Improving Thermoelectric Properties of Chemically Synthesized

Jun 16, 2010 - The power factors of chemically synthesized Bi2Te3 and Bi0.5Sb1.5Te3 nanocrystals (NCs) were .... Case Western Reserve University...
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J. Phys. Chem. C 2010, 114, 11607–11613

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Improving Thermoelectric Properties of Chemically Synthesized Bi2Te3-Based Nanocrystals by Annealing Yixin Zhao,† Jeffrey S. Dyck,*,‡ Brett M. Hernandez,‡ and Clemens Burda*,† Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western ReserVe UniVersity, 10900 Euclid AVenue, CleVeland, Ohio 44106, and Department of Physics, John Carroll UniVersity, 20700 North Park BlVd., UniVersity Heights, Ohio 44118 ReceiVed: March 29, 2010; ReVised Manuscript ReceiVed: May 18, 2010

The power factors of chemically synthesized Bi2Te3 and Bi0.5Sb1.5Te3 nanocrystals (NCs) were improved up to 2.4 and 7.8 µW cm-1 K-2, respectively, which are significantly higher than previously reported values for chemically synthesized Bi2Te3 NCs and even comparable to the recently reported highest power factor of 5 µW cm-1 K-2 for Bi2Te3 NCs consolidated by spark plasma sintering. This improvement was achieved by annealing the NCs under argon protection, and the crystal structures and morphologies of these annealed NCs were characterized via XRD, SEM, and TEM measurements. The temperature-dependent thermoelectric properties of these modified NCs were explored on cold-pressed pellets of NCs. Improvement of the thermoelectric performances of the pellets resulted primarily from an increase in electrical conductivity (σ), while only weakly increasing the lattice thermal conductivity (κL), which was still kept lower than bulk values. Hall carrier concentration studies suggest that the improvement of the electrical conductivity is caused primarily by modification of the charge carrier mobility rather than the carrier concentration. A mechanism is proposed to explain a large increase of electrical conductivity by annealing related to a decrease of activation energy for the mobility after the removal of organic capping ligands through annealing. Introduction The thermoelectric (TE) energy conversion efficiency is directly related to the dimensionless figure of merit expressed by ZT ) S2Tσ/κ ) S2T/κF (S, thermopower or Seebeck coefficient; T, temperature; σ, electrical conductivity; F, electrical resistivity; and κ, thermal conductivity).1–5 The higher ZT values achieved by nanostructured thermoelectric materials compared with the corresponding bulk materials6–10 have inspired enthusiasm to develop new techniques to prepare nanostructured thermoelectric materials.10–26 In the case of solid-state synthesis approaches, the improvement on ZT for optimized bulk material has been achieved by lower thermal conductivities induced by nanoscale grain boundaries, which is realized by breaking up the optimized bulk materials into nanograins by “top-down” methods, such as ball-milling and mechanical alloying.6,9,21,27–29 On the other hand, “bottom-up” methods, such as chemical synthesis, have generally not provided nanomaterials with TE properties as good as those prepared by “top-down” approaches. However, chemical synthesis methods have the potential of producing nanoparticles with a smaller diameter, narrower size distribution, and with control over the shape (spheres, rods, wires, etc.).30 This experimental control over the nanostructure can result in increased grain boundary scattering to reduce the thermal conductivity as well as in quantum confinement effects for charge carriers, resulting in larger Seebeck coefficients.31–33 Bismuth telluride based nanostructured materials have been prepared by numerous chemical approaches, including surfactant-directed synthesis,14 hydro/solvo-thermal,18,20 thermolytic reduction methods,17 and sonochemistry.19,23 Nanostructured * To whom correspondence should be addressed. E-mail: [email protected] (C.B.), [email protected] (J.S.D.). † Case Western Reserve University. ‡ John Carroll University.

Bi2Te3 thin films have been prepared by electrochemical deposition.11,12,34,35 Recent work has shown that size-tuned Bi2Te3 nanocrystals could be prepared by ligand-based chemical synthesis and assembled into bulk samples by pressing, enabling measurement of TE transport properties.32 They have shown that size tuning can significantly lower the thermal conductivity. However, the electrical conductivities are too low to result in a high ZT. Unfortunately, significantly lower electrical conductivities were commonly observed in many previous reported chemically synthesized nanocomposite materials11,32,34–37 compared with the state-of-the-art electrical conductivity of optimized bulk ones. To ultimately utilize the advantages of thermoelectric nanomaterials by reducing lattice thermal conductivity,8,10,37 the electrical conductivity of these chemically synthesized nanomaterials has to be improved. The observed lower electrical conductivities are mainly due to the surface oxidation and capping surfactants.32 To increase the electrical conductivities, these chemically synthesized thermoelectric nanomaterials were treated by hydrazine or annealing.11,18,24,38–41 Although annealing is important for the thermoelectric performance of chemically synthesized thermoelectric nanomaterials, little attention has been given to the careful study of annealing effects on thermoelectric properties. In particular, the effect of annealing on the thermal conductivity is much less studied compared with effects on electrical conductivity and the Seebeck coefficient. In our recent work, we have prepared sub-80 nm Bi2Te3based Bi0.5Sb1.5Te3 NCs through ligand-based chemical synthesis under mild reaction conditions.22 Here, we present a study of temperature-dependent thermoelectric transport properties, including thermal conductivities, on condensed NC pellets. These NC pellets were prepared by cold-pressing vacuum-dried or annealed Bi0.5Sb1.5Te3 NCs under high pressure at room temperature. Chemically synthesized Bi2Te3 NCs were also inves-

10.1021/jp102816x  2010 American Chemical Society Published on Web 06/16/2010

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Figure 1. (A) XRD pattern evolution of Bi2Te3 NCs, annealed at different temperatures for 5 h under an argon atmosphere. The bars are JCPDS standards for rhombohedral Bi2Te3 (72-2036). (B) XRD pattern evolution of Bi0.5Sb1.5Te3 NCs, annealed at different temperatures for 5 h under an argon atmosphere. The bars are JCPDS standards for rhombohedral Bi0.5Sb1.5Te3 (49-1713).

tigated for comparison.22,42,43 Room-temperature carrier concentration and Hall mobility measurements were also examined. This study reveals that annealing significantly increases electrical conductivities of Bi2Te3-based NCs, while maintaining relative lower lattice thermal conductivities compared with bulk, and the increase of electrical conductivity was achieved by the modification of charge mobility rather than the carrier concentration. Experimental Section Synthesis of Bi2Te3 and Bi0.5Sb1.5Te3 Nanocrystals.22,32 Bi2Te3:2 mmol of bismuth acetate (>99%) and 8 mL of dodecanethiol were dissolved in 50 mL of phenyl ether (Bi0.5Sb1.5Te3:0.5 mmol of bismuth acetate (>99%), 1.5 mmol of antimony chloride (>99%), and 3 mL of dodecanethiol were dissolved in 50 mL of phenyl ether). The solution was heated to 150 °C for 60 min and flushed with argon gas. A 2 mL portion of 1.5 M trioctylphosphine Te (TOPTe) solution, which was freshly prepared by dissolving 3 mmol of tellurium powder (>99% pure) in 2 mL of trioctylphosphine (TOP) with moderate stirring for 12 h under the protection of Ar, was injected into the reaction solution and maintained for 30 min. The resulting black precipitate was washed with toluene, chloroform, and ethanol, then vacuum-dried and stored in vacuum. Annealing and Pressing. The Bi2Te3 and Bi0.5Sb1.5Te3 NC vacuum-dried powders were annealed for 5 h in an Ar atmosphere at a steady flow rate of 200 mL/min in a quartz tube oven at 300, 350, 380, and 400 °C. The annealed powder was pressed with a 10 000 lb load on a quarter of an inch diameter steel die, equal to around 1.0 GPa of pressure, for 30 min. The cold-pressing could make pellets with a density of 88%, similar to densities previously reported by others using this method.32 Characterization. The crystal structure of these nanoparticles was examined with a Scintag X-1 Advanced X-ray powder diffractometer (XRD, 2.4°/min, Cu KR radiation), and their morphology was characterized by using a JEOL 1200CX transmission electron microscope (TEM), with an accelerating voltage of 80 kV, and a Hitachi S4500 scanning electron microscope (SEM), with an accelerating voltage of 5 kV. Samples for transport measurements were cut from the ∼1 mm thick pressed disks with a diamond wheel saw to approximate dimensions of 6 mm ×2 mm ×1 mm. The electrical resistivity (F) [or conductivity (σ)], thermal conductivity (κ), and Seebeck coefficient (S) of the cut pellets were measured as a function of temperature. The sample temperature was controlled with a closed-cycle helium refrigerator equipped with a radiation shield, and the sample space was kept at pressures below 10-4 Torr.

Electrical resistivity was measured in a four-probe configuration using a Lakeshore 370 AC Resistance Bridge operating at 13.7 Hz. Hall measurements were performed in magnetic fields of -1.8 to +1.8 T. For S and κ measurements, one end of the sample was thermally contacted to a copper cold sink with woods metal, and a miniature strain gauge mounted at the other end of the sample served as a heater. Thermal gradients with a magnitude of less than 5% of the absolute temperature were measured with the aid of fine copper-constantan thermocouples, and the copper legs of the thermocouples were used to measure the sample voltage to determine S. Keithley 2182 nanovoltmeters (Keithley Instruments Inc., USA) were used to measure the sample voltage and thermocouple voltages. Seebeck coefficients were determined from the slope of the sample voltage versus T while holding the cold end of the sample at a constant temperature. Electrical contacts to the sample were soldered using indium. The thermal conductivity data were not corrected for radiation losses, which were measured44 to be substantial (∼30%) near room temperature due to the relatively small thickness of the samples. The uncertainty in dimension measurements results in about 5% uncertainty for the σ and κ data. Results and Discussion We selected 300-400 °C as a temperature range for annealing, which could remove organic residues but did not significantly change the morphologies and structure of these NCs based on previous reports.32,45 Because previous work has shown that the melting point of Bi2Te3 NCs could be lowered to around 400 °C,32 the annealing temperatures for Bi2Te3 NCs were selected as 300, 350, and 380 °C. To explore the possible eliminating of nanoscaled grain boundaries by annealing, the Bi0.5Sb1.5Te3 NCs were further annealed under a higher temperature of 400 °C. X-ray Diffraction. The XRD studies of the annealing effect on Bi2Te3 and Bi0.5Sb1.5Te3 NCs in Figure 1 indicated that the samples possess the rhombohedral structure up to the highest annealing temperatures. Stoichiometric ratio studies on previous surfactant-covered Bi2Te3 and Bi0.5Sb1.5Te3 NCs are consistent with the obtained XRD pattern.22 The XRD assignment of these annealed Bi2Te3 and Bi0.5Sb1.5Te3 NCs is also consistent with the stoichiometric ratio of the precursor in the synthesis. The XRD pattern evolution of the Bi2Te3 (Figure 1A) and Bi0.5Sb1.5Te3 (Figure 1B) NCs shows that the peak intensity increases and the peak width narrows with increasing annealing temperature. We note that an intensity increase of the peak at 44.5° was observed for Bi0.5Sb1.5Te3 NCs annealed at 400 °C. This peak could be assigned to either the (0015) peak of

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Figure 2. FESEM images of the cross section of pressed Bi2Te3 NC and Bi0.5Sb1.5Te3 NC pellets: (A) vacuum-dried Bi2Te3 NCs, (B) 300 °C annealed Bi2Te3 NCs, (C) 350 °C annealed Bi2Te3 NCs, (D) 380 °C annealed Bi2Te3 NCs, (E) vacuum-dried Bi0.5Sb1.5Te3 NCs, (F) 300 °C annealed Bi0.5Sb1.5Te3 NCs, (G) 350 °C annealed Bi0.5Sb1.5Te3 NCs, (H) 380 °C annealed Bi0.5Sb1.5Te3 NCs, and (I) 400 °C annealed Bi0.5Sb1.5Te3 NCs.

Bi0.5Sb1.5Te3 or the possible (0015) peak of BiSbTe3, but there are no other intense peaks related to BiSbTe3 found in the XRD pattern, which suggests that the peak at 44.5° should be the (0015) peak of Bi0.5Sb1.5Te3 and no phase change takes place when annealed at 400 °C. Furthermore, there were no significant X-ray diffraction peaks related to oxidation or impurity found in these XRD patterns. In separate annealing experiments in N2 rather than Ar atmospheres, oxidation peaks were clearly evident. This observation is consistent with a previous report32 and suggests that Ar offers better protection than N2. Electron Microscopy. The sizes of the chemically synthesized Bi0.5Sb1.5Te3 and Bi2Te3 NCs vacuum-dried are ∼50 nm, as indicated in Figure S1 in the Supporting Information. The effect of annealing of these NC powders is to increase the size of the particles as the temperature increases. The cross-sectional SEM images of the high-pressure pressed vacuum-dried NC pellets are shown in Figure 2A,E and reveal the high density of grain boundaries consistent with TEM images of individual NCs before pressing in Figure S1 in the Supporting Information. The average grain size of vacuum-dried Bi2Te3 NC pellets, as seen in Figure 2, is 53 nm; the average grain sizes of 300, 350, and 380 °C annealed Bi2Te3 NC pellets increased to 62, 71, and 100 nm, respectively, although a wider size distribution of smaller and somewhat larger grains is observed at higher annealing temperatures. These observations are consistent with the narrowing of the XRD in Figure 1. The trend of average grain size increasing with annealing temperature is also observed

in annealed Bi0.5Sb1.5Te3 NCs. The vacuum-dried Bi0.5Sb1.5Te3 NCs have an average grain size of ∼61 nm; once annealed at 300 and 350 °C, the average sizes increase to 73 and 84 nm, respectively. The average size of 380 °C annealed samples increased to 150 nm or even more. In particular, the 400 °C annealed Bi0.5Sb1.5Te3 grain size is found to change from nanocrystalline to microgranular, as shown in Figure 2I. This observable morphology change suggests that annealing will increase the particle size of NCs and an appropriate annealing temperature is very important for maintaining a nanoscale grain structure in the case of these chemically synthesized Bi0.5Sb1.5Te3 and Bi2Te3 NCs. Transport Properties of Bi2Te3 and Bi0.5Sb1.5Te3 NC Pellets. Bi2Te3 NCs. Electrical conductivity versus (σ) temperature data are presented in Figure 3. The vacuum-dried Bi2Te3 NC pellet (no annealing) has a room-temperature electrical conductivity value of 30 S/cm. Although lower than the reported value for optimized bulk Bi2Te3 (around 1000 S/cm),37 this value is comparable to previously reported annealed chemically synthesized Bi2Te3 NCs.32 The electrical conductivity increases with temperature, indicating semiconducting or activated behavior. The room-temperature Seebeck coefficient (S) value is -69.2 µV/K; this negative value indicates that these Bi2Te3 NCs are n-type semiconductors. After 300 °C annealing, the roomtemperature values of σ and S both increase to 59 S/cm and -120 µV/K, respectively. As a result, the room-temperature power factor (σ*S2) increases about 6 times compared with the

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Figure 3. Electrical conductivity (σ), Seebeck coefficient (S), and power factor (σ*S2) dependence on temperature of the Bi2Te3 NCs and Bi0.5Sb1.5Te3 NCs annealed at different temperatures for 5 h under the protection of an Ar atmosphere.

vacuum-dried sample. We note that the nominal expectation is that S should decrease if σ increases, assuming the cause is due to an increase in carrier concentration. A simultaneous increase in σ and S was also seen in the other chemically synthesized Bi2Te3 NCs.32 We propose an alternative mechanism, as will be discussed further in the next section: as the thiol capping layers are removed from the individual NPs with annealing, the carrier mobility in the pressed pellet is drastically increased, leading to much improved properties. After 350 °C annealing, the room-temperature σ increases to 112 S/cm but the roomtemperature S value decreases. The power factor of 350 °C annealed Bi2Te3 NCs is almost the same as that of 300 °C annealed Bi2Te3 NCs. The room-temperature electrical conductivity of 380 °C annealed Bi2Te3 NCs jumps to 570 S/cm, only a factor of 2 lower than that of state-of-the-art TE material. Note that this is an unusually high value for chemical synthesized Bi2Te3 NCs to date. In addition, the electrical conductivity for this annealing temperature showed a decrease with increasing temperature, different from the Bi2Te3 NCs annealed at lower temperatures. While S decreased further, the 380 °C annealed sample still has the highest power factor, 16 times larger than that of vacuum-dried Bi2Te3 NCs, due to the large σ. The power factor value of around 2.4 µW cm-1 K-2, which we have achieved, is much higher than previously reported values for

chemically synthesized Bi2Te3 NCs18,32 and even comparable to the recently reported highest power factor value of 5 µW cm-1 K-2 for other chemically synthesized Bi2Te3 NCs prepared by both hydrazine treatment and plasma spark sintering pressing.46 Bi0.5Sb1.5Te3 NCs. The vacuum-dried Bi0.5Sb1.5Te3 NCs also have a relatively lower room-temperature electrical conductivity value, 26 S/cm, as shown in Figure 3, compared with the stateof-the-art reported value of optimized bulk Bi0.5Sb1.5Te3, which is around 1000 S/cm.6 The room-temperature Seebeck coefficient value of the vacuum-dried Bi0.5Sb1.5Te3 NCs is 127.2 µV/K. This positive value indicates that these Bi0.5Sb1.5Te3 NCs are p-type semiconductors. Both σ and S increase in annealed samples at each temperature step up to 380 °C. The Seebeck coefficient reaches its maximum value of 251.2 µV/K for this annealing temperature. The power factor of 380 °C annealed Bi0.5Sb1.5Te3 NCs is around 7.8 µW cm-1 K-2, which is over 15 times greater than that of the vacuum-dried Bi0.5Sb1.5Te3 NCs, and this value is even higher than the recently reported highest power factor value of 5 µW cm-1 K-2 for chemically synthesized Bi2Te3 NCs.46 The 400 °C annealed Bi0.5Sb1.5Te3 NC sample has a much higher room-temperature σ of 252 S/cm, while the room temperature S value decreases slightly. Like the Bi2Te3 NC samples, the electrical conductivity has an activated

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Figure 4. Thermal conductivity (κ) dependence on temperature at different annealing temperatures of (A) Bi2Te3 NC pellets and (B) Bi0.5Sb1.5Te3 NC pellets and the lattice thermal conductivity (κ) dependence on temperature of (C) the annealed Bi2Te3 NC pellets and (D) the Bi0.5Sb1.5Te3 NC pellets.

character over the entire temperature range except at the highest annealing temperature, indicating a possible change in conductivity mechanism. The power factor increased monotonically with the annealing temperature, reaching a maximum value of 13.8 µW cm-1 K-2, which is over 30 times greater than that of the unannealed Bi0.5Sb1.5Te3 NCs. Another finding is that all power factors of the Bi0.5Sb1.5Te3 NCs annealed at different temperatures are much higher than those of their corresponding annealed Bi2Te3 NCs. Thermal Conductivities of Bi2Te3 and Bi0.5Sb1.5Te3 NC Pellets. One of the main advantages of utilizing nanomaterials for thermoelectric applications is to reduce the thermal conductivity. Therefore, to study the annealing effect on the thermal conductivities of these Bi0.5Sb1.5Te3 and Bi2Te3 NCs is important and useful for further thermoelectric research. The temperaturedependent thermal conductivities and lattice thermal conductivities of these NC pellets are plotted in Figure 4. As mentioned above, the data are uncorrected for radiation loss. Our radiation loss experiments44 reveal that corrected data would have values of roughly 0.3-0.4 W m-1 K-1 lower at room temperature than the values reported, whereas the data below 150 K would need only a negligible correction. The total thermal conductivity, shown in Figure 4A,B, of the Bi2Te3 and Bi0.5Sb1.5Te3 NCs pellets was found to increase with temperature. This increase would not be as high if the data were corrected for radiation loss. The total thermal conductivity increases very weakly over the entire temperature range with annealing temperature until the highest annealing temperatures of 380 °C for Bi2Te3 and 400 °C for Bi0.5Sb1.5Te3, where a large increase is seen. To clarify whether the increase of thermal conductivity comes mainly from the increase of carrier thermal conductivity κE or the lattice thermal conductivity κL, we employ the relationship κ ) κE + κL to separate κL from the total measured thermal conductivity κ. The electronic thermal

conductivity was calculated using the Wiedemann-Franz law, κE ) L0σT, where L0 is the Lorentz number, L0 ) 2.45 × 10-8 W Ω K-2.37 The lattice thermal conductivity of all the NC pellets is shown in Figure 4C,D. For crystalline solids, κL typically decreases with temperature, whereas the data in Figure 4 show a weakly increasing dependence. This suggests significant phonon scattering due to the nanostructuring. All annealed Bi2Te3 NC pellets show about the same affect on the lattice thermal conductivity, increasing the value at 200 K (where radiation losses are small) from roughly 0.5 W m-1 K-1 to around 0.8 W m-1 K-1. These annealed Bi2Te3 NC pellets have significantly lower κL values than a direction averaged value for bulk Bi2Te3 and as well as for other nanostructured Bi2Te3.32 For the Bi0.5Sb1.5Te3 NCs pellets, 300-380 °C annealing also leads to an increase of lattice thermal conductivity similar to the annealed Bi2Te3 NCs. On the other hand, 400 °C annealing not only increases the value of the lattice thermal conductivity significantly but also changes the lattice thermal conductivity temperature dependence behavior, which suggests that 400 °C annealing may have introduced significant change in structure or size. The above-mentioned SEM indicated that the Bi0.5Sb1.5Te3 NCs have grown into microgranules during 400 °C annealing, which results in a lattice thermal conductivity similar to bulk values. Finally, we point out that, for lowertemperature annealing, the Bi0.5Sb1.5Te3 NC pellets have roomtemperature thermal conductivity values comparable to that of the best ball-milled nanocomposite BiSbTe bulk alloy material7 and values that are lower when considering the overestimation due to experimental radiation losses. Discussion The thermopower S and, especially, the electrical conductivity σ were significantly modified by annealing. A possible reason

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TABLE 1: Room-Temperature Carrier Concentration (N; here, Bi2Te3 NCs are n-Type and Bi0.5Sb1.5Te3 NCs are p-Type), Electrical Conductivity (σ), Hall Mobility (µ), and Seebeck Coefficient (S) for Bi2Te3 NCs Annealed at 350 and 380 °C and Bi0.5Sb1.5Te3 NCs Annealed at 380 and 400 °C Bi2Te3 NCs 350 °C -3

N (cm ) σ (S cm-1) µ (cm2 V-1 S-1) S (µV/K)

1.2 × 10 112 6.0 -80.2

20

Bi2Te3 NCs 380 °C 1.9 × 10 570 18.4 -65.5

can be the significant change in either the carrier concentration or the charge carrier mobility, or both. The measured Hall carrier concentrations of Bi2Te3 NCs annealed at 350 and 380 °C and Bi0.5Sb1.5Te3 NCs annealed at 380 and 400 °C are listed in Table 1. The carrier concentration values of the annealed Bi2Te3 NCs, around 1.2 × 1020 and 1.9 × 1020 cm-3, are consistent with the previously reported value for chemically synthesized Bi2Te3 NCs.32 The increase in carrier concentration after annealing is also consistent with the observed decrease in the Seebeck coefficient of the Bi2Te3 and Bi0.5Sb1.5Te3 NCs. The mobility value of the charge carriers of these annealed Bi2Te3 and Bi0.5Sb1.5Te3 NCs has been calculated from the Hall carrier concentration and electrical conductivity, σ ) N × µ × e, which is also listed in Table 1. It can be clearly seen that the improvement of electrical conductivity is more affected by modification of the mobility rather than the increase of the carrier concentration. We now provide a plausible mechanism for the modification of the charge transport character from semiconducting to metallic and for the large increase of the mobility by annealing. We argue that all of the samples measured have carrier concentrations at least as high as those listed in Table 1. At the lower annealing temperatures, our difficulty in obtaining Hall data (weak signal due to larger carrier concentration) is consistent with the lower S values that we measure for the higher annealing temperatures. We, therefore, assume that all samples are degenerately doped semiconductors, which, in principle, should lead to metallic electrical conductivity character and a carrier concentration that is roughly constant with temperature. In this case, the activated character of the conductivity is likely due to grain boundary scattering. When grain boundaries are the main carrier scattering mechanism, the expression for mobility is followed as eq 147

µ ) (el/ √8kBTπm*)exp(-EB /kBT)

(1)

where e is the charge of an electron, l the average grain size, kB the Boltzmann constant, T the temperature, m* the effective mass, and EB the energy needed to overcome the boundary. Annealing removes the capping layers and it promotes grain growth, which was confirmed by the XRD, TEM, and SEM results. As the capping layers are removed, the grain boundary potential may drop, yielding an EB that decreases with annealing. At the same time, annealing promotes grain growth (as seen in XRD data), which increases the l, which, in turn, increases mobility at all temperatures. Our chemically synthesized Bi2Te3 NCs show a higher electrical conductivity compared with recently reported chemically synthesized Bi2Te3 NCs, which were synthesized by following a similar synthesis procedure and also cold-pressed.32 The difference may come from two factors: First, annealing under Ar protection introduces less possible surface oxidation than N2 protection, which was confirmed by the fact that there are much fewer to no observable oxidation peaks found in our

Bi0.5Sb1.5Te3 NCs 380 °C 1.4 × 10 105 49.1 251.2

20

19

Bi0.5Sb1.5Te3 NCs 400 °C 1.6 × 1019 252 95.9 234.6

XRD results. The second factor is the purity of Bi precursors used; we used >99% pure Bi precursor, whereas previous work used technical grade pure Bi precursor in their synthesis.32 The lesser oxidation and higher purity can both reduce possible crystal defects and surface trapping in these Bi2Te3 NCs, which may reduce the grain boundary potential, yielding the decrease of the EB and leading to the increased mobility. Because of the significant increase in the electrical conductivity after annealing (Figure 3), Bi2Te3 NCs improved the power factor dramatically after annealing compared with the Bi2Te3 NCs that were only vacuum-dried. Furthermore, the Bi2Te3based NCs, Bi0.5Sb1.5Te3, showed even higher power factors compared with Bi2Te3 NCs, and this value is even higher than the best reported power factor value for chemically synthesized Bi2Te3 NCs. Conclusion In summary, temperature-dependent thermoelectric properties of chemically synthesized Bi0.5Sb1.5Te3 and Bi2Te3 NCs have been studied, and the crystal structure and morphologies of these annealed Bi0.5Sb1.5Te3 and Bi2Te3 NCs were characterized by XRD, TEM, and SEM. Thermoelectric performances of these chemically synthesized Bi0.5Sb1.5Te3 and Bi2Te3 NCs are improved by appropriate annealing under the protection of Ar gas. The improvement is realized by a significant increase in electrical conductivity, modification of Seebeck, and maintaining a lower lattice thermal conductivity compared with bulk ones. Furthermore, we propose a plausible mechanism for the modification of the charge transport character from semiconducting to metallic and for a large increase of the mobility caused by a decreased activation EB, which is due to the grain boundary potential drop as the capping layers are removed during the annealing process. Our study exemplifies the large potential of drastically improving the thermoelectric performance of chemically synthesized NCs by annealing and suggests that an appropriate post-treatment on chemically synthesized thermoelectric nanomaterials is critical to realizing a higher thermoelectric performance. Acknowledgment. C.B. acknowledges support from the NSF (No. CHE-0239688), ACS-PRF (No. 45359-AC10), and the Ohio Board of Regents. Supporting Information Available: TEM images of these Bi2Te3 and Bi0.5Sb1.5Te3 NCs are shown in Figure S1, and the XPS of 350 °C annealed Bi0.5Sb1.5Te3 NCs is shown in Figure S2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) DiSalvo, F. J. Science 1999, 285, 703. (2) Nolas, G. S.; Sharp, J.; Goldsmid, H. J. Thermoelectrics: Basic Principles and New Materials DeVelopments; Springer: Berlin, 2001. (3) Service, R. F. Science 2004, 306, 806. (4) Tritt, T. M.; Subramanian, M. A. MRS Bull. 2006, 31, 188. (5) Vining, C. B. Nat. Mater. 2009, 8, 83.

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