Ethylene-Octene-Copolymer with Embedded Carbon and Organic Conductive Nanostructures for Thermoelectric Applications

polymers

Article Ethylene-Octene-Copolymer with Embedded Carbon and Organic Conductive Nanostructures for Thermoelectric Applications

Petr Slobodian 1,2,*, Pavel Riha 3,*, Robert Olejnik 1,4 and Michal Sedlacik 1

1 Centre of Polymer Systems, University Institute, Tomas Bata University, Tr. T. Bati 5678, 760 01 Zlin, Czech Republic; [email protected] (R.O.); [email protected] (M.S.) 2 Faculty of Technology, Polymer Centre, Tomas Bata University, T.G.M. 275, 760 01 Zlin, Czech Republic 3 The Czech Academy of Sciences, Institute of Hydrodynamics, Pod Patankou 5, 166 12 Prague 6, Czech Republic 4 Department of Production Engineering, Faculty of Technology, Tomas Bata University in Zlin, T. G. Masaryk nam. 275, 762 72 Zlin, Czech Republic * Correspondence: [email protected] (P.S.); [email protected] (P.R.)

Received: 7 May 2020; Accepted: 4 June 2020; Published: 9 June 2020

Abstract: Hybrid thermoelectric composites consisting of organic ethylene-octene-copolymer matrices (EOC) and embedded inorganic pristine and functionalized multiwalled carbon nanotubes, carbon nanofibers or organic polyaniline and polypyrrole particles were used to form conductive nanostructures with thermoelectric properties, which at the same time had sufficient strength, elasticity, and stability. Oxygen doping of carbon nanotubes increased the concentration of carboxyl and C–O functional groups on the nanotube surfaces and enhanced the thermoelectric power of the respective composites by up to 150%. A thermocouple assembled from EOC composites generated electric current by heat supplied with a mere short touch of the finger. A practical application of this thermocouple was provided by a self-powered vapor sensor, for operation of which an electric current in the range of microvolts sufficed, and was readily induced by (waste) heat. The heat-induced energy ensured the functioning of this novel sensor device, which converted chemical signals elicited by the presence of heptane vapors to the electrical domain through the resistance changes of the comprising EOC composites.

Keywords: ethylene-octene-copolymer; carbon nanotubes; carbon ﬁbers; polyaniline; polypyrrole; thermoelectric composites

1. Introduction Thermoelectric conductive polymer composites convert thermal energy into electricity when there is a difference in temperatures between the hot and cold junctions of such two dissimilar conductive or semiconductive composites. The conversion is based on a phenomenon called the Seebeck effect. The heat supplied at the hot junction (hot side) of the thermoelectrics causes a flow of electric current to the cold side that can be harnessed as useful voltage. The classical thermoelectrics are made from inorganic materials such as metals (Al, Cu, Ni), metallic alloys (chromel, alumel) and semiconductors (PbT, Bi2Te3), which are thermoelectrically efficient, but at the same time expensive, heavy, and some of them materially in short supply. Consequently, alternative organic thermoelectrics are being developed. These organic thermoelectrics do not produce as much energy as the metal ones. The thermoelectric figure of merit is typically only in the range 0.001–0.01 at room temperature. However, the mechanical flexibility, processability,

Polymers 2020, 12, 1316; doi:10.3390/polym12061316 www.mdpi.com/journal/polymers Polymers 2020, 12, 1316 2 of 14 light weight, and low manufacturing costs are attractive features for potential applications for electricity microgeneration e.g., in sensors and electronics. Petsagkourakis et al. [1] thoroughly reviewed principles and advances in the development of those thermoelectric materials. The electronic properties of the conductive (conjugated) polymers are based on their macromolecular structure and morphology, since they affect the electronic conditions, and therefore the charge transport and the thermoelectric properties. Such conductive polymers have been used mostly for thermoelectric generators and temperature sensors. Hybridizing of such polymers with inorganic thermoelectric materials is another way to enhance their thermoelectric performance. Such hybrids have low thermal conductivity, which is advantageous in energy harvesting. Among others, metals, Bi2Te3 or carbon nanoparticles have been used as the inorganic portions of such composites. Polyaniline (PANI) and polypyrrole (PPy) are two of the most studied conductive polymers owing to their easy and low-cost synthesis, good environmental stability, and simple doping/dedoping processes based on acid–base reactions [2,3]. The PANI conductivity apparently depends on its ability to form polarons, cation radicals [4]. The four double bonds constituting the quinonediimine unit in PANI emeraldine salt convert to three double bonds in a benzene ring and two unpaired electrons, which act as charge carriers. The polarons can eventually spread over the polymer chain to produce a polaron lattice. The polymerization condition has an important effect on the final electrical properties (the conductivity and dielectric loss) of conductive polymers. In the case of PANI, the most used synthesis is the polymerization initiated by an addition of an oxidant, generally ammonium persulfate. This synthetic route leads to the conductive emeraldine salt of PANI, which is formed by the head-to-tail coupling of the monomers. Various fillers such as Ag nanoparticles [5], Bi2Se3 nanoplates [6], or thermally reduced graphene [7] can be embedded to improve the properties of PANI. The thermoelectric properties of PPy may be improved by a controlled synthesis of various nanostructures [8]. Nanotubular-type PPy are synthetized through a chemical polymerization route and treated with various dopants such as hydrochloric acid, p-toluenesulfonic acid monohydrate or tetrabutylammonium hexafluorophosphate, which affect its electrical and thermal properties [9]. Also hybrid PPy thermoelectrics have been prepared such as the PPy nanowire/graphene composite [10], the PPy/multiwalled carbon nanotube composite [11], the PPy/Ag nanocomposite film [12], or the PPy/graphene/PANI nanocomposite with a high thermoelectric power factor [13]. Thanks to the content of oxygen functional groups on their surfaces, the multiwalled carbon nanotubes (MWCNTs) and the carbon nanofibers (CNFs) embedded into ethylene-octene-copolymer (EOC) may affect its thermoelectric power [14]. Besides the oxygen groups, thermoelectric properties of carbon nanotubes can be changed by doping with many other electron donors [15]. Waste heat dissipated from homoiothermic human bodies can be readily used as the source of electrical power for polymer thermoelectrics, which can be used as unobtrusive low cost self-powered sensors and integrated devices for biometric monitoring [16]. Similarly, thermoelectric self-powered temperature sensors based on Te nanowire/poly(3-hexyl thiophene) polymer composite are described in [17]. Alternatively, such thermoelectrics can be used also as a wearable energy harvester turning radiated human body heat into a source of electric energy. The current research progress on flexible thermoelectric devices and conducting polymer thermoelectric materials is reviewed in [18]. In this paper, hybrid thermoelectrics consisting of EOC matrices and embedded pristine or functionalized MWCNTs, carbon nanofibers, and organic PANI and PPy particles were used to form conductive nanostructures with thermoelectric properties, which at the same time had sufficient strength, elasticity, and stability. Their thermoelectric power was measured and discussed in the light of the oxygen content of their inorganic fillers. A practical application of the prepared thermoelectric composites for electricity microgeneration for a self-powered temperature signaling sensor and a self-powered vapor sensor was introduced. Polymers 2020, 12, 1316 3 of 14

2. Materials and Methods Purified MWCNTs produced by a chemical vapor deposition of acetylene were supplied by Sun Nanotech Co. Ltd., Jiangxi, China. According to the supplier, the nanotube diameter is 10–30 nm, the length 1–10 µm, the purity ~90% and the volume resistivity 0.12 Ωcm. The diameter of individual nanotubes is between 10 and 60 nm (100 measurements) at the average diameter and the standard deviation is 15 6 nm. The nanotube length is from about 0.2 µm up to 3 µm. They consist of 15 to ± 35 rolled layers of graphene at the interlayer distance of ca 0.35 nm [19]. The pristine nanotubes are denoted further on as MWCNT(Sun)s. MWCNTs (BAYTUBES C70 P) were supplied by Bayer MaterialScience AG, Leverkusen, Germany. The nanotube purity is >95 wt %, outer mean diameter ~13 nm, inner mean diameter ~4 nm, length > 1 µm and declared bulk density of MWCNT of agglomerates of micrometric size 45–95 kg/m3. The pristine nanotubes are denoted further on as MWCNT(Bayer)s. The carbon nanofibers (CNFs) with trade name VGCF® (Vapor Grown Carbon Fibers) were supplied by Showa Denko K.K., Tokyo, Japan. The fiber diameter was 150 nm, length 10 µm and electrical resistivity 0.012 Ωcm. The oxygenated MWCNTs were prepared in a glass reactor with a reflux condenser filled with 250 mL of 0.5 M H2SO4, into which 5 g KMnO4 and 2 g MWCNTs were added. Then the dispersion was sonicated at 85 ◦C for 15 h using a thermostatic ultrasonic bath (Bandelin electronic DT 103H, Merck spol. s r. o., Prague, Czech Republic). Thereafter, the product was filtered, washed with concentrated HCl to remove MnO2, thoroughly rinsed with deionized water and dried [19]. Alternatively, the MWCNTs were oxygenated by HNO3 as follows: 2 g of the MWCNTs were added to 250 mL of HNO3 (concentrated) and heated at 140 ◦C for 2 h. After that, the dispersion was cooled and filtered. The sediment was washed by deionized water and dried at 40 ◦C for 24 h. The corresponding oxygenated MWCNTs are denoted MWCNT(Sun)H2SO4 or MWCNT(Sun)HNO3 and MWCNT(Bayer)H2SO4 or MWCNT(Sun)HNO3. For preparing PANI emeraldine salt, 0.2 M aniline hydrochloride was mixed with 0.25 M ammonium persulfate (APS) in water, briefly stirred, and left to polymerize for 24 h at room temperature. Then the PANI precipitate was collected on a filter and washed with 0.2 M HCl and acetone. Polyaniline emeraldine particles were in turn dried in air and then under vacuum at 60 ◦C for 24 h [20]. Pyrrole monomer (Py, purity 98%, Sigma–Aldrich Inc., St. Louis, MO, USA) was distilled ≥ twice under reduced pressure and stored below 4 ◦C. Polypyrrole particles were prepared via in situ polymerization of the precooled Py in a system containing surfactant CTAB. The precooled initiator APS was added into the system dropwise and the polymerization was allowed to proceed with stirring for 2 h at 0–5 ◦C. After being washed with water and ethanol, the PPy powder was dried in air and then under vacuum at 60 ◦C for 24 h. After drying, both types of polymers were gently ground with mortar and pestle. The reagents APS (purity = 98%) and HCl (concentration 35%) were purchased from Sigma– ≈ Aldrich Inc., St. Louis, MO, USA. The aniline hydrochloride (purity 99%) was purchased from ≥ Fluka, Buchs, Switzerland, cetyltrimethylammonium bromide (CTAB, purity = 98%) from Lach–Ner Ltd., Neratovice, Czech Republic, acetone and ethanol from Penta Ltd., Chrudim, Czech Republic. The reagents were used without further purification. The carbon allotrope/EOC composites were prepared by an ultrasonication of dispersions of MWCNTs or CNFs in EOC/toluene solution (5% of EOC in toluene). The chosen concentration of the filler in the composites was 30 wt %, which was well above the percolation threshold. The ultrasonication of the dispersion was done in the thermostatic ultrasonic bath (Bandelin electronic DT 103H, Merck spol. s r. o., Prague, Czech Republic) for 3 h at 80 ◦C. Then the dispersion was poured into acetone to form a precipitate. The final composite sheets were prepared by compression molding at 100 ◦C[14]. The organic PANI (PPy)/EOC composites were prepared in the same way as the carbon allotrope composites except for filler concentration, which was 70 wt %. Polymers 2020, 12, 1316 4 of 14 Polymers 2020, 12, x FOR PEER REVIEW 4 of 14 sample,MWCNTs the MWCNT were dispersion analyzed in by acetone means prepared of transmission by ultrasonication, electron microscopywas deposited (TEM) on a 300 using mesh the coppermicroscope grid with JEOL a carbon JEM 2010 film (Jeol(SPI, Ltd.,Washington, Freising, D.C., Germany) USA) and at andried. accelerating The structure voltage of CNFs of 160 was kV. observedThe sample, by means the MWCNT of a scanning dispersion electron in microscope acetone prepared (SEM) byNova ultrasonication, NanoSEM 450, was FEI, deposited Lincoln, NE, on a USA,300 meshat operating copper gridvoltage with 10 a kV. carbon The filmsample, (SPI, CNF Washington, dispersion DC, in USA)acetone and prepared dried. The by structureultrasonication, of CNFs waswas deposited observed on by meanscarbon oftargets a scanning and covered electron with microscope a thin Au/Pd (SEM) layer. Nova For NanoSEM the observations, 450, FEI, Lincoln, a regime NE, ofUSA, secondary at operating electrons voltage was 10used. kV. The sample,same scanning CNF dispersion electron inmicroscope acetone prepared was used by to ultrasonication, observe the morphologywas deposited of PANI on carbon emeraldine targets salt and and covered of PPy with particles. a thin Au /Pd layer. For the observations, a regime of secondaryThe X-ray electronsphotoelectron was used. spectroscopy The same (XPS) scanning signals electron were measured microscope to wasobtain used information to observe on the functionalmorphology groups of PANI attached emeraldine onto saltthe andnanotube of PPy particles.surfaces. The XPS signals from MWCNT(Sun), MWCNT(Sun)KMnOThe X-ray photoelectron4 and MWCNT(Sun)HNO spectroscopy (XPS)3 network signals surfaces were were measured recorded to obtainusing the information Thermo Scientificon functional K-Alpha groups System attached TFA ontoXPS the(Physical nanotube Electronics surfaces. Instrument, The XPS signals Chanhassen, from MWCNT(Sun), MN, USA) equippedMWCNT(Sun)KMnO with a micro-focused,4 and MWCNT(Sun)HNO monochromatic 3A1network Ka X-ray surfaces source were (1486.6 recorded eV). An using X-ray the beam Thermo of 400Scientific μm size K-Alpha was used System at 6 mA TFA x XPS12 kV (Physical [21]. The Electronics spectra were Instrument, acquired Chanhassen, in the constant MN, analyzer USA) equipped energy modewith with a micro-focused, pass energy of monochromatic 200 eV for the A1survey. Ka X-ray Narrow source regions (1486.6 were eV). collected An X-ray using beam the snapshot of 400 µm acquisitionsize was usedmode at (150 6 mA eV pass12 energy) kV [21]. enabling The spectra rapid were collection acquired of data in the(5 s constantper region). analyzer The narrow energy × regionmode data with were pass energyas post-processed of 200 eV for using the survey. the Jansson’s Narrow algorithm regions were to remove collected the using analyzer the snapshot point spreadacquisition function, mode which (150 resulted eV pass energy)in an improved enabling re rapidsolution collection of the spectra of data for (5 sthe per peak region). deconvolution The narrow [22].region The data concentration were as post-processed of elements was using determined the Jansson’s from algorithm survey spectra to remove by MultiPak the analyzer v7.3.1 point software spread (Physicalfunction, Electronics which resulted Inc., Chanhassen, in an improved MN, resolution USA). of the spectra for the peak deconvolution [22].

TheFourier-transform concentration of elementsinfrared was (FTIR) determined analyses from of survey MWCNTs, spectra byMWCNT(KMnO MultiPak v7.3.14)s software and (Physical Electronics Inc., Chanhassen, MN, USA). MWCNT(HNO3)s were performed on the FTIR spectrometer Nicolet 6700 (Thermo Scientific, Fourier-transform infrared (FTIR) analyses of MWCNTs, MWCNT(KMnO )s and MWCNT(HNO )s Waltham, MA, USA). The transmission accessory was used for pristine4 MWCNT(Sun)s,3 were performed on the FTIR spectrometer Nicolet 6700 (Thermo Scientific, Waltham, MA, USA). MWCNT(Sun)KMnO4 and MWCNT(Sun)HNO3 samples in powder form prepared by potassium The transmission accessory was used for pristine MWCNT(Sun)s, MWCNT(Sun)KMnO4 and bromide.MWCNT(Sun)HNO FTIR analyses3 samples of chemical in powder compositio form preparedn of PANI by emeraldine potassium bromide.and PPy FTIRparticles analyses were of examinedchemical compositionby the above of PANImentioned emeraldine FTIR andspectrom PPy particleseter Nicolet were 6700 examined using by the the attenuated above mentioned total reflectanceFTIR spectrometer technique Nicolet with a 6700germanium using the crystal attenuated in the range total reflectance600–4000 cm technique–1 at 64 scans with per a germanium spectrum crystal in the range 600–4000 cm 1 at 64 scans per spectrum at 2 cm 1 resolution. at 2 cm–1 resolution. − − The Hall coefficient, the resistivity, and the conductivity of the samples as well as the charge The Hall coefficient, the resistivity, and the conductivity of the samples as well as the charge mobility and the charge carrier concentration were measured by means of the HCS 1 apparatus (Linseis mobility and the charge carrier concentration were measured by means of the HCS 1 apparatus Messgeräte GmbH, Selb, Germany) equipped with static 0.7 T field permanent magnets for bipolar (Linseis Messgeräte GmbH, Selb, Germany) equipped with static 0.7 T field permanent magnets for measurement. The disc form samples had diameter 20 mm and thickness 2.65 mm. The sample current bipolar measurement. The disc form samples had diameter 20 mm and thickness 2.65 mm. The was set to 4 mA. The thermoelectric power measurement was carried out for all the samples using sample current was set to 4 mA. The thermoelectric power measurement was carried out for all the the set-up illustrated in Figure1. The schematic diagram shows that the circular composite sample samples using the set-up illustrated in Figure 1. The schematic diagram shows that the circular (diameter 20 mm, thickness 2 mm) was placed between two copper electrodes. The ends of each composite sample (diameter 20 mm, thickness 2 mm) was placed between two copper electrodes. The of the Cu electrodes were immersed in thermostatic silicone oil baths set at different temperatures. ends of each of the Cu electrodes were immersed in thermostatic silicone oil baths set at different The temperature at the copper/composite interfaces was measured by a Pt100 temperature sensor. temperatures. The temperature at the copper/composite interfaces was measured by a Pt100 The arising thermoelectric current was measured by the Keithley 2000 Digital Multimeter (Tektronix, temperature sensor. The arising thermoelectric current was measured by the Keithley 2000 Digital Inc. Beaverton, OR, USA). Multimeter (Tektronix, Inc. Beaverton, OR, USA).

Figure 1. A schematic illustration of the set up for the measurement of electric voltage generated between the hot and cold ends of the Cu/composite /Cu thermoelectric device as a response to a temperature diﬀerence. Polymers 2020, 12, x FOR PEER REVIEW 5 of 14

FigurePolymers 2020 1. ,A12 ,schematic 1316 illustration of the set up for the measurement of electric 5 of 14 voltage generated between the hot and cold ends of the Cu/composite/Cu thermoelectric3. Results device as a response to a temperature difference.3. Results

3.1. Characterization Characterization of of Fillers Fillers and Composites The detailed detailed view view of of individual individual pr pristineistine MWCNT(Bayer)s, MWCNT(Bayer)s, their their cluste clustersrs as well as well as clusters as clusters of the of the oxidized MWCNT(Bayer)KMnO as obtained by the TEM are shown in Figure2. The wall of the oxidized MWCNT(Bayer)KMnO4 as4 obtained by the TEM are shown in Figure 2. The wall of the MWCNT(Bayer) consistedconsisted of of about about 15 rolled15 rolled layers layers of graphene. of graphene. The nanotube The nanotube outer and outer inner and diameter inner wasdiameter about was 20 nmabout and 20 4–10 nm and nm, 4–10 respectively. nm, respectively There were. There also were defects also obstructing defects obstructing nanotube nanotube interiors, whichinteriors, were which commonly were commonly seen in the MWCNTseen in the structures. MWCNT When structures. a cluster When of pristine a cluster MWCNT(Sun)s of pristine was compared with a cluster of wet oxidized nanotubes, a diﬀerence in the tube length was visible. MWCNT(Sun)s was compared with a cluster of wet oxidized nanotubes, a difference in the tube The oxidation of MWCNT(Sun)s by KMnO caused shortening of the nanotubes, creation of defect length was visible. The oxidation of MWCNT(Sun)s4 by KMnO4 caused shortening of the nanotubes, sites, and opened ends. A small amount of amorphous carbon after the KMnO oxidation process creation of defect sites, and opened ends. A small amount of amorphous carbon4 after the KMnO4 oxidationcan be also process expected can [23 be], also although expected another [23], report although showed another that oxygenationreport showed by that KMnO oxygenation4 in an acidic by suspension provides nanotubes free of amorphous carbon [24]. KMnO4 in an acidic suspension provides nanotubes free of amorphous carbon [24].

Figure 2. The upper upper set: set: Transmission Transmission electron electron microscopy microscopy micrographs micrographs ofof MWCNT(Sun)s.MWCNT(Sun)s. (a) The structure of of an an individu individualal pristine pristine nanotube, nanotube, (b (b) )the the detail detail of of a nanotube a nanotube crossing, crossing, (c) (thec) the cluster cluster of pristineof pristine nanotubes. nanotubes. The The lower lower set: set:TEM TEM micrographs micrographs of MWCNT(Bayer)s. of MWCNT(Bayer)s. (d) (Thed) The structure structure of an of individualan individual pristine pristine nanotube, nanotube, (e) (ethe) the cluster cluster of ofpristine pristine nanotubes, nanotubes, (f ()f )the the cluster cluster of of oxidized

MWCNT(Bayer)KMnO4..

The micrographs ofof thethe surfaces surfaces of of filler filler layers—PANI layers—PANI and and PPy PPy particles particles as wellas well as MWCNTs as MWCNTs and andCNFs—are CNFs—are shown shown in Figure in Figure3. The 3. layers The layers were formedwere formed from thefrom respective the respective aqueous aqueous dispersion dispersion of the offillers the onfillers the on surface the surface of the interdigitated of the interdigitated electrode electrode by the drop by the method. drop method. The particles The particles were globular were withglobular a diameter with a diameter 0.5–1 µm 0.5–1 and the μm PPy and particles the PPy hadparticles narrower had narrower size distribution. size distribution. The networks of CNFs, MWCNT(Su MWCNT(Sun)sn)s and MWCNT(Bayer)s resulted from the filtration filtration of their dispersions through non-woven polyurethane membranes.membranes. The dispersion, which consisted of CNFs (0.8 mg), or the same amount of nanotubes, dispersed in 530 mL of water with 15.4 g of the the surfactant surfactant (sodium dodecyl sulphate) and 8.5 mL of the co-sur co-surfactantfactant (1-pentanol), was properly sonicated and the pHpH adjustedadjusted to to 10 10 using using an an aqueous aqueous solution solution of NaOH. of NaOH. The filtrateThe filtrate layer waslayer washed was washed in situ severalin situ severaltimes with times deionized with deionized water and water finally and with finally methanol. with methanol. The cross-section of the composites presented in Figure4 showed a uniform distribution of the MWCNT(Sun) and CNF filler in the EOC matrix, which together with the filler concentration 30% ensured the electrical and thermal conductivity of the composites. Polymers 2020, 12, x FOR PEER REVIEW 6 of 14

Polymers 2020, 12, 1316 6 of 14 Polymers 2020, 12, x FOR PEER REVIEW 6 of 14

Figure 3. SEM micrographs of a surface of layers made of organic and inorganic fillers. The respective fillers are denoted in the images.

The cross-section of the composites presented in Figure 4 showed a uniform distribution of the MWCNT(Sun)Figure 3. SEMSEM and micrographs micrographs CNF filler of in a surfacethe EOC of layers matrix, made which of organic together and withinorganic the filler ﬁllers.fillers. concentration The respective 30% ensuredﬁllersfillers the are electrical denoted inand the thermal images.images. conductivity of the composites.

The cross-section of the composites presented in Figure 4 showed a uniform distribution of the MWCNT(Sun) and CNF filler in the EOC matrix, which together with the filler concentration 30% ensured the electrical and thermal conductivity of the composites.

Figure 4.4. SEMSEM images images of of cross-sections cross-sections of carbonof carbon allotrope allotrope/ethylene-octene-copolymer/ethylene-octene-copolymer matrices matrices (EOC) composites.(EOC) composites. The respective The respective composites comp areosites denoted are denoted in the images.in the images.

3.2. XPS Data Figure 4. SEM images of cross-sections of carbon allotrope/ethylene-octene-copolymer matrices The mainmain bindingbinding energy energy peak peak (284.5 (284.5 eV) ineV) the in XPS the spectra XPS spectra of pristine of pristine MWCNT(Sun)s, MWCNT(Sun)s, Figure5, (EOC) composites. The respective composites are denoted in the images. wasFigure assigned 5, was to assigned the C1s-sp2, to the while C1s-sp2, the other while ones the were other assigned ones were to the assigned C–O (286.2 to the eV), C–O C=O (286.2 (287.1 eV), eV), O–CC=O= (287.1O (288.6–289 eV), O–C=O eV) and (288.6–289 C1s-π-π eV)* (291.1–291.5 and C1s-π eV).-π* (291.1–291.5 After the oxidation eV). After treatment, the oxidation the intensities treatment, of 3.2. XPS Data the peaksintensities corresponding of the peaks to thecorresponding oxidized carbon to the bonds oxidized increased carbon as bonds seen in increased Figure5. as FTIR seen data in Figure of the MWCNTs5. FTIRThe datamain also of conﬁrmed binding the MWCNTs theenergy presence also peak confirmed of (284.5 the C–O, eV) the C =presenceinO the and O–CXPS of the=spectraO C–O, functional C=Oof pristine groupsand O– MWCNT(Sun)s, on C=O their functional surfaces. ItgroupsFigure is also on5, stated wastheir assigned insurfaces. other studiesto It the is alsoC1s-sp2, that stated MWCNTs while in other the treated otherstudies with ones that (NH were MWCNTs4) 2assignedS2O8,H treated2 toO2 the, or with C–O O3 have(NH (286.24 higher)2S eV),2O8, concentrationsHC=O2O2 ,(287.1 or O 3eV), have of O–C=O carbonylhigher (288.6–289 concentrations and hydroxyl eV) and functionalof C1s- carbonylπ-π* groups, (291.1–291.5 and hydroxyl while eV). more functional After aggressive the oxidationgroups, oxidants while treatment, (HNO more3, KMnOtheaggressive intensities4) form oxidants higherof the (HNO fractionalpeaks3 ,corresponding KMnO concentrations4) form tohigher ofthe carboxyl ox fractionalidized groups carbon concentrations [25 bonds]. Acidic increased potassiumof carboxyl as seen permanganate groups in Figure [25]. (KMnO5.Acidic FTIR potassium4) data is a strongof the permanganate MWCNTs oxidizing agentalso (KMnO confirmed and4 produces) is a the strong presence more oxidizing surface of the agent acidicC–O, C=Oand groups producesand than O– C=O nitric more functional acid surface [26]. Thegroupsacidic increase groups on their numberthan surfaces. nitric of oxygenatedacid It is [26]. also The stated functional increase in other number groups studies of attached oxygenated that MWCNTs on MWCNT(Sun)KMnO functi treatedonal groups with attached(NH4 surfaces4)2S2 Oon8, signiﬁcantlyHMWCNT(Sun)KMnO2O2, or O3 increaseshave higher4 thesurfaces concentrations contact significantly resistance of inincreasescarbonyl the MWCNT theand co hydroxylntact junctions resistance functional of the in network the groups, MWCNT structure while junctions [more27]. aggressiveof the network oxidants structure (HNO [27].3, KMnO 4) form higher fractional concentrations of carboxyl groups [25]. Acidic potassium permanganate (KMnO4) is a strong oxidizing agent and produces more surface acidic groups than nitric acid [26]. The increase number of oxygenated functional groups attached on MWCNT(Sun)KMnO4 surfaces significantly increases the contact resistance in the MWCNT junctions of the network structure [27]. Polymers 2020, 12, 1316 7 of 14 Polymers 2020, 12, x FOR PEER REVIEW 7 of 14

Figure 5. XPS C1s spectra for MWCNT(pristine) (A), MWCNT(HNO )(B) and MWCNT (KMnO ) Figure 5. XPS C1s spectra for MWCNT(pristine) (A), MWCNT(HNO3) (3B) and MWCNT (KMnO4) (C4) samples.(C) samples. 3.3. FTIR Measurements 3.3. FTIR Measurements Table1 and Figure6 present the frequencies of some of the functional groups in the FTIR Table 1 and Figure 6 present the frequencies of some of the functional groups in the FTIR spectra spectra of MWCNT networks. FTIR spectra in Figure6 from MWCNTs showed a broad peak about −1 of MWCNT1 networks. FTIR spectra in Figure 6 from MWCNTs showed a broad peak about 3430 cm 3430 cm− which was characteristic of the O–H stretch of hydroxyl group. C–H stretching of the which was characteristic of the O–H stretch of hydroxyl group. C–H stretching of the pristine pristine MWCNT(Sun) sample was shifted to lower wavelengths for all oxidation treatments. A weak MWCNT(Sun) sample1 was shifted to lower wavelengths for all oxidation treatments. A weak C=O C=O peak at 1705 cm− , Figure6, was observed in the pristine MWCNT(Sun) sample, which showed peak at 1705 cm−1, Figure 6, was observed in the pristine MWCNT(Sun) sample, which showed that that there was a carbonyl or carboxylic group on its surface. The reason why the pristine MWCNT(Sun) there was a carbonyl or carboxylic group on its surface. The reason why the pristine MWCNT(Sun) sample had carbonyl and OH groups could be a partial oxidation of the surfaces of MWCNTs during sample had carbonyl and OH groups could be a partial oxidation of the surfaces of MWCNTs during the puriﬁcation by the manufacturer [28]. A higher shift in the carbonyl stretching mode was seen the purification by the manufacturer [28]. A higher shift in the carbonyl stretching mode was seen for for MWCNT(HNO3) than for MWCNT(KMnO4). The reason could be a C=O group or other groups MWCNT(HNO3) than for MWCNT(KMnO4). The reason could be a C=O group or other groups that that interacted with the C=O group. FTIR spectra in Figure6 also indicated that there were probably interacted with the C=O group. FTIR spectra in Figure 6 also indicated that there were probably no no anhydride/lactone groups on the surfaces of MWCNTs since these groups are usually observed at anhydride/lactone1 groups on the surfaces of MWCNTs since these groups are usually observed at around 1750 cm− or higher wavenumber [26,29,30]. around 1750 cm−1 or higher wavenumber [26,29,30]. Table 1. Summary of FTIR measurements for the pristine and oxidized MWCNT networks. Table 1. Summary of FTIR measurements for the pristine and oxidized MWCNT networks. Wavenumber (cm 1) Wavenumber (cm−1)− MWCNT MWCNT Possible Assignments MWCNT MWCNT MWCNT Possible Assignments MWCNT (HNO ) (KMnO ) (HNO3 3) (KMnO4 4) OHOH stretch stretch 3435 3435 3428 3428 3427 3427 C–HC–H stretch stretch (CH (CH2, CH2, CH3) 3)2908,2840 2908,2840 2980,28802980,2880 2978,28902978,2890 C=OC= stretchO stretch (carboxyl (carboxyl or ketone) or ketone) 1705 1705 1726 1726 1710 1710 Intermediate Intermediate 1652 1661,1635 1641 oxidizedoxidized products—quinone products—quinone groups 1652 1661,1635 1641 C=C stretch 1559 1580 1569 groups CH /CH bending 1460 1437 1440 C=C2 stretch3 1559 1580 1569 Skeletal C-C CHtangential2/CH3 bending motions 14601222 1184 1437 1190 1440 Skeletal+C–O C-C stretch tangentialC–O motions stretch 1222 1082 1084,10491184 1087,10461190 + C–O stretch C–O stretch 1082 1084,1049 1087,1046 1 The peak assigned to the quinone group at 1652 cm− in MWCNT(pristine) sample was usually shiftedThe to peak a higher assigned wavelength to the inquinone oxidized group MWCNTs. at 1652 Coupling cm−1 in MWCNT(pristine) eﬀects (i.e., both of sample inter-molecular was usually and shifted to a higher wavelength in oxidized MWCNTs. Coupling effects (i.e., both of inter-molecular and intra-molecular hydrogen bonding with hydroxyl groups) also might be responsible for the Polymers 2020, 12, 1316 8 of 14

Polymers 2020, 12, x FOR PEER REVIEW 8 of 14 intra-molecular hydrogen bonding with hydroxyl groups) also might be responsible for the downshift downshiftin the C=O in stretching the C=O stretching mode, besides mode, the besides production the production of surface-bound of surface-bound quinone groups quinone with groups extended with extendedconjugation conjugation [31]. [31]. The up-shift in the C=C C=C stretching stretching mode mode of of MWCNTs MWCNTs was observed for both oxidized carbon nanotubes. The highest shift was observed for MWCNT(HNO3) compared to MWCNT(KMnO4). This nanotubes. The highest shift was observed for MWCNT(HNO3) compared to MWCNT(KMnO4). treatmentThis treatment suggested suggested a change a change in the in structure the structure of the of MWCNTs the MWCNTs [32]. [32]. The C–H (CH2/CH3) bending at 1460 cm−11 and the peak at 1222 cm−11 for the MWCNT(pristine) The C–H (CH2/CH3) bending at 1460 cm− and the peak at 1222 cm− for the MWCNT(pristine) sample were shifted to a higher wavelength for all oxidized MWCNTs. There There was also observed a new band around 1050 cm−11 in the FTIR spectra of oxidized MWCNTs, which could be assigned to new band around 1050 cm− in the FTIR spectra of oxidized MWCNTs, which could be assigned to thethe alcoholic C–O stretching vibration [33]. [33]. Overall, Overall, the the observed observed changes changes in in the the FTIR FTIR spectra spectra of of the oxidized MWCNTs MWCNTs confirmed conﬁrmed the efficiency eﬃciency of th thee oxidizing process and the formation of of the new oxygen-containing functional groups on the surfaces of the carbon nanotubes.

−1 −11 FigureFigure 6.6. FTIRFTIR spectraspectra ofof thethe MWCNTMWCNT samplessamples inin thethe rangerange ofof 1000–18001000–1800 cmcm− and 2800–3500 cm −. .

FTIR analysis was also carried out to identify whether the the prepared powder was was indeed PANI PANI emeraldine oror PPy PPy (Figure (Figure7). For7). theFor FTIR the absorptionFTIR absorption spectroscopy spectroscopy of the prepared of the PANIprepared emeraldine, PANI 1 1 emeraldine,the vibration the at 1578 vibration cm− wasat 1578 attributed cm–1 was to the attributed quinoid ring, to the while quinoid the vibration ring, while at 1489 the cm vibration− depicted at 1 1489the presence cm–1 depicted of a benzoid the presence ring unit of a [34 benzoid]. The peak ring atunit 1306 [34]. cm The− is peak assigned at 1306 to thecm–1 C–N is assigned stretching to ofthe a C–Nsecondary stretching aromatic of amine.a secondary Furthermore, aromatic the peakamine. characteristic Furthermore, for thethe electrically peak characteristic conductive for form the of 1 + electricallyPANI emeraldine conductive was observedform of PANI at 1245 emeraldine cm− . This was peak observed was attributed at 1245 cm to− the1. This stretching peak was of theattributed C–N • • 1 topolaron the stretching structure [of35 the]. The C–N peak+ polaron at 1160 cmstructure− corresponding [35]. The peak to the at vibrations 1160 cm associated–1 corresponding with the to C–H the vibrationsof N=Q=N associated (Q = quinoid with ring) the alsoC–H appeared of N=Q=N in the(Q = spectrum quinoid ofring) PANI also emeraldine. appeared in The the band spectrum between of 1 1 PANI913–680 emeraldine. cm− with The a maximum band between at 823 913–680 cm− was cm characteristic–1 with a maximum for an aromatic at 823 cm ring–1 was deformation characteristic and forC–H an bond aromatic vibrations ring deformation out of the plane and ofC–H the bond ring [vibrations36]. These out results of the suggested plane of thatthe ring the synthesized [36]. These resultsparticles suggested were PANI that in the the synthesized emeraldine state.particles For we there FTIR PANI spectroscopy in the emeraldine of the prepared state. For PPy, the all FTIR the spectroscopycharacteristic of peaks the prepared were in good PPy, agreement all the characterist with theic earlier peaks investigations were in good ofagreement the same with product the [earlier37,38]. 1 1 investigationsThe vibration peak of the at same 1704 cmproduct− was [37,38]. assigned The to thevibration C=N bond. peak Peaksat 1704 at cm 1550–1 was and assigned 1477 cm− toattributed the C=N 1 bond.to the Peaks in-plane at 1550 vibrations and 1477 of the cm PPy–1 attributed ring, and to the thepeaks in-plane at1178, vibrations 1037 andof the 783 PPy cm ring,− attributed and the peaks to the 1 atin-plane 1178, 1037 bending and of783 the cm PPy–1 attributed ring were to also the observed. in-plane Finally,bending the of characteristic the PPy ring peakwere at also 900 observed. cm− was Finally,assigned the to thecharacteristic out-of-plane peak vibration at 900 ofcm the–1 was Cβ–H assigned group. to the out-of-plane vibration of the Cβ–H group. Polymers 2020, 12, 1316 9 of 14 Polymers 2020, 12, x FOR PEER REVIEW 9 of 14

FigureFigure 7.7. FTIR spectra ofof thethe PANIPANI emeraldineemeraldine andand PPyPPy particles.particles.

TheThe experimentalexperimental testtest ofof thethe electricelectric transporttransport propertiesproperties ofof thethe MWCNT(Sun)pristineMWCNT(Sun)pristine/EOC/EOC compositecomposite at room room temperature temperature specified specified the the fo followingllowing values: values: the the sample sample resistance resistance 0.87 0.87 Ω, theΩ, 1 1 theresistivity resistivity 0.23 0.23 Ωcm,Ωcm, the the conductivity conductivity 4.3 4.3 ΩΩ−−1cmcm−1− andand the the magneto-resistance magneto-resistance 0.249 0.249 mΩΩ.. FurtherFurther 2 determineddetermined properties were were the the Hall-mobility Hall-mobility 7.2 7.2 cm cm2/Vs,/Vs, the the charge charge carrier carrier concentration concentration (bulk) (bulk) 3.7× 18 3 3 3.718 10 −3 cm and the average Hall coefficient +1.67 cm3 /C. The positive sign of the Hall coefficient 10 × cm and− the average Hall coefficient +1.67 cm /C. The positive sign of the Hall coefficient identifiedidentified thethe naturenature ofof thethe composite,composite, i.e.,i.e., thethe p-typep-type ofof driftdrift currentcurrent whenwhen thethe holesholes areare thethe dominantdominant currentcurrent carriers.carriers. 3.4. Thermoelectric Power Measurement 3.4. Thermoelectric Power Measurement Values of induced voltage in response to a temperature difference across the measured samples of Values of induced voltage in response to a temperature difference across the measured samples the investigated thermoelectric composites are presented in Figure8. The slope of the linear dependence of the investigated thermoelectric composites are presented in Figure 8. The slope of the linear of the resulting voltage VTEV on the temperature difference ∆T defined the thermoelectric power dependence of the resulting voltage VTEV on the temperature difference ΔT defined the thermoelectric (Seebeck coefficient): power (Seebeck coefficient): S = (VTEV/∆T)open circuit 𝑆=𝑉⁄∆𝑇 which evaluated a potential thermoelectric performance. The corresponding thermoelectric power valueswhich areevaluated summarized a potential in Figure thermoelectric9. The thermoelectric performance. power The corresponding of MWCNT /EOC thermoelectric composites power was substantiallyvalues are summarized enhanced by in the Figure nanotube 9. The oxygenation. thermoelectric When power compared of MWCNT/EOC to the composite composites with pristine was nanotubessubstantially (MWCNT(Bayer)pristine enhanced by the nanotube/EOC), oxygenation. the respective When compositecompared to with the oxygenatedcomposite with nanotubes pristine achievednanotubes a 150%(MWCNT(Bayer)pristine/EOC), increase of thermoelectric power. the respective composite with oxygenated nanotubes achievedThe X-raya 150% photoelectron increase of thermoelectric spectroscopy power. (XPS) was performed on MWCNTs to ascertain the functionalThe X-ray groups photoelectron attached onto spectroscopy the nanotube (XPS) surfaces. was The performed oxygen contentson MWCNTs (%) of theto ascertain pristine andthe oxidizedfunctional MWCNT groups samples,attached asonto calculated the nanotube from the surfaces. XPS spectra, The oxygen are shown contents in Figure (%) 9of. The the comparisonpristine and ofoxidized results indicatedMWCNT thatsamples, all MWCNTs as calculated have C–O, from C= theO and XPS O–C spectra,=O groups are onshown their in surfaces Figure and 9. thatThe comparison of results indicated that all MWCNTs have C–O, C=O and O–C=O groups on their MWCNT(HNO3)s have the maximum percentage of all the oxygen-containing groups. Moreover, thesurfaces more and oxygenated that MWCNT(HNO functional groups3)s have at the surfacemaximum of thepercentage embedded of MWCNTs,all the oxygen-containing the higher the generatedgroups. Moreover, electric voltage the more and oxygenated the thermoelectric functional power groups of at the the EOC surface composite of the embedded as follows MWCNTs, from the resultsthe higher in Figure the generated9. According electric to the voltage published and resultsthe thermoelectric on carbon nanotube power of/ thermoplasticthe EOC composite polymer as composites,follows from positive the thermoelectricresults in Figure powers 9. have According always been to determinedthe published [39]. results on carbon nanotube/thermoplastic polymer composites, positive thermoelectric powers have always been determined [39]. Polymers 2020, 12, 1316 10 of 14 PolymersPolymers 2020 2020, 12, 12, x, xFOR FOR PEER PEER REVIEW REVIEW 1010 of of 14 14

FigureFigureFigure 8. 8.8. The The relations relationsrelations between betweenbetween generated generatedgenerated voltage voltagevoltage and andand temperature temperaturetemperature difference didifferenceﬀerence for forfor all allall investigated investigatedinvestigated EOCEOCEOC composites. composites.composites.

Figure 9. left panel FigureFigure 9. 9. Thermoelectric Thermoelectric powerpower power for for for tested tested tested EOC EOC EOC composites composit composit witheses with indicatedwith indicated indicated embedded embedded embedded fillers (fillers fillers (left (left). Oxygen content (%) on surfaces of pristine and differently oxygenated MWCNTs (right panel). panel).panel). Oxygen Oxygen content content (%) (%) on on surfaces surfaces of of pristine pristine and and differently differently oxygenated oxygenated MWCNTs MWCNTs (right (right panel). panel). TO denotes the total oxygen amount and the C–O, C=O, and O–C=O the amount of the particular TOTO denotes denotes the the total total oxygen oxygen amount amount and and the the C–O, C–O, C=O, C=O, and and O–C=O O–C=O the the amount amount of of the the particular particular functional groups on the surface of the respective MWCNTs. functionalfunctional groups groups on on the the surf surfaceace of of the the respective respective MWCNTs. MWCNTs. 3.5. Self-Powered Signaling Sensor of Temperature Change 3.5.3.5. Self-Powered Self-Powered Signaling Signaling Sensor Sensor of of Temperature Temperature Change Change To demonstrate a possible use of the conductive MWCNT/EOC composites as thermoelectrical To demonstrate a possible use of the conductive MWCNT/EOC composites as thermoelectrical materials,To demonstrate a self-powered a possible signaling use of the sensor conducti of temperatureve MWCNT/EOC change composites was assembled. as thermoelectrical The sensor materials, a self-powered signaling sensor of temperature change was assembled. The sensor materials,consisted ofa self-powered three conductive signaling strips sensor (thermoelements): of temperature one change MWCNT(Sun)pristine was assembled./EOC The and sensor two consistedconsisted ofof three three conductiveconductive stripsstrips (thermoe (thermoelements):lements): oneone MWCNT(Sun)pristine/EOC MWCNT(Sun)pristine/EOC and and twotwo MWCNT(Sun)KMnO4/EOC composites, Figure 10. The sticky strips were stuck on a PET foil so that the MWCNT(Sun)KMnO4 4/EOC composites, Figure 10. The sticky strips were stuck on a PET foil so that MWCNT(Sun)KMnOends overlapped at points/EOC A composites, and B. The temperatureFigure 10. The signal sticky was strips induced were bystuck a finger on a touchPET foil at pointso that A the ends overlapped at points A and B. The temperature signal was induced by a finger touch at point theor B,ends which overlapped in turn heated at points the A connection and B. The of temperat the strips.ure The signal generated was induced electricity by a wasfinger monitored touch at bypoint the A or B, which in turn heated the connection of the strips. The generated electricity was monitored by AMultiplex or B, which datalogger in turn heated 34980A. the Even connection a temperature of the strips. gradient The induced generated by aelectricity mere short was touch monitored of a finger by the Multiplex datalogger 34980A. Even a temperature gradient induced by a mere short touch of a thesuffi Multiplexced to elicit datalogger a detectable 34980A. signal. Even The a temperatur illustrativee record gradient of repeatedinduced by finger a mere touches short is touch presented of a finger sufficed to elicit a detectable signal. The illustrative record of repeated finger touches is fingerin Figure sufficed 11. Heat to elicit transfer a detectable from the finger signal. to The the hotillustrative junction record of the sensorof repeated modeled finger a technological touches is presented in Figure 11. Heat transfer from the finger to the hot junction of the sensor modeled a presentedsituation whenin Figure such 11. a sensorHeat transfer could monitor from the for finger example to the changes hot junction in the generationof the sensor of modeled waste heat, a technological situation when such a sensor could monitor for example changes in the generation of technologicalchanges of technological situation when temperature, such a sensor warming could of monitor packaged for grocery example products, changes etc.in the generation of wastewaste heat, heat, changes changes of of technological technological temperature, temperature, warming warming of of packaged packaged grocery grocery products, products, etc. etc. Polymers 2020, 12, 1316x FOR PEER REVIEW 1111 of of 14 Polymers 2020, 12, x FOR PEER REVIEW 11 of 14

Figure 10. The self-powered signaling sensor of temperature change assembled from FigureFigure 10.10.The The self-powered self-powered signaling signaling sensor sensor of temperature of temperature change assembled change fromassembled MWCNT(Sun) from MWCNT(Sun)pristine/EOC and MWCNT(Sn)KMnO4/EOC composites. MWCNT(Sun)pristine/EOCpristine/EOC and MWCNT(Sn)KMnO4 and MWCNT(Sn)KMnO4/EOC/EOC composites. composites.

Figure 11. The illustrative time-dependent record of induced voltage in the self-powered signaling Figure 11. The illustrative time-dependent record of induced voltage in the self-powered signaling sensor by short finger finger touching at the point A (the finger finger touch is denoted by open red circles and the sensor by short finger touching at the point A (the finger touch is denoted by open red circles and the fingerfinger lift by filled filled red circles) or at the point B (the finger finger touch is denoted by open blue triangles and finger lift by filled red circles) or at the point B (the finger touch is denoted by open blue triangles and the finger finger lift by filledfilled blue triangles) of thethe sensor.sensor. the finger lift by filled blue triangles) of the sensor. 3.6. Self-Powered Vapor Sensor 3.6. Self-Powered Vapor Sensor 3.6. Self-Powered Vapor Sensor The induced voltage in the self-powered sensor (Figure 10)10) was changed changed not not only only by by heating, The induced voltage in the self-powered sensor (Figure 10) was changed not only by heating, but also by ambient orga organicnic vapors, Figure 12.12. As in our paper [[14],14], the self-powered sensor was but also by ambient organic vapors, Figure 12. As in our paper [14], the self-powered sensor was exposed to to saturated saturated organic organic solvent solvent vapors. vapors. In particular, In particular, the sensor the sensor was placed was placed in a glass in a bell, glass which bell, exposed to saturated organic solvent vapors. In particular, the sensor was placed in a glass bell, which whichenclosed enclosed a layer aof layer liquid of organic liquid organicsolvent solventand its re andspective its respective saturated saturated vapor. After vapor. a chosen After time, a chosen the enclosed a layer of liquid organic solvent and its respective saturated vapor. After a chosen time, the sensortime, the was sensor removed was from removed the bell from and the the bell effect and of thevapor eff ectdesorption of vapor on desorption induced voltage on induced was assessed. voltage sensor was removed from the bell and the effect of vapor desorption on induced voltage was assessed. wasSuch assessed.measurements Such were measurements conducted werein saturated conducted vapors in at saturated atmospheric vapors pressure, at atmospheric temperature pressure, 22 °C, Such measurements were conducted in saturated vapors at atmospheric pressure, temperature 22 °C, andtemperature relative humidity 22 ◦C, and 60%. relative The humidity temperature 60%. gradient The temperature in the sensor gradient was ininduced the sensor by means was induced of the and relative humidity 60%. The temperature gradient in the sensor was induced by means of the resistiveby means heating of the resistiveof the MWCNT heating ofnetwork/epoxy the MWCNT co networkmposite/epoxy unit placed composite under unit the placed thermoelement under the resistive heating of the MWCNT network/epoxy composite unit placed under the thermoelement junction.thermoelement The heating junction. DC The voltage heating was DC 13 voltage V and the was current 13 V and 0.09 the A. current The temperature 0.09 A. The temperatureof the hot end of junction. The heating DC voltage was 13 V and the current 0.09 A. The temperature of the hot end wasthe hot40.6 end °C and was of 40.6 the◦C cold and end of theambient cold end room ambient temperature. room temperature. was 40.6 °C and of the cold end ambient room temperature. The variations of the voltage of the self-poweredself-powered sensor were caused by the changes of the The variations of the voltage of the self-powered sensor were caused by the changes of the electric resistance of the forming EOC composites through the chemical signals during the respective electric resistance of the forming EOC composites through the chemical signals during the respective adsorption/desorptionadsorption/desorption cycles as is illustrated in Fi Figuregure 99.. WhenWhen thethe sensorsensor waswas subjectedsubjected toto heptane,heptane, adsorption/desorption cycles as is illustrated in Figure 9. When the sensor was subjected to heptane, a larger increase of relative resistance resulted in a larger larger decrease decrease of of induced induced voltage voltage and and vice vice versa. versa. a larger increase of relative resistance resulted in a larger decrease of induced voltage and vice versa. Thus, the sensor output in terms of variation of voltage indicated a good response to the vapor Thus, the sensor output in terms of variation of voltage indicated a good response to the vapor occurrence. Moreover,Moreover, considering considering that that the the sensor sensor did notdid requirenot require a power a powe supply,r supply, but it self-produced but it self- occurrence. Moreover, considering that the sensor did not require a power supply, but it self- producedthermoelectricity thermoelectricity from a heat from source, a heat which source, could whic be wasteh could heat be ofwaste industrial heat of processes, industrial solar processes, energy, produced thermoelectricity from a heat source, which could be waste heat of industrial processes, solar energy, or body heat, etc., then the self-powered vapor sensor appears to be an advantageous solar energy, or body heat, etc., then the self-powered vapor sensor appears to be an advantageous Polymers 2020, 12, 1316 12 of 14

Polymers 2020, 12, x FOR PEER REVIEW 12 of 14 or body heat, etc., then the self-powered vapor sensor appears to be an advantageous alternative to alternativepowered vapor to powered sensors. vapor The illustrativesensors. The thermoelement illustrative thermoelement resistance changes resistance plotted changes in Figure plotted 10 were in Figurequantiﬁed 10 were by the quantified relative resistanceby the relative change, resistance change, − Rg Ra ΔR S = Rg Ra = ∆R S = R− = R Raa Raa wherewhere RRaa representedrepresented the the initial initial thermoelement thermoelement resistance resistance in in the the air, air, RRgg thethe resistance resistance when when the the thermoelementthermoelement was was exposed exposed to to a a vapor, vapor, and and Δ∆RR denoteddenoted the measured resistance change.

FigureFigure 12.12. TheThe time-dependent time-dependent relative relative resistance resistance change change of the MWCNT(Sun)pristineof the MWCNT(Sun)pristine/EOC/EOC composite

composite(in %) during (in %) adsorption during adsorption/desorption/desorption cycles of cycles exposition of exposition to saturated to saturated vapors vapors of heptane of heptane at 22 ◦atC, 22the °C, cycle the lengthcycle length 600 s. 600 The s. changes The changes of the of thermogenerated the thermogenerated voltage volt inducedage induced by the by self-powered the self-powered vapor

vaporsensor sensor in the in course the course of three of heptanethree heptane adsorption adsorption and desorption and desorption cycles cycles at 22 ◦ atC. 22 °C.

4.4. Discussion Discussion and Conclusions ThisThis study study presents presents hybrid hybrid thermoelectric thermoelectric comp compositesosites consisting consisting of of organic organic EOC EOC matrices matrices and and embeddedembedded conductive conductive inorganic inorganic pristine pristine and and functi functionalizedonalized multiwalled multiwalled carbon carbon nanotubes, nanotubes, carbon carbon nanofibersnanofibers or or conductive conductive organic organic PANI PANI and and PPy PPy particles. All All these these composites composites induced induced voltage voltage in in responseresponse to to a a temperature temperature difference difference across across the the resp respectiveective samples. samples. This This response response was was affected affected in in the the casecase of of the the composites composites with with inorganic inorganic MWCNT MWCNT nano nanostructuresstructures by by the the amount amount of of oxygen oxygen containing containing functionalfunctional groups groups attached attached to to their their surfaces. surfaces. The The more more oxygenated oxygenated were were the the functional functional groups groups at at the the surfacessurfaces of of the the embedded embedded MWCNTs, MWCNTs, the the higher higher was was the the thermoelectric thermoelectric power power of of that that EOC EOC composite. composite. AccordingAccording to to the the published published results results about about carbon carbon nanotube/thermoplastic nanotube/thermoplastic polymer polymer composites, composites, positivepositive thermoelectric thermoelectric powers powers have have always always been been determined determined indicating indicating pp-type-type composites, composites, in in which which holesholes are are dominant dominant current current carriers. carriers. The The composit compositeses with with embedded embedded organic organic PANI PANI a a PPy PPy particles particles induceinduce compensating compensating negative negative voltage. voltage. According According to to [38], [38], the the composites composites exhibit exhibit n-n-typetype conductivity conductivity specifiedspecified by by dominating dominating electrons electrons as as charge charge carriers. carriers. TheThe chosen chosen combination combination of of the the two two p-p-typetype EOC EOC composites composites in in the the experimental experimental thermoelectric thermoelectric microgenerator,microgenerator, which generatedgenerated susufficientfficient electric electric current current to to make make the the sensors sensors self-powered, self-powered, was was one oneof manyof many other other possible possible combinations. combinations. Other Other combinations combinations suchsuch asas p-p-typetype together with with n-n-typetype thermoelementsthermoelements or two n-n-typetype EOCEOC compositescomposites could could have have been been chosen chosen as as well. well. An An illustration illustration of theof thepractical practical use use of the of EOCthe EOC composites composites as a thermoelectric as a thermoelectric generator generator was provided was provided by the self-poweredby the self- poweredsignaling signaling sensor of sensor temperature of temperature change and change the self-powered and the self-powered vapor sensor. vapor The sensor. voltage The induced voltage in inducedthe sensors, in the which sensors, was which in the was range in of the microvolts, range of microvolts, did not approach did not byapproa far thech magnitudeby far the magnitude of induced ofvoltage induced in thevoltage classical in the metal classical thermocouples, metal thermocouples, yet it was su yetfficient it was to powersufficient the to novel power sensor the device,novel sensorwhich device, converted which chemical converted signals chemical to electric signals ones. to Inelectric the self-powered ones. In the self-powered vapor sensor, vapor the resistance sensor, thevaried resistance according varied to theaccording ambient to chemicalthe ambient signals chemical and changedsignals and the changed voltage the induced voltage by induced a source by of a(waste) source heat.of (waste) The EOCheat. thermoelectricThe EOC thermoelectric composites composites thus offer athus novel offer and a novel unique and set unique of properties, set of properties,which are notwhich readily are not available readily in available any other in material. any other material.

Polymers 2020, 12, 1316 13 of 14

Author Contributions: Data curation, R.O.; Investigation, P.S. and M.S.; Writing—original draft, P.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This work was supported by the Operational Program Research and Development for Innovations co-funded by the European Regional Development Fund (ERDF), the Operational Program Education for Competitiveness co-funded by the European Social Fund (ESF), the National Budget of the Czech Republic within the framework of the Centre of Polymer Systems project (reg. number: CZ.1.05/2.1.00/03.0111), the project Advanced Theoretical and Experimental Studies of Polymer Systems (reg. number: CZ.1.07/2.3.00/20.0104) and by the Fund of Institute of Hydrodynamics AV0Z20600510. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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