Controllable Growth of Conducting Polymers Shell for Constructing

Aug 26, 2013 - Guofa Cai , Jiangping Tu , Ding Zhou , Lu Li , Jiaheng Zhang , Xiuli Wang , and Changdong Gu. The Journal of Physical Chemistry C 2014 ...
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Controllable Growth of Conducting Polymers Shell for Constructing High-Quality Organic/Inorganic Core/Shell Nanostructures and Their Optical-Electrochemical Properties Xinhui Xia,†,§ Dongliang Chao,† Xiaoying Qi,‡ Qinqin Xiong,§ Yongqi Zhang,§ Jiangping Tu,§ Hua Zhang,‡ and Hong Jin Fan†,* †

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore § State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: High-quality metal oxide/conducting polymer (CP) heterostructured nanoarrays are fabricated by controllable electrochemical polymerization of CP shells on preformed metal oxides nanostructures for both electrochromic and electrochemical energy storage applications. Coaxial and branched CP shells can be obtained on different backbones (nanowire, nanorod, and nanoflake) simply by controlling the electrodeposition time. “Solvophobic” and “electrostatic” interactions are proposed to account for the preferential growth of CP along metal oxides to form core/ shell heterostructures. The coaxial TiO2/polyaniline core/shell nanorod arrays exhibit remarkable electrochromic performance with rich color changes, fast optical modulation, and superior cycling stability. In addition, the Co3O4/polyaniline core/shell nanowire arrays are evaluated as an anode material of Li ion battery and exhibit enhanced electrochemical property with higher and more stable capacity than the bare Co3O4 nanowires electrode. These unique organic−inorganic heterostructures with synergy pave the way for developing new functional materials with enhanced properties or new applications. KEYWORDS: Metal oxides, conducting polymer, core/shell, nanowire arrays, electrochromic, lithium ion battery

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chemical capacity/capacitance and high working efficiency for these devices. The mainstream routes for the construction of high-performance oxides/CP composites can be classified to two: (1) Creation of highly porous or hollow nanostructure. The open porous structure is favorable for easy electrolyte penetration and thus sufficient contact between active materials and electrolyte, leading to more efficient ion/electron transports. This is also the case for a nanowire or nanorod array structure, whose vertical alignment makes itself an effectively porous film. (2) Direct growth of CP and metal oxides on conductive substrates. This direct growth ensures a good mechanical adhesion and electric connection of the active material to the current collector. Moreover, a nanowire array can more effectively accommodate the strain as compared to continuous films. On the basis of above considerations, it is found that core/ shell heterostructured nanoarray architecture can combine all

ybrid inorganic−organic materials represent the natural interface between two worlds of chemistry each with very significant contributions to the field of materials science, and each with characteristic properties that result in distinct advantages and limitations.1 Typically, a judicious combination of conducting polymers (such as polyaniline and polythiophene) with selected metal oxides with tailored structures is fascinating due to their underlying synergistic effect or added functionalities.2−6 Construction of high-performance metal oxide/conducting polymer (CP) composites entails both opportunities and challenges. The main challenge is to controllably synthesize metal oxide/CP composite with bespoke architectures for developing advanced functional materials with enhanced device performance or new useful properties.7−11 Over recent years, free-standing oxide/CP composites with oriented open nanostructures have been highly desirable for sophisticated modern optical, electronic, and electrochemical devices including photovoltaic cells, batteries, supercapacitors, and displays.3,10,12−14 The high surface area and high porosity associated with the open nanostructures, as well as their favorable orientations, usually translate into high electro© XXXX American Chemical Society

Received: July 24, 2013 Revised: August 23, 2013

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Figure 1. (a) Schematics of the fabrication of metal oxide/CP core/shell nanoarrays, exemplified by Co3O4 and PANI; (b−d) Top-view and profile SEM images of coaxial Co3O4/PANI core/shell nanowire arrays (fine structure and cross-sectional image in inset); (e) TEM image of one coaxial Co3O4/PANI core/shell nanowire (low-magnification image and SAED pattern in inset); (f) FTIR spectra of both pure Co3O4 and coaxial Co3O4/ PANI core/shell nanowires; (g) EDS maps of O, Co, C, and N.

tedious and involved a long reaction time of 24 h. On the whole, it is still a challenge to fabricate metal oxide/CP core/ shell nanoarrays with a good morphology control and function diversity. CP is a unique class of conjugated polymer with interesting optical properties due to its multiple redox states accompanied by rich color changes, which make it suitable for electrochromic applications in displays and smart windows. Unfortunately, the rate of interconversion between redox states is limited by the slow transport of counter ions into the electrochromic layer to balance charges. This bottleneck can be circumvented by integrating CP into self-supported core/shell nanoarrays, which combine the merits of high surface area and short diffusion distances for ion/electron transport leading to fast reaction kinetics with fast switching speed.

these advantages and fulfill the requirements for designing highperformance metal oxide/CP composites electrode materials for advanced optical and electrochemical devices. In fact, core/ shell nanostructures have been a hotspot in the field of nanomaterial science. Various materials such as metal/ metal,15−18 semiconductor/semiconductor,19−21 metal oxide/ conducting polymer,22,23 metal/metal oxide,24,25 and oxide/ oxide,26−28 have been explored as core/shell nanostructures and demonstrated improved properties. However, there are only a few reports on metal oxide/CP core/shell nanoarrays, which were realized with the aid of sacrificial templates (anodic aluminum oxide) and anodic TiO2 nanotubes.22,29−32 However, the sacrificial template method is not applicable to prepare core/shell nanoarrays with hierarchical porous structure. Liu et al. adopted a chemical oxidation method to assemble polypyrrole (PPY) on CoO,23 but the reaction process was B

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in complete agreement in showing the formation of coaxial Co3O4/PANI core/shell nanowire arrays.

In the present work, we report a facile strategy for the construction of self-supported hierarchical metal oxide/CP core/shell nanoarrays with adjustable components and heterostructures on different conductive substrates (carbon cloth, FTO glass, and nickel foam). Demonstrated examples include different nanoarray cores (nanowire, nanorod, and nanoflake) of metal oxides (Co3O4, TiO2, and NiO) and different conducting polymer (polyaniline, PANI, and poly(3,4ethylenedioxythiophene, PEDOT) shells with a hierarchical and porous morphology. The growth mechanism and morphology evolution of the conducting polymer shells are discussed. Given the unique compositions and spatial characteristics of metal oxide/CP core/shell nanoarrays, they are expected to show great promise for applications in optical display, electrochemical energy storage, catalysis, and gas sensing. We first present the results of Co3O4/PANI and TiO2/PANI core/shell nanoarrays grown on carbon cloth as the representative for characterization. Results of PEDOT and on FTO substrates are illustrated in Supporting Information. Figure 1a illustrates the growth of two kinds of Co3O4/PANI core/shell nanowire arrays on carbon cloth. The first step is to prepare Co3O4 nanowire array backbone by hydrothermal method. Next PANI shell is directly assembled on the Co3O4 nanowire surface by electrochemical polymerization. The crystalline Co3O4 nanowire is composed of nanoparticles of ∼5 nm and has an average diameter of ∼80 nm (Supporting Information Figure S1a−c). After electrochemical polymerization for 500 s at 2.5 mA cm−2, a thin PANI film is uniformly coated on each individual Co3O4 nanowire, forming coaxial Co3O4/PANI core/shell nanowire arrays (Figure 1b,c). This coaxial core/shell nanowire heterostructure can be clearly revealed by the sectional image (Figure 1d) and TEM image (Figure 1e). The surface of the coaxial Co3O4/PANI core/shell nanowire becomes much smoother than the bare Co3O4 nanowires. The selected area electronic diffraction (SAED) patterns in the inset reveal the crystalline Co3O4 phase (JCPDS 42-1467) after electropolymerization; no obvious diffraction rings of PANI are observed (Supporting Information Figure S1c,e), indicating the amorphous nature of PANI deposited by the electrodeposition, as supported by the XRD results (Supporting Information Figure S2a,b). In addition, we conducted FTIR (Figure 1f) measurement to further check the components of the coaxial core/shell nanowires. For bare Co3O4 nanowire, only two strong peaks centered at 668 and 579 cm−1 characteristic of spinel Co3O4 phase are observed. For the coaxial Co3O4/PANI nanowires, in addition to the peaks of Co3O4 several characteristic peaks of PANI are noticed. The broad band at 3400 cm−1 is attributed to the stretching vibration N−H of an aromatic amine as well as the stretching vibration of absorbed water. Two bands at 1578 and 1487 cm−1 are due to the CC stretching vibrations of quinoid (Q) ring and benzenoid ring (quinoid ring and benzenoid ring are the basic molecular units of PANI), respectively.33 The band at 1306 cm−1 belongs to the C−N stretching mode of an aromatic amine. The typical NQN stretching band of PANI is at 1130 cm−1.34 The presence of PANI is also confirmed by the energy dispersive X-ray spectroscopic (EDS) analysis (Supporting Information Figure S2c). Moreover, this unique coaxial core/shell nanowire heterostructure is also clearly distinguished by EDS elemental maps (Supporting Information Figure S2d) of Co, O, N, and C from the designated area (Figure 2g). Therefore, all results are

Figure 2. Branched Co3O4/PANI core/shell nanowire array obtained after a longer electrodeposition. (a−c) Top-view and profile SEM images (fine structure and cross-sectional image in inset); (d) TEM image of one branched Co3O4/PANI core/shell nanowire (lowmagnification image and SAED pattern in inset).

Interestingly, in our experiment the morphology of PANI shell can be tuned from coaxial shell to branched shell by elongating the electrodeposition time. The above smooth shell is obtained from 500 s electrodeposition. When the time increases to 1500 s, the surface roughness increases and PANI nanospikes of ∼15 nm in average diameter are formed resulting in a branched Co3O4/PANI core/shell nanowire structure (Figure 2). The PANI nanospikes are also amorphous according to the SAED pattern (Figure 2d). TEM image shows that the branched core/shell nanowires have a total diameter of up to ∼300 nm. Our results show that the PANI preferentially nucleates and grows along the Co3O4 nanowire template. The formation process of the PANI shell is proposed to be related to the solvophobic and electrostatic interactions.35−37 For the first factor, it is well-known that the reactant monomer (aniline) is soluble in solution, but the product PANI is completely insoluble. In this case, the Co3O4 nanowires in our experiment provide the substrate for heterogeneous nucleation of the solvophobic PANI. As for the second contribution, an electrostatic interaction may exist between the Co3O4 nanowire surfaces and the nascent PANI, which is cationic. It is inferred that the anionic sites on the Co3O4 surface may act as the anchors for the nascent polymers to bind due to a columbic interaction between the Co3O4 and the heterocyclic polycation, resulting in a preferential nucleation on the oxide surface. These two factors work together and contribute to the preferential nucleation and growth of PANI along the oxide nanostructure surfaces to form a uniform coating. At the initial stage, the PANI nucleates fast on the Co3O4 nanowires and protects the latter from dissolving in the acid reaction solution. The homogeneous nucleation provides centers to minimize the interfacial energy barrier for the subsequent growth of PANI, leading to the coaxial oxide/PANI core/shell structures. When the electrodeposition time increases, some reactive aniline cation-radicals and oligomeric intermediates are consumed resulting in the suppression of the growth rate of PANI. This C

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will interrupt the growth homogeneity and lead to the nanospike structure of PANI,38,39 as observed in Figure 2. Our developed synthesis strategy is powerful and general and can be easily extended to fabricate other metal oxide/CP core/ shell nanoarrays by simply choosing different metal oxide backbones (such as nanowire, nanorod, and nanoflake) and CP shell. To demonstrate the versatility, we also fabricated coaxial and branched TiO2/PANI core/shell nanorod arrays, coaxial and branched Co3O4/PANI core/shell nanoflake arrays, NiO/ PANI core/shell nanoflake arrays, and Co3O4/PEDOT core/ shell nanowire arrays. Figure 3 and Supporting Information

Figure 4. Formation of conducting polymer shells on 2D nanoflake arrays. SEM images of (a) porous Co3O4 nanoflake arrays, (b) coaxial Co3O4/PANI core/shell nanoflake arrays, and (c) branched Co3O4/ PANI core/shell nanoflake arrays. SEM images of (d) porous NiO nanoflake arrays, (e) coaxial NiO/PANI core/shell nanoflake arrays, and (f) branched NiO/PANI core/shell nanoflake arrays.

preparation condition. Supporting Information Figure S5 shows the example of obtained Co3O4/PEDOT core/shell nanowire arrays. Taking all, it is concluded that the developed methodology is general and powerful for preparing various metal oxide/CP core/shell nanoarrays with tunable heterostructures, components, and morphologies. As the growth of the CP shells is based on electrodeposition, the working electrode must be electrical conductive. The details about the electrical conductivity of these metal oxide nanoarrays grown on conductive substrates are provided after Figure S5 in the Supporting Information. We would like to highlight the unique aspects of our results as follows: (1) Both coaxial and branched metal oxide/CP core/shell heterostructures are realized via specific control of electrodeposition. (2) Shell and core components can be combined by choosing different CP (PANI, PEDOT, etc.) shells and metal oxide backbones (Co3O4, TiO2, NiO, etc.). The method appears to be little dependent on the substrate morphology and therefore works well with 1D nanowire, nanorod, and 2D nanoflakes. (3) A direct deposition of the CP on planar substrate such as FTO results in only a nonuniform low coverage and a weak adhesion of the polymer structures to the substrate. A nanorod template facilitates the polymer nucleation and also mechanical support. (4) The unique architecture and material combination will make the 3D heteronanostructure arrays useful in various applications, as to be demonstrated below.

Figure 3. TiO2/PANI core/shell nanorod arrays. (a,b) SEM and TEM images of TiO2 nanorods (low-magnification TEM and SAED pattern in inset); (c,d) SEM and TEM images of coaxial TiO2/PANI core/ shell nanorod arrays (low-magnification TEM and SAED pattern in inset); (e,f) SEM and TEM images of branched TiO2/PANI core/shell nanorod arrays after extended electrodeposition (low-magnification TEM and SAED pattern in inset).

Figure S3a−c show the results of TiO2/PANI core/shell arrays on carbon cloth via the similar polymer assembly on the TiO2 nanorod arrays. The nanorod diameter increases from 250 to 350 and finally 550 nm after polymer growth, and the morphology evolution of PANI shell is found rather reproducible. The PANI nanospikes are clearly distinguished especially at the later stage of electrodeposition (Figure 3e,f). The presence of PANI and its amorphous nature is confirmed by comparing the SAED patterns of nanorods in the inset at different stages (Figure 3b,d,f), supported by the XRD (Supporting Information Figure S3d) and FTIR (Figure 3e) results. The core/shell heterostructure is also obviously shown in EDS mapping (Supporting Information Figure S4). In another example, porous Co3O4 or NiO nanoflake arrays are selected as the backbone for the subsequent electrodeposition of smooth or branched PANI shells (Figure 4). Furthermore, the CP shell can be changed to PEDOT under similar D

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Figure 5. Electrochromic characterizations of coaxial TiO2/PANI core/shell nanorod arrays grown on FTO substrate: (a) CV curve in the potential range from −0.2 to 1 V at a scanning rate of 50 mV s−1; (b) transmittance spectra of coaxial TiO2/PANI core/shell nanorod arrays under different applied potentials; (c) photographs of sample with a size of 3 × 3 cm2 under different potentials; (d) chronoamperometric curve by voltage switch between −0.2 and 0.8 V; (e) peak currents evolution of coaxial TiO2/PANI core/shell nanorod arrays during the step chronoamperometric cycles. (f) SEM image of coaxial TiO2/PANI core/shell nanorod arrays after 5000 cycles.

array support, we could not obtain uniform films of PANI onto bare FTO substrates due to poor adhesion but only some random unbonded microparticles (Supporting Information Figure S7e). This justifies the essential role of TiO2 nanorod array in the electrochromism of PANI films in this work. The electrochromism of PANI is closely related to its unique redox doping/dedoping processes, which can be easily controlled by applying electrical potentials. The typical cyclic voltammetry (CV) curve of the coaxial TiO2/PANI core/shell nanorod arrays on FTO is shown in Figure 5a. The first redox couple A1/C1 corresponds to the change between leucoemeraldine base (LB) and emeraldine salt (ES) with anion doping upon oxidation and dedoping upon reduction, simply expressed as follows40−42

In view of the unique compositions and architectures, the asprepared metal oxide/CP core/shell nanoarrays are anticipated to display promising applications in different areas such as electrochromics, electrochemical energy storage, catalysis, and sensors. Herein, we investigate the electrochromic properties of the coaxial TiO2/PANI core/shell nanorod arrays on FTO substrate for electrochromism. Characterization of the electrochemical properties of the coaxial Co3O4/PANI core/shell nanowire arrays on nickel foam for lithium ion battery application is provided in Supporting Information. The TiO2 nanorods grown on FTO exhibit a diameter of ∼90 nm (Supporting Information Figure S7a,b), smaller than those grown on carbon cloth above. As shown in the top and side view SEM images, the PANI shell confines the TiO2 nanorods to form uniform coaxial core/shell nanorod (diameter of ∼130 nm) arrays with a thickness of ∼600 nm (Supporting Information Figure S7c,d). It is important to note that the TiO2 nanorods act as a strong and stable support for PANI for the electrochromic application. Without the TiO2 nanorod

PANI + nCl− ↔ (PANIn +)(Cl−)n + ne− (LB,yellow)

(ES,green)

(1)

The second redox couple A2/C2 is due to the conversion between emeraldine base (EB) and pernigraniline salt (PS) E

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positively after 5000 cycles. Though the redox potential changes, two obvious redox couples are still observed and the current density maintains ∼94% of the highest value (Figure 5a). The noticeable electrochromic performance of the coaxial TiO2/PANI core/shell nanorod arrays mainly come from the unique core/shell nanorod arrays architecture. (1) Strong mechanical stability and favorably morphological stability. The TiO2 nanorod arrays provide not only a stable mechanical support for the active PANI shell but also a template for homogeneous coverage of PANI (note that a direct electrodeposition of PANI on FTO substrates is unsuccessful). In addition, the mechanical stability of the core/shell nanorod architecture is beneficial to alleviating the structure damage caused by volume expansion during the cycling process, leading to an enhanced cycle life. (2) Highly porous structure and homogeneous thin PANI shell. A nanoporous structure provides large reaction surfaces (Supporting Information Figure S9) and inner space that favor an efficient contact between active materials and the electrolyte. As a result, it provides more active sites for electrochemical reactions as well as shortens the transportation/diffusion path for both electrons and ions, thus leading to fast modulation. Meanwhile, the homogeneous thin PANI shell might also be favorable for a fast counterion transport into the PANI film contributing to enhanced electrochromic performance.8 In summary, we have demonstrated the fabrication of conducting polymer shells and branches onto various metal oxide nanoarrays by a facile and controllable electrochemical polymerization. The fabrication is applicable to different oxide nanostructures including Co3O4, TiO2, and NiO nanowires, rods and flakes. The morphology of CP (PANI and PEDOT) is dependent on the deposition time. Because of their unique composition and architecture, the coaxial TiO2/PANI core/ shell nanorod arrays display interesting electrochromic property with four color modes and fast optical switching speed (1.3 s with switching voltages of 0.8 and −0.2 V). The performance is superior to conventional inorganic electrochromic thin film materials such as NiO and WO3. In addition, Co3O4/PANI core/shell nanowire arrays are also demonstrated as Li ion batteries with enhanced property (Supporting Information). It is optimistic that this robust and general method could enable the fabrication of other oxide/CP heteronanostructures for selected applications in optical coating, electrochemical energy storage, and optoelectronic devices.

with anion doping/dedoping processes, represented by the following reaction40−42 EB + mCl− ↔ (EBm +)(Cl−)m + me− (EB,blue)

(PS,purple)

(2)

The conversion between ES and EB is due to protonation/ deprotonation processes as illustrated as follows40−42 ES ↔ (EB) + nCl− + nH+ (Green)

(Blue)

(3)

The change between ES and EB does not involve electron transfer process, so no redox peak is reflected in the CV curve. Note the fact that no redox peaks of TiO2 are observed in the potential range. The main molecular structures of LB, ES, EB, and PS are shown in Supporting Information Figure S8. The coaxial TiO2/PANI core/shell nanorod arrays show evident electrochromism with rich reversible color changes ranging from yellow, green, and blue to purple under different applied potentials. The corresponding optical changes are recorded by the transmittance spectra (Figure 5b) and the photographs of sample on the corresponding states are shown in Figure 5c. As the applied potential increases from −0.2 to 0.7 V, the transmittance of the sample decreases and the transmittance edge blueshifts to the ultraviolet region, namely from LB (yellow) to ES (green) and EB (blue) states. When the applied potential increases up to 1 V, the PANI is oxidized into the totally PS state (purple) and the transmittance edge shifts to the near-infrared region. The electrochromic property of CP can be applied for applications such as smart windows, rear-view mirrors, electronic paper, displays, and stealth technology. For these, a fast color switching is a prerequisite especially for display applications. The switching characteristics of the coaxial TiO2/ PANI core/shell nanorod arrays are analyzed by continuously stepping the voltage between 0.8 and −0.2 V with a 5 s delay at each potential. (In this case, the switching time was defined as the time required for a system to reach 90% of its full response.) The resultant current−time response is shown in Figure 5d. The response times for a complete reduction (PS to LB state) and oxidation (LB to PS state) are found to be about 1.2 and 1.3 s, respectively, implying that the average switching time between four PANI redox states (LB→ES→EB→PS) is approximately 400−430 ms. This switching speed is evidently faster than other inorganic electrochromic films such as WO3 and NiO (more than 3 s),43−45 PANI/WO3 dense film (9.5 s),46 PANI film embedded in WO3 nanorods (0.9 s),47 sulfonated-graphene/polyaniline film (6 s),48 PANI film grown on graphene substrate (0.6−7 s),49 and spin-coated sulfonated polyaniline film (15 s),50 and comparable to that of in situ prepared WO3/PANI hybrid film (0.4 s).42 In addition, the coaxial TiO2/PANI core/shell nanorod arrays exhibit fairly good cycling stability. The electrochemical stability is characterized by chronoamperometry with potentiostatic cycling at 0.8 and −0.2 V. The evolution of anodic and cathodic peak currents is presented in Figure 5e. The peak currents maintain ∼95% of the highest values after 5000 cycles, also better than inorganic electrochromic NiO and WO3 films in the literature (∼75% after 1600 cycles),43,51 PANI film embedded in WO3 nanorod (∼77% after 1000 cycles),47 and comparable to the in situ prepared WO3/PANI hybrid film (∼90% after 5000 cycles).42 Moreover, the core/shell nanorod architecture is well preserved after 5000 cycles and does not show evident degradation (Figure 5f). The CV curve shift

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details on synthesis of different kinds of metal oxide/CP core/shell nanoarrays including Co3O4/PANI, TiO2/ PANI, NiO/PANI, Co3O4/PEDOT. Details of electrochromic measurements and Li ion battery test. Figure S1, SEM-TEM images of single Co3O4 nanowire arrays. Figure S2, XRD-EDS results of Co3O4/PANI core/shell nanowire arrays. Figure S3, SEM images and XRD patterns of TiO2/PANI core/shell nanorod arrays on carbon cloth. Figure S4, EDS mapping of TiO2/PANI core/shell nanorod. Figure S5, SEM image and FTIR of Co3O4/PEDOT core/shell nanowire arrays. Figure 6, electrochemical impedance measurements of the metal oxide nanoarrays on carbon cloth. Figure S7, SEM images of TiO2/ PANI core/shell nanorod arrays on FTO. Figure S8, Molecular structure of PANI at different states. Figure S9, BET measurement. Figure S10, Electrochemical properties as the F

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lithium-ion battery anode. Figure S11, EIS analysis of Co3O4/ PANI core/shell nanowire arrays. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by SERC Public Sector Research Funding (Grant 1121202012), Agency for Science, Technology, and Research (A*STAR). H.J.F. acknowledges the support from Singapore Ministry of Education Academic Research Fund Tier 3 (MOE2011-T3-1-005).

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dx.doi.org/10.1021/nl402741j | Nano Lett. XXXX, XXX, XXX−XXX