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Nanostructured Semiconducting PEDOT–TiO2/ ZnO Hybrid Composite for Nanodevice Applications Rajaraman Ramakrishnan, Sudha Janardhanan Devaki, Aravind AAshish, Senoy Thomas, Manoj Raama Varma, and Naajiya KPP J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10744 • Publication Date (Web): 29 Jan 2016 Downloaded from http://pubs.acs.org on February 8, 2016

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The Journal of Physical Chemistry

TOC 254x155mm (150 x 150 DPI)

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Nanostructured

Semiconducting

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PEDOT–TiO2/ZnO

Hybrid

Composite for Nanodevice Applications Rajaraman Ramakrishnan, Sudha J Devaki*, A. Aashish, Senoy Thomas, Manoj Raama Varma, Najiya KPP Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695 019, India.

Abstract: In this paper, we have developed a simple strategy for the preparation of hybrid nanocomposite of poly (3, 4-ethylenedioxythiophene) -TiO2/ZnO (PZT) and demonstrated its application in thermoelectric device. Hierarchical hetero nanostructured TiO2/ZnO (ZTO) was prepared by a facile sol-gel process in presence of a bio-capping agent. PZT was prepared by in-situ polymerization of EDOT in presence of ZTO. They were characterized by UV-Visible, FT-IR, XRD, Raman, SEM, TEM and AFM analysis.

Results revealed

homogeneous distribution of ZTO with hetero-crystalline phase of wurtzite ZnO/anataseTiO2 having high density of various defects in the hybrid nanocomposite. Studies showed that ZTO is excellently interfaced with highly ordered self assembled extended π-layers of PEDOT chains which could enhance the charge carrier concentration (3.92x1020 cm-3) and charge carrier mobility (0.83cm2 V-1 S-1). Further, we have demonstrated its application as a thermoelectric material by fabricating the device (Cu/PZT/Cu) which showed low thermal conductivity of 0.0495 W/mK, Seebeck coefficient 19.05µ.V.K-1, power factor of 1.28 µW. K-2.m-1and figure of merit 4.8x10 -3 at ambient temperature. All these excellent material properties of PZT suggest its application as an active material for the fabrication of nanoelectronic devices with large area. Keywords: Hetero nanostructure, Zinc oxide, Titania, PEDOT, Thermal conductivity, Seebeck Coefficient, Figure of merit.


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1. Introduction: Recent developments and studies on the nanostructured semiconducting polymer inorganic hybrid nanocomposites are receiving tremendous importance among researchers from academia and industries since it find applications in various high tech applications. This is due to the synergistic properties arising from the molecular level mixing of semiconducting polymer and inorganic counter parts combined with its flexibility, processability, light weight along with appreciable electrical conductivity and low thermal conductivity.1-4 In hybrid semiconducting nanocomposites, conducting polymers possess unique features such as high mechanical flexibility, light weight, low cost synthesis, solution processability and printability over a large area which may favour for their application in light-emitting diodes,5 transistors,6 memory devices,7 thermoelectric devices,8 and photovoltaic cells.9 Physical and chemical properties of conducting polymers can be controlled by tuning their chemical design and synthesis. Conducting polymers such as polyaniline, polythiophene, PEDOT-PSS, polyacetylene, polypyrrole and their derivatives have a great potential to use in electronic and energy device applications.10-13 Among the various conducting polymers, PEDOT is having the advantages such as optical transparency, ease of preparation, flexibility, thermal stability and also tunable electrical conductivity by doping.14 Hierarchical nanostructured metal oxides have attracted considerable interest because of their potential applications in optoelectronic devices. Among the metal oxides, TiO2 and ZnO find advantages such as ease of preparation, high charge carrier concentration, and mobility as well as controlled morphology.15-17 Furthermore they possess many inherent properties such as band gap of 3 eV to 3.39 eV, large exciton binding energy due to their electronic, chemical, and optical properties.18-20 Development of hierarchical hetero nanostructured metal oxides are receiving importance because of their improved opto-electronic properties and also formation of various crystal defects (such as oxygen vacancies, zinc interstitials, titanium



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interstitials) which plays an important role in improving the electronic structure, charge carrier and surface properties of metal oxide.21-24 It can be prepared by several methods like doping, metal deposition, surface sensitization and coupling of semiconductors.25-27 Recently our group reported the development of hierarchical self assembled porous microspheres of ZnO with wurtzite crystal phase endowed with large number defect states prepared by biocapping agent through the sol-gel process.28 Resulting ZnO with a large number of oxygen and interstitials defects could enhance the scattering of the photon with relatively short wavelength and help to improve the performance of dye sensitized solar cell. So preparation of hetero nanostructures materials with defect expected to create a large density of interfaces in which phonon scattering can occur without compromising their charge carrier concentration and carrier mobility. Thus it reduces the electron phonons mean free path and resulting hetero nanostrucutres may reduce thermal conductivity which is a gaining parameter in the development of electronic and energy devices.29, 30 Recently researchers have taken great efforts towards enhancing the properties of hybrid nanocomposite by excellent percolation of nanoparticles such as semiconductor nanoparticles, nanorods and carbon nanostructures inside the polymer matrix.31-33 Apart from achieving excellent percolation, enhancement in charge carrier mobility and charge carrier concentration is also a great challenge in the materials community for development of efficient energy conversion and nanoelectronic devices. During processing of hybrid nanocomposites, an intimate contact between polymer-nanoparticles is to be established which may help to increase the interfacial area between the junction, controlled distribution of the nanoparticles within the polymeric matrix and also facilitate to transfer of charge carriers across the interfaces which results hike in the charge carrier mobility and also enhance the electrical conductivity.34 In addition to the interfaces, ordered structure, well defined morphologies and interaction between the semiconducting polymer-nanoparticle also


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play a role in hybrid nanocomposites. In this context, considerable efforts have been made to develop a facile low-cost and environmentally friendly fabrication of nanostructured hybrid materials with organic–inorganic semiconductor interfaces for tuning the morphology, crystallinity, optical and electrical properties.35 Thermoelectric materials (TE) are receiving a tremendous potential application in both power generation and solid-state cooling or heating. They are employed as novel energy harvesting systems from waste heat recovery. The conversion efficiency of a TE can be calculated by dimensionless figure of merit (ZT) is given in equation (1), 36-40 ZT = S2σT/κ,

(1)

Where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, temperature and thermal conductivity respectively. Therefore, simultaneous increases of the electrical conductivity, Seebeck coefficient along with the reduction of thermal conductivity are favourable for the enhancement of ZT. Recently significant improvements on ZT have been achieved in nanostructured materials like superlattices, nanoinclusions and nanocomposites etc by phonon scattering to reduce the thermal conductivity and without losing of Seebeck coefficient and electrical conductivity.41-42 In this paper, we are demonstrating the preparation of hybrid nanocomposite based on ZnO/TiO2 –PEDOT through a facile strategy by in-situ polymerisation of EDOT in presence of ZTO. Hetero nanostructured ZnO/TiO2 (ZTO) having mixed crystalline phase of wurtzite ZnO and anatase TiO2 was prepared by the sol-gel process in presence of a bio-capping agent. The interaction between the PEDOT and ZTO was studied by FT-IR, XRD, Raman analysis and morphological observation using microscopic techniques such as SEM, TEM, and AFM. Further we have studied the effect of ZTO on the electrical conductivity, carrier concentration, carrier mobility, Seebeck coefficient and power factor. Then demonstrated its application as a thermoelectric device by fabricating the device having configuration



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Cu/PZT/Cu and measured the figure of merit of PZT hybrid nanocomposites using physical property measurement system (PPMS). 2. Experimental Section 2.1. Materials: Zinc acetate.dihydrate and xylene were purchased from Sd.fine Chem. Ltd., Mumbai, India. Titanium isopropoxide, ferric chloride hexahydrate (FeCl3.6H2O) and 3,4ethylenedioxythiophene (EDOT) were purchased from Sigma Aldrich Chemical, Bangalore, India. Butanol and all solvents of analytical grade were purchased from Merck India Ltd., Mumbai, India. 2.2. Synthesis of zinc oxide (ZnO) and hetero-nanostructured ZnO-TiO2 (ZTO): ZnO was prepared by the procedure earlier reported from our group. In a typical procedure, 0.173g (0.45 mmol) of PDPPA was dissolved in a mixture of m-xylene (25 ml) and butanol (50 ml). 3.29 g (14.9 mmol) of zinc acetate was added to the above solution and the mixture was refluxed for 3 hours. The solution turned into a milky suspension and the white precipitate formed was separated by centrifugation at 2500 rpm for 10 min. Then it was rinsed with distilled water and ethanol. The solvent was evaporated under reduced pressure at 40

o

C to obtain ZnO nanoparticles. Experimental procedure for preparation of

pentadecylphenyl phosphoric acid (PDPPA) was given in supporting information S1.43 ZTO nanostructured materials were prepared as follows; initially 0.88ml (2.97 mmol) of titanium isopropoxide in 10 mL ethanol and 10 mL of water solution refluxed at 70 °C for 2 hours. Finally titania solution was added drop wise to the ZnO solution (0.05g) in 2.5 mL ethanol and 1.5mL of water dispersed by ultra-sonication under vigorous stirring and the reaction was continued for 20 hours. The solution turned into a cloudy white precipitate and


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formed white precipitate was separated by centrifugation at 2500 rpm for 20 min. Then it was rinsed with distilled water and ethanol. Further the prepared ZTO was calcined at 600 °C. 2.3. Preparation of PEDOT-ZTO hybrid nanocomposites (PZT): PZTs were prepared by oxidative emulsion polymerization of EDOT in the presence of ZTO using ferric chloride hexahydrate as initiator at room temperature. 0.033g of ZTO nanostructure was ultrasonically dispersed in 25 mL of double distilled water. EDOT 0.67 mL (6.2 mmol) was added to the dispersion and continued stirring for 10 minutes. FeCl3.6H2O (2.30g, 8.5mmol in 10 mL water) was added dropwise to the solution under vigorous stirring in an ice bath and the reaction was continued for 24 hours. After completion of the reaction, the resulting precipitate was separated by centrifugation at 5000 rpm for 20 minutes and washed several times with distilled water to remove the unreacted monomer. Finally the hybrid composite was dried at 60 oC under vacuum and designated as PZT-1. Similarly PZTs were prepared with different weight percentage of ZTO in the EDOT under same the condition and designated as PZT-2, PZT-3 and PZT-4. For comparison, PEDOT was prepared under the same condition without ZTO. Experimental details are given in Table S1. 2.4. Characterization Techniques UV-Visible absorption spectra of the nanocomposites were studied by dispersing the sample in chloroform and recorded using UV-Visible spectrophotometer (Shimadzu model 2100) in the range of 200-900 nm. FT-IR measurements were done with a fully computerized Nicolet impact 400D FT-IR spectrophotometer. Polymers were mixed thoroughly with potassium bromide and compressed into pellets before recording. All the spectra were corrected for the presence of moisture and carbon dioxide in the optical path. Powder X-ray diffraction studies were performed with a X- ray diffractometer (Philip’s X’pert Pro) with Cu



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Kα radiation (λ=0.154) employing an X’celarator detector and a monochromator at the diffraction beam side. The d-spacing of the nano composites were calculated from the angular position 2θ based on the Bragg’s formula nλ = 2dsinθ, where λ is the wavelength of the X-ray beam and θ is diffraction angle. Raman confocal microscope analyses of zinc oxide, ZTO and hybrid composites were done using WITec alpha 300R. Raman measurement was done at room temperature by using 633 nm Ar+ laser source for excitation. The number of accumulation 5 and integration time was 1 second. The BET surface area measurements of the samples were measured using Micrometritics tristar 3000 model surface area analyser. 0.5 g of the sample was heated to 200 oC for 3 hours under the nitrogen atmosphere for degassing. Then it was subjected to absorption of nitrogen using sorptometer at liquid nitrogen temperature. The morphology of the samples were studied using a Zesis EVO 18 cyro Scanning Electron Microscope (SEM) with variable pressure working at 20-30KV. High Resolution Transmission electron microscopy (HR-TEM) was performed in an FEI, TECNAI S Twin microscopy with an accelerating voltage of 100KV. For TEM measurements, the sample solutions were prepared by dispersion under an ultrasonic vibrator. Then sample solutions were drop casted on a formvar coated copper grid and dried at room temperature before observation. Atomic force microscopic images were carried under ambient conditions using a (Bruker Multimode AFM-3COCF, Germany) operating in tapping mode. Samples for the analysis were prepared by drop casting the hybrid composite solution on mica sheet. The cyclic voltammetry studies carried out using CHI6211B Electrochemical Analyzer, in a threeelectrode one-compartment electrochemical cell in which GCE used as working electrode and a platinum wire used as a counter electrode. All the potentials were recorded using Ag/AgCl as

the

reference

tetrabutylammonium

electrode.

The

experiments

hexafluorophosphate

were

conducted

(Bu4NPF6)-acetonitrile

in

solution.

0.1

mol/L

Electrical

conductivity measurements were performed with a standard four-probe conductivity meter


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using a keithely 6221 programmable current source and a 2128A nanovoltmeter. The samples were pressed in to an 11 mm diameter disk for the measurement. Thermal conductivity and Seebeck coefficient measurements were performed using a thermal transport puck attached to a physical property measurement system (Quantum Design, Dynacool). For measurements, the samples were pressed in the form of pellets of approximately 6.5 mm diameter. The pellet was then sandwiched in between gold coated copper disks from which heater, thermometer, sample current and voltage connections are drawn. For thermal conductivity and Seebeck coefficient measurements, heat is applied to one end of the sample by running current through the heater. The temperature Thot and Tcold are measured at the thermometer shoes. Also during the heat pulse, Seebeck voltage is monitored (∆V = V+ - V-). The heat of the sample flows to the cold foot. The measurements were performed in the temperature range 300 to 385K. The room temperature Hall measurement was carried out using Physical Property Measurement System (Quantum Design). Hall Effect measurement system with a magnetic field of (-9 T to +9 T) and an electric current of 1 mA with a good Ohm contact established using silver electrodes. Hall Effect measurements were carried out on rectangular pieces of the samples. The voltage leads were connected perpendicular to the current leads and the center keeping the applied field perpendicular to the plane of the sample. 3. Results and Discussion: PZT hybrid nanocomposite containing semiconducting PEDOT and ZTO prepared through a facile strategy as shown in Scheme 1. Initially ZTO was prepared by a two step sol gel process: In the first step, hierarchical self-assembled porous ZnO in the wurtzite phase prepared by a bio-capping mechanism. Then ZTO hetero nanostructure phase with titania was prepared by the sol-gel hydrolysis of titanium isopropoxide in the presence of the previously prepared ZnO. The formed ZTO was purified and the isolated white powder was characterized by its particle size (50 nm), zeta potential (23 mV) and surface area (45.3 m2/g).



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Crystalline phase and size of ZTO crystallites were studied by XRD, defect phase of hetero nanostrucutres (ZTO) was confirmed by Raman analysis and morphology of prepared ZTO was studied by various microscopic techniques which will be discussing in the later sessions. Finally hybrid composites were (PZT) prepared by emulsion polymerization of EDOT using ferric chloride hexahydrate as oxidative initiator at room temperature. The interaction between the PEDOT and ZTO was confirmed by UV-Visible, FT-IR and Raman spectroscopy.

Scheme 1 Preparation of PZT hybrid nanocomposites

3.1. UV-Visible Spectra The photo-physical properties of the ZnO, ZTO, PEDOT, PZT-1, PZT-2, PZT-3 and PZT4 hybrid nanocomposites were studied by UV-Visible absorption spectroscopy. The UVVisible spectra of the ZnO and ZTO nanostructures in ethanol are shown in Figure S1A (a and b) in the supporting information. ZnO nanoparticles showed absorption maximum at 354


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nm where as ZTO exhibited the shift in the absorption maxima to 343 nm. The UV-Visible spectra of PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 are shown in Figure 1(a to e), respectively. The UV-Visible spectra of PEDOT exhibited bands at 378 nm and exhibited a broad strong absorption band at 800-900 nm due to π- π * and polaron- π * transition. It indicates that PEDOT is in a bipolaronic state and the formation of a sufficient number of charge carriers. The UV-Vis spectra of hybrid composites PZT-1, PZT-2, PZT-3 and PZT-4 showed the band at 378 nm, 376 nm, 374 nm and 373 nm. The onset of the free carrier tail is shifted to higher wavelength 826 nm, 829 nm, 832 nm, 838 nm and 842 nm revealing the presence of the metallic state.44 It was also observed that UV-Visible spectra of PZTs exhibited free carrier tail with shift in the polaron band revealing the presence of high density of charge carriers due to PEDOT and ZTO interactions. This is further supported during the measurement of electrical conductivity.

Figure 1 UV-Visible spectrum of (a) PEDOT, (b) PZT-1, (c) PZT-2, (d) PZT-3 and (e) PZT-4 3.2. FT-IR spectroscopy The chemical structure and interaction among the moieties of ZnO, ZTO, PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 hybrid nanocomposites were studied by the FT-IR spectroscopy. FT-IR spectra of the ZnO and ZTO are shown in Figure S1B (a and b) respectively. Figure



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S1B (a) shows the FT-IR spectra of ZnO exhibited a broad peak at 3472cm−1 which is assigned to hydroxyl groups and water molecules. The bands observed around 1572 cm-1, 1412 cm-1 and 1336 cm−1 were attributed to the asymmetric and symmetric C=O stretching. The band appeared at 2922 cm-1, 2855 cm-1 and 1032 cm-1 corresponds to CH2 vibrations. The characteristic band of P = O group was observed at 1246 cm-1 and the aromatic vibration peak observed at 695 cm-1.The weak band observed at 1740 cm-1 is corresponds to the stretching vibration of C=O groups. All these results suggested the formation of phosphate capped ZnO.28 Figure S1B (b) shows a FT-IR spectrum of ZTO showed bands at 664 cm−1 are due to the stretching of Ti–O–Ti. The sharp peak at 1412 cm−1 can be attributed to the lattice vibrations of TiO2. The absorption peak at 1627 cm−1 was caused by a bending vibration of coordinated H2O as well as Ti–OH. It can be observed that silence peaks at 3400 cm-1, 1627 cm−1 and 1246 cm-1 corresponding to the complete removal of the organic matter and the broad band observed at 573 cm-1 confirmed the formation of ZTO hetero nanostructures.16,17 FT-IR spectra of PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 are shown in Figure S2 (a to e) respectively (supporting information). Figure S2a shows the FT-IR spectra of PEDOT. The bands observed at 688 cm-1, 845 cm-1, 926 cm-1, and 983 cm-1 which are attributed to the deformation modes of C-S-C in the thiophene ring. The bands at 1095 cm-1, 1146 cm-1 and 1212 cm-1 are associated with the C-O-C bending vibration of the ethylenedioxy moiety. The band at 1353 cm-1 is assigned to C-C stretching of the quinoidal structure. The band at 1473 cm-1 and 1517 cm-1 are due to the C=C stretching of the quinoid structure of the thiophene ring.45,46 The absorption band at 1629 cm-1 can be attributed from the polarons present in PEDOT. Figure S2b shows the FT-IR spectra of PZT-1 showed the band at 1361 cm-1 are corresponds to C-C stretching of the quinoidal structure. The observed band at 1481 cm-1 and 1524 cm-1 are due to the C=C stretching of the quinoid structure of the thiophene ring. Similar observations were made for PZT-2, PZT-3 and PZT-4 is shows in


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Figure S2 (c to e), respectively. All PZT showed red shift with the increase of ZTO ratio in the composites compared to the pure PEDOT, which is due to an interaction of ZTO particles with PEDOT chains and further it was confirmed from Raman spectroscopic analysis.44 3.3. Raman Spectra The interactions and the presence of density variation in the defect states are further confirmed by Raman spectroscopy which was measured using 633 nm Ar+ laser line at room temperature. Figure S1C (a and b) showed Raman spectra of ZnO and ZTO, respectively. Raman spectra of ZnO exhibited vibration bands at 332 cm-1, 438 cm-1, 583 cm-1 and 660 cm-1 corresponding to the active phonon modes of ZnO. The highest vibration band observed at 438 cm-1 is characteristic ZnO wurtzite structure. The phonon modes of ZnO were observed at 332 cm-1 and nonpolar phonons at 438 cm-1 and 584 cm-1. These two nonpolar phonon modes are arising from oxygen deficiency and defect structure formed from Zn sublattice.28 Figure S1C (b) shows the Raman spectra of ZTO which exhibited bands at 144 cm-1, 197 cm-1, 400 cm-1, 522 cm-1, and 640 cm-1 (Eg1, Eg2, Bg1, A1g, B1g, and E g3) can be correlated to those of the anatase phase of titania. The band at 144 cm-1, another weak band at 197 cm-1 and 400 cm-1 corresponds to the formation of oxygen vacancy in ZTO hetero nanostructures 47, 48 and it was further supported by EDS analysis. Figure 2(a to e) shows the Raman spectra of PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 respectively. Figure 2a shows Raman spectra of PEDOT, showed band at 1007 cm-1 corresponds to the deformations of oxyethylene ring, the band observed at 1282 cm-1 attributed to the C-C inter ring symmetric stretching, 1348 cm-1 related to the C-C symmetric stretching and the band at 1544 cm−1 are attributed to out of plane bending of the ethylene dioxy ring of PEDOT chains. The strongest symmetric stretching band at 1449 cm-1 can be ascribed due to C=C symmetric stretching of PEDOT chains. Further we observed peaks at 446, 566, 724 and 850 cm-1 for PEDOT.44,49, 50 Figure 2b shows Raman spectra of PZT-1, we observed that major shift in the peak position



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in conjugated C=C structure of PEDOT. The red shift in the peak position at 1449 and 1544 cm-1 are due to the oxidized structure of PZT-1 segments compared with PEDOT. The peak position at 1282 and 1348 cm-1 are also slightly shifted in the C-C structure of PEDOT. Further peaks at 446, 566, 724 and 850 cm-1 are also with broadening of Raman signal. It confirms the interaction between PEDOT and ZTO hetero nanostructures in the hybrid nanocomposites. These results indicate that the resonant structure of the PEDOT chain changes benzenoid structure to a quinoid structure. The benzenoid structure favours coiled conformation because of the conjugated π-electrons is not completely delocalized. However, in the quinoid structure PEDOT favours a linear or expanded coil conformation with the thiophene rings oriented in nearly the same plane, which results in the delocalization of conjugated π-electrons throughout the entire chain.44 Hence, the resulting quinoid structures favour for increased conjugation length of the PEDOT chain and also improve the electrical conductivity of hybrid nanocomposites.50 Similar observations were made for PZT-2, PZT-3 and PZT-4 are shown in Figure 2 (c to e) respectively. Details shift in the wave number of ZTO, PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 are given in Table S2 (supporting information).These interactions modify the chain conformation of PEDOT chain and improve the charge transport properties of hybrid nanocomposites. The intensities of these symmetric stretching peaks vary with the hybrid nanocomposites and it confirms the electrons are more delocalized on the conjugated backbone of PEDOT which agrees with the result observed in UV-Visible spectra.44


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Figure 2 Raman (a) PEDOT, (b) PZT-1, (c) PZT-2, (d) PZT-3 and (e) PZT-4 (inset figure showed wave number 1300 to 1600 cm-1)

3.4. X-Ray Diffraction Studies Crystalline phase and crystallite size of the prepared ZTO and PZT were studied by XRD technique and the XRD patterns of the ZnO, ZTO, PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 are shown in Figure 3(a to g). Figure 3a shows the XRD pattern of ZnO, which exhibited sharp peaks at 2θ = 31.5 °, 34.42 °, 36.25 °, 39.3 °, 47.54 o, 56.58 o , 62.9 o, 64.2 o, 67.96 o, and 69.1°. These peaks are assigned to the wurtzite phase of zinc oxide (JCPDS 36-1451).28 XRD patterns of ZTO nanostrucutres (Figure 3b) showed characteristic sharp peaks at 2θ = 25.2 °, 30.3 °, 32.3 o, 34.9 o, 37.8 °, 47.6 o, 53.6 o, 54.5 o, 56.7 o, 69.1o. These peaks are indexed by Miller indices as (101), (100), (002), (004), (200), (105), (211) and (103) planes with the d-spacing characteristic of hetero crystalline anatase and wurtzite phase of titania and ZnO.16,17 Figure 3c shows the XRD profile of PEDOT which observed peaks at 2θ = 6.2 o, 12.2 o, 18.2 o and the broad peak at 25.5 o (d = 3.5 Å) attributed to the crystalline nature of PEDOT with highly ordered π-π interaction of conjugated units of the PEDOT chains.51 Figure 3d shows XRD pattern of PZT-1 hybrid composites, which exhibited peaks at 2θ = 6.2 o, 12.3 o, 18.7 o, 25.3 o, 30.5 o, 34.9o, 37.8 o,47.7 o, 54.2 o and 62.3 o corresponding to the



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characteristic peaks of PEDOT and ZTO. However as the amount of ZTO increased in PZTs, it exhibited only a small deviation in the assigned peaks of ZTO as shown in Figure 3 (e to g). Wide angle x-ray scattering of PEDOT and PZTs are shown in Figure S3 (supporting information).The observed results suggest that the molecular arrangement or interchain stacking of PEDOT chains in the hybrid nanocomposites become more ordered and also it reduced the interchain hopping distance which can attribute to the measured hike in the electrical conductivity of hybrid composites which will be discussing in the later in the conductivity part of the manuscript.

Figure 3 XRD of (a) ZnO (b) ZTO (c) PEDOT, (d) PZT-1, (e) PZT-2, (f) PZT-3 and (g) PZT- 4 3.5. Morphology Morphology of the prepared ZTO hetero nanostructure was characterized by HR-TEM and AFM images are shown in Figure 4 (a and b). HR-TEM image of ZTO exhibited hierarchical aggregates of tetragonally oriented well defined cluster of spheres.AFM image showed that each tetragonal column is composed of well aggregates of spheres. Figure 4c shows EDS of ZTO confirms the presence of ZnO and titania in ZTO. The grain size or particle size of ZTO nanostructure measured as 15-20 nm and the grain boundaries between the nanoparticles are well defined and very close together. The HR-TEM image of ZTO is shown in the Figure 4d


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which showed clear and identical crystallographic orientations of the titania and ZnO with lattice fringes of 0.35 and 0.26 nm concur well with interplanar spacing of (101) and (002) crystallographic planes of anatase titania and hexagonal wurzite phase of ZnO. These results strengthened the observation made during XRD analysis. Figure 4 (e and f) shows the SAED and FFT images of ZTO revealing polycrystalline type electron diffraction pattern of ZTO hetero nanostrucutres.

Figure 4 (a) HR-TEM (inset image shows higher magnification) (b) AFM (c) EDS (d) HRTEM (e) SAED and (f) FFT image of ZTO nanostructures Further, morphological analyses of PZTs were studied by using SEM, HR-TEM and AFM. SEM and TEM pictures of PZTs hybrid nanocomposites are shown in Figure 5 and Figure 6, respectively. Figure 5a shows the SEM image of PEDOT which consists of self-assembled nanosheets like features. Further morphology of PEDOT was confirmed by HR-TEM as shown in Figure 6a which shows the formation of nanosheets. SEM and HR-TEM images of PZT-1, PZT-2, PZT-3 and PZT-4 are shown in Figure 5 (b to e) and Figure 6 (b to e) respectively. PZT-1 with low concentration of ZTO exhibited uniform dispersion of ZTO in PZT-1. As the amount of ZTO increased it shows well defined self-assembled PEDOT-ZTO



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nanostructures. The grain size of ZTO nanostructures inside the polymer matrix measured as 8-10 nm. The contrast in the image of TEM is due to difference in the electron migration through the organic and inorganic particles present in PZTS. Dark portion corresponds to the presence of ZTO while light part related to the presence of PEDOT in PZT. As the amount of ZTO increased in the PZT composites, the size of the ZTO hetero nanostructures inside the PZT increased to 10-12 nm. Similarly the size of ZTO hetero nanostructure enlarged to 16 nm and 20 nm in PTZ-3 and PZT-4. Figure 5f shows the energy dispersive spectrum (EDS) of PZT-4 which showed the presence of sulphur, Zn and Ti. Figure 6 (f and g) shows the HRTEM image of PTZ-4 recorded at higher magnification which confirms the preservation of crystalline phase of ZTO in PZT and FFT pattern of PZT-4 showed bright doted circle corresponds to the ordered phase of ZTO in PZT.

Figure 5 SEM images of (a) PEDOT (b) PZT-1 (c) PZT-2 (d) PZT-3 (e) PZT-4 and (f) EDS of PZT-4


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Figure 6 HR-TEM images of (a) PEDOT (b) PZT-1 (c) PZT-2 (d) PZT-3 (e) PZT-4 (f) High magnification image of PZT-4 and (g) SAED pattern of PZT-4. Further morphology was confirmed by recording AFM images and the height profile of PTZs hybrid nanocomposites are shown in Figure 7 (a to f) respectively. AFM image of PEDOT reveals a highly ordered nanosheet like structures. The height profile of the same is shown in the order of 30-40 nm with root mean square surface roughness of 3.8 nm. AFM image of PZT-1, PZT-2, PZT-3 and PZT-4 are matched with the observation made by HR-TEM and SEM. The height profile of the same measured as ~30 nm with root mean square surface roughness of ~5.2 nm. Similar observations were made with PZT-2, PZT-3 and PZT-4 respectively. The height profile of hybrid composites increased from 55 nm to 63 nm and decreases to 52 nm for PZT-2, PZT-3 and PZT-4 respectively. The root mean square surface 8roughness value measured 6.4 nm, 7.2 nm and 7.8 nm for PZT-2, PZT-3 and PZT-4. Morphology studies revealed that ZTO nanostructures embedded into the PEDOT matrix effectively reduced the grain size, hopping barriers and increased the number of the grain boundary for scattering of long wavelength phonons.29, 32, 34 At the same time the HR-TEM picture of PZTs showed the extended sheets of PEDOT chains decorated with well ordered



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ZTO hetero nanostructures. In PZTs the PEDOT chains are in the extended-aggregated or linear morphology thereby enhancing the hopping rate in the polymer chain and improve the electrical conductivity without compromising the seebeck coefficient as well as reduce the thermal conductivity.

Figure 7 AFM images of (a) PEDOT (b) PZT-1 (c) PZT-2 (d) PZT-3 (e) PZT-4 (inset image shows higher magnification of PZT-4) and (f) height profile of PTZs

3.6. Electrical conductivity Electrical conductivity of hybrid nanocomposites was measured at variable temperature using four probe conductivity meter. Temperature dependent electrical conductivity was investigated in the range from 303 to 383 K. Figure 8A (a to e) shows the temperature dependent electrical conductivity of PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4, respectively. Electrical conductivity of PEDOT measured in the order of 14.47 S/cm @ 303 K and 19.27 S/cm @ 383 K. Electrical conductivity measurement of PZTs revealed that conductivity showed hike in its values with the increasing amount of ZTO in the hybrid nanocomposite. The electrical conductivity of PZT-1 showed values of 19.32 S/cm @ 303 K and 34.09 S/cm


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@ 383 K compared to PEDOT. Electrical conductivity of PZT-2, PZT-3 and PZT-4 showed values in the range of 27.08 S/cm, 40.62 S/cm and 52.03S/cm @ 303 K and 53.79 S/cm, 66.34 S/cm and 71.08 S/cm @ 383 K, respectively. The electrical conductivity of PZT-4 @ 303 K measured as 3 orders of higher magnitude than the PEDOT. The reason for improved electrical conductivity is mainly because of increasing the amount of ZTO in hybrid nanocomposites which may allow the PEDOT molecular chains to undergo a transition from compact coil conformation to extended or linear conformation thereby enhancing the hopping rate in the polymer chain. Another possible reason is presence of charge carriers induced by hetero nanostrucutres of ZTO. The interaction between the π-system of PEDOT chains with ZTO expected to enhances the charge carrier mobility and electrical conductivity of hybrid nanocomposites.52-54 Figure 8B (a to e) shows the logarithmic plot of temperature dependent electrical conductivity of PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4, respectively. Temperature dependent electrical conductivity can be described by the Mott’s variable range hopping (VRH) model.12 The general form of Variable Range Hopping (VRH) mechanism is given in equation (2),  = 0 exp[-(T/T0)1/n]

(2)

Where T0 is the characteristic Mott temperature related to electron wave function localization degree and 0 is the high temperature limit of conductivity. The value of n is assigned as 2, 3 and 4 for one, two and three dimensional systems respectively. To are estimated by fitting the above equation (2). It is observed that the To decreases with increasing amount of ZTO revealing that the carrier hopping barrier decreases in PZTs which results increases in the electrical conductivity. According to the VRH model, the average hopping distance (Rhop) and the activation energy (Ehop) are given in equation (3) and (4),



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Rhop = (3/8).(To/T)1/4L

(3)

Ehop = (1/4).KT (To/T)1/4

(4)

Where L is the localization length. The localization length of EDOT monomer unit is ~ 2.29 Å.55At room temperature (Rhop) for the PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 are calculated from equation (3) and the average hopping distances are 0.66 Å, 0.64 Å, 0.60 Å, 0.54 Å 0.53 Å, respectively. At room temperature (Ehop) is estimated for PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 are in the range of 64.60 meV, 76.62 meV, 79.94 meV, 83.31 meV and 84.9 meV. Results suggested that decrease in the hopping distance and increased activation energy is responsible for the observed enhancement in the conductivity of prepared PZTs hybrid nanocomposites.12, 55

Figure 8 (A) Electrical conductivity (Vs) temperature and (B) Logarithmic plot showing the temperature dependent electrical conductivity of (a) PEDOT, (b) PZT-1, (c) PZT-2, (d) PZT3 and (e) PZT- 4 3.7. Hall measurement: The interaction between the π-system of PEDOT chains with ZTO expected to enhances the charge carrier mobility and electrical conductivity of hybrid nanocomposites. In order to gain a scientific understanding for the observed enhancement in the electrical conductivity of hybrid nanocomposites was studied by measuring the Hall coefficient (R H), carrier


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concentration (n) and carrier mobility (μ) using Hall measurement (Physical Property Measurement System, Quantum Design). Hall Effect measurement system with a magnetic field of (-9 T to + 9 T) and an electric current of 1 mA with a good Ohm contact established using silver electrodes. Hall Effect measurements were carried out on rectangular pieces of the samples. The voltage leads were connected perpendicular to the current leads and the center keeping the applied field perpendicular to the plane of the sample. From the slope value of resistivity Vs applied magnetic field plot we can calculate the Hall coefficient (RH). On the basis of Band model, the carrier concentration can be estimated by substituting the Hall coefficient (RH) values in equation. n = 1/ (e.RH) Where, e = electronic charge and RH = Hall coefficient. Charge carrier mobility calculated from the values of electrical conductivity using the equation. σ = n. e. µ σ = electrical conductivity, n = charge carrier concentration, e = electronic charge and µ = carrier mobility Figure 9 shows the room temperature Hall measurement of carrier concentration and mobility as a function of the ZTO (wt %). The Hall coefficient (RH) was found to be PEDOT (4.13x10-2 m3C-1), PZT-1 (5.16x10-2 m3C-1), PZT-2 (2.37x10-3 m3C-1), PZT-3 (1.8x10-3 m3C-1) and PZT-4 (1.637x10 -3 m3C-1) respectively. The positive RH values indicate that the majority charge carriers are holes (p-type).The positive sign value of the Seebeck coefficients in both PEDOT and PZTS was related to the holes (p-type) character of the semiconductor. The charge carrier concentration calculated for PEDOT (1.52x1020 cm-3), PZT-1 (1.97x1020 cm-3 ), PZT-2 (2.63x1020 cm-3), PZT-3 (3.48x1020 cm-3) and PZT-4 (3.92x1020 cm-3) respectively. The carrier mobility was calculated for PEDOT (0.58 cm2 V-1 S-1), PZT-1 (0.62 cm2 V-1 S-1),



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PZT-2 (0.65 cm2 V-1 S-1), PZT-3 (0.74cm2 V-1 S-1) and PZT-4 (0.83cm2 V-1 S-1) respectively. The observed higher electrical conductivity was mainly because of increases charge carrier concentration and charge carrier mobility in the PZTs.56-58 The increase of charge concentration in the PZTs reduces the energy band gap, which leads to the decreases of the Seebeck coefficient.34 The band gap of PEDOT and PZTs were calculated from electrochemical studies showed that the band gap is decreasing from 2.24eV, 2.13eV, 2.04eV, 1.896eV and 1.72 eV which confirmed the effective reduction of band gap in hybrid composites.

Figure 9 Carrier concentration and mobility as a function of the ZTO (wt %) 3.8. Thermo electric properties Thermoelectric properties such as Seebeck coefficient, thermal conductivity, power factor and figure of merit of PEDOT and PZTs were evaluated from the data obtained from the temperature dependent measurement. We have performed all these experiments with five devices of each sample and the results are reproducible. The Seebeck coefficient, thermal conductivity, power factor and ZT were plotted with standard deviations error. Seebeck coefficient is a measure of the magnitude of an induced thermoelectric voltage in response to the temperature difference across the material. This value can be positive for holes and is


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negative for electron as charge carriers. The temperature dependent seebeck coefficients of PEDOT and PZTs are shown in Figure 10A. The seebeck coefficient of PEDOT increased with temperature in the range of 303 K to 383 K. The seebeck coefficient of PEDOT was observed 17.3698 μV/K @303 K and 21.3399 μV/K @383 K. However, hybrid nanocomposites showed enhanced value of 18.3561 μV/K, 19.0568 μV/K @ 303 K and 21.8514 μV/K, 24.0723 μV/K @ 383 K for PZT-1 and PZT-2 respectively. Further increasing the amount of ZTO, it exhibited decrease in the value of Seebeck coefficients for PZT-3 (16.9258 μV/K @303 K, 15.7356 μV/K @383 K) and PZT-4 (15.8356 μV/K @303 K, 19.1669 μV/K @383 K). Due to thermal fluctuations, the thermally excited charge carriers constantly diffuse away from the hot end and produce entropy by drifting towards the cold end of the device. The observed initial increase and decreases in Seebeck coefficient of PZTs may be due to presence of ZTO with large number of grain boundaries which may generate higher surface roughness in the PZTs and reduced band gap with increasing the ZTO (wt %) in the PZTs. This higher surface roughness leads to strong carrier scattering and also caused the charge carrier filtering effect.59,60 This result is substantiated by the observation made during the morphological studies. Figure 10B shows the temperature dependent thermal conductivity of PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 hybrid nanocomposites. The temperature dependent thermal conductivity increases with hike in temperature and also showed variation with respect to increase in amount of ZTO. The thermal conductivity of the PEDOT was evaluated as 0.1893 W/mK @ 303 K and 0.1896 W/mK @ 383 K. In the case of hybrid nanocomposites, the thermal conductivity was 0.0495, 0.0617, 0.1308, 0.1485 W/mK @ 303 K and 0.0506, 0.0796, 0.1149, 0.1335 W/mK @ 383 K for PZT-1, PZT-2, PZT-3 and PZT-4 respectively. In the present work, PZTs hybrid nanocomposites form well defined nanointerfaces which may act as the effective scattering centers of phonons and then decrease thermal conductivity. This



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observation is supported by the previous theoretical and experimental investigations made by other researchers and they suggested that the nanostructures, including nanoinclusions and nanointerfaces in composites can scatter phonons and reduce thermal conductivity.61 Another possible reason for reduced thermal conductivity behaviour is due to the large number of grain boundaries created by ZTO hetero nanostructures on PEDOT layers which act as effective phonon scattering centres and thereby reducing thermal conductivity.34 The intrinsically low thermal conductivity of hybrid nanocomposites provides it a great potential for TE applications. The performance of the TE depends on the power factor. Power factor is calculated from the values obtained from the Seebeck coefficient and thermal conductivity.

The PF is

described as PF = α2σ. The temperature dependent PF values of PEDOT and PZTs composites increased with respect to temperature and are shown in Figure 10C. The PF of PEDOT was calculated as 0.4367 μW/K2.m @ 303 K and 0.8779 μW/K2.m @ 383 K. The PF value increased with increasing amount of ZTO hetero nanostrucutres content in the PZTs. The PF value of PZT-1, PZT-2, PZT-3 and PZT-4 are analysed in the range of 0.6511 μW/K2.m, 1.0165 μW/K2.m, 1.1637 μW/K2.m, 1.2885 μW/K2.m @ 303 K and 1.6673 μW/K2.m, 2.1173 μW/K2.m, 2.3806 μW/K2.m, 2.6115 μW/K2.m @ 383 K. The highest PF was observed for PZT-4 and it is 2 times higher than that of PEDOT. These results demonstrate that the presence of ZTO have a great influence on the power factor of hybrid nanocomposites.62, 63


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Figure 10 (A) Seebeck coefficient (B) Thermal conductivity (C) Power factor and (D) Figure of Merit (a) PEDOT, (b) PZT-1, (c) PZT-2, (d) PZT-3 and (e) PZT-4.

The efficiency of a thermoelectric material is determined by its figure of merit (ZT). ZT =α 2. σ T/ K Where ZT depends on the Seebeck coefficient (α), thermal conductivity (k) and electrical resistivity (σ). Figure 10D shows the effect of temperature on the figure of merit of PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 hybrid nanocomposites. ZT value of PEDOT and PZTs increased with the rising temperature. The ZT value observed for PEDOT, PZT-1, PZT-2, PZT-3 and PZT-4 were 7.2x10 -4, 3.9x10-3, 4.8x10-3, 2.7x10 -3, 2.21x10-3 @ 303 K and 1.77x10-3, 1.26x10-2, 1.49x10 -2, 9.17x10 -3, 7.49x10-3 @ 383 k respectively. In PZT the presence conductive ZTO along with PEDOT produces more charge carriers and hence



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observed higher electrical conductivity. Also ZTO created large number of grain boundaries which may generate more number of phonon scattering centres and reduced thermal conductivity. Further, the extensive interface created by the p-n junction shows PEDOT chains embedded with the n-type ZTO hetero nanostructures for enhancing the electrical conductivity and also reduce the thermal conductivity by phonon scattering. Our observations are supported by the work done by other researchers.64-66 4. Conclusion: In summary, we have successfully prepared semiconducting PZTs hybrid nanocomposite having highly extended PEDOT chains with interfacing ZTO hetero nanostructures through a facile strategy. The synergistic effect of hybrid nanocomposite exhibited efficient active material for nanoelectronic device with excellent charge carrier concentration (3.92x1020 cm-3), charge carrier mobility (0.83cm2 V-1S-1), electrical conductivity of (52 S.cm-1), low thermal conductivity of (0.0495 W/mK), Seebeck coefficient (19.05 µ V.K-1) and power factor (1.28 µW. K-2.m-1). The power factor evaluated 2 orders higher magnitude than the pure PEDOT. The hybrid composites showed the maximum ZT of merit of 4.8x10-3 @ 303 K and 1.49x10-2 @ 383 K in thermoelectric device fabricated with configuration Cu/PZT/Cu. ASSOCIATED CONTENT Supporting Information S1.Preparation of pentadecylphenyl phosphoric acid (PDPPA), Table S1 Experimental details of preparation of hybrid nanocomposites, Table S2 Details shift in the wave number of ZTO, PEDOT and PZTs, Table S3 Thermoelectric performance of recently reported PEDOT based system, Figure S1 (A) UV-Visible spectra of (a) ZnO and (b) ZTO (B) FT-IR spectra of (a) ZnO and (b) ZTO, (C) Raman spectra of (a) ZnO and (b) ZTO, Figure S2 FT-IR spectra of


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(a) PEDOT, (b) PZT-1, (c) PZT-2, (d) PZT-3 and (e) PZT-4, Figure S3 WAXS of (a) PEDOT, (b) PZT-1, (c) PZT-2, (d) PZT-3 and (e) PZT-4. AUTHOR INFORMATION Corresponding Author: Corresponding Author: Dr. J. D. Sudha; Email. ID: [email protected]; Tel: 0471 2515316 Acknowledgements We thank Dr. A. Ajayaghosh, Director, CSIR-NIIST, TVM for his constant encouragement and support. We also thank TAPSUN, CSIR-NWP-54 for finance support. We are also thankful to Dr. Prabhakar Rao, Mrs. Sowmya, Mr. Kiran Mohan, Mr. Aswin, Mr. Ajeesh Paulose, Ms. P. M. Aswathy and Ms. Remya, XRD, SEM, TEM, AFM, PPMS measurement and Raman analysis. R. Ramakrishnan is thankful to CSIR-New Delhi, India, for the senior research fellowship. References: (1). Agranovich, V. M.; Gartstein. Yu. N.; and Litinskaya. M. Hybrid Resonant OrganicInorganic Nanostructures for Optoelectronic Applications. Chem. Rev. 2011, 111, 5179– 5214. (2). McGrail, B. T.; Sehirlioglu, A.; Pentzer, E. Polymer Composites for Thermoelectric Applications. Angew. Chem. Int. Ed. 2015, 54, 1710-1723. (3). Janáky, C.; Rajeshwar, K. The Role of (Photo) Electrochemistry in the Rational Design of Hybrid Conducting Polymer/Semiconductor Assemblies: From Fundamental Concepts to Practical Applications. Prog. Polym. Sci. 2015, 43, 96−135. (4). Du, Y.; Shen, S. Z.; Cai, K.; Casey, P. S. Research Progress on Polymer–Inorganic Thermoelectric Nanocomposite Materials. Prog. Polym. Sci. 2012, 37, 820-841.



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