Anisotropic Growth of Cubic PbTe Nanoparticles to Nanosheets

Jul 12, 2010 - Synopsis. PbTe single crystal nanosheets of 20−80 nm thickness and 0.2−5 μm length along the edge have been synthesized by a hydro...
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DOI: 10.1021/cg100563x

Anisotropic Growth of Cubic PbTe Nanoparticles to Nanosheets: Controlled Synthesis and Growth Mechanisms

2010, Vol. 10 3727–3731

T. J. Zhu,*,† X. Chen,† X. Y. Meng,† X. B. Zhao,† and J. He‡ †

State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China, and ‡Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634 Received April 28, 2010; Revised Manuscript Received June 30, 2010

ABSTRACT: Fabricating topologically anisotropic nanostructures of a cubic structured material may shed important light on the growth mechanisms that are crucial for fabricating self-assembled nanodevices. PbTe is a cubic structured narrow gap semiconductor that is technically important for thermoelectric energy conversion and infrared devices. We herein report the alkaline hydrothermal synthesis of PbTe single crystal nanosheets of 20-80 nm thickness and 0.2-5 μm length along the edge. The reaction parameters such time, temperature, NaOH concentration, type of surfactants and reductant have been systematically varied in order to establish the correlation between the recipe and the micromorphology of the final products. The timedependent experiments suggest that the formation of PbTe nanosheets involves the spontaneous arrangement and alignment of PbTe nanoparticles via an oriented attachment process, and finally they form a single crystal upon fusion of the nanoparticles. The possible reasons for self-assembly of PbTe nanoparticles have been discussed. We also measured the Seebeck coefficient on a thin film sample made of these nanosheets, which reached the highest value of 120 μV 3 K-1 at 650 K.

Introduction Low-dimensional nanostructures have stimulated intensive research activities because of their contribution to the understanding of novel physical properties and potential applications in nanoelectronic and optoelectronic devices. Generally, the morphology of a nanosized single crystal largely reflects the symmetry of its crystal structure, and the morphology complexity profoundly affects the electrical, magnetic, mechanical, optical, and chemical properties of the nanostructure.1,2 Therefore, there has been a great effort devoted to fabricating nanocrystals with desired and controlled micromorphologies (sizes, shapes, and the way of interconnection) using the so-called “bottom-up” approach.3,4 Various materials with a wide variety of micromorphologies, such as nanowires,5 nanotubes,6 nanohelices,7 zigzag nanobelts,8 and hierarchical dendrite structures,9 have been reported. The study of materials at the nanoscale has also yielded another great contribution, that is, the understanding of fundamental aspects. One clear example is the process by which crystals form and grow. Anisotropic growth is a widely adopted strategy for achieving low-dimensional nanostructures in solution. On the basis of manipulation of the kinetics of nanoparticle growth, a surfactant is generally introduced to the reaction system that preferentially binds to crystal faces of the growing particles resulting in growthinhibited faces.1,10,11 The inherent anisotropy of crystal structure or crystal surface reactivity was identified as the driving force for the low-dimensional growth in previous studies.12-14 However, anisotropic growth of nanocrystals with highly symmetric cubic lattice is counterintuitive, such as PbSe nanowires and nanosheets.15,16 It is of great interest to understand the anisotropic growth mechanisms of such materials system with centrosymmetric lattice.

Lead telluride with cubic rocksalt structure is a narrow bandgap semiconductor showing great promise in the fields of thermoelectric (TE) and infrared (IR) photoelectric devices.17 Various methods18-28 such as the sonoelectrochemical method, the hydrothermal/solvothermal method, and the chemical vapor transport method have been utilized to prepare PbTe nanostructures with different morphologies, such as nanocubes,18 nanowires,19-22 nanorods,23-25 nanotubes,26,27 nanoboxes,28 and other nanostructures. PbTe compounds have a highly symmetric cubic crystal structure and can hardly spontaneously grow into anisotropic nanostructures. It is a great challenge and wide interest to controllably fabricate anisotropic PbTe nanostructures and understand their growth mechanisms. However, to date, there is almost no report on the fabrication of PbTe two-dimensional (2D) nanosheets. We herein report an alkaline hydrothermal synthesis of PbTe nanostructures, including nanosheets, nanoparticles, and hierarchical superstructures. In particular, PbTe 2D nanosheets have been successfully synthesized using polyvinyl pyrrolidine (PVP) as the surfactant at the appropriate growth temperature and NaOH concentration. The possible formation mechanisms of the PbTe nanosheets are discussed. Experimental Section

*To whom correspondence should be addressed. E-mail: zhutj@zju. edu.cn. Phone: þ86-571-87952181. Fax: þ86-571-87952181;

Chemicals. All the chemicals used for the synthesis of the PbTe nanostructures in this work are analytical grade without further purification. Pb(NO3)2, Na2TeO3, and NaBH4 were purchased from Shanghai Chemical Reagent Co.. Ethanol, acetone, cetyl trimethyl ammonium bromide (CTAB), polyvinyl pyrrolidine (PVP), sodium dodecylbenzene sulfonate (SDBS), and NaOH were purchased from various sources. Hydrothermal Synthesis. In a typical procedure, 1 mmol of Pb(NO3)2 and an appropriate amount of NaOH were dissolved into 15 mL of distilled water by stirring. Meanwhile, 1 mmol of Na2TeO3 and 0.5 g of PVP were dissolved into 10 mL of distilled water. Subsequently, the two solutions were added together, and 10 mL of N2H4 3 H2O and distilled water were put in to form an aqueous solution (40 mL). After being stirred for 5 min, the mixture was

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Figure 2. XRD pattern of PbTe powder synthesized by a hydrothermal method at 160 °C for 24 h with 2 M NaOH and PVP.

Figure 3. FESEM images of PbTe crystals synthesizd in 2 M NaOH at (a) 100 °C and (b) 200 °C for 24 h with PVP. Figure 1. (a) Typical FESEM image of PbTe nanosheets synthesized by a hydrothermal method in 2 M NaOH at 160 °C for 24 h with PVP, (b) magnified FESEM image of PbTe nanosheets, (c-e) TEM images of PbTe nansheets, and (f) high-resolution TEM image of the portion marked by the rectangle in (e). transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was held at 160 °C for 24 h and then cooled to room temperature. The gray powders were collected by filtering, washed with distilled water and ethanol, and finally air-dried for characterization. To study the growth mechanism and establish the correlation between the recipe and micromorphology of the final product, the reaction time, temperature, the concentration of NaOH, and the kind of surfactants (PVP, CTAB, and SDBS) and reductant (NaBH4) were systematically varied. Characterizations. The phase structure of the final product was investigated by X-ray diffraction (XRD) on a Rigaku-D/MAX2550PC diffractometer using Cu KR radiation. The morphology and composition of the products were analyzed by a Hitachi S-4800 field emission scanning electron microscopy (FESEM) with the energy-dispersive X-ray (EDX) spectrometer. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) of the nanostructures were performed on a JEOL JEM-2010 microscope. Seebeck Coefficient Measurement. To measure the Seebeck coefficient (thermopower) of the PbTe nanosheets, a thin film sample was prepared by spin-coating the nanosheets from dispersed ethanol solutions on a glass substrate and then drying the sample at 373 K for 10 h. The similar method can be found in the previous work by Tai et al.20 and Yan et al.22 The Seebeck coefficient was measured on a computer-aided apparatus using a differential voltage/temperature technique. A small auxiliary heater was powered to produce a temperature difference ΔT between both ends of the sample. The thermoelectric voltage ΔV and temperature difference ΔT were measured from ΔT = 0 K to about 5 K by an Agilent 34401A multimeter. The Seebeck coefficient was then calculated by the slope of the line of ΔV versus ΔT fitted using more than 100 data points.29

Results and Discussion Structure and Morphology of PbTe Nanosheets. Figure 1a,b presents the FESEM images of the products with different

magnifications, showing that PbTe nanosheets are successfully synthesized with 20-80 nm in thickness and 0.2-5 μm in inplane sizes. These nanosheets exhibit a flat surface and straight boundaries. Figure 1c-e is the TEM images of the synthesized PbTe nanosheets. The thinner nanosheets in Figure 1c,d have voids within themselves, which are related to the formation mechanism and will be discussed in the following section. The ripplelike contrast should result from strain or small thickness variation. The electron diffraction pattern shown in the inset of Figure 1e can be indexed to be the [001] zone axis of the fcc PbTe, indicating that the nanosheet is single crystalline and has a preferential (001) orientation. The high-resolution TEM image in Figure 1f clearly shows that the 2D lattice fringes are structurally uniform with a spacing of 0.32 nm, which is in good agreement with the d value of the (200) planes of f.c.c. PbTe. From Figure 1e,f, it is concluded that the single-crystal nanosheets grow along the (100) planes. Figure 2 is the powder XRD pattern of the product obtained at 160 °C for 24 h using PVP as the surfactant. All the diffraction peaks can be indexed to face-centered-cubic (f.c.c.) structure with space group Fm3m (225) (JCPDS: 38-1435), indicating that pure PbTe compound has been synthesized. The calculated lattice constant (a = 6.4621 A˚) is in good agreement with the standard literature value of 6.459 A˚, and the slight increase in a possibly results from the relaxation of the crystal structure due to the small sizes in thickness of synthesized PbTe nanosheets. Influences of Reaction Parameters. The morphologies of the PbTe products can be affected by many factors, such as reaction temperature, NaOH concentration, and the kind of surfactants and reductants etc. Among them, temperature is one of the most crucial factors. We compared products synthesized at a lower temperature (100 °C) and at a higher temperature (200 °C) with all other reaction conditions unchanged. At 100 °C, the main products are incomplete nanosheets and some nanoparticles, as shown in Figure 3a.

Article

Figure 4. FESEM images of nanocrystals synthesized with different concentrations of NaOH at 160 °C for 24 h with PVP: (a) 0.25, (b) 0.5 M, (c) 1 M, and (d) 2 M.

These nanosheets have smaller sizes compared to those synthesized at 160 °C (Figure 1). When the reaction temperature was increased to 200 °C, no nanosheets were found, but instead, some microcubes and dendritic structures were observed. This result agrees well with our previous work.30 These observations suggest that the formation of PbTe nanosheets is subject to an appropriate temperature range. Figure 4 shows the FESEM images of the as-synthesized PbTe nanostructures prepared with different NaOH concentrations at 160 °C for 24 h with PVP added. When the concentration of NaOH was as low as 0.25 M, only PbTe cubes with sizes of dozens of nanometers were observed (Figure 4a). A large amount of PbTe nanoparticles with larger sizes were obtained when the NaOH concentration was 0.5 M (Figure 4b). With increasing NaOH concentration to 1 M, the nanosheets formed (Figure 4c). When the NaOH concentration was up to 2 M, a high yield of PbTe nanosheets was obtained (Figure 4d). The above fact agrees well with the role of NaOH that we found in the growth of PbTe hierarchical superstructures:30 The results indicate that a high NaOH concentration promotes the formation PbTe nanosheets. The study on PbSe nanosheets by Wang et al. also demonstrated a similar phenomenon.16 They suggested that a high concentration of OH- makes the release of Pb2þ easier, which can facilitate anisotropic growth between different crystallographic surfaces under nonequilibruim kinetic growth conditions with a high monomer concentration.10,11 Now we proceed to the effects of surfactants on the micromorphology. In this line, we synthesized PbTe products without PVP and with other surfactants (CTAB and SDBS) while keeping other reaction conditions unchanged. When no PVP was added, the main products were PbTe nanoparticles. This indicates that PVP is crucial for the growth of PbTe nanosheets. PVP is widely used as a surfactant. Zhang et al.31 synthesized flower-liked PbTe structures using PVP as a capping agent and concluded that the adsorption and desorption of PVP molecules on different faces of PbTe nuclei may kinetically control the growth rates along different crystal directions. As PVP was replaced by a mass equivalent of CTAB and SDBS, hierarchical structures (Figure 5b) and well-defined microcubes (Figure 5c) were obtained respectively, which further proves the important

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Figure 5. FESEM image of PbTe nanocrystals synthesized without any surfactant (a), synthesized with different surfactants: CTAB (b) and SDBS (c), and synthesized using NaBH4 as reductant with PVP as a surfactant (d).

role of PVP in the formation of PbTe nanosheets though we do not exactly understand its action mechanism. PVP may act as some face-inhibited function surfactant during the anisotropic growth of PbTe nanosheets, which can modulate the kinetics of the crystal growth and determine the subsequent shape of the crystals.10,11,32 By removing PVP from the reaction system, growth-inhibited faces disappeared and the probability of anisotropic growth could be weakened greatly. This supposition can be corroborated by the morphology of the products obtained with and without PVP while keep other conditions unchanged (Figures 4d and 5a). In addition, we used NaBH4 as a reducing agent instead of N2H4 3 H2O. Subsequently, PbTe particles with different sizes were obtained (Figure 5d). Careful scrutiny found that the bigger particles were actually agglomerations of small crystals. All in all, both the surfactant (PVP) and the reductant (N2H4 3 H2O) are indispensable to the formation of PbTe nanosheets. Evolution of Morphology and Growth Mechanisms. To further understand the formation mechanisms of the PbTe nanosheets, a time-dependent experiment was carried out at 160 °C for 3, 6, 12, and 24 h in 2 M NaOH with the presence of PVP. The evolution of the micromorphology is shown in Figure 6. At 3 h, some sheet-like aggregates can be seen among many quasi-spherical nanoparticles (Figure 6a). These initial nanosheets are formed by self-assembly of nanoparticles. With increasing the reaction time, these quasispherical nanoparticles grow into nanocubes (Figure 6b). After 12 h, the nanocubes in the nanosheets grow big enough that they border on each other closely to form flat planes (Figure 6c). Finally, extending time to 24 h, the pure nanosheets with smooth planes are obtained, which have regular shapes (Figure 6d). It is well-known that, as an fcc crystal, the shape evolution of a PbTe crystallite is determined by the ratio of the growth rate of the [100] to that of the [111].33,34 In general, the {111} planes of PbTe have higher energy than the {100} ones and hence a faster growth rate along the [111] direction than along the [100] direction, which tends to form a cubic structure. On the basis of the micromorphology evolution addressed above, it can be seen that the PbTe nanosheets are

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Figure 7. Temperature dependence of Seebeck coefficient of PbTe nanosheets measured on the film sample.

Figure 6. FESEM images of PbTe nanocrystals synthesized by a hydrothermal method at 160 °C with PVP for different time intervals: (a) 3, (b) 6, (c) 12, and (d) 24 h.

formed by the self-assembly of the PbTe nanoparticles. The main pathways of the formation of PbTe nanosheets involve the spontaneous arrangement and alignment of PbTe nanoparticles via an oriented attachment process,12,15,35,36 and finally form a single crystal upon fusion of the nanoparticles with some voids left inside the sheet (Figure 1d). A similar phenomenon was also observed in the studies of platelike silver mesocrystal37 and In(OH)3 nanosheets.38 The formation mechanism of PbTe nanosheets seems to be different from the Ostwald ripening mechanism of PbSe nanosheets suggested by Wang et al. The inherent anisotropy of crystal structure or crystal surface reactivity as identified in previous studies as the driving force for the low dimensional growth.12-14,39 However, nanosheet formation of PbTe rocksalt nanocrystals with their highly symmetric cubic lattice should be different. Dipolar interactions are possibly one of the most probable candidates for the driving force directing PbTe nanocrystals to assemble into sheets. The origin of a dipole moment for rocksalt PbSe nanocrystals with their centrosymmetric lattice has been identified for designing PbSe nanowires and nanorings through oriented attachment process by Cho et al.,15 which should also be applicable to rocksalt PbTe nanocrystals. On the other hand, a “soft template” may be formed to facilitate the formation of PbTe nanosheets due to the effect of hydrogen bonds between -NH2 in N2H4 3 H2O and OH-,40 since our result showed that no PbTe nanosheets were formed as NaBH4 was used as the reducing agent instead of N2H4 3 H2O. PbTe nucleates and attaches to each other among this 2D network array template, grows into big cubes by Ostwald ripening,41,42 and finally forms a single crystal sheet by fusion of nanocrystals. Meanwhile, PVP also plays an important role during the process, as discussed above. The products synthesized at different temperatures (Figure 3) further support our argument: high reaction temperature tends to decompose the “soft template”. Thermoelectric Property of PbTe Nanosheets. PbTe is one of best intermediate temperature thermoelectric materials. The thermoelectric properties of nanostructured PbTe have been extensively studied.20,22,43 In this work, the Seebeck coefficient of the thin film of the PbTe nanosheets was measured in the temperature range of 300-650 K. As shown in Figure 7, the Seebeck coefficient is very small and has a low value of about 20 μV 3 K-1 at room temperature. The Seebeck

coefficient rises dramatically after 500 K and reaches the highest value of 120 μV 3 K-1 at 650 K. The measured Seebeck coefficient values over the whole temperature range are lower than those of PbTe nanowire film by Yan et al.,20,22 possibly because it was the in-plane Seebeck coefficient of PbTe nanosheets that was measured in this work and hence no obvious nanoscale effects were observed. On the other hand, since the PbTe nanosheets were grown in a NaOH solution, the nanostructure may include a high concentration Na, resulting in the high hole concentration and hence the small Seebeck coefficient at room temperature. With increasing temperature, the Na impurity included in the nanostructure may be decreased. Conclusions In summary, PbTe nanosheets have been synthesized via an alkaline hydrothermal method. These nanosheets are of 20-80 nm in thickness and 0.2-5 μm in-plane sizes. Reaction time and temperature, concentration of NaOH, and kinds of surfactants and reductants play important roles in the growth of PbTe nanocrystals. PVP, NaOH, and N2H4 3 H2O are essential for the formation of PbTe nanosheets. The growth of PbTe nanosheets involve the spontaneous arrangement and alignment of PbTe nanoparticles via an oriented attachment process, and they finally form a single crystal upon fusion of the nanoparticles. The growth mechanism should also be extendable to nanosheet formation of other cubic materials systems with centrosymmetric lattice. The Seebeck coefficient was measured on the film sample of PbTe nanosheets and showed a maximum Seebeck coefficient value of 120 μV 3 K-1 at 650 K. Acknowledgment. The work is financially supported at Zhejiang University by the Natural Science Foundation of China (50731006 and 50971115) and the National “973” Basic Research Program (2007CB607502), at Clemson University by the DOE/EPSCoR Implementation Grant (#DE-FG0204ER-46139) and the SC EPSCoR cost sharing program.

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