Conducting Polymer 1-D Composites: Formation, Structure and Application

Abstract

Recent advances in the study of the synthesis, structure and applications of 1-D composites containing conducting polymers are discussed in this review. Conducting composites can form 1-D structures with metal and metal oxides, 1-D carbon nanomaterials, semiconducting materials, crystals of metalloorganic complexes. Advanced synthetic approaches allow for the formation of well-organized structures with polymeric phase deposited both on the surface of 1-D material and inside of the 1-D tubes. 1-D polymeric wires can also serve as a matrix for the formation 1-D composites with other materials. 1-D nanocomposites containing conducting polymers exhibit many exceptional properties which allow for various practical applications including energy converting and energy storage devices, electronic nanodevices, chemical, electrochemical and biochemical sensors, catalysis and electrocatalysis.

1. Introduction

The combination of 1-D nanostructures with conducting polymers creates nanocomposites with good processability and improved physical, electrical, and mechanical properties such as conductivity, solubility, optoelectronic and magnetic properties. These systems draw considerable attention in a wide range of applications including supercapacitors, batteries, energy conversion systems, catalysts and sensors.

Many synthetic strategies, such as template-directed, template-free chemical and electrochemical method, solvothermal syntheses, electrospinning techniques, vapor-phase approaches, have been developed to prepare several classes of 1-D nanostructures including metals, metal oxides, metal complexes, and semiconductors [1, 2, 3, 4].

Typical conducting polymers such as polypyrrole (PPY), polyaniline (PANI), polythiophene (PTH), and poly(3,4-ethylenedioxythophene) (PEDOT) are attractive polymers for composites synthesis given their low cost, easy processability, a large area of fabrication, and environmental stability. While, interest in 1-D nanostructures has increased due to their efficiency in electron transport, and their potential use in nanoelectronic devices [5, 6]. For example, structures such as nanowires, nanorods, nanotubes, or nanobelts with unique electric transporting characteristics would be more essential than irregular particles to be used as composites of solar cell devices after the introduction of conducting polymers [7, 8].

There are two main kinds of nanocomposites of conducting polymers with 1-D materials:

1. 1-D nanostructures covered with a conducting polymer. There are many techniques known for the deposition of conducting polymers onto 1-D nanomaterials. However, the encapsulation of 1-D nanostructures into the core of conducting polymers shell to obtain novel core-shell nanomaterials has become the most attractive aspect of nanocomposite synthesis. These composites are usually formed by the chemical or electrochemical polymerization of a thin layer of a conducting polymer onto different nanostructures.

2. Conducting polymers encapsulated of the 1-D nanostructures. However, there is much less work done on this type of nanocomposites.

This review aims to present general synthesis and characterization of the conducting polymers with 1-D nanostructures including the various methods used in these materials’ preparation. Finally, different aspects of the practical applications of these materials are presented.

2. Synthesis and characteristic of conducting polymers with 1-D nanostructures

The synthetic methods for the fabrication of conducting polymer with 1-D nanostructures are of great importance because the structure of conducting polymers and secondary components will affect the properties of formed nanocomposites. Several template-based and template-free methods have been used to form 1-D nanostructure/polymer composites.

2.1 The conducting polymers with metal and metal oxides nanomaterials

1-D nanostructures of metal and metal oxides are of great importance for their electrical, optical, and catalytical properties as well as a wide range of applications in nanoelectronics and sensing devices [9, 10]. However, these nanostructures are very sensitive to air and moisture, which degrade the performance of the nanodevices. A polymer envelope would protect nanostructures from oxidation and corrosion, giving a good performance for a long time.

Nanostructured composites of silver nanowires with polypyrrole (Ag/PPY) have been prepared by the redox reaction in an aqueous solution at room temperature between silver nitrate and pyrrole using poly(vinyl pyrrolidone) (PVP) as assistant agent [11]. Under these conditions, a metallic nanowire coated with conducting polymer is formed. PVP is used both as a capping agent to form silver nanowires, and as a dispersant of pyrrole monomer. Silver nanowire is formed during silver nitrate reduction with pyrrole and monomer polymerizes on the surface of nanorode at the same time. A typical TEM image of these nanocables is shown in Figure 1. The diameter of the layer is ∼50 nm, and the diameter of the silver core is ∼20 nm. The lengths of these nanocables are in the range of several to several tens’ micrometers [11].

Figure 1.

TEM image of Ag/PPY nanocables (reproduced with permission from Ref. [11]).

Recently, novel three-dimensional (3-D) silver nanowires@polypyrrole nanocomposites have been created [12, 13]. In the fabrication of the silver nanowires@polypyrrole sponge, the silver nanowires with the length of about 10 μm and diameter of 48.3 nm were synthesized trough solvothermal method and were used as the skeleton of the composite material. Polypyrrole was coated on the surface of silver nanowires by in situ chemical polymerization as shown in Figure 2a [12]. The prepared sponge exhibit good elasticity, mechanical strength and water absorption properties. The results showed that the complex permittivity and microwave absorption properties of the silver nanowires@polypyrrole sponge can be modulated by regulating the water amount. Another procedure 3-D core-shell silver nanowires@polypyyrole nanocomposite fabrication was proposed by Yuksel et al. [13]. Silver nanowires were prepared according to the polyol method and polypyrrole was synthesized using in-situ chemical polymerization (Figure 2b) [13]. These nanocomposites were applied for supercapacitors fabrication.

Figure 2.

Schematic illustrations of the formation of the (a) silver nanowires@polypyrrole sponge (reproduced with permission from Ref. [12]) (b) silver nanowires@polypyrrole nanocomposite (reproduced with permission from Ref. [13]).

In addition to one-dimensional structures of silver and conducting polymer, 1-D composites of conducting polymers with copper and gold nanowires were also created. The combination of these nanowires and conducting polymer can endow new properties and exhibit synergistic effects of the nanocomposite components. For example, copper nanowires with polypyrrole [14] and polyaniline [15] have been prepared by a facile liquid-phase reduction with copper (II) chloride as precursor [14] and using a simple and reproducible approach by spontaneous chemisorption of polyaniline on the copper surface [15].

Composites containing conducting polymers with metal oxides such as ZnO, RuO₂, MnO₂, Co₃O₄, V₂O₅, MgO, Fe₂O₃, TiO₂, and NiO have been produced. For example, Fan et al. [16] adopted an electrochemical polymerization method to assemble PANI and PEDOT on the surface of different oxides nanostructures including Co₃O₄, TiO₂, and NiO nanowires, nanorods and nanoflakes. These metal oxides structures were fabricated by the hydrothermal method. The typical cyclic voltammetry (CV) curve of the TiO₂/PANI nanorods on FTO (fluorine-doped tin oxide) glass is shown in Figure 3a. The first redox peaks A₁ and C₁ corresponds to the change between leucoemeraldine base and emeraldine salt with anion doping upon oxidation and dedoping upon reduction. The second pair of redox peaks A₂ and C₂ is due to the conversion between emeraldine base and pernigraniline salt. The change between emeraldine salt and emeraldine base does not involve an electron transfer process, and the redox peak is not reflected in the CV curve. Also, redox peaks of TiO₂ are not observed in the studied potential range. The TiO₂/PANI nanorods display interesting electrochromic properties. Namely, these nanorods show evident electrochromism with rich reversible color changes ranging from yellow for leucoemeraldine base, green for emeraldine salt, and blue for emeraldine-base to purple for pernigraniline salt under different applied potentials. In Figure 3b transmittance spectra are shown for different potentials applied to the electrode covered with a thin layer of composite. Moreover, the architecture of this material is well preserved after prolonged potential cycling, and does not show evident degradation (Figure 3c) [16].

Figure 3.

Electrochromic characterization of coaxial TiO₂/PANI nanorods 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 nanorods under different applied potentials, (c) SEM image nanorods after 5000 cycles (reproduced with permission from Ref. [16]).

Multicomponent 1-D nanostructures and conducting polymers have also been made. These materials can be tailored to exhibit, besides novel electrical, magnetic and optical properties, also good processing properties. For example, the composite of Co₃O₄@PPY@MnO₂ “core-shell-shell” nanowires exhibited prominent electrochemical performance and remarkable long-term cyclic stability [17]. Co₃O₄ nanowire core backbone was grown on nickel foam by the hydrothermal and post-annealing method. Next, a conductive polypyrrole film was assembled on Co₃O₄ nanowire surface by potentiostatic deposition. The final product Co₃O₄@PPY@MnO₂ was formed by soaking Co₃O₄@PPY in aqueous KMnO₄. In this case, a redox reaction occurred in 3-D ordered nanowire interface. Such nanocomposites showed an effective pathway for fast electron transport and accelerates the reaction kinetics between the electroactive center and current collector.

It is also possible to deposit 1-D nanostructures of metal within or around a preformed polymer nanotube. Polyaniline nanotubes prepared using AAO template were coated with gold to form PANI/Au composite [18]. The morphology of these structures in a different stage of formation is shown in Figure 4.

Figure 4.

SEM images of nanostructures grown in the AAO membrane (after the dissolution of AAO in 1 M NaOH for 1.5 h). (a) PANI fiber, (b–d) PANI/Au nanostructures for: (b) 1 h, (c) 1.5 h, (d) 2.5 h (reproduced with permission from Ref. [18]).

The metallic phase can be also deposited within the polymeric nanotube structure. For example, cobalt nanowires were produced within the PANI tubes [19]. Such a system exhibits unique magnetic properties. Cobalt nanowires show greatly enhanced magnetic coercivity.

2.2 The conducting polymers with carbon nanotubes or carbon fibers

For the formation of 1-D composites containing conducting polymers, carbon nanotubes and carbon fibers was used as a carbon component of composites.

Synthesis of carbon nanotubes (CNTs) and conducting polymer composite was firstly reported by Ajayan et al. [20]. Since then, a lot of attention has been paid to the fabrication of such 1-D functional composite materials with desirable electrical and mechanical properties. Composites of single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) with polyaniline have been the most intensively studied. For example, aniline has been polymerized on MWCNTs electrodes to obtain PANI films with novel surface characteristics including higher current densities and more effective polymerization [21]. Nanotube electrodes were constructed with whiskers of loosely packed MWCNTs. These nanotube whiskers with typical dimensions of 0.15 cm long and 0.028 cm in diameter were used as an electrode by attaching them to the tips of copper wire covered with conductive paint. Carbon nanotubes were prepared by the electric-arc process. The PANI films formed on the nanotube electrodes were prepared by electrochemical polymerization of aniline in H₂SO₄ solution. The morphology of polyaniline film deposited on carbon nanotube electrode and corresponding cyclic voltammetric response are shown in Figure 5 [21].

Figure 5.

PANI film deposited on carbon nanotube electrode: (a) SEM image, (b) CV curves showing much larger background currents compared to the Pt electrode. Sweep rate used 20 mV s⁻¹. The geometrical area of the CNTs is 0.016 cm² compared to 0.16 cm² for the Pt electrode (reproduced with permission from Ref. [21]).

Wu et al. [22, 23] have shown that the conductivity of MWCNTs/PANI composites received by in-situ chemical polymerization is 50–70% higher than that of the pure PANI. Whereas, with the increase of the MWCNTs nanotubes content to 24.8 wt% the conductivity increases by two orders of magnitude. Also, the change in the sign from positive to negative of the magnetoresistance at low temperatures is observed revealing the strong coupling between the carbon nanotubes and polyaniline in these composites. A similar effect on the conductivity of PANI was reported by Karim et al. [24]. They studied composite formed by deposition of PANI on the surface SWCNTs by using the in-situ chemical polymerization method [24]. The characterization of this material indicated that the conductivity and thermal stability of complex nanotubes were higher than polyaniline but lower than CNTs.

Many other conducting polymers such as PPY, PEDOT, and PTH were deposited onto the carbon nanotubes to form 1-D composites. Both in-situ and ex-situ chemical and electrochemical methods were used for polymer deposition. Similarly, such as polyaniline, these composites show considerably better electrochemical properties than that of the pristine polymer. The increase of polymeric phase conductivity (Table 1) and specific capacitance (Table 2) is observed for these materials.

Material	Method	Conductivity (S cm^−1)	References
PANI	Chemical polymerization	∼0.35	[24]
SWCNTs/PANI		3.41
PPY	Chemical polymerization using DBSA as surfactant	22.3	[25]
MWCNTs/PPY		26
PPY	Chemical polymerization	7.3 × 10^−3	[26]
MWCNTs (9.1 wt%)/PPY		5.6 × 10^−2
MWCNTs (13.04 wt%)/PPY		9.6 × 10^−2
MWCNTs (23.1 wt%)/PPY		0.23
PTH	Chemical polymerization	∼1.67 × 10^−6	[27]
SWCNTs/PTH		0.41
PTH	γ-radiation-induced chemical polymerization	∼1.23 × 10^−4	[28]
MWCNTs/PTH		3.71

Table 1.

Average values of conductivity of pure polymers and carbon nanotubes/polymer composites.

Material	Method	Specific capacitance (F g^−1)	References
MWCNTs/PEDOT 85/15 wt%	Chemical polymerization	95	[29]
MWCNTs/PEDOT 30/70 wt%		120
MWCNTs/PTH	Electrochemical polymerization	110	[30]
Px-MWCNTs/PANI	Chemical polymerization	809.6	[31]
MWCNTs/PANI	Chemical polymerization	446.89	[32]
MWCNTs/PPY	Electrochemical polymerization	600	[33]

Table 2.

Average values of specific capacitance of pure polymers and carbon nanotubes/polymer composites.

There is also possible to incorporate conducting polymer into inside of carbon nanotubes. Steinmetz et al. [34] produced polyacetylene (PA) filled MWCNTs by in-situ polymerization using supercritical carbon dioxide (scCO₂) and Ziegler-Natta catalyst. The supercritical fluid method can be also used to fill nanotubes with various organic substances to polymerize photo-conducting poly(N-vinyl carbazole) (PNVC) and the conducting polypyrrole inside MWCNTs and double-walled carbon nanotubes (DWCNTs) [35]. MWCNTs and DWCNTs were opened by refluxing them in concentrated HNO₃. The monomer together with an initiator was filled into these carbon nanotubes using scCO₂. In the case of N-vinyl carbazole, 2,2′-azobis-isobutyronitrile (AIBN) was used as a monomer polymerization initiator, while polymerization of pyrrole was made using FeCl₃ as the initiator of this process.

The structure and properties of composites of carbon 1-D nanomaterials and conducting polymers can be significantly improved by using well-organized structures of carbon nanotubes, such as aligned carbon nanotubes (ACNTs) network. In the case of ACNTs, the 1-D carbon cylinders are oriented in a parallel fashion perpendicular to the substrate. The aligned carbon nanotubes allow the polymer to be deposited on the walls of separated carbon nanotubes, limiting the thickness of the formed composite and the produced material has an open, and porous structure with a high surface area. Formation of ACNTs/conducting polymer composites was also carried out using both chemical and electrochemical polymerization. Feng et al. [36] aligned multi-walled carbon nanotubes (AMWCNTs) encapsulated by polyaniline by in-situ chemical polymerization. AMWCNTs grown on quartz glass sheet using catalytic pyrolysis. Next, these AMWCNTs were dipped in HCl solution containing aniline monomer for 12 h at 0°C. Aniline was adsorbed on the surface of the nanotubes. The polymerization process occurred after the addition dropwise of ammonium peroxydisulfate (APS) dissolved in HCl solution at 0°C for 4 h. Figure 6 shows the preparation procedure for organizing AMWCNTs/PANI nanocomposite [36].

Figure 6.

The preparation procedure of organizing AMWCNTs/PANI nanotubes (reproduced with permission from Ref. [36]).

Compared to CNTs-based composites, carbon nanofibers (CNFs) have received much less attention as a component of 1-D composites, because CNTs have better mechanical properties, smaller diameter, and lower density than CNFs. However, because of their availability, relatively low price, and much easier production produce, carbon nanofibers are an excellent alternative to the more expensive carbon nanotubes [37]. Jang et al. [38] demonstrated that vapor deposition polymerization method could be effectively used for the introduction of polyaniline onto the carbon nanofibers. This process has allowed the formation of a uniform and ultrathin PANI layer of which the thickness-dependent on the amount of monomer (Figure 7). Besides, the increasing of PANI layer thickness results in CNFs significant increase in the specific capacitance of these composites. Good electrochemical properties are also observed for CNFs/PANI composites prepared by functionalizing carbon nanofibers with toluenediisocyanate trough amidation followed by reaction with an excess of aniline to form urea derivative and residual aniline, which was subsequently polymerized and grafted with a urea derivative [39].

Figure 7.

SEM: (a and b), TEM: (c–e), images of pristine CNFs (a), and CNFs/PANI nanocomposites (b–e) with different the thickness of PANI layer deposited on the CNFs controlled by changing the amount of monomer: (c) 0.05 ml, (b) 0.1 ml, (c) 0.2 ml (reproduced with permission from Ref. [38]).

Recently, graphene nanoribbons were used to make nanocomposites with polypyrrole [40]. Graphene nanoribbons were synthesized by unzipping and exfoliation of MWCNTs, while polypyrrole was prepared using a chemical polymerization process in the presence of graphene oxide nanoribbons. These nanocomposites had a higher surface area than pure polypyrrole, which improved the charge storage capacity of the nanocomposites.

2.3 The other 1-D nanocomposites

To enhance processability, electrochemical, thermal, and mechanical stability, conducting polymers are often combined with other 1-D nanostructures, such as semiconducting materials, crystals of metalloorganic complexes.

Similar to the preparation of 1-D metal or metal oxide/conducting polymer nanocomposites, semiconducting selenides and sulfides can be incorporated into conducting polymers trough chemical or electrochemical methods. Alivisatos et al. [41] obtained CdSe/poly-3(hexylthiophene) (P3HT) nanorods. CdSe nanorods were dispersed with P3HT in a mixture of pyridine and chloroform and spin-cast to create a uniform film consisting of dispersed nanorods in the polymer. Such material was used to fabricate efficient hybrid solar cells with an external quantum efficiency of over 54% and monochromatic power conversion efficiency of 6.9%. Template techniques of synthesis 1-D nanomaterials, have been demonstrated for the preparation of sulfides with conducting polymers. Thus, Lin et al. [42] reported the preparation of CdS/PANI coaxial nanocables by the electrochemical synthesis in the AAO membrane. The diameter of the CdS nanowires was about 70 nm, which was the same as the pore diameter of the AAO membrane. The outer diameter of the PANI was about 90 nm. Guo et al. [43] synthesized CdS/PPY heterojunction nanowires by template technique using also porous AAO membrane. These nanowires had a smooth surface with diameters in the range of 200–400 nm. In addition to the template technique, an in-situ polymerization at the interfacial layer between chloroform and water has been developed for the preparation of Cu₂S nanorods coated with polypyrrole layer [44]. Smooth and uniform coaxial Cu₂S/PPY nanocables have been fabricated by controlling the reaction conditions, such as the molar ratio of pyrrole to oxidant and concentration of pyrrole in chloroform. The thickness of the PPY layer on the surface of Cu₂S nanorods depends on the polymerization time (Figure 8). The hydrothermal reaction was also applied to prepare Bi₂S₃/PPY nanocomposites [45]. The PPY coating on the surface Bi₂S₃/PPY nanorods was smooth and uniform in thickness.

Figure 8.

TEM images of Cu₂S/PPY nanorods were obtained with a pyrrole polymerization time of: (a) 1 h, (b) 2 h, (c) 3.5 h, (d) 5 h. inset of (B): SAED of the single Cu₂S/PPY nanorod (reproduced with permission from Ref. [44]).

1-D nanocrystals and conducting polymer composites were also created. Crystals of β-akaganeite (β-Fe³⁺O(OH,Cl)) with PEDOT [46, 47] and crystals of iridium complex ([IrCl₂(CO)₂]⁻) with PPY [48] are examples of 1-D composites containing conducting polymer and crystal metalloorganic complexes. The advantage of fabrication of these materials is the possibility of their preparation by in-situ one-step or two-step approach, respectively. Namely, PEDOT/β-Fe³⁺O(OH,Cl) nanospindles (Figure 9a) were synthesized in an aqueous solution trough one-step chemical oxidation polymerization using monomer EDOT, and FeCl₃∙6H₂O as an oxidant in the presence of CTAB and poly(acrylic acid) (PAA). Under these conditions, the polymerization of EDOT, and the hydrolyzation of FeCl₃ to form β-Fe³⁺O(OH,Cl) occur at the same time, leading to the formation PEDOT/β-Fe³⁺O(OH,Cl) nanocomposite [46, 47]. The 1-D-IrCl₂(CO)₂/PPY nanocomposites (Figure 9b) were synthesized in a dichloromethane solution by in-situ two-step electrodeposition. First, needles of the iridium complex were prepared by electrochemical oxidation of (AsPh₄)[IrCl₂(CO)₂]. Next, pyrrole was electropolymerized on the surface of the iridium needles [48]. Both the crystals size and thickness of the polymer can be easily controlled very simply using the reaction conditions such as concentration of compounds, different reaction times, and kind of solvent.

Figure 9.

SEM and inset TEM images of: (a) PEDOT/β-Fe³⁺O(OH,Cl) nanospindles (reproduced with permission from Ref. [46]), and (b) 1-D-IrCl₂(CO)₂/PPY nanocomposites (reproduced with permission from Ref. [48]).

3. Applications of conducting polymers and 1-D nanostructures composites

Many of the targeted applications for 1-D nanostructures require their incorporation into conducting polymers. Therefore, such nanocomposites are expected to find applications in nanoelectronic devices, sensors, catalysis or electrocatalysis and energy.

3.1 Energy conversion and energy storage applications

Incorporating 1-D nanostructure into conducting polymers play an important role in fabricating materials in energy conversion devices such as solar cells, and fuel cells, and energy storage devices such as lithium-ion batteries and supercapacitors. These materials have improved conductivity, cycleability, mechanical stability, processability, and specific capacitance.

Solar cells are energy conversion devices that convert sun light to electric energy. In comparison to nanoparticles, 1-D nanostructures are excellent candidates for the preparation of solar cells because they provide high electron mobility along these nanostructures. Their hole-transporting properties may contribute to the improvement of the photovoltaic efficiency performance. Solar cell devices fabricated with aligned ZnO/P3TH and ZnO/didodecylquaterthiophene (QT) composites exhibit well-resolved characteristics with the efficiency of 0.036%, a circuit current density (J_sc) of 0.32 mA cm⁻², and a fill factor (FF) of 0.28 [49]. Additional enhancement of photovoltaic properties of ZnO/P3TH-based solar cells can be achieved by incorporation of TiO₂ into the composite structure. Yang et al. [8] demonstrated that solar cells based on ZnO-TiO₂/P3TH composites exhibited an efficiency of 0.29% for devices stored in air for 1 month. While, these devices without TiO₂ layer had an efficient of only 0.04%. CNTs/PEDOT-PSS (polystyrenesulfonate) nanotubes were also used as dye-sensitized solar cells (DSCs), which exhibit very high performance with the energy conversion efficiency of 6.5%, J_sc = 15.5 cm⁻², and FF = 0.63 [50].

Fuel cells, which convert the chemical energy of a fuel directly into electricity by electrochemical reactions, have attracted attention for applications in electric vehicles [51]. Also in this case, the introduction of 1-D composites containing polymers can significantly improve the performance of such systems. For example, Co/PPY/MWCNTs nanotube composites were used as the cathode electrocatalysts for the reduction of oxygen in polymer electrolyte fuel cells (PEMFCs), direct ethanol fuel cells (DEFCs), and direct methanol fuel cells (DMFCs) [52]. The stability of these composites for the reduction of oxygen was excellent without any noticeable loss in performance over long PEMFC operating time, which is shown in Figure 10.

Figure 10.

Cell performance showing the stability of PEMFC at 90°C with cathode catalyst containing Co/PPY/MWCNTs and anode catalyst containing Pt/Ru/MWCNTs at ambient pressure (reproduced with permission from Ref. [52]).

Rechargeable batteries are widely used in daily life such as in cell phones, laptop computers, and electric vehicles. 1-D nanofibers composite of V₂O₅/PANI has been used as cathode materials in ion-Li batteries [53]. The composite showed enhanced capacitance properties in comparison to vanadium oxide nanotubes. The charge capacity of V₂O₅/PANI composite nanofibers was about 150 Ah kg⁻¹ during the 10 initial charge/discharge cycles, while a charge capacity of 100 Ah kg⁻¹ was obtained for V₂O₅ nanotubes. The composite also exhibits much better cyclability in comparison to V₂O₅ nanotubes. A significant enhancement in electrochemical performance has been also found for the silver vanadium oxides (SVO)/PANI triaxial nanowires [54]. It has been observed that the SVO/PANI triaxial nanowires exhibited a much higher current density than that of β-AgVO₃ nanowires, due to faster kinetics of charge transfers and higher capacity. Therefore, these composites can be used as cathode material for Li⁺ ion batteries with high specific capacity and good cycle performance [54]. The composite of polypyrrole@CNTs has been used as pseudocapacitive cathodes and Fe₃O₄@carbon as anode for nonaqueous lithium-ion capacitor applications [55]. Due to the synergistic effect of the remarkable pseudocapacity of the polypyrrole and the high electrical conductivity of CNTs, the polypyrrole@CNTs composite exhibited enhanced capacitive properties and cycle life in comparison to the pristine CNTs. The rechargeable sulfate- and sodium-ion batteries based on MWCNTs-polypyrrole core-shell nanowire anode and a Na_0.44MnO₂ nanorod cathode were also studied [56]. This MWCNTs-polypyrrole core-shell nanowire//Na_0.44MnO₂ nanorod full cell delivered discharge capacities of 99.2 mAh g⁻¹ and 87.2 mAh g⁻¹ with a high voltage of 1.6 V at the charge-discharge current densities of 100 mA g⁻¹ and 3000 mA g⁻¹, respectively, making it suitable for large scale energy storage application [56].

Supercapacitors also called electrochemical capacitors with high specific power, exceptional long cycle life compared with rechargeable batteries, and higher specific energy compared to conventional capacitors are of great interest for their potential applications in portable electronics, hybrid electronic vehicles, memory protection of computer electronics and renewable energy system [57]. Active electrode materials used in supercapacitors can be classified into three main categories: carbon, transition metal or metal oxide, and conducting polymer. Among these third groups, carbon has a relatively low specific capacitance usually under 200 F g⁻¹, metal oxides are either expensive, for example, ruthenium oxide, or poor conductors, for examples MnO₂, NiO, etc., while conducting polymers have a high specific capacitance, but their cyclic stability is poor. Therefore, nanocomposites can be useful for the construction of electrochemical capacitors with both high capacitance and good cyclic stability. PANI/CNFs composite was tested as an electrochemically active component for supercapacitors [38, 39]. The specific capacitance of PANI layer-coated CNFs showed a maximum value of 264 F g⁻¹ at 20 nm thickness of PANI, whereas that of pure CNFs were as 100 F g⁻¹ [38]. The specific capacitance of 557 F g⁻¹ and good cycling stability were reported for CNFs/PANI by Kotal et al. [39]. The electrochemical measurements of graphene nanoribbons with PPY showed the highest specific capacitance of 2066 F g⁻¹. Therefore, these nanocomposites could be used as an electrode material for the fabrication of high-capacity supercapacitors [40]. The nanocomposite of metal wires and conducting polymers can be also used in supercapacitors [12, 13]. In the case of 3-D Ag nanowires/PPY the maximum specific capacitance, maximum power and energy density 509 F g⁻¹, 60.7 W kg⁻¹, and 4.27 Wh kg⁻¹ was reported, respectively. The fabricated supercapacitors showed excellent stability of almost 90% after 10,000 charge/discharge cycles [13]. The capacitance performance of a variety of conducting polymer and 1-D carbon nanostructure composites is summarized in Table 3. In addition to CNTs, nanocomposites containing metal, metal oxide or metal hydroxide and conducting polymers have also been intensively investigated as building components in supercapacitors [62, 63, 64, 65].

Composite	Formation method	Capacitance	performance	References
		Specific capacitance	Stability
Porous PANI/CNTs	Chemical grafting and creating interpenetrating pores via templating using CaCO3 nanoparticles	1266 F g^−1 at 1 A g^−1 in 1 M H2SO4	83% (10,000 cycles)	[58]
CNFs/PANI	One-step vapor deposition polymerization	264 F g^−1 in 1 M H2SO4	—	[38]
PANI/TCNF	In-situ polymerization	557 F g^−1; in 0.5 M H2SO4	86% (2000) at 0.3 A g^−1	[39]
CNTs-GO/PPY	Facile electrochemical synthesis	6.3 F cm^−3 at 0.043 A cm^−3 with PVA/H3PO4 gel as the solid electrolyte	87.7% (10,000 cycles)	[59]
PPY/GO/CNTs	Electrochemi-cal co-deposition	196.7 mF cm^−2 at 0.5 mA cm^−2	98.1% (5000 cycles)	[60]
PPY/GO/MWCNTs	Facile one-step potentiostatic technique	358.69 F g^−1 at 100 mV s^−1 in 1 M H2SO4	88.69% (2000 cycles)	[61]

Table 3.

Capacitance performance of the composites of conducting polymers with 1-D carbon nanostructures.

CNTs, carbon nanotubes.

CNFs, carbon nanofibers.

TCNF, isocyanate-functionalized CNF.

3.2 Electronic nanodevices

It is generally accepted that 1-D nanostructures provide a good system to investigate the dependence of electrical transport or mechanical properties on dimensionality and size reduction. Most conducting polymers are suited for the construction of electronic devices because of their high electrical conductivity, and mechanical flexibility. Therefore, materials combined of 1-D nanostructures and conducting polymers can be potentially applicable in diodes, memory, transistors, and photovoltaic devices.

Woo et al. [66] reported the fabrication of organic light-emitting diode (OLED) using a conjugated emissive copolymer, poly(3,6-N-2-ethylhexyl carbazolyl cyanoterephthalidence) (PECCP) and SWCNTs dispersed in a hole conducting PEDOT in the buffer. The schematic of this device construction is shown in Figure 11. This composite deposited on the ITO served as an anode in LED. A cathode was a bi-layer consisting of a LiF and Al.

Figure 11.

Schematic of OLED construction.

By a combination of electrochemical polymerization of pyrrole and electrophoretic deposition of CNTs, new composite material has been prepared and tested for application in a triode-type field emission array (FEA) [67]. This triode-type FEA showed an emission current of 35 mA at an anode voltage of 1000 V and the gate voltage of 60 V. The emission current of the FEA was modulated by the gate voltage of 30 V. For photovoltaic applications, nanocomposite material consisting of CNTs and PANI as highly conductive and transparent has also been prepared [68]. Organic photovoltaic cells were built using this film as an anode in flexible ITO-free devices. These results indicated that novel ITO-free optoelectronic devices can be optimized with very high performance using transparent films of conjugated polymers and carbon 1-D nanomaterials.

3.3 Sensors

Conducting polymers are good candidates for chemical and biological sensors because the interactions with various analytes may influence the redox and doping states of these materials. Adding a second nanocomponent, such as carbon nanotubes, metal and metal oxide nanostructures, and biological materials, into conducting polymer is another way to increase the charge mobility of conducting polymers or to change the affinity of these composites.

Because of the large specific surface areas, these nanocomposites are good candidates for gas sensors. For example, CNTs/conducting polymer nanocomposites exhibit high sensitivity in NH₃ detection. Ammonia is one of the important industrial exhaust gases with high toxicity. Liu et al. [69] demonstrated a simple and effective method of NH₃ detection by at sensors based on MWCNTs/PANI nanocomposites. The results showed that MWCNTs/PANI had high sensitivity and quick sensor response, good reproducibility and repeatability for NH₃ detection. The mechanism of the enhanced sensitivity may be attributed to the increased surface area of PANI, providing more active sites for the adsorption of NH₃ molecules. Similar properties were also observed for MWCNTs/Au/PANI nanocomposites [70]. However, the high cost discourages its extensive application. The sensing properties of the CNFs/PPY coaxial nanocables for toxic gases, such as NH₃ and HCl detection were also studied [71]. These materials were fabricated by one-step vapor deposition polymerization. This simple process allowed the formation of ultrathin and uniform PPY layer on the CNFs surface, which thickness was dependent on the loaded amount of the monomer. The responses of the CNFs/PPY coaxial nanocables after interaction with NH₃ and HCl were dependent on the thickness of the PPY layer on CNFs and exhibited reversible and reproducible performance (Figure 12). The resistance change of the CNFs/PPY coaxial nanocables was negligible when the thickness of the PPY layer was smaller than 10 nm. The sensitivity of these nanocables increased significantly with increasing the PPY thickness and then stopped increasing when the PPY layer thickness was larger than the growth limit thickness point (22 nm) [71].

Figure 12.

Variation in normalized resistance change (absolute value) of the CNFs/PPY nanocable sensors after exposure to (a) NH₃ (20 ppm), and (b) HCl (20 ppm) vapors as a function of the PPY layer thickness. Inset TEM image of the CNFs/PPY nanocable (reproduced with permission from Ref. [71]).

In recent years, conducting polymers with 1-D nanostructured composites have also been used to construct a variety of biosensors because of their large surface area and unique electronic, chemical, and mechanical properties. In biosensors, the detection of H₂O₂ is important because it is often a product in enzymatic reactions. The sensing performance of coaxial nanowires consisting of a layer of PPY uniformly coated onto aligned CNTs in H₂O₂ detection makes it attractive for the fabrication of oxidase-based glucose biosensors, because H₂O₂ is generated in the reaction between glucose and oxygen in the presence of glucose oxidase (GOX) [72]. In these materials the aligned structure of CNTs plays a significant part in glucose determination, shifting its oxidation potential toward less positive values and enhancing the sensitivity of glucose determination. The immunosensor was also constructed based on an antibody/conducting polymer/TiO₂ nanowires film [73]. First, TiO₂ nanowires were made by hydrothermal synthesis and spin-coated on Au/Ti microelectrodes surface patterned Si/SiO₂ substrate. Next, polypyrrole propylic acid (PPA) and antibody composite films were immobilized on the surface of TiO₂ nanowires by electrochemically polymerized using pyrrole propylic acid (PA) and anti-rabbit IgG (1^oAB) mixture solution, as illustrated in Figure 13. The devices designed in these studies showed a linear concentration range of antigen determination between 11.2 μg/mL to 112 μg/mL. The detection sensitivity of these immunosensors was −0.64 A/(g/mL) for the 5 V of the applied voltage and the sensitivity for this voltage was better than that of 6 V and 7 V [73].

Figure 13.

The experiment setup of the electrochemical polymerization of porypyrrole propilic acid/anti-rabbit IgG immobilized TiO₂ nanowires immunosensor system (reproduced with permission from Ref. [73]).

3.4 Catalysis

Catalytic materials are important for the industry and the development of various sensors. Therefore, composites of conducting polymer and 1-D nanostructures also have been studied in this area of research.

The nanowires consisting of gold-coated PANI film exhibit excellent catalytic behavior for the chemical reduction of organic dyes such as methylene blue (MB) and rhodamine B (RhB) [74]. Most of these dyes are not biodegradable and persist in the environment, but they can be disposed of by chemical reduction using a strong reducing agent as an economical route. Unfortunately, the chemical reduction of dyes is a very slow process under ambient conditions. Therefore, these Au/PANI nanowires are used as catalysts for the reduction of MB and RhB dyes in the presence of NaBH₄. A total catalytical reduction of MB and RhB was observed. The catalytic activity of composite was much better in comparison to the catalytic performance of individual components [75, 76].

Several 1-D semiconductor materials such as TiO₂, ZnO, MnO₂, CdS, etc., have also been used as semiconductor photocatalysists. However, their wide band gap and the low quantum yield largely limited the overall photocatalytic efficiency. The photocatalytic performance of this semiconductor can be significantly improved by incorporating them into 1-D structures with conducting polymers. For example, a novel photocatalyst, polypyrrole coated Ag/TiO₂ nanofibers, was synthesized using an electrospinning technique, followed by a surfactant in-situ chemical polymerization method [77]. This photocatalyst showed obvious visible-light photocatalytic activity in the decomposition of gaseous acetone. The 1.0 wt% PPY/Ag/TiO₂ sample provided the optimum photocatalytic activity. In Figure 14, photocurrent transient responses of PPY/Ag/TiO₂, PPY/TiO₂, Ag/TiO₂ are compared. The photocurrent followed the order: PPY/Ag/TiO₂ > PPY/TiO₂ > Ag/TiO₂ > pure TiO₂. The enhancement of PPY/Ag/TiO₂ in photocurrent indicates smaller recombination and more efficient separation of photogenerated electron-hole pairs at its interface. Besides, the recycling test revealed that the PPY/Ag/TiO₂ nanofibers were stable and effective for the removal of organic pollutants [77]. The high photoactivity of the PPY/Ag/TiO₂ nanofibers can be attributed to the synergistic effect originating from the excited-state electrons in PPY, which can be readily injected into the TiO₂ conduction band and next transported to the Fermi level of Ag (Figure 15) [77]. Therefore, the combination of conducting polymers with semiconducting materials is an effective strategy for improving photocatalytic activity.

Figure 14.

(a) Photocurrent transient responses and (b) photocatalytical activity of PPY/Ag/TiO₂ and of single and two-component samples (reproduced with permission from Ref. [77]).

Figure 15.

Postulate mechanism of the visible-light-induced photodegradation of acetone with PPY/Ag/TiO₂ nanofibers (reproduced with permission from Ref. [77]).

The composites consisting of two or more components containing CNTs and the conducting polymers can be also used as electrocatalysts for hydrogen and alcohol fuel cells [52], as well as bio and microbial fuel cells [78]. Using these materials as a cathode or anode catalysts is an important step in reducing the use of high-cost platinum and platinum-based electrocatalysts to promote practical applications. Additional optimization of the catalyst structure and stability may improve catalysts performance and reduce the total cost.

4. Summary

This paper reviews study of the formation, properties and applications of conducting polymer 1-D nanocomposites. These materials have attracted much attention due to their unique physical, mechanical, chemical and electrochemical properties that provide nanostructures formed from them with multi-functionality. As described in this review, many synthetic approaches have been developed, including in-situ and ex-situ chemical and electrochemical method, template and template-free method, vapor-phase polymerization, emulsion polymerization, hydrothermal reaction, redox reaction, electrospinning, and so on, for the fabrication of conducting polymer nanocomposites with carbon nanotubes and nanofibers, 1-D metals, metal oxides, metal complexes, semiconductors, and porphyrins. Nanocomposites synthesized by these methods exhibit many exceptional properties. The size, shape and structure morphology of these composites can be controlled by the conditions of composite formation. 1-D structures exhibit high real surface area. The organization of these structures on the substrate surface provides unique conditions for reactant transport within the 1-D composite film. These composites usually exhibit good conductivity. Such properties enable wide practical applications of 1-D composites. They can be used for charge storage devices construction and as a component of electronic devices. They are applying in a chemical, biochemical and electrochemical sensors. Their catalytic and photocatalytic properties allow the use of these systems to remove environmental pollutants. So, the improvement of the syntheses is sought, which would lead to: (i) highly conducting, (ii) macroscopically uniform materials, (iii) produced at reasonable reaction time, and (iv) in high yield.

Therefore, conducting polymer 1-D nanostructures are useful materials in basic research and technology. Important of synthesis conditions and the production of new 1-D structures can have a significant impact on scientific development. Studies for the preparation of such materials are still desired. It is believed, that the combined chemical, physical, and mechanical properties of these nanocomposite materials are crucial in the development in the future more discoveries will be made in this field.