1 Introduction
As a new type of conductive materials, conductive polymers have attracted much attention since they were discovered in 1977
[1]. Because of its outstanding properties such as high conductivity, low density, easy synthesis, adjustable molecular structure and good flexibility, it has been widely used in mechanics, electricity, optoelectronics, thermoelectricity and other fields
[2⇓⇓⇓~6]. Conductive polymer is a kind of polymer material with conjugated π bond after chemical or electrochemical doping. Common conductive polymers and their first discovery years are polyacetylene (1977), polypyrrole (1977), polyaniline (1980), polythiophene (1981) and its derivatives. Among them, polyaniline has potential toxicity, polypyrrole has structural defects, and poly (3,4-ethylenedioxythiophene) (PEDOT) has high conductivity (4380 S/cm), low sheet resistance (< 100 Ω/sq), high optical transmittance (80% ~ 95%) and good flexibility after treatment, which has attracted much attention in academic research and industrial applications
[7][8][8,9][8⇓~10][9,11,12]. However, the low solubility of PEDOT in water limits its large-scale application. In order to solve this problem, a more widely used method is to use water-soluble polymer polystyrene sulfonic acid (PSS) as the counterion cosolvent of PEDOT and the polymerization template of PEDOT monomer, so that it can be easily dispersed in water. At present, the PEDOT: PSS water-soluble system has been successfully developed into various commercial products (Figure 1). PEDOT ∶ PSS is the best conductive polymer system for its high flexibility, good aqueous solution and easy film formation, which can replace the expensive and brittle ITO inorganic oxide in sensors, solar cells, supercapacitors, light-emitting diodes and other fields to reduce costs
[13,14][15][16,17][18⇓~20][21,22][23].
In order to further expand the application scope of PEDOT ∶ PSS and improve its comprehensive performance, it is an effective method to introduce inorganic nano-materials into the system. The obtained PEDOT: PSS/inorganic nano composite material not only has the excellent conductivity and flexibility of the conductive polymer, but also has the unique physical and chemical properties endowed by the inorganic nano material, such as high mechanical strength, thermal stability, catalytic activity, high conductivity, magnetism, special optical properties and the like
[24]. In addition, nano-fillers also have special effects, such as small size effect, surface and interface effect, quantum size effect, quantum tunneling effect and so on, which can be used to cooperate with conductive polymers to optimize the structure of composites from the level of molecular design, so as to obtain products with excellent properties
[25]. At present, a large number of reports show that the introduction of inorganic nanomaterials into PEDOT: PSS is an effective means to enhance its use value, and the PEDOT: PSS/inorganic nanocomposites prepared can be used in different fields and show good performance. Previous researchers have made a more comprehensive exposition on the optimization of PEDOT conductive properties and its use as conductive coatings and electrode film structures of organic optoelectronic devices, but there is still a lack of systematic summary on the preparation technology and related applications of PEDOT ∶ PSS/inorganic nanocomposites
[26⇓~28][29][30⇓~32]. In this paper, the preparation methods of PEDOT ∶ PSS/inorganic nanocomposites are introduced in detail, and the latest applications of PEDOT ∶ PSS/inorganic nanocomposites in sensors, solar cells, supercapacitors, thermoelectric generators and other devices are comprehensively reviewed.The development prospect of PEDOT ∶ PSS/inorganic nanocomposites was prospected, which has important reference value for more comprehensive and profound understanding and promoting the application of PEDOT ∶ PSS/inorganic nanocomposites in various fields.
2 Preparation of PEDOT ∶ PSS/Inorganic Nanocomposit
Polymer/nanoparticle composites are generally composed of polymer matrix and nano-fillers, which have a special structure of "macroscopically homogeneous and microscopically heterogeneous", so specific technical means are needed to introduce nanoparticles into polymer matrix. With the increasing demand for conducting polymer/inorganic nanocomposites in many practical applications, there are many ways to prepare PEDOT: PSS/inorganic nanocomposites, such as in-situ method, blending method, self-assembly method and intercalation method.
2.1 In-situ method
In-situ method is the most effective method to prepare stable nanocomposites, which can make nanoparticles have good dispersion in the polymer matrix, further enhance the interaction between fillers and matrix, and significantly improve the properties of composites. There are three common ways to obtain nanocomposites in situ: (1) in-situ growth of nanoparticles in the polymer matrix; (2) polymerizing the polymer in situ in the presence of nanoparticles; And (3) a double-in-situ method, namely simultaneously generate that polymer matrix and the nano particles.
2.1.1 Nanoparticle in-situ growth
The polymer is first dissolved in a suitable solvent, then the nanoparticle precursor is added, and then the polymer is subjected to heat treatment or electrochemical treatment to induce the formation of nanoparticles, thereby obtaining the polymer nanocomposite material. This method can be used to prepare nanoparticles with different morphologies and uniform dispersion in polymer matrix, so as to endow materials with expected properties by means of molecular design.
The nano-particle in-situ growth method can be used to prepare PEDOT ∶ PSS/inorganic nanocomposites containing metal nano-particle fillers. In the preparation process, metal inorganic salts with stable chemical properties and good water solubility are generally selected as precursors, which are directly dissolved in PEDOT ∶ PSS system, and then uniformly dispersed metal nanoparticles are obtained by adding reducing agents. In the previous research work, noble metals such as platinum, gold and silver are usually selected as nano-fillers to be added into the PEDOT medium to improve the photoelectric conversion efficiency by using the surface plasmon resonance effect of metal nanoparticles, so the composite material is particularly suitable for the preparation of photoelectric conversion materials such as solar cells
[33,34]. As platinum and gold are inert metals, they can not react with common acids to form salts, so water-soluble metal complexes such as chloroplatinic acid and chloroauric acid can be used as precursors of this type of metals, and the corresponding metal nanoparticles can be generated by using NaBH
4. The advantage of this method is that it is simple to operate and does not require the use of stabilizers such as polyvinylpyrrolidone (PVP) or cetyltrimethylammonium bromide (CTAB) to disperse the nanoparticles
[35,36]. Compared with the pure PEDOT ∶ PSS film, the prepared PEDOT ∶ PSS/metal particle composite film has higher catalytic activity when applied to the counter electrode of a dye-sensitized solar cell. Taking the composite film doped with Pt nanoparticles as an example, the addition of Pt improves the power conversion efficiency by about 80%
[33].
In addition to coating, PEDOT ∶ PSS can also be used as a coating material to prepare inorganic nanocomposites with core-shell structure. For example, the PEDOT: PSS solution is added into the AgNO
3 solution, the NaBH
4 is used as a reducing agent, and the Ag
+ is reduced into Ag nanoparticles by controlling the adding concentration and rate of the reducing agent, so that the core-shell PEDOT: PSS/Ag nanocomposite material can be prepared
[37]. A limitation of this material is that it has a strong tendency to aggregate even at high dilutions due to the π-π interactions between PEDOT molecules. However, the presence of metal nanoparticles increases the conductivity of the polymer by three orders of magnitude, and the electrochromic properties of the composite imply the possibility of its application in many fields, such as light-emitting diodes, solar cells, smart windows, electrochemical sensors and capacitors.
Based on the existing mature technology of in-situ growth of metal particles, the preparation of metal alloy nano-fillers containing two or more components has become a new trend in the development of PEDOT ∶ PSS/inorganic nano-composites. Different from the single component metal nanoparticles, the different components in the alloy nanoparticles can realize the interface effect in the nanometer dimension, thus further enhancing the carrier filtering ability, that is, they can more effectively allow high-energy carriers to pass through and participate in energy transmission. This special property makes it possible to improve the Seebeck coefficient of materials and develop new thermoelectric materials. Fig. 1 is a technical route for preparing a thulium ternary composite film
[38]. Te has a high Seebeck coefficient and is easy to synthesize Te nanocrystals in aqueous solution, while Cu
7Te
4 has a high conductivity, so the thermoelectric performance can be effectively improved by combining the two. Due to the synergistic effect between Te and Cu
7Te
4 and PEDOT: PSS, the thermoelectric properties of the film are significantly improved, and the film can be used for assembling flexible thermoelectric devices.
2.1.2 In-situ polymerization of polymer monomer
Polymer monomer in-situ polymerization is generally used to prepare PEDOT: PSS/inorganic nanocomposites containing carbon nanofillers. In this method, the nanoparticles are generally subjected to surface treatment or modification to achieve good dispersion in the PEDOT prepolymer solution, and then the oxidant and/or dopant are added to achieve polymerization. Because the monomer molecule is small, the viscosity of the prepared solution is low, the nano particles can be uniformly dispersed in the prepolymer solution, and a polymer network with a relatively uniform structure can be further formed to entrap the nano filler. The method leads to stronger interaction between the polymer matrix and the nano-filler, thereby being beneficial to the improvement of mechanical strength and other properties. The method has mild reaction conditions, is simple and easy to operate, retains the characteristics of nanoparticles, and is an important method for preparing PEDOT: PSS/inorganic nanocomposites with good dispersion effect.
As a new type of carbon nanomaterial, graphene has attracted much attention due to its unique electrical and optical properties, high transparency, good conductivity, high bending resistance and excellent stability
[39⇓⇓⇓⇓⇓⇓~46]. The related technical route for preparing the PEDOT: PSS/graphene composite material with high conductivity by adopting an in-situ polymerization method can be briefly summarized as follows: firstly, the graphene is dispersed in the PSS solution, then the EDOT monomer is added into the dispersion, and ferric iron is used as an oxidant to carry out oxidative polymerization at a low temperature to obtain the PEDOT: PSS/graphene nanocomposite material
[47]. The addition of graphene can effectively improve the conductivity of the composite material, so the composite material has a good application prospect in energy collection and energy storage devices.
Graphene oxide (GO) has strong hydrophilicity, but its conductivity is low due to the destruction of its conjugated structure. Reduced graphene oxide (rGO) is considered to be an ideal filler for preparing PEDOT ∶ PSS/inorganic nanocomposites because it can improve the conductivity and maintain good dispersibility in aqueous solution. In the preparation process, a strong reducing agent such as N
2H
4·4H
2O is generally used to reduce GO to obtain rGO. Subsequently, the EDOT monomer was oxidized by electrochemical method and polymerized on the surface of the electrode to form PEDOT ∶ PSS/GO composite film
[48]. In the presence of polyelectrolyte PSS and oxidants such as FeCl
3 and ammonium persulfate (APS), EDOT can also be polymerized in situ on the rGO nanosheets
[49]. Among them, PSS can be used as an ionic stabilizer due to its negatively charged-
, and there is a π-π bond between its aromatic ring and the rGO plane, so it can be adsorbed on the rGO nanosheet. The carbon atoms on the rGO nanosheets can interact with C2 and C5 in the EDOT ring, and can also form π bonds with the delocalized electrons of the sulfur atoms in the EDOT thiophene ring. In addition, functional groups such as carboxyl and hydroxyl remain in the rGO nanosheets, which can be used as growth sites of PEDOT ∶ PSS nanofibers on the plane and edge of the rGO nanosheets. Therefore, the π-π interaction between rGO nanosheets and PEDOT ∶ PSS is beneficial to the formation of rGO/PEDOT ∶ PSS nanocomposites.
Carbon nanotube (CNT) is another important carbon nanomaterial different from graphene. In addition to its good electrical conductivity, its unique one-dimensional (linear) structure endows the material with other properties.For example, CNT has excellent conductivity and high Seebeck coefficient, which is expected to become an ideal filler in polymer materials and greatly improve the conductivity and thermoelectric properties of PEDOT ∶ PSS/inorganic nanocomposites. In previous reports, PEDOT: PSS/CNT nanocomposites were generally prepared by simply physically mixing commercial PEDOT: PSS products with CNTs (single wall or a mixture of single wall, double wall, and triple wall CNTs)
[50⇓⇓~53][50]. In recent years, it has been reported that PEDOT ∶ PSS/CNT nanocomposites were prepared by in-situ polymerization. The main points of the related technology are as follows: (1) Multi-walled carbon nanotubes (MWCNTs) were added into PSS aqueous solution, and after stirring and ultrasonic treatment, MWCNTs were well dispersed and a PSS adsorption layer was formed on their surface
[54]; (2) EDOT is tightly adsorbed on the surface of MWCNTs/PSS due to π-π attraction and van der Waals interaction; (3) After adding oxidant FeCl
3 and APS, PEDOT ∶ PSS/MWCNTs nanocomposites were prepared by in-situ oxidative polymerization of EDOT on the surface of dispersed MWCNTs. Compared with pure PEDOT: PSS, the obtained PEDOT: PSS has better thermal stability,
The PEDOT ∶ PSS/MWCNTs nanocomposites have higher electrical conductivity, Seebeck coefficient and power factor. In addition, in the process of preparing PEDOT: PSS/graphene/MWCNTs (multi-walled carbon nanotubes) ternary nanocomposites (Figure 2), the removal of hetero ions in PSS and the removal of dissolved oxygen in the aqueous solution system by inert gas, thereby reducing the unnecessary consumption of EDOT, have also been proved to be the key factors to improve the polymerization efficiency of EDOT and the overall performance of the material
[55].
2.1.3 Double in situ method
Bi-in-situ method refers to the simultaneous formation of polymer and nanoparticles when monomer and nanoparticle precursors coexist. This method can not only make the nanoparticles uniformly dispersed in the polymer matrix synthesized synchronously during the formation process, but also simplify the synthesis process. Therefore, this method has been considered as the most efficient way to synthesize polymer/inorganic nanocomposites with good stability. The PEDOT ∶ PSS/inorganic nano-particle composite material is prepared by adopting a double in-situ method, and a specific experimental scheme can be designed according to different fillers.
(1) Doped metal nanoparticles
When the metal nanoparticles are doped, the EDOT monomer is firstly dissolved to form a prepolymer solution, and then an oxidizing metal nanoparticle precursor is added as an oxidant to promote the polymerization of EDOT, and the metal nanoparticles are generated by the reduction reaction of the EDOT.
Au nanoparticles have the advantages of high catalytic activity, excellent stability and high conductivity. The doping of Au nanoparticles can effectively improve the electrochemical response of conductive polymers, so the related composites are most widely used in biosensors
[56]. To synthesize the PEDOT: PSS/Au nanocomposite, firstly, PSSNa can be added into the aqueous solution of EDOT, and after PSSNa is completely dissolved, the HAuCl
4 solution is slowly added into the above mixed solution, so as to ensure that the Au
3+ is fully reduced by EDOT into Au
0. The specific method is shown in fig. 3
[57]. During the preparation, it was observed that the solution changed from initially colorless to bright green, and when the reaction was continued for 30 min, the solution became dark brown. When the solution appears purple, it indicates that the Au
3+ has been completely reduced to Au
0, and the solution is acidic. In the preparation process, EDOT is used as a reducing agent, and PSS
- is used as a doping agent and a stabilizing agent of PEDOT; and the PEDOT: PSS/Au aqueous dispersion finally formed can be used as an effective catalyst for reducing 4-nitrophenol to 4-aminophenol under the condition of excessive NaBH
4, and can be extended to other catalytic reactions.
Compared with gold, palladium, platinum and other precious metals, Ag nanoparticles are a relatively low price material, and its inherent characteristics have attracted wide attention. According to the previous research, Wang et al. Directly prepared PEDOT ∶ PSS @ Ag nanocomposites by using AgNO
3 as oxidant, EDOT as reducing agent and PSS-Na as stabilizer
[58][59]. The synthesis method avoids the use of severe oxides such as APS to prepare PEDOT: PSS. In addition, the morphology of PEDOT ∶ PSS @ Ag composites can be affected by adjusting the pH value by introducing HNO
3 at the end of the synthesis process. This is because the Ag nanoparticles may originate from the decomposition of the AgNO
3, so the Ag nanoparticles have poor monodispersity and are irregularly dispersed in the PEDOT ∶ PSS colloidal particles. In the last step, nitric acid was introduced to make the redox reaction between AgNO
3 and EDOT take place, and the obtained Ag nanoparticles were modified by PEDOT and uniformly dispersed in PEDOT ∶ PSS with high stability. It was found that the conductivity of PEDOT ∶ PSS @ Ag nanocomposites could be improved by changing the morphology of nano-fillers. The related technical experience may be extended to PEDOT ∶ PSS/inorganic nanocomposites containing other types of metal particles.Besides pH, temperature, reaction time and additives (such as surfactants) may also affect the morphology of nanofillers. It is a new idea to improve the application value of composite materials by controlling the morphology of fillers, which is worthy of being widely used by other researchers in this field.
(2) dope nano metal oxides
The nano metal oxide is mainly prepared by a sol-gel method, namely, inorganic salt or metal alkoxide is used as a precursor, the precursor is dissolved in water or an organic solvent, a dispersion liquid of EDOT dispersed in PSS is mixed with the precursor, PEDOT and nano particle sol are generated by electrochemical polymerization, and the nano particle sol and the PEDOT: PSS composite are obtained by drying to volatilize the solvent.
Sol-gel method is an important method to prepare inorganic nanoparticles, which has been studied for more than 20 years. It has been used to prepare
, ZnO and
nanoparticles
[60,61][63]. The advantage of this method is that it is simple and easy to operate at room temperature, but the related technology for the preparation of PEDOT ∶ PSS/inorganic nanocomposites is still in its infancy, and there are few reports on it. Taking the preparation of PEDOT∶PSS/TiO
2 nanocomposite membranes by the dual in-situ method as an example, the aqueous solution of EDTA-2Na is first dropped into the ethanol solution of tetrabutyl titanate, while the EDOT monomer is dispersed in the aqueous solution of PSS to prepare the precursor solutions of nano-metal oxides and polymers, respectively
[66]. The two solutions were then mixed and the PEDOT∶PSS/TiO
2 film was prepared by electropolymerization. The doping of the metal oxide can improve the electrochromic capability of the PEDOT polymer matrix, and compared with a pure PEDOT: PSS film, different components of the doped composite film material can cooperate in a nanometer dimension, so that the composite film material has higher color contrast and better stability.
The in situ formation of nanoparticles during monomer polymerization is beneficial to the formation of hydrogen bonds between the organic phase and the inorganic phase, so that the synthesized nanocomposites have good stability and significant nano-interface effect. Therefore, the bi-in-situ method has broad application prospects in the preparation of functional conductive polymer/inorganic nanocomposites.
2.2 Blending method
Blending is considered to be the simplest and most direct way to prepare organic-inorganic nanocomposites, which has been widely used in industrial production
[67]. Blending refers to the direct mixing of nanoparticles with a conductive polymer dissolved in a suitable solvent by mechanical action. According to the different properties of polymers, blending can be completed by mechanical blending, melt blending, solution blending or emulsion blending
[68,69]. PEDOT ∶ PSS/inorganic nanocomposites prepared by blending method have no strict restrictions on the type and morphology of fillers because of the simple process and easy operation, but the problem is that the dispersion of fillers is not enough and it is easy to agglomerate, so it may be necessary to add specific types of additives to obtain nanocomposites with uniform distribution of nanomaterials and stable properties.
The blending of CNT and PEDOT ∶ PSS system is a common technical means to prepare thermoelectric materials
[70⇓~72]. The results show that PEDOT ∶ PSS can be effectively blended with CNT by using special solvents (such as dimethyl sulfoxide (DMSO)), and stable conductive channels can be established
[73]. The post-treatment of the composite membrane material with alkaline reagents such as sodium hydroxide can also play a role in adjusting the concentration of charge carriers
[73]. Due to the excellent conductivity of CNT, the application fields of related composites can also be extended to other fields such as supercapacitors, sensors, smart films and so on. In addition to the use of special solvent treatment, the introduction of surfactant can also play a certain role in promoting the dispersion of CNT, so that the performance of the composite material is improved, and the prepared slurry can be formed into a film in a variety of ways, including dripping, blade coating, deposition and so on
[74].
Two-dimensional carbon nanomaterials graphene and graphene oxide can also be used as fillers to blend with PEDOT ∶ PSS to improve the electrical properties of the materials
[75⇓~77]. The related technical means is similar to that when CNT is used as the filler, after the filler and PEDOT: PSS are blended, DMSO needs to be added, and ultrasound is used to improve the dispersion of the filler in the slurry
[78]. Due to the physical properties of the graphene, the film formed by the composite material also has good light transmittance and can be used as an electrode of a photoelectric conversion material
[77]. In addition, because pure graphene is not sensitive to hydrogen, PEDOT ∶ PSS system doped with graphene oxide and reduced graphene oxide can be used in gas sensing devices
[78⇓~80].
As mentioned above, the blending method has no strict restrictions on the types of fillers, so metal nanoparticles can also be used as fillers to blend with PEDOT: PSS through this method. The morphology of the material includes nanoparticles, nanowires, etc. The electrical properties of PEDOT after doping show a significant improvement trend, which can be used for the development of various electronic materials. For example, after the doping of Ag nanoparticles, Ag atoms have a high affinity with the functional groups on the PEDOT: PSS skeleton, which can effectively improve the conductivity of PEDOT: PSS without damaging its morphology. As shown in fig. 4A, Ag nanowires with different contents are dispersed in a PEDOT: PSS aqueous solution by a simple solution blending method, and after being uniformly mixed, a uniformly distributed Ag/PEDOT: PSS composite film is formed by post-treatment such as annealing, and is used as an electrode of a non-fullerene organic solar cell
[81]. The composite electrode has excellent light transmittance and film forming property, and the work function and conductivity are improved (Figure 4 C ~ e), indicating that the composite electrode can effectively improve the performance of non-fullerene organic solar cells.
The PEDOT: PSS system is blended with other types of inorganic nano-materials to improve the electrical properties of the material and endow the composite material with new additional functions by utilizing the characteristics of related nano-materials. The functional materials used include magnetic nano-particles (Fe
3O
4), semiconductor nano-particles (Si) and nano-particles with special electrical properties (black phosphorus, MXene, etc.)
[82][83][84⇓⇓⇓⇓⇓⇓~91]. The PEDOT: PSS/CNT composite material prepared by the blending method has the characteristics of simplicity, good repeatability and the like, so that the PEDOT: PSS/CNT composite material is particularly suitable for large-scale industrial production and has good social and economic values.
2.3 Self-assembly method
In recent years, the preparation of nanocomposites by self-assembly technology has attracted wide attention, mainly including template self-assembly method, layer-by-layer self-assembly method and so on. For the preparation of PEDOT ∶ PSS/inorganic nanocomposites, researchers mostly use the layer-by-layer self-assembly method. It refers to the self-assembly technology in which organic molecules and inorganic nanoparticles are alternately deposited to form multilayer organic-inorganic films through electrostatic interaction, hydrogen bonding interaction and external field driving force. The method not only can ensure that the nano-fillers are orderly distributed in a layered structure, but also can lead the polymer chains to be directionally arranged through the induction effect of the nano-fillers, thereby obtaining the PEDOT: PSS/inorganic nano-composite material with special functions.
Layered double hydroxides (LDHs), as typical two-dimensional materials, have attracted more and more attention due to their adjustable composition and morphology, and easy exfoliation into monolayer nanosheets. By adjusting the components and using different metal ions to prepare the LDHs, the electronic structure of the layer plate can be effectively adjusted, and then the LDHs and the PEDOT: PSS system are compounded and synergistically acted to generate specific electrical properties, thereby developing inorganic nanocomposite materials suitable for different electronic devices.
The uniform interface and close interaction between the monolayer Co-Ni LDH nanosheet and PEDOT ∶ PSS contribute to the excellent charge storage properties of the composite. Fig. 5 is a schematic diagram of a superlattice heterostructure composed of Co-Ni LDHs and PEDOT: PSS by a layer-by-layer self-assembly method, and a long-range ordered Co-Ni LDH/PEDOT: PSS nanocomposite can be obtained through electrostatic interaction
[92]. As an electrochemically active material, the monolayer LDH nanosheet provides a confined microenvironment for PEDOT ∶ PSS with ideal spatial arrangement and doping level, and leads to resonance and conformational changes of PEDOT chains, thus improving the charge mobility. At the same time, PEDOT ∶ PSS molecules act as a multilayer conductive network, which promotes charge collection and transport. In addition, the long-range periodic stacking provides good ion accessibility to the LDH active center. The supercapacitor prepared by the composite material has high capacity, remarkable cycle life and good mechanical flexibility, which shows that the molecular scale design has made an important contribution to the excellent overall performance of the film supercapacitor and is worth learning.
The positively charged LDH nanosheets (such as MgAl-CO
3LDH) can also be effectively combined with the negatively charged PEDOT ∶ PSS system to change the molecular structure of the conducting polymer through layer-by-layer self-assembly, thereby improving the electrical properties of the conducting polymer. For example, reducing the entanglement and aggregation between molecular chains can effectively improve the conductivity of PEDOT and further improve the response rate of the electrochromic process
[93]. In addition, LDH nanosheets can provide rigid laminates and two-dimensional confined microenvironment for PEDOT to improve its dispersibility, and fully designing and utilizing the synergistic effect of these special nanodimensions will play a constructive role in the development of new functional electronic materials.
Besides LDH, inorganic nanocomposites with excellent electrical properties can also be obtained by self-assembly of PEDOT with other types of nanomaterials. For example, the conductivity of PEDOT∶PSS/MoS
2 nanocomposites is significantly improved compared with PEDOT: PSS, and the electrical properties are significantly enhanced, which can be directly used as a flexible substrate for thermoelectric energy harvesting systems
[94]. In recent years, the number of reports on the self-assembly of PEDOT ∶ PSS with different types of nanomaterials is on the rise, and it has gradually become the focus of attention in the preparation of PEDOT ∶ PSS/inorganic nanocomposites. There are various types of materials used, including metals and oxides, elemental substances and even composite materials directly self-assembled on the surface of natural biomaterials
[95⇓~97]. Self-assembly method has become one of the frontier methods in the field of materials research and has attracted more and more attention because of its advantages such as simple equipment, simple operation, easy to implement the structural design at the molecular level, and the uniform and controllable structure of the prepared composites.
2.4 Intercalation compound method
The intercalation method is to insert PEDOT into the layers of two-dimensional layered nano-fillers by specific technical means, so as to realize the nanoscale composite of PEDOT and two-dimensional layered nano-fillers. According to the compounding process, the intercalation compounding method can be divided into intercalation polymerization method and polymer intercalation method. The intercalation polymerization method is to disperse the EDOT monomer in the PSS solution, embed the prepolymer into the lamella, and then initiate the polymerization under the action of the initiator, so that the two-dimensional layered filler reaches nanoscale size and is dispersed in the PEDOT, and then the PEDOT ∶ PSS/inorganic nanocomposite is prepared. Polymer intercalation refers to the direct intercalation of polymers into two-dimensional layered fillers to achieve nanoscale composites.
At present, graphite is the only two-dimensional layered filler that can help the polymerization and intercalation of EDOT monomer with the help of polymer cosolvent PSS. The main technical means comprises the following steps of: partially intercalating an EDOT monomer into colloidal graphite by adopting an intercalation polymerization method, and polymerizing the EDOT in situ by using PSS as a template under the action of oxidants of ferric sulfate and sodium persulfate to prepare the PEDOT ∶ PSS/graphite nanocomposite material
[98]. Intercalation polymerization of EDOT in other systems, such as intercalation into the interlayer of a V
2O
5, is usually accomplished with the aid of small molecule cosolvents/surfactants
[99]. PEDOT is a kind of polymer with high rigidity, which is not easy to curl and fold, and it is difficult to enter the two-dimensional filler with small interlayer spacing, so PEDOT can not be directly intercalated into two-dimensional layered materials.
The intercalation compounding method has the advantages of simple process, wide source of raw materials and low price. The stacked layered nano-filler is only in the nanometer level in one dimension direction, so the particles are not easy to agglomerate and are easy to disperse, and the PEDOT: PSS/inorganic nano-composite material with stable performance can be prepared. At present, there is little research on the intercalation polymerization of EDOT monomer. Based on a series of advantages of this method, it has broad application prospects in the preparation of PEDOT ∶ PSS/inorganic nanocomposites.
3 Application of PEDOT ∶ PSS/Inorganic Nanocomposit
3.1 Sensor
Sensor is a kind of detection device that can convert the measured information into electrical signal output according to certain rules to meet the requirements of information transmission, processing, storage, display, recording and control. PEDOT ∶ PSS/inorganic nanocomposites are especially suitable for the development of flexible electronic sensors as conductive materials because of their conductivity, flexibility, good chemical stability and thermal stability. In addition, the addition of specific nanomaterials can also endow them with special physical, chemical or biological properties, which can be used to prepare sensing devices with special functions or under extreme conditions.
In recent years, the preparation of conductive paper-based biosensors has become a research hotspot. Conductive paper can be prepared from a wide range of materials, such as metals, metal oxides, carbon nanomaterials and conductive polymers, among which conductive polymers have attracted much attention due to their low cost, high conductivity, solution processability and high mechanical flexibility
[100][101][102,103][104,105]. Among the conductive polymers, PEDOT ∶ PSS can form a uniform coating on the paper, while maintaining the characteristics of flexibility, stability, high conductivity and visible light transmittance, so it can be used as an ideal material for the preparation of conductive paper
[106⇓~108]. In the study of Kumar et al., nFe
2O
3@PEDOT∶PSS nanocomposites were obtained by dispersing nano-Fe
2O
3 in PEDOT ∶ PSS solution
[109]. The introduction of nano-sized Fe
2O
3 is aimed at improving the conductivity, electrochemical performance and signal stability of PEDOT ∶ PSS, and the synergistic effect between nano-sized Fe
2O
3 and PEDOT ∶ PSS can improve its performance. The Whatman paper (WP) was modified with nFe
2O
3@PEDOT∶PSS to prepare nFe
2O
3@PEDOT∶PSS/WP, which can be used to obtain flexible, lightweight, disposable conductive paper-based electrodes with high sensitivity, good storage stability and applicability to patient samples. The final cancer biosensor can detect carcinoembryonic antigen (CEA), a biomarker of cancer. This has important clinical value in the differential diagnosis of malignant tumors, disease monitoring, efficacy evaluation and so on. Taccola et al. introduced iron oxide nanoparticles into PEDOT: PSS by a simple and rapid one-step process, and prepared a PEDOT: PSS/iron oxide multifunctional nanocomposite film, which can be used as a resistive humidity sensor. The SEM photos of the film transferred to various substrates are shown in Figure 6
[110]. The sensitivity of the composite film to humidity increases with the increase of the concentration of iron oxide nanoparticles. The humidity sensing properties of PEDOT ∶ PSS/Fe _ 2O _ 3 composite films are better than that of pure PEDOT ∶ PSS according to the measurement results of the films on different solid substrates. This work results in simple and reliable humidity sensors that can be easily transferred to rigid and flexible substrates with arbitrary shapes and surface topographies. In addition, the introduction of iron oxide nanoparticles into the conductive polymer matrix can also be applied to the fields of electronics, optical communication and optics.
In addition, the Fe
3O
4/PEDOT∶PSS conductive nanocomposite composed of PEDOT ∶ PSS modified magnetic europium nanoparticles can be used to prepare a sensitive, fast and simple sandwich-type biosensor.To detect prostate specific antigen (PSA), a biomarker of prostate cancer, in an unprocessed human plasma sample, wherein magnetic Fe
3O
4 nanoparticles are introduced to enhance the signal of the biosensor
[111]. The sensor can also be used for the detection and clinical application of other biomarkers.
In addition to biosensors and humidity sensors, PEDOT: PSS/inorganic nanocomposites can also be used for gas sensors. NO
2 is a toxic air pollutant, which is the source of acid rain and photochemical smog and poses a great threat to environmental safety
[112]. Therefore, it is necessary to develop a low-cost and high-sensitivity sensor for detecting the concentration of the NO
2 gas. It has been shown that PEDOT: PSS-based chemical resistors can detect NO
x at room temperature, because PEDOT: PSS changes its resistance when it is in contact with the NO
x, and when it is used as a sensing layer, it can provide a conductive channel for charge carriers
[113,114]. The composite material prepared by the semiconductor metal oxide and the conductive polymer can be used for effectively detecting the NO
2 gas at room temperature
[115]. Lin et al. Synthesized PEDOT∶PSS/WO
3 nanocomposite ink by dispersing WO
3 nanoparticles in PEDOT ∶ PSS solution without any surfactant
[116]. A flexible chemical resistance type NO
2 gas sensor based on PEDOT∶PSS/WO
3 nanocomposites was fabricated by printing Ag staggered electrodes and a PEDOT∶PSS/WO
3 sensing layer on a polyimide substrate.The sensor has high sensitivity to low concentration of NO
2 gas at room temperature. The excellent performance of the sensor is mainly attributed to the conductive channel provided by PEDOT ∶ PSS for charge carriers and the heterojunction formed at the PEDOT∶PSS/WO
3 interface.
3.2 Solar Cell
Solar cells, also known as "solar chips" or "photovoltaic cells", are devices that directly convert light energy into electrical energy through photoelectric effect or photochemical effect. Among them, perovskite solar cells, as the third generation of new thin film solar cells, have attracted wide attention in the photovoltaic field because of their low cost, high photoelectric conversion efficiency and flexible devices
[117]. However, one of the main problems facing this battery is instability. The results show that the inverted planar perovskite solar cells with PEDOT ∶ PSS as the hole transport material have good stability
[118]. The properties of PEDOT ∶ PSS can be further improved by preparing nanocomposites
[119]. DMSO was doped into PEDOT ∶ PSS to improve its conductivity, and then PEDOT ∶ PSS/f-MWCTs nanocomposites were obtained by mixing with DMSO solution of functionalized multi-walled carbon nanotubes (f-MWCTs)
[120]. Among them, PEDOT ∶ PSS is a reliable organic hole transport material, and f-MWCTs can improve the fill factor of perovskite solar cells. The composite material was used as a hole transport material for an inverted perovskite solar cell and was cast onto a FTO (fluorine-doped tin oxide) glass substrate, and the cell structure is shown in fig. 7.
In addition to perovskite solar cells, dye-sensitized solar cells are also the third generation of solar cell products. Compared with traditional silicon-based solar cells, its greatest advantages are simple preparation, low production cost and acceptable conversion efficiency. The halloysite nanotubes (HNTs) have been successfully used as an effective reinforcement for PEDOT nanocomposites due to their large aspect ratio (nanotube length of 0.2 ~ 1.5 μm, inner and outer diameters of 10 ~ 30 nm and 40 ~ 70 nm, respectively), good mechanical strength and thermal stability. Viscous PEDOT ∶ PSS/HNTs inks were prepared by a simple homogenization process with different contents of insulating HNTs as fillers
[121]. The ink is screen-printed on an FTO substrate and can be used as a counter electrode of a dye-sensitized solar cell. TGA test showed that the introduction of HNT
S significantly improved the thermal stability of the ink. Compared with the pure PEDOT ∶ PSS, the PEDOT∶PSS/HNT
S applied to the counter electrode of the dye-sensitized solar cell has more excellent photovoltaic performance.
PEDOT: PSS can also be applied to the buffer layer of organic solar cells (OSCs) to modify the ITO electrode. In order to further improve the charge transport properties of PEDOT ∶ PSS, GO was prepared by Hummers method.After thermal reduction of GO, PEDOT ∶ PSS/rGO nanocomposites were prepared by ultrasonic mixing
[122]. It was used as a hole transport layer in P3HT ∶ PCB-based flexible organic solar cells, and the power conversion efficiency was 0. 43%. Since incident light must pass through the nanocomposite film in OSCs, the optical properties of PEDOT ∶ PSS/rGO are critical for P3HT ∶ PCBM-based organic solar cells. The PEDOT ∶ PSS/rGO nanocomposite film has high transparency (> 85%) in the whole visible region, which is confirmed by UV-vis spectroscopy.
3.3 Upercapacitor
In recent years, supercapacitors have become an alternative energy storage and conversion device in wearable electronic devices due to their low cost, environmental friendliness and energy storage characteristics. Flexible supercapacitors have attracted much attention due to their compatibility and portability, fast charging and discharging response, and long-term operation stability in electronic devices. PEDOT: PSS has been used as an active material in supercapacitors due to its adjustable conductivity, high transparency, high flexibility, good stability and processability. Cho et al. Prepared RuO
2/PEDOT∶PSS/ graphene ternary nanocomposites, which can be used as electrode materials for supercapacitors through screen printing. The preparation process is shown in Figure 8
[123]. In this work, the effects of graphene and RuO
2 nanoparticles on the electrical and electrochemical properties of PEDOT ∶ PSS were systematically investigated. The graphene sheet enhances the charge transport properties through a strong π-π stacking interaction with PEDOT. However, the gadolinium nanoparticles attached to the surface of PEDOT ∶ PSS/graphene promoted the redox reaction with the electrolyte, which significantly improved the specific capacitance of the electrode material. Benefiting from the synergistic effect of PEDOT ∶ PSS, graphene and RuO
2, the RuO
2/PEDOT∶PSS/ graphene electrode has high conductivity (1570 S/cm), high specific capacitance (820 F/G) and good cycling stability (81.5% of the specific capacitance is retained after 1000 cycles). The method can be used as an effective way for preparing high-conductivity electrode materials with electrochemical activity to be applied to electronic devices such as solar cells, chemical sensors, thin film transistors and the like.
Khasim et al. prepared GO by Hummers method, and reduced it with ascorbic acid with strong reducibility to obtain rGO
[124]. PEDOT ∶ PSS was doped with ethylene glycol (EG) for the second time, and the PEDOT-PSS ∶ EG suspension was mixed with rGO to prepare PEDOT-PSS ∶ EG/rGO nanocomposite, which was used to prepare the flexible electrode through a series of steps. Compared with pure PEDOT ∶ PSS, the electrical conductivity of PEDOT ∶ PSS/rGO nanocomposites was improved by several orders of magnitude. When used as a flexible electrode of a symmetric all-solid-state supercapacitor, the supercapacitor has excellent capacitance behavior and good cycle stability (the capacitance retention rate is about 90%, and the coulombic efficiency is about 110%), and can be continuously charged and discharged for 5000 times. This research has a broad application prospect in high-performance all-solid-state flexible supercapacitors.
In addition, PEDOT ∶ PSS was mixed with MnO
2 and CNT to prepare CNT/MnO
2/PEDOT∶PSS nanocomposites based on its affinity for bacteria and bioelectrocatalytic properties, which can be used as anode materials for new biological supercapacitors
[125]. The pseudocapacitance of the MnO
2 and the double-layer capacitance of the CNT increase the overall capacitance of the capacitor.
3.4 Thermoelectric generator
At present, the global energy crisis (supply shortage, pollution to the atmosphere, etc.) Has reduced the demand for fossil fuel consumption, and researchers around the world are committed to developing new technologies for sustainable power generation. thermoelectric generators (TEGs) are devices that can directly convert thermal energy into electrical energy. They have a series of advantages, such as no noise, no vibration, easy maintenance and environmental friendliness.
Compared with traditional inorganic thermoelectric materials, PEDOT ∶ PSS has the advantages of non-toxicity, light weight, low cost, good flexibility and low thermal conductivity
[126,127]. The introduction of inorganic fillers into PEDOT ∶ PSS matrix can improve the thermoelectric properties of PEDOT ∶ PSS through the filtration effect of organic/inorganic interfacial energy and the control of filler morphology
[128][129]. Among them, the preparation of PEDOT ∶ PSS-based composites with graphene as filler is an effective way to improve their thermoelectric properties, because graphene has excellent electrical conductivity, mechanical properties and chemical stability
[130,131]. Liu et al. mixed graphene dispersion with PEDOT: PSS solution by solution blending method, and obtained PEDOT: PSS/graphene nanocomposite thermoelectric film after vacuum filtration and drying
[132]. After it is cut into thin film strips, it can be assembled into TEGs. Tests show that the prepared TEGs have good stability under different bending directions and curvature radii. This also shows that PEDOT ∶ PSS/graphene composite film is expected to be used in flexible thermoelectric devices.
Similarly, Bi
2Te
3 alloys have good thermoelectric properties at room temperature, but their application is limited due to their high cost and difficult processing. Therefore, researchers have considered the preparation of organic-inorganic composites. Among them, one of the most promising systems is PEDOT∶PSS/Bi
2Te
3 composites. However, the oxide on the surface of the Bi
2Te
3 inhibits the improvement of the thermoelectric properties of the composite
[133⇓~135]. Based on the progress made in the bottom-up synthesis of nanomaterials, a method of encapsulating Bi
2Te
3 nanowires with PEDOT ∶ PSS to reduce or even remove the oxides on the surface of Bi
2Te
3 was proposed
[136,137]. Thongkham et al. Prepared hybrid nanowires with Bi
2Te
3 as the core and PEDOT: PSS as the shell to inhibit the aggregation of Bi
2Te
3 and the encapsulation of surface oxides
[138]. PSS acts as a stabilizer for the formation of perylene nanowires and a surfactant for the dispersion of Bi
2Te
3 in the PEDOT ∶ PSS solution. As shown in step (1) of fig. 9, PEDOT∶PSS/Bi
2Te
3 hybrid nanowires were synthesized in situ by a one-pot method. The prepared lightweight flexible thermoelectric foam has the best thermoelectric performance among all the PEDOT ∶ PSS-based thermoelectric materials. Its maximum output power can reach 130 μW at low temperature (< 100 ℃), and it has good operation stability within 15 days. Due to the simple preparation of the hybrid nanowire and the thermoelectric foam, the material can be produced on a large scale.
3.5 Other application
In addition to the above fields, PEDOT ∶ PSS/inorganic nanocomposites also have applications in other fields, such as adsorbents, lithium-sulfur batteries, direct methanol fuel cells, electrochromic films and so on
[139].
In order to improve the storage performance of lithium, PEDOT ∶ PSS/sulfur nanocomposites were prepared by hydrothermal method in the presence of surfactant, and used as the cathode of lithium ion battery
[140]. The polymer coating formed by PEDOT ∶ PSS on the sulfur particles is beneficial to electron conduction and can inhibit the dissolution of lithium polysulfide. The electrochemical study showed that the capacity of the battery remained stable after 200 cycles. This study proves that PEDOT ∶ PSS/sulfur nanocomposites have broad application prospects as cathode materials for next-generation lithium-sulfur batteries with high energy density. In addition, MnO
2 nanorods were prepared by a hydrothermal method, and subsequently, PEDOT∶PSS/MnO
2/rGO ternary nanocomposites were synthesized by an in-situ polymerization method
[141]. Electrochemical studies show that the composite is more stable than PEDOT∶PSS/MnO
2 nanocomposites and MnO
2 nanorods, and has lower onset potential and higher oxidation current density, which can improve the electrocatalytic activity of methanol oxidation and is a potential alternative for platinum-based anode catalysts in direct methanol fuel cells. In the study of Li et al., W
0.71Mo
0.29O
3/PEDOT∶PSS nanocomposite films were prepared on FTO glass by layer-by-layer spray deposition, which can be applied to electrochromism
[142]. It was observed that PEDOT ∶ PSS chains penetrated into the porous nanorod layer of the W
0.71Mo
0.29O
3 and formed an interpenetrating conductive network. Compared with the W
0.71Mo
0.29O
3, the composite film has shorter response time, higher optical contrast and coloring efficiency. In addition, the composites have good cycle stability, which makes them have broad application prospects in industrial applications.
4 Conclusion and prospect
PEDOT ∶ PSS has attracted much attention because of its high conductivity, good flexibility, high transparency, high thermal stability and solution processability after treatment. In recent decades, researchers have not only tried to improve the conductivity of PEDOT ∶ PSS by acid treatment, salt solution treatment, organic solvent treatment, surfactant treatment and other ways.Its conductivity has reached the level of the traditional transparent conductive material ITO, and great efforts have been made to introduce inorganic nanomaterials with unique physical and chemical properties into the PEDOT: PSS matrix. Therefore, PEDOT ∶ PSS/inorganic nanocomposites have been developed rapidly. In addition, the discovery of graphene indicates that the prospect of PEDOT ∶ PSS nanocomposites is broader. Because graphene is a carbon nanomaterial with excellent chemical structure and physical properties, it has a relatively mature synthesis technology. Up to now, PEDOT ∶ PSS/inorganic nanocomposites with excellent properties have been synthesized by introducing inorganic fillers into PEDOT ∶ PSS, which can be widely used in sensors, solar cells, supercapacitors, thermoelectric generators, adsorbents and other fields.
However, PEDOT ∶ PSS/inorganic nanocomposites are currently mainly used in electrical devices, and its application in coatings is relatively small.In the future, PEDOT ∶ PSS/inorganic nanocomposites can be applied to wearable flexible substrates by introducing a series of inorganic nanoparticles with flame retardant, antibacterial, superhydrophobic, self-cleaning and other functions. At the same time, as a new research field, PEDOT ∶ PSS/inorganic nanocomposites still have some problems to be solved. First, the stability of PEDOT: PSS/inorganic nanocomposites is very important to obtain the desired properties. Inorganic nanoparticles are easy to agglomerate, if they can be dispersed uniformly in PEDOT: PSS, a stable interfacial film can be formed, and the respective properties of PEDOT: PSS and inorganic nanoparticles can be brought into play. To solve this problem, we can find a new synthesis method to obtain a better compatibility between the inorganic phase and the organic phase of nanoparticles. Secondly, the composite mechanism and the relationship between structure and properties of nanocomposites need to be further explored, so as to deepen the theoretical level. Finally, we should further explore how to improve the electrical conductivity, optical properties and biological properties of composite materials. If these problems can be solved, it can be predicted that many PEDOT ∶ PSS/inorganic nanocomposites with special functions will be developed, which will bring great sustainable value to society and become one of the greatest achievements in the field of material science. Based on the rapid progress in this field, it is conceivable that PEDOT ∶ PSS/inorganic nanocomposites will be applied in more fields in the near future.
National Key Research and Development Program of China(2017YFB0308602)
National Natural Science Foundation of China(51903143)
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