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Band Engineered I/III/V-VI Binary Metal Selenide/ MWCNT/PANI Nanocomposites for Potential Room Temperature Thermoelectric Applications Anuraj Kshirsagar, Chaitanya Hiragond, Abhijit Dey, Priyesh V. More, and Pawan Kumar Khanna ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00013 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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ACS Applied Energy Materials

Band Engineered I/III/V-VI Binary Metal Selenide/ MWCNT/PANI

Nanocomposites

for

Potential

Room Temperature Thermoelectric Applications Anuraj S. Kshirsagar †, Chaitanya Hiragond†, Abhijit Dey‡, Priyesh V. More†, Pawan K. Khanna†* †Nano

Chemistry and Quantum Dots R & D Lab, Department of Applied Chemistry Defence Institute of Advanced Technology (DIAT), Ministry of Defence, Govt. of India, Girinagar, Pune-411 025, India. ‡Energetic

Materials Research Division, High Energy Materials Research Laboratory (Defence Research & Development Organization), Pune, India-411 021

Keywords: Thermoelectric, conducting polymer, polyaniline, MWCNT, ternary hybrid.

Abstract

Room temperature thermoelectric studies are reported herein of ternary hybrids fabricated by exsitu processing involving polyaniline (PANI), multi walled carbon nanotubes (MWCNT) and binary metal selenide nanoparticles (MSe NPs) such as CuSe, Ag2Se, In2Se3, Sb2Se3 synthesized by using cyclohexeno-1, 2, 3-selenadiazole (SDZ) as selenium precursor. Presence of phase pure MSe NPs in ternary hybrids was confirmed by XRD analysis. FTIR analysis suggested a strong π-π interaction between PANI and MWCNT. Electrical conductivity, Seebeck coefficient and power factor of these band engineered ternary nanocomposites were investigated at room

ACS Paragon Plus Environment

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temperature. Ag2Se NPs/MWCNT/PANI (ASCP) showed p-type behavior with figure of merit (zT) of 0.012 at room temperature while other hybrids exhibited n-type behavior. All the ternary hybrids

showed

high

electrical

conductivities

and

amongst

n-type

hybrids,

CuSe

NPs/MWCNT/PANI showed best thermoelectric performance. The present work opens up an efficient approach to enhance the utility of metal chalcogenides as sensitizers in thermoelectric applications. The possible mechanism for charge transport is also discussed.

Introduction Ever increasing clean energy demand can be met through alternative environmentally benign, sustainable and renewable energy sources1. The current energy requirement is being satisfied by combustion of fossil fuels which are rapidly depleting. Moreover, their use as energy source is known to exhibit adverse effect on environment due to release of greenhouse gases2. Therefore, high efficiency sustainable energy generation and conversion technologies based on clean energy sources are of great importance and attracting attention globally3. Thermoelectric energy conversion is one of the important and promising technology which works on the principle of conversion of thermal energy to electricity. Heat released from heat sources such as, sun, car exhaust, power plants and other sources is considered as waste and is estimated to be in huge amount4. Energy harvesting from such waste lead to generation of electricity and can be considered as an alternative solution as global sustainable energy. Thermoelectric materials are promising candidates and have been widely studied for their application in solid state cooling/heating and power generation5. However; low energy conversion efficiency is major concern which limits their use in widespread applications and therefore, enhancement of energy conversion efficiency of such materials is a great challenge6.


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The performance of the thermoelectric materials (TE) is evaluated by dimensionless figure of merit (zT) which is given by the equation zT = S2σT/(kL+ke) where, S is Seebeck coefficient, σ is electrical conductivity, T is average absolute temperature, kL is lattice/phonon thermal conductivity and ke is electronic thermal conductivity. The figure of merit consists of two distinct contributors such as, a) power factor expressed as, S2σ and b) heat conductivity represented as, k= (kL+ke) which emphasizes on the fact that total thermal conductivity is dependent on both lattice and electronic thermal conductivity of a material1,7. Therefore, to enhance TE performance it is essential to increase power factor (S2σ) and minimize the total thermal conductivity (k) which is a difficult task owing to their interdependency. Elaborately this can be well explained by considering the relation between carrier concentration, Seebeck coefficient and electronic thermal conductivity. Generally, increase in carrier concentration enhances electrical conductivity (σ) which adversely affects Seebeck coefficient (S) and eventually the power factor. On the other hand, Wiedemann-Franz law demonstrates that ke is proportional to the σ and hence increased electrical conductivity eventually increases electronic thermal conductivity which leads to negative impact on figure of merit. Therefore, it is challenging and difficult job to optimize all these parameters in single material to draw high TE performance1, 8. One of the most important and possible way to enhance zT of the thermoelectric materials is to reduce thermal conductivity of material without altering electronic transport properties. Alloying, doping and nanocomposite formation are the main possible options available to enhance power factor. Amongst these, band engineering via nanocomposite formation has attracted recent attention. To date many materials have been explored and tested to generate thermoelectricity, amongst them BiSbTe, SiGe alloys, PbTe, Bi2Te3, SnSe etc. are reported to have great potential in TE devices

9-11.

However, brittleness, high cost, and poor processability of semiconductors


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restricted realization of practical thermoelectric devices12,

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13.

With the current technological

advancement in nanotechnology, processability can be addressed effectively as the disc and the flexible film formation becomes easier and likely improves conversion efficiency. The approach of combining two or more nano-materials irrespective of their nature can show synergetic effect 14-19.

Recently, composites of organic conducting polymers with graphene, carbon nanotubes

(CNTs) and hybrids containing semiconducting NPs as sensitizers have been explored to improve thermoelectric power factor and figure of merit as well as to fabricate flexible thermoelectric devices and therefore are considered as promising candidates20-24. However, conducting polymers possesses relatively low electrical conductivity which is eventually unsuitable for TE devices and therefore, incorporation of 1D conductive template (CNT) can help to improve the performance25,

26.

CNTs are known to exhibit zero band gap, excellent

electrical conductivity, good mechanical properties, and facile processability and are suitable for thermoelectrics27, 28. Therefore, combination of CNT and conducting polymers such as, poly-(3, 4-ethylenedioxythiophene) (PEDOT)29, polypyrole (PPy)30, polythiophene31, polyaniline (PANI) 32, 33

have gained recent popularity. Binary composites of PANI with CNT or graphene or

reduced graphene have been widely studied to improve power factor values. Presence of inorganic semiconductor in such hybrid composites acts as energy filters and further improves thermoelectric performance 23, 25, 27, 34. The combination of semiconductors with CNT and PANI helps to form extra heterojunctions with energy barriers at interfaces that helps to filter out low energy carriers and allow high-energy carriers to cross energy barrier leading to enhancement in the TE performance35,

36.

There are several reports in the literature demonstrating utility of

ternary composites in thermoelectricity. Ternary nanocomposite of polypyrrole/graphene/PANI demonstrated power factor up to 52.5μW/mK2 which was reported to be more than their binary


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and individual counterparts37. Similarly, Choi et al38 have documented ternary hybrid of reduced graphene oxide (RGO)/PEDOT:PSS/Te nanowires exhibiting power factor of 143μW/mK2. Wang et al39 have investigated ternary composite of PANI/SWCNT/Te for thermoelectric performance which showed power factor of 101μW/mK2. Recently, Erden et al40 have reported highest power factor value (114μW/mK2) for TiO2/a-CNT/PANI among PANI based ternary composites. We recently reported thermoelectric performance of p-type RGO/CdS/PANI and PDOT:PSS or PVAc/graphene/titanium dioxide nanocomposites with significant enhancement in the Seebeck coefficient24,

41

due to filtering of the low-energy carriers at junction. The

thermoelectric study of RGO/CdS/PANI composite showed enhancement in electrical conductivity and thermopower by increasing CdS QDs and RGO concentration. The literature thus highlights that the enhancement in TE performance is possible by incorporation of semiconducting NPs as sensitizers which helps to adjust band positions leading to formation of heterojunctions. Such band engineering is considered to be helpful for better TE performance via effective charge transportation as well as filtering of the low-energy carriers38. Recently, highest power factor of 2.2 mW/mK2 has been reported using Ytterbium silicide as thermoelectric material at room temperature42. Layered Compound Yb2-xEuxCdSb2 has studied for its room temperature TE performance and exhibited high Seebeck coefficient and low thermal conductivity43. Ohta et al44 have reported ~9 mW/mK2 power factor of high mobility 2D electron gas. There are some other reports describing promising materials for the room temperature thermoelectric devices 45. The performance of the thermoelectric materials is dependent on the several factors such as, phase purity, stoichiometry, morphology etc. and therefore, chemical or physical tailoring of nanomaterials is often required. Various methods are available in literature for the synthesis of


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binary metal selenide NPs such as, chemical vapor deposition (CVD), chemical bath deposition (CBD), hot injection, sol-gel, solvothermal, thermal decomposition of single molecular precursors etc46-48. Many reports are highlighting these advantages and some of them are briefly discussed here. Choi et al49 have reported synthesis of high-quality nano-discs of copper selenide using imidazoline-2-selenone as novel selenium precursor.

Pyridine selenolate copper and

indium complexes were employed as single molecular precursors for the synthesis of CuSex as well as In2Se3 NPs by Cheng et al

50.

Wherein, they have described decomposition of copper

complex for formation of α-CuSe at low temperature and Cu2-xSe at higher temperature while Incomplex decomposed to form In2Se3. Similarly, Indium (III) (3-methyl-2-pyridyl) selenolate complex have been shown to be excellent precursor by Sharma et al

51

for the formation of

In2Se3 nanocrystals. In recent report coordination polymers of the indium and copper selenolates have been described for the formation of In2Se3, Cu1.8Se and CuInSe2 NPs52. Silver benzoate and cyclohepteno-1,2,3-selenadiazole have also been documented for the facile synthesis of Ag2Se NPs53, 54. In case of antimony selenide, diseleno-phosphate complex Sb[Se2P(OiPr)2]3 has been utilized as molecular precursor for synthesis of NPs of Sb2Se3 55. There are several other reports describing use of different organoselenium compounds as selenium precursor for the synthesis of binary metal selenide NPs56. Chemical synthesis involving use of organoselenium compounds as selenium precursor offers advantages like, formation of less toxic by-products, lower reaction temperature, use of non-toxic reagents, easy extrusion of active selenium and its reaction with metal ions to maintain stoichiometry and phase purity during synthesis of MSe NPs. In present article, we report the synthesis of a series of I/III/V-VI Group (I-Cu, Ag, III-In, VSb and VI-Se) binary metal selenide NPs (CuSe, Ag2Se, In2Se3 and Sb2Se3) using cyclohexeno1,2,3-selenadiazole

and

investigation

of

thermoelectric

performance

of

MSe


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NPs/MWCNT/PANI ternary hybrids at room temperature. Present study is first of its kind and as per our knowledge there is no report till date explaining utility of such ternary hybrids for thermoelectric application. Separately synthesized series of binary metal selenide nanoparticles (MSe NPs) were employed to construct novel ternary hybrids with MWCNT and PANI. MSe NPs were characterized by using advanced characterization tools. As-formed hybrids were then employed for thermoelectric studies by fabricating thick pellets. The Seebeck coefficient and electrical conductivity of ternary nanohybrids were experimentally measured and power factor values were estimated. Thermal conductivity of two representative samples was measured and zT value was estimated. The variation in thermoelectric performance due to change in semiconductor NPs in ternary hybrids is studied in detail and comparatively discussed by considering possible charge transport mechanism. Experimental Section Materials and Methods Silver nitrate (AgNO3), oleic acid, diphenyl ether, ethanol and methanol were purchased from Sigma Aldrich Pvt. Ltd. India, Indium (III) chloride (InCl3) and Antimony (III) chloride (SbCl3) were procured from TCI chemicals Pvt. Ltd. India. Copper chloride dihydrate (CuCl2.2H2O) was purchased from Merck Pvt. Ltd. India while aniline was procured from Spectrochem Pvt. Ltd. India. Multiwalled carbon nanotubes (MWCNTs Type 5, OD 30-50 μm, length 10-30μm) were acquired from SRL Pvt. Ltd. India. All chemicals were reagent or analytical grade and were used without further purification. Cyclohexeno-1,2,3-selenadiazole (SDZ) was synthesized by reported method57,

58.

For powder X-ray diffraction analysis Mini Flex Rigaku X-ray

diffractometer Cu_Kα (λ=1.5406Å) was used. SEM and EDS analysis were performed on Carl Zeiss Scanning electron microscope. Transmission Electron Microscopy (TEM) images were


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taken on FEI-Technai G2 (Czech Republic) with 300 KV instrument. ESCA+ (Oxford Instrument Germany) equipped with mono-chromator Aluminum Source (Al kα radiation hν = 1486.7ev) operated at 15 kV and 20 mA was used for X-ray photoelectron spectroscopy (XPS). UV-Visible absorption spectra were recorded at room temperature using SPECORD 210 PLUS (analytikjena, Germany) UV spectrophotometer. Raman and Infrared (FTIR) spectra were recorded using EZ Raman spectrometer (USA, λEm is 780 nm) and FTIR Perkin Elmer spectrum two (USA) in the range of 4000 cm-1 to 400 cm-1 at room temperature. Th Hall voltage measurement was carried out using Indosaw SK-006 measurement system with magnetic field sweeping up to 2.2 T in both positive and negative directions. The carrier concentration (n) and the Hall carrier mobility (μH) were calculated by n=1/RHe where e is elementary charge Synthesis of Binary Metal Selenide Nanoparticles The synthesis of binary metal selenide NPs involves reaction between respective metal salt and previously synthesized cyclohexeno-1,2,3-selenadiazole (SDZ) as selenium precursor

57, 58.

In a

typical procedure, the mixture of copper (II) chloride dihydrate (0.011 moles)/silver nitrate (0.011 moles)/antimony chloride (0.0087 moles)/indium chloride (0.0090 moles), diphenyl ether (10 mL) and oleic acid (8.0-10 mL) was heated to 85-90°C under nitrogen atmosphere for 10-15 min. Subsequently pre-dissolved SDZ (0.0058 moles for silver, 0.0117 moles for copper salt while 0.0134 moles for In and Sb salt) in 2.0 mL oleic acid was injected in to above reaction mixture. The reaction was then carried out at 180°C for about 3 h. n-Hexane was added after cooling the mixture and was stirred for some time. The black precipitate was obtained after adding ethanol followed by centrifugation process. The precipitate of MySez (M=Cu, Ag, In, Sb, y=1 for Cu and 2 for Ag, In and Sb, z=1 for Cu and Ag while 3 for In and Sb) were the final products and dried in an oven for 3-5 h.


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Synthesis of Polyaniline The chemical oxidative polymerization of aniline was carried out as per the previous literature report59. A two neck round bottom flask was filled with a mixture of aniline (3.06 mL) and 1M HCl solution (50 mL) and continuously stirred at 0-5°C. To this mixture, pre-dissolved ammonium persulphate (7.49 gram (g) in 100 mL water) was added dropwise by ensuring stable reaction temperature. The reaction was continued for about 4 hrs between 0-5°C. The green suspension/solution was centrifuged at 5000 rpm and repeatedly washed with water as well as with methanol to remove traces of HCl. As-formed product was then dried in an oven for 24 hrs. Dark green coloured polyaniline powder is stored in air tight cylindrical vial for further use. Preparation of Ternary Hybrid for Thermoelectric Study Separately synthesized and thoroughly characterized MSe NPs were used as fillers or sensitizers along with commercially procured MWCNT and freshly synthesized PANI for preparation of ternary hybrids. Semiconductor MSe NPs (1.0 g) were homogeneously mixed with MWCNT (0.25 g) and PANI (0.25 g) by using mortar and pestle. The hybrids obtained after proper grinding and mixing were collected and used for the pellet preparation. The pellets of 12 mm diameter were prepared by using hydraulic KBr press and used further for thermoelectric measurements. Thermoelectric Measurements of Ternary Hybrids a) Seebeck Coefficient Measurement In order to measure thermopower, pellets of ternary hybrid having dimensions of 12mm X 6mm X 1mm

60

were sliced and placed on a thermal insulating fibre glass. One end of film was

connected by Peltier cooling module possessing thermally conductive and electrically insulating epoxy (2763 Stycast) and Peltier heater was placed at another end. The contact between Peltier


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cooler and heater was established by piece of copper. The voltage drops and temperature gradient were measured along the film with thermocouples arranged in a series using two copper wires. Cu films with thermally/electrically conducting silver epoxy (Dupont 4929N) were attached to the sample to ensure measurement of thermal gradient and the voltage drop at the same place. The connection was completed by attaching voltage wires and thermocouple to these Cu films. The Peltier cooling module was used for alteration of temperature. The Keithley 2182A nano-voltmeter was used to monitor the thermoelectric voltages with respect to temperature. b) Electrical Conductivity Measurements The pellets of dimensions 12mm X 6mm X 1mm of ternary hybrids were prepared to measure electrical conductivity. The heating of samples at low temperature was avoided by applying lowest possible current. The electrical resistivity of the samples was measured by employing delta mode four probe method using Keithley 6220 nanovoltmeter. Due to high electrical conductivity, the 100 mA current source was applied and voltage was recorded with Keithley nano-voltmeter. Results and Discussions The synthesis of MSe NPs was executed by employing SDZ as selenium precursor. The synthesis of binary metal selenides such as CuSe (CS), In2Se3 (IS), Sb2Se3 (SS) has never been reported by employing SDZ as Se source despite report on silver selenide a decade ago 61. The present article highlights first report on synthesis of copper, indium and antimony selenides employing SDZ. As-synthesized binary metal selenide NPs were thoroughly characterized by UV-Visible, FTIR, Raman spectroscopy, XRD, SEM/EDS, TEM, XPS, particle size analyzer and employed for preparation of ternary hybrid as depicted in scheme 1.


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Optical properties of as-prepared binary metal selenide NPs were studied by the UV-Visible spectroscopy. UV-Visible spectra revealed broad absorption profiles in case of AS, IS and CS covering entire visible region [Figure S1 (a)]. The absorption values of about 605 nm and 617 nm can be approximately predicted for CS and IS while it cannot be predicted for AS because of featureless profile tailing into NIR region. SS showed well defined absorption profile with absorption maxima at 616 nm [Figure S1(a)].

Scheme 1. A schematic presentation of conversion of metal selenide NPs, polyaniline and MWCNT to ternary hybrid The band gaps were estimated by plotting (αhν)2= (1/l)2(Ahν)2 versus hν (Tauc’s plot) where, α is absorption coefficient, hν is photon energy, l is path length in cm, A and h are absorbance and Planck constant respectively62, 63, 64. The band gap energies were much higher as the absorption profile was much blue shifted for each metal selenide nanoparticles. Typically, the band gap of AS, SS, IS, CS were estimated to be 1.24 (bulk Eg 0.15 eV), 2.92 (bulk Eg 1.05 eV), 1.81(bulk Eg 1.45 eV) and 1.70 (bulk Eg ~1.00 eV) respectively65-68. The higher band gap values obtained for all MSe NPs are correlated with decrease in the particle size which indeed was confirmed by


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other spectroscopic tools also. Normally, the effect of quantum confinement is responsible for the increased energy gap between conduction and valence band of the MSe NPs. X-ray photo electron spectroscopy (XPS) was employed to understand oxidation state of various elements present in the samples and to confirm the purity of MSe NPs. XPS survey of CS, AS, IS and SS is presented in Figure 1 (a) which confirms the presence of all elements with expected oxidation states. HRXPS of CS, AS, IS and SS is presented in Figure 1 (b-f). Core level spectrum of CS revealed presence of four different peaks due to Cu. The broad peaks obtained at 936.04eV and 955.94 eV are due to Cu 2p3/2, and Cu 2p1/2 states confirming presence of monovalent Cu. However, these peaks also comprise other two peaks with slight humps at 933.78eV and 957.45 eV due to Cu 2p3/2 and Cu 2p1/2 implying presence of Cu2+ in the sample. Additionally, two satellite shake-up peaks were observed at binding energy of 944.28 eV and 964.11 eV which also confirms the presence of Cu2+ in the sample. A similar observation for Cu is reported by others69. The possibility of slight oxidation of the sample upon exposure to the environment cannot be ruled out during XPS analysis which may lead to observation of peaks for divalent copper also. Based on the observations reported in the literature it is opined that monovalent nature of the copper in sample is due to conversion of Cu2+ to the Cu+ during reaction62. Core level spectrum of Se 3d [Figure 1 (f) CS] showed peak at 56.68 eV which is due to presence of Selenium in -2 oxidation state. Absence of peak near 58 eV confirms that the sample is free from oxidized form of selenium70. HRXPS of Ag [Figure 1 (c)] showed two peaks at binding energy of 368.31 eV and 374.32 eV due to 3d5/2 and 3d3/2 states. The peaks were found to be separated with peak splitting binding energy of 6.0 eV suggesting monovalent silver [Ag+] in the sample.


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The counterion of AS (i. e. selenide) showed peaks at 53.88 eV and 54.58 eV due to 3d5/2 and 3d3/2 states of selenium confirming the -2 oxidation state [Figure 1 (f) AS]64. The change in the peak position of the selenide is attributed to the change in counterion. Another sample IS showed two peaks due to 3d5/2 and 3d3/2 states at binding energy of 446.34 eV and 453.90 eV. These peaks were observed to be separated by binding energy difference of 7.56 eV which implies trivalent nature (In3+) of indium in IS. Selenium showed single broad peak at 55.90 eV confirming the presence of 3d state due to Se2- 62, 63.

Figure 1. (a) XPS survey of CS, AS, IS and SS and core level spectra of (b) copper (Cu 2p) (c) silver (Ag 3d) (d) indium (In 3d) (e) antimony (Sb 3d) (f) selenium (Se 3d) Two different peaks observed in case of Sb at 531.44 eV and 540.82 eV with peak splitting binding energy of 9.38eV. These peaks are indicative of 3d5/2 and 3d3/2 states of Sb and suggesting trivalent antimony in SS69. Core level spectrum of selenium showed peak at 55.93 eV similar to that in case of IS indicating 3d valence with -2 oxidation state. This study highlights


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the purity of all synthesized binary metal selenides. The analysis of XPS data is presented in Table S1 in supporting information. The phase purity of as-formed MSe NPs and their presence in hybrids along with MWCNT and PANI were evaluated by considering XRD results. Figure 2 shows XRD pattern for all MSe NPs and their hybrids. Presence of diffraction peaks due to (100), (101), (103), (006), (106), (110), (108), (202), (116) and (208) in case of copper selenide confirms formation of hexagonal α-CuSe (Klockmannite)71. Absence of diffraction peaks in between 35°-37° rules out the presence of CuO or Cu2O in the sample and thus highlights the purity72. The diffraction peaks present in CuSe were all found in its hybrid along with presence of a broad peak at 25° (Figure 2 (a)) which confirms presence of MWCNT and PANI in the sample73.

Figure 2. XRD pattern of (a) CS (b) AS (c) IS and (d) SS NPs and their respective hybrids (CSCP), (ASCP), (ISCP) and (SSCP). Yellow highlighted area corresponding to peaks due to MWCNT and PANI


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In case of sample AS and ASCP (Figure 2 (b)) all diffraction peaks are matched well with orthorhombic crystal phase of Ag2Se (Naumannite) and observed to be crystalline while presence of MWCNT and PANI was confirmed due to presence of broad peak at 25° similar to that of CSCP. Major intense peaks obtained at 33.4° and 34.7° for (112) and (121) crystal planes indicate formation of phase pure Ag2Se. Synthesis of IS showed formation of hexagonal γ-In2Se3 as it is evident from well matching XRD profile (Figure 2 (c)) with literature. The possibility of presence of InSe, In4Se3 and In6Se7 was eliminated due to absence of intense peaks at 2θ 22° and 35.5° 74. Another binary metal selenide i.e. Sb2Se3 was also well indexed to orthorhombic crystal structure and 2θ values were found to be matching with literature reports75-77. In both cases XRD of hybrid showed merging of peaks near 25° due to broadening caused by presence of MWCNT and PANI. Morphological analysis of as-prepared samples was carried out by TEM and SEM analysis. Figure S2 shows TEM images of all binary metal selenide NPs. The TEM images of CS [Figure S2 (a)] and IS [Figure S2 (c)] revealed NPs of varied shape and size while AS [Figure S2 (b)] and SS [Figure S2 (d)] showed spherical shape with smaller size particles. From TEM images particle size range can be estimated to be about 10-20 nm in all samples. SAED patterns of IS and SS are shown in inset of Figure S2 (c) and (d) respectively. The presence of circular bright spots due to selected area electronic diffraction reveals crystallinity of both samples. These spots are considered due to the diffractions from the respective crystal planes. In case of IS the diffractions from (110), (200), (201) and (116) crystal planes are responsible for the bright circular spots in the SAED while in case of SS (130), (230), (211), (221) and (301) crystal planes are involved. This further confirms the crystallinity of these samples.


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The particle size distribution profile of as-synthesized MSe NPs was obtained from particle size analyzer working on the principle of dynamic light scattering (DLS). The results revealed formation of MSe particles in nano-regime with reasonably narrow size distribution (Figure 3). CS, AS and SS showed nearly similar distribution in the range of 2-20/25 nm which is in good agreement with the FESEM and TEM results while IS showed excellent size distribution with average particle size of 5-8 nm. The result obtained from particle size analyzer closely matches with the TEM result indicating the formation of NPs with different size distribution.

Figure 3. Particle size distribution of the as-synthesized MSe NPs The SEM image of sample CS showed formation of mostly pyramidal to oval shaped NPs along with some spherical NPs. The formation of agglomerates of CuSe NPs can be observed leading to generation of large flake like structures with inhomogeneous size distribution [Figure 4 (a)]. Formation of large globules with rough and uneven surface due to agglomeration of small Ag2Se NPs can be observed from SEM image (b) in Figure 4. In case of sample IS mixed morphology was observed. It is evident from Figure 4 (c) that the growth of IS NPs was nondirectional and lead to form uneven and poly dispersed NPs with different shapes such as, rods,


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flakes, spheres etc. However, IS NPs are observed to be less agglomerated as compared to other samples. Smooth, large globules due to agglomeration of SS NPs are visible from Figure 4 images (d). The agglomerated spherical NPs showed formation of larger structures of varied morphology. Almost similar observations have been made from the TEM images. Overall, results obtained from SEM and TEM images of MSe NPs are closely correlated and found to be well matching. Ternary hybrids prepared by ex-situ method using MWCNT, PANI and binary metal chalcogenides as sensitizer have been characterized by FESEM to understand their morphological changes. The FESEM images are presented in Figure 4 suggesting formation of giant network leading to mesh like appearance of as-prepared hybrids. Since PANI and MWCNT contain number of π-bonds, their interaction within hybrid is well known to exhibit temporary ππ stacking59. Therefore, combination of all three materials expected to show formation of homogeneous hybrid along the surface of MWCNT. As expected FESEM images of hybrids revealed similar pattern. In all samples MWCNT is easily identified along with decoration of NPs and PANI over its surface [Figure 4 (e-h)]. Globular agglomerates around MWCNT are considered due to PANI while careful observation of images have revealed presence of smaller spherical NPs within the hybrid particularly in case of CSCP, ASCP and SSCP while ISCP showed large agglomeration which could be due to PANI. Because of mixed morphology of IS their presence in hybrid is difficult to identify. The decoration of NPs and PANI over MWCNT are clearly seen in case of ASCP and SSCP [Figure 4 (f) and (h)] which are suggesting formation of homogeneous hybrid. Therefore, FESEM images suggesting that metal selenide NPs are firmly seized over the surface of MWCNT by PANI leading to formation of uniform hybrid. Elemental composition of as-prepared binary metal selenide NPs was estimated using EDS analysis (Figure S3).


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All samples showed stoichiometric elemental composition. In case of CS atomic percentage values revealed nearly equal amount of Cu and Se [Figure S3 (a)] confirming the purity of CS. AS represents 2:1 silver to selenium ratio confirming stoichiometric formation [Figure S3 (b)]. Other two samples are also found to be stoichiometric [Figure S3 (c) and (d)]. The EDS analysis of the ternary hybrids showed presence of all expected elements with nearly stoichiometric ratio as it is evident from Figure S3 (e-h). The presence of higher percentage of carbon in all samples of hybrid is due to PANI and MWCNT.

Figure 4. FESEM images of (a-d) CS, AS, IS and SS NPs and (e-h) ternary hybrids of CSCP ASCP, ISCP and SSCP respectively.


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The FTIR analysis of MSe NPs and their hybrids have been carried out in order to understand the presence of different functional groups due to capping agent and presence of PANI as well as vibrational modes of NPs. During the synthesis oleic acid was employed as capping agent and therefore, peaks due to different functional groups such as, C=C, C=O, -O-H etc. are expected in FTIR spectra of MSe NPs. The FTIR spectra of MSe and their hybrids are shown in Figure S4 (a) and (b). The presence of low intensity peaks around 3300 cm-1 are considered due to O-H vibrations. Symmetric and asymmetric C-H vibrations lead to peaks in the range of 2800-3000 cm-1. Low intensity peaks between 1540 cm-1 to 1627 cm-1 are due to stretching vibrations of C=C.

C-H rocking vibrational mode of oleic acid showed strong to medium intensity peak

between 1370 cm-1 to 1445 cm-1. Other low intensity vibrational peaks near 1100 cm-1 are considered due to C-O stretching. All these peaks observed in MSe NPs are due to capping of NPs by oleic acid. In ternary hybrids it is expected that NPs will get deposited over the surface of MWCNT and N-H group in the PANI will hold them firmly by loose/temporary bond since nitrogen atom contains lone pair of electrons. The composites of the MWCNT and PANI are known to possess π-π stacking due to presence of number of π-bonds59 and therefore, addition of MSe NPs in such hybrids may lead to their insertion between MWCNT and PANI resulting to firm network. Therefore, it is essential to understand the different functionalities in the hybrids by FTIR. The presence of broad vibrational stretching near 3450 cm-1 and 3100 cm-1 are confirming presence of -N-H and -C-H symmetric and asymmetric groups from PANI. Most important features for confirmation of PANI in hybrid are presence of C=C stretching peaks due to quinonoid and benzenoid structures. The high intensity peak at about 1620 cm-1 is associated with quinonoid ring while peak near 1390 cm-1 originated from benzenoid 78. The noticeable high intensity peak


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in ternary hybrid is due to delocalization of electrons in PANI as well as π-π interaction between PANI and MWCNT. The low intensity peaks observed between 990-1100 cm-1 are indicative of C-O stretching arising due to presence of oleic acid as capping agent [Figure S4 (b)]. Raman spectroscopy of as-synthesized binary metal selenide reveals presence of all expected peaks. In case of AS characteristic peaks for Ag-Se bond vibrations appeared as low intensity peaks at 137 cm-1 and 157 cm-1

79, 80.

A broad peak at 269 cm-1 in case of CS is attributed to the vibrational

frequency of Cu-Se bond. The peak broadening in Raman scattering is due to the smaller particle size and amorphous nature of the CS81. Sample IS showed different peaks at 142, 225, 266, 335, 424 and 552 cm-1 confirming formation of In2Se3. The low intensity peak at 142 cm-1 is due to In-In bond vibrations generally illustrated as, transverse and longitudinal optical modes (TO-LO states) while broad peak at 225 cm-1 is due to Se chain vibrations. Peak at 254 cm-1 may be assigned to Se8 ring vibrations and strong peaks at 424 and 552 cm-1 are due to second order Raman scattering in In2Se3 NPs 82. Similarly, SS also showed peaks due to Se chain vibrations and Se8 at 260 cm-1 and 550 cm-1. Other characteristic peaks of Sb-Se and Sb-Sb bond vibrations appeared at 199 cm-1 and 142 cm-1 83-85. Hybrids have showed different nature of spectra in lower frequency region which could be due to dominance of MWCNT and PANI stretching vibrations. In order to confirm the different modes due to MWCNT and PANI, Raman spectra were recorded in the range of 1000 to 2000 cm-1. Spectra of all hybrids revealed almost similar pattern with different peaks at 1069, 1258, 1344, 1548, 1674, 1693, 1781, 1877 cm-1. The C=C quinonoid and benzenoid forms of PANI have also been identified in Raman spectra similar to that of FTIR. Peaks at 1548 cm-1 and 1674 cm-1 were due to quinonoid and benzenoid forms of the PANI. The quinonoid form also showed


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peak due to C-N+ at 1344 cm-1

86-88.

However, another contributor of the hybrid is MWCNT

which is known to exhibit peaks at 1340 cm-1 and 1611 cm-1 and corresponding to the D and G band

89

but these peaks are found to be merged with the peaks due to quinonoid and benzenoid

forms of PANI. The merging of the peaks leads to peak broadening as it is evident from the Figure S4 (e). It is also observed that all peaks were matched well with the hybrid of MWCNT and PANI which confirms their presence in all other samples also [Figure S4 (e) CP]. The broadness of the peaks for all these samples hints towards scattering of phonons which may help in lowering the thermal conductivity in them. The low thermal conductivity would be beneficial to obtain higher thermopower from these samples. Thermoelectric performance of ternary hybrid Thermoelectric properties of the materials can be well explained by three different components such as, electrical conductivity (σ), Seebeck coefficient (S) and power factor (PF=S2σ). Since power factor values are good enough to evaluate thermoelectric performance, its estimation is essential and hence measurement of first two factors was carried out at room temperature (RT). Electrical conductivity of CS and SS were found to be higher (4570 S/m and 4975 S/m respectively) than that of AS and IS this could be due to higher charge carrier mobility within hybrid possessing giant network possibly comprised of strong connectivity between PANI, metal selenide NPs and MWCNT [Figure 5 (a)]. FESEM images of the hybrids [Figure 4] are clearly indicative of giant network formation with connections between these three materials which probably helps for easy movement of the charge carriers. PANI comprises benzene and quinoid rings as it is already confirmed from FTIR study in present case. These rings are linked with each other by an amine nitrogen atom. This linkage leads to active π-π conjugated interactions and forms channel for carrier transport.


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Similarly, carbon nanotubes also possess π-bonded surface and may exhibit strong π-π interaction with conjugated PANI. Such interactions can favor charge transport leading to enhanced electrical conductivity27. Moreover, incorporation of the MSe NPs in such system may further help in energy filtering. Nevertheless, all ternary hybrids contain same quantity of MWCNT and PANI therefore, difference observed in electrical conductivities is considered due to change in binary metal selenide NPs in hybrid.

Figure 5. (a) Electrical conductivity (b) Seebeck coefficient and (c) power factor of CSCP, ISCP, SSCP and ASCP (d) Charge transfer mechanism in p and n-type hybrids. Generally, combination of two materials with huge difference in their electrical conductivities obeys percolation law which helps to enhance electrical conductivity and therefore, this possibility also cannot be ruled out in present case since PANI has low electrical conductivity than

MWCNT.

Thus,

good

electrical

conductivity

of

CuSe/MWCNT/PANI

and


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Sb2Se3/MWCNT/PANI is expected to show negative impact on Seebeck coefficient along with increased electronic thermal conductivity (ke), This therefore can be claimed as the main reason for decreased power factor in totality. The thermal conductivity of CSCP was obtained to be 0.54 W/mK at room temperature which affected power factor and thus on zT value (0.002). In view of the above it can be concluded that lower Seebeck coefficient values of CSCP, ISCP and SSCP as compared to ASCP are due to their increased charge carrier concentration as it is evident from electrical conductivity values [Figure 5 (b)] and high thermal conductivity. In contrary, ASCP exhibited moderate σ value (2962 S/m) which is justified by its higher Seebeck coefficient (65 µV/K) and thus power factor (12.5 µWm-1K-2) at room temperature [Figure 5 (c)]. The positive value of Seebeck coefficient for ASCP suggests p-type semiconducting behavior while other hybrids are observed to be n-type owing to their negative S. The thermal conductivity of ASCP found to be lower (0.31 W/mK) as compared to that of CSCP and therefore, zT of ASCP at room temperature is 0.012 and higher than CSCP. Amongst n-type hybrids, CSCP showed higher S (-28µV/K) and PF (3.6µWm-1K-2) and therefore, it is compared with the p-type ASCP by measuring thermal conductivity at room temperature. The higher power factor and zT obtained in case of ASCP could be due to scattering of the charge carriers via energy filtering90, 91. The presence of AS in hybrid could be responsible for decoupling of electrical and thermal conductivities resulting to enhanced performance. Present investigation therefore revealed formation of various hybrids with electrical conductivity in order of 103 with p and n-type behaviors which can become valuable materials in thermoelectric field for better energy conversion. Such p and n-type hybrids can be compiled for efficient flexible thermoelectric devices by further improvement in their performance. Possible charge transport mechanism in ternary hybrid


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As per the literature reports nanostructured material possesses more interfaces and grain boundaries in comparison to their bulk materials92. The formation of hybrids with two or more materials containing NPs leads to further increase in grain boundaries and interfaces. Such increased interfaces and grain boundaries help to scatter electrons resulting in to reduction in electrical conductivity as compared to bulk materials. However, presence of conducting constituents such as MWCNT and PANI with strong π-π interactions nullifies this possibility via electron transport from percolated network resulting to enhancement in electrical conductivity55. It is well understood from the literature93 that in case of nanocomposite systems interfacial scattering is more viable than that of Resonant Levels (RLs) mechanism and therefore, it is apparent to consider interfacial scattering in present case also. Decoration of MSe NPs over the surface of MWCNT and PANI can favor interfacial scattering of phonons and transfer of electrons/holes leading to decreased thermal conductivity and enhancement in electrical conductivity respectively. Since lowest unoccupied molecular orbital (LUMO) of PANI is assumed to be at higher energy than conduction band of MSe NPs (for better understanding examples of Ag2Se (p-type) and CuSe (n-type) are considered and discussed henceforth) while highest occupied molecular orbital (HOMO) of PANI is considered slightly higher in energy than that of valence band of Ag2Se and CuSe NPs. Such type-II heterojunction formation between these two materials with band alignments as presented in Figure 5 (d) helps to favor electron transfer from PANI to Ag2Se NPs and holes in reverse order to MWCNT while in case of CuSe NPs exactly opposite charge transfer can be considered wherein electrons travel from CuSe NPs to MWCNT under the influence of thermal or light energy. In case of n-type hybrids, we believe that MSe NPs, MWCNT and PANI are not connected together as seen above in ASCP nanocomposite. Here, the dense MWCNT network was locally covered with either PANI


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or MSe but not together. Thus, we obtain two separate active interfaces of MWCNT/PANI and MWCNT/MSe where MWCNT may act as a charge carrier throughout the nanocomposite system Figure S5. This arrangement may result in influx of electrons in MWCNT from MSe while transfer of holes in MWCNT from PANI. The end result may be n-type behavior of these hybrids due to increased density of electrons in MWCNT decreasing the density of holes in PANI. The MWCNT possesses energy level higher than the HOMO of PANI41 and therefore, transfer of holes from PANI to MWCNT are expected in p-type hybrid while transfer of electrons from CuSe to MWCNT in n-type hybrid. This clearly illustrates the charge carriers transport mechanism within ternary hybrid, responsible for good electrical conductivity as it is evident from Figure 5 (a). The presence of MWCNT, PANI and binary metal selenide NPs altogether results formation of two interfaces such as, MWCNT/PANI and PANI/MSe or MWCNT/MSe NPs which are favoring charge transfer as well as energy filtering. Filtering of the low energy charge carriers allows high energy carriers to cross energy barrier leading to increase in electrical conductivity and Seebeck coefficient. The higher Seebeck coefficient obtained in case of ASCP is attributed to the synergetic effect of the interfacial electron and phonon scattering while other three ternary hybrids containing CS, IS and SS showed dominance of electronic scattering and lower phonon scattering resulting to overall decreased Seebeck coefficient. Overall, such ternary hybrids show high electrical conductivity and both n and p-type semiconducting behavior depending on presence of binary metal selenide NPs. Conclusion Novel ternary hybrid comprising binary metal selenide nanoparticles, MWCNT and PANI have been synthesized and investigated towards their thermoelectric performance. The


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characterization of initially synthesized MSe NPs revealed their phase pure formation with varied sized nanoparticles. Decoration of MSe NPs between MWCNT and PANI have clearly identified from FESEM images. π-π stacking between MWCNT and PANI could be the responsible for formation of channels for charge transport leading to electrical conductivity in the order of 10K. The room temperature thermoelectric performance of the all hybrids suggests n and p-type behavior. Higher zT of 0.012 and power factor of 12.5 μW/mK2 was estimated in case of Ag2Se/MWCNT/PANI exhibiting p-type behavior while zT of 0.002 and 3.6 μW/mK2 power factor was obtained in case of Cu2Se/MWCNT/PANI showing n-type behavior. The performance of such hybrid composites can be further enhanced by employing highly conductive filler such as reduced graphene oxide (RGO) and/or doping the binary selenides. The fact that these binary selenides and PANI show photovoltaic effect, their thermoelectric efficiency may further increase in presence of visible to near IR light. Such studies to enhance the thermoelectric performance will be performed in the next phase of the work. Corresponding Author *Pawan. K. Khanna Nano Chemistry and Quantum Dots R & D Lab, Department of Applied Chemistry, Defence Institute of Advanced Technology (DIAT), Ministry of Defence, Govt. of India, Girinagar, Pune411025, India. Email ID: [email protected] Author Contributions The designing of schemes, planning of work, draft correction with overall guidance was executed by the Prof. Pawan. K. Khanna. The experimental work was carried out by Anuraj S. Kshirsagar and Chaitanya Hiragond. Characterization, interpretation and draft preparation was done by Anuraj S. Kshirsagar. Dr. Priyesh. V. More was involved in the designing of


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thermoelectric work, its interpretation and draft correction. The thermoelectric work was carried out by Dr. Abhijit Dey at HEMRL, Pune. Acknowledgment Authors thank Vice-Chancellor, Defence Institute of Advanced Technology (DIAT), Girinagar, Pune for encouragement and permission. ASK acknowledges DIAT for a Senior Research Fellowship (SRF). Authors are thankful to Dr. A. Abhyankar, DIAT (DU), Pune and Dr. Sreenu Bhanoth for their help in SEM analysis. Abbreviations NPs, Nanoparticles; MSe, Binary Metal Selenides; MWCNT, Multi Walled Carbon Nanotubes; PANI, Polyaniline; CS, Copper selenide; AS, Silver selenide; IS, Indium selenide; SS, Antimony selenide;

CSCP,

CuSe/MWCNT/PANI;

ASCP,

Ag2Se/MWCNT/PANI;

ISCP,

In2Se3/MWCNT/PANI; SSCP, Sb2Se3/MWCNT/PANI. References 1.

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Metalorganic

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Graphical abstract Synthesis of binary metal selenides (MSe) and their room temperature thermoelectric performance as composites with MWCNT and PANI is discussed.


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