Article pubs.acs.org/JPCC
Adsorption and Electrochemical Polymerization of Pyrrole on Au(100) Electrodes as Examined by In Situ Scanning Tunneling Microscopy Alda Khairunnisa,†,‡ Weichen Liao,† and Shuehlin Yau*,† †
Department of Chemistry, National Central University, 300 Jhongda Road, Jhongli District, Taoyuan City, Taiwan 320, Republic of China ‡ Department of Chemistry, Faculty of Mathematics and Science, Brawijaya University, East Java 65145, Indonesia ABSTRACT: Despite many efforts studying polypyrrole (PPy) with scanning probe techniques in the past several decades, its molecular structure and conformation have been elusive. This study presents the first atomic and molecularresolution scanning tunneling microscopy (STM) images of pyrrole (Py) and PPy electrochemically produced on bare and iodine-modified Au(100) electrodes in pH 1 and 4 sulfate media. First, STM reveals a highly ordered Au(100)(4√2 × 4√2) R45°Py between 0.15 and 0.5 V (vs saturated calomel electrode) in 0.1 M H2SO4 containing 30 mM Py. By contrast, a disordered Py adlayer is seen in 0.1 M HClO4 medium, indicating the coadsorption of Py and an acid. This anion effect is manifested in voltammetric results obtained with Au(100), showing a pair of sharp peaks at 0.12 V in 0.1 M H2SO4 + 30 mM Py and a featureless profile seen with Au(111) or Au(100) in HClO4. Py and bisulfate (and perchloric acid) attract each other and are coadsorbed on Au(100) electrodes. The favorable adsorption of bisulfate anions on Au(100) benefits the adsorption of Py. Raising the potential to 0.6 V causes irreversible oxidation of Py, yielding a dark deposit on the Au(100) electrodes. The IR results indicate that this deposit is PPy. The as-prepared PPy film yields a pair of reversible redox peak at −0.4 V in pH4 K2SO4. Molecular-resolution STM imaging at 0.4 V in 0.1 M H2SO4 reveals the oxidized PPy molecules, which assume loop, circle, and wigging conformations. The main molecular features observed by STM can be explained by structural models, based on α−α coupling and syn-conformation of Py rings; however, the α−β coupling of Py is also possible. Linear segments 2−3 nm long have anti-conformation of Py. The PPy molecules are not stable at E < −0.1 V and >0.7 V, possibly due to degradation and overoxidation.
1. INTRODUCTION Polypyrrole (PPy) is a renowned organic conductor, which can be used as an electrode material for sensors, solar cells, and high-density batteries.1−3 Conductivity and chemical stability are of prime importance to these applications. Shown in Scheme 1 is the chemical structure of the oxidized form of PPy, where positive charges can travel through its backbone−an extended conjugation system. It is found that the oxidation of PPy yields positive charges at the polymeric chain, and triggers occultation of anions.4 Therefore, the redox process of PPy is
affected greatly by the ingress of anions from the solution phase to the deposited polymeric phase. Numerous studies have addressed the redox of PPy in terms of charge, mobility, and hydrophobicity of anions.5 At the microscopic level, anions can also affect the structure and organization of polymeric molecules, which determine the charge mobility in the polymeric chain and charge hopping rate between neighboring chains. The chemical structures of PPy depend on the preparative conditions, including solvent,6 the size and nature of the dopant counter-anions,7 and pH.8 Despite extensive studies using X-ray diffraction9−11 and scanning probe techniques,12,13 there are as yet no reports on the molecular conformation of electrochemically formed PPy. Scanning tunneling or atomic force microscopy (STM and AFM) have been used to study the adsorption of Py and its subsequent electropolymerization on Au(111) and HOPG electrodes.12−16 No ordered Py adlayer has been found on all
Scheme 1. Oxidized Form of Polypyrrolea
Received: September 23, 2016 Revised: October 29, 2016 Published: October 31, 2016
a −
X represents the doped anion. © XXXX American Chemical Society
A
DOI: 10.1021/acs.jpcc.6b09668 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 1. Steady-state CVs obtained at 50 mV/s with Au (100) (a) and Au (111) (b) in 0.1 M H2SO4 without (blue traces) and with (red traces) 30 mM pyrrole. The black trace in panel (a) is recorded in 0.1 M HClO4 + 30 mM Py.
configuration, including a reversible hydrogen electrode (RHE, the reference electrode) and a Pt wire as the counter electrode. The cell is purged with ultrapure N2 stream to prevent pyrrole from reacting with oxygen. All potentials mentioned below are converted to a saturated calomel electrode (SCE) scale. The STM instrument was a Nanoscope E (Veeco, Santa Barbara, CA) and the cell is equipped with two platinum wires as the reference and counter electrodes. The STM tip was made of tungsten electrochemically etched in 1 M KOH. After thorough rinsing with Millipore water and acetone, the tip was coated with Apeazon wax to minimize the faradaic current. All STM images were acquired with the constant-current mode and unfiltered. The IR spectrometer was the Nicolet AUATAR360 FTIR, which was equipped with homemade attachments and a liquidN2-cooled linearized narrow-band HgCdTe detector. The resolution and incident angle were 4 cm−1 and 60°. All spectra are shown in the absorbance unit defined as A = −log (I − Io/ Io), where I and Io represent the intensities of the IR radiation reflected from the electrode at the selected and background potentials, respectively. The prism was a nondoped Si hemicylinder (PASTEC, Japan, Osaka), which was polished before chemical deposition of gold. The gold coating on the Si prism was prepared by a previously reported method.22 The Au-modified Si prism was subjected to potential annealing by sweeping between 0.1 and 1.15 V to remove contamination from the surface.
systems examined thus far. By contrast, aniline, structurally similar to Py, is readily coadsorbed with HSO4− on Au(111) and Au(100) in nicely ordered structures.17,18 This is readily explained by the difference in the adsorbate and substrate interactions. As amine molecules, such as aniline, are frequently coadsorbed with an acid, the interaction of the acid with the electrode can be important. Being a much weaker base than aniline (the pKb of Py and aniline are 13.6 and 9.38, respectively), Py can interact more weakly with acid on the gold electrode. PPy film electrochemically deposited on Au(111) has been examined by STM and AFM, revealing a rough deposit with a cauliflower morphology.14,16,19 Due to complications in the PPy molecular structure, no molecular resolution images have yet been obtained in situ, although molecular STM images of PPy have been obtained in air.20 The lack of ordered spatial arrangement of Py adsorbed on Au(111) makes it difficult to substantiate the understanding of molecular adsorption in terms of the coverage, adsorption orientation, subsequent oxidation reaction, and other features. On the other hand, it is known that the orientation of a substrate can be important to molecular adsorption, and thus the adsorption of Py at Au(100) is explored in this study. Molecular-resolution STM images are obtained here, showing that Py is adsorbed on Au(100) in a highly ordered adlayer, characterized as (4√2 × 4√2), in 0.1 M H2SO4. In addition, the first in situ molecular resolution STM image of oligo- and PPy molecules generated by oxidation of Py was also obtained. In situ STM imaging elucidates conformations and organization of the electrochemically formed PPy molecules. This study is thus considered an important step toward studying the conformation of this complicated conducting polymer.
3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetry. 3.1.1. Adsorption of Py Molecules on the Au(100) and Au(111) Electrodes. Figure 1a,b shows CVs recorded with Au(100) and Au(111) electrodes in 0.1 M H2SO4 without (thin lines) and with (thick lines) 30 mM Py. The sweeping potential of these electrodes at 50 mV/s between −0.35 and 0.4 V results in A and A′ peaks at 0.33 and 0.19 V, as seen in Figure 1a,b, respectively. These are associated with the specific adsorption of bisulfate.23,24 The presence of 30 mM Py results in marked changes in the CV profiles. For Au(100), a pair of symmetric peaks (A1/C1) is seen at 0.12 V, ascribed to a disorder-to-order phase transition of the Py adlayer. This view is supported by the in situ STM results described below. This CV profile is stable against repeated potential cycling if the positive window does not exceed 0.5 V, where Py is irreversibly oxidized. As the aromatic ring of Py molecule is known to interact weakly with the gold electrode, it is most likely that the Py molecules are adsorbed via the N-end. However, Py molecules are mostly protonated in 0.1 M H2SO4, it is surprising that Py is adsorbed on these gold electrodes. Nonetheless, aniline, structurally
2. MATERIALS AND METHODS The pyrrole used in this work was purchased from Acros-Fisher (New Jersey, US) distilled before use, and then stored in a refrigerator (∼4 °C). The sulfuric acid and potassium hydroxide were both ultrapure grade and obtained from Merck (Darmstadt, DFG). They were diluted with Millipore triple-distilled water to the needed concentrations. All solutions were then thoroughly degassed with nitrogen. The Au(100) and Au(111) single crystal electrode beads were made by melting a Au wire (Φ 0.8 mm), as reported previously.21 They were cut and polished with SiC cloth and alumina slurry to expose the oriented faces. The electrodes were then annealed by a hydrogen flame and quenched into ultrapure water saturated with hydrogen. Cyclic voltammetry is performed using a potentiostat (CH 614 Instruments, Texas) with the working electrode hanging in the electrolyte. The electrochemical cell was a three-electrode B
DOI: 10.1021/acs.jpcc.6b09668 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. CVs recorded at 50 mV/s with an Au (100) electrode in 0.1 M H2SO4 containing 30 mM Py (a). Post-polymerization CV obtained with PPy-coated Au(100) electrode in pH 4, 0.1 M K2SO4 (b). The PPy film is prepared by sweeping the potential between −0.3 and 0.9 V for 25 times in 0.1 M H2SO4 + 30 mM Py.
Figure 3. In situ STM images acquired with bare the Au (100) at −0.1 V (a) and 0.4 V (b,c), and iodine-modified Au(100) electrode (d,e) in 0.1 M H2SO4. The feedback current and bias voltage are 1 nA and −226 mV.
In order to see the role of electrode orientation in Py adsorption, we conducted a CV experiment with the Au(111) electrode in 0.1 M H2SO4 + 30 mM Py. As shown in Figure 1b, the CV is mostly featureless, except for a small hump at 0.05 V, which contrasts markedly with that of Au(100). This featureless CV profile again implies a disordered adlayer, as indeed shown by STM. It is proposed that the bisulfate anion is an anchor for the coadsorbed Py molecule. The formation of an ordered Py + HSO4− adlattice on Au(100), but not on Au(111), can be traced to the adsorption of bisulfate on these electrodes. Adsorbed bisulfate anions arrange in an orderly fashion on Au(100) at 0.35 V,26 but in a disordered manner on Au(111) at E < 0.75 V.27 This is compared with the aniline adsorbed on the Au(111) electrode,25,28 where a highly ordered aniline structure forms. This contrast might stem from the different basicities of Py and aniline. As Py is a much weaker base than aniline, it can interact more weakly with acid, not only in the solution, but also on the electrode. 3.1.2. Electropolymerization of Py on Au(100). In order to produce PPy film on the Au(100) electrode, the potential is
similar to Py, is also adsorbed on the Au(100) electrode under the same conditions.25 To understand the effects of anions on the adsorption Py, we conducted CV experiments in 0.1 M HClO4 + 30 mM Py and compared the results with those found when using H2SO4. The resultant profile shown as the dotted line in Figure 1a is mostly featureless and stable against potential cycling between −0.25 and 0.5 V. This is very different from that found in sulfuric acid, indicating that anions are important in the adsorption of Py. If one recalls the model of aniline adsorption on a gold electrode,17 the Py molecules are also coadsorbed with acid molecules, such as HSO4− or HClO4. It is known that perchlorate is not adsorbed on a gold electrode at E < 0.5 V, but the presence of Py evidently enhances the Au−ClO4− interaction. However, the STM results obtained with Au(100) (described below) show that the Py + ClO4− structure is disordered, but Py + HSO4− is highly ordered. This contrast in adsorption structure could be inferred from the difference in CVs obtained in these acids. C
DOI: 10.1021/acs.jpcc.6b09668 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C cycled at 50 mV/s between −0.3 and 0.9 V in 0.1 M H2SO4 + 30 mM Py. The resultant CV profiles are shown in Figure 2a. The positive-going potential scans result in a sharp increase in current at 0.67 V, which is associated with oxidation and polymerization of Py. This is an irreversible process, as no counter reductive feature is seen in the negative-going scans. The sharp peak at 0.12 V due to adsorption of Py then disappears. These CV profiles between −0.3 and 0.6 V are featureless, but the current density increases progressively with potential cycling, signaling growth of a conducting polymer, as seen for PPy and polyaniline.4,25,29,30 Electropolymerization of Py has been extensively studied in various electrolytes. The ingression of anion or egression of cation is a central issue in studies of the redox of PPy. An investigation of the redox PPy conducted in 1 M H2SO4 concluded that expulsion and insertion of protons can be more important than the processes involving anions.31,32 It is possible that protons can be important in the redox of PPy in 0.1 M H2SO4. It is also noted that, depending on the electrolyte, oxidation of Py might not yield PPy. For example, two unknown species are produced in acetonitrile containing a small amount of acid.29,33 Similar CV results are observed with an iodine-modified Au(100) electrode, except the current is lower than 5% (not shown). The iodine adlayer is not oxidized until 1.35 V in 0.1 M H2SO4.34,35 The iodine redox reaction thus contributes little to the CV profiles. The redox of PPy can be influenced by anions6 and the characteristics of PPy (chemical structure, thickness, and porosity).7 For example, the reduction potential of PPy is seen at 0.2 and ∼ −0.2 V in 0.1 M KCl,15 and 0.1 M NaNO3,7 respectively. To characterize the as-prepared PPy film, the pH of the electrolyte was raised from 1 to 4 to widen the potential window at the negative end. The resultant CV profile is shown in Figure 2b, which reveals a reversible redox couple (A2/C2) centered at −0.40 V. The separation of the peak potentials of A2 and C2 is 30 mV. The peak currents of the oxidation and reduction peaks are not identical, and they decrease slightly (
