Shape-Controlled Synthesis and Electrical Conductivities of

Mar 16, 2010 - Department of Chemistry, Harbin Institute of Technology, Harbin 150001, ... The sizes and morphologies of AgPb10SbTe12 were examined in...
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J. Phys. Chem. C 2010, 114, 5827–5834

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Shape-Controlled Synthesis and Electrical Conductivities of AgPb10SbTe12 Materials Lin Wang, Gang Chen,* Qun Wang, Hongjie Zhang, Rencheng Jin, Dahong Chen, and Xiangbin Meng Department of Chemistry, Harbin Institute of Technology, Harbin 150001, P. R. China ReceiVed: NoVember 18, 2009; ReVised Manuscript ReceiVed: February 26, 2010

AgPb10SbTe12 crystals with nanocubic and flower-like morphologies have been successfully fabricated by a facile solution route. The sizes and morphologies of AgPb10SbTe12 were examined in relation to reaction temperature, the molar ratio of KOH/Pb(Ac)2, polyvinyl pyrrolidone (PVP), and solvents. A surface-protected etching growth mechanism has been proposed to elucidate the formation of nanocubes and flower-like crystals. Furthermore, the electrical conductivities of the samples with nanocubic and flower-like shapes were measured to investigate the possible impact of size and morphology on the electrical conductivity. 1. Introduction The architectural control of nanocrystals possesssing welldefined shapes remains a promising topic owing to the distinctive shape-dependent properties that inorganic nanocrystals exhibit.1-3 Much effort has been devoted to the synthesis of crystals with a variety of novel morphologies, such as flowerlike crystals,4 rods,5 belts,6,7 cubes,8 hollow spheres,9 nanoneedles,10 and crystals with dendrite morphologies.11 However, to the best of our knowledge, most of these studies concentrate on the synthesis of binary systems possesssing diverse morphologies and sizes. Literature on the synthesis of quaternary alloy semiconductors with controlled shapes has been rarely reported. Therefore, it is still a challenging and urgent task to manipulate and control the morphologies and sizes of quaternary alloy semiconductors. Among the PbTe-based alloys, AgPbmSbTe2+m is an ideal thermoelectric (TE) material system at elevated temperature that has attracted intense attention because of enhanced TE performance, with a higher dimensionless figure of merit value (ZT) of 2.2 at 800 K.12 Such a good TE property was due to the phase segregation of AgSb-rich nanodots embedded in a PbTerich matrix, which may increase phonon scattering and reduce thermal conductivity, further improving TE performance.13,14 Recently, considerable investigations related to AgPbmSbTe2+m material have been carried out; for example, a series of AgPbmSbTe2+m (m ) 1, 2, 4, 6, 8, 18) with nearly spherical shapes have been fabricated by a solvothermal method.15 Cai’s research group achieved AgPbmSbTe2+m nanocubes via a hydrothermal method.16 Ag0.8Pb18+xSbTe20 bulk materials with micrometer-sized grains were prepared by mechanical alloying and spark plasma sintering,17 and Karkamkar et al. adopted an inverse micellar synthetic route to synthesize approximately spherical AgPbmSbTem+2 powders.18 However, it is noteworthy that the research on the synthesis of AgPbmSbTe2+m semiconductors with controlled shapes have been seldom reported. In this work, we develop a facile and environmentally benign solution route for the preparation of AgPb10SbTe12 with controllable morphologies (nanocubes and flower-like crystals) and tunable sizes. Some experimental parameters affecting the morphologies and sizes of the synthesized specimen, including * To whom correspondence should be addressed. Fax: (+86)-45186413753. E-mail: [email protected].

the reaction temperature, the molar ratio of KOH/Pb(Ac)2, PVP, and the solvents, were investigated. Moreover, a surfaceprotected etching growth mechanism was proposed to clarify the formation of nanocubes and flower-like crystals. In addition, the electrical conductivities of the as-prepared samples with cubic and flower-like morphologies were measured in order to know how morphologies contribute to their electrical conductivities. 2. Experimental Section 2.1. Materials and Preparation Procedures. All reagents were analytically pure and used as received. In a typical procedure, definite amounts of PVP and KOH were added to a solvent mixture of water (2 mL) and ethanol (8 mL) with a magnetic stirrer at room temperature. Afterward, 0.12 mmol of AgNO3, 1.20 mmol of Pb(CH3COO)2 · 3H2O, 0.12 mmol of K(SbO)C4H4O6 · 0.5H2O, 1.44 mmol of Na2TeO3, and 0.3 g of N2H4 · H2O were added, respectively, under constant stirring. After being stirred for 15 min, the mixture was transferred into a 15 mL stainless Teflon-lined autoclave. The autoclave was placed into the oven with the desired reaction temperature (180 °C), held for 20 h, and then cooled to room temperature naturally. The specimen was poured into a clean beaker, and the products were washed six times with ethanol and water. The products synthesized using ethylene glycol (EG) as solvent took about six days to precipitate in water. The precipitate was washed in ethanol and water eight times, and the final product was dried in a vacuum oven at 60 °C for 3 h. The whole process can be readily adjusted to prepare AgPb10SbTe12 with different morphologies by changing the temperature, the amount of PVP, the molar ratio of KOH/Pb(Ac)2, and the solvents while keeping other conditions unchanged. The detailed experimental parameters and corresponding brief results are listed in Figure 6. 2.2. Characterization. The X-ray diffraction (XRD) patterns were collected at room temperature from 20° to 80° with a step of 0.02° and scanning rate of 5°/min on a Rigaku D/max-2000 diffractometer equipped with Cu KR radiation. The morphologies of the samples were characterized by field-emission scanning electron microscopy (FESEM, MX2600FESEM), transmission electron microscopy, and high-resolution transmission electron microscopy (HRTEM, FEI, Tecnai G2 S-Twin); otherwise, energy-dispersive X-ray spectroscopy (EDX) implemented by FESEM was used to analyze the chemical composition of the prepared products. Additionally, the electrical

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Figure 1. (a) Powder X-ray diffraction pattern of the as-prepared AgPb10SbTe12 nanocubes, and the standard pattern of PbTe (JCPDS: 38-1435) is also presented at the bottom for comparison. (b) FESEM image of the as-obtained AgPb10SbTe12 nanocubes. (c) TEM image of a typical nanocube. (d) EDX spectrum of AgPb10SbTe12 nanocubes. The inset of Figure 1b is the size distribution of the as-prepared nanocubes with an average size of 100 nm.

conductivities of the bar-shaped samples (10 × 4.6 × 0.8 mm3) were measured by a DC four-probe technique using a 2400 sourcemeter (Keithley 2700, Keithley Instruments Inc., America) under an argon atmosphere. 3. Results and Discussion 3.1. Morphology and Structure of AgPb10SbTe12 Nanocubes and Flower-like Crystals. The typical XRD pattern of the as-prepared AgPb10SbTe12 nanocubes in the presence of 0.2 g of PVP shows that it is in good agreement with that of PbTe as reported in the literature (JCPDS: 38-1435), confirming that the as-synthesized samples have a typical fcc crystal structure; meanwhile, one tiny diffraction peak of Pb is detected (Figure 1a). The sizes and morphologies of the assynthesized products were characterized by FESEM and TEM. Figure 1b shows the FESEM image of the obtained AgPb10SbTe12 nanocubes, in which nanocubes with good uniformity of shape and size are clearly observed. The average size of the as-obtained products, determined by measuring 100 nanocubes from different regions (inset of Figure 1b), is approximately 100 nm. The nanocubes exhibit a glazed surface and regular shape. Figure 1c displays the typical TEM image of a single nanocube with smooth facets, and the edge length of the nanocube is approximately 100 nm. The composition of the as-prepared products was confirmed by EDX analysis, as shown in Figure 1d, which reveals the coexistence of four elements, Ag, Pb, Sb, and Te. Furthermore, the composition of the prepared nanocubes is Ag0.71Pb10Sb0.33Te11.74. Different from the uniform nanocubes obtained above, flowerlike particles can be acquired without PVP while keeping the other conditions constant. Figure 2a shows the FESEM image

of the sample: the majority of the products are composed of flower-like crystals with eight identical petals which present fascinating cubic symmetric structure, and the petals of each AgPb10SbTe12 grain range from 1.0 to 1.5 µm. Some protrudent steps can be seen on the petals. The XRD pattern of the asprepared products shown in Figure 2b are crystalline with characteristic NaCl structure and Fm3m space group. The diffraction peaks can be indexed to the phase of PbTe while two small diffraction peaks are attributed to Pb. It is not surprising that high purity PbTe is difficult to prepare; literature reports have shown that the as-synthesized PbTe19 and AgPb10SbTe1220 contain some unavoidable Pb impurities in the products, which is in accordance with our results. The TEM image of two typical flower-like crystals with eight symmetric petals of 1 µm is presented in Figure 2c, which agrees with the FESEM result. Moreover, the EDX spectrum shown in Figure 2d is quite similar to that of the nanocubes, in which Ag, Pb, Sb, and Te are detected, and its composition matches Ag0.62Pb10Sb0.27Te11.13. 3.2. Investigation of Parameters Influencing the Formation of AgPb10SbTe12 with Various Morphologies. The experimental parameters affecting the morphologies of AgPb10SbTe12, including temperature, the molar ratio of KOH/ Pb(Ac)2, and the solvent, were investigated. As one significant operation parameter, the synthetic temperature may have considerable effect on the structure and morphologies of the specimen. For instance, it was pointed out that the sizes and morphologies of the PbTe nanowires are sensitive to the reaction temperature; by changing the temperature from 180 °C to 210 °C, longer and wider PbTe nanowires can be obtained.21 Varying the temperature from 80 °C to 200 °C affords nanosheets of varied sizes.22 Thereby, a series of temperature-

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Figure 2. (a) FESEM image of AgPb10SbTe12 flower-like crystals. (b) Powder X-ray diffraction pattern of as-prepared AgPb10SbTe12 crystals with flower-like morphology. (c) TEM image of two typical flower-like crystals. (d) EDX spectrum of AgPb10SbTe12 flower-like crystals.

Figure 3. (a) Powder X-ray diffraction patterns of AgPb10SbTe12 crystals prepared at different temperatures and (b) the FESEM image of the products synthesized at 220 °C.

dependent experiments were carried out to investigate the influence of temperature on the formation of AgPb10SbTe12 nanocubes. The phase structure and morphologies of products vary greatly as the temperature changes from 100 to 220 °C. XRD patterns of the powders fabricated at different temperatures are shown in Figure 3a. The samples do not crystallize completely at temperatures of 100 °C and 140 °C, and the corresponding products are irregular nanoparticles (not shown) while the samples with complete crystallization and typical fcc crystal structure can be achieved at 180 °C. For the specimen synthesized at 220 °C, two diffraction peaks of Pb can be detected, which were also observed in the formation of Ag0.8Pb18+xSbTe20 (x ) 0, 3.5, 4, 4.5, 5).20 Its FESEM image, shown in Figure 3b, indicates that the as-prepared samples consist of nanocubes and some irregular nanoparticles. The percentage of nanocubes in the sample was about 80%. Our results reveal that the synthesis temperature is critical for the construction of AgPb10SbTe12 cubes with a desirable structure and regular morphologies.

In addition to the synthetic temperature, the molar ratio of KOH/Pb(Ac)2 is another experimental parameter which plays paramount roles in the fabrication of the products with designed structure and morphologies. The samples with good qualities can only be obtained by optimizing the concentration of alkali, and the quality of the objective is poorer with higher or lower concentration of alkali.23 Zhao et al. fabricated PbTe hopper cubes, flower-like crystals, and dendrites by varying the concentration of NaOH4 while PbTe nanocubes with sizes ranging from 120-160 nm to 30-35 nm can be acquired via adjusting the concentrations of NaOH from 2 mol/L to 10 mol/L in the system.24 Consequently, possible roles that KOH plays in defining the morphologies of AgPb10SbTe12 in the absence of PVP were systematically investigated via adjusting the molar ratio of KOH/Pb(Ac)2 from 0 to 13.2, 25, and 40, respectively. AgPb10SbTe12 irregular nanoparticles are acquired by reducing the molar ratio of KOH/Pb(Ac)2 to 0 (Figure 4a), and the percentage of the irregular nanoparticles is about 70%. When the molar ratio of KOH/Pb(Ac)2 is increased to 25, ap-

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Figure 4. FESEM images of AgPb10SbTe12 prepared with different molar ratios of KOH/Pb(NO3)2 while all other reaction parameters remain unchanged: (a) 0, (b) 25, and (c) 40. (d) FESEM image and (e) Powder X-ray diffraction patterns of the products synthesized by adding PVP but without the presence of KOH.

proximately 60% of AgPb10SbTe12 cubes with concave facets are observed (Figure 4b), and the average edge length of the cubes is 2.5-3 µm. Further increasing the molar ratio of KOH/ Pb(Ac)2 to 40 leads to AgPb10SbTe12 cubes with cupped faces (around 50%) together with some anomalistic crystals as shown in Figure 4c. The comparative experiment in which PVP was added but without the presence of KOH was implemented in order to better understand the effect of KOH, and the corresponding specimen was quasispherical nanoparticles with a diameter of 50-100 nm (Figure 4d). However, some diffraction peaks of Pb and Pb3O4 can be detected (Figure 4e), which indicates that KOH not only has a striking influence on the morphologies of the samples but also contributes to the purity of the as-synthesized specimen. The organic solvent EG is widely used in various synthetic systems because of the high boiling point (ca. 198 °C), a low saturation vapor pressure, a relatively high viscosity, and reducing action. Recently, Li et al. have adopted EG as the solvent to synthesize uniform colloidal particles and complex anisotropic structures of various metal sulfides.25 PbTe hierarchical flower-like crystals with eight tower-like horns have been achieved using nontoxic EG as solvent.24 Therefore, EG was used to substitute the mixed solvents of water and ethanol to investigate the influence of solvent in our synthesis system when the other conditions are kept fixed as those for the formation of AgPb10SbTe12 nanocubes. Figure 5a and 5b represents the FESEM, TEM image, and the size distribution of the as-prepared specimen; it can be clearly seen that a large quantity of nanocubes with a diameter of 30-50 nm has been obtained. The HRTEM image was recorded on a single nanocube in Figure 5c to provide additional insight into the structure of the

nanocubes. The visible lattice fringe with a d-spacing of 0.322 nm is consistent with that of the (200) planes of AgPb10SbTe12, which is also similar to the lattice spacing of the AgPb18SbTe20 reported by Quarez et al.13 The XRD pattern of the synthesized nanocubes shown in Figure 5f reveals that specimens with the typical fcc crystal structure are acquired, and the broadening diffraction peaks are due to the decreasing size of the nanocubes, which is consistent with the FESEM analysis. Then we prepared the specimen using EG as solvent while the other conditions were kept the same as those used for fabricating flower-like crystals under the mixed solvents of water and ethanol. Excitingly, flower-like crystals can also be obtained only if the molar ratio of KOH/Pb(Ac)2 is augmented to 40, and the size of the crystal is 1-1.5 µm (Figure 5d), which is smaller than that of the samples fabricated with the solvents of water and ethanol. Figure 5e displays the TEM image of the two welldefined flower-like crystals consisting of eight identical petals with a length of 0.6-1.0 µm. Furthermore, from the XRD pattern of the flower-like crystals shown in Figure 5f, we can clearly see that the peaks can be readily indexed to the structure of PbTe, while there are two tiny diffraction peaks of Pb for the as-synthesized specimen. 3.3. Conversion Mechanism of AgPb10SbTe12 Crystals. To shed light on how the experimental conditions result in the assynthesized products having different morphologies, the summary of morphology evolution for the samples is shown in Figure 6. It indicates that changing the experimental parameters (e.g., the amount of alkali, PVP, solvent, etc.) may lead to the formation of AgPb10SbTe12 crystals with diverse shapes and tunable sizes.

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Figure 5. Nanocubes and flower-like crystals fabricated with EG as the solvent. (a) FESEM image and (b) TEM image of the crystals with nanocubic morphology. (c) HRTEM image taken from one typical nanocube. (d) FESEM image. (e) TEM image of flower-like crystals. (f) Powder X-ray diffraction pattern of as-synthesized nanocubes and flower-like crystals. The inset of Figure 5b is the size distribution of the as-prepared nanocubes.

Figure 6. The summary schematic illustration of the experimental parameters and morphology evolution for the samples.

On the basis of the observations from FESEM and TEM images, it can be concluded that PVP acts as a switch between AgPb10SbTe12 nanocubes and flower-like crystals. AgPb10SbTe12 nanocubes can be obtained in the presence of PVP, while flowerlike crystals with eight symmetric petals are the final products without PVP. As a PbTe-based alloy, AgPb10SbTe12 possesses an average NaCl structure (Fm3m symmetry).12 It is well-known that {111} planes have higher surface energy than {100} planes for fcc crystals; thus, cubic morphology is favorable to be formed because of a faster growth rate in the 〈111〉 directions.26,27 Therefore, AgPb10SbTe12 crystals are inclined to form cubic

morphologies in theory. However, AgPb10SbTe12 nanocubes can only be achieved by adding a definite amount of PVP; otherwise, flower-like crystals are the final products. So questions may arise: why does this happen and what are the exact roles that KOH and PVP play in the formation of AgPb10SbTe12 nanocubes and flower-like crystals? Based on the experiments above, a surface-protected etching mechanism is proposed. In our system, KOH and PVP play different roles in the formation of AgPb10SbTe12 crystals; KOH is used as an etching agent and PVP is used as a protecting ligand. KOH as etching agent is apt to attack the {100} planes of AgPb10SbTe12 crystals, which

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Figure 7. Powder X-ray diffraction patterns of PbTe and AgPb10SbTe12 flower-like crystals. The inset is the XRD patterns for the two samples where 2θ ranges from 27° to 28°.

results in the formation of flower-like crystals. A similar effect of OH- has also been reported in the synthesis of PbSe octopod hierarchitectures28 and PbS cubes with pyramidal pits.29 While PVP as capping molecules can adsorb onto the surfaces of crystals via ligand coordination,30 it has been reported that PVP may attach to the {100} facets of PbSe and slow the growth rate of {100} planes.22 Here, PVP plays a protective role in the formation of AgPb10SbTe12 nanocubes; it can absorb on the {100} planes of AgPb10SbTe12 crystals and further weaken the etching role of KOH. Moreover, PVP molecules can be used to regulate the viscosity and diffusion coefficient of the reaction system. By increasing PVP to a certain amount, a specimen with smaller sizes can be acquired,25 and a substantial amount of PVP may also prevent particle agglomeration, which may lead to the formation of cubes with decreasing sizes.31 As to the nanocubes with smaller size fabricated with EG as solvent, it is well-known that EG can act not only as a solvent but also as a capping agent which may absorb onto the {100} facets of the products and further prevent the growth of {100} facets, finally forming nanocubes of smaller size. However, for AgPb10SbTe12, flower-like crystals of relatively small sizes with EG as solvent can only be fabricated via increasing the molar ratio of KOH/Pb(Ac)2 to 40. This may also relate to the capping effect of EG that strengthens the suppression of etching, which is why a higher concentration of alkali is required. 3.4. Electrical Conductivity Comparative Analysis of AgPb10SbTe12 Crystals with Different Morphologies and PbTe Powders. PbTe was also fabricated by using the same method as that for AgPb10SbTe12 flower-like crystals. The XRD patterns of PbTe and AgPb10SbTe12 flower-like crystals shown in Figure 7 suggest that the diffraction spectra for AgPb10SbTe12 crystals shift to larger 2θ angles compared to those for PbTe. Such a shift is expected qualitatively due to the smaller lattice parameters of AgPb10SbTe12, which also indicates incorporation of Ag and Sb into the PbTe lattice and agrees with the EDX result. The lattice parameters calculated via the XRD data for AgPb10SbTe12 and PbTe are 6.450(1) Å and 6.503(3) Å, respectively, and the fact that the lattice parameters of AgPbmSbTem+2 (6.498 Å for m ) 8,15 6.45 Å for m ) 218) is smaller than those of PbTe (6.552 Å,15 6.55 Å18) have been verified in the literature.15,18 Additionally, the two specimens were pressed into a bar with a rectangular shape of about 10 × 4.6 × 0.8 mm3 for measurement of electrical conductivity

Wang et al. under identical circumstances. The temperature dependence of electrical conductivities for AgPb10SbTe12 nanocubes, flowerlike crystals, and PbTe powders is shown in Figure 8a. For nanocubes, the electrical conductivity increases slowly between 25 °C and 350 °C following by a sharp increase, and it may reach 5093.8 S/cm at a temperature of 465 °C. The rising electrical conductivity with increasing temperature in highly doped PbTe materials indicates the typical electrical conductivity of the semiconductor, which have also been reported previously, such as, indium-doped PbTe based on a hopping mechanism,32 and the electrical conductivity of AgPb18Sb1-xTe20 compounds prepared by quenching the melts in liquid nitrogen also monotonically increases with rising temperature.33 A similar process takes place for AgPb10SbTe12 crystals with flower-like morphologies; moreover, the electrical conductivity of flowerlike crystals is much smaller than that of nanocubes. Recently, Martin et al. proposed that the density of the specimen was paramount for research on thermoelectric nanocomposites; the higher the density that the specimen possesses, the larger the electrical conductivity that will be obtained.34 In our experiment, more holes may exist when flower-like crystals consisting of eight identical petals are pressed into a bar compared to nanocubes under identical circumstances, which results in the density of a bar prepared with flower-like crystals being smaller than that of the bar containing nanocubes. Additionally, the electrical conductivities of AgPb10SbTe12 with nanocubic and flower-like morphologies are far superior to that of PbTe, which indicates that codoped Ag and Sb into PbTe is favorable to remarkably enhance the electrical conductivity of PbTe, which is in accordance with the results reported by Hsu et al.12 and Li et al.16 We detected the PXRD of AgPb10SbTe12 nanocubes, flower-like crystals, and PbTe collected after the measurement of electrical conductivities at temperatures under 500 °C for 1.5 h, shown in Figure S1, to see whether there is a phase transformation during the annealing. The results illustrate that the above samples still belong to the PbTe crystal structure as the specimen before annealing, indicating that these samples have good thermal stability and no change of structure. Cai’s research group has reported that sintering AgPbmSbSem+2 does not alter the phase of the specimen, which corresponds with our results.35 The electrical conductivities of AgPb10SbTe12 cubes with concave facets and AgPb10SbTe12 quasispherical crystals were also measured, as shown in Figure S2, which represent that the temperature dependence of electrical conductivities for the above two samples are similar to those of AgPb10SbTe12 with cubic and flower-like morphologies. The electrical conductivities all increase slightly at lower temperatures followed by an obvious increase. Figure 8b displays the temperature dependence of electrical conductivities for AgPb10SbTe12 nanocubes and flower-like crystals using EG as solvent. The curves are analogous to those of the samples synthesized with mixed solvents of water and ethanol, and it is surprising to observe that the specimen prepared with EG as solvent exhibits smaller electrical conductivities compared with those of the same morphologies fabricated with the mixed solvents of water and ethanol. There may be two causes for this; one is the smaller sizes of the samples synthesized by using EG as solvent, therefore resulting in more grain boundaries when the powders are pressed into a bar, which leads to the reduction of mobility by enhancing the carrier scattering on the grain boundaries.36 The influence of grain boundaries in the products to decrease the electrical conductivities has also been reported by Zhao et al.37 The other possible cause for decreased electrical conductivities are EG ligands that possibly remain around the crystals,

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Figure 8. Temperature dependence of the electrical conductivities for the samples: (a) AgPb10SbTe12 nanocubes, flower-like crystals, and PbTe prepared with the mixed solvents of water and ethanol, and (b) AgPb10SbTe12 nanocubes and flower-like crystals synthesized with EG as solvent.

which can passivate the products and kill the electrical conductivities. Accordingly, to elucidate whether EG remains on the samples and the effect it may have, we measured the IR spectra of pure EG, the unwashed products, and the specimen washed eight times for electrical conductivity. As shown in Figure S3, the characteristic stretching and bending vibration of O-H can be seen at about 3362.57 cm-1 and 1659.80 cm-1, and the absorption peaks located at approximately 2945.19 cm-1, 2879.65 cm-1, and 1405.77 cm-1 can be attributed to C-H stretching and bending vibration. Whereas there are plenty of uncertain peaks for the products acquired without washing, as shown in Figure S3b, which may result from the complication of the solvothermal reaction, the reaction may produce some unknown organic products. For the samples washed eight times, as we mentioned in Materials and Preparation Procedures, there only exist two absorption peaks at about 3436.26 cm-1 and 1635.01 cm-1, which can be assigned to the stretching and bending vibration of O-H and may be attributed to the water adsorbed onto the surfaces of the samples as reported by Qian’s research group,22 indicating that no EG surrounds the crystals. Consequently, we have excluded the second cause mentioned above. The real cause for the decreased electrical conductivities for specimens produced with EG results from more grain boundaries in the samples with decreasing sizes. The experimental results above indicate that as-synthesized crystals possess shape- and size-dependent electrical conductivities, which may be helpful for the design of samples with superior electrical conductivities. 4. Conclusion In summary, we have developed a facile solution strategy for the high-yield, controlled synthesis of AgPb10SbTe12 nanocubes and flower-like crystals composed of eight symmetric petals. A surface-protected etching growth mechanism has been proposed to illustrate the formation of AgPb10SbTe12 crystals with nanocubic and flower-like morphologies, where KOH plays an etching role in attacking the {100} planes of AgPb10SbTe12, yet PVP acts as a protective agent of the {100} planes. The competition between the etching and protection of the {100} planes may determine the morphologies of the synthesized products. Notably, the electrical conductivities of AgPb10SbTe12 crystals are dependent on their morphologies. AgPb10SbTe12 nanocubes present superior electrical conductivity compared to

flower-like crystals, which may have potential in the design of AgPb10SbTe12 crystals with desirable shapes for the improved fabrication of advanced TE devices. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Project No. 20871036) and Development Program for Outstanding Young Teachers in Harbin Institute of Technology (HITQNJS. 2009. 001). Supporting Information Available: Powder X-ray diffraction patterns of AgPb10SbTe12 nanocubes collected after the measurement of electrical conductivity. Temperature dependence of the electrical conductivities for AgPb10SbTe12 with concave facets and AgPb10SbTe12 quasispheres. The IR spectra of pure EG, the unwashed specimen, and the products washed eight times. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hsu, Y. J.; Lu, S. Y.; Lin, Y. F. Small 2006, 2, 268. (2) Xu, L.; Shen, J. M.; Lu, C. L.; Chen, Y. P.; Hou, W. H. Cryst. Growth Des. 2009, 9, 3129. (3) Wang, T.; Hu, X. G.; Dong, S. J. J. Phys. Chem. B 2006, 110, 16930. (4) Zhu, T. J.; Chen, X.; Cao, Y. Q.; Zhao, X. B. J. Phys. Chem. C 2009, 113, 8085. (5) Purkayastha, A.; Yan, Q. Y.; Gandhi, D. D.; Li, H. F.; Pattanaik, G.; Borca-Tasciuc, T.; Ravishankar, N.; Ramanath, G. Chem. Mater. 2008, 20, 4791. (6) Wen, X. G.; Fang, Y. P.; Pang, Q.; Yang, C. L.; Wang, J. N.; Ge, W. K.; Wong, K. S.; Yang, S. H. J. Phys. Chem. B 2005, 109, 15303. (7) Shi, W. D.; Yu, J. B.; Wang, H. S.; Zhang, H. J. J. Am. Chem. Soc. 2006, 128, 16490. (8) Chen, G. Z.; Xu, C. X.; Song, X. Y.; Xu, S. L.; Ding, Y.; Sun, S. X. Cryst. Growth Des. 2008, 8, 4449. (9) Gao, J. N.; Li, Q. S.; Zhao, H. B.; Li, L. S.; Liu, C. L.; Gong, Q. H.; Qi, L. M. Chem. Mater. 2008, 20, 6263. (10) Park, C. J.; Choi, D. K.; Yoo, J.; Yi, G. C.; Lee, C. J. Appl. Phys. Lett. 2007, 90, 083107. (11) Li, G. R.; Yao, C. Z.; Lu, X. H.; Zheng, F. L.; Feng, Z. P.; Yu, X. L.; Su, C. Y.; Tong, Y. X. Chem. Mater. 2008, 20, 3306. (12) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Science 2004, 303, 818. (13) Quarez, E.; Hsu, K. F.; Pcionek, R.; Frangis, N.; Polychroniadis, E. K.; Kanatzidis, M. G. J. Am. Chem. Soc. 2005, 127, 9177. (14) Lin, H.; Bozin, E. S.; Billinge, S. J. L. Phys. ReV. B 2005, 72, 174113. (15) Arachchige, I. U.; Wu, J. S.; Dravid, V. P.; Kanatzidis, M. G. AdV. Mater. 2008, 20, 3638.

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