Multiple functionalization of tungsten disulfide inorganic nanotubes by covalently grafted conductive polythiophenes

Abstract

According to the “polymer growth from surface” concept, a variety of dual phase polythiophene–tungsten disulfide inorganic nanotube (PTh–WS₂ INT) composites have been successfully prepared. This concept has made use of a covalent approach of liquid phase oxidative polymerization (LPP) directly from a WS₂ INTs surface. In contrast to a simple bulk LPP, this oxidative polymerization variant enables an effective way to control the interfacial polythiophene-based layering. Various chemically stable nanometric polyTh adlayers have been generated towards morphologically well-defined core–shell WS₂ INTs/polymer composites. The introduced free carboxylic acid groups, arising from the COOH-functionalized thiophene monomers, seriously improve the functional WS₂ INTs dispersability, and thus, provide a versatile route for additional step surface modifications. This study has opened the attractive possibility of achieving high charge mobility via a two-dimensional transport through the WS₂ INTs/polymer interface, combined with mobility anisotropy through the crystalline domains of grafted polymer shells.

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1. Introduction

Inorganic tungsten disulfide fullerene-like nanoparticles (WS₂ IFs) and nanotubes (WS₂ INTs) were first reported in 1992 by Tenne et al.¹ In recent years, the synthesis techniques of such materials have been readily optimized,^2,3 including a scaled-up process.⁴ They are now readily available at a highly pure industrial level towards various commercial applications, such as efficient lubricants in the automobile industry, or as reinforcement nanoscale fillers in polymer nanocomposites. These nanomaterials have become one of the most promising classes of nanomaterials since the discovery of carbon nanotubes (CNTs), due to their unique layered structure^5–8 and their functional properties.^9–11 Moreover, the mechanical properties of WS₂ INTs include an ultra-high strength modulus (a Young's modulus of 150 GPa, a bending modulus of 217 GPa),^12–14 and an outstanding shock absorbing ability.^15–17 In contrast to CNTs, WS₂-based nanomaterials have been established as being biodegradable,^18,19 with a relatively low cytotoxicity.²⁰ Nowadays, researchers are investigating many R&D efforts, focusing on the functionalization of such dichalcogenide-based nanomaterials, for exploiting these nanostructures and their new properties in potential devices.^21–24

Nanocomposites based on hybrid materials, with both organic and inorganic phases, are particularly interesting towards numerous applications and devices. Indeed, the incorporation of inorganic WS₂ NPs into matrices of polymers is expected to increase their polymer strength and their fracture toughness, and also, to enhance their tribological and thermal properties.^25,26 In addition, the organic polymer phases may greatly improve the dispersiveness of functional nanotubes and their adhesion onto other substrates (interfacial chemistry fitting). So far, many researchers have focused on WS₂-based nanocomposites with several polymers/matrices,^27–31 such as epoxy,³² polystyrene/poly(methyl methacrylate),³³ poly(propylene fumarate),³⁴ poly(phenylene)sulphide,³⁵ and nylon 12.³⁶ Electrically conductive polymers, such as polyphenylene PP, polythiophene PTh, polyaniline PANI, polypyrrole PPy, and others, have received considerable attention due to their interesting electronic, magnetic, and optical properties.^37–40 PTh-based polymers are amongst the most studied conductive polymers. This is due to their excellent environmental and thermal stability, allowing for their further use in nanoelectronic and optical devices, such as batteries, solar cells, polymer light-emitting diodes, smart windows, and more.^41,42 Creative designs and development strategies for such new PTh-based composites have led to interesting new materials, disclosing their enhanced performance in selected devices.

In this study, contrary to randomly difficult to control bulk polymerization in the presence of WS₂ INTs, we covalently grew PTh-based conducting polymers, directly from previously developed and optimized polycarboxylated-WS₂ INTs (polyCOOH–WS₂ INTs), by using an effective concept of “growth from the surface of nucleophilized nanotubes”.^43,44 Indeed, the covalent growth of the corresponding PTh polymers from the WS₂ INTs surface combined two key steps (Fig. 1). The first step was the covalent coupling/grafting of polyCOOH–WS₂ INTs with a hydroxylated thiophene linker (2-(3-thienyl)-ethanol), leading to the formation of a polythiophene group's shell onto the surface of the chemically modified WS₂ INTs (“nucleophilized” Th-linker–WS₂ INTs). These decorated thiophene groups then acted as nucleophilic nucleation points for the second step of the in situ liquid phase oxidative polymerization (LPP) of the Th monomers/polyTh polymer's growth from the INTs surface.⁴⁵ This two-step approach provided a chemically stable polymeric nanometric adlayer, which homogeneously covered the WS₂ INTs sidewall, resulting in a morphologically well-defined core–shell WS₂ INTs/polymer dual phase composite that promoted an effective way to control such an interface functional layer. In this study, one focused on three types of thiophene-based monomers: thiophen-3-yl-acetic acid (TAA), 3,4-ethylenedioxythiophene (EDOT), and 3,4-ethylenedioxythiophene carboxylic acid (EDOT_Ac), in order to obtain four types of polyTh polymers that were grown from the WS₂ INTs surface, i.e., the PTAA, the PEDOT, the PEDOT_Ac and the co-polymer PTAA/PEDOT, respectively. The newly introduced free carboxylic groups, arising from both of the TAA and the EDOT_Ac monomers, seriously improve the correspondingly functional WS₂ INTs dispersability, and thus, provided a versatile route for additional 3rd step surface modifications. On the other hand, the PEDOT did not contain any free functional group, but it is considered as one of the most promising conductive polymers for practical applications, due to its well-known strong conductivity properties.⁴⁶

Fig. 1 The two-step fabrication of the polyTh–WS₂ INT nanosized composites via (i) the coupling of the polyCOOH–WS₂ INTs with 2-(3-thienyl)-ethanol and (ii) an in situ oxidative polymerization of the various thiophene monomers, i.e., the TAA, the EDOT, and the EDOT_Ac.

These unique functional PTh–WS₂ INT-based composites were extensively analyzed by using combined analytical, spectroscopic, and microscopic methods, enabling a full characterization of their physical properties, their structure and the morphology of their PTh polymers shells that were oxidatively grown onto the WS₂ INTs surface. The possibility of achieving high charge mobility via a two-dimensional transport through both of the WS₂ INTs/PEDOT interface and the crystalline domains of the PEDOT shell was also demonstrated, by using both a two-electrode powder resistivity measuring system and an EPR methodology. These results have important consequences for a full characterization of the electrical properties of these novel composite WS₂-based materials.

2. Experimental section

2.1. Materials and methods

The tungsten disulfide inorganic nanotubes (WS₂ INTs) were purchased from NanoMaterials Ltd (Yavne, Israel). The thiophen-3-yl-acetic acid (TAA), the 3,4-ethylenedioxythiophene (EDOT), the 2,3-dihydrothieno[3,4-b][1,4]dioxine-2-carboxylic acid (EDOT_Ac) and the thiophene-containing linker 2-(3-thienyl)-ethanol, were all purchased from Sigma-Aldrich and were used as received. All of the solvents were purchased from commercial sources and were used without further purification.

The dry powder samples that were required for the different analyses were prepared (vacuum stove 1 h at 40 °C followed by lyophilization using a FreeZone 2.5 liter bench-top freeze dry system, Labconco, Kansas City, MO, USA).

Transmission Electron Microscopy (TEM) made use of a Tecnai G2 TEM (FEI Company, Eindhoven, Netherlands) high contrast microscope (120 kV acceleration voltage, Gatan CCD camera). The samples for the TEM analyses were prepared by spreading a small drop of aqueous nanotube dispersions on the amorphous carbon-coated copper grids (Formvar carbon 400 mesh grids, SPI® Supplies West Chester, USA). This was followed by air-drying.

The thermogravimetric analysis (TGA) was performed on a TA Q600-0348, model SDT Q600 (Thermofinnigan), by using a temperature profile of 25–1000 °C at 10 °C min⁻¹ under nitrogen flow (180 mL min⁻¹) with sample masses of 5–10 mg.

The FT-IR spectra were recorded by using a Bruker TENSOR 27 spectrometer (Diffuse Reflectance Accessory EasyDiff, PIKE Technologies, 4 cm⁻¹ resolution). The samples were prepared by mixing the product powders with dry IR grade KBr (2% weight).

The Raman spectroscopy was performed at λ = 514 nm by using a Renishaw inVia Raman microscope equipped with RL785 and RL830 Class 3B wavelength-stabilized diode lasers and a Leica DM2500M.

The Elemental Analyses (EA) of all of the nanoscale WS₂ INTs-based composites were afforded atomic percentage values of the C, O, S, N, and H elements for each sample, by using a Thermo flash EA, 1112 CHNS analyzer.

All of the chemically accessible carboxylic group shells that were present on the surface of the corresponding polythiophenes–WS₂ INTs were quantified by using a UV-sensitive spectroscopic Kaiser test after the shell derivatization by using 1,3-diaminopropane (UV-VIS spectrophotometer Cary 1E, Varian Inc., λ_detection = 570 nm).⁴⁷

The X-ray powder diffraction (XRD) (Bruker AXS D8 Advance diffractometer with Bragg–Brentano geometry when using Cu-K_α radiation (λ-1.54 Å)) was operated at 40 mA and 40 kV. The measurements were performed in the 2θ range from 20° to 80°, with a step size of 0.05°, at a 0.5 s per step rate.

The X-band EPR spectra were recorded at both a room temperature and a low temperature by using an X-band Elexsys E500 EPR spectrometer (Bruker, Karlsruhe, DE) with an integrated frequency counter. The polymers and the composite powders were inserted into narrow quartz tubes (2 mm OD 1 mm ID, Wilmad LabGlass), being placed within a standard rectangular Bruker EPR cavity (ER 4119 HS). In order to compare the EPR signals in a quantitative way, the material powders filled the same cavity length (4 cm) of the quartz tubes for all of the measurements. For the low temperature measurements in the 110–280 K range, the spectra were obtained by using a Bruker ER4131VT variable-temperature unit that was equipped with a nitrogen flow cooling system. The EPR device was operated at a microwave frequency of ∼9.8 GHz for the room temperature and of ∼9.3 for the low temperature measurements. The spectra were recorded by using a microwave power of 0.6325 mW across a sweep width of 500 G with a modulation amplitude of 2 G. For the PEDOT and the PEDOT–WS₂ INTs, the curve spectra were recorded by using a microwave power of 0.6325 mW across a sweep width of 1300 G during 5 scans. The integrated EPR spectra of the PEDOT–WS₂ INTs at different temperatures (122–220 K) were recorded by using a microwave power of 6.325 mW. The curve fitting was obtained from the curve fitting function of the X-band EPR acquisition program for suitable line shapes.

The conductivity measurements of both of the polyTh polymers and the polyTh-decorated WS₂ INT composites were carried out by using a Powder Resistivity Measuring System (Fig. SI-1†) that was purchased from Mobichem Scientific Engineering Ltd., Jerusalem, Israel. About 20 mg of powdered samples were compressed under a constant weight of 4 kg into a cylindrical pellet shape (1 cm diameter) and the resistivity of each pellet sample was deduced/calculated from its volumetric resistance.⁴⁸

2.2. Polycarboxylation of WS₂ INTs (polyCOOH–WS₂ INTs)

First, a de-agglomeration procedure was performed in order to de-aggregate the commercial non-functional WS₂ INTs into individual nanotubes.⁴⁹ This was a repetitive process of dispersing the commercial WS₂ INTs in acetone by using a mild sonication for 30 min and collecting only the corresponding de-agglomerated nanotubes before any use. After drying the corresponding de-agglomerated WS₂ INTs, the polycarboxylation process of the WS₂ INTs was performed by using a versatile method that was based on a modified acidic Vilsmeier–Haack reagent, as previously reported.⁴³

2.3. Preparation of nucleophilic Th-linker–WS₂ INTs – covalent coupling of polyCOOH–WS₂ INTs with a thiophene-based linker

The coupling chemistry that was used for the fabrication of the intermediate nucleophilic thiophene-containing-WS₂ INTs made use of an aqueous EDC·HCl-mediated activation of the carboxylate functions that were present on the polyCOOH–WS₂ INTs surface [1400.0 mg polyCOOH–WS₂ INTs, 40.25 mg EDC·HCl (0.21 mmol, 1.5 equiv. COOH groups), 10.0 mL ddH₂O, 1 h, RT] followed by the covalent attachment of the thiophene-containing linker 2-(3-thienyl)-ethanol (0.21 mmol, 23.5 μL, 1.0 mL CH₃CN). The resulting product was washed with CH₃CN (1 × 40 mL) and ddH₂O (3 × 40 mL), decanted by centrifugation (5500 rpm, 10 min, RT), and then dried under vacuum (lyophilization).

2.4. In situ oxidative polymerization of thiophene-based monomers onto the Th-linker-decorated WS₂ INTs (typical procedure)

When the thiophene linkers were covalently attached onto the WS₂ INTs, they readily interfered with the medium bulk oxidative Th-monomer oxidative polymerization and caused a PTh-growth from the nucleophilized WS₂ INTs surface. The Th-linker functions acted as nucleation species for the polymerization of the targeted PTh polymers. The experimental conditions that were used in such an oxidative process were as follows: 1 : 1 weight Th-linker–WS₂ INTs : Th-monomer, 1 : 5 equiv. Th-monomer : oxidizing agent (FeCl₃) for a final concentration of 0.01 M CTAB surfactant. Each polymerization process included the dispersion of the Th-linker–WS₂ INTs (CHCl₃) with the cationic surfactant cetyltrimethyl ammonium bromide (CTAB) by using an ultrasonicator bath (Bransonic, 42 kHz at full power) [200.0 mg Th-linker–WS₂ INTs, 292.0 mg CTAB, 60 mL CHCl₃, 15 min, RT]. This was followed by the addition of the oxidant FeCl₃ [15.0 mL CHCl₃] followed by an additional 30 min sonication. The Th-monomers had been previously dissolved in 10.0 mL CHCl₃ and were added dropwise to the suspension [300.0 mg for the TAA, 150.0 mg for both the TAA and the EDOT, 300.0 mg for the EDOT, 393.0 mg for the EDOT_Ac, the PTAA, the PTAA/PEDOT, the PEDOT, and the PEDOT_Ac–WS₂ INTs]. The polymerization took place for 1 hour in an ultra-sonication bath at RT. In order to discard excess of non reacted materials and the eventual presence of polythiophene oligomers, the resulting polyTh–WS₂ INTs composites were sequentially washed by using CHCl₃ (1 × 40 mL), MeOH (5 × 40 mL) and ddH₂O (1 × 40 mL), then were decanted by centrifugation (5500 rpm, 10 min, RT), before drying under a vacuum (lyophilization).

2.5. Quantification of the COOH group on the surface of the polyTh–WS₂ INT composites

The polyTh–WS₂ INTs were chemically activated with N′-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDC·HCl) in ddH₂O (1 h, RT). Then 1,3-diaminopropane was added in excess (overnight mixing, RT). The resulting product was washed with ddH₂O until there was a neutral pH. Then it was decanted by centrifugation and dried under a vacuum (lyophilization). This derivatization step quantitatively converted the COOH groups into amine (NH₂) ones and resulted in the amino-polyTh–WS₂ INT composites. The NH₂ groups of the corresponding amino-polyTh–WS₂ INTs were then assayed by using an Elemental Analysis and a sensitive ninhydrin-based UV spectroscopic Kaiser test. The duplicated Kaiser measurements provided average values of the NH₂/COOH groups per gram of polyNH₂/polyCOOH–polyTh–WS₂ INTs.

2.6. Synthesis of polythiophene-based polymers

To better investigate and characterize the physic-chemical features of the polyTh–WS₂ INTs composites, all of the corresponding polythiophene polymers were also separately synthesized with the same procedure as was used for the composites preparation [1 : 5 equiv. of Th-monomer : oxidizing agent (FeCl₃), 1 h polymerization under ultrasonication] and with the following experimental modifications: FeCl₃ was added to 1/3 of the dissolved monomer (total of 10.0 mg mL⁻¹ in CHCl₃) and then sonicated for 30 min at RT. Then, the rest of the Th-monomers were added dropwise to the suspension, followed by 1 hour in a bath sonication. The resulting polymers were washed with MeOH (5 × 40 mL) and ddH₂O (1 × 40 mL) until a colorless filtrate was obtained (centrifugation, 11 500 rpm, 10 min, 4 °C), before drying using lyophilization.

3. Results and discussion

The core–shell nanostructures of the covalently grafted polythiophene (the PTAA, the PTAA/PEDOT, the PEDOT, & the PEDOT_Ac) decorated-WS₂ INTs hybrids were fully characterized by a wide variety of analytical, spectroscopy and microscopy methods, thus permitting one to quantify the amount of polymer coating on the WS₂ INTs surface and to characterize its polymer phase features that influenced the overall conductivity of the resulting WS₂ INTs-based composites.

The TEM microphotographs enabled the straightforward characterization of these WS₂-based nanocomposites and showed the formation of a polythiophene shell with a thin nanometric width enveloping the WS₂ INTs (of about 10–25 nm) in a core–shell structure as shown in Fig. 2. For example, and according to the HR-TEM image, the thickness of the illustrated PEDOT shell was measured and it was found to be 15 nm for a WS₂ core of 54 nm (Fig. 2d). The distributive uniformity of the generated polymer shell mainly depends on the starting functional homogeneity of the existing polycarboxylated species/functions onto theWS₂ INTs (polyCOOH–WS₂ INTs) surface. Indeed and following covalent thiophene functionalization, these critical carboxylic groups/species generate the future nucleation points via corresponding thiophene linkers during in situ liquid phase oxidative polymerizations (LPPs) of corresponding thiophene monomers from the WS₂ INTs surface. Control experiments were done in the case of bulk oxidative polymerization in the presence of non-functional non polycarboxylated WS₂ INTs, which resulted in the mass embedment of corresponding non-functional WS₂ INTs in a non-conformal polythiophene polymer matrix as checked by TEM (see Fig. SI-2†).

Fig. 2 The TEM (a–c and e) and the HR-TEM (d) images of the polyCOOH–WS₂ INTs (a) and of the core–shell structured PTh–WS₂ INTs composites: PTAA–WS₂ INTs (b), PTAA/PEDOT–WS₂ INTs (c), PEDOT–WS₂ INTs (d; arrows indicating the shell thicknesses of the PEDOT–WS₂ INTs composites), and the PEDOT_Ac–WS₂ INTs (e).

In order to quantify the amount of the organic polymer phase in each of the polyTh-modified WS₂ INT composites, TGA analyses were performed under nitrogen. In the temperature ranges of 100–1000 °C, the weight losses of the PTAA–WS₂ INTs, the PTAA/PEDOT–WS₂ INTs, the PEDOT–WS₂ INTs and the PEDOT_Ac–WS₂ INTs, were 27.0, 34.0, 44.0, and 46.0%, respectively (Fig. 3). While the de-agglomerated untreated WS₂ INTs,⁴⁴ the polyCOOH–WS₂ INTs and the Th-linker–WS₂ INTs, with the same TGA analyses, disclosed very low weight losses of 1.0, 1.3 and 2.0%, respectively.

Fig. 3 The TGA analyses and the inset 1st derivative (DTG) plots of de-agglomerated WS₂ INTs and of the four polyTh–WS₂ INT composites.

Regarding both of the PEDOT and the PEDOT_Ac-based composites, the TGA analyses indicated a higher amount of polymers when compared to the PTAA-based composites. The differences in the weight losses between these composites were effectively related to the presence of electron donating oxygen atoms in the substituted EDOT monomers, which lowered the oxidation potential of thiophene heterocycle in both the PEDOT and the PEDOT_Ac monomers. This led to a higher polymerization degree for the same reaction time and the same conditions.^50,51

In addition, the DTG plots (derivative thermogravimetry) of the four composites (see inset in Fig. 3) revealed the superior stability of the PEDOT-based composite with a pyrolysis process starting from 370 °C, in contrast to the carboxylic substituted ones, which decomposed at a lower temperature (from 180 °C). In order to compare between all of the polyTh-modified composites and their corresponding polymers, TGA analyses were also performed for all of the crude polymers. According to the DTG plots (see 1st derivative graph, Fig. SI-3†), the same pattern was observed between the polyTh-composites and their respective crude polymers.

The successful Th-monomer polymerization towards the corresponding polythiophene polymers onto the WS₂ INTs was also confirmed by an Elemental Analysis (Table 1). Indeed, the presence of a relatively high amount of C, O and H elements demonstrated a high amount of polymeric phase in all of the polyTh–WS₂ INT composites. The percentage values of the C, O and H elements that were obtained from Elemental Analysis were in agreement with the TGA analyses, i.e., the percentages of the C element were 18.8, 24.7, 26.5 and were 27.4% for the PTAA, the PTAA/PEDOT, the PEDOT, and the PEDOT_Ac–WS₂ INTs, respectively.

Table 1 The elemental analysis of the commercial WS₂ INTs, the polyCOOH–WS₂ INTs, the Th-linker–WS₂ INTs and the polyTh–WS₂ INTs composites RBI

Nanomaterial	C%	O%	S%	H%
WS2 INTs	0	1.389	17.693	0.011
polyCOOH–WS2 INTs	0.121	0.384	18.658	0
Th-linker–WS2 INTs	0.501	0.390	16.628	0.007
PTAA–WS2 INTs	18.842	8.336	17.411	1.139
PTAA/PEDOT–WS2 INTs	24.7421	11.468	19.521	1.407
PEDOT–WS2 INTs	26.566	15.297	18.543	1.735
PEDOTAc–WS2 INTs	27.426	21.387	20.799	2.151

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Additionally to the amount of the polymeric shells, the amount of the carboxylic groups that were present on the surface of the functionalized composites were also important for the physic-chemical feature tracking, since they were the main factors to influence the dispersion quality of the obtained composites. They may also have been covalently modified in a second and later step reaction. Therefore, in order to quantify the outer carboxylic groups of the functional PTh-composites, they were coupled with 1,3-diaminopropane, giving new sets of polyNH₂ and polyTh–WS₂ INTs. A UV-sensitive Kaiser testing was then performed. The average results of the free amine groups at each amine-modified composite surface are reported in Table 2.

Table 2 The quantification of the outer carboxylic groups by the Kaiser testing after the 1,3-diaminopropane coupling/derivatization

After diamine derivitization	Average NH2 [μmol g^−1]	±	Average^N NH2^a [μmol g^−1]
a Average^N NH2 normalized to 100% of the total composite weight loss according to the TGA (Fig. 3).
Amino-PTAA–WS2 INTs	44.07	0.03323	163
Amino-PTAA/PEDOT–WS2 INTs	149.56	0.02018	438
Amino-PEDOT–WS2 INTs	1.42	0	2.2
Amino-PEDOTAc–WS2 INTs	293.13	0.01382	500

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The coupling reaction with the unsubstituted PEDOT composite stood as a control reaction and gave a negligible result of 1.4 μmol g⁻¹. Both of the polycarboxylated substituted PTAA–WS₂ INTs and the PEDOT_Ac–WS₂ INTs yielded 44.0 and 293.1 μmol g⁻¹, respectively, whereas the semi-substituted PTAA/PEDOT–WS₂ INTs disclosed an intermediate Kaiser value of 149.5 μmol g⁻¹. Even though these results fitted well with all of the observed weight loss amounts of the composites, a normalization to 100% of the total weight loss from the TGA analyses disclosed different outputs (163.0, 438.0, 2.2, 500.0 μmol g⁻¹ for the PTAA, the PTAA/PEDOT, the PEDOT, and the PEDOT_Ac–WS₂ INTs, respectively).

Interestingly, and first, the amount of the NH₂/COOH groups that were obtained for the PTAA-based composite was three times lower than the ones that were measured for the PEDOT_Ac-based composite, when it was expected to afford similar values. Second, the measured amounts of the NH₂/COOH groups of the co-polymer PTAA/PEDOT were very high. They were even higher than half of the obtained values for the PEDOT_Ac-based composites. This was when it was expected to be half of the scheduled values for the PTAA-based composites.

Regarding these results, and in accordance with the TGA data for the PTAA/PEDOT–WS₂ INT composites, it seemed that the electronically enriched EDOT monomers that were present during the polymerization process acted as a “TAA polymerization catalyzator”, since it effectively promoted the oxidative polymerization of the TAA monomers. This assumption explained the high amount of NH₂ groups that were present in the polyNH₂ PTAA/PEDOT composites. In addition, it is well known that the EDOT structure causes a strict linear chain growth of the PEDOT polymer. Consequently, this linearity feature that likely exists also existed locally within the co-polymer PTAA/PEDOT composites.

This might lead to one of the following assumptions: (1) thanks to the geometric linearity of the co-polymer, the carboxylic groups on the surface of the PTAA/PEDOT–WS₂ INTs composites might have been sterically more available during the EDC activation/coupling reaction with the 1,3-diaminopropane. (2) On the other hand, the finally generated NH₂ groups from the coupling reaction might have been more available during the UV-Kaiser test, which meant that the amino groups on the surface of the involved PTAA–WS₂ INTs composites were less available, being not fully detected via the Kaiser testing. Both of these assumptions are likely to explain the lower amount of NH₂ groups that were detected on the PTAA composite surfaces when compared to the ones that were measured for the PTAA/PEDOT and the PEDOT_Ac–WS₂ INT composites. Moreover, and in order to better grasp the overall significances of the quantitative Kaiser test results, one should also look at the normalized values of the polyNH₂ PTh–WS₂ INTs from the Elemental Analysis (Table 3). In accordance with the Kaiser test, the N^N% that was obtained from the polyNH₂ PTAA–WS₂ INTs was much lower than the ones that were measured for the polyNH₂ PEDOT_Ac–WS₂, thus strengthening the assumption that most of the carboxylic groups that were present on the PTAA composite surfaces were not fully accessible to the 1,3-diaminopropane coupling reaction. Moreover, the high N^N% of the PTAA/PEDOT–WS₂ INTs supported the first assumption that presented the EDOT as a “TAA polymerization catalyzator”.

Table 3 The elemental analysis of the four polyNH₂ polyTh–WS₂ INTs (following the 1,3-diaminopropane coupling)

After diamine derivitization	C%	O%	S%	H%	N%	N^N^a%
a N^N% normalized to 100% of the total composite weight loss according to the TGA (Fig. 3).
Amino-PTAA INTs	10.641	3.928	22.903	1.206	1.445	5.3
Amino-PTAA/PEDOT–WS2 INTs	28.061	15.549	19.069	2.763	3.234	9.5
Amino-PEDOT–WS2 INTs	28.330	15.000	20.299	1.821	0.307	0.7
Amino-PEDOTAc–WS2 INTs	27.660	20.020	16.000	2.757	3.698	8.0

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The FT-IR spectroscopy of the polyTh–WS₂ INT composites was performed in order to confirm the presence of the specific organic polythiophene phase onto the WS₂ nanotubes. Fig. 4 shows the FT-IR spectrum of the WS₂ INTs, the PEDOT_Ac polymer and the PEDOT_Ac–WS₂ INTs. The spectrum of the untreated WS₂ INTs is nearly featureless, while the two other composite spectra are quite similar and they indicate the clear presence of a PEDOT backbone structure and of the corresponding carboxylic groups. The vibrations observed at 1516, 1402 and 1348 cm⁻¹ corresponds to both the C=C and C–C bond stretchings relating to the thiophene ring, while the C–S bond stretchings can be seen at 850, 700 and 581 cm⁻¹. The ethylenedioxy groups are indicated by the antisymmetric C–O–C bond stretchings of the vinyl ether function at 1213 and 1099 cm⁻¹, and also by the C–H bond stretchings of the ethylene groups in the 2980–3000 cm⁻¹ range. The strong absorption peak at 1734 cm⁻¹ is associated to the C=O bond stretchings from the carboxylic groups.

Fig. 4 The FT-IR spectra of the WS₂ INTs, the PEDOT_Ac–WS₂ INTs and the PEDOT_Ac.

The general overview of the four polyTh–WS₂ INT composites on the FT-IR spectra clearly shows the expected absence of C=O bond stretchings at ∼1700 cm⁻¹ in the PEDOT composite spectrum (Fig. 5). Moreover, the peak corresponding to the C–O–C bond stretchings of the ethylenedioxy groups at ∼1100 cm⁻¹ appears in all of the three types of composites containing the PEDOT structure, while it is missing in the PTAA spectrum. The same characteristics are also observed in the neat polyTh-polymer IR spectra (Fig. SI-4†). In the range of 2980–3000 cm⁻¹, the low intensity peaks correspond to the C–H bond stretchings of the ethylene groups. In addition to these peaks, the 3000–3150 cm⁻¹ broad peak within the PTAA–WS₂ INTs spectrum can be readily attributed to the aromatic C–H bond stretching vibrations of the thiophene ring, while in the EDOT and the EDOT_Ac, all of the C–H positions have been converted to saturated ones when polymerized. By comparing the spectra of the neat polymers, the CH=CR₂ bending at 744 cm⁻¹ is only observed in the PTAA spectrum (Fig. SI-4†).

Fig. 5 The FT-IR spectra of the WS₂ INTs and of the various polyTh–WS₂ INTs composites.

The Raman spectra show the presence of two sharp peaks at 358 and 418 cm⁻¹, assigned to the S–W–S E¹_2g(Γ) and the A_1g(Γ) stretches, respectively (Fig. 6).⁵² These peaks appear in both the WS₂ INTs spectrum and in all of the polyTh–WS₂ INT composite spectra. Moreover, the spectra of the four polyTh-composites correspond well with the relating spectra of the appropriate monophase polyTh polymers. The broad Raman bands at the high frequency can be assigned to the aliphatic C–H bonds from the amorphous organic shell and the C=C bond vibrations arising from the thiophene rings are also observed in the 1300–1600 cm⁻¹ range.⁵³ Above all, the characteristic symmetric C_α=C_β stretching vibration band at 1433 cm⁻¹, close to the 1430 cm⁻¹ peak in the PEDOT–WS₂ INTs spectrum, is attributed to the structural tendency towards the quinoid structure of the PEDOT.⁵⁴ The PEDOT benzoid structure forms a coiled conformation, whereas the quinoid structure favors a linear one. In this last linear conformation, all of the charge carriers are more delocalized and, as a direct result, they can hop easily from one localized site to the other. The presence of such quinoid structures means that the conjugated system of thiophene molecules possess delocalized electron states, while the quinoid-rich structure is a typical case of electron-rich PEDOT chains.⁵⁵ The conformational changes of the PEDOT chains from benzenoid to quinoid configurations increases the inter-chain interactions, thereby, they may strongly enhance the conductivity of the PEDOT phase.

Fig. 6 The Raman spectra of the untreated WS₂ INTs and of all of the polyTh–WS₂ INT composites together, with the inset spectra for all of the neat polyTh polymers.

Quite interestingly, and in order to study the crystalline nature of the polymer shell, XRD measurements were also performed for the untreated WS₂ INTs, all of the neat polymers, and for the whole set of the corresponding composites (Fig. 7 and 8). As can be seen in the XRD diffractograms (Fig. 7), no particular feature form of the polyTh-based polymers is detected, as they are probably being amorphous polymers, except for the PEDOT case, which exhibits a large and sharp diffraction peak at 2θ = 26.2° (d spacing of 3.4 Å), corresponding to the 400 planes of the PEDOT backbone and the inter-chain planar ring-stacking distance.^56,57

Fig. 7 The X-ray diffraction patterns of the polythiophene-based polymers.

Fig. 8 The X-ray diffraction patterns of the polyTh–WS₂ INTs composites.

The enhancement of sharpness of the diffraction peak intensity can likely be ascribed to the increment of polymer chain ordering, leading to a higher crystallinity feature, due to the effective inter-chain stacking. Indeed, the XRD technique has been successfully used for a long time in the crystallographic studies of polymers. It is commonly used for analyzing crystalline phases in solid materials, i.e., determining the extent of their crystallinity and also identifying their exact crystalline structure. All of the crystalline parts give sharp narrow diffraction peaks and the amorphous component gives a very broad peak (halo).⁵⁸ The increased peak intensity over the broad background signifies a partial crystallinity where there exist some crystalline regions embedded in an amorphous matrix.^59,60 The higher degree of crystallinity might be considered as a real indication of a better inter-chain stacking phenomenon.

The XRD pattern of the starting pure untreated WS₂ INTs (Fig. 8) afforded typical peaks and are in a good agreement with previous reports.^61–63 The peaks recorded for the polyTh–WS₂ INTs coincide with those from the WS₂ INTs and no new peaks appear. Otherwise, at 2θ = 26.2°, a sharp and intense characteristic PEDOT diffraction peak is observed in the XRD pattern of the PEDOT–WS₂ INTs, thus demonstrating the successful PEDOT–WS₂ INTs functionalization. More interestingly, this specific diffraction peak also reveals the high crystallinity of the PEDOT shell phase composition. This is an important point to notice in that it might strongly improve the charge transport via chain hopping, and therefore, enhance the conductivity over the whole of these PEDOT–WS₂ INT composites.⁶⁴ The same indication can be slightly observed for both the PEDOT_Ac and the PEDOT_Ac–WS₂ INTs at 2θ = 24.5°. Generally, the peak at 2θ ∼ 13°, observable in both of the PEDOT and the PEDOT_Ac spectra, is to be considered to characterize the distance between the two stacks in the 2D-stacking arrangement of the polymer chains with the incorporated dopant ions.⁶⁵

The paramagnetism features of both polymers and their respective composites were investigated by EPR spectroscopy. The main EPR signal parameters, i.e., the g value, the peak to-peak linewidth (ΔB_pp), the signal symmetry parameter A/B (the ratio of the low to high field peak heights of the measured absorption derivative spectra), the Q value and the double integration (DI) value at room temperature, are summarized in Table 4.

Table 4 The characteristic EPR signal parameters and the EPR parameter values

Entry	Sample name	g value	ΔBpp (G)	A/B	Q value	DI
1	PTAA–WS2 INTs	2.002	4.11	0.98	10 000	664
2	PTAA	2.002	4.4	0.88	10 000	1333
3	PTAA/PEDOT–WS2 INTs	2.003	3.18	0.96	10 000	3397
4	PTAA/PEDOT	2.003	3.88	0.89	10 000	20 410
5	PEDOT–WS2 INTs (narrow)	2.002	6.3	0.98	1500	—
6	PEDOT–WS2 INTs (broad)	1.989	190	1.5		—
7	PEDOT (narrow)	2.002	9	1	7700	—
8	PEDOT (broad)	2.001	209	0.9		—
9	PEDOTAc–WS2 INTs	2.003	4.7	0.91	5000	1361
10	PEDOTAc	2.003	4.88	0.92	10 000	31 562

---

In Fig. 9, the representative X-band EPR spectrum that is observed at RT for each polymer and its respective composite is displayed, showing the presence of a rather narrow signal in all of the samples except for the PEDOT and its composite, which disclose two different signals to be extensively analyzed in the next section. A typical g value of ∼2.003 with ΔB_pp ∼ 3–5 G (Table 4) was measured for all of the narrow signals, which refers to the charge transport in the conjugated polymer phase, attributed to the polarons,⁶⁶ and fully agrees with previously reported polythiophene values.^67,68 This is noteworthy in that the unmodified WS₂ INTs did not generate any significant EPR signal (data not shown). When comparing both the EPR signals of the polymers and of their respective composites, no change of g values was observed. However, there were significant differences in their signal intensities (Fig. 9). In all of the cases, the polymers disclosed a higher DI, at least by a factor of two, when compared to the values that were obtained for their composites (Table 4, entries 1–4 & 9–10), which is likely to be due to the differential relative amounts of the polymer phase present in each EPR sample (i.e. neat polymer vs. ∼25–45% of the polymer into the composites – Fig. 3). Additionally, the signal linewidth ΔB_pp values of the composites were constantly narrower than those of the corresponding monophase polymers. This can be explained by the semi-conductive properties of the WS₂ INTs, which influence a spin exchange and allow for the over sharing of the free electrons along the interface between both the shell polymer and the nanotube surface. Thus, the electrons are spread out over a large region, likely averaging the corresponding magnetic field fluctuations of the samples, which results in an exchange narrowing.

Fig. 9 The EPR spectra of the PTAA, the PTAA/PEDOT, the PEDOT, the PEDOT_Ac polymers and of their respective polymer-modified WS₂ INT composites.

In this research, the EPR signals of the PTAA and the PTAA/PEDOT, and of their respective composites, were found to fit a Lorentzian (L) line shape (Fig. 10), which is usually associated with spins typically located in the predominating amorphous phase of the conducting polymers. Indeed, in the XRD analysis, no particular feature forms were detected for both of these polymers (Fig. 7 and 8). While in the case of the PEDOT_Ac, such a fitting did not completely fit into an L line shape and it also had a Gaussian (G) contribution. This may indicate the crystalline character of the PEDOT_Ac polymer phase, as it can also be slightly deduced from the XRD analysis.^66,69 Unlike the other polymers, in addition to an L narrow signal, the PEDOT and its relating composite exhibit another broad EPR signal (Fig. 9). The broad EPR signal was found to fit a Dysonian (D) line shape, which corresponded to a line shape distortion usually exhibited by the conductive samples (Fig. 11).^70–72 Such a Dysonian phenomenon resulting from the crystalline areas embedded within the polymer was intensified by the large crystalline clusters, causing a significant skin depth thickness.^73,74

Fig. 10 The EPR spectra and its line fitting to a Lorentzian (dashed line) for the PTAA (left) and the PTAA–WS₂ INTs (right).

Fig. 11 The EPR spectra and its line fitting to the Dysonian (broad line) and the Lorentzian (narrow line) for the PEDOT (left) and the PEDOT–WS₂ INTs (right).

Moreover in this study, the linewidth of the broad D-shaped signal of both of the PEDOT and the PEDOT composites was unexpectedly high (∼200 G, Table 4, entries 6 & 8) when compared to previous PEDOT reports (∼4–10 G).^2,11 This unusual EPR linewidth signal may likely result from in situ oxidative polymerization conditions, which result in a high degree of crystallization of the PEDOT polymer, thus significantly increasing the dipolar interactions between the unpaired electrons.

Furthermore, a significant deterioration in the quality Q factor of the EPR cavity from 10 000 to 7700, 5000, and 1500 for the PEDOT, the PEDOT_Ac–WS₂ INTs and the PEDOT–WS₂ INTs, respectively, was also observed (Table 4, entries 5–9) confirming the skin depth effect and the highly conductive property of these materials. It is well-known that the conductive samples can affect the absorption of the EPR microwave (mW) energy incident upon sample, causing a less intense electron absorption per unit volume.⁷⁵ This skin effect phenomenon is complicated to predict, due to the finite penetration of the mW field into the conductive material.⁷⁶

Interestingly, the g value of the PEDOT composite broad EPR signal was negatively shifted to a g value of 1.989, while all of the other signals were close to the free electron g_e (2.00232) (Table 4). This negative shift can be attributed to the delocalization of the unpaired spins within the WS₂ energy states, resulting from the expansion of the spin–orbit coupling in the PEDOT–WS₂ composite with the WS₂-relating unoccupied orbitals. This g value is close to the one reported for the conduction band electrons in other semiconductors,^77,78 thus supporting the assumption that the broad EPR signals originate from the conduction band electrons. Moreover, the apparent EPR D-broad signal asymmetry reflected in the A/B ratio of 1.5 (Table 4, entry 6) most likely arises from a g matrix anisotropy relating to the spin transfers among several crystalline grains,⁷⁹ and/or, from the increment of electron spin relaxation times.⁸⁰ It is logical to assume that the free electrons of the semi-conductive WS₂ INTs influence the spin dynamics in the whole composite, thus increasing the spin relaxation of the PEDOT electrons. Probably, the wave function extension of the free electrons of both of the PEDOT shell and the WS₂ INTs core are responsible for the high mobility of electrons in the PEDOT-relating composite.

The EPR spectra of the PEDOT and the PEDOT–WS₂ INTs were also examined at different temperatures, trying to verify the origin of both the narrow and the broad EPR signals (Fig. 12). It was found that at the lower temperatures (122–180 K), the narrow signal predominates in the integrated EPR spectra, thus supporting the paramagnetic character of this signal. On the other hand, at the higher temperatures (200–220 K), the broad signal increases. This reinforces the conclusion that the broad signal can easily be distinguished and it belongs to the electrons in the conduction band, thus reflecting an increment of the electron population in the conduction band at the higher temperatures.

Fig. 12 The integrated EPR spectra of the PEDOT–WS₂ INTs at different temperatures (122–220 K) at a microwave power of 6.325 mW.

One of the main attractive physic-chemical properties of these unique polyTh-based WS₂ INT composites is their basic electrical conductivity. For this characterization purpose, a sample electrical conductivity was determined by using a resistivity-measuring device, in which powder resistivities of the WS₂ INT composites and of the neat polymers were measured (Fig. 13). In order to line-up with previously reported conductivity measuring systems, the resistivity of the conductive carbon black* (EQ-Lib-SuperP, purchased from MTI Corporation) was also measured (4 × 10⁻² S cm⁻¹) in the same way.

Fig. 13 The table and column diagram for the conductivity values of the bulk polythiophene-based polymers and for the WS₂ INTs with and without the decorating polyTh-shells.

Starting from a conductivity value of 2 × 10⁻³ S cm⁻¹ for the unmodified WS₂ INTs, the incorporation of a covalently grafted polythiophene shell clearly alters in a different manner, the overall conductivity of such new composites. For example, the PTAA–WS₂ INTs composite has been found to be nonconductive like its related neat PTAA polymer. In the case of the grafted hybrid PTAA/PEDOT polymeric phase, it appears that the EDOT units favorably contribute to increase the conductivity of both the PTAA/PEDOT polymer and the PTAA/PEDOT–WS₂ INTs composite as reported above (Fig. 13, inset: table data). However, the measured composite conductivity was still lower than the one for the unmodified WS₂ INTs. Moreover, the monophase PEDOT polymer (outer shell of WS₂ INTs) significantly increased the conductivity of the corresponding PEDOT–WS₂ INTs composite. Indeed, the conductivity value of the PEDOT–WS₂ INTs was 2.2 × 10⁻² S cm⁻¹, which was one order of magnitude higher than the one measured for the unmodified WS₂ INTs. This value is fully compatible with the electrical conductivity values that have been reported for other various PEDOT derivatives and that have been based on the substituted EDOT monomers.^48,81 This conductivity value can also be compared very favorably with the ones that have been measured for conventional radical polymers which show conductivity values of between 10⁻⁵ and 10⁻⁷ S cm⁻¹.⁸² In the case of the PEDOT_Ac and the PEDOT_Ac–WS₂ INTs, their conductivity values were found to be lower than those of the PEDOT and of its composite, respectively, but they were much higher than those of the PTAA and the PTAA/PEDOT polymers and their respective composites. Indeed, both of the XRD and the EPR analyses validated the formation of a more crystalline polymer phase for the PEDOT and the PEDOT_Ac, thus considerably enhancing their conductivity. The measured low conductivity of the PEDOT_Ac, when compared to the PEDOT, most likely results from the presence of electron-withdrawing carboxylic groups within the EDOT_Ac units. This significantly interferes with the conductivity of the whole PEDOT_Ac. Nevertheless, and despite such detrimental conductivity interference, these shell free carboxylic groups might readily be activated for any covalent modification, with a large variety of additional conductive species. For example, they could be chemically modified towards related redox active polymers, by combining the present successfully conjugated polymer backbone, with an active redox side group, such as the stable nitroxide radicals.^82,83

As a matter of conclusion, all of the conductivity values of such dual phase composites were always higher than the ones of the related neat polymers. This readily suggests an effective contribution of the WS₂ INTs to the composite conductivities. Indeed, this conclusive observation fully agrees with the EPR analysis data, since the signal linewidth ΔB_pp of each composite was constantly narrower than the one for the corresponding monophase polymers, as detailed above in the EPR characterization section. Moreover, the highly conductive properties of the PEDOT, the PEDOT–WS₂ INTs and the PEDOT_Ac–WS₂ INTs (Fig. 13) have also been confirmed by the measured low quality Q factors of their EPR cavity (Table 4).

Furthermore, and in this context, both of the PEDOT and the PEDOT_Ac composites led to conductivity values that were higher than the ones for the starting WS₂ INTs. Such promoting conductivity values suggest a synergistic conductivity effect between both of the contacting polymer shell and the WS₂ INTs phases. It would appear that the semi-conducting WS₂ INTs positively act as an active doping nanomaterial for the corresponding polythiophene shells via the related polymer–WS₂ INT interfaces for an efficient charge transfer.⁸⁴ Therefore, and in contrast to the random bulk thiophene monomer liquid phase oxidative polymerization (LPP) in the presence of the WS₂ INTs, this same LPP oxidative polymerization that is executed from the covalently anchored nucleophilized nucleation points, enables an effective way to control such an interfacial polythiophene-based layering (full control of processing conditions). This opened the possibility of achieving high charge mobility via the two-dimensional transport through the WS₂ INTs/polymer interface, combined with mobility anisotropy through the corresponding crystalline domains of the polymer shell. This has important consequences for the relating electrical properties of these novel composite materials.

4. Conclusion

Four types of PTh–WS₂ INT composites were prepared based on a covalent approach of the oxidative thiophene monomer polymerization, directly from the surface of the functional chemically modified polyCOOH–WS₂ INTs. According to this quite effective “polymer growth from surface” concept, various chemically stable nanometric polyTh adlayers were successfully generated towards morphologically well-defined core–shell WS₂ INTs/polymer composites. All of these final dual phase PTh–WS₂ INT composites were fully characterized by using a combined series of analytical and spectroscopic tools. The WS₂ INTs might be considered to be an active inorganic doping nanoscale material for the various polythiophene shells via the polymer–WS₂ INT interfacial efficient charge transfer. In addition, the selected functional (polyCOOH) outer polyTh polymeric shells might be quite useful for a subsequent 2nd step covalent attachment of a wide variety of organic molecules and/or ligands. In this manner, this methodological platform might heavily promote the use of such functional designed conductive polymer–WS₂ INT-based nanocomposites for a wide range of applications.

Acknowledgements

The authors greatly thank the Israel National Nanotechnology Initiative Focal Technology Area (FTA) project “Inorganic Nanotubes: From Nanomechanics to Improved Nanocomposites” (Prof. Reshef Tenne, Weizmann Institute, FTA program coordinator) for the partial funding of this research.

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Footnotes

† Electronic supplementary information (ESI) available: Picture of the powder resistivity measuring system; TEM images of PEDOT bulk polymerization in the presence of non functional-WS₂ INTs; DTG plots of crude polyTh-polymers and of the relating polyTh–WS₂ INT composites; FT-IR spectra of the entire crude polyTh-polymers. See DOI: 10.1039/c6ra19628d

‡ These authors contributed equally to the paper.

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