New Chemical Reaction Process of a Bi2Te2.7Se0.3 Nanomaterial for

Article pubs.acs.org/IECR

New Chemical Reaction Process of a Bi2Te2.7Se0.3 Nanomaterial for Feasible Optimization in Transport Properties Resulting in Predominant n‑Type Thermoelectric Performance Cham Kim,*,† Chang Eun Kim,† Ju Young Baek,† Dong Hwan Kim,† Jong Tae Kim,† Ji Hyeon Ahn,† David Humberto Lopez,‡ Taewook Kim,§ and Hoyoung Kim† †

Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333 Techno Jungang-daero, Daegu 42988, Republic of Korea Department of Chemical and Environmental Engineering, University of Arizona, 1133 East James E. Rogers Way, Tucson, Arizona 85721, United States § Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Pohang 37673, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Various chemical reaction processes have been adopted to synthesize Bi2Te3 thermoelectric nanomaterials for achieving remarkably low thermal conductivities, but chemical contaminations were usually pointed out as flaws, severely deteriorating electrical conductivities. We devised a novel water-based chemical reaction process for a Bi2Te2.7Se0.3 nanocompound in which the possibility for chemical contaminations was reduced. We successfully synthesized a small and highly distributed Bi2Te2.7Se0.3 nanocompound with high purity and adequately packed it via a spark plasma sintering process to produce a nanobulk structure. The resulting nanobulk specimen exhibited a physical density as high as the theoretical one with highly distributed nanograins; thus, we were able to obtain remarkably high electrical conductivity while maintaining thermal conductivity as low as possible. The synergistic effect was greatly induced between the transport properties; thus, the highest reported figure of merit value was achieved for n-type Bi2Te3 in the bulk phase.

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INTRODUCTION Thermoelectrics, the technology for energy conversion between heat and electricity, is gaining widespread attention because of its potential applications, including heat-to-electricity conversion and solid-state cooling.1−7 However, a wide range of thermoelectric applications have been constrained by the low energy conversion efficiency of current thermoelectric materials.8−10 The comprehensive performance of such materials is evaluated via the dimensionless figure of merit (ZT = α2σT/κ), where α is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κ the thermal conductivity.5−7,11−13 To obtain an excellent thermoelectric material, a high ZT value should be achieved through either the enhancement of the power factor (α2σ) or the reduction of the thermal conductivity (κ).8,9,14−23 Many researchers have extensively investigated methods of reducing the thermal conductivities of various thermoelectric materials. In particular, they have focused on synthesizing entire nanostructures or introducing partial nanoinclusions, the presence of which increases phonon−boundary scattering or phonon−impurity scattering, respectively, where both types of scattering can reduce the lattice thermal conductivity. In the case of complete nanostructuring, low-dimensional nanostruc© 2016 American Chemical Society

tures have been widely utilized because they are known to have very low thermal conductivities.15−23 For Bi2Te3-based thermoelectric materials, researchers have reported quantum dots,24,25 nanowires,26−28 and superlattices.29−31 The low-dimensional Bi2Te3 recorded various thermal conductivity values usually below ca. 0.5 W m−1 K−1; to the best of our knowledge, the lowest thermal conductivity is ca. 0.11 W m−1 K−1 of the Bi2Te3/Sb2Te3 superlattice reported by the Yamasaki group.32 However, it is relatively difficult to apply the low-dimensional Bi2Te3 to commercial or industrial uses. For these applications, bulk-type materials are ideal, and researchers have therefore endeavored to develop Bi2Te3 nanobulk materials.33−43 High thermoelectric properties have been often reported for p-type Bi2Te3, especially BixSb2−xTe3.33−37 Researchers have generally reported the maximum ZT values ranging from 1.2 to 1.4 at approximately 100 °C for BixSb2−xTe3 in the bulk phase; to the best of our knowledge, the highest ZT value is ca. 1.86 (at 47 °C) that Kim et al. recently achieved.37 It has been usually Received: Revised: Accepted: Published: 5623

March 8, 2016 April 10, 2016 April 28, 2016 April 28, 2016 DOI: 10.1021/acs.iecr.6b00933 Ind. Eng. Chem. Res. 2016, 55, 5623−5633

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Industrial & Engineering Chemistry Research

utilized to hydrate the dissolved precursors, and hydrazine monohydrate (N2H4·H2O, Samchun, 80.0%) was used to reduce the resulting hydrated precipitate. All chemicals were used without further purification. Sample Preparation. The elemental precursors (60 mmol of Bi with 81 mmol of Te and 9 mmol of Se) were soaked in 300 mL of deionized water, and 100 mL of HNO3 was subsequently added dropwise. The mixture was vigorously stirred until the precursors were completely dissolved and a colorless solution was obtained. NH4OH was slowly added to adjust the solution pH to 7.5. As the pH increased, white precipitates gradually appeared because of the hydration of the precursors. The white precipitates were aged for 6 h under vigorous stirring; then, 50 mL of N2H4·H2O was added. The white precipitates slowly changed to gray and then to black. The resulting black substances were aged overnight and rinsed thoroughly using dry ethanol and deionized water. After the precipitates were dried under vacuum at 40 °C overnight, a black powder was obtained. The powder was compacted using spark plasma sintering equipment (SPS; DR. Sinter, SPS-3, 20MK-IV). Approximately 15 g of the powder was placed into a cylindrical graphite mold (55 mm × 60 mm) with an inner hole of 15 mm in diameter for compaction at 360 °C under a pressure of 50 MPa in an Ar atmosphere. The heating rate was ca. 45 °C min−1, and the time for which the sample was maintained at this sintering temperature was 5 min. This process resulted in the production of a cylindrical specimen (20 mm × 30 mm, see the graphic included in the Abstract). Material Characterizations. Powder X-ray diffraction (XRD) patterns were recorded using a D/MAX-2500 diffractometer (Rigaku) using Cu Kα radiation (λ = 1.5406 Å) and a scintillation counter detector. Relevant patterns were recorded over a 2θ range of 10−80°. Raman spectroscopy was conducted using a Nicolet Almeca XR spectrometer (Thermo Scientific) with an excitation photon energy of 532 nm, which covers a spectral range of Raman shift from 100 to 4000 cm−1. Inductively coupled plasma mass spectrometry (ICP-MS) was examined for quantitative analyses using a CT spectrometer (PerkinElmer Norwalk). Field-emission scanning electron microscopy (FE-SEM) micrographs were obtained using an S-4800 microscope (Hitachi). Electron backscatter diffraction (EBSD) analysis was conducted using an S-4300SE microscope (Hitachi), while transmission electron microscopy (TEM) micrographs were taken from a Titan G2Microscope (FEI Company). Fourier transform infrared (FT-IR) spectra were recorded on a Continuum spectrometer (Thermo Scientific) with the spectral range from 500 to 4000 cm−1. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi spectrometer microprobe with dual anode (Mg Kα/Al Kα) X-ray source; the C 1s photoelectron peak (285 eV) was chosen as energy reference. Ultraviolet−visible−nearinfrared (UV−vis−NIR) diffuse reflectance spectra (DRS) were obtained from a Cary 5000 UV−vis−NIR spectrophotometer (Varian) equipped with an internal diffuse reflectance integrating sphere. Thermoelectric Characterizations. Considering the typically anisotropic nature of Bi2Te3-based materials, we measured the transport properties of only those faces of the cylindrical sintered specimen that were perpendicular to the pressurizing direction. The Hall coefficient was measured with the van der Pauw method at room temperature using a Hall effect measurement system (HMS-3000, Ecopia) to obtain the carrier concentration (n). The electrical resistivity (ρ) and

reported that n-type Bi2Te3 demonstrates lower thermoelectric performance than p-type Bi2Te3; thus, researchers have intensively focused on fabricating n-type Bi2Te3 nanobulk materials. Many researchers have conducted various chemical reaction processes for n-type Bi2Te3 nanobulk materials because the processes are practical routes to easily prepare small and uniform nanostructures, which greatly reduce thermal conductivity. Some researchers have reported n-type Bi2TeySe3‑y nanobulk structures exhibiting remarkably low thermal conductivities below 0.5 W m−1 K−1; however, the nanobulk structures mostly showed low ZT values below 1.0,38−43 which should be caused by their poor electrical conductivities. It has not been clarified why the nanobulk structures through chemical reaction processes recorded low electrical properties. We assumed that the properties might be deteriorated because of the contaminations caused by diverse chemicals, such as organic solvents and chemical stabilizers, used in the processes. In the present study, we devised a novel wet chemical reaction process to synthesize an n-type Bi2Te2.7Se0.3 nanomaterial using neither organic solvents nor chemical stabilizers; thus, we successfully prepared a highly pure Bi2Te2.7Se0.3 nanocompound. In detail, we designed a water-based chemical reaction route for the nanocompound, which consisted of the simultaneous oxidation of bismuth, tellurium, and selenium followed by their coprecipitation and liquid-phase reduction. We theoretically presented a series of chemical reaction mechanisms for each stage and attempted to synthesize via the mechanisms. The resulting nanocompound exhibited very small and highly distributed nanoparticles, which were then packed in a spark plasma sintering process (SPS) to create a Bi2Te2.7Se0.3 nanobulk crystalline structure that consisted of small nanograins of a narrow size distribution. Because we did not use either organic solvents or chemical stabilizers, we could ignore any concerns that residues of the chemicals possibly interfere with grain growth of the nanocompound during the sintering process and that they might hinder carrier transport in the resulting nanobulk structure. An excellent physical density with high carrier mobility was found for the nanobulk structure; thus, it showed predominant electrical conductivity. The high carrier mobility increased carrier thermal conductivity; thus, total thermal conductivity also increased. However, the nanobulk Bi2Te2.7Se0.3 exhibited a considerably low lattice thermal conductivity possibly due to high phonon scattering rate in the nanostructure; thus, the Bi2Te2.7Se0.3 retained competitive low thermal conductivity values as a thermoelectric material. Consequently, we obtained a Bi2Te2.7Se0.3 nanobulk crystalline structure having predominant electrical properties with a low thermal conductivity, which resulted in the highest reported ZT for n-type Bi2Te3 in bulk phase. Considering that the Bi2Te2.7Se0.3 significantly reduced the gap between ZT values of p-type and n-type Bi2Te3, the results of the present study indicate great potential for the production of highperformance thermoelectric materials and devices.

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EXPERIMENTAL SECTION Chemicals. Bismuth (Bi shot, Kojundo Chemical, 99.99%, 2−5 mm), tellurium (Te powder, Kojundo Chemical, 99.9%, 45 μm), and selenium elements (Se powder, Kojundo Chemical, 99.9%, 75 μm) served as raw precursors. Nitric acid (HNO3, Junsei Chemicals, 60−62%) was used to dissolve the precursors in deionized water. A 5.0 M aqueous solution of ammonium hydroxide (NH4OH, J.T. Baker, 28−30%) was 5624

DOI: 10.1021/acs.iecr.6b00933 Ind. Eng. Chem. Res. 2016, 55, 5623−5633

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in eq 2. These two reactions may occur because the standard reduction potentials of Bi3+/Bi (0.308 V)46,47 and H2TeO3/Te (0.589 V)48 are lower than that of N2O4/NO3− (0.803 V),46,49 causing the nitric acid to function as an oxidant. An entirely different reaction occurred when we exposed bismuth shots and tellurium powders to nitric acid simultaneously; the tellurium powders gradually dissolved without forming precipitates soon after the bismuth shots began to dissolve, resulting in a greenish transparent solution (Figure S1c). It is hypothesized that bismuth ions and nitrate formed because of the preferential dissolution of bismuth (eq 1) and that the tellurium was then oxidized by the nitrate, resulting in the generation of tellurite (TeO32−, eq 3) rather than tellurous acid. In other words, the precipitate, tellurous acid, forms through the oxidation of tellurium alone in nitric acid (eq 2), whereas a transparent solution is obtained upon the oxidation of the reactant mixture because nitrate, rather than nitric acid, serves as the oxidant (eq 3). Thus, tellurite is generated in the latter oxidation reaction. We presented the overall oxidation reaction of the reactant mixture of bismuth and telluride in eq 4a. We also tried to replace some portion of tellurium with selenium as shown in eq 4b, possibly resulting in the generation of selenite (SeO32−). The transparent solutions that were produced as a result of the overall reactions (eq 4) were neutralized by adding ammonium hydroxide to coprecipitate the oxidized bismuth ions, tellurite, and selenite (Figure S1d). It is assumed that the ionic substances precipitated as bismuth hydroxide (Bi(OH)3), ammonium tellurite ((NH4)2TeO3), and ammonium selenite ((NH4)2SeO3) (eq 5). (2) Coprecipitation of the oxidized sources

Seebeck coefficient (α) were measured simultaneously in the temperature range between 25 and 300 °C using a commercial instrument (ZEM-3, Ulvac-Rico). Carrier mobility was calculated with the equation μ = 1/(ρne), where e is electric charge, 1.6 × 10−19 C. The thermal conductivity (κ) is related to the thermal diffusivity (λ) through the equation κ = λCpd, where d and Cp denote the physical density and specific heat of the sample, respectively. The thermal diffusivity was measured between 25 and 300 °C using a laser flash tool (LFA447, Netzsch). The density was determined by using the Archimedes immersion method. We measured the specific heat capacity using a differential scanning calorimeter (DSC200, Netzsch). We used these measured values to calculate the thermal conductivity through the equation given above. Consequently, the figure of merit was calculated by the following equation: ZT = α2σT/κ. The thermal conductivity can be approximately determined by summing the carrier thermal conductivity (κc) and the lattice thermal conductivity (κl). The carrier thermal conductivity can be calculated using the Wiedemann−Franz law, κc = L0σT, where L0 and σ are the Lorenz number and the electrical conductivity, respectively. We used the Lorenz number of 2.0 × 10−8 W Ω K−2 for a degenerate semiconductor.44,45 Thus, the lattice thermal conductivity can be estimated from the total conductivity and the carrier thermal conductivity.

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RESULTS AND DISCUSSION In the present study, we designed a new chemical synthesis route in a water-based reaction system under ambient conditions without chemical stabilizers and organic solvents to yield a Bi2Te2.7Se0.3 nanocrystalline powder. This synthesis route can be summarized as follows: (1) oxidizing dissolution of reactant elements, (2) coprecipitation of the oxidized sources, and (3) liquid-phase reduction of the precipitate. Oxidation of reactant elements Bi + 4HNO3 → Bi 3 + + 3NO3− + NO↑ + 2H 2O

(1)

Te + 4HNO3 → H 2TeO3↓ + 4NO2 ↑ + H 2O

(2)

Te +

2NO3−

→ TeO3

2−

+ NO2 ↑ + NO↑

2Bi 3 + + 3TeO32 − + 6NH4OH → 2Bi(OH)3 ↓ + 3(NH4)2 TeO3↓

(5a)

2Bi 3 + + 2.7TeO32 − + 0.3SeO32 − + 6NH4OH

(3)

→ 2Bi(OH)3 ↓ + 2.7(NH4)2 TeO3↓ + 0.3(NH4)2 SeO3↓

(1) Overall reactions for the oxidation of the reactant mixtures (eq 1 + eq 3)

(5b)

We attempted to reduce the resulting white precipitates via liquid-phase reduction. When we used hydrazine monohydrate (N2H4·H2O) as a liquid-phase reducing agent, the precipitates instantly changed from white to gray and then turned black as they aged (Figure S1e,f). We expected the precipitates to be reduced by the hydrazine monohydrate because of its strong reducing power, thus affording bismuth telluride (Bi2Te3, eq 6a) and bismuth tellurium selenide (Bi2Te2.7Se0.3, eq 6b). (3) Reduction of the coprecipitates

2Bi + 3Te + 8HNO3 + 6NO3− → 2Bi 3 + + 3TeO32 − + 12NO2 ↑ + 2NO↑ + H+ + 7OH−

(4a)

2Bi + 2.7Te + 0.3Se + 8HNO3 + 6NO3− → 2Bi 3 + + 2.7TeO32 − + 0.3SeO32 − + 12NO2 ↑ + 2NO↑ + H+ + 7OH−

(4b)

Bismuth shots instantly dissolve in nitric acid, and a transparent solution is subsequently obtained. This observation indicates that bismuth is oxidized by nitric acid to form bismuth ions (Bi3+) and nitrate (NO3−) (eq 1). By contrast, tellurium powders exhibit no initial response in nitric acid (Figure S1a in the Supporting Information), but white precipitates accompanied by a reddish brown gas are generated after approximately 30 min (Figure S1b). This phenomenon indicates that tellurium is oxidized by nitric acid to form tellurous acid (H2TeO3), which is one of the most stable tellurium oxoacids, and nitrogen dioxide gas (NO2), as shown

2Bi(OH)3 + 3(NH4)2 TeO3 + 4.5N2H4 · H 2O → Bi 2Te3 + 6NH4 + + 6OH− + 4.5N2↑ + 13.5H 2O (6a)

2Bi(OH)3 + 2.7(NH4)2 TeO3 + 0.3(NH4)2 SeO3 + 4.5N2H4 ·H 2O → Bi 2Te2.7Se0.3 + 6NH4 + + 6OH− + 4.5N2↑ + 13.5H 2O 5625

(6b) DOI: 10.1021/acs.iecr.6b00933 Ind. Eng. Chem. Res. 2016, 55, 5623−5633

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Raman spectrum of the binary product, the three different lattice vibrations were detected, two Raman-active vibration modes (Eg2 at 112.6 cm−1, A1g2 at 136.7 cm−1) and one infrared-active vibration mode (A1u2 at 124.0 cm−1), which are assigned to in-plane (Eg2) or out-of-plane (A1g2, A1u2) vibrations of Bi−Te(1) (Figure 1b).50,51 Compared to the binary product, the ternary one exhibited clear shifts of the vibrational modes to higher frequency (i.e., Eg2 at 115.9 cm−1, A1g2 at 138.4 cm−1, and A1u2 at 125.8 cm−1). This phenomenon means that some tellurium atoms were replaced with selenium having higher electronegativity; thus, the chemical bonds between Bi and Te(1) should be shortened. This finding well corresponded to the XRD result above, indicating that the smaller unit cell was formed for the ternary product due to the selenium incorporation. Both the XRD results and Raman spectra confirm that a ternary compound can be prepared via our new chemical synthesis route; we regarded the compound as Bi2Te2.7Se0.3, having verified its stoichiometry through the atomic composition analysis, ICP-MS (Table S1). According to the SEM images of the Bi2Te2.7Se0.3 presented in Figure 2, the product consisted of nanoparticles approximately 50 nm in size with a highly uniform size distribution. Therefore, we successfully obtained a Bi2Te2.7Se0.3 nanocompound via the chemical synthesis. The nanocompound was packed via a spark plasma sintering process to obtain a nanobulk structure. We tried to preserve the nanostructure by utilizing a rapid heating rate and short cooling time of SPS and also searched for the sintering conditions to maximize physical density. We manually controlled an electric current in SPS equipment to increase sintering temperature with the heating ramp of ca. 45 °C min−1 (Figure 3). The Z-axis displacement of a graphite mold started to increase after 2 min, indicating that the mold was being contracted; as the temperature was elevated, the displacement increased because of compaction of the nanocompound. The increased rate of the displacement gradually slowed ca. 7 min after the sintering process was initiated, and the displacement became saturated at 8 min. We made the temperature slightly exceed the saturation point to guarantee the completion of sintering. We precisely kept it at 360 °C because the Z-axis displacement proceeded downward above 360 °C, indicative of mold expansion caused by the liquid phase in the nanocompound. After the process was maintained at 360 °C for 5 min, it was terminated. A sintered specimen was finally recovered after the graphite mold cooled to below 100 °C. As shown in the EBSD images (Figure 4a,b), the sintered specimen was composed of randomly oriented crystalline nanograins, which were larger than the nanoparticles observed in Figure 2. Particle growth inevitably occurs during any type of sintering process, but we endeavored to minimize this effect by manually controlling an electric current in SPS equipment. We could accurately regulate the heating ramp and find the appropriate sintering point as elaborated above (Figure 3); thus, we obtained a nanobulk crystalline Bi2Te2.7Se0.3 (denoted as nc-Bi2Te2.7Se0.3). According to the results of applying the linear-intercept method to the EBSD result, the nc-Bi2Te2.7Se0.3 specimen primarily consisted of grains of below 1 μm in size (Figure 4c), and its average grain size was calculated to be approximately 200 nm. We clearly observed that the sintered specimen was composed of much smaller grains with a more highly uniform size distribution compared with the results achieved in our previous study (Figure S2). As confirmed in the inverse pole figure images of EBSD, the nanocompound was also verified to

2Bi(OH)3 + 2.7(NH4)2 TeO3 + 0.3(NH4)2 SeO3 + 4.5N2H4 ·H 2O → x Bi 2Te2.7Se0.3 + (1 − x)Bi 2O3 + 2.7(1 − x)TeO2 + 0.3(1 − x)SeO2 + 6NH4 + + 6OH− + 4.5N2↑ + 9(1 − x)H 2↑ + 4.5(1 + 2x)H 2O (6c)

We observed crystalline structures of the intermediate precipitate and the reduced products, which were produced in processes 2 and 3, respectively. According to the XRD result presented in Figure 1a, the precipitate (from eq 5a) exhibited

Figure 1. (a) XRD patterns of intermediate and end products from the chemical reaction process; enlarged diffraction lines of (015) plane for the end products (inset). (b) Raman spectra of the products: Ramanactive vibrational modes (Eg2, □; A1g2, ○) and infrared-active vibrational mode (A1u2, △) in the lattice of rhombohedral Bi2Te3.

amorphous characteristic while a binary product (from eq 6a) showed the typical rhombohedral Bi2Te3 diffraction pattern, which precisely corresponded to JCPDS card no. 85-0439, and indicated no secondary phases. Therefore, the liquid-phase reduction step (i.e., process 3) can be regarded as an effective process for the desired crystal growth. The diffraction pattern of another product, a ternary one (from eq 6b), exactly coincided with JCPDS card no. 50-0954 (i.e., rhombohedral Bi2Te2.7Se0.3 diffraction pattern). This pattern was similar to that of Bi2Te3 except for the peak shift to a higher-angle side (Figure 1a, inset), indicative of the lattice distortion due to the incorporation of smaller selenium into the larger tellurium site; thus, a smaller unit cell was expected to occur. The rhombohedral structure is basically composed of three quintuple layers (QLs), which are stacked by van der Waals forces. Each QL has the layered structure of alternative five atomic levels, Te(1)−Bi−Te(2)−Bi−Te(1) (Figure 1b), where lattice vibration among the atoms appears. According to the 5626


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Figure 2. SEM images of the Bi2Te2.7Se0.3 nanocompound at different magnifications.

compound. As explained above, we initially generated the oxidized precipitates (i.e., Bi(OH)3 , (NH4 )2TeO3, and (NH4)2SeO3) from the raw elements and then chemically reduced the precipitates to produce the nanocompound. The precipitates should be mostly reduced as desired, but a very small amount of them might not be, thus possibly leaving a trace of oxidized substances (i.e., Bi2O3, TeO2, and SeO2), because x is slightly lower than 1 in eq 6c. As presented in the XPS spectra of Bi 4f (Figure 5b), the nc-Bi2Te2.7Se0.3 showed main peaks for Bi 4f7/2 (157.9 eV) and Bi 4f5/2 (163.2 eV) with shoulders at 159.4 and 164.7 eV, respectively. While the main peaks mean Bi0 in the nc-Bi2Te2.7Se0.3, the shoulders typically indicate Bi3+ of the Bi2O3. We observed similar peak patterns for Te 3d: two main peaks (i.e., at 571.8 and 582.6 eV) with subordinate shoulders (i.e., at 575.4 and 586.1 eV) (Figure 5c), indicative of Te0 (from the nc-Bi2Te2.7Se0.3) with Te4+ (from the TeO2). The spectrum of Se 3d was composed of Se 3d5/2 (52.7 eV) and Se 3d3/2 (55.1 and 57.8 eV) (Figure 5d), where the smallest peak indicates Se4+ from the SeO2 while two larger peaks indicate Se0 from the nc-Bi2Te2.7Se0.3. We did not expect that the oxidized species would seriously deteriorate the physical properties of the nc-Bi2Te2.7Se0.3 because all of the species might be trace substances detected as the small shoulder signals in the XPS spectra, and they were also found in the highly pure raw elements (Figure 5b−d). The nc-Bi2Te2.7Se0.3 recorded the very high physical density of 7.81 g cm−3, which is about 99% of the theoretical density of Bi2Te2.7Se0.3 (ca. 7.86 g cm−3);42,59 thus, the oxidized species might not interfere with grain growth of the Bi2Te2.7Se0.3 nanocompound during the sintering process. The ncBi2Te2.7Se0.3 seemed to exhibit the outstanding carrier mobility (i.e., 193 cm2 V−1 s−1, Table 1) because of the high relative density. This high carrier mobility should exert a more immediate influence on the low electrical resistivity (8.30− 11.0 μΩ m at 25−150 °C, Table 1, Figure 6a) than the moderate carrier concentration (i.e., 3.91 × 1019 cm−3, Table 1). The electrical resistivity was much lower than those of ntype Bi2Te3-based nanobulk structures in previous studies. Many researchers have developed n-type Bi2Te3-based nanomaterials via various bottom-up processes (i.e., chemical syntheses), but they did not seem to successfully densify their nanomaterials because the maximum relative densities for their nanobulk structures were mostly less than ca. 95%.38−42

Figure 3. Variation in z-axis displacement of a graphite mold filled with the Bi2Te2.7Se0.3 nanocompound accompanied by the control of electric current and sintering temperature during an SPS process.

consist of polycrystalline grains through the TEM analyses (Figure 4d,e). We observed highly crystallized lattice fringes having interplanar spacing values (dhkl) of 3.21, 2.35, and 2.23 Å (Figure 4e), which are in the proximity of 3.22, 2.37, and 2.23 Å for the diffraction index of (015), (1010), and (0111) in rhombohedral Bi2Te2.7Se0.3 (JCPDS card no. 50-0954), respectively. In addition, the interplanar spacing values nearly corresponded to the calculated values from each diffraction angle in the XRD result (i.e., 3.20, 2.36, and 2.22 Å using dhkl = λ/(2 sin θ)); thus, the lattice fringes were denoted as presented in Figure 4e. According to the FT-IR result (Figure 5a), the ncBi2Te2.7Se0.3 did not exhibit any traces of chemical residues that have been found in chemically synthesized Bi2Te3, such as CO groups between 1750 and 1550 cm−1,52−54 -NH2 groups around 1650 cm−1,55 −CH3 and −CH2− groups between 3000 and 2800 cm−1,52,54 and −OH groups from 3700 to 3000 cm−1.52,53 In addition, we verified the Raman characteristic peaks for the Bi2Te3-based materials in a spectral range of Raman shift from 100 to 4000 cm−1 (Figure S3), in agreement with those in other studies,51 but did not find any signals for chemical residues mostly observed above 1000 cm−1, such as −CH2− or −CH3 vibration modes (1000−3000 cm−1), −OH stretching modes (3000−4000 cm−1), and NH4+ stretching modes (1400−3300 cm−1).56−58 None of the residues were detected because neither organic solvents nor chemical stabilizers were used to synthesize the Bi2Te2.7Se0.3 nano5627


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Figure 4. Electron microscopy studies for the sintered Bi2Te2.7Se0.3 specimen: image-quality map (a), inverse pole figure (b), from EBSD analyses with the corresponding grain-size distribution (c), TEM image (d), and HR-TEM image (e).

decrease the lattice thermal conductivity of the nanoplatelet because Bi2TeO5 worked as a phonon scattering center; however, it seemed to also decrease the physical density of the nanoplatelet (i.e., relative density of ca. 93%). Compared to the relative density reported by the Son group, the byproduct (i.e., Bi2TeO5) might bring less damage on physical density but it should also decrease carrier mobility; the nanoplatelet showed lower carrier mobility (i.e., 166 cm2 V−1 s−1, Table 1) than the nc-Bi2Te2.7Se0.3. In addition, the nanoplatelet exhibited lower carrier concentration than the nc-Bi2Te2.7Se0.3; thus, it recorded higher electrical resistivity (Table 1, Figure 6a). Mehta et al. fabricated a Bi2Te3 nanoplate via a microwave synthesis using thioglycolic acid as a shape-directing agent.41 They insisted that the thioglycolic acid should act as a sulfur dopant that changed both carrier concentration and majority carrier of the nanoplate. The product was converted to an n-type semiconductor with the moderate carrier concentration (i.e., 4.20 × 1019 cm−3, Table 1) by the doping effect of thioglycolic acid; however, it showed higher resistivity than the ncBi2Te2.7Se0.3 (Figure 6a) because of much lower carrier mobility (Table 1). Considering the relative density (i.e., ca. 95%), the thioglycolic acid might not severely interfere with grain growth, but it should be left in the product as a chemical residue hindering carrier transport.

Chemical residues or second phase byproducts possibly resulting from chemical syntheses might hinder grain growth during sintering process; thus, relative density should decrease. Son et al. used complex organic precursors, such as bismuth dodecanethiolate and tri-n-octylphosphine telluride, and employed dodecanethiol surfactants in alkylamines to form a Bi2Te3 nanoplate.39 They adopted a purification method and also varied sintering conditions to remove latent organic residues, and they insisted that the optimum sintering temperatures should range from 250 to 300 °C; however, their products still showed a low relative density (i.e., ca. 90%) at the sintering temperatures resulting in poor carrier mobility (i.e., 36.5 and 62.6 cm2 V−1 s−1; Table 1). We expected that the organic precursors, surfactants, and solvent were not completely removed but they remained in the nanoplate as organic residues. They might hinder the densification of the nanoplate during the sintering process, which possibly led to low relative density and carrier mobility. Despite the high carrier concentration (i.e., 6.80 and 4.68 × 1019 cm−3, Table 1), the nanoplate recorded much higher electrical resistivity (Table 1, Figure 6a) than the nc-Bi2Te2.7Se0.3 because of the inferior carrier mobility. Li et al. prepared a Bi2Te2.25Se0.75 nanoplatelet through a hydrothermal method followed by an SPS process.40 After the sintering process, the second phase byproduct, Bi2TeO5 was found. They insisted that Bi2TeO5 should 5628


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Figure 5. FT-IR spectrum of the nc-Bi2Te2.7Se0.3 (a) and XPS spectra for Bi (b), Te (c), and Se (d) of the raw elements and the nc-Bi2Te2.7Se0.3. Core levels of the XPS spectra are presented as Bi 4f7/2 (□, panel b), Bi 4f5/2 (▽, panel b), Te 3d5/2 (○, panel c), Te 3d3/2 (△, panel c), Se 3d5/2 (◇, panel d), and Se 3d3/2 (☆ and ◁, panel d).

Table 1. Comparison of Carrier Concentration (n), Carrier Mobility (μ), and Electrical Resistivity (ρ) for the ncBi2Te2.7Se0.3 to Those for the Similar Nanobulk Structures in Previous Works at 25°C sample nc-Bi2Te2.7Se0.3 Bi2Te3 nanoplate39 Bi2Te2.25Se0.75 nanoplatelet40 Bi2Te3 nanoplate41

S250 S300

n (×1019 cm−3)

μ (cm2 V−1 s−1)

ρ (μΩ m)

3.91 6.80 4.68 3.36

193 36.5 62.6 166

8.30 25.6 21.5 11.2

4.20

75.0

25.6

According to the measured Seebeck coefficient (Figure 6b), the nc-Bi2Te2.7Se0.3 was verified as an n-type semiconductor. The Seebeck coefficient increased from 25 to 100 °C because the majority carriers (i.e., electrons) were vigorously excited in the temperature region. Meanwhile, resulting electron−hole pairs should be excited across the band gap of the ncBi2Te2.7Se0.3 above 100 °C and the Seebeck coefficient gradually decreased because of the opposing effects between the two types of carrier. Consequently, the nc-Bi2Te2.7Se0.3 showed the maximum Seebeck coefficient (−160.6 μVK−1) at 100 °C, and we attempted to theoretically prove the maximum value. The Seebeck coefficient behaviors of the nc-Bi2Te2.7Se0.3 comply well with the bipolar conduction in a semiconductor, which involves thermal excitation of both electrons and holes across a band gap. Intensity of the bipolar conduction depends on the band gap energy of a semiconductor. Goldsmid et al. derived an analytical relation between the band gap energy and the maximum Seebeck coefficient of a semiconductor as60,61

|α|max ≅

Figure 6. Electrical resistivity (a) and Seebeck coefficient (b) of the nc-Bi2Te2.7Se0.3 (◆) compared to those of the similar nanobulk structures in previous works: Bi2Te 3 nanoplates (▶, ◀),39 Bi2Te2.25Se0.75 nanoplatelet (★),40 and Bi2Te3 nanoplate (●).41

Eg 2eTmax

where |α|max is the maximum Seebeck coefficient, Eg the band gap energy, e the elementary charge, and Tmax the temperature

(7) 5629


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Industrial & Engineering Chemistry Research at which |α|max occurs in the bipolar regime. To obtain the band gap energy of the nc-Bi2Te2.7Se0.3, we adopted the Tauc plot as (F(R )hν)1/ n = A(hν − Eg )

other n-type Bi2Te3 nanobulk materials below 150 °C; in particular, the nc-Bi2Te2.7Se0.3 exhibited power factors from 25 to 100 °C that were much superior to those of the others. As shown in Figure 6b, the Seebeck coefficient values of the ncBi2Te2.7Se0.3 were not predominant compared to those of other nanobulk materials; thus, we were assured that the high power factor was achieved because of the significantly low electrical resistivity of the nc-Bi2Te2.7Se0.3. Considering that Bi2Te3 is known to be the most appropriate material for low-temperature application in terms of thermoelectric generation, the power factors especially below 100 °C should be very encouraging values. As verified in the electron microscopy results (Figure 4), the average grain size of the nc-Bi2Te2.7Se0.3 was calculated to be approximately 200 nm. Electron scattering would not be severe because the grain size is greater than the mean free path of electrons in semiconductors (i.e., below ca. 100 nm62); thus, the carrier mobility should be preserved. Considering the fact that the mean free path of phonons in semiconductors generally ranges from 0.1 to 10 μm,62,63 vigorous phonon scattering could occur at numerous grain boundaries of the ncBi2Te2.7Se0.3. Therefore, we expected the nc-Bi2Te2.7Se0.3 to demonstrate competitively low thermal conductivity. The lattice contribution of the nc-Bi2Te2.7Se0.3 was found to be only ca. 40% of that in bulk single crystalline Bi2Te2.85Se0.15 (denoted as sc-Bi2Te2.85Se0.15, Table 2) at 25 °C.64 We also

(8)

where F(R) is the Kubelka−Munk function, hv the incident phonon energy, n the constant determined by the transition nature that a sample undergoes, A the proportional constant, and Eg the band gap energy. The Kubelka−Munk function (F(R) = (1−R)2/2R) allows the optical absorbance of the ncBi2Te2.7Se0.3 to be approximated from its diffuse reflectance spectrum (Figure 7 inset). The n values are 1/2, 3/2, 2, and 3

Figure 7. Tauc plot of (F(R)hν)1/2 versus band gap energy for indirect allowed transition of the nc-Bi2Te2.7Se0.3 and its optical reflectance spectrum (inset).

Table 2. Total Thermal Conductivity (κt) of the ncBi2Te2.7Se0.3 and the sc-Bi2Te2.85Se0.15 at 25°Ca

for direct allowed, direct forbidden, indirect allowed, and indirect forbidden transitions, respectively. Because a Bi2Te3based material is an indirect band gap semiconductor with allowed transition, we used 2 as the n value. We generated the plot of (F(R)hv)1/2 versus hv and found the band gap energy of the nc-Bi2Te2.7Se0.3 to be 0.12 eV using a least-squares fit (Figure 7). When we substituted this band gap energy with the maximum temperature, 373 K, in eq 7, we derived 160.9 μV K−1 as the magnitude of the maximum Seebeck coefficient, which corresponded to the experimental value within 0.2% of error range. We found that temperature-dependent behavior of the power factor (Figure 8) seemed to be somewhat similar to that of the Seebeck coefficient, which was closely related to the bipolar conduction of the nc-Bi2Te2.7Se0.3 as elaborated above. The ncBi2Te2.7Se0.3 recorded power factors higher than those of any

sample

κt (W m−1 K−1)

κl (W m−1 K−1)

κc (W m−1 K−1)

nc-Bi2Te2.7Se0.3 sc-Bi2Te2.85Se0.1564

0.88 1.70

0.35 0.95

0.53 0.75

a

The lattice (κl) and carrier (κc) terms of the total thermal conductivity were calculated using the Wiedemann−Franz law.

verified that the lattice contribution was one of the lowest reported values for n-type Bi2Te3 nanobulk materials; the ncBi2Te2.7Se0.3 recorded values lower than 0.35 W m−1 K−1 at all measured temperatures (Figure 9a). Meanwhile, we obtained less decrease in the carrier contribution; the carrier one of the nc-Bi2Te2.7Se0.3 was found to be approximately 70% of that in sc-Bi2Te2.85Se0.15 (Table 2) at 25 °C. Compared to the other nanobulk materials, the nc-Bi2Te2.7Se0.3 exhibited a somewhat high carrier contribution, which ranged from 0.5 to 0.7 W m−1 K−1 at the measured temperatures (Figure 9b). It is generally known that carrier thermal conductivity is dependent on both carrier concentration and mobility. As indicated in Table 1, the nc-Bi2Te2.7Se0.3 showed an ordinary carrier concentration (i.e., 3.91 × 1019 cm−3) compared with the other nanobulk materials, while it recorded a considerably higher carrier mobility. The nc-Bi2Te2.7Se0.3 might exhibit carrier thermal conductivity higher than the other nanobulk materials because of the high carrier mobility. The carrier contribution increased the overall thermal conductivity; thus, the nc-Bi2Te2.7Se0.3 normally showed high thermal conductivity values compared to other nanobulk materials (Figure 9c). However, the nc-Bi2Te2.7Se0.3 still maintained competitively low thermal conductivity values as a thermoelectric material (i.e., 0.80−0.88 W m−1 K−1 at all measured temperatures); the thermal conductivity was only 50% of that of sc-Bi2Te2.85Se0.15 at 25 °C (Table 2). We confirmed that this small value of thermal conductivity was attributable to the relatively low

Figure 8. Temperature dependence of power factor of the ncBi2Te2.7Se0.3 (◆) compared to those of the Bi2Te3 nanoplates (▶, ◀),39 Bi2Te2.25Se0.75 nanoplatelet (★),40 and Bi2Te3 nanoplate (●).41 5630


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Figure 10. ZT values of the nc-Bi2Te2.7Se0.3 (◆), Bi2Te3 nanoplates (▶, ◀),39 Bi2Te2.25Se0.75 nanoplatelet (★),40 and Bi2Te3 nanoplate (●).41

uniform size distribution. We packed the nanocompound via spark plasma sintering and optimized the sintering conditions to preserve its nanostructure as well as to secure sufficient physical density. The resulting nanobulk crystalline Bi2Te2.7Se0.3 (nc-Bi2Te2.7Se0.3) specimen exhibited very high physical density in close proximity to its theoretical density; thus, it recorded higher carrier mobility than any other n-type Bi2Te3 nanomaterials in the bulk phase. The high carrier mobility enhanced the electrical conductivity, resulting in a predominant power factor. The carrier mobility also increased the carrier thermal conductivity; however, the nc-Bi2Te2.7Se0.3 still recorded the low thermal conductivity because of its low lattice contribution, which might result from a high phonon scattering rate in the nanostructure. The high electrical conductivity was achieved while the thermal conductivity was retained as low as possible; thus, the synergistic effect of the conductivities should be induced, which led to the highest ZT value for n-type Bi2Te3 in the bulk phase.

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Figure 9. Thermal conductivities of the nc-Bi2Te2.7Se0.3 (◆), Bi2Te3 nanoplates (▶, ◀),39 Bi2Te2.25Se0.75 nanoplatelet (★),40 and Bi2Te3 nanoplate (●):41 lattice (κl, panel a), carrier (κc, panel b), and total (κt, panel c) thermal conductivities.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00933. ICP-MS results (Table S1), synthesis procedures (Figure S1), EBSD images (Figure S2), and Raman spectra (Figure S3) of the Bi2Te3 and Bi2Te2.7Se0.3 nanocompounds (PDF)

lattice thermal conductivity, which should result from the phonon scattering effect in the nc-Bi2Te2.7Se0.3. The nc-Bi2Te2.7Se0.3 was not biased to low thermal conductivity, but it ideally kept a balance between the electrical and thermal conductivities; thus, the synergistic effect of the conductivities should be induced. This led to the predominant ZT values at the temperatures ranging from 25 to 150 °C; we accomplished an average ZT of 1.03 in the temperature region and the maximum ZT of 1.15 at 100 °C (Figure 10), which is the highest performance for n-type Bi2Te3 in the bulk phase. Because Bi2Te3 is known to be the only thermoelectric material for low-temperature operations below 200 °C, the ZT values in the temperature region are significant. We also expect the ncBi2Te2.7Se0.3 to contribute to the reduction of ZT gaps between n- and p-type Bi2Te3, resulting in the great opportunity to constitute high-performance thermoelectric devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +82-53-785-3602. Fax: +82-53-785-3609. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the DGIST R&D Program of the Ministry of Science, ICT and Technology of Korea (16-EN02). This work was also supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20152020001210).

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CONCLUSION We designed a novel wet chemical process in which no chemical additives and organic solvents were used for the synthesis of a highly pure Bi2Te2.7Se0.3 nanocompound with 5631


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Soft X-Ray Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 085203. (22) Hermann, R. P.; Grandjean, F.; Keppens, V.; Schweika, W.; Nolas, G. S.; Mandrus, D. G.; Sales, B. C.; Christen, H. M.; Bonville, P.; Long, G. J. The Dynamics of the Guests in Filled Germanium Clathrates. MRS Online Proc. Libr. 2005, 886, DOI: 10.1557/PROC0886-F10-01. (23) Paschen, S.; Pacheco, V.; Bentien, A.; Sanchez, A.; CarrilloCabrera, W.; Baenitz, M.; Iversen, B. B.; Grin, Y.; Steglich, F. Are Type-I Clathrates Zintl Phases and ‘Phonon Glasses and Electron Single Crystals’? Phys. B 2003, 328, 39. (24) Panmand, R. P.; Kumar, G.; Mahajan, S. M.; Shroff, N.; Kale, B. B.; Gosavi, S. W. Growth of Bi2Te3 Quantum Dots/Rods in Glass: a Unique Highly Stable Nanosystem with Novel Functionality for High Performance Magneto Optical Devices. Phys. Chem. Chem. Phys. 2012, 14, 16236. (25) Zhou, J.; Wang, Y.; Sharp, J.; Yang, R. Optimal Thermoelectric Figure of Merit in Bi2Te3/Sb2Te3 Quantum Dot Nanocomposites. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 115320. (26) Wang, K.; Liang, H. W.; Yao, W. T.; Yu, S. H. Templating Synthesis of Uniform Bi2Te3 Nanowires with High Aspect Ratio in Triethylene Glycol (TEG) and Their Thermoelectric Performance. J. Mater. Chem. 2011, 21, 15057. (27) Kim, J. J.; Kim, S. H.; Suh, S. W.; Choe, D. U.; Park, B. K.; Lee, J. R.; Lee, Y. S. Hydrothermal Synthesis of Bi2Te3 Nanowires through the Solid-State Interdiffusion of Bi and Te Atoms on the Surface of Te Nanowires. J. Cryst. Growth 2010, 312, 3410. (28) Zhang, G.; Kirk, B.; Jauregui, L. A.; Yang, H.; Xu, X.; Chen, Y. P.; Wu, Y. Rational Synthesis of Ultrathin n-Type Bi2Te3 Nanowires with Enhanced Thermoelectric Properties. Nano Lett. 2012, 12, 56. (29) Wang, Y.; Liebig, C.; Xu, X.; Venkatasubramanian, R. Acoustic Phonon Scattering in Bi2Te3/Sb2Te3 Superlattices. Appl. Phys. Lett. 2010, 97, 083103−1. (30) Tan, M.; Deng, Y.; Hao, Y. Enhanced Thermoelectric Properties and Superlattice Structure of a Bi2Te3/ZrB2 Film Prepared by IonBeam-Assisted Deposition. J. Phys. Chem. C 2013, 117, 20415. (31) Hansen, A. L.; Dankwort, T.; Winkler, M.; Ditto, J.; Johnson, D. C.; Koenig, J. D.; Bartholomé, K.; Kienle, L.; Bensch, W. Synthesis and Thermal Instability of High-Quality Bi2Te3/Sb2Te3 Superlattice Thin Film Thermoelectrics. Chem. Mater. 2014, 26, 6518. (32) Yamasaki, I.; Yamanaka, R.; Mikami, M.; Sonobe, H.; Mori, Y.; Sasaki, T. Thermoelectric Properties of Bi2Te3/Sb2Te3 Superlattice Structure. Proc. 17th Int. Thermoelectrics Conf. 1998, 210. (33) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys. Science 2008, 320, 634. (34) Finefrock, S. W.; Yang, H.; Fang, H.; Wu, Y. Thermoelectric Properties of Solution Synthesized Nanostructured Materials. Annu. Rev. Chem. Biomol. Eng. 2015, 6, 247. (35) Scheele, M.; Oeschler, N.; Veremchuk, I.; Reinsberg, K. G.; Kreuziger, A. M.; Kornowski, A.; Broekaert, J.; Klinke, C.; Weller, H. ZT Enhancement in Solution-Grown Sb(2−x)BixTe3 Nanoplatelets. ACS Nano 2010, 4, 4283. (36) Hasan, S. W.; Mun, H.; Kim, S. I.; Cho, J. Y.; Roh, J. W.; Yang, S.; Choi, S. M.; Lee, K. H.; Kim, S. W. Formation of Dense Pore Structure by Te Addition in Bi0.5Sb1.5Te3: an Approach to Minimize Lattice Thermal Conductivity. J. Nanomater. 2013, 2013, 1. (37) Kim, S. I.; Lee, K. H.; Mun, H. A.; Kim, H. S.; Hwang, S. W.; Roh, J. W.; Yang, D. J.; Shin, W. H.; Li, X. S.; Lee, Y. H.; Snyder, G. J.; Kim, S. W. Dense Dislocation Arrays Embedded in Grain Boundaries for High-Performance Bulk Thermoelectrics. Science 2015, 348, 109. (38) Yang, L.; Chen, Z. G.; Hong, M.; Han, G.; Zou, J. Enhanced Thermoelectric Performance of Nanostructured Bi2Te3 through Significant Phonon Scattering. ACS Appl. Mater. Interfaces 2015, 7, 23694. (39) Son, J. S.; Choi, M. K.; Han, M. K.; Park, K.; Kim, J. Y.; Lim, S. J.; Oh, M.; Kuk, Y.; Park, C.; Kim, S. J.; Hyeon, T. n-Type

REFERENCES

(1) Chen, Z. G.; Han, G.; Yang, L.; Cheng, L.; Zou, J. Nanostructured Thermoelectric Materials: Current Research and Future Challenge. Prog. Nat. Sci. 2012, 22, 535. (2) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. New Directions for Nanoscale Thermoelectric Materials Research. MRS Online Proc. Libr. 2005, 886, DOI: 10.1557/PROC-0886-F01-01. (3) Byrnes, D. F.; Heshmatpour, B. High Efficiency Thermoelectric Power Conversion. MRS Online Proc. Libr. 2005, 886, DOI: 10.1557/ PROC-0886-F12-03. (4) Tritt, T. M.; Zhang, B.; Gothard, N.; He, J.; Ji, X.; Thompson, D.; Kolis, J. W. New Directions in Bulk Thermoelectric Materials Research: Synthesis of Nanoscale Precursors for “Bulk-Composite” Thermoelectric Materials. Mater. Res. Soc. Symp. Proc. 2005, 886, DOI: 10.1557/PROC-0886-F02-01. (5) Sootsman, J.; Kong, H.; Uher, C.; Downey, A.; D’Angelo, J. J.; Wu, C. I.; Hogan, T.; Caillat, T.; Kanatzidis, M. Transport Behavior and Thermal Conductivity Reduction in the Composite System PbTePb-Sb. MRS Online Proc. Libr. 2007, 1044, DOI: 10.1557/PROC1044-U08-01. (6) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit. Science 2004, 303, 818. (7) Ji, X.; Zhang, B.; Tritt, T. M.; Kolis, J. W.; Kumbhar, A. SolutionChemical Syntheses of Nano-Structured Bi2Te3 and PbTe Thermoelectric Materials. J. Electron. Mater. 2007, 36, 721. (8) Bell, L. E. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science 2008, 321, 1457. (9) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. New Directions for Low-Dimensional Thermoelectric Materials. Adv. Mater. 2007, 19, 1043. (10) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105. (11) Zhu, T. J.; Liu, Y. Q.; Zhao, X. B. Synthesis of PbTe Thermoelectric Materials by Alkaline Reducing Chemical Routes. Mater. Res. Bull. 2008, 43, 2850. (12) Li, H.; Cai, K. F.; Wang, H. F.; Wang, L.; Li, X. L. Preparation and Thermoelectric Properties of AgPb18SbTe20 Materials via Hydrothermal Synthesis Method. Key Eng. Mater. 2008, 368−372, 550. (13) Martin, J.; Nolas, G. S.; Zhang, W.; Chen, L. PbTe nanocomposites synthesized from PbTe nanocrystals. Appl. Phys. Lett. 2007, 90, 222112−1. (14) Rowe, D. W. Thermoelectrics Handbook: Macro to Nano; CRC/ Taylor & Francis: Boca Raton, FL, 2006. (15) Kurosaki, K.; Matsuda, T.; Uno, M.; Kobayashi, S.; Yamanaka, S. Thermoelectric Properties of BaUO3. J. Alloys Compd. 2001, 319, 271. (16) Goldsmid, H. J. Introduction to Thermoelectricity; Springer: Berlin, 2009. (17) Peng, J.; He, J.; Alboni, P. N.; Tritt, T. M. Synthesis and Thermoelectric Properties of the Double-Filled Skutterudite Yb0.2InyCo4Sb12. J. Electron. Mater. 2009, 38, 981. (18) Nolas, G. S.; Morelli, D. T.; Tritt, T. M. SKUTTERUDITES: A Phonon-Glass-Electron Crystal Approach to Advanced Thermoelectric Energy Conversion Applications. Annu. Rev. Mater. Sci. 1999, 29, 89. (19) Min, G.; Rowe, D. M. A Serious Limitation to the Phonon Glass Electron crystal (PGEC) Approach to Improved Thermoelectric Materials. J. Mater. Sci. Lett. 1999, 18, 1305. (20) Grytsiv, A.; Rogl, P.; Berger, S.; Paul, C.; Michor, H.; Bauer, E.; Hilscher, G.; Godart, C.; Knoll, P.; Musso, M.; Lottermoser, W.; Saccone, A.; Ferro, R.; Roisnel, T.; Noel, H. A Novel Skutterudite Phase in the Ni-Sb-Sn System: Phase Equilibria and Physical Properties. J. Phys.: Condens. Matter 2002, 14, 7071. (21) Tang, J.; Rachi, T.; Kumashiro, R.; Avila, M. A.; Suekuni, K.; Takabatake, T.; Guo, F. Z.; Kobayashi, K.; Akai, K.; Tanigaki, K. Energetics of Endohedral Atoms in Type-I Clathrates Observed by 5632


Article

Industrial & Engineering Chemistry Research Nanostructured Thermoelectric Materials Prepared from Chemically Synthesized Ultrathin Bi2Te3 Nanoplates. Nano Lett. 2012, 12, 640. (40) Li, D.; Qin, X. Y.; Liu, Y. F.; Wang, N. N.; Song, C. J.; Sun, R. R. Improved Thermoelectric Properties for Solution Grown Bi2Te3‑xSex Nanoplatelet Composites. RSC Adv. 2013, 3, 2632. (41) Mehta, R. J.; Zhang, Y.; Karthik, C.; Singh, B.; Siegel, R. W.; Borca-Tasciuc, T.; Ramanath, G. A New Class of Doped Nanobulk High-Figure-of-Merit Thermoelectrics by Scalable Bottom-Up Assembly. Nat. Mater. 2012, 11, 233. (42) Soni, A.; Yanyuan, Z.; Ligen, Y.; Aik, M. K. K.; Dresselhaus, M. S.; Xiong, Q. Enhanced Thermoelectric Properties of Solution Grown Bi2Te3−xSex Nanoplatelet Composites. Nano Lett. 2012, 12, 1203. (43) Kim, C.; Kim, D. H.; Kim, J. T.; Han, Y. S.; Kim, H. Investigation of Reaction Mechanisms of Bismuth Tellurium Selenide Nanomaterials for Simple Reaction Manipulation Causing Effective Adjustment of Thermoelectric Properties. ACS Appl. Mater. Interfaces 2014, 6, 778−785. (44) Goldsmid, H. J. Electronic Refrigeration; Pion Limited: London, 1986. (45) Sales, B. C.; Mandrus, D.; Chakoumakos, B. C.; Keppens, V.; Thompson, J. R. Filled Skutterudite Antimonides: Electron Crystals and Phonon Glasses. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, 15081. (46) Haynes, W. M. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2011. (47) Bratsch, S. G. Standard Electrode Potentials and Temperature Coefficients in Water at 298.15 K. J. Phys. Chem. Ref. Data 1989, 18, 1. (48) Bouroushian, M. Electrochemistry of Metal Chalcogenides; Springer: Berlin, 2010. (49) Sun, Z.-m.; Zheng, Y.-j. Preparation of High Pure Tellurium from Raw Tellurium Containing Cu and Se by Chemical Method. Trans. Nonferrous Met. Soc. China 2011, 21, 665. (50) Pan, Y.; Wei, T. R.; Wu, C. F.; Li, J. F. Electrical and Thermal Transport Properties of Spark Plasma Sintered n-type Bi2Te3‑xSex Alloys: the Combined Effect of Point Defect and Se Content. J. Mater. Chem. C 2015, 3, 10583. (51) Puneet, P.; Podila, R.; Karakaya, M.; Zhu, S.; He, J.; Tritt, T. M.; Dresselhaus, M. S.; Rao, A. M. Preferential Scattering by Interfacial Charged Defects for Enhanced Thermoelectric Performance in Fewlayered n-type Bi2Te3. Sci. Rep. 2013, DOI: 10.1038/srep03212. (52) Sharma, K.; Kumar, A.; Goyal, N.; Lal, M. Characterization and Optical Properties of Bi2Te3 and (Bi.20Sb.80)2Te3. AIP Conf. Proc. 2013, 603. (53) Yokoyama, S.; Sato, K.; Muramatsu, M.; Yamasuge, T.; Itoh, T.; Motomiya, K.; Takahashi, H.; Tohji, K. Green Synthesis and Formation Mechanism of Nanostructured Bi2Te3 Using Ascorbic Acid in Aqueous Solution. Adv. Powder Technol. 2015, 26, 789. (54) Xu, Y.; Ren, Z.; Cao, G.; Ren, W.; Deng, K.; Zhong, Y. Synthesis of Single Crystal Bi2Te3 Nanoplates via an Inorganic-SurfactantAssisted Solvothermal Route. Mater. Lett. 2008, 62, 4525. (55) Salavati-Niasari, M.; Bazarganipour, M.; Davar, F. Hydrothermal Preparation and Characterization of Based-Alloy Bi2Te3 Nanostructure with Different Morphology. J. Alloys Compd. 2010, 489, 530. (56) Šebek, J.; Pele, L.; Potma, E. O.; Gerber, R. B. Raman Spectra of Long Chain Hydrocarbons: Anharmonic Calculations, Experiment and Implications for Imaging of Biomembranes. Phys. Chem. Chem. Phys. 2011, 13, 12724. (57) Kolesov, B. Raman Investigation of H2O Molecule and Hydroxyl Groups in the Channels of Hemimorphite. Am. Mineral. 2006, 91, 1355. (58) Carter, R. L. Raman Spectra of the Double-Anion Salts M3ZnCl4NO3 (M+ = K+, Rb+, NH4+). Spectrochim. Acta, Part A 2000, 56, 2351. (59) Nolas, G. S.; Sharp, J.; Goldsmid, H. J. Thermoelectrics: Basic Principles and New Materials Developments; Springer: New York, 2001. (60) Goldsmid, H. J.; Sharp, J. W. Estimation of the Thermal Band Gap of a Semiconductor from Seebeck Measurements. J. Electron. Mater. 1999, 28, 869.

(61) Gibbs, Z. M.; Kim, H. S.; Wang, H.; Snyder, G. J. Band Gap Estimation from Temperature Dependent Seebeck Measurement − Deviations from the 2e|S|maxTmax Relation. Appl. Phys. Lett. 2015, 106, 022112−1. (62) Blakemore, J. S. Solid State Physics; Cambridge University Press: Cambridge, 1985. (63) Kim, C.; Kim, D. H.; Kim, H.; Chung, J. S. Significant Enhancement in the Thermoelectric Performance of a Bismuth Telluride Nanocompound through Brief Fabrication Procedures. ACS Appl. Mater. Interfaces 2012, 4, 2949−2954. (64) Fleurial, J. P.; Gailliard, L.; Triboulet, R.; Scherrer, H.; Scherrer, S. Thermal Properties of High Quality Single Crystals of Bismuth Telluride − Part I: Experimental Characterization. J. Phys. Chem. Solids 1988, 49, 1237.

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New Chemical Reaction Process of a Bi2Te2.7Se0.3 Nanomaterial for

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