Surfactant-Induced Structural Phase Transitions and Enhanced Room

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Surfactant-Induced Structural Phase Transitions and Enhanced Room Temperature Thermoelectric Performance in n‑Type Bi2Te3 Nanostructures Synthesized via Chemical Route V. R. Akshay,†,‡ B. Arun,†,‡ M. V. Suneesh,† and M. Vasundhara*,†,‡ †

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

ABSTRACT: A systematic study of surfactant-assisted aqueous-based low-temperature chemical method for the synthesis of different phases of Bi−Te-based nanostructures with different morphologies ranging from nanocrystals to nanorods/nanosheets is investigated. The n-type Bi 2Te3 nanostructures are inherited from the low-temperature reflux reaction, and a structural phase transition is established for different surfactant concentrations and reaction time. Simultaneous optimization of reaction time and surfactant concentration yields the formation of hexagonal Bi2Te3 nanocrystals even with lower reaction time, which is the desirable crystal structure for obtaining enhanced thermoelectric properties. Tuning the surfactant concentration from 50 to 100 mmol facilitates the formation of low-dimensional structures of Bi2Te3, which is evident from the refined X-ray diffraction results and high-resolution transmission electron microscopy analysis. Bi2Te3 nanostructures inherited from 24 h reaction time with 100 mmol surfactant concentration exhibit a promising figure of merit of 0.75 at 300 K. An in-depth understanding of the reaction mechanism to form BT nanostructures is explained. The present study provides an efficient and simple method to develop low dimensional nanostructures for improved thermoelectric performance. KEYWORDS: chalcogenide materials, nanostructuring, nanorods/nanosheets, low thermal conductivity, figure of merit application only when it possesses a high S and σ along with a glass-like κ. The inherent material conflict reflects in S, σ, and κ that make it difficult to tune all these properties simultaneously. Nowadays, significant enhancement in the TE efficiency has been achieved by increasing ZT within nanostructured materials,7−10 where the major target lies in the drastic reduction of κ by encouraging phonon scattering with nanograins. In this context, achieving an enhanced power factor, PF (S2σ), along with a significant reduction in κ becomes so crucial to further enhance ZT. Among the commercially available chalcogenide based TE materials, Bi2Te3 (BT) based materials are the suitable thermoelectric candidates for room temperature applications.11−18 The narrow energy gap and layered crystal structure make BT a potential candidate that meets the criteria of high PF and low κ. In comparison to its bulk counterpart, nanostructured BT-based material is expected to exhibit

1. INTRODUCTION Thermoelectric (TE) materials are the efficient class of materials, which could reversibly convert heat to electricity and offers a suitable solution to balance the energy crisis and environmental pollution caused by the excessive consumption of conventional fossil fuels.1−3 The world’s increasing energy demand raises the usage of fossil fuel and environmental impact on the global climate change is becoming increasingly alarming. This key issue forces us to think about devices that can tackle energy issues not only with high efficiency but also with less carbon emission. One of the possible ways to achieve this goal is to develop technologies based on any of the renewable energy sources.4,5 TE devices hence attract the attention of scientific community which consists of both p-type and n-type materials that can enable a direct and reversible conversion of thermal energy to electrical energy. Energy conversion efficiency of a TE material is directly related with the dimensionless figure of merit, ZT = S2σT/κ, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and the absolute temperature, respectively.1−3,6 Hence, a material can be considered suitable for TE © XXXX American Chemical Society

Received: March 23, 2018 Accepted: May 31, 2018

A

DOI: 10.1021/acsanm.8b00464 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Scheme 1. Schematic Representation of Overall Synthesis Process and Different Achievements during the EDTA-Assisted Reflux Method

much lower κ and provides new pathways to tailor the TE properties.13 Especially, nanocrystals of BT structures with R3m ̅ space group and hexagonal stacking exhibit excellent phonon scattering because of the dense grain boundaries after sintered into pellets. To further strengthen the phonon scattering and to achieve a significant reduction in κ, we can optimize the synthesis conditions of BT nanostructures, where crystal structure, particle size, and morphology could favorably boost up the TE properties.19−22 The widely used synthesis techniques of nanocrystalline BT structures are solvothermal, sonochemical, melt spinning, exfoliation techniques, etc., and which requires highly desirable synthesis conditions often together with toxic and hazardous reaction medium.23−29 Recently, a facile and aqueous-based green synthesis of nanocrystalline BT material is reported under ambient conditions with control over crystal structure and morphology, which is quite good enough to perform as an efficient thermoelectric material.22 Previous reports suggest that it is desired to develop a procedure to synthesize shape and size-controlled BT nanostructures to further improve the TE properties.21,22 The synthesis conditions could favorably affect the chemistry of these materials, where sufficient reduction in κ is possible by point defects, voids, and rattlers and grain boundary effect. Thus, the phonon glass electron crystal model is well expected for materials having long c-axis, where inherent κ reduction is expected as in the case of bismuth chalcogenides.30 Unfortunately, there is an alloy limit of κ in crystalline solids and to cross this limit, nanostructuring and structural changes are the recommended remedies.31 Thus, previous reports on chalcogenide materials have demonstrated that κ can be decreased significantly by nanostructuring.22,32−35

Grain boundary effects allow extensive phonon scattering, which is one of the key reasons for employing nanostructured TE materials to have low κ. Almost all the synthesis techniques have focused on phonon scattering at the grain boundaries by quantum confinement effects.36−38 Apart from the quantum confinement effects, the mean free path of electrons and phonons plays a crucial role when nanostructuring is employed in these materials. As a result, a large density of interfaces are formed, which enables the preferential scattering of phonons over electrons, and thus the lattice contribution toward κ is decreased, while the carrier concentration and σ are preserved. In the present study, we report the temperature dependence of TE properties and efficiency of chemically synthesized BT nanostructures in terms of ZT. Here we used a simple refluxing method in accordance with our recent report22 and obtained the required product with different crystal structures, morphologies and resistance toward oxidation. We attempted to investigate the combined effect of surfactant concentration and reaction time involved in the synthesis on the crystal structure and morphologies of BT nanostructures. To the best of our knowledge, there have been almost no reports on the preparation of low-dimensional structures (1D/2D) of BT nanostructures using an aqueous-based low-temperature refluxing method. Here ethylenediaminetetraacetic acid (EDTA) acts as the molecular capping agent, responsible for inhibiting the oxidation apart from its role as a surfactant, and hence stable BT 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 and interestingly EDTA at higher concentration B


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Figure 1. Refined XRD patterns of BT nanostructures (a) BT50-1, (b) BT50-12, (c) BT50-24, (d) BT100-1, (e) BT100-12, and (f) BT100-24. and a schematic representation of the same is shown in Scheme 1 in which EDTA concentration was fixed at 75 mmol. In the present synthesis, the reaction was carried out both below and above 75 mmol EDTA concentrations (i.e., 50 and 100 mmol) where reaction times varied from 1 to 24 h to get a clear idea of the growth mechanism and TE properties of BT nanostructures. The reaction time is restricted to 24 h because beyond 24 h the structural transition from hexagonal to rhombohedral takes place, results in the deterioration of TE properties as mentioned in our previous work.22 It can be noted that the experimental data present in this work is on 50 and 100 mmol series of EDTA concentrations, whereas 75 mmol series data is taken as a reference from our previous work for the comparative study.22 The BT nanostructures prepared for different EDTA concentrations (50, 75, and 100 mmol) and reaction times (1, 12, and 24 h) are labeled as (BT50-1, BT50-12, BT50-24, BT75-1, BT75-12, BT75-24, BT100-1, BT100-12, and BT100-24) respectively. The precipitated BT nanostructures after refluxing is washed with acetone, ethanol, and deionized water several times until a clear solution is obtained. Vacuum dried BT powders at 393 K were subjected to high-pressure pelletizing40 with a uniaxial pressure of 1.2 GPa. These pellets were sealed in an evacuated quartz tube and subjected to sintering at 600 K for 3 h. Sintering at a temperature above 600 K is not favorable here due to the tendency of Te to vaporize especially at a sintering temperature above 600 K because of its high vapor pressure, and hence an insufficient amount of Te can significantly affect both stoichiometric chemical composition and material properties. The sintered pellets from each batch were powdered for X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray spectroscopy (EDS) analysis, whereas Xray photoelectron spectroscopy (XPS) and TE measurements were

could act as a soft template for the formation of low dimensional structures as in the case of high-temperature chemical reactions.13,39 Hence, reproducible samples of n-type BT nanostructures with a variety of structural, as well as morphological variations could be synthesized, which is one of the significant achievements of the present study. A reaction mechanism suggesting the formation of these low dimensional nanostructures has also been presented and we have tried to demonstrate that both structural and morphological changes are appropriate ways to enhance the overall TE properties of materials belonging to the BT category. Thus, the lowtemperature reflux technique provides potential opportunities for the synthesis of low dimensional, oxidation resistant and highly efficient stable nanostructured materials through the control of both EDTA concentration and reaction time. This study reveals a significant reduction in κ in comparison to that of the reported bulk values while preserving enhanced σ and overall ZT value in the synthesized BT nanostructures. Thus, significant enhancement in the overall TE performance by introducing low-dimensional nanostructures has been demonstrated in this work.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. In the synthesis of BT nanostructures, a refluxing technique is employed using BiCl3 and Te powder where NaBH4 is the reducing agent and EDTA is the molecular capping agent. This synthesis procedure is adopted from our previous studies22 C


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Figure 2. BT50 and BT100 nanostructures: (a−o) TEM images showing the formation of different nanostructures for 50 and 100 mmol EDTA concentration with reaction time, (p−r) HR-TEM images showing lattice fringes of BT100 series with FFT shown in the inset, and (s−x) SAED patterns of BT50 and BT100 series 2.2. Materials Characterizations. Crystal structures of the synthesized BT nanostructures were determined by XRD (Bruker D8 Advance X-ray diffractometer), using Cu Kα radiation with an Xray 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 GSASEXPGUI software, and the crystal structures were obtained from Crystal Maker software. The morphologies of BT nanostructures were investigated by HR-TEM (FEI Tecnai F20, operated at 200 kV). The elemental compositions of all BT nanostructures were determined from TEM equipped with EDS. To understand the effect of EDTA concentration on nanosheet formation of the synthesized samples, atomic force microscopy (AFM) analysis in the tapping mode was

carried out on sintered pellets. The careful optimization of EDTA concentration could enable an ambient condition for the formation of low dimensional BT structures by decreasing the surface energy. Thus, optimizing both EDTA concentration and reaction time could deliver stable, phase pure compounds of BT nanostructures. The stability of the prepared samples are directly related to the concentration of capping agent and was confirmed by exposing them to open atmosphere for two months before measuring the transport properties and XPS measurements have been done even after six months of sample preparation. The overall synthesis process and different achievements during the EDTA assisted reflux method is represented in Scheme. 1. D


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ACS Applied Nano Materials carried out (Bruker Multimode, Germany). XPS measurements were carried out using a Kratos Analytical Instrument, UK (SHIMADZU group, Model: AXIS Supra). A monochromated Al Kα source (Energy= 1486.6 eV) was used to probe the BT samples. The binding energies were corrected by C 1s as reference energy (C 1s = 284.8 eV). A wide scan was collected to ensure that no foreign materials were present on the sample surface. Narrow scans of Bi 4f and Te 3d regions were collected. Curve fitting to the XPS spectrum was done using XPSPEAK 4.1 software. Background subtraction was done using the Shirley method. BT powders were subjected to high-pressure and high-temperature sintering (HPHTS) and density of the BT pellets was determined using an Archimedes setup and was found to be near 95% for all the samples.22,40 To avoid the influence of nanostructures orientation on the TE properties of the synthesized BT material, the measurements for electrical resistivity (ρ), S, and κ were measured with the samples oriented identically (i.e., perpendicular to the press direction in this study). The ρ values of the BT rectangular pellets (8 × 2 × 2 mm) were measured using a closed-cycle cryocooler integrated cryostat (Janis Research Co., SHI SRDK-408 SW, Wilmington, NA) by DC four-probe technique in the temperature range 20−300 K. The temperature of the sample was precisely controlled and monitored using two temperature controllers (Lakeshore-340 and 332, Westerville, OH). The κ and S values of the pellets were measured using the thermal transport option of physical property measurement system supplied by Quantum Design, USA, in the temperature range 260−380 K. The uncertainty of the transport measurements S, σ, and κ is estimated to be about 5%.

Bragg’s reflections of BiTe and pink bars indicating the Bragg’s reflections of Bi2Te3. The expanded view of the refinement for both BT50-24 and BT100-24 is shown in Figure S2, which indicates the absence of impurity peaks in the single-phase Bi2Te3 sample. The crystal structures of BT50, BT75, and BT100 series has been obtained using the refined parameters and is depicted in Figure S3, and it shows the structural transition from mixed phases of different BT structures to Bi2Te3 single phase material. Hence the structural variations have resulted from the aqueous-based reflux method where EDTA concentration and reaction time acts as the key reaction parameters. As mentioned above, the shifting of stable Bi2Te3 phase formation toward lower reaction time with the increase in surfactant concentration is also clearly demonstrated in the crystal structure diagram (Figure S3). 3.2. Morphological Analysis. Detailed TEM micrographs are shown in Figure 2 for both BT50 series and BT100 series. From the morphology of BT50 series, it is evident that nanostructuring gets facilitated with reaction time even for the same EDTA concentration. Interestingly, the morphology is highly dependent on EDTA concentration as well, which is evident from the previous reports and these reports suggest the formation of BT nanostructures with a variety of dimensions like nanorods, nanosheets and nanoplates.41−45 Figure 2a,g,m, 2b,h,n , and 2c,i,o shows the morphology of the BT50-1, BT5012, and BT50-24, respectively. Both BT50-1 and BT50-12 shows an agglomerated morphology and is evident in Figure 2m and 2n. Upon further increasing the reaction time to 24 h, that is, in BT50-24, a complete bridging among the adjacent nanocrystals are observed with maximum particle size ranging up to 50 nm (Figure 2o). The corresponding SAED patterns of BT50 series are shown in Figure 2s−u. Reflections corresponding to the (214), (208), and (0012) in SAED pattern correspond to the BiTe and Te phases as shown in Figure 2s, which is in corroboration with the XRD refinement results. Nanocrystals of BT50-12 samples show three different structures from the SAED patterns as indicated in Figure 2t, those correspond to Bi4Te3, Bi2Te3, and Te, which is well in agreement with the XRD analysis. It is reported that the BT compounds can have mixed phases under some preferable conditions where reaction time, surfactant concentration and sintering temperature, etc. could play a crucial role in determining the reaction conditions and formation mechanisms20,22 which will be discussed in the later sections. An increase in the reaction time to 24 h yields the formation of Bi2Te3 hexagonal nanocrystals in a connected morphology which is in accordance with the earlier reports20,46 and SAED patterns of BT50-24 is shown in Figure 2u, further, confirm the hexagonal phase. The BT75 series exhibited a plate-like morphology up to 24 h reaction time22 and we could suggest that EDTA plays an inevitable role in nanocrystal formation. Furthermore, it is evident from the TEM images of BT100 series that rod-like structures have formed in comparison with the other series of BT nanostructures. Figure 2d−f shows the morphology of the BT100-1, BT100-12, and BT100-24, respectively. BT100-1 shows a combination of nanorods and nanosheets, along with some nanocrystals where the nanorod dimensions are found to be of 95 nm width and 250 nm length (shown in Figure S4). Nanorod and nanosheet formation is facilitated with increased reaction time as well in BT100 series, which could be seen in BT100-12 (Figure 2e and 2k). In BT100-12 samples, nanorod morphology having around 35 nm width, which is intercalated among nanosheets is noticed

3. RESULTS AND DISCUSSION 3.1. Structural Analysis. Figure 1 shows the Rietveld refined XRD patterns of the BT samples prepared for different EDTA concentrations and reaction times and the refined parameters are tabulated in Table S1. The quality and goodness of refinement is represented using the residual parameters which are well in accordance with the expected results. The Rietveld refinement of BT50 series (shown in Figure 1a−1c) reveals the structural changes with the increase in reaction time. A combination of BiTe and Te is observed for BT50-1, whereas Bi2Te3, Bi4Te3, and Te is observed for BT50-12. For lower reaction time (i.e., BT50-1 and BT50-12), Te impurity is seen to be present which disappears for higher reaction time, that is, for 24 h and forms a stable Bi2Te3 structure. An increase in EDTA concentration from 50 to 75 mmol, the Te impurity completely disappears and facilitates the BT structure formation in the early stage, but the 24 h reaction seems to be the best optimization for the formation of hexagonally stacked Bi2Te322 in accordance with our previous report. When the EDTA concentration is further increased to 100 mmol, a similar observation is noticed, but Bi2Te3 formation is facilitated even at the lower reaction time (i.e., BT100-1) along with BiTe phase. Finally, a stable Bi2Te3 phase is obtained for both 12 and 24 h reaction time samples. The Rietveld refinement of BT100 series (shown in Figure 1d−1f) gives the observed structural changes. It is confirmed that structural transitions from mixed phases of different BT structures to hexagonal phase of Bi2Te3 take place with the reaction parameters from BT50-1 to BT100-24. There is a diffraction peak shift for the synthesized single phase Bi2Te3 and mixed phases of BiTe and Bi2Te3, which is because solution-based chemical synthesis can cause such phenomenon.34,35 We tried to emphasize this peak shifts from the detailed refined XRD patterns and Figure S1 represents the peak shift associated with different samples of BT100 series. Again, considering Figure 1d, where both BiTe and Bi2Te3 phases are coexisting, a clear shift is indicated for both BiTe and Bi2Te3 phases, brown bars representing the E


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alloying process. In such a case Bi3+ ions will become Bi with the action of BH4− ions and elemental Bi could combine with elemental Te to form bulk Bi2Te3 as in the case of typical alloying which is represented in eqs 1 and 2.

(Figures 2k and S5). Upon increasing the reaction time to 24 h, that is, in BT100-24, a perfect combination of nanorods, nanosheets and nanoflakes are observed which is evident from the TEM images (Figures 2f and 2l and S6). Hence, the reaction time optimization enables the formation of nanorods along with nanosheets as given in Figure 2l, the dimensions of which are represented in Supporting Information (shown in Figure S6). The high-resolution image (FFT is shown in the inset) and SAED patterns of BT100 series are shown in Figure 2p−r and 2v−x. Reflections corresponding to the (1010), (002), (003), and (104) planes of BiTe phase are observed in BT100-1, which is represented in Figure 2p and FFT shows (inset of Figure 2p) the crystalline nature of the BT100-1 sample. Nanorods in the BT100-12 samples grew with a preferred orientation, as their morphology constituted both rod and sheet-like structures as shown in Figures 2k and 2q. The (003) and (006) planes of Bi2Te3 phase is evident from both lattice fringes and SAED patterns as indicated in Figures 2q and 2w. It is evident from the FFT and SAED patterns of BT100-12 that the formation of the nanorod/nanosheet and their crystallinity has improved considerably with reaction time. The important observation is that Bi2Te3 single phase is achieved quickly as compared with BT-50 and BT-75 series, which could be due to the reaction mechanism favored with EDTA concentration which will be discussed in later sections. A further increase in the reaction time to 24 h yields the formation of perfectly intercalated nanorods (Figure 2l) which is in accordance with the earlier reports, where synthesis conditions were different.41,47−49 The high-resolution image and SAED patterns of BT100-24 is shown in Figure 2r and 2x, respectively. It is very much evident from the FFT and SAED patterns of BT100-24 that highly crystalline nanorods and nanosheets have been formed with 24 h reaction time. The lattice fringes correspond to the (015) plane of the Bi2Te3 phase is shown in Figure 2r, in which it can be observed that nanorods have been grown in the preferred orientation. From TEM analysis, we could conclude that the desirable phase of BT nanostructures has facilitated with EDTA concentration. However, BT100-1 exhibited multiphases as in the case of BT50 and BT75 series. Both 12 and 24 h reaction time enables the formation of Bi2Te3 phase, which further corroborates the mixed phases of BT100-1 and Bi2Te3 single phase of BT100-12 and BT100-24 samples as discussed in the XRD analysis, which is further in accordance with the EDS analysis which is represented in Figure S7 and Table S2. To explore more on the reaction mechanism and morphological evolution, a deep understanding of the chemistry of these materials have discussed below. 3.3. Reaction Mechanism for BT Nanostructure Formation. In our previous report,22 we presented a reflux reaction technique using Bi source (Bi3+ ions) from a bismuth precursor, bismuth chloride (BiCl3), which could then be reacted with anions like Te2− to obtain a nanostructured Bi2Te3 material. Unlike most of the chemical reactions using organic solvents as the reaction medium, we used deionized water to carry out the reaction where BiCl3 reacts with water to form bismuth(III) dihydroxide-chloride. When the temperature of the medium increases, this bismuth(III) dihydroxide-chloride could act as Bi3+ ion contributor and Te powder could give away two electrons upon reduction where Bi and Te ions could combine to form Bi 2 Te 3 structures. Another possible mechanism for the formation of these nanostructures could be a direct combination of metals as in the case of a typical

BiCl3 + 2H 2O → Bi(OH)2 Cl + 2HCl

(1)

3BH4 − + Bi 3 + + 24OH− → 3H 2BO−3 + 15H 2O + Bi (2)

2Bi + 3Te → Bi 2Te3

(3)

Because of the extensive precipitation and formation of Bi(OH)2Cl, it is impossible to synthesize the desired nanomaterial in an aqueous medium without selecting a proper capping agent. In the absence of such a complementary agent, inorganic reducing agents like sodium borohydride (NaBH4), which are commonly used for the reduction of metal ions, have significant reducing power to convert Bi(OH)2Cl directly to bismuth particles (Bi0), and the reaction in eq 3 is more favorable in the chemical reaction. This reaction mechanism can be directed either toward unreacted metal ions or the existence of some unstable phases of these BT structures which could be the reason for impurity phases present in the BT50 series where an insufficient capping agent concentration exists. Hence reaction represented in eq 3 can be treated as an undesirable one and which should be hindered especially in the synthesis of BT nanostructures. Ligands such as ethylenediaminetetraacetic acid (EDTA) can coordinate with several inorganic ions to form multinuclear complexes. It is well reported that such structures can also act as a template in self-assembly process.50−52 It is expected that an oriented attachment may occur with the template effect of EDTA by adding appropriate ligands to the reaction mixtures. Here, we show that nanorods and nanosheets of Bi2Te3 structures (as shown in Figure 2) can grow via varying the EDTA concentration. In the refluxing process, reaction occurs at the boiling point of water and ambient atmospheric conditions, it is expected that complete formation of 1D structure will be a tedious job46 but still a combination of 1D and 2D structures could be developed where we could effectively control the κ and overall TE performance of these BT nanostructures. These rods and sheets are arranged through template action of the EDTA additive which is a favorable mechanism as per the recent reports where expensive techniques such as solvothermal/hydrothermal methods and high reaction temperature were used to develop the desired nanomaterials.13,19,47 Here, we could successfully lower the reaction temperature and executed an aqueous based reaction to deliver the nanostructured material with desired crystal structure and morphology. The TEM images in Figure 2 shows the various morphologies of the BT nanostructures prepared using NaBH4 as the reductant with EDTA as a surfactant. The BT nanostructures synthesized by reflux method with less amount of EDTA concentration (Figure 2a) mainly contained irregular nanocrystals of about 50−100 nm width. The formation of nonuniform nanocrystals could be due to the lack of sufficient concentration of EDTA where the abovementioned reaction mechanism will lead to the formation of agglomerated morphologies. When EDTA is used as a capping agent, the following reactions could be favorable on Bi since, upon reaction with metals, EDTA shows a typical tendency to form EDTA-metal ion complexes as shown in Figure S8. F


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Figure 3. Schematic diagram showing the proposed reaction mechanism for the formation of BT nanostructures.

BiCl3 + 2H 2O → Bi 3 + + 2OH− + Cl− + 2HCl

(4)

C10H16N2O8 → [C10H12N2O8]4 − + 4H+

(5)

Uniform thin nanorods along with nanosheets of a few hundred nanometers were formed for BT100 series, as shown in Figure 2d−f and 2j−l. As shown in Figures S4−S6, the nanosheets and nanorods formation among BT nanostructures is occurring with different reaction time in BT100 samples. It is interesting to note that reaction time facilitates the formation of nanorods and nanosheets which is in accordance with our previous report where nanocrystal formation was optimized with reaction time alone.22 From Figure 2a and 2g, it can be seen that nanocrystals with agglomerated morphology for BT50 series could be because the amount of EDTA is not sufficient to cap the Bi3+ ions, so that ionic reaction process will occur partially but the reactions mentioned in eqs 1−3 will allow the formation of BT agglomerates. When EDTA reaches a critical concentration, a sufficient amount of which could cap the entire Bi3+ ion, and it will enable the formation of uniformly distributed nanocrystals of BT. If EDTA concentration exceeds the critical concentration, these EDTA agglomerates can act as soft templates for the formation of nanorods/nanosheets, which is schematically represented in Figure 3 and which suggests the formation of 1D and 2D structures even in an aqueous medium by adopting a low temperature refluxing technique under ambient conditions. The above-mentioned reaction processes dominate the formation of BT nanostructures under different reaction conditions, which results in different morphologies of BT nanostructures as shown in Figure 3. As discussed in the previous section, morphological control was difficult when less amount of EDTA was used for BT50 series and increasing the concentration of which would likely to facilitate the formation of 1D/2D structures as nanorods or combination of sheet-rod structures. It is well-known that EDTA is a multidentate ligand with polyfunctional groups and it could effectively serve as bridging ligands to form multinuclear complexes with metal ions above some critical concentration. Then chains of crystalline seeds would form in the nucleation process especially with reaction time, which finally yields sheet/rod-like structures of our desired compound. Here, EDTA acts as a structure directing agent in the formation of BT nanostructures. The present approach using reflux technique could favor the reaction mechanism through nucleation and growth process. The crystalline BT nanostruc-

[C10H12N2O8]4 − + 2NaOH → [Na 2C10H12N2O8]2 − + OH−

(6)

Bi 3 + + [Na 2C10H12N2O8]2 − → [Bi(C10H12N2O8)]+ + 2NaOH

(7)

The chemical structures of EDTA are shown in Figure S8, where Figure S8a shows C10H16N2O8, which is not completely soluble in water but in the presence of NaOH and H2O, where reactions mentioned in eqs 5 and 6 is liable to occur. Na-EDTA can effectively involve in the chemical reaction, where it could act as a capping agent and the structure of which is represented in Figure S8b. The real capping mechanism of Na-EDTA with Bi3+ ion is mentioned in eq 7 and is diagrammatically depicted in Figure S8c. Now the reduction mechanism of Te in the ionic reaction process may be considered by the following chemical equations: 3Te + 6OH− → 2Te2 − + TeO32 − + 3H 2O

(8)

2TeO32 − + NaBH4 → Te 2 − + Te + NaBO3 + H 2O + 2OH−

(9)

5Te + 10OH− + NaBH4 → 5Te 2 − + NaBO3 + 7H 2O (10)

It is interesting to note that complete reduction of Te is not occurring in a single step. The reduction reaction can be subdivided into three, where Te partially reduces to Te2− and TeO32−. This TeO32− further reacts with NaBH4 to form Te2− but a portion of it will be reduced to its zero oxidation state that again undergoes the reduction reaction with NaBH4 to further form the Te2− ions, which is represented in eqs 8−10.53 Figure 2 shows that the morphologies of the BT nanostructures synthesized in the presence of EDTA include nanorods with diameter of 35 nm and length of about 150 nm. G


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Figure 4. Schematic diagram showing the overall morphological prediction for different EDTA concentrations.

Figure 5. Overview of AFM images (a) showing sheet formation for BT100-12, (b) height profile pattern of BT100-12, (c) TEM image showing sheet formation corresponding to the BT100-12, (d) nanoflakes formed for BT100-24, (e) height profile pattern of BT100-24, and (f) HR-TEM image of BT 100-24 with (015) plane.

tures could form in the reflux reaction process through a homogeneous nucleation process if reaction time is a varying parameter along with EDTA concentration. It is well-known that Bi−Te-based materials have a highly anisotropic structure, which favors the growth primarily confined to a particular direction and the crystalline seeds of the material tend to grow into the rod/sheet shape under the influence of EDTA which would be a soft template, and induce the formation of 1D/2D structures. Hence EDTA could promote a preferential directional growth under the template effect where the formation of inorganic nanoparticles in liquid media is associated with the monomer growth. Previous reports suggest that the nucleus is characterized by various shapes and facets with different surface energies, and grows by bonding with other monomers exist in the solution.43 Crystal surface energy and facet attachment are highly influencing in the nanoparticle growth and shape formation. If any capping agent is present in the solution, they can bind to specific facets of the nucleus to coat it with a monolayer which is precisely happening between Bi and EDTA in the present technique. These attached surfactants could lower the total surface energy when Bi2Te3

nanocrystals are formed which is specifically by blocking high energy facets and exposing low energy facets. The formation of BT nanostructures during the refluxing should be related to the layered anisotropic hexagonal lattice structure of Bi2Te3. During the refluxing process, covalent bonding enables free Te atoms or Te2− ions to bond with the atoms on the growing crystal surface. The overall refluxing process at 24 h reaction time and different EDTA concentrations is shown in Figure 4, where hexagonally stacked Bi2Te3 is considered. Due to the fact that van der Waals force is insufficient to attach a Te2− ion to a Te (1) layer crystal surface will probably remain in the reaction medium and which is not strong enough to hold the atoms onto the atomic surface. Hence a Bi2Te3 structure will always tend to grow significantly faster in a-axis/b-axis directions than in the c direction. This growth mechanism should lead to a crystalline morphology of agglomerated and connected flakes for BT50 series as shown in the Figures 3, 4, and S9a. When EDTA is present in the aqueous medium, this monolayer of surfactant could control the growth rate and facilitate the formation of individual nanocrystals by holding the H


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Figure 6. XPS spectra of BT100-24 sample: (a) Bi 4f and (b) Te 3d.

reaction time in BT100 series, the single-phase BT100-24 sample is exhibiting an ultralow thickness of about 1 nm and which can be considered as the nanoflakes of Bi2Te3. Figure 5c shows the evidence of a nanosheet formation from the TEM analysis and a combination of rod, sheet and flake like structures are obtained by the increase of EDTA concentration. Figure 5f indicates the (015) plane of the nanorod structure formed for BT100-24. Further, we have recorded the XPS spectra of the Bi 4f and Te 3d regions for BT100−24 sample are shown in Figure 6. The wide scan spectrum of the same is shown in Figure S10 which suggests the formation of stable and oxidation free surfaces of BT nanostructures specifically for higher EDTA concentration. Figure 6a shows the photoelectron spectrum where core level XPS signals arising out of Bi 4f. The two distinct and highly intense peaks at an energy of 159.4 and 164.8 eV, corresponding to the binding energy of Bi 4f7/2 and Bi 4f5/2 of Bi2Te3, are in good agreement with the data observed from a BT nanostructured material.20,47 Moreover, Figure 6b shows the photoelectron spectrum of the Te 3d core level which is also exhibiting a distinct doublet peaks. These doublet peaks at energies of 575.3 and 587.3 eV are in good agreement with the binding energies seen in the Te 3d spectra observed from Bi2Te3 single phase material. The previous reports suggest that the formation of BT nanostructures without surface oxidation is a tedious process54 and here we could achieve an excellent oxidation inhibition specifically due to the higher EDTA concentration which acts as a capping agent to prevent the oxidation. From quantitative analyses of the XPS spectra, the BT100-24 sample had 41.23 and 59.77 atomic percentage of Bi and Te, respectively. Hence, it can be confirmed that the single phase formation of Bi2Te3 is evident which corroborates with XRD, TEM and EDS analyses. Meanwhile, the BT50-24 sample which was synthesized with a lower concentration of EDTA exhibit surface oxidation which could be due to the insufficient EDTA concentration to cap the individual ions as discussed in the formation mechanism and the wide scan spectrum of BT50-24 is shown in Figure S11. The core level spectra of both Bi and Te, in this case, are also clearly depicted in Figure S12a and S12b, respectively, which confirm the impact of EDTA concentration for the formation of Bi2Te3 nanostructures using the low-temperature reflux technique. 3.4. Thermoelectric Properties. 3.4.1. Seebeck Coefficient, Electrical Resistivity, and Thermal Conductivity. The temperature variation of S for BT100-12 and BT100-24 samples was measured in the temperature range 150−350 K,

surface energy of the nanocrystal facets. It is important to note that the morphologies of the synthesized BT nanostructures with various surfactant concentrations will be different from that without surfactants. EDTA acts as anion surfactant in the reaction medium, which could connect with Bi3+ ions to form large molecular groups as discussed in the previous sections, which facilitates Bi2Te3 nuclei to grow along the surfaces of EDTA agglomerates when EDTA exceeds the critical concentration, directing the growth structure in the preferred orientation of the Bi2Te3 crystals. When sufficient amount of surfactant exists in the reaction medium, it could selectively bind to one of the facets and interestingly, which kinetically slows down growth rate in a- or b-axis directions with a relative increase of growth in the c-axis direction. This reaction mechanism is applicable even in the low temperature reaction process as in the present case where the movements of fine crystals of BT capped with molecular groups in the aqueous medium make it possible for uniformly distributed nanocrystals of BT nanostructures to connect with each other by suspended bonds according to definite epitaxy in the c-axis direction as mentioned above and consequently to form nanorods or nanosheets of Bi2Te3 structures (Figure S9b). The HR-TEM image of a nanosheet and nanorod in the BT100 series with EDTA as a surfactant (Figure S9b, c) shows the lattice structure of the Bi2Te3 nanocrystal. The distance between two neighboring (015) planes measured from Figure S9c is about 0.32 nm, which corroborates with the lattice parameter of desired Bi2Te3 given by ICDD 82-0358. The nanorods were formed during reflux technique like an intercalation in the nanosheet indicating a combination of rods/sheets of Bi2Te3 nanostructures where complete conversion of nanocrystals to form nanorods may not be possible due to the fact that low reaction temperature provided by the aqueous medium is insufficient to promote the entire nanorod formation even with the addition of a surfactant. HR-TEM observation of a nanorod with EDTA as the surfactant (Figure S9c) also shows a width of 33 nm and a further high-resolution image along with AFM images are illustrated in Figure 5 to demonstrate the nanosheet formation and (015) plane of the Bi2Te3 nanorod. AFM was employed to have an idea about the impact of both surfactant concentration and reaction time on the height profile of nanosheets formed especially for BT100 series. BT100-12 and BT100−24 samples were subjected to AFM analysis. Figure 5a and 5d represent the typical AFM images and Figure 5b and 5e shows the corresponding height profile in which the maximum thickness observed is less than 8 nm. With increasing the I


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ACS Applied Nano Materials and the results are shown in Figure 7a. A negative value of S has resulted in both the samples indicating an n-type behavior of

300 K which is also comparable with recently reported materials of the same class.56 However, it is expected that the inverse relationship between S and σ could favorably enhance the electrical conduction1,57 thus maintaining an overall power factor comparable to some of the recent reports.58 To understand the electrical transport of the synthesized Bi2Te3 nanostructures, detailed resistivity analysis was performed on BT100-12 and BT100-24 samples in the temperature range of 20−300 K. Figure 7b presents the temperaturedependent ρ of these samples, and reveals a metal to semiconducting transition near 225 K. BT100-12 exhibits a ρ value of 1.91 × 10−5 Ω-m and BT100-24 exhibits 1.85 × 10−5 Ω-m at 300 K which could be a promising figure for a TE material. The σ versus T is represented in Figure S13a, where a peak value of 53600 S m−1 is achieved for BT100-24, which is comparable and much higher than the recently reported n-type BT based materials prepared using different chemical approaches.55,56 Such a moderate value of σ can be attributed to the highly crystalline nanostructures present in the BT10012 and BT100-24 as represented in TEM and AFM analyses that performs a high density of interfaces of Bi 2 Te 3 nanostructures as demonstrated in the previous sections (Figures 2, 3, and 5). High density of interfaces is very much required for improving the phonon scattering mechanisms and to obtain a better reduction in κ. The various phonon scattering mechanisms include Umklapp processes, electrons, grain boundaries, point defects and dislocations.34 These mechanisms will be predominant in nanostructures in comparison to the bulk counterparts.59 Hence, the reduction in κ is obviously observed in nanostructured materials. At the same time, it is really challenging to maintain a moderate value of σ as mentioned above without affecting κ so that a moderate value of PF can be maintained. In the present investigation, we tried to demonstrate a system where a combination of nanoflakes, nanorods and nanosheets that constitutes widely distributed grain sizes having different crystallographic orientations to tailor the overall TE performance. Interestingly, a combination of different nanostructures (nanorods, nanoflakes, and nanosheets) can provide the means for getting high density interfaces without compromising the conflicting TE properties which is similar to the fine and coarse structure model discussed in recent reports.20,22 Figure 7c gives the variation in κ of the BT100-12 and BT100-24 as a function of temperature. As can be seen, a very low κ for BT100-12 and BT100-24 (a peak value of 0.34 and 0.38 W/m-K, respectively, at 350 K) has been obtained, which is significantly lower than that of bulk Bi2Te3 materials and recently reported nanostructures of the same family synthesized via complex and expensive techniques.60,61 Especially, low κ values of 0.30−0.35 W/m-K at room temperature have been obtained which is close to the lowest value reported for BT nanostructures, which was 0.28 W/m-K synthesized at a relatively high reaction temperature using hydrothermal technique.62 Despite its inherent structural properties of Bi2Te3, the lattice κ strongly depends on the presence of additional sources of phonon scattering. This scattering can be increased by reducing the phonon mean free path by decreasing the dimensionality of the lattice. The phonon scattering will be excellent here due to the fact that ultrathin nanosheets and nanoflakes are improving the scattering mechanism whereas nanorod structures are helpful in maintaining the σ. Hence, the combination of different low dimensional nanostructures could behave like a “phonon glass and electron crystal” to decrease

Figure 7. Temperature dependence of (a) Seebeck coefficient S, (b) resistivity (ρ), and (c) thermal conductivity, κ, of BT100-12 and BT100-24.

the material. It is demonstrated that the S value increases linearly with the temperature and the same is enhanced with the reaction time. The S value observed for BT100-12 is −119.53 μV/K at 350 K and for BT100-24 is 127 μV/K at 350 K. This S value obtained at 350 K is comparable to the recently reported S values for other binary BT nanostructures which includes nanostring-cluster structures, mesoporous structures, ultrathin nanosheets, and polycrystalline nanotubes.19,55,56 It is evident from the previous reports that the influence of the reaction time on BT nanostructures is positively attributed to the combination of enhanced crystal quality and increased TE properties.22 Also, previous reports suggest that different parameters such as annealing temperature, nature of surfactants, etc. had a favorable effect on the improvement in overall TE performance of n-type BT nano and bulk compounds. It is interesting to note that the S measured for the BT nanostructures formed for an EDTA concentration of 100 mmol was decreased to about 50% compared to those prepared with 75 mmol concentration. But a room temperature S value of 121 μV/K makes sense for the practical applications near J


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ACS Applied Nano Materials the κ. Hence, the reason for low κ value may be due to the overlapping of different nanostructures, where the particle interfaces could significantly scatter phonons to achieve a maximum reduction in κ, while preserving the σ in the range 55000−60000 S/m which is clearly demonstrated in Figure S13a from which a nondegenerative behavior of the synthesized nanostructures have been confirmed. The hiking trend in κ value, especially above 200 K, is specifically due to this electronic contribution toward κ as shown in Figure S13b. Hence, from the thermal transport studies together with HRTEM and AFM analysis suggests the low dimensional and highly crystalline nanostructures inherited from the low temperature reflux reaction and their corresponding highdensity interfaces in the HPHTS nanostructures can significantly scatter the phonons and thereby contribute to the significant reduction in κ to maximize the TE efficiency. The temperature variation of PF, which is calculated as S2σ is plotted and shown in Figure 8a. The PF value of BT100-12

ultimately reflecting in the PF values of BT nanostructures prepared by varying the EDTA concentration. It is expected that the considerably enhanced σ in the BT100-24 nanostructures can compensate the moderately deteriorated S and, in turn, lead to the enhancement of overall S2σ value. Finally, the temperature variation of ZT has been determined and is depicted in Figure 8b, which shows a promising range of ZT values for BT nanostructures prepared for BT100-12 and BT100-24. The ZT value of BT100-12 shows a linear dependence with the temperature and achieved a maximum value of 0.52 at 300 K, which is comparable with the Bi2Te3 nanostructured bulk materials. A significant improvement in ZT is observed for BT100-24 nanostructures showing a maximum value of 0.75 at 300 K. As explained in the reaction mechanism for the formation of BT nanostructures, complete conversion of 1D/2D structures may not be possible because of the very low reaction temperature of the aqueous medium. Still, the enhanced ZT value of BT100-24 at room temperature is comparable to the previously reported BT nanostructures synthesized especially via different chemical approaches.22,32,33 Furthermore, both the moderate values of S and σ along with a significantly reduced κ comparable to that of the other materials in the chalcogenide family result in an enhanced ZT of the BT100-24 sample. The variation in TE properties can be well explained with grain boundary scattering due to the formation of nanorods and nanosheets of BT nanostructures with increased EDTA concentration and the phase transitions and scattering mechanism of which is represented in Figure 9. The single phase samples of BT with morphological control are very much essential for high-performance TE material where both reaction time and EDTA concentration favors the formation of the desired structure using the reflux method and is depicted in Figure 9. The mixed-phase samples with coarse structures are formed when EDTA concentration is very less, that is, in 50 mmol series as shown in the schematic representation where scattering centers will be minimum and may ultimately result in a drastic decrease of overall TE efficiency due to which we excluded the BT50 series from TE performance analysis. The XRD and TEM images confirm the mixed phases and coarse structures respectively for different samples in the BT50 series with lower reaction time. Among the BT50 series, BT50-24 is the only sample that formed with desired BT structure (hexagonal Bi2Te3) but still the insufficient EDTA concentration resulted in the formation of agglomerated morphology which is not recommended to decrease the κ. When the surfactant concentration is increased to 75 mmol, the formation of nanocrystals get facilitated and an excellent scattering mechanism was proposed in our earlier work by which κ value decreased to 0.34 W/m-K at room temperature for the sample synthesized for 1 h but this low reaction time favors the BiTe formation rather than Bi2Te3. The 12 h reaction time promotes the initiation of Bi2Te3 formation along with Bi4Te3 as shown in Figure 9. Here, in 75 mmol EDTA concentration with 24 h reaction time is helping in the formation of single phase Bi2Te3 structures where significant phonon scattering reduces κ to 0.42 W/m-K which is less compared to the conventional TE materials. In BT75-24, uniformly distributed fine nanocrystals have been formed which considerably enhanced the PF and ZT value.22 When EDTA concentration increased to 100 mmol, EDTA agglomerates act as a soft template for the formation of low dimensional structures of BT as mentioned in the ionic reaction mechanism.

Figure 8. Temperature dependence of (a) power factor, PF, and (b) figure of merit, ZT, of BT100-12 and BT100-24.

sample gave a maximum of 600 μW/m-K2 at 300 K and this moderate P.F value is arising due to the decreased S value compared to the previous work reported in our earlier studies. The structural and morphological variations were the key factors discussed in the previous report;22 hence, we selectively carried out the TE performance on BT100-12 and BT100-24, where structural as well as morphological control has been achieved. The PF value increased significantly with reaction time, which also corroborates the previous report22 and shows the maximum for BT100-24 that increased with temperature from 320 to 750 μW/m-K2 as shown in Figure 8a. The S and PF values of the reported samples are comparable with most of the nanostructured BT materials, but around 50% reduction is observed for S in comparison to our recent report on BT nanocrystals prepared using the same method by varying the reaction time.22 Again, it is interesting to note that a 2-fold increase in σ than the state-of-the-art nanostructured bulk BT is K


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Figure 9. Schematic diagram showing structural phase transitions and model explaining enhanced electrical conductivity.

In a nutshell, Figure 10 represents the systematic variation of S, σ, and κ, along with ZT and morphological variations. Among the mixed phases, BT75-12 is the best candidate with coarse and fine structures that showed a room temperature ZT value of 0.38. The BT75-24 was best among the series in the previous report which exhibited a ZT of 0.67 at room temperature where reaction time was the only varying parameter. In comparison to BT75-12, BT100-12 is exhibiting significant enhancement of about 40% reaching a ZT value of 0.54 at room temperature. Similarly, in BT100-24 where nanorods are formed partially, the κ reduces significantly by maintaining a moderate σ, which provides the improvement in ZT value to a maximum of 0.75 at 300 K which is about 12% higher than that of our previous report BT75-24.22 These results demonstrate that both reaction time and surfactant concentration could carefully optimize the desired Bi2Te3 structures and could tailor the morphology to enhance the overall TE performance of different BT nanostructures. On the basis of ionic reaction mechanisms, we attempted to manipulate the reaction conditions where reaction time and EDTA concentrations were the crucial factors regulating the stability, structure, and morphology of BT nanomaterials. As mentioned in the previous discussions, we obtained a reactive Bi source (i.e., Bi3+ ions) with EDTA capping through the simple manipulation of EDTA concentrations. By reacting this EDTA capped Bi ions with anions (Te2−), we obtained stable oxidation resistant nanomaterials with desired stoichiometry, structure and morphology. We optimized the TE transport properties with the desired structural and morphological tailoring, which maximized the ZT value. A comparison of room temperature σ, S, κ, and ZT of our present study with some of the recent reports are represented in Table 1 and careful optimization of reaction conditions could favor the formation mechanism of BT nanostructures that enhance the overall TE properties. It indicates that even though the PF is least, the overall room temperature ZT value is enhanced by around 12% compared to our recent report22 as shown in Table 1.

Interestingly, Bi2Te3 phase formation initiated even in the 1 h reaction here, and it is demonstrated in Figure 9 that the formation of desired Bi2Te3 structure is facilitated both by reaction time and EDTA concentration. Both BT100-12 and BT100-24 exactly allow the desired Bi2Te3 formation along with low-dimensional nanostructures, where phonon scattering could be efficient to significantly decrease the κ value that exhibits 0.31 W/m-K at room temperature. When the low dimensional structures are forming in the system (especially nanosheets and nanoflakes of 1−10 nm thickness), a high pressure could sufficiently stack these nanostructures in press direction that increase the scattering centers at the interfaces and could provide an ultralow κ. The previous report suggests the mechanism for enhancement in σ by the formation of coarse structures and reduction in κ by the formation of fine structures.20,22 Here, in the present study, our compound forms as a combination of rod, sheet and flake like morphology that facilitates the phonon scattering and reduction in κ, whereas the combination of these nanostructures allows the electrical conduction; the combined effect is the overall improvement in the TE efficiency in terms of ZT, which is depicted in Figure 10.

Figure 10. Comparison of overall room temperature TE performance for different BT nanostructures. L


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formation in BT100-1 sample, combination of sheet and rod formation in BT100-12 sample, combination of sheet-, rod-, and flake-like structures in BT100-24 sample, EDS spectra obtained for BT100-24 sample showing the elemental peaks, chemical structures of EDTA, Na-EDTA, and bicapped EDTA, overview of TEM images showing dimensions of nanocrystals formed for BT50-24, HR-TEM image showing lattice fringes corresponding to the Bi2Te3 nanosheets, HR-TEM image showing lattice fringes corresponding to the Bi2Te3 nanorod of 33 nm width, wide scan XPS spectra of BT100-24 sample showing Bi and Te elemental peaks, wide scan XPS spectra of BT50-24 sample showing the surface oxidation, XPS spectra of BT100-24 sample Bi 4f and Te 3d with surface oxidation, variation of electrical conductivity with temperature for BT100-12 and BT10024, variation of electronic contribution toward thermal conductivity with temperature, and EDS data for BT50-1, BT50-12, BT50-24, BT100-1, BT100-12, and BT100-24 (PDF)

Table 1. Comparison Showing Electrical conductivity (σ), Seebeck Coefficient (S), Thermal Conductivity (κ), and Figure of Merit (ZT) of Undoped Bi2Te3 Nanostructures of the Present Study with Recent Trends SL no. 1 2 3 4 5 6 7 8 9 10

σ S κ (× 104 S/m) (μV/K) (W/m-K) 6.30 1.40 8.00 5.60 7.30 7.70 3.43 7.50 1.82 5.36

125 −258 −148 −142 −131 −151 −135 −185 −126 −121

1.10 0.42 0.99 0.75 0.58 0.97 0.49 1.10 0.28 0.31

ZT 0.27 0.67 0.53 0.45 0.65 0.54 0.36 0.70 0.31 0.75

(at (at (at (at (at (at (at (at (at (at

300 300 300 300 300 300 313 298 300 298

ref K) K) K) K) K) K) K) K) K) K)

19 22 31 32 33 34 53 58 61 Present work

4. CONCLUSION In summary, Bi−Te-based nanostructures have been synthesized successfully by an aqueous based simple and cost-effective low-temperature reflux method and proposed a favorable reaction mechanism for the formation of these nanostructures. The EDTA concentration along with reaction time facilitates the formation of desired Bi2Te3 phase and EDTA behaves not only as a capping agent but also as a soft template for lowering the surface energy to facilitate the formation of rod and sheetlike structures. The BT sample synthesized for an EDTA concentration of 100 mmol and reaction time of 24 h exhibits the best thermoelectric figure of merit among the synthesized samples. The grain boundary scattering along with low dimensional structure’s high-density interfaces could successfully explain the thermoelectric properties of the synthesized nanostructures. The significant reduction in thermal conductivity, considerably high electrical conductivity and ultimately a high ZT of 0.75 makes n-type hexagonal Bi2Te3 a potential candidate for thermoelectric applications near room temperature. These results indicate that optimization of reaction conditions could provide defect free, stoichiometric and stable products of Bi2Te3 in which 1D/2D structures could be successfully synthesized at low temperature and lower reaction time using a simple aqueous based reflux technique. Both structural and morphological changes can be achieved through variation in surfactant concentration and reaction time that provides appropriate ways for enhancing the overall thermoelectric performance of nanostructured Bi2Te3 materials. Thus, our synthesis approach offers potential opportunities for tailoring the reaction mechanism for different morphologies of Bi2Te3 nanostructures with improved thermoelectric properties. Our research provides a simple, cost-effective and lowtemperature method through green chemistry approach to fabricate stable Bi2Te3 1D/2D nanostructures for room temperature thermoelectric applications.

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AUTHOR INFORMATION

Corresponding Author

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

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support received from Council of Scientific and Industrial Research (CSIR), Govt. of India. V.R.A. and B.A. are thankful to Academy of Scientific and Innovative Research and CSIR for granting the Fellowship. The authors would also like to thank Board of Research in Nuclear Sciences, sponsored project number GAP 218939 and Department of Science and Technology sponsored project number GAP 232339 for partially supporting this work.

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REFERENCES

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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/acsanm.8b00464. Peak shift associated with different samples of BT100 series, expanded view of the refinement for both BT5024 and BT100-24, crystal structures of BT nanostructures with varying reaction times, Initiation of rod M


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