Nanocomposites based on graphene analogous materials and conducting polymers: a review

Abstract

Interest in nanocomposite materials has grown rapidly over the past decade. In this time, graphene, graphene analogs, and polymers have increasingly found use as components in nanocomposite materials. Combining multiple materials at the nanoscale provides an opportunity to produce nanocomposites with unique and in many cases enhanced properties. Conductivity is one such property, which can be enhanced through the interaction of multiple materials at the nanoscale. In this review, we highlight nanocomposites derived from graphene analogous materials (MoS₂, WS₂, BN, MXenes) and electronically conducting polymers (polyaniline, polypyrrole, polythiophene, and derivatives). Methods of nanocomposite preparations including solution or nanodispersion mixing, melt mixing, and in situ polymerization are discussed, as well as specific applications of nanocomposite materials.

Introduction

A nanocomposite can be broadly defined as any solid material of multiple phases, where at least one of the phases has one or more dimensions of less than 100 nm. In addition, chemical structures which have repeating distances between different phases at the nanoscale can also be termed nanocomposites. Nanocomposites are useful because the combination of different materials at the nanoscale leads to markedly different optical, electrical, electrochemical, thermal, and mechanical properties arising from the combined components compared to the components on their own. The change in properties can usually be explained by physical processes which occur at the interface between the components in the nanocomposites and by considering the relative size of these components. The mechanisms involved in such phenomenon are among others: mechanical load transfer, percolation, and electrical charge transfer through doping. The definition of nanocomposites is obviously very broad, including colloids, copolymers, gels, general porous media, and many other nanomaterials. In this review, the term nanocomposite will be generally applied to composites in which a nanoscale graphene analogous material (MoS₂, WS₂, BN, MXenes) is combined with a polymer, in particular a polymer which is electronically conducting.

As previously stated, when a nanocomposite material is prepared, it tends to have different properties than the materials it is created from. In some cases, this leads to a new material that can fulfill a purpose that other materials cannot, or that can perform better than other materials over a wider range of conditions. This ability to improve a material or to tune it for a certain task is one of the main motivations for scientists researching on nanocomposites.

There is an enormous amount of applications for nanomaterials over a broad range of areas, and there is without a doubt going to be many more as this emerging field continues to grow. An example of an application is provided in Ref. [1], where the authors describe how a small amount of nanostructure can lead to a great improvement in the fire-resistant properties of certain materials. According to the National Institute of Standards and Technology [2], a nanodispersion of mica-type clays in common polymers such as polystyrene ‘exhibits the unusual combination of reduced flammability in the form of lower peak heat release rates, and improved physical properties’ [3].

Nanomaterials are also very useful for increasing the strength of common materials. A good example is the use of carbon nanotubes to strengthen ceramic or metal matrices, a practice which is commonly done today [4].

One of the most exciting ways nanomaterials are being utilized is as anode and cathode materials in high-output lithium-ion batteries. One study produced a material that was able to deliver a reversible capacity of 209 mAh g⁻¹ [5]. With the rising interest in green energy, a more efficient lithium-ion battery would be of great value. Nanocomposites appear to offer much promise in this growing field. Nanocomposites have also found extensive use in semiconducting applications, particularly in the areas of integrated circuitry, printed electronics, and through-silicon via technology [6].

The impetus for working on graphene analogous materials is due to the discovery of graphene in 2004 by Geim and Novoselov. This discovery earned Geim and Novoselov the 2010 Nobel Prize in Physics for groundbreaking experiments on graphene [7]. Graphene is a zero band gap two-dimensional material with a carbon honeycomb structure which is as thin as a single carbon atom. Soon after its discovery, graphene was found to exhibit some very unique and interesting properties. For example, the electrons in graphene behave as if they are massless relativistic particles. The effect of this is an anomalous quantum Hall effect resulting from Dirac-type low-energy electron excitations [8, 9]. Key to the electronic applications of graphene is quantum (2D) confinement which gives it the ability to conduct both electrons and holes, unlike many conventional metals typically used in electronics. Graphene also exhibits very good thermal conductivity on the order of 5000 Wm⁻¹ K⁻¹, as well as exceptional electron mobility of almost 250,000 cm²/Vs, at room temperature [10, 11]. It is also worthy to note that graphene exhibits a Young’s modulus of 1 TPa which speaks to the material’s superior mechanical properties [12]. There are many excellent reviews on polymer nanocomposites based on graphene [13,14,15,16,17,18,19,20,21,22]. Hence, this review will focus primarily on polymer nanocomposites based on graphene analogous materials (GAMs), although wherever appropriate, references will be made to graphene due to its similarity to GAMs. In particular, the focus will be on nanocomposites based on graphene analogous materials (GAMs) and electronically conducting polymers such as polyaniline, polypyrrole, polythiophene, and its derivatives. Due to the vast diversity of GAMs, this review will focus only on MoS₂, WS₂, BN, and MXenes. Comprehensive summaries on the synthesis, characterization, and applications of nanocomposites based on the aforementioned GAMs and conducting polymers are provided, including pioneering works in the field as well with up-to-date reports. While there are excellent reviews on nanocomposites based on graphene and polymers in general [13,14,15,16,17,18,19,20,21,22], our review distinguishes itself from the others since our focus is on the fabrication of nanocomposites based on GAMs (MoS₂, WS₂, BN, MXenes) and conducting polymers (polyaniline, polypyrrole, polythiophene, and derivatives). We also pay particular attention to the structural characteristics of these nanocomposites, as to whether they are intercalated, exfoliated or the polymer chains simply adsorbed on the surface of the GAM.

Preparation of nanocomposites based on GAMs

The techniques used for preparing GAMs–polymer nanocomposites are similar to those used for preparing graphene-based polymer nanocomposites. The most popular and practical approaches include solution mixing, melt mixing, and in situ polymerization.

Solution mixing and melt mixing

Solution-based methods usually involve suspending the GAM in a solvent, usually with the help of ultrasonication, and then mixing the colloidal suspension with a solution of the polymer. The solvent used for dissolving the polymer should be miscible with that used in the formation of the colloidal suspension of the GAM. The solvent system can then be left to evaporate, or the solution can be centrifuged and the product collected [23]. However, this can lead to aggregation of the GAM material in the nanocomposite, which can negatively affect the properties of the material. This problem can be alleviated by adding a ‘non-solvent’ for the select polymer–solvent colloidal suspension. A non-solvent addition to a miscible solvent decreases the solubility of the polymer in solution. This is generally due to the solvent–non-solvent interactions becoming more favorable than the solvent–polymer interactions, upon addition of the non-solvent. This can cause the filler material to become encapsulated within the polymer chains upon precipitation. This technique leads to a less aggregated product [24].

In general, solution mixing is seen as a simple way to prepare nanocomposite materials based on graphene analogous materials. The primary reason besides the simple procedure is that the degree of dispersion of the filler in the composite is largely due to the degree of exfoliation the filler possesses before the mixing takes place. If poorly exfoliated filler is used, then the nanocomposite will contain larger, more crystalline components. A well-exfoliated filler material will produce a finer less crystalline dispersion throughout the polymer matrix.

Melt mixing is a process that involves taking the filler in the solid state and a polymer melt and mixing them in a high-shear mixer, which mechanically forces the filler into the polymer matrix. Melt mixing is a common procedure in industry, primarily because it is cheaper than solution mixing, and it is easier to mass produce the desired product. There is also no need for solvents when melt mixing is utilized [25]. The downfall of this technique is that it does not disperse the filler material as well as in in situ polymerization or solution mixing processes [26]. As well, there is no practical way to produce true nanoscale composites using melt mixing without first exfoliating the layered material [27].

In situ polymerization

The in situ method of preparing nanocomposites involves mixing a monomer solution with the desired filler material and then polymerizing the monomer to form the desired product. The product is then generally precipitated out and extracted or cast as thin films. It is possible to form both covalently linked and non-covalently linked nanocomposites using a variety of polymers and fillers by utilizing the in situ polymerization method. In situ polymerization can be used to promote non-covalent polymer–filler interactions. For example, the in situ polymerization of pyrrole in the presence of the graphene analogue MoS₂ resulted in a nanocomposite with a high specific capacitance of 553.7 F g⁻¹, which still retained 90% of its capacitance after 500 cycles at a current density of 1 A g⁻¹ [28]. The in situ polymerization technique was used to produce nanocomposites consisting of polystyrene and graphene-like layered MoS₂ modified with cetyl trimethyl ammonium bromide (CTAB), as well as nanoscale materials comprised of poly(methyl methacrylate) and MoS₂ functionalized with CTAB. Both nanocomposites exhibited enhancement in thermal stability and fire resistance properties [29].

One of the advantages of in situ polymerization is that unlike melt mixing or solution mixing, the filler does not always have to be exfoliated prior to preparation of the nanocomposite. Nonetheless, with the in situ technique, the filler becomes highly dispersed in the polymer matrix. Monomers can be intercalated into layered hosts and polymerized inside the gallery space of the hosts. If the expanded layers remain ordered, the product is an intercalated nanocomposite. However, in this method the layered host could also be exfoliated into single layers during the polymerization of the monomer. This will typically produce an exfoliated nanocomposite. In situ polymerization is a common practice when preparing, e.g., V₂O₅-based nanocomposites [30]. An illustration of solution mixing, melt mixing, and in situ polymerization is shown in Fig. 1.

Figure 1

a Solution mixing, b melt mixing, and c in situ polymerization

Additional methods of nanocomposite preparation

There are several other methods for dispersing graphene analogous materials in a polymer matrix to form nanocomposites. Those listed in "Solution mixing and melt mixing" and "Additional Methods of Nanocomposite Preparation" are the most common; however, many other methods exist; these are usually described in the literature as pertaining to one specific composite system or a closely related family of composites [31]. However, it should be noted that there is insufficient research in this area of nanocomposite preparation. Therefore, these additional methods which have been proven to work for specific composite systems may also work as general procedures for novel nanocomposite preparations.

These additional methods include non-covalent grafting of polymers onto the filler material via π–π stacking [32, 33]. In the case of conducting polymers, the non-covalent nature of the grafting may help to better preserve the conjugated nature (conductivity) of some graphene analogous substrates, which in turn may help to increase the conductivity of the nanocomposite material as a whole [34]. Other methods include several adaptations of in situ polymerization, melt mixing, and solution mixing. These include but are not limited to emulsion polymerization [35], phase transfer techniques [36], lyophilization methods [37], shear pulverization [38], layer-by-layer assembly [39], and mechanochemical treatment [40]. All of these methods have been successfully used in preparing various nanocomposite materials.

Graphene analogous material-based nanocomposites

Overview and history of GAMs

Extensive research on graphene has opened the door to a wealth of new discoveries related to graphene analogous materials. Graphene analogous materials (GAMs) are described as ‘analogous to graphene’ due to their graphene-like two-dimensional structure. Like graphene, GAMs are formed of individual atomic layers which can be exfoliated from a three-dimensional bulk material comprised of stacked two-dimensional layers. This leads to exceptional properties arising from low dimensionality and edge states. They have found applications in industry, research, and electronic devices. GAMs are van der Waals materials which are easily obtained via exfoliation along the z-axis of their three-dimensional counterpart layered structures. In many cases, these exfoliated individual layers can be dispersed into a conducting polymer matrix, thus forming nanocomposites based on GAMs and conducting polymers.

In 2005, Novoselov et al. [41] isolated single layers of several types of GAMs, and these included h-BN, dichalcogenides such as NbSe₂ and MoS₂, and complex oxides, e.g., Bi₂Sr₂CaCu₂O_x. Since 2005, several other GAMs have been successfully isolated or synthesized in increasingly efficient ways. These include but are not limited to silicene, BC₃, transition metal dichalcogenides such as WS₂, and many others.

Nanocomposites based on common GAMs

Since the discovery of graphene in 2004, there has been an increased interest in graphene-based nanocomposites. As this field continues to grow, so does the emerging field of nanocomposites based on graphene analogous materials.

While many GAMs do exist [42], in this section we will limit ourselves to MoS₂, WS₂, BN, and MXenes and explore their utilization as fillers in polymer-based nanocomposites. Particular attention is paid to nanocomposites which contain conducting polymer matrices.

Transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) are a family of compounds which have the general formula MX₂ where M=Mo, W, V, Nb, Ta, Ti, Zr, Hf, etc., and X=S, Se, Te. Some common examples of TMDs are TiS₂, WS₂, MoS₂, MoSe₂, and VS₂; however, many others exist. We will focus our attention on two of the most common TMDs used in polymer-based nanocomposites. These are molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) [43]. A summary of monolayer transition metal dichalcogenide properties is provided in Fig. 2.

Figure 2

Reproduced with permission from Ref. [44]

A summary of the structural and electronic properties of monolayer transition metal dichalcogenides.

MoS₂

MoS₂ is the principal ore of molybdenum and is mined in vast quantities as molybdenite. It is cheap to buy high-grade MoS₂, making it an attractive candidate for research and a cost-effective material for use in industry. Single-layered MoS₂ is a semiconductor possessing a direct band gap of 1.8 eV. The monolayer band gap of MoS₂ in addition to the layer dependence of the band structure provides complimentary electronic properties to nanocomposite materials. As an alternative to the zero band gap associated with graphene, the finite band gap of single-layer MoS₂ is scientifically and industrially important for applications such as in transistors, optoelectronics, energy harvesting, and other nanomaterial fields [45]. When incorporated as a filler into polymeric matrices, its electronic properties can be altered [42]. When the nanocomposites require flexibility and must bear an applied load this becomes readily apparent. A transition from direct to indirect gap can be observed upon the application of relatively small strain (∼ 2%). A large tensile strain of about 8% or a compressive strain of about 15% should lead to a semiconductor–metal transition. Application of uniaxial strain to single-layered MoS₂ also leads to a reduction of its band gap and transforms it to an indirect band gap semiconductor [46]. The layered structure of MoS₂ is shown in Fig. 3.

Figure 3

Reproduced with permission from Ref. [44]

a The atomic structure of layered MoS₂. b A top view of the honeycomb lattice, emphasizing the inversion symmetry breaking.

A number of studies have been undertaken in an attempt to entrap different polymers in the layered structure of MoS₂. The intercalation of polyaniline (PANI) into MoS₂ was first reported by Kanatzidis et al. [47]. The methodology involved the pre-intercalation of Li⁺ into MoS₂, followed by reaction with water to form exfoliated MoS₂ nanosheets. Addition of an NMP solution of PANI in the emeraldine base form to the separated layers resulted in the formation of an intercalated nanocomposite. Years later, the intercalation of polyaniline, poly(N-methyl aniline), poly(2-ethyl aniline), and poly(2-propyl aniline)) into layered MoS₂ was achieved by Bissessur and White [48]. The polymers were utilized in their protonated form as colloidal suspensions in NMF. The ramification of this work is that complete solubility of the guest species is not a requirement for performing intercalation chemistry on MoS₂ monolayers.

The PANI–MoS₂ nanocomposites prepared by Kanatzidis et al. and by Bissessur et al. were shown to be intercalated by powder X-ray diffraction. Since then, there have been other reports on PANI–MoS₂ nanocomposites. For example, Wang et al. [34] reported on the synthesis of PANI–MoS₂ nanocomposite by exfoliating 2H–MoS₂, followed by in situ polymerization of aniline with ammonium peroxydisulfate (Fig. 4).

Figure 4

Reproduced with permission from Ref. [34]

Intercalation of PANI into MoS₂.

As already demonstrated by previous researchers [47,48,49], MoS₂ undergoes complete lithiation when reacted with n-BuLi to form LiMoS₂. The LiMoS₂ undergoes a redox reaction with water to produce single layers of MoS₂. The single layers have a tendency to re-stack; however, in the presence of foreign species such as polymers [47, 48] or even small molecules [49], the layers re-stack with these species trapped in between. The XRD data in the article of Wang et al. showed only partial intercalation of PANI into MoS₂ (peak at 2θ = 7.58°), and strong evidence of unintercalated MoS₂ (peak at 2θ = 14.3°). By varying the amount of MoS₂ in the reaction mixture, a series of compositions were obtained with specific capacitance several orders of magnitude larger than that of pure PANI, showing the potential of these nanocomposites as electrodes in supercapacitors.

A further example is that of Wang et al. [50], where the authors first and foremost intercalated oleic acid into MoS₂. Aniline was then introduced into the gallery space through an amidation reaction with the interlamellar oleic acid. Subsequently, treatment with an external oxidizing agent led to the production of an intercalation compound of PANI in MoS₂. Evidence of intercalation was obtained from XRD data, and FTIR spectroscopy confirmed that the identity of the final encapsulated species was, in fact, polyaniline in the protonated, conducting form (Fig. 5). The PANI–MoS₂ exhibited excellent lithium and sodium storage capacity.

Figure 5

Reproduced with permission from Ref. [50]

a The synthetic steps, b FTIR spectrum of MoS₂–OA and PANI–MoS₂, c XRD patterns of MoS₂–OA, PANI–MoS₂, and annealed MoS₂.

Zhang et al. [51] reported on the synthesis of MoS₂–PANI nanocomposites with three different morphologies, namely aggregated particles (MoS₂–PANI-1), nanowires (MoS₂–PANI-2), and nanotubes (MoS₂–PANI-3). The morphology of the final product obtained was dictated by the pH used in the second step of the synthesis as outlined in Fig. 6. Due to its hollow structure, MoS₂–PANI-3 exhibited the highest specific capacitance of 375 F g⁻¹ at a current density of 50 A g⁻¹ and also demonstrated good stability with respect to charge–discharge cycling.

Figure 6

Reproduced with permission from Ref. [51]

Preparation of MoS₂–PANI–1, MoS₂–PANI-2, and MoS₂–PANI-3. APS: (NH₄)₂S₂O₈.

Zeng et al. [52] have covalently bonded polyaniline to MoS₂ nanosheets. The synthetic procedure is illustrated in Fig. 7. The first step involved lithiation of bulk MoS₂ with n-BuLi to form Li_XMoS₂. Reaction of Li_xMoS₂ with water resulted in the formation of MoS₂ monolayers (CE–MoS₂). The CE–MoS₂ was then functionalized with 4-aminophenyl (MoS₂–NH₂) by reacting with 4-aminophenyl diazonium salt. Treatment of MoS₂–NH₂ (100 mg) with varying amounts of aniline (100–300 μL) in the presence of (NH₄)₂S₂O₈ in acid medium led to the formation of MoS₂–NH₂–polyaniline nanocomposites of varying compositions. The authors found out that the use of 150 μL of aniline resulted in an optimum composition in terms of electrochemical performance in supercapacitor applications. The optimized composition exhibited a high capacitance of 326.4 F g⁻¹ at 0.5 A g⁻¹ and retained 63.1% of its original capacity even at 1000 A g⁻¹.

Figure 7

Reproduced with permission from Ref. [52]

Steps leading to the formation of MoS₂–NH₂–PANI nanocomposites.

There are other reports of PANI–MoS₂ nanocomposites. However, these are not intercalation compounds; the polymer is simply adsorbed on the surface of the MoS₂ nanosheets, or the authors do not provide evidence of intercalation by powder X-ray diffraction [53,54,55,56,57,58,59] Yet, these nanocomposites have been shown to be beneficial for use as electrodes in supercapacitor applications [53], sensing platform for simultaneous determination of adenine and guanine [54], modified electrode for detection of bisphenol A [55], electrode with applications in DNA detection [56, 57], as biosensor for detection of chronic myelogenous leukemia [58], and photocatalyst for the degradation of organic pollutants such as methylene blue (MB) and 4-chlorophenol [59].

It is worthy to note that there are few reports of PANI–MoS₂ nanocomposites that are genuinely exfoliated and therefore non-crystalline or amorphous. For instance, Ren et al. [60] synthesized exfoliated nanocomposites consisting of PANI and MoS₂ nanosheets. When used as an electrode, the nanocomposite containing 53% by weight of PANI showed a specific capacitance of 478 F g⁻¹ at 0.5 A g⁻¹, which is three times higher than that of pristine MoS₂. Moreover, the nanocomposite retained a specific capacitance of 378 F g⁻¹ when the current density was increased to 30 A g⁻¹, indicating fast ion and electron transport within the nanocomposite (Fig. 8).

Figure 8

Reproduced with permission from Ref. [60]

Capacitance retention from 0.5 to 30 A g⁻¹.

Furthermore, exfoliated PANI–MoS₂ nanocomposites with varying percentage by mass of exfoliated MoS₂ have recently been reported by Lyle et al. [61]. It is interesting to observe that the electronic conductivity of the nanocomposites containing low % by mass of MoS₂ (e.g., 1% and 10%) exhibited higher conductivity than pure PANI (Fig. 9).

Figure 9

Reproduced with permission from Ref. [61]

Conductivity versus temperature for PANI, and exfoliated MoS₂–PANI nanocomposites. The percentage of MoS₂ in the nanocomposites is indicated in the legend.

While the majority of the work in the literature has been on the synthesis, characterization, and device applications of nanocomposite based on polyaniline and MoS₂, there has also been an effort toward the development of polypyrrole (ppy)–MoS₂ nanomaterials.

The intercalation of polypyrrole into MoS₂ was first reported by Wang et al. [62]. The methodology of intercalation was similar to that used by Wang et al. from Ref. 34 (Fig. 4), the only difference was that pyrrole was used in lieu of aniline, and iron(III) chloride was employed as the oxidizing agent instead of ammonium peroxydisulfate. The authors provided strong evidence of complete intercalation from powder X-ray diffraction measurements (Fig. 10). An increase of ca 4.5 Å of d-spacing of the host material was observed in order to accommodate a single layer of the polymer chain.

Figure 10

Reproduced with permission from Ref. [62]

XRD pattern of a (ppy)_xMoS₂, b MoS₂ (restacked).

In 2006, Bissessur and Liu achieved the direct intercalation of polypyrrole (ppy) into MoS₂. Polypyrrole was first synthesized and then dispersed in NMF with the help of ultrasonication. The colloidal suspension of the polymer was then added to exfoliated layers of MoS₂ in water. This procedure led to the insertion of the polymer within the restacked MoS₂. The interlayer expansion value of the sandwiched compound with respect to pristine layered MoS₂ was typically around 3.5 Å. This is consistent with having a single layer of the polymer chain lying parallel with respect to the dichalcogenide sheet [63].

There are several other reports describing the synthesis of ppy–MoS₂ nanocomposites; however, the polypyrrole is coated on the surface of the MoS₂ flakes, and the resulting nanocomposites are not, strictly speaking, of the intercalated type as per the reported XRD data [28, 64, 65]. However, these nanocomposites can exhibit higher conductivity in comparison with the pure components. For example, nanocomposites consisting of MoS₂ and 15–30 wt% ppy showed conductivity value as high as 13 S cm⁻¹ [65]. Hence, ppy–MoS₂ nanocomposites, although not of the intercalated-type, can be used as electrode materials for high-performance supercapacitor applications [28, 64, 65].

Of particular interest is the recent report of Asan et al. [66] where the authors studied the doping effect and amount of exfoliated MoS₂ on polypyrrole coatings. The corrosion rates of these coatings were measured in 0.1 M H₂SO₄. Polypyrrole coating containing 25 ppm MoS₂ gave the best corrosion protection for brass, with a corrosion protection efficiency of 99.38%, higher than polypyrrole coating with a corrosion protection efficiency of 77.68%.

Hong et al. [67] synthesized exfoliated polypyrrole–MoS₂ nanocomposites. The ppy–MoS₂ nanocomposites were prepared by polymerization of pyrrole with ammonium peroxydisulfate, in the presence of the exfoliated MoS₂ (prepared in the solid state by reacting molybdic acid with excess thiourea at 500 °C, under inert atmosphere).The amount of MoS₂ in the reaction mixture was systematically varied to produce a range of nanocomposite materials ranging from 1 to 50% by mass of MoS₂. Enhanced conductivity over the pure polymer was observed when the MoS₂ content was 10% and 35% by mass.

Polythiophene (PT) has also been intercalated into MoS₂. By using a similar methodology as described in Ref. 34 (Fig. 4), Lin et al. [68] were able to prepare PT–MoS₂ intercalated nanocomposites, where thiophene was used as the monomer. The electronic conductivity of the PT–MoS₂ nanocomposite was found to be greater than that of pristine layered MoS₂ by a factor of 16.

Of particular interest has been the coating of polythiophene onto MoS₂ nanotubes [69]. MoS₂ nanotubes (average diameter 285 nm) were first prepared by thermal decomposition of hexadecylamine–MoS₂ lamellar precursor on anodized aluminum oxide which served as a template. The MoS₂ nanotubes were dispersed in ethanol, cast as a thin film onto a glass slide, and then allowed to dry at room temperature. Thiophene monomer was then cast onto the dry MoS₂ nanotube film. Application of atmospheric pressure glow discharge RF plasma at various power (117 W, 234 W, 360 W) resulted in the fabrication of polythiophene-coated MoS₂ nanotubes (Fig. 11). The use of 360 W plasma power resulted in a more uniform alignment of the PT–MoS₂ nanotubes. The plasma polymerization technique is a green efficient approach that requires no solvents.

Figure 11

Reproduced with permission from Ref. [69]

SEM of a powdered MoS₂, b MoS₂ nanotubes.

Poly(3,4-ethylenedioxythiophene) (PEDOT) has also been intercalated into MoS₂ [70]. To MoS₂ single layers was added 3,4-ethylenedioxythiophene monomer. The reaction mixture was refluxed, and an aqueous solution of iron(III) chloride was added. This experimental procedure led to the formation of PEDOT–MoS₂ intercalated material. Evidence of intercalation was obtained from powder X-ray diffraction. The conductivity of the PEDOT–MoS₂ was found to be one order of magnitude larger than that of pristine MoS₂. The improved conductivity of the PEDOT–MoS₂ and its larger interlayer spacing of 13.76 Å contributed to a significantly enhanced discharge capacity when used as a cathode in a lithium cell (Fig. 12).

Figure 12

Reproduced with permission from Ref. [70]

Discharge capacity as a function of number of cycles for MoS₂ and PEDOT–MoS₂, data obtained at a current density of 15 mA g⁻¹.

WS₂

Tungsten disulfide (WS₂) is another common transition metal dichalcogenide, and it is isostructural to MoS₂. Similar to MoS₂, single-layered WS₂ is a semiconductor. When going from the three-dimensional bulk 2H-structure to a single 1H-WS₂ layer, the system becomes a direct band gap semiconductor with an increase in the energy gap from 1.3 to 1.8 eV, respectively [46, 71].

Conducting polymers have been intercalated into layered WS₂, albeit to a lesser extent in comparison with MoS₂. For example, Xu et al. reported on the encapsulation of polypyrrole into layered WS₂. Li_xWS₂ was first prepared by reaction of WS₂ with n-BuLi in an autoclave at 100 °C for 5 h, using hexane as the solvent. The Li_XWS₂ was treated with water to create exfoliated WS₂ layers. The monomer pyrrole was then introduced to the single WS₂ layers, followed by the addition of iron(III) chloride in a methanol/H₂O mixture. The collected product was an intercalation compound of polypyrrole within WS₂, as evidenced by XRD. The observed interlayer expansion of 3.43–3.76 Å was consistent with having a monolayer of the polypyrrole sandwiched parallel between the WS₂ sheets as shown in Fig. 13 [72].

Figure 13

Reproduced with permission from Ref. [72]

Lamellar representation of polypyrrole intercalated in WS₂.

Lane et al. reported on the synthesis and characterization of exfoliated nanocomposites consisting of PANI and WS₂. The authors reported an eightfold increase (24.5 S cm⁻¹) in conductivity when 15% by mass of exfoliated WS₂ was dispersed in the PANI polymer matrix [73].

Stejskal et al. [74] were able to deposit polypyrrole on the surface of WS₂ by oxidation of pyrrole with iron(III) chloride hexahydrate in water at ambient temperature. They did not make any attempt to exfoliate the WS₂ flakes. The polypyrrole-coated WS₂ nanocomposite showed enhancement in electrical conductivity compared to the components all themselves. Nanocomposites consisting of various contents of ppy were created, and their efficiency in hydrogen evolution reaction (HER) was evaluated. The authors noted that HER took place primarily at the edge of the WS₂, and that coating the WS₂ with ppy decreased HER activity by blocking the edge site. However, a mere physical mixture of the two components enhanced HER activity (Fig. 14).

Figure 14

Reproduced with permission from Ref. [74]

Illustration of HER activity a occurring at the edges of WS₂, b diminished due to excessive coating with ppy, c enhanced by using a physical mixture of WS₂ and ppy; CB = carbon black.

Recently, Arsenault et al. [75] showed that exfoliated tungsten disulfide-polythiophene nanocomposites can be synthesized. They found out that the nanocomposite with 5% by mass of WS₂ exhibited an order of magnitude higher in conductivity compared to the pure polythiophene.

Nanocomposites materials consisting of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) and WS₂ nanotubes have also been fabricated. It is noteworthy that PFO is a semiconducting polymer that has been widely studied due to its high mobility, good solubility, and characteristic blue light emission. PFO has been applied as a light source in organic light-emitting diodes (OLEDs). On the other hand, WS₂ nanotubes possess excellent tribological and mechanical properties. In order to tap into the characteristics of both materials, Luccio et al. [76] prepared nanocomposites consisting of PFO and WS₂ nanotubes at 10, 30, and 50% by weight WS₂. The nanocomposites and pure PFO were used in OLEDs devices, and the electro-optical properties of these devices were studied. The authors found that the efficiency of OLEDs devices utilizing the nanocomposites was lower than that of the OLED containing only pure PFO; however, the observed decrease in efficiency was not too dramatic (Fig. 15).

Figure 15

Reproduced with permission from Ref. [76]

Electro-optical characterization of OLED devices: threshold voltage (a) and efficiency (b) as a function of WS₂ nanotube concentration.

Boron nitride

Boron nitride (BN) can exist in the form of nanosheets (BNNS) consisting of hexagonal arrangement of boron and nitrogen atoms covalently bonded to form a two-dimensional material of great strength [77] (Fig. 16). BNNS is analogous to graphene. However, BN can also exist in the form of nanotubes (BNNT), analogous to carbon nanotubes [77, 78]. BNNS possesses a bang gap of 4.60 eV [79], whereas BNNT has a band gap of about 5.5 eV [78, 79].

Figure 16

Reproduced with permission from Ref. [77]

The structure of typical BNNS.

Nanocomposites containing BN and conducting polymers have been explored. For instance, there are several reports on PANI–BN systems. For example, self-organized BNNTs–PANI nanocomposite films were obtained by mixing and sonicating the two components in N,N dimethylformamide [80]. The PANI was actually in the emeraldine base form (EB). Favorable interaction of EB with BNNT led to its ordering within the composite material, as denoted by a sharpening of the broad peak corresponding to the EB (2θ = 19°) in the XRD pattern of the composite (Fig. 17).

Figure 17

Reproduced with permission from Ref. [80]

XRD pattern of BNNT, EB and EB/BNNT composite.

Raman shifts observed in the composite were also consistent with interaction between the EB and BNNT. The Raman spectrum of EB displayed two major peaks, at 1350 and 1579 cm⁻¹. In the Raman spectrum for the composite, the EB peak at 1350 cm⁻¹ could not be detected due to strong overlap with the BNNT peak; the BNNT itself shifted from 1365 to 1352 cm⁻¹. However, the EB peak at 1579 cm⁻¹ was shifted to 1589 cm⁻¹ (Fig. 18).

Figure 18

Reproduced with permission from Ref. [80]

Raman spectrum of BNNT, EB, and EB/BNNT.

An in situ polymerization method was also used to prepare BNNTs–PANI nanocomposites, and it was found that presence of BNNTs induced the formation of crystalline phases within the PANI [81]. The nanocomposites were prepared by absorbing aniline on BNNTs, followed by immersion in an aqueous solution of ammonium peroxydisulfate (APS) for 24 h. From TEM, it was observed that the average thickness of the PANI film on the surface of the BNNT was 2–5 nm, and by using excess aniline, the thickness could reach 20 nm. TEM also revealed that some of the BNNTs are filled with PANI chains, as aniline molecules and APS solution were siphoned inside the nanotubes where the polymerization reaction took place, with the encapsulated PANI interacting with the inner surface of the BNNTs (Fig. 19).

Figure 19

Reproduced with permission from Ref. [81]

a Low-magnification TEM image of PANI–BNNTs, b high-resolution TEM image of PANI–BNNTs and c high-resolution TEM image showing BNNT filled with PANI.

Powder X-ray diffraction showed evidence of an increase in crystallinity of the PANI in the PANI–BNNTs nanocomposites. The XRD of pure PANI showed two broad peaks at 21° and 25°, characteristic of the amorphous nature of the polymer. In the XRD pattern of the nanocomposite, the peak corresponding to PANI at 25° could not be identified due to strong overlap with the peak from the BNNT component. However, the PANI peak at 21° was split into three sharp peaks at 19.9°, 21.3°, and 22.1°, indicative of a structural ordering of the PANI when embedded within the BNNTs (Fig. 20).

Figure 20

Reproduced with permission from Ref. [81]

XRD pattern of PANI, BNNT and PANI–BNNT.

Wu and Lin fabricated a BNNT–PANI–Pt–GOD nanocomposite as outlined in Fig. 21 [82]. To a DMF suspension of BNNTs was added an acidified solution of aniline, followed by a solution of ammonium peroxydisulfate. This resulted in the formation of a BNNT–PANI hybrid nanomaterial, onto which was deposited Pt nanoparticles via the addition of H₂PtCl₂ and NaBH₄. Glucose oxidate (GOD) was then attached to the BNNT–PANI–Pt hybrid in a phosphate saline buffered (PBS) solution at PH 6.5 to form the bio-nanocomposite, BNNT–PANI–Pt–GOD. The BNNT–PANI–Pt–GOD was deposited on glassy carbon (GC), and the GC/BNNT–PANI–Pt–GOD was used as an electrode for the detection of glucose. It was found that the response current increases linearly from 0.01 to 5.5 mM glucose, reaching saturation at 13 mM glucose (Fig. 22). The sensitivity of detection was estimated to be 19.02 mA M⁻¹ cm⁻².

Figure 21

Reproduced with permission from Ref. [82]

Schematic showing the fabrication of BNNT–PANI–Pt–GOD nanocomposite.

Figure 22

Reproduced with permission from Ref. [82]

Current response for GC/BNNTs–PANI–Pt–GOD electrode upon successive addition of glucose in 0.2 M PBS (pH 6.5) at an applied potential of 0.55 V.

Çakmakçı and Madakbaş [83] were able to prepare PANI/h-BN composites (1, 3, and 5 wt% h-BN) by using in situ chemical oxidative polymerization of aniline in the presence of h-BN particles. XRD of PANI/h-BN showed that the PANI was actually on the surface of the h-BN sheets and was not intercalated, although the authors were inspired to show the potential of intercalative polymerization of aniline in h-BN.

Ahmad et al. [84] also synthesized PANI/h-BN nanocomposites. However, similar to Çakmakçı and Madakbaş’s work, the PANI was on the surface of the h-BN sheets as opposed to being intercalated. Ahmad et al. proposed that the interaction between PANI and h-BN is coulombic since h-BN has partially polar bonds and can interact with the lone pairs on the PANI chain, as depicted in Fig. 23. In comparison with pure PANI, the PANI/h-BN nanocomposite showed greater extent of degradation of methylene blue and rhodamine B dyes under UV radiation. The enhancement in the degree of degradation can be explained by an increase in the surface area of the PANI/h-BN nanocomposite due to coulombic interactions of the two components as shown in Fig. 23. However, the improvement can also be explained by electron transfer from the excited PANI to h-BN, leading to an increase in charge separation between the promoted electron and hole, and thus decreasing the electron–hole recombination (Fig. 24). Co-linear four-probe technique showed that the conductivity of PANI/h-BN was 0.045 S cm⁻¹, slightly lower than that of pure PANI (0.062 S cm⁻¹).

Figure 23

Reproduced with permission from Ref. [84]

Coulombic interactions between PANI and h-BN.

Figure 24

Reproduced with permission from Ref. [84]

Proposed degradation process of methylene blue and rhodamine B over PANI/h-BN nanocomposite.

Shahabuddin et al. [85] also explored the photocatalytic activity of PANI/h-BN (1, 2, and 5 wt% h-BN) for the degradation of methylene blue (MB) and methyl orange (MO). The authors found that the nanocomposites showed improved photocatalytic degradation of MB and MO. PANI and h-BN on their own showed lower photocatalytic activities, with PANI being far better than h-BN. The PANI/h-BN nanocomposite containing 2 wt% h-BN proved to be the best photocatalyst, with 93% and 95% degradation for MB and MO, respectively (Fig. 25).

Figure 25

Reproduced with permission from Ref. [85]

a and c Photodegradation rate of MB and MO as a function of time in the presence of various catalysts, b and d percentage degradation of MB and MO in the presence of various photocatalysts.

Zhi et al. [86] reported on the use of PANI–h-BN nanocomposites in order to improve the thermal stability and fire safety performance of polystyrene (PS) and thermoplastic polyurethane (TPU). The h-BN was first thermally exfoliated by heating in air at 1000 °C. This process led to an oxidized form of BN, denoted as BNO. PANI was then deposited on the BNO by an in situ process to form PANI–BNO. A suspension of PANI–BNO and polymer (PS or TPU) was created. Addition of deionized water to the suspension resulted in the precipitation of PANI–BNO–PS or PANI–BNO–TPU nanocomposites (Fig. 26). The authors noted on the uniform dispersion of the PANI–BNO nanosheets within PS and TPU which improved the thermal stability of the polymeric materials. In particular, PANI–BNO–PS and PANI–BNO–TPU nanocomposites that contained 3 wt% PANI–BNO showed suppressed fire hazards with respect to reduced peak of heat release rate and low smoke yield. Incorporation of 3 wt% PANI–BNO into PS and TPU actually led to an enhancement in the thermal stability of the PS and TPU. Further, the presence of PANI was crucial because of its high charring properties and its synergistic effect with BNO resulted in ternary composites with fire resistance characteristics.

Figure 26

Reproduced with permission from Ref. [86]

Schematic representation showing: a the formation of PANI–BNO, b formation of polymer/PANI–BNO nanocomposites.

Maity et al. explored the feasibility of ternary nanocomposites consisting of h-BN, PANI, and carbon-based materials [graphite oxide (GO), reduced graphite oxide (RGO), carbon nanotube (CNT)] as electrodes in supercapacitor applications. Among the ternary nanocomposites studied, h-BN/PANI/CNT proved to be the best material delivering a specific capacitance of 387.5 F g⁻¹ at a current density of 1 A g⁻¹ in 1 M KCl, as well as exhibiting a high energy density of 34.4 W h kg⁻¹ at a power density of 400 W kg⁻¹. In addition, h-BN/PANI/CNT retained 87% of its specific capacitance after 6000 charge/discharge cycles. These data were found to be excellent, clearly demonstrating the possibility of incorporating the ternary nanohybrids in supercapacitors [87].

Cui et al. incorporated h-BN/poly(2-butylaniline) as an anticorrosive reinforcement of epoxy coating (EC) for metallic substrates. h-BN was first dispersed in THF with the presence of poly(2-butylaniline) (PBA). PBA was used to assist the h-BN layers to exfoliate as shown in Fig. 27. Epoxy resin was then added to the mixture with ultrasonication. After solvent removal, polyamide hardener (curing agent) was added, followed by vigorous stirring along with pumping for removal of air bubbles. The resulting mixture was coated to a thickness of 20 µm on Q250 steel electrodes. Epoxy coatings consisting of 0.5, 1.0, and 2.0 wt% h-BN were created. Corrosion studies showed that epoxy coatings consisting of 0.5 and 1.0 wt% h-BN could offer a relatively stable protection against corrosion compared to the pure epoxy over long exposure time. The PBA used in the coating mixture had a dual role. First, the PBA helped to disperse the h-BN layers, and the randomly exfoliated layers within the coating provided a torturous diffusion pathway for the corrosive medium (H₂O, O₂) to travel through, thus hampering corrosion (Fig. 28). Second, the PBA being an electroactive polymer passivated the surface of the metal and provided protection against corrosion [88].

Figure 27

Reproduced with permission from Ref. [90]

Schematic illustrating the dispersion of h-BN with the aid of PBA as a dispersant. Left: h-BN in THF; right: h-BN in THF with the assistance of BN.

Figure 28

Reproduced with permission from Ref. [88]

Illustration of protection mechanism in h-BN/poly(2-butylaniline)/epoxy coating.

There are also reports on polypyrrole (ppy)/h-BN nanocomposites. For instance, with the aim of improving the thermal stability of ppy in addition to tuning its dielectric properties, Madakbaş et al. [89] prepared ppy/h-BN nanocomposites containing 1, 2, 3, and 4 wt% h-BN. This was achieved by dispersing h-BN in water, followed by addition of pyrrole. Thereafter, an aqueous solution of ammonium peroxydisulfate was added to oxidize the pyrrole to polypyrrole. It should be noted that the polypyrrole was deposited on the surface of the stacked h-BN nanosheets, as evidenced by powder X-ray diffraction measurements. The powder patterns of the nanocomposites are similar to that of h-BN. The thermal stability of the polypyrrole dramatically increased with the presence of h-BN in the range of 1–4 wt% (Fig. 29). The authors performed dielectric constant measurements on pressed pellets of PPy and nanocomposites and found out that the dielectric constant decreases with increasing frequency. The dielectric constant of PPy and of the nanocomposites was found to increase with an increase in temperature. In addition, the dielectric constant of the nanocomposites decreased with increasing h-BN content (Fig. 30).

Figure 29

Reproduced with permission from Ref. [89]

Thermograms of ppy and ppy/h-BN nanocomposites containing 1–4 wt% h-BN.

Figure 30

Reproduced with permission from Ref. [89]

Dielectric constant of ppy and ppy/h-BN (1–4 wt%) as a function of temperature at 100 kHz frequency.

Sultan et al. [90] prepared ppy/h-BN nanocomposite for application in detection of liquefied petroleum gas (LPG) which consists of butane, propane, and traces of ethyl mercaptan and other hydrocarbons. In ppy/h-BN, the ppy was doubly doped since its preparation involved the oxidation of pyrrole with FeCl₃, in the presence of camphor sulfonic acid (CSA) and h-BN. The conductivity of the resulting nanocomposite (denoted as ppy/BN-2) was found to be 0.234 S cm⁻¹. ppy/h-BN without using CSA was also prepared (denoted as ppy/BN-1), and this nanocomposite showed no conductivity. Polypyrrole with CSA was also prepared (denoted as ppy-2), and its conductivity was 0.466 S cm⁻¹, higher than that of ppy/BN-2 (0.234 S cm⁻¹). Since ppy-2 and ppy/BN-2 were found to be electronically conducting, the authors demonstrated their use in the detection of LPG by measuring changes in conductivity upon exposure to LPG, followed by exposure to air. When in contact with LPG, the conductivity of ppy-2 decreased with an increase in time (0–60 s), and upon exposure to air, the conductivity value increased with time (60–120 s), almost re-attaining its initial value. The reversibility of the response was tested by 10-s exposure to LPG, followed by 10-s exposure to air for a duration of 120 s, and it was found that ppy-2 showed good reversibility. In the case of ppy/BN-2, its conductivity also decreased as a function of time when exposed to LPG, but upon exposure to air, the conductivity remained almost the same. The reversibility response of ppy/BN-2 was also measured in a manner similar to that of ppy-2. It was found that the reversibility response of ppy/BN-2 was rather poor. The poor reversibility of ppy/BN-2 led the authors to conclude it can only be used as a sensor for LPG detection for a single time use only.

Sultan and Mohammad [91] synthesized a nanocomposite material consisting of BN, ppy, and Ag nanoparticles, denoted as ppy/Ag@BN. Silver nanoparticles were first incorporated into BN by reacting AgNO₃ with BN in a 1:1 mol ratio in the presence of DMF which served as solvent and also as a reducing agent to reduce Ag⁺ to Ag. To the reaction mixture was also added PVP (0.1 mmol) which functioned as stabilizer for the Ag nanoparticles. The reaction mixture was heated in an autoclave at 160 °C for 8 h. The product (Ag@BN)) was washed with DI H₂O, followed by EtOH, and then dried at 80 °C for 12 h. An aqueous solution of pyrrole was then added to an aqueous dispersion of Ag@BN, followed by the dropwise addition of an aqueous solution of FeCl₃. The reaction mixture was stirred for 12 h which led to the formation of ppy/Ag@BN. The authors also prepared ppy via chemical oxidation of pyrrole with FeCl₃. Four-probe electrical conductivity measurements were performed on pelletized samples of ppy and ppy/Ag@BN. It was found that the conductivity of ppy/Ag@BN was greater than that of ppy. Both ppy and ppy/Ag@BN were explored as sensors for CO₂ by observing changes in electrical conductivity when exposed to the gas. Pelletized samples of both materials were exposed to CO₂ for 1 min, followed by exposure to air for 1 min. The reversibility response for both samples was also assessed. This was done by exposing the samples to CO₂ for 5 s, followed by exposure to air for 5 s, and repeating the process for a duration of 45 s. For both samples, the conductivity dropped when in contact with CO₂ and rose back up to its initial value when in contact with air. However, the change in conductivity for ppy/Ag@BN was more rapid compared to ppy. In addition, the reversibility response of ppy/Ag@BN was far better than that of ppy. Hence, ppy/Ag@BN is a promising material for application in CO₂ sensing device [Fig. 31].

Figure 31

Reproduced with permission from Ref. [91]

Variation in electrical conductivity response of PPy/Ag@BN on alternate exposure to CO₂ and air.

Velayudham et al. [92] were able to form nanocomposites of boron nitride nanotubes (BNNT) with poly(p-phenylene-ethylene)s (PPEs) and a substituted polythiophene (SPT). The structure of the polymers is shown in Fig. 32. The resulting nanocomposites were found to be soluble in common organic solvents such as THF, CH₂Cl₂, and CHCl₃. The solubilization of the nanocomposites was due to strong π–π interactions between the polymers and BNNT. These strong non-covalent interactions led to the planarization of the PPEs in BNNT–PPEs, resulting in redshifts in the absorbance and emission spectra of the nanocomposites in comparison with the pure PPEs (Fig. 33). On the other hand, for BNNT–SPT, their absorbance and emission spectra are blueshifted due to disruption of the π system in the SPT (Fig. 34). HRTEM and SEM revealed that structure of the BNNTs was not affected by functionalization with the polymers, and this approach of solubilizing the BNNTs can open door toward the development of other nanocomposite materials, and in particular those which are mechanically strong.

Figure 32

Reproduced with permission from Ref. [92]

Structure of PPEs (polymer A and polymer B), and substituted polythiophene (polymer C).

Figure 33

Reproduced with permission from Ref. [92]

a Fluorescence spectra of polymer A (dark line) and BNNT–polymer A nanocomposite (dash line) in CHCl₃, b absorption spectra of polymer A (dark line) and BNNT–polymer A nanocomposite (dash line) in CHCl₃.

Figure 34

Reproduced with permission from Ref. [92]

a Fluorescence spectra of polymer C (dark line) and BNNT–polymer C nanocomposite (dash line) in CHCl₃. b Absorption spectra of polymer C (dark line) and BNNT–polymer C nanocomposite (dash line) in CHCl₃.

Martinez-Rubi et al. [93] were able to non-covalently bound regiorandom poly(3-hexylthiophene) (rra-P3HT) to the surface of BNNTs to form rra-P3HT/BNNT nanocomposite. The rra-P3HT/BNNT nanocomposite was prepared by suspending the BNNTs in CHCl₃ with the aid of bath sonication, followed by dropwise addition of a solution of rra-P3HT in CHCl₃ (orange color). A purple suspension was observed suggesting the functionalization of the BNNTs with the rra-P3HT (Fig. 35). As shown in Fig. 35, BNNTs did not absorb visible light in the range of 300–700 nm, owing to its wide band gap residing in the UV range. rra-P3HT in CHCl₃ exhibited a broad envelope (λ_max = 432 nm), and this was assigned to intrachain π–π* transition due to twisting of the polymer chain, resulting in a coil-like conformation. Interestingly enough, the absorption band for the rra-P3HT/BNNT nanocomposite was redshifted and was partially resolved, displaying weak peaks at 520, 555, and 603 nm. The observed redshift was attributed to the favorable and strong π–π stacking interaction between P3HT and BNNT, thereby discouraging chain twisting of the rra-P3HT. Consequently, this led to planarization of the polymer chain with extended conjugation on the surface of the BNNT. By using polarized excitation fluorescence microscopy, the authors performed conformational analysis of the P3HT chains on the surface of BNNT and concluded that the chains were aligned along the BNNT axis.

Figure 35

Reproduced with permission from Ref. [93]

a Photographs of rra-P3HT/BNNT, b absorption spectra of BNNT (0.2 mg/mL), rra-P3HT solution, and rra-P3HT/BNNT suspensions in CHCl₃ (rra-P3HT to BNNT weight ratio = 0.08).

Petrelli et al. synthesized polythiophene derivatives with pendant groups, e.g., poly[3-(4-methoxycarbonylphenyl)thiophene] (PTME). Saponification of PTME at high temperature, followed by acid treatment, resulted in the formation of the derivatized polythiophene in the free acid form, poly[3-(4-benzoic acid)thiophene] (PTBA) (Fig. 36) [94].

Figure 36

Reproduced with permission from Ref. [94]

Synthetic steps leading to the formation of PTME and PTBA.

PTME and PTBA were then used to form nanocomposite materials with BNNS. The driving force for the formation of the nanocomposites was due to π–π interaction between the polymer and the nanosheet as illustrated in Fig. 37. Evidence of π–π stacking was obtained from UV–visible spectroscopy. While the PTBA–BNNS nanocomposite showed a UV shift compared to the free polymer, no shift was observed in the PTME–BNNS system.

Figure 37

Reproduced with permission from Ref. [94]

Illustration of thiophene backbone lying flat against BNNS for π–π stacking.

Giambrome et al. prepared a nanocomposite material consisting of regioregular polyhexylthiophene (RRP3HT) and h-BN. h-BN was dispersed in CHCl₃ and mixed with a solution of RRP3HT in CHCl₃. The resulting suspension was used to fabricate thin films of RRP3HT/h-BN nanocomposites. For comparative studies, thin films consisting of RRP3HT and microdiamond (MD), as well as those of RRP3HT and nanodiamond (ND), were also fabricated. In particular, the UV–Vis spectrum of RRP3HT/h-BN showed a redshift to 494 nm which confirmed interaction between the polythiophene and h-BN. Photoelectrochemical studies were performed on thin films of RRP3HT/h-BN, RRP3HT/MD, and RRP3HT/ND deposited on FTO glass plates which served as the working electrode. In these studies, Pt was used as the auxiliary electrode, 0.1 M tetraethylammonium tetrafluoroborate in CHCl₃ was used as the electrolyte, and the voltage was scanned from − 1 V to 1 V. When exposed to light, photocurrent was measured for all three systems. However, RRP3HT/h-BN displayed the highest photocurrent in the potential window of − 1 to +1 V [95].

MXenes

MXenes are a family of layered ternary carbides, carbonitrides, and nitrides with the general chemical formula, M_n+1AX₄, where M is an early transition metal, and A is a main group element, mostly from group 13 or 14. The first MXene, Ti₃C₂, was produced in 2011 by Naguib et al. [96], by the treatment of Ti₃AlC₂ (MAX) with HF. To date, there are about 30 MXenes that have been synthesized [97]. Similar to MoS₂, WS_2, and BN, nanocomposites based on conducting polymers and MXenes have been developed albeit to a lesser extent, owing to the relatively new synthesis of this particular type of 2D structures.

Lu et al. [98] synthesized ternary composite consisting of PANI, TiO₂, and MXene (Ti₃C₂T_x) as illustrated in Fig. 38. First, Ti₃C₂T_x was obtained by chemical etching of Ti₃AlC₂ with an HF solution. The etching process led to the removal of Al, and the introduction of functional groups such as −F,=O, or –OH, denoted by ‘T,’ with the subscript ‘x’ representing the number of such groups. The TiO₂/Ti₃C₂T_x composite was then obtained via a hydrothermal process at PH 0.5. Thereafter, addition of aniline followed by ammonium peroxydisulfate afforded the ternary composite, PANI/TiO₂/Ti₃C₂T_x.

Figure 38

Reproduced with permission from Ref. [98]

Steps showing formation of ternary nanocomposite, PANI/TiO₂/Ti₃C₂T_x.

The ternary composite exhibited a high specific capacitance of 188.3 F g⁻¹ at 10 mV s⁻¹, which was roughly two times higher than that of the binary TiO₂/Ti₃C₂T_x composite. It also showed excellent cycling stability as it retained 94% of its initial capacity after 8000 cycles at 1 A g⁻¹.

Ren et al. [99] synthesized PANI–Ti₃C₂ nanocomposite for application in high-performance supercapacitors. The Ti₃C₂ was prepared by etching the Al in Ti₃AlC₂. To an acidified dispersion of Ti₃C₂ was added aniline, followed by an acidified solution of ammonium peroxydisulfate. The reaction mixture was stirred at 2 °C for 6 h to yield PANI–Ti₃C₂. Electron microscopy revealed that the PANI particles were adsorbed on the surface and edge of the Ti₃C₂ nanosheets (Fig. 39). Powder X-ray diffraction data corroborated with the electron microscopy results.

Figure 39

Reproduced with permission from Ref. [99]

SEM and TEM images of Ti₃C₂ (a),(d), PANI (b), (e), PANI–Ti₃C₂ (c), (f).

The PANI–Ti₃C₂ nanocomposite exhibited excellent electrochemical performance with a maximum specific capacitance of 164 F g⁻¹, which is 1.25 times greater than that of Ti₃C₂. In addition, 96% of its initial capacity was retained after 3000 cycles, thereby demonstrating its superior cycling stability.

Fu et al. [100] reported the synthesis of a ternary composite consisting of graphene, Ti₂CT_x, and polyaniline as illustrated in Fig. 40. First, the selective etching of an Al layer from the MAX phase (Ti₂AlC) led to the formation of the MXene phase (Ti₂CT_x). PANI was then grown on the surface of the MXene phase by following a protocol from the literature [101]. The produced binary composite, PANI–Ti₂CT_x (MP), was chemically modified with cetyltrimethylammonium bromide (CTAB) and then treated with a chemically converted graphene (CCG) sheet to yield the ternary composite, graphene/Ti₂CT_x/PANI (GMP). The ternary composite exhibited excellent electrochemical performance with a specific capacitance of 635 F g⁻¹ (at 1 A g⁻¹), as well as excellent cycling stability of 97.54% after 10,000 cycles (at 10 A g⁻¹).

Figure 40

Reproduced with permission from Ref. [100]

Steps leading to the formation of graphene/Ti₂CT_x/PANI (GMP).

Zhang et al. [102] fabricated composite films made of Ti₃C₂T_x and polyaniline co-doped with dodecylbenzenesulfonic acid (DBSA) and hydrochloric acid. The procedure used for constructing these films is outlined in Fig. 41. Ti₃C₂T_x was prepared by etching T_i3AlC₂, and the co-doped polyaniline (c-PANI) by reacting aniline with ammonium persulfate in the presence of HCl and DBSA. Ti₃C₂T_x–cPANI films in 1:1, 3:1, 5:1, and 7:1 weight ratios were fabricated via vacuum-assisted filtration. The authors found that the increasing the mass fraction of Ti₃C₂T_x in the composite increased the electronic conductivity, electromagnetic shielding effectiveness, and tensile strength of the composite material. Ti₃C₂T_x–cPANI film (7:1) exhibited an electronic conductivity of 24.5 S cm⁻¹, electromagnetic shielding effective of 36 dB, and a tensile strength of 19.9 MPa, all of which were higher than those of c-PANI showing values of 0.3 S/cm, 16 dB, and 2.6 MPa, respectively.

Figure 41

Reproduced with permission from Ref. [102]

Steps leading to the formation of Ti₃C₂T_x–cPANI film.

Kumar et al. [103] prepared PANI–Ti₃C₂ composite in view of studying its electromagnetic shielding effectiveness in the microwave X-band region (8.2–12.2 GHz). Ti₃C₂ was prepared from Ti₃AlC₂ by chemical etching with HF. The PANI–Ti₃C₂ composite was prepared by mechanically blending chemically synthesized PANI in the emeraldine form with Ti₃C₂ in a 1:1 weight ratio. The resulting composite with a sample thickness of 1.5 mm exhibited excellent electromagnetic shielding in the microwave X-band region with a value of ca 23 dB. Wei et al. [104] also prepared PANI–Ti₃C₂ composites which showed good absorption for microwave radiation. However, in Wei et al’s PANI–Ti₃C₂, the polymer was in the protonated form synthesized by in situ polymerization of aniline with ammonium persulfate in acidic medium, with the presence of Ti₃C₂. The Ti₃C₂ was synthesized by treating Ti₃AlC₂ with a solution of LiF in 6 M HCl. The nanocomposite synthesized by Wei et al. showed great potential for microwave absorption, with a maximum reflection coefficient reaching − 56.30 dB at 13.80 GHz for a sample with a thickness of 1.8 mm. In addition, the effective absorption bandwidth (> 90%) ranged from the X-band (8–12.4 GHz) to the Ku-band (12.4–18 GHz) with the tunable thickness of 1.5–2.6 mm.

Zhao et al. [105] prepared PANI/Ti₃C₂T_x nanocomposite. Delaminated Ti₃C₂T_x was first prepared by etching and exfoliating Ti₃AlC₂ with HF. Treatment of the Ti₃C₂T_x nanosheets with aniline in the presence of ammonium persulfate resulted in the formation of polyaniline particles on the surface of the nanosheets. The PANI/Ti₃C₂T_x nanocomposite showed rapid detection response at room temperature for volatile gases such as ethanol, methanol, ammonia, at 200 ppm level. Among the gases tested, the nanocomposite was most selective for the detection of ethanol, and its sensitivity was higher compared to the bare MXene.

Wu et al. [106] synthesized a nanocomposite based on Ti₃C₂ and PANI. The details of the synthesis are outlined in Fig. 42. Ti₃C₂, obtained by chemical etching of Ti₃AlC₂ with HF, was reacted with a mixture of p-phenylenediamine and isoamyl nitrite to produce the amino-functionalized Ti₃C₂ (H₂N–Ti₃C₂), referred to as N–Ti₃C₂. The N–Ti₃C₂ was dispersed in acetone with the help of ultrasonication, and to the resulting suspension was added I₂. The N–Ti₃C₂ suspension was then electrochemically deposited on fluorine tin oxide (FTO) glass electrode. Aniline was then electrochemically polymerized to PANI onto the N–Ti₃C₂–FTO glass electrode. The duration of the electropolymerization reaction was varied, and it was found that 420 s was the optimum length of time, resulting in a composite that demonstrated the highest capacitance, 228 mF cm⁻² at 5mV s⁻¹, 34 times greater than that of N–Ti₃C₂. The composite also exhibited excellent cycling stability (85% capacitance retention after 1000 cycles at the current density of 1 mA cm⁻²).

Figure 42

Reproduced with permission from Ref. [106]

Synthetic steps leading to the formation of N–Ti₃C₂/PANI on FTO.

Boota et al. fabricated polypyrrole–Ti₃C₂T_x intercalated nanocomposite [107]. This was achieved by mixing a colloidal suspension of delaminated Ti₃C₂T_x layers with pyrrole in 2:1 and 1:1 mass-to-volume ratio. The authors noted that the highly acidic Ti₃C₂T_x protonated a pyrrole molecule which reacted with another pyrrole to form a dimer. The dimer then reacted with either a protonated or unprotonated pyrrole, and the process continued to form a longer polypyrrole chain. The polypyrrole was doped by the fluorine atom from the Ti₃C₂T_x. Evidence for the formation of doped polypyrrole was obtained from Raman and FTIR spectroscopy. The powder pattern of Ti₃C₂T_x–PPy (2:1) showed that Ti₃C₂T_x layers expanded by 10 Å, suggesting formation of an intercalated nanocomposite. TEM corroborated with the XRD data. The nanocomposite exhibited a high volumetric capacitance of ca 1000 F cm⁻³ and retained 92% of its capacitance after 25,000 cycles.

Zhu et al. [108] prepared polypyrrole–Ti₃C₂ by electrochemically polymerizing pyrrole onto the surface of the Ti₃C₂. The Ti₃C₂ was prepared by etching Ti₃AlC₂ with HF. To a suspension of the Ti₃C₂ in acetone, was added I₂. The Ti₃C₂ was then electrochemically deposited onto a fluorine tin oxide (FTO) glass electrode. Pyrrole was then electropolymerized onto the Ti₃C₂-coated FTO glass. The polypyrrole–Ti₃C₂ was peeled off as a free-standing film from the FTO glass. The free-standing film was used directly as a supercapacitor electrode. The volumetric capacitance of the nanocomposite film reached 406 F cm⁻³, which was over 30% higher than that of pristine polypyrrole film.

Yan et al. [109] electroplated polypyrrole (PPy) onto the surface of textile coated with Ti₃C₂T_x. The Ti₃C₂T_x nanosheets were obtained via chemical etching of Ti₃AlC₂ with an HCl/LiF solution mixture. Coating of the textile with Ti₃C₂T_x was achieved by using a simple dipping–drying technique. The electrochemical deposition of PPy on the MXene–textile prevented the formation of TiO₂ on the edge of the MXene. The PPy–Mxene–textile system served as a flexible electrode in supercapacitor application, exhibiting a specific capacitance of 343.20 F g⁻¹.

Wu et al. [110] intercalated polypyrrole into Ti₃C₂. First, chemical etching of Ti₃AlC₂ with HF produced the MXene, Ti₃C₂. The Ti₃C₂ was suspended in 1 M HCl and dispersed with the help of ultrasonication. Pyrrole was then added, followed by ammonium peroxydisulfate. Evidence for the formation of polypyrrole was provided by FTIR and Raman spectroscopy. XRD showed proof that the polypyrrole was in the gallery space of the Ti₃C₂. At a scan rate of 2 mV s⁻¹, the ppy/Ti₃C₂ nanocomposite exhibited a specific capacitance of 184.36 F g⁻¹, which was 37% higher than that of Ti₃C₂. The nanocomposite also retained 83.33% of its capacitance after 4000 charge/discharge cycles at a current density of 1 A g⁻¹.

Zhang et al. [111] fabricated planar supercapacitor based on polypyrrole and Ti₃C₂. The steps involved are outlined in Fig. 43. First, by using laser ablation, an interdigital pattern was created on an ITO glass electrode. A dispersion of Ti₃C₂ was then deposited on the ITO glass by means of electrophoretic deposition, followed by deposition of polypyrrole via electropolymerization of pyrrole. Areal capacitance of the resulting composite microelectrode was measured by using two different setups. When 2 M H₂SO₄ was used as the liquid electrolyte, 2 platinum electrodes were clamped to the ends of the interdigital pattern and immersed in the acid solution. With PVA/H₂SO₄ as the gel electrolyte, it was first cast onto the interdigital pattern and then dried, followed by making connection with silver paste to two silver strings to the electrochemical apparatus. At a current density of 1.05 mA cm⁻², the interdigital electrodes showed high areal capacitance of 109.4 mF cm⁻² in 2 M H₂SO₄, and 86.7 mF cm⁻² in PVA/H₂SO₄ gel electrolyte.

Figure 43

Reproduced with permission from Ref. [111]

Schematic illustrating the fabrication of planar interdigited electrode systems based on Ti₃C₂/PPy.

Biological applications

Most of the nanocomposite applications described in this review have been with regard to enhancement in materials performance such as energy storage (batteries and supercapacitors), corrosion protection efficiency, hydrogen evolution reaction (HER), light-emitting diodes, photocatalytic degradation of dyes, fire resistance, tuning of dielectric properties, detection of CO₂ and volatile organic compounds, and electromagnetic shielding efficiency. In this section, we will focus on the biological applications of the nanocomposites. A few biological applications have already been highlighted in the review, and these include the use of PANI–MoS₂ as a sensing platform for important biomolecules such as adenine and guanine [54], DNA [56, 57], and chronic myelogenous leukemia [58]. In addition, we highlighted the BNNT–PANI–Pt–GOD nanocomposite as a sensing electrode for glucose [82]. Another example of biosensing application includes the work of Hun et al. [112] where they used a PANI–MoS₂ nanocomposite modified with gold as an electrode for cancer cell glutathione detection. A linear response was observed in the concentration range from 1.0 × 10⁻¹⁰ to 1.0 × 10⁻⁴ mol L⁻¹, with a detection limit of 3.1 × 10⁻¹¹ mol L⁻¹.

In this section, we will further expand on the biological applications of the nanocomposites. For instance, Cheng et al. [113] functionalized the surface of WS₂ with polyethylene glycol, which was used as a photothermal therapy agent to completely ablate cancer cells under irradiation with a NIR laser beam at a relatively low power density. The authors demonstrated in animal experiments that PEGylated WS₂ was also effective for in vivo enhanced X-ray computed tomography (CT) and photoacoustic tomography (PAT) bimodal imaging of tumors. Along similar lines, Meng et al. developed an aptamer-PEGylated MoS₂/Cu_1.8S nanoplatform with photoluminescence, photoacoustic, and photothermal imaging capabilities for in vivo and in vitro tumor cells. The nanoplatform also allowed target-cell-specific delivery gene probe to detect intracellular microRNA related to cancer cells, and anti-cancer doxorubicin (DOX) drug for chemotherapy. The nanoconstruct loaded with DOX showed excellent in vivo and in vitro antitumor efficiency as a result of NIR-trigged targeted chemo-photothermal therapy [114].

Summary

Nanocomposite materials consisting of graphene analogous materials and conducting polymers are a revolutionary new class of materials with a wide range of applications. The ability to tailor the optical, electrical, electrochemical, thermal, and mechanical properties of a synthesized material gives scientists the ability to literally build a material to fit a certain specific task. Challenges in evenly dispersing two-dimensional nanomaterials within polymer matrices remain. However, recent progress has enhanced understanding of the chemical and mechanical contributions to matrix–filler interactions, creating exciting opportunities to utilize nanocomposites derived from conducting polymers and two-dimensional nanomaterials, in novel applications.