Dopant-Enabled Supramolecular Approach for Controlled Synthesis of

Letter pubs.acs.org/NanoLett

Dopant-Enabled Supramolecular Approach for Controlled Synthesis of Nanostructured Conductive Polymer Hydrogels Yaqun Wang,†,‡ Ye Shi,‡ Lijia Pan,*,† Yu Ding,‡ Yu Zhao,† Yun Li,† Yi Shi,*,† and Guihua Yu*,‡ †

Collaborative Innovation Center of Advanced Microstructures, Jiangsu Provincial Key Laboratory of Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China ‡ Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Conducting polymer hydrogels emerge as a novel class of polymeric materials that show great potential in many energy, environmental, and biomedical devices. We describe here for the first time a general supramolecular approach toward controlled in situ synthesis of one-dimensional nanostructured conductive hydrogels (polypyrrole (PPy) as a model system) using a rational dopant counterion, which is a disc-shaped liquid crystal molecular copper phthalocyanine3,4′,4″,4‴-tetrasulfonic acid tetrasodium salt (CuPcTs). The dopant molecule CuPcTs cross-linked the PPy chains to form a three-dimensional network that gelated into a hydrogel. The PPy hydrogel could be synthesized in bulk quantities with uniform morphology of self-assembled interconnected nanofibers. The tetrafunctional dopant favors a supramolecular self-assembly mechanism to form onedimensional PPy nanostructures. Furthermore, the enhanced interchain charge transport of CuPcTs doped PPy resulted in greatly enhanced conductivity and pseudocapacitance compared with pristine PPy. KEYWORDS: Supramolecular, one-dimensional, nanostructured polymers, hydrogel, polypyrrole, conductive polymers In this Letter, we report for the first time a novel yet general supramolecular strategy to prepare morphology-controlled nanostructured conductive hydrogels (with polypyrrole (PPy) as a model system) using a dopant counterion of a disc-shaped liquid crystal, copper phthalocyanine-3,4′,4″,4‴-tetrasulfonic acid tetrasodium salt (CuPcTs). The CuPcTs acts as both the dopant and gelator to self-assemble the PPy into nanostructured hydrogels. Through electrostatic interaction and hydrogen bonding between the tetra-functional CuPcTs and PPy chain, a self-sorting mechanism acts to align the PPy chains to form 1D nanostructured PPy, as shown in Figure 1. Our approach offers a method of in situ generating hierarchically interconnected nanofiber monolithic, which is distinct from other syntheses of conducting polymer nanofibers.19 It is necessary to point out that although PAni was proved to intrinsically favor its growth into fiber-like structures,21 PPy tends to grow into nanoparticles due to preferred formation of branched and cross-linked chains.22 Our supramolecular selfassembly strategy exhibits several advantages, proving to be a facile approach to prepare nanostructured PPy hydrogels with tunable morphology by changing either the dopant used or its concentration. As comparison, our previous report of phytic acid doped PAni does not have the morphological tunability by

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onducting polymer hydrogels represent a novel polymeric material platform that synergizes the advantages of both organic conductors and hydrogels.1−5 They exhibit great potential in energy devices, biosensors, and medical electrodes because they combine several advantageous features, such as high conductivity, high water content/porous, biocompatibility, hierarchical interconnected micro/nanostructure, and high permeability to ions and molecules due to their hydrophilic nature and solvated surface.3 Conducting polymer hydrogels offer excellent processability to form nanostructured films by solution processing and can be micropatterned by inkjet printing or masked spray.6−8 Recently, nanostructured polyaniline (PAni) hydrogels were used as a structural template for fabrication of highly graphitic and porous carbon with ultrahigh specific surface for energy generation and storage, respectively.9−11 Generally, applications involving the structurerelated properties of nanostructured materials call for scalable and effective method for morphologically tunable synthesis. Supramolecular self-assembly provides a powerful tool that allows the design of functional nanomaterials via noncovalent interactions.12−14 Molecular scale design of conducting polymer hydrogels will be essential for morphological control and realizing specific applications. Among various kinds of nanostructures, one-dimensional (1D) nanofiber-like structures have attracted a great deal of attention because they represent the smallest structures that can still provide efficient charge and energy transport.15−20 © XXXX American Chemical Society

Received: September 24, 2015 Revised: October 11, 2015

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Figure 1. Illustration depicting controlled synthesis of the CuPcTs doped PPy hydrogel. The CuPcTs act as both the cross-linker and protonic dopant for the PPy hydrogel and align the PPy chains into ordered nanostructure through a steric effect. Note that the special disc-shaped structure and the tetra-sulfonic acid functional groups enabled the CuPcTs for a supramolecular self-sorting mechanism that align the PPy chains to form 1D nanostructured PPy.

Figure 2. (a) FTIR spectra of CuPcTs-doped PPy nanofiber sample (PPy−CuPcTs) and granular PPy sample (PPy). (b) XRD of CuPcTs-doped PPy nanofiber sample (PPy−CuPcTs), granular PPy (PPy) sample, and CuPcTs-only sample (inset, deconvoluted XRD of PPy−CuPcTs sample).

solution of an oxidative initiator and CuPcTs (see Supporting Information for details). As the polymerization of pyrrole progressed, the color of the solution changed from blue (the color of the CuPcTs) to black (the color of PPy) within a few seconds. The mixed solution gelated to form a hydrogel, losing its fluidic properties in approximately 2 min. The PPy hydrogel was rinsed with excess deionized (DI) water to remove residual ions and oligomers. The water content of the PPy hydrogel was measured to be as high as 94% (w/w) by lyophilization.

concentration of dopant, as well as phytic acid is an insulator in solid state.7 Moreover, the dopant CuPcTs enhanced the interchain charge transport of PPy and resulted in much improved conductivity (two orders increase) and electrochemical capacitance compared with pristine PPy because copper phthalocyanine molecule is a good organic semiconductor. In a typical synthesis, two precursor solutions were prepared and mixed: a pyrrole solution in isopropanol, and an aqueous B

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Figure 3. SEM images of nanostructured PPy hydrogels with various morphologies prepared by chemical oxidative polymerization with different concentrations of CuPcTs dopant: (a) 0.015 M CuPcTs; the CuPcTs concentration in (b−f) is 1/2, 1/4, 1/8, 1/16, and none that of sample (a), respectively. Scale bar: 1 μm (inset, the diameter distribution of the PPy−CuPcTs samples).

consisted of interconnected nanofibers with diameter of ∼60 nm (Figure 3a). The PPy−CuPcTs hydrogel can form largearea thin films with interconnected nanofibers rather than agglomerated bundles (Supporting Information Figure S2). This type of interconnected nanoscale conducting matrix offers a greater effective surface area and porosity compared with those of agglomerated nanoparticles and may facilitate the transport of electrons and ions. In contrast, the control sample synthesized without the CuPcTs consisted of only granular particles (Figure 3f). The morphologies of the CuPcTs-PPy hydrogels collected at different polymerization times were investigated (Supporting Information Figure S3). The CuPcTsdoped PPy tended to form nanofibers, and the size of the nanofibers increased with increasing polymerization time. The diameter of the nanofibers increased from 30 to 60 nm when the polymerization time was increased from 15 to 30 s. Note that this general synthetic route can be extended to synthesize PAni and poly(3,4-ethylenedioxythiophene) (PEDOT) hydrogels with interconnected nanofiber morphologies (Supporting Information Figure S4). The CuPcTs dopant played an indispensable role in the supramolecular self-assembly of PPy, as revealed by the relationship between the morphology of the PPy hydrogel and the concentration of CuPcTs (Figure 3). The diameter of the PPy nanofibers increased with decreasing CuPcTs concentration. The average diameter of the fibers increased to approximately 400−500 nm and some granular particles appeared on the nanofiber surface when the concentration of the CuPcTs was decreased to 0.45 mM. As indicated by EDS spectrum, the concentration of CuPcTs in the PPy samples was proportional to the concentration of the CuPcTs in the precursor solution, revealing the counterion of CuPcTs played an important role in the supramolecular self-assembly and morphologic tuning of PPy. We deduced that the rigid disc shape and the sulfonic acid functional groups of the CuPcTs favor for a supramolecular self-sorting mechanism that align the PPy chains to form 1D nanostructured PPy, as shown in Figure 1. Moreover, pristine PPy tends to adopt a granular morphology because of the formation of side chains and cross-linked structures between the PPy main chains, which

Figure 2 shows the Fourier transform infrared (FTIR) spectra of as-synthesized samples (both CuPcTs-doped and pristine PPy), where the absorption peak at 1548 cm−1 can be assigned to the in-plane bending of CN bonds and the 1472 cm−1 is due to the stretching vibrations of CC bonds.23 The peak around 1340 cm−1 (D-band) is related to the ring stretching mode, while the 1592 cm−1 (G-band) is due to the CC backbone stretching of PPy.24 The 1297 cm−1 was attributed to the C−C stretching. The in-plane deformation of the C−H bond and N−H bond showed its characteristic peak at 1042 cm−1. The peak at 1174 and 903 cm−1 attributed to the stretching vibrations of C−N+ bonds and CN+−C bonds, respectively.23 The absorption of these two peaks of PPy− CuPcTs has a blue shift in contrast to that of the PPy without being doped, indicating the PPy−CuPcTs is in a doped state (Figure 2a). The energy-dispersive (EDS) spectrum of the PPy nanofibers revealed that the copper concentration arising from the CuPcTs in the PPy hydrogel was proportional to the concentration of the CuPcTs in the precursor solution (Supporting Information Figure S1). The X-ray diffraction (XRD) patterns revealed the amorphous nature of the pristine PPy and PPy−CuPcTs (Figure 2b). A characteristic broad peak of PPy is centered at approximately 22.8°, and the high angle asymmetric scattering peak of PPy−CuPcTs from 20 to 30°, which implies that the polymer is oriented along the molecular-chain direction.25 This peak of PPy−CuPcTs could be deconvoluted to two separate peaks situated at ∼22° and 27° (Figure 2b), giving d-spacing of 0.404 and 0.330 nm, respectively. The peak at 22° is attributed to the π−π stacking between polypyrrole units. The intensity of high angle peak at 27° depends on the dopant, and it is related to the interplanar distance of pyrrole−counterions.26 The peak at 10.7°, which is associated with crystalline CuPcTs, disappeared in the PPy curves, implying the CuPcTs reorganized in the PPy matrix. Moreover, a peak at ∼0.8° showed up for PPy−CuPcTs only, indicating the long-range ordering structure of PPy−CuPcTs sample (Supporting Information Figure S1). The morphology of the PPy hydrogel was investigated by scanning electron microscopy (SEM). The CuPcTs-doped PPy C

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Figure 4. SEM images of different nanostructured PPy hydrogels with various dopants: (a) CuPcTs as dopant, (b) indigo carmine as dopant, and (c) isatin-5-sulfonic acid sodium salt dehydrate as dopant. Scale bar: 1 μm. The morphology of the as-synthesized PPy hydrogels changed from 1D nanofibers, a nanoparticle necklace (quasi-1D), to granular particles in the presence of different dopant molecules, corresponding to the change of steric effect from strong to weak between the dopants and PPy chains, respectively.

hinders the 1D growth of PPy.27 Notably, the relatively large size of the disc-shaped CuPcTs may sterically hinder the formation of these cross-linked structures and enhance the 1D growth of PPy. To further investigate the effect of the dopant molecule on the supramolecular self-assembly of the PPy hydrogels, we chose indigo carmine and indigo carmine dehydrate as the control dopants (Figure 4); these dopants have molecular structures similar to a half and a quarter of CuPcTs molecule, respectively. As shown in Figure 4a−c, the morphology of the as-synthesized PPy hydrogels changed from 1D nanofibers, to a nanoparticle necklace (quasi-1D), to granular particles in the presence of these dopant molecules. When the disc-shaped CuPcTs was used as the dopant, the PPy hydrogel exhibited a smooth nanofiber morphology. When the binary-functional sulfonic acid group of indigo carmine was used, the PPy hydrogel consisted of agglomerated bundles of nanofibers with particles present on the surface of the nanofibers. This observation indicates that the binary-functional sulfonic acid group still favors the alignment of the PPy chains to form a PPy 1D nanostructure through a steric effect (Figure 1) but is less effective that some particles were produced. The PPy doped using the isatin-5-sulfonic acid sodium salt formed a granular nanostructure with a morphology similar to that of the PPy synthesized without dopants (Figure 4c). This similarity implies that the single sulfonic acid group molecule does not exert the same steric effect as that CuPcTs, and isatin-5-sulfonic acid sodium salt act on the supramolecular self-assembly. The diameter of the CuPcTs-PPy nanofibers can be tuned using different oxidative initiators. PPy appeared to form a nanofiber morphology irrespective of the oxidative initiator used in the polymerization, including ammonium persulfate (APS), iron(III) chloride, iron(III) sulfate, and iron(III) nitrate (Supporting Information Figure S5). The average diameter of the nanofibers produced with APS was about 60 nm (Figure 3a), whereas those produced with iron(III) chloride (Support-

ing Information Figure S5), iron(III) nitrate, and iron(III) sulfate were approximately 42, 40, and 38 nm, respectively. The conductivities of the samples were measured by standard four probe method and comparably listed in Table 1. The Table 1. Measured Conductivities of Different PPy Samples with Different Dopants samples conductivity (S/cm)

PPy−CuPcTs

PPy−indigo

PPy−isatin

pristine PPy

7.8

0.4

0.06

0.07

improved ordering of the unidirectional polymer chains resulted in greatly increased conductivity. The conductivities of CuPcTs−PPy, indigo carmine−PPy, isatin-5-sulfonic acid sodium salt dehydrate−PPy, and pristine PPy were measured to be 7.8, 0.4, 0.06, and 0.07 S·cm−1, respectively. Note that these samples were synthesized in solutions with similar pH values. The results confirmed that the CuPcTs dopant molecule favors for the enhancement of interchain charge transport between conducting polymer chains.28 To evaluate the electrochemical characteristics of the PPy hydrogel, we performed a combined measurement using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) in a conventional three-electrode system. Figure 5a shows impedance curves of electrodes containing CuPcTs-doped PPy hydrogel and pristine PPy; the curves were measured with the electrodes immersed in a 1 M H2SO4 electrolyte. The equivalent series resistance (ESR) extracted from high-frequency (100 kHz) data indicated that the impedance of the CuPcTs−PPy hydrogel is substantially smaller than that of the pristine PPy. This smaller impedance of the CuPcTs−PPy hydrogel stems from the alignment and orientation of the PPy chains in the hydrogel’s nanofibers. The nearly vertical shape of the obtained curve at lower frequencies indicates an ideal capacitive behavior of the CuPcTs−PPy hydrogel. CuPcTs−PPy exhibited greatly enhanced electroD

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Figure 5. Electrochemical characteristics of different PPy samples showing greatly enhanced electrochemical activity of CuPcTs-doped PPy compared with pristine PPy. (a) Electrochemical impedance plot (inset, zoom in of the plot), (b) cyclic voltammogram (scan rate: 10 mV/s), (c) charge−discharge curves (current density: 0.2 A/g), and (d) specific capacitance as a function of current density for CuPcTs−PPy and pristine PPy.

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chemical activity compared with that of pristine PPy, as indicated by the boosted redox peak of CuPcTs-PPy compared with pristine PPy shown in Figure 5b. Notably, the CuPcTsPPy exhibits a higher conductivity, 1D morphology, and more porous structure than the pristine PPy, which favors the transfer of electrons and ions. Figure 5c shows the galvanostatic charge/ discharge curves of the CuPcTs−PPy and pristine PPy electrodes at the same current density. The specific capacitance of the CuPcTs-doped PPy was calculated to be as high as ∼400 F·g−1 at 0.2 A·g−1, whereas that of pristine PPy was only 232 F· g−1 (Figure 5d). The rate performance of CuPcTs−PPy is also shown to be higher than that of pristine PPy. In summary, we demonstrated a supramolecular selfassembly approach to controllably synthesize nanostructured conductive hydrogels using a disc-shaped liquid crystal molecular CuPcTs as a dopant and cross-linking gelator. The steric and electrostatic interactions between CuPcTs and PPy favors the self-assembly of PPy chains, which promotes the 1D growth of PPy and results in the formation of interconnected nanofibers. We rationally tuned the morphology and structural parameters of the PPy hydrogels by using different oxidative initiators, varying the dopant concentration, or changing the dopant molecules. This supramolecular self-assembly method could be extended to the synthesis of other nanostructured conducting polymer hydrogels such as PAni and PEDOT. This general supramolecular self-assembly strategy with tunable morphology could be used to design and produce conducting polymer hydrogels with low-dimensional structures as material platforms for applications based on various structure-related properties, such as energy conversion and storage materials, sensors, catalysts, and actuators.29

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b03891.

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Detailed experimental procedures and supplementary characterization methods including angle XRD, SEM, and TGA of the conducting polymer (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

Y.W. and Y.S. equally contributed to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.Y. acknowledges the financial support from the National Science Foundation award (NSF-CMMI-1537894) and 3M Nontenured Faculty Award. L.P. and Y.S. are thankful for financial support from Chinese National Key Fundamental Research Project (2013CB932900, 2011CB92210), National Natural Science Foundation of China (41401257, 61229401, 61076017, and 60990314), Natural Science Foundation of Jiangsu Province of China (BK20141054), and Programs of NCET and the PAPD. E

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