ARTICLE pubs.acs.org/IECR
Modulating Electrochemical Activity in Polyaniline/Titanium Oxide Hybrid Nanostructured Ultrathin Films Antonio F. Frau, Thomas J. Lane, Andrea E. Schlather, Jin Young Park, and Rigoberto C. Advincula* Department of Chemistry and Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204-5003, United States
bS Supporting Information ABSTRACT: Hybrid and nanostructured ultrathin films of polyaniline (PANI) were fabricated using combined layer-by-layer (LbL) and surface solgel (SSG) processing with titanium oxide (TiOx) layers. This enabled modulation of the electrochemical and the dopingdedoping process of the electroactive conjugated polymer with respect to thickness, presence of another polyelectrolyte, and intercalation of the inorganic slabs. The structure, composition, and viscoelastic behavior were proven by UVvis absorbance, FT-IR, XPS, and QCM-D measurements. Spectroelectrochemical behavior showed that the oxidative stability of the films brought about the nanostructure control of the LbL process. On the other hand, the presence of the inorganic layer resulted in preventing electron transfer based on quinoid to benzenoid, Q f B, transitions. Thus, pairing the LbL assembly and the SSG process yielded a highly ordered, tunable structure in which the electrochemical behavior was modulated and correlated with diffusion-related arguments (Cottrell equation) of a blocking effect of the solgel layer. Further studies will be made on evaluating possible applications in thin film battery and capacitor devices.
1. INTRODUCTION Polyaniline, PANI, is one of the most well-studied conducting polymers due to its low cost, ease of preparation, and interesting electrooptical characteristics.13 The oxidation states displayed by PANI are both chemically (by pH adjustment) and electrochemically (by applied voltage) dependent.1c PANI shows dramatic enhancement of its electrical conductivity when doped (from 1010 up to 101 S cm1), and the process is reversible. This can involve acidic treatment (Scheme 1) of the emeraldine base (EB), the most useful form of PANI at room temperature. The nitrogen atoms can be fully or partially protonated to give the corresponding salts. Such doping is termed “nonredox” in the absence of electron transfer within the polymer backbone. The improved conductivity is due to the intimate rearrangement of the molecular levels and a half-filled polaron energy band.1c Extended spin and charge delocalization are achieved as a consequence of the spreading out of the polaronic wave function.1b Electrochemical doping of PANI (e.g., by cyclic voltammetry, CV) is another means to improve its electrical conductivity;4 the oxidation is reversible and two redox peaks can be seen (circa þ0.13 and þ0.70 V vs SCE) in acidic solution.4c pH-related issues are known to potentially interfere with electroactivity.4a,b A number of efforts have been made5 to tune the conductive capabilities toward neutral pH values to render PANI more suitable for bioengineering uses.5ac Synergistic incorporation of inorganic material has been reported with PANI.6 Such hybrid materials have potential applications as gas sensors.7 One viable method allowing for facile assembly of diverse components within the same structure is the layer-by-layer (LbL) electrostatic deposition technique.8 This typically involves the sequential adsorption of oppositely charged polyelectrolytes on suitable surfaces, creating heterogeneous structures. Our group has previously shown that nanostructure r 2011 American Chemical Society
control of the film architecture can be realized.9 The feasibility of applying LbL to PANI-based nanostructures provides a facile route toward device applications. For instance, the incorporation of transition metal oxides in PANI-based materials10 enables nanocomposites with enhanced properties. The mutual organicinorganic implementation gives rise to superior coatings,11 conducting nanofibers,12 and intriguing architectural variations such as nanotubes.13 The simultaneous incorporation of dissimilar materials within the same layered nanostructure is challenging, but can be accomplished by means of a compatible process such as the surface solgel (SSG) method.14 Activated surfaces enable the chemisorption of metal alkoxides which, upon successive hydrolysispolycondensation reactions, collapse in an extended oxide network. Kunitake et al. have demonstrated the general applicability of this method15 for self-supporting thin films,16 polymer/metal oxide composites,17 biocompatible surfaces,18 enantioselective binding of amino acids,19 and wrapping of macromolecules with ultrathin silicate layers.20 With the goal of modulating electrochemical activity in PANI ultrathin films, we have investigated the LbL self-assembly of hybrid, nanostructured ultrathin films that incorporate PANI and titanium oxide (TiOx) using poly(acrylic acid), PAA, as a heterojunction layer. PAA has a suitable affinity for heterogeneous interfaces, especially for metal oxides in SSG processes.21 Recent works have shown that PANI/acidic polyelectrolyte films on optically transparent electrodes are electroactive even at neutral pH.5b,22 On the other hand, the unique interplay between PANI and TiOx at the nanoscale is well documented; Received: August 28, 2010 Accepted: March 13, 2011 Revised: January 22, 2011 Published: March 23, 2011 5532
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Industrial & Engineering Chemistry Research Scheme 1. Chemical Doping of Emeraldine Base (EB PANI) with Aqueous HCl To Afford the Emeraldine Salt, ES PANI, According to MacDiarmid1c
photocatalytic enhancement is possible,23 as well as NH3/CO sensing,24 alcohol dehydration by pervaporation,25 fine-tuning of conductivity,26 and methanol-assisted redox activity.27 Our ultimate goal was a quantitative understanding of a nanostructured PAAPANITiOx system and understanding how the electroactivity of PANI is affected by intercalating dielectric oxide network layers. Such films may eventually have applications as ultrathin film electrochemical capacitors enabling energy storage and frequency dependent charging behavior.28 The film growth was monitored on solid support substrates including quartz slides (UVvis spectroscopy), Si wafers (optical ellipsometry), and Au-coated quartz crystals (quartz crystal microbalance with dissipation monitoring, QCM-D). The amount and placement of titania layers within the hybrid thin film was varied in order to determine its effect on the overall electrochemical activity. X-ray photoelectron spectroscopy (XPS) allowed for quantification of Ti and its oxide in the nanostructure. Spectroelectrochemical properties of the π-conjugated polymer were investigated in situ with a combined UVvis spectroscopy and electrochemistry setup. In principle, the use of the SSG process modulates the electrochemical and spectroelectrochemical activity of PANI by confining electron transfer within sandwiched dielectric layers, i.e., a capacitor effect. The dielectric TiOx and its deposition within the LbL architecture were achieved under mild conditions. This ensured that the process was procedurally simple and can therefore be adapted in the preparation of sandwich devices.
2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were used as received unless otherwise stated. Absolute ethanol (EtOH), dimethylacetamide (DMAC), and methylpyrrolidone (MPP) were used as received. Toluene was distilled over metallic sodium prior to use according to normal laboratory procedures. 11-Mercaptoundecanoic acid (Aldrich, 95%) was utilized for functionalizing the QCM gold surface. PAA (MW = 1 250 000 g/mol, Aldrich) was dissolved in deionized water (resistivity = 18.3 MΩ) as 1 mg/mL solution. The solgel precursor was Ti(OEt)4 (Alfa Aesar, 99þ%) as a 102 M solution in 50 mL of toluene/EtOH (1/1, v/v). One milligram per milliliter solutions of PANI (MW = ca. 65 000, Aldrich) in H2O/DMAC were prepared according to a slight modification of the procedure reported by Ram et al.8b Twenty milligrams of PANI was dissolved in 1 mL of DMAC, sonicated
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overnight, and then filtered through a 0.7 μL Whatman filter. Nineteen milliliters of acidic deionized water (pH adjusted to 3 by 1 M HCl, dropwise) was added to the DMAC solution; then the pH was further lowered to 2.52.6 by 1 M HCl (dropwise) and checked with a pH meter (Eutech pHTestr 30). Prior to LbL deposition, the solution was filtered through three passes on a 0.45 μL Whatman filter. 2.2. General Instrumentation. All the measurements were recorded at room temperature unless otherwise stated. UVvis spectra were recorded on a HP-8453 UVvis spectrometer within the 260850 nm range. Cyclic voltammetry (CV), spectroelectrochemical, and potentiostatic measurements were executed using a one-compartment, three-electrode cell driven by a Princeton Applied Research Parstat 2263 instrument and AMEL potentiostat (Model 2049). Spectroelectrochemical measurements were carried out by pairing the aforementioned UVvis spectrometer and potentiostat. Fourier transform infrared (FTIR) spectra were recorded with a FTS 7000 Digilab spectrometer within the 3750750 cm1 range in multiple attenuated total reflection (ATR) geometry at the surface of the waveguide prism (ZnSe window, transmits down to 650 cm1) equipped with a liquid N2-cooled MCT detector. QCM-D experiments were performed on a KSV QCM-Z500 system using an AT-cut, Aucoated quartz crystal (f = 5 MHz, frequency resolution 102 Hz, mass resolution 1.77 109 cm2 in liquid, D sensitivity ca. 3 108 in liquid). Null ellipsometry was performed on Si wafers using an Optrel GmbH Multiskop (incidence angle 60°) and a 632.8-nm HeNe laser as a light source. Thickness values were estimated from the obtained Δ and ψ ellipsometric angles using the Elli modeling software provided with the instrument. Five measurements were taken for each layer and averaged out. The refractive indexes utilized for ellipsometric profiling were 1.60 (solgel deposited TiOx), 1.80 (PANI solutions), and 1.53 (PAA). X-ray photoelectron spectroscopy (XPS) was carried out on a Physical Electronics 5700 instrument with photoelectrons generated by nonmonochromatic Al KR irradiation (1486.6 eV). Photoelectrons were collected at a takeoff angle of 45° using a hemispherical analyzer operated in the fixed retard ratio mode (energy resolution setting = 11.75 eV). The binding energy scale was calibrated prior to analysis using the Cu 2p3/2 and Ag 3d5/2 lines. Charge neutralization was ensured through cobombardment of the irradiated area with an electron beam and the use of the nonmonochromatic Al KR source, placing the C 1s peak at a binding energy of 284.6 (0.2) eV. Water contact angle measurements were taken at ambient conditions by a camera goniometer setup (KSV CAM 200 instrument, KSV Ltd. now Biolin) on an Au-coated glass substrate, with TiOx and PAA as topmost layers. The angles were calculated using a Young/ Laplace method on a droplet of water. 2.3. Substrate Preparation. Indium tin oxide (ITO) glasses were cut into 1 1 cm (CV) or 0.5 5 cm (spectroelectrochemistry) slides, sonicated in 2-propanol (10 min), hexane (10 min), and toluene (10 min) and then gently dried under N2. Subsequent plasma cleaning (3.5 min) produced hydroxylated surfaces. Silicon wafers (N100 type) and quartz glass were cut into 1 2 cm slides, sonicated in deionized H2O (15 min), acetone (5 min), then soaked in piranha solution (30% H2O2, concentrated H2SO4, 3/7, v/v; CAUTION: Piranha solution is a strong oxidizer!) for 50 min. After a thorough rinse in deionized H2O, they were gently dried under N2 and plasma cleaned for 3 min. The gold-coated QCM resonator was rinsed in deionized H2O, dried under N2 stream, plasma cleaned (3 min), and then soaked 5533
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Figure 1. LbL self-assembly of multilayered, hybrid [(TiOx/PAA/PANI/PAA)nTiOx] ultrathin films.
in NH3/H2O2/H2O solution (1/1/5, v/v) for 10 min. Subsequent rinsing in deionized H2O, drying under N2 stream, and plasma cleaning (3 min) yielded clean surfaces that were finally dipped in a 1 mg/mL ethanolic solution of 11-mercaptoundecanoic acid (coverage >95%) overnight. All these activations afforded suitable surfaces for the chemisorption of the materials according to the film architecture. 2.4. Multilayer Assembly. In order to evaluate the effect of the intercalated TiOx slabs within a purely organic (PAA/PANI)n nanocomposite, three different heterostructures were assembled and compared of the form (PAA/PANI/PAA)n or [(TiOx/PAA/ PANI/PAA)nTiOx] (note: here, n indicates repeated layer structures). In the latter film, each SSG process was carried out either once or twice, respectively, to increase the amount of deposited TiOx. Fabrication of [(TiOx/PAA/PANI/PAA)nTiOx] thin films was executed as follows: functionalized substrates (Si wafers, quartz, ITO) were immersed in the Ti(OEt)4 solution for 20 min, rinsed in toluene to remove the physically adsorbed alkoxide, hydrolyzed in deionized H2O (2 min), and gently dried under N2 stream. After that, the substrates were alternatively placed in PAA and PANI solutions (15 min each). Each adsorption was followed by a H2O rinse and gentle drying under N2 stream. This process, iterated n times, produced a multilayered, hybrid ultrathin film. (PAA/PANI/PAA)n thin films were similarly fabricated except for the SSG process, which was omitted. The substrates were immersed in PAA solution first to deposit the initial layer. Care was taken to ensure consistency of the rinsing/drying steps (vide infra). The chemisorption/dipping time used is typical for most surface solgel/LbL processes reported in the literature.29 From here on, we shall refer to the (PAA/PANI)n and [(TiOxPAA/ PANI/PAA)nTiOx] architectures as PPP, PPPT1 (single SSG deposition), and PPPT2 (double SSG deposition), respectively. Although we did monitor and confirm the growth of thicker PPP
(up to 14 bilayers), PPPT1, and PPPT2 (five and four “sandwiches”, respectively) films, the optimal number of PANI depositions for electrochemical and spectroelectrochemical measurements was found with three SSG layers. Therefore, the architectures we focused on for electrochemical/spectroelectrochemical measurements were (PAA/PANI/PAA)3 and [(TiOx/ PAA/PANI/PAA)3TiOx] (single and double SSG deposition). A representative scheme of the fabrication of a typical PPPT1/ PPPT2 film is depicted in Figure 1. 2.5. Multilayered Film Characterization. PPP Films. Ex situ UVvis and ellipsometry on suitable surfaces were carried out to evaluate the final magnitude of film deposition, while in situ QCM with dissipation monitoring (QCM-D) was utilized to investigate the mechanical/viscoelastic properties of PPP in terms of mass deposition per layer (measurements carried out at 20.0 ( 0.1 °C). This is accomplished by following the frequency change, Δf, of a quartz crystal when excited to resonance by an ac potential. Mass deposition was evaluated by means of the Sauerbrey equation, which assumes that the deposited film is rigid, evenly distributed, and thin.30 PAA and PANI solutions (5 mL each) were injected into the QCM-D chamber according to the LbL deposition sequences, with distilled water rinse afterward (2 mL, see section 3.2). The seventh harmonic was utilized for monitoring Δf and D, to minimize noise. Both values were retrieved by means of the impedance-based QCM Browse software (KSV). PPPT1 and PPPT2 Films. A crucial issue we tackled was to prove that the SSG process did yield TiOx deposition and not desorb existing material. Moreover, it was necessary to verify that PPPT2 films contained more titania than PPPT1 films. XPS spectroscopy was chosen as a suitable analytical tool to enable elemental analysis and determination of surface content ratio; i.e., high-resolution scans of the titanium (Ti) region were retrieved and compared. 5534
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Figure 2. UVvis spectra of PANI depositions and ellipsometric profile (inset) of overall PANI/PAA depositions within PPP thin film. Fourteen bilayers in both cases with PANI (even) and PAA (odd) adsorption steps plotted in the inset.
2.6. Electrochemistry and Spectroelectrochemistry. PPP, PPPT1, and PPPT2 thin films were evaluated for their optical (UVvis) and electrochemical (CV) features independently and then compared by the spectroelectrochemical responses (see section 3.4). LbL films for CV/spectroelectrochemical measurements were fabricated onto activated ITO slides (In2[Snx]O3y, sheet resistance e 30 Ω cm2) which served as working electrodes. CV was performed in a one-compartment, threeelectrode cell with a Pt wire as counter electrode and an aqueous Ag/AgCl electrode (3.5 M in NaCl) as reference. The potential was scanned between 0.2 and 0.9 V at rates between 50 and 150 mV s1. Spectroelectrochemical experiments were executed in a 1 cm path length quartz cuvette; the cuvette was 90% filled with 0.1 HCl as support electrolyte wherein the counter electrode, reference electrode, and electrolyte were the same as the CV ones. ITO slides were placed perpendicularly to the light path. The potential was stepped from 0.2 to 0.9 V and back (the electrode was maintained at each potential for 5 s).
3. RESULTS AND DISCUSSION 3.1. Film Fabrication/Growth. Various layered nanostructures of PANI/TiOx were optimized by electrostatic LbL selfassembly and the SSG process. This is a crucial and nontrivial step for the following reasons: first, LbL self-assembly of PANI/ polyelectrolyte multilayered films is sensitive to pH, which is crucial for applications such as biosensing/electrocatalysis.31 Moreover, P/PANI films (where P = PAA, PEI, PAH) display UV/thickness-related features that vary with pH.31c Ge et al. reported31b on the electroactivity of PANI/PAA ultrathin (
