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r-GO)
Physica B xxx (xxxx) xxx
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Physica B: Physics of Condensed Matter journal homepage: http://www.elsevier.com/locate/physb
Enhanced thermoelectric power factor in wet chemical synthesized Sb2Te3 by the incorporation of (GO/r-GO) Khalid Bin Masood a, Sharmistha Anwar b, R.A. Singh a, Jai Singh a, * a b
Department of Physics, Dr. Harisingh Gour University, Sagar, M.P, India Advanced Materials Technology Department, CSIR- Institute of Minerals and Materials Technology, Bhubaneswar, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanocomposites Sb2Te3 Electrical conductivity Seebeck coefficient Power factor
Present work deals with the thermoelectric transport properties of bare and (GO/r-GO) composites of Sb2Te3 synthesized via simple wet chemical method at the temperature of 100 � C. The presence of (GO/r-GO) in the as synthesized samples has been confirmed by Raman spectra, FE-SEM (NOVA NANOSEM 450) images and reduction in the intensity of XRD (Advanced D8 Bruker) diffraction peaks of the composites. Due to the addition of (GO/r-GO), power factor increased with the maximum value of 2.14 μW/cmK2 and 2.48 μW/cmK2 at 50 � C for Sb2Te3@GO and Sb2Te3@r-GO respectively. The results show that the maximum power factor of the Sb2Te3@rGO composite is about 35% higher than the bare Sb2Te3. This study provides the simple and effective method for the synthesis of graphene based composite thermoelectric materials and is a promising means of achieving synergistic enhancement of the thermoelectric performance levels by increasing the power factor.
1. Introduction Thermoelectric materials are the promising solution to overcome the growing energy crises and pollution related problems in the modern world. The efficiency of thermoelectric materials is determined by the dimensionless figure of merit (zT ¼ S2σT/κ), where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal con ductivity and T is the absolute temperature [1,2]. The quantity associ ated with electrical transport of thermoelectric materials is the power factor (S2σ). The thermoelectric power generation efficiency is given by the [3,4]. � pffiffiffiffiffiffiffiffiffiffiffiffiffi ΔT 1 þ zT 1 � � (1) η¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi Th 1 þ zT þ TThc Th is the temperature of hot side, Tc is the temperature of cold side, ΔT is the temperature difference between hot and cold side, zT is the figure of merit, and ΔT/Th gives the Carnot efficiency. Thus to enhance the efficiency, the factors like large zT and substantial difference be tween Tc and Th should be taken into consideration. In order to enhance zT, low thermal conductivity (κ) should be maintained while maxi mizing Seebeck coefficient (S), electrical conductivity (σ). However, these quantities are dependent on each other and altering one definitely
changes the other two. The thermal conductivity (κ) and electrical conductivity (σ) are related to each other by Weidman Franz law which states that the ratio of electrical and thermal conductivity of a solid remains constant [5]. This implies that increasing electrical conductivity will increase thermal conductivity. The relation between Seebeck coef ficient (S) and conductivity (σ) is given by equation (2) [6]. S¼
� π �2=3 8π2 K 2B * mDOS T 2 3n 3eh
(2)
Where m*DOS is the density of states effective mass of charge carriers and n is the concentration of charge carriers. In order to decouple Seebeck coefficient (S), electrical conductivity (σ) and thermal conductivity (κ) some special strategies have been introduced like nanostructuring [7,8], nanocomposite formation [9,10], doping [4,11], band convergence [12, 13], nanoinclusion in the matrix [14]. Nanostructuring enhances zT by strengthening DOS near Fermi level through quantum confinement thereby increasing power factor [15,16] and also by the efficient scat tering of phonons at the grain boundaries thereby reducing thermal conductivity [7,8]. Nano-composites enhance zT because the power factor is much higher that the constituent phases and also the thermal conductivity is reduced by the scattering of phonons at the boundaries of neighbouring phases [2,17]. Antimony telluride is a p-type narrow band semi conductor bearing
* Corresponding author. Department of Physics, Dr. Harisingh Gour Central University, Sagar, M.P, 47003, India. E-mail addresses: [email protected], [email protected] (J. Singh). https://doi.org/10.1016/j.physb.2019.411795 Received 29 May 2019; Received in revised form 30 September 2019; Accepted 17 October 2019 Available online 22 October 2019 0921-4526/© 2019 Published by Elsevier B.V.
Please cite this article as: Khalid Bin Masood, Physica B, https://doi.org/10.1016/j.physb.2019.411795
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layered hexagonal structure consisting of the quintuple layers of mostly covalently bonded Te (1)-Sb-Te (2)-Sb-Te (1) layer with adjacent layers connected by weak van der Waals bonds between Te (1) and Te (1) atoms of the adjacent layers. Antimony telluride has the reputation of being an excellent thermoelectric material in the temperature range up to 200 � C along with its doped derivatives. Due to this, the use of anti mony telluride as thermoelectric material is limited to low temperatures only. Also due to the anti-site defects, Sb2Te3 shows low Seebeck coef ficient which results in the low power factor [18]. Various efforts have been made to enhance the thermoelectric properties of bare and doped derivatives of Sb2Te3 in the recent years. Jin et al. synthesized hexagonal Sb2Te3 nanoplates with varying thickness via solvothermal approach [19]. The samples show maximum Seebeck coefficient of 119.9 μV/K, 142.4 μV/K, and 161.7 μV/K at 600 K for 500 nm, 60 nm and 40 nm particle sizes. Im et al. synthesized Sb2Te3 hexagonal nanoplates by a simple solvothermal and consolidated the as prepared samples by spark plasma sintering (SPS) [20]. The nanoplates show the maximum See beck coefficient of (300–350 μV/K) with the electrical conductivity of (0.8–1.8 Scm 1) in the temperature range of 300–575 K. Yan et al. synthesized Sb2Te3 nanoplatelets in the thickness range of 10 nm–100 nm by solvothermal method and consolidated the samples using SPS [21]. The thick platelets show the power factor of (2.7–3.0 μW/cmK2) and the thin platelets show power factor of (8.6–9.1 μW/cmK2) in the temperature range of 300–550 K with the maximum zT of 0.2 for thick platelets and 1 for thin platelets. Various studies has also been focused on the thermoelectric properties BiSbTe/graphene com posites synthesized via zone melting followed by SPS [22], melt spinning process followed by SPS [23] etc. Consolidation by SPS has its role in enhancing the thermoelectric properties by preserving the microstruc ture of samples as compared to the conventional sintering. However, maximum research is mainly focused on (BiSbTe)/graphene composites and there is a lack of research on composites of pristine Sb2Te3. The studies on separate effects of GO and r-GO on the thermoelectric prop erties of materials is also very rare. In the present studies, bare and (GO/r-GO) composites of Sb2Te3 have been synthesized by wet chemical method. The as synthesized samples were consolidated by cold pressing followed by sintering. The phase of as prepared samples is rhombohedral which has been analysed by XRD. Various vibrational modes of Sb2Te3 bonds have been analysed by Raman spectra which also shows the presence of GO/r-GO in the matrix. The morphology and compositional analysis has been analysed by SEM (scanning electron microscopy) and EDX (energy dispersive Xray analysis) analysis. Electrical transport properties measurements show that Sb2Te3@(GO/r-GO) composites have enhanced power factor due to increased electrical conductivity.
Rest of the process for the synthesis of composites remains same as discussed above. For thermoelectric measurements, as prepared samples were consolidated into pellets of 13 mm diameter and 1–2 mm thickness by cold pressing at 80 MPa followed by sintering at 350 � C in inert gas at mosphere for 4 h. 2.3. Characterization techniques The phase and quality of as prepared samples was analysed by Advanced D8 Bruker X-ray diffractometer (XRD) having Ni-filtered CuKα (1.5405 Å) with the step size 0.02� . The morphology of as synthe sized materials was observed by using scanning electron microscope (NOVA-NANOSEM 450). The Raman spectra of the samples were recorded by Renishaw micro-Raman spectrometer with laser excitation source of 488 nm. High temperature thermoelectric (Seebeck) and resistivity mea surements were performed using circular shaped samples using a ULVAC ZEM-3 setup. Thermoelectric measurement mode employ steady state differential method i.e. entire sample is heated in steps to successively higher temperatures and for each step small temperature differentials were maintained across two ends. The electrical resistivity measure ments were performed by four probe method. Four fine contacts were prepared linearly in a straight line at equal distance. The four probe arrangement eliminates the error induced by test lead resistances. The measurements were carried out under low pressure He environment from room temperature to 300 � C at an interval of 50 � C. 3. Results and discussions The phase of as prepared bare Sb2Te3 and composites has been examined by XRD spectra. Fig. 1(a) represents XRD pattern of the as prepared samples. All the diffraction peaks of as prepared samples match with JCPDS card no. 71–0393 and have rhombohedral phase with space group R-3m. The diffraction intensity reduced in the composites which might be due to the presence of (GO/r-GO) in the matrix of Sb2Te3. Also, the lattice parameter c shows increment in the values in case of Sb2Te3 composites. The lattice parameter c has the values of 30.304, 30.438 and 30.403 Å for bare Sb2Te3, Sb2Te3@GO and Sb2Te3@r-GO respectively. The volume of lattice also increases from 499.73 Å3 for bare Sb2Te3 to 504.91 Å3 for Sb2Te3 composites. Hence, from the XRD patterns we can confirm that the Sb2Te3@(GO/r-GO) composites have been successfully fabricated using the wet chemical synthesis method. No new diffraction peaks corresponding to GO/rGO have been observed due to very small amounts (GO/r-GO) [25]. The average crystallite size was estimated by Scherer formula which is given as
2. Experimental section 2.1. Synthesis of Sb2Te3
D¼
In the typical synthesis of Sb2Te3, highly pure antimony chloride (4 g m) and tellurium metal powder (2 g m) were taken with deionized water, ethylene glycol and hydrazine hydrate in the ratio of 6:3:1. The solution is then kept on stirrer (600 rpm) at 100 � C for 6 h until a black grey precipitate is formed. The precipitate removed from the solution and then washed with hot distilled water and ethanol several times to remove the impurities. The precipitate is then dried in vacuum at 60 � C to obtain the powdered sample of Sb2Te3.
0:9λ βcosθ
(3)
Where, D is average crystalline size, λ is wavelength of X-rays (0.15405 nm), β is full width at half maximum and θ is the diffraction angle. Crystalline size has been calculated by taking 2θ values 27.59 (0 1 5), 31.96 (1 0 7), 34.88 (0 0 12), 45.81 (1 0 13), 54.35 (1 0 16), 57.10 (0 1 17) and 58.96 (2 0 11). It could be easily observed from table (a) that crystallite size increases with the incorporation of GO and r-GO but the increase in Sb2Te3/GO is more than that of Sb2Te3/r-GO. The strain caused by change in the bond length between two atoms by the incorporation of (GO/r-GO) in the crystal was calculated by Williamson-Hall (W–H) fitting method.The equation for strain calcula tion is given as:
2.2. Synthesis of Sb2Te3 @(GO/r-GO) composites Graphene oxide (GO) was synthesized by modified Hummer’s method [24]. GO was converted to reduced graphene oxide (r-GO) by annealing GO at the temperature of 300 � C for 1 h. For the synthesis of Sb2Te3 composites, 20 mg (GO/r-GO) (0.5 wt %) was added to antimony chloride in 20 ml deionized water followed by ultra-sonication for 1 h.
βhkl cosθ 0:9 sinθ ¼ þ εhkl λ Dhkl λ
(4)
Where, βhkl is full width at half maximum, θ is Bragg’s diffraction angles, 2
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Fig. 1. (a)XRD pattern and (b) Williamson-Hall plot to measure the micro-strain bare and (GO/r-GO) composites of Sb2Te3.
Dhkl is the effective crystalline site and εhkl is the micro-strain. The micro-strain is estimated by the slope of sinθ/λ Vs βcosθ/λ plot which is shown in Fig. 1(b) and the positive slope of this plot reflects the presence of tensile strain in the crystal. The values of micro-strain for bare and (GO/r-GO) composites of Sb2Te3 are summarized in Table 1 (a) and it could be inferred that the micro-strain increases in case of Sb2Te3 composites as compared to bare Sb2Te3 and is almost same for Sb2Te3@ (GO/r-GO) composites. The vibrational modes of bare Sb2Te3 and composites have been analysed by Raman spectra which is shown in Fig. 2. From the spectra, the characteristic vibrational peaks of Sb2Te3 are obtained at 70 and 120 cm 1 as is also mentioned in earlier reports [26,27]. The presence of (GO/r-GO) in the matrix of Sb2Te3 was confirmed by D and G bands present in the range 1150–1700 cm 1 of Raman spectra. The portion enclosed in the rectangular boxes are the characteristic Raman bands of GO and r-GO. The characteristic bands of GO/r-GO have been provided as insets in Fig. 2 and these insets are the magnified image of the portion of Raman spectra enclosed in rectangular boxes. The morphology of as-prepared samples was observed by scanning electron microscopy (SEM), and the resulting images are shown in Fig. 3 (a and b). The electron micrographs displayed are taken at the scale of 3 μm. The bare Sb2Te3 particles have irregular and non-uniform shape. The shape of the particles in Sb2Te3@r-GO composite is more uniform. SEM image of Sb2Te3@r-GO composite indicates that the r-GO flakes are present on the surface of Sb2Te3 particles. EDX spectra shown as the inset of Fig. 3 (a) confirms the presence of antimony and tellurium in the as prepared samples. The electrical conductivity as the function of temperature is given in Fig. 4 (a) and the electrical conductivity of all the samples increases with the increasing temperature which is the behaviour of typical semi conductors. The same trend in electrical conductivity has been reported in some recent studies on binary chalcogenides like Bi2Se3 [28]. The electrical conductivity of Sb2Te3 composites is larger than that of bare Sb2Te3 in the entire temperature range of 40–300 � C. The enhancement in electrical conductivity can be credited to the increment in carrier mobility and carrier concentration due to the incorporation of Fig. 2. Raman spectra of bare and (GO/r-GO) composites of Sb2Te3 with the inset of Raman spectra of GO and r-GO.
Table 1 (a): Crystallite size and micro-strain of Sb2Te3 and Sb2Te3(GO/r-GO) composites by Scherer and W–H fitting method. Sample
By Scherer formula Kλ (nm) Bcrystallite ¼ Lcosθ
By W–H method Kλ Br ¼ þ η tan θ(nm) L cos θ
Micro-strain
Sb2Te3 Sb2Te3/GO Sb2Te3/rGO
45.40 60.69 53.09
48.20 73.39 67.66
0.0009 0.025 0.026
(GO/r-GO) in the matrix [22,23]. The value of mobility in Sb2Te3 composites increases because of the high mobility of graphene [29]. Increase in electrical conductivity in case of Sb2Te3 composites is sug gested because of the extra transmission lines for electrons provided by the introduction of graphene [30]. Moreover, the electrical conductivity 3
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Fig. 3. (a) SEM image of bare Sb2Te3 with the inset of EDX of Sb2Te3 (b) SEM image of Sb2Te3@r-GO composite. The presence of r-GO is marked by arrows.
Fig. 4. Temperature dependence of (a) Electrical conductivity, (b) Seebeck coefficient and (c) Power factor of bare and (GO/r-GO) composites of Sb2Te3.
of Sb2Te3@r-GO composite is more than that of Sb2Te3/GO composite which can be due to the more conducting nature of r-GO. Although, GO gets converted to r-GO after the sintering of samples at 350 � C but still the sample in which r-GO has been added during synthesis process shows more conductivity. This may be because when GO is added during synthesis process, it gets attached to the matrix and might not get fully converted into r-GO upon sintering. It can be concluded that the pres ence of extra O atoms in GO is the reason of low electrical conductivity of Sb2Te3/GO as compared to Sb2Te3/r-GO composite. The room tem perature electrical conductivity of bare Sb2Te3, Sb2Te3@GO and Sb2Te3@r-GO is 2437, 4517 and 4738 S/m respectively. The dependence of Seebeck coefficient on temperature is given in Fig. 4(b). The Seebeck coefficient of all the samples shows positive values implying that the samples behave as p-type semiconductors. Seebeck coefficient of all the samples shows a small increment in the temperature range from room temperature to 50 � C. Seebeck coefficient of the samples decreases with further increase in temperature in accordance with equation (2) which implies that by increasing the electrical conductivity caused mainly by the increment of carrier con centration ‘n’, Seebeck coefficient decreases. The Seebeck coefficient of the composites has lower value than bare Sb2Te3 is because of the higher
conductivity of Sb2Te3 composites which is obvious form equation (2). The decrease in Seebeck coefficient with increasing temperature can also be due to the bipolar transition of minority charge carriers at elevated temperatures [31]. The maximum value of Seebeck coefficient is 246, 214 and 220 μV/K at 50 � C for bare Sb2Te3, Sb2Te3/GO and Sb2Te3/r-GO respectively. (GO/r-GO) incorporation enhances the elec trical conductivity and simultaneously is not harmful for Seebeck coef ficient which is because the electrical conductivity in the composites is mainly increased by the increase in carrier mobility while there is slight increase in the carrier concentration [23]. The power factor of all the samples first increases and then decreases with increasing temperature with the maximum value at 50 � C as is clear from Fig. 4 (c). The power factor first increases from room temperature up to 50 � C because of the small increment in Seebeck coefficient in the same temperature range. This small increment is reflected in the power factor because it is proportional to the square of Seebeck coefficient. The maximum value of power factor for bare Sb2Te3, Sb2Te3@GO, Sb2Te3@r-GO is 1.6, 2.14 and 2.48 μW/cmK2 respectively. Hence, the electric transport properties of Sb2Te3 have been optimized by the incorporation of (GO/r-GO) in the matrix with r-GO being better option than GO. 4
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4. Conclusion
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Bare and (GO/r-GO) composites of p-type Sb2Te3 have been syn thesized by low cost facile wet chemical method. The individual effects of GO and r-GO on the thermoelectric power factor of Sb2Te3 have been investigated in detail. The incorporation of (GO/r-GO) in the matrix of Sb2Te3 resulted in the increase in electrical conductivity which resulted in the increment of power factor. The Sb2Te3@ (GO/r-GO) composites show maximum power factor of 2.14 and 2.48 μW/cmK2 respectively at 50 � C. This study provides a simple and efficient route for incorporating (GO/r-GO) in the matrix of thermoelectric materials and also, the electrical transport properties are optimized by the incorporation of rGO in the matrix. Declaration of competing interest The authors declare that there is no conflict of interest regarding the publication of this paper. Acknowledgements Authors are thankful to Sophisticated Instrument Centre (SIC) of the University for providing various characterization facilities. Jai Singh would like to acknowledge UGC-India and DST for providing project under UGC Start-up Grant FT30 [HYPHEN] 56/2014 (BSR)3(A)a and DST Fast track Grant no. SR/FTP/PS [HYPHEN] 144/2012. Khalid Bin Masood would like to thank Neha Jain for her valuable suggestions in manuscript writing. References [1] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2) (2008) 105–114, 2008. [2] K.B. Masood, P. Kumar, R.A. Singh, J. Singh, Odyssey of thermoelectric materials: foundation of the complex structure, J. Phys. Commun. 2 (6) (2018), 062001. [3] K. Biswas, Advances in thermoelectric materials and devices for energy harnessing and utilization, Natn. Sci. Acad. 81 (2015) 903–913, 2015. [4] E.S. Toberer, A.F. May, G.J. Synder, Zintl chemistry for designing high efficiency thermoelectric materials, Chem. Mater. 22 (2010) 624–634. [5] R. Franz, G. Wiedemann, Ueber die W€ arme-Leitungsf€ ahigkeit der Metalle, Ann. Phys. 165 (8) (1853) 497–531. [6] M. Cutler, J.F. Leavy, R.L. Fitzpatrick, Electronic transport in semimetallic cerium sulfide, Phys. Rev. 133 (4A) (1964) A1143. [7] Y. Wang, H. Huang, X. Ruan, Decomposition of coherent and incoherent phonon conduction in superlattices and random multilayers, Phys. Rev. B 90 (2014) 165406. [8] M.G. Kanatzidis, Nanostructured thermoelectrics: the new paradigm? Chem. Mater. 22 (3) (2009) 648–659. [9] C. Li, X. Qin, Y. Li, D. Li, J. Zhang, H. Guo, H. Xin, C. Song, Simultaneous increase in conductivity and phonon scattering in a graphene nanosheets/ (Bi2Te3)0.2(Sb2Te3)0.8 thermoelectric nanocomposite, J. All. Com. 661 (2016) 389–395.
5
Contents lists available at ScienceDirect
Physica B: Physics of Condensed Matter journal homepage: http://www.elsevier.com/locate/physb
Enhanced thermoelectric power factor in wet chemical synthesized Sb2Te3 by the incorporation of (GO/r-GO) Khalid Bin Masood a, Sharmistha Anwar b, R.A. Singh a, Jai Singh a, * a b
Department of Physics, Dr. Harisingh Gour University, Sagar, M.P, India Advanced Materials Technology Department, CSIR- Institute of Minerals and Materials Technology, Bhubaneswar, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanocomposites Sb2Te3 Electrical conductivity Seebeck coefficient Power factor
Present work deals with the thermoelectric transport properties of bare and (GO/r-GO) composites of Sb2Te3 synthesized via simple wet chemical method at the temperature of 100 � C. The presence of (GO/r-GO) in the as synthesized samples has been confirmed by Raman spectra, FE-SEM (NOVA NANOSEM 450) images and reduction in the intensity of XRD (Advanced D8 Bruker) diffraction peaks of the composites. Due to the addition of (GO/r-GO), power factor increased with the maximum value of 2.14 μW/cmK2 and 2.48 μW/cmK2 at 50 � C for Sb2Te3@GO and Sb2Te3@r-GO respectively. The results show that the maximum power factor of the Sb2Te3@rGO composite is about 35% higher than the bare Sb2Te3. This study provides the simple and effective method for the synthesis of graphene based composite thermoelectric materials and is a promising means of achieving synergistic enhancement of the thermoelectric performance levels by increasing the power factor.
1. Introduction Thermoelectric materials are the promising solution to overcome the growing energy crises and pollution related problems in the modern world. The efficiency of thermoelectric materials is determined by the dimensionless figure of merit (zT ¼ S2σT/κ), where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal con ductivity and T is the absolute temperature [1,2]. The quantity associ ated with electrical transport of thermoelectric materials is the power factor (S2σ). The thermoelectric power generation efficiency is given by the [3,4]. � pffiffiffiffiffiffiffiffiffiffiffiffiffi ΔT 1 þ zT 1 � � (1) η¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi Th 1 þ zT þ TThc Th is the temperature of hot side, Tc is the temperature of cold side, ΔT is the temperature difference between hot and cold side, zT is the figure of merit, and ΔT/Th gives the Carnot efficiency. Thus to enhance the efficiency, the factors like large zT and substantial difference be tween Tc and Th should be taken into consideration. In order to enhance zT, low thermal conductivity (κ) should be maintained while maxi mizing Seebeck coefficient (S), electrical conductivity (σ). However, these quantities are dependent on each other and altering one definitely
changes the other two. The thermal conductivity (κ) and electrical conductivity (σ) are related to each other by Weidman Franz law which states that the ratio of electrical and thermal conductivity of a solid remains constant [5]. This implies that increasing electrical conductivity will increase thermal conductivity. The relation between Seebeck coef ficient (S) and conductivity (σ) is given by equation (2) [6]. S¼
� π �2=3 8π2 K 2B * mDOS T 2 3n 3eh
(2)
Where m*DOS is the density of states effective mass of charge carriers and n is the concentration of charge carriers. In order to decouple Seebeck coefficient (S), electrical conductivity (σ) and thermal conductivity (κ) some special strategies have been introduced like nanostructuring [7,8], nanocomposite formation [9,10], doping [4,11], band convergence [12, 13], nanoinclusion in the matrix [14]. Nanostructuring enhances zT by strengthening DOS near Fermi level through quantum confinement thereby increasing power factor [15,16] and also by the efficient scat tering of phonons at the grain boundaries thereby reducing thermal conductivity [7,8]. Nano-composites enhance zT because the power factor is much higher that the constituent phases and also the thermal conductivity is reduced by the scattering of phonons at the boundaries of neighbouring phases [2,17]. Antimony telluride is a p-type narrow band semi conductor bearing
* Corresponding author. Department of Physics, Dr. Harisingh Gour Central University, Sagar, M.P, 47003, India. E-mail addresses: [email protected], [email protected] (J. Singh). https://doi.org/10.1016/j.physb.2019.411795 Received 29 May 2019; Received in revised form 30 September 2019; Accepted 17 October 2019 Available online 22 October 2019 0921-4526/© 2019 Published by Elsevier B.V.
Please cite this article as: Khalid Bin Masood, Physica B, https://doi.org/10.1016/j.physb.2019.411795
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layered hexagonal structure consisting of the quintuple layers of mostly covalently bonded Te (1)-Sb-Te (2)-Sb-Te (1) layer with adjacent layers connected by weak van der Waals bonds between Te (1) and Te (1) atoms of the adjacent layers. Antimony telluride has the reputation of being an excellent thermoelectric material in the temperature range up to 200 � C along with its doped derivatives. Due to this, the use of anti mony telluride as thermoelectric material is limited to low temperatures only. Also due to the anti-site defects, Sb2Te3 shows low Seebeck coef ficient which results in the low power factor [18]. Various efforts have been made to enhance the thermoelectric properties of bare and doped derivatives of Sb2Te3 in the recent years. Jin et al. synthesized hexagonal Sb2Te3 nanoplates with varying thickness via solvothermal approach [19]. The samples show maximum Seebeck coefficient of 119.9 μV/K, 142.4 μV/K, and 161.7 μV/K at 600 K for 500 nm, 60 nm and 40 nm particle sizes. Im et al. synthesized Sb2Te3 hexagonal nanoplates by a simple solvothermal and consolidated the as prepared samples by spark plasma sintering (SPS) [20]. The nanoplates show the maximum See beck coefficient of (300–350 μV/K) with the electrical conductivity of (0.8–1.8 Scm 1) in the temperature range of 300–575 K. Yan et al. synthesized Sb2Te3 nanoplatelets in the thickness range of 10 nm–100 nm by solvothermal method and consolidated the samples using SPS [21]. The thick platelets show the power factor of (2.7–3.0 μW/cmK2) and the thin platelets show power factor of (8.6–9.1 μW/cmK2) in the temperature range of 300–550 K with the maximum zT of 0.2 for thick platelets and 1 for thin platelets. Various studies has also been focused on the thermoelectric properties BiSbTe/graphene com posites synthesized via zone melting followed by SPS [22], melt spinning process followed by SPS [23] etc. Consolidation by SPS has its role in enhancing the thermoelectric properties by preserving the microstruc ture of samples as compared to the conventional sintering. However, maximum research is mainly focused on (BiSbTe)/graphene composites and there is a lack of research on composites of pristine Sb2Te3. The studies on separate effects of GO and r-GO on the thermoelectric prop erties of materials is also very rare. In the present studies, bare and (GO/r-GO) composites of Sb2Te3 have been synthesized by wet chemical method. The as synthesized samples were consolidated by cold pressing followed by sintering. The phase of as prepared samples is rhombohedral which has been analysed by XRD. Various vibrational modes of Sb2Te3 bonds have been analysed by Raman spectra which also shows the presence of GO/r-GO in the matrix. The morphology and compositional analysis has been analysed by SEM (scanning electron microscopy) and EDX (energy dispersive Xray analysis) analysis. Electrical transport properties measurements show that Sb2Te3@(GO/r-GO) composites have enhanced power factor due to increased electrical conductivity.
Rest of the process for the synthesis of composites remains same as discussed above. For thermoelectric measurements, as prepared samples were consolidated into pellets of 13 mm diameter and 1–2 mm thickness by cold pressing at 80 MPa followed by sintering at 350 � C in inert gas at mosphere for 4 h. 2.3. Characterization techniques The phase and quality of as prepared samples was analysed by Advanced D8 Bruker X-ray diffractometer (XRD) having Ni-filtered CuKα (1.5405 Å) with the step size 0.02� . The morphology of as synthe sized materials was observed by using scanning electron microscope (NOVA-NANOSEM 450). The Raman spectra of the samples were recorded by Renishaw micro-Raman spectrometer with laser excitation source of 488 nm. High temperature thermoelectric (Seebeck) and resistivity mea surements were performed using circular shaped samples using a ULVAC ZEM-3 setup. Thermoelectric measurement mode employ steady state differential method i.e. entire sample is heated in steps to successively higher temperatures and for each step small temperature differentials were maintained across two ends. The electrical resistivity measure ments were performed by four probe method. Four fine contacts were prepared linearly in a straight line at equal distance. The four probe arrangement eliminates the error induced by test lead resistances. The measurements were carried out under low pressure He environment from room temperature to 300 � C at an interval of 50 � C. 3. Results and discussions The phase of as prepared bare Sb2Te3 and composites has been examined by XRD spectra. Fig. 1(a) represents XRD pattern of the as prepared samples. All the diffraction peaks of as prepared samples match with JCPDS card no. 71–0393 and have rhombohedral phase with space group R-3m. The diffraction intensity reduced in the composites which might be due to the presence of (GO/r-GO) in the matrix of Sb2Te3. Also, the lattice parameter c shows increment in the values in case of Sb2Te3 composites. The lattice parameter c has the values of 30.304, 30.438 and 30.403 Å for bare Sb2Te3, Sb2Te3@GO and Sb2Te3@r-GO respectively. The volume of lattice also increases from 499.73 Å3 for bare Sb2Te3 to 504.91 Å3 for Sb2Te3 composites. Hence, from the XRD patterns we can confirm that the Sb2Te3@(GO/r-GO) composites have been successfully fabricated using the wet chemical synthesis method. No new diffraction peaks corresponding to GO/rGO have been observed due to very small amounts (GO/r-GO) [25]. The average crystallite size was estimated by Scherer formula which is given as
2. Experimental section 2.1. Synthesis of Sb2Te3
D¼
In the typical synthesis of Sb2Te3, highly pure antimony chloride (4 g m) and tellurium metal powder (2 g m) were taken with deionized water, ethylene glycol and hydrazine hydrate in the ratio of 6:3:1. The solution is then kept on stirrer (600 rpm) at 100 � C for 6 h until a black grey precipitate is formed. The precipitate removed from the solution and then washed with hot distilled water and ethanol several times to remove the impurities. The precipitate is then dried in vacuum at 60 � C to obtain the powdered sample of Sb2Te3.
0:9λ βcosθ
(3)
Where, D is average crystalline size, λ is wavelength of X-rays (0.15405 nm), β is full width at half maximum and θ is the diffraction angle. Crystalline size has been calculated by taking 2θ values 27.59 (0 1 5), 31.96 (1 0 7), 34.88 (0 0 12), 45.81 (1 0 13), 54.35 (1 0 16), 57.10 (0 1 17) and 58.96 (2 0 11). It could be easily observed from table (a) that crystallite size increases with the incorporation of GO and r-GO but the increase in Sb2Te3/GO is more than that of Sb2Te3/r-GO. The strain caused by change in the bond length between two atoms by the incorporation of (GO/r-GO) in the crystal was calculated by Williamson-Hall (W–H) fitting method.The equation for strain calcula tion is given as:
2.2. Synthesis of Sb2Te3 @(GO/r-GO) composites Graphene oxide (GO) was synthesized by modified Hummer’s method [24]. GO was converted to reduced graphene oxide (r-GO) by annealing GO at the temperature of 300 � C for 1 h. For the synthesis of Sb2Te3 composites, 20 mg (GO/r-GO) (0.5 wt %) was added to antimony chloride in 20 ml deionized water followed by ultra-sonication for 1 h.
βhkl cosθ 0:9 sinθ ¼ þ εhkl λ Dhkl λ
(4)
Where, βhkl is full width at half maximum, θ is Bragg’s diffraction angles, 2
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Fig. 1. (a)XRD pattern and (b) Williamson-Hall plot to measure the micro-strain bare and (GO/r-GO) composites of Sb2Te3.
Dhkl is the effective crystalline site and εhkl is the micro-strain. The micro-strain is estimated by the slope of sinθ/λ Vs βcosθ/λ plot which is shown in Fig. 1(b) and the positive slope of this plot reflects the presence of tensile strain in the crystal. The values of micro-strain for bare and (GO/r-GO) composites of Sb2Te3 are summarized in Table 1 (a) and it could be inferred that the micro-strain increases in case of Sb2Te3 composites as compared to bare Sb2Te3 and is almost same for Sb2Te3@ (GO/r-GO) composites. The vibrational modes of bare Sb2Te3 and composites have been analysed by Raman spectra which is shown in Fig. 2. From the spectra, the characteristic vibrational peaks of Sb2Te3 are obtained at 70 and 120 cm 1 as is also mentioned in earlier reports [26,27]. The presence of (GO/r-GO) in the matrix of Sb2Te3 was confirmed by D and G bands present in the range 1150–1700 cm 1 of Raman spectra. The portion enclosed in the rectangular boxes are the characteristic Raman bands of GO and r-GO. The characteristic bands of GO/r-GO have been provided as insets in Fig. 2 and these insets are the magnified image of the portion of Raman spectra enclosed in rectangular boxes. The morphology of as-prepared samples was observed by scanning electron microscopy (SEM), and the resulting images are shown in Fig. 3 (a and b). The electron micrographs displayed are taken at the scale of 3 μm. The bare Sb2Te3 particles have irregular and non-uniform shape. The shape of the particles in Sb2Te3@r-GO composite is more uniform. SEM image of Sb2Te3@r-GO composite indicates that the r-GO flakes are present on the surface of Sb2Te3 particles. EDX spectra shown as the inset of Fig. 3 (a) confirms the presence of antimony and tellurium in the as prepared samples. The electrical conductivity as the function of temperature is given in Fig. 4 (a) and the electrical conductivity of all the samples increases with the increasing temperature which is the behaviour of typical semi conductors. The same trend in electrical conductivity has been reported in some recent studies on binary chalcogenides like Bi2Se3 [28]. The electrical conductivity of Sb2Te3 composites is larger than that of bare Sb2Te3 in the entire temperature range of 40–300 � C. The enhancement in electrical conductivity can be credited to the increment in carrier mobility and carrier concentration due to the incorporation of Fig. 2. Raman spectra of bare and (GO/r-GO) composites of Sb2Te3 with the inset of Raman spectra of GO and r-GO.
Table 1 (a): Crystallite size and micro-strain of Sb2Te3 and Sb2Te3(GO/r-GO) composites by Scherer and W–H fitting method. Sample
By Scherer formula Kλ (nm) Bcrystallite ¼ Lcosθ
By W–H method Kλ Br ¼ þ η tan θ(nm) L cos θ
Micro-strain
Sb2Te3 Sb2Te3/GO Sb2Te3/rGO
45.40 60.69 53.09
48.20 73.39 67.66
0.0009 0.025 0.026
(GO/r-GO) in the matrix [22,23]. The value of mobility in Sb2Te3 composites increases because of the high mobility of graphene [29]. Increase in electrical conductivity in case of Sb2Te3 composites is sug gested because of the extra transmission lines for electrons provided by the introduction of graphene [30]. Moreover, the electrical conductivity 3
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Fig. 3. (a) SEM image of bare Sb2Te3 with the inset of EDX of Sb2Te3 (b) SEM image of Sb2Te3@r-GO composite. The presence of r-GO is marked by arrows.
Fig. 4. Temperature dependence of (a) Electrical conductivity, (b) Seebeck coefficient and (c) Power factor of bare and (GO/r-GO) composites of Sb2Te3.
of Sb2Te3@r-GO composite is more than that of Sb2Te3/GO composite which can be due to the more conducting nature of r-GO. Although, GO gets converted to r-GO after the sintering of samples at 350 � C but still the sample in which r-GO has been added during synthesis process shows more conductivity. This may be because when GO is added during synthesis process, it gets attached to the matrix and might not get fully converted into r-GO upon sintering. It can be concluded that the pres ence of extra O atoms in GO is the reason of low electrical conductivity of Sb2Te3/GO as compared to Sb2Te3/r-GO composite. The room tem perature electrical conductivity of bare Sb2Te3, Sb2Te3@GO and Sb2Te3@r-GO is 2437, 4517 and 4738 S/m respectively. The dependence of Seebeck coefficient on temperature is given in Fig. 4(b). The Seebeck coefficient of all the samples shows positive values implying that the samples behave as p-type semiconductors. Seebeck coefficient of all the samples shows a small increment in the temperature range from room temperature to 50 � C. Seebeck coefficient of the samples decreases with further increase in temperature in accordance with equation (2) which implies that by increasing the electrical conductivity caused mainly by the increment of carrier con centration ‘n’, Seebeck coefficient decreases. The Seebeck coefficient of the composites has lower value than bare Sb2Te3 is because of the higher
conductivity of Sb2Te3 composites which is obvious form equation (2). The decrease in Seebeck coefficient with increasing temperature can also be due to the bipolar transition of minority charge carriers at elevated temperatures [31]. The maximum value of Seebeck coefficient is 246, 214 and 220 μV/K at 50 � C for bare Sb2Te3, Sb2Te3/GO and Sb2Te3/r-GO respectively. (GO/r-GO) incorporation enhances the elec trical conductivity and simultaneously is not harmful for Seebeck coef ficient which is because the electrical conductivity in the composites is mainly increased by the increase in carrier mobility while there is slight increase in the carrier concentration [23]. The power factor of all the samples first increases and then decreases with increasing temperature with the maximum value at 50 � C as is clear from Fig. 4 (c). The power factor first increases from room temperature up to 50 � C because of the small increment in Seebeck coefficient in the same temperature range. This small increment is reflected in the power factor because it is proportional to the square of Seebeck coefficient. The maximum value of power factor for bare Sb2Te3, Sb2Te3@GO, Sb2Te3@r-GO is 1.6, 2.14 and 2.48 μW/cmK2 respectively. Hence, the electric transport properties of Sb2Te3 have been optimized by the incorporation of (GO/r-GO) in the matrix with r-GO being better option than GO. 4
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4. Conclusion
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Bare and (GO/r-GO) composites of p-type Sb2Te3 have been syn thesized by low cost facile wet chemical method. The individual effects of GO and r-GO on the thermoelectric power factor of Sb2Te3 have been investigated in detail. The incorporation of (GO/r-GO) in the matrix of Sb2Te3 resulted in the increase in electrical conductivity which resulted in the increment of power factor. The Sb2Te3@ (GO/r-GO) composites show maximum power factor of 2.14 and 2.48 μW/cmK2 respectively at 50 � C. This study provides a simple and efficient route for incorporating (GO/r-GO) in the matrix of thermoelectric materials and also, the electrical transport properties are optimized by the incorporation of rGO in the matrix. Declaration of competing interest The authors declare that there is no conflict of interest regarding the publication of this paper. Acknowledgements Authors are thankful to Sophisticated Instrument Centre (SIC) of the University for providing various characterization facilities. Jai Singh would like to acknowledge UGC-India and DST for providing project under UGC Start-up Grant FT30 [HYPHEN] 56/2014 (BSR)3(A)a and DST Fast track Grant no. SR/FTP/PS [HYPHEN] 144/2012. Khalid Bin Masood would like to thank Neha Jain for her valuable suggestions in manuscript writing. References [1] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2) (2008) 105–114, 2008. [2] K.B. Masood, P. Kumar, R.A. Singh, J. Singh, Odyssey of thermoelectric materials: foundation of the complex structure, J. Phys. Commun. 2 (6) (2018), 062001. [3] K. Biswas, Advances in thermoelectric materials and devices for energy harnessing and utilization, Natn. Sci. Acad. 81 (2015) 903–913, 2015. [4] E.S. Toberer, A.F. May, G.J. Synder, Zintl chemistry for designing high efficiency thermoelectric materials, Chem. Mater. 22 (2010) 624–634. [5] R. Franz, G. Wiedemann, Ueber die W€ arme-Leitungsf€ ahigkeit der Metalle, Ann. Phys. 165 (8) (1853) 497–531. [6] M. Cutler, J.F. Leavy, R.L. Fitzpatrick, Electronic transport in semimetallic cerium sulfide, Phys. Rev. 133 (4A) (1964) A1143. [7] Y. Wang, H. Huang, X. Ruan, Decomposition of coherent and incoherent phonon conduction in superlattices and random multilayers, Phys. Rev. B 90 (2014) 165406. [8] M.G. Kanatzidis, Nanostructured thermoelectrics: the new paradigm? Chem. Mater. 22 (3) (2009) 648–659. [9] C. Li, X. Qin, Y. Li, D. Li, J. Zhang, H. Guo, H. Xin, C. Song, Simultaneous increase in conductivity and phonon scattering in a graphene nanosheets/ (Bi2Te3)0.2(Sb2Te3)0.8 thermoelectric nanocomposite, J. All. Com. 661 (2016) 389–395.
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