Tailoring Thermoelectric Properties through Structure and Morphology

May 10, 2017 - †Materials Science and Technology Division and ‡Academy of Scientific and Innovative Research, CSIR-National Institute for Interdis...
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Tailoring Thermoelectric Properties through Structure and Morphology in Chemically Synthesized n‑Type Bismuth Telluride Nanostructures V. R. Akshay,†,‡ M. V. Suneesh,† and M. Vasundhara*,†,‡ †

Materials Science and Technology Division and ‡Academy of Scientific and Innovative Research, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695019, India S Supporting Information *

ABSTRACT: Here, we report a simple, cost-effective, surfactantassisted, and aqueous-based low-temperature reflux method for the synthesis of Bi2Te3 nanocrystals. Thermoelectric properties of ntype bismuth telluride (BT) nanostructures are reported by varying the morphology and crystal structure. Tuning the reaction time from 1 to 36 h enables the phase transformation from BiTe with a hexagonal crystal structure to Bi2Te3 with a rhombohedral crystal structure, which is evident from the refined X-ray diffraction results and high-resolution transmission electron microscopy analysis. A perfect stoichiometric balance is achieved for all the compositions, and temperature variation of the electrical resistivity of all BT nanostructures shows the typical metal to semiconducting transition near room temperature. Seebeck coefficient and Hall measurements confirm electrons as the majority carriers and show the typical characteristics of n-type BT nanostructures. The nanocrystals inherited from the optimized reaction conditions and high densification of nanoparticle interfaces contribute to the considerable reduction of thermal conductivity in BT nanostructures. Highly crystalline, uniformly distributed nanocrystals of Bi2Te3 formed for 24 h reaction time demonstrate a promising figure of merit of 0.81 at 350 K, which can be attributed to their low thermal conductivity while the high electrical conductivity is maintained. Our research could provide new possibilities in low-temperature synthesis where structural, compositional, and morphological tuning of BT nanostructures could promote practical thermoelectric applications near room temperature. structured materials,3−10 and hence the nanostructured TE materials have attracted much attention for achieving enhanced ZT for practical applications. Among the available class of TE materials, group V chalcogenides such as Bi2Te3-based materials have been investigated as good TE materials known to exhibit very high ZT values over the temperature range of 200−400 K.8 Again, it is interesting to note that Bi2Te3 has a stable structure at room temperature, and hence BT structures can be considered to be a magnificent system for understanding the preparation of nanostructured TE materials,11 which in bulk form exhibit a ZT of around 0.5 at room temperature.12 Recent reports emphasize the importance of a variety of synthesis techniques such as solvothermal methods,13 sonochemical methods,14 aqueous chemical methods,15 refluxing methods,16 etc. as bottom-up approaches and ball milling,17−19 melt spinning,20,21 and exfoliation techniques22 as top-down approaches for obtaining nanostructured TE materials. Among the two approaches, the bottom-up approach is the best method to achieve uniformity in the synthesized nanoparticles and to

1. INTRODUCTION Thermoelectric (TE) materials are a new class of renewable energy resources having the capability of direct conversion of heat to electricity reversibly, which could be utilized for power generation and cooling applications. Even though the complete conversion of low-grade waste heat energy into electricity is not possible, a considerable amount of the same can be converted to useful electricity using efficient TE materials.1 For this reason, research has been mostly driven in such a direction, so that improvement in the quality of materials and synthesis methods can contribute toward the development of a module or device. These devices have significant features such as no moving parts, no noise, easy maintenance, environmental friendliness, long life, and high reliability. The dimensionless figure of merit (ZT), an indication of the overall TE efficiency, determines the thermoelectric performance of a material and is represented as ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity.2 The complex relationship among the thermoelectric parameters makes it difficult to tune the individual properties beyond a certain limit. Current research on TE materials is focused on improving the power factor with a drastic reduction of κ using mixtures of multiphase nano© 2017 American Chemical Society

Received: February 8, 2017 Published: May 10, 2017 6264

DOI: 10.1021/acs.inorgchem.7b00336 Inorg. Chem. 2017, 56, 6264−6274

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Inorganic Chemistry

that of the reported bulk values while preserving enhanced S and power factor in the synthesized BT nanostructures. Thus, significant enhancement in the overall thermoelectric performance by introducing multiphasic nanostructures has been demonstrated.

enable a morphological tuning. Earlier methods suggested a sufficient reduction in κ by preparing solid solutions of materials where point defects in solid solutions aided the phonon scattering.23 Thus, the phonon glass electron crystal model is well suited for materials having complex crystal structures24 where voids and rattlers could be the phonon scattering centers. Almost all of the modern approaches have aimed at enhancing the phonon scattering at the grain boundaries by quantum confinement effects. The nanostructuring approach enhances the density of states near the Fermi level and decouples the inverse relationship among S and σ.25 Despite the quantum confinement effects, the mean free path of electrons and phonons plays a crucial role when nanostructuring is employed, as it introduces a large density of interfaces. This large density of interfaces enables the preferential scattering of phonons over electrons, and thus the lattice contribution toward κ is reduced while the carrier concentration and σ are preserved.26 Structural changes can affect the TE properties of a material, and one of the main advantages of nanostructuring and introduction of structural changes is the reduced thermal conductivity κ and improvement in the overall thermoelectric figure of merit.8,27 There is an alloy limit in crystalline solids below which it is too difficult to reduce κ without introducing nanostructuring or other structural changes.28 Previous reports have demonstrated that thermal conductivity can be reduced drastically by nanostructuring. Extensive phonon scattering is one of the key reasons for employing nanostructured thermoelectric materials to have a low κ due to the grain boundary effect.29,30 In the present study, we report the temperature dependence of thermoelectric properties and efficiency of chemically synthesized BT nanostructures in terms of ZT. Here we used a refluxing method, one of the bottom-up, surfactant-directed (EDTA as the surfactant), simple, low-cost methods, and obtained the required product with precise control over the stoichiometric balance between Bi and Te. To the best of our knowledge, there have been almost no reports on the preparation of different phases of bismuth telluride nanostructures with control over morphology using an aqueous-based low-temperature refluxing method. To fill this gap, we present a systematic study to observe the effect of reaction time on the crystal structure, morphology, and thermoelectric properties of bismuth telluride nanostructures. Here EDTA is the molecular capping agent, responsible for inhibiting the oxidation apart from its role as a surfactant, and hence stable bismuth telluride nanostructures could successfully be synthesized. In addition to the control over crystal structure and morphology, the stoichiometry of the synthesized samples can be maintained, as the method is carried out at low temperature. Hence, reproducible samples of n-type Bi2Te3 with perfect stoichiometry can be synthesized, which is one of the significant achievements of the present work. This will help in the future to keep the stoichiometry even after the suitable dopants are added for enhanced thermoelectric performance. Hence, we have tried to demonstrate that both structural and morphological changes are appropriate ways to enhance the overall transport properties of materials belonging to the BT category and that the synthesis technique plays a crucial role in maintaining the stoichiometry of the synthesized samples. Thus, our low-temperature refluxing provides potential opportunities for the synthesis of defect-free, stable nanostructured materials through the control of reaction parameters. This study reveals a significant reduction in κ in comparison to

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. In the synthesis of BT nanostructures, 0.2 mmol of BiCl3 and 0.3 mmol of Te powder were used as starting materials. NaBH4 acts as the reducing agent and ethylenediaminetetraacetic acid (EDTA) as the surfactant. Deionized water was used as the reaction medium, and the reaction was carried out at the boiling point of the medium. After being stirred for 10 min, the solution was refluxed, which constitutes the primary stage of reaction, where BiCl3 splits up into bismuth and chloride ions. EDTA is essential to cap the bismuth ions and to control the morphology over different reaction times. Once the reaction temperature reached the boiling point of the medium, NaBH4 was introduced into the mixture to facilitate the reduction reaction. The reaction was carried out for different molarities of NaBH4 (0.5−4.4 M) and different reaction times from 1 to 36 h to get a clear idea of the growth mechanism and thermoelectric properties of BT bulk and nanostructures. As soon as the reaction was completed, the synthesized products were collected, centrifuged, washed with acetone, ethanol, and deionized water several times, and then dried under a vacuum environment. All of the samples were synthesized in powder form and later were sintered into a rectangular shape. After sintering, a pellet from the same batch was powdered for XRD and TEM analysis. For SEM, EDS, and rest of the transport property measurements, sintered pellets were used. Te has a tendency to vaporize at higher reaction timings and especially at a sintering temperature above 600 K due to its high vapor pressure, and hence an insufficient amount of Te can result in the formation of nonstoichiometric compositions which may, in turn, influence the material properties.31,32 Refluxing enables a perfect and ambient condition for the formation of BT structures by controlling the Te evaporation to a certain extent even for prolonged reaction times.16 Thus, precise control over Te evaporation and Te deficiency is a challenging task due to its high vapor pressure in comparison to Bi. The stability of the prepared samples was confirmed by exposing them to open atmosphere for 2 weeks. 2.2. Materials Characterizations. Crystal structures of the synthesized BT nanostructures were determined by X-ray diffraction techniques, XRD (Bruker D8 Advance X-ray diffractometer), using Cu Kα radiation with an X-ray wavelength of 1.5406 Å in the 2θ range of 20−90° with a step size of 0.0167°. Structural refinement was carried out using GSAS-EXPGUI software, and the crystal structures were obtained from Crystal Maker software. The morphologies of the synthesized BT nanostructures were investigated by scanning electron microscopy, SEM (JEOL 7800, operated at 5 kV), and high-resolution transmission electron microscopy, HR-TEM (FEI Tecnai F20, operated at 200 kV). The elemental compositions of all BT nanostructures were determined from SEM equipped with energydispersive X-ray spectroscopy (EDS). BT powders were subjected to high-pressure and high-temperature sintering (HPHTS), where highpressure pelletizing33,34 has been carried out with a uniaxial pressure of 1.2 GPa and sintering at a temperature of 600 K. To avoid the influence of nanoplate orientation on the thermoelectric properties of the synthesized BT nanostructures, the measurements for electrical resistivity (ρ), S, and κ were performed with the samples oriented identically (i.e., perpendicular to the press direction in this study). The density of the BT pellets was determined using an Archimedes setup and was found to be above 95% for all the samples with a maximum of 97% for higher reaction time samples: i.e., samples synthesized above 12 h. The ρ values of the BT rectangular pellets (8 × 2 × 2 mm) were measured using the four-probe method in electrical transport option (ETO) attached to a Physical Property Measurement System (PPMS) supplied by Quantum Design (USA) in the temperature range 2−350 K. Hall resistivities of the BT rectangular pellets (8 × 3 × 2 mm) were measured at room temperature using the ETO probe of PPMS. The κ 6265

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Inorganic Chemistry and S values of the pellets were measured using the thermal transport option of PPMS in the temperature range 260−380 K.

3. RESULTS AND DISCUSSION 3.1. XRD Analysis. The X-ray diffraction patterns of the samples prepared for different molarities of NaBH4 and 1 h reaction time are shown in Figure S1 in the Supporting Information. The synthesized samples were of molarities ranging from 0.5 to 4.4 M of NaBH4. The sample prepared with 4.4 M NaBH4 was confirmed to be BiTe as per the standard powder diffraction data for JCPDS: 83-1749 (ICSD code 30525). From the results obtained, it was confirmed that stable samples of BT nanostructures could be synthesized using the reflux method. Figure 1 shows the XRD patterns along with Rietveld refinement of the BT samples prepared for different reaction times of 1, 6, 12, 24, and 36 h (labeled as BT-1−BT-5, respectively) by keeping the molarity of NaBH4 at 4.4 M. The Rietveld refinement (shown in Figure 1) gives a clear idea of the structural changes, and the structural parameters were determined and are given in Table 1. The χ2 (goodness of fit) is defined as the square of the ratio between the weighted profile R factor (Rwp) and expected R factor (Rexp). A low value of χ2 has been observed, which justifies the quality and goodness of refinement. It is confirmed that a series of structural transition from the BiTe hexagonal phase to the Bi2Te3 rhombohedral phase with JCPDS 85-0439 (ICSD code 15753) takes place with reaction times from 1 to 36 h, and further it is noticed that a combination of different BT nanostructures dominates in BT2 and BT-3 samples: i.e., BiTe and Bi2Te3 phases for the BT-2 sample but Bi2Te3 and Bi4Te3 phases for the BT-3 sample. Finally, a stable Bi2Te3 phase with hexagonal stacking is obtained with a reaction time of 24 h. However, upon further increase in the reaction time, i.e., for 36 h, the Bi2Te3 phase with a rhombohedral crystal structure is formed. The crystal structure has been obtained using the refined parameters and is depicted in Figure 2, and it clearly shows the structural transition from the BiTe phase to the Bi2Te3 phase for 1 to 24 h reaction time. 3.2. Morphological Analysis. 3.2.1. SEM Micrographs. SEM images and EDS spectra of all the samples were taken, and as a representative of the series, BT-4 sample data are shown in Figure S2 in the Supporting Information. All samples show well-formed grains with less porosity, revealing the high densification. The EDS spectrum of the BT-4 sample and the composition data for all the studied samples are given in Table S1 in the Supporting Information. As can be seen from the spectrum and Table S1, the compositions of the lower reaction time sample matches with the BiTe phase, as the sample is exposed to the reaction temperature for a shorter time, in which unreacted Bi and Te ions will be washed off resulting in the formation of BiTe, whereas samples synthesized at a longer reaction time of above 24 and 36 h experience the reaction temperature for a prolonged time and ambient reaction conditions to give the formation of the Bi2Te3 phase. An intermediate composition is observed from the compositional data for BT-2 and BT-3 and reveals the coexistence of different BT phases, which has been confirmed by HR-TEM analysis and is discussed below. 3.2.2. TEM Analysis. Figure 3 shows the TEM images of the BT nanostructures synthesized for different reaction times. The crystal structure of most of the chemically synthesized BT nanocrystals has two-dimensional platelike form.8,35 Figure 3a− j shows the morphology of the BT nanostructures. BT-1 has

Figure 1. Refined XRD patterns of BT nanostructures (a) BT-1, (b) BT-2, (c) BT-3, (d) BT-4, and (e) BT-5.

nanocrystals with sizes ranging from 20 to 25 nm (shown in Figure S3a in the Supporting Information). Initiation of the growth mechanism is clearly seen in BT-2 (Figure 3b,g). In BT2 samples, a platelike morphology having around 100 nm width 6266

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Inorganic Chemistry Table 1. Refined Parameters of BT-1−BT-5 BT structure BT-2

BT-3

BT-1 phase crystal structure space group

BiTe hexagonal P3m ̅ 1

a (Å) b (Å) c (Å) γ (deg) volume (Å3)

4.38(4) 4.38(4) 24.01(4) 120 399.08(5)

χ2

1.05

Bi2Te3 + BiTe hexagonal R3m ̅ + P3m ̅ 1 4.40(3) 4.40(3) 30.08(5) 120 504.99(6) 1.12

Bi2Te3 + Bi4Te3 hexagonal + rhombohedral R3m ̅ Lattice Parameters 4.35(8) 4.46(7) 4.43(8) 4.35(8) 4.46(7) 4.43(8) 24.80(9) 29.57(7) 42.41(1) 120 120 120 406.50(1) 508.80(1) 719.30(1) Residual Parameters 1.10

BT-4

BT-5

Bi2Te3 - H hexagonal R3m ̅

Bi2Te3 - R rhombohedral R3m ̅

4.39(1) 4.39(1) 30.39(3) 120 506.51(4)

10.44(1) 10.44(1) 10.44(1) 24.30(1) 169.22(2)

1.08

1.14

Figure 2. Crystal structures of BT-1 and BT-4 samples.

along the planes perpendicular to the c axis,28,36 as shown in Figure 2. Reflections corresponding to the (011), (104), (018), and (005) planes of the BiTe phase are observed in Figure 3k, and FFT shows the crystalline nature of the BT-1 sample. Nanocrystals in the BT-2 samples synthesized for 6 h reaction time grew with a preferred orientation, as their morphology constituted coarse structures along with the fine structures, as shown in Figure 3b,g. The (015) plane of the grown Bi2Te3 phase is evident from both lattice fringes and SAED patterns, as indicated in Figure 3l,q. An increase in the reaction time yields the formation of nanocrystals as a combination of fine and coarse structures which is in accordance with the earlier reports, where sintering temperature was the varying parameter.11 The high-resolution image and SAED patterns of BT-3 and BT-4 are shown in Figure 3m,r and Figure 3n,s, respectively. It is evident from the FFT and SAED patterns of BT-4 (Figure 3n,s) that the size of the crystals and crystallinity have improved considerably with the reaction time. The lattice

is noticed (Figure S4a in the Supporting Information). When the reaction time is increased to 12 h, i.e. in BT-3, fine and coarse nanostructures are observed with maximum particle size ranging up to 70 nm (Figure 3c,h and Figure S4b). Interestingly, a 24 h reaction time enables the formation of nanocrystals with an average particle size of 20 nm (shown in Figure S3b) and Figure 3d,i clearly illustrates the effect of reaction time on morphological aspects of the BT-4 samples synthesized for 24 h. A further increase in the reaction time, i.e., for the BT-5 sample, causes morphological changes, as shown in Figure 3e,j. The BT-5 sample shows an agglomerated kind of morphology where a significant change in the lattice parameters is seen with a transition to the Bi2Te3 rhombohedral structure. The corresponding lattice fringes and SAED patterns are shown in Figure 3k−t (FFT is shown in the inset). It is well reported that the BT compounds have a layered structure with a Te− Bi−Te−Bi−Te sequence showing weak van der Waals interactions among the Te atoms which allow the cleavage 6267

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Figure 3. BT nanostructures: (a−j) TEM images showing platelike nanocrystals; (k−o) HR-TEM images with lattice fringes (FFT is in the inset); (p−t) SAED patterns.

rates the mixed phases of BT-2 and BT-3 samples as discussed in the XRD analysis. 3.3. Thermoelectric Properties. 3.3.1. Seebeck Coefficient and Thermal Conductivity. Figure 4 shows the temperature dependence of S and κ in the temperature range of 260−380 K. A negative value of S obtained for all the samples (shown in Figure 4a) indicates an n-type semiconducting behavior. BT-1 with a hexagonal BiTe phase shows the highest value of S of −145 μV K−1 around room temperature, whereas a considerable reduction of the same is observed for the BT-2 sample, which shows a maximum S value of −85 μV K−1 at 380 K. When the reaction time was increased further, S values of BT-3 and BT-4 improved further and achieved a peak values of −147 and −270 μV K−1, respectively. It is noted that the S value of the BT-5 sample was reduced to a

fringes corresponding to the (006) plane of the Bi2Te3 phase are shown in Figure 3n, in which it can be observed that uniformly distributed cubelike nanocrystals have been formed for BT-4. Figure 3e,j shows the morphological changes of BT-5 samples. Lattice fringes and diffraction patterns of BT-5 samples, shown in Figure 3o,t, confirm the rhombohedral crystal structure of the BT-5 sample. In order to further confirm the existence of different phases in BT-2 and BT-3, highresolution TEM images showing lattice planes of the corresponding phases are shown in Figure S5 in the Supporting Information. In BT-2, the (015) plane of Bi2Te3 and (104) and (018) planes of BiTe are coexisting, whereas in BT-3, the (015) plane of Bi2Te3 and (009) plane of Bi4Te3 are coexisting (shown in Figure S5a,b, respectively), which further corrobo6268

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Figure 4. Temperature dependence of (a) the Seebeck coefficient S and (b) thermal conductivity κ of BT nanostructures.

Figure 5. BT nanostructures: (a) temperature dependence of resistivity ρ and (b) field dependence of Hall resisitivity ρxy at 300 K.

value below −130 μV K−1. BT-1 shows the typical range of S values for BiTe nanocrystals, and it has been well reported that the Bi2Te3 phase of BT structures can exhibit the maximum value of S in comparison to other phases of the same.6,37 The reduction in S for the BT-2 sample may be due to the transition of crystal structure from the BiTe hexagonal phase to a combination of BiTe and Bi2Te3 phases with variation in the lattice parameters. Furthermore, the temperature dependence of S in the case of BT-3 and BT-4 samples reveals that a significant enhancement in S is achieved as a result of nanostructuring and crystal structure variation. However, BT5 synthesized at 36 h shows S values in the range −80 to −130 μV K−1, which may be due to the Bi2Te3 rhombohedral crystal structure.11 In the present study the BT-4 sample, Bi2Te3 with hexagonal stacking, possesses the highest value of S and is found to be near −260 μV K−1 at room temperature, which is still higher in comparison to values for the conventional state of the art materials.29,32,35 It has been well reported that S is in inverse proportion to carrier concentration, and thus a decrease in S from BT-1 to BT-5 is expected. On the contrary, there is a deviation from the general trend, and this could be due to the different phases present in different BT nanostructures and

potential barrier scattering. These potential barriers will aid the filtering of low-energy carriers at the interface which is responsible for the reduction of S. Thus, filtering of these low-energy carriers could successfully enhance the S value,38−40 and another reason for the enhanced value of S in the BT-4 sample is the improved interface density. We have performed Hall resistivity measurements to confirm that the majority charge carriers are electrons in all the samples, which are discussed in later sections. Figure 4b shows the measured κ values of the BT samples, which are lower than those of the reported bulk ingots Bi2Te3 (1.5 W m−1 K−1) and are comparable to those of recently reported Bi2Te3 nanostructures.29,41 The BT-1 sample shows the lowest κ among the synthesized samples and exhibist a κ value of 0.34 W m−1 K−1 at room temperature. BT-3 and BT-4 also exhibit lower values of κ well below 0.5 W m−1 K−1, which are much less than that of bulk values (0.8−1.1 W m−1 K−1) in the temperature range of 260−380 K and approaches close to the minimum κ among the reported values of Bi2Te3 series.42,43 Thus, for all the samples prepared at different reaction times, it was possible to tune the κ value in the range 0.32−0.68 W m−1 K−1 from 260 to 380 K except in the case of the BT-2 sample. 6269

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a decrease in the ρ values of these samples. However, both electrical and thermal properties strongly depend on the crystal structure and the different combination of BT structures arising from different reaction times could be the reason for the lack of uniformity in the transport properties of different samples.11 The ρ achieved a decreased value of 3.37 × 10−5 Ω m for the BT-3 sample with an n value of 5 × 1019 e cm−3 and a μ value of 35 cm2 V−1 s−1. The n values for BT-4 and BT-5 samples has increased to 8 × 1019 and 14 × 1019 e cm−3 with a decrease in the μ and σ values. Thus, the electron concentration of different BT structures increases from BT-1 to BT-5 and an increase in σ is expected, as σ is proportional to n. The variation of σ in the temperature range 260−350 K has been plotted for all the BT samples and is shown in Figure S6 in the Supporting Information. The inverse proportionality among the n and σ values can be observed in the conventional nanostructured thermoelectric materials synthesized by the bottom-up approach.38,43 The increased μ is expected in samples with coarse structures, due to the fact that the reduced interface density will favor the mean free path of the carriers. Above 260 K, it is observed that σ has increased with an increase in temperature for all of the BT structures. This behavior is in contradiction with the conventional BT structures, where degenerate semiconductor behavior is exhibited. However, if a nondegenerate semiconducting behavior is exhibited by the system, this kind of unusual electrical behavior can be expected. If potential barrier scattering is the dominant scattering mechanism as mentioned earlier, the σ values of nanostructured materials increase with an increase in temperature in a particular temperature range, as indicated in Figure S6 and exhibited by some of the recently reported systems.43−45 If n increases, the concept of mean free path of electrons can be successfully explained by a decrease in μ. Due to the drastic decrease in the μ values and densification in comparison to spark plasma sintered samples,29,46,47 the results achieved for σ are less in comparison to the reported values but there is still an enhanced S with a peak value of about −260 μV K−1 at room temperature, resulting in the overall improvement of power factor and ZT for BT-4, as illustrated in Figure 7. 3.3.3. Power Factor and Figure of Merit. The power factor, which is calculated as S2σ ,is plotted with the variation of temperature and shown in Figure 7a. The power factor value of the BT-1 sample was a maximum of 160 μW m−1 K−2 at 350 K which is decreased to 100 μW m−1 K−2 for the BT-2 sample because of the reduction in S value of BT-2 as discussed in the earlier sections. The structural and morphological variation of BT-2 could also be another reason for the decreased value of power factor. The power factor increased significantly for samples BT-3 and BT-4 when the reaction time increased beyond 6 h. The power factor is at a maximum for BT-4, which increased with temperature from 900 to 1000 μW m−1 K−2, as shown in Figure 7a. In comparison to the bulk Bi2Te3 ingots, the S value of the present sample is around 50% higher and it is also about 2−3 times higher than those of the state of the art nanostructured bulk bismuth telluride structures prepared by arc melting, ball milling, and HPHTS.11,30,48 It is expected that the considerably enhanced S in the BT-4 nanostructures can compensate for the moderately deteriorated σ and in turn lead to the S2σ improvement. Finally, ZT as a function of temperature has been determined and is depicted in Figure 7b, which shows the range of ZT values for different BT structures. The ZT value of BT-1 showsed a linear dependence with the temperature and

The BT-2 sample, which has formed as a combination of BiTe and Bi2Te3 phases, exhibits a κ value of 1.5 W m−1 K−1 as in the case of bulk alloys. This is due to the grown Bi2Te3 coarse structures, as shown in Figure S5a in the Supporting Information, which has a κ value close to that of its bulk counterparts. Among the BT-4 and BT-5 samples, BT-4 has a lower κ value, and this could be possibly due to the fact that uniform nanocrystals with a large number of grain boundaries effectively enhances the scattering of the short-wavelength phonons. Moreover, the fine nanostructures promote the specified orientation along the press direction, which in turn further enhance the phonon scattering in the direction that is parallel to the press direction. Again, the BT-5 sample has a rhombohedral crystal structure with distorted geometry, where atomic packing is less in comparison to hexagonal stacking. A significant reduction in the κ value is observed in the nanostructured single-phase samples with hexagonal stacking, especially in BT-1 and BT-4, which is understood to be due to extensive phonon scattering.29 3.3.2. Electrical and Hall Resistivity. Figure 5a shows the temperature variation of ρ for all the BT nanostructures in the range 5−350 K. All of the synthesized samples show a metal to semiconductor like transition near room temperature. It is observed that the ρ value of BT-1 at 300 K is 1.39 × 10−4 Ω m, which is decreased to 6.82 × 10−5 Ω m for BT-2 and 3.37 × 10−5 Ω m for BT-3. It should be noted that the BT-1 sample has a single phase of BiTe, whereas the BT-2 and BT-3 samples contains multiphase and coarse structures, which results in the drastic decrease of ρ with an increase in reaction time. Again, in the case of BT-4 and BT-5 samples where the single phase of Bi2Te3 is formed but with different crystal structures (i.e., hexagonal for BT-4 and rhombohedral for BT-5) is observed, the ρ values are found to be 7.12 × 10−5 and 8.22 × 10−5 Ω m, respectively, which are close to that of BT-2. Further, Hall measurements were conducted on the BT pellets, and the variation of Hall resistivity (ρxy) with temperature is shown in Figure 5b. All of the samples show negative Hall coefficients (RH), confirming that electrons are the majority charge carriers in the BT samples. This result is concordant with the values of S discussed in the earlier sections. The carrier concentration (n) has been calculated from the relation n = 1/RH and the mobility (μ) from the relation ρ = 1/(neμ), where e is the electronic charge (1.6 × 10−19 C). The n and μ values calculated from the Hall measurements are plotted in Figure 6. BT-1 has an n value of 5 × 1019 e cm−3, which is increased from 2 × 1019 e cm−3 for BT-2 to 14 × 1019 e cm−3 for BT-5, and there is expected to be

Figure 6. Mobility and carrier concentration versus reaction time of BT nanostructures. 6270

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Figure 7. Temperature dependence of (a) power factor (PF) and (b) figure of merit (ZT) of BT nanostructures.

Table 2. Thermoelectric Parameters of BT Nanostructures at 300 K sample

S (μW K−1)

κ (W m−1 K−1)

ρ (10−5 Ω m)

σ (S m−1)

μ (cm2 V−1 s−1)

n (1019 e cm−3)

power factor (μW m−1 K−2)

ZT

BT-1 BT-2 BT-3 BT-4 BT-5

−141 −57 −140 −258 −113

0.34 1.49 0.47 0.42 0.51

13.99 6.82 3.37 7.12 8.22

7148 14663 29674 14045 12165

10 40 35 10 5

5 2 5 8 14

142 48 582 935 155

0.12 0.01 0.37 0.67 0.09

achieved a peak value of 0.15 at 350 K. As a result of decrease in S and power factor, the BT-2 sample shows a drastic reduction in the ZT value below 0.10 and is less effective for use as a TE material. Significant improvement in ZT is observed for BT-3 and BT-4 samples synthesized for 12 and 24 h, showing peak values of 0.45 and 0.81, respectively, at 350 K. A further increase in reaction time causes a decrease in the TE properties in BT-5 and overall reduction in the ZT value to 0.11 at 350 K. The enhanced ZT values of BT-3 and BT-4 are comparable to those of the previously reported Bi2Te3 nanostructures.49−51 The measured ZT value for BT-4 is higher than those of recently reported, solution-synthesized Bi2Te3 samples.52 Furthermore, both the increased power factor and the reduced κ along with a resistivity range comparable to that of semimetallic materials result in an enhanced ZT in the BT-3 and BT-4 samples (Figure 7b). The thermoelectric parameters of all BT samples at room temperature are given in Table 2. The variation in TE properties can be well explained by grain boundary scattering and formation of coarse structures.11 The single-phase samples form as fine nanocrystals, whereas mixedphase samples form coarse structures along with fine structures, as shown schematically in Figure 8. XRD and TEM images confirm the phase as well as structural changes of synthesized samples with reaction time. The BT-1 sample forms the BiTe phase with an average size of 24 nm. Due to the formation of nanocrystals, BT-1 exhibits a κ value of 0.34 W m−1 K−1 at room temperature and grain boundary scattering facilitates the reduction in κ for BT-1 sample. S and σ values exhibited by the BT-1 sample are −141 μV K−1 and 7148 S m−1 at 300 K, which are in the typical range of a BiTe phase and are confirmed by the XRD refinement results. The formation of coarse structures is noticed for mixed phases (i.e., for BT-2 and BT-3 samples),

which favors electron transport, as the number of grain boundaries have decreased due to the growth of coarse structures. Coarse and fine structures for BT-2 and BT-3 are clearly shown in Figure S4 in the Supporting Information. Electrons can move easily in coarse structures and through the electrically neutral grain boundaries, unlike phonon scattering, which enhances electron transport in fine and coarse structures. This is the reason for the enhancement of σ in BT-2, and on the other side S is reduced drastically to a value near −60 μV K−1 at room temperature. As the grain boundary effect is less in comparison to that for fine nanocrystals of BT-1, the BT-2 sample exhibits the maximum κ and the microstructures are predominant over the fine structures, as illustrated in Figure S4a. When the reaction time is increased to 12 h, a perfect combination of fine and coarse structures is observed for BT-3 and as a result both κ and σ are enhanced, where σ is maintained by the coarse structure and κ is maintained by the fine structure. A compromise is observed in the case of S as well, and thereby the overall ZT is tuned to 0.5 for the BT-3 sample at 350 K. A further increase in reaction time to 24 h enables the formation of nanocrystals of single-phase hexagonally stacked Bi2Te3 (BT-4), where the κ value is less in comparison to those for BT-2 and BT-3 but slightly higher in comparison to that for BT-1. BT-4 nanostructures could considerably enhance the power factor and preferentially scatter the phonons due to high densification and efficient stacking of the nanoplates. The slight increase in κ of BT-4 in comparison to that of BT-1 single phase could be due to the different phases exhibited by BT-1 and BT-4. High crystallinity and hexagonal stacking53 are evident from the diffraction pattern, as depicted in Figure S7 in the Supporting Information for BT-4. As a result, the TE characteristics of the BT-4 sample have been 6271

DOI: 10.1021/acs.inorgchem.7b00336 Inorg. Chem. 2017, 56, 6264−6274

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Inorganic Chemistry

4. CONCLUSION In summary, bismuth telluride (BT) nanostructures have been synthesized successfully by a simple, low-cost, and lowtemperature reflux method using deionized water as the solvent. Structural and morphological changes were obtained by varying the reaction time, which is confirmed from XRD, SEM, and TEM analysis and energy dispersive X-ray spectroscopy, revealing a perfect stoichiometry of the synthesized nanostructures. The reaction time facilitates the formation of different BT structures transforming from the BiTe phase to Bi2Te3 phase, and BT-4 samples synthesized for a reaction time of 24 h show hexagonal stacking and exhibit the best thermoelectric properties among the synthesized samples. Grain boundary scattering along with fine and coarse structures could successfully explain the thermoelectric properties of the synthesized samples. Even though ρ is considerably higher in comparison to the previously reported materials, temperaturedependent transport studies reveal that the increase in S is significant, thus increasing the power factor ,and ultimately a high ZT of 0.81 is observed for BT-4 at 350 K. These results indicate that optimization of the reaction method and conditions could provide defect-free, stoichiometric products, and both structural and morphological changes are appropriate to enhance the overall thermoelectric properties of materials belonging to the BT category using reflux techniques. Thus, our approach offers potential opportunities for tuning the thermoelectric properties of stable and defect-free nanostructures in the future by suitable substitutions and through the control of reaction parameters. Our research offers a simple and economical method to fabricate stable Bi2Te3 nanostructures for low-temperature thermoelectric applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00336. XRD patterns of the BT structures prepared for different molarities of NaBH4, BT-4 sample SEM image and EDS spectra, particle size distributions of BT-1 and BT-4, TEM images giving coarse and fine structures of BT nanocrystals BT-2 and BT-3, HR-TEM images showing planes corresponding to different phases of BT nanostructures BT-2 and BT-3, temperature variation of electrical conductivity of BT nanostructures, and SAED pattern of BT-4 showing hexagonal stacking (PDF)

Figure 8. Schematic diagram explaining the thermal and electrical properties of BT nanostructures.

Accession Codes

CCDC 1546845−1546851 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

enhanced considerably. A further increase in reaction time to 36 h induces structural and morphological changes which are responsible for the inferior TE properties of BT-5 sample in comparison to BT-3 and BT-4. Again, the hexagonal structure of the BT-4 sample has more atomic packing in comparison to that of the rhombohedral structure of the BT-5 sample, which results in enhanced TE properties in spite of having the same Bi2Te3 phase in both samples. These results demonstrate that nanostructuring and careful optimization of coarse structures along with fine grains could enhance the overall TE properties of different BT nanocrystals.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.V.: [email protected], [email protected]. ORCID

M. Vasundhara: 0000-0002-4004-8186 6272

DOI: 10.1021/acs.inorgchem.7b00336 Inorg. Chem. 2017, 56, 6264−6274

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Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support received from the Council of Scientific and Industrial Research (CSIR), Government of India, sponsored projects CSC0132 and CSC0114. The authors acknowledge the various instrumental facilities of CSIR-NIIST. V.R.A. is grateful to the Academy of Scientific and Innovative Research and CSIR for granting a Fellowship. The authors also thank the Board of Research in Nuclear Sciences, sponsored project GAP 218939, for partially supporting this work.

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