Electrodeposited Polyaniline in a Nanoporous ... - ACS Publications

Article pubs.acs.org/JPCC

Electrodeposited Polyaniline in a Nanoporous WO3 Matrix: An Organic/Inorganic Hybrid Exhibiting Both p- and n-Type Photoelectrochemical Activity Csaba Janáky,*,†,‡ Norma R. de Tacconi,† Wilaiwan Chanmanee,† and Krishnan Rajeshwar*,† †

Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019, United States Department of Physical Chemistry and Materials Science, University of Szeged, Szeged H6720, Hungary

‡

S Supporting Information *

ABSTRACT: This study focuses on the electrodeposition of a conducting polymer such as polyaniline (PANI) into a tungsten trioxide nanoporous host framework. Nanoporous WO3 films were initially electrosynthesized on tungsten foil by anodization at different voltages in a fluoride-containing medium. The PANI layer was electrografted onto the entire surface of the WO3 using potentiodynamic electrodeposition in sulfuric acid electrolyte. The morphological features of oligomer/polymer formed in the nanoporous oxide template were monitored by field-emission scanning electron microscopy. Systematic changes in the morphology afforded insights into the evolution of the WO3/PANI hybrid assembly. This assembly was subsequently characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and photoelectrochemical measurements. Photovoltammetric data indicated the complex behavior of the hybrid, featuring the properties of both of its components: namely, p-type behavior in the cathodic polarization regime and n-type behavior at the anodic end. Moreover, much higher cathodic photocurrents were observed for PANI in the hybrid configuration (compared to neat PANI itself), in which effective charge separation manifested in the shape of the photocurrent transients. thus, its absorption edge is in the visible light region.12,13 It has been successfully utilized in photoelectrocatalytic processes,14,15 electrochromic devices,16−18 dye-sensitized solar cells,19,20 gas sensors,21,22 and electrocatalysis.23 Preparation methods involve vapor- and solution-phase procedures, including sputtering, thermal evaporation, sol−gel methods, and others. It is worth mentioning that particular efforts have been devoted to synthesize this oxide with various morphologies covering a wide range of nanostructures including nanoparticles, nanodisks, nanorods, nanowires, nanofibers, nanoplatelets, nanotrees, etc.13 A recently developed method is electrochemical etching or anodization of W foil. The process occurs at either constant voltage24,25 or constant current.26 For the preparation of inorganic/organic hybrid materials, anodization can be a

1. INTRODUCTION Hybrid materials based on organic conjugated polymers (CP) and inorganic nanostructures have been actively studied in recent years.1,2 A large variety of composites have been realized, including CP-containing assemblies based on metal nanoparticles, carbon nanostructures, and inorganic compounds. Metal oxide semiconductors are particularly interesting materials in this regard, since a wide range of properties can be combined with complementary properties of the polymer.3 For example, TiO24,5 and ZnO6 were combined with thiophene derivatives to form p/n junctions and utilized in solar cells, in organic electronics, or for photocatalysis. Materials with enhanced charge storage capacity were prepared by incorporating RuO2 and V2O5 into the matrix of CPs such as polyaniline and polypyrrole.7,8 Combination of MoO3 and Fe3O4 with CPs has led to hybrid materials with advanced sensing, magnetic, and catalytic properties.9−11 Tungsten oxide (WO3) is an n-type semiconductor, having an indirect band gap of E = 2.7 eV (for the monoclinic phase); © 2012 American Chemical Society

Received: December 5, 2011 Revised: January 11, 2012 Published: January 19, 2012 4234

dx.doi.org/10.1021/jp211698j | J. Phys. Chem. C 2012, 116, 4234−4242

The Journal of Physical Chemistry C

Article

solutions. Sodium fluoride (Alfa Aesar, 98.0%) was used as received. Tungsten foil (Alfa Aesar, 0.25 mm thick, 99.95%) was used as the substrate for oxide film growth. Before use, the foil was cut (1.4 cm × 1.4 cm), mechanically polished to mirror finish using silicon carbide sandpaper of successively finer roughness (220, 240, 400, 800, 1000, and 1500 grit), and cleaned in three 5 min steps in ultrasonicated acetone, 2propanol, and finally ultrapure water. Subsequently, the substrate was dried in ultrapure N2 stream and used immediately. For polyaniline (PANI) growth, aniline monomer (EM Science, 99.5%) was freshly distilled under vacuum before use. Sulfuric acid (Alfa Aesar, 98.0%) and sodium sulfate (J.T. Baker, 99.0%) were used as received. 2.2. Anodic Growth of Nanoporous WO3. Nanoporous films of WO3 were grown in a two-electrode electrochemical cell using a large Pt foil as counter electrode and tungsten foil as the working electrode. NaF was used as the electrolyte (0.15 M). The W foil was pressed between a set of O-rings in the electrochemical cell, leaving 0.63 cm2 exposed to the electrolyte, and the electric contact was located on the backside of the sample. Anodization employed a 420X power supply (The Electrosynthesis Co., Inc., Lancaster, NY). The voltage was held at the preselected level (40 or 60 V) for 3 h. After the oxide film was grown, the anodized W foil was removed from the O-ring assembly, carefully washed by immersion in deionized water, and then dried in a N2 stream. Before use as substrates for aniline electropolymerization, the W/WO3 electrodes were annealed at 450 °C (under air atmosphere) for 3 h at a heating rate of 20 °C/min (Fisher Scientific, Model 650-14 Isotemp Programmable Muffle Furnace) and allowed to cool gradually back to the ambient temperature. Other details of anodic growth are given elsewhere.30 2.3. Electrosynthesis of PANI/WO3 Hybrid. After a series of optimization experiments, a polymerization medium consisting of 0.2 M aniline monomer and 0.5 M aqueous sulfuric acid was chosen. Optimization of the electrochemical polymerization was carried out in multiple steps. Particular care was devoted to optimize the hybrid morphology (e.g., ensure homogeneous distribution of the polymer) and electroactivity of the deposited polymer simultaneously. As a first step, both potentio- and galvanostatic procedures were utilized, but neither homogeneity nor electroactivity of the resulted hybrid was satisfactory. Then potentiodynamic methods were employed, where the effect of the scan rate was also studied (10, 25, 50, and 100 mV s−1), and 100 mV s−1 was proved to be optimal. Finally, polymerization of aniline was carried out by a simple, but carefully optimized potential cycling protocol between −0.2 and 1.1 V. As a part of the optimization, both samples containing polyaniline grown by 3, 5, 10, and 20 cycles were prepared. The working electrode was the previously prepared nanoporous WO3 in all cases. The reference electrode was Ag/ AgCl/satd KCl (Microelectrode Inc., Bedford, NH); all potentials in this paper are given with respect to this reference. For further voltammetric studies, the solution was changed after polymerization to a monomer-free 0.5 M H 2 SO 4 electrolyte. 2.4. Characterization Methodology. The morphology of WO3 and the WO3/PANI hybrid samples was studied using a Hitachi S-5000H field emission scanning electron microscope (SEM) at an accelarating voltage of 20 kV. Images were taken at different magnifications between 10K and 200K. Raman

particularly interesting process since the resulted oxide has an ordered nanoscale structure, which greatly increases the specific surface and semiconductor behavior. This opens a route to prepare hybrid materials with a large contact interface and effective p/n junction. Nanoporous WO3 prepared via anodization proved to be more efficient compared to materials counterparts synthesized by other methods in terms of photoelectrochemical behavior,27 sensing properties,28 water splitting ability,29 electrocatalyic property,30 or electrochromism.31 Combination of WO3 with CPs to form inorganic/organic hybrid materials can be advantageous for numerous reasons. Such composite materials possessing interesting electrical properties32,33 can be utilized in electrocatalysis,34 charge storage,35 electrochromism,36−38 or for sensing: humidity39 and gases as hydrogen40 or NO2.41 These hybrids have been synthesized by different synthetic procedures ranging from simple mechanical mixing of the components, polymerization in the presence of nanoparticulate WO3, to simultaneous codeposition of the polymer and the oxide. However, these methods have significant drawbacks and limitations, two most important being the uncontrolled, random distribution of the (nano)particles within the polymer and inadequate electric contact. Thus, use of the hybrid as an electrode (which is the case in most of the applications that have been considered) is hampered by the fact that only the polymer matrix has direct electrical connection to the electrode. In contrast, by infiltrating a conducting polymer into an organized nanoporous WO3 framework, hybrid materials with large area of organic/ inorganic junction with well-defined morphologies can be obtained where both components have electrical contact with the supporting electrode. Despite the obvious advantages in using an organized nanostructured WO3, according to the best of our knowledge, there is no precedence in the literature for combination of WO3 prepared through anodization with any conjugated polymer. Our aim was to study the feasibility of combining nanoporous tungsten oxide with a semiconducting polymer, namely polyaniline (PANI). This candidate has enjoyed prime importance among CPs, due to its versatile redox behavior (involving many different intrinsic oxidation states), excellent chemical and electrochemical stability, electrochromic properties, and large capacitance.42−44 Combination of inorganic semiconductors (CdS, ZnO) with polyaniline was recently shown to be useful for photoelectrocatalysis.45,46 The selection of polyaniline was also motivated by the fact that its ion exchange properties show similarity to WO3 in many aspects; most importantly, both of them are electron and proton conductors as well. Moreover, our study goes one step beyond the state of the art of electrodeposition as well, since electrochemical grafting of conducting polymers into nanostructured semiconductors was heretofore limited to TiO247−50 and ZnO.51 Thus, the aim of this study is to prove the feasibility of electrodeposition of PANI onto the nanoporous WO3 template, to tune the morphology of the hybrid, and to investigate the photoelectrochemical properties of the resultant organic/inorganic junction.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. All chemicals were from commercial sources and were of the highest purity available. Deionized water (18 MΩ cm) was used in all cases for making 4235



Article

spectra were recorded with a HORIBA Jobin Yvon LabRam ARAMIS instrument (incident power ≤300 mW) using an excitation wavelength of 473 nm and an 1800 line/mm grating. In all the cases the slit width was 10 μm, and 10 scans were accumulated for each spectrum. X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis Ultra X-ray photoelectron spectrometer at room temperature using an Al source. The gun was operated at 15 kV and 7 mA. Data were acquired with 0.1 eV step size and 0.1 s dwell time. The pass energy for high-resolution spectra (e.g., N 1s, W 4f) was 10 eV. Binding energy data were calibrated using the standard value for C 1s at 285 eV. All electrochemical measurements were performed on a CHI electrochemical workstation 440A instrument, in a classical one-compartment, three-electrode electrochemical cell. Cyclic voltammograms of the WO3/PANI hybrid were registered in 0.5 M H2SO4 solutions at four different potential sweep rates between 10 and 100 mV/s. For the photoelectrochemical measurements a standard single-compartment, three-electrode electrochemical cell was used. A large Pt coil and the reference electrode mentioned earlier, along with the working electrode, completed the cell setup. The light source was a 150 W xenon arc lamp (Oriel, Stratford, CT). The radiation source was placed 8 cm away from the working electrode surface. Photovoltammetry profiles were recorded with the same potentiostat, in 0.1 M Na2SO4 electrolyte. Photovoltammograms were obtained using a slow potential sweep (2 mV s−1) in conjunction with interrupted irradiation (0.2 Hz) on the semiconductor and hybrid film electrodes. For the chronoamperometric measurements, the working electrode was set at preselected potential values (in the range −0.1 to +0.8 V), and the photocurrent transients under illumination were monitored for 20 s. All procedures described below were performed at the laboratory ambient temperature (25 ± 2 °C).

Figure 1. Polymerization of aniline over 10 potentiodynamic cycles at a sweep rate of 100 mV s−1 in a solution containing 0.2 M aniline and 0.5 M aqueous H2SO4 as electrolyte.

morphological aspects of electrochemical infiltration of the polymer, SEM images were taken at various magnifications, both with the bare WO3 nanostructures (prepared at 40 or 60 V) and the hybrid material. In Figure 2A−C and Figure 3A−C, SEM images of the bare oxide materials are presented. As can be clearly seen, the overall morphology of the anodized WO3 samples is similar in the two cases. However, some differences in the pore size and porosity of the two oxide materials can be revealed, reflecting more complete overetching of the tungsten surface by the fluoride species at the higher voltage.30 More precisely, the oxide film obtained by anodization at 40 V gave a nanohole array morphology for the oxide surface. On the other hand, the oxide layer obtained from an anodization voltage of 60 V gave a very porous structure with high surface area. Figure 2D,E and Figure 3D,E show representative SEM images for the hybrid materials, obtained for the deposition of PANI with five voltammetric cycles onto the tungsten oxide matrix obtained at 40 and 60 V, respectively. During electrochemical polymerization of aniline, the monomers are initially oxidized, and correspondingly, oligomers are formed in the solution phase. After this fast initial step, the formed oligomers are deposited onto the surface of the tungsten oxide nanostructure. When the electrosynthesis proceeds further, chain propagation of the deposited oligomers takes place in the solid phase. This process evolution manifests in the cloudiness seen for the WO3 surface on the whole sample. Expansion of the grain size is also seen relative to the parent situation in Figures 2 and 3A−C. It is also important to mention that at this stage the nanostructured support is covered by PANI, and the space between the grains (the pore size) is seen to be much smaller than the neat tungsten oxide case, although most of the grains are not completely fused to one another. The ∼30 nm difference in the diameter between the neat WO3 grains and the coated samples suggests that oligomer/polymer electrodeposition results in a ∼15 nm thick PANI layer on the surface. Regardless of the different morphology of the WO3 substrates anodized at different voltages, the polymer is seen to deposit in a very similar fashion on both samples, resulting in a comparable coating, which covers the entire surface of the nanoporous oxide semiconductor (Scheme 1). Further advancement of the polymerization process was also investigated. In Figure 4, we present the evolution of the structure of the hybrid material as a series of SEM images, at a

3. RESULTS AND DISCUSSION 3.1. Potentiodynamic Growth of PANI into the Nanoporous WO3 Matrix. Potentiodynamic deposition is one of the most frequently used methods for the electrochemical synthesis of polyaniline.44 Since diffusion of the monomer during electrodeposition is usually the limiting step for nanostructured working electrodes,52 the use of a potentiodynamic growth waveform is indeed effective. Figure 1 contains a series of cyclic voltammograms (10) between −0.20 and +1.10 V, recorded during the electrodeposition of PANI onto the nanostructured WO3 electrode. During the first cycle of polymerization, the anodic current starts to increase only above 1.00 V, and then it rises continuously, exhibiting a hysteresis loop on the return scan. This behavior can be explained by the fact that at the beginning of the sweep only monomer oxidation and oligomer formation take place, while after this initial step polymerization also occurs. As the oxidation potential of the oligomers is lower as compared to that of the monomer (and it decreases by the increasing segment number), the currents for monomer and oligomer oxidation cross each other. The initial oxidation potential on the W/WO3 film is ∼0.2 V more positive than at a Au electrode, presumably stemming from a larger surface resistance on the WO3 working electrode. Otherwise, the shapes of the curves are identical to those obtained with gold (not presented here). 3.2.1. Physical Characterization. PANI/WO3 Hybrid Morphology As Probed by SEM. To gain insights into the 4236



Article

Figure 2. SEM images of the bare WO3 synthesized at E = 40 V (A−C) and of the corresponding WO3/PANI hybrid samples (D−F, synthesized using five cycles) at 50K, 100K, and 150K magnification.

Scheme 1. Illustration of the Two-Step Synthetic Procedure of the WO3/PANI Hybrid

Since the samples prepared by five potentiodynamic cycles (prepared on WO3 anodized at 60 V) show the largest electrolyte-exposed area (considered as a prerequisite for many applications), all further characterization is presented for these hybrid samples. 3.2.2. Raman Spectroscopy. To confirm the chemical identity of the polymer deposited into the WO3 framework, Raman spectra were taken. This method is rather useful for this purpose since both WO3 and PANI have characteristic bands, although polyaniline exhibits much higher intensities. Figure 5 contains Raman spectra of a WO 3 sample and the corresponding WO3/PANI hybrid. As for the bare oxide material, the existence of typical monoclinic vibrational modes14 at 272, 325, 715, and 807 cm−1 confirms the formation of WO3 during the anodization and the subsequent heat treatment. For the hybrid sample in Figure 5, several new bands can be observed compared to the support oxide framework. All the characteristic bands can be assigned to PANI.43,44 The appearance of specific bands (see assignments on the spectrum) all confirms the formation of the targeted conducting polymer. Moreover, despite some overlap with the

selected magnification (25000×). The compared samples contain different amounts of polymer deposited; more specifically, the polymerization was allowed to proceed for 0, 5, 10, and 20 cycles. In Figure 4A, B similar images can be seen as in Figure 3. As the polymeric layers on the grains get progressively thicker, the layers grown on different parts of the substrate abut, and the pores of the WO3 template becomes almost completely filled. In this manner, a “skinlike” conjugated polymer network is formed on the oxide template. When all the WO3 surface is covered and almost all of the polymer covered grains are attached to each other, the further growth of polyaniline does not result in a compact layer on the top of the substrate as in the case of polymer deposition on TiO2 nanotube arrays.47−49 On the other hand, growth occurs in specific directions, and nanofiber formation can be detected starting from the grains to all directions outward (Figure 4C). Finally, if the polymerization is allowed to progress even further, the density of these nanofibers increases, and the nanoporous substrate gets fully covered with a network of PANI fibers, as can be clearly seen in Figure 4D. 4237



Article

Figure 3. SEM images of the bare WO3 synthesized at E = 60 V (A−C) and of the corresponding WO3/PANI hybrid samples (D−F, synthesized by five potentiodynamic cycles) at 50K, 100K, and 150K magnification.

Figure 4. Electron microscopic visualization of the morphology development of the WO3/PANI hybrid (0, 5, 10, and 20 potentiodynamic cycles) at 25000× magnification. The insets present representative enlarged areas.

N signal, presented in Figure 6B. The XPS results presented above also suggest that the WO3 matrix is homogenously covered by the polymeric film, and uncovered areas are decreased by the longer polymerization time. These observations are coherent with the SEM images presented in Figure 3, where the polymer coverage appears over the entire sample. 3.3.1. Electrochemical and Photoelectrochemical Measurements. Voltammetric Behavior. The cyclic voltammograms in Figure 1 above suggest that the deposited polymer exhibits good electroactivity within the nanoporous WO3 framework. In order to acquire further information on the electrochemical activity of the hybrid material, cyclic voltammograms were recorded in aqueous H2SO4. Cycling in this electrolyte is indeed advantageous, since both WO3 and PANI are known as H+ exchangers, thus showing a quasi-reversible electroactivity in this media. The data in Figure 7 bear out the

PANI bands, the vibrations from the WO3 matrix can be also identified in the spectrum of the hybrid material. 3.2.3. X-ray Photoelectron Spectroscopy. In Figure 6, XPS data for three samples are presented: the anodically grown WO3 matrix and two hybrid samples (5 and 10 cycles of electropolymerization). In Figure 6A, the W 4f signal is shown, and the well-resolved doublet peaks corresponding to W 4f5/2 and W 4f7/2 can be seen. The binding energies of these peaks are 37.8 and 35.8 eV, respectively, indicating that the sample surface is close to the chemical stoichiometry of WO3.22 Moreover, the peak locations are unchanged diagnosing that the WO3 stoichiometry at the surface is not affected by oxidative deposition of the polymer. One can also see the stepby-step diminution of these peak amplitudes by covering the substrate with increasing amounts of PANI. A similar conclusion can be drawn for the gradual development of the 4238



Article

Figure 7. Cyclic voltammograms of the WO3 substrate and the hybrid layer in 0.5 M sulfuric acid, at a sweep rate of 100 mV s−1.

Figure 5. Raman spectra of the nanoporous WO3 and the WO3/PANI hybrid prepared by five CVs.

indicates the limiting role of mass transport, namely, the diffusion of H+ ions within the WO3 matrix.13 3.3.2. Photoelectrochemical Characterization. Figure 9 compares linear sweep photovoltammetry data for the WO3/ PANI hybrid material, with its components namely the bare anodic oxide, and a neat PANI film deposited on Au electrode (with identical charge density). Note that the photovoltammograms are scaled differently to highlight the features of each individual curve. This voltammetry technique is described elsewhere56 but briefly consists of a slow scan of the potential while the film irradiation is periodically interrupted. In this manner, both the “dark” and the light-induced photoresponse of the samples can be assessed in a single experiment. As for the WO3 matrix, the photocurrents are anodic in polarity consistent with the n-type semiconductor behavior of this semiconductor. The photocurrents arise mainly from the photooxidation of adsorbed hydroxyl groups, water molecules, or even the electrolyte sulfate ions. On the other hand, in case of the neat PANI film, at the beginning of the scan, a cathodic photocurrent can be detected at negative potentials, related to the p-type behavior of the polymer. This photocurrent flow is sustained by the reduction of dissolved O 2 , as was

fact that both the WO3 support and PANI exhibit well-defined and reversible electroactivity in the hybrid configuration. Moreover the voltammogram of the hybrid translates to the exact sum of the electroactivity of its components. It is important to point out that while the inorganic component possesses its redox activity at the cathodic (negative potential) end, the organic part’s redox transformation takes place at the anodic (positive potential) end. This separation of the electroactivity within the applied potential window (without losing any electroactivity due to the presence of the other component) is advantageous for the utilization of such hybrids, for example, in electrochromic applications. In Figure 8A, cyclic voltammograms recorded at four different sweep rates are presented. As for PANI, the peak currents are directly proportional to the potential scan rate for both the anodic and cathodic current flow (Figure 8B), attesting to the lack of diffusion limitations at least for potential sweep rates up to 0.1 V/s.55 Oppositely, for WO3, the characteristic cathodic currents (at E = −0.45 V) are directly proportional to square root of the scan rate (Figure 8C). This

Figure 6. Detailed XPS scans of (A) W 4f and (B) N 1s lines for the bare WO3 and two WO3/PANI hybrids (prepared by 5 and 10 CVs). 4239



Article

Figure 8. (A) Cyclic voltammograms for the WO3/PANI hybrid in a solution of 0.5 M H2SO4 at different scan rates. (B, C) Analysis of main anodic and respective cathodic current peak for PANI and a cathodic peak (for WO3) current vs scan rate data from (A).

and this trend is further elaborated in the transient photocurrent data in Figure 10. The cathodic photocurrents related

Figure 9. Representative photovoltammograms of WO3, PANI, and the WO3/PANI hybrid, recorded between −0.5 and 0.8 V, in 0.1 M Na2SO4, at a sweep rate of 2 mV s−1 using the full output of a 150 W Xe arc lamp.

Figure 10. Comparison of photocurrent transients at three bias potentials selected from the photovoltammograms presented in Figure 9. Each transient lasted for 5 s.

demonstrated for other conducting polymers, such as PEDOT.57 At positive potentials, superimposed on the dark current related to oxidation of the polymer, some anodic (ntype) photocurrent can also be observed. This is not surprising because although semiconducting polymers (such as PANI) are generally known to be p-type, both p-type and n-type behaviors have been reported depending on its oxidation state.58−62 The moderate n-type behavior of the oxidized PANI (emeraldine form) was discussed and described by different authors.58−62 The photovoltammogram of the hybrid material shows some features of both of its components (Figure 9). While the appearance of the cathodic photocurrents below E = 0.0 V proves the p-type photoactivity of PANI in the hybrid configuration, the n-type behavior with anodic photocurrents at higher potentials is related to the inorganic part. However, the absolute value of these photocurrents is rather interesting,

to polyaniline are significantly (more than 3 times) larger in the WO3/PANI hybrid, compared to a pure PANI film deposited on the Au electrode. This effect can be interpreted by the nanostructured character of the WO3 electrode, which is covered by a homogeneous, thin (15 nm) PANI film (cf. Figure 3). Thus, collection of the photogenerated holes is not limited by transport of the charge carriers in the organic polymer as is the case with a bulk layer. Moreover, WO3 is conducting and negatively polarized in this region and is thus able to receive the photogenerated holes, promoting enhancement of the photocurrent. In addition, beyond the obvious difference in the value of the photocurrents, careful examination of the shape of the photocurrent transients reveals another significant change. Namely, for the cathodic photocurrents, the hybrid sample exhibits a plateauor a moderately linear decrease, depending 4240



Article

on the potential rangeinstead of a spiked transient profile as is the case for neat PANI (Figure 10). This is diagnostic of the carriers being effectively separated before they undergo recombination. The plateau-like shape supports the advantage of both the nanostructured semiconductor support and the homogeneous distribution of the deposited polymer. This significant increase of the cathodic photocurrents may be exploited in the future in photoelectrocatalytic applications such as O2 and CO2 reduction. As for the anodic photocurrent magnitude, interestingly a decrease is observed compared to the bare WO3 case (Figure 10). More careful examination reveals that this decrease is not uniform over the whole potential window. That is, while up to 0.4 V, the photocurrent is almost completely diminished, at a 0.7 V potential bias, only a partial decrease is seen (Figure 10). Note also the coincidence between development of the anodic photocurrent (starting from 0.4 V) and the cessation of dark current flow related to oxidation of the polymer (Figure 9). This indicates that the “quenching” effect of the polymer is strongly dependent on its oxidation state. Moreover, the amount of the deposited polymer plays a key role as well, since the effect is more and more expressed with the gradual increase in the amount of PANI (see Supporting Information Figure S1). The important point to note is that electrochemical infiltration of PANI into a nanoporous WO3 framework results in a hybrid material exhibiting both p-type and n-type behavior, with relatively large photocurrent densities. To gain deeper understanding into the unexpected anodic behavior, stationary (chronoamperometric) photocurrent measurements were performed with both the bare oxide and the hybrid material at various potentials (see an example in the inset of Figure 11). In Figure 11, the relative decrease in the

be qualitatively explained by the fact that in its reduced (leucoemeraldine) form PANI is an insulator (with photoconductivity) while in the oxidized form (emeraldine) it is conducting and behaves as an n-type semiconductor. Apparently this dual character results in the interesting behavior presented above: namely, in its reduced form, PANI drains all the photoelectrons from the conduction band of WO3 (which thus cannot access the back-contact, and consequently no photocurrent can be detected) while in the oxidized form this effect is not expressed anymore, and the holes can reach the solution (since PANI is conducting). Finally, although the polymeric film deposited on the WO3 nanostructured matrix is very thin, the decrease in the anodic photocurrent (related to WO3) can be also related to the optical shielding effect from the polymer. It is worth noting that this notion would be in perfect agreement with the data trends seen in the Supporting Information Figure S1). This possibility would be in accordance with the observations presented in Figure 11, since the optical spectrum of the fully reduced polyaniline (leucoemeraldine)41 almost completely overlaps with the spectrum of WO3.13 On the other hand, after oxidation to emeraldine and coupled with the consequent spectral changes, the overlap is not that significant anymore, and thus the shielding effect is not expressed.

4. CONCLUDING REMARKS In this study, the feasibility of electrochemical infiltration of a conjugated polymer, namely polyaniline (PANI), into a nanoporous WO3 host matrix prepared by anodization was demonstrated. Development of the hybrid structure was monitored by SEM, and evidence for a homogeneous distribution of the polymer was presented. Spectroscopic probes such as Raman spectroscopy and XPS not only confirmed the chemical identity of the components of the hybrid material, but the nanoscale homogeneity was also reflected in these data. The hybrid assembly exhibited a quasireversible voltammetric behavior, which was the exact sum of its components. Photoelectrochemical data revealed a complex behavior of the WO3/PANI composite. Specifically the hybrid exhibited the properties of both individual components: cathodic (p-type) photocurrents at negative potentials and anodic (n-type) photoactivity at positive potentials. Importantly, the cathodic photocurrents related to PANI were ca. 3− 4 times larger in the hybrid configuration compared to values obtained for the neat polymeric film on a Au electrode. This enhanced photocurrent may be utilized in applications; for example, the photoelectrocatalytic reduction of O2 and CO2 and such studies are in progress.

■

Figure 11. Relative decrease in the stationary anodic photocurrent at different potentials in the range 0−0.8 V, as determined by chronoamperometric measurements. The inset shows representative chronoamperometric data at a selected potential (E = 0.4 V).

ASSOCIATED CONTENT

S Supporting Information *

Effect of the amount of the deposited polymer on the photovoltammetry data. This material is available free of charge via the Internet at http://pubs.acs.org.

■

photocurrents ((Iphoto,WO3 − Iphoto,hybrid)/Iphoto,WO3), measured under chronoamperometric circumstance, is shown as a function of the bias potential. Clearly, at low potentials, there is an almost complete quenching of the photocurrent, while during the oxidation of the polymer (between E = 0.2 and 0.5 V) this effect decreases and then remains constant at potentials where PANI is oxidized to its emeraldine form. This trend can

AUTHOR INFORMATION

Corresponding Author

*Tel: 817 272 5421; e-mail: [email protected] (C.J.), [email protected] (K.R.). Notes

The authors declare no competing financial interest. 4241



■

ACKNOWLEDGMENTS

■

REFERENCES

Article

(33) Lee, K.; Zhang, L.; Lui, H.; Hui, R.; Shi, Z.; Zhang, J. Electrochim. Acta 2009, 54, 4704−4711. (34) Zou, B. X.; Liu, X. X.; Diamond, D.; Lau, K. T. Electrochim. Acta 2010, 55, 3915−3920. (35) Zou, B.-X.; Liang, Y.; Liu, X.-X.; Diamond, D.; Lau, K.-T. J. Power Sources 2011, 196, 4842−4848. (36) Zhang, L.; Xiong, S.; Ma, J.; Lu, X. Sol. Energy Mater. Sol. Cells 2009, 93, 625−629. (37) Kalagi, S. S.; Mali, S. S.; Dalavi, D. S.; Inamdar, I.; Im, H.; Patil, P. S. Synth. Met. 2011, 161, 1105−1112. (38) Zhang, J.; Tu, J. P.; Zhang, D.; Qiao, Y. Q.; Xia, X. H.; Wang, X. L.; Gu, C. D. J. Mater. Chem. 2011, 21, 17316. (39) Parvatikar, N.; Jain, S.; Khasim, S.; Revansiddappa, M.; Bhoraskar, S. V.; Prasad, M. V. N. A. Sens. Actuators, B 2006, 114, 599−603. (40) Sadek, A.; Wlodarski, W.; Shin, K.; Kaner, R.; Kalantarzadeh, K. Synth. Met. 2008, 158, 29−32. (41) Tiwari, A.; Gong, S. Electroanalysis 2008, 20, 1775−1781. (42) Kang, E.; Neoh, K.; Tan, K. Prog. Polym. Sci. 1998, 23, 277−324. (43) Huang, W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2385. (44) Inzelt, G.; Pineri, M.; Schultze, J.; Vorotyntsev, M. Electrochim. Acta 2000, 45, 2403−2421. (45) Zhang, H.; Zong, R.; Zhu, Y. J. Phys. Chem. C 2009, 113, 4605− 4611. (46) Zhang, H.; Zhu, Y. J. Phys. Chem. C 2010, 5822−5826. (47) Wang, D.; Liu, Y.; Wang, C.; Zhou, F.; Liu, W. ACS Nano 2009, 3, 1249−57. (48) Kowalski, D.; Schmuki, P. Chem. Commun. 2010, 46, 8585−7. (49) Janáky, C.; Bencsik, G.; Rácz, A.; Visy, C.; de Tacconi, N. R.; Chanmanee, W.; Rajeshwar, K. Langmuir 2010, 26, 13697−702. (50) Kowalski, D.; Tighineanu, A.; Schmuki, P. J. Mater. Chem. 2011, 17909−17915. (51) Döbbelin, M.; Tena-Zaera, R.; Carrasco, P. M.; Sarasua, J.-R.; Cabañero, G.; Mecerreyes, D. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 4648−4653. (52) Cho, S. I.; Lee, S. B. Acc. Chem. Res. 2008, 699−707. (53) Quillard, S.; Louran, G.; Lefrant, S.; MacDiarmid, A. G. Phys. Rev. B 1994, 50, 496−508. (54) Lapkowski, M.; Berrada, K.; Quillard, S.; Louarn, G.; Lefrant, S.; Pron, A. Macromolecules 1995, 28, 1233−1238. (55) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; JohnWiley & Sons: New York, 2001. (56) Rajeshwar, K. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001. (57) Bencsik, G.; Lukács, Z.; Visy, C. Analyst 2010, 135, 375−80. (58) Miquelino, F. L.; C. Depaoli, M. A.; Genies, E. M. Synth. Met. 1994, 68, 91−96. (59) Kilmartin, P.; Wright, G. Electrochim. Acta 1996, 41, 1677− 1687. (60) Kilmartin, P. A.; Wright, G. A. Electrochim. Acta 1998, 43, 3091−3103. (61) Maia, D. J.; Neves, S.; das; Alves, O. L.; Paoli, M.-De Electrochim. Acta 1999, 44, 1945−1952. (62) Dasneves, S.; Dafonseca, C.; Depaoli, M. Synth. Met. 1997, 89, 167−169.

C. Janáky gratefully acknowledges the support of the European Union under FP7-PEOPLE-2010-IOF, Grant 274046.

(1) Gomez-Romero, P. Adv. Mater. 2001, 13, 163−174. (2) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608−622. (3) Rajeshwar, K.; de Tacconi, N. R.; Chenthamarakshan, C. R. Chem. Mater. 2001, 12, 2765. (4) Coakley, K. M.; Liu, Y.; McGehee, M. D.; Frindell, K. L.; Stucky, G. D. Adv. Funct. Mater. 2003, 13, 301−306. (5) Zhang, W.; Cheng, Y.; Yin, X.; Liu, B. Macromol. Chem. Phys. 2011, 212, 15−23. (6) Beek, W. J. E.; Wienk, M. M.; Janssen, R. J. Adv. Funct. Mater. 2006, 16, 1112−1116. (7) Zang, J.; Bao, S. J.; Li, C. M.; Bian, H.; Cui, X.; Bao, Q.; Sun, C. Q.; Guo, J.; Lian, K. J. Phys. Chem. C 2008, 112, 14843−14847. (8) Huguenin, F.; Torresi, R. M. J. Phys. Chem. C 2008, 112, 2202− 2209. (9) Kerr, T.; Wu, H.; Nazar, L. Chem. Mater. 1996, 8, 2005−2015. (10) Janáky, C.; Endrő di, B.; Berkesi, O.; Visy, C. J. Phys. Chem. C 2010, 114, 19338−19344. (11) Janáky, C.; Visy, C.; Berkesi, O.; Tombácz, E. J. Phys. Chem. C 2009, 113, 1352−1358. (12) Deb, S. K. Sol. Energy Mater. Sol. Cells 2008, 92, 245−258. (13) Zheng, H.; Ou, J. Z.; Strano, M. S.; Kaner, R. B.; Mitchell, A.; Kalantarzadeh, K. Adv. Funct. Mater. 2011, 21, 2175−2196. (14) Monllor-Satoca, D.; Borja, L.; Rodes, A.; Gómez, R.; Salvador, P. ChemPhysChem 2006, 7, 2540−51. (15) Baeck, S.-H.; Choi, K.-S.; Jaramillo, T. F.; Stucky, G. D.; McFarland, E. W. Adv. Mater. 2003, 15, 1269−1273. (16) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2000, 60, 201−262. (17) Zhang, J.; Wang, X. L.; Xia, X. H.; Gu, C. D.; Zhao, Z. J.; Tu, J. P. Electrochim. Acta 2010, 55, 6953−6958. (18) Cheng, W.; Baudrin, E.; Dunn, B.; Zink, J. I. J. Mater. Chem. 2000, 11, 92−97. (19) Sadek, A. Z.; Zheng, H.; Breedon, M.; Bansal, V.; Bhargava, S. K.; Latham, K.; Zhu, J.; Yu, L.; Hu, Z.; Spizzirri, P. G.; Wlodarski, W.; Kalantarzadeh, K. Langmuir 2009, 25, 9545−51. (20) Zheng, H.; Tachibana, Y.; Kalantar-Zadeh, K. Langmuir 2010, 26, 19148−52. (21) Akiyama, M.; Tamaki, J.; Miura, N.; Yamazoe, N. Chem. Lett. 1991, 20, 1611−1614. (22) Sun, H.; Cantalini, C.; Lozzi, L.; Passacantando, M.; Santucci, S.; Pelino, M. Thin Solid Films 1996, 287, 258−265. (23) Endo, N.; Miho, Y.; Ogura, K. J. Mol. Catal. 1997, 1169, xxxx. (24) Tsuchiya, H.; Macak, J. M.; Sieber, I.; Taveira, L.; Ghicov, A.; Sirotna, K.; Schmuki, P. Electrochem. Commun. 2005, 7, 295−298. (25) de Tacconi, N. R.; Chenthamarakshan, C. R.; Yogeeswaran, G.; Watcharenwong, A.; de Zoysa, R. S.; Basit, N.; Rajeshwar, K. J. Phys. Chem. B 2006, 110, 25347−55. (26) Mukherjee, N.; Paulose, M.; Varghese, O.; Mor, G. K.; Grimes, C. A. J. Mater. Res. 2003, 18, 2296−2299. (27) Berger, S.; Tsuchiya, H.; Ghicov, A.; Schmuki, P. Appl. Phys. Lett. 2006, 88, 203119. (28) Kukkola, J.; Mäklin, J.; Halonen, N.; Kyllönen, T.; Tóth, G.; Szabó, M.; Shchukarev, A.; Mikkola, J.-P.; Jantunen, H.; Kordás, K. Sens. Actuators, B 2011, 153, 293−300. (29) Cristino, V.; Caramori, S.; Argazzi, R.; Meda, L.; Marra, G. L.; Bignozzi, C. A. Langmuir 2011, 27, 7276−84. (30) Watcharenwong, A.; Chanmanee, W.; de Tacconi, N. R.; Chenthamarakshan, C. R.; Kajitvichyanukul, P.; Rajeshwar, K. J. Electroanal. Chem. 2008, 612, 112−120. (31) Nah, Y.-C.; Ghicov, A.; Kim, D.; Schmuki, P. Electrochem. Commun. 2008, 10, 1777−1780. (32) Zhu, J.; Wei, S.; Zhang, L.; Mao, Y.; Ryu, J.; Karki, A. B.; Young, D. P.; Guo, Z. J. Mater. Chem. 2011, 21, 342. 4242


Electrodeposited Polyaniline in a Nanoporous ... - ACS Publications

Recommend Documents