Introduction

The fundamental understanding of intra-chain and inter-chain charge transfer processes in conjugated polymers opened up an inspiring area of research nowadays. The electronic structure of π-conjugated molecules is of specific interest to scientists and researchers, particularly in semiconductor science and technology, in order to bring a tuning and alignment of energy levels to control the optical, electrical, and chemical catalytic properties [1,2,3,4]. In addition, the extensive delocalization and localization of defect states in the polymer chain provide outstanding opportunities in tuning their optical and electrical properties [5]. Thus, such π-conjugated molecules have been appearing with excellent physical properties, and as a result, conducting polymers such as polyaniline [6,7,8], polypyrrole [9, 10], polythiophene [11], and their derivatives have been widely investigated for application in electronic devices such as light-emitting diodes [12,13,14], solar cells [15,16,17], energy storage devices [18,19,20], catalytic applications [21, 22], thermoelectric devices [23,24,25], supercapacitors [26,27,28,29], and sensors [30,31,32,33] as active materials. Among different π-conjugated molecules, polyaniline is one with excellent physical properties and suitable electronic band structures [34], providing its possibility to be employed in electronic devices [35,36,37,38,39]. Moreover, polyaniline has advantages of its simpler polymerization process, low cost, and tunability of physical properties like electrical and optical properties [40,41,42,43].

Dye molecules are of special significance for their optical absorption properties, and they have chromophore for generating color in organic compounds. The chromophores are part of a conjugate system and thus likely to interact with π-conjugated molecules. Based on the fact that the partially oxidized polyaniline has positive charged defect states [44, 45], we approached to investigate the interaction of a dye with polyaniline. In this work, an attempt has been taken to observe the effect of methylene blue (MB) dye interaction with polyaniline, in tuning its optical and electrical properties. Especially, this polyaniline dye interaction has been observed on cellulose system, considering the advantages of flexibility and high hydrophilicity of cellulose, which provides opportunities of better chemical interaction with dye solution and also during synthesis.

With the recent rapid progress of functionality of polymer-based electronic materials, one can also observe parallel developments in their synthesis techniques, which included a number of polymerization processes such as photo-induced polymerization [46, 47], interfacial polymerization [48, 49], electrochemical polymerization [50,51,52], solution polymerization [53, 54], seeding polymerization [55], emulsion polymerization [56], vapor phase self-assembling polymerization [57, 58], plasma polymerization [59, 60], and sonochemical synthesis process [61, 62] techniques. Considering the point of better control and uniformity over cellulose substrate, the vapor phase polymerization process has been chosen and employed for polyaniline synthesis in this article. Here, polymerization of aniline has been carried out using FeCl3 as polymerizing agent. Subsequently, the polyaniline-coated cellulose sheet was used for MB dye interaction. The prepared MB dye–implanted polyaniline samples have been studied for structural analysis, optical properties, and electrical properties. A remarkable tuning has been achieved in the electrical transport properties along with optical properties, thus opening a wider and simpler way of functionalizing π-conjugated molecules on cellulose as an electronic material system.

Experimental details

Materials

Highly purified aniline, methanol, anhydrous ferric chloride (Merck), and methylene blue (Sigma-Aldrich) were used in the chemical synthesis process. Cellulose papers of thickness of 127 μm were used as the flexible substrates.

Synthesis of dye-implanted polyaniline on cellulose

Polymerization of aniline on cellulose was carried out from double-distilled aniline through vapor phase polymerization technique as detailed in our earlier report [63]. Partially oxidized emeraldine salt form of polyaniline has been obtained through this process using FeCl3 as polymerizing agent. The prepared polyaniline-implanted cellulose sheets were then used for MB dye interaction. For that, powdered MB dye was dissolved in distilled water to prepare solutions of MB dye at four different concentrations of 1 mg/l, 3 mg/l, 7 mg/l, and 11 mg/l in four different beakers. Then, four pieces of polyaniline-implanted cellulose sheets were emerged into those beakers for desired interactions with MB dye at four different concentrations. All the samples were taken out from the dye solution and were dried on air at ambient temperature. The dye-interacted polyaniline-implanted cellulose sheets were thus produced and then used for different physical characterizations to study their functionality as electronic materials.

Characterizations

The prepared samples were studied for their physical properties through different characterization techniques. X-ray diffraction (XRD) measurements have been done to analyze crystal structure using XRD (Bruker D-8 Advance) system. Field emission scanning electron microscopy (FESEM) has been performed using JEOL (JSM-7200F) model for its morphological investigations. The optical absorbance has been measured by UV-Vis-NIR spectrophotometer (Shimadzu 3600 plus). The electrical properties were studied by electrical conductivity (I-V) measurements using Agilent (B2912A, USA) precision source/measurement.

Results and discussion

Structural and morphological characterizations

The crystal structure of the polyaniline-loaded cellulose has been investigated for both the cases before and after MB dye incorporation. The X-ray diffraction pattern has been depicted in Fig. 1, which shows one prominent peak appeared at 2θ = 22.4∘ and two small peaks at 2θ = 38∘and 2θ = 44.26∘ corresponding to the Miller planes (020), (300), and (222) of polyaniline [5].

Fig. 1
figure 1

X-ray diffraction pattern of dye-interacted polyaniline on cellulose

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The X-ray diffraction measurements show that the degree of crystallinity of the polyaniline-loaded paper does not decline significantly due to MB dye interaction. The XRD peaks also indicate that the emeraldine salt form of polyaniline has been obtained during polymerization process, as intended during its synthesis process. The basic oxidation state of the polyaniline remains unaltered during dye interaction without further oxidation or reduction of the polymer chain by the interaction of MB dye.

Morphological and compositional analyses

The morphological investigation of the prepared polyaniline-coated cellulose has been done from field emission scanning electron microscopy (FESEM) measurements. Figure 2a represents the FESEM image of the polyaniline-implanted cellulose.

Fig. 2
figure 2

a FESEM image of the polyaniline-implanted cellulose. b FESEM image taken after dye loading

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The FESEM image clearly depicts that the polyaniline has been successfully implanted over the cellulose sheet with excellent uniformity. In order to investigate the incorporation of MB dye on cellulose sheet, the FESEM image has been taken after dye loading as well, which is shown in Fig. 2b. This demonstrates uniform distribution of the loaded dye over the cellulose sheet. Further, the compositional analysis was performed from energy dispersive X-ray (EDX) measurement, in order to confirm the incorporation of MB dye. The EDX spectra of polyaniline-implanted cellulose with MB dye incorporation are shown in Fig. 3. The elemental detection by EDX measurements confirms the presence of MB dye over the polyaniline-implanted cellulose. In the EDX spectra, the presence of S, Cl, C, N, and O has been observed. Among these elements, C and N have appeared for both the compounds polyaniline and MB dye. But S and Cl have been observed due to the presence of MB dye only, since no other sources of S and Cl were used. Further, the UV-vis-NIR spectra (Fig. 4) of the dye-loaded polyaniline show the characteristic optical absorption of MB dye confirming the presence of MB dyes.

Fig. 3
figure 3

The EDX spectra of the dye-loaded polyaniline-implanted cellulose

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Fig. 4
figure 4

Optical absorbance spectra of MB dye–interacted polyaniline on cellulose

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Optical absorption spectroscopy

The interaction of organic π-conjugated molecules with electromagnetic radiation provides a convincing tool to investigate the energy band structure of such macromolecular systems. In order to understand the effect of MB dye on the fundamental optical absorption properties of polyaniline, optical absorption spectroscopic measurements have been done for polyaniline-implanted cellulose sheets interacted for different times with MB dye. The optical absorption spectra of polyaniline interacted with MB dye at 1 mg/l concentration are demonstrated in Fig. 4.

For the pure polyaniline, the absorption bands are centered at 255 nm, 355 nm, 400 nm, and 780 nm respectively. The absorption bands are due to the characteristic optical transitions of the emeraldine form of polyaniline. The weak peak appearing at 255 nm is related to the molecule conjugation [64]. The broad peak centered at 355 nm is due to π-π* transition within the benzenoid segments [65]. The absorption peak at 400 nm corresponds to polaron–π* transition and is related to doping level and formation of polaron [66]. Another broad characteristic absorption peak of polyaniline appearing at 780 nm attributes for π–polaron transition [65]. The MB dye has its characteristic absorptions occurring at 605 nm and 668 nm [67]. This fundamental absorption of MB dye occurs due to vibronic transitions and π-π* transitions.

The effect of MB dye treatment on the optical band gap of polyaniline has also been studied from optical absorption measurements. The optical band gap has been calculated from the fundamental relationship between the optical absorption coefficient (α) and the incident photon energy (hν) as given below

$$ {\left(\alpha h\nu \right)}^{\frac{1}{n}}=A\left( h\nu -{E}_g\right) $$
(1)

where α, ν, Eg, and A denote the absorption coefficient, linear frequency of the incident photon, energy band gap, and an energy-independent constant, respectively. The index n is related to the different types of transition. \( n=\frac{1}{2} \) for a direct band allowed transition energy gap, n = 2 for an indirect allowed transition, \( n=\frac{3}{2} \) for forbidden direct transition, and n = 3 for forbidden indirect transition energy gap. The optical energy band gap is obtained from the (αhν)2 vs. (hν) plot by extrapolating the linear portion of the plot to the (hν) axis, as illustrated in Fig. 5.

Fig. 5
figure 5

Variation of optical band gap of polyaniline interacted with MB dye for different interaction times

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It has been observed that the energy band gap of polyaniline suffers a red shift during interaction with MB dye. Some inter-band defect levels have been created in the polyaniline band structure due to MB dye interaction, and this causes the decrease in energy band gap of polyaniline. This is also responsible for localizations of carriers affecting the electrical behavior of polyaniline interacted with MB dye, discussed in the next section.

Electrical properties

The polyaniline as prepared on cellulose shows electrical conductivity, and the charge transport process of the polyaniline-implanted cellulose is based on the formation of polaron and bipolaron charged defect states on the polymer backbone [63]. The overall electrical transport of polyaniline depends on the performance of two different transport processes, generally involved in such polymeric systems: intra-chain and inter-chain transport processes. The intra-chain charge transport is based on the delocalization of the carriers on the polymer chain and the effective conjugation length [68]. The inter-chain charge transport in polymeric system is based on the hopping mechanism, which is dependent on the ordered molecular packing. In crystalline polymer system, the hopping mechanism can play a significant contribution in electrical charge transport. In this work, the crystallinity of the polyaniline remains almost unaltered even after the MB dye interaction as observed from XRD results. Thus, the interaction of the polyaniline with dye mainly affects the intra-chain charge transport process. Furthermore, the intra-chain transport is much faster than the inter-chain one, and thus, a substantial effect can be observed in the electrical conductivity of polyaniline after MB dye interaction. The electrical conductivity variation of polyaniline with MB dye interaction is depicted in Fig. 6. It has been observed that the electrical conductivity is reduced significantly with increasing concentration of MB dye, indicating a convenient way of controlling the electrical conductivity and tuning it towards lower conductivity or higher resistivity.

Fig. 6
figure 6

Current vs. voltage plot of MB dye–interacted polyaniline on cellulose

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The MB dye interaction brings an operative localization of the charge carriers bringing a control over the intra-chain carrier movement, thus increasing the resistivity. The mechanism of localization of polarons in the polymer backbone is based on the interaction of the cationic MB dye with the polymer chain as illustrated in Fig. 7.

Fig. 7
figure 7

Schematic representation of MB dye interaction with polyaniline

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The delocalization of the electronic states relies on the resonance-stabilized structure of the polymer. Molecular arrangement must be conjugated and allow the charge defect states to propagate through the polymer chain for the charge transfer. The interaction of the MB dye with the polymer chain at first prevents the formation of polaron at the interaction site of the chain and thus reduces the number carrier. Secondly, it does not allow alteration of the σ and π bonds in the polymer backbone. As a result, the polyaniline interacted with the MB dye is not a good medium for the propagation of the charged defect states through its chain, affecting the intra-chain charge transport.

As a result, the resistivity increases, and the variation of sheet resistance of the polyaniline-implanted cellulose after MB dye interaction at different concentrations is depicted in Fig. 8. One can clearly observe a remarkable increase in sheet resistance with the increasing concentration of MB dye with an excellent tuning over a wide range.

Fig. 8
figure 8

Variation of sheet resistance of MB dye–interacted polyaniline with concentration of MB dye

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Conclusions

Interaction of MB dye with polyaniline has been studied in this article with an objective to bring a control over the electrical transport properties of the polyaniline prepared on flexible cellulose sheet. MB dye mainly affects the intra-chain transport properties by localizing the polarons in the polymer backbone. The chemical attachment of MB dye with the polyaniline prevents the alteration of σ and π bonds in that part of the chain, and thus, the polaron already existing in the chain becomes localized. The electrical studies of the polyaniline interacted with MB dye on cellulose sheet show an excellent control of its electrical transport properties with a remarkable 10,000 times increase in sheet resistance. This wide-range control and tuning in its electrical transport have achieved keeping its optical band gap within the range of 2.47 to 2.63 eV, which lies on a suitable range of values for electronic and optoelectronic applications.