to the electrode surface, as revealed by Figure 7c. Those poorly defined blotches, measured 1.0 nm in height by the STM, indicate that they could comprise of three layers of PAN. However, these PAN deposits were so rough microscopically that it was not possible to achieve molecular resolution. It seems then a long-range ordered aniline adlayer does not guarantee well-defined linear PAN molecules, which contrasts markedly with results seen in H2SO4 and HClO4. These STM results imply that polymerization of aniline in BSA might not occur preferentially on the electrode surface. At (26) Poirier, G. E. Chem. Rev. 1997, 97, 1117.
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proximity of the Au(111) electrode aniline molecules in the solution phase could also be oxidized and polymerized. Which route, the surface or solution process, eventually prevailed depended on the stability of the adlayer of aniline þ anions, the concentration of aniline, the lyophilicity of anions, etc. Presumably, surface polymerization on a properly ordered structure would be beneficial to the production of well-defined PAN molecules. However, polymerization in the solution phase could predominate under the conditions of Figure 7, resulting in PAN molecules in the solution. Because of the poor solubility of PAN molecules in the aqueous electrolyte, they precipitated on the solid support, giving rise to blotches of PAN imaged by the STM in Figure 7c. It is not clear why polymerization in solution was favored in BSA, but not in HClO4. The effects of anion on aniline’s electropolymerization have been studies.3 It is shown that the lyophilicity of anions, as outlined by the Hofmeister series,3,12 can affect the rate of polymerization and the texture of PAN. It was proposed that anions form complexes with anilinium in the solution as well as on the electrode surface. These anilinium-anion species would have to dissociate prior to the oxidation reaction. Being more lyophilic than HSO4-, ClO4- clings to anilinium cation more strongly, which slows down the rate of polymerzation. In addition, anions involved in polymerization of aniline could also affect the texture of PAN.3 This idea could apply to explain results observed in the present study of polymerization in BSA. We contend that BSA could be even more lyophilic than perchlorate anion, because of the phenyl group in its molecular structure. This character could enhance the intermolecular interactions between PAN molecules and favor their aggregations when they were produced in the solution phase or on the electrode surface. This could be the reason why we observed pronounced cluster formation of PAN on the Au(111) electrode in Figure 7. Langmuir 2010, 26(8), 5576–5582
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Figure 8. In situ STM images showing the initial stage of aniline’s polymerization on Au(111) at 0.85 V in 0.1 M BSA þ 0.030 mM aniline (a). Aniline admolecules located at domain boundaries seemed to be oxidized first (b), producing crooked PAN molecules. Meanwhile, reactions also occurred on ordered domains to yield linear PAN chains, whose internal molecular structure is seen in part c. Arrows marked in the images indicate the main axes of the Au(111) surface.
Figure 9. Potential-dependent STM images showing the production of polyaniline molecules on Au(111) electrode immersed in 0.1 M BSA þ 0.030 mM aniline. The images were obtained at 0.9 (a), 0.95 (b), 0.98 (c), and 1.05 V (d). It appears that aniline polymerized much more slowly than that found in 30 mM aniline (Figure 7) and linear growth of PAN prevailed. These conditions were still unable to avert aggregation of PAN molecules.
On the other hand, the extent of PAN aggregation varied with experimental conditions, for example, the concentration of aniline. In particular, we examined aniline polymerization in 0.1 M BSA using 0.3 mM aniline, one-hundredth of that used in Figure √ 7. First, STM imaging (not shown) indicated that the (4 2 3)rect aniline structure, found previously in 30 mM aniline, still prevailed in 0.3 mM aniline. The potential was then raised to 0.95 V to initiate the reactions of aniline. Figure 8a reveals a 200 200 nm topography STM scan recorded shortly after the shift of potential, showing linear and meandering PAN threads. They tended to grow near defects, such as domain boundaries, protruded islands, and step ledges. Higher resolution STM scans shown in Figure 8, parts b and c, reveal that internal molecular features√of PAN molecules, which appeared to perch on the (4 2 3)rect-aniline adlayer. Figure 8c is a further close-up STM image showing the internal structure of a PAN molecules made of segmented lines spanning 15 nm in length. In situ STM was able to discern corrugated oval protrusions with two nearest neighbors separated by 0.55 nm in the PAN chain. These features are associated with phenyl groups, which could be tilted from the Au(111) plane by ∼50, as previously observed for PAN in sulfuric acid.9 On average PAN molecules were higher than the aniline monomers by 0.34 nm, which agrees with the c dimension of a PAN crystal.27,28 Ultimately, it was the zigzag (27) A. Ivanova, G. M. A. T. N. G. Int. J. Quantum Chem. 2006, 106, 1383. (28) Pouget, J. P.; Jozefowicz, M. E.; Epstein, A. J.; Tang, X.; MacDiarmid, A. G. Macromolecules 1991, 24, 779.
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arrangement of aniline monomers that guided this linear growth of PAN chains.10 In comparison with the Æ110æ aligned PAN observed in sulfuric and perchloric acid, PAN chains were preferentially aligned in the Æ121æ directions in BSA. This is consistent with the spatial arrangement of aniline molecules seen in these media; aniline admolecules zigzagged in the Æ110æ directions in H2SO4 and HClO4, but in the Æ121æ directions in BSA. Shown in Figure 9a-d are time-dependent STM images, revealing the growth dynamics of PAN at 0.95 V in 0.1 M BSA containing 0.3 mM aniline. The rate of reaction was so slow that only 30% of the Au(111) surface was covered with PAN in 20√min. PAN first appeared at defects as well as on ordered (4 2 3)rect, producing crooked and linear PAN molecules, respectively. Multilayer PAN imaged as brighter linear segments were found on the top of the first yet-to-complete PAN layer. Aggregations of PAN were responsible for the local protrusions seen in the STM images (Figure 9d). Compared to the results seen in Figure 4, it is fair to state that lowering [aniline] from 30 to 0.3 mM was effective in reducing molecular aggregations. The fact that PAN molecules preferred to nucleate at surface defects, rather than occurred randomly, suggests that it was the nature, for example, the lyophilicity, of the BSA anion that eventually controlled how PAN molecules were formed. Polymerization of aniline could occur in the solution phase, but it was not responsible for the STM results presented here. These results are important to uncover the structures of electrochemically produced PAN films in different acidic media.
Conclusions In situ electrochemical STM has disclosed the adsorption and the subsequent polymerization of aniline on Au(111) electrodes in aniline-containing perchloric acid and BSA. Aniline admolecules intermingled with ClO √ √4 and BSA and formed well-ordered (3 2 3)rect and (4 2 3)rect, respectively, at the onset potentials for polymerization. Both structures had aniline admolecules arranging in zigzagged chains in the Æ110æ and Æ121æ directions of Au(111). PAN molecules produced in HClO4 assumed mainly linear conformations, but contained more structural defects of bending and folding, as compared with those found in sulfuric acid. The specific zigzag molecular arrangement served as a template for the subsequent polymerization at E > 0.9 V. Depending on the concentration of aniline, PAN molecules could be produced on the electrode or in the solution, yielding rough and smooth film morphology on the Au(111) electrode. The lyophilicity of anion could be the reason why PAN grew in different fashions in perchloric acid and in BSA. The more lyophilic anion of BSA resulted in stronger intermolecular DOI: 10.1021/la903857x
5581
Article
interactions between PAN molecules, which favored molecular aggregation on the electrode surface. Overall, this study indicates the important roles of anions in molding PAN molecules and in controlling the texture of electrodeposited PAN films on Au(111) electrode.
5582 DOI: 10.1021/la903857x
Lee et al.
Acknowledgment. The authors thank technical help from Prof. C. C. Su (Institute of Organic and Polymeric Materials, National Taipei University of Technology). This research is supported by the National Science Council of Taiwan (NSC 982113-M-008-001).
Langmuir 2010, 26(8), 5576–5582
