A Facile Chemical Synthesis of Pbte Nanostructures at Room Temperature

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Article A Facile Chemical Synthesis of PbTe Nanostructures at Room Temperature

Anil B. Gite 1,2 , Balasaheb M. Palve 1, Vishwasrao B. Gaikwad 3, Gotan H. Jain 2,*, Habib M. Pathan 1 , Samir Haj Bloukh 4 and Zehra Edis 5,*

1 Advance Physics Laboratory, Department of Physics, Savitribai Phule Pune University, Pune 411 007, India; [email protected] (A.B.G.); [email protected] (B.M.P.); [email protected] (H.M.P.) 2 SNJB’sArts, Commerce Science College, Chandwad, Nashik 423 101, India 3 Department of Chemistry, KTHM College, Nashik 422 002, Maharashtra, India; dr.gaikwadvb@rediﬀmail.com 4 Department of Clinical Sciences, College of Pharmacy and Health Science, Ajman University, Ajman PO Box 346, UAE; [email protected] 5 Department of Pharmaceutical Sciences, College of Pharmacy and Health Science, Ajman University, Ajman PO Box 346, UAE * Correspondence: gotanjain@rediﬀmail.com (G.H.J.); [email protected] (Z.E.); Tel.: +971-56-694-7751 (Z.E.)

Received: 12 August 2020; Accepted: 23 September 2020; Published: 25 September 2020

Abstract: Thermoelectric (TE) materials are possible solutions of the current problems in the energy sector to overcome environmental pollution, increasing energy demand and the decline of natural resources. Thermoelectric materials are a promising alternative for the conversion of waste heat to electricity. Nanocrystalline PbTe powder was synthesized by a simple chemical method at room temperature and systematically investigated at various durations as samples A1–A5. Fourier Transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), microstructural analysis by scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) confirmed the composition of the samples. TE parameters as thermo-emf of samples A1–A5 and electrical conductivity were measured. The cyclic voltammetry gives a band gap of 0.25 eV, which is in agreement with the optical band gap of the material. The A4 sample has an average crystal size of 36 nm with preferred orientation in (200) verifying the cubic morphology. The obtained TE parameters are beneficial for the non-uniform TE materials which might be due to strong current boundary scattering and extremely low thermal conductivity of the samples.

Keywords: lead telluride; PbTe; thermoelectric materials; chemical synthesis; nanostructures; cyclic voltammetry; electrical conductivity

1. Introduction Thermoelectric materials can convert heat to electricity [1]. These materials are environment friendly and mostly used for the development of sustainable energy materials. It has been reported that the narrow band material like PbTe has shown superior thermoelectric (TE) properties and is currently used in different applications [2,3]. These materials have prospective uses in thermal power generation and thermal sensing equipments. Diverse theoretical calculations and experimental data point to the upgrading in TE properties, which can be achieved by reducing the dimensionality of TE materials to a certain lowest amount level [2,4–8]. The dimensionless parameter figure of merit, which determines the efficiency TE material is ZT = (S2σT)/K, where S is Seebeck coefficient, σ is electrical conductivity, T is working temperature in Kelvin, and K is thermal conductivity of TE material [9]. A good TE material simultaneously demands large Seebeck coefficient (S), high electrical conductivity (σ), and low

Nanomaterials 2020, 10, 1915; doi:10.3390/nano10101915 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1915 2 of 15 thermal conductivity (K). The thermal conductivity is due to the electron motion (Ke) and lattice vibration produced within the material (Kl). This complex relationship between these TE parameters makes it difficult to enhance the ZT value of the TE materials. For that reason many efforts are aimed to improve the ZT value by balancing these interdependent TE parameters. Reducing the thermal conductivity via nanostructuring plays an important role for the improvement of the ZT of the material. For high ZT, high Seebeck coefficient is needed, but due to the complicated electronic band structures, superior electrical conductivity from high-symmetry cubic crystal structures, and low thermal conductivity caused by strong anharmonicity in TE materials, also due to the Pb local off-center, plays an important role in variations of TE parameters [10–12]. The quantum confinements will produce significant effects on charge and phonon transports in thermoelectric materials. Quantum confinement deals special way to manipulate carrier transports due to close connection between electronic band structure and dimensionality of the materials [13]. The Seebeck coefficient increased because of larger effective mass due to the band structure. PbTe material, indicating that reducing dimensionality of a given material is a potential possibility to enhance thermoelectric performance [14]. Studies of the film show a great interest in high performance and low dimensional TE materials since these materials perform better as compared to bulk materials [15]. PbTe is a great material for optoelectronic and in mid-infra-red ranges, also widely used in a large number of various TE devices [16]. The PbTe material also useful for the energy harvesting power generation [17,18]. Many thermoelectric materials are being explored for power generation applications, such as GeTe [19], PbTe [20,21], half-Heusler [22,23], and skutterudites [24,25]. Nanostructured materials that appear to be the most promising from a commercial point of view by virtue of their excellent thermoelectric performance and their high efficiency. The study thermoelectric nanomaterials are explored due to various approaches such as narrow bandgaps, heavy elements doping, point defects loading, and nanostructuring [26]. PbTe thin films were synthesized by various methods and have been utilized for research of PbTe material [27–34]. Wang et al. reported the simplistic chemical synthesis of PbTe on the glass substrate material at normal atmospheric conditions with temperatures around 300 K [35]. Chemical synthesis is one of the easiest methods for the synthesis of the material since it is a low-cost technique, which does not require any kind of sophisticated equipment, as well as specific kind of substrate materials. In the lead chalcogenide synthesis, PbX (PbX, X = Te, S, Se) material films are synthesized by chemical technique [36–38]. Seleno-sulfate (Na2SeSO3) is used widely as Se source by various groups for the synthesis of PbSe on glass substrate material [39–41]. Thiourea SC(NH2)2 or sodium thiosulfate (Na2S2O3xH2O) as sources of S are used for the synthesis of PbS on glass and silicon substrate materials [42–44]. Unfortunately, the corresponding Te source is difficult to obtain due to the scarce solubility of Te. Precursors like TeO2, Na2TeO3, and Na2TeSO3 are available, but unstable under natural conditions and therefore very rarely studies on chemical synthesis of PbTe are reported. In comparison to S or Se, the dis-proportionating reactions and hydrolysis of Te in different pH solutions are much more difficult [45]. Further problems are the cost effectiveness, a maximum ZTmax, which is greater than 2.4 by the band convergence, and strain is produced in the lattice of PbTe [46]. Scarce and expensive material needed for the synthesis strain the opportunities to produce useful solutions for future energy problems. Recently, PbSe and PbS gained further attention due to their lower cost and higher operation temperature compared to Te [47,48]. However, the chemical synthesis of the PbTe material is easy and not frequently used due to the problem of hydrolysis of Te when compared to hydrothermal and solvothermal techniques, as well as solution phase synthesis and electro-deposition techniques [16,45,47,49–55]. In this study, we synthesized of PbTe powder by utilizing lead nitrate Pb(NO3)2 and tellurium oxide TeO2 as the precursors for Pb and Te from an alkaline aqueous solution bath. Lead telluride powder was prepared by a chemical method. High yield of PbTe for the various durations and formation mechanism of the PbTe powder and pellet was proposed and its TE properties of the film Nanomaterials 2020, 10, 1915 3 of 15 Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 17

were measured for the better thermometric materials. We investigated in this work the synthesis of lead successfully obtained at high yield. XRD reveals cubic nanocrystals of PbTe. The synthesized PbTe telluride powder by a chemical route technique. Silver-gray metallic powder of PbTe was successfully powder reveals homogeneous grains and agglomeration. The obtained values of the electrochemical obtained at high yield. XRD reveals cubic nanocrystals of PbTe. The synthesized PbTe powder reveals band gap from the Cyclic Voltammetry are in agreement with the optical band gap. The formation of homogeneous grains and agglomeration. The obtained values of the electrochemical band gap from PbTe by chemical synthesis shall be a promising material that can be used as thermoelectric the Cyclic Voltammetry are in agreement with the optical band gap. The formation of PbTe by chemical applications. Among the various synthesis techniques employed for the formation of PbTe synthesis shall be a promising material that can be used as thermoelectric applications. Among the nanostructures, chemical synthesis process has attracted much interest due to the advantage of high various synthesis techniques employed for the formation of PbTe nanostructures, chemical synthesis yield, low synthesis temperature, high purity, and high crystallinity. process has attracted much interest due to the advantage of high yield, low synthesis temperature, 2.high Materials purity, and highMethods crystallinity. 2. Materials and Methods 2.1. Materials

2.1. MaterialsLead (II), telluride (TeO2, 99.998% trace metals basis), potassium hydroxide (KOH), trisodium-citrate (TSC) and potassium borohydride (KBH4) were purchased from Sigma Aldrich (St. Lead (II), telluride (TeO2, 99.998% trace metals basis), potassium hydroxide (KOH), Louis, MO, USA). Lead Nitrate (Pb(NO3)2, 99% pure) was obtained from Fisher Scientific trisodium-citrate (TSC) and potassium borohydride (KBH4) were purchased from Sigma Aldrich (Pittsburgh, PA, USA). All the materials were pure and used as received. Additional purification (St. Louis, MO, USA). Lead Nitrate (Pb(NO3)2, 99% pure) was obtained from Fisher Scientiﬁc was(Pittsburgh, not done, PA, when USA). analytical All the materialsgrade precursors were pure were and utilized. used as Double received. distilled Additional deionized puriﬁcation water was was used.not done, when analytical grade precursors were utilized. Double distilled deionized water was used.

2.2.2.2. Preparation Preparation of of Samples Samples

InIn a a typical typical run, run, 0.1 0.1 M MPb(NO Pb(NO3)2, 30.1)2, M 0.1 TeO M2 TeO, 2 M2, KOH, 2 M KOH,0.2 M tri-sodium 0.2 M tri-sodium citrate (TSC), citrate and (TSC), 0.8 Mand KBH 0.8 M4 were KBH 4dissolvedwere dissolved in sequence in sequence in 50 in mL 50 mLdeionized deionized water. water. Initially, Initially, a aclean, clean, colorless, colorless, transparenttransparent solution solution was was formed. formed. As As time time progress progressed,ed, the the color color of of the the solution solution became became dark dark gray gray andand precipitation precipitation of of dark dark gray gray material material collected collected in in the the beaker beaker for for different different durations durations as as 72, 72, 144, 144, 216, 216, 288,288, and and 360 360 h. h. TheThe solutions solutions werewere then then centrifuged centrifuged for for 10 10 min min at at 2000 2000 rpm rpm in in a a centrifuge centrifuge machine. machine. AfterAfter centrifugation, centrifugation, the the samples samples were were collected collected in in dry dry crucibles. crucibles. Finally, Finally, the the sample sample was was dried dried in ina vacuuma vacuum at ata temperature a temperature of of 500 500 °C◦ Cfor for 30 30 min. min. After After the the annealing, annealing, the the collected collected grains grains were were crushed crushed toto fine fine powder powder and and the the powder powder samples samples of of different different time time durations durations were were labeled labeled as as A1, A1, A2, A2, A3, A3, A4, A4, andand A5. A5. The The samples were collectedcollected andand analyzed analyzed for for di differentfferent characterization characterization techniques techniques and and the the TE TEproperties properties of the of samplesthe samples were were studied. studied. Figure Figure1 represents 1 represents the step-by-step the step-by-step procedure procedure for the chemical for the chemicalbath deposition bath deposition methodused method for theused synthesis for the synthesis of PbTe. of PbTe.

FigureFigure 1. 1. StepStep by by step step procedure procedure for for the the synthesis synthesis of of PbTe PbTe powder. powder.

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2.3. Characterization of Samples 2.3. Characterization of Samples The samples A1–A5 were characterized by SEM/EDS, x-ray diffraction (XRD), FTIR, and cyclic voltammetryThe samples (CV). A1–A5 These were methods characterized confirmed by the SEM composition/EDS, x-ray of di ourffraction samples. (XRD), FTIR, and cyclic voltammetryThe PbTe (CV). structure These was methods obtained confirmed by using the the composition XRD–D8 Advance of our samples. (Bruker, Karlsruhe, Germany) withThe Cu-K PbTeα line structure wavelength was obtained 1.54 Å. by The using morphologica the XRD–D8l study Advance of the (Bruker, synthesized Karlsruhe, material Germany) under withhigher Cu-K magnificationα line wavelength was done 1.54 Å.by The SEM morphological from JEOL (JSM-6400, study of the Tokyo, synthesized Japan) material with an under accelerating higher magnificationvoltage of 20 was kV done of A1 by SEMand fromA5. JEOLThe energy (JSM-6400, dispersive Tokyo, Japan)X-ray withspectroscopy an accelerating is used voltage for the of 20compositional kV of A1 and analysis A5. The energyof the sample dispersive A1 X-rayand A5. spectroscopy For the FTIR is used analysis, for the JASCO compositional FT/IR 6100 analysis (Tokyo, of theJapan), sample with A1 a and range A5. from For the 400 FTIR to 4000 analysis, cm−1 JASCOwas used FT /IRto 6100find the (Tokyo, presence Japan), of withvarious a range modes from in 400 the 1 tomaterial. 4000 cm −Cyclicwas voltammetry used to find the (CV) presence measurements of various we modesre carried in the out material. by K-Lyte Cyclic 1.2 voltammetry with the research (CV) measurementsapplications of were K-Lyte carried hardware out by K-Lyte(K-Lyte 1.2 1.2, with from the Knopy research Techno applications Solutions, of K-Lyte Kanpur, hardware India). (K-LyteSeebeck 1.2,coefficient from Knopy measurements Techno Solutions, were done Kanpur, by a India). TEP unit Seebeck TYPE-2, coeffi purchasedcient measurements from Borade were Embedded done by aSolutions, TEP unit TYPE-2,Kolhapur, purchased India. Finally, from for Borade each Embeddedsample, the Solutions,pellets with Kolhapur, dimensions India. 12 mmFinally, × 3 mm for eachwere sample, the pellets with dimensions 12 mm 3 mm were prepared by a hydraulic press machine to prepared by a hydraulic press machine to find× the thermo-emf of the synthesized material. find the thermo-emf of the synthesized material. 3. Results and Discussion 3. Results and Discussion 3.1. Characterization of Samples 3.1. Characterization of Samples Qualitative analysis of the obtained yield was carried out by plotting the reaction time against Qualitative analysis of the obtained yield was carried out by plotting the reaction time against the the mass of the product (Figure 2). mass of the product (Figure2).

Figure 2. Yield of collected samples A1, A2, A3, A4, and A5 against the number of hours. Figure 2. Yield of collected samples A1, A2, A3, A4, and A5 against the number of hours. 2 2 Lead nitrate and tellurium oxide were dissolved in excess alkali and formed HPbO − and TeO3 − Lead nitrate and tellurium oxide were dissolved in excess alkali and formed HPbO2− and TeO32− ions during the reaction time. These ions further precipitate at the bottom. As time progressed, the rate ions during the reaction time. These ions further precipitate at the bottom. As time progressed, the of reaction and formation of PbTe may have been enhanced. This can be evident from the maximum rate of reaction and formation of PbTe may have been enhanced. This can be evident from the yield in the case of sample A4. However, a further increase in the reaction time may have led to the maximum yield in the case of sample A4. However, a further increase in the reaction time may have overgrowth and thus, resulted in the deteriorated yield in the case of sample A5. led to the overgrowth and thus, resulted in the deteriorated yield in the case of sample A5. 3.2. Cyclic Voltammetry (CV) Studies 3.2. Cyclic Voltammetry (CV) Studies CV studies revealed the appropriate potential ranges of the ions in the given electrolyte solutions. The curvesCV studies give the revealed scanning the of appropriate the electrolyte potential in the cathodicranges of direction the ions and in the negativegiven electrolyte current producedsolutions. is The called curves the cathodicgive the currentsscanning (Figure of the 3elec). trolyte in the cathodic direction and the negative current produced is called the cathodic currents (Figure 3).

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Figure 3. Cyclic Voltammogram of 0.1 M Pb(NO3)2 at a scan rate of 50 mV/s versus Pt tip working Figure 3. Cyclic Voltammogram of 0.1 M Pb(NO33))22 atat aa scanscan raterate ofof 5050 mV/smV/s versusversus PtPt tiptip workingworking electrode and Ag/AgCl as a reference electrode. electrode and Ag/AgCl as a reference electrode. Figure3 shows the deposition on the working electrode at a potential around 0.87 V versus Figure 3 shows the deposition on the working electrode at a potential around− −0.87 V versus Ag/AgCl,Figure in the3 shows cathodic the scan.deposition The potential on the wo becomesrking electrode more negative at a potential to the anodic around scan −0.87 and V a strongversus Ag/AgCl, in the cathodic scan. The potential becomes more negative to the anodic scan and a strong oxidationAg/AgCl, peakin the was cathodic observed scan. at The0.11 potential V. The precursorbecomes more solution negative of lead to nitrate the anodic in water scan as: and a strong oxidation peak was observed at− −0.11 V. The precursor solution of lead nitrate in water as: 2+ Pb(NOPb(NO3)2(s)3)2(s)Pb Pbaqaq2+2+ ++ 2 NO3−−aqaq, ,(1) (1) Pb(NO3)2(s)→ Pbaq + 2 NO3 aq, (1) with Pbaq22+2++ ion has standard reduction potential (E000) of—0.125 V and NO3−−aq ion in acidic conditions withwith Pb Pbaqaq ionion has standard reductionreduction potentialpotential (E (E)) of—0.125of—0.125 V V and and NO NO3−3 aqaq ion inin acidicacidic conditionsconditions hashas EE000 ofofof ++0.956+0.9560.956 VVV [[43,56].[43,56].43,56]. TheThe hysteresishystereshysteresisis waswaswas observedobservedobserved betweenbetweenbetween thethethe potentialpotentialpotential −0.870.87 VV toto ++0.900.90 VV indicating that reduction of Pbaq22+2++ occurs more rapidly on the Pt tip working electrode.− The potential indicatingindicating that that reduction reduction of of Pb Pbaqaq occurs more rapidly on thethe PtPt tiptip workingworking electrode.electrode. TheThe potentialpotential waswas negatively negatively shifted shifted versus versus Ag Ag/AgCl/AgCl and and the the reduction reduction peak peak revealed revealed the reduction the reduction of Te as of shown Te as inshown Figure in4 .Figure 4.

Figure 4. Cyclic Voltammogram of 0.1 mM HTeO2+ at scan rate of 50 mV/s versus Pt tip working 2+ electrodeFigure 4. and Cyclic Ag /VoltammogramAgCl as a reference of 0.1 electrode. mM HTeO 2+ atat scanscan raterate ofof 5050 mV/smV/s versusversus PtPt tiptip workingworking electrode and Ag/AgCl as a reference electrode. The reduction of Te can be represented as: The reduction of Te can be represented as: + HTeO2− +− 3H + 4e−− Teads + 2H2O, (2) HTeO22− ++ 3H3H+ ++ 4e4e−→→ TeTeadsads ++ 2H2H22O, (2) wherewhere Te Teadsadsadsindicates indicatesindicates that thatthat Tellurium TelluriumTellurium atoms atomsatoms are absorbed areare absorbabsorb in aned electrolytein an electrolyte solution. solution. When the When potential the reachespotential 0.17reaches V, another −0.17 V, reduction another reduction wave started, wave which started, is attributed which is attributed to the production to the production of H Te [41 of]. potential− reaches −0.17 V, another reduction wave started, which is attributed to the production2 of H22Te [41]. Each of the reduction peaks were some sort of limited deposition. This behavior involves

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Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 17 Each of the reduction peaks were some sort of limited deposition. This behavior involves the slow reductionthe slow reduction of HTeO2 +of. HTeO Cyclic2+ Voltammetry. Cyclic Voltammetry shows the shows presence the presence of semiconductor of semiconductor material material that has athat discrete has aand discrete ﬁxed and energy fixed level. energy The level. correlation The correlation between thebetween optical the and optical electrochemical and electrochemical band gap wasband reported gap was the reported ﬁrst time the by first Haram time etby al. Haram [56]. Figure et al. 5[56]. gives Figure the electrochemical 5 gives the electrochemical band gap and band it is calculatedgap and it fromis calculated the peak from values. the peak values.

Figure 5. Cyclic Voltammetry of all the solutions when they are mixed together. Pt is used as working electrode,Figure 5. PtCyclic mesh Voltammetry as a counter electrode of all the and solutions Ag/AgCl when as reference they are electrode mixed for together. sweeprate Pt ofis 50used mV as/s fromworking1.0 electrode, V to +1.0 VPt atmesh the currentas a counter limit ofelectrode 1 mA by and using Ag/AgCl K-Lyte as 1.2 reference Potentiostat. electrode Cyclic for Voltagram sweep rate of − 0.1of 50 M mV/s Pb(NO from3)2, 0.1−1.0 M V TeO to +1.02, 2 MV KOH,at the current 0.2 M trisodium limit of 1 citrate mA by (TSC), using and K-Lyte 0.8 M 1.2 KBH Potentiostat.4 were dissolved Cyclic inVoltagram sequence of in 0.1 50 M mL Pb(NO double3)2 distilled, 0.1 M TeO deionized2, 2 M KOH, water. 0.2 M trisodium citrate (TSC), and 0.8 M KBH4 were dissolved in sequence in 50 mL double distilled deionized water. The obtained values are very well in agreement with an optical band gap: The obtained values are very well in agreement with an optical band gap: ∆E = Eox Ered (eV), (3) ΔE = Eox− − Ered (eV), (3)

Values from the cyclic voltammetry curve Eox = 0.59 V and E = 0.84 V give ∆E = 0.25 eV. Values from the cyclic voltammetry curve Eox = −0.59− V and Ered red= −0.84− V give ΔE = 0.25 eV. The Theband band gap gapof the of PbTe the PbTe is well is well matched matched with with the observed the observed band band gap gapfor the for thePbTe PbTe material. material.

3.3. Structural Analysis 3.3. Structural Analysis The average crystallite size was calculated from XRD data which is based on Debye Scherrer’s The average crystallite size was calculated from XRD data which is based on Debye Scherrer’s formula [57]: formula [57]: D = 0.9λ/βcosθ, (4) D = 0.9λ/βcosθ, (4) where D = average crystallite size, β = broadening of the diﬀraction line measured at half maximum intensitywhere D (FWHM),= averageλ crystallite= wavelength size, ofβ = X-ray broadening radiation, of andthe diffractionθ = Bragg’s line angle. measured The calculated at half maximum d-spacing andintensity crystallite (FWHM), sizes correspondingλ = wavelength to variousof X-ray crystal radiation, planes and are presentedθ = Bragg’s inTable angle.1. FigureThe calculated6 shows d-spacing and crystallite sizes corresponding to various crystal planes are presented in Table 1. the typical XRD pattern for samples A3, A4, and A5 annealed after 500 ◦C. Figure 6 shows the typical XRD pattern for samples A3, A4, and A5 annealed after 500 °C.

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Table 1. X-ray diﬀraction (XRD) analysis of the PbTe powder for sample A4.

(h k l) 2 (Degree) d Spacing (Å) FWHM ( 10 3) Crystallite Size (nm) × − (1 1 0) 24.15 3.72 2.004 42.37 (2 0 0) 27.94 3.22 2.004 42.70 (2 2 0) 39.91 2.28 4.004 22.04 (3 1 1) 49.10 1.86 2.004 45.55 (2 2 1) 57.30 1.61 4.008 23.61 (4 2 0) 64.60 1.44 2.004 49.02 Nanomaterials(4 22020 2), 10, x FOR PEER 72.04 REVIEW 1.31 4.008 25.62 7 of 17

50,000 (2 0 0) A5 (2 2 0) A4 A3 40,000 (3 1 1) (4 2 0) (1 0 0) (2 2 1) (4 2 2) A3

30,000

20,000 A4 Intensity (a.u.)

10,000

A5 0 20 30 40 50 60 70 80 2theta Figure 6. X-ray diffraction (XRD) analysis of samples A3, A4, and A5. Figure 6. X-ray diffraction (XRD) analysis of samples A3, A4, and A5. All the diffraction signatures along the (h k l) plane can be attributed to the PbTe crystals. All the diffraction signatures along the (h k l) plane can be attributed to the PbTe crystals. The The powder samples A3, A4, and A5 give the well-matched peaks of their XRD patterns. The peak powder samples A3, A4, and A5 give the well-matched peaks of their XRD patterns. The peak corresponding to (2 0 0) and (2 2 0) are the most prominent peaks as per the JCPDS file number corresponding to (2 0 0) and (2 2 0) are the most prominent peaks as per the JCPDS file number 077-02460. Further strong intense peaks and other weak peaks are also indexed in the XRD pattern 077-02460. Further strong intense peaks and other weak peaks are also indexed in the XRD pattern matching with the corresponding JCPDS file. The observed peaks were well matched with the XRD of matching with the corresponding JCPDS file. The observed peaks were well matched with the XRD undoped PbTe synthesized by solvothermal/hydrothermal process [58]. With the XRD of maximum of undoped PbTe synthesized by solvothermal/hydrothermal process [58]. With the XRD of yield sample A4 shows the formation of PbTe and reveals an average crystallite size of 36 nm for the maximum yield sample A4 shows the formation of PbTe and reveals an average crystallite size of 36 A4 sample (Table1). nm for the A4 sample (Table 1). The intensity and FWHM of XRD along (2 0 0) matches with the standard XRD pattern and thus, can be attributed to the prominent peak indicating the formation of the PbTe crystal in all the samples. Table 1. X-ray diffraction (XRD) analysis of the PbTe powder for sample A4. This suggests that few crystals have followed the preferred orientation along (2 0 0). This may be evident(h from k l) the 2independent (Degree) small d Spacing cubic (Å) morphologies FWHM shown (× 10−3) in scanningCrystallite electron Size (nm) micrographs. However,(1 the1 0) XRD reveals 24.15 additional weak 3.72 orientations along 2.004 (4 2 0) and (4 2 2) planes, 42.37 which may have led to the(2 agglomeration 0 0) 27.94 of the cubes in 3.22 the bunch like morphologies. 2.004 The dislocation 42.70 density is defined as the length(2 2 0) of dislocations 39.91 lines per 2.28unit volume of crystal 4.004 and calculated from 22.04 the formula [59] as δ = 1/D2(3where 1 1) D is the 49.10 crystal size and 1.86 the micro-strain ε 2.004is given by ε = βcosθ/4. 45.55 For the synthesized material(2 the 2 1) calculated 57.30 values for dislocation 1.61 density are 4.008 7.74 1014 and micro-strain 23.61 produced in the × synthesized(4 2 0) material 64.60 is 2.41 10 3. 1.44 2.004 49.02 × − (4 2 2) 72.04 1.31 4.008 25.62

The intensity and FWHM of XRD along (2 0 0) matches with the standard XRD pattern and thus, can be attributed to the prominent peak indicating the formation of the PbTe crystal in all the samples. This suggests that few crystals have followed the preferred orientation along (2 0 0). This may be evident from the independent small cubic morphologies shown in scanning electron micrographs. However, the XRD reveals additional weak orientations along (4 2 0) and (4 2 2) planes, which may have led to the agglomeration of the cubes in the bunch like morphologies. The dislocation density is defined as the length of dislocations lines per unit volume of crystal and calculated from the formula [59] as δ = 1/D2 where D is the crystal size and the micro-strain ε is given by ε = βcosθ/4. For the synthesized material the calculated values for dislocation density are 7.74 × 1014 and micro-strain produced in the synthesized material is 2.41 × 10−3.

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3.4. Scanning ElectronNanomaterials Microscope 2020, 10, x FOR PEER (SEM) REVIEW and Elemental Studies 8 of 17 Microstructural3.4. Scanning analysis Electron by Microscope SEM/ EDS(SEM) wasand Elemental utilized Studies for the morphological analysis of the samples, their sizes, and theirMicrostructural composition. analysis Figure by SEM/EDS7 gives was typical utilized SEM for imagesthe morphological of sample analysis A1 of and the A5 obtained at samples, their sizes, and their composition. Figure 7 gives typical SEM images of sample A1 and A5 diﬀerent magniﬁcations.obtained at different magnifications.

Figure 7. ScanningFigure 7. Electron Scanning Electron Microscope Microscope (SEM) (SEM) images of the of sample the sample at various at magnifications various magniﬁcations of of sample preparation.sample preparation. SEM reveals the formation of bunches of independent cubes. This may be attributed to the SEM revealsenhanced the rate formation of reaction as of a bunchesfunction of reaction of independent time. Agglomerat cubes.ions of PbTe This particles may take be place attributed to the enhanced rate ofas reactionthe time progress as a functionas seen in the of SEM reaction images of time. sample Agglomerations A5. Elemental spectra of of PbTeboth the particles sample take place as A1 and A5 reveal the strong peaks of Pb and Te as shown in Figure 8. the time progress as seen in the SEM images of sample A5. Elemental spectra of both the sample A1 and A5 revealNanomaterials the strong 2020, 10 peaks, x FOR PEER of PbREVIEW and Te as shown in Figure8. 9 of 17

(a)

(b)

Figure 8. EnergyFigure 8. dispersive Energy dispersive spectroscopy spectroscopy (EDS) (EDS) and co correspondingrresponding scanning scanning electron microscopy electron microscopy (SEM) of two samples. From up to down: (a) A1; (b) A5. (SEM) of two samples. From up to down: (a) A1; (b) A5. Besides, the observed additional peaks may be attributed to the physical adsorption of oxygen in the sample in air. Table 2 shows the composition of the A1 and A5 samples.

Table 2. Elemental content of samples A1 and A5.

Sample Element Te (L) Pb (M) Total A1 Weight % 7.82 15.25 23.06 Atomic % 45.43 54.57 100 A5 Weight % 30.76 16.69 46.47 Atomic % 53.15 46.85 100

At shorter durations in the sample A1, the amount of Te was less than the amount of Pb with 45.43% and 54.57%, respectively. With increasing duration in sample A5, this changed to the opposite with increasing atomic percent of Te (53.15%) compared to Pb (46.85%), according to Table 2. The stoichiometric composition with atomic percentages should be in samples A1 and A5.

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Besides, the observed additional peaks may be attributed to the physical adsorption of oxygen in the sample in air. Table2 shows the composition of the A1 and A5 samples.

Table 2. Elemental content of samples A1 and A5.

Sample Element Te (L) Pb (M) Total A1 Weight % 7.82 15.25 23.06 Atomic % 45.43 54.57 100 A5 Weight % 30.76 16.69 46.47 Atomic % 53.15 46.85 100

At shorter durations in the sample A1, the amount of Te was less than the amount of Pb with 45.43% and 54.57%, respectively. With increasing duration in sample A5, this changed to the opposite with increasing atomic percent of Te (53.15%) compared to Pb (46.85%), according to Table2. The stoichiometric composition with atomic percentages should be in samples A1 and A5.

3.5. Fourier-Transform Infrared Spectroscopy (FTIR) The optical characterization of prepared samples was performed with FTIR transmission (%T) measurements. FTIR analysis of the synthesized PbTe samples confirmed their composition and indicates the presence of various modes of PbTe in samples A1 and A5 (Figure9). The organic or inorganic nature of the samples can be identified by using this technique for sample A1. The bands at 1 1 1 1 1409 cm− , 1562 cm− , 2885 cm− , and 2813 cm− belong to the –CH2 outplane swinging, asymmetric 1 1 (COO–) and symmetric –CH2 stretches in sample A1 (Figure9). The bands at 2961 cm − and 2813 cm− are due to the –C–H stretching and anti-stretching vibrations of the –CH2 group, respectively, for the sample A1.Nanomaterials This confirms 2020, 10, x FOR the PEER presence REVIEW of stretching and bending modes of PbTe in the 10 of synthesized 17 1 1 films. The bands around 1107 cm− to 1409 cm− might originate from –C–N stretching, –CH2 scissoring 1 mode of vibration, and –OH bending mode. The bands between 781–907 cm− reveal vibrations of ternary amines with –CH2 rocking modes [54,60,61].

Figure 9.FigureFourier-transform-infrared 9. Fourier-transform-infrared (FTIR) (FTIR) spectrometricspectrometric analysis analysis of samples of samples A1 and A5. A1 and A5.

−1 −1 FTIR spectraFTIR of spectra the sample of the sample A5 show A5 show that that the the strong strong absorption absorption bandsbands at at 1572 1572 cm cm, 12701, 1270 cm , cm 1, and −1 − − 1 and 1385 cm are attributed to stretching vibrations of the carboxylate groups in the film. Various 1385 cm− bandsare attributed at 2980 cm− to1, 2882 stretching cm−1, and vibrations 2776 cm−1 are of thedue carboxylateto –CH stretching groups vibrations in the present ﬁlm. in Various the bands 1 1 1 at 2980 cmstructure− , 2882 of cm the− PbTe, and [60]. 2776 The cmFTIR− spectrumare due ofto the –CH PbTe stretchingsample prepared vibrations by chemical present precipitation in the structure at room temperature for 288 h of sample A5, indicates the presence of the bond of Pb–Te. The FTIR spectrum does not show strong bands associated with Pb–Se stretching, and bending vibrations, nor –CH2 Scissoring mode vibration, –C–N stretching, and –OH bending mode [61]. The vibrations of ternary amines are available at 842 to 943 cm−1 and the –CH2 rocking modes are observed between the 1270–1572 cm−1. FTIR transmittance bands are summarized in Table 3. The FTIR spectra of sample well matched with those reported earlier [54,60–63].

Table 3. Fourier-transform-infrared (FTIR) spectrometric analysis of PbTe samples of sample A5.

X Y X Y Peak No. Peak No. (cm−1) (%T) (cm−1) (%T) 1 2980.04 52.10 6 1270.56 58.49 2 2882.32 56.13 7 1152.32 62.51 3 2776.28 56.17 8 943.53 62.38 4 1572.67 58.11 9 908.60 63.56 5 1385.47 50.94 10 842.71 61.07

3.6. Thermo-Electromotive Force Measurements The thermo-electrical properties were studied in previous investigations [63]. The Seebeck coefficient of PbTe was measured by mounting the sample on two metal blocks, which enabled the generation of a temperature gradient, as reported in the previous studies [64]. For all measurements,

Nanomaterials 2020, 10, 1915 10 of 15 of the PbTe [60]. The FTIR spectrum of the PbTe sample prepared by chemical precipitation at room temperature for 288 h of sample A5, indicates the presence of the bond of Pb–Te. The FTIR spectrum does not show strong bands associated with Pb–Se stretching, and bending vibrations, nor –CH2 Scissoring mode vibration, –C–N stretching, and –OH bending mode [61]. The vibrations of ternary 1 amines are available at 842 to 943 cm− and the –CH2 rocking modes are observed between the 1 1270–1572 cm− . FTIR transmittance bands are summarized in Table3. The FTIR spectra of sample well matched with those reported earlier [54,60–63].

Table 3. Fourier-transform-infrared (FTIR) spectrometric analysis of PbTe samples of sample A5.

X Y X Y Peak No. 1 Peak No. 1 (cm− ) (%T) (cm− ) (%T) 1 2980.04 52.10 6 1270.56 58.49 2 2882.32 56.13 7 1152.32 62.51 3 2776.28 56.17 8 943.53 62.38 4 1572.67 58.11 9 908.60 63.56 5 1385.47 50.94 10 842.71 61.07

3.6. Thermo-Electromotive Force Measurements The thermo-electrical properties were studied in previous investigations [63]. The Seebeck coefficientNanomaterials of PbTe was2020, 10 measured, x FOR PEER REVIEW by mounting the sample on two metal blocks, which 11 ofenabled 17 the generation of aThe temperature thermo-electrical gradient, properties as reportedwere studied in thein previous previous investigations studies [64 [63].]. For The all Seebeck measurements, one side contactcoefficient was of keptPbTe atwas room measured temperature by mounting (approximately the sample on two 25 ◦metalC) with blocks, the which help enabled of circulating the water at one sidegeneration contact, of while a temperature the other gradient, one was as reported heated in allowing the previous the studies generation [64]. For of all a measurements, temperature gradient. one side contact was kept at room temperature (approximately 25 °C) with the help of circulating A copper contactwater at one has side been contact, around while thethe other samples one was contact, heated allowing in order the to gene conductration of the a temperature thermoelectrically driven voltagegradient. measurements. A copper contact Silver has pastebeen around was present the samples between contact, the in copper order contactto conduct and the the sample surface. Thethermoelectrically copper contact driven from voltage the two measurements ends were. Silver fed to paste a multimeter-voltmeter was present between the and copper the generated thermo-voltagecontact (V)and was the measured.sample surface. The The temperature copper contact gradient from ∆theT wastwo measured.ends were fed Then to thea Seebeck multimeter-voltmeter and the generated thermo-voltage (V) was measured. The temperature coefficientgradient (S) was ΔT derived was measured. from Then the the ratio Seebeck of ∆ coefficiV/∆T.ent In (S) the was synthesis derived from of the the ratio powder, of ΔV/ΔT. pellets In of the dimensionsthe 12 synthesis (diameter) of the powder,3 (height) pellets mmof the were dimensions used 12 in (diameter) the measurements × 3 (height) mm of were the Seebeckused in the coe fficient × (S) and electricalmeasurements conductivity of the Seebeck (σ) (Figure coefficien 10t). (S) Seebeck and electrical coeffi conductivitycient, as determined (σ) (Figure 10). by laboratorySeebeck made measurementscoefficient, system as purchaseddetermined by from laboratory the Borade made Embadedmeasurements Solutions, system purchased provided from the contactsthe Borade mentioned Embaded Solutions, provided the contacts mentioned above by using pellets formed from an exact above by usingsize of the pellets samples formed (A1–A5) from respectively. an exact The size circul ofar the pellets samples of the (A1–A5)dimensions respectively. were prepared Theby circular pellets of thethe hydraulic dimensions press were machine, prepared by application by the of hydraulic high stress. pressThen, the machine, thermo-emf by of application the samples ofwas high stress. Then, the thermo-emfmeasured. of the samples was measured.

Figure 10.FigurePlot 10. of Plot thermo-electromotive of thermo-electromotive force versus versus temperature temperature difference diﬀ fromerence two points from on two the points on the pellets.pellets.

Seebeck coefficient (S) for the samples A1 to A5 shows the p-type behavior with the variation of S from 50 µVK−1 to 380 µVK−1. The values of σ and S for the high yield sample were 23 Sm−1 and 314 µVK−1, respectively, and varied with temperature. Figure 10 shows the non-linear behavior of S with temperature T. These values are much in comparison to the gas evaporation method reported [65]. From the samples A1 to A5, there is increase in the thermo-emf as seen, since sample A5 shows the agglomeration at longer time durations in the synthesized sample. The sample A4 resulted in these values of σ and S due to strong grain boundary scattering of carriers, which may lead to extremely low thermal conductivity K for the synthesized material. This is more beneficial for the non-dimensional TE materials. With increase in thermo-emf, the electrical conductivity should also increase. The figure of merit, ZT (ZT = S2σT/k), determines the TE conversion efficiency of the TE material. The TE properties of the material could be increased by doping the other elements such as Se, Sn, Sb, and Ag. The decreased value of S is also observed when the PbTe is synthesized by the other methods and increase in the power factor of the material [66,67]. Figure 11 shows the schematic diagram of the Seebeck coefficient measurement.

Nanomaterials 2020, 10, 1915 11 of 15

Seebeck coefficient (S) for the samples A1 to A5 shows the p-type behavior with the variation of 1 1 1 S from 50 µVK− to 380 µVK− . The values of σ and S for the high yield sample were 23 Sm− and 1 314 µVK− , respectively, and varied with temperature. Figure 10 shows the non-linear behavior of S with temperature T. These values are much in comparison to the gas evaporation method reported [65]. From the samples A1 to A5, there is increase in the thermo-emf as seen, since sample A5 shows the agglomeration at longer time durations in the synthesized sample. The sample A4 resulted in these values of σ and S due to strong grain boundary scattering of carriers, which may lead to extremely low thermal conductivity K for the synthesized material. This is more beneficial for the non-dimensional TE materials. With increase in thermo-emf, the electrical conductivity should also increase. The figure of merit, ZT (ZT = S2σT/k), determines the TE conversion efficiency of the TE material. The TE properties of the material could be increased by doping the other elements such as Se, Sn, Sb, and Ag. The decreased value of S is also observed when the PbTe is synthesized by the other methods and increase in the power factor of the material [66,67]. Figure 11 shows the schematic diagram of the Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 17 Seebeck coefficient measurement.

FigureFigure 11. 11. SchematicSchematic diagram diagram of of the the Seebeck Seebeck coefficient coeﬃcient measurement. measurement.

AsAs a a result, result, the the sample sample A4 A4 is is the the best best sample sample considering considering the the yield yield produced produced and and the the measured measured valuesvalues of thethe σσand and S S of of the the high high yield yield sample. sample. The The synthesized synthesized PbTe PbTe pellet pellet of high of yieldhigh sampleyield sample shows 1 1 showsp-type p-type conduction conduction with electrical with electrical conductivity conductivity and Seebeck and Seebeck coefficient coefficient (S) as 23 (S) Sm as− 23and Sm 314−1 andµVK 314− , µVKrespectively.−1, respectively. This verifies This verifies that the that higher the higher quality qualit of leady of telluride lead telluride material material is formed is formed at high at yield.high yield.The plots The ofplots thermo-electromotive of thermo-electromotive force force versus vers temperatureus temperature difference difference for all for the all samplesthe samples from from two twopoints points on the onpellet the pellet film materialfilm material indicate indicate that the that material the material shows shows the typical the typical semiconducting semiconducting of p-type of p-typebehavior, behavior, and at higherand at temperature, higher temperature, the Seebeck the coe Seebeckfficient coefficient of the samples of the almost samples remains almost the remains constant theand constant decreasing and nature. decreasing The sudden nature. increase The andsudden decrease increase in the valueand decrease of the thermo-emf in the value indicates of thatthe thermo-emfthe structural indicates phase transition that the structural in the material phase will tran besition observed in the during material these will temperature be observed ranges during of these500 K temperature to 700 K (Figure ranges 10). of 500 K to 700 K (Figure 10).

4. Conclusions 4. Conclusions Nanocrystalline PbTe powder was synthesized by a simple chemical method at room temperature Nanocrystalline PbTe powder was synthesized by a simple chemical method at room and systematically investigated at various durations. This is a novel technique to synthesize the PbTe temperature and systematically investigated at various durations. This is a novel technique to synthesizematerial which the PbTe shows material noble physico-chemical which shows noble properties. physico-chemical Depending proper on theties. duration, Depending samples on were the duration,labelled assamples A1, A2, were A3, A4,labelled and A5.as A1, There A2, isA3, a directA4, and relation A5. There between is a duration direct relation and yield between of the durationsample until and 288yield h. of After the sample 288 h, the until yield 288 of h. sample After 288 A4 h, declined the yield due of to sample agglomeration. A4 declined The due cyclic to agglomeration.voltammetry gives The acyclic band voltammetry gap of 0.25 eV, gives which a band is very gap well of 0.25 in agreementeV, which withis very the well optical in agreement band gap withof the the material. optical Asband compared gap of the to thematerial. other samples,As compared the most to the prominent other samples, peaks ofthe A4 most sample prominent in XRD peakspattern of give A4 sample the average in XRD crystal pattern and sizegive ofthe 36 average nm having crystal preferred and size orientation of 36 nm in having (2 0 0), preferred verifying orientationthe cubic morphology. in (2 0 0), verifying The dislocation the cubic density morphology. of the A4The sample dislocation is calculated density asof havingthe A4 asample value ofis 7.74 1014 and the micro strain produced in the sample is 2.41 10 3. The lower yield of A1 and high calculated× as having a value of 7.74 × 1014 and the micro strain× produced− in the sample is 2.41 × 10−3. Theyield lower of A5 yield sample of A1 surface and high is a resultyield of of A5 the sample formation surface of independent is a result of cubesthe formation of PbTe havingof independent diﬀerent cubesmagniﬁcations of PbTe having in SEM diff studies.erent magnifications In the sample A5,in SEM as time studies. progressed, In the sample the agglomeration A5, as time progressed, of particles thetook agglomeration place with the of decrease particles in thetook yield. place The with various the decrease stretches in produced the yield. in theThe lower various yield stretches A1 and producedmaximum in time the duration lower yield A5 samples A1 and are maximum observed time in FTIR duration stretches, A5 givingsamples the are presence observed of PbTe in FTIR with stretches,various modes giving of the vibrations presence in of transmission PbTe with variou spectra.s modes The plots of vibrations of thermo-electromotive in transmission spectra. force versus The plots of thermo-electromotive force versus temperature difference for all the samples from two points on the films indicate that the film is a p-type conductor. The thermo-emf of all samples were measured to show the constant thermo-emf for all the samples, but higher yield sample A4 gives the well agreement values of electrical conductivity and thermo-emf as compared to the other samples. These values are beneficial for the non-uniform TE materials which might be due to strong current boundary scattering and extremely low thermal conductivity of the sample.

Funding: This research received no external funding.

Nanomaterials 2020, 10, 1915 12 of 15 temperature difference for all the samples from two points on the films indicate that the film is a p-type conductor. The thermo-emf of all samples were measured to show the constant thermo-emf for all the samples, but higher yield sample A4 gives the well agreement values of electrical conductivity and thermo-emf as compared to the other samples. These values are beneficial for the non-uniform TE materials which might be due to strong current boundary scattering and extremely low thermal conductivity of the sample.

Author Contributions: Conceptualization, H.M.P., G.H.J., and A.B.G.; methodology, A.B.G. and H.M.P.; software, B.M.P. and H.M.P.; validation, Z.E., S.H.B., and H.M.P.; formal analysis, H.M.P., G.H.J., and V.B.G.; investigation, H.M.P., G.H.J., A.B.G., and V.B.G.; resources, Z.E. and S.H.B.; data curation, H.M.P., S.H.B., and Z.E.; writing—original draft preparation, Z.E. and A.B.G.; writing—review and editing, Z.E., S.H.B., A.B.G., and H.M.P.; visualization, H.M.P., Z.E., and S.H.B.; supervision, H.M.P., S.H.B., and Z.E.; project administration, H.M.P., S.H.B., and Z.E. All authors within this team, conducted literature research and data interpretation additionally to the above mentioned contributions. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: Authors are thankful to Savitribai Phule Pune University Department of Physics for various characterization facilities; A.B.G. is grateful to Principal SNJB’s ACS College for providing the necessary facility to carry out the research work; Principal Sandip Institute of Pharmaceutical Sciences (SIPS) for the use of the FTIR facility. Z.E. and S.H.B. are grateful to the Deanship of Graduate Studies and Research, AU, Ajman, United Arab Emirates. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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