Bi2Te3-MWCNT nanocomposite: An efficient thermoelectric material

Author’s Accepted Manuscript Bi2Te3-MWCNT nanocomposite: An efficient thermoelectric material Sunil Kumar, Deepti Chaudhary, Punit Kumar Dhawan, R.R. Yadav, Neeraj Khare www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)31708-X http://dx.doi.org/10.1016/j.ceramint.2017.08.017 CERI15971

To appear in: Ceramics International Received date: 14 July 2017 Accepted date: 2 August 2017 Cite this article as: Sunil Kumar, Deepti Chaudhary, Punit Kumar Dhawan, R.R. Yadav and Neeraj Khare, Bi 2Te3-MWCNT nanocomposite: An efficient thermoelectric material, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.08.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Bi2Te3-MWCNT nanocomposite: An efficient thermoelectric material Sunil Kumar1, Deepti Chaudhary1, Punit Kumar Dhawan2, R.R. Yadav2, Neeraj Khare1,* 1

Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-

110016, India. 2

Department of Physics, University of Allahabad-211002, Uttar Pradesh, India.

*

Corresponding Author Electronic mail: [email protected]

Abstract Bi2Te3–MWCNT nanocomposite has been synthesized by hydrothermal technique and demonstrate the role of MWCNT for thermoelectric properties. Herein, MWCNT has been used as conducting filler, which leads to the enhancement in the electrical conductivity in the case of nanocomposite. Bi2Te3–MWCNT nanocomposite shows ~22% decrease in the thermal conductivity as compared to Bi2Te3 nanostructures, which is attributed to the enhanced phonon scattering at the interfaces of Bi2Te3–MWCNT nanocomposite. Due to the increase in the electrical conductivity and decrease in the thermal conductivity, the overall enhancement in the figure of merit is ~45% in Bi2Te3–MWCNT nanocomposite as compared to Bi2Te3 nanostructures.

Key words: Electrical conductivity; Thermal conductivity; Bi2Te3; Nanocomposites

1. Introduction In recent years, generation of green energy has attracted considerable attention to fulfil the demand of energy and reduction of emission of harmful gases [1]. Thermal energy can be a potential source of sustainable energy due to its abundance, environment friendly and its conversion into electrical energy using a thermoelectric devices [2, 3]. The performance of thermoelectric materials is quantified by figure of merit (ZT) [4],

ZT 

S 2



T

(1)

where S is the Seebeck coefficient,  is the electrical conductivity, κ is the thermal conductivity and T is absolute temperature. S2 is defined as thermoelectric power factor (TEP). To enhance the performance of a thermoelectric device, thermoelectric materials needs to have a higher value of TEP, low thermal conductivity and high value of Seebeck coefficient [5]. In order to reduce joule heating the electrical conductivity should be high [6]. However, Seebeck coefficient, thermal conductivity and electrical conductivity are dependent to each other, therefore it is very difficult to achieve high conversion efficiency in bulk materials [7]. The increase in the carrier concentration will increase the electrical conductivity but decrease the value of Seebeck coefficient. Therefore, to achieve higher value of TEP, the optimised value of carrier concentration is required [8]. Recently, significant improvement has been observed in the thermal energy conversion efficiency of nanocomposites by quantum confinement and charge carrier filtering to increase the Seebeck coefficient without reducing the electrical conductivity [9, 10]. In the case of conducting filler based thermoelectric materials, the carrier concentration can be optimized for decoupling both the parameters (S and ) using interfaces [11]. At interfaces, low energy

charge carrier filter out and high energy charge carriers can travel through the conducting path in the nanocomposites [12]. The work function of conducting filler and nanostructures should be similar to transport high energy charge carriers across the interfaces and to construct conducting channel and low energy filtering surface [13]. The conducting filler introduces many conducting path between inorganic matrix and thereby increases the mobility to promote S and  [14, 15]. In order to avoid thermal shorting, low thermal conductivity can be achieved in the case of nanocomposite thermoelectric material in which lattice scattering of phonons increases due to a large number of interfaces [1]. Ju et al. [16], reported a significant reduction in thermal conductivity of Bi2Te3 nanowire/graphene nanocomposite. Generally, nanocomposites and superlattices are prepared at higher temperature using molecular beam epitaxy, melt spinning techniques etc. [12, 17]. Recently, there have been a lot of interest to develop a low cost and simple technique such as sol-gel, wet-chemical and hydrothermal technique for the formation of nanocomposite materials [9, 13, 14]. Bismuth Telluride (Bi2Te3) is a promising thermoelectric material with superior energy conversion efficiency at room temperature [18]. Me et al. reported the interfacial barrier height to filter out the low energy carrier by introducing conducting filler (P3HT) into Bi2Te3 nanocomposites matrix [5]. Enhanced figure of merit in Bi2Te3-P3HT, MoS2-Bi2Te3, Bi2Te3graphene and Sb2Te3-Pt nanocomposites are also reported [5, 19-21]. Lai et al. [22] introduced MWNCT into rutile TiO2 to significantly enhance thermoelectric power factor of the nanocomposite. Among the various conducting filler, CNTs as the conducting filler is attracting because it has extraordinary electrical, mechanical and thermal properties even at room temperature [23]. However, it is very difficult to get good dispersion of conducting filler into the inorganic matrix by sol-gel or ball milling techniques [24]. Notably,

hydrothermal technique is an easy technique to get homogeneous dispersion of CNT filler into the inorganic matrix [25]. In this report, a thermoelectric study for enhancing figure of merit of Bi2Te3 by incorporation of MWCNT using hydrothermal technique has been carried out. In Bi2Te3MWCNT nanocomposite, Bi2Te3 nanostructures are decorated over the surface of MWCNT. Bi2Te3-MWCNT nanocomposite showed an enhanced figure of merit as compared to Bi2Te3. In addition, the surface potential study of nanocomposite has also been carried out to study the nature of different phases present in the nanocomposite, affecting the thermoelectric properties. 2. Experimental details 2.1 Synthesis of Bi2Te3 nanostructures Bi2Te3 nanostructures were synthesized by hydrothermal technique. Initially, an appropriate amount of tellurium was added to the aqueous solution of sodium borohydride and sodium hydroxide and the solution was heated with continuous stirring at 65C for 15–20 min till the formation of hydrogen stop (solution A). An aqueous solution containing bismuth chloride was also prepared by dissolving bismuth chloride in deionized water (solution B). Afterwards, solution A and B were mixed together and stirred for 1h at 65C. Excess sodium hydroxide was added to maintain the pH of the solution at 12 during the synthesis. The final solution was filled in a stainless steel autoclave with Teflon liner and the autoclave assembly was kept in an electric oven at 180C for 12h. The resulting black precipitate was centrifuged and washed with distilled water, ethanol and acetone respectively. Finally, the dark product (Bi2Te3) was dried in an oven at 70C for 5h. 2.2 Synthesis of Bi2Te3-MWCNT nanocomposite Figure 1 shows the schematic of the synthesis process of Bi2Te3–MWCNT nanocomposite. MWCNT surface was used as substrate for the growth of Bi2Te3–MWCNT

nanocomposite. Initially, MWCNT were added to an aqueous solution of bismuth chloride and sonicated for 1h which is named as solution C. Afterward, appropriate amount of tellurium was added to the aqueous solution of sodium borohydride and sodium hydroxide and then resulting solution was heated to 65C for 15–20 min, till the formation of hydrogen stops (Solution D). Afterwards, Solution C and solution D were mixed together and stirred for 1 h at 65C. The resulting solution was transferred to a stainless steel vessel (autoclave) with Teflon liner. The autoclave was kept in an oven at 180C for 12 h. The final product was filtered, washed and dried at 70C for 5 h, and denoted as Bi2Te3–MWCNT nanocomposite. 2.3 Characterization techniques The structural properties of Bi2Te3-MWCNT nanocomposite were studied using X-ray diffractometer (Rigaku Ultima-IV with Cu-Kα radiation source). The morphology of the samples was studied by scanning electron microscopy (SEM) [Model No. -Quanta 3D FEG, FEI] and transmission electron microscope (TEM) [JEOL JEM-2200 FS]. For electrical measurements, pellets of the Bi2Te3 and Bi2Te3-MWCNT nanocomposite powder were prepared using the hydrothermal press at 5 ton/inch2 pressure. Renishaw plc Micro Raman Spectrometer was used for the measuring the Raman spectra using 514 nm laser. Surface potential measurements were carried out using the Atomic Force Microscope (AFM) [Bruker Dimension Icon]. A Pt-Ir coated antimony doped Si cantilever (Model: SCM-PIT from Bruker) of curvature 30 nm at 75 kHz frequency was used for Kelvin Probe Force Microscopy (KPFM) measurements. A scan rate of 0.75 Hz and lift height of 60 nm was used for both the samples to obtain the surface potential image. Electrical contact pads of area ~2 mm2 were deposited by e-beam evaporation technique. The temperature dependent electrical conductivity () was measured in the four probe configuration set up under vacuum (10-3 Torr) condition. The charge carrier concentration and mobility was measured by Hall measurement. The Seebeck coefficient was measured by a differential method using home

designed Seebeck coefficient measurement setup [26]. The temperature difference between the hot side and cold side was maintained at 5K during the entire measurement. Seebeck voltage was measured using Keithley 2182A nanovoltmeter. The hot disc thermal constants analyzer (Hot Disc Inc., Sweden) is used for the measurement of thermal conductivity by TPS (transient plane source) method in which wide range (0.02-200 W/mK) of the thermal conductivities can be measured. 3. Results and discussion XRD pattern of Bi2Te3, and Bi2Te3-MWCNT nanocomposite is shown in Figure 2. The XRD pattern of Bi2Te3 nanostructure has diffraction peaks corresponding to (101), (015), (1010), (110), (116), (205), (0210), (1115) and (125) planes of single phase Bi2Te3 [JCPDS card no. 15-0863]. No visible peak corresponding to MWCNT in Bi2Te3-MWCNT nanocomposite was observed due to the low amount of MWCNT in the nanocomposite. The particle size of the sample is estimated using Debye–Scherer equation, d

k 

 cos 

(2)

where kβ is 0.9, λ is Cu-Kɑ wavelength (1.54 Å), β is the full width half maximum of the intense XRD peak and θ is the Bragg angle [27]. The average crystallite size of Bi2Te3 nanostructures is estimated as ~15±1 nm. Figure 3 shows the Raman spectra of Bi2Te3 nanostructures and Bi2Te3-MWCNT nanocomposite. The peak at 95 cm-1, 111 cm-1 and 135 cm-1 corresponds to single phase Bi2Te3 due to E22g, A1u and A21g mode respectively. Bi2Te3 is centro-symmetric and therefore A1u (classical Raman-active mode) is forbidden in the Raman spectra of Bi2Te3 bulk, however the appearance of A1u peak at 111 cm-1 in the Raman spectra of present Bi2Te3 sample confirms the breakdown of the centro-symmetric nature in Bi2Te3 nanostructures. Raman spectra of the MWCNT-Bi2Te3 nanocomposite shows two peaks at 1354 cm−1 and 1582 cm−1,

which corresponds to the D and G band of MWCNT, confirming the presence of MWCNT in the nanocomposite. Figure 4 shows the SEM image of Bi2Te3 and Bi2Te3-MWCNT nanocomposite. The SEM image of Bi2Te3 shows flower like morphology. The SEM image of the Bi2Te3-MWCNT nanocomposite shows that Bi2Te3 nanoflowers morphology are decorated over the surface of MWCNT. Figure 5 shows the TEM and high-resolution TEM images of synthesized Bi2Te3MWCNT nanocomposite. It can be clearly seen from TEM image that the Bi2Te3 nanostructures grow over the surface of MWCNT. The high-resolution TEM image demonstrates a good interface between MWCNT and Bi2Te3, indicating good contacts with Bi2Te3 and MWCNT. The measured surface potential of Bi2Te3 and Bi2Te3-MWCNT nanocomposites using Kelvin Probe Force Microscopy (KPFM) technique is shown in Figure 6(a) and (b). The variation in surface potential is denoted in the surface potential map by different colours. In addition, the presence of different charge carrier density in nanocomposite samples corresponds to the presence of secondary phase, which are MWCNT in the present case [28, 29]. The surface potential statistical distribution of the surface potential image of the Bi2Te3 and Bi2Te3-MWCNT nanocomposites is shown in figure 6(c) and 6(d) respectively. Gaussian fitting of the surface potential distribution showed a peak at 325 mV for Bi2Te3, whereas Gaussian fitting for Bi2Te3-MWCNT nanocomposite showed two peaks at 390 mV and 550 mV. Peak obtained at 325 mV in Bi2Te3 nanostructure clearly indicates that there is no secondary phase in the sample. It can be seen clearly from Figure 6(d) that Bi2Te3-MWCNT nanocomposite shows a drastic change in surface potential due to incorporation of MWCNT in Bi2Te3. Peaks obtained at 390 mV and 550 mV in Bi2Te3-MWCNT nanocomposite sample are attributed to Bi2Te3 and MWCNT, respectively. A large difference between peaks (390

mV and 550 mV) in the surface potential histogram of Bi2Te3-MWCNT nanocomposite indicates the formation of interfacial barrier height. In order to see the effect of temperature on a figure of merit of Bi2Te3 and Bi2 Te3MWCNT nanocomposites, electrical, thermal conductivity and Seebeck coefficient were measured in the temperature range of 295-340K. Figure 7 shows the temperature dependent electrical conductivity of Bi2Te3 and Bi2Te3-MWCNT nanocomposite. The electrical conductivity of both samples decreases monotonically with the increase of temperature, confirming the degenerate semiconductor behaviour. In Bi2Te3-MWCNT nanocomposite, nanostructure Bi2Te3 are decorated over the surface of MWCNT and MWCNT act as a conducting filler between Bi2Te3 matrix, which allows for electrical network to remain intact. At 300K, the electrical conductivity of Bi2Te3 nanoflowers material increases from ~495 S/cm to ~650 S/cm after the formation of its nanocomposite with 1wt % MWCNT. Using Hall measurements, the carrier concentration of Bi2Te3 and Bi2Te3-MWCNT nanocomposite at 300K is obtained as 2.431019 cm-3 and 2.781019 cm-3, respectively. The dependence of electrical conductivity () on carrier concentration (n) and mobility (µ) is given as

  ne

(3)

The value of mobility of Bi2Te3 and Bi2Te3-MWCNT nanocomposite at 300K is found as 127 cm2V-1s-1 and 149 cm2V-1s-1, respectively. The enhanced value of mobility (~17%) in Bi2Te3-MWCNT nanocomposite is attributed to the formation of conducting channel between Bi2Te3 nanostructures via MWCNT. Figure 8 shows temperature dependent charge carrier mobility of Bi2Te3 and Bi2Te3-MWCNT nanocomposite which decreases with increase in the temperature. The decrease in mobility with increase in temperature in both samples is due to more scattering of charge carriers. The higher value of electrical conductivity of Bi2Te3 nanoflowers in the presence of MWCNT is due to the increase in the carrier concentration as well as in carrier mobility.

Seebeck coefficient of Bi2Te3 and Bi2Te3-MWCNT nanocomposite is shown in Figure 9. The negative value of Seebeck coefficient shows that electrons are in the majority, which confirms n-type semiconducting nature of the samples. In addition, the increase in Seebeck coefficient was observed in both the samples with the increase in temperature. The relation between Seebeck coefficient and the carrier concentration is expressed by Mott formula, [30]

8 2 k 2

  S m T  2 3eh  3n 

2/3

*

(4)

where S is the Seebeck coefficient, n is the carrier concentration, kβ is the Boltzmann constant, e is the charge of carrier, h is Plank’s constant and m* is the effective mass of the charge carrier. It is expected that Seebeck coefficient will decrease with increase in carrier concentration. We found that the carrier concentration of Bi2Te3 increases from 2.431019 cm-3 to 2.781019 cm-3 in the case of nanocomposite. Therefore, Seebeck coefficient is expected to decrease for Bi2Te3-MWCNT nanocomposite sample. Using the carrier concentration values and equation 4, the Seebeck coefficient of Bi2Te3 (-130 µVK-1) was found to decrease -119 µVK-1, which is very close to the observed value of -121 µVK-1, in the case of the nanocomposite. Temperature dependent thermal conductivity of Bi2Te3 nanostructures and Bi2Te3MWCNT nanocomposite is shown in Figure 10. Similar behaviour in thermal conductivity with the increase in the temperature is observed for both the samples. However, the thermal conductivity is smaller in the Bi2Te3-MWCNT nanocomposite sample. The thermal conductivity has the contribution of charge carriers (electronic thermal conductivity, electronic) and lattice vibration (lattice thermal conductivity, lattice), which can be written as

   electronic   lattice

(5)

The electronic thermal conductivity (electronic) is defined by Weidman Franz law,

 electronic  L

(6)

where  is the electrical conductivity, T is the absolute temperature and L is the Lorentz number. The value of electronic thermal conductivity is obtained using a Lorentz number (2.4410-8 V2K‐2) in the degenerate limit [20]. The Figure 11(a) shows the variation of electronic thermal conductivity of Bi2Te3 and Bi2Te3-MWCNT nanocomposite. The higher value of electronic thermal conductivity in Bi2Te3-MWCNT nanocomposite is due to higher electrical conductivity. The lattice part of thermal conductivity can be obtained using equation (5) and from the measured value of total thermal conductivity. Figure 11(b) shows the variation of lattice part of thermal conductivity. The lower value of lattice thermal conductivity for Bi2Te3-MWCNT nanocomposite confirms more phonon scattering at interfaces, which result in the reduction of total thermal conductivity of the nanocomposite. A proposed model to explain the enhancement in electrical conductivity and decrease in thermal conductivity in Bi2Te3-MWCNT nanocomposite is shown in Figure 12, which describes the lattice phonon scattering at the interfaces and conduction of electrons through the MWCNT channels. Interfaces present in the Bi2Te3-MWCNT nanocomposite between MWCNT and Bi2Te3 nanostructures, act as a barrier for phonons resulting in enhanced scattering at the interfaces, preventing phonons to travel longer distance and thus the resulting reduction of lattice part of thermal conductivity. Temperature dependent figure of merit is shown in Figure 13, which shows that in the case of Bi2Te3-MWCNT nanocomposite, the figure of merit is increased by ~45%. This confirms that the incorporation of MWCNT in Bi2Te3 matrix decouple the Seebeck coefficient, electrical conductivity and thermal conductivity with the help of interfaces and improves the ZT parameter. The addition of MWCNT in Bi2Te3 matrix gives rise to the formation of compact conductive networks, which generates higher surface area for contact among the grains of Bi2Te3 nanoflowers and resulted in improvement of the pathway of electrons. Therefore, the improvement in electrical

conductivity and decrease in thermal conductivity for the Bi2Te3-MWCNT nanocomposite resulted in the enhancement of the figure of merit of Bi2Te3.

Conclusion Bi2Te3-MWCNT nanocomposite is synthesized by hydrothermal technique. Bi2Te3MWCNT nanocomposite is found to exhibit higher electrical conductivity, lower value of thermal conductivity and higher figure of merit as compared to Bi2Te3 nanostructures. The enhanced electrical conductivity of the nanocomposite has been attributed to increase in the carrier concentration and charge carrier mobility. The reduction in thermal conductivity of the nanocomposite is due to the reduction in lattice part of thermal conductivity which has been attributed to more phonon scattering at interfaces, present in the case of nanocomposite. This work demonstrates the use of MWCNT as conducting filler, as a powerful tool to optimize the electrical conductivity, Seebeck coefficient and thermal conductivity by introducing interfaces in Bi2Te3-MWCNT nanocomposite. Acknowledgments We gratefully acknowledge the MeitY (Govt. of India) for financial support. One of us (SK) is thankful to the University Grant Commission (UGC) for providing the senior research fellowship.

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Figure 1 Schematic illustration of the synthesis process of Bi2Te3-MWCNT nanocomposite.

Figure 2 XRD pattern of Bi2Te3 nanostructures and Bi2Te3-MWCNT nanocomposite.

Figure 3 Raman spectra of Bi2Te3 and Bi2Te3-MWCNT nanocomposite.

Figure 4 SEM image of (a) Bi2Te3 and (b) Bi2Te3-MWCNT nanocomposite.

Figure 5 (a) TEM and (b) HRTEM images of Bi2Te3-MWCNT nanocomposite.

Figure 6 KPFM images of (a) Bi2Te3 and (b) Bi2Te3-MWCNT nanocomposite. Gaussian fit peaks of surface potential distribution of (c) Bi2Te3 and (d) Bi2Te3-MWCNT nanocomposite.

Figure 7 Variation of electrical conductivity with temperature of Bi2Te3 and Bi2Te3MWCNT nanocomposite.

Figure 8 Temperature dependent charge carrier mobility of Bi2Te3 and Bi2Te3-MWCNT nanocomposite in the temperature regime (300-340K).

Figure 9 Variation of Seebeck coefficient with temperature for Bi2Te3 and Bi2Te3-MWCNT nanocomposite.

Figure 10 Temperature dependent thermal conductivity of Bi2Te3 and Bi2Te3-MWCNT nanocomposite.

Figure 11 Temperature dependent (a) lattice and (b) electronic thermal conductivity of Bi2Te3 and Bi2Te3-MWCNT nanocomposite.

Figure 12 Schematic of charge carrier conduction and phonon scattering mechanism in the Bi2Te3-MWCNT nanocomposite.

Figure 13 Temperature dependent figure of merit for Bi2Te3 and Bi2Te3-MWCNT nanocomposite.