Polyol-Mediated Synthesis of PbS Crystals: Shape Evolution and

Polyol-Mediated Synthesis of PbS Crystals: Shape Evolution and Growth Mechanism Zewei Quan, Chunxia Li, Xiaoming Zhang, Jun Yang, Piaoping Yang, Cuimiao Zhang, and Jun Lin*

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2384–2392

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed December 17, 2007; ReVised Manuscript ReceiVed March 4, 2008

ABSTRACT: An interesting shape evolution of PbS crystals, that is, from cubes to (truncated) octahedra and finally to stable star-shaped multipods with six arms along the directions is first realized via a facile polyol-mediated reaction between lead acetate and sulfur powder in the absence of surfactants. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Fourier transform infrared (FT-IR) techniques were employed to characterize the samples. We elucidate the important parameters (including reaction temperature and sulfur sources) responsible for the shapecontrolled synthesis of PbS crystals. The possible formation mechanism for products with various architectures are presented, which is mainly based on the variation of the ratio (R) of the growth rates along the direction and direction. In addition, the effect of the diffusion-controlled branching growth on the formation of the multiarmed structure is also taken into account. This polyol-mediated method should be readily extended to the controlled synthesis of other metal chalcogenides and the proposed growth model could also be used to explain and direct the growth of crystals with a face-centered cubic (fcc) structure.

1. Introduction The architectural control of nano- and microcrystals with welldefined shapes remains an important goal of modern materials chemistry because of the importance of the shape and textures of materials in determining their widely varying properties.1 Although there has been an increasing number of excellent studies on novel nano- and microstructured materials with various shapes, such as low-dimensional structures (e.g., rods,2 wires,3 belts,4 tubes,5 cubes6) and hierarchical structures (e.g., dendrites,7 branches,8 urchins,9 networks10), the ability to understand and predict the final structures is still limited. If we could understand the growth mechanism and the shape-guiding process, it will be possible for us to program the system to yield the crystals with desired shape and crystallinity.11 Lead sulfide (PbS), as an important π-π semiconductor material with a small band gap (0.41 eV) and a large exciton Bohr radius (18 nm),12 has aroused intensive interest due to its wide potential applications in many fields such as sensors,13 photography,14 IR detectors,15 and solar absorbers.16 Notably, the exceptional third-order nonlinear optical property of PbS nanoparticles renders them highly desirable for photonic and optical switching device applications.17 Additionally, the availability of PbS crystals with well-defined morphologies should be able to bring in new types of applications or to enhance the performance of the currently existing devices. Until recently, various synthetic approaches have been successfully employed to prepare PbS nano- and microstructured materials so far.11,18 Among them, the liquid synthesis is an especially powerful tool for the convenient and reproducible shape-controlled synthesis of nano- and microcrystals. Note that various surfactants acting as “soft templates” are usually adopted in these reaction systems, and they all play critical roles in the morphological control of PbS crystals. However, it is known that the use of surfactants will inevitably increase the reaction complexity, cause impurity in the products, and is disadvantageous from the viewpoint of * Corresponding author. E-mail: [email protected].

green chemistry. Thus, the development of a facile, effective, and surfactant-free approach for the controlled preparation of PbS crystals is highly desirable for the detailed investigation of its growth behavior and the potential large-scale production. Recently, the polyol method has been demonstrated to be a promising route for the preparation of nano- and microscale materials and is considered to be a relatively green chemical alternative of practical significance.19 This process is a type of liquid-phase synthesis method that results in precipitation while heating suitable precursors in a high boiling alcohol.20 Our group has demonstrated that the polyol synthesis of a series of nanoand microstructured materials, including CeF3, ZnO, LaPO4, and CaWO4, is readily achieved.21 These results have inspired us to extend this polyol method to prepare other kinds of functional materials. In this paper, we present a polyol route for the shapecontrolled synthesis of PbS crystals through the direct reaction between lead acetate and sulfur powder. The alcohol involved in this system is diethylene glycol (DEG), considering its many intrinsic merits.19a First of all, due to the strong reduction ability of DEG, the environmentally benign and cheap sulfur sources (i.e., sulfur powder) can be easily converted to S2-, and thus this reaction can be readily performed at a much lower temperature (above 80 °C) compared to that required through the gas-phase route (about 650 °C).22 Second, the selective interaction of DEG with PbS surfaces can be used to alter the surface energies of different facets and thus to control the final morphologies. Third, DEG exhibits a high boiling point of 246 °C and consequently allows us to achieve kinetic control of the growth processes through careful regulation of reaction temperature, and therefore well-crystallized PbS crystals with various morphologies can be realized. At last, one-pot synthesis, short reaction time, and low toxicity can make it ideal for largescale production. In our system, an interesting shape evolution of PbS crystasls, that is, from cubes to (truncated) octahedra and finally to stable star-shaped multipods with six arms along the directions was first realized by only adjusting the reaction temperature,

10.1021/cg701236v CCC: $40.75  2008 American Chemical Society Published on Web 06/03/2008

Polyol-Mediated Synthesis of PbS Crystals

Crystal Growth & Design, Vol. 8, No. 7, 2008 2385

Table 1. Sample Denotations and Their Corresponding Detailed Experimental Conditions and Final Morphologies sample

S/Pb ratio

T (°C)

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12

3:1 3:1 3:1 3:1 3:1 3:1 3:1 2:1 1:1 1:2 3:1 3:1

80 120 160 200 240 240 240 240 240 240 240 240

1 1 1 1 1 1 1 1 1 1 0 1

h h h h h h h h h h min min

S powder S powder S powder S powder S powder thiourea Na2S S powder S powder S powder S powder S powder

S13 S14

3:1 3:1

240 240

3 min 5 min

S powder S powder

time

morphology characteristics

S source

cubes truncated octahedra (truncated) octahedra star-shaped multipods star-shaped multipods star-shaped multipods cubes star-shaped multipods star-shaped multipods irregular morphologies spherical particles spherical particles and octahedra octahedra star-shaped multipods

and the corresponding growth mechanism based on the selective interaction of DEG with {111} facets is proposed to explain this shape evolution. In addition, the formation of star-shaped multipods is closely related to the diffusion-controlled growth. Moreover, the formation process of star-shaped multipods has been investigated through the shape evolution at different intervals. Finally, the effect of several experimental factors including different sulfur sources and a molar ratio of [Pb2+]/ [S2-] on the final morphologies of PbS crystals is explored in detail.

2. Experimental Section Synthesis. All the reagents were purchased from Beijing Chemical Reagent Co. and used as received. In a typical experiment, lead acetate (2 mmol), sulfur powder (6 mmol), and DEG (25 mL) were mixed together in a three-necked vessel equipped with a condenser at room temperature. Then the mixture was rapidly heated to the desired temperatures and held for 1 h under continuous vigorous stirring and nitrogen atmosphere. After the sample was cooled to room temperature naturally, carbon bisulfide was added to the reaction vessel to dissolve the unreacted sulfur powder, and then the product was collected by centrifugation and washed repeatedly with ethanol. The obtained products were stored in ethanol for further characterization. The morphologies of final products together with the corresponding detailed experimental conditions are denoted as S1-S14, which are listed in Table 1. It should be noted that samples S11-S14 were prepared through this procedure: 2 mL of solution was drawn out from the reaction vessel as soon as the solution turned black (t ) 0), and finally the resultant sample was denoted as S11. Samples S12-S14 was harvested at the following different intervals (t ) 1, 3, and 5 min). Characterization. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer with graphite monochromatized Cu KR radiation (λ ) 0.15405 nm). SEM micrographs were obtained using a field emission scanning electron microscope (FE-SEM, XL 30, Philips). Transmission electron microscopy (TEM) was performed using FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV and images were acquired digitally on a Gatan multiople CCD camera. Samples for TEM were prepared by depositing a drop of samples dispersed in ethanol onto a carbon grid. The excess liquid was wicked away with a filter paper, and the grid was dried at 70 °C for 1 h in a vacuum dryer. Fourier transform infrared (FT-IR) spectrum was measured with Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. All the measurements were performed at room temperature.

3. Results and Discussion 3.1. Phase Formation. The composition and phase purity of the products were first examined by XRD, and the results reveal that pure PbS was obtained in all samples (S1-S14). Figure 1 displays the representative XRD patterns of the star-

Figure 1. XRD patterns of samples S5 (a) and S7 (b), as well as the standard data for PbS (JCPDS 05-0592).

shaped multipods (S5, a) and cubes (S7, b) as well as the standard card (JCPDS No. 05-0592), indicating that the asprepared samples present a well-defined fcc structure (space group: Fm3m, a ) 0.5936 nm). The strong and sharp peaks indicate that the obtained PbS crystals are highly crystalline. Meanwhile, we observe that the intensity ratio between the (111) and (200) diffraction peaks is higher than the literature value (1.21 versus 0.84) for star-shaped multipods (S5), whereas the value (0.61) for cubes (S7) is lower. These results indicate that star-shaped multipods are abundant in {111} facets, whereas {100} facets should dominate cubes. It has been known that the facets with a slower growth rate will be exposed more on the crystal surface and consequently exhibit relatively stronger diffraction intensity in the corresponding XRD patterns.23 Therefore, it can be concluded that the relatively slower growth rate for star-shaped multipods and cubes of PbS are {111} and {100} facets, respectively. These results are confirmed by the following discussions. 3.2. Morphologies and Growth Mechanism of PbS. 3.2.1. Morphology Results. Generally, the growth process of crystals is a kinetically and thermodynamically controlled process that can form different shapes with some degree of shape tenability through changes in the reaction parameters. In our system, we focus on the effects of reaction temperature, kinds of precursors, and the reactant ratios on the final shapes of PbS crystals. Effects of Reaction Temperature. In the present system, as soon as the solution was increased to temperatures above 80 °C, the solution would immediately turn black, indicating the formation of PbS. Therefore, to investigate the morphological evolution process of the PbS crystals, the reaction temperature was changed from 80 to 120, 160, 200, and 240 °C under otherwise the same experimental conditions; a series of PbS morphologies including cubes (Figure 2a), (truncated) octahedra (Figure 2b,c), and star-shaped multipods (Figure 2d,e) were obtained. Figure 2 clearly shows the shape evolution process of the products (S1-S5) by varying the reaction temperature. Figure 2a shows the SEM image of S1 prepared at 80 °C, indicating the exclusive formation of PbS cubes, of which the edge lengths range from 200 to 800 nm. When the temperature was elevated to 120 °C, truncated octahedra with an average edge length of 700 nm were obtained in S2 (Figure 2b). Through the magnified SEM image of an individual truncated octahedron (inset of Figure 2b), we can observe that it is mainly composed of eight smooth surfaces accompanied by six deeply truncated corners, and the overall structure shares 24 identical edges in a mecon way. Increasing the temperature to 160 °C, the obtained products (Figure 2c) are octahedra with slightly truncated corners and even perfect octahedra begin to appear in S3. A large

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Figure 2. SEM images of samples S1 (a), S2 (b), S3 (c), S4 (d), and S5 (e). (f) is the SEM image of an individual star-shaped multipods (sample S5).

amount of star-shaped multipods are observed in S4 when the reaction temperature was elevated at 200 °C (Figure 2d). Through careful examination of the structure of a single particle (inset of Figure 2d), it is clearly shown that six symmetric rods along the directions are in prism shape with square cross-section and grow on the center of six {100} facets of the truncated cubes, and the average edge length of the prism rods and truncated cubes are estimated to be 500 and 900 nm, respectively. Finally, the presence of well-defined star-shaped multipods with an octagonal cross-section is observed in S5 when the temperature reached 240 °C (Figure 2e). The typical SEM image of an individual PbS multipods (Figure 2f) with three pods standing on the silicon substrate shows a clear view of its three-dimensional (3D) structure with six symmetric arms that grow along the directions, and the diameter of the edge length of square cross sections is about 500 nm. It is worth noting that the as-obtained star-shaped PbS multipods will not disband under irradiation of strong ultrasonic wave, indicating that this morphology is very stable. From the above results, we reasonably believe that shape-controlled synthesis of PbS crystals can be readily achieved through the delicate control of the reaction temperature in our case.

The star-shaped PbS multipods were further characterized by transmission electron microscopy (TEM) technique. A typical TEM image of an individual PbS crystal together with the corresponding selected area electron diffraction (SAED) pattern is shown in Figure 3a. The SAED pattern (inset in Figure 3a) taken from the arm can be indexed to the fcc phase of PbS viewed along the [110] zone axis, and all the patterns taken from different parts of the arm are almost identical, indicative of their single crystal nature. This can also be proven by the high-resolution transmission electron microscopy (HRTEM) image taken from the arm (marked with a square area), which is shown in Figure 3b. Lattice resolved fringes with a constant spacing of 0.302 nm ascribed to the (200) planes of PbS can be clearly observed, indicating that the crystal growth of the arm is preferential in the direction. Effects of Sulfur Sources. To explore the influence of sulfur sources on the morphologies of final PbS products under otherwise same reaction conditions, thiourea and Na2S were used as sulfur sources instead of sulfur powder. When thiourea was adopted, a large amount of six-arm star-shaped PbS crystals with the arm length about 800 nm was present in S6 (Figure 4a). Careful examination of the top-view SEM image of an



whole parts of multipods. When the ratio was lowered to 1/2, besides the presence of star-shaped PbS multipods, large quantities of other morphologies, such as dendrites, spheres, and triangular plates, etc., appear in the final products (S10, Figure 7). One may be led to believe that excess S2- is favorable for the formation of star-shaped PbS crystals with smooth surfaces. 3.2.2. Growth Mechanism. In a typical synthesis, based on the reduction mechanism of polyols,24 the formation of PbS crystals can be described by the following three steps:

HO - C4H8O - OH f 2CH3CHO + H2O Figure 3. TEM image (a) and HRTEM image (b) of an individual starshaped PbS multipods of sample S5. Note that HRTEM image and SAED pattern (inset of a) are both obtained from the area marked with square in the arm.

individual PbS crystal (Figure 4b) reveals that each arm appears like a tetrahedron with four obvious concave corners at the bottom. However, large amounts of PbS cubes with edge length of about 100 nm are obtained in S7 (Figure 4c,d), when Na2S was used as the sulfur source. Above results demonstrate the strong dependence of final PbS morphology on the kinds of sulfur sources. Effects of Reactant Ratios. Comparative experiments were carried out, in which other parameters were kept constant, to investigate the influence of the reactant ratios of S2- to Pb2+ on the formation of star-shaped PbS crystals. The morphologies of the corresponding products are shown in Figure 5. When the ratio was decreased to 2, the obtained products (S8) show similar morphologies, that is, star-shaped mutlipods with smooth surface (Figure 5a,b). As for S9, it is obvious that a large amount of concave parts can be observed in all the arms of the starshaped multipods (Figure 5c,d), when an equal amount of Pb2+ and S2- was employed. However, through the elemental analysis of the obtained crystals (S9) by SEM (Figure 6), we can also observe that both Pb and S elements evenly distribute in the

Figure 4. SEM images of samples S6 (a, b) and S7 (c, d).

(1)

2S + 2CH3CHO f CH3CO - OCCH3 + 2S2- + 2H+ (2) Pb2+ + S2- f PbS

(3)

Acetaldehyde is produced by the dehydration of DEG at elevated temperatures, where the acetaldehyde can donate a hydrogen atom and act as a reducing agent to convert sulfur powder to S2-, as shown in reactions 1 and 2. Then it reacts with Pb2+ released from lead acetate to form PbS nuclei, as shown in reaction 3. It is widely accepted that formation of crystals is mainly achieved through two stages: nucleation and growth. In the solution-phase synthesis of nanocrystals, “nucleation” is generally referred to the formation of seeds with a stable structure and well-defined crystallinity, and the shape of seeds is primarily determined by the minimization of surface energy.23 As dictated by thermodynamics (i.e., Wulff construction), the PbS nuclei in the vacuum are expected to nucleate and grow into cuboctahedral or quasi-spherical seeds enclosed by a mix of {111} and {100} facets to minimize the total surface energy.23 It has also been demonstrated by Cheon et al. rock-salt phase (fcc) PbS crystals generally nucleate as tetradecahedron seeds, exposing six {100} facets and eight {111} facets,18k as shown


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Figure 5. SEM images of samples S8 (a, b) and S9 (c, d).

Figure 6. Element analysis of sample S9.

in Figure 8 (left). Subsequent competitive growth on these two different types of crystalline facets of the as-formed seeds (i.e., the growth rates of two facets) determines the final shape. As previously illustrated by Wang,25 the shape of an fcc nanocrystal is mainly determined by the ratio (R) between the growth rates along the and directions. Cubes bounded by six equivalent {100} planes will be formed when R ) 0.58, and octahedra bounded by the eight equivalent {111} planes will result if R is increased to 1.73. For the truncated octahedra, the ratio R should have a value close to 1.15. Generally, a basic crystal shape is determined by two growth process: habit formation and branching growth. The former is determined by the relative order of surface energies of crystallographic planes of a crystal, while the later is determined by a diffusion effect.26–29 It is well-known that surface energies associated with different crystallographic planes are usually different, and the growth rates on different facets are dominated by the surface energy. In a solution-phase process, impurities or capping agents are usually adopted to alter the surface free energies via adsorption or chemical interaction and thus induce new shapes.23 For example, as for the synthesis of fcc structured Pd, poly(vinyl pyrrolidone) (PVP) is a capping polymer, of which the oxygen (and/or nitrogen) atoms bind most strongly to the {100} facets of Pd, and thus efficiently lower the surface energy of {100} facets, leading to the formation of nanocubes with truncated corners mainly bounded by six {100}facets.30 As for the synthesis of fcc structured PbS, it has been demonstrated that the surface energy of the {111} facets of PbS

can be selectively lowered relative to that of the {100} facets in the presence of dodecanethiol that can bind strongly to {111} facets via a the µ3-Pb3-SR bridging mode.18i In the present system, besides one function of reducing sulfur powder, another function of DEG molecule is that it can effectively interact with {111} facets of PbS similar to dodecanthiol to alter the relative order of surface free energies for different facets. Alternatively, branching growth usually plays an important role in the formation of dendrites or multiarmed structures of facetted crystals.31 As a crystal grows, ions or molecules near the surface are consumed and a concentric diffusion field forms around the crystal. This makes the apexes of a polyhedral crystal, which protrude further into regions of higher concentration within the diffusion layer present around each nanocrystal, grow faster than the central parts of facets, and eventually deplete further the concentration of monomers close to those regions, therefore suppressing further their growth rate.28,29 As a consequence, branches can start forming out the initial crystal through development of new set of facets. This process can be selfsustained, because fast growth of these branches can push them into regions of even higher concentrations of monomers with respect to the remaining regions of the nanocrystal surface, therefore contributing to increase their growth further. During the crystal growth, preference for habit formation and branching growth can be altered by growth conditions such as concentration and temperature that amplify or minimize the diffusion effect. In the present system, since the intrinsic surface energy of {111} facets is higher than that of the {100} facets for PbS crystals,18a relatively fast growth along eight equivalent directions (R ) 0.58) from the tetradecahedron seeds under conditions of low temperature (80 °C) results in the formation of cube-shaped nanocrystals, a shape more favorable from the viewpoint of thermodynamics. This is because the interaction strength of DEG with {111} facets at low temperature is relatively weak, and the effects on the surface energies of PbS



Figure 7. SEM image of sample S10.

Figure 8. Schematic illustration of the shape evolution process of PbS crystals as a function of the reaction temperature.

crystals can be ignored. When the reaction temperature was increased to 120 °C, this interaction strength was greatly enhanced, and it could efficiently lower the surface energies of {111} facets, which thus would block the growth on the {111} facets and facilitate the growth on the {100} facets. The obviously different changes of the growth rates on the {100} and {111} facets possibly induced the ratio R to have a value close to 1.15, leading to the formation of the truncated octahedral PbS crystals. Further enhancement in the reaction temperature to 160 °C, truncated corners ascribed to {100} facets began to disappear gradually, and even perfect octahedra (R ) 1.73) were obtained due to the further increase of the growth rate on {100} facets relative to the {111} facets. According to this tendency, we can speculate that truncated cubes and other kinds of truncated octahedra (0.58 < R < 1.15) as intermediate products can be achieved through the regulation of the reaction temperature in the range of 80-120 °C. Furthermore, in our system, it is worth noting that when the temperature is further elevated to above 200 °C, the greatly enhanced growth rate on the {100} facets induces the ratio R to have a value of much more than 1.73, and thus the rapid anisotropic growth of the six equivalent {100} facets into the

six prominent prisms, resulting in a six-armed structure. It can be understood based on the following argument: the very strong interaction of DEG with {111} facets due to the higher reaction temperature (>200 °C) can sufficiently prevent further addition of PbS nuclei from solution to the {111} facets of PbS seeds, and thus the {100} facets will become the most favorable and unique sites for the addition of PbS nuclei. This preferential growth at six equivalent {100} facets eventually leads to the formation of six-armed structures on the truncated octahedra (i.e., star-shaped multipods). Furthermore, by fine-tuning the reaction temperature, we can selectively generate two kinds of star-shaped multipods (denoted as only one structure in Figure 8): one structure with square cross-section and enclosed by {100} facets (200 °C, S4), and the other structure with an octagonal cross-section whose side surface is bounded by a mix of {100} and {110} facets (240 °C, S5). The whole shape evolution of PbS crystals from cubes to (truncated) octahedra and finally star-shaped multipods as a function of the reaction temperature is schematically illustrated in Figure 8. The change of interaction strength as a function of the reaction temperature can to some extent be proven by the FT-IR spectra of the samples (S1-S5), as shown in Figure 9a. The FT-IR spectra of samples (S2-S5) display two characteristic peaks of DEG: one is at 1165 cm-1 arising from the stretching vibration of C-O-C band in the vinyl ether and the other peak is at 1061 cm-1 attributing to the stretching vibration of C-O(H) band,32 which proves the presence of DEG in these samples after extensive washing with ethanol and water, indicative of the interaction between DEG and the crystal surface in theses samples. The peak intensity of the FT-IR spectrum can reflect the amount of corresponding molecules (i.e., DEG in our system) in the samples, and thus can indicate the interaction strength of DEG with crystal surface under otherwise the same conditions. As for the FT-IR spectrum of sample S1, there are no obvious characteristic peaks of DEG, illustrating the relatively weak interaction strength at relatively low temperature. Along with the increase of the reaction temperature, two characteristic FT-IR peaks of DEG are present, and their intensities increase gradually and reach the maximum at 200 °C, indicative of the increase of the interaction strength in the region of 80-200 °C (samples S1-S4). However, when the temperature is elevated to 240 °C, this interaction strength begins


Figure 9. FT-IR spectra (a) of samples (S1-S5) and EDS (b) of sample S5.

to decrease, which is observed from the decrease of the intensity of characteristic peaks of DEG in the FT-IR spectrum of sample S5. As stated above, the increase of this interaction strength between the DEG and {111} facets directly results in the increase of the growth rate on {100} facets relative to the {111} facets, therefore leading to the formation of different forms of PbS crystals with an increased ratio R. On the other hand, when the reaction temperature is raised, the effect of diffusioncontrolled branching growth becomes prominent for facetted PbS crystal. In particular, when the reaction temperature reaches above 200 °C, this effect may dominate the growth process, therefore resulting in the formation of multiarmed structures, even though the interaction strength was a little reduced at 240 °C. Additionally, the presence of DEG on the surface of PbS crystals was also confirmed by the EDS analyses. Figure 9b shows the EDS spectrum taken from sample S5 after extensive washing with ethanol and denionized water. The presence of carbon and oxygen signals implies the interaction of DEG with PbS. Previously, Cheon et al. demonstrated an inverse shape evolution of PbS crystals, that is, from multipods and star-shaped structures to truncated octahedra and cubes along with the increased temperature, and they explained these results mainly based on the shift from the kinetic growth to thermodynamic growth regimes.18a Many other groups have also reported that the most thermodynamically favorable form of PbS should be cubelike structures mainly bounded by six equivalent {100} facets.18 However, through the selective interaction of DEG with PbS surfaces, the formation of star-shaped multipods is favored at elevated temperatures, and thus it will be the most stable

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form under this condition in our system. It is worth noting that the use of the DEG to control the shape of crystals is a thermodynamic means, because it makes some facets thermodynamically favorable by reducing their surface energies through chemical interaction. To investigate the formation process of stable star-shaped multipods, samples (S11-S14) at different intervals were prepared under otherwise the same experimental conditions with S5. It should be noted that sample S11 (t ) 0) was harvested as soon as the solution turned black, and samples S12-S14 were obtained at different reaction times (t ) 1, 3, and 5 min), and the corresponding SEM images are shown in Figure 10. The crystal growth is a completed process. Generally, the intermediates obtained at different reaction intervals are be used to shed light on the growth mechanism of the crystals. So far, this method is widely used to study the morphological formation of various kinds of crystals under different reaction conditions.33 It reveals that uniform spherical particles with an average size of 300 nm are present in sample S11 (Figure 10a), and rough and concaved surfaces can be clearly observed at a higher magnification (Figure 10b). When the reaction proceeded 1 min, certain amounts of stars that exhibit a well-defined star-shaped geometry with six symmetrical horns (about 600 nm) began to appear in the sample S12 (Figure 10c,d). As the reaction was extended to 3 min, spherical particles disappeared completely, and only stars existed in sample S13 (Figure 10e,f). It is to be noted that when the sample S13 was drawn out from the reaction vessel, its temperature reached about 160 °C. As stated above, the sample S2 prepared under this temperature is octahedra, which can be regarded as the stars with depressed portion of the stars being filled. However, along with a further increase of reaction time to 5 min, the reaction temperature was changed from 160 to 240 °C, and finally star-shaped multipods with an average arm length of 2 µm dominated the morphology of sample S14 (Figure 10g,h), and little amounts of stars can still be observed as marked with a square. It demonstrates that the transformation from stars to star-shaped multipods occurs via the rapid addition of PbS species, whereas the slow addition of PbS species kept at 160 °C will result in the formation of octahedra. On the basis of the above results, the shapes of PbS crystals evolve from spherical particles through stars as a transient species to star-shaped multipods within 5 min, and thus it can be concluded the higher reaction temperature is preferable for the growth of {100} facets and as a consequence the formation of star-shaped multipods. This above growth model can also be used to explain the sulfur sources related effects on the final morphologies of PbS crystals. Once Na2S is introduced into the DEG solution, it dissociates into Na+ and S2- immediately, and therefore the formation of PbS crystals occurs rapidly at lower temperature. For sulfur powder and thiourea as sulfur sources, the release of S2- only occurs when the reaction temperature reaches a certain value to make sulfur sources to produce S2-, and thus the nucleation and growth of PbS crystals are mainly completed at relatively higher temperatures. On the basis of the above analysis, we can conclude that the obvious difference in the reaction temperature can cause the different growth patterns of PbS crystals: at lower temperature, the growth on the facets is favored and thus the formation of cubes is achieved; however, under conditions of higher reaction temperature, the considerably enhanced growth rate of facets to that of facets and diffusion-controlled branching growth induce the formation of six-arm star-shaped PbS crystals. These results indicate that the controlled release of S2- is critical for the



Figure 10. SEM images of samples S11 (a, b), S12 (c, d), S13 (e, f), and S14 (g, h).

formation of star-shaped PbS crystals, proving that sulfur sources play an important role in the final morphologies of PbS crystals.

4. Conclusions In this paper, we present a facile and effective route for the shape-controlled synthesis of PbS crystals through the reaction between the lead acetate and sulfur powder by virtue of its strong reduction and selective interaction abilities of DEG. A series of morphologies, such as cubes, truncated octahedra, and starshaped multipods with six arms along the directions,

are easily obtained through the delicate manipulation of the reaction temperature and sulfur sources. On the basis of relatively clear understanding of shape evolution and corresponding growth mechanism, this polyol-mediated approach can be readily extended to the controlled synthesis of other fcc structured crystals. Acknowledgment. This work is financially supported by the “Bairen Jihua” of Chinese Academy of Sciences, the National Natural Science Foundation of China (50572103,


20431030, 00610227) and the MOST of China (Nos. 2003CB314707, 2007CB935502).

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