Self-Assembled, Nanostructured Polypyrrole Films Grown in a High

Article pubs.acs.org/Langmuir

Self-Assembled, Nanostructured Polypyrrole Films Grown in a HighGravity Environment Jean H. Chang,*,† Christian R. Aleman de Leon,‡ and Ian W. Hunter† †

BioInstrumentation Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology (MIT), 77 Massachusetts Avenue, 3-147, Cambridge, Massachusetts 02139, United States ‡ Universidad del Turabo, Gurabo 00778, Puerto Rico S Supporting Information *

ABSTRACT: A simple, novel method of synthesizing self-assembled, nanostructured conducting polymer films has been developed. Applying an increased centrifugal force on the electrodes during the electrochemical deposition process yields high surface area, micro- or nanostructured polymer films. Scanning electron microscopy showed that as the applied g-force increased, the polymers progressed from having smooth, “cauliflower” morphologies, to intermediate microstructured surfaces, to finally dense nanostructured surfaces with pore sizes as small as 50 nm. Cyclic voltammetry revealed that films grown at higher centrifugal accelerations (higher than 500g) exhibited less degradation after electrochemical cycling and more capacitive behavior.

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INTRODUCTION Conducting polymers, in particular polypyrrole (PPy), are a promising class of materials that possess unique properties that allow them to be used in a wide variety of applications. PPy is one of the more useful conducting polymers because of its high conductivity (can be on the order of 104 S/m), ease of fabrication, and stability in ambient conditions. PPy is typically grown electrochemically in an electrochemical cell, which consists of a working electrode (WE) and a counter electrode (CE). When an electric potential is applied between the electrodes, the polymer grows as a thin film on the WE. PPy is currently being developed as artificial muscle actuators,1 flexible electrodes,2 sensors,3 supercapacitors,4 battery electrodes,5 microfluidic pumps,6 and cell and tissue culture platforms.7 Recently, conducting polymers have been shown to switch wetting states when an electric stimulus is applied.8,9 For many of the aforementioned applications, it is desirable to use a polymer with a high surface/volume ratio. Because the electrochemical behavior of these electroactive polymers highly depends upon the influx and efflux of ions between the polymer and electrolyte solution, an increased surface area can greatly improve the response of the polymer. Additionally, nanostructuring has been shown to enhance the hydrophobicity or hydrophilicty of a material, resulting in a superhydrophobic or © 2012 American Chemical Society

superhydrophilic surface. Furthermore, roughened surfaces are known to promote cell adhesion. There are several methods of achieving high surface area polymers, including hard-template10 and soft-template methods,11 as well as electrospinning techniques. Soft-template methods are typically preferred over hard-template methods because of the ease of fabrication and lack of micromachining/ microfabrication steps. Current soft-template methods combine the electrochemical and chemical polymerization of the polymer by adding a catalyst, such as ferric chloride (FeCl3) or ammonium persulfate (APS), to the electrochemical deposition.11,12 Here, we present a new method of creating nanostructured conducting polymer surfaces. We have discovered that performing a simple electrochemical deposition in an increased gravity environment results in a high surface area, nanostructured PPy film. This technique removes the need to use potentially corrosive catalysts in soft-template depositions. The high-gravity environment was created by use of a centrifuge. The amount of acceleration applied to the sample is typically described by the dimensionless quantity “relative centrifugal Received: November 22, 2011 Revised: January 25, 2012 Published: February 2, 2012 4805

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Figure 1. (a) Side view of the custom-built deposition chamber (wires and deposition solution not shown). The Teflon chamber is shown to be transparent for visualization purposes. A Teflon-filled Delrin cover was used to prevent spillage of the deposition solution. The chambers were positioned such that the centrifugal forces were perpendicular to and pushing against the WE, and PPy was electrochemically grown on the WE. (b) Three-dimensional view of the deposition chamber. (c) Image of deposition chambers and circuitry placement inside the centrifuge. The chambers and circuitry were placed to maintain rotor balance. (d) Image of the custom-built PCB used to maintain constant current between the WE and CE (wires not shown).

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force” (RCF), expressed in multiples of g, the standard acceleration due to gravity at the Earth’s surface (9.8 m/s2)

RCF =

EXPERIMENTAL SECTION

Materials. Potassium perfluorooctanesulfonate (KPFOS, SigmaAldrich) and acetonitrile (anhydrous, 99%, Sigma-Aldrich) were of analytical grade and used as received. Pyrrole (99%, Sigma-Aldrich) was vacuum-distilled and stored under nitrogen at −20 °C. Electrochemical Deposition of PPy in a High-Gravity Environment. PPy samples doped with KPFOS were synthesized via galvanostatic polymerization with a current density of 1.5 A/m2 for 90 min at 22 °C. The deposition solution contained 15 mM KPFOS with 0.1 M pyrrole in acetonitrile. While electrochemical polymerizations are typically conducted using a potentiostat, because of size contraints, a custom-made battery-powered printed circuit board (PCB) was built to provide constant current between the electrodes (see Figure 1d for the PCB and the Supporting Information for the schematic). The electrochemical deposition cells were constructed out of Teflon, and gold-plated stainless-steel foil was used as the CE and WE. Nylon standoffs were used to maintain a constant distance of 16 mm between the CE and WE, and the electrodes were affixed to the bottom of the Teflon chamber using nylon screws and a Teflonfilled Delrin clamp (panels a and b of Figure 1). Teflon-coated wires were used to make electrical contacts between the PCB and the electrodes. Two deposition cells were placed opposite each other (to maintain rotor balance) in swinging buckets inside an Eppendorf Centrifuge 5804 R and positioned such that the centrifugal forces were perpendicular to and pushing against the WE (Figure 1c). The circuitry was placed in separate swinging buckets. The polymers were grown inside the centrifuge, subjected to accelerations ranging from 50g to 2250g. Polymer Characterization. The surface morphologies of the polymers were characterized using a JEOL JSM-6060 scanning electron microscope. The electrochemical behavior of the polymers was characterized by cyclic voltammetry (CV). The PPy was placed as

r ω2 g

where r is the rotational radius (in meters), and ω is the angular speed (in rad/s). Studies on PPy grown in increased gravity environments are limited. It has been shown that the electrochemical growth of PPy in the presence of an applied centrifugal force results in a denser, more electrochemically stable film.13 Other studies have shown that the surface morphology depends upon the placement of the WE and CE relative to the applied centrifugal field; that is, a different surface structure arises if the g-force is directed toward or away from the WE.14−16 However, to the best of the authors’ knowledge, no studies have shown the extent of the nanostructuring as we present in this paper nor has PPy been synthesized at centrifugal forces higher than several hundred g. In this work, we explore the effects of current density and RCF on the surface morphology of PPy. The electrochemical deposition chambers will be subjected to centrifugal forces as high as several thousand g. We will present a set of guidelines that will allow researchers to use this technique to tune surface morphologies to desired roughnesses and porosities. We will conclude with a proposed deposition mechanism that explains the nanostructuring phenomena. 4806

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Figure 2. Scanning electron microscopy (SEM) images of PPy samples grown galvanostatically with a current density of 1.5 A/m2, submitted to centrifugal accelerations of (a) 1g, (b) 50g, (c) 250g, (d) 2250g, and (e) 4500g. (f) SEM image of the PPy film grown at 2250g, with the locations of the WE and CE switched.

Figure 3. Summary of the progression of the nanostructuring on the surface of PPy as the RCF is increased. As the applied centrifugal force was increased, the extent of the nanostructuring of the PPy surface also increased. the WE in an electrochemical cell that contained a gold-plated stainless-steel foil as the counter electrode, a silver wire as the pseudoreference electrode, and 15 mM KPFOS in acetonitrile as the electrolyte solution. A VMP2 multichannel potentiostat (Princeton Applied Research) with EC-Lab software v9.55 (Bio-Logic Science Instruments) was used to perform cyclic voltammograms on the films between −0.8 and +0.8 V at a scan rate of 15 mV/s. The voltage window was selected to avoid damage to the polymer by overoxidation. Conductivity was measured using the four-wire resistance method.

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centrifugal field was increased, the extent of the nanostructuring of the PPy surface also increased. Polymers grown at normal gravitatation accelerations (1g; Figure 2a) had a cauliflower morphology that is typical of galvanostatically deposited films.17,18 As the centrifugal acceleration was increased to 50g (Figure 2b), microstructures on the order of 1−10 μm in size grew sparsely on the surface. As the acceleration was further increased to 250g (Figure 2c), the microstructures grew very densely on the surface and had a surface morphology that resembled films grown electrochemically with a small amount of ferric chloride in the deposition solution at normal gravitational acceleration.12 At high centrifugal accelerations (2250g and 4500g; panels d and e of Figure 2), a thick (1−10 μm) layer of nanostructures with pores from 50 nm to 1 μm in

RESULTS AND DISCUSSION

Electrochemical growth of PPy in an increased gravity environment resulted in highly roughened surfaces (Figure 2). The extent of the nanostructuring was dependent upon the strength of the applied centrifugal field. In general, as the strength of the 4807

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diameter grew densely on the film surface. It was observed that the pore sizes became smaller as the centrifugal acceleration was increased (Table 1). Table 1. Pore Sizes of Micro- or Nanostructured Layers of PPy Films Grown at Different Centrifugal Accelerations centrifugal acceleration (g) 500 1000 2250 4500

pore size range 160 100 85 50

nm−3 μm nm−3 μm nm−2.5 μm nm−2 μm

Figure 3 shows a summary of the transition from the cauliflower morphology to micro- and nanostructured PPy with increasing applied centrifugal force. The graphic in Figure 3 can aid researchers in tuning surface roughness by selecting the strength of the applied centrifugal field. Four zones were identified: I, “Cauliflower” morphology; II, transition zone, where sparse microstructures (1−10 μm in size, spaced 5−10 μm apart) have begun to form on the surface of the PPy; III, microstructured PPy zone, where spherical globules 1 μm in diameter have amassed on the PPy surface, completely covering the underlying cauliflower morphology; and IV, nanostructured PPy zone, where a thick (2−10 μm) layer of nanostructures has formed on the surface. The transitions between zones were observed to be gradual rather than distinct. The conductivities of the polymer samples were measured and are shown in Table 2. The magnitude of the centrifugal field was found to have no significant effect on the conductivities. Table 2. Conductivities of PPy Samples Grown at Different Centrifugal Accelerationsa centrifugal acceleration (g) 1 100 250 1000 2250

conductivity (S/m) 1116 724 766 1063 564

± ± ± ± ±

473 297 153 198 332

a

The errors are the standard deviations of measurements taken from different locations on multiple samples. Four to eight conductivity measurements for each centrifugal acceleration were taken.

CV of polymer samples grown with a current density of 1.5 A/m2 (shown in Figure 4) revealed that the electrochemical behavior of these electroactive films changed as a function of the applied centrifugal force. Films grown at higher centrifugal accelerations were found to be more reversible than films grown at lower centrifugal accelerations. Specifically, CV of polymers grown at low centrifugal accelerations (zone I) showed that the redox process was irreversible, as electrochemical activity (measured by the current across the WE and reference electrode) decreased significantly after each cycle (Figure 4a), suggesting degradation. Polymers grown in zone II showed a slight oxidative peak around +0.3 V (Figure 4b). They also showed less degradation upon electrochemical cycling. Polymers grown in zone IV did not have this oxidative peak and demonstrated more capacitive behavior, as exhibited by a more “box-like” shape when compared to the other polymers (Figure 4c). The cyclic voltammagram for an ideal redox supercapacitor is a perfect rectangle, where the current remains constant across the entire voltage range. Conversely, the cyclic voltammogram

Figure 4. Cyclic voltammograms of PPy samples grown at (a) 1g, (b) 100g, and (c) 2250g.

for a resistor with no storage capabilities is a straight line. The nanostructured polymers also experienced very little change in electrochemical behavior between cycles, indicating that the redox process was fully reversible. Cyclic voltammograms of polymer films grown at different current densities showed similar behavior. The increase in electrochemical reversibility in the nanostructured polymers may be due to the nanopores, allowing the polymer to tolerate volumetric changes during redox processes. It was observed that some tears had formed in polymer films grown in zones I and II during the cycling process. Polymers grown in zone IV (nanostructured polymers) were more 4808

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Figure 5. (a) At normal gravitational acceleration (1g), PPy is synthesized on the WE with nearby micelles. (b) As the centrifugal force is increased, the concentration of micelles near the WE increases and they act as soft templates. (c) At high centrifugal fields (>1000g), there is a dense layer of micelles near the WE, resulting in a nanostructured surface. (d) Adding a catalyst also allows for increased PPy synthesis around micelles near the WE, yielding polymers with similar morphologies as in panel b.

amphiphilic dopants, such as cetryltrimethylammonium bromide, poly(ethylene glycol), and perfluorooctanesulfonate, among others.11,12,21 At normal gravitational accelerations and in the absence of an oxidizing catalyst, PPy is synthesized electrochemically on the WE with few nearby micelles (Figure 5a). We propose that when a centrifugal force is applied, the micelles are pushed toward the WE. When a potential is applied between the WE and CE, PPy is polymerized with these soft templates accumulating on the surface (Figure 5b). As the centrifugal force is increased, the concentration of micelles near the surface of the WE is further increased, resulting in a highly rough surface (Figure 5c). It is well-known from sedimentation theory (derived by balancing the buoyant force, centrifugal force, and Stokes’ drag) that the settling velocity of small spherical particles in a fluid is dependent upon the size and density of the particle

durable and did not exhibit any tears after cycling. It is possible that the existence of nanopores allowed for the mechanical swelling and contraction of the film upon the influx and efflux of ions during the redox process, while films without nanopores experienced too much mechanical stress during cycling. Switching the location of the WE and CE (such that the centrifugal forces were perpendicular to and directed away from the WE) resulted in films with no nanostructuring (Figure 2f). The polymers had the cauliflower morphology typical of films grown at 1g. Additionally, placing the electrodes in a vertical configuration (such that the electrodes were parallel to the direction of the gravitational field) resulted in no nanostructuring. The PPy films grown in the vertical configuration also had the cauliflower morphology typical of films grown at 1g (see the Supporting Information). Proposed Deposition Mechanism. From the results, we hypothesize that increasing the centrifugal force on the deposition cell results in the accumulation of micelles at the WE that act as a soft template. It is well-known that surfactants can form micelles that act as templates for synthesizing porous materials.19 It has also been shown that using a surfactant as the dopant for a conducting polymer results in the formation of micelles composed of the dopant, the dopant and the monomer, and even the monomer itself in the deposition solution.20 The addition of an oxidizing catalyst causes chemical synthesis of PPy around micelles, resulting in the formation of nanostructures. This phenomenon has been observed with

2rp2(ρp − ρf )ω2r v= 9μ

where v is the settling velocity, rp is the radius of the particle, ρp is the density of the particle, ρf is the density of the fluid, ω is the angular velocity of the centrifuge (in rad/s), r is the distance of the particle from the axis of rotation, and μ is the fluid velocity. As the angular velocity of the centrifuge (and, correspondingly, the applied centrifugal force) is increased, smaller and less dense micelles are able to reach the WE during the 4809

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material is available free of charge via the Internet at http:// pubs.acs.org.

deposition period. Thus, as the centrifugal force is increased, smaller micelles are pushed toward the WE, leading to the formation of nanostructures that are more densely packed (and, hence, smaller pores). SEM images of the cross-section of the nanostructured film illustrate the sedimentation theory. Figure 6 shows larger polymer

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded in part by the Institute of Soldier Nanotechnologies supported by the U.S. Army Research Office under Grant W911NF-07-D-0004, T.O.4., by the Center for Materials Science and Engineering (CMSE) Research Experience for Undergraduates Program, as part of the Materials Research Science and Engineering Center (MRSEC) Program of the National Science Foundation under Grant DMR-08-19762, and by the MIT Materials Processing Center. The authors also thank Dr. Cathy Hogan for her aid with the centrifugation, Adam Wahab for his help with the electronic circuit design, and the rest of the members of the BioInstrumentation Laboratory at MIT for their advice and support with this project.

Figure 6. SEM image of film cross-section (grown at 2250g).

globules near the bottom of the film and smaller nanometer-sized features near the top of the film. This is to be expected, because larger micelles will be deposited before smaller micelles. This mechanism is similar to the mechanism by which rough hierarchial PPy surfaces are grown by adding a catalyst (such as FeCl3 or APS) into an electrochemical deposition. The catalyst allows for the increased chemical synthesis of PPy around micelles near the WE, resulting in a roughened surface (Figure 5d).12 Instead of using a chemical catalyst, the “catalyst” in this case can be thought of as the centrifugal force. It is important to note that the micro- and nanostructuring phenomena occur only with amphiphilic dopants that are able to form micelles in the deposition solution. There was no nanostructuring observed for polymers grown with a deposition solution that contained a salt that was not amphiphilic (e.g., tetrabutylammonium hexafluorophophate). However, similar surface morphologies were observed for polymers grown with potassium nonafluorobutanesulfonate (see the Supporting Information). It is possible that previous works13−16 have not presented the micro- and nanostructuring phenomena because of the type of dopant that was used.

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CONCLUSION We have presented a novel method of creating self-assembled nanostructured PPy surfaces. Subjecting the electrochemical deposition cell to a centrifugal force causes micelles of the monomer and dopant to congregate at the surface of the WE and act as soft templates during polymerization. This method consistently yields high surface area polymers with improved electrochemical characteristics, which can be used in a wide variety of applications. The extent of the nanostructuring can be tuned by selecting the magnitude of the applied centrifugal force.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

SEM micrographs of films grown with a current density of 0.5 A/m2, submitted to different centrifugal forces (Figure S-1), SEM micrographs of films grown in the vertical configuration with electrodes parallel to the direction of the centrifugal field (Figure S-2), SEM micrographs of films grown with potassium nonafluorobutanesulfonate as the dopant (Figure S-3), and schematic of the constant-current circuit (Figure S-4). This 4810

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