Shape-Controlled Synthesis of PbS Nanocrystals via a Simple One

Article pubs.acs.org/Langmuir

Shape-Controlled Synthesis of PbS Nanocrystals via a Simple OneStep Process Yu Wang,† Aiwei Tang,*,†,‡ Kai Li,§ Chunhe Yang,† Miao Wang,† Haihang Ye,† Yanbing Hou,‡ and Feng Teng*,‡ †

Department of Chemistry and ‡Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing JiaoTong University, Beijing 100044, PR China § Department of Chemistry, Capital Normal University, Beijing 100048, PR China S Supporting Information *

ABSTRACT: A one-step colloidal process was adopted to prepare face-centered-cubic PbS nanocrystals with different shapes such as octahedral, starlike, cubic, truncated octahedral, and truncated cubic. The features of this approach avoid the presynthesis of any organometallic precursor and the injection of a toxic phosphine agent. A layered intermediate compound (lead thiolate) forms in the initial stage of the reaction, which effectively acts as the precursor to decompose into the PbS nanocrystals. The size and shape of the PbS nanocrystals can be easily controlled by varying the reaction time, the reactant concentrations, the reaction temperatures, and the amount of surfactants. In particular, additional surfactants other than dodecanethiol, such as oleylamine, oleic acid, and octadecene, play an important role in the shape control of the products. The possible formation mechanism for the PbS nanocrystals with various shapes is presented on the basis of the different growth directions of the nanocrystals with the assistance of the different surfactants. This method provides a facile, low-cost, highly reproducible process for the synthesis of PbS nanocrystals that may have potential applications in the fabrication of photovoltaic devices and photodetectors.

1. INTRODUCTION Over the past more than two decades, semiconductor nanocrystals have received a great deal of attention owing to their interesting size-, phase-, and shape-dependent optical and electrical properties.1,2 As an important type of direct band gap semiconductor nanocrystals, lead sulfide (PbS) nanocrystals have been intensively studied because of their narrow band gap (0.41 eV) and large exciton Bohr radius (18 nm).3 These distinguished characteristics make PbS nanocrystals widely used in near-infrared (NIR) communication, thermal and biological imaging, infrared light-emitting devices, and infrared photodetectors and photovoltaics.4−10 Because there has been an increasing demand for applications in various optoelectronic and biological fields for PbS nanocrystals, different synthesis approaches have been developed to control the morphology and size of PbS nanocrystals precisely with well-defined shapes including spheres, rods, stars, flowers, cubes, wires, octahedrons, and so on.3,11−30 Among these obtained differently shaped PbS nanocrystals, the overall size of some samples are relatively large (40−100 nm), and quantum-confinement effects are often not expected.11−16 However, quantum-confinement effects can be observed near the edges of the nanocrystals because the size is less than or comparable to the PbS Bohr radius.3 In addition, the larger nanocrystals are efficient for charge transport in the optoelectronic devices, and the nanocrystals can span the entire device thickness to form a built-in percolation pathway for charge transport.31−33 More© 2012 American Chemical Society

over, larger nanocrystals have good crystallinity and lower surface energy, which are important issues for studying the shape-controlled synthesis and self-assembly of PbS nanocrystals. Conventionally, high-quality PbS nanocrystals are prepared by a hot-injection method, which has also shown surprising success in tuning the shape and size of PbS nanocrystals with a narrow size distribution.20,21 In this method, however, hazardous phosphine agents such as trioctylphosphine oxide (TOPO) and tributylphosphine (TBP) are often used as organic solvents and capping agents.22 Because of the highly reactive agents used in the hot-injection method, air-free manipulation and fast injection rates are required, which limit large-scale production in industrialization.19 A single-source precursor approach was reported to synthesize differently shaped PbS nanocrystals by the thermal decomposition of a molecular precursor in a hot phenyl ether solvent at a variety of reaction temperatures.11 Xue et al. also prepared cube-shaped PbS nanocrystals by virtue of a solvothermal single-source method under mild reaction conditions.14 Nevertheless, such a single-source method adds extra steps for the synthesis of special lead precursors. Therefore, it is still a major challenge to develop a simple, Received: September 18, 2012 Revised: November 4, 2012 Published: November 4, 2012 16436

dx.doi.org/10.1021/la303738u | Langmuir 2012, 28, 16436−16443

Langmuir

Article

low-cost, environmentally benign approach for the controllable synthesis of differently shaped PbS nanocrystals. In this article, we report a simple one-step route to the preparation of PbS nanocrystals with precise control of both the size and shape at high temperature. This process was designed by directly heating metal salts in n-dodecanethiol without any precursor injection. A layered intermediate lead thiolate compound was formed in the initial stage of the reaction, which then decomposed into PbS species upon heating, eventually leading to the nucleation and growth of PbS nanocrystals. The size and shape of the PbS nanocrystals could be easily tailored by tuning the reaction conditions, such as the reaction time, reaction temperatures, and different surfactants. Because of the preferential adsorption with specific crystal facets for different surfactants, the growth kinetics of PbS nanocrystals was influenced significantly, which considerably influenced the morphology of the products. With the assistance of the various surfactants, diversely shaped PbS nanocrystals, such as octahedral, star-shaped, cubic, and truncated octahedral, have been successfully obtained.

Table 1. Experimental Conditions for Synthesis of Differently Shaped PbS Nanocrystals and the Corresponding Results

2. EXPERIMENTAL SECTION

(XRD) was performed using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). The Fourier transform infrared spectroscopy (FTIR) spectrum was measured using a Varian Excalibur 3100 spectrometer. Thermogravimetric analysis (TGA) was taken on a Netzsch TG209 F3 instrument.

Chemicals. Lead acetate (Pb(OAc)2·3H2O, 99%), 1-oleic acid (OA, 85%), and 1-oleylamine (OLA, 90%) were purchased from Aladdin Reagent Company; 1-octadecene (ODE, 90%) was obtained from Sigma-Aldrich; and n-dodecanethiol (DDT, 98%) was obtained from Beijing Sinopharm Chemical Reagent Company. Other solvents including ethanol and chloroform were commercially available analytical-grade products, which were purchased from Beijing Chemical Reagent Company. All chemicals were used in our experiments as received without any further purification. Synthesis of PbS Nanocrystals. In a typical experiment, 1.14 g (3 mmol) of Pb(OAc)2·3H2O and 20 mL of DDT were mixed in a three-necked flask at room temperature under magnetic stirring and then degassed with nitrogen gas for about 20 min. Afterwards, the reaction mixture was heated to the desired temperature and continuously vigorously stirred and kept under a nitrogen atmosphere for a fixed time. Then the reaction was terminated by naturally cooling to room temperature after the removal of the heating equipment, and ethanol was added to the resultant solution to precipitate nanocrystals, which were subsequently collected by centrifugation at 6000 rpm for 10 min. After centrifugation, the precipitates were washed using chloroform to remove residual precursor and surfactant. In general, the redispersion/centrifugation procedures were repeated three times. Finally, the as-obtained products were dispersed in chloroform or dried in vacuum for further characterization. The morphologies of the final products together with the corresponding detailed experimental conditions are denoted as samples A−I, which are summarized in Table 1. It should be noted that samples A−I with different morphologies were achieved by controlling the amount of surfactants (such as ODE, OA, and OLA) and the reaction temperatures. The assynthesized PbS nanocrystals under different reaction conditions can be well dispersed in chloroform to form colloidal solutions, and the corresponding digital photographs are shown in Figure S1 in the Supporting Information. However, the colloidal solutions are not stable, and the nanocrystals can precipitate on the bottom when they are kept for several hours because of relatively larger particle sizes. Most of them exhibit a black color at room temperature, but two samples display green colors that may result from the incomplete decomposition of the lead thiolate compound. Characterization. Transmission electron microscopy (TEM) images were recorded on a Hitachi-7650 electron microscope at an acceleration voltage of 100 kV. High-resolution TEM images and the selected-area electron diffraction (SAED) patterns were taken on a JEM-2010 at an acceleration voltage of 200 kV. Scanning electron micrographs (SEM) were obtained using an S4800 field-emission scanning electron microscope (FE-SEM). Powder X-ray diffraction

samples

Pb(OAc)2 (mmol)

DDT

surfactant

A B C

3 1 l

20 20 12

D

1

5

E

l

1.5

F

1

10

G

l

5

H

1

5

I

l

5

N/A N/A 8 mL ODE 15 mL ODE 18.5 mL ODE 10 mL OA 15 mL OA 15 mL OLA 15 mL OLA

reaction temperature (°C)

morphology

200 200 200

octahedron octahedron octahedron

200

star

200

star

200

truncated octahedron cube

240 200 240

anomalous polyhedron truncated cube

3. RESULTS AND DISCUSSION Formation of the Lead Thiolate Compound. Our synthesis of colloidal PbS nanocrystals involves directly heating Pb(OAc)2 in a mixed hot solvent of DDT and surfactants such as ODE, OLA, and OA. In this case, Pb(OAc)2 and DDT are selected as the lead source and sulfur source raw materials, which react at high temperature to produce PbS nanocrystals. Additionally, DDT also acts as a surfactant that is similar to other surfactants including OA and OLA and plays two important roles in the synthesis of PbS nanocrystals: capping agent and reaction media. These surfactants are often used in the shape-controlled synthesis of inorganic nanocrystals in colloidal systems because they significantly influence the growth kinetics of the colloidal nanocrystals and direct the growth of the nanocrystals.34 In our reaction system, the surfactants also play an important role in the synthesis of PbS nanocrystals with tailored morphologies, which will be discussed in detail in the following discussion. As stated in previous work, metal thiolate compounds are often generated by the reaction between metal salts and DDT in the initial stage of the reaction.19,35−37 Herein, the reaction of Pb(OAc)2 and DDT at a relatively low temperature leads to the formation of the lead thiolate [Pbm(SC12H25)n] compound, as suggested by the change from a turbid mixture to a pellucid yellow solution. To study the structure of the Pbm(SC12H25)n compound, the compound was separated to be analyzed by the XRD pattern, which is shown in Figure 1a. A series of intense, sharp diffraction peaks at lower angles appeared, which correspond to successive orders of diffraction from a periodic layered structure of the Pbm(SC12H25)n compound and can be indexed as (0k0) reflections with k ≤ 20, which have been observed in the copper thiolate compound reported previously.38−40 According to Bragg’s law, the average interlayer spacing between the adjacent sharp diffraction peaks is estimated to be about 3.747 nm, matching one layer of Pb2+ (rPb2+ = 0.119 nm) and a double layer of DDT molecules whose 16437


Langmuir

Article

Figure 2. XRD patterns of PbS nanocrystals for different reaction conditions, with the lines at the bottom representing the corresponding standard diffraction lines from the JCPDS card (no. 77-0422).

cubic (fcc) structure of pure PbS (JCPDS no. 77-0422). In particular, there is a small difference in the intensity ratio between the (111) and (200) diffraction peaks for samples B and G, which may come from the preferred orientation of the differently shaped PbS nanocrystals on the substrate.19 However, the changes in the relative intensities of the (111) and (200) planes confirm that the octahedral nanocrystals (sample B) are abundant in {111} facets whereas the cubic nanocrystals (sample G) are dominated by {100} facets.19,24 Size Control and Shape Control of PbS Nanocrystals. Generally, the growth process of the inorganic nanocrystals is often governed by the thermodynamic and kinetic parameters, leading to the formation of differently shaped products.43 Herein, we focus on the effects of the surfactants, reaction time, reactant concentrations, and reaction temperatures on the shape and size of the PbS nanocrystals. Octahedral PbS Nanocrystals. We first prepared PbS nanocrystals by directly heating Pb(OAc)2 in DDT without any other surfactants, and the size was adjusted easily by controlling the reaction time and the reactant concentrations. Figure 3 gives the TEM images of samples A and B. As shown in Figure 3a−d, the average diameter of PbS nanocrystals is about 47.1 nm at 10 min, and a uniform population of PbS nanocrystals is produced with an increased size of ∼55.8 nm as the reaction time is increased to 30 min. Further increasing the reaction time to 80 min leads to the formation of larger nanocrystals with an average size of ∼73.9 nm. The size distribution histograms of the PbS nanocrystals obtained at different reaction times are shown in Figure S3 in the Supporting Information. At first glance, the PbS nanocrystals possess a hexagonal shape. The careful observation of these nanocrystals suggests that they are octahedral in shape, which can be further proven by the SEM results (Supporting Information, Figure S4). As a matter of fact, the TEM image mainly shows a hexagonal projection of 3D octahedrons. It is interesting that the nanocrystals are so monodisperse at longer reaction time that the nanocrystals can self-assemble into a close-packed hexagonal array in which one nanocrystal is surrounded symmetrically by six other nanocrystals. The selected-area electron diffraction (SAED) pattern of the individual PbS nanocrystal, as depicted in the inset of Figure 3c, demonstrates the single-crystalline nature of octahedral PbS nanocrystals. It was reported previously that a higher concentration of reactant promotes the formation of larger nanocrystals.19 When the amount of Pb(OAc)2 is increased from 1 mmol (sample B) to 3 mmol (sample A), the average diameter of PbS nanocrystals at 60 min is increased from 65.6 ± 2.9 to 93.6 ± 3.6 nm and the

Figure 1. (a) XRD pattern and (b) FTIR spectrum of the intermediate compound collected in the initial stage of the reaction between Pb(OAc)2 and DDT at 200 °C. (a, inset) Schematic illumination of the lead thiolate compound. (b, inset) FTIR of pure DDT.

length is calculated to be about 1.77 nm from Bain’s empirical equation.41 Therefore, one period of the layered structure of Pbm(SC12H25)n consists of stacked layers of Pb and S atoms separated by two layers of dodecyl chains,38 and the schematic structure of the Pbm(SC12H25)n compound is shown in the inset of Figure 1a. The detailed analysis is given in Table S1 in the Supporting Information. Apart from the XRD pattern, the FTIR technique has been used to study the formed Pbm(SC12H25)n compound, which is depicted in Figure 1b. For the sake of comparison, the FTIR result for pure DDT is also given in the inset of Figure 1b. By comparing the FTIR spectra of the as-obtained Pbm(SC12H25)n compound and pure DDT, two sharp bands at 2915 and 2847 cm−1 are found, which can be ascribed to the asymmetric methyl stretching vibration and the asymmetric and symmetric methylene stretching modes, respectively.42 It should be noted that the band at 2578 cm−1 belonging to the S−H vibration of pure DDT disappears in the FTIR spectrum of the lead thiolate compound, which indicates the cleavage of S−H and the formation of the Pbm(SC12H25)n compound. The thermal decomposition behavior of lead thiolate compound was studied by thermogravimetric analysis (TGA) (Supporting Information, Figure S2). The TGA result indicates that the Pbm(SC12H25)n compound begins to decompose at about 235 °C and then a rapid weight loss takes place and becomes almost stable after 330 °C, which is caused by the cleavage of the C−S bond or the evaporation and volatilization of pure DDT.32 Structural Characterization of PbS Nanocrystals. The composition and structure of the PbS nanocrystals obtained under different conditions are first characterized by XRD patterns, and the results are shown in Figure 2. These three obvious diffraction peaks of the (111), (200), and (220) planes reveal that the as-obtained samples present the face-centered16438


Langmuir

Article

Figure 4. TEM images of sample D obtained at (a) 10 and (b) 60 min. (c) Typical HRTEM image. (d) SAED pattern of sample D obtained at 60 min. (e) Typical SEM image of sample D.

shaped morphology is still unchanged. The star-shaped morphology can be further confirmed by the SEM result (Figure 4e), where a well-defined star-shaped geometry with six highly symmetric horns is clearly observed. A low-magnification SEM image is also given in Figure S7a in the Supporting Information, which indicates the formation of uniform starlike nanocrystals. Energy-dispersive X-ray spectroscopy (EDS) (Supporting Information, Figure S7b) suggests that the atomic ratio of Pb to S is about 1:1.4, and excess S in the nanocrystals can be attributed to the surface-capping ligand of DDT. Figure 4c depicts a typical HRTEM image of the PbS nanocrystals obtained at 60 min, which reveals that the as-obtained PbS nanocrystals are single-crystalline with clear lattice fringes with an interplanar spacing of 0.207 nm, confirming the formation of PbS nanocrystals with fcc structure. The single crystallinity is further confirmed by the corresponding SAED pattern shown in Figure 4d, which shows the diffraction spots, indicating that the orientation of the products is the [111] zone axis of cubicstructured PbS. This result confirms that the starlike nanocrystals are single crystals with the six horns grown along the ⟨100⟩ directions.3 Effects of the Amount of DDT. To further study the effects of the amount of DDT on the shape control of PbS nanocrystals, different PbS nanocrystals were synthesized by using different volumes of DDT while other reaction conditions were kept the same. Figure 5 shows the corresponding TEM images of PbS nanocrystals synthesized by using different volumes of DDT for the reaction time of 30 min. The TEM image of sample E is shown in Figure 5a, and the samples exhibit a well-defined starlike shape with six symmetrical sharp horns. With the volume of DDT increasing from 1.5 to 5 mL, the average size is increased from 107.1 to 144.8 nm (Supporting Information, Figure S8), but the well-defined star-shaped geometry is still unchanged except for the change of the horns from sharp to smooth. It is surprising that the morphology of the sample evolves toward octahedron as the volume of DDT is increased to 12 mL (Figure 5c). When the volume is increased to 20 mL without any ODE, the exclusive

Figure 3. TEM images of sample B for different reaction times: (a) 10, (b) 30, (c) 60, and (d) 80 min. (e) TEM image and (f) a typical HRTEM image of sample A obtained at 60 min. (c, inset) Corresponding SAED pattern of sample B.

shape remains octahedral (Figure 3e), which indicates that the size of the nanocrystals can be tuned easily by changing the reactant concentrations. The relationship of the average diameter to the reaction time for samples A and B is given in Figure S3f in the Supporting Information. Figure 3f presents a typical HRTEM image of octahedral PbS nanocrystals, and the obvious lattice fringes indicate that the nanocrystals are single crystals with interplanar spacings of 0.34 and 0.29 nm, which can be indexed to the (111) and (200) planes of an fcc PbS phase, respectively. The HRTEM images for the different parts of an individual PbS nanocrystal are also given in Figure S5 in the Supporting Information. Starlike PbS Nanocrystals. When we reduced the volume of DDT to 5 mL and 15 mL of a noncoordinating surfactant ODE was introduced into our reaction system, it was unexpected that starlike PbS nanocrystals would be achieved. Figure 4a,b presents the TEM images of sample D obtained at 10 and 60 min, respectively. An obvious 2D projection of the 3D stereographics of PbS nanostars is observed, which indicates that most of the samples stood stably on the substrate with three horns when a drop of sample solution was deposited onto a carbon-coated copper grid.3 For the sample obtained at 10 min, the exclusive star-shaped PbS nanocrystals can be clearly seen through the zone axis of ⟨111⟩.11 The average overall size of the PbS nanostars can be determined from the opposite horns, and the average size is 125.1 ± 5.5 nm at 10 min (Supporting Information, Figure S6a). As the reaction time is prolonged to 60 min, the average size further grows to 158.3 ± 6.0 nm (Supporting Information, Figure S6b), but the star16439


Langmuir

Article

DDT. OA and OLA can collaborate with DDT to offer suitable coordination and affect the growth process, thus the shape of the final products can be controlled.47 OA is one of the most widely used surfactants in the synthesis of different types of inorganic nanocrystals.49−53 Upon introducing OA into the reaction system, the morphology of the as-obtained PbS nanocrystals transformed from octahedrons to truncated octahedrons at 200 °C (sample F) and cubes at 240 °C (sample G), and the corresponding TEM results are presented in Figure 6. When 10 mL of DDT

Figure 5. TEM images of PbS nanocrystals synthesized for different volumes of DDT: (a) sample E, (b) sample D, (c) sample C, and (d) sample B obtained at a reaction time of 30 min.

octahedral PbS nanocrystals with a uniform size distribution are observed (Figure 5d). The aforementioned results show that the morphology of PbS nanocrystals can be easily transformed from octahedrons to stars by changing the amount of DDT. As reported previously, the monomers from the decomposition of Pbm(SC12H25)n would grow into truncated octahedral nuclei with an fcc structure once the solution was oversaturated and the truncated octahedral seeds often had six {100} facets and eight {111} facets.11,44,45 When only DDT acts as the capping agent without any surfactant, the excess DDT molecules can effectively control the growth of the PbS nanocrystals by selectively stabilizing the facets. The Pb ion on the {100} facets is five-coordinated, and the DDT molecules weakly bind to the {100} facets via a single bonding mode. However, the Pb ion on the {111} facets is three-coordinated, and the DDT molecules tend to stabilize the {111} facets via a relatively stronger μ3-Pb3−SR bridging bond.19,44 Therefore, the growth on the {111} facets is restricted, and the nanocrystals grow along the ⟨100⟩ directions from truncated octahedral seeds, resulting in the formation of octahedral PbS nanocrystals.44 As the amount of DDT is decreased by substitution with nonselective surfactant ODE, the insufficient DDT molecules prefer to bind to {111} facets so that the {100} facets become the most favorable sites. Thus, the enhanced growth rate of {100} facets induces the shrinking of the six {100} facets into sharp corners.11 With the amount of DDT changing, the relative growth rate between the {111} facets and the {100} facets can be modulated. As a result, the morphology of the PbS nanocrystals is gradually changed from octahedral to starlike. Effects of Surfactants. It is well known that the surface energy associated with different facets is different, which often determines the growth rates of different facets.34 The surface energy can be modulated by using different surfactants with various end functional groups, thus influencing the growth kinetics, which determines the morphology of the final products.34,44−46 To gain further control over the shape of the PbS nanocrystals, additional organic surfactants, such as OA and OLA, are introduced into the reaction system besides

Figure 6. (a) TEM image and (b) typical HRTEM image of sample F obtained at 30 min. (c−e) TEM images of sample G for different reaction times: (c) 10, (d) 20, and (e) 60 min.

was replaced by an equivalent amount of OA and the other reaction conditions were unchanged, truncated octahedral PbS nanocrystals were obtained (Figure 6a). These look quasispherical with blurred edges and corners as compared to sample B. The corresponding HRTEM image displayed in Figure 6b shows their good crystallinity. The obvious lattice fringes have an interplanar spacing of 0.34 nm, which can be indexed as the (111) plane of fcc structure of PbS. When 15 mL of OA was used to replace an equivalent amount of DDT and the reaction temperature was increased to 240 °C, cubic PbS nanocrystals were obtained (Figure 6c−e). The projections of these cubes in the TEM images are squarelike; however, the corners are slightly smooth and they are cubic in 3D SEM images (Supporting Information, Figure S9). As the reaction time of sample G is prolonged from 10 to 60 min, the average size of sample G is increased from 65.1 ± 5.4 to 182.0 ± 4.6 nm. On the basis of the above TEM results in combination with the XRD results, it can be confirmed that the octahedral nanocrystals prefer to grow along the ⟨100⟩ directions and expose mostly {111} facets and cubic nanocrystals prefer to grow along the ⟨111⟩ directions and expose mostly {100} facets.11,19 In the growth process of PbS nanocrystals in the presence of OA and DDT, competition between the growth along the ⟨100⟩ and ⟨111⟩ directions often exists. The binding energy of OA molecules on the {111} facets is relatively weak 16440


Langmuir

Article

kinetics and hence play an important role in the shape control of PbS nanocrystals. Such nanocrystals with controllable shape and size will be valuable for further applications in photovolatics or photodetectors. Moreover, this one-pot approach may be extended to the shape-controlled synthesis of other metal sulfide nanocrystals.

as compared to that of DDT, which makes the growth rate along the ⟨111⟩ directions increase.19,48 As a result, the truncated octahedral PbS nanocrystals are inclined to be formed. However, the thermodynamics governs the major growth process at a relatively higher temperature, and fast growth along the ⟨111⟩ directions from the truncated octahedral seeds results in the formation of cubic nanocrystals.44 Apart from OA, OLA was also often used in the synthesis of various types of inorganic nanocrystals.48,54−57 When OLA was introduced into this reaction system, some differently shaped PbS nanocrystals were obtained at different reaction temperatures. Figure 7 shows the TEM images of PbS nanocrystals

■

ASSOCIATED CONTENT

* Supporting Information S

TGA curve of the lead thiolate compound, and digital photographs, size distribution histograms, SEM images, and HRTEM images of differently shaped PbS nanocrystals. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*(A.T.) Tel: +86 10 51683627. E-mail: [email protected]. (F.T.) Tel: +86 10 51684860. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant no. 61108063), the National Science Foundation for Distinguished Young Scholars of China (no. 61125505), and Basic Scientific Research Fund of Beijing JiaoTong University (2010JBZ006 and 2011JBM301).

Figure 7. TEM images of (a) sample H and (b) sample I obtained at a reaction time of 60 min.

synthesized with the assistance of OLA at 200 °C (sample H) and cubes at 240 °C (sample I). As shown in Figure 7a, the edges and corners of the as-obtained samples become rounded and the samples exhibit an anomalous polyhedron shape. As the reaction temperature is elevated to 240 °C, the edges and corners of sample I are not as clear as the cubic or octahedral nanocrystals in the TEM image (Figure 7b), and the samples are nearly truncated cubes in shape. Meanwhile, the size of the as-obtained samples is increased greatly when the temperature is elevated to 240 °C, and the aggregation of some nanocrystals can be observed. This phenomena may be attributed to the following reasons: First, the higher temperature makes the stability of the lead thiolate compound decrease greatly, leading to more monomer species to accelerate the nucleation and growth rates. Second, increasing the reaction temperature makes the binding of OLA to the surface of the nanocrystals become weaker, resulting in the acceleration of the nanocrystal growth rate.38 However, the nanocrystals are prone to aggregation because of the weaker binding of OLA to the nanocrystal surface.58

■

REFERENCES

(1) Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996, 271, 933−937. (2) Murray, C. B.; Noms, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodiesperse CdE (E = Sulfur, Selenium, Tellurium) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706−8715. (3) Zhao, N. N.; Qi, L. M. Low-Temperature Synthesis of StarShaped PbS Nanocrystals in Aqueous Solutions of Mixed Cationic/ Anionic Surfactants. Adv. Mater. 2006, 18, 359−362. (4) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X. H.; Debnath, R.; Cha, D.; Chou, K. W.; Fischer, A.; Amassian, A.; Asbury, J. B.; Sargent, E. H. ColloidalQuantum-Dot Photovoltaics Using Atomic-Ligand Passivation. Nat. Mater. 2011, 10, 765−771. (5) McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Solution-Processed PbS Quantum Dot Infrared Photodetectors and Photovoltaics. Nat. Mater. 2005, 4, 138−142. (6) Seo, J.; Cho, M. J.; Lee, D.; Cartwright, A. N.; Prasad, P. N. Efficient Heterojunction Photovoltaic Cell Utilizing Nanocomposites of Lead Sulfide Nanocrystals and a Low-Bandgap Polymer. Adv. Mater. 2011, 23, 3984−3988. (7) Sun, L. F.; Choi, J. J.; Stachnik, D.; Bartnik, A. C.; Hyun, B.-R.; Malliaras, G. G.; Hanrath, T.; Wise, F. W. Bright Infrared QuantumDot Light-Emitting Diodes through Inter-Dot Spacing Control. Nat. Nanotechnol. 2012, 7, 369−373. (8) Osedach, T. P.; Zhao, N.; Andrew, T. L.; Brown, P. R.; Wanger, D. D.; Strasfeld, D. B.; Chang, L. Y.; Bawendi, M. G.; Bulović, V. BiasStress Effect in 1,2-Ethanedithiol-Treated PbS Quantum Dot FieldEffect Transistors. ACS Nano 2012, 6, 3121−3127. (9) Kim, H. S.; Jeong, N. C.; Yoon, K. B. Photovoltaic Effects of CdS and PbS Quantum Dots Encapsulated in Zeolite Y. Langmuir 2011, 27, 14678−14688.

4. CONCLUSIONS We have demonstrated a simple and effective one-pot approach to synthesizing high-quality PbS nanocrystals using DDT as a sulfur source and capping agent with the assistance of different surfactants including ODE, OLA, and OA. This approach avoids the presynthesis of single-source precursors and the injection of any phosphine agent. A series differently shaped PbS nanocrystals, such as octahedrons, cubes, stars, truncated octahedrons, and truncated cubes, can be easily obtained by manipulating experimental conditions such as the reaction time, reactant concentrations, reaction temperature, and different surfactants. In particular, the surfactants as capping agents can bind to the surfaces of the nanocrystals to direct the growth 16441


Langmuir

Article

(10) Peterson, J. J.; Krauss, T. D. Fluorescence Spectroscopy of Single Lead Sulfide Quantum Dots. Nano Lett. 2006, 6, 510−514. (11) Lee, S. M.; Jun, Y. W.; Cho, S. N.; Cheon, J. Single-Crystalline Star-Shaped Nanocrystals and Their Evolution: Programming the Geometry of Nano-Building Blocks. J. Am. Chem. Soc. 2002, 124, 11244−11245. (12) Plante, I. J-L.; Zeid, T. W.; Yang, P. D.; Mokari, T. Synthesis of Metal Sulfide Nanomaterials via Thermal Decomposition of SingleSource Precursors. J. Mater. Chem. 2010, 20, 6612−6617. (13) Luther, J. M.; Zheng, H. M.; Sadtler, B.; Alivisatos, A. P. Synthesis of PbS Nanorods and Other Ionic Nanocrystals of Complex Morphology by Sequential Cation Exchange Reactions. J. Am. Chem. Soc. 2009, 131, 16851−16857. (14) Duan, T. P.; Lou, W. J.; Wang, X. B.; Xue, Q. Size-Controlled Synthesis of Orderly Organized Cube-Shaped Lead Sulfide Nanocrystals via a Solvothermal Single-Source Precursor Method. Colloids Surf. A 2007, 310, 86−93. (15) Zhang, C.; Kang, Z. H.; Shen, E. H.; Wang, E. B.; Gao, L.; Luo, F.; Tian, C. G.; Wang, C. L.; Lan, Y.; Li, J. X.; Cao, X. J. Synthesis and Evolution of PbS Nanocrystals through a Surfactant-Assisted Solvothermal Route. J. Phys. Chem. B 2006, 110, 184−189. (16) Bao, H. B.; Han, W.; Gao, M. Y. Shape Controllable Preparations of PbS Nanocrystals Using Cysteine as the Precursor of S2− Ions. Chin. Sci. Bull. 2006, 51, 2576−2580. (17) Wang, Y. N.; Dai, Q. Q.; Yang, X. Y.; Zou, B.; Li, D. M.; Liu, B. B.; Hu, M. Z.; Zou, G. T. A Facile Approach to PbS Nanoflowers and Their Shape-Tunable Single Crystal Hollow Nanostructures: Morphology Evolution. CrystEngComm. 2011, 13, 199−203. (18) Ge, J. P.; Wang, J.; Zhang, H. X.; Wang, X.; Peng, Q.; Li, Y. D. Orthogonal PbS Nanowire Arrays and Networks and Their Raman Scattering Behavior. Chem.Eur. J. 2005, 11, 1889−1894. (19) Zhuang, Z. B.; Lu, X. T.; Peng, Q.; Li, Y. D. A Facile “Dispersion−Decomposition” Route to Metal Sulfide Nanocrystals. Chem.Eur. J. 2011, 17, 10445−10452. (20) Warner, J. H.; Cao, H. Q. Shape Control of PbS Nanocrystals Using Multiple Surfactants. Nanotechnology 2008, 19, 305605. (21) Abel, K. A.; Shan, J. N.; Boyer, J.-C.; Harris, F.; van Veggel, F. C. J. M. Highly Photoluminescent PbS Nanocrystals: The Beneficial Effect of Trioctylphosphine. Chem. Mater. 2008, 20, 3794−3796. (22) Qiu, W. M.; Xu, M. S.; Chen, F.; Yang, X.; Nan, Y. X.; Chen, H. Z. Morphology Evolution Route of PbS Crystals via EnvironmentFriendly Electrochemical Deposition. CrystEngComm. 2011, 13, 4689− 4694. (23) Wang, S. F.; Gu, F.; Lv, M. K. Sonochemical Synthesis of Hollow PbS Nanospheres. Langmuir 2006, 22, 398−401. (24) Quan, Z. W.; Li, C. X.; Zhang, X. M.; Yang, J.; Yang, P. P.; Zhang, C. M.; Lin, J. Polyol-Mediated Synthesis of PbS Crystals: Shape Evolution and Growth Mechanism. Cryst. Growth. Des. 2008, 8, 2384− 2392. (25) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. A General Strategy for Nanocrystal Synthesis. Nature 2005, 437, 121−124. (26) Cho, K.-S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. Designing PbSe Nanowires and Nanorings through Oriented Attachment of Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7140−7147. (27) Lu, W. G.; Fang, J. Y.; Ding, Y.; Wang, Z. L. Formation of PbSe Nanocrystals: A Growth toward Nanotubes. J. Phys. Chem. B 2005, 109, 19219−19222. (28) Houtepen, A. J.; Koole, R.; Vanmaekelbergh, D.; Meeldijk, J.; Hickey, S. G. The Hidden Role of Acetate in the PbSe Nanocrystal Synthesis. J. Am. Chem. Soc. 2006, 128, 6792−6793. (29) Acharya, S.; Sarma, D. D.; Golan, Y.; Sengupta, S.; Ariga, K. Shape-Dependent Confinement in Ultrasmall Zero-, One-, and TwoDimensional PbS Nanostructures. J. Am. Chem. Soc. 2009, 131, 11282−11283. (30) Wang, N.; Cao, X.; Guo, L.; Yang, S. H.; Wu, Z. Y. Facile Synthesis of PbS Truncated Octahedron Crystals with High Symmetry and Their Large-Scale Assembly into Regular Patterns by a Simple Solution Route. ACS Nano 2008, 2, 184−190.

(31) Querejeta-Fernández, A.; Hernández-Garrido, J. C.; Yang, H. X.; Zhou, Y. L.; Varela, A.; Parras, M.; Calvino-Gámez, J. J.; GonzálezCalbet, J. M.; Green, P. F.; Kotov, N. A. Unknown Aspects of SelfAssembly of PbS Microscale Superstructures. ACS Nano 2012, 6, 3800−3812. (32) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nanowire Dye-Sensitized Solar Cells. Nat. Mater. 2005, 4, 455−459. (33) Gur, I.; Fromer, N. A.; Chen, C. P.; Kanaras, A. G.; Alivisatos, A. P. Hybrid Solar Cells with Prescribed Nanoscale Morphologies Based on Hyperbranched Semiconductor Nanocrystals. Nano Lett 2007, 7, 409−414. (34) Xiao, J. Y.; Qi, L. M. Surfactant-Assisted, Shape-Controlled Synthesis of Gold Nanocrystals. Nanoscale 2011, 3, 1383−1396. (35) Tang, A. W.; Qu, S. C.; Li, K.; Hou, Y. B.; Teng, F.; Cao, J.; Wang, Y. S.; Wang, Z. G. One-Pot Synthesis and Self-Assembly of Colloidal Copper(I) Sulfide Nanocrystals. Nanotechnology 2010, 21, 285602−285602/9. (36) Yi, L. X.; Gao, M. Y. From Ultrathin 2D Djurleite Nanosheets to 1D Nanorods Comprised of Djurleite Nanoplates: Synthesis, Characterization, and Formation Mechanism. Cryst. Growth. Des. 2011, 9, 4793−4796. (37) Han, W; Yi, L. X.; Zhao, N.; Tang, A. W.; Gao, M. Y.; Tang, Z. Y. Synthesis and Shape-Tailoring of Copper Sulfide/Indium SulfideBased Nanocrystals. J. Am. Chem. Soc. 2008, 130, 13152−13161. (38) Wang, Y.; Hu, Y. X.; Zhang, Q.; Ge, J. P.; Lu, Z. D.; Hou, Y. B.; Yin, Y. D. One-Pot Synthesis and Optical Property of Copper(I) Sulfide Nanodisks. Inorg. Chem. 2010, 49, 6601−6608. (39) Espinet, P.; Lequerica, M. C.; Martín-Alvarez, J. M. Synthesis, Structural Characterization and Mesogenic Behavior of Copper(I) nAlkylthiolates. Chem.Eur. J. 1999, 5, 1982−1986. (40) Chen, Y. B.; Chen, L.; Wu, L. M. The Structure-Controlling Solventless Synthesis and Optical Properties of Uniform Cu2S Nanodisks. Chem.Eur. J. 2008, 14, 11069−11075. (41) Taleb, A.; Petit, C.; Pileni, M. P. Synthesis of Highly Monodisperse Silver Nanoparticles from AOT Reverse Micelles: A Way to 2D and 3D Self-Organization. Chem. Mater. 1997, 9, 950−959. (42) Zhong, H. Z.; Zhou, Y.; Ye, M. F.; He, Y. J.; Ye, J. P.; He, C.; Yang, C. H.; Li, Y. F. Controlled Synthesis and Optical Properties of Colloidal Ternary Chalcogenide CuInS2 Nanocrystals. Chem. Mater. 2008, 20, 6434−6443. (43) Tao, A. R.; Habas, S.; Yang, P. D. Shape Control of Colloidal Metal Nanocrystals. Small 2008, 4, 310−325. (44) Jun, Y. W.; Lee, J. H.; Choi, J. S.; Cheon, J. W. SymmetryControlled Colloidal Nanocrystals: Nonhydrolytic Chemical Synthesis and Shape Determining Parameters. J. Phys. Chem. B 2005, 109, 14795−14806. (45) Wang, Z. L. Transmission Electron Microscopy of ShapeControlled Nanocrystals and Their Assemblies. J. Phys. Chem. B 2000, 104, 1153−1175. (46) Zhou, G. J.; Lü, M. K.; Xiu, Z. L.; Wang, S. F.; Zhang, H. P.; Zhou, Y. Y.; Wang, S. W. Controlled Synthesis of High-Quality PbS Star-Shaped Dendrites, Multipods, Truncated Nanocubes, and Nanocubes and Their Shape Evolution Process. J. Phys. Chem. B 2006, 110, 6543−6548. (47) Lu, X. T.; Zhuang, Z. B.; Peng, Q.; Li, Y. D. Controlled Synthesis of Wurtzite CuInS2 Nanocrystals and Their Side-By-Side Nanorod Assemblies. CrystEngComm 2011, 13, 4039−4045. (48) Sun, S.; Liu, H. M.; Gao, Y. P.; Qian, D. H.; Chen, J. W. Controlled Synthesis of CdTe Nanocrystals for High Performanced Schottky Thin Film Solar Cells. J. Mater. Chem. 2012, 22, 19207− 19212. (49) Hines, M. A.; Scholes, G. D. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Adv. Mater. 2003, 15, 1844−1849. (50) Cozzoli, P. D.; Kornowski, A.; Weller, H. Low-Temperature Synthesis of Soluble and Processable Organic-Capped Anatase TiO2 Nanorods. J. Am. Chem. Soc. 2003, 125, 14539−14548. 16442


Langmuir

Article

(51) Yang, X. W.; Zeng, Y. W.; Mo, L. Q.; Han, L. X. Oleic Acid Assisted Glycothermal Synthesis of Cuboidal Ba0.6Sr0.4TiO3 Nanocrystals and Their Ordered Architectures via Self-Assembly. J. Colloid Interface Sci. 2011, 357, 308−316. (52) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.; Festin, O.; Svedlindh, P.; Haase, M.; Weller, H. Study of Nucleation and Growth in the Organometallic Synthesis of Magnetic Alloy Nanocrystals: The Role of Nucleation Rate in Size Control of CoPt3 Nanocrystals. J. Am. Chem. Soc. 2003, 125, 9090−9101. (53) Mizuno, M.; Sasaki, Y.; Lee, S.; Katakura, H. High-Yield Sol-Gel Synthesis of Well-Dispersed Colorless ZrO2 Nanocrystals. Langmuir 2006, 22, 7137−7140. (54) Zhang, H. T.; Ding, J.; Chow, G. M. Morphological Control of Synthesis and Anomalous Magnetic Properties of 3-D Branched Pt Nanoparticles. Langmuir 2008, 24, 375−378. (55) Zhang, H. T.; Chen, X. H. Controlled Synthesis and Anomalous Magnetic Properties of Relatively Monodisperse CoO Nanocrystals. Nanotechnology 2005, 16, 2288−2294. (56) Liu, Y; Yao, D.; Shen, L.; Zhang, H.; Zhang, X. D.; Yang, B. Alkylthiol-Enabled Se Powder Dissolution in Oleylamine at Room Temperature for the Phosphine-Free Synthesis of Copper-Based Quaternary Selenide Nanocrystals. J. Am. Chem. Soc. 2012, 134, 7207− 7210. (57) Quan, Z. W.; Yang, D. D.; Li, C. X.; Kong, D. Y.; Yang, P. P.; Cheng, Z. Y.; Lin, J. Multicolor Tuning of Manganese-Doped ZnS Colloidal Nanocrystals. Langmuir 2009, 25, 10259−10262. (58) He, J. B.; Kanjanaboos, P.; Frazer, N. L.; Weis, A.; Lin, X. M.; Jaeger, H. M. Fabrication and Mechanical Properties of Large-Scale Freestanding Nanoparticle Membranes. Small 2010, 6, 1449−1456.

16443


Shape-Controlled Synthesis of PbS Nanocrystals via a Simple One

Recommend Documents