WO2014076693A1

WO2014076693A1 - Materials and composites of conductive polymers and inorganic nanostructures

Info

Publication number: WO2014076693A1
Application number: PCT/IL2013/050933
Authority: WO
Inventors: Reshef Tenne; David Zbaida; Alla VOLDMAN
Original assignee: Yeda Research And Development Co. Ltd.
Priority date: 2012-11-13
Filing date: 2013-11-12
Publication date: 2014-05-22

Abstract

The invention generally relates to novel composites of conductive polymers and inorganic nanostructures, such as inorganic nanotubes and fullerene-like materials, and their uses in the preparation of electronic or opto-electric devices.

Description

MATERIALS AND COMPOSITES OF CONDUCTIVE POLYMERS AND INORGANIC NANOSTRUCTURES

TECHNOLOGICAL FIELD

This invention generally relates to novel composites of conductive polymers and inorganic nanostructures, such as inorganic nanotubes and fullerene-like materials.

BACKGROUND OF THE INVENTION

Transition metal dichalcogenides and halides such as MX_n (e.g., M=W, Mo; X=S, CI etc) possess a layered structure, i.e. strong covalent bonds occur between atoms within the layer and weak van der Waals forces stack the planar sheets together. The driving force for the formation of hollow closed nanostructures stems from the high energy of the dangling bonds at the periphery of the nanoparticles, a property which is common to highly anisotropic layered materials, such as WS₂, M0S₂, NiBr₂ and others. These materials have been shown to create a variety of close caged nanostructures such as inorganic nanotubes (INTs) and inorganic fullerenes (IFs).

The number of experimental studies on the electronic, optical and magnetic properties of such INTs and IFs is still very small. In particular, the issue of contacting carbon nanotubes (CNT) still involves a large amount of ongoing research with a focus on matching the contact material work-function with that of the measured material, as well as matching the Fermi surfaces in order to reduce electron scattering at the interface.

Intrinsically conducting polymers combine many advantages of plastics, e.g. flexibility and processing from solution, with the additional advantage of conductivity either in the metallic or semiconducting regimes. This class of materials is of great interest for the development of thin film plastic opto-electronic devices including light- emitting devices and photovoltaic cells.

Among the conducting polymers, poly aniline (PANI) is one of the most studied intrinsically conducting polymers, owing to its ease of synthesis, remarkable environmental stability and high conductivity in the doped form. While it is long known that proton-doping can make PANI conducting, the possibility of using non-protonic dopants such as electron-deficient Lewis acids was demonstrated only very recently [1]. These composites are expected to be different from the conventional protonated PANI owing to a qualitatively different chemical interaction between the dopant and the polymer.

PANI exists in a variety of forms that differ in chemical and physical properties. It is already established that the addition of acid (HCl, HCIO₄) to the basic form of PANI, Emeraldine base (EB), leads to protonation of the imine moieties rather than the amine moieties, thus forming the conductive Emeraldine salt (ES) (as shown in Fig. 1- a transition from structure 2 to structure 3).

The resulting material appears to have a significant electron derealization in the polymer backbone. Previous studies have also showed conductivity of PANI in the presence of CNT [2-5].

REFERENCES

[I] D. Chaudhuri, D. D. Sarma, Pramana - J Phys 2006, 67, 135-139.

[2] H. Guo, H. Zhu, H. Lin, J. Zhang, Materials Utters 2008, 62, 3919-3921.

[3] R. Sainz, Nanotechnology 2005, 16, 4.

[4] E. Zelikman, M. Narkis, A. Siegmann, L. Valentini, J. M. Kenny, Polymer

Engineering & Science 2008, 48, 1872-1877.

[5] P. G. a. R. Saraswathi, Pure Appl. Chem. 2008, 80, 23.

[6] Y. Feldman, A. Zak, R. Popovitz-Biro, R. Tenne, Solid State Sciences 2000, 2, 663-672.

[7] C. Shahar, D. Zbaida, L. Rapoport, H. Cohen, T. Bendikov, J. Tannous, F. Dassenoy, R. Tenne, Langmuir 2009, 26, 4409-4414.

[8] J. Stejskal, R. G. Gilbert, Pure Appl Chem 2002, 74, 857-867.

[9] aA. Zak, L. Sallacan-Ecker, A. Margolin, M. Genut, R. Tenne, NANO 2009, 4, 91-98; bA. Zak, L. Sallacan Ecker, R. Efrati, L. Drangai, N. Fleischer, R. Tenne, Sensors ^Transducers 2011, 12, 1-10.

[10] A. ZAK, L. SALLACAN-ECKER, A. MARGOLIN, M. GENUT, R. TENNE, Nano 2009, 04, 91-98.

[II] G. L. Frey, MATERIALS RESEARCH 1998, 13, 5.

[12] S. N. KUMAR, Synthetic Metals 1990, 36, 16. SUMMARY OF THE INVENTION

Electronic and opto-electric devices based on plastic materials are of increasing interest in this industry, as they permit to exploit the advantages of plastics such as flexibility, easy processing and cost. Conducting polymers play a great role in this field as they posses the metallic-like or semiconducting-like properties.

The invention disclosed herein concerns conductive materials, and compositions of matter, having improved electronic properties (conductivity). The materials of the invention comprise conductive polymers and inorganic nanoparticles as doping components (dopants), such as inorganic fullerenes and nanotubes. The inorganic nanoparticles have the advantage of a desirable physical structure, which permits percolation of charge carriers, as well as the desired electronic structure, which permits charge transfer within the conductive material.

Thus, in one aspect the invention provides a conductive material comprising at least one polymer doped with at least one inorganic nanostructure. By some embodiments, the inorganic nanostructure has a closed-cage structure. The conductive polymer and the nanostructures may be associated through coordinative or ionic association (bond or interaction).

In another aspect, the invention provides a composite comprising a conductive material of the invention.

In a further aspect, the invention provides a composite comprising at least one conductive polymer and at least one inorganic closed-cage nanostructure. In a composite of the invention, the conductive polymer and the nanostructures exhibit induction of charge carriers therebetween.

In yet a further aspect, the invention provides a conductive polymer associated with at least one inorganic nanostructure.

Without wishing to be bound by theory, the association between the at least one conductive polymer and the at least one (i.e. one or more) inorganic closed-cage nanostructure in a material or composite according to the invention, results from an acid-base interaction, causing association, via chemical bond (e.g., ionic or coordinative bond) between the two moieties. This association results in a charge carrier transfer between the polymer and the inorganic closed-cage nanostructures.

The term "doping" , or any lingual variation thereof, will be used in the context of the invention to denote the introduction of inorganic nanostructures, as defined, into the conductive material for the purpose of altering the polymer's electronic/electric properties.

In some embodiments, the induced charge carrier transfer is an electron transfer process from the polymer (which acts as a donor or a Lewis base) to the inorganic nanostructure (which acts as an acceptor or a Lewis acid). In some embodiments, the induced charge carrier transfer is an electron transfer process from the inorganic nanostructure (which acts as a donor or a Lewis base) to the polymer (which acts as an acceptor or a Lewis acid).

The "conductive polymer" employed in the materials and composites according to the invention, is any conductive polymer known in the art. Typically, the conductive polymer employed is selected to comprise at least one moiety permitting charge transfer between the components of the composition. In some embodiments, the conductive polymers are selected amongst polymers comprising an atom such as O, N, and S or a moiety comprising such an atom. In some embodiments, the conductive polymer is selected from polypyrrole, polythiophene and polyaniline.

In some other embodiments, the conductive polymer is polyaniline (PANI).

The interaction between the polymer and the inorganic nanostructure, as described above, may results in changes (typically improvement) in the polymer electronic properties (e.g., conductivity). Thus, in another aspect the invention provides a conductive material comprising a conductive polymer doped with inorganic nanostructures, typically closed-cage nanostructures, wherein the conductivity of the material is enhanced as compared to the un-doped conductive polymer.

In yet another aspect, the invention provides a composite comprising a conductive polymer doped with inorganic nanostructures, typically closed-cage nanostructures, wherein the conductivity of the material is enhanced as compared to the un-doped conductive polymer.

Without wishing to be bound by theory, the improved conductivity of the doped polymer (composite) may be attributed to the characteristics of the inorganic nanoparticles. The "inorganic nanoparticles" (for brevity used interchangeably hereinforth with the term "nanoparticles") are hollow, in some cases closed-cage nanoparticles of transition metal chalcogenides, metal dichalcogenides or metal halides, which may be single or multi-layered, having structures such as nanospheres, nanotubes, nested polyhedra, onion-like (multiwalled and singlewalled) and the like. As a person skilled in the art would appreciate, the term " nanoparticle" should not be regarded as limiting the average size of the particles to the nanoscale. While in some embodiments the nanoparticles employed in accordance with the invention are fully in the nanoscale regime, in some other embodiments, particularly those relating to nanotubes, at least one of the particles' dimensions is in the nanoscale (e.g., width) while other dimensions (e.g., length) may be at the microscale.

In some embodiments, the nanoparticles are inorganic nanotubes (INT) or inorganic fullerene-like nanoparticles (IF).

In some embodiments, the nanoparticles are of the general formula MLn, wherein M is a transition metal, L is a chalcogen and n is the number of chalcogen atoms L per each atom of the transition metal M. A transition metal includes all the metals in the periodic table from titanium to copper, from zirconium to silver and from hafnium to gold. In some embodiments, the transition metals are selected from Sn, In, Ga, Bi, Mo, W, V, Zr, Hf, Pt, Pd, Re, Nb, Ta, Ti, Cr and Ru. The chalcogen is selected from S, Se and Te.

In some embodiments, the metal chalcogenides and dichalcogenides are selected from TiS₂, TiSe₂, TiTe₂, WS₂, WSe₂, WTe₂, MoS₂, MoSe₂, MoTe₂, SnS₂, SnSe₂, SnTe₂, RuS₂, RuSe₂, RuTe₂, GaS, GaSe, GaTe, InS, InSe, HfS₂, ZrS₂, VS₂, ReS₂ and NbS₂. In some other embodiments, the metal chalcogenides and dichalcogenides are selected from WS₂ and MoS₂.

In additional embodiments, the inorganic nanoparticles are selected from WS₂, MoS₂, NiBr₂, NiCl₂, VS₂, TiS₂ and InS.

In some embodiments, the inorganic nanoparticles are of the general formula Aj_ _x-B_x-chalcognide, wherein A is either a metal or a transition metal or an alloy of such a metal/transition metal, B is a metal or a transition metal, and x being < 0.3 and different from zero, provided that: A≠B.

The metal or transition metal or an alloy of metals or transition metals are selected from the following atoms: Mo, W, Re, Ti, Zr, Hf, Nb, Ta, Pt, Ru, Rh, In, Ga, InS, InSe, GaS, GaSe, WMo, and TiW.

B is a metal or transition metal selected from the following: Si, Nb, Ta, W, Mo,

Sc, Y, La, Hf, Ir, Mn, Ru, Re, Os, V, Au, Rh, Pd, Cr, Co, Fe, and Ni.

Within the nanostructure A_!__x-B_x-chalcognide, B and/or B-chalcogenide are typically incorporated within the Aj-x-chalcogenide. The chalcogenide is selected from S, Se and Te. For example, IF nanostructure to be used in the preparation of materials and composites of the invention may be IF-Moi_-xNb_xS2, IF-Mo(W)i_-xRe_xS2, the alloys of WMoS₂, WMoSe₂, TiWS₂ and TiWSe₂, where Nb or Re are doped therein.

The term " incorporated' means that the B and/or B-chalcogenide are doped or alloyed uniformly within the Aj-x-chalcogenide lattice. The B and/or B-chalcogenide substitute the A atom within the lattice. Such substitution may be continuous or alternate substitutions. Continuous substitution are spreads of A and B within each layer alternating randomly (e.g., (A)_n-(B)_n, n>l). Depending on the concentration of incorporated B, it may replace a single A atom within Aj-x-chalcogenide matrix forming a structure of (...A)_n-B-(A)_n-B ...). Alternate substitution means that A and B are alternately incorporated into the Aj-x-chalcogenide lattice (...A-B-A-B ...). It should be noted that other modes of substitution of the B in the A-chalcogenide lattice are possible according to the invention. Since the A-chalcogenide has a layered structure, the substitution may be done randomly in the lattice or every 2, 3, 4, 5, 6, 7, 8, 9 or 10 layers.

The conductive materials and composites of the invention may comprise any weight % of nanoparticles. In some embodiments, the conductive materials or composites of the invention comprise between 0.5 and 5 wt nanoparticles. In some embodiments, the amount of nanoparticles in the composite is between 0.5 and 3 wt . In further embodiments, the amount of nanoparticles is between 0.5 and 1.5 wt . In additional embodiments, the amount of nanoparticles is 0.5 and lwt .

In some embodiments, the conductive material or composite of the invention comprises PANI and inorganic nanotubes.

In some embodiments, the conductive material or composite comprises PANI and WS2 inorganic nanotubes (INT-WS2).

In some embodiments, the conductive material or composite of the invention comprises at least one conductive polymer and INT-WS2.

In another aspect of the invention, there is provided use of a conductive material or composite according to the invention for the manufacture of a device. The device may be any electronic or optoelectronic device.

Thus, the invention also provides a device comprising a layer or an element of a conductive material or composite thereof. In some embodiments, the device is an electronic device comprising a material or a composite of the invention. In other embodiments, the device is an opto-electric device comprising a material or a composite of the invention.

In some embodiments, the device may comprise an element or a layer made of or comprising a conductive material or a composite of the invention. The device may be a flexible or a rigid device.

In some embodiments, the device incorporating a conductive material or composite according to the invention is a sensor device.

The structure of the conductive polymers of the invention renders them with increased sensitivity to chemical or electrochemical redox states. As the fluctuations from an oxidation to a reduction state may be reversible, the conductive materials of the invention permit control over electrical and optical properties. Thus, the materials of the invention may additionally be used in a variety of applications such as in electrostatic materials, in electromagnetic shielding, conductive materials, in molecular electronics, in printing circuit boards, electronic displays, as sensors for a variety of chemical or biochemical applications, in energy storage such as rechargeable batteries and solid electrolytes, in the construction of electronic elements such as transistors and diodes and switches, and so forth.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 shows molecular structures of PANI in its various forms: (a) PANI- leucoemeraldine, white/clear to colorless, fully reduced, insulating; (b) PANI- Emeraldine base, blue, partially oxidized, insulating; (c) PANI-Emeraldine salt, green, partially oxidized, conducting; and (d) PANI-pernigraniline (PNB), fully oxidized, violet, insulating.

Fig. 2 shows the molecular structure of PANI in its emeraldine base (EB) form and in its doped Emeraldine salt (ES) form.

Fig. 3 is a SEM image of PANI powder on carbon tape as purchased from Sigma Aldrich, showing worm-like structures. Fig. 4 is a UV-Vis spectrum of PANI in DMF: (■) corresponds to Emeraldine base; (— ) corresponds to Emeraldine salt pHl.

Fig. 5 is a picture of 3 solutions: (a) PANI solution in DMF; (b) PANI solution after addition to IF-WS₂; (c) clear DMF solution after centrifugation, the IF-WS₂ particles precipitated with the absorbed PANI.

Fig. 6A presents a SEM image of reference sample of IF-WS₂ powder; Fig. 6B shows IF-WS₂ treated with acidic pHl solution and with PANI; and Fig. 6C shows IF- WS₂ treated with basic solution of pH9 and with PANI.

Figs. 7A-B present close up images of fullerenes coated with PANI.

Figs. 8A-B are TEM images of PANI/IF-WS₂ composite.

Figs. 9A-D are SEM images of PANI/Re:IF-MoS₂ composites exhibiting rejection between the two components.

Fig. 10 shows an absorbance curve: (— ) synthesized PANI, and (■) purchased

PANI.

Figs. 11A-B are SEM images of synthesized PANI.

Fig. 12A is a SEM image of PANI/INT 0.8 wt% using the in-lens detector; Fig. 12B is a SEM image of PANI/INT 3 wt% of embedded nanotube in PANI matrix, using the backscattering detector.

Figs. 13A-B are TEM images of PANI/WS2 cast from a suspension of the powder in ethanol.

Fig. 14 shows Raman spectra: (a) PANI; (b) INT-WS₂; and (c) PANI/INT-WS₂ sample.

Fig. 15 is a UV-Vis spectrum of EB (— ) and ES (— ), the arrow points the intensity for -633 nm.

Fig. 16 shows the conductivity vs. IF wt%.

Fig. 17 shows the conductivity vs. Re:IF-MoS₂ wt%.

Fig. 18 presents oxidized N contents as a function of W contents (at%).

Fig. 19 is an energy diagram of Fermi levels of INT-WS2, PANI and the produced composite material containing both materials.

Figs. 20A-20B show PANI/INT-WS₂ composites conductivity vs. INT weight percent.

Fig. 21 is the averaged conductivity of each wt% and its deviation.

Fig. 22 presents chemical structure of PANI doped with Lewis acid. Fig. 23A shows a post treatment suspension; and Fig. 23B shows suspension prior to treatment.

DETAILED DESCRIPTION OF EMBODIMENTS

Synthesis

IF-PANI hybrid experiment

In the first step the possibility of adsorbing PANI onto the pristine IF-WS2 was explored. IF-WS2 nanoparticles were synthesized according to a published procedure

[6]. Poly aniline M_w~20,000(g/mole) was purchased at Sigma- Aldrich Co. N,N- dimethylformamide (DMF) was selected as the solvent of choice permitting high solubility of the polymer.

IF-WS2 (20mg) powder was added to PANI solution in DMF (1.3mg/30ml) and suspended using ultrasonic bath for 1 hour. The sample was centrifuged for 1.30 min at rpm=27 to receive a sediment of the synthesized product. The solvent was removed and the product was washed with water and ethanol by shaking and centrifugation repeatedly. The solid residue was dried in a vacuum oven over night at room temperature.

PANI/WS₂ "in situ " composite synthesis

The synthesis of PANI was based on a common procedure [8]. Aniline hydrochloride was used as a monomer due to its solubility in aqueous solutions. In addition, the handling of solid anilinium salt is preferred compared to on liquid aniline from the toxic hazards point of view and the high tendency of the liquid aniline to oxidize. Peroxydisulfate is the most commonly used oxidant, and its ammonium salt was preferred to the potassium counter cation because of its better solubility in water. The concentration of aniline hydrochloride was set to be 0.025 M.

To minimize the presence of residual aniline and to obtain the best yield of PANI, the stoichiometric peroxydisulfate/aniline ratio of 1.25 was used, so the concentration of ammonium peroxysulfate was set to 0.031 M.

Aniline hydrochloride (Sigma; 0.259 g, 2 mmol) was dissolved in distilled water in a volumetric flask to 10 mL of solution. Ammonium peroxydisulfate (Sigma; 0.571g, 2.5 mmol) was dissolved in 10 mL of water. Both solutions were kept for 1 h at room temperature (-18-24 °C). Inorganic nanoparticles of WS2 (IF-WS₂) were suspended in 20 mL of water for 15 min. Anilinium hydrochloride was added to this suspension in order to keep the particles suspended homogeneously with the aniline molecules and was further dispersed in an ultrasonic bath for 1 h. The ultrasonic bath was cooled with ice during this process in order to avoid an increase in the temperature of the water inside the bath and keep it at room temperature. A solution of the oxidant, ammonium persulphate was added to this reaction mixture drop wise with continuous stirring for 30 min at 0-5°C. A color change from yellow to pale green to deep green was observed.

The mixture was left to polymerize for 2 h with continuing stirring at room temperature. The PANI/INT-WS2 precipitated out and was collected on a vacuum filter, washed with water (3x20 mL) followed by (3x20 mL) ethanol wash to remove the excess of the water. The PANI/INT-WS2 powder was dried in air and then in vacuum oven at 100°C for 24 h until a constant weight was obtain.

In order to calculate the wt% amount of nanoparticles for each of the samples, PANI was synthesized several times. The average yields of PANI experiments, using the same concentrations of the monomer and oxidizing agent without nanoparticles was found to be 176.3 ± 1.3 (mg) (68% chemical yield, average of 6 samples). The different amounts of INT and IF were calculated according to this weight. Inorganic Fullerenes

IF-WS2 samples were prepared according to a published procedure. Table 1 presents the composite samples prepared.

Table 1: IF-WS2 content for PANI/IF-WS2 composites samples; the amount of anilinium chloride (259 mg) and ammonium peroxy sulfate (571 mg) was constant

The total volume of each experiment was 40 mL yielding concentration of 0.025M and 0.031M for anilinium chloride and ammonium peroxysulfate respectively. Doping of the IF nanoparticles of M0S2 with minute amounts (<0.1 wt%) of Re atoms has been demonstrated. The Re (rhenium) atoms reside in substitutional (Mo) sites leading to an excess of negative charge carriers which are trapped at the nanoparticle's surface. It was considered that this surface charge may affect the obtained composite material in terms of electrical properties.

PANI/Re:IF-MoS2 was synthesized according to the described procedure above. Table 2 presents the composite sample prepared.

chloride and ammonium peroxyslfate was constant and the total volume of 40 mL

Inorganic Nanotubes

A composite material of PANI containing INT-WS2 was prepared by the same procedure as described above (except for the chemical treatment). Pure multiwall INT- WS2 was obtained from NanoMaterials Ltd. (Israel), synthesized according to a published procedure [9-10]. The nanotubes were typically 30-100 nm (nanometers) in diameter and 1-20 μπι (micrometers) long, comprising of approximately 30 WS2 layers and frequently containing a WC>3__X sub oxide core.

PANI/INT-WS2 composites were prepared in weight percent ratio in which the concentration of INT-WS₂ was varied (0.5, 0.85, 1.5, 2.5, 3, 5, 20, 40 and 100 wt %) where the concentration of anilinium chloride and ammonium peroxysulfate were kept constant. Table 3 below presents the wt% amount of INT taken for preparation of each sample.

Table 3: WS2-INT contents for PANI/INT-WS2 composites samples; the amount of monomer (259 mg) and oxidizing agent (571 mg) remained constant for all

experiments, all dissolved at a final volume of 40 mL Results and Discussion

IF Characterization

Polyaniline (PANI) exists in a variety of forms that differ in chemical and physical properties. It is already established that the addition of acid (HC1, HCIO₄) to the basic form of PANI, emeraldine base (EB) leads to protonation of the imine moiety rather than the amine one to form the conductive emeraldine salt (ES) (Fig. 2). PANI was commercially available, in its reduced state-emeraldine base.

The resulting material appears to have a significant electron derealization in the polymer backbone. It results in the formation of an environmentally stable nitrogen base salt.

Initial characterization of the as-purchased PANI by UV-Vis absorption spectroscopy was done by dissolving the material in DMF in various concentrations. The UV-Vis spectrum of PANI, in the base form is dominated by two absorption peaks at ca. 320 nm (Band I) and ca. 610 nm (Band II), respectively. Band I is often assigned to p-p* transition in the benzenoid structure. The absorption in the visible range, i.e. Band II, is ascribed to exciton formation in the quinonoid rings (Fig. 2). These results are concurrent with previous optical studies made on PANI. PANI can be produced as granular or fibrous shape as shown in Fig. 3.

At acidic pH 1, the band at 620 nm disappears and a new absorption band at 430 nm is found together a with the band at 850 nm (also existed at the EB). These bands correspond to a certain, possibly partial, protonation stage of the imine structural units in the PANI chain. In the literature, it is often interpreted as excitations to the polaron band (Fig. 4).

A bluish color solution (EB) was obtained when dissolving PANI in DMF which converted to green color (ES) when a few drops of HC1 0.1M were added (pH 1) (see Fig. 2).

PANI solution in DMF (1.3mg/30ml) was added directly to IF-WS2 powder and the bluish color of the solution turned green. This is due to the acidic pH of IF-WS2 in solution (pH 1-2). The mixture suspended by ultrasonic bath for 1 hour at room temperature followed by centrifugation (rpm-5, 5min). The resulting colorless solution (Fig. 5) suggests complete adsorption of PANI to the WS2 nanoparticles. In order to further study the adsorption process of PANI onto the IF-WS2 and examine the possibility of chemical linkage (or interaction) between them, the adsorption process was performed at two different pH solutions: acidic and basic.

In both pH samples, colorless solutions were observed after centrifuging, indicating that the adsorption step is independent of the initial pH of the WS2 nanoparticles.

SEM images of the matrix of PANI/IF were taken for both basic and acidic pretreatment of the IF nanoparticles. There were no significant structural differences shown in the nanocomposites; the initial pH of the nanoparticles does not affect the morphology of the obtained composite. In both cases the polymer (PANI) is within the WS2 particles, as contrast to the bare WS2 that were not treated with PANI (Figs. 6A- C).

Using SEM and TEM at a higher magnification of PANI treated with WS2 NP clearly show that the WS2 nanoparticles are submersed in the PANI film (Figs. 7-8).

Table 4 below presents the atomic concentration (%) of PANI-IF as derived from XPS elemental analysis. The same results were obtained for the acidic and basic pretreatment.

Table 4: Elemental analysis of PANI-IF sample (at%)

In the above Table 4, W_s stands for the tungsten signal associated with the WS2 phase. The W_ox signal is associated with the WC>3__X core of the particles (uncompleted oxygen to sulfur conversion). The C and N signals show the ratio of 1 :6 as expected from the PANI's structure (Fig. 2) and the existences of Ws and S at the ratio of approximately 1 :2. The atomic concentration of the oxygen can be accounted for from the WO₃ core of the particles and from water molecules that are chemisorbed to the defect sites on the surface or inner location of the fullerenes or polymer. The S: Ws ratio suggests a higher amount of S which will be further checked.

Re: IF-M0S₂

Electron microscopy characterization of PANI/Re:IF-MoS2 composite material shows coated particles resembling to the images obtained from PANI/IF- WS2. A closer view of these coatings reveals existence of gaps between the IF and the PANI coatings as can be seen by Fig. 9.

INT-WS₂/PANI nanocomposite

Considering the possibility that the structure of the particle can affect the layout of the composite material and later on the conductivity, introducing inorganic nanotubes to the polymer was suggested. The INT-WS₂ with their high aspect ratio (ca. 10 ) may increase the conductivity of the composite material by arranging with certain directionality, thus facilitating the electron transport across the sample.

Prior to characterizing the composite material, UV-Vis measurement of the polymerized PANI without nanoparticles was done and compared with the purchased one to verify the success of the polymerization. UV-Vis spectra clearly indicate the synthesis of PANI. Two fundamental absorption bands appear in the spectra centered at; 320 nm and 610 nm. SEM images of the produced PANI reveal granular morphology of the polymer. (Fig. 11).

The SEM studies performed with all the composite samples of PANI/INT-WS₂ reveal that the WS₂ tubes are embedded in the PANI matrix (Fig. 12A). In the lower weight samples, the nanotubes are mostly separated or in small bundles. As the amount of nanoparticles increases, the clusters become more frequently seen in the sample and the nanotubes are less separated (Fig. 12B).

TEM images (Figs. 13A and B) of PANI/WS₂ nanocomposite suggest that the WS₂ nanotubes are coated with PANI and thus create an interface layer between the tubes and the polymer.

Raman spectroscopy was used to verify the contents of INT-WS₂ and PANI in the composite material and compared with the pristine INT and PANI. Fig. 14 shows the Raman spectra of PANI/WS₂ composite material. The bands at 354-587 cm^"1 region are associated with the E_2g and the A_lg lines, respectively, of the WS₂ nanotubes as was shown before [11]. The bands from 1167 cm^"1 to 1600 cm^"1 are assigned to the PANI structure according to Table 5. Raman band (cm^"1) Possible assignment

354 WS2 nanotubes

420

523

587

1167 C-H stretch

1027-1170 (1/cm)

1348 C-N stretch

1324-1375 (1/cm)

1477 C=N stretch

1450-1500 (1/cm)

1560 Quinonoid ring

C=C stretch

1600 Benzoid ring

C=C

Table 5: Main Raman bands and their possible assignments

These observations offered strong evidence for the presence of PANI and INT- WS2 in the composite material and revealing that PANI was successfully synthesized.

By observing the intensities of PANI in the composite and the pristine PANI, one can notice a large difference in the intensity of the signals and their clarity. The pristine PANI (a) reaches intensities of approximately 2000 compared to the PANI/INT-WS2 (c) which its intensities reach almost 8000. This large difference may be explained by the UV-Vis absorption of the two forms of PANI, EB and ES as seen by Fig. 15.

The wavelength of the Laser beam at the Raman measurement instrument was 633 nm. As seen by Fig. 15, the absorption of EB at this wavelength is very high compared to the absorption of the ES. This fact causes most of the coming light to be absorbed by the sample, get lower penetration depth into the sample and as a result, to receive smaller scattering intensities. The ES absorption at 633 nm is much lower (for the same concentration), thus the intensity of the scattered light is expected to be larger, as seen by Fig. 14.

XPS analysis of the surface of PANI/INT was conducted to investigate the elemental composition of the composites. Table 6 presents the results of three different samples. Each sample was tested in various areas and the quoted result represents an average of the three separate measurements. o C N Sox SiNT w_ox CI C N S/W

PANI 13.03 72.88 10.92 2.57 - - - 0.59 6.7 -

PANI + 9.18 76.61 11.75 2.02 0.06 0.03 0.03 0.35 6.5 2 3% INT

PANI + 13.25 68.28 10.07 2.67 3.68 1.75 0.12 - 6.8 2.1 40% INT

Table 6: Elemental analysis

The results of elemental analysis clearly indicate the presence of oxygen in PANI (520 eV) (which is far more than the oxygen originating from the core of the nanotubes, as verified by analysis of the W line). This may be due to bound water molecules or, more likely, to partial oxidation of the PANI chains. The presence of sulfur in the PANI sample is partly due to the residual sulfate counter ions produced by the reduction of peroxydisulfate during polymerization.

There is also evidence for some chlorine residues in PANI from using anilinium chloride monomer even after several washings of the samples, indicating a partial benzene -ring substitution with chlorine.

Another noticeable fact is that the INT wt% in the 3% and 40% samples is not compatible with the actual amount of INT nanoparticles which were added to each of the experiments. This deviation can be explained by the existence of INT in bundles; thus creating areas depleted of particles contrary to large bundles of them. The size of the scanning beam in the XPS is around 500 μ and it averages the information it collects at this scanning radius. This implies that the above sample are highly non homogenous. Properties

IF-WS₂

PANI/IF conductivity was measured for the 15 samples. Fig. 16 presents the conductivity behavior of the composites. There exists a clear increase in conductivity of the composites compared with the pristine PANI. The highest conductivity achieved was for 1 wt% IF (10.46 ± 0.32 S/cm), two orders of magnitude higher than the conductivity of pristine PANI (0.16 ± 0.05 S/cm), as can be seen in Table 7.

Table 7: Conductivity values for PANI/IF-WS₂ composite Important note to consider is the value of the deviation for each of the measured samples. Although 3-4 experiments were performed for each composition and 3 pellets were tested for each experiment, repeatability could not be easily achieved.

IF-M0S2

Conductivity values of the PANI/Re: IF-M0S₂ composite material were plotted vs. the wt% of IF nanoparticles in each sample. Fig. 17 presents the results.

The appeared conductivity of the samples clearly indicates that the existence of Re: IF-M0S₂ NP decreases the conductivity of PANI tremendously. The greatest decrease obtained for the 20 wt% sample, lowering the conductivity by approximately 5 orders of magnitude as can be seen by Table 8.

Table 8: Conductivity values for PANI Re: IF-M0S₂ composite These results, combined with electron spectroscopy characterization demonstrate that the negative charge accumulated on the surface of the particle has a large affect on the composite material obtain, both in terms of morphology and conductivity. Creating a hybrid material between PANI and Re:IF-MoS2 nanoparticles is not favorable due to, presumably, electrostatic repulsion between the two components. PANI exists in its undoped, nonconductive form (EB) in the composite material. Addition of Re: IF-M0S2 nanoparticles to EB results in additional interference to the transport of electrons, presumably, due to creation of grain boundaries and barriers.

INT

XPS analyses of the surfaces of PANI/INT-WS2 were conducted to investigate variations at nitrogen binding sites. A sample of 40 wt% PANI/INT-WS2 was examined by XPS. Three different grains of the sample were studied. Table 9 presents the elemental contents of the examined grains and compared with the undoped PANI sample.

Table 9: Elemental composition of 3 samples of PANI/INT-WS₂ 40% wt and of PANI The Carbon/Nitrogen ratio is approximately as predicted from the PANI structure (Fig. 1), 6 Carbon atoms for each Nitrogen atom. Shifts from the value of 6 may be due to contaminations of Carbon or Nitrogen absorbed on the sample from the air prior to measurements. The S/W is also compatible with the WS2 phase.

It is important to notice the large differences in atomic percentage of W and S between the three spots of the same 40% wt sample. These large differences arise from the fact that a sample of composite material containing a great amount of nanoparticles is bound to create large aggregates of the nanoparticles which in turn creates areas depleted of nanoparticles, areas abundant with INT and a measured average of particles in the polymer. Observing the PANI/INT-WS2 40 wt% sample which contains the greatest amount of nanotubes, revealed that the nitrogen signal was split into two. To understand this interesting observation, the band of N_ls was deconvoluted and the different sub- bands were fitted with peaks at 398.8, 399.6, 400.7, and 401.8 eV, which corresponded to quinonoid imine (=N-), benzenoid amine (-NH-), protonated amine (-N⁺), and protonated imine (=N⁺), respectively.

Kumar et al. [12] attributed the last two peaks to the presence of polarons (radical cations) and bipolarons (dications), respectively. The ratio of these two N components at 400.7 and 401.8 eV (positively charged nitrogen atoms) could be considered as a direct manifestation of the doping level of PANI (see also Fig. 2). The area ratios of the four nitrogen constituents in PANI were calculated; the results were listed in Table 10.

Table 10: Four Nitrogen constituents of PANI for PANI/INT-WS2 and PANI (=N⁺ protonated imine; -N+ protonated amine; -NH- benzenoid amine; =N- quinonoid imine)

The pure PANI was synthesized in the presence of HC1 (by using anilinium chloride as a monomer), hence the nitrogen atoms are partially protonated in the polymer, representing a certain degree of doping. To estimate the doping level of PANI originating from the presence of the INT-WS2 we compared it to the undoped PANI. To calculate the doping level which occurred only by the presence of INT, we subtracted the ratios of the protonated amine (-N⁺) and protonated imine (=N⁺) components which were received from the pure PANI (as a result of protonation by HC1) from the results received by the composite material. The doping level for PANI/INT-WS2 (27 -56 ) was higher than that for the undoped PANI (26%).

Fig. 18 presents the correlation between the INT-WS2 content and the degree of doping of the PANI. CREM can provide us with the work function of a sample. Work functions for both INT and PANI were measured separately. The obtained work functions are displayed in Fig. 19. The energy of the valence band was obtained from XPS for both components. The band gap was obtained from UV-Vis spectra together with previous studies.

The obtained work functions for both INT-WS2 and PANI reveal a significant difference, larger than 0.5 eV. Therefore, it was suggested that an electron transfer process from PANI to INT-WS2 should occur naturally. The obtained work function for PANI/INT-WS2 was 4.72 eV which stands in the middle between the work functions of PANI and INT/WS2. The work function values support the probability of electron transfer process from PANI to INT-WS2.

The splitting of the nitrogen signal of PANI verifies the oxidation process of one of the nitrogen atoms in PANI structure. This oxidation is known as the process of doping which took place with priority on one site (the imine nitrogen). No evidence of acceptance of electrons on the tungsten or sulfur atoms of INT was observed. It is assumed this may be due to the fact that the reference measurements of INT were done mostly on aggregates of INT and surroundings of them. In the measurement of INT in the composite of PANI/INT-WS2, the chemical surrounding of the INT was different, smaller aggregates and PANI setting.

The electrical conductivity of PANI and PANI/WS2 nanocomposites are presented in Fig. 20. The addition of INT-WS2 nanoparticles into polyaniline shows a very peculiar behavior. First the addition of the nanotubes leads to increasing electrical conductivity of the nanocomposites, which peaks at -0.85 wt% nanoparticles. At higher nanotube concentration the conductivity decreases as the amount of INT-WS2 becomes larger. Fig. 20 presents the conductivity data. Three samples were synthesized for each concentration. Three different pellets were prepared from each synthesized sample and their conductivity was measured. Thus each point in the graph represents an average from three tested pellets. Fig. 22 presents the average of each set of points and their standard deviation.

The inset on Fig. 20 presents the conductivity for the samples with wt% greater than 5% up to 100% of INT. Table 11 summarizes the conductivity data using the average conductivities of each sample as also presented in Fig. 20.

Table 11: Conductivity and standart deviation of synthesized composite samples

The effect of the INT-WS2 nanotubes on the PANI is clearly seen from the conductivity behavior of the composite. The average electrical conductivity of PANI thus prepared (average of 10 separate experiments and 30 samples, synthesized and measured at the same conditions described at the experimental section) was 0.15 ± 0.04 S/cm. The largest increase in conductivity of the composite is achieved for the -0.85 wt INT-WS2 concentration which gave 12.73 ± 0.96 S/cm, two orders of magnitude higher than the pristine PANI hydrochloride. As the amount of nanoparticles in the composite increases, the conductivity is decreases even below the conductivity of undoped PANI.

To explain the obtained results, an "acid-base" reaction between PANI and INT was proposed. PANI can be considered as Lewis base due to a lone electron pair on the nitrogen atoms of PANI (Fig. 2). Recently, it was evidenced that the PANI can be doped with Lewis acids, like BF₃, FeCL, or SnCL_t and form acid- base complexes. The general structure of PANI doped with Lewis acid (LA) is presented in Fig. 21. Similarly to the case of protonation, one molecule of dopant is coordinated to nitrogen atom of PANI, but contrary to the protonation, both types of nitrogen atoms of PANI (amine as well as imine ones) may be coordinated by LA.

Lewis acid-doped PANI systems are expected to be different from the conventional protonated PANI owing to a qualitatively different chemical interaction between the dopant and the polymer; for instance, the absence of any counter ion in these systems may have different influence on the properties of the polymer. In this work, we suggested that, INT-WS2 can also form a complex with PANI. This may explain the increase in conductivity for the -0.85% INT. WS2 nanotubes, due to their structure and high aspect ratio; have strong van der Waal forces which results in their tendency to form bundles. At polymer samples with 0.5%-3% INT content, the tendency to form bundles is smaller due to the larger average distance between the nanotubes and their strong acid-base interaction with the polymer. As the INT concentration increases, the chance of creating large bundles also increases. This in turn lowers the active surface area of the tubes and in addition, creates grain boundaries which scatter the conduction electrons and results in lowering of the composite conductivity.

It has been shown that INT-WS2 nanoparticles were successfully embedded in a conductive, organic polymer matrix. In the scope of the experimental work it was shown that these particles may serve as an inorganic dopant material for PANI and thus increase its conductivity. The conductivity of PANI/INT-WS2 was investigated and compared to PANI without furthered doping. It was further shown that doping PANI with 0.8 wt% INT-WS2 increases the conductivity of the polymer by two orders of magnitude compared with the non-treated polymer (i.e., undoped polymer).

It was also found that, the type of additive has a considerable effect on morphology of resultant products. When using a prepared polymer (commercially purchased), the resulting composite material was not homogeneous and it appears as there is no connection between the two species.

XPS and conductivity measurements results from the in situ polymerized composite material reveal an electron transfer process from the nitrogen atoms of the polymer to the INT. The anilinium chloride monomer molecules can interact with the nanotubes via electron transfer from the nitrogen atoms to the nanoparticle; creating an electron donor acceptor complex, followed by the subsequent formation of PANI. The fact that only a small amount of INT may increase the conductivity of the polymer by two orders of magnitude suggests that the IF/INT nanoparticles can serve as promising candidates for other composites of conductive polymers such as polypyrrole and polythiophenes. The improvements made in the conductivity properties of the present PANI/INT-WS₂ enhance the application potential of the conducting polymer without hampering the chemical and physical properties of both moieties.

Claims

CLAIMS:

1. A conductive material comprising at least one conductive polymer doped with at least one inorganic closed-cage nanostructure.

2. The conductive material of claim 1 , wherein the conductive polymer are associated with the nanostructures through coordinative or ionic association.

3. The conductive material of claim 1 or 2, wherein the conductive polymer and the at least one inorganic closed-cage nanostructure are associated by acid-base interaction.

4. The conductive material of any one of the preceding claims, wherein the conductive polymer has at least one moiety permitting charge transfer between the conductive polymer and the inorganic closed-cage nanostructures.

5. The conductive material of claim 4, wherein the conductive polymer comprises an atom selected from O, N, and S, or comprises a moiety comprising such an atom.

6. The conductive material of claim 5, wherein the conductive polymer is selected from polypyrrole, polythiophene and polyaniline.

7. The conductive material of claim 6, wherein the conductive polymer is polyaniline (PANI).

8. The conductive material of any one of the preceding claims, wherein the inorganic closed-cage nanoparticles are hollow nanoparticles selected from metal chalcogenides, metal dichalcogenides, and metal halides.

9. The conductive material of any one of the preceding claims, wherein the inorganic closed-cage nanoparticles are single or multi-layered nanoparticles.

10. The conductive material of claim 9, wherein the inorganic closed-cage nanoparticles are selected from nanospheres, nanotubes, nested polyhedra, and onionlike.

11. The conductive material of claim 10, wherein the inorganic closed-cage nanoparticles are inorganic nanotubes (INT) or inorganic fullerene-like nanoparticles (IF).

12. The conductive material of any one of the preceding claims, wherein the inorganic closed-cage nanoparticles are of the general formula ML_n, wherein M is a transition metal, L is a chalcogen, and n is the number of chalcogen atoms L per each atom of the transition metal M.

13. The conductive material of any one of the preceding claims, wherein the inorganic closed-cage nanoparticles are of the general formula A_!__x-B_x-chalcognide, wherein A is either a metal or a transition metal or an alloy of such a metal/transition metal, B is a metal or a transition metal, and x being < 0.3 and different from zero, provided that A≠B.

14. The conductive material of any one of the preceding claims, wherein the material comprises between 0.5 and 5 wt% nanoparticles.

15. The conductive material of claim 14, wherein the material comprises between 0.5 and 3 wt% nanoparticles.

16. The conductive material of claim 15, wherein the amount of nanoparticles in the material is between 0.5 and 1.5% wt.

17. The conductive material of claim 16, the amount of nanoparticles in the material is between 0.5 and 1% wt.

18. The conductive material of any one of the preceding claims, comprising PANI and inorganic nanotubes.

19. The conductive material of claim 18, comprising PANI and WS2 nanotubes (INT-WS2).

20. The conductive material of any one of claims 1 to 17, comprising at least one conductive polymer and INT-WS2.

21. The conductive material of any one of the preceding claims, wherein the conductivity of the conductive material is enhanced as compared to un-doped conductive polymer.

22. A conductive material comprising a conductive polymer doped with inorganic closed-cage nanostructures, wherein the conductivity of the conductive material is enhanced as compared to non-doped conductive polymer.

23. A composite comprising at least one conductive material according to any one of claims 1-22.

24. A composite comprising at least one conductive polymer and at least one inorganic closed-cage nanostructure.

25. A composite comprising at least one conductive polymer and at least one inorganic closed-cage nanostructure, exhibiting induction of charge carriers therebetween.

26. The composite of claim 24 or 25, wherein the conductive polymer and the nanostructures are associated with the nanostructures through coordinative or ionic association.

27. The composite of claim 24 or 25, wherein the conductive polymer and the at least one inorganic closed-cage nanostructure are associated by acid-base interaction.

28. The composite of any one of claims 24 to 27, wherein the conductive polymer comprises an atom selected from O, N, and S, or comprises a moiety comprising such an atom.

29. The composite of claim 28, wherein the conductive polymer is selected from polypyrrole, polythiophene and poly aniline (PANI).

30. The composite of any one of claims 24 to 29, wherein the inorganic closed-cage nanoparticles are hollow nanoparticles selected from transition metal chalcogenides, transition metal dichalcogenides, and transition metal halides.

31. The composite of claim 30, wherein the inorganic closed-cage nanoparticles are inorganic nanotubes (INT) or inorganic fullerene-like nanoparticles (IF).

32. The composite of claim 30 or 31, wherein the inorganic closed-cage nanoparticles are of the general formula ML_n, wherein M is a transition metal, L is a chalcogen, and n is the number of chalcogen atoms L per each atom of the transition metal M.

33. The composite of claim 30 or 31, wherein the inorganic closed-cage nanoparticles are of the general formula Aj-_x-B_x-chalcognide, wherein A is either a metal or a transition metal or an alloy of such a metal/transition metal, B is a metal or a transition metal, and x being < 0.3 and different from zero, provided that A≠B.

34. The composite of any one of claims 24-33, comprising PANI and WS₂ nanotubes (INT-WS2).

35. The composite of any one or claims 24 to 34, comprising between 0.5 and 5 wt nanoparticles.

36. An electronic device comprising the material of any one of claims 1 to 22 or the composite of any one of claims 23 to 35.

37. An opto-electric device comprising the material of any one of claims 1 to 22 or the composite of any one of claims 23 to 35.

38. A device comprising a layer or an element of a conductive material of any one of claims 1 to 22 or a composite of any one of claims 23 to 35.

39. The device of any one of claims 36 to 38, being a flexible device.

40. Use of a conductive material of any one of claims 1 to 22 or a composite of any one of claims 23 to 35 in the manufacture of a device.

41. The use of claim 40, wherein the device is selected from an electronic device and an opto-electric device.