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Preparation and investigation of Montmorillonite-K10 Polyaniline nanocomposites for optoelectronic applications.

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  • 1. Department of Chemistry, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad 45650, Pakistan.
      Authors
    • Idrees R 1
    • Shah SAA 1
    • Omer S 1
    • Mehmood Z 1
    • Saeed S 1
    (5 authors)

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Heliyon, 04 Mar 2024, 10(6):e27573
https://doi.org/10.1016/j.heliyon.2024.e27573 PMID: 38545176 PMCID: PMC10965523

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Abstract 

One-dimensional polyaniline (PANI) nanostructures were synthesized in situ in the presence of two-dimensional (2D) Montmorillonite (MMT) clay nanosheets. Strong interactions between the polymer and MMT platelets in the nanocomposites were confirmed through spectroscopic studies. X-ray diffraction and scanning electron microscopic studies revealed the clay's profound effect on the polymer's crystallinity and morphology. The clay nanosheets induced higher crystallinity and well-defined nanorod morphology in the polymer structure. Consequently, the nanocomposite showed an electrical conductivity of 8.72 S/cm, closer to that of the pristine polymer (8.97 S/cm), despite the presence of highly insulting clay material. Surprisingly, a notable decrease in the optical bandgap of the polymer from 3.73 to 2.88 eV of the nanocomposite was also observed. This novel integration of a narrow band gap and high conductivity in PANI/MMT nanocomposites can expand their utility for visible light interactions in areas encompassing photocatalysis, photovoltaics, electro/photochromism, and related technologies.

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Heliyon. 2024 Mar 30; 10(6): e27573.
PMCID: PMC10965523
PMID: 38545176

Preparation and investigation of Montmorillonite-K10 Polyaniline nanocomposites for optoelectronic applications

Ramsha Idrees, Syed Aizaz Ali Shah,** Saeed Omer, Zahid Mehmood, and Shaukat Saeed[low asterisk]
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Abstract

One-dimensional polyaniline (PANI) nanostructures were synthesized in situ in the presence of two-dimensional (2D) Montmorillonite (MMT) clay nanosheets. Strong interactions between the polymer and MMT platelets in the nanocomposites were confirmed through spectroscopic studies. X-ray diffraction and scanning electron microscopic studies revealed the clay's profound effect on the polymer's crystallinity and morphology. The clay nanosheets induced higher crystallinity and well-defined nanorod morphology in the polymer structure. Consequently, the nanocomposite showed an electrical conductivity of 8.72 S/cm, closer to that of the pristine polymer (8.97 S/cm), despite the presence of highly insulting clay material. Surprisingly, a notable decrease in the optical bandgap of the polymer from 3.73 to 2.88 eV of the nanocomposite was also observed. This novel integration of a narrow band gap and high conductivity in PANI/MMT nanocomposites can expand their utility for visible light interactions in areas encompassing photocatalysis, photovoltaics, electro/photochromism, and related technologies.

Keywords: Polyaniline nanorods, Clay nanocomposites, Optical bandgap, Optioelectronics, Conducting polymers

Graphical abstract

1. Introduction

Due to their promise in progressive applications such as energy storage [1], electrochemical sensors [2], electrochromic displays [3], microwave absorption [4], photocatalysis [5], photovoltaics [6], and corrosion protection [7], conducting polymers have received much attention [8]. Polyaniline (PANI), because of its simplicity of synthesis, low-cost monomers, tunable properties, and relative stability [9,10], has been comprehensively studied as a semiconducting polymeric material [11,12]. Tremendous efforts are being put into tailoring the properties of PANI for applications in futuristic semiconductor devices such as light-emitting diodes, transistors, and solar cells, among others [13]. Such applications demand the polymer to possess an optimum optical bandgap associated with an appreciable conductivity [14]. PANI nanocomposites, incorporating metals, metal oxides, carbon, and two-dimensional (2D) materials, are promising for controlled properties [[15], [16], [17]]. For instance, Amasi et al. [18] incorporated multi-walled carbon nanotubes in the PANI matrix to decrease its band gap from 3 to 2.84 eV, along with a 10-fold increase in conductivity. The smaller band gap associated with higher conductivity was suggested to be more suitable for solar cell applications. More recently, Behkti et al. [19] reduced the band gap of PANI from 3.22 eV to 2.88 eV by incorporating nano alumina (Al2O3) into the polymer. Sankar and Ramesan [20] noted that increased crystallinity improved PANI hybrids' electrical conductivity and optical bandgap, achieving a minimum value of 2.101 eV. The direct current (DC) conductivity, although increased, remained at a lower order of magnitude (~0.001 S/cm). It is, however, highly desirable that lower band gap PANI hybrids/composites with appreciable conductivity are prepared.

High aspect ratio polyaniline is reported to possess higher electrical conductivity [21]. Synthesizing PANI with controlled nanostructured morphologies can, therefore, alleviate the problem of lower conductivity. Various physical methods, such as mechanical stretching [22] and electrospinning [23], are employed to draw PANI chains into 1D nanofibers and nanorods. These methods are, however, highly sophisticated and cumbersome. Self-assembling PANI chains into desired morphologies during polymerization is a relatively uncomplicated and straightforward methodology. It usually requires the use of a “hard template” (such as anodized alumina channels, track-etched polycarbonate channels, and zeolite channels [24,25]), “soft template” (such as surfactants, micelles, liquid crystals, and poly acids [26]), or optimized synthetic conditions [27] employed during the synthesis of PANI from its monomers.

Montmorillonite (MMT) K-10 is a widely used nanofiller. It is a naturally occurring 2D clay material with a high specific surface area, low cost, high adsorption capacity, and catalytic activity [28]. Combining PANI with MMT results in a composite material with synergistic properties and performance in terms of physical, electrical, thermal, and catalytic properties [29]. The preparation, electrical conductivity, and thermal stability of the composite material are widely studied, which has enabled its utility primarily in anticorrosive coatings [30] and pollution absorbents [31]. However, only limited attention is paid to applying the material in emerging fields such as photocatalysis, photovoltaics, electrochromics, and semiconductor devices. These applications demand a thorough investigation into the optoelectronic properties of the PANI/MMT nanocomposites [32]. Liu et al. [33] noted that the electrical conductivity of PANI/MMT composite is highly sensitive to the synthetic conditions and the amount of MMT used. For optimum conditions, the researchers reported a maximum conductivity of 11.5 S/cm. Although subsequent studies were carried out to enhance the conductivity of PANI/MMT nanocomposite, the optical band gap of the material remains unexplored to the best of the authors' knowledge. Therefore, a study to investigate the nanocomposite's optical bandgap could open new arenas of its applications in the emerging fields of photovoltaics, light-emitting diodes, and photo/electrochromics. The current research, thus, adopted a comprehensive approach to studying the optoelectronic properties and the underlying structural and morphological characteristics of PANI/MMT nanocomposites prepared under suitable experimental conditions. Interestingly, a pretreatment of the MMT clay resulted in conductivity of the same order as the pristine polymer but with a drastically narrower band gap suitable for optoelectronic applications. This study, therefore, establishes PANI-MMT nanocomposites as novel, promising candidates for future applications in photocatalysis, photovoltaics, photo/electrochromism, and supercapacitors, among others.

2. Experimental

2.1. Materials

Aniline (C6H7N, 99%) from Riedel-deHaan (C·O·O., Germany), Ammonium persulphate [(NH4)2 S2O8, 98%] from Chem-Lab NV, Montmorillonite-K10 clay from Sigma-Aldrich, Hydrochloric acid (HCl, 37%), Ethanol (C2H5OH, ≥99.8%) and Acetone (C3H6O, ≥99.8%) from BHD AnalaR, Copper sulfate petahydrate (CuSO4·5H2O, ≥98.0%) from Merck and Distilled water (H2O) were used as obtained, with no further purification.

2.2. Synthesis of polyaniline

Pristine polyaniline was synthesized by magnetically swirling a solution of 2 mL aniline in 50 mL of 1 M HCl for 20 min at 0°C. 5 g of APS (dissolved in 50 mL of 1 M HCl) was slowly added to the solution over a 10-min interval during which the solution temperature was allowed to rise to 20 °C. The mixture was continually agitated for 2 h to complete the polymerization and then left stagnant for 24 h. Using vacuum filtration, a dark green precipitate was obtained and denoted as PANI. To eliminate contaminants and excess reactants, the product was washed sequentially with 1 M HCl, distilled water, and acetone. The solid substance was then dried at 50°C overnight.

2.3. Preparation of polyaniline/montmorillonite-K10 nanoomposites

Polyaniline/Montmorillonite-K10 Clay nanocomposites were prepared through in-situ polymerization of aniline monomers in the presence of Montmorillonite K10 (MMT-K10) Clay nanoplatelets. The synthesis followed the same protocol as for the pristine PANI, except for the addition of a fixed quantity (1 wt% of aniline) of MMT-K10 clay suspension in the reaction mixture.

Two different approaches were adopted to treat the clay suspension before its introduction to the reaction mixture. In one approach, 1 g (per 100 mL aniline) of MMT-K10 without prior chemical treatment was added into 100 mL of distilled water, sonicated for 30 min, and stored overnight for the particles to disperse. The resulting nanocomposite was named PANI-C. In the second approach adopting the method reported in Ref. [34], MMT-K10 was first chemically treated by mixing and agitating 5 g of the clay in 250 mL of 0.5 M aqueous CuSO4·5H2O solution for 5 h. The resulting suspension following overnight storage was filtered, rinsed with distilled water to remove excess ions, and dried at 110°C for 2 h. A calculated amount (1 g/100 mL of aniline) of the chemically treated clay was subsequently sonicated in 50 mL of water for 30 min, stored overnight, and introduced into the reaction mixture. The resulting precipitated material was named PANI-EC.

2.4. Characterization

The synthesized/prepared materials were analyzed for chemical structure using a Fourier transform infrared spectrometer (FTIR, Shimadzu IR Affinity-1S). X-ray powder diffractometry (XRD) was performed using Cu-Kα radiations (1.54187 Å) of Malvern Pananalytical X'Pert Pro MPD at a scanning rate of 0.06 deg/sec. The morphological study of the samples was carried out under a field emission scanning electron microscope (FESEM, MIRA-III, TESCAN, Czech Republic). A UV–visible spectrophotometer (Thermoscientific Evolution 201) was utilized for the optical evaluation of the prepared samples. The materials' DC conductivity was measured on compressed pellets using the Osila BV four-probe conductivity meter. All pellets were made to a 4 mm thickness and 10 mm diameter under pressure of 4 tons using a Stenhoj hydraulic H frame. Thermal gravimetric analysis (TGA) of dried samples powder was carried out using METTLER TOLEDO TGA/SDTA851e at a 5 °C/min heating rate under an air atmosphere.

3. Results and discussion

To understand the behavior, the prepared materials were analyzed for their chemical structure, molecular arrangements, nanoscale morphology, and thermal oxidative degradation. The results of the studies are discussed in the following sub-sections.

3.1. FTIR analysis

PANI consists of benzenoid and quinoid rings [36] joined together in a polymeric fashion, as shown in Fig. 1. The FTIR spectra (Fig. 3) of the nanocomposites (PANI-C, PANI-EC) indicated the presence of the characteristic CN (1545 cm−1) and C–N (1288 cm−1) along with C–H (2650, 1083, 789 cm−1) and CC (1630 cm−1) vibrational bands, suggesting the successful synthesis of PANI [27] in the presence of the MMT-K10. While comparing the FTIR spectra of the three materials, the vibrational peaks of PANI in the nanocomposites appeared slightly broadened compared to the corresponding peaks of the pristine polymer. This broadening of peaks can be indicative of physical interactions existing between the functional groups of the polymer (Fig. 1) and the clay surface (Fig. 2) [27].

Fig. 1

Chemical structure of PANI composed of benzenoid and quinoid rings joined together in a polymeric chain.

Fig. 2

Chemical structure of MMT-K10 Clay showing layered morphology. Reproduced under Creative Commons license from ref. [35].

Fig. 3

a) Full and b) resolved FTIR spectra of PANI, PANI-C, and PANI-EC showing the characteristic peaks of polyaniline.

A distinguishing feature in the case of both PANI and its clay nanocomposites was the presence of a peak corresponding to the polaron, i.e., C–N.+ (1239 cm−1) [38]. This suggested that polyaniline was synthesized in the conducting emeraldine salt form in all three cases. Spectra in Fig. 3a also showed that the peak intensity ratio of C–N.+ to C–N increased from PANI to PANI-C and PANI-EC, signifying increased doping levels in the nanocomposites, that could contribute to the enhancement of conductivity of the nanocomposite materials, remarkably the PANI-EC. This increased doping level in the case of the nanocomposites can be attributed to the stabilizing effect of the negative surface charge of the K10 clay [37] nanoplatelets.

3.2. XRD analysis

Fig. 4 shows and compares the X-ray diffractograms of the synthesized PANI, and its nanocomposites. The results indicated semi-crystallinity in the PANI sample, as evident from the XRD broad band with peaks at 2θ of 15.4⁰ (100), 20.3⁰ (110) and 25.2⁰ (111) attributed to the periodicities parallel and perpendicular to the polymer chain [39,40]. PANI-C also depicted a similar XRD to PANI, suggesting minimal effect of the untreated MMT clay on the crystallinity of the polymer. In contrast, the PANI-EC exhibited a higher degree of crystallinity similar to metallic polyaniline as signified by the increased sharpness of the peaks at about ~20⁰,~21⁰,~26⁰,~27⁰,~29⁰ and ~32⁰ of crystalline PANI [41,42]. Furthermore, the presence of intense sharp peaks below and above 25⁰ suggests enhanced periodicity in both the perpendicular and parallel directions of the polymer chains [43]. It, therefore, can be deducted from the XRD that a minute amount of MMT-K10 can induce crystallinity in PANI, when the clay is chemically treated prior to its incorporation in the polymer.

Fig. 4

Xray diffractograms of PANI, PANI-C, and PANI-EC, showing enhanced crystallinity in PANI-EC nanocomposite.

3.3. SEM analysis

SEM micrographs of PANI, PANI-C, and PANI-EC at different magnifications are shown in Fig. 5, Fig. 6, Fig. 7. Unlike the widely reported spherical morphology [[44], [45], [46]] of the chemically synthesized PANI, the current study observed a closely packed agglomerated morphology (Fig. 5b) with numerous interlinked fibers (Fig. 5c) that could be beneficial for efficient charge transport. This unique morphology can be attributed to the controlled synthetic conditions (oxidant to monomer molar ratio of 1.25, initial temperature of 0 °C and subsequent temperature of 20 °C with light stirring) that could provide a stagnant environment to the growing polymer chain to self-assemble and adopt the interconnected morphology [44]. A lower oxidant-to-monomer ratio can result in secondary nucleation due to insufficient monomers available for primary nucleation. Similarly, the low initial, followed by the increased temperature, can ensure only sufficient and controlled production of the active centers without the risk of aggregation due to the sudden sporadic onset of nucleation. Additionally, the low-intensity stirring can facilitate a homogenous nucleation without externally disrupting the interpenetrated growth that can be hindered by the mechanical shear of vigorous stirring [44]. As a result, the polymer sample appeared to consist of interconnected grains (Fig. 7a) that could easily be pressed together under pressure to form a highly conducting continuous bulk.

Fig. 5

SEM micrograph of PANI showing a) a fibrous morphology of b) elongated nanostructures connected through c) interlinks.

Fig. 6

SEM micrographs of PANI-C showing a) large agglomerates of clay particles covered with nanostructured granules of PANI at b) surface and c) within the agglomerates.

Fig. 7

SEM micrographs of PANI-EC showing a) loosely packed uniform nanostructured morphology consisting of b) interconnected PANI globules and c) well-defined PANI nanorods.

On the other hand, the polymer morphology in the PANI-C nanocomposite appeared densely granular (Fig. 6b) and remained primarily confined within the clay agglomerates (Fig. 6c). Consequently, the sample consisted of separated chunks of agglomerated polymer-decorated insulating clay platelets and lacked the well-defined fibrous morphology of PANI (Fig. 6a), which could hinder charge transport across the bulk of the material. Since the clay in PANI-C was not well dispersed, the disruption of the interconnected morphology can be attributed to the spatial hindrance [46] caused by the clay particles to the growing PANI chains within their agglomerates.

Contrary to PANI-C, the PANI-EC nanocomposite showed a highly uniform and continuous submicron-sized morphology (Fig. 7a) with well-defined PANI nanorods (Fig. 7c), which abundantly appeared along with nanosized PANI globules (Fig. 7b). The clay particles did not appear agglomerated like in PANI-C, and it is suggested that they existed well dispersed within the PANI matrix. The exceptional well-defined nanorod formation in PANI-EC can be attributed to some possible physiochemical interaction of the nanoscopic 2D layers of MMT-K10 with the propagating aniline chains to grow in a specified manner. Aniline monomers itself, have the tendency to grow into nanofibers [44,46], which may have been further enhanced by the interacting clay surfaces by preventing chain branching of the growing polymer chains. This is consistent with the XRD observation where PANI-EC showed the highest degree of crystallinity, suggesting the presence of linear PANI chains that could easily be folded into regular geometric shapes. Interestingly, the uniform and continuous matrix with numerous high-aspect-ratio nanorods could be highly conducting, which is a unique feature of 1D materials [44].

3.4. TGA analysis

TGA thermograms (Fig. 8) showed the typical three-step weight loss of polyaniline for all three samples but with different onset temperatures and weight loss values [42,47,48]. The PANI-C composite showed a closely similar degradation profile to the PANI except for its more excellent stability at higher temperatures. The behavior is typical of polymer nanocomposites, where the higher thermal stability can be attributed to the insulating and charring effect of the MMT-K10 phase. It is suggested that a significant amount (~25 wt%) of the polymer was protected inside the clay agglomerates against O2 penetration for its thermo-oxidative degradation, resulting in the formation of thermally stable char at higher temperatures. The TGA results supported the SEM findings, showing that PANI was grown inside the free space of the MMT-K10 agglomerates.

Fig. 8

TGA thermograms of PANI, PANI-C, and PANI-EC, showing a markedly different degradation profile for the latter.

PANI-EC, on the other hand, showed the most peculiar degradation profile among the three samples. The material resulted in about 22 wt% loss in the temperature range of 119–313°C, indicating a significantly large amount of the dopant, as suggested by the FTIR results. Moreover, the unique degradation profile suggested a different mechanism with altered kinetics. The decrease in the onset degradation temperature may indicate possible chemical involvement of the 2D clay nanosheets in the degradation process of polyaniline. It can be suggested that the treated MMT nanosheets could more strongly interact with the polymer chains due to their higher surface area and surface energy, which are characteristics of 2D nanomaterials. Moreover, the weight loss during the degradation process was slow-going, which may indicate a sluggish oxidation due to poor diffusion of oxygen in the compact interconnected morphology of the composite as evident from the SEM micrographs. However, unlike the PANI-C nanocomposite, the polymer fraction in PANI-EC composite degraded almost completely at about 700°C, suggesting better dispersion of the filler.

Optical band gap and electrical conductivity are the key parameters required to determine a material's electronic, catalytic, and optical performance. Therefore, the following studies were conducted to reveal the potential of the nanocomposites for the desired optoelectronic applications.

3.5. Spectroscopic studies (UV–visible range)

The pristine PANI showed three absorption maxima (Fig. 9) at 275 nm, 383 nm, and 550 nm corresponding to π-π* transitions of the benzenoid/quinoid rings, polaron- π* absorption peak and charge transfer of the quinoid ring, respectively [38,49]. For both PANI-C and PANI-EC, the characteristic peaks of PANI were retained, with a significant blue and red shift observed for the polaron- π* and the charge transfer peak of the quinoid ring, respectively. Additionally, the relative intensity of the polaron- π* absorption peak appeared markedly high for the nanocomposites compared to the pristine material. The appearance of the three characteristic peaks suggested the presence of a doped emeraldine form of polyaniline in all the prepared materials [40]. However, the observed blue shift coupled with the increased relative intensity of the polaron- π* absorption peak could indicate an increased doping level of the PANI-EC followed by PANI-C compared to the pristine polymer [49]. The same effect of the clay on the doping concentration of PANI was also observed during the FTIR analysis, confirming the UV–visible results. The redshift for the quinoid ring charge transfer peak at 550 nm, without a substantial change in relative intensity, might indicate that the addition of clay had facilitated the charge transfer mechanism with no effect on the polymer's oxidation potential [49]. This suggested that the charge transfer of PANI occurred at relatively lower energy levels in the presence of clay platelets, as indicated by the higher wavelength of the transition in the composite materials. The clay platelets can be attributed to facilitating the removal of charge from the benzenoid segment to generate positive polarons or adding electrons in the quinoid segment to form negative polarons responsible for the red shift [50].

Fig. 9

UV–visible absorption spectra showing characteristic peaks of PANI-C and PANI-EC with reference to the pristine PANI.

The UV–visible spectral analysis, thus, pointed towards strong molecular level interactions between the MMT-K10 and the PANI chains that could lead to synergetic characteristics and performance of the composite materials [49].

Since molecular-level interactions can change the energy band gap of semiconducting materials such as PANI [49], the PANI nanocomposites were anticipated to offer an altered optical band gap. To confirm, Tauc plots of the prepared materials were drawn (Fig. 10a–c) using the relation (α.hν) 1/γ = B (hν−Eg) [51] where Eg is the optical bandgap, ν the incident photon frequency, γ the index value of 0.5 for direct permitted transition, B is the band tailing parameter, a constant, and α the absorption coefficient [52]. The analysis revealed a direct band gap for all the prepared materials with the estimated values compared in Fig. 10d. The comparison showed that the optical band gap of pristine PANI was the highest (3.73 eV), corresponding to the UV region of electromagnetic radiation. Adding and uniformly dispersing the clay nanoplates significantly decreased the band gap of the pristine PANI to 3.08 and 2.88 eV, respectively. This confirmed that the clay particles strongly interacted with the electronic energy levels of the polymer to develop a composite material with synergistic properties.

Fig. 10

Tauc plots indicating a direct band gap of (a) 3.73 eV, (b) 3.08 eV, and 2.88 eV for PANI, PANI-C, and PANI-EC, respectively. (d) Comparison of the direct optical band gaps of the prepared materials showing a decrease for the nanocomposites.

Furthermore, the suggested interaction appeared to be strengthened by the chemical treatment and uniform dispersion of the MMT K-10, as evidenced by the lowest band gap value (2.88 eV) in the case of PANI-EC. It can be proposed that the chemically treated and well-dispersed MMT sheets can provide more reactive surfaces attributed to the higher surface energies associated with 2D materials than their agglomerated assemblies. Thus, the low optical bandgap in PANI-EC can lead to superior optical performance of the nanocomposite. This is highly significant because it provides a competing optical band gap (2.88 eV) to that of PANI-Multiwall Carbon Nanotubes (MWCNTs) nanocomposites (2.84 eV) at a lower mass loading [18]. The PANI/MMT K10 nanocomposites, thus, provide a cost-effective alternative to the PANI-MWCNTs composites for practical applications in organic photovoltaics and catalysis. Since visible light covers the wavelength range of 390–700 nm, or 1.8–3.1 eV, the nanocomposites, particularly PANI-EC, can be employed in the photoelectronic devices with visible light activation in contrast to the requirement of UV light for pristine PANI.

3.6. Conductivity studies (direct current)

DC conductivity results (Fig. 11) revealed that all the three materials were significantly conductive, attributed to the presence of polyaniline in the emeraldine salt form, as evidenced by the FTIR analysis. The pristine PANI showed a significantly high conductivity of 8.97 S/cm, rarely reported in the literature. This high conductivity is attributed to the fibrous morphology of the polymer, as evidenced by the SEM analysis. The three-dimensional connected structure of conducting PANI chains could offer minor contact resistance to the current flow. A significant drop to 3.37 S/cm conductivity was observed for the PANI-C nanocomposite due to the incorporation of the untreated insulated MMT-K10 clay disrupting the fibrous morphology of the conducting polymer. As evident from the material's characterization, a significant portion of the conducting PANI was encapsulated inside the interspaces of the insulating clay agglomerates, which could drastically upset the conductive networked organization of polyaniline by inducing insulating barriers inside it.

Fig. 11

Room Temperature DC conductivities of PANI, PANI-C, and PANI-EC showing similar but very high values for PANI and PANI-EC compared to PANI-C nanocomposite.

Although incorporating the same amount of insulating MMT-K10, the PANI-EC surprisingly showed a conductivity as high as 8.72 S/cm near the pristine PANI. This peculiar high value can be justified based on the FTIR, XRD and SEM analysis. The FTIR analysis predicted a greater number of polarons in PANI-EC, which facilitated the conductivity through the polymer chains. Additionally, the XRD and SEM indicated increased crystallinity and the presence of PANI nanorods of a high aspect ratio, respectively. Both the parameters can enhance the bulk conductivity of the sample [53]. Moreover, the bulk of PANI-EC appeared homogenous and uniformly connected, which could facilitate charge transport across it. Thus, an interpenetrating conductive network across highly dispersed clay nanosheets can be considered responsible for the nanocomposite's higher conductivity. Practically, this high conductivity of PANI-EC associated with its low optical band gap and the characteristic catalytic performance of MMT-K10 can be desirable for various electrochemical and photocatalytic applications.

Table 1 compares the PANI/MMT's optoelectronic attributes (optical band gap and electrical conductivity) with similar composites. It can be seen that the optical band gap of the PANI/MMT (PANI-C and PANI-EC) corresponded to the visible range of electromagnetic radiations contrary to the ultraviolet range of the pristine PANI. It suggests that the prepared PANI/MMT composites can be effectively used for harvesting the visible spectrum of light. Moreover, the band gap was comparable to or even better than the previously reported PANI composites. However, the remarkable feature of the PANI/MMT composites was the associated superior electrical conductivity, surpassing that of the PANI/MWCNTs composites in particular.

Table 1

Comparison of the optical band gap and conductivities of PANI nanocomposites.

SampleBand Gap (eV)Conductivity (S/cm)Reference
PANI-ZnO3.88–[54]
PANI-NiAl0.8Fe1·2O43.84–[55]
PANI-AINPs3.840.000002[56]
PANI-SiO23.830.0092[57]
PANI3.738.97Current Study
PANI-co-PIN-Cu-Al2O33.720.0006[58]
PANI-ITONP3.600.0003[59]
PANI-SnIP3.600.0061[60]
PANI-TiC3.200.682[61]
PANI-C3.083.37Current Study
PANI-Al2O32.88–[62]
PANI-EC2.888.72Current Study
PANI-MWCNTs2.8400000003[18]
PANI-SnO22.73–[63]

4. Conclusion

High aspect ratio PANI (emeraldine form) of high crystallinity, superior electrical conductivity, and optimum optical band gap was successfully synthesized by employing a small amount of MMT clay nanoplates under specific experimental conditions. Control over the synthetic conditions imparted a semicrystalline but fibrous morphology to the pristine polymer without requiring a template, resulting in appreciably high conductivity. Although including MMT-K10 clay in both untreated and treated forms resulted in a narrower optical band gap of the resulting nanocomposites, the aspect ratio, crystallinity, and conductivity were highly pronounced in the latter case. Stronger interaction between the clay and the growing PANI, as evident from the FTIR and UV–visible spectroscopy, might be responsible for producing linear polymer chains and their subsequent folding into highly crystalline and well-defined nanorods. The study provided a synthetic methodology with optimum conditions and mixing ratios for fabricating PANI/MMT nanocomposites of superior conductivity and optimum optical band gap suitable for optoelectronic applications. Moreover, the optical band gap of PANI/MMT composites, investigated for the first time, was comparable to or even better than the reported PANI nanocomposites/hybrids, associated with excellent electrical conductivity. Furthermore, the studies concluded that the pretreatment of MMT-K10, which leads to its exfoliation, significantly improves the optoelectronic properties of PANI/MMT nanocomposites.

Data availability

Data will be made available on request.

CRediT authorship contribution statement

Ramsha Idrees: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Syed Aizaz Ali Shah: Writing – review & editing, Validation, Supervision, Formal analysis, Data curation, Conceptualization. Saeed Omer: Visualization, Validation, Software, Data curation. Zahid Mehmood: Validation, Resources, Funding acquisition, Formal analysis. Shaukat Saeed: Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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