PbSe Hollow Spheres: Solvothermal Synthesis, Growth

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PbS/PbSe Hollow Spheres: Solvothermal Synthesis, Growth Mechanism, and Thermoelectric Transport Property Rencheng Jin, Gang Chen,* and Jian Pei* Department of Chemistry, Harbin Institute of Technology, Harbin 150001, P. R. China S Supporting Information *

ABSTRACT: Uniform PbS/PbSe hollow spheres consisting of PbS and PbSe nanoparticles were synthesized by a facile solvothermal method in mixtures of ethylene glycol and tetrahydrofuran at 120 °C with the assistance of thioglycollic acid. Experimental parameters, such as reaction time, volume of thioglycollic acid, and volume ratio of ethylene glycol to tetrahydrofuran, played crucial roles in determining the morphologies and composites of the final products. Based on the electron microscope observations and X-ray diffraction (XRD) patterns, the reaction process and growth mechanism of such hierarchitectures were proposed. Nitrogen adsorption−desorption measurements and pore size distribution analysis revealed that the mesoporous existed in the product. Moreover, thermoelectric transport measurements demonstrated that the synergistic effects of PbS and PbSe would lead to enhancement of the electrical conductivity; the obtained binary phased PbS/PbSe hollow spheres had the maximum electrical conductivity and Seebeck coefficient of 22.1 S cm−1 and 323.3 μV/K, respectively, which were higher than those of pure PbSe nanoparticles.

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INTRODUCTION In recent years, hollow micro- and nanostructures have attracted considerable attention due to their widespread applications in many fields, such as drug delivery, efficient catalysis, energy-storage media, nanoreactors for catalysis, sensors, etc.1−6 Diverse procedures involving soft templates (e.g., emulsion droplets,7,8 surfactant, and gas bubbles9,10) and hard templates (e.g., silica,11 polymer latex spheres,12 and metal nanoparticles13,14) have been applied to fabricate hollow spheres of inorganic materials. However, these methods require template materials to build sphere architectures and tend to be rather complicated. Meanwhile, the removal of the templates is hard and often destroys the structural integrity of the final products, which limits the practical applications.15 Therefore, it is still highly desirable to develop a facile and cheap synthetic approach for the fabrication of inorganic hollow structures with a defined shape and size. Lead chalcogenides, a kind of very important narrow band gap semiconductor, are highly desirable in potential applications in the fields of nonlinear optical switches,16 solar cell sensitizers,17,18 and thermoelectric devices.19−21 In the past decade, various morphological lead chalcogenides including nanowires,22 nanorods,23 nanocubes,24 nanoplates,25,26 nanooctahedrons,27 hollow spheres, etc.28−30 have been synthesized by different methods. Among them, the hollow spherical lead chalcogenides have been the focus of intensive research due to their unique properties and potential application in many fields. Some literature has reported the synthesis of lead chalcogenides © 2012 American Chemical Society

hollow spheres. For instance, PbS hollow nanospheres have been synthesized by a sonochemical route with the assistance of dodecylbenzenesulfonate.30 Hollow PbSe nanospheres have been fabricated by an in situ cation exchange approach,31 Ostwald ripening process,28 and Kirkendall-effect-induced growth.29 PbTe hollow spheres have been prepared by a solvothermal method via a self-assembly and soft template procedures.21,32 However, to the best of our knowledge, there is no report on the successful synthesis and formation mechanism of binary phased PbS/PbSe hollow spheres. Thioglycollic acid, a multifunctional group agent, used as a surfactant and sulfur source can make some nanostructures with novel morphology form.33−37 In addition, the thioglycollic acid can be removed easily through heat treatment due to its low boiling point. Therefore, the thioglycollic acid technique has broad application in the synthesis of various nanostructures. Herein we demonstrate a simple, low temperature, and onestep strategy for the synthesis of binary phased PbS/PbSe hollow microspheres by a simple thioglycollic acid assisted solvothermal route. To determine the optimal synthesis conditions, some reaction parameters such as reaction time, volume of thioglycollic acid, and volume ratio of ethylene glycol to tetrahydrofuran are systematically investigated. Based on the experimental results, a surfactant directed assembly process is Received: March 7, 2012 Revised: July 2, 2012 Published: July 3, 2012 16207

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Figure 1. (a) XRD pattern of PbS/PbSe hollow spheres prepared at 120 °C, (b) low magnification SEM image of the PbS/PbSe hollow spheres, scale bar: 10 μm, (c) high magnification SEM image of the PbS/PbSe hollow spheres, scale bar: 2 μm, and representative (d) TEM and (e and f) HRTEM images of the PbS/PbSe hollow spheres in the same batch as that in panel b.

prepared precipitate was collected by centrifugation and washed with distilled water and then ethanol for five times. Finally, the precipitate was separated by centrifugation again and dried at 80 °C for 8 h under vacuum for further characterization. Characterization. The X-ray diffraction (XRD) patterns of the products were recorded on Rigaku-D/MAX-2550PC diffractometer using Cu Kα radiation. The morphology of the samples was inspected with a field emission scanning electron microscope (FESEM, FEI Quanta 200F). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained by employing a FEI Tecnai G2 S-Twin transmission electron microscope with a field-emission gun operating at 200 kV. Nitrogen adsorption−desorption measurement was detected at 77.35 K by using an AUTOSORB-1-MP surface analyzer. The specific surface area was determined by the Brunauer− Emmett−Teller (BET) method. Thermoelectric Transport Property Measurement. The thermoelectric transport properties of the samples were performed following the reported method with a slight modification.38 The samples were first pressed into a bar with a rectangular shape with the size of 10 × 4.6 × 1.0 mm3 under 20 MPa pressure at room temperature. For the measurement of electrical conductivity, four-probe method was adopted and silver pastes were used as electrical contacts of the electrodes to

proposed for explaining the formation of PbS/PbSe hollow spheres. Furthermore, the thermoelectric transport property measurements depict that the PbS/PbSe hollow sphere has a simultaneous enhancement in electrical conductivity and Seebeck coefficient compared with that of pure PbSe nanoparticles.

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EXPERIMENTAL SECTION Chemicals. Lead acetate (99.5%), sodium selenite (97%), thioglycollic acid (99.0%), tetrahydrofuran (99.0%), ethylene glycol (99.5%), hydrazine hydrate (80%), and ethanol (99.7%) were purchased from the Shanghai Chemical Company. All of the reagents were used without further purification, and distilled water was used throughout the experiment. Preparation of PbS/PbSe Hollow Spheres. In the typical procedure, 0.5 mmol of lead acetate and 0.5 mmol of sodium selenite were dissolved in ethylene glycol to form 20 mL of solution. Then the mixed solution was transferred into a Teflon bottle (40 mL) held in a stainless steel autoclave (see the Supporting Information in Figure S1). After that, 8 mL of tetrahydrofuran, 2 mL of hydrazine hydrate, and 1 mL of thioglycollic acid were added into the autoclave in turn. The autoclave was sealed and maintained at 120 °C for 12 h. After allowing the autoclave to cool to room temperature, the as16208

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Figure 2. SEM, TEM, and HRTEM images of the products obtained at various reaction stages: (a) 1 h, (b) 2 h, (c−f) 4 h, and (g) 8 h. Scale bar: 5 μm.

the specimen. To obtain the Seebeck coefficient, a microheater that created a temperature difference of about 3−15 K between the cool and hot ends of the specimen was applied. The temperature and temperature differences (ΔT) were determined by calibrated nickel chromium−nickel silicon thermocouples, while the corresponding thermally induced voltage ΔV was recorded by the voltage probes. Then the Seebeck coefficient can be obtained by the formula, S = −ΔV/ΔT. The experimental data of electrical conductivity and Seebeck coefficient were collected by a computer-controlled multifunctional measuring system under the argon atmosphere (Keithley

2400 source meter, Keithley 2700 multimeter, Keithley Instruments Inc., U.S.A.).

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RESULTS AND DISCUSSION The phase composition and phase structure of the sample prepared via solvothermal reaction at 120 °C for 12 h were examined by X-ray powder diffraction (XRD). As shown in Figure 1a, all of the diffraction peaks except PbS (JCPDS, no. 05-0592) can be indexed to the face-centered cubic (fcc) PbSe structure with the Fm3m space group (JCPDS, no. 06-0354). Typical low-magnification SEM image shows that the majority 16209

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Figure 3. (a) Typical XRD pattern of the products produced at different reaction times. (b) Magnified view of the XRD pattern with the 2θ degree ranging from 18 to 35.

Figure 4. SEM, TEM, and HRTEM images of the products prepared by using various amounts of thioglycollic acid: (a−c) 0 mL, (d) 0.5 mL, (e) 1.5 mL, and (f) 2 mL. Scale bar: 5 μm.

of the products are composed of numerous microspheres with the average diameter of 1 μm (Figure 1b). More detailed morphologies, as displayed in Figure 1c, demonstrate that the single microsphere is composed of many nanoparticles. The size of these nanoparticles ranges from 40 to 100 nm. The

broken PbS/PbSe microsheres confirm the hollow structure of the product. The composition of the as-prepared samples characterized by energy dispersive spectroscopy (EDS) is shown in Figure S2. In the representative EDS spectrum, Pb, S and Se with the atomic ratio of 45:5:39 can be found. The 16210

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Figure 5. Schematic illustration of the proposed growth mechanism of PbS/PbSe hollow spheres.

PbS and PbSe nanocrystals. When the reaction time is prolonged to 8−12 h, more complete hollow spheres form instead of the broken hollow spheres and the nanoparticles, which can be seen in Figures 2g and 1b. The XRD patterns of the products prepared at different times at the same hydrothermal temperature (120 °C) are shown in Figure 3. The XRD pattern of the sample obtained at 120 °C for 1 h demonstrates that the product is composed of PbSe and Se elements. The lower diffraction intensity of PbSe indicates that the crystallinity is comparatively poor. As the reaction time increases, the intensity of diffraction peaks for the PbSe becomes higher and higher due to the better crystalline of the samples. Meanwhile, the element Se decreases with the reaction time. Observed carefully, some diffraction peaks corresponding to PbS form when the reaction time is extended to 2 h or longer. According to the above analysis, the formation mechanism of the PbS/PbSe hollow spheres depends on a series of chemical and structural transformations. During the process, thioglycollic acid plays a critical role in controlling the morphology and the composite of the final product. To investigate the effect of thioglycollic acid on shape evolution, a control experiment with different volume of thioglycollic acid was carried out. In the absence of thioglycollic acid, only irregular nanoparticles can be obtained (Figure 4a). The TEM analysis in Figure 4b shows the obtained PbSe is composed of a large amount of nanoparticles, in accordance with the SEM observation. The corresponding HRTEM shows that the interplanar distance between the adjacent lattice fringes is 0.307 nm (Figure 4c). This plane can be indexed as the d spacing of the (200) plane of PbSe. It should be mentioned that only PbSe could be synthesized when no sulfur source was employed. When 0.5 mL of thioglycollic acid was added in the reaction solution, many microspheres of PbS/PbSe composed of some randomly aggregated nanoparticles were produced (Figure 4d). When the addition of thioglycollic acid reached 1−1.5 mL, the well-defined hollow

typical TEM image of the PbS/PbSe hollow spheres demonstrates the stacking of the building blocks and the interior cavity of the sphere, since the contrast at the center is much lower than that of a solid particle, which further confirms the uniform size and hollow structure of the PbS/PbSe spheres. High-resolution TEM (HRTEM) is recorded on the corner of the hollow sphere (labeled with circle in Figure 1d). The interplanar spacings are 0.353 and 0.307 nm, which can be indexed as the d-spacing of (111) and (200) planes of PbSe crystal. Figure 1f shows the HRTEM image taken from another area marked by a square in Figure 1d. Interestingly, the interplanar spacings are 0.298 and 0.307 nm, which correspond to the (200) plane of PbS and the (200) plane of PbSe, respectively. These results further confirm the coexisting of the PbS and PbSe nanocrystals. To reveal the growth process of this binary phased PbS/PbSe hollow sphere, the time-dependent experiments were carried out without changing other reaction parameters. Figure 2 shows the SEM images of the products prepared at different reaction times. After 1 h solvothermal treatment, the obtained product is composed of numerous microrods and some irregular nanoparticles (Figure 2a). Prolonging the reaction time to 2 h, the nanoparticles increase obviously with the decreasing of microrods. Simultaneously, some hollow sphereical morphologies can be observed, which can be seen in Figure 2b. When the reaction time is increased further to 4 h, the broken hollow spheres and complete hollow spheres can be obtained as well as the nanopartices. It should be pointed out that the microrods morphology can be occasionally found (Figure 2c) and that the TEM image (Figure 2d) further reveals that the nanoparticles and the hollow spheres coexist. The corresponding HRTEM images taken from hollow sphere marked with a circle and square are shown in Figure 2, panels e and f. The interplanar spacings are 0.210 and 0.309 nm, which are close to the (220) plane of PbS and (200) plane of PbSe, respectively, indicating that the hollow spheres are composed of 16211

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Figure 6. SEM images of the obtained product after solvothermal treatment at 120 °C in media with different volume ratio of ethylene glycol to tetrahydrofuran: (a) 0:28, scale bar: 10 μm, (b) 8:20 scale bar: 5 μm, (c) 14:14, scale bar: 10 μm, (d) 14:14, scale bar: 2 μm, (e) 24:4, scale bar: 5 μm, and (f) 28:0, scale bar: 10 μm.

thioglycollic acid used as a surfactant can absorb on the surface of the PbSe and PbS nanoparticles due to the existence of the carboxyl and sulfydryl groups. Because of the electrostatic effects of thioglycollic acid, the adsorbed PbSe and PbS nanoparticles aggregated quickly and formed spherical particles to further reduce the surface energy. Such a similar phenomenon has been reported in the fabrication of microsphere bouquets like bismuth telluride; they present that thioglycollic acid served as the shape-and size-directing agent can promote the nanoplates aggregation.35 A similar molecule, L-cysteine, has three functional groups, −NH2, −COOH, and −SH, which has a strong tendency to promote the aggregations of the nanoparticles, resulting in the formation of 3D hierarchical structures. For example, in the presence of Lcysteine, pagoda-like PbS, flowerlike SnS2, and In2S3 microflowers can be obtained.42−44 In addition, thioglycollic acid can also be applied for the fabrication of metal sulfides, and such methods have been reported in the previous work.34 In our case, the PbS can be synthesized by pyrolyzing the thioglycollic acid. However, the pyrolysis rate of thioglycollic acid is comparatively low because of the low reaction temperature. Therefore, little PbS was obtained after 2 h of solvothermal

spheres could be fabricated, which were shown in Figures 1b and 4e. Further increasing the addition of thioglycollic acid to 2 mL, some assembled spherical morphologies of PbS/PbSe were obtained except some hollow spheres. On the basis of the above discussion, we considered that the formation mechanism of the binary phased PbS/PbSe hollow sphere was dominated by a surfactant directed assembly process. The whole process was illustrated in Figure 5. In the early stage of the reaction, the rod-like Se element and PbSe nanoparticles formed, as confirmed by XRD and SEM images (Figures 2a and 3). As reported in the literature, the Se element shows a trigonal crystal structure and is highly anisotropic with a long c axis. The anisotropic crystal structure of Se has a strong tendency toward 1D growth along the c axis.39−41 Thus, the rod-like Se element is easily achieved. But the obtained Se element was not stable in the reducing aqueous solution and was easily transformed into Se2‑ ions with the assistance of hydrazine hydrate. As Se2‑ ions could be released from the intermediate (Se element), the PbSe nuclei were fabricated. As the reaction continued, the intermediate was consumed gradually and the formed PbSe nuclei grew into PbSe nanoparticles for reducing the surface energy. Furthermore, 16212

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the volume of ethylene glycol in mixed solvent is very important for the synthesis of PbS/PbSe hollow spheres. This may be attributed to the different physical and chemical properties such as the viscosity and concentration of ethylene glycol, which affect the solubility, reactivity, and diffusion behavior of the reagents and the intermediates, resulting in different morphologies of the final products.47−50 To investigate the texture properties of the PbS/PbSe hollow spheres, the nitrogen adsorption−desorption isotherm analysis was carried out. The results are depicted in Figure 7, it can be

treatment. The probable reaction process for the formation of PbS/PbSe hollow spheres may be summarized as eqs 1−5. With further reaction, uniform PbS/PbSe hollow spheres were obtained by consuming the nanoparticles, which is believed to be the result of the surfactant directed assembly mechanism. Generally, the formation mechanism of the hollow sphere with the assistance of surfactant is defined as self-assembly accompanied by Ostwald ripening.2,9,21,45,46 They depict that the nanoparticles aggregate and form spherical particles at the first stage. Due to the steric effect of surfactant molecules, the aggregated particles are loosely compacted and many voids may exist in the interior of spherical particles. In this stage, these aggregated nanoparticles are not in thermodynamic equilibrium and become metastable for their quite large surface energy.15 Thus, larger crystallites are thermodynamically preferable. That is, the transformation from loose spheres to hollow spheres through Ostwald ripening can be observed. In our case, the formation of the hollow sphere is different. In the initial stage, the PbS and PbSe nanoparticles were obtained. Then the nanoparticles aggregate gradually with the assistance of thioglycollic acid. Then the broken hollow structures were obtained, as can be seen in Figure 2c−f. The broken structures were composed of PbS and PbSe nanoparticles. Different from the reported literature, the loosely compacted solid spheres can not be observed in this process. As the time is prolonged, the larger crystals become thermodynamically preferred. So the nanoparticles in the solution continue to grow onto the larger crystals for reducing their surface energy. As a result, the completed hollow spheres can be observed. Na 2SeO3 + 2NH 2NH 2 → Se + 2NaOH + 3H 2O + 2N2 (1)

Se + NH 2NH 2 + 4OH− → Se 2 − + 4H 2O + N2

(2)

Pb2 + + Se 2 − → PbSe

(3)

HSCH 2COOH + H 2O → HOCH 2COOH + H 2S

(4)

Pb2 + + H 2S → PbS + 2H+

(5)

In addition to the effect of thioglycollic acid on the morphologies of the products, studies also find that the volume ratio of ethylene glycol to tetrahydrofuran in the solvothermal system is another important influencing factor. Figure 6 displays the samples with different morphologies that can be obtained by changing the volume ratio of the solvents. When no ethylene glycol was introduced in the reaction system, only irregular nanoparticles were obtained (Figure 6a). If 8 mL of ethylene glycol was added into the solution, some microsheres with wide size distribution and a large quantities of nanoparticles were synthesized (Figure 6b). Controlling the ethylene glycol volume at 50 vol % (14 mL), the major products were composed of nanoparticles (Figure 6c). The high magnification SEM image illustrated that some microspheres and hollow spheres could be fabricated via this method (Figure 6d). Further increasing the ethylene glycol volume to 20 mL, uniform hollow spheres formed, as shown in Figure 1b. When 24 mL of ethylene glycol was employed in the solution, the majority of the product was composed of microspheres with coarse surfaces, which were assembled by numerous nanoparticles. However, no obvious hollow structures were observed (Figure 6e). If the volume of ethylene glycol was increased to 28 mL, only irregular nanoparticles were obtained (Figure 6f). According to the above experiments, it can be concluded that

Figure 7. (a) Typical nitrogen adsorption−desorption isotherm of the PbS/PbSe hollow spheres and (b) the corresponding pore-size distribution.

seen that the PbS/PbSe hollow spheres show a type of IV isotherm with a type of H4 hysteresis loop (at 0.4 < P/P0 < 1), which are properties of typical mesoporous materials. The result is further confirmed by the corresponding pore size distributions, as can be seen in Figure 7b. The pores are possibly attributable to the interstitial spaces between nanocrystals, which is in agreement with SEM and TEM observations in Figure 2. The BET specific surface area of PbS/PbSe hollow spheres calculated from nitrogen adsorption is about 15.6 m2 g−1, and the pore volume is 0.243 cm3 g−1. This result indicates that the as-synthesized PbS/PbSe hollow spheres have porous structures. Thermoelectric transport properties. To elucidate the thermoelectric transport properties of the samples, we pressed the products into a strip with a rectangular shape (10 × 4.6 × 1 mm3) under 20 MPa pressure at room temperature. The electrical conductivity was measured by a four probe technique in the temperature range of 300−600 K. The electrical conductivity of the two samples (PbS/PbSe hollow spheres and PbSe nanoparticles) increases slowly with increasing 16213

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Figure 8. (a) Electrical conductivities, (b) Seebeck coefficients, and (c) power factors of the obtained samples in the temperature range of 300−600 K.

close to those of solution-route PbSe films.19 With increasing the temperature, the electrical conductivity reaches to 13.1 S cm−1 at 500 K. However, for the binary phased PbS/PbSe hollow spheres, the electrical conductivity is 15.5 S cm−1 at room temperature, which is 2 times higher than that of pure PbSe nanoparticles, and the maximum electrical conductivity of 22.1 S cm−1 can be obtained at 500 K. The enhanced electrical property may be attributed to the synergism effect of the PbS and PbSe nanocrystals. The enhanced p-type conductivity in self-assembled PbTe/Ag2Te thin films has been reported previously.54 They demonstrate that nanocrystals can behave as dopants in nanostructured assemblies, which increase the concentration of the charge carriers and lead to the high electrical conductivity. Recently, some literature reported that the size and grain boundaries have much effect on the electrical conductivity,55 the electrical conductivity is higher for products with larger particle size. To acertain the exact role of the particle size, the SEM images of fracture surface of PbSe nanoparticles and PbS/PbSe hollow spheres after annealing are measured and shown in Figure S4, as indicated in the images, the grain sizes of the products have not obvious difference, implying that difference of the electrical conductivity is not attributed to the grain size. It should be pointed out that the PbS/PbSe hollow microspheres collapse at the pressure of 20 MPa. Meanwhile, the density of the samples was measured by Archimedes principle. The density of PbS/PbSe hollow spheres is 80.3%, a little lower than that of the PbSe nanoparticles (82.6%); thus, the density of the products can not be used to interpret the higher electrical conductivity of the PbS/PbSe

temperature (300−500 K) and decreases sharply after 500 K (Figure 8a). This may be attributed to two possible mechanisms: (1) For air-exposed PbSe nanoparticles or PbS/ PbSe hollow spheres, oxygen atoms and/or molecules can be adsorbed on the surface of the products, which act as p-type dopants.51,52 When the temperature increases, desorption of oxygen from the surface of the products may occur, resulting in the decrease of the carrier concentration in a p-type sample. Therefore, the electrical conductivity decreases at higher temperature. (2) The other mechanism is the creation of defects at elevated temperature as proposed by Sun and Smith et al.52,53 In our experiment, the defects may generate at high temperature, and these defects can act as scattering centers for the carriers. The reducing mobility of the carriers finally leads to the decrease of the electrical conductivity. Comparing the two mechanisms, we find that the first one (desorption of oxygen) is typically reversible, whereas the second one (creation of defects) is irreversible. In order to determine the reason for the variation of the electrical conductivity, we remeasured the electrical conductivities of the two specimens after annealing at 600 K for 4 h. The electrical conductivity of the product after annealing is shown in Figure S3. As can be seen in Figure S3, the electrical conductivities of the two samples showed a similar variation trend. The result indicates that the major reason of the variation of the electrical conductivity is the adsorption of the foreign atoms. It should be mentioned that the creation of defects is not to be neglected. Moreover, the measured electrical conductivity of the pure PbSe nanoparticles is 7.1 S cm−1 at room temperature, which is 16214

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after annealing at 600 K for 4 h. (4) FESEM images of typical fractured surface of the pressed strips. This material is available free of charge via the Internet at http://pubs.acs.org.

hollow spheres. From the above discussion, one can conclude that the higher electrical is attributed to the synergism effect of the PbS and PbSe nanocrystals. In addition, some other factors such as the substitution of element between S and Se that leading to the higher charge carrier mobility may also exist.56 Thermoelectric characterization of samples shows that pure PbSe nanoparticle p-type with a Seebeck coefficient, S, of 170− 310 μV/K in the temperature range 300−600 K. The S value increases with the increasing temperature and decreases sharply after 575 K. A similar process takes place for the binary phased PbS/PbSe hollow spheres. Moreover, the Seebeck coefficient of the PbS/PbSe hollow spheres is 230.8 μV/K at room temperature and reaches to maximum at 575 K (323.2 μV/ K), which is higher than that of pure PbSe (Figure 8b). The enhancement of the Seebeck coefficient may originate from the electron energy filtering induced by an alteration of the scattering mechanism.57,58 Figure 8c is the corresponding power factors of PbSe nanoparticles and PbS/PbSe hollow spheres. A maximum power factor of 106.5 μW/(K2·m) can be obtained at 550 K for PbSe nanoparticles, which is smaller than those of the solution route of PbSe films with the grain size of 100−400 nm and solid state sintering bulk PbSe.52,59 As discussed above, the electrical conductivity is higher for products with larger particle size. In our case, the size of PbSe is only tens to dozens of nanometers, which will scatter the charging carriers. Thus, the electrical conductivity will decrease sharply and then decrease the power factor. Because of the improved electrical conductivity and Seebeck coefficient, the power factor of PbS/PbSe hollow spheres increases significantly. A maximum power factor of 205.4 μW/(K2·m) can be obtained at 500 K, which is much higher than that of pure PbSe nanoparticles (106.5 μW/K2·m at 550 K). Such an increase of the power factor can be a promising route to achieve highly efficient thermoelectric energy conversion devices. In addition, effective phonon scattering at the interface between PbS/PbSe and PbSe/PbSe boundaries of the nanograins will lower the thermal conductivity; thus, the enhancement of ZT value can be expected.

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Corresponding Author

*Fax: (+86)-451-86413753. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Science Foundation of China (Project Nos. 20871036 and 21071036), Province Natural Science Foundation of Heilongjiang Province (ZD201011), Scientific Research Innovation Foundation of Harbin Institute of Technology (Project No. GFCQ98332122), and the China Postdoctoral Science Foundation (Project No. AUGA4130915112).

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CONCLUSIONS In summary, a facile solvothermal strategy has been developed for the synthesis of binary phased PbS/PbSe hollow spheres. By properly monitoring the experimental conditions, such as reaction time, volume of thioglycollic acid, and the volume ratio of ethylene glycol to tetrahydrofuran, the PbS/PbSe with different morphologies can be obtained. Based on the observation of the products in the different reaction time, a surfactant directed assembly process for explaining the mechanism of the PbS/PbSe hollow spheres is proposed. Moreover, the thermoelectric transport properties measurements show that the obtained PbS/PbSe hollow spheres have simultaneous enhancement in electrical conductivity and Seebeck coefficient. The present work not only opens new strategies for the controllable synthesis of hollow spheres but also provides a step forward in the design of thermoelectric materials with controllable morphology and enhanced power factors.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

(1) The manufacturer and model of the autoclave used in the experiment. (2) Representative EDS spectrum of the PbS/PbSe hollow spheres. (3) The electrical conductivity of the product 16215

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