Synthesis and Characterization of Telluride Aerogels: Effect of

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Synthesis and Characterization of Telluride Aerogels: Effect of Gelation on Thermoelectric Performance of Bi2Te3 and Bi2−xSbxTe3 Nanostructures Shreyashi Ganguly,† Chen Zhou,‡ Donald Morelli,*,‡ Jeff Sakamoto,*,‡ and Stephanie L. Brock*,† †

Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States Chemical Engineering and Material Science, Michigan State University, East Lansing, Michigan 48824, United States

‡

S Supporting Information *

ABSTRACT: The synthesis and characterization of Bi2Te3 and Bi2−xSbxTe3 aerogel materials, and the effect of gelation on thermoelectrically relevant properties, is reported. Aerogels are prepared from oxidation of discrete thiolate-capped nanoparticles to yield a wet gel, followed by supercritical CO2 drying. The resultant aerogels have surface areas between 36 and 45 m2/g. Characterization of the thermoelectric properties of hot-pressed pellets of Bi2Te3 aerogels suggested a decrease in lattice thermal conductivity with respect to the bulk materials, attributed to the effect of nanostructuring, but the power factor (S2σ) was also reduced due to the effect of adventitious doping. In the case of Bi2−xSbxTe3 aerogels, there was no change in the lattice thermal conductivity upon nanostructuring, but again the power factor was reduced with respect to bulk materials. This is attributed to the presence of excess tellurium, which led to compensation of the majority charge carriers. Proper carrier concentration optimization of the chalcogenide aerogel materials is needed if these materials are to be exploited in thermoelectrics.

1. INTRODUCTION Thermoelectric (TE) devices are those that convert thermal energy into electrical energy and vice versa. The efficiency of a TE device is related to its dimensionless figure of merit ZT = (S2σ)T/κ (where S = Seebeck coefficient, σ = electrical conductivity, and κ = total thermal conductivity, which consists of both an electronic contribution, κe, as well as a lattice contribution, κl).1 Since higher ZT means higher device efficiency, a good TE material should possess high values for S and σ and a small κ. In the past, the ZT of state-of-the-art bulk TE materials has remained stagnant, near unity, but in order to be commercially useful, ZT > 3 is required.2 Nanostructuring of already established TE materials has gained increasing attention over the years as both theoretically and experimentally it was found that nanostructuring was helpful in increasing the ZT beyond unity.2,3 Enhancement in ZT for nanostructured TE materials can arise via increased density of states near the Fermi level, leading to enhancement of thermopower, S, and also scattering of heat carrying phonons by the nanoscale interfaces, leading to reduction in κl.1−3 Among thermoelectric materials, nanostructures of Bi2Te3 and alloys like Bi2−xSbxTe3 have been extensively studied because of their relatively high ZT (∼1) near room temperature.4−6 Different forms of nanostructuring of these materials have been reported where the ZT ≈ 1 barrier was overcome. This includes superlattices of Bi2Te3/Sb2Te3, prepared via chemical vapor deposition, where ZT = 1.4−2.4 was obtained.7 In another © 2012 American Chemical Society

case, a ZT of 1.4 was reported for nanostructures of BixSb2−xTe3 prepared by ball milling.8 Decreased lattice thermal conductivity has also been demonstrated in Bi2Te3 infiltrated with Bi nanoparticles, the latter inducing additional phonon scattering.9 The enhancements in ZT for the above systems were observed primarily as a result of reduction in lattice thermal conductivity. Our attempts to use a solution-based approach of generating heterostructured nanocomposites of PbTe nanoparticles in a BixSb2−xTe3 bulk matrix via incipient wetness processing led to good control of the size of the nanoparticles incorporated. However, while reduction in κl was seen, Pb acted as a dopant and compromised the electronic properties, resulting in low ZT.10 In general, researchers have turned to nanoparticle assembly methods for generating nanostructures as the synthesis of discrete nanoparticles enables exquisite control of size. Several solution-based approaches have been introduced for assembling nanoparticles. For example, superlattices of lead selenide (PbSe) were achieved via self-assembly.11 However, insulating organic ligands at the nanoparticle interfaces remains a challenge for achieving optimal device conductivity in these systems.11 This has recently been addressed by the use of inorganic ligands.12 Another method to potentially improve the efficiency of a TE device in a solution-derived nanostructure is by connecting the Received: June 6, 2012 Revised: July 25, 2012 Published: August 2, 2012 17431

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min (growth time), followed by quenching of the reaction in a cold water bath. The product was isolated as described for Bi2Te3. 2.3. Generation of Bi2Te3 and BixSb2−xTe3 Aerogels. A 500-mg sample of Bi2Te3 was dispersed in 20 mL of acetone targeting a final concentration of Bi3+ of 0.06 M; alternatively, 200 mg of BixSb2−xTe3 was dispersed in the same volume of acetone for a combined concentration of Bi3+ and Sb3+ of 0.03 M, assuming a Bi0.5Sb1.5Te3 composition. The concentrations were calculated based on the bulk molecular weight of each species (i.e., neglecting ligating groups). The solutions were then each divided in four different vials (5 mL each) and gelation was induced by adding 0.1 mL of 3% TNM in acetone (v/v). The solution was shaken and gelation was observed within 1 h. The wet gels were aged for 4−5 days under ambient conditions and then were exchanged with acetone twice a day for another 2 days. A portion of the wet gel was transferred to fresh vials and supercritically dried with CO2, using a SPI-DRY model critical point dryer (CPD) to produce an aerogel, by a procedure previously reported.15

nanoparticles together so that the interfaces consist of the same chemical constituents as the interior of the material. This can potentially be achieved via the sol−gel method.13,14 In addition to removing interfacial organics, the gel obtained by this process provides additional pore−matter interfaces that will further scatter heat-carrying phonons, leading to a decrease of κl. The sol−gel method has been extensively studied for oxide materials.14−16 Gacoin and co-workers were the first to apply the nanoparticle assembly sol−gel method to a metal chalcogenide material, CdS.17 Our lab and others have extended the sol−gel methodology to other metal chalcogenides, including ZnS, CdSe, CdTe, and PbTe materials, as well as composites of noble metals and chalcogenide aerogels, and even discrete Zintl ions.18−30 The nanoparticle assembly sol−gel method for metal chalcogenides employed here consists of three major steps: (1) the synthesis of discrete ligand-capped metal chalcogenide nanoparticles; (2) ligand replacement with thiolate groups, and (3) oxidative condensation of the nanoparticles with concomitant ligand oxidative loss. This paper describes the synthesis and characterization of aerogels of Bi2Te3 and Bi2−xSbxTe3 from discrete nanoparticles with use of the sol−gel methodology. Preliminary work has been reported on the synthesis and characterization of Bi2Te3 aerogels, along with Seebeck coefficient and resistivity measurements on hot-pressed pellets of these materials, yielding impressive power factor values (6.06 × 10−4 W/(m·K2)), but thermal conductivity values were not measured.31 This paper describes a systematic study of the synthesis and characterization of nanoparticles and aerogels of Bi2Te3 as well as a doped system, Bi2−xSbxTe3. The thermoelectric property measurements on hot-pressed pellets of these materials, including thermal conductivity, were obtained and will be discussed.

3. CHARACTERIZATION TECHNIQUES 3.1. Powder X-ray Diffraction. Powder X-ray diffraction was collected on a Rigaku RU 200B (40 kV, 150 mA, Cu Kα radiation) diffractometer. Samples were deposited on a zero background quartz holder coated with a thin layer of grease and the data were acquired in the 2θ range 20−70° with a step size of 1.2°. The PXRD data obtained were processed by using the Jade software package and compared to powder diffraction files (PDFs) from the International Center for Diffraction Data database (ICDD). 3.2. Transmission Electron Microscopy and Energy Dispersive Spectroscopy. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) were performed with a JEOL 2010 transmission electron microscope operated at a voltage of 200 kV and a current of 106−108 μA with a coupled EDS detector (EDAX Inc.). Images obtained from TEM measurements were analyzed by Amtv600 software (Advanced Microscopy Techniques Corporation). Samples for TEM were prepared by sonicating Bi2Te3 and BixSb2−xTe3 nanoparticles and aerogels in acetone, followed by deposition of a drop from each solution on a carbon-coated 200 mesh Cu grid (SPI) and air drying for 1 day. 3.3. Surface Area Analysis. Surface area analysis was done by nitrogen physisorption with use of a TRISTAR II 3020 porosimetry analyzer from Micromeritics. The samples were purged at 150 °C for 24 h before analysis by nitrogen gas. The surface area was evaluated by using the Brunauer, Emmett, and Teller (BET) method, whereas the pore size was calculated by the BJH adsorption method developed by Barrett, Joyner, and Halenda.34 3.4. Infrared Spectroscopy. A Varian FTS 3000 MX FTIR spectrometer was used to probe surface organic groups on the nanoparticles and aerogel samples. Powdered samples were ground with KBr and pressed into a transparent pellet in a 13 mm die with 2000 psi pressure from a Carver Hydraulic pellet press. 3.5. Thermoelectric Transport Properties. Bi2Te3 and BixSb2−xTe3 nanoparticles and aerogels were hot pressed at 450 °C under Argon to form pellets from which rectangular parallelepipeds of approximately 2.5 mm ×2.5 mm ×8 mm were cut with a diamond saw. Seebeck coefficient, electrical resistivity, and thermal conductivity were measured from 80 to 380 K under vacuum, using a steady state technique in a

2. EXPERIMENTAL SECTION 2.1. Materials. Bismuth neodecanoate (technical grade), bismuth acetate (≥99.99% metal basis), antimony acetate (≥99.99% metal basis), tellurium powder (Te, 200 mesh, 99.8%), 1-dodecanethiol (DDT, 98%), diphenyl ether (DPE, technical grade 90%), and tetranitromethane (TNM) were obtained from Sigma-Aldrich; trioctylphosphine (TOP, technical grade 97%) and triethyl amine (TEA, reagent) were obtained from Strem chemicals; and oleic acid (OA, technical grade, 90%) was obtained from Fisher. 2.2. Synthesis of Thiolate-Capped Bi 2 Te 3 and Bi2−xSbxTe3 Nanoparticles. The synthesis of the Bi2Te3 nanoparticles employed in the sol−gel assembly was adapted with minor variations from Dirmyer and co-workers.32 The method includes heating a mixture of bismuth neodecanoate (0.63 mL, 0.9 mmol), diphenyl ether (50 mL, 290 mmol), and thiol capping agent 1-dodecanethiol (1-DDT) (10 mL, 40 mmol) at 120 °C, followed by injection of anion monomer, trioctylphosphine telluride (TOPTe) (1.5 mL, 1 M). The temperature was maintained for 1 h (growth time), followed by quenching of the reaction in a cold water bath and isolation by centrifugation. The black product was washed twice with toluene and dried under vacuum. The synthesis of the BixSb2−xTe3 nanoparticles (where, x = 0.5) employed in the sol−gel assembly was adapted from Burda and co-workers.33 The method includes heating a mixture of bismuth acetate (0.193 g, 0.4 mmol), antimony acetate (0.342 g, 1 mmol), diphenyl ether (50 mL, 290 mmol), and thiol capping agent 1-DDT (10 mL, 40 mmol) at 120 °C followed by injection of TOPTe (2 mL, 1 M). The temperature was maintained for 30 17432

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continuous flow cryostat with liquid nitrogen as a refrigerant. More details on the measurements and instrumentation can be found in refs 35−37.

4. RESULTS AND DISCUSSION 4.1. Synthesis and Characterization of Bi2Te3 and BixSb2−xTe3 Nanoparticles and Aerogels. a. Synthesis. Bi2Te3 and BixSb2−xTe3 materials were chosen because they are well-established thermoelectric materials in the bulk form, and nanostructuring of these materials is expected to enhance ZT by reducing the lattice thermal conductivity via scattering of heat carrying phonons from the nanoscale interfaces, as has been observed in related systems.38,39 Moreover, the presence of pore−matter interfaces in aerogels may lead to enhanced phonon scattering beyond that from matter−matter interfacial scattering. The synthetic procedure for Bi2Te3 nanoparticles was adapted from Dirmyer et al., 32 whereas that for the BixSb2−xTe3 nanoparticles was adapted from Burda et al.33 Both procedures employed 1-dodecanethiol (1-DDT) capping groups. Gelation of the Bi2Te3 and BixSb2−xTe3 nanoparticles was achieved by dispersing the thiolate-capped nanoparticles in acetone to form a sol, followed by addition of 0.1 mL of 3% TNM as oxidizing agent, removing the surface-bound thiolate groups and also condensing the nanoparticles together, forming wet gels (Figure 1). These wet gels were further aged for 4 days under

Figure 2. Powder X-ray diffraction patterns of (a) Bi2Te3 and (b) BixSb2−xTe3 nanoparticles (nps), (c) Bi2Te3 aerogel, and (d) BixSb2−xTe3 aerogels. The vertical lines correspond to the ICDD-PDF #15-0863 for Bi2Te3 and ICDD-PDF #49-1713 for Bi0.5Sb1.5Te3, both adopting the rhombohedral phase. Peaks due to Si in panels b and d are indicated by an asterisk.

Figure 1. Image of Bi2Te3 aerogel, wet gel, and sol.

ambient conditions, and then exchanged with fresh acetone twice daily for 2 days to remove byproducts of gelation. At this point, monolithic wet gels were obtained and were transferred carefully into new vials and then supercritically dried to form aerogels (Figure 1). The resultant aerogels for both the samples were monolithic in nature and black in color. b. Characterization. The phase and crystallite size of the products obtained above were determined by powder X-ray diffraction experiments. The PXRD patterns of the Bi2Te3 and BixSb2−xTe3 nanoparticles matched the rhombohedral phase of Bi2Te3 and Bi0.5Sb1.5Te3, respectively (Figure 2). The PXRD data for BixSb2−xTe3 were collected along with an internal silicon standard to observe any shifts in the lattice parameter due to formation of a more Sb-rich or less Sb-rich phase than the targeted Bi0.5Sb1.5Te3. However, the pattern was a perfect match to ICDD-PDF #49-1713 of Bi0.5Sb1.5Te3. Application of the Scherrer equation rendered a value of 35 nm for Bi2Te3 and 50 nm for Bi0.5Sb1.5Te3 nanoparticles, based on the (015) plane in both cases. TEM was employed to verify the size and determine the morphology of the Bi2Te3 and BixSb2−xTe3 nanoparticles. Both the Bi2Te3 and BixSb2−xTe3 nanoparticles manifested as aggregates of plate-like morphology with an overall aggregate size of ∼40 nm (Figure 3). The plate-like morphology is inherent to Bi2Te3 materials, as Bi2Te3 has layered anisotropic crystal

Figure 3. TEM images of (a) Bi2Te3 and (b) BixSb2−xTe3 nanoparticles and (c) Bi2Te3 and (d) BixSb2−xTe3 aerogels obtained from the corresponding nanoparticles. The insets in panels a and b show highresolution TEM images of nanoparticles (HRTEM).

structure. The crystal structure with preferred growth direction in the ab plane of Bi2Te3 consists of Bi and Te layers with rhombohedral-hexagonal symmetry.4,40,41 Strong covalent interactions exist within the Bi and Te layers whereas the adjacent Te layers are held together by weak Van der Waals interactions. BixSb2−xTe3 has a similar lamellar structure. The PXRDs for the Bi2Te3 and BixSb2−xTe3 aerogels obtained from the nanoparticles were similar to those of the starting precursor nanoparticles, suggesting both the crystallite size and phase are retained in the gelation and drying process (Figure 2). The Bi2Te3 and BixSb2−xTe3 aerogels displayed a colloidal morphology consisting of an interconnected network of nanoparticles with pores clearly evident in the TEM images 17433

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(Figure 3). This shows that the sol−gel method was successful in connecting the discrete nanoparticles together, forming a gel network. A semiquantitative EDS study of the Bi2Te3 and BixSb2−xTe3 nanoparticles and aerogels was performed to determine the elemental compositions of the samples. The elemental composition of Bi2Te3 nanoparticles reveals a Bi:Te:S ratio of 1.8:3:0.9 for the nanoparticles, corresponding to a Te-rich Bi2Te3 phase. The sulfur content is attributed to surface thiolate groups. The aerogels have a Bi:Te:S ratio of 2.2:3:0.4, suggesting some Te loss in the gel and aerogel formation process, yielding a Bi-rich phase (Figure 4). The decrease in sulfur content is indicative of loss of surface thiolates, consistent with the oxidative gelation mechanism.42

Table 1. BET Surface Area, Average Pore Diameter, and Cumulative Pore Volume of the Nanoparticles and Aerogels of Bi2Te3 and BixSb2−xTe3 Samples samples Bi2Te3 nanoparticles Bi2Te3 aerogel BixSb2−xTe3 nanoparticles BixSb2−xTe3 aerogel

BET surface area (m2/g)

av pore diameter (nm)

micro to meso cumulative pore vol (cm3/g)

16

24

0.10

45 15

9 13

0.10 0.06

36

9

0.10

Figure 5. N2 adsorption (■, green)/desorption (●, blue) isotherms of (a) Bi2Te3 and (b) BixSb2−xTe3 nanoparticles (nps) and (c) Bi2Te3 and (d) BixSb2−xTe3 aerogel samples. The inset shows the corresponding BJH pore size distributions.

calculated by using the Barrett, Joyner, and Halenda (BJH) model, is consistent with a broad range of pores extending into the macropore region (>50 nm). Intriguingly, BJH plots for nanoparticles reveal very similar features. As there is no pore structure evident in TEM images, we attribute these to the filling of interstices in the solid nanoparticle samples.44 Relative to Bi2Te3, similar pore sizes were observed for the BixSb2−xTe3 aerogels and nanoparticles (Figure 5, inset), where the nanoparticles again showed lower surface area (15 m2/g) relative to the aerogels (36 m2/g) (Table 1). The nanoparticles and aerogels of Bi2Te3 and BixSb2−xTe3 were annealed at 200 °C for 2 h under Ar atmosphere in a flow furnace to remove residual organic ligands. This is done because the organic ligands are expected to act as trap centers for the electrons and hamper the electrical transport in the system. IR was employed to probe the removal of the surface organic groups. Figure 6 shows the IR before and after heating the aerogels of Bi2Te3 samples. The peaks at 2924 and 2846 cm−1 correspond to the aliphatic−CH stretches from the capping agent (1-DDT) which is present before heating (Figure 6a). The disappearance of organic peaks, as evident in the IR spectra after annealing the samples, suggests that heating removes almost all the organic surface capping groups. The same observation was noted for the nanoparticles and aerogels of BixSb2−xTe3 (Supporting Information). 4.2. Thermoelectric Property Measurements. To evaluate thermoelectric properties, the samples (nanoparticles and aerogels) were hot pressed at 450 °C. TEM/EDS data were

Figure 4. EDS of (a) Bi2Te3 and (b) BixSb2−xTe3 nanoparticles and (c) Bi2Te3 and (d) BixSb2−xTe3 aerogels. The Cu signal arises from the copper grid used to support the sample in the TEM.

In the case of BixSb2−xTe3 nanoparticles and aerogels, the Bi:Sb:Te:S ratio was found to be 0.6:1.6:3:0.1 for nanoparticles and 0.5:1.5:3:0.1 for aerogels, respectively. The values obtained are close to the desired composition of Bi0.5Sb1.5Te3. The data also show that there was less residual sulfur from the dodecanethiol for the nanoparticles as well as aerogels, when compared to the Bi2Te3 nanoparticles and aerogels (Figure 4). The presence of a lower amount of organic groups is a good sign for thermoelectric measurements as the organic groups are expected to act as insulators that reduce the electrical conductivity of the system.43 The surface area analysis of Bi2Te3 materials, modeled by the Brunauer, Emmet, and Teller (BET) method, yielded a value of ca. 45 m2/g for aerogels and 16 m2/g for nanoparticles (Table 1). The surface area of the nanoparticles was lower than that of the aerogels suggesting that supercritical drying, and the consequent pore structure, enables better access to the particle surface. The nitrogen adsorption/desorption isotherms illustrated that the aerogels and xerogels display type IV curves (Figure 5) that are characteristic of a mesoporous material (2−50 nm pore diameter), and the pore size distribution (Figure 5, inset), 17434

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S value is comparable to that of the bulk material of similar composition (Table 2).9 The electrical resistivity for both Bi2Te3 and BixSb2−xTe3 nanoparticles exhibits a similar trend; the value increases with the increase in temperature as shown in Figure 9. Although the electrical resistivity increases with the increase in temperature for both the nanoparticles, due to the strong influence of S in the power factor term, S2σ (S2/ρ) increases with the increase in temperature (Figure 9). While the room temperature value of S (Table 2) is higher for the BixSb2−xTe3 nanoparticles relative to Bi2Te3 nanoparticles, the greater electrical resistivity values in the former result in a lower power factor for BixSb2−xTe3 nanoparticles. The power factor value obtained at 300 K for Bi2Te3 nanoparticles (5.0 × 10−4 W/ (m·K2)) is either higher or comparable to other chemically synthesized Bi2Te3 nanoparticles (for example, 2.4 × 10−4 W/ (m·K2)38 and 5.0 × 10−4 W/(m·K2)39 at 300 K). However, for BixSb2−xTe3 nanoparticles, the power factor value obtained at 300 K (3.3 × 10−4 W/(m·K2)) is lower than those reported in the literature (for example, 7.8 × 10−4 W/(m·K2)38 and 7.0 × 10−4 W/(m·K2)45 at 300 K). Power factors for bulk materials are a factor of 5−10 times greater than those obtained for nanoparticles (Table 2). One of the main benefits of using nanoparticles for thermoelectric applications is to achieve reduction in thermal conductivity.38 The total thermal conductivities of the Bi2Te3 and BixSb2−xTe3 nanoparticles are shown in Figure 9. The lattice thermal conductivity is estimated by subtracting the electronic thermal conductivity from the total thermal conductivity. The electronic thermal conductivity is calculated by the Wiedemann−Franz law, κl = LσT, where σ is the electrical conductivity, T is the absolute temperature, and L is the Lorenz number, which assumes a value of L = 2.44 × 10−8 V2/K2. The total, as well as the lattice thermal conductivity, decreases with the increase in temperature for Bi2Te3 nanoparticles, whereas for BixSb2−xTe3 nanoparticles, both remain constant over the entire temperature range measured. The lattice thermal conductivity value obtained for the Bi2Te3 nanoparticle sample at 300 K is lower than the reported value for bulk material (Table 2).46 This result justifies the use of the reduced dimension (less than 100 nm) of Bi2Te3 nanoparticles to effectively cap the phonon mean free path. For the case of BixSb2−xTe3, addition of nanostructuring has little effect on the lattice thermal conductivity, which is already low due to point scattering effects from the disordered cation lattice.6 Overall, the lattice thermal conductivity value obtained for Bi2Te3 and BixSb2−xTe3 nanoparticles is similar to that found in prior reports for nanoparticles of these materials (1.0 and 1.1 W/ (m·K) at 300 K for Bi2Te3 and BixSb2−xTe3 nanoparticles, respectively).38 Finally, the overall ZT obtained after combining all the parameters increases with temperature and at 300 K the ZT obtained was 0.18 for Bi2Te3 and 0.1 for BixSb2−xTe3 nanoparticles. The value of ZT obtained for Bi2Te3 nanoparticles was either higher (previously reported as 0.03)32 or comparable (previously reported as 0.2)39 to other chemically synthesized nanoparticles, whereas for BixSb2−xTe3 nanoparticles, the ZT is lower than the other chemically synthesized BixSb2−xTe3 nanoparticles (previously reported as 0.5).45 For Bi2Te3 and BixSb2−xTe3 aerogels, surprising changes in electronic properties are observed. In Bi2Te3 aerogels, the value of S changed from negative at low temperature to positive at high temperatures during the measurement, suggesting the presence of two types of carriers in the system. This result is distinct from that observed for the nanoparticles of Bi2Te3, where the type of conduction was p-type over the whole temperature range (Figure

Figure 6. IR of Bi2Te3 aerogel (a) before and (b) after heating under Ar in a flow furnace at 200 °C.

acquired in order to probe the effect of hot pressing on crystallite size and composition, TEM images of the nanoparticles show nanoscopic features which are