Electrodeposition of Polypyrrole in TiO2 Nanotube Arrays by Pulsed

Article pubs.acs.org/JPCC

Electrodeposition of Polypyrrole in TiO2 Nanotube Arrays by PulsedLight and Pulsed-Potential Methods E. Ngaboyamahina,†,‡ C. Debiemme-Chouvy,†,‡ A. Pailleret,†,‡ and E. M. M. Sutter*,†,‡ †

Sorbonne Universités, UPMC Université Paris VI, UMR 8235, Laboratoire Interfaces et Systèmes Electrochimiques (LISE), 4, place Jussieu, case courrier 133, F-75005, Paris, France ‡ CNRS, UMR 8235, LISE, F-75005, Paris, France S Supporting Information *

ABSTRACT: Using highly ordered TiO2 nanotube arrays as the substrate for electropolymerization provides a high surface area for polymer deposit and vertical pathways for electron transport. The challenge is to deposit the polymer on the inner and outer walls of the tubes and to avoid the sealing of the tubes at the mouth. Ideally, this situation could be reached by filling the tubes from the bottom. We demonstrate here that by using pulsed methods (light pulses, potential pulses), the deposition rate can be controlled, leading to a better monomer supply within the nanotubular matrix. The parameters to apply for the deposition are greatly dependent on the nature of the anion in the electrolyte, which determines the rate and the location of the polymer growth at the electrolytic solution/semiconductor interface.

1. INTRODUCTION Highly ordered titanium oxide nanotube arrays (TiO2 NTAs) have been shown to be promising materials for applications in many areas, such as energy conversion, sensors, heterojunction solar cells, and photocatalysis.1−11 TiO2 NTAs can easily be fabricated by anodizing a titanium foil.12−20 The main advantages of such structures are to provide a high surface area and also a vertical pathway for electron transport, which should reduce the rate of recombination between photogenerated charges. Unfortunately the walls of the synthesized nanotubes often show insulating properties, and further treatments are then needed for enhancing the conductivity of the tube walls. One of them is to deposit an electronically conducting polymer (ECP), since it has been demonstrated that the charge transfer in heterostructures, such as nanostructured TiO2/ECP, is enhanced in comparison with the same structures considered separately.3 Among the conducting polymers, polypyrrole (PPy) has attracted considerable attention because of its good stability and simple electrodeposition from aqueous media.21 During electropolymerization, different parameters, such as the applied potential, the substrate geometry, or the choice of the electrolyte, influence the ECP film properties, and in particular their ion exchange behavior has to be selected carefully. Within the vast list of supporting electrolytes used during the electrodeposition of polypyrrole, and electronically conducting polymers generally speaking, anionic surfactants have been the target of very special attention. According to several reports, aqueous micellar solutions are better solubilizing media than nonmicellar aqueous electrolytic solutions toward pyrrole and thiophene derivatives. As such, they allow a substantial increase © 2014 American Chemical Society

of the current density resulting from the electrochemical oxidation of monomers possibly released by the micelle opening in the vicinity of the polarized electrode surface. Anionic surfactants also have the ability to lower the oxidation potential of various electropolymerisable monomers as a consequence of their specific interactions with the electrooxydation product of these monomers, i.e., their radical cation. Other reports point out that ECPs can take a columnar and porous morphology when they are electrogenerated in the presence of anionic surfactants,22,23 possibly as a consequence of the growth of the polymer chains perpendicularly to the electrode surface. One should also notice that anionic surfactants may form micellar structures (hemimicelles, bilayer micelles) adsorbed on the working electrode surface whose geometry and dimensions are influenced by the applied anodic potential and surfactant concentration. Such micelles are expected to influence either the initial growth stages of ECP electrodeposition and/or their adhesion on the working electrode surface. Let us also emphasize that anionic surfactants were also found to affect the polymerization kinetics of electropolymerizable monomers as well as the viscoelastic properties of the resulting ECPs.24 For PPy deposition on a n-type semiconductor such as TiO2 NTA, two difficulties have to be overcome.25−30 (1) Electropolymerization of pyrrole has to be performed in a potential range in which titanium oxide is depleted from its majority charge carriers. Consequently a light-assisted proceReceived: July 25, 2014 Revised: October 13, 2014 Published: October 22, 2014 26341

dx.doi.org/10.1021/jp507491x | J. Phys. Chem. C 2014, 118, 26341−26350

The Journal of Physical Chemistry C

Article

525 °C for 2 h. The anatase structure thus obtained was confirmed by XRD spectroscopy and Raman spectroscopy. A geometric model formerly used28,31,32 enabled a rough estimation of the developed surface to be made. Taking into account the tube parameters estimated by FEG-SEM (length, diameter, wall thickness, and intertube space) and a geometric surface of 0.78 cm2, the model leads to a developed surface of around 50 cm2. Electrodeposition of polypyrrole was performed at 0.8 V/ SCE in aqueous solutions of 0.1 M pyrrole and 0.1 M sodium dodecyl benzenesulfonate (SDBS), or 0.1 M LiClO4. Only a 0.28 cm2 portion of the titania substrate was illuminated and in direct contact with the solution during electropolymerization. After polymer electrodeposition, the characterization of the hybrid structure was performed in the same supporting electrolyte as that used for the synthesis but without Py monomer. Electrochemical measurements made on hybrid structures were performed at a potential value selected inside a potential range situated between 0.5 and −1.1 V vs SCE shown to encompass the oxidation/reduction (i.e., doping/undoping) of polypyrrole.

dure can be necessary. Several authors28,30 used light during electrodeposition of conjugated polymers on nanostructured oxides and obtained hybrid structures with high electroactivity. The role of UV light is to provide the photoholes necessary for monomer oxidation but also to increase the doping level of the TiO2 tube walls31 in an aqueous medium, making the charge transfer easier between the semiconductor and the monomer. Nevertheless the need for light for PPy deposition on TiO2 NTA has been shown28 to depend on the nature of the solvent: whereas UV light is necessary to induce PPy electrodeposition in SDBS aqueous electrolyte, electrodeposition in the presence of LiClO4 occurs in the dark as well. (2) The second challenge is to deposit the polymer on the inner and outer walls of the tubes and to avoid the sealing of the tubes at the mouth, taking into account that the pyrrole monomer and counterion must diffuse over the nanotube length. Kowalski et al.27 depicted a procedure in which they used current pulses with high intensity (26 mA·cm−2) in order to achieve self-organized polymeric layers using TiO2 nanotubes as template. They showed the benefit of micelles to facilitate the transport of the monomer and counterions by using sodium dodecyl sulfate (SDS) solutions whose surfactant concentration was higher than the critical micellar concentration of SDS. Jia et al.25 used a potentiostatic method at 1 V in the dark and obtained PPy modified TiO2 NTAs with better photogenerated charges separation under visible light compared to pristine titania nanotubes. The aim of the present work is to perform the electropolymerization of pyrrole within TiO2 NTAs under potentiostatic conditions using pulses of either light or potential. Indeed, by alternating oxidation phase with rest time, a good control of the electrodeposition rate should be achieved. As a consequence, monomers renewal by diffusion is allowed within the tubes, preventing pyrrole from being electropolymerized at the mouth of the tubes. Cyclic voltammetry experiments have been performed to control the electroactivity of the hybrid structures and to compare the yields of the different synthesis methods. The characterization of the obtained composite structures was also carried out by field emission gun-scanning electron microscopy (FEG-SEM) so as to determine the morphology of the polymer deposit.

3. RESULTS AND DISCUSSION 3.1. Deposition in the Presence of Sodium Dodecylbenzenesulfonate (SDBS). Electropolymerization at 0.8 V/ SCE was performed in SDBS aqueous solution, in the dark, under continuous UV illumination, and by a pulsed-light method. In the Dark. First, attempts to deposit polypyrrole in TiO2 nanotube arrays were performed by applying a potential of 0.8 V/SCE in a SDBS containing pyrrole aqueous solution, in the dark. The chronoamperogram recorded during this experiment (Figure 1A,B) shows first a current drop assumed to be related to the decrease of monomer concentration in the diffusion layer. The current value reaches a minimum after 90 s of polarization, and then it increases quickly for the next 300 s and keeps increasing but much more slowly when the polymer grows at a constant rate. Nevertheless, the anodic current is always low, and the anodic charge after 1800 s is only about 3 mC (Figure 1C). From Figure 2 (inset) obtained after transfer of the resulting supposedly hybrid structure, one can hardly identify the peaks related to the oxidation/reduction (i.e., doping/undoping) of polypyrrole. This confirms that only a negligible amount of PPy has been deposited at the surface of the TiO2 NTA which is depleted from its majority charge carriers at such an anodic potential. Under UV Illumination. To improve the electropolymerization rate of pyrrole on TiO2 NTA, the same experiment was performed under UV-light (Figure 1A). As expected, the deposition rate was enhanced due to the contribution of the photoelectropolymerization reaction.28,29 Under UV illumination, the photogenerated holes oxidize pyrrole monomers, leading to the formation of radical cations, which enables initiation of the electropolymerization. Simultaneously, photogenerated holes are partially consumed for water oxidation leading to hydroxyl radicals. Compared to the chronoamperogram recorded in the dark, at the start of the experiment, the photochronoamperogram (Figure 1A) displays a decreasing current that lasts 600 s (Phase 1). The decrease in current intensity in this case can be explained by the decrease of the photocurrent related to the TiO2 NTAs substrate. Since the photocurrent is proportional to the TiO2 surface exposed to the UV illumination, as polypyrrole deposition

2. EXPERIMENTAL SECTION 2.1. Materials and Apparatus. Pyrrole was distilled and conditioned in darkness, under nitrogen and at 4 °C. All the other reagents were used as received. Electrochemical syntheses and analyses were performed in a three-electrode cell, using a Solartron SI 1287 as the electrochemical interface. All the potentials were measured against and referred to a saturated calomel reference electrode (SCE), with a Pt spiral wire as a counter electrode. An HPR 125-W mercury vapor lamp was used as UV source (wavelength range: 300−400 nm). It was placed at a distance of 25 cm from the working electrode, and the UV beam was perpendicular to the working electrode surface. The morphologies of samples were studied by FEGSEM (Zeiss Ultra 55). 2.2. Synthesis and Characterization of Hybrid Structures. TiO2 nanotube arrays were synthesized in a 3 wt % NH4F solution in ethylene glycol containing 2 vol % deionized water using a large area platinum counter electrode in a two electrode setup. A constant potential of 20 V was applied on a Ti foil for 45 min. After anodization, samples were ultrasonically rinsed in ethanol and water and then annealed at 26342



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Figure 2. CV characterization of TiO2 NTA/PPy(DBS) junctions in an aqueous electrolytic solution containing SDBS (0.1 M) at 10 mV/s in the dark: (a) PPy obtained in the dark, (b) under UV illumination, (c) with pulses of UV-light. Inset: Enlargement of (a) and (c).

photosynthesized TiO2 NTA/PPy(DBS) junction to contain a greater amount of polypyrrole than that obtained without UV illumination. The voltammogram of the former hybrid structure after transfer in a SDBS electrolytic aqueous solution is shown in Figure 2. It clearly displays two strongly intense peaks at 0.05 V and −0.90 V/SCE corresponding to the oxidation and reduction (i.e., doping and undoping) of polypyrrole, respectively. The influence of UV light illumination on the rate of electrodeposition of polypyrrole on TiO2 NTAs is therefore emphasized, in agreement with the results of references.28,29 FEG-SEM analysis was performed on the sample synthesized by photoassisted electrodeposition at 0.8 V/SCE during 1800 s, and a cauliflower-like structure was obtained (Figure 3A), due to a very high electrodeposition rate. When the electropolymerization was performed for a shorter UV exposure duration, corresponding to Phase I (Figure 1A), side view images show polypyrrole covering TiO2 nanotubes throughout their length (Figure 3B). However, images taken from the top do not enable the morphology of the substrate to be discerned very well (Figure 3C). For purpose of comparison, Figure 3D,E illustrates respectively the side and top views of pristine TiO2 NTAs before electropolymerization. Deposition by a Pulsed-Light Method. Since the electrodeposition rate of PPy on TiO2 NTA in SDBS aqueous solution was found to be insignificant in the dark and too high under UV exposure, attempts were made to decrease the UV exposure time by using light pulses. While being polarized at 0.8 V/SCE, the electrode was successively maintained in the dark for 40 s and then exposed to UV-light for 5 s. The procedure was repeated 40 times. Attention is drawn to the light pulses duration that have to last long enough to activate the whole TiO2 NTAs surface. In between each pulse, the step in the dark is assumed to allow the deposition rate to slow down, in order to provide the bottom of the tubes with new monomers from the bulk of the solution by diffusion. From Figure 4 one can notice that at the beginning of the electrode polarization, the current is negligible. The oxidation of pyrrole and hence the electropolymerization occur slowly. When the UV source is switched on, an increase in current is provoked. This increase is mainly due to the photocurrent of

Figure 1. (A) Chronoamperogram registered at a TiO2 NTA electrode polarized at 0.8 V/SCE in an aqueous solution containing 0.1 M SDBS + 0.1 M Py, (a) in the dark, (b) under UV illumination, (c) with pulses of UV-light. (B) Enlargement of panel A. (C) Chronocoulometric curves registered during the corresponding potentiostatic electropolymerizations (same code). Inset: Enlargement of panel C.

progresses, the resulting optical shielding hinders further exciton generation. After 600 s of polarization, the current rises sharply and tends to reach a plateau. This increase begins when the contribution of the photocurrent to the total current starts to be neglected (Phase 2). When a sufficient amount of polypyrrole has been photodeposited, the electrochemical polymerization then takes the control of the polymer growth. Additionally, when the titania surface is fully covered by a PPy film, a sharp charge increase can be observed after around 800 s in the corresponding chronocoulometric curve (Figure 1C) which corresponds to an increase in electropolymerization rate. This is in agreement with the fact that there is no more competition between monomer and water oxidation since the latter occurs only on the uncovered TiO2 surface. The overall charge consumed during the photopotentiostatic deposition is around 665 mC, a 200 times increase as compared to the synthesis in the dark. Consequently, one has to expect the 26343



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Figure 3. FEG-SEM images of TiO2 NTA/PPy(DBS) junctions obtained by chronoamperometry at 0.8 V/SCE under UV: (A) After 1800 s and (B) and (C) at the end of the Phase 1 (cf. Figure 1A). (D) Side view image of pristine TiO2 NTAs before electropolymerization. (E) Top view image of pristine TiO2 NTAs before electropolymerization. (F) Top view image of TiO2 NTA/PPy(DBS) junctions obtained by chronoamperometry at 0.8 V/SCE with pulses of UV-light.

the TiO2 substrate. One can also point out that during the 5 s illumination of the first pulse, the current slightly decreases from 128 μA to 109 μA. This current drop can be assigned to (i) the recombination of photogenerated excitons at the surface of the semiconductor. When illuminated, TiO2 NTA requires a short period of time to accommodate. Consequently, the photocurrent decreases to reach a constant value, (ii) the optical shielding of the substrate surface. Polypyrrole photodeposited during the pulse acts as an obstacle for photons; thus less excitons can be produced. When the UV-light source is switched off, the current intensity drops drastically to reach a value lower than 1 μA. The consecutive pulse gives rise to a current of 108 μA, a value close to that obtained at the end of the previous pulse. That confirms that only a very small amount of polypyrrole can be deposited in the dark. We distinguish here two phases. The first, under UV, involves photoelectropolymerization of pyrrole monomers. The second one, in the dark, corresponds to the electro-

polymerization being likely to occur on the seed layer of polypyrrole deposited under UV. As one can also see in Figure 1B, the current baseline obtained in the dark keeps increasing. Moreover the effect of illumination is less and less pronounced over time, as it can be deduced from the decrease of the current intensity resulting from pulses. This is an expected result arising from polypyrrole electrodeposition. Nevertheless, the current baseline only reaches 7 μA after 1800 s. If we take a look at the corresponding chronocoulometric curve (Figure 1C), it appears that the process is still in Phase 1, as defined for electrodeposition under constant UV light (see Figure 1C curve a) even after 1800 s, which corresponds to 40 pulses following our procedure. As a consequence, one cannot expect a large amount of polymer to have been deposited. It is confirmed by cyclic voltammetry performed in a SDBS blank solution (Figure 2). However, compared to the sample synthesized at 0.8 V/SCE in the dark, the voltammogram now displays oxidation/reduction peaks respectively at −0.1 V and −0.6 V/SCE (Figure 2, inset). 26344



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Figure 4. Chronoamperogram registered during electropolymerization of pyrrole on a TiO2 NTA at 0.8 V/SCE in an aqueous electrolytic solution containing 0.1 M SDBS + 0.1 M Py.

Regarding the structure obtained by pulsed light, Figure 3F shows an enlargement of the tube walls thickness compared to pristine TiO 2 NTA (Figure 3E). In fact, pristine TiO2 nanotubes have on average 10-nm-thick walls. After electropolymerization assisted by pulses of light, SEM-FEG images indicate that the walls thickness increases and reaches a mean value of between 15 and 20 nm, thus suggesting that electropolymerization started in the void between the tubes and/or inside them. The nanotube profile can still be discerned, thus tending to show that polypyrrole deposit is likely to follow the shape of the TiO2 NTAs, which corresponds to one of the targeted outcomes. With regard to the structure obtained in the dark, it still displays the same morphology as an uncovered pristine TiO2 NTA substrate (not shown here). 3.2. Deposition in the Presence of Lithium Perchlorate (LiClO4). In a previous work,28 it has been shown that PPy synthesis on TiO2 NTA in a LiClO4 aqueous solution does not need UV exposure and that significant amount of polymer can be deposited in the dark though the deposition rate is further enhanced under UV exposure. Synthesis in potentiostatic conditions at 0.8 V/SCE in the dark allows reaching 116 μA after 1800 s (Figure 5A) and an anodic charge of about 157 mC (Figure 5B). Under UV exposure the current increases sharply, reaching a plateau of around 695 μA (Figure 5A) and an anodic charge close to 1200 mC (Figure 5B) within the same polarization time. Here again, illumination during the synthesis increases the overall electrodeposition rate. The junctions made respectively in the dark and under UV illumination were characterized by voltammetry (Figure 6), and it clearly appears that the hybrid structure made under UV-light displays a better electroactivity than that realized in the dark. In order to understand the different behavior of TiO2 NTA in the two electrolytes, the evolution with time of the photocurrent was followed as a function of the applied potential. As an example, Figure 7 shows the response of the TiO2 NTA substrate at 0.4 V/SCE, in the two electrolytes in the absence of Py monomer. Whereas in SDBS solution the steady state is reached within a few seconds, a significant decrease of the photocurrent is observed in LiClO4 solution during the first 200 s. Such a decrease can be attributed to the filling of surface states present at the surface of TiO2 NTA by photogenerated

Figure 5. (A) Chronoamperogram registered on a TiO2 NTA at 0.8 V/SCE in an aqueous solution containing 0.1 M LiClO4 + 0.1 M Py and under different conditions: (a) in the dark, (b) under UV illumination, and (c) with pulses of UV-light. (B) Chronocoulometric curves registered during the corresponding potentiostatic electropolymerizations (same code).

Figure 6. CV characterization of TiO2 NTA/PPy(ClO4) junctions in an aqueous electrolytic solution containing LiClO4 (0.1 M) at 10 mV/ s in the dark corresponding to electrodeposition of PPy under different conditions: (a) in the dark, (b) under UV illumination, and (c) with pulses of UV-light.

charges. It is admitted that surface states at the surface of TiO2 result from interaction between the substrate and the electrolyte and that water adsorption on TiO2 generates energy states in the band gap which can store photo charges.31 Such surface states act as electron traps, and the trap density has been shown to be strongly pH dependent.33 It appears from Figure 7 that the density of states is higher in aqueous LiClO4 electrolyte than in a micellar SDBS solution. This is corroborated by electrochemical impedance spectroscopy measurements and Mott−Schottky diagrams.34 Accordingly TiO2 NTA in aqueous LiClO4 solutions has a flat-band 26345



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It can be seen in Table 1 that in a LiClO4 aqueous electrolyte, the potentiostatic method in the dark leads to a Table 1. Current Efficiency for Electropolymerization of Pyrrole in LiClO4 synthesis mode potentiostatic in the dark photopotentiostatic pulsed-light potentiostatic

potential of −0.45 V/SCE, which is a usual value in aqueous solutions of neutral pH, in the presence of water induced surface states. In comparison, for the same substrate in SDBS, the flat band potential has been found34 to be close to 0.1 V/ SCE, as a result of the absence of such states, since SDBS adsorption is likely to occur in preference to water. We can thus assume that, during electropolymerization in LiClO4 electrolytic solution in the dark, the Py monomer can inject electrons into the polarized semiconductor via those surface states, enabling electropolymerization without light excitation, contrary to what is observed in SDBS. Deposition by a Pulsed-Light Method. As it has been done above in a SDBS aqueous electrolyte, attempts to electropolymerize pyrrole by the pulsed-light method have been realized in Py solution containing LiClO4. The current baseline in the dark (Figure 5A) increases here significantly. This is an expected result, given that pyrrole can be electropolymerize in the dark in a perchlorate containing electrolyte at 0.8 V/SCE. Besides, one can also notice that the photocurrent decreases rapidly as well, which is in good agreement with a high polypyrrole growth rate on the titania surface. No effect of illumination is further observed beyond 900 s as a consequence of UV-light absorption by the polypyrrole layer. At this moment we can assume that a minimum thickness of 50 nm was deposited on TiO2 NTA, considering an absorption coefficient value35 for PPy of about 2 × 105 cm−1. Finally, the current reached a steady value of 585 μA, when the electrodeposition rate became constant. The obtained structures were transferred in a blank electrolyte and characterized by voltammetry. Figure 6 shows that the sample made by pulses of UV-light displays greater oxidation/reduction currents than those related to syntheses at the same potential under constant UV-light and in the dark. In order to compare in a greater depth the three synthesis methods, the electropolymerization yields for each procedure were calculated from chronocoulometric (Figure 5B) and cyclic voltammetry curves (Figure 6) as follows: Q CV /γ Q synth /(2 + γ )

Qsynth (mC)

yield (%) with γ = 0.25

yield (%) with γ = 0.33

14.2

157

81

63

32.0 63.4

1105 819

26 70

20 55

higher efficiency (81% or 63% for γ = 0.25 or 0.33 respectively) in comparison with pulsed-light and continuous light potentiostatic methods. Moreover it appears that the pulsedlight method is more efficient than the continuous light method (70% vs 26% or 55% vs 20% for γ = 0.25 or 0.33 respectively). The detrimental effect of light on the yield is because during illumination at 0.8 V/SCE water and monomer oxidations occur simultaneously, thus limiting electropolymerization efficiency. This competition is not expected during deposition in the dark. In all cases, yields do not reach 100% because of different side-reactions such as local overoxidation, α−β cross-linking, oligomer dissolution, or formation of electronically isolated polymeric chains, during the synthesis. A rough estimation can be made for the deposition charges reached during the lightpulse deposition method, if we assume that the deposition rates under UV and in the dark are constant within the 1800 s of the experiment. Since during the light-pulse method the sample was exposed to UV light for 200 s (40 pulses of 5 s) and maintained for 1600 s in the dark, electropolymerization would have generated 123 mC plus 140 mC respectively, which equals 263 mC. In comparison, for the light-pulse method, the synthesis charge determined experimentally is 3 times higher, i.e., 819 mC (Table 1). The same tendency is evidenced if we compare the Qcv values (measured in the dark) obtained for the light-pulse electrodeposition to that of continuous-light electrodeposition (Table 1), since Qcv (pulse) is about twice Qcv (continuous). This emphasizes the beneficial effect of pulses of light and is likely to show that in these conditions a higher surface of TiO2 NTA is covered by polymer, resulting from a better supply of pyrrole monomers at the electrode/solution interface by diffusion. FEG-SEM analyses were also performed on the hybrid structures obtained in the presence of LiClO4. If we compare the side views of hybrid junctions (Figure 8A,B) to that of a pristine TiO2 NTA (Figure 8C), we can see that in the former case only 450 nm of the tubes length are visible (Figure 8A). Considering 960 nm long pristine nanotubes (Figure 8C), it can be assumed that PPy starts growing within the TiO2 NTAs. Figure 8B shows that in the case of pulsed-light method, a thicker polypyrrole layer (about 2.8 μm) is deposited over a TiO2 NTA substrate of about 150 nm thickness. (At the scale of Figure 8B, the uncovered TiO2 NTA substrate appears as a bright and very thin layer.) 3.3. Deposition by a Pulsed-Potential Method in SDBS and LiClO4. Since attempts to alternate oxidation phases with rest phases seem to favor diffusion of monomer species into the nanotubes, but since the previous light-pulse method led to deposition rates which were still too high for a good control of the localization of polypyrrole deposition in lithium perchlorate

Figure 7. Photochronoamperograms registered on a TiO2−NTA at 0.4 V/SCE in an aqueous, electrolytic solution containing (a) 0.1 M LiClO4 and (b) 0.1 M SDBS.

yield (%) =

QCV (mC)

× 100

where Qcv is the average charge obtained by cyclic voltammetry ((Qox + |Qred|)/2) and Qsynth is the synthesis charge. In this calculation, we consider that polypyrrole oxidation involves γ e− (γ is the doping level) and that two electrons are used per monomeric unit for the electropolymerization.21 26346



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Figure 8. FEG-SEM side view images of TiO2 NTA/PPy(ClO4) junctions obtained by chronoamperometry at 0.8 V/SCE: (A) under constant UV illumination and (B) with 40 pulses of UV-light. (C) FEG-SEM side view image of a pristine TiO2 NTA.

aqueous solutions, a pulsed-potential method was developed. After preliminary tests in both aqueous electrolytes (SDBS and LiClO4), the selected procedure was to alternate anodic and cathodic pulses with 0.5 s duration, separated by a rest step of 5 s. The anodic and cathodic pulse amplitudes were 0.8 V/SCE and −0.2 V/SCE, respectively. Contrary to Kowalski et al.27 who used pulses of current, the choice in this work to fix a constant potential allows overoxidation of PPy to be avoided. Rest between pulses (5 s) is an important step allowing the renewal of monomers along the nanotubes by diffusion. In the Dark. Figure 9A shows the overall shape of a chronoamperogram registered in the dark during the first potential pulses for TiO2 NTA in SDBS electrolyte containing pyrrole. The first pulse of potential gives rise to a current of 35 μA. The decrease in monomers concentration in the diffusion layer leads to a current decrease until 6.3 μA. The second pulse, which is cathodic at −0.2 V/SCE (−26 μA), has been recommended27 for obtaining homogeneous polymer deposition. In LiClO4, the chronoamperogram has the same shape, yet with higher current values, since, as expected, pyrrole electropolymerization in this medium can be performed in the dark, contrary to SDBS. The overall chronoamperograms in the two electrolytes are shown in Figure 9B. In SDBS, from one cycle to the other, the maximum current does not really evolve, both in the anodic and cathodic parts. The average anodic current intensity is only around 315 μA. Moreover, the current baseline, corresponding to the current at the end of each pulse, only reaches 27 μA after 200 pulses, confirming that neither the oxidation nor the reduction of polypyrrole occurs at a significant rate in SDBS aqueous solution in the dark. Accordingly in Figure 10, the voltammogram performed on the hybrid structure for characterization confirms that the PPy amount deposited in these conditions is negligible. In a similar way, we investigated the electropolymerization of pyrrole by pulses of potential in an aqueous electrolyte containing LiClO4 (Figure 9C). In this case, contrary to electrodeposition in the presence of SDBS, the current significantly increases and reaches 1500 μA following the 24th pulse. The anodic current then decreases and starts to increase again following the 95th pulse. Concerning the cathodic current, it corresponds to the reduction of newly formed oligomers/polymers. Its evolution in time in absolute value follows exactly that of the anodic current. It can therefore be assumed that the decrease in current in absolute value around the 95th pulse is due to oligomer dissolution. If one now takes a look at the overall chronoamperogram in the

Figure 9. (A) Chronoamperogram registered in the dark in an aqueous electrolytic solution containing pyrrole (0.1 M) and SDBS (0.1 M): Effect of potential pulses upon the current response. Chronoamperograms registered during electropolymerization of pyrrole on a TiO2 NTA in the dark, with pulses of potential (E = 0.8 V/SCE) and in an aqueous electrolytic solution containing either (B) SDBS (0.1 M) or (C) LiClO4 (0.1 M).

anodic part, it is interesting to see that the current baseline follows the same variations as during a continuous potentiostatic method at 0.8 V/SCE in the dark (Figure 5A) but with a higher amplitude in the former case. It can be concluded that the synthesis mechanism does not change, and only the deposition currents are affected by this method. The higher dark current observed when potential pulses are applied can be attributed to an enhanced active surface enabled by the diffusion of the monomer into and between the nanotubes of the substrate, very similarly to the effect of light pulses. During characterization of the hybrid structure obtained after 200 potential pulses, the voltammogram in Figure 10 exhibits the features of a reversible TiO2 NTA/PPy(ClO4) junction, with an anodic peak at 0.27 V/SCE and two cathodic peaks at −0.20 V and −0.65 V/SCE. 26347



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Figure 10. CV characterization of TiO2−NTA/PPy junctions obtained using a potential-pulsed potentiostatic method after 200 pulses (a) in the presence of SDBS (0.1 M) in the dark, (b) in the presence of SDBS (0.1 M) under continuous light exposure, and (c) in the presence of LiClO4 (0.1 M) in the dark.

Concerning the localization of PPy within the nanotubular TiO2 matrix, Figure 11B shows a side view of the hybrid junction synthesized in an electrolyte containing LiClO4. It shows the presence of PPy not only at the top of the substrate but also within the nanotubes. In our experimental conditions, pristine nanotubes were 960 nm long on average. Consequently, we can assume that PPy penetrated into almost half of the nanotubes layer depth, starting to fill the voids between the nanotubes. Under UV Exposure in SDBS. Since in SDBS solutions, PPy deposition on TiO2 NTA can hardly be performed in the dark, preliminary experiments were performed by applying potential pulses at 0.8 V/SCE under continuous UV exposure. The experimental conditions were the same as previously (0.5 s at 0.8 V/SCE, 5 s at rest, 0.5 s at −0.2 V/SCE). The hybrid structure was analyzed after 200 potential pulses by voltammetry and FEG-SEM observation, and the results are shown in Figures 10 and 11. In comparison with the voltammogram obtained after 200 potential pulses in the same electrolyte but in the dark, no significant change can be observed, and the current values in the two cases are much smaller than for the deposit obtained by potential pulses in the dark in LiClO4 (Figure 10 curve c). Nevertheless the SEM image for the corresponding hybrid structure clearly shows the presence of a PPy deposit located at the walls of the nanotubes (Figure 11A). That means that an accurate control of the deposition rate and the deposition location of PPy on and into TiO2 nanotubes can be performed by combining the duration of polarization and light exposure. Though the parameters in the present work were not optimized to obtain a perfect coverage of the tube walls by polymer, an important conclusion is that the parameters should be selected so as to allow slow electrodeposition rate of the polymer and that they greatly depend on the nature of the anion present in the electrolyte.

Figure 11. (A) FEG-SEM top view image of a hybrid structure obtained after 200 pulses of potential (t = 0.5 s, E = 0.8 V/SCE) under UV-light in an aqueous electrolytic solution containing pyrrole (0.1 M) and SDBS (0.1 M). (B) FEG-SEM side view image of a hybrid structure obtained after 200 pulses of potential (t = 0.5 s, E = 0.8 V/ SCE) in the dark in an aqueous electrolytic solution containing pyrrole (0.1 M) and LiClO4 (0.1 M).

UV illumination during electropolymerization and the greater deposition rate in a LiClO4 aqueous electrolyte compared to an aqueous electrolytic solution containing SDBS have been highlighted according to previous works. The main outcomes of this work are the following: • The control of the deposition rate using pulses of light enables higher electropolymerization yields to be reached than under a constant illumination electropolymerization. • The short duration of light pulses limits the competition between Py and water oxidation, leading to a higher yield of electrodeposition. • Pulses of light and/or potential allow the monomer to diffuse into and between the nanotubes and favor the coverage of the tube walls. It appears that controlling the deposition rate by means of pulses of potential or UV-light is the route to follow, in order to have a great benefit from the high developed surface provided by TiO2 nanotube arrays. Nevertheless the parameters have to be adapted to the nature of the electrolyte in which

4. CONCLUSION The aim of this work was to investigate the electropolymerization of pyrrole within TiO2−NTAs using different methods including pulses of light and potential. The beneficial effect of 26348



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electropolymerization is performed, since charge transfer between the monomer and the semiconductor is determined by the interface between TiO2 and the electrolyte. Indeed pyrrole oxidation in aqueous solutions can occur via energy states located at the TiO2 /solution interface and a lightassisted procedure is thus not necessary. On the contrary, the presence of SDBS in high amounts inhibits water adsorption and consequently decreases the surface state density. UV-light is then necessary to provide the photoholes for pyrrole oxidation at the surface of the semiconductor.

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ASSOCIATED CONTENT

S Supporting Information *

Electrochemical properties of a TiO2-NTA/PPy junction. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 33 1 44 27 41 68. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Mrs. F. Pillier is warmly acknowledged for her expertise in FEG-SEM experiments. In a similar manner, the authors would also like to thank A. Desnoyers de Marbaix for his help related to mechanical works.

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REFERENCES

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Electrodeposition of Polypyrrole in TiO2 Nanotube Arrays by Pulsed

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